Handbook of Biodegradable Materials 3031097092, 9783031097096

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Gomaa A. M. Ali Abdel Salam H. Makhlouf Editors

Handbook of Biodegradable Materials

Handbook of Biodegradable Materials

Gomaa A. M. Ali • Abdel Salam H. Makhlouf Editors

Handbook of Biodegradable Materials With 566 Figures and 137 Tables

Editors Gomaa A. M. Ali Chemistry Department Al-Azhar University Assiut, Egypt

Abdel Salam H. Makhlouf Vice President of Advanced Material Research, Stanley Black & Decker, Inc Connecticut, USA

ISBN 978-3-031-09709-6 ISBN 978-3-031-09710-2 (eBook) https://doi.org/10.1007/978-3-031-09710-2 © Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The Handbook of Biodegradable Materials is one of the most comprehensive and insightful references that challenges the most recent and critical topics in materials biodegradability and sustainability. The book discusses the biodegradation process, including its fundamentals, types, efficiency, importance, and impacts on many strategic areas, such as the environment, health, industry, technology, food, and agriculture. The book explores a wide spectrum of biodegradable materials including, but limited to, ceramics, glasses, polymers, composites, glass-ceramics, and metal alloys. The Handbook of Biodegradable Materials consists of eight main parts. Part I: Fundamentals of Biodegradations This part introduces the basic principles of biodegradable materials. It includes nine chapters and discusses in detail the biodegradation process and the factors affecting the process with some examples from industrial applications. It also discusses the concept and significance of the microbial consortium in the biodegradation process, the role of microorganisms in biodegradation of pollutants, and the most common types of microorganisms for biodegradation. The mechanism of microbial biodegradation and the secrets behind efficient biodegradation are explored. The recent advances and future trends in microbial biodegradation are also discussed. Part II: Polymer Biodegradation This part contains seven chapters that discuss in detail the biodegradable polymers, types, and industrial applications. It explores the electrically conducting smart biodegradable polymers, the biocompatibility of nanomaterials reinforced polymerbased nanocomposites, sustainable biopolymers, biodegradable polysaccharides nanocomposites, and their industrial applications. Part III: Plastic Biodegradation This part contains four chapters that discuss in detail the biodegradation and biofragmentation of plastics wastes, the emerging technologies in biodegradable plastics for sustainability, and the impacts of biodegradable plastic on the environment. Part IV: Other Materials Biodegradation This part contains three chapters that provide other examples for materials degradation such as biodegradable inorganic nanocomposites, carbon nanotubes, and chitin and chitosan. v

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Preface

Part V: Environmental Impacts of Biodegradation This part contains seven chapters that discuss in detail the environmental impacts of biodegradation with some industrial examples such as biodegradation of azo dye pollutants using microorganisms, genetically engineered bacteria used in bioremediation applications, biowaste materials for biodegradable packaging applications, biodegradation of industrial pollutants, and biodegradable nanocelluloses for removal of hazardous organic pollutants from wastewater. Part VI: Medical and Health Impacts of Biodegradation This part contains eight chapters that discuss in detail the biodegradation of medical wastes. It highlights some critical examples such as biodegradable polymers and hybrid biodegradable polymeric scaffolds for cardiac tissue engineering, biodegradable polymer-based inorganic nanoparticles, biodegradable Mg alloys for orthopedic implant materials, and biodegradable nanocomposite as bone tissue scaffold. Additionally, this part highlights other biodegradable polymers for skin and bone regeneration, stem cell technologies, and pharmaceuticals and personal care products. Part VII: Foods and Agricultural Impacts of Biodegradation This part contains five chapters that discuss several examples of biodegradable materials in the food industry. Topics including biodegradation versus composting, biochar and chicken manure compost, biodegradable food packaging materials, ecological sustainability of biodegradable materials for healthy food storage, and biodegradable alkyd materials for eco-friendly coating applications are discussed. Part VIII: Industrial and Technological Impacts of Biodegradation This part contains eight chapters that discuss several examples of biodegradable materials in the industry. It discusses the biodegradable electrode materials for sustainable supercapacitors as future energy storage devices, biodegradation of crude oil and surfactants biodegradation, and oil industry waste materials. This part also explores the new trends for using the biodegradation for metal extraction and biodegradable textiles. The future trends in biodegradable materials and sustainability are also discussed as final remarks and conclusions in each chapter. Assiut, Egypt Connecticut, USA February 2023

Gomaa A. M. Ali Abdel Salam H. Makhlouf

Contents

Volume 1 Part I 1

2

Fundamentals of Biodegradations

.....................

Biodegradable Materials: Fundamentals, Importance, and Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gomaa A. M. Ali, Mohammad R. Thalji, and Abdel Salam Hamdy Makhlouf Biodegradation Process: Basics, Factors Affecting, and Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lai Mun Koh and Sook Mei Khor

1

3

19

3

Fundamentals of Biodegradation Process . . . . . . . . . . . . . . . . . . . . Koula Doukani, Dyhia Boukirat, Assia Boumezrag, Hasna Bouhenni, and Yassine Bounouira

57

4

Anaerobic Biodegradation: The Anaerobic Digestion Process . . . . Ouahid El Asri

85

5

Recent Advances in Microbial Biodegradation . . . . . . . . . . . . . . . Samah Husseiny, Nada Elgiddawy, Gharieb S. El-Sayyad, and Waleed M. A. El Rouby

111

6

Concept and Significance of Microbial Consortium in the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lai Mun Koh and Sook Mei Khor

137

Mechanism of Microbial Biodegradation: Secrets of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Doaa A. R. Mahmoud

179

7

vii

viii

Contents

8

Types of Microorganisms for Biodegradation Shaimaa A. Khalid and Walaa M. Elsherif

................

195

9

Role of Microorganisms in Biodegradation of Pollutants . . . . . . . . Farida Ah. Fouad, Donia G. Youssef, Fatma M. Shahat, and Mohamed N. Abd El-Ghany

221

Part II

Polymer Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . .

261

10

Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atika Alhanish and Gomaa A. M. Ali

263

11

Biodegradable Polymer Challenges . . . . . . . . . . . . . . . . . . . . . . . . Sumaira Naeem, Jawayria Najeeb, Sheikh Muhammad Usman, and Hummera Rafique

293

12

Sustainable Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mustafa K. Ismael

319

13

Biocompatibility of Nanomaterials Reinforced Polymer-Based Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Farida Ah. Fouad, Donia G. Youssef, Fatma A. Refay, and Fakiha El-Taib Heakal

14

Electrically Conducting Smart Biodegradable Polymers and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meena Bhandari, Dilraj Preet Kaur, Seema Raj, Tejpal Yadav, Mohammed A. S. Abourehab, and Md Sabir Alam

351

391

15

Biodegradable Polysaccharides Nanocomposites . . . . . . . . . . . . . . Hagar F. Forsan and Randa S. Hasan

415

16

Biodegradable Polymers for Industrial Applications . . . . . . . . . . . Abdelaal S. A. Ahmed, Ahmed N. R. M. Negm, M. Mohammed, M. Abd El-Majeed, A. K. Ali, and M. Abdelmotalleib

451

Part III 17

18

Plastic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

477

Biodegradable Plastics as a Solution to the Challenging Situation of Plastic Waste Management . . . . . . . . . . . . . . . . . . . . . Hafsa Javaid, Mahrukh Khan, Kiran Mustafa, and Sara Musaddiq

479

Biodegradable Plastics Based on Algal Polymers: Recent Advances and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohamed Gomaa

501

Contents

19

20

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Emerging and Advanced Technologies in Biodegradable Plastics for Sustainability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nor Izati Che Ab Aziz, Yusmazura Zakaria, Noor Zuhartini Md Muslim, and Nik Fakhuruddin Nik Hassan Plastics Biodegradation and Biofragmentation . . . . . . . . . . . . . . . Nagwan Galal El Menofy and Abdelrahman Mossad Khattab

Part IV

Other Materials Biodegradation . . . . . . . . . . . . . . . . . . . . .

533

571

601

21

Biodegradable Inorganic Nanocomposites . . . . . . . . . . . . . . . . . . . Juan Matmin, Nik Ahmad Nizam Nik Malek, and Nor Suriani Sani

603

22

Biodegradation of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . Amany Saad Ibrahim, Dina A. M. Farage, and Gomaa A. M. Ali

643

23

Biodegradation, Biosynthesis, Isolation, and Applications of Chitin and Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Showket Ahmad Dar and Fahd Mohammed Abd Al Galil

Part V

Environmental Impacts of Biodegradation . . . . . . . . . . . . .

677

719

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Environmental Impact of Biodegradation Esraa E. Ammar

...................

721

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Biodegradable Nanocelluloses for Removal of Hazardous Organic Pollutants from Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saikumar Manchala, Ambedkar Gandamalla, and Aravind Rudrarapu

761

Biodegradation of Azo Dye Pollutants Using Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hassanien Gomaa, Mohammed Y. Emran, and Marwa A. El-Gammal

781

26

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Impacts of Biodegradable Plastic on the Environment . . . . . . . . . Nayera Awny Mahmoud, Alaa Mohamed Yasien, Dina Hamada Swilam, Mai Muhammed Gamil, and Shimaa Tarek Ahmed

28

Genetically Engineered Bacteria Used in Bioremediation Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rana Tarek and Gomaa A. M. Ali

29

Biowaste Materials for Advanced Biodegradable Packaging Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohammed Y. Emran, Waheed Miran, Hassanien Gomaa, Islam Ibrahim, George V. Belessiotis, Adel A. Abdelwahab, and Mahmoud Ben Othman

811

839

861

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30

Contents

Biodegradation of Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Koula Doukani, Dyhia Boukirat, Assia Boumezrag, Hasna Bouhenni, and Yassine Bounouira

899

Volume 2 Part VI 31

Medical and Health Impacts of Biodegradation

........

Biodegradable Nanocomposite as Advanced Bone Tissue Scaffold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yara A. Kammoun and Amal Ashry

927

929

32

Biodegradable Polymers for Cardiac Tissue Engineering . . . . . . . Aymieza Yaacob and Nazzatush Shimar Jamaludin

33

Biodegradable Polymers in Biomedical Applications: A Focus on Skin and Bone Regeneration . . . . . . . . . . . . . . . . . . . . 1015 Mai Abdelgawad, M. Abd Elkodous, and Waleed M. A. El Rouby

34

Hybrid Biodegradable Polymeric Scaffolds for Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045 Hussein M. El-Husseiny, Eman A. Mady, Yasmine Radwan, Maria Nagy, Amira Abugomaa, Mohamed Elbadawy, and Ryou Tanaka

35

Biodegradation Method of Pharmaceuticals and Personal Care Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093 Marwa A. El-Gammal, Ahmed Saad Elsaeidy, Hamid Ashry, and Afnan W. M. Jobran

36

Biodegradable Materials from Natural Origin for Tissue Engineering and Stem Cells Technologies . . . . . . . . . . . . . . . . . . . 1133 Ahmed Atwa, Mahmoud R. Sofy, Sara M. Fakhrelden, Ola Darwish, Ahmed B. M. Mehany, Ahmed R. Sofy, and Sayed Bakry

37

Medical Waste Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173 Boon Hong Lee and Sook Mei Khor

38

Biodegradable Mg Alloys for Orthopedic Implant Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211 Salah Salman and Mohammed K. Gouda

Part VII

Foods and Agricultural Impacts of Biodegradation . . . . .

979

1241

39

Biochar and Chicken Manure Compost . . . . . . . . . . . . . . . . . . . . . 1243 Nur Zulaikha Izzati binti Rosman and Nazzatush Shimar Jamaludin

40

Biodegradation Versus Composting . . . . . . . . . . . . . . . . . . . . . . . . 1275 Boon Hong Lee and Sook Mei Khor

Contents

xi

. . . . . . . . . . . . . . . . . . . 1307

41

Biodegradable Food Packaging Materials Jawayria Najeeb and Sumaira Naeem

42

Ecological Sustainability of Biodegradable Materials for Food Healthy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1337 Mahmoud Said Rashed

43

Vegetable Oil-Based Biodegradable Alkyd Materials for Eco-friendly Coating Applications . . . . . . . . . . . . . . . . . . . . . . . . . 1369 Mohamed S. Selim, Sherif A. El-Safty, Mohamed A. Shenashen, Shimaa A. Higazy, and Ahmed I. Hashem

Part VIII Industrial and Technological Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1405

. . . . . . . . . . . . . . . . . . . . . 1407

44

Biodegradation of Industrial Materials Md. Jahidul Haque and M. S. Rahman

45

Biodegradable Textiles, Recycling, and Sustainability Achievement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1449 Reem Mohamed Nofal

46

Biodegradation of Crude Oil and Biodegradation of Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1487 Lai Mun Koh and Sook Mei Khor

47

Biodegradation for Metal Extraction . . . . . . . . . . . . . . . . . . . . . . . 1533 Boon Hong Lee and Sook Mei Khor

48

Biodegradable Electrode Materials for Sustainable Supercapacitors as Future Energy Storage Devices . . . . . . . . . . . . 1569 Himadri Tanaya Das, Swapnamoy Dutta, Elango Balaji T, Payaswini Das, Nigamananda Das, and Gomaa A. M. Ali

49

Biodegradable Inorganic Nanocomposites for Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595 Abdelaal S. A. Ahmed and Fatma S. M. Hashem

50

Surfactant Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1621 Wan Hazman Danial

51

Insight into the Environmental Applications in the Biodegradation of Oil Industry Waste Materials . . . . . . . . . . . . . . 1651 Tahany Mahmoud, Walaa S. Gado, A. H. Mady, and Khalid I. Kabel

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1679

About the Editors

Gomaa A. M. Ali, (Ph.D.) Associate Professor Chemistry Department, Faculty of Science Al-Azhar University Assiut, Egypt Dr. Gomaa A. M. Ali is an associate professor at the Chemistry Department, Faculty of Science, Al-Azhar University, Egypt. He has 15 years of experience working in the research areas of materials science, humidity sensing, graphene, supercapacitors, water treatment, and drug delivery. He was awarded his Ph.D. in Advanced Nanomaterials for Energy Storage from UMP, Malaysia. He is the recipient of some national and international prizes and awards such as TWAS-AREP (2018), Obada International Prize (2021), Arab Water Council Award 2022, Gold Medal (Archimedes, Russia, 2014), Green Technology Award (CITREX, Malaysia, 2015), and Gold Medal (British Invention Show, UK, 2015). Dr Gomaa has been included in Stanford University’s List of World’s Top 2% of Scientists, Egypt. Dr. Gomaa has published over 137 journal articles and 22 book chapters on a broad range of cross-disciplinary research fields, including multifunctional materials, nanotechnology, supercapacitor, water treatment, humidity sensing, biosensing, corrosion, and drug delivery. So far, he has more than 4372 citations and an h-index of 41. Dr. Gomaa is an editor of many international journals and a reviewer for more than 80 WoS journals. Dr. Gomaa is a member of national and international scientific societies, such as TWAS Affiliate, AAS Affiliate, the American Chemical Society, the Royal Society of Chemistry, the National Committee of Pure and Applied Chemistry, xiii

xiv

About the Editors

and the Egyptian Young Academy of Sciences, ASRT. He is an editor of many handbooks such as Waste Recycling Technologies for Nanomaterials Manufacturing, Springer, 2021. Prof. Dr. Abdelsalam H. Makhlouf is an internationally recognized leader with more than 29 years of independent research project management, teaching, and consulting in industry and academia. He is the Vice President of Advanced Material Research, Stanley Black & Decker, Inc., USA. Before joining SBD, he was the President of EMC3, USA; VP of IM2C – USA; Full Professor of Materials Science at Central Metallurgical Research and Development Institute, Egypt; and Full Professor of Manufacturing and Industrial Engineering at the University of Texas, USA. Prof. Makhlouf has been included in Stanford University’s List of World’s Top 2% of Scientists, USA, 2020 and 2021. He is the recipient of numerous national and international prizes and awards including the Humboldt Research Award for Experienced Scientists, at Max Planck Institute, Germany; Fulbright Scholar, NSF, and Dept. of Energy Fellowships, USA; Shoman Award in Engineering Science; the State Prize of Egypt in Advanced Science and Technology; and more. He is a member of TMS-USA, EPSRC-UK, European Science Foundation, College of Expert Reviewers, Fulbright Alumni, Max Planck Institute Alumni, and Alexander von Humboldt Alumni. He has excellent knowledge of the US, EU, and UK research landscape. Prof. Makhlouf is a Board Member, Consultant, Advisor, and Reviewer for several universities worldwide. He is a member of the Industrial Advisory Council for Coppin State University, USA, and Advisory Editor for Elsevier, USA. He has served as both a Senior Editor and board member of many international journals, as well as a reviewer for several international funding agencies in USA, EU, UK, and more. Prof. Makhlouf is the author of over 200 peerreviewed journals and 21 books and handbooks. Many of his publications have been ranked among the world’s best in the fields of Nanostructures, Nanomaterials, Biomedical Engineering, Materials Science, Coatings, Environmental Science, and Nuclear Materials.

Contributors

Fahd Mohammed Abd Al Galil Department of Biology, Faculty of Science, University of Bisha, Bisha, Saudi Arabia Mohamed N. Abd El-Ghany Botany and Microbiology Department, Faculty of Science, Cairo University, Giza, Egypt M. Abd El-Majeed Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Mai Abdelgawad Biotechnology and Life Sciences Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt M. Abdelmotalleib Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Adel A. Abdelwahab Department of Chemistry, Faculty of Science and Arts, Jouf University, Al Qurayyat, Saudi Arabia Mohammed A. S. Abourehab Department of Pharmaceutics, Faculty of Pharmacy, Umm Al-Qura University, Makkah, Saudi Arabia Department of Pharmaceutics, Faculty of Pharmacy, Minia University, El-Minia, Egypt Amira Abugomaa Faculty of Veterinary Medicine, Mansoura University, Mansoura, Dakahliya, Egypt Showket Ahmad Dar Department of Entomology, Sher-e-Kashmir University of Agricultural Sciences &Technology of Kashmir, Srinagar, India Abdelaal S. A. Ahmed Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Shimaa Tarek Ahmed Chemistry/Geology Department, Faculty of Science, Cairo University, Cairo, Egypt Md Sabir Alam SGT College of Pharmacy, SGT University, Gurugram, Haryana, India xv

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Contributors

Atika Alhanish Chemical Engineering Department, Faculty of Petroleum and Natural Gas Engineering, University of Zawia, Zawiya, Libya A. K. Ali Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Gomaa A. M. Ali Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Esraa E. Ammar Plant Ecology, Botany Department, Faculty of Science, Tanta University, Tanta, Egypt Amal Ashry Prosthodontics Department, Faculty of Dentistry, Damanhour University, Damanhour, Egypt Hamid Ashry Biochemistry branch, Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Ahmed Atwa Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt Sayed Bakry Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt George V. Belessiotis School of Chemical Engineering, National Technical University of Athens, Zografou, Athens, Greece Meena Bhandari School of Basic and Applied Sciences, K. R. Mangalam University, Gurgaon, Haryana, India Hasna Bouhenni Faculty of Nature and Life Sciences, University of Ibn Khaldoun, Tiaret, Algeria Dyhia Boukirat Faculty of Science and Technology, Department of Nature and Life Sciences, University of Tissemsilt, Tissemsilt, Algeria Assia Boumezrag Institute of Veterinary Sciences, University of Ibn Khaldoun, Tiaret, Algeria Yassine Bounouira Faculty of Science and Technology, Department of Nature and Life Sciences, University of Tissemsilt, Tissemsilt, Algeria Ecology and Management of Natural Ecosystems Laboratory, Department of Ecology and Environment, University of Tlemcen, Tlemcen, Algeria Nor Izati Che Ab Aziz Forensic Science Programme, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia Wan Hazman Danial Department of Chemistry, Kulliyyah of Science, International Islamic University Malaysia, Kuantan, Pahang, Malaysia Ola Darwish Clinical Microbiology Labs Department, Fayoum General Hospital, Egyptian Ministry of Health, Fayoum, Egypt

Contributors

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Himadri Tanaya Das Centre of Excellence for Advanced Materials and Applications, Utkal University, Bhubaneswar, Odisha, India Nigamananda Das Department of Chemistry, Utkal University, Bhubaneswar, Odisha, India Payaswini Das CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Koula Doukani Faculty of Nature and Life Sciences, University of Ibn Khaldoun, Tiaret, Algeria Swapnamoy Dutta Department of Chemistry, Utkal University, Bhubaneswar, Odisha, India Ouahid El Asri Microbial Biotechnology and Plant Protection Laboratory, Faculty of Sciences, Ibn Zohr University, Agadir, Morocco Nagwan Galal El Menofy Microbiology and Immunology Department, Faculty of Pharmacy (Girls), Al-Azhar University, Cairo, Egypt Mohamed Elbadawy Department of Pharmacology, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt Marwa A. El-Gammal Nanotechnology Department, School of Science and Engineering, American University of Cairo, Cairo, Egypt Nada Elgiddawy Department of Biotechnology and Life Science, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt Hussein M. El-Husseiny Laboratory of Veterinary Surgery, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan Department of Surgery, Anesthesiology, and Radiology, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt M. Abd Elkodous Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan Ahmed Saad Elsaeidy Faculty of Medicine, Benha University, Benha, Egypt Sherif A. El-Safty National Institute for Materials Science (NIMS), Ibaraki, Japan Gharieb S. El-Sayyad Department of Microbiology & Immunology, Faculty of Pharmacy, Galala University, New Galala city, Suez, Egypt Drug Microbiology Lab, Drug Radiation Research Department, National Center for Radiation Research and Technology (NCRRT), Egyptian Atomic Energy Authority (EAEA), Cairo, Egypt Chemical Engineering Department, Military Technical College (MTC), Egyptian Armed Forces, Cairo, Egypt

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Contributors

Walaa M. Elsherif Food Hygiene Department, Animal Health Research Institute, Agricultural Research Centre, Cairo, Egypt Mohammed Y. Emran Department of Chemistry, Faculty of Science, Al-Azhar University, Assiut, Egypt Sara M. Fakhrelden Tropical Department, Faculty of Medicine, Fayoum University, Fayoum, Egypt Dina A. M. Farage Institute of Graduate Studies and Research, Environmental Studies Department, Alexandria University, Alexandria, Egypt Hagar F. Forsan Animal Production Research Institute, Agricultural Research Center (ARC), Dokki, Giza, Egypt Farida Ah. Fouad Biophysics Department, Faculty of Women for Arts, Science and Education, Ain Shams University, Cairo, Egypt Walaa S. Gado Petrochemical Technology Laboratory, Petrochemicals Department, Egyptian Petroleum Research Institute, Cairo, Egypt Mai Muhammed Gamil Chemistry/Zoology Department, Faculty of Science, Cairo University, Cairo, Egypt Ambedkar Gandamalla Department of Chemistry, National Institute of Technology, Warangal, Telangana, India Hassanien Gomaa Department of Chemistry, Faculty of Science, Al-Azhar University, Assiut, Egypt Mohamed Gomaa Botany and Microbiology Department, Faculty of Science, Assiut University, Assiut, Egypt Mohammed K. Gouda Department of Mining & Petroleum Engineering, Faculty of Engineering, Al-Azhar University, Cairo, Egypt Randa S. Hasan Regional Centre for Food and Feed (RCFF), Agricultural Research Center (ARC), Orman, Giza, Egypt Ahmed I. Hashem Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Fatma S. M. Hashem Chemistry Department, Faculty of Science, Assuit University, Assiut, Egypt Md. Jahidul Haque Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi, Bangladesh Fakiha El-Taib Heakal Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Shimaa A. Higazy Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

Contributors

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Samah Husseiny Department of Biotechnology and Life Science, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt Amany Saad Ibrahim Institute of Graduate Studies and Research, Environmental Studies Department, Alexandria University, Alexandria, Egypt Islam Ibrahim Department of Chemistry, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt Mustafa K. Ismael Middle Technical University, Institute of Technology, Baghdad, Iraq Nazzatush Shimar Jamaludin Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Hafsa Javaid Department of Chemistry, The Women University Multan, Multan, Pakistan Afnan W. M. Jobran Faculty of Medicine, Al Quds University, Jerusalem, Palestine Khalid I. Kabel Additives Laboratory, Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo, Egypt Yara A. Kammoun Prosthodontics Department, Faculty of Dentistry, Damanhour University, Damanhour, Egypt Dilraj Preet Kaur School of Basic and Applied Sciences, K. R. Mangalam University, Gurgaon, Haryana, India Shaimaa A. Khalid Food Hygiene Department, Animal Health Research Institute, Agricultural Research Centre, Cairo, Egypt Mahrukh Khan Department of Chemistry, The Women University Multan, Multan, Pakistan Abdelrahman Mossad Khattab Botany and Microbiology Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt Sook Mei Khor Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia Lai Mun Koh Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia Boon Hong Lee Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia A. H. Mady Petrochemical Technology Laboratory, Petrochemicals Department, Egyptian Petroleum Research Institute, Cairo, Egypt

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Eman A. Mady Laboratory of Veterinary Physiology, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan Department of Animal Hygiene, Behavior and Management, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt Doaa A. R. Mahmoud Department of Chemistry of Natural and Microbial Products, National Research Centre, Giza, Egypt Nayera Awny Mahmoud Biophysics Department, Faculty of Science, Cairo University, Cairo, Egypt Tahany Mahmoud Special Application laboratory, Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo, Egypt Abdel Salam Hamdy Makhlouf Stanley Black & Decker, Inc., New Britain, CT, USA Nik Ahmad Nizam Nik Malek Department of Biosciences, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Saikumar Manchala Department of Chemistry, Indian Institute of Technology, New Delhi, India Juan Matmin Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Noor Zuhartini Md Muslim Forensic Science Programme, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia Nur Zulaikha Izzati binti Rosman Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia Ahmed B. M. Mehany Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt Waheed Miran School of Chemical and Materials Engineering, National University of Sciences and Technology, Islamabad, Pakistan M. Mohammed Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Sara Musaddiq Department of Chemistry, The Women University Multan, Multan, Pakistan Kiran Mustafa Department of Chemistry, The Women University Multan, Multan, Pakistan Govt. Graduate College (W), Higher Education Department, Punjab, Pakistan Sumaira Naeem Department of Chemistry, University of Gujrat, Gujrat, Pakistan

Contributors

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Maria Nagy Biotechnology/Biomolecular Chemistry Program, Faculty of Science, Cairo University, Giza, Egypt Jawayria Najeeb Department of Chemistry, University of Gujrat, Gujrat, Pakistan Ahmed N. R. M. Negm Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt Nik Fakhuruddin Nik Hassan Forensic Science Programme, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia Reem Mohamed Nofal Women’s college for Arts, Science, and Education- Ain Shams University, Cairo, Egypt Mahmoud Ben Othman Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan Yasmine Radwan Center for Materials Science, Zewail City of Science and Technology, Giza, Egypt Nanoscale Science Program, Department of Chemistry, University of North Carolina, Charlotte, NC, USA Hummera Rafique Department of Chemistry, University of Gujrat, Gujrat, Pakistan M. S. Rahman Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi, Bangladesh Seema Raj School of Basic and Applied Sciences, K. R. Mangalam University, Gurgaon, Haryana, India Mahmoud Said Rashed Food Science and Technology Department, Faculty of Agriculture, Alexandria University, Alexandria, Egypt Fatma A. Refay Chemistry/Micro-biology Department, Faculty of Science, Cairo University, Giza, Egypt Waleed M. A. El Rouby Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt Aravind Rudrarapu Department of Chemistry, Faculty of Science and Technology, The ICFAI Foundation for Higher Education, Hyderabad, Telangana, India Salah Salman Department of Mining & Petroleum Engineering, Faculty of Engineering, Al-Azhar University, Cairo, Egypt Nor Suriani Sani Office of Deputy Vice-Chancellor (Research & Innovation), Universiti Teknologi Malaysia, Johor Bahru, Malaysia Mohamed S. Selim National Institute for Materials Science (NIMS), Ibaraki, Japan Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

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Fatma M. Shahat Chemistry/Microbiology Department, Faculty of Science, Cairo University, Giza, Egypt Mohamed A. Shenashen National Institute for Materials Science (NIMS), Ibaraki, Japan Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt Ahmed R. Sofy Botany and Microbiology Department, Faculty of Science, AlAzhar University, Cairo, Egypt Mahmoud R. Sofy Botany and Microbiology Department, Faculty of Science, AlAzhar University, Cairo, Egypt Dina Hamada Swilam Chemistry/Zoology Department, Faculty of Science, Cairo University, Cairo, Egypt Elango Balaji T PG & Research Department of Chemistry, Bishop Heber College, Tiruchirappalli, Tamil Nadu, India Ryou Tanaka Laboratory of Veterinary Surgery, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan Rana Tarek Biotechnology Department, Faculty of Science, Cairo University, Cairo, Egypt Mohammad R. Thalji School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, South Korea Sheikh Muhammad Usman Hunza Sugar Mills Private Limited (Distillery Division), Lahore, Pakistan Aymieza Yaacob Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Tejpal Yadav NIMS Institute of Pharmacy, NIMS University, Jaipur, Rajasthan, India Alaa Mohamed Yasien Biophysics Department, Faculty of Science, Cairo University, Cairo, Egypt Donia G. Youssef School of Biotechnology and Science Academy, Badr University in Cairo, Badr City, Cairo, Egypt Yusmazura Zakaria Biomedicine Programme, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia

Part I Fundamentals of Biodegradations

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Biodegradable Materials: Fundamentals, Importance, and Impacts Gomaa A. M. Ali, Mohammad R. Thalji, and Abdel Salam Hamdy Makhlouf

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biodegradable Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymers Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastics Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Materials Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Industrial and Technological Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Foods and Agricultural Impacts of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter covers the fundamentals of the biodegradation process, including its importance and impacts on many strategic areas, such as the environment, health, industry, technology, food, and agriculture. The future trends in this area were also discussed. Significant advancements have been achieved in biomaterials in

G. A. M. Ali (*) Chemistry Department, Faculty of Science, Al–Azhar University, Assiut, 71524, Egypt e-mail: [email protected] M. R. Thalji School of Chemical Engineering, Yeungnam University, Gyeongsan, 38541, Gyeongbuk, South Korea A. S. H. Makhlouf Stanley Black & Decker, Inc., New Britain, CT, USA © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_74

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the last few decades. These biomaterials include ceramics, glasses, polymers, composites, glass-ceramics, and metal alloys. A wide range of biodegradable materials is currently in use. Examples of these materials are the bio-medical implants that are intended to break down or be resorbed by the body so that the implant does not need to be removed after its purpose has been fulfilled. Therefore, several parameters, including mechanical properties, non-toxicity, surface modification, degradation rate, biocompatibility, corrosion rate, and design, should be considered when designing a scaffold. Keywords

Biodegradable materials · Biocompatibility · Biodegradation processes · Bio-medical implants · Biodegradable sensors

Introduction Nowadays, one of the most interesting topics in biomaterials fields is the study of innovative biodegradable materials, particularly how they disintegrate over time after being transplanted into a living creature [1, 2]. Several names are known as biodegradable materials, including absorbable, resorbable, and degradable [3]. They are a form of biomaterial that interacts with biological processes to treat or enhance tissues, organs, or functions in the body [4]. It can be biologically inert or biodegradable. Biocompatibility, defined as the ability of a substance to perform specific functions while eliciting an acceptable host response [5], is one of the most important characteristics of biological materials [6]. As a result, bio-inert materials such as titanium (Ti), stainless steel, nickel (Ni) alloys, and cobalt-chromium (Co-Cr) alloys elicit no host reaction when transplanted into live tissue. They are now routinely employed in orthopedic surgery and other medical fields [7]. However, although their principal benefit is mechanical strength [8], they also have various downsides, including the influence of stress shielding, metalwork failure needing further surgery, metal ion penetration into neighboring tissues, and imaging modality distortion [9, 10]. Because of these issues, biodegradable alternatives have been developed. Not only are the benefits of bio-inert materials outweighed by their drawbacks, but they may also be destroyed and replaced by host tissues in the long run. To be helpful, biodegradable materials must have several qualities. When implanted in the body, it should not cause any pain or injury, and it should be able to execute the duties it was designed to do and be utilized or adapted for something else. It must also be usable or convertible into another item or object type [11, 12]. In this chapter, we will highlight the fundamentals, importance, and impacts of biodegradation processes and the significant research advances made on various biodegradable materials.

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Fundamentals of Biodegradation Biodegradation is when chemical compounds are decayed by living organisms [13]. CO2 formation, called mineralization, is considered the early step in the biodegradation process [14]. The term “biodegradation” refers to the process through which biologically induced changes occur in a substrate. However, to completely appreciate the biodegradation process, one must first understand the microorganisms involved. Microbial organisms alter the material through metabolic or enzymatic pathways [15]. Growth and metabolism are critical components of this system. Organic pollutants generated during the development process are converted to carbon and energy [16]. As a result of this treatment, organic pollutants are removed (mineralized). Metabolism occurs when organic molecules are metabolized in the presence of the primary source of nutrients and energy, i.e., a growth substrate. In this case, organic substances degrade in both the presence and absence of oxygen [17]. In environmental rehabilitation and waste management scenarios, the “biodegradation process” concept is often used [18, 19]. This process can be divided into three main stages. To begin with, indigenous microorganisms reduce poisons via natural attenuation, all without the assistance of humans and with the presence of several environmental conditions such as temperature, pH, and humidity [20]. Following that, biostimulation is used, in which nutrients and oxygen are delivered into the systems to improve their function and speed up waste breakdown [21]. Eventually, during bioaugmentation, microorganisms are introduced into the systems. These extra organisms should break down the target contaminant faster than natural plants. Realistic remediation solutions need microorganisms that can swiftly adapt to and efficiently use pollutants of interest in a given context within a reasonable time to accomplish their work. Many factors impact microorganisms’ ability to use contaminants as substrates or metabolize them. The rate and extent of degradation seem to be regulated by genetic potential and environmental factors like temperature, pH, and the availability of nitrogen and phosphorus sources. There has been much interest in using genetically engineered microbes in bioremediation (GEMs) [22]. It has been shown that, under certain conditions, these GEMs can degrade a broad spectrum of pollutants. However, it is difficult to test GEM in the field due to environmental and ecological issues and norms and regulations.

Importance of Biodegradation Waste accumulation in the environment could cause serious problems to humans and all living species [23–31]. Recycling technologies could help in waste management and control and therefore protect the environment [32–37]. As mentioned earlier, biodegradation is natural waste disposal and recycling strategy. It can be used to mine everything from garden waste to crude oil. To keep our planet clean and safe, we should rely on this natural process. However, natural biodegradation can no

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longer keep up with our waste production rate, and we are currently in an unsustainable situation. As a result, landfills are overflowing their capacity, and air, water, and soil pollution increase. Unlike very persistent chemicals, which retain their harmful effects for an extraordinarily long time due to their low biodegradation rate, hazardous chemicals break down quickly and, therefore, quickly reduce their concentration and toxic effects. For instance, metabolic fate or biodegradation of polymers as drug delivery carriers is an essential factor [38]. The molecular weight of hydrophilic polymers such as chitosan should be enough for renal clearance in the event of systemic absorption. Polymer degradation will occur if the administered polymer is bigger than this. Fragments appropriate for renal clearance might be generated by chemical and enzymatic biodegradation. Acid-catalyzed degradation, in this situation, refers to the breakdown of chemical compounds in the gastrointestinal tract. Depolymerization by oxidation-reduction and free radical degradation have also been seen, although these processes are unlikely to be important in vivo [39, 40]. Another important example is biodegradable sensors based on two-dimensional (2D) materials that can be used as temporary implants and have attracted substantial interest in the last decade [41]. Biodegradable sensors provide specific benefits over traditional non-degradable ones when monitoring the postoperative rehabilitation of serious brain injuries, preventing persistent inflammation and additional surgery, and minimizing medical risks and expenses [42]. Biodegradable sensors can benefit from the ultra-thinness, excellent mechanical characteristics, and biosafety of 2D materials. Remarkably, they can also adjust band gaps and degradation rates [41].

Types of Biodegradable Materials Polymers Biodegradation Synthetic polymers are used in various industries [23, 43–46], including the packaging market. However, they harm the environment and generate difficulties with waste deposition and usage. As a result, there is a growing trend to substitute such polymers with biodegradable polymers. Scientists have become more interested in polymers made from natural resources like cellulose and starch [47]. They are investing a great deal of time and energy in developing innovative biodegradable polymers and the improvement of test techniques for evaluating their breakdown [48]. The biodegradation process of polymer materials is multi-staged and can be stopped at any stage [49]. Figure 1 shows the different steps involved in biodegradation (fragmentation, hydrolysis, and assimilation) [50]. These stages can be summarized as follows: (1) Microbial communities, decomposer species, or abiotic forces combine to fracture biodegradable materials (biodegradation). (2) The catalytic agents produced by bacteria may break polymer materials, lowering their molecular weight and resulting in monomers, dimers, and oligomers. Depolymerization occurs in this stage. (3) Some molecules cross the plasmic membrane and are

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Fig. 1 The biodegradation process of polymer materials. (Adapted with permission from Ref. [50], copyright 2021, Wiley-VCH)

recognized by microbial receptors. The other molecules remain outside the cell and may be modified. (4) Transported molecules create energy, new biomass, storage vesicles, and various main and secondary metabolites in the cytoplasm (assimilation). (5) Some simple and complex metabolites may be discharged into the extracellular environment (e.g., organic acids, terpenes, antibiotics, aldehydes). Simple molecules including CO2, N2, CH4, H2O, and salts from oxidized intracellular metabolites are discharged into the atmosphere (mineralization) [51]. For example, when it comes to removing endocrine-disrupting pollutants from wastewater, biodegradable polymeric materials have attracted significant interest [52]. Using chitin-based nanohydrogels, Sharma et al. [53] studied the removal of adsorbed atrazine from wastewater. They showed how the removal process works through a certain interaction mechanism, shown in Fig. 2. Optimal adsorption at neutral pH was achieved with a contact time of 180 min. The highest adsorption capacity was given as 204.08 mg g 1.

Plastics Biodegradation There has been a progressive shift from conventional metal, wood, and leather materials to new plastics during the last 60 years. Unfortunately, plastics’ most prized quality and long-term use also pose a significant environmental risk. Plastic waste recycling – especially for the plastics originated from non-recyclable and

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Fig. 2 The interaction mechanism between atrazine on chitin-cl-poly(acrylamide-co-itaconic acid) nanohydrogel. (Adapted with permission from Ref. [53], copyright 2020, Elsevier)

non-biodegradable petroleum-based products – has almost completely failed to offer a safe solution [54]. To reduce the amount of plastic waste in the environment, it is necessary to reduce the build-up of polyethylene waste, which accounts for around 140 million tons of garbage each year [55, 56]. Attempts have been made to identify bacteria that can use synthetic polymers despite polyethylene’s reputation as essentially inert. Using fungi or microbial strains to biodegrade plastic waste is now a realistic option. According to several studies, the ability of bacteria to degrade plastic is based on their innate propensity to digest long-chain fatty acids [57]. The bacterial breakdown of plastics is greatly aided by biofilm development, which helps the colonies stick to and survive on the plastic surface. As an example, Ideonella sakaiensis, a newly discovered species of the bacterium [58], can break down poly(ethylene terephthalate) (PET) because it uses the polymer as its primary source of energy and carbon [59] (Fig. 3). According to the most published findings, Aspergillus fungi are the most significant synthetic plastic biodegrades. Aspergillus clavatus [60], Aspergillus niger [61], and Aspergillus fumigatus [62] are the three Aspergillus species that have been

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Fig. 3 The pathway of Ideonella sakaiensis poly(ethylene terephthalate) degradation. (Adapted with permission from Ref. [58], copyright 2016, Science)

observed to degrade polyethylene, polyurethane, and polypropylene, respectively. In many investigations under solid-state and submerged fermentation conditions, polyurethane breakdown was seen in endophytic fungal isolates from various plants. Figure 4 shows the microbial degradation pathway of polyurethane [63].

Other Materials Biodegradation Food, pharmaceuticals, textiles, cosmetics, and leather sectors are seeing a surge in dye demand fuelling the current industrialization wave. As azo dyes degrade and are removed from the environment, concerns have been raised about these colors’ environmental and human health risks [64–67]. Excessive energy and economic expenditures, high sludge output, and chemical handling have stifled the use of physical-chemical treatments [66–70]. Comparatively, the bioremediation process resolves the issue eco-friendly, efficient, and affordable. Recent research has found several bioremediation strategies that are effective, affordable, and environmentally friendly which have been the subject of recent research. These studies have focused

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Fig. 4 The pathway microbial degradation of polyurethane. (Adapted with permission from Ref. [63], copyright 2021, Elsevier)

on developing novel bioremediation procedures. There have been reports of biological methods for dye removal under diverse circumstances. Pure microorganisms, like bacteria, fungus, and microalgae, or mixed microbial groups that have been moved can be used in this biological technique to make things right. Figure 5 summarizes the azo dye biodegradation process by bacteria [71]. Coconut husk fiber-reinforced biocomposites are becoming more popular owing to the rising need for biodegradable materials [72]. The husks and shells of coconuts are often thrown away as rubbish. Because of this, biocomposite manufacturers may benefit from using these resources. Coir fiber-reinforced biocomposites materials must be robust and stiff, which coir fibers provide. However, due to the inhomogeneous nature of the coir material, the biocomposites made from it exhibit a wide range of performance characteristics. Biocomposites are made by combining coir materials with various thermoplastic, thermosetting, and cement-based polymers. The mechanical, thermal, and physical qualities of synthetic fiber-reinforced

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Fig. 5 Biodegradation process of azo dyes under aerobic treatment. (Adapted with permission from Ref. [71], copyright 2021, Elsevier)

composites are surpassed by those of coir fiber-reinforced composites. They are superior to synthetic fiber-reinforced composites in this regard.

Impacts of Biodegradation Environmental Impacts of Biodegradation Although biodegradation is regarded as the most environmentally beneficial method of disposing of plastic, one of the most significant environmental consequences of plastic is the production of the micro- (less than 5 mm) and nano-sized plastics (less than 0.1 μm) [73]. Microscopic plastic particles can be generated as a by-product of biodegradation by the action of microbes, or they can be formed because of the abonnement of single-use plastics in the environment and exposure to UV, and heat, or physical forces. It does not matter how the micro- and nano-plastics are made. They could end up in the soil, food, and water systems, which could cause problems for both marine and land-based organisms. For instance, Ziajahromi et al. [74] evaluated the effects of polyethylene microplastics on the survival and development of Chironomus tepperi. They observed that a high concentration of polyethylene microplastics had a detrimental effect on the growth and shape of the Chironomus tepperi body. At the molecular level, Hwang et al. [75] studied the probable toxicity of micro- and nano-polystyrene plastic particles in humans at a dosage of roughly 500 μg mL 1. They observed that microplastic polystyrene particles at such a low dosage had no adverse effect on

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human cells. On the other hand, nanoparticles (460 nm–1.0 μm) harmed red blood cells and caused them to hemolyze.

Health Impacts of Biodegradation Biodegradable materials such as polymers and hydrolyzable metals make up the bulk of temporary and implantable medical devices [76]. Host reaction, specifically the immunological response to degradable devices, is an essential component in determining the effectiveness of these devices [2]. The material characteristics and degradation processes utilized in their fabrication impact this response greatly. Each step in the decomposition process produces a different degradation product that can impact the host’s reaction. These include metal ions (basal and alloying), hydroxides, oxides, and calcium-phosphate precipitates [77]. Creating a dynamic interface formed by a bio-imitating calcification matrix on the deteriorating implant is one of the distinctive features of biodegradable-metal deterioration. The implantation location, which is linked to creating this interfacial layer, strongly influences the host’s response to biodegradable metals. New bone may typically develop right on top of the biomimicking interfacial layer formed when biodegradable metals are implanted in bone environments without the need for a fibrous layer or gap as the study by Xiao et al. [78] using pure Zn and Zn-0.05wt%Mg alloy (Fig. 6). As a result, this is known as “indirect bone development.”

Industrial and Technological Impacts of Biodegradation In the last decade, there has been a considerable increase in public and governmental recognition of the need for more environmentally friendly products, driving future research into the next generation of materials and processes. In such an environment, biodegradable materials are considered one of the innovations and current market development since they provide an extra end-of-life choice for the products that employ them. By looking at the patterns of how these new technologies are growing, academics, investors, and policymakers can better judge the potential of the technology and understand how it is changing. Policies can change companies’ emphasis away from one method and toward another, whether on a local level or via international conventions [79]. Since the United Nations’ Sustainable Development Goals (SDGs) were established, many companies have lobbied for environmentally friendly practices (SDGs). Following the objectives, plastics manufacturing will consider utilizing renewable resources that have no negative effect on human health (SDG3), climate change (SDG13), life below water (SDG14), and life on land (SDG15). For SDG11 (sustainable cities and communities) and SDG12 (sustainable production and consumption), circularity (responsible production and consumption) should also be considered.

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Fig. 6 Microscopy cross-sectional images for examining representative histology of the boneimplant contact stained with toluidine blue. Zn for (a) 12 weeks and (b) 24 weeks. Zn-0.05 Mg for (c) 12 weeks and (d) 24 weeks. The blue arrow represents normal cortical bones; the white arrow represents the implant; the red arrow represents newly formed bone fractions around the implant; the green arrow represents the bone junction between cortical bone and new bone formation. (Adapted with permission from Ref. [78], copyright 2018, Elsevier)

Foods and Agricultural Impacts of Biodegradation Agricultural waste is a significant raw material source for developing plasticizers, bio-based polymers, and antioxidant supplements [80]. There are three fundamental categories of agricultural waste: organic, inert, and poisonous. Polysaccharides may be found in agricultural waste derived from plants{Maraveas, #450}. In producing natural plasticizers, polysaccharides are crucial, and vegetable-based agricultural wastes constitute a significant source of these polysaccharides. Plasticizers can significantly boost the elasticity and mechanical strength of bio-based polymers. In contrast to glycerol and other synthetic plasticizers, the performance of polysaccharide plasticizers generated from plants has not been researched, and their commercial use is restricted. To produce antioxidant supplements, it is usual to use agricultural by-products such as mango kernel extracts, green tea extracts, and essential oils. Food processing factories supply these constituents. Antioxidantrich agro-waste includes pomegranate peel extract, mint plant extracts, Thymus vulgaris L., and oregano, among many others. It is vital for antioxidation activity

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because natural antioxidants include phenols that operate as Lewis bases and electron donors. In addition to phenols, pomegranates also contain gallic acid and gallates, natural stabilizers, and age indicators.

Conclusions This chapter emphasizes the fundamentals and importance of the biodegradation process. It addresses numerous forms of biodegradable materials, including polymer biodegradation and plastic biodegradation. Biodegradation impacts on the environment, health, industry, technology, food, and agriculture were explored. We concluded that the research and application of biodegradable materials are an important part of achieving sustainable development and ecological protection. Researchers try to find the best balance between how well a biodegradable material works, how useful it is, and how much it costs.

Future Perspectives Biodegradable materials and their progress will be critical in enhancing human health and achieving the aim of sustainable materials strategy and the global efforts to meet 2035 Net Zero Strategy for carbon footprint. Employing biodegradable implanted and wearable sensors to monitor bodily states continuously shifts illness management in healthcare systems toward preventive, predictive, and customized disease management. Furthermore, concerns about electronic waste might be alleviated by developing electrical devices and sensors that degrade naturally in the environment. Although some key principles of biodegradable electronics have been proven in vitro and in vivo, the technology is still in its early stages, with several difficulties ahead. The biodegradable materials database should be developed further with thorough investigations of materials chemistry, materials-biology interaction, and related biocompatibility. Stimulus-reactive materials that may interact with electrical devices are particularly appealing since they can be altered in various ways and allow you to regulate how long they function.

References 1. Gorejová R, Haverová L, Oriňaková R, Oriňak A, and Oriňak M (2019) Recent advancements in Fe-based biodegradable materials for bone repair. Journal of Materials Science 54(3): 1913 1947 2. Li C, Guo C, Fitzpatrick V, Ibrahim A, Zwierstra MJ, Hanna P, Lechtig A, Nazarian A, Lin SJ, and Kaplan DL (2020) Design of biodegradable, implantable devices towards clinical translation. Nature Reviews Materials 5(1):61 81 3. Seitz JM, Durisin M, Goldman J, and Drelich JW (2015) Recent Advances in Biodegradable Metals for Medical Sutures: A Critical Review. Advanced Healthcare Materials 4(13): 1915 1936

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4. Biswal T, BadJena SK, and Pradhan D (2020) Sustainable biomaterials and their applications: A short review. Materials Today: Proceedings 30:274 282 5. Jurak M, Wiącek AE, Ładniak A, Przykaza K, and Szafran K (2021) What affects the biocompatibility of polymers? Advances in Colloid and Interface Science 294 6. Rahmati M and Mozafari M (2019) Biocompatibility of alumina-based biomaterials–A review. Journal of Cellular Physiology 234(4):3321 3335 7. Wang Z, Wang C, Li C, Qin Y, Zhong L, Chen B, Li Z, Liu H, Chang F, and Wang J (2017) Analysis of factors influencing bone ingrowth into three-dimensional printed porous metal scaffolds: A review. Journal of Alloys and Compounds 717:271 285 8. Matsunaga J, Watanabe I, Nakao N, Watanabe E, Elshahawy W, and Yoshida N (2015) Joining characteristics of titanium-based orthodontic wires connected by laser and electrical welding methods. Journal of Materials Science: Materials in Medicine 26:50 9. Rautray TR, Narayanan R, and Kim KH (2011) Ion implantation of titanium based biomaterials. Progress in Materials Science 56(8):1137 1177 10. Sonnow L, Könneker S, Vogt PM, Wacker F, and von Falck C (2017) Biodegradable magnesium Herbert screw image quality and artifacts with radiography, CT and MRI, BMC Medical Imaging 17(1):1 9 11. Girotto F, Alibardi L, and Cossu R (2015) Food waste generation and industrial uses: A review. Waste Management 45:32 41 12. Bowen PK, Shearier ER, Zhao S, Guillory RJ, Zhao F, Goldman J, and Drelich JW (2016) Biodegradable Metals for Cardiovascular Stents: From Clinical Concerns to Recent Zn-Alloys. Advanced Healthcare Materials 5(10):1121 1140 13. Juan C. Sanchez-Hernandez YC and Kyoung SR (2020) Potential Use of Earthworms to Enhance Decaying of Biodegradable Plastics. ACS Sustainable Chem. Eng 8:4292-4316 14. Tosin M, Pischedda A, and Degli-Innocenti F (2019) Biodegradation kinetics in soil of a multiconstituent biodegradable plastic. Polymer Degradation and Stability 166:213 218 15. Cravens A, Payne J, and Smolke CD (2019) Synthetic biology strategies for microbial biosynthesis of plant natural products. Nature Communications 10(1):1 12 16. Liu Q, Wu L, Jackstell R, and Beller M (2015) Using carbon dioxide as a building block in organic synthesis. Nature Communications 6:5933 17. Bouwer EJ and Zehnder AJB (1993) Bioremediation of organic compounds - putting microbial metabolism to work. Trends in Biotechnology 11(August):360 367 18. Davis G and Song JH (2006) Biodegradable packaging based on raw materials from crops and their impact on waste management. Industrial Crops and Products 23(2):147 161 19. Emadian SM, Onay TT, and Demirel B (2017) Biodegradation of bioplastics in natural environments. Waste Management 59:526 536 20. Trigo A, Valencia A, and Cases I (2009) Systemic approaches to biodegradation. FEMS Microbiology Reviews 33(1):98 108 21. Azubuike CC, Chikere CB, and Okpokwasili GC (2016) Bioremediation techniques–classification based on site of application: principles, advantages, limitations and prospects. World Journal of Microbiology and Biotechnology 32(11):1 18 22. Pant G, Garlapati D, Agrawal U, Prasuna RG, Mathimani T, and Pugazhendhi A (2021) Biological approaches practised using genetically engineered microbes for a sustainable environment: A review. Journal of Hazardous Materials 405:124631 23. Agarwal S, Sadegh H, Monajjemi Majid, Makhlouf ASH, Ali GAM, Memar AOH, Shahryarighoshekandi R, Tyagi I, and Gupta VK (2016) Efficient removal of toxic bromothymol blue and methylene blue from wastewater by polyvinyl alcohol. Journal of Molecular Liquids 218:191 197 24. Alhasan HS, Alahmadi N, Yasin SA, Khalaf MY, and Ali GAM (2022) Low-Cost and Eco-Friendly Hydroxyapatite Nanoparticles Derived from Eggshell Waste for Cephalexin Removal. Separations 9(1):10 25. Ali GAM, Habeeb OA, Algarni H, and Chong KF (2018) CaO impregnated highly porous honeycomb activated carbon from agriculture waste: symmetrical supercapacitor study. Journal of Materials Science 54:683 692 26. El-Maghrabi HH, Nada AA, Soliman FS, Raynaud P, Moustafa YM, Ali GAM, and Bekheet MF, Recovery of Metal Oxide Nanomaterials from Electronic Waste Materials, in Waste

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Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 203 227. 27. Habeeb OA, Ramesh K, Ali GAM, and Yunus RM (2017) Low-cost and eco-friendly activated carbon from modified palm kernel shell for hydrogen sulfide removal from wastewater: adsorption and kinetic studies. Desalination and Water Treatment 84:205 214 28. Hegde G, Abdul Manaf SA, Kumar A, Ali GAM, Chong KF, Ngaini Z, and Sharma KV (2015) Biowaste sago bark based catalyst free carbon nanospheres: waste to wealth approach. ACS Sustainable Chemistry & Engineering 5(9):2247–2253 29. Nada AA, Soliman FS, Ali GAM, Hamdy A, Selim H, Elsayed MA, Elmowafy ME, and El-Maghrabi HH, Conversion of Waste Cheap Petroleum Paraffinic Wax By-Products to Expensive Valuable Multiple Carbon Nanomaterials, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 729 751. 30. Ali GAM, Divyashree A, Supriya S, Chong KF, Ethiraj AS, Reddy MV, Algarni H, and Hegde G (2017) Carbon nanospheres derived from Lablab purpureus for high performance supercapacitor electrodes: a green approach. Dalton Transactions 46(40):14034 14044 31. Ali GAM, Manaf SAA, A D, Chong KF, and Hegde G (2016) Superior supercapacitive performance in porous nanocarbons. Journal of Energy Chemistry 25(4):734 739 32. Aboelazm EAA, Mohamed N, Ali GAM, Makhlouf ASH, and Chong KF, Recycling of Cobalt Oxides Electrodes from Spent Lithium-Ion Batteries by Electrochemical Method, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 91 123. 33. Ali GAM (2020) Recycled MnO2 Nanoflowers and Graphene Nanosheets for Low-Cost and High Performance Asymmetric Supercapacitor. Journal of Electronic Materials 49:5411– 5421 34. Ali GAM, Bakr ZH, Safarifard V, and Chong KF, Recycled Nanomaterials for Energy Storage (Supercapacitor) Applications, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 175 202. 35. Ali GAM and Makhlouf ASH, Fundamentals of Waste Recycling for Nanomaterial Manufacturing, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 3 24. 36. Ali GAM, Yusoff MM, Shaaban ER, and Chong KF (2017) High performance MnO2 nanoflower supercapacitor electrode by electrochemical recycling of spent batteries. Ceramics International 43:8440–8448 37. Makhlouf ASH and Ali GAM, Waste Recycling Technologies for Nanomaterials Manufacturing. Topics in Mining, Metallurgy and Materials Engineering. 2021, Springer: Springer. 38. Kean T and Thanou M (2010) Biodegradation, biodistribution and toxicity of chitosan. Advanced Drug Delivery Reviews 62(1):3 11 39. Hsu SC, Don TM, and Chiu WY (2002) Free radical degradation of chitosan with potassium persulfate. Polymer Degradation and Stability 75(1):73 83 40. Zoldners J, Kiseleva T, and Kaiminsh I (2005) Influence of ascorbic acid on the stability of chitosan solutions. Carbohydrate Polymers 60(2):215 218 41. Chen X and Ahn JH (2020) Biodegradable and bioabsorbable sensors based on two-dimensional materials. Journal of Materials Chemistry B 8(6):1082 1092 42. Tan MJ, Owh C, Chee PL, Kyaw AKK, Kai D, and Loh XJ (2016) Biodegradable electronics: Cornerstone for sustainable electronics and transient applications. Journal of Materials Chemistry C 4(24):5531 5558 43. Alhanish A and Ali GAM, Recent Developments in Wastewater Treatment Using Polymer/Clay Nanocomposites, in Advances in Nanocomposite Materials for Environmental and Energy Harvesting Applications, AE Shalan, AS Hamdy Makhlouf, S Lanceros-Méndez, Editors. 2022, Springer International Publishing: Cham. p. 419 451.

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44. Haroun AAA, Rabie AGM, Ali GAM, and Abdelrahim MYM (2019) Improving the mechanical and thermal properties of chlorinated poly (vinyl chloride) by incorporating modified CaCO3 nanoparticles as a filler. Turkish Journal of Chemistry 43(3):750 759 45. Mahmoodi Z, Abhari AR, Lalehloo RS, Bakr ZH, and Ali GAM (2022) Thermodynamic Studies on the Adsorption of Organophosphate Pesticides (Diazinon) onto ZnO/Polyethersulfone Nanocomposites. ChemistrySelect 7(2):e202103619 46. Thalji MR, Ibrahim AA, and Ali GAM (2021) Cutting-edge development in dendritic polymeric materials for biomedical and energy applications. European Polymer Journal 160:110770 47. Vroman I and Tighzert L (2009) Biodegradable polymers. Materials 2(2):307 344 48. Albright VC and Chai Y (2021) Knowledge Gaps in Polymer Biodegradation Research. Environmental Science and Technology 55(17):11476 11488 49. Lucas N, Bienaime C, Belloy C, Queneudec M, Silvestre F, and Nava-Saucedo JE (2008) Polymer biodegradation: Mechanisms and estimation techniques - A review. Chemosphere 73 (4):429 442 50. Wang GX, Huang D, Ji JH, Völker C, and Wurm FR (2021) Seawater-Degradable Polymers— Fighting the Marine Plastic Pollution. Advanced Science 8(1):1 26 51. Thirmizir MZA, Ishak ZAM, and Salim MS, Compatibilization and Crosslinking in Biodegradable Thermoplastic Polyester Blends, in Reactive and Functional Polymers Volume Two: Modification Reactions, Compatibility and Blends, TJ Gutiérrez, Editor. 2020, Springer International Publishing: Cham. p. 23 89. 52. Al Sharabati M, Abokwiek R, Al-Othman A, Tawalbeh M, Karaman C, Orooji Y, and Karimi F (2021) Biodegradable polymers and their nano-composites for the removal of endocrinedisrupting chemicals (EDCs) from wastewater: A review. Environmental Research 202 (July):111694 111694 53. Sharma G, Thakur B, Kumar A, Sharma S, Naushad M, and Stadler FJ (2020) Atrazine removal using chitin-cl-poly(acrylamide-co-itaconic acid) nanohydrogel: Isotherms and pH responsive nature. Carbohydrate Polymers 241(May):116258 116258 54. Sivan A (2011) New perspectives in plastic biodegradation. Current Opinion in Biotechnology 22(3):422-426 55. Alhanish A and Ali GAM, Recycling the Plastic Wastes to Carbon Nanotubes, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 701 727. 56. Yasin S, Bakr ZH, Ali GAM, and Saeed I, Recycling Nanofibers from Polyethylene Terephthalate Waste Using Electrospinning Technique, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 805 821. 57. Amobonye A, Bhagwat P, Singh S, and Pillai S (2021) Plastic biodegradation: Frontline microbes and their enzymes. Science of the Total Environment 759:143536 143536 58. Yang Y, Yang J, and Jiang L (2016) Comment on “a bacterium that degrades and assimilates poly(ethylene terephthalate) ”. Science 353(6301):759 759 59. Essa WK, Yasin SA, Abdullah AH, Thalji MR, Saeed IA, Assiri MA, Chong KF, and Ali GAM (2022) Taguchi L25 (54) Approach for Methylene Blue Removal by Polyethylene Terephthalate Nanofiber-Multi-Walled Carbon Nanotube Composite. Water 14(8):1242 1242 60. Gajendiran A, Krishnamoorthy S, and Abraham J (2016) Microbial degradation of low-density polyethylene (LDPE) by Aspergillus clavatus strain JASK1 isolated from landfill soil. 3 Biotech 6(1):1 6 61. Usman MA, Momohjimoh I, and Usman AO (2020) Mechanical, physical and biodegradability performances of treated and untreated groundnut shell powder recycled polypropylene composites. Materials Research Express 7(3):035302 62. Osman M, Satti SM, Luqman A, Hasan F, Shah Z, and Shah AA (2018) Degradation of Polyester Polyurethane by Aspergillus sp. Strain S45 Isolated from Soil. Journal of Polymers and the Environment 26(1):301 310

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63. Ali SS, Elsamahy T, Al-Tohamy R, Zhu D, Mahmoud YAG, Koutra E, Metwally MA, Kornaros M, and Sun J (2021) Plastic wastes biodegradation: Mechanisms, challenges and future prospects. Science of the Total Environment 780:146590 146590 64. Badawi AK, Abd Elkodous M, and Ali GAM (2021) Recent advances in dye and metal ion removal using efficient adsorbents and novel nano-based materials: an overview. RSC Advances 11(58):36528 36553 65. Ethiraj AS, Rhen DS, Soldatov AV, Ali GAM, and Bakr ZH (2021) Efficient and recyclable Cu incorporated TiO2 nanoparticle catalyst for organic dye photodegradation. International Journal of Thin Film Science and Technology 10(3):169 182 66. Seyed Arabi SM, Lalehloo RS, Olyai MRTB, Ali GAM, and Sadegh H (2019) Removal of Congo red azo dye from aqueous solution by ZnO nanoparticles loaded on multiwall carbon nanotubes. Physica E: Low-dimensional Systems and Nanostructures 106:150 155 67. Solehudin M, Sirimahachai U, Ali GAM, Chong KF, and Wongnawa S (2020) One-pot synthesis of isotype heterojunction g-C3N4-MU photocatalyst for effective tetracycline hydrochloride antibiotic and reactive orange 16 dye removal. Advanced Powder Technology 31(5): 1891 1902 68. Shi Y, Yang Z, Xing L, Zhang X, Li X, and Zhang D (2021) Recent advances in the biodegradation of azo dyes. World Journal of Microbiology and Biotechnology 37(8):1 18 69. Laouini SE, Bouafia A, Soldatov AV, Algarni H, Tedjani ML, Ali GAM, and Barhoum A (2021) Green Synthesized of Ag/Ag2O Nanoparticles Using Aqueous Leaves Extracts of Phoenix dactylifera L. and Their Azo Dye Photodegradation. Membranes 11(7):468 70. Naeimi A, Sharifi A, Montazerghaem L, Abhari AR, Mahmoodi Z, Bakr ZH, Soldatov AV, and Ali GAM (2022) Transition metals doped WO3 photocatalyst towards high efficiency decolourization of azo dye. Journal of Molecular Structure 1250:131800 71. Selvaraj V, Swarna Karthika T, Mansiya C, and Alagar M (2021) An over review on recently developed techniques, mechanisms and intermediate involved in the advanced azo dye degradation for industrial applications. Journal of Molecular Structure 1224 72. Hasan KMF, Horváth PG, Bak M, and Alpár T (2021) A state-of-the-art review on coir fiberreinforced biocomposites. RSC Advances 11(18):10548 10571 73. Taghavi N, Udugama IA, Zhuang WQ, and Baroutian S (2021) Challenges in biodegradation of non-degradable thermoplastic waste: From environmental impact to operational readiness. Biotechnology Advances 49(February):107731 107731 74. Ziajahromi S, Kumar A, Neale PA, and Leusch FDL (2018) Environmentally relevant concentrations of polyethylene microplastics negatively impact the survival, growth and emergence of sediment-dwelling invertebrates. Environmental Pollution 236:425 431 75. Hwang J, Choi D, Han S, Jung SY, Choi J, and Hong J (2020) Potential toxicity of polystyrene microplastic particles. Scientific Reports 10(1):1 12 76. Hosseini ES, Dervin S, Ganguly P, and Dahiya R (2021) Biodegradable Materials for Sustainable Health Monitoring Devices. ACS Applied Bio Materials 4(1):163 194 77. Seitz JM, Eifler R, Bach FW, and Maier HJ (2014) Magnesium degradation products: Effects on tissue and human metabolism. Journal of Biomedical Materials Research - Part A 102(10): 3744 3753 78. Xiao C, Wang L, Ren Y, Sun S, Zhang E, Yan C, Liu Q, Sun X, Shou F, Duan J, Wang H, and Qin G (2018) Indirectly extruded biodegradable Zn-0.05wt%Mg alloy with improved strength and ductility: In vitro and in vivo studies. Journal of Materials Science and Technology 34(9): 1618 1627 79. Filiciotto L and Rothenberg G (2021) Biodegradable Plastics: Standards, Policies, and Impacts. ChemSusChem 14(1):56 72 80. Xie Y, Niu X, Yang J, Fan R, Shi J, Ullah N, Feng X, and Chen L (2020) Active biodegradable films based on the whole potato peel incorporated with bacterial cellulose and curcumin. International Journal of Biological Macromolecules 150:480 491

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Biodegradation Process: Basics, Factors Affecting, and Industrial Applications Lai Mun Koh and Sook Mei Khor

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition of Biodegradation and Biodegradable Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abiotic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abiotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Industrial Applications of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioremediation of Crude Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Industrial Applications of Anaerobic Digestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The natural environment is seriously threatened by plastic accumulation due to increasing plastic demand and production. This scenario has driven increased interest in the research of the biodegradation process and the development of biodegradable materials to replace conventional plastics. Nevertheless, the knowledge of biodegradation is limited, and biodegradable materials remain new to the public. Because of this, the current chapter presents the definitions of biodegradation and biodegradable materials stated by different standards and

L. M. Koh · S. M. Khor (*) Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_66

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organizations. Besides, an insight into biodegradation with emphasis on abiotic and biotic mechanisms is also included. Understanding the biodegradation process is significant in controlling the biodegradation rate and inventing biodegradable materials with the desired biodegradability. Here, several polymer characteristics and abiotic and biotic environmental factors are reviewed. Moreover, three classification methods of biodegradable materials are discussed, along with the examples, advantages, and disadvantages. This chapter describes industrial applications of biodegradation, focusing on oil bioremediation and anaerobic digestion. Oil bioremediation is imperative for removing oil hydrocarbons, while the latter is widely utilized for organic waste treatment and production of biogas and digestates. Keywords

Biodegradation · Biodegradable polymers · Abiotic degradation · Biotic degradation · Oil bioremediation · Anaerobic digestion Abbreviations

ASTM ATP BPCs BTEX CEN CIS DIN ISO JBPA NAFTA PA PBS PBSTMG PCL PCL/TPS PE PEG PET PHA PLA PMDI PP PTMG PS PU PVC PVA

American Society for Testing and Materials Adenosine triphosphate Biodegradable polymer composites Benzene, toluene, ethylene, and xylene European Committee for Standardization Commonwealth of Independent States German Institute for Standardization International Organization for Standardization Japan BioPlastics Association North American Free Trade Agreement Polyamide Poly(butylene succinate) Poly[(butylene succinate)-co-poly(tetramethylene glycol)] Poly-ε-caprolactone Poly-ε-caprolactone/thermoplastic starch blends Polyethylene Polyethylene glycol Polyethylene terephthalate Polyhydroxyalkanoates Polylactic acid Polyisocyanate Polypropylene Poly(tetramethylene glycol) Polystyrene Polyurethane Polyvinyl chloride Polyvinyl alcohol

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Introduction The advantages of plastics, such as lightweight, low cost, ease of processing, and durability, have contributed to increasing manufacturing and wide applications of plastics in various sectors [1]. Between 1950 and 2015, over 6,300 million metric tons of plastic were produced, including both primary and secondary plastic waste [2]. In 2019 alone, approximately 370 million tons of plastic were produced globally, with Asia topping the plastic production, contributing 51% of the world’s plastic production (Fig. 1) [3]. Plastic waste and fragmented plastic debris, microplastics, are widely distributed in both terrestrial and aquatic environments due to increasing plastic production [2]. Moreover, research estimates that more than 1,000,000 tons of plastic have accumulated in the Mediterranean sea and about 200,000 tons of plastic have flowed into the Mediterranean every year [4]. The buildup of plastic in the environment arises due to most plastics’ nonbiodegradability [5]. The persistence of plastic waste and its adverse effects on the environment have led to an increase in environmental consciousness, triggering the development of biodegradable materials. In 2019, over 1.2 million tons of biodegradable plastics were produced globally, and the production is anticipated to increase to 1.8 million by 2025 (Fig. 2) [6]. Despite the rapid development of biodegradable materials, their production and application are still limited due to cost constraints and safety concerns [7]. For instance, the biodegradation of plastic may lead to microplastics that threaten marine life. Besides, the higher manufacturing costs of biodegradable plastics than conventional plastics contribute to the higher market price for biodegradable plastics [8].

Fig. 1 Distribution of global plastic production by regions. (Adapted with permission from Ref. [3], Copyright 2020, Plastics Europe Association of Plastics Manufacturers)

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Fig. 2 Bioplastic production capacity on a global scale. (Adapted with permission from Ref. [6], Copyright, European Bioplastic)

Biodegradation or biological degradation refers to the reduction in complexity of chemical compounds via chemical and biological processes with the aid of microorganisms. Bacteria, fungi, and algae are the most common plastic-degrading microorganisms which are compulsory in transforming organic substrates into simpler forms [9]. During biodegradation, the primary role of microorganisms is to secrete both intracellular and extracellular enzymes that catalyze the depolymerization and metabolism of organic compounds [9]. Due to the significant role of microorganisms, biodegradation is also known as microbial degradation [9]. The microorganisms consume the organic substrates to gain energy and carbon for growth and reproduction. Materials that microorganisms can degrade are known as biodegradable materials. Biodegradable materials are not limited to plastics but include biodegradable composites, polymeric materials, fabrics, etc. The materials undergoing biodegradation may experience some common physical changes such as reductions in weight loss and tensile strength and surface changes such as cracking and the formation of pores and holes. Besides, changes in functional groups due to breaking and forming bond linkages are common after biodegradation [10]. Biodegradable materials are subjected to different biodegradation pathways under different environmental conditions. Aerobic degradation is the predominant pathway when oxygen is present. The degradation products are often water, carbon dioxide, mineral salts, and biomass [11]. In contrast to aerobic degradation, anaerobic degradation is more likely to occur under anoxic (or oxygen-deficient) and anaerobic (or absence of oxygen) conditions. This degradation produces methane, carbon dioxide, and digestate as the main products.

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Definition of Biodegradation and Biodegradable Materials In the literature, biodegradation and biodegradable materials are defined in various ways. A few national and international standards and organizations define biodegradation and biodegradable plastics. The national standards include the German Institute for Standardization (DIN), the American Society for Testing and Materials (ASTM), and the Japan BioPlastics Association (JBPA), while the International Organization for Standardization (ISO) is accepted worldwide. Besides, the European Committee for Standardization (CEN) is a regional standard commonly applied in Europe [12]. According to both CEN and DIN, biodegradation is a biological process that alters the chemical structure. CEN also emphasizes that biodegradation is often driven by enzymes. Marinescu et al. [13] mentioned that living microorganisms play a significant role in breaking down the organic matter into smaller units during biodegradation. Based on these definitions, biodegradation must include a few features: biological processes, chemical changes, and actions by microorganisms. Across the standards discussed earlier, the definition of biodegradable materials is not provided. Instead, the term biodegradable polymers or plastics are widely defined. Biodegradable plastics are polymers degraded into lower molecular weight compounds, and microbial metabolism must be involved in the degradation, as defined by the Japanese Biodegradable Plastics Society. This highlights that biodegradable polymer must be degraded by naturally occurring microorganisms such as fungi, bacteria, and algae, which is also emphasized by ASTM D 6400-99 and CEN. Also, CEN specifies that the end products formed from biodegradation must be water, carbon dioxide and/or methane, and biomass [12]. According to ISO 472:2013, a significant change will occur to the chemical structure of degradable plastics under specific environmental conditions, depriving these plastics of some properties. The changes in the properties of the plastics over time are measured using an appropriate standard test method, which aids in determining if the plastic is biodegradable. One of the standard test methods established to measure the ultimate aerobic biodegradability of plastic under controlled composting settings is ISO 14855-2:2018. Gravimetric analysis is used to test the biodegradability of plastics by detecting carbon dioxide evolution. According to this test procedure, the plastic is biodegradable if 90% of the carbon is transformed into CO2 in 180 days [14]. Based on the definitions above, biodegradable polymers or plastics can be commonly defined as polymeric materials degraded by naturally occurring microorganisms. Nevertheless, the term biodegradable is incomplete as the information on the environment, the duration of decomposition, and biodegradability are not stated. From the definition of biodegradable polymers or plastics, it can be inferred that biodegradable materials can be degraded by naturally occurring microorganisms, which are then converted into metabolites.

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Principles of the Biodegradation Process As discussed in section “Definition of Biodegradation and Biodegradable Materials,” various standards define biodegradation as a biological activity involving microbial actions. Several studies, however, indicate that biodegradation cannot occur in the absence of abiotic involvement [15]. Biodegradation is complex; it involves biological, chemical, and mechanical actions [15]. Biological actions predominate in the biodegradation process and are called biotic degradation or simply biodegradation. These involve the actions of enzymes secreted by microorganisms. Abiotic degradation, including mechanical and chemical activities, usually occurs simultaneously with biological activity. They may enhance biotic degradation or sometimes be needed to initiate the biodegradation process [16]. This happens as biotic and abiotic features exert synergistic effects on the degrading organic matter. Nevertheless, abiotic degradation is excluded from biodegradation and is considered one of the degradation routes that aids in biodegradation. Karak [15] clarified that the degradation of polymers might also be known as environmental degradation due to the involvement of both biotic and abiotic processes. Despite the roles and importance of abiotic degradation, it is necessary to include them in the discussion of the biodegradation process. The mechanisms of both abiotic and biotic degradation are presented in Fig. 3.

Fig. 3 Biodegradation mechanisms

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Abiotic Degradation Abiotic degradation occurs naturally in the environment, which involves only physical and chemical activities but not biological activities [9]. The exposure of polymers to environmental factors, such as air pollutants, humidity, and weather, allows for their transformation via mechanical, light, chemical, and thermal means [16]. The polymers may be fragmented into smaller pieces, or their structures may be weakened for subsequent metabolism via a biotic pathway, ensuring their complete breakdown. Mechanical degradation of polymers occurs due to the external forces acting on the polymers. Compression, tension, and/or shear forces are examples of these forces [16]. For instance, polymeric materials are subjected to strong shear forces during polymer processing like agitation, grinding, and extrusion, causing the molecules to break. Mechanical degradation can also occur naturally, resulting from mechanical weathering where the polymers collide with rocks during abrasion [17]. The mechanical degradation of plastics often results in micro- and nano-plastic fragments forming without reducing molecular weight (Fig. 4) [9]. Photodegradation of polymers involves photochemical reactions, usually mediated by free radicals, which are induced by the absorption of UV radiation [19]. Photodegradation requires the presence of light-absorbing chromophores in the polymer [17]. For example, the radiation is absorbed by carbonyl groups on the carbonyl backbone of polymers. The absorption of UV radiation fragments the polymer chains into smaller chains and introduces oxygenated (e.g., carboxylic acid, carbonyl, or alcohol) end groups to these chains. The photodegradation of polymers with carbonyl groups proceeds either through the Norrish types 1 or 2 photochemical reactions or by hydrogen abstraction reactions. The Norrish type 1 reaction transforms polymers by photoionization, while the type 2 reaction transforms polymers by chain scission [16]. At elevated temperatures, energy is supplied to break down plastics into smaller molecules. This process is known as thermal degradation [17]. Thermal degradation of thermoplastic polymers occurs at the melting temperature [16]. Like photodegradation, thermal degradation is a free-radical-mediated process. The degradation is initiated when the temperature exceeds the activation energy. Fragmentation of polymers occurs through depolymerization, random chain scission, and/or sidegroup elimination, producing smaller chains with reduced molecular weight and decreased mechanical strength. However, cross-linking or cyclization may also recombine the fragments and lead to the enlargement of the polymers [17]. Kopinke

Fig. 4 Mechanical degradation of polymers leads to main chain scissions, producing two daughter chains. (Adapted with permission from Ref. [18], Copyright 2020, Springer Nature)

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et al. [20] studied the thermal degradation of a thermoplastic biodegradable polymer, polylactic acid (PLA). The study found that at temperatures above 200
C (the melting temperature of PLA is 180
C [21]), PLA degraded through intra- and intermolecular ester exchange, radical and non-concerted radical reactions, and cis-elimination, producing CO, CO2, methyl ketene, and acetaldehyde [20]. Another important parameter in abiotic degradation is chemical degradation. This arises due to the chemical reactions between the polymers with atmospheric pollutants and agrochemicals [16]. The main ways that chemical degradation can occur are oxidative degradation and hydrolysis. This occurs since hydrolyzable or oxidizable bonds are found in all biodegradable polymers. Oxidative degradation occurs when oxygen or ozone from the atmosphere attacks the covalent bonds of the polymers, producing free radicals [16]. For instance, the ester groups in aliphatic polyesters are broken down to form their carboxylic and alcohol end groups. These abiotic parameters are important in the biodegradation of polymers. Hence, the biodegradation process depends strongly on the abiotic parameters and the environmental conditions to which the materials are exposed.

Biotic Degradation During the biodegradation of polymers, four key steps occur. First is biodeterioration, a step in which microorganisms colonize the target substrates, and their aggregates form a microbial biofilm on the substrate surface [22]. This leads to superficial degradation, and polymers are fragmented into smaller particles. Naturally occurring organisms act by chemical, physical, and/or enzymatic means [16]. There are two important mechanisms so that the biodeterioration of thermoplastic polymers could proceed, i.e., bulk and surface erosion (Fig. 5). The erosion process is described by the diffusion of water through a polymer and triggering hydrolysis, resulting in bond cleavage and a reduction in polymer size [23]. In surface erosion, the mass loss occurs to the polymer surface, and the erosion progresses gradually toward the center of the polymer. While in bulk erosion, a polymer is degraded simultaneously throughout the entire structure, which is initiated by chemicals (e.g., water, acids, bases, transition metals, and radicals) or radiation. One distinct difference between the two erosions is an early slow reduction in molecular weight in surface erosion, but an early rapid reduction is observed for bulk erosion [16]. The surface and bulk erosion occurrence depends on the relative speed of degradation and water diffusion into the polymer. When the degradation is faster than the water diffusion, degradation can only be made on the surface, and surface erosion is exhibited. In contrast, bulk erosion is shown when water diffusion happens faster than degradation. Karak [15] mentioned that biodegradation usually involves surface erosion when the extracellular enzymes produced by microorganisms are too large to penetrate through the polymer and can only adhere to the surface. The second step, depolymerization, occurs when microorganisms in biofilm secrete extracellular enzymes (or exoenzymes), which catalyze the depolymerization of polymer chains [24]. These exoenzymes will digest the large polymers into

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Fig. 5 Surface and bulk erosion during biodeterioration. (Adapted with permission from Ref. [24], Copyright 2019, John Wiley & Sons)

smaller molecules such as oligomers, dimers, and monomers. Once the fragments are sufficiently small, they will penetrate through the semipermeable outer bacterial membranes into the cells. Subsequently, these fragments will be used as carbon and energy sources to support bacterial growth. The third step during biodegradation is bioassimilation, a step in which the small molecules are integrated into the microbial cells. As a result, primary and secondary metabolites are produced. The last step, mineralization, involves the conversion of the metabolites into end products (CO2, CH4, H2O, and N2) [24]. Besides gases, salts, minerals, biomass, and energy in adenosine triphosphate (ATP) are also produced [25]. The microorganisms benefit from biodegradation as the energy generated can be used to grow and reproduce. Depending on the environmental conditions (aerobic or anaerobic), energy can be produced via either one of the three possible catabolic pathways: aerobic and anaerobic respiration and fermentation. A summary of biotic degradation is illustrated in Fig. 6 [24].

Factors Affecting Biodegradation In general, the biodegradation of polymers is influenced by two key factors: their characteristics and the conditions to which they are exposed. The exposure conditions can be classified into biotic and abiotic factors (Fig. 7).

Abiotic Factors The abiotic factors include temperature, moisture, pH, UV radiation, etc. [23]. These factors can affect abiotic degradation processes such as oxidation, hydrolysis,

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Fig. 6 Schematic diagram of the steps involved in biotic degradation. (Adapted with permission from Ref. [24], Copyright 2019, John Wiley & Sons)

Fig. 7 Factors affecting biodegradation

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mechanical degradation, photodegradation, and thermal degradation. Temperature can strongly influence the enzymatic activity of microorganisms, as enzymes are heat sensitive. Usually, an increase in temperature increases enzymatic activity, fostering the biodegradation process [26]. At 15, 20, and 28
C, Pischedda et al. [27] incubated Mater-Bi HF03V1, a commercial, biodegradable plastic consisting mainly of polyesters and starch. The study found that the mineralization rate of plastic increases with temperature. At 28
C, the greatest CO2 evolution and the least amount of carbon remaining in the plastic to be mineralized into CO2 during biodegradation were recorded. However, further temperature increases may cause denaturation of the enzyme and irreversible loss of its activity when the optimum temperature has been exceeded [26]. Ohtaki et al. [28] studied the biodegradation of poly-ε-caprolactone (PCL) under controlled laboratory composting conditions at 40, 50, and 60
C. The study discovered that the optimum temperature for PCL degradation was around 50
C. Glaser [29] mentioned that the combination of moisture and temperature effects could help microbial colonization. This happens as more sites are available for microorganisms to attack the polymer chains when high moisture speeds up the hydrolysis reaction, producing more chain scission [23]. In addition, the high moisture content in the pore spaces of soil enhances the exchange of oxygen. In contrast, biodegradation can be retarded when the soil becomes anaerobic under saturated conditions. Soil pH can influence the availability of nutrients for microbial metabolism, thereby affecting microbial activity and biodegradation rate. For example, phosphorus has the greatest solubility at pH 6.5, and it begins to decrease at pH values lower or higher than 6.5 [30]. Often, the biodegradation rate is maximal when the soil pH is optimal for the growth of substrate-degrading microbial specimens. Most microorganisms grow at a pH of 7.4, resulting in faster biodegradation at this pH [15]. Nevertheless, weakly acidic soil is suitable for the growth of many other microbes, like fungi. The biodegradation of polyhydroxyalkanoates (PHA) in soil, mainly by fungi, was higher at a weakly acidic pH (e.g., pH 5.48) compared to that in soil with a nearly neutral pH (pH 6.63) [31]. Irradiation of polymers by UV light affects photodegradation by cleaving the C-H bonds, particularly in conjugated polymers. However, irradiation does not always result in a faster rate of biodegradation. Kijchavengkul et al. [23] revealed that the biodegradation rate might be increased or decreased by UV light because the radiation may provoke either main chain scission or cross-linking in polymeric materials. The rate is expected to increase when main chain scission occurs and decrease when cross-linking occurs, leading to more giant molecules.

Biotic Factors In addition to abiotic factors, biotic factors can alter the biodegradation rate in nature. One of these factors is the presence of extracellular enzymes produced by microorganisms. Enzymes are highly selective and have active sites with different shapes that bind with different polymers. Extracellular enzymes are the main hydrolysis and

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oxidation reactions that depolymerize polymers into smaller fragments such as oligomers, dimers, and monomers [24]. For example, the degradation of PLA is driven by both lipase and protease [24]. Among biotic factors, hydrophobic microbes can accumulate on organic pollutants via biofilm formation, which subsequently facilitates biodegradation. Hydrophobic cells tend to adsorb more strongly to hydrophobic surfaces, whereas hydrophilic cells adsorb strongly to hydrophilic surfaces. However, some results demonstrate that hydrophilic microorganisms play an important role in biodegradation.

Characteristics of Polymers Biodegradation of polymers is affected by polymer characteristics which influence the accessibility of the polymer chain to microbes and water [23]. There are various chemical and physical properties of polymers that can limit biodegradation, and these include molecular weight, crystallinity, morphology, functional groups, conformational flexibility, cross-linking, etc. Generally, polymers with a larger molecular weight are more difficult to degrade, and microorganisms can only assimilate small fragments of low molecular weight polymers and convert them into metabolites within the cells [12]. In large polymers, more bonds are cleaved by microorganisms before being converted into smaller fragments [23], which slows down the biodegradation process. Besides, biodegradation efficiency also depends on the conformational flexibility of polymers. It is always easier for flexible polymers to access microbes and water. The conformational flexibility of polymers is influenced by two factors: bulky side groups and the types of linkages in the polymer backbone. Polymers with bulk side groups often have limited movement and are less flexible. On the other hand, polymers with unsaturated bonds can rotate and are hence more flexible. Apart from these two factors, the flexibility of polymers is also affected by their crystallinity [23]. In the context of crystallinity, amorphous regions of polymers (Fig. 8) are more accessible to water and microbes, endowing them with greater biodegradability [32]. In contrast, the closely packed molecules in crystalline regions (Fig. 8) hinder the diffusion of water, thus limiting their access to water, and the hydrolysis rate decreases [23]. According to Pantani and Sorrentino [33], the PLA’s denser

Fig. 8 A semicrystalline polymer with crystalline and amorphous regions is depicted schematically. (Adapted with permission from Ref. [34], Copyright 2013, Elsevier)

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crystalline regions were impermeable to enzymatic attachment, preventing the oligomers formed from diffusing out of the polymer and eventually reducing PLA’s biodegradability. Biodegradation efficiency is improved with hydrolyzable functional groups in the polymer chains, such as amide, carbonate, ester, and ether. These functional groups are more susceptible to surface erosion via enzymatic hydrolysis and abiotic hydrolysis than plastics without hydrolyzable groups. Polymers with hydrolyzable ester bonds in their backbones, such as polyethylene terephthalate (PET) and polyurethane (PU), are easier to biodegrade than polymers with carbon chain backbones, such as polyethylene (PE), polystyrene (PS), polypropylene (PP), and polyvinyl chloride (PVC) [35]. The chemical structures of these polymers are shown in Table 1. Copolymerization is an efficient method to improve the biodegradability of polymers. Copolymers are heteropolymers that are synthesized from more than one type of monomer unit. Copolymers often biodegrade faster than homopolymers due to the reduced crystallinity and improved flexibility achieved by adding comonomers containing hydrolyzable groups. However, the biodegradability of copolymers can be decreased when comonomers with a rigid aromatic structure are added. Hence, the biodegradability of copolymers strongly depends on the flexibility and linkages of the comonomers. Wu et al. [36] investigated the biodegradability of poly (butylene succinate) (PBS) and a series of its copolymers, poly[(butylene succinate)co-poly(tetramethylene glycol)]s (PBSTMGs) (Fig. 9) with varying poly(tetramethylene glycol) (PTMG) content. It was found that the biodegradation rates were significantly improved with the addition of more PTMG. This could be attributed to the introduction of hydrophilic soft segment PTMG that decreased crystallinity and improved the hydrophilicity and flexibility of copolymers. Besides copolymers, polymer blends are another alternative to obtain materials with enhanced properties. Polymer blends are a class of materials formed by blending two or more polymers. Depending on the components of the blends, biodegradation efficiency can be influenced in different ways. A biodegradable blend is PLA/PHA blends with reduced crystallinity and improved biodegradability [37]. Poly-ε-caprolactone/thermoplastic starch blends (PCL/TPS) and PLA/PCL blends are two other examples of biodegradable polymer blends [38]. The chemical structures of these polymers are presented in Fig. 10. In the industry, additives are often added to modify the properties of polymers, including their biodegradability. Wilkes and Aristilde [40] stated that prooxidant additives introduce carbonyl groups into PE, increasing their hydrophilicity and generating lower molecular weight components. Thus, PE added with prooxidants is more biodegradable than pure PE. In contrast to prooxidants that enhance the biodegradation rate, chain extenders inhibit the onset of PHA biodegradation by increasing the molecular weight of the polymer and inhibiting the extracellular enzyme activity [37]. As additives, the presence of cross-linking agents can either increase or decrease the biodegradation rate. Dalev et al. [41] found that gelatin cross-linked with hexamethylene diisocyanate degraded faster than pure gelatin, and more significant weight loss was observed. Using its six methylene groups,

Carbon chain backbones

Functional groups Hydrolyzable ester bonds

Polyvinyl chloride (PVC)

Polyethylene (PE)

Polypropylene (PP)

Polystyrene (PS)

Polyethylene terephthalate (PET)

Polymers Polyurethane (PU)

Structural formula

Table 1 Examples of polymers with hydrolyzable ester bonds and carbon chain backbones

32 L. M. Koh and S. M. Khor

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Fig. 9 Chemical structure of poly[(butylene succinate)-co-poly(tetramethylene glycol)]s (PBSTMGs). (Adapted with permission from Ref. [36], Copyright 2017, American Chemical Society) Fig. 10 Chemical structures of (a) polyhydroxyalkanoates (PHA) (where x¼1–8 and n¼100–1000), (b) polylactic acid (PLA), and (c) polycaprolactone (PCL). (Adapted with permission from Ref. [39], Copyright 2016, Nature)

hexamethylene diisocyanate contributed to relatively long cross-linkages in the polymer, making the polymer more flexible and thus allowing easier access of the polymer to water and enzyme. Yue-Hong et al. [42] studied the effects of crosslinking on the biodegradability of soybean protein-based composites and found that the combination cross-linker of glyoxal and polyisocyanate (PMDI) improved the inter- and intramolecular interactions in the structure and decreased the biodegradation. All the factors affecting biodegradation discussed above are summarized in Table 2. In general, any factors that favor microbial growth and the accessibility of polymers to microbes and water will increase the susceptibility of the polymers to biodegradation. To sum up, low molecular weight, amorphousness, better flexibility, and hydrolyzable functional groups can increase the biodegradation rate of polymers. The biodegradability of copolymers, polymer blends, and polymers with additives mainly depends on the nature of the components, which affects their flexibility, crystallinity, and molecular weight. One or more factors must be considered to study the rate of biodegradation as they are interrelated and affect one another.

Classification of Biodegradable Polymers Biodegradable polymers can be classified based on their chemical composition, processing method, origin, synthesis method, economic importance, application, etc. [44]. The most common classification of biodegradable polymers is based on

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Table 2 Factors affecting biodegradation Factors 1. Abiotic factor (a) Temperature

(b) Moisture

(c) pH

(d) UV light

2. Biotic factor (a) Extracellular enzymes (b) Hydrophobicity

3. Polymer characteristics (a) Molecular weight

(a) Conformational flexibility

(b) Crystallinity

(c) Functional group

Effects on the biodegradation rate ● Higher temperatures contribute to greater enzyme activity and a higher biodegradation rate ● Excessive supply of heat over the optimum temperature may cause rapid degradation of the enzymes, lowering the biodegradation rate ● High moisture speeds up the hydrolysis reaction, causing more polymer chain scission and enhancing microbial colonization, ultimately leading to a higher biodegradation rate ● Important in an anaerobic soil environment by enhancing oxygen exchange between soil pores and the atmosphere ● The biodegradation rate is maximal at the soil pH optimal for microbial growth, i.e., at pH 7.4 for most microbes ● However, a greater biodegradation rate can also be achieved at a weakly acidic pH, which is suitable for the growth of fungi ● Influences photodegradation ● Increases the biodegradation rate by provoking main chain scission ● Decreases the biodegradation rate by inducing cross-linking ● Affects hydrolysis and oxidation ● Specific enzymes are needed for the biodegradation of a certain polymer ● Hydrophobic microbes can form biofilms on the polymer, facilitating biodegradation ● Hydrophobic cells tend to adsorb more strongly to hydrophobic surfaces ● Hydrophilic cells adsorb strongly to hydrophilic surfaces ● The biodegradation rate of a polymer decreases with its increasing molecular weight ● Microorganisms can only assimilate low molecular weight fragments of polymers ● Polymers with either bulky side groups and/or saturated bonds are less flexible ● The rate of biodegradation increases with polymer flexibility due to improved access to microbes and water ● Amorphous polymer regions have a higher biodegradation rate than crystalline polymer regions ● Greater access to water and microbes in amorphous regions, while the diffusion of water is hindered in crystalline regions ● The presence of hydrolyzable functional groups in polymers increases the biodegradation rate ● Hydrolyzable functional groups provide sites for hydrolysis and increase the flexibility of the chains

Reference [43]

[23]

[15]

[23]

[24]

[12]

[23]

[23]

(continued)

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Table 2 (continued) Factors (d) Copolymers

(e) Blend (f) Additives

Effects on the biodegradation rate ● The biodegradability of copolymers strongly depends on the flexibility and functional groups of the comonomers ● Enhanced biodegradability of polymers is achieved by incorporating comonomers with hydrolyzable groups ● Lower biodegradability when comonomers with rigid aromatic structures are added ● The influence on biodegradation rate depends on the components of the polymer blends ● The presence of prooxidants increases the rate of biodegradation, while the presence of chain extenders in the polymers inhibits it ● The presence of cross-linking agents can either increase or decrease the rate of biodegradation

Reference [23]

[37]

their origin and synthesis process. As stated in literature [44], they can be categorized into natural (or bio-based) and synthetic (or petrochemical-based). Natural biodegradable polymers or biopolymers are formed naturally from natural origins (plants, animals, or microorganisms) during their growth cycle [44]. Within the living cells, these polymers are usually synthesized from polymerizing activated monomers, catalyzed by enzymes secreted by microorganisms. These polymers can then be subdivided into biopolymers extracted directly from biomass (e.g., polysaccharides, proteins or polypeptides, and lipids) and biopolymers produced directly by natural or genetically modified organisms (e.g., PLA and PHAs) [44]. In industries and laboratories, synthetic or petrochemical-based biodegradable polymers are produced via polymerization (e.g., chain growth, step growth, and ring-opening polymerization) [45]. They mainly consist of aliphatic polyesters, aliphatic/aromatic co-polyesters, and polyvinyl alcohol (PVA) [14]. Aromatic polyesters are excluded from synthetic biodegradable polymers since they are highly stable and resistant to microbial attack, which is contributed by many aromatic rings [46]. The classification of biodegradable polymers is illustrated in Fig. 11. Natural biodegradable polymers offer advantages over synthetic biodegradable polymers due to their renewability, while synthetic polymers are often made from oil, which is nonrenewable. However, some synthetic polymers are made from renewable resources. For instance, PLA is synthesized from agricultural resources via fermentation. These polymers may also be semisynthetic polymers derived from natural resources but undergo chemical modifications to obtain the desired properties. Semisynthetic polymers overcome the shortcomings of natural polymers, for instance, poor mechanical properties and difficulty in processing, while possessing outstanding properties such as high biocompatibility, low toxicity, and enhanced biodegradability [45]. Avérous and Pollet [48] proposed a different classification method according to their synthesis process, where biodegradable polymers are first categorized into agro-polymers and biodegradable bio-polyesters. Agro-polymers are extracted

Fig. 11 Classification of biodegradable polymers based on their origin and production method. (Adapted with permission from Ref. [47], Copyright 2019, AIMS Press)

36 L. M. Koh and S. M. Khor

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from biomass, such as starch and cellulose. Polymers formed by microbial actions (e.g., PHA), polymers chemically synthesized from monomers derived from natural resources (e.g., PLA), and polymers derived from nonrenewable fossil resources are all examples of bio-polyesters [48]. Biodegradable polymers can also be categorized according to their biodegrade capacity, as Karak mentioned [15]. There are two types: completely biodegradable and partially biodegradable polymers [15]. The capacity of biodegradable polymers to biodegrade is closely related to their origin, in which the completely biodegradable polymers are mostly natural or naturally derived. In contrast, the partially biodegradable polymers are usually synthetic polymers and blends of natural and synthetic polymers. Based on the main functional groups, usually hydrolyzable backbones present in the entirely biodegradable polymers, they can be subdivided into polysaccharides, polypeptides, polyesters, lipids, natural rubber, and natural composite. Besides, completely biodegradable polymers can also be subdivided based on their origins; either they are derived from fossil-based or bio-based materials, or they are blends of fossil- and bio-based polymers [46]. Polyesters, polyureas, PU, polyamides (PA), PVA, polyethylene glycol (PEG), and some composite materials are examples of partially biodegradable polymers [15]. Table 3 compares and contrasts the three classification methods mentioned above regarding their advantages and disadvantages. Apart from biodegradable polymers, another important class of biodegradable materials is biodegradable polymer composites (BPCs) or biocomposites. Polymer composites are made up of multiple components with two or more phases, usually consisting of matrix or fillers. In most composites, fillers, as the minority phase, are embedded in a continuous matrix, reinforcing the materials [49]. Biocomposites can be divided into partially biodegradable and completely biodegradable biocomposites. In completely biodegradable composites, the matrices must be made of biodegradable polymers, either from renewable or petrochemical-based biopolymers. At least one of the phases, either fibers or matrix, must be bio-based in partially biodegradable biocomposites [49]. Besides partially and completely biodegradable composites, hybrid composite materials are also included as biocomposites, and they are polymer blends reinforced with one or more fibers [49]. A schematic presentation of the classification of biocomposites is shown in Fig. 12.

Industrial Applications of Biodegradation Biodegradation is not merely naturally occurring; it is similarly applied in the industry on a commercial scale. In the industry, biodegradation is critically significant in addressing soil and water pollution due to the discharge and buildup of contaminants such as petroleum hydrocarbons and solid waste [50]. Biodegradation is paramount in environmental conservation and preservation because it can break down and detoxify organic contaminants. The removal of contaminants using the biodegradation approach is often known as microbial bioremediation or simply as

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Table 3 Comparison of biodegradable polymer classification methods Method of classification Natural (or bio-based) versus synthetic (or petrochemical-based)

Advantages ● The easiest and most direct classification method ● The origins and synthesis methods of biodegradable polymers are known

Agro-polymers versus biodegradable polyesters/ bio-polyesters

● It provides information on the synthesis methods of polymers ● The biodegradability of polymers is known ● Based on their biodegradability, the polymers’ properties, chemical structure, and origins may be predicted ● Useful in selecting the appropriate polymers for processing into end products in the industry

Completely biodegradable polymers versus polymers that are partially biodegradable

Disadvantages ● Biodegradable polymer formulations that combine both classes exist, such as starch-based blends (e.g., starch-polyvinyl alcohol) and cellulose-based blends (e.g., cellulose-lowdensity polyethylene) ● The distinction between renewable and nonrenewable polymers is frequently misunderstood. The origins of the polymers do not always reflect the renewability of their structure ● Lack of information on the properties of polymers

Reference [44]

● Biodegradation tests are necessary to measure the biodegradability of polymers, making it the most complex classification method ● The precise standards and definitions for the terms “partially biodegradable” and “completely biodegradable” do not exist

[15]

[48]

bioremediation from the environmental perspective. Bioremediation relies upon biological systems to detoxify, transform, and mineralize contaminants to an innocuous state, with the ultimate purpose of cleaning up and restoring contaminated sites. Bioremediation techniques can be classified into two major types: in situ and ex situ bioremediations, which differ in treatment sites. In situ bioremediation refers to the removal of contaminants at polluted sites. In contrast, ex situ bioremediation requires the excavation of the contaminants from the contaminated sites and successive transportation of them to other sites suitable for biological treatment [51]. The operational differences result in relatively low transportation costs for in situ bioremediation and little or no disruption to soil construction [52]. Various practices for in situ and ex situ bioremediation are presented in Table 4. This section emphasizes the bioremediation of crude oil with

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Fig. 12 Classification of biocomposites. (Adapted with permission from Ref. [49], Copyright 2020, MDPI)

practices, i.e., biostimulation and bioaugmentation. These bioremediation practices are among the two most known and major strategies in crude oil bioremediation [53]. Some other practices (e.g., bioreactors, phytoremediation, land farming, bioattenuation, etc.) are also applied to remove crude oil hydrocarbons. However, they are excluded from this discussion due to their comparatively limited applications. Indigenous microorganisms, readily available on the contaminated site and containing genes with inherent degradation capabilities, can be employed in bioremediation. They are significant in intrinsic bioremediation, natural attenuation, or bioattenuation. Intrinsic bioremediation is the aerobic or anaerobic degradation of organic contaminants by indigenous microorganisms without any external force (e.g., supplementing external microorganisms or nutrients) to enhance the process efficiency [51]. Apart from indigenous microorganisms, increasing environmental pollution has led to the field applications of nonindigenous microorganisms, including genetically modified microorganisms, to treat contaminated sites [55]. Bioremediation possesses greater advantages than conventional treatment methods such as incineration, landfilling, and catalytical cracking in waste management. Bioremediation is a green technology that is eco-friendly and cost-saving [58]. The biological treatment of contaminants is thus an effective method to solve water and soil pollution stemming from the human population explosion and excessive anthropogenic activities. Like biodegradation, the selection and efficiency of bioremediation technologies are affected by various factors. These include the nature of

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Table 4 Examples of in situ and ex situ bioremediation practices Type of bioremediation In situ bioremediation

Bioremediation practices a) Biosparging

b) Bioattenuation

c) Bioventing

d) Bioaugmentation

e) Biostimulation

f) Phytoremediation

g) Bioslurping

Ex situ bioremediation

a) Bioreactors

b) Land farming

c) Composting

d) Biopiling

Basic principles Aeration of the saturated zone to promote the volatilization of volatile organic compounds into the upper unsaturated zone for biodegradation, meanwhile triggering natural biodegradation for the removal of contaminants from the contaminated site Aerobic or anaerobic degradation of organic contaminants by indigenous microorganisms without any external force (e.g., the addition of external microorganisms or nutrients) to enhance the process efficiency Air and nutrients are injected into the unsaturated zone through a well to trigger indigenous microbial activities Enhanced biodegradation via the addition of external oil-degrading microorganisms when the indigenous oil-degrading microorganisms are scarce Stimulation of indigenous microbial growth by adding nutrients, electron donors and acceptors, and surfactants Interactions between plants and rhizospheric microorganisms immobilize contaminants on the roots or translocate them to shoots or leaves Vacuum extraction of contaminants in the groundwater and subsoil while promoting aerobic biodegradation in the unsaturated zone via bioventing A reaction vessel in which the polluted samples are homogenized, amended with nutrients, and aerated under controlled environmental parameters Excavation and spreading of contaminated soil over a specific area are followed by periodic nutrient amendment and aeration to promote aerobic biodegradation Mixing raw organic wastes with contaminated soil to produce stable, nutrient-rich products suitable as a soil amendment A modified land farming bioremediation involves excavation and stacking of contaminated soils above ground, and

Reference [51]

[51]

[51]

[54]

[54]

[55]

[56]

[55]

[55]

[57]

[51]

(continued)

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Table 4 (continued) Type of bioremediation

Bioremediation practices

e) Windrows

Basic principles operations such as aeration, nutrient amendment, and irrigation are conducted in a treatment bed to enhance the growth of aerobic and anaerobic microorganisms A modified biopiling bioremediation involves periodic rotation of piled contaminated soil for irrigation and aeration so that the contaminated soil is homogenized with nutrients, besides ensuring the uniform distribution of water and air over the soil

Reference

[56]

contaminants (e.g., toxicity, solubility, chemical properties, and concentration of substrates) and environmental conditions of the contaminated sites such as pH, oxygen and nutrient availability, temperature, and soil texture and permeability [59]. Besides, the commercial application of bioremediation must also consider the expenditures involved [51]. Aside from these factors, microorganisms with the desired degradation genes and capabilities are the most critical ingredient that ensures the feasibility of bioremediation [58]. Besides detoxifying toxic waste, biodegradation has also emerged as a powerful and sustainable platform for forming biogas and nutrient-rich digestate as anaerobic biodegradation’s end product [50]. Due to their promising properties and economic significance, there have been extensive studies and wide applications of biogas and digestates, mainly in energy recovery and agricultural industries, respectively.

Bioremediation of Crude Oil The decomposition and mineralization of oil hydrocarbons occur mainly through microbial biodegradation. Hence bioremediation is extensively practiced in removing crude oil and petroleum from soil and marine environments. The entry of oil hydrocarbons into the environment is confined to crude oil spills and may also be due to the discharge of petroleum refinery effluents from industrial activities [54]. A large-scale oil spill can have devastating effects on marine ecosystems, and exposing aquatic organisms to hydrocarbons may cause acute toxicity and mortality among aquatic species. In 1989, cleanup of oil spills via bioremediation was successfully applied to the Exxon Valdez spill. The bioremediation was performed by adding fertilizers (i.e., biostimulation) which enhanced the biodegradation rates of oil hydrocarbons by indigenous microorganisms [60]. Many studies have been carried out to analyze the biodegradation of hydrocarbons, encompassing both aliphatic and aromatic hydrocarbons, and many of these compounds can be biologically degraded or completely

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mineralized. Across the studies, common hydrocarbon substrates are benzene, toluene, ethylbenzene, and xylene (BTEX) because they are relatively resistant to biodegradation among aromatic compounds and their adverse effects on human health via ingestion, inhalation, and skin contact [61]. Hence, the successful biodegradation of these compounds is significant. Chiu et al. [62] asserted that BTEX and methyl tert-butyl ether were effectively removed from groundwater contaminated with gasoline oil via intrinsic bioremediation using naturally occurring microorganisms. The effectiveness of the bioremediation process was greater with increasing concentrations of the contaminants. The highest biodegradation capabilities of the contaminants were observed in the spill location due to (1) the great carbon content, which increased the bacterial populations, and (2) increased bacterial diversity since some of the bacterial species responsible for the contaminant biodegradation were only found in this area. Biostimulation stimulates indigenous microbial growth by adding nutrients, electron donors and acceptors, and surfactants [54]. External oil-degrading microorganisms are added when the indigenous counterparts are scarce in bioaugmentation. Wu et al. [53] have evaluated the efficiency of these two strategies for degrading petroleum hydrocarbons. It was shown that biostimulation is a more efficient bioremediation strategy with greater removal efficiency. The lower efficiency of bioaugmentation was due to the reduced growth of indigenous microbes and a narrowing of microbial diversity as the added oil-degrading microbes predominated. The opposite finding also suggests that bioaugmentation was more effective in the bioremediation of contaminated crude oil seawater. This happens when the recovery of the contaminated site is favored by the presence of a single microbial species, and the microbial consortium suppresses the bioremediation. To complement the limitations of both strategies, biostimulation and bioaugmentation have also been shown to accelerate the bioremediation process. From these studies, it is clear that the efficiency of bioremediation technologies varies from site to site. Therefore, the characteristics of the contaminated sites should be well understood before implementing bioremediation. Hydrocarbon-degrading microorganisms, residual total petroleum hydrocarbon composition, and soil and sea physiochemical properties must be evaluated. To reduce risks associated with the failure of bioremediation in oil-contaminated sites, lab or bench-scale experiments should be carried out first before the bioremediation at pilot scale and ultimately at full scale. At the lab or bench scale, process optimization is often conducted by adjusting and selecting the appropriate process parameters that could show the maximum contaminant degradation rate and extent. Furthermore, preliminary tests before full-scale application of bioremediation are needed for determining the operational expenditures and evaluating the cost-effectiveness of the process. Successive scaling-up processes help to decide the commercial viability of the bioremediation strategy in the treatment of oil-contaminated sites and avoid money wastage due to failure or ineffectiveness of bioremediation. From the environmental perspective, bioremediation shows greater advantages than physical and chemical treatments of oil spills. The physical treatment method, in situ burning of spilled oil, is easier to perform. However, their particulate and

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carbon dioxide emissions may degrade air quality and contribute to air pollution [63]. Removal of oil spills performed with chemical dispersants may cause adverse effects due to the toxicity of the dispersants. In contrast, bioremediation will not lead to secondary pollution, making it an eco-friendly treatment option [63]. Moreover, the expenses associated with the physical and chemical remediation methods are higher than for bioremediation. However, a research gap exists in bioremediation using engineered microorganisms. The potential long-term effects and the persistency of engineering microorganisms are unclear [64]. Mutations of microorganisms may be possible, potentially forming novel chemical substances that threaten the environment. Therefore, risk assessments for identifying the hazards of oil spill response technology are important to avoid catastrophic effects on living populations in the contaminated sites. In selecting the treatment technology, environmental safety should be prioritized instead of the economic costs.

Industrial Applications of Anaerobic Digestions Worldwide, many anaerobic digesters have been established and put into operation [65]. Anaerobic digestion is when the organic substrate is decomposed when oxygen is deficient or absent. The main outputs of anaerobic digestion are biogas and nutrient-rich residues, called digestate [66]. In anaerobic digestion, four main stages are involved: hydrolysis, fermentation or acidogenesis, acetogenesis, and methanogenesis [67]. The operation of anaerobic digestion consists of substrate loading, pretreatment, and the digestion process, as presented in Fig. 13 [68]. In addition, a posttreatment of biogas and digestates is needed to transform them into usable products that comply with the standards and regulations. In the industry, anaerobic digestion systems are often found in single-phase and two-phase anaerobic digestion. In a single phase, anaerobic digestion system, the four main stages of digestion are all carried out in the same reactors. In contrast, the two-stage anaerobic digestion is more sophisticated, has the methanogenesis process separated from other stages, and works individually. Often, anaerobic digestion is considered a sustainable process in the industry, and its main applications include organic waste treatment, energy recovery, and the

Fig. 13 Anaerobic digestion process flow diagram. (Adapted with permission from [68], Copyright 2019, SAGE Publications)

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production of biofertilizers. Their sustainability is mainly contributed to by the availability of a wide array of suitable substrates as feedstock. These include agricultural wastes, animal manures [69], food wastes, sewage sludge [70], and municipal solid wastes [71]. The applications of anaerobic digestion subsequently result in some general environmental benefits, such as waste minimization, mitigation of greenhouse gases, and soil quality improvement.

Organic Waste Treatment and Resource Recovery According to the World Bank, the annual global generation of municipal solid waste in 2016 reached approximately 2 billion tons, with one-third of the waste not handled properly and safely. By 2050, global waste production is estimated to be elevated to 3.40 billion tons (Fig. 14) [72]. Negative environmental impacts are frequently reported for massive waste generation, closely related to poor management [73, 74]. The end-of-life options for municipal solid waste are landfilling, open dumping, and treatment via modern incineration. Meanwhile, recycling and composting offer alternative options for material recovery [75–79]. Inadequate waste management can lead to issues comprising water contamination, flooding, and landscape changes. Hazardous effects are not limited to the environment, but human and animal health may be threatened due to vector transmission, inhalation of airborne particulates, and unintentional ingestion of toxic wastes [78–81]. An emerging concern for sustainable waste management has led to enhanced anaerobic digestion applications for organic disposal and recovery of organic wastes. Nevertheless, the decomposition of some organic wastes in an anaerobic digester is difficult to control, such as municipal solid wastes, specifically lignocellulosic

Fig. 14 Projected global waste generation. (Adapted with permission from Ref. [72], Copyright 2018, World Bank)

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wastes [75]. This happens due to the high solid content, large particle size, and heterogeneity of the waste [75]. On the other hand, municipal solid wastes have a high organic and moisture content, making them suitable substrates for anaerobic digestion. Food waste, kitchen leftovers, and lignocellulosic wastes (yard wastes) make up most of the organic content of municipal solid wastes [82]. Chen et al. [83] demonstrated that food wastes taken from five different sources (e.g., soup processing plant, cafeteria, commercial kitchen, fish farm, and grease trap collection service) were able to be digested in a centralized anaerobic digester at both mesophilic and thermophilic temperatures using batch and continuous digestion modes. Figure 15 shows the treatment of food waste using anaerobic digestion, coupled with the generation of electricity from biogas and fertilizer from digestate. Advancement is also witnessed by co-degrading food waste with other substrates anaerobically to maximize biogas yield. In anaerobic co-digestion, two or more feedstocks are broken down concurrently in a single digester [84]. For example, simultaneous digestion of food waste and lignocellulosic waste in the anaerobic digester improved the stability of the system and enhanced the methane yield. The research found that thermophilic co-digestion of food waste, corn stover, and prairie cordgrass provided a large methane yield. Anaerobic digestion plays a remarkably important role in wastewater treatment plants by turning sludges into biogas and digestate. Hallaji et al. [86] studied the anaerobic digestion of pretreated, mixed activated sludges and primary sludges collected from wastewater treatment plants. In the study, mixed sludges are pretreated with individual and combined free nitrous acid and Fenton reactions. These pretreatment methods led to a substantial increase in methane production and a significant decline in chemical oxygen demand, with the greatest effects being

Fig. 15 Anaerobic digestion is used to treat food waste. (Adapted with permission from Ref. [85], Copyright 2021, Elsevier)

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observed for the combined pretreatment (the methane production increased by about 72%, and the removal efficiency of chemical oxygen demand increased to 59% compared to 34% for the control). The pretreatment approaches released free radicals (e.g., peroxide (H2O2), peroxynitrite (ONOO), nitrogen dioxide (NO2), and hydroxide ion (OH)), which deranged the substrates’ cell walls and extracellular polymeric substances, subsequently expanding soluble fractions of protein and polysaccharides. As a result, the sewage was more biodegradable and contributed to enhanced methane yield. An additional benefit that can be gained from applying anaerobic digestion is that wastewater treatment plants can operate using the electricity supplied from biogas processing. In other words, no external power supply is needed for the daily operation of the plants, and operational costs are lower. These conditions increase the commercial feasibility of anaerobic digestion in wastewater treatment plants.

Production and Applications of Biogas Increased urbanization has contributed to a massive energy demand for supplying heat and electricity to support various economic and industrial activities. The increasing energy consumption results in the exploration and extraction of fossil fuel resources and eventually exhaustion of these unrenewable resources. In 2018, almost 80% of the primary energy supply came from fossil fuel resources such as coal, crude oil, and natural gas, while renewable energy only contributed 14% [87]. Therefore, many countries have invested in exploring renewable energy and constructing renewable power plants to ensure a sustainable energy source that can support human activities. Therefore, biogas produced from anaerobic digestion is emerging as a renewable and ideal energy source that can be utilized to supply heat and electricity. Biogas is constituted mainly of methane, followed by carbon dioxide, while trace gases (e.g., ammonia (NH3), nitrogen (N2), hydrogen sulfide (H2S)) and water vapor are also found [66]. However, the exact contents of biogas vary according to organic substrates used in anaerobic digestion and the operational parameters of the digesters. An anaerobic digestion is more viable than sanitary landfills in producing heat, energy, and power from biogas. Landfilling releases greater levels of greenhouse gases and leachates than anaerobic digestion. Therefore, anaerobic digestion is an eco-friendly method used in energy recovery, while contributing to pollution control. Enhanced biogas production can be achieved via anaerobic co-digestion of two or more different biomasses. The anaerobic co-digestion of several different lignocellulosic biomass was significantly greater (20–35% increase in the biogas yield) than the mono-digestion [69]. Anaerobic co-digestion offers several other advantages over mono-digestion. One of the benefits is the ability of co-digestion to dilute inhibitory chemicals such as ammonia, sulfate, and organics, which are often found in feedstocks in large quantities and could reduce or inhibit microbial population or growth and ultimately lead to a decrease in methane gas production [84]. Using co-digestion, recalcitrant substrates or those with low nutrient levels can degrade, while accelerating methane production. Other advantages of anaerobic co-digestion include increased stability of the digestion process, improved nutrient balance [88],

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and regulation of digestion pH [84]. Given its superior performance, anaerobic co-digestion is gaining more attention from researchers and is recognized as an efficient technology for producing biogas. The posttreatment of biogas is necessary for the upgrading to biomethane. Biogas upgrading helps remove undesired carbon dioxide and other trace gases, while producing biomethane with greater methane content (80–95%) [89]. Biomethane has a greater calorific value and energy content (approximately 10 kWh/m3), imparting better economic values than raw biogas [65]. Biomethane is considered a renewable natural gas used as vehicular biofuel for the transportation sector. According to Global Bioenergy Statistics by the World Bioenergy Association [87], liquid biofuels and biogas represent the most suitable and sustainable energy sources for the transportation sector, despite crude oil and oil products dominating the energy sources. Besides, biomethane can also substitute traditional solid biomass as a clean cooking oil. Biogas upgrading also promotes the mitigation of carbon dioxide emissions due to the lower carbon dioxide content in biomethane. Biogas conversion into biomethane is carried out via a chemoautotrophic carbon dioxide bioconversion process, mediated by hydrogen gas as the electron acceptor. Methanogens are the microorganisms responsible for the biotransformation of carbon dioxide into methane, either through direct conversion of carbon dioxide into methane or through indirect production of methane from the intermediate acetic acid. To maximize the biogas yield, the establishment of a biogas plant requires careful consideration of several operational parameters (e.g., temperature, mixing modes [71], pH [69], etc.) and the appropriate concentration of substrates [69]. Other approaches to increasing biogas yield include pretreatment and the addition of additives (e.g., biochar). Nava-Valente et al. [90] studied the effects of thermal and acid pretreatments on anaerobic digestion of coffee pulp and discovered that both methods aided in the hydrolysis process, favoring organic matter solubilization and thus improving biodegradability and biogas yield.

Production and Applications of Digestate Digestate is well characterized by its multielement properties and high nutrient and organic content. Another by-product of anaerobic digestion, nutrient-rich digestate, shows its main applications in the agricultural sector. Compared to biogas, fewer investigations encompass the properties and applications of digestates. Nevertheless, digestates still show few promising properties that endow them with agronomic values. Only in the last decade, the first study investigating the properties of digestates was demonstrated [91]. It was found that the digestate possesses the greatest nutrient content (i.e., nitrogen (N), phosphorus (P), potassium (K)) in their available forms (i.e., NH3, P2O5, and K2O, respectively) over other biomasses, namely, compost, estates, and digested sludge. This was confirmed by a later study [92], which showed that the digestate from anaerobic digestion contained a significant number of macronutrients, particularly N, P, K, and Ca, and a trace amount of micronutrients (e.g., iron, zinc, manganese, copper, lithium, strontium, and barium). From the same study, high organic matter content was also found in the digestate. Based on these two studies, the high nutrient and organic contents outline the

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potential of digestate as an effective biofertilizer that provides nutrients for plant growth. In a similar study [91], digestates show promising amendment properties justified by their biological stability [51]. The applications of soil amendments affect the soil properties by increasing water retention capacity and improving aeration, meanwhile decreasing bulk density. The combined applications of thermophilic digestate and solarization (which boosts passive solar heating by covering the soil with a plastic tarp) increased weed seed mortality and water retention capacity [93]. In another study, Calamai et al. [94] evaluated the application of biochar generated from the pyrolysis of solid digestate as the soil amendment for Pelargonium graveolens. The study mentioned that the biochar reduced the bulk density by 52% and increased the pH from pH 7.1 to pH 8.5, significantly eliminating nutrient stress on plant growth in acidic soil. Besides the common applications in the agricultural field, nutrient-rich digestates show surprising roles in ecological restoration. Asaoka et al. [95] revealed that a fertilization composite consisting of an anaerobic digestate combined with blast furnace cement released solubilized, inorganic nitrogen, which was then taken up by the primary producers in the marine ecosystem, microalgae, and seaweed. The nutrient uptake promotes the growth of the primary producers, which are significant in overcoming cultural oligotrophication of the marine system owing to nutrient deficiency [95]. Despite the practical applications of digestates, criticism arises regarding their applications since organic contaminants are found in the digestates. These contaminants can cause damage to the environment when they are applied as soil fertilizers. For instance, Ali et al. [96] identified several organic contaminants of emerging concern (e.g., acetaminophen, caffeine, flame-retardant substances, octocrylene, etc.) in the solid and liquid digestates collected from biogas production plants. Therefore, other technologies must be implemented in synergy with anaerobic digestion for the decontamination of the digestates directly after their production. In some European countries, digestates without posttreatment are prohibited as soil amendment and fertilizer. Instead, posttreatments such as pasteurization or sterilization and composting are essential for ensuring the generation of high-quality digestates that meet the regulations [97]. To sum up, anaerobic digestion can be part of the circular economy. Anaerobic digestion creates further value from various organic wastes by turning them into value-added fertilizers, soil amendments, and biomethane. The paradigm shift from landfilling to anaerobic digestion also leads to waste and pollution minimization with the mitigation of greenhouse gas emissions. A summary of the industrial applications of biodegradation is illustrated in Fig. 16. Although extensive studies on anaerobic digestion have been conducted, most are on a laboratory scale, with fewer pilot-scale and full-scale studies due to cost and time constraints. The complexity of substrates’ composition and properties complicates their mineralization and makes larger-scale experiments difficult to carry out. Besides, waste treatment using anaerobic digestion remains low due to a lack of legislation and regulations that promote this technology, except in the European Union and California. Both the European Union and California enforce the diversion

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Fig. 16 Industrial applications of biodegradation

of organic waste from landfills and encourage the utilization of anaerobic digestion. Nevertheless, the commercial application of anaerobic digestion is still impeded by several challenges. Anaerobic digestion is a comparatively new technology, and special skills and experiences are required for its operation and maintenance. Besides, an anaerobic digestion plant is characterized by complicated mechanical equipment that needs higher energy demand [98]. Both pretreatment and posttreatment contribute to higher energy demand and manufacturing costs despite increasing the biogas yield.

Conclusions Increasing environmental awareness has led to rapid progress in biodegradation and the invention of new biodegradable materials. Biodegradation is composed of abiotic and biotic processes which degrade and mineralize biodegradable materials, either under aerobic or anaerobic conditions. In abiotic degradation, these materials are degraded by exposure to external forces (mechanical degradation), UV radiation (photodegradation), chemicals (chemical degradation), and heat from elevated temperatures (thermal degradation). In contrast, biotic degradation is commenced with the biofilm formation on the substrate’s surface (via biodeterioration), which is then fragmented (via depolymerization), assimilated into microbial cells (via bioassimilation), and eventually mineralized to give the final metabolites. The biodegradation rate is influenced by polymer characteristics and abiotic and biotic environmental conditions. An enhanced rate is obtained with the increased

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accessibility of polymers to microbes and water and under conditions that favor microbial growth. The three classification methods discussed provide different types of information regarding biodegradable polymers, such as their origins, synthesis methods, and biodegradability. However, these methods are restricted by limited information and complex and confusing classification. The industrial applications of biodegradation are obvious in oil bioremediation and the establishment of anaerobic digestion plants. Biostimulation and bioaugmentation are utilized to remove oil hydrocarbons via the addition of nutrients and external oil-degrading microbes, respectively. Anaerobic digestion is important for organic waste treatment, such as disposal of municipal solid wastes and removal of sludges from wastewater treatment plants, besides producing biogas and digestates, which are mainly used for energy recovery and as biofertilizers, respectively. The efficacy of anaerobic digestion is primarily reflected by the feedstock’s biogas yield and degradation rate, which can be improved with co-digestion, additives, pretreatment, and posttreatment.

Future Perspectives Despite the expanding penetration of biodegradable materials into the market, the term “biodegradable” remains vague, and many still believe the myths which biodegradable materials can be fully degraded without any remaining residues. Confusion also arises due to the diversity of standards for biodegradable materials under different environments with distinct testing procedures and degradation rates within a certain incubation period. Because of this, a standardized and globally accepted definition for biodegradable providing information on the degradation test method and environment, biodegradability, and the duration of decomposition should be established. Furthermore, we recommend intense studies on biodegradation behaviors under the influence of two or more factors (e.g., temperature, enzymes, molecular weight, etc.) to investigate how one factor influences another and their combined effects on the degradation behavior of the substrates. These studies should then be utilized for controlling the biodegradation process in the natural environment, which is altered by fluctuating environmental factors (e.g., temperature, pH, moisture, UV light intensity, etc.). Besides, the factors should also be considered in the design and operation of bioremediation strategies and anaerobic digesters. Knowledge encompassing these factors should be incorporated into the process optimization of an anaerobic digester to maximize the biogas and digestate yields. Driven by technological advancement, future studies can be implemented by utilizing computational modeling to study the biodegradation mechanisms and predict the biodegradability of biodegradable materials. This approach is timesaving and low cost [99]. Cheng et al. [99] used an in silico method to predict the chemical biodegradability of 1631 chemicals, classify them into readily and not readily biodegradable chemicals, and identify the privileged substructures of these chemicals. Using a mathematical model, Watanabe et al. [100] identified that the small polyethylene molecules are directly assimilated into microbial cells, while the

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large molecules are subjected to β-oxidation before bioassimilation. The advancement of anaerobic digestion can be achieved by using plastics as the feedstock for energy generation, while providing a sustainable end-of-life option. Moreover, future work on biodegradation can be expanded to heavy metals. As compared to plastic pollution, heavy metal pollution is a much more chronic environmental issue, threatening the ecosystem and human health. Heavy metals show a high degree of toxicity and persistency due to their recalcitrant nature and bioaccumulative potential. Hence, future studies of biodegradation should concentrate on the degradation of heavy metals and their bioremediation strategies by exploring novel heavy metaldegrading microorganisms 101, 102 using genomic approaches, such as 16S rRNA.

Cross-References ▶ Anaerobic Biodegradation: The Anaerobic Digestion Process ▶ Biodegradable Materials: Fundamentals, Importance, and Impacts ▶ Biodegradable Polymers ▶ Fundamentals of Biodegradation Process ▶ Mechanism of Microbial Biodegradation: Secrets of Biodegradation ▶ Plastics Biodegradation and Biofragmentation ▶ Role of Microorganisms in Biodegradation of Pollutants Acknowledgments This work was financially supported by the Fundamental Research GrantScheme (FRGS) from the Ministry of Higher Education of Malaysia (MOHE) (FRGS/1/2019/STG01/UM/02/6).

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50. Roopnarain A, Rama H, Ndaba B, Bello-Akinosho M, Bamuza-Pemu E, and Adeleke R (2021) Unravelling the anaerobic digestion ‘black box’: Biotechnological approaches for process optimization. Renewable and Sustainable Energy Reviews 152 51. Azubuike CC, Chikere CB, and Okpokwasili GC (2016) Bioremediation techniques–classification based on site of application: Principles, advantages, limitations and prospects. World Journal of Microbiology and Biotechnology 32(11):180 52. Sharma I, Bioremediation techniques for polluted environment: Concept, advantages, limitations, and prospects, in Trace Metals in the Environment - New Approaches and Recent Advances, MA Murillo-Tovar, H Saldarriaga-Noreña, A Saeid, Editors. 2021, IntechOpen. p. 221–236. 53. Wu M, Wu J, Zhang X, and Ye X (2019) Effect of bioaugmentation and biostimulation on hydrocarbon degradation and microbial community composition in petroleum-contaminated loessal soil. Chemosphere 237 54. Sayed K, Baloo L, and Sharma NK (2021) Bioremediation of total petroleum hydrocarbons (TPH) by bioaugmentation and biostimulation in water with floating oil spill containment booms as bioreactor basin. International Journal of Environmental Research and Public Health 18(5) 55. Zouboulis AI, Moussas PA, and Psaltou SG, Groundwater and soil pollution: Bioremediation☆, in Encyclopedia of Environmental Health J Nriagu, Editor. 2019, Elsevier: Oxford. p. 369–381. 56. Gogoi N, Baroowa B, and Gogoi N, Ecological tools for remediation of soil pollutants, in Bioremediation Science: From Theory to Practice, A Rakshit, M Parihar, B Sarkar, HF B. Singh, Leonardo Fernandes Editors. 2021, CRC Press. p. 57–78. 57. Kästner M and Miltner A (2016) Application of compost for effective bioremediation of organic contaminants and pollutants in soil. Applied Microbiology and Biotechnology 100 (8):3433–3449 58. Ojha N, Karn R, Abbas S, and Bhugra S (2021) Bioremediation of industrial wastewater: A review. IOP Conference Series: Earth and Environmental Science 796(1) 59. Mishra M, Singh SK, and Kumar A, Environmental factors affecting the bioremediation potential of microbes, in Microbe Mediated Remediation of Environmental Contaminants, A Kumar, VK Singh, P Singh, VK Mishra, Editors. 2021, Woodhead Publishing. p. 47–58. 60. Atlas RM and Hazen TC (2011) Oil biodegradation and bioremediation: A tale of the two worst spills in U.S. history. Environmental Science & Technology 45(16):6709–6715 61. Stasik S, Wick LY, and Wendt-Potthoff K (2015) Anaerobic BTEX degradation in oil sands tailings ponds: Impact of labile organic carbon and sulfate-reducing bacteria. Chemosphere 138:133–139 62. Chiu HY, Verpoort F, Liu JK, Chang YM, and Kao CM (2017) Using intrinsic bioremediation for petroleum–hydrocarbon contaminated groundwater cleanup and migration containment: Effectiveness and mechanism evaluation. Journal of the Taiwan Institute of Chemical Engineers 72:53–61 63. Khade S and Srivastava S, Genetically modified microbes for bio-remediation of oil spills in marine environment, in Bioremediation: Current Research and Applications AK Rathoure, Editor. 2017, IK International Publishing House: New Delhi, India. p. 275–292. 64. French KE, Zhou Z, and Terry N (2020) Horizontal ‘gene drives’ harness indigenous bacteria for bioremediation. Scientific Reports 10(1):1–11 65. Lai C-Y, Zhou L, Yuan Z, and Guo J (2021) Hydrogen-driven microbial biogas upgrading: Advances, challenges and solutions. Water Research 197 66. Gould MC and Kirk D, M. , Chapter 18 - Bioenergy and Anaerobic Digestion, in Bioenergy, A Dahiya, Editor. 2015, Academic Press: Boston. p. 297–317. 67. Rajin M, A current review on the application of enzymes in anaerobic digestion, in Anaerobic Digestion Processes: Applications and Effluent Treatment N Horan, AZ Yaser, N Wid, Editors. 2018, Springer Nature Singapore Pte Ltd. p. 55–70.

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68. Logan M and Visvanathan C (2019) Management strategies for anaerobic digestate of organic fraction of municipal solid waste: Current status and future prospects. Waste Management & Research 37(1):27–39 69. Buragohain S, Mahanta P, and Mohanty K (2021) Biogas production from anaerobic monoand co-digestion of lignocellulosic feedstock: Process optimization and its implementation at community level. Environmental Technology & Innovation 24 70. Kumar M, Dutta S, You S, Luo G, Zhang S, Show PL, Sawarkar A, Singh L, and Tsang DCW (2021) A critical review on biochar for enhancing biogas production from anaerobic digestion of food waste and sludge. Journal of Cleaner Production 305 71. Babaei A and Shayegan J (2019) Effects of temperature and mixing modes on the performance of municipal solid waste anaerobic slurry digester. Journal of Environmental Health Science and Engineering 17(2):1077–1084 72. Kaza S, Yao L, Bhada-Tata P, and Van Woerden F, What a waste 2.0: A global snapshot of solid waste management to 2050, in Urban Development Series 2018: Washington, DC. 73. El-Maghrabi HH, Nada AA, Soliman FS, Raynaud P, Moustafa YM, Ali GAM, and Bekheet MF, Recovery of Metal Oxide Nanomaterials from Electronic Waste Materials, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 203–227. 74. Nada AA, Soliman FS, Ali GAM, Hamdy A, Selim H, Elsayed MA, Elmowafy ME, and El-Maghrabi HH, Conversion of Waste Cheap Petroleum Paraffinic Wax By-Products to Expensive Valuable Multiple Carbon Nanomaterials, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 729–751. 75. Modak P, Wilson DC, and Velis C, Waste management: Global status, in Global waste management outlook T Cannon, Editor. 2015. p. 51–120. 76. Ali GAM, Yusoff MM, Shaaban ER, and Chong KF (2017) High performance MnO2 nanoflower supercapacitor electrode by electrochemical recycling of spent batteries. Ceramics International 43:8440–8448 77. Ali GAM (2020) Recycled MnO2 Nanoflowers and Graphene Nanosheets for Low-Cost and High Performance Asymmetric Supercapacitor. Journal of Electronic Materials 49: 5411–5421 78. Aboelazm EAA, Mohamed N, Ali GAM, Makhlouf ASH, and Chong KF, Recycling of Cobalt Oxides Electrodes from Spent Lithium-Ion Batteries by Electrochemical Method, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 91–123. 79. Alhanish A and Ali GAM, Recycling the Plastic Wastes to Carbon Nanotubes, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 701–727. 80. Ali GAM, Bakr ZH, Safarifard V, and Chong KF, Recycled Nanomaterials for Energy Storage (Supercapacitor) Applications, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 175–202. 81. Ali GAM and Makhlouf ASH, Fundamentals of Waste Recycling for Nanomaterial Manufacturing, in Waste Recycling Technologies for Nanomaterials Manufacturing, ASH Makhlouf, GAM Ali, Editors. 2021, Springer International Publishing: Cham. p. 3–24. 82. Paritosh K, Yadav M, Mathur S, Balan V, Liao W, Pareek N, and Vivekanand V (2018) Organic fraction of municipal solid waste: overview of treatment methodologies to enhance anaerobic biodegradability. Frontiers in Energy Research 6(75) 83. Chen X, Romano RT, and Zhang R (2010) Anaerobic digestion of food wastes for biogas production. International Journal of Agricultural and Biological Engineering 3(4):61–72 84. Tait S, Harris PW, and McCabe BK (2021) Biogas recovery by anaerobic digestion of Australian agro-industry waste: A review. Journal of Cleaner Production 299

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85. Jin C, Sun S, Yang D, Sheng W, Ma Y, He W, and Li G (2021) Anaerobic digestion: An alternative resource treatment option for food waste in China. Science of The Total Environment 779 86. Hallaji SM, Torabian A, Aminzadeh B, Zahedi S, and Eshtiaghi N (2018) Improvement of anaerobic digestion of sewage mixed sludge using free nitrous acid and Fenton pre-treatment. Biotechnology for Biofuels 11(1):1–12 87. World Bioenergy Association, Global bioenergy statistics 2020. 88. Hanum F, Yuan LC, Kamahara H, Aziz HA, Atsuta Y, Yamada T, and Daimon H (2019) Treatment of sewage sludge using anaerobic digestion in Malaysia: Current state and challenges. Frontiers in Energy Research 7(19) 89. Wu L, Wei W, Song L, Woźniak-Karczewska M, Chrzanowski Ł, and Ni B-J (2021) Upgrading biogas produced in anaerobic digestion: Biological removal and bioconversion of CO2 in biogas. Renewable and Sustainable Energy Reviews 150 90. Nava-Valente N, Del Ángel-Coronel OA, Atenodoro-Alonso J, and López-Escobar LA (2021) Effect of thermal and acid pre-treatment on increasing organic loading rate of anaerobic digestion of coffee pulp for biogas production. Biomass Conversion and Biorefinery:1–14 91. Tambone F, Scaglia B, D’Imporzano G, Schievano A, Orzi V, Salati S, and Adani F (2010) Assessing amendment and fertilizing properties of digestates from anaerobic digestion through a comparative study with digested sludge and compost. Chemosphere 81(5):577–583 92. Torrisi B, Allegra M, Amenta M, Gentile F, Rapisarda P, Fabroni S, and Ferlito F (2021) Physico-chemical and multielemental traits of anaerobic digestate from Mediterranean agroindustrial wastes and assessment as fertiliser for citrus nurseries. Waste Management 131:201– 213 93. Fernández-Bayo JD, Achmon Y, Harrold DR, McCurry DG, Hernandez K, Dahlquist-Willard RM, Stapleton JJ, VanderGheynst JS, and Simmons CW (2017) Assessment of two solid anaerobic digestate soil amendments for effects on soil quality and biosolarization efficacy. Journal of Agricultural and Food Chemistry 65(17):3434–3442 94. Calamai A, Palchetti E, Masoni A, Marini L, Chiaramonti D, Dibari C, and Brilli L (2019) The influence of biochar and solid digestate on rose-scented geranium (Pelargonium graveolens L’Hér.) productivity and essential oil quality. Agronomy 9(5):260 95. Asaoka S, Yoshida G, Ihara I, Umehara A, and Yoneyama H (2021) Terrestrial anaerobic digestate composite for fertilization of oligotrophic coastal seas. Journal of Environmental Management 293 96. Ali AM, Nesse AS, Eich-Greatorex S, Sogn TA, Aanrud SG, Aasen Bunæs JA, Lyche JL, and Kallenborn R (2019) Organic contaminants of emerging concern in Norwegian digestates from biogas production. Environmental Science: Processes & Impacts 21(9):1498–1508 97. Fan YV, Klemeš JJ, Lee CT, and Perry S (2018) Anaerobic digestion of municipal solid waste: Energy and carbon emission footprint. Journal of Environmental Management 223:888–897 98. Mavridis S and Voudrias EA (2021) Using biogas from municipal solid waste for energy production: Comparison between anaerobic digestion and sanitary landfilling. Energy Conversion and Management 247 99. Cheng F, Ikenaga Y, Zhou Y, Yu Y, Li W, Shen J, Du Z, Chen L, Xu C, Liu G, Lee PW, and Tang Y (2012) In silico assessment of chemical biodegradability. Journal of Chemical Information and Modeling 52(3):655–669 100. Watanabe M, Kawai F, Shibata M, Yokoyama S, and Sudate Y (2003) Computational method for analysis of polyethylene biodegradation. Journal of Computational and Applied Mathematics 161(1):133–144 101. Qomariyah, L., Ira Kumalasari, P., Suprapto, S., Fajar Puspita, N., Agustiani, E., Altway, S., & Hardi, H. (2022). Bioremediation of heavy metals in petroleum sludge through bacterial mixtures. Materials Today: Proceedings, 63:S140-S142 102. Li, M., Cheng, X., & Guo, H. (2013). Heavy metal removal by biomineralization of urease producing bacteria isolated from soil. International Biodeterioration & Biodegradation, 76: 81–85

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Fundamentals of Biodegradation Process Koula Doukani, Dyhia Boukirat, Assia Boumezrag, Hasna Bouhenni, and Yassine Bounouira

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamental Biodegradation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Interaction with Inorganic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotransformation of Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metabolic Mechanisms in Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metabolic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cometabolic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Microbial Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aliphatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Halogenated Aliphatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 60 61 62 63 64 67 68 70 71 72 73 73 73 73

K. Doukani (*) · H. Bouhenni Faculty of Nature and Life Sciences, University of Ibn Khaldoun, Tiaret, Algeria e-mail: [email protected] D. Boukirat Faculty of Science and Technology, Department of Nature and Life Sciences, University of Tissemsilt, Tissemsilt, Algeria A. Boumezrag Institute of Veterinary Sciences, University of Ibn Khaldoun, Tiaret, Algeria Y. Bounouira Faculty of Science and Technology, Department of Nature and Life Sciences, University of Tissemsilt, Tissemsilt, Algeria Ecology and Management of Natural Ecosystems Laboratory, Department of Ecology and Environment, University of Tlemcen, Tlemcen, Algeria © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_73

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Alicyclics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dioxins and PCBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterocyclic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation by Genetically Modified Microbes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

73 74 74 74 75 76 77 78 78 79

Abstract

Environmental pollution is a major problem that has accompanied the development of human society, placing it in front of a major challenge which is the decontamination of soil and water. Biological remediation, a technique that attempts to use the ability of certain microorganisms to degrade and transform different components, seems to be an excellent alternative for the remediation of pollutants. Biodegradation is a major process in the recycling of matter. It often depends on the nature of the compound and the microorganisms involved in the process and several other factors. The function of microbes in the biodegradation and biotransformation of pollutants into benign forms is well known, and knowing the molecular process of pollutant accumulation has significant biotechnological implications for contaminated site bioremediation. The present chapter will attempt to promote a thorough understanding of the biodegradation process, which employs microorganisms’ critical abilities in bioremediation, as well as the factors that influence this process and degradation by genetically modified microorganisms’ role and implications in an efficient biodegradation of contaminants. Keywords

Biodegradation · Pollutants · Fundamentals · Factors · Metabolic Abbreviations

BHC DDE DDT MMO OM PAHs PCBs PCE TCA TCE TEA

Benzene hexachloride Dichlorodiphenyldichloroethylene Dichlorodiphenyltrichloroethane Methane monooxygenase Organic matter Polyaromatic hydrocarbons Polychlorinated biphenyls Tetrachloroethene Tricarboxylic acid cycle Trichloroethylene Terminal electron acceptor

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Introduction Biodegradation is the process of transforming complex chemicals into simpler ones [1], and it is a critical part of natural systems mediated largely by microorganisms (e.g., bacteria, fungi, protozoa, and other organisms) [2]. Microorganisms play a major role in the biogeochemical cycling of organic and inorganic elements [3]. Microorganisms are the most important biodegradation agents in nature regarding the quantity of material transformed and the degradation rate [4]. The degradation of organic compounds by microbes is a natural process used in environmental remediation [2]. Microorganisms are the main mediators of biodegradation and mineralization of organic substances [3], and they are mainly responsible for the degradation of contaminants in soil and groundwater of both synthetic and natural substances [5]. Bioremediation techniques strive to exploit microbial catabolic diversity to biodegrade, biotransform, or bioaccumulate a wide range of pollutants [6]. Microbes are appropriate for the decontamination process. They can use environmental pollutants as a growth substance due to the enzymes that allow them to degrade several substances [7]. In addition to that, they can transform or mineralize compounds either chemically or physically [8]. Microorganisms can restore the environment through various processes, including oxidation-reduction, binding, volatilization, immobilization, and chemical modification of contaminants [9]. By exploiting the biodegradation capacity of microorganisms, a wide range of contaminants can be completely removed or at least transformed into less dangerous compounds [10]. This chapter provides an overview of the process of biodegradation and biotransformation of contaminants and emphasizes the factors that influence it.

Fundamental Biodegradation Reactions Biodegradation is based on the metabolic ability of microbes to degrade, transform, or mineralize contaminants into nontoxic or less toxic substances and even into simple inorganic products such as water, carbon dioxide (CO2), and minerals, which are then integrated into natural biogeochemical cycles [11]. This ability can be profitable to remove contaminants and hazardous materials such as hydrocarbons, heavy metals, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and pharmaceutical substances from soil and water [12]. While this is often very effective, sometimes such degradation processes do not reduce toxicity but produce other toxic substances. Microorganisms can carry many biochemical reactions resulting in biomass production and the degradation, transformation, or production of organic or mineral substances. They can adapt to almost every environment and have the ability to decompose different organic pollutants [13]. Biodegradation of contaminants can be partial or complete depending on the substance and/or on the ability of the microorganism to degrade it. Degradation of

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complex substances involves various distinct steps, often carried out by different microorganisms (consortium) [2]. This is related to the large enzymatic ability of the consortium, which improves the degradation rate [14]. Generally, the biodegradation process can lead to three different changes in the molecule [2]: (1) Minor change in the chemical structure or a loss in the functional groups attached to the organic compound, altering the chemical and physical properties of the compound, e.g., the substitution of a hydroxyl group for a chlorine group (biotransformation). (2) Actual breaking of the compound into fragments in such a way that can be reversible (the original molecule could be reassembled) (biofragmentation). (3) Complete degradation of an organic compound to minerals, which usually involves several organisms and the addition of carbon substrates. This biodegradation process is called complete biodegradation, mineralization, or biomineralization [1]. Depending on the nature of the substrate being degraded or removed, two types of microbial decontamination can be distinguished: (1) organic pollutants and (2) inorganic pollutants [15]. Organic contaminants are biodegradable, but inorganic ones (heavy metals) cannot be biodegraded; nevertheless, they can either be transformed into a stable form or eliminated from the medium [6].

Biodegradation of Organic Pollutants Biodegradation of organic molecules may be considered a succession of biological degradation stages (pathway) that eventually ends in the oxidation of the parent molecule [12]. And it can lead to complete biodegradation of the compound, a mechanism that supports cell growth and reproduction by providing both carbon and energy [16]. The mineralization process of any substrate involves the same succession of degrading stages, whether it is a simple sugar, a plant polymer, or a contaminant molecule, and each stage in the process is catalyzed by a particular enzyme produced by the degrading organism [17]. The first step of degradation includes using the substance, by microbial enzymes, as the only source of nutrients [16]. Biodegraded contaminants or one of their metabolites may be blocked because of the absence of appropriate degrading enzymes [17]. However, the products resulting from the breakdown of compounds may be further attacked by other groups of microorganisms (consortium), leading to the destruction of the contaminant. The complete breakdown of contaminates is the advantage that can offer a biodegradation process for the decontamination of water and soil. However, this process is not always carried out completely. Contaminates can be transformed or fragmented by enzymatic reactions to other forms (intermediates/ metabolites), often less complex; however, it is also possible that these metabolites become more toxic than the

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original compound (e.g., 1,1-dichloro-2,2-bis-(p-chlorophenyl) ethylene (DDE) one of the metabolites of the organochlorine pesticide 1,1,1-trichloro-2,2-bis (p-chlorophenyl)-ethane (DDT) and vinyl chloride a result of the degradation of TCE) [2]. Biotransformation, also known as biomodification, can reduce the persistence and toxicity of the pollutant [16]. These transformations depend on enzymes produced by microorganisms. The reactions that pollutants undergo during their transformation are like those occurring in nature. They can be categorized into seven classes: hydrolysis, oxidation, reduction, condensation, isomerization, new carbon bond formation, and functional group incorporation [16], which are the bases of the biogeochemical cycles of different elements in nature [16]. According to Saravana et al. [18] and Rao et al. [19], the main classes of enzymes responsible for the breakdown of the majority of hazardous contaminants in the environment are as follows: hydrolases, transferases, oxidoreductases, dehalogenases, and oxygenases. Biotransformation of the compound occurs in the cell, after its uptake, or outside the cell through the interaction of surfactant and extracellular enzymes released by the cell with the compound. These exudates bind to the compound and change its properties [16]. The extracellular enzymes initiate the breakdown of compounds into smaller subunits (oligomers), so they can cross the cell membrane. [12], to integrate the classical catabolic pathways [20]. Inside the cell, the chemical structure of a molecule governs the reactions that it can undergo. Practically, all of the reactions implicated in the biodegradation process may be categorized as oxidative, reductive, hydrolytic, or conjugative (Table 1) [4].

Microbial Interaction with Inorganic Pollutants On the other hand, metals are inorganic pollutants not derived from living organisms [16]. They cannot be destroyed but can be converted into innocuous molecules [18] or converted from one state of oxidation or organic compound to another [6, 21]. Microorganisms have been reported to have an exceptional capacity to remove heavy metals [22]. Metals can be solubilized by microbial activities, increasing their bioavailability and potential toxicity, or they can be immobilized, which reduces their bioavailability. These transformations are part of metal biogeochemical cycles and may be exploited in bioremediation [23]. In general, microorganisms can act on heavy metals by several mechanisms, including (1) biosorption (metal sorption to the surface of the cell through physicochemical mechanisms), (2) bioleaching (metal mobilization via methylation reactions or the excretion of organic acids), (3) biomineralization (metal immobilization via the formation of polymeric complexes or insoluble sulfides), (4) intracellular accumulation, and (5) enzyme-catalyzed transformation (oxidation-reduction reactions) [23].

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Table 1 Examples of biodegradation reactions [4] Type of reaction β-oxidationa C

C

Example of the chemical subject to reaction Fatty acids

COOH

C

C

CH2OH + H

C

COOH

Epoxidation C

Aldrin, heptachlor, styrene O2

C

C

2H

+ H2O

C O

Nitro reductiona R

R

NO2

Parathion, other nitro compounds NH2

Reductive dehalogenation Cl

H2

C

C

DDT, BHC H

C

+ HCl

C Cl

Cl

Hydrolytic dehalogenation Cl R

C

COOH

H2O

Chlorobenzoates, dalapon OH

R

C

COOH + HCl H

H

Esters, amides, carbamates

Hydrolysis (ester shown) O R1

C

O

R2

H2O

R1

COOH + R2

a

OH

Some intermediates are not shown

Many species of bacteria, fungi, and yeast can be used to transform heavy metals from polluted soil and water [16]. Bacteria can interact with metals in many ways. They can reduce, oxidize, sequester, volatilize, or degrade the pollutants [24]. Microalgae, through their metabolism, can also transform, detoxify, and volatilize metals [25].

Biotransformation of Metals Bioremediation of contaminated environment with metals may be achieved through biotransformation [6]. Metals and metal-containing compounds can go through transformation reactions [17] based on the utilization of microorganisms or their enzymes to convert metal from a harmful to a less harmful form [6]. Metal biotransformation refers to several mechanisms, the change of the metal chemical form through oxidation or reduction, by substitution of metal ligands (complexing agents), or by phase change (e.g., volatilization of Hg) [24]. Metals are biotransformed through redox conversions of inorganic forms or organic conversions to inorganic forms and vice versa [26]. Metal ions can be reduced by bacteria and microscopic fungi to metallic form [27]. The reduced form is much

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less soluble than the oxidized form for most metals. It is the case in particular for uranium and chromium [24]. According to Ramasamy et al. [26] and Matocha et al. [28], metals can be reduced by direct enzymatic reduction, where the metal ions get reduced during the oxidation of organic compounds, or indirect enzymatic reduction, where metal ions get reduced during iron or sulfur oxidation processes. Through abiotic reactions with other reduced products such as iron(II) and hydrogen sulfide, that interact chemically with the pollutants and form insoluble species, for example, Cu(II) reduction to Cu (I) by Fe(II). Microorganisms can directly reduce a wide range of highly toxic metals (e.g., Cr, Hg, U), through dissimilatory reduction or detoxification pathways [29]. During dissimilatory metal reduction, the metal is used by microbes as a terminal electron acceptor (TEA) in anaerobic respiration [6] or as an energy substrate in which the metal is oxidized [29]. For example, bacteria such as Aeromonas, Bacillus, Citrobacter, Escherichia, Pseudomonas, Rhodococcus, and Staphylococcus are resistant to mercury. They can reduce organic Hg that is toxic to metallic Hg, a less toxic form. Mercuric reductase is the enzyme that catalyzes the reduction of Hg [30]. Microbes can use reduction processes that are not linked to respiration and that do not provide them with energy [29] but are rather believed to confer resistance to metals [26]; [6]. The reduction of metals is through common detoxifying processes seen in microorganisms [26], for example, reduction of Hg(II) to Hg(0) [31] and reduction of U(VI) to U(IV) [32]. Microbial methylation of heavy metals is essential in bioremediation; this reaction often increases metals’ mobility and transformation into volatile compounds [6] [26], e.g., biomethylation of Hg(II) to gaseous methylmercury by several species of bacteria Alcaligenes faecalis, Bacillus pumilus, Brevibacterium iodinum, Pseudomonas aeruginosa [33], Pseudomonas sp., Escherichia sp., Bacillus sp., Clostridium sp. [34]. Methylation can either increase metal’s toxicity, as for Hg the methylated form is extremely toxic and the most accumulated form of mercury [35], or decrease it, as for Se [26] it is transformed to volatile dimethyl selenide (CH3)2Se [26]. Along with redox and methylation processes, it has been reported that iron and sulfur-oxidizing bacteria [36] are capable of leaching significant levels of Cu, Zn, As, Co, and Cd from polluted soils [6]. And precipitation of heavy metal compounds may happen due to microbe activity, resulting in the metals biotransformation into form sparingly, decreasing their toxicity at the same time [27].

Metabolic Mechanisms in Biodegradation Biodegradation process mediated by microorganisms may be driven by energy or nutritional requirements, a need to detoxify the immediate environment from pollutants, or it may occur fortuitously. In this case, the organism receives no nutritional or energy benefit [37]. Microorganisms are the exclusive owners of several

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metabolic pathways that exploit toxic substances as a source of energy for different cell functions [22]. Biodegradation of compounds (pollutants) can be classified into two main mechanisms: (i) metabolic biodegradation, the use of the pollutant as a primary subtract for growth (source of carbon and energy) in which it is a part of the metabolism, or (ii) cometabolic biodegradation, where microorganisms need another source of carbon and energy for growth and other activities, while the transformation of contaminants occurs as a simultaneous process [38].

Metabolic Biodegradation Biodegradation of pollutants is recognized as part of the metabolism when the energy, carbon, and/or nitrogen produced by this process can be used by microbes [39]. Microorganisms are the exclusive owners of several metabolic pathways that exploit toxic substances as a source of energy for different cell functions [22]. Microorganisms use three essential catabolic pathways for energy production: aerobic respiration, anaerobic respiration, and fermentation [40]. Aerobic biodegradation is generally a growth-related process in which the concentration of the substance decreases and the biomass of microorganisms increases [41]. It is a process that can completely catabolize an organic compound to CO2 using the glycolytic pathways and the tricarboxylic acid cycle with O2 as the TEA [2]. The organic substrate (natural or pollutant) is oxidized, reducing the oxygen to water (H2O). The byproducts of this process are carbon dioxide, water, an increase in the number of microorganisms, and other (inorganic/organic) products [42]. Several studies reported that different pollutants and xenobiotics could be used as primary subtract for growth by different bacteria and fungi under aerobic conditions, and it can lead to the complete degradation of the pollutant [41]. For example, microorganisms can use PAHs up to 4-ring units as energy and carbon sources, leading to complete mineralization [43]. The herbicide 2,2-dichloropropionic acid, known as dalapon [44], can be transformed into pyruvate by Arthrobacter species [45]. Low chlorinated compounds (mono- and dechlorinated biphenyls) such as PCBs can be mineralized, while highly chlorinated ones (four or more) are recalcitrant under aerobic conditions [12]. The biodegradation of the pollutants often produces metabolites that are encountered in the metabolism of the cell (e.g., pyruvic acid, acetyl coenzyme A, and succinyl coenzyme A) that are integrated with the different metabolic pathways (e.g., TCA cycle, glycolic pathways) and lead to a complete degradation [41], for example, aerobic biodegradation of aliphatic and aromatic hydrocarbons (Fig. 1). Anaerobic biodegradation is the degradation process of compounds without oxygen [42]. In anaerobic conditions, microorganisms use substitutes for O2 as a TEA for electron transport [2]. This process is carried out by many bacteria and archaea and some eukaryotic microbes [2]. The most common TEA used during anaerobic respiration are nitrate (NO3–), sulfate (SO42–), and CO2, but oxidized metal ions such as iron (Fe3+) and manganese (Mn4+) and a few organic molecules

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Fig. 1 General overview of aerobic biodegradation of aliphatic and aromatic hydrocarbons in cold environments; all end products from aerobic biodegradation will be used in the TCA cycle. (Adapted from [46] (Copyright: © 2021, MDPI, Basel, Switzerland))

can also be reduced [10, 47–49]. The byproducts of anaerobic biodegradation depend on the TEA. It may be methane (CH4), molecular nitrogen (N2), hydrogen sulfide (H2S), ammonia (NH3), and reduced forms of metals and biomass [10, 42, 49]. Many pollutants (e.g., pesticides) are subject to biodegradation under anaerobic conditions [41]. Some PAHs have been mineralized by facultative and strict anaerobic bacterial strains and archaea in sulfated, nitrated, iron-reducing, and methanogenic environments [50, 51]. For many pollutants, including alkyl solvents, aryl halides, and organochloride pesticides, the initial step in biodegradation is reductive dehalogenation

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(dehalorespiration or halorespiration) [2, 12], an essential anaerobic biodegradation mechanism [52]; it is the process of eliminating a halogen substituent (e.g., bromine, chlorine, fluorine). Chlorinated pollutants such as PCE, chlorinated solvents, PCBs, and several organochlorine pesticides [41, 47] are converted to less chlorinated species through reductive dechlorination, which might be biodegradable as primary growth substrates under anaerobic or, more often, aerobic conditions [38]. Several studies have detailed the biodegradation of pollutants (e.g., alkanes, PAH, halogenated aliphatics, PCBs, pesticides) under aerobic and anaerobic conditions such as [12, 50, 52–54] and others. The aerobic biodegradation process is more efficient than the anaerobic [55] (Fig. 2). However, some contaminants, such as chlorinated ethenes and PCBs with four or more chlorine atoms like PCE, may resist aerobic microbial biodegradation and preferably undergo anaerobic biodegradation (reductive dechlorination) [12, 38, 47]. As reported by Urbaniak [56], microbial biotransformation activity includes anaerobic, aerobic, and sequential anaerobic-aerobic transformation. Xenobiotics are poorly biodegradable under strict aerobic conditions; the sequential anaerobic-aerobic processes represent a promising alternative for the degradation of these compounds. Under anaerobic conditions, xenobiotics can be reduced, generally in a cometabolic process, to metabolites that may be significantly easier to biodegrade than the parent compounds under aerobic conditions [38]. Fermentation is a process that necessitates neither O2 nor other electron acceptors like NO3–, Mn4+, Fe3+, SO42–, or CO2 and depends on the capability of microbes to use part of the organic molecule (often a metabolite) as an electron acceptor [47]. Fermentation is an incomplete oxidation pathway [40, 57]. The substrate (even natural one) is not completely catabolized [2]. The fermentation products can be mineral and/or organic molecules (CO2, acetate, ethanol, butanediol, and lactate) [58, 59]. Other organisms can use these products as carbon sources since they still have reduced power [40]. Few studies reported microorganism growth using

Fig. 2 Aerobic and anaerobic biodegradation pathways of hydrocarbon compounds. Two arrows represent more than one reaction. (Adapted from [60] (Copyright: © 2013, IntechOpen))

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pollutants and xenobiotics, such as benzene, toluene, o-xylene, phenol, and others, as growth substrates under fermentative conditions compared to aerobic conditions [38]. Metabolism of organic compounds by respiration leads to significantly more efficient use of potential chemical energy than transformation through fermentation [47]. Both mineralization and cometabolic reactions occur in aerobic and anaerobic environments [41].

Cometabolic Biodegradation A wide range of pollutants, such as polychlorinated pollutants and high molecular weight PAHs, resist metabolic biodegradation when microorganisms are enabled to use them as a source of energy and carbon to support their growth [1]. However, they can be biodegraded due to the ability of some microorganisms to transform them fortuitously during their metabolic activities [1]. This process is cometabolic biodegradation [1]. Cometabolism is defined as a fortuitous microbial transformation of a compound that cannot be used as a growth substrate in the obligate presence of another biodegradable substrate that can be used for cell growth and maintenance [1]. Cometabolism is a nongrowth-linked process [41], catalyzed by an enzyme or cofactor with a nonspecific active site [61]. The first example of cometabolism was reported by Leadbetter and Foster in 1959 [62] and was defined as co-oxidation, where a growth substrate might facilitate oxidation of nongrowth substrate in microbes. Later, the concept of cometabolism was highlighted by Jensen [63], where the transformation of the nongrowth substrate, either via oxidation or reduction, was considered cometabolism [64]. The process of cometabolism can occur during the metabolic activity of the microorganisms (active growth phases) or by the interaction of microorganisms with the substance during inactive phases (non-growing cells) [65]. A wide variety of synthetic and organic compounds (pollutants) can be biotransformed through cometabolism [41], such as pesticides, petroleum, halogenated hydrocarbons, aromatic compounds, synthetic detergent, and chemical plastic [64]. The cometabolic degradation process is crucial for most organochlorine contaminants [66]. Dehalogenation reactions are important cometabolism reactions that may make pollutants such as pesticides accessible for further breakdown [67]. The pollutant does not serve as the primary (essential) source of carbon or energy for the microorganism but is considered as a secondary or non-growth substrate, which makes the presence of a growth substrate necessary for microorganism activities [41]. An example of a cometabolized alkane is cyclohexane degraded by Mycobacterium austroafricanum in the presence of isooctane [68]. The pollutant is degraded during the metabolism of the primary substrate [1], by a nonselective enzyme (or more) produced by the microorganism to catabolize a growth substrate, or for another function. This enzyme can transform the pollutant simultaneously with its established function [41].

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The transformation of the two substrates occurs simultaneously by the same enzyme. The presence of the pollutants does not induce the activity of the enzymes involved in its transformation since it is a collateral transformation to the degradation of the primary substrate, which induces the enzymatic activity [69]. The microorganism transforms the pollutant but does not benefit from the cometabolic reaction [41]. This process may lead to the complete breakdown of the contaminants. During the initial oxidative reaction of the process, the produced intermediates can be easily degradable and mineralized [41]. Microorganisms with cometabolic functions extend from prokaryotes to eukaryotes, including bacteria, fungi, and actinomyces [64]. Numerous groups of soil aerobic bacteria have been reported to cometabolize a wide range of pollutants, such as nitrifying bacteria, methane-degrading bacteria, and propane-, toluene-, and phenol-oxidizing bacteria [41]. The main important cometabolic reactions involve microbial oxygenase enzymes (mono- and dioxygenase), e.g., methane monooxygenase (MMO), toluene mono- and dioxygenase, ammonia monooxygenase, and biphenyl oxygenase, among others [1]. However, cometabolism transformation can lead to the incomplete degradation of the pollutant [17], which sometimes accumulates persistent intermediates [41], due to microorganisms’ incapability to integrate into central metabolism pathways [53]. Usually, the cometabolized contaminant is only partially modified during the process, and the resulting product can no longer be assimilated by the microorganism [1]. These molecular modifications can lead to a decrease in the toxicity of the pollutants or, on the contrary, to an increase [5]. Maier [17] reported that the biodegradation process can stop at any stage due to the absence of suitable biodegrading enzymes, thus confirming that it is the common cause of the persistence of certain contaminants. In some cases, the generated intermediates can be toxic to microorganisms and can inhibit cometabolic degradation [1]. It can be by affecting the production of the key enzymes, due to the inhibition of cell (growth or respiration) or by inhibiting enzyme activity [64], and by the inactivation of the enzyme due to the modification of the active sites by the toxic intermediates [70]. In several cases, the activity of a single microorganism can lead to complete degradation of the contaminant during the cometabolic reaction [1]. However, several studies reported that biodegradation of numerous contaminants occurs in the presence of a consortium of microorganisms [71], where cometabolism products can be further transformed or mineralized by other microorganisms [5] due to their wide enzymatic diversity [14], allowing them to process a wide range of reactions. Biodegradation of contaminants can be improved by adding growth substrates (energy and carbon) to enhance microorganism population and their activities [64].

Factors Affecting Microbial Degradation The vast degradation of the environment has come from the global discharge of industrial and agricultural pollutants. During the processing of coal and oil and the generation of nuclear energy, the energy industry creates a substantial quantity of

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waste. For example, according to the US Environmental Protection Agency (USEPA), the country uses around one million underground storage tanks (USTs) that are largely used to store petrol. In 2013, over 510,000 verified unintentional discharges from these USTs were reported [72]. This is recognized as a point source of contamination. Pesticide and fertilizer application across wide tracts of land, on the other hand, might result in nonpoint source pollution. Pesticides have been found to pollute 90 percent of monitored streams and 55 percent of shallow groundwater on agricultural and urban lands [73]. Microorganisms can destroy a wide spectrum of organic contaminants due to their metabolic machinery and capacity to adapt to harsh settings. As a result, microbes are crucial in site restoration. However, their efficiency is determined by several parameters, including the chemical type and quantity of pollutants, their availability to microorganisms, and the physicochemical features of the environment [74]. As a result, the variables influencing the ratio of pollutant breakdown by microbes are either connected to the microbes and their nutritional needs (biological factors) or are related to the environment (environmental factors) (Fig. 3).

Fig. 3 Bioremediation of contaminants using biodegradation capabilities of microbes includes the natural attenuation, although it can be improved by engineered techniques, by adding a selection of microbes (bioaugmentation), or by the addition of nutrients (biostimulation). Genetic engineering may also be used to enhance microbes’ degradation abilities by GEM. However, numerous factors influence the effectiveness of this process and risks related to the use of GEM in the field. (Adapted from [6] (Copyright: © 2013, IntechOpen))

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Environmental Factors Many factors impact microorganism survival and activity in any given habitat. Organic matter (OM), the principal carbon source for heterotrophic microbes in most settings, is one component that has a considerable impact on microbial activity. Because of their high and fluctuating OM concentration, surface soils feature a diversified microbial community and metabolic activity. OM content and variety are often substantially lower in the subsurface unsaturated (vadose) and saturated zones, resulting in reduced microbial populations and activity. Areas of the saturated zone with a significant debit or recharging rates are an exception to this norm since they might result in microorganism populations and activity equal to those observed in surface soils [75]. The prevalence and abundance of microbes in a medium are determined by accessible carbon and several physical and chemical variables. O2, temperature, pH, nutrition availability, water activity, and salinity are examples. Any of these conditions can impede biodegradation, but pinpointing the reason for a contaminant’s persistence can be challenging. Contaminant bioavailability, oxygen, nitrogen availability, and OM content are some of the most critical parameters influencing contaminant biodegradation in the environment. Surprisingly, the first three of these factors can change greatly depending on the location of the contamination [75]. Soil type and OM concentration influence the possibility for organic compound adsorption to the surface of a solid. A comparable process is an absorption, in which a pollutant enters the soil matrix’s bulk mass. Both absorption and adsorption lower contaminant’s availability to most bacteria and the pace at which the chemical is degraded [76]. Variations may influence fluid flow and pollutant migration in groundwater in the porosity of the aquifer matrix’s unsaturated and saturated zones. The capacity of the matrix to transfer gases like CH4, O2, and CO2 is diminished in sediments with fine grains and in water-saturated soils. This can potentially affect the rate and kind of biodegradation that happens. A soil’s oxidation-reduction potential indicates the system’s electron density. Biological energy is generated by molecule oxidation, in which electrons are transported to a variety of more oxidized substances known as electron acceptors. The low density of electrons (Eh > 50 mV) suggests aerobic oxidation, whereas a high electron density (Eh 50 mV) indicates anaerobic reducing conditions [76].

Organic Matter Content Microorganisms flourish on dirt surfaces. Bacterial counts in soil normally vary from 107 to 1010 per gram, whereas fungal numbers range from 105 to 106 per gram. In profound locations, such as the groundwater regions and the deep vadose zone, microbial populations are usually two orders of magnitude or fewer. The considerable decline in microbial populations with depth is caused mostly by changes in OM concentration. Both the groundwater area and the vadose zone are deficient in organic materials. A tiny population of contaminant degraders may be present initially due to the low overall number of microorganisms. As a result, unless a

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substantial biodegrading population is created, biodegradation of a single pollutant may be delayed. The organisms in the vadose zone and groundwater region are typically inactive due to the low quantity of OM present, another reason for sluggish biodegradation. This can lead them to react slowly to the addition of a carbon source, particularly if the source of carbon is a pollutant substance with poor bioavailability or in the case where the microorganisms have never been exposed to it. Numerous generalizations regarding the vadose zone, soil surface, and groundwater area may be drawn based on these patterns in oxygen availability and OM concentration [77].

Nitrogen Microorganisms use organic pollutants, mainly hydrocarbons consisting mostly of hydrogen and carbon, to produce a need for important nutrients like phosphorus and nitrogen. As a result, nitrogen fertilizers can commonly improve biodegradation. This is especially true in the case of petroleum oil spill biodegradation when nitrogen shortages can be severe. The typical C:N ratio in microbe biomass ranges from 5:1 to 10:1, depending on the bacterium. As a result, a C:N:P ratio of roughly 100:10:1 is commonly utilized in such areas. However, significantly different ratios were applied in several circumstances. Wang and Bartha [78] reported that the efficient remediation of hydrocarbons in the soil necessitated keeping a C:N ratio of 200:1 and a C:P ratio of 1000:1, by injecting phosphorus and nitrogen. The C:N and C:P ratios are kept so high compared to the cell C:N and C:P ratios because most of the metabolized hydrocarbon is released as CO2 and lost from the system. On the other hand, nearly all the metabolized phosphorus and nitrogen are integrated into microorganism biomass and remain in the system. Redox Conditions Redox conditions heavily influence the amount and pace of contaminant biodegradation. Aerobic biodegradation rates outweigh anaerobic biodegradation rates for most pollutants. Petroleum-based hydrocarbons, for example, are normally degradable once they enter the aerobic zones of freshwater lakes and rivers, while oil that accumulates in anaerobic sediments can be exceedingly tenacious. It is impossible to overestimate the role of oxygen in the breakdown of highly reduced hydrocarbons. Anaerobically, alkanes with low molecular weight (CH4) do not decompose. And alkanes with higher molecular weight (hexadecane C16H34) can break down but only in locations already polluted by petroleum. Some highly chlorinated chemicals, on the other hand, are resistant to biotransformation under aerobic settings but susceptible to biotransformation under anaerobic conditions (e.g., perchloroethene (PCE)) [77].

Biological Factors The metabolic capacity of microorganisms is an abiotic component. Direct inhibition of the growth processes and the enzymatic activities of degrading microorganisms are two biotic variables that impact the microbial degradation of organic molecules

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[79]. This suppression might occur due to microbes vying for restricted sources of carbon, antagonistic relationships between microbes, or predation on microbes by bacteriophages and protozoa [76]. The pollutant concentration typically determines the pace of contamination biodegradation and the quantity of “catalyst” present, which in this context refers to the number of microorganisms able to metabolize the contaminant and the number of enzymes(s) generated by each microbe. Additionally, the amount of contamination metabolized is significantly controlled by the specific enzymes involved, their “affinity” for the pollutant, and the pollutant’s availability. Furthermore, appropriate conditions must be present for unlimited microbial development, including appropriate quantities of nutrients and energy sources [76]. Temperature, pH, and moisture are other elements that impact biodegradation rates by regulating the rates of enzyme-catalyzed processes. Biological enzymes in the degradation process have an optimal temperature and do not have a similar metabolic yield at all temperatures [79]. Indeed, the biodegradation rate is lowered in half for every 10  C fall in temperature. Although biodegradation may occur in a wide pH range, a pH of 6.5 to 8.5 is ideal for biodegradation in most aquatic and terrestrial systems. Moisture influences contaminant metabolism by changing the type and quantity of the available soluble elements, pH, and the osmotic pressure of terrestrial and aquatic systems [80].

Other Environmental Factors pH The rate of hydrocarbon breakdown in soils is regulated by pH, with neutral pH exhibiting the quickest rates. Microorganisms isolated from previously contaminated locations, on the other hand, have gained the ability to thrive on hydrocarbons even at highly acidic pH values. These microorganisms’ diversity is lower than their counterparts that thrive at neutral pH [81]. Salinity Congestion caused by moderate-to-high salinity levels delays hydrocarbon breakdown in normal terrestrial or freshwater habitats [82]. Hydrocarbons are commonly introduced into marine ecosystems naturally through deposits of natural gas and oil seeps and anthropogenically using oil tanker spills and discharges; as a result, those ecosystems tend to contain microorganisms suitable for hydrocarbon degradation. On the other hand, increased salinity has decreased deterioration in marine ecosystems [83]. Temperature Temperatures ranging from near freezing to more than 30 degrees Celsius have been documented to cause hydrocarbon breakdown. Bacteria can adjust to harsh temperatures to keep the metabolic activity going; nevertheless, in the natural environment, the seasonal changes in temperature have been demonstrated to impact the pace of

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deterioration [84]. The biodegradation rates of naphthalene and hexadecane in river silt were reduced roughly 4.5-fold in winter (0–4  C) samples against 40-fold in summer (8–21  C) samples [85].

Biodegradation of Organic Pollutants Aliphatics Aliphatic hydrocarbons enter the environment via several different routes; linear alkyl benzenesulfonate (LAS) detergents; straight and branched-chain structures of petroleum hydrocarbons; and one- and two-carbon halogenated chemicals used as industrial solvents such as chloroform and TCE [75].

Alkanes Many microbes may use n-alkanes as their primary carbon and energy source due to their structural similarities to fatty acids and plant paraffin, which are plentiful in nature. Alkane-degrading microorganisms may be easily identified from any sample. Alkanes are typically regarded as the most biodegradable hydrocarbon. Alkane biodegradation occurs when a significant biological oxygen demand (BOD) occurs. A monooxygenase enzyme directly integrates one oxygen atom onto one of the end carbons of an alkane, leading to the creation of primary alcohol. On the other hand, a dioxygenase enzyme may integrate both oxygen atoms into the alkane to generate a hydroperoxide. Both routes result in the synthesis of a primary fatty acid. There are other cases of diterminal oxidation, in which both extremities of an alkane are oxidized, and subterminal oxidation, in which an internal carbon is oxidized, in the literature [86].

Halogenated Aliphatics Chlorinated solvents like TCE (Cl25CHCl) and PCE (Cl2C5CCl2) have been widely employed as solvents in industries. As a result of incorrect usage and removal, they are among the most discovered forms of organic pollutants in groundwater. The necessity for fast and economical cleanup of the polluted sites has generated attention to the biodegradation of these pollutants [75].

Alicyclics Cyclopentane and cyclohexane are simple components, but trimethylcyclopentane and different cycloparaffins are more complicated. Alicyclic compounds are less regulated in their usage in the chemical industry and their discharge into the

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environment by means of different industrial processes than petroleum treatment and use than aliphatics and aromatics. As a result, alicyclic health hazards in humans have received less attention than other types of chemicals, notably aromatics. Bicyclic hydrocarbons account for 20–70% of crude oil volume. They can be found in nature as plant oils and paraffin, microbial lipids, and insecticides [87].

Aromatics Aromatics are unsaturated cyclic compounds composed of a minimum of one ring. Their common structure is C6R6, with R representing any functional group. Cyclohexatriene, commonly known as benzene, represents the parent hydrocarbon of this class (C6H6). Polyaromatic hydrocarbons are benzene rings with two or more fused rings (PAHs). Aromatic hydrocarbons are natural chemicals generated when organic materials burn, such as in forest fires. However, because of activities such as petroleum processing and the use and combustion of wood and coal, the release of aromatic chemicals into the environment has grown substantially. The amount and content of aromatic hydrocarbons are crucial when examining a polluted site since they have been demonstrated to be carcinogenic to humans. Aromatic chemicals have also been hazardous to microbes [75].

Dioxins and PCBs Dibenzofurans and dioxins are emitted during garbage incineration and can be discovered in the smoke released from chimneys [88]. Considered among the most dangerous carcinogenic agent known, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been connected to the synthesis of 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), hexachlorophene, and other pesticides that use 2,4,5trichlorophenoxyacetic acid as a precursor. According to present understanding, TCDD is less harmful in carcinogenicity and teratogenicity than formerly supposed. However, noncancerous concerns like diabetes, decreased IQ, and behavioral consequences can be more severe. TCDD’s structure and its poor water solubility (0.002 mg/L) result in an exceptionally stable molecule in the environment. Although bacteria and fungus have proved TCDD biodegradation, the level of biodegradation is quite restricted. A combination of six isolated bacteria strains from TCDDcontaminated soil in Seveso, Italy, has been capable of generating a metabolite assumed to be 1-hydroxy-TCDD [89].

Heterocyclic Compounds They represent cyclic compounds that include a single heteroatom or more (oxygen, nitrogen, or sulfur) and carbon atoms. Heterocyclic compounds, in general, are more difficult to break down than aromatics simply having carbon. This is most likely due

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to the increased electronegativity of oxygen and nitrogen atoms compared to the carbon atom, which causes molecular deactivation toward electrophilic replacement. Heterocyclic compounds with five-membered rings and one heteroatom are easily biodegradable, most likely since compounds with five members are more reactive to electrophilic agents and therefore easier to be hydroxylated physiologically. The increase in the number of heteroatoms in the molecules reduces the biodegradability of these compounds [75].

Pesticides Pesticides are the most prevalent nonpoint source of pollutants (Fig. 4). Most organic insecticides are heavily mineralized within one growing season or fewer. The chemical structures of synthetic pesticides are dizzying, but the vast majority may

Fig. 4 The fate of pesticides in the environment. Adapted from [90] (Copyright: © 2013, IntechOpen)

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be traced back to comparatively simple aliphatic, alicyclic, and aromatic base structures. These base structures contain amino, carboxyl, halogen, hydroxyl, nitro, and phosphorus substituents. Herbicides containing chlorophenoxyacetates, like 2,4-D and 2,4,5-T, have been discharged into the environment in the previous 50 years, for example [75].

Biodegradation by Genetically Modified Microbes Bioaugmentation and biostimulation, as previously indicated, are strategies that may be utilized to speed up the remediation of contaminated environments. Bacterial genes encoding catabolic enzymes for resistant substances were first cloned and described during the late 1970s and early 1980s. Several microbiologists and molecular biologists soon recognized the biodegradation potential of genetic engineering [80]. Genetically modified microorganisms (GMMs) or genetically engineered microorganisms (GEMs) are microbes whose genetic composition has been transformed through genetic engineering methods, also known as recombinant DNA technology, based on the naturally occurring exchanges of genetic material among microbes. GEMs are promising for groundwater, activated sludge, and soil bioremediation, with increased degrading abilities of a large range of pollutants [91]. When the idea of discharging GMM for bioremediation became a reality, a large portion of the work in field’s research was devoted to risk assessment and biosafety [80]. At least four primary techniques for generating GEMs for bioremediation applications exist [92]. Examples of these are modifying enzyme specificity and affinity; creating and regulating pathways; generating, monitoring, and controlling bioprocesses; and employing bioaffinity bioreporter sensors for chemical detection, the decrease of toxicity, and endpoint analysis. By combining catabolic segments from multiple bacteria, molecular biology provides the means to enhance microorganisms’ biodegradative capabilities, expedite the evolution of new activities, and construct wholly new pathways [93]. Genes have been shown to digest environmental pollutants such as toluene, chlorobenzene acids, and other halogenated pesticides and hazardous wastes. Each chemical demands the use of its plasmid. It is improbable that a single plasmid will be able to digest all hazardous chemicals from all classes. There are four kinds of plasmids: (1) The OCT plasmid degrades decane, hexane, and octane; (2) the XYL plasmid destroys toluene and xylene; (3) the CAM plasmid degrades camphor; and (4) the NAH plasmid destroys naphthalene [93]. The capacity of genetic modification to produce microbial strains is capable of decomposing a wide spectrum of hydrocarbons [94]. They succeeded in engineering a Pseudomonas strain with several plasmids able to oxidize terpenic, aromatic, PAHs, and aliphatic. P. putida that contains the XYL and NAH plasmids, along with a hybrid plasmid resulting from the recombination of fragments of CAM and OCT established by conjugation, can degrade naphthalene, salicylate, camphor, and octane and grew quickly on crude oil because it could metabolize hydrocarbons better than any other bacteria [94]. This genetically modified product has been nicknamed a superbug (oil-eating bug). Several plasmids were identified in P. putida that can degrade a

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variety of chemicals TOL (toluene and xylene), RA500 (3,5-xylene), pAC 25 (for 3-cne chlorobenzoate), and pKF439 (toluene, salicylate). It was the first live organism subject to an intellectual property. At the time, molecular approaches, whether through plasmid breeding or pure genetic engineering, might swiftly develop microorganisms with increased enzymatic capacities capable of digesting practically any pollution [92]. Reports on the breakdown of environmental contaminants by GMM have mostly focused on genetically altered bacteria generated utilizing various genetic engineering technologies: Comamonas testosteroni VP44 alters the route and alters substrate specificity. The idea of genetic engineering to remove heavy metals has piqued the curiosity of many people. Alcaligenes eutrophus AE104 (pEBZ141) [95] and engineered Rhodopseudomonas palustris were utilized to remove chromium and heavy metals, respectively, from wastewater. PCB catabolic genes from Achromobacter sp. LBS1C1, A. denitrificans JB1, and R. eutropha A5 were spontaneously conjugated into the heavy metal-resistant strain R. eutropha CH34 to break down PCBs [96]. Genetic engineering of endophytic and rhizospheric bacteria for use in plant-associated degradation of soil’s hazardous chemicals is among the most promising emerging methods for rehabilitating damaged environmental locations [97]. Three criteria have been proposed for selecting a suitable strain for gene recombination and rhizosphere inoculation: first, the strain should be stable after cloning, and the target gene should have a high expression; second, the strain should be tolerant or insensitive to the contaminant; and, third, some strains can only survive in a few specific plant rhizospheres. Many rhizosphere bacteria can only digest organic contaminants to a limited extent. As molecular biology advances, genetically modified rhizobacteria containing pollutant-degrading genes are being produced for use in rhizoremediation [98]. Some contaminants, such as TCE and PCBs, have been examined for molecular pathways involved in their breakdown. Sriprang et al. [99] inserted the Arabidopsis thaliana phytochelatin synthase (PCS; PCSAt) gene into Mesorhizobium huakuii subsp. rengei strain B3 that creates phytochelatins and accumulates cadmium under the control of a bacteroid-specific promoter, nifH gene. Finally, the inoculation of GMM during seeding would alleviate the strain competition issues in a mixed culture. However, there is much controversy surrounding the release of such genetically altered microbes into the environment; therefore, field testing of these organisms must be postponed until safety questions and the potential for environmental risks are addressed.

Conclusion Microbial activity is essential for environmental regeneration and the global carbon cycle. All these actions are included in biodegradation. Microorganisms may break down or change a wide range of chemicals, even synthetic ones with ecotoxicological consequences, such as heavy metals and hydrocarbons. However, in most situations, this phrase relates to prospective degradabilities computed in the laboratory using chosen cultures and optimal growth circumstances. Natural

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biodegradation is less effective due to many problems, including competition with microbes, limited stock of critical substrates, adverse external circumstances (temperature, moisture, pH, aeration), and poor contaminant bioavailability. As a result, environmental biotechnology aims to address and resolve these challenges so that microorganisms can be used in bioremediation systems. As a result, bioaugmentation or biostimulation is essential to promote the activities of indigenous microbes and boost their biodegradative capacities in contaminated biotopes. Genetic engineering can also be used to boost the biodegradation capacities of microbes. The employment of GEM in the field, on the other hand, is fraught with peril. Whether such techniques for bioremediation of contaminants are eventually effective may impact our capacity to decrease waste, remove industrial pollution, and appreciate a sustainable future.

Future Perspectives As a result, the evaluation will concentrate on the future potential of changed bacterial strains under field circumstances, which may help us analyze the challenges of using genetically modified bacteria in environmental bioremediation. Adverse field conditions for synthetic bacteria are the primary hurdle in effective bioremediation technology. Furthermore, most molecular applications are restricted to a few well-known bacteria, such as B. subtilis, P. putida, E. coli, etc. Other bacterial strains must be studied to develop the changed microorganisms. The unique properties of open biotechnological applications have undoubtedly driven the development of modified bacterial strains to handle the new challenges. The major objective is to develop GE bacteria for bioremediation field release while retaining a high degree of environmental assurance. Efforts should be made to investigate the survival of synthetic bacteria and the possibility of horizontal gene transfer, which might affect the indigenous microflora in a complicated environmental situation. Unexpected scientific findings can pose even more compelling public-interest issues. Bacteria for bioremediation methods were created in the lab for a specific purpose, neglecting field needs and other complicated conditions in most situations. There is, however, no proof that the intentional introduction of GE bacteria for bioremediation has had a substantial detrimental impact on the native microbial population. At the very least, the inflated concept of risk assessment has sparked much discussion and a variety of research activities, many of which have made major contributions to the science of environmental microbiology. However, given recent discoveries, the survival of GE bacteria in complex environmental conditions remains a significant barrier that must be solved [100].

Cross-References ▶ Biodegradable Materials: Fundamentals, Importance, and Impacts ▶ Biodegradation of Pollutants ▶ Biodegradation Process: Basics, Factors Affecting, and Industrial Applications

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Anaerobic Biodegradation: The Anaerobic Digestion Process Ouahid El Asri

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Digestion Is the Principal Anaerobic Biodegradation Process . . . . . . . . . . . . . . . . . . . . . Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Digestion: Concept and Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbiology and Metabolic Pathways of Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy and Economic Recovery of Biogas Produced by Anaerobic Digestion . . . . . . . . . . . . . . . What Is Biogas? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roles of the Constituent Gases of Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Purification of Produced Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Digestion Assessment Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochemical Methane Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of Biochemical Methane Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Kinetics of Biogas and Methane Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-to-Nitrogen Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Load and Organic Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design of the Digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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O. El Asri (*) Microbial Biotechnology and Plant Protection Laboratory, Faculty of Sciences, Ibn Zohr University, Agadir, Morocco e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_4

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Abstract

Our planet suffers from different types of pollution and the destruction of ecosystems due to the rapid proliferation of industrial units and demography, leading to significant organic waste production. On the other hand, fossil fuels progress toward depletion spreading an energy deficit over several nations. Thus, we need biotechnology that combines management, treatment, and energy generation but respects the environment. Anaerobic biodegradation is among the current biotechnologies that are gaining momentum to meet the previous requirements. It is a microbial process in an oxygen-free environment. We can currently distinguish several types of procedures in anaerobic biodegradation. First, we have anaerobic digestion, which makes anaerobic biodegradation unique because it combines organic conversion matter into green energy in an anaerobic context. This chapter will discuss the different roles of anaerobic biodegradation as a new treatment, management, and generation of green energy. On the other hand, we present the various concepts of anaerobic digestion as models, metabolic pathways, evaluation techniques, and the microbial applications of this solution. We also discussed the significant parameters that disturb this process to make it more efficient. Finally, we will highlight the energetic and economic importance of this biotechnology. Keywords

Anaerobe · Biogas · Degradation · Green energy · Waste Abbreviations

C1 C3 CH4 CO2 GDP NO2 TS VS

One carbon Three carbon Methane Carbon dioxide Gross domestic product Nitrogen dioxide Total solid Volatile solid

Introduction Currently, the socioeconomic development of countries depends on some parameters. Firstly, it is challenging to assure the energy supply; none of the European countries possesses a positive energy balance. They have linked considerably to other countries for their energy procurement; the European Union imports 54% of its energy. In the Asian area, technological countries like Japan, South Korea, and Singapore also depend on energy needs. The wealthy Asian countries of fuel resources, such as China and India, import widely energy than they export [1].

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Morocco is subject to a severe energy deficit that worsens over time in Africa. It was about 73% in 1970 and increased to 83% in 1980, which further increased to 97% in 2007 [2]. In 2021, Taoufik and Fekri confirmed that the country imports more than 96% of its energy [3]. So, most countries on our planet show an energy deficit. On the other hand, 1.1 billion citizens worldwide fail to access electricity [4]. Furthermore, several houses and farms in rural areas and agricultural units in Africa, Asia, and South America do not yet have access to electricity. As a result, most farmers and herders in their countries rely on fossil fuels for their activities. However, these activities will soon be limited due to the depletion of petroleum, increased fuel prices (gas, diesel, premium fuel, and fuel oil), and removing subsidies on specific energy like petroleum products [5]. Therefore, economic activities and normal daily life are vulnerable to fossil fuels. Consequently, it is obligatory to provide reliable and affordable access to electrification and energy. In parallel with this increase in dependence on fossil energy, there is a growth in organic waste production. Several researchers have confirmed the relationship between gross domestic product and municipal solid waste production [6]. So, the production of this waste type strongly correlates with the citizen’s income level and the size of the municipality. The global production of municipal solid waste is about two billion tons, but this amount will increase to 3.4 billion tons in 2050 [7]. Munawar et al. declared that one-third of municipal solid waste produced is not collected and transported to the treatment units [8]. As well as, organic waste treatment measures are poorly used and often deposited haphazardly in public landfills. However, the problem of the management, recovery, and treatment of its organic waste remains a permanent challenge for municipalities and decision-makers. Additionally, sometimes organic wastes such as manure, droppings, and agricultural residues are recycled directly into the soil to improve the structure and fertility of agricultural land as an essential source of nutrients for crop production. Arifin et al. have confirmed that this direct application contaminates the entire environment [9]. Ganoulis stated that organic waste presents a real risk of pollution of water resources near the accumulation and storage areas of this waste [10]. As well as, healthy water from agricultural and livestock farms may contain nitrates and ammonium from this waste. In addition, organic waste can be a source of transmission of pathogenic microorganisms to humans and livestock. It can occur through several routes of contamination: contact, ingestion, and inhalation. Anupoju et al. confirmed that organic waste slowly degrades and produces different gases: ammonia, a hazardous gas for animals and human health. In addition, methane (CH4) and carbon dioxide (CO2) are a source of greenhouse gases [11]. Dunkley and Dunkley have shown that the most significant contributors to CH4 and nitrogen dioxide (NO2) emissions in the USA are enteric fermentation and management of farm animal feces [12]. The adoption of technology that allows for treating and managing the daily production of waste to convert it into green energy is necessary for each country’s development. So, improving the energy balance of certain countries that suffer from a deficiency is possible. Anaerobic biodegradation is among the emerging technologies that implicate daily in our environmental programs. Several researchers have

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described that anaerobic biodegradation is among the sustainable mitigation trends of environmental pollutants and is also cost-effective. So, it is also a time-saving way that could improve the current conventional bioremediation like the alternative emerging application. This chapter is an excellent tool that presents the different roles of anaerobic biodegradation as a new solution for the treatment, management, and generation of green energy. It is divided into five axes: (a) the first part discusses the different concepts of anaerobic biodegradation. (b) The second axis shows anaerobic digestion as the primary anaerobic biodegradation type. Then, we decorticate the different definitions, models, metabolic pathways, and microbial applications in this field. (c) The last part underlines this biotechnology’s energetic and economic importance. (d) Then it presents the different techniques for evaluating the anaerobic biodegradability. (e) Finally, it presents the main factors influencing this process. These axes allow industrialists, researchers, and operators who operate in anaerobic biodegradation to control and optimize this process to derive great energy and socioeconomic benefit.

Anaerobic Biodegradation Anaerobic biodegradation is the decomposition of substrates by microorganisms without an oxygenated medium. For example, glucose’s aerobic degradation produces a higher energy yield of 2900 kJ/mol than anaerobic degradation (400 kJ/mol). Another difference between these two metabolisms is that single aerobic organisms can fully mineralize complicated organic matter to produce a significant energy yield. Still, no single anaerobic organism can mineralize organic substrate [13]. Hence, the need for several organisms, i.e., a consortium of microbes, to mineralize a substrate is a stage succession. Furthermore, the researchers have long believed that there are no growing anaerobic microbes in oxygenation; this gas is toxic. However, some studies have confirmed that the anaerobic microbes can tolerate substantial oxygen levels and have developed tactics that minimize the extent of oxygen conditions [14]. So, the anaerobic exercise is the optimal condition for anaerobic degradation. The availability of electron acceptors and the specific energy production are two principal conditions of microbe’s types of anaerobic biodegradation. Among the alternative electron acceptor, we can detect inorganic elements like nitrate, manganese, sulfate, ferric iron oxyhydroxides, and carbonate. The natural organic matter that plays this role is often a metabolite subtract, and there are manufactured chemicals such as dinoseb, toluene, phenoxyethanol, dimethylsulphoxide, chlorobenzoates, and chlorophenols [15]. Several types of anaerobic biodegradation processes are: nitrate reduction or denitrification, sulfate reduction or sulfidogenesis, manganese reduction, iron reduction, and anaerobic digestion [16]. But, the term “anaerobic digestion” is widely used synonymously with anaerobic degradation. Some researchers say that treating organic waste through anaerobic digestion is a technology that has become increasingly common as an essential component of the

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global search for sustainable energy sources that stabilize waste and minimize the impact on the environment and ecological system [17]. So, anaerobic digestion is an anaerobic biodegradation technology for treating, managing, and recovering organic waste worldwide. We will frame this degradation process by focusing on anaerobic digestion as single anaerobic biodegradation.

Anaerobic Digestion Is the Principal Anaerobic Biodegradation Process Anaerobic Digestion Anaerobic digestion is when organic material microbiologically is converted into carbon dioxide and methane under anaerobic conditions, but small amounts of nitrogen, hydrogen, ammonia, and hydrogen sulfide are also generated. The mixture of gaseous products is called biogas, and the process of anaerobic degradation is often called digestion (Fig. 1). However, many researchers have used the word anaerobic biodegradation for a long time instead of anaerobic digestion. Today, a wide substrate can be used to generate biogas. Anaerobic digestion occurs in natural landfills, rice paddies, ocean sediments, lakes, rivers, swamps, marshes, and animal intestines. These ecosystems are characterized by light, and inorganic electron acceptors (oxygen, nitrate, sulfate, iron, etc.) are not present or limiting [18]. Therefore, various anaerobic digestion application fields, such as wastewater, sludge, and solid waste, are developed. These applications use different bioreactors (fully mixed, plug-flow, biofilm, etc.) and process conditions (retention time, concentration, loading rate, temperatures, etc.) to improve the process’s stability and maximize the biogas production process.

Fig. 1 The transition from organic waste to energy production by anaerobic digestion

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Moreover, anaerobic digestion biogas production is gaining much attention as an increasingly attractive renewable and sustainable energy technology that can replace fossil fuels. Therefore, anaerobic digestion is a promising technology to solve the problems of managing organic waste and the impoverishment of foreigners, thus works toward an energy self-insufficiency of our country.

Anaerobic Digestion: Concept and Models To understand the mechanism of anaerobic digestion, we will present an overview of the evolution of the construction of the different models explaining the process. The first investigation described biogas formation in one step in 1906 by Omelianski. Then, after 50 years, the first more detailed reaction scheme for methane fermentation was proposed by Barker in 1956 [19]. This researcher has divided this process into two phases, an “acidification” or “acid” phase, and a “methanogenic” or “methane” phase. Finally, after discovering acetogenic hydrogen-producing bacteria and a better appreciation of the limited capacities of methanogenic bacteria, the multistage model appeared with Bryant in 1977 [19]. So the early studies on modeling anaerobic digestion paid particular attention to the last stage of anaerobic digestion, methanogenesis, which was also considered the most critical stage. Thus, these models were elementary and consisted of a limited number of equations. Since 1997 and up to this chapter’s writing, many patterns can be found in the scientific literature; each has its reasoning and state of worth. According to Gavala et al., there is not yet a unified modeling framework for anaerobic digestion [20]. One of the excellent works is that of Lyberatos and Skiadas in 1999, which grouped all the essential models for anaerobic digestion to trace the historical evolution of these models by demonstrating the complexity of these designs [21]. Kythreotou et al. have described biological and physicochemical discovery. The kinetics of bacterial growth, substrate type, and product production during this process must be considered to produce a good model. In the face of this diversity of models, we can cite the famous model proposed by Hill in 1982. This model was developed to simulate the anaerobic digestion of animal waste. It contains five bacterial groups and four stages. During this model’s first step (hydrolysis), complex organic matter enters the digester and is converted by extracellular enzymes into soluble, biodegradable organic matter. During the second step (acidogenesis), the soluble organic matter is mainly biodegraded into butyrate, propionate, and acetate. Finally, in the third step (acetogenesis), acetate is produced from butyrate and propionate. In comparison, the fourth step (methanogenesis) refers to the production of methane from acetate and hydrogen [22]. The five bacterial groups catalyzing the last three steps are believed to be inhibited by the total volatile fatty acid. This model has constituted a platform where each researcher has added these parameters. Some researchers have modified this model by integrating the effect of the concentration of dissolved hydrogen on the regulation of the redox potential inside bacterial cells and the mixture of volatile fatty acids produced during anaerobic digestion

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[23]. Angelidaki et al. (1993) described a new model based on the ammonia effect; this model applies to anaerobic biodegradation fed with a substrate with a high ammonia content, such as animal manure [24]. For this, Angelidaki’s model established an equilibrium relationship between ammonia, carbon dioxide, pH, gas-phase dynamics, and temperature effects.

Microbiology and Metabolic Pathways of Anaerobic Digestion The formation of combustible biogas in nature has already been known since 1776 by Allessandro Volta; it was not until the end of the nineteenth century in 1868 that the formation of biogas was associated with the activity of microorganisms by Bechamp [19]. Currently, anaerobic digestion is described as microbiological degradation that effectively treats organic waste without oxygen [25]. Several researchers have confirmed that anaerobic digestion is a four-step process (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) with several interrelationships and microbial dependencies [18]. In this passage, we will dissect each step.

Hydrolysis Hydrolysis describes a wide range of depolymerization and solubilization processes in anaerobic digestion. Three major substrates are present in this stage (carbohydrates, lipids, and proteins), which are hydrolyzed to monosaccharides, long fatty acids, glycerol, and amino acids, respectively. This process is mediated by extracellular enzymes attached to the microbial cell or secreted into the bulk solution. Therefore, the hydrolysis phase results in the decomposition of the organic substrate into elementary constituent molecules, hence the qualification of these bacteria is hydrolytic. Several researchers have qualified this step as a pioneering step in this anaerobic biodegradation type. For example, Pavlostathis and Gomez have trained and distinguished this hydrolysis phase as the limiting step. Furthermore, Nopharatana et al. confirmed that the insoluble fraction of an organic substrate intended for this anaerobic biodegradation type could only be consumed after being solubilized to produce the precursors of acidogenesis and methanogenesis [26]. Therefore, the rate of hydrolysis is the limiting step in the biodegradation of the organic substrate in the digester. Acidogenesis The second stage of anaerobic digestion is called acidogenesis, defined as the set of anaerobic acid-producing biological reactions without acceptors or additional inorganic electron donors. Batstone et al. described this step as fermentation and renamed it in their model “fermentation acidogenesis” [27]. A wide range of substrates can be fermented in this step, including the monosaccharides, amino acids, unsaturated fatty acids, and glycerol produced during hydrolysis [23]. Optimizing acidogenic fermentation requires determining a few environmental parameters (temperature, pH, substrate, inoculum, etc.) that

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significantly impact the profile of metabolites [28]. However, monosaccharides and amino acids are the most abundant substrates for this fermentation, a primary pathway for carbon flow [18]. The two fermentation processes of these two substrates are entirely different, although the possibility of using these substrates is ubiquitous [29]. Many microorganisms, mainly Clostridia, can utilize these two substrates. Wagner et al. confirmed that substrates rich in amino acids and monosaccharides are relatively high in energy and rapidly biodegradable [30]. Monosaccharides convert to three carbon (C3) products, such as lactate and propionate, or 2-, 4-, and 6-carbon products, such as ethanol, acetate, and butyrate via acetyl-CoA. The C3 products are rare, except under conditions of overload, and the most common products are acetate, butyrate, and ethanol; carbon releases turn into carbon dioxide [31]. Amino acids are different, hence their fermentations are also divergent. But all amino acids ferment in an oxidation/reduction couple called the Stickland reaction. This way, one amino acid is oxidized while the other is reduced. For example, the acidogenesis of alanine and glycine [32] is as follows: Alanine: CH3CHNH2COO
+ 2H2O ! CH3COO
+ CO2 + NH3 + 4H+ + 4e
Glycine: 2CH2NH2COO
+ 4H+ + 4e
! 2CH3COO
+ 2NH3 In the case of uncoupled oxidation, alanine would produce two hydrogen molecules. Therefore, acidogenesis is a step that breaks down monosaccharides and amino acids into mixed organic acids, hydrogen, and carbon dioxide [33]. These are then converted to acetate in the next step.

Acetogenesis The acetogenesis phase is named because acetate is its main carbon product. Andriani et al. stated that substrates such as short organic acids, alcohols, and long-chain fatty acids produced in previous phases are converted into acetate, hydrogen, and carbon dioxide. Therefore, butyrate and ethanol oxidation to acetate are added by acetogens producing hydrogen. These last microorganisms are qualified those obligatory hydrogen producers because the electrons produced from this oxidation reaction are transferred to protons (H+) to produce H2 [34]. Therefore, we have grouped the equations for acetate production by obligate hydrogen-producing bacteria in Table 1. In this phase, another group of bacteria is detected, called the hydrogenconsuming acetogens, previously called homoacetogens. They are strictly anaerobic bacteria using the acetyl-CoA pathway as a terminal electron acceptance process and energy conservation [20]. Mata-Alvarez states that the catabolism of homoacetogenic bacteria is mixotrophic because these bacteria catabolize mixtures of carbon dioxide, hydrogen, and multicarbonate compounds to produce acetate [34]: 2HCO3
+ 4H2 + H+ ! CH3COO
+ 4H2O So, this phase is provided by two types of acetogenic bacteria (homoacetogenic and obligatory hydrogen producers). Thus it is controlled by the concentration of hydrogen produced because some researchers, such as Kotsyurbenko et al., have confirmed that homoacetogens are limited to high concentrations of hydrogen

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Table 1 Different metabolic pathways of obligate hydrogen-producing bacteria Conversion type Propionate ! Acetate Butyrate ! Acetate Ethanol ! Acetate Lactate ! Acetate Lactate ! Propionate Lactate ! Butyrate

Chemical equations CH3CH2COO
+ 3H2O ! CH3COO
+ H+ + HCO3
+ 3H2 CH3CH2CH2COO
+ 2H2O ! 2CH3COO
+ H+ + 2H2 CH3CH2OH + H2O ! CH3COO
+ H+ + 2H2 HCHOHCOO
+ 2H2O ! CH3COO
+ HCO3
+ H+ + 2H2 2CHCHOHCOO
+ 2H2O ! 2CH3CH2COO
+ CH3COO
+ H+ + HCO3
2CHCHOHCOO
+ 2H2O ! CH3CH2CH2COO
+ 2HCO3
+ 2H2

Fig. 2 Diagram of interspecies hydrogen transfer

[35]. Hence excellent progress in this phase requires a low concentration of hydrogen. Thus, Mata-Alvarez qualified methanogenic bacteria as competitors for hydrogen with homoacetogens [34]. So this researcher revealed the competition between these two bacteria on the same substrate. We can describe another concept in this phase of biodegradation as anaerobic. It is the presence of a syntrophic coupling called the transfer of interspecies hydrogen between the bacteria producing obligatory hydrogen and methanogens producing methane (Fig. 2). Several researchers studied and developed this concept. Batstone et al. have interpreted this coupling as the negative energy of the conversion processes in acetogenic bacteria [36]. Therefore, this phenomenon is an obligatory dependence of these bacteria on methanogens to ensure the conversions described above.

Methanogenesis Methanogenesis is the last stage of anaerobic digestion. The methanogenic archaeobacteria are responsible for the progress of this step. Six phylogenetic orders of methanogens are identified (Methanosarcinales, Methanocellales, Methanobacteriales, Methanomicrobiales, Methanococcales, and Methanopyrales). Each phylogenetic order has specific physiological characteristics such as carbon sources,

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Table 2 Different properties of methanogens bacteria Types Methanosarcinales

Methanomicrobiales Methanobacteriales Methanococcales Methanopyrales Methanocellales

Carbon sources used Acetate, H2 + CO2, CO, methanol, methylamines, methyl mercaptopropionate, dimethylsulfide H2 + CO2, formate, ethanol, 2-propanol,2butanol, cyclopentanol H2 + CO2, CO, formate, C1-methyl compounds H2 + CO2, formate H2 + CO2 H2 + CO2, formate

Temperature 1–70

pH 4–10

15–60

6.1–8

20–88 ˂20–88 84–110 25–40

5–8.8 4.5–9.8 5.5–7 6.5–7.8

temperature, and pH [18] (Table 2). This archaeobacteria can form methane from acetate or carbon dioxide and hydrogen, alcohols, and methyl compounds in a single carbon atom. Therefore, three main routes for the formation of methane are known: + Acetoclastic methanogenesis cleave the acetate into methane and carbon dioxide [37]: CH3COO
+ H+ ! CH4 + CO2 + Hydrogenotrophic methanogenesis reduces the carbon dioxide and hydrogen to methane [36]: 4H2 + CO2 ! CH4 + 2H2O + Methylotrophic methanogenesis converts methylated compounds (methanol, methylamines, etc.) into methane. This type of methanogenesis is limited to species of the order Methanosarcinales except for Methanosphaera sp., which belongs to the order Methanobacteriales [38]: Méthanol: 4CH3OH ! 3CH4 + CO2 + 2H2O Monométhylamine: 4(CH3)NH2 + 2H2O ! 3 CH4 + CO2 + NH3 Among these three methane production pathways, acetoclastic methanogenesis is the primary pathway for methane production. In contrast, hydrogen and carbon dioxide produce the global remaining 30%. Braun stated that the course of these pathways depends on hydrogen’s partial pressure, hence a sudden increase in this partial pressure above 20 Pa indicates a poor function of this phase [19]. As well as, some researchers confirmed that high partial pressure of H2 showed a negative effect on methanogenesis while CO2 had no impact.

Energy and Economic Recovery of Biogas Produced by Anaerobic Digestion What Is Biogas? Anaerobic digestion of a biodegradable organic substrate leads to biogas as a renewable energy source. Biogas is a mixture of gases (CO2, CH4, H2S, H2 . . .). It is composed of approximately 55–70% of CH4, 30–45% of CO2, traces of NH3

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Table 3 Several fuel energy values

Fuels Natural gas Liquefied petroleum gas Kerosene Diesel Biogas

95 Calorific value 8600 kcal.m
3 10,800 kcal.kg
1 10,300 kcal.kg
1 10,700 kcal.kg
1 5000 l.m
1

(0–100 ppm), and H2S (0–10,000 ppm) [39]. Among all these gases produced, methane is the most critical gas from an economic point of view because it presents a significant energy interest by its high calorific value (9.94–9.97 kWh.m
3) [40]. The other constituent gases do not contain any calorific value, i.e., the absence of energy production. Therefore, the calorific value of biogas is proportional to its methane content. The term “biogas” has become exclusively used to designate the combustible mixture of CH4 and CO2. However, it produces less energy than other fuels (Table 3). Biogas is gaining much attention as an increasingly attractive renewable and sustainable energy technology [41]. Awe et al. consider that biogas can be an alternative solution to the insatiable global energy demands and, simultaneously, reduce waste and greenhouse gas emissions [42].

Roles of the Constituent Gases of Biogas Researchers consider the gases (CO2 and H2S) produced by anaerobic digestion as biogas contaminants. CO2 is a recalcitrant gas that reduces the density and lowers the calorific value of biogas, but it is not corrosive like H2S. Therefore, CO2 is a gas of no energy interest, and a presence in biogas in large quantities can reduce its energy value. On the other hand, hydrogen sulfide (H2S) is an element harmful to the environment and corrosive to the metal parts of cogeneration engines, pumps, compressors, gas storage tanks, and valves [43]. Therefore, when the content of this element is high in the biogas produced, it reduces the life of the biodegradation anaerobic plant and causes health problems due to the continuous inhalation of this element. Furthermore, Chen et al. described H2S as toxic to methanogens because it inhibits them during anaerobic digestion [44].

Purification of Produced Biogas Currently, biogas requires a purification phase on an industrial scale [45]. Tippayawong and Thanompongchart state that the elimination of CO2 leads to increased CH4 content of the produced biogas [46]. Other researchers recommended the manufacturers of biogas production install H2S elimination units because of its corrosive power in the metal parts inside the cogeneration engine and the user’s channels of the anaerobic digestion installation. Reducing the CO2 and

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H2S content will significantly improve the biogas quality. Therefore, biogas purification has become an obligation to obtain good biogas quality. Biogas’ high calorific value leads to excellent economic valorization. It has high energy quality and an absence of corrosive impurities (CO2 and H2S) to protect cogeneration engines, the anaerobic digestion process, and the unit environment. Different emergent technologies are used to purify the biogas produced by digesters: Firstly, the CO2 removal technologies: absorption by chemical solvents, physical adsorption, cryogenic separation, membrane separation, and CO2 fixation by biological or chemical methods [43]. There are also many techniques for removing H2S: the chemical reactions in aqueous solutions, physical adsorption on solid adsorbents such as activated carbon and steel wool rust, and conversion to base S or low-solubility metal sulfides [45, 47]. The purification technologies of biogas have significant industrial importance generally; the softening of natural gas and the removal of CO2 from the fumes of power plants are inspirational sources for these biogas purification technologies. Among these technologies described above, the biogas water washing system is the simplest and cheapest method involving water as an absorbent [48]. On the other hand, several researchers have criticized this technology [49]. Therefore, they have presented new technologies such as green lung technology that uses carboanhydrase and methane from the waste gas.

Anaerobic Digestion Assessment Techniques Biochemical Methane Potential The biogas potential produced is defined as the volume (or quantity) of biogas per tonne of dry volatile matter digested from the substrate during their anaerobic biodegradation [17]. This potential is called the methanogenic or biochemical methane potential (BMP) because the methane is measured only in the produced biogas. Methanogenic potential is a relatively simple, reliable method for obtaining the estimate and rate of conversion of organic matter to methane. The amount of biogas produced and the methane content in the gas phase depend on the degradation of the waste. Although the economy of biogas plant is highly dependent on the methanogenic potential of the biomass used in these digesters, the prediction of the biogas (methane) potential is a vital issue in anaerobic digestion [18].

Determination of Biochemical Methane Potential The theoretical potential of biogas and methane is predicted widely by elemental composition. The estimation of the theoretical value of this potential is according to the composition of carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS) according to the equations of Buswell and Boyle:

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     a b n a b n a b þ
CH4 þ
þ CO2 Cn Ha Ob þ n

H2 O ! 4 2 2 8 4 2 8 4   a b c d Cn Ha Ob Nc Sd þ n

þ 3 þ H2 O 4 2 4 2     n a b c d n a b c d !
þ þ 3 þ CO2 þ þ

3
CH4 þ cNH3 þ dH2 S 2 8 4 8 4 2 8 4 8 4 Here, n: number of carbon atoms; a: number of hydrogen atoms; and b: number of oxygen atoms. Nuchdang and Phalakornkule used these equations to determine the biogas composition produced by glucose and glycerol [50]. So, suppose the exact composition of the substrate is known. In that case, the production of biogas and methane can be predicted by these two stoichiometric equations, balancing the total conversion of the organic matter of the substrate to CH4 and CO2 with H2O as the only external source. PBiogas ¼ PBiogas ¼

n:22400 12:n þ a þ 16:b

n:22400 12:n þ a þ 16:b þ 14:c þ 32:d

So, using this approach for 1 g of glucose, we have a potential of around 746 ml.gVS
1 of biogas composed of CO2 and CH4 in equal proportions. On the other hand, ethanol produces a quantity of biogas (974 ml.gVS
1) with different ratios of CO2 and CH4. We compared the experimental biogas potentials, and the biogas calculated from the Buswell and Boyle equations of the six monomers (glucose, fructose, valine, cysteine, maltose, and lactose) can see that there is a big gap between these two types of potentials. This gap exists between the identical monomers with the same chemical composition and molecular weight despite being theoretically considered similar in biogas production [51]. Therefore, the experimental potentials remain lower than the calculated potentials. So, the biogas potential is an exaggerated calculation by Buswell and Boyle’s stereochemical equation. It does not consider several parameters that intervene in biogas production, such as the isometric shape, the spatial configuration, and the intermediate metabolites of the intended substrates in biodegradation anaerobic. Therefore, anaerobic digestion is a complex phenomenon that considers factors other than the number of atoms in the monomers. This complexity increases when studying the anaerobic digestion of difficult organic waste like chicken droppings. At the industrial level, the experimental potential takes the majority and essential place in installing an anaerobic digestion unit because it reflects the economic gain in green energy. Biogas and methane’s practical and theoretical potential are expressed in ml of biogas or ml CH4 at standard temperature, and pressure per quantity of organic matter added (gTS, gVS, and gDCO). Although, it can also be expressed as organic matter removed.

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The Kinetics of Biogas and Methane Production Measuring the experimental potential of the methanogenic potential (BMP) is simple. So, it puts the organic substrate with or without anaerobic inoculum under defined operating conditions. Then, the biogas released is quantified by a specific measurement system until that gas production is practically stopped. The techniques and experimental materials for measuring biogas and methane produced in BMP tests are very diverse (Fig. 3). Currently, several techniques are distinguished: (a) In the technique presented by Esposito (2012) and Gaur, the gasometer is a vertical test tube that receives and measures the amount of water or an alkaline solution (NaOH) [52]. These solutions are evacuated by the biogas and methane produced from an inverted glass bottle connected by a capillary tube to the digester [53]. (b) The gasometer used by Juntarasiri et al. is an inverted test tube filled with water that receives the biogas directly from the digester using a capillary tube [54]. (c) Another technique uses a syringe as a gasometer that measures the volume of biogas produced. This syringe is introduced into the free space of the digesters and then allowed to equilibrate with atmospheric pressure [55]. (d) This technique uses a manometric for biogas measurement, i.e., the gasometer is a pressure gauge that measures the pressure in the gas space of the digester. Then, it converts it into the volume of biogas produced. Whatever technique quantifies produced biogas must be corrected under standard pressure and temperature conditions [56]. Biogas composition is usually determined by biogas analyzers or gas chromatography. The daily monitoring of methane produced by anaerobic digestion leads to cumulative methane production curves, representing the cumulative production of this specific gas as a function of time, which constitutes a direct result of the BMP

Fig. 3 The main biogas measurement techniques. (a) Measure by volume, (b) pressure measurement, and (c) displacement by water

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tests. Moreover, the shapes of these curves are far from negligible. So, according to the shape of the curve, we can distinguish several parameters: the biodegradability characteristics of the substrate, production of intermediate substrates, inhibitors, and the performance of methanogenic bacterial populations [57]. Thus, observing the shape of the production curve makes it possible to extract information on the biodegradation mechanism.

Factors Affecting Anaerobic Digestion Temperature The temperature inside the digester significantly affects the biogas production process. This parameter plays a vital role in anaerobic digestion as it directly influences the growth rates of bacteria. Thus, it also affects physical parameters such as viscosity, surface tension, and mass transfer properties inside the digester. We can distinguish different temperature ranges during which anaerobic digestion can be carried out: psychrophilic (below 20  C), mesophilic (30–40  C), and thermophilic (50–60  C). Some researchers have compared anaerobic digestion in two temperatures: mesophilic (35  C) and thermophilic (55  C). They found that anaerobic units at 55  C produce more biogas than those incubated at 35  C. But the only disadvantage of thermophilic incubation is the high energy consumption to heat the digesters. Therefore, the energy gains in the thermophilic system are small compared to the mesophilic system. Other researchers, such as Komemoto et al., studied two parameters (solubilization and acidogenesis) which serve as parameters for evaluating the efficiency of AD at different temperatures (15  C, 25  C, 35  C, 45  C, 55  C, and 65  C) [58]. Thus, they found a fast solubilization rate under mesophilic conditions. Therefore, biogas production was higher under the latter mesophilic conditions. Cioabla et al. stated that the optimum mesophilic digester temperature for biogas production is 35  C. Mata-Alvarez shows that the optimal temperature of thermophilic and mesophilic digesters is 35  C and 55  C, respectively [59]. The chosen temperature should be constant in the digesters because the stability of the temperature is also crucial for installing the acclimatization of the bacteria (Fig. 4). The choice of the mesophilic or thermophilic system is according to the treatment goals. For example, we choose the thermophilic process if we need maximum degradation of a complex waste. But, if we need to save energy, we choose the mesophile.

Potential of Hydrogen Hydrogen (pH) potential is one of the most critical parameters of anaerobic digestion. It uses as an indicator the stability of the biodegradation medium. Therefore, the change in pH can be both an indicator and the cause of the imbalance of this process [51]. According to Angelidaki et al., anaerobic digestion presents a relatively narrow

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Fig. 4 Anaerobic digestion temperature ranges

pH range of 6.0–8.5 [60]. So, a pH value outside this range can lead to an imbalance and inhibition of the biodegradation process. On the other hand, Yadvika et al. state that the pH of the digester should be maintained within the desired range of 6.8–7.2 by feeding it at an optimum loading rate [61]. The pH reacts on several pillars of anaerobic digestion: (a) The ratio of CH4 and CO2 in the produced biogas also depends on the pH. (b) CO2 dissolution depends on pH. (c) Microbial community diversity, therefore pH fluctuation can also change the composition of the biogas produced. pH is an important parameter affecting the growth of microorganisms during the AD process. Several researchers have confirmed that each microbial group involved in anaerobic degradation has an optimal pH. Therefore, they can grow in a specific pH range. For example, methanogens and acetogens bacteria have an optimal pH of around 7. In contrast, acidogenic bacteria have a lower pH of about 6. Kalyuzhnyi proposed the famous model of anaerobic digestion of glucose because all the bacterial steps of the model were assumed to be pH dependent (Fig. 5) [62]. Zhou et al. investigated the relationship between maximum methane production and microbial community structure by electrophoresis in three pH values (6, 7, and 8) involved in the anaerobic digestion [63]. This study found that biogas production and methane content at neutral pH 7.0 were significantly higher. Additionally, microbial cultures maintained at a pH of 7.0 could support increased biogas production. Therefore, a pH equal to 7 is recommended in digesters to stabilize the bacterial community and increase the methane content produced. Installation of pH measurement sensors is required. Many factors influence the pH: (i) The pH is closely related to the concentration of volatile fatty acid

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Fig. 5 The main flow lines of organic substrates and their inhibition in the anaerobic digestion of organic waste

(VFA) produced during the anaerobic digestion of the organic substrate. In general, increased production of VFAs leads to a drop in pH in a digester, and their toxicities are steeper when the pH is below 7. (ii) CO2 dissolution is highly pH dependent. An increase in ammonium (NH4+) during anaerobic digestion leads to increased toxicity due to increased pH (Fig. 6). So, the organic acids, volatile fatty acids, carbon dioxide, and ammonia concentration influence the pH in the digester.

Ammonia The most common inhibitor for anaerobic digestion is ammonia (NH3). It is produced by the biodegradation of nitrogenous matter, mainly in the waste’s proteins and amino acids content. Therefore, the amount of ammonia generated by anaerobic digestion of organic substrate can be estimated using the stoichiometric relationship of Boyle, 1977. The released ammonium can diffuse passively in the bacteria of the anaerobic digestion thanks to its hydrophobic character, thus creating several problems: (a) an imbalance of protons, (b) leading to potassium loss [44], and (c) blocking enzymatic reactions. High-concentration ammonia can inhibit specific biological processes and methanogenesis at concentrations exceeding 100 mM [64]. Khalid et al. reported that methanogenic activity is reduced by 10% at ammonium concentrations of

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Fig. 6 Influence of pH and temperature on the dissociation of NH3

Fig. 7 Dissociation equilibrium between ammonia and ammonium as a function of pH and temperature

1670–3720 mg/l, but it showed 50% of reduction at 4090–5550 mg/l. They recorded a complete cessation at a concentration of 5880–6000 mg/l [65]. Angelidaki and Ahring found that the results concerning ammonia’s inhibition level are contradictory since they depend on specific parameters such as pH, temperature, and adaptation of the inoculums [66]. When pH and temperature increase, the fraction of free ammonia also increases because pH strongly influences the degree of ionization (Fig. 7). So, ammonia inhibition is controlled by installing certain conditions: the

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choice of inoculum, the need for acclimatization of the bacterial consortium, the period of operation, the pH, and the temperature.

Sulfide After ammonium, sulfide is the second suspect in inhibiting anaerobic digestion. This term includes three chemical species (H2S, HS
, and S2
). The bad smell we can feel in the biogas produced as an operational parameter is due to a hydrogen sulfide concentration (H2S). The formation of hydrogen sulfide in anaerobic digesters results from reducing oxidized sulfur compounds and hiding sulfur amino acids such as cysteine. Below is the dissolution of H2S in water forming the equilibrium system [67]: H2 S $ Hþ þ HS
$ 2Hþ þ S2
There are two levels of inhibition resulting from sulfate production: Primary inhibition is due to competition for standard organic and inorganic between sulfate-reducing bacteria and methanogenic bacteria [68]. Secondary inhibition results from sulfide toxicity to various groups of bacteria. It diffuses across the cell membrane; once in the cytoplasm, it denatures native proteins. Also, it interferes with the multiple sulfide-binding coenzymes, causing an imbalance in the sulfurassimilating metabolism [69]. Several studies have stated inhibitory concentrations between 100 and 800 mg/l for dissolved sulfide or between 50 and 400 mg/l for undissociated H2S [69].

Carbon-to-Nitrogen Ratio Carbon and nitrogen are essential for bacterial growth because they serve microorganisms’ nutrition and development during anaerobic digestion. These two elements present in organic waste are estimated using Giroux and Audesse and the Kjeldahl method [70]. As the C/N ratio balances the food, a microbe needs to grow because an imbalance of nutrients is considered a limiting factor in the anaerobic digestion of organic waste. Some researchers have confirmed that the C/N ratio for optimal anaerobic digestion is 25–30 [71]. Therefore, the chicken’s droppings have a low C/N ratio than other manure [17]. So, this ratio is controlled by adding another waste to the digester; this is co-digestion. Iacovidou et al. and Zhang et al. recommended organic wastes with low C/N, such as chicken droppings co-digestion with other wastes like cow dung, to increase this ratio [72]. But, some researchers criticized this latter recommendation, noting that even low C/N waste produces a significant amount of biogas. So preferably, before starting anaerobic digestion, determining the C/N ratio present in waste is necessary to optimize anaerobic biodegradation.

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Load and Organic Composition The solid concentration is a feed fermentable organic matter load in a digester suspension volume. Usually, the solids concentration in a digester is 7–9%. Budiyono et al. demonstrated that the more organic load is in 7.4–9.2% of dry matter, and biogas production is optimum [73]. Weiland states that wet digestion is operated with total solids concentrations below 10% in fermenters, allowing complete agitation and the digested material to be pumpable and spread over fields for fertilization [74]. Some researchers declare that the organic load is not a sufficient parameter to assess anaerobic biodegradation. Hence, it is necessary to evaluate its biodegradability and availability. Hamilton recommended using chemical oxygen demand (COD) to evaluate this biodegradability [75]. Thus, several works present the production of methane and biogas by anaerobic digestion with COD. With technological progress, we can now determine the organic composition of substrates intended for anaerobic digestion. Organic matter has three chemical natures (lipids, carbohydrates, and proteins). Recently Ohemeng-Ntiamoah and Datta stated in their previous work that lipids, proteins, and carbohydrates are the critical parameters of anaerobic digestion because the biogas produced depends on these three components [76]. Cirne et al. confirmed that organic waste rich in lipids produces more biogas than those rich in carbohydrates and proteins [77]. On the other hand, Cui and Jahng stated that introducing protein-rich waste in anaerobic digestion is not recommended, given ammonia’s increased risk of inhibition [78]. This last recommendation is rejected by Wagner et al., who studied the impact of lipids, proteins, and cellulose (a complex carbohydrate) on biogas production. They found that protein-rich substrates can be benign to the anaerobic digestion process by stabilizing methanogens. Some researchers have pushed their investigation to the atomic scale, such as Mulka et al. demonstrated that the amount of carbon and hydrogen available in the organic substrate determines the maximum methane formed by anaerobic digestion [79]. So, the content and composition of the substrate intended for anaerobic digestion influence this process, hence there is the need to know it before putting it in the digester.

Pretreatment Substrate pretreatment to enhance anaerobic digestion has been extensively presented in the scientific literature. The effects of pretreatment on this process are very different as they depend on the characteristics of the substrates and the type of pretreatment chosen. For example, the presence of some substances, such as lignocellulosic, requires pretreatments. The pretreatment techniques used in anaerobic digestion are very diverse and depend on the types of waste and the operating principles. We can quote thermal, freezing/thawing, ultrasonic, mechanical, chemical, wet oxidation, microwaves, and pulsed electric fields. Rodriguez et al. stated that performing a pretreatment makes the process more complicated and more expensive.

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Still, it can improve its efficiency and reduce the total cost to achieve a positive energy balance compared to a process not pretreated. Pretreatments can achieve the improvement of this process by affecting the different biological and physicochemical pathways: (a) particle size reduction, (b) solubilization, (c) improved biodegradability, (d) formation of refractory compounds, and (e) loss of nonbiodegradable organic matter. Therefore, the central goal of “pretreatment” is to facilitate the anaerobic digestion process by removing blocking all barriers and making the organic content of the substrate easily accessible and usable by the microbial community.

Design of the Digesters In addition to these biotic factors affecting the performance of anaerobic digestion, the design of the digester is essential in anaerobic biodegradation. Different digesters exist for the anaerobic digestion of organic waste: (i) Covered lagoon digester is for liquid effluents containing less than 2% solids. (ii) Complete-mix digester suitable for manure containing between 2% and 10% solids. (iii) Plug-flow is for animal feces with 11–13% solids [59]. So, before starting the process, we must be choice of the digester type.

Conclusion Every year, waste production and energy demand increase worldwide. These trends and difficulties increase the risk of destroying our planet’s natural ecosystems. Therefore, it is necessary to convert the organic matter of wastes into biogas by anaerobic biodegradation like anaerobic digestion. This field has seen different metabolic pathways, energetic concepts, intervening microbes, affecting factors, and experimental evaluation techniques. This arsenal of information in this chapter allows us to provide a suitable framework for using this technology.

Future Perspectives We have succeeded in this chapter in highlighting the role of anaerobic digestion as better sustainable development biotechnology. The mastery and reasonable control of different concepts and parameters that react to this process ensure the proper functioning of this technology. This control produces a large quantity of convertible methane into thermal and electrical energy. Thus, we can apply this process mainly for the residues and the organic waste, which are continuously renewed every day because of socioeconomic development. Therefore, this application-optimized biodegradation process makes it possible to achieve the management, treatment, and energy recovery of organic waste worldwide.

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On the other hand, anaerobic biodegradation made it possible to propose a new solution to remove environmental and energy constraints. Thus, installing digesters based on anaerobic digestion makes it possible to improve the national energy balance, mainly for countries that do not have energy resources. Therefore, anaerobic digestion makes it possible to reduce the energy taming of certain countries, reducing the import of energy and saving national reserves. We can therefore declare that anaerobic digestion makes it possible to consolidate several pillars of the socioeconomic environment: • • • •

Obtain green energy that can improve the energy balance of the country Reduce the risk of pollution Correlate agricultural and industrial sectors with other socioeconomic sectors Improve the aesthetic and hygienic aspects of all the surroundings of the waste storage areas units • Increase the gross domestic product (GDP) of each country To achieve these main axes, the creation of companies for the installation of units based on anaerobic digestion allows the creation of value and the improvement of these pillars.

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Recent Advances in Microbial Biodegradation Samah Husseiny, Nada Elgiddawy, Gharieb S. El-Sayyad Waleed M. A. El Rouby

, and

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacterial-Mediated Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fungal-Mediated Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Algal-Mediated Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymes Involved in Microbial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Microbial Degradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adaptation of Microorganisms to the Toxic Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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S. Husseiny · N. Elgiddawy Department of Biotechnology and Life Science, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt G. S. El-Sayyad (*) Department of Microbiology & Immunology, Faculty of Pharmacy, Galala University, New Galala city, Suez, Egypt Drug Microbiology Lab, Drug Radiation Research Department, National Center for Radiation Research and Technology (NCRRT), Egyptian Atomic Energy Authority (EAEA), Cairo, Egypt Chemical Engineering Department, Military Technical College (MTC), Egyptian Armed Forces, Cairo, Egypt e-mail: [email protected]; [email protected] W. M. A. El Rouby (*) Material Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_3

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Application of Microbial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Degradation of Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Degradation of Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Degradation of Antibiotic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanobiodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles Enhance Microbial Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticles for Immobilization of Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

With the widespread consumption of synthetic materials around the world, the amount of waste produced has increased, resulting in higher levels of toxicity in marine and soil ecosystems and causing severe health issues. Research on waste material degradation was focused on physical, chemical, biological, and enzymatic techniques to waste degradation. Interestingly, microorganisms like algae, fungi, and bacteria have attracted scientists’ interest as a bioremediation technique due to their cost-effectiveness and affordability. This work discusses many features of biodegradation in natural environments and variables that influence microbial waste degradation and application.

Keywords

Biodegradation · Plastics · Pesticides · Antibiotics · Nanobiodegradation Abbreviation

Ag NP BES CNTs DDT IET LAC LDPE Lip nZVI PAHs PCL PCP PES PHA PLA sp. TEC WWTPs

Silver nanoparticle 2-bromoethanesulfonate Carbon nanotubes Dichloro-diphenyl trichloroethane Interspecies electron transfer Laccase Low-density polyethylene Lignin peroxidase Zero valent iron polycyclic aromatic hydrocarbons Polycaprolactone Pentachlorophenol Polyethylene succinate Polyhydroxyalkanoates Polylactic acid Species Tetracycline Wastewater treatment plants

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Introduction Microbes are ubiquitous in the biosphere, interacting with their environment, each other, plant, and animal species. Microorganisms have distinct habitats in every ecosystem due to their metabolic features. Their presence always affects the environment in which they grow, as they have a variety of essential functions. As a result, microbes may be considered an integral part of “ecosystem services,” which are critical to the environment, well-being, and productivity [1, 2]. Many microbes are uniquely adapted to specific environmental niches. Microbes can be found everywhere and in the world’s most dangerous sites, such as those that inhabit the Dead Sea (Halobacterium), the bacteria and cyanobacteria that inhabit the boiling water springs in Yellowstone National Park, and Chlamydomonas nivalis, the algae that cause “pink snow” [3]. Their hidden interaction with the surrounding environment has yielded beneficial significance, such as terraforming of earth and transforming it into the livable green world we have now. The ecosystem remained in the balance due to its ability to utilize and adapt to any available substrate to gain energy. Microbes also play an essential role in the natural recycling of living materials [3]. Biodegradability refers to the ability to live organisms, such as bacteria or fungus to break down naturally occurring compounds. Human activity has developed a wide spectrum of hazardous compounds to which bacteria can naturally adapt since the industrial revolution. The issue is that biodegradation cannot keep up with the increasing volume of chemicals generated. Thankfully, current science has developed technology that uses bacterial flexibility to adapt. It’s referred to as bioremediation [4]. The word biodegradation is frequently used in ecology, waste management, and environmental remediation (bioremediation). Bacterial ability to recycle essential components of all living systems, particularly carbon, oxygen, and nitrogen is positively impacting the planet. Photosynthetic organisms convert CO2 from the atmosphere to organic material (CO2 fixation), representing a large amount of available organic carbon. The breakdown of complex organic materials into various forms of carbon that other organisms may utilize is known as decomposition or biodegradation. While microbes could degrade all natural, organic substances, some manufactured chemicals such as plastics, Teflon, and pesticides are not biodegradable or broken down very slowly [5, 6]. In addition to all these benefits of microbes, they play an important role in environmental sustainability. This chapter highlights mainly the helpful microorganisms, their effects on the environment, and their role in maintaining quality, health, and environmental sustainability by contributing to biodegradation with important parameters illustrated in Fig. 1.

Microbial Biodegradation Bacterial-Mediated Biodegradation The employment of bacteria for the biodegradation of different natural and manufactured chemicals, and therefore reducing risks, is gaining popularity. Bacteria have a wide range of bioremediation capabilities that are helpful to both the

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Fig. 1 Schematic representation regarding microbial degradation, the enzymes involved, and the factors affecting microbial degradation

environment and the economy (Fig. 2). Microorganisms’ naturally occurring metabolic capacity to break down, convert, or accumulate hazardous chemicals such as hydrocarbons, heterocyclic compounds, medicinal drugs, and toxic metals has been used in biodegradation. Many bacteria from the genera Mycobacterium, Corynebacterium, Aeromonas, and Bacillus have also exhibited biodegradation ability [7]. Bacteria, such as Staphylococcus sp. and Rhodococcus sp., help to remove pollutants (Table 1). A wide range of chemical substances, including biphenyls and benzene, can be degraded by members of the Rhodococcus, Bacillus, Arthrobacter, Nocardia Streptomyces, and Gordonia genera [8]. Microbes such as Azotobacter sp., Bacillus megaterium, Ralstonia eutropha, Pseudomonas sp., Halomonas sp., etc. are used in plastic biodegradation [8]. Gram-negative bacteria are more resistant to a hazardous combination of saturated, mono-aromatic, and poly-aromatic hydrocarbons than Gram-positive bacteria. It also may tolerate a wider range of organic compounds and metal pollutants [9], such as genus Pseudomonas, isolated mostly from soil to degrade aromatic hydrocarbons.

Fungal-Mediated Biodegradation Fungi can tolerate a wide range of environmental surroundings and substrates due to their various enzymatic systems (Fig. 2). Fungi are always heterotrophic, meaning

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Fig. 2 Schematic representation of bacteria, fungi, and algae in the microbial degradation

Table 1 Examples of different bacteria capable of biodegradation in the laboratory and environmental conditions Bacteria Rhizobacterium Ensifer meliloti Acidogenic bacteria Stenotrophomonas maltophilia (WZ2) Bacillus cereus Pseudomonas fluorescens Mycobacterium gilvum Saccharomyces cerevisiae Bacillus cereus Pseudomonas putida Streptomyces sp.

Type of pollutant Phenanthrene Nonylphenol Quinclorac (herbicide) ß cypermethrin Direct orange 102 dye Pyrene and benzo pyrene Methyl red, chrysene, benzopyrene Polyethylene Milk cover Low-density polyethylene

References [10] [11] [12] [13] [14] [15] [16] [60] [61] [62]

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they get their nutrition from parasitic or saprotrophic sources, and they do so by performing a series of enzyme reactions on the substrate they grow so it can convert a very complex chemical composition into a simpler form for fungi nutrition [1]. Phanerochaete chrysosporium (a white rot fungus) metabolizes and degrades lignocellulose compounds using enzymes such as laccase (LAC) and lignin peroxidase (Lip) [17]. Fungi can naturally break down organic materials. It converts organic residue to various simple products has been used to produce valuable products for humanity, such as bread, wine, medicine, and other industrial applications. Fungi are essential in the environment because they serve as decomposers and recycle organic matter in nature, providing nutrition to plants by mycorrhization, and the enzymes produced by fungi have been studied to create various by-products from waste. Most research on microbial biodegradation has focused on bacteria, with fungal applications just becoming prominent in the last two decades. On the other hand, the filamentous fungus has characteristics such specific bioactivity and growth form that makes them more likely to degrade a wide range of organic compounds than bacteria. Furthermore, new research has revealed that filamentous fungi can cooperate with bacteria to promote pollutant degradation by converting pollutants into simpler forms for bacteria to degrade. Also, fungi provide bacteria with the means to reach inaccessible pollutant compounds through hyphal development [9]. As mentioned above, fungi have numerous advantages over bacteria in degradation processes, such as the capacity to survive in greater concentrations of poisonous organics and the ability to degrade more complex organic compounds, for example, fungal genera, pyrethroids. Aspergillus, Candida, Trichoderma, Eurotium, and Cunninghamella are among the microbial strains that can degrade pesticides like pyrethroids and use them as their only source of carbon and nitrogen [4]. Consequently, fungi can be applied to the biodegradation of pesticides in soil and aquatic environments [10] (Table 2).

Table 2 Examples of different fungi capable of biodegradation in the laboratory and environmental conditions Fungi Zalerion maritimum Fusarium solani, Spicaria spp., Alternaria solani, Aspergillus flavus Gliocladium virens Myceliophthora thermophila Aspergillus sydowii Cladosporium sp. Phanerochaete chrysosporium Pleurotus sajorcaju (MTCC 141) Aspergillus niger, Penicillium pinophilum Aspergillus oryzae

Type of pollutant Polyethylene pellets Polyester polyurethane

References [18] [19]

Lignocellulosic compounds Polycyclic aromatic hydrocarbons Methyl parathion Anthracene Polyethylene, polypropylene Decoloration of effluent Powdered low-density polyethylene High-density polyethylene films

[20] [21] [22] [23] [24] [25] [26] [27]

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Algal-Mediated Biodegradation In aquatic and terrestrial environments, algae are significant components of the microbial community [28]. Organic pollutant breakdown by algae is a natural process that guarantees a long-term, eco-friendly means of breakdown pollutants with no secondary contamination (Fig. 2). Many algal enzymes are involved in the biodegradation of various organic contaminants. One of the benefits of algae-based biodegradation is the capacity to collect, detoxify, or break down xenobiotics and contaminants. Furthermore, biomass products of bioremediation could be used as an additional renewable source to produce biofuel, which has both environmental and economic impacts [29]. Therefore, phytoremediation is considered cost-effective, environmentally beneficial, and relatively safe. Recently, bio surfactant-producing microalgae have been discovered to play a key role in the rapid bioremediation of hydrocarbon-contaminated locations. Madadi et al. [30] proposed using a mixture of surfactants and Chlorella vulgaris to enhance the efficiency of pre-treat and nutrient removal from petrochemical wastewaters. Phytoremediation has been applied in the treatment of a wide range of pollutants, including petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), chlorinated solvents, metals, radionuclides, nutrients, and pentachlorophenol (PCP) [31]. Many micro algal species have been reported to be used in bioremediation, including Chlamydomonas, Arthrospira, Cyanothece, Chlorella, Phormidium, Scenedesmus, Botryococcus, Spirulina, Desmodesmus, Nodularia, and Oscillatoria [32]. On the other hand, some macro algae have been investigated for biodegradation, including Ulva lactuca and Kappaphycus alvarezii. Macroalgae can consume large amounts of nutrients during their normal growth and development. Hence, macroalgae culture positively affects marine animals and human activities [33]. Algae can remove heavy metals through two processes: bioaccumulation and bio-sorption. Bio-sorption refers to the passive removal of heavy metals by binding to nonliving biomass; however, bioaccumulation refers to an active process in which metal removal needs the metabolic activity of a living organism [34]. Moreover, algae have also demonstrated a hopeful potential for reducing CO2 levels in the atmosphere, allowing them to reduce the consequences of global warming.

Enzymes Involved in Microbial Biodegradation Microbial biodegradation is mostly based on the participation of various intracellular and extracellular enzymes. These enzymes could break down the contaminants and turn them into harmless compounds [35], as shown in Fig. 3. For example, proteases can be generated by Brevibacillus spp. and Bacillus spp., which aid in the breakdown of different polymers. Laccases are found in fungi that biologically degrade lignin and catalyze the oxidation of aromatic and nonaromatic substances [47]. Different enzymes that involved biodegradation are illustrated in Table 3. Because bacterial cells can be grown and genetically modified more easily than other higher

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Fig. 3 Illustration of the microbial biodegradation process and the outcomes

Table 3 Examples of enzymes involved in bioremediation produced by different microbes Enzyme Oxidoreductases Oxygenases, monooxygenases, and dioxygenases

Laccases

Peroxidases Lipases Cellulases Proteases

Contribution in bioremediation Degradation of xenobiotics, such as phenolic or anilinic compounds Removal of sulfur, halogens, nitrate, and nitrite, production of ammonia Biodegradation and biotransformation of organic compounds aromatic as methanes, alkanes, cycloalkanes, alkenes, haloalkene Capable of catalyzing the oxidation of bisphenol A, ortho-, and para- polyphenols, diphenols, aminophenols, polyamines, lignins, and aryl diamine Utilize H2O2 in the oxidation of lignin, methoxybenzenes, and other phenolic compounds Biodegradation of polyethylene, polycaprolactone, polybutylene succinate, and adipate Biodegrade waste material manly (textile industry) used in paper, textile industry, and pulp industry Breakdown of substances contain protein through hydrolyzation of the peptide bonds

References [36] [37]

[38]

[39] [40] [41] [42]

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species, bacteria are thought to be superior suppliers of these enzymes among microorganisms [29].

Factors Affecting Microbial Degradation Process Many factors influence biodegradation efficacy, including the type and concentration of contaminants, their availability to microorganisms, and the physicochemical features of the environment [28]. The biodegradation process is a multifactored system that requires optimization and management. The availability of a microbial population capable of degrading pollutants; pollutants characteristics like surface features, shape, and molecular weight; and the accessibility of contaminants to the microbial population, in addition to environmental conditions (such as temperature, pH, and the presence of oxygen or other electron acceptors and nutrients), are all considered as crucial factors to control the process [43] (Fig. 4).

Moisture Water is an essential requirement for the development and multiplication of microorganisms and can affect pollutant breakdown speed [44]. The moisture content of the soil should be between 25 and 85 percent of the water holding capacity, with a

Fig. 4 Factors affecting the rate of microbial degradation

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range of 50 to 80 percent being ideal for biodegradation. Moisture-rich environments help the hydrolysis process by causing additional chain scission reactions. However, under high saturated conditions, oxygen in the soil vapor space can be used faster than supplied, causing the soil to become anaerobic. This can slow down the biodegradation rate and induce significant changes in microbial metabolic activity [45].

pH Environmental pH is an important parameter for biodegradation. Also, the pH of the substrate has a direct impact on microbial development [46]. Likewise, the pH conditions and the rate of degradation and microbial growth are all affected by the degradation products. Biodegradation may occur in a wide pH range; however, in most aquatic and terrestrial systems, pH of 6.5–8.5 is typically optimum, with values ranging from 5 to 9 considered acceptable. By altering the acidic or basic conditions, the pH can influence the rate of hydrolysis processes. For example, the rate of hydrolysis of PLA (polylactide) capsules is optimum at pH 5 [47], according to King et al. [48]. At higher pH, polymer breakdown increased. The pH of the soil may influence the availability of nutrients. For instance, the optimum pH for the solubility of phosphorus, an essential nutrient in biological systems, is 6.5 and declines at pH values greater or lower than this value.

Temperature Temperature plays a vital role in bioremediation [49]. The rate of biodegradation is directly influenced by temperature, which regulates the rates of enzyme-catalyzed processes and the physical and chemical state of substrate and microbes [50]. It affects microbial growth rate, metabolism, gas solubility, and soil matrix [51]. At 0  C, biodegradation rates are typically quite slow. For every 10  C drop in temperature, the biodegradation rate drops by about one-half. On the other hand, higher soil temperatures increase microbial metabolic activity and biodegradation rates up to around 65  C. With extremely high temperatures, the potential enzymatic degradability diminishes [52]. As a result, the biodegradation rate may vary seasonally [53], and the microbial metabolic activity can increase soil temperature. Vedrtnam et al. [53] reported that the softening temperature of the organic compound has a considerable impact on enzymatic degradability. Biodegradation is less likely in polyester with a higher melting point. Also, purified lipase from Rhizopus delemar, for example, hydrolyzed polyesters with low melting points like PCL [54]. According to J. Aislabie et al.’s study [56], temperature enhances the solubility of hydrophobic pollutants, lowers viscosity, improves diffusion, and accelerates the transfer of long-chain n-alkanes from the solid to the liquid phase. For example, the increase in temperature enhances the rate of oil breakdown by Bacillus sp., as

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reported in Adulrazag et al. [56]. In contrast, the decrease in temperature increases oil viscosity and reduces the volatilization of harmful low-molecular-weight chemicals, delaying biodegradation [55].

Microbes The two most crucial criteria for microorganisms’ growth are an energy source and carbon supply; otherwise, microorganisms can be isolated from any type of environment. Microbes can adapt and grow in a wide range of conditions, including temperatures, the presence of toxic chemicals, waste waters, and aerobic and anaerobic conditions.

Exogenous Versus Indigenous Exogenously applied microorganisms typically fail to operate at the intended level in a foreign environment; therefore, biodegradation methods’ effectiveness depends on the biodegrading capabilities of local microbial populations (Fig. 4) [24]. The indigenous populations of microbes constantly exposed to pollutants became acquainted, displaying selective enrichment and genetic changes. Adapted microbial communities can respond to contaminants such as hydrocarbons quickly and have greater biodegradation rates than communities that have never been exposed to similar conditions [26]. As a result, it is a priority to isolate indigenous microorganisms with significant degrading potential. Consortium Versus Individual (Pure) Microbe Microbial communities of bacteria and fungi are major groups reported to have been used to biodegrade pollutants [58]. Individual microbes in a consortium exchange metabolites or signal molecules to interact with each one. As a result of numerous species’ combined interactions, microbial consortia have more flexible and adaptive skills to handle complex environmental challenges by executing complex enzymatic catalysis [57]. In general, microbial species perform effectively in a community rather than a single one due to the interactions between microorganisms in the community, which guarantee the transmission of genetic information between microbial species, resulting in increased chemical-degrading capacity, resistance, and tolerance (Fig. 4).

Adaptation of Microorganisms to the Toxic Environment Adaptation is an evolutionary process characterized by a change in the microbial population or species, which is indicated by the increase in the rate of chemical biodegradation [47]. The emergence of microbial cells capable of metabolizing unusual chemicals could result from community adaptation to the molecule itself or a structurally similar one [44]. As a result, community members can increasingly endure or even degrade the potentially toxic effects of surrounding chemicals [45].

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Fig. 5 Adaptation of microorganisms to a toxic environment and in situ bioremediation

Once exposed to novel environmental contaminants, microorganisms evolve resistance and novel metabolic mechanisms [44]. The development of resistance mechanisms to metals or antibiotics (pollutants in the laboratory) in the natural environment of microbial species is a well-known example. Bacteria evolve in reaction to environmental changes, resulting in new enzymes and even catabolic pathways, as shown in Fig. 5. Microbial adaptation occurs via microbial interactions within the community, genetic transformation, and the microbial community’s interaction with the environment [48]. A well-known example is the evolution of microbial species’ metal or antibiotic resistance mechanisms in the natural environment. Bacteria evolve in reaction to environmental changes, resulting in the creation of new enzymes and even catabolic pathways.

Application of Microbial Biodegradation Microbial Degradation of Plastics Plastic is a polymeric compound generated mostly from hydrocarbon and petroleum byproducts. Plastics are being manufactured and used at an increasing rate due to their numerous industrial and domestic applications. Most of these polymers are nonbiodegradable, with a few exceptions [42].

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Nondegradable polymer buildup over time in soil has led to the loss of soil fertility and environmental and health issues. According to worldwide estimation, annually, around 57 million tons of plastic waste are created [43]. Furthermore, the accumulation of plastic polymers has increased sixfold in comparison to plankton in the oceans, placing the lives of aquatic birds and animals in danger. Humans are also suffering from the effects of plastics and their additives [44]. Biodegradable plastics are intended to degrade fast by microorganisms due to their capacity to degrade most organic and inorganic components, such as lignin, starch, cellulose, and hemicelluloses [59]. Biodegradable plastics can help alleviate the waste problem to some extent; as a result, there is a growing interest in biodegradable plastics. Microbial and/or enzymatic depolymerization of waste plastics into monomers for recycling is a viable technique for overcoming this problem and creating higher-value bio products [45]. The plastic biodegradation process includes the microbe excreting extracellular enzymes. Subsequently, the enzyme is attached to the plastic’s surface and hydrolyzed into short polymer intermediates, which are then consumed as a carbon source by microbial cells, producing CO2 (Fig. 6). Recently numerous microorganisms capable of metabolizing plastics have been discovered. Plastic degradation has been demonstrated in vitro by more than 90 microorganisms, including bacteria and fungi [46]. Bacteria and fungi use a variety of metabolic and enzymatic processes to degrade plastic polymers into CO2 and H2O. Different enzymes break down different polymer types due to their selectivity utilized in plastic degradation. Depending on the microbial species and even among strains, enzymes vary in nature and catalytic activity. Azotobacter sp.,

Fig. 6 Diagramed illustrations regarding the steps for the microbial degradation of plastic

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Bacillus megaterium, Halomonas sp., Pseudomonas sp., Ralstonia eutropha, Pseudomonas fluorescens, Pseudomonas aeruginosa, etc. are used to degrade plastics [49]. In recent years, fungal strains such as Aspergillus versicolor [52], Aspergillus flavus [53], and Chaetomium spp. [54] have been identified for plastic degradation. Also, fungi (Aspergillus niger, Aspergillus glaucus) were identified as the microbial species associated with the degrading plastic [55]. Table 4 summarizes some biodegradable polymers and microorganisms involved in biodegradation.

Microbial Degradation of Pesticides Pesticides are commonly utilized in agriculture to prevent and control crop diseases and pests. However, it is a recalcitrant toxic compound, and it remains in the environment for a long time, contaminate food systems, and builds up in the soil for decades after application, posing major health and environmental risks [64]. Bacteria are the most often used microorganisms in pesticide biodegradation because they are inexpensive and ecologically favorable and do not create secondary pollution [65]; particularly Bacillus, Pseudomonas, Arthrobacter, and Micrococcus are among the most common bacteria found in soil [3] (Table 5). Pseudomonas species

Table 4 Some biodegradable polymers and microorganisms involved in biodegradation Type of biodegradable plastics Polylactic acid (PLA) Polyhydroxyalkanoates (PHA) Polyethylene succinate (PES) Polycaprolactone (PCL)

Microorganisms involved in biodegradation Amycolatopsis sp., Bacillus licheniformis Bacillus, Burkholderia, Nocardiopsis, Cupriavidus, Mycobacterium, Micromycetes Bacillus sp. TT96, Paenibacillus genera Aspergillus sp. ST-0

References [60] [61] [40] [62] [63]

Table 5 Degrading microorganisms for some important classes of pesticides Class of pesticides Organochlorine

Degrading microorganisms Pseudomonas putida, Klebsiella sp., pseudomonas stutzeri, Aspergillus sydowii, Penicillium raistrickii

Organophosphates

Sphingomonas sp., Penicillium miczynskii, Trichoderma sp., Bacillus pumilus, Proteus vulgaris, Vibrio sp.

Carbamates

Pseudomonas, Mesorhizobium, Ralstonia, Rhodococcus, Ochrobactrum Bacillus licheniformis CY-012, Acinetobacter baumannii ZH-14, Brevibacillus parabrevis BCP-09

Pyrethroids

Examples DDT, aldrin, lindane, chlordane, mirex Malathion, methyl parathion, diazinon Sevin, carbaryl

References [66] [67] [35]

Pyrethrins

[72] [73] [74]

[68] [69] [70] [71]

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is the most effective bacterial genus for breaking down toxic compounds. The capacity of these bacteria to breakdown these chemicals are dependent on the amount of time they have in touch with the molecule, the environment in which they grow, and their physiological flexibility [14]. Studies showed the ability of fungi to biodegrade organochlorine pesticides. For example, the pesticide cypermethrin in the soil produce some major effect on the plant, but after biodegradation by plant growth-promoting rhizobacteria (PGPR; degraded microbes), the pesticide will convert to 3-phenoxy benzoic acid by the microbial enzyme through three different degradation mechanisms (adsorption, entering, and enzyme rection) and finally to the safe inorganic matter as shown in Fig. 7. Pesticide degradation has been reported in marine fungi such as Aspergillus sydowii, Trichoderma sp., Penicillium miczynskii, and Bionectria. Penicillium raistrickii sp. [75]. Bordjiba et al. [76] isolated various fungal species from

Fig. 7 Microbial degradation of some pesticide

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pesticide-contaminated soils in Algeria, including Fumigatus, Aspergillus terreus, Absidia, Rhizopus microsporus var. corymberifera microsporis, and Aspergillus niger.

Microbial Degradation of Antibiotic Antibiotics are pharmaceutically active chemicals that eliminate microorganisms in the host’s body. Antibiotics are excreted as active compounds or metabolites by animals (and humans) through urine or feces. It enters and accumulates in the environment because of direct discharges of untreated wastewater to surface water or groundwater and leaky sewers disposal in agricultural regions. Antibiotics discharged in wastewater cannot be removed using traditional treatment methods. Furthermore, antibiotic release and persistence in the environment may rise in antibiotic-resistant microorganisms and their high microbial growth inhibitory activity to other microbes [77]. One of the most promising strategies to break down antibiotics in water or soil sediments into simple compounds is to utilize microorganism [78]. In pure bacterial cultures and microbial consortiums, microorganisms from various environmental conditions have degraded a wide range of pharmaceutically active substances such as paracetamols, anti-inflammatory medication, Ibuprofen, and antibiotics. Several simple and complex metabolic pathways used in metabolizing a variety of antibiotics have been recognized and described. Thus, a thorough understanding of “indigenous microbial communities,” as well as the frequency with which they have antibioticresistant genes, redox mechanisms, biodegradation ability, nutritional, and carbon requirements, will aid in the discovery or design of an environmentally friendly microbial consortium capable of effectively reducing antibiotics and related contaminants. Many studies have found that supplemented nutrients, particularly carbon, nitrogen, and phosphorus sources, aid in the biodegradation and elimination of xenobiotics [78]. The association between antimicrobial-resistant genes and the ability of the microbes to degrade antimicrobials usually exists; however, this is not always the case. For example, Candida and Escherichia coli were able to degrade doxycycline without the presence of an antimicrobial-resistant gene. In addition to bacteria, antibiotic breakdown has also been described using fungal extracellular enzymes isolated from wasted mushroom composts [79]. It was also discovered that bacteria and fungus from the aquatic environment could degrade a combination of two antibiotics, ciprofloxacin, and erythromycin [80]. Many studies have reported the microbial degradation of antibiotics, for example, Aspergillus niger and Rhodococcus rhodochrous, degrade pharmaceuticals via co-metabolism [6]. Klebsiella sp. SQY5 and Stenotrophomonas maltophilia DT1 biotransform tetracycline. Sulfanilamide and chlortetracycline biodegradation was performed by acclimated microbial populations, which use them as a sole carbon and nitrogen source. Sulfamethoxazole is biodegraded by individual and mixed bacteria [78]. Finally, Fig. 8 illustrates the mechanism of ciprofloxacin biodegradation.

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Efflux pump

Biodegradation

Desethylation Hydroxylation NH2 N

NH

Ciprofloxacin HO F O

N

O HO NH N

N

N F

O

O

HO OH O

NH

OH

NH N

O

N

HO O

Fig. 8 Microbial degradation of ciprofloxacin

Nanobiodegradation Nanoparticles have begun to be newly used environmental remediation technologies that have the potential to provide low-cost solutions to some of the most complex environmental cleaning problems [12]. Nanoscale particles give considerable environmental flexibility due to their biocompatibility, huge surface areas, and strong surface reactivity [81]. These benefits applications will help improve the biocatalyst’s stability and its recovery and reuse processes in biodegradation (Table 6).

Nanoparticles Enhance Microbial Growth Nanoparticles act as biodegradation enhancers. When nanoparticles and microorganisms are involved in the process, nanoparticles either enhance the process or improve microbial growth profiles [98]. SiO2 nanoparticles enhance the growth of

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Table 6 Different applications of nanomaterials used in biodegradation processes Nanoparticles/ material SiO2 Fullerene 60

Microorganisms Pseudomonas sp. C 5, Bacillus sp. V8 Bacterial consortium

Magnetic Fe3O4

Bacterial consortium

Zero valent iron (nZVI)

The organochlorine respiring bacteria Bacteria reduce sulfate Bacteria reduce iron Sphingomonas sp.

Zero-valent iron (nZVI); Ti, Mn, Ag, Au Iron oxide (Fe3O4) Microbacterium sp. Pseudomonas putida α-Fe2O3

Bacillus spp.

Fe3O4

Bacillus subtilis

CNTs

Arthrobacter sp., Saccharomyces cerevisiae, Actinomycetes Escherichia coli

Cadmium zinc sulfide quantum dots Nanocellulose composites

Arthrobacter globiformis D47

Role in biodegradation Improve growth and increase biodegradation of polyethylene Increase LDPE degradation and it acts as a growth accelerator Enhancing the consortium growth and enhancing degradation Optimization of growth conditions and biodegradation rate

References [82] [83] [84]

[85]

Degradation of chlorinated hydrocarbons and decarbominated diphenyl ether Enhance the consortium growth and increase low-density polyethylene degradation Augmentation of atrazine biodegradation Ability to decolorize and degrade several azo dyes Easily separation and reuse Increase dye biodegradation, immobilization of microbes, bioremediation of heavy metals Degradation of nitro aromatic compounds

[86]

Herbicide (diuron)

[93]

[87]

[88] [89]

[90] [91] [92]

Pseudomonas sp. V8 and C25, and Bacillus sp. improve the degradation of polyethylene. Recent studies showed that magnetite nanoparticles might speed up the biodegradation of organic compounds (e.g., ethanol, acetate, propionate, and butyrate). Magnetite may promote interspecies electron transfer (IET) between heterotrophic partners [94]. Magnetite has also been shown to enhance the biodegradation of organic contaminants (such as trichloroethene and benzoate) [95]. In ciprofloxacin biodegradation, Yang et al. [96] reported that biodegradation was considerably enhanced by magnetite-supplemented enrichments, especially in the presence of 2-bromoethanesulfonate (BES). When BES was added to the magnetite-supplemented enrichments, ciprofloxacin biodegradation was 67 percent higher than in the magnetite-un supplemented enrichments which overcame the suppression of methanogenic activity and changing in the bacterial community composition. In bioremediation of crude oil by

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rhizosphere fungal, the crude oil degradation rate was significantly affected by adding silver nanoparticles (Ag NPs) [97].

Nanoparticles for Immobilization of Microorganisms Free cell biodegradation has many disadvantages, such as low biodegradation rate, cell separation, and substrate inhibition [98]. Thus, immobilized cell technology can be used to solve these problems by allowing a variety of microorganisms to be stabilized for a long time [37]. When comparing immobilized microorganisms to free cells in biodegradation, there is an improvement in the biocatalyst’s stability and facility of the recovery and reuse processes. The entrapment method is the conventional method for immobilizing microorganisms [99]. Xia Wang et al. [100] succeeded in developing efficient biocatalysts using magnetically immobilized cells by using a modified traditional entrapment method to entrap Sphingomonas sp. XLDN2–5, in a mixture of Fe3O4 nanoparticles and gellan gum in carbazole degradation. The resulting microbial cell/Fe3O4 bio-composite exhibited good biodegradation activity and reusability. Immobilization of microbial cells has been shown to improve microbial activity and lifespan and microbial cell reuse and recovery and protection from harmful environmental conditions [16].

Conclusions Bacteria, fungi, and algae have a wide range of enzymatic systems which help them grow in a wide range of environmental conditions and utilize a wide variety of substrates, including organic pollutants. Due to the increase of waste produced from extensive consumption of synthetic materials to higher levels of toxicity in whole ecosystems, the microbial consortium’s biodegradation of various organic pollutants is an efficient ecofriendly and sustainable technology. The efficiency of biodegradation depends on the chemical nature, the concentration of pollutants and their availability to microorganisms, and the physicochemical characteristics of the environment (such as temperature, pH, and the presence of oxygen or other electron acceptors and nutrients). Emerging nanotechnology to biodegradation offers low-cost solutions to several of the most complex environmental cleaning problems. Nanoparticles improve the biocatalyst’s stability and its recovery and reuse processes in biodegradation.

Future Perspectives As biodegradation technology advances to design and engineer microbial consortia, it is now possible to increase microbial productivity and achieve productivity beyond what is possible with pure culture. These programed microbial cells can be further employed to construct various synthetic and semisynthetic microbial

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consortia that may result in higher productivity. So, microbial consortia require more interdisciplinary research, including computational modeling, metabolomics, and proteomics, to overcome barriers such as cross talk, side products, and resource sharing. To make these consortia more applicable in real life, there is a need to focus on the following issues: • Creation of microbial consortia databases that contains complete information about nutrients, culture, and metabolite profiles of microbes to help in the selection process. • Introduction of novel techniques to study microbial consortia and their interactions using functional genomics (metabolomics, proteomics, and transcriptomics). • Synthetic biology and metabolome approaches should be explored to construct new microbial consortia with unique functions. • Using algae in the biodegradation process could be considered better than or with bacteria and fungi due to less specific pretreatment, and a rich carbon source is required. • The use of nanotechnology in biodegradation may enhance overcome the limitations of the degradation process. • To accelerate the microbial biodegradation of organic contaminants and reduce decontamination time, the physicochemical characteristics of environmental systems must be altered. Effective design of the growing system is critical to maximizing growth rates and lower costs.

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Concept and Significance of Microbial Consortium in the Biodegradation Process Lai Mun Koh and Sook Mei Khor

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Infallibility Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roles of Microorganisms in Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Consortium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Environmental pollution due to the discharge and accumulation of chemicals is a rising concern. Biodegradation, which utilizes microorganisms to degrade and detoxify a broad variety of substrates, is a critical research field dealing with contaminant attenuation. Continued efforts have been devoted to exploring the microbial diversity concerned with the biodegradation process. This chapter attempts to review the microbial infallibility hypothesis, which provides the backbone for biodegradation studies. In addition, the involvement of microorganisms in each of the biodegradation stages, i.e., biodeterioration, depolymerization, bio-assimilation, and mineralization, is described. This is followed by the concept and significance of microbial consortiums in biodegradation and the microbial interactions within the consortium. Also, advances and applications of different classes of microbes, comprising bacteria, fungi, and algae in biodegradation are considered. Here, specialized features of these microorganisms that L. M. Koh · S. M. Khor (*) Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_67

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facilitate efficient biodegradation and their dominant species engaged in the process are included. Lastly, different classes of enzymes capable of stimulating transformation are described, with oxidoreductases, hydrolases, and lyases being highlighted. These enzymes are compared by providing insights into their enzymatic functions and the pertinent substrates. Keywords

Biodegradation · Microbial infallibility · Microbial consortium · Bacteria · Fungi · Algae · Enzymes Abbreviations

AM AMO BIS CHEER ChrR FTIR HA HAO mRNA m/z NADH NMR NPnEO PAM PCR POP RNA rRNA SA sp. or spp. TBP TCA TEMED

Acrylamide Ammonia monooxygenase Bis-acrylamide HierarCHical taxonomic classification for viral mEtagEnomic data via deep leaRning Chromate reductase Fourier transform infrared Hydroxyapatite Hydroxylamine oxidoreductase Messenger RNA Mass-to-charge ratio Nicotinamide adenine dinucleotide Nuclear magnetic resonance Nonylphenol polyethoxylate Polyacrylamide Polymerase chain reaction Persistent organic pollutants Ribonucleic acid Ribosomal RNA Sodium alginate Species Tribromophenol Tricarboxylic acid N, N, N 0 , N 0 -tetramethylethylenediamine

Introduction Biodegradation is also known as microbial degradation, which implies the role of microorganisms in transforming organic substrates into simpler forms. Modern microbiology has focused on investigating microorganisms with degrading capabilities, and a wide array of microbial species have been discovered to be involved in biodegradation.

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Analysis of ribosomal ribonucleic acid (rRNA) genes provides the basis for the three-domain system to classify living organisms, including microorganisms. Carl Woese proposed three domains of life which are bacteria, archaea, and eukaryotes [1]. Both bacteria and archaea are prokaryotes, which lack a nucleus and membranebound cell organelles. For instance, fungi, protozoa, and algae are microorganisms that possess both a nucleus and cell organelles [2]. In biodegradation studies, it has been proven that both prokaryotes and eukaryotes can act as decomposers to break down various types of substrates (Fig. 1). Among these, biodegradation studies about archaea and algae are scarce, while the predominant decomposers include bacteria and fungi [3]. Archaea are found in all kinds of environments, with the majority of them being able to survive under extreme environmental conditions, besides being common in oceans and freshwater bodies [4]. The diversity of archaea is little known [1], and the most widely studied archaea in biodegradation are methanogens, which participate in anaerobic digestion by producing methane from substrates, such as acetate, and carbon dioxide and methylated compounds [5]. Protozoa are the least studied, and their associations with biodegradation are mainly predatory effects on bacterial populations [6]. The bacterivorous protozoa have either promotive or inhibitory effects on the biodegradation rate [6]. In some studies, selective grazing of protozoa on bacteria has enhanced the biodegradation rate. Protozoa reduce the abundance of some bacterial species and their competition for limited nutrients, besides stimulating the growth of some other species by releasing growth-limiting nutrients [6]. However, the direct involvement of protozoa in the biodegradation process has been shown in a study by Kachieng’a and Momba [7]. In this study, petroleum hydrocarbons, mostly aliphatic hydrocarbons and some aromatic hydrocarbons, were successfully degraded by individual protozoan species, Aspidisca sp.,

Fig. 1 Microorganisms participating in biodegradation

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Fig. 2 Magnification images of protozoan species: (a) Paramecium sp., (b) Vorticella sp., and (c) Epistylis sp./Opercularia sp. (Adapted with permission from Ref. [7] (2018, Elsevier))

Trachelophyllum sp., and Peranema sp., which decomposed almost 70% of the hydrocarbons. A greater biodegradation capacity (more than 80%) was also shown using a consortium of these species. Their later study [7] also demonstrated the biodegradation of petroleum hydrocarbons using other protozoan species, i.e., Paramecium sp., Vorticella sp., Epistylis sp., and Opercularia sp. (Fig. 2), and their consortium. Bacteria show diverse morphology and are present in different shapes such as rods, spheres (cocci), commas (vibrio), spirals, etc. [8]. Bacteria are known to thrive under harsh environmental conditions such as extreme pH, temperatures, salinity, pressure, and arid and nutrient-deficient areas. Fungi are non-photosynthetic eukaryotic organisms that comprise yeasts (unicellular) and molds (filamentous and multicellular). Most species of fungi are found in terrestrial environments, and fewer live in aquatic habitats [4]. Algae predominantly include photosynthetic organisms which obtain sunlight energy to grow, although some of the phyla are heterotrophic and rely on organic substrates for carbon and energy sources [9]. Algae can be classified into microalgae and macroalgae based on their cellular sizes. Microalgae possess microscopic, unicellular structures and are actively involved in biodegradation compared to macroalgae with multicellular and large-size structures. Like bacteria, algae can tolerate many environmental conditions and are mainly found in aquatic habitats.

Microbial Infallibility Hypothesis Gale (1951) proposed the microbial infallibility hypothesis, in which any organic compound, regardless of its complexity, can be degraded by a specific microorganism under suitable conditions. Otherwise, it can be synthesized by evolution and adaptation [10]. However, there is controversy about this hypothesis in the biodegradation study, where Alexander et al. [10] argues that microorganisms are fallible and some naturally occurring compounds are recalcitrant to biodegradation, even under suitable circumstances.

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On the other hand, Alexander’s concept had also received critique, where Horvath [11] mentioned that the concept of recalcitrance was due to human infallibility instead of microbial infallibility. This happened since the biodegradability tests overlooked the degradation of some recalcitrant compounds via co-metabolism, which produces neither carbon nor energy for the microorganisms involved [11]. Technological advancement has continuously led to the invention of synthetic materials that are foreign to microorganisms and thus remain persistent in the environment. The degrading capabilities are relatively scarce in these cases, and the relevant biodegradation pathways have not emerged. For example, per- and polyfluoroalkyl substances cannot be completely defluorinated by any microorganisms, though partial defluorination has been demonstrated. In line with Gale’s thought, many publications have successfully shown that the exposure of microorganisms to novel compounds encourages microbial adaptation and evolution [12]. This suggests that a higher concentration of a substance in the environment may stimulate the manipulation of genes in microorganisms, causing the genes encoding the enzymes for particular biotransformations to evolve. Consequently, several previously considered recalcitrant compounds are now proven to be biodegradable, which is attributed to the changes in the microbial community. Undoubtedly, the microbial infallibility hypothesis inspires the discoveries and investigations of microorganisms to degrade recalcitrant compounds. However, this belief should not be the reason for manufacturing and using environmentally unfriendly materials. The concept of microbial infallibility may collapse when increasing amounts of xenobiotic compounds are constantly discharged into the environment, complicating microbial evolution and adaptation and ultimately lengthening the duration needed for complete biodegradation or biodegradation impossible. Indeed, microbial adaptation and evolution cannot address the problems of pollution caused by nonbiodegradable materials due to the overwhelming introduction of xenobiotics into the environment.

Roles of Microorganisms in Biodegradation For biodegradation to occur, it is apparent that microorganisms are the most pivotal requirements that must be satisfied. The presence of microorganisms reduces the mass of contaminants in the environment and, in some circumstances, enables the complete removal of environmental pollutants, thus preventing their bioaccumulation. The main role of microorganisms is to secrete enzymes with the desired capabilities to break down particular substrates. Microorganisms engage throughout the biodegradation steps (i.e., biodeterioration, depolymerization, bio-assimilation, and mineralization) by demonstrating different functions and roles (Fig. 3). Microorganisms can perform chemical, physical, and/or enzymatic biodeterioration. In the initial stage, microorganisms colonize and survive on the polymer surface by forming biofilms [13]. The biofilm is a microbial aggregate that is formed when microorganisms secrete a viscous matrix of extracellular polymeric substances, such as polysaccharides,

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Fig. 3 Illustration of roles of microorganisms in different biodegradation stages of polymers. (Adapted with permission from Ref. [17] (2021, Elsevier))

proteins (amyloid, fibronectin, fibrin, laminin, and lipoprotein), nucleic acids, and lipids that enhance the microbial cells’ adhesion [13]. The microbial adhesion on the polymer surface usually turns the smooth surface into a rough surface and creates cracking [13, 14]. According to Sangeetha et al. [13], adding mineral oil increased the cellular surface hydrophobicity, favored the attachment of fungal strains like Aspergillus tubingensis and Aspergillus flavus to the high-density polyethylene surface, and eventually sped up the biofilm formation and biodegradation process.

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Fig. 4 Stages involved in biofilm development. (Adapted with permission from Ref. [15] (2021, Elsevier))

The microbial biofilm can be composed of a single or multiple microbial species, with multispecies biofilms possessing greater microbial diversity and complex matrixes being more common [15]. As demonstrated in Fig. 4, biofilm formation is a complex process that involves five stages: adhesion, multiplication, exodus, maturation, and active dispersal [15]. Biofilm formation starts with the reversible adherence of microorganisms to a substrate’s outer surface and occurs via transient interactions. Rapid attachments and detachments happen several times, making the microbial cells surface-adaptive and lengthening their adherence time on the substrates’ surface [16]. This eventually directs the microbial cells to attach irreversibly to the surface using adhesins such as flagella, pili/fimbriae, lipopolysaccharides, exopolysaccharides, and surface proteins. After that, the microbial cells accumulate to form a microcolony, producing extracellular polymeric substances that enhance cell-cell adhesion and their adherence to the surface [15]. New cells will adhere to the newly formed biofilm in the following exodus stage, while some cells may also escape from the biofilm [15]. This is followed by the biofilm maturation stage, where the multiplication of microbial cells occurs within the embedded extracellular polymeric substances, leading to the formation of three-dimensional, multicellular microcolonies. Besides, water-filled channels that transfer nutrients to the cells and eliminate waste products are formed [16]. Finally, the biofilm formation is terminated with the microbial cells detaching and moving to other substrates for new biofilm formation [16]. Figure 5 shows the formation of a microbial biofilm on the polystyrene film after 30 and 60 days of incubation.

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Fig. 5 Scanning electron micrograph image of (a) control polystyrene film, (b) attachment of the bacterial species Bacillus paralicheniformis G1 to polystyrene film after 30 days of incubation, and (c) formation of a microbial biofilm on polystyrene film after 60 days of incubation. (Adapted with permission from Ref. [18] (2021, Elsevier))

A variety of microorganisms are shown to initiate the colonization and biodeterioration processes. These include photosynthetic (e.g., algae and cyanobacteria), chemolithotrophic (e.g., sulfur-, iron-, and hydrogen-oxidizing and nitrifying bacteria), and chemoorganotrophic microorganisms [19]. Chemoorganotrophic microorganisms can release organic acids (e.g., malic, citric, tartaric, acetic, lactic, glucuronic, and oxalic) [20], whereas chemolithotrophic microorganisms can secrete inorganic acids (e.g., nitrous acids, nitric acids, and sulfuric acids) to support their growth. The actions of chemolithotrophs and chemoorganotrophs are referred to as chemical biodeterioration. For instance, in physical biodeterioration, microorganisms and fungi belonging to genera such as Alternaria, Cladosporium, Penicillium, and many others use their hyphae to penetrate deeply into the polymer, inducing cracking on the surface and decreasing their physical strength [21]. Enzymatic biodeterioration occurs when microorganisms secrete extracellular enzymes to catalyze chemical conversions involved in the degradation of materials.

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In the depolymerization step, microorganisms’ function is to secrete extracellular enzymes and/or free radicals. These enzymes are usually hydrolases and oxidoreductases. These enzymes will not diffuse into the polymer structure but attach to specific bonds that are vulnerable to an enzymatic attack, leading to bond cleavage [22]. Following the depolymerization step, the microorganisms assimilate transitional metabolites, and the metabolism occurs inside the cells via β-oxidation and the tricarboxylic acid (TCA) cycle, both catalyzed by intracellular enzymes [23]. The biodegradation process is finished with the mineralization of the metabolites within the microbial cells. Biodegradation involves mutual interaction between the microbes and the degradable substrates. These substrates act as energy and carbon sources, ensuring microbial growth and reproduction. A series of redox reactions occur during biodegradation, and energy is generated along with these reactions [24]. Carbons supplied from biodegradation are imperative as the primary fuels for energy generation and substrates to synthesize new cell constituents. Several studies have also shown that microbial growth is limited mainly by carbon sources [24].

Microbial Consortium The microorganisms involved do not always originate from the same strains or species in biodegradation. Instead, a microbial consortium consisting of several microbial groups is observed in many cases [25]. Individual microbial strains often possess lower degradation capabilities, attributed to the limited genes. In contrast, microbial consortia with varying genes provide synergistic effects and can degrade more complex structures and xenobiotic compounds. According to Shariati et al. [25], the biodegradation of xenobiotics, a mixture of phthalic acid esters (i.e., dimethyl phthalate, diethyl phthalate, dibutyl phthalate, monoethylhexyl phthalate, dioctyl phthalate, and diethylhexyl phthalate), was successfully shown using a consortium An6 which is composed of two bacterial strains, Pseudomonas putida strain ShA and Gordonia alkanivorans strain Sh6 which effectively degrade short-chained and long-chained phthalates, respectively. Besides, the consortium almost mineralized the individual diethylhexyl phthalate in less than 3 days. Also, Chen et al. [26] reported that a bacterial consortium mineralized polyvinyl alcohol within 36 h. On the contrary, individual bacterial strains isolated from the consortium could not break down the polymer completely even after 72 h, though they showed degradation capabilities. These two studies prove that a microbial consortium may be a more effective degrader than an individual strain. In microbial consortia, the microbes cooperate by providing nutrients to other microbes or by removing and neutralizing inhibitors to the degradation activities of other microbes. The microbial interaction occurring in a microbial consortium can be co-metabolism, syntrophism, and/or interspecies hydrogen transfer [27]. Among these, co-metabolism is discussed most frequently. In co-metabolism, the compound (nongrowth substrate) is transformed by a microorganism grown on a primary

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substrate, but the metabolites generated cannot be used as an energy source for microbial growth. The primary substrate produces enzymes that catalyze the transformation, besides acting as a carbon and energy source for the nongrowth substrate [28]. In addition, the metabolites cannot be mineralized, and biodegradation is halted [27]. Hence, co-metabolism is analogous to commensalism, where the nongrowth substrate is metabolized by the biodegradation of the primary substrate, while the biodegradation derives neither benefit nor harm from the co-metabolism. An example of co-metabolism is shown in Fig. 6, where monochloramine is co-metabolized by ammonia monooxygenases (AMO) and reducing power from ammonia is needed to drive the transformation [29]. A microbial consortium must not contain genes from the same microbial classes, genera, or species; however, the microbes can be associated with different microbial species. Rodrigues et al. [30] verified that a bacteria-algae consortium improved the biodegradation efficiency of an antibiotic, sulfamethoxazole, via symbiotic interaction where photosynthetic microalgae provide oxygen for the aerobic biodegradation of contaminants by the bacteria. In return, microalgal growth relies on carbon dioxide generated from bacterial respiration. An example of biodegradation involving microbial consortia is anaerobic biodegradation, where hydrolytic bacteria, acidogens, acetogens, and methanogens (i.e., acetoclastic or acetotrophic, hydrogenotrophic, and methylotrophic methanogens) work symbiotically to affect biodegradation under anoxic conditions. Each of these microorganisms has a distinct role in different stages of the degradation process. In anaerobic degradation, the microorganism supplies substrates needed for the subsequent degradation step performed by other microorganisms. Therefore, the

Fig. 6 Monochloramine co-metabolism pathways with ammonia in Nitrosomonas europaea strain. (Adapted with permission from Ref. [29] (2013, Elsevier))

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degradation step cannot proceed without any microorganism, even under suitable conditions. In microbial consortia, different combinations of microbial strains can give rise to different biodegradation rates, and some strains may be resistant to biodegradation rather than promote the process [31]. Hence, there are also cases where the consortium demonstrates lower degrading ability than the individual strain. Across the literature, the cultivation of microorganisms for biodegradation studies has focused on microbial consortia rather than individual strains in recent years. This is particularly obvious in the biodegradation of recalcitrant and synthetic compounds. Studies on microbial consortia are imperative as they are highly identical to the environmental conditions. However, the different rates arising from the distinct combinations of strains have complicated the bioremediation work, and the proper choice of strains plays a dominant role in ensuring the success of the removal of pollutants.

Bacteria Many studies have indicated that bacteria are effective degraders for many substrates in a diverse environment, such as activated sludge, contaminated soil, and marine. Several features contribute to the degradation potential of bacteria. First, a bacterial cell enables the passage of soluble substances into the cell and the transport of metabolites out of the cell. The easy transport of these substances is mainly attributed to the semipermeable phospholipid bilayer of the bacterial cells [27]. Second, due to their ability to use virtually all organic and inorganic substrates as energy and nutrient sources, bacteria have the greatest metabolic variability [32]. Third, the hydrophobic fimbriae and fibrils of bacteria enable the adherence of microbes to the water-insoluble hydrocarbons [33]. This feature makes bacteria effective petroleumdegrading microbes. Falkiewicz-Dulik et al. listed 13 out of 22 bacterial phyla, which apply to the deterioration of materials. These phyla are Actinobacteria, Cyanobacteria, Chloroflexi, Planctomycetes, Firmicutes, etc. [34]. Cyanobacteria (or blue-green algae) are photosynthetic microorganisms increasingly important in bioremediation. Numerous studies have shown that cyanobacteria can be utilized to remove heavy metals, aliphatic and aromatic hydrocarbons, and xenobiotic compounds. The study showed that the biodegradation efficiency of cyanobacteria decreased in the order of Cyanothece, Dunaliella, and Chlorella pyrenoidosa. A later work [35] showed that cyanobacteria species Phormidium lucidum and Oscillatoria subbrevis, isolated from domestic sewage water effectively degraded low-density polyethylene and about 30% of the polymer weight loss was achieved only after 42 days. As shown in Table 1, the dominant bacterial species are from the genera Pseudomonas and Bacillus. Many studies have shown that Pseudomonas spp. and Bacillus spp. can degrade a wide range of polymers, including polyvinyl chloride [36], polystyrene [18], low-density polyethylene [37], high-density polyethylene [38], polyvinyl alcohol [26], and vulcanized styrene-butadiene rubber [39]. In

Microorganisms Consortium An6 is composed of Pseudomonas putida strain ShA and Gordonia alkanivorans strain Sh6

Pseudomonas sp. YJB6

Pseudomonas aeruginosa (bacteria) and Meyerozyma sp. (yeast)

Pseudomonas citronellolis and Bacillus flexus

Pseudomonas lini and Acinetobacter johnsonii

Source Bacteria

Bacteria

Bacteria and yeast

Bacteria

Bacteria

Polystyrene

Crude oil includes aliphatic hydrocarbons (e.g., octane, nonane, decane, etc.), cyclic hydrocarbons (e.g., 2,4-ditertbutyl-7,7-dimethyl1,3,5-cycloheptatriene), and halogen-containing petroleum hydrocarbons (bromododecane, octadecane, 1-chloro, and tridecane, 1-iodo) Polyvinyl chloride

Target substrate Phthalic acid esters (a mixture of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, monoethylhexyl phthalate, dioctyl phthalate, and diethylhexyl phthalate) Di-n-butyl phthalate

Table 1 List of microorganisms associated with biodegradation

30

90

21

5

The incubation period (days) 56

The average molecular weight was decreased by about 10% and 17% with Pseudomonas citronellolis and Bacillus flexus, respectively The weight loss was about 1–2%

The weight loss was greater than or equal to 80% Pseudomonas aeruginosa removed 91% and over 80% of crude oil and chlorinated hydrocarbons, respectively, while 87% and 75–90% of crude oil and chlorinated hydrocarbons were degraded by Meyerozyma sp.

Remarks A 58–100% loss of phthalic acid esters was recorded at a high concentration of 2000 mg/L for each phthalic acid ester

[45]

[36]

[44]

[40]

References [25]

148 L. M. Koh and S. M. Khor

Bacillus spp. and Pseudomonas spp.

A consortium of Pseudomonas sp., Flavobacterium sp., Streptococcus sp., Micrococcus sp., Leptothrix sp., Paenibacillus sp., Brevibacterium sp., Bacillus sp., Sphingomonas sp., and Rhodococcus sp. Bacillus subtilis, Pseudomonas aeruginosa, and Streptomyces sp. Bacillus paralicheniformis G1

Bacteria

Bacteria

Bacteria

Bacteria

Bacteria belonging to genera Pseudomonas, Bacillus, Brevibacillus, Cellulosimicrobium, and Lysinibacillus, and fungi of genus Aspergillus

Bacteria and fungi

Polystyrene

Vulcanized styrene-butadiene rubber

Polyvinyl alcohol

High-density polyethylene

Low-density polyethylene

60

28

36 h

30

112

Significant weight loss was not shown. The carbon loss was about 15–20% The weight loss of polystyrene was reduced by 34%

Aspergillus oryzae strain A5 resulted in a 36.4% reduction in polymer weight The greatest weight loss (35.7%) among the bacterial species was caused by Bacillus cereus strain A5, an (MG645264) Bacillus aryabhattai VRKPV15 and Pseudomonas aeruginosa VRKPC5 were the most effective degraders with polymer film weight losses of 23.14% and 13.73%, respectively Polyvinyl alcohol was completely degraded

(continued)

[18]

[39]

[26]

[38]

[37]

6 Concept and Significance of Microbial Consortium in the Biodegradation Process 149

Enterobacter sp. D1

Consortium CAS6 (Exiguobacterium sp., Halomonas sp., and Ochrobactrum sp.).

Alcanivorax sp., Hyphomonas sp., and Cycloclasticus sp.

Bacteria

Bacteria

Microorganisms A consortium composed of Paenibacillus alvei CBMAI 2221, Bacillus sp. CBMAI 2222, Bacillus safensis CBMA 2220, Bacillus aryabhattai CBMAI 2223, and Bacillus sp. CSA-13 Phormidium lucidum and Oscillatoria subbrevis

Bacteria

Bacteria

Source Bacteria

Table 1 (continued)

Polyethylene terephthalate and hydrocarbons (tetradecane, diesel, and naphthalene/ phenanthrene)

Polyethylene terephthalate and polyethylene

Polyethylene

Low-density polyethylene

Target substrate Pyraclostrobin

20 days for hydrocarbons and 45 days for polyethylene terephthalate

28

31

42

The incubation period (days) 28

The weight loss was estimated at 30% for both bacterial strains The formation of biofilm was observed, along with the appearance of cracks, roughness, and depressions In polyethylene terephthalate and polyethylene, the proportion of large molecules decreased while the proportion of small molecules increased. The crystallinity index was decreased The hydrocarbons were degraded by 20–90%. The surface of the degraded polyethylene terephthalate was highly uneven, with cracks and cavities; additionally, the quantities of ester bonds decreased due to hydrolysis

Remarks The consortium, together with the native microbiome, caused almost 50% of the weight loss

[48]

[47]

[46]

[35]

References [31]

150 L. M. Koh and S. M. Khor

Streptomyces sp.

Rhodococcus fascians strain NKCM 2511. Cyanothece sp. PCC7822, Synechocystis sp. PCC6803, and Synechococcus sp. PCC7942

Proteiniphilum acetatigenes and Propionibacterium acidipropionici

Citrobacter sp. Y3.

The dominant microalga is Chlorella sorokiniana (78.8%), and the bacterial species was not characterized

Bacteria

Bacteria

Bacteria

Bacteria

Microalgae and bacteria

Bacteria

Ideonella sakaiensis 201-F6

Bacteria

Sulfamethoxazole

Hexabromocyclododecane

Nonylphenol

Poly(butylene adipate-coterephthalate) Dimethyl phthalate

Polyethylene terephthalate

Polyethylene terephthalate

7

8

8

5

20

18

42

Complete degradation of dimethyl phthalate (20 mg/L) was recorded for Cyanothece sp. PCC7822 and Synechocystis sp. PCC6803 in 4 and 5 days, respectively. Synechococcus sp. PCC7942 caused a weight loss of more than 75% for dimethyl phthalate (20 mg/L) The anaerobic degradation efficiency of nonylphenol was 55–70% at pH 10 and in the presence of a nonionic surfactant, Brij 35 More than 90% of the hexabromocyclododecane was removed Under light illumination, approximately 50% of the mass loss was reported

The polyethylene terephthalate film was almost completely decomposed The weight loss recorded was about 50–70% The loss of weight was 9%

(continued)

[30]

[54]

[53]

[52]

[51]

[50]

[49]

6 Concept and Significance of Microbial Consortium in the Biodegradation Process 151

Aspergillus tubingensis VRKPT1 and Aspergillus flavus VRKPT2 Aspergillus bertholletiae BIORG 4, Aspergillus sp. BIORG 5, Penicillium paxilli BIORG6, Trichoderma sp. BIORG 7, and Aspergillus sp. BIORG 9 Coriolopsis caperata BM-172, Pluteus chysophaeus BM-792, and Fomes fomentarius BM-745

Fungi

Fungi

Fungi

Fungi

Microorganisms Rhodococcus sp. (bacteria), Trichoderma tomentosum (fungi), and Fusarium oxysporum (fungi) Aspergillus terreus strain MANGF1 and Aspergillus sydowii strain PNPF15

Source Bacteria and fungi

Table 1 (continued)

Polycyclic aromatic hydrocarbons (phenanthrene, pyrene, anthracene)

Chloramphenicol

High-density polyethylene

Target substrate Polycyclic aromatic hydrocarbons (e.g., anthracene, phenanthrene, fluorine, and pyrene) Polyethylene

25

9

30

60

The incubation period (days) 49

The degradation rate was 85–95% with Pluteus chysophaeus, more than 75% for Fomes fomentarius, and more than 65% for Coriolopsis caperata

The initial concentration (100 mg/L) of chloramphenicol was declined by 25–30%

Aspergillus terreus strain MANGF1 degraded more than 50% of polyethylene, while Aspergillus sydowii strain PNPF15 reduced the tensile strength of the polymer by 2.4% The weight loss ranged from 6 to 9%

Remarks The mass loss ranged from 8 to 17%

[58]

[57]

[13]

[56]

References [55]

152 L. M. Koh and S. M. Khor

A consortium of Aspergillus niger, Aspergillus flavus, and Aspergillus oryzae

Polyporus versicolor, Pleurotus sajor-caju, Phanerochaete chrysosporium, ME 446, Pleurotus ostreatus, Pleurotus sapidus, Pleurotus eryngii, and Pleurotus florida Gymnopus luxurians and Hypholoma fasciculare Penicillium oxalicum

Fungi

Fungi

Fungi

Fungi

Phanerochaete chrysosporium

Fungi

Nonylphenol polyethoxylates (NPnEO). Polycyclic aromatic hydrocarbons (e.g., anthracene, dibenzothiophene, phenanthrene, and dibenzofuran)

Polyvinylchloride

Low-density polyethylene

Polycyclic aromatic hydrocarbons in the asphalt shingle binder

5

15

30

55

56

The weight loss was estimated at 35–75% Both anthracene and dibenzothiophene were mineralized (100% degradation), while phenanthrene and dibenzofuran were removed by 30% and 19%, respectively

The reduction in weight loss of asphalt was recorded at 21%. A decrease in the aromaticity index was observed Low-density polyethylene (2 cm  2 cm) was reduced by 26% of its initial weight when incubated in a potato dextrose broth medium Significant weight loss was not seen; however, the proportions of C-H, C-Cl, and C-O bonds were reduced drastically

(continued)

[63]

[62]

[61]

[60]

[59]

6 Concept and Significance of Microbial Consortium in the Biodegradation Process 153

Microalgae

Microalgae

Source Fungi

Chlorella vulgaris and Scenedesmus oblique

Microorganisms Pleurotus ostreatus, Bjerkandera adusta, Phanerochaete chrysosporium, Phanerochaete magnoliae, Trametes versicolor, Pycnoporus cinnabarinus, and Dichomitus squalens Chlorella vulgaris

Table 1 (continued)

Crude oil hydrocarbons (indole-3-acetic acid, decane, benzene, phenanthracene, cyclohexane undecyl, naphthalene, etc.)

Crude oil hydrocarbons include light hydrocarbons (e.g., naphthalene, benzene, eicosane, dodecane, etc.) and heavy hydrocarbons

Target substrate Polychlorinated biphenyl

15

14

The incubation period (days) 42

Complete degradation of light hydrocarbons (10 g/L oil concentration) while 78% of heavy hydrocarbons (10 g/L initial oil concentration) were removed Complete degradation of indole-3-acetic acid for both strains

Remarks The greatest removal rate (over 98%) was caused by Pleurotus ostreatus

[66]

[65]

References [64]

154 L. M. Koh and S. M. Khor

Chlorella sp. and Scenedesmus sp.

Cymbella sp. and Scenedesmus quadricauda

Uronema africanum Borge

Microalgae

Microalgae

Microalgae

Low-density polyethylene

Pesticides, such as chlorpyrifos, cypermethrin, and oxadiazon Naproxen

30

30

7

Naproxen was removed by 97% and 58% for Cymbella sp. and Scenedesmus quadricauda, respectively, when the cultures were spiked with 1 mg/L of naproxen Damage to the polymer surface with erosions, abrasions, grooves, and ridges was observed. The formation of carboxylic acids, esters, and nitro and amino compounds was shown

The pesticides were removed by 10–55%

[69]

[68]

[67]

6 Concept and Significance of Microbial Consortium in the Biodegradation Process 155

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L. M. Koh and S. M. Khor

addition, the degradation capabilities of Pseudomonas spp. are not exclusive to polymers but also phthalic acid esters [25] and petroleum hydrocarbons. Figure 7 shows the metabolic pathways of di-n-butyl phthalate where the initial activation step involves β-oxidation and de-esterification. The mono-butyl phthalate thus formed is then transformed into phthalic acid via another de-esterification reaction. Subsequent degradation steps generate intermediates such as benzoic acid, 4,5-dihydroxy phthalate, catechol, and protocatechuic acid, which produce organic acids using ring-cleavage enzymes. This eventually drives the organic acids into the TCA cycle. Bacillus spp. have also been shown to degrade xenobiotics such as strobilurin and the fungicide pyraclostrobin [31]. Table 1 also lists other bacterial strains that are degraders. To date, numerous studies have been carried out to investigate the biodegradation of polymers using bacteria from diverse phyla. The different rates arising from the inoculum of these bacteria indicate that phylogenetic origins impact biodegradation, with certain phyla demonstrating remarkable degrading efficiency. The broad range of species for each genus also makes it difficult to determine the genus with the greatest and least degrading capabilities. Hence, current research calls for rapid microbial identification approaches that provide accurate information for the characterization and classification of microorganisms in a shorter time. In this context, molecular methods (e.g., polymerase chain reaction (PCR)) and spectroscopic methods (e.g., Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy) are of increasing importance in providing profiles of microbial communities through gene amplification and the interaction of electromagnetic radiation with light-absorbing macromolecular composition in microorganisms, respectively. These methods are culture-independent and thus time-saving than the conventional culture method. Recently, sophisticated omics approaches, including metagenomics, proteomics, transcriptomics, and metabolomics, have opened a new window for the characterization of microbial communities. Multiple omics approaches can provide information about genes, protein abundance, messenger ribonucleic acid (mRNA) transcript levels, and metabolites [41]. Another multidisciplinary approach, i.e., machine learning, has also emerged to handle and analyze extensive sets of microbial community data by creating and evaluating models that use algorithms to recognize, classify, and predict the data outcomes [42]. For instance, a hierarchical classification model, namely, CHEER, enables the read-level taxonomic classification of microorganisms into order, family, and genus for novel ribonucleic acid (RNA) viral species via a deep learning classifier [43].

Fungi Many fungi from different classes have been discovered to be degraders due to their widespread availability in air, water, and soil, mainly caused by tiny spores formed on the fungal hyphae that enable fungi to be dispersed over long distances

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Fig. 7 Suggested biodegradation pathways of di-n-butyl phthalate by Pseudomonas sp. strain YJB6. (Adapted with permission from Ref. [40] (2021, Elsevier))

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[32]. Besides, filamentous fungi have mycelium consisting of branching threads (hyphae) capable of penetrating deeply into the polymer, enabling them to assimilate the polymer. Besides that, filamentous fungi produce proteins known as hydrophobins that cover the hyphal wall, enhancing their attachment to hydrophobic substrates. Across the literature, fungi are demonstrated to degrade a wide variety of substrates, including polymers [13], hydrocarbons [59], surfactants [62], and emerging organic contaminants such as antibiotics [57] and organochlorine pesticides [64]. An example of the degradation mechanisms of a nonionic surfactant, nonylphenol polyethoxylates (NPnEO), by fungal species, Gymnopus luxurians and Hypholoma fasciculare, is shown in Fig. 8 [62]. Most studies have shown that the Aspergillus genus is the most common degrader (Table 1). For example, A. sydowii [56], as well as A. tubingensis and A. flavus [13], has been demonstrated to degrade polyethylene and high-density polyethylene, respectively. Apart from the Aspergillus genus, fungal species comprising Trichoderma tomentosum, Fusarium oxysporum [55], Coriolopsis caperata, Pluteus chysophaeus, Fomes fomentarius [58], and Phanerochaete chrysosporium [59] were also shown to degrade polycyclic aromatic hydrocarbons. Table 1 provides other examples of fungi that are involved in biodegradation.

Fig. 8 Enzymatic degradation mechanisms of NPnEO by fungi NPnEO is taken into the fungal cell through biosorption, which is then degraded by intracellular enzymes through catabolism. The fungi also secrete extracellular enzymes to oxidize the surfactant, such as manganese peroxidase and laccase. (Adapted with permission from Ref. [62] (2019, Elsevier))

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Algae Algae predominantly include photosynthetic organisms which obtain sunlight energy to grow, although some of the phyla are heterotrophic and rely on organic substrates for carbon and energy sources [9]. Compared to bacteria and fungi, few studies have concentrated on biodegradation using algal species, and most of their degradation mechanisms are not yet clarified. However, algae are superior to bacteria and fungi in biodegradation from the perspective of environmental safety. Both bacteria and fungi produce their respective toxic substances, endotoxin, and mycotoxin, which make these microbes biological pollutants, whereas neither endotoxin nor mycotoxin is found in microalgae. The most notable algal species are green algae from the genera Chlorella and Scenedesmus. This species is frequently used as a degrader in the biodegradation of crude oil hydrocarbons. Besides, Avila et al. [67] highlighted the potential of a microalgae consortium to decompose pesticides, namely, chlorpyrifos, cypermethrin, and oxadiazon, via biodegradation and/or photodegradation. After 7 days of incubation, about 55%, 35%, and 14% of chlorpyrifos, cypermethrin, and oxadiazon were removed, respectively. In a study conducted by Ding et al. [68], two freshwater microalgal species, Cymbella sp. and Scenedesmus quadricauda, both demonstrated the ability to degrade the pharmaceutical drug naproxen; however, removal efficiency decreased as naproxen concentration increased. Many biodegradation studies have been focused on phycoremediation, which refers to the employment of microalgae to treat wastes and wastewaters [70], mainly attributed to their ability to produce algal biomass. Algal biomass can generate a variety of biofuels such as biodiesel, biogas, biohydrogen, biochar (carbon-rich charcoal produced via pyrolysis), and bioelectricity. Moondra et al. [71] also recommended microalgae-based wastewater treatment plants as a cost-effective and energy-saving alternative. The study assessed the efficiency of Chlorella vulgaris and found that the removal efficiencies reached more than 80% for the nutrients (i.e., phosphate and ammonia) and chemical oxygen demand (COD) in 7 h without any external aeration, for both filtered and unfiltered effluents. In the wastewater treatment plants, algae also supply oxygen obtained from photosynthesis for aerobic degradation of organic matter by bacteria while consuming carbon dioxide released from the aerobic degradation for their growth (Fig. 9) [14]. The metagenomic analysis provides insights into the complex microbial communities responsible for degrading specific substrates in the biodegradation study. Table 2 summarizes the features of microorganisms that facilitate efficient biodegradation and the dominant species engaged in the process. Microorganisms from different classes, genera, and species often demonstrate varying biodegradation efficiency. Bacteria, fungi, and algae all possess certain structural features that enable them to survive under different conditions and acquire certain degradation capabilities. In addition, the potential to biodegrade a material may also vary when the microbial strains are isolated from different environments (e.g., aerobic, methanogenic, sulfate-reducing, nitrate-reducing, etc.). Recent advancements in genetic engineering have also contributed significantly to the invention of

160

L. M. Koh and S. M. Khor

Fig. 9 Roles of microalgae in the wastewater treatment plant and the applications of algal biomass. (Adapted with permission from Ref. [14] (2021, Elsevier))

Table 2 An overview of the features of microorganisms and the dominant genus/genera that enable effective biodegradation Classes of microorganisms Bacteria

Fungi

Algae

Features that enable effective biodegradation • A semipermeable bacterial cell membrane enables easy transport of substances in and out of the cell • The greatest metabolic variability • Easy adhesion of hydrophobic materials to bacterial walls due to hydrophobic fimbriae and fibrils • Tiny spores formed on the hyphae enable easy dispersal of fungi in various environments • Hyphae in mycelium allow penetration into polymers • Secrete hydrophobins, which permit easy adhesion of hydrophobic materials • Algae do not produce endotoxin or mycotoxin, making them environmentally friendly • Photosynthetic activity supplies oxygen, which accelerates aerobic degradation

Dominant genus/ genera involved in biodegradation • Pseudomonas • Bacillus

References [32]

• Aspergillus

[32]

• Chlorella • Scenedesmus

[14]

biodegradable materials. Identifying the microbial strains or genes with outstanding degradation capabilities allows the microbial communities to be tailored, and the genes needed for biodegradation can be incorporated into the microbial strains to enhance the material’s biodegradability.

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Enzymes During biodegradation, the enzymes secreted by microorganisms mediate a certain metabolic reaction. The enzymes can work inside (intracellular) or outside (extracellular) the microbial cells. Intracellular enzymes influence intracellular metabolism, whereas extracellular enzymes mainly digest large substrates into smaller substrates for assimilation into the cell [32]. Enzymes are particular; they only bind to specific substrates at their active sites and thus catalyze specific transformation reactions. The conventional classification categorizes enzymes into six classes: oxidoreductases, hydrolases, transferases, lyases, isomerases, and ligases, as shown in Fig. 10. Among these enzymes, oxidoreductases, hydrolases, and lyases are supreme enzymes since the degradation mechanisms of most materials involve oxidation, reduction, decarboxylation, etherification, and esterification [32]. Thus, this section introduces the enzymes oxidoreductases, hydrolases, and lyases, emphasizing their general roles and specific examples of the enzymes. Oxidoreductases are the largest class of enzymes with the function of catalyzing redox reactions. A wide diversity of oxidoreductases can be produced by microbes, including oxygenases, dehydrogenases, oxidases, peroxidases, hydroxylases, and reductases. The involvement of oxygenases and dehydrogenases in the aerobic biodegradation of crude oil hydrocarbons has been widely described. Oxygenases, i.e., monooxygenases and dioxygenases, catalyze the addition of one or two oxygen atoms to the substrates [22]. Oxygenases activate the aerobic biodegradation process by catalyzing monoterminal, biterminal, and subterminal oxidations of aliphatic hydrocarbons [22]. For aromatic hydrocarbons, aerobic biodegradation is catalyzed by a greater variety of oxygenases, each performing different roles in the biodegradation mechanisms. For instance, the biodegradation of aromatics is kicked off with the presence of ring-hydroxylating dioxygenases, which catalyze the hydroxylation

Fig. 10 Classification of enzymes involved in biodegradation

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of the aromatic ring. Apart from that, intradiol or extradiol ring-cleavage dioxygenases are needed for the ring opening [72]. Hydrogenases oxidize substrates by transferring hydrogen to electron acceptors. In the aerobic biodegradation pathway, hydrogenases often catalyze the oxidation of alcohols into aldehydes and later carboxylic acids [22]. These enzymes are also used for the dehydrogenation and aromatization of cis-dihydrodiols to produce catechol [72]. Hydroxylases catalyze the addition of hydroxyl groups to substrates [73]. Many studies have recognized alkane hydroxylases are the key enzymes for the biodegradation of various materials, for example, petroleum [74] and polymers such as polystyrene [75]. Peroxidases use hydrogen peroxide and organic hydroperoxides to stimulate the conversion of organic compounds, mainly phenols, amines, and heterocyclic compounds, into the oxidized form [32]. Based on the availability of the heme group, peroxidases are categorized into two main classes: heme and non-heme peroxidases (Fig. 11). Heme peroxidases are more dominant and include animal peroxidase, catalase, haloperoxidase, and nonanimal peroxidase. Nonanimal peroxidases can be further categorized into three subclasses, each originating from different sources. Class 1 peroxidases are mainly secreted by bacteria and eukaryotes and include catalase-peroxidase. Class 2 is fungal peroxidase, including manganese peroxidase, lignin peroxidase, and versatile peroxidase, which have been extensively studied for biodegradation. Class 3 peroxidases are synthesized by plants [76]. In biodegradation, laccases, manganese peroxidase, lignin peroxidase, and versatile peroxidase are commonly known as ligninolytic or lignin-modifying enzymes which are used to degrade lignocellulosic biomasses. One of the main applications of

Fig. 11 Classification of peroxidases. (Adapted with permission from Ref. [76] (2017, Elsevier))

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Fig. 12 Decolorization of azo dyes using enzymes like laccase and azoreductases synthesized by microorganisms. (Adapted with permission from Ref. [80] (2021, Elsevier))

these ligninolytic enzymes is the decolorization of synthetic dyes (Fig. 12), as shown in Table 3. The degrading abilities of these enzymes enable them to be utilized to remove synthetic dyes from wastewater treatment plants [77]. An example is depicted in Fig. 13, which shows the degradation pathway of Acid Orange 7 by manganese peroxidases [78]. Though these enzymes are mainly secreted by fungal mycelium, which is responsible for their degradation ability, some peroxidases are also secreted by many bacterial species, such as Bacillus and Pseudomonas spp. [79]. Nayanashree and Thippeswamy [79] identified Bacillus subtilis as the dominant rubber-degrading bacteria, and the bacterial activity was contributed mainly by manganese peroxidase, followed by laccase. Oxidases utilize oxygen as an electron or hydrogen acceptor, while reductases catalyze reduction reactions. Laccase is one of the oxidases that have received much attention in biodegradation studies due to their low substrate specificity, enabling them to oxidize a broad range of substrates such as phenolic and non-phenolic compounds as some inorganic ions [86]. Laccases are multi-copper oxidases commonly secreted by fungi that catalyze the single-electron oxidation of hydrogendonating substrates concurrently with reducing molecular oxygen to water. Laccases do not require external sources of hydrogen peroxide and Mn2+ as cofactors, as in the case of lignin peroxidase and manganese peroxidase; besides, water is the only

Anthraquinone dyes, i.e., C.I. Acid Blue 225 and C.I. Acid Violet 109

Azo dyes such as Acid Orange 7, Reactive Violet 5, methyl orange, Reactive Black 5, methyl red, etc. Lignin (i.e., 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid, 2,6-dimethlphenol, guaiacol, and veratryl alcohol) Synthetic dyes are composed of azo, triphenylmethane, heterocyclic, polymeric, and metal complexes (e.g., methyl orange, cotton blue, methyl violet, amido black, etc.) Azo dyes (i.e., C.I. Acid Black 210 and C.I. Acid Black 234) Diazo, heterocyclic, triphenylmethane, and anthraquinone dyes such as bromophenol blue, Coomassie brilliant blue, methylene blue (heterocyclic), Rimazol brilliant blue R, methylene blue, panseu-S, and Congo red

Horseradish peroxidases

Manganese peroxidases

Laccases

Laccases

Lignin peroxidases

Dyedecolorizing peroxidase

Substrates Textile dye, i.e., Drimarine blue K2R

Enzymes Manganese peroxidases

Whiterot fungi

Bacteria

Bacteria

Whiterot fungus

Yeast

Sources Whiterot fungi No data

Micrococcus luteus Paraconiothyrium variabile

Korcuria rosea MTCC 1532

Irpex lacteus CD2

Meyerozyma caribbica

Microorganisms Phanerochaete chrysosporium IBL-03 No data

Table 3 Examples of peroxidases and laccases used for the decolorization of dyes

3h

96 h

More than 90% 18–100%

60–100%

No data

No data

5h

87–99%

More than 85%

15 (for C.I. Acid Violet 109) and 32 minutes (C.I. Acid Blue 225). 6–24 h

Incubation period 7 days

Percentages of decolorization 65%

[85]

[77]

[84]

[83]

[78]

[82]

Ref. [81]

164 L. M. Koh and S. M. Khor

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Fig. 13 Degradation pathway of Acid Orange 7 by manganese peroxidases secreted by Meyerozyma caribbica. (Adapted with permission from Ref. [78] (2021, Elsevier))

by-product formed [87]. Hence, laccases are ideal green catalysts that aid in bioremediation, predominantly removing synthetic dyes from water bodies [86]. Inspired by the importance of laccases in industrial applications, many researchers are searching for laccase immobilization technologies to overcome high production costs and other shortcomings of the enzymes. It is noteworthy that Liu et al. [87] found a novel immobilization technology using 3D bioprinting, and the immobilized laccases were shown to be effective catalysts for the biodegradation of p-chlorophenol (Fig. 14). Reductases are often utilized in the bioremediation of heavy metals, such as chromium and mercury. For instance, chromate (Cr6+) reductases are effective in the bioremediation of hexavalent chromium species (Cr6+) under both aerobic and anaerobic conditions [88]. Cr6+ ions are extremely toxic to living organisms due to their mutagenic, carcinogenic, and teratogenic properties [88] and their high bioavailability due to their high water solubility at all pH levels [89]. Hence, bioremediation often employs enzymatic reduction of Cr6+ to trivalent chromium species (Cr3+), which is less toxic and bioavailable [88]. With Cr6+ reductase, a chromiumresistant bacterial species, Staphylococcus sciuri A-HS1 eliminated about 90% of Cr6+ in 6 days of incubation by reducing Cr6+ to Cr3+ [89]. Figure 15 demonstrates how chromate is removed by reductases [90]. Other than that, mercuric reductase (MerA) genes purified from Citrobacter freundii strain MM7 have been shown to promote the reduction of Hg2+ to Hg0, degrading about 80% of the Hg2+ [91].

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Fig. 14 Immobilization of laccases on a 3D-bioprinted hydrogel containing calcium chloridecross-linked sodium alginate (SA), hydroxyapatite (HA), and polyacrylamide (PAM) formed by cross-linking of acrylamide (AM) by bis-acrylamide (BIS) as cross-linkers, catalyzed by N, N, N 0 , N 0 -tetramethylethylenediamine (TEMED). (Adapted with permission from Ref. [87] (2020, Elsevier))

Fig. 15 Mechanisms involved in Cr(VI) enzymatic reduction. Cr(VI) enters the microbial cells either through biosorption on the extracellular polymeric substances of the microbial cell surface or direct uptake. Inside the microbial cell, Cr(VI) is transformed into Cr(III) by chromate reductases, and excess Cr(VI) is delivered away from the cell. To catalyze the Cr(VI) reduction, chromate reductases may also be bound on the cell surface. (Adapted with permission from Ref. [90] (2021, Elsevier))

Enzymes such as esterases, depolymerases, and dehalogenases belong to the second-largest class of enzymes, hydrolases. These enzymes catalyze the hydrolysis of the specific active bonds of the target compounds [27]. Gunawan et al. [92] indicated that cholesterol esterase derived from Pseudomonas spp. could hydrolyze ester and urethane bonds in polyurethane, decomposing the polymer into a combination of monomers and water-soluble oligomers. Liang et al. [93] successfully identified novel dehalogenases released by Bacillus sp. GZT catalyzes the debromination of 2,4,6-tribromophenol (TBP) by replacing the three bromine atoms with hydrogen atoms in a stepwise mechanism (Fig. 16). Another study by Nguyen et al. [94] discovered that L-2-haloacid dehalogenase extracted from

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Fig. 16 A suggested debromination pathway of TBP by the TBP dehalogenase. The presence of the metabolites (2-bromophenol and phenol) drawn inside the red boxes is not yet confirmed. (Adapted with permission from Ref. [93] (2017, Elsevier))

Burkholderia cenocepacia 869T2 dechlorinated dioxin congener 2,3,7,8tetrachlorinated dibenzo-p-dioxin. Lyases are enzymes catalyzing non-hydrolytic and non-oxidative cleavage of the bonds between carbon and atoms such as carbon, oxygen, nitrogen, sulfur, halides, and phosphate (P-O) bonds. These enzymes commonly display high substrate specificity and selectivity [32]. An example of a lyase is alginate lyases, which hydrolyze the glycosidic bonds in the alginate (polysaccharides present in the algal cell wall) via β-elimination into oligomers and monomers. The alginate degradation often involves a recombinant of endotype and exotype lyases [95]. In a study by Fischer and Wefers [95], two out of the five alginate lyases cloned from Cellulophaga algicola DSM 14237 hydrolyzed alginates endolytically and the rest worked exolytically. The two types of lyases were found to have different substrate specificity, with exolytic lyases having broader substrate specificity. The studies described in this section on the enzymatic degradation of materials demonstrate the promising roles of enzymes in the biodegradation process. The

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Table 4 A summary of the main enzymes involved in biodegradation with their functions Classes of enzymes Oxidoreductases

Examples Oxygenases

Dehydrogenases

Oxidases

Peroxidases

Hydroxylases Reductases Hydrolases

Esterases

Depolymerases

Dehalogenases

Lyases

Polysaccharide lyases

Enzymatic function Catalyze the addition of an oxygen atom or atoms to the carbon chain, resulting in alcohol or phenol Oxidize substrates by transferring hydrogen to electron acceptors Oxidation reactions can be catalyzed by using oxygen as the electron or hydrogen acceptor Oxidize substrates like phenol, amine, and heterocyclic compounds using hydrogen peroxide and organic hydroperoxides Catalyze the addition of hydroxyl groups It catalyzes reduction reactions Catalyze ester groups in a compound to break ester bonds It breaks down large, complex polymers into smaller oligomeric and monomeric units Catalyze the cleavage of the carbon-halogen bonds Catalyze the non-hydrolytic and non-oxidative cleavage of carbonatom bonds, including carbon, oxygen, nitrogen, sulfur, and halides, as well as phosphate (P-O) bonds

The type of substrate Aliphatic and aromatic hydrocarbons

Reference [22]

Alcohols

Phenolic and non-phenolic compounds and some inorganic ions Phenols, amines, heterocyclic compounds, synthetic dyes, lignin, and rubber

[73]

Petroleum and polystyrene Heavy metals

[73]

Esters and polymers containing ester linkages Polymers

[32, 76]

[73] [92]

[98]

Halogenated compounds

[93]

Polysaccharides

[99]

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functions of each enzyme, along with its target substrates, are summarized in Table 4. In waste management, enzymatic degradation may be developed as an alternative to chemical treatment, considering enzymes’ reusability and high selectivity. However, several pitfalls must be overcome before the applications of this technology can be used, particularly the high cost and low stability of enzymes. Therefore, future research should prioritize the immobilization of enzymes and the discovery of low-cost enzymes to realize the industrial removal of pollutants via enzymatic actions. Apart from enzymes, a variety of homogeneous and heterogeneous catalysis has received much attention in wastewater treatment. The anaerobic oxidation process, which uses hydroxyl radicals to oxidize organic contaminants into CO2 and H2O, requires these catalysts to generate the radicals [96]. For example, wet catalytic peroxidation focuses on transition metals as homogeneous catalysts, while noble metals are used as the most common heterogeneous catalysts. The transition metals include Cu, Fe, Co, Mn, and Ni, while nano-gold-based catalysts (i.e., gold dispersed on nanoparticles) are heterogeneous catalysts that have received much interest [97].

Conclusion Varieties of microorganisms, comprising prokaryotes and eukaryotes, have been justified as powerful biological decomposers. The leading classes of microorganisms are bacteria and fungi, whereas algae, specifically microalgae, are likewise identified as biological decomposers, yet they are less studied. Protozoa alter biodegradation rates either through their predatory effects on the substrate-degrading bacteria or their hydrocarbon-degrading capabilities. Archaea are most pronounced, with several types of methanogens participating in the mineralization stage of anaerobic digestion. The current progress in microbial degradation is attributed to the microbial infallibility hypothesis, where a specific microorganism can degrade all organic compounds. Otherwise, microbial evolution and adaption will synthesize the active microbial decomposers. This proposal is criticized, and the reasons against it are that microorganisms are fallible and recalcitrant natural compounds exist. However, the hypothesis is true to a certain extent that the recalcitrant nature arising since the proposed biodegradability test does not identify co-metabolism. Besides, evidence also shows that some recalcitrant compounds are now proven to be degraded by newly evolved microorganisms. Microorganisms consume carbon and energy from organic substrates by demonstrating different functions in every stage of biodegradation. In biodeterioration, microorganisms aggregate and form biofilms through processes, i.e., attachment, colonization, development, maturation, and active dispersal. After the biofilm formation, the target substrates can deteriorate via chemical, physical, and/or enzymatic actions. Chemolithotrophic and chemoorganotrophic microorganisms induce chemical biodeterioration by secreting inorganic and organic acids. In addition,

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microorganisms secrete extracellular and intracellular enzymes to mineralize substrates inside their microbial cells. Microorganisms concerned with biodegradation can be individual strains or consortiums of several strains. A microbial consortium, usually with greater degrading capacities, has been revealed to degrade xenobiotic compounds (e.g., phthalic acid esters, pesticides, and polycyclic aromatic hydrocarbons). In the consortium, microorganisms interact via co-metabolism, syntrophism, and/or interspecies hydrogen transfer. A broad range of bacterial species, particularly Pseudomonas and Bacillus, has been shown to decompose polymers (e.g., polyvinyl chloride, polystyrene, low-density polyethylene, high-density polyethylene, polyvinyl alcohol, etc.), crude oil hydrocarbons (e.g., aliphatic, cyclic, aromatic, and halogenated hydrocarbons), and so on. The biodegradation efficiency is facilitated due to the semipermeable bacterial cell membrane containing hydrophobic fimbriae and fibrils. Similar to bacteria, Aspergillus, the most well-known fungal genera, has been discovered to degrade polymers, such as polyethylene and high-density polyethylene. In addition, polycyclic aromatic hydrocarbons are common substrates consumed by fungi. Fungal hyphae enable easy dispersal and deep penetration into the substrates, thus enabling biodegradation to occur. Algae have been demonstrated to effectively degrade crude oil hydrocarbons and recalcitrant compounds such as pesticides and pharmaceutical drugs. Chlorella and Scenedesmus are the two major species of microalgae involved in biodegradation. As photosynthetic organisms, algae lead to degradation via photosynthetic and biological activities. Besides, microalgae contribute to wastewater treatment plants by producing algal biomass, a feedstock for biofuels. Enzymes are present as components for successful biodegradation, with common oxidoreductases, hydrolases, and lyases. They are precise in catalyzing a particular reaction and demonstrate superior substrate specificity. Oxidoreductases, including oxygenases, hydroxylases, peroxidases, oxidases, and reductases, catalyze different oxidation or reduction reactions. Among these enzymes, peroxidases possess significant roles in decolorizing dyes, which helps remove synthetic dyes from wastewater. Esterases, depolymerases, and dehalogenases belong to hydrolases that catalyze the hydrolysis of ester, depolymerization, and cleavage of carbon-halogen bonds. Polysaccharide lyases are enzymes that catalyze the cleavage of polysaccharide glycosidic bonds.

Future Perspectives This study has marked the degrading capabilities of various microorganisms to decompose distinct substrates, ranging from polymers, heavy metals, petroleum hydrocarbons, and many more. Though the discussions are mainly constrained to bacteria, fungi, and algae, there is limited information on the less studied protozoa and archaea. Accordingly, further efforts are vital to screen the potential of protozoa and archaea in biodegradation using combined omics technologies, such as

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metagenomics, proteomics, transcriptomics, and metabolomics. Artificial intelligence, such as machine learning, can be employed concurrently with omics technologies to acquire a more precise picture of the microbial communities, while facilitating the efficiency of the microbial recognition practice. Further studies should probe extremophilic microorganisms that can tolerate harsh environmental conditions such as metal richness, nutrient deficiency, intense salinity, temperature, pH, and pressure. The extremophiles and their enzymes, extremozymes, should be further exploited in bioremediation operations to deal with heavily polluted sites that often feature extreme environmental parameters. The introduction of recalcitrant and novel chemicals into the environment complicates bioremediation. In the absence of degrading microorganisms, the biodegradation of these compounds may only be initiated after many years of microbial adaptation and evolution. Their limited biodegradability may enhance the bioconcentration, bioaccumulation, and biomagnification of the compounds in the biotic components of the ecosystems. The retention and increased concentration of recalcitrant compounds may provoke acute and chronic toxicity among living organisms. Given this, prevention at the source represents the priority in reducing the release of recalcitrant compounds. One of the preventive measures is the replacement of synthetic materials with their natural counterparts. For instance, bio-based and biodegradable plastics should be used instead of synthetic, nonbiodegradable plastics. Multidisciplinary approaches integrating nanotechnology, biotechnology, and microbiology should be promoted to improve the performance of microorganisms and enzymes in biodegradation. Using gene-editing tools such as CRISPR-Cas, genetic engineering is utilized to alter the genetic makeup of microorganisms and design microbial communities with functional genes of interest [100]. Besides, nanotechnology can be incorporated into enzymes by immobilizing enzymes on nanoparticles, making them viable waste treatment options. Other than that, the applications of biological decomposers in waste treatment can be expanded by incorporating microalgae-based wastewater treatment plants into conventional treatment plants [101]. This is particularly significant in promoting the development of power and energy sectors via biofuels from algae biomass.

Cross-References ▶ Mechanism of Microbial Biodegradation: Secrets of Biodegradation ▶ Recent Advances in Microbial Biodegradation ▶ Role of Microorganisms in Biodegradation of Pollutants ▶ Types of Microorganisms for Biodegradation Acknowledgments This work was financially supported by the Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education of Malaysia (MOHE) (FRGS/1/2019/ STG01/UM/02/6).

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93. Liang Z, Li G, and An T (2017) Purifying, cloning and characterizing a novel dehalogenase from Bacillus sp. GZT to enhance the biodegradation of 2,4,6-tribromophenol in water. Environmental Pollution 225:104–111 94. Nguyen B-AT, Hsieh J-L, Lo S-C, Wang S-Y, Hung C-H, Huang E, Hung S-H, Chin W-C, and Huang C-C (2021) Biodegradation of dioxins by Burkholderia cenocepacia strain 869T2: Role of 2-haloacid dehalogenase. Journal of Hazardous Materials 401:123347 95. Fischer A and Wefers D (2019) Chromatographic analysis of alginate degradation by five recombinant alginate lyases from Cellulophaga algicola DSM 14237. Food Chemistry 299: 125142 96. Rodrigues CSD, Silva RM, Carabineiro SAC, Maldonado-Hódar FJ, and Madeira LM (2019) Wastewater treatment by catalytic eet peroxidation using nano gold-based catalysts: A review. Catalysts 9(5):478 97. Jing G, Luan M, and Chen T (2016) Progress of catalytic wet air oxidation technology. Arabian Journal of Chemistry 9:1208–1213 98. Haider TP, Völker C, Kramm J, Landfester K, and Wurm FR (2019) Plastics of the future? The impact of biodegradable polymers on the environment and on society. Angewandte Chemie International Edition 58(1):50–62 99. Illanes A, Introduction in Enzyme Biocatalysis, A Illanes, Editor. 2008, Springer: Dordrecht. p. 1–56. 100. Jaiswal S, Singh DK, and Shukla P (2019) Gene editing and systems biology tools for pesticide bioremediation: A review. Frontiers in Microbiology 10:87 101. LIFE Project Biosol Water Recycling. Biosol Water Recycling. n.d.; Available from http:// www.life-biosol.eu/

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Mechanism of Microbial Biodegradation: Secrets of Biodegradation Doaa A. R. Mahmoud

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Microbial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Absorption Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Breakdown Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Air Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacterial Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aerobic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fungal Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Algal Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yeast Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Microbial Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

180 180 180 181 182 183 183 183 183 184 184 185 185 186 186 187 187 187 188 188 189 189 190

Abstract

Microbes are the magic solution to solve most environmental problems. Those microbes that you cannot see with the naked eye have amazing tools inside them. One of the tools they have is digestive enzymes that can digest anything you D. A. R. Mahmoud (*) Department of Chemistry of Natural and Microbial Products, National Research Centre, Giza, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_6

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would not think of. There is no organic substance found in nature that can not be decomposed by microorganisms. Microbes heal the environment naturally. The biodegradation action of these amazing creatures is discussed in this chapter. Keywords

Biodegradation · Bioremediation · Environment · Aerobic biodegradation · Anaerobic biodegradation

Introduction Recently, the problems of contaminants and pollutants that threaten our environment and our public health have increased [1]. If you look at the type and nature of pollutants, you will think it is impossible to get rid of them. These pollutants may be plastic, petroleum, wood residues, metals, rocks, and many more [2]. The cleaningup strategy needs a magic solution. If you are looking for a solution to solve environmental problems, you are looking for microbes. Microbial degradation is an important part of cleaning up because they utilize contaminants as food. Microbiology is an interesting, ever-evolving branch of science with a wide range of applications since microbes play an important role in our daily lives. Those microbes have amazing tools inside them. They have digestive enzymes that can digest any substrate. Microbes heal the environment naturally because they aid in decomposing hazardous compounds into harmless ones. Microbial products have been marketed as commercial bioremediation agents to dispose of waste. You can read how these amazing creatures do this mission in the current chapter. You can recognize the secrets of biodegradation from a microbiological point of view. Microbes, despite their small size, have the potential to tackle major global problems and open new routes for progress toward a green economy.

Microbial Biodegradation The term biodegradation generally refers to any biological change in a substrate. In general, biodegradation is the decomposition of organic matter by living organisms [3]. If the decomposition occurs by microorganisms like bacteria, fungi, yeasts, etc., it will be called microbial biodegradation. Nature created this biodegradation process to benefit living organisms as the wastes are recycled into nutrients.

Mechanism of Microbial Biodegradation Before we get into the mechanism of microbial decomposition, it is important to understand that decomposition has two different names: microbial biodegradation

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and microbial bioremediation. The decomposition of pollutants by microbes is known as biodegradation, and it is a natural process. Bioremediation is a humanengineered method for removing pollutants from the environment by assisting microbes in the degradation process. That is, microbes are introduced to the polluted area. Whether by nature or man, the crucial element is that microorganisms are the true nutrient recyclers. Microbial communities are characterized by their ability to live in an extreme environment. Some microbes can survive in harsh conditions unsuitable for other life forms; their habitat prepares them for bioremediation potential [4]. Microorganisms have two mechanisms for handling contaminants. Absorption of inorganic substances is one mechanism, and breakdown of organic compounds is another. The important end consequence of breakdown is the conversion of organic or inorganic nutrients initially tied up in the basic component of any substance into simple elements that can be used again.

The Absorption Mechanism The accumulation of inorganic materials by absorption is the significant and principal process by which microorganisms collect their requirements. The first line of defense is the attachment of microbial cells to the substrates, followed by the absorption process [5]. The inorganic materials are found in the form of metals. These metals are necessary for bacteria and fungi in modest amounts. It is fascinating that metals cannot be destroyed; instead, they must be transformed into a stable form or discarded. In natural habitats, microorganisms use four different ways to get metals: biosorption, biotransformation, bioleaching, and biomineralization. The characteristic of microbial cells retaining cations is known as “biosorption.” Their ability for absorption differs from one species to another. Some microbial species can completely absorb inorganic compounds. Other species can only convert toxic compounds into less toxic ones in a process known as “biotransformation.” For example, Cr(VI) is very toxic; it can be converted into Cr(III), which is less toxic [6, 7]. Interestingly, electronic waste is easy to dispose of by shredding and grinding into small, digestible pieces. Although this waste is hazardous to the environment, it includes important minerals of high purity. As a result, this waste can be used to encourage the growth of fungus, causing them to create organic acids, which aid in the processing or recovery of these metals [8]. This process is known as” bioleaching.” The immobilization of heavy metals by forming a polymeric complex or insoluble sulfides is known as “biomineralization” [9]. Table 1 shows the percentage range of metals removed by different fungi. This demonstrates the ability of microorganisms to assist in removing hazardous metals from the environment.

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Metal Mercury Chromium Cadmium Copper Cobalt Lead Nickel

Percentage of removal (%) 10 25 98 60 95 60 55

Reference [10] [11] [12] [13] [14] [15] [16]

Table 2 Some enzymes and their role in bioremediation Enzyme Laccase, dioxygenase, Tyrosinase Dehalogenase Peroxidase Nitrilase Nitroreductase Phosphatase Cellulase, xylanase, Hemicellulase Esterases, lipases, Depolymerase Oxidase, decarboxylase Lignin, and azoreductase

Role of enzyme Different aromatic compounds are degraded

Ref. [18, 19]

Hydrolysis of halogenated hydrocarbon to produce chlorine and fluorine Cleaves polycyclic hydrocarbons Break down cyanide groups from aromatic and aliphatic nitriles Reduction of nitro groups on nitro-aromatic compounds Cleaves phosphate groups from organophosphates Degradation of soft and hard wood

[20]

[23] [24] [25, 26]

Cleaves the carbon backbone of plastic

[27–30]

Help leaching out of mineral ions from stone and cement Removal of industrial dyes, textile dyes, and azo dye

[31–33]

[21] [22]

[34]

The Breakdown Mechanism Microorganisms degrade pollutants by their metabolic processes, with or without minor route alterations to allow the pollutant to enter the normal microbial metabolic pathway for degradation. They consume or decompose organic pollutants to produce carbon dioxide and water as end products or metabolic intermediates that can be employed as primary substrates for cell growth in bioremediation processes [17]. A particular enzyme system can mediate the breakdown pathway. Enzymes involved in degradation come in various forms, depending on the substrate or the type of pollutants to be broken down. That is to say, many enzymes have a diverse degradation capacity because of their substrate affinity. Therefore, they are involved in the metabolic reactions of microorganisms. These enzymes would be hydrolases, transferases, oxidoreductases, lyases, isomerases, and ligases. Table 2 shows examples of some enzymes and their role in bioremediation.

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Types of Bioremediation Bioremediation processes are divided into biofiltration of the air, onsite land treatment for soil, and bioreactors for water treatment.

Air Bioremediation Several volatile organic molecules produced by various industrial operations pollute the air. Particulate matter, ground-level ozone (O3), sulfur dioxide (SO2), nitrogen dioxides (NO2), and volatile organic compounds are all major air pollutants [35]. Although various technologies have been implemented to address air pollution, the air remains contaminated. While chemical scrubbing has been used to clean gases released by chimneys, a newer approach known as “biofiltration” is now being utilized to clean industrial emissions. This approach entails passing polluted air through a replacement culture medium containing microorganisms that break down contaminants into carbon dioxide, water, or salts [36]. Biofiltration is the only biological approach for removing contaminants from the air that is currently accessible.

Soil Bioremediation Soil contamination extends beyond soil and its life forms, affecting every part of the environment and every organism. Pollutants spread in the soil, whether agricultural or industrial. The type of pollutants varies according to the nature of the soil. Agricultural soils are polluted due to heavy metal contamination in agricultural goods or pesticide use. Industrial soils are contaminated by Petroleum hydrocarbons, chemical spills, and heavy metal deposition [37]. Microorganisms play a critical part in the degradation process. They are the true nutrient recyclers in the soil. We can stimulate the growth of microorganisms already existing in the soil by a technique known as biostimulation. Or we can introduce new microorganisms into the soil. Both techniques can overcome the factors that might hinder the natural biodegradation process. They address these constraints by providing microorganisms with the resources they require, allowing them to multiply and degrade faster. Bioremediation can be adapted to the demands of the polluted site in question. It has been proved to be a feasible option for recovering polluted areas cost-effectively.

Water Bioremediation The world’s largest and most important bioremediation sector is sewage treatment bioreactors. Suspended particles, organic debris, nitrogen, and phosphorus are the main components of raw sewage. Bioreactors for water treatment are based on the aeration of water entering a sewer system. This aeration provides oxygen to

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Fig. 1 Diagram of microbial biodegradation

microbes that decompose organic matter and contaminants [38]. The organic pollutants are consumed by microbes, which bind the less soluble portions and may be filtered out. Toxic ammonia is converted to nitrogen gas, then discharged into the atmosphere. The impact of bioremediation of contaminated water is the preservation of water resources (Fig. 1).

Bacterial Biodegradation Bacteria are used in bioremediation to minimize pollution by biologically degrading contaminants into nontoxic compounds. This can include aerobic and anaerobic bacteria that participate in this breakdown and utilize it as an energy source.

Aerobic Biodegradation The term “aerobic biodegradation“refers to decomposing organic compounds when there is oxygen present. As a result, oxidative conditions characterize the chemistry of the organism, system, or environment. The metabolism of aerobic bacteria

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depends on oxygen. In their catabolic reactions, one molecule is oxidized while another is reduced. It is a kind of cellular respiration that converts food energy into a substance that all cells can use. When cells cannot respire due to a lack of oxygen, they use a process known as fermentation to liberate some of the stored energy. The elimination of most organic pollutants is more effective under aerobic conditions [39]. Plastics can be biodegraded by allowing aerobic bacteria to metabolize the molecular structure of plastic films, resulting in an inert humus-like material that is less damaging to the environment. The aromatic and chloro-aromatic chemicals can be overcome by producing central intermediates with a biphenolic structure. Enzymes that utilize molecular oxygen then cleave these molecules. These reactions are interesting because they provide energy to organisms, but they also play a role in the ecological biodegradation of organic compounds [40].

Anaerobic Biodegradation The term “anaerobic biodegradation“refers to degrading organic compounds when there is no oxygen present. Various reactions are included in the decomposition strategy of anaerobic bacteria. For example, some chlorinated chemicals, such as chloro-benzoates, chloro-phenols, or tetrachloroethene, have been shown to act as a physiologically functioning electron acceptor in anaerobic respiration, resulting in non-chlorinated or reduced chlorinated products [40]. Anaerobic biodegradation is a natural process that occurs in the long run. It is slow and time-consuming. Usually, the paper takes from 1 to 3 months to degrade. Degradation time for cotton is 5 months, while tin cans require 50–100 months. Aluminum cans are very slowly degradable material; it takes from 150 to 200 months. Because it reduces volume and mass, anaerobic digestion is commonly employed to treat wastewater sediment and digestible waste [41].

Fungal Biodegradation There are ecological relationships between microorganisms and contaminants. It has been discovered that fungi, particularly ligninolytic ones, utilize unique catabolic routes and enzymes. They convert oxygen into hydrogen peroxide, which is subsequently used to produce an aryl cation radical, which undergoes spontaneous alternations and decay. Some fungal species can grow freely and abundantly in soil and waste sites in a short incubation time. Therefore, they are commonly used in the biodegradation of low-density polyethylene [42]. In various research fields, such as the degradation of textile colors, studies have demonstrated that using a consortium of fungal species yields better outcomes than using singular fungi [43, 44]. Bioactive pharmaceutical compounds are difficult to break down. Even at modest dosages, their release into the environment is extremely hazardous. Lignolytic fungi are one treatment option [45]. Petroleum contamination is unavoidable at any stage of the oil production process and severely influences the environment

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[46, 47]. Due to their resilience to Petroleum Hydrocarbons (PHs), they have an advantage over bacteria in biodegradation. Fungi can utilize PHs as carbon and energy sources, transformed into carbon dioxide and water and low- or nontoxic compounds [48].

Algal Biodegradation Algal bioremediation, or the use of algae to remove contaminants from water, has been extensively studied, and the commercial usage of algal cultures has a 75-year history [49]. Microalgae culture is an important phase in wastewater treatment since it provides a tertiary bio-treatment while also producing great potential biomass that may be used for various applications. Inorganic nitrogen and phosphorus can be used by microalgae cultures to help them grow. Furthermore, it can remove heavy metals and some harmful organic compounds, resulting in no secondary contamination [50]. Accidental spilling of hazardous chemicals into the environment has increased as global transportation of hazardous chemicals has increased. Phenol is a frequent chemical that is connected with spills [51]. To eliminate phenolic chemicals, various wastewater treatment procedures have been developed. Due to the unique properties of algae in assimilating numerous harmful contaminants in aromatic hydrocarbons, phenols, heavy metals, and organochlorine [52], algae are successfully able to use phenol as a sole carbon source [53].

Yeast Biodegradation Traditional approaches use physical and chemical processes to clean up oil contamination. Booms, skimmers, incineration, brick manufacturing, adsorbents, and other physical cleaning methods are used. Chemical surfactants are not recommended as a remediation agent because of their deleterious effects on the existing biota in the polluted area. On the other hand, the applicability of the traditional techniques to cleanse oil-polluted areas has been limited. It is not possible to remove more than 10–15% of the spilled oil [54]. Yeast has been implicated in the breakdown of a variety of substances. They can use a variety of petroleum hydrocarbon substrates, such as oil and oil-related chemicals. Petroleum and its byproducts have contaminated the environment, and strategies for managing this have been developed. Therefore techniques for removing or reducing the impacts of pollutants by utilizing different kinds of yeasts have been developed [55]. Due to the abundant availability and inexpensive cost of yeast suspension, yeast cells have the potential to transition fast from respiration to fermentation. This encourages Saccharomyces cerevisiae as a dinitrophenol biodegrading and bio-accumulating material [56]. Dinitrophenols and their derivatives are powerful uncoupling agents that are hazardous to the environment [57]. Bioremediation, which uses biological materials such as yeasts, is a promising, safe, and cost-effective option for treating contaminated water and soil. Yeasts are characterized by adaptability to harsh environments such as temperature, pH, and

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high amounts of organic and inorganic pollutants. As a result, they are capable of overcoming heavy metal pollution. Yeasts have developed ways to adapt to heavy metal stress, such as diffusion across the cell membrane, bioaccumulation to cell walls and trapping in extracellular vesicles, precipitation, complexation, and oxidation-reduction reactions [58]. Because of their fast growth rate and cell wall structure, yeasts are a better bio-sorbent for removing heavy metal ions from wastewater. Interestingly, cell wall phosphates and carboxyl groups were identified as the primary determinants of negative yeast cell surface charge, which improves yeast cells’ ability to bind heavy metallic ions. Electrostatic interactions are most likely to be responsible [59].

Factors Affecting Microbial Degradation Biostimulation and bioaugmentation are two primary ways of conducting bioremediation procedures. Biostimulation is a type of bioremediation that involves providing an electron acceptor, nutrient, or other material to a contaminated site to stimulate the growth of the microbial population already present [60]. A procedure known as bio-augmentation occurs when microorganisms are brought to a contaminated site to aid degradation. As a result, the microorganisms used in bioremediation could be native to the contaminated area or imported from elsewhere. The physical and chemical properties of the environment, the chemical type of pollutants and their concentration, and their availability to microorganisms are all factors that influence the efficiency of biodegradation. Specifically, it can be said that the rate of biodegradation is influenced by four factors: water, oxygen, temperature, and light. Because microorganisms and pollutants do not spread equally in the environment and bacteria and pollutants do not come into contact, the decomposition rate is altered.

Water Bioremediation is only effective when environmental conditions allow for microbial growth and activity; it is frequently used to manipulate environmental factors to speed up microbial growth and degradation. Microorganisms require a sufficient amount of water to grow. This water affects the type and quantity of soluble materials available and hence affects biodegradation rate. The moisture content of the soil hurts biodegradation agents due to nutrient availability [61]. Mold will grow on foods like bread and fruit if the moisture content is high enough. For example, cereal grains must be kept dry in order to avoid biodegradation during storage.

Oxygen To grow and reproduce, microbes require energy, oxygen, and a variety of metals. Different organisms require oxygen, while others do not. Exothermic

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(heat-releasing) processes include the oxidation of organic compounds to carbon dioxide and water. About 104 kilocalories (435 kJ) of energy are potentially available for each mole of oxygen employed as an electron acceptor (oxidant). Depending on their needs, the biodegradation rate might be accelerated. Because oxygen is a gaseous necessity for most living organisms, biological degradation occurs in aerobic and anaerobic conditions. In most circumstances, the presence of oxygen can improve hydrocarbon metabolism [62].

Temperature The temperature of a substance is a crucial factor; keeping it frozen helps inhibit biodegradation. The majority of biodegradation takes place between 10  C and 35  C. It is the most important physical element in affecting the survival of microorganisms and the composition of hydrocarbons. Oil deterioration by natural processes is very sluggish in freezing places like the Arctic, putting microbes under extra pressure to clean up the spilled petroleum. Most oleophilic bacteria are metabolically inactive due to the subzero temperature of the water in this location, which causes transport channels within microbial cells to shut down or even freeze the entire cytoplasm, rendering them metabolically dormant [63]. The degradation of a certain substance necessitates a specific temperature. The metabolic cycle of biological enzymes involved in the degradation process has an optimal temperature and will not be the same at all temperatures. Temperature increases the rate of microbial activity. It begins to diminish abruptly when the temperature increases or decreases, and it eventually comes to a halt after reaching a certain degree. Temperature influences microbial physiological features; hence it can speed up or slow down the bioremediation process.

Light The effect of light on the biodegradation process is very important. Simply when some nutrients are exposed to sunshine, they degrade. The amount of biodegradable organic matter in dissolved organic matter can be increased or decreased by exposing it to sunlight. When photo- and biodegradation of organic matter overlap, various scenarios emerge in which sunlight and bacteria work together and compete to decompose organic matter. Organisms and sunlight work together to break it down. Degradation of dissolved organic materials by sunlight produces biodegradable low-molecular-weight acids or aldehydes. In general, the modification of materials caused by light is known as photodegradation. The combined action of sunshine and air results in oxidation and hydrolysis, for example, the effects of light on microcrystal degradation in agricultural soils [64]. Aerobic mineralization in surface water assesses the persistence of chemicals in surface water that is now utilized in regulation. The destiny of a chemical in the surface water is affected by direct and indirect photolysis processes. In comparison

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Fig. 2 Factors affecting microbial degradation

to dark studies, the influence of sunshine on the destiny of Pendimethalin in surface water subjected to simulated sunlight indicated a significantly faster deterioration [65]. Finally, the factors affecting microbial degradation can be summarized in (Fig 2).

Conclusion Bioremediation is a technology that has existed for a long time. Although it provides technological and financial advantages, it takes longer time to be completed than traditional approaches. Bioremediation uses fewer resources and less energy than traditional technologies, and it does not produce hazardous waste. Pollution endangers our health and harms the environment, threatening species and the planet’s long-term viability. Bioremediation can aid in the reduction and removal of pollution produced by humans, ensuring clean water, air, and soils for future generations.

Future Perspectives Given that adsorption is reversible process, there must be some concerns. As pollutant desorption may takes place when waste sludge’s are disposed of to the environment. The interference of active transport and biodegradation processes must be minimized by inactivating the microbial cells to research bio-sorption as organics uptake. The disadvantage that must be modified in the future is: 1. Biological processes are often

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highly specific. 2. Important site factors required for success include the presence of metabolically capable microbial populations, suitable environmental growth conditions, and appropriate levels of nutrients and contaminants.

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Types of Microorganisms for Biodegradation Shaimaa A. Khalid and Walaa M. Elsherif

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer-Degrading Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pesticide-Degrading Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanisms of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodeterioration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Biofilm Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biofragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Involvement of Microbial Enzymes in the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors That Affect the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Species and Their Metabolic Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Substrate Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

196 197 207 209 210 210 211 211 212 213 213 214 214 214 215 215 215

Abstract

Human activities are the leading contributors to global contamination. Approximately two billion tons of waste are produced each year. Waste accumulation represents an environmental challenge causing a serious problem in modern societies since it affects all life forms. Biodegradation is an effective and eco-friendly technique for waste recycling. Certain types of microorganisms play a key role in the ecosystem through waste transformation and the complete removal of contaminants. Enzymatic activities expressed by microorganisms play an important role in the biodegradation process by breaking down the waste S. A. Khalid (*) · W. M. Elsherif Food Hygiene Department, Animal Health Research Institute, Agricultural Research Centre, Cairo, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_2

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material into safe environmental compounds. However, numerous factors can influence microbial biodegradation. This chapter will introduce different types of microbial biodegradation, their mechanisms in the degradation of different environmental pollutants, and the factors that may affect them. Keywords

Microorganisms · Biodegradation · Biodeterioration · Biofragmentation · Mineralization · Plastic waste · Pesticide biodegradation Abbreviations

EPS HDPE Lac LDPE LiP MnP OPs PAHs PAHs PE PET PHB PLA PP PS PU PUR PVA PVADH PVC SH TPS VP

Extracellular polymeric matrix High-density polyethylene Laccase Low-density polyethylene Lignin peroxidase Manganese peroxidase Organophosphorus compounds Polycyclic aromatic hydrocarbon Polycyclic aromatic hydrocarbons Polyethylene Polyethylene terephthalate Polyhydroxy butyrate Polylactic acid Polypropylene Polystyrene Polyurethane Polyurethane Polyvinyl alcohol Polyvinyl alcohol dehydrogenase Polyvinyl chloride Serine hydrolase Thermoplastic starch Versatile peroxidase

Introduction Biodegradation is the mineralization of organic compounds by microorganisms such as bacteria, fungi, and archaea, resulting in carbon dioxide and water. Biodegradation falls into a category of degradation processes for waste materials that are affordable, low-cost, and eco-friendly. Microorganisms, categorized into bacteria, fungi, viruses, archaea, protozoa, and algae, are widely distributed in nature and grow in various environmental conditions. Many natural and synthetic compounds are biodegraded by microbial capacity. Materials are termed biodegradable if

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decomposed by bacteria and other microorganisms. The release of extracellular enzymes by microorganisms for cleavage of polymers and liberation of monomers is important in the biodegradation process. The efficiency of the biodegradation process is relatively slow, as the complexity and changeability of the natural environment affect its feasibility [1]. Fungi are major consumers of plant and animal waste and chemical pollutants, which play an important role in polymeric material degradation. Fungal mycelia can easily penetrate polymeric material, causing maximum substrate degradation [2]. Many extracellular and intracellular enzymes are found in fungi, such as proteases, nucleases, glycoside hydrolases (GHs), and lipases. Extracellular enzymes are involved in the degradation of polymers into monomers, dimers, and oligomers, which are mineralized or assimilated by intracellular enzymatic systems. Components of the fungal enzymatic system, such as lignin peroxidase (LiP), laccase, manganese peroxidase (MnP), and versatile peroxidase (VPO), contribute to their degrading capability [2].

Polymer-Degrading Microorganisms Plastics are compounds consisting of long carbon chains of hydrogen, sulfur, and nitrogen. They are classified into natural, semi-synthetic, synthetic, thermoplastic, and thermosetting plastics. Synthetic plastics play a critically important role in modern society due to their outstanding stability, low price, and resilience [3]. Around 140 million tons of synthetic polymers are manufactured globally per year, and international plastic usage grows 12% annually. From 1950 to 2018, the total production of plastics reached approximately 8.3 billion metric tons. Plastics are widely used in the packaging of food, pharmaceuticals, beverages, chemicals, and detergents (Fig. 1). Plastic bags represent the most common form of plastic used in daily life, and they are considered one of the main environmental pollutants. Unfortunately, 76% of overall production accumulates as waste that poses a serious threat to wildlife, ecosystems, and the environment [4]. Plastic accumulation is considered a major environmental problem because plastics are nonbiodegradable under natural environmental conditions; recalcitrant insoluble polymers may take years to degrade naturally. Of global concern is that 500 billion to 1 trillion plastic bags are used worldwide every year, disrupting ecosystems, leading to land and air pollution, and creating serious environmental hazards and threatening organisms. Despite increasing efforts to reduce plastic waste through recycling or segregated collection, eco-friendly and efficient waste management strategies are still lacking. Table 1 summarizes several studies reporting the possible microbial degradation of plastic wastes as a promising, effective, economical, and environmentally friendly processing method for plastic waste [5]. The ability of microorganisms or their enzymes to degrade plastic polymers presents an ideal solution for plastic waste accumulation. Plastic biodegradation involves an enzymatic process in which intra- and/or extracellular enzymes of microorganisms attach to the plastic surface and catalyze C-C bonds of plastic

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Fig. 1 Overview of the application of synthetic plastic. (Adapted with permission from Ref. [8])

polymers, thereby converting them into smaller fragments [6]. Plastic biodegradation is a complex process that includes biotic and abiotic factors in several stages, including biodeterioration, depolymerization, and assimilation. The biodeterioration process facilitates the biodegradation process through a cooperative mechanism of microorganisms and abiotic factors, leading to the fragmentation of bulk polymers into smaller ones. The deterioration process is followed by depolymerization, which involves the secretion of catalytic enzymes and free radicals to break down the polymer chain and form biofilm [7]. Polystyrene (PS) is an aromatic polymer whose unique characteristics of transparency and rigidity at room temperature make it suitable for use in food packaging. Researchers proved the toxicity effect of PS on plants, animals, and humans. Furthermore, DNA damage in brain tissue and erythrocytes through its use has been shown [9]. PS degradation involves a group of enzymatic reactions; for example, serine hydrolase (SH) reportedly degrades PS. At a low concentration of

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Table 1 Efficiency of microbial degradation on different polymers Incubation time/days 8

Tested microorganisms Xanthomonas sp., Sphingobacterium sp., and Bacillus sp. Rhodococcus sp. and Bacillus sp.

Isolated source Soil

Mangrove environment

40

Pseudomonas sp.

Soil

30

Phanerochaete chrysosporium and Engyodontium album Alcanivorax borkumensis

Plastic dumping site

365

Mediterranean Sea

7

Bacillus weihenstephanens

Hydrocarbon enriched soil

180

Brevibacillus agri, Brevibacillus sp., Aneurinibacillus aneurinilyticus, and Brevibacillus brevis Burkholderia cepacia

Sewage and landfills

140

Hydrocarbon enriched soil

180

Pseudomonas putida

Garden soil

30

Bacillus flexus

Plastic dumbing site Sea water

365

Soil

30

Staphylococcus pyogenes

Soil

30

Pseudomonas aeruginosa

Soil

30

Pseudomonas sp.

Digester sludge

40

Aspergillus caespitosus, A. terreus, Paecilomyces variotii, Alternaria alternate, Phialophora alba, and Eupenicillium hirayamae Bacillus subtilis

28

Biodegradation observation Reduction in PS weight by 40–56% Reduction in PP weight by 4–6.4% Reduction in PS weight by 10% Reduction in PP weight by 8%.

References [11]

[19]

[14] [17]

Reduction in LDPE weight by 3.5% Reduction in LDPE weight by 35.64% Reduction in PP film weight by 22.8–27%

[69]

Reduction in LDPE weight by 31.43% Reduction in PE weight by 10% Reduction in PP weight by 2.5% Deterioration effect on PE films with CO2 emission

[39]

Reduction of PET weight by 8.33% Reduction of PS weight by 8.33% Reduction of PS weight by 5% Morphological damage of PLA

[39]

[70]

[25]

[24] [45]

[52]

[52]

[52]

[48] (continued)

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Table 1 (continued) Tested microorganisms Bacillus cereus

Incubation time/days 40

Pseudomonas aeruginosa

Isolated source Mangrove sediment Mangrove sediment Landfill soil

Stenotropphomonas sp.

Soil

30

Ochrobacterum anthropi

Landfill soil

45

Aneurinibacillus sp. and Brevibacillus sp.

Plants and sewage treatment

140

Arthrobacter sp. and Pseudomonas sp.

Plastic waste

30

Bacillus amylolyticus

Soil

30

Bacillus subtilis

Soil

30

Pseudomonas putida

Garbage soil

30

Pseudomonas sp.

Soil

45

Pseudomonas fluorescens

Soil

30

Alcanivorax borkumensis

Plastic waste of marine

80

Bacillus carbonipphilus

Soil

60

Bacillus nedei

Soil

60

Bacillus megaterium

Soil

60

Bacillus smithii

Soil

60

Sporosacrina globispora

40 45

Biodegradation observation Loss of weight of PP by 12% Loss of weight of PP by 11% Loss of weight of LDPE by 32% Damage of plastic surface Loss of weight of HDPE by 20% Loss of plastic weight between 37.2% and 45.7% Loss of weight of LDPE by 12–15% Loss of weight of PE by 32% Loss of weight of PE by 14% Loss of weight of PE by 18% Loss of weight of LDPE by 5% Loss of weight of LDPE by 22% Loss of weight of LDPE by 3.5% Loss of weight of LDPE by 25% Loss of weight of LDPE by 36.07% Loss of weight of LDPE by 34.48% Loss of weight of LDPE by 16.4%

References [20] [20] [40]

[41] [40]

[70]

[46]

[26] [26] [26] [37] [26]

[42]

[38]

[38]

[38]

[38]

(continued)

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Table 1 (continued) Tested microorganisms Bacillus sporothermodurans

Isolated source Soil

Incubation time/days 60

Vibrio sp.

Seawater

42

Aspergillus sp.

Seawater

42

Biodegradation observation Loss of weight of LDPE by 36.54% Reduction in plastic weight by 35% Reduction in plastic weight by 22%

References [38]

[5]

[5]

SH inhibitor (10 μM), a 1.3% reduction in PS was observed after 2 weeks of incubation. Conversely, no degradation effect was observed at a high concentration of SH inhibitor (50 μM) [10]. Sphingobacterium sp., Xanthomonas sp., and Bacillus sp. exhibited (40–56%) biodegrading effects on PS [11]. A 50% reduction in the molecular mass of polystyrene sulfonate was investigated after 20 days of incubation with Gloeophyllum trabeum DSM 1398 [12]. Sekhar et al. [13] isolated four bacterial species (Alcaligenes sp., Enterobacter sp., Brevundimonas diminuta, and Citrobacter sedlakii) and found a maximum PS degradation rate of 12.4% was achieved by Enterobacter sp. After 30 days. Additionally, Mohan et al. [14] found that Bacillus sp. can degrade PS film with a 23% reduction after 30 days. The toxicity effect of polyvinyl chloride (PVC) has been established; moreover, PVC is difficult to degrade with microbial activity due to its hydrophobic nature. However, several microbial strains, such as B. cereus, Bacillus aerius, P. putida, Pseudomonas otitidis, Acanthopleurobacter pedis, Bacterium Te68R, Microbacterium sp., and Pseudomonas aeruginosa, have shown a capability to damage the PVC surface within 70 days [15]. Polyvinyl alcohol (PVA) degradation can occur by Pseudomonas sp., producing polyvinyl dehydrogenase. The degradation process is carried out in two stages. Hydroxyl-group oxidation occurs in the first stage, including converting the 1,3-glycol structure to β-ketones, which is facilitated by the alcohol oxidase enzyme. The second stage includes converting the ketone group to carboxylic acid and the breakdown of the C-C bond [16]. Polypropylene (PP) and polyethylene (PE) represent about 92% of synthetic plastics produced, and they are used to produce disposable containers, plastic bags, packaging material, bottles, etc. PP exhibits a rough surface and is hydrophobic in nature, making it recalcitrant and resilient in the environment. Researchers reported that microbial degradation of PP occurs by different bacteria isolated from different sources. Jeyakumar et al. [17] found that Engyodontium album MTP091 and P. chrysosporium NCIM 1170 degraded PP by 9.42% and 18.8%, respectively. Stenotrophomonas panacihumi PA3-2 was confirmed to degrade low- and highdensity PP by 12.7–20.3%, respectively, resulting in a decrease in molecular weight (MW) and release of CO2 after 90 days of incubation [18]. Furthermore, Bacillus sp.,

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Bacillus gottheilii, and Rhodococcus sp. reportedly degrade PP [38]. Bacillus cereus and Sporosarcina globispora degraded PP at rates of 0.003 and 0.002 g/day, respectively [20]. Meanwhile, Lasiodiplodia theobromae was isolated from two plants (Humboldtia brunonis and Psychotria flavida) and exhibited a degradation effect on irradiated PP films [21]. PE is an inert material considered one of the largest commodities in the industry. However, it is difficult to be degraded in the environment. PE recalcitrance is due to its high MW and insolubility in water [22]. PE can be classified according to density into low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Different types of Gram-positive and Gram-negative bacteria can degrade PE, such as Ralstonia sp., Klebsiella sp., Rhodococcus sp., Staphylococcus sp., Stenotrophomonas sp., Bacillus sp., Pseudomonas sp., Acinetobacter sp., Streptococcus sp., and Streptomyces sp. [23]. Microorganism fixation on the PE surface is the first step of biodegradation, followed by the growth of microorganisms using PE as a carbon source, and finally, final polymer degradation [23]. Arkatkar et al. [24] reported the complete degradation of PE with P. fluorescens in the presence of surfactants and biosurfactants. Different bacterial species demonstrate the capacity to degrade PE, such as Enterobacter sp. and Pseudomonas putida [25]. Patil [26] isolated bacteria degrading PE from the soil, such as Bacillus subtilis, B. amylolyticus, Pseudomonas putida, and Pseudomonas fluorescens and investigated the subsequent reductions in PE weight by 14%, 32%, 18%, and 22%, respectively. Yang et al. [27] reported the capability of Bacillus sp. YP1 and Enterobacter asburiae YT1 to degrade PE film; the morphotypes of these bacteria on the PE surface are shown in (Fig. 2). Bacterial species, such as Bacillus sp., Pseudomonas sp. and Rhodococcus sp., microalgae (Uronema africanum Borge), and fungi (Fusarium and Aspergillus), were reported to biodegrade and depolymerize PE after pretreatment with UV and/or thermal treatment [28]. Biodegradation of PE starts with converting its carbonyl group into alcohol by the monooxygenase enzyme. Then, the aldehyde is formed by an alcohol dehydrogenase enzyme converted to fatty acids by aldehyde dehydrogenase. It enters the microbial cells for the oxidation process by a laccase enzyme that breaks down PE into carboxylic acid. Next, the formed acids are fragmented by the action of coenzymeA into two carbons, which enter the citric acid cycle. The process ends with carbon dioxide and water [30]. Some lignin-degrading enzymes participate in PE breakdown, making the initial cleavage into oligomers of five to ten carbon atoms, then transferring into the cell for further metabolism. These enzymes include laccases (EC 1.10.3.2.), manganese peroxidase (MnP, EC1.11.1.13), and lignin peroxidase (LiP, EC 1.11.1.14) [31]. Moreover, some enzymes, such as hydrolases, lipase, esterase, and cutinase, also contribute to PE degradation [14] (Fig. 3). A copper-dependent laccase produced by R. ruber C208 degraded PE film after UV treatment [32]. Laccase-mediated oxidative cleavage of the amorphous region of PE film leads to the formation of the easily accessible carbonyl group and a significant reduction in the weight of PE film. Fungi are reportedly more efficient in PE degradation than bacteria because their ability to attach to the hydrophobic surface of polymers releases extracellular enzymes targeting the insoluble fibers [33].

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Fig. 2 Morphotypes of bacterial cells (Bacillus sp. YP1 and Enterobacter asburiae YT1; a and b, respectively) on the surface of PE [27]. Fluorescent photomicrographs show the colonization of viable fungi (A. flavus VRKPT2 and A. tubingensis VRKPT1; c and d, respectively) and biofilm formation on the surface of HDPE. (Adapted with permission from Ref. [29])

LDPE is produced through the polymerization of ethylene under high pressure. It is used for food packaging, plastic bags, manufacturing trays, and coating for paper and textiles. Exposure to strong oxidizing agents and some solvents results in the swelling or softening of LDPE. The highest LDPE degradation was reported from B. cereus and B. borstelensis by 36% and 20%, respectively [35]. Brevibacillus borstelensis was shown to utilize LDPE as a sole source of energy and carbon, and it reduced the LDPE film by 30% after 30 days of incubation [36]. The addition of mineral oil to LDPE degradation media containing Pseudomonas sp. AKS2 facilitates the hydrophobic interaction and degrades 5%
1% of the film after 45 days [37]. Additionally, in the presence of agar minerals, Bacillus carbonipphilus was able to degrade LDPE by 34.55%; B. coagulans degraded LDPE by 18.37%. Meanwhile, B. sporothermodurans degraded it by 36.54%, B. smithii by 16.0%, B. megaterium by 34.48%, and B. neidei by 36.07% [38]. B. weihenstephanensis degraded LDPE plastic bags by 32.61% and thin plastic bags by 35.64% after

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Fig. 3 Mechanism of polyethylene degradation. (Adapted with permission from Ref. [34])

6 months of incubation. B. cepacia degraded thick plastic bags by 31.43% and thin plastic bags by 36.34% [39], whereas Pseudomonas aeruginosa reduced LDPE weight by 18.75% after 1 month [40]. Moreover, E. coli reduced the weight of thick plastic bags by 23.72% and that of thin plastic bags by 23.75% [39]. Furthermore, Bacillus licheniformis, Serratia sp., Stenotrophomonas sp., and Bacillus sp. were isolated from soil and damaged the surface of the plastic film made from LDPE after 1 month of incubation [41]. Alcanivorax borkumensis was able to form biofilm on and degrade the LDPE surface [42]. As reported by Ref. [41], biodegradation processes with enzymatic reactions are involved in the chemical changes in plastic polymers, such as reduction, oxidation, esterification, and hydrolysis. First, the depolymerization stage occurs through extracellular enzymes, wherein LDPE is broken down into small fragments. This stage facilitates LDPE absorption into the cell. The second stage is the mineralization process, where LDPE is mineralized into H2O, CH4, and CO2. Several fungal species (Aspergillus fumigatus, Aspergillus terreus, and Fusarium solani) have been isolated from soil and utilized LDPE as a carbon source [43]. Furthermore, researchers observed [35] that these strains exhibit high efficacy in

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LDPE degradation with a subsequent weight reduction in LDPE sheets. The dry weight of LDPE decreased by 5.8% and 11.1% through the effects of A. niger and A. japonicas, respectively, after 1 month of incubation [44]. A variety of fungal species isolated from seawater (Aspergillus caespitosus, A. terreus, Paecilomyces variotii, Alternaria alternate, Phialophora alba, and Eupenicillium hirayamae) showed a deterioration effect on PE films and CO2 emission after 28 days [45]. HDPE is produced through the catalytic process and possesses high tensile strength and strong intermolecular forces. Due to its hardness, durability at high temperatures, and opacity are commonly used in different industrial applications, such as detergent bottles, garbage containers, water pipes, and margarine tubes [46]. A study found that Klebsiella pneumoniae adheres strongly to the HDPE surface, leading to a 60% and 18.4% decrease in its tensile strength and weight, respectively, within 60 days of incubation [47]. Ochrobacterum anthropic is a bacterium that degrades HDPE film by 20% after 45 days of incubation [40]. Bacillus sp. MKY2 and Pseudomonas sp. MKY1 reportedly degrades PLA [48]. Pseudomonas sp., Psychrobacter sp., Shewanella sp., and Moritella sp. play an important role in diagnosing PCL [49]. The degradation process of PCL can be accomplished by different types of enzymes, such as lipase, esterase, and hydrolases [16]. Furthermore, lipase, polyurethane depolymerase, and lipase enzymes are important for hydrolyzing PLA [16]. Polyethylene terephthalate (PET) is classified as a thermoplastic and represents about 10% of the plastic market share. It is considered one of the main polyester plastic polymers in many industries. However, research showed that PET demonstrates a toxicity effect on living cells, and it affects the human endocrine system; furthermore, the breast cancer rate increases by 78% after PET exposure. PET can endure in a normal environment for 700 years. The degradation process of PET, which is categorized into abiotic and biotic degradation, received attention for reducing environmental contamination [50]. Abiotic degradation consists of chemical, thermal, and hydrolytic degradation [51]. Biotic degradation (biodegradation) exhibits the advantage of complete mineralization through microorganism effects. Ideonella sakaiensis is considered a PET-degrading bacterium that produces MHETase and PETase enzymes. PETase is an enzyme that hydrolyzes PET to produce mono-(2-hydroxyethyl) terephthalic acid and polyethylene terephthalic acid. Then, these are converted by MHETase into terephthalic acid and ethylene glycol [51]. Moreover, S. aureus and B. subtilis are believed to degrade PET and PS [52]. Under proper environmental conditions, different types of bacteria can accumulate polyhydroxy butyrate (PHB) in their cells, followed by its breakdown in polyhydroxy butyrate depolymerase enzyme [53]. Streptomyces lydicus MM10 is a type of actinomycete filamentous bacteria that secrete the PHB depolymerase enzyme, which can break down polyhydroxy butyrate [53]. Moreover, other bacterial species like Bacillus sp. and those in the Azotobacter genus have reportedly been able to degrade PHB [54] (Fig. 4). Polyurethane can be depolymerized with degrading bacteria, like Pseudomonas chlororaphis. Papain and urease are two enzymes involved in polyurethane polyester

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Fig. 4 Scanning electron micrograph of a pure culture of bacteria on (a) PEG, (b) PHB, (c) watersoluble PUR, and (d) HDPE. (Adapted with permission from Ref. [56])

biodegradation. Papain plays an important role in the hydrolysis of urea and urethane, and it produces the hydroxyl groups and free amines [55]. Next, lignin is considered one of the most abundant polymers in nature, mainly present in plant cell walls. Lignin is insoluble and complex, which is speculated in the degradation process. Laccase and peroxidase are crucial extracellular enzymes in lignin biodegradation. Because it demonstrates a higher redox potential, the lignin peroxidase enzyme demonstrates greater capability in the degradation process. In the case of bacterial degradation, lignin can serve as the only carbon source for bacterial growth. Various bacterial strains have been extensively studied for the possibility of utilizing Kraft lignin for simultaneous growth and degradation [57]. In another investigation by the same research group [58], Kraft lignin was utilized as the only carbon source by Pandoraea sp. B-6, resulting in a 46.5% weight reduction after a 7-day incubation, while the lignin removal rate reached 795.7 mg/L within the first 2 days of treatment. After 6 days of incubation in a medium supplemented with glucose and peptone as carbon and nitrogen sources, Raj et al. [59] reduced the Kraft lignin weight by 65% using Bacillus sp. ITRC-S8. Both natural and manufactured lignins are equally susceptible to fungal breakdown by their extracellular enzymes that decompose and mineralize lignin. Pleurotus ostreatus is a white rot saprophyte that lives in the wood and is reported to degrade lignin, cellulose, and hemicellulose. This fungus has additionally been utilized in the biodegradation of organic pollutants, xenobiotics, industrial effluents, cellulose bleaching, and food and enzyme synthesis [60]. In the presence of a

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medium supplemented with carbohydrates, lignin degradation occurs simultaneously with Phanerochaete chrysosporium, a type of white fungus. Several authors find that white-rot fungi break down lignin in the presence of extra nitrogen or carbon sources in the medium. Ligninolytic enzyme secretion can be activated in response to a nitrogen deficiency, carbon, manganese, or sulfur. Nitrogen repression may cause lignin biodegradation in several fungal species, including Trametes versicolor and Phanerochaete chrysosporium, but this is not necessary [61]. Due to its availability in nature and low cost, starch is considered one of the main resources for bioplastic production. It is mostly used in agriculture for mulching film, plastic films, and bags. Starch is commonly cleaved by α-amylase into short chains that are consequently hydrolyzed by glucoamylase, α-glucosidase, and β-amylase. Furthermore, Aspergillus oryzae, Klebsiella pneumonica, Bacillus stearothermophilus, and Bacillus circulans produce different types of enzymes that can degrade long starch polymers [62]. Starch degradation ranges from about 3 days after introduction to soil to weeks or 1 year. In addition, the biodegradation of thermoplastic sugar (TPS) was studied [63] in a soiled setting, and the number of degrading bacteria was found to increase after 3 months of incubation. Bacteria that degrade TPS have been mentioned in different studies, including Streptococcus sp., Staphylococcus sp., Bacillus sp., Pseudomonas sp., and Bacillus licheniformis [64]. Cellulose is a polysaccharide abundant in nature as the main component of plant cell walls. It is commonly present in crystalline form, and only 15% is found in the amorphous form. Amorphous cellulose biodegrades faster than crystalline cellulose [65]. Cellulases are a group of extracellular enzymes responsible for cellulose biodegradation [62]. These enzymes are produced by the hydrolytic system of microorganisms which can break the β-glycosidic links of cellulose. Pseudomana cellulomonas and Streptomyces sp. are reported to degrade cellulose through their enzymes [66]. Furthermore, Phanerochaete chrysosporium and Trichoderma reesei are a class of fungi that can degrade cellulose. Laetisaria arvali is more efficient in the biodegradation of cellulose due to the production of hydrolytic lytic polysaccharide monooxygenase enzymes [67]. The filamentous algae can colonize plastic waste surfaces depending on several crucial environmental factors for algae growth, such as nutrients, water, and sunlight. Algae can degrade polycyclic aromatic hydrocarbons (PAHs) by using dioxygenase enzymes that lead to cis-trans hydroxyl groups. After attachment, the degradation process starts with the production of exopolysaccharides and ligninolytic enzymes. The secreted extracellular enzymes react with macromolecules on plastic surfaces and initiate biodegradation. Additionally, the metabolic reactions are dependent on light, and the metabolites depend on the type of light radiation [68].

Pesticide-Degrading Microorganisms Pesticides are commonly used to control and prevent crop diseases. Simultaneously, pesticide residues seriously affect human and environmental health due to their environmental stability [1]. Bacteria could degrade pesticide residues in an eco-friendly and cost-effective manner by using these compounds for their metabolic

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Table 2 Efficiency of different microbial degradation on different polymers Species of microorganisms Bacillus sp.

Pseudomonas sp.

Streptomyces sp. Pseudomonas oleovorans Klebsiella sp. Brevibacterium frigoritolerans, Rhodopseudomonas palustris, Pseudomonas fulv, and Bacillus aerophilus

Pesticide name Glyphosate, chlorpyrifos, coumaphos, DDT, diazinon, methyl parathion, dieldrin, endosulfan, endrin, monocrotophos, parathion, polycyclic aromatic hydrocarbons, chlorpyrifos Endrin, diazinon Aldrin, chlorpyrifos, DDT, endosulfan, coumaphos, monocrotophos, parathion hexachlorocyclohexane, methyl parathion Carbofuran, Aldrin, chlorpyrifos Lambda-cyhalothrin

References [85]

Fentopropathrin Phorate

[87]

[85]

[85] [86]

reactions, as shown in (Table 2) [71]. Biodegradation of pesticides is carried out mainly by microbial enzymes, such as oxygenase, peroxidase, and hydrolase. Pesticides are transformed into water-soluble compounds through hydrolysis, reduction, or oxidation reactions. Subsequently, the compounds are transformed into amino acids and sugar, demonstrating lower toxicity and high water solubility. The degradation time depends on pollutant concentration, water content, temperature, and the pesticide leaching from the soil [72]. PAHs are a large group of chemicals with two or more aromatic rings. They are colorless solids with low solubility in water and a high melting point. According to recent research, the extracellular peroxidase enzyme of fungi is responsible for the primary oxidation of fungi. Several studies found that naturally occurring fungi in soil play an important role in biodegrading PAHs from solid matrix. Fungi that belong to genera Penicillium, Aspergillus, Fomitopsis, Coriolus, Daedalea, Pycnoporus, and Pleurotus can degrade PAHs in soil and the aquatic environment [73]. Moreover, the genera belonging to white-rot fungi, such as Phanerochaete, Pleurotus, Irpex, Bjerkandera, Stereum, Lentinus, and Polyporus have reportedly been responsible for the remediation of contaminated soil [74]. The white-rot fungus (P. chrysosporium) showed high biodegrading ability for anthracene (40% reduction) in comparison to Irpex lacteus (38% reduction) [75]. Trichoderma asperellum degraded 74% of phenanthrene, 81% of benzo[a]pyrene, and 63% of pyrene within 14 days of incubation [76]. Lee et al. [77] reported that Peniophora incarnata KUC8836 showed the ability to biodegrade phenanthrene by 95.3% and pyrene by 97.9% after 14 days of incubation. The Trichoderma/Hypocrea genus degraded PAHs by 27% after 2 weeks of incubation [78]. The degradation of pyrene, benzo[a]pyrene, and phenanthrene in soil occurred from Phanerochaete chrysosporium metabolism [79]. Furthermore, 1,2,3,4-tetrahydronaphthalene (THN) biodegradation was reported by producing 3,4-dihydro-4-

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hydroxy-1(2H)-naphthalenone, 1,2,3,4-tetrahydro-1,2-naphthalenediol, 3,4-dihydro-1 (2H)-naphthalenone, and 1,2,3,4-tetrahydro-1-naphthalenol [80]. There are wide applications of organophosphorus pesticides (OPs) in human activities. OPs are classified into two categories: one has P ¼ O and includes phosphothiolates and phosphotriesterates, and another category has P ¼ S and includes phosphorothionothiate and phosphorothionate. Malathion, profenofos, moncrotophos, dimethoate, diazinon, methyl parathion, and chlorpyrifos are commonly used OPs. Although the organophosphorus compounds can be degraded, they exhibit a toxicity effect on animals and humans. Overuse of these compounds leads to soil and water pollution. OPs are susceptible to degradation by bacteria through the production of hydrolases enzymes, which break down the double bond between P and O [81]. Furthermore, Pleurotus ostreatus reportedly degrades malathion through manganese peroxidase, lignin peroxidase, and laccase. Biodegradation results in the following: methyl l,2-(dimethoxyphosphoryl)-2-(1,2,3-thiadiazol-4-yl) acetate; 2,20 -Thiodisuccinic acid; 3,4-Dihydrothienyl (3,4,b)-5-carboxythiophene; 2,20 Thiodisuccinic acid; butanedioic acid; diethyl mercaptosuccinate; diethyl mercaptosuccinate; disulfide; di-tertdodecyl; and 1,1-dimethyltetradecyl hydrosulfide [82]. Many microorganisms use herbicides as a carbon source for survival and growth in the environment and thus demonstrate great potential in herbicide degradation. The massive use of herbicides results in their accumulation in agricultural soil, groundwater, and river systems, changing the structure and function of the soil microbial system. Herbicides are classified into about 25 groups based on their targets, inhabited proteins, or induced symptoms. Herbicide overuse leads to different degradation metabolites in nature that persist in water, plants, and soil and pose potential toxicological problems. The herbicide chloroacetamide could be transformed in soil by microbial metabolism into 4,2-methyl-6-e-thylaniline, an intermediate compound that can be used by Sphingobium sp. as a carbon source. This compound undergoes enzymatic reaction, leading to 4-hydroxy-2-methyl-6ethylaniline and 2-methyl-6-ethylhy-droquinone [83]. Microbial consortia, which consist of Testosterone sp., Comamonas sp., Variovorax sp., and Hyphomicrobium sulfonivorans, demonstrate the ability to degrade the phenylurea herbicide linuron [84].

Mechanisms of Biodegradation The biodegradation of plastics occurs aerobically in nature, anaerobically in landfills and sediments, or partially aerobically in soil and compost (Fig. 5) [88]. In aerobic degradation, aerobic microbes utilize oxygen as an electron acceptor to break down organic molecules into smaller organic compounds, CO2, and water. In contrast, anaerobic biodegradation is facilitated different types of microorganisms in the absence of oxygen. For example, some anaerobic bacteria use sulfate, nitrate, iron, carbon dioxide, and manganese as electron acceptors to break down the organic compounds into smaller ones [88]. The general mechanisms of biodegradation are shown in Fig. 6.

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Polymer Depolymerases

Microbial Biomass CO2 H2O

Aerobic degradation

Dimers Monomers Oligomers

Anaerobic degradation

Microbial Biomass CO2 H2O CH4/H2S

Fig. 5 Polymers biodegradation in aerobic and anaerobic conditions. (Adapted with permission from Ref. [56])

Fig. 6 General mechanism of the biodegradation process

Biodeterioration Modifying or changing a polymer surface with microbial colonization and biofilm formation facilitates the biodeterioration process, including physical, mechanical, and chemical changes. Therefore, plastic polymers can be degraded by endoenzymes and exoenzymes secreted by microorganisms [89].

Microbial Biofilm Formation Polymer biodegradation is enhanced by the initial attachment of microorganisms and biofilm formation on the polymer surface [69]. Biofilms, formed from a community of

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living cells, accelerate the biodeterioration process. Microbial biofilms develop more rapidly on plastics with less hydrophobicity and buoyancy. The extracellular polymeric matrix (EPS) produced by microbial cells during biofilm formation increases microbial adhesion and facilitates the plastic surface’s breakdown. EPS penetrates the pores of the plastic surface, facilitating microbial entry and the deterioration of the polymer surface. Moreover, biofilm formation leads to the generation of different acidic compounds, such as sulfuric, gluconic, citric, oxalic, glutaric, oxaloacetic, and glyoxylic acids, which alter the polymer microstructure [90].

Biofragmentation The biofragmentation process includes the conversion of polymers into simpler forms and the subsequent assimilation. Polymeric substances are broken down into oligomers, dimers, and monomers by enzymes that facilitate fragmentation by destabilizing the carbon skeletons. Microorganisms can easily use monomers as a carbon source, leading to an increase in microbial biomass [91]. In addition, microbial enzymes that break polymers down into short-chain molecules, such as esterase, depolymerase, and lignolytic enzymes, may be utilized in the biodegradation of plastics. Lastly, aerobic degradation produces H2O, CO2, and microbial biomass, whereas anaerobic degradation produces H2O, CO, CH4, or H2S and microbial biomass [92].

Mineralization Mineralization, the final biodegradation step, converts the compounds into environmentally friendlier materials. When polymers are completely degraded, final products, such as water, salt, gases, and minerals, are released. The released gases include methane, nitrogen compounds, and carbon dioxide. The mineralization step is complete when all biodegradable compounds have been consumed by microbial cells, and all the carbon has been converted to carbon dioxide [93]. Meanwhile, co-metabolism and mineralization are the main mechanisms in the biodegradation of pesticides. Co-metabolism transforms pesticides into useful compounds via the synergy of microbial enzyme activities and other physical, biological, chemical, and transformations [93]. The enzymes involved in co-metabolism include transferases such as glucosyltransferases, glutathione, and S-transferase; hydrolytic enzymes, such as esterases, nitrilases, and amidases; reductases, such as reductive dehalogenases and nitroreductases; and oxidases, such as peroxidase and cytochrome P-450s. On the other hand, mineralization mechanisms include the breakdown of pesticides into carbon dioxide, water, salts, and minerals. The microbial community mainly influences the rate of mineralization. Moreover, soil properties affect the mineralization process. For example, according to a previous study, different soil parameters such as pH, soil organic matter content, ions content, and soil texture affect glyphosate mineralization [94].

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Involvement of Microbial Enzymes in the Biodegradation Process Enzymes are macromolecules that are involved in cell regulation and cell functions and biodegradation. Intracellular and extracellular enzymes secreted by microorganisms play an important role in biodegradation, showing a unique interaction mechanism (Table 3) [95]. For example, enzymes like lipase, laccase, esterase, amylase, lignin peroxidase, and manganese oxidase are important catalysts for biodegradation. The released enzymes will cleave primary polymeric chains into lowmolecular-weight compounds during degradation. Then, these smaller compounds will be efficiently dissolved in water and absorbed through the microbial cell membrane to be utilized as carbon and energy sources. In another way, the small fragments find their way and diffuse through the organisms with consequent degradation forming energy and biomass [95]. Several enzymes are involved in the degradation of plastics indirectly or directly. Their activities include hydrolysis, oxidation, or hydroxylation, which converts polymers into monomers or small oligomers following assimilation by microorganisms. For example, depolymerase and hydrolase are extracellular enzymes that can break down large plastic polymers by attacking the long polymeric chain with hydrolytic cleavage or the end of the chain. In addition, hydrolase catalyzes the hydrolysis of the amides, carbonates, esters, and glycosidic bonds of different polymers to produce monomers. Moreover, oxidoreductase catalyzes carbonates, urethanes, ethylenes, and amides [96]. Meanwhile, Pseudomonas can degrade polymers using extracellular oxidative or activities of hydrolytic enzymes, or both, that enhance polymeric uptake and degradation [97]. In addition, soybean peroxidase and hydrogen peroxidase have demonstrated their abilities to decrease PE’s hydrophobicity and facilitate its degradation [34]. Table 3 Different types of microbial enzymes for biodegradation of different compounds Microorganisms Clostridium botulinum Ideonella sakaiensis Rhodococcus ruber Pseudomonas sp. Aspergillus caespitosus and A. terreus Phanerochaete chrysosporium Pseudomonas sp. Aspergillus caespitosus and A. terreus Streptomyces lydicus MM10 Trichoderma reesei and Laetisaria arvalis Phanerochaete chrysosporium

Enzyme Lipase MHETase PETase Laccase PVADH Lac, MnP, and LiP

Polymer/ compounds PCL PET

References [65] [51]

LDPE PVA PE

[32] [16] [45]

Manganese peroxidase Oxidase Lac, MnP, and LiP

PE PVA PE

[34] [97] [45]

PHB depolymerase Lytic polysaccharide monooxygenases β-glucosidase

PHB Cellulose

[53] [67]

Cellulose

[66]

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On the other hand, laccases, also called blue-copper oxidases, have four copper atoms per enzyme. It is commonly found in bacteria, fungi, and higher plants. Laccase is a glycosylated enzyme; with a molecular weight of around 60 kDa. It is larger than peroxidase. Laccases enzymes are involved in the oxidation of phenolic compounds, anilines, polyphenols, and aromatic amines and reducing oxygen to water. Also, it oxidizes non-phenolic compounds in the presence of redox mediators [98]. Laccase is the most studied enzyme connected to HDPE degradation. Laccase makes the carbonyl group easily accessible via the oxidative cleavage of the amorphous region of HPDE [34]. In addition, fungi have been discovered to produce various carbohydrate-active enzymes (CAZymes), which break down complex plant polymers into digestible and assimilable compounds for other ecosystem members. Almost 200 CAZyme families with more than 300 members have been identified [99]. CAZymes degrading plants such as ligninases, hemicellulases, pectinases, and cellulases, as well as accessory debranching enzymes, belong to the following groups, polysaccharide lyases (PLs), glycosyltransferases (GTs), glycoside hydrolases (GHs), carbohydrate esterases (CEs), and auxiliary activities (AAs), that can be connected to carbohydrate-binding modules (CBMs). The development of auxiliary depolymerizing enzymes in filamentous fungi, which act in tandem with backbone-degrading enzymes to further degrade plant polysaccharide complexes, is primarily controlled at the gene transcription level [100]. Many fungi are believed to be capable of removing chlorinated phenolic chemicals from contaminated environments. For example, extracellular oxidoreductase enzymes, such as laccase, manganese peroxidase (MnP), and lignin peroxidase released from fungal mycelium into the surrounding environment, are primarily responsible for fungal activities. Moreover, Phanerochaete chrysosporium produces MnP that can reduce the tensile strength of PE and its molecular weight [34]. Lastly, specific oxygenases are responsible for breaking down different environmental contaminants, such as insecticides, herbicides, plasticizers, fungicides, and hydraulic compounds. In collaboration with multifunctional enzymes, oxygenase also mediates the dehalogenation of halogenated methane, ethane, and ethylene. In addition, laccase and oxygenase are incorporated into the oxidation process of these contaminants utilizing electron transfer from reductants to oxidants. Laccase enzyme cleaves the aromatic ring, converts oxygen to water, and generates free radicals, releasing energy utilized by microorganisms in their metabolic reactions. Moreover, esterase, cellulase, and lipase are involved in the hydrolysis of pesticides by cleaving the compounds into smaller chains [87].

Factors That Affect the Biodegradation Process Microbial Species and Their Metabolic Activities The reaction of different species of microorganisms or different strains of the same species toward the same organic matter.

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Substrate Characteristics The spatial structure, molecular weight, type, and the number of substituents and characteristics of substitution affect the rate and efficiency of microbial degradation. For instance, composites are more resistant to biodegradation than compounds with simpler structures. Polymer biodegradation is affected by changes in molecular weight, melting temperature, tensile strength, chemical structure, phase separation, corrosion, cracking, degree of crystallinity of a polymer, and the type of microorganisms involved. For example, polymers with side chains are more difficult to decompose than those without. In addition, polymers with high molecular weights are more challenging to degrade. Also, amorphous polymers are more easily degraded than crystalline polymers. The number of benzene rings in PAHs will also affect the degradation process. The compounds with two or three rings are quickly mineralized as a carbon source for microorganisms. In contrast, four-ring or multi-ring PAHs with high molecular weights are more stable in the environment and difficult to degrade. Meanwhile, the polymers with hydrolyzable ester bonds in their backbones, such as PUR and PET, are more susceptible to biodegradation than carbon chains, such as PVC, PP, PS, and PE. Furthermore, blending plastics with certain natural polymers, like starch, facilitates biodegradation because the rapid hydrolysis of natural polymers creates pores in the plastic material, making it more susceptible to abiotic and biotic degradation.

Environmental Factors Microorganisms and their enzymes require suitable conditions, such as humidity, temperature, oxygen, carbon dioxide, salinity, pH, surfactant, and polymer concentrations, affecting biodegradation. A lack of nutrients limits microbial growth. Besides, the time required for biodegradation mostly depends on the type of microorganisms, the polymer type, and its concentration. Moreover, the presence of surfactants can change a compound’s solubility and the balance between compound adsorption and desorption, further changing its bioavailability. In addition, the optimal temperatures for different enzymatic activities have been studied. Enzymatic activity is highest at mesophilic temperatures and declines at very high and low temperatures. For example, at 5  C, only laccase activity is detected; the optimal temperature for its activity is 45  C. In contrast, the activity of Mn-dependent peroxidase was detected at a higher temperature (75  C). In addition, the degradation rate can be enhanced with a pretreatment at a high temperature, which results in volatilization and a reduction in the soil-water partition coefficient. Indeed, with every decrease of 10  C, the biodegradation rate is reduced by nearly half.

Conclusion The modern economy is growing quickly, leading to the accumulation of waste materials, a global threat to the environment. Waste materials have undesirable effects on life via their accumulation in soils, landfills, and other locations. Recent

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research has been focused on waste management using microbial and enzymatic strategies. For example, some microorganisms can degrade plastics and other waste materials. Furthermore, different enzymes synthesized by various microorganisms can play a vital role in biodegradation, during which compounds are broken down into simpler molecules to produce water and carbon dioxide.

Future Perspectives The production of high-value products from plastic wastes and valuable compounds from the oligomers or monomers formed during biodegradation represent a future potential that can improve the use of plastics. Furthermore, implementing active biodegraded enzymes in the biodegradation process is a high research task that will significantly decrease the global waste problem. Also, further research is required to increase the diversity of microorganisms and enzymes for biodegradation opportunities.

Cross-References ▶ Mechanism of Microbial Biodegradation: Secrets of Biodegradation

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Role of Microorganisms in Biodegradation of Pollutants Farida Ah. Fouad, Donia G. Youssef, Fatma M. Shahat, and Mohamed N. Abd El-Ghany

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacterial Biodegradations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant Growth-Promoting Rhizobacterial Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Role in Nitrogen Fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Role in Phosphorous Solubilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Hormone Regulation by Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Protection from Phytopathogenic Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microfungi and Mycorrhiza Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filamentous Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yeast Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of Algae and Protozoa in the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Microbial Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioremediation and Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Farida Ah. Fouad, Donia G. Youssef, Fatma M. Shahat and Mohamed N. Abd El-Ghany contributed equally with all other contributors. F. A. Fouad Biophysics Department, Faculty of Women for Arts, Science and Education, Ain Shams University, Cairo, Egypt D. G. Youssef School of Biotechnology and Science Academy, Badr University in Cairo, Badr City, Cairo, Egypt F. M. Shahat Chemistry/Microbiology Department, Faculty of Science, Cairo University, Giza, Egypt M. N. Abd El-Ghany (*) Botany and Microbiology Department, Faculty of Science, Cairo University, Giza, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_11

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Degradation by Genetically Engineered Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of GEM in Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Enzymes in Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidoreductases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrolases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Environmental pollution through industrial development has led to the generation of a variety of lethal materials, especially recalcitrant classes such as polycyclic aromatic hydrocarbons, toxic dyes, pesticides, and heavy metals, which are now of critical environmental importance due to their harmful and mutagenic effects on humans, plants, and aquatic organisms. Biodegradation is naturally performed by microorganisms, implying the decomposition of complex organic compounds into a more straightforward inorganic form. These organisms will harness these organisms as a source of energy, while bioremediation is a human engineering technology that reduces pollutants using microorganisms through techniques of natural attenuation, biostimulation, or bioaugmentation to strengthen the ability of microorganisms. Various microorganisms can degrade environmental pollutants with promising skills like bacteria, fungi, algae, and protozoa. Certain parameters must be established to provide the highest biodegradation rate of degradable microorganisms under the optimum conditions. These factors are biological factors such as bioavailability, nutrient availability, culture type and type of microorganism, and environmental factors such as pH, temperature, oxygen availability, and pollutant concentration. Biodegradation mechanisms depend mainly on microbial enzymes such as oxidoreductases, hydrolases, peroxidase, oxygenase, proteases, lipases, and lacquers. Genetically modified microorganisms have been applied to further improve the remediation of pollutants and ensure safe biodegradation using symbiotic microorganisms. Keywords

Biodegradation · Pollutants · Biofertilizer · Genetically modified microorganisms · Bioremediation · Eco-friendly · Sustainability · Polycyclic aromatic hydrocarbons · Heavy metals · Plant growth-promoting rhizobacteria Abbreviations

ABGs AMF ATP ATPase BR

Aerobic bacterial granules Arbuscular mycorrhizal fungi Adenosine triphosphate Adenosine triphosphatase Brassino steroids

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CAM plasmid CK CoA COD EC50 EcM EcO 157 EPA ET FQ GA GE GEMs gfp GMMs GTAs HGT HMs IAA lacZ LDPE LEV LGT lux/luc MO MTs N¼N NADH NADPH NAH plasmid PAHs PASHs PCR PCs PET PGPR PSMs PU S-GEMS UV VP WWTPs XYL plasmid

transmissible camphor plasmid Cytokinin Coenzyme A Chemical oxygen demand Effective concentration Ericoid mycorrhiza Escherichia coli O157:H7 Environmental Protection Agency Ethylene Fluoroquinolone Gibberellin Genetic engineering Genetically engineered microorganisms encoding green fluorescent protein Genetically modified microorganisms Gene transfer agents Horizontal gene transfer Heavy metals Auxins indole-3-acetic acid Encoding β-galactosidase genes Low-density polyethylene Levofloxacin Lateral gene transfer Encoding bacterial/firefly luciferase Microorganisms Metallothioneins Atmospheric dinitrogen Nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide phosphate Naphthalene plasmid Polycyclic aromatic hydrocarbons Polycyclic aromatic sulfur heterocycles Polymerase chain reaction Phytochelatins Polyethylene terephthalate Plant growth-promoting rhizobacteria Phosphate-solubilizing microorganisms Polyurethane Suicide gene-mediated GEM transfer control Ultraviolet Versatile peroxidase Wastewater treatment plants A nonconjugative xylene-degradative plasmid

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Introduction Nowadays, environmental problems are growing rapidly; the unsolved problems resulting from environmental pollution explain the inability to control local problems. Globally, the environment is always under threat due to climatic disorders such as unexpected, damaging heavy rains, and recurring floods from intense tropical storms. Environmental problems have become intense due to uncontrolled human activity [1]. Generally, pollutants are classified according to their physical or chemical properties (shown in Fig. 1). First: Chemical pollutants result from (1) artificial factors such as toxic substances and nuisance substances like plastic or glass, and (2) natural factors including (2.1) critical pollutants such as carbon and nitrate also include (2.2) inessential pollutants like lead and copper. Second: Energy and pollutants result from the heat, noise, and ionizing radiation [1]. Environmental pollutants are among the most critical global problems leading to miscellaneous health problems for living beings, damage, and changes in the natural environment. Fungicides, herbicides, and pesticides are of substantial consideration on a large scale since they are enormously used in agriculture. Also, volatile pesticides can be introduced to the atmosphere due to the usage, emission of the residual environmental substances from a previous existence. Other pollutants have been found in major environmental sectors, air, water, and soil, like polycyclic aromatic hydrocarbons (PAHs), termed anthropogenic pollutants, resulting from the inadequate combustion of organic matters [1]. Some microbial species can do a better acceptable experiment for the degradation rate of chemicals. Microorganisms (MO) can be termed as biosensor devices with a low cost to determine the presence of a specific chemical and its toxicity according to the standard preferences and risk assessments [2]. Biological methods for degradation by microorganisms are usually preferred as observed; the use of microorganisms is eco-friendlier and reduces toxicity in a suitable way like Aspergillus terreus showed high degradation capability of pollutants, produced by polycyclic aromatic hydrocarbons (PAHs) resistant to biodegradation because of their hydrophobic property, due to manganese peroxidase, lignin peroxidase, and laccase production [3]. In contrast, chemical degradation leads to the depolymerization of polymers Fig. 1 Pollution classification. (Adapted with permission from Ref. [1] (Copyright © 2010, John Wiley and Sons))

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depending on the hydrolysis process or the catalyzed enzyme hydrolysis. Chemical waste degradation, like pesticides, has diverse methods like photolysis, hydrolysis, oxidation, and dehalogenation. Chemical fertilizers and pesticides highly affect environmental health; repetitive usage without knowing their side effects leads to deterioration and reduces the ecosystem’s abilities and functions. The ideal solution for eliminating toxicity and providing a safe environment is biodegradation using microorganisms like Azotobacter species that work against rhizosphere and pollutants. It also helps to improve fertility and works as chemical removal of pollutants [4]. Another example of water treatment is with MO, capable of biodegradation of bisphenol, which results in no toxicity of water [5]. Plastic or microplastics are treated as a residual waste that can be thrown into the water (steps: 1, 2, 5), or even it can be treated as a residual of the manufacturing process thrown in the water by the industry (steps: 1, 3, 6, 7). So, the best solution is to transform the waste into a functional product like fertilizers to initiate the biodegradation process (steps: 1, 3, 8), as explained in Fig. 2 [6].

Bacterial Biodegradations Various pollutants increase significantly due to different industries and have lethal consequences for the ecosystem and living things [7]. Among various environmental pollutants, anthraquinone dyes such as Remazol Brilliant Blue R and Reactive Blue 19 are the second most widely applied dyes after azo dyes. These dyes are low-cost, have high dye performance, and have easy accessibility. Anthraquinone dyes are Fig. 2 Microplastic is a global problem. (Adapted with permission from Ref. [6] (Copyright © 2020, Elsevier))

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harmful to people and microorganisms due to their sophisticated and stable structure. These dyes are more toxic than azo dyes. A variety of bacterial strains may break down anthraquinone dyes. Aerobic bacterial granules (ABGs), mainly containing Proteobacteria and Firmicutes, can biodegrade significant amounts of Reactive Blue 4. Reactive Blue 4 was 1000 mg/L with maximum discoloration (6.16 0.82 mg/(L h). Serratia liquefaciens PT01 showed noticeable activity for discoloration of Remazol Brilliant Blue R up to 50–60% through 336 h. Bacterial adsorption of dyes generally takes place before the degradation process. It occurs when the electrostatic, molecular, and covalent forces induce the adhesion of the dyes to the bacterial surface. Bacterial degradation of anthraquinone dyes is produced by a reduction reaction in which the use of the reductase enzyme cracks the conjugated dye bonds. After the loss of chromophores, the remaining complex polycyclic aromatic hydrocarbons are divided into simple or polycyclic rings. Single or polycyclic rings that are further decomposed as single rings or as single polycyclic aromatic hydrocarbons such as naphthalene, anthracene, etc., are ultimately fully degraded to carbon dioxide and water under aerobic conditions [8]. The composition of diesel hydrocarbons consists of olefins, paraffins, aromatic hydrocarbons (naphthene), nonmetals (S, O, N), and metals (Ni, Fe, and V). It is found to be toxic, mutagenic, and carcinogenic. Consequently, their accumulations are harmful and toxic to the environment and different living things. Hydrocarbon toxicity is based on several factors such as compound properties, surface tension, volatility, viscosity, and additives. Several bacterial strains have been established as a potential bioremediation agent for diesel by decreasing the diesel composition. These strains can convert into CO2 and H2O (nontoxic compounds). These bacterial strains can be Pseudomonas sp., Acetobacter sp., Achromobacter sp., Micrococcus sp., Staphylococcus sp., Bacillus sp., Flavobacterium sp., Klebsiella sp., Actinomycetes sp., and Rhodococcus sp. [9]. These bacterial strains may tolerate the toxicity of diesel pollutants through certain metabolic or enzymatic reactions subjected to a biodegradation process to use end products as a carbon source. Alkanes or paraffin (n-alkanes, iso-alkanes, and cycloalkane) are saturated chain hydrocarbons. The bond between C and C is single (C-C) and comprises most of the diesel under aliphatic hydrocarbons. In aliphatic hydrocarbons (n-alkanes), biodegradation is initially carried out under aerobic conditions using monooxygenase and dioxygenase enzymes. The oxygen atom is added to the terminal or subterminal carbon. This process (shown in Fig. 3) involves the conversion of aliphatic compounds into a few central intermediate primary and secondary alcohols. In this case, the alcohol dehydrogenase enzyme converts the alcohol into an aldehyde. After this, the alcohol dehydrogenase enzyme interacts with aldehydes. Because of this, fatty acids will form and react with coenzyme A (CoA) to yield acetyl CoA. Acetyl CoA enters the central metabolic pathway (Krebs cycle) after the β-oxidation process. N-alkane and iso-alkane have the same pathways. Cycloalkanes are biodegraded by Acidovorax sp. CHX100, Chelatococcus sp. CHX1100, and Rhodococcus aetherivorans BCP1 since cycloalkanes are their only carbon source [9–10].

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Fig. 3 Biodegradation of cyclohexane. (Adapted with permission from Ref. [9] (Copyright © 2019, Elsevier))

H+, Hg2+ R

C

C

R

R1

OH R

R

O

R1 Alkynes

Keto form

Enol form

KM OH

O R

R1

+ OH

nO , 4 OHHO 2

O

+

CO2

Carboxylic acid cleavage mixtures b -oxidation Acetyl-CoA

Fig. 4 Alkyne biodegradation. (Adapted with permission from Ref. [9] (Copyright © 2019, Elsevier)

Aquincola tertiaricarbonis L108 and Methylibium petroleiphilum PM1 have been optimized to acquire potential biodegradation capability to alkynes from diesel. Alkynes have a similar cycloalkane biodegradation pathway, as shown in Fig. 4. Aromatic hydrocarbons: the second most important substance in diesel. The biodegradation of aromatic compounds is more complicated than aliphatic compounds because of many enzymes and complex pathways, as shown in Fig. 5. When the bacteria completely biodegrade, the hydrocarbon compounds that drift to generate the ATP energy source for their metabolism. CO2 and H2O are also produced because the biodegradation paths are subjected to aerobic conditions [9].

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O2

OH

Polycyclic aromatic hydrocarbons

OH

Dioxygenase Cis-dihydrodiol

H

OH

OH

Catechol 1,2Ortho or intradiol pathway

Catechol

Catechol 2,3dioxygenase

dioxygenase O2

Metha or extradiol pathway

OH

O2

COOH COOH COOH CHO

Cis, cis- Muconic

2- Hydroxymuconic semialdehyde COOH

O COOH COOH

b -Ketoadipic acid

O

HO

4-Hydroxy-2oxovaleric acid

O COSCoA

CH3

COSCoA

C

b-ketoadiply-CoA

CH3 O

CHO

Acetaldehyde

COOH

Pvruvic acid CH3

CH3

COSCoA + COSCoA

COSCoA COSCoA

Acetyl-CoA

Acetyl-CoA

Succinyl-CoA Fig. 5 Aromatic hydrocarbons biodegradation. (Adapted with permission from Ref. [9] (Copyright © 2019, Elsevier))

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The accumulation of plastic has altered the appearance of different landscapes. Therefore, it is necessary to biodegrade. Cyanobacteria (Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942) and photosynthetic microorganisms produce high-valuable products by capturing solar energy and the greenhouse gas carbon dioxide. Cyanobacteria and microalgae (Chlamydomonas reinhardtii and Phaeodactylum tricornutum) are vital to the biodegradation of plastics. Biosorption involves two steps. Firstly, the pollutant attaches to the cell surface or vice versa, depending on the size ratio. Second, the polluter is transported passively or actively into the cell. Immobilization and biomass and membrane separation have a significant role in improving plastic uptake. The biodegradation process may be summarized into four processes: (A) Biodeterioration or initiation of biodegradation which takes place by superficial degradation by abiotic parameters. The parameters can be mechanical, thermal, light, or chemical. These parameters may influence the mechanical and chemical properties of the macromolecular structure and extracellular polymeric substances. Microbial communities produce the macromolecular structure and extracellular polymeric substances. These substances enter the plastic pores and weaken them and if chemolithotrophic bacteria are present and produce acids such that nitrous would further weaken the structure. (B) Biofragmentation. Oxygenase (mono- and di-oxygenase) enzymes are produced and interact with long carbon chains. Alcohols are formed. (C) Microorganism assimilation occurs when specific carriers (receptors) are used to pass through the cytoplasm membrane. Once the building blocks are inside, they are oxidized and produce energy and biomass through catabolic pathways (anaerobic respiration, fermentation, and aerobic respiration). (D) Mineralization indicate complete degradation; as the assimilation step derives the metabolites from the biodegradation organism, other organisms may use the materials. The products are also oxidized metabolites (CO2, H2O, N2, CH4) [11].

Plant Growth-Promoting Rhizobacterial Degradation Microbial Role in Nitrogen Fixation Nitrogen is highly necessary for plant growth, as it is considered one of the major nutrients and components of chlorophyll and the crucial pigment needed for photosynthesis. Plants do not have a direct mechanism to convert atmospheric dinitrogen into ammonia (NH3). It is reported that some PGPR strains such as Azoarcus sp., Beijerinckia sp., Klebsiella pneumoniae, Pantoea agglomerans, and Rhizobium sp. can bind atmospheric N2 into the soil; therefore, it may be available for use by plants [12]. Nitrogen-fixing bacteria and beneficial microorganisms have been

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widely used in agriculture to reduce chemical or organic fertilizers. This means a low cost of production with high productivity and thus the elimination of pollutants that destroy the environment. Two primary types of bacteria can fix nitrogen in a form available for use by plants. They are symbiotic bacteria and free-living. An example of interdependent bacteria includes Rhizobium, whereas nonsymbiotics have Cyanobacteria and Azotobacter. In symbiotic nitrogen binding, 80% of the nitrogen absorbed by plants comes from these types of bacteria, and this occurs due to the mutual relationship in the rhizosphere between legume crops and beneficial microbes. Plants that produce certain substances are called exudates that boost beneficial microbes to move to the roots of plants. When they enter, nodules will be formed and act as N2-fixing factories. As a result, unused atmospheric nitrogen becomes available for use by plants. The method for converting atmospheric nitrogen to ammonia (NH3) is highly sensitive. This sensitivity is closely related to specific bacterial enzymes. These bacterial enzymes are termed nonsymbiotic freeliving nitrogen-fixing bacteria. Only a limited portion of the fixed nitrogen (20%) is formed by nonsymbiotic bacteria [13]. Unlike symbiotic bacteria, they do not have a mutual relationship to plant roots, but they survive through plant residues. N2-fixing bacteria use a complex enzyme system termed nitrogenase in the N2 fixation process. Nitrogenase is a metalloenzyme composed of iron protein (di-nitrogénase reductase) and metal cofactor (di-nitrogenase). The electrons are supplied with an enormous reduction potential by di-nitrogenase reductase, and di-nitrogenase uses these electrons to reduce N2 to NH3. Three separate N-fastening systems are available: (a) mo-nitrogenase, (b) V-nitrogenase, and (c) Fe-nitrogenase. And these different systems have different metal cofactors. However, the common metalloenzyme in the nitrogen binding process is Mo-nitrogenase. Genes used in the nitrogen fixation process are known as Nif-genes. Nif-genes exist in both symbiotic and nonsymbiotic free-living systems. Nif genes consist of both structural genes and regulatory genes. Structural genes play a part in the Fe protein, activation of iron-molybdenum cofactor biosynthesis, and electron donation. In contrast, regulatory genes contribute to the biosynthesis and functionality of the enzyme, as shown in Fig. 6. In Rhizobium, the Nif gene activation is based on a reduced oxygen level regulated by fix genes. Fix genes are found in symbiotic and free-living nitrogen fixation systems. The oxidative phosphorylation of bacterial carbon resources for ATP production rather than glycogen synthesis. While energy storage is in the form of glycogen. These bacteria assist in the nitrogen fixation process. This process requires at least 16 mol of ATP for every mole of reduced nitrogen [14].

Microbial Role in Phosphorous Solubilization Phosphorus is regarded as one of the main macronutrients essential to plants. Chemical fertilizers provide soluble inorganic phosphate to the soil, but rapid immobilization occurs, making phosphate unavailable to plants. Microorganisms are capable of transformation p in available forms. As a result, the microorganisms

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En

Atmospheric N2

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erg y photosynthesis Ammon ia

Energy

Flavonoids

Nif & fix genes

rhizobia N2

NH3

Nod gene

Fig. 6 Symbiotic nitrogen fixation in legumes by rhizobia. (Adapted with permission from Ref. [15] (Copyright © 2020, Wiley Online Library))

optimize crop production and reduce soil P loss [16]. The mechanism can be divided into two processes, inorganic and organic mechanisms, as shown in Fig. 7. Organic acid production is a primary mechanism for solubilizing inorganic phosphates. Organic acids such as acetic and formic (monocarboxylic acids); lactic, gluconic, and glycolic (monocarboxylic hydroxy acids); 2-keto gluconic (monocarboxylic keto acid); oxalic and succinic (dicarboxylic acids); malic (dicarboxylic hydroxy acids); and citric (tricarboxylic hydroxy acids) acids that are synthesized by microorganisms have been reported to be vital in phosphate solubilization because the organic acid production make the microbial cells as well as the surrounding to be acidic. Acidification facilitates the phosphate solubilization process and can be supplied to soils by acids or H+ excretion, which arises via respiratory H2CO3 production, organic acid anion extrusion, and NH4 + assimilation. Gluconic acid is the key acid in this process [17]. Acids such as 2-keto-gluconic acid, fulvic acid, and humic acid are potential chelation cations of aluminum, calcium, and iron. They, therefore, contribute to the solubilization of the inorganic phosphate complex by these cations [18]. Inorganic acids (sulfuric acid, nitric acid, and carbonic acids) have been established to solubilize phosphate by transforming insoluble phosphate compounds into soluble forms [19]. Microbial exopolysaccharides may influence the solubility of metal phosphates such as tricalcium phosphate in soils. Many phosphate-solubilizing microorganisms produce siderophores, which also contribute to the solubility of iron phosphates in

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Fig. 7 Mechanisms of inorganic and organic phosphate solubilization by microorganisms. (Adapted with permission from Ref. [17] (Copyright © 2019, Elsevier))

the soil. The solubilization of organic phosphates occurs in soils via microbial enzymes such as phosphohydrolase or phosphatase, phytase, phosphatase, and C-P lyase. As phosphatase, hydrolyze phosphate ester bonds to transform high molecular weight organic phosphate into lower forms. As a result, phosphate ions are released. Phytase interacts with phytic acid or myoinositol phosphate compounds through the hydrolysis mechanism. C-P lyase and phosphatase convert phosphonates to phosphate ions and hydrocarbons by hydrolysis of phosphonates ester bonds to be assimilated [18].

Growth Hormone Regulation by Plant Soils can be considered a sustainable source of nutrients; they can be described as the basic component of agriculture. The soil is considered an important factor in climate change, affecting environmental health. A nutrient increase is needed for soil every day [20]. Chemical fertilizer is therefore required, but they have a catastrophic effect on soil; not just the degradation phenomena has increased in the last century but also the environmental, water, and air pollution, leading to the deterioration of the ecosystem and living beings’ health in addition to the elevated cost of many products. Many solutions were invented to narrow down the damage in a suitable way [4]. Direct plant growth is based on the profit from certain microorganisms in diverse mechanizes [13]. Plant growth-promoting rhizobacteria (PGPR), known as

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phytohormone that deals with the sustainability of agriculture, are safely used more than chemicals. Biofertilizers are a heterogeneous bacterial group that plays the main role by offering soil fertility and sustainability through the fixation of atmospheric dinitrogen “N¼N” and- macro/micronutrients or even convert insoluble phosphorus in the soil into simplified forms that can be assimilated by plants; also biofertilizer functions together to protect the plant crop by the growth of the rhizosphere directly or indirectly. This allows not only the faster seed germination by modifying the ability of the water and nutrient absorption of the plant but also the protection against microbial pathogens. Usually, hormones originated from the Greek meaning “to arouse and to activity” [20]; hormones can be defined as the natural secretion of a chemical substance or even signals to regulate their growth factor. Hormones may be organic promoters like gibberellic acid, auxins, and cytokinin or inhibitors as ethylene and primarily abscisic acid, as shown in Fig. 8. Their path of transportation starts at a high concentration to the lower one. Hormones are secreted in small amounts, although a higher one can play an inhibitory role or more by retarding the plant growth. Together, one or more hormones are related to performing a specific growth operation [21]. After many studies, it was shown that hormones could regulate plant growth even with their expansion along a longitudinal axis like auxins, gibberellins

Fig. 8 Role of plant growth hormones. (Adapted with permission from Ref. [22] (Copyright © 2019, Springer))

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“GA,” and brassinosteroids “BR” in addition to their ability to control the organ size or by their expansion along the transversal axis by reducing the stature of darkgrown seedlings like ethylene “ET” and cytokinin “CK” [22]. Plant hormones can perform an exceptional role in pollutant biodegradation like the biodegradation of polycyclic aromatic sulfur heterocycles (PASHs) and diesel fuel in soil by the gibberellin hormone. Also, it was found that GA can reduce the toxicity of cadmium, especially in arabidopsis, which is the most important heavy metal pollutant in agriculture [23]. Besides other factors, including temperature, pH, soil moisture, and hormone concentration, must be considered. Ethylene hormone is produced due to the abiotic stress of air pollution SO2 ozone and heavy metals. This production allows the decrease in ozone-fumigated zinnia and Cd2+-treated bean plants. Other hormones like abscisic acid, cytokinin, indole-3acetic acid (IAA), and most used auxins can also be used to protect against toxic ozone pollutants during plant growth [24].

Protection from Phytopathogenic Microorganisms Phytopathogenic microorganisms induce plant diseases and can be divided into two categories: biotic as fungi, bacteria, nematodes, insects, and viruses and abiotic such as the sudden change in the surrounding environment, pH, temperature, light, nutrients, salinity, or even chemical products (pesticides). This affects the plant productivity by a disease infection; the microorganisms may penetrate inside the plants by two ways: traumatic surface, damage occurs to the plant in a biological, mechanical or physical process, or through a natural hole formation in the plant structure, the infection affects the productivity of agriculture and ay lead to its loss [25]. Plants have many defense strategies against pathogens in a direct as in hyperparasitism, indirect through nutrients competitions, or even a mixed-method way by signaling pathway using antibiotics or by the usage of one of the diverse hormones; hormonal secretion release may be in the soil or the air for a better protection method [26]. Other methods can protect against pathogens, such as chemical pesticides, but the catastrophic damage will be higher than the benefits. An eco-friendly method is preferred such as commercial bio-controller PGPR, which has a specific environmental, chemical, or metabolic strategy [27]. Antibiotic production is the most efficient method of microbial defense; lately, it has been improved by the gene’s modifications, and many microorganisms produce it as bacteria, fungi, and actinomycetes to provide protection. Antibiotics are heterogeneous organic components characterized by a low molecular weight; their mechanisms of action have three stages: replication and transcription, protein synthesis inhibitors, and cell wall protein inhibitors. Production of hydrolytic enzymes has a high capacity to prevent pathogen existence, including chitinase, protease or proteinase, glucanase, and cellulase. All enzymes have a specific protection mechanism, specifically on the cell wall, without any harm to the plant tissue in a sustainable, environmentally friendly way [27].

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Iron is a vital element for all living forms on earth. Despite its availability, plants cannot easily use it due to its insolubility. Siderophores, derived from the Greek word “iron carriers,” are secreted by microorganisms characterized by a low molecular weight and strong susceptibility to iron. Siderophores are used in the chelating of ferric iron processes. The indirect method explains how pathogens and nonpathogens compete for nutrient resources. They improve the plant growth ability by active transport as they allow the swath of iron from the soil. The production of siderophores can be considered phytopathogens inhibitors. The presence of siderophores prevents fungal diseases so that plants can profit from it as an inhibitor or as a growth nutrition factor. Pseudomonas strain GRP3 represents how siderophore can promote the iron and chlorophyll in the Vigna plant [27]. One natural way to protect against insects is pyrethrin secretion which acts as a biodegradable insecticide. It is characterized by low toxicity and improved environmental and human health. At first, pyrethrin accumulates at the floral disc at the lower concentration at the ray floret. Besides the defensive role, pyrethrin is used for wound healing since jasmonate or volatile organic components are present. Natural pesticides reduce the ecological and environmental damages that have increased in the last century due to the high usage of synthetic or chemical pesticides [28]. Chitosan, a biodegradable natural polymer with high biocompatibility, has a crucial function as a biopesticide against pathogen or sporulation besides its defensive role in plant-pathogen interaction [29].

Microfungi and Mycorrhiza Biodegradation Microfungi are a broad group of eukaryotic organisms distributed worldwide and have the distinct characteristic of containing microscopic mycelium. They are distinguished by the secretion of broad-based extracellular enzymes, which allow them to break down bulky compounds; based on this character, microfungi have been exploited in the degradation of various pollutants that represent a threat to the environment and public health such as toxic dyes, pesticides, polycyclic aromatic hydrocarbons, explosives, and chlorophenols [30]. Microfungi are adaptable to the inconsistent conditions in the ecosystem. Mycorrhizal fungi are a group of microorganisms that live in a symbiotic correlation with plant roots, most probably formed during the early stages of plant colonization of soil, and found in over 90% of plant families. Mycorrhizal fungi are extremely diverse and make a substantial contribution to fungal diversity. Symbiotic mycorrhizae occur mainly in Ascomycota and Basidiomycota. There are two groups of mycorrhizae, which are endomycorrhizae and ectomycorrhizae; endomycorrhizae, as shown in Fig. 9, are further subdivided depending on the plant species, and the structural character of the symbiosis into three categories varies in their structure which are arbuscular mycorrhizal fungi (AMF), ericoid mycorrhizas (EcM), and orchidaceous endomycorrhizas, in addition to arbutoid and monostrophic mycorrhiza [31].

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EndomycorrhizaeArbuscular mycorrhizae Spores Vesicle

Mantle External hyphae

Ectoendomycorrhizae

Fig. 9 Structures of arbuscular endomycorrhiza and ectomycorrhiza. (Adapted with permission from Ref. [32] (copyright © 2019, MDPI))

Arbuscular mycorrhiza has a strong tendency to absorb substantial amounts of heavy metals, which can be achieved through different mechanisms. From these mechanisms is heavy metal immobilization, in which they transform metal ions into a form that cannot be accessible to the plant. Because of its larger surface area, the outer mycelium of roots has a greater influence on the immobilization of metals in stressed soil. This allows them to accumulate metals than plant roots more efficiently. Another mechanism is the sequestration of metals in their vacuoles by stimulating polyphosphate granules. Moreover, the chelation of metals in mycorrhizal cytosol is a potential mechanism for eliminating heavy metals where chelating chemicals reduce their solubility and reactivity, and chelation also decreases the availability of HMS in the cytoplasm. Numerous chelating agents such as metallothionein (MT), organic acids, phytochelatin (PC), and amino acids are found in AMF and plants [33].

Filamentous Fungi Fungi are eukaryotic microorganisms that feed in a heterotrophic manner and use digestive enzymes to absorb food from their environment/host. Many saprophytic fungi contribute to bioremediation and obtain nutrients by degrading pollutants [34]. Fungi-based biodegradation is eco-sustainable and one of the latest alternatives; it can detoxify pollutants such as polycyclic aromatic hydrocarbons, plastics, toxic dyes, and other environmental contaminants.

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Fig. 10 Mechanism of polycyclic aromatic hydrocarbon (PAH) degradation by fungi. (Adapted with permission from Ref. [34] (copyright © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG))

Polycyclic aromatic hydrocarbons (PAHs) are molecules that regularly disperse into the environment. Unfortunately, they induce cancer in humans and aquatic animals; in addition to their mutagenic activity, they are also characterized by environmental persistence and are resistant to degradation due to their hydrophobic behavior. Aspergillus terreus, which belongs to the phylum Ascomycota, has shown great capacity to effectively degrade anthracene and naphthalene by producing lignin-degrading enzymes, lignin peroxidase, and manganese peroxidase [3]. PAHs are degraded by microbial enzymes to quinines and then to phthalates, which are converted to carbon dioxide by ring fission by ligninolytic enzymes, e.g., laccases or manganese peroxidase. On the other hand, PAHs can be transformed into oxidized compounds such as trans-dihydrodiol by non-ligninolytic enzymes, e.g., cytochrome P450 monooxygenase enzyme (Fig. 10) [34]. Acrylamide is a carcinogenic compound produced in foodstuffs subjected to a thermal treatment process. Acrylamide is generally produced by a thermal reaction known as the Maillard reaction between the asparagine amino group and carbonyl reducing sugar. Penicillium crustosum NMKA 511 was reported to yield the highest concentrations of L-asparaginase, which can be used in the pre-treatment of coffee to diminish the production of acrylamide by 80.7% and 75.8% according to the roasting of coffee [35]. Colored substances known as reactive dyes are used to dye cellulose fibers. Synthetic dyes are used in textiles, leather, personal care products, food products, and paper production. Substantial amounts of highly colored effluent are produced due to these dyes. The fact that synthetic dyes resist biodegradation is a special concern in their use. The laccase enzyme produced by Aspergillus flavus NG85 has the potential to degrade synthetic dyes efficiently. In the presence of copper ions, the dye degradation efficacy against synthetic dyes was exceptional and the detoxification of real textile wastewater by purified laccase [36].

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Plastics are synthetic and semisynthetic polymeric materials derived from petrochemicals. Most environmental issues arise from the widespread use of plastics. Plastic waste contaminates soil, air, and water ecosystems, and it is a persistent polymer that accumulates in the environment over time. Natural decomposition results from sunlight, microbial contact, and other environmental conditions. UV rays and high-intensity sunlight can oxidize the plastic, increasing microbial contact. Polyurethane (PU) is used as a carbon source by wood-degrading fungi Pleurotus spp., Lentinus, and Rigidoporus species. Endophytic and filamentous fungi can degrade low-density polyethylene (LDPE); in addition, in laboratory tests, Aspergillus japonicas degrade low-density polyethylene [34].

Yeast Biodegradation Yeasts are unicellular members of microfungi; they are divided into two phyla, Ascomycota and Basidiomycota. There are numerous advantages to yeast, including large-scale production and easy handling; consequently, it has been ubiquitous in the elimination of numerous pollutants, such as the removal of toxic dyes that have accumulated heavily in the environment as a result of growing industrial activities that pose a serious problem to the ecosystem and human health. Pseudozyma rugulosa Y-48 and Candida krusei G-1 have shown a promising Reactive Brilliant Red K-2BP elimination rate reached 99% within 24 h [37]. Hydrocarbons are hazardous contaminants, and large quantities of these compounds are released into the environment due to mismanagement during oil extraction or in the effluents of companies that use petroleum or its derivatives. Yarrowia lipolytica W29 can degrade the hydrocarbons (hexadecane and hexadecane) and uses them as an essential carbon source for its growth. It produces economically beneficial compounds, including lipase, used in food industries, detergent industries, biofuel synthesis, and microbial lipids used in biodiesel production [38]. Heavy metal contamination is currently one of the most critical concerns facing the industrialized world. Microorganisms such as innovative biotechnology can greatly benefit, especially in the food industry. Saccharomyces cerevisiae received much interest as a biosorbent for heavy metal removal. Hydroxyl and carboxyl groups in S. cerevisiae’s cell walls are primarily responsible for the biosorption mechanism; they are the major contaminant binding agents. As a result, heavy metals accumulate in the cell wall, and metals can enclose themselves in cell molecules [39].

Role of Algae and Protozoa in the Biodegradation Process Algae are photosynthetic creatures that can be found in almost any living environment. Algae play an important role in the aquatic ecology since they release most of the dimethyl sulfide and half of the oxygen, and they are the main food source for

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bivalve mollusks, zooplankton, crustacean larval stages, and some fish. Algae also facilitate nutrient separation and remediation of wastewater contaminants [37]. Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester. PET is mainly used in the food and beverage processing industry and in domestic appliances, the automotive industry, and many other industries. Plastic pollution has multiplied as a result of growing plastic consumption. Every year, around 100 million tons of plastic are produced worldwide. Because of its persistence, it adversely affects the environment. Chlorella vulgaris has demonstrated notable biodegradation of PET to less toxic products, which are pentanoic acid, 4-methyl-, d-galactono-1,4-lactone,5,6-O(ethylboranediyl)-, hexanoic acid, 1-cyclopentylethyl ester, 2-[pentafluorophenyl]-4[N-aziridyl]-2-butanol, 5,8,11-hepta-decatrien-1-ol, benzenemethanol,3-hydroxy-5methoxy-, 5,8,11-heptadecatrien-1-ol, pentadecyl ester, octadecyl ester, and 1-monolinoleoylglycerol trimethylsilyl ether. The algae produce a mucilaginous layer that enhances the degradation of the plastic. However, it has been detected that the rate of biodegradation is promoted when the polymer is pretreatment physiochemically [40]. Antibiotic fluoroquinolone (QF) called levofloxacin (LEV) has raised concerns among researchers as a possible emerging pollutant, causing serious effects on human and aquatic organisms. The average removal efficiency of LEVs from WWTPs is typically less than 10%, as WWTPs are not used specifically to eliminate insignificant levels of permanent emerging contaminants. Scenedesmus obliquus green alga could degrade levofloxacin in artificial saline wastewater into low molecular weight products, as shown in Fig. 11; however, inhibition of S. obliquus observed at concentrations 20, 50, and 100 mg L1 LEV. The LEV elimination rate was 93.4%, including 0.36% through bioadsorption, 0.8% through bioaccumulation, and 92.24% through biodegradation mechanisms in 171 mM NaCl in S. obliquus cultures after 11 days of cultivation [41]. Several algal species can be used for bioremediation of organic dyes through biodegradation, biosorption, or bioaccumulation mechanisms. Because of the presence of various functional groups on the cell surface, such as proteins, amino acids, sulfates, carboxyl phosphate, and polysaccharides, algal cells have a large binding affinity with synthetic dyes during the biosorption process. Algal cells also produce a variety of extracellular biopolymeric compounds with functional surface groups. The produced biopolymer improves the adsorption and stability of remaining dye particles on the biomass surface. Furthermore, the discharged metabolic intermediates have a high coagulation and absorption capacity for remaining dyes in wastewater on the extracellular biopolymer’s surface. Algae use three different mechanisms to biodegrade dyes from complicated molecules to simple forms: (1) harvesting algal biomass, carbon dioxide, and water using dyes chromophores as nutrients; (2) assisting in the transformation of chromophores substance to non-chromophores substance; (3) accumulating chromophores onto the algal surface of the cell [37]. Protozoans are well known for their role in pathogen removal and their contribution to reducing the chemical oxygen demand (COD) of wastewaters. They are particularly valuable biomarkers of an activated sludge’s state. Protozoa can also

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Fig. 11 Levofloxacin biodegradation by S. obliquus. (Adapted with permission from Ref. [41] (Copyright © 2017, Elsevier))

vary and develop into various species, subspecies, or morphotypes, which help them respond to diverse prey sizes and establish prey identification mechanisms based on cell surface epitopes [42]. Escherichia coli O157:H7 (EcO157) is a pathogenic bacterial species commonly found in contaminated food and dairy lagoon wastewater from stomach microbial populations of ruminants. It causes the fatal hemolyticuremic syndrome, and a consortium of protozoa Vorticella microstoma, Platyophyra sp., and Colpoda aspera found in dairy wastewater could ingest EcO157 keep in their food vacuoles, but preying of EcO157 was limited to Platyophyra and Colpoda. However, these protozoa failed to eliminate EcO157. They only assisted in the reduction of their numbers; also, their function ceases when bacterial concentration decreases to 104/mL [43].

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Mixed culture of Aspidisca sp., Trachelophyllum sp., and Peranema sp., as well as their separate cultures, were studied for their capacity to degrade hydrocarbons in polluted crude-oil water, with a degradation rate reaching 85.2%, 71.0%, 68.94%, and 56.03%, respectively; however, a reduction in degradation was observed when the hydrocarbon concentration increased by more than 50 mg/L [42].

Factors Affecting Microbial Degradation Biodegradation is a process that is impacted by a variety of factors. By controlling these factors, the biodegradation process will be more rapid and much more effective, decreasing the cost and reducing the time of remediation procedures [44].

Biological Factors Biological factors determine the capability of certain microorganisms to catabolize pollutants through bioavailability, type of microorganism, type of culture, and nutrient availability. The bioavailability of pollutants to microorganisms is a critical factor in the biodegradation process in various contaminated areas of the environment; it depends on pollutant properties such as chemical structure, volatility, hydrophobicity, and lipophilicity. Bioavailability is concerned with physical conditions, whether an organic pollutant is present in a physical condition that permits it to be absorbed and destroyed by the biodegrading microorganism. Various factors can influence the bioavailability of the organic pollutant, such as sorption/desorption kinetics and equilibrium, spatial mismatches between degrading microorganisms and contaminant molecules, and specialized microbial growth strategies to ensure maximum access to the contaminated substrate. Biosurfactant solubilization is the most prevalent way that microorganisms improve the bioavailability of organic pollutants through the mechanism (as shown in Fig. 12) [45]. Bacterial strains such as Acinetobacter sp., Pseudomonas sp., and Rhodococcus sp. secrete biosurfactants to degrade oil-based contaminants [9]. It was stated that fungi produce more biosurfactants than bacteria due to their stiff cell walls. For example, Aspergillus ustus produces a glycolipoprotein biosurfactant, while Aspergillus niger produces a glycolipid biosurfactant [46]. Another factor is the nature of culture inoculum used in biodegradation, whether mixed or single culture; it is preferable to use a consortium of microorganisms, as this can lead to community interactions that reduce stress conditions [47]. After 120 hours of culture, mixed culture of Candida rugosa and Yarrowia lipolytica was able to successfully eliminate triglycerides from the concentrated palm oil mill effluent by up to 98.5% in comparison with their separate cultures, which degraded triglycerides with a rate reached only 94.5% and 83.9%, respectively [48]. The process of pollutant biodegradation depends mainly on the ability of microorganisms to metabolize pollutants; bacteria, fungi, and algae can degrade various

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Soil/sediment- associated contaminant

Cell membrane C

A

Enhanced phase exchange

Released contaminant

D

Absorbed contaminant in organism

E

Site of biological response

B

Microbial mobilization

Fig. 12 Microbial modes of action on bioavailability processes: (A) solubilization process by surfactant’s phase exchange. (B) Microbial mobilization by chemotactic bacteria. (C) Attachment of organic pollutants to interfaces. (D) pollutant uptake by the microbial cells. (E) Assimilation of pollutants by microorganisms resulting in biodegradation. (Adapted with permission from Ref. [45]) (copyright © 2020, Springer Nature Switzerland AG))

contaminants such as petroleum hydrocarbons and use them as a source of energy [49]. The type of microorganism used in biodegradation is an essential factor influencing biodegradation. Some researchers tend to select degrading microorganisms based on the metagenomic analysis of local microbes in the contaminated sites, and this analysis made it possible to identify the microbial community at contaminated sites and to gain a better understanding of the degradation function of the microbes and genes responsible for their biodegradability [50]. Nutrient availability significantly affects the biodegradation process; microorganisms could obtain carbon through the degradation of organic pollutants, nutrient availability highly depends on contaminated site; for example, oil-contaminated water contains extreme quantities of carbon and a low percentage of nitrogen and phosphorous; therefore, the supply of nutrients is essential to attain a high biodegradation rate, for example, Lasiodiplodia theobromae, Mucor racemosus, and Penicillium chrysogenum when tested for lipase production using different carbon substrates, sucrose, and cellulose were proven to cause the maximum activity in those species. Despite the importance of nutrients to microorganisms, it was found that extreme amounts of nutrients could lead to toxic effects on microorganisms [49].

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Environmental Factors Microbial growth is influenced by pH, and when pH falls below the ideal range for microbial species growth, cellular growth ceases, and the ability to produce enzymes is diminished. Additionally, the pH impacts the number of available nutrients [51]. Many heterotrophic bacteria demand a pH range of neutral to alkaline for optimal growth; for instance, under neutral conditions, Acinetobacter baumannii was capable of degrading diesel with an efficiency reached 99%, while Enterobacter cloacae degraded diesel with a degradation rate of 55 to 58% at the same pH condition [9]. Fungal growth and production of enzymes and, in turn, their biodegradation are highly dependent on pH. It was observed that the pH ranges between 4.5 and 7 (acidic to neutral pH) [30]. Two strains of Trichoderma (Td85 and Td50) were evaluated under different pH ranges from 4.5 to 8.5, and identified strains could tolerate an extensive diversity of pH values. However, growth was inhibited on alkaline media between 7.5 and 8.5 [46]. In algae, pH value affects biosorption of pollutants as if the pollutants are cationic at lower pH increased competition between H+ ions and dye cations, which results in lower adsorption of pollutant. When the pH rises, the algae’s surface becomes negatively charged, enhancing the electrostatic forces of attraction between the positively charged pollutant cations and the algae’s surface [52]. Temperature is another crucial factor that influences different biodegradation processes. When the temperature drops, the activity of an enzyme decreases since it is a protein substance. When the temperature rises, the activity of an enzyme increases. It was indicated that isolated bacteria degraded diesel best from 30 to 37  C. The maximum rates of diesel biodegradation were also found at 30–40  C in soil, 20–30  C in freshwater, and 15–20  C in the marine environment [9], while Trichoderma fungal strains (Td85 and Td50) grow best at 25–30  C, with slow growth seen at temperatures lower than 15  C [46]. Biodegradation can be either aerobic or anaerobic, and in the case of aerobic biodegradation, oxygen is utilized as an electron receptor by microorganisms to perform their metabolism. Therefore, the microbial population and the amount of oxygen are directly related. Bacteria can degrade aliphatic and aromatic compounds through enzymatic oxidation. Thus, the degree of bioremediation can be accelerated by adding oxygen [51]. Also, fungi require oxygen to biodegrade pollutants as they use non-ligninolytic enzymes, which require oxygen [46]. Water availability is one of the important parameters that influence microbial development and their biodegradation rate. It was reported that the biodegradation efficiency of hydrocarbons was utmost with the existence of 30–90% water saturation in the treatment system [48]. Another factor that impacts biodegradation, especially dye elimination, is the agitation of treatment systems due to the elevation of oxygen level on agitation and activation of oxidative enzymes. This was proved in the case of Aspergillus ochraceus, which eliminated malachite green dye with efficiency reached 83% with agitation, while the percentage reduced to 55% in the case of the stagnant system [30]. In addition, it was reported that salinity has a great impact on the biodegradation process; because it influences the growth of

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microorganisms and alters the activity of enzymes responsible for biodegradation. For example, a decrease in the hydrocarbon biodegradation efficiency ranged between 3.3% and 28.4% due to elevated salinity levels [49]. The concentration of contaminations is a crucial factor during strain selection; for example, the growth of Scenedesmus obliquus was constant on a low concentration of levofloxacin residues, although at levofloxacin concentrations of 20, 50, and 100 mg L1 LEV, the growth was declined by 14, 35, and 52%, among the most frequently used toxicity metrics is the effective concentration (EC50). It is described as the concentration at which an organism’s growth rate decreases by 50%. During the later stages of the cultivation, the microalgae began to recover from the stress caused by LEV, leading to an increase in the EC50 of LEV. The diverse variety of LEV EC50 values for distinct microalgal species was attributable to differences in sensitivity to toxic environmental contaminants across microalgal species [41]. In another study of biodegradation of oil using protozoa, it was reported that the increase of protozoan biomasses of 1.00  105 cells/mL and 1.00  106 cells/mL has shown an effect on the rate of biodegradation and resulted in being from 15.4% to 71.0%, 15.0% to 68.9%, 13.0% to 56.0% and 28.0% to 85.2%, respectively. While the increase of oil concentration level in wastewater mixed liquor (>50 mg/L) has been studied, it showed the decrease of their capacity for biodegradation to >60% [42].

Bioremediation and Biodegradation Many solutions were studied to maintain the balance and sustainability of the ecological system. It was observed that the suitable solution for the global pollution problems is bioremediation as it gives a multiple choice to destroy the contamination by a biological process (Fig. 13). The diverse categories of bioremediation. Bioremediation is more likely used than traditional conditions as it is based on many techniques to reduce the contamination risks as natural attenuation, bio-stimulation, and bio-augmentation [53]. The Environmental Protection Agency (EPA) defined natural attenuation as a biological, chemical, or physical process that reduces the mass of toxicity in soil and groundwater without human intervention. The concept of natural attenuation definition differs according to the organization’s objective, but mainly it reduces the cost of the cleanup substantially, environmental pollution, and chemical usage due to a natural process occurring within the ground. Bio-stimulation, the second technique of the bioremediation process, is derived according to the enhanced bioremediation in which a specific MO is enhanced for the contamination degradation; the process stimulates the natural microbial population by nutrient and electron acceptors’\donors’ addition [54]. The technology in which microorganism MO degradation is enhanced due to the presence of organic wastes under controlled conditions is known as bio-augmentation. It is considered the third step in the healing strategy and is only applied if the bio-stimulation and bio-attenuation fail [55].

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Fig. 13 Classification of bioremediation. (Adapted with permission from Ref. [55]) (copyright © 2009, Elsevier GmbH))

Besides all the advantages of bioremediation, some limitations are also present: (1) the mechanism only occurs for the biodegradable components; (2) factors affecting the bioremediation process like environment, microorganisms, and contaminants, should be maintained in high specificity; (3) the process takes longer time than other handling options, so much research is needed to develop the diverse techniques of usage. Bio-stimulation and bio-augmentation are bioremediation’s basic technology and help develop many commercial fields [56].

Degradation by Genetically Engineered Microorganisms Ecological contamination is one of the biggest problems facing the global world; air, soil, and water are exposed to diverse pollutants from human activities, making the planet earth lose all its qualifications and donations by the time. Therefore, many regulations and precautions must be followed to minimize the damage. Microbial biosensors and biomarkers are two methods used to detect the number of pollutants. Generally, microbial biosensors are used to detect chemical and physiological signals. The process was developed to use a variety of MOs as the sensing elements in the biosensor, by the time the method was developed by the alteration of the DNA through the fusion of natural regulatory genes associated with a reporter gene as “Lux/Luc (encoding bacterial/firefly luciferase), gfp (encoding green fluorescent protein), and lacZ (encoding β-galactosidase) genes.” Biomarkers like induction of bio-transforming enzymes and certain isoenzymes are used to indicate the response of bioindicators or even pollutants [57]. The diverse advantages of the process, low cost, and high efficacy will allow its multiple usages shortly. Nowadays, all solutions are targeted to solve the environmental problem using bioremediation in which microorganisms like fungi, yeast, and bacteria degrade the

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organic or inorganic toxicity; these anthropogenic pollutions are unable to degrade naturally due to the presence of chemical bonds or even functional groups resistant to the natural degradation by the MOs. Therefore, discoveries were made, and new methods were developed to use modified microorganisms leading to a new evolution in science strategies and allowing the production of diverse applications to solve existent catastrophic problems even with a high production yield and faster evolution [58]. Genetically engineered microorganisms’ “GEMs” have been known since the last century and developed over time to obtain a broad application using it, especially when the DNA altering concept was discovered. The concept was inspired by the natural genetic exchange between microorganisms “insertion of a desired gene or removal of unwanted gene” or based on the change of DNA sequence “mutagenesis” leading to new organisms’ capability and functions. GMMs improve the traditional method or solve environmental problems by modifying certain organisms. Microorganisms like Bacillus idriensis, Ralstonia eutropha, Sphingomonas desiccabilis, Pseudomonas putida, Escherichia coli, and Mycobacterium marinum can be modified by the inclusion of a functional gene to obtain the desired efficacy [58]. Various techniques are available to produce genetically engineered microorganisms like electroporation or electro-transformation. The method depends on a highvoltage electrical pulse to enhance the saturation of the uptake DNA and allow the gene transfer to the host-microbe. The biolistic transformation technique is the firing gun of a micro-projectile coated with gold or tungsten through a stopping screen to the host cell; the method is simply used, but it has some limitations as expensive material and no specific target to the DNA [58]. Molecular cloning is generally based on the formation of multiple gene copies; the process requires the presence of a certain DNA fragment known as a plasmid [59]. The GEMs showed a huge success in many fields due to many factors as stability, survival of GEMs after modification, and the transmission of GEMs into an appropriate MO. Gene transfer method in which gene movement occurs between or in the interior organisms is widely used to enhance the pollutant degradation by placing the desired gene\DNA fragment into (plasmid, transposons, or bacteriophage or remnants of these) vectors like horizontal gene transfer HGT known also as lateral gene transfer LGT which occurs in unicellular or multicellular organisms (as shown in Table 1) [59]. LGT may happen in a few ways, such as conjugation as the DNA is transmitted from one cell to another, and both are alive further explanation. The DNA transfer is due to the physical connection between cells even by a plasmid conjugation or integration and conjugative element ICE. The transfer is observed in the cargo gene, chromosomal DNA. Transformation transfer is similar to the conjugation mechanism but differs from DNA obtained from any organism using protein on its outer membrane [59]. The LGT (as shown in Fig. 14a) can occur by an agent. Gene transfer agents (GTAs) are similar to the phage particle in their structure, containing an insufficient encoded protein located on the host chromosomes to be transported by a similar mechanism of the transformation process for the DNA exchange. Randomly DNA fragments (shown as blue particles) are delivered from the

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Table 1 Characteristics that correlate with different frequencies of HGT [59] Property Type of organism

Genetic relation between donor and recipient Ecological relationship between donor and recipient Function of gene transferred

Relative frequency of HGT Low Moderate Multicellular Prokaryote/ eukaryote single-celled eukaryote Distantly Closely related related species species Separated in Parasitic/ space or time symbiotic Toxic/ informational

a

Structural/ metabolic

High Virus

Same species or closely related strains Same ecological niche

MGE associated/pathogenic/ defense/ecological opportunistic

b

GTA genes

Expression

Prophage or phage

Expression and replication

Fig. 14 Comparison of gene transfer agent and transducing phage production. (Adapted with permission from Ref. [60] (copyright © 2012, Nature Publishing Group, a division of Macmillan Publishers Limited))

producing cell in addition to the GTA (red particle). GTAs require lysis (displayed as a dashed line) for release from cells [60]. Transduction (as shown in Fig. 14b) is a unique process by phages (black), or prophages occur as the DNA is delivered from the host cell to another cell using their

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Fig. 15 A network of intercellular and extending nanotubes in Bacillus subtlis. (Adapted with permission from Ref. [61] (copyright ©, 2016 Elsevier))

adaptive capsid to a different environment. Packaging of the complete phage genome then occurs (orange phage heads), with some packaging of non-phage DNA (blue phage head). The process can lead to a huge improvement in diverse medical applications [60]. The bacterial nanotube gene transfer method allows the transfer between two cells by a bridge formation for the exchange of molecules or even for the fusion of different bacterial cells (as shown in Fig. 15); an electron microscope is highly required for this type of transfer for a better resolution [61]. Finally, besides all the usage of bacterial membrane vesicles like pathogen or cell killing and biogenesis also, it can be used for the lateral gene transfer process by DNA exchange by a specific transport mechanism [62]. Despite all the advantages of GT methods, plasmid transfer is not stable enough, so to obtain the most accurate results, gene transfer must be controlled; various strategies can be applied like minitransposon-mediated; mini-transposon is simply a membrane collection characterized by transposons Tn10 and Tn5 properties in which a recombinant gene receive the package from a stable host chromosome, the merge between nonantibiotic resistance selection with the mini-transposons allows the removal of the gene into the environment; P. putida was modified to degraded toluene associated with the mini-transposon leading to the removal of the antibiotic resistance markers after the insertion of a specific gene into the host chromosome [63]. Suicide gene-mediated GEM transfer control and S-GEMS are considered a control strategy after the death of the GEM. GEMs contain killer and anti-killer genes, each having a specific role; in the presence of pollutant or at the contamination site, anti-killer genes produce cognate after the pollutant removal, and the

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Table 2 Different tracking techniques and their functions for the gene transfer process Tracking\monitoring technique DNA hybridization Colony hybridization Slot blot and southern blot hybridization PCR technique

Bioluminescence

DNA microarray

Fluorescent-based DNA hybridization

Function Usage of a marker DNA as a specific probe Can only be used for the detection of culturable cells Not sensitive for the detection of low cell numbers

References [67] [67] [67]

More sensitive than the DNA hybridization method. Allow the detection of the non-culturable GEMs or cells whose count is below the threshold by amplifying a particular nucleic acid Some particular organisms are characterized by their energy emission in the form of visible light used to detect the phenotypic characteristic of GEMs Both DNA and rRNA are used as a probe to count nonrecombinant cells of GEMs. The specificity of the technique depends on the degree of quantification Fluorescence of a DNA in a specific strain is detected, but the method has limitations as considerable time is required due to the hybridization mechanisms

[68]

[68]

[69]

[68]

anti-killer genes are shut down, and the “killer” destroys the GEM [64]. gef expression is considered the third method of HT control and function with the help of the suicide gene system [65]. The safer method used to control the LGT is composting, in which the cell temperature increases above 90  C by reducing the pH by GEMs. This allows the reduction of the lysis followed by the DNA release to control the transfer [66]. After the transfer process, analysis is made to confirm the consequences of the growth rate, gene activity, and survival and also to evaluate the performance and the risk factors of GEMs, such as tracking techniques (as shown in Table 2). Toxic material requires a specific strategy to be removed safely. Arsenic has been considered the most toxic substance on earth and a global problem for many years and carcinogenic material with a harmful effect on many organs. Arsenic is produced from human activities and other natural consequences such as volcanoes. So microorganisms can be employed to reduce their undesirable effects, even by a resistance property or physiological need like respiration [70]. Rhodopseudomonas palustris encoding bacterial and archaeal homologs of the mammalian Cytl9 As (III) S-adenosylmethionine methyltransferase was regulated by arsenic found that the sample of soil was healthy and contained no arsenic by 2.2–4.5% in a bio-volatilization process during a month [71]. Despite GEM advantages, there are rules and risk assessments made to provide safety usage called biosafety regulatory frameworks for GEMs which can lead to ecological and economic damages impossible to be eliminated if no precautions were taken at the risk of horizontal gene transfer (HGT) concern; HGT is based on the gene transfer from one organism to the other without external involvement. The transfer mechanisms can occur by conjugation, transformation, and transduction or

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DNA or RNA found in the natural environment. Other precautions like inoculant survival, translocation, and competition are important [59]. Issues can be described by three major categories: the first type relates to the risk issue with the nonrecombinant bacterium host, the second type has no easy solution in which the potential risk is due to the DNA fragment from the recombinant DNA release like the degradation of toluene gene of P. putida or the nitrogen fixation genes of Sinorhizobium meliloti. Finally, the third type relates the risk issue to the recombinant constructed DNA. So, by improving genetic engineering, new tools are now available to remove the unwanted gene and appropriately reduce instability [72].

Role of GEM in Bioremediation Bioremediation is seen as an important process for resolving environmental contamination as well as toxic organic and inorganic pollutants that get worse day after day due to the huge population increase, industrialization, and urbanization, so the use of microbial systems to relieve various contaminations that threaten the environment is most often necessary. Several microorganisms (Pseudomonas, Burkholderia, Sphingomonas, Ralstonia, Comamonas, Achromobacter, Alcaligenes, Rhodococcus, Dehalococcoides) have the potential to degrade xenobiotics or to accumulate or detoxify pollutants from heavy metals such as Cd, Hg, Pb, Zn, and U. Genetic engineering (GE) is considered a powerful technological tool in bioremediation. Genetic engineering is used to amend the genetic material of microorganisms such as Ralstonia eutropha, Pseudomonas putida, Mycobacterium marinum, Escherichia coli, Sphingomonas desiccabilis, and Bacillus idriensis by inserting certain genes into their genomes to produce a specific enzyme necessary to degrade various wastes. This process of insertion depends on the nature of the microorganism. This systematic approach is primarily known as recombinant DNA technology. To provide the ability to eliminate undesirable waste, there are four strategies are made which are (1) adjustment of enzyme specificity and affinity; (2) pathway construction and regulation; (3) bioprocess development, control, and monitoring; (4) bio-affinity bioreporter sensor applications for toxicity decrease, chemical sensing, and endpoint analysis. GE provides the construction of various microorganism strains to withstand challenging environmental conditions that affect their survival and bioremediation rates. Essential bacterial genes are carried on a single chromosome, while genes specifying enzymes required for the catabolism of some unusual substrates such as toluene, benzene, xylene, chlorobenzene acids, and persistent organic contaminants may be carried on plasmids that are implicated in the catabolism [73]. Plasmids are divided into four categories: (1) XYL plasmid, which degrades xylene and toluene; (2) OCT plasmid, which degrades octane, hexane, and decane, (3) NAH plasmid, which degrades naphthalene; and (4) CAM plasmid that decomposes camphor [74]. GE P. putida strains, including muti-plasmid (XYL, NAH, CAM, and OCT), can degrade various pollutants naphthalene, octane, camphor, and salicylate. Genetically modified Deinococcus radiodurans degraded toluene, and modified Pseudomonas strains are identified in various types of contaminated pollutants [75].

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GEM Application in Biodegradation of Dye Pollutants Azo dyes are highly carcinogenic and toxic and are generally produced to prevent the loss of colors under different conditions, which may be chemical, physical, and biological. Therefore, these dyes do not easily degrade through traditional systems, used in various applications and industries such as paper, food, and drugs. Dye degradation processes can be accomplished by treatments, including single or physical, chemical, or genetic integration. Under anaerobic environmental conditions, azoreductase-mediated dye degradation has been estimated in different bacterial strains such as Pseudomonas luteola, Klebsiella pneumonia RS-13, Xenophilus azovorans KF46, and Clostridium perfringens. GE Escherichia coli JM109 (pGEXAZR) has been reported to increase azoreductase, and GE E. coli JM109 (pGEXAZR) also demonstrated degradation activity of the azo colorant Direct Blue 71 (DB 71) [75]. GEM in Industrial Food Enzyme Production Enzymes exist in all living organisms and are proteins in nature. Enzymes act as biocatalysts to increase the speed of biochemical reactions. They have been used in various food industries, including beer and wine, clarification and filtration, cooking processes, baking processes, different types of cheese, and other industries like paper, pulp, textiles, and pharmaceuticals. Modifications increase the ability to produce proteins, limiting the production of unwanted secondary metabolites such as mycotoxins. In general, these modifications may be made using genetically modified strains or recombinant enzymes in which the characteristics of the enzyme produced are enhanced, such as its activity, optimal temperature, and pH stability. In these modifications, it is important to select a host microorganism with appropriate characteristics for enzyme production and expression of the desired enzyme. Because indigenous genes are suppressed to prevent their expression, necessary enzymes are expressed in microorganisms that do not naturally possess them. The most selected bacterial stranis are Escherichia coli, Bacillus subtilis, Bacillus licheniformis, and Bacillus amyloliquefaciens while in case of filamentous fungi are Aspergillus niger, Aspergillus oryzae, and Trichoderma reesei [76]. Other Applications It has also been reported that GM microorganisms have enormous bioremediation capabilities for toxic organic compounds. The most radioresistant organism known until now, the genetically modified bacterium Deinococcus radiodurans, can digest and degrade toluene and ionic mercury. Their biodegradation is required as radioactive compounds can cause severe environmental problems (Table 3 and Fig. 16).

Microbial Enzymes in Biodegradation Enzymes are large biomolecules that accelerate diverse biochemical reactions by reducing the activation energy of reactants. They are proteins or glycoproteins enclosing active sites in which the substrate binds, and catalytic reaction occurs;

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Table 3 GEM application in heavy metal removal [73] Engineered bacteria Ralstonia eutropha CH34 Deinococcus radiodurans strains

Expression of modified gene Metallothionein (MT) Hg (II) resistance gene (merA)

Synechococcus strain PCC 6301 HgR E. coli Salmonella choleraesuis strain 4A Proteus penneri strain GM10 E. coli strain

smtA and smtB merA smtAB smtAB PCS gene expression (SpPCS) Cd transport system and MT (viz., M4) Phytochelatin synthase (PCS) gene expression Chromate reductase (ChrR) PCS Expression of a metal-binding peptide (EC20)

E. coli JM109 Mesorhizobium huakuii B3 P. Putida strain P. Putida 06909

Pseudomonas fluorescens OS8; Escherichia coli MC1061; Bacillus subtilis BR151; Staphylococcus aureus RN4220 Achromobacter sp AO22

MerR/CadC/ZntR/Pmer/ PcadA/PzntA (expression of luxCDABE genes)

Sphingomonas desiccabilis and bacillus Idriensis strains Methylococcus capsulatus (Bath)

Overexpression of arsM gene

E. coli strain

Metalloregulatory protein ArsR (overexpressing ELP153AR) Ni transport system and MT Hg2+ Gcsgs

E. coli SE5000 Enterobacter sp. CBSB1 (endophyte of Brassica juncea) B. subtilis BR151 (pTOO24) Pseudomonas strain K-62

Acidithiobacillus ferrooxidans strain Escherichia coli and Moraxella sp. P. fluorescens 4F39

Hg reductase expressing mer gene

CrR genes for Cr (VI) reductase activity

Luminescent cd sensors MerE protein encoded by transposon Tn21 (broad hg transporter) Hg ion transporter gene expression Expressing EC20 (with 20 cysteines) Ni transport system

Heavy metals Cd2+ (in laboratory) Hg (radioactive waste sites from nuclear weapons) Zn and cd Hg Pb Pb Cd2+ (microbial sorbents for Cd removal) Cd (removal of cd from aqueous solution) Cd2+ (from rice fields) Cr (bacterial cultures, as well as cell suspensions) Cd (mixed organicmetal-contaminated sites) Cd, Zn, hg, and Pb (water-suspensions and extracts of soils) Hg (in situ bioremediation of contaminated sites) As (laboratory conditions) Cr (VI) (cell-associated Cr removal in laboratory conditions) As (contaminated drinking and groundwater) Ni (accumulation of Ni2+ from aqueous solution) Pb, cd Cd (naturally polluted soils) Hg (across the bacterial membrane) Hg (in laboratory) Cd and hg (in laboratory) Ni (in laboratory)

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Fig. 16 Various Genetically modified organism’s applications. (Adapted with permission from Ref. [77], (copyright © 2020, Elsevier B.V))

complex enzymes are composed of two units which are apoenzyme (proteins or glycoprotein part) and prosthetic group (nonproteins). Since microbial enzymes were discovered in the twentieth century, they have been continually developed in their extraction methods and applications; one of the emerging applications of microbial enzymes is their use in the biodegradation of environmental contaminants. Applications of enzymes have various advantages and disadvantages (shown in Table 4). One of the advantages of applying microbial enzymes in biodegradation is their wide specificity, enabling them to degrade a large variety of pollutants and convert them into simple and nontoxic products. In addition, they can transport easily due to their small size and can effectively degrade pollutants in the presence of microbial grazers or competitors [78]. Another advantage of using microbial enzymes is that they do not require an adaptation period, unlike microbial cells. Moreover, the pollutant concentration does not affect their degradation ability. Also, enzymes do not result in toxic metabolites like some microbial biodegrade [79]. Various enzymes are involved in biodegradation, such as the oxidoreductase group, which includes oxygenases (monooxygenases and dioxygenases), laccases, and peroxidases, in addition to hydrolases such as lipases, cellulase, and protease [79].

Oxidoreductases Microbial oxidoreductases use the oxidative coupling mechanism to degrade harmful chemical molecules. Microorganisms obtain energy by breaking chemical bonds with their enzymes and facilitating the electron transfer from the organic substrate,

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Table 4 Advantages and disadvantages of enzymes in bioremediation [79] Advantages Work as catalysts for all sorts of transformations They can be used in the bioremediation of a wide range of pollutants Converts more harmful contaminants into safe end products Enzymes are eco-friendly catalysts, and so the process is environmentally friendly For most of the soil types, immobilized enzymes are used as it increases the life of the enzyme

Disadvantages Enzyme concentration cannot be maintained for a long time It is an expensive technique and limits the overall success Process optimization is sometimes complicated Some enzymes lose reactivity and become completely inactive during the process Crude enzymes are cheaper to use but have side effects

which is reduced to another chemical compound. Contaminants are converted into nontoxic molecules by these oxidation-reduction processes [80]. Oxygenases are one of the enzymes that belong to oxidoreductases. They are classified according to the number of oxygen atoms transported in the catalytic reaction into monooxygenase and dioxygenases. Monooxygenase action is represented by transforming one oxygen atom into the substrate. Cytochrome P450 is a type of monooxygenase that can degrade a wide range of substrates such as alkanes, steroids, fatty acids, and aromatic and aliphatic hydrocarbons through reactions desulfurization, dehalogenation, denitrification ammonification, and hydroxylation. Monooxygenases degrade hydrocarbons by oxidizing a methyl group to primary alcohol, then oxidizing to an aldehyde, ultimately transforming into fatty acid. In contrast, the NADH or NADPH electron coenzymes reduce the remaining oxygen in the water. Dioxygenases catalyze by adding a molecule of oxygen to their substrate. They are essential enzymes in the microbial degradation of aromatic hydrocarbons. For instance, catechol dioxygenases isolated from soil bacteria catalyze the biodegradation of aromatic substrates into aliphatic compounds. Iron can be used as a cofactor by dioxygenases; these enzymes are classified into three groups based on how iron is integrated into the active site: the first group is mononuclear iron dioxygenases, also known as non-heme iron-dependent dioxygenases, and uses a single iron to introduce one or two oxygen atoms into the substrate, cleaving C-C bonds, C-hydro peroxidation of fatty acids, and thiol oxidation. Catechol dioxygenases are an example of these enzymes. It is secreted by many microorganisms such as Pseudomonas bacteria. The second group is Rieske dioxygenases secreted by soil bacteria, which speed up the conversion of cis-dihydroxylation of aromatic chemicals to cis-dihydro-diol. The third group is heme dioxygenases which exploit iron through a heme prosthetic group, such as oxidation of L (and D-)-tryptophan to N-formyl kynurenine [81]. Versatile peroxidases (VP) contain an accessible tryptophan amino acid structure. They have high substrate specificity and a potential to oxidize substrates in the lack of manganese. Laccases are another class of oxidoreductases. They are multicopper proteins that are members of the blue oxidase enzyme family. They are used to

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biodegrade many recalcitrant compounds such as pesticides, poly-nitrated aromatic compounds, dyes, and PAHs. Many bacterial and fungal strains secrete laccases, such as fungal strains Trametes versicolor and Pleurotus ostreatus. Peroxidases oxidize an electron donor to reduce peroxides. They are mainly involved in the biodegradation of phenolic compounds and are classified into several categories based on their activity and production source [80]. For example, manganese peroxidases, which are heme-containing extracellular enzymes produced by lignindegrading fungi, can undergo a multistep reaction to convert Mn2+ to Mn3+. These enzymes degrade several phenols, amine-containing aromatic compounds, and toxic dyes [78]. VP can degrade both phenolic and non-phenolic lignin mimic dimmers. As a result, biotechnological applications demand a highly efficient VP production system to eliminate persistent contaminants [79].

Hydrolases Hydrolases are another group of microbial enzymes; they function by using a water molecule to break the functional group bond. They are used in the biodegradation of organophosphate-based and carbamate-based insecticides. Parathion hydrolases are distinctive microbial hydrolases for the degradation of organophosphates; lipases or triacylglycerol acyl hydrolases are one of the hydrolases that are involved in biodegradation of oil contaminants through hydrolysis of fatty acids; lipase isolated from Pseudomonas aeruginosa showed significant efficacy in degrading crude oil in wastewater with elimination ratio reached 80% within 7 days [81]. Moreover, cellulose is another example of hydrolases specified in the hydrolysis of cellulosic substrates. Microbial cellulases can be intracellular or extracellular. Bacillus strains produce alkaline cellulases, while Trichoderma and Humicola fungi produce neutral and acidic cellulases applied to degrade wastes discharged from paper industries [78]. Microbial proteases are a class of enzymes that catalyze hydrolysis and generate peptide bonds in aqueous and nonaqueous environments, respectively. They are categorized according to the peptide chain’s catalysis into endopeptidases and exopeptidases and degrade protein-based pollutants generated as a result of industries such as leather industries [81]. Despite the significant efficacy in applying free microbial enzymes in the biodegradation of pollutants, they have some limitations, such as low stability, hardness to achieve extraction conditions, and resistance to recycling, so many approaches have been made to reduce these limitations. One of these approaches is enzyme immobilization. Covalent immobilization creates a stronger bond between the enzymes and the matrix and a high level of efficiency. Unlike more difficult adsorption, most covalent binding requires different pretreatment processes. Although adsorption approaches have a lower enzyme-to-support binding than that of covalent immobilization, they are usually more straightforward and less costly, and adsorbed enzymes have shown great results and reusability in many applications compared to covalently immobilized enzymes. New materials, such as nanoparticles, magnetic particles, biopolymers, and composites, have developed new enzyme

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immobilization techniques. Immobilized oxidative enzymes can deal with micropollutants, and these enzymes are often regarded as an eco-friendly technique to deal with micropollutants in our environment. The immobilization of manganese peroxidase from Anthracophyllum fungi on magnetic nanocomposite Fe3O4/chitosan has shown impressive performance in textile wastewater treatment [80].

Conclusions Microorganisms exhibit a tremendous biodegradation ability in reducing ecotoxicological pollutants, causing both environments and living things to suffer. Also, they promote the growth of plants through direct or indirect mechanisms, as shown in PGBR. Bacteria and fungi have a valuable role in the biodegradation of many emerging environmental pollutions. In addition, algae also showed high efficiency in eliminating pollutants characterized by their safety, easy handling, and high yield production. Moreover, protozoa’s ability as bacterial predators was used to eliminate pathogenic bacteria such as Escherichia coli from wastewater. All these organisms perform their beneficial function of biodegradation under optimum factors related to the system: environmental factors, factors related to the organism itself, and biological factors. The biodegradation function is mainly performed through different microbial enzyme mechanisms enhanced through enzyme immobilization techniques. Biodegradation in natural conditions is insufficient. Nevertheless, using GEM may result in many risks; improving with reducing these risks will make highly significant differences in the environment more sustainable. Supporting the genetic engineering field is needed to enhance microbial degradative abilities.

Future Perspective Modified genetically engineered microorganisms allowed a new field to sparkle over the last century. GEMs are highly required and needed as they can be used in various applications and minimize dangerous hazards due to their characteristic properties. According to the biodegradation of the GEMs, the enzyme immobilization technique also needs further improvement regarding cost and optimization. The global pollution problem could vanish at a faster rate. All of this hard work is done to control the results and yield a better future, but challenges are overcome.

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67. Atlas RM (1992) Molecular methods for environmental monitoring and containment of genetically engineered microorganisms. Biodegradation 3(2-3):137–146. 68. Rilling JI, Acuña JJ, Nannipieri P, Cassan F, Maruyama F, Jorquera MA (2019) Current opinion and perspectives on the methods for tracking and monitoring plant growth–promoting bacteria. Soil Biology and Biochemistry 130: 205–219. 69. Cho JC, Tiedje JM (2002) Quantitative detection of microbial genes by using DNA microarrays. Applied and Environmental Microbiology 68(3):1425–1430. 70. Páez-Espino D, Tamames J, de Lorenzo V, Cánovas D (2009) Microbial responses to environmental arsenic. Biometals 22(1):117–130. 71. Liu S, Zhang F, Chen J, Sun G (2011) Arsenic removal from contaminated soil via biovolatilization by genetically engineered bacteria under laboratory conditions. Journal of Environmental Sciences 23(9) :1544–1550. 72. Davison J (2002) Towards safer vectors for the field release of recombinant bacteria. Environmental Biosafety Research 1(1): 9–18. 73. Singh JS, Abhilash PC, Singh HB, Singh RP, Singh DP (2011) Genetically engineered bacteria: an emerging tool for environmental remediation and future research perspectives. Gene 480(1–2): 1–9. 74. Joutey NT, Bahafid W, Sayel H, El Ghachtouli N (2013) Biodegradation: involved microorganisms and genetically engineered microorganisms. Biodegradation-life of Science 1: 289–320. 75. Kumar A, Kumar A, Singh R, Singh R, Pandey S, Rai A, Rahul B (2020) Genetically engineered bacteria for the degradation of dye and other organic compounds. In: Abatement of Environmental Pollutants. Elsevier, pp. 331–350 76. Deckers M, Deforce D, Fraiture MA, Roosens NH (2020) Genetically modified microorganisms for industrial food enzyme production: an overview. Foods 9(3): 326. 77. Pant G, Garlapati D, Agrawal U, Prasuna RG, Mathimani T, Pugazhendhi A (2021) Biological approaches practiced using genetically engineered microbes for a sustainable environment: a review. Journal of Hazardous Materials 405: 124631. 78. Bhandari G, Sharma M (2021) Microbial Enzymes in the Bioremediation of Pollutants: Emerging Potential and Challenges. In: Mycoremediation and Environmental Sustainability. Springer, Cham, pp. 75–94. 79. Shah NA, Kaleem I, Shabbir A, Moneeba S, Khattak AH (2018). Bioremediation of Contaminated Paddy Soil. In: Environmental Pollution of Paddy Soils. Springer, Cham 53: 245–271. 80. Saini R, Rani V, Sharma S, Verma ML (2021) Screening of Microbial Enzymes and Their Potential Applications in the Bioremediation Process. In: Microbial Products for Health, Environment and Agriculture. Springer, Singapore 31: 359–378. 81. Okino-Delgado CH, Zanutto-Elgui MR, do Prado DZ, Pereira MS, Fleuri LF (2019) Enzymatic bioremediation: current status, challenges of obtaining process, and applications. In: Microbial metabolism of xenobiotic compounds. Springer, Singapore 10: 79–101.

Part II Polymer Biodegradation

Biodegradable Polymers

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymers Derived from Petroleum Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymers Derived from Natural Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting the Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nowadays, synthetic polymers (plastics) are unavoidable as they are used in nearly every sector in our daily life, including bottles, packaging, boxes, households, cars, medical devices, and phones, to name a few. The sustained consumption of these materials will cause a tremendous accumulation of plastic waste in land and water bodies based on synthetic counterparts that are not biodegradable. Due to the necessity of plastics, numerous efforts were devoted to explore more sustainable materials with similar performance. Biodegradable polymers (BPs) were raised as a promising eco-friendly alternative as they are susceptible to hydrolytic or enzymatic cleavage. Many BPs have been developed until now, and several microorganisms/microorganisms capable of degrading them have been found in nature, invented, and even commercialized. In recent years, there has been an abundance of literature related to the BPs, which covered different A. Alhanish (*) Chemical Engineering Department, Faculty of Petroleum and Natural Gas Engineering, University of Zawia, Zawiya, Libya e-mail: [email protected] G. A. M. Ali Chemistry Department, Faculty of Science, Al–Azhar University, Assiut, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_13

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aspects of this subject. At the same time, some focused on a specified polymer, specific application for a certain type; others covered biodegradation mechanisms in a particular environment or even the biodegradation of one polymer. This chapter is aimed to provide insight into the latest advances to the new researchers in the field with a general overview of the basic knowledge in the subject, including the various types and their capability to biodegrade in different environments with an emphasis on the most known BPs. Also, the factors affecting the biodegradation of BPs, in general, will be highlighted, and the future trends are also briefly summarized.

Introduction Polymer materials are one of the main pillars of the current civilization, as their applications are endless and cover every aspect of our life, from advanced technologies to food packaging. These materials are durable and have a long life even after exceeding their usage life. The accumulation of their waste is still a nightmare for the whole world since few of them reach the recycling facility due to the high cost of recycling, while the rest ended in the landfills and water bodies or even in incineration, which raises the threat of the global warming [1]. About 50% of the plastics’ hazardous monomers and additives can percolate easily into the environment [2]. Several studies reported the adverse effects of various products of plastic fragments in the environment, especially microplastics [3]. Thus, numerous endeavors have been devoted to find ways to mitigate this issue. One of these solutions is developing biodegradable polymers (BPs) with cost and performance comparable to commodity plastics [4]. Since the 1970s, BPs have been extensively explored [5]. Many of these materials are commercially available, yet more are introduced. According to a business report from Markets and Markets ®, the market size of BPs was estimated at 7.7 billion USD in 2021 and is expected to grow to 23.3 billion USD by 2026. They are used in a wide range of applications as many of them showed good performance like the conventional polymers. Their use has expanded over the years, with special attention paid to the packaging industry [6]. Currently, developed countries release different edible food packages in the market to replace the nondegradable packages. Stonyfield Farm, Inc., is one of the first companies that already sell edible packaging such as WikiPearls™ that contain Stonyfield frozen yogurt. Pepsi Co., Nestle, and Bacardi are other companies that adopted BPs in their product packaging. Very recently, Starbucks ® started trials of compostable cups synthesized from Bio-PBS in different countries to shift toward sustainable cups eventually. For example, they found an application in advanced biomedical as nanocarriers for drug delivery [7]. Various types of starting materials have been used to develop new BPs through different processes (Fig. 1). The hallmark of the feedstock used in the production of BPs is polysaccharides or the presence of ester bonds, which allow the attack of microbes and, eventually, depolymerization [2]. Other types of polymers such as

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Fig. 1 Synthetic BPs based on natural and petroleum resources

polyamides (e.g., nylon 4) [8], poly(amino acids) [9], and polyurethanes (PUs) [10] are also found to be susceptible to microbial attack. Generally, BPs are natural, microbiologically synthesized, and chemically synthesized polymers [11]. A more general classification is used in this chapter based on their origins. They are categorized into natural and synthetic BPs, where some of them are derived from renewable resources such as agricultural wastes and others from non-renewable resources such as petroleum products. Many of them already showed a promising potential for several applications, in particular, poly-lactic acid (PLA), polycaprolactone (PCL), and poly-butylene adipate-co-terephthalate (PBAT) [12]. These polymers are used individually, in composite or blended with other materials (mostly a more biodegradable material), or even synthesized as nanoparticles to reduce the production cost and control their biodegradation rate depending on the targeted application. A more durable polymer is desired in the structural application, while a more degradable polymer is favorable for disposal goods. For example, PLA is used to synthesize long-term orthopedic implants, including screws and plates, for biomedical applications due to the slow degradation. In contrast, polyglycolic acid (PGA) produces drug delivery carriers and sutures due to its fast degradation [13]. Hence, many reports deal with designing and exploring various BPs under aerobic and anaerobic conditions based on their most probable disposal environment. Among these materials, polyesters, especially aliphatic polyesters, are the most BPs investigated for various applications. The biodegradation process can occur in the natural environment, such as sea, soil, and inside the human body, or an industrial environment including composting facilities; in aerobic conditions (with the presence of oxygen) such as soil, compost, and some aquatic environments; and anaerobic conditions (without oxygen) including digestion plants and some aquatic habitats. Anaerobic digestion (e.g., landfills) and aerobic composting are currently used for the final disposal of BPs wastes; thus, most studies explored the biodegradability of these polymers under

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anaerobic-aerobic conditions for decades [14]. It is necessary to underline and clarify the concept of biodegradation before going into the details of this chapter. Biodegradation is a biological process where microorganisms, including bacteria, enzymes, and fungi, utilize polymeric materials as a food source. These species are responsible for breaking macromolecules through cleaving specific chemical bonds or performing a specified chemical reaction, resulting in fragments with lower molecular weight. The digest of the polymer occurs through hydrolysis or enzymatic cleavage initiated by microorganisms. Polymers with hydrolyzable moieties including polyesters and polyanhydrides containing easily oxidizable functional groups are susceptible to enzymatic and hydrolysis biodegradation. Usually, this process is assisted by other degradation processes such as thermal, mechanical, etc. The BPs can be degraded aerobically where water, carbon dioxide, hydrocarbon biomass, hydrocarbon residue, and salts are produced, or methane is added to the biodegradation products. This process is a complex phenomenon where it involves various steps, such as in the case of PLA composting where four main phenomena (water absorption, ester cleavage followed by the oligomer fragments formation, solubilization of the oligomers, and finally diffusion of oligomers by bacteria). The biodegradation generally depends on the polymer’s chemical structure, which could be tailored to other factors that will be covered later. To evaluate the degradation of BPs, a wide variety of methods developed by the International Standard Organization (ISO) and other organizations, including ASTM, JIS, and CEN, are available, including CO2 evolution, burial tests, weight loss measurements, O2 consumption, enzymatic degradation test, change in mechanical properties (e.g., loss of tensile strength), and clear zone formation [15]. In contrast, the mechanism differs in the context of the disposal environment, e.g., compost, marine, etc. Thus, different tools have been developed, and this aspect has been covered elsewhere among other sources in literature. In addition, the assessment of biodegradability to test how BPs will perform under certain conditions was reviewed [16]. In this chapter, we focused only on providing insight into the various types of BPs, emphasizing biodegradation behavior in different environments for the most common BPs, and the factors that may affect it will be briefly provided.

Biodegradable Polymers Derived from Petroleum Resources Most famous polymers derived from petroleum resources such as polyolefins and polystyrenes are nondegradable. Other series of synthetic polymers those derived from petroleum resources that contain hydrolyzable functions, including amide, ester, and urethane, are biodegradable and commercialized under various trademarks, including aliphatic polyesters, such as PCL, poly(propylene carbonate) (PPC), PGA, poly(lactic-co-glycolic acid) (PLGA), poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), polibutilensebacato tereftalato (PBST), polyvinyl alcohol (PVA), and aliphatic-aromatic polyesters such as PBAT, as well as polyurethanes (PUs). These materials reveal different degradation behaviors in other conditions. For instance, the degradation rate of PBAT showed a faster

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Table 1 Degradation behavior of the several common petroleum-based BPs in different environments Polymer type PCL

Period (days) 365 28

Biodegradability (%) 0.5 100 for bay; 67 for ocean

Ref. [19] [20]

35

34

[21]

6 28

36 7.6

[22] [23]

28

53

[23]

28

1 (film) 16.8 (powdered) 90

[24]

[24]

PBS

Environment Artificial seawater at 25oC Seawater in lab. Tokyo bay and the Pacific Ocean, Japan) 25 C Pacific coast of the main island of Japan 19–26 C Compost at 55oC Inoculum from a municipal wastewater treatment plant, 30 C, aerobic Inoculum from a municipal wastewater treatment plant, 30 C, aerobic Soil, 25oC, and 60% humidity

160

PBS/starch

Compost, 58–65oC, 50–55% humidity Soil, 25oC, 60% humidity Compost, 58–65oC, 50–55% humidity Compost, 58–65oC, 50–55% humidity Compost, 58–65oC, 50–55% humidity Compost, 58–65oC, 50–55% humidity Compost, 55oC Artificial seawater and freshwater under controlled conditions

100

24.4 (film) 7.6 (powdered) 90

100

90

100

90

170

90

22 365

99 100

PCL/starch

PBS/corn gluten meal (75/25%) PBS/soy meal (75/25%) PBS/canola meal (75/25%) PBS/switch grass (75/25%) PBAT PLGA

28

[25]

[25]

[26] [19]

degradation rate when buried in natural soil (1 month) compared to the laboratory conditions (6 months) in terms of depression in elongation at break [17]. On the other hand, the hydrolytic degradation of PCL was found to be slower than the enzymatic degradation by Pseudomonas lipase; as the former follows the bulk erosion mechanism, the latter follows the superficial erosion mechanism [18]. Table 1 shows the biodegradation behavior of several common petroleumbased BPs in various environments. These biodegradation performance differences result from various factors, even for polymers with similar chemical structures. These factors will be discussed later in this chapter. As a first step, this study focuses on comparative degradation studies of six polymers (five taken from the so-called biodegradable polyesters, including poly (lactic-co-glycolic acid) (PLGA), PCL, PLA, poly(3-hydroxybutyrate) (PHB), and

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Ecoflex, and one well-known nondegradable polymer poly(ethylene terephthalate) (PET) in artificial seawater and freshwater under controlled conditions for 1 year). Only amorphous PLGA shows 100% degradation as determined by weight loss, change in molar mass with time, NMR, electron microscopy, and high-performance liquid chromatography. This is a step forward in understanding the degradability of polyesters required to design environmentally friendly novel polymers for future use. Nowadays, several commercial petroleum-based BPs are available globally, as various companies produce different grades for various applications, especially those requiring specified biodegradation conditions. Tone™ was first created by Union Carbide, a PCL for coating and elastomer application, and, years later, it showed a complete biodegradability that broadened its application as tree planting containers, agricultural films, and drug delivery systems (Table 2). Among the petroleum-based BPs, PCL is one of the most important due to its diverse application and disposal. PCL is synthesized via ring-opening polymerization of ε-caprolactone. It has molecular weights ranging from 3000 to 90,000 g/mol [27]. It exhibits unusual properties not found in other aliphatic polyesters and is recognized for its slow degradation than other aliphatic polyesters [18]. In contrast, in a compost environment, it showed a higher degradation level than other BPs, including PLA, PHB, and PBS [28]. This polymer has been used extensively in biomedical applications such as longterm implants, slow-releasing drug delivery, and bone tissue engineering. Furthermore, PCL recently emerged as a suitable candidate to replace conventional polymers in food packaging applications [29]. It also has the privilege of a large controllable set of biodegradation and mechanical characteristics by regulating the environmental driving forces such as enzymes, microorganisms, and hydrolysis. For example, the enzymatic degradation of PCL in 4 days was possible in the presence of pseudomonas lipase, while it was slower in the same condition of the experiment when using porcine pancreatic lipase and Candida cylindracea lipase [30]. Several studies revealed that PCL is degradable in various natural environments, especially in biotic environments, including seawater, soil, compost, and active sludge [31]. Generally, the rate of PCL is considered low, which limits its application [54, 55]. It was reported that the biodegradation period takes from a few months to several years based on its degree of crystallinity, molecular weight, porosity, morphology, less frequent ester linkage per monomer, sample thickness, and the environment [31]. A parallel interest in the field is developing inexpensive BPs with suitable biodegradation characteristics. Most of the efforts explored polymer blends containing starch [31]. Mater-Bi™ (Novamont Co., Italy) is the most famous commercial starch blend that degrades soil under aerobic and anaerobic conditions. Starch is a natural biopolymer found in plants composed of polysaccharide (amylose and amylopectin) with minor components such as proteins and lipids [32]. It is cheap, with low manufacturing cost and low O2 permeability, and is easy to handle,

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Table 2 Commercial petroleum-based BPs Polymer type PCL

Trade mark Purasorb™ PC 12 Capa™ 6500C Capromer™ PT1-05 eMate-PCL™ Tone™ Celgreen™

Poly(glycolideco-ε-caprolactone) copolymer

PBAT

Cellink™ PCL Monocryl™, Monocryl Plus™ Suruglyde™

PBS

Company Corbion

Adhesives and master batches

Ingevity

Production of PUs

BASF

Filaments Coatings, elastomers, agricultural films, and drug delivery systems Mulch films, developing foam products, loose fill packaging, etc.

eSUN Union Carbide

Tissue engineering applications Suture

Suture

Ecoflex™ Series Tunhe™ Th801t

Flexible film packaging, prosthetic appliances, cushioning for orthotics Film bags, mulching films, paper coating, labels

Biomax™ Series Origo-Bi™

PLA modifier, packaging and industrial applications Coffee cups and pods, carrier bags, cutlery, straws, and food packaging Food service ware markets for cutlery and single-use disposable packaging, protective packaging, compost bags, and organic waste bin liners Trash bags

Eastar bio™

PCL/TPS/glycerol blends PCL and starch

Application Medical devices

Mater-Bi™

Tunhe™ TH803S

Biofiller for the automotive sector, tableware, carrier bags, waste bags, food packaging Tablewares, packaging (including food packaging, cosmetic bottles, medicine bottles), trays, disposable medical supplies, agricultural films, pesticides, and fertilizers

Daicel Chemical Industries, Ltd. Bico Co. Ethicon, Inc.; Somerville, USA Suru International BASF Xinjiang Blueridge Tunhe Chemical Industry DuPont Novamont Eastman Chemical Co.

Yukong Company Novamont

Xinjiang Blueridge Tunhe Chemical Industry (continued)

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Table 2 (continued) Polymer type

Trade mark TH801T™

Application Film, bottles, flexible hose, and spinning

Bionolle™ 1000 and 3000 BioPBS™

Disposable medical products

PVA

Celvol™

PBSA

Bionolle™

PLGA

Vicryl Rapide™ and Panacryl™ Sandostatin™ Lar Depot (Glu-PLGA) Lupron Depot™ Risperdal Consta™

Adhesives, textiles, building products, paper Agricultural purposes, shopping bags, compost bags Sutures

Flexible packaging

Drug delivery applications

Company Xinjiang Blueridge Tunhe Chemical Industry Showa Denko Mitsubishi Chemical Celanese Showa Highpolymer Ethicon Inc.

Novartis Pharmaceuticals AbbVie Alkermes, Inc. (Product A, Wilmington, OH)

but it is brittle and hydrophilic [6]. Starch can be converted to an enhanced form called thermoplastic starch (TPS) with improved flexibility and low hydrophilicity. This plasticized starch is produced by heat and pressure to destroy the crystalline phase of the starch to form an amorphous TPS. Generally, starch is usually blended with either synthetic or natural polymers as it can increase the biodegradability of the less BPs [33]. Since the 1980s, PCL/starch blend has attracted the attention of academia and industry [34]. PCL/starch is one of the most common PCL derivatives produced at industrial scale. This combination made a more economical and biodegradable product, among other features [16]. With appropriate blending, PCL/starch blend can mitigate the limitations of both polymers and promote a controllable characteristic; for example, starch improves the biodegradation of PCL, while PCL adjusts the sensitivity of starch to humidity [34]. A study conducted by Cho et al. [14] showed the ability of PCL/starch blend to degrade under aerobic and anaerobic conditions; for example, 88% of the PCL/starch blend degraded in 44 days under aerobic conditions; this indicates that wastes made by PCL/blend can be handled in landfilling, composting, and anaerobic digestion. TPS also enhanced the biodegradation of PCL under composting conditions [31]. In addition, different fillers such as

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grapefruit seed extract [35] and hydroxyapatite [36] proved to be an effective strategy to enhance the biodegradability of PCL. This subject has comprehensively been covered by [37]. PVA is another famous petroleum-based biodegradable polyester, which has hydrolyzable ester linkage like PCL that makes it susceptible to microbial attack via esterase and lipase. Different grades of PVA with varied water solubility and mechanical properties have found their applications in various fields, including coating, food packaging, and paper [4]. It can be biodegraded under aerobic and anaerobic conditions [4]. However, the microorganisms that can degrade PVA efficiently are not ubiquitous [38]. For instance, the number of microorganisms that degrade PVA in seawater is less than those able to degrade aliphatic polyesters [4]. As the water-soluble polymer is readily degradable in wastewater sludge [32], the degradation of PVA in the natural conditions is difficult as it causes extensive foaming in an aqueous system that results in O2 enrichment, which affects the activates of the aquatic organisms and distribution the ecological balance of water systems [39]. Introducing functional groups susceptible to specific enzyme action proved to be an effective strategy for dealing with such polymer. It was widely blended or used in composites with other readily degradable materials such as biodegradable polyesters from natural and petroleum resources to promote the degradation process. PVA exhibits good compatibility with starch, where various blends were developed and showed great potential as biodegradable packaging materials [15]. Increasing the starch content in the blend also proved to increase microorganism growth and hence the biodegradation level of PVA [40]. Also, PVA reinforced with bacterial cellulose demonstrates a fine ability for biodegradable packaging [41]. This subject has been covered comprehensively by [38]. On the other hand, PVA is utilized as a hydrolysis accelerator for PCL in natural seawater [42]. After 3 months in natural seawater, rapid dissolution of PVA in PVA/PCL blend was recorded much higher than the pure PCL. Despite the improvements in PVA blends, studies on their toxicity are still limited. A recent study raised concerns about the potential toxicity of the degradation fragments (i.e., microplastics) of PVA combined with glycerol in marine conditions [43]; this subject needs further investigations. In addition to PCL and PVA, it is synthesized from fossil sources, other BPs whose monomers are totally or partly obtained from petroleum sources. Polybutylene terephthalate (PBT), PBS, PBAT, PUs, poly(ethylene succinate) (PES), PBST, polyethylene-co-isorbite terephthalate, PBSA, polytrimethylene terephthalate (PTT), and epoxy resins (thermoset plastic) are the most common BPs that are partly synthesized from petroleum resources. Most of these materials have a great interest in the industry, and their production capacity is daily expanding. PBAT belongs to biodegradable aliphatic-aromatic polyesters produced from 1,4-butanediol adipic acid and terephthalic acid, which are known for their mechanical properties comparable to those of low-density polyethylene (LDPE) [44]. Its excellent ductility and softness have been used in food and agricultural packaging [12]. It is usually used in blends with other BPs such as PLA, TPS, PCL, PBS, and polyhydroxyalkanoates (PHAs), especially poly(3-hydroxybutyrate) (PHB) and with other fillers to obtain

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products with excellent properties and acceptable costs. For instance, it is blended with PLA to produce disposal items that meet composability criteria [45]. There are many commercial PBAT, and its blends are present in the market; the commonly known is produced by DuPont (Biomax™), by Eastman Chemical (Eastar Bio™), and by BASF (Ecoflex™) (Jiang and Zhang 2013). The biodegradation of PBAT and its blends was confirmed by many types of research [46]. The benzene ring in PBAT provides excellent physical properties. In contrast, aliphatic chains endorse degradation in different conditions, including composting without temperature control [45]. PBAT undergoes hydrolytic degradation due to the cleavage of ester linkage under the effect of microbial enzymes [45]. It is also known for its high degradability in soil conditions, where fungi were more effective than bacteria [17]. BPs have a similar structure to PBAT produced from succinic acid and butanediol [44]. It has comparable mechanical properties to traditional nondegradable polymers such as high-density polyethylene (HDPE) and degrades under multiple environments. With the emerging of bio-based 1,4-butanediol and succinic acid, PBS can be synthesized based on natural resources with melting temperatures comparable to polyethylene (PE) and polypropylene (PP) [47]. It showed a faster degradation in compost than natural soil due to the controlled compost’s increased humidity and temperature conditions [48]. The addition of fillers and blending with other BPs have enhanced the biodegradation of PBS buried in compost soil [24]. Although a few reports on the biodegradation of PBS in the marine environments are existed, researchers are focusing on studying the degradation of PBS and its copolymers by examining new degradation conditions, microorganisms, and enzymes [49]. Another commercial BPs type is PBST, which is a modification of the PBAT where sebacic acid from a vegetable source is used to replace adipic acid (Fig. 2). It has gained interest due to its excellent mechanical characteristics exhibited by the presence of the aromatic group in its structure. Enzymes also characterize sufficient degradability due to their low crystallinity [49]. Compared to PBAT, PBSA is significantly degraded by several types of lipase [12]. Among the synthetic PBs, PUs are considered the most versatile for fabricating scaffolds with a wide range of architecture, pore size, and mechanical properties [51]. PUs are segmented versatile polymers that have gained considerable interest for decades, with some types already commercialized. It is produced by polymerizing polyols and polyisocyanates, with a carbon chain linked by a urethane bond. The urethane group links the polyether and/or polyester sequence. It is produced mainly from petroleum materials, although bio-based materials derived from natural resources are widely reported [10]. These building blocks control the final properties of the PU, especially the biodegradation characteristic. Segmented PU comprises soft and hard segments, which degrades slower than the soft segments due to the urethane group. By the appropriate choice of the soft segment chemistries, the degradation rate can be amended from weeks to years [52]. For instance, conventional thermoplastic polyurethane (TPU) is not intended to degrade, except in vivo, although TPU comes from ester diol that showed a good biodegradability under composting conditions [53]. Normally, biodegradable TPUs

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Fig. 2 Chemical structures of (a) PBAT, (b) PBS, and (c) PBSA. (Adapted with permission from Ref. [50], Copyright 2016, MDPI)

are recognized as biodegradable when polyester diols or degradable chain extenders are used, which are used as various tissue engineering materials [54]. Generally, polyester-based PU is more biodegradable than those based on polyether due to the hydrolyzable ester bond [55]. Thus, polyesters are commonly used as soft segments for synthesizing biodegradable PUs. However, hydrolysis of these polyesters occurs at elevated temperatures, which causes a reduction in tensile strength and release of acidic species [56]. PUs can be designed to have different biodegradation rates by using different strategies such as mixing polyether with polyester, among other strategies. In essence, PUs showed the ability to biodegrade by naturally occurring microorganisms [57]. Various types of enzymes such as protease, urease, and esterase were effective on the biodegradation of PUs; where esterase attacks the ester bond, protease attacks the urethane bond [57]. Although enzymes are not very effective alone [58], they can also be degraded via different processes, including oxidation, hydrolysis, environmental attack, and even with high temperatures. The number of publications investigating other PU materials’ biodegradation increases [55]. Although not covered here, the biodegradation of PUs with different degradation mediums, including soil, compost, and enzymes, was

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researched. The subject of the biodegradation of PUs is not simple to be covered in paragraph due to the diversity of the sources of the building blocks, especially from natural resources, as well as the wide range of types and forms (e.g., foam, elastomer, rigid, waterborne, blends, and composites) in various environments. Various works in literature covered the biodegradation of different PUs, including the biodegradation of polyurethane elastomers [59] and the biodegradation of bio-thermoplastic polyurethane (Bio-TPU) [60]. Nylon 4 is another promising biodegradable polyamide that could be synthesized from a petroleum-derived 2-pyr-rolidone monomer or 2-pyr-rolidone derived from natural resources such as gamma-aminobutyric acid (GABA) [61]. Several studies confirmed the degradation of nylon 4 in an artificial environment such as soil, activated sludge, and seawater through microbial degradation [8]. Poly(p-dioxanone) is another petroleum-based BPs from the polyester families with good biodegradability in vivo and in vitro [62]. It also showed a remarkable performance compared to the common BPs such as PLA and PBAT in natural conditions (i.e., air and soil) [63], although it showed a lower degradation level via hydrolysis than PLA due to the lower ester groups. One of the modern approaches is the development of BPs from the non-BPs by using natural biodegradable plasticizers. For instance, using a biodegradable additive proved to accelerate the biodegradation of polyvinyl chloride (PVC) [64]. Despite the merits of the petroleum-based BPs, even in the biodegradation performance compared to the bio-based analogs (e.g., the rate of biodegradation of PCL was faster than PLA, PHB, and PBS and fully degraded during 91 days in soil and compost [28]). They still represent a small portion of the total produced petroleum-based plastics, and with the sustained depletion of crude oil, frequent oscillation in oil price, besides the environmental concerns. Hence, research efforts are devoted to exploring more sustainable alternatives by designing BPs from renewable resources that can be eco-friendly, sustainable, and biodegradable.

Biodegradable Polymers Derived from Natural Resources Natural biopolymers, which generally form in nature during the growth cycles of organisms from plants, microorganisms, and animals, should be distinguished from synthetic biopolymers synthesized using bio-based materials such as PLA. These polymers may be biodegradable as well as compostable [65]. Many commercial products have been developed using these materials, including polybags, compost bags, garbage bags, and agriculture mulch films. So far, various renewable materials, including vegetable oils, polysaccharides, and proteins, have been explored to synthesize BPs. Among these resources, agricultural products (e.g., cassava, corn, and cotton) and agricultural by-products (e.g., rice straw) are the most common. About 47% of the commercial bioplastics produced are used for packaging applications, including Bio-on’s Minerv-PHA™, Mitsubishi Chemical’s GS PLA™, and Mater-Bi™, extensively used for compost bags and food packaging [66].

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PLA and PHAs are the most common BPs derived from natural resources [5]. PLA is considered the most representative biodegradable and bio-based in the market. This aliphatic polyester is synthesized by fermentation of natural materials such as starch and agricultural wastes, representing 24% of the world production of BPs [67]. It is a versatile polymer and utilized in various applications; it also exhibits characters similar to polypropylene (PP) and polystyrene (PS); thus, it is considered a good replacement of these commodity polymers in packaging and other applications [68]. Furthermore, a new study suggested that the world reduce 800 tons of gas emissions every year by replacing synthetic plastics such as PE, PET, and PVC with more cost-effective PLA materials, especially PLA pellets [69]. It is synthesized from lactic acid (2-hydroxypropionic acid) as a monomer is made via microbial fermentation of carbohydrates, including corn, potatoes, sugarcanes, and other biomass. There are three active optical configurations of the lactic acid: poly (L-lactide) or PLLA, poly(D-lactide) or PDLA, and poly(D,L-lactide) or PLLA (Fig. 3), and by varying the ratio of these isomers, a crystalline or amorphous high-molecular-weight PLA can be obtained [64, 70]. There are three different methods for synthesizing high-molecular-weight PLA: direct condensation, azeotropic dehydration condensation, and ring-opening, the most commonly used [5]. Generally, it demonstrates the ability to biodegrade by a wide range of microorganisms in abiotic and biotic environments [15]. It takes only 6 to 24 months to degrade in the natural environment [71]. Thus, it has been extensively investigated for its ability to biodegrade in numerous applications covering food packaging, disposal of household items, agriculture films, implantable biomedical devices,

Fig. 3 Chemical structures of DLA, PDLA, and PLLA

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and drug delivery systems. Due to its high glass transition temperature, the degradation of PLA mainly depends on environmental conditions (above the room temperature). Hence, PLA biodegradation in the soil is relatively slow [46]. PLA is considered a non-home compostable polymer, and to enhance its degradation, it is generally blended with other compostable polymers, including PBAT [46] and PCL [47], among others, although additives used in these blends tend to reduce the blend’s ability to biodegrade. Palsikowski et al. [46] reported a reduction in the biodegradation rate of PLA/PBAT to blend in soil due to chain extender. The authors claimed that the functional group of chain extenders might react with the products produced during the degradation. As a result, a reduction in molecular weight occurs, and, consequently, a slowing in the degradation process will be observed. Another study reported a reduction in biodegradation PLA/starch blend due to the presence of plasticizers [6]. Several other ways to increase the biodegradation rate of PLA are reported in literature such as modifications in synthesizing methods of PLA [66] and incorporating biodegradable fillers in PLA composites [72]. Next to PLA, PHAs receive great interest revealed by many patents [68]. Their global production represents 6% of the world production of BPs [67]. Depending on the potential combination of the building blocks, many PHAs and their copolymers can be formed, as more than 91 of polyhydroxyalkanoic acids (the building block of the PHAs (Fig. 4)) have been reported, and the number is still increasing [73]. Unlike PLA, this polymer is obtained from 100% renewable resources; they are directly produced by microbial fermentation using different microorganisms. Depending on the kind of bacteria and carbon source, different structures of PHAs are produced, and based on the number of carbon atoms in the monomer, PHAs are classified into short-chain PHAs (5 carbons), which behave like PP, medium-chain PHAs (6–14 carbons), and long-chain PHAs (15 carbons) which is an elastomer [70]. PHB, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and their copolymer PHBVPHB are the most common forms of PHAs family. The ability of PHAs to biodegradation in different environments made them an attractive option for various sensing applications. They are utilized in many applications, especially in biomedical because of their biodegradability, adjustable mechanical properties, and biocompatibility [13, 74–76]. Another key advantage is their excellent barrier properties comparable to PET and better than PLA; thus, they are also utilized in food and cosmetic packaging [53]. The synthesis of PHAs with microbes is matched by the abundance of microbes that form PHA-degrading enzymes [77]. In general, PHAs show biodegradable behavior in aerobic and anaerobic conditions and can be used to Fig. 4 Chemical structure of PHAs. (Adapted with permission from Ref. [73], Copyright 2016, Nature)

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produce completely soil, compostable, and marine biodegradable products [73]. For example, Kasuya et al. study [20] showed the ability of different types of PHAs to biodegrade in 28 days and at 25oC under aerobic conditions in different natural aquatic environments. Despite the merits of PLA and PHAs, they are relatively expensive compared to their petroleum analogs. To reduce the cost, PLA and PHAs are blended with cost-effective polymers or used in biocomposites reinforced with cheap fillers, which are widely explored in the literature. Great efforts have also been devoted to exploring a wide range of occurring natural BPs, including chitosan, gelatin, alginate, cellulose, derivatives and blends, and starch with their well-known blends, among others commonly used in biomedical and packaging sectors. They are recognized as being eco-friendly, biodegradable, and cheap. The global production capacity of these polymers has been accelerated in the past years as some of these polymers are replacing some traditional plastics, particularly in flexible film packaging [68]. Among the natural BPs, polysaccharides such as starch, cellulose, and protein, including silk fibroin and zein, have found applications in a wide range of applications, especially in the food packaging sector. At the same time, some of them are considered suitable for triggering the release of natural antioxidants, as being eco-friendly and fully biodegrade [12]. Some of these polymers can also be converted into fully biodegradable form with the help of co-blending, as in the case of starch previously discussed. Currently, the main polysaccharides of interest are cellulose and starch (Fig. 5) [78]. However, great attention was paid to the more complex carbohydrates produced by fungi and bacteria, including pullulan, curdlan, xanthan, and hyaluronic [32]. In nature, cellulose is associated with hemicellulose and lignin in the cellular cell wall in plants. Cellulose (along with lignin) is the most abundant natural polymer in the ecosystem. Its structure is similar to starch, although one of the main differences is the more vital linkage between the monomers in the cellulose, making it more resistant to decompose [33]. Due to this difference, enzymes involved in the biodegradation process of each of these two polysaccharides are different [32], although the hydrolyzed starch is considered less diverse than those enzymes that perform cellulose degradation [33]. Aerobic, anaerobic, mesophilic, filamentous OH

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Fig. 5 Chemical structure of starch and cellulose. (Adapted with permission from Ref. [78], Copyright 2017, MDPI)

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fungi, thermophilic bacteria, actinomycetes, and basidiomycetes are celluloseutilizing species. It was demonstrated that fungi are the best microorganism capable of degrading cellulose and lignin [79]. Various mesophilic microorganisms have been isolated from different environments such as compost, soil, sewage, sediments, and anaerobic digesters. Under anaerobic conditions, about 5–10% of cellulose is degraded by some well-known microorganisms that degrade vast amounts of cellulose, including Clostridium thermocellum, fungi, rumen bacteria, and protozoa. However, high temperature is needed for the growth of these organisms [79]. Cellulose is a semicrystalline polymer; it possesses crystalline and amorphous regions. It is usually converted to various forms through chemical or enzymatic methods to crystalline form or amorphous cellulose. It was reported that the biodegradation of crystalline cellulose is faster than amorphous cellulose [80]. In some cases, it showed a better biodegradation performance than other BPs such as PCL/starch blend and copolyesters in composting. Natural cellulose is considered the most common choice as a natural filler in plastic composites for being cheap and biodegradable and has strong mechanical properties [81]. However, the low biodegradability of some types, such as cellulose acetate [82] and nanocellulose [83], can limit their use in some applications, such as the biomedical sector. To tailor their biodegradation, chemical modification of cellulose can make them degradable under certain conditions; for example, replacing the glucopyranosyl hydroxyls with hydrophobic esters in the esterification process may boost the degradation level; also, the rate of biodegradation of cellulose esters could be increased with decreasing degrees of acetate substitutions [81]. It was also reported that the functionalization form of cellulose, such as cellulose nanocrystals, is vital to produce biodegradable nanocomposites with better performance [84]. Another strategy of integrating enzyme carriers within cellulose nanofiberfilled tissue was developed and succeeded in enabling the control of their biodegradation in vivo [83]. Nanocellulose materials, in general, exhibit excellent biodegradability and are actively utilized in synthesizing films, hydrogels, composites, and different other prospectuses, where numerous studies claimed the biodegradability of these polymers. At pilot-scale composting, cellulose nanofibers were fully degraded in 3 weeks without detecting toxicity [85]. Over decades, bacterial cellulose (microbial cellulose) was one of the most investigated types of nanocellulose compared with cellulose derived from plant and other renewable sources [86]. Recent studies propose the replacement of plant cellulose with bacterial cellulose that possesses a better biodegradation rate [82]. Depending on the intended application, bacterial cellulose could be made either stable or degradable via modification with inorganic nanoparticles. Generally, it is fully biodegradable and easily degraded with no negative impact on the environment; furthermore, biodegradation products may be reused in diverse ways, such as animal food and remediation processes. Chitosan is another important polysaccharide member, which gained significant interest during the last two decades [87]. It finds application in various medical and environmental sectors [12]. It is widely explored as a nanocarrier for different drug molecules, proteins, and genes, among other biomedical applications

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CH2OH

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Fig. 6 Chemical structure of chitosan. (Adapted with permission from Ref. [90], Copyright 2014, MDPI)

[88]. According to the global industry analysis report, the market size of chitosan is assumed to reach €4 billion by 2027 [89]. It is synthesized through chemical or enzymatic deacetylation of chitin, the most abundant amino polysaccharide in nature that is mainly found in shrimp and crab shells (Fig. 6) [90]. When the deacetylation process is modified, various materials with new functional properties are produced [77]. Thus, the rate and the extent of biodegradability of the chitosan can be tailored by controlling the degree of deacetylation (i.e., increasing the degree of deacetylation decreases the degradation rate) [87]. It is biodegradable in the natural environment and showed better biodegradation performance in aquatic systems than PBAT [91]. It can be biodegraded by enzymes due to the action of chitinase enzymes that are widely distributed in nature, although it has unsatisfied mechanical properties; thus, it is often used in combination with additives or other polymers [12]. In general, various BPs blends are mixed with two or more BPs to obtain different functional properties, affecting the degradation performance [47]. However, blending with BPs does not always guarantee an improvement in the biodegradability of the mixed polymers. For example, when PLA is blended with cellulose acetate butyrate or PBAT, the biodegradation rate becomes lower than pure PLA [92]. Other research also confirmed this phenomenon [63]. The present production of BPs has adopted several approaches: BPs from natural resources (natural or synthetized by using monomers from biological resources) and bio-based polymers synthesized via the modification of non-BPs. In recent years, biodegradable nanocomposites (consisting of either filler or matrix or both from biodegradable sources) have become a topic of interest [72]. The addition of nanofillers into BPs can effectively produce a polymer with controlled properties and degradation rates. Various types of bio-based fillers showed an enhancement in the biodegradability of biopolymers. For instance, increasing the soluble sugar content in composite by introducing fillers with high protein content showed an improvement in the biodegradability of the biopolymer matrix [93]. For instance, bio-PBS’s biodegradability was enhanced by using meal-based filler compared to pure bio-PBS due to the high concentration of sugar in meal filler. Similarly, Kim et al. [94] found that introducing rice husk flour (RHF) filler to commercial PBS matrix can promote the biodegradation in both natural soil and compost as shown in Fig. 7 [94].

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Fig. 7 Weight loss of PBS and PBS/RHF composites. (Adapted with permission from Ref. [94], Copyright 2006, Elsevier)

Composites made from starch or cellulose matls the most preferable as they biodegrade easily by various microorganisms [73], where the content is one primary criterion here. A higher cellulose content can enhance the mechanical strength but limits the biodegradation level [82]. However, a decline in the biodegradation of a blend can be recorded when starch or any other bio-based polymer content decreases in the mixture. Cellulose nanocrystals have been extensively studied as reinforcement in BPs in the past few years due to their ability to improve the thermal, rheological, barrier, mechanical, and biodegradation properties. For further reading, the authors would like to recommend a recent comprehensive work in the biodegradation of PHAs [73], biodegradation of Bio-TPU [60], and the biodegradation of cellulose derivatives, lignin, and TPS [33]. There is a common belief that polymers derived from natural resources are biodegradable. However, there is no guarantee that these polymers are fully biodegradable. For example, bio-polyethylene (bio-PE), bio-propylene (bio-PP), and bio-polyethylene terephthalate (bio-PET) are analogs to petroleum ones which are synthesized from natural and renewable resources that are not biodegradable, although a recent study demonstrated a microbial degradation of bio-PET [95]. Despite the origin of the BPs (i.e., bio-based or petroleum-based), it is essential to consider that each polymer biodegrades better under certain conditions. For instance, the enzymatic biodegradation of the most famous bio-based and petroleum-based BPs (i.e., PLA and PCL) is extremely fast in soil. In contrast, the enzymatic biodegradation of both polymers in the human body is extremely slow (the period for PLA is in the range of 6–12 months and 2–4 years for PCL) depending on molecular weight and the starting material. Except for PCL and PHA, the biodegradation rate of BPs in the marine environment is generally low; thus, these polymers may damage the marine ecosystem similarly to commodity plastics [96]. These variations could be attributed to various factors influencing their behavior.

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Factors Affecting the Biodegradation Biodegradation is an irreversible process initiated by internal or external stimuli, including the environment, that is chemically tuned and directly engineered [86]. Generally, the biodegradation process for different polymers is influenced by intrinsic factors of the BPs, including but not limited to the molecular weight (a shorter carbon chain is easier to biodegrade), structure (a more complex structure requires various microorganisms to attack different functional groups), crystallinity (crystalline phase is harder to degrade than amorphous), and abiotic and biotic factors (i.e., environmental), like type and the amount of microorganisms, mechanical stress, pH, UV exposure, humidity, temperature, and salinity. In essence, the decomposition of a polymer takes place by synergy abiotic (pH, water, UV, weathering, etc.) and biotic (enzymes, microbes) factors [2]. Among the factors, the chemical structure, complexity, and compositions of the BPs play a crucial role [6]. For instance, BPs with a less complex structure is more susceptible to microbes, where the enzymatic hydrolysis increases with decreasing the crystallinity [24]. Increasing the molecular weight causes a decrease in degradability. This was confirmed by Tokiwa et al. study [80], which showed a faster degradation of PCL with low molecular weight than the one with high molecular weight in the presence of Rhizopus delemar lipase. On the contrary, a recent study found no influence of the molecular weight of PCL films on the degradation behavior by Candida antarctica lipase [27]. For water-soluble polymers with heteroatoms in the backbone, molecular weight is not considered an insurmountable limiting factor for biodegradation [38]. The functional group present in the polymer backbone is an important factor in abiotic and biotic environments. Functional groups like amides, esters, urethanes, and carbonates allow faster surface erosion by the action of enzymes than polymers without functional groups. For example, amide and ester groups in the backbone promote the enzymatic attack, while this behavior is not observed in aromatic polyesters and polyamides. The distance between the bonds, such as ester bonds, and the absence of branching in the polymer also facilitate biodegradation [64]. Thus, when the chemical structure of the polymer is altered, various microorganisms can be involved in the biodegradation process. The stereochemistry of the polymer is also vital as many enzymes are stereochemical selective. Furthermore, the materials used to fabricate the polymer influence the ability of microorganisms to attack the polymer structure. For instance, polymers obtained from the natural organism (e.g., PHAs) are easily degraded compared to synthetic ones, as the number of microorganisms to attack the substrate is limited. The influence of the polymer characteristics in their biodegradation behavior also varied according to the type of microorganisms involved. For example, the hydrolytic degradation of PCL network is proven to be faster than the linear one. In contrast, the enzymatic degradation of the linear PCL is faster than the PCL network [18]. Also, for the high crystalline PCL that hydrolytically degrade in years, it is reported to degrade in 4 days in the presence of lipase [30]. The presence of

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Fig. 8 Weight loss of PCL. Adapted with permission from Ref. [97], Copyright 2019, Nature

pores in the polymer structure also accelerates the degradation rate due to enzyme penetration into the polymer chains. An important factor for determining microbial attachment related to the polymer structure is hydrophobicity, especially for BPs prone to water-induced degradation mechanisms. The more hydrophilic the polymer is, the higher the water sorption on the surface; consequently, a higher hydrolysis rate occurs in polyesters. Thus, increasing the hydrophilicity of the polymer increases the hydrolysis rate. For instance, alkali-treated PCL and poly[(R)-3-hydro-xybutyrate)] (R-PHB) showed better enzymatic hydrolyzabilities than the neat polymers (Fig. 8) [97]. Functionalization of the polymer also affected their biodegradation rate in various ways. For example, copolymerization can accelerate the degradation rate and reduce it in some cases [7]. The effect of the shape of the polymer sample in the biodegradation behavior was also reported. Yang et al. [98] have demonstrated that the biodegradation of BPs in its powder form is faster in the early stage of the degradation than its film form due to the larger aspect ratio of the powder. The study also reported no significant effects were noticed in the later stage of degradation, especially in the case of easily degraded plastics (Fig. 9). The presence of synthetic additives such as plasticizers in the production of BPs for commercial use to enhance other properties like the mechanical properties could also play a crucial role in tailoring their biodegradation performance. Other polymer features, including its mobility, tacticity, and the presence of any substituents in its structure, all play a crucial role in its degradation. Generally, polymers with shorter chains, complex structures, and amorphous parts are more susceptible to biodegradation. Other environmental key aspects of BP must be taken into consideration. For instance, the biodegradation behavior of some chemosynthetic polyesters (e.g., PCL, PBS) differs according to the source of the natural water (e.g., bay, river, ocean, sea)

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Fig. 9 Biodegradation of PCL films with different shapes in an animal fodder compost. (Adapted with permission from Ref. [99], Copyright 2005, Elsevier)

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[20]. The biodegradation rate in seawater is higher than in freshwater ponds and rivers. This could be attributed to various factors such as the type of microorganisms and the environmental conditions. For example, near the ocean floor, there is no oxygen, and the water movement is minimal conditions can be found near bottom sediment that contains more microorganism’s consortium [73]. The abundance of microorganisms also has a strong impact on the degradation rate. These species are highly adaptive to the environment. Biofragmentation is generally concerned with enzymes that belong to hydrolases and oxidoreductases. Several types of hydrolases such as amylases, cellulases, and cutinases are produced by soil microorganisms to decompose natural polymers, including starch, cellulose, and cutin; also various enzymes that can break down copolyesters such as lipases and esterases have been identified. The biodegradation performance differs according to different soil conditions due to the diversity of the soil microorganisms that have different growth conditions [15]. More bacteria in soil are accompanied by an enhancement in the rate of biodegradation [24]. Among the microorganisms that can degrade bioplastics and isolate from different environments including soil, compost, river, and marine water, Pseudomonas, Bacillus, Paenibacillus, and Burkholderia as bacterial species and Fusarium, Aspergillus, and Penicillium as fungal species are the most known [93]. Several factors influence microorganisms’ growth, including temperature, the availability of water, pH, oxygen usage, redox potential, minerals, carbon, and energy source. In addition, the microorganisms in soil are sensitive to pollution as several soil contaminations come from degradation fragments (e.g., commodity plastics) that affect the microbial diversity, although the degradation of bioplastics has no influence on the bacterial diversity in soil [24]. The role of microorganisms in context of the degradation of BPs was discussed from various points of view elsewhere [93].

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The temperature and moisture required for degrading BPs are relatively high and cannot be found in the natural environment; for example, the degradation in compost requires 70% moisture and 55oC [92]. Among the environmental conditions taken into account for the degradation of BPs, soil and compost are mostly studied due to their high microbial diversity [25]. Soil and compost are the aerobic environments for the biodegradation of bioplastics. They also differ in the microbial population, with soil exhibiting a much lower abundance of microorganisms [47]. Also, the chemical, physical, and biological conditions are different from those encountered in soil and aquatic environments. The composting process conditions (temperature, pH, humidity) are optimal during the controlled process. Thus, BPs such as PCL and PLA showed a better biodegradation performance than soil and aquatic environments, although not all BPs are suitable for composting. Generally, a few BPs show a very promising biodegradability in soil and compost both in laboratory and field, encouraging further investigation in this field. Other factors that affect biodegradability are related to the test conditions, including the shape and the volume of the vessel, open or closed, way of mixing, temperature, test duration, and oxygen supply [23]. Furthermore, the nature and the mechanism of the process itself is influential factor. There is a huge difference in the biodegradation level between the natural and synthetic biodegradation conditions. It is also seen in comparing industrial and home composting, leading to a huge difference in bioplastic degradation [93]. It is not easy to assess the biodegradation behavior of BPs in natural conditions as experiments designed to obtain data under standard conditions, which are not found in open environments, although it helps to examine their suitability for recycling (e.g., composting).

Conclusions There is an increasing demand for replacing nondegradable polymers and fossilbased plastics with degradable ones in many sectors to address sustained environmental concerns. Most companies focus on biomaterials to offer a good service life and easy disposal. The current challenge is creating BPs that combines the merits of conventional plastics with minimal environmental impact. BPs from renewable resources present an attractive alternative to overcome the uncertainty of the petroleum market. Thus, they gain more and more space in the global plastic market. To date, several types of BPs have been developed in laboratories, although there are few commodity products. Some limitations restrict scaling up, such as the high cost of production and the uncertainty of their toxicological impact on the environment in the context of the degradation products. However, in the upcoming years, and due to the sustained efforts in designing and exploring various types and combinations (e.g., blends, composites) of these polymers, the number of commercial BPs that replace commodity plastics will be increased.

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Future Prospective Despite the promising performance for both bio-based and petroleum-based BPs, the balance between durable and BPs is not easy to reach for both. The current trend is investigating BPs from natural resources as a promising alternative over their petroleum-based analogs in the context of their biodegradability and performance. The future of these polymers depends basically on their cost competitiveness, although BPs derived from natural resources are still hampered by higher production costs. Thus, developing more cost-effective synthesis routes, especially biosynthesis, is vital. Furthermore, many of these polymers are synthesized from edible resources, raising concerns about increasing the cost of food. Thus, exploring agriculture, food, and industrial wastes is a more sustainable choice. Another dark side of these promising polymers represents the formation of thousands of micro-/nano-plastic fragments as degradation products. This was observed by many researchers from the most common BPs such as PLA and PBAT [63], and PHB [100]. In comparison, other works demonstrate some polymers’ safety from a toxicology point in the same condition [46]. Thus, promoting the large scale of BPs before a critical assessment of their ecotoxicology is yet to come. A greater effort is needed to address the biological and economic effects of BPs by subjecting them to life cycle assessment (LCA), although the diversity of the possible bio-based feedstock and environments makes it difficult to make a generic assessment. Commercial BPs in the market may claim to meet biodegradation criteria standards, although the lack of governmental regulations on the biodegradability standards may drown the local market with false products. A recent study raises our concerns regarding the reality of this fact. They discovered the non-degradability of claimed BPs used for a local frozen yoghurt in Chile which exhibit a similar performance of nondegradable petroleum plastics under the same conditions [65]. This requires consistent monitoring of the BPs used in the local markets by specialized authorities. Still, there is a lack of knowledge about the time required to full degradation of those materials in the different conditions that allow efficient degradation.

Cross-References ▶ Biocompatibility of Nanomaterials Reinforced Polymer-Based Nanocomposites ▶ Biodegradable Plastics as a Solution to the Challenging Situation of Plastic Waste Management ▶ Biodegradable Plastics Based on Algal Polymers: Recent Advances and Applications ▶ Biodegradable Polymer Challenges ▶ Biodegradable Polymers for Industrial Applications ▶ Biodegradable Polysaccharides Nanocomposites ▶ Electrically Conducting Smart Biodegradable Polymers and Their Applications

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▶ Emerging and Advanced Technologies in Biodegradable Plastics for Sustainability ▶ Plastics Biodegradation and Biofragmentation ▶ Sustainable Biopolymers

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Materials: Challenges and Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyhydroxyalkanoates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polybutylene Succinate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic Acid/Polylactide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polycarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential Challenges and Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modification in Synthetic Strategies for Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . Banning of Problematic Conventional Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Implementation of Extended Producer Responsibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Implementation of Deposit Refund Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

After their intended use, the natural breakdown of synthetic polymeric materials into byproducts has always been regarded as a foremost challenge in recent years from the biodegradation perspective. The current chapter presents a comprehensive overview of the biodegradable polymeric materials that are now considered an alternative to synthetic non-biodegradable materials in terms of their waste management advantages for building an ultimate pollution-free environment. The major benefits of biodegradable polymers are that they can be composted with

S. Naeem (*) · J. Najeeb · H. Rafique Department of Chemistry, University of Gujrat, Gujrat, Pakistan e-mail: [email protected]; [email protected]; humera_rafi[email protected] S. M. Usman Hunza Sugar Mills Private Limited (Distillery Division), Lahore, Pakistan © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_14

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organic wastes and returned to the environment to enrich the soil, which will not only reduce injuries to wild animals caused by the dumping of conventional polymeric wastes but will also lessen the labor cost for the removal of such wastes in the environment. As they are degraded naturally, their decomposition will help increase the longevity and stability of landfills by reducing the volume of garbage. Recently, tremendous interest has been reported in the academic literature to develop such biodegradable polymers for various applications. Owing to their diverse market value and applications, the biodegradable polymers represent a dynamically growing research field, and organizing the documented academic case studies associated with this research domain is the need of an hour. The future of these materials also seems promising with their potentially significant contribution to the biomedical industry, drug/gene delivery, nanotechnology, agriculture, and an exceptional role in waste management to help protect the natural environment. Keywords

Biodegradable polymers · Challenges · Plastics waste management · Biodegradation · Plastic pollution Abbreviations

BPA CC BY DRS EPR LDPE P3HB PA PBAT PBS PCs PCL PE PEF PET PHAs PLA PP PPC PS PTT PVC TPS

Bisphenol-A Creative Commons Attribution Deposit Refund Scheme Extended producer responsibility Low-density polyethylene Poly-3-hydroxybutyrate Polyamide Poly(butylene adipate-co-terephthalate) Poly(butylene succinate) Polycarbonates Polycaprolactone Polyethylene Polyethylene furanoate Polyethylene terephthalate Polyhydroxyalkanoates Polylactic acid/polylactide Polypropylene Polypropylene carbonate Polystyrene Polytrimethylene terephthalate Polyvinyl chloride Thermoplastic starch

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Introduction Polymer-based materials have gained enormous scientific and industrial interest due to their extensive use in almost every material used in our daily lives, such as textile fibers, bottling, food packaging, auto parts, plastic bags, computers, gadgets, and many other areas in medicine [1–5]. Being made up of long repeating chains of molecules, these polymeric materials have unique properties based on the type and arrangement of the combining molecules. Their use in broad spectrum (i.e., from basic applications to biopolymers and therapeutic polymers) has brought them into the limelight in recent years, especially concerning environmental mitigation tools for environmental pollution [5–7]. The term biodegradable refers to a class of polymers decomposed into natural byproducts by bacteria/microbes after their intended use. The byproducts, such as water (H2O), carbon dioxide (CO2), nitrogen (N2), methane (CH4), biomass and inorganic salts, etc., are produced because of this end-of-life cycle. Biodegradable plastics and polymers (introduced in the 1980s), especially from renewable sources, are highly regarded based on the capability of these materials to address environmental concerns and the realization of finite petroleum resources. Natural polymers are called biopolymers because they are naturally formed during the growth cycles of all organisms [8]. Complex metabolic processes govern the formation of synthetic polymers, generally involving enzymecatalyzed chain-growth polymerization reaction of activated monomers. There is a group of biopolymers that are directly extracted from biomass, such as polysaccharides (cellulose and starch), but those produced by bacteria and fungi (such as xanthan, curdlan, pullulan, and hyaluronic acid) are gaining more attention these days [9]. The non-biodegradable materials are resistant to microbial degradation and contribute to environmental problems in dangerous proportions [10]. Many polymer-based materials obtained from petroleum resources are non-biodegradable, which end up in landfills and pose serious concerns for human beings as the causative agents of numerous infections and diseases. Similarly, plastic production also experienced rapid growth worldwide in the nineteenth century due to facile processing, utilization potential, and high-end market value, yet plastic production has created serious environmental concerns due to its non-biodegradable nature [11]. With a global market value of several billions per year, the strong dynamic growth of the current market for bioplastics is due to many applications of these biodegradable polymers. The generalized mechanism of degradation is presented in Fig. 1. Currently, vigorous research and development are being furnished to make biodegradable polymeric materials on a large scale, which is considered a cure for the planet but still has a long way to go owing to the challenges faced [12]. It represents a growing field with wide-ranging properties, including both natural and polymeric materials [13].

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Fig. 1 General biodegradation cycle of any discarded biodegradable polymer. (Adapted with permission from Ref. , Copyright 2018, CC BY 4.0, Springer Nature)

Biodegradable Materials: Challenges and Opportunities Because of the adverse health and environmental effects of non-biodegradable polymers, the concept of introducing biodegradable and/or bio-based polymers has been found as a remarkable perspective for the industries and consumers alike [14]. Due to their degradable nature, these polymers have shown promising applications in tissue engineering, drug delivery, imaging, and many other related fields [15]. Their mode of action in an organized format is similar to macromolecular complexes, which synergistically initiate the interaction between macromolecular science including cell biology and pharmacology to play a vital role in developing various suitable nanostructures with potential biomedical applications [16]. The medical procedures which require only short-term interventions (such as the introduction of stent, composed of non-biodegradable plastics to properly heal up the arteries) where the introduced material become just like a vestigial organ after its intended use (i.e., after healing up of arteries, the introduced stent has no function in the human body) are predominately looked to be settled by using biodegradable materials. According to a study, only a short-term support is required by stents to heal up the arteries in practice because no long-term advantage or utility of these stents have been found medically [17]. The materials used to prepare these stents must receive regulatory approvals before any biomedical application, such as polylactic acid (PLA), commonly used in biodegradable sutures [18]. Although the utilization of biodegradable polymer solves the non-biodegradable problem of conventional plastics, it should be kept in mind that the impact of these biodegradable polymer-based stents should be checked thoroughly in terms of their design processes (complex geometries) and specific loading conditions [19]. Starch-based biomaterials have also found great attraction owing to their wide applications. But, the door is still open to overcome the challenges faced by these

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starch-based materials, such as moisture sensitivity and mechanical properties. There have been certain safety concerns in developing various blends and composites concerning additives for food packaging applications. On the way to improve performance and decrease manufacturing cost (as two top priority strategies), the techniques such as reinforcement by natural fillers, starch-based nanocomposites, self-reinforced composites, blending with other natural polymers, and functionalized composites have been utilized to obtain these goals. The hydrophilic nature of the hydroxyl group in starch contributes to its enhanced moisture sensitivity resulting in lower tensile strengths of starch films [20]. Various blends and composites include hydrophobic polylactic acids/hydrophilic starch blends [21] and gelatinized/biodegradable polyester blends [22]. As disposable plastics have been banned due to the new regulations, starch-based foam has found extensive applications worldwide in its replacement. In agriculture, conventional plastic films are very common in specialty crop production. But the majority of the films are non-biodegradable, which needs greater public awareness concerning the adverse global plastic pollution problem [23]. Moreover, its ecological and societal impact is the major driving force in changing the current policies related to plastic use. The future perspectives also include new technology for recycling such materials [24–27]. Researchers are currently developing sprayable, biodegradable polymer membranes using renewable and biodegradable raw materials, including protein polymers, polysaccharide polymers, natural polymers, and polyethylene glycols acrylics polymers. These polymers adhere to the soil surface directly and may degrade more efficiently as they have weaker linkages between polymeric molecules than the previously formed conventional plastic mulch films [28]. A few models reported biodegradable polymeric materials had been mentioned in Table 1 to combat the challenges in the best possible way. Polymers and biopolymers are macromolecules formed by the covalent linkage of small and simple repeating units whose ultimate properties remain unaffected by the addition or deletion of a few such units due to their gigantic structures. Biopolymers have a biological origin and are formed by biochemical processes with or without the help of specific enzymes or catalysts [30]. Their resources include renewable agricultural, forestry, or biomass-based feedstocks and have properties like manufactured polymers. Their additional and most promising characteristic is the ability to degrade biologically after their intended use (also called bio-derived or bio-based polymers) [31]. The degradation process is very important in waste management options when dealing with degradable or non-degradable polymeric materials. The term “biodegradation” refers to the degradation provoked by the action of metabolism by naturally available microorganisms under normal environmental conditions (involving biological activity). This degradation and assimilation of polymers are done by living organisms such as bacteria, algae, and fungi to produce degradation products [32]. Processes of oxidation and hydrolysis lead to the degradation of natural polymers such as proteins, nucleic acids, and polysaccharides, whereas biomass, carbon dioxide, and methane are the degradation products of biodegradable materials [33].

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Table 1 Summarizes a few models that reported biodegradable polymeric materials to combat the challenges in the best possible way Polymeric materials Plastics

Starchbased films, capsules, sheets, or foams

Plastic mulch films

Edible polymers

Challenge The benefit of the market economy and reoccurring environmental hazards. Poor gas/moisture barrier issues related to current biodegradable polymers Weakness of pure starch-based materials, e.g., lower mechanical properties in natural polymers, moisture sensitivity, safety issues in food packaging applications Commonly used plastic mulch films must be removed after every growing season due to their non-biodegradability. Plastic mulch remnants cause pollution by entering the food chain and water systems, recycling is hampered by soil particles adhered to the plastics

To get improved recyclability of materials compared to the more traditional non-environmentally friendly materials, edible polymers may offer many advantages for delivering drugs and tissue engineering

Biodegradable polymer composition Chain architecture tailoring, crystallinity, melt blending/multilayer co-extrusion, nanotechnology, and surface coating are considered effective strategies

Function/application Sustainable packaging applications

Ref [11]

Starch-based, natural fibers, starch or cellulose crystals, and laver

Waste management, food packaging, medicine capsules to cartilage tissue engineering, products to feed animals or even edibles, and starch-based foaming

[20]

Sprayable biodegradable polymer membrane (SBPM) technology (polysaccharide polymers, natural polymers (e.g., seaweed), protein polymers, polyethylene glycols, polysiloxane, polyurethane, and acrylic polymers) including natural products (e.g., sugar cane, wood-cellulose microfibers, lignin, gum, and leather) Synthetic hydrogel polymer as a new possibility for developing films, coatings, extrudable pellets, and synthetic nanopolymers, particularly designed for medical, agricultural, and industrial fields

It improves soil microclimate, helps to conserve soil moisture, and suppresses weed growth

[28]

Can expand the organoleptic properties of packaged foods, food industry, and medical industries

[29]

(continued)

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Table 1 (continued) Polymeric materials Absorbable polymer stents

Challenge Material composition and design (geometry and loading conditions)

Biodegradable polymer composition PLA polymers and contains two radioopaque markers

Function/application Biomedical: to support artery to heal in shorter periods

Ref [19]

Factors affecting Biodegradation Exposure Characteristics

Abiotic

• Moisture • Temperature • PH

Polymer Characteristics

Biotic

• Enzymes • Hydrophobity

• Molecular weight • Size and shape • Additives • Bio surfactants

Fig. 2 Factors affecting the biodegradation process. (Adapted with permission from Ref. [36], Copyright 2018, Springer Nature)

The factors affecting microorganism growth, including temperature and carbon sources, nitrogen, and pH, are presented in Fig. 2. The degradation pathways, including aerobic and anaerobic processes, are also indicated in Eqs. 1 and 2. When the degradation product is water, gases, salts, minerals, and residual biomass, the process is referred to as mineralization which is considered complete when all the biodegradable mass is consumed and all the carbon has been converted into carbon dioxide [34]. Some of the factors affecting the biodegradability of the polymeric materials include chemical structure, which is considered responsible for functional group stability, its reactivity, hydrophilicity, swelling behavior, and physical and physico-mechanical properties like molecular weight, porosity, morphology, and elasticity [35]. Polymer þ O2 ! CO2 þ H2 O þ Biomass þ Residue ðAerobic degradationÞ ð1Þ Polymer ! CO2 þ CH 4 þ H2 O þ Biomass þ Residue ðAnaerobic degradationÞ

ð2Þ

Some of the factors affecting the biodegradability of polymeric materials include chemical structure, which is considered responsible for functional group stability, reactivity, hydrophilicity, swelling behavior, and physical and physico-mechanical

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1. Bio-deterioration

CO2, CH4 and H2O

2. Bio-fragmentation

Intermediates used by microbes

Biofilm 4. Mineralization

3. Assimilation

Fig. 3 Biodegradation mechanism for the biodegradable plastics: (1) bio-deterioration, the process of alterations in the physical/chemical characteristics of discarded plastics owing to environmental factors; (2) bio-fragmentation, enzymatic cleavage results in the breakdown of polymer into its simpler forms; (3) assimilation, the process of up-taking of fragmented molecules/byproducts by the microbial life; and (4) mineralization, generation of end-of-life oxidized products including CO2, H2O, CH4, etc. (Adapted with permission from Ref. [37], Copyright 2017, CC BY 4.0, Springer Nature)

properties like molecular weight, porosity, morphology, and elasticity. The degradation procedure for the disposal of biodegradable plastics is also presented in Fig. 3.

Biodegradable Polymers Generally, biodegradable polymers can be divided into two main categories: natural biodegradable polymers and designed to be biodegradable polymers. Another class of commercially viable polymers includes non-biodegradable biopolymers [13]. These polymers are derived from natural resources but are non-biodegradable/ partially degradable due to some non-biodegradable moiety introduced into the system to enhance the properties of the base materials [38]. These materials are excluded from the discussion presented in this book chapter. However, this should be kept in mind that these non-biodegradable biopolymers are regarded as the most commercially successful bio-based polymeric materials [39]. Coca-Cola’s plant bottle containing mono ethylene glycol (acquired from sugarcane) and terephthalic

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acid (derived from petrochemicals) is the prime example of one such material which is successfully utilized at the commercial scale. These bio-based or partially bio-based non-biodegradable polymers are regarded as “drop-in solutions” for synthetic plastics due to their enhanced characteristics that are comparable with the fossil fuel-based polymers/plastics and their recycling potential (i.e., these materials are mechanically partially recyclable) [40]. Usually, these non-biodegradable biopolymers are structurally regarded as variants of polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), and polypropylene carbonate (PPC), but other non-biodegradable materials including polyamide (PA), polyethylene furanoate (PEF), and polytrimethylene terephthalate (PTT) are also included in this category [41]. Natural biodegradable polymers include thermoplastic starch (TPS), cellulose acetate, starch blends, polyhydroxyalkanoates (PHAs), polycarbonates (PCs), etc. Others designed to be biodegradable polymers include the polycaprolactone (PCL), poly(butylene adipate-co-terephthalate) (PBAT), poly (butylene succinate) (PBS), and polylactide (PLA), etc. [42]. One advantage of using these biodegradable materials is that they do not utilize non-renewable resources (particularly fossil fuel) as the feedstock while conventional plastics depend heavily on fossil fuels to provide raw materials [43]. As with the passage of time, the issue of the fossil fuel depletion is emerging as one of the serious global concerns; therefore, the use of biodegradable materials for numerous applications is widely appreciated and encouraged at all level levels (government, global, and scientific community). According to the report presented by European Bioplastics in collaboration with technical market experts from the Nova-Institute (Hürth, Germany), the global production of bioplastics which was found to be 2.41 million tons by 2021 is set to approximately reach 7.59 million tons by 2026 [44]. Figure 4a represents the bioplastics production in 2021, where the non-biodegradable bio-based polymeric materials account for 0.86 million tons (35.8%), and the biodegradable polymers account for 1.55 million tons (64.2%) of the total documented bioplastics production values. From the 64.2% of biodegradable polymers, materials of PBAT (19.2%), PLA (18.9%), and starch blends (16.4%) were found to be the main contributors in this regard. The global production of biodegradable polymers is expected to increase 69.9% (5.28 million tons). In comparison, the production of the non-biodegradable polymeric materials is expected to reduce from 35.8% to 30.4% of the expected global production of bio-based plastics by the year 2026, as indicated in Figure 4b. Another interesting observation presented in this report was that the Asia region is the top producer of bioplastics for the year 2021, accounting for the 49.9% of the total production, and this share is expected to increase up to 70.8% of the total expected production of the bioplastics by the year 2026 as [44]. This observation is truly a sentiment of the available workforce power of the Asia region and further cement the enhanced efforts placed by these innovators in achieving the practical applicability of the synthesized materials. These projections indicate that the research community’s interest has shifted to the generation of the biodegradable polymers rather than the non-biodegradable bio-based polymers and the biodegradable polymers are ready to replace the conventional polymeric materials/plastics

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Fig. 4 (a) Global market data regarding biodegradable polymers for the year 2021 and (b) global forecasted data for producing biodegradable polymers for the year 2026. (Adopted with the permission from Ref. [44], Copyright 2021, European Bioplastics)

shortly. Commercially viable biodegradable polymers are discussed individually in the subsection. Although the PBAT is the most extensively produced bioplastic, it is synthetically designed and not completely biodegradable. Therefore, the discussion on PBAT is not included in this book chapter.

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Polyhydroxyalkanoates PHAs are structurally considered polyesters acquired from the bacterial fermentation of certain lipids (oils or sugars) [45]. More than 150 monomeric units (lipids) and 300 bacterial species (for fermentation) are documented in the academic literature for the synthesis of these polymers [46]. The PHAs are generated as granules or vesicles inside the cells, making the presence of a proper extraction mechanism essential for the acquisition of PHAs. This post-synthesis extraction protocol makes this biodegradable polymer’s synthesis process quite expensive. This is indicated by the fact that the production costs calculated for the PHAs were five to ten times higher than the petroleum-based plastics [47]. A critical overview of the PHA synthetic process also reveals that 50% of this cost was associated with the feedstock taken for the extraction of the PHAs, indicating that the alternative feedstock should be explored for the commercial production of the PHAs [48]. It was further observed that the need for utilization of pure aseptic cultures (needing expensive carbon sources, usage of the expensive organic solvents, and sterile working conditions) is the focal point in terms of the production cost that can be explored for the further improvement in the synthesis protocol. Figure 5 represents the key process of acquisition of the PHAs from the lignocellulose waste and highlights the significance of PHAs for the formation of degradable plastic bottles. Poly-3-hydroxybutyrate (P3HB) is one of the most common and extensively used PHAs known for its high mechanical and barrier properties, making it an excellent bioplastic material for food packaging applications [50]. Although the mechanical properties of the P3HB is comparable with that of the commercial non-biodegradable PP, the P3HB is a rigid material with the lower elongation

Fig. 5 Production of PHAs from the agro-lignocellulosic waste and its application in food packaging applications. (Adapted with permission from Ref. [49], Copyright 2022, Elsevier)

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break values of 5% [51] (in comparison to 400% value in the case of PP [52]) indicating that overcoming the brittle nature of the P3HB is necessary for acquiring the long-term stability for this particular material [53]. Researchers have proposed incrementing the valeric acid dose during the production protocol to make the copolymers of the hydroxybutyrate and hydroxy valerate to address this issue of brittleness. Comparatively, it was also observed that the copolymer-based assemblies of P3HB exhibited faster degradation rates than the pure P3HB, which also enhances the practical applicability of the P3HB [54]. The blending of the base matrix of the PHAs with reinforcing materials is also one such approach that is also utilized for making the density and mechanical properties of the PHAs comparable with the commercial plastics of polystyrene (PS), polyvinyl chloride (PVC), and low-density polyethylene (LDPE) [55]. Consequently, these blends of the PHAs are regarded as copacetic materials with potential applications in the agriculture, biomedical, and packaging sectors. However, due to their limitation of mechanical properties, the PHAs are best suitable for food packaging applications [53]. In other applications where the mechanical properties of plastics play an important role (such as the construction sector and automobile applications), the PHAs are not suitable because of their brittle nature [56]. The low thermal degradation properties (thermal degradation temperature for P3HB ¼ 185  C) of the PHAs also indicate that even in food packaging applications, PHAs are not suitable for the processes where thermal sterilization of the product is involved owing to its low melting temperatures. Addressing these challenges is essential for achieving the commercialization of this biodegradable material.

Polybutylene Succinate Microorganisms-based fermentation process carried on the renewable feedstock is utilized to produce PBS via bio-based 1,4-butanediol and succinic acid [57]. In terms of mechanical strength and structural stability, PBS is one of the most commercially viable materials as the yield strength of the PBS is 1.1 and 3.64 times more than that of LDPE and PP, respectively [58]. This thermoplastic material has high ductility (elongation break value of 560%) and high thermal degradation values of 200  C, which is almost similar to the LDPE [59]. Consequently, the PBS can be conveniently processed by using conventional plastic protocols. The PBS is now commonly utilized for the synthesis of compostable bags, supermarket retail bags, packaging films [60], mulching (agriculture sector), catering products [61], thermal insulation foams [62], etc. Owing to its melt processability, the PBS is utilized in the textile sector for the formation of nonwoven fabric [63] renowned for its high absorptivity potential making these PBS-based fabrics excellent materials for sanitary towels, filtration applications, disposable diapers [64], etc. Figure 6 represents the scheme for the preparations of the PBS-based films having quercetin incorporated into it in different concentrations and utilize these films for food packaging applications. Although PBS is regarded as an excellent alternative to the commercial non-biodegradable plastics, certain issues such as poor barrier properties, lower

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Fig. 6 Preparation of quercetin modified PBS thin films with excellent barrier properties and lower mechanical properties for food packaging applications. (Adapted with the permission from Ref. [68], Copyright 2021, CC BY 4.0, MDPI)

modulus of elasticity (500 to 700 MPA) [65], and slow degradation rate (particularly in the natural environment) are needed to be removed for further improving the practical applicability of PBS [66]. The high price of PBS (US$ 4660 per ton [67]) as compared to the LDPE price (US$ 1000 per ton [58]) also hinders the large-scale production of the PBS.

Polylactic Acid/Polylactide Bio-based lactic acid acquired via the bacterial fermentation of the sugars or carbohydrates is utilized to synthesize the biodegradable PLA [69]. The utilization of PLA is advantageous for achieving certain sustainability goals (i.e., environmental impact is negligible, reduced carbon dioxide emissions, and reduced dependency on petroleum-based plastics) [70]. The first usage of the PLA was documented in the case of biomedical applications as the PLA is degraded into the lactic acid (a non-toxic compound) within the period of 6 to 24 months [69]. PLA is utilized to manufacture take-away food containers, disposable cups, tea bags [71], flexible degradable packaging, disposable diapers, sanitary towels [72], etc. The PLA packaging improves the packaged foods’ shelf-life due to its enhanced barrier properties and the packaged food remains fresh for a longer period. The synthetic

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Fig. 7 Formation of modified PLA films for food packaging applications. (Adapted with the permission from Ref. [74], Copyright 2020, Elsevier)

protocols and required manufacturing equipment are similar to that of the manufacturing tools utilized for conventional non-biodegradable plastics. Therefore, the PLA can be easily commercially manufactured with slight modifications in the synthetic protocols [73] (Fig. 7). One of the major concerns associated with the PLA is the density values of the PLA (i.e., 1.24 g/cm3) which makes the mechanical separation/recycling quite difficult while processing and the advanced sorting methodologies (such as the near-infrared technique) are required for the PLA usage [75]. These advanced sorting techniques would not be available in low-income countries where hand sorting and density sorting are more prevalent [76]. In addition, the degradation cycle of the PLA is complicated because the degradation speeds of the PLA are faster above the glass transition temperature (55 to 60  C) in the presence of high content of moisture and microbial dose. The degradation speeds are not favorable at lower temperatures than these temperature values, making the PLA’s degradation quite a difficult task. Optimum and modified degradation setups are required to achieve proper composting at home and industrial scales. Synthesizing the blends of PLA with PCL and other copolymers has been known to improve the degradation cycle of the PLA [76]. The production cost of the PLA is also higher (i.e., US$ 3500 to US$ 4500 per ton [58]) as compared to that of the PET (i.e., US$ 1000 per ton). This also limits the commercialization potential of the PLA. Furthermore, the high brittleness (elongation break at 4 to 7%) and lower glass transition temperature (55  C) limits the applicability potential of the PLA as well because PLA cannot be utilized for the applications where higher temperature values are needed (such as automobile industry, a container for the storage of hot drinks/beverages/packaged foods). Therefore,

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blending the PLA with other reinforcing materials is needed to enhance the practical applicability of the PLA [77].

Polycarbonates The PCs are also among the most commercially viable bioplastics that are extensively exploited at the industrial scale for numerous applications [78]. The PCs are known for their high impact-resistance and flammability-resistance values. Consequently, the PCs are widely utilized in the construction sector (for the manufacturing of safety helmets, skylights, windows, power tools, stadium roofs, etc.) and the automobile sector (for the manufacturing of headlamps, bumpers, wheel covers, etc.) [79]. The PCs are also extremely transparent materials, making these materials a potential candidate as an alternative to conventional glass [80]. The conventional glass suffers from low strength and breaks quite easily. The glass prepared from the PCs was extremely stable compared to the conventional glass [81]. Figure 8 represents the PC-based transparent roofs prepared for greenhouses, and it was found that the synthesized roofs were more effective than PE-based roofs. One of the major points of debate in the practical applicability of the PCs is the manufacturing process of conventional PCs which requires the raw materials of phosgene gas and bisphenol-A (BPA). Both these materials are known carcinogenic/ poisonous materials, and occupational exposure of the workers to these chemicals is considered the point of concern whenever the PC manufacturing is involved [83].

Fig. 8 Synthesis of transparent polycarbonate sheets for greenhouses. (Adapted with the permission from Ref. [82], Copyright 2020, CC-BY-NC-ND-4.0, Elsevier)

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Although the production cost of the PCs is quite low (owing to the ease of the processing of the polymers, facile synthetic routes, and moderate operating conditions), phosgene is an extremely toxic material. The bio-based PCs remove all these disadvantages as a safer synthetic route utilizing the renewable feedstock materials used to produce biodegradable PCs [84]. The bio-based PCs market is still in its early stages in terms of its global production, but a huge interest exists at the laboratory scale for PCs production. The Japanese company, Mitsubishi, is the world’s first company to mass-produce stiff transparent PCs, and the market for their transparent roof-based products has been exponentially growing in recent years. Another interesting feature of these bio-based PCs is the thermal and mechanical properties of this material, as the tensile strength value of the bio-based PCs is 93MPa which is higher in comparison to the tensile strength values recorded for petroleum-based PCs (55 to 75 MPa) and fossil-based commercialized PCs (64 to 79 MPa) [85]. Therefore, the bio-based PCs can be utilized to perform all the functions and applications of the conventional plastics more effectively and remove all the negative environmental impacts of the conventional plastics owing to their biodegradable nature [86].

Potential Challenges and Mitigation The commercialization of biodegradable plastics as alternative tools to conventional plastics is a significant prospect that is gaining much attention in recent years, as indicated by the tremendous increase in the global production of these bioplastics [87]. However, certain challenges associated with these materials still need to be addressed to get the bigger picture of this research domain. A comprehensive summary of the limitations associated with the biodegradable plastics is provided as follows [45]: • Understanding the research domain and properly differentiating between biodegradable, non-biodegradable, and compostable plastics • Improving the innate characteristics of the biodegradable polymers to make them comparable with the utility of conventional plastics • Finding a proper compromise between achieving the optimum utility and biodegradation of these biodegradable plastics • Developing an acceptance regarding this new research domain among consumers • Developing an acceptance regarding biodegradable plastics at the governmental level • Development and implementation of global frameworks and declarations These challenges require multi-dimensional and collective efforts from the consumers, manufacturers, governmental stakeholders, and non-governmental stakeholders for their mitigation [57]. Numerous strategies/steps/perspectives that can act as the potential mitigation tools concerning these challenges are discussed below.

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Modification in Synthetic Strategies for Biodegradable Polymers An honest assessment of the innate shortcomings of the biodegradable polymers in terms of their mechanical strengths, production costs, utility, comparative utility against conventional plastics, etc. is essential for sustaining the industrial boom observed in this domain [88]. To address the specific shortcomings of the particular biodegradable polymer (as discussed in the above section), the synthetic protocols are modified to address those shortcomings either by the processes of blending or copolymerization. Both these properties modify the characteristics of the base materials. In the case of the blending, the base matrix of biodegradable polymer is reinforced with the reinforcing material. The reinforcing material is selected so that it should address the shortcomings of the base matrix. For example, Nazrin et al. [89] documented an excellent study where the addition of the different amounts of TPS in the PLA-based matrix was utilized to modulate the properties of tensile strength and flexural strength for food packaging applications. Ahmed et al. [90] added the foamy material of thermoplastic polyurethane as a blending material into the PLA matrix for achieving the enhanced foamability for developing shape-memory foams. Hazer et al. [91] engineered the blend of PLA/PCs, which remarked enhanced thermal stability, flammable resistance, and rigid polymeric materials by utilizing the blending approach. All these enhanced properties are not available in the simple base matrix. Although the blending technique is quite effective, it should be kept in mind that the reinforcing material should also be biodegradable in nature. Most of the academic studies documented in the literature utilize the non-biodegradable reinforcing material as the characteristics enhancer which although provide significant increase in the utility potential but the degradation property suffers a lot in this regard [92]. Therefore, adequate selection of reinforcing material is essential for addressing the limitation of improving the innate characteristics of the base matrix. Similar to the blending, an approach of copolymerization can also be utilized for achieving the same purpose. In copolymerization, two different monomeric units are polymerized in a single pot to achieve a hetero component polymeric material [93]. It is synthetically different from the blending approach as both components (base matrix and reinforcing materials) are separately prepared and are then mechanically mixed to achieve the uniform blend [94]. Moreover, the copolymer amount is substantially greater than the amounts generally utilized for reinforcing material, indicating that proper homogenization, uniformity, and integration are achieved in copolymerization rather than blending [95].

Banning of Problematic Conventional Plastics Banning non-biodegradable plastics is one such perspective that is opted by several other countries to primarily reduce plastic waste, but this approach has also directly resulted in the boom of biodegradable plastics. Banning single-use plastics (non-biodegradable plastics, mulches, and multi-layered plastics) has always been an

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approach presented as an option at the global level. Australia has placed a ban on single-use plastics and has set a target of eliminating their usage by 2025 concerning the National Waste Policy Action Plan-2019 [96]. Canada is also following the example set by Australia and has implemented a ban on single-use plastics in the year 2021 [97]. In the Asia region, India, Bangladesh, and Indonesia have also banned single-use plastics. However, the impact of these bans is only as good as the enforcement of these bans. The single-use plastics are still in use and mismanaged due to the lack of proper enforcement by the concerned authorities [98]. Therefore, the consumers should be given the option of alternative biodegradable plastics in order to replace these unsustainable non-biodegradable plastics. The proper implementation of this ban will develop an acceptance in the general public for these biodegradable polymers and increase the market value of these biodegradable polymers [13].

Implementation of Extended Producer Responsibility The EPR is regarded as a policy initiative that can also be utilized to convince the manufacturers to adopt the biodegradable polymers as an alternative to the conventional plastics [99]. This policy initiative extends the burden from waste management municipalities back to the manufacturer as, according to this initiative, the manufacturer is solely responsible for the synthesized product from the cradle to the grave [100]. Additionally, the manufacturer is also regarded as fiscally responsible for recycling, collecting, treating, and disposing of the product till the proper degradation of the product. This EPR encourages manufacturers to produce sustainable products. The non-biodegradable products are discouraged concerning this policy initiative as disposing of the non-biodegradable product is commercially unfavorable for the industries. Although the major advantage of EPR is the management of plastic waste, making the EPR initiative compulsory for plastic manufacturers also serves as a good mitigation strategy for encouraging the production of bio-based plastics [97]. Moreover, the industrial sector can also be made responsible for developing the modified recycling and degradation chambers for biodegradable polymeric materials.

Implementation of Deposit Refund Schemes The DRS policy is another policy initiative implemented alongside the EPR policy that has indirectly resulted in the increment of the industrial boom of the biodegradable polymers market. In this scheme, the consumer has to pay the advance disposal fees while buying the plastic product, and this disposal fee is returned to the consumer when the used product is returned/collected by the manufacturer. This scheme has been in practice for 40 years, and approximately 350 million people are using this scheme [58]. DRS essentially increases the targeted plastic’s capture rate and reverts the responsibility to the manufacturer for its disposal. This scheme also encourages the manufacturers to adopt biodegradable polymers instead of plastics,

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as disposing of these polymeric materials is quite easy compared to non-biodegradable polymers.

Conclusions Due to their environment-friendly nature and biodegradability, biodegradable polymers are regarded as excellent alternatives to replace conventional non-biodegradable plastics. Although these biodegradable polymers suffer from some innate shortcomings due to their physicochemical nature, the recent improvements in synthetic strategies (such as blending and copolymerization processes) have enhanced the commercial utility of these biodegradable polymers. The PHAs, PLA, PBS, and PCs are among the most commercially viable biodegradable polymers explored in recent times for numerous applications. Due to its biocompatible nature, the PLA has found copacetic potential for medical applications, whereas the PHAs and PBS have exhibited excellent potential for food packaging applications. The mechanical properties of PCs are comparable with that of conventional plastics and industrial as well as automobile sector applications that can be effectively carried out by utilizing this biodegradable polymer. Another challenge, such as developing an acceptance among the consumers regarding the biodegradable polymers, is also addressed by taking policy initiatives at the governmental level in recent times. The EPR and DRS are two successful policy initiatives widely utilized to address this issue.

Future Prospectives The inclusive overview of the studies discussed in this book chapter highlights the need to follow future directions to be implemented shortly. The identified research gaps are documented as follows: • Proper documentation of the market data regarding bioplastic production requires an hour. The country and continent-based analysis is required for ascertaining a clear picture of the bioplastics research domain. • Academic literature regarding the manufacturing cost of bioplastics should be carefully studied to improve the commercialization potential of bioplastics. • Awareness schemes highlighting the negative impacts of conventional non-biodegradable plastics should be highlighted to enhance the consumers’ understanding of the bioplastics. • Novel and facile synthetic methodologies should be explored to enhance biodegradable materials’ innate shortcomings. • A systematic meta-analysis of all the stakeholders (government, global agencies, plastic/polymer manufacturers, municipalities, and consumers) should be performed to develop a multi-dimensional solution to this plastic problem.

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• Special emphasis should be given to selecting the reinforcing materials (for blending assemblies), and copolymers (for co-polymerized assemblies) as these materials should be biodegradable and preserve the overall biodegradation advantage the whole assembly. • Global frameworks and governmental level policy initiatives are required for sustaining the recent industrial boom of biodegradable polymers.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biofibers and Their Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant-Based Biofibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Animal-Based Biofibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biopolymers for Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitin/Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Collagen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyaluronic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elastin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polylactic Acid, Polyglycolic Acid, and Their Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Poly(ε-caprolactone) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Poly(orthoesters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyphosphazene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polydioxanone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Durability of Biocomposite Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The water purification industry promotes biopolymers for health, safety, and ecological consideration. Biopolymers selected for environmental sustainability require lower production rates, encouraging some products to progress using new sustainable polymers and fibers. This chapter is concerns biopolymers, biofibers, and composites for water purification, tissue engineering, and other environmental uses. M. K. Ismael (*) Middle Technical University, Institute of Technology, Baghdad, Iraq e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_15

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Keywords

Biopolymer · Tissue engineering · Biocomposite · Biofibers · Bone scaffolds · Biocompatibility Abbreviations

AM ALP BCs PLGA BTE CNs CNF CNS CS CS/PVP CHS CAD Td ELPs ECM GAGs GO HA HDA HAP HBO MO n-HAP PNS PCL PDS PE PEO PGA PHB PHA PLA PP PPO PS PVC SBAM SFF

Additive manufacturing Alkaline phosphatase cells Biocomposites Poly Lactic-co-Glycolic Acid Bone and tissue engineering Cellulose nanocrystallites Cellulose nanofibers Central nervous system Chitosan Chitosan/polyvinylpyrrolidone Chitosan-silica Computer-aided design Decomposition temperature Elastin polypeptides Extracellular matrix Glycosaminoglycans Graphene oxide Hyaluronic acid Hydrogels Hydroxyapatite Hyperbaric oxygen Methyl orange Nano-hydroxyapatite Peripheral nerve system Poly(ε-caprolactone) Polydioxanone Polyethylene Polyethylene oxide Polyglycolic acid Polyhydroxybutyrate Polyhydroxyalkanoate Polylactic acid Polypropylene Polypropylene oxide Polystyrene Polyvinyl chloride Slurry-based additive manufacturing Solid freeform fabrication

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SLA t-CDM TEM TPP

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Stereolithography Trans-cyclohexyldimethanol Transmission electron microscope Tripolyphosphate

Introduction Biologically produced, renewable materials are gaining traction as an alternative to traditional composite materials sourced only from petrochemical or mineral sources for their composition. For example, biocomposites (BCs) are less expensive and less environmentally damaging and maybe destroyed at the end of their useful life [1]. BCs blend renewable and nonrenewable polymer matrices with natural fibers (biofibers), such as wood or nonwood fibers. In the definition of biofibers [2], plants are a source of fibrous materials such as trees or shrubs. Straw, bast, leaf, seed, or fruit fibers and grass fibers are all examples of natural fibers. Although the most common fibers are jute, hemp, sisal, and coir, practically any cellulosic material maybe employed. For BC reinforcement, straw fibers are a low-cost option that is widely accessible. The forest or recycled materials produce wood fibers (newspaper and magazine fibers) [3]. Today, polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are the most common synthetic resins used in BCs [3]. However, natural materials, including polylactic acid (PLA), polyhydroxybutyrate (PHB), and resins such as cellulose acetate, are used significantly [4]. Renewable resins like amber and balm of Gilead may also be helpful [5]. In Fig. 1, a flowchart for fibers is illustrated. Nevertheless, it is necessary to thoroughly comprehend natural resins’ mechanical and physical properties and their ability to resist deterioration by microbes and animals; additional study is required. Here, we examine the many raw materials utilized in BCs, discuss their impact on the materials’ qualities, and go through the tests necessary to prove that they are up to the task [3].

Biodegradable Polymers Biodegradable polymers stand out in the title of numerous industrial and scientific interests because of their unique properties such as biocompatibility and being environmentally friendly. Biodegradable polymers play a more significant part in consumable parts such as packaging materials, which comprise 60% of plastic products [7]. Polymers have become a substantial concern globally because of their pollution; it takes hundreds of years to break down the polymers into harmless soil components [8]. Therefore, biodegradable polymer safety plays a crucial role in medical applications such as tissue engineering product development. Biopolymer and bioplastic synthesized from Argo waste become sustainable, nontoxic, and alternative to the polymer [9].

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Fig. 1 General classifications of natural fibers. (Adapted with permission from Ref. [6], Copyright 2008, MDPI)

Biofibers and Their Properties The diameter of natural fibers, as well as the length of individual filaments, varies greatly. The size, maturity, and extraction procedures determine the fibers’ quality and rates [10]. The modulus of the fiber reduces as its diameter grows. The internal structure and chemical composition of fibers influence attributes like density, electrical resistivity, ultimate tensile strength, initial modulus, etc. Table 1 compares the qualities of various natural fibers with those of standard synthetic fibers. The angle between the axis and the fibril of the fiber correlates with the strength and stiffness of the fiber; the smaller the angle, the better the mechanical characteristics; the chemical contents and complex chemical structure of natural fibers also have a significant impact on the qualities [1]. Biofibers, or natural polymers, are biodegradable in most cases, but they lack the thermal and mechanical qualities required for engineering plastics. On the other hand, synthetic polymers provide the most significant technical plastics, but they are nonbiodegradable. Biofiber-reinforced synthetic polymers have been the subject of much study and development. Natural fiber composites with nonbiodegradable synthetic polymers may provide a new class of materials. However, they are not entirely biodegradable. Government restrictions and increased environmental awareness throughout the globe have prompted a paradigm shift toward environmentally friendly materials [11]. As reinforcing fibers in thermoplastic and thermoset matrix composites, biofibers obtained from yearly renewable resources have considerable environmental advantages in final disability and raw material utilization [12]. Natural fiber reinforcements currently hold much potential for automakers [1]. Several

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Table 1 Properties for some natural and conventional synthetic fibers Fiber Cotton Jute Flax Hemp Ramie Sisal PALF Coir E-glass S-glass Aramid Carbon

Density (g/cm3) 1.5–1.6 1.3–1.45 1.5 – 1.5 1.45 – 1.15 2.5 2.5 1.4 1.7

Diameter (μm) – 25–200 – – – 50–200 20–80 100–450 – – – –

Tensile strength (MPa) 287–800 393–773 345–1100 690 400–938 468–640 413–1627 131–175 2000–3500 4570 3000–3150 4000

Young’s modulus (GPs) 5.5–12.6 13–26.5 27.6 – 61.4–128 9.4–22 34.5–82.51 4–6 70 86 63–67 230–240

Elongation at break (%) 7–8 1.16–1.5 2.7–3.2 1.6 1.2–3.8 3–7 1.6 15–40 2.5 2.8 3.3–3.7 1.4–1.8

Adapted with permission from Ref. [1], Copyright 2000, John Wiley and Sons

reports on natural fiber composites in automobile applications [13]. There are also various reviews on cellulosic and lignocellulosic fiber-based composites [1]. Compared to traditional reinforcing materials like glass fibers, talc, and mica, biofibers offer many advantages over their conventional counterparts in terms of low cost, low density, high toughness, and acceptable specific strength properties. They also have reduced tool wear and irritation to the skin and lungs and good thermal properties. The hydrophilic nature of biofibers makes it challenging to use them in composites because of the reduced compatibility with the hydrophobic polymeric matrix. Due to the possibility of fiber deterioration and/or volatile emissions that might affect composite qualities, required to process at a low temperature. Most biofibers can only process at around 200 C; however, greater temperatures are utilized for limited periods [14]. The general term “biofiber” encompasses lignocellulose and cellulose from plant biomass and microorganisms such as bacteria and tunicates that create cellulose (sea snails). Hair, feathers, wool, and silk are fibers and other natural materials such as cotton and linen [15].

Plant-Based Biofibers The physical, morphological, chemical, and mechanical properties of cellulose, hemicellulose, lignin, pectin, and other fibers derived from plants and trees are influenced by a variety of variables, including location, the portion of the plant or tree from which the fibers are extracted, and the method of extraction. Lignocellulose fibers are obtained from plants and are available in the form of hair (kapok, cotton, etc.), thick fibers (coir), and bast fibers (jute, kenaf, kudzu, linden, etc.). Plant fibers classify into primary and secondary utility fibers based on their use to the plant or tree. The utility plant fibers most often include sisal, jute, hemp, cotton, kenaf, and

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other fiber-producing plants. Secondary utility fibers are produced by-products of various plants, including banana, pineapple, coir, and oil palm. Plant fibers classify according to their origin and physiological characteristics: bast fibers (flax, hemp, kenaf, jute, and isora, among others), leaf fibers (sisal, abaca, curaua, and palm, among others), seed fibers (cotton, soya, kapok, and Calotropis procera, among others), and grass fibers (bamboo, wheat straw, and bagasse, among others) [16].

Lignocellulose Because of their low density, high specific properties, low weight, low cost, outstanding thermal properties, and biodegradability, the lignocellulose fibers outperform conventional inorganic fibers obtained from fossil fuels. In composites, they may take the place of glass fibers. Many studies have demonstrated that natural fiber-reinforcing phases in composite materials including sisal, flax, kenaf, and bamboo have great potential. Because natural fibers have low moisture resistance, they absorb much water and have poor mechanical qualities and dimensional stability. Alkali is a popular fiber treatment that improves fiber/matrix adhesion and composite mechanical characteristics. Bamboo belongs to the grass family Poaceae, subfamily Bambusoideae, and tribe Bambuseae and is the most prominent member of the tribe Bambuseae. Bamboo is a fundamental natural resource that plays an essential role in people’s everyday lives and cultures in tropical locations, particularly in Asian nations, due to its quick growth, outstanding flexibility, high strength, weight ratio, excellent specific strength, and high specific modulus [17]. Biofibers and their composites include cellulose, hemicelluloses, and lignin from lignocellulose. Lignocellulose is cellulose encased in a lignin matrix and linked by hemicelluloses in Fig. 2. Cellulose is the most common organic polymer on the planet, and its properties are discussed individually below. After cellulose, lignin is the second most prevalent biopolymer in the world. Lignin copolymerization is a random radical copolymerization of sinapyl, coniferyl, and coumaryl alcohol. Most organic solvents cannot dissolve lignin, a hydrophobic material that only dissolves in alkaline solutions. This compound has a high carbon content and low hydrogen content, with the most critical functional groups being hydroxyl, methoxyl, and ethylenic. Extreme condensation is employed to polymerize lignin even further if desired. It keeps the cell wall together; lignocellulose acts as a matrix component. There is more lignin in more demanding plant biomass, which means more lignin. It is lignin that is responsible for the UV degradation of lignocellulosic material. Hemicelluloses are C5 and C6 sugar heteropolysaccharides that are amorphous and hydrophilic. Hemicellulose carbohydrate chains are highly branched. Hemicelluloses have a degree of polymerization of 300–500. Dilute acids, alkalis, and some enzymes may readily hydrolyze hemicelluloses. They degrade lignocellulose because of their hydroxyl and acetyl functional groups [15]. Cellulose The most plentiful raw material on the planet is cellulose. Over a billion tons of cellulose are generated by diverse plants every year. Cellulose is renewable, biodegradable, inexpensive, thermally stable, and lightweight and has various additional

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Fig. 2 Structure of lignocellulosic lignin. (Adapted with permission from Ref. [18], Copyright 2014, Springer Nature)

advantages. Plant cellulose fibers consist of highly crystalline microfibrils known as cellulose nanofibers (CNF), each with its own set of features and sizes. Because of its wide range of applications in medicines, cellulose has recently attracted much interest in its potential use in various products, including paints, food, fabrics, and stretchable electronics. The width of CNF is typically approximately 520nm, with a length of several micrometers. CNF separation includes homogenization, grinding, microfluidization, acid hydrolysis, and oxidation [19]. CNF is not soluble in water but is exceptionally dispersible, creating a viscous solution. That implies the CNF suspension is spinning. The physical characteristics of CNF are anisotropic. Highly crystalline longitudinal elastic material modulus CNF was reported to be about 150 GPa and the transverse elastic modulus of around 18–50 GPa [20]. Although CNF has excellent mechanical qualities, it is too short to be employed as fibers in both strong and ecologically acceptable composites, such as CNF fiber-reinforced composites. As a result, fabricating a long cellulose fiber using CNF is difficult without losing CNF’s superior mechanical qualities. In general, nanofiber alignment

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may increase mechanical properties. Magnetic, electric, shear, and mechanical stretching have all described aligning CNF. The use of shear force to align CNFs is relatively simple and has the potential for large-scale manufacturing. Spinning is an effective and promising method for obtaining uniaxially oriented CNFs. In addition to spinning, mechanical stretching may increase the CNF orientation, resulting in good mechanical characteristics [19]. All plant lignocellulose cell walls consist of cellulose, the primary component. Glucose units are linked head to tail in a linear monosaccharide with the molecular formula (C6H10O5)n. Lignocellulose’s cell walls include mostly crystalline cellulose, with minimal amorphous cellulose. A dense network of hydrogen bonds exists both inside and between molecules to keep the crystalline cellulose chains stable. All cellulose polymorphs are interchangeable because crystalline cellulose’s hydrogen bond network and molecular orientation may alter significantly. Cellulose I, which exists in two crystalline forms in varying amounts, is assumed to make up the vast majority of native cellulose [15].

Cellulose Nanocrystallites Bacteria can produce cellulose nanocrystallites (CNs), lignocellulose, and tunicates (tunicate cellulose). Thin and long crystals, called whiskers, make up the CNs. Acetobacter xylinum has bacterial cellulose on a glucose-rich feeding medium. The protein encases in the matrix that is devoid of lignin and hemicellulose. The bulk density of bacterial cellulose’s network structure is about 1.6 g/cm3. Because it is biocompatible to make technical paper, diaphragms for speakers, and other things for the medical field, these are just some of the high-performance composites. Lignocellulosic sources can make the width of the CNs about 550 nm, and they can make the length of the CNs about 100–500 nm. Tunicate cellulose has a width of 10–20 nm and a length of 500–2000 nm. Bacterial cellulose has a width of 10–50 nm and a length of 100–1000 nm. Figure 3 shows transmission electron microscope (TEM) figures of chosen CNs. A flat surface and a uniplanar-axial orientation in CNs isolated from lignocellulose. Bacterial and tunicate cellulose exhibit twisted

Fig. 3 The network structure of CNs that are separated from one another. (a) Ramie lignocellulose. (Adapted with permission from Ref. [21], Copyright 2016, Elsevier), (b) tunicates. (Adapted with permission from Ref. [22], Copyright 2008, American Chemical Society)

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ribbonlike structures with half-helical pitches of 600–800 nm and 1.2–1.6m, respectively. The modulus of the CNs is about 100–140 GPa. Cotton has a length-to-width ratio of roughly 10–30, whereas tunicate has a ratio of about 50–100. CNs’ unusual mechanical, optical, electrical, and chemical capability demonstrated by their enormous aspect ratio and nanoscale size are not revealed in macroscopic cellulose [15].

Animal-Based Biofibers Animal fibers classify according to their origin: hair and secretions. Because of their exceptional physical, chemical, and mechanical qualities, animal fibers can be employed as reinforcement in polymer matrices [23]. Silk and wool are two of the most often used materials in the textile industry. The wool comes from alpaca, bison, sheep, angora, and other animal hair; silk comes from various insect sources, mostly from butterfly larvae (approximately 14000 species), spiders, and other creepy crawlies (approx. 4000 species). According to recent research, wool-knitted fabrics are utilized as touch capacitances in LED lighting electronic applications covered with silver nanoparticles; biomedical equipment requires further investigation to detect muscle and bone joint motions [24]. Biomedical uses for silks include surgical implants and tissue engineering. On the other hand, spider silk research belays to be more promising than other silk proteins due to its durability and mechanical capability. Soon, synthetic spider silk polymer composites might be employed as biological implants to make durable, highstrength composites. The primary element of chicken feathers is keratin, a by-product waste from slaughterhouses. Antimicrobial, purification, and corrosion purposes resort to the absorbent sponges made from keratin. In recent research, keratin sponges, which are more susceptible to oil absorption, were created. More research into keratin-based materials is needed to develop large-scale composite sponges for cleaning lenses, polished surfaces, and precision engineering applications [25]. Wool is a well-known textile fiber. Wool’s physicochemical properties vary depending on its origin. For example, alpaca, angora, and qiviut wool fiber diameters range from 12 to 29 meters, 12 to 16 meters, and 15 to 20 meters, respectively [26]. Keratin, an animal protein, makes up most fur, hair, and feather fibers. Wool keratin graphene oxide-based composite fibers with a maximum strength of 157 MPa were produced from waste wool and their textile applications as a sustainable waste management solution. In polymer composites, fiber composite might replace natural fibers such as coir, banana, etc. [16]. Figure 4 depicts the keratin structure schematically.

Biopolymers for Tissue Engineering Tissue engineering is a multidisciplinary subject that combines engineering, material science, cell biology, and medical science concepts to construct scaffolds that help sick or injured tissues and organs regenerate. The primary purpose of these scaffolds

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Fig. 4 The keratin structure in human hair. (Adapted with permission from Ref. [18], Copyright 2014, Springer Nature)

is to provide the wounded tissue with temporary structural and mechanical support. These scaffolds should also offer physical (topographic) and biochemical signaling cues that replicate natural extracellular matrix (ECM) architecture to encourage progenitor cells to create functional tissue [27]. Fibrillar elements, grouped into hierarchically ordered constructions, are the primary structural components of ECM in biological tissue. Among the proteins that make up these fibrillar components are type I collagen and fibrin, with diameters 20–200 nm. Fibril structure varies from tissue to tissue to maximize their beneficial mechanical properties. The original tissue’s unique arrangements and biochemical compositions guide cell-mediated tissue regeneration after damage or trauma [28]. By interacting directly with cells or via the ECM, inflammatory, proliferative, and/or remodeling healing processes are targeted by bioactive compounds. They influence cellular signaling pathways, causing critical participants in the healing process such as fibroblasts, keratinocytes, macrophages, and endothelial cells to grow, differentiate, and function [29]. Biopolymers of various origins are among the bioactive materials due to their outstanding biocompatibility, capacity to promote cell development, regenerative potential, biodegradability, and durability. Also, it had a broad range of applications in wound care materials (Fig. 5). The achievements in using bioactive materials address the tissue healing process [30]. Organic substances produced by living organisms are known as biopolymers or natural polymers. Covalent bonds bind repeating units/monomers of amino acids, monosaccharides, nucleotides, or esters to form peptides, polysaccharides, polyphenols, and polyesters. Biopolymers from a wide range of sources, including plant (starch, cellulose, and natural rubber), animal (collagen, hyaluronic acid, chitosan), fungal (chitin), bacteria (bacterial cellulose, exopolysaccharides), and algae (algae

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Fig. 5 The stages of wound healing, as well as the role of biopolymers at each step. (Adapted with permission from Ref. [30], Copyright 2018, Springer Nature)

cellulose, cellulose), are used to meet the needs of a wide range of applications (alginate). Biocompatibility, biodegradability, reduced antigenicity, and renewable nature of these polymers make them superior to synthetic materials in many respects [31]. The central nervous system (CNS) and the peripheral nervous system (PNS) regeneration and repair are significant difficulties in tissue engineering. It is possible to construct tissue substitutes structurally and physiologically comparable to actual tissue by integrating cell biology, engineering ideas, methodologies, and material science. The objective is to restore an injured or sick tissue [32]. It is typical for animals to suffer from long-term functional impairments after sickness or injury since the central nervous system (CNS) and the peripheral nerve system (PNS) both have a limited capacity to recover on their own in humans [33]. These issues required tissue engineering to overcome them. Whether caused by accident or by a disease, neuronal injury may have a substantial or even fatal impact on one’s health. Injured neural systems are complicated to repair because of their complex physiological systems and limited regenerating capacity. Whether synthetic or natural, several investigation on polymers for their ability to heal injured brain tissue and the regaining of cognitive function in patients. If you look at polymers’ form and mechanical characteristics, they are unequaled by other biomaterials like metals and ceramics in terms of their adaptability. In several investigations and discovery, forming polymers into appropriate support structures, such as nerve conduit grafting, scaffolds, and electrospun matrices, is capable of aiding the regeneration of injured neural tissues. On the other hand, synthetic polymers offer more extraordinary mechanical properties and structural stability since they are not natural polymers. As a result, polymeric conduits that mimic the typical physiological environment of healthy

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neural tissues may regulate cell activity and aid in the regeneration of wounded nerve tissues. Collagen polymer conduits intended for regenerating peripheral nerves have previously undergone clinical trials, with promising results, while the bulk of neural tissue engineering applications is now preclinical [34]. Novel materials are utilized to generate implants with specific porosity and controlled chemistry on the site of use utilizing additive manufacturing (AM) and bone and tissue engineering (BTE). In recent decades, AM in bone and tissue engineering has been extensively employed [35]. AM overcomes the limits of conventional production and traditional processing by using computer-aided design (CAD). In 1987, stereolithography (SLA), a 3D printing technique, was first employed to make AM [36]. It is common to associate rapid prototyping with AM processing, where the final product is incrementally built. One of the most common terms used to describe rapid prototyping, which uses computer-generated models to create solid items without any human intervention, is solid freeform fabrication (SFF). Liquid-, solid-, powder-, and gas-based rapid prototyping are all types of rapid prototyping [37]. Slurry-based additive manufacturing (SBAM) is the result of combining two or more fast prototyping procedures. Two of the many subcategories of additive manufacturing (AM) include rapid prototyping and solid freeform fabrication (SFF). AM can control the final part’s chemistry, porosity, and complexity, unlike other additive manufacturing technologies. Since AM is flexible and valuable, natural polymeric systems enhance the usefulness of scaffold-based methods [38]. Specialized responsibility increased for natural materials in the past century since it has a higher biomechanical bonding capacity than bone. Ivory made the first hip replacement in Germany in 1891. Bone plates for fixing fractures were invented in the early 1900s, as were blood vessel replacements and natural prosthetic heart valves in the mid-1900s [39]. Biopolymers derived from nature, particularly those produced by biological processes, are once again a favored alternative for primary bioactive chemicals in medical material applications. Natural biopolymers such as chitin and chitosan, collagen alginate, hyaluronic acid, gelatin, and fibrinogen are used extensively in biomaterial applications [40]. Using natural biomaterials to construct bone and tissue scaffolds is now possible. Traditional processing techniques have offered several chances for creating different forms of bone scaffolds; however, additive manufacturing has grown into a diversified technology platform that allows for the construction of implants and scaffolds of various shapes and sizes. Another recent clinical use of AM of natural polymers in wound treatment is the fabrication of translucent films that employ bioink films [41]. Silk, for example, uses natural polymers in flexible sensors and biodegradable electrical circuits [42]. Through bioprinting, a novel platform for producing skin structures without donor transplants [43]. Natural polymer usage in AM-assisted plastic and reconstructive surgery (Fig. 6) has also brought advancements. The usage of these biopolymers in scaffold processing, in general, will be explored with an emphasis on natural polymer systems in traditional and additive manufacturing processes.

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Fig. 6 (a) 3D image model created a haptic 3D print of the ankle defect made in PLA filaments (adapted with permission from Ref. [41], Copyright 2015, Frontiers in Surgery), (b) TEMPO and C-periodate nanocellulose bioinks were used to build wound dressings with very porous structures (adapted with permission from Ref. [44], Copyright 2015, Hindawi)

Chitin/Chitosan Chitosan (CS) is an N-deacetylated derivative of chitin, a biopolymer composed of monomers of N-acetyl and N-glucosamine. The source of chitin is invertebrates, mushrooms, yeast, green, and algae. Chitin is considered the second most abundant

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biopolymer [45]. The inclusion of N-acetyl glucosamine in chitin and chitosan speeds up tissue healing and prevents the development of scar tissue. These materials used as powder are now films, membranes, and gels in burn control applications [3]. Chitosan in gauzes and sprays has proven to be hemostatic because of its ability to cross-link red blood cells. Due to its ease of processing into film and fiber forms. Nontoxic N-glucosamine, a by-product of the decomposition of the polymers, exists outside of eukaryotic cells in its extracellular matrix [46]. Depending on the molecular weight of chitosan, this biopolymer’s breakdown rate might vary from 3105 to 106 g/mol. Chitosan’s functional and biological performance promotes new processing methods proposed for vascular stents and bone scaffolds. Chitosan-based products may be degraded in specific areas using post-processing procedures. Chitosan scaffolds have also proven to produce calcium phosphate nucleation sites when immersed in simulated human bodily fluid. As a consequence, create a naturally covered HA scaffold. In the biomaterial industry, chitin is becoming more common [47]. Synthesis of nanoparticles, such as chitosan nanoparticles using a variety of ways. Glutaraldehyde maybe used to cross-link chitosan nanoparticles generated by the water in the oil emulsion (W/O) process, followed by cross-linking. Chitosan nanospheres containing 5-fluorouracil, a new anticancer medication, were created in research by cross-linking amino groups of chitosan. It has a remarkable tendency to gel when it comes into contact with particular polyanions, such as chitosan. Ionically linked nanoparticles play a significant role in gel formation by the complexation process by the cationic character of chitosan. Chitosan gelation in the presence of tripolyphosphate (TPP) is suggested by Bodmeier et al. [48].

Collagen About 30 percent of the human body’s protein is collagen, which exists in connective tissue. Strength and flexibility maybe present in all primary tissues that need it. The most common collagen is type I, which accounts for around 80% of all collagens in the human body. As a result of its abundance, type I collagen has been employed extensively in creating biomedical materials [49]. Consequently, the skin, fascia, bone, and tendon are excellent sources of type I collagen because of the high quantities of this natural polymer, and it has various forms, macroscopic bundles, fibers, and fibrils of collagen [50]. Include tendons and ligaments, heavily crosslinked with orientated type I collagen fibrils in the extracellular space. It is possible to improve the cross-linking of collagen in vitro using a variety of well-described physical or chemical methods. Intermolecular cross-links are strengthened when the biodegradation is extended for a period time. That refers to the collagen being less sensitive to enzyme degradation. Collagen’s water absorption capacity decreases, and the solubility reduces, enhancing the collagen fiber tensile strength [9]. Since these chemical modifications are straightforward, type I collagen is utilized in various tissue engineering contexts. For example, when cross-linking, the amine groups on collagen lysine residues maybe employed to connect or sequester active

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compounds. Substrate attachment sites are needed for most cell types in culture to grow, change, replicate, and use energy. That has been known for a long time. Because of collagen and its integrin-binding domains, it is easier to keep attachment-dependent cell types alive in a culture dish (RGD sequences). Examples include fibroblasts that seem to develop in ways that resemble the activity of living cells and have shapes and metabolism that are substantially comparable [51]. Chondrocytes preserve their phenotypic and cellular activity. In light of these findings, type I collagen might be utilized as a scaffold for any number of cell types [52]. Collagen is composed of a triple helix structure held together by hydrogen bonds, with glycine, proline, and hydroxyproline as primary amino acid groups (Fig. 7). These triple helices generate fibrils that are both strong and flexible, and they can be cross-linked to improve the mechanical characteristics of various forms of collagen [35].

Hyaluronic Acid Glycosaminoglycans (GAGs), such as a fourth class hyaluronic acid (HA), exist throughout the body’s extracellular tissues, serving an essential role in lubrication.

Fig. 7 Schematic of triple helix, tropocollagen molecule construction, collagen fibril, and collagen fibril aggregation. (Adapted with permission from Ref. [35], Copyright 2022, Royal Society of Chemistry)

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Because of its biodegradability, biocompatibility, and bioresorbability, HA is widely used in tissue engineering [53]. Neuronal tissue engineering aided by HA promotes the formation of neurites, differentiation, and proliferation on many substrates. Peripheral nerve regeneration and CNS treatments should benefit significantly from using HA hydrogels, which have increased neural precursor survival and proliferation rates [54]. When it comes to treating neurodegenerative illnesses, HA hydrogels provide a novel avenue because of their mechanical qualities that impact the development of brain progenitors. Natural biopolymers, mainly collagen, maybe mixed with HA because of their comparable properties. It is found that neural stem cells in an HA/collagen conduit may improve the repair of a 5mm facial nerve gap in rabbits [55]. The use of HA and chitosan may also aid peripheral nerve regeneration. Researchers employed an injectable biodegradable chitosan/HA hydrogel to regenerate the nerves when treating peripheral nerve crush damage in rats [56]. Hydrogels (HDA) and biodegradable synthetic polymers (PLGA and poly-L-lysine) have proven to work well together to manage the distribution of drugs that aid in axonal regeneration after spinal cord injury in further investigations as well [57]. The biocompatibility of HA has proved excellent in brain tissue engineering and valuable in reducing the inflammatory reaction triggered by electroconductive polymers. A chitosan/gelatin scaffold including HA nanoparticles doped with PEDOT has shown remarkable PC12 cell adhesion and proliferation [58]; conjugates of pyrrole and HYA, on the other hand, showed great promise in protecting conducting electrodes against glial response during implantation [34].

Elastin Tissue engineering is taking notice of the remarkable properties of elastin-based biomaterials. Elastin is the principal extracellular matrix component (ECM) protein for tissues and organs that need elasticity. Structural proteins like elastin are known for elongating and self-assemble over time. Applicable for blood vessels, elastic ligaments, lungs, and skin. Elasticity effects maybe utilized by incorporating elastin into biomaterials; therefore, its most prominent uses are for skin and blood vessel regeneration [59]. Although elastin polypeptides (ELPs) have been utilized in brain tissue engineering, they have specialized uses. ELPs maybe used as robust drug delivery methods for the brain to enhance the biocompatibility and durability of polymeric structures. Thermally sensitive and passively targeting particular parts of the CNS for therapy of neurodegenerative illnesses maybe achieved by tailoring ELPs [60]. A drug depot made of ELPs and neurotrophin controls drug delivery and reduces the loss of neurotrophins due to diffusion [61]. As a result, intranasal administration of CNS-targeting medicines was made possible by ELPs’ ability to be tunable. Although elastin is not typically used in brain tissue engineering, ELPs have recently been researched for novel drug delivery strategies. They have shown promising results for thermal inhibition of neurodegenerative diseases. Indicates that Elastin has a role in brain tissue engineering and that its applications maybe broadened to include a range of devices and regeneration processes [34].

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Fig. 8 Structure of PLA, PGA, and their copolymer PLGA, n represents the number

Polylactic Acid, Polyglycolic Acid, and Their Copolymers Polylactic acid (PLA), polyglycolic acid (PGA), and their copolymers poly(lacticco-glycolic acid) (PLGA) are among these synthetic biodegradable polymers (Fig. 8). PGA is the most hydrophilic of the linear aliphatic polyesters in this class. Inorganic solvents cannot dissolve it since it is very crystalline; therefore, its melting point and solubility are high and low, respectively. The first completely synthetic absorbable suture was developed using PGA [62]. When it comes to surgical sutures, PGA crystallinity is usually in 46–52 percent. Because of their hydrophilic nature and quick water absorption, PGA surgical sutures lose their mechanical strength after 2 to 4 weeks of insertion [63]. Researchers conducted a thorough analysis of PGA-polylactic acid (PLA) copolymers to widen the spectrum of applications for PGA’s material features. Vicryl is a brand name for sutures made of glycolic acid and lactic acid copolymers that are now available. PLA is more hydrophobic than PGA because it has one additional methyl group. High molecular weight PLA’s hydrophobicity restricts thin films’ water absorption to around 2% and leads to a lower rate of backbone hydrolysis than PGA [63]. PGA is less soluble in organic solvents than PLA, so PLA is better. It is essential to highlight that the physicomechanical characteristics of their copolymers are not linearly related to the ratio of glycolic acid to lactic acid. Crystallinity is quickly lost in PGA-PLA copolymers, unlike PGA, which has a high crystallinity. Hydration and hydrolysis rates rise because of these morphological alterations. Because copolymers deteriorate more quickly than either PGA or PLA, they are more susceptible to degradation [9]. Particulate drug delivery methods also employ a copolymer that combines polyethylene oxide (PEO) with polypropylene oxide (PPO). Because PEO-PPO block copolymers are amphiphilic, they can form stable micellar systems in the presence of water because of their unique behavior. These polymers have been extensively explored as hydro-solubilizers [64]. Anticancer drugs, nucleic acids, and proteins may all be transported by these supramolecular block copolymers based on

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Fig. 9 Attached ligand and drug load inhibit self-assembly and micelle formation. (Adapted with permission from Ref. [48], Copyright 2021, Elsevier)

polyethylene oxide and polylactic acid. These materials can provide precise medicine targeting and a controlled-release pattern by emulating biological transport systems [65]. The diagram (Fig. 9) demonstrates the production of micelles by the self-assembly of ligand-attached block copolymers containing medicines or biomolecules [48].

Poly(ε-caprolactone) Poly(ε-caprolactone) (PCL) is made by polymerizing caprolactone of metal alkoxides as catalysts (such as tin octoate) [66]. Although PCL has been widely used as a scaffold matrix material in tissue engineering [67], its slow degradation and poor water solubility restrict its usage in particle delivery systems [68], where it is a promising drug delivery method [69]. However, by incorporating PLA and PLGA into PCL, the degradation of block polymers maybe improved [70]. PEG, PAA, PNIPAAm, and other hydrophobic polymers maybe coupled with hydrophilic PCL to create polymeric micelles. It is possible to tailor PCL block copolymers’ physical and mechanical characteristics by adjusting their polymer ratios [71]. In the biomaterial field, poly(ε-caprolactone) (PCL) (Fig. 10) has been extensively studied. The fact that microorganisms may destroy PCL led to its study as a biodegradable packaging material [72]. The PCL reveals after hydrolytic degradation. These low molecular weight fragments are taken up by macrophages and destroyed intracellularly, like other poly(hydroxy acids) [73]. Enzymatic degradation of cross-linked PCL may lead to “enzymatic surface erosion” under certain situations [74]. PCL degradation is much more gradual than that of PGA or PLA. Because of this, PCL is ideal for long-term implanted devices like Capronor, a contraceptive implant that maybe used for 1 year [72]. Many unique features

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Fig. 10 Chemical structure of PCL; “n” denotes number

distinguish poly(ε-caprolactone) from other aliphatic polyesters, its melting and glass transition temperatures of 62 and 57  C. Poly(ε-caprolactone)’s exceptional heat stability is another oddity. Its decomposition temperature (Td) is 350  C, which is more characteristic of poly(orthoester) polymers than aliphatic polyesters, while the other examined aliphatic polyesters had Tds of between 235 and 255  C [75]. Sixty degrees Celsius is the glass transition temperature of PCL; hence, PCL is a semicrystalline polymer. The rubbery nature of PCL is maintained at room temperature. A unique feature for aliphatic polyesters contributes to PCL’s excellent permeability to a wide variety of medicinal medicines [76]. Other polymers blended with PCL include nylon, polyester, and polycarbonate. The copolymerization of ε-caprolactone with various additional monomers (e.g., ethylene oxide, chloroprene, THF, valerolactone, 4-vinylanisole, styrene, methyl methacrylate, and vinyl acetate) is also possible. Lactic acid and ε-caprolactone copolymers have been intensively explored. PCL and copolymers with PLA were electrospun to generate nanofibrous tissue-engineered scaffolds with potential for vascular applications [77]. As part of the Capronor, considerable research into PCL’s toxicity was conducted. The monomer caprolactone and the polymer PCL have undergone many tests and are now considered harmless and tissue friendly. As a result, Capronor’s clinical trials are presently underway [78]. It’s noteworthy to note that despite its flexibility, PCL has only been studied for controlled-release medication delivery applications. Considering PCL’s widespread usage in Europe as a biodegradable staple, it’s reasonable to assume that it (or PCL blends and copolymers) may find utility in medicine in the future as well. Pitt [79] conducted a complete assessment of PCL’s current state (1990).

Poly(orthoesters) Since 1990, poly(orthoesters) have been researched as a series of synthetic degradable polymers (Fig. 11). It is possible to design poly(orthoester) devices to deteriorate just at their surface rather than breaking into fragments so that the device becomes thinner over time rather than crumbling into smaller bits. Several examples of controlled-release medication administration using poly(orthoesters) are in the literature. This interest is mirrored in the numerous descriptions of these uses in the scientific literature [80]. In terms of polymers, there are two main categories of poly (orthoesters). Alzamer and Chronomer, two of the first commercial names for poly (orthoesters), were made by condensing 2,2-diethoxytetrahydrofuran with alcohol.

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Fig. 11 The exact composition of poly(orthoesters) given here is a terpolymer of 1,6-hexadecanol (HD), trans-cyclohexyldimethanol (t-CDM), and DETOSU

When these polymers are hydrolyzed, acidic by-products act as autocatalysts, speeding up the breakdown. It wasn’t until 1980 that Heller et al. [81] created a novel form of poly(orthoester) based on the reaction of DETOSU with different alcohols that were able to produce the poly(orthoester). As a result of their lack of acidic by-products during hydrolysis, these poly(orthoesters) do not display autocatalytically rising breakdown rates [77]. Since 1970, poly(orthoesters) have been developed as a class of synthetic biodegradable polymers [9]. So that only the surface of the polymeric device degrades, it is possible to construct poly(orthoesters) devices so that they only undergo erosion at their surface, rather than collapsing into pieces over time. Poly(orthoesters) seem to be especially helpful for controlled-release medication administration because of surface erosion; slab-like devices release pharmaceuticals incorporated in the polymer at a steady pace [82], as evidenced by the numerous descriptions of these applications in the literature [83]. Polymers maybe divided into organic and inorganic orthoesters. Condensing 2,2-diethoxytetrahydrofuran and a dialcohol resulted in the first poly (orthoesters) [84], and Chronomer and Alzamer are trademarked names for the products. Because acidic by-products are generated during the hydrolysis process, these polymers degrade at an increasing rate over time. Heller et al. [81] recently made a novel form of poly(orthoester) by reacting 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro [5,5]undecane) (DETOSU) with a variety of alcohols. Due to the absence of acidic by-products during hydrolysis, this poly(orthoester) does not display autocatalytically rising breakdown rates. By choosing diols with varying degrees of chain flexibility, polymers ranging from hard, brittle, to gel-like materials may be synthesized. Between 2000 and 2005, phase 2 clinical studies examined a drug delivery system for mepivacaine (a postoperative pain medication), while phase 3 clinical trials tested a fourth-generation poly(orthoester)-based drug delivery system for the prevention of chemotherapy-induced nausea and vomiting as of 2012 [81].

Polyphosphazene Polyphosphazene is an artificial polymer in which nitrogen atoms are connected to phosphorus atoms by single and double bonds of alternating lengths [85]. Due to two substitution sites (two phosphorus side groups), as shown in Fig. 12. In this polymer,

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Fig. 12 Polyphosphazene structural unit of poly (organophosphazenes)

side chains may be attached to control the biodegradation process [86]. Furthermore, the polyphosphazene may be modified by adding targeted ligands to the drugpolymer system [87]. As a result of this polymer’s ease of manipulation, researchers have created controlled-release devices to deliver medications such as insulin [88]. Several investigations have shown that polyphosphazene has adjuvant immunological properties and perhaps constructs particulate systems to provide antigenic compounds [48]. In contrast to the typically used hydrocarbon-based polymers, polyphosphazene has an inorganic phosphorous nitrogen backbone [89]. As a result, the phosphazene backbone is hydrolyzed to form phosphate and ammonium ions, releasing the side group concurrently. Among the various polyphosphazene produced, those with medicinal promise are replaced with low-pKa amines and activated alcohol moieties [90]. Singh et al. (2006) [91] change the side groups to modify the glass transition temperature, degradation rate, surface wettability, tensile strength, and polyphosphazene’s elastic modulus and allow these polymers to be explored for a wider variety of biomedical applications. Hydrophobic polyphosphazene with fluoroalkoxy side groups is the most widely researched. These materials are attractive because of their predicted low tissue contact, comparable to Teflon. Additionally, aryloxyphosphazenes and their closely related derivatives have been widely investigated. Due to the polyelectrolytic nature, one polymer may be cross-linked with dissolved cations such as calcium to produce a hydrogel matrix [92]. Microspheres of aryloxyphosphazene have been utilized to encapsulate hybridoma cells without impairing their survival or ability to generate antibodies. Poly(l-lysine) interaction resulted in the formation of a semipermeable membrane. Similar materials have been produced and used in blood contact and new drug delivery applications [77].

Polydioxanone This poly(ether-ester) is derived from p-dioxanone by ring-opening polymerization (Fig. 13). Due to its in vivo breakdown to low-toxicity monomers, poly

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Fig. 13 Chemical structure of monomer p-dioxanone and polymer polydioxanone

(p-dioxanone) (PDS) has attracted growing attention in the medical and pharmaceutical fields. Because PDS has a lower modulus than PLA or PGA, it was the first biodegradable polymer to be utilized to manufacture a monofilament suture [93]. Poly(p-dioxanone) (PDS) is a moderately soft, fast biodegradable polymer brought to the market in the 1980s as a novel degradable suture material. When weighed (e.g., per 1 gram of implant), poly(p-dioxanone) produces fewer acidic breakdown products than PGA or PLA. That potential benefit for orthopedic applications prompted the creation of tiny bone pins to repair non-load-bearing bone fractures. In the United States, bone pins made of poly(p-dioxanone) are sold under the brand name Orthosorb; in Europe, they are offered under Ethipin. Suture clips are also made of poly(p-dioxanone). Regrettably, poly(p-dioxanone) is a relatively soft material that lacks the rigidity and strength necessary for most orthopedic applications. As a result, its applicability range is restricted. Poly(p-dioxanone) has been studied in academic labs primarily as a polymer for implantable drug delivery applications. Some research investigate this polymer as a scaffold for tissue engineering, most likely because it provides no evident benefits in cell adhesion and tissue compatibility over the commonly utilized polylactic acid and lactic acid-glycolic acid copolymers [9].

Durability of Biocomposite Polymers Polystyrene (PS), PP, PE, and PVC are standard components found in many composites, all generated from petrochemicals and come with significant environmental consequences. These materials have a robust biological resistance but a low photochemical disintegration rate for long-term outdoor exposure. Although BCs are better for the environment, they have poorer durability; high moisture absorption, which promotes fiber swelling; a lower impact strength; and a greater sensitivity to photochemical deterioration than other materials. Even though product decay resistance is an essential aspect of the performance of these materials, very few researchers have examined how long these materials may be expected to last. However, the durability of biofiber/biopolymer composites has received very little

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research attention [3]. For example, field and laboratory studies were used by Sistani [3] and Bari et al. [94] to study the biological degradation of composites. According to these investigations, plastic-based composites are naturally resistant to physical assault, but deterioration is feasible under the right circumstances. Other materials are blended with natural fibers such as hemp, kenaf, jute, bamboo, and biopolymers such as PLA, polyhydroxyalkanoates (PHAs), polyhydroxybutyrate (PHB), and chitosan, to make biological composites. These polymers were dubbed “biodegradable composites” since they are biologically derived building blocks and the fibers they contained were both natural or environmentally friendly. PLA, for example, is based on the kernels of maize. The mechanisms of degradation are at the heart of the BC durability debate. Water is essential for the development and destruction of all cellulose-degrading organisms [3]. Cellulosic materials in their natural state are all hygroscopic, meaning they can absorb water easily. By reducing the pace at which BCs absorb water, the inherent moisture resistance of synthetic or bioplastics may significantly lower the rate at which these products degrade [95]. On the other hand, ultraviolet radiation can cause significant damage to both synthetic and natural textiles. When it comes to biodegradability, any BC must prevent photodegradation of the plastic component and biological attack on the cellulose. While photostabilizers and ultraviolet inhibitors may help reduce the chance of plastic degradation, wood presents a more challenging issue [3]. Biodegradation protection can be achieved in one of four ways: 1. By reducing the enzyme accessibility to the fibers 2. By altering the substrate-specific configuration so that enzymes no longer recognize the polysaccharide polymers in the fibers 3. By removing the biodegradation-sensitive components 4. By decreasing the hygroscopicity of the fibers However, the decomposition of these materials should occur naturally, with no harmful effects on the environment. Microbial degradation of these composites has not been extensively studied. According to many experts, studies have shown that BCs disintegrate entirely toward the end of their service life [1]. Two reports on biological composites are available, however. This research resulted in a variety of bamboo/PLA composite boards. These materials were tested for their physical, mechanical, chemical, and biological qualities. In a 60-day experiment, these materials were exposed to three wood-decaying fungi, yet only the mixed composites were degraded, but not the pure PLA polymer composite. After 1500 hours of immersion, water absorption and thickness swelling were close to zero. Despite these findings, additional study is required to understand better the long-term durability of PLA-based composite materials [3]. On the other hand, much research is being done to enhance chitosan and increase the number of active sites by adding organic or inorganic nanomaterial. Pan Hu et al. [96] created a cross-linked quaternized chitosan/bentonite composite to remove methyl orange (MO) from wastewater. Because of the cross-linked structure of

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Fig. 14 Morphology of nanocomposite of CHS/SiO2, (a) chitosan (CH), (b) chitosan-SiO2 nanocomposite (CHS). (Adapted with permission from Ref. [97], Copyright 2022, IOP)

chitosan/B-cyclodextrin composite, the cross-linked composite has a high selectivity for the methyl orange (MO) dye [97]. After combining chitosan with silica, chitosan’s adsorbent capabilities were enhanced, making it more appropriate to remove MO dye. The elimination effectiveness of MO was shown to rise when the contact duration and chitosan-silica (CHS) dose were increased as the starting MO concentration was increased. Simple filtering removal and a high degree of stability throughout the adsorption process may be attributed to the SiO2 and CHS compositions. In Fig. 14, CHS/SiO2 nanocomposite seems to be fused and decorated by SiO2 [97]. Adsorbent’s protonated amino group and the amino group’s interaction with anionic dye molecules, which favors adsorption, were both thought to be responsible for the high removal efficiency in the acidic pH range with high-velocity water [98]. Several studies have shown that the three components of nano-biocomposites mix smoothly under controlled conditions, resulting in the formation of thick films with a high level of quality, a smooth surface, and exceptional elasticity. The fabrication of nano-biocomposite films constructed of chitosan/polyvinylpyrrolidone (CS/PVP) and graphene oxide (GO) has been shown effective. Structural and morphological analyses indicated that the GO sheets are evenly distributed throughout the CS/PVP mix, resulting in solid interfacial contacts that promote load transfer between the polymer chains and the GO sheets, thereby improving the properties of the final product. The addition of 0.75 and 2% wt. GO improves the water resistance of the CS/PVP mix and limits hydrolytic breakdown. Modulus, strength, elongation, and toughness of nano-biocomposites are all enhanced simultaneously [99]. Its successful use of antibacterial nano-biocomposite scaffolds composed of chitosan, carboxymethyl cellulose, and zinc- and Fe-coated hydroxyapatite (HAP) for bone tissue engineering. Polymeric scaffolds are one of the most extensively researched biomedical systems. Nonetheless, their enzymatic breakdown and lack of antibacterial activity pose significant difficulties for researchers. In this context,

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research developed Fe(III)-doped ZnO-integrated hydroxyapatite ZFHAP nanoparticles and chitosan-carboxymethyl cellulose-ZFHAP scaffolds for bone tissue engineering. Porosity (60–80 percent), swelling (21–50 percent), and enzymatic degradation (21–50 percent) of the nano-biocomposite scaffolds were all dependent on the ZFHAP concentrations in the nano-biocomposite scaffolds. The scaffolds were formed inflated and deteriorated linearly. A 5 percent ZFHAP concentration in the scaffolds (SCA-5) resulted in pore sizes of 92 microns and a half-life of 42 days for 50 percent degradation [100]. Lyophilization, nano-hydroxyapatite (nHAP), and nano-hydroxyapatite/ carboxymethyl-chitosan/zoledronic acid (nHAP/CMC/ZOL) biocomposite for bone tissue engineering. All these techniques are successfully applied in bone tissue engineering. The biocomposite increased the activity of alkaline phosphatase cells (ALP) and accelerated the mineralization of osteoblast cells in vitro. Hyperbaric oxygen (HBO) treatment seems to have had a more beneficial impact on the production of new bone than conventional therapy. A favorable impact of HBO on bone repair around the implant has been shown. HBO’s potential to increase the partial oxygen pressure across the implants early, thereby increasing the transmission gap between oxygen and tissue and local oxygen levels, promoting osteoblast growth and related cytokines and enzymes and reducing the risk of implant failure. Various bone grafts and a firm border layer may be explored and other procedures. It is also possible to notice the mending pattern when the healing intervals are longer than usual.

Conclusion One of the primary benefits of natural composites is their environmental friendliness since they are composed entirely of natural fibers and biopolymers and may be decomposed at the end of their useful life. These characteristics would mitigate the possible negative consequences of adopting composite materials. However, a critical aspect of their utilization will be ensuring that they are resistant to physical and biological deterioration but can be decomposed efficaciously once they are no longer helpful.

Future Prospective For oral administration, the delivery mechanism should transport medications and antigens in stable forms and safeguard them from degradation in the harsh environment of the stomach and intestinal lumen. In this context, biodegradable polymerbased particle systems derived from natural and synthetic polymers have emerged as a potential and safe option for medication and vaccine administration. These formulations outperform conventional ones in targeted administration, controlled release, dose reduction, and stability of biomolecular medicines. Advances in particle systems and the availability of biodegradable polymers with unique features such as

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innate immune control, biocompatibility, mucosal adherence, and low toxicity have made it feasible to develop safe and effective carriers. Most of the time, the immunogenicity of polymer-based particulate systems that include or serve as adjuvants exceeds that of antigen alone. Despite their many benefits, biodegradable polymers have significant drawbacks, including production difficulties, unpredictable drug release kinetics, and limited organic solvent solubility. As a result, much terrain remains unexplored to expand the use of particle systems. With developments in technology and more excellent knowledge of the physicochemical characteristics of polymer-based particle systems and the processes behind immune system activation or therapeutic response, significantly more effective medication and vaccine delivery systems may be devised.

Cross-References ▶ Biodegradable Food Packaging Materials ▶ Biodegradable Materials from Natural Origin for Tissue Engineering and Stem Cells Technologies ▶ Biodegradable Nanocelluloses for Removal of Hazardous Organic Pollutants from Wastewater ▶ Biodegradable Nanocomposite as Advanced Bone Tissue Scaffold ▶ Biodegradable Polymers ▶ Biodegradable Polymers for Cardiac Tissue Engineering ▶ Biodegradable Polymers in Biomedical Applications: A Focus on Skin and Bone Regeneration ▶ Ecological Sustainability of Biodegradable Materials for Food Healthy Storage ▶ Hybrid Biodegradable Polymeric Scaffolds for Cardiac Tissue Engineering

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94. Bari E, Sistani A, Taghiyari HR, Morrell JJ, and Cappellazzi J (2017) Influence of test method on biodegradation of bamboo-plastic composites by fungi. Maderas. Ciencia y tecnología 19 (4):455–462 95. Wei L, McDonald AG, Freitag C, and Morrell JJ (2013) Effects of wood fiber esterification on properties, weatherability and biodurability of wood plastic composites. Polymer Degradation and Stability 98(7):1348–1361 96. Hu P, Zhang L, Wang J, and Huang R (2017) Removal of methyl orange from aqueous solution with crosslinked quaternized chitosan/bentonite composite. Desalination and Water Treatment 80:370–379 97. Mohammed MI, Ismael MK, and Gönen M. Synthesis of Chitosan-Silica Nanocomposite for Removal of Methyl Orange from Water: Composite Characterization and Adsorption Performance. in IOP Conference Series: Materials Science and Engineering. 2020. IOP Publishing. 98. Agarwal S, Sadegh H, Monajjemi Majid, Makhlouf ASH, Ali GAM, Memar AOH, Shahryarighoshekandi R, Tyagi I, and Gupta VK (2016) Efficient removal of toxic bromothymol blue and methylene blue from wastewater by polyvinyl alcohol. Journal of Molecular Liquids 218: 191–197 99. El Achaby M, Essamlali Y, El Miri N, Snik A, Abdelouahdi K, Fihri A, Zahouily M, and Solhy A (2014) Graphene oxide reinforced chitosan/polyvinylpyrrolidone polymer bio-nanocomposites. Journal of Applied Polymer Science 131(22) 100. Saxena V, Hasan A, and Pandey LM (2021) Antibacterial nano-biocomposite scaffolds of Chitosan, Carboxymethyl Cellulose and Zn & Fe integrated Hydroxyapatite (Chitosan-CMCFZO@ HAp) for bone tissue engineering. Cellulose 28(14):9207–9226

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Farida Ah. Fouad, Donia G. Youssef, Fatma A. Refay, and Fakiha El-Taib Heakal

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis and Fabrication Methods of Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation Methods of Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intercalation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Melt Intercalation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Situ Polymerization Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol-Gel Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct Mixing of Polymer and Nanofillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Melt Compounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solvent Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer Nanocomposite Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical and Dynamic Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Properties of Polymer-Based Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer–Nanocomposite Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biocompatibility and Non-toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation by Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodeterioration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

354 357 358 359 360 360 361 362 363 363 364 364 366 366 367 368 368 370 370

Farida Ah. Fouad, Donia G. Youssef and Fatma A. Refay contributed equally with all other contributors. F. A. Fouad (*) · D. G. Youssef Biophysics Department, Faculty of Women for Arts; Science and Education, Ain Shams University, Cairo, Egypt F. A. Refay Chemistry/Micro-biology Department, Faculty of Science, Cairo University, Giza, Egypt F. E.-T. Heakal Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_17

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Bio-fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Assimilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation by Body Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Decomposition Rate of Biopolymeric Substance . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical and Enzymatic Oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymatic Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymatic Hydrolysis Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of Enzymatic Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of the Biodegradation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of Polymer-Nanocomposites Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/Organophilic Montmorillonite Nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Polylactic Acid Accompanied by Nanocomposites . . . . . . . . . . . . . . . . . . . . Biodegradation of Poly(ε-caprolactone) Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Graphene Oxide-Bio-chitosan Nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . Aliphatic Polyesters Biotic and Abiotic Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Degradation of Poly(hydroxybutyrate) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Polymers Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wound Dressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bone Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan-Based Nanohydroxyapatite Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applicability and Safety of Polymer-Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Nowadays, biodegradable materials are considered the suitable solution for most global problems. Therefore, many types of research were made to study their properties to develop the applied methods, and introducing the concept of biodegradable polymer nanocomposites leads to the improvement of several applications like wound dressing, drug delivery, bone tissue engineering, etc. The biodegradation process generally breaks an extensive material into simpler and less complex substances, and the prefix bio refers to the reliance on vital ways. The breakdown of the material may occur through microbial enzymes, as it is one crucial biological method that analyzes polymeric materials in the environment owing to the dependence of microbes on them as a source of their nutrition. Decomposition may occur by enzymes located in the body due to their ability to catalyze the breakdown of materials. The presence of additives such as nanomaterials with the polymer affects the rate of its decomposition and the non-biotic factors present, affecting the biodegradation rate. Keywords

Polymer nanocomposites · Fabrication methods · Characterization · Biodegradation · Enzymes · Mechanisms · Biocompatibility · Applications

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Abbreviations

AFM ASTM BTE CNT CS CS-GO DMA DSC ECM EDAX FTIR HA H2O2 HOCl HPLC JCPDS LDH LDPE MS MWCNT nHA NMR OMMT PCL PDLLA PEEK PEO PGA PHB P3HB PHBV PHV PLA PLG PLGA PLLA PLS PNCs PS-CNTs ROS SAXS SEC SEM

Atomic force microscope American Society for Testing and Materials Bone tissue engineering Carbon nanotubes Chitosan Graphene oxide-bio-chitosan Dynamic modulus analysis Differential scanning calorimeter Extracellular matrix Energy dispersive analysis by X-rays Fourier transform infrared spectroscopy Hydroxyapatite Hydrogen peroxide Hypochlorous acid High-performance liquid chromatography Joint Committee on Powder Diffraction Standards Lactate dehydrogenase Low-density polyethylene Mass spectrometry Multi-walled carbon nanotube Nanohydroxyapatite Nuclear magnetic resonance spectroscopy Organophilic montmorillonite Polycaprolactone Poly(D, L-lactic acid) Poly(ether-ether-ketone) Polyethylene oxide Glycolic acid Poly(hydroxybutyrate) Poly(3-hydroxybutyrate) Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) Polyhydroxy valerate Polylactides Poly(lactide-coglycolide) Poly(lactic-co-glycolic acid) Poly-L-lactic acid Photoluminescence spectroscopy Polymer nanocomposites Polystyrene-carbon nanotubes Reactive oxygen species Small-angle X-ray scattering Size exclusion chromatography Scanning electron microscope

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Silicon dioxide Tricalcium phosphate Transmission electron microscopy Thermogravimetric analysis Thermomechanical analysis Wide-angle X-ray diffraction X-ray photoelectron spectroscopy X-ray diffraction

Introduction Biomaterials have had wide-ranging applications since the last century; it is defined as the use of a device designed to heal a part of the body or replace it entirely. This is by imitating the organ’s function in a compatible manner, economically and physiologically safe. The origin of biomaterials may be either biologically known as natural or synthetic, but their target is to treat and enhance the healing process. A good biomaterial must be nontoxic, resistant to infections, size, balance weight, and density, and easy to manufacture. The development of the field allows its use in nanotechnology, which helps solve many problems, particularly in the regeneration of tissues making a scaffolding. Most scaffold materials are natural or synthetic polymers; they are characterized by their biocompatibility and biodegradability for a new interconnection between tissue and cells. In general, biomaterials are classified as metals, ceramics, or polymers [1, 2]. Biomaterial classification can be described as follows (Fig. 1) [3]. Polymeric biomaterials consist of long-chain atoms of organic materials. These are two major categories: collagen, chitosan, silk fibroin, cellulose, alginate, fucoidan, and gelatin; or synthetic polymers such as polycaprolactone, polyurethane, and polyethylene. Polymers are widely used because they are easy to manufacture, flexible, and biocompatible, and have a wide range of mechanical, chemical, and thermal properties [3]. The composite textile material is formed of avital matrix or nonliving and reinforcement phases known as avital or avital composite, but if it is made by vital or living and avital (nonliving) materials, it is called vital/avital composite. The avital/avital composites have similarities to polymer composites known to engineers. Polymeric composites are further separated into non-resorbable, fractionally resorbable, and fully resorbable composite biomaterials. The non-resorbable composites are designed not to break down in in vivo environments. The resorbable composites are targeted to lose their mechanical integrity under in vivo conditions. Many studies have been made of composite polymers and their uses [4]. Improvement of the material characteristics at the nanoscale gave the chance to solve many problems in the medical field with high efficiency compared to the conventional therapies, although this may have a dark side due to its side effect. Many methods for nanomaterial synthesis have been used and have two approaches of synthesis: top-down or bottom-up methods [5], as shown in Fig. 2.

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Fig. 1 Biomaterials classification. (Adapted with permission from Ref. [3])

Fig. 2 Nanoparticles synthesis. (Adapted with permission from Ref. [6]. Copyright © 2020, Elsevier Ltd)

Methods may include coprecipitation, hydrothermal synthesis, microemulsion, ultrasound, microwave, spark discharge, inert gas condensation, template synthesis, Laser ablation, sputtering, sol-gel, and biological synthesis [7]. Another important material that has a huge value in nanomedicine and can be more sustainable with higher advantages is nanocomposite. The research group of Toyotas first discovered a nanocomposite in the 1990s. By the time it was developed and became more advanced, it could be used professionally. A nanocomposite is a mixture of polymer and inorganic material at the nanoscale. It is also a multiphase solid material with dimensions less than 100 nm, or structures having nanoscale with a repeated distance between the different phases belonging to the material. This definition may include

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porous media, gels, colloids, and copolymers in the widest sense. However, it is more usually taken to mean the solid mixture of a bulk matrix and a nanodimensional phase(s) with different properties and non-similarities in the structure and chemistry [5]. Their success in many fields shows the need to find or figure out novel nanocomposites with more benefits. The physical and mechanical properties of the novel nanocomposites have been a point of interest for many fields, especially the medical one, as they have higher capacity, bending strength, and flexibility than other composites. Their properties can be categorized depending on both the material used in the nanoscale and the fabrication methods of the nanocomposites [5]. Previous studies have been observed that postimplantation of chitosan-based scaffolds produced no complications such as inflammation or an allergic reaction. Several improvements, like mechanical strength as well as the thermal balance and stability of pure chitosan, can be increased by chitosan-based nanocomposites formulation with natural polymers (collagen), synthetic polymers, or with nanofiller bioactive glass, zeolite, hydroxyapatite (HA), copper nanoparticles, carbon fill, and tricalcium phosphate (TCP). Another example that can be mentioned herein is alginate, which is a type of natural, biodegradable polymers that originated from algae (brown seaweed) and can be chemically modified for being utilized in bone regeneration. Dealing with functional groups like heparin, for example, yields the best results of three-dimensional construction of micropatterning by the adhesion of ligands to promote the cell attachment and for the growth factor. Pure alginate is unstable and fragile, but it gives surprising results by adding divalent calcium to form a nanocomposite at the nanoscale. The novel nanocomposite has new mechanical, physical, and biological properties compared to the pure one. The novel nanocomposite properties gave a new treatment with a new technique. Nanocomposite biomaterials are the relation between the biopolymer and biodegradable in addition to bioactive and resorbable filler at the nanoscale. Types of nanocomposites are divided into ceramic, metals, and polymers, each differing in their uses, properties, and fabrication methods [8]. By taking specifically the polymer nanocomposites (PNCs) to be the point of focus in this chapter, polymer-based nanocomposites are considered multiphase hybrid solid materials that contain in their juncture fillers, at least one phase having a minimum of one coordinate less than or equal to 100 nanometers (nm) diffused within a polymer matrix. Due to the nanometer-sized particles separated in the polymer textile matrix, these nanocomposites exhibit enhanced mechanical, optical, thermal, and physicochemical properties relative to the unmixed polymers or conventional composites with a minimal filler loading, mostly as 5 wt.% or below. Many studies have shown a positive impact on stumbling block properties of polymer nanocomposites after reinforcing with nanofillers. Polymer nanocomposites are mainly made of polymer, textile matrix, plasticizers, nanofillers, and compatibilizers [9]. Also, polymer nanocomposites can be determined by the junction of the dominant length scales related to the polymer matrix, the nanoparticle, or the complex microstructure. The continuous success for PNCs in the long term would allow the break of the work barriers based on the measurable quantities [10].

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Fig. 3 Polymer nanocomposite formation. (Adapted with permission from Ref. [9]. Copyright © 2016, John Wiley and Sons)

A polymer nanocomposite, as shown in Fig. 3, is considered to be a system in which at least one of its fillers has a monomeric scale dimension. Within the polymer matrix, a uniform filler distribution is always required. The formation of a favorable interaction between the polymer and the nanofiller is necessary to avoid phase separation and agglomeration. Besides, for developing and improving the compatibility of the ingredients, two main potential solutions can be used: (i) chemical transformation of one or more of the components or (ii) launching an equivalent compatibilizer [9].

Synthesis and Fabrication Methods of Polymer Nanocomposites Logically, the dispersion of nanofillers is very different from that of traditional microfillers. At the nanometric level, uniform dispersion becomes increasingly difficult. Uniform arrangement and distribution within the matrix are generally required to maximize fillers’ reinforcing effect. A large amount of nanofillers is necessary because the nanoscale dimension exhibits a given volume fraction of fillers in the polymer matrix and a tremendous surface area, which gives rise to a large interface or the

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Fig. 4 The combustion method for the nanomaterial preparation. (Adapted with permission from Ref. [12]. Copyright © 2021, Elsevier)

interphase area polymeric matrices. The distances between inner particle nanofillers are close, which enlarge the van der Waals force or increase the electrostatic interactions between nanoparticles present, making it extremely difficult to add closely spaced. Fabrication methods aim to integrate and stabilize nanofillers into the polymer textile matrix and avoid their tendency to agglomeration. Therefore, various external energy forms are required to control the energy barrier in nanofiller bundles [11]. Several methods have been developed for nanomaterial synthesis, but the main focus was the combustion method as it produces a pure, chemically homogeneous material like phosphorus powder. The resulting product can be used in numerous applications and various filings. This can be observed in Fig. 4 [12].

Preparation Methods of Polymer Nanocomposites As explained above, nanomaterials may be detailed into nanostructured materials and nanophase materials or nanoparticles. Thus, nanostructured materials are described as condensed bulk substances composed of aggregates and combined with the nanoscale phases, where typically nanophase materials are primarily dispersive NPs. The inorganic nanoparticle loading as an additive within a polymer, the textile matrix allows better performance in polymer nanocomposites’ properties than in conventional polymer-filled composites; as a result, thermal resistance, moisture resistance, decreased permeability, flame resistance, charge dissipation, and chemical resistance are all improved. The homogeneous and unitary dissipation of nanofillers allows a very large conformational area per volume between the nanoparticle and the host matrix of the initial polymer. The huge internal interfacial area and the nanoscopic coordinates between nanoparticles distinguish polymer nanocomposites from conventional filled polymer composites.

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Recently, a considerable effort was made in the preparation methods of polymer nanocomposites, and those can be generally described in the diverse preparation processes for the polymer-reinforced nanocomposites, including (1) polymer intercalation from solution, (2) melt intercalation, (3) in situ polymerization, (4) sol-gel process, (5) direct mixing of polymer and fillers, and (6) template synthesis. Besides, the new method like melt blending is commonly used for preparing clay/polymer nanocomposites of a thermoplastic and elastomeric polymeric array. Here, the polymer is melted and mixed with the appropriate amount of the intercalar clay using a Banbury or coating. Melt blending is accomplished in the presence of inert gas, such as nitrogen, argon, or neon. Otherwise, the polymer may eventually be dry mixed with the intercalant, followed by a heating step in a mixer and subject to enough shear to shape the desired clay polymer nanocomposites. Melt blending has more benefits over in situ intercalative polymerization or even a polymer solution. Also used for the nanocomposite fabrications, the polymer is melted into a viscous liquid, and the nanoparticles are dispersed into the polymer matrix by the facility of high shear proportion rate through diffusion at high temperature. Nanocomposites are subsequently manufactured by compression molding or injection molding [13]. The most common fabrication methods can be described briefly as follows:

Intercalation Methods Intercalation methods have four categories: gas phase, electrochemical, wet chemical, and ion exchange intercalations, which can be observed in Fig. 5. These types allow the modification of physical and chemical properties due to the layered intercalation of the used material, especially from the point of view of electronics, superconductivity, catalysis, energy storage or even thermoelectricity, etc. [14]. The manufacturing stages of polymer-reinforced nanocomposites require a homogeneous modification of the nanomaterial used. Intercalation is known as the eco-friendly top-down method. The polymer chain is mainly diffused into layered structures with a respective polymer ratio. The dispersion of nanoplatelet types of nanomaterials into the polymer matrix is essential as it generally modifies and improves the surface energies of polymer properties from the point of view of stiffness, shrinkage, and flammability. Also, the prepared material has a higher storage modulus, increased tensile and flexural properties, heat distortion, temperature, and reduced gas permeability than matrix material or conventional micro- and macro-composite materials. The dispersion of nanoplatelets can be carried out uniformly using one of the following two techniques. Mechanical technique, in which a mixed solution allows the direct intercalation of polymer with particles of nanoplatelets. The chemical approach covers the in situ polymerization method as the nanoparticles are fragmented into monomers followed by the polymerization reaction. An additional polymerization process occurs after the nanoplatelets are dispersed into the polymer so that the nanoplatelets are bloated in the monomer solution, and the polymer formation arises among the intercalated sheets by polymerization method. In other words, the polymer is melted in a

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Fig. 5 Intercalation methods. (Adapted with permission from Ref. [14]. Copyright © 2021, WileyVCH GmbH)

cosolvent, and nanoplatelet sheets are distended in the solvent so that the solutions can be mixed and the polymer chains in the solution displace the solvent after its intercalation into the nanoplatelet layers [15].

Melt Intercalation Method Melt intercalation is primarily used in industry. This method involves mixing nanofillers such as clays, for instance, in the polymer matrix at the melting temperature. Annellation of the polymer mixture and nanofibers occurs statically or even in shear. A similar process known as melt blending allows the melting of polymer powder to form a viscous solution by adding nanofillers into the polymer solution with a combination of high shear rate and high-temperature diffusion. Several techniques may be used to obtain the final shape, such as compression molding, injection molding, or fiber production [15].

In Situ Polymerization Method This was the primary method used to synthesize and make polymer clay nanocomposites under nylon 6. The process is based on the low molecular weight of the

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Fig. 6 Schematic of in situ polymerization method. (Adapted with permission from Ref. [16]. Copyright © 2021, Elsevier Ltd)

monomers in the solution; the monomers can easily seep between the layers, causing the nanofillers’ swelling. The polymerization of the mixture occurs by radiation, heat, initiator diffusion, or even by an organic promoter. Therefore, the intercalated nanocomposite or exfoliated formation is due to the polymerized monomer between layers. A similar process is known as the in situ template, as shown in Fig. 6 [16], in which the nanocomposite layers are synthesized and made in the existence of polymer chains where the polymer matrix and nanocomposite layers are dissolved and dissociated in an aqueous solution, and generally, the gel is refluxed at high temperature. The process leads to the trapping and capture of polymer inside the layers so that the nucleation and growth of layers take place on the polymer chains at high temperatures [13]. The weak point is that polymers can decompose due to the high temperature during the synthesis process.

Sol-Gel Method Based on an opposite concept, sol-gel is a bottom-up method (Fig. 7). The method allows the dispersion of the solid nanoparticles in the monomer solution; consequently, a colloidal suspension of solid NPs (sol) and an interconnecting link between stages (gel) by polymerization reactions are constructed, followed by the hydrolysis procedure [17, 18]. The relation between sol and gel can be observed from the process term. In the first part, the sol describes the solid nanoparticle suspension in the monomer solution, and in the second part, the gel describes the three-dimensional (3D) interconnecting network shaped between phases. The derived polymer can expand through the liquid too. It can serve as a nucleation agent and favors the growth of laminated crystals. During crystal growth, the polymer diffuses between layers, and as a result, the nanocomposite is formed [13].

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Fig. 7 Schematic diagram of the sol-gel method. (Adapted with permission from Ref. [19]. Copyright © 2019, Elsevier Ltd)

Direct Mixing of Polymer and Nanofillers A suitable top-down fabricating polymer matrix nanocomposite is the direct mixing (as shown in Fig. 8) of a polymer textile matrix and nanofillers approach. The method depends on the aggregated nanofillers collapse all along the process. The method covers two broad strategies for mixing nanofillers and polymer. Melt compounding method without any solvents; the polymer can be combined with nanofillers with a transition temperature above the glass. Solvent method/solution mixing involves mixing polymer and nanofillers in solution using solvents [13].

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Fig. 8 Schematic diagram of direct mixing . (Adapted with permission from Ref. [19]. Copyright © 2019, Elsevier Ltd)

Melt Compounding The process involves adding nanofibers to the polymer having a higher glass transition temperature. In this method, the shear stress (hydrodynamic force), induced in the polymer melt by viscous drag, is used to break down the nanofiller aggregates, promoting homogeneous and uniform nanofiller dispersion in the polymer matrix [13].

Solvent Method The process depends on the dispersion of the nanoparticles into a solvent and the dissolving of the polymer into the cosolvent. The resulting nanocomposites are recovered from the solvent using the evaporation of solvent process or by the coagulation of the solvent method in which the shear stresses induced and generated in the polymer textile matrix are reduced compared to that in the process of melt compounding. The nanofillers are pre-dispersed in the solvent by sonication to break down the nanofiller aggregates [16, 19]. The formation of a polymer nanocomposite (PNC) after mixing nanoparticles (NPs) to a polymer matrix aims to modify the microscopic dynamic processes for both kinds of materials to reach a special macroscopic property of the final nanocomposite. Many PNCs sustain the desired characteristics of the polymer such as processability or capacity to process and low mass density, in addition to functional properties like optic and electronic, as they can lead to augmentation of the electrical conductivity of the insulating polymer matrix by introducing a percolated network of silver nanowires, CNTs, or other conductive fillers. The mechanical properties like stiffness, strength, toughness, and stress relaxation are affected by the segmental dynamics, but NPs addition can change the segmental dynamics to obtain various mechanical properties in a controlled way. Therefore, it is essential to be aware of the dynamics of polymers to anticipate, control, and have a clear vision of their final performance. Also, while dealing with a polymer nanocomposite, it is crucial to consider that the sizes of NPs are similar to or smaller than the characteristic size of polymers (Rg) [1–8].

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Polymer Nanocomposite Properties The advantages of the ideal design of nanocomposites include nanoparticles homogeneously dispersed in a polymeric matrix in addition to obtaining tremendous properties such as lighter weight than conventional composites, thermal, mechanical, optical, barrier, flammability, functionalities, and biocompatibility without affecting their processability that match the requirements to be used in various applications with high efficiency [16]. The interactions between nanofillers and a polymeric matrix can impact a huge interfacial area inside polymer nanocomposites by uniform dispersion, as shown in Fig. 9. Interactions in the polymer nanocomposite are curial in addition to reducing interspecific voids to maintain intact matrix components and improving the performance of nanocomposites [20, 21]. Interactions within polymer nanocomposites may exhibit weak or strong forces. Weak forces may be electrostatic, Van der Waals’ sterile, or Lewis’ acid-base interactions, while strong forces are the existing covalent bonding. Additionally, the polymer nanocomposites interaction can be affected by some factors such as intrinsic polymer properties, like net mass, terminal functional groups, polarity, and hydrophobicity; nanofiller properties like type, size, aspect ratio, orientation, distribution, the concentration of polymer, and nanofiller; as well as the type of solvent used. An interface junction is created by doping the nanostructured metal oxide with different morphologies in a conductor polymer with essential features. The interface junction, thus formed, exhibits completely new physicochemical properties due to the participating components, the mechanical strength as the surface area increases, the improved electrical characteristics of the conducting polymers, and the creation of a unique p-n junction with fair electron conductivity [16].

Electrical and Dynamic Mechanical Properties MXenes is a new important member of promising 2D materials. It is a transition metal carbonitrides, carbides, or nitrides with the chemical structure of Mn + 1XnTx

Interface

Interface

Polymer matrix

Diffuse layer

Polymer matrix

Nanoparticles

Nanoparticle surface

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Fig. 9 Schematic polymer nanocomposites interface. (Adapted with permission from Ref. [16]. Copyright © 2021, Elsevier)

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where M can be transition metal elements (such as Ti). Due to their very excellent electrical conductivity (9880 S/cm), the addition of MXene into a polymer matrix produces a significant improvement in its electrical conductivity. For example, Ti3C2Tx/polyethylene oxide (PEO) nanocomposites have been conducive to an excellent electrical conductivity of 210 μS/cm, i.e., an increase of 73.6% relative to the polymer host. Figure 10 shows the various applications of MXenes [22]. Regarding the DMA, it is defined as a function used to indicate the ability of polymer nanocomposites to react with any type of deformation, load, or force, such as tensile, compression, shear, and torsion, in the presence of temperature. Three important aspects obtain DMA outcomes: (a) the modulus storage (E0 or G0 ) being a determination of the elastic reaction and response of polymer nanocomposites; (b) the modulus loss (E0 , G0 ) representing the plastic stage upon the deformation; and (c) the tan or δ that expressing the ratio between the loss and storage modulus [23]. Incorporating nanomaterials in the biopolymer matrix is conducive to very significant t-mechanical properties. For example, carbon nanotubes (CNT) have been used as nanofillers in a polymer matrix for their excellent properties. Incorporating CNT into polymer matrices has been optimized for superior mechanical, thermal, and electrical properties. However, their dispersion in the polymer matrix is not entirely straightforward, as agglomeration usually occurs during processing due to their low interaction with the polymer matrix. Furthermore, the treatment conditions may affect the state of dispersion, so the modification and development of the CNT are necessary significantly. CNT can be modified by chemical methods to add functional groups on their sidewalls, increasing the chemical reactivity among the filler and matrix [24]. The researchers found that surface-functionalized nanofillers and the hybridization of nanofillers successfully improve the features and mechanical properties of chitosan-based biopolymer nanocomposites. CNT may be used as a single CNT or multiple CNT by adding carboxyl or a hydroxyl group. These properties allow its usage in scaffold formation for bone tissue engineering and increase its electrical and mechanical strength. In this

Fig. 10 Surface modification of MXenes allows its usage in various applications. (Adapted with permission from Ref. [22])

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connection, alginate was studied with a multi-walled carbon nanotube, MWCNT alginate solution. The mixture acts as a non-Newtonian fluid by increasing the shear rate at a constant temperature, with no alteration in its viscosity, despite that of the MWCNT-alginate characteristic, where the shear rate increases with the decrease in viscosity [25].

Thermal Stability It has been specified that incorporating nanoparticles into a polymeric matrix improve the thermal stability of polymers. Pure polymers or biopolymers cannot resist high temperatures because of their low coefficient of thermal expansion for dimensional consistency. Chitosan (CS), the most nearly critical derivative of chitin, is obtained by partial deacetylation of chitin in the solid-state under alkaline conditions (concentrated NaOH) or by enzymatic hydrolysis (chitin deacetylase). Because of the semicrystalline morphology of chitin, chitosan obtained by solid-state reaction has a heterogeneous distribution of acetyl groups along the chains. CS cannot cope with high-temperature conditions ranging from 200  C to 220  C. However, CS-based nanocomposites, like calcium carbonate, hydroxyapatite, nanofibrillated cellulose, and nanoclay, have been ensured excellent significant thermal properties. It has been reported that the thermal stability property of poly(ether-ether-ketone) (PEEK) polymer was improved by 40  C when using nanocomposites with alumina or silica nanoparticles. However, incorporating metallic nanoparticles in the polymer, the textile matrix can have special characteristics. For example, incorporation of Cu nanoparticles beyond 2 wt.% levels into low-density polyethylene (LDPE) has been shown to clear deterioration in the thermal stability of LDPE polymer [26].

Other Properties of Polymer-Based Nanocomposites In addition to the abovementioned excellent properties, the presence of nanofillers in a polymer matrix may produce other properties, as described in the following examples. (1) Space, charge reduction feature, exhibited by epoxy nanocomposites is thought to be of utmost importance for improving erosion of nanocomposites, as well as their dielectric strength [21]. (2) Optical clarity properties such as polymerbased nanoclays (butyl rubber, polyamide, and acrylic rubber). These polymer-based nanoclays exhibit significant enhancement in the transparency, haze reduction, and optical clarity in the visible region, owing to the modifications in their crystallization behavior [27]. (3) Tribological properties were also recognized in polystyrenecarbon nanotubes (PS-CNTs) with 0–4.0 wt.% CNT content, which has been prepared by radical polymerization of styrene. CNTs enhance the wear resistance behaviors of the nanocomposites by increasing their microhardness and decreasing friction. (4) Luminescence property of single isolated and outlying semiconducting nanoparticles or aromatic organic molecules is considered primarily important in biological, pharmaceutical, and medical applications. (5) Catalytic properties found

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in thin polypyrrole films based TiO2 nanoparticles with an air-water interface having greater oxidative catalytic activity in the matter of light presence than in aqueous suspension of TiO2 nanoparticles. (6) To catalyze reactions and magnetic properties, a perfluorinated sulfo-cation membrane (MF-4SK) polymer-based nickel nanoparticles with d ¼ 3–6 nm, as well as narrow size distribution, display super magnetic properties. Due to such properties, polymer-based nanocomposites are used in various industrial and biomedical applications [26, 28].

Polymer–Nanocomposite Characterization Various techniques characterize and determine the chemical and physical properties of polymer-based nanocomposites and their structure before being used in applications. In this respect, X-ray diffraction (XRD) is used, where X-rays interfere with the crystalline material, producing a unique characteristic diffraction pattern that may be described as a fingerprint for identifying the solid besides calculating the intrinsic parameters crystalline structure [29]. The X-ray energy dispersive analysis (EDAX) is a nondestructive technique performed to analyze the elemental chemical composition of the material like EDAX can give a simultaneous recording of all elements. X-ray photoelectron spectroscopy (XPS) is an analytical technique for the composite’s chemical composition, oxidation states, and electronic configuration. Nuclear magnetic resonance (NMR) is another method of validation of the structural characteristics of the material. 1H and 13C solid-state spectroscopic analysis are the two common NMR types are used to characterize the organic structure. 1H NMR is generally used to specify the type and the number of hydrogens in a molecule. 13C NMR is also used to obtain information about the carbon-based polymer nanomaterials. Fourier transform infrared spectroscopy (FTIR) reveals the numerous functional and active adsorption sites through interaction between bonds present in polymer nanocomposite and infrared radiation. Raman spectroscopy provides chemical structure, crystallinity, polymorph, phase, and molecular interactions. In addition, various techniques are used to determine other properties such as shape, nanocomposite size, etc. Transmission electron microscopy (TEM) defines the physical shape of nanoparticles. Planes in the crystal lattice, the distance between them related to the d spacing, the orientation, and direction of the planes can be identified by comparing the obtained d spacing values with the Joint Committee on Powder Diffraction Standards (JCPDS) data. UV-Vis-NIR spectroscopy (UV-Vis) enables the determination of the wavelength, the intensity of absorption nearultraviolet and visible light was taken by a sample, and the band-gap energy levels of the composite. A scanning electron microscope (SEM) is used to analyze and study the composite’s surface morphology, quickly visualizing structural changes and effectively testing both the amorphous or crystalline texture of nanomaterials. Atomic force microscope (AFM) is established to get high-resolution HR at the nanoscale images and study localized sites in liquid (electrochemical AFM) or air surrounding (conventional AFM) that is used mainly for micro/nanostructured coatings. Photoluminescence spectroscopy (PLS) is a nondestructive technique

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widely used for semiconductors and molecules to characterize their optical and electronic properties [16, 30]. Wide-angle X-ray diffraction (WAXD) is widely used for probing the structure of nanocomposites and the kinetics of the polymer melt intercalation when layered silicates are used. Small-angle X-ray scattering (SAXS) is a technique for observing structures on a scale of 10 Å or larger. For further and more characterization of polymer nanocomposites, rheometric analysis, thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), thermomechanical analysis (TMA), and dynamic modulus analysis (DMA) have been used. When the nano- and mesoscale polymer structures are combined with TEM, DSC WAXD, TGA, and DMA, they provide a fundamental explanation and understanding of the state and dispersion mechanism of the NPs in the matrix. Understanding the rheological properties of nanocomposites is one of the crucial features in understanding polymer processability and application development [24]. In addition to the characterization techniques, testing and standard tools are used to verify the physical and mechanical properties. Various tests from the American Society for Testing and Materials (ASTM) are used to evaluate the properties of nanocomposites [16].

Biocompatibility and Non-toxicity Biocompatibility is thought to be one of the most critical requirements that polymer nanocomposites must exhibit, especially when used in biological systems such as tissue engineering applications. Typically, a polymer nanocomposite should be selected with nontoxic, nonallergenic, noncarcinogenic, and non-immunogenic properties. Biocompatible materials can interact with the host cells without producing any complications or adverse effects. Consequently, biocompatibility characteristics may be tested under in vitro and in vivo conditions. In an in vitro assay, the experimental material reacts with the cultured cells and determines the toxic properties of the chosen material. In in vivo conditions, rats or rabbits can be used as the material is implanted in the subcutaneous or intramuscular region for a specific duration to observe an interaction between implanted materials and biological system and their response. Chitosan-based nanofibers in the tissue of bone engineering are a well-known example to explain [31].

Biodegradable Polymers Biodegradable polymers (Fig. 11) decompose under specific aerobic or anaerobic conditions such as oxygenation, pH, and humidity. The presence of some metals is required to ensure biodegradation when they are present in the degradation media or environment that contains catalytic substances or molecules such as enzymes and alkalis [31]. The degradation process is dependent on the surface erosion mechanism in which the ions interact only on the surface of the materials, whereas the central part remains as such. In contrast, the bulk erosion mechanism is performed due to the

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Fig. 11 Classification of biodegradable polymers. (Adapted with permission from Ref. [36]. Copyright © 2021, Elsevier Inc.)

lack of catalytic molecules in which biodegradable polymers are, for the most part, degraded. Products resulting from the biodegradation process must be biocompatible, nontoxic, and smaller in size to dissolve in bodily fluids with problems. The rate of biodegradation is one of the crucial parameters that should be considered when polymeric biomaterials are chosen in applications, like tissue engineering or other biological applications, where polymeric scaffolds biodegradation generates suitable space for new tissue and cells to grow [32]. Biodegradable polymers may originate from organic sources (corn, wood cellulose, etc.). Also, small molecules like butyric acid or valeric acid are produced by bacteria, such as polyhydroxy butyrate (PHB) and polyhydroxy valerate (PHV). They can be derived from petroleum sources or obtained from mixed biomass and petroleum sources. Biodegradable polyesters are obtained from different sources; it may be from a natural source (i.e., estolide, suberin, cutin, and PHB), a synthetic (i.e., PCL, PBS), or a renewable source (i.e., PLA, PHB) [31–33]. Aliphatic polyester or aliphatic-aromatic co-polyesters are the most widely known biodegradable petroleum-derived polymers. Recently, biodegradable polymers that originate from renewable resources like polylactides (PLA) have attracted much attention for exhibiting tremendous features such as greater eco-friendliness, maintaining carbon dioxide (CO2) balance after their composing, and reducing CO2 content, which assists in climate protection. Their complete biological degradation minimizes the amount of garbage and produces green materials in agriculture. The most common biodegradable polymers used in various applications are polylactide (PLA), poly(3-hydroxybutyrate) (P3HB), and its copolymers, thermoplastic starch, plant oils, cellulose, gelatin, and chitosan [34, 35]. The process of material degradation may be due to various factors such as photocatalysis, hydrolytic, thermal, and microbial degradation. Furthermore, the substance resulting from the decomposition process has different properties from the originating substance.

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The biodegradation process may occur by microbes in nature, as microbial enzymes that significantly affect the decomposition of polymeric materials. Decomposition also occurs by enzymes in the human body because they catalyze the breaking bond in the substance and turn it into smaller pieces. This is the case for polymers with applications in the human body, such as those used to manufacture a scaffold in tissue engineering applications. Materials resulting from the decomposition process must not cause any harm to the cells. The presence of polymer-related nanomaterials significantly influences the compound’s overall properties, such as antimicrobial properties and water resistance. The concentration of the nanocomposite controls the rate at which its properties affect the matrix [37, 38]. Their presence also affects the decomposition rate of polymers, whether they may increase or decrease the rate of degradation. The next section discusses the process of biodegradation of polymers, whether by microbes or by enzymes in the human body, and explaining the mechanism of action of these degraders, as well as substances resulting from the biodegradation process, and mentions some examples illustrating the process of decomposing polymer nanocomposites [39].

Biodegradation by Microorganisms Microbial enzymes are some of the most important biological factors that decompose polymeric materials into the environment. The process of biodegradation by microbes can be considered in three steps, the result of which is to break the large polymeric material into smaller pieces that the cell can benefit from. The three steps encompass biodeterioration, bio-fragmentation, and assimilation. Figure 12 shows a diagram of the process of microorganisms biodegrading polymers [40]. Enzyme response may result in extracellular depolymerase and intracellular depolymerase enzyme. Exoenzymes can break down polymers into small monomers and dimers used by microbes as a source of energy and carbon. In the case of aerobic conditions, the biodegradation of the polymer produces CO2 and H2O and is accompanied by the presence of the cellular biomass of microorganisms. In the case of anaerobic conditions, the biodegradation of the polymer results in organic acids, H2O, and gases (CO2 and CH4) [40]. A schematic presentation of polymer biodegradation by microorganisms under aerobic and anaerobic conditions is also presented in Fig. 13.

Biodeterioration The term biodeterioration refers to the failure of materials by microorganisms, always being a superficial process targeting the superficial part of the material. It should be noted that, during the degradation process, the progression of deterioration is performed by a physical or chemical method. In other words, microbes have various ways to complete the biodeterioration process, which may be by mechanical, chemical, or enzymatic means. This process is affected by various factors such as

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Fig. 12 Polymer biodegradation by soil microorganism scheme. (Adapted with permission from Ref. [40]. Copyright © 2008, Elsevier Ltd) Fig. 13 Biodegradation of polymers by microorganisms in aerobic and anaerobic conditions. (Adapted with permission from Ref. [41])

humidity, temperature, and the atmosphere. Biodegradation depends on different organisms such as bacteria, algae, and fungi, each of which has its way of decomposing materials. Those organisms work together in the form of a network (biofilm), causing breakage of the large complex materials into simpler ones, and simultaneously use them as a carbon and nitrogen source for their nutrition [40, 41].

Methods of the Biodeterioration Process Physical The adhesion of microbes is regarded as the basis of the physical deterioration process. This is efficient through substances secreted by organisms (polysaccharides and proteins) which allow them to stick to matter. These substances result in cracks and gaps in the material, making them brittle [42].

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Chemicals Microbes are graded according to different characteristics, including the chemicals they use. In this respect, chemoorganotrophic microorganisms’ break down organic matter and then uses carbon as a nutrition source, producing organic acids, such as oxalic, citric, gluconic, glutaric, glyoxalic, oxaloacetic, and fumaric acids. These substances can affect the pH of the media according to their acidity or basicity, which have a role in the degradation process and the ability to interact with the substance and increase the rate of its surface erosion [41, 42].

Enzymes Microbes secrete enzymes, such as lipases, esterases, ureases, and proteases, which break down polymeric materials and convert them into smaller compounds to benefit microbes [43]. The process involves both bulk and surface erosion [44]. Table 1 shows the difference between bulk erosion and surface erosion to explain the material decomposition process.

Assessment of Biodeterioration Use the microscope to follow-up the material in terms of changes in surface roughness, color, and the formation of holes. The microscopes include photonic microscopy, electronic microscopy [32, 45], polarization microscopy, and atomic force microscopy (observation of surface topography of the polymer) [46]. Mechanical changes in tensile strength, elongation, and elasticity are measured based on dynamic mechanical and thermal analyses. Determination of the number of materials generated by the polymer biodegradation process. They are tracing the decrease in the mass of the material. However, it is not very effective, because, in some cases, the decrease in mass may be caused by the disappearance of volatile and soluble impurities [47].

Table 1 Differences between bulk and surface erosion [44] Bulk Fragments are lost from the entire polymer mass. Bond cleavage occurs. Change in mass and molecular weight. Originator: chemicals (e.g., H2O, acids, bases, transition metals and radicals) or radiation. The rate of entry of chemicals into the material is faster than the cleavage rate of polymer bonds.

Surface The matter is lost from the surface. Material conserves the original geometric shape. No change in molecular weight of matrix polymer Originator: enzymes. The rate of polymer bond cleavage is faster than the rate of entry of chemicals into the material.

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Bio-fragmentation The bio-fragmentation process is cutting a large substance into small pieces by breaking the bonds in the substance. This process requires energy obtained from many sources such as heat, chemicals, and light, or through biological means that will be dealt with here in certain details. If the goal of microbes is to break down the material for using it as a source of food, then the material must be in the form of oligomers and monomers that allow them to penetrate the microbe cell walls and cytoplasm. Microorganisms may break down these materials by aerobic (methane production) or anaerobic means (no methane formed), wherein both cases water and carbon dioxide are also generated. The fragmentation process is performed using the enzymes produced by the microbes. These enzymes are mainly proteins acting as catalysts that reduce the chemical reaction’s activation energy. Herein, the enzyme is the depolymerase that breaks the polymer into smaller pieces on which the microbe may or may not have the ability to use it in its metabolism. For example, actinomycetes have a high ability to break down polymers, albeit they cannot use the resulting substances in their metabolism. This process is done by relying on hydrolytic enzymes (hydrolases) and oxidative enzymes (oxidoreductases) [40].

Assessment of Bio-fragmentation This process can be inferred by testing the microbial activity of the materials in the media, which results in the presence of a clear zone around the microbes, evidence of the breaking down of materials in the media. The presence of small pieces with low molecular weight in the media is also indicative of the bio-fragmentation process. It can be inferred through analytical methods including: (1) size exclusion chromatography (SEC) [45], (2) high-performance liquid chromatography (HPLC) to identify monomers in a liquid medium, (3) mass spectrometry (MS) for the intermediate monomers, (4) nuclear magnetic resonance spectroscopy (NMR) to know the shape of the monomer, and (5) Fourier transform infrared spectroscopy (FTIR) to detect any functional chemical changes [48].

Assimilation The end follows the material biodegradation process, where small parts resulting from the biodeterioration and bio-fragmentation methods may enter the cell through the cell membrane. Considering this fact, not all materials can penetrate through the cell membranes, as they must be of an appropriate size to allow them to pass through. Microbes use these small substances to provide energy for their cells to carry out various activities. These activities are assessed by measuring the consumption of oxygen or the evolution of carbon dioxide (in aerobic conditions) [49], as well as the augmentation of the pressure after gas release [50].

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Biodegradation by Body Fluids Biodegradable polymers have many applications in the medical field, such as resorbable surgical sutures, matrices for the controlled release of drugs, scaffolds for tissue engineering, and resorbable orthopedic devices such as bone cement, pins, screws, and plates. These materials are diverse, and their use in a specific application depends greatly on their stability. Besides, they are distinctive in their medical applications depending on their ability to decompose, which makes the time of their presence in the body a specific period, so there is no need for a second surgery to remove it from the body. These polymers break down into the body after performing their intended function; they should be nontoxic and biocompatible [51].

Factors Affecting Decomposition Rate of Biopolymeric Substance The factors affecting the decomposition rate of biopolymeric substances include (1) method and conditions of construction of composites, (2) sterilization of biomedical materials before their use in the human body because heat or high-energy radiation can affect the rate of decomposition of materials, (3) additives also influence the decomposition of polymers, such as materials such as plasticizers, lubricants, antioxidants, salts, and stabilizers added during polymer treatment may increase or decrease the rate of decomposition, (4) mechanical stress is caused by loading under service or by residual stress arising during manufacturing, and this is related to substances which are exposed to mechanical stress such as sutures, scaffolds for tissue engineering, and fixation devices, and (5) adding certain materials to the polymer affects its decomposition; for example, adding sodium, or calcium, or zinc carbonate into poly(lactide-coglycolide) (PLG) may lead to a delay in its decomposition rate resulting from the neutralization of carboxylic acid groups formed during PLG hydrolysis by the added basic salt. Instead, the inclusion of an increasing quantity of epirubicin HCl in poly(lactic-co-glycolic acid) (PLGA) nanospheres can accelerate its rate of degradation [49–51].

Chemical and Enzymatic Oxidations Body fluids can break down biomedical polymers by chemical and enzymatic oxidation. The body releases substances that break down the chains of the polymeric substance and accelerate the degradation process of the polymer caused by its ability to oxidize these substances. Examples of oxidizing substances include superoxide (O2), hydrogen peroxide (H2O2), nitric oxide (NO), and hypochlorous acid (HOCl) [52].

Enzymatic Hydrolysis Enzymes play a role in the catalytic process of the degradation of polymeric materials, and the process is called enzymatic hydrolysis. These enzymes are

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diverse, and all play a role in different decomposing substances. These include proteases, esterases, glycosidases, and phosphatases [37–52].

Enzymatic Hydrolysis Mechanism As a first step, the enzyme moves to the surface of the polymeric material and is attached to the polymer surface (substrate), forming a complex with it (enzymesubstrate complex), then the enzyme starts to catalyze the process of breaking down the polymer over the time. Finally, the breakdown process produces materials released into the surrounding solution, and the enzyme that affects the hydrolysis process is presented in Table 2 [52].

Examples of Enzymatic Hydrolysis Proteinase is an endopeptidase enzyme that hydrolyzes peptides and amides in proteins. This enzyme can accelerate the degradation process of certain polymers such as poly(L-lactic acid) (PLLA) and poly(glycolic acid) (PGA). The enzyme can also analyze these polymers as they are made from materials found in the human body [53]. Lactate dehydrogenase (LDH) is an enzyme that plays a role in converting lactic acid. This enzyme can decompose polylactic acid (PLA) and form monomers and oligomers of L-lactic acid. The process is conducted using NADH-reductase coenzyme. Lipase is an enzyme that can hydrolyze Table 2 Factors affecting enzymatic hydrolysis [37] Biopolymer(substrate) Physicochemical properties (molecular weight, chemical composition, crystallinity, surface area, etc.). Polymer chemical modifications (crosslinking).

Introduction of chemical groups in the polymer chain. Removal of chemical groups in the polymer chain. Hard segments are distributed at the surface of the polymer, where the increase of hard segments can decrease the activity of hydrolytic enzymes.

Enzyme Activity and stability.

Host body The pH and temperature of the medium can affect both the enzyme and substrate.

Saturation of enzyme, as the rate of decomposition, decreases when the polymer is surrounded by the enzyme on all sides. 3D conformation.

Effect of stabilizers, activators, or inhibitory products resulting from the degradation products and additives. Variation of biological systems, e.g., rate of metabolism. Inflamed tissues around the implant. The complex chemical nature of body fluids.

Amino acid composition. Local concentration.

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polycaprolactone (PCL). The analysis process is done by surface erosion of the material according to the nature of the enzyme substance and the substrate. Since PCL is a hydrophobic substance and the enzyme is a hydrophilic one, so the enzyme cannot diffuse into the polymer. Material analysis is performed by breaking the ester bonds present in polyester in a liquid medium [53].

Mechanism of the Biodegradation Process Material decomposition begins with the diffusion of water solution into the polymer matrix, resulting in chemical changes and roughness in the material surface and eventually producing decomposition of the material. When the material starts to break down, the diffused pieces of large molecular weight come out into the solution, leading to a decrease in the molecular weight of the substance. By following the decomposition process of the material, the holes in the matrix increase, which completely affects the material’s mechanical properties. For example, the tensile forces of the material decrease, and the material becomes noncrystalline, so the crystallinity disappears. When the enzyme is large, the breakdown of the material by the enzymes results in surface erosion, as it cannot pass through the polymeric material. When the material is hydrophobic, the enzyme catalyzes the hydrolysis of the material from the external side; over time, the degree of degradation of the material increases, and the rate of fragmentation is also increasing, which outcomes in augmenting the surface area and boosting the effect of enzymes [39].

Examples of Polymer-Nanocomposites Biodegradation Biodegradation of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/Organophilic Montmorillonite Nanocomposite Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, is classified as a microbial biopolymer. This polymer is both biocompatible and biodegradable [54]. The biodegradation of PHBV/OMMT depends on the degrading effect of microbes found in natural soil. Different kinds of degraders can vary from bacteria and molds isolated from the soil. The biodegradation process is performed by an extracellular depolymerase enzyme secreted by microbial degrading agents to use PHBV as a nutrient. The process involves two major steps: hydrolysis and microbial metabolism. As in the first step, the nanocomposite decomposes by enzymes secreted from degraders, leading to hydrolysis of the PHBV and transforming it from a high molecular weight to a low molecular weight. During the biodegradation process, water-soluble products, including organic acids, are produced through the metabolism of microbes which cause a change in pH. Therefore, we can consider these substances as evidence for the efficiency of the decomposition process. Therefore, the decrease in molecular weight is an indication of the decrease in pH, confirming

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that biodegradation is carried out correctly. Another evidence that the biodegradation process takes place effectively is the increase of microbial number during the degradation, as the increase in decomposition increases the number of degraders during the process [55]. Factors that affect the biodegradation process are generally the properties and processing of the material, physical, and chemical factors of the ecosystem, and microbial parameters. In addition, the structure of PHBV/OMMT nanocomposites is another important factor that is critical to the decomposition process. The interaction and adhesion of PHBV and layered silicate surfaces of OMMT can decrease the biodegradation process, because the adhesion prevents the movement of the segments. In another way, increasing the amount of OMMT can reduce the decomposition process, because it can prevent the permeability of water and its access from the outer surface to the bulk of the film, so the PHBV matrix’s hydrolysis would be decreased. This shows that OMMT has antimicrobial activity, so its presence can break down the normal activities of PHBV degraders. According to an experiment conducted and explained in Ref. [55], the decomposition process can also be affected by other factors, such as the degree of adhesion between the PHBV and OMMT, water permeability, crystallization, and the antimicrobial resistance effect of OMMT.

Biodegradation of Polylactic Acid Accompanied by Nanocomposites PLA is considered a critical polymer, especially in the agricultural field, owing to its possession of several properties that exist as an essential substance in the industry, including its biodegradability. The preparation of lactic acid can be from a natural source, such as the fermentation of sugar found in renewable sources such as sugarcane and corn, so it is considered a nontoxic substance and can be used in medical applications that enter the human body [56]. It can be decomposed naturally by compost or in the soil, and the products resulting from the hydrolytic decomposition process can be completely assimilated by bacteria and fungi [57]. Examples of some degraders are: (1) Amycolatopsis sp. is a bacterial strain classified as a degrading microorganism of PLA. (2) Proteinase K is a fungal serine protease enzyme produced by Tritirachium album, being used in the biodegradation process of PLA. (3) Amycolatopsis KT-s-9, a silk degrader, is a soil-isolated bacterial strain [59]. Microbes do not necessarily have to break down PLA, while some microbial strains can utilize lactic acid and oligomeric products of PLA resulting from the decomposition process. The decomposition process of PLA is affected by the presence of nanomaterials, including nanoclays. These nanoclays can increase the rate of PLA decomposition as they are hydrophilic materials; hence they can promote water diffusion into the polymer matrix and facilitate its decomposition [60]. Compost decomposition of PLA can be observed visually due to form and color changes. Such changes appear as deformation of the surface of the material. Whitening especially appears more clearly in the presence of nanocomposites. Thus, we can consider the white color’s appearance on the surface as a sign of water absorption and breakdown of the matrix.

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Fig. 14 SEM micrographs of PLLA surface films, biotically aged polylactide films: (a) surface erosion, (b) in the presence of bacteria, and (c) fungi. (Adapted with permission from Ref. [61]. Copyright © 2001, Elsevier)

This indicates that PLA is converted from a high molecular weight compound into lower molecular weight fragments by the hydrolytic process. Some factors help degrade PLA faster by compost, including the nature of nanoclay, because it can catalyze the hydrolysis process of the ester bonds present in the PLA matrix. This is attributed to the presence of hydroxyl groups within the silicate layers of nanoclays [61]. SEM micrographs for the surface erosion of PLLA in the presence of bacteria and fungi are shown in Fig. 14.

Biodegradation of Poly(ε-caprolactone) Nanocomposites One of the means of decomposing PCL, whether alone or with nanocomposites, is bacterial strains, e.g., PG01. According to an experiment carried out by PG01 strain on capsules made of PCL and PCL nanocomposites, there is a relationship between the decomposition of PCL nanocomposites and their mechanical properties, e.g., the tensile strength of the material decreases as the material breaks down. The type of the

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70 PCL/clay(with cell) PCL(with cell) PCL/Starch/clay(with cell) PCL/Starch(with cell)

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substance accompanying the PCL also affects the rate of its decomposition, e.g., in this experiment, starch increases the rate of decomposition of the capsule, in contrast to the presence of clay, which reduces the rate of decomposition [62]. Figure 9.16 shows a relation between the incubation time of PCL with Bacillus sp. PG01 and the breakdown strength of PCL in the presence of different types of materials accompanying the polymer. Figure 15 shows the effect of different nanocomposites on the biodegradation of PLC. The presence of starch accompanying the PLC results in a faster weight loss than PLC alone. Thus, the process of biodegradation of the substance is accelerated. It is also clear that the addition of clay reduces the biodegradation process. The ranking of biodegradability decreases in the order: PCL/starch > PCL/starch/clay > PCL > PCL/clay [62, 63]. Biodegradation shows very high efficiency in the decomposition process of PCLs, and a large percentage of this is degraded superficially by biotic degradation. Decomposition appears as a surface erosion process, followed by a slight decrease in molecular mass, which is a fast process. Biodegradation methods for CPL are varied; for example, they may depend on compost, fertilizer, fungi, or anaerobic bacteria. The decomposition process is also affected by factors specific to the degraders, the composite itself, or environmental factors. For example, (1) Fungi such as Aspergillus sp., Penicillium funiculosum, Chaetomium globosum, and Fusarium sp., their use in the decomposition process is affected

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by the molecular weight of the substance and the degree of crystallinity. (2) The use of different microbes also points to different forms and mechanisms for the decomposition process. (3) Temperature is another important parameter that affects the decomposition process in the compost or anaerobic medium. (4) The matrix composition also plays a major role; for example, the presence of phthalic end groups as end-cappers reduces the rate of PCL decomposition in the presence of microbes, whether pure cultures or mixed. (5) The presence of additives to treat PCL affects its decomposition rate and may differ from its pure decomposition. (6) Crosslinking affects enzyme decomposition of PCL, which may reduce the decomposition rate [45].

Biodegradation of Graphene Oxide-Bio-chitosan Nanocomposite This example shows how microbes in wastewater biodegrade chitosan. The graphene oxide present with chitosan can decompose due to the ability of microbes existing in polluted water to grow on the nanocomposite surface film as biofilms. This process is associated with a decrease in film mass due to polymer decomposition. In addition, an important property is that the presence of GO affects the antimicrobial activity that impacts the biodegradation process. Chitosan cannot affect microbes and shows them no antimicrobial results, as this process depends on the degree of pH in the medium. For example, chitosan has a stronger antibacterial action in an acidic medium than in a neutral medium. The bacteria break down chitosan to use it as a carbon source by the chitosanase enzyme, which breaks down chitosan into glucosamine oligomers. As for GO, its biodegradation rate appears relatively very small compared to the decomposition rate of chitosan. The toxicity of CS-GO nanocomposite is attributed to the amount of GO present, given that its increasing concentration in the compound increases bacterial toxicity. The process of stopping biodegradation due to the inactivation of microbes is caused by graphene oxide, as once the polymer decomposes, the microbes become exposed to graphene oxide, which is present on the surface of the nanocomposite. Similarly, the production of substances such as reactive oxygen species (ROS) during the biodegradation process results in the cessation of bacterial activity and hinders biofilm growth. Therefore, an increase in the proportion of GO in the compound is considered to reduce chitosan biodegradation due to bacterial inhibition. Besides, this process is accompanied by morphological changes of the compound; for example, the degree of surface roughness increases due to the appearance of sharp edges of GO, which have a role in the antimicrobial process. However, it is not necessary in all cases for bacterial growth to stop, as bacteria can be present in GO-resistant contaminated water. Another reason is that a percentage of the bacteria that die can form a barrier over the film’s surface, which prevents the effect of GO on other bacterial cells. Given the reasons explained in this example, it is evident that certain nanocomposites, despite their economic importance, can be dangerous to the environment and public health [64].

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Aliphatic Polyesters Biotic and Abiotic Degradation Among the essential polymer materials are aliphatic polymers. These polymers are used in a wide range of applications and are among the most commonly used polymers in the medical field. The process of their degradation may be carried out in various ways by biotic or abiotic factors. Biological relying on microbes or enzymes. For example, the decomposition of a composite that includes an aliphatic ester may require heat. Each method has its mechanisms of degradation. The degradation may be superficial and external (homogeneous) or be in the bulk compound (heterogeneous). This is because the decomposition process is restricted by various factors, including the chain’s location to be broken, the chemical composition, the molecular weight, and the morphology of the matrix. Examples of aliphatic polymers are diverse, and each case has its method and conditions for the decomposition, and biodegradation plays a very important role in the decomposition of each of them [65].

Degradation of Poly(hydroxybutyrate) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) The decomposition environments of PHB and PHBV vary with soil, compost, or seawater. The decomposition appears in the form of a rapid loss of molecular mass followed by a weakness in the mechanical strength of the compound being lost during the hydrolysis process of the compound. Abiotic factors contribute to the biodegradation process, such as heat, which plays a role in speeding up the biodegradation process. Bacterial enzymes are among the vital factors that are relied upon in the bio-breaking process, such as polyhydroxyalkanoates. This bacterial enzyme breaks down the polymer by surface corrosion (heterogeneous). Surface decomposition is based on two steps: endo-scissions that are random through the polymeric chain and exo-scissions at the ends of the chain [65].

Biodegradation Products The substances resulting from the polymer degradation process differ in molecular weight and chemical composition. These materials are very important in identifying the decomposition process and the form. It expresses the extent of the polymer’s interaction with the surrounding environment or the human body. Usually, some of these products are not required to settle in the material, while others move to the surrounding environment. Therefore, reliance is placed on extracting these substances and separating them from the polymeric material or the environment to which they are transferred, whether it is soil, water, or others. Degradation of some polymers can form complex product mixtures which contain hundreds of different products [66].

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The nature of the resulting material or matrix determines the method used in the extraction process. Materials resulting from the biodegradation of PLA may be identified using analyses showing these materials. For example, when studying the biodegradation of the PLA matrix using the bacteria present in the compost, they produce different materials either after a long period of decomposition process or as intermediate compounds that appear and disappear during the biodegradation process. Acetic acid, propanoic acid, and lactoyllactic acid ethyl ester are all substances that develop following a biodegradation period of PLA. Acetic and propanoic acids are considered intermediate compounds which appear but do not occur after a period of decomposition. The duration of the released substances from the decomposition process varies from one to another and it may begin after 5 weeks associated with a decrease in the molecular mass of the decomposed matrix. For example, in the case of massive PDLLA lactic acid appears after 5 weeks and develops in greater quantities after the 8th week until the 11th week and 35th week. But in the case of semicrystalline PLLA, L-lactic acid does not start to appear until the 31st week [58]. The addition of substances to a polymeric compound influences its properties and decomposition rate in various ways. For example, materials such as ER are a primary fatty acid amide used as a slip agent. Also SiO2 and CaCO3 are inorganic materials used with polymeric materials to increase their surface roughness. On the other hand, the addition of CaCO3 to PLA delays its biodegradation concerning PLA alone or in the presence of SiO2 [45].

Applications of Polymers Nanocomposites Wound Dressing In general, the wound is a term applied to a laceration or discontinuity in the skin. According to the rate of healing, wounds can classify into two categories: acute and chronic wounds. An acute wound takes a short time to be healed, unlike a chronic wound which needs more time due to several factors such as severe discontinuity and a bacterial infection that affects the healing rate. The regeneration of wounds is considered a dynamic process that includes four main stages: hemostasis, inflammation, cell proliferation, and remodeling. Various biomaterials have been used to heal both types of wounds, and their healing rates depend on the used biomaterials characteristics. However, there are crucial requirements that should be existing in the selected biomaterial, such as an antimicrobial activity that supports the tissue mechanically, as well as cell differentiation, preventing dehydration of wound, protecting the wound from any infected microbes, and having the ability to reach the fluids of the wound as shown in Fig. 16. Chitosan-based nanocomposites (silver, iron, copper, or zinc hydroxyapatite) formed as hydrogels, thin films, or fiber-meshes have been demonstrated as a wonderful wound dressing. This is due to chitosanbased nanocomposites’ additional features, such as O2 permeability and stimuli responsiveness [67].

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Fig. 16 Carbohydrate polymer-based composites embedded silver nanoparticles enhance wound healing by preventing bacteria penetration and releasing antibacterial agents. (Adapted with permission from Ref. [67]. © 2019, Elsevier Ltd)

Drug Delivery In general, drug delivery is an application used to treat several diseases, including osteoarthritis or cancer. Drug embedded nanocomposites should include outstanding features and requirements such as excellent pharmacokinetics, ability to drive drug to the targeted site, and stimuli-responsiveness, which aid in an appropriate rate of resale of the drug [68]. One of the advantages of using biodegradable polymers in drug delivery applications is that they deliver the drug to the desired organ accurately and, after a specified stimulation, release the drug, which results in the drug being used in lower doses and with fewer side effects. Overall, the fabricated drug can occur in different processes such as diffusion, erosion, or swelling, like the chitosanbased nanocomposite as shown in Fig. 17 [57]. As a result of the easy chemical modification of chitosan, drugs can be effectively incorporated into chitosan-based nanomaterials. Composite films of chitosan/silver nanoparticles produced with moxifloxacin medicines incorporated by in situ coprecipitation method would be biocompatible. These films exhibit appropriate biodegradation, excellent antimicrobial activity against several pathogenic microorganisms, high mechanical characteristics, excellent swelling, and prolonged drug release of up to 36 h. Another chitosan/gold nanoparticle thin-films encapsulated 5-fluorouracil (5-FU) has been fabricated using the solution casting method. Their controlled drug release rate via an external dc electric field in an electrolyte solution was estimated to be with higher proficiency and extremely high death of cancer cells (above 90%), indicating its proper effectiveness on cancer [68].

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Fig. 17 The three-process included in the drug delivery. (Adapted with permission from Ref. [57])

Bone Tissue Engineering Bone performs crucial functions such as protection from damage and mechanical support for different tissues, ligaments, tendons, and joints. It is considered a reservoir of minerals, particularly phosphate and calcium. Therefore, normal mineral homeostasis is preserved. Generally, the bone is subject to a life-long renewal and repair process, such as bone remodeling. Problems and diseases in the skeleton bone are associated with abnormalities in the remodeling process, mechanical strength, abnormalities of homeostasis (calcium and phosphate), which lead to pain, structure deformation, or fracture. The bone’s ability to regenerate includes small cracks, certain fractures, and bone defects to cure the site for damage. The standard surgical method generally depends on using a metal device, including bone autografts or allografts. These methods help to reduce important bone defects but are associated with disease transmission or an increased risk of morbidity. The risk factor increases with aging so that the material used to regenerate bone tissues is developed to enhance their healing ability [8]. Bone tissue engineering (BTE) is one of the most multidisciplinary areas of research launched in the 1990s. It is developed to repair, replace, and maintain or improve the function of a tissue or organ’s function and defects due to different pathologies. Consequently, tissue engineering manipulates living cells through their extracellular or genetic environment. BTE focuses primarily on scaffold preparation methods and characterization. This strategy depends on three important key factors: osteogenic cells, osteoconductive scaffolds, as well as osteoinductive growth factors. The new tissue is generated by cells that are then supported by a suitable scaffold with the required characteristics and a growth factor that provides the tissue for regeneration. Depending on the patient’s health and the defect site, biomaterials are chosen with appropriate properties [69]. Bone has self-regeneration capability of only minor defects within a few weeks, but scaffolds are mostly required in case of severe defects, complications, and volume loss. Those should have relevant characteristics to be used in BTE strategy appropriate for the treatment of the bone defect having damaging living tissues, as the bone in severe complications cannot heal by itself. There are many good clinical therapies, such as autotransplantation and allograft, but they have limits. In this

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context, the need for biocompatible biomaterials is considered a multidisciplinary research area [70]. It is reported that the materials used in designing bone scaffolds, usually made from a polymer having the structure of a supportive cellular attachment and the deposition of the mineral matrix to temporarily fulfill the role of the extracellular matrix (ECM) in the forming tissue until the damage reduction occurs. Certain features should be considered during bone scaffolds’ fabrication method, including crystallinity, porosity, and surface-to-volume ratio, to enable bone tissue to regenerate [71]. In addition, various requirements should exist, such as appropriate mechanical, chemical, and biological features to stimulate and support the bone cell proliferation process [70]. Furthermore, non-thrombogenic, osteoinduction, osseointegration, biocompatibility, required compressive strength should be fulfilled in the scaffold used in BTC. Besides, it should possess optimum pore size for allowing cell adhesion, proliferation, and nutrients and oxygen transport, controlled bioresorbable (degradation), delivery with controlling the release of growth factors, and recreating the natural hierarchical bone structure [70, 71].

Chitosan-Based Nanohydroxyapatite Composite Nanohydroxyapatite (Ca10(PO4)6(OH)2) (nHA) is considered a common nanofiller for chitosan-based composite in bone engineering technology. This is due to its appropriate characteristics like thermal and mechanical properties required to make flexible chitosan used in bone repair, cellular activity, high bioactivity, and osteoconductive characteristics. The main reason is that its chemical structure is similar to the natural matter of bone apatite, and nHA can penetrate the chitosan layer to prevent inflammatory reactions. The form and size of nanohydroxyapatite should be uniform on the composite surface to match the regular structural features of natural bone apatite. It has been documented that increasing the content of nanohydroxyapatite can increase the surface hardness and roughness of the nanocomposite and make a faster rate of degradation. Composite mesoporous silica nanoparticle/CS nanofibers have been optimized to be effective scaffolds in bone tissue applications owing to their excellent promotion of osteoblast attachment, proliferation osteogenic differentiation, and bone formation. Another study showed that nano-silica in CS/chondroitin sulfate nanocomposites improves the mechanical property of scaffolds, mineralization rate, and serum protein adsorption [72].

Other Applications Polymer nanocomposites have excellent properties that attract their use in various applications such as photodiodes, light-emitting diodes, photovoltaic solar cells, gas sensors, photocatalytic and water treatment, as well as electronics and automobile sectors, coatings, gas barriers, plastic bottles, packing and packaging, sports goods, and optical glasses [23]. Environmental applications include water disinfection membranes, polymers, energy storage, sensors, and adsorption (Fig. 18).

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Fig. 18 Overview of environmental applications of polymer nanocomposites. (Adapted with permission from Ref. [16]. Copyright © 2021, Elsevier)

Applicability and Safety of Polymer-Nanocomposites Nanoparticles (NPs) have a size of less than 10 nm that can enter human tissues and disrupt different cells and organs, such as the liver, spleen, brain, gastrointestinal tract, heart, and lung. Their toxicity has been proved to depend on their size and dose. As a result, aggregation of nanoparticles at high concentrations can reduce their toxicity by limiting the translocation of particles and reducing the reactive surface [51].

Conclusions Polymers are biodegradable materials in the environment, some of which are degraded by enzymes or microbes, where they secrete enzymes that decompose the material into smaller pieces. Polymers are used in many applications, some of which enter the body and are deposited in body tissues for some time. Therefore, biodegradable materials are used because they are not toxic to tissues, making them safe within the body. The body’s enzymes play a role in analyzing these substances that leads to erosion of the substance after it has performed the task required of it inside the body. The presence of nano-particulates with the polymer affects the rate of its decomposition, in addition to the influence of other abiotic factors that have a role in the rate of biodegradation. Various synthetic methods can produce nanoparticles and nanocomposites using the various techniques mentioned above in this chapter. But it is important to consider

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certain factors to improve reproducibility and quantity. It is important to deal with the interaction between the polymer and the synthesized nanoparticles and improve the incorporation of the nanoparticles in the polymer matrix. This chapter provided an overview of the various stages of nanoparticle synthesis and methods of manufacturing nanocomposites.

Future Perspectives The usage of an advanced and innovative nanocomposite is highly needed as it can be used in various applications. However, there are further challenges and more research discoveries required, and still, there is hope for improving nanocomposites properties, shape, and design and their compatibility with the matrix. Nevertheless, researchers worldwide are making rapid progress toward the development of novel nanocomposites for different usages and to control them precisely. More substantial breakthroughs are expected to occur shortly concerning the practical applications of nanocomposites.

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Meena Bhandari, Dilraj Preet Kaur, Seema Raj, Tejpal Yadav, Mohammed A. S. Abourehab, and Md Sabir Alam

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Conductive Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemical Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metathesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentrated Emulsion Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solid-State Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plasma Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Biodegradable Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biodegradable Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Biodegradable Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic Devices, Sensors, and Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochromic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M. Bhandari · D. P. Kaur · S. Raj School of Basic and Applied Sciences, K. R. Mangalam University, Gurgaon, Haryana, India T. Yadav NIMS Institute of Pharmacy, NIMS University, Jaipur, Rajasthan, India M. A. S. Abourehab Department of Pharmaceutics, Faculty of Pharmacy, Umm Al-Qura University, Makkah, Saudi Arabia Department of Pharmaceutics, Faculty of Pharmacy, Minia University, El-Minia, Egypt M. S. Alam (*) SGT College of Pharmacy, SGT University, Gurugram, Haryana, India © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_64

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Water and Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy Conservation and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Smart biomaterials are designed on the principle of intrinsically conducting polymers. Conducting polymers based on polypyrrole (PPy), polylactic acid (PLA), poly(3,4-ethylenedioxythiophene) (PEDOT), and polyaniline (PANI) exhibit an excellent response to electrical conductivity. Several strategies are involved in biodegradable conducting polymers synthesis. This chapter summarizes recent breakthroughs in the synthesis and design of biodegradable and biocompatible polymers with better electrochemical responsiveness and could be used as substrates, dielectrics, conductors, or semiconductor materials in electronics. These polymers also find distinctive biomedical applications such as electronic medical equipment, surgical implants, drug delivery, tissue engineering, cancer application, etc. Keywords

Biodegradable polymers · Biocompatible · Conducting polymers · Biomedical applications Abbreviations

CNT CPCs CPs CQDs DD GEM LED NLC NSCLS PANI PCL PEDOT PLA PTX PU PVA PVP TE

Carbon nanotubes Conducting polymer containers Conducting polymer Carbon quantum dots Drug delivery Gemcitabine Light-emitting diodes Nano lipid carrier Non-small cell lung cancer Polyaniline Polycaprolactone Poly(3,4-ethylenedioxythiophene) Polylactic acid Paclitaxel Polyurethane Poly(vinyl alcohol) Poly(vinyl pyrrolidone) Tissue engineering

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Introduction Biodegradability reflects the interaction of any natural and synthetic substance with nature [1]. Nowadays, there is a boom of biomaterials in different applications because of their potential and biodegradability. For the formation of biomaterials, a variety of polymers are available in the chemical industry, possessing good physical and mechanical properties. The biocompatibility of such substances also helps towards sustainable development. These materials are utilized as vascular grafts, delivery of drugs, skin surgeries, equipment to fix bones, gene delivery, tissue engineering, surgical sutures and diagnostic applications, and many more biomedical applications. As biomaterials/biopolymers are insulators, their applications are limited [2]. For long-term stability, such materials are also used in bone fixing, dentistry, bone replacement, etc. Within the starting of the twentieth century, a new segment of substances such as synthetic polymers, composites, ceramics, alloys, metals, and carbons [3] have been added to biomaterials to impart them good mechanical properties, better biological and chemical behavior, and functionality, but a few limitations are still there which hinder their applications. The important application of these biocompatible biomaterials is conducting polymers, which possess electrical and optical properties [4]. Conducting polymers are majorly used in biomedical applications, batteries, microelectronics, lightemitting diodes, electrochromic displays, etc. [5]. Researchers are continuously working on biomaterials in different areas, and combinations/doping is done to overcome the challenges of biomaterials like weak molecular interaction, conductivity, biocompatibility, biodegradability, mechanics, and topography [6]. This chapter is a cumulative description of various biodegradable conducting polymers, preparation, and design of conducting biodegradable polymers with the characterization of graft polymers, block polymers, polymeric composites, and polymer hydrogels. This chapter also provides the applications of electrically conducting biocompatible polymers in electronic devices, sensors and actuators, materials for electrodes, wastewater treatment, energy storage devices, and biomedical applications.

Biomaterials Biomaterials are biodegradable substances mostly used in biological systems such as biosensors, surgical tools, tissue engineering, medical implants, and other devices [7]. Aliphatic polyesters, such as polyglycolide (PGA), polylactide (PLA), polycaprolactone (PCL), and polyurethane (PU), etc., [8] are used as biodegradable materials. They exhibit biocompatibility and degrade easily because of the breakage in ester linkage, which leads to small molecules and are automatically eliminated from the body. Condensation reactions mostly obtain biodegradable polymers, ring-opening reactions of lactones, microbiological synthesis, chemical synthesis in the presence

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of a catalyst, enzyme-mediated synthesis, chemo-enzymatic synthesis, or modification of the natural polymeric products [9]. Polymers are used to synthesize various nanocarriers like polymeric nanoparticles, metallic nanoparticles, lipid nanocarriers, NLC, liposome, etc. [10]. They find applications as scaffoldings, biomedical implants, sensors, and drug delivery systems [11].

Conducting Polymers Conducting polymers are mostly linear in structure and possess high conjugation. Examples include polyacetylene, polypyrrole, polyaniline, and polyphenylene, etc. Conductivity in the polymers is because of the presence of pie bonds which is mostly increased by doping them suitable substances which bring out oxidation, i.e., p-doping, which induces a positive charge in the polymer and reduction, i.e., n-doping, which generates negatively charged polymer. Doping is a reversible process that transforms insulators into highly conductive polymers, depending on how much dopant is present. Conjugation, doping level, and morphology of the polymer impact its properties. Conducting polymers possess the polymeric behavior, i.e., ability to flex and bend and electroactive behavior like metals and semiconductors. They can be prepared by different processes such as electrosynthesis, chemical synthesis, photochemical synthesis, bio-catalyzed synthesis, pyrolysis, composites, inclusion method, solid-state method, and plasma polymerization [12].

Synthesis of Conductive Polymers Chemical Methods Polymers are made by oxidizing or reducing monomers and polymerizing the monomers, e.g., poly(3-hexylthiophene) is mainly synthesized by chemical methods.

Electrochemical Methods PANIis most commonly prepared by chemical oxidative technique, which involves anodic oxidation of aniline in a single glass cell containing aniline and hydrochloric acid, resulting in cost-effective films of desirable uniform thickness.

Photochemical Polymerization Polymerization takes place in the presence of light and a photosensitizer, e.g., aniline polymerization in the presence of horseradish peroxidase.

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Metathesis Methods The chemical reaction between two compounds that leads to the exchange of one part of one molecule with another part of the second molecule is known as metathesis. In the literature, ring-opening metathesis of cycloolefins, acyclic or cyclic metathesis of alkynes, and diolefin metathesis have been documented.

Concentrated Emulsion Method This is radical polymerization that takes place in three hetero phases like water, latex particle, and monomer droplets. A polymer formed shows solubility in either solvent or monomer. Initially, an emulsion of monomer droplets in water is formed, resulting in the formation of the product showing latex particles distribution. This process is used in the acrylic industry.

Solid-State Methods Solid-state polymerization is a straightforward and cheap process carried out by heating the monomers without oxygen or in a vacuum to get polymers having good mechanical strength. Temperature and pressure impact the properties of the polymer. Bottle grade PET films are produced by this method.

Plasma Polymerization It results in the formation of thermally and chemically stable, coherent, pinhole-free polymer thin films possessing good mechanical properties. They are mostly used for biomedical, optical, and electronic devices.

Pyrolysis Method It is referred to as decay of organic material at elevated temperature, leading to polymer formation [12] (Fig. 1).

Biodegradable Conducting Polymers Conducting polymers shows electrical and optical properties, while biodegradability is essential for biomedical applications. Biodegradable conducting polymers are composed of intrinsically conducting polymers connected with biodegradable moiety.

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Fig. 1 Synthesis methods of conductive polymers

Synthesis of Biodegradable Conducting Polymers Blending,copolymerization, and composite arrangement from biodegradable and conducting polymers could be used to coordinate biodegradable conducting polymers. Electroactive polyaniline, polypyrrole, and polythiophene molecules can be combined with biodegradable polymers like poly(D, L-lactic acid) (PDLLA) and polycaprolactone (PCL) to create biomaterials that are both conductive and biocompatible. They can be made as (i) block copolymers made by combining electroactive oligomers with biodegradable ester linkages and (ii) copolymerization of altered biodegradable and electroactive macromonomers made from polyesters made in the first step with conducting polymers [6]. Poly(D, L-lactic acid) (PDLLA), polycaprolactone (PCL), and poly(glycolic acid) (PGA) have been mostly studied for their usage in the pharmaceutical industry, because they exhibit good biocompatibility [8]. The degradation, mechanical, thermal, and other biological properties of the biomaterials are designed to depend on the polymers’ shape and size [13].

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They can be linear [14], star-like [15], hyperbranched [16], and cross-linked [17], which were combined by combining the degradability of aliphatic polyesters with the electroactivity of conducting polymers based on PCL, PLA, and aniline oligomers. Conducting polymers like polypyrrole and polythiophene biodegradable polymers show electrical activities because of the presence of linear π-conjugated electrons. They can further be doped with negatively charged dopants like hyaluronan to design them for specific purposes [18]. The Nobel Prize in chemistry was awarded to Shirakawa in 2000 to discover iodine doped polyacetylene (CH)x conducting polymer [19]. Several conducting polymers based on π-conjugated systems have pyrrole and thiophene and carboxylic acid units that were synthesized, which exhibited biodegradable properties. Conducting oligomers can be eliminated from the body through macrophages. However, it is size- and dose-dependent. Small particles stimulated macrophages activity [20].

Types of Biodegradable Conducting Polymers Block Polymer Triblock copolymer, PLA-b-AP-b-PLA showed the formation of self-assembly and electrical conductivity between aniline pentamer (AP) occurred through polylactide (PLA) [23]. PLA acted as the soft matrix for bridging AP. Rivers et al. [21] devised novel biodegradable, biocompatible conducting polymers having ester linkages using polylactic acid (PLA), which was responsible for biodegradability and three π-conjugated rings, i.e., thiophene in conjugation with two other pyrroles responsible for conducting properties. Enlargement of aromatic chain, as well as defects in π conjugation present in three pyrrole units, induced conductivity, thiophene, introduced stability to the molecule and ester linkage in polymer introduced biodegradability as they were lysed by enzymes, like cholesterol esterase, which is mostly discharged by cells at the time of injury [21]. After the degradation of polymers, oligomers released would be ingested by white blood cells, limiting unpropitious riposte. Later on, oligoaniline-based biodegradable conducting polymers were devised by Hardy et al. [24]. Degradable conducting block polymer having aniline tetramer end groups was obtained in two step process: (i) aniline dimer (AD) opens caprolactone ring (CL) leading to the formation of polyester with aniline dimer end group and (ii) oxidative coupling reaction leads to the formation of aniline tetramer polycaprolactone AT-PCL having conductance between 6.3  10 7 and 1.03  10 5 S/cm depending upon aniline tetramer amount. Multiblock copolymer (PLA-co-AP), as well as triblock copolymer PLA-b-AP-bPLA, was designed by using aniline pentamer (AP) and polylactic acid (PLA), which exhibited both electrical and biodegradable properties, biocompatibility, good mechanical properties due to which it can be used as scaffolding material [23]. Nucleophilic substitution process with polyphosphazene yielded aniline pentamer (AP) and glycine ethyl ester (GEE) block polymers with improved solubility

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and film-forming properties, as well as an electroconductivity value of ~2  10 5 S/cm [24]. It can be used as scaffolds for neuronal and cardiovascular tissue engineering. The biodegradable, electroactive diblock copolymer blend PGAT was reported to be synthesized by mixing poly(ethylene glycol) methyl ether (mPEG) and aniline tetramer (AT), which was further blended with poly(L-lactic acid) (PLLA) [25].

Graft Polymers Polymers are generally either homopolymers or copolymers. Graft polymers are obtained by adding different macromonomers to the homopolymer/copolymer chain [26]. These macromonomers mostly enhance solubility, ease of processability, biodegradability, biocompatibility, etc. But sometimes, they may also hamper the conducting properties of homopolymers [27] by interfering with π bond conjugation, which is responsible for conductivity. Catalytic or enzymatic pathways can synthesize them. Evidence suggests that the amount of macromonomer also impact the properties of graft polymer, e.g., copolymers of (3,4-ethylenedioxythiophene) PEDOT-co-PDLLA (polylactides) in three different portions (1:05, 1:25, and 1:50) exhibit altogether different properties. The polymer having composition 1:50 have been found to exhibit better-conducting properties and biointerface characteristics with cells than the other two polymers [28]. Many polymers have divergent properties like biosensors, biointerfaces, and biomimetic conducting polymers have been reported in the literature [29]. Aniline pentamer cross-linking chitosan (AP-cs-CS) has been synthesized, exhibiting better water solubility and biocompatibility [30]. The incorporation of chitosan is responsible for the biodegradable properties of molecules. Aniline pentamers with a carboxylic group at the chain end were prepared and subsequently condensed with amine groups of chitosan using N-hydroxysuccinimide. These amphiphilic polymers can form micelles and can be used for drug delivery. Synthesis of the multialdehyde sodium alginate (MASA) and the tetraaniline-graft-multialdehyde sodium alginate (MASA-AT) graft copolymer were synthesized by crosslinking polysaccharide multialdehydic sodium alginate with phenyl/amino-capped tetraaniline (AT). The graft polymers obtained showed solubility in water at pH 1–14 and possess biodegradability, conductivity, and noncytotoxic [31]. Polymeric Composites PANi-gelatin blend nanofibers, PANi-based conducting nanofibers having poly(Llactide) matrix, HCl-doped poly(aniline-co-3-aminobenzoic acid) (3ABAPANI) copolymer and poly(lactic acid) (PLA) nanofibers, polyaniline was prepared and have been utilized for tissue engineering [32]. Polycaprolactone fumarate (PCLF) and polypyrrole (PPy) have been processed to give three-dimensional polymer composites (PCLF-PPy) having good mechanical strength and biocompatibility [33]. Several novel electrically conductive biodegradable composites based on poly(D,L-lactide) PDLLA and polypyrrole nanoparticles were synthesized in which nanoparticles were embedded in lactide matrix [34, 35]. Similarly, polypyrrole/polycaprolactone (PCL)/ gelatin nanofibers have been developed by Boutry et al. [36].

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Three-dimensional composites possessing good mechanical strength, good electrical properties, biocompatibility, biodegradability from polycaprolactone fumarate (PCLF), and polypyrrole have been produced for their usage in nerve regeneration [36]. The linkage of biodegradable and conducting segments with dopant molecules into one polymeric chain provides dopant-free conductive elastomer. Biodegradable electroactive polymer dopant-free conductive polyurethane elastomer (DCPU) made up of polycaprolactone diol (PCL), aniline trimer, and dimethylolpropionic acid (DMPA), which was added as a dopant into a polyurethane chain through hexadiisocyanate, was synthesized by Runge et al. [37]. The DCPU polymer films exhibited good mechanical properties, good elasticity, and good conductivity, and they can be degraded hydrolytically and enzymatically. To increase mechanical strength, non-toxicity, biodegradability, processability, several polymeric composites have been synthesized which find applications in tissue engineering. Conducting and degradable polymeric composites such as polylactide (PLA) [37], polycaprolactone (PCL) [39], poly(lactide-co-glycolide) (PLGA) [40], polycaprolactone fumarate [41], poly(lactide-co-polycaprolactone) (PLA-coPCL) [42], polyurethane [43] chitosan [44], gelatin [45], collagen [46], and heparin [47] have been synthesized which show biodegradability and biocompatibility. After expanding aqueous sodium salt of dodecylbenzene sulfonic acid to chloroform, followed by a specific measure of pyrrole, a biocomposite of conductive PPy and biodegradable poly(L-lactide) (PPy/PLLA) was integrated through water in an oil emulsion. Using a co-solution casting process, a biodegradable conductive membrane based on conductive PPy nanoparticles (PPy, 2.5 wt%) and biodegradable chitosan (97.5 wt%) was created [48]. As PPy present is toxic, the lesser the substance of PPy in the polymer composite, the better it is.

Polymer Hydrogels A cross-linked polymer with hydrophilicity and rubber-like characteristics exhibiting biocompatibility is greatly in demand as it can be used for tissue engineering purposes [49]. Polymeric hydrogels based on photopolymerizable macromer acrylated poly(D, L-lactide)-poly(ethylene glycol)-poly(D, L-lactide) (AC-PLA-PEGPLA-AC), glycidyl methacrylate (GMA), and ethylene glycol dimethacrylate (EGDMA) were synthesized into a degradable bio-network. Later on, glycidyl methacrylate (GMA), which was modified by the addition of aniline tetramer (AT), was used to increase conductance [50]. Hydroxyethyl methacrylate (HEMA) having hydroxyl group were photopolymerized with glycidyl methacrylate (GMA). They were processed into polycaprolactone-poly(ethylene glycol)-polycaprolactone (GMA-PCL-PEG-PCL) network using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC
HCl) as water-condensing agent and 4-dimethyl aminopyridine (DMAP) as the catalyst to give better hydrogel possessing better conductivity and lesser toxicity [50]. Hydrogels composed of aniline oligomer and gelatin were prepared to generate scaffolds by grafting N-hydroxysuccinimide-capped aniline pentamer to the amino group of gelatin chain was then cross-linked by 1-ethyl-3-(3-dimethylaminopropyl)

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carbodiimide (EDC) in ethanol. The hydrogel structure varied from a honeycomb to a bamboo raft with an increasing amount of aniline pentamer and possessed biocompatibility conductivity [51].

Applications of Biodegradable Conducting Polymers Electronic Devices, Sensors, and Actuators In today’s fast-paced life, humankind’s increasing dependence on electronic gadgets has also raised concerns for disposing of them safely after using manifold. Generally made up of petroleum-derived polymers such as polypropylene (PP), polystyrene (PS), poly(ethylene terephthalate) (PET), polyimide (PI), and epoxy, the improper disposal of these nonbiodegradable materials can leak hazardous chemicals into the soil, water stream, and ambient air. Considering today’s technological advancements and fast-paced life, it is nearly impossible to reduce the use of electronic equipment. Hence the only practical and sustainable solution is the fabrication of electronic devices from biodegradable polymers. Recently, much research has been directed towards the bio-integration of electronics, emphasizing green, biodegradable, and sustainable materials for electronic gadgets. Biocompatible and biodegradable polymers help make environment-friendly electronic gadgets and revolutionize the field of wearable or skin electronics, medical electronics, and sensors. The limitations of biodegradable materials, such as low thermal stability poor electrical and mechanical properties, hamper their utilization in electronic devices. However, there are many fabrication techniques such as copolymerization, blending, and cross-linking through which the performance of biodegradable polymers can be improved. An efficient and successful strategy is the addition of nanofillers to prepare polymer nanocomposites. It has been proved by different researchers that the incorporation of nanofillers in small amounts can effectively improve the thermal, electrical, and strength properties of biodegradable polymers. Following polymer nanocomposites are receiving great attention for application in electronic devices and sensors.

Polylactide Being an intrinsic insulator, PLA has found applications in electronic devices as a substrate or dielectric layer. The advantage of PLA is its processability and compatibility with various nanofillers for different applications. Modern and advanced electronic devices employ materials based on carbon nanotubes (CNTs), graphene, and carbon quantum dots (CQDs), because they possess high aspect ratio, conductivity, and thermal stability. Electrochemical sensors, prepared with PLA nanocomposites by 3D printing technique, can detect different types of molecules. PLA mixed with carbon nanotubes (4% by weight) through melt spinning technique is used to fabricate electrically conducting PLA textile sensor to detect moisture content [52]. PLA/graphene

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hybrid filament-based sensor is used to detect 2,4,6-trinitrotoluene (TNT) among other nitro-explosives; different solvent such as dichloromethane, chloroform, tetrahydrofuran, acetone, ethyl acetate, and ethanol can be identified through PLA/carbon black-based sensors; electrochemical sensor with PLA/graphite electrode is used to check the presence of Pb2+ and Cd2+ in the jewelry [53]. In organic light-emitting diodes (OLED), electrodes of blended poly(L-lactide) (PLLA) with its enantiomer poly(D-lactide) (PDLA) and embedded with silver nanowires show high transparency and electrical conductivity, even after 10,000 bending cycles or 100 tape testing cycles [54].

Poly(vinyl alcohol) It has been shown that aqueous solutions of PVA with inorganic nanofillers work well as dielectric layers due to their chemical resistance to most organic solvents. Flexible thin-film transistors (TFTs) based on PVA/SiO2 have shown excellent mechanical properties even after 2000 cycles bending at 3 mm bending radius [55]. Resistance random access memory (RRAM) based on PVA nanocomposites shows excellent performance. PVA nanocomposites are deposited as active layers in RRAM. TiO2 nanotubes incorporated in PVA matrix has shown retention of 104 s with On/Off ratio of greater than 104 [56]. ZnO nanoparticles incorporated in PVA and PEDOT: PSS blended matrix is used in fabricating memristive devices (electrical resistance switches) with excellent switching current ratio [57]. PVA/SrTiO3 hybrid nanocomposites as an active layer in memristive devices have bipolar, rewritable, and nonvolatile properties [58]. Polyvinylpyrrolidone In addition to PVA nanocomposites, PVP nanocomposites have also shown excellent performance as an active layer in memristor devices. These devices containing PVP/CdSe composite as an active material has shown bistable properties [59]. All printed memory devices containing PVP/graphene quantum dots exhibited a stable retention time of more than 30 days and robust behavior even after 1000 cycles bend at the diameter of 8 mm [60]. PVP matrix with various nanofillers such as inorganic ceramic nanofiller, silicon nanowires, graphitic carbon nitride (g-C3N4) has also found application in an organic thin-film transistor (OTFT) with higher dielectric constant and lower leakage current [61]. Cellulose Cellulose is used as a substrate for sensors, memristors, transistors, etc. Cellulose fiber as a dielectric layer has shown compatible results concerning the electronic properties of transistors, although these devices operate at a comparatively higher voltage. However, the inclusion of nanoparticles considerably improves the performance of such devices. Cellulose and its derivatives have also been utilized to fabricate actuators and intelligent sensors. When incorporated in cellulosic materials, metal, metal oxides, and carbon nanofillers have shown good results as flexible sensors, e.g., CNT incorporated in carboxymethyl cellulose, are utilized in humidity and temperature sensors [62].

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Electrochromic Applications Conducting biodegradable polymers such as polythiophene (PTh), polypyrrole (PPy), polyaniline (PANI) are also finding applications as electrode materials. These produce polymer-based light-emitting diodes (LED) and electrochromic devices. High power density has been exhibited by PANI based electrodes in bio electrochemical systems. Composite of PANI and MWCNT has also been used as a cathode in the microbial electrochemical cell, which showed an increase in hydrogen production. ChitosanPANI electrode has also been used to detect early breast cancer [63].

Water and Wastewater Treatment Heavy metals and hazardous organic compounds are being removed from water and wastewater using nanocomposites made of biodegradable polymers. The large surface area of nanocomposite materials assists the absorption and removal of heavy ions and toxic compounds. Starch-PANI nanocomposites efficiently removed dyes such as Reactive Black 5 and Reactive Violet 4. PANI-dextrin nanocomposite is used for the efficient adsorption of Cu (II), Cd(II), and Pb(II) in aqueous media [64]. A nanocomposite synthesized from polyacrylamide, xanthan gum, and nanosilica (PAc-XG-SiO2) has shown an excellent result for removing Pb2+ in aqueous media with an aqueous media removal capacity of 99.54% within 25 min [65]. Another natural polymer exhibiting excellent results for water treatment is cellulose. Nanocomposites based on cellulose are being used in membranes and as absorbents to remove heavy metals and chemical compounds due to their hydrophilicity, functional characteristics, and large surface area. Nanocellulose and chitosan composites have been used to remove flexographic ink, tylosin antibody in wastewater from pharma industries, petroleum substances like diclofenac sodium (DS) from water, etc. [66].

Energy Conservation and Storage Another area where biodegradable polymers are gaining popularity is energy storage devices. The conventionally used mineral-based materials for energy storage devices are expensive and difficult to dispose of. As a natural alternative, nanocellulose is being utilized by various researchers for fabricating organic photovoltaic devices for solar energy applications, lithium-ion batteries, sodium-ion batteries, electrodes, and supercapacitors [67].

Biomedical Applications Conducting polymers (CPs) are attracting a lot of interest because of their ability to match the performance requirements of various neuronal therapies, such as recording

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Table 1 Properties and applications of some biodegradable polymers Conventional conducting polymer Polypyrrole (PPy)

Conductivity 10 5 s m 1

Polyaniline (PANI)

10

s cm

1

Polythiophenes (PT)

1000 S cm

1

Poly (3,4-ethylenedioxythiophene) (PEDOT) Poly(ε-caprolactone)

Up to 210 S cm

Poly(D, L-lactideco-ε-caprolactone) [PDLLA/ CL] Poly(2-hydroxyethyl methacrylate)

10

1.79  10 to 6.50  10

11

6

Applications Biosensors, antioxidants, drug delivery, neural prosthetics, cardiovascular applications, and tissue engineering Biosensors, antioxidants, drug delivery, bioactuators, food industry, conductive fiber, and tissue engineering Electrical conductivity and optical property, biosensors, food industry, and tissue engineering Antioxidants, drug delivery, and neural prosthetics

1

S cm

S cm

1

1

Mechanical strength, biological property (Schwann cell adhesion, migration, and differentiation cosmetic use Electrical cue for multitude of cell functions and tissue engineering Oriented scaffold; physical characteristics similar to soft tissue, and mechanical property similar to spinal cord glass fiber filter media

and stimulating brain activity and targeting various neurological diseases [68] (Table 1). Conducting-polymer containers (CPCs) are being explored as viable possibilities for encapsulation, drug delivery, and controlled release applications. Implantable bioelectrodes for stimulating or recording tissues engineering are the most prevalent biomedical applications for which CPs have been investigated. Tissue engineering is a multifaceted and versatile area. The objective of tissue engineering to create biological agents to reestablish, prolong, and increase tissue act by accumulating cells, biological entities, and a scaffold [69]. The main biomaterials for tissue engineering as a scaffold includes polymers from natural sources are gelatine, alginate, chitosan, collagen, etc., and polyurethane (PU), polylactic-coglycolic acid (PLGA), polycaprolactone (PCL), and polylactide (PLA) are synthetic polymers [70]. Electrically responsive cells like muscle, nerve, bone, skin and heart muscles are excited by the conductive polymer matrix, and these polymers are highly useful in electrically adaptive tissue regeneration [71] (Fig. 2).

Tissue Engineering for Skin Skinis directly in contact with the open environment, so microbial contamination and chances to break are immense. Many biomaterials have been developed for

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Fig. 2 Conductive polymer matrix electrically adaptive tissue regeneration

wound dressing with antibacterial activity like polyaniline (PANI) [72]. Conductive polymers have also been established to stimulate cellular functions like keratinocytes and fibroblasts.

Tissue Engineering for Heart Myocardial infarction and cardiac failure cause high morbidity and mortality rates. Tissue engineering for the heart depends on natural or synthetic biodegradable polymers that highly regenerate damaged heart muscles, so it is a positive and trustable approach to make entire cardiac tissues or substitutes. Cardiac grafts are effectively and safely implanted in this muscular organism. The heart produces electrical signals for contraction and excitation, so the conductive biodegradable polymers create a scope to regenerate the cardiac tissues. Examples are: (I) Conductive nanofibrous sheets of PLA/PANI are used to increase the cell-tocell contact and maturation [73]. (II) Shen et al. studied that a platelet fibrin gel is useful for heart repairing with matrix metalloproteinases (MMPs) [74]. Tissue Engineering for Nerve Neurons are electrically regulated cells that transfer signals between one to another nerve cell [75]. PPY and PANI are conducting polymers developed by researchers as

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conductive nanofiber frameworks to promote nerve (axon) tissue [76]. Electroactive PANI/PSS/(TMV) nanofibers provide progress in the length of neuritis and neuronal cells.

Tissue Engineering for Skeletal Muscles Skeletal muscles are the part of the body that provides strength to the bones and voluntary control [77], so their fast regeneration after any injury is very important for a quality life. Tissue engineering generates contrast and development of muscle stem cells by in vitro approach on the scaffold by various ways, e.g., advancement of conductive PEG [poly(ethylene glycol)] – co-PGES [co-poly(glycerol sebacate)] united aniline pentamer (AP) copolymer for myotubes arrangement by myoblast cells differentiation and development of new electroactive and biodegradable copolymers of polyurethane-urea (PUU) [78]. Tissue Engineering for Bone Support, movement, and organ protection are very important roles of bones. But bones have various problems that weaken bone strength, such as fracture, bone infection, trauma injuries, ageing, and diseases. Presently bone fixing is accomplished by different leading polymers like the advancement of injectable nanocomposites with biodegradable polymers. Mechanical properties of biodegradable polymers find their usage in interior obsession rather than tempered steel for bone mending. Tissue Engineering for Cancer Treatment Malignancy has been the most injurious illness of late, and shockingly its spread is expanding. Considering this grave well-being circumstance, researchers are attempting to foster new carriers for anticancer medication conveyance explicitly focused on malignancy growths. New approaches for the treatment cancer by using conducting polymers, e.g., paclitaxel (PTX) loaded hybrid microparticles (PTX-Hyb-MPs) for further developed conveyance in ovarian malignancy of chemotherapy. The anticancer prodrug was planned by tying gemcitabine (GEM) to polyfurfuryl methacrylate (PFMA) using N-(3-maleimidopropionyloxy) succinimide as a linker by DA response (PFMA-L-GEM). Another class of covalent organic polymers was manufactured by cross-connecting the photosensitizer mesotetra( p-hydroxyphenyl) porphine (THPP) to chemotherapeutic support of medication, cis-platinum (IV); the last additionally goes about as a decrease responsive linker, etc. (Table 2).

Conclusion From synthesis to applications, a detailed overview of various conducting polymers (CPs), their benefits, and associated problems was presented in detail. The motivation and purpose of CPs in biological applications such as biosensors, tissue engineering, artificial muscles, and drug administration were initially discussed. Over the

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Table 2 Biomedical applications of biodegradable conducting polymers Polymers/polymers with drug Polyhydroxyalkanoates (PHA)

Poly(lactic acid) (PLA)

Poly(glycolic acid) PGA Poly(e-caprolactone) (PCL)

2-hydroxyethyl-cellulose/ polyaniline (HEC/PANI) nanocomposite cryogels Chitin/poly (3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) hydrogel

Dosage form/devices use Skin substitutes, sutures, nerve cuffs, surgical meshes, staples, and swabs Cartilage tissue engineering, spinal cages, bone graft substitutes, internal fixation devices Micro- and nanospheres for anticancer therapy Heart valves, cardiovascular fabrics, pericardial patches, vascular grafts Cartilage Urological stents Barrier material for guided tissue regeneration in periodontitis Contrast agents

Biomedical application Management of wound

Reference [79]

Orthopedic

[80]

Drug delivery

[81]

Vascular system

[82]

Urological Dental care

[83] [84]

Computer-assisted tomography and ultrasound imaging Surgery, wound management, drug delivery, tissue engineering

[85]

[86]

Tissue engineering

[87]

Surgical implants, tissue culture, resorbable surgical sutures, wound closure, controlled release systems, and prosthetic devices Tissue scaffold application Provide mechanical support, shape, and cell-scale architecture for neo-tissue construction Cryogels

Tissue engineering

[88]

Tissue engineering

[89]

Hydrogel

Tissue engineering

[90]

(continued)

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Table 2 (continued) Polymers/polymers with drug Chitin-poly(caprolactone) composite nanogels (chitinPCL CNGs) Methoxy polyethylene glycol (mPEG) Polymethacrylic acid– chitosan–polyethylene glycol (PCP)

Chitosan-graft-poly (e-caprolactone) (CS-g-PCL) Ethylene vinyl acetate polymer system loaded with L-dopamine Silicone elastomer as well as resorbable polyester pellets loaded with dopamine Polypyrrole and poly(L-lactic acid-co-ε-caprolactone) (PPY/PLCL) Hyaluronic acid (HA) dopedpoly (3,4-ethylenedioxythiophen) (PEDOT-HA) Poly(lactic-co-glycolic) acid (PLGA) microparticles

Poly (3,4-ethylenedioxythiophen)poly(styrenesulfonate) (PEDOT/PSS)

Dosage form/devices use Nanogels

Nanoparticles Nanoparticles

Biomedical application Drug delivery in non-small cell lung cancer (NSCLC) Drug delivery

Reference [91]

[92]

Oral drug delivery of metoprolol (MTP) tartrate was incorporated as a model drug Delivery of 5-fluorouracil (5-Fu) Drug delivery

[95]

Pellet

Nerve regeneration

[96]

Conductive polymer biocomposites

Peripheral nerve regeneration

[97]

Nanoparticles

Nerve tissue regeneration

[98]

Microparticles for drug delivery and tissue engineering applications Nano-bio interfaces for medical applications such as nucleic acid detection

Drug delivery

[99]

Biomedical engineering, biomaterials and tissue engineering (TE), bioinstrumentation, clinical and rehabilitation engineering

[100]

Amphiphilic copolymer micelles Controlled release polymer matrix

[93]

[94]

last few decades, CPs have been extensively produced and explored in biomedical engineering; they can give unique optical and electrical properties and the advantages of easy morphological control, high chemical and environmental stability, and biocompatibility. PPy, PANI, PT, and PEDOT are the most commonly utilized CPs, and they are easily made via chemical or electrochemical redox polymerizations. Despite tremendous breakthroughs in biomedical engineering due to CPs as a biomaterial, numerous hurdles remain, including poor biodegradability, low hydrophilicity, low charge-carrier mobility, and small biomolecule loading capacity.

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However, we expect CPs to become more promising biomedical research materials shortly, as a range of strategies to address the CPs’ shortcomings are being investigated.

Future Prospective The synthesis of biodegradable CPs, specifically optimizing their electrical conductivity and biodegradability, is a major challenge. It is still difficult to maintain the conductivity of biodegradable CPs while also ensuring their biodegradability for practical applications. One of the main reasons for this limitation is that, in contrast to nonconjugated degradable polymers, the molecular building blocks of biodegradable fully conjugated CPs are extremely limited. Researchers could investigate diversifying macromolecular architectures and biodegradable linkers used to assemble decomposable CPs to improve their conductivity and biodegradability. Another significant challenge is related to biomedical applications of biodegradable CPs, particularly for tissue engineering applications. Many literatures reported the applications of biodegradable CPs for improving the adhesion, proliferation, and differentiation of electroresponsive cells and tissues, however, the in vitro and in vivo biodegradation profiles of these polymeric structures are still largely unknown. In fact, it is still largely unknown how well the temporary CP-based scaffolds degrade, whether the tissues or cells are still supporting each other or have already repaired and rebuilt themselves, as well as how well the cells and tissues integrate with their surrounding microenvironment both before and after the temporary scaffolds break down.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer Nanocomposites and Their Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Interface’s Role . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer Nanocomposites as Matrices for Biomolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer Nanocomposites: Methods of Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Barrier Characteristics of Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer Nanocomposites of Polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polysaccharides from Lignocellulose Plants and Woods Sources . . . . . . . . . . . . . . . . . . . . . . . . . . Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marine Biomass Polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan and Chitosan Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Semolina with Embedded Nanokaolin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymers Biodegradability After the Formation of Nanocomposite/Composite . . . . . . . . . . . . . . . Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch and Thermoplastic Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Composites with Nanosized Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lignocellulosic Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose Nano-crystallites Are a Type of Crystal (Bacterial Cellulose) . . . . . . . . . . . . . . . . . . . Cellulose That Has Been Regenerated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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H. F. Forsan (*) Animal Production Research Institute, Agricultural Research Center (ARC), Dokki, Giza, Egypt e-mail: [email protected]; [email protected] R. S. Hasan Regional Centre for Food and Feed (RCFF), Agricultural Research Center (ARC), Orman, Giza, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_38

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Other Varieties of Bio Fibers Are Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Migration of Various Nanoparticles into Diverse Foodstuffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Every day, a significant quantity of non-biodegradable materials is generated for use in food packaging. These products have a real and significant impact on human health and the environment. As the concept of a bio-based economy started to gain popularity in the social and financial sectors, there are many areas where scientific methods may be enhanced. Thus, synthesizing biopolymers from natural sources (polysaccharides or proteins) has generated considerable attention as a non-toxic and safe packaging material. It is a feasible alternative to harmful traditional food packaging materials. The present chapter will discuss key recent advancements in biodegradable Nano food packaging materials that use nanotechnology to improve their properties. Nanomaterials will almost certainly be used in most new features for food packaging goods, either directly or indirectly, because they are required to enhance the antibacterial, thermal, and packaging materials’ mechanical properties. Finally, the safety associated with using these nonmetric materials will be reviewed. Keywords

Biodegradable · Nanoparticles · Polysaccharides · Polymers · Migration Abbreviations

BC CNTs DP EVOH HCs HDPE LDPE NFC/Ag OTR PA PE PHAs PHBV PP PVA PVC

Bacterial cellulose Carbon nanotubes Degree of polymerization Ethylene vinyl alcohol Hemicelluloses High density polyethylene Low- density polyethylene Cellulose nanofibers coated with silver nanoparticles Oxygen transfer rate Polyamide Polyethylene Poly (hydroxyl alkanoates) Poly (3 hydroxybutyrate-co-3 hydroxy valerate) Polypropylene Polyvinyl acetate Polyvinyl chloride

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Introduction This chapter discusses the usage of polystyrene and polyethylene, two petroleumderived polymers used in various applications packaging ingredients made from plastics. Several novel polymer compounds, including polylactic acid, polyvinyl acetate (PVA), poly (3 hydroxybutyrate-co-3 hydroxy valerates) (PHBV), and related biopolymer blends, are being developed. Plastic materials that are novel to the market include polyvinyl acetate (PVA), poly (3 hydroxybutyrate-co-3 hydroxy valerates) (PHBV), and biopolymer blends that are related to PVA (biodegradable polymerbased plastics). Even a minor change in the surface-to-volume ratio of a polymer matrix can have a considerable impact on the fundamental properties of the matrix, which in turn can impact the performance of the material. It is the in-situ synthesis of inorganic particles (for example, metal oxides) and the integration of fillers (for example, layered nano clays) that can be accomplished through the formalized synthesis in a polymerized matrix There are different production techniques for polymer nanocomposites: In situ polymerization, Melt mixing, In situ inorganic synthesis, from the solution, and in situ synthesis of nanoparticles of interest [1].

Polymer Nanocomposites and Their Chemistry The Interface’s Role The interface between the polymer and the nanoparticle is critical to the final properties of nanocomposites. In agreement with Kickelbick, 16 16 16 packed atoms in a cubic have a total of 4096 atoms in its structure. 1352 are placed on the surface, accounting for approximately 33%. The total surface atoms number remains the same when the cube is divided into eight equal halves, but the number of atoms on the surface increases to 2368, accounting for 58% of the total. We get 3584 surface atoms by extending the cube’s subdivision, representing an 88% surface atom density for the cube [1] (Fig. 1). Material qualities will be directly affected by the newly exposed inner interface. Referring to the chemistry of the interfacial layer, two types of interfaces can be

Fig. 1 The surface area of a cubic particle after subdivision. Atom’s total numbers are N, while surface atoms are n. (Adapted with permission from Ref. [1]. Copyright 2021, Elsevier)

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distinguished: weak connections such as hydrogen bonds, Van der Waals, or ionic contact, and covalent interactions between organic and inorganic components 584 surface atoms by extending the cube’s subdivision, representing an 88% surface atom density for the cube [1]. Several studies on nanocomposites have been published since their discovery but getting nanoparticles to disperse evenly in a polymeric matrix is still a challenge. Deficient interfacial adhesion, poor dispersion, and limited properties emerge from the mix of hydrophilic nanoparticles with hydrophobic polymers. Most of the time, small forces between nanoparticles, including Van der Waals, attract agglomerates’ formation. As a result, nanocomposites may have poorer characteristics than regular polymers without nanoparticles, limiting their usefulness. Scientists have focused on synthesis techniques and innovative combination tactics to control polymer composition and morphology to address interfacial issues. Additionally, particle functionalization or novel procedures, such as reactive extrusion, are now being developed to reap the benefits of nanotechnology [1].

Polymer Nanocomposites as Matrices for Biomolecules Several studies on nanocomposites have been published since their discovery but getting nanoparticles to disperse evenly in a polymeric matrix is still a challenge. For multilayered systems, it is common to use additional materials as ethylene-vinyl alcohol (EVOH) or polyamide (PA), among others. Aside from the inherent qualities of the polymer matrix, many additives might affect the final product’s performance. Highdensity polyethylene (HDPE) is a polymer used for milk containers or bottles that require a good moisture barrier, chemical resistance, and mechanical properties are all important considerations. On the other hand, transparency isn’t one of the prerequisites [2]. Contrary to the authors’ claims, low-silver nanocomposites outperformed high silver micro composites. After 100 days in water, Polyamide 6 containing 2% AgNPs proved efficient against E. coli. Nanocomposite polyethylene (PE) sheets with AgNPs aided jujube, a Chinese fruit, and photocatalytic disinfectant surface coatings. Titanium dioxide (TiO2) has been proven to inactivate harmful foodborne germs [3]. According to Choi et al., silver doping increases the visible light absorption of TiO2 and its photocatalytic activity when exposed to ultraviolet radiation that showed TiO2/Ag+ nanoparticles in a nanocomposite with PVC had demonstrated excellent antibacterial activity [4]. Hypothesized additional antibacterial strategies based on chitosan nanocomposites with carbon nanotubes (CNTs). CNTs can irreversibly harm microbial cells. The inexpensive, flexible, and good barrier qualities of low-density polyethylene (LDPE), make it ideal for food storage bags [5]. Polypropylene (PP) is a high-strength polyolefin polymer used in rigid containers. After processing, polyethylene terephthalate (PET) is strong, clear, and has good oxygen barrier qualities. So, it’s utilized for cold drinks [6]. PVC is low cost and has a high stretch ability, making it the most extensively used transparent plastic wrap. Plasticizers essential for optimum processing behavior such

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as phthalates or adipates are likely to replace PVC in principal package constructions in the next years [7]. Food containers and egg trays are also made of PS foamed. Multilayered systems of these materials can improve packaging solutions like water jugs and oven-bake bags. These polymers have had the most characteristics modified through nanotechnology [8]. Since the 1990s, most research has concentrated on enhancing the HDPE oxygen barrier, mechanical performance, and thermal resistance [9] and PP [6]. The bulk of PET investigations aims to improve barrier performance. Polyamide and its nanocomposites are another promising material for improving food packaging. No contemporary packaging plastic addresses biodegradability or resource use. The limited disposal options for persistent plastic packaging waste raise global environmental concerns. Demand for renewable, biodegradable polymers has increased as well-alternatives to gasoline, especially for short-term and throwaway packaging. Bio-based polymers contain constituent units derived entirely or partly from biomass. In addition to saving resources, these goods help farm and rural economies while creating green jobs. Biodegradable polymers are materials that degrade via natural metabolic processes [10]. When the moisture, temperature, and concentration of oxygen conditions are all perfect. There are no potentially hazardous byproducts of biodegradation. Bio-based polymers are classified according to their production techniques. The synthetic biopolymers accessible today include poly(L-lactide), poly (L-caprolactone), poly (glycolic acid), poly (butylene succinate), and various natural biopolymers, for example, starch, cellulose, and chitosan, as well as a range of synthetic biopolymers [11]. Poly (hydroxy alkanoates) (PHAs), starch, and PLA are the most often used materials for packaging. Making a polymer nanocomposite with better qualities can be accomplished by combining elements that are either raw materials or serve as a matrix for the polymer [1]. The tensile strength and modulus of polyamide 6–clay hybrids were increased due to the delaminated nanocomposite’s structure. The tensile strength improved by 55%, and the modulus increased by 90% when only 4% clay was used. PMMA-clay nanocomposites with 20% clay outperformed pure polymer by 60%, according to Lee and Jang. This method is most commonly used in petroleum-based matrices such as PA, PE, or PET. But, due to health concerns, their use in food packaging is still unknown. Carbon-based nanoparticles such as carbon nanofibers and CNTs have improved mechanical strength for packaging applications. CaCO3 nanocomposite calcium carbonate or kaolinite in polyolefin matrix enhanced tensile strength and modulus [12]. In semi-crystalline polymers, improving mechanical properties often means improving thermal resistance. Thermal distortion temperature jumps of up to 87  C have been achieved in PA/clay nanocomposites. Copper nanoparticles increase the thermal conductivity and heat capacity of LDPE and HDPE, raising the breakdown temperature and optical properties and gas scavenging [13].

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Polymer Nanocomposites: Methods of Preparation Polymer nanocomposites can be manufactured using one of two fundamental processes. The production of inorganic particles as metal oxide nanoparticles in situ and fillers dispersion such as multilayered nanoclays in a polymeric matrix makes it possible to fabricate them. To generate nanomaterials with desirable qualities based on a uniform distribution of nanodomains inside a continuous polymer matrix, suitable preparation process selection is critical. Approaches to polymer nanocomposites synthesis are bottom-up or top-down, both viable options. A precursor is the beginning point for developing well-defined structures at the nanoscale level in a bottom-up approach to nanoscale engineering. The method, which is distinct from block assembly, refers to combining a large number of Nanosized materials developed. The target material can be created by hierarchically combining nanostructures that have already been formed [14]. Bulk material is broken down into tiny (occasionally designed) parts (nanoparticles) using largely physical means in the top-down approach. When stacked Nano-sized silicates are suspended inside a polymer matrix, the resulting dispersion is the most prevalent type of dispersion seen in nature (Fig. 2). Five major approaches (1–5) are employed to prepare polymer matrices containing dispersed inorganic nanoparticles. Alternatively, a top-down methodology is used in the first two situations to inject nanoparticles into an unsolidified polymer matrix; in these cases, a solution-based method is used to mix the filler with the polymeric matrix, and a melt-based method is used to melt the polymeric matrix (2). In situ polymerization of monomers in the presence of nanoparticles (3) and situ production of organic particles in the presence of a polymer (4). Finally, inorganic and organic components (polymer matrix) can be manufactured in situ during composite preparation, saving money and time (5) [1].

a

Layered Silicate

Polymer

Polymer Nanocomposites

b Cluster formation

Polymerization In organic precursor

Polymer

Polymer Nanocomposites

Fig. 2 Outline of (a) top-down and (b) bottom-up approaches. (Adapted with permission from Ref. [1]. Copyright 2021, Elsevier)

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Preparation from Solution When a dispersed polymer is disseminated into an aqueous solution containing layered silicon and allowed to react with the silicon, this process is known as exfoliation adsorption from a solution or polymer or pre-polymer intercalation from solution. Another step involves inflating and dispersing the layered silicate (Fig. 3) in a solvent solution before mixing it with the polymer solution in a reaction tank. Intercalating polymer chains displace the solvent between the silicate interlayers by intercalating and dissolving it in the space between the silicate interlayers, resulting in the displacement of the solvent between the silicate interlayers. The sheets reconstruct and encircle the polymer chains once the solvent is removed, resulting in creating a multilayer structure [1]. Intercalated nanocomposites are made up of water-soluble polymers that have little or no polarity. These include polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrole was done, as well as polyacrylic acid, which is generally employed in intercalated nanocomposites production. It isn’t ecologically friendly, even though this approach is not harmful to the environment because of the large amounts of solvents used and the fact that only about 10% of the original material was solid at the time of the experiment. Embedded emulsion polymerization is a term used in the science of polymerization to describe the polymerization of an organic monomer dispersed in an aqueous medium after being spread on a surface. It is essential to add

Fig. 3 Layered silicate’s structure. (Adapted with permission from Ref. [1]. Copyright 2021, Elsevier)

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various concentrations of silicate typically reaching between 1% and 5% to the monomers, mostly composed of methyl methacrylate and styrene monomers. When working with water, it is required to spread the monomers with the help of emulsifiers, which are often nonionic polyethoxylated or ionic alpha-oléfin sulfonates. Once the monomer has been polymerized, it results in a nanocomposite containing silicate within the polymer particle itself and adsorbs on its surface [1].

Preparation by Melt Mixing This method comprises heating a polymer-nanoparticle mixture over the polymer’s softening point, commonly under shear stress, to dissolve the nanoparticles and produce nanocomposites multi-layer polymer. Polymer chains can spread between host monolayers during annealing because of the flexibility of melted polymer chains. As a result, it is possible to obtain an exfoliated Nano-material distribution. An organic polymer matrix and an inorganic nanomaterial with differing polarity degrees may experience significant dispersion inhibition, worse intercalated layout, or aggregates (tactoid confirmation), as shown in Fig. 4 because different chemical components tend to stick together [15].

Fig. 4 Schematic representation of different layered silicates-polymer composites based on their dispersion effectiveness. (A) tactoid (aggregates), (B) intercalated, and (C) exfoliated are the three types of exfoliation (nanocomposites). (Adapted with permission from Ref. [1]. Copyright 2021, Elsevier)

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A popular strategy for relieving this difficulty is functionalizing nanoparticles with organic groups compatible with polymers [16]. Because of the low heat resistance of nanoparticles and grafted functional groups and the high temperatures required to melt a polymer, it is possible that nanomaterials will be destroyed, resulting in poor final composite performance, which is regrettable. For the nanocomposite to be successful, several variables must be addressed, including the type of functional groups on the nanoparticles, the processing temperature, and the amount of shear stress applied [16]. Compared to other methodologies that rely on material solubility or synthetic processes, the melt mixing process has many advantages, including its environmental friendliness, compatibility with a varied range of polymer matrices, and compatibility with industrial processes due to the lack of the need for an organic solvent.

Preparation via In Situ Polymerization Compared to other methodologies that rely on material solubility or synthetic processes, the melt mixing process has many advantages, including its environmental friendliness, suitability for a varied range of polymer matrices, and industrial processes due to the lack of the need for an organic solvent. It is probable that significant dispersion will be hampered as an effect of the polarity mismatch among the organic polymer matrix and the inorganic nanomaterial, resulting in a less desirable intercalated layout or, worse intercalated layout or aggregates (tactoid confirmation) because different chemical components tend to stick together in the presence of a polarity mismatch [17]. A popular strategy for relieving this difficulty is functionalizing nanoparticles with organic groups compatible with polymers. Because the interaction between nanoparticles and grafted functional groups has low heat resistance, along with the high temperatures required to melt a polymer, it is possible that nanomaterials will be destroyed, resulting in poor final composite performance, which is regrettable. For the nanocomposite to be successful, several variables must be addressed, including the type of functional groups on the nanoparticles, the processing temperature, and the amount of shear stress applied. In Situ Synthesis Nanoparticle Preparation Because of this bottom-up method, nanocomposite constituents can be produced in situ, allowing for the fabrication of well-defined structures with attributes unique from their original predecessors. When it comes to the synthesis of nanoparticles, a polymer is frequently used as a reaction medium. The following step creates the necessary nanoparticles by chemically converting their precursor [18]. On the other hand, polymers are organic molecules that are both strong and chemically stable. Due to the synergetic qualities of the resulting nanocomposites, they can be used in applications that would be impossible to do with either the polymeric matrix or the nanoparticles alone. Ex-situ or in situ procedures can be used when incorporating metal nanoparticles into a polymeric matrix. Organic nanoparticles are created in the ex-situ process and then introduced into a polymer solution or melt, where they dissolve and become a polymer matrix component. This phenomenon is caused by the physical trapping of

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metal or metal oxide nanoparticles in a polymer network, which is the underlying mechanism of how it occurs. However, achieving a homogeneous dispersion of the nanoparticles in the polymer matrix is a difficult task. However, the in situ approach can be utilized to overcome this obstacle. Using a metal precursor during the polymerization process is necessary to form metallic or metallic oxide particles within the polymer phase, which are then converted to the desired nanoparticle size. Particle size and shape can be regulated more precisely when using an in-situ approach [18].

Preparation by Inorganic Synthesis and In Situ Polymerization To create polymer nanocomposites, it is necessary to build a polymeric matrix while also scattering nanoparticles that would later become incorporated into the polymer. The sol-gel process used for the in-situ production of polymer nanocomposites in various applications is the most extensively utilized technology. This method of transitioning from a solution state (a colloidal suspension of solid particles in a liquid phase) to a gel state relies on two chemical pathways to accomplish the task (a network of interconnected phases) [18]. A network of interconnected networks between phases exists in the solution state instead of the gel state, which is a colloidal suspension of solid particles in a liquid phase. Gels are composed of interconnected networks that span several phases transitions, while solutions are composed of colloidal suspensions of solid particles suspended in a liquid phase. When a solution is formed, solid particles are suspended in a colloidal suspension in a liquid phase. Still, the gel state is formed when a network of interconnected networks forms and spans the transition between the two phases. Precursors are first dissolved in water and hydrolyzed to make reactive groups used in the subsequent process; next, the reactive groups that have just been formed are condensed to form the final network [19].

High Barrier Characteristics of Polymer Nanocomposites Because of the reduction in oxygen, water vapor, and scent transfer through the matrix due to better gas barrier qualities, food can be kept fresher for longer periods. Nanoparticles, often in stacked Nano-platelets, act as a gas barrier, preventing gas infiltration. After adsorbing on the polymer surface, gas molecules diffuse through the bulk polymer matrix, permeating polymer nanocomposite materials and allowing them to function properly. The authors of 2009 describe the process as follows: As a result, most studies in this field focus on estimating diffusion and permeability coefficients rather than solubility coefficients. Exfoliation degree in the polymer nanocomposite influences gas diffusion in the nanocomposite [20]. This phenomenon is based on the “tortuous path” (Fig. 5). The particles are surrounded by a polymer matrix that allows gas to pass through these layered structured materials, lengthening the effective route length. Thus, platelets with a 1000 aspect ratio were found to be exfoliated, and with a volume

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Fig. 5 Tortuous pathway for Gas molecule diagram in exfoliated platelets in polymer nanocomposites. (Adapted with permission from Ref. [1]. Copyright 2021, Elsevier)

fraction of 0.07, total oxygen transfer can be reduced by 97%. Over 2 days, the oxygen transfers rate of the neat polyamide-6 (PA-6) was lowered from 2.2 to 0.45 cc mil/100 by Gupta et al. [21]. By adding oxygen scavengers into the matrix, OTR was also lowered. Chemical modification of natural kaolinite improved exfoliation in Ethylene vinyl alcohol EVOH-kaolinite nanocomposites. According to Cabedo et al., the treatment improved the nanocomposites’ oxygen barrier qualities compared to a neat polymer. Also superior to pure polymer in terms of heat resistance, glass transition temperature, and crystallinity were the nanocomposites’ heat resistance, glass transition temperature, and crystallinity. Also superior to pure polymer in terms of heat resistance, glass transition temperature, and crystallinity were the nanocomposites’ heat resistance, glass transition temperature, and crystallinity. According to their, de Abreu et al. discovered that 5% layered nano clay in polypropylene and OTR was decreased from 480 to 374 cc/m2 per day using polyethylene terephthalate composites [22]. Thellen et al. developed coextrusion multilayer films with an oxygen barrier layer of montmorillonite layered silicate and a hydrophobic layer of LDPE. The OTR of the neat film is reduced from 3.7 to 1.1 cc/m2 per day with a nanofiller level of 3.3–3.6%. Using polymer melt intercalation, the researchers created PET nanocomposites and two nano clay platelets. The lowest oxygen permeability was found in nanoclay/PET nanocomposite films containing 1% nano clay. Cloisite 15 A has the best barrier characteristics at a concentration of 2% a commercial montmorillonite type layered silicate. In both situations, exfoliation lowered permeability by a significant amount [23].

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Polymer Nanocomposites of Polysaccharides Polysaccharides from Lignocellulose Plants and Woods Sources Cellulose is the richest biopolymer found in nature, and it provides both a renewable and biodegradable source of energy. It is utilized as a structural component by plants and bacteria [24]. It is classified as a linear polysaccharide [25]. There are repeating cellobiose units in this polysaccharide, a mixture of two anhydroglucose rings joined together by a 1,4 glycosidic bond. Since the turn of the century, Cellulose derivatives have been used to produce food packaging. Cellulose and its derivatives can mechanically strengthen and improve the barrier qualities of polymer materials, and they are commonly used in this capacity. A further advantage of cellulose derivatives is that they are more resistant to microbes and enzyme digestion, meaning that they are more durable than native cellulose [26].

Cellulose Ethers Cellulose ethers (CE) are a water-soluble polymer that is extensively used to modify cement mortars due to their unique water retention and thickening properties. The raw ingredient used to make these items is cellulose, which is frequently sourced from cotton linter or wood pulp, among other sources. Even though each unit of the long repeating anhydroglucose molecular chain has three hydrophilic hydroxyl groups, cellulose itself is not water-soluble due to its extremely crystalline structure. During the manufacturing process, caustic soda is used to reactivate it. It combines with functional polymers such as methyl chloride, ethylene oxide, or propylene oxide to generate cellulose ethers, various organic compounds. In Fig. 6, a typical technique of producing cellulose ethers is illustrated [27]. These high-molecular-weight compounds function by substituting alkyl groups for the hydroxyl groups in anhydroglucose units, causing cellulose ethers synthesis due to the substitution. Additionally, it is vital to know the solubility and viscosity of these cellulose derivatives in solution as well as their surface activity, thermoplastic film properties, and stability against heat that are influenced by the substituent group distribution of these cellulose derivatives as well as their stability against heat and other environmental factors. MC, EC, HEC, HPC, HPMC, and CMC (Fig. 7) are just a few of the abbreviations often used in medical coding and administration. When employed as a specific binder, it has great Variety and usual applications: rheology modifiers and water-holding agents [27]. High-purity microcrystalline cellulose (HPMC) is an edible film-forming substance that is odorless, flavorless, clear, stable, oil-resistant, and nontoxic. Biodegradable thermoplastic polymers MC, HPMC, and HPC, create a hard gel when

Cellulose

Grinding

Reaction

Purification

Drying

Milling

Screening

packaging

Fig. 6 Cellulose ethers manufacturing process. (Adapted with permission from Ref. [27], Copyright (2021), Elsevier)

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OH

OH

OH

O

O

O O

HO

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OMe

O

O HO

HO OMe

OH

Fig. 7 Molecular structure of typical CE: (a) HEMC; (b) HPMC; (c) HEC. (Adapted with permission from Ref. [27]. Copyright 2021, Elsevier)

heated to 50–80  C. HPMC can also be used as a film-forming material for the barrier and mechanical control paper coating properties, among other uses. Additionally, plasticizers or inorganic nanoparticles, for example, starch, glycerol, graphene oxide, can be included in the polymer structure to increase these qualities [27]. Comparing coated paper with HPMC without plasticizers to uncoated paper without plasticizers exhibits a 25% drop in WVP and water absorption capacity. Using plasticizers in HPMC coatings enhances the water vapor pressure (WVP) and coating flexibility. HPMC is also utilized to adjust the coating’s barrier and mechanical qualities. Beeswax can also improve the barrier properties and smoothness of HPMC-coated sheets and their smoothness.

Cellulose Esters Inorganic cellulose esters (cellulose acetate) are distinguishable from organic cellulose esters (cellulose acetic acid) (cellulose nitrate and cellulose sulfate). In conjunction with cellulose ethers, these water-insoluble polymers with good filmforming characteristics are commonly used to fabricate microporous membranes for various applications, including biomedical devices. Since the 1960s, it has become increasingly popular in commercial and pharmaceutical applications. Inorganic cellulose esters (ex, cellulose nitrate) are clear chemicals with good film-forming capabilities, but they are rarely used alone due to their limited solubility and high combustibility. However, they are seldom used alone due to their low solubility and high combustibility. Food packaging is one application for inorganic cellulose esters employed in a range of industries. Its nontoxicity, edibility, and biocompatibility are all outstanding characteristics. Cellulose acetate is a chemical compound widely found in films and fabrics. Cellulose esters are used in paper packaging to laminate the paper, which makes it more durable for example, cellulose acetate foils can be applied to the surface of paper or paperboard and heat laminated or bonded to the surface to produce food packaging. Nanofibers made of cellulose acetate were created, with possible applications in fresh fruits and vegetables. Despite the high cost of derivatization, cellulose derivatives show promise in visually appealing production, food packaging, and packaging materials that are biodegradable and functional (e.g., membranes, edible films, or paper coatings) [28].

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Hemicelluloses

Hemicellulose-based bio-adsorbents

Heat Cross-linking

Hydrogels

• • • •

Activated carbons

Heavy metal ions Dyes Water Salt solutions

• • • •

Heavy metal ions Dyes Organics Gases

pollutants

Fig. 8 Schematic illustration of hemicelluloses used to make bio-adsorbents. (Adapted with permission from Ref. [31]. Copyright 2021, Elsevier)

Cellulose Micro (Nano) Fibrillated Structures In recent years, interest in the manufacture of micro (nano) scale cellulose fibers has increased due to the unique qualities of cellulose fibers (high strength and stiffness, low weight, biodegradability, and renewability). Amini and colleagues [29] created antimicrobial packing papers utilizing cellulose nanofibers coated with silver nanoparticles (NFC/Ag). These thin coatings of paint were applied to both kraft and greaseproof sheets of paper. Despite the hydrophilic nature of NFC, the coatings on kraft paper demonstrated excellent resistance to both water vapor movement and absorption of water. Results demonstrate that covering kraft and greaseproof paper with NFC/Ag increases their C potential as solid and oily goods environmentally-friendly antimicrobial packaging. When applied to greaseproof paper, the coating did not increase water resistance. This difference is due to water sorption on the two substrates.

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Hemicelluloses Hemicelluloses (HCs) (Fig. 8) are the second most prevalent polymer in plant cell walls, behind cellulose [30]. Hemicellulose-based bio adsorbents are fast evolving into super adsorbents. Figure 6 depicts the utilization of hemicelluloses in the manufacture of super adsorbent materials. Hemicellulose-based materials are promising bio adsorbents in the carbon cycle, with the primary goal of removing heavy metal ions or dyes during wastewater purification and water uptake. To better comprehend and provide future perspectives on hemicellulose-based bioadsorbents, this chapter covers these hemicellulose-based materials from the perspectives of adsorbates, manufacturing methods, the mechanism of adsorption, and effects on adsorption capacity. HCs in Food Packaging according to research, the market for HCs is growing, and this polysaccharide class has various potential applications. Based on research, hemicelluloses from cereal straws can be used as a fermentation feedstock in a chemical manufacturing platform (furfural, ethanol, acetone, butanol, xylose, and xylitol). Furfural is a chemical compound generated from pentosanes found in agricultural waste and hardwoods, and it is utilized in the production of nylon. In the commercial world, xylitol and mannitol are highly valuable commercial chemicals generated from xylan and mannan [30]. There have also been reports of other hemicellulosederived oligosaccharides used as prebiotics in food and feed. Hemicelluloses, either naturally occurring or synthetically modified, have been employed in the paper industry to reinforce cellulose nanocomposites. While most HCs used as barrier materials are for standalone packaging films, others are used as coatings for packaging paper [32]. Water-soluble and film-forming HCs are produced by extracting HCs that retain most of their side chains. Xylan/ glucomannan films provide effective oxygen barriers, other gases, grease, and scent, making them suitable for food paper packaging. Wet conditions reduce the barrier and strength properties of HCs films. The structural structure of hemicelluloses causes many challenges when employed as packaging materials. This includes thermos stability, mechanical characteristics, and water vapor permeability [33]. Hemicelluloses have a high concentration of hydroxyl groups on their backbone and side chains, naturally hydrophilic. When used in packaging materials, this results in moisture barriers and water protection that are not as effective. Because of the low molecular weight and degree of polymerization (DP) of hemicellulosebased polymers have poor mechanical strength and flexibility. In addition to having limited mechanical strength, Hemicellulose films are often moisture-sensitive [34]. Because of its affinity for cellulose, xylan hemicellulose is utilized to reinforce paper and biocomposites. Its features, such as hydrophobicity, thermal formability, and film formation, can be chemically altered. This last property is required for self-supporting barrier films used in food packaging. Derivatives with better filmforming characteristics come from plasticizing and hydroxypropylating xylan. Internal hydroxyl propyl plasticization can be supplemented with external glycerol or sorbitol plasticization. A xylan-based product (Skalax) with barrier qualities against

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oxygen, scent, grease, and mineral oil has been developed in Sweden [7]. The limited stretch ability of xylan-based films for food packaging reduces water vapor permeability. It is used to coat food or packaging with an oxygen barrier and provide mechanical support (like paper). Xylan’s hemicellulose can also be used to wrap food. While the raw material for xylan hemicellulose is plentiful, commercial availability and large-scale extraction are limited. Xylan HC was also employed to increase water absorption in Nano-fibrillated cellulose-based nanocomposite films. The results show that hemicellulose can be used as a plasticizer in NFC films and that bioinspired nanocomposite films for food packaging are feasible [35]. Glucomannan hemicellulose is abundant in coniferous (spruce) wood and has remarkable film-forming capabilities. Paper mills and fiberboard manufacturers that produce newspapers can supply you with this material. Glucomannan films provide excellent gas barrier properties. Therefore, they could be employed as barrier coatings for food packaging made of cardboard or paper. Cross-linked glucomannan is soluble in both water and organic solvents, and it has excellent water absorption capabilities (gels, etc.). It was discovered that using plasticizers enhanced the film’s flexibility and moisture sensitivity as Glycerol, sorbitol, xylitol, alginate, and carboxy methyl cellulose are among the substances used. Increased mechanical strength and humidity resistance were achieved by combining biopolymers with other materials. Hydrophobic softwood galacto-glucomannan films with low moisture sensitivity and good barrier properties were made using plasma and vapor phase treatments [35].

Starch Starch is the most commonly used bio additive in the paper industry, and it is found in wet-end, surface/coatings, and coatings applications in particular. Figure 9 shows the four stages of biodegradation of starch–PU hybrid films. The initial stage included the hybrid films absorbing water, allowing microbes to bind to their surface. In the second stage, films began to degrade, resulting in surface erosion. In the second stage, microorganisms degraded starch as a primary carbon source. Microbial hydrolysis of polymeric chains produced water, CO2, and other metabolic biomass. Stage 3 saw the microorganisms invading the film matrix and breaking down the starch–PU hybrid polymers into smaller molecular units. The third stage ended with a rough and porous structure. Microorganisms destroyed most of the starch component of the starch–PU hybrid film leaving only the urethane component. The third stage required more time to degrade the ester/ether urethane chain. Because of the low starch concentration and high starch-PEG-PU interaction, the chemically crosslinked HS-20PEG-PU film degraded the slowest [36]. Starch is a scaling agent (paper machine size press) or as a coating ingredient in various applications (after chemical, physical, or enzymatic changes). Starch can be found in various foods, including potatoes, corn, wheat, tapioca, and rice [36]. Mineral nanoparticles (ZnO, MgO, metallic ions, nanoclays) or nano-fibrillated cellulose

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Fig. 9 Degradation of starch–PU hybrid films in soil burial (54% relative humidity, 22 3  C, dark storage). (Adapted with permission from Ref. [36]. Copyright 2021, Elsevier)

are good transporters for compounds with a barrier or active antibacterial characteristics (ZnO, MgO, metallic ions, nanoclays). The optical characteristics of coated paper are improved using carboxymethyl starch and ZnO nanoparticle composite coatings. Compared to bulk ZnO-coated paper, these coated papers also had outstanding antifungal and UV protection. This is explained by the sluggish breakdown of ZnO in wet settings, resulting in immobilized Zn2+ ions. This improves paper life. According to Johansson et al. [37], they improved starch hydrophobicity in paper coatings. The starch suspension is colored using mineral pigments (hyper platy nano clays) (highly plate particles). As a result of good water, oil, and air barriers, the coating color is applied thinly to the surface of the paper and board.

Marine Biomass Polysaccharides Chitosan and Chitosan Derivatives Other polymers (such PLA), Nano-fibrillated cellulose, or inorganic nanoparticles (Nano clays, metallic oxides, and silver nanoparticles) can increase the antibacterial and barrier properties of chitosan. Adding chitosan and propolis to paper and cellulose packaging boosted antibacterial and antioxidant properties. Other studies used gelatin-chitosan as edible coatings on fresh-cut melons as food models. Two-component edible coatings reduce microbiological (bacteria and fungi) spoilage [38].

Alginates Alginate is a polysaccharide found in brown algae as sodium and calcium salts of alginic acid. Alginate is one of the most promising and extensively studied biopolymers due to its film-forming capabilities, nontoxicity, biodegradability, and

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biocompatibility. Alginates are made up of (1–4)-linked L-guluronic acid and M-d-mannuronic acid. Because alginates and their derivatives are already widely used as food additives, these biopolymers are deemed acceptable as functional barriers for food-contact materials. On the market are also water-soluble alginate formulations that can be utilized with traditional coating processes in the paper and packaging industries [39]. However, alginate coatings, either alone or in combination with other biopolymers, have been shown to increase water barrier qualities of coated paper (i.e., chitosan). The results reveal that this biopolymer does not reduce paper water resistance but has a synergistic impact when combined with chitosan [40].

Semolina with Embedded Nanokaolin Semolina flour is a type of wheat flour that contains a substantial amount of gluten, which modifies the nutritional qualities of a range of recipes when used as a substitute for regular flour. In addition to having antioxidant characteristics, semolina is a light-colored, extra-hard, fine grain. It has been claimed that nano kaolinenhanced semolina exhibits favorable qualities for use as an edible film and the ability to improve its mechanical, physicochemical, and barrier properties, among other things. Nano kaoline increases the sensitivity to water, the water vapor permeability (WVP), the permeability to oxygen, the mechanical properties, and the barrier characteristics of materials. Regarding water susceptibility, it was discovered that a high concentration of Nano-kaolin was associated with a gradual decrease in the film’s moisture content. When the plasticizers, biopolymers, and Nano-kaolin were combined, the hydroxyl group’s interface with water was reduced. This resulted in a less hydrophilic matrix, which is assumed to be the origin of this characteristic. First and foremost, the water vapor permeability (WVP) was 8.61 10 7; however, this value can be decreased by increasing the proportion of kaolin in the mixture. WVP is described as the transfer of moisture from ambient air to food or between two components of a heterogeneous product with varying humidity values when it comes to moisture transfer. The WVP can be lowered to 4.58 10 7 by, for example, adding 5% kaolin to the mixture. A lower oxygen permeability coefficient is achieved by including nanoparticles in the film path because they create a more complicated path for the oxygen molecule to travel through. This has the consequence that the oxygen permeability is reduced by increasing the kaolin in the matrix. After adding kaolin nanoparticles to a polymer matrix, mechanical characteristics such as tensile strength (TS), elongation-at-break (EB), and Young’s modulus become more favorable [41]. Bacteria, algae, and higher plants produce biopolymer cellulose (b-1, 4-Dglucopyranose polymer), which is used in the production of paper. A unique property of this material is its capacity to minimize water loss from dry areas while also absorbing undesirable liquids from a wound. This characteristic has the effect of speeding up the healing of deep ulcers. However, because cellulose’s

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feature of encouraging bacteria growth allows it to be used in wound dressings, antimicrobial therapies including cellulose are recommended [42].

Cellulose As a support material for some nanomaterials, cellulose is used to allow the exposed surface of the nanoparticles to expand, resulting in increased performance. Aside from that, the antibacterial activity of adsorbed silver nanoparticles on cellulose fibers against S. aureus and E. coli is extremely strong, with antimicrobial activity reaching up to 99.99% due to interactions between oxygen (derived from cellulose) and the silver. In this study, silver, gold, and platinum nanoparticles were generated and embedded in a cellulose gel using the hydrothermal reduction method, subsequently dried using supercritical CO2. Apart from having extraordinary porosity and surface area, the aerogels produced also possessed excellent mechanical strength and moderate thermal stability, amongst other properties. Previous research examined the crystallinity index of the newly produced nano cellulose material, which was shown to be lower when compared to microcrystalline cellulose, enhancing the prospect of employing it as a biodegradable composite film enhancer in composite films. Another study discovered that by mixing cellulose nanocrystals with starch-based nanocomposite films, the permeability of Dlimonene could be modulated. It tested for its antibacterial properties using cellulose, which was successful (E. coli and S.aureus) [43, 44].

Probiotic Cellulose Antibacterial Activity Several studies have examined the antibacterial activity of probiotic cellulose against SA and PA, two opportunistic pathogens that cause a wide spectrum of skin diseases, some of which are potentially lethal (severe and chronic wounds). Both Lf and Lg have demonstrated antibacterial action against SA and PA in a medium that promotes probiotics’ growth. When tested on ideal pathogenic media, such as TSA, we discovered that neither Lf nor Lg could prevent the growth of SA or PA. Since the virus and the probiotic meet in a real-life infection scenario, this minor subtlety is critical since the pathogen and probiotic are in an environment that is favorable for the former but not optimal for the latter. To assess the antibacterial activity of probiotic cellulose, we conducted the agar diffusion studies indicated, in which the pathogens were exposed to the probiotic cellulose. These intriguing findings prompted us to investigate the anti-MRSA potential of probiotic cellulose using agar diffusion experiments, which we conducted (Fig. 10) [45]. Experiments using time-kill supported these findings. When SA or PA was grown on TSB (an unfavorable medium for probiotics) [36, 43], pathogen growth was observed, with initial loads of 10 6–10 7 CFU increasing to 10 9 CFU after 24 hours (Fig. 9a). The addition of bacterial cellulose did not affect pathogen proliferation in a control experiment (Fig. 9a, SA + BC or PA + BC bars). However, pathogen viability was significantly reduced when probiotic cellulose (Lf- or Lg cellulose) was used instead of bacterial cellulose. Lf- cellulose inhibited PA and SA growth after 24 h,

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Fig. 10 Probiotic cellulose inhibitors: (a) Experimental technique for determining inhibitory activity. (b) The efficacy of probiotic cellulose (Lf- and Lg-cellulose) and BC with adsorbed probiotics (BC + Lf and BC + Lg) against SA and PA. (Adapted with permission from Ref. [45]. Copyright 2022, Elsevier)

but Lg- cellulose effectively killed PA and significantly reduced SA viability (Figs. 9b and 12) [45]. As shown in Fig. 12, both Lf- and Lg-cellulose were effective in inhibiting the growth of MRSA. In addition to probiotic celluloses for BC + Lf and BC + Lg (biomaterials generated using an adsorption incubation process). However, the inhibition zones for these biomaterials were significantly smaller than those observed for probiotic celluloses (Fig. 12) [45].

Polymers Biodegradability After the Formation of Nanocomposite/Composite Chitosan Chitosan is a linear polysaccharide compound formed of β-(1–4)-linked D-glucosamine and N-acetyl-glucosamine. The advantages of chitosan have mineral complexation, biocompatibility, and non-toxic properties used in commercial perspective. Chitosan has active sites bonding and forms aldimines with chitosan to produce N-alkyl chitosan used in prepared films. Pandey et al. [46] stated that meat packaged in polyvinyl alcohol (PVA) composed with chitosan and silver nano-layers is better than packaged in conventional plastic. Where chitosan has an antimicrobial

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Fig. 11 shows probiotic cellulose’s antibacterial action. (a) Survival of PA and SA following co-incubation in TSB with BC or probiotic celluloses (Lf- and Lg-cellulose). Statistical significance (p 0.001) and no significance (ns) are indicated by asterisks and ns, respectively. (b) Comparison of the bactericidal characteristics of Lf- and Lg-cellulose with the equivalent control assays. (Adapted with permission from Ref. [45]. Copyright 2022, Elsevier)

effect and extends keeping quality food on shelf-life for 1 week by an eco-friendly method. Kumar et al. [47] reported that the hybrid nanocomposite film of chitosan and silver nanoparticles used in packaging extended the shelf life of red grape for extra 2 weeks. Chitosan enters for synthesizes microneedles which use to deliver drugs through transdermal. Where nanoparticles prepared from the chitosan-1 wt% graphene quantum dots have many advantages, such as low cytotoxicity, strong for inserting drugs into the body and stimulating drug release behavior through iontophoretic for small and large molecular weight drugs. Kumar et al. [48] showed that chitosan – gelatin Nano-compound hybrid films composite of zinc oxide nanoparticles has antimicrobial activity for Escherichia coli and Staphylococcus aureus in Cassia fistula fruit. So, hybrid nanocomposite films can package fresh fruits and vegetables after postharvest. Chitosan/TiO2 film is considered a good food packaging material that inhibits the growth of pathogenic microorganisms in red grapes. Chitosan–TiO2 film is

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Fig. 12 Probiotic cellulose (a, Lf -cellulose and b, Lg -cellulose) inhibits MRSA growth on TSA plates. BC + Lg (a) and pure BC (b) were also tested. Despite the pathogen-friendly medium, clear inhibitory zones emerged around the probiotic cellulose and to a lesser extent around the adsorption-incubation biomaterials (BC + Lf and BC + Lg). (Adapted with permission from Ref. [45]. Copyright 2022, Elsevier)

considered a good packaging material used in Gingko Biloba seeds and black plum peel packing that prevented fungi growth, antioxidant capacity, and decreased ethylene production, which is food preservation. Zhang et al. [49] and Yuan et al. [50] showed that Chitosan–TiO2 coating film could elongate the shelf life of red grapes and Stauntonvine fruit at 37 C up to (15 days, 45 days respectively), where keep mechanical properties, ascorbic acid ratio, pH change compared to the chitosan coating film. Gohargani et al. [51] reported that corporate whey protein -chitosan-TiO2-Zataria multiflora oil composite film noted improved antimicrobial and mechanical properties of food as good packaging material. Stephen [52] reported the advantages of the combination of Titanium dioxide (TiO2) with chitosan matrix, which can use as antibacterial and antifungal in packing food, photocatalytic properties, purification of water from heavy metal, wastewater treatment, and manufacturing microneedles for drug delivery. Chitosan–TiO2 composite can be used for purification water from hexavalent chromium by reducing Cr(VI) to Cr(III), which is lower toxicity as reported by [53]. Chitosan–TiO2 nanocomposites hybrid can eliminate Cu (II) and Pb (II) from water. Where Chitosan can absorb Cu (II), Pb (II) (710 and 526 mg/g respectively) while TiO2 absorbs (Cu (II) 579 mg/g, Pb (II) 475 mg/g) after 30 min at 45  C, it is due to the concentration of metal ion. It has been noted that the anti-pathogenic activity of chitosan increase when combined with TiO2 in a nanocomposite matrix. Chitosan–TiO2 composite membrane work as a wound dressing where Behera et al. [54] revealed that CS–TiO2 composite membrane has no cytotoxicity effect on L929 cells growth and excellent

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antioxidant capability, subsequently increasing reproduction and endurance of fibroblast cells. Chitosan–TiO2 composite and Chitosan–pectin–TiO2 showed antiinflammatory effects by decreasing inflammatory factors and stimulating blood coagulation which helps wounds heal. Evidence proves that chitosan-Titanium dioxide composite has a good effect on the nutrition and growth of plants. Titanium dioxide and chitosan nanoparticles are extracted from (pomegranate rind and Aspergillus brasiliensis respectively), so they can apply as liquid spray or powder for leaf feeding and soil Nano-fertilizers [55].

Starch and Thermoplastic Starch Crop starch, a polysaccharide made up of glucose molecules and classed as such, can be obtained from various sources. The biggest advantage of starch is that it can be converted to thermoplastic starch by plasticizer example, sorbitol, fructose, or glycerol. Starch is commonly used as a food packaging material because of its nontoxicity, availability, antibacterial activity, and stability at room temperature. The modified starch-based films’ mechanical characteristics and thermal stability have been improved. Many studies showed that biodegradable starch films are used for food preservation by prolonged food shelf life [56]. Müller et al. [57] synthesized thermoplastic starch modifies films by blending starch with 5% hygroscopic nanoparticles by extrusion and thermos pressing method to strengthen hydrophilic, mechanical, hydrophobic properties, dispersion and reduce the permeability to water vapor in the polymer matrix, so these films can use in industrial manufacture. Ayana et al. [58] reported nanocomposite films prepared by thermoplastic (potato) starch mixing with polylactic acid, clay, and sodium montmorillonite during eco-friendly, which has improved mechanical and thermomechanical properties so could use in packaging food. Chung et al. prepared the dispersed starch–clay nanocomposites by using a concentration starch sol to lower the viscosity and promote the miscibility with clay and adding clay suspension to increase clay dispersion, then pour ethanol showing in Fig. 11. López et al. [59] suggest using thermoplastic corn starch, copper nanoparticles, and silica-coated copper microparticles to form biodegradable films for packaging meat products because copper has an antimicrobial effect increases tensile strength, thus more elasticity.

Thermoplastic Starch with Silver Nanoparticles Silver nanoparticles have an antimicrobial effect and optical properties, applied in many chemical and biological implementations. Many metal nanocomposites like silver are most used to form film nanoparticles due to their stability, thermal, optical properties and reinforce biopolymer matrices, in addition to the antimicrobial effect against bacteria, viruses, and fungi. Rozilah et al. [60] reported that starch-silver nanocomposite film could use as antibacterial coatings when using 3 wt. % silver nanoparticles in the mechanical and

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antimicrobial properties. Yahia et al. [61] showed that using modified nanocomposite films from starch-polyvinyl alcohol-sorbitol-cardanol oil with silver nanoparticles is better than using starch. That was due to lower water vapor, more plasticization, increased film elongation, and decreased transmittance; in addition, silver nanoparticles work as anti-gram-positive and negative bacteria used as food preservation and packaging materials. Smart-biodegradable packaging is newly used for the food packaging industry, so Knitter et al. used nanocomposite modified with Mater-Bi. Pseudomonas aeruginosa, Escherichia coli, and Listeria monocytogenes were tested on silver, produced using an injection molding approach and showed improved mechanical and thermal capabilities against these bacteria. As a result, food packaging can be used instead of polyethylene polymers in the Mater-Bi/Ag composite Ponsanti et al. [62] studied the structure of nanoparticle films by silver nanoparticles (flowers shaped) and starch from (corn, cassava, and sago), where starch reduces silver and silver ions from silver nitrate. Transmission electron microscopy appears silver as flower shape and found silver nanoparticles from cassava starch, and sago starch is bigger than corn starch. Ortega et al. [63] tested silver nanoparticles as an antibacterial effect, where prepared films from low concentrations of silver nanoparticles with genic film suspensions (gelatinized starch) and determined by the spectrophotometric method. Silver nanoparticles improved film thickness opacity, reduced water vapor permeability, increased solid material with a soft surface, so suggest using 143 ppm of silver nanoparticles in packaging cheese where an increased shelf life for 21 days and inhibited bacteria growth like Salmonella spp and E. coli. Da Silva Rodrigues et al. [64] stated that the best method for preserving and packing pork with film from thermoplastic starch and polylactic acid covered with silver nanoparticles where silver prevented the growth of bacteria more than 7 days from storage and increased lipid oxidation comparison to without silver in addition to maintain mechanical properties and water vapor permeability.

Thermoplastic Starch with Talc Nanoparticles Talc is considered the strongest filler in polymeric composite due to its layered mineral with particle diameter and nonmetric thickness. Thermoplastic starch–talc film is enhancement torque more and fixed until the end of the mixing time, and hydrogen bonds need to interact with starch granules [65]. López et al. [66] developed packaging bags made from thermoplastic starch – talc nanoparticles by concentrating 3% w/w, where the addition of Talc to starch makes opening bags easily for packaging food tested by tensile and quasi-static essays analysis. Similarly agree with Castillo et al. [67], who revealed that films of thermoplastic corn starch- Talc nanoparticles made by thermo-compression, where talc increased softening resistance when using 3% w/w concentration of nanocomposite. Thermoplastic starch films can be synthesized by injection molding, blowing, or thermo- compression but have disadvantages like hydrophilic and less mechanical properties. Many methods can reduce these defects by using many organic or mineral metrics to reinforce and develop bio polymeric due to the synergic effect [68].

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Starch molecules

+ Clay

+ Ethanol

Starch film

+ Glycerol Hot compression

Fig. 13 Graphic showing preparation method for well-dispersed starch–clay nanocomposites. (Adapted with permission from Ref. [56]. Copyright 2021, Elsevier)

Castillo et al. [69] studied the effect of talc nanoparticles on thermoplastic starch by melt and compression mixing method (Fig. 9) and observed a decrease in the degree of crystallinity due to homogenous thermoplastic starch while talc helped to nanoparticles alignment confirmed. Thermal processing of starch and the presence of talc cause structural changes. It is necessary to have a thorough understanding of the molecular architecture of polysaccharide granules to comprehend the influence of heat processing on a starch structure. Figure 13 depicts the main starch ingredients and their distribution inside the granules structure of the granules of starch. The process of starch production begins at the hilum and progresses via subsequent layers stacking [69] (Fig. 14). A simplified illustration of plasticizer molecules entrapped within a single amylose helix is shown in Fig. 15. (Frontal and transversal views). These amylose-glycerol complexes are arranged in a V-shaped configuration to facilitate their transport. Depending on their hydration state, intra-helical inclusion complex crystals can be divided into V-hydrated (Vh) and V-anhydrous (Va) subtypes. The presence of water molecules within the crystalline structure distinguishes these two V-crystalline phases, which is the primary distinction between them. When water molecules are lost between the amylose helices, the hydrated subtype might convert into anhydrous. Particularly, the glycerol-amylose combination crystallizes in a Vh-type structure most of the time [69].

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Fig. 14 The following two methods of preparing thermoplastic starch films were depicted in the graphic: (a) melt-mixing and (b) thermo-compression. TPS-M and TPS-F (melt-mixtures and films of thermoplastic corn starch), as well as TPS/T-M and TPS/T-F (thermoplastic corn starch/meltmixture and film of thermoplastic corn starch) (melt-mixtures and films of nanocomposites containing talc particles). (Adapted with permission from Ref. [69]. Copyright 2022, Elsevier)

Biodegradable Composites with Nanosized Fillers Lignocellulosic Fibers In general, plant biomass consists of two types: primary biomass, which produces lignocellulose, and secondary biomass, which does not. The jute, sisal, cotton, kenaf, and hemp fibers are classified as main plant biomass since they are grown specifically for their lignocellulose content [70]. Among the secondary plant biomass, there

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Fig. 15 (a) Corn starch granules architecture and (b) A- and Vh-type crystalline structures. (Adapted with permission from Ref. [69]. Copyright 2022, Elsevier)

are oil palms, agave, and PALF, which make use of lignocellulose and other secondary byproducts to provide energy. Lignocellulose is a form of biocomposites that happens naturally and contains hemicellulose, cellulose, and lignin constituents. Lignocellulose is a type of biocomposites that occurs naturally and contains hemicellulose, cellulose, and lignin [71]. Lignocelluloses are becoming increasingly general and beneficial for various reasons, including their biodegradable nature, a large supply source, lower density, increased stiffness and specific strength, and improved mechanical properties. Several disadvantages include their hydrophilic nature, limits of processing temperature, and modest mechanical features. Lignin matrix components and hemicellulose bind natural biocomposites together, which act as a binding agent.

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Cellulose is a naturally occurring organic fiber found worldwide that serves as the principal cell wall for all lignocellulose fibers. The majority of cellulose is found in crystalline form, with the formula (C6H10O5) n indicating that it is a polymer. The creation of hydrogen bonds between the molecules of crystalline cellulose results in the development of stabilized cellulose. It also entails the formation of liner links between polysaccharide units of glucose [72]. It is possible to have cellulose polymerization degrees ranging between 10,000 and 15,000. The chemical makeup of cellulose remains constant regardless of the source of lignocellulose, although variations in mechanical qualities have been observed due to differences in crystallinity, polymerization, and crystalline morphologies. There are various mechanical properties of lignocellulose/bio fibers and their composition. The percentage of cellulose in lignocellulose determines its mechanical strength, with a higher cellulose percentage resulting in greater mechanical strength. It is present in cellulose at moisture % of 8% to 12.6%, indicating the presence of OH functional groups [73]. Hemicellulose is the second most abundant organic ingredient after cellulose availability. It comprises the polysaccharides group and a carbon group of sugars with C-5 and C-6 ring configurations. Comparing polysaccharides to cellulose, the molecular weight of polysaccharides is lower [73]. Hemicelluloses provide various functions, including viscosity modification, gelling agent, and binding agents for other ingredients. The degree of hemicellulose polymerization varies between 300 and 500 units. It is necessary to use limited enzymes, alkalis, and dilute acids to hydrolyze hemicellulose. In addition to cellulose and hemicellulose, lignin is a part of the material. In its three-dimensional structure, lignin is composed of a combination of aromatic and aliphatic components. The production of lignin can also be accomplished through the use of monomeric units such as p-coumaryl alcohol, coniferyl alcohol, and synapyl alcohol, among others. Despite eliminating lignin, hemicellulose coexists with cellulose in a natural state. Lignin can be generated as a waste byproduct of cellulosic bioethanol production and the pulp and paper sector. Primary lignin function is to shield the hemicellulose and cellulose from exposure to high moisture levels in the environment. Lignin is also employed as an aerogel, a reinforcement, a coating, and a coupling agent in the fabrication of sustainable biocomposites, as well as a possible matrix in the fabrication of sustainable biocomposites. Sustainable developments, which incorporate lignin with a biopolymer matrix for biocomposites reinforcement, are currently a topic of great interest in society. It is an excellent material for the matrix because it allows the matrix to bind with other nanomaterials. Compared to plant celluloses, the additional unusual properties of regenerated cellulose include higher moisture regain properties, dimensional stability to heat application, dye compatibility, wetted stage strength retaining capacity, and morphological and mechanical predictable characteristics. For example, when compared to hemp, jute, flax, and sisal fibers, the Lyocell regenerated cellulose brand has superior mechanical properties to these fibers [73].

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Cellulose Nano-crystallites Are a Type of Crystal (Bacterial Cellulose) Acetobacter xylinum is a bacterium species that produces cellulose, which contains glucose, and it is found in the soil. Bacterial cellulose (BC) is also called Cellulose Nano crystallites or whiskers since it is crystalline and has a long and thin network structure with a long and thin network structure. This crystallinity characteristic contributes to the good mechanical properties of the material. Hemicellulose and lignin are not present because they are implanted with a protein matrix. BC has a crystallinity degree of 60–90% and an isotropic young’s modulus of 15–35 GPa, with a tensile strength of 450 MPa. BC has a crystallinity degree of 60–90% and an isotropic young’s modulus of 15–35 GPa. The width ranges between 5 and 50 nm, the length ranges between 100 and 500 nm, and the bulk density is 1.6 g/cm3. The increased nanoscale dimension and aspect ratio of cellulose-based composites (BC) instead of macroscopic cellulose result in BC exhibiting different chemical, optical, electrical, and mechanical properties. Because of their excellent biocompatibility, among other things, these BCs can be used to make speaker diaphragms, artificial skin, wound dressings, and other biomedical equipment [74].

Cellulose That Has Been Regenerated It is an excellent material for the matrix because it allows the matrix to bind with other nanomaterials. Compared to plant celluloses, the other extraordinary properties of regenerated cellulose include higher moisture regain properties, dimensional stability to heat application, dye compatibility, wetted stage strength retaining capacity, and predictable morphological and mechanical characteristics. For example, compared to sisal fibers, flax, jute, and hemp, the mechanical properties of Lyocell regenerated cellulose are superior those of these fibers [75].

Other Varieties of Bio Fibers Are Available Chitin is the name given to the bio fibers produced from crustacean and fungi exoskeletons that rank second on the list of the most abundant natural polymers after cellulose. This structural polymer can be found in abundance in plant and animal kingdoms. Krill shells, crab, fungus, and shrimp are some of the commercial food industry waste sources used to make chitin. In the chitin framework, protein serves as a matrix, and polysaccharide serves as reinforcement, whereas in the collagen framework, protein serves as a matrix, and polysaccharide serves as reinforcement. An increase in the DE acetylation of chitin by more than 75% results in the formation of chitosan (CS). Because of its exceptional biocompatibility and biodegradability, it is particularly well suited for use in the production of drug delivery systems, artificial skins, and kidney membranes, among other applications. Chemistry can be used to alter the physical, chemical, and network structure of both chitin and cellulose without altering the physical, chemical, or network structure [72].

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Migration of Various Nanoparticles into Diverse Foodstuffs To biodegrade polymers, microbes must first cleave the polymer chains to lower their molecular weight, allowing transit into cells, where most biochemical reactions occur. Extracellular enzymes depolymerize polymers outside the cells to break down polymeric compounds (Fig. 16). Then the substance degrades either aerobically or anaerobically [76]. Storage of items in nanoparticle-containing packaging can increase items shelf life, resulting in less waste and negative environmental trach reduction. Although meals can contaminate by chemicals during the packaging process, the proportion of packaging substances that migrate to the food surface is an important food packaging consideration. Numerous research has shown that nanoparticles migrate from their packing into the food matrix and cause food contamination. A few of this research, on the other hand, have used experimental methods to demonstrate that the levels of migration are lower than the allowable threshold. Research on silver nanoparticle migration has found that they are not always consistent with one another; nonetheless, in acidic conditions, silver nanoparticles migrate quickly, according to all research in the field. Many important topics are currently being investigated, including the physical and chemical nanoparticles properties, methods for detecting nanoparticle migration and determining their

Fig. 16 Plastic biodegradation mechanism. (Adapted with permission from Ref. [76]. Copyright 2022, Elsevier)

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food concentrations, and relationships between nanoparticle attributes and potentially harmful consequences. Studies have also revealed that the rate of material migration is affected by various factors, including remaining segments density, additives thickness, nanoparticles that contact with foodstuff, the solubility of the substances found in food, and time length and temperature for each packaging material. The overall conclusion is that micro-and nanoparticles can get into the cell units of meals in various scenarios. While the information on the toxicity of these substances is continuously revised and updated, there remains a paucity of knowledge about their effects on humans [77].

Conclusion There are significantly different biological applications because of smart nanocomposites or nanomaterials accepted in the environment. The application of nanotechnology in food packaging is particularly encouraging because this technique can increase the protection and superiority of food and minimize the consumption of valuable raw materials and garbage generation. For the most part, nanomaterials will be responsible for the majority of the new features in food packaging products; they are essential for enhancing the gas barrier, both directly and indirectly, the packaging materials’ antimicrobial characteristics, as well as their mechanical characteristics.

Future Perspectives Nanomaterials have a significant role in developing biodegradable food packaging materials properties, including mechanical characteristics, barriers made of water or gas, and activity of antimicrobials. This results in stored shelf-life food increased and extended by spoilage prevention due to nanomaterials used. Substituting nonbiodegradable resources such as polyethylene with biodegradable alternatives will help protect both the environment and our health. However, it is vital to highlight that, as of this writing, there has not been any discovery in biodegradable food packaging materials development that consider a true substitute for traditional ones. As a result, additional research into increasing the qualities of biodegradable food packaging materials is still required.

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Contents Introductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Natural Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Technological Applications of Biodegradable Natural Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Technological Applications of Biodegradable Synthetic Polymers . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

These times, great attention in academia have been devoted to developing biodegradable materials to solve the problem of white pollution. Currently, biodegradable materials have a significant interest in various technological aspects, including food backing, drug delivery, regenerative medicine, orthopedic, medicine, and modern technological applications. This is due to their eco-friendly, biodegradability, biocompatibility, and high viability at low cost. There are many kinds of these polymers, including natural or synthetic. However, the widespread application of biodegradable materials still has more effort to go. This chapter will highlight and review the recent progress of utilizing these promising materials to provide readers with an intuitive and systematic understanding of biodegradable polymers.

A. S. A. Ahmed (*) · A. N. R. M. Negm · M. Mohammed · M. Abd El-Majeed · A. K. Ali · M. Abdelmotalleib (*) Chemistry Department, Faculty of Science, Al-Azhar University, Assiut, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_37

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Keywords

Biodegradable polymers · Natural polymers · Synthetic polymers · Biomedicine · Tissue engineering · Energy conversion · Energy storage · Biosensors Abbreviations

BPMs CRCF CS CAB GTR GBR MFCs m-TCPP OSCs PDT

Biodegradable polymer materials Conductive regenerated cellulose film Chitosan Cellulose acetate butyrate Guided tissue regeneration Guided bone regeneration Microbial fuel cells Meso-tetra(4-carboxyphenyl)porphyrin Organic solar cells Photodynamic therapy

Introductions Over the years, the world has seen significant progress in polymeric materials for various uses, including food packaging, technological gadgets, medical sciences, and so on. Unfortunately, this leads to increased pollution from nonbiodegradable synthetic polymer wastes in the environment [1, 2]. As a result, creating biopolymers to replace these polymers in specific applications is highly encouraged [3]. Biodegradable polymer materials (BPMs) are polymers with a high molecular weight that can be broken down into smaller molecular weight species. This occurs because of the action of micro- and/or macroorganisms and enzymes. Currently, polymeric materials are widely penetrated almost in all elements of our lives, such as packaging supplies, building materials, and basic commodities. Since the 1970s, the first synthetic polymer was declared worldwide due to its wide variety, great performance, and low prices. Polymer materials have become an indispensable component of the global economy, with global output expected to exceed 500 million metric tons in 30 years. Synthetic polymeric materials or plastics, on the other hand, have a high degree of durability and are not easily biodegradable; therefore, they tend to collect in the environment, resulting in white pollution [1]. There are two strategies to mitigate the environmental effects of this problem: (i) sensible management of solid waste plastics produced from food packaging bags or plastic containers, which relies on disposal in landfills and incineration, and (ii) postconsumer of waste solid plastic materials [4]. However, these strategies are not considered a sustainable environmental improvement. This is due to landfills resulting in foam groundwater contamination and the overheated process, releasing toxic gases into the atmosphere. Furthermore, recycling-used plastic materials have not kept pace with recent use, resulting in a massive accumulation over time that has worsened the discarding problem and added to the environmental difficulty [5]. According to reports, only

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about 13% of all plastic waste materials were recycled from all used plastics, with the rest going unmanaged or rejected in the recycling process [6, 7]. Plastic pollution has become a severe problem since the invention of synthetic polymers. As a result, in academia and industry, much emphasis has been paid to producing relevant biodegradable polymers and designing novel catalysts or polymerization procedures to reduce the total cost of manufacturing the prepared materials. BPMs are typically divided into two categories: natural and synthetic polymers. We hope to provide a recent update on natural and synthetic polymers and their progress in contemporary applications in this chapter.

Biodegradable Natural Polymers These polymers, also called biopolymers, can be defined as polymeric materials found in abundance in nature. These polymers are usually prepared through addition polymerization or condensation polymerization. These polymers have some unique properties, including (i) biocompatibility (the ability to be completely broken down by microorganisms) and (ii) low toxicity levels, and thus have attracted significant attention in a variety of applications [8]. Many subgroups of these polymers exist, including aliphatic polyesters and proteins like albumin, gelatin, and collagen. Polysaccharides, including chitosan, agarose, dextran, hyaluronic acid, alginate, carrageenan, and cyclodextrin, are also significant components of this polymer [8]. Because of their long-term viability and environmental friendliness, biopolymers have been proposed for various applications, including medical and electronics.

Technological Applications of Biodegradable Natural Polymers Chitin/Chitosan Chitin is a linear polymer composed of repeated (1,4)-N-acetylglucosamine units. It is the second most prevalent natural polysaccharide after cellulose (Fig. 1) [9]. Chitin is usually obtained from the shells of arthropods, including crabs, shrimps, insects, fungi, and bacteria. Even though chitin is a cellulose derivative, it is not found in the same species that make cellulose [10]. Despite the chitin structure’s resemblance to cellulose, the C2 position in chitin contains an acetamide group (-NHCOCH3) [11]. Chitosan, also known as a deacetylated derivative of chitin, is the most important chitin derivative created by full or partial deacetylation (removal of the acetate component) of chitin [12]. Chitosan is derived mostly from the hard exterior skeletons of shellfish (e.g., crab, lobster, shrimp) and fungus cell walls [13]. Biocompatibility, biodegradation, antibacterial capabilities, nontoxicity, and nonallergic qualities are among the biological properties of chitin and chitosan. It can also be made into hydrogels, films, fibers, beads, support matrices, and mixes [14]. The presence of hydroxyl and amino groups in the structure provides a high ability for chemical changes, which is useful for the assembly of both sensors and electrochemical devices, but it also provides good ion attraction. Furthermore, the proton

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Fig. 1 Chitin and chitosan processing. (Adapted from Ref. [11] with permission, Copyright 2015, John Wiley & Sons, Inc)

Fig. 2 Molecular structures of chitin and chitosan via deacetylation

binding sites in chitosan derivatives allow for proton mobility, which results in high electrical conductivity [11]. Because of these qualities, chitin and chitosan have received a lot of attention for their use in various applications [15]. The presence of positively charged N-acetyl glucosamine units in chitosan causes it to operate as a highly basic cationic polymer with mucoadhesive properties, as seen in Fig. 2. Both chitin and chitosan molecules have been found to have antibacterial and painkilling effects in recent decades. In addition, they encourage faster wound healing and connective tissue repair. Li et al., for example, used the derived chitin from shrimp shells as a biocompatible substrate for the photosensitizer meso-tetra(4-carboxyphenyl)porphyrin (m-TCPP) in photodynamic treatment (PDT). By dissolving deacetylated chitin (DA-chitin) in ionic liquid and casting

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DA-chitin/m-TCPP films, the deacetylated chitin (DA-chitin) reacted with m-TCPP. The produced films had a high optical absorbance, but the absorbance intensity was directly impacted by m-TCPP component, as were the degrees of DA-chitin deacetylation. Blending metal ions such as Cu2+, Zn2+, Gd3+, and Fe3+ dramatically modified the optical characteristics. Furthermore, the DA-chitin/m-TCPP composite films under light radiation create singlet oxygen and might thus be employed as a photosensitizer in PDT. More recently, chitin and chitosan biopolymers exhibit potential ability in various applications, including biomedical devices and electronics. The use of such green materials in electronics has ushered in a new era in microelectronics, including electronic gadgets, batteries, and sensors. Chitosan can be manufactured as films by casting, spin-coating, or electrochemical deposition processes as a carrier or substrate for various electronic devices in some applications. The sensor is a device that can receive and respond to signals before converting the falling signals into electrical or magnetic energy that can be analyzed [16]. A biosensor is a device that uses signals proportional to the concentration of an analyte in a reaction to assess biological or chemical responses. Disease monitoring, drug discovery, and detecting contaminants, disease-causing microorganisms, and disease markers in physiological fluids are applications where biosensors are used (blood, urine, saliva, sweat). These elements are commonly attached to the transducer to detect extremely low concentrations of specific chemical moieties due to a chemical or biological change. Chitosan-based materials have attracted great attention as biosensors based on their biodegradability, high water permeability, and sticky capabilities [17]. This is due to numerous oxygen- and nitrogen-based functional groups that can be chemically changed to generate a composite with sensor-friendly characteristics [17]. For example, Vilouras et al. employed a drop-casting method to construct an ultrathin graphene oxide (GO)-chitosan film on a Si wafer coated with cellulose acetate butyrate to act as a sacrificial layer, as well as mechanical support. By electrochemical recognition of serotonin (5-HT) in blood samples, the developed GO-chitosan film was applied as a sensor-on-probe (SoP) platform to identify carcinoid syndrome. The limit of detection (LOD) was 0.2 M, with great stability across multiple cyclic voltammogram (CV) cycles. The sensor was deemed successful. The fabrication methods, as well as a chemiresistor fitted on the probe and redox reactions in a DMEM droplet, are shown in Fig. 3a–h. Zou et al. recently reported on ball milling nitrogen-doped graphene (NG) as a highly electroactive material with carboxylated chitosan to produce carboxylated chitosan-functionalized nitrogencontaining graphene (GC-OOH) (Fig. 4a) [18]. Casted composite coatings on glass carbon electrodes were used to make the ophthalmic electrode in an eyeball biosensor to detect glucose biosensors. The built biosensor had a sensitivity of 9.7 A/mMcm2; after 30 days of storage, the LOD was 9.5 M. A new type of sensor known as a spectroscopic sensor has been constructed using chitosan and its derivatives. Han et al., for example, look at the electrochemiluminescence (ECL) behavior of Cu2+/cysteine complexes and N-(aminobutyl)N-(ethylisoluminol) (ABEI) functionalized gold nanoparticles mixed with chitosan (Cu2+-cysteine-ABEI-GNPs-chitosan) (Fig. 5a) [20]. According to the calculations,

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Fig. 3 (a-h) Fabrication steps and (i) GO-chitosan chemiresistor mounted on probe and redox reactions undergoing in a DMEM droplet (red color). (Adapted from Ref. [19] with permission, Copyright 2018, IEEE Sensors Council)

Fig. 4 (a) Synthetic road of GC-COOH, (b) digital picture of the synthesized GC-COOH in DI water before (left) and after (right) ball milling, (c) water contact angle of a hydrophobic CD plate (top) and the same CD plate coated with GC-COOH (bottom), (d) AFM of the GC-COOH, (e) CVs with 10 mM glucose in 0.1 M PBS (pH¼7.4) buffer solution at 100 mV/s, and (f) amperometric response of the GC-GOx enzyme electrode to the successive additions of 0.5 mM glucose in 0.1 M PBS. (Adapted from Ref. [18] with permission, Copyright 2020, American Chemical Society)

Cu2+-cysteine-ABEI-GNPs-chitosan has a higher ECL intensity than Cu2+-cysteineABEI-GNPs (Fig. 5b). A co-reactant-free and label-free ECL immunosensor to determine early acute myocardial infarction biomarker copeptin was also developed using

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Fig. 5 (a) Schematic illustration of proposed co-reactant-free and label-free ECL immunosensor for copeptin based on luminescent immuno-gold nanoassemblies and (b) relationship between ECL response and logarithm of copeptin concentration. (Adapted from Ref. [20] with permission, Copyright 2018, American Chemical Society)

luminescent immuno-gold nanoassemblies involving Cu2+-cysteine-ABEI-GNPschitosan and immuno-gold nanoparticles synthesized by bonding copeptin antibody with trisodium citrate stabilized gold nanoparticles. In the presence of copeptin, ECL intensity decreased, indicating that it might be utilized to detect copeptin. The LOD was 5.18  1015 mol/L which is about two orders of magnitude lower than sandwich immunoassays based on labeling technology. Chitosan and its composites have also been used to build several electrical devices that require specialized materials. This has to do with biocompatibility, which impacts the composition of wound-healing materials in various ways. Chitosan-based polymers have also been employed for nerve tissue wound healing and regeneration, according to Bu et al. [21]. A sodium alginate (SA) and carboxymethyl chitosan (CM-chitosan) polymer hydrogel doped with polypyrrole (SA/CM-chitosan/PPy) was created and cross-linked with calcium ions (Fig. 6) [37]. The polymer hydrogel that results is beneficial in aiding peripheral nerve regeneration. Changing the PPy content of the hydrogel enhanced its conductivity, which ranged from 2.41  105 to 8.03  103 S/cm. The SA/CM-chitosan/PPy conductive hydrogel displayed biocompatibility and repair capabilities as a bioactive biomaterial. In the development of electrochemical devices such as supercapacitors, chitosan and its derivatives have gained much attention [22]. Hosseini et al. used in situ polymerization aniline in the presence of chitosan/GM to develop a chitosan/graphene oxide-MWCNT/polyaniline (chitosan/GM/PANI) ternary composite [23]. The specific capacitance of the chitosan/ GM/PANI nanocomposite was 609.2 F/g, which was much higher than that of virgin chitosan and chitosan/GM. Furthermore, after 500 charge-discharge cycles at a current density of 5 A/g, the synthesized nanocomposite retains around 96% of its initial capacitance. Tseng et al. [24] created a PEDOT/reduced graphene oxide (rGO)chitosan composite by combining chitosan with poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon components. To make a PEDOT/reduced graphene oxide

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Fig. 6 Possible molecular structures of alginate [37]

(rGO)-chitosan composite, chitosan was introduced into poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon materials. PEDOT/rGO-chitosan was screen printed on carbon fabric to create electrodes for gel-electrolyte flexible supercapacitors. The built electrode has a broad contact surface with the electrolyte due to its high porosity and hydrophilicity. PEDOT/rGO-chitosan composite showed 2 mW/cm2 power density. A dye-sensitized solar cell (DSSC) is a promising solar energy conversion technique that employs a redox reaction and is constructed using a sandwich method. A photoanode, a counter electrode, and an electrolyte are all part of a DSSC device [25]. Many research organizations are currently working to improve their photoanode, counter electrode, and electrolyte [26, 27]. There are just a few investigations using chitosan as an electrolyte-based material, despite the usage of numerous synthetic polymers to construct quasi-solid-state electrolytes [28]. For instance, Yahya et al, utlized the quaternization process with iodopropane for preparing tripropyl chitosan iodide as gel polymer electrolyte for DSSC applications [29]. The assembled DSSC device displayed overall power conversion efficiency (PCE) of 0.415% by utlizing tripropyl chitosan iodide gel electrolyte and ionic liquid, which is greater than that achived by the device with gel polymer electrolyte film without ionic liquid (PCE ¼ 0.075%). In another study, Yusuf et al. employed tetrapropylammonium iodide to synthesize phthaloylchitosan-based gel electrolytes [30]. The conductivity of the generated polymer electrolyte can be changed by altering the concentrations of the individual electrolyte components. Phosphoryl chitosan (15.70 wt%), ethylene carbonate (31.7 wt%), propylene carbonate (3.17 wt%), Pr4NI (19.0 wt%), and ethylene carbonate (31.7 wt%) are all present in the electrolyte (31.7 wt%). By adjusting the concentrations of the various electrolyte components, the conductivity of the produced polymer electrolyte may be modified. The electrolyte includes phthaloylchitosan (15.70 wt%), ethylene carbonate (31.70 wt%), propylene carbonate (3.17 wt%), Pr4NI (19.0 wt%), and iodine (1.9 wt%) which had the best ionic conductivity of 5.27  103 S/cm. The completed DSSC gadget had a PCE value of 3.71%. Chawla et al. built a DSSC device employing a chitosan-based polymer gel electrolyte [31]. Chitosan is used as the host

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polymer, LiI:I2 as the redox couple, and TiO2 as the filler in this study. Using phthaloylchitosan (PhCh) and potassium selenocyanate as salts and selenocyanogen as a redox mediator, Zulkifli et al. [32] produced gel polymer electrolytes for DSSCs. The ionic conductivity increased by increasing the amount of salt in the electrolyte, reaching 4.76  102 S/cm. The associated DSSC device had a PCE of 2.28%. Conductive polymers and/or their composites are a well-known material for CEs in the counter electrode (a crucial component that consists of a catalytic layer on a conductive substrate) [33–35]. However, no reports of employing chitosan as a counter electrode have been reported, related to the chitosan’s inferior conductivity. Chitosan was employed as a TiO2 binder in the development of a photoanode, according to Jin et al. [36]. After a small amount of chitosan was added, the pore size of the TiO2 film became smaller, and the pores were very equally spread. The assembled DSSC device with 2.0 wt% chitosan-TiO2 composite film showed a PCE of 4.18%.

Sodium Alginates Sodium alginate (NaC6H7O6) (SA) is a polysaccharide alginic acid derivative made up of 1,4-D-mannuronic (M) and 1,4-L-guluronic (G) acids. SA is a component of the cell wall of marine brown algae that contains 30 to 60% alginic acid. The conversion of alginic acid to SA makes it water-soluble and easier to extract. Only two bacterial genera, Pseudomonas and Azotobacter, produce bacterial alginates, which are employed for environmental protection and the formation of biofilms that attach to surfaces. This synthesis method allows bacteria to produce alginates with a welldefined monomer composition, potentially allowing “tailor-made” bacterial alginates. Alginate is a water-soluble polysaccharide found in nature that was first discovered in 1881. The chemical structure of alginates is made up of groups of linear copolymers made up of repeated monomeric units of (1,4)-D-mannuronic acid (M) and (1,4)-Lguluronic acid (G) units [37]. The chemical composition of M and G units is influenced by the biological source, growth, and stationary conditions. There are three types of dead sequences in these polysaccharides: MM, GG, and MG block alginate are typically recovered from the cell wall of marine brown seaweed as sodium, magnesium, and calcium salts of alginic acid [38]. However, some bacteria, such as Azotobacter vinelandii and Pseudomonas aeruginosa, can also produce it. Alginate is the only polysaccharide with carboxyl groups in each basic residue and can form gels in polyvalent cations like Ca2+, resulting in strong gels or insoluble polymers [39]. Alginate is widely utilized in various industries, including the food industry, textile printing, paper manufacturing, wound dressings, and medicine formulation, owing to its biocompatible, nontoxic, and cost-effective qualities. Yagishita et al. created electrochemical cells using Anabaena variabilis M-3 trapped within alginate beads in 1988, with a conversion of light energy into electricity of 0.2% [40]. In other experiments, immobilized microalgae were introduced into the cathode chamber of microbial fuel cells to form the assembling microbial carbon capture cell (MFCs). MFCs with immobilized Chlorella vulgaris in alginate beads had coulombic efficiency of 9.40% and 14.1%, respectively [41]. Ng et al. have reported using sodium alginate immobilized Chlorella cells coated on the ITO anode surface as an

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algal-alginate biofilm for a bio-photovoltaic (BPV) device [42]. Compared to the standard suspension culture BPV device, a peak power production of 0.289 mW/m2 was reported, representing an increase of 18% in power output. According to Li et al., SA was used to improve the capacitive performance of polymer-based supercapacitors in energy storage [43]. To make a porous mat-like polyaniline/sodium alginate (PANI/SA) composite, sodium sulfate was used as a template. The PANI/SA composite has a fibrous, homogeneous structure with sizes ranging from 50 to 100 nm (Fig. 7a, b). The PANI/SA electrode has a high specific capacitance, a long cycle life, and quick oxidation/reduction reflection on high-current fluctuations (Fig. 7c, d). Furthermore, as previously noted, oxygendoped porous carbon was generated by carbonization of SA using a green synthesis technique, as described by Xia et al. [44]. The shape, oxygen content, and electrical conductivity of the as-obtained carbonaceous electrode are balanced by adjusting the mole ratio of L-guluronic acid units/D-mannuronic acid units in sodium alginate.

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This electrode material has a capacitance of up to 424.6 F/g in a 6 M potassium hydroxide electrolyte at 1A/g, which is roughly double that of previous carbonaceous electrodes derived from biomass precursors. After 20,000 charge-discharge cycles, the constructed electrodes showed good cyclic stability and capacitance retention of >90%. Alginate hydrogel has recently been used to create electrolyte sheets for quasi-solid-state QS-QDSCs [45]. The TiO2-sol interconnecting-modified photoanodes increase the surface area and roughness simultaneously. The crosslinking network structure of the alginate hydrogel-based electrolyte films had good interfacial contact with the modified TiO2 surface, which immediately reflected in improved ion transport and redox reaction of the polysulfide pair (Fig. 8a–b). As shown in Fig. 8c, the completed device had a PCE of 8.87%, which is higher than pristine devices (8.01%) and comparable to liquid-based electrolyte QS-DSCs (9.06%). According to Shi et al., converting sodium alginate to a porous carbon coupled with a bimetal as a counter electrode in DSSCs results in a significant improvement in total performance. In this study, SA was employed to make a Co-Mo bimetal/carbon composite via a simple carbonization procedure to use them as CEs of the future. By controlling the Co2+/Mo2+ molar ratio, the analysis showed that a 1: 3 ratio provides THE best photoelectric conversion with a PCE close to the device

Fig. 8 (a) TEM and (b) HR-TEM images of P25 NPs and TiO2-sol; (c) current-voltage (J-V) curves of the assembled QDSCs with or without TiO2-sol. (Adapted from Ref. [45] with permission, Copyright 2021, Elsevier. SEM image of the Co2+/Mo2+ with a molar ratio of 1:3 (d) and 3:1 (e). (f–h) EDS of Co2+/Mo2+ with a molar ratio of 1:3 carbon aerogel powder carbonized at 700  C and (k) J-V curves of the assembled DSSCs. Adapted from Ref. [46] with permission, Copyright 2020, Springer)

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with Pt-CE (Fig. 8d–k) [46]. According to Jiao et al., SA is making headway in 3D printing, which produced a double cross-linked ink by refining the rheological properties of SA, transglutaminase (TG), CaCl2, and gelatin/CaCl2 solutions [47]. The findings revealed that a two-component ink mixture containing SA (4% w/v)/TG (0.8% w/v) and gelatin (4% w/v)/CaCl2 (3% w/v) provided the best printability. In a hydrogel containing 4.5% (w/v) gelatin, the maximum cell growth rate (497%) was achieved.

Cellulose Cellulose (wood pulp) is a solid bio-based material that acts as one of the most abundant biomaterials. Cellulose is a linear biopolymer comprising alternating amorphous and crystalline domains of D-anhydro-glucopyranose units linked by β1!4-glycosidic bonds [48]. As a result, it can be used in various disciplines, including biology, pharmaceuticals, electronics, supercapacitors, and so on. Plants create it in most cases; however, bacteria can also make it. The plant cell wall comprises cellulose grouped in microfibrils of polysaccharides structured in fibrils. This arrangement helps keep plant structures stable, showing that cellulose is a biomaterial with high strength and other mechanical qualities. Generally, cellulose is synthesized by many living organisms, such as Acetobacter xylinum (A. xylinum) [48]. Many bacteria, Dictyostelium discoideum, and higher plants have been found to have cellulose synthase genes. Cellulose is a homopolymer of glucose, like starch, but unlike starch, glucose monomers are linked by 1,4 links. Cellulose, a strong, fibrous, water-insoluble polymer, is essential for maintaining plant cell wall integrity. Cellulose films are being used in biosensing applications with considerable care [3]. For example, Barandun et al. created a printed electrical gas sensor [49]. The built sensors showed low LOD for NH3 < 200PPb with a quick and reversible response, this indicating great sensitivity to water-soluble gases. At a fraction of the cost, the sensors work as well as or better than most commercial ammonia sensors (particularly at high relative humidity). Furthermore, Fukuhara et al. introduce an amorphous cellulose nanofiber (ACF) supercapacitor [50]. The supercapacitor that had been produced had a large storage capacity. After charging 2 mA at 10 V, the supercapacitor could light a red LED for 1 second. Then cellulose/GO networks are covered with PANI nanoclusters using in situ aniline monomer polymerization. PANI nanoclusters are subsequently coated onto cellulose/GO networks using in situ aniline monomer polymerization. The ternary cellulose/GO3.5/PANI aerogel film exhibits well-defined three-dimensional porous structures with high specific capacitance (1218 mF/cm2) at 1.0 mA/cm2 due to an optimum weight ratio of GO and PANI. The supercapacitor has a high energy density (258.2 Wh/cm2) and a power density of 1201.4 W/cm2 due to cellulose/ GO3.5/PANI aerogel film as electrodes in symmetric configuration. According to Wu et al., CNP was successfully used to improve the conversion efficiency of organic solar cells (OSCs), with the completed device showing a PCE of 16.18%. Gao et al. also produced a transparent CNP with an acrylic resin coating as a substrate for making flexible perovskite solar cell PSCs. After 50 times of bending, the built PSCs demonstrated a good PCE value (4.25%) and good stability, losing only around 20%

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of their initial efficiency. In DSSCs, cellulose-based materials were utilized as gelling agents in the electrolyte to create quasi-solid-state devices that outperformed liquid-state devices. Willgert et al. developed a solvent-free nanocomposite gel electrolyte for QS-DSSC in this regard [51].  The electrolyte was made up of polyethylene oxide (PEO), CNCs, and an I 3 =I redox pair. Increasing the amount of CNC in the electrolyte improved the overall performance of the built QS-DSSCs substantially. Furthermore, the constructed devices maintained a high level of stability for 2 months. Tyler et al. created an aerogel film made of covalently cross-linked CNCs and poly(oligoethylene glycol methacrylate) (POEGMA) for use as electrolyte absorbers in DSSCs in another work [51]. The built DSSCs with CNC-POEGMA aerogels have a total conversion efficiency comparable to liquid electrolyte devices. Cellulose was also utlized as a CE as reported by by Li et al. [52]. In this work a conductive regenerated cellulose film (CRCF) was prepared and utlized as an CE for DSSCs. The fabricated DSSC showed a PCE of 8.11%, which is comparable to Pt-CE (8.57%).

Synthetic Biodegradable Polymers These polymers are synthetic materials adapted to tissue engineering applications, making them extremely helpful in the biomedical industry. Synthetic polymers’ physicochemical and mechanical properties are equivalent to biological tissues. Moreover, hydrolysis destroys polymers, and the breakdown rate is consistent across hosts. Synthetic biopolymers are preferred despite the advantages of natural biodegradable polymers in mechanical and thermal qualities. Aliphatic polyesters, polyanhydrides, polyphosphazenes, polyurethane, and polyurethane are all examples of synthetic biodegradable polymers (glycerol sebacate). These polymers, unlike natural polymers, typically have regulated structures and a higher degree of flexibility [53]. Furthermore, depending on the composition, structure, and molecular weight, they can form stable porous materials and be destroyed via hydrolysis of the ester groups in their backbones. The most employed synthetic biodegradable polymers in tissue regeneration are aliphatic polyesters. The most important biodegradable materials are polyvinyl alcohol (PVA), polylactide (PLA), polyglycolide (PGA), and their copolymer poly(lactide-co-glycolide) (PLGA). Their chemical structures are shown in Fig. 9 [54]. The following section will highlight and review the recent progress of the technological applications for these polymers.

Technological Applications of Biodegradable Synthetic Polymers Polyvinyl Alcohol Polyvinyl alcohol (PVA) is a well-known biodegradable synthetic polymer with good mechanical characteristics. PVA is a water-soluble and biodegradable polymer

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Fig. 9 Structure of some biodegradable synthetic polymers. Adopted from Ref. [54] with permission, copyright 2016, Elsevier

with great biocompatibility. Moreover, it can self-cross-link due to the high density of hydroxyl groups on its side chains [55]. PVA usually rapid hydrolysis and is bioinert, limiting protein and cell attachment to the pure substance. PVA is a vinyl polymer with solely carbon-carbon bonds connecting it. This linkage is like plastics such as polyethylene, polypropylene, polystyrene, and water-soluble polymers like polyacrylamide and polyacrylic acid, which have the same connection. PVA is considered the only vinyl polymer known to be mineralized by microbes among the industrially manufactured vinyl polymers. PVA can be made by hydrolysis of a variety of polyvinyl esters and polyvinyl ethers and has many applications in pharmaceuticals, cosmetics, and the paper and food industries, either alone or in blends with other polymers such as poly (3-hydroxy butyrate). Because PVA is water-soluble and biodegradable, it is utilized to create water-soluble and biodegradable carriers that could be beneficial in the delivery of chemicals like fertilizers, insecticides, and herbicides. Moreover, PVA strongly interacts with carbon nanotubes (CNTs), giving it unique features not found in other polymer/CNT nanocomposites (NCs). CNTs offer exceptional features like huge surface areas and aspect ratios, strong electrical conductivity, superior thermal conductivity, and high mechanical strength along their axes [56], making them suitable reinforcing agents in the manufacture of polymer NCs. The biocompatible, biodegradable, bioinert, and semicrystalline nature of polyvinyl alcohol (PVA) is due to vinyl alcohol and acetate in the manufacturing process. PVA, like PEG, is a watersoluble polymer that is utilized in the SLS printing procedure. PVA has a tensile strength like that of human articular cartilage. PVA can also be combined with the right adhesive to create a matrix for bone cell ingrowth. The chemical structure of PVA is shown in Fig. 9 [57]. PVA is a thermoplastic biopolymer made from the hydrolysis of the polyvinyl acetate precursor and can be degraded by biological organisms and in water as a solubilized crystalline structure polymer [58]. Hydrolysis

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increases the degree of degradability due to hydroxyl groups (O-H) on the carbon atoms [59]. The proportion of hydrolysis, which determines the PVA grade and molecular weight, determines the chemical and physical properties of PVA [60]. PVA, unlike other polymers, is an oxygen barrier; however, it must be kept under free moisture to avoid gas breakdown [53]. PVA has high tensile strength, flexibility, and hardness, making it suitable for a variety of commercial, medical, and food uses and industrial applications [61]. Due to its film-forming capabilities, PVA is commonly combined with various biopolymers, synthetic polymers with hydrophilic properties, and some lab-made polymers generated from nonrenewable and nonbiodegradable sources. PVA has much interest in various applications. For example, PVA has been used as a gel polymer electrolyte in flexible solid-state supercapacitors with great potential achievements [62]. Aval et al. [63] produced a gel-based electrolyte made of PVA/H3PO4 as a gel electrolyte and the BaTiO3 film as a separator film for three symmetric paper supercapacitors based on the carbon nanotubes, graphite nanoparticles, and graphene electrodes. The assembled symmetric paper supercapacitor based on the CNT electrode showed a specific capacitance of 411 F/g, higher than the others. Wu et al. have reported employing diol borate ester-cross-linked PVA gel electrolyte for smart double-layer capacitors. The hydroxy groups on PVA showed a strong interaction with the 1-ethyl-3-methylimidazolium chloride (EMIMCl), which immediately decreased crystallinity. The produced gel demonstrated an amorphous form with outstanding ionic conductivity (2.43  103 S/cm) and high flexibility, owing to the EMIMCl’s ability to operate as an ion supplier that improves ionic conductivity as a plasticizer that improves chain mobility. At a current density of 2 A/g, the constructed chitosan with PVA-boric acid/EMIMCl gels had a specific capacitance of 90 F/g at 0.1 A/g and retained 98% capacitance after 3000 charge/ discharge cycles. Fuel cells have received great attention in the last decades as an alternative energy conversion technology due to their high electrical energy generation efficiency and high power density. However, some ion exchange membrane disadvantages, such as the high cost and high fuel crossover dehydration of the Nafion membrane, have hampered their commercialization. Finding alternatives to Nafion membranes is thus strongly advised. PVA-based membranes are one of these promising materials. We will discuss current advances in PVA-based polymer membranes in the next section. Kulasekaran et al. also developed a series of polymer electrolyte membranes using chemically cross-linked PVA and sulfonated poly(ether sulfone) (SPES) polymer [64]. A post-sulfonation method with chlorosulfonic acid was used in this study, followed by blending the SPES polymers with PVA. With a proton conductivity of 0.0367 S/cm at 30 oC, the PVA-SPES-20 membranes had a greater ion exchange capacity than virgin PVA (0.0259 S/cm). Gouda et al. have created a binary polymer by combining polyethylene oxide (PEO) with polyvinyl alcohol (PVA) doped with phosphate titanium oxide nanotube (PO4TiO2) in a weightpercentage ratio of 1:3 (Fig. 10a) [65]. The fabricated membrane PVA/PEO/ PO4TiO2-3 achieved higher ability as an exchange membrane for developing a green and low-cost direct borohydride fuel cell (DBFC) (Fig. 10b).

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Fig. 10 (a) Possible structure of the SPVA/PEO/PO4TiO2 membrane and (b) power density and polarization of DBFCs using PVA/PEO/PO4TiO2-3 and Nafion117 membranes, at room temperature [65]

Polyglycolic Acid The most basic aliphatic polyester is polyglycolide, often known as polyglycolic acid (PGA). PGA was one of the first degradable polymers employed in biomedical applications. PGA has been utilized to help with face nerve regeneration in the past. This was accomplished by putting bone marrow stem cells into a PGA tube and watching for neuronal regeneration. Because it is absorbable and has FDA approval for nerve grafting, PGA is a good choice for neural regeneration [66]. PGA is a stiff, high-crystallinity substance that is insoluble in most organic solvents. It is a biodegradable polymer with high-modulus fibers and excellent strength. It is a biodegradable polymer with high strength and modulus fibers [67]. PGA is a polymer that belongs to the polyhydroxy acid family of degradable synthetic aliphatic polyesters. PGA is frequently employed in the fields of tissue engineering and medication delivery. PGA is only soluble in strongly fluorinated solvents due to its high crystallinity (e.g., hexafluoroisopropanol) [68]. Because PGA is a type of polyester; it is easily dissolved by hydrolysis (bulk degradation), and the rate of breakdown can be customized by adjusting the crystallinity and hydrophilicity of the polymer. According to the literature, not only does PGA degrade faster than PLA and PLGA, but it also has better mechanical qualities [67]. The chemical structure of PGA is shown in Fig. 9a. PGA has been widely used in biomedical applications as scaffolds for tissues of bones, tendon, cartilage, and tooth. Polymer barrier membranes for guided tissue regeneration (GTR) and guided bone regeneration (GBR) are two prominent applications of PGA. In both GTR and GBR applications, a barrier membrane is typically employed to avoid unwanted tissue migration during the healing process [69]. Different barrier membranes for clinical applications have been produced with a specific property (e.g., biocompatibility, tissue integration, and clinical manageability) [70]. These membranes are divided into resorbable and non-resorbable membranes based on their breakdown. The final membrane comprises expanded polytetrafluoroethylene and

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titanium mesh, and it was limited since it required a second surgical treatment to remove the biomaterial. Nowadays, collagen and polyhydroxy acids (e.g., PGA or PLA) are commonly utilized polymers in degradable membranes [71]. These materials did not require a second surgical procedure to remove the biomaterial, and the possible consequences of stress shielding on the regenerated tissue were also eliminated [72]. These membranes are often designed to retain humidity while also being resistant to infection. Due to their great antibacterial capabilities, bio-based polymers like collagen and chitosan are still favored. On the other hand, PGA is a biodegradable synthetic polymer that can be used to make artificial skin [80]. In a 12-patient study, Vicar et al. used a self-reinforced PGA membrane to repair 15 orbital floor fractures [73]. Furthermore, due to the replacement of the PGA membrane by the rearranged tissue, the tensile strength of self-reinforced PGA decreases after 4 weeks. Drug delivery is another biological application of PGA. Inertness, additional purity, proper physical structure, and processability are some of the parameters for drug delivery materials. Biodegradable polymers have been employed as medication carriers because of their ability to degrade in the human body [74]. The main reason for their use in this industry is the capacity to modify their basic structure by mixing with other molecules such as ester, anhydride, carbonate, amide, urea, and urethane [75]. PGA has been frequently used as a medication delivery vehicle because of its capacity to be broken down into metabolized molecules by the body and quickly eliminated by normal metabolic routes. However, PGA has some drawbacks as a drug delivery system, including insolubility in many common solvents; a high melting point; inability to form films, rods, or capsules; incompatibility with solvent or melting-based procedures; and a quick degradation process [76]. Moll and Köller employed biodegradable homo- and copolymers of lactic and glycolic acids [77]. Hurrel and Cameron look at how buffer concentration, pH, and specific buffer ions affect PGA degradation in different buffer solutions [78]. The results revealed that the release of a model drug was influenced by buffer concentration, pH, and specific buffer ions. These findings suggest that PGA has much promise and is frequently utilized for wound closure, surgical sutures, and tissue engineering scaffolds. Biodegradable polymers have shown promise as a void filler and a guided tissue regeneration membrane in dental applications [79]. PGA has limitations in dental applications because of its crystallinity and large molecular weight, making it sensitive to deterioration [80]. According to the literature, complete bone healing takes roughly 2 months, although PGA loses its mechanical strength between 4 to 7 weeks. Okuyama et al. recently published a fibrin glue study to patch open wounds with PGA sheets after removing oral mucosal lesions in 100 patients [81]. When PGA was applied to the raw surface of the tongue, a substantial risk of granulomalike neoplasm (GLN) formation was discovered.

Polylactic Acid Polylactic acid (PLA) is an aliphatic polyester manufactured primarily via the synthesis of lactic acid from renewable resources such as corn, flour, sugar, or

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other biomass. It is a biodegradable thermoplastic polyester with many potential thanks to its unique physical qualities, making it useful in a wide range of applications like surgical and medical applications, paper coating, fibers, films, and packaging. PLA blends take use of PLA-consistent miscibility with other polyesters. Generally, the properties of PLA, such as heat stability and impact resistance, are inferior to those of other thermoplastic polymers. The US Food and Drug Administration has rated PLA as generally regarded as safe in 1970s. PLA is one of the most utilized synthetic biodegradable polymers in medical applications due to its outstanding biocompatibility [82]. PLA is made using a variety of procedures, the most common of which are (1) direct condensation polymerization, (2) azeotropic dehydrative condensation, and (3) ring-opening polymerization (ROP) of lactide [83, 84]. PLA has several advantages over other synthetic materials: (1) PLA can be made by converting renewable agricultural sources, such as corn, or other carbohydrate sources to dextrose, which is then fermented into lactic acid (LA); and (2) PLA is recyclable and compostable. PLA takes a few years to degrade completely. PLA is a more sustainable alternative to petrochemical-derived products since the lactides used to make it come from the fermentation of agricultural by-products like corn starch or other carbohydrate-rich foods like maize, sugar, or wheat. Due to copolymers that can reinforce PLA plastic, PLA can be used as a substitute for high-impact polystyrene with as little as 1% non-PLA [85]. PLA is widely used in packaging because of its mechanical and physical qualities, equivalent to and better than existing petroleum-based polymers. PLA films and packages are commercially available and have qualities comparable to PET and better mechanical properties than PS. Usually, the usage of PLA food packaging including compostable cups, deli containers, cutlery, straws, bags, and more [86]. Similarly, some products, such as yogurts, bottled water, and juices, are packaged in PLA, and PLA has recently been employed as a food packaging material for products with a limited shelf life, such as fruits and vegetables [87]. Many review articles are highlighted [82]. According to Silva et al., PLA can also be employed for biosensors [88]. In this study, an rGO was synthesized and employed for biosensing within 3D-printed PLA electrodes. Compared to the identical surface treated with dimethylformamide immersion with the lowest charge transfer resistance, the as-prepared rGO-PLA electrodes demonstrated a significant current increase for the redox probe ferrocenemethanol. Palenzuela et al. published another study in which 3D-printed graphene electrodes for electrochemical sensing were created in ring and disc configurations (Fig. 11a) [89]. When the produced electrodes were used for electrochemical detection of picric and ascorbic acids, the results revealed a nearly two-order-of-magnitude response (Fig. 11d, e). PLA has also been utilized to store electrochemical energy. Baskakov et al., for example, built a metal-free supercapacitor using microwave exfoliated graphene oxide electrodes [90]. As a separator, the GO membrane is used. Compared to the prepared chitosan with the Nafion separator, the total performance of the prepared chitosan with the GO separator exceeds capacity. Ghosh et al. recently developed a supercapacitor and Li-ion battery electrode made of 3D-printed graphene/PLA [91]. Due to the intrinsic metal-based impurities in graphene/ PLA composite, the generated 3D-printed electrode shows pseudo-capacitance.

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Fig. 11 (a) 3D-printed electrode dimensions and shapes. Full lines from light gray to black: activated electrodes in the presence of increasing analyte level. SEM image of the (b) nonactivated and (c) activated ring structure. CVs 3D-printed for different concentration levels of (d) picric acid in acetate buffer 0.1 M and (e) ascorbic acid in KCl 0.1 M. Dashed line: nonactivated electrodes in the highest concentration of the analyte. Discontinuous line: blank current in the supporting electrolyte. (Adapted from Ref. [89] with permission, Copyright 2016, Royal Society of Chemistry)

PLA is utilized as a biodegradable substrate for constructing flexible devices in solar cells. Lou et al. used PLA and silver nanowires (AgNWs) modified by 3,4-ethylenedioxythiophene:polystyrene-sulfonic acid (PEDOT:PSS) to create an ultra-flexible and transparent biomass-derived conductive substrate [92]. The produced composite electrode was inserted (F-PSCs). The results revealed a low sheet resistance of 25/sq, strong transmittance (>82% in the 400–800 nm range), and high mechanical durability. The greatest PCE of the built F-PSCs was 11.44%. As recently reported by Gunasekaran et al., PLA is employed as a gel electrolyte for DSSCs, where PLA with a varied polydispersity index (PDI) is prepared through the aluminum ring-opening method from polymer gel electrolyte for DSSC device.

Poly(lactide-co-glycolide) Poly(lactide-co-glycolide) (PLGA) is a polylactic acid/polyglycolic acid copolymer. PLGA has gotten much attention in the last two decades as a synthetic biodegradable polymer [93]. Drug delivery and tissue engineering are two of the most common

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applications for PLGA [94]. PGA is hydrophilic, has high crystallinity, and degrades quickly in the PLGA structure. PLA has distinct physiochemical and mechanical properties due to the inclusion of a methyl group on the alpha carbon. Therefore, the copolymer PLGA is chosen to produce bone substitute constructs over its constituent homopolymers. Figure 9 depicts the chemical structure of PLGA. Because of its great compatibility and degradability, PLGA is gaining popularity in biomedical applications [95]. The medication rose bengal was added to the nanoparticles during the production process. Because of the polar nature of the medicine and the small size of the nanoparticles, the drug loading was found to be quite low. However, the produced composite’s half-life in the bloodstream was significantly extended, with roughly 30% remaining after 1 h compared to only 8% of rose bengal surviving 5 minutes after administration. Qi et al. created a PLGA/poly(trimethylene carbonate) (PTMC) composite porous polymeric structure for tissue engineering by combining PLGA polymers with salt particles [96]. According to the results, the porosity of the created composite was 80–98%, and the compression strength was in the range of 0.56–7.98 MPa. Furthermore, the best mechanical properties were found at 85 wt% NaCl and 20 wt% solution concentration, indicating that this composite has much potential as a bone repair material. Further research revealed that PLGA might be utilized to make biosensors. For example, Sun et al. developed a PLGA-F127 nanosphere that can be used as an amperometric glucose biosensor for whole blood [97]. According to electrochemical measurements, glucose oxide immobilized on PLGA-F127 NSs showed a direct electron transfer process. This resulted in stable glucose amperometric biosensing with an LOD of 5.57  106 M. Currently, Qiao et al. have reported a fluorescent biosensor for detecting nitroaromatic compounds based on porous biocompatible microspheres loaded with a bioreporter [98]. The bacterial bioreporters were inserted in PLGA microbeads as biosensors. The changed surface structure in microbeads gives a high surface area easy penetration and increases the number of connected bioreporters for improved fluorescent signals of biosensors, allowing for easier detection.

Conclusion One of the major difficulties our society currently faces is using plastic materials that are not degradable under our current environmental conditions. As a result, discovering polymers that can be digested and degraded in natural environments is highly suggested for most daily usage. Biodegradable polymers are environmentally friendly products that can help us save money and reduce pollution in our environment. In recent decades, significant progress has been made in developing a wide range of biodegradable materials for a variety of applications. Natural biodegradable polymers (e.g., alginates, cellulose, and chitosan) and synthetic biodegradable polymers (e.g., alginates, cellulose, and chitosan) are two types of these materials (e.g., PLA, PGA, and PVA).

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Future Perspectives Natural biodegradable polymers have been widely used as alternative materials in the food industry, biomedicine, and current technology applications, particularly for energy storage and conversion, due to their low cost, biocompatibility, and biodegradability. However, using these polymers in the design of green electronic devices is still a work in progress, and much more research is needed in this field. In contrast to these polymers, synthetic biodegradable polymers usually have a regulated structure while also having more flexibility and strong film-forming capabilities. As a result, they have been widely used for more particular applications, or they can be combined with a variety of natural or synthetic polymers to improve qualities through a synergistic effect. Finally, biodegradable polymers can be concluded to protect our environment and be a natural cycle in the world’s biological system. The primary purpose of this research is to develop environmentally friendly products that reduce pollution in the environment. (C)

Cross-References ▶ Biodegradable Polymers ▶ Biodegradable Textiles, Recycling, and Sustainability Achievement ▶ Biodegradation, Biosynthesis, Isolation, and Applications of Chitin and Chitosan ▶ Sustainable Biopolymers

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Part III Plastic Biodegradation

Biodegradable Plastics as a Solution to the Challenging Situation of Plastic Waste Management

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Hafsa Javaid, Mahrukh Khan, Kiran Mustafa, and Sara Musaddiq

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process of Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Plastics are used heavily for packaging in the consumer industry. Nonbiodegradable plastics are causing potential hazards to the environment. Plastic waste management is challenging due to its non-biodegradability, waste accumulation, and escalating water and land contamination problems in the twenty-first century. The biodegradability of plastics is additionally favored because of the increased use of plastics and mounting demands on the existing capacity for plastic waste disposal. Environmental issues are becoming more popular, so there is a need for materials that are a low impact on the environment. Thus, in recent years, degradable polymers have drawn attention to nullify the environmental problems associated with nonbiodegradable polymers. Water-soluble or waterimmiscible polymers require biodegradation since they eventually end up in streams that cannot be recycled or burnt. These biodegradable materials leave H. Javaid · M. Khan · S. Musaddiq Department of Chemistry, The Women University Multan, Multan, Pakistan K. Mustafa (*) Department of Chemistry, The Women University Multan, Multan, Pakistan Govt. Graduate College (W) near GPO Khanewal, Higher Education Department, Punjab, Pakistan © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_18

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no toxin or cannot be distinguished after degradation. Understanding the interactions between materials and microbes and the metabolic changes is essential. Numerous studies on the biodegradation of plastics have been performed to address the environmental issues related to synthetic plastic waste. The current study will cover environment-friendly biodegradable plastics’ characteristics, degradation process, synthesis, and applications. Keywords

Alginate bioplastics · Carrageenan · Bioplastics · Food packaging · Algae polymers

Introduction It is impossible to access clean water, food, and air in this era. Everything, not only humans but all other creatures, is also under constant threat. Among other types of pollution, plastic pollution is a persistent form of pollution, requiring an influential solution. Plastics shaped our society in the twentieth century [1]. The word plastic derives from the Greek word “plastikos,” meaning the “ability to be reshaped or molded into different forms/shapes” [2]. Plastics are inexpensive, lightweight, inert, durable, and long-chain-polymeric materials, which can readily be molded into various products used in various applications. Plastics are everywhere and have become a very crucial part of our lives. About 33% of plastics are used for packaging, 7% in automobiles, 20% in building materials and paints, 10% for electronics, 5% in agriculture, and 25% in storage and transport of life-saving fluids, biomedical implantations, textile, kitchenware, engineering, aircraft, household, and children toys [3]. Conventional plastics are made from petroleum, coal, oil, and natural gas. About 4% of petroleum is converted into plastic every year. Around 45% of all polymer resin production has contributed to packaging materials [4], 19% to construction and buildings, and consumer and institutional products with 12% of production. The consumer plastics are made from polyethylene (LDPE, MDPE, HDPE, and LLDPE), polypropylene, polystyrene, polyvinylchloride, polyurethane, polyethylene terephthalate, polyester, polybutylene terephthalate (PBT), polyamide, and acrylic (PP&A) fiber. These are inherent and nonbiodegradable and have persisted in the environment for decades [5, 6]. The ever-increasing demands and production of plastic products have endangered the lives of species living on land and in oceans. The negative impact of plastics was firstly noticed and addressed by Sir David Attenborough, who narrated the impact to the public through Blue Planet II. About 80–85% of oceanic pollution is caused by plastics, containing eight million tons of plastics [7]. Plasticizers leak out from the litter and cause harm to live organisms [8]. Exposure to persistent organic pollutants (POPs) [9] has caused ingestion, stress problems [10], physical blockages, behavioral changes [10], metabolic disruptions [11], diminished growth, blocked photosynthesis in producers [12],

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and reproductive impacts on crustaceans [13]. On the coast of the Philippines, a dead whale was found with 40 kg of plastic, which was found in his stomach. Plastics have killed the coral reefs, severely affecting the marine ecosystem. A single plastic bag can kill many species but remain the same. About 90% of birds are found with plastic in their stomach. When humans and other animals eat these species, it has a carcinogenic effect and thus has blocked the food chain. Plastics have challenged the sanitary conditions and caused the contamination of underground water [14]. So, plastic disposal has become a challenge for the authorities in recent decades. Conventional methods to dispose of plastics are landfill, incineration, and recycling [15, 16]. About 11% of plastics are disposed of by landfills and incineration [17]. Incineration releases hazardous greenhouse gases into the air, causing air pollution. Plastic recycling is another solution to plastic pollution. However, many steps involve the collection, transportation, processing, and remanufacturing. Due to these recycled plastics’ low cost and low commercial value, recycling plastic products is not economically beneficial [18]. Moreover, plastics can be recycled 2–3 times before their quality is lost [19]. Every recycling process shortens polymer chains and reduces their quality and mass. Long-chain polymeric plastic degradation requires the specific action of light, heat, moisture, and other physical and chemical conditions. Degradation changes material’s physical, mechanical, chemical, optical, and electrical properties. Degradation causes crazing, cracking, discoloration, bond breakage, functional group formation and removal, phase change, and separation [20]. However, artificial degradation mode is very expensive and releases carcinogenic products into the environment. Most of the plastics are nonbiodegradable. It persists in the atmosphere for decades. If they are degradable, they require specific temperatures, exposure to light, oxygen, and other factors to be degraded. For biodegradation, microorganisms-host suitability enzyme-substrate specificity is required. It is studied that microbes present in the mangrove tree soil are capable of plastic biodegradation [21]. Microbes release different extracellular and intracellular polymerases that degrade the polymer and end products: carbon dioxide, water, and methane [22]. Degradation rates change for the type of environment [23]. So, to minimize the hazards of plastics, the synthesis of eco-friendly biodegradable plastic is a topic of great interest these days. Bioplastics are degradable plastics that meet the needs of society and remain unchanged during usage, and they can be degraded after use and disappear from the environment. At present, biodegradable and commercially available natural polymers on the market mainly include polylactic acid (PLA), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyhydroxybutyrate valerate (PHBV), and polyhydroxybutyrate (PHV). BPs can be biodegraded without any harmful effects caused by their persistence [21]. About 80–90% of bioplastics available in the market are starch-based. The current study is focused on the synthesis, types, and processing of biodegradable plastics. The biodegradability of bioplastics is assessed quantitatively using activated sludge, compost, marine samples, single microbial strains, microbial consortia, etc., while CO2 and CH4 evolution and O2 consumption are frequently

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measured. Biodegradation may be aerobic or non-aerobic. Moreover, thermal conditions, moisture, chemical environment, exposed surface, crystallinity and amorphic of the substrate, hydrophilicity, pendant groups, density, chemical linkages, melting temperature, stereochemistry, and glass transition temperature also affect the biodegradability.

Properties of Biodegradable Plastics Problems stemming from conventional plastic manufacture are becoming a major issue as public demand for ecologically friendly items grows, as does public concern about global warming. As a result, plastics made from renewable agricultural commodities are being created, which will be the next phase of plastics. One of the basic materials with much potential for novel bioplastics is soybean oil (SO). Enhanced SO is a possible alternative in various applications, such as plastic sheetmolding adhesives, coatings, and compounds [24]. Epoxidized soybean oil (ESO) with 100 mol% epoxidations (ESO100) has been commercialized under numerous brand names, which is used as a lubricant and a polymer plasticizer of vinyl chloride [25]. Because of its flexible hydrocarbon backbone, ESO was utilized as a hardening agent for thermosetting plastics, notably phenolic resins [26]. ESO can cross-link with an appropriate hardener due to the strong reactivity of the epoxide ring. The term “bioplastic” refers to a thermosetting material made from cross-linked ESO [27]. The impact of various curing processes on the viscoelastic characteristics of cross-linked ESO has been investigated [28]. Zhang and Wang [29] have developed a soy protein plastic with good mechanical strength, thermal stability, and water resistance using anionic waterborne polyurethane as a novel plasticizer. Additionally, Zhong and Sun [30] discovered that the plasticizers sodium dodecyl sulfate (SDS) and guanidine hydrochloride (GuHCL) improved the elongation, water resistance, and tensile strength of the soy protein 11S. Mechanical properties of soy protein plastic can be controlled and optimized by adjusting processing parameters such as the molding temperature, pressure, and the initial moisture content [31]. Because of its poor strength, the usage of soy protein plastic is limited. As a result, combining soy protein with biodegradable polyester can result in more effective soya-based bioplastics. Polyester amide, polycaprolactone, and polycaprolactone are currently utilized in biodegradable polyester blends with soy polymers (tetramethylene adipate-co-terephthalate) [32], whose processing windows are identical to those of soy protein plastic. Sorbitol-plasticized soy-based bioplastic (SSBP) was used to have a higher tensile modulus and elastic modulus than glycerol-plasticized soy-based bioplastic (GSBP), while mixed plasticized soy-based bioplastic (MSBP) was used to have a range of tensile modulus and tensile strength values that were halfway between GSBP and SSBP. MSBP and SSBP had tensile strengths that were about 45% and 50% greater than GSBP, and their tensile moduli increased by around 135% and 255%, respectively, compared to GSBP. GSBP had more extension and less strain, whereas SSBP had more strain and less stretch. MSBP exhibited minor stretching and equivalent plastic strain to SSBP.

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The flexural strengths of MSBP and SSBP are about 70% and 160% greater, respectively, than those of GSBP, while the flexural moduli of MSBP and SSBP are about 100% and 235% higher, respectively, than those of GSBP. GSBP had higher elongation, higher impact strength, lower modulus, and lower strength than SSBP, which had lower elongation, lower impact strength, higher modulus, and higher strength. Starch, a natural renewable carbohydrate polymer and a poor resource, is one of the most investigated and promising raw ingredients for creating biodegradable plastics. However, starch-based films are stiff and hard to handle; plasticizers are typically added to the film-forming solution during casting and drying methods to mitigate the brittleness of the films. Many researchers have shown interest in using filler as encouragement in thermoplastic matrixes to enhance starch-based plastic properties and mechanical resistance. They have shown that fiber incorporation can boost films’ tensile strength and elasticity modulus while decreasing their elastic capacity. Nanosilica, organoclay, nanocalcium carbonate, and carbon nanotube are some of the intriguing nanofillers that have piqued researchers’ curiosity. According to studies, the large surface area of these nanofillers promotes stronger interfacial interactions with the polymer matrix than micrometer-sized particles, resulting in superior property enhancement. Polypropylene contains a variety of inorganic mineral fillers. Calcium carbonate, talc, and barium sulfate are the most frequent mineral fillers; wollastonite and mica are employed. Mineral fillers are significantly less expensive than polypropylene resin. Mineral fillers lower the cost of a polypropylene-based compound while also improving rigidity. Mineral fillers also strengthen the polymer matrix. To improve usability and effectiveness, some mineral fillers are chemically treated. Commercially, stearates, glycols, and silanes increase dispersion and treatment and react with contaminants [33].

Synthesis of Biodegradable Plastics The ability of bacteria’s PHA-biosynthetic pathways to incorporate new components has been investigated primarily in the laboratories of Lenz and Fuller in the Amherst [34]. The first study relied on the inclusion of 3-hydroxyalkanoic acids with an aromatic substituent into PHAs (polyhydroxyalkanoates) of P. oleovorans cells cultured on 5-phenylvaleric acid. This significantly increases the number of constituents identified in biosynthetic PHA [35]. They discovered unsaturated 3-hydroxyalkanoic acids with 5–14 carbon atoms and 1 or 2 double bonds, saturated 3-hydroxyalkanoic acids with 3–12 carbon atoms in straight and branched chains, length 3-hydroxy-5-phenylvaleric acid, and halogenated 3-hydroxyalkanoic acids with a medium chain. 5-hydroxyvaleric acid (HV) and 4-hydroxybutyric acid (HB) were already known as components in addition to 3-hydroxyalkanoic acids. Lately, 4-hydroxyvaleric acid was also detected [36]. PHAs accumulated by Alcaligenes eutrophus and Pseudomonas oleovorans contained most of these constituents. There are already 40 different biosynthetic PHA constituents (Fig. 1) [37]. Others are on the way, and many more will almost certainly be discovered shortly. As a result,

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Fig. 1 Structure of PHA O HO

C

R CH2

x

C

O

H

H

n

Table 1 Different kinds of microorganisms produce PHA Microorganism P. oleovorans P. putida BM01 M. extorquens (ATCC55366) P. denitrificans Methylobacterium rhodesianum MB 1267

PHA Polyhydroxy butyric acid Poly(3-hydroxy-5phenylvalerate) Polyhydroxy butyric acid Polyhydroxy valeric acid Polyhydroxy butyric acid

Source of Carbon Gluconate 11-Phenoxyundecanoic acid Methanol Pentanol Fructose/methanol

Ref. [41] [42] [43] [44] [45]

bacterial PHA synthases, which act as catalysts in the committed step of the PHA biosynthetic pathway, are extremely adaptable to the sidechain and the carboncarbon backbone of the produced polyester molecule. Many microorganisms have now been found to accumulate PHA. PHA can only be synthesized by a few types of bacteria (like lactic acid bacteria and methanogenic bacteria) [37]. Pseudomonas [38] and anoxygenic phototrophic bacteria have been subjected to extensive surveys [39]. The buildup of PHA in Syntrophomonas wolfei, a completely anaerobic bacteria, has been examined in depth [40] (Table 1). The Imperial Chemical Industries process for producing Biopol™, a poly (3HB-co-3 HV) co-polyester of 3-hydroxyvaleric acid and 3-hydroxybutyric acid, has been scaled up even more. The co-polyester is made with a glucose-using mutant of A. eutrophus strain H16 in a fed-batch procedure using glucose and propionic acid, and the polymer is made from enzymatically disintegrated cells without the need for organic solvents [37]. Wella, a German hair-care firm, released the first retail Biopol™ product in 1990, and a perfume bottle built from this combination was developed and commercialized in Japan in 1991. The only advertising products made from bacterial PHA are still unknown. All other potential agricultural and medical applications are now in the early stages of development. PHA has an interesting application in the denitrification of drinkable water as an immobilized electron donor [46]. Except for polyhydroxybutyrate (PHB), the majority of 40 biosynthetic PHAs can only be made from substrates with a carbon skeleton connected to the framework of the constitutive monomer. Moreover, the hydroxyl group, a component of the ester bond, must be present or replaced by another reactive group. According to the latest studies, some polyesters can now be synthesized using basic, unrelated substrates.

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Some Pseudomonas makes co-polyesters from gluconate, with 3-hydroxydecanoic acid as the primary component and 3-hydroxyoctanoic acid and 3-hydroxydodecanoic acid as minor components [38]. Only pseudomonads belonging to the ribosomal RNA homology group I have this PHA-biosynthetic potential. In addition to the saturated ingredients stated above, Witholt and collaborators [47] discovered a minor fraction of unsaturated 3-hydroxyalkanoic acids in PHA accumulated from glucose by a Pseudomonas putida. This suggests that the contents are obtained from de novo fatty acid biosynthesis intermediates produced from acetyl coenzyme A. Moreover, it is unclear whether the acyl moieties of 3-hydroxyalkanoate-acyl carrier protein are promptly integrated into the polymer or if a transfer to coenzyme A is necessary prior to polymerization. Rhodococcus ruber and other Gram-positive bacteria may now manufacture co-polyesters composed of 3 HV and 3HB from basic sources like glucose, acetate, fructose, or succinate [48]. The molar fraction of 3 HV in the collected PHA is 90% or higher than the substrates mentioned above. The creation of an acyl-coenzyme A thioester, which results in the integration of 3 HV into these bacteria, is currently unclear and needs to be explored. The issue is relatively clear in the A. eutrophus R3 mutant, which has modified branched-chain amino acid anabolism. R3 is a secondary mutant developed from an isoleucine auxotrophic mutant that overproduces x-acetolactate synthase to adjust for the initial mutation’s deficient threonine dehydratase. R3 cells grown with fructose or gluconate developed poly(3HB-co-3 HV) with up to 7 mol% 3 HV [49]. Propionyl-coenzyme A, which is generated as an intermediary during the catabolism of valine and isoleucine or their deaminated predecessors, is the source of 3 HV in this mutation. Due to an ammonium deficit, the latter will most likely be upregulated if the appropriate amino acids cannot be synthesized. This mutant’s metabolism can be used to create mutants that produce poly (3 HV-co-3 HV) from solitary unconnected sources. New procedures must be devised to replace dangerous reactions with harmless bio-based ones. Some polyamides, such as polyamide 11 synthesized from castor oil and sold under the brand name Rilsan by the Arkema Group, are among these novel breakthroughs [50]. Polyols are seeing increased industrial development in various uses, including polyurethanes. Polyurethanes (PUs), with a history of just over 70 years, have become one of the most dynamic groups of polymers, with versatile properties suitable for use in virtually all fields of polymer applications, such as foams, thermoplastics, elastomers, adhesives, thermorigids, sealants, coatings, fibers, and so on [51]. They are also employed in biomedical surgery and other specialized applications. An oligomeric polyol (low molecular weight polymer with terminal hydroxyl groups) reacts with a diisocyanate to produce PUs (polyisocyanate). These basic ingredients are derived from petroleum, although the chemical industry has recently focused on developing bio-based polyols, mostly from vegetable oils [52]. Polyether polyols (75%) are the most often used polyols for polyurethane production, originating from a reaction between a “starter” polyol and an alkylene oxide, both of which are petrol based. Polyester polyols (25%) are another polyol

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utilized in the production of polyurethanes. They are produced via step development polycondensation of dicarboxylic acid and polyol in abundance [53]. The use of a bio-based polyol derived from saccharides (sorbitol, sucrose) as a “launch” polyol in the synthesis of partially bio-based polyether polyols becomes a way to boost the renewable content of polyurethanes (30% renewable carbon). It does, however, results in polyurethanes with low recyclable content (around 8%). As a result, working on prepolymers with a higher amount of renewable content is more appealing. Polycondensation of propane-1,3-diol derived from glycerin can also provide polyether polyol. Moreover, bio-based polyester polyols combine bio-based dicarboxylic acids like adipic or succinic acid with bio-based polyols (propane-1, 3-diol). Finally, natural oils (vegetable or animal) are cheap and abundant renewable organic materials that can be used to produce bio-based polyols [54]. Vegetable oils are derived from various plants (palm, soybean, rapeseed, and so on) and primarily consist of triglyceride molecules, in which the three hydroxyl functionalities of glycerin are esterified with fatty acids. These fatty acids might be saturated, with non-reactive aliphatic chains (stearic or palmitic acids), or unsaturated, with double-bonded aliphatic chains (linoleic, oleic, ricinoleic acid, linolenic, etc.). These essential oils, especially the unsaturated ones, are important because diverse reactions can be carried out from their various groups to obtain bio-based polyols, which can then be used in reactions with diisocyanates to produce polyurethanes. Table 2 shows some applications and characteristics of polyether polyols. Bio-based starch has been used to create plastics. The approach originated with the separation of starch from starch-rich tubers such as potato and yam. To get crude starch, the materials were grated, ground, and filtered, after which they were centrifuged and rinsed to obtain pure starch. To keep the starch from becoming plastic-like, it was treated with hydrochloric acid to decompose amylopectin. Finally, propan-1, 2, 3-triol was introduced as a plasticizer to improve the product’s elasticity. Fourier transform infrared (FTIR), tensile strength meter, and thermogravimetric analysis examined the items’ mechanical, chemical, and thermal properties (TGA). C-H, C¼O, C-O, and O-H absorption peaks in the product’s FTIR spectra indicate that bioplastic formation occurred. Potato and yam starch-based bioplastics have tensile strengths of 0.6 MPa and 1.9 MPa, respectively. TGA results revealed that at Table 2 Applications and characteristics of polyether polyols [55] Starting molecule Trimethylolpropane Trimethylolpropane Propylene glycol Propylene glycolTrimethylolpropane Sucrose-propylene glycol-H2O

Alkylene oxide (AO) Propylene oxideEthylene oxide Propylene oxide Propylene oxide Propylene oxideEthylene oxide Propylene oxide

Hydroxyl number 35

Molecular weight 4800

550 56 49

306 2000 3180

380

856

Application Flexible foam (can be molded) Hard foam Elastomer Slabstock foam with significant flexibility Hard foam

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250  C for potato and 310  C for yam-based plastic, 50% weight loss happened. The plastic’s strong biodegradability was shown in a soil burial test, which revealed 43% and 26% of soil biodegradation for potato and yam-based bioplastics, respectively, in 1 week. These bio-based plastics offer strong thermal and mechanical qualities and great biodegradability, making them a viable alternative to currently used traditional plastics [56]. Solution intercalation was used to make bioplastic. The 10 g of tapioca flour was dissolved in the 100 ml aquadest and cooked at 600C for a few minutes while mixing. The glycerol (as a plasticizer) and graphene oxide were mixed with the tapioca solution and swirled with a stirring time variation. After that, a homogeneous solution was poured into the glass mold and allowed to dry for 3 days at room temperature. The bioplastic that had been created was removed from the mold and characterized. The characteristics were successfully improved by adding graphene oxide (GO) filler to a cassava starch-based bioplastic. The mechanical, water absorption, and biodegradation impacts of GO content and mixing time were investigated. The production of GO and its incorporation into a bioplastic composite were also investigated. Scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy revealed that increasing the GO concentration and mixing duration enhanced the mechanical properties of the composite, owing to good homogeneity among the elements in the composite. With a tensile strength of 3.92 MPa, elongation of 13.22%, and modulus young of 29.66 MPa, the bioplastic made with 15% GO and 60 minutes mixing time exhibited the best mechanical properties. Water uptake and biodegradation improved as the GO level increased but reduced as the mixing duration increased. Graphene oxide is a good filler for future cassava starch-based bioplastics development [57].

Process of Biodegradation Bioplastics are degradable, requiring different environmental conditions, which the natural environment fails to provide [58]. Biodegradation depends on oxygen, water availability, UV light, optimum temperature and pH [59], the thickness of biodegradable material, etc. [18]. Usually, environmental degradation involves chemical, physicochemical, and biological degradation. Biodegradation is more natural and pollution-free. The process of biodegradation can be assessed by the following steps: analyzing the products of the reaction, the destruction of reaction, the microbial growth, and the reaction conditions and properties [60]. Biodegradation in aquatic environments varies according to different marine microorganisms present in different cones of the sea. This is due to different temperatures, pH, and light availability [59]. In the benthic zone, light is available, so the degradation is aerobic due to oxygen consumption [61]. The biodegradation rate is higher in marine environments than in freshwater [62]. The raw materials used for polymer formation also affect its biodegradability. The anaerobic degradation releases water, H2S, NH3, digestate residue, and methane gas. Methane gas acts as a source of energy, and digestate residue can be used as fertilizer. The rate of

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Fig. 2 Process of biodegradation

biodegradation is analyzed by mL CH4 VS1 day1. The schematic diagram is shown in Fig. 2 [63]. Organic matter þ H2 O þ Nutrients ! CH4 þ CO2 þ NH3 þ H2 S þ Digestate þ heat Polymeric plastics are long-chain and complex and have high molecular weight, so the degradation is not straightforward. Polymers cannot be engulfed or degraded by the outer cell membrane or cell wall. So, the degradation process consists of three steps: biodeterioration, fragmentation, and bioassimilation [59]. In biodeterioration, the microorganisms accumulate on the surface of bioplastics. Microorganisms grow continuously and form biofilms and coatings, causing bioplastics’ properties to change thoroughly. In the second step of biofragmentation, microorganism releases depolymerase, and polymer is broken down into oligomers and monomers. The third step, namely, bioassimilation, is characterized by the assimilation of de-polymerization products as a carbon and energy source. Afterwards, the microorganism releases CO2 and H2O as assimilation products, which again becomes part of the ecosystem’s carbon cycle. Biodegradation of bioplastics is also carried out by composting. It converts the heterogenous waste and biodegradable plastic into homogenous beneficial material [64]. Compost is the major stable product with physical and nutritional value [65]. Industrial composting is carried out at 60  C, with moisture 50% and oxygen over 5% [66]. A biodegradable material necessarily must not be a compostable material. For a material to be compostable, it must possess the following characteristics [67]:

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(i) organic or inorganic, (ii) biodegradability, (iii) disintegration under composting conditions, (iv) compost should not promote plant growth. The degradation of lignin-based polymer has been studied by microorganisms [68]. The plastic with 6% starch and pro-oxidant was degraded by Streptomyces viridosporus, S. setonii, S. badius, and fungus Phanerochaete chrysosporium. The plastic sheet was heated at 70  C, and the relative activity of bacterial and fungus species was observed. The experiment revealed that all species except fungus showed degradation activity [68]. Degradation by microalgae was studied by Kumar [69]. The plastic polyethylene bags were dumped in suburban water bodies, and three algae species (green algae, blue-green algae, and diatoms) were selected for LD (low density) and HD (high density) polyethylene bags. When these previously dumped polyethylene bags were obtained, algal blooms were found on them, which had caused degradation. The Scenedesmus dimorphus, Anabaena spiroides, and Navicula pupula showed maximum efficiency. The results proved that LD plastics showed more degradation by algae with 8.81% degradation by Anabaena spiroides treatment [69]. The hydrolysable plastics are polyamides like nylon and polyethylene terephthalate (PET). They are formed by the combination of diamine and dicarboxylic acid. There are two modes of degradation for nylon, i.e., hydrolysis and oxidative cleavage. Enzymes responsible for hydrolysis are found in Flavobacterium, Pseudomonas, Arthrobacter, and Agromyces strains [70]. Oxidative cleavage of nylon6,6 has also been reported [71]. The white-rot fungi IZU-154, Trametes versicolor, and Phanerochaete chrysosporium, when incubated with nylon 6,6, underwent oxidative degradation. MnP modified and improved the biodegradation of nylon by an enzymatic system of fungi. The biodegradation of nylon-4 was a fast process in soil and activated sludge [72]. The biodegradation of PET is reported at 55  C with 50% weight loss of polymer by Thermobifida fusca actinomycete. Cutinase enzymes have also been found efficient in the hydrolytic degradation of PET [73]. The degradation of non-hydrolysable polymers has been reported by Pseudomonas species within 45 days [74]. Oxo-biodegradation is characterized by the action of oxygen along with light and heat. It causes the breakage of chemical bonds in the polymeric chain. The degraded products undergo bioassimilation and mineralization by microorganisms [75].

Types of Biodegradable Plastics Bioplastics are classified as bio-based and fossil-based bioplastics (Fig. 3). The bio-based plastics are further divided into biodegradable and nonbiodegradable plastics. The bio-based nonbiodegradable materials, also known as “drop-ins” or “bio-blend,” also contain some biodegradable material to accelerate the breakdown of fragments. The “PlantBottle” is an example of drop-ins introduced by Coca-Cola Company in 2019. It is reported that “PlantBottle” has prevented 365,000 metric tons of CO2 emissions. Conventional plastics and drop-ins differ in their costs and environmental effects. The only advantage is the reduction in environmental hazards

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Fig. 3 Classification of bioplastics and their examples

[76]. The bio-based biodegradable plastics are produced from animal, plants, and microbial sources. Plant sources include fibers from pineapple, jute, hump, and banana trees, providing starch, cellulose, and other biopolymers [77]. The utilization of microbial fermentation for bioplastic synthesis is limited. Silk, wool gelatin, and other animal-based polymers are used for bioplastic production [78]. Commercial bioplastics belonging to this category include polylactic acid (PLA), polybutylene succinate (Bio-PBS), polyhydroxy butyrate (PHB), and polybutylene adipate terephthalate (PBAT). Fossil-based biodegradable materials bear a certain level of biodegradability. They are not bio-based but degraded after a specific period. For example, poly-caprolactone is degraded after 6 weeks, although it is not bio-based. Guilbert [79] has classified biodegradable materials into three general categories. These are agricultural polymers, microbial polymers, and chemical polymers. Averous [80] has added the fourth category to this classification, i.e., synthetic derivatives. Agricultural polymers are used in isolated form or combined with synthetic polymers. The fermented agricultural products are formed by microbial synthesis, e.g., polyhydroxyalkanoates (PHA). These are microbial polymers. The monomers or oligomers synthesized by fermentation are combined by a chemical approach known as chemical polymers. The fourth category is synthesized by synthetic method from petrochemicals, e.g., polycaprolactones (PCL), polyester amide (PEA), polybutylene succinate adipate (PBSA), and polybutylene adipate co-terephthalate (PBAT). Based on source, biodegradable materials are classified as follows: Biodegradable plastics can be classified as completely biodegradable, semibiodegradable, and photodegradable. Completely biodegradable plastics are novel and the most utilizing materials because biopolymer is obtained from bacteria, e.g.,

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polyhydroxyalkanoates (PHA), polylactides (PLA), aliphatic polyesters, polysaccharides, copolymers, or blends. Semi-biodegradable plastics are the materials in which starch is added as the backbone to short fragments to polythene. When the starch-based polymers are disposed of in landfills, the bacteria present in the soil depolymerize the starch in the polymer, and the polymer is thus degraded. However, the polythene fragments remain nondegradable [81]. Photodegradable plastics contain light-sensitive groups incorporated into fragments of polymers. When light falls, photosensitive groups break, making the polymer available for bacterial degradation. However, these plastics require exposure to light. Based on the extent of degradation, biodegradable plastics can be divided into two groups: completely biodegradable and destructive biodegradable. Completely biodegradable materials come from purely natural materials like starch, cellulose, chitin, and other agricultural and microbial products [82]. Starch and cellulose are not plastics but can be converted to plastics by modification. The destructive biodegradable materials are formed by natural and synthetic plastic. They are not fully degraded. However, the natural material present is fully degraded and, thus, can destroy the copolymer, e.g., oxo-biodegradable plastics [83]. The copolymerization of synthetic and natural polymers involves blending in molten, solution, or dispersion system. Based on the source of raw material, biodegradable plastics are categorized as synthetic microbial plastics, synthetic biodegradable plastics, and natural polymeric plastic. Synthetic microbial plastics are formed by special microbial fermentation of sugar and acid. The plastics mainly contain an aliphatic chain with polyester linkage [84]. The natural polymeric plastics are polysaccharide-based, mainly containing starch. Starch-based plastics are thermoplastic formed by heating starch and plasticizer together. The synthetic polymers containing hydrolysable structures as their backbone are called hydrolysable bioplastics. They are degraded by hydrolases [39]. The biodegradable plastics that are inert to hydrolysis are called non-hydrolysable plastics. The thermoplastic biodegradable polymers include polylactide, bio-polyesters, starch, polyhydroxyalkanoate (PHA), cellulose, etc. Polylactide has replaced low density (LD) and high density (HD) polyethylene, PET, etc. The natural carbohydrates are fermented into lactic acid with three different stereochemical isomers, i.e., L,L-lactide, D,D-lactide, and L,D-lactide, offering a good combination of polymer formation with a wide range of applications as transparent materials [85]. The bio-polyesters have replaced the miner oil-based plastics [77]. The fermentation of renewable sources forms them. The starch-based biodegradable material with high water and plasticizers as ingredients is thermoplastic starch (TPS). They are often blended with synthetic or natural polymers starch. Starch-based thermoplastics have dominated the packaging market during the last few years [86]. The starch-based bioplastics include polyethylene-vinyl alcohol or polyvinyl alcohol and polycaprolactone [87]. Starch-based bioplastics are formed by plasticization, blending with other materials, genetic or chemical modification, or combinations of different approaches [88]. Polyhydroxyalkanoates (PHA) are another family of thermoplastic biodegradable polymers that are elastomers. They are naturally prepared by 75 genera of gram-negative and gram-positive bacteria [89]. More than 100 different PHAs

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have been identified, and poly(3-hydroxybutyrate) (PHB) is common. PHB produced by Alcaligenes eutrophus is utilized as an energy storage molecule within the microorganism’s cellular structure. PHB possesses good mechanical properties due to melting temperature at 175–180  C [90]. Cellulose is a biodegradable polysaccharide that can form cellophane film as cellulose xanthate. Cellulose esters and others act as biodegradable thermoplastics materials [91]. Algae-based bioplastics have been reported to be more durable, toxic-free, less expensive, and cause fewer environmental problems. Microalgae cannot be harvested easily, but algae-like seaweeds can be grown and cultivated easily, so algae have emerged as a potential source of bioplastic manufacturing. Algal plastics are non-toxic and have reduced the consumption of fossil fuels. Natural polymers like starch, cellulose, PHA, PHB, PLA, and proteins can be modified with algae to develop biodegradable plastics [92]. The seaweeds like red, green, and blue algae are mainly used. The main component of algae is polysaccharides like carrageenan, agar, Floridean starch, and alginate. When separated from seaweeds, the polysaccharides have been used to manufacture bioplastics [93]. The polymerization of Chlorella sp. and Spirulina sp. to form algal bioplastics has been reported [94]. Moreover, seaweed, Laminaria japonica, and Enteromorpha criniteo obtained after producing bioenergy prove effective to form algal-based bioplastic nanocomposites.

Applications of Biodegradable Plastics Biodegradable plastics are being used in the packaging industry, agriculture, and horticulture because of their biodegradability. Bio-based and fossil-based biodegradable polymers and mixes of the two are used to make such items. Bioplastics are very important in the production of compostable trash bags and shopping bags that can be utilized as organic waste bags and can be used to collect organic waste. They can enhance the volume of organic waste collected, reducing landfill trash and improving the composting process and compost quality. Due to their large market volume and compelling reasons in favor of their use, such bags are frequently viewed as a crucial application for biodegradable polymers. Biodegradable mulch film is another important application of bioplastics. Biodegradable mulch films play an important part in modern agriculture since they help to boost yield, improve crop quality, improve weed control, minimize water irrigation, and reduce pesticide use. They can be left on the field and ploughed into after the crop cycle, reducing labor and disposal costs. Figure 4 shows the main applications of biodegradable plastics. Bioplastics are being used in the catering products manufacturing process on a large scale. Cups, plates, bowls, trays, food-to-go boxes, cutlery, bags, and straws are examples of catering products for major events or service packaging for snack food sales. They can be composted with any leftover food after use; they merely need to be collected and delivered to a waste stream due to their compostability. In March of this year, the European Parliament passed a new regulation prohibiting the use of single-use plastics to minimize marine litter. Single-use plastic cutlery (knives, forks, spoons, and chopsticks), cotton bud sticks, oxo-degradable plastics, single-use

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Fig. 4 Applications of biodegradable plastics

plastic plates, plastic balloon sticks, plastic straws, and food containers and expanded PS cups will all be banned in the European Union by 2021. For some of the applications above (e.g., plates or cutlery), biodegradable, i.e., compostable plastics, can provide an organically recyclable alternative [15, 16]. Bioplastics play an essential role in the packaging business as well. Bioplastics have unique properties and a wide range of uses in agriculture, medicine, and various other industries [95–98]. Such plastics are employed in the packaging of goods with a short shelf life that requires a beautiful presentation or to prolong the shelf life of meals with a long shelf life. Netting, biodegradable bags, and trays for vegetables, fruit, eggs, and fresh meat are just a few examples [99]. PHAs are extensively appreciated in food packaging because of their distinctive properties: processability, high structural integrity, smell barriers, flavor, oil resilience, grease, and temperature stability [100]. Bioplastics are often used in rigid packagings such as containers and bottles. PLA is the most common biodegradable plastic used in non-alcoholic beverages and dairy products. Wound stitches and compostable screws, staples, pins, or plates for pinning and polymer tissues, healing sponges, ligaments, and substances for capsules and tablet packaging are just a few of their medicinal applications [99]. PHAs are critical in the medical field. Suture fasteners, meniscus repair, sutures, bone plating systems, surgical mesh regeneration devices, rivets, tacks, staples, screws, bone plates, cardiovascular patches, repair patches, vein valves, bone marrow scaffolds, skin substitutes, ocular cell implants, wound dressings, and bone graft substitutes are just a few of the applications [100]. One of the unique applications of bioplastics is 3D printing. PLA is a typical biodegradable substance used in manufacturing fused filaments and is available in various colors (FFF). PCL is another biodegradable polymeric substance utilized in FFF procedures [99].

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Conclusion Plastic materials have always been engineered to withstand deterioration. This has produced a waste management dilemma for plastic objects with a brief usage phase, such as packaging materials. Using biomolecules and constructing microbial factories to produce monomers to produce biodegradable polymers with a short use period seems like a reasonable strategy. As a result, the mission is to establish biodegradable materials that have the necessary components during use while also mineralizing completely after each use, leaving minimal hazardous leftovers. Researchers are particularly interested in producing biopolymers such as PHA and various polyols. These bioplastics have unique properties and a wide range of uses in agriculture, medicine, and other industries.

Future Perspectives The innovative bioplastic materials are made from renewable or non-renewable resources. So, all bio-based plastics are not fully biodegradable. They may cause environmental problems. Moreover, synthesizing biodegradable plastic is more expensive than commonly used plastics. In 2019, the Nova institute had estimated the global bioplastic production capacity to be 2.11 million tons and could reach around 2.43 million tons in 2024. The techno-economical life cycle revealed that bioplastics could be sold as low as $970 per ton, and it can reduce the emission of greenhouse gases by up to 116%. Algae-based bioplastics contain algae as a major constituent containing hydrocarbon, proteins, lipids, and biopolymers used as food. So these polymers must be free from pollutants, especially microplastics. Along with additives, bioplastics themselves have proved their environmental hazards. The energy requirement of bioplastics is still reported to be controversial. Moreover, the environmental effects of the bioplastic during their production could be hazardous and demand further research. Bioplastics lack recycling ability; hence the degradation products may accumulate in the ecosystem and create pollution problems. The recycled bioplastics lack good mechanical properties; thus, recycling is avoided. Replacing petro-based plastics with bioplastics requires further research about bioplastic waste disposal, recycling, and disposal.

Cross-References ▶ Biodegradable Plastics Based on Algal Polymers: Recent Advances and Applications ▶ Biodegradable Polymers for Industrial Applications ▶ Emerging and Advanced Technologies in Biodegradable Plastics for Sustainability ▶ Plastics Biodegradation and Biofragmentation

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Biodegradable Plastics Based on Algal Polymers: Recent Advances and Applications

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginate Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carrageenan Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agar Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ulvan-Based Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porphyran-Based Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fucoidan-Based Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyhydroxyalkanoates Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioplastics Based on Algal Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioplastics Based on Algal Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioplastics Based on Algal Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioplastics Based on Algal Extracellular Polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging and Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmaceutical and Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Purification and Desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mulching Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of Bioplastics in Electronic Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fire-Retardant Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

There is an upsurge need for bioplastics as a sustainable and eco-friendly alternative to petroleum-based plastics. Algae polysaccharides such as alginate, carrageenan, agar, starch, ulvan, porphyran, fucoidan, and cellulose are M. Gomaa (*) Botany and Microbiology Department, Faculty of Science, Assiut University, Assiut, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_20

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promising candidates for the development of biobased plastics. Similarly, macromolecules such as polyhydroxyalkanoates and proteins could be utilized in bioplastic preparation. These bioplastics have been exploited in the food industry, medicine, water treatment and desalination, and agriculture. In these fields, bioplastics are mainly applied as food coatings, packages, drug-delivery materials, wound dressings, adsorbents, filters, mulching films and membranes for fuel cells and batteries, etc. This chapter provides a general overview on recent advances and applications of bioplastics derived from algae polymers and composites. Keywords

Alginate · Carrageenan · Ulvan · Porphyran · Polyhydroxyalkanoates · Edible films · Food packaging · Mulching films · Fire retardants · Electrical conduction Abbreviations

CMC CNCs EPS G M PHAs PVA WVP

Carboxymethyl cellulose Cellulose nanocrystals Exopolysaccharides Guluronic acid Mannuronic acid Polyhydroxyalkanoates Polyvinyl alcohol Water vapor permeability

Introduction Nowadays, great attention has been paid to the serious environmental problems caused by the extensive use of petroleum-based plastics [1–4]. Tons of plastics are used every year around the world, and these plastics have been persistent for many years and are resistant to microbial degradation. The methods for disposal of waste plastic materials are challenging, and their recycling can be very time-consuming and can alter the properties of the recycled material [1]. Consequently, using biopolymers from natural resources to prepare biobased plastics is a promising sustainable alternative and an eco-friendly process. The term bioplastics refers to the use of living macro- and microorganisms and their polymers such as polysaccharides, proteins, and lipids in the development of plastics. The commodity bioplastics are mainly based on utilizing plants and crops with several disadvantages, such as the need for fertile land, fresh water, and fertilizers [5]. Therefore, algae have emerged as a promising alternative to produce bioplastics since they can be easily grown without the need for arable land, freshwater, and fertilizers, as well as they are characterized by a fast growth rate and environmental tolerance [6]. Furthermore, algae, as a source of bioplastics, can directly reduce carbon dioxide in the atmosphere through algal photosynthesis.

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Algae can be classified into microalgae and macroalgae. Macroalgae are usually rich in polysaccharides, while microalgae are rich in lipids, proteins, and carbohydrates. Algal macromolecules include alginate and fucoidan from members of Phaeophyceae, carrageenan, porphyran and agar from Rhodophyceae genera, and ulvan and starch from Chlorophyceae members [7, 8]. In addition, cellulose, lipids, proteins, and extracellular polysaccharides are found in different algal groups, and several green and blue-green algae can produce polyesters [6, 9]. These polymers have gained a growing interest as bioplastics and have found several applications. Macroalgal polysaccharides, namely, alginate, carrageenan, and agar, are the most widely exploited hydrocolloids in bioplastic manufacturing. However, the use of different algal polymers as bioplastics remains highly conceptual. Accordingly, this chapter aims to discuss the current status and progress in developing bioplastics based on algal polymers and their recent applications.

Alginate Bioplastics Alginates (salts of alginic acid) are important structural polysaccharides of brown macroalgae, which provide both strength and flexibility to the algal cells. Alginic acid is a linear polysaccharide consisting of 1,4-linked β-D-guluronic acid (G) and its C-5 epimer α-L-mannuronic acid (M), which are arranged as homopolymeric blocks (poly-M or Poly-G) or heteropolymeric blocks (poly-MG) within the same chain [10–13] (Fig. 1). Generally, alginates from brown algae have several excellent properties, such as biodegradability, biosafety, biocompatibility, reproducibility, and gel-forming ability. Accordingly, alginates have been exploited in different fields such as food, textile, pharmaceutical, medical, and environmental industries, where they can be used as a thickening, stabilizing, emulsifying, chelating, encapsulating, and swelling agent or can be used to form gels, films, and membranes [14–17]. The sodium salt of alginate is water-soluble and can be used to increase the viscosity of solutions or to develop films and membranes. In contrast, alginate has a strong affinity to chelate bivalent cations to form insoluble products, and its affinity increases in the following order Ca2+ < Sr2+ < Ba2+ [18]. The bivalent salts of alginate can form films with higher mechanical and barrier properties than Naalginate-based films [14, 15]. Alginate can also chelate several heavy metals such as cadmium, copper, iron, nickel, zinc, lead, and manganese [13, 19]. In general, alginate can react with divalent or trivalent cations through their guluronic acid blocks, leading to an egg-box structure (Fig. 1). The formation of egg-box structure firstly involves the interaction between the divalent or the trivalent cations with the G monomer, followed by the alignment and association of the adjacent polymer chains and the formation of egg-box dimers in which diamond-shaped cavities are formed to surround the cation. The cation connects with the oxygen atoms of the carboxyl groups of alginates through multiple coordinations. Further associations can induce the formation of multidimers [20]. Accordingly, molecular weight and M/G ratio of alginate are crucial features that determine the efficiency of its application in different fields. For instance, alginate

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Fig. 1 The chemical structure of alginate and its conformational changes in the presence of divalent cations. M: β-D-mannuronic acid, G: α-L-guluronic acid

with high viscosity and low M/G ratio can form firm and dense gels suitable for food and cosmetic industries [10]. At the same time, alginate with low viscosity and high M/G ratio can build flexible and porous gels, which are exploited in the preparation of polyelectrolyte complexes or used in textile paper, or paper dyeing industries [8]. Alginate with a low M/G ratio leads to robust films with low water solubility, and these properties can be further optimized based on the concentration of the crosslinking Ca2+ ions [20]. Alginate with a low M/G ratio (0.45) produced films with low water vapor permeability (WVP) compared to those with a high M/G ratio (1.5) [21]. Generally, alginate bioplastic films are produced by casting and solvent evaporation methods. Ionic crosslinking of alginate films using divalent cations (i.e., Ca2+) can produce bioplastic films with the improved mechanical, barrier, and water resistance properties compared to univalent cations [14, 15]. The ionic crosslinking process can be performed by different techniques such as direct mixing, internal gelation, interfacial gelation, and external gelation (Fig. 2). Direct mixing of the desired cation in powder or solution and Na-alginate aqueous solution can form instantaneous gels, which can impede the film casting in some cases [22]. In the internal gelation method, the crosslinking cations are introduced in the form of insoluble salts (i.e., CaCO3); then the gelation is proceeded by immersing the alginate films containing the insoluble salt in an acid solution. In the interfacial gelation process, the Na-alginate solution is separated from the crosslinking solution by a selective semipermeable membrane where the ions can diffuse unidirectionally

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Fig. 2 Different methods for ionic-crosslinking and gelation of sodium alginate

to induce alginate gelation. While, in the external gelation process, also known as dry-cast gelation, the dried Na-alginate films are immersed in a crosslinking solution or sprayed by the crosslinking solution. The external gelation is usually preferred from an industrial and biotechnological viewpoint. However, alginate’s fast crosslinking and swelling can produce polymer folding lumps in some cases. The main phenomenon during external gelation is the formation of dry Na-alginate films before gelation, whereas, in other methods, Ca-alginate gels are produced within the Na-alginate solution (Fig. 2). External gelation can produce thin films with smooth surfaces and high mechanical strength and stiffness in the internal or interfacial gelation processes. Moreover, the external gelation technique can be improved in several ways. For instance, ethanol as a cosolvent with CaCl2 solution produced homogenous Ca-alginate films with greater thickness and strength, which was related to reducing the swelling state during gelation [23]. The incorporation of non-gelling Na+ ions improved the mechanical strength of the Ca2+-crosslinked alginate films, but in the case of Cu2+ or Zn2+-crosslinking, the tensile strength was reduced [24]. Another mechanism of ionic crosslinking of alginate is positively charged organic compounds such as chitosan. Chitosan is a polyamine polysaccharide composed of β-(1,4)-linked N-acetyl-D-glucosamine and D-glucosamine [25]. The electrostatic interaction is attributed to chitosan’s positively charged amine groups and alginates’ negatively charged carboxyl groups. Incorporating chitosan into alginate films can promote their mechanical properties and reduce their solubility [14]. However, chitosan is insoluble in water; thus, it is dissolved in dilute acids such as acetic and hydrochloric acids during film preparation. Furthermore, when chitosan is mixed with alginate, instantaneous crosslinking and gelation are formed. Thus, careful homogenization is necessary. Multilayer films can be developed with excellent properties. Covalent crosslinking is another mechanism for the preparation of alginate hydrogels. In covalent crosslinking, irreversible chemical links are formed compared

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to ionic crosslinks that can be dissociated and reformed. Recently, ferulic acid developed covalently crosslinked alginate films with enhanced physical and mechanical properties. In another study, citric and tartaric acids were used as covalent crosslinkers to develop alginate films with good thermal stability, biodegradability, flexibility, and transparency. Alginate bioplastics can also be fabricated using thermo-mechanical mixing and compression molding. In these processes, the alginate solution and a plasticizer are mechanically homogenized at high temperatures and then cast into sheets under specific temperatures and pressure [26]. These methods can also develop composite bioplastics with different characteristics and applications, i.e., by mixing common thermoplastic polymers with alginate. Therefore, this process can overcome the major drawbacks, such as limited thickness, productivity, and blending with other polymers or fillers. During thermo-chemical mixing, plasticizers are highly recommended to increase the flexibility and stability of the end-product. Glycerol plasticization of alginate during thermo-mechanical mixing increased the flexibility of the final product by decreasing tensile strength and enhancing elongation at break [26].

Carrageenan Bioplastics Carrageenans are a group of sulfated polysaccharides that build up the cell wall and the intracellular matrix of different red seaweeds. Their structure consists of linear disaccharide units of α-(1,4)-linked D-galactose or 3,6-anhydrogalactose and β-(1,3)-linked D-galactose, and the sulfate groups are located in the carbon atoms C-2, C-4, and C-6 of galactose [27] (Fig. 3). Red macroalgae, which contain carrageenan as a structural component, belong to the families Solieriaceae, Rhabdoniaceae, Phyllophoraceae, Gigartinaceae, Rhodophilidaceae, and Thichocarpaceae [28]. Based on the presence or absence of the 3,6-anhydrogalctose and the number of sulfate groups, carrageenans are classified into six different forms designated by Latin letters: kappa (κ), lambda (λ), iota (ι), nu (ν), mu (μ), and theta (θ) (Fig. 3). The 3,6-anhydrogalactose residues are absent in λ-, μ-, and ν-carrageenan, while present in the other forms (Fig. 3). The number of sulfate groups is one (κ-type), two (ι-, μ-, and θ-type), or three (λ- and ν-type) [27] (Fig. 3). These structural properties are associated with wide variations in physicochemical properties and helical structure formation and potential applications [18]. However, only κ-, λ-, and ι-carrageenans are produced commercially, and their biotechnological aspects are confirmed. In general, the biosynthesis of carrageenan in the algal cells may be discontinuous. Therefore, isolated carrageenan may have a hybrid structure with disaccharide units and different structural patterns [27]. Commercially, carrageenan is obtained from genera Kappaphycus and Eucheuma. The extraction and purification methods of the product lead to either refined or semi-refined carrageenan. The main difference between these two products is the presence of cellulose in the semi-refined carrageenan, while the refined carrageenan does not contain cellulose and is characterized by high purity. Thus,

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Biodegradable Plastics Based on Algal Polymers: Recent Advances and Applications 507

Fig. 3 Chemical structure of the disaccharide repeating residues of the most common carrageenan types. The main difference is the presence of 3,6-anhydrogalctose and the number and location of sulfate groups

extra steps of extraction and purification are mandatory to remove cellulose, which increases the total production cost. Therefore, using the semi-refined products can reduce the production cost of carrageenan-based bioplastics. Semi-refined carrageenan can produce strong but less clear gels than refined carrageenan. Compared to alginate bioplastics, carrageenan-based bioplastics are generally less explored. The bioplastic films of carrageenan are typically obtained by casting and solvent evaporation techniques [29]. In general, κ-, θ-, and ι-type can form gels when certain cations co-exist in the solution, while ν-, μ-, and λ-carrageenan are non-gel forming due to the absence of 3,6-anhrdrogalgctose units [18]. Cations such as K+ in the case of κ-carrageenan and Ca2+ in the case of ι-carrageenan can induce conformational shifts in the coiled carrageenan chains to form the double helix conformation followed by aggregation of these helices and gel formation [30] (Fig. 4). The double helices are generated by crosslinking of the close spiral chains when the sulfate groups are directed to the outside upon decreasing the temperature or the existence of cations. Generally, the ability of cations to promote gelation follows the order of lithium < sodium  magnesium, calcium < ammonium < potassium for κ-type and ammonium, and lithium < sodium < potassium < calcium for ι-carrageenan [31]. The variations in gelling properties of these different types of carrageenan considerably influence the characteristics of the developed films. Generally, gels based on κ-carrageenan are hard, strong, and brittle, while those based on ι-carrageenan are soft, elastic, and weak. Edible films prepared from κ-carrageenan

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Fig. 4 Mechanism of carrageenan gelation in the presence of certain cations, i.e., K+ for κ-carrageenan and Ca2+ for ι-carrageenan

and plasticized with glycerol demonstrated low WVP and oxygen permeability and high mechanical strength and transparency about ι-carrageenan films. In another study, the ι-carrageenan membranes prepared with CaCl2 exhibited higher mechanical strength than those of κ-carrageenan, due to the specificity of ι-carrageenan to Ca2+ [29]. Furthermore, the exposure of CaCl2-carrageenan wet membranes to CO2 gas can lead to the formation of CaCO3 minerals within the polymer matrix. The formation of CaCO3 after 5 min of exposure to CO2 gas enhanced the tensile strength of the hybrid κ-carrageenan by ~ twofold, while this effect was not obvious in the case of ι-carrageenan hybrid membranes due to less available Ca2+ [29]. Plasticizers are practically important to increase the mechanical properties of semi-refined carrageenan films. However, sorbitol-plasticized films exhibited homogenous and smooth surface and low oxygen permeability compared to glycerol. Additionally, sorbitol can decrease film thickness, WVP and water solubility, and water uptake compared to glycerol or polyethyleneglycol-300 [32].

Agar Bioplastics Agar is a structural polysaccharide in different red seaweeds which is chemically composed of two different polymers known as agarose and agaropectin. Agarose is an unbranched polysaccharide that contains D-galactose and 3–6, anhydro-L-galactose as the main building blocks, linked by alternating α-(1,3) and β-(1,4) glycosidic bonds [33–35] (Fig. 5). Agaropectin has similar composition but is highly branched and sulfated. Agarose represents the gelling fraction in agar, while agaropectin is the non-gelling fraction. Therefore, agaropectin is usually removed during the industrial production of agar to produce a product with higher gel strength. Commercially, agar is extracted from the red algae Gelidium sp. and Gracilaria sp. [36]. The mechanism of producing bioplastic membranes based on agar relies on its thermo-reversible gelation by decreasing the temperature of its viscous solution beyond its gelling temperature. During gelation, the agarose coils are transformed

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Fig. 5 Chemical structure of the disaccharide repeating residues of agar. R can be a hydrogen atom or a certain group such as ester sulfate, methyl, or pyruvic acid

into double helices stabilized by water molecules and aggregated by their external hydroxyl groups [37]. To produce agar films by casting method, the hot agar solution should be cast onto a warm surface at temperatures higher than the agar’s gelling temperature to avoid instant gelation and formation of clumps. After water evaporation, the dried films contain numerous hydrogen-bonding interactions between the agarose chains, enabling the formation of easily peeled continuous films [36]. Agar bioplastic films are generally characterized by high transparency and retraction ratio during drying due to syneresis between agarose chains, and more importantly, they are biologically inert, which facilities their modification by incorporating bioactive compounds. However, pure agar films are usually brittle and have high water susceptibility [36]. Accordingly, these drawbacks can be overcome by incorporating different compounds or producing composite films. For instance, blending κ-carrageenan and konjac glucomannan promoted both tensile strength and transparency of agar films [36]. Sousa and coauthors utilized deep eutectic mixtures (choline chloride/urea and choline chloride/glycerol) as plasticizers for agar films. The films were developed based on three steps: solubilization of agar in a deep eutectic solvent, compression molding to produce films, and film drying in ethanol and air. The films showed superior mechanical properties compared to aqueous agar films.

Ulvan-Based Bioplastics Ulvan is a sulfated heteropolysaccharide that constitutes one of macroalgae’s main cell wall components in the order Ulvales (Chlorophyta), such as Ulva and Enteromorpha sp. [38]. Ulvan consists of repeating disaccharide units of rhamnose, uronic acid, iduronic acid, and xylose. The major repeating disaccharides are aldobiuronic acids, designated as ulvanobiouronic acid 3-sulfate type A3S and B3S (Fig. 6). The type A3S consists of repeating β-D-guluronic acid (1,4)-α-Lrhamnose-3-sulphate, while the type B3S has to repeat α-L-iduronic acid (1,4)-α-L-

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Fig. 6 Chemical structure of the main disaccharide units of ulvan

rhamnose-3-sulphate units [39] (Fig. 6). Other monosaccharides such as xylose, glucose, and galactose are also present in minor proportions. The utilization of ulvan in the development of bioplastics is generally limited. One study developed antioxidant ulvan films from the green seaweed Ulva lactuca and evaluated the role of extraction conditions [40]. The results indicated that enzymatic extraction could produce films with enhanced light barrier, thermostability, and antioxidant properties. The ability of ulvan to produce thermo-reversible gels and films depends mainly on its physicochemical features such as the co-existence of hydrophilic (hydroxyl, carboxyl, and sulfate) hydrophobic (methyl) functional groups as well as its anionic properties. Additionally, the structural features of ulvan films can be enhanced by using plasticizers during film preparation. Plasticization with glycerol showed superior structural properties and improved the antioxidant capacity compared to sorbitol [40]. Furthermore, glycerol in ulvan films promoted transparency, solubility, and mechanical properties, while the use of sorbitol improved moisture resistance and decreased WVP and visible and UV light transmission [41].

Porphyran-Based Bioplastics Porphyran is a sulfated polysaccharide in the cell wall and the intercellular spaces of the seaweeds in the order Bangiales (Rhodophyta), especially from the genus Porphyra. Porphyran is a linear polysaccharide that consists of alternating 1,3-linked β-D-galactose and 1,4-linked α-L-galactose 6-sulfate units accompanied by smaller number of 3,6-anhydro-α-L-galactose units (Fig. 7). Partial methylation

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Fig. 7 Chemical structure of the disaccharide units of porphyran

can be found at the C-6 position of the galactose units and the C-2 position of the anhydro-galactose units [27]. Generally, porphyran is mainly utilized to prepare composite bioplastics to enhance their functional and bioactive properties. Porphyran from Porphyra dioica was used as inner coating of polyvinyl alcohol (PVA) films using electrospinning and electrospraying technologies [42]. The developed films were applied as a coating to enhance the shelf-life of poultry products by reducing microbial growth and enhancing color stability and sensory parameters. Cian et al. [43] utilized porphyran and phycobiliprotein-enriched fractions from the red seaweed Porphyra columbina to develop antioxidant films. The authors demonstrated that incorporating the phycobiliprotein extract into the porphyran film had a plasticizing effect, and it promoted water solubility, mechanical properties, and antioxidant characteristics of the resulting films, and further, the WVP was reduced. Another investigation prepared composite films using carrageenan and porphyran from the red seaweed Pyropia columbina [44]. The water susceptibility and WVP of these films were enhanced by glycerol plasticization. Furthermore, calcium-crosslinking diminished the influence of glycerol in some features (water solubility and WVP) but exhibited a synergism with glycerol on mechanical properties [44].

Fucoidan-Based Bioplastics Fucoidans are a group of fucose-rich-sulfated polysaccharides which represent one of the main structural components in the cell wall and the intracellular matrix of brown algae. Fucoidans are heteropolysaccharides with α-L-fucose (F) as the main monomer, and other residues may include glucose, galactose, guluronic acid, xylose, mannose, arabinose, and rhamnose [14, 17, 45]. Fucoidans are classified based on their structural variations into different groups as follows [46]:

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(a) Fucoidans or fucans in which F residues are linked by α-(1,3)-glycosidic bonds and sulfate groups are mainly linked to C-4 and partially at C-2 (Fig. 8). They mainly occur in the order Laminariales. (b) Fucoidans in which F residues are linked by alternating α-(1,3) and α-(1,4)glycosidic bonds and sulfate groups are mainly linked to C-2 and partially at C-4 (Fig. 8). They mainly occur in the order Fucales. (c) Fucoidans or galctofucans in which F and galactose residues are the main components. They can be found in the members of the family Sargassaceae and Laminariaceae. (d) Fucoidans with different molar ratios of fucose, mannose, xylose, galactose, arabinose, and uronic acids. Countless promising bioactivities characterize fucoidans. Therefore, they may be recognized as a treasure of the sea [8, 11, 14–16, 45]. However, fucoidans cannot form gels by themselves; thereby, they are blended with other gel-forming polymers such as alginate [15], chitosan [14, 47], PVA [48], sodium hyaluronate [49], and collagen [50]. In these films, fucoidan can alter the physicochemical properties of the composite films and boost their bioactive properties.

Polyhydroxyalkanoates Bioplastics Polyhydroxyalkanoates (PHAs) are polymers belonging to polyesters and can be produced by several bacteria, cyanobacteria, and microalgae. They are considered a promising alternative to petroleum-based plastics since they have similar mechanical properties to synthetic polymers and are biodegradable. PHAs are predominately linear polymers composed of hydroxy alkalotic acid residues, which connect by ester

Fig. 8 Chemical structure of the main repeating units of fucoidans. R may be a hydrogen atom or sulfate group

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bonds through the hydroxyl groups in one unit and the carboxyl groups of the adjacent unit [5]. PHAs are characterized by different physicochemical properties based on the length of the carbon chain within the individual monomers and the final polymer composition, which can be modified based on the carbon source in the culture medium of the producer microorganism [51]. Based on the length of the monomer chain, PHAs are classified into three categories: (i) short-chain length: with 3–5 carbons; (ii) medium chain length: with 6–14 carbons; and (iii) long-chain length: with more than 14 carbon atoms. In general, short-chain PHAs have close properties to synthetic plastics, while medium-chain PHAs and long-chain PHAs are considered elastomers and rubbers [51]. Several microalgae and cyanobacteria have accumulated PHAs as storage materials such as Chlorella and Botryococcus from Chlorophyta and Spirulina, Synechocystis, and Synechococcus from Cyanobacteria. Generally, the production of PHAs by cyanobacteria or microalgae is more economical and advantageous than bacterial production. This is mainly related to the autotrophic nutritional mode of these microorganisms, which can markedly reduce the cost of the organic carbon sources used for heterotrophic bacterial production [51]. Film casting followed by solvent evaporation is the basic method for developing PHA-based bioplastic films. Generally, PHAs are soluble in chloroform; however, a pretreatment step for the polymer is important to remove the residual moisture [52]. Other methods for preparing PHA-based bioplastics are compression molding, injection molding, and extrusion blowing. In compression molding, the polymer is placed in molds with a desirable shape, and temperature and pressure are maintained at certain values. While injection molding involves complete polymer melting before casting into shapes, it solidifies inside the molds [51]. In extrusion blowing, the polymer is extruded into a tubular-shaped die, and air pressure is applied to further expand the bioplastic [57]. In general, the mechanical properties of PHAs can be promoted by blending with glycerol, polyethylene glycol, or polyvinyl acetate [53].

Bioplastics Based on Algal Proteins Some microalgae such as Spirulina sp. and Chlorella sp. can accumulate high concentrations of proteins inside their cells [9]. These proteins can be utilized as an alternative source to plant and animal-derived proteins to develop innovative bioplastic materials. The protein concentrate from Spirulina platensis was used to develop bioplastic films using sorbitol as a plasticizer but not glycerol [54]. The formulated films were opaque, brown, and thick and had more water susceptibility and less elasticity than the Na-caseinate films [54]. It was also reported that incorporating 2% of Spirulina platensis-protein concentrate to the gelatin-based films markedly increased their thickness and mechanical strength and decreased WVP, which was related to the interaction between matrix components algal protein [55].

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On the other hand, the whole protein-rich microalgal biomass can be used to develop biocomposite films or membranes, reducing the cost of using chemicals and materials to extract and purify algal proteins. Yan et al. [56] utilized the deoiled Nannochloropsis biomass to develop cellulose/microalgae biocompatible films using ionic liquid as solvent. The microalgal biomass showed strong hydrogenbonding interactions with cellulose, and the developed composite films exhibited good mechanical properties and thermal stability. In another study, thermoplastic blends were developed using Chlorella (57% protein) or Spirulina (58% protein) biomass and polyethylene [57]. In general, blending compatibilizers is fundamental to effectively utilize the whole microalgal biomass in bioplastics for commercial applications. Compatibilizers are compounds that can bind and interact with two different polymers leading to better stability and blending. Zhang and coauthors used maleic anhydride as a compatibilizer between the hydrophilic biomass of Chlorella and the hydrophobic polypropylene to produce bioplastics with promoted tensile strength [58]. In another study, the residual biomass of Spirulina after the extraction of phycobiliproteins was utilized for the development of bioplastics using PVA with good mechanical properties. The presence of salts in the residual microalgal biomass acted as filler in alkali conditions and as a crosslinker in acidic conditions.

Bioplastics Based on Algal Cellulose Cellulose is a homopolysaccharide that consists of glucose residues connected by β-1,4-acetal linkages. It is widely exploited in developing bioplastics and can be used as a filler with different polymers owing to its rigidity and high resistance. Cellulose can be a cell wall component in different microalgae and macroalgae species in Chlorophyta, Charophyta, Rhodophyta, and Ochrophyta [38]. Cellulose is usually characterized by a high degree of polymerization (up to 15,000 glucose units), which can limit its applications. Therefore, cellulose is usually converted into nanocellulosic materials with enhanced mechanical properties suitable for bioplastic production. Based on the extraction and conversion processes, these cellulosic nanomaterials can be classified into cellulose nanocrystals and cellulose nanofibrils. In general, limited information is available in the literature regarding the utilization of algal cellulose or its nanomaterials in developing bioplastic materials compared to plant-based cellulose. Lakshmi et al. [59] synthesized carboxymethyl cellulose (CMC) from the cellulose extract of the green seaweed Ulva fasciata and developed biodegradable bioplastic films by solution casting and evaporation method. The synthesized CMC produced films with similar characteristics to the commercial CMC. Sucaldito and Camacho [60] isolated cellulose from Cladophora rupestris and prepared cellulose nanocrystals (CNCs) by HBr hydrolysis of cellulose. Incorporating CNCs into the starch films increased its mechanical strength by 78%. In another study, CNCs were prepared by acid hydrolysis of Enteromorpha proliferaderived cellulose and were utilized to prepare biocomposite films using the watersoluble polysaccharides that were isolated from the same seaweed biomass [61].

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Similarly, CNCs derived from the waste red algae industry exhibited an enhancing effect on PVA-composite films’ tensile, thermal, and transparency properties.

Bioplastics Based on Algal Starch Starch is a glucose-based homopolysaccharide composed of two main polymeric chains, namely, amylose and amylopectin. Amylose is unbranched and consists of α (1–4)-linked glucose units, while amylopectin is branched and contains both α (1–4)and α (1–6)-linked chains. Starch represents the main reserve food material in various species of algae. Ramli and coworkers showed that the structural features of algaederived starch surpass plant-derived starch, despite a similar amylose/amylopectin ratio [62]. Therefore, algal starch can be used as a promising alternative to the foodbased starch to develop biodegradable plastic materials. However, little information is available in the literature regarding the use and economic feasibility of algal starch in the production of bioplastics. Recently, Mathiot et al. demonstrated that starchaccumulating microalgae such as Chlamydomonas reinhardtii 11-32A could be utilized in bioplastic production when cultivated under sulfur-deprived conditions the presence of glycerol as a plasticizer and using a twin-screw extruder [63].

Bioplastics Based on Algal Extracellular Polysaccharides Several microalgae can produce extracellular polysaccharides (EPS) in their cultivation medium. These EPS are unique in their physicochemical properties, which can be directly altered by culture conditions [9]. However, the utilization of these EPS in the preparation of bioplastics is limited. One study indicated that EPS of the cyanobacterium Nostoc sp. and the red microalga Porphyridium purpureum can produce transparent and flexible biofilms, but they contain cracks and pores [64]. In a recent study, the green thermophilic microalga Graesiella sp. EPS was utilized to develop homogenous elastic films with high thermostability and low WVP and oxygen permeability [65]. Furthermore, these films were characterized by natural antioxidant properties suitable for food packaging.

Applications Algae-derived polymers and their bioplastic formulations can find promising applications in the industrial sector in different fields such as food, medicine, agriculture, environment, electronics, etc. (Fig. 9).

Food Packaging and Coatings One of the most crucial applications of bioplastics is to enhance the quality and shelf-life of different food products where they can be made into thin edible films for

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Fig. 9 Different applications of algal polymers and their bioplastics

food packaging or directly applied as a coating over the food. Furthermore, this function can protect food products from external physicochemical and biological factors. Physicochemical deterioration includes gases, moisture, and light, while biological deterioration involves microbes, insects, and the senescence of food itself. Moreover, edible films based on algal polysaccharides can be suitable for improving the delivery and dispersion of food additives or controlling the release of bioactive agents during packaging [14, 15]. The development of edible bioplastics for food packaging and coating depends on many physicochemical and bioactive properties. Physicochemical characteristics include film thickness, density, WVP, CO2 and O2 permeability, mechanical properties (tensile strength, elongation at break and Young’s modulus), optical properties (color, opacity, and light transmission), and thermostability. While bioactive properties include antibacterial, antifungal, and antioxidant properties [15]. Hydrophilic phycocolloids usually produce films with superior mechanical properties compared to those from hydrophobic compounds. Furthermore, these water-soluble polymers can also impede gases due to their hydrogen-bonded network structures, resulting in compact and tightly packed films and coatings [66]. The solubility of edible films can also reflect their superior biodegradability in the environment. Generally, edible films based on natural polymers cannot provide all desirable food packaging and coatings characteristics. Therefore, the main purpose is to select integrated and synergistic film components and use better plasticizers and gelling agents. The casting method is the most common method for preparing edible films for food packaging. The casting surface for drying plays a fundamental role in the final physical properties of the developed films. Silicone, Teflon, and glass are the most common casting surfaces, but silicon surfaces are commonly used owing to

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their low adhesiveness. Biopolymers can be applied as coatings by wrapping, dipping, or spraying the food surface using the film-forming solutions. Food and coatings are usually applied to fruits, vegetables, or meat products. They can delay fruit ripening, prevent microbial spoilage and shrinkage, and delay the cooking time of microwavable food products. Recent advances in the preparation of active food packaging include incorporating bioactive natural extracts and nanoparticles [15]. The incorporated compounds can be slowly released into the food products leading to extending shelf-life and enhancing functional properties. The release of active compounds from edible films and coatings depends on several factors related to the film components, the nature of the bioactive compounds, and the possible interactions between the film matrix and the bioactive compounds. Bioactive compounds’ related factors include molecular weight, solubility, density, and polarity in food components. Polymer-related factors include molecular weight, density, and swelling behavior, which determine the orientation and micropores of the film matrix as well as its diffusion-related properties. Additionally, bioactive compounds can induce plasticizing or antiplasticizing effects with the polymeric film, and their interactions can determine the rate of diffusion [14, 15]. Algal-derived polymers such as alginate, ulvan, carrageenan, fucoidan, and porphyran are characterized by natural bioactivities, including antioxidant and antimicrobial activities. The development of edible films based on these polysaccharides can support active food packaging and coating. Furthermore, the development of edible films based on the crude extracts of these polymers can reduce the cost of using chemicals and equipment for purification. In addition, these crude phycocolloids are usually coextracted with impurities such as polyphenolic compounds. The co-existence of these phenolics can provide additional advantages regarding the bioactive properties of the developed films. Gomaa et al. [15] utilized the crude alginate extract from the brown alga Sargassum latifolium to prepare calcium alginate films rich in co-extracted phlorotannins, polyphenols of brown macroalgae. The developed edible films were characterized by good antioxidant, light and gas barrier, moisture sorption, and WVP and can be applied as packages of perishable food products. Furthermore, these films were characterized by the fast release of phlorotannins in acidic, aqueous, and alcoholic food simulants. Blending different algal-derived polysaccharides to produce composite films is an effective strategy to obtain desirable characteristics. For instance, carrageenan and calcium alginate showed synergistic interactions and produced composite films with promoted tensile strength, thermostability and transparency, and low water solubility. In these films, the alginate chains were arranged at the outside surface due to their higher hydrophilic properties than carrageenan. Their content and a crosslinking agent can control the pore size of carrageenan/alginate films. It was reported that a high carrageenan/alginate ratio could produce films with small pore size, which can be further reduced by calcium chloride-crosslinking. In another study, combining alginate to two different types of carrageenan (κ- and ι-carrageenan) at different ratios produced films with different physical properties. In these films, alginate

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contributed to high uniformity and transparency, κ-carrageenan increased moisture barrier and tensile strength, while ι-carrageenan induced the formation of aggregates and increased film opacity [30]. On the other hand, the blending of carrageenan with agar decreased WVP and water solubility. These films’ mechanical properties and water susceptibility can be enhanced by incorporating nanoclay. Edible composite films with antioxidant properties suitable for food packaging were prepared using semi-refined carrageenan from the red seaweed Kappaphycus alvarezii and ulvan from the green seaweed Ulva fasciata [7]. The composite films were plasticized with glycerol, which improved their mechanical properties and decreased WVP. On the other hand, edible films with an electrical conduction property provide an additional advantage to food packaging since they allow low-cost sterilization of food products by a pulsed electric field. Recently, Alves and coauthors utilized reduced graphene oxide and zinc oxide as an electrically conducting and active filler in alginate films and used sepiolite for compatibility [67]. The developed nanocomposite film was characterized by bioactive (antioxidant and antimicrobial) and electric conducting properties, allowing extending the shelf-life of food through low-temperature sterilization and preventing post-contamination and spoilage.

Pharmaceutical and Biomedical Applications Algal polysaccharides have been extensively investigated for various pharmaceutical and biomedical applications. The utilization of alginate, carrageenan, ulvan, fucoidan, agar, and PHA bioplastics in these fields relied mainly on the production of wound dressings and drug delivery systems. The thin films of these polysaccharides are ideal biodegradable wound dressings since they are biocompatible, flexible, and permeable to water vapor, have adequate mechanical properties, can adhere to the skin, and can create a moist environment in the wound as well as can protect against microbial infections and maceration [68]. Furthermore, these wound dressings are suitable for carrying and controlling the release of therapeutic agents such as synthetic or natural antimicrobial compounds to the wound surface. In wound dressings, the polysaccharides are usually crosslinked to enhance their water holding capacity to dehydrate/hydrate the wounds upon contact to create a moist environment [66]. Among algae-derived polymers, alginate is currently used commercially by several manufacturers to prepare wound dressings. The common method for the preparation of polymer wound dressings is casting and solvent evaporation. This method is simple and can be effectively utilized to develop medicated dressings [69]. Bergonzi et al. [70] developed a wound dressing by incorporating human elastin-like polypeptide with alginate and crosslinked with curcumin and calcium chloride. The developed films controlled the release of curcumin for up to 10 days, and the polypeptide induced an enhancing effect not only for the release rate but also for the antioxidant capacity. In another study, ulvan/ chitosan layer-by-layer films showed the ability to stabilize the adhesion of viable

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primary hippocampal neurons, as well as expedited neurite outgrowth and selective suppression of astrocytes. Therefore, these membranes can be utilized for neural implants and devices. In another study, ulvan was crosslinked using 1,4-Butanediol diglycidyl ether and was exploited to develop wound dressing with good water uptake and mechanical strength for the controlled release of a steroid antiinflammatory drug (dexamethasone) [69]. However, chemical crosslinking can induce cytotoxic responses related to the unreacted crosslinker, which can be overcome by physical crosslinking. Sulastri and coauthors utilized boric acid as a physical and ionic crosslinker for glycerol-plasticized ulvan films [71]. The developed films were characterized by natural antioxidant and antimicrobial activities and good swelling and WVP, implying their potential use as wound dressings. The blending of sodium alginate and κ-carrageenan in the presence of potassium ions as crosslinker produced topical wound dressings with good swelling and bioactivity. Furthermore, incorporating the antibiotic chloramphenicol and silver nanoparticles into these films boosted their broad-spectrum antibiotic properties. In another study, κ-carrageenan was blended with agar, and biocompatible wound dressings were produced [72]. The authors showed that incorporating montmorillonite can control the release of analgesic lidocaine hydrochloride and antibiotic chloramphenicol into the medicated dressing. Several studies incorporated fucoidan in wound dressings due to its healing, antioxidant, antimicrobial, anticoagulant, antithrombotic, and hemostatic properties, and it showed promising effectiveness. Thus, Sezer et al. [47] developed chitosan/ fucoidan porous film as a wound dressing. The film induced faster cell generation and wound closure than the control. Similarly, fucoidan incorporation benefited PVA-based films and vascular grafts by promoting endothelial cell adhesion and lowering intimal hyperplasia [48]. Furthermore, fucoidan-based films can find potential application in tissue regeneration, as reported in the case of collagenfucoidan blend films. Generally, for drug delivery, multilayered composite films are usually developed. The targeted drug is incorporated into the innermost layer to manipulate its release in the human body based on certain conditions such as temperature, pH, enzymes, or ionic strength [66]. The selection of polymers as scaffolds for drug delivery depends on mechanical properties to sustain the in vivo stress and adequate diffusion and release rate. Furthermore, impurities must be controlled as they may induce immunogenic responses. Algal polymers can also be used to develop membranes for cosmeceutical purposes due to their antioxidant and whitening properties. Recently, Don et al. [73] prepared chitosan/ulvan composite membranes further crosslinked by tripolyphosphate and glycerol to increase their tensile strength from 1.09 to 2.67 MPa. The composite films exhibited a higher swelling degree and WVP and were characterized by promising antioxidant and whitening properties, which was attributed to ulvan incorporation. Furthermore, ulvan release from these films (40–65% for 12-h releasing) was observed, which was enhanced to 64.8% by incorporating chlorophyll into the films [73]. The results indicated that the films

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had good antioxidant and antimicrobial activities and adequate swelling behavior and water vapor transmission, which is suitable for the development of wound dressings. On the other hand, PHAs can prepare various medical and pharmaceutical tools such as sutures, stents, nerve guides, orthopedic pens, wound dressings, skin substitutes, pericardial patches, ocular cell implants, hemostats, and bone marrow substitutes.

Water Purification and Desalination The contamination of natural water with various inorganic and organic pollutants is a serious global problem. The membrane-based water treatment can play a fundamental role in water treatment by reducing costs and energy. Thin membranes based on algal polymers can be used to remove both inorganic (i.e., heavy metals) and organic pollutants (i.e., dyes, pharmaceuticals, pesticides, etc.). The removal mechanism relies mainly on adsorption; thus, these membranes are easily applied with no generation of toxic intermediates. However, several limiting factors related to the adsorbent, adsorbate, and environment must be considered during adsorption. The adsorptive interactions between the polymeric membranes and the adsorbate molecules depend on several mechanisms. One mechanism is associated with the electrostatic interactions that emerge when the polymeric matrix and the adsorbate molecules have opposite charges. Another mechanism can be related to hydrogen bonding interactions between the hydrogen donating groups (i.e., CH3, CH2, NH2, COOH, NH2) in the polymer structure and the hydrogen accepting groups of the adsorbate molecules. Further interactions can be indicated by electron-donating and accepting groups. On the other hand, removing heavy metal ions by polymeric membranes can occur by ion-exchange, metal coordination and complexation, and microprecipitation. In general, thin polymeric membranes as adsorbents have superior advantages than other adsorbents. One key advantage is the increase in diffusion across the membrane owing to a decrease in resistance to mass transfer based on Fick’s second low [74]. Alginate and carrageenan are the most exploited algae-derived polymers in preparing membranes for water treatment. Alginate is rich in carboxyl groups and has a strong affinity to cationic pollutants. Mohammed et al. [74] prepared thin calcium alginate films for the selective adsorptive removal of heavy metals from an aqueous solution. The thin films showed high adsorption capacity, and its selectivity followed the following order: lead > copper > cadmium. To decrease swelling and solubility and enhance mechanical properties, polymer composites can be used to prepare thin membranes for water purification. Jo et al. [75] developed anionic nanocomposite film using cellulose, carrageenan, and TiO2. The developed composite has a strong adsorption capacity to methylene blue, which can be promoted by increasing carrageenan content within the film matrix. Furthermore, this nanocomposite catalyzed the removal of methylene blue by photodegradation. In another study, composite membranes based on alginate and activated carbon effectively

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removed the analgesic drug diclofenac from an aqueous solution with a maximum adsorption capacity of 29.9 mg g 1 [76]. On the other hand, bioplastics can be used as filters for separation purposes and water purification. For instance, Alam and coauthors incorporated κ-carrageenan into polyvinylidene fluoride membrane and reported an enhancement of water separation from methyl orange dye solution. In another study, a nanocomposite based on alginate and bacterial cellulose was utilized to separate ethanol/water mixtures. The developed membranes showed hydrophilic properties and effective separation of water from ethanol. Similarly, Alshahrani et al. [77] developed multiwalled carbon nanotubes/chitosan/ι-carrageenan membrane to remove heavy metals. The developed membrane showed effective performance in removing up to 90% of several heavy metals at low pressure as well as it was characterized by high thermostability and porosity. Producing freshwater from saline water (desalination) for human consumption and industrial/domestic purposes is a fundamental challenge in modern societies. The process of desalination is usually performed by utilizing thermal and membranebased processes. Membrane-based techniques are generally more effective and energy-saving than thermal processes. The development of polymeric membranes for water desalination depends on the flux values. Thereby, it is fundamental to develop and modify the polymeric membranes’ components by blending and using fillers and super-permeable materials to enhance the flux values. On the other side, biodegradable polymers in membrane-based desalination can be expected to undergo biodegradation in seawater. Accordingly, membranes with antibacterial properties are preferred. Ugur and Nigiz [78] utilized graphene oxide as a filler in sodium alginate membranes for water desalination by pervaporation. The incorporation of graphene oxide promoted the thermal and mechanical properties of the composite membranes and increased their flux values. Furthermore, graphene oxide inhibited bacterial growth on the membrane surface.

Mulching Films Biodegradable mulching films cover the seeded soil areas to prevent water evaporation, protect the crops from weeds, and increase soil temperature. Thus, mulching films can reap several agricultural benefits and enhance plant growth. Liling et al. [79] investigated the effect of different cations on alginate mulching films’ physical and mechanical properties. The results demonstrated that alginate crosslinking by Ca2+ for 2 min has the best performance regarding tensile strength, elongation at break, and light transmission. Additionally, these biobased-mulching films are suitable carriers for plant nutrients to induce better plant growth. Santos and coworkers prepared composite konjac glucomannan/alginate films enriched with sugarcane vinasse, and the results indicated an enhancement of mechanical strength and a reduction of WVP and swelling degree of these composite films. Generally, to produce mulching films at a low cost, alginate in the preparation of mulching films could be replaced by utilizing the whole brown algal biomass.

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On the other hand, polysaccharide mulching coatings can be applied onto the soil surface by spraying technique. Sprayable Na-alginate mulching coatings were effective for more than 6 months and maintained their capacity to suppress weeds during the whole period. In a recent study, the sprayable Na-alginate mulch suspensions were loaded with microparticles of the brown seaweed Undaria pinnatifida, and the results indicated a stimulation of plant growth [80].

Use of Bioplastics in Electronic Devices Electromagnetic Interference Shielding The growing increase of electronic devices with highly integrated circuits causes serious adverse effects on the nearby electronic components, biosphere, and human health [81–84]. Therefore, there has been a growing interest in the potential application of biodegradable polymers with adequate electromagnetic interference shielding in electronic devices. Biodegradable plastics based on algal polysaccharides are promising materials in this field. Jia and colleagues prepared reduced graphene/calcium alginate thin films with a thickness of 12 μm and adequate mechanical properties [85]. The complex films exhibited an electromagnetic interference shielding effectiveness of 25.7 dB, and the specific shielding was 2142 dB mm 1. In another study, Zhou and coauthors fabricated Ti3C2Tx/Caalginate films with ultrathin and spongy structures [86]. The aerogel film was characterized by promising electromagnetic interference shielding effectiveness of 17,586 dB cm2 g 1, and their spongy structure permitted the dissipation of the electromagnetic waves by reflection and scattering. Electricity Conduction Generally, the compatibility of the algal polysaccharides with different salts and their abundant hydrogen bonds makes them a suitable material for electricity conduction. A recent study developed an alginate hydrogel-polyacrylamide composite embedded with silver flakes for soft electronics [87]. The composite was characterized by high electrical conduction (>350 S cm 1), low Young’s modulus ( PBS > PLA [38].

The Background Chemistry of Bioplastic Biodegradation Biodegradable plastics are typically composed of polymers or macromolecules that are predictable by enzymes found in the environment. Esterase enzymes, for example, degrade polyesters that contain an ester functional group in their structure [25, 26]. Plastics can be thought of as the backbone of a monomer of carbon atoms at the molecular level. The functional group or side chain of each monomer leads to differences in the mechanical and chemical properties of the plastic. The complexity differences varied from simple carbon chains to complex side chains. The functional group is very important at the biodegradation level, as some functional groups are degraded easily by biological action [49]. Table 2 Examples of microorganisms can degrade different kinds of polymers Polymer type Polyurethane

Polyethylene

Low density polyethylene (LDPE)

Fungi Aureobasidium pullulans, Curvularia senegalensis, Cladosporium sp., and Fusarium solani [39] Penicillium simplicissmum[43]

Aspergillus Niger, Chaetomium globosum, Gliocladium virens, Penicillium funiculosum, Pullularia pullulans, and Fusarium sp. AF4 [46]

bacteria Pseudomonas cepacia, pseudomonas sp., and Arthrobacter globiformis [40]

Actinomycetes Actinetobacter calcoaceticus [41] Actinetobacter gerneri P7 [42]

Pseudomonas aeruginosa, pseudomonas putida, and Bacillus subtilis [44] Pseudomonas aeruginosa PAO1 (ATCC 15729), Pseudomonas aeruginosa (ATCC 15692), Pseudomonas syringae (DC3000 ATCC 10862), and Pseudomonas putida (KT2440 ATCC 47054) [47]

Streptomyces sp., Pseudonocardia, Actinoplanes, Sporichthya [45] Streptomyces KU8, Streptomyces KU5, Streptomyces KU1, and Streptomyces KU6 [48]

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The carbon-bound utilization in the polymer used as a substrate for various microorganisms is the primary driving force behind biodegradation. In an aerobic environment, the reaction can be simplified as: Cpolymer þ O2 ! CO2 þ H2 O þ Cbiomass So, simply, biodegradation is the complete breakdown of polymers into carbon dioxide, water, hydrogen, ammonium, nitrogen, and biomass due to the biological activity of various microorganisms such as aerobic and anaerobic bacteria algae fungi [50]. The biomass residues from bioplastic biodegradation are usually nontoxic and can be biodegraded further by other living organisms [51]. Temperature, humidity, pH, oxygen content, time [27, 28], nutrient availability [52], sunlight or UV radiation, the thickness of the biodegradable material [23], and the type of microorganisms in the underlaying media [27, 28] are all factors that influence the biodegradation process. The schedule for plastic material biodegradation is dependent on the nature of the starting material, such as plastic composition and additives, chain length, and strength of bonding. It also depends on the underlying environment (water, soil, presence of microorganisms, temperature) in addition to the shape and thickness of the polymer (surface area) [39]. Moreover, molecular weight is a key element in polymer biodegradation as it determines many physical characteristics of the polymer. As a general role, raising the polymer molecular weight decreases its degradability [40]. The physical and chemical properties of biopolymers impact the biodegradation mechanism. The lifespan is managed by designing the chemical structure and the additives, which allow for faster microbial degradation at the end of usage [1]. Polyesters without side chains are often highly biodegradable than those with additional side chains [53]. Furthermore, the degree of crystallinity influences the onset of biodegradability. Since the molecules in the amorphous component are loosely packed, they are more susceptible to spoilage. Otherwise, the crystalline component of polymers is more resistant to biodegradation because it is firmly packed [54].

Factors Affecting the Biodegradability of Plastics The plastic biodegradation process is affected by many factors, including polymer properties, exposure conditions, and enzyme properties, as shown in Fig. 2. Polymer properties include physical and chemical properties, as well as polymer additives.

The Physical Properties of the Polymer The physical properties involve the surface condition such as the surface area, hydrophobic, and hydrophilic properties. Biodegradation reactions occur mainly

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on the polymer surface. The surface area ratio to total volume is one of the most important factors affecting biodegradation [55]. Thus, the degradation rate improves with the valid surface of plastic, as the degradation of finely milled polybutylene occurs faster than valid plastics in real environments [54]. Concerning hydrophobic properties, the hydrophobic polymers that are derived from the petrochemical sources cannot be readily degraded in the environment [43].

The Chemical Properties of the Polymer The first-order structures of the polymer contain the chemical structure, molecular weight, and molecular weight distribution. While the high order structures belong to morphology or polymer shape, melting temperature (Tm), glass transition temperature, and elasticity modulus of polymers. The biodegradation rate decreases with an increase in the molecular weight of the polymer. For example, the fungi Rhizopus delemar lipase can degrade the higher molecular weight of PCL (4000) slower than the PCL with low molecular weight [56]. Moreover, molecular weight distribution affects degradation rate such as PCL polyesters that have side chains are lower degradable than those without any side chains [57]. Regarding polymer shape, the degradation rate increases in the amorphous region more than crystalline one, as the hydrolyzing enzymes mainly attack the amorphous domains of molecules, and the polymers are loosely packed in the amorphous area [16]. Melting temperature is the softening temperature of the polymer. It can affect enzymatic degradability. Increasing the melting temperature of polyester reduces the biodegradation possibility, thus increasing the temperature and decreasing the potency of the enzymatic degradability [58]. For instance, the aliphatic polyesters and polycarbonates with lower Tm showed a higher biodegradability rate than others due to their sensitivity to the lipolytic enzymes. At the same time, the aliphatic polyurethane and polyamides (nylon) with higher Tm values showed more resistance to assimilation [12]. The chemical structures of some aliphatic polymers with their melting temperature are mentioned in Table 3. Where the polycarbonates represent carbonate bond (-O-CO-O-) and aliphatic polyesters [ester bond (-CO-O-)].

The Polymer Additives Using a pure form of plastic polymers in several products is rare for enhancing plastic properties like stability, color, and texture. So, the additives are integrated into the final product released into the environment during degradation [61]. Pro-oxidants are chemicals that can induce polymer oxidative stress [62], as the type of pro-oxidants can control the plastic lifetime in the environment rather than the actual polymer type [63]. The transition metals act as pro-oxidants, and usually, some mixtures of metal salts are added to the polymer, such as cobalt stearate, iron stearate, or manganese stearate. The most desirable biodegradable properties

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Table 3 Chemical structures of some aliphatic polymers with their melting temperatures [59, 60] Name Polyester Polyamide Polycarbonate Polyamide Polyurethane

Chemical structure -O-(CH2)6-O-CO-(CH4)-CO-NH-(CH2)6-NH-CO-(CH2)4-CO-O-(CH2)4-O-CO-O-(CH2)4-O-CO-NH-(CH2)6-NH-CO-(CH2)6-O-CO-NH-(CH2)6-NH-CO-O-(CH2)4-O-CO-

Melting temperature ( C) 60 265 65 240 180

appeared with iron stearate additive via thermo and photodegradation of linear low-density polyethylene [64]. At the same time, the higher concentration of cobalt additive leads to a lower rate of bacterial decomposition due to its toxic effect on microorganisms [63]. Biosurfactant is an amphiphilic molecule, mostly released on living surfaces. The polymers’ biodegradation is enhanced by adding biosurfactants because of their low toxicity and high biodegradability [65]. High biodegradation indicates the presence of specific functional groups in the surfactants, allowing activity to occur under harsh conditions [66].

Enzyme Characteristics Various types of enzymes had specific active sites and the ability to degrade different types of polymers. A fungal species Aspergillus flavus and A. niger can produce enzymes that quickly degrade the straight-chain polyesters obtained from diacid monomers with 6–12 carbon atoms, compared to the same polyester from other monomers [67]. Also, it was reported that the depolymerization mechanism of PHB depends on the producer microorganism [17]. Exposure Conditions Moisture level can influence the rate of polymer biodegradation. Sufficient moisture accelerates microbial action, increasing the rate of biopolymer decomposition [68]. In addition, rich moisture conditions help the hydrolysis process by generating more reactions of chain scission [28]. Additionally, the degree of pH can change the hydrolysis rate by changing the reaction conditions into acidic or basic. For instance, the optimal hydrolysis rate of PLA capsules occurs at pH 5 [69]. Also, the pH conditions are changed by the polymers-degradation products, affecting the microbial growth and biodegradation rate. On the other hand, the exposure temperature can affect the biodegradability rate according to the polymer melting temperature.

Methodology for Testing Plastic Biodegradability The degradation rate of various polymers differs according to different conditions. The degradation rate of polymers is quantified by other methods such as visual inspection, mass loss, gas formation, biomass growth, and soil analysis. Mass losses

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consider the simplest one to determine the material loss, while the soil analysis method is expensive and time-consuming [70]. Visual inspection is too subjective and should be analyzed by automation and accredited software. Mass loss is the simplest way to detect material loss. This method is valid for larger particles of plastics only at the earlier degradation stages regardless of the degradation method (biotic or abiotic). The gas formation (methane and/or carbon dioxide) method depends on the polymer biodegradation under aerobic conditions that releases carbon dioxide (CO2), while under anaerobic conditions release a mixture of methane (CH 4) and CO2. The environmental condition may be neither fully aerobic nor anaerobic. The biomass growth method is useful only when the polymer is the sole carbon source in a highly controlled environment. Soil analysis is an analytical method that can account for the destiny of any additives or any released microplastic [71]. Although most materials are degraded to produce CO2 or methane within a certain timeframe, the rest of the material may release into the environment as plastic or microplastic. The theoretical calculation of the produced CO2 is based on the known amount of the polymer and its carbon content. In contrast, certain enzymes break down the polymers for biomass accumulation rather than CO2 production, especially under aerobic conditions [72].

Variation in Biodegradability Tests The current plastic biodegradability tests are designed for marine and wastewater, lacking standardization [70]. This problem leads to difficulties in comparing the different research results related to plastic biodegradability. Hence it is hard to identify the consensus on biodegradation timelines. Some relevant issues among test methodologies include general conditions such as temperature degree (15–55  C), the incubation period (3–24 months), sources of microorganisms, and oxygen conditions. It is determined according to the source and size of particles, plastic origin, and pretreatment, such as light or heat. Additionally, validity criteria include a percentage of degradation (60–90%), limitation of CO2 production, and pH stability. The type of measurement could be visual through a microscope or interpretation using standardized analytical software or by gas assessment via validated equipment [69].

The Laboratory Conditions Versus the Unmanaged Ecosystem It is a critical issue that the standard based on the laboratory conditions is not working well in the environment. These negatively affect the biodegradation rate of plastic in the open environment that occurs due to several conditions as summarized in Table 4.

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Table 4 Comparision between plastic biodegradability in the laboratory and open ecosystem The parameter The conditions The plastic degradation rate Measuring the biodegradability criteria

Laboratory - more controlled - plastic may show more good degradation - measured accurate

Open ecosystem - less controlled - the plastic may fail to degrade appropriately because of lacking a suitable microorganism or favorable conditions - assessing the biodegradability rate not measured accurately, so the biodegradability criteria are failed

Harrison et al. (2018) mentioned that even when degradation in a certain ecosystem has been partially observed, many researchers still consider long-term degradation as an unsolved problem [66]. They stated that specifications and standards mostly need additional studies for showing a reasonable rate of biodegradability in the ecosystem [70]. Based on the controlled conditions in the laboratory that cannot mimic the open ecosystem, there is a need to test the rate of plastic biodegradability under both controlled conditions and in an unmanaged ecosystem for the possibility of applying practically. Some challenging factors that affect the biodegradation rate of plastics in different environments are shown in Table 5.

Stages of Biodegradation Bioplastics have a life cycle like other polymeric products [74]. However, each type of bioplastics polymer has its life cycle [73]. Biodegradable polymers are fragmented into simpler constituents and reorganized through particular elemental rounds such as the carbon and nitrogen cycles during the biodegradation process, with the liberation of water, carbon dioxide, and biomass as the end products under aerobic conditions and methane, hydrocarbons, and biomass as the end products under anaerobic conditions [19]. Biodegradation is an irreversible process that results in a considerable alteration in the material structure, often defined by a lack of attributes (e.g., mechanical strength, integrity, molecular mass, or structure) and deterioration. Biodegradation is primarily influenced by environmental factors and takes place over time in phases. [63]. There are three major phases in the biodegradation of polymers: The process starts with biodeterioration, that is, the change of mechanical, physical, and chemical properties of the molecule by microorganisms that grow on or inside the surface of the polymer, followed by biofragmentation, in which the polymers are converted to oligomers and monomers by microorganisms, and assimilation finally, in which microorganisms are provided with requisite carbon, nutrient, and energy from biofragmented polymers and bicarbonates. Table 6 demonstrates the biodegradation of a plastic polymer.

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Table 5 Examples of challenging factors that affect the biodegradation rate of plastics in different environments Environment Soil

Composting facilities

Landfill

Fresh or saltwater

Degradation In the soil surface: Plastics entombed in the soil is unexposed to the UV rays by the sun to commence photolysis, which leads to additional degradation [67]. According to certain research, photolysis is a chain process, and degradation in polymers can persist after UV exposure [68]. Biodegradation is triggered by naturally existing bacteria, which change depending on the soil type [69]. Anaerobic conditions limit thermal-oxidative degradation in deeper parts of the soil [1] Customized microorganisms’ cocktails can degrade plastic under specific conditions [73]. Bacteria prefer soluble nutrients to assimilate. However, most plastics are illiquid and lack key components (such as nitrogen, potassium, and phosphorous). As a result, fertilizer aids in the bacterial degradation process [46] The rate of photolysis and thermal oxidation slows down due to a lack of ultraviolet radiation and oxygen, as well as a dark and anaerobic atmosphere. Consequently, anaerobic circumstances encourage the synthesis of both methane and carbon dioxide [71] In surface water: Plastic floats in the water and is exposed to oxygen, mild temperatures, and UV radiation [72], hastening its abiotic decomposition. Because of the restricted UV availability, relatively low temperatures, and oxygen supply, thermal-oxidative degradation and photolysis decrease in deep waters [73]. In saltwater, lower concentrations of microorganisms alter the rate of polymer hydrolysis, depending on the microorganism’s capacity to cling to the polymer surface [64]

Biodeterioration Biodeterioration alters a polymer’s mechanical, physical, and chemical properties [75]. The polymer breaks down into smaller parts through the biodegradation process by abiotic and biotic activity [43]. Abiotic degradation occurs before or simultaneously with biotic deterioration [75].

Abiotic Deterioration Abiotic biodeterioration is defined as physicochemical degradation without the biological action of microorganisms. Because most first degradation phases include physical and chemical activities but not biological actions, they are classified as abiotic biodeterioration [76]. Mechanical stress, temperature, solar exposure, humidity, wind, and rain, among other things, may all damage polymeric materials. The following are the essential considerations: – Mechanical degradation: caused by physical forces operating on plastic. Tension, compression, and shear forces include snow pressure, air, water disturbance, and animal ripping.

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Table 6 Stages of biodegradation, inspired by Kjeldsen et al., 2018 [61] Degradation stages The equation Definition

Quantify by

Degradation mechanism Abiotic mechanism

Biotic degradation

The initial breakdown rate depends on

1. Biotic and Abiotic decay Plastic polymer➔ oligomers It is the end of the plastic’s useable lifetime, and the initial stage of degradation as the plastic begins to lose its structural and physical characteristics

2. Biofragmentation Oligomers➔ monomers Using microbial enzymes converts the oligomers (shorter polymer chains) into monomers

Changes in the material: - tensile - elastic strength - brittleness Abiotic more than biotic

1. Visual inspection by microscope and computational analysis 2. Mass loss Biotic more than abiotic

• mechanical degradation • light degradation • thermal degradation • chemical degradation The presence of microbes is essential for the secretion of enzymes to act on the polymer’s surface

The microorganism’s enzymes may break the polymer into small particles to let them enter the microbial cell - exogenous enzymes that microorganisms can release into their surrounding environments to act directly on the polymers - the endoenzymes that can be released when small species of polymer enter the microbial cell 1. The nature of the polymers 2. The amount of available enzyme, which in turn relies on the present microorganisms

1. The polymer chain length. 2. Molecular mass distribution 3. Crystallinity 4. The shape, size, and geometry of polymer particles 5. Water diffusivity in the polymer matrix 6. The surface porosity, pore size, distribution, and geometry

3. Microbial assimilation and mineralization Monomers➔ CO2/ CH4 + H2O That occurs by assimilating the monomers into the microorganisms to enhance cellular biomass and methane (under anaerobic conditions) or carbon dioxide (under aerobic conditions) 1. Measuring gas evolution 2. Increases in biomass: (in a bioreactor) Biotic only (aerobic or anaerobic) –

Some microorganisms cannot degrade plastic under anaerobic conditions, while other microorganisms need anaerobic conditions

In external uncontrolled environment: 1. Number of available microbial cells 2. Kind of the surrounding ecosystem 3. Environmental factors

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– Light deterioration (photodegradation): UV energy rays from the sun or from an artificial origin starts a chemical process that destabilizes polymers. – Thermal deterioration (thermos-oxidative): caused by exposition to the polymer’s structure. – Chemical degradation: exposure to chemicals from contaminants in the atmosphere or chemical wastes. Atmospheric oxygen may damage various chemical bonds [70]. The rate of degradation is affected by the length of the polymer chain, molecular weight distribution, crystallinity, size, and shape of polymer particles. Furthermore, the surface porosity, distribution, pore size, water diffusion, and shape are considered in the polymer matrix. The deterioration rate is measured by changes in the material’s tensile, elastic, and brittle properties [77]. Oxidation and hydrolysis are the most important abiotic biodegradation reactions, with the combined effect of sunlight decomposing. Free radicals made up of oxygen and ozone molecules in the environment can attack covalent bonds in biopolymers and cross-linking or chain cleavages [75]. Hydrolysis reactions are influenced by several parameters, including the presence of water, time, pH, temperature, and hydrolyzable covalent bonds (carbamine, anhydride, ester amine, or ester amide). The diffusion of oxygen or water within the polymer structure is affected by crystallinity, structure, polarity, and elasticity [78]. Other kinds of abiotic and biotic biodegradation can be accelerated and facilitated by macro- and microscopical mechanical damage, as they increase the available contact surface or create flaws, making them more vulnerable to chemical infiltration and heat damage [12]. In addition, the lack of crystallinity also the change to an elastic case that might raise the penetrability of abiotic and biotic factors into the polymeric matrix, speeding up the biodegradation process. Particularly for polyester polymers such as PLA because the hydrolysis process is mostly subject to hydrolysis processes and will therefore proceed at a much faster rate because water can easily penetrate the polymeric network [79]. The essential mechanisms in abiotic deterioration are oxidative processes caused by molecular oxygen. Oxidation frequently occurs concurrently with UV-light degradation, resulting in the formation of free radicals, which induce chain scission and cross-linking of the polymeric network, resulting in a decrease in molecular weight and increased brittleness. Light deterioration, also known as photodegradation, is fairly frequent in everyday plastics. As previously mentioned, photolysis can go to extremes, resulting in chain dissociation and/or cross-linking, and these processes can occur simultaneously with oxidative destruction. Plastics are normally degraded through photodegradation, which increases the surface area accessible for biodegradation and, as a result, speeds up the biodegradation process [80]. Figure 3 depicts all of the parameters that influence abiotic biodegradation.

Biotic Deterioration Biotic degradation refers to the breakdown of polymeric bonds produced by the enzyme’s action on microorganisms such as bacteria, fungi, algae, and protozoa [62].

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Microorganisms can build biofilms on the polymer surface and release chemicals such as enzymes, polysaccharide viscous, and acids. Then enter the polymer and damage the pore structure [75]. Several enzymes, including endogenous enzymes (acting within the polymer chain), exogenous enzymes (acting at the polymer ends), inductive enzymes (acting on specific substrates), and constitutive enzymes, are the specific stages during biodegradation (acting on the unspecified substrate). The enzymes are solely responsible for surface degeneration due to their poor diffusion properties and their huge molecular size [81]. During the biotic phase, a biofilm is formed on the material surface, consisting of various bacteria in a matrix of water, sugar, and proteins produced by the same microorganisms [80]. Fouling is the colonization process of a polymeric surface by a microbial biofilm. It includes numerous processes that result in the settling microorganisms that can infiltrate the polymer’s surface porosity during and after biofilm growth, causing porous volume changes and perhaps cracks. This process also permits water infiltration and, as a result, hydrolysis. Plasticizers and additives may also seep out of the polymer within this process, causing rupture and embrittlement. Bacteria that live in biofilms produce enzymes roughly classed as intracellular and extracellular depolymerase. These enzymes carry out the biodegradation depolymerization process, in which polymer chains are decomposed into shorter oligomers and then into monomers. When the plastic polymer fragments degrade into shorter chains (oligomers and monomers), it becomes more ready for enzymatic or bio attack [46].

Fig. 3 Abiotic bioplastic biodegradation

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Biofragmentation The second stage is biofragmentation, when microbial activity breaks down polymers into oligomers and monomers. Biofragmentation entails the hydrolysis and/or fragmentation of polymer carbon chains and the release of intermediates, all mediated by microorganism-produced enzymes [82]. The bioavailability of the substance increases since it is chemically and physically accessible to the action of microorganisms and the enzymes that secrete it [46]. Microorganisms can release enzymes into their surroundings to work directly on polymers, or they can rely on abiotic forces to break the polymer down into tiny enough fragments to enter the cell and be hydrolyzed by internal enzymes [83]. The degree of polymer breakdown is determined by its composition. Compared to polymers with multiple functional groups that provide handles for enzymes to work on, non-reactive linear sections would be more difficult for enzymes to access and inactivate [12]. Lipase, proteinase K., pronase, hydrogenase, and other enzymes generated by microorganisms are used in plastic biodegradation. Proteinase K, an enzyme released by the Tritirachium album, was a powerful PLA degrader. Many Amycolatopsis and Saccharothrix were also reported to break down PLA [84]. Another influencing element is the amount of enzyme available, determined by the number of microorganisms present. The more microorganisms that can destroy plastic, the faster it degrades. Some researchers showed that bioaugmentation (adding additional microorganisms) could speed up biodegradation, and a variety of microbial origins have been found as polymers degrader, including PET, PLA, and other polyesters [85].

Microbial Assimilation and Mineralization The assimilation and mineralization processes are the third steps of biodegradation [82], in which microorganisms consume bio-fragmented chemicals and convert them to biodegradation end products such CO2, H2O, and biomass [41]. Monomers and oligomers of the degraded polymer can reach the cytoplasm of microorganisms, where they are converted into metabolites, energy, and biomass while releasing gases, organic compounds, and salts into the environment [75]. In bioassimilattion, small hydrocarbon fragments from fractionation are absorbed and metabolized by fungi or bacteria. Otherwise, mineralization transfers hydrolysis products within the cell wall, followed by intracellular conversion of hydrolysis products into microbial biomass with an associated second release of carbon dioxide and water coming out of the cell. This step is critical given that numerous established approaches rely on the examination of evolved CO2 to evaluate biodegradability [42]. Extracellular and intracellular enzymes are at least two types of enzymes that are actively involved in the biological destruction of polymers [43]. Exoenzymes from microbes degrade complex polymers, resulting in smaller molecules of short chains, such as dimers, oligomers, and monomers, that are tiny enough to pass through the

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outer bacterial membranes and be used as energy and carbon sources. Depolymerization is the name given to this process. Mineralization refers to the deterioration process where the end products are CO2, H2O, or CH4 [12]. Environmental factors frequently determine the dominant types of bacteria and the degradative processes involved with polymer breakdown. When oxygen is present, aerobic bacteria are primarily responsible for the polymer breakdown, producing microbial biomass, water, and carbon dioxide as by-products. While under anoxic circumstances, anaerobic consortia of microbes are responsible for polymer degradation, releasing microbial biomass, water, carbon dioxide, and methane [83] (as shown in Fig. 4). Enzymatic breakdown of plastics includes the release of enzymes by microorganisms, after which enzymes act on polymers’ chains and efficiently dissolve them. Enzymes break down long-chain polymers into monomers. The enzyme attacks the amorphous part of the plastic first, followed by the crystalline region. The properties of the polymer affect the rate of enzymatic breakdown [86]. The first stage in enzyme biodegradation is hydrolysis, which increases the hydrophobicity of the polymer by adding a functional group. As a result, the polymer’s vulnerability to microorganisms is increased. Exoenzymes, released by bacteria, turn polymers into monomers, subsequently taken up by microorganisms’ semi-permeable membranes [76]. Accordingly, if the polymer volume remains large even after hydrolysis, the polymers are first depolymerized to pass through the cell membrane and then are destroyed by intracellular enzymes. In addition, many variables affect the rate of plastic breakdown by enzymes, such as temperature, humidity, pH, and humidity. Hydrolase enzymes carry out the hydrolytic processes. This family of enzymes includes many enzymes such as lipase, phosphatase, glycosidase, esterase, and many more [87].

Waste Management Options for Bioplastic Several technologies are available for the appropriate treatment of plastic waste, including collection and incineration for energy recovery and selective combustion to obtain a high calorific value of plastic as in cement kilns for use as a reducing agent in blast furnaces or as raw materials for recycling [88–92]. Some governments proposed “Waste Hierarchy” as guidance for choosing the best way to control plastic. Plastic control aims to reduce the influence of waste recognized by reusing the produced plastic and reducing material consumption [69]. The management of the biodegradable bioplastics waste when entering the stream of waste is showed in Fig. 5 and is handled by the following:

Recycling Recycling the bioplastic that enters the waste stream has complications for plastic recycling systems. For instance, although adding natural fibers or starch to the traditional polymers can ease the mechanical recycling for some bioplastic polymers

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Fig. 4 Biofragmentation, assimilation, and mineralization of bioplastics

such as PLA, it complicates the plastic recycling system. The deficiency of a continuous and reliable supply of bioplastic waste in large quantities makes recycling less attractive economically than traditional plastics [69].

Energy Recovery by Incineration Most plastics have high calorific values equal to or higher than coal. Thus, energy recovery by incineration is a potential option after removing the important elements. Burning petrochemical carbon for fuel is more environmentally friendly than burning oil directly. Incineration with energy recovery is a suitable solution for all bioplastic polymers. The total calorific values of natural cellulose fibers and starch are lower than charcoal but similar to wood. Moreover, the production of starch materials and fiber consumes significantly low energy in the first place, contributing positively to the total energy balance in the life cycle [69, 93].

Landfill The waste plastics landfill is the least favored choice, although it is extremely simple, cheap, and does not need further treatment, separation, or cleaning. This choice refers to running out the suitable sites for landfills and raising public concerns about its negative impact on health and the environment. Therefore, many countries have a strategic plan to reduce the amount of municipal waste that goes to landfills [69].

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Fig. 5 The life cycle of plastics with the management options

Treatment for Biological Waste (Anaerobic Digestion or Composting) Biodegradable and bioplastic polymers can be composted rather than the traditional petrochemical-based polymers via aerobic waste management systems. This system generates a rich compost with carbon and nutrients ready to fertilize the soil. Some biodegradable polymers are also fit for anaerobic digestors producing methane gas that drive generators for energy production [46].

Advantages and Disadvantages of Bioplastics Biodegradable and bio-based plastics have been on the market since the 2000s, showing similar characteristics to traditional plastics. Although bioplastics are becoming increasingly common, the production volume is still very small in our economy versus conventional plastics. Hence, it is hard to predict all impacts resulting from large-scale adoption [82]. In the current production of bioplastics, some advantages and disadvantages of using bioplastics are illustrated in Fig. 6 and illustrated as follows:

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Advantages – A good solution to reduce petroleum resources that pose a serious threat to the environment. Reduces some bad scenarios for environmental risks from current plastics, such as global warming [94]. – In the coming decades, the price of fossil resources is expected to rise significantly due to the decrease in the total petroleum volume in the world. On the other hand, bioplastic is produced from renewable and sustainable resources. Therefore, bioplastics can help reduce dependence on limited petroleum resources [95]. – Bioplastics can be inoculated and molded into the same form as traditional thermoplastics. Moreover, some types of bioplastics are lighter and stronger in weight. Consequently, bioplastics are not fit for the highly specialized purpose [96]. – Bioplastics are more marketable, with 80% of European customers purchasing products with low environmental impact. Therefore, bioplastics may improve the product’s added value through an environmentally friendly marketing campaign [97]. – It was found that 90% of the beach’s waste was plastic. Accordingly, using biodegradable plastic will reduce plastic waste [98].

Fig. 6 The advantages and disadvantages of bioplastics

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– Major bioplastic polymers are produced naturally and do not own the carbon footprint of petroleum plastics. At the same time, the production of petroleum plastics requires much energy and releases a lot of carbon dioxide reaching 500 million tons per year [99]. – Bioplastics production reduces greenhouse gas emissions or even becomes carbon neutral. Because the plants that produce bioplastics absorb the atmosphere’s carbon dioxide through growing, it causes temporary removal of greenhouse gases (CO2). Otherwise, using bioplastic instead of traditional plastic will keep the atmosphere from the million-tons of released CO2 through petroleum plastic production [100]. – Popular bioplastics, such as PHB and PLA, are nontoxic. This makes it suitable for food packaging, as it saves the food without any contamination, taste, or leech chemicals like BPAs, like in some traditional plastics [101]. – Bioplastics production ability to “close the cycle” and improve resource efficiency. Renewable resources produce durable, bio-based products that can be reused, mechanically recycled, and ultimately burned when producing renewable energy. Or it can be recycled through industrial composting and anaerobic digestion at the end of the product life cycle to produce valuable biomass (humus) that can be used to grow new plants, thus closing the cycle [102]. – In the process, specific energy can be produced by anaerobic digestion of bioplastics, achieving the optimum nitrogen to carbon ratio [103]. – Bioplastics can help turn bio-waste from landfills and improve waste management efficiency by returning them directly to the soil as compost [104]. – Bioplastic use will be an important way to avoid existing traditional plastic wastes [94].

Disadvantages – The cost of bioplastics production is generally not competitive compared to petroleum plastic. Bioplastics are generally two or three times more expensive than traditional plastics. Consequently, The higher cost of bioplastic causes lower plenty of bioplastic versus the conventional one [105]. – There is concern about the potential competition of crops producers of bioplastics with the food supply. However, a new approach to using food waste can be useful. Moreover, it is perhaps that 300,000 hectares will be used to grow bioplasticproducer crops representing about 0.02% of the world’s agricultural land. This percentage may rise to 3% if half of the world’s plastic is made from crops grown on farmland. Thus, these concerns seem unfounded [103]. – Crop-based bioplastics need farmland, fertilizers, water, and weather conditions. Thus, the bioplastics supply is threatened by natural phenomena, such as desiccation [94]. – Some bioplastics have a shorter life than petroleum plastics due to their weaker mechanical properties. For example, water vapor permeability is greater than common plastic, as it is easy to tear like tissue paper or too brittle. Also, some

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algae-based bioplastics will decay within hours when in water. Therefore, bioplastics are highly biodegradable but also brittle [106]. Biodegradable plastics may encourage people to increase their waste [94]. Lack of know-how or lack of bioplastics processability with mutual technologies [107]. Small volume of the bioplastics market, limiting the major investments, redesign the production framework and infrastructure for the waste management [108]. Potential competition of bioplastic production and biofuel industry in feedstock [109]. Need more farming land and freshwater for the massive production, such as PLA production from corn farming [83], may represent a challenge in certain regions [12]. Risk of fouling of waste streams with bioplastics [18]. Lack of logistics and recycling infrastructure for dedicated composting [104]. Risk of landfilling bioplastics results in greenhouse gas emissions, as bioplastics will not decay well in a landfill. Hence, its sustainability might need more assessment [40]. The added cost comes from establishing an additional waste stream [85]. Ensure that composting facilities can cover the increase in volume at the regional level [103]. The bioplastic degradability in different open environments (concentration, quality, and type of microorganisms in use) needs overall life cycle assessment [66]. According to a United Nations report, bioplastics are not the real solution to marine waste. Bioplastics that degrade on land under favorable conditions are much slower to degrade in the ocean contributing to marine waste and undesirable results for the marine ecosystem. The delay in the bioplastic degradation in marine refers to the environmental impacts leading to taking the degradation up to 5 years [110].

Conclusion The depletion of petroleum fuel supplies and the negative environmental impacts of conventional plastics due to their limited biodegradability have motivated researchers to research and develop new alternative plastics. Two primary efforts have been made: the first is to synthesize biodegradable polymers, and the second is to isolate a microorganism that can biodegrade plastic trash. Biodegradable bioplastics are polymers converted into carbon dioxide, water, methane, inorganic compounds, or biomass by the enzymatic action of certain microorganisms. The physical and chemical structure of the biopolymers, the environmental conditions, and the microbial populations to which the bioplastics are exposed are the most important components in biodegradation. This process can occur both in natural and industrial environments and under aerobic or anaerobic conditions. Biodeterioration, biofragmentation, and assimilation are the three basic phases in the polymeric biodegradation process. Methane and carbon dioxide measurements, spectroscopy,

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degree of dissociation, mass loss, optical screening, and scanning electron microscopy are among the many methods proposed and developed to monitor the biodegradation process of various bioplastics. Many options available for bioplastic waste management include collection and incineration for energy recovery and selective incineration of high calorific value polymers such as cement kilns for use as a reducing agent in blast furnaces or as raw materials for recycling. Bioplastic has many advantages, including reducing the accumulation of plastic waste, a gradual reduction in reliance on limited petroleum resources, reducing greenhouse gas emissions, and avoiding segregation of existing traditional plastic wastes. They are also more marketable and nontoxic.

Future Perspective Biodegradable plastic is an innovative way to solve the plastic waste problem from new material development. It is expected that the bioplastics industry will get a growth boost, development of new materials and technologies, and expansion of production on a larger scale. Bioplastics should be propagated and used based on their performance rather than relative costs. The need to commercialize further bioplastics has grown as consumer awareness and sustainability have emerged as new market drivers, boosting features and performance while cutting manufacturing costs. Also, more assessment will be required once the higher bioplastic production understands the fate of these materials. Producing a larger quantity of bioplastic will also need to inform the consumers of the correct way to dispose of the new waste. The biodegradation rate is achieved by optimizing molding techniques and managing the molecular weights of the material and the ordered structure. Moreover, biodegradable polymers must demonstrate better performance throughout their full life cycle. In other words, it is critical to start biodegradation as soon as possible after use and disposal. Biodegradable polymers have been used in food packaging and a variety of disposable items that are used regularly. Furthermore, biodegradable plastics are expected to be used as agro-engineering materials, in a medical context as bio-absorbable materials, such as sanitary goods, surgical scaffolding, and paper diapers.

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Part IV Other Materials Biodegradation

Biodegradable Inorganic Nanocomposites

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Juan Matmin, Nik Ahmad Nizam Nik Malek, and Nor Suriani Sani

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bionanocomposites from Green Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Biodegradable Inorganic Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhanced Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Biodegradable Inorganic Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wet Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dry Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffold Material for Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stem Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bionanocomposites Interaction with Biological Entities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Plant-sourced materials (starch, cellulose, and other polysaccharides), animal products (proteins), and polymers produced chemically from naturally generated monomers (polylactic acid or PLA) are becoming less attractive. Accordingly, the J. Matmin (*) Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Malaysia e-mail: [email protected] N. A. N. N. Malek Department of Biosciences, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Malaysia e-mail: [email protected] N. S. Sani Office of Deputy Vice-Chancellor (Research & Innovation), Universiti Teknologi Malaysia, Johor Bahru, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_23

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development of nanocomposites by combinations of polymers and inorganic compounds from entirely or partially renewable resources allow exceptional attributes and attracted attention in areas ranging from materials science to biology or medicine. It was shown that nanoscale reinforcements in inorganic composite materials delivered the characteristics enhancement such as decreased hydrophilicity, increased mechanical qualities substantially, and enhanced bioactivity compared to synthetic and natural polymers, which did not display these properties on their own. Indeed, the development of novel biodegradable inorganic nanocomposites has reduced the possible human health risks associated with the pharmaceutical and pesticide sectors through nano-based agricultural commodities and the discovery of enhanced eco-friendly bioactive compounds. The chapter aims to provide details of nanocomposites based on biodegradable polymers and inorganic nanocomposites, preparation methods from wet to dry approaches, and recent applications, especially in the biomedical and healthcare sectors. This emerging idea will enhance the scope of bionanocomposites materials for sustainable products development with improved properties than commercially existing synthetic or natural polymer-based biodegradable materials. Keywords

Biodegradable · Nanocomposites · Polymer · Reinforcement · Biomedical · Healthcare Abbreviations

0D 1D 2D 3D AgNPs BPA CNC CNTs CVD DMA DSC FESEM GO hMSC HPLC hUCMSCs KCl-HCl MFC Mg-Zn MMT

Zero-dimensional One-dimensional Two-dimensional Three-dimensional Silver Nanoparticles Bisphenol A Cellulose Nanocrystals Carbon Nanotubes Chemical Vapor Deposition Dynamic mechanical analysis Differential scanning calorimetry Field Emission Scanning Electron Microscope Graphene Oxide Human Mesenchymal Stem Cells High-Performance Liquid Chromatography Human Umbilical Cord Matrix Stem Cells potassium chloride-hydrochloric acid Micro Fibrillated Cellulose Magnesium-zinc Montmorillonite

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MWCNTs PANI PBS PEO PLA PMMA SBF SNC SNP SPR TEM Tg Tm

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Multi-Walled Carbon Nanotubes Polyaniline Phosphate buffer solution Poly(ethylene oxide) Polylactic Acid Poly(methyl methacrylate) Simulated Body Fluids Starch nanocrystals Starch nanoparticles Surface Plasmon Resonances Transmission Electron Microscope Glass transition temperature Melting point

Introduction Synthetic polymers derived from petroleum-based monomers, often known as plastics, are highly required because they have replaced woods and glasses in a wide range of commercial uses. They have gained popularity because of their considerable mechanical strength and thermal stability [1]. Nonetheless, the consumption of these synthetic polymers or plastic items has resulted in a massive amount of waste material, which accumulates in landfills and causes significant environmental concerns over the years. Moreover, plastics contain many hazardous components, such as polyfluorinated chemicals, bisphenol A (BPA), phthalates, and brominated compounds, which can leach out and cause environmental and public health problems [2]. Recently, environmental concerns have driven substantial commercial and academic research initiatives to develop ‘green materials’ based on natural materials such as biopolymer and biodegradable products. This is also due to the depletion of fossil fuels, the primary sources of monomers used to make most synthetic polymers. At the same time, the research in bio-based focuses solely on plant-sourced materials (starch, cellulose, other polysaccharides), microbial products (polyhydroxybutyrate), animal products (proteins, polysaccharides), or polymers produced chemically from naturally generated monomers (polylactic acid, PLA) has remained stagnant and becoming less attractive [3]. For many years, both petrochemical and bio-based resources were used to manufacture bioplastic, a degradable type of plastic. Figure 1 shows the classification of bio-plastic materials with different properties in three main groups: (i) renewable non-biodegradable bioplastics, (ii) renewable bio-based degradable bioplastics, and (iii) non-renewable fossil resources biodegradable plastics [4]. Currently, biodegradable inorganic nanocomposites or bionanocomposite significantly replace all existing bioplastics and their composites. In this respect, the bionanocomposites differ from other composites in the nanosized of inorganic components, and their organic components are substituted by biopolymer.

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Biopolymers or bioplastics are increasingly being replaced by bionanocomposite as a promising alternative for a greener and more sustainable environment. Recently, many bionanocomposites are being intensively explored by developing different reinforced polymer materials with low nanometric particle content, i.e., less than around 5% of nanoclays [5]. Because of this, bionanocomposites are being highlighted as an advanced class of nanostructured hybrid materials with distinct properties because of their small scale and large specific area originating from their biopolymer-nanoparticles interactions (Fig. 2) [5]. Many inorganic bionanocomposites can be prepared in two ways, either by introducing biopolymer matrices or/and using different degradable nano-fillers. In some instances, additives are also mainly used in the form of compatibilizers to enhance their constituents’ interactions, impacting the material cohesiveness and uniformity. Moreover, integrating inorganic nanoparticles into the bio-based polymer matrix can produce high-performance materials for many industrial applications. Compared to their micro- counterparts and conventional composites, the mechanical, thermal, and barrier properties significantly improve. As a result, essential synergetic phenomena such as distinct magnetism, size-dependent band-gap, and phonon transport are discovered. Furthermore, to ensure that the whole composite is biodegradable, the polymeric matrices must be derived from a biological system, generated from various microorganisms, or acquired from renewable resources. Typically, biodegradation is

Fig. 1 Different types of bioplastics and some examples. (Adapted with permission from Ref. [4], Copyright 2020, Elsevier)

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Fig. 2 Type of composites based on their filler and matrix components. (Adapted with permission from Ref. [6], Copyright 2018, Elsevier)

described as the process of deterioration caused by the action of microorganisms such as bacteria, algae, or fungi [7]. There are two stages of biodegradation firstly (i) chain cleavage and followed by (ii) mineralization process. A longer polymer chain is first broken into smaller oligomeric fragments with microorganism-released enzymes in the chain cleavage. Later, the mineralization process within the cell converts all the microscopic oligomeric fragments to biomass components such as carbon dioxide (aerobic) and methane (anaerobic). In this process, enzymes released by microorganisms break down the degradable polymer components of the bionanocomposites [8]. When the polymer is broken down into its monomeric form, it is used as a carbon source for microbial metabolism. Although the biodegradation process is chemical, the attacking molecules (enzymes) are supplied from the microorganisms. In general, the presence of polymer matrices with dimensions ranging from 1–100 nm is referred to as poly-nanocomposites, which is distinct from the other matrices of ceramic nanocomposites or metal nanocomposites [9]. Mainly, polynanocomposites are multiphase systems consisting of both continuous phase of polymer matrix and dispersed phase from the particulate materials, generating different shapes of fibers, platelets, or spheroids structures. This system should undergo a controlled mixing for stable dispersion whereby the dispersed particulate needs to be initially oriented. For the polymers matrices, the use of agricultural resources, such as polysaccharides (starch and cellulose), proteins (PLA), or even biopolymers from petrochemical sources, have been discovered [10]. The progress in bio-renewable resources of these materials can significantly contribute to adopting

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a low-carbon economy in packaging, medical, and automotive engineering. Despite these advantages, it remains challenging to achieve uniform dispersion of the polynanocomposites components. For this reason, this technique appears undeveloped, as it reflects the weaker mechanical and thermal properties of the generated nanocomposites. In the future, their manufacturing procedure must be improved due to the demand for high stiffness, lightweight materials, and other functionality with extremely high qualities offered by the poly-nanocomposites. On the other hand, other ways of producing inorganic bionanocomposites rely on incorporating filler particles. The presence of nano-fillers can modify the qualities of the materials introduced, such as flame retardancy, unique optical features, or good electrical properties [11]. Nonetheless, to attain stable composites, specific nanofillers must be homogenized into the polymer matrix in a particular proportion. Many types of filler, primarily carbon nanotubes, nanoclays, and certain organic nanofillers, are employed in nanocomposites [11]. Among the organic nano-fillers, naturally abundant plant-sourced materials such as cellulose, lignin, and starch are used as green fillers to improve nanocomposites barrier characteristics with significant reinforcing effects. Notably, green fillers are organic compounds added to composites to reduce the number of binding components. Cellulose nanocrystals (CNCs) are obtained from hydrolysis treatment of microfibril cellulose with sulfuric acid. The introduction of CNCs may reinforce any composites because of their remarkable mechanical properties. It has Young’s modulus of roughly 70 GPa, which is considerably greater than glass fibers. Depending on the extraction technique, it has tensile strength equivalent to synthetic fiber (60–125 GPa) and is more robust than steel (200–220 GPa) [12]. As biodegradable nano-fillers, CNCs offer a lot of potential due to their low density, low energy consumption, abrasive character during processing, and reactive surface potentially for grafting certain valuable groups [13]. CNCs are limited to aqueous processing procedures due to their highly hydrophilic nature. Hence, the resulting composites have a high polar surface and certain drawbacks, including high moisture absorbability and incompatibility with most polymeric matrices. As an alternative, lignin, a lignocellulosic substance, can be easily altered in most polymeric systems. Specifically, lignin is a three-dimensional macromolecule made up of three types of substituted phenols with a diverse range of functional groups and linkages [14]. Their source of origin might contribute to their chemical structure and classification. For classification, they are generally grouped according to their arrangements of monomer units into hardwood, softwood, and grass type of lignin. Hardwood lignin is composed of equal amounts of syringyl- and guaiacyl-propane units. Furthermore, softwood lignin comprises guaiacyl-propane units connected to their carbon-carbon linkages, while grass lignin is based on syringyl-, guaiacyl-, and p-hydroxyphenyl-propane units. Specifically, the concentration of lignin varies significantly between species, but usually, it consists of both softwood lignin (25–35%) and hardwood lignans (15–30%) [14]. Several chemical and physical approaches from various resources have been used to develop lignin nanocomposites for thermoplastic, thermoset, or elastomer-based materials. In practice, lignin nanocomposites are prepared in multiple morphologies

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by modulating essential parameters such as lignin concentration, solution pH, and reaction temperature. Moreover, nanofiller lignin is also the standard choice for reinforcing polysaccharides, natural rubber, and synthetic polymers of poly(methyl methacrylate) (PMMA) and polyaniline (PANI) [15]. Interestingly, when incorporated as lignin nanoparticles, the nanocomposites obtained exhibit remarkable thermal stability and biocompatibility, implying that they can be used as nanocarriers in biomedical applications. However, the proclivity of lignin to self-aggregate has a detrimental impact on its dispersion in many composite formulations. Therefore, many approaches have recently focused on obtaining excellent dispersion of lignin particles in a biodegradable matrix. Another type of plant-sourced filler is starch, a polysaccharide composed of D-glucose units found primarily in roots, tubers, and seeds. Typically, plantcontaining starch such as corn, potato, and paddy take on various granular shapes in spherical or polygonal morphology depending on their origin [16]. The starch backbone has several hydroxyl groups, which causes the starch to become hydrophilic and may be further modified or grafted for better composites compatibility. More importantly, starch is modified into nanocrystals or nanoparticles through acid hydrolysis before being used as nano-fillers. Both starch nanocrystals (SNC) and starch nanoparticles (SNP) are effective fillers in increasing the mechanical characteristics and water vapor permeability of composites, as well as their biodegradability. It is worth pointing out that the specific surface area and total surface energy of starch granules rise when their size is reduced to the nanoscale region. As a result, a highly reactive surface with a larger number of hydroxyl groups is ready to be functionalized, allowing for the introduction of new properties. In most cases, chemically modified starch by replacing the hydroxyl with ether or ester groups is an efficient technique to enhance processing and improve its corresponding composites quality. For instance, a higher degree of substituted acetylated starches is very hydrophobic and insoluble in water, while hydroxypropylated starch displays lower viscosity and better processability. However, chemical modification commonly reduces polysaccharide molar mass, resulting in composites with lower mechanical strength. Also, toxic chemical residues may alter the composites’ biodegradability and hurt the final product’s life cycle. Indeed, the addition of biodegradable polymer matrices and/or organic nano-fillers affects the properties of the nanocomposites, especially their degradation rate. For biodegradation to occur, the polymer molecular weight variations reflect the overall rates of composites breakdown and offer information about when primary fragmentation happens. This is because the disintegration by surface hydrolysis immediately occurs, allowing microorganisms to attack the amorphous phase of the degradable substances (polymer matrices or organic fillers) [17]. In the case of composite degradation, there are four typical steps: water absorption, bond breaking and fragment generation, solubilization, and bacterial diffusion [8]. Intriguingly, the degradation rate may be accelerated due to hydroxyl groups that entrapped water on the surfaces of the composites, thus increasing the bacterium affinity. Nonetheless, depending on the eventual uses of the developed bionanocomposites, different biodegradation

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conditions, namely, hydrolytic, composting, and enzymatic, should be considered. Therefore, we must investigate the intercalation interaction between composites components to acquire desired enhanced characteristics, particularly the filler/matrices interrelation. This chapter seeks to provide advancements in bionanocomposites based on their polymers and inorganic components. Specifically, we present the usage of nanofillers, carbon nanostructures, nano-hydroxyapatite, and nano-cellulose fibers as parts of bionanocomposites. These bionanocomposites have enhanced properties and are typically prepared in two ways, either by wet or dry processing. This developing concept will broaden the bionanocomposites materials for sustainable goods with better qualities than commercially available synthetic or natural polymerbased biodegradable materials. Furthermore, the potential; biomedical applications of generated bionanomaterials, such as scaffold materials, stem cells, and their interactions with biological entities, are discussed.

Bionanocomposites from Green Resources Any composites are “green materials” when at least partly incorporated with natural resources, demonstrating biodegradable and renewable properties. In most aspects, these types of composites are appealing because they are environmentally benign, totally degradable, and sustainable [5]. In the end, they are readily decomposed through the biodegradation process without affecting the environment. As highlighted earlier, up until now, the main challenge lies in finding the right green resources that are compatible enough to be used as matrices or fillers to produce the bionanocomposites. As a general rule, green resources are polymers generated from two main resources of agricultural and animal products. Also, not to discriminate the notable presence of biodegradable raw materials produced chemically from naturally generated monomers or microbial products.

Classification of Biodegradable Inorganic Nanocomposites Composites have more than one combined component, resulting in qualities that differ physically and chemically from their single component. Again, when the composites are composed of biopolymers and inorganic solids that lie within the nanometer regime to give a degradable type of materials, they are recognized as bionanocomposites. Notably, two main approaches induce biodegradation properties in any composite’s structure (Fig. 3). The most common method of incorporating nanoscale particles into a polymer matrix is polymer/nanoparticle composites production (top-down approach). Another way is to functionalize the nanomaterials with a biodegradable polymer (bottom-up approach). For many years, both directions have been successfully adopted on various non-biodegradable inorganic systems using different synthesis methods such as pyrolysis, chemical vapor deposition, sol-gel synthesis, and hydrothermal treatment methods [5].

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Fig. 3 Two main approaches to produce biodegradable bionanocomposite

Fig. 4 Classification of bionanomaterials according to the source of biopolymers. (Adapted with permission from Ref. [19], Copyright 2017, Elsevier)

According to Singh et al. [18], the bionanomaterials can be classified according to their biopolymer matrix, organic, biological, and synthetic. Mohanty and Swain [19] used the same classification when describing the biopolymers’ matrices. Figure 4 shows the classification of bionanomaterials concerning their corresponding biopolymers resources. Starch, cellulose, and lignin are mainly examples of biological plant-based bionanomaterials, while protein, lipid, gelatine, and PLA are biological animal-based bionanomaterials. In the case of chemical bionanomaterials, natural minerals and petroleum derivatives constitute the major components. Additionally, microorganisms-derived bionanomaterials are from living microbes (fungi, yeast, and bacteria) and non-living microbes (viruses). Nonetheless, it is worth noting that bionanocomposites are classified in various ways based on the techniques of synthesis, components used, shape, and properties of their product. Based on the current guideline, bionanocomposites are classified into elongated particles (intercalated), multilayer structures (exfoliated), and particulate (phase-

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Fig. 5 Different types of bionanocomposites according to the morphology of the particle reinforced. (Adapted with permission from Ref. [20], Copyright 2017, CC BY license, MDPI)

separated) according to the morphology of the particle reinforced (Fig. 5) [20]. Moreover, bionanocomposites of elongated particles, such as carbon nanofibrils, are superior for a mechanical reason due to their high aspect ratio. In the case of layered-particle-reinforced bionanocomposites, the term comes mainly from polymers that co-existed in many-layered structures. Another form of composites is a particulate structure or phase-separated nanocomposites. These structures are existed due to the presence of particle-particle interactions, whereby there are no defined boundaries between the layers for microcomposites.

Nanofillers Particles Fillers constructed on nanoparticles have been widely used in composites to tackle market consumer demand over the last decades. These include special nano-fillers such as carbon black, fumed silicas, precipitated calcium carbonates, and modified clays. To reinforce a material, these nanoparticles are entrapped at 1–10% wt into the biopolymer matrix, with a considerable impact on other physical characteristics [11]. Notably, nano-fillers particles are defined as materials having at least one dimension of 1–100 nm. The nanoparticles (iso-dimensional) are formed when three dimensions (3D) are in the nano range, nanotubes and nanofibers are formed when two dimensions (2D) are in the nano range, while nanoplates laminas and/or shells are created when one size (1D) is in the nano range (Fig. 6) [21].

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Fig. 6 Different types of bionanocomposites according to the morphology of the particle reinforced. (Adapted with permission from Ref. [21], Copyright 2018, CC BY license, MDPI)

The interactions between nanoparticles and biomolecules cause the changes in many properties, such as alteration in rheological, optical, electrical, thermal, and magnetic properties and improvement in surface qualities and dimensional stability. Furthermore, nanofiller particles’ different dimensions and shapes have increased the surface area and interaction sites within the bionanocomposites structures. The use of nano-fillers in composites improves their mechanical characteristics (tensile, impact, flexural, and tribological), and, more crucially, they gain popularity in selfhealing capability [22]. At present, advanced microscopy methods of Transmission Electron Microscope (TEM) and Field Emission Scanning Electron Microscope (FESEM) can investigate the structural information of bionanocomposites. In contrast, dielectric spectroscopy can investigate the interaction between particles and matrix polymers. In terms of cost-effectiveness, nanoplates fillers from minerals of montmorillonite (MMT) and kaolinite clay are considered more practical because of their lower cost compared to fibers. More recently, nano-clay is being utilized as filler to reinforce composites due to its natural availability and excellent form factor. Preferably, using clay-based nanocomposites from phyllosilicates (smectites) has tremendously improved the overall physical performance. In addition, mica and talc in the same phyllosilicates family are traditional filler choices in paper making, coatings, ceramics, and cosmetics. Apart from clay, alumina nanoparticles (Al2O3) are commonly utilized as inert fillers, but they can have catalytic characteristics in certain conditions. Of interest, silver nanoparticles display unique optical and chemical properties that can be tailored through a precise process by parameter pre-selection. In recent years, silver nanoparticles have gained considerable attention as nanofiller due to their distinct optical, catalytic, electrical, and antibacterial characteristics that differ significantly from bulk ones. Not only could silver nanoparticles reinforce the composite, but the vital antibacterial effect also permits their potential use in several healthcare-related applications, such as drug delivery, biosensing, biomedical, and

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nanodevice fabrication. Additionally, the surface plasmon resonance (SPR) of silver nanoparticles that absorbs and scatters light when activated by electromagnetic radiation is useful in medical diagnostics and bio-imaging fields.

Carbon Nanostructures Carbon is a mass element that may polymerize at the atomic scale, resulting in long carbon chains. Carbon-based minerals, such as graphite and diamond, are allotropes of certain structural arrangements. Their geometrical arrangement and structure of carbon-based nanomaterials are used to classify them, as shown in Fig. 7 [23]. Carbon-based nanomaterials have attracted the interest of many researchers in a wide range of applications, including sensor, catalysis, energy, and water treatment fields [24–29]. For example, the common carbon-based bionanomaterials are activated carbon, fullerene, nanodiamonds, graphene, nanotubes, and nanospheres. The carbon-based materials are classified according to their morphological structures. Zero-dimensional materials like fullerenes, one-dimensional materials like carbon nanotubes (CNTs), two-dimensional materials like graphene, and threedimensional materials like graphite are often referred to for categorization. Noteworthy, the carbon-based tube shape or CNTs has excellent electrical conductivity, which produces conductive polymers, antifouling coatings, micro and nanoelectronics, conductive fabrics, and other items. For CNTs’ synthesis with single

Fig. 7 Classification of carbon allotropes according to their structural dimensions. (Adapted with permission from Ref. [30], Copyright 2015, ACS)

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or multi-walled tubes, metals such as Fe, Ni, and Co are employed as a basis for the production using the chemical vapor deposition method (CVD). On another note, the 2D carbon material of graphene shows both optical light transmittance and electrical conductivity [23, 30–34]. In detail, graphene is covalently bonded in 2D layers held together by van der Waals forces, with a spatially periodic array of individual molecular sheets interacts with one another via covalent and van der Waals intermolecular interactions. Recently, huge efforts have been made to incorporate carbon nanostructures into polymer matrices to alter the polymer characteristics, primarily electrical and mechanical, and thus expand their possible usage in medical and healthcare fields.

Nano-hydroxyapatite Hydroxyapatite or Ca10(PO4)3(OH)2 (Fig. 8) is a common bioactive ceramic implant that has gotten much interest as a bone grafting option [35]. Bone grafting is a surgical procedure that includes repairing or replacing damaged or diseased bones with grafted bone. Hydroxyapatite has a high level of biocompatibility because it is a primary mineral component of natural bone. A recent study discovered that naturally produced nanohydroxyapatite made from waste pigeon bones was biocompatible with osteoblast cells. The osteoblast cells are responsible for the production of new bone. Bone remodeling, also known as bone regeneration, entails the resorption of old bone material and new bone. The hydroxyapatite is degraded during resorption and releases mainly calcium and phosphate ions, which are naturally metabolized and do not cause aberrant calcium or phosphate levels in urine, serum, or organs

Fig. 8 Hydroxyapatite that resembles bone material, and the morphology of rod-shaped hydroxyapatite

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(liver, skin, brain, heart, kidney, lung, and intestine). The biodegradability of hydroxyapatite combines with its resemblance to bone constituents, makes it an ideal material for biomaterials. The interaction between the implant and its “biological surroundings” is highly complex because of the non-equilibrium conditions and the unknown number of molecules involved in this interaction. The creation of a strong link between hydroxyapatite and natural bone can be promoted and supported during bone remodeling, resulting in the formation of new bone [36]. Most bone fractures occur within the hydroxyapatite structure and native bone because of the strong bonding strength at the contact. The pores of hydroxyapatite create a mechanical interlock that secures the material in place. Bone tissue grows effectively into the pores, improving the hydroxyapatite implant’s strength. The bonding strength of hydroxyapatite and bone is substantially higher than that of other metal implants because of this bonding contact [37]. This direct bonding reduces relative micromovement between the implant and the bone, which is critical for the patient’s rehabilitation in the early stages following implantation. Although hydroxyapatite was once thought to be a non-biodegradable substance, various research involved in long-term observation has discovered that it is biodegradable. From the results of the hydroxyapatite implantation on the patient after 6 years and 7 months, Goto et al. [38] concluded that the hydroxyapatite was a biodegradable substance. A new bone develops at the site of hydroxyapatite implantation after such a long time. As a result, numerous studies on the biodegradation of hydroxyapatite following this event have been published. The osteoclasts with ruffled boundaries and acid phosphatase activity destroyed the hydroxyapatite in pigs in an in vivo biodegradation investigation, which eventually improved bone remodeling [39]. Osteoclasts are cells involved in bone breakdown and absorption, usually as a mediator in reshaping a shattered bone. The biomaterial’s biodegradation is investigated by immersing it in simulated body fluids (SBF). The SBF is a similar solution composition to human plasma and is commonly used to research biomaterial bioactivity. The term “bioactive material” usually creates an appetite layer on the biomaterial’s surface (Fig. 9). Hydroxyapatite has several disadvantages compared to alternative bioactive materials despite its advantages as biodegradable material for bone implants. Hydroxyapatite is intrinsically brittle and has low mechanical characteristics regardless of its excellent biocompatibility and bioactivity. Biocompatibility is good interaction with various biological states such as cells, blood, skin, and so on. On the other hand, the bioactivity of biomaterials refers to the material’s ability to interact with human fluids and generate material-like bone formation. The material’s poor impact resistance and low fatigue strength limit its clinical use. Furthermore, stoichiometric hydroxyapatite (hydroxyapatite with a Ca:P ratio of 10:6) has high chemical stability in interaction with bodily fluids, resulting in limited bioactivity and a diminished osteoconductive impact. The ability of biomaterials to provide a scaffold or template for bone growth and formation is referred to as osteoconductivity.

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Fig. 9 Ions and reagents used in simulated body fluid (SBF)

The biodegradable property of hydroxyapatite is influenced by its stability in body fluids. For example, hydroxyapatite with high stability takes longer to dissolve and create new bone. In contrast, calcium-deficient hydroxyapatite is less stable in bodily fluids. As a result, patients need to spend more time in rehabilitation, which raises the risk of implant failure due to poor implant attachment. Hydroxyapatite cannot be employed as a bulk material that can withstand strain or impact. The disadvantages of hydroxyapatite can be solved by mixing it with various biocompatible materials to form a nanocomposite. The primary goal of developing nanocomposites is to improve biocompatibility and bioactivity, and biodegradation properties under appropriate conditions. Hydroxyapatite could be mixed with single, double, or triple other materials for nanocomposite formation. Chitosan, an organic biomolecule generated from biological resources, is one of the materials used in bionanocomposite to improve hydroxyapatite’s biodegradability and other features. Mainly, chitosan is a type of sugar (polysaccharide) generated or acquired from the hard skeletons of shellfish such as shrimp, lobster, and crab. Β-1,4 glycosidic linkages join glucosamine and N-acetyl-glucosamine monomers in chitosan, a biopolymer produced from chitin deacetylation. Chitosan is an ideal material due to its biocompatible and biodegradable qualities and the fact that it is created from biological resources. The chitosan can increase cell adhesion and reabsorb in physiological fluids after hydrolyzing the enzyme. The freeze-dried 3D chitosan/hydroxyapatite scaffold has a high porosity and tensile strength almost identical to cancellous bone. When hydroxyapatite is added to chitosan, it has several advantages, including reducing scaffold swelling while maintaining an appropriate disintegration rate in physiological solution compared to chitosan

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alone. This is owing to the increased protein adsorption on the chitosan/hydroxyapatite due to the hydroxyapatite’s apatite (bone)-like properties. There is also a bionanocomposite of hydroxyapatite with silica aerogels as an alternative. Silica aerogels are silica networks that have been filled with air and entrapped solvents in the silica gels when removed, leaving only the silica network. Because of this framework, silica aerogels are a one-of-a-kind material that is both strong and light, with a large surface area, porous structure, and low thermal conductivity. Sani et al. [40] used a sol-gel ambient-pressure drying approach to make silica aerogels from rice husk ash as a silica source and then tested their bioactivity by immersing them in SBF for 7 days at varied mass concentrations. The silica aerogel was shown to be resorbed (biodegrade) and simultaneously generated apatite layers, replacing the Si-O-Si bonding of silica aerogels, based on the structural and morphological characterization of the samples and examination of phosphorus in the simulated body fluids. The produced silica aerogel was then used as a network for hydroxyapatite, and the bioactivity and biocompatibility of hydroxyapatite-silica aerogels with varying weight ratios were investigated [41]. According to the characterization data, the hydroxyapatite particles were positioned inside the silica networks, and the varied weight ratios exhibited different characteristics. Lower hydroxyapatite/SiO2 weight ratios (0.05, 0.1, and 0.5) indicated silica-rich materials, while higher weight ratios (1.0 and 1.3) indicated silica-deficient biomaterials [41]. In the SBF, the biodegradation activity of the hydroxyapatite-silica aerogels climbed to 0.5 before declining at a higher ratio [41]. This research suggested that mixing hydroxyapatite with silica at the right ratio could improve biodegradation properties. Furthermore, the biocompatibility against normal human fibroblast cells and osteoblast cells in vitro was greatest at a weight ratio of 0.5, indicating that it possessed hydroxyapatite and silica properties. Furthermore, hydroxyapatite may improve the biodegradability of other silica nanoparticles, such as mesoporous silica nanoparticles. The degradability increased in the presence of hydroxyapatite, as did the drug loading, which was up to 5 times that of mesoporous silica nanoparticles alone. Figure 10 shows how apatite is formed on the hydroxyapatite/silica surfaces.

Fig. 10 The apatite layer formation on the hydroxyapatite/SiO2 composite surface. (Adapted with permission from Ref. [42])

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Fig. 11 Schematic diagram of the synthesis of nano-hydroxyapatite. (Adapted with permission from Ref. [45], Copyright 2020, CC BY license, MDPI)

Figure 11 shows the schematic synthesis of nanohydroxyapatite composites for bone implants that mimic the components or content of bone. Antoniac et al. [43] produced a collagen-hydroxyapatite-magnesium nanocomposite that mimicked the basic composition of mammalian bone. The Mg released was favorable for bone repair due to the slower breakdown rate in the SBF. The biodegradation rate is also be affected by the morphology of the hydroxyapatite particles, which either spherical or needle-shaped. The biodegradation rate of Mg/hydroxyapatite with a spherical shape is lower than that of a needle shape [44]. Noteworthy, hydroxyapatite is a substance that can improve the biodegradation of other biomaterials. It is appropriate to be used as one of the active ingredients in a bionanocomposite. The use of nanocomposite for biomedical applications can benefit from a material’s high biodegradation capability.

Nanocellulose Fibres Cellulose is a primary ingredient in plants that microbes can produce. It is one of the most abundant natural polymers on the planet that can be made from renewable resources. Cellulose is classified as an organic chemical, specifically a biomolecule polymer in chemistry. It is made up of long-chain glucose molecules. Nanocellulose is a kind of cellulose that can be categorized into nanostructured cellulose, including microcrystals and microfibrils, and nanofibers, including nanofibrils and nanocrystals, and cellulose generated from bacteria (bacterial cellulose). Nanocellulose

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Fig. 12 Cellulose and nanocellulose

can be made from bacteria or other appropriate microorganisms and natural polymers, which can be degraded [46]. Its unique qualities, such as abundance, high mechanical properties, biocompatibility, renewability, and high aspect ratio, make it a future material for various applications. Nanocellulose can be used as a flocculant, energy storage, and catalyst in environmental engineering [46], particle-stabilized emulsions in food [47], and reinforcement in cement composites [46] (Fig. 12). The better capabilities of nanocellulose can be achieved by mixing it with other appropriate materials to create a composite. Inorganic materials, such as metals, mineral salts, non-metallic elements, and oxides, are some of the materials that can be used. According to Zhang et al. [48], the nanocellulose composite with inorganic materials can be used for immobilizing protein and DNA, as an antibacterial agent, biomedical materials, sensors, catalysts, and electronic materials. The synergistic effects of the inorganic material loaded nanocellulose produced features distinct from those of the inorganic material alone. For example, silver nanoparticles loaded in nanocellulose have a high and long-lasting antimicrobial activity. Depending on the ability and quality of the loaded inorganic material, the nanocomposite of inorganic material and nanocellulose can degrade other items. The photocatalyst action of a nanocomposite of poly(pyrrole-co-aniline)-coated titania/nanocellulose can destroy eosin yellow dye [49]. Since inorganic materials are more stable than organic compounds, the biodegradability of nanocellulose composites with inorganic materials is limited. In most cases, nanocellulose is mixed with other organic materials such as natural rubber [50], starch [51], or polymer [52]. These nanocomposites are primarily employed in food packaging. Cellulase enzymes can break down nanocellulose. Since this enzyme is not found in the human body, it cannot be biodegraded there. Furthermore, bacterial cellulose has small pore diameters that inhibit cell proliferation, making it unsuitable for bone tissue scaffolds. That is why nanocellulose is not commonly employed as a biomaterial, unlike hydroxyapatite. However, nanocellulose can be

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functionalized or modified with buffers, proteins, polymers, and solvents to improve biodegradability. One method for making nanocellulose a biodegradable biomaterial is to oxidize it first and then combine it with hydroxyapatite. According to Favi et al. [53], the bacterial cellulose-hydroxyapatite composite was made by first oxidizing the bacterial cellulose scaffold with sodium periodate. The cellulose scaffold was then immersed in a calcium and phosphate solution to induce hydroxyapatite deposition. The composite comprising oxidized cellulose demonstrated a considerable mass loss in a Tris-HCl-buffered simulated body fluid degradation experiment, indicating its biodegradable property compared to original cellulose. The biodegradable effect of oxidized cellulose could be cellulose chain breakdown in the SBF, a solution containing different ions and elements. Luz et al. [54] made a hydroxyapatitebacterial cellulose composite by oxidizing bacterial cellulose with a potassium chloride-hydrochloric acid (KCl-HCl) solution. Based on the high-performance liquid chromatography (HPLC) analysis, the in vitro degradability test of the composite in phosphate buffer solution (PBS) revealed that the oxidized cellulose undergoes hydrolytic degradation, with the hydrolysis products being 2,4-dihydroxybutyric acid, carbohydrates, and hydroxyacetic acid. In conclusion, the biodegradability of nanocellulose can be improved by oxidizing and combining it with suitable inorganic compounds, primarily hydrocarbons. Table 1 summarizes some studies on the cellulose bionanocomposite with other materials and its biodegradation property.

Table 1 Recent works on biodegradable nanocellulose Source of cellulose Jute fiber

Composite material Natural rubber reinforced nanocellulose

Oil palm empty fruit bunch

Nanocellulose reinforced bionanocomposite tapioca starch films crosslinked with Citrus aurantifolia or lime juice

Bacterial strain Acetobacter xylinus subsp. Sucrofermentans

Mineralized with nanohydroxyapatite

Komagataeibacter hansenii ATCC 53582

Hydroxyapatite-associated bacterial cellulose

Application/important finding Biodegradation by vermicompost worm Eudrilus eugeniae Biodegradable food packaging The crystallinity affects the moisture absorption, thermal properties, and biodegradation The bionanocompoiste can biodegrade Adhere by bone marrow cells Apply as bone tissue engineering scaffolds Good biodegradable and bioactive material with a potential for bone regeneration applications

References [50]

[51]

[53]

[54]

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Enhanced Properties Nanoscale reinforcements in inorganic composite materials dramatically improve properties such as mechanical capabilities and bioactivity compared to synthetic and natural polymers, which do not display these characteristics independently. The enhanced properties are derived from the synergistic effects of different materials in the bionanocomposite.

Tunable Biodegradability Degradation is a continuous procedure in which material attributes deteriorate owing to many stimuli such as humidity, light, heat, and mechanical stresses. When enough degradation occurs, the resulting tiny fragments do not contribute effectively to the mechanical qualities, and the material finally collapses. The degradation of biomedical composite scaffolds in physiological settings, also known as biodegradation, is influenced by various parameters, including composition, pore structure, scaffold geometry, fluid flow, hydrophilicity, and the pH of the surrounding media [8]. For example, hydroxyapatite has a comparable composition to mammalian bone and plays an essential role in improving the biodegradability of other biomaterials. When hydroxyapatite is combined with other materials, a superior composite enhances the biomaterial’s properties. Hydroxyapatite improves the biocompatibility and degradation resistance of magnesium-zinc (Mg-Zn) based metallic implants [55]. More particular, the cylindrical shape of hydroxyapatite has a much more significant impact on degradation resistance than the spherical shape. This improved characteristic could contribute to the large surface area of cylindrical shape, which results in higher interaction with Mg-Zn surfaces [55]. This effect results in a more significant relative density, which increases resistance in simulated body fluids and makes a better temporary orthopedic implant. Furthermore, silica can improve hydroxyapatite biodegradability or vice versa. When the composition of these two materials is identical, the nanocomposite hydroxyapatite, which includes silica aerogel, has the maximum bioactivity, as aforementioned earlier. It contains calcium, phosphate, and silica content [41]. These components oversee desorption in simulated body fluid and forming a new apatite layer on nanocomposite surfaces. Inert materials that have been implanted into bone defects are generally encapsulated by non-adherent fibrous tissue, leading to their isolation from the surrounding living tissues. Since the 1970s, some glasses, such as Bioglass, have shown spontaneous integration within the surrounding bone without forming fibrous tissue at the interface layer, apparently through the formation of biological apatite as an interfacial layer [56]. Since then, several composite materials such as sintered hydroxyapatites, glass-ceramics containing crystalline apatite, wollastonite, and silica-based materials such as silicon-substituted calcium phosphates, have shown a similar in vivo behavior that can integrate with the host tissue. Capabilities to undergo a complete conversion to apatite-like mineral simultaneously with the degradation process were also observed in these materials [57]. Based on these findings, comparable with in vivo observations, some researchers suggested that the apatite-forming ability of biomaterial

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could be measured in simulated body fluid [57]. The ability of biomaterial to form an appetite layer on its surface and its dissolution effects after the immersion can be used to predict its bioactivity in vivo. Sani et al. [41] studied the bioactivity of hydroxyapatite incorporated silica and suggested that the silanol group, which was available abundantly on the material’s surface, was responsible for the active growth apatite layer on its surface. When the hydroxyapatite-silica was immersed in SBF, the silanol groups changed the surface of samples into a negative charge and attracted Ca2+ ions in the SBF, causing them to migrate into its silica-rich layer [58]. Thus, it increased the zeta potential values. The loose chemical bonding between silica networks and Ca2+ ions stimulated the phosphate ion to form a bi-dentate chelates complex within the silica networks. The open-pore structure of the samples allows the diffusion of phosphate ions inside its framework. The hydroxyapatite-silica framework is simultaneously resorbed. This causes the depletion of phosphate ions in SBF and the increment of phosphate content at the silica-rich layer. The increment of the new bi-dentate chelate complex bond at the silica-rich layer breaks Si-O-Si bonds and stimulates the nucleation of hydroxyl carbonate apatite structure [59]. The broken Si-O-Si bonds are eventually replaced by the carbonate phosphate thin layer in the silica network, stabilizing the samples framework. The rapid formation of a carbonate-phosphate thin layer on the hydroxyapatite silica finally allows the apatite to be actively developed. In summary, silica could enhance hydroxyapatite’s biodegradation property and become a good candidate as a bioactive material. It can simultaneously be resorbed and actively form an appetite layer after immersion in SBF.

Antibacterial Activity The integration of antimicrobial fillers in the biodegradable polymer matrix opens the potential of novel multifunctional nanostructured materials with high antimicrobial activity even at low nanoparticle concentrations. Certain materials have antimicrobial activity, such as silver, zinc, or copper, or antibiotics that can act as antimicrobial filler since a typical biodegradable material does not possess antimicrobial activity. Antimicrobial agent refers to any material, compounds, ions, or other material states that can kill or inhibit a wide spectrum of microorganisms, including bacteria, yeast, fungi, and viruses. Table 2 shows previous research on antimicrobial biodegradable products. The antimicrobial property of the biodegradable product is derived from the antimicrobial agents that are loaded together in the bionanocomposites. The solvent casting (wet process) generates bioactive bionanocomposites from silver nanoparticles and synthetic biodegradable polymers such as PLGA and PCL. Their unique microbiological properties have been discovered to inhibit many bacteria, viruses, and fungus, silver nanoparticles, which have been frequently employed for packing materials [3]. The antibacterial activity and tunable degradation are found possible in bionanocomposites. Nevertheless, the interaction of stem cells is important to determine the influence of nanoparticles on the overall biocompatible characteristics. The fundamental challenge in producing antibacterial bionanocomposite materials is to inhibit bacterial growth without impairing cell

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Table 2 Antimicrobial activity of biodegradable materials Material Oxidized nanocellulose

Antimicrobial agent Silver nanoparticles

PVA electrospun fibrous composite nano-layers

Biosynthesized silver nanoparticles by chitosan

Thermoplastic starch film Alginate film

Carvacrol essential oil and montmorillonite ZnO nanoparticles and citronella essential oil

Nanofibrous poly (lactide-co-glycolide) (PLGA) scaffolds

Bioactive antibiotics and platelet-derived growth factor (PDGF)

Application/significant finding Long-lasting antibacterial behavior Biomedical devices Biodegradable food packaging Biodegradable packaging material for meat (extended meat shelf-life) Inhibit microbial degradation of packaged food Synergistic effects between two antimicrobial agents Antimicrobial film for biodegradable food packaging Biodegradable diabetic infectious wounds

References [60]

[61]

[62] [63]

[64]

viability. Furthermore, a surface examination is a beneficial tool to adjust the initial component of the material that comes into touch with biological entities and can induce antibacterial performance without affecting the bulk properties.

Mechanical Properties The size and quantum effects of bionanomaterials help them to exhibit commendable mechanical properties. In theory, when any of the bionanomaterials components are mixed with a common material, they refine the grain and form either an intergranular or an intragranular structure, enhance the grain boundary, and increase the mechanical properties of the common materials. Bionanomaterials exhibit high mechanical stability that allows their potential exploration in the biomedical and healthcare fields. The addition of nano-fillers to biopolymers is commonly done to strengthen the mechanical strength of the resulting bionanocomposite. Bone substitute materials must be mechanically strong enough to be used as a bone replacement and facilitate bone development at the implantation site. Moreover, the mechanical characteristics of the substrate have a significant influence on cell behavior [18]. For example, when particle size decreases, the total strength of the composite system increases, whereas deformability and impact strength decrease [18]. Also, the filler’s reinforcing effects increase with particle anisotropy and can be distinguished by their degree of anisotropy (or aspect ratio). As a result, fillers with 1D structure of the plate-like and 2D structure of fiber-like geometries strengthen polymers more than 0D structure of spherical particles, while particles with 3D complex forms are anticipated to have an even stronger impact.

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On the other hand, certain nano-filler particles may be excluded during polymer crystallization due to precipitation of the polymer matrix, resulting in filler agglomeration at the edges of the spherulites. If this occurs, the effective filler modulus will decrease due to the reduced elastic strength of agglomerated filler grains, but it will also allow for internal voiding and delamination between the matrix and filler [65]. For example, nano-whiskers are a specific nano-cellulose with a large specific area and excellent mechanical capabilities, making them a potential candidate for improving the mechanical properties of the appropriate matrices. Moreover, non-linear mechanical characteristics can be evaluated using traditional tensile measurement or compressive testing [65]. Notably, mechanical characteristics are significantly enhanced based on cellulose filler’s dispersion amount and homogeneity. The solid reinforcing action is commonly attributed to establishing a percolating network structure above the threshold because of hydrogen bonding between nanoparticles. The polymer strength and modulus enhancement above the host polymeric matrix’s glass transition temperature (Tg) are well documented. The addition of incompatible components, on the other hand, tends to reduce the elongation at the break points of the bionanocomposites as compared to the clean matrix. Dynamic mechanical analysis (DMA) is a useful approach for investigating the linear mechanical behavior of nanocomposites across a wide range of frequencies. Previous DMA studies revealed that the storage modulus of bionanocomposites was higher over the whole temperature range when compared to pure components.

Thermal Properties The thermal characteristics of bionanomaterials must be evaluated to identify their processing temperature range and application. Differential scanning calorimetry (DSC) measurement is used to assess the key properties of polymeric systems, such as glass-rubber transition temperature (Tg), melting point (Tm), and thermal stability. Additionally, the Tg of polymers can be determined via DMA studies. In most cases, water molecules with plasticizing impact were responsible for the decrease in Tg values in selected bionanocomposites reinforced with sisal, starch, and cotton [16]. Typically, the Tg decreased as humidity increased, irrespective of the composition of the composites. However, in a certain case, when whiskers were added, the Tg of PVA-based nanocomposites increased significantly in humid conditions. Another example showed that nanocellulose crystallite filler content in glycerol plasticized starch nanocomposite reinforced increased Tg [66]. On the other hand, Tm values in most nanocomposites are almost independent of their filler content. For example, the Tm of the reinforced composites was not affected by cellulose fibers employed as fillers in microfibrillated cellulose (MFC) or unmodified bacterial cellulose nanocrystals [67]. In contrast, the Tm value increased with increasing filler content when bacterial cellulose nanocrystals were treated with silane groups. This sharp contrast might be due to synergistic contacts between the cellulose fibers’ filler and the matrix. Based on the Tm and Tg information, one can tailor the desired composites. For example, PLA-MFC nanocomposites [68] demonstrated a higher crystallinity than pure PLA, implying that

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MFC facilitated PLA crystallization. Aside from that, MFC was also expected to act as a nucleating agent in this work, speeding PLA crystallization. In the case where nanocellulose fibers were used as fillers to reinforce poly(ethylene oxide) (PEO) nanocomposites, the degree of crystallinity [67] was found to be increased consistently up to 10% wt of fillers content while decreased with a higher filler loading (15–30% wt). In these circumstances, the steric behavior of PEO spherulites arose in the presence of the nanocellulose network, which was attributed to this observation. The nucleating impact of cellulose nanocrystals appears to be primarily influenced by surface chemical factors. Proceeding from this track, the application of degradable polymers as medical or bioabsorbable stents has good potential in the future concerning chemical and biochemical functionalities. Despite several limitations such as fragility due to the relatively high glass transition temperature, poor mechanical strength, premature diameter reduction due to degradation, inflammatory reaction, and limited shelf life, all these drawbacks are currently being addressed by progressive developments of their corresponding bionanocomposites. It is anticipated that potential ideas breakthrough technologies of bionanocomposites from renewable resources will continue to advance and that their biomedical use will be widely available 1 day.

Synthesis of Biodegradable Inorganic Nanocomposites The synthesis procedure is critical to ensure all the unique features and benefits of their nanosized constituents complement each other as stable and enhanced bionanocomposites. Their components naturally aggregate and agglomerate to complicate a homogeneously dispersed formulation. Therefore, selecting compatible and degradable substances from bioresources such as plants is one of the essential criteria in synthesizing bionanomaterials. In plant-based synthesis, it is either the nanosized substance introduced inside the plant extract (in vivo) or by utilizing the plant to synthesize the nanostructured composite (in vitro). During the synthesis, the plant extract is washed and filtered before being composed of metal/nonmetal salts. Finally, the resulting plant-based bionanomaterials are typically followed by visible color changes. Different plants may differ in phytochemicals, enzyme activity, and biochemical processing based on their topical origin, so their composites preparation will get more complicated. More importantly, there are two typical ways to synthesize the bionanocomposites, which are through the wet process or dry process.

Wet Process In the wet process, the polymers undergo a continuous casting procedure to produce bionanocomposites from natural resources such as proteins, carbohydrates, and lipids [23]. In the first place, the polymers are solubilized using a suitable solvent for film formation. The solvent must be capable of entirely dissolving the required

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polymer while allowing for optimal nanostructure dispersion in the polymer matrix. As this affects the evaporation rate and viscosity, the concentration between solvent and polymer must be optimum. After that, the desired components such as nano/microparticles, filler, crosslinkers, and plasticizers are added to the final solution. The addition of plasticizers into the formulation increases the product flexibility by reducing the molecular interactions and stiffness. Once the film is spread, the solvent is evaporated completely to avoid toxicity before using the final bionanocomposite. This approach is excellent for developing packaging materials since it increases the mechanical qualities of the final material. In the case of the wet process via solution casting, the approach offers a low-temperature method that produces films with consistent thickness, low haze, and optical clarity. Nonetheless, the downsides of this process include slow processing speed, the cost of solvent usage and recovery, and the fact that the approach is often very limited to laboratory-scale processes. For bulk bionanocomposites, effective distribution of a nanoscale component through simple solution mixing is physically unfeasible. Ultrasonic treatment or vigorous stirring may slightly enhance the mixture homogeneity. To allow a better film formation in the wet process, a slow procedure combined with self-organization in the system is compulsory. In most cases, surface modification is applied before the wet process to give less agglomeration and homogenous distribution by cooperating interfacial adhesion between the matrix and filler. Because of industrial usage, organic solvents such as chloroform are often avoided in their facilities; thus, using water-soluble polymer is an essential technique. Unfortunately, there are limited polymer matrices that can be utilized, restricting the procedure for large-scale production that is commercially feasible. A new technique known as a layer-by-layer assembly has been investigated using oppositely charged nanoparticles and biomacromolecules assembled alternately [69]. This method has prepared many bionanocomposites with a tailored structure for membranes, films, coatings, capsules, and shell materials. Nevertheless, the layer-by-layer approach has drawbacks in producing bulk materials such as hydrogels. Therefore, the wet process should only be utilized in specialized applications.

Dry Process The dry method manipulates the polymer thermoplastic characteristics, particularly for packaging materials from agricultural resources such as whey, keratin, and wheat. In theory, a thermoplastic polymer can be changed from a metastable glassy state into an unsteady rubbery state by applying heat. The polymer’s physical and mechanical properties can be induced through interchange reactions at the glass transition temperature. This could increase the mobility of macromolecules and free volume, resulting in less dense structures used in packaging materials. For packaging materials, the suitable polymer can be shaped as desired at high temperatures in the presence of plasticizer additives. During the heating process, new linkages and

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interactions are formed, which promote changes in the characteristics of the materials. Over the past decades, the polymer industry has employed extrusion or thermoforming as a high-volume production process for many products based on thermoplastic, such as frames, plastic sheets, and films. This commercially feasible process is considered a “green method” due to its solvent-free characteristics. For both methods, the polymer is processed at high temperatures. Therefore, the polymers used should be highly thermal resistant. Each polymer system requires a certain combination of processing conditions depending on the processing efficiency and product qualities to obtain the desired product. Polymer extrusion is a common industrial process because it is a continuous, fast, and simple procedure for converting raw components into finished products. In this process, the polymer is fed from a hopper and pushed toward the screw during the extrusion. The polymer is subjected to high-temperature compression and mechanical shear forces at a certain period to achieve a suitable viscoelastic mass. The final product of a homogeneous film can be produced at a sufficient high extrusion temperature. In the thermoforming process, the raw materials mixture such as soy, gelatin, and keratin are initially placed between the processing plates. The plasticized mixture is then applied with high temperature and pressure to achieve viscoelastic behavior. The reaction mixture is cooled down and stabilized by different chemical reactions to form a film in the final stage. A thermostat plate controls the overall process to alter the thermodynamic parameters such as temperature and pressure. Nonetheless, the dry method using both extrusion and thermoforming processes is environmentally friendly as it avoids the use of organic solvents and is compatible with the existing industrial processes. The overall schematic diagram for both the wet and dry processes is presented in Fig. 13 for better understanding.

Potential Biomedical Applications Despite the availability of materials with suitable structural characteristics, they still require modification to meet the requirements for biomedical applications. Most materials are not mechanically competent, bioactive, and biodegradable becomes a huge hurdle to advancing biomedical technology. Bioinert materials are often not mechanically strong, while bioactive and biodegradable materials are mechanically weak when manufactured due to a high presence of porosity. Bionanocomposite materials provide an ideal possibility for desirable structural properties in biomedical applications by integrating biodegradability, antibacterial activity, and mechanical competence. Particularly, bionanocomposites have numerous advantages in biomedical applications, such as wound dressings, drug release systems, and bioengineering, due to their biocompatibility and intrinsic non-toxic qualities. Furthermore, bionanocomposite films offer several advantages, including less costly, higher efficiency in food packaging, and the highest antibacterial activity. The potential biomedical applications of certain bionanocomposie are further discussed.

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Fig. 13 Schematic diagram of bionanocomposites processing using a wet and dry process

Scaffold Material for Bone One of the significant challenges in orthopedic and maxillofacial surgeries is slow bone regeneration in significant skeletal defects caused by pathological or traumatic conditions. This significant bone defect is typically challenging to heal naturally. Conventional bone-grafting, such as autologous and allograft harvested from a patient’s bone or other human bone skeletons, has been extensively used by surgeons to treat this large fracture bone. However, many difficulties arise with direct transplantation therapy. The transplantation technique is expensive due to extended surgery periods, unavailable autograft, and associated donor-site morbidity. Moreover, allograft allows potential transmissible disease and infection risks that can induce immune responses and rejection to patients or bone receivers. Therefore, constructing an alternative bone substitute is considered an excellent alternative for the direct transplantation of donor organs. Nowadays, tissue engineering has emerged, focusing on developing alternative bionanomaterials that can replace and enhance tissue function at the implantation site. The demand for biomaterial to rehabilitate bone defects has increased in recent years. The bionanomaterial’s performance depends on integrating with the host bone and facilitating new cell and tissue regeneration [70]. Metals such as commercially

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pure titanium, titanium alloys, stainless steel, and chromium have been used as single or multi-components in orthopedic and dental implants. It is due to their superior mechanical properties, such as low weight/high strength ratio and ability to remain inert in vivo [71]. Although these metal implants satisfy the mechanical requirements of bone, they fail clinically due to insufficient bonding with fractured bone or incomplete osseointegration. Thus, the bonding strength between these materials and natural bone is weak. Besides, these materials are easily corroded, releasing toxic ions into our bodies. Therefore, a new bionanomaterial that is biocompatible, mimics the natural bone properties, and forms a stable bond between the implant and natural bone is essential to be fabricated. Crystalline hydroxyapatite is a synthetic material analog to calcium phosphate found in bone and teeth. Hydroxyapatite has been widely used in implantation due to its bio-properties. It is a highly compatible material that has been considered for coating on metallic implants, a porous ceramic that facilitates bone ingrowths, an inorganic component in a ceramic-polymer composite [72], a granulate to fill small bone defects, and for tissue engineering scaffolds. The nanocomposite’s biodegradation property is suitable for being used as a scaffold material for bone. Nowadays, researchers in applied material science have developed a bone scaffold that can degrade in the body fluids, and then the new bone formation occurs naturally. Hydroxyapatite is the most popular material as a bone scaffold due to its almost similar framework to the mammalian bone. Hydroxyapatite is an apatite with calcium and phosphate as its framework structure, and it is always referred to as osteoconductive. Hypothetically, the hydroxyapatite or other biomaterials can be biodegraded by the osteoclasts during the bone remodeling process, and finally, the new bone formation replaces the scaffolds. The biodegradation process depends on the biomaterials, but most result in a slow process that could suffer the patients. Hence, several studies have been carried out to accelerate bone formation by combining various suitable biomaterials of organic and inorganic materials to form nanocomposites. Most studies focused on hydroxyapatite nanocomposite with organic materials such as polymer or biomolecules, while the nanocomposite with inorganic materials is scarce because the hydroxyapatite itself is inorganic materials have higher stability compared to organic material. The high stability material is hard to be biodegraded naturally as it has a higher resistance to harsh environments such as high pH or temperature. However, there are some studies related to the inorganic nanocomposite. The nanocomposite scaffold of hydroxyapatite with diopside, a monoclinic mineral composed of Mg, Ca, and Si, enhanced biodegradation in simulated body fluid compared to the pure hydroxyapatite scaffold. The X-ray imaging of the scaffold made by the molybdenum disulfide nanosheets reinforced hydroxyapatite nanocomposite scaffold showed more rapid bioresorbable ability than the hydroxyapatite. The biocompatibility behavior of hydroxyapatite can be improved by introducing some substitutions in its structure that resemble the chemical composition and the structure of the mineral phase in bones. Recently, there has been more research interest concerning the effects of various ion substitutions such as magnesium [73], silicon [74], fluorine [75], and carbonate [76] on the bioactivity of hydroxyapatite.

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Table 3 Overview of hydroxyapatite modifications for tissue engineering applications Material Bioglass / hydroxyapatite Hydroxyapatite / zeolite Hydroxyapatite / chitosan / silica Poly(D,L-lactic-co-glycolic acid) / hydroxyapatite Single-walled carbon nanotubes / chitosan / hydroxyapatite Silicon / hydroxyapatite Encapsulation of hydroxyapatite microspheres with fluorapatite Hydroxyapatite / collagen / xerogel Hydroxyapatite / Mg-Zn-Ca alloy

Technique Foaming method, sintering Microwave wet precipitation method Co-precipitated method

References [77] [78] [79]

Microsphere sintering method (modification of the emulsion and solvent evaporation method) Lyophilization procedure, scaffold architecture

[80] [81]

Slip foaming technique, scaffold architecture Diffusion process

[82] [75]

Lyophilization procedure, sol-gel technique

[83]

Micro-arc oxidation and electrochemical deposition

[84]

The hydroxyapatite lattice is very accommodative and allows various positive and negative ionic substitutions. Small substitution significantly affected the hydroxyapatite’s thermal stability, solubility, and bioactivity, which enhanced its biological performance. In this sense, an exciting way to improve the bioactivity of hydroxyapatite is the addition of silica to the hydroxyapatite structure, taking into account the importance of silica on bone formation and growth at in vitro and in vivo conditions. The overview of hydroxyapatite modifications to enhance its biocompatibility is tabulated in Table 3.

Stem Cells Stem cells are an essential component with multipotential and self-renewal features. The ability of stem cells to self-renew allows them to offer a reservoir that lasts throughout the tissue/organ life inside a particular setting where the stem cells live (niche). In this case, stem cells can divide asymmetrically to produce two daughter cells, one of which is a stem cell and one of which is a committed cell, or two stem cell daughters (symmetric division). Notably, many studies have been conducted to investigate the effects of bionanomaterials on different stem cells. Table 4 shows the examples of selected bionanomaterials used in stem cells. Several bionanomaterials that incorporated carbon nanostructures have been demonstrated to guide stem cell differentiation in the presence or absence of soluble differentiation factors (Fig. 14) [85]. For example, graphene and its derivative, graphene oxide (GO), have sparked the curiosity of researchers as potential new materials capable of causing the physical stimulation required for stem cell growth.

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Table 4 Examples of the specific biological applications achieved using different types of stem cells and biomaterials in terms of polymers, nanocomposites, and gels Stem cells Embryonic

Adipose Bone marrow mesenchymal

Induced pluripotent Neural

Bionanomaterials Nanostructured poly(D, L-lactide-co-glycolide)/ collagen scaffolds Modified chitosan Layer-by-layer SWCNT Vascular collagen/chitosan Functionalized hyaluronic acid Alginate hydrogel Polylactic acid (PLA)/collagen nanofiber scaffold CNT networks Polylactic acid (PLA)/ hydroxyapatite Polylactic acid (PLA)/nanohydroxyapatite Chitosan hydrogel SWCNT/laminin 2D bionanocomposites poly-L-lysine/modified PLA

Applications Cardiomyocytes

References [87]

Neural differentiation Neurons and astrocytes Prominent vascular artery Wound healing Axonal growth Hepatocyte-like cell generation Proliferation and osteogenic differentiation Chondrogenic differentiation Osteogenic differentiation

[88] [89] [90] [91] [92] [93]

[96]

Neuronal differentiation Neuron cells

[97] [98]

Neuronal differentiation

[99]

[94] [95]

Fig. 14 Schematic illustration of stem cells differentiation using bionanomaterials and their applications. (Adapted with permission from Ref. [85], Copyright 2018, CC BY license, Springer)

The surface structures of GO honeycomb influence the cytoskeletal dynamics of cells adhering to the GO surface, resulting in changes in cell spreading, morphology, and proliferation. More specifically, 2D graphene oxide of micropatterns, incorporated as bionanomaterials hybrid platforms, has been proven to accelerate human

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mesenchymal stem cells (hMSC) differentiation into adipocytes and osteocytes, and chondrocytes [85]. Carbon-based compounds are often synthesized via chemical vapor deposition (CVD), which provides high film quality before being transferred to a range of substrates [23]. For example, graphene is often functionalized before being utilized as a surface coating on biomaterial substrates to increase the bioactivity of the bionanocomposite. Recent studies have found that graphene could influence the osteogenic development of hMSCs. For example, Lee et al. [86] observed that the graphene’s capacity to bind to many osteogenic differentiation factors helped improve hMSC proliferation into the osteogenic lineage. Moreover, when graphene is cultivated without BMP-2, they produce an expected growth factor in bone formation as the cultivation causes osteogenic differentiation. It is learned that two-dimensional materials such as graphene accelerate and direct the adipogenic or osteogenic differentiation of hADMSCs and other stem cells due to their distinct chemical and physical properties. Prior research showed that two-dimensional materials and nanomaterials combined with biological components, such as growth hormones, peptides, and proteins, improve cellular activities such as cell adhesion, proliferation, migration, and differentiation [86]. Surprisingly, these materials are well-performed as an attractive signal for the osteogenesis of hMSCs and the improvement of the bone regeneration process. This indicates that carbon-based materials can be used in regenerative medicine and other biological sectors.

Bionanocomposites Interaction with Biological Entities Bioceramic has been widely developed during the past few decades because of its biocompatibility and excellent stability in the human body. Bioceramic does not release toxic elements to the human body even after a long exposure. Hydroxyapatite or Ca10(PO4)3(OH)2 is a typical bioactive ceramic implant which has attracted much attention as an alternative substance for bone grafting. It is the most prominent bioactive ceramic and is widely used and investigated. Dhert et al. [100] proposed the kinetic of the early events occurring within the first month at the interface between hydroxyapatite-coated implants. Regardless of the biomaterial’s nature, its natural surroundings evolve with time. In the first 3 days, blood invades all the empty spaces between the original bone and the implant. At the end of the first week of implantation, callus and mesenchymal tissues replace the whole blood while the host bone starts to resorb. Finally, callus, mesenchymal tissues, and host bone gradually disappear in favor of newly formed bone, while bone remodeling occurs between the second and fourth week of implantation. All components of biological fluids and cells interact with the biomaterial. Hence, the biomaterials features can affect the molecular and cellular interactions at their surface and consequently affect the process of bone formation. Cell viability assay determines the in vitro toxicity of the synthesized sample. It is an initial compatibility test to evaluate the biological effects of material against cells

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Monolayer of cells Well of microtitre plate

Incubation

a

Disc of sample Cell Response

b

Biocompatible

c

Cytotoxic

Fig. 15 Schematic diagram of the viability evaluation of material using the direct contact test. (a) A piece of test material is placed directly on a monolayer of cells and incubated for some time and observed for cytotoxicity signs. (b) Biocompatible material does not damage the cells, and (c) cytotoxic material is likely to leach harmful substances that can damage the cells. (Adapted with permission from Ref. [42])

and living tissues. Figure 15 shows the schematic diagram of how the evaluation of the direct contact of the biomaterial can be performed on the cells in vitro. Typically, the cell viability can be quantified using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide) assay. MTT is cleaved in the mitochondria of all living cells that are metabolically active and produces the colorless compound known as tetrazolium, changing into a dark purple formazan crystal product. Proliferating cells that are more metabolically active than non-proliferating cells cleave MTT to form a dark purple formazan crystal (Fig. 16). Thus, the formazan generated is directly proportional to the number of living cells in a cell population. In the case of conducting bionanocomposite, carbon nanotubes are employed to establish a 3D-conductive network in the polymer above a certain concentration known as the percolation threshold. This threshold is determined by the nanotube dispersion and the aspect ratio of the nanofiller, in which the increase in aspect ratio reduces the percolation threshold [30]. By incorporating carbon nanotubes into a polymer matrix, their electrical conducting regions increase with more electroactive sites. Notably, electrical behavior and surface topography at the micro and nanoscale influence the interactions between stem cells and polymer nanocomposites.

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Fig. 16 Images of fibroblast cells (HSF 1184) that were actively cleaved MTT to a dark purple formazan crystal at (a) 10/0.25 and (b) 20/0.40 magnifications. Images were obtained using an inverted optical microscope. (Adapted with permission from Ref. [42])

Specifically, culturing human umbilical cord matrix stem cells (hUCMSCs) on PLLA polymer film and PLLA/multi-walled carbon nanotubes (MWCNTs) nanocomposite films promote the specific interaction with stem cells and activate a particular biological response. The electroactive surface may effectively increase intracellular calcium levels, enhancing myoblast development. Therefore, conductive nanomaterials, including carbon nanotubes and gold nanoparticles, have been typically employed to create conductive bionanocomposites for muscle tissue regeneration. It is critical to ensure an appropriate dispersion and solid interfacial contacts between the nanomaterial and the polymer to optimize the load transfer across the nanofiller–polymer matrix interface. The functionalization of nano-fillers has been regarded as one of the most effective methods for preventing aggregation caused by strong van der Waals forces. In the case of CNTs, functionalization is necessary to benefit the optimum qualities that allow for the facile development of novel nanomaterials and nanodevices. It is widely known that functionalization can be either by covalent or non-covalent techniques. Interestingly, the surface functionalization of CNTs by carboxylic acid significantly influences cell parameters and enhances the interface between polymer and CNTs. As a result, more work should be done to materialize these highly potential bionanomaterials as robust medical devices.

Conclusions The goal of this chapter was to explain the current level of research in bionanocomposites. Indeed, this is a fascinating, intriguing, and rapidly evolving field. Today, the world is on the verge of large-scale technological use of biopolymers, which are seen as the only viable alternative to synthetic, petroleum-based polymers. Notably, biopolymers are renewable resources that can serve as the foundation for long-term economic growth. They are also biodegradable and

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environmentally friendly. Also, their commercial usage offers environmental advantages, including lower greenhouse gas emissions, less reliance on fossil carbon, and biodegradation of innocuous compounds by the activity of microorganisms. Indeed, biopolymers are extremely adaptable, with various physical qualities suited for a wide range of applications. Nonetheless, biopolymers have limited potential to replace petroleum-based polymers since their characteristics are inadequate for material production. In terms of mechanical strength, biopolymers are often inferior to their synthetic polymers’ counterparts. Moreover, they have a high gas and water permeability, a low heat degradation temperature, etc. Interestingly, nanostructured materials are being used to improve the properties and functionalities of biopolymers to produce degradable inorganic composites, also known as bionanocomposites. To date, a limited number of biopolymers are feasible for developing bionanocomposites. Nonetheless, obtaining a homogenous dispersion of nanosized fillers or additives in the biopolymer matrix on a large scale is a huge hurdle. The advantages of having nanoparticles are only apparent when they are appropriately disseminated in a polymer matrix due to the high surface area. Numerous interactions and links between the mixed components also modify the macromolecules’ mobility and relaxation behavior, enhancing the final product’s mechanical and thermal characteristics. The developed bionanocomposites are potentially used in biomedical applications such as bone regeneration and stem cell development with constant research progress.

Future Prospective A diverse set of accessible biopolymers and inorganic nanostructures materials allows for the possible realization of bionanocomposites on a large scale. This will be accomplished by combining them in various ways. Biopolymers provide the matrix for bionanocomposites and influence their form, structural structure, and primary functions. Typically, the matrix is altered by dispersed nanofiller particles. By doing this, they enable the modification of the structure, and attributes and, as a result, promote better functioning of the final product. In addition, specific nanofiller particles can also be incorporated to introduce a particular feature that biopolymers do not afford. In theory, for every desired use viable, an extensive range of biopolymers matrices and nanofiller particles can be introduced to develop the bionanocomposite materials. Only a handful of natural resources materials are compatible enough to produce bionanocomposites, such as nanofiller particles, carbon nanostructures, nano-cellulose, and nano-hydroxyapatite. With the rapid growth of interest in the industrial production of bionanocomposites, there is a high hope to circumvent the limitation possessed by conventional composites. Nonetheless, as interdisciplinary sciences progress, it is expected to develop novel bionanomaterials that improve the health and quality of life for millions of people by restoring, maintaining, or enhancing tissue and organ functions. In the future, by continually developing novel bionanomaterials with better properties than existing materials, appropriate waste management features

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with a lower carbon footprint could be outlined to eradicate harmful wastes in the environment contributed by the synthetic petroleum-based polymers.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanotubes Structures and Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arc Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membranes Filtration and Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impact of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of Carbon Nanotubes Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of Carbon Nanotubes Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Economic Cost of Carbon Nanotubes Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A. S. Ibrahim · D. A. M. Farage Institute of Graduate Studies and Research, Environmental Studies Department, Alexandria University, Alexandria, Egypt e-mail: [email protected]; [email protected] G. A. M. Ali (*) Chemistry Department, Faculty of Science, Al–Azhar University, Assiut, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_24

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Abstract

Carbon nanotubes are unique nanomaterials with excellent physicochemical properties commonly used in energy, sensing, biomedical field, and environmental remediation. However, CNTs are inevitably released into the environment while rapidly developing. They are toxic to living organisms in the environment and are problematic to degrade under normal conditions. This chapter systematically describes the properties, applications, and toxicity of CNTs. In addition, the chapter focuses on biodegradation methods of CNTs by microbes and enzymes, along with experimental and molecular simulation methods to explore nanomaterial degradation. Finally, the economic cost of CNTs degradation is estimated. Keywords

Carbon nanotubes · Properties · Applications · Toxicity · Biodegradation List of Abbreviations

1D 2D ABS CNTs CNHs CNM DLS EPO f-MWCNTs FTIR GO HRP HMD LiP MPO MnP MWCNTs OH-SWCNTs Ox-SWCNTs PA 6 PAHs p-MWCNTs p-SWCNTs RGO SCL SL SWCNTs

One dimensional Two dimensional Acrylonitrile-butadiene-styrene Carbon nanotubes Carbon nanohorns Carbon nanomaterials Dynamic light scattering Eosinophil peroxidase Amino-functionalized MWNTs Fourier transform infrared Graphene oxide Horseradish peroxidase Hexamethylenediamine Lignin peroxidase Myeloperoxidase Manganese peroxidase Multi-wall carbon nanotubes Hydroxylated single-wall carbon nanotubes Acid-oxidized single-wall carbon nanotubes Polyamide 6 Polycyclic aromatic hydrocarbons Purified MWNTs Pristine single-wall carbon nanotubes Reduced graphene oxide Sandy clay loam Sandy loam Single wall carbon nanotubes

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Transmission electron microscopy Thermogravimetric analysis Ultraviolet–visible X-ray photoelectron spectroscopy X-ray diffraction

Introduction In recent years, the utilization of carbon nanotubes (CNTs) increased significantly due to their various applications thanks to their unique properties, producing many CNTs in the environment [1]. CNTs are widely used in various applications, including drug carriers, electronics, biosensors, sorbents, and fuel cells [2– 7]. Some studies demonstrated the toxicity of CNTs and their derivatives. Consequently, to lessen their toxicity to living organisms and eliminate them from the environment, CNTs must be degraded [3, 4, 7, 8]. Microbes and enzymes are now of great importance for the biodegradation of carbon nanomaterials, thus removing them from the environment. The biodegradation of CNTs, and their derivatives by fungi, bacteria, and plant, animal, and microbial enzymes and provided an overview of the standard experimental and molecular simulation methods to study the biodegradation of these nanomaterials [9]. When compared to enzymatic degradation, microbial degradation looks to be the most promising for practical applications because enzymatic degradation frequently necessitates the presence of a suitable environment. Many studies have shown that macrophages can biodegrade CNTs through enzymatic oxidation. Neither CNTs undergoing biodegradation nor the by-products of their degradation have been cytotoxic in vitro and in vivo. In addition, many factors such as CNTs type, length, surface functionalization, and impurities can influence the biodegradation of CNTs [10, 11]. Here we review CNTs classification, properties, and applications and determine their toxicity and importance of degradation. In addition to how we can degrade CNTs and the methods for investigating CNTs degradation, we can detect optimal methods for degradation.

Classification of Carbon Nanotubes CNTs are divided into single-walled carbon nanotubes (SWCNTs) and multiplewalled carbon nanotubes (MWCNTs). SWNTs and MWNTs share some matches, yet, in addition, prominent contrasts [12, 13]. They have exceptional strength, can be profoundly electrically leading or semiconducting, have an enormous surface region per unit mass, in addition to remarkable optical properties. While the other type, referred to as MWCNTs, is considered a progression of SWCNTs settled inside each other. They might reach 100 or more concentric dividers plus concentric walls; accordingly, their diameters may be inordinate much as incredible as 50 nm. For the most part, just the external divider contributes fundamentally to the electrical and mechanical properties of MWNTs when utilized in composites [14] (Fig. 1).

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Fig. 1 Differences between SWCNTs and MWCNTs, copied with permission from Ref. [15], copyright, Springer, 2020

Carbon Nanotubes Structures and Morphology Single-Walled Carbon Nanotubes SWCNTs fundamentally comprise carbon; more clearly, they are an allotrope of SP2 hybridized carbon, like fullerenes. The structure can be thought of as a cylindrical tube comprised of benzene-type hexagonal rings of carbon atoms. The cylindrical tubes may have one or both ends capped with a buckyball or fullerene structure [1, 4, 16 ]. Near the periphery, SWCNTs often have just ten atoms, and the tube is only one atom thick. Nanotubes have a length-to-diameter ratio (aspect ratio) of roughly 1000 in general. As a result, they can be deemed nearly one-dimensional structures [17]. Multi-Walled Carbon Nanotubes MWNTs are concentric graphene rolls shaped by moving almost 3–5 sheets of single-walled nanotube more than each other. MWCNTs comprise many graph sheet layers not exceeding ten layers, and. Their thickness is greater than one atom, and they have an interior diameter of almost ten nanometers. Moreover, the arrangement pattern of graphitic sheets results in a variation of MWCNTs from each other [17, 18]. Fundamentally, SWCNTs contrast from MWNTs in structure, as SWNTs have a necessary variable arrangement of carbon atoms, resulting in three different structural formations. Firstly, the zigzag arrangement, in which the tube is depicted as having a V-shape structure, is vertical to the tube’s axis; the armchair arrangement, in which the chiral vector is represented by the chair structure, is perpendicular to the tube’s axis. Finally, the chirality degree of carbon nanotubes

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Fig. 2 Schematic of three typical SWNTs, (a) Armchair (10, 10), (b) Chiral (13, 6), and (c) zigzag (14, 0), copied with permission from Ref. [20], copyright, Elsevier, 2012

is a measure of their electrical conductivity. Additionally, chirality governs the nanotubes’ diameter as well as their elastic or semi-metallic characteristics [12]. The integers n and m determine the number of unit vectors in the honeycomb crystal lattice of graphene in two orientations. When m ¼ 0, nanotubes are known as zigzag nanotubes; when n ¼ m, they are known as armchair nanotubes; and other states are known as chiral nanotubes [19]. SWCNTs forms are dependent on wrapping to a cylinder way, and SWCNTs’ structure is defined by two indices (n, m) that specify the chiral vector and have a direct effect on the electrical characteristics of nanotubes. Despite dissimilarities in structural form, the CNTs also differ concerning the dimensions. Generally, the structural features of SWNTs are elucidated using a (n, m) vector that refers to the chirality and diameter. There are three types of SWNTs: (a) armchair (10, 10), (b) Chiral (13, 6), and (c) zigzag (14, 0) (Fig. 2). The nanotube is described as “metallic” when n  m is a multiple of 3, and if not, then the nanotube is semi-metallic or a semiconductor. MWCNTs can be shaped into two primary models: The Parchment and the Russian Doll models. When a single graphene sheet is folded over itself various times, the same as a wrapped up scroll of paper, it is called the Parchment model. On the other hand, when one more nanotube is within a CNT and the external nanotube has a larger diameter than the smaller nanotube [19, 21]. SWNTs offer fundamentally better optical and electrical characteristics and many other characteristics; consequently, they have been used in various applications. Moreover, there are many common variants of CNTs, such as carbon nanohorns (CNHs), Fig. 3, nanotorus, and nanobuds, Fig. 4 [2, 10]. CNHs have the property to change their diameter as their length increases. CNHs are cone-shaped, with one narrow and one broad open end to the horn-like structure. Recently, Carbon nanobuds have been discovered; they synthesize from CNTs and fullerenes and have features between them. Preparing the CNHs is like preparing CNTs, by the laser ablation method. It is like CNTs in structure, but the only difference exists in its external growth. Nevertheless, nanoporous theoretically are CNTs wrapped up in a doughnut- or torus-shaped structure [2, 12]. Another striking aspect of CNTs is their elasticity. Under strong force and press sitting, and when exposed to enormous axial compressive forces, it can bend, twist, kink, and finally buckle without destroying the nanotube, and the nanotube will

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Fig. 3 (a) Schematic illustration, and (b) HRTEM image of carbon nanohorn “dahlia” aggregate (scale bar 10 nm). (c) HRTEM image of an individual carbon nanohorn (scale bar 2 nm) copied with permission from Ref. [22], copyright, MPDI, 2020

Fig. 4 (a) Molecular graph of a nanotorus, copied with permission from Ref. [23], copyright, Research Publication, 2020; (b) A model of a nanobud, copied with permission from Ref. [24], copyright, Elsevier, 2015

revert to its original structure. However, nanotube elasticity has a limit, and it is possible to deform the shape of a nanotube temporarily under extremely intense physical forces. Some faults in the nanotube’s structure, such as atomic vacancies or carbon bond rearrangement, can impair the nanotube’s strength. Elastic modulus, or module, determines the elasticity of single and multi-walled nanotubes. [19, 21].

Properties of Carbon Nanotubes Chemical Properties As mentioned above, CNTs are composed of only one or more hexagonal graphene sheets of carbon atoms twisted into tubes [25]. Several remaining impurities may be present in CNTs. The manufacture of carbon nanotubes typically necessitates the use

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of catalytic metals., and concentrations of remaining impurities may be relatively high, especially in industrial-grade CNTs [26]. Metal catalysts are often used to manufacture SWCNT, particularly cobalt, iron, nickel, and molybdenum [26]. Consequently, authentic SWCNTs usually contain higher trace metal concentrations than MWCNTs [25]. In addition, support materials are frequently used to promote the catalyst or the tube’s growth zone, including fine alumina, silica, or magnesium oxide [26]. In general, the types and levels of impurity depend on the production process. Gas-phase processes tend to produce CNTs with less impurities and are suitable for large-scale production. Post-production purification processes such as mechanical handling and strong acids can eliminate any leftover impurities or flaws in graphene sheets., but these tend to shorten the CNTs [25, 27].

Physical Properties Physical properties of CNTs include atomic structure; surface chemistry; length, diameter, optical, mechanical, thermal, and electrical characteristics and their aggregation state; the specific area; and bulk density.

Atomic Structure The atomic structure of CNTs is termed with chirality of tube, which is determined by graphene sheet alignment during synthesis of the tube. The chirality of tubes may be armchair and zigzag conformations, which are common types, and they can occur in a combination of different conformations [28]. However, the chiral axis is described as the axis of the carbon hexagon orientation relative to the CNTs axis, affecting nanotubes diameter [29]. Chirality in MWCNTs also impacts the electrical and optical properties of the CNTs due to different chiralities of the adjoining graphene layers [30]. CNTs can be metallic or semiconducting even though graphene is a semi-metal, but it depends on the chiral angle; however, chirality does not amend the mechanical properties of CNTs [31]. Thickness Principally, the thickness of CNTs is based on the number of graphene layers besides the tubes’ chirality. The SWCNTs generally have an outer diameter ranging from 1 to 3 nm [32], but the outer diameter of MWCNTs is 10–200 nm [33]. In general, the alterations in the diameter are determined by the synthetic process because the catalytic metal’s diameter plays an important role, especially in the case of SWCNTs [30]. Length The length of an exemplary CNT is a few micrometers, although it can range from a few hundred nanometers to tens of micrometers. The tubes usually have a length of 50 m, although some can be hundreds of micrometers long. In most CNTs preparations, variations in tube length are the norm rather than the exception. [27].

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Specific Surface Area Due to their small size and structure, each CNTs has a very high surface-to-mass ratio, which is denoted as a specific surface area. The diameter, concentric graphene layers, and degree of bundling influence the particular surface area. SWCNTs have a specific surface area of 1300 m2/g on average, whereas bundles of SWCNTs have a value that is six times lower. [34]. Bulk Density CNTs have a low bulk density that changes depending on how they are made. Graphene powder has a bulk density of 200–600 mg/cm3, whereas pure graphite has 2200 mg/cm3 [26, 27]. Thermal and Optical Properties The optical and thermal properties of carbon nanotubes are unique. Near-infrared light (800–1600 nm) is readily absorbed by SWCNTs [35], which involves the wavelength that passes through biological tissues without scattering, heating, absorption, or damaging the tissue. Therefore SWCNTs can be used in photothermal therapy [29, 30]. CNTs also display unique thermal conductivity. At room temperature, the thermal conductivities of SWCNTs can reach 6000 W/(m.K), while diamonds have a value of 3320 W/(m.K) (m.K). SWCNTs can also withstand up to 2800  C in a vacuum and 750  C in air [36]. Electrical Characteristics CNTs have a conducting nature due to the sp2 bonds between carbon atoms. Depending on their chirality, CNTs can operate as semiconductors or conductors, which is directly related to their electrical properties; curvature also plays a role in the case of thin SWCNTs [30, 37]. They can also resist strong electric currents because of the strong nature of bonds [30]. CNTs have several potential electric components and device applications, and SWCNTs can make a diode with different electrical properties. CNTs can also be applied in sensors thanks to CNTs modification by deformation and stretching [38].

Synthesis of Carbon Nanotubes Various techniques for preparing carbon nanotubes include arc discharge, laser ablation, and chemical vapor deposition (CVD).

Arc Discharge The most famous method for carbon nanotubes preparation is arc discharge, which produces high-quality nanotubes. During this method, the plasma is formed through the electrical breakdown of a gas, and carbon atoms in the plasma begin to evaporate

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Gas Inlet

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Cathode

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Power Supply

Fig. 5 An arc discharge setup illustration copied with permission from Ref. [43], copyright, Elsevier, 2014

by using higher temperatures (above 1700  C). The chamber comprises two electrodes, one anode, and the other a cathode. A graphite powder and catalyst combination is used to fill the anode [39, 40]. The process begins with retaining the electrodes under a gaseous atmosphere, then the distance between the electrodes is reduced until it is kept at 1 mm, and a current of around 50 amps is transmitted between two graphite rods in an inert gas-filled enclosure (like helium or argon) at low pressure (between 50 mbar and 700 mbar) related to a potential drop of 25 V [41]. The plasma is formed when the temperature in the inter-electrodes exceeds 1700–2400  C, and the electrodes become red hot. Due to the temperature gradient, carbon sublimes from the consumed positive anode and condenses on the cathode as filamentous carbon. The whole duration of the process needs about 1–2 min. The CNTs with the by-products settled on the chamber’s walls after cooling the chamber. Most nanotubes deposit on the cathode Fig. 5 [39, 42]. The fundamental process parameters for obtaining high yields of CNTs are flow rate, inert gas pressure, and metal concentration. This method can produce multiwalled CNTs as the process is tuned with two standard graphite electrodes to produce single-walled carbon nanotubes using metallic catalysts such as cobalt, iron, molybdenum, or nickel doped at the electrode. The breakdown of the gaseous molecule into carbon is catalyzed by these metals, resulting in the formation of a tube with a metal particle at the tip [44, 45], but the utilization of high temperatures in the fabrication process, low pressure, and pricey noble gases are all defects of this approach [46].

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Laser Ablation Laser ablation is one of the most effective techniques for producing SWNTs. In general, Intense laser pulses such as YAG or continuous wave- (cw-) CO2 focuses on the target, which is situated in a tube-furnace containing pure graphite for MWNTs production or a catalyst metal-graphite composite (Ni, Co, Co/Ni, Co/Pt, Rh/Pd), in trace amount (1%, 2%), targeted for SWNTs under high temperature around 1200  C [47]. During the process, inert gas or a mixture of gases flows through the chamber through the quartz tube, carrying the produced nanotubes to the copper collector located at the furnace’s exit, which filters them to deposit CNTs. Finally, the nanotubes and the by-products can be collected after cooling the chamber, Fig. 6. In addition to the fact that laser ablation is a superior method, it can also be tuned to produce either single-walled CNTs or MWCNTs depending on various parameters in the production process, such as laser pulse duration, furnace temperature, laser power, and laser wavelength as well as graphite target composition. The greater amounts of metal in the target create multi-walled CNTs, whereas a low metal in the target and a high furnace temperature is related to high-quality crystalline SWCNTs with higher yield through a few moments. Moreover, due to the high cost of this method, it is not gaining significance for the synthesis of MWNTs [12].

Collector SWCNT Catom Laser Target Gas Inlet

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Laser ND-YAG Laser

Inner Quartz Tube

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Nanotube Felt Graphite + Catalysts

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Fig. 6 A LA setup using an ND:YAG laser system is illustrated schematically. When the target rod is pure graphite or a graphite catalyst mixture, MWCNT and SWCNT are synthesized, copied with permission from Ref. [48], copyright, Springer, 2016

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Chemical Vapor Deposition Chemical vapor deposition (CVD) is the most extensively utilized method for CNTs production. In principle, chemical vapor deposition catalyzes hydrocarbon decomposition with supported transition metal catalysts [49]. Various CVD procedures include radiofrequency CVD, plasma-enhanced, floating catalyst, microwave plasma, water-assisted CVD, hot-filament CVD, and oxygen-assisted CVD method [50]. Generally, the process is carried out by inserting the catalyst into a quartz boat, the reaction temperature ranging from 500  C to 1200  C, and takes place in a quartz tube [51]. Then, a mixture of reactions, including hydrocarbon sources such as acetylene, carbon monoxide, methane, and ortho-xylene [52], as well as an inert gas flow, is moved through the catalyst bed, and the chemical reactions occur on or near the hot surfaces in an inert atmosphere. Carbon atoms are created after the carbon precursor decomposes, and a thin layer is formed on the catalyst particles surface by carbon nanotubes. Eventually, CNTs are collected from walls and supported surfaces after cooling the system to room temperature, Fig. 7 [53]. Since the structure of CNTs is governed by the size and chemical composition of metal catalysts, several of the most critical parameters are catalyst selection. They are determining CNT development and morphological characteristics. Platinum, iron, palladium, manganese, and aluminum isopropoxide are considered efficient catalysts in the form of nanoparticles less than 3 nm [55, 56]. Various parameters

Fig. 7 The chemical vapor deposition process is illustrated schematically. (a) A simplified scheme for a CVD reactor for the production of carbon nanotubes; (b) base growth model of CNT growth mechanism; (c) tip-growth model of CNT growth mechanism, copied with permission from Ref. [54], copyright, Springer, 2016

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such as the metals’ physical properties and supports, the flow of gas, reaction time, temperature, and hydrocarbon sources are used to regulate the nature and yield of CNTs produced by the CVD technique [53, 57]. The main advantages of CVD method include high yield, low impurity production, easy control of the reaction, relatively low operating temperature lower than 1200  C, and low cost of CNTs [39, 40].

Applications of Carbon Nanotubes In recent years the utilization of CNTs increased significantly due to their various applications thanks to their unique properties. They have numerous applications in the textile industry such as waterproof and tear the proof fabric and sensors based on remarkable thermal conductivity, extraordinary heat, and electrical conductivity. CNTs can be used in membranes filtration and adsorption, biomedical fields, and nanoelectronics. The following are some of the most important applications of carbon nanotubes [1].

Biomedical Field CNTs are a potential material for a variety of biomedical applications because of their unique properties. Several of their exceptional features comprise their novel structure resulting in an unprecedented marvelous combination of optical, mechanical, and electrical properties [58]. Scientifically, before using CNTs in biotechnology and biomedical-based industries, three parameters may act as barriers. These barriers have to be overcome: toxicity, pharmacology, and functionalization perspectives of CNTs [59]. The most noticeable barrier is the toxic nature of CNTs. Generally, aggregated nanoparticles are toxic due to the synchronization of maximum surface area condensed by CNTs and the inherent toxic feature of nano surface [60]. Overall, the particle size of designed nanotubes can have an impact on the toxicity of CNTs. Another significant barrier with CNTs is the pharmacokinetics and biodistribution of agglomerated nanoparticles, which are influenced by several physicochemical properties. When exposed to any media, the most prominent disadvantage of CNTs is that the aqueous solubility is scarce [60]. CNTs can be recommended for drug and gene delivery, tissue engineering, biomedical imaging, cancer cell tracing, and artificial implant scopes, as Fig. 8.

Nanoelectronics Thanks to CNTs being highly conductive, it has a significant role in nanoelectronics. Whereas the chemical bonds of the C atom are gratified without dangling bonds in CNTs, a high crystalline insulator and dielectric constant can be applied successfully in CNT-based devices [36, 61]. Besides, the naturally small geometric size for

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Drug and Gene delivery

CNTs Applications Tissue Engineering

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Artificial Implant

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Cancer Cells Tracing

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Fig. 8 Biomedical Applications of CNTs

SWCNTs of 1 nm prompts enhancement and optimization of the coupling between that gateway and the transistor channel and gives extraordinary applications of nanoscale devices [36]. The most conductive carbon fiber known is single-walled nanotube ropes. Alternative formations of CNTs may cause consequential material being semi-conductive such as silicon [62]. Fundamentally, nanotubes’ conductivity relies on the degree of chirality – i.e., the degree of twist and size of the actual nanotube – which results in a nanotube that is actually highly conductive or nonconductive making it suitable as the basis for semiconductors [38, 63]. In general, CNTs can be used in nanoelectronics for various services such as in transistors, solar cells, interconnect, batteries, and energy production and storage, Fig. 9.

Membranes Filtration and Adsorption Carbon nanotubes (CNTs) would be employed in filtration due to their high aspect ratio as well as mechanical and chemical stability. A layered self-supporting nanotube film produced by dead-end filtration of a CNTs suspension is the most basic CNTs filter. Nonetheless, filtration could be one of the applications of nanotubes that can be commercialized. [59, 64]. The adsorption of heavy metal ions by multi-walled carbon nanotubes is unique. The sorption process is affected by nanotube functionalization, pH, solution ionic strength, temperature, and other factors. Furthermore,

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Transistors

Energy production and storage

Solar cells Nanoelectronics

Batteries

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Fig. 9 Applications of CNTs as nanoelectronics

MWCNT adsorbents can be tuned to specific ions like Pb2+, Cu2+, Zn2+, and Cd2+. [65]. Furthermore, CNTs can be used in composites; for example, MWCNTs can be combined with MnO2 to remove Pb2+ and Cd2+ [66], and with iron/iron oxide to adsorb Ni2+, Sr2 +, and Cu2+ [67]. CNTs can remove harmful gases from mobile sources, but their proficiency depends on many variants [68]. In addition, MWCNTs exhibited filtration efficiencies exceeding 99% for fine airborne particles when coated with cellulose fiber filters [69].

Environmental Impact of Carbon Nanotubes The toxicity and reactivity of carbon nanotubes and their derivatives in in vivo systems and the environment contrast with the numerous benefits of chemical functionalization of carbon nanotubes. SWNTs influence electron transport in cytochrome c and influence mitochondrial activity. SWNTs have been shown in mice to cause pulmonary damage by activating alveolar macrophages, inducing a great number of proinflammatory genes, recruiting leukocytes, and forming severe pulmonary granulomas. The toxicity of MWNTs in the lungs was investigated, and it was discovered that they cause protein exudation, interstitial granulomas, as well as inflammatory and fibrotic responses. Immunological responses to CNTs (SWNTs and MWNTs) were studied, and allergic immune responses were discovered [70]. Lanone and Boczkowski [71] demonstrated that the toxicity of CNTs is caused by their size, surface area, content, and form.

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CNTs, GRA, and their derivatives are increasingly being shown to negatively affect human health and the environment [9]. Ema et al. [72] reported that CNMs can harm pregnancy and embryo and fetus development. The developmental and reproductive toxicity depends on dimensions, structure, surface features, chemistry concentration, agglomeration, dose, and preparation of CNMs. Because of the great range of CNMs, the data available is still restricted and may not be extrapolated to all forms of CNMs. In the natural environment, biodegradation is an important mechanism for removing contaminants. It is determined by the concentration and qualities of CNTs, the physicochemical properties of microbes and contaminants, and the state of the environment. It is closely linked to microorganisms and enzyme function. CNTs may promote or decrease biodegradation, except few known environmental situations. This made determining the effects of CNTs on biodegradation more difficult. CNTs inhibit microbial growth in various harmful ways. On the other hand, microorganisms have adaptive and defensive mechanisms to counteract such negative impacts. CNTs attach to the activity center of the enzyme and help with the electron transfer process between the substrate and the enzyme. As a result, the enzyme’s activity in oxidizing or reducing substrates was boosted, while CNTs prevented enzymatic oxidation of substrates through effects on the enzyme–substrate interaction. The adsorption and desorption behavior of contaminants was also linked to the effects of CNTs on biodegradation. Pollutant availability to functional microorganisms was reduced due to adsorption on CNTs [73] (Figs. 10 and 11).

Cell membrane Cell membrane/membrane potential disruption

DNA damage

Protein Nucleus

efflux of cytoplasm materials

Ox co idize mp d on cel pro ent, lular tei e.g n .,

Reactive oxygen species (ROS) Interrupted transmembrane electron transport

+

Ag+ Zn2+

-

Release hazardous constituents, e.g., metals, ions

e-

e+

Mitochondria damage

Fig. 10 Different toxicity mechanisms to eukaryotic cells of CNTs, copied with permission from Ref. [73], copyright, Elsevier, 2020

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Adsorption increased

d ate Co h t i w c ani org er tt a m

Dispersion

Fu

nct

ion

aliz

ed

cid

A

d ize

Adsorption decreased

Adsorption of hydrophobic contaminants decreased; Adsorption of hydrophilic contaminants increased

id ox

s

re po er Inn ked c blo

Fig. 11 Factors affecting the sorption of pollutants on CNTs copied with permission from Ref. [73], copyright, Elsevier, 2020

In vitro transformation of SWCNTs and MWCNTs was aided by plant-derived horseradish peroxidase, manganese peroxidase from white-rot fungus, and human myeloperoxidase and eosinophil peroxidase. Given the abundance and diversity of bacteria in the environment, as well as their broad metabolic potentials, microbial breakdown of carbon nanotubes is most likely the most important biotransformation activity in the environment, and it deserves more investigation. [74]. CNTs collected in the environment may have reached concentrations of 0.01–3 g/ kg in soils and 30–1000 g/kg in sediments, according to model estimates [75]. The transformation and breakdown of CNTs in the environment may considerably impact their transport, bioavailability, and ecotoxicity. You et al. discovered that the PAH-degrading bacteria Mycobacterium vanbaalenii PYR-1 might partially convert and potentially mineralize p-MWCNTs and c-MWCNTs, despite a low mineralization rate of 8%, 0.25% (carbon mass) per day, utilizing TEM and Raman/confocal-Raman, as well as respiration studies [74]. MWCNTs have been found to have an effect on the destiny of polycyclic aromatic hydrocarbons (PAHs) and other organic pollutants in some investigations. When MWNTs are present in sandy loam (SL) soil, highly accessible PAHs are less damaging to less tolerant microbial species. In the highest treatment in the sandy clay loam (SCL) soil with more organic matter, the addition of MWNTs increased pyrene degradation, which could be linked to increased PAH bioavailability in this soil. According to this study, depending on soil type and organic matter concentration, MWNTs can impact microbial dispersion and PAH decomposition. The findings of this study should be followed up with studies in different soil types (large variations in organic carbon content), periodic monitoring (monitoring at different time intervals for changes in the microbial community, as well as PAH degradation), different PAH concentrations, and aged soil from the field. [76].

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Qian et al. [77] reported that treatment with SWCNTs resulted in an increase in microorganism metabolism related to soil organic compound breakdown, as well as a change in the organization of soil microbial communities, but not a significant change in soil microorganism diversity. The rise in the relative abundance of nitrospira and decrease in soil urease activity following exposure to SWCNTs could be due to soil nitrification induction. After being exposed to SWCNTs, the relative abundances of phosphate-solubilizing bacteria increased, which improved phosphorus bioavailability in the soil. They discovered that exposing soil microorganism communities to SWCNTs at concentrations of 3 g/g and 10 g/g can alter the composition of soil microorganism communities, promote soil organic degradation, and improve soil fertility by increasing N and P availability in a short period. Carbon nanotubes interact strongly with natural organic components in aquatic habitats, considerably enhancing their stability and mobility in that environment. CNTs (SWNT and MWNT) have been shown to have strong adsorptive contacts with nytroaromatic chemicals in an aqueous media, whereas CNTs and nonpolar aromatic compounds have poor interactions and aliphatic molecules have the weakest interactions. Carbon-based nanoparticles poison some microorganisms. Low concentrations of graphene oxide, for example, aid in the growth of Phanerochaete chrysosporium, but large amounts destroy the fiber structure and render the organism inert. Carbon-based nanomaterials are harmful to various creatures, including animals, in addition to bacteria. The toxic molecular mechanism of SWCNTs toward Caenorhabditis elegans was assessed using genome-wide gene expression analysis. It was discovered that amide-modified SWCNTs cause toxicity in the worms through several pathways, including reduced citrate cycle activity and defective endocytosis. Carbon nanoparticles appear to be completely harmless [78].

Importance of Carbon Nanotubes Degradation As mentioned above, CNTs and their derivatives have many unique properties and are prevalently utilized in several products; therefore, the possibility of releasing them to the environment is increasing with widespread applications. The nature of CNTs, whether chemical or physical, makes them stable, recalcitrant, inert, and degrade difficult [65, 79]. Many studies have reported their existence in the environment.. [80–82]. There has been a wide agreement that they present expected risks to the ecosystem systems [83] because of their toxicity to different living organisms [84]. For example, Martinez-Paz et al. [85] reported cytotoxic effects induced by MWCNTs associated with the transcription of genes involved in apoptosis. CNTs have been shown to be toxic and have other negative impacts in many researches. [70–72]. There is a need to identify a safe and effective technique for removing CNTs from the environment, as their harmful impacts and other obscure risks have raised environmental and health concerns among researchers and people in general. Biodegradation technology might have the option to challenge this issue. Chen et al. [86] studied the molecular basis of SWCNTs degradation by two different enzymes.

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It is generally perceived that focusing on the biodegradation of nanomaterials has become fundamentally significant for the design of degradable nanomaterials for practical applications. The investigation of the structural varieties in the materials brought about by enzymatic catalysis, allowing potential problems caused by nanomaterials discharged into the environment to be addressed [87].

Methods of Carbon Nanotubes Degradation CNTs have been employed in sophisticated composites, textiles, and fibers, and even in the manufacture of cables for the space elevator because of their unique graphite structure, which makes them mechanically strong and resistant to the process of deterioration [88]. CNT’s decay was frequently connected with diameter in the following order: SWNTs  CNHs > thinner MWNTs > thicker MWNTs Defects and dispersion also influenced the pace of degradation. However, these were not the key causes [88]. Human exposure to CNTs can occur at any point in the CNTs’ lifecycle, from laboratory study to production, product inclusion, modification of CNT-characteristics and eventually, carbon nanotubes must be disposed of and recycled [89]. By increasing the manufacture and use of SWCNTs, the risk of human exposure to SWCNTs is expected to rise, and there are serious worries about CNTs’ side effects [90]. According to imaging and spectroscopy data, SWCNTs can be taken up, maintained, expelled, or destroyed within nucleated cells [10]. According to other studies, pure SWCNTs persisted in the lungs of mice after being exposed to them for more than 3 months [27]. Nanomaterials’ physical and chemical properties frequently change while they are within animals, increasing or decreasing their toxicity [91]. Internalized CNTs are cytotoxic, causing lung inflammation, granuloma formation, and fibrosis in vivo, primarily through oxidative stress [10].

Thermal Degradation The thermal stability for CNTs is very high. Acrylonitrile-butadiene-styrene (ABS) was destabilized by the inclusion of SWNTs, resulting in the degradation of composites at lower temperatures [88, 89]. Probst et al. reported that MWCNTs could enhance polyvinyl alcohol decomposition. The thermal degradation behavior of CNTs-filled polymer composites may be influenced by the polymer matrix’s structure and the interaction between CNTs and the matrix [92] (Tables 1 and 2). Grafting HMD onto MWNTs improves MWNT dispersion in PA6. Thermogravimetric analysis (TGA) results demonstrate that temperatures with 5% weight loss are 18  C for p-MWNTs/PA6 composites and 14  C for f-MWNTs/PA6 composites under air, but temperatures remained nearly unchanged for all composites under nitrogen. The addition of MWNTs appears to prevent PA6 from thermooxidizing in the presence of air, increasing PA6’s thermal stability. However, in

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Table 1 TGA results of PA6 and MWNTs/PA6 composites with varying MWNT concentrations in an air environment, Copied with permission from Ref. [92], copyright, Elsevier, 2006 Samples MWNTs Cont (wt%) 0 0.1 0.5 1 2

p-MWNTs \ PA6 T10wt T5wt ( C) ( C) 366 388 371 392 372 396 372 393 384 400

Tmax1 ( C) 451 446 454 451 454

Tmax2 ( C) 536 548 539 548 536

Residue at 600  C (%) 0 0.1 0.7 0.3 1.2

Table 2 Results of TGA analysis of PA6 and f-MWNTs/PA6 composites with different MWNTs concentrations under nitrogen environment.(Copied with permission from Ref. [92], copyright, Elsevier, 2006) Samples MWNTs Cont (wt%) 0 0.1 0.5 1 2

f-MWNTs/PA6 T10wt ( C) T5wt ( C) 384 409 383 408 383 408 385 411 388 410

Tmax1 ( C) 457 457 456 456 455

Residue at 600  C (%) 0.1 0.2 1.5 1.1 1.6

the absence of oxygen, this effect is not observed for all MWNTs/PA6 composites in a nitrogen atmosphere; two-step degradation is reported for p-MWNTs/PA6 composites in both the air and nitrogen atmosphere, but only in the air for f-MWNTs/PA6 composites. The lesser influence of f-MWNTs on the second degradation of PA6 under nitrogen atmosphere is thought to be due to their less integrated structure than p-MWNTs. The activation energies for degradation in air are 153, 165, and 169 kJ/ mol, respectively, for neat PA6, p-MWNTs/PA6, and f-MWNTs/PA6 materials. [92].

Biodegradation The biodegradation of CNTs may be one of the most significant elements of hazardous effects in those exposed to them [10]. Carbon nanomaterial biodegradation by microorganisms and enzymes is becoming increasingly important for lowering their toxicity to living creatures and eliminating them from the environment. CNTs, GRA, and their derivatives offer a variety of appealing features and are employed in a numerous applications, such as drug carriers, electronic parts, biosensors, sorbents, and fuel cells. As the result of their extensive use, there is a greater chance that they will enter the environment. Due to their physical and chemical features, CNTs, GRA, and their derived products are all inactive substances, disobedient, stable, and hard to decompose; various studies have proven their presence in the environment. CNTs and graphene could have different fates depending on

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their unique features, such as length, degree of oxidation, and functionalization. Because of their toxicity to numerous living organisms and cells, widespread agreement poses potential threats to living organisms and the ecosystem. Some investigations have found that SWCNTs and graphene have cytotoxic effects related to the nanomaterials’ shapes and concentrations. Several studies have shown that CNTs, GRA, and derivatives are hazardous and have other negative effects [9]. Enzymatic, cell, and bacterial decomposition are the most common biodegradation techniques for carbon nanomaterials. CNTs are usually functionalized before they are degraded since the original CNTs are hard to degrade by enzymes found in animals and plants that occur naturally in environmental conditions, although they can be degraded after they have been functionalized. CNT biodegradation is mostly reliant on a variety of enzymes. The chemical basis for CNT decomposition by functionalization is with two well-known biodegradative enzymes, horseradish peroxidase (HRP) and lactoperoxidase (LPO). The findings reveal that the functionalization energy can alter CNT characteristics, such as improving the stability of complexes formed by enzyme and substrate processes via carboxylation [85]. The enzymatic degradation of CNTs may be correlated with specific properties, such as length, degree of oxidation, and surface functionalization. Comparative studies using different types of CNTs have been limited due to the complex enzymatic degrading process [88]. The oxidation of CNTs by hypochlorite (ClO), which is produced as a by-product of numerous enzymatic activities, is required for CNTs biodegradation by neutrophils or macrophages. Within 2 h, 2  106 active neutrophils produced roughly 100 nmol of HClO. As a result, evaluating the degradation characteristics of CNTs using sodium hypochlorite NaClO as an oxidant is a viable option. NaClO can destroy CNTs; according to recent findings, two-dimensional (2D) graphene oxide (GO) sheets decayed quicker than one-dimensional (1D) oxidized CNTs, while SWNTs degraded faster than MWNTs. The findings also show that CNTs wall thickness may not be the most critical component in hypochlorite degradation and that other factors like contaminants may impact the degradation rate [88]. CNTs biodegradability could be a key factor in determining the harmful effects in people who have been exposed to them. According to research, SWCNTs can be degraded by peroxidases, including MPO and EPO [10, 93] (Fig. 12). From the previous graph, Hou et al. reported that 27% of the ox-SWCNTs were degraded after 1 day exposure with the in vitro enzymatic system [10] (Fig. 13).

Microbial Degradation Bacteria are abundant in nature, and bacteria take a long time to degrade carbon nanotubes. CNTs can adsorb bacteria on the surface because they are biocompatible, and bacteria’s response with CNTs begins at the flaw, edge, or surface. MWCNTs can be stressed by bacteria, which subsequently generate peroxidase. This active site’s conformation has a high affinity, which improves bacteria’s ability to digest MWCNTs. CO2 is produced when MWCNTs are degraded by bacteria [94].

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Fig. 12 SWCNTs are biodegraded by an in vitro enzymatic system over time. The maximum values of the distinctive semiconducting (S2) peaks of SWCNTs present in recorded Vis-NIR spectra are used to calculate the level of biodegradation at different time points, copied with permission from Ref. [10], copyright, MPDI, 2016

The biodegradation of CNTs, GRA, and their derivatives has been studied using a variety of bacteria [9]. Some investigations have shown that bacteria can break down acid-treated MWCNTs in real-world circumstances, reducing their environmental stability [11]. According to some research, a bacterial population can co-metabolize C-labeled MWCNTs into CO2 in the presence of an external carbon supply. Based on genotypic analysis, Burkholderia kururiensis, Delftia acidovorans, and Stenotrophomonas maltophilia were promising microbial degraders. This finding suggests that interactions between microbes and MWCNTs may impact MWCNTs’ long-term fate [11]. Zhang et al. [11] reported that Burkholderia kururiensis, Delftia acidovorans, and Stenotrophomonas maltophilia were discovered as three possible star players in a microbial group’s mineralization of up to 6.8% of MWCNTs in just 1 week through co-metabolism with additional carbon resources. You et al. [74] reported that after 6 months, the white-rot fungus Trametes Versicolor or microbiota from polychlorinated biphenyl-contaminated sediments or aerated sewage sludge treatment mineralized less than 0.1% of SWCNTs. These disparities in degradation rates necessitate greater research into the microbial decomposition of CNTs. M. vanbaalenii PYR-1 likely mineralized some MWCNTs during microbial breakdown. PYR-1 respiration was examined (first ~4  109 cells) in the presence

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CNT

Microbes GRA

Trabusiella guamensis, Naphthalenedegrading bacteria etc.

Enzymes MnP, HRP, MPO, etc.

GRA derivatives: GO, GRA nanoribbons, fluorographene, graphyne, porous GRA, graphdiyne etc. CNT derivatives: SWCNTs functionalized with PEG, PEG and aminoanthracene, or PEG and aminofluorene, etc.

Intermediate products e.g., 2-methoxy naphthalene, 2naphthol, cinnamaldehyde, and isophthalic acid

Final products e.g., CO2

Fig. 13 Carbon nanotubes, graphene, and their derivatives: microbial and enzymatic degradation. Common examples include CNTs and graphene derivatives, bacteria, enzymes, intermediate intermediates, and ultimate products. A study gap has tied microbial degradation to enzyme degradation in many past investigations. MnP stands for manganese peroxidase, HRP stands for horseradish peroxidase, and MPO stands for myeloperoxidase, copied with permission from Ref. [9], copyright, Elsevier, 2017

of various carbon sources to test and quantify the potential mineralization of MWCNTs [74]. To obtain resistant bacteria, soil microorganisms were isolated from a mining site contaminated by nanomaterials (NMs) and enhanced in the presence of MWCNTs. Trabusiella guamensis was identified biochemically and genetically as resistant bacteria. In resistant bacteria exposed to MWCNTs, redox-enzyme activity and cell viability assays revealed molecular adaptability and minimal membrane damage. To explore the biotransformation in the structure of the resistant bacteria, they were allowed to interact with manufactured MWCNTs. The production of C O and COOH groups on the exterior walls of nanotubes was indicated by Raman spectra of biotransformed MWCNTs, which were also validated by FTIR data. Surface oxidation of CNTs was discovered in bio-transformed MWCNTs using XPS, XRD, and UV–vis. Images taken using TEM revealed structural alterations in concentric walls.

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Our findings showed that resistant bacteria were responsible for the biotransformation of MWCNTs via an oxidative process [95] (Fig. 14). Nanomaterials have been reported to be degraded by fungi as well as bacteria. The Sparassis latifolia mushroom, for example, can release LiP, which can destroy carboxylated SWCNTs that have been thermally processed as well as raw-grade SWCNTs. Furthermore, the white-rot fungus (Phanerochaete chrysosporium) has been widely used to break down lignin, PAHs, dyes, and other contaminants [9] (Table 3).

Fig. 14 Schematic representation of isolation and identification of NMs resistant bacteria from field to lab, copied with permission from Ref. [95], copyright, Elsevier, 2016

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Table 3 Degrading Microbes for CNTs, GRA, and their Derivatives, copied with permission from Ref. [9], copyright, Elsevier, 2017 Microorganism Naphthalene-degrading bacteria A bacterial community comprising Burkholderia kururiensis, Delftia acidovorans, and Stenotrophomonas maltophilia Trabusiella guamensis Sparassis Sparassis latifolia White-rot White-rot fungi (Phanerochaete chrysosporium

Taxonomy Bacteria Bacteria

Materials GO, graphite, and RGO MWCNTs

Bacteria Fungi

MWCNTs SWCNTs

Fungi

Trametes versicolor and natural microbial cultures

Fungi

MWCNTs SWCNTs SWCNTs, oxidized and reduced graphene nanoribbons SWCNTs

OH-SWCNTs

ox-SWCNTs

p-SWCNTs

Microbial degradation decrease Resistance increase

Fig. 15 The relation between microbial degradation and resistance

SWCNTs come in three forms (p-, ox-, and OH-SWCNTs). Macrophages could breakdown SWCNTs with defect sites (ox-SWCNTs and OH-SWCNTs), but p-SWCNTs were resistant to biodegradation due to poor reactive sites for oxidative attack; the rate of degradation is OH-SWCNTs > ox-SWCNTs > > p-SWCNTs. The use of PMA to activate macrophages could expedite or delay the biodegradation of SWCNTs. The in vitro enzymatic system that mimicked respiratory burst demonstrated that respiratory burst played a crucial role in speeding up the decomposition of SWCNTs [10] (Fig. 15). According to research, clean and minimally oxidized carbon nanomaterials are difficult to biodegrade when exposed to fungi or bacteria in contaminated sediments or aerated sludge. Because of SWNT’s long-term persistence in the environment, it is critical to understand what changes they could go through that would result in more hydrophilic breakdown products that are easier to transport and potentially more bioavailable. The most relevant researchers have confirmed drug administration potentially valuable as a purposely constructed mechanism for cell-based biodegradation after a medication has been delivered [96].

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Enzymatic Degradation In Zhao et al.’s [97] study, the presence of HRP and H2O2, the enzymatic degradation of carboxylated and nitrogen-doped MWNTs, were examined. Controlling the time of oxidative acid treatment resulted in different degrees of carboxylation on MWNTs, and the resulting degradation rate was linked to the degree of carboxylation, proving that it is the hydrophilic interaction between HRP’s heme active site and the oxygen-containing defective sites on nanotubes that causes them to be oxidized and degraded. GC-MS analysis of CO2 gas as a final oxidation product validated the deterioration. Furthermore, MWNTs are more resistant to HRP degradation due to their multilayer graphitic structures, and in the same experimental settings, MWNTs take substantially longer to deteriorate than SWNTs. The fact that MWNTs with smaller diameters and lengths remained stable after 80 days of disintegration suggests a layer-by-layer degradation mechanism, as revealed by TEM and Raman spectroscopy. The MWNTs are degraded at the faulty locations of the outer graphitic walls, which are exfoliated layer by layer, leaving the pure inner walls more resistant to HRP oxidation. Nitrogen-doped MWNTs, including intrinsic nitrogen-functionalized faulty sites in all graphitic walls, showed 100% enzymatic breakdown within 80 days, confirming the suggested process. SWCNTs have been demonstrated to be oxidized by myeloperoxidase. This enzyme’s CNTs degradation process was further examined and it was discovered that in vivo, the degradation was dependent on the generation of hypochlorite by myeloperoxidase. Electrostatic interactions between SWCNTs carboxyl groups and the protein’s Arg substances and p–p stacking interactions between SWCNTs and the protein’s Tyr substances were discovered to greatly enhance the decomposition of SWCNT, as the interactions increased myeloperoxidase production and hypochlorite formation. SWCNTs treated with poly(ethylene glycol) (PEG) molecules of various molecular weights could be degraded by myeloperoxidase. Finally, antioxidants like glutathione and ascorbic acid can stop myeloperoxidase from degrading CNTs. SWCNTs biodegrades in the presence of human eosinophil peroxidase and H2O2, in addition to myeloperoxidase. Because NaBr can reduce enzyme activity over time and stimulate the enzyme, it improves SWCNTs biodegradation. Lactoperoxidase, a secreted peroxidase enzyme present in the airways, has also been shown to degrade oxidized SWCNTs, both with and without the presence of a pulmonary surfactant. The authors of this study oxidized SWCNTs first, then used X-ray photoelectron spectroscopy to demonstrate the synthesis of oxidized SWCNTs. After that, they used ultra violet (UV)–visible light (Vis)–near-infra red (NIR) spectra, Raman spectra, scanning electron microscopy (SEM), and atomic force microscopy to perform biodegradation tests to explore the biodegradation chemistry of the oxidized SWCNTs. Surface modification (e.g., the insertion of carboxyl groups) is widely believed to be a precondition for CNTs biodegradation in the nanotechnology industry. They discovered that MnP from P. chrysosporium may damage pure CNTs using transmission electron microscopy (TEM), NIR spectroscopy, and Raman spectroscopy. Due to the carboxyl groups of oxidized SWCNTs adhering to Mn2+ at the MnP binding site, inhibiting the catalytic cycle between Mn2+ and Mn3+, which is required for MnP activity, MnP is unable to attack surface-oxidized SWCNTs [9] (Table 4).

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Table 4 Biodegradation of CNTs, GRA, and their derivatives by enzymes, copied with permission from Ref. [9], copyright, Elsevier, 2017 Enzymes Substrates CNTs, GRA, or their derivatives can be degraded by enzymes reported. Lactoperoxidase SWCNTs Horseradish peroxidase SWCNTs, MWCNTs, and GO Myloperoxidase SWCNTs and GO Xanthine oxidase MWCNTs Eosinophil peroxidase SWCNTs LiP S WCNTs, oxidized and reduced graphene nanoribones MnP SWCNTs Enzymes have been shown to be incapable of degrading CNTs, GRA, or their derivatives. Tyrosinase MWCNTs Laccase SWCNTs, MWCNTs

Allen et al. [98] showed SWCNTs biodegradation via natural enzymatic catalysis. They demonstrate the enhanced breakdown of nanotube structure by incubating nanotubes with natural horseradish peroxidase (HRP) and low quantities of H2O2 (~40 μM) at 4  C for 12 weeks under static conditions. These findings suggest that HRP can destroy CNTs in an environmentally relevant situation. They showed that HRP/H2O2 biodegraded CNTs over several weeks. This raises the possibility that HRP will degrade nanotubes in real-world settings. Other peroxidases found in plants and animals (e.g., myloperoxidase) may be helpful in the oxidative destruction of CNTs. In 2008, it was reported for the first time that at a low concentration of hydrogen peroxide, HRP causes SWCNTs to degrade. This finding revealed the possibility of degrading CNTs in the environment via biotechnological and natural methods [98]. In Seabra et al.’s [99] study, to remove remaining metal catalysts, SWCNTs were purified using H2SO4/H2O2, synthesized with carboxylic acid groups (diameter 517–372 nm, assessed by dynamic light scattering), and treated with HRP and hydrogen peroxide at pH 7. Following 2 months of incubation, the typical length of CNTs had fallen to 231  94 nm, and after the last treatment (4 months of incubation), there were no nanotubes visible. Both SWCNTs and MWCNTs decompose by the influence of the HRP/H2O2 system, other peroxidases such as myeloand lacto- or a model peroxidase system (hemin); Fenton solution (FeCl3), or even induced biological fluids [99] (Fig. 16).

Economic Cost of Carbon Nanotubes Degradation Biologically oxidizing graphitic materials is becoming increasingly crucial for practical applications as an environmentally benign and cost-effective technique. Bacterial oxidation of graphitic materials can make GO and degrade carbon nanomaterials in the environment [100].

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Air environment

Thermal degradation

Nitrogen

environment Methods of CNTs Enzymatic

degradations

Biodegradation Bacteria Microbial Fungi Fig. 16 Methods of carbon nanotubes degradation

Conclusions Carbon nanotubes are used nowadays in numerous fields such as industrial fields, medical fields, and environmental fields due to their unique properties. CNTs are characterized by their outer diameter ranging from 1:200 nm. The difference in diameter depends on the preparation method. CNTs are classified as SWCNTs and MWCNTs. SWCNTs have a single graphene layer, but MWCNTs have several graphene layers. Although the importance of CNTs is well known, they negatively affect the ecosystem. Some research reported that CNTs have toxicity in the lungs and cause pulmonary damage. This work provides some ideas on the importance of CNTs degradation. There are different methods for degradation, like thermal degradation and biodegradation. Thermal degradation may occur in an air environment or under a nitrogen environment. On the other hand, carbon nanomaterials undergo biodegradation by microorganisms and enzymes. Various bacteria are used to break down MWCNTs to reduce their environmental stability. The genotypic analysis identified Burkholderia kururiensis, Delftia acidovorans, and Stenotrophomonas maltophilia as promising microbial degraders. There is a relation between microbial degradation and the resistance of SWCNTs. By increasing the resistance microbial degradation

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increases. So, the degradation rate is OH-SWCNTs >Ox-SWCNTs>> p-SWCNTs. SWCNTs can also be degraded in the presence of human eosinophil peroxidase and H2O in addition to myeloperoxidase. Each type of CNT has a specific enzyme to degrade it. As an environmentally friendly and cost-effective approach, biologically oxidizing graphitic materials is becoming increasingly essential for practical applications. The potential for bacterial oxidation of graphitic materials to produce GO and degrade carbon nanomaterials in the environment is enormous.

Future Perspectives CNMs’ toxicity prevents them from being used in animals. Studying carbon nanomaterials’ degradation aims to minimize biological toxicity and make biomedical applications of carbon nanomaterials more accessible. Carbon nanomaterials will absorb compounds in the environment under natural conditions, and their degradability will change as a result. As a result, we must also investigate the degradation of CNMs in the presence of various pollutants to develop a highly efficient green degradation approach. The amount of waste produced is proportional to global economic development [101–104]. Environmental damage is caused by garbage, particularly synthetic polymer waste. As a result, the European Community has proposed a waste management concept based on two complementary strategies: reducing waste through better product design and enhancing waste recycling and reuse, with a focus on life cycle assessment (LCA) to generate transparent and complete assessments of the effect on the environment resulting from all stages of the product or activity’s life cycle and to use this information to assess it. The challenge of synthesizing green nanocomposites employing biodegradable polymers as part of the wave of nextgeneration materials has been spurred by the viewpoint of the future in terms of resolving waste treatment problems. Because of their degradability, biodegradable polymers have attracted researchers’ curiosity as an alternative to nonrenewable petroleum-based polymers. However, most biodegradable polymers have weak mechanical properties and low heat distortion temperatures, making them unsuitable for a wide range of applications. As a result, carbon nanotubes could be used as nanoreinforcements in biodegradable polymers to develop a variety of composite materials that have superior mechanical properties, durability, and thermal stability. The biodegradable polymer/CN nanocomposite’s quality is affected by CNT alignment, CNT-biodegradable polymer adhesion, and CNT dispersion in the biodegradable polymer matrix. The biodegradable polymer can be broken down by microbial or enzymatic degradation under certain pH and temperature conditions. After the CNTs have decayed, they may be used as reinforcing fillers in new composites. Carbon nanotube reuse and recycling could help to reduce waste while also making material processing more cost-effective.

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Biodegradation, Biosynthesis, Isolation, and Applications of Chitin and Chitosan

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Showket Ahmad Dar and Fahd Mohammed Abd Al Galil

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Characteristics of Chitin and Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Structure and Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitin Biosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isolation of Chitin at Industrial Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitin Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Significance of Chitin and Chitosan in Biomedical and Nanotechnology . . . . . . . . . . . . . . . . . . . . . Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wound Healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cancer Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitin- and Chitosan-Based Dressings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitin- and Chitosan-Based Applications in Ophthalmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antibacterial Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antithrombogenic and Hemostatic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antiaging Cosmetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antitumor Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vaccine Adjuvant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decomposition, Regeneration, Repair, and Damage of Cuticle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Chitin is a most abundant fibrous matter comprised of polysaccharide carbohydrates. Chitin is a dominant and main building material in the exoskeleton of many living organisms, including arthropods, crustaceans, fungi, and fishes. S. Ahmad Dar (*) Department of Entomology, Sher-e-Kashmir University of Agricultural Sciences &Technology of Kashmir, Srinagar, India F. M. Abd Al Galil Department of Biology, Faculty of Science, University of Bisha, Bisha, Saudi Arabia © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_72

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Chitin strengthens the exoskeleton, and enzymatic and chemical deacetylation by removing an acetyl group is converted into linear polysaccharide chitosan. In nature, chitosan is a carbohydrate acquired from the degradation of the hard skeleton of shellfish, arthropods, and crustaceans. Crustaceans and shellfishes contribute a significant proportion to total chitin used in the food-processing industry, besides having considerable applications in the biomedical field. The organisms that synthesize chitin employed a rigorous and complex enzymatic mechanism for degradation and body homeostasis. The enzyme uridine diphosphate-N-acetylglucosamine (UDPGlcNAc) is important for chitin synthesis; it brings out small chitin polymers, while a hydrolytic chitinase enzyme breaks down the chitin. In nature, the major biotic factor that degrades breaks down and mediates chitin hydrolysis is bacteria. Chitin and chitosan have multiple properties and features, including translucence, pliability, resilience, toughness, biodegradability, biocompatibility, innocuous film formation, revolutionizing the biomedical field. The emerging application of nanotechnology has utilized chitin and chitosan-originated materials to achieve innovations to transform the biomedical field. The multiple chitins and chitosan applications have contributed a major role in the polymer industry, especially in fabricating polymer scaffolds. Biomedical sciences face many challenges, and the major role chitin and chitosan played in terms of their nano-/microparticles and encapsulation of cargos are interesting. The uniquely designed nanocarriers and microencapsulation techniques are very interesting based on chitin-based materials for effectiveness in delivering drugs, biologics, and vaccines. The encapsulated drugs and nanoparticles are specific to applications, dimension, and cargo-release properties. Chitosan has been used effectively and efficiently in hydrogel solutions, nano-/ microparticles, drug and vaccine delivery, antibacterial, wound healing, anticancer, cancer diagnosis, chitin- and chitosan-based dressings, ophthalmology, antibacterial properties, antithrombogenic and hemostatic materials, antiaging cosmetics, antitumor activity, and vaccine adjuvant as customized biochemical properties; therefore it is one of the most critical, essential, and well-researched biomaterials. This book chapter is aimed to thoroughly discuss the biosynthesis, isolation, and applications of chitin and chitosan under various headings. Keywords

Chitin · Chitosan · Insects · Shellfishes · Nanoparticles · Drugs · Vaccines · Encapsulated Abbreviations

(GlcNac)2 ADP B cell BPN BTTG CD4+Th2 cell

(N-acetylglucosamine)2 Adenosine diphosphate B cells Block-copolymer nanoparticles British Textile Technology Group Th2 subset of CD4+ T cells synthesizing cytokines

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T cell CMCS CO2 COS CS CS CSN CTL DC DD DNA FA GH GH-18 GH-19 GlcN GlcNAc GlNac GPIa-Iia GPIb-IX-V GPIIa-IIIb GPIIb-IIIa GPVI HCPT HCPT HPLC IFNs IgA IgG IgN IL-13 IL-4 IL-6 IR LMPOs LPL M cells MAPK M-cell MHC-I and MHC-II MR NCBI

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The cluster of differentiation in T cell Cell-mediated cytotoxicity Carbon dioxide Chitosan oligosaccharide Chitin synthase Cytokine storm Chitosan glutamate Cytotoxic T lymphocytes Dendritic cells Degree of deacetylation Deoxyribonucleic acid Fatty acid Glycosyl hydrolases Glycoside hydrolase family 18 Glycoside hydrolase family 19 D-glucosamine, a 2-amino-2-deoxy-D-glucopyranose N-acetylglucosamine N-acetylgalactosamine Integrin alpha(2)beta(1)-very late antigen 2 GPIb-IX-V complex Glycoprotein IIb/IIIa integrin αIIbβ3 Immune-mediated thrombocytopenia Immunoglobulin receptor-very late glycoprotein antigen (VI) 10-hydroxycamptothecin Hyperosmolar conjunctival provocation test High-pressure liquid chromatography Type I interferons Immunoglobulin A Immunoglobulin G Immunoglobulin N Interleukin-13 Interleukin-14 Interleukin 6 function as pro-inflammatory cytokine and an anti-inflammatory myokine Infrared Lytic polysaccharide monooxygenases Lipoprotein lipase Mucosa-associated lymphoid cells Mitogen-activated protein kinases Microfold cells Major histocompatibility complex (MHC) class I and class II proteins Magnetic resonance National Center for Biotechnology Information

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NF-kB NIBRG-14 NK NMR PCL PDGF PELCL (PDGF) pH PHHYCN PLC PLGA (VEGF) QD RNA SARS-COV-2 STING-cGAS Syk T cell TMC TNF-α TNF-γ TXA2 UDPGlcNAc UDP-N UV VEC VEGF VSMC VWF ZnS

S. Ahmad Dar and F. M. Abd Al Galil

Nuclear factor-κB (NF-κB) National Institute of Biomedical Genomics-14 Natural killer Nuclear magnetic resonance Poly-E-caprolactone Platelet-derived growth factor Collagen/poly(L-lactic acid-co-ε-caprolactone) Power of H+ ion concentration which measures acidic/ basic response of medium Phosphatidylcholine hyaluronic acid chitin Phospholipase C, an assembly of enzymes lading to hydrolysis of phosphatidylinositol 4,5-bisphosphate Poly(lactic-co-glycolic acid Quantum dot Ribonucleic acid Severe acute respiratory syndrome coronavirus 2 Stimulator of interferon genes-cytosolic cyclic GMP– AMP synthase Spleen tyrosine kinase Cells originated from thymus N, N, N-trimethylated chitosan Tumor necrosis factor α Tumor necrosis factor γ Thromboxane A2/prostaglandin H2 Uridine diphosphate-N-acetylglucosamine glycosyltransferases Uridine diphosphate N-acetylglucosamine Ultraviolet Vascular endothelial cells Vascular endothelial growth factor Vascular smooth muscle cells von Willebrand factor Doped-zinc sulfide

Introduction Chitin which, according to the International Union of Pure and Applied Chemistry, is called β-(1–4)-poly-N-acetyl-D-glucosamine; it occurs extensively dispersed in nature [1] and is the second most abundant polysaccharide after cellulose [2]. Chitin, called covering, is present in nature as an organized macrofibrils and is an important systematic unit in the covering capsule of the crustaceans, crabs, shrimps, fishes, and fungi [1]. Usually, chitin is deacetylated into chitosan, which has a huge biomedical significance [3, 4]. Both chitin and chitosan are biological derivatives and have biocompatibility, biodegradability, and sustainability with nontoxic nature [5].

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Both biopolymers work as antimicrobial and hydrating agents [6]. In nature, chitin occurs in two major forms, namely α and β forms, characterized by infrared and solidstate NMR spectroscopy [7], together with X-ray diffraction [8]. The three chained γ-chitin (two chains in the same direction, third is in the opposite direction) is a third allomorph, an important structural component in exoskeleton described by many researchers [7]. All three chitin allomorphs vary in their bond orientation in individual microfibril units [7]. The biosynthesis of chitin is hydrolyzed by a wide range of enzymes in nature, called chitin synthase, a glycosyltransferase enzyme that is manufactured in the endoplasmic reticulum and present in all chitin organisms. This enzyme exists in every chitin-synthesizing organism. In living organisms, the enzyme chitin synthase is responsible for growing the polymer chain [9, 10] through many polymerization steps. The polymerization sequentially adds a single GlcNAc entity to the non-oxidized terminal end of the elongated chitin chain through transcriptional and non-transcriptional levels [9]. The linear polymers of chitin that are first obtained are voluntarily added into microfibrils with different diameters and lengths [11]. The whole fibrils are transported to the extracellular space [4]. Glycosyl hydrolases (GH) are a broad category of enzymes responsible for hydrolyzing polysaccharides. Further, the GH is categorized into 130 families subject to their similarities in amino acid sequences [12]. The dominant families include GH-18 and GH-19 recorded from many living organisms such as bacteria, fungi, insects, plants, and animals [13]. Chitin and its acetylated derivative chitosan have diverse applications in biomedical science. The prominent applications involved tissue engineering, drug and gene delivery, wound healing, and stem cell technology [14]. These biopolymers can be easily converted to essential other biomedical items such as hydrogels, membranes, nanofibers, beads, micro/nanoparticles, scaffolds, and sponges [15]. Tissue engineering is one of the emerging scientific fields where both chitin and its derivative chitosan have been dominantly used to make up polymeric frames [16], which later are used for tissue repair and regeneration either to replace or restore damaged tissues [17]. High porosity, maximum biodegradability, fast decomposition rate, organized and integrated structural nature, zero toxicity, and compatibility [18] with biological systems have made chitin a very important polymer in nature [19]. Loading chitin-based nanoparticles with various drugs such as lamivudine and 5-fluorouracil are just two examples of successful chitin and chitosan applications in drug delivery. Recently, semiconductor nanocrystals have been coated with chitosan for bio-imaging applications in cancer diagnosis. The enhancement of immune responses against pathogenic microorganisms by using chitosan derivatives as vaccine adjuvants has been reported [17]. The various other uses of chitin and chitosan shall be reviewed in the following sections.

General Characteristics of Chitin and Chitosan Greek word “chiton” gives birth to the name chitin, meaning etymologically as a coat of mail. Chitin is also an elongated polyplacophora bilaterally symmetrical marine mollusk having a hard dorsal shell of calcareous plates that are synonymous

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with chitin [7]. According to the biological dictionary, chemically, chitin is represented by formula (C8H13O5N)n, named poly(β-(1–4)-N-acetyl-D-glucosamine), which is a universal polycarbonate of huge significance [7] with a huge contribution of it to marine carbon and nitrogen that dominantly comes from marine organisms. The applications of chitin were earlier defined by Henri Braconnot in 1811 [20]. Chitin and cellulose are similar structurally; however, chitosan is a deacetylated form with higher solubility in acid solution [21, 22]. There are two general ways, including the enzymatic method and chemical hydrolysis, responsible for transforming chitin to chitosan; worldwide, this amount is estimated to be almost 1011 t annually [21]. According to Hahn [23], the annual production of crustaceans for various purposes was estimated to be 8 million tons, with 40% contributed by waste exoskeletons having a chitin content of 15–40%. In an experiment, Triunfo et al. [24] found that chitin’s material form looks like a white, hard, and rigid nitrogenous polysaccharide inelastic in nature. Chitin contributes much to beach pollution in coastal areas of seas [25]. The crystalline microfibrils of chitin form structural elements of arthropods’ exoskeletons and fungal cell walls, contributing rigidity and toughness [26], with other diverse functions and roles. The exact quantity and buildup of chitin are not estimated annually in oceans and earth, demanding that its accumulation, degeneration, quantity, and utilization be balanced. The content and omnipresence of chitin and associated hydrolyzing enzymes were found in many living creatures from land to water. The degradation of chitin in nature owes to the actions of chitinase enzymes (chitinolytic enzymes), which are dominantly occurring in nature [26]. Diversity in the occurrence of chitin suggests that it is present in fossils in huge content; for instance, in Pogonophora, there is evidence of fossilization of crystalline amber-like chitinous wing material. Over millions of years, the accumulation and preservation of this crystalline material suggested its insolubility in water under high temperatures. The chitin products and their role in provocation of immune system immunogenicity are nonsignificant but controversial despite being nontoxic. The higher insolubility and non-reactivity of chitin and chitinous materials in nature had suggested its high applications in biomedical sciences. Since chitin is similar in structure to cellulose, therefore the exchange of chemical groups and interchange of bonding elements such as the hydroxyl group at position C-2 has been replaced by an acetamido group [7], which has added exceptional features to chitin. Chitin is sometimes a cellulose derivative but is not present in celluloseproducing living organisms. There is no fixed and accepted IUPAC naming regarding the degree of nucleophilic N-deacetylation of chitin and its derivatives [27]. Ssekatawa et al. [28] suggested higher content of 6.89% of the nitrogen in chitin and chitosan in contrast to 1.25% in synthetically produced cellulose. Polysaccharides, for instance, e.g., cellulose (high pH stability), dextran (stability pH range 6–7), pectin (high stability at pH 4), alginic acid (high stability at pH < 7), agar (high stability at 4.5–9), agarose (high stable at pH 4), and carrageenan (high stability at pH 9) are neutral or slightly acidic to acidic in nature, in comparison to chitosan, a highly basic polysaccharide [26]. The specialty is also due to their properties, such as the formation of polyoxy salts, films, biocompatibility,

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biodegradability, non-toxicity, and absorption properties [29, 30]. Despite several reports describing the preparation of important utilized chitosan-based derivatives by physiochemical changes among the amino acid groups, few derivatives have acceptable solubility in general organic solvents or binary solvent systems comprised of only chitin and solvent systems [31]. Researchers worldwide have explored many derivatives possessing higher solubility in most organic solvents, such as acetic acid, I-glutamic acid, lactic acid, and succinic acid [31].

Chemical Structure and Properties Figure 1 shows sugar unites are linked and rotated at 180 among themselves, resulting into a disaccharide namely N,N0 -diacetylchitobiose [(GlcNAc)] [32]. Chemically chitin is poly-(1 ! 4)-β- N-acetyl-D-glucosamine [32, 33]. Each polymer chain is like helices, having inverted sugar units opposite in adjacent chains [34]. This structural configuration adds to higher stability as strong and rigid chains are bound by 03-H ! 05 and 06 ! 07 in a hydrogen-bonding pattern. In nature, the chitin occurs in three forms called crystalline morphs such as α-, β-, and γ-forms [35]. All three morphs differ in the orientation of individual microfibrils, with the common most being α-chitin as a structural component comprised of two N,N0 -diacetylchitobiose units designed in an antiparallel fashion [31]. Therefore, the antiparallel running of microfibrils is tightened together by 06-H ! 06 hydrogen bonds, arranged in a sheet-like structure bonded in 07 ! H-N linking. The orientation of -CH2OH groups is equivalent to orientations of half oxygen molecule, resulting in the formation of inter- and intramolecular hydrogen bonding [36]. The interchaining in C¼O ! H-N bonds form two amide groups; however, half of the

O

CH3

HO

O O

O

O

CH3

O

OH

NH

O

NH

HO

O

HO

OH

Chitin

O NH

O

OH

CH3

Deacetylation

OH

NH+3

HO O

O OH

O HO

O

CH3 NH O

O

O HO NH+3

Chitosan

O OH

Fig. 1 Chemical structure of chitin and its acetylated derivative. (Figure adapted and expanded from Boroumand et al. [40], Journal [Front. Cell. Infect. Microbiol])

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amide group work as a receptor as 06-H ! O¼C bonding. The amide groups are linked by hydrogen bonding and are held together tightly. The hydrogen bonding gives solid structure stability, and the polymer chains assemble into microfibrils by self-bonding, which later leads to crystallization. Among all morphs, the β-chitin structure is less abundant and rear with basic unit N, N0 -Di-acetylchitobiose. The α-chitin is comparatively rigid, touch, and more common having a similar structure as β-chitin with 03 ! 05 intramolecular H-bonds [37]. The opposite fibril chains are held together in an organized plane bed of C¼O ! H-N H-bindings between the amide groups through -CH2OH as antiparallel strands that lead to more bonding across H and the carbonyl oxygen of adjacent microfibrils through 06-H ! 07. With the consequences of bonding between oxygen and hydrogen molecules, the poly-N-acetylglucosamine results in the formation of sheets with inter-sheet H-bonds. This polymer chain in β-chitin allows for higher flexibility of antiparallel α-chitin, with higher toughness in the resultant polymer. The γ-chitin is the third allomorph, possessing mixed parallel and antiparallel orientations. γ-chitin has been reported to occur in mushrooms [37]. Chitin has cross-linking behavior in most structures except β-chitin found in diatoms [37]. The bonding structure of chitin is found strong with glucans in fungal cell walls, for instance, in Candida albicans. While, among organisms like arthropods and other invertebrates, there is particular linking with proteins either by covalent or non-covalent bonding. This association means it produces the observed ordered structures in the chitin of insect cuticles. Mineralization links phenolic and lipid molecules, especially calcification and sclerotization [38]. In fungi and other invertebrates, a different extent of deacetylation resulted in specific chitin structures with full acetylation and chitosan molecules with full deacetylation [38]. Deacetylation of chitin occurs in two chemical ways, both by acids or alkalis; the acids are more effective at breaking and destroying the glycosidic bonds in chitin fibrils; however, alkali deacetylation is more common [4]. Fatima [39] found that N-deacetylation in chitin occurred through heterogeneous and homogeneous ways, and the distinction between chitin and chitosan with different degrees of deacetylation is not strict. In nature, the polysaccharide chitin occurred in bounding form with other polymers to the extent of 50% of the total mass of tissues. The N-deacetylation of chitin makes it soluble in various media, especially in dilute acetic acid and formic acid [40]. Chemical structure of chitin and its deacetylation (Fig. 1) and the corresponding enzymatic pathways are discussed in Fig. 2. The extent of deacetylation in chitin and chitosan is either complete or incomplete, varying from 0.9 to more than 0.65 degrees. However, new methods of determining the deacetylation range, including X-ray power diffraction, showed deacetylation varying from 17% to 94% DD in α-chitin and chitosan, respectively. Many advanced scientific tools and analytical procedures were used for evaluation of degree of removal of acetyl groups (deacetylation) from chitin fibrils, which included infrared (IR) spectroscopy (interaction of IR with matter through absorption, radiation, and reflection), pyrolysis gas chromatography (determination of involatile compounds), gel permeation chromatography (analysis the material on basis of size), UV-vis spectrophotometry (for absorption

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CREUSTACEAN SHELLS

ORGANIC ACIDS - PRODUCING BACTERIA

Demineralization Deproteinization

PROTEASES - PRODUCING BACTERIA Chitin deacetylase (EC 3.5.1.41)

CHITOSAN

CHITIN reductant and O2 or H2O2

LPMOs (EC 1.14.99.53-56)

Chitinase (EC 3.2.1.14)

(GlcNac)n

Chitin deacetylase (EC 3.5.1.41)

Chitosanase (EC 3.2.1.132)

(GlcN)n

Chitooligosaccharides deacetylase (EC 3.5.1.105) b -N-acetylhexosaminidase (EC 3.2.1.52)

(GlcNac)2

GlcNac

Chitosanase (EC 3.2.1.132)

(GlcN)2 Exo-1,4-b-D-glucosaminidase (EC 3.2.1.165)

GlcN

Fig. 2 Enzymatic pathways and chemical structure of chitin and its derivative. (Figure adapted and expanded from Jung and Park [41], Journal [Mar. Drugs]). GlcN D-glucosamine, a 2-amino-2deoxy-D-glucopyranose, (GlcNac)2 N-acetylglucosamine, GlNac N-acetylgalactosamine, LPMOs lytic polysaccharide monooxygenases

spectrum in UV and IR wavelengths) [42], 1H NMR spectroscopy (structural determination), 13C solid-state NMR (determine atomic environment), thermal analysis (determine the properties of material sensitive to temperatures), various titration schemes (concentration of analyte), acid hydrolysis (nucleophilic substitution to using water), HPLC (separate, identify, and quantify individual constituents), separation spectrometry methods (2D-electrophoresis using soft-ionization), and more recently near-infrared spectroscopy using near electromagnetic spectrum of range 780–2500 mm wavelengths [43].

Chitin Biosynthesis Chitin biosynthesis and cellular processing [44–46] is based on twisted, intricate, diversified, and interlinked steps of development starting from inside the cell and terminates with the inclusion of the structure of chitin in the outside of complex biomolecules (proteins, lipids, fats, carbohydrates), from arthropod integument especially cuticles and fungi cell walls [47, 48] (Fig. 3). The whole process encompasses several individual steps: (a) Sequential biotransformation is similar to

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mineralization (with CO2, NH4+, and H2O as main end products) of sugars (mainly trehalose or glucose). The biotransformation comprised chemical interactions with significant steps, for instance, phosphorylation, amination, and formation of the enzymes substrate, UDP-N-acetylglucosamine [49]. (b) Chitin synthase (CS) synthesizes the chains. Chitin synthase enzyme plays an essential role in protein and carbohydrate cluster formation using chemically related molecules and bindings to pack molecules [49]. This structure improves the coalescence and merging of new chitin units to form a crystalline fibril. (c) Orientation and structural arrangement of chitin units having long chains. (d) One-dimensional activated process comprises entropic barriers known as polymer translocation that works across the plasma membrane of cells. (e) Hydrogen bonding results in crystallization and microfibril interchain formation. (f) Coupling and an association with insect cuticular proteins and fungal walls by linking to other carbohydrates [50]. The significant reservoirs of chitin synthetase in fungi are called chitosomes, small intracellular cytoplasmic microvesicles of size 116 nm with enzymatic activity in the zymogenic form [51]. These microvesicles have been identified through highlevel electron microscopy employing fungal cell walls and other cellular systems [52]. Content of chitosomes in cells at the hyphal tip implies their crucial role in CS trafficking and transport across members and to predetermined locations [53]. Chitosomes originate from organelles such as the endoplasmic reticulum, Golgi bodies, and ribosomes. Chitosome structures are storehouses of zymogenic CS clusters, which later on get highly activated once fusion of the chitosomes with the plasma membrane occurs. CS insertion into plasma membranes followed the targeting and recognition of specific proteins. The exact process of such vesicular fusion with proteins had remained a matter of debate in the scientific world. The structures like chitosomes are found in the cell-free system of insects. However, Fig. 3 Chemical structure representation of synthesis of chitosan from chitin. (Figure adapted and expanded from Hares et al. [48], Journal [Carbohydr.Polym])

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these chitosomes are chitin fibril production [53]. Moreover, in the epidermis of arthropods (insects), the structures like vesicles have not been described so far [53]. Chitin is synthesized and arranged in different forms depending on enzymes. The most important enzymes involved include chitin synthase (UDPN-acetyl-Dglucosamine: chitin4-β-N-acetylglucosaminyl-transferase) [54, 55]. This highly conserved enzyme is expressed in every organism possessing chitin-synthesizing activity. The enzyme CS utilizes UDP-N-acetylglucosamine (UDP-GlcNAc) to an activated sugar contributor to produce the sheet-like multi-fibril polymer. In insects, chitin biosyntheses start with trehalose comprised of eight different enzymes. In this regard, the first work was proposed by Candy and Kilby [56, 57] explained that chitin synthesis in insects started with glucose (trehalose) and terminated with UDP-GlcNAc. The enzymes UDP-N-acetylglucosamine were utilized to determine the enzymatic pathways in armyworm (Spodoptera eridania). Further, Schmid et al. [57] found that insects, crustaceans, and arachnids rely on chitin synthesis to undergo metamorphosis. Since, the process of partial synthesis of chitin in insects had great significance and applications in IPM as growth regulators. However, the nontarget organisms of chitin synthesis inhibitors, including natural enemies and pollinators, suffer from population decline and therefore have a negative influence on all insect biodiversity.

Isolation of Chitin at Industrial Level Crustaceans are major sources of chitin [58], and therefore, the isolation and extraction from organisms such as crayfish, crab, shrimp, and fungi is a timeconsuming process [59]. It requires 17–72 h including 1–24 h of treatment with HCl and 16–48 h of NaOH processing [60, 61]. Lengthy procedures for chitin isolation require more energy and in return increase the cost of production. Like crabs and shrimps, barnacles also belong to the crustacean family. The shell structures of barnacle species are less crystalline, and they are reported to have abundant minerals compared to other members of the crustacean family. Generally, the exoskeleton composition of crustacean includes 30–40% proteins, 30–50% minerals (calcium carbonate), 20% of chitin and other compounds (astaxanthin), and some percent of lipids [12]. Among various minerals present, the calcium carbonate, calcium phosphate, and calcite are dominant [12]. The minerals are obtained from the carapace using HCL, since in crustaceans, the protein is also present in the carapace and occupies maximum space but bound weakly; therefore, chitin is obtained with ease because of low and thin shell crystalline structure, e.g., barnacle species. This suggests a quicker removal of chitin from barnacle species due to weaker shell structure [62]. The barnacle Chelonibia patula is found to be present epizoically on other sea animals or rocks especially when host is present in shallow water. The most common animals’ barnacle use includes turtles, crabs, whales, and mollusks [63]. In Chelonibia patula, the procedure for the isolation involved demineralization of chitin in 1 M HCl for 10 min followed by deproteinization in

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2 M NaOH for 20 min till complete breakdown from shell and isolation. Completion of this whole process consumes only half an hour. It starts with dripping 1 M HCl solution onto 10 g of shell remains and dust of C. patula followed by a fast stirring for 10–15 min by means of a magnetic stirrer at room temperature. If the HCl is added too quickly, a vigorous effervescence occurs that may lead to overflowing. The samples then should be washed out using a neutral medium (pH > 7.0). Considering that the maximum proportion of original mass of crystalline shell consisted of minerals isolated by acid hydrolysis process using hydrochloric acid. Other constituents like proteins and lipids are isolated by a process of deproteinization. In barnacles, the comparative proportion of chitin by weight (3.11%) compared to shell is higher; however, the protein content is less [64]. The chitin obtained from shells of shrimps is associated with various editable items. The shrimp processing adds various chitosan-glucan nutritious complexes obtained from the mycelia of many fungi. Among fungi, Aspergillus niger producing citric acid is used in shrimp processing as a fermentation agent. Generally, at the industrial scale, the removal of protein and calcium carbonates after acidic dissolution from crustacean shell is obtained at higher concentrations [65, 66]. Protocols for procuring chitin derivatives and chitosan comprise deacetylation at 40% NaOH at 120C for 1–3 h, yielding 70% of chitosan. Recently, a “green conversion” of agroindustrial by products and house and farm wastes using Rhizopus arrhizus [67] and Cunninghamella elegans strains has been reported to produce chitin and chitosan [68]. Such industrial chitin-based sources have significant advantages, including prevention of allergic reactions from agents most vulnerable to shellfish antigens and decline in duration of production and processing [69].

Chitin Degradation It is believed that bacteria are the major mediators of chitin degradation in nature [70, 71] (Fig. 4). Their role can be demonstrated in both soil and water systems. In soil systems, the chitin hydrolysis rate correlates with bacterial population and abundance. The degradation in this case depends on factors such as temperature and PH. Not only the bacteria but also fungi may be considered quantitatively important agents in the chitin degradation process. The results of experiments in aquatic systems demonstrated convincingly that bacteria are the main mediators of chitin degradation [72–75]. Dense fungal colonization of the carapaces of chitinous zooplankton has been observed, and some diatoms were found to be able to hydrolyze chitin oligomers [76]. Furthermore, enzymes released during molting of planktonic crustaceans are believed to be another source of enzymes that can metabolize chitin in aquatic systems. The degradation of chitin is a highly regulated process, and the hydrolytic enzymes have been found to be induced by the actual products of the chitin hydrolysis (GlcNAc) and soluble chitin oligomers (GlcNAc). Unlike (GlcNAc)-2, the enzyme GlcNAc is found to overcome the expression of enzyme chitinase in bacteria Streptomyces [77]. This suppressor activity could be connected with GlcNAc occurring in the murein found in fungal cell walls rather

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Fig. 4 Degradation of shrimp shell by bacteria under different treatments such as before sonication (liquid with shrimp shell) represented by long dash lines, after sonication (liquid with shrimp shell) shown by solid lines, and third without shrimp shell (sea water) represented by short dash lines recorded different levels of proliferation and linking of bacteria Pseudoalteromonas rubra S4059 wild and mutant GH19 (black) on marine shrimp shell. (Figure adapted and expanded from Raimundo et al. [70]. Journal [Microbiome]). Note: X-axis showed duration of degradation under different treatment effect; Y-axis represents log10 of degree of degradation under given treatment

than in chitin. Despite the insoluble nature of chitin and its relatively high resistance to degradation because of its complex configuration, many bacteria from soil and marine ecosystems like Bacillus circulans WL-12 [78], Serratia marcescens [79], Streptomyces coelicolor A3, Aeromonas caviae, Pseudoalteromonas spp., strain S91, and Vibrio harveyi [80] are able to degrade, transport, and utilize chitin as an energy source through the action of chitinases. The Paenibacillus spp. strain FPU-7 was found responsible for degradation using the enzyme chitinase from intercellular and those present immediately on the cell surface [81]. The bacteria Paenibacillus completely hydrolyze chitin flakes from spine-like armored crab shells. Possible chitin degradation pathways have been discussed by Ibrahim and El-Zairy [82]. The term chitinoclastic is used when process of degradation pathway is not exactly clear, whereas it is best termed chitinolytic in which process comprised of starting hydrolysis of (1 ! 4)-β-glycosidic bond linked through carbon atoms having different stereochemistry. In chitin, the alpha and beta confirmation is best defined by position of main building units called Haworth projection. In all chitin materials, the breakdown of -OH molecule called hydrolysis works well using an enzyme chitinase [83]. In cells, the exo-chitinase (outer-surface chitin) enzyme breaks diacetylchitobiose units using nonreducing a terminal end of chitin. In living

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organisms, the enzyme chitinase is broadly classified into endo- and exo-chitinase [84], which, if work together, enhance the chitinolytic reaction [85], cleaving glycosidic bonding in a random and uneven manner throughout the length of chitin string [86]. The common product of this enzyme is diacetylchitobiose, associated with triacetylchitotriose. Disaccharide and monosaccharide units are obtained after degradation of triacetylchitotriose which is chemically beta-D-GlcpNAc-(1-> 4)beta-D-GlcpNAc-(1-> 4)-beta-D-GlcpNAc. The triacetylchitotriose has a linear configuration of amino trisaccharide comprised of three N-acetyl-beta-D-glucosamine residual groups which are linked (1-> 4) [87]. The chitin degradation by enzymes strongly depends on the nature of the substrate. The prominent example of this enzyme is Streptomyces chitinase that degrades the pure crystalline β-chitin of diatom spines from its end yielding only diacetylchitobiose [88], while colloidal chitin is degraded to oligomers and disaccharide [88–100]. Researchers found that the higher secretory production by Bacillus subtilis of diacetylchitobiose deacetylase, in addition to molecular engineering, is responsible for enhanced catalytic performance to produce chitosan oligosaccharides and monosaccharides.

Significance of Chitin and Chitosan in Biomedical and Nanotechnology In the current era of advanced science and technology, a parallel and rapid advancement in using various bioactive materials in various fields of science has occurred. The naturally occurring bioactive materials are present in varied structures and compositions hinged upon ceramic, metallic, or polymeric materials. These materials induce different specific responses in tissues in a compatible manner. Therefore, there is an increasing demand and scope for applications in manufacturing and preparation of these materials, possessing very compatible physical, biological, and mechanical properties, besides being biodegradable in nature to protect the ecosystem. The most important among all bioactive materials is chitin and its acid derivative chitosan, having exceptional qualities. In Table 1, we highlight briefly the various applications of chitin and chitosan in medical and allied fields.

Tissue Engineering Tissue engineering is defined as the manipulation of living cells followed by their substitution into body of target organism using combination of cells, methods, and biochemical and physiochemical factors for restoration, improvement, and maintenance of target organism. The tissues are manipulated, controlled, and customized through their extracellular environment to design and fabricate tissue as the substitutes for implantation into an organism. The main purposes of tissue engineering are categorized as to restore, replace, maintain, or enhance and improve the functions of specific target (tissue or an organism). Mostly the chitinous materials are fabricated into tubular forms and are successfully used in tissue engineering of nerves and

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Table 1 Applications of chitinous scaffolds/fibrils incorporated with naturally occurring polymers, bioactive and other biocompatible components in tissue engineering Chitin/chitosan scaffold/fibrils Chitosan/gelatin

Various application Applied for skin treatment and bone, impart strengthen to epidermal membrane, increase tensile strength, thermal and heat stability, control degradation, and cell culture breakdown Chitosan/gelatin Applied as an antibacterial agent, hemostasis, wound healing, tissue regeneration. Used in blood vessel, for increasing thermostability, mechanical and tensile properties CollagenScaffolds, biocompatible glycosaminoglycan- coatings, cosmetics, and chitosan as porcine oral mucosa for facial trauma or malignant lesion surgery, as regenerative medicine Collagen-chitosan/ fibrin glue

Chitosan/collagen

Chitosan/collagen

Specified effects Cell survival, shape, viability, and crate cell wall

Reference [24]

Cell growth, formation of carbohydrate skeleton, activation and attenuation of immune system Cell migration and activates inflammatory cells to enhance the cell immune system

[100]

Cell differentiation, proliferation in fibroblasts and keratinocytes. Extracellular matrix production, deposition and maintenance of extracellular matrix, formation of antheroprone, wound repairing, and signaling Tissue engineered skin Cell growth in three-dimensional (asymmetric scaffolds in (3D) aspects, cell proliferation, skin tissue), as porous nutrition metabolism, and membrane with chitosan- extracellular matrix secretion fibrin glue, construction of tissue adhesives Skin (improved Cell proliferation, improves surface biostability for skin property, pore size, porosity, and tissues) -construction of connectivity of pores, and cell fibroblasts and a human infiltration keratinocyte line (HaCaT), for reduced intensities in the –OH and amide groups Wound dressing, adipose Vascularization and tissue (glutaraldehydeneovascularization induction, crosslinked collagenadipose tissue formation. Induction chitosan) – of invasion to endothelial cells by epididymal fat pads cells basement membrane in tumor cells. and subcutaneous pocket Inhibition of antimetastatic sulfated of male Lewis rat, chitin and inhibition of SCM-chitin biocompatibility, III. No effect on cell viability and

[90]

[96]

[91]

[67]

(continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

antimicrobial, thermoresponsive, tensile strength, and elongation Chitosan/collagen Blood vessel (porous collagen/chitosan scaffold for vascular tissue), increase the number of fibroblasts, vascular graft, high suitability, fabricate, poly-L-lactic acid/ chitosan/collagen as electrospun tube ChitosanLigament (chitosanhyaluronan based hyaluronan hybrid polymer fibers), drug delivery, wound healing, tissue design, and physicochemical properties Chitosan/silk Musculofascial (silk fibroin fibroin and chitosan blend scaffolds), films as biomaterial, nanohydroxyapatite composite scaffold, matrix as a biosensor, scaffolds, nanoparticles, microparticles, hydrogels Chitosan + Hybrid polymer fiber, hyaluronan drug delivery, wound healing and tissue engineering, bacteriostatic, fungistatic, and applied in tumoral extracellular as treatment Collagen-chitosan + Skin tissue, asymmetric fibrin glue porous scaffold, porous membrane with fibrin glue, tissue adhesives, chitosan -tripolyphosphate nano/microfibrous scaffolds for tissue engineering Chitosan + alginate Polyelectrolyte multilayer film, drug interaction behaviors, dermal wound healing,

Specified effects

Reference

the growth of tumor cells and endothelial cells in vitro Induces cell adhesion, proliferation, and ECM production

[40]

Activates and increases cellular adhesion, proliferation, and ECM production

[69]

ECM deposition, production, and maintenance Increases vascularization that improves oxygen and nutrient supply to tissues. Cellular infiltration, migration, extension, and multiplication

[70]

Strengthen the ligament by formation of fibroblasts from the patellar tendon, especially in the Japanese white rabbit

[59]

Formation of human dermal fibroblasts and keratinocytes and formation of skin epidermis

[61]

Strengthen muscle tissue. Also activates the alginate-polyurethane scaffolds and fibrils that support in skeletal muscle tissue formation

[63]

(continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

Specified effects

Reference

Involved in skin regeneration, healing burns, anti-inflammatory, protective layer formation, and wound protection

[73]

Improves growth of salivary gland, epithelial-mesenchymal interactions, and basement membrane (BM); activates collagen, laminin, heparan sulfate proteoglycan and their respective receptors. Promotes salivary glands branching

[20]

Helps in growth of adipose tissue proliferation, enhance microstructure, pore size, bibulous ability, water content, interval porosity, enzyme degradation and affinity, pluripotency, adipose tissue formation from stem cells (ADSCs), adhesion, expansion, and maintenance of pluripotency Chitosan + Applied in bovine Strengthen the cartilage, cell polyester articular chondrocytes, attachment, colonization, controlled extracellular matrix formation, biodegradability, cytoproteoglycans and collagen type II compatibility, production, enhancement of microporous structures, (glycosaminoglycan) GAG and excellent mechanical production properties; form strong, tough, and functional scaffolds with potential applications in cartilage tissue engineering Chitosan + collagen Form cross-linked porous Form cartilage, reduced cartilage + genipin membrane, cartilage and pore size, degradation rate, and bone tissue engineering, swelling ratio. Increases crossalternative to collagen, linking, glycosaminoglycans, and tissue fabrication, differentiation of chondrocytes

[33]

dressing, coating, drug release, natural polymer Chitosan + aloe vera Blended membrane (BM), wound healing, have efficient suitable biological and physicochemical properties for wound dressings and crosslinking applications in scaffolds Chitosan Embryonal submandibular gland cells, embryonic stem cells (buffalo, mouse embryos), thermoresponsive, neural differentiation, scaffolds for stem cell engineering, 3D cell culture and in nanomaterials for therapeutic applications Chitosan + collagen Epididymal fat pads cells and subcutaneous pocket of male Lewis rat, antimicrobial, wound dressing, wound healing, and as nanofibrous

[49]

[58]

(continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Chitosan + chondroitin sulfate

Various application artificial skin, and tissue regeneration Chondrocyte and human mesenchymal stem cells culture, cartilage tissue engineering, nanocomplexes for HIV-1 infection inhibition, complex microcapsules and controlled release of 5-fluorouracil, fabricate scaffolds, and ultrasound-triggered drug release

Specified effects

Chitosan enhances the cartilage growth by increased expression of cartilage extracellular matrix (ECM) proteins in chondrocytes. Cartilage membranes and chondroitin sulfate enhances water and salt absorption, and reduces the stiffness. Chitosan and chondroitin sulfate sustain the adhesion of ATDC5 pre-chondrocyte cells, and increases the cellular aggregations and chondrogenesis through increased expression of cartilage markers Chitosan + adipose- Sprague-Dawley rats Accelerate therapeutic potential derived stem cells sciatic nerve transaction, and facilitate nerve regeneration. treat ischemic tissue, cell Heals the spinal cord injury. migration, co-culturing Improves neural tube regeneration fibroblasts, adiposeand neural induction derived stem cells encapsulation, biomimetic matrix, and regenerative medicine Chitosan alone Beagle dogs (males) Improves nerve fiber regeneration, phrenic nerve resection, therapeutic potential, and nerve peripheral nerve regeneration regeneration, nerve defects, peripheral nerve repair (Bungner bands), graft, and guide. Spinal cord repair and nerve tissue engineering Chitosan alone Female minipigs thirdProduces hybrid and porous nerve degree burns, functional chitosan-γmaterial to prevent or glycidoxypropyltrimethoxysilane treat wound and burn (chitosan-GPTMS) membranes. infections, antimicrobial Improves microfibril regeneration, properties, wound heals axonotmetic and neurotmetic healing, antibacterial sciatic nerve injuries. Increases the hydrogel, silver regeneration of myelinated nerve nanoparticles, wound fibers and myelin thickness dressings Chitosan + silk Female guinea pigs Helps in nerve reconstruction, fibroin ventral hernia, bridging regeneration and healing of injured sciatic nerve gaps, form sciatic nerve. Increases the biocompatible restoration of nerve continuity and

Reference

[41]

[55]

[39]

[34]

[28]

(continued)

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Biodegradation, Biosynthesis, Isolation, and Applications of Chitin and Chitosan

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

membranes, scaffolds, composites, prevent vessels damage, and sciatic nerve injury. Chitosan + -sodium Articular defect (male glycerophosphate + and female), high and hydroxyethyl controlled cellulose (CH-GP- biodegradability, HEC) cytocompatibility, microporous structures, and posses excellent mechanical properties, cartilage engineering, thermosensitive, liver transplant, mesenchymal stem cells Chitosan + calcium Applied as cementing phosphate cement paste (chitosan microspheres), least biodegradability, high cytocompatibility, microporous structures, cartilage tissue engineering, efficient flexibility, efficient siRNA delivery, osteogenic differentiation, bone void fillers, bone-mimicking effect Scaffold Application Chitosan Periodontal tissue hydroxyapatite (bFGF-loaded HA-chitosan), nanocomposite for bone tissue engineering, in vitro bone cell models, nanotube membranes, mechanical properties, mimicking osteoblast, bone regeneration

Chitosan hydrogel

Wound healing (controlled releases of

Specified effects

Reference

function recovery in target skeletal muscle which are highly reinnervated Improves chondrogenic differentiation, form higher scaffolds, and had higher therapeutic potential

[39]

Helps in in-situ hardening of cartilage, improves controlled release of bioactive agents and proteins, boosts the cell proliferation, differentiation, and provides protective effect from moxifloxacin HCL toxicity

[11]

Effect Forms cellular structure, increases cell proliferation, mineralization, enhance attachment and proliferation of mescenchymal stem cells, stimulation of osteoblasts, work as bone substitution Helps in an efficient wound

Factor FGF2

[34]

FGF2

[40] (continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

Specified effects

FGF-2 and paclitaxel from chitosan hydrogels and their subsequent effects on wound repair, angiogenesis, and tumor growth), wound dressing, hemostatic, thermally responsive, antibacterial

healing, treatment, and provides safety from new infections. Accelerates tissue regeneration, fast cell growth, and removal of wound exudates. It has high hemostatic effect that helps in inhibition of microbial growth and thereby improves wound healing The implanted collagenchitosan bioactive microfibrils activate the higher vascular growth and proliferation, thereby helps in more subcutaneous attachment of tissue. It improves the physical strength of collagen, support endothelial cells, and therefore angiogenesis for use heart vesicles Stimulates and increases

Chitosan hydrogel

Vascularization induced by FGF-2-incorporated chitosan, cell therapy, cardiovascular tissue engineering, high durability, cell retention, neovascularization, vasculogenesis, angiogenesis, blood flow recovery in ischemic hind limbs, cartilage tissue engineering

Chitosan

Wound healing (fibroblast growth factor),

Reference

FGF2

[48]

FGF2

[75] (continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

Specified effects

functional material, hemostatic effect, membranes, hydrogels, fibers, sponges, burn therapy, tissue regeneration, stimulates hemostasis

fibroblast proliferation at higher degree of chitosan deacetylation. Increases vasculogenesis and expression of HUVECs cell growth factor Responsible for keratinocyte differentiation, wound healing, adhesive properties, and increases and improves the epidermal differentiation in injured tissues and wounds Improves wound healing by biomimetic and chemotactic effects, releasing Nacetyl glucosamine slowly that helps in growth of dermal tissues. Chitosan promotes fibroblast proliferation, collagen deposition, activates hyaluronic acid biosynthesis Supports anatomical and

Chitosan-pluronic

Wound healing (pluronic/ chitosan possess epidermal growth factor), wound adhesive, photocross-linking, tissue remodeling, granulation, collagen deposition

Chitosan-collagen gel

Wound healing by using PDGF – novel gel for wound healing¸ inhibit microbial growth, membranes, hydrogels, fibers, sponges

Collagen-chitosan/ silicone

Vascularization, angiogenesis, dermal

Reference

EGF

[71]

PDGF

[21]

VEFG

[22] (continued)

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Table 1 (continued) Chitin/chitosan scaffold/fibrils

Various application

Specified effects

Reference

gene activation, porcine model, silicone membrane, full-thickness skin defects, injectable hydrogels applied in field of tissue engineering

Methacryl amide chitosan scaffold

Chitosan/collagen

physical structure and vasculogenesis of cells to increase angiogenesis. Preservation of cellular phenotypes, the binding and enhancement of bioactive factors, controls gene expression, synthesis and deposition of extracellular matrix, scaffold regulation. Tissue reconstruction, supporting nerves and vascular tissues Nerve regeneration, Neural immobilization, neuronal progenitor cell differentiation, differentiation, macroporous scaffolds enhance employed in cell culture, oxygen cell adhesion, crossconcentration, linking, drug delivery, maintain amine coupling, scaffolds oxygen mimicking gradient, cell proliferation and growth Periodontal tissue, Improves cell chitosan/collagen regeneration, scaffold, growth factor- formation and beta1 DNA, chitosan/ proliferation of collagen films, collagen, periodontal tissue formation of engineering, stem cell extracellular culture, cartilage, bone matrix, tissue regeneration, migration and tensile strength, sponge, differentiation wound dressing of osteoblast cells

IFN-γ

[28]

TGF-β1

[18]

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blood vessels as a template. The fibril scaffolds from chitin are flexible, multifaceted, high viability, and proliferated product, and are optimized for diverse reanimation and rejuvenation causes (Fig. 5). In various techniques of tissue engineering, the fabrication of polymer fibrils using chitin and chitosan has shown great success and has been applied successfully. Various initial perquisites to design polymer scaffolds include high porosity (suitable pore dimensions), biodegradability (breakdown rate should equate with rate of neo-cell production), integrity of designed (to withstand and prevent the chitin fibrils and scaffold); harmless to tissues, biocompatibility (behavior of chitin once subjected to multiple biotic factors), and improvement in cell adhesion and function (proliferation, migration, and differentiation). There are many procedures employed to produce chitosan scaffolds including: phase separation and lyophilization technique, particulate leaching techniques, freeze gelation technique, rapid prototyping technology, and formation of microparticles and microspheres. The gas foaming technique involves creating synthetic matrices that avoid the use of solvents, without changing the bioactivity of the molecules. Technique employs the utilization of chitosan solution comprised of glutaraldehyde as

Fig. 5 Multilayered graft of PCL/gelatin layer (outer), PLGA/gelatin + PDGF (middle), and PLCL/gelatin + PDGF (inner). The layers support high porosity and growth (Figure adapted and expanded from Han et al. [72]. Journal [Biomaterials]). V1 and V2 electric voltage connection, PCL poly-e-caprolactone, PLGA (VEGF) poly(lactic-co-glycolic acid, PELCL (PDGF) collagen/poly (L-lactic acid-co-ε-caprolactone, VEC vascular endothelial cells, VSMC vascular smooth muscle cells, VEGF vascular endothelial growth factor, PDGF platelet-derived growth factor

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cross-linker, supersaturated with CO2, and high pressure responsible for making huge cross-linking in polymer. Gas bubbles are generated; for example, by using NaHCO3 which work as blowing agent, which on chemical degeneration, prepolymer produces gas bubbles. Once system pressure is reduced below normal level, nucleation and gas bubble production is observed due to thermodynamic instability or due to formation of new instable thermodynamic phase. Pores are generated while the bubbles escape the polymer solution. Freeze casting, also known as freeze gelation technique, is a process in which gel turn into ceramic product. This technique starts work with phase separation because of deep freezing. Chitin scaffold produced in the process are put into solution of NaOH and C2H5OH at the temperature below the chitin freezing point. After gelation, the air-drying of scaffold involves the complete removal of moisture. The chitosan polymer fibrils and scaffolds are further manufactured by fusion or compacting the solid mass of scaffold using heat and pressure on chitosan microspheres [72] (Fig. 6). Zhou et al. [37] observed that electrospun is gaining huge demand for production of effective antibacterial wound-healing dressings, for instance, polycaprolactone, chitosan oligosaccharides, and Qe/Rutin. Further, the electrospun has emerged as the latest technique in which nanofiber scaffolds work as an important and potential alternative for tissue autologous grafts. The technique has huge significance in biomedical engineering, especially in tendon, vascular, nerve, bone, and cartilage treatments. An electrospun of nanofibers consisted of a water-soluble blend of carboxymethyl chitin CMC (which works to protect mammalian HepG2 and HeLa cells from oxidative DNA lesions) with poly(vinyl alcohol), a synthetic polymer for biomedical applications. Scaffold properties such as cytotoxicity and cell attachment were determined by employing medicinal signaling cells by the colorimetric assay of metabolic cell (MTT assay) viability through succinate dehydrogenase activity. These studies revealed the ability of cells to attach and spread on the nanofibrous scaffolds.

Fig. 6 The PLGA in combination with fibrin glue form a multiple layered fibrous scaffolds, expressed good viability, proliferation, and reduced leakage. (Figure adapted and expanded from Wang et al. [36]. Journal [Polym. Basel]). PLGA (VEGF) ¼ poly(lactic-co-glycolic acid)

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Wound Healing Gul et al. [67] and Feng et al. [59] expressed in detail the wound healing, and it is represented in Fig. 7. Feng et al. [59] formulated α-chitin/nano-silver composite scaffolds for wound healing and dressing discomfort caused due to infectious bacteria. Gul et al. [67] in an experiment found that chitin scaffolds and fibrils are very effective as they possess antibacterial activity against S. aureus and E. coli. It was further recorded that α-chitin/nano-silver composite also had blood-clotting ability [16]. Such properties have made them useful nanostructures for chronic wound dressing use. Similarly, it was found that β-chitin/nano-silver composite fibrils and complex scaffolds of chitin were also used for wound dressing and healing utilizing β-chitin hydrogel embedded with silver nanoparticles [16]. In addition, these scaffolds were evaluated for adhesive in cell culture using Vero cells, and it was found that nano-silver incorporated chitin scaffolds were ideal

A Promoting platelet adhesion and B Promoting Enhancing the erythrocyte aggregation aggregation expression of GPIIb-IIIa

Electrostatic interaction with negatively charged molecules on activated platelets

C Inhibiting fibrinolysis

Electrostatic interaction with negatively charged molecules on erythrocytes Suppressing the secretion of plasminogen activator to inhibit fibrinolysis, extending the time of hemostasis

Forming a 3D network to capture erythrocytes

collagen GPIIb-IIIa GP Ia-IIa GP Ib-IX-V GP VI GP Ib-IX-V

GPIIb-IIIa Platelet

chitosan

GPVI

VWF

Activated platelets

Fibrinogen

TXA2

ADP

Insoluble fibrin polymer

Erythrocyte leukocyte

GPIIb-IIIa

glycoproteins(GP Ia-IIa, GP VI ,GP Ib-IX-V, GPIIb-IIIa) on the platelet membrane

plasmin

Fig. 7 Role of chitin in wound healing through hemostatic effect. (a) The wound healing expressed by increased chitosan expression of GPIIb-IIIa (immune-mediated thrombocytopenia) by platelet cells, the opposite charged platelet cells enhance cell aggregation and clumping. Expression of cell aggregation by attraction among appositively charged chitosan molecules. (b) The clot formation by chitosan molecules using fibrin that is framed into a 3D shaped network structure, to capture erythrocytes. (c) The inhibitions of fibrinolysis enhance the hemostatic effect. (Figure adapted and expanded from Feng et al. [59], Journal [Front. Bioeng. Biotechnol.]). GPIIb-IIIa immunemediated thrombocytopenia, GPIa-IIa very late antigen 2, GPIb-IX-V GPIb-IX-V complex, GPVI immunoglobulin receptor very late glycoprotein antigen (glycoprotein VI), GPIIa-IIIb glycoprotein IIb/IIIa integrin αIIbβ3, VWF von Willebrand factor, TXA2 thromboxane A2/prostaglandin H2, ADP adenosine diphosphate

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wound dressing agents. Carboxymethyl chitin was found useful in drug delivery systems – carboxymethyl chitin (CMC) was used for drug delivery applications, antibiotics, absorption of metal ions, cosmetics, antitumor, and tissue engineering. CMC nanoparticles are produced using cross-linking methods, in combination with CaCl2 and FeCl3 minerals. Structurally, the CMC is round to spherical in morphology, possessing a diameter of 200–250 nm. Further, CMC is similar in morphology and functions such as drug delivery to 5-fluorouracil (5-FU). MTT assay results showed their less toxic nature and safety to most sensitive cells such as fibroblast L929 mouse tissue lines. The multipurpose drug 5-FU was found hydrophobic and had showed good results as an anticancer agent. The 5-FU (fluorouracil) is an anticancer drug used as cytotoxic chemotherapy medication applied as an intravenous load into CMC nanoparticles and entangled into polycaprolactone nanoparticles via an emulsion cross-linking method, releasing the drug in a gradual and sustained manner at a slightly alkaline pH of 6.8. The drug delivery mechanism by an anti-HIV drug using poly(lactic acid) (PLA)/CS nanoparticles, which are biocompatible blended polymers efficient in drug administration for efficient application, were reported. Further, lamivudine, a drug used for the treatment of HIV and chronic Hepatitis-B, worked as hydrophilic antiretroviral in action and was loaded into PLA/CS nanoparticles for an effective delivery. The absorption spectrophotometry was found useful to determine the encapsulation efficiency [40] while delivering the drug lamivudine.

Cancer Diagnosis Semiconductor nanocrystals (or quantum dots) can be bio-conjugated to a variety of biological recognition ligands (Fig. 8), substituting the regular organic fluorescent dyes in immune-staining (to determine specific proteins) applications and bio-imaging (process to noninvasively visualize the processes in real time) of cancerous cells and tissues. In more recent times, many researchers use light wavelengths for degrading and destroying the tumor tissues (Fig. 9). However, quantum dots, including the heavy metals, cadmium sulfide, cadmium selenide, zinc selenide, etc., and can be cytotoxic and even hazardous, have shown promising results. Nanotechnology is a forefront tool to diagnose and treat cancer and other chronic diseases, based on the use of nanoparticles not only to solve limitations of conventional methods, but it has created new ways and prospective and the most advanced techniques to diagnose and treat tumors. The particles differ in size due to quantum mechanics. The size of particle defines its optoelectronics. The semiconductor particles (quantum dots) of nanometer in size are metal-free luminescent quantum dot (QD) based on doped-zinc sulfide (ZnS). The heavy metal particles were conjugated with a cancer-targeting ligand, folic acid (FA) for targeting the cancer imaging. Roy et al. [11] found that folate receptors have high expression in malignant cells and can mediate receptor-oriented endocytosis (injected into target cell) on interaction with folate-conjugated nanoparticles, offering an intracellular uptake of particles. Likewise, the mannose receptors have been used for cancer

Biodegradation, Biosynthesis, Isolation, and Applications of Chitin and Chitosan

Chitosan-Drug nanoparticle

23

Cancer cells

Blood vessel

703

Anticancer activity

Alternating magnetic field

Controlled and sustained release effect

Fig. 8 Nanoparticle delivery of chemotherapeutic drugs, using bioactive carriers, expressed sustained release of drugs at target sites to minimize the toxic effects on the normal cells. (Figure adapted and expanded from Liu et al. [14], Journal [Wiley Online Library])

Light irradiation

O2

1

O2

Chitosan-based nanoparticles with Photosensitizers

Fig. 9 Tumor treatment using photodynamic therapy involving chitosan-based complexes, which are comprised of photosensitizers which localize in the tumor tissues. The mechanism is based on the photosynthetic degradation of tumor tissues releasing molecular oxygen species at specific light wavelengths. (Figure adapted and expanded from Calixto et al. [28], Journal [AAPS PharmSciTech])

diagnosis, providing an appealing that MR work as bioagent for diagnosis and therapy of cancer. Das et al. [12] in book reviewed that carboxymethyl chitosan is highly useful in biomedical and pharmaceutical applications and the FA-conjugatedCMCS used manganese coupled with zinc sulfide quantum dots as composite were successfully used for cancer treatment. This FA-conjugated-CMCS system has high efficiency to be used for targeting, sustained and regulated drug-delivery, and target malignant cell imaging. However, the selected anticancer drug 5-FU that was used for breast cancer treatment showed a comparatively high success percentage. Truszkiewicz et al. [13] used MCF-7 breast cancer cell line for studying cell imaging, specific targeting, and cytotoxicity by conjugated nanoparticles. Most

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commonly in recent times, fluorescence microscopy has been employed in vitro imaging of cancer cells with nanoparticles.

Chitin- and Chitosan-Based Dressings Omer et al. [16] conducted an experiment on pH-sensitive aminated chitosan-gelatin hydrogel as a surgical dressing. The chitosan is solubilized in water and acid is added to regulate the pH of the solution between 2 and 3, and later the gelatin is added. The ratio between 3:1 and 1:3 of chitosan and gelatin was prepared, and a plasticizer like glycerol or sorbitol was added to the solution for reducing the stiffness and adding stickiness to the mixture. Both glycerol and sorbitol have antibacterial and antiviral properties added to chitosan, thereby fastening the wound treatment. The solution was turned into a thin film and dried at 25 degree temperature, and later used for wound dressing. The thin film showed good adhesive subcutaneous fat tissue. Sánchez-Cardona et al. [17] conducted an experiment and showed that chitin scaffolds and fibrils made by blending with gel had efficient moisture absorption and sucking properties, therefore found very suitable for wound dressing and drug delivery. Since chitin-based scaffolds blended with gel have efficient utilization in drug delivery and dressing, therefore, British Textile Technology Group (BTTG) patented a methodology for making these dressing materials at commercial level. Ta et al. [33] reviewed that chitosan comprised of huge bioactive compounds suitable for chemical reactions with other bioactive materials, modifications, and preparation of advanced biomaterials for use as antibiotics and drug delivery system against osteomyelitis and various types of malignants. Similarly, Narmani and Jafari [18] reviewed the chitosan properties and suggested that it has huge significance in preparation, modification, and delivery of various kinds of drugs useful for cancer diagnosis and therapy. Fibrils from both chitin/chitosan are obtained from microfungi (instead of shrimp shells), are not fabricated by the traditional fiber-spinning technique and are bio-safe, and functional materials with high porosity to enhance their applications in the biomedical field.

Chitin- and Chitosan-Based Applications in Ophthalmology Chitosan has played a significant role in eye disease treatment. From ophthalmological point of view, chitosan has overwhelming properties like optical clarity, mechanical stability, sufficient optical correction, and gas permeability. Further, the other optical features found include wettability and immunological compatibility that make it suitable for use as fabricating contact lenses used in the eye. These lenses are made from monomer units and chitosan which are clear, tough, having various physical properties like Young’s modulus (degree of elasticity), high resistance against stress (tensile strength), tear strength, elongation and stretch, water content, and oxygen permeability. The antimicrobial activity, film-forming capability, and

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wound healing properties of chitosan also make it suitable for development of ocular bandage lenses for traumatic injuries.

Antibacterial Properties Studies showed that almost 100% was inhibited by chitosan concentration within 24 h of application. Hoda et al. [19] found that chitin in association with essential oils did not showed an inhibition of growth in E. coli. However, Ke et al. [6] was at this opinion that E. coli growth was inhibited in the presence of chitosan at the concentration >0.025%. Chitosan is also able to inhibit the growth of other microbial species such as Fusarium, Alternaria, and Helminthosporium. The growth of E. coli was inhibited by cationic amino of chitosan that binds with anionic terminal from microbes thus reducing the growth. The chitosan of lower molecular mass once penetrated into the cell wall of bacteria, binds with cellular DNA and hinders DNA transcription into RNA and synthesis of messenger (m) RNA. Naturally, the chitosan and its derivatives have their role as commercial disinfectants and topical antimicrobials. All over the world, the allegoric diseases are most widespread, and the allergic disorder is an immune system disease and weakness caused by many agents such as pathogen, pollen, animal dander, foods, and dust. The mechanism of allergic reaction is given below (Fig. 10). Singh et al. [48] showed that mucosal administration of microparticle-loaded antibodies provides a starting defense against various pathogens, showing that local antibodies act faster than serum antibodies (Fig. 11).

Antithrombogenic and Hemostatic Materials Blood clot formation is an area where chitosan may have many effects depending on its functional type and formulation. Researchers have found that chitosan is helpful in wound care as a hemostatic agent [75]. Chitosan has been shown to be a highly efficient as preventing wound bleeding, blood loss from vessels, and triggers blood clotting. Therefore, chitosan and its derivatives have been formulated into microspheres, bandages, dressings, and absorbable sponges which help in quick haling of wounds and prevention of blood loss. In fact, a chitosan dressing was able to control an experimental arterial hemorrhage in dogs. The efficiency of chitosan as a hemostatic agent was shown to be dependent on its degree of hydration. However, there are also reports that different chitosan preparations can be antithrombogenic and anticoagulant. N-hexanoyl and N-octanoyl chitosan fibers can be used as antithrombogenic materials [50]. A phthalized chitosan derivative was used to prepare an antithrombogenic material for the fabrication of vascular grafts. It was found that sulfate derivatives of both chitin and chitosan exhibited anticoagulant and lipase (LPL)-releasing activities. Chitin and its combination with other drugs has huge physiochemical, intra- and intercellular drug delivery, and dressing properties; especially the chitin 3,6-sulfate exhibited two times more anticoagulant and a drug

706

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Endoplasmic reticulum

Chitosan and its derivatives

Chitosan and its derivatives Cell degranulation

Syk

MAPK3

PLC

NF-kB

Chitosan and its derivatives Chitosan and its derivatives

Ca+2 Nucleus Ca+2 Ca+2 Ca+2 Ca+2

Synthesis of IL-13 and IL-4

Chitosan and its derivatives

Fig. 10 Anti-allergic effect of chitin and chitosan. (Figure adapted and expanded from Rajoka et al. [49] Journal [Critical Reviews in Biotechnology]). Syk spleen tyrosine kinase, PLC phospholipase C, an assembly of enzymes lading to hydrolysis of phosphatidylinositol 4,5-bisphosphate, NF-kB nuclear factor-κB (NF-κB), MAPK mitogen-activated protein kinases, IL-13 interleukin-13, IL-4 interleukin-14

releasing activity over heparin; however, the sulfate derivatives heparinoids are unable to function for artificial blood dialysis.

Antiaging Cosmetics Chitosan derivatives have been used as antiaging cosmetics [24, 100] (Fig. 12). The crystal morphologies are modified and new properties are added by using blockcopolymer nanoparticles (BPN). BPN is a versatile crystal compound work as growth additives composed of phosphatidylcholine and linoleic acid. The acids are nano-complexed with hyaluronan and chitin nanofibrils (PHHYCN), which both had huge cosmetic and pharmaceutical properties. The nanoconstructs were used to encapsulate cargos comprising of various bioactive molecules such as cholesterol, creatine, caffeine, melatonin, vitamins E and C, and the amino acids (glycine and arginine) [10]. The idea was to use these carriers which are loaded with nanoparticles for skin rejuvenation, since various ingredients used had shown some activity in this

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IgA IgA+ Plasma cell Mucosal surface

Mucosal response

IgG antibodies

IgA+ B cell CD4+Th2 cell Naive T cell Antigen

M cell

B cell

IL-6

Cellular response

Peyer’s Patch

TNF-a DC Microparticle

Macrophage

+

CD4 Th1 cell

T cell

IFN-g Epithelial cell

+ CD8 T cell

Humoral response CTL

NK

Fig. 11 Presentation of chitosan derivative nanoparticles in mucosal immunity against allergic reactions. (Figure adapted and expanded from Singh et al. [48], Journal [International Journal of Biological Macromolecules]). IgA immunoglobulin A, IgG immunoglobulin G, CD4+Th2 cell Th2 subset of CD4+ T cells synthesizing cytokines, T cell cells originated from thymus, CDS
T cell the cluster of differentiation in T cell, IL-6 interleukin 6 function as pro-inflammatory cytokine and an anti-inflammatory myokine, TNF-α tumor necrosis factor α, INF-γ interferon γ, B cell B cells, CTL cytotoxic T lymphocytes, NK natural killer, M cells mucosa-associated lymphoid cells

regard. Subjects whose skin was treated with the active chitin nanofibrils containing BPN were shown to be softer after 30–45 days of treatment. Immediately after 2 weeks of treatment by active chitin nanofibrils loaded with BPN, both fine wrinkles and crease lines appear to be reduced. Further, the telangiectasia were found reduced and the overall look of the face was improved. Kong et al. [50] in an experiment found that chitosan oligosaccharide (COS) had preventive effect for D-gal-inducing aging due to serum IgG and IgM concentration in mice. It was found that COS mainly effect thymus and spleen thereby activated the release of antioxidant enzymes from liver and kidneys, and also prevented the pathogen-induced bowl diseases [75]. Therefore, author concluded that COS work as therapeutic product for age-related disorders and diseases.

Antitumor Activity Chitosan and its derivatives have exhibited antitumor activity (Fig. 13) in both in vitro tests and in vivo animal models. Antitumor activity was subjected to an enhanced release of interleukins-1 and 2 responsible for inducing maturation and infiltration of tumor in cytolytic T-lymphocytes. It was further suggested that

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Fig. 12 Presentation of antiaging role of stratum corneum lamellae organization increasing the skin penetrability of the active ingredients entrapped in chitin microfibrils. (Figure adapted and expanded from Morganti et al. [51], Journal [Cosmetics MDPI])

HCPT Penetration

CS

Fig. 13 Penetration and enhancement of nanoparticles, permeability, encapsulation, and antitumor drug NPs/10-hydroxycamptothecin (HCPT) delivery. (Figure adapted and expanded from Guo et al. [52] Journal [Frontiers in Pharmacology]). HCPT hyperosmolar conjunctival provocation test, CS cytokine storm

chitosan could increase lymphokine production and enhance proliferation of cytolytic T-lymphocytes.

Vaccine Adjuvant Researchers have found that intranasal chitosan glutamate significantly impacted antibody response in mammals (mice and guinea pigs) against diphtheria antigen. The diphtheria toxin is produced from a gram positive bacteria, Corynebacterium diphtheria, affecting mainly the nose and throat, comprised of common symptoms of a thick gray coating on throat and tonsils, fatigue, fever, difficulty in swallowing, rashes in the skin, and coughing. Impacts of this chitosan adjuvant to a diphtheria vaccine in healthy human volunteers were studied world over, as shown in Fig. 14 [40]. The chitosan glutamate (CSN) is a cationic polysaccharide and also an important adjuvant for cell-mediated immunity; however, the exact mechanism of action is unclear to the scientific world. It was found that chitosan increases dendritic cell maturation through Type I interferons (IFNs) which promotes antigen-specific T

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helper activities in other receptor interferons. Further, it was found that mechanism of IFNs response depends on mitochondrial reactive oxygen species and cytoplasmic DNA. The other chitosan derivatives which work as an efficient adjuvant is N, N, Ntrimethylated chitosan (TMC). This is a biodegradable polymer compound with emerging properties and huge significance in biomedical sciences, with encouraging properties to work as nonviral vector for nucleic acid and protein delivery. TMC was also found to work as adjuvant for inactivated and inoperative NIBRG-14 subunit antigen that is obtained from influenza antigen for SARS-COV-2 [59].

Decomposition, Regeneration, Repair, and Damage of Cuticle The process by which dead insect or its body shedding during development process are broken down into its constituents is called decomposition. At the end of death of an organism, it is gradually disintegrated till chemical structure is recognized. In nature, the decomposition of an organism is comprised of four major processes such as photooxidation, leaching, comminution, and mineralization. Rate of decomposition is directly proportional to respiration rate as rate of litter

Free Antigen

M-cell

Sustained Release MHC-I pathway Promote Antigen uptake Chitosan

iDC

STING-cGAS pathway

DC

Promote DC maturation MHC-II pathway

Fig. 14 Use of chitosan as an adjuvant for vaccine delivery. (Figure adapted and expanded from Boroumand et al. [40] Journal [Frontiers in Cellular and Infection Microbiology]). DC dendritic cells, STING-cGAS stimulator of interferon genes-cytosolic cyclic GMP–AMP synthase, M-cell microfold cells, MHC-I and MHC-II major histocompatibility complex (MHC) class I and class II proteins

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disappearance. The rate of decomposition is also determined by isotopic tracers and is found higher under mesic (moderate moisture percentage) than in arid ecosystem. Decomposition is actually a multitype negative exponential decay function over time proportional to total organic matter content in environment. End product of decomposition is not always converted into CO2, but under low O2 content characterized by warm and humid conditions, incomplete decomposition occurs that leads to methane gas production. Nitrogen is immediately immobilized by microorganisms after decomposition by detritivore organisms. Soil-burrowing insects are helpful by redistributing soil and organic matter inside soil surface. Ants, termites, soil-dwelling hymenopteran insects, and wood-boring coleopteran pests dig huge masses of soil and wood and assemble organic matter in the substrate, later incorporated into soil material to balance the distribution of nutrients and organic content. The exoskeleton of arthropods is an outer covering comprised of limitless specializations varying across species, stage of development, and taxa. Cuticle is responsible for performing multiple functions and activities, displaying a multiple range of material and chemical properties. The major properties are accounted to amount and orientation of cuticle fibers, constituents and degree of cross-linking, hydration of protein matrix, relative proportion of exo- and endocuticle, and shape of cuticle and structures. In insects, cuticle is comprised of two layers, endocuticle and exocuticle. Chitin and protein are the main constituents of endocuticle, while the outer covering is harder due to tanning and forms a dark brown exocuticle. Compared to other natural materials like wood and bone, little research has been carried over the mechanical properties of cuticle, regeneration, damage, repair, and importance in balancing and restoring mechanical stability and body posture. In nature, the insect body is exposed to extended and repeated cyclic loads while under motion in air, on the ground, or in water. The hard cuticle is continuously exposed to extremes during emergency behaviors and survival fitness such as jumping, wedging, righting, predating, pollination, digging, food gathering, and while escaping from extremes of environment. Decomposition and degradation of insect cuticle especially wings (tegmina) of Orthoptera was a question of debate for many entomologists. As an insect wing is complex, its constituents are examined by dewaxing through ether or deproteinizing dewaxed wings in sodium hydroxide. Normally, various microbes such as fungi, bacteria, actinomycetes, and protozoa are found degrading wings, insect cuticles and their residues in wood and soil. Ecological observations and physiological properties have showed that chitin from wings is degraded by many organisms, especially such as the Mortierella spp. bacterium Pseudomonas and the Actinomycetes genus Streptomyces. The degrees of decomposition are determined through release of CO2. Many questions regarding insect motions, dynamics, and cuticles remain unanswered, such as the actual impacts of various activities on the insect cuticle, limits of failure when an insect subjects its body to extremes of environment, recovery from injury to the exoskeleton, and the degree of flexibility and stretching of the cuticle and the body.

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Conclusion The dead insects are turned into organic matter by the process of decomposition. The end result of any organism after death is its disintegration into its chemical components, and the process occurs through a series of steps. In insects, the chitin of the exoskeleton is the hardest material, and the unique biochemical properties of chitin and chitosan suggest that they could be seen as almost ideal biopolymers with numerous applications in the field of scientific research. These materials can be processed into various products, and on the other hand, it is possible to fabricate scaffolds and nanoparticles with increasing applications in the burgeoning field of nanomedicine and biomedical research. In this chapter, we have presented an overview of biodegradation, biosynthesis, isolation, applications, and natural degradation and decomposition of chitin and chitosan.

Future Prospects In everyday life, most people use plastic packaging films, which have polluted every aspect of the ecosystem due to their nonbiodegradable nature. The nonbiodegradable materials have limited applications and also need to be fully abounded to protect the environment in all aspects. Awareness regarding merits and demerits of nonbiodegradable synthetic materials and naturally occurring biodegradable ones needs to be shared among the masses. In nature, the definition of biodegradability of material pleases the environment in many ways, like user-friendly and eco-friendly, besides being available in high abundance. Among all biodegradable materials available, chitin is one of the abundant polysaccharides produced from crustaceans as an industrial waste. In spite of its limited use, researchers have found its huge applications in the biomedical, agrochemical, food, and cosmetic industries. The conversion of chitin into chitosan and acetylated chitosan oligomers has opened its diverse ways for application in many fields of research and applied sciences. Besides global efforts for mitigation of climate change, the concept of organic farming and good agricultural practices have a high thrust too, and therefore, more research needs to be done on chitin and its derivatives to save the environment, health, and maintain sustainable agricultural production.

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68. Guo H, Li F, Qiu H, Liu J, Qin S, Hou Y and Wang C (2020) Preparation and Characterization of Chitosan Nanoparticles for Chemotherapy of Melanoma Through Enhancing Tumor Penetration. Front. Pharmacol. 11:317, https://doi.org/10.3389/fphar.2020.00317 69. Gushiken LFS, Beserra FP, Bastos JK., Jackson CJ, Pellizzon CH (2021) Cutaneous Wound Healing: An Update from Physiopathology to Current Therapies. Life 11: 665. https://doi.org/ 10.3390/life11070665 70. Raimundo I, Silva R and Meunier L (2021) Functional metagenomics reveals differential chitin degradation and utilization features across free-living and host-associated marine microbiomes. Microbiome 9: 43. https://doi.org/10.1186/s40168-020-00970-2 71. Hahn T, Tafi E, Paul A, Salva R, Falabella P and Zibek S (2020) Current state of chitin purification and chitosan production from insects. J. Chem. Technol. Biote. 95 (11): 2775–2795. https://doi.org/10.1002/jctb.6533 72. Han F, Jia X, Dai D, Yang X, Zhao J, Zhao Y, Fan Y, Yuan X (2013) Performance of a multilayered small-diameter vascular scaffold dual-loaded with VEGF and PDGF, Biomate. 34 7302–7313, https://doi.org/10.1016/j.biomaterials.2013.06.006 73. Haque ST, Saha SK, Haque ME and Biswas N (2021) Nanotechnology-based therapeutic applications: in vitro and in vivo clinical studies for diabetic wound healing. Biomater. Sci. 23, https://doi.org/10.1039/D1BM01211H 74. Hasibuan Z, Yuandani PA, Tanjung M (2021) Antimicrobial and antihemolytic properties of a CNF/AgNP-chitosan film: A potential wound dressing material. Heliyon 7(10):e08197, https://doi.org/10.1016/j.heliyon.2021.e08197 75. He W, Huang X, Zhang J, Zhu Y, Liu Y, Liu B, Wang Q, Huang X and He D (2021) CaCO3Chitosan Composites Granules for Instant Hemostasis and Wound Healing. Materials 14: 3350. https://doi.org/10.3390/ma14123350 76. Heras A., Rodríguez N.M., Ramos V.M., Agulló E (2001). N-methylene phosphonic chitosan: A novel soluble derivative. Carbohydr. Polym. 2001;44:1–8. https://doi.org/10.1016/S01448617(00)00195-8. 77. Hoda RA, El-Zehery ZRA, Abdel-Rahman HM, Salem AA and El-Dougdoug KA (2021). Novel strategies of essential oils, chitosan, and nano- chitosan for inhibition of multi-drug resistant: E. coli O157:H7 and Listeria monocytogenes, Saudi J. Biol. Sci ISSN 1319-562X, https://doi.org/10.1016/j.sjbs.2021.12.036. 78. Horn SJ, Sørbotten A, Synstad B, Sikorski P, Sørlie M, Vårum KM and Eijsink VG (2006). Endo/exo mechanism and processivity of family 18 chitinases produced by Serratia marcescens. FEBS J.273(3):491–503. https://doi.org/10.1111/j.1742-4658.2005.05079.x. 79. Hou J, Aydemir BE and Dumanli AG (2021) Understanding the structural diversity of chitinsas a versatile biomaterial. Philos. Trans. Royal Soc, 379: 20200331. https://doi.org/ 10.1098/rsta.2020.0331 80. Hugo MG, Adriana FS and Erik JVM (2021) Sustainable chitosan production by mucoralean fungi using waste post-frying oils and corn steep liquor as substrates Int. J. Dev. Res, 11: (01), 43185–43194 81. Ibe C and Munro CA (2021) Fungal cell wall: An underexploited target for antifungal therapies. PLoS Pathog. 17(4): e1009470. https://doi.org/10.1371/journal.ppat.1009470 82. Ibrahim HM and El-Zairy EMR (2015) Chitosan as a Biomaterial-Structure, Properties, and Electrospun Nanofibers, Concepts, Compounds and the Alternatives of Antibacterials, Varaprasad Bobbarala, IntechOpen, Ltd. United Kingdom, London https://doi.org/10.5772/ 61300. https://www.intechopen.com/chapters/49246 83. Iesa MA (2021a) Biology of Brinjal Shoot and Fruit Borer (Leucinodes orbonalis Guenee) and screening of various genotypes for resistance. Turk. Online J. Qual. Inq. 12:6.6025–6032. 84. Iesa MA (2021b) Studies on Banana Insect Pest complex in tropical and subtropical areas of Asia. Turk. Online J. Qual. Inq. 12:6: 10039–10047 85. Iesa MA (2021c) predatory role of green lacewing Chrysoperla nipponensis larvae (Neuroptera: Chrysopidae) reared on different diets. Tianjin Daxue Xuebao. https://doi.org/ 10.17605/osf.io/tfxhp. 64: 08

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86. Iesa MA (2021d) Bio efficacy check of different synthetic chemicals applied against whitefly (bemisia tabaci Gennadius) in tomato to enhance vegetable production for growing human populations. Tianjin Daxue Xuebao 54: (08) https://doi.org/10.17605/OSF.IO/S4BGX 87. Iesa MA (2021e) Foraging behaviour of apidae bees on rapeseed flowers Brassica napusunder open conditions. Tianjin Daxue Xuebao. https://doi.org/10.17605/osf.io/vbqze. 64: 08 88. Iesa MA (2021f) Rise of health related issues and management of health services in Asia. Journal of Tianjin University Science and Technology, 54. 08. https://doi.org/10.17605/osf.io/ vz8yk 89. Ingenieur (2021) Biomimetic adhesives from natural polymers. Vom Fachbereich Maschinenbau und Verfahrenstechnik der Technischen Universität Kaiserslautern zur Verleihung des akademischen Grades. Datum der mündlichen Prüfung, Tag der mündlichen Prüfung 22.03: 1–227. 90. International Conference on Tissue Engineering (ICTG) (2021) Enhancing the Recent Advancements and Innovations in Tissue Engineering. Euro Sci Con, Tissue engineering 2021 on February 25–26, 2021 in London, UK. 91. Itoh T (2021) Structures and functions of carbohydrate-active enzymes of chitinolytic bacteria Paenibacillus sp. str. FPU-7, Biosci. Biotechnol. Biochem. 85: 6. 1314–1323, https://doi.org/ 10.1093/bbb/zbab058 92. Jack CI, Qiu J and Benny KKC (2021) Genomic insights into the sessile life and biofouling of barnacles (Crustacea:Cirripedia). Heliyon 7(6), e07291. https://doi.org/10.1016/j.heliyon. 2021.e07291. 93. James TB, Kylie AR, Alan T, Marshall TLD and Heloise G (2021) A cross-species test of thefunction of cuticular traits in ants (Hymenoptera: Formicidae).Myrmecol. News. 31: 31–46 https://doi.org/10.25849/myrmecol.news_031:031 94. Jeong CB, Lee BY, Choi BS, Kim MS, Park JC, Kim DH, Wang MH, Park HG and Lee JS (2020) The genome of the harpacticoid copepod Tigriopus japonicus: Potential for its use in marine molecular ecotoxicology. Aquat. Toxicol. 222, 105462. 95. Johnson A, Neelakandan M, Jose J, Thomas S and Kalarikkal N (2021) Cellulose and Chitin Nanofibers: Potential Applications on Wound Healing. In: Nayak AK, Hasnain MS (Eds.) Biomedical Composites. Materials Horizons: From Nature to Nanomaterials. Springer, Singapore. https://doi.org/10.1007/978-981-33-4753-3-6 96. Joseph SM, Krishnamoorthy S, Paranthaman R, Moses JA and Anandharamakrishnan C (2021) A review on source-specific chemistry, functionality, and applications of chitin and chitosan. Carbohydr. Polym. 2: 100036, ISSN 2666-8939, https://doi.org/10.1016/j.carpta. 2021.100036. 97. Jung, W. J., and Park, R. D. (2014). Bioproduction of chitooligosaccharides: present and perspectives. Mar. Drugs. 12, 5328–5356. https://doi.org/10.3390/md12115328 98. Jungprasertchai N, Chuysinuan P and Ekabutr P (2021) Freeze-Dried Carboxymethyl Chitosan/Starch Foam for Use as a Haemostatic Wound Dressing. J Polym Environ J POLYM ENVIRON. https://doi.org/10.1007/s10924-021-02260-w 99. Kalem MC, Subbiah H, Leipheimer J, Glazier VE and Panepinto JC (2021) Puf4 Mediates Post-transcriptional Regulation of Cell Wall Biosynthesis and Caspo-fungin Resistance in Cryptococcus neoformans. mBio 12(1):e03225–20, https://doi.org/10.1128/mBio. 03225-20. PMID: 33436441; PMCID: PMC7844544. 100. Kallenbach EMF, Hurley RR, Lusher A and Friberg N (2021) Chitinase digestion for the analysis of microplastics in chitinaceous organisms using the terrestrial isopod Oniscus asellus L. as a model organism. Sci. Total Environ, 786: 147455, 0048-9697, https://doi.org/10.1016/ j.scitotenv.2021.147455

Part V Environmental Impacts of Biodegradation

Environmental Impact of Biodegradation

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impacts of Biodegradation on Soil Fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Plastics/Bioplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Herbicides, Pesticides, and Insecticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation Agricultural Crop Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impacts of Biodegradation on Air Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impacts of Biodegradation on Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation and Improvement of Productivity of Plants and Animals . . . . . . . . . . . . . . . . . . . . . Biodegradation: Ecosystem Balancing Viewpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation and Facilities of Human Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Globally, several indicators recorded that in 2025 about 4.3 billion urban areas may generate 2.2 billion tonnes of solid waste, e.g., nonbiodegradable plastics. This will cause cultural, social, environmental, and public health troubles. The most urgent problem is “How can countries get rid of waste?”, especially new poor industrial countries. The using of low amount of collected wastes for composing or recycling and hunge amount still find as wastes which poor countries can not rid get of them. Plastic packages are considered complex because they contain the highest composition of polymers and low recycling rates. Bioplastics are regarded as alternatives owing to their ability to biodegrade. Accumulating the chemical residues of pesticides, herbicides, fungicides, and fertilizers as heavy metals will damage the soil structure and fertility, causing E. E. Ammar (*) Plant Ecology, Botany Department, Faculty of Science, Tanta University, Tanta, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_27

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desertification and deforestation. Moreover, a gathering of oil residues from ships in marine habitats will destroy marine health. Also, the accumulation of pollutants in the air from car exhaust and factory chimneys harms the health of wildlife; thus, the nonstability of the ecosystem and human life is difficult. Undoubtedly, biodegradation will create a revolution to preserve the environment by purifying the soil and reducing the proportion of heavy elements in it, as well as purifying water and air, then improving the productivity of plants and animals and creating an environment conducive to the growth of all living creatures, especially humans. Keywords

Biodegradation · Environmental impacts · Plant and animal productivity · Ecosystem · Human life Abbreviations

Bio-PE Bio-PET CADR GC/MS GHG OPs OSA PAH PAs PBAT PBS PET PHAs PLA PP TPHs WAFs

Bio-polyethylene Bio-polyethylene terephthalate Clean air delivery rate Gas chromatography/mass spectrometry Greenhouse gas Organophosphates Oil-suspended particulate substances aggregation Polycyclic aromatic hydrocarbon Pyrrolizidine alkaloids Adipate terephthalate Polybutylene succinate Polyethylene terephthalate Polyhydroxy alkanoates Polylactic acid Polypropylene Total petroleum hydrocarbons Water-accommodated fractions

Introduction Pollution, overexploitation of natural resources, and environmental degradation, exacerbated by pollution, will result in severe, pervasive, and possibly irreversible changes for people, assets, economies, and ecosystems worldwide. The negative impact of climate change on our lives is becoming more pronounced. Global warming is already affecting us. The severity of negative consequences will worsen as climate change continues. Deep climate change mitigation, such as zero emissions by 2050, necessitates fundamental changes in our way of life [1–3]. These changes are required to halt the continuation of global warming and prevent its worst

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consequences. Climate and environmental actions have gained public support, and now they are major concerns for citizens. We are still living outside of the planet’s boundaries. Human activity hurts the environment. We pollute natural ecosystems and deplete resources at such a rate that ecosystems cannot regenerate. These trends have been identified in the past and may have grown or faded over time, such as: (1) lower greenhouse gas (GHG) emissions, (2) growth of cities, (3) pollution reduction techniques, (4) resilience building, and (5) sustainable development goals. All of these trends can be met through the safe and regular biodegradation of residues [4]. Naturally, infinite interactions of environmental factors affect the biological community in the soil as pH, temperature, oxidoreductase potential, granulometric composition, the presence of metal cation, various ions, and organic substances (pesticides, humic acids, polychlorinated biphenyls, etc.). All the parameters mentioned affect the activity of the soil microorganisms and enzymes [5, 6]. Organochlorine pesticides have a severe environmental effect owing to their long-term degradation [7–9]. They may make bounds with organic and organic compounds, which have cytotoxic effects on microorganisms and higher organisms [10, 11]. Elimination of toxic plant residues is a difficulty for land management and farmers dealing with polluted biomasses. Current studies confirm the threat of the transfer of toxic plant components such as pyrrolizidine alkaloids (PAs) supplied from plant remains to poison crops through the soil [8, 12, 13]. Eighty percent of the eight million tonnes of plastic that are thrown into the ocean yearly are single-use plastics, such as bottles, shopping bags, and cigarette lighters. Plastic destroyed ecosystems, decreasing of poor communities from thier natural resources due to pollution and a safe, healthy living environment. Marine life is the first to suffer from plastic pollution, such as the albatross, a sea bird that suffers from hunger because of plastic pollution in distant areas around the world. It is known that plastic micro-particles enter the food chain by fish, which transmit the plastic micro-beads to humans. Humans are getting large quantities of plastic from fish and consuming the absorbed poisons from the plastic micro-particles. Cleaning up is not a choice, when plastic pollution is spread out over the ecosystem. In addition, recycling increases carbon emissions. It is necessary to decrease, then stop the use and production of plastics, especially single-use plastics, and then reconnect with our discarded culture toward nature, with time the evaluation of natural resources in the earth become useless [14, 15]. In organic fertilizers, pesticides, herbicides, and fungicides have a carcinogenic effect in humans, and most pesticides consist of cancer-causing ingredients. Furthermore, nutrient imponderables occur owing to the impetuous abuse of inorganic fertilizers. This reduces the uptake ability of other vital nutrients, causing soil acidity, leading to the deficiency of crop yields. On the other hand, the common nitrogen phosphorus and potassium fertilizers lead to depression of secondary and micronutrients in both the soil and the crops. Moreover, continuous use of inorganic fertilizers only causes the depression of the soil organic content, degradation of the soil physical structure and features, increasing soil acidity, and soil erosion. At least agricultural chemicals have polluted the soil surface, the ground water, and damaged

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wildlife. In addition, they increase dependence on fossil fuel resources in agriculture. Inorganic fertilizers also need great buying power, especially in distant areas with very high and low rainfall [16, 17]. Heavy metals are those such as Pb, Hg, Cr, Cd, Cu, Mn, Zn, Ag, Ni, etc. Cd, As, Hg, and Pb are most toxic to plants, animals, fishes, the environment, and humans. High concentrations of heavy metals are harmful. They decrease the stability of ecosystems owing to the bio accumulation inside bodies of organisms and toxic impacts on biota, leading to death in the majority of living organisms [18–20]. However, all heavy metals are considered necessary micro-nutrients; they have toxic impacts on living beings through mutagenesis and metabolic interference. The bio accumulation of toxic metals occurs inside the body and through the food chain so that the toxic metals decrease in chronic toxicity. Hg and Pb have vital toxic impacts. The heavy metals are killer pollutants for fishes, where these are not removed from aquatic systems naturally as organic contaminants, and are enriched with mineral organic components. The concentration of heavy metals in fish bodies depends on their development, age, and physiological processes. Fishes are highly affected by toxic contaminants. Heavy metals have toxic impacts on various organs, passing to the water through the atmosphere, drainage, soil erosion, and various human activities [21–23]. Owing to the high concentration of heavy metals in the environment, they can enter the biogeochemical cycle, causing toxicity [22, 24–26]. Because the daily clear and unclear harmful impacts of solid waste residues, pesticide or fungicide, chemical remains, and heavy metals occur in the current. Their future predicts long-term effects, and it is significant to focus on the roles of biodegradation in the environment, especially in soil, air, and water. It is effective in giving light to plants, animals, and humans. Now and shortly, biodegradation will be ecosystem recovery and stability [27, 28].

Environmental Impacts of Biodegradation on Soil Fertility Biodegradation of Plastics/Bioplastics Nowadays, bioplastics constitute only 1% of about 368 million tonnes of plastics produced per year. By increasing human needs, and with more creation of biopolymers, advanced applications, and emerging products, the bioplastics market is diversifying and growing daily. According to the most recent market data reported by European Bioplastics with the nova-Institute, the production capacities of global bioplastics will increase from 2.11 million tonnes in 2020 to their maximum value of 2.87 million tonnes in 2025. Innovative and new biopolymers, such as polylactic acid (PLA), bio-based polypropylene (PP), and polyhydroxy alkanoates (PHAs), seem to reach high growth rates. The most frequently recorded biopolymers are PHAs, PLAs, plant-based components, and thermoplastic starch [29]. Biodegradable and renewable materials made from biomass are more attractive than

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Fossil fuel:

Polyethylene Polypropylene Polystyrene Polyvinyl chloride Fossil fuel: A polycarbonate Renewables: Bisphenol vs. Polyethylene Poly(methyl methacrylate) Polypropylene Polystyrene Polyvinyl chloride Bisphenol A polycarbonate Poly( methyl methacrylate) Feedstock Renewabality

Renewables:

Renewables:

Slow Degradation

vs.

Polymer Degradability

Feedstock Renewabality

Polyactide Bio-PBS Starch composites (Mater-Bi) Cellulose acetate

Bio-polyethylene Bio-PET (bio-based ethylene glycole Zytel (bio-sourced Nylon) Sorona (partially bio-based polyester)

vs.

Feedstock Renewabality

Fossil fule:

Fast Degradation

Poly(butylene succinate) Polyglycolide Polycaprolactone Poly(ethylene oxide) Poly(propylene oxide)

Fig. 1 The polymer can degrade and renew into new polymers. (Adapted with permission from Ref. [13], copyright 2018, Elsevier)

nonbiodegradable petrochemicals [30–34]. The degradation and renewal of various polymers are demonstrated in Fig. 1 [35–37]. The global production of bioplastics escalates yearly; for example, in 2018, it reached 43.2%. In 2019, bio-based PP was first used commercially. It is predicted that it will more than quadruple by the end of 2025 because of its widespread application in various sectors. PHAs are a vital polymer category whose production capacities are predicted to increase throughout the next 5 years. Polyesters are totally bio-based, so they are biodegradable in various environments. They have a wide choice of mechanical and physical features, which depend on their chemical structure. However, the wide use of biodegradable plastics represents only about 60% (over 1.2 million tonnes) of their global production capacities as PLA, PHA, and starch mixture. It is expected that the production of biodegradable plastics will surge up to 1.8 million in 2025 because of the clear growth rates of PHAs and renewed investments in PLA production in Europe and the USA (Fig. 2). This means that the world is seeking more ecofriendly bioplastics to obtain a clean environment (Fig. 3) [38]. Nonbiodegradable and bio-based plastics represent over 40% (about 890 thousand tonnes) of the global production capacities of bioplastics, including bio-based polyethylene (PE) and bio-based polyethylene terephthalate (PET), as well as bio-based polyamides. It is predicted that they will decrease slightly to over 37% by 2025 (about 1 million tonnes), when the expectations of biodegradable plastics production appear to be at a higher growth level. The capacities of bio-based PET production will continue to be delayed; it has not realized the predicted rate over the last few years. This is in comparison with PET (100% bio-based); in addition, it has a superior barrier feature and thermal advantages, so that it will be an ideal material for

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In 1000 Tonnes 0

500

1500

1000

2000 1051

2025

1952

901 1227

2024

2111

884 1356

2230

1533

2579

865

2022

1046 1571

2021

Total capacity

2023

2643

1072 1775

2020

2856

1081 2871

1800

2019

1071 Biodegradable

Bio-based/Non-biodegradable

Fig. 2 Production capacities of bioplastics globally from 2019 to 2025. (Modified from: European Bioplastics and the nova-Institute report, 2020, [39])

Total produced plastics: 2.11 million tonnes Other (Bio-based / Non-biodegradable)

1% 41%

Bio-based / Non-diodegradable

58% Biodegradable

PBAT

PE PET

PBS

PA

23% PLA

26%

3% 23%

PHA

32%

PP PEF* PTT

0% 3%

7% Starch blends

3%

29%

19%

32% Other (Biodegradable)

Fig. 3 Capacities of global production of bioplastics in 2020. (Modified from European bioplastics and the nova-Institute report 2020, [39])

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the manufacturing of drinks packaging, nonfood, and food products. Thus, in 2020, the manufacturing of biodegradable plastic constituted 58.1% of the total amount of plastics produced (2.11 million tonnes) (Fig. 3) [39]. In 2020, the cultivated land that uses renewable feedstock for the bioplastic production reached 0.7 million hectares. This approach represents 0.015% of the area used globally for agriculture (4.8 billion hectares of ¼ 94% of pasture, food, and feeding. Despite the market’s growth expected over 5 years, the land used for bioplastic production will increase slightly to 0.02%. This refers to the absence of competition between the renewable feedstock for bioplastic production, feeding, and

(a)

Global Agricultural Area

1.4 billion ha = 30% 3.3 billion ha = 70%

Pasture

Arable land

(b)

30%

Perentages (%)

25%

1.24 billion ha

20% 15%

Expected in 2025: 1.1 million ha = 0.020 %

10% 5%

106 million ha

53 million ha

0.7 million ha

0% Food and feed

Material use

Biofuel

Bioplastics

Arable land in 2020 Fig. 4 Estimated land use for bioplastic production globally (a) and on arable land (b) in 2020. (Modified from the European Bio-plastics and nova-Institute report 2020, [39])

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food (Fig. 4) [39]. Europe represented the main hub for the industry of bioplastics, where it records the greatest production of research and development; thus, it has a considerable bioplastic market with the whole world. Up to now, a quarter of the global production capacity of bioplastics is in Europe (Fig. 5) [39]. Numerous studies have projected that by 2025, 4.3 billion urban occupants will generate about 2.2 billion tonnes of metropolitan solid waste, e.g., plastics per year. Owing to the relative inexpensiveness and availability of Zea may L. in many African countries such as Nigeria, this natural polymer is the safe alternative, blended with low-density polyethylene to synthetic plastics. In recent years, biodegradable packaging produced from renewable natural resources has increased considerably, especially in the EU [40, 41]. The usage of biodegradable substances impacts the environment more positively than traditional materials, especially those made from plants or animals. Recent trends have sought to preserve the ecosystem balance via the biodegradation of materials so that a group of new industries, dependent on the recycling of plant, animal, or microbial residues, are on the way to binge all over the world [42]. There is no doubt that the biological decomposition of solid substances, especially materials that are soluble between the elements of the soil, may increase its fertility. This may be indicated by the growth of plants within it when the rest of the factors are stable. Long-term biodegradation in the prophecy of soil fertility is one of the inspiring research points for future study. Few norms are accessible with regard to biodegradable plastics in soil internationally to govern the biodegradability within the soil of biodegradable plastics, especially biodegradable agricultural plastics. The biodegradability of plastics is contingent on the raw

Fig. 5 Bioplastic production capacities on the world continents in 2020. (Modified from the European Bio-plastics and nova-Institute report 2020, [39])

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materials, the chemical structure of the end product, and the environment under which the outcome is predicted to biodegrade. Moreover, the degradation time of some polymers varies from a few weeks to numerous months under the same environment. Degradable compostable plastic is designed to change the chemical structure under environmental conditions, resulting in a decline of some properties that may alter, such as the nature of the microorganism and the time of use. Bioplastics are classified into five types: biodegradable, photodegradable, oxidative degradable, hydrolytically degradable, and compostable plastics (Table 1) [43]. The biodegradation of PLA needs specific artificial composting conditions, a temperature above 58  C, systematic management, and detection of sites of composting and recycling facilities. Under accurate circumstances, microbes can convert the bioplastic into CO2 and water within 2 weeks. This will normally save a suitable growth condition for plants and micro fauna, in addition to increasing soil fertility. Nevertheless, if it becomes besieged or is dumped, PLA remains nearby for much longer. However, if it becomes littered as tiny pieces, PLA remains longer. When pure PLA is thrown into seawater, it does not biodegrade totally [44]. Plastics are used daily in our lives, in electronic textiles, toys, health care products, and packaging products [40, 44–46]. Synthetic plastics are made of semi-artificial or artificial organic matter (flexible nature). In the 1940s, synthetic plastics spread in society because of their mechanical strength, durability and lightness, flexibility, low cost, and the ability to be recycled into products manufactured from other substances such as glass, metals, and paper [47]. Plastic degradation is extremely complicated; it emits CO2 and many severely toxic substances [40, 48]. About 2.8 kg of CO2 is estimated to evolve from burning 1 kg of plastics [29]. The evolution of bioplastic is elevated because of the environmental impacts of nonbiodegradable plastics. The basis of bioplastic (biodegradable plastics) is starch, cellulose, sugar, etc., which are the first renewable materials in nature [23]. Biodegradable plastic is mainly degraded via the biological processes by microorganisms that occur naturally, such as algae, fungi, and bacteria [49] (Fig. 6). These days, bio-bioplastic degradation has reported that their biopolymers Table 1 Types of degradable plastics according to the mechanism of degradation [43] Type of degradable plastics Biodegradable plastic Photo-degradable plastic Oxidative degradable plastic Hydrolytically degradable plastic Compostable plastic

Mechanism of degradation Degradation occurs by the action of natural microorganisms such as bacteria, fungi, and algae Degradation occurs by the action of natural sun light daily Degradation occurs by oxidation Degradation occurs by hydrolysis Degradation occurs during composting, CO2, biomass, and inorganic compounds at a rate consistent with biological processes

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Fig. 6 Enzymatic degradation of bioplastics. (Adapted with permission from Ref. [49], copyright 2016, Elsevier)

have received great attention in markets, because they meet the requirements of an environmental impact on the life cycle or assessments of the life cycle to achieve proper removal (Fig. 7) [40, 44]. Recently, bioplastics have been mainly generated from agricultural feedstock such as carbohydrate ricing plants, for example, sugarcane, corn, legumes, and cassava. The bioplastics industry is currently invested in diversity development and research. The biogenic availability of feed stock for bio-based plastics production is necessary (Fig. 8). The industry is especially aiming for the development of fermentation technologies that utilize the sources of lignocellulosic feedstock as nonfood crops acting as agricultural waste remains. The estimate of European Bioplastics published at the end of 2020 indicates that the land area used for growing the biomass and the bioplastic production in 2017 represented 0.016% of the global agricultural area, 97% of the land used for food and feeding. Regarding the expected elevated growth rates of the bioplastics industry throughout future years, the land use will slightly increases to 0.021% of the total agricultural lands starting in 2020 [50]. The experiments of compost soil proved the ability of the bacteria (Arthrobacter sulfonivorans and Serratia plymuthica) and the fungi (Clitocybe sp. and Laccaria laccata) to biodegrade PLA and PET. Combining these microorganisms with the plants studied (Brassica napus L., Salix viminalis var. gigantea, and Miscanthus x giganteus) increased the microbial residents in the soil, in addition to the degradation of plastic films. The occurrence of the microorganisms in soil with PLA decreased

24

Environmental Impact of Biodegradation

731

Fig. 7 Bioplastic reuse via photosynthesis. (Adapted with permission from Ref. [49], copyright 2016, Elsevier)

the soil pH value. In compost and soil, significant polymer degradation changes were detected in S. plymuthica [40, 51] (Fig. 9). Three kinetics were recognized by measuring the evolved CO2: 1. The biodegradation of low molecular weight elements, 2. The self-degradation of the formed biomass in the first phase, 3. the biodegradation of the rest of polyesters. The plastic material with nano-plastics (100 nm diameter) can support biodegradation fully for 15–20 days [52]. Bioplastics are usually used for two different things: bioplastics (plastics made partially from a biological material) and biodegradable plastics (plastics that can be completely broken down by microbes within a suitable time, under certain conditions). Not all bioplastics or plastics are biodegradable, and even biodegradable plastics cannot biodegrade in all environments [53]. In addition, not all bio-permeable plastics are found in every environment, for example, PLA. It is used to manufacture transparent cups, shopping bags, 3D printing material, and various other products. It can be derived from plant materials such as potato, corn sugar, or sugarcane; it

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Fig. 8 Bioplastic production capacities in the world continents in 2020. (Source: European Bioplastics [50])

Catalytic Domain

1. Expression of extracellular enzymes

4. Assimilation through membrane for digestion

Substrate Binding Domain

2. Enzyme adsorption

5. Mineralization CO2, H2O, CH4...

Low molar mass, solubel intermediates

3. Hydrolysis

Surface Erosion

Fig. 9 Biodegradation of water-insoluble polymers by bacteria. (Adapted with permission from Ref. [40], copyright 2016, Elsevier)

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Environmental Impact of Biodegradation

733

decreases the need for fossil fuels to make conventional plastics. PLA is biodegradable, recyclable, and compostable. But it is impossible to use it in all environments. According to Frederik Wurm, there are drinking straws made from PLA that are perfect for greenwashing. Although they are more expensive than other plastic drinking straws, they do not entirely biodegrade in the sea or on beaches [54]. Are the bio-plastic components toxic or safe? After the extraction, 43 daily biodegradable or bio-based products or their precursors were analyzed, especially those with most contact with foods. The estimated results were 42% toxicity, 23% oxidative stress, and only one substance produced an anti-androgen effect. The chemical and toxicological features of polyethylene terephthalate (Bio-PET), polyethylene (Bio-PE), PHA, polybutylene succinate (PBS), PLA, polybutylene adipa teterephthalate (PBAT), and bamboo-based substances spread according to the expert product rather than the substance. Toxicity spread less and was not as strong in raw substances than in the end product material. Current plastics, referred to as plant-based materials and bioplastics, have the same toxic impacts. So it is necessary to focus on types of chemical safety in the future (Fig. 10, Table 2) [54–57].

Biodegradation of Herbicides, Pesticides, and Insecticides Naturally, pesticides and their degradable products can be degraded or transformed by microorganisms such as bacteria, fungi, and algae in the presence of stimulating factors such as the growth of some plant species, e.g., Luffa cylindrica (L.) M. Roem. (Loofa), Agave tequilana F.A.C. Weber, and the beans of Coffea arabica L. that lead to total degradation by the microbial interactions, although continuous xenobiotics such as metabolic end substances and pesticides accumulate gradually in the environment and enter the food chain, causing bio-magnification or forming soil humus (Figs. 11, 12, 13, and 14) [58]. Organophosphates (OPs) are one of the major constituents of herbicides, pesticides, and insecticides. OPs disturb the growth mechanism of herbs, pests, or insects by inhibiting their enzymatic system, permeability, and diffusion, which is crucial for plant growth. The use of OPs regularly decreases microbial fauna and reduces soil fertility. Owing to environmental fears regarding the gathering of OPs in foods and water sources new eco-friendly economical biodegradable herbicides, pesticides, and insecticides have been developed [59]. Biodegradable products have specific fundamental and environmental characteristics related to the soil for agriculture. The use of mulch films is common in agricultural biodegradable plastic nowadays. The following list shows the main effects of mulch films in agriculture regarding NFU52-001: controlling the weed growth (if the mulch film is vague), reducing the loss of soil water, reducing the leaching of soil chemicals, protecting fruits and leaves from diseases of soil, protection of fruits from dirt (soil dirt), protection of soil from crusting, and the elevation of soil heat for vague infrared products [43, 60–62]. In soil, effective degradation of pesticides occurs when organisms cannot metabolize and degrade them and survive in the polluted environment. The degradation

734

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AREc32

YES

YAAS

EC20 EL ECIR2 EL hERa Cyto hAR Cyto

PLA

PHA

Starch

Cellulose

Bamboo

1 2 3 4 5 6 7 8 9 10 1 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7

Bio-PE

1 1 2 3 4 5 6 7 8 9 10

Bio-PET

1 2

PBS

1 2

PBAT

1 2

(%) 100 80 60 40 20 0

PB1 PB2 C PB3 PB4 SB

Fig. 10 Toxicological indicator of plant-based substance and bioplastics according to oxidative stress response (AREc32), toxicity baseline (Microtox), anti-androgenic activities (YAAS), and estrogenic (YES). The results are represented according to the effect levels (ELs), concentrations

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Environmental Impact of Biodegradation

735

Table 2 Plant-based substances and bioplastics. Detection of chemical properties by UPLCQTOF-MS/MS. FCM: Indication to detect that a substance is relevant to food contact. Raw material/substance (RM), end/final product (P), where n.a. not analyzed [54] Plastic category Bio-based, biodegradable

Sample and plastic type PLA 1 PLA 2 PLA 3 PLA 4 PLA 5 PLA 6 PLA 7

Petroleum-based, biodegradable

Plant-based

PLA 8 PLA 9 PLA 10 PHA 1 PBS 1 PBS 2 PBAT 1 PBAT 2 Starch 1 Starch 2 Starch 3 Starch 4 Starch 5 Starch 6 Starch 7 Starch 8 Cellulose 1 Cellulose 2 Cellulose 3 Cellulose 4

Plastic product Single-use drinking cup Disposable cutlery Film Food tray Coffee capsule Bag for foodstuff Single-use bottle Film Pellet Pellet Pellet Plastic bar Food tray Waste bag Pellet Disposable cutlery Bag for foodstuff Film Film Pellet Pellet Waste bag Film Tea bag wrapping Chocolate wrapping Cigarette filter Pellet

FCM +

Type P

Number of detected features 3755

+

P

3479

+ + + +

P P P P

8648 6465 6121 17,224

+

P

3002

+ + + +

P RM RM RM RM P P RM P

10,958 3667 880 614 3864 10,959 15,843 9161 1065

+

P

18,198

+

P P RM RM P P P

15,770 16,857 9118 8325 20,965 11,901 14,456

+

P

3378

 +

P RM

15,719 2953

+

+ + + 

(continued) ä Fig. 10 (continued) (EC20, ECIR2), EC20 for cytotoxicity (Cyto), and relative receptor activation/ inhibition. Results are represented according to a gradient from 0 ¼ green to 100% ¼ red. The other results were symbolized to the lowest effect, and the highest one noticed to the end point using endocrine activities. For AREc32 ELs, the highest value was noncytotoxic, by concentrations. (Adapted with permission from Ref. [54], copyright, 2020, Elsevier)

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Table 2 (continued) Plastic category

Sample and plastic type Cellulose 5 Cellulose 6 Cellulose 7 Bamboo 1

Bio-based, non-biodegradable

Bio-PE 1 Bio-PE 2 Bio-PE 3 Bio-PE 4 Bio-PE 5 Bio-PE 6 Bio-PE 7 Bio-PE 8 Bio-PE 9 Bio-PE 10 Bio-PET 1 Bio-PET 2

a

Plastic product Bag for foodstuff Bag for foodstuff Bag for foodstuff Reusable coffee cup Bag for foodstuff Wine closure Bag for foodstuff Pellet Food tray Film Wine closure Pellet Bag for foodstuff Film Reusable bottle Box

FCM +

Type P

Number of detected features 20,416

+

P

14,031

+

P

17,495

+

P

5426

+

P

5272

+ +

P P

1629 n.a.a

+

RM P P P RM P

819 290 928 947 186 19,028

+ +

P P

13,381 390

P

5625

+ +

where n.a. not analyzed

rate depends on various features, e.g., physical/chemical features of the residue, character of the soil, climatic factors (rainfall, temperature, and humidity), and the number of microorganisms able to analyze pesticides. Amazingly, microorganisms exposed to pollution develop mechanisms for adaptation to the predominant conditions. This activity is thought to have developed so that microorganisms can use the active component of the pesticides in the soil environment [40].

Biodegradation Agricultural Crop Residues Crop residues, the secondary products in the crop production process, are precious natural wealth, so it is urgent to manage them to maximize the various inputs. Crop residue management is a widely and well-defined accepted knowledge, so it is a backbone component of agricultural conservation. The speedy change of conventional agriculture into input-severe modern agriculture usually causes an elevation of crop residue production. Cultivation of food in large quantities for facing a high

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Environmental Impact of Biodegradation

737

Fig. 11 A scheme of pesticides in the environment. (Reproduced from Ref. [58], copyright 2013, Intech Open)

Fig. 12 Microbial communities and pesticides, an overview of biodegradation [58]

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OH

O HO NH

O

OH O

HO

O O

HN

OH

O

O O

HN O

OH

Fig. 13 Biodegradation pathway of carbofuran. Presence of various bacteria by metabolic hydrolysis. (Reproduced from Ref. [58], copyright 2013, Intech Open)

O O

HN O

O

O HN

O

HN

O

OH

O

O OH

O O

HN

O

O

Fig. 14 Fungal biodegradation of carbofuran. (Reproduced from Ref. [58], copyright 2013, Intech Open)

population leads to the fast generation residues. Ecosystem services of crop residues enhance necessary plants and soil health (Fig. 15) [63–65]. Tropical crops such as wheat, rice, and maize have 40% C, 1.3% K, 0.8% N, and 0.1% P, providing shelter and food leading to suitable soil microbial biological

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Fig. 15 Benefits of crop residue conservation and degradation for enhancing soil fertility. (Reproduced from Ref. [61], copyright 2018, Frontiers)

processes [66]. Additionally, removing crop residues for industrial purposes and cattle feeding increases removal of nutrients from croplands and harming of the soil with numerous negative impacts, erosion, then degradation of the soil, hampering soil, water, and air quality. Therefore, the conservation of crop residues in the land after harvesting can successfully preserve the components of the soil and maintain productivity. Degradation of residues reduces water flow, water conservation, and sediment losses or transport [66]. The degradation of residue mulching can decrease the soil loss up to 43 times compared with exposed land. Mulching also helps to reduce the water flow, presented sediment in the water flow, and nutrient loss. The highest ground cover of crop residue can decrease the surface soil losses up to 30%. Using crop residues affects water flow, conserves water and sediment transport with increasing plant density and mulching residues, and the potential for water flow decline [67]. Soil erosion, then removal of the fertile surface soil layer depends mainly on a cropping system. Mono crop cultivation with decay permits crops to accelerate water and soil loss yearly. Soybean residue can decrease the soil loss up to 50% in comparison with no residue soil. Conservation of crop residues and their management effectively protect against soil loss in Ethiopia’s wheat cropping systems [36]. The effect of straw mulching on zero-tillage land for potato in the coastal saline soil of West Bengal, India, showed that the development of salinity in fallow rice fields was faster than in a no-dig-mulched field and demonstrates a positive impact on the control of soil salinity via a decrease in evapotranspiration [68]; the level of soil salinity (0.44%) was reduced to 0.07% after straw mulching over 2 years [69]. Under the mulched condition, the soil salinity level in the surface

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layers decreased compared with the controlled conditions (no mulching) [69]. The transformation of plant carbon by degradation of agricultural residues into soil carbon by sequestration of farming soils is necessary to decrease GHG emissions, especially in greenhouses [70]. The nitrogen content increased from 75% to 1.23% in 100% agricultural compost, and the positive impact of oily waste was significant for extra humidity because of the hydrophobic feature of the special waste. Agricultural residues and oily waste play the role of bulking agents, so that they can be decomposed by cow manure as compost. Compost is a certified ingredient that allows bacteria to have a growth medium that requires the growth of micro-organisms. Aerobic composting was adopted as a method of composting to avoid bad odors. The type of soil, the bio-inoculum, and the method of composting affect crop production [71]. By studying of wet fermentation and composting for biogas production of contaminated crop residues with noxious pyrrolizidine alkaloids. In addition to testing of the ability for toxic pyrrolizidine alkaloids in plant biomass to degrade. A sum parameter method was applied to identify and quantify toxic PA (all 1,2-unsaturated retronecine- and heliotridine-type ester PAs). In addition, the common PAs of the initial PA-plant material, all similar structures (e.g., metabolized forms of a PA including all modifications of Pas as yet unknown) were covered, then the known and relevant features of PA toxicity still existed (1,2-unsaturation and esterification) [72]. They manufactured tableware from biomass , from the bagasse left over from sugarcane after sugar production, was low-cost, biodegradable, and eco-friendly compared with synthetic plastics [73]. Results of biodegradation of agricultural residues and oily waste; the nitrogen content was elevated from 75% to 1.23% in 100% agricultural compost, and the positive impact of oily waste was important for extra humidity because of the hydrophobic feature of the special waste. Agricultural residues and oily waste play bulking agents to be decomposed by cow manure as composting. Compost is a certified ingredient that allows bacteria to have a growth medium that requires the growth of microorganisms.

Biodegradation of Oil Many microorganisms can metabolize oil components such as bacteria, yeasts, molds, fungi, algae, and protozoa. These microorganisms can convert oil into carbon and energy; they are naturally distributed in different areas of the world’s oceans, especially chronic oil-polluted coasts that receive untreated sewage and industrial discharges of ships, bacterial biodegradation hydrocarbon (Fig. 16) [72, 74]. Numerous interacting biological, chemical, and physical factors affect the biodegradation of oil in seawater, such as the occurrence of a specific microorganism, its suitable habitat, presence of its nutrients, wind-direction sea movement, the velocity of oil, type of oil components, salinity, chemical bulk in the area, amount of accumulated oil, and long term mechanism of the natural degradation of fat [75, 76].

24

Environmental Impact of Biodegradation

N, P, Fe, Cu, ...

Biosurfactant

environmental

741

T, P, O2

Biologically dispersed oil

Chemical dispersants

Chemically dispersed oil

Metabolic products

grazing

Metabolic products

viral infection

Fig. 16 Oil ecology, oil dispersion, and dispersant biodegradation. (Reproduced from Ref. [77], copyright 2002)

Hydrocarbon-oxidizing microbes can give bio-surfactants to exhibit oil degradation, which is symbolized in blue. Environmental features, appearing in orange, were responsible for the regulation of biodegradation as pressure, temperature, nutrient, and availability of electron acceptors. The activity of microorganisms may be stimulated and inhibited by chemical dispersants. Various hydrocarbon degraders, which appeared in red, can degrade chemically dispersed oil and dispersants (Colwelliasp.RC25). Secondary consumers for metabolism, compounds are produced during oil biodegradation, (dispersant impacts are greatly unknown, and appeared in gray). Some of this network (availability, nutrient, viruses, or grazers) may affect all the above microorganisms. Oil pollution is a worldwide threat to the environment, so removing contaminated oil from soils, sediments, and wastewater is a dominant challenge for environmental research. One of the effective methods of soil recovery under moderate concentrations of pollutants is bioremediation, where nonbiological techniques are not achieving economic benefits. Earthworms and additive organic material increase the degradation of petroleum hydrocarbons in soil by microbes. By treating Eisenia fetida, Lumbricus terrestris, and

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Allolobophora chlorotica, the total petroleum hydrocarbon (TPH) concentration decreased significantly in the degradation of crude oil-polluted soil. A depression of phytane and pristane components under earthworm treatments referred to microbial degradation. Also, soil respiration was increased owing to earthworm stimulation. Mixing of the soil (simulation burrowing activities by the earthworms) [72]. The elevated rate of oil pollution of the Niger Delta, refining crude oil led the destruction of farm lands and aquaculture and the loss of biodiversity. By determination of the effect for granite dust, poultry waste, and palm bunch ash on the hydrocarbon degradation rate. Oil-polluted soil was gathered from a site of hand workers in the Niger Delta with mixing of additional combinations (poultry waste + granite dust + palm bunch ash (1:2:1), poultry waste only, and granite dust only) of the waste. Chemical features observed over 70 days included TPHs, nitrate, phosphorus, potassium polycyclic aromatic hydrocarbons (PAHs), and soil pH. The 16S rRNA gene detected bacterial differentiations of molecular markers. The hydrocarbon rate removal was higher under treatment with the combined waste substances (0.04 d1) than with a single waste substance (0.02 d1). TPHs in the combined waste treatment decreased from 16,0 0 0 mg/kg to 533.85 mg/kg on day 70, whereas pH increased from 6.1 to between 6.8 and 7.1 in all treatments mixed with nutrients. Effective elevations in hydrocarbons utilizing bacterial counts in correlation with pH (R ¼ 0.82; p ¼ 0.04), TPH (R ¼ 0.88; p ¼ 0.02) reduction, and phosphate (R ¼ 0.93; p ¼ 0.006). The refined diesel was less biodegradable, with the genera Providencia and Lysinibacillus showing the hardest biodegradation potential [78]. The dispensability and stability of oil drops in the water column were effective factors affecting the biodegradation of oils. Because of the stability of oil drops with interactions of particles, oil-suspended particulate matter aggregate (OSA) appeared as a five-fold enhancement of biodegradation regarding nondispersed oil. The decrease in TPHs was greatest in OSA, about 51.45%, then 33.5% by chemically dispersed oil, 21.6% by an oil film, and 14.3% by water-accommodated fractions (WAFs). PAH profiles and percentage weathering plots showed that decreases in PAHs in oil film, WAFs, and chemically dispersed oil were caused by evaporation (41.5–69.5%) and only some were caused by biodegradation (7.4–16.3%). A reduction of 36.8% of PAHs in OSA occurred because of biodegradation, and then 29.7% by evaporation. The strong PAH–particle interactions in OSA can inhibit evaporation of PAHs and elevate the biodegradation of microorganisms in the water column [75]. The spread of oil on water is affected by the interaction of oil type, waves, water currents, winds, temperature, and enhanced evaporation of the volatile parts such as non-aromatic hydrocarbons and alkanes with low molecular weight. Splitting oil into droplets, then dispersing it along the water column led the enhancement of the biodegradation of hydrocarbons, and then the dissolution of the water-soluble particles of oil. A turbulent sea causes the suspending of water drops in the oil, forming emulsions or water-in-oil, which like chocolate mousse, is hard to degrade owing to its higher viscosity and reduction of the surface area. Photo oxidation forces hydrocarbons (PAHs) to react with oxygen during sunlight, causing structural

24

Environmental Impact of Biodegradation

743

Fig. 17 Scheme of a marine oil spill. (Reproduced from Ref. [76], copyright 2012, Springer)

changes, thus increasing water conversely or solubility, increasing resistance toward biodegradation. Sedimentation will generally occur when oil adsorbs the particles near crude oils that have a lower density than this sea water (Fig. 17) [76].

Environmental Impacts of Biodegradation on Air Purification Acetone biodegradation in a trickle-bed biofilter was performed using two similarsized columns, one filled with coconut fibers, and the other with plexi-glass chips. The columns were treated with two strains of bacteria: Acinetobacter baumannii and Burkholderia cepacia. The continuous air purification process was performed at gradual acetone concentrations ranging from 0.3 to 2.5 g3 air and air flow rates ranging from 0.1 to 0.3 m3/h. An acetone elimination has its maximum capacity of 95.8 gm3/h at an air flow rate of 36 m/h (Figs. 18 and 19) [79]. Botanical filtration has effectively eliminated indoor gas pollutant by using Epipremnum aureum (golden pothos) within its normal soil or activated root bed.

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Fig. 18 Evidence for acetone-removing efficiency on the inlet load in a coconut fiber-containing biofilter (a) and a plexiglass chips-containing biofilter. (Adapted with permission from Ref. [79], copyright 2016, Elsevier)

It was observed that the air flows dynamically through the root bed in the presence of microbes. It was vital for the removal efficiency of meaningful formaldehyde to be achieved. Regarding static plants in pots as in closed rooms, the clean air delivery rate (CADR), which is usually used to detect the air quality and has the cleaning ability of mobile air cleaners, was 5.1 m3/h/m2 bed only, whereas in the air flowing

24

Environmental Impact of Biodegradation

745

Elimination capacity (g m-3h-1)

a

Elimination capacity (g m-3h-1)

b

120 Experimental data

100

100% removal Regression curve for experimental data

80 60 40 20 0 0

20

60 80 40 Inlet load of acetone (g m-3h-1)

100

120

120 Experimental data

100

100% removal Regression curve for experimental data

80 60 40 20 0 0

20

40

60

80

100

120

Inlet load of acetone (g m-3h-1) Fig. 19 The capacity of biofilters containing plexiglass (a) versus coconut fiber (b) to remove an inlet load of acetone. (Adapted with permission from Ref. [79], copyright 2016, Elsevier)

through the bed, the CADR increased up to 233 m3/h/m2 bed. The estimated CADR as the microbial activity is 108 m3/h/m2 bed. Moisture in the root bed also played a vital role in maintaining suitable living conditions for microbes and absorbing water-soluble compounds such as formaldehyde. The plant was responsible for introducing and supporting a suitable microbe community that effectively degraded the unstable absorbed organic compounds. The presence of the plant increased the removal efficiency [80]. Glycine max (L.) Merr. (Soybean) consists of a large number of functional chemical groups, about 18 different amino groups. Each chemical group has the potential to hold the pass of pollutants at the molecular level. Acrylic acid was used to disentangle the rigid soy protein to expose the chemical groups to pollution. The manufactured filter could remove all small particles and chemical pollutants (Fig. 20) [81].

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Fig. 20 Purification of air pollutants by manufactured filters from Glycine max (L.) Merr. (soybean). (Reproduced from Ref. [81], copyright 2013, WSU INSIDER)

Environmental Impacts of Biodegradation on Water Purification Biodegradation is defined as the biological treatment of sewage/wastewater that uses microorganisms such as bacteria, algae, fungus, protozoa, and green plants in the presence of enzymes to remove the pollutants from the natural domain or by modifying them into beneficial forms. The biodegradation process catalyzes biological, chemical substituents by organic matter broken into more minor compounds by living microorganisms that are easily removable [80, 82]. Biodegradation is a natural phenomenon recently used in sewage purification because it is a very sustainable technology for cleaning contaminated water, where it offers many economic and safe applications. After all, it is superior to other technologies such as physical and chemical removal technologies [82]. Thus, autotrophic strains couple the oxidation of hydrogen gas with reduced perchlorate reduction. Perchlorate reduction gives oxygen and chloride as products of degradation as follows: ClO4 ! ClO3 ! ClO2 ! Cl þ O2 A two-enzyme system shows the catalysis of the reaction. The initial enzyme, a reductase, reduces perchlorate into chlorate and chlorite. Another highly preserved chlorite dismutases into incommensurable chlorite then into oxygen and chloride as a detoxification reaction. The broad diversity and wide environmental distribution of perchlorate-reducing strains are related to their capability to metabolize perchlorate to oxygen and chloride and the rapid growth of ex situ and in situ bioremediation

24

Environmental Impact of Biodegradation

747

scenarios for perchlorate, such as enzymology of perchlorate-reducing strains, ecology, and the phylogeny [83]. Sewage polluted with hydrocarbons is considered an environmental problem that damages human health and the ecosystem. Researchers developed bioreactors to treat these contaminated waters (Fig. 21) [84]. A laboratory-scale plant consisting of three 1-L bioreactors with nonsimilar sorbent materials inside (granulated cork and melt-blown polypropylene) was established. Wastewater was recirculated to be treated by each bioreactor for 7 days. The hydrocarbon retention rate in the three bioreactors is 92.6–94.5% of the TPHs after a cycle of straightforward recirculation. No hydrocarbon fractions were discovered from the gas chromatography/mass spectrometry (GC/MS) in the streaming sewage. After wastewater treatment, the sorbent substances were removed

Fig. 21 Biodegradation of sewage treatment: pathway (a) and scheme (b) by a newly designed system. (Adapted from Ref. [84], copyright 2020, MDPI)

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E. E. Ammar

from the bioreactors and deposited in vessels to examine the biodegradation of the remaining hydrocarbons by the native sewage adhered microbiota to sorbents through the sewage treatment. Removal of TPH was detected after 1 month of use of a Pad Sentec™ carrier (41.2%). Moreover, the transformations detected in the percentage of hydrocarbon fractions show that biodegradation is engaged in hydrocarbon removal. Thus, this system for treating hydrocarbon in polluted waters was effective (Figs. 22, 23, 24 and 25) [84].

600 500

mg/L

400 300 200 100 0 Initial time

st

1

recirc. cycle

Pad SentecTM

1 day

CorksorbTM 01025

7 days

3 days Barrier SentecTM

Fig. 22 The time cycle of total petroleum hydrocarbons in water (mg/L water) before the initial time in sewage after the first recirculation and at days 1, 3, and 7 of operation. (Adapted from Ref. [84], copyright 2020, MDPI)

9 8

Log (UFC)

7 6 5 4 3 2 1 0 7 days Pad SentecTM

1 month CorksorbTM 01025

4 months Barrier SentecTM

Fig. 23 Native adhered microbiota through the carrier bio treatment, expressing as log (UFC/g carrier). (Adapted from Ref. [84], copyright 2020, MDPI)

24

Environmental Impact of Biodegradation

749

160 140 120 mg/Kg

100 80 60 40 20 0 7 days Pad SentecTM

1 month

4 months

CorksorbTM 01025

8 months

Barrier SentecTM

Fig. 24 Time cycle of TPH concentrations through the carrier bio treatment at day 7 (the end of treatment) and after 1, 4, and 8 months. (Adapted from Ref. [84], copyright 2020, MDPI)

Fig. 25 Time cycle of hydrocarbon fraction concentrations determined by GC/MS, absorbed by the carrier through the first month of the bio treatment phase: (a) Pad Sentec™ carrier, (b) Barrier Sentec™ carrier, and (c) Corksorb™ 01025 carrier. (Adapted from Ref. [84], copyright 2020, MDPI)

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Biodegradation and Improvement of Productivity of Plants and Animals The comparative analysis of chickpea remains, as well as unchopped and chopped and wheat straw was studied for 120 days. The following were studied to evaluate their response with adding inorganic nitrogen and mixed fungi; Aspergillus nidulans, Phanerochaete chrysosporium, and Trichoderma viride: C/N ratio, pH, microbial biomass, electrical conductivity, dehydrogenase, total phenol, alkaline phosphatase, xylanase, cellulase, and soluble protein. The evaluation of the organic matter, organic carbon, N, C/N, EC, pH, and microbial biomass) detected that by supplementing unchopped wheat straw with 1% urea and mixed fungi, the smallest C/N (10.7), the smallest biomass of about 9.54, and the highest humus percentage of 13% can be achieved within 3 months. Lepidium sativum (cress seeds) were germinated with an index >60%. Dehydrogenase referred to the maximum microbial activity in inadequate treatments compared with fungal therapy in the second month of composting, although xylanase activity and cellulose showed an upward direction during the curing phase of composting. Chickpea remained composted; although it gave in a C/N (17.3), its germination record was less than 60%. The rapid quality tests recorded for H2S, NH3, NO3, and starch provided stability, whereas the maturity of the last compost was extracted from wheat straw through microbial treatments [85]. The mulch film has vital impacts on the production and growth of crops, where the agricultural biodegradable film should be able to respond with imitative mulch film behavior during this time [86, 87]. After the end of the vital time, the agricultural biodegradable films are rototilled or buried into the soil with the remains of the plants. Then, these biodegradable mulch films are predicted to be entirely biodegraded during a definite time under actual field conditions. Additionally, a mulch film should have the ability to deal with various conditions and media. It is necessary for the film substance degradation to evolve during the time that the same substance of the film is exposed to two different media throughout the crop period [88]. In Canada, management strategies from huge livestock feeding techniques involve the pre-storage of manure. Long-term manure storage takes place on the farmland, then the manure is applied to the field as a fertilizer. Natural biodegradation may cause bad emissions, although with proper use, biodegradation will be a benefit and decrease the pollutants of the animal wastes. During their life cycle, numerous microorganisms use different compounds of manure, either in the absence or the presence of oxygen. Microorganisms can be grouped as their consumed compounds. Organic compounds are considered a vital part of swine manure and consist of many different substances that can be differentiated into four parts: readily biodegradable (SS), inert soluble (SI), inert particulate (XI), and slowly biodegradable (XS). The SS part is usually soluble, composed of relatively small molecules such as valeric acids, monosaccharides (sugar), alcohols, and volatile fatty acids (acetic, butyric).

24

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On the other hand, the XS part is considered to be particles and consists of high molecular weight organic dead biomass or polymers. This organic substance part cannot be directly absorbed by microorganisms and must first be hydrolyzed to SS. The distribution of the organic components can be changeable among the different parts, also depending on other factors such as the manure storage time and the type of feed. SS values from the literature vary between 8% and 30% of the total chemical oxygen demand, whereas XS values were between 10% to 60% for the inert parts (SI and XI). Microorganisms can degrade organic compounds: bacteria, fungi, and protozoa, and the process products are water, CO2, and biomass. Organic matter þ O2 þ Nutrients ! Biomass þ CO2 þ H2 O Aerobic biological analysis of manure treatment may be simple through shortterm manure aeration, removing up to 90% of the biodegradable organic substances. This process can decrease odors (up to 96%, released with volatile fatty acids) through manure storage for up to 190 days. Biological processes usually used suspended biomass as activated sludge reactors developed for sewage and aerated lagoons treatment for the treatment of manure [89]. The main mechanisms of the pollutant biodegradation are similar for all biological systems of treatment: the pollutant is absorbed as a gas phase (polluted air) to a liquid phase, the biological degradation is initiated [56]. Oxidation reactions (in addition to some reductions) convert the pollutants into water vapor, CO2, and biomass. The air contaminants (inorganic or organic) are used as energy and/or carbon to improve microbial communities. There are three sorts of bioreactors with different structures that transfer the gas and the liquid phases of air pollutants and exhibit the microbial metabolic processes: biofilters, bio-trickling filters, and bioscrubbers. Each piece of equipment has a specific structure according to the nature of the microbiological case (microorganisms suspended in the liquid or attached to the filter bed) and also by the circulation of the liquid (flowing or stationary) (Table 3) [56].

Biodegradation: Ecosystem Balancing Viewpoint Natural microbial communities offer numerous regulated ecosystem services, water quality, and maintaining the soil. Recovery of water and soil and the ecosystem from pollution is shown in Fig. 25 [90–92]. One of the most important ecosystem services is air purification for organic pollutants by urban trees via the photodegradation and biodegradation processes on the leaves. Photodegradation and biodegradation showed that the biodegradation processes occurring on leaf surfaces should not be ignored when predicting the uptake and release of PAH by plants. Neglecting this phenomenon may lead to an increase in PAH fluxes to soil and air by 15–30% based on the chemical type, the plant species, and the season [93].

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Table 3 Biological reactors for air purification on farms that use natural biodegradation of manure for energy generation [56] Reactor Biofilter

Biotrickling filter

Bioscrubber

Description Biofiltration is the most widespread and oldest biotechnology for gas emissions treatment. The polluted gases flow via humid porous substances made of organic waste, where microorganisms degrade the pollutants present In contrast to biofilters, bio-trickling filters use an inorganic packing substance with the liquid medium continuously recirculating above the filter bed Advantages: concentration of toxic substances), easy control of essential operating conditions (pH, temperature, nutrient supply, in addition to low-pressure dropping, and reduced space requirements by allowing high flow rates In a bioscrubber, the steps are separated: the contaminants are transferred first to a liquid medium in an absorption system, and then the washing liquid is regenerated in a biological reactor, which is generally like an activated sludge reactor. There are many types of absorbers: the spray tower, the packed tower, the wet cyclone, and the venturi scrubber

Microorganisms Fixed

Liquid phase Stationary

Fixed

Flowing

Suspended

Flowing

Urban trees play an influential role in enhancement of air purification to increase its quality in cities offering a model ecosystem service (ES) (e.g., purifying clear air, psychological comfort for humans and improvement of environmental health. The removal of the widespread inorganic air contaminants (NOx, O3, CO, and SO2) and particulate matter (PM) was gauged for various cities, and the total pecuniary value of this ES was predicted. Urban trees bestow integrated ecosystem services such as air quality improvement. PAHs are among the most substantial air pollutants owing to their maximum concentrations forming toxicity. Plants may act as chemical reactors for pollutant elimination, in addition to filters of PAHs, thus lowering air concentrations. Therefore, the first estimation of photodegradation versus biodegradation of PAHs on leaves of urban trees becomes clear. A dynamic air–vegetation–soil model (Soil Plus Veg) was developed to simulate the fate of two representative PAHs with contradictory physiochemical features (benzo [a] pyrene and phenanthrene). Simulations were carried out in two different environmental scenarios in Italy (Naples and Como), with different meteorological characters, emission levels, and plant species. The effect of photodegradation and biodegradation on leaf concentrations and air flux directions and soil was observed by comparing deciduous trees (cornel, maple, and hazelnut) and evergreen trees (holm oak) that constitute broadleaf woods. It

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resulted in biodegradation in the phyllo-sphere never being ignored during the evaluation of urban tree ecosystem services. This process contributed significantly to the reductions (above 25% on average) in PAH leaf concentrations and fluidities to air and soil; however, the reductions revealed great variations over time (above two orders of magnitude), showing the dependence on meteorological parameters, air compartment structure, as well as the type of woods. These definitions enhance the ecological realism of the simulations and obtained accurate results when predicting organic contaminant uptake and release by plant leaves, including potential for food chain transfer and long-range transport [93, 94].

Biodegradation and Facilities of Human Life Undoubtedly, biodegradation offers various facilities to human life directly or indirectly. Nowadays, biodegradable plastics enter directly into the manufacturing of different eco-friendly products used daily, which are made from plant or algae sources such as seaweeds, for example, bags, water bottles, office tools, furniture, kitchen tools, agricultural beds, agricultural mulch films, medical tools, water filters, cosmetic products, food packages, sachets, food containers and utensils, and drinking cups (Fig. 26) [95–99]. Western countries do not use seaweed as ordinary greens in diets, but since the eighth century, high-protein seaweeds have been eaten daily in Japan (in Asian cuisines). In Indonesia, some creative companies manufactured packaging applications of bioplastics that were made from farmed seaweed. Therefore, this decreases the use of plastics, especially in the manufacture of packaging, where the bioplastics formed from seaweeds are ecofriendly. Thus, it is important to conserve their habitats. Plastic made from seaweeds is also used for manufacturing wrappers, such as for power protein bars, waffles, and burgers, and this packaging is then edible. In addition, the seaweed reused for packaging dry foods such as instant tea, noodles, and cereals can quickly dissolve when liquid is added. Finally, these bioplastics are used for soaps and sanitary napkin packaging, which can biodegrade over time. More applications will be developed in the future as this option is explored [100]. The usage of bioplastics has an increasing number of markets such as catering products, packaging, automotive, consumer electronics, horticulture/agriculture, toys, furniture, and textiles. Packaging represented the most consuming field of bioplastic applications, 47% (0.99 million tonnes) of the bioplastics market in 2020. In addition, the usage of bioplastic applications is marked by diversification, such as building and construction, automotive and transport, and electronics with the increase in the growing functional capacities of polymers (Fig. 27) [38]. On the other hand, the usage of bioplastic, biodegradation of complex oils, chemical pesticides, and fungicides decrease the accumulation of heavy metals in soil, air, and water, saving and increasing the soil fertility and the purity of air and water, thereby increasing the crop yield productivity. Despite the biodegradation of oil in the air, especially in growing trees, will reduce the volatile pollutants such as fumes from cars and factories. This represents an indirect outcome to humans, where

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Fig. 26 Examples of current bioplastic products. (Adapted from Writer [95], copyright 2019, Eco friend)

decreases the costs of recovery of damaged soil, water treatment, or air purification. Moreover, this creates a healthy environment and decreases pollution, leading to the development of human health gradually and lowering costs of medical treatment so that the speed of production will be increased, and then a rise in the standard of living will be achieved. Besides, managed biodegradation of oil discarded from ships in marine habitats will decrease the accumulation of tiny particles in the bodies of fish that cause diseases to humans eating them. Also, this degradation will clarify the water transparency, allowing lighting to penetrate the water layer to preserve the life of deep marine organisms. All previous benefits of facilities of biodegradation to human life ensure the balance of the ecosystem, stability of the food chain, and preservation of wild life on land, in the soil, and in the water, and then ensure the maintenance of the human life.

Conclusions This chapter focused on the environmental impacts of the biodegradation of bioplastics, pesticides, chemical treatments, oil, etc., concerning soil fertility, air purification, water purification, natural biodegradation of manure and mulch,

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Fig. 27 Global production capacities of bioplastics versus various uses in manufacturing. (Source: European Bioplastics and the nova-Institute report 2020 [39])

enhancement of plant and animal productivity, and also improvement of the natural ecosystem, thereby improving human life. It also shows future predictions for using biodegradable products in different aspects of human life directly and indirectly.

Future Prospective Decreasing and then stopping the use and production of plastics, especially singleuse plastics, and finally reconnecting with our discarded culture toward nature. In this way, we return the sense of the earth’s resources, where more products of bioplastic industries in the majority of life activities will be applied in the future. In addition, more research on the degradation of organic pollutants on leaves of different plant species is required. In research on the depositing of PM on leaves, their biodegradation for air purification will accelerate. Undoubtedly, biofertilizers are the future of agriculture globally, where they are expected to replace chemical fertilizers. Because it is safer for the soil and facilitates the process of biodegradation

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carried out by microorganisms, it leads to an increase in soil fertility safely without leaving chemical residues. We also expect to add nanomaterials to biofertilizer, as nanotechnology would provide green and efficient alternatives for managing plant diseases, improve plant resistance to environmental stress, and increase plant growth, productivity, and the quality and quantity of plants.

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Biodegradable Nanocelluloses for Removal of Hazardous Organic Pollutants from Wastewater

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Saikumar Manchala, Ambedkar Gandamalla, and Aravind Rudrarapu

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic Types of Biodegradable Nanocelluloses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymatic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of Biodegradable Nanocelluloses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Biodegradable Nanocelluloses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Specific Surface Area and Surface Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Aspect Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Chemical Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Good Mechanical Strength and Rigidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Nanocelluloses for the Removal of Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . Removal of Dye Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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S. Manchala (*) Department of Chemistry, Indian Institute of Technology, New Delhi, India e-mail: [email protected] A. Gandamalla Department of Chemistry, National Institute of Technology, Warangal, Telangana, India A. Rudrarapu Department of Chemistry, Faculty of Science and Technology, The ICFAI Foundation for Higher Education, Hyderabad, Telangana, India © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_29

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Abstract

Sustainable nanotechnology-enabled contributions have made considerable and impressive solutions to provide contaminant-free water to the global society. Mainly, nanocellulose-based composites are widely employed in wastewater treatment technologies due to their unusual properties like high mechanical strength, eco-friendly, high surface area, functional ability, low cost of production, lightweight, abundant in nature, chemically inert, biodegradable, and regeneration. Moreover, nanocellulosebased composites are used to remove organic pollutants, including dyes, pesticides, fertilizers, organic chemicals, and drugs in adsorbents, catalysts, photocatalysts, flocculants, thin films, aerogels, and membranes. Here, we discussed fundamentals and potential applications concerning organic pollutants removal from water. Besides, various sustainable technologies developed based on the nanocellulosebased composites, and future perspectives are also explained. Keywords

Nanocelluloses · Nanocellulose-based composites · Adsorbents · Membranes · Wastewater treatment

Introduction The availability of safe drinking water is a significant issue worldwide in many countries. Water pollution has become a critical problem in many regions due to rapid human population growth, industrialization, and urbanization. Many domestic, industrial, agricultural, chemical, and pharmaceutical effluents such as dyes, organic compounds, pesticides, fertilizers, and harmful heavy metal ions must be eliminated from polluted water to obtain safe drinking water worldwide [1, 2]. There are many methods reported over the years to introduce several wastewater treatment technologies to remove harmful contaminants from the water: distillation, filtration, adsorption, biological treatment, membrane separation technology, catalytic, and photocatalytic processes [3–8]. The methods mentioned above have good elimination efficiency and are cost-effective for water purification processes. In recent years, enormous research has been examined for eco-friendly and novel materials for water purification with low energy requirements, low lost, and no harmful by-products. In this sense, biodegradable nanocellulose (BNC) is an essential biopolymer/nanomaterial. Because of its hydrophilic nature, insolubility in neutral water pH environments, inexpensive, reusable, biodegradability, interconnected porosity, high tensile strength, stiffness, tunable porosity, and more availability of OH surface functional groups, which helps the incorporation of different chemical moieties, these incorporated moieties can support the remove different contaminants such as toxic heavy metals, toxic dyes, pesticides, and so on. It concludes that BNC is an enormous potential material for water filtration [9, 10]. Moreover, BNCs possess many potential applications in batteries, fuel cells, supercapacitors, pharmaceutics, medicine, solar panels, plastics, cosmetics, flexible electronics, sensing devices, polymer industry, construction industry, car, etc. [11].

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Basic Types of Biodegradable Nanocelluloses Generally, BNCs are categorized into three types; as shown in Table 1, those are bacterial (1), micro-fibrillated (2), and nanocrystalline (3) BNCs [12].

Overview of Synthesis Methods The synthesis of BNCs requires more energy. To avoid this, the pre-treatment methods like refining, steam explosion, chemical pulping, ultrahigh friction grinding, twin-screw extrusion, enzymatic, alkali, and TEMPO-oxidation have been important to minimize the energy consumption before proceeding to further synthesis steps. Moreover, the production quantity can also increase to commercialize the generation of BNCs. Majorly three routes have been used for the synthesis of BNCs, which are chemical (3.1.1), enzymatic (3.1.2), and mechanical (3.1.3) treatments. Figure 1 depicts the extraction BNCs from trees [12–15].

Mechanical Methods It involves mechanical energy to produce BNCs from the pulp. Pre-treatment should be performed to simplify the process to proceed with mechanical fibrillation. Several mechanical methods were developed to produce BNCs from wood fibers, including microfluidization, high-intensity ultrasonication (HIUS), high-pressure homogenization (HPH), cryo-crushing, grinding, and ball milling.

Table 1 Types of biodegradable nanocelluloses Type Bacterial

Other names Biocellulose, microbial cellulose, bacterial cellulose

Sources Bacteria

Microfibrillated

Nanofibrils, microfibrils, and nano-fibrillated cellulose

Wood, tuber, sugar, flax, beet, potato, and hemp

Nanocrystalline

Cellulose nanocrystals, rod-like cellulose microcrystals, whiskers, and crystallites

Wood, wheat straw, cotton, Avicel, tunicin, hemp, flax, mulberry bark, ramie, bacteria, and algae

Size Diameter: 20–100 nm Length: Different types of networks Diameter: 5–70 nm Length: Several micrometers Diameter: 5–70 nm Length: 100–several nanometers

Synthesis Bacterial synthesis

Chemical and/or mechanical

Acid hydrolysis

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Fig. 1 The process of extracting BNCs from trees and micrographs of wood fibers/cellulose forms cellulose nanofibrils and nanocrystals [16]

Chemical Methods The chemical method is the preferable, energy-efficient, and powerful process that takes less time for the BNC production than the mechanical method. Generally, the mechanical process yields amorphous domains of polysaccharides which can be destroyed by acid hydrolysis to produce rodlike cellulose nanocrystals, resulting in high crystallinity. Furthermore, it increases the space between the surface hydroxyl groups and enlarges the inner surface by disintegrating the cellulose hydrogen bonds. These all result in improved surface areas, further enhancing the reactivity of the BNCs. Several strong acids can be used in hydrochloric, hydrochloric, hydrobromic, phosphoric, phosphotungstic, sulfuric, nitric, formic, and maleic acids. Some other chemicals, including ethylenediamine and urea, were also reported to isolate cellulose structures effectively from kraft pulp.

Enzymatic Method It is also called a biological method. Here, enzymes can hydrolyze the cellulose microfibrils. Generally, laccase break down the amorphous and lignin contents without damaging cellulose structure. Cellulose fibers have a composite structure and compose of several types of organic compounds. But a single type of enzyme is not sufficient to decompose the fibers. There is some set of enzymes that can degrade the fibers. Those are:

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(i) Cellobiohydrolases: divides into A and B type cellulases – significantly attack crystalline cellulosic fibers. (ii) Endoglucanases: divides into C and D type cellulases – an attack on the disordered structure of cellulosic fibers. At present, most researchers are concentrated on optimizing the existing synthesis methods to begin eco-friendly methods; those can help in the commercialization process for the production of BNCs with new properties.

Characterization of Biodegradable Nanocelluloses Biodegradable nanocelluloses can be characterized by using different techniques like X-ray scattering, CP/MAS 13C NMR, electron microscopy, atomic force microscopy, laser light scattering, infrared, Raman spectroscopy, thermo-analytical methods, BET, calorimetry, conductometry, potentiometric titration, and standard chemical methods to evaluate the properties [17], as listed in Table 2.

Properties of Biodegradable Nanocelluloses Biodegradable nanocelluloses possess several anomalous properties, which are the main reason to keep BNCs in the top place for wastewater treatment. Those are depicted in Fig. 2.

High Specific Surface Area and Surface Tension If the material has a high surface area, then the number of active sites on that material has increased, leading to bio-sorbent to restrain metal ions. Generally, BNC has a high specific surface area. The surface energy of BNC is nearly 60 m Jm 2; basically, the BNC surface is highly hydrophilic and readily soaked with polar solvents and water [18].

High Aspect Ratio BNC showed a high aspect ratio, which favors the setup of entangled and percolated networks composed of heavy hydroxyl groups due to its fibrillar nature and smaller size.

High Chemical Resistance The solubility of BNC is limited even in high polar solvents due to its high crystallinity, which varies between approximately 60 and 80%.

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Table 2 Summary of characterization techniques utilized to investigate the biodegradable nanocelluloses Technique Wide-angle X-ray scattering

Solid-state cross-polarization magic angle spinning 13C NMR spectroscopy Infrared and Raman spectroscopy Electron microscopy Atomic force microscopy Laser light scattering Thermogravimetric analysis Differential scanning calorimetry Differential thermal analysis BET Calorimetry Potentiometric titration, conductometry, and standard chemical methods Zeta potential

Information obtained Structural investigation (types and content of allomorphs, distortion degree of the lattice, interplanar distances, sizes of nanocrystallites, and crystallinity) Crystallinity, lateral size of nanocrystallites and their aggregates, type of crystalline allomorph, and degree of substitution Some structural characteristics, functional group identification, and degree of crystallinity Size, morphology, and composition Thick ness Particle size Changes in the mass with respective to temperature Specific heat capacity and heat of fusion Endothermic and exothermic phase transitions Surface area Measuring the amount of heat Know the quantity of carboxylic and sulfonic functional groups Electric potential based on the size

Good Mechanical Strength and Rigidity Generally, BNC materials show good mechanical strength and high rigidity, and this high rigidity is expected to enhance the mechanical properties of the adsorbent, allowing the potential for regeneration and reuse without the risk of disintegration. The intrinsic mechanical strength of BNC was 120–170 GPa in modules [19].

Surface Functionalization The surface modifications of BNC through the different functional groups were straightforward because of having high-density hydroxyl groups on their surface. This surface modification could be achieved by oxidation of hydroxyl groups into the carboxylic group, etherification, esterification, silylation, radical grafting, and addition [20, 21].

Biodegradable Nanocelluloses for the Removal of Organic Pollutants Wastewater contains drugs, pesticides, dyes, antibiotics, organic chemicals, etc. These are responsible for many diseases in humans and animals. Recently, nanotechnology has been recognized as a tremendous potential, cost-effective, and

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Fig. 2 The beneficial effects of biodegradable nanocelluloses

environmentally friendly method in wastewater treatment and control. BNCs have an increasing interest, and specific characteristics of renewable energy sources have led to substantial research aiming for current generations of materials. Numerous features have led to an interest in BNC as a new family of materials in wastewater treatment. Based on the BNCs, various methodologies have been emerged, such as adsorption, absorption, membrane technology, disinfectants, flocculants, catalyst, and photocatalysts for wastewater treatment, as depicted in Fig. 3. The key two uses for BNC in this field are its usage as a useful sorbent material for pollutants and as a stabilizing agent for many other pollutant particles [22].

Removal of Dye Pollutants In recent years, the usage of synthetic dyes in the textile, paper, food, and pharmaceutical industries has increased significantly, which has led to the release and deposition of industrial dye-containing pollutants into aquatic environments. This dye contaminates toxic and very harmful human and aquatic animals even in minute quantities because most of the dyes have a complex aromatic structure that is strong to light, ozone, and biological activities and is not readily eliminated by the conventional wastewater treatment technologies. Previous researchers report it the

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Fig. 3 Pictorial representation consists of various water/wastewater treatment processes based on cellulosic systems

annual worldwide production of dyes is 700,000–1,000,000 tons, and it has been noted that more than 100,000 different types of dyes are commercially available [23]. For removing industrial dye effluents from the aquatic environment, adsorption is one of the most efficient methods. In recent years, researchers reported using alternative materials other than metal oxides, metal nitrides, and metal sulfides with low cost and synthesized from the natural biological method as a suitable adsorbent for industrial dyes. BNC has gained tremendous interest among those alternatively developed materials because of its ready availability of surface functional groups, high strength, and abundance in nature [9]. BNC-based composites are the most important promising materials for dye degradation. This chapter mainly focused on the membrane-based nanocellulose composites, adsorbent-based nanocellulose-composites, catalyst-based nanocellulose composites, and photocatalysis-based nanocellulose composites for dye degradation. The most important for the current researchers are membrane-based nanocellulose composites for dye degradation and wastewater treatment process. Different types of membranes are being used to remove dyes from wastewater. Water treatment processes employ several types of membranes. They include ultrafiltration (UF), microfiltration (MF), reverse osmosis (RO), and nanofiltration (NF) membranes. Even though recently, Derami and co-workers have been reported a membranebased composite such as polydopamine/bacterial nanocellulose (PDA/BCNC)

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hybrid membrane for the effectively removing heavy metal ions such as lead and cadmium and organic dyes as surrogate markers of organic pollutants such as rhodamine 6G (R6G), methylene blue (MB), and methyl orange (MO) [24]. The results conclude that the PDA/BCNC membrane showed effective contaminant removal from feedwater containing heavy metal ions and positively charged organic dyes at high concentrations (40–60 ppm). The membrane was working effectively from all directions to remove a single dye pollutant or group of pollutants. More importantly, the stability and reusability of the PDA/BCNC membrane were excellent (Fig. 4), and it can be used several times without altering its adsorption capacity. It has extensive properties such as porosity, hydrophobic/hydrophilic nature, low capital cost, and surface charge. These fantastic properties of membrane technologies play an essential role in filtration. Singh and Hankins state that membrane technology has become a bridge between sustainability and economics [25], which is no chemical usage, environmentally friendly, and easy to handle. That is why membrane technology has become a more encouraging process in wastewater treatment. For efficient removal of wastewater process, filtration membrane should be high strength and selectivity. In recent years, cellulose has become a valuable filtration material because it is sustainable, economic, inert, hydrophilic, and stable in all pH strength solutions. Recently, in 2019, Lesedi Lebogang and co-workers reported that Ag3PO4/ nanocellulose (Ag3PO4/NC) composite was synthesized by the facile in-situ casting method [26]. Moreover, the synthesized Ag3PO4/NC composite showed high visible-light-driven photocatalytic performance towards the methylene blue (MB), methyl orange (MO) dye degradation, and the complex matrix under sunlight irradiation. Furthermore, the results prove that BNC can act as excellent supporting material for the dispersion of Ag3PO4 particles, and it avoids aggregation. Therefore, a sufficient supporting material can enhance the visible light absorption capacity. And also, this work demonstrates a good heterojunction between cellulose and Ag3PO4. They also discuss the suitable photocatalytic dye degradation mechanism.

Fig. 4 Pictorial representation of PDA/BCNC membrane for collecting organic dyes [24]

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Organic Compounds Many volatile organic compounds (VOCs) like phthalates, phenolic compounds, alkanes, oils, and so on are released from the research labs and several industries polluting the water. Zheng et al. reported the one-step facile synthesis of highly porous, flexible, hydrophobic, and ultralightweight nanocellulose (NC) sponges using methyltrimethoxysilane solvent in Fig. 5. The hydrophobicity of sponges due to silane used. The silylated NC sponges showed high efficiency for dodecane’s selective removal from wastewater with outstanding recyclability. Furthermore, the sponges can withdraw various VOCs and oils from wastewater with excellent absorption capacities according to their densities [27]. Hydrophobicity is the main property of the sponge and aerogel to adsorb the organic pollutants efficiently [28]. If hydrophobicity is more, the absorption capacity is more for the oil like organic substances. Huazheng et al. developed bacterial cellulose aerogel (BCA) to collect colored plant oil from wastewater efficiently, and BCA has been modified by trimethylchlorosilane for high hydrophobicity. And the obtained hydrophobic BCA (HBCA) are shown in Fig. 6. They possess a high surface area (169.1 m2/g), low density (6.77 mg/cm3), and high porosity (≈ 99.6%) and further able adsorb various organic solvents from wastewater with an efficiency of up to 185 g/g.

Pesticides The most intelligent species on earth poisons its food before eating it. These great words by Rishi Miranhshah demonstrate the harsh truth of the devastation we are

Fig. 5 Schematic diagram showing the general scheme for the synthesis of silylated CNF sponges and their simultaneous use in the removal of red-colored dodecane spill from water [27]

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Fig. 6 SEM images of BCA (a), HBCA-1 (b), HBCA-2 (c), and HBCA-3 (d) [28]

witnessing today owing to the unjust usage of pesticides [29]. A pesticide is a mixture of the highly toxic chemical substance used to destroy or control livestock, fungi, or plants that cause economic damage to crops or ornamental plants or are harmful to the health of domestic livestock or humans. Agriculture was an integral aspect of human culture and so was also the assault on crops by numerous forms of species. Crops benefit from pesticides; however, they can have a significant detrimental effect on the climate. Water has a wide influence on all facets of life, including healthcare, food, energy, and economies, though not constrained. In addition to the environmental, social, and economic impacts of inadequate water supply and sanitation [30–33], it is necessary for the welfare of children and the poor to supply freshwater [34]. The flow of pesticides through processes such as spray drift, oozing, and subsurface draining into watercourses has become an important concern for many water utilizing industries such as textile, paper, food, chip designing, automobile, etc. [35, 36]. BNC was of paramount importance owing to its novel properties such as buoyant, biodegradable, low-cost, strong mechanical strength, recyclability, safe and easy handling, broad surface area, and many potential environmental remediation applications. Most pollutants, such as heavy metals, dissolved organic pollutants, dyes, oil, and undesired runoff, can be adsorbed by BNC [37, 38]. The regenerative ability of the BNC adsorbent is also another reason to take maximum advantage of the emerging nanostructured biobased material.

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Adsorption is a method that has been widely used for the extraction of pollutants in water. BNC, owing to its affinity with chemical compounds, is known as an outstanding natural biomaterial adsorbent for treating wastewater. Referring to microcellulose, the nano-dimensional counterparts are significantly smaller and have a larger porosity surface which constrains internal dissemination and quantum size implications [39]. The enhanced surface area, crystalline structure, and the number of efficient moieties that are accessible will strengthen the adsorption removal competence of BNC. The high modulus and strength of cellulose nanofibrils (CNFs) are now in use in industrial applications. Cellulose nanofibrils (CNFs) are promising BNC materials and have recently been focused on due to their excellent properties for removing various forms of pesticides from aqueous solutions [37, 41– 44]. Some of the hazardous organic pollutants such as herbicides can be effectively removed by these CNFs. Maatar et al. [40] explored the role of organo-gel cellulose nanofibrils with hydrophobic hydrocarbon chains. The adsorption capacity was highly increased by introducing modifications in CNFs (Fig. 7). The adsorption process is the result of the dissemination of the organic solvent within the grafted hydrocarbon chain functioning as a reservoir for the accumulation of organic compounds. The findings revealed, without loss of adsorption capacity, that modified cellulose organogels could be quickly regenerated and reused, which is one of the great advantages of this class of adsorbents from renewable resources. Chlorpyrifos (O,O-diethyl-O-3,5,6-trichloro-2-pyridyl phosphorothioate) is non biodegradable organophosphate pesticide [45, 46]. It can be removed from an aqueous solution using BNC and nanocellulose/graphene composite (NCGC). Nayak and Vashishta synthesized NC and NCGC with potential chlorpyrifos adsorption property. The adsorption studies of chlorpyrifos were carried out using

Fig. 7 Schematic illustration of herbicides physisorption on modified cellulose nanofibrils, (a) diffusion from the bulk solution, (b) accumulation on the organo-gel exterior surface, and (c) diffusion within hexadecyl grafted chains [40]

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UV-visible spectroscopy chlorpyrifos and show absorption around 290 nm. In the case of NCGC, the rate of adsorption increased over time. However, in the case of BNC, absorption increases around 290 nm over time, indicating that the pesticide is desorbed after 15 minutes. Thus, this study has demonstrated that RGO-modified cellulose/nanocellulose could be an effective and stable adsorbent in the water of chlorpyrifos pesticides. However, further research on the practical usefulness of such adsorbents must still be carried out on a large scale leading to improved emission control and numerous other factors. Chlorpyrifos can also be eliminated by the biosorption process using rodlike cellulose nanoparticles by batch method. Moradeeya et al. [47] fabricated a 3.0–3.8nm-diameter ranged BNC fiber, and it is considerably less than previously reported values, i.e., 14.6–80 nm [48, 49]. A regression equation, biosorption (percent), was constructed using three independent operating variables (time, NC, and CP concentration) against response. For the various experimental formulations of the operational parameters provided by the CCD matrix, biosorption efficiency was observed from 2.28 to 99.31%.

Fertilizers Fertilizers are chemicals used in crops to maximize their production. Fertilizers provide the essential nutrients required by plants, including nitrogen, potassium, and phosphorus. Nitrogen is the primary nutrient, and plants take up more nitrogen than other elements. Nitrate is a colorless, odorless, tasteless substance with high water solubility and is a key element of the nitrogen cycle. Nitrate is highly leachable and passes quickly through the surface soil with water. If excessive rainfall or irrigation occurs, nitrate leaches under the root zone and enters the groundwater. Elevated nitrate levels in potable water (over permissible) lead to methemoglobinemia and disorders such as high blood pressure, increased child mortality, stomach carcinoma, meningitis, hypertrophy of the thyroid, cytogenetic disorders, and Parkinson’s disease [50–55]. Ayyasamy et al. [56] attempted to remove nitrates from groundwater collected from Jodhpur and Poli districts in Rajasthan, India. They treated the groundwater in a two-step mechanism, (i) bacterial isolation and (ii) biological treatment. By using the first method, the nitrates present in the groundwater were reduced to ammonia, and the addition of Nessler’s reagent confirms it. In the second step, workers used cellulose at different concentrations and other chemicals. The impact of cellulose on nitrate removal increases tremendously when it is scaled down to nano-size. Quaternary trimethylammonium group-modified cellulose nanofibrils are most effective in removing nitrates from freshwater. The nano paper ion-exchanger produced from cellulose nanofibrils efficiently removes the nitrates from freshwater. Mautner et al. [57] produced three types of cellulose nanofibril materials, unmodified CNF (CNF-0), modified CNF (CNF-A) with quaternary trimethylammonium groups, and the mixture of both (CNF-H). The filtration experiments demonstrated the efficacy of cationic CNF as a nano paper ion-exchanger for nitrate removal.

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Similarly, cationic cellulose nanofibrillated (CCNF) paper membranes were proven to be excellent adsorption material for removing nitrogen from an aqueous medium. The membrane efficiency of CCNF nano papers was analyzed by nano paper permeability measurement and investigations of nitrate adsorption. The membrane efficiency was determined in a dead-end cell at 2 bar head pressure by evaluating membrane permeance (P) and a rejection of contaminants (nitrogen). The permeance of CCNF was compared with previously reported unmodified CNF [57, 58]. Phosphate is another important nutrient present in fertilizers. BNC materials are also capable of removing phosphate from wastewater [56]. Cationic nanofibers of cellulose can even be fabricated from waste pulp residues, and these exhibit potential adsorption properties towards nitrates and phosphates when modified with glycidyl trimethylammonium chloride (positively charged quaternary ammonium group). The unmodified and chemically treated CNF morphology was examined using scanning electron microscopy. The micrographs demonstrated a strong aspect ratio of nanofibers and CNF’s typical network structure (Fig. 8). The diameter of the nanofibers measured in the SEM images is between 10 and 100 nm. Cationic CNF is stronger fibrillated than unmodified CNF, due to charges in cationic CNF that have allowed disintegration by fibril repulsion [59]. Nitrate adsorption to cationic CNF was examined at different pH values. The maximum nitrate adsorption in the pH range 5–7 for all cationic CNFs was envisaged.

Drugs The phenomenon of pharmaceutical occurrences in water sources was first found in the West, especially in the USA, Canada, and Europe. The first pharmaceutical detection study was produced in Germany in 1990 when environmental scientists detected clofibric acid, a cholesterol-reduction drug under a water treatment plant in groundwater. Further studies found that up to 90% of the medicinal compounds existed in untreated wastewater, surface water, groundwater, and potable water; hence, they concluded that only a few sewage treatment plants (STPs) were equipped to eliminate pharmaceuticals at the time. In municipal wastewater, the most commonly identified drugs include anti-inflammatory agents, lipid regulators,

Fig. 8 SEM images of CNF and modified CNFs [56]

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antidepressants, X-ray-related stuff, antibiotics, and tranquilizers [60–62], while the typical wastewater hormones are natural estrogens, estrone, and 17b-estradiol, as well as contraceptive agents such as 17a-ethinylestradiol [63]. Antibiotics make up a significant group of compounds among pharmaceutical and personal care products extensively used for human therapy, breeding, and aquaculture. Remnant antibiotics reach the watercourses by surface runoff, laundry, and industrial waste disposal systems and their ineffective treatment from traditional wastewater plants. Tetracycline (TC, C22H24N2O8) is the second most widely used antibiotic for disease prevention and animal feed due to its high therapeutic [64, 65]. Recently, it has become a global environmental problem as only a limited portion (20%) is consumed in metabolism, and incompletely metabolized TC has been found in farm soils, surface waters, and wastewater. Research has shown that TCs in sewage treatment plants are still partially removed [66–68]. In 2015, Rathod et al. [69] reported a biosorption method to remove TC from aqueous layers using nanocrystalline cellulose. The effects of pH, contact time, the concentration of TC, and desorption were investigated in batch mode. BNC was analyzed by FTIR technique before and after TC biosorption, as displayed in Fig. 9. The interaction between surface functional groups and absorbed TC is revealed by assessing peak demeanor. TC biosorption kinetics were very rapid and reached equilibrium in almost 2 hours. The kinetic parameters were also well defined by double exponential kinetics and pseudo-second-order kinetics. The findings from the result of this work have important consequences for the removal of tetracycline hydrochloride (TC) with crystalline BNC from an aqueous solution. An efficient carboxylated cellulose nanofiber/montmorillonite nanocomposite was fabricated as an effective adsorbent for removing levofloxacin hydrochloride (Levo-HCl) for aqueous solutions. It has also been reported that CMNF-MMT can be synthesized by two-step synthesis [70]. This study intended to use BNC and alkalized montmorillonite as raw materials to fabricate a new composite (CMNF-MMT) material and investigate adsorption efficiency for a typical antibiotic pollutant using levofloxacin hydrochloride (Levo-HCl). The adsorption characteristics were explored using adsorption kinetics and isotherms. It is evident that the reaction followed pseudo-secondorder and the Sips models provide the best fit with experimental data. It is evident that the reaction followed pseudo-second-order and the Sips models provide the best fit with experimental data. In contrast to the adsorption of levofloxacin in pure water, when adsorption equilibrium has been achieved, river water availability (90.37%) was marginally less than that of pure water (93.97%), indicating that CMNF-MMT is not easily affected by the ecological parameters. Reusability studies have shown that CMNFMMMT can retain a certain Levo-HCl adsorption capacity for six uses. Many studies are exploring the efficacy of BNC as a drug carrier, but there are only fewer publications on drug removal using BNC materials. Although studies on BNC pharmaceutical materials have been relatively low, it is clear that BNC can be an efficient drug adsorbent, similar to the far more commonly reported field of dye adsorption.

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Fig. 9 FT-IR spectra of (A) BNC and (B) TC biosorbed BNC [69]

Conclusions Biodegradable nanocelluloses have extraordinary applications in various industries due to their peculiar chemical, biological, and physical properties. Additionally, they are renewable and biodegradable. They are widely explored as adsorbents, flocculants, membranes, photocatalysts, catalysts, absorbents, and disinfectants for wastewater treatment. In addition to wastewater treatment, several commercial wound dressing products are also developed based on the BNC-based nanocomposites. Due to the wide scope to use the BNCs in wastewater treatment and the other nanomaterials, including carbon nanomaterials, noble metal nanoparticles, metal oxides, and sulfides, extensive research should be conducted on the BNC-based nanocomposites and polymers.

Future Perspective There are enormous opportunities to develop commercial BNC-based nanocomposites in various applications. They will provide a route to answer the environmental problems and constitute sustainable and recycle-based societies. Even though there are many applications, there are still some challenges to realize commercial applications like large quantities and high-quality production of BNCs and BNC-based nanocomposites.

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Hassanien Gomaa, Mohammed Y. Emran, and Marwa A. El-Gammal

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of Safe Water and Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Water Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inorganic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation of Textile Manufacturing-Generated Dyes Using Microorganisms . . . . . . . . . . . Azo Dye Biodegradation Using Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Azo Dye Biodegradation Using Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Azo Dye Biodegradation Using Yeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Algae Use for Azo Dye Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Water is a crucial need for all living organisms, and it must be protected against contaminations such as inorganic and organic hazardous compounds and pathogens to keep public health and ecosystems safe. The inorganic hazardous compounds of toxic metals are found in the industrial wastewater from the metal processing, mining, and chemical sectors containing toxic metals, compounds, organic and inorganic compounds, and chlorinated derivatives. Several organic artificial dyes are used for material coloration in the textile industry and are major contributors to organic pollutants in industrial wastewater. The release of H. Gomaa (*) · M. Y. Emran Department of Chemistry, Faculty of Science, Al-Azhar University, Assiut, Egypt e-mail: [email protected] M. A. El-Gammal Biochemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_33

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untreated colored wastewater, including organic pollutants such as dyes and phenolic compounds, into the ecosystem harms the environment’s aesthetics and impacts marine life. Global climate change, industrialization, overcrowding, and agricultural practices have all been related to a lack of appropriate and sustainable sources of clean drinking water. The expanding worldwide drinking water shortage necessitates the development of novel advanced, high-tech, and cost-effective solutions for water treatment to enhance the existing conventional methods. Bioremediation is one of the methods used for wastewater treatment and is known as the breakdown of pollutants by microorganisms into nonhazardous or less dangerous compounds. Moreover, bioremediation reduces or neutralizes hazardous compounds in contaminated water using biological agents such as fungi, bacteria, or green plants. The biodegradation of toxic organic pollutants has been linked to many fungi, plants, and bacteria enzymes. Bioremediation of toxic compounds presents a low-cost, environmentally benign biotechnology fueled by microbial enzymes. Further using and expanding the application of this technology advance the natural and eco-friendly substances for degradation of organic pollutants (dyes). The successful implantation of bioremediation technology achieves the global goals of a safe environment and increases the economic impact. Keywords

Biodegradation · Textile wastewater · Organic pollutants · Azo dyes · Microorganisms Abbreviations

ADs AOPs BOD COD CPCB LiP MnP TDS TSS WHO

Azo dyes Advanced oxidation processes Biological oxygen demand Chemical Oxygen Demand Central Pollution Control Board Lignin peroxidase Manganese peroxidase Total dissolved solids Total suspended solids World Health Organization

Introduction Currently, azo dyes (ADs) are among the main products of the chemicals industry, and the need for ADs will grow more over time. They play a significant role in influencing the industries of print and dyeing manufacturing. ADs are the most harmful synthetic dyes used in cloth manufacturing and industries, resulting in their chemical structures, which contain amino groups, azoic linkages, and aromatic rings,

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so they are permanent in the aquatic environment [1]. Chemical, biological, and physical processes have been employed to remove these ADs from the environment. Biological methods include bacteria, microbes, algae, fungus, vegetation, and enzymes that have been widely applied to remove these dyes from various industrial wastes and resources. These techniques are popular because they are friendly to the environment and low cost [2]. Dyes can be biodegraded under aerobic, anaerobic, and anaerobic-aerobic conditions with the support of microbes. The additional process in the azo group includes hydrogenation, which results in hydrazobenzene formation, and oxidation with the help of peracids and hydrogen peroxide [3]. They are both helpful in promoting the molecule. Nanoparticle microbe enzyme conjugates are technological improvements that are very beneficial in reducing ADs from textile effluent in just a few minutes. This chapter will demonstrate the AD’s classification, sources, toxicity, and causes. The AD’s biodegradation uses various microorganisms of bacteria, algae, fungi, and yeast that produce some enzymes and co-enzymes due to the metabolic activity of these microorganisms. Environmentally safe and biocompatible technology will be discussed. The biodegradation of longchain and hazardous organic compounds such as ADs shows various economic and environmental benefits and may act as the implanted technology in industrial plants.

Importance of Safe Water and Wastewater Treatment Water is a critical concern for making life exist on Earth. It is necessary, yet many people would suffer worldwide without clean drinking water. Due to the several human uses of water, water demand has increased by seven times due to the quadrupling of the world population in the twentieth century. Domestic and industrial water demand has risen dramatically in cities because of community growth and industrialization, increasing wastewater volume released into the sewage system. As a result, wastewater recycling and reuse have become increasingly important for improving water availability and safeguarding water resources [4]. Water makes up 60–70% of the human body (42 L). Surface water, lakes, rivers, dug dams, springs, rainwater artesian bores, tanks, wells, and other drinking water sources are the most common. Each person requires at least 20 L of clean, safe water for drinking, personal hygiene, and cooking. Women drink less water than men, and pregnant women drink less than breastfeeding women. Therefore, the amount of healthful water necessary for the body varies by sex and age. On average, a middle-aged man may consume roughly 4 L of water daily. Drinking plenty of good water can help with various biological functions, including the following (see Fig. 1) [5]: (i) Water is required for appropriate cell activity, as water is one of the basic components of cell composition. (ii) Water, as a main component of blood, helps transport nutrients to the cells. (iii) Water hydrates the body and helps reduce headaches and back pain by regulating body temperature and hydration.

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Fig. 1 The provided diagram shows the importance of safe water to the human body; healthy water helps in different bodily functions such as digestion system, skin protection, dissolution of minerals and other nutrients, etc.

(iv) Drinking plenty of good water regularly boosts metabolism and helps digestion, replenishing critical nutrients in the muscles. (v) Our digestive systems require enough water to digest meals and avoid constipation properly. (vi) Drinking plenty of water will help you avoid skin conditions like eczema, dry skin, psoriasis, wrinkles, and spots. (vii) Drinking adequate water helps purify the kidney’s toxins, reducing the risk of kidney stones and other disorders. (viii) Water keeps tissues like those in the mouth, nose, and eyes moist. (ix) Water aids in the lubrication of joints. (x) Water aids in the dissolution of minerals and other nutrients, making them available to the body and preventing joint roughness. (xi) Water aids weight loss by assisting fat decomposition and raising metabolic rate.

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Treatment of contaminated water is considered one of the important human goals based on these principles. It has remained a major universal challenge until now, owing to the continual expansion in human numbers. With the increase in population, agricultural, domestic, industrial, and energy-generating processes, wastes will expand. Drinking safe water is not available for about 1.1 billion people globally; by 2023, 3.9 billion people will live in water-scarce areas [6]. Nowadays, water pollution affects approximately 844 million people worldwide, primarily in underdeveloped nations, due to a lack of modern facilities and technology to clean drinking water. The World Health Organization (WHO) says that almost 500,000 people die yearly due to diarrhea caused by contaminated drinking water [7]. According to WHO data, impure water can spread nearly 80% of diseases. In addition to diarrhea, cholera, typhoid fever, dysentery, gastroenteritis, vomiting, typhoid, cirrhosis, and polio, contaminated water can cause immune deficiency, genital insufficiency, cancer, and death. Organic contaminants, pathogenic microorganisms, and toxic metals are commonly found in untreated water, which causes environmental and health threats to increase [8]. Polluted water management’s ultimate goal is to protect biosystems using cost-effective methods.

Types of Water Pollutants The four main forms are domestic, industrial, agricultural, and urban wastewater. Urban wastewater is generated through industrial, domestic, and surrounding sewage infiltration and rainwater. In contrast, wastewater is generated through farm activities, surrounding farms, and sometimes contaminated groundwater. Various infectious pathogens (i.e., protozoa, viruses, and bacteria), organic molecules, and inorganic contaminants of cations and anions exist in the contaminated waters. Over their permitted limits, these compounds lead to transformation into extremely harmful agents, causing significant illnesses in humans and other environmental organisms [9]. Various mechanisms can mix pollutants with water contents (ground and surface), including surface runoff, discharge, subsurface infiltration, and atmospheric precipitation. When these contaminants become mixed into water contents, they quickly enter the global water cycle. Waterborne (microbial water contamination) can cause various gastrointestinal diseases and damage to the liver, kidneys, neurological system, and immunity [10]. Society has termed such pollutants “conventional pollutants” for a long time since they are familiar to society. Numerous researchers have detailed reports on their sources and negative effects on ecosystems [11]. People are aware of conventional pollutants because they have a desirable and allowed standard limit, and their causal impacts on ecosystems and human health are widely documented in the literature. As previously stated, rapid worldwide industrial development and massive population growth are linked to an increase in the discharge of various dangerous and destructive compounds. Hazardous pollutants such as microbiological, organic, and inorganic pollutants rapidly destabilize nature’s balance and create environmental poisons at an alarming rate [12]. As a

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Fig. 2 Broad classification and effects of hazardous pollutants

result, suitable sewage treatment facilities must be installed before public water use. The broad categories and consequences of hazardous pollutants are depicted in Fig. 2. The WHO proposes internationally accepted criteria and guidelines for maximum permissible amounts of certain chemicals. Also, the United States Environmental Protection Agency and the European Union have adopted similar health and environmental regulations and many regulatory procedures for the analysis of contaminants in drinking water that have been published globally [13].

Microbial Pollutants Microbial agents are becoming increasingly crucial among wastewater contaminants, and their removal efficiency in several wastewater treatment systems should be documented. Some types of bacteria are biologically polluting the water. Several types of bacteria have been found in wastewater, including (Salmonella, Fecal coliforms, Shigella, Escherichia coli, and Vibrio cholerae) viruses and parasitic protozoa. Depending on the type and amount, they can harm humans and the environment [14]. Cholera, typhoid fever, and tuberculosis are caused by bacteria in wastewater, while hepatitis and dysentery are caused by viruses and protozoa [15]. Many microbiological agents associated with suspended solids in wastewater will

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risk humans and aquatic organisms if wastewater is not properly treated and discharged into the environment, such as river water, crops, and green space. One of the microbial contamination sources is the sewage and wastewater of laboratories, medical centers, and hospitals that may be mixed with the drinking and/or agricultural water. Moreover, these wastes also contain various components of antibiotics, antibiotic-resistant bacteria, and disinfectants due to their widespread use in medical settings [16]. According to epidemiological studies, viruses are the principal cause of infectious diseases due to their tiny size, low infective doses, and discrete dispersion. Above 150 enteric viruses have been identified in domestic wastewater, including Enteroviruses, noroviruses, hepatitis A virus, adenoviruses, astroviruses, and rotaviruses. Infectious disorders such as gastroenteritis, poliomyelitis, aseptic meningitis, myocarditis, conjunctivitis, hepatitis, respiratory infections, systemic neonatal infection, and diabetes mellitus are more common in those exposed to these viruses through contaminated water or food [17]. Although the dangers of viruses to human health have long been recognized, and that indeed, viruses should be removed from domestic wastewater before it is discharged into the environment. Many environmental engineers are still debating the best and most effective way to reduce the risk of microbial infection to an acceptable level. So, accomplishing appropriate wastewater treatment methods customized to remove various microbiological agents is required. The two main goals of wastewater treatment are to prevent contamination of water assets and to preserve human health by protecting water supplies from the spread of harmful organisms [18].

Inorganic Pollutants Wastewaters may contain high inorganic pollutants such as heavy metals and cations accumulated in living organisms, as they are not biodegradable. And due to the rapid development of enterprises such as metal plating facilities, tanneries, fertilizer industries, batteries, mining operations, paper industries, and pesticides, wastewater is increasingly released into the environment, particularly in developing countries [19]. The presence of high levels of heavy metal cations/anions in water streams is categorized as poisonous and cancerous agents and is regarded as a high health risk factor. In industrial wastewater treatment, toxic heavy metals such as zinc, nickel, lead, copper, mercury, chromium, and cadmium should be avoided. Certain metals, especially hazardous heavy metals, serve no biological purpose and are frequently detected in trace amounts. The largest inorganic pollutants (i.e., heavy metals) come from untreated industrial and agricultural wastes, mining operations, sewage water, and rock erosion. The treatment of these liquid wastes is a major concern around the world. Heavy metals possess atomic masses varying between 63.5 to 200.6 a.m.u and a density of more than 5 g per cubic centimeter [20]. Lead, chromium, mercury, selenium, cadmium, nickel, copper, iron, aluminum, uranium, arsenic, and zinc, among other contaminants, are hazardous to ecological systems. Heavy metal severe poisoning can harm the lungs, central nervous system,

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kidneys, liver, bones, and endocrine glands and cause cancer and death. Toxic metals primarily enter the body through breathing, eating contaminated food, or drinking contaminated water. The presence of high levels of heavy metals like magnesium, zinc, and selenium in the human body disables some cellular enzymes’ functions. Even though certain metals, such as iron, manganese, chromium, zinc, and copper, are required by the human body, ingesting an excessive amount of them can be lethal [21]. Other metals have no functional role in the human body and can create issues even at ultra-trace concentrations. Scientists have looked at the negative impacts of these hazardous components on humans and ways to get rid of them. The WHO has determined acceptable quantities of these components in water and food [22]. Through diverse operations, a considerable amount of radioactive waste has been produced from the lack or disasters of nuclear plants. Moreover, the development and widespread use of nuclear plants for electricity lead to new nuclear technologies worldwide. It is critical to admit that rising levels of radioactive pollution constitute a significant warning to the health and the environment, as evidenced by the four catastrophic nuclear disasters in history that developed high-level governmental concerns. The mobile dissolved radionuclides in the natural environment permeate aquatic environments in rivers and groundwater. As a result, the risk of human radionuclide exposure will surely increase. Evidence shows that the radionuclides accumulation in the human body leads to prolonged exposure to high levels of radioactivity, potentially raising the risk of bone cancer, anemia, leukemia, metabolic disorders, and mortality [23]. Treating the contamination of radioactive wastewater has evolved into a revolved technological, societal, and political challenge with grave implications for human health. Hazardous anions and heavy metals and radioactive species are other types of pollutants in drinking water that are harmful and carcinogenic. These anions in ground and surface waters have caused serious pollution and health consequences. As a result, there is a pressing need to improve sustainable and cost-effective engineering systems to effectively remove surplus anion species from wastewater streams before dumping or reusing them. Arsenite (As(III)), fluoride (F
), arsenate (As(V)), nitrate (NO3
), phosphate (PO43
), and other hazardous anions have caused concern in wastewater and drinking water treatment. Arsenic pollution has become the most widespread environmental problem among these species. Arsenic concentrations in natural streams are dangerous to millions of people’s health, which has gotten much attention [24, 25]. Because of the high hazard potential of these poisonous species, there is still much interest for future researchers about anion contamination clearance.

Organic Pollutants The organic compounds of food, textile, leather industries, oil and gas production companies, vegetable oil processing industry, and domestic sewage are major pollutants. Their discharged wastewaters frequently contain a broad spectrum of

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organic compounds. In addition to being potentially carcinogenic to people, organic pollution can harm flora and fauna of the affected area significantly. Moreover, organic pollutants such as phenols, pesticides, food processing waste, medicines, cosmetics, oils, detergents, and dyes increase water contamination. These compounds in water contents are a significant threat to all living organisms. The need for sustainability and environmental preservation has prompted increased research focusing on water reuse by removing these impurities during the last few decades [26]. As will be detailed later, numerous treatment procedures have been used to remove organics in general. Water regulations will continue to drive the implementation of water treatment techniques.

Phenols Phenols are one of the most significant contaminants in water streams. Phenolic substances including phenol, chlorophenol, ethylphenol, isopropyl phenol, nitrophenol, and others can be found in industrial effluents such as petrochemicals, oil refining, coking operations, pharmaceuticals, paint, resin manufacture, pulp, wood goods, plastics, and paper. It is a corrosive and nerve-damaging substance. Human toxicity values typically vary from 10 to 24 mg/L, while fish toxicity levels typically range from 9 to 25 mg/L. Phenol has a 150 mg/100 mL [27]. Prolonged exposure to phenols can cause breathing irregularities, tremors, muscle weakness, and unconsciousness. In addition, respiratory arrest may occur at lethal doses. Direct exposure to phenols irritates the skin, eyes, and mucous membranes. In addition, weight loss, anorexia, diarrhea, salivation, vertigo, and dark urine coloring are chronic and long-term effects of phenol exposure. In animals, chronic phenol exposure irritates the liver, gastrointestinal and central nervous systems, and kidney and cardiovascular tissues [28]. However, there are a variety of methods for recovering and removing phenol. The most used processes belong to one of two categories: (i) conventional methods such as biodegradation, adsorption, steam distillation, liquid-liquid extraction, solid-phase extraction, and catalytic wet air oxidation and (ii) advanced methods such as ozonation, electrochemical oxidation, UV/H2O2, photo-oxidation, enzymatic treatment, membrane processes, and Fenton reaction. These techniques are effective for removing phenol compounds [27]. Biological methods are considered practical, cost-effective, and environmentally acceptable alternatives to physiochemical technologies to treat industrial wastewaters. This boosts and encourages biodegradation by microorganisms, fungi, bacteria, algae, yeast, and/or enzymes. This method involves aerobic, anaerobic, or a combination of aerobic-anaerobic processes [29]. Pesticides Extensive agricultural usage of plant protection agents, also known as “pesticides,” has significantly influenced air, soil, and water. The WHO defines pesticide as “any chemical agent intended to eliminate pests” (weeds, insects, rodents, fungi) [30]. Pesticides have been widely used in recent decades, resulting in substantial water pollution. Pesticide residues of various sorts were regularly identified in surface water, causing widespread alarm. Pesticides threaten human health even in small

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quantities (pg/L to ng/L). Annually, millions of tons of pesticides are used. Still, it is estimated that only about 5% of these amounts make it to the target organism, while the rest settle on the soil, go to non-target organisms, and flow into the water and atmosphere. Over time, pesticides are stable, so they can be carried by air and water, causing harm in areas far from their source. Wildlife, birds, fish, domestic animals, livestock, and humans have been harmed by pesticide use. Chronic exposure to herbicides like atrazine (1-chloro-3-ethylamino-5-isopropylamine-2, 4, 6-triazine) causes cardiovascular issues, muscle degeneration, retinal degeneration, and human cancer and also slows photosynthesis in sensitive plants. Protoporphyrinogen oxidase is inhibited by oxyfluorfen-containing herbicides, resulting in irreversible cell membrane damage. Liver and blood count problems (anemia) can occur due to human exposure to oxyfluorfen [31]. Unfortunately, it will not be possible to replace all harmful pesticides with clean alternatives in a reasonable amount of time. As a result, treating pesticides at the source and after release must be considered as alternative in many circumstances. Photocatalytic degradation, biological oxidation, nanofiltration membrane, photo Fenton, enhanced oxidation process, fluid extraction, ozonation, aerobic degradation, coagulation, solid-phase extraction, and adsorption are some pesticide removal methods. One of these approaches is using living organisms to decompose pollutants (i.e., Bioremediation) to convert pesticides to water, carbon dioxide, and more microbial biomass. The ability of bioremediation to be carried out in an opened, non-sterile environment with a diverse range of species is one of the most distinguishing features of this process [32].

Food Processing Waste Food wastewater is a sort of waste from homes, farms, hotels, restaurants, and the food industry. Food wastewater consists of organic substances such as protein, organic acids, carbohydrates, and lipids. Food processing is one of the most waterintensive industries, and it is critical to reaching sustainable development goals. The consumption of water in food processing is split into two categories: (i) process functions, where water is used as a raw material in the process, and (ii) non-process functions, where water is used for washing, chilling, and heating. For limited freshwater resources, food processing and other water-intensive industries have become a threat, prompting significant efforts to design and implement novel water management methods in these industries. Pollution of the environment is mainly caused by wastewater from food production and agricultural operations. For example, to produce 1 g of animal protein from milk, egg, or meat, you need approximately 31, 29, and 112 L of water, respectively, while we need approximately 21 L of water to produce 1 g of cereal protein. Food processing and production require a lot of biochemical and chemical oxygen, making it one of the most challenging and costly wastes to manage. There is a rising interest in developing more effective and ecologically friendly food processing wastewater treatments to withstand these issues [33, 34].

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Pharmaceuticals Pharmaceuticals are biologically active chemicals with a specific mechanism of action in humans and animals. Due to advancements in medical science research and discoveries, better and improved human health and a safer life are aimed to be maintained and provided, resulting in high demand for pharmaceuticals production with a concurrent increase in a population. The body did not fully metabolize these pharmacological (biologically active) chemicals; therefore, they were expelled in wastewater. This micropollutant remains unmodified throughout wastewater treatment plant operation and enters the receiving environment via treated water outflow. Four of the 24 medicines typically found in water include antibiotics, non-steroidal anti-inflammatory medications, anticonvulsants, and lipid regulators. Pharmaceuticals can be found in environments in various sources, including pharmaceutical plants, hospitals, landfills, wastewater treatment plants, and even cemeteries. As the human population rises, antibiotics, analgesics, anti-inflammatory, antihistaminic, antiepileptic, and other drugs are more common in aquatic ecosystems. This contributes to an increase in lake and river ecotoxicology, which can be detrimental to aquatic creatures [35–37]. Furthermore, medications might cause antibiotic resistance gene development, resulting in antibiotic-resistant bacteria or so-called superbugs. Causing high and chronic toxicity on the biota, the proliferation of antibiotic-resistant microorganisms, endocrine disruption in humans and animals, and the presence of pharmaceuticals and metabolites in various aquatic ecosystems are all potential adverse effects on the environment. As a result, practical strategies for resolving or removing these pollutants are required. The methods of conventional wastewater treatment may involve chemical, physical, or a mix of physicochemical processes: filtration, coagulation, flocculation, sedimentation, membrane filtration, biological processes, adsorption, chlorination, ozonation, ultrasonication, photocatalysis ultraviolet irradiation, and others [38, 39]. Cosmetics In recent years, the cosmetics manufacturing business has experienced significant expansion due to the rising demand and usage of cosmetic products. Cosmetics consumers have been spending a higher percentage of their disposable income on cosmetics than in the past. According to the report, the worldwide cosmetic goods industry was worth $532 billion in 2017 and $863 billion by 2024. The growing use of cosmetics for human beauty has aided in forming wastewater containing a wide range of toxins from manufacturing facilities. In recent years, cosmetics and several other non-cosmetic personal care products (disinfectants, dietary supplements, insect repellents) have rising crucial concern as one of the emerging pollutants classes that are greatly important as they are continually discharged into the aquatic environment; their environmental impact is linked to the large quantities used and that they may be bioactive, persistent in the environment, and potentially able to bioaccumulate [40, 41].

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Cosmetic wastewater has been identified as having extremely high biological oxygen demand (BOD), chemical oxygen demand (COD) (>100,000 mg/L), total organic carbon levels, high quantities of petroleum ether extract, organic phosphorus, organic nitrogen, suspended particles, oils, fats, and detergents over the years. Most of the contaminants that are dominant in cosmetic wastewater are poorly biodegradable. The cosmetic industry usually uses various chemicals such as surfactants, dyes, perfumes, and sunscreen UV light. They are non-polar and difficult for microorganisms to degrade biologically. The UV filters (benzophenone-3, octocrylene, 4-methyl benzylidene camphor), synthetic masks (galaxolide, tonalide), and antimicrobials (triclosan, triclocarban, para-aminobenzoic acid) are dangerous for the environment and estrogenic to human health. The prolonged use of these chemicals poses a major health risk to humans. Wastewater is hazardous, persistent, and complex due to the complex structure of these substances, including highsuspended solids and lipids and a high surfactant concentration. More advanced, practical, and economically viable treatment solutions are needed to lessen cosmetic wastewater discharges’ environmental impact and potential dangers. Many remediation studies based on dissolved air flotation, biological process, coupled electrocoagulation with TiO2, and advanced oxidation processes (AOPs) such as photo-Fenton, Fenton, etc. have been described for the mineralization of cosmetic wastewater [42, 43].

Oils Oily wastewater is the waste mixed with oils, fats, greases, and many dissolved elements (inorganic and/or organic) found in high concentrations. Furthermore, such wastes contain high total petroleum hydrocarbon, BOD, sulfides, total suspended solids (TSS), ammonia, COD, total organic carbon, and other hazardous compounds depending on the products and operations of the industries. Various negative impacts are observed when oily effluents are dumped into water streams, land, and sewer lines without treatment. The hazardous environmental impact of oil wastewater could be assigned to i) killing of fish and other aquatic creatures, ii) the blockage of photosynthesis of algae and aquatic plants, iii) harmful ecological impacts on the aquatic environment’s floral and faunal distribution, iv) mutagenic and carcinogenic for human, and v) inhibit plants growth. The main negative impact of these wastes may be building an impermeable layer on the water’s surface due to the increased BOD and COD of water bodies. Oil-contaminated wastewater is produced by metal processing companies, slaughterhouses, restaurants, dairy industries, poultry processing industries, petrochemical industries, edible oil refineries, tannery industries, and other businesses. In 2012, much oily wastewater was produced worldwide, ranging from 9 to 14 billion m3. So, the proper treatment to decrease and limit its effect on humans and the environment has attracted great interest. Also, oil recovery from oily wastewater treatment could have economic benefits [44, 45]. Other manifestations are the destruction of natural landscapes, a decline in crop output, soil pollution, surface water and ground water resources, sewage blockage, and a reduction in light penetration and dissolved oxygen. Some elements of oily wastewater are extremely toxic and carcinogenic, damaging the kidneys, liver, and

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blood, ultimately increasing cancer risk. As a result, one of the most critical concerns is the remediation and treatment of oily wastewater. Given the current state of oily wastewater contamination in China, the maximum permitted oily wastewater concentration is 10 mg/L. As a result, in today’s field of environmental engineering concerns, oily wastewater treatment is critical. For the better cleanup of oil-containing wastewater and water resource recovery, advanced and cost-effective treatment technologies are needed. Oil removal has been accomplished by gravity separation, chemical precipitation, cyclone separation, sorption, chemical oxidation, and membrane filtering [46, 47].

Detergents and Surfactants Surfactants (10–15% of the detergent) and other auxiliaries and adjuvants make up most detergent. Non-ionic and anionic surfactants are the most commonly utilized in domestic and industrial applications. One of the most significant sources of water contamination is the ever-increasing volume of laundry wastewater generated by the world’s ever-growing population. Laundry detergent wastewater produced by home, industrial, and institutional sectors, ranging from 200 to 2100 L per day, represents a potentially renewable resource for water reuse in various agriculture, industrial, and urban uses, augmenting existing water shortages. To assist ease water shortages, laundry detergent wastewater can be recovered and utilized. The treatment of laundry detergent wastewater is particularly complex due to its multicomponent composition, massive discharge volumes to the environment because of increased detergent usage as the global population rises, and the ineffectiveness of conventional treatment technologies [48]. Surfactant environmental risk is determined by the ultimate concentration in the aquatic medium. Surfactants degradation by microbial activity reduces surfactant concentration and thus its potential harmful effect. However, the hazardous chemicals generated during biodegradation can bioaccumulate, and their long-term effects are not fully understood. Also, soil absorption is crucial, as surfactant high soil levels lead to groundwater contamination. Surfactants are removed from wastewater using various processes, including physical, chemical, biological, and membrane treatment. The most appropriate wastewater treatment procedure is determined by several parameters, including effluent quality, influent quality, environmental footprint, energy consumption, and treatment cost [49]. Textile Dyes and Azo Dyes Textile, pharmaceutical industry, food, cosmetics, photography, plastics, and paper industries use dyes. Figure 3 represents the sources and pathways of various dyes that contribute to environmental pollution by coming from diverse sources such as wastewater treatment plants, industry, and residences. Textile companies use much freshwater for their numerous wet processes and produce many effluents. Not all dyes are attached to the materials throughout the dyeing process, so a fraction of unfixed dye is constantly discharged into the wastewater. These pollutants are the most significant contaminants in this effluent. More than half of the world’s dye and organic pigments are used in textiles, and their demand is predicted to rise by more

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Fig. 3 The environment’s sources and pathways of dyes. (Adapted with permission from Ref. [50], copyright 2021, RSC)

than $30 billion in 2019. Dissolved solids, printing gums (detergents, pentachlorophenol), colors, toxic metals (chromium), sequestering agents (trisodium polyphosphate), ADs, chlorine, sodium hexametaphosphate, stain removers (CCl4, chlorine), and fixing agents (benzidine and formaldehyde) are all present in the water released after fabric preparation. Though fast industrialization is the most efficient means of achieving economic expansion, it directly or indirectly impacts human health by releasing wastes into water streams [50]. Unfortunately, most dyes make their way into the wastewater treatment process. Textile plants are the most polluting industrial sector, notwithstanding that dye effluent escapes into water bodies from various sources. Some dyes are extremely poisonous, carcinogenic, and mutagenic. They also reduce the photosynthetic activity of aquatic organisms, preventing light penetration, resulting in oxygen deprivation, and limiting downstream beneficial uses, including drinking water, recreation, and irrigation. According to a recent estimate, around 70 lakh tons of dyes are generated annually. As a result, research in this area is ongoing, as seen by the massive increase in the research publications published about the removal of dyes between 2016 and 2020, with around 3689 articles published [50]. The allowed amount of textile dye wastewater release, according to the Central Pollution Control Board (CPCB), is as follows: COD: 250 mg/L, TSS: 100 mg/L, color: 5 mg/L, BOD: 30 mg/L, and total dissolved solids (TDS): 2000 mg/L [51]. The by-products of dye degradation and other related substances in the wastewater can harm human health and cause serious problems such as hemorrhage, skin ulceration, mucous membrane damage, and nausea. In addition to harming some of the most important human body organs and systems, such as the

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brain, liver, kidneys, central nervous system, and reproductive system, all may suffer devastating and long-term consequences. For example, the basic red-9 dye used in textile, ink formation, and paper has toxic environmental effects and carcinogenic potential in humans; crystal violet dye irritates the skin and digestive tract and causes chemical cystitis and respiratory and renal system failure in humans; and the azure-B dye can insinuate nucleic acids (DNA and RNA) [52]. Dye molecules may contain two organic functional groups: chromophores and auxochromes [100]. Direct, reactive, acid, basic, dispersion, vat, mordant, and sulfur are the classes of dyes based on the dye’s application technique. The dyes have been classified according to their chemical structure and the kind of molecular chromophore into anthraquinone, azo, nitro, and other dyes. The dye sources and extractions can be produced from a) natural sources either from plant sources such as weld, madder, indigo, beetroot red (betanin), cornflower (Centaurea cyanus L.), dyer’s broom (Genista tinctoria L.), Deptford pink (Dianthus armeria L.), rose, Indian pokeweed (Phytolacca acinosa Roxburgh), etc. or from animal sources such as sawfly larva, cochineal, shellfish, snout caterpillar, etc. b) Synthetic dyes (chemical synthesized dyes) are divided into two categories of ADs and non-azo dyes. Figure 4 shows the classifications of dyes according to the chemical constituents, behavior, and applications. ADs (R – N ¼ N
R0 ) now account for most of the dye chemical production capacity and are widely used, and their relevance may grow over time. As a result, ADs have a cost-effective and simple manufacturing process. They are also stable, have a long shelf life, and have various colors compared to the dyes extracted from natural sources. ADs account for more than half of all commercial

Fig. 4 Classification of dyes based on ionic charge and based on chemical structure

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dyes on the market and are the most often used dye class in textile processing. During the dyeing or textile coloring process, 4–12% of ADs are lost in the industrial effluents. Globally, about 280,000 t of textile dyes are released annually into wastewater. Yellow (reactive yellow 3 and acid yellow 117), orange (orange I, orange G, and reactive orange 16), red (methyl red, Sudan I–IV, acid red 151, and acid red 18) are all colors that are frequently made with the synthetic dye ADs [2]. ADs negatively influence the environment by affecting the water pollution indicators of COD, TOC, and BOD. Furthermore, ADs break down into various poisonous, mutagenic, and carcinogenic chemicals. The European Union has banned the use of ADs due to their toxicity. Figure 4 shows the classification of ADs. ADs have been linked to various malignancies in the bladder, liver, and spleen and many human chromosomal abnormalities. According to studies, ADs can connect to DNA molecules easily. Reactive orange 16, for example, causes major health concerns such as kidney failure, eye burns, tachycardia, cyanosis, skin irritation, and shortness of breath. They also cause damage to the aquatic environment as they prevent the passage of sunlight through water [53]. As a result, removing ADs from effluent has been prioritized. Many physicochemical methods have been used, including electrocoagulation, coagulation/ flocculation, adsorption, filtration, AOPs, ion-exchange, activated sludge processes, sequencing batch reactors, constructed wetlands, membrane bioreactors, and moving bed biofilm reactors. Still, they have been plagued by being economically unfeasible and generating toxic by-products. Bioremediation has become a unique challenge since it is environmentally benign and cost-effective and does not generate much sludge [54]. Biotechnological approaches can gradually improve biotreatment by producing more resistant microbes that effectively degrade dyes.

Biodegradation of Textile Manufacturing-Generated Dyes Using Microorganisms Due to the substantial health dangers to individuals and the environment, removing ADs from contaminated water is now regarded as one of the most pressing environmental issues. This global need compelled scientists to develop practical strategies for treating polluted water and making it healthy and potable to protect the environment. Adsorption, photocatalytic degradation, membrane processes, magnetic separation, chemical oxidation, advanced oxidation processes, electrochemical treatment, precipitation, solvent extraction, reverse osmosis, and coagulation/flocculation are just a few of the physicochemical methods that have been developed to remove ADs from textile manufacturing wastewater. However, these technologies are inefficient, as they use many chemicals, require much energy to run, and produce secondary pollutants. Consequently, it is critical to develop low-cost, high-efficiency technologies for treating these dyes for environmental remediation. Microorganisms reduce ADs by secreting enzymes such as laccase, azo reductase, hydrogenase, and peroxidase during bioremediation, which uses the ability of bacteria, fungi, or a combination system, so it has emerged as a viable method for the treatment of textile wastewaters [55]. Biological dye decolorization procedures are more cost-effective, efficient, environmentally benign, and adaptable than physicochemical approaches. Biological

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agents, such as microbes, offer significant benefits over traditional approaches. The methods are relatively low cost and low maintenance. Furthermore, this process ensures that all organic contaminants are entirely mineralized. ADs are mineralized into simpler compounds after being reduced. Microbes, fungi, algae, vegetation, bacteria, and their enzymes are examples of biological approaches gaining popularity as they have low cost and are friendly to the environment. Dye decolorization by microorganisms can occur through adsorption of dye molecules on microbial biomass or dye breakdown by microbes. ADs can be biodegraded in anaerobic, aerobic, and anaerobic-aerobic settings and with the help of microbes. Bacteria, actinomycetes, fungus, and yeast are among the microorganisms that can break down a variety of ADs [56]. Anaerobic conditions are usually required for AD reduction, whereas bacterial biodegradation of aromatic amines is almost entirely aerobic. As a result, a biological treatment technique for azo chemicals that combines anaerobic and aerobic conditions is proposed [57]. Biological treatment systems are very different, and various elements can affect the biodegradation of ADs in textile effluents. Temperature, pH, the type of reducing agents utilized, aeration, microbial consortia, and other variables can all influence dye biodegradation. Furthermore, while examining the wastewater treatment approach, the type of dye, its concentration, the functional groups of the dye, additives such as mordants, and their effluent content are all crucial factors to consider. Although specific enzymatic machinery for aerobic degradation of particular ADs has been identified, ADs are not easily destroyed in aerobic circumstances [58]. Figure 5 shows various possible methods of synthetic AD degradation. Living and dead cell mass can decolorize ADs using bacterial or fungal cells. Most of the enzymes involved are azo and nitro reductases, which require one or more co-factors such as FAD and NAD for their catalytic activity [111]. Non-specific

Fig. 5 Different possible methods of synthetic AD degradation

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azo-reductase enzymes degrade ADs in anaerobic systems, converting them to aromatic amines, which may or may not be degraded in anaerobic environments. The aromatic amines must be aerobically biodegraded, as their accumulation could be fatal to the treatment plant’s biological system. In research, several bacteria have been discovered to be capable of dye decolorization. Biodegradation appears to be a more natural process that can eliminate organic contaminants. On the other hand, organic dyes are degraded into fragments that may be xenobiotic and must be mineralized into CO2 and H2O during the biodegradation process. Gas chromatography/mass spectrometry, high-performance liquid chromatography, and liquid chromatography-mass spectroscopy can be used to identify biodegradation products. Decolorization experiments on ADs, for example, were conducted in 100 ml flasks containing 50 ml dyes (500 mg/l), traces of yeast extract, sucrose, and glucose. Sodium hydroxide and hydrochloric acid solution were used to alter the pH to be 7.0  0.2. The flasks were then autoclaved for 15 min at 121  C. Then, 5 ml of bacterial inoculums of each isolate were injected into the autoclaved flasks. The flasks were shaken and incubated for 4 days at 37  2  C. 10 mL of the dye solution was filtered and centrifuged for 20 min at 8000 rpm. For observation, samples were taken at 24 h intervals. The efficiency of decolorization was assessed by measuring the absorbance of the supernatant with the help of a spectrophotometer at the maximum wavelength (λmax) of the dye [59].

Azo Dye Biodegradation Using Bacteria Bacteria effectively process dangerous organic pollutants; they are frequently used to digest ADs among microorganisms as they have a fast-growing ability and long hydraulic retention time. The role of several bacteria groups in the decolorization of ADs has been the subject of extensive research. Bacterial degradation can be divided into two types: a) using a single bacterial strain or consortia and b) collecting diverse bacterial strains. The ADs are frequently non-specifically reduced, and decolorization occurs more quickly throughout the bacterial degradation. A wide range of aerobic and anaerobic bacteria has been reported for having good results in AD biodegradation, it includes Staphylococcus sp., Pseudomonas sp., Bacillus subtilis, Geobacillus sp., Escherichia coli, Rhabdobacter sp., Enterococcus sp., Corynebacterium sp., Lactobacillus sp., Alishewanella sp., Xenophilus sp., Dermacoccus sp., Clostridium sp., Acinetobacter sp., Micrococcus sp., Rhizobium sp., Proteus sp., Morganella sp., Aeromonas sp., Alcaligenes sp., Klebsiella sp., and Shewanella sp. Because of its ability to degrade a variety of ADs (red HE7B, reactive blue 172, reactive red 22, reactive red 2, orange I and II). Interestingly, Pseudomonas sp. is commonly used in decolorization studies; it also degrades commercial ADs used in textile wastewater. The first phase in bacterial decolorization is aerobic or anaerobic fermentation or, by the sequential way, followed by reductive azo bond breaking. ADs are reduced to a carcinogenic colorless amine under anaerobic conditions and are degraded by aerobic mechanisms [60–81]. Table 1 illustrates the biodegradation of ADs using different bacteria-based microorganisms published in 2021.

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Table 1 Biodegradation of ADs using different bacteria-based microorganisms Name of bacteria-based microorganisms Citrobacter sp. strain EBT-2 Bacterial biofilm consortia Bacillus sp. Indigenous bacteria-acclimated microbial fuel cells Acinetobacter baumannii JC359 Indigenous siderophoresproducing bacteria immobilized in chitosan Acinetobacter baumannii strain VITVB Pseudomonas geniculate strain Ka38, Bacillus cereus strain 1FFF and Klebsiella variicola strain RVEV3 Enterobacter hormaechei Marinobacterium Laccase-producing bacteria of B. cereus strain 1 and P. parafulva strain 2 Nesterenkonia lacusekhoensis EMLA3

Stenotrophomonas sp. TepeL and Stenotrophomonas sp. TepeS Intracellular azo-reductase enzyme from alkaliphilic Bacillus subtilis Biofilm consortia such as C1 (Vitreoscilla sp. ENSG301, Acinetobacter lwoffii ENSG302, Klebsiella pneumoniae ENSG303, and Pseudomonas fluorescens ENSG304), C2 (Escherichia coli ENSD101, Enterobacter asburiae ENSD102, and E. ludwigii ENSH201), C3 (E. asburiae ENSD102, Vitreoscilla

ADs with chemical structure Methyl orange, congo red, and eriochrome black T Congo red

Decolorization (%) 100% (100 mg/l, 96–120 h) > 96.9% (100 mg/L, 72 h at 28  C) 98.7% 93%, 96.6%, and 91.41%, respectively 98.8%, 96%, and 96.2%, respectively 90% at 300 mgL
1 dyes initial concentration 90% and 87%

Ref. [60]

89%, 78%, and 73%

[67]

98% and 98.8%

[68]

98.3% 44–86.4%

[69] [70]

Methyl red, tartrazine, Ponceau S, reactive red 35, Evans blue, acid red 3R, acid red, violet C BL, reactive violet, red AG and methyl orange Acidic red, methyl orange, reactive green, acidic yellow, and reactive black Reactive red, reactive brown, and reactive black

More than 90% of these ADs (100 mg l
1 each) within 72 to 192 h

[71]

65–82%

[72]

87.35%

[73]

Methyl orange

97.30, 98.75, 99.51, and 99.29%

[74]

Acid orange 7 Sunset yellow FCF, allura red, and tartrazine Reactive black 5, reactive Red 120, reactive blue 19 Acid black 1 and reactive black 5 Reactive blue 221 and reactive black 5 Methyl red, methyl orange, and tartrazine

Reactive yellow 145 and Reactive red 180 Metanil yellow G Orange 3R, yellow GR, and T blue

[61] [62] [63] [64] [65]

[66]

(continued)

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Table 1 (continued) Name of bacteria-based microorganisms sp. ENSG301, and Bacillus thuringiensis ENSW401), and C4 (E. coli ENSD101, E. ludwigii ENSH201, and B. thuringiensis ENSW401) Halo-thermophilic bacterial consortium Micrococcus yunnanensis Proteobacteria Isolated Pseudomonas sp. Thermophilic Anoxybacillus sp. PDR2 Aeromonas hydrophila MTCC 1739 and Aeromonas hydrophila SK16 Bacterium Providencia rettgeri

ADs with chemical structure

Acid black ATT, acid orange 7, and Brilliant Crocein GR Methyl orange Acid blue 29 Procion red H-3B Direct black G Remazol yellow RR, Joyfix red RB, and reactive yellow F3R Brilliant Crocein

Decolorization (%)

Ref.

69.5–85%

[75]

90% 91–94% 57–96% 98%

[76] [77] [78] [79]

88.7–90.4%

[80]

92.52%

[81]

Azo Dye Biodegradation Using Fungi Fungi can use an extracellular enzymatic system to transform aromatic substances, including pesticides, lignin, and polycyclic aromatic hydrocarbons. Decolorization techniques are currently the subject of much research. Fungal biomass is used as a source of enzymes involved in biotransformation and biodegradation and a sorbent for them. Manganese peroxidase, laccase, and lignin peroxidase are extracellular ligninolytic enzymes that assist fungus break down complicated chemical compounds. Pleurotus ostreatus, Penicillium sp., Pichia sp., and Candida tropicalis have decolorized ADs through adsorption and/or degradation. Fungi are an efficient system due to their large surface area and simplicity of solid-liquid separation. Fungi can degrade both organic and inorganic pollutants in a variety of ways. Though the use of specific contaminant-degrading fungi in the wastewater treatment system can improve the degradation of toxic organic pollutants, the high degradation rate of pollutants in wastewater treatment systems does not continue for a long time due to the loss of degrading microorganisms. Other fungi, such as Aspergillus ochraceus, Trametes versicolor, Pleurotus, Bjerkandera adusta, and Phlebia, have also been studied for dye biodegradation. Due to their cheap cost, safe, and full mineralization of dye, fungi for dye decolorization is a preferred strategy. White-rot fungi like Phanerochaete chrysosporium, Bjerkandera sp., Trametes versicolor, Irpex lacteus, and Pleurotus ostreatus, which generate enzymes including lignin peroxidase, manganese peroxidase, and laccase, are the most studied. Because the mycelium is more exposed to external stressors, the fungal free-cell therapy has some disadvantages. As a result, biomass immobilization on various

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Table 2 Biodegradation of azo dyes using different fungi-based microorganisms Name of fungi-based microorganisms Tyrosinase from Agaricus bisporus fungal tissue White-rot fungi Trametes villosa laccase Aspergillus flavus CR500 Aspergillus salinarus isolated from textile effluents Extracellular laccase from white-rot fungus Fungal consortium incorporated with Penicillium oxalicum and Aspergillus tubingensis Aspergillus arcoverdensis SSSIHL-01 Phanerochaete chrysosporium ME-446

Pycnoporus cinnabarinus, Pleurotus ostreatus, and Trametes hirsuta Penicillium crustosum PWWS-6

ADs with chemical structure Methyl orange, and Congo red Acid blue 277 and acid black 172 Congo red Reactive red HE7B

Decolorization (%) 94% 99.00% and 98.00% 95% 82–97%

[84] [85]

Congo red

~97%

[86]

Congo red

~97%

[87]

Congo red Direct yellow 27, reactive black 5, and reactive red 120 Allura red AC

98.61% 82%, 89%, and 94%

[88] [89]

>70%

[90]

Congo red

99.85%

[91]

Ref. [82] [83]

supports could be a viable solution. The biomass is protected, and fungal activity is improved by immobilization. Furthermore, immobilization of fungal biomass improves fungal tolerance to environmental challenges such as high concentrations of harmful chemicals. In comparison to free fungal biomass, immobilization improves the decolorization effectiveness of biomass due to the less dense fiber packing. The synthesis of lignin modifying enzymes, laccase, manganese peroxidase (MnP), and lignin peroxidase (LiP) is the main process for dye decolorization in live cells (LiP). The proportional contributions of LiP, MnP, and laccase to dye decolorization may differ for each fungus. Several operational parameters can affect the efficiency and rate of degradation of the organic molecule, such as nutrients, oxygen, enzyme, and dye concentration, pH, temperature, presence of redox mediators, and AD structure, which all influence the enzymatic decolorization of the dye for living cells [82–91]. Table 2 illustrates the biodegradation of ADs using different fungibased microorganisms published in 2021.

Azo Dye Biodegradation Using Yeast Since the 1990s, a realistic approach has been evolved to use yeast for AD degradation, and enzymatic decolorization in many yeast species has been illustrated. In 1992, Kakuta, with the help of his colleagues, used Candida curvet, an immobilized yeast in cleaning dye wastewater. On the other hand, yeasts have many advantages compared to bacteria and filamentous fungi, as they can withstand harsh wastewater

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Table 3 Biodegradation of azo dyes using different yeast-based microorganisms

Name of yeast-based microorganisms Meyerozyma guilliermondii, Yarrowia sp., and Sterigmatomyces halophilus Candida tropicalis A1 Candida sake 41E Yarrowia sp. SSA1642, Barnettozyma californica SSA1518, and Sterigmatomyces halophilus SSA1511 Manganese peroxidase-producing oleaginous yeasts

ADs with chemical structure Reactive red 120 Acid red B Reactive orange 16 Red HE3B

Acid orange 7

Decolorization (%) 100%

Ref. [92]

97.50% 94.3%

[93] [94]

82%

[95]

99%

[96]

conditions in a salty or acidic habitat. They have a fast growth rate and special non-pathogenicity. Although they cannot grow as fast as bacteria, they grow faster than filamentous fungi. A few ascomycetous yeast species, including Candida zeylanoides, Debaryomyces polymorphus, Candida tropicalis, Issatchenkia occidentalis, Candida albicans, Candida oleophila, and Galactomyces geotrichum, have been found to biodegrade and decolorize ADs. ADs can be biodegraded in reductive or oxidative processes using yeast-mediated enzymatic biodegradation. ADs are broken down into aromatic amines by reductive mechanisms, then mineralized by yeasts. The ligninolytic enzymes laccase, lignin peroxidase, and manganesedependent peroxidase are responsible for the oxidative breakdown of ADs. In general, ligninolytic enzymes oxidized ADs to yield a carbonium ion and then converted into a benzoquinone and a diazene derivative by nucleophilic water attack. Finally, the diazene is oxidized, releasing molecular nitrogen and producing a hydroperoxide derivative. The growth and metabolism of yeast are linked to the breakdown of ADs. For the breakdown of ADs, yeast cells require a carbon source of glucose [92–96]. ADs cause the yeast to produce oxidases and reductases and nicotinamide adenine dinucleotide hydrogen reductase [52]. Table 3 illustrates the biodegradation of ADs using different yeast-based microorganisms published in 2021.

Algae Use for Azo Dye Biodegradation The use of algae for effluent treatment has gained popularity due to its low cost and ability to be grown on a big scale. Algae are photosynthetic creatures that may thrive in a variety of environments. Algae are cost-effective to grow in large quantities since they get their energy from sunlight and produce food [52]. Both living and dead algal cells can be employed for biosorption and dye removal. Azoreductase, which is found in algae, destroys ADs by producing aromatic amines. Algae may degrade various dyes, with varying degradation rates depending on algal species and dye molecule structure [52]. To degrade ADs, yeasts such as Oscillatoria, Chlorella

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Table 4 Biodegradation of ADs using different algae-based microorganisms Name of algae-based microorganisms Chlorococcum sp. and Scenedesmus obliquus Pseudomonas putida, Chlorella, and Lactobacillus plantarum Chlorella vulgaris Enteromorpha intestinalis

ADs with chemical structure Reactive orange 122 and reactive red 194 CI reactive blue 40

Decolorization (%) 97%

Ref. [97]

99%

[98]

Congo red Congo red

75% 99%

[99] [100]

pyrenoidosa, and Chlorella vulgaris were utilized. Encourage the cells to proliferate in the presence of the pollutant to investigate the biodegradation of organic contaminants by algae. In industrial effluents, algae have been discovered to grow. As a result, microalgae are an excellent choice for bioremediation of colored wastewater; also, microalgae are not required. Unlike fungi and bacteria, algae acquire their energy from sunlight, their carbon from the air, and some even scavenge nitrogen from the atmosphere, making mass growth of algae less expensive. Algal decoloration can be caused by enzymatic breakdown, adsorption, or both. Algae, like bacteria, can degrade ADs by breaking the azo bond using an induced azoreductase, resulting in the generation of aromatic amines. Oxidative enzymes also aid the decoloration process. Another option for decolorization is immobilized microalgae. For example, C. vulgaris and S. quadricauda can remove a higher percentage of color from textile dyes when immobilized on alginate than suspended algae [97–100]. Table 4 illustrates the biodegradation of ADs using different algaebased microorganisms published in 2021.

Conclusions ADs are chemicals used globally and are of the most studied pollutants and recalcitrant compounds as they were synthesized to be stable and persistent. Although several types of physicochemical treatment procedures could be used to treat and degrade them, biological treatment methods are preferred due to their several environmental benefits. Microbes such as bacteria, fungus, yeast, and algae and their enzymes are widely helpful for dye adsorption, decolorization, catalysis, and degradation. Enzymes that are biological catalysts have been extensively examined and found both ecologically acceptable and cost-effective.

Future Perspectives Finally, we can take advantage of the biodegradation of azo dye pollutants using microorganisms. In that case, this will make it possible to develop new approaches to rapidly and efficiently removing ADs from wastewater. The biodegradation of azo

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dye pollutants using microorganisms provides a general direction for future research on AD degradation/removal.

Cross-References ▶ Biodegradable Materials: Fundamentals, Importance, and Impacts ▶ Biodegradation of Pollutants ▶ Types of Microorganisms for Biodegradation

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Nayera Awny Mahmoud, Alaa Mohamed Yasien, Dina Hamada Swilam, Mai Muhammed Gamil, and Shimaa Tarek Ahmed

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biobased Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fossil-Based Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production of Biodegradable Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Plastics Produced with Renewable Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Plastics Produced with Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors that Impact the Plastics’ Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impacts of Biodegradable Plastic Mulches on Soil Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastic Films for Agricultural Mulching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Assessment of the Ecotoxicity of Biodegradable Plastic Mulches . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradability of Plastics in the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Waste Management Options of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages and Disadvantages of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of Using Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of Biodegradable Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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N. A. Mahmoud (*) · A. M. Yasien Biophysics Department, Faculty of Science, Cairo University, Cairo, Egypt D. H. Swilam · M. M. Gamil Chemistry/zoology Department, Faculty of Science, Cairo University, Cairo, Egypt S. T. Ahmed Chemistry/geology Department, Faculty of Science, Cairo University, Cairo, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_34

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Abstract

Plastic items have become widely used worldwide in food, shelter, and clothing, but also in buildings, medical industries, and transportation, like sacks, refreshment bottles, food packages, components, and furniture. The great physical properties and the low price are the most important reasons behind the high usage rates of plastics. Synthetic plastics were developed as durable substitute products. Recently, there has been an increase in the need for biodegradable plastics to overcome global environmental and solid waste management problems. Biodegradable plastics’ primary function is to replace traditional plastics that affect the environment. As a result, the ability of microorganisms to degrade these polymers is a substantial environmental benefit. Globally, research on biodegradable plastics and polymers has been performed to balance human activity and the natural environment. Keywords

Biobased plastics · Biodegradation · Fossil resources · Renewable raw materials · Production of biodegradable plastic Abbreviations

Bio-PE Bio-PE Bio-PET BL CA DS EHT LDH PBA/T PBAT PBS PBS/A PCL PE PET PHA PHAs/ CW/Gelatin PHB PLA PLA PP PS PVA ROP

Bio-based poly(ethylene) Bio-Polyethylene Bio-poly (ethylene terephthalate) β-butyrolactone Cellulose acetate Degree of substitution Ethyl hexanoate Layered double hydroxide Poly (butylene adipate-co-terephthalate) Poly butyrate adipate terephthalate Polybutylene succinate Poly (butylene succinate/adipate) Poly(ε-caprolactone) Polyethylene Poly (ethylene terephthalate) Polyhydroxyl alkanoate Polyhydroxyalkanoates/cheese whey/gelatin Poly-3-hydroxybutyrate Poly (lactic acid) Poly lactic acid Polypropylene Polystyrene Polymerization of polyvinyl alcohol Ring-opening polymerization

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SDGs Tm TPS WVTR

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Sustainable development goals The melting point temperature Thermoplastic starch Water vapor permeability coefficient

Introduction One of the quickest categories of the global plastics industry is biodegradable plastics. In 2020, the worldwide biodegradable plastics capacity was 1.2 million tons per year, which is predicted to rise substantially [1, 2]. Switching from typical plastics to environmentally friendly polymers is difficult [3]. All polymers degrade somehow, whether physiochemically or biologically [3, 4]. Biodegradable plastics could be decomposed by microorganisms (bacteria; fungal enzymes) [3, 5]. The definition of biodegradable plastics is their ability to be mineralized into gaseous end products under the surrounding conditions, such as temperature, moisture, and microbial populations, which match biodegradability standards [1]. Plastics should break down to CO2, methane, water, and nutritious biomass/compost using aerobic and anaerobic organisms. For biodegradable plastics, the organic material produced must be safe for animals and plants. It is known that most commercially available biodegradable polymers are composted rather than gaseous products [3, 6]. However, the public was distracted to differentiate between biodegradable plastics and bioplastics (e.g., bio-based, oxo-biodegradable plastic, biodegradable, compostable, etc.) by people [1, 7]. For example, bio-based plastics are generated from renewable resources, so they may be called green [3, 6]. Bioplastics are one of the biodegradable plastic types, which refers to biodegradable synthetic polymers [e.g., polylactic acid (PLA)] generated from bio-based materials [e.g., bio-based polyethylene (bio-PE)] [1]. Furthermore, one prevalent misconception about bioplastics is that they will decompose in the natural surroundings [1, 8], but biodegradable plastics are degraded in natural surroundings, and many “so-called” biodegradable polymers are nonbiodegradable in aquatic ecosystems [1, 8]. Biodegradable plastics are circular because plants utilize CO2 for growth, and CO2 is released during aerobic degradation [3, 6]. It is circular if plastic components are reused or recycled during waste management. Biodegradation is a natural mechanism in which microorganisms degrade polymers into end products, either partially or completely [1, 9, 10]. The decomposition of bioplastics occurs in four steps: biodeterioration, depolymerization, assimilation, and, finally, mineralization [1, 11]. First, microbiological biofilms grow on the surface of the substances, which are then fragmented into microscopic fragments by decomposers and/or abiotic forces, while polymers miss their basic physicochemical characteristics. Second, extracellular enzymes are expelled from the biofilm. Many enzymatic compounds depolymerize polymers into simpler parts like dimers, oligomers, and monomers, lowering their molecular mass. Third, those molecules are physiologically absorbed in the cytoplasm, resulting in new biomass, energy, and primary and secondary metabolites. Eventually, these metabolites are mineralized,

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resulting in carbon dioxide, methane, water, and mineral salts. Biodecomposition of biodegradable polymers is influenced by several variables, such as the materials’ physicochemical structure, ambient circumstances, and the microbiological communities engaged in the process [1, 12]. Biodegradation rates vary depending on temperature, humidity, and other factors [3, 6]. It is important to note that degradation and biodegradation are not the same things [1, 13]. Nonbiodegradable plastics’ degradation stops at the fragmenting stage, resulting in more durable microplastics [1]. This chapter covers a wide range of biodegradable plastics and highlights some of their types and the factors that impact them.

Types of Biodegradable Plastics Biodegradable plastics are one of the largest materials called “bioplastics.” Bioplastics are made from biomass, but not all are degraded; nonbiodegradable bio-based plastics, such as bio-based polyethylene [PE], and others are not biobased, like fossil-based ones polycaprolactone (PCL). Biodegradable plastics can be classified according to their raw material origin (renewable resource (bio-based), fossil resource, or mixture of bio-based and fossil resources) and biodegradability into four categories mentioned in Table 1 [17]. Biobased plastics are plastics produced from biomass resources and have a biodegradable property [17]. “Biomass” refers to non-fossilized, biodegradable organic matter derived from plants, animals, and microbes [14]. However, those polysaccharide derivatives with a high degree of substitution (DS) are nonbiodegradable such as cellulose acetate. However, cellulose is a natural polysaccharide, and polyol-polyurethane bio-polyethylene (bio-PE) made from bio-ethanol is not degraded even though bio-PE has also been synthesized from biomass by using bio-based ethylene glycol, so not all bio-based plastics have to be biodegradable. There are poly(lactic acid) (PLA) and microbial polyesters which are produced from Table 1 Classification of biodegradable plastics Biodegradability Biodegradable plastics

Nonbiodegradable plastics

Material Bio-based plastics Polylactic acid (PLA) Polyhydroxyl alkanoate (PHA) Polysaccharide derivatives (low DS) Poly(amino acid) Polysaccharide derivatives (high DS) Polyol-polyurethane Bio-polyethylene (bio-PE) Bio-poly (ethylene Terephthalate) (bio-PET)

Fossil resources Poly(ε-caprolactone) (PCL) Poly(butylene succinate/adipate) (PBS/A) Poly(butylene adipate-coterephthalate) (PBA/T) Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Poly (ethylene terephthalate) (PET)

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lactic acid that gained from glucose extracted from corn starch and sugar after fermentation, that accumulate in microorganisms, respectively [15]. Plastics from fossil resources are plastics that can be synthesized from fossil resources [17]. Most of this classified as non-degradable (e.g., PE and PET) and poly (butylene adipate-co-terephthalate) (PBA/T), poly(ε-caprolactone) (PCL), and poly (butylene succinate/adipate) (PBS/A) are degradable [15]. This small group is most commonly utilized in conjunction with bio-based biodegradable polymers (mainly starch) [17]. Chemical or biotechnological processes manufacture plastic without changing polymer biodegradability [17, 16]. Chemical synthesis of a polymer from a monomer based on renewable resources is obtained through biotechnological converting of a renewable resource (e.g., the manufacture of polylactic acid [PLA] from lactic acid generated from sugar fermentation) or chemical synthesis of a polymer using non-renewable resources and components produced through petrochemical processes (e.g., polymerization of polyvinyl alcohol [PVA]). Moreover, biotechnological technique based on a renewable resource for producing a polymer (production of thermoplastic aliphatic polyesters like PHB) by fermentation of sugars with natural microorganisms [17].

Biobased Biodegradable Plastics The increased consumption of fossil-based raw materials in the past, which are non-renewable resources, led to increases in fossil raw material prices, so the economy has been threatened. It also has bad side effects on the world’s climate with increasing atmospheric carbon dioxide concentration. In the 1980s, the world is becoming aware of environmental protection. They started to use starch to produce bioplastic materials instead of fossil-based raw materials. PLA was used in nonmedical and biodegradable PHA packaging materials [14]. Bio-based plastics are produced by the chemical or physical functionalization of natural polymers. Polymers such as polysaccharides, proteins, lignin, and nucleic acids are manufactured by living organisms (animals, plants, algae, and microorganisms). They serve a variety of purposes in organisms, including energy storage (starch, proteins, polyhydroxy alkanoates), metabolism (proteins – enzymes, nucleic acids), structural materials (lignin, cellulose, chitin, proteins), and information storage (nucleic acids). One biodegradable plastics is polyhydroxy alkanoates (PHA), or cellulose acetate (CA). Not all bio-based plastics are obtained from natural polymers; for example, poly(lactic acid) is among the most significant biodegradable plastics, but it is produced by chemical polymerization of the bio-based monomer, lactic acid [14].

Polylactic Acid or Polylactide Polylactide is a biodegradable thermoplastic [18]. The first industrially manufactured polymer was derived from renewable resources [17]. It was produced by heating lactic acid under a vacuum. Lactic acid, the monomer, is a metabolic

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product of glucose metabolism that can be detected in blood and muscle tissue [17]. Lactic acid bacteria (mainly of the genus Lactobacilli) may synthesize it by converting simple carbohydrates into lactic acids, such as sucrose, galactose, or glucose [18]. Polylactic acid is water-resistant, and it is the most common biodegradable aliphatic polyester [17]. PLA takes 6 months to 2 years to degrade in the environment, depending on the product’s size and form, the isomer ratio, and the temperature. PLA’s tensile characteristics vary greatly depending on whether it is annealed or orientated and its crystallinity [17, 19]. PLA is available in two varieties. LLA or PLDA, L (þ)-lactic acid or (S)-lactic acid and D()-lactic acid or (R)-lactic acid are the two optical isomers of lactic acid. Because lactic acid is formed in the L/S configuration, the identical PLA is known as poly-L-lactic acid (PLLA). Because lactic acid is formed in the L/S configuration, the identical PLA is known as poly-L-lactic acid (PLLA). Furthermore, if lactic acid is created in the D/R configuration, the equivalent PLA, poly-D-lactic acid (PDLA), but (PLLA) is the most often generated PLA. PLA’s qualities are greatly reliant on the ratio of D/R and L/S enantiomers of lactic acid used in its generation and its chain arrangement [18]. PLA has several applications in food packaging (cups, bowls, foils, and food storage containers) [17, 18]. It takes 3 weeks for degradation. Because of its transparency, the other application in agriculture can be used to make mulching films and pesticide and fertilizer delayed-release materials. PLA has also been applied as implants, stents, and bone support splints in the medical sector; its use is safe where PLA decomposition via hydrolysis results in lactic acid, a natural substance that the body’s normal metabolism can easily degrade [18]. Foamed polylactide is an insulator that can be used instead of foamed PS. The mechanical properties of polylactides are similar to those of PET and PP. However, they have disadvantages, including brittleness and stiffness [17].

Polyhydroxy Alkanoates Polyhydroxy alkanoates are natural aliphatic polyesters. A big group of polymers, both homopolymers and copolymers, form more than 150 monomers. The structure of polyhydroxyalkanoates is shown in Fig. 1a [17]. PHAs are produced through bacterial fermentation of sugar and fats (glucose, vegetable oils, sucrose, and glycerine). The first step to producing PHA is fermentation when the foodstuff is added to the reactor until all of the bacteria have grown and the PHA has accumulated internally. The PHA is extracted after the cells have been concentrated with organic solvents, and the dissolved PHA is precipitated with the addition of alcohol, and the damaged cells formed during this process are removed by solid-liquid separation. Isolation and purification are the remaining processes for PHA production [18]. PHA synthesis is frequently promoted in shortage conditions (such as phosphorus, nitrogen, trace elements, or a lack of oxygen) and abundant carbon [17]. PHA biodegradability is primarily determined by crystallinity and polymer type; copolymers degrade more quickly than homopolymers [17, 20]. The type of polymer depends on the type of bacteria culture [17].

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Fig. 1 Chemical formula representation of the biodegradable plastic types (a) polyhydroxyalkanoates (n ¼ 1, 2, 3, 4, usually ‘n’ is 1; ‘R’ is the n-alkyl group of different chain length with or without the side group, unsaturated bonds or substituent groups). Adapted with permission from Ref. [64] (Copyright 2000, Elsevier)). (b) Polybutyrate adipate terephthalate (PBAT). This image is shared under the CC BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/) [65]. (c) Polycaprolactone (PCL). This image is shared under the CC BY-NC-ND 4.0 license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) [66]. (d) Polycondensation of succinic acid and 1,4-butanediol (Adapted with permission from Ref. [67] (Copyright 2018, Elsevier). (e) PVA: (A) partially hydrolyzed, (B) fully hydrolyzed (adapted with permission from Ref. [68] (2002, Elsevier)), and (f) starch. This image is shared under the CC BY-NC-ND 4.0 license (http:// creativecommons.org/licenses/by-nc-nd/4.0/)

PHA group has two members, PHB and PHBV [17]. PHB (Poly-3hydroxybutyrate) was one of the first PHAs to be identified [18]. PHB is a moisture and odor barrier similar to PP in terms of its properties. It is a highly crystalline homopolymer that is extremely brittle. Although it can be biodegraded by microorganisms such as fungi, algae, and bacteria, it has significant drawbacks such as low heat stability, brittleness, and processing complexity. PHBV is a copolymer created by adding propionic acid to a nutrient-rich feedstock provided by bacteria. It is less brittle and has a larger crystalline concentration than PHB. The rate of deterioration is affected by crystallinity, processing conditions, and structure [17, 20]. PHA polymers are thermoplastics that can be processed with standard plastics processing equipment. Thus, they have various packaging, medical, and disposal applications. They are also used in cosmetics, hygiene products, and golf tees as hardeners. Due to its renewability, biodegradability, and water vapor barrier qualities, PHA-based films have received interest for use in food packaging. PHBV could well be applied in film packaging, paper coatings, blow-molded bottles, and medical applications due to its slow hydrolytic degradation and biocompatibility. PHB is regarded as a more effective light barrier than PHB PLA in the visible and ultraviolet

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light ranges. Because of its low thermomechanical qualities, it can be modified by mixing PHA with other polymers, inorganic materials, or enzymes, giving them a broader range of applications [17]. So, the PHA application focused on using copolymer instead of the homopolymer due to the weak mechanical characteristics [17].

Cellulose-Based Plastics (Polysaccharide Derivatives) Cellulose is starch, and it is a structural polysaccharide, shown in Fig. 1f; cellulose is made up of glucose units that range in size from a few hundred to over ten thousand and are bonded together with a glycosidic bond, similar to starch. Cellulose is synthesized by trees and other plants (cotton, cereals, flax, sugarcane, jute, etc.) [17] as well as acetic acid bacteria [17]. Cellulose produced by bacteria has various physicochemical and mechanical properties that are unique such as a high degree of polymerization; water-holding capacities and high water-absorbing, high crystallinity, high tensile strength, and high elasticity; good biocompatibility; and biodegradability. It is extremely pure from cellulose from plants [17, 21]. Cellulose can generate fibers, films, and cellulose derivatives [17]. The first industrial cellulose polymers are celluloid and cellophane. Protein-Based Plastics (Poly Amino Acid) Proteins are natural polymers that noncovalent interactions can stabilize. These materials have functional properties depending on the thermal stability, structural heterogeneity, and hydrophilic behavior of proteins. Wheat gluten, corn zein, peanut protein, and soy protein are examples of plant-derived proteins that have been studied for use in the production of biodegradable polymers [17, 22]. Protein-based plastics are used in food packaging applications, but there are drawbacks such as poor processability, high cost, low mechanical properties, and brittleness [17, 23]. Soya protein-based plastics increase the biodegradability of plastics more than plastics from casein, zein, and glycine [17, 22]. They can be used as packaging materials due to their excellent properties as oxygen and UV radiation barriers and biodegradability. They also utilize foil in agriculture and, therefore, foam products of different densities and insulation materials [17, 30]. Zein proteins look like beeswax. They are soft, ductile, elastic, and tenacious. They are low water-soluble, greaseproof, tough, hydrophobic, and resistant to microbial attack, but are brittle; as a result, they are employed as a raw material for medicinal applications, film, and coatings for food [17, 24].

Fossil-Based Biodegradable Plastics These plastics are derived not only from crude oil but also from coal and natural gas. However, they do not occur in nature. The degradability is popular among these polymers by the hydrolytic attack to bonds into the polymer (ether, ester, amide bonds) [17, 25]. These polymers represent a tiny category of fossil-based bioplastics used to improve the properties of bio-based biodegradable polymers. Poly butyrate

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adipate terephthalate (PBAT), PBS, PCL, and PVA are the most important in fossilbased biodegradable plastics [17].

Poly Butyrate Adipate Terephthalate PBAT is generated from butanediol, adipic acid, and terephthalic acid using a polycondensation reaction. It is a biodegradable aliphatic-aromatic polyester. Because PBAT has excellent properties such as toughness, biodegradability, and processability, it may be utilized to make agricultural mulch films, compostable organic waste bags, packaging, disposable packaging, and tableware [17, 26], also used to improve the properties of biodegradable plastics. The structure of PBAT is shown in Fig. 1b [17]. Polycaprolactone PCL is derived from caprolactone by ring-opening polymerization. It is a linear aliphatic biodegradable polyester. PCL is useful in medical applications, like drug delivery, sutures, dentistry, and tissue engineering, and is also used with bio-based biodegradable plastics as blends [17]. PCL is a fossil-based polymer that is flexible, biodegradable, non-toxic, easy to produce, and hydrophobic. When the hydrophobic polymer (PCL) is mixed with the hydrophilic polymer (chitosan), the overall hydrophobicity of the blended films increases while the film with a composition ratio of 80:20 (PCL: chitosan) improves the barrier qualities and maintains the mechanical properties. The structure of PCL is shown in Fig. 1c. Pure PCL has a good crystalline ability, but blends of PCL have been synthesized to improve this feature. Because of its reinforcing properties, the existence of starch increased the blend’s crystallinity [38]. So PCL/TPS blends are utilized for food packaging applications [73]. Two factors affecting PCL are layered double hydroxide (LDH) and 2-ethyl hexanoate (EHT) by mixing an ionic liquid constituted of positively charged phosphonium ions electrostatically with the negative ions of 2-ethyl hexanoate (EHT) [73] wherefore the characteristics of PCL can be improved by adding LDH-EHT into PCL matrix. Take the consideration of WVPC to refer to the water vapor permeability coefficient equation [73, 76]: WVPC ¼ WVTR  Thickness of film where (WVTR) is the water vapor transmission rate in gm2 per day, the film’s thickness in meters, and the difference between water vapor partial pressure across the film in Pascal and WVTR is shown in kg mm2 Pa1. By blended LDH and PCL 1:3 wt%, the WVPC values were reduced, and the polar component of the matrix was observed to decrease, ranging from 5 mN m1 for pure PCL and 0.4 mN m1 for LDH-PCL. The LDH concentration in the matrix can form a large aggregation of portions in the matrix, but the EHT combination in the PCL films can cause the thermal stability of the PCL-EHT blend to decrease [73].

Polybutylene Succinate PBS is derived from succinic acid and 1,4-butanediol by condensation polymerization with catalyst, as shown in Fig. 1d. It is a biodegradable aliphatic thermoplastic

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polyester. PBS is mostly composed of fossil-based monomers, but it also can be composed of bio-based monomers or a mix of bio-based and fossil-based monomers [17, 27]. PBS can be used for many purposes, including shopping bags, agricultural mulch films, packaging films and sheets, plant pots, hygiene items, and food contact applications [17].

Polyvinyl Alcohol PVA is water-soluble biodegradable with good biocompatibility. Because of the dense concentration of hydroxyl groups on its side chains, it has the capacity to self-cross-link. PVA is also bioinert and rapidly hydrolyzes; therefore, protein and cell attachment are restricted on the pure material. So, it has various uses, including water treatment chemicals, dyes, industrial cleaning chemicals, agricultural chemicals, laundry detergents, disinfectants, and the manufacture of water-soluble films. The structure of PVA is shown in Fig. 1e [17, 35].

Production of Biodegradable Plastic Biodegradable plastics are commonly produced with renewable raw materials, microorganisms, or combinations [28]. The agricultural waste-derived renewable resources are known as “waste agricultural biomass” or “biomass resources” [37]. Biodegradable plastics are classified based on their generational order as follows: First generation: Biomass derived from carbohydrates-rich plants may be utilized as food or animal feed (e.g., sugar corn, wheat, and cane). Second generation: Biomass derived from unsuitable plants for animal feed production or food. They can be non-food crops (cellulose) or waste products from firstgeneration feedstock (e.g., waste vegetable oil, corn Stover, or bagasse). Third generation: Biomass generated from algae has more progress than the first- or second-generation feedstock and has its classification [29, 30].

Biodegradable Plastics Produced with Renewable Raw Materials Biodegradable plastic produced from renewable resources is an essential material innovation because it lowers reliance on petroleum and waste material while producing a product similar to normal plastics [31]. Synthetic polymers are used to make petroleum-derived plastics. On the other hand, polymer chains are found in nature. Cellulose, lignin, and starch all have this type of chain. Cellulose is found in all plants; however, some produce more than others. Lignin is usually found in wood, whereas starch can be found in corn, potatoes, and wheat. Plants, wheat, and other fundamental resources are renewable and readily available. The main difference between synthetic and natural polymers is that natural polymers contain nitrogen and oxygen. Because of the oxygen and nitrogen atoms found in polymer structures, they can biodegrade [32].

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Plants can be utilized as an indirect nutrition source in bioreactors to manufacture biodegradable plastics. Plant resources can be collected and utilized directly to make plastics (natural rubber is a good example), or plant polymers can be derivatized to make plastics. Plants can also be genetically modified to synthesize new polymers directly. The advantage of utilizing corn is that discarded corn may be used because the corn is not immediately consumed. The raw material does not need to be in great condition [33]. Polymers are separated from basic materials to produce plastic from natural sources. Different methods are required to create plastic depending on the material utilized. Polymers are often produced using chemicals or by fermentation sugars. Lactic acid is derived from corn when it is used to make plastic. The starches in corn are broken down into sugars during the purification process. The carbons in the sugar are removed after the sugar is fermented. The plastic is then made from the carbons in the same way that plastic is made from petroleum carbons [34].

Biodegradable Plastics Produced with Microorganisms Polyhydroxy Alkanoates Polyhydroxy alkanoates (PHAs) are microbial polyester belonging to the thermoplastic polymer’s family. They are generated as carbon and energy storage compounds by various prokaryotic bacteria under unbalanced feeding circumstances. Poly-3-hydroxybutyrate (PHB) is one example of PHAs [35]. Poly-3-hydroxybutyrate Synthesis PHB is generated in the cells of the microorganisms as a byproduct of secondary microbial metabolism, typically in situations of nutritional stress or in an unfavorable environment, such as a carbon-excessive environment with insufficient nutrients, which is available in both gram-positive and gram-negative bacteria. When critical nutrient sources are unbalanced or low, bacteria employ a natural material accumulation method to store carbon and energy [36]. There are many methods for extracting and recovering PHB components and derivatives from bacterial cells. This issue is quite interesting since different PHB materials may be produced based on the microorganism utilized and the method used to produce the substance. When a bacterial method is used, for example, an isotactic PHB with little to no stereoregularity is produced, all in the R-configuration because of the polymerizing enzyme’s stereospecificity and PHA synthase. A small proportion of the S-configuration may be identified in extremely rare situations, whereas the syndiotactic PHB with stereoregularity can be produced with chemical synthesis [37, 38]. PHB materials may be synthesized in three ways. The first is by ring-opening polymerization (ROP) of β-butyrolactone (BL). The other strategy is to use natural/ transgenic plants. PHA production in transgenic plant cells is feasible because acetyl-CoA, the primary substrate in PHA biosynthesis, is widely available, as in the case of Linum usitatissimum L., also known as flax [37].

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2 Acetyl-CoA CoA-SH ATP+ CoA-SH

CoA-SH NADH

Acetyl-CoA

DEGRADATION

Acetoacetate 3-Hydroxybutyryl-CoA

NADH

A1 CoA-SH

SYNTHESIS

NAD+

NAD+ 3-Hydroxybutyryl-A1

3-Hydroxybutyrate

Oligomers

PHB

A1

Fig. 2 The biosynthetic pathway of PHB from acetyl-CoA [42]. (This image is shared under the CC BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/))

People have used this plant since ancient times; however, improvements in transgenesis technology have permitted the potential of changing flax plants and allowing for better biomass growth, with a yield increase of about 20% compared to control cultures. Compared to control callus, the cellulose in transgenic callus plant cell walls was structurally different, with little order, resulting in a lower degree of crystallinity [39]. With a rising need for biodegradable resources in the future, the use of transgenic plants is a methodology that is being further explored to produce extremely effective bioprocesses, and it is an area with great opportunity. Bacterial fermentation is the third method of producing PHB compounds. When ideal fermentation conditions are met, more than 90% of the dry weight of the cells is made up of PHA components [40]. This third method is the most widely utilized in PHB synthesis (Fig. 2). PHB production is dependent on the main carbon metabolite derived from acetyl-CoA through a series of three enzymatic reactions: condensation of two acetyl-CoA moieties in a reversible manner, mediated by β-ketothiolase, to create acetoacetylCoA (Phase A); acetoacetyl-CoA reductase transforms acetoacetyl-CoA to (R)-3hydroxybutyryl-CoA (Phase B); and the polymerization of (R)-3-hydroxybutyrylCoA to generate PHB, catalyzed by the enzyme PHB synthase (phbC gene). The acetyl-CoA biosynthesis cycle for PHB is shown in Fig. 2 [41, 42].

Cyanobacterial Systems and Their Capability of Producing PHB Cyanobacteria are the only photosynthetic prokaryotes that can produce oxygen, and they get their energy from photosynthesis. The word “cyanobacteria” comes from the Greek word “blue,” which describes the color of the bacteria. Cyanobacteria are prokaryotes, also known as “blue-green algae,” though the term “algae” is technically incorrect because it only refers to eukaryotes [80]. The advantage of employing cyanobacteria to manufacture PHB over traditional fermentation techniques that use

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sugar or another organic (waste) materials as feedstock is that it is more sustainable, as CO2 is the only carbon source and sunlight is the only source of energy [81]. Cyanobacteria can produce PHB as an intracellular energy and carbon storage molecule [82]. Cyanobacteria have various industrially significant advantages compared to their plant counterparts, including a faster growth rate, stronger CO2 utilization, and greater genetic engineering amenability [83]. Cyanobacteria can store a wide range of potential reserve resources. Cyanophycin, polyphosphate, polysaccharide, and PHB are among them. Polyphosphate is a potential energy or phosphate reserve, and cyanophycin is a nitrogen reserve with the potential to supply limited amounts of carbon and energy [84]. PHB and polysaccharides are commonly thought of as carbon and energy stores.

Detection and Analysis of Poly-β-hydroxybutyrate It is important to screen a large collection of bacteria quickly while isolating PHB accumulating cyanobacteria from nature. PHB-specific stains are used in the detection of granules. A viable colony staining technique has been proposed to screen PHB-accumulating bacteria quickly. Sudan black B was first proposed as a test cyanobacterial fat stain. The larger usefulness of this dye was understood, and the approach was adjusted for displaying intracellular fatty material in cyanobacteria by making microscopic slides stained with alcoholic Sudan black B solution and counterstained with safranin [85]. When poly-β-hydroxybutyrate granules were stained with Nile blue, they showed a bright orange fluorescence. Heat-fixed cells were treated for 10 minutes with 1 percent Nile blue A, and excitation was measured at 460 nm. Polyphosphate and glycogen did not stain. Nile blue stain was more specific for polyhydroxybutyrate than Sudan black B stain [86]. PHB producers and non-producers were separated using Nile red dissolved in acetone screening. For PHB detection, the recommended use of a sensitive viable colony-staining approach utilizing Nile red for direct screening of cyanobacteria that accumulate PHB has been largely employed. In addition, when viewed under UV light, the PHB producers showed high fluorescence [87]. Biodegradability and Biological Considerations of Poly-β-hydroxybutyrate The biodegradability of PHB distinguishes it from petroleum-based plastics. Under aerobic conditions, PHB biodegradation produces CO2 and H2O, but the breakdown products are CO2 and CH4 under anaerobic conditions. PHB is biodegradable over a wide temperature range, even at a maximum of roughly 60  C, with moisture levels around 55%. According to studies, 85 percent of PHA is destroyed in 7 weeks. PHA has been shown to deteriorate in aquatic environments (Lake Lugano, Switzerland) in 254 days, even at temperatures below 60  C [88]. Under nitrogen constraint, PHA breakdown by Ralstonia eutropha could proceed concurrently with its biosynthesis. This is known as the “cyclic character of PHA metabolism.” When the substrate was changed from butyric acid to pentanoic acid, the author stated that the polymer composition changed from PHB homopolymer to

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PHB 49 percent PHV copolymer. The density of PHB differs from that of other common plastic polymers. PHB does not float in an aquatic environment because of its high density. As a result, PHB-based plastics sink and disintegrate in surface sediment via biogeochemical mechanisms once discarded. Microorganisms use PHB, P (HB-HV), and other PHA energy sources. P (HB-HV) is biodegraded [89]. Microorganisms colonize the polymer’s surface and produce enzymes that break down P (HB-HV) into HB and HV units. The cells then utilize these units as a carbon source for biomass expansion. The rate of polymer decomposition is affected by several parameters such as surface area, microbial activity in the disposal environment, pH, temperature, moisture, and the presence of other nutritional elements. P (HB HV) is insoluble in water and is unaffected by moisture. It does not decay under typical storage conditions and is indefinitely stable in the air [90]. CO2 and H2O are the end products of PHA breakdown in aerobic environments, while methane is generated in anaerobic circumstances. Several researchers have investigated the influence of different settings on the degradation rate of PHB and P (HB-HV) [91, 92]. Anaerobic sewage degrades the fastest, while sew water degrades

Carbon sources Sunlight Energy

Carbondioxide

Oxygen water

Fermentation process Plants

Extraction, Purification

Moulding

Bioplastic products

Polyhydroxybutyrates

Recycling

Fig. 3 Biodegradability process of poly-β-hydroxybutyrates. (Adapted with permission from Ref. [95] (2013, Elsevier))

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the most slowly. In anaerobic sewage, soil, and seawater, P (HB-HV) decomposed completely after 6, 75, and 350 weeks. When PHAs are exposed to soil, compost, or sea sediment, they decompose. Biodegradation is affected by various parameters, including environmental microbial activity and exposed surface area, moisture, temperature, and pH. Figure 3 depicts the biodegradability of polyhydroxybutyrates [93–95].

Factors that Impact the Plastics’ Biodegradability Biodegradable polymers are decomposed in ecological systems by biotic and abiotic mechanisms. Bacteria, fungus, archaea, and algae are biotic agents that decompose plastic [1, 43, 44]. Temperature, mechanical impact (weathering), humidity, oxygen, and acidity are abiotic factors [1, 9, 45]. The major degrading phase is abiotic hydrolysis, which occurs when temperature and relative humidity allow the ester bond to be cleaved [1, 46]. When the heat reaches the polymer’s glass temperature, the rate of hydrolysis and biodegradation increases [1, 47]. The presence of oxygen determines the kind of decomposers and biological reactions. Aerobic organisms use polymers as carbon and energy sources when oxygen is available [1, 48]. Anaerobic organisms break down polymers and produce biogas, primarily in methane, in anaerobic environments [1, 49, 50]. Biodegradability is also determined by the plastic’s chemical structure and crystallinity [3, 6]. The biodegradability of plastics is linked to their characteristics. Plastics’ chemical and physical features have an impact on the biodegradation pathway. Surface properties (hydrophilic, surface area, and hydrophobic properties), first-order structures (molecular weight, chemical structure, and molecular weight distribution), and high-order structures (melting temperature, glass transition temperature, crystal structure, crystalline structure, and modulus of elasticity) of polymers all play a role in biodecomposition processes [49]. In general, polyesters with side chains absorb less than polyesters without side chains [49, 51]. Because it impacts several physical features of the polymer, molecular weight is also essential for biodegradability. The degradability of the polymer was reduced when the molecular weight of the polymer was increased. Rhizopus Delmar lipase (endo-cleavage type) decomposed polycaprolactone (PCL) with greater molecular weight (Mn > 4000) more slowly than PCL with low Mn [49, 52]. Furthermore, the shape of polymers has a significant impact on their biodegradation kinetics. Because enzymes primarily target the amorphous regions, crystallinity is a critical factor impacting biodegradability. The loosely packed molecules in the amorphous area make it more sensitive to disintegration. Polymer crystalline regions are more resistant to damage than amorphous regions. Polylactide (PLA) breakdown rates decrease as the crystallization of the polymer increases [49, 53, 54]. Polyesters’ melting point temperature (Tm) significantly impacts polymer enzymatic breakdown. The lower the Tm, the slower the polymer biodegrades [49, 52, 54, 55]. The following equations are used to express Tm generally.

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Tm ¼ ΔH=ΔS where ΔH was the change in melting enthalpy and ΔS is the change in melting entropy. It is generally known that reactions between polymer chains have the greatest impact on the ΔH value, whereas the energy of internal rotation related to the stiffness (flexibility) of the polymer molecule has a significant impact on the ΔS value [49].

Impacts of Biodegradable Plastic Mulches on Soil Health Plastic pollution in agricultural soils is a growing environmental hazard requiring both science-based and strong policy solutions. Plastic pollution in agricultural soils is a major environmental hazard caused by the insufficient removal of polyethylene mulch after use. As a result, biodegradable plastic mulches have gained popularity as a polyethylene mulch alternative; nevertheless, little is known about how they affect soil health. Plastic mulches that disintegrate water, carbon dioxide, and microbial biomass are biodegradable. Biodegradable plastic mulches have been available for over 15 years, but data on their influence on soil health is scarce. It must be proved that regularly using and incorporating biodegradable mulches into the soil is sustainable and does not negatively influence soil health [69]. The use of biodegradable polymers should be thoroughly scrutinized from an environmental standpoint. The so-called global warming potential is the most significant contributor to numerous indicators (GWP). This examines the process’s CO2 footprint at each stage [24].

Plastic Films for Agricultural Mulching Plastics are ubiquitous in our environment and have a significant effect on all human activities and lives; as a result, the current Anthropocene period is known as “The Plastic Age.” Governments and businesses throughout the world are pushing the development of biodegradable polymers, such as mulches, while taxing or limiting the use of nonbiodegradable plastics. Biodegradable plastic mulches to alleviate plastic pollution have been suggested to reduce the excess of LDPE and other permanent plastic trash in the surroundings [70]. Plant-based biodegradable polymers are frequently assumed to have a zero or negative carbon impact. However, the losses are much higher when life cycle assessments (LCAs) compute carbon emissions [70].

Assessment of the Ecotoxicity of Biodegradable Plastic Mulches To avoid environmental and health concerns, a marketable product’s ecotoxicity assessment is required. The assessment focuses on biodegradable plastic mulches, which are totally and frequently integrated into agricultural soil [70]. Plant development in soils containing biodegradable plastic film fragments has been monitored to determine the toxicity of biodegradable mulches on plants. Potential plant growth effects of

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biodegradable plastic mulches have been studied after the spent mulch fragments have been buried in the soil and from the beginning of the mulch operation [71]. Compounds released from biodegradable plastic mulches into soils may move to different ecosystems, and the use of model organisms adds to the knowledge of mechanisms behind toxicity on living beings, addressing the consequences of biodegradable plastic mulches. As far as the authors are aware, only two articles have evaluated the effects of bioplastic mulch fragments buried in the soil on a battery of species. The sensitivity of plants to mulch materials varies by species. Several studies have revealed that some BDM and specific mulch parts may affect plant growth, while others have found that specific mulches are likely to be more suitable for usage in agricultural settings. However, only a few BDM have been thoroughly studied on a wide range of organisms, and long-term impact studies lasting more than a few months are needed [70]. Because admixtures make up a small percentage of the mulch, they can go unreported in biodegradation and ecotoxicity investigations. Because of the significant effects of some plastic additives, it is recommended that they be detected in the biodegradable mulch composition and tested for safety (Fig. 4) [70].

Fig. 4 Biodegradable plastic mulch film agricultural cycle during use. (Adapted with permission from Ref. [70] (2020, Elsevier))

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Biodegradability of Plastics in the Environment All plastics degrade in some way, whether physiochemically or biologically. Weathering (degradation caused by sunlight, wind, or waves) and hydrolysis/oxidation are examples of physicochemical processes. All plastics are affected by these processes, and microplastics result from them. Oxo-degradable and hydrodegradable polymers are designed to decompose through oxidation and hydrolysis processes. They are usually nonbiodegradable in their natural state and must be modified. Plastics originating from fossil carbon (e.g., polyolefins) that degrade in oxygen are known as oxo-degradable plastics [71, 96]. Around 20,000 tons of plastic enter the oceans every day because of landfill leaks and plastic pellet losses during pre-product shipment (Fig. 5) [71].

Waste Management Options of Biodegradable Plastics The level of biodegradation of biodegradable materials is determined by the end-oflife options and the biotic environment. The material, market volume, and collection and processing facilities influence the optimum end-of-life option for any waste product. Aside from the end-of-life choices available for standard plastics, biodegradable plastic products offer another choice: organic recovery or recycling. They can be composted in a commercial or residential environment, anaerobically digested, or biodegraded on farmland, with commercial composting being the most frequent approach. Composting is the most environmentally friendly alternative for biodegradable plastics towards the end of their lives. Of course, this assumes that biodegradable/compostable plastics are collected separately and delivered to a commercial composting facility; none of which is currently accessible in many nations. Landfilling is the least desirable choice for any trash, not just bioplastics. Others 17% Agriculture 3% Packaging 40% Household, Leisure & Sports 4% Electrical & Electronics 6%

Automotive 10%

Building & Construction 20%

Fig. 5 Plastic production by sector in Europe. Others include furniture, medical appliances, and machine construction [71]. (This image is shared under the CC BY-NC-ND 4.0 license (http:// creativecommons.org/licenses/by-nc-nd/4.0/))

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Table 2 Biological waste treatment options [72] Conditions Aerobic

Anaerobic

Waste treatment method Composting

Anaerobic digestion

Microorganisms Fungi, bacteria, and actinomycetes Bacteria, no fungi

Temperatures cyclic alkanes > n-alkanes > branched alkanes > branched alkanes > cyclic alkanes [73]. The entry of molecular oxygen into the hydrocarbon is usually the first step in bacteria’s aerobic breakdown of hydrocarbons. Bacteria convert aromatic hydrocarbons to trans-dihydrodiols, which are then oxidized to dihydroxy compounds, such as catechol in the case of benzene, following an initial dioxygenase attack [75]. Terminal oxidation of alkanes (short or long chains) produces alcohols, aldehydes, and monobasic fatty acids, which are then beta oxidized (Fig. 2). Although the enzymology of alkane oxidation is not clear, dehydrogenation, hydroxylation, and hydroperoxidation are commonly accepted processes [76]. n-Alkanes followed by cycloalkanes are the most easily degradable petroleum components, whereas the aromatic portion is the most resistant to microbial attack [75]. Indeed, an oxygen-dependent enzyme called oxygenases initiates the attack on alkanes by incorporating molecular oxygen into the alkane molecule. When two or more carbon atoms are present in an n-alkane, aerobic breakdown usually starts with the oxidation of a terminal methyl group to form primary alcohol [73]. After that, the alcohol is oxidized to produce the corresponding aldehyde and subsequently converted into fatty acid. Fatty acids are activated and then metabolized via the β-oxidation sequence to produce acetyl-CoA. Both ends of the alkane molecule are oxidized via di-terminal or ω-hydroxylation of fatty acids to produce ω-hydroxy fatty acid, which is subsequently transformed into a dicarboxylic acid processed via

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Fig. 2 Alkane degradation mechanisms in the periphery. The terminal oxidation of alkanes to fatty acids is performed by n-alkane monooxygenase, alcohol dehydrogenase, and aldehyde dehydrogenase

β-oxidation sequence to generate acetyl-CoA [14, 75]. Acetyl-CoA enters then the TCA cycle, where it will be mineralized to carbon dioxide (CO2) and water (H2O). Moreover, n-alkanes have been reported to undergo subterminal oxidation [77]. Secondary alcohol is formed, then reduced to a ketone, and oxidized to generate

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an ester by a Baeyer-Villiger monooxygenase. The resultant ester is subsequently hydrolyzed by an esterase, producing alcohol and fatty acid, entering the ß-oxidation cascade. In some bacteria, terminal and subterminal oxidation can coexist [74]. Minor mineral oil components, such as cyclic alkanes, are generally resistant to microbial attack. The principal attack is complicated by the lack of an exposed terminal methyl group. Few microbial species can use cyclohexane as their primary carbon source; mixed cultures are more likely to utilize it as a cometabolite. Figure 3 depicts the mechanism of cyclohexane degradation. Their alkyl side chains aid the

Fig. 3 Peripheric metabolic pathway of cycloaliphatic compounds (cycloparaffins)

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breakdown of cycloalkanes in general. As the chain length of an aliphatic hydrocarbon increases, it becomes less water-soluble. In addition, hydrocarbons with a chain length of C12 and above are virtually water-insoluble [74].

Anaerobic Biodegradation Anaerobic degradation is a process in which microorganisms break down biodegradable materials without oxygen. Because it reduces the volume and bulk of the input material, anaerobic degradation is commonly used to treat wastewater sludge and biodegradable waste. Sulfate-reducing bacteria or other anaerobes that utilize various electron acceptors as an oxidant are mostly responsible for oil biodegradation in the subsurface [74]. Under anaerobic denitrifying conditions, hydrocarbon biodegradation follows an oxidative strategy. Hydrocarbon substrates, such as toluene, are converted to oxidative intermediates in the presence of nitrates before further biodegradation [75]. The rate of anaerobic microbial breakdown of petroleum hydrocarbons in natural environments has been demonstrated to be minimal, and its ecological significance has been considered minor [78]. Microbial degradation of oxidized aromatic compounds such as benzoate and halogenated aromatic compounds such as halobenzoates, chlorophenols, and polychlorinated biphenyls has been demonstrated under anaerobic circumstances [75]. There have been numerous reports of bacterial strains utilizing alkanes as carbon sources in anoxic conditions [75]. As electron acceptors, these microbes utilize nitrate or sulfate. Compared to aerobic alkane degraders, the rate of growth is substantially slower. On the other hand, anaerobic alkane degradation is significant in the recycling of hydrocarbons in the environment. For example, strain Bus5, a Desulfosarcina/Desulfococcus sulfate-reducing bacteria, only assimilates propane and butane [75]. Denitrifying bacteria Azoarcus sp. HxNI uses C6-C8 alkanes, while Desulfobacterium HdX3 uses C12-C20 alkanes [75]. The metabolic pathways for anaerobic degradation of alkanes have been investigated for some strains [71]. Two main processes are involved: in the first one, the alkane is activated at a subterminal location by adding a fumarate molecule, resulting in an alkyl-succinate derivative. The production of an organic radical intermediate, most likely a glycyl radical, is thought to be the catalyst for this reaction [71]. The reaction product is then linked to CoA and converted into an acyl-CoA, which can be further metabolized by oxidation. In the second reaction, which has only been observed for propane, the fumarate molecule is linked to one of the alkane’s terminal carbon atoms (Fig. 4).

Microfungi and Mycorrhiza Degradation Microfungi are eukaryotic, aerobic microorganisms that range from unicellular yeasts to mycelial molds. Molds normally grow as mycelia-forming real hyphae, whereas yeasts prefer to grow as single cells or pseudomycelia [71]. Plant roots and soil fungi form a symbiotic relationship known as mycorrhiza. The arbuscular

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Fig. 4 Anaerobic degradation of alkanes by addition of fumarate molecule

mycorrhizal fungi (AMF), which colonize the host plant’s roots intracellularly, and the ectomycorrhizal fungi, which colonize the plant root extracellularly, are the two most prevalent types of mycorrhizal associations [14].

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Yeast Degradation Several yeasts may use aromatic compounds as growth substrates (only carbon and energy sources), but their ability to break down aromatic compounds cometabolically is the most significant. Degradation by cometabolism means the transformation of a substrate that cannot be used as a carbon source if a second utilizable substrate is present [79]. The biodegradation of aliphatic hydrocarbons found in crude oil and petroleum products, particularly in yeasts, has been extensively studied. The most frequent hydrocarbons are n-alkanes, with those between C10 and C20 providing the best microfungi substrates [80]. Additionally, biological degradation of n-alkanes with chain lengths up to n-C24 has been confirmed [71, 81]. Candida lipolytica, Candida tropicalis, Rhodotorula rubra, and Aureobasidium (Trichosporon) pullulans are examples of alkane-using yeasts. Diesel oil can be degraded by Rhodotorula aurantiaca and Candida ernobii [82]. Candida methanosorbosa BP-6 is an example of yeast that has been reported for aniline biodegradation [83]. PCB metabolism has also been studied for Candida boidinii and Candida lipolytica and Saccharomyces cerevisiae. S. cerevisiae can also adsorb insecticides and fungicides [84]. Furthermore, yeasts are well known for their role in detoxifying toxic heavy metals. Many studies have shown that yeasts can accumulate heavy metals such as Cu(II), Ni(II), Co(II), and Cd (II) when compared to certain bacteria. They are superior metal accumulators [84]. Moreover, it has been reported that Pichia anomala can remove hexavalent chromium (Cr(VI)) [71], whereas Pichia guilliermondii, Rhodotorula pilimanae, Yarrowia lipolytica, and Hansenula polymorpha can reduce Cr(VI) to Cr(III) [85]. Also, Pichia guilliermondii’s tolerance to chromate was also found to be dependent on its ability to reduce Cr(VI) and Cr(III) chelation outside the cell [71].

Fungi Degradation Fungi are significant members of the degrading microflora because they metabolize dissolved organic material; they are the main agents of carbon decomposition in the biosphere. On the other hand, fungi can flourish in low-moisture environments and in acidic solutions, which helps them break down organic waste [86]. Fungi are the most effective in degrading natural polymeric materials due to their extracellular multienzyme complexes. Their hyphal systems allow them to easily invade and quickly penetrate substrates and transport and redistribute nutrients within their mycelium [87]. Filamentous fungi are considered good biodegrades due to their mycelium and degradative enzymes. The growing mycelial structure of fungi provides a competitive advantage over individual cells like bacteria and yeasts, particularly when colonizing insoluble substrates. Fungi can quickly develop ramification into substrates by producing a set of extracellular degrading enzymes. Hyphal penetration is a mechanical complement to the chemical degradation caused by

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released enzymes. The filaments’ high surface-to-cell ratio enhances mechanical and enzymatic contact with the environment. Furthermore, because the degradative enzymes are extracellular, fungi can resist higher quantities of hazardous compounds than they could if they were introduced inside the cell. In addition, insoluble substances that cannot pass across a cell membrane are vulnerable to attack [88]. Bioremediation strategies are classified into three groups: (1) the pollutant is utilized as a source of carbon; (2) the pollutant undergoes enzymatic degradation; however, it does not serve as a carbon source (cometabolism); and (3) the pollutant is not metabolized but is instead concentrated within the organism (bioaccumulation). Even though fungi are involved in all three processes, they are often better at cometabolism and bioaccumulation than utilizing xenobiotics as a sole source of carbon [88]. Cladophialophora, Exophiala, Leptodontium deuteromycetes, and the ascomycete Pseudeurotium zonatum are toluene-degrading fungi that rely only on toluene for carbon and energy [89]. In addition, when bacteria are used, most filamentous fungi are incapable of entirely mineralizing aromatic hydrocarbons; instead, they only convert them into less toxic products with higher degradation susceptibility, implying that fungi-bacterial interactions are beneficial for the mineralization of petroleum hydrocarbons [87]. Cladosporium and Aspergillus are filamentous fungi that participate in aliphatic hydrocarbon biodegradation, whereas Aspergillus, Cunninghamella, Fusarium, and Penicillium can participate in aromatic hydrocarbon biodegradation [90]. Three fungal species Amorphoteca, Neosartorya, and Talaromyces were isolated from petroleum-polluted soil and found to be promising hydrocarbon degraders [91]. Pseudeurotium zonatum is a toluene-degrading ascomycete that relies solely on toluene for carbon and energy [71]. Likewise, Penicillium, Cephalosporium, and Aspergillus have also been identified as potential crude oil hydrocarbon degraders [71]. The ligninolytic fungi have been particularly studied due to their extracellular, specific oxido-reductive enzymes, which have already been successfully used in the breakdown of numerous aromatic contaminants [92]. Fungi have also been found to decompose finished wood products, standing timber, fibers, and a broad range of noncellulosic materials, such as glues, plastics, paints, fuels, pharmaceuticals, and others [93]. Fungi have identified several mechanisms for tolerating and detoxifying metals, such as valence conversion, extracellular and intracellular precipitation, and active uptake. Cadmium, copper, lead, mercury, and zinc can all be absorbed by the mycelium and spores of many fungi species [94]. The cell surface functional groups of Aspergillus niger AB10 and Rhizopus arrhizus M1 during cadmium and lead biosorptions revealed that these surface functional groups may serve as ligands that sequestrate metals, thus resulting in metal removal from liquid culture medium [95]. Besides, Deng [12] reported that Aspergillus niger YAT could completely break down beta-CY(ß-CY) and its intermediates by mineralization and cometabolism. Ligninolytic fungi are the most extensively studied fungus in dye degradation [71]. Exocellular (cytochromes P450) and intracellular (peroxidases and lactases) enzymatic systems are used by the filamentous fungus to break down insecticides. Pesticides could also activate or inhibit each of these systems, allowing them to control their metabolism [96].

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Algae and Protozoa Degradation Although algae and protozoa are major parts of the microbial community in terrestrial and aquatic ecosystems, there has been little research on their function in hydrocarbon biodegradation. It has been reported that Prototheca zopfi, a competent alga, was able to degrade n-alkanes and isoalkanes and aromatic hydrocarbons using crude oil and a mixed hydrocarbon substrate [71]. Naphthalene can be metabolized by nine cyanobacteria, five green algae, one brown alga, one red alga, and two diatoms. On the other hand, protozoa have not been demonstrated to use hydrocarbons; however, their presence in a biodegradation system has been found to lower the number of bacteria available for hydrocarbon removal dramatically; thus, their presence may not always be helpful [97]. Protozoa cultivated on hydrocarbon-using yeasts and bacteria did not use crude oil directly. Overall, the scant evidence suggests that algae and protozoa play no ecologically relevant role in breaking down hydrocarbons in the environment. Fresh algae, including Scenedesmus platydiscus, Scenedesmus capricornutum, Scenedesmus quadricauda, and Chlorella vulgaris, have been shown to absorb and decompose polycyclic aromatic hydrocarbons [98]. Chlorella vulgaris and Chlorella pyrenoidosa utilized dyes as carbon and nitrogen sources to degrade azo dyes; however, this was reliant on the dyes’ chemical structure [99]. The degradation was found to be an inducible catabolic process. Chlorella vulgaris, Nostoc linckia, Lyngbyala gerlerimi, Volvox aureus, Oscillatoria rubescens and Elkatothrix viridis decolored and removed methyl red, G-Red (FN-3G), orange II, basic fuchsin, and basic cationic. Although heavy metals are tolerated by Anabaena inaequalis, Chlorella, Stigeoclonium lenue, Synechococcus sp., and Westiellopsis prolifica, they are removed by numerous Chlorella, Anabaena, and marine algae species, but the operational conditions limit their use [100]. Algae absorb metals via adsorption, and unicellular algae have been found to chelate metals. Brown algae biosorb heavy metals by the sorption of alginate and fucoidan, two cell wall elements, and most of the research studies in this field have focused on marine and soil algae. In continuous cultures, the microalga Scenedesmus incrassatulus was found to remove Cr (VI), Cd (II), and Cu (II) [71]. Heavy metal bioremediation has been used in green algae, with Chlorella sorokiniana being removed from Cr (III) [100]. Protozoa are primary grazers of organic pollutants on degrading bacteria; hence, interactions between protozoa and degrading bacteria directly impact the outcome of bacteria degradation. The effects of protozoa flagellate Heteromita globosa grazing bacteria on benzene and methylbenzene biodegradation were investigated. The results showed that the rate of benzene and methylbenzene degradation during the growth period of the flagellate population had been enhanced 8.5 times by bacteria. Protozoa infusorians can accelerate the biodegradation of heterogeneous compounds in the environment, like polycyclic aromatic hydrocarbons. The rate of decomposition of naphthalene, for example, can be improved by four times [71]. There are various explanations for protozoa’s role in accelerating the biodegradation of organic pollutants, the most common of which are divided into six parts: (1) nutrient

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mineralization, which improves nutrient turnover; (2) bacteria activation, which regulates quantity, grazes old cells, or produces active material; (3) selective grazing, which minimizes competition for resources and space, allowing degrading bacteria to thrive; (4) physical disruption, which increases O2 content and the surface of degraded substance material; (5) direct degradation, which requires the cooperation of specific enzymes; (6) and metabolism, which provides bacteria with energy and carbon during decomposition [71].

Conclusion Bioremediation is gaining popularity to remove inorganic pollutant contamination because it has several advantages that make it a suitable and effective technology. Its research is very interdisciplinary in nature and requires additional studies to improve our knowledge of plant physiology, biochemistry, and absorption of these pollutants in plants, ecology, soil/water microbiology, soil/water chemistry, and environmental engineering, as well as a proper assessment of the potential synergistic effects of numerous contaminants.

Future Perspectives Chemical pollutants have been the subject of many studies, providing the essential body of knowledge to comprehend their recalcitrance and hazardous character. With this important data, policymakers must determine if cleanup is essential and feasible. Bioremediation solutions are becoming increasingly appealing as regulations limit chemical disposal and the costs of physical and chemical treatments rise. The increased use of bioremediation technologies necessitates new government rules, including risk-based criteria in cleaning methods. Each bioremediation procedure technique has its benefits and drawbacks to be examined in each case. Various factors restrict the effectiveness of microbial breakdown of organic contaminants. Low temperature, anaerobic circumstances, low quantities of nutrients and co-substrates, the presence of hazardous compounds, and the physiological potential of microbes are all essential factors in polluted areas and the pollutant’s bioavailability.

Cross-References ▶ Anaerobic Biodegradation: The Anaerobic Digestion Process ▶ Biodegradation of Azo Dye Pollutants Using Microorganisms ▶ Fundamentals of Biodegradation Process ▶ Role of Microorganisms in Biodegradation of Pollutants ▶ Types of Microorganisms for Biodegradation

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80. Miranda RdC, Souza CSd, Gomes EdB, Lovaglio RB, Lopes CE, and Sousa MdFVdQ (2007) Biodegradation of diesel oil by yeasts isolated from the vicinity of suape port in the state of Pernambuco-Brazil. Brazilian Archives of Biology and Technology 50:147–152. 81. Mucha K, Kwapisz E, Kucharska U, and Okruszeki A (2010) Mechanism of aniline degradation by yeast strain Candida methanosorbosa BP-6. Polish Journal of Microbiology 59(4): 311–315. 82. Wang J and Chen C (2006) Biosorption of heavy metals by Saccharomyces cerevisiae: a review. Biotechnology Advances 24(5):427–451. 83. Bahafid W, Sayel H, Joutey NT, and Ghachtouli NE (2011) Removal mechanism of hexavalent chromium by a novel strain of Pichia anomala isolated from industrial effluents of Fez (Morocco). Journal of Environmental Science and Engineering 5(8). 84. Ksheminska H, Honchar T, Gayda G, and Gonchar M (2006) Extra-cellular chromate-reducing activity of the yeast cultures. Open Life Sciences 1(1):137–149. 85. Matavulj M and Molitoris HP (2009) Marine fungi: degraders of poly-3-hydroxyalkanoate based plastic materials. Zbornik Matice srpske za prirodne nauke (116):253–265. 86. Abrusci C, Marquina D, del Amo A, and Catalina F (2007) Biodegradation of cinematographic gelatin emulsion by bacteria and filamentous fungi using indirect impedance technique. International Biodeterioration and Biodegradation 60(3):137–143. 87. Boldú FP, (2002) Growth of fungi on volatile aromatic hydrocarbons: Environmental technology perspectives PhD Thesis Wageningen University, Wageningen, The Netherlands. 88. Steliga T (2012) Role of fungi in biodegradation of petroleum hydrocarbons in drill waste. Polish Journal of Environmental Studies 21(2):471–479. 89. Chaillan F, Le Flèche A, Bury E, Phantavong Y-h, Grimont P, Saliot A, and Oudot J (2004) Identification and biodegradation potential of tropical aerobic hydrocarbon-degrading microorganisms. Research in Microbiology 155(7):587–595. 90. Singh H (2006) Mycoremediation: fungal bioremediation Ed.John Wiley & Sons. 91. Tigini V, Di Toro S, Belardo A, Prigione VP, Fava F, and Varese G (2008) Identification and characterisation of the active mycoflora of a consortium in the bioremediation of a soil historically contaminated by polychlorinated biphenyls (PCBs). in 4th European bioremediation conference. Technical University of Crete. 92. Pal TK, Sauryya B, and Arunabha B (2010) Cellular distribution of bioaccumulated toxic heavy metals in Aspergillus niger and Rhizopus arrhizus. International Journal of Pharma and Biosciences 1(2). 93. Deng W, Lin D, Yao K, Yuan H, Wang Z, Li J, Zou L, Han X, Zhou K, and He L (2015) Characterization of a novel β-cypermethrin-degrading Aspergillus niger YAT strain and the biochemical degradation pathway of β-cypermethrin. Applied Microbiology and Biotechnology 99(19):8187–8198. 94. Bumpus JA (2004) Biodegradation of azo dyes by fungi. Mycology Series 21:457–470. 95. Chaplain V, Mamy L, Vieublé L, Mougin C, Benoit P, Barriuso E, and Nélieu S (2011) Fate of pesticides in soils: Toward an integrated approach of influential factors. Ed. InTech. 96. Cerniglia CE and Gibson DT (1977) Metabolism of naphthalene by Cunninghamella elegans. Applied and Environmental Microbiology 34(4):363–370. 97. Jinqi L and Houtian L (1992) Degradation of azo dyes by algae. Environmental Pollution 75(3):273–278. 98. Dwivedi S (2012) Bioremediation of heavy metal by algae: current and future perspective. Journal of Advanced Laboratory Research in Biology 3(3):195–199. 99. Davis TA, Volesky B, and Mucci A (2003) A review of the biochemistry of heavy metal biosorption by brown algae. Water Research 37(18):4311–4330. 100. Akhtar N, Iqbal M, Zafar SI, and Iqbal J (2008) Biosorption characteristics of unicellular green algae Chlorella sorokiniana immobilized in loofa sponge for removal of Cr (III). Journal of Environmental Sciences 20(2):231–239.

Part VI Medical and Health Impacts of Biodegradation

Biodegradable Nanocomposite as Advanced Bone Tissue Scaffold Yara A. Kammoun

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bone Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure and Properties of Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bone Defects and Healing Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Materials for Bone Scaffolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioactive Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Nanocomposites Scaffolds Applied in Bone Tissue Engineering . . . . . . . . . . . . . Biodegradable Nanostructured Calcium-Phosphate Based Composites . . . . . . . . . . . . . . . . . . . . Nanostructured Bioglasse-Based Bone Scaffolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piezoelectric Polymer-Ceramic Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inorganic Piezoelectric Materials: Piezoelectric Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piezoelectric Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piezoelectric Ceramic-Polymer Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric Conductive Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetically Responsive Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3D Printed and Biomorphic Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffolds Synthesized by 3D Printing Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffolds Synthesized Through Biomorphic Transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composite Nanostructured Delivery Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct Incorporation of Nanodelivery Systems in 3D Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Modification and Cross-Linking of Nano-delivery Systems to 3D Constructs . . . Multifunctional Nanofiber Scaffolds as Drug Delivery Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . Intelligent Materials and Modular Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barriers to Clinical Translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scientific and Technological Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Translational Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The continuous advancements in nanotechnology, understanding of reinforcement procedures of nanoscale materials, and related fabrication technologies were reflected in bone tissue engineering progress. Nanocomposites bone scaffolds can provide for outstanding effects in the repairing process of bone defects. In the bone regeneration area, the biodegradability of nanocomposites bone scaffolds allows a superior structural performance. This chapter describes the properties and applications of biodegradable nanocomposites scaffold materials commonly used in bone defect repair. Keywords

Nanocomposites · Tissue engineering · Nanotechnology · Biodegradation · Bone scaffold Abbreviations

3D AAL AP-g-GA BCP BMP BMSCs BN BT BTE CAD CS CT DCM DCPs ECPs FDM FEA HA hBMSCs HBPA Hep hPDCs HYAFF-11

Three Dimensional L-Alanine Aniline pentamer-graft-gelatin Biphasic calcium phosphate Bone morphogenetic proteins Bone marrow mesenchymal stem cell Boron nitride Barium titanate Bone tissue engineering Computer-aided designing Chitosan Computed tomography Decellularized cartilage matrix Dicalcium phosphates Electroconductive polymers Fused Deposition Model Finite Element Analysis Hydroxylapatite Human bone marrow mesenchymal stem cells Heparin-binding peptide amphiphile Heparin Human Periosteum-derived cells Benzyl ester of hyaluronic acid

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IM LAB MC3T3-E1 M-CSF MES MNPs MRI MWCNTs nBG n-HA NPt PANI PCL PCL PCL/FeHA PDS PEDOT PGA PHB PHBV PLA PLGA PLLA PPF PPy PSS PTh PVDF PVDF RUNX2 SAP P11–4 SLA SLS TCP TGF- β TGF ZnO

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Immobilized Laser-assisted bioprinting Pre-osteoblasts cells Macrophage colony-stimulating factor 2-(N-morpholino) ethanesulfonic acid Magnetic nanoparticles Magnetic Resonance Imaging Multi-walled carbon nanotubes Nano-Bioglass Nano-hydroxyapatite Platinum nanoparticles Polyaniline Polycaprolactone Poly-caprolactone Poly(3 caprolactone)/iron-doped hydroxyapatite Poly-para-dioxanone Poly(3,4-ethylenedioxythiophene) Polyglycolic acid Poly 3-hydroxybutyrate Poly-3-hydrox ybutyrate-3-hydroxy valerate Poly-lactic acid Poly (lactic-co-glycolic acid) Poly-L-lactic acid Polypropylene fumarate Polypyrrole Poly(4-styrene sulfonate) Polythiophene Poly(vinyl idene fluoride) Poly(vinylidene fluoride Runt-related transcription factor 2 Self-assembling β peptides Stereolithography Selective Laser Sintering Tri-calcium phosphate Transforming growth factor-β Transforming growth factor Zinc oxide

Introduction Bone Tissue Engineering Tissue engineering is a technology that incorporates cells, engineering materials, and physicochemical factors to augment or regain the biological functions of damaged tissues or organs [1]. Bone tissue engineering (BTE) was developed [1]. Bone tissue

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engineering requires the cooperative work of experts, researchers, engineers, and medical practitioners to achieve the desired objectives of creating bone grafts that improve bone regeneration [2]. Bone defects repair maneuvers based mainly on two bone grafting procedures that showed some drawbacks: the first procedure is the auto-graft approach which is not only restricted by the bone quantity that can be collected from the iliac crest bone but also exhibit several surgical jeopardies, including infection, pain, donor site morbidity [2]. The other procedure is the allograft approach (implants from a donor) similarly had revealed several disadvantages, for example: the unavailability of donors, extraordinary expenses, the necessity for sterilization, and the possibility of infection or successions immunological reactions that ends up with tissue rejection [2]. The previously mentioned disadvantages indicated alternative therapeutic strategies for bone defects repair. Consequently, BTE had developed new biomaterials/scaffolds in association with stem cells, growth factors, as well as novel biological cues to promote bone repair (Fig. 1). Nanotechnology improves the scaffold bioactivity owing to the greater surface area offered by the nanoparticles [3, 4]. Thus, bone scaffolds can promote bone cell adhesion, proliferation, and differentiation [3, 4]. At the moment, nanocomposites have gained great attention in bone tissue engineering, as the native bone itself is considered a nanocomposite. As living bone cells naturally respond to nanostructured materials exhibiting rough surfaces with pores size of about 2100 nm [3, 4]. Innovation of new biodegradable nanocomposites that simulate the physicochemical properties and biological activity has consequently attained high gratitude attention in BTE. Biodegradable nanocomposites had been fabricated utilizing various strategies, including hydrogels, microspheres, and 3D scaffolds. These scaffolds demonstrated a promising future for biomedical applications [3, 4].

Fig. 1 The main properties of bone should be presented into bone regenerative strategies for improved outcomes. (Copied with permission from Ref. [3], Frontiers Media S.A, 2020)

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Structure and Properties of Bone Bone Architecture Bone is a connective tissue that seems extremely dynamic and provides adequate mechanical strength and structural support. The macroscopic scale contains mainly two classifications, cortical (compact) bone and trabecular (cancellous) bone [4]. These two categories are distinguished by unique orchestrated 3D architecture with a high level of structural complexity (Figs. 2 and 3a, b). Furthermore, bone tissue is made up of both inorganic and organic components. Hydroxylapatite (HA; Ca10(PO4)6(OH)2) is the main ingredient of the inorganic component. Citrate, carbonate, and ions such as F , K+, Sr2+, Pb2+, Zn2+, Cu2+, and Fe2+ are presented in the inorganic phase of natural bone. On the other hand, type I collagen and non-collagenous proteins such as osteonectin, osteocalcin, bone sialoproteins, in addition to numerous proteoglycans, are found in the organic compartment of native bone and play a key role in matrix maturation and bone cell functional activity regulation [4]. Bone Cells There are four specialized cells in bone tissue: osteoblasts, osteocytes, osteoclasts, and bone lining cells. Osteoblasts are bone-forming cells whose differentiation is primarily regulated by runt-related transcription factor 2 (RUNX2) and other transcription factors [6]. Osteocytes are the cells with the longest life expectancy in the bone matrix. Osteocytes emerge when the osteoblasts become surrounded by the bone matrix. This switchover happens when the extracellular matrix produced by osteoblasts is inhibited; thus, subsequently, osteoblasts differentiate into osteocytes. Osteocytes are primarily responsible for removing defected organelles and macro-molecules utilizing quality-control paths [1, 6]. Sclerostin, a protein expressed by osteocytes,

Fig. 2 Hierarchical architecture of bone tissue from the macro scale to the nanometric scale. (Copied with permission from [5], Scientific Research Publishing Inc., 2016)

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Fig. 3 Represent the bone inter-scale. (a) A view of macroscopic and microscopic of both cancellous and cortical bone. (b) A nanometric scale view of bone tissues includes collagen fibers assembled of collagen triple helix structures that give the collagen fibril. (Copied with permission from [4], Elsevier, 2018)

inhibits Wnt signaling. Parathyroid hormone signaling can inhibit the expression of this protein, allowing Wnt-directed bone formation to occur. Furthermore, osteocytes can inhibit osteoclastogenesis by secreting the transforming growth factor (TGF-). However, when stimulated, osteoblasts and osteocytes induce bone remodeling by producing osteoclastogenic factors such as CSF-1 and RANKL, the NF-B receptor activator ligand [6]. Osteoclasts can absorb mineralized bone matrix and degrade collagen, resulting in bone absorption. In addition, they regulate the synthesis of matrix enzymes that play a significant role in bone resorption. Tumor necrosis factor ligand superfamily member 11 (RANKL) and macrophage colony-stimulating factor (M-CSF or CSF-1) predominate pathways leading to osteoclast formation and activity. RANKL is engaged in osteoclast differentiation, whereas CSF-1 is necessary for osteoclast precursor cell proliferation and survival. Understanding this mechanism contributes to the emergence of therapeutic agents that can inhibit osteoclastogenesis, thereby reducing bone loss [7–9] (Fig. 4). The fourth specialized cells in bone tissue are bone lining cells that are dormant osteoblasts that cover bone surfaces where bone resorption or formation is not desired [10]. Bone multicellular units comprise osteoblasts, precursor cells, and

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Fig. 4 RANKL/OPG balance is an essential element that controls bone resorption in periodontal and periapical tissues. RANK (receptor activator of nuclear factor-kB) with its ligand, RANKL regulates osteoclast differentiation and activation. Osteoprotegerin, OPG, is a decoy receptor of RANKL that obstructs RANK-RANKL engagement. In homeostatic conditions (left side), RANKL and OPG values are assumed to be in equilibrium so that there is limited osteoclastogenesis and bone resorption. In inflammatory conditions, the RANKL/OPG proportion upturns in periodontal and periapical environments and induces osteoclastic activity leading to pathologic bone resorption. (Copied with permission from [6], Taylor and Francis Ltd., 2011)

associated cells such as endothelial cells and nerve cells [9]. The most significant feature of these unique multicellular units in the adult skeleton is to mediate bone remodeling, a skeleton-maintenance mechanism [9]. This process of bone remodeling is required for fracture healing, adaptation to mechanical use, and calcium homeostasis. Alterations in bone resorption/formation in this process result in various skeletal diseases. For instance, osteopetrosis is characterized by excessive bone formation due to osteoblasts’ increased activity without an equivalent amount of bone resorption by osteoclasts, whereas osteoporosis is the opposite.

Bone Defects and Healing Mechanism Normally, bone tissue has a unique healing capacity and unique regenerative properties. Every year, more than 20 million patients globally suffer from bone tissue loss caused by trauma. In addition to trauma, there are other common bone healing diseases, including osteopenia, osteoporosis, and severe dental problems related to tooth loss [4]. For example, osteoporosis, hypertension, and diabetes mellitus have

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been identified as some of the most important diseases afflicting globally [11]. Patients with osteoporotic fractures have a significantly lower quality of life due to decreased functional mobility and an indirect increase in professional homecare services. As a result, it is critical to limit the consequences of this pathology through appropriate and precise prevention and treatment [12]. Cancer is another chronic disease that affects the bones. Despite advances in prevention, early detection, and treatment protocols, this disease remains the world’s second-leading cause of death [11]. Besides hematological and solid tumors, typically diagnosed in the elderly [11] osteosarcoma primarily affects children, adolescents, and young adults. Surgical resection in conjunction with chemotherapy is typically used to treat osteosarcoma [11]. Over the course of treatment of osteosarcoma, a variety of vigorous agents such as great quantities of methotrexate with leucovorin rescue, doxorubicin, and cisplatin could be used for 30 weeks [11]. Thus, several innovative biomaterials have been examined as bone scaffolds and local drug-delivery carriers to promote bone healing and enhance patients’ quality of life [11].

Scaffolds Human bone is a self-healing tissue with a tremendous ability to regenerate. Unfortunately, this healing capacity isn’t infinite and has its limitations. Bone defects beyond critical size can’t heal on their own. The traditional treatment options for these defects usually require surgical interventions [13]. Different graft materials and metal work can be used, but they have limitations. Despite being the gold standard graft material, autogenous grafts have a limited harvested graft size, and the increased morbidity of the second surgical site are the major restrictions of autogenous grafts. In contrast, allogenic and xenografts have limitations in inducing immune responses or infection transmission. On the other hand, using metal plates, pins, or frameworks to replace or support the bone defect leads to bone thinning and bone resorption due to stress shielding [14]. Bone tissue engineering based on the triad of cells, scaffolds, and signals has been developed to overcome the limitations of traditional treatments [14, 15]. Scaffolds work as a temporary skeleton in which cells, nutrients, and growth factors are diffused, forming the regenerated tissue [16]. Despite the significant research in the medical field of bone tissue engineering and scaffolds in the last decades, the ideal scaffold hasn’t been reached yet.

Properties of an Ideal Scaffold The ideal bone scaffold should mimic the natural bone in all its features. It should be biocompatible, bioactive, and biodegradable with enough mechanical strength to resemble the bone [14] (Scheme 1). Biocompatibility The scaffold material and the breakdown byproducts should not cause any cytotoxicity. They should be easily removed from the body without triggering an immune response [17, 18].

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Biodegradability The ideal scaffolds’ biodegradability should have the same rate of tissue regeneration while maintaining the required mechanical support for the tissues. An excessively quick degradation might cause mechanical failure, while too slow degradation triggers inflammatory responses towards the scaffold, impairs the healing and regeneration process [19]. Bioactivity The bone scaffold should provide both osteoconduction and osteoinduction. Osteoinduction means that the scaffold can stimulate the undifferentiated pluripotent cells to develop into bone-forming cells. While osteoconduction means that the scaffold material doesn’t induce cell differentiation, the cells can grow on the surface and inside the pores and channels of the scaffold [20]. The ideal bone scaffold should also include different cues such as growth factors, nano-bioactive particles, and nano-topography that promote cells recruitment, attachment, differentiation, and proliferation [14]. Scaffold Micro-architecture Scaffolds must be engineered with advantageous parameters for the targeted tissue cells, so the bone scaffold should contain a mixture of interconnected micro- and macropores with sufficient overall porosity. The interconnections between pores allow oxygen, nutrients, and waste products between the scaffold and the surrounding tissues. Also, individual pore size significantly impacts cellular growth and attachment. Pore sizes ranging from 300–400 Mm are preferred by bone cells and promote osteogenesis, while scaffold macroporosity improves angiogenesis [14].

Scheme 1 Represented the main requirements of the bone scaffold

Properties of an ideal scaffold

Mechanical Properties Scaffolds, particularly load-bearing areas, should have mechanical properties comparable to the surrounding bone. This would provide enough support to complete defect regeneration and enhance bone growth. The compressive strength of the ideal

Biocompatibility Biodegradability Bioactivity Scaffold microarchitecture Mechanical Properties

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bone scaffold to mimic the cancellous bone should be 100–230 MPa, the modulus of elasticity should be 7–30 GPa, the tensile strength should be 50–150 MPa, with a fracture toughness of 2–12 MPam1/2 and a strain to failure range of 1–3%. The overall porosity of the scaffold also affects the mechanical properties, especially the compressive strength, as it was found that a 60–90% overall porosity can complement the compressive strength [14].

Scaffold Fabrication Methods The main methods of scaffold fabrication are presented in Scheme 2. Conventional Scaffold Fabrication Techniques Solvent Casting/Particulate Leaching

In this technique, porogens (materials used to create pores, e.g., sodium chloride salt) are dispersed into the prepared scaffold material. Once the setting of the scaffold material is completed, the porogen can be dissolved, leaving pores all over the scaffold. Even though this technique is basic and easy, it is uncontrollable. The pores’ size, shape, distribution, and interconnectivity cannot be controlled [21]. Gas Foaming

In this technique, after creating a solid disc of scaffold material, pores are created by saturation of the scaffold carbon dioxide under high pressure of 5.5 Mpa for 72 h at room temperature. Then, the pressure of carbon dioxide is decreased to atmospheric pressure to create pores all over the scaffold. Although in this technique, there is no need for porogens or solvents like in the previous technique, the pores’ size, shape, distribution, and inter-connectivity are still uncontrollable [22, 23]. Freeze-Drying

There are two techniques to create pores by freeze-drying. The first one is to freeze the scaffold material solution, e.g., polymer solution, so the solvent forms ice crystals within the polymer and then decrease the surrounding pressure until all the solvent crystals turn into gas. When all the solvent evaporates, this leaves a network of pores all over the scaffold. The second technique is to dissolve the scaffold material in a solvent and then add water to this mixture to form an emulsion. This emulsion is freeze-dried to remove the water and solvent, creating pores [24]. Phase Separation

This technique depends on the demixing of the polymer/solvent mixture depending on temperature changes. First, the polymer is dissolved in a low molecular weight solvent at a high temperature, forming a homogenous mixture. Then this mixture is cooled to induce phase separation, forming a polymer-rich phase and a solvent-rich phase. The solvent is then removed via evaporation, creating a porous scaffold [14].

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Solvent casting/particulate leaching Gas foaming

Conventional scaffold fabrication techniques

Freeze-drying

Phase separation

Bone scaffolds fabrication techniques

Electrospinning

Stereolithography (SLA)

Fused deposition modeling (FDM)

3D printing techniques

Selective laser sintering (SLS)

Inkjet bioprinting

Laser-assisted bioprinting (LAB)

3D bioprinting Micro-valve bio-printing

4D printing Extrusion bio-printers

Scheme 2 The common bone scaffold fabrication methods

Electrospinning

Electrospinning depends on extruding the polymer solution from a tube towards a collector plate controlled by an electric field [25, 26]. In contrast to the previous techniques, in this technique, by controlling the electric field, micro- and nanofibers can be extruded. Also, the formed scaffold’s fiber orientation, porosity, topography, and mechanical properties can be controlled [27].

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3D Printing Techniques New techniques were needed to overcome the limitations of the traditional manufacturing methods of bone scaffolds and fabricate scaffolds with precise characteristics and more predictable bone regeneration. 3D printing techniques have presented new prospects for scaffold fabrication. It provides the ability to predesign the scaffold using computer-aided design (CAD) depending on the patients’ computed tomography (CT) or magnetic resonance imaging (MRI). Also, through 3D printing techniques, the micro and nano-architecture of the scaffold (the pores’ size, orientation, interconnections, and mechanical strength) can be tailored. In addition, in the case of composite scaffold fabrication, the orientation and ratios of the used materials can be controlled as G-code optimizes all the printing parameters [14]. Stereolithography (SLA), Fused Deposition Model (FDM), and Selective Laser Sintering (SLS) are the most common 3D printing techniques that have been used in bone tissue engineering (BTE).

Stereolithography

Stereolithography is a technique in which photopolymerizable polymers are cured by a laser beam in a layer-by-layer pattern according to the personalized 3D predesigned architecture of the scaffold [28] (Fig. 5).

Fig. 5 Schematics of the stereolithography (SLA) method. (Copied with permission from [5], Scientific Research Publishing Inc., 2016)

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Fused Deposition Modeling

In fused deposition modeling (FDM), thermoplastic materials are 3D printed through thermally controlled printing tips onto a platform in a layer-by-layer pattern (Fig. 6). This technique eliminates the need for organic solvents and the cytotoxicity resulting from them, despite the elevated heat used for printing, limiting the inclusion of cells or biomolecules [29].

Selective Laser Sintering

In this technique, the scaffold material is used in a powder form spread and sintered by a laser beam in successive layers according to the design until the 3D scaffold is formed (Fig. 7). This technique yields scaffolds with high mechanical strength and highly controllable micro and nano-architectures. While the main drawback is the high heat resulting from laser sintering, it must be considered during material choice to withstand the laser heat without dimensional changes [29].

Fig. 6 Schematics of the fused deposition modeling method. (Copied with permission from [5], Scientific Research Publishing Inc., 2016)

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Fig. 7 Schematics of the selective laser sintering method. (Copied with permission from [5], Scientific Research Publishing Inc., 2016)

3D Bioprinting The 3D Bioprinting technique is one of the developments of the 3D printing technique that permits using cells and/or bioactive molecules loaded material (which is called bioink) as a scaffold material. Besides the advantages of the previously mentioned 3D printing techniques, bioprinting provides the ability to create an intrinsic vascular system into the scaffold. Also, unlike the old cell seeding technique on prefabricated scaffolds, bioprinting allows the precise distribution of cells and biomolecules in hierarchical 3D scaffolds that mimic human bone [14, 30]. The most common bioprinting techniques are inkjet, laser-assisted, microvalve, and extrusion bioprinting. Inkjet Bioprinting

The inkjet technique depends on thermal, acoustic, or electromagnetic forces to create pressure in the print tip to eject droplets of the bioink from the nozzle according to the 3D design of the scaffold (Fig. 8). Despite inkjet being an easy, low coast, and available technique, it has some limitations such as the repeated clogging of the nozzle and the possible excessive pressure or heat and their effect on the cells and biomolecules that are loaded in the bioink [14, 30]. Laser-Assisted Bioprinting

This technique utilizes a pulsed laser beam to create pressure that drives the bioink from a ribbon of scaffold material towards a collector according to the determined

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Fig. 8 Schematics of the inkjet method. (Copied with permission from [5], Scientific Research Publishing Inc., 2016)

scaffold design. Nonetheless, laser-assisted bioprinting has drawbacks such as potential cell damage, high cost, and extended time of printing [14, 30]. Micro-Valve Bioprinting

In micro-valve bioprinting, the bioink is constantly under pneumatic or mechanical pressure, and droplets are extruded via the microvalve, which can be mechanical, magnetic, or electrically controlled. This technique has a minimal effect on the loaded cells or bioactive molecules, which is considered its main advantage [14, 30]. Extrusion Bioprinters

The main advantages of extrusion bioprinting are printing a wide range of bioinks and the ability to print high cell densities of bioinks or even pure cells. This technique prints the scaffold materials in filaments or strands instead of droplets. This is done by applying continuous pneumatic or mechanical pressure on the bioink cartridge. Although extrusion bioprinting has a relatively low printing resolution, it might apply shear stress on cells during extrusion and needs continuous progression on new bioinks for more safe delivery of cells [31, 32] (Fig. 9). 4D Printing The 4D printing technique progresses to 3D printing that incorporates time as the fourth dimension. 4D printing uses stimuli-responsive materials to construct 3D scaffolds that modify their shape according to the applied stimulus (thermal, chemical, biological, or physical) depending on their shape memory property. Recently, the 4D scaffolds’ responses to the applied stimuli are not limited to shape changes

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Fig. 9 Advanced approaches for scaffold production used in BTE include nanofibers, hydrogels, and 3D printing. (Copied with permission from [33], Dove Medical Press Limited, 2019)

but include chemical, biological, physical, and maturation and functional transformation [34, 35].

Biodegradable Materials for Bone Scaffolds A wide range of materials can be used for bone tissue engineering. These materials are mainly metals, bioceramics, bioactive glass, polymers, and related subcategories of materials and composites [14] (Scheme 3 and Fig. 10). The next section presents an overview of most biodegradable materials that can be utilized in bone regeneration.

Metals Although metals have been used for a long time in bone repair, their use has been restricted mainly to fixation or joint replacement. Their lack of biodegradability

Calcium sulfate and silicate-based bioceramics

Dicalcium phosphate

Tri-calcium phosphate

Hydroxyapatite

Bioceramics

Bioactive glass

Silk

Fibrin

Hyaluronic acid

Chitosan

Collagen

Natural polymers

Polymers

Poly (lactic-co-glycolic acid)/PLGA

Poly-lactic acid (PLA)

Polycaprolactone (PCL)

Synthetic polymer

Biodegradable Nanocomposite as Advanced Bone Tissue Scaffold

Scheme 3 Represents the most common biodegradable materials for bone scaffolds

composite scaffolds

Biodegradable Strontium

composite scaffolds

Biodegradable zinc

Biodegradable iron composite scaffolds

Biodegradable magnesium composite scaffolds

Metals

Biodegradable materials for bone scaffolds

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Fig. 10 An illustration of the main biodegradable materials used for BTE. (Copied with permission from [34], BMC, 2020)

hinders their use in bone tissue engineering as scaffolds. Recently, there have been multiple trials to fabricate metal composite bone scaffolds.

Biodegradable Magnesium Composite Scaffolds Magnesium’s biodegradability, bioactivity, and suitable mechanical strength with density and modulus of elasticity that all are relatively comparable to human bone make it a more convenient metal to be incorporated into bone scaffolds than other metals such as titanium and stainless steel. Early magnesium scaffold trials revealed an unfavorable rapid degradation, which resulted in issues such as insufficient bone support until complete bone healing, the increased pH that could affect bone regeneration, and the accumulation of released hydrogen, forming air bubbles that could lead to implant failure. Nowadays, magnesium alloys such as ZEK100 and tricalcium phosphate coated magnesium alloy AZ31 have a slower, more suitable degradation rate [34]. Also, hydrothermal treatment, electrolytic oxidation, and different coating techniques have been used to decrease magnesium and magnesium alloys [36, 37]. Biodegradable Iron Composite Scaffolds Iron is one of the biocompatible and biodegradable metals used in bone tissue engineering. Compared to magnesium, iron has stronger mechanical properties that allow using them in load-bearing areas [38, 39]. Iron oxide nanoparticles have also been used to deliver different biomolecules such as drugs and growth factors. They have also been used for mechano-sensitive signaling, magneto-mechanical regulation of cells, and creating a magnetic composite bone scaffold for enhanced osteogenic effect [40].

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Biodegradable Zinc Composite Scaffolds Zinc is also an important metal that affects growth, wound healing, and immunity. Many studies have discovered that using zinc in bone scaffolds increases cell proliferation and differentiation and enhances osteogenesis and angiogenesis [41, 42]. Biodegradable Strontium Composite Scaffolds Strontium is also one of the metals that have been used in biodegradable bone scaffolds. Lei et al. successfully fabricated and tested strontium hydroxyapatite/ chitosan nano-hybrid scaffolds. Upon testing, the scaffolds had favorable biocompatibility, bone cell proliferation, and osteogenic differentiation [43]. Also, Xu et al. added Ag nanoparticles to the strontium hydroxyapatite/chitosan (CS) porous scaffold, which increased the osteoinductivity and antibacterial activity of the scaffold without any negative effect on the human bone marrow mesenchymal stem cells (hBMSCs) adhesion, spreading, or proliferation [44] (Fig. 11).

Bioceramics Bioceramics have been widely used in bone regeneration due to their biocompatibility, biodegradability, and corrosion resistance, with the main benefit being that they mimic the inorganic crystals of human bone. The most common bioceramics for bone tissue engineering are hydroxyapatite, tricalcium phosphate, and dicalcium phosphate [34]. Bone defect Osteoprogenitors Bone formation Differentiation replacement

Paracrine pathway

Bone vascullarization Signaling molecule FGF

VEFG

Osteoblasts Growth factors MSCs

BMP

IGF

Osteoclasts Hydrogel

Osteoblasts

Bone formation

Fig. 11 Bone tissue damage repair by mesenchymal stem cells (MSCs) and growth factors (MSCs, marrow mesenchymal stem cells; BMP, bone morphogenetic proteins; IGF, insulin-like growth factor; FGF, fibroblast growth factor; VEGF, vascular growth factor. (Copied with permission from [15], MDPI, 2020)

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Hydroxyapatite Hydroxyapatite has excellent biocompatibility, osteoinductivity, osteoconductivity and can form a strong bond with the bone. This is mainly due to their similarity to the natural hydroxyapatite crystals in the bone. However, the brittleness and slow degradation of the hydroxyapatite limit its use. So, in most bone scaffolds, hydroxyapatites are combined with other materials to form composite scaffolds [34]. Tri-calcium Phosphate Tri-calcium phosphate (TCP) is one of the bio-ceramics commonly used in bone scaffolds. Tri-calcium phosphates have three crystalline forms, α-TCP, β-TCP, and α-TCP. The β-TCP is the most common form used in bone tissue engineering due to its similarity to the inorganic bone minerals. Tri-calcium phosphates have a better degradation rate than hydroxyapatites. Because of their biocompatibility, biodegradability, and ability to enhance bone formation, tri-calcium phosphates have been combined with different materials such as collagen and polycaprolactone to form 3D bone scaffolds [14, 34, 45]. Di-calcium Phosphate Di-calcium phosphate is also a bio-ceramic that has been used in bone engineering, but to a lesser extent than hydroxyapatite and tri-calcium phosphate. This is mainly due to its brittleness and weak mechanical properties. Many studies successfully combined di-calcium phosphates with other materials (mainly polymers) to enhance their mechanical strength and make them suitable for bone scaffolding [46, 47]. Calcium Sulfate and Silicate-Based Bioceramics Calcium sulfates are naturally found in nature as calcium sulfate dehydrate, which upon calcination, converts to calcium sulfates hemihydrate (plaster of Paris). Calcium sulfates have been used for decades as cement for bone repair as they have a regulatory effect on bone formation-related genes and enhance osteogenesis. Similarly, silicates are common and have great potential in connective tissue regeneration, including bone, so calcium silicates have always been an interesting material for bone tissue engineering [34]. Despite their advantages, both calcium sulfates and calcium silicates have mechanical strength and fast degradation rates that are inappropriate to be used individually as bone scaffolds. Upon mixing calcium sulfate hemihydrate and tricalcium silicate, Hao et al. noticed that the degradation rate and osteogenesis improved [48]. While Chen et al. slowed the degradation rate by mixing calcium sulfate hemihydrates with nanohydroxyapatite mineralized collagen [49]. On the other hand, Yang et al. used the 3D printing technique to create a tri-calcium silicate bone scaffold with controllable nano-topography and enhanced bone regeneration compared to pure tri-calcium silicate scaffolds [50].

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Bioactive Glasses Bioglass is one of the bioceramic materials that has been extensively investigated for bone regeneration since Professor Larry Hench introduced it in 1969. Bioglass gained its fame as a bone regenerative material. It mimics the trabecular bone and is one of the most bioactive materials (it is a class A bioactive material while other bioceramics are class B) as it is both osteoconductive and osteoinductive biomaterial. Bioglass also forms a strong direct bond with the bone, making its removal nearly impossible without cutting into the surrounding bone [16, 51, 52]. The first bioglass was the 45S5 silicate bioglass composed of (weight %): 45 SiO2, 24.5 Na2O, 24.5% CaO, and 6 P2O5. Later on, another silicate bioglass (13–93) was introduced with a composition of 53 SiO2, 6 Na2O, 12 K2O, 5 MgO, 20 CaO, and 4 P2O5. Both forms of silicate bioglass had a limitation of slow and incomplete conversion into hydroxyapatites, so borate bioglass has been developed for more controllable degradation and faster and complete conversion into hydroxyapatites. However, composites with other materials have regularly been used to improve the mechanical properties of bioglasses, such as porosity, degradation rate, and elastic modulus as bone scaffolds [16]. The main nanostructured calcium-phosphate-based materials for bone regeneration, with their respective advantages and disadvantages, are summarized in Table 1.

Polymers Polymers can be divided based on their origin into natural and synthetic polymers [53]. Natural polymers have always attracted bone tissue engineering researchers as scaffold materials for their biocompatibility, biodegradability, bioactivity, availability, and simulation of the bone. Despite this, natural polymers have limitations, such as an unfavorable degradation rate, and they might trigger immunity reactions. On the other hand, synthetic polymers have more controllable biodegradation, design, and mechanical properties. They have a favorable hydrophobic surface for cell adhesion, and their degradation products are acidic, which increases pH and inflammation around the scaffold [14, 34].

Natural Polymers Collagen Collagen (especially type I) is the most frequently used natural polymer in bone scaffolds. This is most probably because collagen forms the majority of the organic extracellular matrix of the bone. Besides, collagen is biocompatible, biodegradable, and improves bone cell differentiation and proliferation. As most natural polymers, Collagen’s mechanical properties are inappropriate to be used alone as a bone scaffold. However, the degradation rate and the mechanical properties of collagen can be controlled by crosslinking or by combination with other materials [14, 34].

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Table 1 Summary of the main nanostructured calcium-phosphate-based materials for bone regeneration, their respective advantages, and disadvantages Nanostructured Bioceramics

Nano-bioglasses

Hydroxyapatite

Tricalcium phosphate

Nanocomposites

Ceramic/polymer composites (e.g., HA/PLGA, HA/Alginate)

Bio-hybrid composites

Advantages Improved biocompatibility. Outstanding bone integration performance. Enhanced biodegradability. Extraordinary biocompatible behavior. Act like inorganic (mineral) components of bone. Enhanced Osteoconductivity. Extraordinary biocompatible performance. Enhance new bone matrix formation. Improved bioactivity as well as biodegradation. Proper Osteoconductive properties. Rapid in vivo biodegradation behavior. Improved biocompatibility. Flexibility of production. Can be employed to fabricate 3D scaffolds. Enhanced mechanical features. Appropriate biodegradation level. Great compatibility. No need for organic solvents during manufacturing. Outstanding bonelike structure. Outstanding bioactive behavior.

Copied with permission from [3], Frontiers Media S.A, 2020

Disadvantages Suboptimal Biodegradation Inferior mechanical Performance. Undesirable mechanical Behavior. Poor biodegradation degrees. Inadequate osteoinductivity.

Inadequate mechanical features.

Manufacture involved the use of organic thinners.

Poor mechanical properties (not load bearing)

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Chitosan Chitosan is a natural polysaccharide polymer that is extracted from crustaceans’ shells. Chitosan has bio-adhesive and antibacterial features in addition to its bioactivity, biodegradability, and biocompatibility. To enhance the mechanical strength of chitosan, it has been cross-linked and combined with other materials in composite scaffolds [14, 34]. Hyaluronic Acid Hyaluronic acid is a natural biopolymer that is one of the natural components of the extracellular matrix of the connective tissue, synovial fluid, dental pulp, and other tissues in our bodies. Hyaluronic acid is composed of D-glucoronic acid and N-acetyl glucosamine disaccharide units. It has a positive influence on cell adhesion, migration, and proliferation. The exact mechanism of the hyaluronic effect has not yet been completely discovered. Some studies mentioned that hyaluronic acid could interact with cell surface marker CD44, facilitating stem cell migration and motility. Others suggested the molecular weight of hyaluronic acid as a reason for enhanced differentiation and early angiogenesis, and the sulfated glycosaminoglycan has also been mentioned as a reason for enhanced differentiation of early osteoblasts. Like many other natural polymers, hyaluronic acid cannot be used alone as a bone scaffold. However, its properties can be controlled and enhanced by crosslinking, manufacturing, designing its derivatives, coating other materials, and combining to form composite scaffolds [54, 55]. Fibrin Fibrin is also a natural biopolymer that forms a nano-scaffold as the last step of the clotting cascade and initiates hemostasis after any injury in our bodies. Fibrin is a promising candidate for bone regeneration as it has excellent biocompatibility, is available in our bodies, and enhances cell migration, adhesion, differentiation, and proliferation. On the other hand, similar to other natural polymers, its weak mechanical properties and rapid degradation are considered major drawbacks. Despite this, the biodegradation rate can be controlled by changing its precursors (fibrinogen and thrombin) ratios. Also, the mechanical properties can be enhanced if combined with other materials. Another advantage of fibrin is that it can be used in different forms; it can be used as a coating, injectable hydrogel, micro-beads, drug and biomolecule delivery, or as a nano-composite scaffold material [56]. Silk Silk is naturally spun by silkworms (the dominant source) and some arachnids (e.g., spiders). Besides its biodegradability and biocompatibility, the main advantage of silk is that it is controllable. Different processing techniques can control both the degradation rate and mechanical characteristics. Also, silk can be formed into many forms, including films, non-woven mats, hydrogels, or 3D scaffolds. Many studies investigated silk in different forms and combinations with other materials and found that it improves osteogenic differentiation and bone formation.

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Although most studies are in vivo and in vitro stages, further human studies are needed before clinical use [57].

Synthetic Polymer Polycaprolactone Polycaprolactone is synthetic polyester that is biocompatible, biodegradable (despite having a slow rate of degradation), has fewer acidic breakdown byproducts than other synthetic polymers, and has good mechanical strength that makes it suitable for load-bearing areas. However, its slow degradation rate and unfavorable cell adhesion make its use alone as a bone scaffold unlikely [14]. Several studies attempted to overcome these limitations by combining polycaprolactone with natural polymers, bioceramics, or bioactive glass. Nadini et al. developed a multilayered nano-fibrous scaffold from polycaprolactone, poly-vinyl alcohol, and sodium alginate. Upon examination, the scaffold proved to be appropriate as a bone scaffold [58]. While Gonçalves et al. created their 3D printed bone scaffold by mixing polycaprolactone with nano-hydroxyapatite crystals and carbon nanotubes [59]. Polylactic Acid Polylactic acid is also synthetic polyester that is bioactive and biocompatible and used for many years in medical implants. Like polycaprolactone, polylactic acid has been integrated into composite scaffolds to enhance its bioactivity and mechanical properties [14]. Yang et al. mixed polylactic acid, polycaprolactone, and hydrophobically modified nano-silica particles to produce a hierarchical scaffold with high porosity, enhanced cell adhesion, and proliferation [60]. Also, Holmes et al. fabricated micro, and nano-featured scaffolds with microvascular channels by nano-hydroxyapatite conjugation of 3D printed polycaprolactone that had improved vascular cell growth and osteogenic activity [61]. Poly (lactic-co-glycolic acid)/PLGA Poly (lactic-co-glycolic acid) is a synthetic copolymer of poly-L-lactic acid (PLLA) and polyglycolic acid (PGA) in which, according to their ratios, the biodegradation rate of the poly (lactic-co-glycolic acid) becomes controllable. Despite this, it still has limitations such as low modulus of elasticity, easy deformation, and low osteoconductivity [14]. So, to overcome these limitations, some researchers combined nano-hydroxyapatites with (PLGA), while others combined them with other polymers [62].

Biodegradable Nanocomposites Scaffolds Applied in Bone Tissue Engineering From the previous section, we disclose that no single material can be used individually as a bone scaffold. Most probably, to mimic the natural bone properties and architecture, composites of different materials have to be the choice. That makes

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sense, as the bone is a hierarchical, heterogeneous composite structure containing an inorganic phase (hydroxyapatite crystals) and an organic phase (collagen fibers).

Biodegradable Nanostructured Calcium-Phosphate Based Composites Bioceramics gained their importance in bone tissue engineering due to their resemblance to the inorganic hydroxyapatite crystals of the bone. Many studies incorporated different bioceramics into nanocomposite scaffolds to overcome the limitations of bioceramics and produce biomimetic bone scaffolds [14]. Kim et al. used the rapid prototyping technique to fabricate a biocompatible, bioactive nano-omposite scaffold that consists of polylactic-polyglycolic/ hydroxyapatite tricalcium phosphate [63]. Also, Mohseni et al. created a tricalcium phosphate/collagen nano-composite scaffold that enhanced bone regeneration in a rabbit 5–6 mm femur bone defect compared to hydroxyapatites and spontaneous healing [64].

Nanostructured Bioglasse-Based Bone Scaffolds Although bioglass is a highly bioactive material that improves osteogenesis and forms a strong bond with the bone, its mechanical properties are suboptimal for being used alone as a bone scaffold, especially in load-bearing areas. So, to enhance the mechanical properties and form bone-mimicking scaffolds, bio-glass has been incorporated into different composite scaffolds with all other classes of material (metals, bio-ceramics, and polymers) [14, 16, 34].

Bioglass-Metal Nano-composite Scaffolds Metals were always convenient candidates for load-bearing areas. They have adequate mechanical strength, but they lack enough bioactivity. Bioglass is a highly bioactive material was an interesting option for many researchers to examine its effects on bone regeneration when combined with different metals [65]. Rau et al. successfully coated the Mg-Ca alloy using pulsed laser deposition with a glassceramic layer. The coating layer had micro-nano roughness, and both the corrosion resistance and the biodegradation rate of the Mg-Ca alloy have been improved [66]. Kaczmarek et al. prepared nano-composite Ti–10 wt % 45S5 Bioglass scaffolds with cytocompatibility more than the conventional microcrystalline titanium [67]. Recently, Matter et al. coated the titanium dental implants with bioglass/ceriabased layer using scalable liquid-feed flame spray pyrolysis technique that produced a super-hydrophilic surface which promoted cell adhesion and provided an antimicrobial effect [68]. Bioglass-Bioceramics Nanocomposite Scaffolds Bioglass has been mixed with other bioceramics to improve the resulting structure’s bioactivity, biodegradability, and mechanical properties [16]. For example, some

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studies co-sintered the hydroxyapatites with 45S5 bioglass to gain better cell proliferation, better apatite deposition, and better mechanical properties in comparison to pure hydroxyapatites [69, 70]. Others prepared silicon nitride- bioglass composite scaffold to add bioactivity to the bioinert silicon nitride (Si3N4) [71].

Bioglass-Polymers Nanocomposite Scaffolds The polymer-bioglass composite might be an ideal choice for bone tissue engineering as it mimics the natural bone composite (the bioglass mimics the inorganic phase, and the polymer mimics the organic phase of the bone). Also, the combination of bioglass and polymers enhances the final scaffold’s bioactivity, biodegradability, and mechanical characteristics [16]. For example, Marelli et al. prepared a nano-bioglass/collagen scaffold for bone tissue regeneration [72]. Montazeri et al. prepared a nano-bioglass scaffold with poly-3-hydroxybutyrate as a coating [73]. Nawaz et al. prepared gelatinmanganese-doped mesoporous bioglass nanoparticles for the same purpose [74]. While Parvizifard et al. evaluated the influence of coating the nano-bioglass (nBG)-titania (nTiO2) scaffolds with poly (3-hydroxybutyrate) PHB-Chitosan (Cs)/ multi-walled carbon nanotubes (MWCNTs) and found that both the bioactivity and the mechanical strength increased in comparison to non-coated nano-bioglass (nBG)titania (nTiO2) scaffolds [75].

Hydrogels Hydrogels are cross-linked, hydrophilic polymers that might have natural or synthetic origins. Hydrogels are biocompatible materials that mimic the extracellular matrix, enhancing both angiogenesis and osteogenesis. They are hydrophilic structures that absorb water up to 1000 times their weight without disintegration. They are also highly permeable, making oxygen, nutrients, and any water-soluble material exchange easily between tissues and the scaffold. Natural hydrogels might be derived from natural polymers such as collagen, gelatin, agarose, alginate, hyaluronic acid, and chitosan. In contrast, synthetic hydrogels such as polyethylene glycol, polyvinyl alcohol, pluronic hydrogels, and other synthetic polymer hydrogels might also be derived from natural polymers. Despite the higher bioactivity and biocompatibility of natural hydrogels, their mechanical strength and biodegradability rate might be compromised compared to synthetic hydrogels. Hydrogels are categorized according to their crosslinking into four categories; homo-polymeric hydrogels, which are formed by polymerization of a single hydrophilic monomer, co-polymeric hydrogels that consist of two or more different monomers, semi-interpenetrating networks where one linear polymer penetrates a cross-linked network of another polymer without a real chemical bond, and interpenetrating hydrogels where there is real bonding between two different polymers. Besides, hydrogels have versatile forms, they can be used as bio-ink for bio-printing, for drugs and biomolecule delivery, as an injectable cell-loaded hydrogel, or as a scaffold in combination with other materials [15, 76, 77] (Figs. 12 and 13).

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Drug release

Su st

d ne ai

Long dody circulation

Re sp on si

ve

se lea re

Physical modification

Compartmentalization

ery liv de

H+

Targeted delivery

955

Hydrog el

Dox loaded micelle

Biocompatible/ safe

Dox

se

m

rf

m od ific ati on

ed

Time

su

Biodegradable

le a

sy

ac e

High penetration

re

Ea

Cargo protection/ encapsulation

Drug

Chemical conjugation

p Re

ro

gr

am

Enhanced biodistribution

Fig. 12 Properties of hydrogel scaffolds used in (BTE) utilizing growth factors and cells with varying delivery mechanisms. (Copied with permission from [33], Dove Medical Press Limited, 2019)

Fig. 13 Hydrogel for (BTE) research methods. (Copied with permission from [15], MDPI, 2020)

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Piezoelectric Polymer-Ceramic Composites When a mechanical deformation results in creating a net dipole moment and polarization of the material, it is said to be piezoelectric (Fig. 14) [8, 34, 78]. The Curie brothers are credited with discovering piezoelectric phenomena in 1880 when they discovered electrical charges generated on the surface of quartz crystals and ochelle salt when these materials were subjected to pressure [78]. Unfortunately, it wasn’t until 1950 that this phenomenon was discovered in biological tissues [78]. Living bone is a piezoelectric nanostructured material. The negative charges that already exist on the surface of the bone increase when subjected to external mechanical loads (compression and/or stretching), and the increased negative charges can enhance defect regeneration by promoting the function of osteoblasts [79] (Figs. 15 and 16). Applied Tension

Applied Compression

- 0 + v

- 0+ v

- 0+ v

Direct piezoelectric effect

Converse piezoelectric effect

0V

Compression due to applied voltage

Tension due to reversed polarity

Fig. 14 Schematic of direct and converse piezoelectric effects. (Copied with permission from [78], Elsevier, 2018)

Fig. 15 The scheme depicts the dual effect of mechanical stress on tissue regeneration. Either through a direct signaling pathway, as applied mechanical force can improve tissue regeneration, or through an indirect pathway. (Copied with permission from [79], Springer Nature, 2018)

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Mechanical stimulation

Electrical signal

Polarized scaffold

Piezoelectric scaffold

Ca2+

a5b1 integrin

Connexin

2+ Ca2+ Ca Ca2+

Ca2+ Ca2+

Ca2+ Activated Calmodulin

MAPK PKC

Calcineurin NF-AT-PO4

NF-AT

NF-AT

Release of IL-4

Gene transcription Nucleus

Aggrecan MMP-3

Gene expression Differentiation Cell Survival

Fig. 16 The Ca2+ signal transduction pathway and other pathways that activate in response to electrical and mechanical stimulation are depicted schematically. The mechanical stress application on the piezoelectric scaffold will lead to an electrical signal that activates the voltage-gated Ca2+ channel. The advanced elevation of the intracellular Ca2+ concentration activates the calciummodulated protein (calmodulin), activating the calcium and calmodulin-dependent serine/threonine protein phosphatase (calcineurin). This activation cascade will result in bone regeneration. Furthermore, the mechanical stimulation can directly activate the mechanoreceptors present in the membrane, which will result in the activation of signaling cascades. These cascades will lead to the synthesis of proteoglycan and inhibition of IL-1, responsible for proteoglycan breakdown. (Copied with permission from [79], Springer Nature, 2018)

Recent piezoelectric resource innovations, such as lead zirconia titanate, barium titanate, and poly(vinylidene fluoride (PVDF), have paved the way for their use in a variety of fields [78, 79]. Due to their outstanding ability to produce charges/ potentials in response to mechanical stresses similar to living bone, piezoelectric

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materials have exhibited a distinct potential for producing stimulatory scaffolds to enhance bone tissue regeneration [3, 78, 79].

Inorganic Piezoelectric Materials: Piezoelectric Ceramics Nanopiezoelectric ceramic materials have attracted growing attention for bone tissue engineering. The most studied nanopiezoceramics involve boron nitride (BN), barium titanate (BT), and zinc oxide (ZnO). Although these materials have extraordinary piezoelectric properties, some exhibit inferior biocompatibility when used in great quantities, limiting their use in bone engineering applications [78, 79].

Piezoelectric Polymers Piezoelectric polymers have also been investigated for bone regeneration applications, including PVDF (poly vinylidene fluoride) and its copolymers, PHBV (Poly3- hydroxy butyrate-3 –hydroxy valerate), and PLLA [1, 3]. Naturally, they have been fabricated either as rods, tubes, or films [1, 78, 79]. Piezoelectric polymers display several advantages, including biocompatibility, piezoelectric activity, and significant osteogenic capacity [80]. Among these, PVDF has several advantages, including the ability to improve cell adhesion and proliferation in myoblast cells and to stimulate the osteogenic differentiation of human adipose-derived stem cells [81], stimulating bone growth at the bone-implant interface, exhibiting both high strength and impact resistance [78, 79], exhibit both high strength and impact resistance [1, 3]. Moreover, it provides the essential electromechanical prompt for differentiating human mesenchymal cells into the osteogenic lineage in vitro [78, 79]. However, the lack of biodegradability of PVDF limits its applicability in tissue engineering and clinical potential [78, 79]. PHBV is an excellent biocompatible, biodegradable, and thermoplastic piezoelectric polymer. PHBV has a piezoelectric coefficient (1.3 pC/N) similar to human bone, demonstrating osteogenic capacity both in vitro and in vivo [3]. Moreover, it has a longer degradation time than another biocompatible polymer. PLLA is a biodegradable and biocompatible polymer with a great shear piezoelectric coefficient. Due to its helical structure and the mechanical orientation of molecules in the crystals and the quasi-crystalline region. PLLA has been welldocumented for rapid bone regeneration. Due to its outstanding mechanical properties [2, 5]. PLLA has great application in orthopedics biodegradable fixation devices such as pins and screws [6, 34] .

Piezoelectric Ceramic-Polymer Composite Materials Piezoelectric ceramics and polymers have been used to formulate a diversity of composite materials that improve bone scaffold’s piezoelectricity, porosity,

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mechanical properties, and biodegradability [1, 3]. Of the polymer matrix composites, PVDF and PVDF-TrFE have both been combined with starch or cellulose-like natural polymer to fabricate an appropriate porous structured scaffold that promotes mutual bone regeneration and repair [82]. PVDF-TrFE and barium titanate piezoelectric composite membranes have been designated as proper charge generators. The improved osteogenic capability was demonstrated at the level of in vitro and in vivo (in rodent bone defect models) studies [5, 34]. Similarly, PLLA-based nanocomposites have been extensively investigated in the field of bone regeneration [3, 14] . For the ceramic matrix composites, HA/BT-based materials are the most investigated composite. The piezoelectric charge constant of these composites was close to bone [83]. Tang et al. (2017) observed that HA/BT piezoelectric composites’ biocompatibility and bone-inducing activity were greater than HA under cyclic loading [83]. Several studies demonstrate the promoted osteogenesis and the osteoinductive capability of these composites [82, 83]. Piezoelectric nanocomposites are promising and gaining significant attention in the bone engineering field due to their capability to simulate the bone environment under static and dynamic conditions. However, biodegradable piezoelectric nano-composite materials require additional research and development before they can be used clinically in bone defect repair [83, 84].

Electric Conductive Nanocomposites Bioelectrical signals, [85] endogenous electrical fields, and external electrical stimulation influenced cellular behavior and tissue function during bone regeneration [78].remove Electroconducive scaffolds are not as piezoelectric as piezoelectric scaffolds as they require exposure to an external power source to discharge electrical signals. In certain circumstances, due to fracture or any other non-load-bearing healing situation, the patient is immobilized, so the mechanical stimulation is prohibited, and the desired outcomes of piezoelectric materials are consequently not achieved [78]. Consequently, several investigations have attempted to investigate the effect of conductive nanocomposites on bone regeneration regulation in non-load-bearing healing conditions through electrical signals. The conductivity of scaffolds has two important effects: (a) it improves cell signaling, and (b) it allows for the local delivery of external electrical stimuli to the site of defects, which improves the healing process [85]. Although it seems challenging to assess the efficacy of the electroconductive scaffolds as they require the control of several variables such as amplitude, frequency, duration, and nature (alternating/direct) of the signals, those scaffolds exhibit a better regulation of bone healing upon stimulus when compared to piezoelectric scaffolds [78] . Different conductive materials were fabricating electroconductive nanocomposites bone scaffolds, such as conductive fillers and electroconductive polymers [1, 3]. Several conductive fillers are widely used to increase conductivity in bone tissue engineering applications, including conductive carbon-based solutions, for instance, carbon nanotubes, carbon nanofibers, carbon black, and graphene. Pang et al. [86] in 2017 revealed that grapheme scaffolds possess antimicrobial behavior.

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Hydroxyapatite–graphene nanocomposites and 45S5 bioglass–graphene nanoplatelet composites [1, 3] have been evaluated. The results revealed that both nanocomposites increased the electrical conductivity, thus enhancing bone formation. Moreover, the lack of degradability and unproven biocompatibility, together with the nonexistence of evidence regarding the long-term safety of graphene, all pose outstanding disadvantages that limit its application in vivo [1, 3]. The other example of conductive filler is metal-based solutions (e.g., platinum nanoparticles (NPt)). Platinum Nanoparticles (NPts) scaffolds significantly affect cell growth and promote cell proliferation. Furthermore, it has anti-inflammatory action [87]. There are several electroconductive polymers (ECPs) that are used in the biomedical field. The polyheterocycle group, which includes polypyrrole (PPy), polyaniline (PANI), and polythiophene (PTh) and its derivative poly (3,4-ethylene dioxythiophene) (PEDOT), has received the most attention for bone tissue engineering applications [3].They exhibit several advantages, including biocompatibility, biodegradability, and electrical conductivity, with greater control of cell proliferation and osteogenic differentiation [1]. Unfortunately, electroconductive polymers offer poor mechanical properties encouraging the innovation of electroconductive polymeric composites. For this reason, ECPs have been blended with several other natural and/or synthetic non-ECPs to enhance their mechanical properties [3, 34, 87]. Furthermore, the biodegradable electroactive copolymers derived from polylactide and the tunable contents of the aniline tetramer were effectively manufactured. The electroactive copolymers significantly augmented the cell proliferation of BMSCs and MC3T3-E1 cells compared to polylactide [1, 4, 88]. In 2015, Becker et al. [88] investigated using polymer-mineral scaffolds composed of a copolymer of polypropylene fumarate (PPF) and polycaprolactone (PCL) as a matrix for attachment of osteoblasts and their maturation into nano-hydroxyapatite (n-HA). Different concentrations of n-HA were added to an injectable preparation of PPF-co-PCL, and both the thermal and mechanical performance of the scaffolds was assessed. The findings of their study suggest that HA/PPF-co-PCL composite scaffolds can induce the maturation of pre-osteoblasts and have the potential for use in bone tissue engineering.

Magnetically Responsive Composites The magnetic attitude of a material (ferro- and ferrimagnetism) is characterized by the ability of a material to become magnetized when subjected to an external magnetic field and remain magnetized after removal. On the other hand, paramagnetism can be defined as a material’s ability to magnetize when exposed to an external magnetic field and the loss of this magnetism upon removal of stimuli. Of particular interest is paramagnetism, as this ability is an outstanding and appropriate property in the bone tissue engineering area, as the aggregation of the material’s magnetic particles in vivo may induce local toxicity [1, 3]. Magnetic field stimulation therapy has been thoroughly investigated for clinical applications for many years [88]. Magnetic field stimulation has been shown in studies to regulate cell responses, enhance cell activity, promote the integration of

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scaffolds, increase calcium content, and accelerate bone healing. The biological effect of static and pulsed magnetic fields on bone scaffolds has become a topic gaining great attention. In vitro studies revealed that both pulsed and static magnetic fields promote osteoblast differentiation, and in vivo animal studies demonstrated promising effects of bone healing and mechanical graft properties [1, 3]. D’Amora et al. [89] explored the effect of a time-dependent magnetic field on the metabolic performance of (hMSCs) seeded on 3D additive-manufactured poly(3 caprolactone)/ iron-doped hydroxyapatite (PCL/FeHA) nanocomposite scaffolds. The authors recommended that poly (E-caprolactone)/iron-doped hydroxyapatite scaffolds positively affect cell proliferation, adhesion, and viability. Magnetic nanoparticles (MNPs) are gaining great attention due to their paramagnetic properties [1, 3, 13]. Among MNPs, iron oxide nanoparticles, typically maghemite (Fe2O3) or magnetite (Fe3O4), have been the most commonly investigated, as they have enhanced osteoinductivity and the formation of new bone in vitro, even with a lack of external magnetic stimulation [3]. Despite its merits, the cytotoxic effects of iron oxides have been reported [34]. However, Scialla and colleagues [90] integrated dextran grafted maghemite nan-architectures into chitosan-based scaffolds. The results revealed enhanced proliferation of both human osteoblast-like cells and human mesenchymal stem cells. In 2020, Shuai and colleagues [91] reported the production of magnetic scaffolds by using SLS. The Fe3O4 MNPs were incorporated in PLLA/PGA composites. Results revealed that the combination of PLLA and PGA positively improved the degradation rate of scaffolds. Researchers developed new bone scaffold materials with magnetoelectric and piezoelectric properties due to both BTE continuous advancement [1, 3]. These scaffolds react to magnetic stimulation with mechanical deformation, resulting in electrical polarization. Consequently, the cellular environment is precisely regulated by bioelectrical signals. The outcomes up till now are promising for such scaffolds in bone regeneration applications. Table 2 summarizes the benefits and drawbacks of nanostructured materials that harness physical stimuli for bone regeneration.

3D Printed and Biomorphic Ceramics Simulating the construction of natural bone is a crucial element of scaffolds for bone tissue regeneration applications. 3D printing and biomorphic transformation are the known approaches for fabricating scaffolds that mimic bone topography.

Scaffolds Synthesized by 3D Printing Systems As mentioned earlier in this chapter, there are various 3D printing systems that enhance the manufacture of biodegradable nanocomposite bone scaffolds that resemble the micro to nano-natural bone environment. 3D printing systems have not only empowered the creation of scaffolds with greater three-dimensional

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Table 2 Advantages and disadvantages of nanostructured materials harnessing physical stimuli for bone regeneration. (Copied with permission from [3], Frontiers Media S.A, 2020) Advantages Nanostructured piezoelectric materials Piezoelectric Ceramics Positive piezoelectric features. (e.g., BT, BN, ZnO) Appropriate osteoinductive behavior. Piezoelectric Polymers Biocompatibility and (e.g., PVDF and its non-toxicity copolymers, PLLA, PHBV) Fabrication flexibility Improved mechanical properties Piezoelectric PolymerEnhanced mechanical Ceramic Composites properties performance and piezoelectric features. Desirable biodegradability and bioactivity. Nanostructured electrically conductive materials Conductive Nanomaterials Improved mechanical non-polymeric, performance. (e.g., graphene, gold Outstanding electrical nanoparticles) conductivity confirming consistent transfer of bio-electric signals Conductive Nanopolymers Better biocompatibility and (e.g., polyheterocycle family biodegradability. of conductive polymers) Fabrication flexibility. Nanostructured Magnetically Responsive Materials Magnetic Nanoparticles Improved paramagnetic (MNPs) and performance. Magnetoelectric Capability of delivering Composites signals when subjected to external stimulus.

Disadvantages Probable for cytotoxicity.

Reduced biodegradability.

Concerns regarding the piezoelectric performance of some composites.

Non-degradability. Lack of scientific evidence concerning both biocompatibility and longterm safety. Undesirable mechanical performance. Deficiency of animal studies assessing in vivo behavior. Unclear evidence about biocompatibility and longterm safety

resolution but have also enabled the introduction of particular pore gradients that more successfully simulate the physical cues for growth present in natural bone tissue [30, 31]. Creating micro and/or nanoscale grooves on the substrate material, which allows cells to grow and instinctively extend alongside the groove orientation, is one technique for achieving the normal cell and extracellular matrix alignment of native bone [3, 31, 32]. While 3D printing techniques for fabricating bone scaffolds are promising, more research is needed to determine their effectiveness, durability, biocompatibility, and reaction with surrounding tissues, among other properties, before they can be used in biomedical applications.

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Scaffolds Synthesized Through Biomorphic Transformation Biomorphic transformation is a synthetic procedure that comprises a sequence of various pyrolysis in addition to compound chemical reactions (predominantly liquid or gas infiltration procedures) that leads to the production of ceramic scaffolds through the chemical transformation of natural substrates while maintaining their original nano and microstructural design scale [3]. Researchers reported that wood had a unique bone-like hierarchical architecture on both cellular micro and nanostructure scale that may aid in long bone defect healing [92]. A prime example of biomorphic transformed scaffolds was proposed in 2009 by Tampieri and coworkers. They create a biomimetic HA bone scaffold from natural wood with a bone-like categorized configuration, demonstrating a mineral substitute for a bone graft that improves cellular adhesion, invasion, and vascularization [92]. In another study, using the process of wood bioceramization, the authors prepared a hollow cylinder-shaped bone scaffold similar to compact bone and occupied with a bio-hybrid, sponge-like hydroxyapatite-collagen scaffold to be similar to natural bone. Significant osteointegration at the bone/scaffold interface was reported in sheep with defects in load-bearing bone greater than or equal to 2 cm in metatarsus defect [1, 4]. Later in 2019, Filardo and colleagues [93] conducted a follow-up study using the sheep critical size load bearing model, increasing the lumen size of the bone-like scaffold. The findings revealed improved osteointegration in all the models, but the model with an 11-mm diameter (the greatest inner diameter) displayed significant encouraging results regarding both bone-to-implant interaction as well as new bone tissue formation. Biomorphic HA scaffolds exhibit outstanding mechanical properties owing to their unique hierarchically organized multiscale bone-like architecture, which is not yet present in other ceramic scaffolds [3, 4]. Unfortunately, the process of biomorphic transformation relies on critical complex gas-solid reactions that are remarkably affected by various phenomena relating to adsorption of the gaseous reactant by the solid. The strict control of reaction kinetics is required to limit the distortions and architectural flaws and preserve the variant multi-size scales of scaffold porosity [3, 4].

Composite Nanostructured Delivery Systems Biodegradable nanocomposites, drug nanocarriers, and bone scaffolds are extensively investigated to determine their role in treating bone cancer. This approach enhances the local administration of antitumor drugs. Moreover, nanocomposite delivery systems for dexamethasone, antibiotics, and other pharmacological agents were fabricated [1, 2, 76]. Ideal drug delivery systems should exhibit high biocompatibility and good biodegradability to ensure high biosafety for in vivo applications with more efficient and defined control over release [1, 34].

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Many inorganic nanostructure delivery systems were investigated in both ceramics and included CaPs, such as HA and TCP, and nanoscales of metallic or metalloid oxides, for example, silica (SiO2) and titanium oxide (TiO2) nanotubes as well. Unfortunately, there are concerns about inorganic nanocarriers regarding degradation and cytotoxicity that restrict their in vivo application [1]. In addition, both synthetic and natural polymers have been used to produce polymer-based drug delivery systems. Some synthetic polymers exhibit cytotoxic acidic degradation. Despite the improved biodegradability of natural polymers, they yield fast degradation rates that are challenging for controlling the release of molecules. Furthermore, natural polymers have other limitations, such as expensive manufacturing costs, batch variability, and harvesting [3]. Nanocomposite drug delivery systems were developed to overcome the limitations mentioned above. Delivery of osteogenic growth factors by nanocomposite delivery systems for bone regeneration applications has been investigated [3]. Bone morphogenetic proteins (BMP-2 and BMP-7) and the transforming growth factor-β (TGF- β) family are well known for their regulatory effects on cell proliferation, differentiation, and osteogenesis [1, 3, 14]. Some nanocomposite drug delivery systems include metal/metalloid/polymer nanocomposites. Poly (lactic-coglycolic acid) and mesoporous silicon composites have been studied as drug delivery systems, respectively. Both in vitro and in vivo, PLGAmesoporous silicon composites exhibit biocompatibility, sustained drug release, and positive osteogenic activity [3, 34, 94]. Singh et al. fabricated a PCL nanofiber composite with a mesoporous silica shell coating. The results demonstrated the sustained release of several bioactive molecules and osteogenic differentiation in vitro [94]. Several techniques have been accomplished to incorporate biomolecules and other drugs into bone scaffolds, such as directly incorporating nano-delivery systems into 3D constructs, surface alteration, and cross-linking of the nano-drug carriers into 3D constructs and multifunctional nanofiber scaffolds as drug delivery systems (Fig. 17).

Fig. 17 A diagram depicting the various drug delivery systems used in bone regeneration. (Copied with permission from [3], Frontiers Media S.A, 2020)

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Direct Incorporation of Nanodelivery Systems in 3D Constructs Bioactive molecules’ direct incorporation into nanodelivery systems has been proposed via variant approaches. The most popular approach is a hydrogel. Several hydrogels have been industrialized and investigated as drug delivery systems. They exhibit biocompatibility, enhance osteoconductivity, and yield the controllable release of bioactive molecules and several drugs. Despite their outstanding biocompatibility, hydrogels exhibit several shortcomings that limit the clinical application of these materials. They lack a solid framework and are concerned about handling and sterilization [15, 43].

Surface Modification and Cross-Linking of Nano-delivery Systems to 3D Constructs Surface alterations as well as cross-linking are further methods for nanodelivery system fabrication. Such methods improve the sustained control release of biomolecules. Sun et al. [95] used sintered nano-HA particles functionalized with BMP-2 or BMP-2-related peptides to create a porous scaffold. The results revealed enhanced osteogenic differentiation to treat cranial defects in rats. Later on, Kim et al. [96] used the ε-polycaprolactone polymer emulsion to coat the surface of a 3D-printed hydroxyapatite scaffold with BMP-2 nanoparticles. This coating technique was useful not only to incorporate BMP-2/NPs onto the surface of the scaffold but also to enhance the compressive strength, which improved bone regeneration. Angiogenic factor drug delivery systems were also developed. For example, VEGF and PDGF delivery carriers were proposed. Moreover, surface modification with immunomodulatory molecules, including IL-10, IFN-g, IL-4, and IL-33, has been described in the literature (Fig. 18) [33].

Multifunctional Nanofiber Scaffolds as Drug Delivery Systems Many nanostructured materials could be used to load and release bioactive molecules, besides binding endogenous growth factors. In 2019, Liang et al. [97] developed hydrogels containing BMP-2 mimicking peptides capable of inducing and accelerating the maturation of osteoblast precursors and thus enhancing the osteogenic differentiation of rat MSCs in vitro. A rat cranial defect model was used to confirm this osteogenic capacity in vivo. Based on the previous research findings, using PA-based scaffolds in (BTE) is encouraging yet still needs further development for clinical application. The main varieties of nanostructured drug carriers used in (BTE), with their particular advantages and disadvantages, are summarized in Table 3.

Intelligent Materials and Modular Fabrication The bone tissue microenvironment is regulated by a series of complex environmental stimuli, including chemical stimuli (such as pH, ionic strength, and oxidation),

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Fig. 18 Surface functionalization of 3D printing scaffolds with active biological molecules to enhance scaffold bioactive behavior: BCP conjugated with protein immobilized on a PCL 3D printed scaffold. (Copied with permission from [33], Dove Medical Press Limited, 2019)

physical stimuli (such as mechanical signals (stress/strain), electrical stimulation, temperature, and magnetic fields), and biological signals (such as enzymatic reactions and receptor-ligand recognition) [34]. Intelligent materials, commonly polymers, biohybrid materials, or cells, are unique materials that can respond and communicate with the surrounding via dynamic environmental stimulation. Innovative bioactive scaffolds should have an appropriate porous configuration, transfer growth factors, enhance cell migration and proliferation, and possess acceptable mechanical behavior to deal with the previously mentioned complex environmental stimuli [34] . Various biodegradable materials (collagen, elastin, fibrin, chitosan, and hyaluronic acid) can regulate cell adhesion through their natural adhesion ligands. Additionally, it has been reported that many natural polymers, such as cellulose, chitosan, and gelatin, can respond to the change in temperature after implantation in the human body [3, 5, 34]. Similarly, iron oxide nanomaterials can sense and respond to the magnetic stimuli in the microenvironment of tissue regeneration. The results of evaluating the dip-coating of HA/collagen scaffolds with iron oxide nanoparticles revealed that human bone marrow stem cells exhibited appropriate adhesion and proliferation [3]. Similarly, self-assembling structures based on peptides can also respond to microenvironmental stimuli. p11-4 is a self-assembling peptide that can form a self-supporting hydrogel by responding to the physiological pH of the body.

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Table 3 Main types of nanostructured delivery systems used in bone regeneration, with their respective advantages and disadvantages. Copied with permission from [3], Frontiers Media S.A, 2020 Inorganic nanostructured delivery systems

Ceramics (e.g., HA, TCP)

Metallic or metalloid oxides (e.g., silica)

Organic nanostructured delivery systems

Composite nanostructured delivery systems

Synthetic polymers (e.g., PLA, PLGA)

Advantages Inherent osteoconductive behavior. Availability.

Mesoporous architecture and topography can be modified and customized. Proper surface functionalization features. Improved cell adhesion together with proliferation. Availability. Promising biocompatible performance. Numerous adjustable properties such as L/G ratio and molecular weight.

Natural polymers (e.g., gelatin, chitosan)

Availability. Appropriate biocompatibility and Biodegradability. Biomimetic features. Adjustable with cross-linkers or surface functionalization.

Composites

Great loading efficiency. Significant release kinetics. Sustained release Inclusion of exceptional properties of several materials.

Disadvantages Undesirable biodegradability performance. Reduced yield of payload loading. Cytotoxic behavior.

Reduced yield of payload loading. Burst release. Concerns regarding achieving sustained release. Cytotoxic biodegradation byproducts may be performed. Reduced yield of payload loading. Fast biodegradation in vivo. Burst release. Concerns regarding achieving sustained release. Complex production.

In 2018, Sun et al. [98] developed a new scaffold by conjoining self-assembled peptide nanofibres to decellularized cartilage matrix (DCM), which positively encouraged cartilage and subchondral bone reconstruction in rabbit cartilage defects (Figs. 19 and 20). In 2019, Saha and colleagues [99] prepared pH-sensitive, selfassembling β peptides (SAP P11–4) that can be transformed from liquid to gel and gel to liquid phases reversibly as a reaction pH changes in the microenvironment. The in vivo findings in rat skull defects revealed bone new regeneration.

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Fig. 19 An illustration of a research strategy in the form of a diagram. (Copied with permission from [98], Elsevier, 2018)

Furthermore, due to new BTE approaches, cell-based products can also be modularly fabricated [1, 3]. Cells with the proper size scale can perform temporary tissues by self-assembly, forming self-regulating progressive procedures, enhancing organ formation [1, 3]. In 2020 (Fig. 21), as a result of using the self-assembly strategy of human periosteumderived cells (hPDCs), researchers developed a callus organism that can be used to repair critical-size bone defects in mice [100] (Fig. 22).

Barriers to Clinical Translation Despite extensive research and the emergence of new promising technologies in bone tissue engineering, many issues and barriers may lead to undesirable effects on

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Fig. 20 Magnetic resonance imaging evaluation of repaired knees. (a) Representative T2 sequence images at different time points. (b) Examples of T2 mapping images. (Copied with permission from [98], Elsevier, 2018)

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Fig. 21 A diagram summary of the bioengineering process begins with cellular aggregation, condensation, and differentiation, followed by callus organoid assembly and implantation in ectopic and orthotopic environments. (Copied with permission from [100], WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020)

Fig. 22 (a) X-ray images of a tibia defect with a day 21 construct; (b) Nano-CT 3D rendering images with a day 21 construct over time. (Copied with permission from [100], WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020)

in vitro studies that consequently hinder the clinical application. These issues include technical, collaborative, and regulatory concerns [14].

Scientific and Technological Challenges Several challenges confront the scientific and technological communities, including the selection of scaffold cell content, the introduction of sufficient scaffold vascularization, the achievement of precise control over scaffold degradation, the

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improvement of structural biomimetic properties [14], and the scaling up of biofabrication and scaffold production to treat critical-size bone defects [14]. The bioprinting technology for bone scaffold fabrication is promising and offers a largerscale method for manufacturing structures suitable for (BTE) applications. However, the choice of the ideal scaffold manufacturing technique is still challenging, owing to the presence of extensive categories of fabrication approaches, biomaterials, cell types, and growth factors. These have been thoroughly investigated, with inconsistent data available in some cases. Moreover, the fabrication of a scaffold that can match all the living bone’s technical, mechanical, and biological performance is correspondingly tremendously challenging [1, 3, 14].

Translational Challenges Besides technical and scientific obstacles, scientists still face research and improvement impediments to translation. Among these, gaining external funding and grants for product development represents a significant barrier. Despite the time, cost, and effort, there are no guarantees of product approval and clinical applications. Moreover, there is a probable lack of a “first-mover advantage.” Research leaders and innovators can be challenged by regulatory and funding constraints, together with their effort to resolve scientific and technical problems that emerge.

Ethical Issues Although biofabrication technologies, including 3D bioprinting, are promising, these approaches are expensive, and ethical issues may be associated with their upcoming clinical application. Implantation of bioprinted cells into the human body may be associated with hazards due to a deficiency in long-term in vivo studies concerning biodegradability and biocompatibility. Firm regulation and government organization are mandatory to guarantee responsible, safe, and ethical application [1, 14].

Conclusions The development of biodegradable nanocomposites scaffolds that mimic bone architecture, bone composition, and/or the electrical environment of bone (piezoelectricity, conductivity, and magnetism) yielded promising results but requires further investigation and long-term in vivo studies. Scientists will never lose faith and continue developing their research despite all the challenges and barriers.

Future Prospective The fabrication of biodegradable nanocomposite scaffolds promotes bone tissue generation and matches all biological and technical prerequisites. Continuous improvement and innovations are advantageous to overcome these challenges.

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A scalable method of directing osteogenic differentiation would be advantageous. Dalby’s team in Glasgow used nanoscale vibrations delivered by a nanovibrational bioreactor to differentiate MSCs seeded in collagen gels into mineralized tissue in 3D. This technology seems to be readily scalable, compatible with 3D scaffolds, easy to maintain compared to rotary/perfusion chamber bioreactors, and could be used “off the shelf.” Moreover, the inclusion of organoids and spheroids into 3D bioprinted scaffolds could accelerate the growth of printed constructs towards clinically relevant, functional tissue. Investigations confirmed that MSC spheroids promote in vitro and in vivo osteoregenerative activity. Similarly, incorporating novel biological cues into bioprinted scaffolds enhances their maturation towards complex, biologically functional tissue. As well as growth factors, microRNAs could have a significant role in the bone tissue engineering field. The role of microRNAs in regulating the osteogenic differentiation of mesenchymal stem cells has been demonstrated in the literature. Furthermore, the Development of smart hydrogels with customized and adjustable properties such as biocompatibility, biodegradation rates, and mechanical properties may aid in the innovation of 3D scaffolds [1, 14]. Various methods have been investigated to enhance cell homing to defect sites by the controlled release of chemokines [3, 4, 76]. Finally, Turnbull et al. mentioned that to improve 3D bioprinting technologies, long-term, cross-discipline collaboration is necessary to enable sharing of expertise and increase research efficiency [14].

Cross-References ▶ Biocompatibility of Nanomaterials Reinforced Polymer-Based Nanocomposites ▶ Biodegradable Inorganic Nanocomposites ▶ Biodegradation of Carbon Nanotubes ▶ Biodegradable Polymers ▶ Biodegradable Polymer Challenges ▶ Electrically Conducting Smart Biodegradable Polymers and Their Applications

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Aymieza Yaacob and Nazzatush Shimar Jamaludin

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Biodegradable Polymers in Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silk Fibroin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Collagen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fibrin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Matrigel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyaluronic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Scaffolds in Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biocompatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fabrication Methods of Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Melting-Based Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solvent-Based Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas Foaming Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rapid Prototyping Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A. Yaacob · N. S. Jamaludin (*) Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_44

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Abstract

Biodegradation is a process of degradation and deterioration of material due to exposure to microorganisms aerobic and anaerobic processes. Biodegradable polymers have been used widely in tissue engineering, and their application varies from bone tissue, epithelium, vascular, and cardiac tissue. This is important to treat patients by mimicking cardiac tissue’s extracellular matrix structure, which helps it regenerate the new cell and degrade after some time. Some polymers are biocompatible, are nontoxic, and have good mechanical integrity and thermal plasticity but longer degradable periods. This chapter will highlight the various types of polymers used in the cardiac tissue scaffolds, biodegradable properties of the polymers, and cost-effective synthesis methods. Keywords

Biodegradable polymers · Tissue engineering · Biodegradation · Biocompatibility cardiac cell · Electrospinning Abbreviations

AMSF BDP BMSC CVD ECM F6P FDM HA hiPSC-CM hPSC LV MI MTE SF TE Tg Tm

Acid-modified silk fibroin Biodegradable polymer Bone marrow mesenchymal stem cell Cardiovascular disease Extracellular matrix Fructose-6-phosphate Fused deposition modeling Hyaluronic acid Human-induced pluripotent stem cell-derived cardiomyocyte Human pluripotent stem cell Left ventricular Myocardial infarction Myocardial tissue engineering Silk fibroin Tissue engineering Glass transition temperature Melting temperature

Introduction In 1987, biomaterial was a nonviable substance used in a medical device, intended to interact with the biological systems. Then, the term biomaterial has evolved to materials designed for evaluation, treatment, augmentation, or replacement of any tissue, organ, or other body function [1]. However, unlimited usage of nonbiodegradable materials created lots of problems, especially when the amount of

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these materials continuingly added up and caused too much disruption to the environment [2]. Nonbiodegradable materials do not decompose but remain unchanged in the soil and ocean. Their presence negatively impacts the life of terrestrial and marine organisms, resulting in an imbalance ecosystem. Due to the increasing awareness in assuring the sustainability of the environment, there have been tremendous demands in switching from nonbiodegradable material to biodegradable type without losing all existing properties [3]. Biodegradable polymers (BDPs) are natural or synthetic macromolecules capable of undergoing physical or chemical changes [4]. Under suitable conditions, the breaking down of the polymeric chain to carbon dioxide, methane, water, inorganic compounds, or biomass is known as biodegradation. These organic residues will be mineralized and redistributed through the carbon, nitrogen, and sulfur cycles [5]. Biodegradable polymers are designed to deteriorate under environmental conditions or in municipal and industrial biological waste treatment facilities. The biodegradation process is divided into aerobic (in nature), anaerobic (in sediments and landfills), and partly aerobic (in compost and soil). Carbon dioxide and water are released during aerobic biodegradation, while anaerobic biodegradation yields carbon dioxide, water, and methane. In the case of plastics, the biodegradation process undergoes mechanistic environmental pathways such as light, heat, moisture, chemical changes, or biological activity. These are examples of biochemical processes that refer to the degradation and assimilation of polymers by biological activity in the environment [6]. For example, a compostable bioplastic degrades at a similar rate to other compostable material without visible toxic remaining. BDPs can be classified as either bio-based or petrochemical-based polymers, according to their origin of polymer. Some examples of polymers from plants, animals, or microorganisms origins are starch, cellulose, gelatine, casein, wheat gluten, silk, plant oils, and animal fats. “Plastics” derived from renewable agricultural and forestry resources such as corn starch, soybean protein, and cellulose are known as bio-based bioplastics [7]. Biomaterials can be divided into ceramic, polymers, and composites. A study on a mixture of ceramic and polymer as the potential biomaterial in tissue engineering has been conducted [8]. The thermal properties of a polymer determine the strength of a material [7]. Some polymers are thermally stable, and some are thermally unstable. The photodegradable bioplastic has light-sensitive groups connected directly to the polymer’s backbone. Prolonged exposure to ultraviolet radiation will cause the polymer to disintegrate, thus rendering them open for biological activity. However, the limited amount of sunlight exposure in several areas, such as landfills, will cause the polymer to be nondegradable. Thermoplastic polymers [2] such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly(vinylchloride) (PVC) are petrochemical-based polymers produced through multiple molding processes as heat does not affect their chemical composition. However, a thermoset polymer forms a highly cross-linked structure upon heating and is temperature resistant. The irreversible reaction makes material from thermoset polymer nonrecyclable. Thus, modification is forbidden [6].

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Cardiac Tissue Engineering Cardiovascular diseases (CVD) such as myocardial infarction (MI) and heart attack are conditions reflecting impairment of the pumping efficiency of the heart, which could lead to death [9–12]. Obstruction of one or more branches of the coronary arteries, underlying diseases, including ischemic heart disease with or without an episode of acute myocardial infarction, hypertensive heart disease, valvular heart disease, and primary myocardial disease are the various factors known in causing cardiovascular-related illness [13]. Cardiomyocytes are the mature contracting cardiac cells in an adult’s heart structure and have a limited capacity to proliferate. The cell cannot be repaired immediately if damage occurs, forming scars [14, 15]. Other inabilities include progressive heart failure in which no available therapies could prevent or reverse cardiac damage [8]. Other scarring examples of diseases that could be accounted for would be arrhythmias, ventricular dilation, and heart failure. There were cases in pediatric patients where the treatment options involved surgical heart transplant to avoid any high risk of diminished cardiac function [16]. To improve the quality of cardiovascular function among patients with heart disease, biomaterial tissue engineering and regenerative medicine are introduced as cheaper and less inventive treatment options [17, 18]. The damaged or infected heart will have limited cell or tissue regeneration [19]. However, with the aid of a novel biodegradable scaffold’s fabrication, the regeneration of the damaged tissue could be accomplished. This could provide an alternative to patients besides tissue transplant or permanent implant insertion [8]. A biodegradable scaffold is a temporary structure that promotes cell growth and diversification. It will simultaneously degrade and replace new tissues [16, 20, 21].Thus, the idea of scaffold is to allow sufficient time for the native tissues to grow as similar as the cell matrix [22]. To ensure the efficiency of the cardiac tissue regeneration process, there are a few important parameters to be contemplated. This includes selecting cells, establishing a cardiac tissue matrix, similar cell structure, architecture, electromechanical, robust and stable contractile tissue function, and functional vascularization. By adhering to the parameters above, the natural extracellular matrix (ECM) of the heart could be mimicked [21, 23, 24] as ECM plays a critical role in the regulation, proliferation, and differentiation of body cells and tissues [11, 25]. Additionally, the engineered cardiac tissue must comply with the cardiac structure’s chemical, biological, and conductive properties [26]. The replacing scar tissue cannot conduct electricity for mechanical stimulation of the pumping heart [8]. This results in a limited volume of blood flowing to the atrium and ventricle, thus lowering the efficiency of blood transportation throughout the body [27]. Furthermore, due to insufficient organ donors, the biodegradable scaffold could provide a better alternative to organ transplants in treating cardiovascular disease [19, 20, 22, 28–30]. The scaffold is cultured and synthesized for application on the damaged areas (Fig. 1). The yellow patches on the heart resemble a temporary location of the

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Fig. 1 Biopolymer scaffold tissue replacing damaged jaw bone, nerve, heart, cartilage, and skin tissues

biodegradable scaffold on the affected area until new cells are regenerated [25]. The scaffold can also be applied for regenerating cartilage around the knee bones, jaw bone tissue, skin tissue, and nerve tissue. Many irreversible damages to the heart are caused by myocardial infarction (MI), which eventually leads to heart failure. Cardiac transplantation was the only option to treat the MI-damaged heart tissue [1]. However, due to the inadequacy of heart donation, researchers have thought of ways to regenerate heart tissues by exploring the tissue engineering field [31]. Tissue engineering (TE) is a modern technique that can treat damaged tissues [5]. The bio-resorbable stents and vascular grafts were tissue engineering products and served many patients with cardiovascular illnesses [32]. There are three advantages of which tissue engineering could be an alternative for a better future in the cardiovascular field (Fig. 2). Firstly, the engineered tissue provides a 3D environment to the cell that confers resemblance to endogenous cardiac tissue. This environment helps the cell to survive and avoid any proteolytic dissociation that requires prior injection. Secondly, engineered tissue is also designed to carry out the role of the stem cells that support the transportation of multiple cell populations. As the use of stem cells is currently under consideration, the role of the “support” cell will be aimed at providing their survival, differentiation, and migration; hence, tissue engineering came to the rescue. Thirdly, tissue engineering renders a medium for growth factors. The impact of the growth factor delivery on the grafted cell will be ensured to react similarly to the targeted myocardial environment [14].

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Fig. 2 Advantages of tissue engineering

Types of Biodegradable Polymers in Cardiac Tissue Engineering Biodegradable polymers are natural and synthetic materials that decompose in the presence of the surrounding microorganisms under certain conditions. Natural polymers are obtained from parts of plants and animals. Starch [24], fibrin [32], silk fibroin [22, 33], alginate [34, 35], chitosan [11, 32], collagen [22, 32], gelatin [22, 23, 28], and hyaluronic acid [21, 36–39] are isolated from their corresponding natural resources. Polysaccharides, proteins, polynucleotides, polyisoprenes, and polyesters are other examples of natural polymers [40]. In general, natural polymers are used in bones, cartilages, tendons, nerve, and skin tissues because they possess good biocompatibility, low toxicity, and high degradable rate and are abundantly available for diverse applications in tissue engineering [41–44]. Each polymer is authentic. Fibrin is highly elastic; silk is rich in polymeric protein, and matrigel is cytocompatible with the native tissues. They are also widely used in adhesive bandages, absorbents, prepared cosmetics, drug delivery, and medical scaffolds [43–45]. The characteristics of a polymer decide the usage of the tissue scaffold for treatment in a particular area. Natural polymer is temperature-sensitive [46], so its chemical structure will easily be disrupted before its melting point is reached [31]. During the extraction of a natural polymer from plants or animals, the pathogens carried along with the polymer could be transmitted to humans. Despite the advantages, their low mechanical properties [46] and heat resistance open up various opportunities for synthetic polymers to be part of tissue engineering.

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Silk Fibroin Bombyx mori (B. mori) [52, 53] or silkworm produces silk fibers in the form of silk cocoons (Fig. 3). The food consumed by these worms affects the quality and reproducibility of the biomaterials. Silk fibers or silk thread is made up of two protein components that are the structural component named fibroin and the adhesive protein that helps to form a cocoon called sericin. Fibroin and sericin must first be separated to obtain a pure silk fibroin solution through a process known as degumming. Sericin is a protein that comprises a series of amino acids such as serine, glycine, glutamic acid, aspartic acid, threonine, and tyrosine. Sericin accounts for approximately 30% of the total mass [52]. Silk cocoons are boiled in 0.02 M sodium carbonate solution for approximately 30–60 minutes to remove sericin. A high concentration of sodium carbonate and a prolonged boiling time will cause fragmentation to the silk fibroins’ molecular structure. This will eventually lead to a change in molecular weight and a degrading in the physical properties of the degummed silk fibers [52]. Secondly, the degummed silk fibers are dissolved to produce a pure silk fibroin solution. The silk fibroin is an amphiphilic compound. It consists of repetitive hydrophobic β-sheet crystallites and disperses the non-repetitive hydrophilic amorphous region [21]. The strength of hydrogen bonds and the hydrophobic nature of the fiber inhibit the dissolution step. Therefore, the degummed silk fiber requires a high ionic strength aqueous or organic salt system to ensure complete dissolution. Calcium chloride/formic acid mixtures, calcium nitrate/methanol mixtures, calcium chloride/water/ethanol mixtures (Ajisawa’s reagent), N-methylmorpholine-Noxide, ionic liquids, and lithium thiocyanate solution are a few choices of aqueous and organic salts that can be used during the degumming process [54]. Among the solvents above, lithium bromide solution (9.3 M) and Ajisawa’s reagent are the most frequently used solvent systems for degumming silk fibroin fibers. Dissolution of silk fibers in lithium bromide occurs at 60  C for 4 h, while Ajisawa’s reagent takes 3 h to dissolve the fibers at 65  C. This shows that the dissolution time can be cut off by 1 h using salts and organic solvent mixtures. Subsequently, the highly viscous silk

Fig. 3 Degumming process of silk cocoons

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fibroin solution is desalted. The removal of salt ions is carried out by dialyzing the solution with ultrapure water for 48 h [21]. Silk fibroin protein that remained after the degumming process is used for tissue engineering applications. It has a tunable degradation rate in which various biomaterials such as fibers, foams, hydrogels, films, and 3D printed structures are produced [55]. Silk is biocompatible and thermally stable up to 250  C and is convenient for processing in a wide range of temperatures [21]. The usage of silk fibroin in myocardial infarction (MI) was reported as early as 2012. A combination of silk fibroin and hyaluronic acid (SF/HA) patch containing bone marrow mesenchymal stem cells (BMSC) was created and investigated in an MI rat model for 8 weeks. The patches were found to be well-adhered, intact, and show little to no immunological responses. Besides, the BMSC’s survival was significantly enhanced. The apoptosis of cardiomyocytes was prevented, and the secretion of important growth factors was well stimulated for cardiac repair. From the observations, a tunable scaffold for cardiac repair can be made from silk-based composite materials as the composite is mechanically strong, shows poor immunological responses, and exhibits good cell adhesion and proliferation [35]. There has also been an increasing interest in developing silk-based scaffold materials. The structure and functions of the cultured human pluripotent stem cell (hPSC)-derived cardiomyocytes were intensified with an electroconductive acid-modified silk fibroin-poly(pyrrole) (AMSF/PPy) scaffolds patterned with nanoscale ridges. Silk with greater mechanical properties has been found produced by spiders. In comparison to silk fibroins secreted by B. mori, the non-mulberry silk fibroins extracted from Antheraea assama (A. assama) possessed a better mechanical strength, elasticity, lower immunogenicity, and better cytocompatibility with cardiomyocytes. Intriguingly, a recombinant spider silk protein from Araneus diadematus known as eADF4(κ16) is studied to overcome the reproducibility issues. The eADF4(κ16) silk solution can be printed without a crosslinking agent [54] and may serve as an alternative for future cardiac applications. Its functionality and processability could be optimized through some genetic modification [46].

Collagen Collagen is the most abundant protein found in animals, and it is the principal component of the extracellular matrix [56]. Collagen helps bind the cell and assists in proliferation, differentiation, and secretion of the extracellular matrix [54]. It is isolated from a wide range of natural resources such as the skin, blood vessels, tendons, and cartilage of human tissue. The protein is extracted in large quantities with relatively high purity at a lower cost using acid neutralization. In the biomedical field, collagen has been utilized for various treatments as it displays low antigenicity, low level of inflammation, and low cytotoxic responses regardless of its low elasticity and poor mechanical strength [21, 54]. Until today, there are over 20 different members in the collagen family. The right triple helix constructed by three α-chains is characteristic of structural features. They

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consist of homotrimers as in type II, III, VII, VIII, and X or two or more heterotrimers as in type I, IV, V, VI, IX, and XI. Collagen of type I is proven clinical of its effectiveness for short or medium term. Nevertheless, this type of collagen has poor mechanical strength, limited chondrogen output, and shrinks considerably, reducing the clinical effectiveness of the scaffold in the long run. Regarding chondrogenic capacity, type II collagen is a good substitute for typing I collagen-based scaffold because of its inherent flexibility. Enzymes such as collagenases and metalloproteins break collagen down to subsequent amino acids. To regenerate cartilage, collagen can be combined with hyaluronic acid and chitosan to produce a final scaffold with improved biological activity and enhanced mechanical properties [34]. Owing to its inherent biocompatibility and bioactivity, collagen has been viewed as a potential resource in many tissue engineering applications. Its ability to provide appropriate binding ligands with cardiac cells, low antigenicity, and modifiable biodegradability have become the key factors for developing myocardial 3D biomimetic substrates. The tailored versatility of collagen also confers to specific functions of a scaffold. By controlling the fabrication conditions, porosity, alignment, cell infiltration, and nutrient diffusion can be facilitated. On the other hand, the composition of the added proteins and crosslinking will turn a scaffold into mimicking the properties of the native tissue [54]. Collagen is undeniably an important biomaterial for the reconstruction of many tissues and organs. Although several collagen-based materials are used for clinical treatments, some remain preliminary. The challenging part about the collagen-based scaffold is how its biodegradation rate could be controlled, and the robustness can be strengthened so that its limitation in application to load-bearing tissues could be resolved [15].

Chitosan Chitosan is derived by deacetylation of chitin [54, 57]. This natural cationic polysaccharide is glycosaminoglycan-like known to contribute to many applications in the medical sciences due to its high blood compatibility, unique interaction with extracellular matrix components, and less active macrophages [58]. Thus, chitosan is widely used in implantable and injectable orthopedic and periodontal devices, lung surfactant additives, drug delivery systems, and wound healing agents. Chitosan and its derivatives deteriorate into nontoxic [5] substances via enzymatic hydrolysis. Their cytocompatibility, nontoxicity, biodegradability, biocompatibility, and excellent cell recognition make them a suitable choice of biomaterial for coating or grafting onto scaffold surfaces in tissue engineering applications [43]. The high charge density, broad hydrogen bond capacities, and reactive hydroxyl and amino groups promote a wide range of properties. The biodegradability of chitosan-based hydrogels is controllable, and their degradation product is biocompatible. Thus, this material had undergone screening for cardiac tissue engineering applications. In a study conducted in vivo, an enhancement in the graft size was observed when the mesenchymal stem cell was delivered in the form of an injectable scaffold using a temperature-responsive chitosan hydrogel. Moreover, the cell retention in an

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ischemic heart was improved, and the mesenchymal stem cell differentiation into cardiomyocytes was stimulated [46]. In another study, an augmentation in the cardiac function and hemodynamics in the infarcted area in rats was shown 5 weeks after receiving cell transplantation via temperature-responsive chitosan hydrogels. Several in vitro studies on a smalldiameter vascular graft constructed using chitosan-based hydrogels exhibited the encouraging biocompatibility of chitosan hydrogels. A satisfactory level of hemocompatibility of the hydrogel was also seen in rats and sheep in vivo studies. Chitin and chitosan undergo slow depolymerization by lysozymes where the rate of biodegradation depends on acetyl. Nevertheless, the electrical properties of the implanted grafts for the regeneration of infarcted heart tissue are challenging. Chitin fibers have produced artificial skin and absorbable sutures [21, 58]. The properties of chitosan in the bone tissue scaffold can be upgraded by combining chitosan with other polymer and inorganic materials. For example, the synthesis of chitosan with chicken feather keratin nanoparticles in osteoblastic cells showed improvement in protein adsorption and promoted degradation within 8 weeks [21]. The hydrophilicity of other biomaterials and their biocompatibility can be improved by chitosan coating and encouraging cell proliferation and adherence.

Alginate Alginate is a linear and homogeneous polysaccharide obtained from bacterial biosynthesis [33] and isolated from the cell walls of brown algae [21]. Since alginate is an exopolysaccharide, it is produced by several bacteria, including Azotobacter and Pseudomonas aeruginosa. During the Krebs cycle, bacteria oxidize a carbon source to acetyl coenzyme A (acetyl-CoA), which is converted to fructose-6-phosphate (F6P) via gluconeogenesis. Commercially, alginate is produced by extracting it from various marine macroalgae, brown algae (seaweeds), Macrocystis pyrifera, Laminaria hyperborean, and Ascophyllum nodosum. Kelps, a type of seaweed from the Laminariales order, are the common raw materials in the worldwide production of alginate [21, 44]. Due to its extremely biocompatible, biodegradable, non-thrombogenic, and economic factors, alginate becomes one of the preferred candidates for scaffold development in cardiac tissue engineering applications. A study conducted in rats showed that rats receiving a direct injection of alginate hydrogel into the infarcted myocardium exhibited persevere improvement in fractional shortening of the left ventricular (LV), resulting in prevention in enlargement of the LV volume. Alginate is well known for possessing many carboxylate and carboxyl groups [44] responsible for its high level of hydrophilicity. So, alginate is easily converted to hydrogels by performing crosslinking with Ca2+ ions [58]. The cross-linking with Ca2+ ions generates a stable alginate hydrogel and lowers the rate of alginate degradation in physiological conditions [21, 58]. However, the highly hydrophilic properties of alginate lead to poor cell regeneration because the cell adhesiveness and proliferation are weakened [51].

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Consequently, alginate’s gelation and mechanical properties were refined through conjugating alginate with other materials, immobilizing specific ligands, and formation of cross-linking [43]. A controlled degree of cross-linking is carried out by introducing a proper cross-linking agent that will covalently cross-linked alginate. The covalent bond offers permanent gelation with better degradation rates and controllable mechanical stiffness [59]. For example, a cell-matrix interaction can be strengthened through alginate modification by instigating arginine-glycine-asparagine (Arg-Gly-Asn). As alginate is a nondegradable polymer in mammals, using a partially oxidized alginate chain could foster alginate degradation under physiological conditions. Bioabsorbable alginate-based hydrogel with additional sequential growth factor is designed for mechanical and physical support to the damaged cardiac tissue [30]. Besides tissue engineering, the natural structure of alginate confers to other various medical applications. Alginate helps in blood detoxification through toxic metal absorption by forming chelation with the metals. Alginate also assists in drug processing, helps in molding materials for dental prosthetics cell stabilization, promotes healing on the burnt area, and acts as a pain reducer [54].

Fibrin Fibrin gel (Fig. 4) is a biodegradable polymer derived from fibrinogen in which the fibrinopeptides are cleaved down by thrombin. Thrombin is a naturally occurring enzyme that triggers coagulation through fibrin clots besides regulating the activation of protein C and controls fibrinolysis. Fibrinogen attractively supports 3D fibrous structure and provides cellular attachment and nano-textured surfaces with a fibrous cell signaling network. Fibrin gel is exceptionally biocompatible, produces nontoxic degradation products, and matches tissue regeneration. It forms a clot by mimicking the final stage of blood coagulation. Its monomers assembled into insoluble fibrin gel that eventually decomposes in plasmin-mediated fibrinolysis [54]. The concentration and ionic strength of the precursor can be controlled to improve the stability of fibrin hydrogel and strengthen its mechanical properties and morphology. Fibrin gel can cure in situ, making it suitable for developing injectable and compatible material with a minimally invasive delivery approach. Apart from

Fig. 4 Fibrinogen is cleaved by thrombin to give fibrin

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that, fibrin gel is also used as a sealant in cardiovascular, neuro-, and thoracic surgeries and is absorbable during the normal wound healing process [30]. Like collagen-based scaffolds, fibrinogen- and fibrin-based scaffolds promote cell proliferation, cell migration, and cell differentiation into specific tissues or organs via secretion of the extracellular matrix [13]. In tissue engineering, the usage of fibrinogen- and fibrin-based scaffolds instigate supports for connective tissues such as nerves, blood vessels, cartilages, tendons, and ligaments, speeding up wound healing and reducing the formation of scars [54]. Fibrin gel increases the survival rate of transplanted cells, reduces the infarct size, and intensifies blood supply to the damaged tissue. A study in myocardial infarcted (MI) model involving female Sprague-Dawley rats demonstrated that, based on histology and echocardiography results, the groups injected with cell-fibrin gel mixture demonstrated impairment in thickness of infarct wall and preserved cardiac functions after 5 weeks of implantations. Furthermore, by treating the infarction area with cell-fibrin gel mixture, a larger amount of viable cells and a small amount of fibrous tissue were found on the area compared to the infarction site treated with no fibrin gel [21]. Despite these wonderful benefits and advantages, fibrin exhibits some drawbacks for using fibrin in tissue engineering, such as shrinkage of the gel, weaker mechanical properties for the regeneration of skeletal tissues, fibrin deformation, and potential disease transmission [23, 58].

Matrigel Matrigel is a wonderful and cytocompatible material for cell culture matrix development and beneficial as a substrate in engineered cardiac tissue. It is a gelatinous extracellular matrix protein secreted by the Engelbreth-Holm-Swarm mouse’s sarcoma cells (Fig. 5). Usually, a matrigel-based hydrogel is used as a coating to improve cell adhesion. However, the critical growth factors and cytokines for cell growth content promoted the use of matrigel-based hydrogel in cardiac tissue repair. A human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) is an example of a cell generated on a matrigel mattress. A mattress of 0.4–0.8 mm thick was used for culturing rod-shaped with aligned myofilaments hiPSC-CM [36]. These cultured hiPSC-CM cells were compared with the control hiPSC-CM group. The timeefficient method produced increased sarcomere length, enhanced a more rod-shaped morphology, and improved the performance of robust contractile responses towards positive inotropic agents at the single-cell level. Despite all the propitious qualities of matrigel, its animal origin and lacking vascularity make matrigel unfit for clinical use, thus giving rise to impediments in cardiac tissue engineering [37].

Hyaluronic Acid Hyaluronic acid (HA) is a naturally present and ubiquitous linear polysaccharide found in many body areas in the extracellular tissue [23, 37, 54, 60]. It is a

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Fig. 5 Matrigel-based hydrogel derived from secretion by EngelbrethHolm-Swarm mouse’s sarcoma cells

glycosaminoglycan built with a repeated disaccharide unit of N-glucuronic acid and N-acetyglucosamine. HA is an essential part of connective tissue where it mediates various cellular functions such as the development of the brain helps in cell growth, cell differentiation, and lubrication [36, 37, 44, 54, 60]. The important usage of HA stretches from tissue scaffolds to cosmetic ingredients. Due to its nonallergic and non-inflammatory characteristics, HA is the most adaptable biomaterial involved in tissue engineering and medicine. HA is applied in visco-supplementation, drug delivery, eye surgery, tissue regeneration, and embryo protection [38, 51]. HA can form a new bioactive and biodegradable scaffold for hard tissue, soft tissue, and scar curing treatment because it contains carboxylic acids and alcohols that can be used to develop a hydrogel. Research conducted on hyaluronic acid showed that biomaterial helps to repair tissue damage caused by an acute ischemic stroke. HA shows low nonspecific protein adsorption; thus, the polysaccharide can be customized to accelerate tissue growth and repair via cell receptors [61]. A synthetic polymer is a structure synthesized via polymerization of its respective monomer in a controlled condition [43]. As displayed in Table 2, in tissue engineering, synthetic polymers are used in restorable sutures, drug delivery systems, artificial skin, wound healing, and orthopedic implants [49, 56], thus the porosity of a scaffold plays an important role in synthesizing biocompatible and biodegradable polymer [26]. The polymer properties are important in communicating with the biological system by mimicking the extracellular matrix of the host tissue. Due to exposure to different environment, its chemical and physical properties need to be focused during designing the polymers for applicable treatment. Synthetic polymers include poly(4-hydroxybutyrate) (P4HB) [22, 42], poly

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Table 1 Types of natural polymers, their uses in tissue scaffold, advantages, and disadvantages Types of polymer Silk fibroin

Chitosan

Hyaluronic acid

Fibrin

Alginate

Matrigel

Gelatin

Uses in tissue scaffold In the form of gel, sponges, and film TE for cartilage, bone, tendon, nerve, and ligament regeneration [22]

Advantages Polymeric protein, biocompatible, cellular adhesion, high tensile strength [46] Elasticity, impressive mechanical strength, morphologically flexible, biodegradable [47] In the form of gel, sponge, Similar structural or fiber [46] characteristics with Nerve regeneration glycosaminoglycan, Skin, bone, corneal, blood possess antimicrobial vessel [43], connective activity [12] tissue, liver, neural and Cellular adhesive, stent regeneration [49] nontoxic, nonimmunogenicity, biocompatible, biodegradable [46] Cytocompatible, processable, renewable, haemostatic property Form hydrogel material Nonadhesive Hard tissue and soft tissue glycosaminoglycan [46], [49] similar physical and Bone tissue engineering biological functions [30] [46] Biodegradable[50] Scar treatment [22] Bio-adhesive in surgeries Nontoxic degradation for haemostasis, wound products, high elasticity, closure, and a sealant [21] excellent biocompatibility [10], controllable degradation rate, promote cell attachment [51] Cardiac tissue [21] High biocompatibility, Drug delivery [10] high biodegradability, non-thrombogenic, nontoxic, cost-effective, mild and ionotropic gelation process, non-antigenicity chelating ability [21] Substrate coating to Cytocompatible, intrinsic improve cell adhesion stiffness [43] Myocardial tissue regeneration [35] Potential biomaterial in Excellent food, cosmetic, and biocompatibility [21],

Disadvantages Slow degradation rate [48]

Too fragile, limited solubility at physiological pH [36]

Low mechanical strength, low structural integrity or toughness [21]

Tendency to shrink Weak mechanical properties Potential disease transmission [21] High hydrophilicity, low protein adsorption [21]

Derivation from murine sarcoma cell [35]

Lower melting temperature, rapid (continued)

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Table 1 (continued) Types of polymer

Uses in tissue scaffold

Advantages

pharmaceutical application [33] Drug delivery and cell culture [21]

biodegradable [27], dissolution in water, lack nontoxic, absorbable [35] of 3D structural integrity [14] Poor mechanical resistance [24], thermal stability, and short degradation rate [52]

Disadvantages

(3-hydroxyoctonoate) (PHO) [59], poly(glycerol sebacate) (PGS) [19, 32, 42], poly (1,8-octanediol) (POC) [70], poly (trimethylene carbonate) (PTMC) [61], polyethylene glycol (PEG) [22, 42, 43], polyglycolide (PGA) [22, 32, 42, 51], polylactic acid (PLA) [32, 51, 68, 71], polyvinyl alcohol (PVA) [22, 42], polycaprolactone (PCL) [21, 39, 43, 59, 69], poly(L-lactic acid) (PLLA) [56], polyhydroxyalkanoates (PHA) [34, 43, 51, 72], poly(lactic-co-glycolic acid) (PLGA) [59, 73] and polyurethane (PU) [51, 74]. Other biodegradable synthetic polymers are aliphatic polymers, poly(p-dioxanone), copolymer soft trimethylene carbonate, glycolide, polyanhydrides, polyphosphazenes, and synthetic hydrogels. Poly(caprolactose), poly(L-lactic acid), poly(trimethylene carbonate), polyurethane, and poly(glycerol sebacate) are used in making cardiovascular catheters, vascular tissue engineering, heart patches, scaffolds supporting cardiomyocytes, and other cardiovascular-related applications (Table 1). Other than these, poly (hydroxyalkanoate) and poly(glycolic acid) are materials used in scaffold suture and drug delivery systems. Nevertheless, synthetic polymer affects the longer and slower degradation process, exposing cells to toxic biodegradation products, causing cell inflammation and consumption of high cost of production. PLA is a biodegradable polyester of lactic acid produced through the polymerization of D-lactic acid (DLA) and L-lactic acid (LLA) enantiomers [75]. It can also be synthesized from renewable and nonrenewable resources such as fermentation and chemical processes. To support environmental sustainability, the fermentation process is preferred over chemical processes as the fermentation process contributes to lower environmental impact, reduces fossil fuels usage, and higher optical clarity in the final product. PLA produces carbon dioxide and water through biodegradation. It is compatible, absorbable, highly heat-stable, relatively resistant, pollutionfree, nontoxic, and nonirritant, thus making the polymer one of the most important materials synthesized for its advantages. Although PHA, PHB (poly(hydroxybutyrate)), and P4HB are synthetics, they can be categorized as natural polymers because the synthesis process involves fermentation by microorganisms [76]. P4HB is used as a trileaflet heart valve scaffold [40], considering the plastic behavior of P4HB. On the other hand, PHB is highly solid and brittle [77].

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Table 2 Types of synthetic polymers, their uses in tissue scaffold, advantages, and disadvantages Types of polymer Polycarprolactone (PCL)

Polylactic acid (PLA)

Poly (trimethylene carbonate) (PTMC)

Uses in tissue scaffold Fabrication of 3D scaffolds for TE [21] Drug delivery [56] Scaffold forming mesh to support cardiomyocytes, Musculoskeletal and skin [21]

Plate for jaw bone [49] Heart valves, vascular grafts, cardiac patches [42] Use as copolymer Vascular tissue engineering [63]

Poly(glycolic acid) (PGA)

Temporary scaffold or support substrate Drug delivery system [63] Tissue scaffold sutures [21]

Poly(L-lactic acid) (PLLA)

Endothelial cell Smooth muscle cell Cardiomyocyte marker proteins

Polyurethane (PU)

Artificial heart Bladders Heart assist balloon pumps Vascular grafts [21] Cardiovascular catheters and cardiac valves [64]

Advantages Elastic Consist nonpolar methylene and semipolar ester group High drug permeability Less acidic byproduct Strong solubility High biocompatibility Nontoxicity High mechanical and viscoelastic properties [49, 62] High mechanical strength, biodegradable, biocompatible, good thermal stability [42] Good thermal stability, biodegradable [63] Biocompatible, biodegradable, poorly soluble in organic solvents, good mechanical stability [21] Nontoxic degradation product [21, 28] Biocompatible, biodegradable, nontoxic, good mechanical integrity, good thermal plasticity, highly versatile [40] Excellent biocompatibility, nontoxic [42] Good elasticity and mechanical properties [31]

Disadvantages Relatively slow degradation rate [27] Poor cellular adhesion [22]

Brittle compound, cause inflammation [42]

Poor mechanical performance, limited application [63] Poor solubility, higher degradation rate (lose mechanical strength after 2–4 weeks) [21] Subsequent inflammation [64] Long degradation time [28]

Long degradation time [65]

(continued)

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Table 2 (continued) Types of polymer

Poly(glycerol sebacate) (PGS)

Uses in tissue scaffold Breast implants Wound dressing membranes Bone adhesives Condoms [21] Cardiac patches [66] Soft tissue engineering [65] Nerve, cartilage, and retina [34]

Polyhydroxyalkanoate (PHA)

Cardiovascular and bone tissue engineering [21] Soft tissue engineering [68]

Polyvinyl alcohol (PVA)

Pharmaceutics and biomaterial areas, tissue-mimicking, vascular treatment, heart valves, cartilage substitute, soft contact lenses, and membranes for bioseparation [22]

Advantages

Disadvantages

Easily customized, controllable mechanical, degradation properties [66], flexible and elastomeric [10, 67] Biodegradable, biocompatible, naturally produced by microorganisms [68]

Cytotoxicity and less biocompatibility [21]

Biocompatibility, highly hydrophobic, mechanical strength and flexible, thermal stability, nontoxic, and cheap [42]

Slow degradation, high hydrophobicity, and crystallinity, poor drug load, high-cost production [68] Lack of sufficient mechanical properties as a replacement tissue [69]

Properties of Polymers Natural and synthetic polymers are distinguished by their mechanical strength, degradation rate, biocompatibility, cost of production, and other side effect [45, 78]. Natural polymers exhibit higher biocompatibility because of their natural properties that are harmonically blended with cells and any biological process (Fig. 6). Their degradation rate is also higher than synthetic polymers due to their molecular weights, surfacearea-to-volume ratio, and crystallinities [55]. The various mechanical properties of natural polymers depend on the different structures, while synthetic polymers’ mechanical properties are easy to control. A polymer that is extracted from natural sources acquires high production costs compared to a synthetic polymer that does not require high costs. However, the risk of side effects is higher where the slow rate of biodegradation gives a low immune response toward polymer. This could lead to inflammation and potential disease risk, whereas natural polymer only affects the contamination risk of antifungals, especially when the polymer is obtained from a plant source.

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Fig. 6 The properties of natural and synthetic polymer

The demand for polymers in the external environment increased the use of controlled-lifetime polymers in biomedical applications [70]. In the medical field [78], the application for drug delivery, tissue engineering, scaffolds, and prosthetics are widely used on a particular human’s or animal’s body part. The challenging part of using biodegradable polymer in the biomedical application is how compatible is the polymer metabolite with the cells and the efficiency of a scaffold in regenerating new tissue [55, 79]. The regenerated tissue should resume and continue the biological function after being replaced by the implant [54]. The polymer should be able to undergo enzymatic degradation, and the degradation product must not produce toxic substances that will harm the cells and host [54]. In the polymer-controlled drug delivery, carrier polymer degradation will affect the kinetics release or the migration following water uptake or swelling [10]. But if a polymer is administered orally or via pulmonary delivery, the degradation product may not affect much. The “lifetime” of a polymer is depending on the polymer’s performance towards body reaction and its functionality.

Properties of Scaffolds in Cardiac Tissue Engineering The properties of the scaffolding polymers are classified based on their intrinsic nature, processing characteristics, and final product. Their chemical structures, crystallinity, solubility, transparency, and mechanical and transition temperatures are some of the intrinsic properties of the scaffolding polymers. Meanwhile, their melt flow index, viscosity, and the strength of the molten phase will decide which processing technique is the best for them. A different processing method will be selected for manufacturing scaffolds with different characteristics. The final product will have a combination of intrinsic properties, degradation rate, esthetic characteristic, heat resistance, and water resistance. In tissue engineering, a biopolymer’s bioactivity, biodegradability, biocompatibility, mechanical strength, porosity, and morphology are the key constraints when constructing a scaffold (Fig. 7) [80].

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Fig. 7 Properties of biodegradable scaffolds

Bioactivity Biomaterial has been used in cardiac tissue engineering because it supports cell adhesion and propagation. It contains biological signs that help to reinforce or strengthen the attachment and physical signs that determine cell morphology and orientation. In this regard, the cell must recognize the pore properties to promote new cell growth that links to the cellular network and the interconnected pathways. This ensures normal cell signal, nutrient transportation to the replacement area, and proliferation of the new cell. The scaffold can act as a delivery vehicle or a repository of exogenous growth-stimulating signals, so it must be compatible with biomolecules and the encapsulation technique [55].

Biocompatibility A bioengineered scaffold designed intentionally for medical purposes must be biocompatible so that the therapy that utilizes the scaffold will not cause suffering

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of any adverse or systemic effects. The fostered cell must be compatible during attachment, differentiation, and proliferation [77]. Therefore, the material should produce the most suitable cellular or tissue response and maximize clinical efficiency. The biocompatibility depends on the scaffold’s cytotoxicity and the immunological response against body fluids or cells [54]. The body develops immunological responses to foreign substances. Thus, the body’s immune response should accept the scaffold [48]. This ensures that no scaffolds or implants are rejected upon entering the body. The scaffolds’ structure must possess the biomimetic binding sites to comply with the cells at the damaged tissue area by providing structural support and tensile strength, attachment site, or cell surface receptors that affect tissue formation and regeneration [76]. The scaffold must also be able to mimic the natural extracellular matrix biocompatibility can also be defined as adsorption and desorption activities of polymeric materials depending on the surface roughness, rigidity, hydrophilicity-lipophilicity ratio, surface charge, and charge distribution. Some modifications made to the surfaces in contact with living tissue will enhance the biocompatibility properties. The unwanted residues can be sealed by employing a coating while the excretion or absorption can be controlled using a selectively permeable surface [21].

Biodegradability Essential parameters such as the physicochemical, mechanical, and biological characteristics of a scaffolding agent control the rate of its biodegradability. To replace the scaffold with new cells, a biodegradable scaffold must self-degrade by undergoing several mechanisms such as hydrolysis, oxidation, and enzymatic and physical degradation. Natural polymers were the first biodegradable scaffolding material used clinically due to their overall interactions with different cells and lower immune response [54]. Although natural polymer degrades quicker than synthetic polymer, natural polymers possess lesser strength and durability to overcome elasticity, compression, and stretching [10]. The residues must not stay longer in the body as the regeneration of the tissue will be suppressed. The disturbance releases toxic substances to tissue cells leading to organ failure. The scaffold tissue should be degraded at a specific rate of period and regenerated accordingly [54]. The degradation rate for the cardiac tissue engineering material is within 6 to 8 weeks in vivo. The biodegradation rate depends on the polymer’s chemical structure, molecular weight, hydrophilicity/hydrophobicity, crystallinity/ amorphous, glass transition temperatures, and the presence of hydrolytically unstable bonds [81]. An ideal biodegradable material produces noncytotoxic degradation products, easily metabolized and excreted from the body. The absorption kinetics is subjected to the regenerating tissue [82]. The deterioration of a biomaterial employed for skeletal system tissue engineering can occur gradually because the mechanical strength must be conserved before the tissue reconstruction is almost complete. Meanwhile, skin tissue engineering needs less than a month to deteriorate [80].

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Porosity Porous scaffolds have an open and completely interconnected geometric structure. The porosity ensures cell penetration, tissue ingrowth, cell proliferation and migration, vascularization, and nutrient supply to the structures under or near the scaffolds. Pore size, distribution, volume, interconnectivity, shape, and wall roughness are a few major parameters that need to be considered when constructing scaffold for tissue engineering [54, 83]. For example, the cellulose-based scaffolds can be refined with microspheres as the microparticles improve scaffolds through the shape of pores [84]. The optimum pore size for full oxygen and nutrient diffusion should be at least 100 μm to promote cell survival. The porous requirement could differ from one tissue to another; however, a scaffold must neither be too porous nor lack porosity. If the surface-to-volume ratio is too high, the scaffold will be too porous and weaken the mechanical strength. On the contrary, lacking porosity will result in poor cellular ingress, vascularization, and signaling. For instance, porosity is unnecessarily essential but the porous structure help in the growth of vascular cells [85]. One of the drawbacks of increasing porosity is reduced compressive and tensile properties [55].

Morphology The chemical and topographical properties on the surface of a scaffold influence cellular adhesion and proliferation [54]. The surface is the interface that surrounds the cell and tissue. A scaffold used in tissue engineering must have a large surface area and should be fabricated consistently with the structure of the tissue. It should be built in a 3D shape, highly porous, structurally strong, and fully interconnected geometry. The type of tissue decides the optimum porosity of a scaffold. The morphology of a scaffolding tissue is also affected by its pore size, the rate of tissue ingrowth, and the performance of the implanted matrix [33]. Apart from that, a scaffold should be biocompatible for long-term substitution of a newly developed tissue. When constructing a scaffold for tissue engineering, cell dimensions must be contemplated along with phenotypic expression, cell activity, and extracellular matrix production [76].

Mechanical Mechanical factors such as elasticity, strength, and absorption of a biomaterial are essential and depend on the stability of the scaffolding agents, type of material, processing method, molecular weight, and crystallinity of a polymer [54]. The mechanical strength is influenced by the tensile and compressive performance of the final product [33]. In addition, the scaffold’s structural integrity must be able to comply with the surrounding tissue and environmental stresses [86]. Supposed that a scaffold tissue engineering is to be applied in bone tissue regeneration, the scaffold

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should possess similar mechanical characteristics to the bone. Since the regenerated tissue fills the spaces occupied by the degraded scaffold, the mechanical strength of the growing tissue and the deteriorating scaffold are comparable. Thus, one or more rheological parameters such as maximum strain, flexural modulus, Young’s modulus, compressive strength, and tensile strength are assessed [87]. In cardiac tissue regeneration, the mechanical strength of a polymer needs to be considered due to the action of blood circulation from the pumping action of the heart [21, 77]. The scaffold must mimic the native structure of cardiac tissue that can withstand cyclic strains and stresses exerted to the tissue [70]. The process could aid the mechanical properties during the synthesis of a new extracellular matrix by the native cell [76]. One of the studies has shown that the mechanical properties of a cellulose-based scaffold depend on its chemical structure, the crystallinity of the material, and degree of crystallization. This is important in surgical implantation, where the tissue-engineered scaffold must exhibit stronger mechanical properties. On the other hand, synthetic polymers show high durability toward stress and strength due to the alteration of the chemical compound [27]. However, there is a risk during transplantation, and the degradation process could affect local inflammation [76].

Fabrication Methods of Biodegradable Polymers A scaffold is a polymer used for cardiac tissue engineering that provides similar properties to the native cells [59, 79]. It supports the cells until the cells can produce their extracellular matrix (ECM) and tissue cells [51, 81, 88]. After a while, the scaffold should degrade and eliminate any unnecessary material. Thus, it is important that the scaffold produced is compatible, suitable, possesses good mechanical efficiency, and is cost-productive. Due to several limitations, a blended combination of alginate and gelatine could be used as a scaffold in myocardial tissue engineering [16]. Furthermore, a similar technique could be applied for blending natural and synthetic polymers as properties enhancement will increase the scaffold’s efficiency. Distinguished scaffolds designed for various applications in tissue engineering can be produced via different approaches. A leached scaffold is highly porous and carries important roles in cell differentiation, proliferation, and migration. Other than that, fibrous scaffold such as aligned fiber, is produced in different shapes and sizes to provide a better cell migration than the non-aligned fiber. This scaffold is fabricated using fiber spinning methods such as electrospinning, wet spinning, jet spinning, and melt spinning [89]. These methods are also being used to produce nano-sized fiber, which improves motor neurons compared to the micro-sized fiber. On the other hand, a hydrogen scaffold is a 3D network made from a natural or synthetic polymer that absorbs a large amount of water, pH-sensitive, thermal-sensitive, injectable, and biodegradable [90]. Lastly, the scaffold produced from the latest 3D technology using bio-ink is the printed scaffold [24, 39]. This method allows researchers to produce a scaffold of desired pore size and structures for treatment applications, although the cost of production is high [91].

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Table 3 Fabrication methods to produce biopolymer scaffolds Melting-based Solvent-based

Gas foaming Rapid prototyping

Melt molding Extrusion/particle leaching Solvent casting/particle leaching Paraffin template Freeze drying Thermally induced phase separation Electrospinning Membrane lamination Chemical reactant gas foaming Physical induced gas foaming/particle leaching 3D printing Fused deposition modeling Direct rapid prototyping

There are various methods to synthesize biodegradable scaffolds [10, 54, 89], such as electrospinning, electrospray, leaching, phase inversion, self-assembly of synthetic polymers in a matrix, gas foaming, vacuum drying, polymerization of hydrogels, sintering technique, and immersion and extrusion. These methods are divided into four different techniques, as displayed in Table 3: melting-based, solvent-based, gas foaming, and rapid prototyping [10, 90].

Melting-Based Technique The melting-based technique comprises the melt molding and extrusion/particle leaching methods. A suitable porogen is dissolved in water to give a porous scaffold. The porogen solution is mixed with the polymers, and the mixture is melted. The molten mixture is cooled to give the scaffold [89]. The use of salt or sugar during the leaching process makes it difficult to control the quality of the scaffold. As a result, irregular particle shapes will be produced, and porous structures’ quality is less satisfying. The hassles caused during these processes could be solved by applying nearly perfect microspheres such as paraffin microspheres [92]. Scaffolds produced via the melting-based technique are PA-, PLA-, PGA-, and PLGA/gelatin-based.

Solvent-Based Technique Solvent casting or particle leaching, freeze-drying, thermally induced phase separation, and electrospinning are some methods of the solvent-based technique. The freeze-drying and electrospinning methods produced products with aligned texture and porous structure among all of these methods. This type of material is the most suitable to be used as a scaffold in myocardial treatment because of its ability to control the tissue’s properties, maintain the phenotype, and lay the extracellular matrix appropriately [93].

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Solvent Casting or Particle Leaching Solvent casting or particle leaching are the cheapest methods of the solvent-based technique to produce porous scaffold [38]. These methods involve using solvents, mainly sodium chloride and sodium citrate, that dissolve in water at 70  C. The fabrication comprises three sections: (1) mixing of the solution, (2) molding, and (3) drying and water cleaning. The salt particles must be specific to obtain a porous structure [93]. Firstly, the polymer is dissolved in an organic solvent and then mixed with the salt solution. Then, it is molded into the desired shape, and the solvent is removed via lyophilisation or evaporation to leach salt particles off the polymer matrix [89]. Lastly, the mold will be cleaned by dipping it into the water to allow the salts to dissolve. These methods are cheap, and the porosity and pore size is easily controlled, depending on the amount and particle size of salts, but the leached salt particle is difficult to be removed from the matrix [93]. Freeze-Drying In the freeze-drying method, the polymer is dissolved in a mixture of solvent/water and homogenized to form an emulsion [38]. The two-phase separation process of freeze and drying will be applied after the emulsion has rapidly cooled down. This is to ensure the emulsion structure is locked in the liquid state. As the structure’s shape is locked, the solvent and water are removed through freeze-drying. The advantages of this method are its ability to control the porosity and pore size of a scaffold to meet its target application. About 90% of the porosity level is achieved by controlling the polymer concentration, solvent, water phase percentage, and freeze-drying parameters [54]. The leaching stage can be skimmed off, but solvent has become a parameter to be considered in tissue engineering applications. A study comparing the relationship between porosity with polymer concentration and water volume fraction was done using a fabricated hydroxyapatite/poly (hydroxybutyrate-co-valerate) composite scaffold [89]. An increase in polymer concentration in the same volume of water phase fraction resulted in decreased porosity. On the contrary, the porosity is enlarged from several microns to 300 microns when the volume of the water phase fraction is increased [38]. However, it requires a longer processing time and is cost-ineffective. Biomaterials produced via freezedrying technique are PGA, PLLA, PLGA, alginate, agarose, gelatin, and chitosan [54]. Thermal-Induced Phase Separation Another simple and versatile method is the thermal-induced phase separation prepared from microporous membranes [64]. This method is widely applied for polymers with poor solubility. A homogeneous polymer solution is prepared by dissolving the polymer at a high temperature in a low molecular weight but high boiling point solvent. After that, the hot polymer is cast into molding, followed by cooling. Next, solidification is induced, and the polymer-rich phase is separated from the polymer-deficient phase. A microporous structure is obtained after removing the

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solvent via extraction or freeze-drying. Thermal induced phase separation is a simple method that offers high reproducibility, low defect rates, high porosity, and narrow pore size by utilizing fewer influence factors such as diluent, cooling rate, polymer, and additive [94]. Despite all the advantages, the processing time is longer and produces mainly thin membranes. The product has limited mechanical properties and residues of solvents and porogen in the material. Common biomaterials produced using thermal-induced phase separation techniques are PLLA, PEG, PLGA, and several natural polymers [21].

Electrospinning Electrospinning is the most ranked and popular method that has been widely studied from the range of polymeric materials of biodegradable and nondegradable as further as synthetic and natural biopolymer [83, 95–98]. In this method, the mechanical and biochemical of a scaffold can be engineered by controlling nanofiber’s thickness, composition, and porosity [18, 24, 69]. The electrospinning system (Fig. 8) consists of a syringe needle, high-voltage (5–50 kV) direct current power, and a grounded collector [39, 92]. The syringe needle will be placed on the syringe pump connected Fig. 8 Electrospinning structure

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with the voltage and collector. As the strand of polymer is pumped out of the syringe, the ground collector will collect the synthesized fiber of the scaffold. The electrospinning method for producing synthetic scaffolds can tune the mechanical properties to meet the desired properties [18, 99]. It is also a simple process that produces scaffolds at a low cost [100]. Although the properties could be constructed as desired, the electrospun polymer resulted in an imperfect or bulky structure due to the collector plate, pump power, and different polymer material. To obtain the perfect texture, the choice of polymer, the amount of power supplied to the pump, and the distance between the collector plate and the syringe pump needs to be observed. Scaffolds produced from natural polymers via the electrospinning method are nontoxic and show a low immune response (Fig. 9). The scaffolds possess enhanced mechanical and structure strength [64]. During property enhancement, the extra

Fig. 9 Pros and cons of producing scaffolds via electrospinning [18]

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cross-linking will affect the 3D structure and porosity of the scaffold [99]. The structure and porosity are important to provide a fixed shape to ensure its efficient function for cell replacement. Furthermore, using an unsuitable solvent to dissolve the polymer alters the degradation process [101]. From this process, the porous structure can be beneficial for cells as it provides effective cell infiltration, nutrient, and metabolite diffusion to and from cells [95, 101, 102]. Besides being a simple process, electrospinning is highly economical in manufacturing scaffolds from various polymers [51, 99]. Furthermore, this method is commonly used in myocardial tissue engineering (MTE) that uses biomaterials such as PLA, PEG, and collagen [54]. For example, gelatine/PCL hybrid fibrous scaffolds were synthesized through this method. This ensures that the fiber diameter, pore size, and strength, promoting and developing constructed tissue, are optimal for cardiovascular tissue regeneration [90]. Nevertheless, electrospinning is not applicable for all types of polymers. The product produced is also insufficient for cell seeding and cell infiltration. Besides having limited mechanical properties, the pore sizes decrease along with the increment in fiber thickness. Electrospinning can be used to produce scaffolds made from PEO, PLGA, PLLA, PCL, PVA, collagen, silk fibroin, fibrinogen, chitosan, and their composites [89].

Gas Foaming Technique Scaffolds produced via the gas foaming technique utilize biomaterials such as PLA, PLLA, and PLGA [38]. The chemical reactant and physical induced gas foaming methods are solvent-free formations of the porous structure via the generation of gas bubbles within a polymer [38, 89]. The polymer is saturated in an isolated chamber at high pressure (800 psi) of CO2. At high pressure, the intermolecular interactions between CO2 and the polymer increase, thus reducing the polymer’s glass transition temperature. The pore inside the polymer matrix occurs from the rapid depressurization process causing thermodynamic instability and forming nucleated gas cells. It is the most suitable process to produce an amorphous and semicrystalline polymer with low Tg or Tm and a high affinity for CO2 [90]. Nitrogen gas also can be used to replace CO2 for this technique. However, the substitution of CO2 with nitrogen yielded a nonporous skin layer and a closed pore structure [54]. Thus, the porogen leaching process is performed where a porogen such as NaCl particles is introduced to the polymeric material before gas begins to foam. The salt particle helps form an interconnected open pore structure in the polymer matrix during this process. Like the melting-based method, the porosity can be controlled simply by altering the salt or polymer ratio and the salt particle [8]. Although the gas foaming technique is easy to operate, controllable, suitable for all soluble polymers, and most importantly cost-effective [61], the products have limited mechanical properties and inadequate pore interconnectivity as well as long processing time [27].

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Rapid Prototyping Technique In contrast to the melting-based and solvent-based techniques, 3D printing, fused deposition modeling, and direct rapid prototyping are the fastest and most efficient methods in producing tissue scaffolds [103]. 3D printing manufactures various tissues such as the liver, kidney, and heart tissue [90]. Apart from that, due to the high accuracy, precise deposition, and heterogeneous structural feature, 3D printing has contributed to producing teaching materials used in surgical and anatomical studies [61, 104]. The printed biopolymer consists of two extruders used for molding cells and a hydrogel molder that acts as the temporary nonadhesive support [79, 105]. Bio-printing involved the electrically heated bio-printers in producing air-pressure pulses, which force the formation of droplets from the nozzle [106]. Pulses are also formed by piezoelectric or ultrasound pressure using acoustic bio-printers. A study has shown where a uniform structure of PLA fabrication can be achieved by using a 3D printer. The surface of PLA is coated by synthetic hyaluronic acid (HA) as the coating material is porous with excellent biocompatibility and bioactivity [10, 107]. To produce a 3D tissue scaffold, a gel or solid form polymer is preferred [100]. Other than the 3D porous structures, sheets, film, or knitted meshes are also formed via these methods. 3D porous printing is one of the rapid prototyping methods that has been developed through fabrication technologies to produce mostly porous polymeric, ceramic, and composite scaffold [86, 107]. The fused deposition modeling (FDM) [8, 105, 108] method produces material in film and is mostly used to fabricate polymer composite. This will be produced at high speed at a low cost. However, this method is suitable only for thermoplastics, and the undesirable particle or white powder formed will appear in the unprinted volumes [10]. There are also conductive blends and mixtures of composites or nanofibers, usually in the form of a film or a membrane [8]. A 3D conductive scaffold produced by fabricating poly(3,4-ethylenedioxythiophene)poly(4-styrene sultonate) (PEDOT:PSS) with nanocomposite of gelatine and bioactive glass enhances thermal stability, resistance towards enzymatic degradation, and mechanical properties of the scaffold [108]. Although rapid prototyping has a higher advantage and polymer quality, these techniques require higher costs than electrospinning [10].

Conclusion In summary, natural polymers have advantages due to their abundant natural resources, whereas synthetic polymer synthesis from chemical substances could harm the biological system. However, synthetic polymer does give various advantages for tissue engineering in a controlled condition. The biopolymers can be structured and controlled to obtain the desired scaffold tissue using a suitable synthetic polymer. In natural polymer, fibrin, alginate, and matrigel are preferred in tissue scaffold and sugary treatment. However, most natural polymers lack mechanical and structural strength. As for synthetic polymer, PCL, PGA, PLLA,

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and PGS are favored in cardiac tissue production, including cardiac patches, cardiomyocytes, and cardiac tissue. All of the mentioned examples for synthetic polymer indicated that these polymers are the most suitable replacement for cardiac tissue due to their biocompatibility, biodegradability, and mechanical strength. The suggested polymer has controllable side effects, and the efficiency of the scaffold tissue and native cell is higher. However, the disadvantage of using synthetic polymers would be the longer degradation time. Hence, to produce an effective biopolymer, it is suggested that the biopolymer be synthesized using both natural and synthetic sources so that the harmful effect of the polymer could be reduced the biodegradability could be enhanced. The method of synthesis is varied according to the types of polymer. Electrospinning is the most suitable approach in producing cardiac tissue scaffolds, mainly because it is the simplest method and economical. Although 3D printing has more benefits in forming desired structures, production and the technology applied are expensive. In addition, most of the suggested biopolymers can be synthesized through electrospinning.

Future Perspective The whole idea of synthesizing biopolymer products is another alternative to provide cell or tissue treatment options. Different organisms will respond differently to the same scaffold. Physiological conditions and environment around the tissue could affect the immune, nerve, muscle, and any system inside the body during treatment. The stages of treatment starting from diagnosing until curing the disease involve drug carrier, tissue transplant, and medication. Although it is proven that biopolymer is nontoxic, the amount of nontoxic effect toward body reaction is not well discussed. Furthermore, the degradation process needs to be controlled, but the human body system is complicated. The environment of the extracellular matrix could change due to a change in temperature, which then results in the reaction of the immune system, thus affecting the degradation time. The longer time it takes to degrade, the host might be exposed to accumulation of toxicity derived from the biopolymer, inflammation, and infection in the treatment area. Further study on the side effects and consequences is needed as tissue engineering offers the fastest treatment than transplantation.

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Biodegradable Polymers in Biomedical Applications: A Focus on Skin and Bone Regeneration Mai Abdelgawad , M. Abd Elkodous Waleed M. A. El Rouby

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, and

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffold Main Features for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Natural Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Collagens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fibrin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyaluronic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gelatin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications of Natural Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Skin Regeneration and Wound Healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bone Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Implants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M. Abdelgawad (*) Biotechnology and Life Sciences Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt e-mail: [email protected] M. A. Elkodous Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan W. M. A. El Rouby (*) Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_45

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Abstract

Natural biodegradable polymers have attracted a lot of attention over the past decade because of their outstanding physical, chemical, mechanical, and physiological properties. Consequently, they are widely employed in many biomedical applications including wound healing, skin regeneration, and bone regeneration as bioscaffolds that mimic the complex 3D environment of cells in vivo. Thus, these natural polymer–based bioscaffolds enhance cell adhesion, proliferation, and differentiation, to replace the dead or damaged parts of the body without inducing inflammation or immune response. In addition, according to their biodegradability, they are promising candidates for short-term implants. This book chapter covers the features of many natural biodegradable polymers and the standards that should be considered upon designing 3D-based bioscaffolds for biomedical applications. Then, various synthetic routes of these polymers with their properties are also summarized, Finally, four common applications (wound healing, skin regeneration, bone regeneration, and implants) are disused in the light of future trends. Keywords

Natural biodegradable polymers · Wound healing · Skin regeneration · Bone regeneration · Tissue engineering · Implant · Bioscaffolds · Regenerative medicine Abbreviations

AT CMC D.W. ECM FXIIIa GLcA GLcNAc HA MSCs OHA/CEC OHA-AT OHA-AT10/CEC PCL PEG PGA PLA-DX-PEG PLGA PLLA

Aniline tetramer Carboxymethylcellulose Distilled water Extracellular matrix Activated factor XIII Glucosamine N-acetyl-glucosamine Hyaluronic acid Mesenchymal stromal cells Oxidized hyaluronic acid/N-carboxyethyl chitosan Oxidized hyaluronic acid-graft-aniline tetramer Oxidized hyaluronic acid-graft-aniline tetramer 10/Ncarboxyethyl chitosan Polycaprolactone Polyethylene glycol Polyglycolic acid Poly-D, L-lactic acid–p-dioxanone polyethylene glycol block copolymer Poly(lactide-co-glycolide) Poly(L-lactic acid)

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PP PVA rhBMP-2 SEM UDP VEGF

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Polypropylene Poly(vinyl alcohol) Bone morphogenetic protein 2 Scanning electron microscope Uridine diphosphate Vascular endothelial growth factor

Introduction Biodegradable polymers have received considerable attention over the past decade due to their outstanding physical, chemical, and physiological properties which promote them to be suitable candidates for biomedical applications [1]. There are two distinguished categories of biodegradable polymers: natural and synthetic polymers. In general, natural polymers possess superior advantages over synthetic polymers. The word “natural” refers to the origin of these polymers; they are derived from natural (renewable) or biological resources such as plants, animals, marine sources, and other microorganisms. In addition, the term “biodegradable” implies that these polymers break down into biologically benign molecules [2]. After disposal, biodegradable polymers can be degraded by the assistance of living organisms [3]. Outstanding efforts have been dedicated for developing effective products from polymers such as lactic acid, cellulose, and starch. In addition, the practical need to produce versatile biodegradable and water-soluble polymer-based products like cosmetics and detergents is of increasing significance. These natural biodegradable polymers possess good biocompatibility and biodegradability. Biodegradable polymers employed in biomedical applications including scaffold-based tissue engineering, orthopedic replacements, and wound healing have a demanding impact because they degrade into products that can be either digested (acids and other components) or eliminated by the animal body [4]. In addition, they exhibit other advantages like the ease of tailoring of their different properties (thermal, mechanical, and chemical) during their synthesis or extraction [5]. To further improve the characteristics of biodegradable polymers, many numbers of methods were developed including random and block copolymerization and/or grafting. In addition, they are often chemically tuned [6]. These methods enhanced not only their biodegradation rate but also their mechanical properties. Another technique is the physical blending to obtain biodegradable products with various physical properties and morphologies. While synthetic polymers are derived from nonrenewable resources such as petroleum resources (feedstocks). Currently, natural biodegradable polymers are widely used in many applications such as sutures, cosmetics, food packaging, dental applications including filling pastes and artificial teeth, material manufacturing, and photography [7]. Consequently, this book chapter presents insightful information about features, biosynthesis, extraction methods, and biomedical applications of some of the most significant natural biodegradable polymers in tissue engineering and regenerative medicine.

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Scaffold Main Features for Biomedical Applications Scaffolds generally and polymeric scaffolds particularly are essential elements in tissue engineering. Its main purpose is to mimic the natural environment and stimulate signaling pathways for regeneration. When it comes to selecting scaffolds and scaffold materials for tissue engineering, scientists are frequently confronted with a vast array of options. The key feature for successful tissue engineering is to select suitable materials to design the appropriate scaffold which is then seeded with target cells (stem cells or specialized cells) and finally supported by growth factors, chemical compounds, drugs, or other bioactive molecules [8, 9]. To design a scaffold for tissue regeneration requires specific features in order to achieve its purpose. The scaffold should mimic the extracellular matrix (ECM) of the native tissue. Therefore, it should provide 3D geometry as what presents in the natural environment. Biocompatible scaffolds that promote tissue development and perform the desired actions with no toxicity are optimal. The used materials should not cause any types of rejection or immunological response. They also should not cause an inflammatory or allergic reaction. The scaffold should also provide cell-cell interaction, and attachment. It should allow cells to proliferate, home, and migrate to the targeted area. In scaffold seeded with stem cells, it should allow cell differentiation to occur [10]. Better cell attachment, proliferation, and growth can be achieved by developing a scaffold with high porosity and a large surface area. The porous scaffold also allows material exchange and waste removal. It plays a role in the degradation rate of the scaffold [11]. A biodegradable scaffold is another feature needed in the developed scaffold. The developed scaffold should be disintegrated progressively, and it should be intact until regeneration takes place. It represents a temporary template for cells to regenerate, it permits automatic gradual degradation of the biomaterials. Therefore, using a biodegradable scaffold leads to avoiding any outside interference for scaffold removal, avoiding inflammation or immunologic reaction [12, 13]. Scaffolds should provide mechanical stability and structural support to maintain the natural tissue shape with avoidance of any irregularity in the shape. Mechanical stable scaffolds help to handle harsh biological conditions that occur in vivo. Economically and commercially, it should be cost-effective with the easiest way for its sterilization to facilitate its use clinically. Moreover, easy manufacturing and handling give scaffold advantages as this allows the scaffold its ease to be selected for clinical application [14]. There are extra specific features required according to the target tissue to be engineered. In bone tissue regeneration and implantation, the scaffold should be strong, osteoconductive, and osteoinductive, with better mechanical stability [10]. While in skin regeneration and wound healing, a scaffold with a larger surface area and matrix shape acts as a net to provide cell proliferation with mechanical support, particularly in chronic and deep wounds. It should be with the suitable geometry that fits the wound shape to permit their growth, in the same manner the skin does [15]. Scaffolds design varies according to the fabrication technique, the materials used,

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and the desired target. For instance, a scaffold can be in form of a nanofiber, core/ shell nanofiber, foam, sponge, hydrogel, or a mesh network [16].

Synthesis of Natural Biodegradable Polymers Natural biodegradable polymers are repeated units connected by covalent bonds and often synthesized by living organisms. Natural biodegradable polymers are known as biopolymers and can be classified into three main types: polysaccharides (like starch, chitin, chitosan, and cellulose) representing the most popular type of these natural polymers, proteins (such as collagen), and lipids. These complex polymers are often formed by plants or animals.

Collagens Collagens belong to proteins which are the essential parts of various tissues, as they are extensively used as biomaterials for scaffolds, hemostatic agents, sutures, drug delivery systems, and tissue engineering. Collagens are multiple (nearly 28 types), insoluble fibrous proteins possessing a triple helix configuration of three coiled polypeptide alpha chains [2]. Each alpha chain contains 1000 amino acids with a sequence of proline, glycine, and hydroxyproline. Collagens are important structural proteins for the bodies of many animals. Collagens can be found in skin bones, cartilages, ligaments, muscle tissues, blood vessels, lungs, and cornea. Collagens are often classified according to their polypeptide chains, lengths and interruptions of their helix structure, and variations in the helix terminations. The biosynthesis of collagen is a complex combination of intracellular and extracellular procedures as shown in Fig. 1, occurring in osteoblasts, chondrocytes, ondoblasts, epithelial cells, fibroblasts, and cementoblasts [17]. In addition, collagens can be isolated from many waste products such as fish skin, bovine limed split wastes, solid wastes of the leather industry, skins of young and adult Nile perch, the skin of threadfin bream, the outer skin of cuttlefish, skin of deep-sea redfish, skins of Baltic cod, swim bladder of catfish, chicken skin, skin and bone of bigeye snapper, mangrove archaeogastropod, natural marine source jelly fish species, and short tendons of slaughtered cattle [18]. Petcharat et al. reported collagen extraction from the skin of clown featherback (Chitalaornata) using an ultrasound-assisted method [19]. The isolated skin (10 g) was immersed in acetic acid solution (0.5 mol L
1) with a ratio of 1:15 w/v under constant stirring in a 250 mL beaker for 48 h at 4  C. Then, double-layered cheesecloth was used to filter the mixture. After that, the filtrate was salted out to 2.6 mol L
1 using NaCl. Then, the mixture was centrifuged for 60 min at 10000 rpm at 4  C. Then, the obtained pellet was dialyzed against acetic acid for 24 h and freeze-dried. After that, acetic-acid-dispersed treated skin was subjected to ultrasonication (750 W–100 max amplitude) for different times (10, 20, and 30 min) and

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Fig. 1 Biosynthesis of collagens. (Adapted with permission from Ref. [2] (Copyright 2018, Elsevier Ltd.))

amplitudes (20, 40, 60, and 80%). Finally, the pellet was filtered, precipitated, dialyzed, and freeze-dried as previously mentioned.

Chitosan Chitosan is the N-deacetylated derivative of chitin. It possesses good antioxidative and scavenging abilities against radicals. Chitosan is a natural abundant polysaccharide that can be mainly found in invertebrates, yeasts, and fungi [20]. Chitosan is soluble only in acidic solvents like (diluted acetic acid, ascorbic acid, propionic acid,

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and hydrochloric acid), meaning its use as a biomaterial in wound dressings or biological adhesion treatments is challenging. However, disaccharides-assisted derivatization (by maltose, cellobiose, and lactose) and changes to carbohydrate branching make it water soluble. Chitosan can be extracted from its resources (mainly crustacean shells) by many methods such as chemical extraction, biological extraction, and microwave-assisted extraction [21]. The common extraction protocol of chitosan includes demineralization, deproteinization, and deacetylation as shown in Fig. 2. The chemical extraction process of chitosan includes the following processes: first, washing by water for any impurities from the shells; second, drying under the sun or inside the oven to make washed shells brittle; third, powdering of the dried shells; fourth, acid treatment for demineralization (mainly with HCl); fifth, deproteinization by NaOH; and sixth, acetone decolorization. While in the biological extraction of chitosan, different kinds of microorganisms and enzymes are used. Herein, demineralization is done through organic acids–producing bacteria, whereas deproteinization is performed via enzyme protease activity. Sometimes, both chemical and biological extraction methods can be combined, as reported by Younes et al. [22]. Finally, under microwave irradiation for 3 h, chitosan can be extracted from chitin whiskers as reported by Lertwattanaseri et al. [23] in the presence of 60% NaOH solution which achieved 95% degree of deacetylation. In addition, the microwaveassisted extraction method produces chitosan with a high molecular weight with respect to the conventional methods [24].

Drying Demineralisation By acid treatment

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Fig. 2 Chitosan extraction steps. (Adapted with permission from Ref. [21] (Copyright 2021, Elsevier))

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Fibrin During tissue or vascular injury, the blood-circulating fibrinogen which is a liverproduced glycoprotein complex is converted by thrombin into fibrin and fibrin-based blood clot which is responsible for occluding blood vessels and thus cease bleeding [25]. Fibrin polymerization starts with the thrombin-catalyzed cleavage of fibrinopeptides from fibrinogen and proceeds through several assembly steps to form an insoluble fibrin clot [26]. Stabenfeldt et al. mixed purified human fibrinogen with activated factor XIII (FXIIIa) and human α-thrombin to launch the clot formation [27]. The resulting fibrin gels possessed porous networks which can be further improved by the conjugation with knob A (GPRPFPAC-PEG) and knob B (AHRPYAAC-PEG) as shown in Fig. 3.

Hyaluronic Acid Hyaluronic acid (HA) is a significant mucopolysaccharide prepared by the alternate connection of N-acetylglucosamine (disaccharide units) and glucuronic acid [28]. HA’s network structure does not possess covalent cross-linking bonds. However, it has good viscoelasticity (elastic and viscous properties). HA can be prepared by tissue extraction and microbial fermentation processes as summarized in Table 1. Prehm suggested the HA biosynthesis mechanism according to enzyme kinetics [29]. On the plasma membrane, HA is synthesized. N-acetyl-glucosamine (GLcNAc) and UDP-glucuronic acid are alternately linked to the uridine diphosphate (UDP)-linked glucosamine (GLcA), by HA synthase on the serosa, then molecules of HA are gradually formed.

Fig. 3 (a–d) Confocal microscopy images (10 μm z-stacks) and (e–h) SEM images obtained from no peptide control group (a and e), control peptide conjugate (GPSPFPAC-PEG) (b and f), knob A conjugate (GPRPFPAC-PEG) (c and g), and knob B conjugate (AHRPYAAC-PEG) (d and h). (Adapted with permission from Ref. [27] (Copyright 2011, Elsevier))

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Table 1 Possible preparation methods of hyaluronic acid

Raw material Existence Extraction method Quality Yield

Methods Tissue extraction Fresh and frozen animal tissues such as whale cartilage, cow’s eye, cockscomb, and human umbilical cord In tissues, with proteins and other polysaccharides, HA forms complexes The extraction method is complicated, and a lot of organic solvents and enzymes are required Depends largely on the quality of used animal tissues Depends on the supplied animal raw materials

Microbial fermentation Microbial fermentation broth by Streptococci and Pseudomonas aeruginosa HA is located free in the fermentation broth The extraction method is simple and easy Depends on both strain and used fermentation culture conditions Large, unlimited yield

Alginate Alginates are polysaccharides that are consist of linear binary copolymers of (1-4)linked ß-D-mannuronic acid (M) and α-L-guluronic acid (G) monomers [30]. Alginate gels are widely used in the controlled release in multiple drug delivery systems. Alginate biopolymers are mainly extracted from the cell walls of brown algae species (Phaeophyceae) [31]. In addition, alginates are present as a mixed salt of cations that can be found in seawater, including sodium, calcium, and magnesium ions [32]. Mohammed et al. reported an optimized method for the extraction and purification of waste Sargassum natans for the high yield production of sodium alginate [33]. First, seaweed was washed continuously to remove any impurities or debris (sand, plastics, silt, and microorganisms). Second, the Sargassum was pretreated by formaldehyde solution 2% (w/v) overnight using the method reported by Fertah et al. [34]. This pretreatment step was important to ensure the removal of all phenolic compounds and to give purer products. Third, the Sargassum was washed and dried at 40  C for 48 h. Finally, the seaweed was pulverized to increase its surface area and extraction rates as shown in Fig. 4.

Starch Starch polymer is often extracted from cell walls and plant tissues. The extraction method of starch from plants depends on α-glucosidase-assisted digestion of amylose and amylopectin [35]. Then, starch content is quantitatively determined by either colorimetric assays or enzymatic methods. Finally, the conversion of starch into glucose by agents such as dimethyl sulfoxide takes place. Lee et al.

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Fig. 4 Sodium alginate extraction steps from Sargassum natans. (Adapted with permission from Ref. [33] (Copyright 2018, Elsevier))

reported an alkaline extraction method for starch from Australian Lens culinaris: Matilda (Green Lentil) and Digger (Red Lentil) containing 40–45% starch and 30–33% protein [36]. Flour of each variety was used as the source of starch with water at different pH conditions (D.W.8, 8.5, 9, and 9.5) adjusted by NaOH and at various temperatures (ambient temp. 22, 30, 35, and 40  C). For both varieties at pH 9.5 and all temperatures, the extraction process achieved the highest starch yield of 85–95%. It is worth mentioning that, for all temperatures and with D.W., Digger gave a lower yield of starch compared with Matilda. In addition, Digger starch yield was not significantly changed at the four pHs values. By contrast, Matilda flour exhibited a gradual increase in starch percentage yield with the increase in temperatures and pH values (up to 95%). While starch damage percentage increased for both varieties upon extraction at high pH and temperature leading to a smoother and more

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symmetrical peak, indicating the absence of protein adhering to the starch surface. Although lower pH and temperature resulted in low starch damage, these extraction parameters gave low starch and protein yields. Their results showed that pH 9 at 30  C was the optimum extraction condition for Matilda and pH 8.5 at 35  C for Digger. Figures 5 and 6 show the extraction yield at different temperatures and pH for both varieties.

Gelatin Gelatin is derived from collagen, the most common structural protein found in bones and skin of animals [37]. Gelatin is famous for coatings and the microencapsulating of many drugs for biomedical uses [38]. In addition, it is used for the preparation of various biodegradable hydrogels [39].

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Fig. 7 Gelatin preparations’ electrophoretic analysis in 2-β-mercaptoethanol presence: a) control, b) S250, c) E250-100 , d) E400-100 , e) E400-2  50 , and f) E250-200 . (Adapted with permission from Ref. [37] (Copyright 2005, Elsevier))

Gómez-Guillén et al. used high pressure–assisted method for the extraction of gelatin from fish skins. The electrophoretic profiles (SDS-PAGE) of the different gelatin preparations are presented in Fig. 7. Dover sole (Solea vulgaris) were collected and frozen at
20  C until use. Briefly, the swelling step was carried out by a mild acid (50 mM acetic acid) for 3 h followed by another step overnight (16–18 h). Then, gelatin was extracted in distilled water (D.W.) at 45  C. Extracted gelatin was dried in the air until reaching moisture of less than 15% (control gelatin). However, for pressure-assisted gelatin extraction, frozen fish skins were thawed and washed as mentioned above. The pretreatment of 50 mM acetic acid was carried out at 10  C for 10 min at 250 MPa, followed by extraction in D.W at 45  C for 16–18  C at atmospheric pressure (S250). Extracted samples in D.W. at 45  C by high pressure were all preceded by a pretreatment (at atmospheric pressure) in 50 mM acetic acid at 10  C for 3 h. Instead of the conventional extraction overnight for 16–18 h, high pressure (250 MPa) was applied for 10 or 20 min or at 400 MPa for 10 min or by two pulses of 5 min each.

Biomedical Applications of Natural Biodegradable Polymers Tissue engineering has remained an attractive and multidisciplinary area with the goal of developing alternatives that achieve replacement or regeneration to the damaged tissues. Polymers have been utilized in tissue engineering and regenerative

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Fig. 8 Schematic illustration of scaffold application in wound healing, skin regeneration, bone regeneration, and implantation. The scaffold is fabricated from biodegradable polymers alone or integrated with other materials or metals. The scaffold can then be seeded with stem cells or differentiated cells as osteoblast, and, finally, growth factors or bioactive molecules can be added to induce better regeneration. After scaffold administration or implantation topically or surgically in skin and bone, respectively, the healing and regeneration are illustrated with better skin and bone formation. (The figure was created using some images from Servier Medical Art http://smart. servier.com, licensed under a Creative Commons Attribution 3.0 Unported License. https:// creativecommons.org/licenses/by/3.0/)

medicine extensively. They are used to regenerate damaged or nonfunctional tissues and organs; they are used to enhance tissue repair, replacement, or regeneration. Polymers have been used in tissue engineering of different organs such as cartilage, hollow organs, ligaments, tendons, liver, kidney, heart, bone, pancreas, skin, and wound healing [40]. In this chapter, we will focus on their applications in wound healing, skin and bone regeneration, as well as bone implantation. The scaffold can be fabricated from biodegradable polymers alone or integrated with other materials or metals. It can then be seeded with stem cells or differentiated cells as osteoblast, and, finally, growth factors or bioactive molecules can be added to induce better regeneration. After scaffold administration or implantation topically or surgically in skin and bone, respectively, the healing and regeneration occur with better skin and bone formation as illustrated in Fig. 8.

Skin Regeneration and Wound Healing The human skin is designed to offer electrolytes, water, and bacteria-resistant barrier to the outside. Therefore, it is the first body defense. Chronic ulcers and burns lead to bacterial infections and electrolyte imbalance which consequently impair the skin’s

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unique function [41]. Chronic wounds, such as ulcers, and diabetic feet impose a large financial burden worldwide. If neglected, they may lead to gangrene incidence and eventually amputation [42]. Skin tissue engineering entails using cells, growth factors, and biomaterials including polymers to be able to reconstruct a functional tissue instead of the damaged or diseased one. Biomaterial should provide a significant physical barrier against fluid loss and infection to be able to accelerate and induce skin regeneration [43]. Groeber et al. have summarized skin regeneration and wound healing for both in vitro and in vivo models. They discussed how skin models can be used to further understand the development and regeneration processes and to discover the corrosive disease situations in the skin. Donor tissues are used to separate keratinocytes and fibroblasts, then they are grown in vitro, then these cells are seeded onto the designed scaffold which includes polymers. The fibroblasts and the matrix play a role in establishing the dermal component of a full-thickness skin analog. While keratinocytes are implanted on the top of the dermis to produce the epidermal layer of the skin as illustrated in Fig. 9 [44]. Bioengineered skin substitutes resolve the shortage of donor grafts from allogeneic sources. They can be permanent or temporary until the availability of the autograft. The skin substitutes represent a barrier from microorganisms, a vehicle for growth factors, mediators, and other components necessary for healing and regeneration processes. Skin substitutes either cover the dermal layer or replace

Fig. 9 A diagram depicts the process underlying using scaffolds for skin regeneration and using them in the in vitro models. Donor tissues are used to separate keratinocytes and fibroblasts, then they are grown in vitro, then these cells are seeded onto the designed scaffold which includes polymers. The fibroblasts and the matrix play a role in establishing the dermal component of a fullthickness skin analog. While keratinocytes are implanted on the top of the dermis to produce the epidermal layer of the skin. (Adapted with permission from Ref. [44] (Copyright 2011, Elsevier [44]))

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the epidermal layer or both. Substitutes that imitate epidermal/dermal layers of the skin are the most sophisticated structures available for therapeutic use. There are commercial skin substitutes that are available and can be used clinically. They represent scaffolds seeded with keratinocytes and fibroblasts from either autologous or allogeneic sources [41]. For instance, Epicel [45] and Epidex are used for epidermal coverage with a permanent life span. While Permacol [46], Dermagraft [47], and Tansyte are used for dermal coverage and Apligraf, Permaderm [48], and Orce are used for both dermal and epidermal replacement. Skin regeneration and tissue engineering polymers and biomaterials should provide the skin with elasticity support; they should also provide a protective barrier (prevent water, microorganism passages, and electrolytes from outside to inside the body) and should secure attachment to the dermis and to be well vascularized [41]. Polymers offer a new avenue to introduce new wound healing treatments via providing novel ways to enhance the rate of wound healing. They are used in the forms of hydrogel and scaffold. The use of polymers in wound healing and skin regeneration opens up new possibilities for faster and better healing with antibacterial activity. It also permits supplying the injured or affected area with growth factors and drugs [49]. Polymers are required to provide mechanical stability to the skin and permit cell migration, differentiation, and/or proliferation. Biomaterials with high biodegradability and biocompatibility with no toxicity should be included in wound healing polymers such as chitosan [50]. Conductive polymers such as polypyrrole and polyaniline can be used to control the release of medications or any mediators to the wound site via their electrical impulses when they are incorporated into the wound dressing or the scaffold. Moreover, this integration allows uniform delivery of the electric impulse to the target tissue. Interestingly, the application of conductive polymers can accelerate the healing process even without the application of electric stimuli [51]. The application of electrical stimulation allows better control over cellular differentiation and proliferation, which, in return, will accelerate the wound healing process [52]. Jin Qua et al. designed a conductive hydrogel that has antibacterial (Fig. 10) and antioxidant properties when mixed with amoxicillin and oxidized hyaluronic acid-graftaniline tetramer (OHA-AT). The hydrogel also exhibited reparative and regeneration features, as the addition of aniline tetramer (AT) enhanced the wound healing rate. Wound healing and skin regeneration assessment were measured on the 5th, 10th, and 15th days, among TegadermTM which is a commercial dressing, oxidized hyaluronic acid/N-carboxyethyl chitosan (OHA/CEC) hydrogel, oxidized hyaluronic acid-graftaniline tetramer 10/N-carboxyethyl chitosan (OHA-AT10/CEC), and D-OHA-AT10/ CEC hydrogel. In a full-thickness skin defect model, investigations revealed that hydrogels with AT added (OHA-AT/CEC hydrogels) dramatically accelerated wound healing with an increase in collagen distribution, granulation tissue thickness, and with more angiogenesis as illustrated in Fig. 11 [53]. Natural and synthetic polymers have a substantial role in wound dressing and skin tissue engineering. Natural polymers have become widely used in the fabrication of scaffolds and hydrogels. They participate in providing the scaffolds the biocompatible and biodegradable properties and they facilitate the chemical integrity of the

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Fig. 10 Illustrates the antibacterial effect of injectable conductive hydrogel against S. aureus and E. coli on days 1, 2, and 3. (Adapted with permission from Ref. [53] (Copyright 2019, Elsevier))

tissue. While synthetic polymers are affordable, easily manipulated, and modified, as they can be incorporated into different structures such as mats, films, nanofibers, and scaffolds. A synthetic polymer represents a suitable agent for cross-linking. Both synthetic and natural polymers can be incorporated together to achieve better mechanical stability. The widely used natural polymers in skin tissue engineering are cellulose, alginate, collagen, gelatin, chitosan, starch, and hyaluronic acid. While synthetic polymers that showed their efficacy in wound healing and skin regeneration are poly(lactide-co-glycolide) (PLGA), polyethylene glycol (PEG), poly (vinyl alcohol) (PVA), and polycaprolactone (PCL) [54]. Starch is a natural polymer that is considered a hemostatic agent used in wound healing and skin regeneration. It is biocompatible, degraded enzymatically, easily obtained, and has a low cost. Starch-based scaffolds are easily manufactured and modified to enhance their mechanical properties and have the ability to induce cellular proliferation without any toxicity to the cells [55–57]. Similarly, chitosan is considered a hemostatic agent; it is a natural biodegradable, biocompatible, and nontoxic polymer that is dissolved in diluted acetic acid. The chitosan is characterized by primary amines which acquire properties that play a significant role in wound healing and tissue regeneration. It participates in skin regeneration fibroblast proliferation and collagen deposition with anti-inflammatory and antimicrobial properties [58]. Gelatin protein is also another type of natural polymer which is considered a derivative from collagen. It is used in wound and burns dressings, either in solid forms as Gelfoam (Upjohn, Kalamazoo, MI), or powder forms as Sugarifoam powder

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Fig. 11 Schematic illustration of the regeneration and the wound healing rate (a) macroscopically and (b) microscopically. Wound healing and skin regeneration assessment were measured on the 5th, 10th, and 15th days, among TegadermTM which is a commercial dressing, OHA/CEC hydrogel, OHA-AT10/CEC, and D-OHA-AT10/CEC hydrogel. In a full-thickness skin defect model, investigations revealed that hydrogels with AT added (OHA-AT/CEC hydrogels) dramatically accelerated wound healing with an increase in collagen distribution, granulation tissue thickness, and with more angiogenesis. (Adapted with permission from Ref. [53] (Copyright 2019, Elsevier))

[59]. Hyaluronic acid is a part of the extracellular matrix in the body that is used in wound dressings due to its less antigenicity. It induces cell proliferation and differentiation. HA scaffold is a hemostatic agent with cell carrier properties [60].

Bone Regeneration Bone has the ability to restructure, regenerate, and heal itself following fracture or injury. It also can restructure and remodel in response to changes in local pressures.

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After the fracture, regenerated bone is produced to connect broken bone fragments with each other, the new bone is modified by the body to improve its mechanical function in the area where the fracture took place. Maintaining and restoring bone mechanical functions are the major goals after bone injury or fracture. However, when the bone repair system fails in response to fracture, an alternative attempt should be executed to induce repair. Bone sometimes needs an external source to fix the site of the fracture, provide mechanical strength, and enhance proliferation and regeneration, otherwise it will not heal properly. In severe injury cases, bone’s regenerative capacity is limited, and healing does not occur completely and correctly, resulting in impaired mechanical function, which necessitates surgical intervention to restore bone function, but it exposes patients to high risks. Medical surgeons try to create circumstances and environments that are similar to those that the body would meet in a fracture repair system to trigger it [10, 14, 61, 62]. When serious damage occurs, an autologous bone transplant is necessary and is taken from various parts of the body to treat any produced bone defects. It is characterized by osteoconductive, osteoinductive, osteogenic, and absence of immune response [63, 64]. However, it is not easily available, will put the patient under stress and risk, and there are surgical constraints. On the other hand, allograft and xenograft were addressed in bone surgery as well. However, an immune rejection can be produced [65]. Designing a porous scaffold imitating bone features represents a new avenue to avoid employing grafts that may be not available or may cause immunological response, graft rejection, and infection. Developing osteoconductive and osteoinductive materials that enhance cell adhesion, proliferation, differentiation, and regeneration have been studied extensively [66–68]. Bone tissue regeneration represents a challenge in designing a suitable scaffold that permits angiogenesis, vascularization, and supplying of oxygen. Bone contains calcium phosphate and collagen type I; it is considered a hard connective tissue with high porosity internally while less porosity externally. On the contrary, it has high mechanical strength externally and low internally in order to support the bone with strength and shape. Therefore, designing a scaffold with all these features is not easily obtained [67]. Scaffolds including polymers, stem cells, or osteoblast transplantation, and bioactive molecules or growth factors are the three main constituents for bone tissue engineering and are considered an alternative therapy. They trigger osteoblasts to form osteoid which is the bone matrix [69, 70]. A wide range of natural and synthetic biodegradable polymers have been studied widely, developed, and used either in research or clinically for the purpose of bone tissue engineering and repair. Employing mechanical strength to a scaffold designed for bone tissue regeneration is done by using polymeric materials such as chitosan, silk protein, and poly(L-lactic acid) PLLA. Different polymers vary mechanically, biologically, and according to their biodegradation mechanisms [65, 71, 72]. PLGA, PLLA, and poly(glycolic acid) (PGA) are used in bone regeneration as they have been considered suitable substrates for osteoblasts that exhibited better attachment, proliferation, and differentiation. 2D and 3D cultures of osteoblasts have been used in vitro with the help of polyphosphazene and PGA polymers,

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respectively, where the cells showed polymer adherence and cell proliferation and differentiation capabilities. On the other hand, PGA polymer meshes have been used in vivo where regeneration took place within 3.5 months [12, 70, 73]. In a study done to enhance scaffold biocompatibility and regenerative capacity, PLGA was combined with silk in a scaffold; the combination affected the PLGA degradation [74]. Chitosan has been widely used in bone tissue regeneration. A study assessed the effect of sulfated chitosan bone morphogenetic protein 2 and consequently bone regeneration. The results revealed that low doses of sulfated chitosan were able to induce osteoblast development, differentiation, and bone formation in both in vitro and in vivo [75]. Chitosan combined with silica xerogels exhibited better regeneration and osteogenesis capacity when compared to using it alone as shown in Fig. 12. The combined chitosan silica xerogel disclosed complete healing within 3 weeks unlike pure chitosan alone where almost only partial healing and closure took place [77, 78]. Moreover, chitosan microspheres and collagen sponge were combined to form a copolymer which has been applied in bone tissue regeneration. Then it was

®

Fig. 12 HOB cells (human osteoblast) grown in SCS8_E1 xerogel after (a) 48 h, (b) 72 h, and (c) ® 1 week in culture. Cultured HOB cells grown in xerogel SCS8_E1 after (d) 48 h, (e) 72 h, and (f) 1 week. Control cells after (g) 48 h, (h) 72 h, and (i) 1 week. (Adapted with permission from Ref. [76] (Copyright 2021, MDPI))

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Fig. 13 Bone initiation by osteogenic cells in micropores resulting in a massive osteointegration: (a) pink/red color shows staining of micropores confirming the formation of bone; (b) corresponding BSE image reveals micropores-filled mineralized bone (light grey) and scaffold (white) density contrast near to the interface of both scaffold and rod bone; (c) cells occupying the micropores can be revealed by light blue color for the cytoplasm and dark blue color of the nuclei; (d) BSE SEM representing cells of the bone matrix; (e) morphology of micropores’ cells; and (f) BSE SEM depicts cell processes extending into surrounding bone matrix. (Adapted with permission from Ref. [82] (Copyright 2010, Elsevier))

integrated with bone morphogenetic protein 2 (rhBMP-2) to form a composite that managed to regulate the release of rhBMP-2 which is injected into a rabbit animal model with bone defects. Bone regeneration is observed after 3 months [79].

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Calcium phosphate–based scaffolds exhibited a high potentiality of bone tissue regeneration as depicted in Fig. 13. When combined with either biodegradable or nondegradable polymers, they overcome the disadvantages accompanying the usage of calcium phosphate alone, which shows weak mechanical stability and less biodegradability. The calcium phosphate–based scaffolds are characterized by high safety with the absence of toxicity [80, 81]. Clinically, a scaffold made by bioceramic polymers and seeded with stem cells has been used in clinical trials in patients suffering from skull injury, a healing process occurred, and a new bone has been formed (NCT01742260). Biodegradable polymers are combined with inorganic material for better conductivity and bioactivity. PCL and other polymers have been combined with hydroxyapatite to form a spiral scaffold, then the scaffold was seeded with cellular elements, and the results revealed an increase in the mineralized extracellular matrix [83]. Since vascularization represents a major obstacle in terms of bone tissue regeneration, scientists tried extensively to overcome such a problem which may lead to insufficient and irregular cell differentiation [84]. The addition of certain biomolecules and growth factors was found to induce vascularization. For instance, the addition of vascular endothelial growth factor (VEGF) recruits osteoprogenitor cells to the affected area, which in turn leads to bone regeneration and repair [85]. Growth factors can be merged within polymeric scaffolds. For example, Kempen et al. showed that VEGF can be combined with gelatin hydrogel while BMP-2 with PLGA microspheres, and all are combined with a polypropylene (PP) scaffold. The results revealed that the addition of VEGF enhanced vascularization and BMP-2 enhanced bone formation as presented in Fig. 14 [86].

Implants Developing biomaterials for the purpose of bone implantation is considered a challenge in terms of their design and biological application. Since bones are susceptible to severe fractures (either from trauma or any pathological conditions), or tumors that necessitate amputation, this, in turn, requires an external implant to be inserted surgically to induce bone fixation and repair with new bone formation [87, 88]. Traditional interference was implantation with materials made of metals such as stainless steel and titanium. These materials are used clinically for years and have exhibited a substantial remarkable role in fixation and repair. However, they need a have their disadvantages [89]. Using biodegradable materials paid the attention of the researchers in bone regeneration and repair, and implantation because this will avoid patients to undergo a second surgery in order to remove the inserted implants which expose patients to health care, high cost, risk of infection, inflammation, pain, complications, and stress shielding [90]. Therefore, developing and using biodegradable material with mechanical, chemical, and compatible properties aids in the success of bone regeneration [91].

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Fig. 14 Bone and vascular network formation analysis in a subcutaneous model rat, 8 weeks postimplantation. (a) Bone (white) and blood vessels (red) images using 3D volume-rendered μCT obtained by running μCT imaging and microangiography at a resolution of about 20 μm where the renderings were used to measure (b) vessels total number (surrounding the implant), and (c) volume of the newly generated bone. With respect to the nonloaded implants with VEGF, VEGF-loaded scaffolds possessed a significantly larger volume of local vessels ( p ¼ 0.040). Similarly, implants having VEGF/BMP-2 exhibited larger bone volume compared to implants with only BMP-2 ( p ¼ 0.009). (Adapted with permission from Ref. [86] (Copyright 2009, Elsevier))

Natural polymers, as collagen, alginate, and hyaluronic acid, are used in implant fabrication because they are biodegradable, nontoxic, and bioabsorbable. While synthetic polymers as PLLA, PCL, PLGA, and PVA are biocompatible. The implant degradation rate is an important factor because upon it the bone regeneration and formation occur; gradual degradation is required to act as a mechanical supporter and at the same time gives suitable spaces to cells to be proliferated. The degradation rates depend on the chemical and physical properties of the polymers and other materials that form the implant. For example, the hydrophilic nature of the materials, the chemical structure, molecular weight, and morphology [92]. A study done by Saito and colleagues used biodegradable materials as a delivery system. rhBMP-2 was delivered by using biodegradable poly-D, L-lactic acid–p-dioxanone–polyethylene glycol block copolymer (PLA-DX-PEG) to form a PLA-DX-PEG/rhBMP-2 composite which enhanced the BMP-2 delivery, as well as the biodegradability

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and biocompatibility characteristics; it has better bone formation and repair properties [93]. Polymers have been also used with other traditional materials to enhance implants properties. For instance, collagen polymers were used as a coating material for implants and calcium phosphate and exhibited a role in cell attachment, proliferation, and new bone formation rather than using these implants alone [94]. Similarly, a polymeric membrane made from polycaprolactone and dichloromethane was used to coat implants made of magnesium and its alloys to reduce and control the extensive corrosion effect in addition to the undesirable hydrogen gas produced upon implant degradation. Polymeric membrane reduced the degradation rate of magnesium with the maintenance of the implant mechanical property, which in turn allowed a higher volume of new bone formation with the absence of inflammation or H2 gas accumulation and even necrosis, indicating the importance of polymeric membranes in resolving problems produced from clinically used magnesium implants and their alloys [95]. Collagen can also be integrated into implant formation to form a composite, such as its integration with calcium phosphate as shown in Fig. 15 [96]. A fabricated scaffold developed from n-HA and polyurethane with different percentages resulted in the production of a scaffold with porosity and high mechanical strength which resulted in cell attachment and compatibility with new bone formation [97]. Similarly, n-HA was fabricated also with polyamide, and this was implanted in vivo either alone or with mesenchymal stromal cells (MSCs). The results revealed that the n-HA-polyamide scaffold was biocompatible and caused no inflammation with distinct new bone formation [98]. While n-HA was fabricated with chitosan and carboxymethylcellulose (CMC), it provided better mechanical stability and strength, and biodegradability. It exhibited remarkable biocompatibility after its implantation with blood vessel formation [99]. Polymers exhibited less tensile strength, brittleness, and regeneration capacity, unlike bioceramics. Moreover, inflammation is usually induced by many polymeric materials; however, this is not the case with bioceramics. The deterioration of the implant polymers is Fig. 15 Fluorescent microscope image revealing the new deposited bone over a composite of β-TCP/collagen. Newly generated and matured bone is filling all bone defective sites and composite surroundings. (Adapted with permission from Ref. [96]) (Copyright 2005, Elsevier))

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accompanied by a loss in mechanical characteristics. However, the stress shield can be prevented when the implant degradation occurs gradually and is under control [100].

Conclusion Tissue engineering and regenerative medicine have been widely used and developed extensively in research studies and clinical use for the purpose of producing biofunctional tissues as well as organs to repair and compensate diseased, damaged, or amputated organs. Wound healing, skin regeneration, bone regeneration, and implantation are among the most widely applied techniques clinically. Bone transplantation and grafting (allogeneic, autologous, and xenogeneic) are considered the second most transplanted tissue annually; it has several limitations as the shortage in donor availability, high-risk morbidity, immune rejection, and others. This, in turn, necessities the development of biomimetic scaffolds. Scaffold development and manufacturing have become increasingly crucial in determining the mechanical stability and scaffold properties. The scaffold’s main purpose is to mimic the target organ ECM in physiological normal native conditions. They can be applied alone or in combination with stem cells/progenitor cells or other cellular elements and bioactive molecules. Recently, there has been wide application of biopolymers, particularly natural polymers, in tissue engineering; they are inserted in scaffolds, wound dressings, hydrogels, and implants. This is because of their biodegradability, biocompatibility, nontoxic nature, and ease of chemical modification. Biodegradable polymeric scaffolds also are considered supports for cells to be able to attach, proliferate, and maintain their specialized function.

Future Perspectives Currently, there are many developed and suitable candidates for biopolymer-based scaffolds. However, many major challenges and obstacles need to be addressed. Vascularization is one of the major challenges scientists face in tissue engineering as in wound healing, as well as in bone and skin regeneration. In addition, safety concerns still exist in some biomaterials, developing scaffolds cope with the contractile activity of the native body. It is still necessary to develop more advanced sophisticated fabrication procedures and tools. 3D printing is very promising tool for the accurate, fast, and easy development of bioscaffolds. However, designing mechanically stable and porous degradable scaffolds that can tolerate harsh loads and conditions in the native environment is still challenging. More research is required before the clinical stage of personalized whole organ transplantation.

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Hybrid Biodegradable Polymeric Scaffolds for Cardiac Tissue Engineering Hussein M. El-Husseiny , Eman A. Mady , Yasmine Radwan, Maria Nagy, Amira Abugomaa, Mohamed Elbadawy, and Ryou Tanaka

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Regeneration Strategies for Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffolds and Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Engineering of the Heart Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scaffoldless Cell Sheet/Cell Patch Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Cell Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decellularization of the Cardiac Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neovascularization Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Hussein M. El-Husseiny and Eman A. Mady contributed equally with all other contributors. H. M. El-Husseiny (*) Laboratory of Veterinary Surgery, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan Department of Surgery, Anesthesiology, and Radiology, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt e-mail: [email protected]; [email protected] E. A. Mady (*) Laboratory of Veterinary Physiology, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan Department of Animal Hygiene, Behavior and Management, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt e-mail: [email protected]; [email protected] Y. Radwan Center for Materials Science, Zewail City of Science and Technology, Giza, Egypt Nanoscale Science Program, Department of Chemistry, University of North Carolina, Charlotte, NC, USA M. Nagy Biotechnology/Biomolecular Chemistry Program, Faculty of Science, Cairo University, Giza, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_48

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Mechanism of Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Degradation of Natural Biodegradation Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Degradation of Synthetic Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation Polymers Employed for Cardiac Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . Natural Biodegradation Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Biodegradation Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natural/Synthetic Hybrid Biodegradation Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Cardiovascular diseases are accounted the number one cause of death worldwide. Besides, this great threat will continue to arise in the future. Cardiac transplantation is an efficient option to fix this issue. However, the insufficiency of voluntary donors following circulatory death rendered this option a highly significant challenge. Recent approaches for cardiac tissue engineering (CTE) are presenting substantial promises in this track. Revitalization of the heart relied over the years on using many sorts of available conventional prosthetic materials. However, being non-biodegradable, they permanently remain inside the body as permanent foreign substances that can act as an infection nidus. To handle these problems, diverse strategies have been adopted to fabricate polymeric scaffolds for CTE. Biodegradation polymers (BPs) have been presented as successful alternatives in different forms like heart patches, injectable polymeric gels, or artificial vessels. Several natural and synthetic BPs have been utilized. While the merge of both types to produce hybrid natural/synthetic BPs comes at the forefront. Moreover, the recent evolution of nano-designed polymers and polymeric nanocomposites has presented remarkable advances to the area of CTE. They can support tissue regeneration via their outstanding surface, electrical, and mechanical features. A brief outline of the existing approaches employed for CTE is presented in the present chapter. Furthermore, different natural, synthetic, and hybrid BPs exploited for the same purpose are highlighted. Their merits and demerits, in addition to the directions of future research, are discussed.

A. Abugomaa Faculty of Veterinary Medicine, Mansoura University, Mansoura, Dakahliya, Egypt M. Elbadawy Department of Pharmacology, Faculty of Veterinary Medicine, Benha University, Moshtohor, Toukh, Elqaliobiya, Egypt R. Tanaka (*) Laboratory of Veterinary Surgery, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-shi, Tokyo, Japan e-mail: [email protected]

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Keywords

Biodegradation · Cardiac tissue engineering · Hybrid polymers · Nanocomposites · Nanopolymers · Tissue engineering · Scaffolds · Decellularized matrix · Extracellular matrix · ECM Abbreviations

AuNPs AVL BADSCs BPs CD/MPEG–PCL–MPEG

CMs CNFs CNTs CPCs CS CSCs CTE CVDs dECM ECM EHT ESCs GFs Gt HA LV MI MSCs PANI PCL PDLA PGS PLA PLGA PLLA RGD rGO SCs SDF-1alpha SeNP

Gold nanoparticles Arteriovenous loop Brown adipose-derived stem cells Biodegradation polymers α-Cyclodextrin/poly(ethylene glycol)–b-polycaprolactone-(dodecanedioic acid)-polycaprolactone–poly (ethylene glycol) Cardiomyocytes Carbon nanofibers Carbon nanotubes Cardiac progenitor cells Chitosan Cardiac stem cells Cardiac tissue engineering Cardiovascular diseases Decellularized extracellular matrix Extracellular matrix Engineered heart tissue Embryonic stem cells Growth factors Gelatin Hyaluronic acid Left Ventricular Myocardial infarction Mesenchymal stem cells Polyaniline Polycaprolactone Poly(D-lactic acid) Poly(glycerol-sebacate) Poly(lactic acid) Poly(lactic-co-glycolic acid) Poly(L-lactic acid) Arginine-glycine-asparagine Reduced graphene oxide Stem cells Stromal-derived factor 1alpha Selenium nanoparticles

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Tissue engineering Tissue-engineered cardiac pacemaker Vascular endothelial growth factor

Introduction Cardiovascular diseases (CVDs) are accounted the number one cause of mortality in the globe; the statistics of people who die due to CVDs are increasing every year [1]. Different tissue engineering (TE) strategies were employed using various materials to address this problem considering the limited potency of human cardiac cells to complete regeneration. The main objective of TE is the renovation of the injured tissues and replacing them with new biological ones [2, 3]. This multidisciplinary process requires studying cell biology and biochemistry. Moreover, clinical medical and material science studies are incorporated for clinical applications. The biologically active networks are porous, three-dimensional (3D) structures that can support the attachment of biologically active components as biomolecules, proteins, and growth factors (GFs) to their surface. The capacity of these biosystems to provide specific bioactivity to the assembly of the scaffold confirms their unique promises in TE applications. They can also function as cargos for the conveyance of drugs and bioactive peptides, filling agents, and 3D structures to the seat of the defect. In addition, they can control the regeneration processes and promote the development of the required tissue. Herein, it is highly imperative for the biomaterial to present the ideal characters matching the necessities of tissue regeneration. Throughout the years, several platforms have been fabricated from diverse natural sources like algae and animal tissues [4, 5], or synthetic sources like lactic acid, glycoside monomers, and caprolactone. Even though many polymeric scaffolds used for TE could provide essential support and assets necessary for that purpose, they lack some crucial properties as adequate cell mimicking and sufficient interface with stromal cells. The engineering of the damaged heart tissue has recently presented substantial progress with high promises to develop cardiac patches with outstanding capacity to regenerate the injured heart and restore the complete cardiac function without being rejected by the recipient body. Moreover, seeding of these patches with bioactive materials like cells, proteins, drugs, and/or GFs plays a key role to provoke the formation of new tissues via guiding self-seeded cell growth and migration. Furthermore, they mediate the fidelity of newly developed tissue structure and function. On the other hand, the current progress in the cardiac function assessment tools has been shared effectively in this track [6–12]. The development of an appropriate patch with physicochemical, conductive, and biological assets mimicry to those of the extracellular matrix (ECM) of the heart is still the main challenge to CTE. Recently, the utilization of biodegradation polymers (BPs) for CTE has attracted significant attention due to their unique properties as compatibility, tunable rate of degradation, unique porosity, and elasticity. In addition, they can maintain their mechanical strength throughout the bioengineering process and then undergo gradual disintegration with the formation of biocompatible substances. Furthermore, they are

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elegant vehicles for the discharge of many biomolecules. The BPs are categorized into two major classes: natural polymers that are present in nature and synthetic ones that are fabricated from different resources particularly petroleum. For use in CTE, these two categories have their special merits and weaknesses. The amalgamation of these two sets of BPs could produce another category called natural/synthetic or hybrid BPS that merges the advantages and avoid the disadvantages of the utilization of natural or artificial polymers alone. Nanotechnology has recently emerged substantial advances via the synthesis of different environmentally friendly nanomaterials from diverse sources and using them in different biomedical applications [13]. Nanocomposite polymers, attributed to their potency to imitate different mechanical, surface, and electrical characters of the connate heart structure, were also fabricated to augment the process of CTE. In the present chapter, the existing strategies to revitalize the cardiac tissue will be discussed. The diverse mechanisms of polymers’ degradation will be highlighted. Moreover, a detailed treatise of the BPs employed for CTE will be conducted. Besides, we will throw light on the future directions regarding CTE and the role of BPs in this track.

Current Regeneration Strategies for Cardiac Tissue Engineering For CTE, diverse techniques have been employed successfully as illustrated in Fig. 1. In this section, we will discuss them in detail with an emphasis on the materials used and limitations for each strategy as presented in Table 1.

Scaffolds and Cells Multipotent cells capable to self-renew are called stem cells (SCs). They are crucial for tissue homeostasis and regeneration. However, adult hearts have a limited capacity for regeneration, and cardiac SCs (CSCs) could not replace the missing cardiomyocytes (CMs). Recent advancement has been made in inducing pluripotent SCs (iPSCs) out of adult cells via reprogramming with specified transcription factors. Various cardiovascular cells have been obtained from iPSCs inside the cardiovascular system, and a recent study found that postnatal cardiac or dermal fibroblasts could be directly converted into cardiomyocyte-like cells [14]. While personalized iPSCs or cardiomyocyte-like cells opened up a new avenue for autologous stem cell therapy, the risk of tumor development has arisen [15]. Several sorts of SCs, including mesenchymal SCs (MSCs), embryonic SCs (ESCs), skeletal progenitor cells, hematopoietic SCs, and cardiac progenitor cells (CPCs), have presented a unique potential to develop functional CMs in the heart [14, 16]. Even though exogenous stem cell transplantation therapy has received a lot of attention, most allogeneic cells go through necrosis or apoptosis after transplantation due to immunoreaction and inadequate surroundings. Furthermore, the employment of ESC therapy has been restricted by ethical issues [14].

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Fig. 1 Schematic illustration of different strategies adopted for cardiac tissue engineering

Although several investigations have reported that cytokines can mobilize autologous SCs, there is no functional way to selectively attract SCs to cardiac damage sites. Furthermore, SCs need a sequence of coordinated interactions with their biological surroundings, whether they were injected directly or attracted by mobilization factors. Compared to the original environment of SCs, the microenvironment changed dramatically, making it harsher for these SCs to survive in the region of injury [17]. Heart failure was treated with tissue-engineered cardiac patches such as ECM; however, the potency of myocardial healing was restricted due to the restricted ability for cell infiltration. To imitate the various interactions between heart cells and the ECM, decellularized platforms from porcine or rat myocardium have recently been used. Although decellularized matrices are found to be favorable when it comes to CTE, their application in transplantation has high risks, including a scarcity of human donors, and other immunological concerns. Hence, researchers are now focusing their efforts on developing ECM biomimetic platforms made of synthetic and/or xeno-free biomaterials [18]. Biomaterials are being used more and more to target repair [4, 5]. They could function as a network for cell endurance, propagation, and multiplication and as a guide for reestablishing 3D tissue [14, 17]. Biomaterials are being established as cardiac patches to increase heart function via supporting and repairing damaged regions by replacing damaged myocardial tissues or scar tissue.

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Table 1 Outline of different strategies utilized currently for cardiac tissue engineering Current strategies for cardiac tissue engineering 1. Stem cells and scaffolds

2. Engineering of the heart tissue

3. Scaffoldless cell sheet/cell patch technology

4. Biological cell assembly

5. Decellularized matrices

Limitations 1. Ability for cell infiltration. 2. Decellularized matrices application has high risks, including a scarcity of human donors, immunological concerns, and viral contamination 1. Intrinsic tissue regeneration for the heart is not a part of existing therapy for a multifaceted cardiovascular injury. 2. Relationships between tissue regeneration, engineered biomaterials, and the immune system have yet to be entirely understood The contemporary need for certain vascularization strategies and nutrient diffusion supply to maintain the patch’s durability and adaptability for the fabrication of thick myocardial tissue constructs Hydrogels having only nanoporous meshes within the cross-linked networks and lacking micropores results in insufficient nutrient transfer and cell viability within the hydrogel

The decellularization process could alter the ECM’s biochemical and morphological components

Materials used Scaffold material (e.g., collagen, alginate). Autologous stem cells Stem cell-specific antibody Arginine-glycineaspartate (RGD) Heparin-binding peptides (HBPs) 3D matrix as a scaffold PGS/fibrinogen core/ shell fibers PLGA/Gel nanofibers Matrigel™ and a mechanical stretching device

References [14, 18, 49]

Cell sheets Poly (N-isopropylacrylamide) (PNIPAAM)

[19, 26–28]

Hydrogel-based scaffolds Microporous annealed particle (MAP) Mesenchymal stem cells (MSCs) Vascular endothelial growth factor VEGFencapsulated MSCs Alginate and gelatin polyelectrolytes RAD16-II peptide gels Decellularized ECM (dECM) Human-induced pluripotent stem cells (HiPSCs) dECM-rGO hydrogel systems

[30–32]

[20, 50]

[29, 51]

(continued)

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Table 1 (continued) Current strategies for cardiac tissue engineering 6. Neovascularization

Limitations 1. There are only 100–200 mm layers of 6–12 cm thick that can survive in an in vitro designed construct implanted with CMs and relies only on diffusion for oxygenation and nutrition. 2. Infarcted myocardium’s epicardial surface will be significantly more challenging to neovascularize. 3. For the transplant to survive, it is crucial to be infiltrated by the host’s blood vessels. As a result, the core of the “transplant” will go through necrosis because of the time it requires for the host vessels to vascularize it

Materials used Intrinsic microvasculature Patient’s ectopic vasculature Arteriovenous loop (AVL)

References [19, 39, 41]

Engineering of the Heart Tissue The classic TE process involves seeding target cells into a scaffold in vitro, sometimes with modification (e.g., special conditioned culture [19]), and then implanting the construct in vivo. Biomedical alternatives as biomaterials are constantly being explored in the field of TE for the entire (or partial) replacement of injured tissue. The advance of a 3D matrix as a platform is a marked role for biomimetic materials. The biomaterials must also be suitable for the conservation of the cells and the signals necessary for tissue or organ regeneration. Following that, regenerated tissues need to maintain, reinstate, and augment function [20]. Whenever cardiac constructs had been implanted to a damaged myocardial area, neovascularization from the epicardium infiltrated the graft and distributed fetal CMs survived the implantation process [19]. Upon implantation in the patient, tissue-engineered materials may become functional at the implantation time or be capable to integrate and accomplish the predicted function following implantation. As in either instance, the biomaterial needs to integrate well with the recipient or transplanted cells to effectively share in the tissue regeneration via cell-cell signaling and the release of GFs, propagation, multiplication, and development of ECM [20]. Intrinsic tissue regeneration for the heart is not a portion of existing therapy for multifaceted cardiovascular injury [20]. Relationships between tissue regeneration,

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engineered biomaterials, and our immune system have to be entirely understood. The objective of cellular and TE is the evolvement of treatments that will stabilize, alter, or improve cardiovascular physiology and anatomy. Polymeric systems used in CTE have been outlined and constructed using different approaches [20]. They might be employed in the fabrication of degradable cardiac patches for example. In the long term, these polymeric biodegradable cardiac patches can provide excellent circumstances for cellular growth. Studies on elastomeric biodegradable poly(glycerol sebacate) (PGS), like gelatin (Gt) nanofibrous scaffolds, and PGS/fibrinogen core/shell fibers, have been conducted. Anisotropy was established in these materials, imitating the left ventricular (LV) myocardium. This can be employed as a construct for myocardial regeneration [21]. The cells’ cytoskeletal organization was influenced by the scaffolds’ structural features. For example, the amalgamation of synthetic poly(lactic-coglycolic acid) (PLGA) with natural Gt polymers was produced via electrospinning by Prabhakaran et al. to create PLGA/gel nanofibers. The potential of these scaffolds as biomimetic cardiac patches was highlighted by culturing the cardiomyocyte cells on them [20]. Engineered heart tissue (EHT) is a spontaneously contractile construct created using neonatal rat CMs, collagen I matrix, Matrigel™, and a mechanical stretching device. It is among the most promising CTE approaches [19, 22, 23]. It was originally designed as a planar structure [23], which was then transformed into a circular structure that had greater contractile qualities and a more distinct CM structure. For in vivo testing, the construct’s 3D geometry was further changed into a pouch-like structure [22], which was then used to “encase” a failing heart to function as a biological ventricular assist system. There have been improvements to the in vitro states to lessen the utilization of serum and Matrigel™ as well. Although the model looks to exhibit a significant potential, core tissue viability is minimal, and a more advanced version of the EHT is still needed [19].

Scaffoldless Cell Sheet/Cell Patch Technology The use of biomaterial systems has faced many challenges such as integration impairment with host tissues, immunogenicity, and undesired degradation products [24, 25]. Such dangers cannot be totally eliminated even though innovative biomaterial features have been devised to fix these issues. Scaffold-free engineered tissues known as cell sheets could be created using intact cell monolayers non-enzymatically isolated from substrates in vitro as an alternative [25]. It is a strategy that has therapeutic application potential. Poly(N-isopropyl acrylamide) (PNIPAAm) is used for covering a thermosensitive cell culture layer. This polymer is cell adherent at 37  C and changes characteristics in reverse below 32  C. It is also possible to extract cells off a culture dish as a cell sheet once they have aggregated and developed gap junctions by lowering the temperature [19, 26]. Furthermore, the capability to layer individual CM cell sheets into a 3D contractile cardiac tissue was

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established in vivo [27]. The tissue endured subcutaneous implantation for 1 year, and cell sheets were functionally integrated with the host’s heart when applied to rat hearts [28]. Also using an orbital shaker, Stevens and his group have created a scaffold-free CM cell patch. In both scenarios, the result is a tissue that is identical to a compact myocardium but without the scaffold. Limitations of this method are the contemporary need for certain vascularization strategies and nutrient diffusion supply to maintain the patch’s durability and adaptability for the development of thick myocardial tissue constructs [19].

Biological Cell Assembly It is not necessary to plant cells into 3D porous scaffolds. Instead, cells can be suspended in hydrogel-based scaffolds that provide a suitable environment for them to travel and arrange into contractile tissues whether in vitro, by gravity-enforced methods to form sphere-like tissues [19], or in vivo, by an arteriovenous loop (AVL) embedded chamber for the vascularization of the arranged CMs [19]. The viscoelastic properties of hydrogels and their adaptability to chemical and physical changes have attracted substantial attention as cardiac tissue constructions [29]. They are water-insoluble polymers that can absorb a large quantity of water or biofluids, causing swelling and an expansion of their dimensions while keeping their shape. This feature makes them very close to soft tissues regarding their structure and function [29]. It is likely to modify the surface of a hydrogel to have it respond to a certain stimulus, such as temperature, pH, molecules, magnetic or electric signals, and ionic strength [2, 3]. Since typical hydrogels are quite often formed into larger sizes that have low surface-to-volume ratios, they have slow degradation values and limited cell infiltration along with weak vascularization. Hydrogels of this type have only nanoporous meshes within the cross-linked networks and lack micropores, indicating tht nutrient transfer and cell vitality are insufficient within hydrogels [30–32]. It was found that replacing bulk hydrogel with microporous annealed particle (MAP) that possesses a larger surface/volume percent and shorter diffusion distance can boost the mass movement of nutrients and promote long-term cells’ survival. Its pores can help guide cell multiplication and tissue development before the hydrogel breaks down [31]. Blood-derived MSCs are essential infiltrating cells that have a predisposition to relocate to the myocardial infarction (MI) region. It was hypothesized that vascular endothelial growth factor (VEGF)-encapsulated MSCs aimed at MI tissue could enhance the cardiac activity via angiogenesis and the MSC’s tropism to the MI area. Angiogenesis and heart function were improved by employing self-assembled alginate and gelatin polyelectrolytes in the first stages of development. SDF-1 was found to be an attractive target for the VEGF-encapsulated MSCs in vitro with a stable release of VEGF. In vivo, angiogenesis was stimulated in the MI region by VEGF-encapsulated MSCs, and cardiac functions were enhanced. For MI treatment, these preclinical data imply that this VEGF-loaded layer-by-layer self-assembled

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encapsulated MSCs may be an effective and minimally invasive treatment option for MI [33]. When self-assembling peptides are situated in a physiological environment, they create stable nanofiber hydrogels [29]. As a consequence of the in situ injection of RAD16-II peptide gels that self-assembled in the myocardial, it induced an appropriate microenvironment [17, 29]. Endothelial cells, nonvascular cells, and smooth myocytes were recruited by this microenvironment. RGDSP sequence with a celladhesive domain was connected to the self-assembling peptide RAD16-I to produce a biomimetic self-assembling peptide. The produced scaffold enhanced the adherence and viability of marrow-derived CSCs and facilitated their propagation to develop CMs, which, as a result, improved heart activity and repair [34].

Decellularization of the Cardiac Matrix When cells are removed from organs or tissues while the ECM is left intact, it is called decellularization [35]. Decellularized ECM has been established based on the idea that native ECM might be a better substitute to a tissue’s complex environment [29]. This can be achieved in diverse ways: chemically, physically, and enzymatically [4, 5, 35]. Although this could alter the ECM’s biochemical and morphological components, it has the advantage of removing cellular antigens which can trigger a foreign body reaction via inflammation, antibody activation, and probable transplant rejection [29]. Biologic scaffolds used in clinical applications are produced using this technique. However, it has been proven that using perfusion decellularization preserves the organs’ 3D geometry while removing the cells in a more uniform manner [35]. To develop an entire-heart ECM scaffold, rats’ hearts were decellularized. The scaffold maintained its 3D structure, with the vascular tubing skeleton preserved by the presence of vascular basement membranes [19]. A spontaneous contractile complete heart was generated after planting neonatal rat EC and CMs under physiological circumstances. This method has the potential of creating a big human or animal heart to replace the human heart functionally. Hydrogels made from the decellularized extracellular matrix (dECM) have gained a lot of interest in recent years due to significant advances in hydrogel technology and theory, as well as advances in the use of dECM hydrogels as a novel regenerative and TE medicine approach. There are structural and stimulatory features of hydrogel responsiveness that are retained along with ECM functionality. dECM hydrogel preserves cell GFs such as transforming GF, fibroblast GF, and hepatocyte GF, which can improve the seed cell’s proliferation, migration, differentiation, and angiogenesis [36]. They offer the following benefits: 1. The capability to inject. At physiological temperatures, viscous fluids can be polymerized to create hydrogels that adjust to the form of the defective location. 2. The bioactivity of the native matrix is found in dECM hydrogels [36]. 3. There is no immunogenic cellular content in dECM hydrogels.

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4. The mechanical assets can be altered. 5. Crosslinking or modifying the hydrogel concentration can be utilized to modify the mechanical strength of the dECM. 6. A gelled dECM has a 3D conformation that is beneficial to cell growth. 7. dECM hydrogels are machinability friendly. It is likely to customize 3D geometric shapes with 3D printing [37]. Human iPSC-derived CMs have a significant potency for disease categorizing and drug monitoring when they are exploited to construct human tissues. Hybrid hydrogels comprised of porcine cardiac dECM and reduced graphene oxide (rGO) were produced to support the normal development of cells and tissues. EHTs developed using hiPSC-derived CMs and dECM-rGO hydrogels showed a significant increase in the tension powers and the expression of genes that control contractile function. It also improved many electrophysiological functions, including calcium handling, conductance speed, and action potential period [38].

Neovascularization Strategy The most novel blood vessel development occurs in mature organisms because of angiogenesis. Angiogenesis is described as the sprouting and formation of novel microcirculatory vessels from preexisting vessels [19]. This happens by the breakdown of basement membranes and EC proliferation, which is stimulated by a large variety of growth agents. There are only 100–200 mm layers of 6–12 cm thick that can survive in an in vitro designed construct that is implanted with CMs and relies only on diffusion for oxygenation and nutrition. Infarcted myocardium’s epicardial surface will be significantly more difficult to neovascularize [19, 39]. There have been diverse ways for vascularizing a clinically significant-sized construct. In general, these procedures can be divided into two categories: in vitro and in vivo vascularization approaches [19, 40].

In Vitro Vascularization In bioengineered tissue constructs, this is described as the growth and manipulation of biological components to generate microvasculature outside of a patient’s own body. This technique is extensively employed in classical TE [19, 40]. A CM-seeded cardiac construct is similar to an avascular transplant. For the transplant to survive, it must be infiltrated by the host’s blood vessels. Consequently, the core of the “transplant” will go through necrosis owing to the time it requires for the host vessels to vascularize it. By creating the intrinsic microvasculature in vitro, it is conceivable to rejoin the steward vasculature by either inosculation or surgically connecting the graft to host vessels [19, 41]. In Vivo Vascularization It can be further divided into extrinsic/external and intrinsic/internal vascularization: Extrinsic vascularization is the utilization of the patient’s ectopic vasculature as subcutaneous fat, omentum, peritoneum, and axial vessels having the high-

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angiogenic potential to generate functional microvessels in a non-vascularized engineered construct [19, 40]. Omentum was successful in CTE, while peritoneum failed in this regard. As a clinically significant vascularized graft still requires 3–4 days of “taking,” it relies on diffusion for survival. Therefore, the perfusion timing of these transplants should be carefully considered. If the ECT’s size is large, this may be insufficient to support it [19]. Regarding intrinsic vascularization, a central macrovascular conduit put in a protected region to vascularize an endogenously produced or transplanted scaffold. It is employed to develop a microcirculatory network. During reconstructive surgery, the principle of prefabrication of flaps prompted the development of intrinsic vascularization in CTEs. When an arterial-vein pedicle was implanted within or under the tissue graft, the tissue would become vascularized and generate an entirely new flap that is based on the pedicle. According to research, the AVL design is the most angiogenic of all pedicle implant configurations [19].

Mechanism of Degradation Biopolymers are polymers attained from natural sources such as plants and animals. BPs are those that can be degraded to form biologically acceptable compounds. They can be categorized into synthetic and natural polymers. Animal, plant, marine, and microbial biopolymers including proteins and polysaccharides are examples of natural biopolymers. Artificial polymers include aliphatic polymers, and polyesters, on the contrary, are chemically manufactured [42].

Degradation of Natural Biodegradation Polymers They are macromolecular structures that are present in a vast variety of organisms. Fungi, bacteria, and algae contain natural polymers. Furthermore, animals contain complex polymers like proteins, fats, nucleic acid, and hydrocarbons, while lower organisms and plants contain natural polymers such as oils, starches, cellulose, and even polyesters [42]. Biological activity, as the main element, can be exploited to explain the biodegradation phenomenon. Also, because of a synergy of abiotic and biotic processes, organic matter decomposes over time. Outdoor conditions as weather, age, and burial can cause BPs to undergo various changes such as chemical, mechanical, thermal, etc. [43]. BP’s performance is reduced owing to this exposure. However, abiotic involvement tends to weaken polymer macromolecular structures, which aids in the degradation process [43]. Living cells’ metabolism (storage compounds, enzyme proteins), structure (proteins, cell wall components), and genetic information (nucleic acid) rely on polymers. To be available for other living organisms and the environmental changes, these polymers must be degraded within cells. The fact that animals have evolved a range of mechanisms to break down naturally produced polymers is therefore not surprising. Yet, these mechanisms might not have been evolved for a large number of

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novel and diverse synthetic polymers which have entered the environment just in the previous 70 years [44]. Polymers are degraded by diverse degradation mechanisms that work together in nature to break down them. Degradation by microorganisms can be conducted by secretion of their enzymes or by-products (like peroxides and acids). Macroorganisms can also ingest and digest polymers, which can cause chemical, mechanical, or enzymatic aging [44]. The microbial degeneration process of a polymer involves two major steps: chain cleavage or depolymerization step and mineralization step. On account of their bulk and insoluble nature, most polymers undergo their first step outside of the organism. Either exo- (sequential fragmentation of terminal monomers on the main chain) or endo- (arbitrary fragmentation of internal links of polymer chains) enzymes are responsible for this phase. A cell’s mineralization process begins when monomeric or oligomeric pieces of small suitable size have been generated. This process produces gases (e.g., CH4, CO2, H2, and N2) as well as minerals, water, and salts. As the organisms, polymer, and environment change, so may the biodegradation process. Oxo-biodegradation occurs in natural polymers [45]. Aldehydes, alcohols, and ketones having low molecular mass are formed during the oxocarboxylic acid degradation by a peroxidation process that is activated by light or heat, which is the main cause of loss of the mechanical characteristics of the carbohydrate polymers. Due to bioassimilation, the increased amount of CO2 produced from bacteria, fungi, and enzymes leads to the development of cavities [45]. Hydro-biodegradation can be defined as a method that converts polyesters, starch, and cellulose into bioassimilable compounds. In aqueous solutions, aliphatic polyesters are hydrolyzed and bioassimilated quickly [45]. For example, drug conveyance devices rely on chitosan (CS), a cationic polymer of non-human sources derived from the crustacean skeleton [46]. CS is degraded in vitro by the enzyme papain, lysozyme, and chitosanase. Its degradability is influenced by the level of crystallinity and acylation of its polymeric backbone [46]. Also, protein-based biomaterials are noted for their capability to degrade under nature circumstances [46]. Because of the breakage of phosphoester linkages in the polymer backbone and side chains, these polymers undergo hydrolytic degradation [46]. Biodegradability is determined mainly by the chemical structure, which has a direct impact on the degradation potential. The biodegradation ability of functional groups may descend in the following order: aliphatic ester, peptide bond, carbamate, aliphatic ether, and methylene. Polymers containing hydrophilic groups, such as hydroxyl, carboxyl, amide, and amino groups, seem to be more biodegradable under particular conditions of humidity. Those with amphiphilic diblock copolymer show more biodegradability than those with one hydrophobic block. Compared to cross-linked and branched polymers, linear polymers seem to be more biodegradable [43]. A polymer’s molecular weight has a significant impact on its biodegradation. Higher molecular weight polymers degrade at a slower pace than those with smaller molecular weights. Polymers will not biodegrade if the

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molecular weight hits a limit or exceeds the scope which microbial cells can accept. Natural polymers do not have such problems because they are smaller than synthetic ones [43]. Researchers have shown that polymer breakdown is dependent not only upon polymer characteristics, such as chemical structure and shape, but also upon certain environmental circumstances, e.g., humidity, temperature, radiation, pH value, etc. [43]. Since microorganisms require water to grow, the biodegradation level of the polymeric materials is limited to certain humidity levels. Regarding biodegradation, the temperature has a double impact. Temperature accelerates metabolic activities in microorganisms and stimulates growth, which is advantageous for degradation. However, for enzyme and protein-related biological activities, the temperature has a bigger impact. To grow and reproduce, each type of microbe has its ideal temperature range [43]. In addition, the value of pH has a significant impact on the microorganisms’ growth rate. Optimized pH value speeds up microbial metabolism which consequently accelerates the degradation process [43].

Degradation of Synthetic Biodegradable Polymers Since it is difficult to attain reproducibility with natural polymers, synthetic biodegradable polymeric materials (BPMs) have gained growing interest. A comprehensive array of BPMs can be generated through the alteration of natural polymers utilizing microbiological, chemical, chemo-enzymatic, and enzyme-mediated synthesis. Because of ecological deficiencies and the fact that existing natural resources are limited, synthetic BPMs have attracted attention through the past two decades. Few polymers are generated from natural renewable resources, like polylactic acid (PLA). They can therefore be clarified in the biopolymers category. A significant source of biodegradability is the weak and hydrolyzable linkages that compose the synthetic biopolymers backbone. They degrade into their monomer components, enzymatically or chemically, which human bodies can be biologically acceptable, making them significant for many biomedical uses [47]. Polyglycolic acid (PGA) has the controlled patterns for drug discharge and the capability to reinforce the efficacy of formulations of drugs for drug conveyance. PGA is employed in drug conveyance devices because of the existence of perfect mechanical assets, non-toxicity, and physiological conditions compatibility. Polyglycolide becomes degraded in physiological environments by an unspecified breakup of its ester backbone. Polyglycolides are degraded into glycine and eliminated out of the body through urine or changed into carbon dioxide or water. Because PGA is rapidly degraded and insoluble in several common solvents, its use in drug conveyance devices applications is limited [46]. Poly(lactic acid) (PLA) may be produced simply from sugarcane or starch. The polylactic acid-co-glycolic acid (PLGA), which is a PGA and PLA copolymer, is widely employed as a biodegradable, non-toxic, and non-immunogenic block

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polymer and provides significant bioavailability, controlled, and sustained release patterns. Because of its stimuli-sensitive characteristics, PLGA is referred to as the “smart polymer.” PLGA is degraded by the hydrolytic reaction that happens in an aqueous medium by the ester linkages cleavage, while this cannot happen in the case of PLGA having an ester end group. Auto-catalytic degradation may also occur in PLGA. The acidic by-product that is still strapped in polymer bulk catalyzes the degradation. As PLG and PLA are homopolymers, PLGA degrades faster than them [46]. Copolymer’s lactide concentration is inversely proportional to its rate of degradation and also to the polymer hydrophobicity [47]. Polyanhydrides are special biodegradable synthetic surface degrading hydrophobic polymers that include two carbonyl groups bonded through an ether linkage. When packaged inside these BPs, drugs can be preserved because most of the water will not access the drug until the polymer degrades. The hydrolytic decomposition of anhydride bonds generally generates the non-toxic diacid units that could either be ejected from the human body or get metabolized [47]. The poly(anhydride-coimides) are first degraded through anhydride bonds after imide bond hydrolysis. At low pH values, the polyanhydrides slowly deteriorate, but in elevated pH solutions, they degrade more rapidly. Changing the various parameters of pH, surrounding medium crystallinity and molecular weight significantly alters the pace of degradation. For example, over long periods, aromatic polyanhydrides slowly break down, while aliphatic polyanhydrides break down in a few days. Monomer units having higher hydrophobicity break down at a slower pace compared to higher hydrophilic monomer units [47]. Surface degradation of the polymer and the degradation level control allow for a steady release of payloads which have shown notable advantages for the drug release using polyanhydrides [46]. Polycarbonates are BPs that have drawn recognition to the evolution of different drug conveyance devices and medical equipment. It is non-enzymatic hydrolysisresistant and also does not generate acidic compounds after degradation. This is a valuable unique characteristic for drug transport [46]. Slow degradation improves the existence of the product, and the lack of acidic by-products reduces the risk of in vivo negative effects [48].

Biodegradation Polymers Employed for Cardiac Tissue Engineering BPs are being extensively employed for CTE. The biodegradable polymeric materials could be naturally obtained or synthesized [52]. The ideal BPs should have antithrombogenic, non-immunogenic, and non-adhesive properties and outstanding mechanical assets to transmit the mechanical stimuli for the heart beating and simultaneously provide stiffness to the scar. They should also induce angiogenesis upon injection. Furthermore, the ideal polymer ought to be biocompatible and possesses the potency to convey bioactive materials or cells. The BPs are exploited as injectable materials, patches, hydrogels, scaffolds, 3D printed scaffolds, and

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Fig. 2 Schematic illustration of different formulations of biodegradable polymers employed for cardiac tissue engineering. (a) cardiac patch, (b) scaffold, (c) hydrogel, (d) injectable material, (e) sheet, and (f) 3D printed scaffold

sheets for CTE intentions (Fig. 2). In this section, various BPs used for CTE (Fig. 3) are described [53].

Natural Biodegradation Polymers Polymers attained from natural origins are extensively utilized as biodegradable compounds in the field of TE including CTE. This is owed to the bioactive characteristics that allow better interactions between them and cells reinforcing the cells’ execution in the biological environment. The categories of natural BPs include polynucleotides, polysaccharides, and proteins [54]. The chemical structure of the natural BPs employed for CTE is presented in Fig. 4.

Proteins Collagen Collagen is an ample protein in mammals that is formed of α-chains folding into a left-handed helix, each three left-handed helices fold into a right-handed coil [55]. It could be attained and purified from tissues of humans and different animals as bovine, horses, and pigs. The heart tissue contains six kinds of collagen (I, III, IV, V, VI, and VIII). Type I is the most plentiful type in the heart and organs such as

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Fig. 3 Schematic illustration of variable types of biodegradation polymers employed for cardiac tissue engineering

tendons, skin, and brain. Thus, it is extensively researched as a bio-polymeric material for TE implementations. In the myocardium, types I and III represent 90% of its collagen and are major elements of the ECM that support and maintain the structure of the heart [53, 55]. Collagen type I is responsible for the structural support of the heart, while type III links contractile elements of neighboring CMs [55]. Collagen is a substantial candidate for CTE being biodegradable and biocompatible. The biodegradation products of collagen are cytocompatible and non-immunogenic; thus, they are absorbed and excreted from the body without any surgical intervention [55]. Collagen serves a pivotal role in providing mechanical strength, flexibility, and resilience of myocardium tissue (Fig. 5I) [56]. It is being used in CTE implementations owing to the numerous advantages it upholds. Since it is well conserved among different species, it does not induce an immunogenic response when transplanted to the host body. In addition, collagen-based biomaterials reduce the TNF-α and macrophage count in the MI heart tissue with consequent reduction of the inflammation (Fig. 5II. A–C) [57]. Furthermore, the large surface area provided by collagen biomaterials allows easy integration of GFs or SCs. Also, collagen is found to enrich the growth and permanence of cardiocytes in vitro [53]. Moreover, the porosity of collagen allows the flow of oxygen and nutrients to cells and promotes the exchange of cytokines, chemokines, and GFs between cells. In addition, collagen allows electric signal transfer from CMs to the cells attributed to

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Fig. 4 Schematic illustration of the chemical structure of natural BPs employed for cardiac tissue engineering

its share in the electrical conductivity, providing synchronization with host tissue. A study conducted by Dai et al. (2005) investigated the influence of injectable collagen on the function of LV in animals. The study demonstrated that the implanted collagen remarkably elevated the thickness of the MI tissue and enhanced the geometry and function of the heart [53]. Another study compared the leverage of three biomaterials, collagen I, fibrin, and matrigel in MI rats. The results indicated that the implanted biomaterials enhanced angiogenesis after 5 weeks of treatment. There was no reported difference in angiogenesis induced from the three biomaterials; however, collagen caused a significant myofibroblasts infiltration in the infarct tissue [53]. Callegari et al. investigated the impact of collagen scaffold on angiogenesis on rat models with infarcted LVs. Results demonstrated that collagen scaffold markedly promoted the vessel density for both arterioles and capillaries and enhanced angiogenetic response in the rat’s heart [53]. A clinical trial named MAGNUM investigated the potency of bone marrow originated cells and seeded on collagen scaffolds on ischemic cardiomyopathic patients, by implanting it on the scar during surgery. The findings revealed that the seeded collagen scaffold increased the ventricle wall thickness thus limiting the

Fig. 5 Employment of collagen biodegradable polymers for the revitalization of the cardiac tissue after myocardial infarction. (I) Scanning electron microscopy of different collagen platforms: (a, b) non-crosslinked collagen, (c, d) 1:2, (e, f) 1:5, and (g, h) 1:10 crosslinked collagen. (Copyright © 2013, Elsevier

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remodeling of LV. Furthermore, it helped in normalizing the stress on the LV wall and enhancing the diastolic function. This treatment was proved safe on the patients and highlighted the potency of using collagen-based scaffolds in CTE [58]. Sun et al. investigated the influence of adding carbon nanotubes (CNTs) to collagen hydrogels. The study used rats with MI. The findings declared that the incorporated CNTs enhanced myocardial regeneration and amended the electric conductance of the material. Moreover, the results indicated significant enhancement in the alignment of neonatal ventricular myocytes and a marked amelioration of heart functions. The supplement of CNTs provided better mechanistic backing and boosted the adhesion and elongation of cells. The promising outcomes of the study made the CNTs/collagen hydrogel a potential cardiac construct for CTE [54, 59]. Gelatin Gelatin is a biodegradable, non-immunogenic, and biocompatible polymer acquired from the disintegration of collagen. It approximately consists of 90% proteins, water, and mineral salts. Random coiled domains of Gt are developed via non-reversible collagen hydrolyzation and enzymatic or heat denaturation. Hence, Gt is very congruent to the molecular composition of collagen and can support functions analogous to the biomaterial. Consequently, it is utilized as a collagen replacer for development in vitro and TE [53, 60]. It is extensively studied as a replacement for intact ECM proteins as it is more naturally abundant, easily found, and inexpensive compared to ECM. In addition, Gt obtained from diverse sources is non-toxic and non-antigenic for cells. Moreover, it is more soluble than ECM and, hence, more popular in biomedical utilities. Besides, the high similarity of Gt to collagen allows enhanced cell attachment owing to the existence of important binding moieties [60]. A study conducted by Shao et al. (2006) investigated the ability of Gt hydrogel in delivering basic fibroblast growth factor (bFGF) known for enhancing the heart functionality by inducing angiogenesis in the MI heart of rat model. The Gt hydrogel caused a sustained release of bFGF into the infarction section. Consequently, blood flow to the heart increased, enhancing the LV remodeling and improving cardiac function [53]. Gt usually degrades faster than collagen, and during its degradation, unspecific inflammatory response could be induced. However, the quick degradation of Gt could be valuable for CTE. Sakai et al. seeded fetal CMs on Gt patches and used them as a replacement to the removed right ventricular outflow tract (ROVT) of syngeneic adult rats. 12 weeks post-implantation, the Gt scaffold is dissolved leaving the surviving endothelium on the endocardial surfaces in the ROVT [61]. ä Fig. 5 (continued) Publishing Group. Replicated with permission from [56]). The implanted collagen scaffold is shared effectively to reconstruct the cardiac tissue following MI via (IIa) control of the inflammation and reducing the secretion of inflammatory cytokines, (IIb) minimizing cardiac remodeling via reduction of formed scar tissue and cardiac fibrosis (%), (IIc) amelioration of the cardiac functions assessed by echocardiography. (Adapted with permission from [57]. Copyright © 2015, Elsevier Publishing Group)

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Gt is exploited as a 3D hydrogel scaffold to provide increased spatial control and functionality. This advancement is being realized owing to the 3D printing technology that permits the printing of scaffolds containing biomaterials including GFs and cells. This resulted in controlling the biomaterials spatially, to mimic the microenvironment of the body in higher precision and better functionality [60]. Gt is exploited as a bioink in 3D printing as it has significant structural intensity and tunable chemical structure. The tunable viscosity of Gt is another reason for using it as bioink to print high-resolution 3D scaffolds that are employed in TE including cardiac tissue [62]. Gaetani et al. (2015) developed a 3D-printed Gt/hyaluronic acid (HA)-based scaffold seeded with human cardiomyocyte progenitor cells (hCMPCs) and investigated its efficacy in cardiac remodeling in mice with cardiac infarction. The transplanted patch succeeded in increasing the proliferation and pluripotency of CMs in the mice and promoting cardiac functionality preservation [62]. Another study recreated aortic valves through 3D bioprinting by using Gt/alginate-based scaffold. The results indicated that the 3D-printed scaffold induced the proliferation of aortic valve leaflet interstitial cells and namely aortic root sinus smooth myocytes; these 2 types of cardiocytes are essential. In addition, the results highlighted that the cells comprised a similar cellular phenotype to those in the heart’s physiological microenvironment and they were viable [63]. Kharaziha et al. developed PGS/Gt-based scaffolds with various stiffness, chemical composition, and anisotropy. PGS addition played a pivotal role in providing nanofibrous scaffolds with distinct anisotropy resembling the LV myocardial structure. The results exhibited that scaffold of 33 wt% PGS promoted cellular alignment and caused ideal concurrent contractions of CMs. Hence, the PGS/Gt nanofibrous scaffold has pronounced potential in CTE applications [64]. Fibrin Fibrin is a natural polymer formed of fibrinogen monomers that polymerize via thrombin during the hemostatic coagulation process [52]. Fibrin got FDA approved for several applications including CTE applications [53]. Fibrin could be acquired naturally from an autologous source, or synthetically. It is extensively utilized in CTE applications as cardiac cell encapsulation and 3D scaffolds for myocardial reconstruction as they are biocompatible, non-inflammatory, and non-toxic. Its controlled degradation rate and useful degradation products also made fibrin an interesting candidate for CTE applications; some degradation products enhance the cardiac healing process and protect against myocardial reperfusion injury. Fibrin gel is an excellent candidate for CTE applications as it improves cell adherence and survival because of the existence of the cell adhesion motifs, arginine-glycineasparagine (RGD), and its capacity to support suitable circumstances for the differentiation of marrow-derived CSCs into CMs [53]. Fibrin, however, has some shortcomings including reduction of the gel, poor mechanical characters, and the hazard of causing intravascular thrombosis when utilized as a glue in CTE [52, 53]. Martens et al. studied the employment of fibrin gel as a cargo to deliver bone marrow-derived MSCs to treat rats with ischemic hearts. The results indicated that

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the non-invasive injection of fibrin could preserve heart activity. In addition, when fibrin gel delivered bone marrow-derived SCs, a significant enhancement in cell attachment, cell survival, and cardiac function was shown [53]. In another elegant study, Shen et al. investigated the influence of matrix metalloproteinases (MMPs) on the platelet fibrin gel loaded with CSC performance in the repair of MI in rats. MMP inhibitor GM6001 was exploited, and the findings revealed that the inhibitor diminished the therapeutic power of the platelet fibrin gel loaded with CSCs compared with the group that did not receive the inhibitor. Hence, confirming the significance of MMPs addition to the platelet fibrin gel loaded with CSCs for the cure of MI [52]. A study conducted by Zhang et al. used poly(ethylene glycol) (PEG) (PEGylated) fibrin patch loaded with stromal-derived factor 1alpha (SDF-1alpha), which is a stem cell factor that has a major role in mobilizing SCs from the bone marrow to the injured tissue and is upregulated following MI. The loaded patch was transplanted into acute MI mice. The fibrosis % was significantly diminished, and the function of the LV was markedly ameliorated compared to the unloaded patch [53, 65]. Melly et al. studied the impact of fibrin hydrogel laden with platelet-derived growth factor-BB (PDGF-BB) and VEGF on cardiac angiogenesis. The optimized fibrin hydrogel had a great ability to provoke angiogenesis without the need for any genetic material, in a very short period safely and effectively [52]. Matrigel Matrigel is a natural ECM gelatinous protein obtained from Engelbreth-HolmSwarm (EHS) mouse sarcoma cells. It is commercially available and mostly used as a cell culture mixture [52, 53]. Matrigel is cytocompatible and contains cytokines and GFs; thus it is used in CTE and as a coating to improve cell adhesion [52]. However, its animal origin hinders its clinical use. In CTE applications, Matrigel showed great potential. A study conducted by Kofidis et al. investigated the effectiveness of liquid injectable tissue compounds composed of Matrigel and ESCs on repairing cardiac tissue after MI. The mice model was injected with the liquid compound which solidified at the body temperature reserving the LV thickness and enhancing the heart function [53]. In a different study, Matrigel was employed as a mattress for culturing humaninduced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Results reported that single contracting hiPSC-CMs that are rod-shaped with aligned myofilaments were generated. Compared to hiPSC-CMs cultured on the control substrate, the Matrigel mattress enhanced the morphology, increased the extension of the sarcomere, and enhanced contractile response. This study highlights the potential role of Matrigel in preclinical cardiotoxicity testing, drug discovery, and disease categorizing as it provided the potential to quantify the contractile performance at the level of a single cell [52]. Zhang et al. constructed a TE cardiac pacemaker (TECP) made of endothelial progenitor cells (EPCs) and CPCs loaded onto Matrigel. The constructed TECP was implanted in rats with cardiac sinus node dysfunction. The fabricated TECP displayed electrical activity and improved the endurance of the damaged sinus

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node. Besides, the electrical activity was improved in vivo. However, one major obstacle hindering its use in clinical settings for CTE applications is the absence of optimum vascularization in the engineered tissue [52]. A study assessed the capability of Matrigel to induce angiogenesis in the MI rat model. The Matrigel enhanced the activity of the LV, increased the wall size, and enlarged the capillary density. In addition, the injection of Matrigel ameliorated cardiac function owing to the increased recruitment of c-Kit+ SCs and CD34+ cells to the seat of myocardial ischemia. However, no significant alteration in the size of the fibrosis area was observed. Further studies are required to translate these auspicious findings into clinical applications of CTE [66].

Polysaccharides Polysaccharides were evaluated for CTE applications for various advantages they uphold. Some of the major biodegradable polysaccharides investigated are CS, chitin, and alginate. The recent advances for each polymer are discussed. Chitin/Chitosan Chitosan is a natural linear polysaccharide obtained from chitin via deacetylation [52, 53]. CS’s structure resembles glycosaminoglycans in the ECM; thus, it is highly biocompatible and has low cytotoxicity. In addition, CS is easily conjugated as it has amino groups on its backbone. The biodegradable products of CS are nontoxic; so, it is safe to use in vivo. Accounted to the advantages CS upholds, it is a distinguished candidate for CTE applications [52]. A study investigated the leverage of thermally responsive CS hydrogel for delivery of SCs to the ischemic hearts and its ability to scavenging of the reactive oxygen species (ROS) ability in the surrounding microenvironment. The CS hydrogel was injected with adipose-derived MSCs (ADSCs) into rats with infarcted myocardium. The findings declared that the ADSC/CS hydrogel was capable of scavenging ROS generated by ischemic conditions that would normally impair adhesion of the cells and recruiting SDF-1, a major chemokine for ADSC homing. Hence, the addition of CS enhanced the microenvironment by scavenging the ROS, and improved SC engraftment, homing, and endurance in the infarcted tissue, leading to myocardial repair [53]. A more recent investigation used thermally responsive CS hydrogel for the delivery of MSCs into rat models with MI. Results demonstrated that the CS hydrogel enhanced cell attachment and increased graft size in the scar. In addition, it promoted the pluripotency of MSCs into CMs and enhanced the impacts of MSCs on neovascularization. Finally, the findings confirmed that the heart function and hemodynamics in infarcted tissue were improved as a result of the injection of CS hydrogel. Hence, CS presented a substantial potency in the conveyance of MSCs for the cure of MI [52]. Another study investigated the employment of CS hydrogels for the development of small-size vascular grafts. Many investigations were held to study the biocompatibility and hemocompatibility of the developed vascular graft. In vitro study concluded that CS improved the adhesion, growth, and endurance of EPCs. In

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addition, it showed that CS hydrogel did not promote platelet activation and is hemocompatible. In vivo researches conducted on rats and sheep demonstrated that CS did not induce chronic inflammation in rats and did not lead to flow obstruction in sheep. In addition, CS could withstand the arterial pressure when implanted in sheep for 3 days. Finally, this study demonstrated that small-diameter vascular graft developed from CS-based hydrogel is biocompatible and hemocompatible and promising for further investigations for clinical use [67]. To enhance the electrical properties and conductivity of BPs used in CTE applications, Baei et al. developed a thermosensitive CS-based porous injectable hydrogel incorporated with AuNPs to enhance its conductivity. The AuNPs enhanced electric cues between the neighboring cells in the CS porous scaffold. Multiple concentrations of AuNPs controlled the gelation and conductive characters of the hydrogel. MSCs were seeded on the AuNPs-CS hydrogel without electrical stimulation for 2 weeks. The developed scaffold maintained the viability, immigration, and growth of the MSCs in vitro. Besides, it stimulated the MSCs to differentiate to cardiac lineages and allowed uniform cellular constructs development that could be utilized in CTE [52]. Selenium is an essential element produced in the body; a deficit of selenium is observed in people with CVDs. It has a therapeutic property and is exploited in the protection against and treatment of CVDs. Another study developed a biocompatible CS-selenium nanoparticle (SeNP) film having high electric conductance and optimum tensile strength to function as a cardiac scaffold. In vitro studies were performed on the H9C2 cell line, and findings declared that the cell growth was enhanced. Furthermore, the SeNPs-CS film enhanced the electrical transmission and cell interactions, resulting in ameliorated cardiocyte activity and cardiac repair. Hence, SeNPs-CS film has the potency to be employed as a biocompatible substrate for cell growth in CTE applications [52]. Reis et al. developed a CS-collagen-based thermoresponsive hydrogel conjugated with angiopoietin-1-derived peptide (QHREDGS) to enhance the endurance and the metabolic activity of transplanted CMs. There was no interference detected in the structure, gelation, and mechanical characters of the gel upon supply of the QHREDGS. The constructs with high peptide concentration provided beating constructs with a higher rate compared to the control. Also, in vivo investigations using Lewis rats indicated that the injected hydrogel was localized at the seat of injection and maintained the cells that developed beating constructs after 1 week. In addition, when using the MI mouse model, the injected hydrogel was localized successfully at the site of injection [53]. A study conducted by Binsalamah et al. developed alginate-CS nanoparticles loaded with placental growth factor (PlGF). PIGF is known to enhance heart function and stimulate angiogenesis. In vivo researches depended on injection of the loaded alginate-CS nanoparticles into rats with acute MI. Results reported that the slow sustained release of PIGF from the nanoparticles into the injured myocardial tissue enhanced the LV function markedly, elevated the neovascularity, and decreased scar area formation. In addition, a sustained discharge of the PIGF was achieved over 120 h, thus maintaining the positive impacts of the PIGF in the damaged tissue [53].

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Alginate Alginate is a natural linear block co-polymer formed of linking β-D-mannuronate and α-L-guluronate, obtained from cell walls of brown algae and seaweed. In the presence of calcium ions, alginate is a negatively charged polysaccharide-based gel. Alginate is a non-thrombogenic, biocompatible, biodegradable, and non-toxic material. In addition, alginate has a similar structure to ECM. Thus, it is highly promising to be exploited for CTE applications [52–54]. A study developed an injectable calcium-cross-linked alginate hydrogel to reduce adverse cardiac dysfunction and remodeling due to MI. The absorbable in situforming alginate hydrogel was injected in infarcted tissue in rats after 1 week of anterior MI and into rats with old MI. The results revealed that the hydrogel reduced the scar thickness and decreased the diastolic and systolic dilatation and dysfunction in the LV in both old and recent MI rat models [53]. Another study developed an alginate-based release device for the conveyance of hepatocyte growth factor (HGF) and insulin-like growth factor-1 (IGF-1) to provoke myocardial reconstruction following MI. The IGF-1/HGF affinity-bound alginate biomaterial was injected into the infarcts of the acute MI rat model. The findings presented that the scar thickness was preserved, scar fibrosis was reduced, and its expansion was diminished. Furthermore, the angiogenesis was enhanced, cell apoptosis was attenuated, and endogenous revitalization of heart tissue was induced [53]. 3D alginate scaffold was produced by freeze-drying technique and was seeded with fetal heart cells to be transplanted in rats with MI. The outcomes showed that the implanted scaffold enhanced neovascularization, decreased LV dilation, and ameliorated the growth of embedded cardiac cells [68]. To improve cell attachment to alginate scaffolds, alginate conjugated to RGD was utilized to enhance cell adhesion and accelerate cardiac reconstruction in rodent models with ischemic cardiomyopathy. Results confirmed that the angiogenesis was enhanced, arteriole density was increased, and cardiac activity was improved [69]. Another study covalently attached a synthetic cyclic peptide Arg-Gly-Asp-DPhe-Lys (RGDfK) to improve the endurance of seeded cells and increase angiogenesis in infarct tissue. hMPCs were seeded on the amended alginate scaffold and transplanted in MI rats. Results revealed that no immunogenic response was induced, cell viability was enhanced, and angiogenesis in infarct was improved. In addition, it enhanced cardiac function. Thus, the modified seeded scaffold has a substantial potential to be exploited in the cure of MI [70]. A study developed alginate blended with polypyrrole (a conductive polymer) to study its therapeutic effects in CTE applications. The alginate-polypyrrole blend was injected in the MI in rats. The presence of polypyrrole enhanced arteriogenesis and increased the infiltration of myofibroblasts into the infarction site. This study highlights the potency to use an alginate-polypyrrole polymer as a successful scaffold for CTE [53]. To improve cell adherence and growth, fullerenol nanoparticles were embedded into alginate hydrogel with antioxidant function. The fullerenol/alginate hydrogel

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was seeded with brown adipose-derived SCs (BADSCs). The seeded hydrogel was transplanted in MI in rats. Results revealed that the hydrogel successfully scavenged ROS in the infarction microenvironment, thus enhancing the retention and survival level of BADSCs. The fullerenol nanoparticles laden to the hydrogel increased the differentiation of BADSCs. Finally, the hydrogel had no cytotoxicity of BADSCs [71].

Synthetic Biodegradation Polymers Synthetic BPs have unique chemicophysical assets that could be easily tuned to fit the target application. They have a controlled structure, strong mechanical properties, and high processing flexibility. Moreover, they do not have immunological concerns. These properties make them excellent candidates for CTE applications. Some examples of synthetic BPs include poly(glycolic acid) (PGA), poly(lactic acid) (PLA), poly(lactic-glycolic acid) (PLGA), polyethylene glycol (PEG), polycaprolactone (PCL), and polyurethanes (PUs) [52]. PLA, PGA, and PCL are semicrystalline aliphatic polyesters that are FDA approved for use in immediate contact with biological fluids. Furthermore, they are biocompatible, thermally stable, and cost-effective. Hence, they are broadly employed in medical applications including CTE applications [54]. The chemical structure of the synthetic BPs employed for CTE is presented in Fig. 6.

Poly(Lactic Acid) (PLA) Poly(Lactic Acid) (PLA) is a synthetic semi-crystalline aliphatic hydrolyzable polyester, obtained by polymerization of lactic acid. It is a biocompatible, biodegradable, and hydrophobic polymer with a low degradation rate. The degradation product is the lactic acid that is non-cytotoxic and is easily metabolized by neurons or oxidized into pyruvate which is converted to glucose [54]. There are four stereoisomers of PLA known as poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), poly(D, L-lactic acid) (PDLLA), and meso-PLA. PDLLA and meso-PLA have an amorphous structure, while PLLA has a semi-crystalline structure, and PDLA has a crystalline structure. The most extensively used form in CTE applications is the PLLA, as it has favorable biodegradability, biocompatibility, and mechanical integrity and is non-toxic. However, one main drawback of PLLA when used in CTE applications is the long degradation time [52]. A study developed a porous nanofibrous PLLA scaffold that mimics the ECM, seeded with cardiovascular progenitor cells (CPCs) originating from mouse embryonic stem cells (ESCs) to develop cardiac tissue in vitro. The scaffold enhanced cell attachment, multiplication, and pluripotency. In vivo study included implanting the developed scaffold subcutaneously in a nude mouse model. The scaffold sustained the cell endurance and enhanced their differentiation into CMs, smooth myocytes, and endothelial cells. Hence, the developed scaffold enhanced the fabrication of the cardiac constructs from CPCs. This study highlighted the potential use of nanofibrous PLLA scaffolds seeded with CPCs for CTE [72].

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H C H

O= C =N

O C N H

N = C =O +

H C H

H H HO C C OH H H

O H H N C O C C O H H H n

Fig. 6 Schematic illustration of the chemical structure of synthetic BPs employed for cardiac tissue engineering

Another study developed aligned PLLA sheets seeded with diaphragmatic myoblasts (DM) overexpressing connexin-43 (cx43) to diminish the infarction and ameliorate the heart activity. Sheep model with acute MI was utilized in the in vivo investigations. The results demonstrated that PLLA sheets with DM overexpressing cx43 markedly reduced the infarct size compared to the other groups and the control. In addition, it increased the capillary density and enhanced angiogenesis and decreased fibrosis in the scar borders. Finally, it significantly enhanced ventricular function. Thus, the findings demonstrate the potential use of the developed PLLA sheets with DM overexpressing cx43 in clinical settings [73]. Wang et al. developed conductive nanofibrous sheets from a blend of PLLA and polyaniline (PANI) for CM-based 3D bioactuators. PANI is a highly electrical conductive synthetic polymer. The developed nanofibrous sheet mimicked the ECM structure and showed improved conductivity. Also, the increase of PANI from 0 to 3 wt% enhanced the conductivity. In in vitro studies using H9C2 rats, cardiomyoblasts were performed. The results presented improved cell viability, differentiation, and maturation. In addition, cell-cell interaction was enhanced, and beating CMs were developed. When comparing the developed sheets with PLA nanofibrous sheets, results showed an enhanced conductivity, higher frequency of beating, and displacement. Hence, the CMs seeded PLA/PANI nanofibrous can form 3D bioactuators with fibrous and folding shapes and have potential use in CTE [74].

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Poly(Glycolic Acid) (PGA) Poly(glycolic acid) (PGA) is a synthetic linear aliphatic biodegradable polyester. PGA is a biocompatible, hydrophilic, highly crystalline biomaterial that has non-toxic degradation products. Glycolic acid is the degradation product of PGA, and it is easily metabolized and excreted from the body [54]. Its hydrophilicity and rapid water uptake prevent it from being used in vivo, due to its mechanical strength loss within 4 weeks after implantation. However, this characteristic is exploited in CTE applications as PGA is used as temporary support or scaffold. In addition, PGA has low cell adhesion capability when its surface is untreated; hence, the surface amendment is introduced to reinforce the PGA’s properties and promote cell adherence and proliferation [52]. Bursac and Papadaki and other colleagues developed a 3-D PGA scaffold for electrophysiological studies on the in vitro model system of the cardiac muscle. The PGA scaffold was seeded with primary neonatal rat ventricular cells containing different fractions of CMs; then the seeded scaffold was cultured in bioreactors developing the cardiac muscle constructs. The scaffold supported cardiac myocytes’ differentiation and organization in 3-D configuration. In addition, metabolic activity and cell size were similar to native neonatal rat ventricles. Furthermore, the developed 3D seeded scaffold exhibited significant electrical assets and maintained impulse propagation. Finally, the fraction of CMs seeded altered the electrophysiological properties of the construct, in which enriched constructs showed higher electrophysiological than regular constructs. The study highlights that tailoring of the 3-D heart muscle constructs to have specific electrophysiological and structural properties makes them of high potential use for CTE [75]. Another recent study combined PGA with trimethylene carbonate (TMC) to develop a suture. The study investigated their optical, mechanical, structural, and biological assets for their use in CTE. The results of the stress-strain study proved that the suture has high mechanistic power and has similar compatibility with the sternum; hence, it could be exploited in heart operations [76]. Poly(Lactic-co-Glycolic Acid) (PLGA) PGA and PLA do not have the optimum elasticity that matches the elasticity of the native cardiac tissue. Thus, PGA and PLA are combined forming a block copolymer called PLGA to overcome the shortcomings of each polymer alone and reach the desired properties. PLGA is non-toxic, biocompatible, and biodegradable and has tunable mechanical characters and an adjustable degradation rate. Therefore, it is extensively used in CTE applications [52, 53]. However, PLGA has some drawbacks including its acidic degradation yields and weak cell attachment [52]. A study developed an SDF-1 alpha/PLGA core-shell particle by coaxial electrospraying. The developed delivery system is aimed to be an injectable treatment for MI, as SDF-1 alpha promotes the recruitment of endogenous SCs into infarct tissue. In vitro studies used MSCs for the assessment of migration and multiplication. The results revealed that there was sustained discharge of SDF-1 alpha for at least 40 days. In addition, bovine serum albumin (BSA) was co-encapsulated in the

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core of the particle to tailor the discharge rate. PLGA/BSA-SDF-1 alpha particles showed enhanced migration and multiplication of the MSCs. Hence, the developed system has a great potential to be used in CTE [65]. Another study used carbon nanofibers (CNFs) with PLGA to develop composites for CTE applications. Human CMs were used in the in vitro studies. The results revealed that CMs’ function was enhanced when seeded on the hybrid composite as the aligned CNFs provided better cell adsorption and attachment and higher hydrophobicity when compared to pure PLGA. Important cardiomyocyte biomarkers including Connexin-43, Troponin-T, and α-sarcomeric actin were highly expressed indicating the enhanced cardiomyocyte function when using the aligned CNFs in PLGA as compared to randomly oriented CNFs in PLGA. Finally, the hydrophobic aligned CNFs in PLGA attracted the key proteins for adsorption mediating the CM’s adhesion and multiplication. The results stage the floor for exploring the CNFs/ PLGA composite in CTE applications [52]. A study by Huang et al. prepared porous PLGA beads and seeded them with human amniotic fluid SCs (hAFSCs) to be investigated as cellularized microscaffold. The in vivo studies used a rat model with MI. The seeded PLGA porous beads were injected into the infarct. The assays demonstrated that the cells in the beads were viable and formed a 3D organization complex with well-preserved ECM. The implantation of the seeded beads was localized in the site of injection and cells differentiated into cardiomyogenic and angiogenic lineages. In addition, results showed that the implant improved cardiac function, inhibited ventricular expansion, and maintained the gross morphology [53].

Poly(ethylene glycol) (PEG) Poly(ethylene glycol) (PEG) is a biodegradable synthetic polymer. It is acquired from ethylene oxide ring-opening polymerization. It is non-toxic, has low protein binding and cell adherence, and is soluble in both organic and inorganic solvents. However, PEG is bioinert, so it does not support cell adherence and survival. However, it is still investigated as a biomaterial for CTE as it has adjustable chemical, structural, and mechanical characteristics [52]. Somekawa et al. developed a thermoresponsive biodegradable injectable gel made of PLLA-PEG and PDLA-PEG mixed giving PLLA-PEG/PDLA-PEG suspension, to assess its efficacy on the remodeling of the LV after MI. In vivo studies use rat model with MI. The positive control in the experiment was alginate gel, while the experimental control was saline. PLLA-PEG/PDLA-PEG gel and alginate gel enhanced the percent fractional shortening and decreased the infarct size as compared to the experimental control. In addition, the PLLA-PEG/PDLA-PEG gel showed a significant reduction in the LV cavity area as compared to both other groups. Thus, the PLLA-PEG/PDLA-PEG gel has great potential as a gel therapy for remodeling the LV after MI [77]. Another study developed a thermosensitive-based PEG hydrogel comprised of poly(δ-valerolactone)-block-poly(ethylene glycol)-block-poly(δ-valerolactone) (PVL-b-PEG-b-PVL) combined with VEGF. Rat model with MI was used for in vivo assessment of the developed hydrogel. The developed hydrogel provided

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a sustained discharge of VEGF. The results presented an elevated vessel density, the thickness of the infarction, and cardiac function. In addition, further cardiac remodeling was reduced [78]. A study investigated the delivery of bone marrow-derived SCs (BMSCs) through an injectable hydrogel composed of α-cyclodextrin/poly(ethylene glycol)–b-polycaprolactone-(dodecanedioic acid)-polycaprolactone–poly(ethylene glycol) (CD/MPEG–PCL–MPEG) into infarcted myocardium. In vivo studies were performed on the MI rabbit model. The hydrogel injected alone did not induce angiogenesis; however, it supported the infarct and inhibited the remodeling of the LV. After 1 month of injection of the BMSCs with the CD/MPEG–PCL–MPEG hydrogel, the gel was absorbed, and the cell retention and survival rate increased. In addition, vessel density increased, and scar expansion was prevented. Furthermore, the ejection function of the LV was enhanced, and LV dilatation was prevented. The study showed the prospect of using the CD/MPEG–PCL–MPEG hydrogel for cellular transplantation for MI treatment [79]. Dobner et al. investigated the effect of PEG hydrogel on preventing cardiac remodeling after MI in the short and long terms. The in vivo studies were conducted on a rat model with MI. Over the short term (4 weeks), the infarct-induced LV end-diastolic diameter (LVEDD) decreased significantly, and LV wall thinning was inhibited. However, in the long run (13 weeks), the expansion of the LVEDD was not prevented, and the inflammatory reaction was induced against the injection material. The study highlights the effect of PEG hydrogel in preventing cardiac remodeling in the short-term healing phase only [80]. Another study developed a responsive PEG-based hydrogel-forming scaffold in situ, to transport thymosin β4, and vascular cells originating from human ESCs. The study used a rat model with heart ischemia for the in vivo experiments. The hydrogel substituted the ECM in the infarct and provided structural organization for the native endothelial cells. Also, the delivered vascular cells enhanced angiogenesis in the infarction. Furthermore, the contractile performance was enhanced, the infarct size decreased, the vessel density increased, and LV dilation reduced. This study highlights the role of dual conveyance of drugs and cells through a biodegradable scaffold for CTE [81].

Polycaprolactone (PCL) PCL is a biodegradable synthetic polymer. It is obtained by polymerization of ε-caprolactone. It has outstanding mechanical characters, unique biocompatibility, and outstanding toughness and elasticity; thus it is widely explored as a biomaterial in CTE applications. However, its long degradation time hinders its use in CTE applications. Hence, PCL is mainly blended with another material, used with a filler material, or used as a copolymer to adjust its degradation properties [52]. Figure 7 elaborates the pivotal role of PCL nanocomposites in the amelioration of CTE. A study developed an organized ultrafine fiber scaffold by using hydroxylfunctionalized PCL named (poly(hydroxymethylglycolide-co-ε-caprolactone) (pHMGCL) via melt electrospinning writing (MEW) for CTE. The developed

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Fig. 7 Role of biodegradable polycaprolactone nanocomposites in cardiac tissue engineering

scaffold via MEW enhanced cellular reaction to mechanical anisotropy. In addition, improved alignment of the CPCs on the pHMGCL scaffold was obtained as compared to PCL scaffolds. Eventually, this study proved the reproducibility of the scaffold via the MEW fabrication technique and the ability of the highly organized scaffold to support cellular growth and survival [82]. Another elegant study by Ghaziof et al. developed a composite scaffold comprised of PCL and different amounts of multiwalled carbon nanotube (MWCNT). The scaffold was prepared by solvent casting and vacuum drying technique. The platform had a highly interconnected porous structure. In addition, the supply of 1 wt % of MWCNT enhanced the conductivity and mechanical assets of the platform. This study highlights the potential use of PCL/MWCNT scaffolds for CTE; however, further in vitro and in vivo investigations are still required [83]. In one work, a highly porous, hydrophilic, biodegradable, elastic, and structurally stable scaffold was developed from a composite of poly(dl-lactide-co-caprolactone), poly(dl-lactide-co-glycolide) (PLGA), and type I collagen. The developed scaffold was seeded with neonatal rat heart cells seeded in Matrigel. As a control for this study, PLGA and collagen sponges were used for comparison. The developed composite scaffold enhanced contractility and cellularity and improved the expression of heart markers compared with the controls [84]. In that study, polypyrrole was blended with PCL to enhance its electrical conductivity. The blend was exploited as a conductive electroactive substrate for cardiomyocyte culture to develop functional cardiac cells sheets. The polypyrrole-

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PCL had similar resistivity to the native cardiac tissue. The developed composite enhanced the CM’s attachment and cell density. Furthermore, the cells showed enhanced peripheral localized of the gap junction protein Cx43. In addition, the calcium transitory period was reduced, and calcium wave propagation velocity was improved, as compared to the control PCL-sodium-treated sheet. Finally, the polypyrrole-PCL showed high potential in being used as a conductive substrate for cardiac cell growth with improved functional properties; thus it could be further investigated for CTE applications [85]. In one study, a 3-D cardiac graft was developed by seeding neonatal rats’ CMs on electrospun nanofibrous poly(epsilon-caprolactone) meshes that have an ECM-like topography. After 2 weeks of cell culture, the CMs had improved attachment to the mesh and had a strong beating. The developed mesh could be layered up to 5 layers without causing ischemia in the core of the constructs and keeping the electrical communication between layers. The individual layers were well adhered to. Furthermore, results demonstrated synchronized beating. This study highlights the development of thick grafts in vitro that could be used in CTE [86]. In another sophisticated study, a poly-glycolide-co-caprolactone (PGCL) scaffold was seeded with bone marrow-derived mononuclear cells (BMMNCs). The seeded scaffold was then implanted into a rat model with MI. BMMNCs successfully migrated into the epicardial area, promoting neovascularization. Moreover, the LV systolic function was improved, and the LV dilatation and remodeling were attenuated. Finally, the BMMNCs promoted differentiation into CMs [87].

Polyurethanes (PUs) Polyurethane (PU) is a class of synthetic biocompatible materials used extensively in biomedical applications. PUs are acquired from the polymerization of diisocyanates and diols with different configurations. PUs have unique stability, mechanical properties, biocompatibility, and long durability. Moreover, PU’s durability could be tailored from a few days up to years. However, PUs have a very long degradation time limiting their usage in CTE applications. Hence, modifications could be done to PUs to enhance their degradation time to match the desired application [52]. A study incorporated reduced graphene oxide-silver (rGO-Ag) as a nanostructure filler material into PU via electrospinning technique to enhance its electromechanical characters. The platform was seeded by human cardiac progenitor cells (hCPCs) for in vitro studies. The addition of rGO-Ag into the PU nanofibers enhanced the tensile strength, electrical conductivity, and wettability, significantly. Additionally, results of in vitro investigations indicated good cytocompatibility of the platform and an improved cellular attachment. Furthermore, the results exhibited that the platform induced cardiogenic differentiation and upregulated cardiac-specific genes such as cardiac troponin T (cTnT), alpha-myosin heavy chain (α-MHC), GATA-4, and T-box 18 (TBX 18). Therefore, the developed rGO-Ag/PU scaffold could be employed in CTE applications [88]. In another work, PU films were coated with microcontact printing of laminin lanes. The laminin-coated PU was exploited to investigate the spatially organized cardiac cell cultures. The patterned CMs exhibited similar morphology as compared

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to other substrates. In addition, secondary and tertiary cell populations grew and aligned on top of the primary patterned cells, providing an organized multilayered construct with a thickness of two to three cell layers. This study highlights the potential of transplanting organized cardiac tissue constructs for CTE [89]. Another study by Shokraei et al. incorporated different weight ratios of MWCNTs into PU nanofibers to develop a patch via simultaneous electrospray/spinning technique. The MWCNTs improved the tensile strength, Young’s modulus, hydrophilicity, and electric conductance of the patch. Besides, the cytocompatibility of the patch for human umbilical vein endothelial cells (HUVECs) and H9c2 cells, with enhanced cell attachment, viability, and proliferation, was confirmed. Furthermore, the fabrication method used enhanced the interactions between the cardiomyoblasts and the MWCNTs/PU patch [90]. In another study, a highly porous pretreated PU scaffold was seeded with rat skeletal myoblasts and then implanted into the heart of the rat model with MI. Laminin coating on the PU scaffold provided the highest cell attachment. Moreover, the spatial distribution of myogenic cells and cell survival was maintained in vitro and in vivo. However, there was no found evidence of differentiation in vivo [91]. Another study investigated the functionality of the myoblast seeded PU scaffold for CTE in a rat model with MI. The ejection fraction was maintained stable, and the contractility of the LV increased. Moreover, high cell density was observed in the scaffold with no migration to host tissue; however, there were no indications of cardiac cells differentiation [92]. These studies highlighted the leverage of the PU scaffold to deliver myoblasts into infarcted cardiac tissue and prevent the progression of post-MI. Further studies using cell differentiation promoting agents are still required to promote the implementation of these scaffolds in CTE.

Natural/Synthetic Hybrid Biodegradation Polymers BPs are highly studied for their applications in CTE. Natural BPs have various advantages including low cytotoxicity, high biodegradability, and renewability. Furthermore, synthetic BPs have enhanced characteristics including better conductivity, controlled degradation rate, and higher mechanical strength. Thus, forming a hybrid of natural and synthetic BPs allows the benefit from the advantages of both polymer categories and provides better composites with enhanced properties for CTE applications [52]. They also exhibit an outstanding capacity to improve cell viability, maturation, differentiation, and interactions between the cells themselves or between the cells and the BP scaffolds as illustrated in Fig. 8. Table 2 provides an overview of some natural/synthetic hybrid BPs employed for CTE.

Poly(Lactic Acid)/Chitosan Wang et al. developed poly(lactic acid) (PLA) and CS core/shell nanofibers by electrospinning to be exploited as a vascular gasket. Various concentrations of PLA/CS were tested for the optimum surface morphology. CS was amended by heparin and cross-linked with genipin. The developed PLA/CS core/shell nanofibers

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Fig. 8 Schematic illustration of the multifunctional roles of the hybrid biodegradable polymeric scaffolds to enhance cardiac tissue engineering outcomes

had better elasticity and mechanical power than commercial vascular patches and pure PLA nanofibers. Moreover, the developed PLA/CS core/shell nanofibers showed longer durations of activated partial thromboplastin time (APTT) and prothrombin time (PT) when compared to refined PLA fibers. Finally, in vitro blood flow studies revealed that there was no significant adherence of red blood cells on the PLA/CS core/shell nanofibers, making it a potential hybrid polymer for vascular gaskets [52, 93]. Another study investigated the structure, hydrophilicity, mechanical strength, and interactions of the PLA/CS fibrous scaffold with cells. Results showed that hydrophilicity and mechanistic power increased with decreasing CS concentration in the hybrid scaffold. Furthermore, the orientation of fibers in the scaffold affected the biocompatibility and the mechanical strength, where aligned fiber scaffolds showed superior biocompatibility and higher mechanical power than randomly oriented fiber scaffolds. In addition, aligned fiber scaffolds with a PLA/CS ratio of 7:1 (w/w) promoted the regeneration of myocardia and enhanced the CM’s attachment and viability [52, 94].

Gelatin/Polycaprolactone/Graphene A study investigated the use of electrospun Gt/PCL/graphene nanofibrous scaffolds for their outstanding electric conduction property in CTE especially bypass tracts for

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Table 2 Employment of natural/synthetic hybrid biodegradation polymers for cardiac tissue engineering Composition of hybrid composite PLA/ chitosan

Model of study In vitro study (UE7T-13 cells)

Merits Better elasticity Improved mechanical strength

PLA/ chitosan

In vitro study (primary cardiomyocytes isolated from neonatal rats)

Enhanced hydrophilicity, mechanical strength, and cell-scaffold interaction

Gelatin/PCL/ graphene

In vitro study (neonatal rat ventricular myocytes) -In vivo study (rat model) In vitro study (neonatal rat cardiomyocytes)

Enhanced electrical conductivity, and biological properties

TiO2-PEG/ chitosan

Collagen/ CNTs

In vivo study (rat model with MI)

Enhanced biological properties, swelling behavior, and physicochemical characters

Better mechanical support, enhanced adhesion, and elongation of cells

Significance of work Longer durations of activated partial thromboplastin time and prothrombin time No attachment of blood cells Potential use as vascular gaskets Aligned fibers scaffolds with a PLA/CS ratio of 7:1 (w/w) promoted the regeneration of the myocardium and enhanced the cardiomyocytes’ attachment and viability. -Potential use in cardiac tissue repair Enhanced cell growth, and cell survival rate. Potential use as atrioventricular block treatment Favorable cellhydrogel matrix interaction with an enhanced synchronous activity Better cell adhesion and cell retention activity Potential use as cardiac patches CNTs enhanced myocardial regeneration CNTs improved the electrical conductivity of the scaffold Enhanced alignment of neonatal

References [93]

[94]

[95]

[96]

[59]

(continued)

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Table 2 (continued) Composition of hybrid composite

Model of study

Merits

Collagen/ AuNPs

In vivo study (rat model with MI)

Enhanced electrical conductivity

Collagen/ Fibrin

In vitro study (hiPSCs derived cardiomyocytes)

Controlled scaffold composition and seeding density

Gelatin/ hyaluronic acid

In vitro study (mice L929 fibroblasts)

Fibrin/PEG

In vivo study (mouse model with acute MI)

Controlled porosity, increased mechanical strength, and enhanced degradation resistance of the scaffold Loaded with SDF-1alpha

Significance of work ventricular myocytes and cardiac function Potential use as cardiac constructs Improved cardiac function Reduced the scar size Increased the blood vessel density Pure collagen scaffolds with higher cardiomyocytes purity resulted in beating tissues Highest tissue compaction was achieved by using decreased collagen concentration, increased fibrin concentration, moderate cardiac purity, and increased seeding density Appropriate for cell growth Cytocompatible Potential use in cardiac tissue repair

Reduced scar area An enhanced function of the left ventricle

References

[98]

[99]

[100]

[65]

treating atrioventricular block. In vitro experiments included seeding the hybrid scaffolds of different graphene concentrations with neonatal rat ventricular myocytes. The in vivo phase included implanting the hybrid scaffolds of different graphene concentrations into rats. Cell growth and survival were promoted on hybrid scaffolds with graphene mass fractions lower than 0.5%. In addition, no inflammatory response was induced in the rats with implanted scaffolds after 4, 8, or 12 weeks. Finally, the hybrid scaffold showed promising biological characters and

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electric conductance, allowing them to be furtherly studied for exploitation in atrioventricular block treatment [52, 95].

Titanium Dioxide-Polyethylene Glycol/Chitosan Liu et al. developed a hybrid nanocomposite with enhanced efficiency and biocompatibility for CTE. The hybrid nanocomposite is composed of titanium dioxide (TiO2) nanoparticles incorporated in PEGylated CS hydrogels (Fig. 9a1–2). It provided enhanced biological and physicochemical properties and promoted cell adhesion and cell retention activity of the CMs on the hydrogel. The TiO2 nanoparticles in the PEGylated CS hydrogels provided better young modulus and swelling properties. It also provided better cell-hydrogel matrix interaction with enhanced synchronous activity as compared to PEGylated CS hydrogels without TiO2 nanoparticles (Fig. 9b). Hence, the developed hybrid hydrogel has a high capacity to be employed in cardiac patches for CTE [52, 96].

Fig. 9 The potency of titanium dioxide-polyethylene glycol/chitosan (TiO2-PEG/CS) hybrid biodegradable nanopolymers for cardiac tissue engineering. (a) Preparation scheme of TiO2-PEG/ CS composites (A.1) with the evaluation of their morphology using SEM (A.2). (b) Evaluation of the ability of TiO2-PEG/CS to enhance cardiomyocyte growth and adhesion using SEM and confocal microscopy. (Adapted with permission from [96]. Copyright © 2015, Elsevier Publishing Group)

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Collagen/Carbon Nanotubes Diverse studies have investigated the effect of adding CNTs to collagen hydrogels. The incorporated CNTs enhanced myocardial regeneration and improved the electric conductance of the platform. In addition, the results indicated significant enhancement in the alignment of neonatal ventricular myocytes and a remarkable amelioration of cardiac function [59]. The supply of CNTs provided better mechanical support and enhanced the adherence and elongation of cells. Moreover, the CNTs/ collagen scaffolds assisted to enhance CTE via enhancement of cardiomyocytes viability, adhesion force, and contractility [97] as presented in Fig. 10. The promising outcomes of the study made the CNTs/ collagen hydrogel a promising candidate for cardiac constructs in CTE [54]. Collagen/Gold Nanoparticles A study conducted by Hosoyama et al. indicated that the use of electro-conductive materials such as gold nanoparticles (AuNPs) and silver nanoparticles in preparing collagen-based cardiac patches enhanced its therapeutic efficacy in the treatment of MI in rat models. Results showed that only collagen patches containing AuNPs were

Fig. 10 The potency of collagen/carbon nanotubes (Col/CNTs) hybrid biodegradable nanopolymers for cardiac tissue engineering. (a) Preparation scheme of Col/CNT hybrid nanocomposites and their SEM evaluation followed by their seeding with cardiomyocytes. (b) Amelioration of the cell viability using Col/CNT scaffolds. (c) Amelioration of the cell contractility using Col/CNT scaffolds. (Adapted with permission from [97]. Copyright © 2017, American Chemical Society (ACS) Publications)

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stable, and ameliorated the cardiac activity, as there was a marked decrease of the scar size, along with a prominent rise in the blood vessel density [52, 98].

Collagen/Fibrin Kaiser et al. designed a blended scaffold made of collagen and fibrin and seeded it with CMs derived from iPSCs. Rat tissue samples were exploited to assess the interactions among the cell-scaffold and the treated tissue. Results indicated that the concentration of fibrin and collagen, seeding intensity, and cardiac purity impacted tissue compaction. In case of increased compaction, the fibrin concentration and seeding density were high. While in the case of decreased compaction, the collagen concentration was low. In addition, the highest tissue compaction was achieved by using decreased collagen concentration, increased fibrin concentration, moderate cardiac purity, and increased seeding density. However, only pure collagen scaffolds with higher CM purity resulted in beating tissues. This study highlighted that the development of tissues with optimal function and efficiency, the scaffold composition, and seeding intensity should be highly investigated for their potential in CTE applications [99]. Gelatin/Hyaluronic Acid Zhang et al. (2011) investigated the cytocompatibility of hybrid 3D macroporous platforms prepared from Gt/HA (GE/HA) via freeze-drying and then crosslinked using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Mice L929 fibroblasts were employed for the evaluation of in vitro compatibility of the platform. The scaffold’s porosity could be managed by varying ratios of HA to Gt; by increasing the HA content, the degradation level and swelling ratios of the scaffold increased. In addition, the use of EDC to crosslink the scaffold increased the mechanical power significantly and enhanced the degradation resistance in the culture media. The study indicated that the hybrid scaffolds are appropriate for cell growth and confirmed their cytocompatibility. Hence, the GE/HA scaffold holds enormous potential for TE applications including CTE [53, 100]. Fibrin/Polyethylene Glycol A study conducted by Zhang et al. used a PEGylated fibrin patch loaded with SDF-1alpha, which is an SC factor that emerges a major role in mobilizing SCs from bone marrow to damaged tissue and is upregulated after MI. The loaded patch was implanted into a mouse model with acute MI. Results presented that the scar size was markedly reduced and the activity of the LV was significantly enhanced compared with the unloaded patch [53, 65].

Conclusion To summarize, in this chapter, we have thrown the light on the existing strategies employed for the CTE that includes the exploit of patches and cells, heart TE approaches, scaffoldless techniques, cell assembly, ECM decellularization, and

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neovascularization approach. The degradation methods of the natural and synthetic BPs were highlighted. Then, we presented in detail the BPs utilized for CTE. Many natural (proteins and polysaccharides) and synthetic polymeric systems were exploited to revitalize the heart tissue. Moreover, a detailed discussion of the hybrid polymers prepared through the mixture of these two types was outlined. Their potential for the reconstruction of the heart was highlighted. Hybrid BPs exhibited pivotal roles in the amelioration of CTE outcomes via enhancement of new tissue formation and neovascularization in vivo. Besides, they improved seeded cell proliferation, alignment, migration, and adhesion in vitro.

Future Perspectives Over the few past decades, many investigations have been conducted to ameliorate the outcomes of CTE. Consequently, a paradigm shift in the designing, fabrication techniques, and characters of biodegradation polymeric scaffolds has been displayed specially for CTE. Furthermore, several investigations confirmed that the nanocomposite polymeric scaffolds are highly promising for the same purpose. However, the ideal biodegradation polymer that fits all the needs of CTE is yet not present. Hence, biocompatibility is an additional pivotal asset that is essential to be deemed during the choice of the appropriate patch. So, additional in vivo, in vitro, and clinical studies on the biocompatible polymers are requested. Besides, the fabrication of polymeric composites via the combination of BPs and other substances is an auspicious option in the same track. Such hopeful composites are promising to ameliorate the CTE, attain a strong and healthy heart, and confirm the notion that a healthy heart equals happy life.

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Marwa A. El-Gammal, Ahmed Saad Elsaeidy, Hamid Ashry, and Afnan W. M. Jobran

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmaceutical and Personal Care Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wastewater Treatment Plants and Pharmaceutical and Personal Care Products . . . . . . . . . . . . . . Pharmaceutical and Personal Care Products and Human Interactions . . . . . . . . . . . . . . . . . . . . . . . . Biological Transformation of PPCPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Parent Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Aquifer Treatment and Activated Sludge Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bacterial Species Included in MFC A/O System’s Biodegradation of PPCPs and Aromatic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradability of Pharmaceutical and Personal Care Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmaceutical and Personal Care Products Biodegradability Categories . . . . . . . . . . . . . . . . . . . . Factors Affecting Pharmaceutical and Personal Care Products Biodegradability . . . . . . . . . . . . . Methods to Analysis Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ready Biodegradability: OECD 301 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inherent Biodegradability: OECD 302 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automated Determination of Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diclofenac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M. A. El-Gammal (*) Nanotechnology Department, School of Science and Engineering, American University of Cairo, Cairo, Egypt e-mail: [email protected] A. S. Elsaeidy Faculty of Medicine, Benha University, Benha, Egypt H. Ashry Biochemistry branch, Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt A. W. M. Jobran Faculty of medicine, Al Quds University, Jerusalem, Palestine © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_49

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Ibuprofen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbamazepine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The biodegradation process is defined as the breakdown of the chemical and synthetic materials, converting them to environmentally acceptable, nontoxic products, where with the help of microorganisms like fungi and bacteria, these materials are converted through a series of intermediates into an end product that is friendly to the environment, which shares in the solving of many critical problems that are related to environmental pollution. One of the most important biodegradable materials is pharmaceuticals and personal care products (PPCPs). PPCPs are a group of micropollutants that threaten the environment and include drugs (e.g., antibiotics, painkillers, depressors, contraceptive drugs). Also included are personal care products (e.g., perfumes, cosmetics, sunscreen, hair spray), and researchers have paid extensive attention to PPCPs biodegradation and the effect of their prevalence in rivers and their severe detrimental effects on the environment and human health. Keywords

Biodegradation · PPCPs · Pharmaceutical biodegradation · Pathways of PPCPs biodegradation · PPCPs biotransformation Abbreviations

ACE AST ATE BEZ BOD BPA BTEX CA-HA CA-Ibu CAS CBT CBX-IBF CBZ COD CYP DCF

Acesulfame Activated sludge treatment Atenolol Bezafibrate Biological Oxygen Demand Bisphenol A Xylene Carboxy-hydratropic acid Carboxy-ibuprofen Conventional activated sludge treatment plant Closed bottle test Carboxy-ibuprofen Carbamazepine Chemical Oxygen Demand Cytochrome P450 Diclofenac

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DEET DIA DOC E1 E2 EC50 EE2 EPCBZ FBR HPLC-MS/MS HRT IBF IBU IMS LEV MBR MFC MI TI MTBE NP NPEOs NSAID OH-Ibu OM OP OSE PAH PPCPs SAT SBR SCAS SMX SPE SRT STPs TBA TCS ThOD TPs VAL WTPs WWTPs

N, N- Diethyl-meta-toluamide Didiazepam Dissolved oxygen concentration Estrone 17b-estradiol Half maximal effective concentration 17a-ethinylestradiol Epoxycarbamazepine Fixed bed reactor High-performance liquid chromatography-tandem spectroscopy Hydraulic retention time Ibuprofen Ibuprofen Iminostilbene Levetiracetam Membrane bioreactor Microbial Fuel Cell Ministry of International Trade and Industry Methyl tert-butyl ether Nonylphenol Nonylphenol polyethoxylated surfactants Non-steroidal anti-inflammatory medications Hydroxy-ibuprofen Organic material Octylphenol Oseltamivir Polycyclic aromatic hydrocarbon Pharmaceutical and personal care products Soil aquifer treatment Sequencing batch reactor Semi-continuous activated sludge test Sulfamethoxazole Solid-phase extraction Solids retention time Sewage treatment plants Tert-butyl alcohol Triclosan Theoretical Oxygen Demand Transformation products Valsartan Water treatment plants Wastewater treatment plants

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Introduction Pharmaceutical and personal care products (PPCPs) are essential for maintaining our lives. Due to the growth of the population, the industries of PPCPs manufacturing have become more popular, but despite the great benefits of these products, and that they are helping us by playing their roles as medicines for humans and animals, hygiene products, and even as perfumes and cosmetics, they can cause significant harm to our health during their post-use stage. If these PPCPs did not get the proper mechanisms for their removal, they would prevail in the environment. As a result, they would reach the surface water groundwater and cause contamination to the drinking water. They would also enter the food chain due to water contamination, which would then be drunk by humans, animals, or even used in agriculture. This kind of contamination can affect human health by affecting the major human systems and organs like the liver, kidneys, and central nervous system, and could even be lethal if they exceed certain limits. They can affect the aquatic environment and even the microorganisms by developing more resistant bacteria due to the release of antibiotics to the environment without proper treatment, that is why it is crucial to find and add developed and cost-effective techniques in addition to the conversational techniques that are not sufficient to be used alone in the removal of PPCPs from the environment. One of the most important methods is biodegradation, which is carried out with the help of microorganisms to degrade these compounds into simpler and safer compounds that are friendly to the environment. So, in this chapter, we will represent more about the biodegradation of PPCPs, their biotransformation, their pathways of biodegradation, and the microorganisms specific for these processes to occur (Fig. 1).

Pharmaceutical and Personal Care Products Pharmaceutical and personal care products are materials used for medical or personal care purposes; the metabolites produced from the transformation of these substances can also be considered PPCPs. Examples of PPCPS are sunscreen agents (e.g., Oxybenzone), medicines (e.g. Ibuprofen), and antidepressants (e.g., Diazepam), antibiotics (e.g., sulfamethoxazole (SMX)), and antineoplastics that are the drugs Fig. 1 The main sources of PPCPs that cause environmental pollution

Large farms PPCPs factories laboratories and hospitals Sewage systems

sources of PPCPs found in the environment

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Table 1 Pharmaceutical and personal care products (PPCPs). (Adapted with permission from Ref. [1], Copyright 2019, Elsevier) PPCPs category Pharmaceuticals

Personal care products

Subcategory Antibiotics, analgesics, antimalarial drugs, antiseptics, hormones, steroid and endocrine-disrupting products, anti-inflammatory, antifungal, antiepileptic and antianxiety drugs, cytotoxic drugs, anticancer drugs, cytostatic drugs, beta-blockers, estrogen, lipid regulators, and anticonvulsants Moisturizers, hair colors, deodorants, toothpastes, sunscreen, detergents, disinfectants, preservatives, fragrances and perfumes

used for cancer treatment and preservative treatments such as 17a-ethinylestradiol (EE2) that are used as birth control pills (Table 1). The products used for hygiene are also considered PPCPS, such as fragrances, synthetic masks, and antibacterial hand soap. Protective products such as N, N- Diethylmeta-toluamide (DEET), the active compound of most insect repellents, is considered such as Triclosan (TCS), and veterinary drugs can also be included in PPCPS [2]. Due to the widespread use PPCPs, it is not easy to be metabolized inside the human body. Their improper disposal can be found along with their metabolites in water treatment plants (WTPs), sewage treatment plants (STPs), seawater, and drinking water. PPCPs are carried into STPs they become partially digested, they disrupt biological treatment processes, rendering conventional STPs ineffective for their removal [3]. Although PPCPs have been tightly regulated to reduce their use [4], considering their benefits for humans and animals, the use of these items is difficult to restrict [5]. Till now, the production of PPCPs can reach 28
107 tons per year, and more than 3000 PPCPs have been employed in the medical treatment of animals and humans and the enhancement of their health and existence [6]. PPCPS have no similar chemical characteristics, but they all have at least one aromatic group in their chemical structure. They may be simple in a structure like acetylsalicylic acid or be more complex like Iopromide. They may be synthetic like TCS or come from natural sources like many antibiotics. One of the main sources of PPCPs discharged into the environment is domestic sewage. In the end, we can find the drugs used by humans or animals in the environment directly or indirectly. Some pharmaceutical chemicals that cannot be metabolized or dissolve in the body (such as methotrexate) are expelled in the feces and urine [7]. Eventually, the wastewater is discharged into the sewerage systems [8] (Fig. 2).

Wastewater Treatment Plants and Pharmaceutical and Personal Care Products The detected number of PPCPs increases continuously due to the increase in the usage of PPCPs, the growth of their industries, and the development of the ways of detection. PPCPS are found in small concentrations in wastewater treatment plants (WWTPs) as part of a billion or parts of a trillion. However, these concentrations are

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Direct source

Household/Runoffs

Industry/Hospitals

Catabolic transformation Sorption/ Bioaccumulation

Landfill/Open dumps

Wastewater treatment plant

Raw water

Major/direct path Minor path

Animal husbandry

Surface water Soil

Groundwater

Aquaculture

Agriculture

Fig. 2 PPCPs pathways in the environment. PPCPs, pharmaceuticals, and personal care products. (Adapted with permission from Ref. [9], Copyright 2019, Elsevier)

still under interest as, unfortunately, these small concentrations have effects as most of these compounds have biological activity in very small concentrations [2] (Fig. 3). WWTPs (wastewater treatment plants) are considered the primary units for treating or dissolving pollutants from wastewater. The standard biological remediation technique throughout the activated sludge system is used in WWTPs. It can slough off the PPCPs from wastewater. Catabolism (compound uses carbon or energy resource) or cometabolism (compound conversion without using carbon or energy resource) might be the cause of microbiological modification of PPCPs [11]. In either situation, full mineralization (complete breakdown to CO2 and water), degradation to lower molecular weight products, or having only a minor chemical change are all possible outcomes. However, Using only the parent molecule, it is hard to determine which fate a PPCP undergoes during biological degradation [12, 13]. The term “biological transformation “will be used instead of “removal” because the metabolic fate of PPCPs may impact aquatic environments by absorbing WWTP effluent. If the PPCP is completely mineralized, it is no longer present in the system and has no effect. However, as previously indicated, conjugation acts as a parent PPCP reservoir that can then be cleaved and discharged into the environment, having the same consequences as the untransformed parent molecule. The different removal effectiveness by WWPTs and other removal techniques might be due to various factors, including the properties of activated sludge (a mixed liquid suspended microbial and solid population) and the wastewater composition. It is crucial to note that the removal efficiency of one of PPCPs might vary significantly. The equivalent adsorption and absorption contributions in biodegradation were also looked into. The behavior of PPCPs during their treatment process can be influenced by multiple factors, including the pH of the surrounding environment. Ciprofloxacin and ceftazidime, for example, displayed distinct chemical types depending on pH.

Biodegradation Method of Pharmaceuticals and Personal Care Products

Primary source

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Pharmaceuticals personal care product and Illicut drugs

Farm animals

Medicine

PPCPS cycle

Industrial sector

Domestic sector

Disseminating hotspot

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Landfill site

Agriculture sector

Wastewater treatment plant

Leaching of PPCPS

Terrestrial ecosystem

Accumulation of PPCP

Aquatic ecosystem

Fig. 3 Sources and pathways of PPCPs in the environment. PPCPs, pharmaceuticals, and personal care products. (Adapted with permission from Ref. [10], Copyright 2019, Elsevier)

Furthermore, the pH of the aqueous phase was shown to affect the breakdown of penicillin [13]. To summarize, traditional treatment processes cannot eliminate PPCPs from wastewater. Although their quantities are modest, the PPCPs can still be found in the effluents following the standard treatment procedure. Ozonation and Fenton oxidation are examples of the advanced treatment procedures used in PPCPs breakdown in wastewater to assure the security of the drinking water supply. Furthermore, PPCPs were founded in a drinking water source [14], representing a health concern to humans. WWTP effluent is suspected of being a source of PPCPs in drinking water (Table 2).

Pharmaceutical and Personal Care Products and Human Interactions The US Environmental Working Group (EWG, 2008) detected the known carcinogen, 1,4-dioxane, in 28% of the PPCPs. Also, the EWG polled 20 young girls aged 14–19; they detected 16 hazardous compounds in the girls’ bodies due to their use of cosmetics, including TCS, synthetic musk, and 2-benzene dicarboxylic salt. According to a study undertaken by the United States Environmental Protection Agency (USEPA),

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Table 2 Names, compound classifications, and WWTP effluent concentrations from the literature for the PPCPs. (Adapted with permission from Ref. [14, 15], Copyright 2019, Elsevier) PPCP Chlorophene Biosol Biphenylol Diclofenac (sodium) Fluorouracil Ibuprofen Triclosan

Classification Antiseptic Antiseptic Antiseptic NSAID Anticancer NSAID Antiseptic

WWTP effluent concentration(s) (ng liter1) 750 250 900 110 Not detected 1900 800

antimicrobials, estrogenic steroids, antibiotics, and antiepileptic medicines were found in the water sources of the USA (EWG, 2009; USEPA, 2009). This shows how far this kind of pollution has devastating effects on humans, as it may cause the development of antibiotic-resistant bacteria; in addition, some drugs with the same targets may have synergic effects that may lead to unexpected effects or cause toxicity [2]. Bioaccumulation of some PPCPs in fish and other aquatic species can result in a range of unintended consequences. Fish liver enlargement can be caused by chronic exposure in water [16]. Because PPCP residues have been found in our food chain, including drinking water, vegetables, and fruits, they have an impact on human health [17]. The destiny of PPCPs depends partially on the product application, its chemical properties, and user physiology. Some products are for external use, like fragrances and hygiene products. These products commonly enter the wastewater stream as graywater components in the form of unaltered parent compounds. In contrast, the PPCPs that are internally administered are exposed to the human body’s detoxification mechanisms, stimulating their excretion and affecting biological activity. These biochemical processes are divided into two stages. Primarily in stage I, monooxygenases like Flavin monooxygenase and cytochrome P450s, reductases, or hydrolases act on the compounds of PPCPs to increase their reactivity. Stage II commonly leads to the conjugation of the compound with sugars in glucuronidation or conjugating the compounds with peptides to improve their solubility and make their excretion easier. The degree of conjunction differs largely across PPCPs and depends primarily on its chemical properties. Ibuprofen, for example, is hydroxylated or carboxylated primarily. Then its metabolites or the parent compound can be glucuronidated, and after that, the parent compound and all of these metabolites are excreted out of the body. More than 90% of certain PPCPs are excreted as conjugates [2]. High-performance liquid chromatography-tandem mass spectroscopy (HPLCMS/MS) and solid-phase extraction (SPE) were used for analyzing 26 seawater samples that were collected from stations that were in the red seawater of Saudi Arabia, and it was found that 13 PPCPs were detected. The most abundant PPCPs were acetaminophen, caffeine, metaformin, and diclofenac. They were found in a concentration above 3 μg/L [3]. PPCPs and their metabolites have poisonous effects; octylphenols and nonylphenols, which are the metabolites of alkylphenols, are toxic and estrogenic.

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1101

Pharmaceuticals and personal care products

Sewage (pharmaceuticals through household discharge, straight disposal, excreta, etc. and PCPs through washing sinks, shower waste, etc.)

Ocean, river, lake, pond, etc.

PPCP production and industries Veterinary discharge (aquaculture, livestock, poultry)

Raw wastewater

Solid waste/ Manure

Wastewater treatment plant

Open dumping and landfill

Treated wastewater

Drinking water

Drinking water treatment plants

Sludge

Soil/ Agricultural land

Groundwater

Food chain

Fig. 4 Sources, environmental fate, and transport of PPCPs. PPCPs, pharmaceuticals, and personal care products. (Adapted with permission from Ref. [18], copyright 2019, Elsevier)

Also, tonalide and galaxolide, which are found in some fragrances with estrogenic effects when introduced at certain concentrations, gain acute toxicity in some aquatic living organisms [2] (Fig. 4).

Biological Transformation of PPCPs Aquatic habitats are affected by PPCPs’ metabolic fate by receiving WWTP effluent, and then, instead of removal, it will be mentioned as biological transformation. If the PPCP is completely mineralized, it has been removed completely from the system

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and has no effect anymore. But in the case of conjugation, this phase acts as a parent PPCP reservoir. It can be discharged into the environment by cleavage and have the same biological effects as the untransformed parent molecule. Furthermore, the degradation of one PPCP might sometimes result in creating another one. This occurs in 17b-estradiol (E2) and (E1) hormones, where the degradation of E2 causes the formation of estrone (E1) as a metabolite [19, 20]. Although the amount of PPCPs present in the environment are found between ng/L and low mg/L levels, toxicity research reveals that even at these tiny amounts, PPCPs can be harmful in some tested mixes [21]. WWTPs are considered one of PPCP’s major sources in the environment. For example, WWTP procedures successfully removed caffeine and acetaminophen in several investigations. However, they are exceptions, as most PPCPs documented in studies cannot be removed completely by WWTPs. In addition to standard wastewater treatment technologies, new techniques for PPCP removal from the water must be used to reduce the PPCP mass released to the environment. The benefits of these methods include decreasing the taste and bad smell problems associated with the contamination of surface water, which will help achieve greater water treatment during resupposition and soil transport, helping reduce the PPCPs mass currently being released to the environment [22]. Indirectly, other microbes may enhance the pollutants’ degradation without transforming them in WWTPs, so it is not just the microbes responsible for the actual metabolism that are crucial. For example, Bacillus was discovered to involve in floc-forming, which is a type of microbial aggregate that increases the degradation of nonylphenol polyethoxylate surfactants (NPEOs) by helping two other bacteria to grow on NPEOs as their source of energy and only carbon source. through the breakdown of NPEOs (Table 3).

The Parent Compounds Although these metabolites were absent in the WWTP effluent, these findings highlight the fact that the absence of the parent compound does not imply the absence of the PPCP compounds and that research into the metabolic pathways by which degradation of PPCPs occurs will be important to understand their fate in wastewater treatment and the environment [24].

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Table 3 Examples of biotransformations of pharmaceuticals. (Adapted with permission from Ref. [23], copyright 2021, Elsevier) Pharmaceutical Biotransformation product Analgesics and anti-inflammatories Ibuprofen Hydroxy-ibuprofen (OH-Ibu) Carboxy-ibuprofen (CA-Ibu) Carboxy-hydratropic acid (CA-HA) Naproxen

Antihypertensives Atenolol ATE-268 Bezafibrate

BEZ-224

Valsartan

BEZ-256 BEZ-360 VAL-336 VAL-252 VAL-267

Primary transformations Hydroxylation of alkyl group Alkyl/alcohol oxidation β-Oxidation O-demethylation Decarboxylation

Primary amide hydrolysis Secondary amide hydrolysis Dechlorination

Hydroxylation Dehydrogenation Tertiary amide hydrolysis Secondary amide hydrolysis Amine oxidation (continued)

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Table 3 (continued) Pharmaceutical Antimicrobials Trimethoprim

Biotransformation product

Oseltamivir Psychiatric Carbamazepine

OSE-285

Diazepam

DIA-271

Primary transformations Hydroxylations

Dealkylation Hydroxylations

N-dealkylation Hydroxylation

(continued)

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Table 3 (continued) Pharmaceutical Levetiracetam Contrast media Iohexol

Biotransformation product LEV-172

Primary transformations Primary amide hydrolysis Alcohol oxidation

Amide hydrolysis

Oestrogens 17β-estradiol

Alcohol oxidation

Mestranol

O-demethylation

(continued)

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Table 3 (continued) Pharmaceutical Others Diphenhydramine

Biotransformation product

Primary transformations N-demethylation

Guaifenesin

O-demethylation

Oxybenzone

O-demethylation

Second, even when the right organisms are found in sufficient numbers, the genes required for PPCP degradation are often not constitutive and must be activated [11]. The lack of gene induction could explain the observed delay in ibuprofen breakdown in sewage sludge before fast transformation [25]. Sphingomonas sp. Strain RD1 is another example; a WWTP isolate requires at least 2 mg of Triclosan (TCS) per liter to transcribe the genes that encode TCS degradation [26]. Although the TCS biotransformation potential in WWTPs exists, biotransformation may be limited by the low concentrations normally observed in WWTPs [27], and it has been documented as the “threshold” effect, which has been

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demonstrated to have a substantial influence on the low concentration persistence of industrial and agricultural pollutants in the environment. Natural disasters can potentially obstruct a WWTP’s ability to remove PPCPs successfully. Ternes (1998) observed that rain runoff from a storm could reduce the clearance of lipid regulators and anti-inflammatory medications (NSAIDs) from 60% to 5%, demonstrating the impact of the homeostasis of WWTP on the amount of PPCP transformation [28].

Soil Some PPCPs can inhibit the actions of soil microorganisms that are responsible for their degradation, which decreases the activity of these microorganisms, thus making PPCPs last longer without being degraded, which may harm humans and the environment. The introduction of PPCPs effluent into agricultural soils could result in pollution of the soil and groundwater contamination, which varies according to the PPCP’s chemical structures and the type of soil. In terms of chemical mobility, the residence time of ibuprofen is short in various soils, indicating that it has a great potential for downward movement with percolating water. However, triclosan and bisphenol are easily retarded in soil. Soil parameters were associated with the degradation of the selected PPCPs, where the higher the organic material (OM) and clay content of the soil, the lower the degradation rate constants. High starting soil concentrations similarly slowed the breakdown of PPCPs. This proves that as the soil gets to have a lower initial concentration for a given incubation time, a higher percentage of PPCPs could be degraded in these soils [29]. Finally, many refractory molecules are not used until alternative carbon sources that are more easily decomposed have been consumed [11]. The most well-known example is the Escherichia coli growth on a compilation of sorbitol, lactose, and glucose. Although E. coli can use either as a growth substrate, lactose catabolism begins only after consuming glucose [30]. For the organic contaminants 4-chlorophenol and 2,4-dichlorophenol, this effect has also been seen in environmental samples [31]. That carbon source preference occurs with PPCPs is supported by the fact that providing additional carbon sources inhibits PPCP degradation [32].

Soil Aquifer Treatment and Activated Sludge Treatment Soil aquifer treatment (SAT) is used for recycling water. It depends on the soil’s physical, chemical, and biological processes to clean WWTP effluent as it infiltrates and then from a spreading basin, it percolates through the soil to an underlying aquifer. Finally, water from the aquifer is withdrawn months or years later for use afterward. SAT helps in halogenated carboxylic PPCPs biodegradation; SAT is also effective in removing antibiotics like macrolides and fluroquinolones through sorption by more than 80%. At the same time, activated sludge treatment (AST) is used to degrade carboxylic PPCPs, and they both show potential effects in the removal of

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Table 4 Anaerobic and aerobic transformation reactions may lead to persistent metabolites. Specific examples of pharmaceutical and personal care products with corresponding transformation products are shown [39] Parent compound N-demethylation Diphenhydramine Anti-histamine O-methylation Bisphenol A (BPA) Plastic precursor De-ethoxylation Octylphenol and nonylphenol polyethoxylate nonionic surfactant

Transformation product N-Desmethyl diphenhydramine BPA monomethyl ether (left) BPA dimethyl ether (right) Octylphenol or nonylphenol

micropollutants, like PPCPs, from the environment. But they both showed poor removals for amide pharmaceuticals like crotamiton and carbamazepine. Acidobacteria were found in higher abundance (>15.7%) in SAT than in SBR, demonstrating that the organization of microbial communities in SAT differs from those in activated sludge and natural soil. Biodegradation might be aided by the sequential mixing of ASTs and SATs. These therapies are effective because of the mixing of several microbial populations [33]. Furthermore, UV light is an important environmental factor for degrading PPCPs by photolysis in surface waters. Although light exposure may increase resource competition between heterotrophic bacteria and algae [34], it is more likely to result in the abiotic modification of pollutants into intermediates that can subsequently be degraded further. Many PPCPs can be photolyzed, including TCS and sulfonamides [35]. TCS is rapidly photolyzed at pH > 8 [27]. This is the major TCS breakdown pathway in neutral to slightly acidic surface waters [36, 37]. On the other hand, photolysis can produce transformation products that are more poisonous than the parent chemical. The development of chlorinated dioxins due to UV exposure demonstrated TCS [38]. In contrast, the photolysis of sulfonamides produces phototransformation metabolites that are often more susceptible to biological transformation than the parent chemical [35] (Table 4).

Bacterial Species Included in MFC A/O System’s Biodegradation of PPCPs and Aromatic Compounds Microorganisms involved with aerobic ring cleavage and anaerobic biotransformation are among the bacteria capable of biodegrading aromatic compounds such as PPCPs, as shown in Table 1. Three species are well known to have a forthright link to the biodegradation of PPCP. Dechloromonas sp., which they consider proteobacteria, was found in an A/O-MBR that displayed good antibiotic elimination efficiency (88.5–99.5%) at various HRTs when using 500 g/L SMX [40]. Multiple

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pharmaceutical contaminants have been reported to be biodegraded by Pseudomonas sp. For example, Pseudomonas aeruginosa isolates from an SBR treatment facility that treated ACE-polluted wastewater completely biodegraded a high concentration of nearly 2000 mg/L ACE as a sole carbon resource [41]. When that antibiotic is employed as the single carbon source, Pseudomonas aeruginosa may biodegrade a 1.3% of 6 mg/L SMX, or 5.6% of 6 mg/L SMX when a 0.5 g per L glucose appears as a collective [42]. Furthermore, a Sphingomonas sp. strain Ibu-2 appeared to be capable of decomposing 500 mg per L IBU as the only energy and carbon source for 80 h in a wastewater remediation system [43]. Anoxic conditions appear to be the optimum for bacterial communities to adapt to IBU biodegradation. The biological disintegration constant rate for IBU grew from 16% at the start to 75% after nearly 350 days in such settings. Benzene-ring molecules dominate the chemical construction of biological metabolic products produced from PPCPs. Geobacter sp. has been found to perform anaerobic benzene biodegradation in a petroleum-polluted aquifer [44]. The biological remediation of BTEX groundwater revealed a Hydrogenophaga sp., which consider a part of a heterogeneous aerobic benzene-injuring bacterial community [45]. If the ferulic acid is utilized as insole energy and carbon exporter, a Cupriavidus sp. has been observed to biodegrade the lignin-related aromatic compound 95% of the time [46]. The Zoogloea sp. was presented to be capable of biodegrading 98.6% of lubricating oil in 12 days associated with nearly HRT of 6 h and an ingress rate of nearly 33 liters per hour [47]. Through PAH bioremediation (3–5 aromatic rings) in soil, and Acidobacteria bacterium was identified as the dominating bacterial group, and it was proven to be capable of breaking down benzene polluted groundwater [48, 49]. When the Staphylococcus sp. congealed on vermiculite, it was utilized to slough hydrocarbons; this system relied solely on a fluidized bed bioreactor and artificial water tainted with naphthalene, toluene, or benzene as an energy and carbon source [50]. A part of ethylbenzene-degrading sulfate-remission association has been identified as a Sphingobacteriales bacterium [51]. Microbial enrichment has identified Prolixibacter sp. as having the ability to degenerate chlorinated chemicals prevalent in polluted locations across India [52]. A Burkholderiales bacterium has been shown to decompose benzene, toluene, ethylbenzene, a combination of methyl tert-butyl ether (MTBE) and xylene (BTEX), and tert-butyl alcohol (TBA) [53]. The study of non-steroidal anti-inflammatory medications (NSAIDs) considered aquatic contaminants has grown in popularity through the last decade. Following the publication of Ternes’ paper in 1998 [28], which described the appearance of 32 pharmaceuticals and their metabolites in an aquatic circumference, the number of studies indicating pharmaceuticals in sediments, soil samples, river and stream waters, wastewater treatment plants, and also drinking water increased dramatically. At the environment, NSAIDs have been found in concentrations ranging from nanograms per liter to micrograms per liter [54]. Because of their longevity, many of them will be slowly digested mostly in the environment. Hydroxylated and Demethylated derivatives, parent medicines, and acyl glucuronide conjugates are all excreted into the environment.

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Table 5 Bacteria identified by nucleic acid sequencing of 16S gene clones and by searching the GenBank database; these are associated with the biodegradation of PPCP and aromatic compounds in the MFC A/O system [56] Accession no (Closest match) KC871534 AJ620198 AF170354 KC871534 AB636293 HE662651 HQ184339 JQ795417 KC310815 JQ723636

Sequences similarity 99% 99% 99% 96% 97% 98% 98% 96% 99% 96%

JN540151 JF808996

95% 99%

Species Pseudomonas sp. Sphingomonas sp. Dechloromonas sp. Uncultured Geobacter sp. Uncultured Hydrogenophaga sp. Cupriavidus sp. Uncultured Zoogloea sp. Uncultured Acidobacteria bacterium Staphylococcus sp. Uncultured Sphingobacteriales bacterium Uncultured Prolixibacter sp. Uncultured Burkholderiaies bacterium

Aside from white-rot fungus, bacteria are effective degraders and cleansers of non-steroidal anti-inflammatory medicines found in the environment. A Grampositive bacteria, Planococcus sp. S5, decomposed naproxen at rates of 19 and 11 g/h in the presence of glucose and phenol, respectively, under metabolic conditions. Naproxen degradation was demonstrated to be faster in the presence of an easily assimilable carbon source. As a result, selecting the appropriate circumstances (for example, the type and concentration of the carbon resource) may accelerate the treatment process. The presence of hydroxylating and aromatic-ring-cleaving enzymes in bacterial naproxen breakdown pathways confirms their importance. The findings point to the possibility of utilizing metabolic systems to treat naproxen-contaminated wastewater. It is worth noting that naproxen levels in environmental waters vary from nanograms per liter up to micrograms per liter. As a result, further Planococcus sp. S5 naproxen biodegradation tests at the microgram per liter of naproxen level would be considered highly essential [55] (Table 5).

Biodegradability of Pharmaceutical and Personal Care Products The Biodegradability of PPCP means the ability of PPCP to get decomposed through the biological activity of microorganisms such as bacteria or fungi. Also, during the degradation process of other organic compounds, the PPCP biodegradation may occur co-metabolically. The biodegradation process may be a complete transformation of the compounds into carbon dioxide and water called mineralization. Partial biodegradation is the other type in which the compound is transformed into metabolites or conjugated with glucuronic acid or sulfate. PPCP biodegradability depends on their bioavailability, allowing enzymatic degradation to degrade compounds

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[57]. The biodegradation process is the most important removal mechanism of many PPCP in wastewater. However, many PPCP micropollutants have very low enzyme affinities opposing microbial removal. In wastewater treatment plants (WWTP), copiotrophs must biodegrade most of the compounds as they exist at an enzymesaturating level providing nutrient-rich environments. However, PPCP provides a nutrient-poor environment as they exist at an enzyme-sub saturating level, so oligotrophs are required to degrade PPCP. Because oligotrophs exist in a small population in WWTP, PPCP is not completely degraded [58].

Pharmaceutical and Personal Care Products Biodegradability Categories Joss et al. have used the following equation to calculate the pseudo first-order kinetics of 35 compounds, including hormones, pharmaceuticals, and personal care products in nutrient-eliminating sludge shown in Table 2. dC=dt ¼ K biol XSS Ssoluble: where Kbiol is the pseudo-first-order reaction rate constant (L/g/day), XSS is the concentration of the suspended solid (g/L), Ssoluble is the concentration of the soluble part of the compound (μg/L), C is the concentration of the total compound (μg/L), and t is time (day). Joss et al. have categorized the PPCP according to their biodegradability based on the calculated pseudo-first-order reaction rate constant into three categories: 1- Kbiol < 0.1 L/ g/day is the poorly biodegradable category expected to be biodegraded by less than 20%; 2–0.1 < Kbiol < 10 L/g/day is the moderately biodegradable category expected to be biodegraded in between 20% and 90%; 3- Kbiol > 10 L/g/day is the highly biodegradable category expected to be biodegraded by more than 90% [59] (Table 6). Although some literature is in good consistency with the data of Table 2 [28, 60– 62], some studies declared different data about ibuprofen, fenoprofen, gemfibrozil, indomethacin, estrogens [63], indomethacin, and diclofenac [28]. Joss et al. have declared the causes of that variation because of (1) sludge age, flow scheme, wastewater composition (sludge origin); (2) different approaches of sludge handling before the batch experiments as sludge storage and artificial substrate dosing; (3) the wide difference in experimental pharmaceutical concentrations [59]. Furthermore, biodegradability was studied using batch tests under different conditions. Under aerobic conditions using batch tests containing activated sludge and carbon source, Quintana (2005) studied the mineralization of five pharmaceuticals within 28 days (ibuprofen, ketoprofen, bezafibrate, naproxen, and diclofenac). Quintana declared that ibuprofen 96% was mineralized, 30% mineralization for ketoprofen, bezafibrate, and naproxen, but diclofenac was not mineralized [24]. Also, Joss (2005) determined the biodegradation reaction constants of some pharmaceuticals using batch tests under aerobic conditions. Joss’ batch tests contained sewage sludge from a conventional activated sludge treatment plant

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Table 6 Categorization of 35 PPCP and hormones in nutrient-eliminating sludge as mentioned in Joss et al. (2006) [59] PPCP Category Antibiotics

Antidepressant Antiepileptic Antiphlogistics

Iodinated contrast agents

Estrogens

Lipid regulators

Nootropic Fragrances

Compound Azithromycin Clarithromycin (Anhydro-)erythromycin Roxithromycin N4-acetyl-Sulfam Sulfamethoxazole Sulfamethazin Diazepam Carbamazepine Acetylsalicylic acid Diclofenac Fenoprofen Ibuprofen Indometacine Naproxen Paracetamol ATH DAMI Diatrizoate Iohexol Iomeprol Iopamidol Iopromide Iothalamic acid Ioxithalamic acid Estradiol Estrone Ethinylestradiol. Bezafibrate Clofibric acid Fenofibric acid Gemfibrozil Piracetam Tonalide Galaxolide

Predicted biodegradability Low Low Low Low Moderate Low Low Low Low Moderate Low Moderate High Low Moderate High Moderate Moderate Low Moderate Moderate Low Moderate Low Moderate High High Moderate Moderate Moderate Moderate Moderate Moderate Low Low

(CAS) or a membrane bioreactor (MBR) [13]. The first-order degradation constants calculated based on Joss’ batch tests are shown in Table 7. On the other hand, little is known about the pharmaceuticals’ biodegradability under anoxic conditions. But in general, biodegradation processes in aerobic conditions are faster than in anoxic conditions. Zwiener (2002) studied the

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Table 7 Biological degradation constants of some pharmaceuticals using sludge from CAS and MBR in batch experiments. T ¼ 17  1oC. (Adapted with permission from Ref. [59], copyright 2006, Elsevier) Pharmaceutical Diclofenac Ibuprofen Bezafibrate Clofibric acid

kbiol (L/gSS/d) for CAS 100 mg/l. Also, this method is suitable for higher microbial cell concentrations. The compound concentration of DOC is measured in an inoculated medium in dark or diffuse light at 22  2  C for 28 days. The biodegradation is calculated as the percentage of the removed concentration of DOC from the initial concentration. Also, primary biodegradation can be calculated by supplemental chemical analysis for the parent compound at the beginning and the end of the test [84]. CO2 Evolution Test (ISO 9439, OECD 301 B) – Modified Sturm Test Compound’s properties: Nonvolatile, known carbon content and purity. Also, this method is suitable for poorly soluble and adsorbing compounds. The inoculated medium of this method contains a sole source of organic carbon, which is the known concentration of the test substance. This system is aerated by the controlled rate of carbon dioxide-free air in dark or diffuse light for 28 days. The microbial activity produces CO2 trapped by Ba(OH)2 or NaOH then measured by the residual hydroxide titration. Biodegradability is calculated as the percentage of CO2, which is the amount of CO2 produced from the test substance [84]. MITI (OECD 301 c) Compound’s properties: Known formula and purity to calculate ThOD. Also, this method can assess volatile and insoluble compounds. The biodegradation can be calculated as the percentage of comparing the value of oxygen uptake and OD. The value of oxygen uptake is measured automatically in an enclosed respirometer at 25  1  C within a period of 28 days in a dark inoculated mineral medium containing specially grown, unadopted microorganisms [84]. Closed Bottle Test (CBT) (ISO 10707, OECD 301 D) Compound’s properties: Known formula and purity to calculate ThOD. Also, this method can assess the biodegradability of volatile and insoluble compounds. In this method, the inoculated mineral medium contains small microorganisms from a mixed population with the tested substance infull, closed containers in the dark and at constant temperature for 28 days. Biodegradability is the percentage of the consumed oxygen by the microbial population during the biodegradation from the ThOD [84].

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Modified OECD Screening (OECD 301 E) Compound’s properties: Nonvolatile, solubility is at least 100 mg/L. This method is like the DOC Die-Away test (301 A) but contains a lower concentration of microorganisms relatively [84]. Manometric Respirometry Test (ISO 9408, OECD 301 F) Compound’s properties: Known formula and purity to calculate ThOD. Also, this method can assess the biodegradability of volatile and insoluble compounds. Compared to the previous methods, this method is used to assess higher contestations of the test substance (100 mg /l giving at least 50–100 mg ThOD/l) as the sole source of organic carbon, in a closed flask and at a constant temperature (+ 1  C or closer) for 28 days. In this method, the biodegradability of the test substance is determined by the oxygen amount added to the system to provide a constant gas volume inside the system or by evolved CO2, which is determined by trapping using KOH or another suitable CO2 absorbent then titration [84]. Combined CO2/DOC Test This method provides reliable results and information about the aerobic biodegradability of the test substance in aquatic ecosystems. Because it studies biodegradability through two parameters, the removal of DOC and CO2 production, it gives larger and more reliable data than the DOC die-away test performed alone, CO2 evolution test, or the respiratory test [85, 86].

Inherent Biodegradability: OECD 302 In 1981, OECD 302 B method was adopted to test the inherent biodegradability [87]. Then, OECD 302 B method was merged with the tests of Swiss Federal Laboratories for Material Testing and Research (EMPA), and the final composition of the mineral medium was in 1992. The new mineral medium composition is identical to the OECD 301 guideline [86].

Semi-Continuous Activated Sludge Test (SCAS): OECD 302 a In SCAS (ISO 9887, OECD 302 A), activated sludge is used as inoculum in ZahnWellens test [85]. Both methods can determine the inherent biodegradability because of their high degradability potential [86]. Zahn-Wellens/EMPA: OECD 302 B Compound’s properties: Nonvolatile, at least 50 mg DOC/l water solubility, not significantly adsorbable, not lost by foaming, and do not inhibit bacteria at the tasted concentration. Also, it is necessary to know vapor pressure and its chemical formula to estimate the values of DOC or chemical oxygen demand (COD) [87]. Like ISO 9888, is standard to evaluate the inherent biodegradability. This method system contains the mineral medium, a large amount of activated sludge, and the test

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substance. This system is aerated in the dark at 20–25  C for 28 days. The DOC determination estimates the biodegradability in the filtered samples taken daily. Usually, this test is not performed unless ready biodegradability fails [87].

Modified Zahn-Wellens Test Compound’s properties: Poorly soluble, adsorbing or volatile substances (Fig. 6). Based on Zahn-Wellens/EMPA (OECD 302 B), the modification is a closed system apparatus of inexpensive laboratory equipment that allows the continuous and parallel measurement to the parameters for Biodegradability: 1) oxygen consumption using the pressure change measurement; 2) CO2 production using the conductivity measurement [40].

Automated Determination of Biodegradability The two systems (Sapromat and OxiTop-C) are based on the principle of the monomeric respirometry measuring through the determination of biological oxygen demand (BOD) in a closed bottle system. BOD value (mg/L) represents the amount of oxygen required to degrade an organic matter in water by aerobic digestion. Also, it represents the water quality and the behavior of WWTP. BOD is used to assess the substance removal efficiency during the biological treatment from wastewater and the substance toxicity on the microorganisms or inhibitory effect [52] (Fig. 7).

Pressure sensor

Membrane pump

Conduction electrode

Culture flask Absorption bottle Fig. 6 Modified Zahn-Wellens test apparatus. (Adapted with permission from Ref. [40], Copyright 2001, Elsevier)

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5 3 4

2

Controller Unit

4

3

2

Computer

6 5 1 1: magnetic stirrer 2: culture medium 3: CO2 trap (A)

4: incubation flask 5: pressure sensor

1: magnetic stirrer 2: incubation flask 3: CO2 trap (B)

1 4: pressure indicator 5: electrolysis cell 6: culture medium

Fig. 7 Schematic views of respirometers. (a) Oxitop system, (b) Sapromat system. (Adapted with permission from Ref. [83], Copyright 2003, Elsevier)

OxiTop-C is a BOD self-measuring mercury-free system. The CO2 produced during the microbial activities is trapped using NaOH, creating a vacuum, and any pressure change is read by the operating unit and presented by mg/l BOD. The system of Sapromat consists of 12 measuring vessels (500 ml). During biodegradation, the system oxygen is reduced, making the manometer and the electrochemical unit produce oxygen to return the initial pressure to the system. The biodegradability is measured as the difference between the ThOD and BOD [83, 86].

Other Methods An activated sludge simulation test (ISO 11733, OECD 303) is used to estimate the kinetic biodegradability parameters in wastewater plants, which clears more details about the biodegradation. Similarly, the Low test concentrations test (ISO 14592) is used to estimate the kinetic parameters in natural lakes and ponds. Anaerobic biodegradability test (ISO 11734) is a method to enhance the environment in digesters of WWTP. The aerobic composting test (ISO 14855) is used to study the biodegradability of the polymer. The soil test (OECD 304) is the only internationally harmonized method studying soil degradation or sediments [41, 42].

Metabolism Diclofenac 2-[2-(2,6-dichloroanilino)phenyl]acetic acid is often detected in the environment, known commercially diclofenac, and classified as a non-steroidal anti-inflammatory drug (NSAID) [43–46]. Microorganisms can degrade diclofenac by influencing mineralization through a variety of metabolic pathways. This enhance adaptation of microbes to environmental changes [47]. Few bacterial strains show the ability to

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Fig. 8 Structure of diclofenac

degrade diclofenac including Labrys portucalensis F11, Pseudomonas moorei KB4, Klebsiella sp., Rhodococcus ruber strain IEGM 346, Pseudoxanthomonas sp. DIN-3, Brevibacterium sp. D4, and Enterobacter cloacae (D16). Here we demonstrate the metabolic pathways and metabolites formed by diclofenac degradation through various microbial strains (Fig. 8).

Bacillus Subtilis and Brevibacillus Laterosporus By isolating BC2 and TK3 strains from micropollutant aqueous stock solutions, Grandclément et al. (2020) completely removed diclofenac (DCF) after 17 h. He also used vibrio fischeri bioassay to determine the ecotoxicity of different metabolites formed through the degradation pathway and estimated that EC50 for diclofenac was (23  4 mg L1), and for 40 -OH-DCF was (19  2 mg L1), others showed more toxicity. This indicates that the two compounds, diclofenac and 40 -hydroxydiclofenac, cannot differ significantly in ecotoxicity so that 40 -OH DCF can be considered the main metabolite in the pathway [48]. Enterobacter Hormaechei D15 By isolating from active sludge, Enterobacter hormaechei D15 can degrade 52.8% of diclofenac after 48 h. When diclofenac is used as the sole carbon source with glucose as an external source of carbon for the enhancement of co-metabolism of the drug, the removal rate of diclofenac increased to 82% and 1-(2,6 dichlorophenol)-1.3-dihydro2H-indol-2-one (TP278) was produced and identified as the main metabolite in the degradation pathway [49]. Labrys Portucalensis F11 Labrys portucalensis F11 strain can degrade 70% of diclofenac through its co-metabolism with acetate as external carbon source and achieved complete removal of diclofenac after 6 days; 12 metabolic intermediates were detected in this pathway. Hydroxylation reaction is the main process in DCF degradation that results in the formation of four isomers from which it was reasonable to deduce the structure of hyroxydiclofenac compounds; 40 -OH DCF and 50 -OH DCF. Benzoquinone imine formation was remarkable in diclofenac degradation pathway [50]. Rhodococcus Ruber IEGM 346 Rhodococcus ruber IEGM 346 achieved complete removal of diclofenac after 6 days by cleavage of the C-N bond in the structure of diclofenac, which led to the formation

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of phenylacetic acid and opening of the aromatic ring in diclofenac [51]. However, these bacteria differ in the genus; all pass through hydroxylation, which is considered the first stage in diclofenac degradation pathways, and as a result 5-OH-DCF, 40 -OH-DCF, and 6-OH-DCF were produced. Thirty-two intermediates were formed through diclofenac degradation pathways in hydroxylated, methylhydroxylated, hydroxylated, oxidized, and decarboxylated compounds. Hydroxylation contributes to producing three isomers 5-OH-DCF, 40 -OH-DCF, and 6-OH-DCF. (E)-(6-((2,6-dichloro-3,4dihydroxyphenyl) imino)-3-oxocyclohexa-1,4-dien-1-yl) methanolate produced from 5-OH-DCF via catechol 1,2-dioxygenase enzyme attack. The production of 3,5-dichlorobenzene-1,2,4-triol was achieved through (1) the cleavage of stable p-quinone derivative forming 4-amino-3,5-dichlorobenzene-1,2-diol and (2) the replacement of the amino group with a hydroxyl. The metabolism of 40 -OH-DCF through the attack of protocatechuate 3,4-dioxygenase led to formation of metabolites such as 2-(2-hydroxyphenyl)acetic acid, 2-(2-(((1E,3Z)-1-carboxy-2,4-dichloro-5oxopenta-1,3-dien-3-yl)amino)phenyl) acetic acid, and 2-(2-((2,6-dichloro-3,4dihydroxyphenyl)amino)phenyl) acetic acid. Therefore, 6-OH-DCF served as the parent compound to synthesize different hydroxylated diclofenac degradation products. Another diclofenac degradation process that led to the production of the primary metabolite in bacterial cells begins with hydroxyphenyl acetic acid undergoing oxidation and 2-(2,3-dihydroxyphenyl) acetic acid being formed, and then metabolized to 3-(carboxycarbonyl)-2-hydroxypent-4-enoic acid, 3-(carboxycarbonyl)-2 oxopent-4enoic acid, and 3-(carboxycarbonyl)pent-4-enoic acid. In addition, major metabolites like malonic acid, hydroxy-oxo-valeric acid, oxoglutaric acid, and 3-hydroxyglutaric acid have been found due to sequential oxidation during metabolism. Proteomic studies have shown that the presence of diclofenac induces the activation of 24 enzymes involved in degradation, including protocatechuate-3,4-dioxygenase, catechol-1,2-dioxygenase, and quercetin-2,3-dioxygenase [52].

Ibuprofen Ibuprofen (IBU) is a commonly used nonsteroidal anti-inflammatory drug found in effluents at the g L1 level and is being noticed as an essential micropollutant in wastewater treatment plants (WWTP). The IBU metabolites produced by biological degradation are poorly known and represent a risk to aquatic ecosystems. Only a few bacterial species, including Sphingomonas Ibu-2, Pseudoxanthomonas sp., Sphingobium yanoikuyae, Variovorax Ibu-1, and Bacillus thuringiensis B1 have been shown to degrade ibuprofen [53, 55, 88–90] (Figs. 9 and 10).

Sphingomonas sp. Ibu-2 Strain IBF is used as the sole carbon source under aerobic conditions. By its isolation from a sewage treatment plant, the biodegradation of IBF was studied. The authors proposed a microbiological biodegradation mechanism of IBF using catechol as a major metabolite that is degraded to 5-formyl-2-hydroxy-7-methylocta-2, 4-dienoic acid by the effect of estradiol dioxygenase enzyme. Genetic studies to determine the metabolic

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Fig. 9 Proposed degradation pathway of diclofenac. (Adapted with permission from Ref. [52], Copyright 2021, Elsevier)

Fig. 10 Structure of ibuprofen

mechanism of IBF identified five ipfABDEF gene clusters that contributed to the IBF conversion to isobutylcatechol. It was found that with the aid of genetic and biochemical research Sphinomonas sp. Ibu2 has a significant role in the following: 1. linking IBF to CoA using CoA ligase IpfF, 2. Dihydroxylation of ibuprofen-CoA using multicomponent oxygenase IpfABHI and 1,2-cis-diol-2-hydroibuprofen CoA was formed. 3. IpfD and IpfE remove the acyl-CoA group, and 4- isobutylcatechol is formed. The presence of ipfABDEF gene clusters in the bacterial genome enhances the ability to degrade ibuprofen [52].

Variovorax Ibu-1 Bacteria identified by 16S rDNA sequencing analysis isolated from sewage sludge as the sole source of carbon and energy through a meta-cleavage metabolic pathway. Adding 3-fluorocatechol to ibuprofen-spiked sewage sludge caused Poisoning of meta cleavage enzymes, which led to the accumulation of the catholic metabolites and two OH-IBF isomers and CBX-IBF [55].

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Bacillus Thuringiensis B1 A novel pathway for the degradation of IBF has been described by the hydroxylation of aromatic rings and aliphatic IBF chains characterized by the high activity of phenols monooxygenases, aliphatic monooxygenases, and hydroquinones monooxygenases. Monooxygenase analysis revealed the formation of several intermediates such as 1,4-hydroquinone, 2-hydroxyquinol, 2-OH-IBF, and 2-(4-hydroxyphenyl) propionic acid, 2-OH-IBF was as a result of aliphatic monooxygenase activity. In turn, 2-(4-hydroxyphenyl) propionic acid can be a substrate for acyl-CoA synthetase [52, 89]. The maximum concentration of this conversion product was observed after 42 h when no IBF was detected. The authors proposed that these movements limit the rate of degradation of IBF. By the activity of hydroquinone monooxygenase, 1,4-hydroquinone, which is a product of acyl-CoA synthetase activity, can be converted to 2-hydroxy-1,4-quinol. Hydroxyquinol-1,2-dioxygenase, an enzyme involved in ortho-cleavage of aromatic rings, combines with 2-hydroxy-1,4-quinol to form 3-hydroxy-cis, cis-muconic acid. Presumably, the final product of IBF degradation is cis-muconic acid, 3-hydroxy-cis. After IBF induction, the activities of all enzymes involved in the putative degradation pathway (acetyl coenzyme A synthetase, monooxygenase, hydroxyquinol 1,2 dioxygenase, and hydroquinone monooxygenase) were observed. In further toxicity studies, B. thuringiensis B1 (2015b) for IBF, set EC50 value to 809 mg/L [52] (Fig. 11).

Fig. 11 Proposed degradation pathway of ibuprofen. (Adapted with permission from Ref. [52], Copyright 2021, Elsevier)

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Carbamazepine 5H-dibenzo[b,f]azepine-5-carboxamide, known as carbamazepine commercially, is an anticonvulsant drug commonly used to treat epileptic disorders, bipolar disorder, schizophrenia, and trigeminal neuralgia [91, 92]. When used by patients, approximately 72% of carbamazepine was absorbed, metabolized, and excreted in the urine. However, 28% failed to transform and discharge into the water through feces. Over time, this compound ends up in wastewater. Carbamazepine is often found in wastewater and is difficult to degrade due to its complex structure and biodegradation resistance [91, 93] (Fig. 12). The degradation of CBZ is based on oxidation, loss of CHNO groups, and ketone formation reactions. There are two ways to initiate degradation of carbamazepine, Fig. 13. The first begins with the elimination of CHNO group to form metabolite 1 that can be attributed to iminostilbene (IMS), which is considered one of the major metabolites produced by carbamazepine by the cytochrome P450 system (CYP) in the human liver [95–98]. In the second degradation pathway, CBZ is oxidized to form: Reactive 10,11epoxycarbamazepine (EPCBZ) (metabolite 2) by cytochrome P450, or manganese peroxidase, or both [95, 99]. Three possible reactions can occur in EPCBZ. The first is the formation of ketones with the formation of unstable Transition compounds of CBZ treated with ionizing radiation during the photocatalytic and ultrasonic photocatalytic treatment of CBZ; metabolite 4 is oxidized to form metabolite 6, which can be attributed to the structure of hydroxyketones. 10,11-dihydro-10,11-dihydroxycarbamazepine is a well-known metabolite in human metabolism [95, 100, 101]. In strain F11 Labrys portucalensis, 10,11-dihydro10,11-dihydroxy-carbamazepine (metabolite 6) is further oxidized to form 10,11-diketone-CBZ (Metabolite-7), which is reported as a TP in carbamazepine degradation [95, 100, 102]. An increase of 10.11 diketone CBZ undergoing hydrolyzation and metabolite8 will be formed with the predicted formula C15H10N2O4 and molecular weight 282. Another compound with a molecular weight of 137 was identified: metabolite-14. Based on both predictions, the MS/MS formulation and fragmentation suggested the molecular formula C7H7NO2. A third possible EP-CBZ reaction involves the Acridine pathway. The loss of the carbamoyl moiety from Metabolite-2 accompanied by a contraction of the seven-membered ring into a six-membered ring forms the Metabolite-9, which can be assigned as 9-Acridine carboxaldeyde, followed by another addition of –CO group to form Metabolite-13. 9-Acridine carboxaldeyde, Fig. 12 Structure of carbamazepine

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Fig. 13 Proposed Carbamazepine degradation pathway. (Adapted with permission from Ref. [94, 95], Copyright 2019, Elsevier)

formed through CBZ bio-transformation, is highly toxic and reactive [94, 95]. Acridine (metabolite-11) is formed by the subsequent cleavage of the aldehyde moiety of 9-Acridine carboxaldehyde. Acridine can then follow the ketone formation reaction, resulting in metabolite-12 (acridone), which has been reported to be formed in the same process described above for acridine. It is a genotoxic compound. Finally, further oxidation of 9-acridine carboxaldhyde (Metabolite-9) yields 9-acridine carboxylic acid (Metabolite-10) [95, 99].

Conclusion Biodegradation is the breakdown of the parent compound into small molecules through the biological activity of microorganisms. Biodegradability of the parent compound means its ability to be degraded by the biological activities of microorganisms. PPCPs have variable biodegradability. In the current literature, this variability is summarized into three categories according to each parent compound’s pseudo-firstorder reaction rate constant (poor-moderate-high). The Biodegradability of PPCPs

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depends mainly on their physicochemical properties and molecular structure, which causes this wide variability in the PPCPs’ biodegradability. This variability is enlarged by the classification of PPCPs depending on their application rather than their chemical structure. Furthermore, many factors affect biodegradability as temperature, retention time, and oxygenation. There are many methods to analyze biodegradability. However, each method is based on specific systems and techniques as the composition of the mineral media of each test, the temperature details, and the time of the test. Also, to select the appropriate analysis method, you have to know some properties of the test substance, such as adsorption, solubility, vapor pressure, volatility, and test compound concentration.

Future Perspectives PPCPs have a great harmful effect on our health. We need to create and focus on using new technologies to help get rid of their interactions with the environment and metabolites. However, using advanced technologies like ozonation and chlorination for PPCPs removal, we need to develop new, cost-effective ways to dispose of PPCPs and treat waste water safely. Knowing more about the degradation of PPCPs will help us improve technologies and techniques for their removal and estimate their effects on our bodies and the environment. The effects of microorganisms on surface and groundwater, black and yellow water, and soils need more wide analysis to develop more effective biological treatment systems. The current literature has declared that the removal of pharmaceutics may be associated with compound electrophilicity and hydrophobicity. The more electrophilic and hydrophobic compounds, the better the removal percentage. But the intermediate biodegradable products of the biodegradation process may be mainly affected by SRT and HRT changes, low pH (increase in hydrophobicity), and higher temperatures, enhancing removal performance. However, the biotransformation products may have higher toxicity than the parent compound, so future research studying the degradation of pharmaceuticals should evaluate the biotransformation intermediates’ properties. Experiments to analyze the Biodegradability of PPCPs are suggested using bacteria that have not been exposed to these chemicals before. Also, such experiments evaluate the accommodation ability of the sludge bacteria. Furthermore, there are toxic and inhibitory effects of PPCPs on the bacteria in WWPTs. These effects should be more studied because this toxicity reduces the removal efficacy. That makes the PPCPs more perseverant.

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86. Vanková M (2010) Biodegradability analysis of pharmaceuticals used in developing countries; screening with OxiTop C-110 87. OECD (1992), Test No. 302B: Inherent Biodegradability: Zahn-Wellens/ EVPA Test, OECD Guidelines for the Testing of Chemicals, Section 3, OECD Publishing, Paris. 88. Lu Z, Sun W, Li C, Ao X, Yang C, Li S (2019) Bioremoval of non-steroidal anti-inflammatory drugs by Pseudoxanthomonas sp. DIN-3 isolated from biological activated carbon process. Water Res 161:459–472 89. Marchlewicz A, Guzik U, Smułek W, Wojcieszyńska D (2017) Exploring the Degradation of Ibuprofen by Bacillus thuringiensis B1(2015b): The New Pathway and Factors Affecting Degradation. Molecules 22(10):1676 90. Balciunas EM, Kappelmeyer U, Harms H, Heipieper HJ (2020) Increasing ibuprofen degradation in constructed wetlands by bioaugmentation with gravel containing biofilms of an ibuprofen-degrading Sphingobium yanoikuyae. Eng Life Sci 20:160–167 91. Nasir NM, Talib SA, Hashim SN, Tay CC (2018) Biodegradation of carbamazepine using fungi and bacteria. J Fundam Appl Sci 9:124 92. Garcia SN, Foster M, Constantine LA, Huggett DB (2012) Field and laboratory fish tissue accumulation of the anti-convulsant drug carbamazepine. Ecotoxicol Environ Saf 84:207–211 93. Li A, Cai R, Di C, Qiu T, Pang C, Yang J, Ma F, Ren N (2013) Characterization and biodegradation kinetics of a new cold-adapted carbamazepine-degrading bacterium, Pseudomonas sp. CBZ-4. J Environ Sci (China) 25:2281–90 94. Seiwert B, Golan-Rozen N, Weidauer C, Riemenschneider C, Chefetz B, Hadar Y, Reemtsma T (2015) Electrochemistry Combined with LC-HRMS: Elucidating Transformation Products of the Recalcitrant Pharmaceutical Compound Carbamazepine Generated by the White-Rot Fungus Pleurotus ostreatus. Environ Sci Technol 49:12342–50 95. Bessa VS, Moreira IS, Murgolo S, Mascolo G, Castro PML (2019) Carbamazepine is degraded by the bacterial strain Labrys portucalensis F11. Sci Total Environ 690:739–747 96. BRETON H, COCIGLIO M, BRESSOLLE F, PEYRIERE H, BLAYAC J, HILLAIREBUYS D (2005) Liquid chromatography–electrospray mass spectrometry determination of carbamazepine, oxcarbazepine and eight of their metabolites in human plasma. J Chromatogr B 828(1-2):80–90 97. Leclercq M, Mathieu O, Gomez E, Casellas C, Fenet H, Hillaire-Buys D (2009) Presence and Fate of Carbamazepine, Oxcarbazepine, and Seven of Their Metabolites at Wastewater Treatment Plants. Arch Environ Contam Toxicol 56(3):408–415 98. Mathieu O, Dereure O, Hillaire-Buys D (2011) Presence and ex vivo formation of acridone in blood of patients routinely treated with carbamazepine: exploration of the 9-acridinecarboxaldehyde pathway. Xenobiotica 41(2):91–100 99. Golan-Rozen N, Seiwert B, Riemenschneider C, Reemtsma T, Chefetz B, Hadar Y (2015) Transformation Pathways of the Recalcitrant Pharmaceutical Compound Carbamazepine by the White-Rot Fungus Pleurotus ostreatus: Effects of Growth Conditions. Environ Sci Technol 49(20), 12351–12362 100. Jelic A, Michael I, Achilleos A, Hapeshi E, Lambropoulou D, Perez S, Petrovic M, FattaKassinos D, Barcelo D (2013) Transformation products and reaction pathways of carbamazepine during photocatalytic and sonophotocatalytic treatment. J Hazard Mater 263:177–186 101. Liu N, Lei Z-D, Wang T, Wang J-J, Zhang X-D, Xu G, Tang L (2016) Radiolysis of carbamazepine aqueous solution using electron beam irradiation combining with hydrogen peroxide: Efficiency and mechanism. Chem Eng J 295:484–493 102. Zhu Z, Chen Y, Gu Y, Wu F, Lu W, Xu T, Chen W (2016) Catalytic degradation of recalcitrant pollutants by Fenton-like process using polyacrylonitrile-supported iron (II) phthalocyanine nanofibers: Intermediates and pathway. Water Res 93:296–305

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Biodegradable Materials from Natural Origin for Tissue Engineering and Stem Cells Technologies

Ahmed Atwa, Mahmoud R. Sofy, Sara M. Fakhrelden, Ola Darwish, Ahmed B. M. Mehany, Ahmed R. Sofy, and Sayed Bakry

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioprinting Technologies and Cell Sheet Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3D Bioprinting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4D Bioprinting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3D Engineered Cardiac Tissue Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natural Polymers–Based Biocomposites: State of the Art, New Challenges, and Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Characteristics of Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carboxymethyl Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of CMC–Based Scaffolds for Use in Tissue Engineering . . . . . . . . . . . . . . . . . . . . Chitosan-Based Biomaterials in Tissue Engineering Applications . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cardiovascular Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomaterials and Nanomedicine for Bone Repair and Bone Regeneration Strategies . . . . Biomaterial Scaffolds and Stem Cell for Skin Tissue Engineering in Wound Healing . . . Combination Therapy: Biomaterials and Stem Cells in Wound Healing and Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A. Atwa (*) · A. B. M. Mehany · S. Bakry Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt e-mail: [email protected] M. R. Sofy · A. R. Sofy Botany and Microbiology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt S. M. Fakhrelden Tropical Department, Faculty of Medicine, Fayoum University, Fayoum, Egypt O. Darwish Clinical Microbiology Labs Department, Fayoum General Hospital, Egyptian Ministry of Health, Fayoum, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_63

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Abstract

Tissue engineering is emerging as an interdisciplinary field in biomedical engineering that aims to regenerate new biological material for replacing diseased or damaged tissues or organs. So far, some research has been carried out to find medicines that may halt the advancement of these diseases. However, despite recent improvements, existing medicines are still limited by inadequate translation into practical applications. This chapter provides an up-to-date overview of the most recent advances in the applications of biodegradable polymers with high processing flexibility as the primary scaffolding materials for therapeutically relevant regenerative tissue methods. In addition, their successes, limits, and future clinical translation potential are highlighted. The possibility of applying them to treat some incurable diseases, such as osteoporosis, cardiovascular diseases, and skin wounds, is also discussed. Aside from chemical functionalization, scaffold designs that mirror the micro- and nano-characteristics of the extracellular matrix (ECM) will be described as composite and nanocomposite scaffolds. Keywords

Extracellular matrix · Osteoporosis · Induced pluripotent stem cells · Myocardial infarction Abbreviations

ADSCs BGSs BMP-2 BMSCs b-TCP CD DMECs ECM EHT HAp iPSCs MI MSCs NPs PCL PLGA SDF-1α VSEL

Adipose-derived stem cells Bone graft substitutes Bone morphogenetic protein-2 Bone marrow stem cells Beta-tricalcium phosphate Clusters of differentiation Dermal microvascular endothelial cells Extracellular matrix Engineered heart tissue Hydroxyapatite Induced pluripotent stem cells Myocardial infractions Mesenchymal stem cells Nanoparticles Polycaprolactone Poly(lactic-co-glycolic) acid Stromal-derived factor-1 alpha Very small embryonic-like stem cells

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Introduction Organ manufacture has been a long-held human ambition. Since then, many novel organ-manufacturing methods, technologies, and ideas have surfaced. With the fast advancement of rapid prototyping (RP) technology, we can now create bioartificial organs with multicellular components, hierarchical structures (particularly branching vascular networks), and complex functions that resemble their natural counterparts. Organ manufacturing might theoretically be described in two ways: in a wide sense and/or limited sense [1]. Organ manufacturers are creating organ replacements from any molecular components on a broad scale. In a broad sense, organ manufacture is creating bioartificial organs applying heterogeneous adult stem cell types/diverse growth factors and various biomaterials and processing technologies, including sophisticated RP and integrated mold technologies [2]. Unlike in vivo organ creation at the molecular level, it is a bridging subject that encompasses many disciplines of science and technology and may be finished in vitro at the cellular level (Fig. 1) [3].

Regenerative medicine Cell therapies and stem cells technologies

Tissue engeneering

Organ manufacturing Materials processing technologies

Drug screeing and delivery

Nanotechnolo gies

Fig. 1 Relationship between organ manufacturing and other sciences and technologies

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Tissue engineering technologies aim to create tissue and organ replacements that can preserve, restore, or increase the functions of their wounded counterparts in vivo [1]. Various issues that have hampered tissue engineering technology’s therapeutic uses have been previously discussed. These limitations included scarcity of immunologically compatible renewable sources of functional cells, a scarcity of suitable biomaterials with the requisite biological, mechanical, and chemical properties, and the loss of the ability to create huge, vascularized tissues. One of these difficulties was the lack of native tissues that could readily integrate into the host’s internal body environment [4]. However, because of our better knowledge of materials science, chemistry, biology, and engineering methodologies and the confluence of these fields, the area of tissue engineering has made remarkable progress in solving the hurdles during the last decade (Fig. 2). Cardiac tissue engineering strives to build functional tissue constructions to help the wounded heart recover its form and function. Engineered constructions may also be used to create high-fidelity heart development and illness models for research. The cell’s biological potential – the true “tissue engineer” – is unleashed by creating highly regulated three-dimensional settings that mediate cell differentiation and functional assembly [5]. A few definitions and ideas must be addressed to understand sustainable tissue engineering. Tissue engineering is the purposeful and controlled stimulation of specified target cells using a systematic mix of mechanical and molecular signals to produce more tissue for the human body’s treatment

Fig. 2 Tissue engineering summary (progress in the past decade)

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regeneration [3]. Therefore, there will almost always be a vehicle to precisely regulate the required processes. Scaffolds are a common term for such vehicles. However, the word “template” is favored since it refers to a distinct concept from scaffold biomaterials and avoids the outdated notions of scaffold biomaterials. Tissue engineering is one of the regenerative medicine research platforms, of which the remaining two are: cell treatments and gene therapies [6]. Tissue engineering has been around since the late 1980s. Therefore, there have been 30 years of advancements in this discipline. However, many scholars have written on tissue engineering’s “potential” with caveats regarding the challenges of reaching accomplishment. The year 2004, alluding to the lack of clarity around its commercial and clinical use, looked at the disadvantages and advantages of tissue engineering [7]. Several companies had been formed to commercialize tissue engineering during the previous decade. However, with a slew of high-profile bankruptcies and shifts in corporate leadership, the tide had turned, leaving little hope that the investments would be recovered [8]. The discipline has failed to achieve the broad clinical and economic success promised; there were also significant issues with clinical research conduct that had put the field back. Tissue engineering has struggled to deliver on its early promises for various reasons, many of which, as mentioned above, are infrastructure related, such as reimbursement, health economics, legislation, bioprocessing, and ethics. However, ways ahead of or around these practical barriers would have been discovered if research concepts underlying promises had been sufficiently developed [9]. The scientific variables mainly concern the capacity to convey the mechanical and molecular signals discussed earlier, allowing for the new generation and maintenance of new tissue with acceptable functional and morphological features. In this complicated context, the roles and effects of target cells and a wide range of biomaterial and biomolecules templates must all be addressed [10]. Nonspecialized cells with limitless or prolonged self-renewal ability, capable of differentiating into numerous cell types and functioning as a reservoir to manufacture, repair, maintain, and regenerate tissues while also maintaining their population, are referred to as stem cells (SCs) [11]. SCs are classified into six groups depending on the biology of the source from which they were derived, each of which corresponds to a different stage in the formation of a whole organism (Fig. 3). Embryonic SCs (ESCs) are SCs that are discovered in the early stages of embryonic development, fetal SCs (FSC), or FSCs from extraembryonic tissues, which may be acquired from the tenth-week postfertilization in the fetal stage; adult SCs (ASCs), including hematopoietic, mesenchymal, and stromal cells; VSELs (very tiny embryonic-like SCs) are seen in both the adult and embryonic phases of development; and, finally, there are induced pluripotent SCs (iPSCs) and nuclear transfer SCs that are obtained by reprogramming (NTSC) [12, 13]. MSCs’ therapeutic potential is mostly linked to two factors: first, the replacement of injured tissue by differentiation into multiple cell lineages, and, second, an immunomodulatory activity that regulates immune responses. An increasing body of research shows that the protective benefits of MSCs for injured and diseased tissues are due to alternative immunomodulatory modalities rather than long-term

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Fig. 3 Stem cell sources in the different stages of human development and stem cell extraction by reprogramming. (Adapted with permission from Ref. [13], Copyright 2022, Advance online publication)

engraftment and differentiation of the integrated MSCs [13]. Table 1 summarizes information from research published in the previous 5 years on a specific biologic trait of each SC. The International Society for Cellular Therapy (ISCT) established basic criteria for characterizing human MSCs, including adherence to plastic and in vitro trilineage differentiation potential into chondrogenic, osteogenic, and adipogenic lineages (Table 2). The properties of the most studied MSCs produced from various tissues have been summarized in this chapter (Fig. 4). This is partly due to accessibility, isolation ease, and MSC-based healing effectiveness.

Bioprinting Technologies and Cell Sheet Tissue Engineering Tissues and organs comprise several cell types surrounded by tissue-specific ECM secreted. This highly populated milieu enables effective cell-cell and cell-ECM interactions, which affect cell destiny and function [17]. This unique technique has been utilized to build implantable devices from various human cells, with or without a temperature-responsive polymer. Their safety and effectiveness have been

References

Source

Potency

Characteristic Definition

[12–14]

Embryonic stem cells (ESC) Apart from the zygote, this is formed from the blastocyst’s inner cell mass or cells accessible from the morula stage Totipotent (Zygote) Pluripotent (Morula, blastocyst) Zygote Morula Blastocyst

Extra foetal tissues (umbilical cord, amniotic fluid, and placenta)

Pluripotent Multipotent

Fetal stem cells Directly produced pluripotent and multipotent stem cells from the fetus or extra fetal tissues

Peripheral blood, adult bone marrow, and umbilical cord blood

Very small embryonic-like stem cells (VSEL) Nonhematopoietic pluripotent stem cells that are negative to the lineage marker CD133 Pluripotent

Table 1 offers a summary of research findings on a certain biologic feature

Bone marrow, adipose tissue, skin, skeletal muscle, heart, liver, and blood are all examples of tissues

Pluripotent Multipotent

Adult stem cells (ASC) Quiescent stem cells that are present in an adult organism’s tissues

Somatic cells

Induced pluripotent stem cells (iPSC) Produced with simultaneous expression of inductor genes in somatic adult cells in vitro Pluripotent

Somatic cell nuclei and enucleated oocytes

Nuclear transfer stem cells (NTSC) The insertion of a nucleus from a somatic cell into an enucleated oocyte produces a single cell Totipotent

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Table 2 Characteristics of the common different tissue-derived MSCs Source tissue Bone marrow Adipose tissue Umbilical cord Peripheral blood Amniotic fluid Dental pulp Skin Endometrium Synovium

Characteristics CD105, CD90, STRO-1, CD73, CD73, CD90, CD13, CD29, STRO-1, CD44, CD166, CD71, CD105 CD29, CD73, CD44, CD105, CD90 CD105, CD44, HLA-ABC, CD90

References [15, 16]

CD44, CD105, CD90, CD13, CD29, CD120a, CD71 CD44, CD29, CD105, CD90 CD44, SSEA-4, CD73, CD105, CD90, CD166, Vimentin CD90, CD29, CD105, CD73 CD44, CD147, CD90, STRO-1, CD105

Fig. 4 The methodology proposed to characterize mesenchymal and stromal cell cultures based on their collection site (specific surface markers according to the literature) and the three minimum general techniques to characterize MSCs after differentiation into the target cell population. (Adapted with permission from Ref. [13], Copyright 2022, Advance online publication)

established in preclinical and clinical settings for various therapeutic reasons (Table 3). Significant progress has been made in producing tissue analogs with high degrees of architectural biomimicry. For example, the use of bidirectionally aligned temperature-responsive electrospun scaffolds has been used to recapitulate natural anisotropic tissue topographies, photolithography on non-cell-adhesive anisotropic patterns, microcontact printing of aligned fibronectin patterns, and temperatureresponsive polymers are grafted onto micropatterned poly(dimethysiloxane) substrates or unidirectional mechanical stimulation [18]. Multilayered cell sheet stacking has been suggested for the synthesis of 3D tissue-like assemblies, which

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Table 3 Indicative examples of human scaffold-free cell system success stories in the experimental and clinical context Clinical indication Skin

Technology description On temperature-responsive plates, three layers of human ADSCs sheets were grown

In three cellular constructions, human EKs, DF, and DMECs were grown on temperatureresponsive plates

Cartilage

Three layers of autologous human CCs were cocultured with synovial cells on temperature-responsive plates

Bone

Human DPSCs developed toward the osteogenic lineage using a helioxanthin derivative in temperatureresponsive pans

Cornea

Human autologous oral mucosal epithelial cells were grown on temperatureresponsive plates with mitomycin C-treated 3 T3 feeder cells

Preclinical/Clinical outcome Transplantation into mice with full-thickness wounds resulted in new vascularization, thicker epidermal regeneration, and new hair follicle production, 21 days after the procedure Cells engrafted into the host wound bed were present in the new tissue generated up to 14 days after implantation when transplanted into mice with full-thickness wounds. The 3D constructions played an important role in reepithelialization and neovascularization Eight individuals with knee osteoarthritis participated in a clinical investigation. Thirtysix months after surgery, cell sheets enhanced hyaline cartilage repair Transplantation of DPSC sheets treated with helioxanthin derivatives into mice calvaria defects revealed that 8 weeks after transplantation, DPSC sheets treated with helioxanthin derivatives caused more widespread bone regeneration than control sheets Four individuals with an entire limbal deficit were studied in a clinical investigation. The ocular surfaces were completely epithelialized 1 week following cell implantation. Corneal transparency was restored, and visual acuity increased dramatically after surgery. All ocular surfaces remained clear throughout a 14-month follow-up period

References [17, 20]

(continued)

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Table 3 (continued) Clinical indication Heart

Technology description Temperature-responsive dishes were used to cultivate human autologous skeletal stem cell sheets

Sheets of human BMSCs grown on temperatureresponsive culture dishes

Esophagus

Periodontal ligament

Three layers of human iPSCs (cardiomyocytes, endothelium cells, and vascular mural cells) were cultivated on temperatureresponsive culture plates to differentiate into cardiovascular cell populations (cardiomyocytes, endothelial cells, and vascular mural cells) On temperature-responsive culture trays, human oral mucosal epithelial cells were grown

Three layers of human autologous periodontal ligament–derived cell sheets were grown on temperatureresponsive culture plates with

Preclinical/Clinical outcome A clinical investigation included 15 individuals with ischemic cardiomyopathy and 12 dilated cardiomyopathy patients. The exercise capacity and symptoms of individuals with ischemic cardiomyopathy were improved when cell sheets were implanted. However, in individuals with dilated cardiomyopathy, effectiveness was limited Transplantation of myocardium from pig ischemic cardiomyopathy models over the infarct myocardium reduced left ventricular remodeling and enhanced heart function 8 weeks after the procedure Transplantation of athymic nude rats’ infarcted hearts increased cardiac function and neovascularization considerably 8 weeks after the transplant

References

Nine patients received esophageal endoscopic submucosal dissection in a clinical study to remove superficial esophageal neoplasms. Complete reepithelialization happened within 3–4 weeks after cell sheet implantation, and no patients developed dysphagia, stricture, or other problems resulting from the surgery Ten individuals with periodontitis were studied in a clinical investigation. The periodontal probing depth, clinical attachment gain, and (continued)

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Table 3 (continued) Clinical indication

Liver

Facial nerve injury

Technology description

Preclinical/Clinical outcome

a medium containing autologous serum until they reached confluence Human BMSCs were differentiated into three layers of hepatic cell sheets and cultivated on temperatureresponsive culture plates after being treated with hexachlorophene

radiographic bone height improved 6 months after the transplant Transplantation into nonobese diabetic severe immunodeficient mice with acute liver damage reduced the severity of the lesion, improved regeneration, and increased the mice’s survival rates The crushed buccal branch of the facial nerve was transplanted into immunocompromised rats. The injury was covered with a cell sheet. The cell sheets preserved nerve structure after nerve damage, increased axon regeneration and extension, and improved electrophysiological functioning

For 10–12 days, human DPSCs were plated in six-well plates at 200,000 cells per well in a medium containing 20% fetal bovine serum and 5 ng/ml fibroblast growth factor 2

References

has resulted in the formation of complex microtissues in vitro (e.g., skeletal musclelike tissue from myoblasts), myocardial-like tissue out of cardiomyocytes, annulus fibrous-like tissue out of bone marrow MSCs, tubular neural-like tissue out of astrocytes, and iPSC-derived neurons [19].

3D Bioprinting The ability to generate tightly controlled 3D structures for tissue engineering has advanced significantly due to two important modular self-assembly technologies and 3D bioprinting. Programmable modular self-assembly makes it simple to create complex synthetic designs. Under the correct physical conditions, pairs of DNA strands with sequence complementarity may create self-organizing structures. This approach may induce biomaterial and tissue building pieces connected using specifically constructed programmable DNA glues to design and build at length scales ranging from a few hundred micrometers to centimeters [3]. Three-dimensional (3D) bioprinting is a kind of 3D printing that involves the deposition of noncellular materials, and allows for unparalleled flexibility in manipulating biomolecules and cells (e.g., ECMs and proteins, while maintaining exact spatial distribution and control over composition to imitate genuine tissues’ exquisite

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form, structure, and architecture). Since its introduction in cell-laden inkjet printing, biofabrication technology has advanced to the point that it is now widely applied in tissue engineering. Bioprinting may be done using a wide range of biomaterials, allowing it to be used in many tissue types. In hydrogel matrices, sacrificial bioprinting has enabled the creation of linked vascular networks. By preventing freeform shapes from collapsing due to gravity during the bioink deposition process [3, 21], the capacity to generate complex tissues with spatial heterogeneity in cell and matrix compositions was increased using bioprinters with several nozzles extruding diverse biomaterials.

4D Bioprinting Smart biomaterials that can change their morphologies over time controlled due to environmental stimulation as humidity, temperature, and pH have recently been merged to form a novel technique known as four-dimensional (4D) bioprinting [22]. In addition to the spatial hierarchy, 4D bioprinting’s unique additional time dimension promises to give generated tissues dynamic temporal control. Natural tissue architectures, such as decellularized tissues, have also gained favor as a source of scaffolds [23]. Decellularized entire organs and their treatments in whole-organ engineering, on the other hand, have only been around for roughly a decade. To eliminate all cellular and immunogenic species while conserving the underlying ECM, detergents are injected into separated donor organs and maybe an integrated vascular network in this approach [22]. To repopulate and functionalize these decellularized organs, desired cell types and/or stem cells (i.e., iPSCs from patients) may be injected. Several organs, including blood arteries, lung, heart, kidney, liver, pancreas, and bladder, have been produced using this method [24]. Although decellularized organs may best approximate the morphological complexity of pristine organs and, presumably, their function, the rarity of donor sources precludes the technology’s widespread adoption in organ transplant surgery [2].

3D Engineered Cardiac Tissue Models Engineered cardiac tissue models are emerging as potential tools for restoring damaged heart tissue and testing drugs and toxicity. Implantable supports that mirror the architecture and content of the cardiac ECM, which is critical for improving cell differentiation, survival, and proliferation in vivo, are conceivable using the synthetic or natural biomaterials described previously. Engineered cardiac tissue models have been designed to mimic the heart’s environment while also providing the necessary electrical and mechanical support for cell distribution within the heart [25]. Improvements in PSC differentiation strategies toward the cardiac lineage inspired the human heart surrogates for autologous cell therapy, disease modeling, and pharmacological testing [26]. In this context, embedding PSC-derived CMs within artificial structures has been shown to have variable degrees of beneficial

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effects on cell maturation, dependent on the 3D structures’ unique characteristics. Even though the quality of human iPSCs and ESCs have improved over time, cell maturation after cardiac stimulation in vitro remains a significant challenge. The first data demonstrating the development of CMs in 3D engineered constructions was provided [27], and CMs generated from human ESCs were also used to produce an EHT. When ESC-derived CMs were merged with fibrin-based 3D constructs, they exhibited a more organized sarcomeric structure than those cultured in 2D under usual conditions while having an immature electrophysiological profile. EHTs generated from human iPSC-CMs were studied for their morphology and function and their potential for drug screening [28]. The EHTs were attached to two flexible silicone supports, which served as a preload against which the EHTs may contract auxotonically. The CMs developed muscle bundles with better alignment and sarcomeric organization due to this therapy. Overall, EHT constructions are appropriate supports for generating CMs generated from human PSCs because they provide the cells with a physiologically similar environment and stimuli [26]. Due to their improved capabilities, EHTs may be used as a model platform for in vitro investigation of human heart functions in terms of force, peacemaking activity, contractile properties, and electrophysiological parameters [28]. Furthermore, it is a viable resource for future heart regeneration treatments. In preclinical studies, transplantation of iPSC-derived EHTs enhanced left ventricular function in a guinea pig model of MI. In certain circumstances, these 3D-EHT structures were also related to the host heart tissue. Other research looked at the raised risk of cardiac arrhythmia due to EHT implantation [29]. After the EHT transplant, telemetric devices were implanted. The heart’s electric activity was monitored for 28 days; the results revealed that the EHT transplant did not affect the occurrence of ventricular arrhythmias [30]. On the other hand, other research discovered a link between the transplantation of iPSC derived CMs and transitory aberrant beating, leading to malignant ventricular arrhythmia [31]. Additionally, notwithstanding the encouraging results obtained from in vivo cell transplantation using heart-engineered tissues, other research found that cardiac patches had a limited impact, dampening early optimism about their quick translation to the clinic. For example, tissue-engineered biodegradable patches seeded with iPSC-CMs were employed [32] to evaluate its applicability in vivo. Researchers employed tissue-engineered biodegradable patches consisting of the polyglycolic acid poly-copolymer. Their findings revealed that no iPSC-CMs remained in the patch 4 weeks after implantation, highlighting the need to conduct thorough tests that account for all possible factors (biomaterial, kind of cells, and degree of heart damage) before using as tissue replacements in patients. In a rat model of MI, Park et al. [33] observed that iPSC-CMs and human mesenchymal stem cell–loaded patches might improve cell retention and tissue regeneration in the heart. The patch’s human mesenchymal stem cells should provide a favorable environment for vascular repair. A new layer of complexity to tissue-created constructions emphasizes the need for vascularization. One of the biggest issues with using EHT for cardiovascular regeneration is a lack of vascularization [34]. To construct an engineered perfusable microvasculature system, the scientists merged stem cell methods with sophisticated tissue engineering. They inserted ECs produced from ESCs into patterned collagen

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matrix and microchannels, indicating that ECs may produce de novo vasculature in these specified constructs by vascular remodeling and neo-angiogenesis [35]. These investigations found that cardiac engineering constructions improve iPSC-CM intercellular organization and interaction, improve cardiac performance after transplantation in vivo, and maturation in vitro, hinting that they could be employed to achieve cardiac regeneration in people 1 day [26].

Natural Polymers–Based Biocomposites: State of the Art, New Challenges, and Opportunities Polymers derived from animals and plants are known as natural polymers. Natural polymers contain chitin, starch, cellulose, alginates, casein, soy protein, polyhydroxyalkanoates, polylactic acid, and hemicelluloses alginates. Many researchers have been attracted to natural polymers because of their appealing properties for medicinal applications [36]. In addition, they are biocompatible, biodegradable, renewable, plentiful, and natural. The production of innovative polymeric materials as nanocomposites, composites, and blends by mixing natural polymers with additional fillers and polymers is now the focus of study [37]. Natural polymers have been studied for medication and bioactive chemical delivery. They are easily changed for drug administration, have specialized interactions with biomolecules, and are degraded by enzymes in a regulated manner. Aside from tiny molecular weight medications, wound healing, and anticancer treatments, natural polymers may be utilized to transport proteins, DNA, and tissue engineering [36]. Natural polymers have a variety of reactive groups, allowing for the introduction of activity-specific functional groups with a variety of changed yet desired physiochemical characteristics. Enzyme degradation qualities, biocompatibility, nontoxicity, stability, gel-forming ability, harmlessness, and the capacity to be chemically and biochemically changed as needed are all advantages of polysaccharides [37]. Polysaccharides include chitosan, dextran, agarose, alginate, hyaluronic acid, cyclodextrin, carrageenan, and protein-based polymers like albumin, collagen, gelatin, and soy that are examples of natural polymers. These polymers have a wide molecular distribution and batch-to-batch variability, posing several problems [38]. After reviewing the types of scaffolds and their various sources, an introduction to the definition of the scope of the promising applications of tissue engineering and scaffolding from various sources in solving intractable dilemmas for tissue damage treatments and engineering laboratory organs with stem cells, we will focus on the kind of scaffolds, which are natural cellulose scaffolds, and the potential for their application in the treatment of cardiovascular disorders [2].

The Characteristics of Biodegradable Polymers Due to features like tunable breakdown rates, optimum porosity, biocompatibility, and elastomeric qualities, biodegradable polymers have drawn much interest in

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cardiac tissue engineering regeneration (which can mechanically favor inherent tissue contraction to cardiac function) [39]. Features like scaffolds’ ability to maintain mechanical qualities during tissue formation, their delayed disintegration into biocompatible chemicals, and their ability to accept cells, growth agents, and other substances are only a few. Natural and synthetic polymers are the two primary groups of biodegradable polymers. Natural polymers are polymers that occur naturally in the environment. On the other hand, humans create synthetic polymers from nonrenewable petroleum resources [40]. Despite these advantages, conventional tissue engineering has proved difficult to restore cardiovascular tissue, a problem that nanomedicine may be able to solve.

Carboxymethyl Cellulose With an annual production scale of around 75–100 billion tons for industrial and therapeutic purposes, cellulose is the most plentiful natural polymer and is renewable [41]. Cellulose is a glucose-derived linear polymer that provides mechanical support to bacteria, plant cells, fungi, and algae. The majority of commercial cellulose fibers are derived as pulp (natural fibers) or as a synthesis from natural cellulose found in wood, cotton, barks, jute, and leaves (synthetic fibers) [42]. Because of its intermolecular connections, cellulose may be semicrystalline, water insoluble, rigid, and form long fibrillar parallel threads. A particular enzyme termed terminal complexes catalyzes the supramolecular assembly of cellulose macromolecules (TCs). Established procedures with numerous consecutive phases have been used (e.g., cellulose sugarcane bagasse). Preheating fibers at 37  C, bleaching with sodium hypochlorite, washing with potassium chloride (KCl), and removing hemicellulose with potassium hydroxide were all part of the process [43]. Despite its potential uses in various industries, cellulose’s original chemical structure has significant drawbacks, including low solubility, wrinkle resistance, nonthermo-plasticity, and high hydrophilicity [44]. Furthermore, only cellulases (exoglucanases, glucosidases, and endoglucanases) produced by certain microorganisms, like symbiotic fungi, protozoans, and bacteria, can degrade natural cellulose. Although cellulose breakdown in an acidic environment produces human stomach enzymes, D-glucose cannot break the beta bonds between the glucose units [45]. Chemical changes of cellulose’s hydroxyl groups have been proven to considerably enhance solubility, making them a viable choice for expanding cellulose’s uses. Furthermore, these changes have shown to be quite beneficial in improving the processability and handleability of the product, which may be adjusted for certain purposes. Certain modified celluloses have thermoplastic properties, which may be used in bioplastics, injection molding, and other applications [21]. Natural cellulose exists in four polymorphs: cellulose I, II, III, and IV, in which its crystalline form (cellulose I and II) undergoes chemical processes to become amorphous (cellulose III and IV) [42]. To manufacture various derived forms such as cellulose II and cellulose III/IV, alkalis such as sodium hydroxide (NaOH) and cuprammonium hydroxide (CuH14N4O2), as well as reagents such as amines and

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glycerol, can be used. Based on the chemical group that has been transformed, cellulose derivatives are separated into cellulose esters and ethers [44]. In the 1920s, carboxymethyl cellulose (CMC) became the first cellulose ether to achieve commercial and industrial significance. It maintained its dominance in the worldwide market until 2024 (Carboxymethyl Cellulose Market Analysis) [42]. Simple synthesis processes and low-cost, basic ingredients may be used to make CMC. Compared to other cellulose derivatives, this is the primary reason behind CMC’s large-scale commercial potential. CMC is used in the textile, paper, pharmaceutical, cosmetics, and food sectors, much as cellulose and other cellulose derivatives. CMC is preferred over other cellulose derivatives because of its thickening, suspending, binding, and emulsifying capabilities [46]. Because of its outstanding functional properties, CMC is utilized in pharmaceutical, nonfood, and food applications like toothpaste, ice creams, tablets, beverages, oil drilling mixtures, detergents, and paints [45]. The Food and Drug Administration in the USA has approved CMC as an inactive or active component in different dosage forms like capsules, tablets, suspensions, paste, gels, and injections for buccal, ocular, intramuscular, and intraarticular purposes. Because it is a polyelectrolyte, CMC is sensitive to pH and ionic strength [42]. As a result, it is compatible when CMC is mixed with other polymeric solutions. This characteristic is crucial for biomaterial scaffolds, hydrogels, and nanoparticles transporting drugs (Fig. 5) [47].

Fig. 5 Carboxymethyl cellulose: properties and biological uses. (Adapted with permission from Ref. [45], Copyright 2021, Elsevier)

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CMC tissue scaffolds benefit from biocompatibility, delayed degradability, mechanical stiffness, and surface characteristics that cell aggregation facilitates protein adsorption and differentiation. Compact CMC chains have also been shown to facilitate cell alignment and improve electric conductivity for heart cell regeneration [48]. Utilizing a mix of cells, scaffolds, and growth factors is common in traditional tissue engineering procedures. Growth-promoting chemicals and cells are introduced to the platform to encourage regeneration. These approaches have resulted in effective scaffolds for skin, bone, myocardium, liver, trachea, and esophagus. Although tissue engineering has had some therapeutic success, most scaffolds do not accurately replicate genuine tissues’ complex and complicated architecture [49]. Furthermore, tissue engineering scaffold manufacturing procedures may not entirely replicate the ability of biological tissues to locate cells in particular 3D planes and necessitate the use of chemical solvents, which may inhibit cell growth. Due to the absence of original architecture and vasculature, these scaffolds may not be able to support biomimetic cell development in 3D completely [50]. Threedimensional (3D) bioprinting has been considered a promising source for producing biomaterial with well-defined micro/nano-designs that include cells, bioactive growth factors, and scaffolds with well-defined micro/nano-designs [23]. In 3D bioprinting, tissue scaffolds are created with high-precision and human-scale cellular structure. Using computer-aided programming tools, the 3D bioprinting process creates precise biological structures that completely replicate the natural anatomical design of organs/tissue. Due to this advantage, 3D bioprinting technology has sparked much interest in regenerative medicine applications [21]. CMC has recently gained traction as a possible bioink for 3D bioprinting of biological and organs structures. Bioinks blend hydrogel polymers and cells that may be used to print stiff biological structures. Bioinks are classed as structural, sacrificial, supporting, 4D printable, and functional bioinks based on their purpose and properties [51]. CMC and its mixed polymers have been widely employed as a scaffold biomaterial in various tissue engineering applications, including skin, bone, cartilage, and the liver. The role of CMC in tissue engineering applications is examined in detail, emphasizing scaffold formation, biocompatibility, and regeneration efficiency [45].

Preparation of CMC–Based Scaffolds for Use in Tissue Engineering Although autografting is the gold standard tissue regeneration approach, it has several limitations, including donor availability, site morbidity after surgery, and patient discomfort. In addition, xenografts and allografts, for example, produce significant immunological rejection, tissue remodeling, and bacterial contamination, severely limiting their clinical viability [47]. The optimum properties of the scaffold materials and scaffold for effective tissue regeneration are: (I) The finished scaffold should have the right amount of porosity and connectedness and pore size for each tissue type, facilitating cellular infiltration, nutrient transport, gas, and ECM metabolic wafering. (II) The polymeric materials utilized to produce the scaffolds also

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provide a viable and hospitable setting for cells to perform cellular modulatory tasks such as cell adhesion, proliferation, maturation, and differentiation in vitro and in vivo. The scaffold’s degraded components should be naturally eliminated and not cause any injury or inflammation. (III) The scaffold should be mechanically sound and resistant to various pressures, such as stress and shear forces comparable to natural tissues, allowing tissue remodeling and stress shielding. (V) Scaffolds must be nonallergic, noncarcinogenic, and compatible with optimum conditions [45, 52]. Some of the traditional and rapid prototyping (RP) scaffold fabrication techniques that can be used to create scaffolds with various characteristics include phase separation, solvent casting, freeze-drying, electrospinning, stereolithography (SLA), emulsion, fused filament fabrication (FFF), selective laser sintering (SLS), and 3D bioprinting [53]. These approaches are used to create CMC-based scaffolds in specific shapes such as microspheres, hydrogels, films, fibers, and sponges to be used in a variety of tissue engineering applications. CMC-based scaffolds were constructed either physically or chemically to boost the requisite features for tissue engineering applications [54].

Chitosan-Based Biomaterials in Tissue Engineering Applications Several natural and synthetic materials have been used in tissue engineering applications, including chitosan, collagen, gelatin (GL), alginate (Alg), silk fibroin, hydroxyapatite (HAp), hyaluronic acid (HA), polyethylene glycol (PEG), polylactic acid (PLA), poly(lactic-co-glycolic) acid (PGLA), and polycaprolactone (PCL) [55]. Chitosan is made from chitin, which is present in the shells of live animals such as crabs, lobsters, tortoises, shrimps, and insects [56]. It stands out because of its abundance, adaptability, and unique qualities, such as biodegradability, biocompatibility, nontoxicity, hydrophilicity, antibacterial and antifungal capabilities, and wound-healing activities of other biomaterials. [57]. Chitosan is made by partially deacetylating chitin via a chemical or biological technique or by combining the two. Although there are no specific guidelines for defining chitosan, it is often defined as chitin with a degree of deacetylation (DD) of 70% or above. Nonetheless, most commercially available chitosan has a DD of 70–90%; even DDs greater than 95% may be achieved by further deacetylation procedures [58]. However, these materials do not match the criteria for tissue engineering due to limitations such as uncontrolled breakdown, infection risk, poor mechanical qualities, difficulty in bioaccumulation of degradation products, and local acidic environments [59]. Researchers have created hybrid biocomposites with outstanding characteristics to solve these issues. To overcome the limitations of single-component composites, they mixed two or more biopolymers with inorganic elements [46]. Because of its unique characteristics and availability, chitosan has been extensively researched as a possible bioactive material among biopolymers. Chitosan has recently been shown to be a potential bioactive material for tissue engineering (bone, skin, cartilage, intervertebral disc, blood vessel, and so on) that may be utilized to heal sick and injured tissue [59]. Several chitosan and chitin derivatives

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Fig. 6 Applications of chitosan-based bioactive compounds are shown in this diagram. (Adapted with permission from Ref. [59], Copyright 2020, Elsevier)

and their composites and a broad range of functional materials have been studied in tissue engineering applications to introduce desirable features [45]. Figure 6 shows a schematic representation of several chitosan-based bioactive materials applications.

Tissue Engineering Applications Cardiovascular Disease Myocardial infarction (MI) is characterized by cardiac tissue necrosis caused by coronary artery blockage, a disease that permanently reduces oxygen and nutrition supply to the heart [50]. Even though effective medicines involving surgical techniques are increasingly being used to alleviate the symptoms of cardiac illnesses such as valve and artery problems, therapeutic options for injured myocardium remain restricted and inefficient. Furthermore, fibrotic scar occurs following an ischemic insult, interfering with the electrical and mechanical functions of the heart tissue [9]. Because of the increased stiffness of the myocardial matrix and

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the altered compliance of the heart chambers, the presence of fibrotic scar tissue reduces the ejection fraction, compromising electrical characteristics of the heart and, as a result, the cardiac outcome [60]. Currently, none of the regularly applied treatments can eliminate such fibrotic scars, allowing new functioning cardiac cells to replace the destroyed heart tissue. Due to these clinical circumstances, heart failure is associated with a poor outcome in individuals who survived a heart attack because the cardiac muscle undergoes unfavorable remodeling. The poor regenerating ability of heart tissue is the fundamental disadvantage of all prospective treatment options for HF [43]. The adult human heart’s regeneration ability is severely limited, with a cardiomyocyte (CM) renewal rate of less than 1% each year. As a result, the best way to heal the injured myocardium is to boost local CM proliferation in vivo or deliver new CMs to directly replace necrotic tissue [6]. Heart failure and cardiovascular disease (CVD), such as myocardial infarction (MI), continue to be the leading causes of morbidity and mortality in developed countries despite considerable medical advances [44]. The adult heart muscle cannot heal wounded myocardial tissue, which is replaced by noncontacting scar tissue, resulting in heart failure. Unfortunately, other than full heart transplantation and pharmaceutical medications that thin blood to make it easier to pump by a failing heart, there is no conventional therapy option for repairing the injured myocardial. As a result, there is a pressing need to discover innovative techniques for repairing injured heart tissue [61]. Cardiac tissue engineering (CTE) has recently emerged as a feasible approach for generating cardiac grafts, whether whole heart replacements or tissue replacements. This might be successfully implanted in an organism, rebuilding tissues for a completely functional heart without the risk of immunological rejection [62]. In addition, tissue engineers may create highly controlled three-dimensional environments to enhance cell differentiation and functional assembly in heart development and cardiovascular disease studies (Fig. 7). The main problem in cardiac tissue engineering restoration of an infarct myocardial is to provide a bioactive substrate with acceptable biological, chemical, and electrical characteristics, thereby physically and functionally matching the normal heart extracellular matrix (ECM) [63]. In this approach, two important tissue engineering and tissue regeneration methods are useful: (1) 3D porous scaffolds made using biomaterial technologies to guide and support tissue growth from dissociated cells and (2) ex vivo tissue engineering bioreactor culturing of 3D cell structures to simulate the normal pressures and fluxes faced by heart tissue [5]. Traditionally, CTE utilized two types of matrices: scaffolds, which facilitate the diffusion of growth factors and donor cells into damaged hydrogels and heart tissue, and a water-insoluble polymer matrix with a high water content applied to construct an array of tissue engineering constructs [64].

Cardiovascular Tissue Engineering The success of regenerative medicine approaches to the cardiovascular system and biomaterials-based tissue engineering treatments is difficult to assess. After two decades of research, Vunjak-Novakovic recently remarked on the reality that the goal of cardiac tissue engineering, establishing a standard of care based on cell therapy, remained unmet [6].

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Fig. 7 In cardiac tissue engineering, stem cell–based techniques are used. (Adapted with permission from Ref. [63], Copyright 2021, Elsevier)

The traditional idea of tissue engineering scaffolds will not apply to the myocardium or myocardial infarction therapy; instead, an injectable biomaterial that distributes signals and possibly cells will be significantly more important. The lack of a three-dimensional adaptable biomimetic milieu and direct exposure of any cells injected into the myocardium to oxygen tension, free radicals, and inflammatory cytokines cause most injected cells to die [65]. This necessitates using synthetic biomaterials to effectively carry cells to the myocardium, with hydrogels being the most plausible option. Hydrogels, both artificial and natural, have been explored. Native ECM molecules, once again, may provide a superior foundation for the needed hydrogels, with collagen and fibrin attracting attention [66]. However, more delicate molecular engineering techniques may be required since generating hydrogels with adequate stiffnesses is challenging. In an animal model, the authors explained the development of “small molecular hydrogels” based on peptides like DFEFKDFEFKYRGD, which have been demonstrated to offer a framework for hepatocyte growth factor–modified mesenchymal stem cells, preserving heart function while reducing ventricular remodeling [67]. Many more unconventional methods, such as cell sheet engineering, will be available, which, despite initial excitement, have taken a long time to reach clinical trial stages [68].

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The area of heart valve replacement provides an interesting and distinct perspective on tissue engineering’s role. This field has no unmet clinical needs since most patients can have fully appropriate implanted devices, either surgically or by a catheter. Tissue-engineered valves are most promising in pediatric applications, where devices that can adapt to growing youngsters are required [52]. Cell penetration and de novo tissue development are weak in most synthetic degradable polymers, such as poly (lactic acid) and polycaprolactone. Combining these polymers with natural biopolymers, such as PCL and chitosan, in a sheep model by Fukunishi may increase performance [58]. Three platforms could define cardiovascular tissue engineering products: (1) cellfree scaffolding techniques, (2) cellular scaffolding techniques, and (3) cell-scaffold hybrid techniques [69]. Based on the strengths and shortcomings of the goods, each platform method incorporates several research areas and products and has exhibited varying degrees of success in advancing therapeutic items to the clinic (Table 4). This study will discuss the three basic cardiac tissue engineering methodologies in this study, emphasizing preclinical models [10] or BioMEMS devices [70]. In a debate, clinical outcomes employing adult stem cells will be contrasted with clinical tissue engineering trials to repair and regenerate failing myocardium. Cardiovascular regenerative medicine may continue to push research activities toward real myocardial regeneration and increased functional activity within the heart by using multidisciplinary approaches [71].

Cardiac Tissue Engineering Products Advancing to the Clinic In preclinical contexts, tissue engineering–based products have exhibited varying development and success. In phase I clinical trials, most of the products that have made it to the clinic are hybrid cells with scaffolds. Pediatric patients with univentricular physiology were the subject of one of the first tissue engineering– based cardiac clinical studies. Patients in this experiment in the USA (clinicaltrials. gov NCT01034007) received tissue-engineered vascular grafts (TEVGs) with autologous bone marrow mononuclear cells implanted [32]. There were seven patients (28%) who had graft stenosis that required balloon angioplasty, one patient (4%) who had graft thrombosis that required anticoagulant treatment after implantation, and no graft-related deaths. This was the first human TEVG trial. Due to their late age and a lack of adequate ventricular contractile efficiency, it proved the feasibility, safety, and potential for long-term implantation success [8]. Biomaterial Scaffolds for Cardiac Tissue Engineering The mammalian heart is a complex, heterocellular environment that contains a high ECM. Structured (collagen I/III) and basement membrane (laminin/collagen IV) proteins and nonstructural glucosoaminoglycans and proteoglycans make up the cardiac ECM. Nearly 10 billion cells are arranged in a compact, well-vascularized habitat in the human cardiac ECM [72]. According to some estimations, human adults’ cardiomyocyte density is approximately 30,000 cells/mm3, with a rich vascular network (3000 capillaries/mm3) needed for optimum nutrition exchange under high metabolic demand [73]. Furthermore, from a tissue and organ standpoint,

Example

Weaknesses/ challenges

Strengths/ benefits

Cell-free scaffolds Structural formation Enhanced vascularization efficiency Synthetic or natural polymers Tunable stiffness Integration risk/host rejection Difficulty with cellular integration Scaring formation affinity Usually inducing inflammatory responses

Scaffolds-free cellular Cellular paracrine effect potential Stronger retention than single-cell injection Autologous or Allogenic Design of cellular architecture Electrical synchronization Integration risk/host rejection Cell combinations require a dosing strategy

Table 4 Tissue engineering products enhance cardiac repairing and function Hybrid cell + scaffolds Ability of scaffolding + cellular products Safety in clinical trails Potential for enhancing cardiac functional improvements More complex design Potentially weaknesses of scaffold and cellular products Not recommended for clinical studies

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the cardiac ECM possesses advanced biophysical characteristics that allow for both stiffness and flexibility in response to dynamic physiology and coordinated electrical and mechanical signal transmission throughout the heart [74]. Currently, no technology seems to be capable of accurately replicating the complexity of the original ECM environment. While cell sheet and spheroid techniques may build tissues without a scaffold, most research uses biomaterial scaffolds to spatially organize cardiomyocytes in a desired 3D form while also delivering important maturation cues. The following characteristics should be included in an ideal 3D scaffold: (1) a biomimetic diversity of binding sites in cardiomyocytes and nonmyocytes to engage functionally relevant integrins; (2) sufficient cell-binding sites to allow for natural tissue density; (3) significant tissue remodeling to facilitate rapid cell spreading, alignment, and replacement of original ECM with cell-secreted ECM; and (4) suitable biomechanical qualities to allow tissue contraction indefinitely (Fig. 8) [75]. Attempts to enhance transplanted cell engraftment by co-delivering biomaterials have found that injecting biomaterials alone, without cells, may also enhance damaged heart function [7]. In addition, multiple therapeutic effects of injectable biomaterials in post-MI hearts are now well understood, including (1) reversal of maladaptive remodeling brought on by a faulty inflammatory response; (2) abnormal mechanical loading generated by tissue thickening is reduced; and (3) inherent regenerative cell recruitment [76].

Fig. 8 Technologies and materials for cardiac tissue engineering

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The Future of Cardiac Regeneration by Tissue Engineering Technologies The intriguing subject of CTE can increase our knowledge of congenital significantly and acquired heart illnesses and aid in the development of innovative regenerative therapeutics for cardiovascular diseases and myocardial violations. A simple PubMed search for “cardiac tissue engineering” yielded almost 2800 papers in the last 2 years. We expect biomaterial research for cardiac tissue engineering and treatments to continue on its present upward trend, given its enormous potential. The introduction of cell-free biomaterials as injectable scaffolds will likely be the first viable therapy for heart illness based on tissue engineering concepts, as indicated by recently active clinical studies previously addressed. In the case of chronic ischemic or infiltrative cardiomyopathies, such scaffolds might offer mechanical stability for acutely wounded myocardium following myocardial infractions (MI) or operate as a substrate to guide positive myocardial remodeling [26]. Modifying injectable biomaterials’ bioactivity, mechanical, and degradation features will enable the development of multifaceted, spatiotemporally controlled therapeutics that precisely modulate immune/inflammatory and regenerative signals, resulting in beneficial clinical results. The next generation of injectable biomaterials will certainly include cell-sourced products to boost bioactivity [77]. Exosomes are exosomes that have been secreted. Name of the trial identifier for clinicaltrials.gov Patient numbers Substrate route of administration Results of Cardiac Injuries Ref NCT01226563 PRESERVATION I Recent STEMI Complete; no functional improvement Sodium alginate + Calcium Gluconate Intracoronary 303 [78]. Standardizing cell sources, handling processes, animal models of illness, and reproducibility throughout multiple research organizations would help speed development. As the area progresses, a strong, scalable platform for future preclinical and clinical cardiovascular research and medicine applications may readily be imagined by combining biomaterial science with iPSC-based technologies [79].

Biomaterials and Nanomedicine for Bone Repair and Bone Regeneration Strategies For orthopedic trauma surgeons, bone defects with poor outcomes (i.e., delayed or unexpected bony healing or high infection rates) remain a major concern [80]. Autogenous bone grafts and allografts are the gold standards for bone restoration in clinics among presently available procedures. However, various flaws, such as a lack of suitable donor sources, restrict their effectiveness [6]. Over the past several decades, the fast development of biomaterials and nanomedicine has opened up new avenues for improving bone regeneration technologies. Bone defects are usually divided into three subfields based on where they occur: long bones and spine, maxillofacial, and craniofacial, i.e., fractures of the femur, shoulder (mainly humerus), hip (femoral neck), wrist (radius/ulna), tibia (distal third), and ankle (above the joint, distal tibia/fibula fractures), as well as vertebral, maxillo-, and cranio-facial (jawbone, calvaria) fractures [80, 81].

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Nanoparticle-Based Strategies Nanoparticles (NPs) are the building blocks of nanomedicine, with unique qualities such as a high surface area to volume ratio and superior chemical and physical performance compared to their bulk counterparts [4]. Nanoscale ceramic particles, poly (lactic-co-glycolic) acid (PLGA), gelatin, collagen, and chitosan are good materials that are often utilized in bone regeneration and have been extensively studied as delivery vehicles or imaging probes for regenerative treatments [82]. Current techniques for treating osteoporosis, for example, include developing osteoporotic bone targeting drug delivery systems with improved therapeutic effectiveness and less nontargeted side effects. Because of their strong affinity for hydroxyapatites, bisphosphonates, one of the bone-targeting medicines, may function as a linker agent between NPs and bone minerals (Fig. 9) [83]. Scaffold-Based Strategies In the clinic, autograft and allograft are often utilized for major bone defect repair. Bone graft replacements (BGSs), such as artificial scaffolds, were created to overcome the limitations (source, size, etc.) and function loss of bones in donor locations. 3D printing of bioinspired materials has been hailed as a promising method for creating customized scaffolds made of multifunctional biocompatible materials (collagen and hydroxyapatite) (Table 5) [84]. For example, a new 3D-printed poly(lactic acid) (PLA) scaffold with bioinspired surface coatings improved the success rate of bone-implanted devices even more [88]. Collagen, minocycline, and citrate-hydroxyapatite nanoparticles might be used to coat surfaces to prevent bacterial biofilm development. Not only did the scaffold offer 3D structural support with a variable disintegration rate, but it also released medicines to encourage cellular infiltration and mineralization. Furthermore, 4D bioprinting has been created to create dynamic 3D-patterned biological constructions using stimuli-responsive materials that alter the form in response to diverse stimuli [51]. Combining scaffolds with NPs or biological morphogenetic molecules like bone morphogenetic protein 2 (BMP-2) may, on the other hand, be a practical way to accomplish several therapeutic aims. In the rehabilitation of various scale damage, several scaffold-based solutions are used: (i) synthetic scaffolds, (ii) scaffolds incorporating active compounds, and (iii) tissue-engineered/cell-seeded scaffolds [16]. Because of their high cost, instability, and side effects, identifying the proper bioactive compounds, optimum concentrations, and release kinetics from the scaffold is a therapeutic problem. In several investigations, biological components inside scaffolds have therapeutic effects in attracting endogenous host cells [65]. The use of bone structure– mimicking scaffolds combined with the insertion of appropriate proteins or growth hormones allows for the complete regeneration of critical-sized bone lesions. Engineered scaffolds with hierarchical topologies and the capacity to supply morphogenetic chemicals like BMP-2 or cell-homing growth factors like VEGF and stromal-derived growth factor-1, for example, have been shown to promote massive bone defect healing. Studying microenvironmental parameters such as mineral composition, crystallinity, microporous architecture, and growth

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Fig. 9 The three main techniques for bone restoration are presently being utilized and explored. Only synthetic bone graft alternatives are used in the first (left column) (BGS). The second (middle column) strategy involves mixing bioactive compounds with a carrier, often an extracellular matrix protein or a ceramic-based carrier. Combining stem cells with a carrier, maybe with the addition of extra bioactive chemicals, is the third (right column) step. Depending on the degree of the bone defect, each of these treatments is more suited for mending. When a defect’s repair is impeded, a biological functioning is required in addition to the BGS, which may be provided by bioactive chemicals, stem cells, or both. (Adapted with permission from Ref. [81], Copyright 2017. Elsevier)

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Table 5 A list of the materials and processes used to make bone tissue engineering scaffolds and their key benefits and drawbacks Scaffold material Metals

Examples NiTi, titanium alloy, magnesium alloy, and porous tantalum

Fabrication methods 3D printing, casting, and powder sintering

Ceramics

TiO2, HAp, β-TCP, and bioglass

3D printing, sol-gel, and selective laser sintering

Natural polymers

Collagen, chitosan, hyaluronic acid, and silk fibroin

Synthetic polymers

PLGA, PCL, PEO, and PPF

Hydrogel crosslinking, electrospinning, freeze-drying, and solvent displacement Electrospinning and cross-linking

Advantages (+) and limitations( ) + High young’s modulus + High compressive strength Not degradable Ion release + Chemically biocompatible + Can be biodegradable Brittle Prone to fracture and fatigue + Biocompatible + Biodegradable + Osteogenic Low mechanical strength + Tunable properties Acidic degradation by-products Rapid strength degradation in vivo

References [85–87]

factor release is crucial to these bone-mimicking building procedures. This strategy cleared the path for creating a “true” biomimetic new nanomaterial for in situ bone regeneration (Fig. 10) [14].

Role of Growth Factors for Bone Regeneration Healing bone fractures is a multistep process governed by a spatiotemporal cytokine lineage that includes vascular, osteocytes, mesenchymal progenitor, and inflammatory cells. Increased production of osteogenic, angiogenic, and proinflammatory growth factors activates this cytokine signaling pathway (Fig. 11). GFs have been shown to improve cell propagation, motility, osteogenic differentiation, and cell adhesion. Bioengineering aims to combine these signaling molecules with materials to speed up the bone healing [89]. TGF-, IGF-, VEGF-, BMPs, PDGFs, and FGF- are only a few GF families implicated in bone tissue regeneration. BMPs have been widely researched, and the Food and Drug Administration (FDA) has authorized devices that employ BMP-2 and BMP-7 in bone regeneration. Exogenous cytokines and growth factors were thought to have angiogenic, pro-osteogenic, cytokine, and inflammatory functions in bone repair [90].

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Fig. 10 By combining optimization of compositional, biomineralization, structural (microenvironmental), and growth factor–related mimicking, a form of “real biomimetic” scaffold may be developed. In situ bone regeneration might be aided by nanoparticles that work as a unique in vivo bioreactor with self-regenerative biological capabilities. Adapted with permission from Ref. [14], Copyright 2017, The Royal Society of Chemistry

Fig. 11 An ECM-inspired scaffold is being used to distribute growth factors for bone tissue creation. (Adapted with permission from Ref. [35], Copyright 2020, Wiley Periodicals, Inc.)

Scaffolds for GF Delivery Tissue engineering is a collaborative endeavor that includes cells, scaffolds, and growth factors. Engineering an excellent scaffold for cartilage and bone tissue should be biodegradable and biocompatible with the ECM shape, allowing cells to connect to it and move to problem locations to participate in the tissue healing

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Fig. 12 Characteristics and kinds of growth factor delivery systems ideal for bone tissue engineering. (Adapted with permission from Ref. [35], Copyright 2020, Wiley Periodicals, Inc.)

process. Aside from biocompatibility, it must endure increased mechanical pressure, be spongy to allow nutrition, oxygen, and metabolites to permeate, and be adaptable to the injured locations (Fig. 12) [84]. Bone tissue engineering is a multistep process that starts with the recruitment of osteoprogenitor cells at injured regions, followed by their adhesion, migration, and propagation to produce a matrix that runs concurrently with bone remodeling. The extremely compressed natural ECM of bone and the macro- and nanoscale configurations, and the bone matrix contributed to increased mechanical stability [66].

Biomaterial Scaffolds and Stem Cell for Skin Tissue Engineering in Wound Healing Skin tissue engineering has made significant advances in wound healing therapy with innovative manufacturing technologies that employ natural/synthetic polymers and stem cells. As a result, many injuries and degenerative skin illnesses are treated using stem cell treatment. Nonetheless, because of the poor survival rate of transplanted cells at the intended wounded location, numerous related investigations showed only minor improvements in organ functioning. Incorporating stem cells into biomaterials provides transplanted stem cells with niches, which improves their transport and therapeutic benefits [91]. New methods like bioprinting, biofabrication, and boinking, together with breakthroughs in DNA microarray, proteomics, and stem cells, have enabled the manufacture of skin substitutes in skin tissue engineering [92]. Biomaterials, cells, growth factors, other signaling molecules, and engineering components such as scaffolds, pumps, tubes, and bioreactors are the essential components of tissue engineering [93]. Currently, biofabrication methods are being used to create 3D scaffold structures that use skin tissue engineering as a critical component in the wound healing process. Scaffolds have a unique function in repairing and restoring disintegrating

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tissue because they provide a favorable platform for many elements related to cell survival, proliferation, and differentiation [94]. Furthermore, it may be made of natural and/or synthetic biomaterials, either stable in a biological setting or degradable in the human body [93]. Several strategies have been employed to build them. Still, the four most common ones are: (i) ECM-secreting sheets; (ii) porosity of synthetic, natural, and biodegradable biomaterial scaffolds; (iii) decellularized ECM scaffolds; and (iii) cells encased in hydrogels [95]. In practice, 3D scaffold manufacturing processes are classified into two categories: traditional and rapid prototyping (RP) approaches (Table 6), each of which produces distinct scaffolds with varied properties [96]. Traditional scaffold fabrication methods include the creation of porous polymeric structures such as cell adhesion substrates; however, obtaining complex microscale (containing an environment suitable for cell survival and function) and macroscale (permitting the coordination of multicellular processes, providing adequate transport of nutrients, and possessing mechanical properties) structures using conventional methods is difficult [97]. The RP scaffold construction process, on the other hand, offers a wide range of possibilities for skin tissue creation. It facilitates the creation of multicellular structures required for complicated tissue functions by allowing autonomous control of macroscale and microscale characteristics [96]. Furthermore, the RP approach allows for the products of 3D vascular beds and the support of vast tissue growth. Moreover, RP allows for combining manufacturing technology with clinical imaging data, boosting the potential of mass-producing a large number of tailored scaffolds in specific designs [82].

Combination Therapy: Biomaterials and Stem Cells in Wound Healing and Regeneration Biomaterials heal malfunctioning tissues and organs in skin tissue engineering and regenerative medicine. Stem cells, as previously said, need a certain environment to survive and proliferate. Biomaterials have created a new avenue for controlling stem cell destiny by simulating the in vivo milieu via cell-matrix interactions. Scaffolds composed of biomaterials may provide cell attachment sites while preserving stem cells’ benefits and features [100]. By assuring physical and chemical signals throughout the ECM, the possible 3D biomaterial scaffold structures offer an acceptable microenvironment for stem cells compared to typical 2D cultivation [95]. Furthermore, by releasing chemical signals or interacting with the cell matrix, a well-designed scaffold with the proper structure may directly govern cell signaling and induce lineage-specific differentiation of stem cells (Fig. 13) [100]. In a diabetic foot ulcer damage, biomaterial-incorporated stem cell treatment paired with growth factors improves wound healing. The growing need for wound healing and tissue repair biomaterials is being changed and utilized in conjunction with stem cells in various wound healing applications, as mentioned in the table below (Table 7).

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Table 6 The role of different GFs in wound healing and their consequences Growth factors bFGF

Origin of secretion Endothelial cells, macrophages, and monocytes

EGF

Function Stimulate proliferation, migration, and angiogenesis

Study In vivo

Platelets, macrophages, and fibroblasts

Epithelialization

In vivo

PDGF

Platelets, keratinocytes, macrophages, endothelial cells, and fibroblasts

Promotes cell proliferation, migration, and angiogenesis

In vivo

TGFβ3

Platelets, keratinocytes, macrophages, lymphocytes, and fibroblasts

In vivo

VGEF

Platelets, macrophages, keratinocytes, and endothelial cells

Inflammation, granulation tissue formation, epithelialization, matrix formation, and remodeling Epithelialization, collagen deposition, and angiogenesis

In vivo

Outcome In a diabetic mouse wound model, improved reepithelialization, angiogenesis, and collagen deposition were seen In a diabetic mouse wound model, increased granulation tissue development, cell migration, and reepithelialization were seen In a full-thickness incision mouse wound model, granulation tissue development and collagen deposition were enhanced Reduced the ratio of type I type III collagen in a rabbit model, which reduced scar formation In a full-thickness incision mouse countermodel, angiogenesis, collagen deposition, macrophage polarization, and granulation tissue development were all enhanced

References [98, 99]

Conclusions In conclusion, stem cell technologies have gotten a lot of interest in tissue engineering and regenerative medicine because of their potential to self-renew, be malleable, and specialize in particular cell types. Immune sensitivity, reduced survival, proliferation, and differentiation rate restrict stem cell use in clinical studies and in vitro

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Fig. 13 Incorporating biomaterials and growth factors into stem cell therapy for diabetic foot ulcers. (Adapted with permission from Ref [91], Copyright 2021, MDPI)

and in vivo applications. The utilization of growth factors, stem cells, and scaffolds to regenerate heart, bone, and skin tissues has significant promise in dealing with the burden of injuries and abnormalities. Due to climate changes, conflicts, and increased vehicle traffic in metropolitan areas, bone injuries and deformities rise. However, there are still many obstacles to overcome to improve the effectiveness of all of these tissue engineering procedures. For example, the capacity of ECM proteins and immune factors to influence GF responses is affected by selecting the appropriate scaffold for various kinds of tissue injuries, optimal concentrations, and an appropriate mix of GFs. These obstacles may be overcome with the use of biomaterials. Furthermore, by providing mechanical and metabolic support, biomaterials are being employed to influence stem cell destiny before and after delivery.

Future Perspectives Despite promising findings in nonclinical investigations, only a few biomaterials have been employed in patients for stem cell–based treatments. As a result, more biomaterial-based clinical studies should be conducted to understand better the impact of biophysical and biochemical characteristics on wound healing,

Scaffold

Scaffold

Gel

Collagen

Collagen with stromalderived factor-1 alpha (SDF-1α) gene

Fibrin

Biomaterials used Chitosan- and argininebased polyester amide

Fabrication method Gel

ASC

ADSC

hiPSCSMC

Stem cells type MSC

Model of rat skins being burned

Diabetic foot ulcer that will not heal

Diabetic mouse wound with full thickness

Application Burn wounds of the third degree in a mouse model

Table 7 Potential applications of biomaterials for wound healing treatment

7 days

14 days

7 days

Curing time 7 days

Treatment outcome Wound healing, reepithelialization, granulation tissue development, and blood vessel regeneration were all improved Angiogenesis, increased cellular proliferation, and production of pro-angiogenic and regenerative cytokines Restored the pro-angiogenic regenerative response in human diabetic ADSCs, as well as activematrix remodeling of fibronectin and the basement membrane protein collagen IV Enhanced local angiogenesis of healing burn wounds without inhibiting wound closure kinetics for up to 21 days, interacts with wound surface to facilitate ASC transmigration into the regenerating wound, and improved granulation tissue development

References [30, 37]

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Hydrogel

Nanofibrous scaffold

Hydrogel

Nanofibrous scaffold

Gelatine

PCL

PEG

PLGA

hASC

ADSC

BMSC

ASC

In the diabetic rat model, a fullthickness excisional wound was created In a mouse model, a fullthickness excisional wound was created

Diabetic mouse with a fullthickness excisional wound

Murine burn model

7 days

7 days

7 days

14 days

Reduced discolouration, roughness, and scab development, as well as the highest wound contraction rate of 55.3% Enhanced granulation tissue formation, angiogenesis, ECM deposition, and elicited pro-regenerative response to accelerate wound healing Inhibit inflammation while encouraging angiogenesis and reepithelialization Better cell adhesion, proliferation, and survival in the PLGA matrix, as well as enhanced wound healing

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osteoporotic fractures, cardiovascular strokes, tissue repair, and regeneration in humans. As a result, we recommend that future efforts be made to improve clinical outcomes by designing and fabricating biomaterials using emerging techniques such as 3D bioprinting, electrospinning, and nanotechnology to meet the specific properties of the components that must be delivered for healing and regeneration. These needs have prompted studies into innovative multifunctional biomaterials, enhanced tissue-targeting methods, and combination therapies to develop more effective and complete treatments.

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Medical Waste Biodegradation

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Environmental Impact of Medical Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Technology to Treat Medical Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Landfilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Incineration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alternative Ways to Solve the Medical Waste Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The 3R Principle (Reduce, Reuse, and Recycle) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Utilization of Biodegradable Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Polymer for Face Shields and Face Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polybutylene Succinate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polybutylene Adipate Terephthalate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polycaprolactone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradable Materials for Face Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrospun Encapsulated Polylactic Acid-Based Nanomembrane . . . . . . . . . . . . . . . . . . . . . . . . Gluten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effectiveness of Biodegradable Face Masks, Face Shields, and Hand Gloves in Preventing Viruses, Bacteria, and Particulate Matter 2.5 Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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B. H. Lee · S. M. Khor (*) Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_70

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Abstract

On the eve of the COVID-19 pandemic, there is an urge to utilize personal protective equipment such as face masks, face shields, and hand gloves. Personal protection equipment comprises solid mechanical properties of petrochemical-based materials such as polyethylene, polypropylene, and polystyrene. Unfortunately, these polymers are nonbiodegradable and persistent in the environment for a long period. The petrochemical by-product polymers were also not environment-friendly, releasing large amounts of greenhouse and hazardous gases. Therefore, the negative impact of medical waste on the environment will be investigated. Numerous research institutions and entrepreneurs recognize the need for biodegradable alternatives that perform similarly to nonbiodegradable materials. The chemical and physical properties of biodegradable polymers such as cellulose, polybutylene succinate, polybutylene adipate terephthalate, and polycaprolactone will be discussed and reviewed as they are used in the manufacture of biodegradable face shields and hand gloves. Meanwhile, the chemical and physical properties of electrospun encapsulated polylactic acid, gluten, chitosan, and starch in manufacturing biodegradable face masks will be discussed. The efficacy of biodegradable face masks, face shields, and hand gloves in protecting against viruses, bacteria, and particulate matter 2.5 is studied in this chapter to understand better whether they can meet the requirements for medicalgrade biodegradable personal protective equipment in terms of particle and bacterial filtration efficiency. Keywords

Biodegradation · Personal protection equipment · Medical waste · Environmental impact · Biodegradable face mask · Biodegradable hand gloves Abbreviations

3R BPA CNC CNF EPA FDA HDPE NPs PBAT PCL PE PLA PM 2.5 PP PS PVA

Reduce, reuse, and recycling Bisphenol A cellulose nanocrystals cellulose nanofibrils Environmental Protection Agency Food and Drug Administration High-density polyethylene Nanoparticles Polybutylene adipate terephthalate Polycaprolactone Polyethylene Polylactic acid Particulate matter 2.5 Polypropylene Polystyrene Polyvinyl alcohol

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ROS SEM Tg

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Reactive oxygen species Scanning electron microscope Glass transition temperature

Introduction The infectious SARS-CoV2 illness outbreak has significantly impacted worldwide public health, causing long-term impacts on human respiratory systems [1]. Using an effective face mask is urgent to prevent infectious aerosol particles from spreading because most infectious respiratory diseases are carried primarily through aerosol particles. For respiratory protection, disposable nonwoven surgical face masks were recommended to limit the transmission of the virus [2, 3]. Thousands of tons of nonbiodegradable petrochemical-based materials face masks are produced that are undoubtedly environmentally unfriendly. Additionally, the manufacture of synthetic petrochemical-based face masks adds to carbon dioxide emissions, which could contribute to global warming because the processes of propene and polypropylene produce a substantial quantity of carbon dioxide emissions to the environment during the manufacturing of polypropylene-based nonwoven face masks [4]. The COVID-19 epidemic had several negative consequences, including increasing medical waste and single-use plastic trash. The globe is dealing with the COVID-19 epidemic, but it is also dealing with an increase in trash, especially from plastic medical waste. Plastic trash is mostly postconsumer, and thermoplastic is the most common type of plastic garbage. Solid plastic trash production continues to rise, yet only a tiny portion of the waste produced is recycled, and most of the waste remaining is landfilled, which is the most convenient way to treat medical waste [5–8]. Currently, most the personal protective equipment is made up of low-density polyethylene (PE), high-density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS). Synthetic plastic is commonly used in personal protective equipment such as face masks, face shields, and hand gloves. These synthetic plastics have extremely strong mechanical strength and durability, notably in the healthcare industry, where they are utilized extensively in single-use medical tools, equipment, and packaging. Because everyone is concerned about their health and cleanliness, disposable plastics have improperly managed discarded personal protective equipment during the COVID-19 epidemic, resulting in widespread pollution. As a result, the global prevalence of medical waste treatment and disposal was not properly addressed. Most countries have experienced incredible growth in medical waste from hospitals with insufficient handling time to deal with it. Although personal protective equipment may be lifesaving, mismanagement of these medical wastes would result in severe degradation of the terrestrial and marine ecosystems. Microplastics have been released into the environment due to the use of single-use disposable non-woven face masks [9]. The widespread use of non-woven face masks and synthetic petrochemical-based face shields and hand gloves to restrict the spread of the COVID-19 pandemic has had major negative

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environmental consequences [10]. These negative environmental effects motivate researchers to design biodegradable materials for manufacturing environmentally benign personal protection equipment [11].

The Environmental Impact of Medical Waste The COVID-19 pandemic has slowed the process and made it more challenging to reduce plastic trash generation. Due to tighter mobility controls in many nations, governments have imposed restrictions prohibiting individuals from traveling between states, reducing traffic on the highways. This may have appeared to mitigate greenhouse gas emissions and sound pollution. However, it ignores the rising usage and consumption of single-use plastics, especially personal protection equipment, detrimental to environmental conservation. It is critical for everyone, especially frontline workers such as medical personnel, to wear appropriate personal protective equipment to prevent disease transmission. Personal protection equipment is necessary for everyone, and hence demand is growing in every part of the globe. Regular folks’ indiscriminate usage of personal protection equipment has become problematic due to inappropriate management and disposal of the products. The surgical mask must be changed and thrown away after a certain amount of time to avoid cross-contamination. However, without any precautions, the used masks litter the public area. Before being disposed of in landfills, contaminated personal protective equipment must be treated with care. Since most of the personal protective equipment is made up of plastics, they are extremely beneficial to society in various ways. Considering the expansion of the plastic market, enormous amounts of plastic garbage are generated and disposed of daily due to human activities. Plastics are made in large quantities every day due to their convenience. However, nonbiodegradability, low melting temperatures, and the short life span of plastics are disadvantages that might negatively affect the environment and human health [12]. Most plastic waste is nonbiodegradable and will take a hundred thousand years to decompose. The intermolecular connections that hold the plastic structure together prevent corroding or decaying. Plastic garbage that is not properly disposed of will choke rivers and be swept away into drains or reservoirs due to the complex disintegration of plastics. This eventually contributes to environmental pollution. When high-temperature chemicals come into contact with plastic materials, the plastic decomposes and releases hazardous fumes. Because of their low melting point, most plastics cannot withstand high temperatures. Because most plastic is composed of petrochemical-based materials, it is highly combustible, posing a fire hazard. The components of plastics, such as crude oil and natural gas, are normally manufactured via addition and condensation polymerization processes [13]. Chemically, plastics are hazardous because they contain hazardous additives, potentially poisonous. When humans are exposed to this for an extended period, their health will be jeopardized. For diverse applications, to acquire the desired qualities of plastic in terms of tensile strength and chemical resistance, colorants, flame retardants,

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peroxides, plasticizers, and stabilizers are all common additives used in the plastic manufacturing process [14]. Phthalates and bisphenol A are additives in plastic that most people are exposed to [15]. These chemicals have permeated every aspect of human life. The long-term accumulation of these compounds in the human body can cause major health problems. For example, the existence of endocrine-disrupting compounds like diisononyl phthalate that act as plasticizers to improve the flexibility and durability of polymers can cause tumors to grow in the liver, spleen, and kidneys of animals, as well as disrupt the development of unborn fetuses [16]. Abiotic and biotic degradation mechanisms may be present in plastic trash dumped in landfills [17]. Plastic medical waste degradation produces many secondary microplastics, which can lead to unrestrained pollution in waste landfills. Furthermore, the decomposition of plastics can result in the release of different additives and secondary microplastics, which can be detrimental to the environment and human health. The improper treatment of plastic garbage and its disposal into oceans endanger the health of aquatic animals, upsetting the ocean’s ecosystem [18]. For example, specific to sea turtles, sea turtles will misinterpret medical waste as their food and consume the microplastic floats on the ocean’s surface. This will eventually affect the buoyancy of sea turtles, and they will eventually die [2]. In particular, about medical waste, all nonbiodegradable synthetic plastic materials can be further fragmented into microplastics, which can operate as transporters of dangerous pollutants, resulting in bioaccumulation and biomagnification in the food web. In microplastics per individual, zooplankton outweighed plastic particles by approximately five to one. Predators like sharks outweigh plastic particles by approximately twenty to one [2, 3]. Figure 1 shows microplastics are transferred

Fig. 1 A schematic diagram showing microplastics in the marine environment and the possible transfer mechanisms. (Adapted with permission from Ref. [19]. Copyright 2021, Elsevier).

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Fig. 2 A representative diagram of the aquatic environment polluted with mask waste due to improper disposal of single-use masks. (Adapted with permission from Ref. [4]. Copyright 2021, Elsevier).

trophically through three mechanisms such as ingestion, bioaccumulation, and biomagnification. Moreover, when plastic medical waste litter is exposed to UV light via photodegradation, secondary microplastics are formed. These have negative consequences for marine species, collecting microplastics after eating them. Microplastics include a variety of toxic, addictive substances, such as pesticides, flame retardants, and polychlorinated biphenyl, which can bioaccumulate in the phytoplankton and zooplankton, which are the primary consumers in the food chain [20]. As humans are the predators with the highest position in the food chain, they may acquire the microplastics and hazardous compounds found in the seafood they eat, potentially causing major health concerns in the long run [21]. Figure 2 shows the mask waste being carried into rivers, polluting the marine environment. It can also produce entanglement, resulting in mortality in aquatic wildlife such as birds and other submerged creatures, as shown in the figure below.

Current Technology to Treat Medical Waste Landfilling and incineration are alternative waste management technologies to mitigate the medical waste issue.

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Landfilling Landfilling is seen as a cost-effective and convenient method of disposing of medical waste, particularly in underdeveloped countries [22]. Postconsumer plastic trash, sometimes mixed with other organic substances, is piled on land without any engineering design for a landfilling dumping site [23]. The photodegradation would be triggered by the abandonment of plastic garbage in an open-type landfill. The ultraviolet radiation from the sun will break down some of the carbon-hydrogen bonds inside the polymeric structure, making the plastic more brittle. As a result, additives and plasticizers are gradually released into the environment. For example, increased hydrogen sulfide generation by sulfate-reducing bacteria in soil populations has been linked to bisphenol A released from trash plastics [23]. Hydrogen sulfide in high amounts can be fatal. Furthermore, more space is required because most plastic components can persist in the landfill for longer. Apart from that, a limited amount of oxygen and anaerobic conditions in landfills will deteriorate the degradation rate and eventually release greenhouse gases such as methane, causing global warming [24].

Incineration Incineration addresses some of the drawbacks of landfilling by requiring little space and even recovering energy from waste. Mass burn is the technique used in incineration [25]. Filters are used to remove large amounts of metal-made waste. Before the ashes go into the incinerator, the other garbage will be fragmented until it can no longer be distinguished as ash. All the ashes are incinerated in the presence of oxygen. At a rather high temperature in the range of 982.22  C to 1093.33  C, the undesired goods are incinerated. The trash will entirely burn out, resulting in ashes and the release of heat and gases. Steam is produced by cooling the gases with water via heat recovery. The steam can be used to power the generators with electricity. Before being released into the environment, the remaining gases are scrubbed by scrubbers to remove contaminants. However, the generation of harmful gases such as dioxins, furans, carbon monoxide, hydrogen sulfide, heavy metals, and polyaromatic hydrocarbons can result from the incomplete burning of plastic and other municipal solid waste [23]. Table 1 summarizes the advantages, disadvantages, challenges, and limitations of incineration and landfilling waste technologies.

Alternative Ways to Solve the Medical Waste Issue Given the damaging effects of plastics on the environment and human health, governments are becoming more aware of the problem and are playing an important role in it. Most countries are experimenting with various waste management

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Table 1 Comparison between landfilling and incineration Landfilling

Incineration

Advantages It is a costeffective and excellent energy source because methane emissions can be collected to generate electricity

Disadvantages It harms the environment as toxic additives may leach into the soil or river, contaminating the underground water system

Challenges More space is required as plastic is strong decomposition resistant and persists in the environment longer

Small space is required compared with landfills, and energy recovery from the waste is allowed

It has high operating costs as it involves high temperatures

Ash waste has the potential to harm the environment and human health. Even though the ash left behind after the operation is

Limitations Global warming results from the emission of greenhouse gases such as methane due to the limited amount of oxygen and anaerobic conditions in landfills. Even though methane can generate electricity, most people do not do so. This is because methane can be released into the environment before it is burned to generate power; while methane has a shorter life span than carbon dioxide, it is more efficient at trapping heat in the atmosphere The emission of harmful gases such as dioxins, furans, and carbon monoxide can result from the incomplete

References [23, 24]

[23, 25]

(continued)

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Table 1 (continued) Advantages

Disadvantages

Challenges

Limitations

usually modest, it includes many toxins and heavy metals that require further treatment. It can cause considerable harm to the public and the environment if it is not properly disposed of

burning of plastic and other municipal solid waste

References

solutions to combat this scourge. Plastic garbage is the most difficult to deal with since it is difficult to biodegrade. However, there will always be a way to deal with the problem to guarantee the Earth’s long-term health. There are several alternative ways to mitigate medical waste, such as the 3R concept, education, and the utilization of biodegradable materials.

The 3R Principle (Reduce, Reuse, and Recycle) Single-use petrochemical-based face masks have come to be seen as a major source of plastic medical waste pollution. This can be seen because most people will throw their used face masks everywhere without treating them properly, and hence, this will have the potential to spread the coronavirus in the air and threaten the environment. As a result, it is strongly recommended that recycling and reusing are viable options to reduce medical waste pollution. For instance, mask rotation is a technique for reusing and recycling used face masks [4]. The masks are worn each day can be rotated in this manner, allowing them to dry for long enough duration that the virus is no longer viable by using various methods, including wet heat, dry heat, UV treatment, and hydrogen peroxide vaporization [4]. One of the options for reducing the medical waste pollution caused by mask waste is to recycle the mask using the right methods. Recycling can be divided into two categories, such as primary recycling and secondary recycling [4]. Primary recycling entails repurposing a product in its original form, whereas the mask made of thermoplastic can be reused in secondary recycling [4]. Because they can be remelted and reprocessed into a variety of end products, such as the same product or composite items, as a result, this approach can be used to recycle the mask efficiently. However, compared to the cost of a new mask, this recycled mask is

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more expensive to process. Its filtering efficiency and quality are worse than those of a new mask. As a result, it is critical to continue looking for other ways to reduce mask waste. Government and nongovernmental organizations are always encouraged to organize the 3R campaign to promote awareness of the importance of waste management [2]. However, this concept is still in its infant stage and has not been fully implemented by waste management practitioners. It is very important to raise public awareness about the impact of plastic on the environment by enforcing the 3R concept on society and the community. To effectively minimize plastic medical waste pollution, the government must have a well-thought-out plan and provision for implementation. For instance, national guidelines for the management of clinical and medical wastes have been published by the Ministry of Health Malaysia. State-level biomedical waste management guidelines have also been developed to rationalize and offer ways to manage healthcare wastes in Malaysia [26]. The Waste Pollution Prevention and Control Law and/or Regulations on the Management of Biomedical Hazardous Wastes were drafted by the Malaysian Department of Environment [26]. On the other hand, hospital garbage is typically collected and disposed of alongside regular home waste according to the rules supplied. According to the Malaysian Department of Environment, the national authority in charge of Malaysia’s clinical or biomedical waste management is the Federal Government, while local municipal councils are also in charge [26]. The Federal Government and the Department of Environment Malaysia manage COVID-19 medical waste management daily. As amended, Malaysia is currently governed under the Environmental Quality Act of 1974. The Environmental Quality (Scheduled Wastes) Regulations 2005 (“Regulations”), which provide a crucial regulatory framework that deals specifically with hazardous waste, classify biomedical, clinical waste, and any other hazardous waste as scheduled waste [26]. Also, governments must enforce rules and regulations. Those who disobey the laws and regulations will face a penalty. To ban the use of plastics (individual packing face masks), many countries, such as the United Kingdom, have taken the lead in enacting plastic bag tax policies. The introduction of this policy could significantly reduce the use of plastic bags and the initiation of new environment-friendly measures.

Education Education is critical to reducing medical waste pollution. Young children who are still forming habits can significantly impact their colleagues in the future generation. According to studies, education impacts a child’s knowledge, attitudes, and behaviors. Medical waste pollution issues, especially mask waste, could be included in the school curriculum, allowing teachers to explore innovative ideas with students to reduce the difficulties related to medical pollution in the larger environment [27]. Several studies have already advocated for increased

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environmental education, outreach, and awareness in schools as a means of safeguarding the environment and diverse behaviors that lead to varied attitudes towards environmental protection may be encouraged [27]. For instance, teachers can spread knowledge about the proper solution for disposable single-use masks by folding the used masks before placing them into sealed plastic bags and throwing them into the waste bins with covers. Because discarded single-use masks increase the risk of COVID-19 infection among the general public, it is critical to disseminate the proper method for disposable single-use masks through education in schools and social media platforms.

Utilization of Biodegradable Materials Polymers are quickly becoming a severe environmental issue, and they are typically disposed of by landfilling, recycling, or incineration. Although incineration is a common technology frequently used to solve the medical waste issue, it harms the environment. At the same time, landfill sites are becoming increasingly limited. In most nations around the world, recycling is still not practiced. Therefore, the development of biodegradable materials is one of the most successful techniques for reducing the problem of plastic medical waste pollution. Researchers have introduced and widely investigated biodegradable materials to solve the current medical waste problem. In terms of breakdown, biodegradable polymers have a significant advantage over nonbiodegradable polymers. This is because microbes can decompose biodegradable polymers and return them to the soil to improve them. Because biodegradable polymers decay naturally, they can reduce the cost of labor necessary to remove traditional plastics from the environment. Furthermore, the dissolution and degradation of biodegradable polymers help to stabilize the ecosystem and extend the life of landfills by reducing garbage volume [22]. Specific to biodegradable plastic, biodegradable plastic is plastic that can produce natural elements such as carbon dioxide or methane, water, and biomass by using the action of microorganisms, fungi, and biological processes [22]. Biodegradable plastic can be classified as bio-based or fossil-based. Even bio-based biodegradable materials are more costly than petroleum-based nonbiodegradable materials. Yet, many researchers are prone to using bio-based biodegradable materials as substitutes for petroleum-based nonbiodegradable materials due to their being environment-friendly. However, the rate of the biodegradation process depends on environmental factors such as temperature, aerobic-anaerobic conditions, microorganisms, and sunlight [22]. Currently, biodegradable materials research is mostly focused on synthetic and natural polymers. Many synthetic polymers are commonly used as biodegradable materials in everyday life, including polylactic acid (PLA), polyvinyl alcohol (PVA), polycaprolactone (PCL), and polybutylene adipate terephthalate (PBAT). Furthermore, a variety of natural polymers are used as raw ingredients to create biodegradable products such as polysaccharides (cellulose, chitosan, and starch).

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Biodegradable Polymer for Face Shields and Face Masks Biodegradable polymers are classified into natural and synthetic polymers. Natural polymers can be found in nature and extracted, such as cellulose, while synthetic polymers can be obtained by humans, mainly from petrochemical-based sources such as polybutylene succinate, polybutylene adipate terephthalate, and polycaprolactone. Both polymers have advantages and disadvantages in manufacturing medical equipment such as face shields, hand gloves, syringes, and medical bottles. The chemical and physical properties of biodegradable polymers such as cellulose, polybutylene succinate, polybutylene adipate terephthalate, and polycaprolactone will be investigated to meet the manufacturing requirement of personal protective equipment, especially face masks and face shields.

Cellulose The most abundant form of live terrestrial biomass is cellulose (Fig. 3a). Cellulose has been discovered to be a long-chain polymer made up of repeating D-glucose units, a simple sugar [28]. Cotton fiber has it in almost pure form. It is found in conjunction with other components such as lignin and hemicelluloses in wood, plant leaves, and stalks. Animals are unable to synthesize cellulose. Although cellulose is most associated with plants, microorganisms produce it. Cellulose is a natural polymer, a long chain made up of smaller molecules linked together [28]. Sugar, β-D-glucose, make up the cellulose chain’s links. The most abundant carbohydrate found in nature is cellulose. It is water-insoluble and can be dissolved in organic solvents. The great tensile strength of cellulose is due to the alternative arrangement of glucose molecules in the molecule. Because of its numerous benefits, cellulose fibers are considered viable reinforcing materials, such as low density, highly biodegradable, and good mechanical properties [28]. However, cellulose also has its drawbacks, such as low thermal stability, and is incompatible with hydrophobic polymers such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene [5, 28, 29]. Cellulose-based masks require antimicrobial treatments to prevent pathogen growth or eliminate pathogens, particularly with antiviral capabilities, because cellulose is a hydrophilic and biodegradable substance. Such characteristics are significant for protective masks because they protect the wearer from dangerous bioaerosols while also providing greater skin comfort, which is important because extended usage of a face mask or hand gloves can cause skin irritation. The innate negative charge of cellulose surfaces, which include numerous carboxylic acid groups, allows for cationic alterations by adding positively charged molecules such as silver ions for antimicrobial functionalization [30]. Technology like nanocellulose technology has a lot of potential for filtering since it gives materials both excellent mechanical filtration effectiveness, which improves the filter’s functionality and enables the creation of more efficient cellulose-based composite filters. Nanocellulose is normally classified into cellulose nanocrystals

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Fig. 3 Chemical structures of (a) cellulose; (b) polybutylene succinate; (c) polybutylene adipate terephthalate; (d) polycaprolactone; (e) polylactic acid; (f) chitosan; (g) amylose; and (h) amylopectin.

(CNC) and cellulose nanofibrils (CNF), which are mostly derived from lignocellulosic fibers, algae, tunicates, and bacteria, among other sources [30]. Because of their microscale dimensions in breadth, cellulose nanostructures allow for better control of pore parameters, resulting in higher filtration effectiveness and mechanical qualities of filters [30]. Nanocellulose has also been shown to be biocompatible and nontoxic in materials that contact human skin. Figure 4 shows nanocellulose such as cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), mostly derived from lignocellulose and used in various applications such as film packaging, biomedical, and others.

Polybutylene Succinate Polybutylene succinate (Fig. 3b) is a semicrystalline polymer having a semicrystalline structure that is more similar to polypropylene and can be used in various

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Fig. 4 A schematic diagram of nanocellulose such as cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), which are mostly derived from lignocellulose (lignin, cellulose, and hemicellulose) and used in various applications such as film packaging, biomedical, and others. (Adapted with permission from Ref. [31]. Copyright 2020, Elsevier).

applications. Polybutylene succinate is a biodegradable, high-performance polymer. Polybutylene succinate has excellent mechanical qualities and can be utilized for various purposes. When polybutylene succinate is exposed to water, an ester group in its chemical structure degrades into low molecular weight polymers [32]. Polybutylene succinate contains ester bonds or polysaccharides vulnerable to hydrolysis by microbial action. Polybutylene succinate can be synthesized from the monomers for polycondensation of succinic acid (or dimethyl succinate), and 1,4-butanediol can come from fossil or renewable sources [33]. Compared to petroleum-based techniques, employing renewable resources to create succinic acid can be more expensive. This will increase the mechanical and thermal properties but is more expensive than petrochemical-based plastics such as polystyrene because the production involves high temperatures of 200  C during polycondensation [33]. However, its high degree of crystallinity restricts the action of degrading microbes in soil and eventually limits biodegradability. Therefore, polybutylene succinate is often blended with polylactic acid or copolymers such as polybutylene succinate adipate to decrease the crystallinity and enhance biodegradability [33]. The developed chitosan-coated polybutylene succinate nanofiber integrated Janus membrane filter of face masks has outstanding qualities such as permanently retained ionic charges, low-pressure drop that allows for easy breathing by the user, and biodegradability [34]. A Janus membrane can be used to make an inner and outer layer of a mask that offers great hydrophobicity and hydrophilicity, respectively. Chitosan, a biodegradable substance, has been found to have beneficial biological features such as biocompatibility, biodegradability, and low toxicity.

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Furthermore, chitosan has strong electrostatic adsorption that contains cationic sites such as ammonium and polar amide groups, which results in capturing PM 2.5 in a highly efficient manner [34]. Polybutylene succinate, a biodegradable polymer wellknown for its ability to physically catch particulate matter while minimizing pressure drop by order of magnitude to allow breathability, was used to make electrospun nanofiber. Figure 5 depicts (a) the working mechanism and disposal of a chitosancoated PBS filter Janus membrane; (b) the electrospinning process for producing PBS filters; and (c) SEM images of electrospun microfiber and nanofiber PBS layers [35].

Polybutylene Adipate Terephthalate When manufacturing hand gloves and face shields, polybutylene adipate terephthalate (Fig. 3c) is the often-used copolymer. Polybutylene adipate terephthalate is a synthetic biodegradable aliphatic-aromatic polymer based on petrochemical resources. Aliphatic polyesters biodegrade efficiently because of ester bonds in the soft chain portion of the polymer. These ester bonds are broken down through hydrolysis, making the polymer degradable in almost any environment. Like polybutylene succinate, polybutylene adipate terephthalate can be synthesized by polycondensation between 1,4-butanediol with both adipic and terephthalic acids (or butylene adipate) [36]. Polybutylene adipate terephthalate is more expensive than other fossil-based plastics because it involves high temperatures, usually more than 190  C during polycondensation [36]. Aromatic groups in polybutylene adipate terephthalate enhance the mechanical properties but are not prone to hydrolysis and hence should be limited to full biodegradation. Therefore, polybutylene adipate terephthalate is often blended with aliphatic chains such as polylactic acid to enhance biodegradability [37]. The desired mechanical properties of face shields and hand gloves will not break down easily and provide superior impact protection against airborne particles.

Polycaprolactone Polycaprolactone (Fig. 3d) is a semicrystalline aliphatic polyester made by polymerizing caprolactone in a ring-opening process [38]. Polycaprolactone can be degraded via hydrolysis of its ester bonds. Polycaprolactone is biodegradable, biocompatible, and nontoxic. Because of its ease of availability and good mechanical properties, polycaprolactone appears the most appealing and practical biodegradable polymer for medical applications. Polycaprolactone has been used to boost the hydrophobicity of other biopolymers as a plasticizer or as a hydrophobic agent [39]. Polycaprolactone has good chemical and solvent resistance as well as hardness. Low melting points of polycaprolactone are possible due to the high mobility of chain segments and low intermolecular interactions. Polycaprolactone is more

Fig. 5 A schematic diagram of the (a) chitosan-coated PBS filter Janus membrane working mechanism and disposal, (b) electrospinning PBS filter manufacturing process, and (c) SEM images of the microfiber and nanofiber PBS layers. (Adapted with permission from Ref. [35]. Copyright 2021, Elsevier).

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thermally stable than polylactic acid and can be degraded by enzymatic activity [39]. Polycaprolactone has a lot of chain flexibility. As a result, polycaprolactone is frequently blended with other polymers such as polyvinyl alcohol, polycarbonate, and polylactic acid to improve the desired properties. Since polycaprolactone is hydrophobic, it has a slow degradation time. A result is that it is frequently blended with starch to improve biodegradability [40]. However, polycaprolactone is not antibacterial in and of itself, but it can be bonded with an antimicrobial chemical to make it antibacterial and be employed in constructing antibacterial masks [41]. The ability to immobilize antimicrobial nanoparticles with polycaprolactone during electrospinning opens up the possibility of developing an antimicrobial face mask with polycaprolactone. Antimicrobial agents such as silver nanoparticles can be immobilized onto polycaprolactone nanofiber, protecting an individual from airborne particles entering the respiratory airways [41]. Silver nanoparticles can continuously discharge silver ions, a microbe-killing mechanism. Attractant silver nanoparticles can penetrate pathogen cell walls, altering cell membrane structure and causing cell death [42]. Polycaprolactone nanofiber with a substantially lower pore diameter can be made and used as a filtering instrument. Also, the polycaprolactone membrane has enough capability to resist maximum inspiratory and expiratory pressures. Figure 6 shows biodegradable antimicrobial nanofibers produced with the electrospinning method of polycaprolactone. Table 2 summarizes the advantages, disadvantages, challenges, and limitations of cellulose, polybutylene succinate, polybutylene adipate terephthalate, and polycaprolactone in the manufacturing of personal protective equipment.

Fig. 6 A schematic diagram of biodegradable polycaprolactone incorporated with antimicrobial nanofiber production using the electrospinning method. (Adapted with permission from Ref. [43]. Copyright 2021, Elsevier).

The aromatic group contributes to better mechanical properties than low-density polyethylene

It has strong mechanical properties and is biocompatible

Polybutylene adipate terephthalate

Polycaprolactone

Polybutylene succinate

Cellulose

Advantages It has low density, low cost and good mechanical properties and is biodegradable It has high mechanical properties and thermal stability

It has a slow degradation rate

It has low melt viscosity and is more expensive than petrochemical-based plastics such as polyethylene because it involves high temperatures during polycondensation Its high production costs limit its large-scale production

Disadvantage It has low thermal stability

Blended with other polymers such as aliphatic chains, polylactic acid is necessary to increase hydrophilicity as well as biodegradability It is often blended with starch to improve its biodegradability

Challenges Chemically transformed cellulose derivatives are necessary to be employed in plastic manufacture When blended with other polymers such as polylactic acid, it is necessary to decrease the crystallinity and enhance biodegradability

A low melting point cannot withstand high temperatures

Aromatic groups are not prone to biodegradation and are not limited to allowing full biodegradation

Because of its high degree of crystallinity, it is only moderately biodegradable in soil due to the action of biodegrading microbes

Limitations It is incompatible with hydrophobic polymers

Table 2 Comparison between cellulose, polybutylene succinate, polybutylene adipate terephthalate, and polycaprolactone

[38, 39]

[36, 37]

[32, 33]

References [28]

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Biodegradable Materials for Face Masks Personal protective equipment such as face masks should be worn to protect healthy people against the coronavirus to prevent the transmission of air droplets and the spread of the disease among the community. In the COVID-19 outbreak, N95 is undoubtedly the most effective face mask in preventing the invasion of coronavirus into the human body. The external, middle, and inner layers are the three polypropylene fibers that make up these structures with a more than 95% bacterial filtration efficiency, capable of blocking particles as small as 0.1 microns [44]. As a result, most frontliners, especially medical practitioners, are encouraged to wear N95 masks to protect themselves against COVID-19. To combat the COVID-19 epidemic, various public health organizations and countries have issued many suggestions for wearing face masks. Furthermore, the World Health Organization and other international health organizations have issued recommendations to wear a facemask from time to time to avoid the spread of airborne infectious diseases such as the flu. Generally, face masks are mainly split into surgical masks and respiratory masks such as N95, mostly made of polypropylene. The melt-blowing process creates these surgical masks from non-woven fabrics [44]. The surgical masks were designed differently depending on the country’s regulations. Bacterial filtration capacity is used to categorize the different face masks available on the market. At the same time, the use of biodegradable materials such as polylactic acid, gluten, chitosan, and starch is worthwhile to develop bio masks due to their high filtration capacity and cost-effectiveness and being environment-friendly; they can be reused and recycled [4]. Therefore, since wearing a face mask is the new normal that we need to practice during the pandemic, face masks should be made of biodegradable alternatives to ensure our environment is sustainable and conserved. Most face masks are polypropylene, polyethylene, and other petrochemical-based nondegradable polymers. The Environmental Protection Agency considers polypropylene, derived from petroleum, to be a safer alternative to other types of plastics. Polymers, mainly polypropylene, are used to make the mask, and the polymers might contain various additives such as bisphenol A and phthalates. Both bisphenol A and phthalates are endocrine disruptors. Among the additives, phthalates are a group of high-volume compounds that should be avoided because they are carcinogenic [45]. Due to their cost-saving properties, phthalates are commonly employed as plasticizers in polymer manufacturing to reduce shear and improve polymer flexibility and adaptability [45]. Because phthalates such as diisononyl phthalate are commonly used as additives and are not chemically bound to materials, they can easily leak into the environment and enter the human body, causing various negative effects. For example, phthalate exposure has been linked to breast cancer, prenatal development, and reproductive damage [45]. Fortunately, the phthalate exposure danger index from all masks was found to be within safe limits [45]. However, a new source of phthalate exposure is a face mask, which can help raise awareness about the use of additives in creating face masks. Because of the rising concern over bisphenol A, various types of bisphenol A-free polymers have been developed. Polypropylene does not cause cancer toxicity

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because it does not contain bisphenol-A. Although the Food and Drug Administration (FDA) has said that modest levels of bisphenol A are safe for humans, some environmental and health experts believe that bisphenol A exposure could cause difficulties with brain development, immunological function, learning capacities, reproductive diseases, and other health problems [46]. However, bisphenol A is unlikely to cause cancer in people, according to the Environmental Protection Agency (EPA) [47]. In short, there is currently no evidence that causes cancer in humans when using existing polypropylene-based face masks.

Electrospun Encapsulated Polylactic Acid-Based Nanomembrane Electrospun biodegradable nanofiber layers based on polylactic acid (Fig. 3e) have shown encouraging results for high filtration efficacy for aerosol particles in the air [48]. The filter of the masks should have a pore size on the nanometer scale, which can be achieved in nanomembranes created by the electrospinning technique, as porosity is the only attribute of the masks for arresting microscopic viruses [49, 50]. This technology applies a strong voltage (5 to 50 kV) to the polymer solution to generate a nano-scale membrane [51]. Numerous studies have employed an organic solvent such as dimethyl sulfoxide or formic acid during the electrospinning process. However, most studies found nanofibers electrospun from organic solutions, even though most organic solutions are hazardous. The majority of organic solvents are volatile, toxic, and difficult to remove [52]. When a person inhales too much volatile gas, it is detrimental to their health. The use of toxic organic solutions and evaporation during the spinning process is common in the electrospinning industry, which poses a risk to the human body and the natural environment. It is critical to produce a greener solution and solvent-free electrospinning to address environmental and human well-being issues [52]. In other words, the invention of green electrospinning lays the groundwork for electrospinning nanofibrous membranes to be widely used. According to the green electrospinning idea, utilizing water as an electrospinning solution is one of the greatest options for fabricating green nanofibrous membranes [52]. Water-soluble polymer materials with polar groups such as carboxyl and hydroxyl groups are the foundation of green solution electrospinning [52]. Hydrogen bonds might form with water in a water solution, and these molecules could then be dissolved in water. For instance, the aqueous solution electrospinning process has previously made polyvinyl alcohol accessible as a water-soluble polymer due to the presence of a hydroxyl group in its repeating unit, which allows it to be cross-linked through hydrogen bonding. Electrospinning is suitable for use in large-scale production and is easily accepted by industries due to its cost-effectiveness [53]. The electrospun polylactic acid nanofiber filter media, which have a high filtration efficiency due to their large specific surface area, are a viable alternative to traditional materials such as non-woven polypropylene fiber, particularly in the production of N95 respirator masks [54]. This technology can process a variety of polymeric materials, which is necessary for biodegradable polymers in the preparation of nanomembranes. For instance,

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chitosan, gluten, polyvinyl alcohol, polylactic acid, and other biopolymers, as well as their mixtures, have been used to make nanomembranes. Fabrication of a three-layered mask with fluorocarbon and cotton in the outer layer, inner layers with cellulosic cotton, and an electrospun nanofibrous polylactic acid layer encapsulated with Neem such as Azadirachta indica in the intermediate layer is useful in the improvement of bacteria filtration efficiency with no adverse effects on the environment [55]. The hydrophobic character of the outer layer is aided by the highly polarized condition and highly electronegative fluorine groups that form a strong bond with the cellulose groups of cotton fabric [56]. The hydrophobic nature of polylactic acid electrospun fibers in the intermediate layer is due to the combined effect of charged molecular groups found in polylactic acid and the porous heterogeneous rough surface of polylactic acid electrospun fibers [57]. The hydrophobicity of the polylactic acid electrospun layer in the intermediate layer is due to polar chemical groups such as the carbonyl group, which have a charged state that repels ions in water [58]. Neem, one of the traditional Indian herbal extracts in the intermediate layer, exhibited high antibacterial and antiviral efficacy against the pathogen by disrupting the bacterial or viral cell membrane [59]. For the inner layer, water rapidly saturated the pristine cotton fabric. For example, abundantly available intrinsic hydroxyl groups in its primary cellulose structure influences its hydrophilic character [58]. Because hydroxyl groups have a strong attraction to hydroxide and hydrogen ion ions, cotton fabric has a water-absorbing property that contributes to its hydrophilic nature [58]. Figure 7 depicts a putative mechanism for water molecules to interact with C6-fluorocarbon spray-coated cotton (outer layer), pristine cotton fabric (inner layer), and PLA/neem (middle layer). Biodegradation of face masks is beneficial in combating the COVID-19 pandemic and being environment-friendly. The biodegradability of used face masks based on polylactic acid was studied by immersing them in fresh cow dung because bacteria in cow dung have been linked to the deterioration of commodity plastics [60]. The nanofibrous polylactic acid layer was destroyed. Polylactic acid degradation has been helped by organic waste that can be digested anaerobically [61]. Since jaggery contains a substantial amount of sucrose, it speeds up polylactic acid and cotton fibers [61]. This is most likely due to the breakage of the bond between fluorocarbon molecules and the cellulose groups present in cotton, which was most likely induced by biodegrading microbes. As a result, the face mask showed enhanced degradation in the presence of jaggery.

Gluten Gluten is a by-product of the grain industry with high cohesive and barrier qualities that can make face masks with good mechanical strength. Since gluten is vulnerable to fire and moisture uptake, gluten can be made fire resistant by adding lanosol, a naturally occurring substance [62]. Gluten, like most commodity polymers such as polyethylene and polypropylene, can be processed in various ways, including compression molding, injection molding, and extrusion to produce thin films. To

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Fig. 7 A representative diagram of the interaction of water molecules with C6-fluorocarbon spraycoated cotton (outer layer), pristine cotton fabric (inner layer), and PLA/neem (middle layer). (Adapted with permission from Ref. [49]. Copyright 2021, Elsevier).

improve the bacterial filtration efficiency, gluten can be electrospun in its native form or in combination with polyvinyl alcohol, a water-soluble polymeric material, to produce a nano-fiber membrane by using a green electrospinning solvent like water. Gluten can be chemically treated to improve the surface charging of ultrafine nanofilter media during electrospinning [63]. Figure 8 illustrates (a) the electrospinning process to produce gluten nanofiber mats, (b) an SEM micrograph of gluten nanofibers, and (c) SEM images of PVA/gluten nanofiber [62]. For the manufacture of facemasks, a carbon-mixed gluten nanofiber filter membrane is also viable [64]. The electrospun gluten nanofiber membrane can then be thermochemically carbonized at high temperatures in an inert atmosphere such as nitrogen to increase the surface area of the individual fibers and make them mechanically stronger. As a result, gluten may be used to generate carbon nanofiber mats, an excellent option for mask development. The new carbon nanofiber mats made from gluten and their integration with gluten biopolymer treated with lanosol could be used to construct unique, totally bio-based masks with improved filter performance. The new masks are expected to be fire-resistant and biodegradable at the same time. Figure 9 shows the carbonization process that can produce carbon nanofiber mats.

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Fig. 8 A schematic diagram shows (a) an electrospinning process to produce gluten nanofibre mats; (b) SEM micrographs of gluten nanofibers; and (c) SEM images of PVA/gluten nanofiber. (Adapted with permission from Ref. [62]. Copyright 2020, Elsevier).

Fig. 9 A schematic diagram of the carbonization process in an inert environment produces carbon nanofiber mats. (Adapted from Ref. [62]. Copyright 2020, Elsevier).

Chitosan Chitosan (Fig. 3f) is the most currently investigated polysaccharide due to its abundance and biodegradability among biopolymers. Chitosan’s antibacterial properties and its ability to form films make it one of the protective personal equipment

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items that are the facemask [65]. Because it is biocompatible and biodegradable, chitosan has potential biomedical applications as a biopolymer. Chitosan has a wide range of industrial applications due to its solubility in acidic aqueous media, and its solubility is determined by the degree of acetylation and position of the acetyl and amino groups throughout the chain. Furthermore, when the amino groups are in a cationic state, chitosan has antibacterial action, meaning that chitosan’s antimicrobial activity is stronger at low pH [66]. Chitosan has positively charged amino groups that may interact with the negatively charged bacterial cell membrane and cause membrane damage in the pathogen, effectively protecting the human respiratory system against bacterial and viral invasion [66]. Chitosan is a natural antibacterial agent with potential use in agriculture, food, biomedicine, and biotechnology because of this feature. Chitosan is a naturally occurring polymer derived by the deacetylation of chitin in various sources, such as fungal cell walls. Nontoxicity, biodegradability, and antioxidant capacity are only a few benefits. As a result, the biomedical and pharmaceutical industries have been studying this polysaccharide for a long time. Furthermore, chitosan is an antibacterial ingredient that can protect the human body from bacteria and deterioration. The biodegradability of chitosan is limited due to the antimicrobial properties that make biodegradable chitosan difficult to decompose. The depolymerization of chitosan leads to oligomers, which are then broken into monomers such as glucosamine and N-acetylglucosamine. Therefore, enzymes are required from several sources to break down this polymer. Lysozyme is one of these hydrolase enzymes, and it is abundant in the human body [67]. Some microbes produce chitosanase, which breaks down chitosan. These bacteria can be found in soil and compost, where they are abundant and can actively engage in chitosan breakdown [67]. Chitosan and chitosan derivates can be used as transporters for flame-retardant chemicals and water-repellent coverings, helping to meet two essential regulatory requirements for medical face masks [68]. Chitosan is frequently combined with other antiviral technologies to create nanocomposite materials, such as silver nanoparticles (NP) combined with chitosan. Chitosan derivates, such as quaternary carboxymethyl chitosan, have been employed as antibacterial agents in medical filter media [68]. Chitosan also improves the affinity of cellulose with antimicrobial compounds and works as a matrix for other antiviral and antibacterial agents, such as silver nanoparticles, to disperse and operate better [68]. Chitosan and cellulose generate hydrogen bonds because their structures are similar. As a result, chitosan improves the bonding and mechanical properties of the medical filter medium in face masks [68]. Due to structural similarities, chitosan-based nanocomposites appear to be one of the most promising antibacterial technologies used to mask and make compatible with cellulose. Many chitosan derivatives are commercially accessible, and they can be coupled with a variety of different antiviral agents to create composite structures that can be used in face masks.

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Starch Starch-based biodegradable compounds are among the most researched and used natural biodegradable materials [69]. Starch is abundant in nature, inexpensive, nontoxic, renewable, and biocompatible and can form films [69]. Starch is a polymer made up of long chains of glucose molecules joined by glycosidic bonds. Starch is made up of two main polymeric elements, amylose (Fig. 3g) and amylopectin (Fig. 3h), which have different characteristics and ratios depending on the source [69]. Furthermore, because starch has many hydrophilic hydroxyl groups, it is a hydrophilic polymer, which makes starch polymers vulnerable to moisture attacks and considerable changes in dimensional stability and mechanical properties [69]. These environment-friendly ingredients are sometimes absorbed along with the product or degraded by microorganisms without releasing harmful emissions into the environment [69]. Electrospinning gives starch and other biopolymers some unique properties that can be employed in various applications, such as the manufacturing of face masks [69]. Because of its biocompatibility, starch offers a lot of potential for electrospinning nanofibers. Other synthetic biodegradable polymers, such as polycaprolactone and polyvinyl alcohol, act as additives and have been electrospun with starch to improve the mechanical properties of starch-based biodegradable materials [70]. Electrospinning was used to create antibacterial properties in potato starch nanofibers, which can be used as face masks [71]. Using gas-phase glutaraldehyde to crosslink electrospun starch nanofibers improves mechanical strength and water stability [72]. Mechanical qualities are significant since they determine how resistant they are to deformation or fracture [73]. Figure 10 shows the electrospinning setup for the formation of electrospun starch nanofibers.

Fig. 10 A representative diagram of the electrospinning setup for the formation of electrospun starch nanofibers. (Adapted with permission from Ref. [74]. Copyright 2017, Elsevier).

Starch

Chitosan

Gluten

Electrospun encapsulated polylactic acid nanomembrane

It is environmentfriendly due to being highly degradable and cost-effective

High cohesive and barrier qualities, abundance, and good biodegradability Excellent antibacterial properties, nontoxic, and abundant

Advantages It saves money and improves filtration efficiency against airborne particles

It has poor mechanical behavior and high water vapor permeability

The formation of gluten nanofiber involves high temperatures, which leads to high operation costs Biodegradation efficiency is limited compared with other biopolymers

Disadvantages Production costs are higher than fossil-based materials because of their intermediate steps involving high temperatures

Poor processing quality resulted from high viscosity

Weak mechanical properties and modifications can be carried out, such as crosslinking

Challenges It is environmentally unfriendly due to the use of organic solvents such as dimethyl sulfoxide and formic acid during nanofiber fabrication. Besides, organic solvents that are difficult to remove are dangerous to human health when an individual inhales a volatile gas It has high water vapor and air permeability and low processibility

Table 3 A comparison between electrospun encapsulated polylactic acid, gluten, chitosan, and starch

[80, 81]

[77–79]

[62, 63]

Poor fire resistance and can be improved by adding lanosol Chitosan’s high hygroscopicity affects the filtration capacity of airborne particles. Chitosan’s high vulnerability to environmental variables and processing conditions can induce structural stress and polymer degradation The glass transition temperature (Tg) is relatively high, followed by brittle behavior at room temperature

References [48, 49]

Limitations The largest pressure drops result in difficulty in breathing

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The amount of carbon dioxide (CO2) released and the final weight loss of the samples in the soil burial are typically used to measure the biodegradability of starch-based films for degradation tests [75]. To improve the biodegradability of starch-based films, acetylation of starch has been proposed [76]. The longer time for acetylation of starch increased the carbon dioxide release rate, resulting in enhanced film degradability. The addition of acetyl groups may have increased the biodegradability of these films by reducing the inter- and intramolecular contacts caused by hydrogen bonds, allowing water to enter and soil microbes to act, which can be facilitated [76]. Table 3 summarizes the advantages, disadvantages, challenges, and limitations for electrospun encapsulated polylactic acid nanomembrane, gluten, chitosan, and starch polymers in manufacturing the biodegradable face mask.

The Effectiveness of Biodegradable Face Masks, Face Shields, and Hand Gloves in Preventing Viruses, Bacteria, and Particulate Matter 2.5 Particles The discarded contaminated face masks, on the other hand, are a potential source of biomedical waste and a vehicle for COVID-19 to spread across the population. Microbial regrowth and survival on traditional face masks after use and incorrect storage can cause secondary infections in people [82]. Especially during pandemics, this type of single-use and discard mentality can lead to a massive shortage and the development of a considerable amount of hazardous waste. Biodegradable face masks can be incorporated with antimicrobial capabilities via electrospinning. This, on the other hand, can aid in the real-time deactivation of microorganisms, allowing for numerous uses and reducing subsequent infections [82]. Given the benefits, various attempts have been made to incorporate antimicrobial agents into face masks that are safer. However, repeated usage of such a face mask could endanger the person and the environment due to nanoparticle leaking and detachment. Considerable thought is essential to avoid leaching and environmental consequences when selecting antimicrobial agents and their integration techniques. For instance, antimicrobial or antiviral agents such as silver and MgO nanoparticles (NP) are typically incorporated into the filter media of biodegradable masks to kill the coronavirus, which might minimize the spread of COVID-19 from discarded biodegradable masks. The possibility of spreading the coronavirus also depends on the surrounding conditions of the discarded biodegradable mask, such as temperature. For instance, the possibility of COVID-19 spreading will be higher when discarded biodegradable masks are placed in a cold ambient environment, even though biodegradable masks are used. Several studies have shown that biodegradable polymers have significant potential as face mask alternatives to traditional materials. Particle filtration efficiency, bacterium filtration efficiency, and viral filtration efficiency are three types of tests used to evaluate the filtration efficiency of face masks to meet the requirements for medical-grade biodegradable personal protective equipment [83]. The size distribution of particulate matter, the pattern of

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Aerosol [2 m

>6 m

Fig. 11 A representative diagram shows (a) the size distribution of particulate matter; (b) a schematic view of aerosol and droplet molecules; and (c) a schematic view of the aerosol filtration mechanism in a common face mask. (Adapted with permission from Ref. [83]. Copyright 2021, Elsevier).

aerosol dispersal during expiration, and the filtration mechanism in surgical face masks are depicted in Fig. 11 [83]. Biodegradable polymeric materials may be ideal choices for manufacturing future filtration medium of face masks, with a long-term perspective and comparable performance to traditional non-woven face masks in terms of the virus, bacteria, and PM 2.5 prevention against the human respiratory system. Various bio-based filtration media, such as cellulose, have recently been trademarked and released as biodegradable face masks. Different biopolymer combinations are being used to produce improved filtration capacity and functions, such as polylactic acid/ polyhydroxybutyrate [83]. These basic needs, such as high filtration efficiency against bioaerosols, fluid barrier qualities [84], air permeability, and superior wearing properties such as comfort, are all requirements that are critical for existing face masks because they help to minimize viral concentrations in the air and disease transmission in congested indoor and outdoor communities. Multifunctional features can be incorporated into traditional surgical face masks by utilizing numerous layers of functional components [83]. High filtration effectiveness and antibacterial activities are all demonstrated by multifunctional TiO2/polyacrylonitrile nanofibers [85]. Electrospun polyimide/metal-organic framework nanofibrous membranes have excellent thermal stability and high mechanical characteristics for efficient particulate matter capture in difficult environments. For instance, zeolitic imidazolate framework-8 (ZIF-8) is a type of metal-organic framework with high chemical and thermal stability and the capacity to trap PM2.5 [86].

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Brownian diffusion, intermolecular interactions, and electrostatic interactions all play a role in air filtration [83]. The interactions between particulate matter, the filter structure, and fluid molecules determine the particle capture capacity of fibrous filtering media [83]. Surgical face masks have been tested for their antibacterial properties, such as efficient suppression of a wide range of bacteria, nonhazardous, compatibility, and avoidance of irritations and allergies with skin [83]. The strongest bacterial filtration efficacy against bacteria was surgical face masks with the smallest mean pore size [87]. Antibacterial electrospun air filtration membranes made up of polylactic acid, and other components like polylactic acid and polyhydroxybutyrate have also demonstrated significant filtration efficiency and antimicrobial activity [83]. Respiratory disorders are caused by viruses, which filter systems’ escape easily and produce serious infections [83]. Different antiviral agents, such as metal and metal oxide nanoparticles (TiO2), can deliver antiviral performance through direct and indirect disinfection mechanisms [85]. The viral filtration efficiency of biodegradable face masks is determined by the ratio of live viruses sampled outside and within the masks [88]. Ideally, the biodegradable face mask materials would trap and inactivate the viruses at the same time because coronavirus could persist on the mask’s surface for up to a day, although biodegradable face masks have been shown to prevent the spread of viruses [88]. Figure 12 shows the antimicrobial action of metal and metal oxide nanoparticles against pathogens. Mechanism (A): The pathogens’ cellular components, such as membrane, DNA, protein, and mitochondria, might be irreversibly damaged and destroyed by the produced reactive oxygen species (ROS), resulting in cell death. Mechanism (B): Free radicals are generated by nanoparticles and their ions, such as titanium and silver, resulting in the production of ROS. Biodegradable face shields and hand gloves have been proven to protect individuals from airborne particles. When worn in conjunction with a correctly fitted surgical mask, they protect from contamination to both others and the wearer. Biodegradable face shields can be used instead of face masks if they are not available since most of the antimicrobial agents will be incorporated into the face shields together with biodegradable materials. For instance, MgO nanoparticles are a promising antimicrobial agent because of their strong resistance to harsh processing conditions and high surface area to volume ratio and unique chemical and physical properties [90]. Biodegradable face shields can be washed, disinfected, and reused, thus reducing medical waste pollution in the long run. For biodegradable hand gloves, adding polysaccharides such as chitosan to the natural rubber latex system is intended to speed up the action of polymer-degrading bacteria, allowing the gloves to decompose more quickly. Also, chitosan has been proven to act as an antimicrobial agent by providing the electropositive charge nanoparticles incorporated into the gloves to introduce a positive charge and enable the gloves to attract microbes more efficiently. As a result, biodegradable hand gloves can protect the wearer against pathogens effectively and speed up the biodegradation rate to avoid medical waste either ending up in landfills or incineration, which are environmentally unfriendly. This is because polysaccharides such as chitosan are advantageous to the biodegradation process. After all, they can act as nutrition for bacteria, enhancing the gloves’ biodegradability.

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Fig. 12 A schematic diagram of the antimicrobial action of metal and metal oxide nanoparticles against pathogens (Adapted with permission from Ref. [89]. Copyright 2018, Elsevier).

Conclusions Following the COVID-19 disaster, the generation of medical waste has increased dramatically. The abrupt surge in medical waste, notably for personal protection equipment such as face masks, was one of the repercussions of the COVID-19 pandemic because face masks are recognized solutions to prevent the spread of coronavirus among the community. These traditional masks are rapidly being

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replaced by biodegradable ones, which are environmentally benign. These nonbiodegradable masks pollute the environment and pose secondary health risks as a result. Because most protective equipment is made of petrochemical-based plastic, plastic plays an important role in the fight against the COVID-19 pandemic. Tons of plastic garbage have been created worldwide, with most of the debris being disposed of in landfills or incinerated and only a small portion being recycled. The use of plastics has increased, which will impact our long-term aims of moving to a circular economy. If medical waste is not correctly handled, long-term implications such as health difficulties and environmental degradation will occur. The current system and infrastructure for managing medical waste are inadequate and ineffective in dealing with trash-generating flows. Therefore, it is time to embrace the biodegradable alternative to mitigate the crisis of plastic waste due to the pandemic of COVID-19. Various biodegradable polymers have been employed as raw materials for electrospun nanofiber membranes as mask materials, including chitosan, gluten, polylactic acid, and other biopolymers. To inhibit virus penetration, nanofiber membranes are used as one of the component layers of the facemask. In addition, as a low-cost and biodegradable alternative, they are used to prepare multilayer masks for general usage. Various herbal ingredients, such as neem, improve the antibacterial effectiveness of such masks. These masks are gentle on the skin and antibacterial as well as antiviral. In addition, some herbal ingredients give the masks a natural color, which improves their visual quality. In the current pandemic, face shields and hand gloves were employed worldwide to protect healthcare personnel and the general population. Using biodegradable materials in the manufacture of face shields and gloves can considerably aid in protecting both people and the environment. Furthermore, high-efficiency face shields and gloves can be further functionalized to include antibacterial, antiviral, and self-sanitizing qualities, as well as fit and comfort, in addition to biodegradability. Existing technology must be inspired by new ideas to increase economic and environmental efficiency. Government backing with more contingency plans will be required to incorporate more efficient, adaptable, and innovative techniques of manufacturing environment-friendly face shields and gloves under crucial circumstances. Every action taken will contribute to the importance of this topic, as it is the community’s primary goal to create a cleaner and greener environment. There is enough information about the manufacture and application of biodegradable personal protection equipment to show that it effectively prevents and destroys pathogen microorganisms. Optimization of the proportions of materials used and conditions of the synthesis process should be focused on to achieve the optimum quality of biodegradable personal protection equipment. Besides, the particle and bacterial filtration efficiency of the biodegradable mask should be up to 95%, and the biodegradable materials should offer good breathability and good water resistance to the mask. The performance of biodegradable materials should be maintained at a high level over time. Research on biodegradable personal protection equipment should be an urgent and important matter. We need to act fast before more and more medical waste ends up in the environment, causing more damage to our planet.

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Future Perspectives In fighting the epidemic, worldwide cooperation is required to address the issue of plastic trash. According to research, biodegradable plastic litter can persist in the soil and marine environment. The degradation of bioplastics into microplastics or nano plastics poses a threat to marine organisms and pollute the air and soil. Microplastics that have accumulated too much in the ecosystem pose a bigger environmental threat. As a result, even if the personal protective equipment were constructed of biodegradable materials, it would not be an appropriate solution considering the current situation. The most crucial aspect is that personal protective equipment and other waste must be appropriately discarded and managed by the instructions established by the authorities in charge. Therefore, improved waste minimization knowledge and laws on illegal dumping activities would help to reduce the massive amount of plastic medical pollution. Apart from that, personal behavioral and social changes are required to redesign the plastic waste management system. Improvements can be seen when both features are present at the same time. Moreover, every family should be provided with specific-colored bags to discard and seal used personal protection equipment. Separating and treating biomedical waste is more practical in this situation. Personal protection equipment should be placed in a specific color bin in public areas. Besides, encouragement of investments in the manufacture of goods and sanitary and recycling objectives will generate new ideas and allow current goods to be adapted to new applications. Research on innovative plastic packaging technologies should be encouraged. Chemical recycling, for example, appears to be capable of producing high-quality compounds from plastic trash. Apart from that, medical waste pollution and its environmental consequences must be addressed in schools. It is a crucial long-term strategy for raising citizen awareness and consciousness. The government also plays an important role by enacting rules and regulations, such as imposing fees for the use of nondegradable plastics. When demand declines, production also decreases. Policy incentives for recycling productivity, which frequently generates incentives for renewable technologies, must be reorganized. It is critical to identify and reward recycling plants that operate efficiently and publicize them to inspire and encourage others. In other words, policies must be changed to reduce multilayer packaging and encourage homogeneous plastic packaging materials that are easier to recycle as part of a long-term effort to increase the efficiency of recycling plastic medical waste. Due to its limited recyclability, a tax could be levied to discourage the use of multilayer packaging. Companies contribute to the funding of the waste management sector and the development of entrepreneurial opportunities in plastic waste management. In short, corporate finances and services should be considered as one of the producer’s duties. Acknowledgments This work was financially supported by the Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education of Malaysia (MOHE) (FRGS/1/2019/ STG01/UM/02/6).

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Mg and Biodegradable Mg Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – Al alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – Zn alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – Ca alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – Zr alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – Sr alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mg – REEs alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Treatments of Biodegradable Mg Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anodization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micro-Arc Oxidation (MAO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ion Implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Magnesium is the lightest structural metal used in materials and is considered one of the most abundant elements. Therefore, magnesium alloys are an excellent choice for many applications when a lightweight design is required. Magnesium alloys are natural biodegradable materials due to their easy susceptibility to corrosion when placed within aqueous media. Among biodegradable materials, magnesium has an essential role in many vital processes in the human body. Furthermore, magnesium alloys have mechanical properties like those of the human body bones. These make magnesium alloys promising and an alternative S. Salman (*) · M. K. Gouda Department of Mining & Petroleum Engineering, Faculty of Engineering, Al-Azhar University, Cairo, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_40

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to the permanent implant materials to avoid a second surgery for implant removal. The rapid degradation of magnesium alloy implants in living organisms limits its widespread usage in this field. This rapid degradation leads to early deterioration of the implant’s mechanical properties before the tissue healing process. Moreover, the high amount of corrosion products and the alkalinity increase in the surrounding area will lead to toxic events. So, there is a significant need to limit the degradation rate of the alloy to fit the rate at which the surrounding tissue heals. Chemicals, physical, mechanical, and biological coatings are among the surface treatments performed to control the degradation rate and ensure the bone healing process. This chapter briefly describes the most biodegradable magnesium alloys and some selected surface treatments for bio-implant applications. Keywords

Magnesium · Orthopedic · Implant Materials · Biodegradable · Corrosion Abbreviations

Ca-PP CVD DCPD FHA HA ODPA PA PBS PEO PVD REEs SA SAM SEM

Calcium phosphatePhosphate Chemical vapor deposition Dicalcium phosphate dehydrate Fluoridate hydroxyapatite Hydroxyapatite Octadecylphosphonic acid Phytic acid Phosphate buffer saline Plasma electrolytic oxidation Physical vapor deposition Rare earth elements Stearic acid Self-assembled monolayers Scanning electron microscope

Introduction Magnesium (Mg) is a silvery-white metal ranked as the sixth most abundant element in the crust. It is the lightest structural metal with a density of 1.74 g cm
3. Mg-based materials are employed in many applications like aerospace, sports, automobile, and medical applications [1, 2]. Magnesium is used in various medical applications because of the abundance of Mg ions in the human physique, and consequently, they are involved in numerous chemical-biological interactions. Mg is a highly biocompatible and biodegradable material in the human physique, where it can be steadily and safely dissolved or absorbed. Moreover, Mg has a greater fracture toughness than ceramic biomaterials like hydroxyapatite, and its compressive yield strength and elastic modulus are more like those found in real bone. This avoids the

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Fig. 1 Graphical comparison between the characteristics of Mg, human bone, and other implant materials

stress shielding effect reported in some other commonly used metallic implants, as explained graphically in Fig. 1 [3, 4]. More explanations about the advantages and disadvantages of Mg alloys compared to other used alloys for human implants are shown in Fig. 2 [5–9]. Nevertheless, a considerable problem such as rapid corrosion, which creates subcutaneous gas bubbles, [3] has to be overcome before Mg and Mg alloys are widely employed in various biomedical applications. Therefore, alloying and surface treatment are the most commonly used strategies to enhance Mg and Mg alloys [10, 11]. The commonly used alloying elements include aluminum (Al), zinc (Zn), manganese (Mn), silicon (Si), rare earth elements (REEs), zirconium (Zr), yttrium (Y), scandium (Sc), and so on [12]. The concept of biodegradable materials necessitates that the materials have adequate mechanical properties for the intended usage and proper corrosion resistance for gradual deterioration. Al, rare earth and Y are considered not to be biocompatible and even toxic to living cells. Moreover, suitable biocompatibility and bioactivity inside the human body are mandatory [13]. Therefore, the element selection should be based on the basic

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Fig. 2 The advantages and disadvantages of Mg alloys compared to commonly used metallic implants

elements present in the human body, such as Ca, Zn, Mn, Sn, and Si [14]. On the other hand, surface treatments techniques provide a solution to improve the corrosion resistance of Mg alloys. Surface treatments provide barrier protective films on the surfaces of the alloys. The most famous surface treatment techniques are chemical conversion coating, anodic oxidation, electroless/electroplating, chemical vapor deposition (CVD), physical vapor deposition (PVD), laser surface treatment, and organic coating. Chemical conversion coatings and anodizing are considered among the most widely approved surface treatment technologies due to their high adhesive strength, simplicity, low cost, good corrosion protection [15]. In this chapter, a brief review of the most biodegradable Mg alloys and selected surface treatments suitable for biodegradable applications.

Properties of Mg and Biodegradable Mg Alloys Table 1 shows pure Mg0 s common atomic, physical, and mechanical properties. However, the properties of Mg alloys are quite different, as will be explained hereafter. Pure Mg is non-toxic and can promote hard tissue recovery after human

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Table 1 Selected atomic, physical, and mechanical properties of pure Mg [12] Atomic Properties and Crystal Structure Atomic number Atomic volume Crystal structure Valence Crystal Structure Physical Properties Density (g cm
3) Melting and Boiling points ( C) Mechanical properties 0.2% Compressive yield strength (MPa) 0.2% Tensile yield strength (Mpa) Ultimate tensile strength (Mpa)

12 14.0 cm3 mol
1 Hexagonal close-packed (HCP) 2+ HCP 1.738 (at 20  C) 650  C and 1090 69–83 (Annealed Sheet) 90–105 (Annealed Sheet) 160–195 (Annealed Sheet)

body implantation and activate new bone formation. However, it degrades rapidly upon implanted inside the human body, causing the mechanical integrity of the implanted alloy to deteriorate quickly before the proper healing of the host tissue [16]. The corrosion or degradation affects the mechanical properties of Mg and Mg alloys, particularly in physiological environments, because corrosion causes a steady loss of structural integrity and mass in Mg alloys [17]. In the case of biodegradable implants, the environment inside the human body is quite complicated. Metal implants, including Mg alloy implants, are susceptible to an extraordinarily complex corrosive environment. The physiological environment generally includes blood, protein, high and low molecular weight substances, dissolved oxygen, and other body fluid elements. In physiological environments, biodegradable metals degrade via anodic and cathodic reactions. In other meaning, degradation occurs due to corrosion, which implies the conversion of the metal to a more stable ion as shown in the anodic and cathodic reactions [18], as described in Fig. 1a, b. At the same time, organic compounds like protein and lipids will adhere to the metal surface and affect the metal’s dissolving process. The extremely aggressive physiological environment with high concentrations of chloride ions causes deterioration of the protective oxide layer and hence accelerates the degradation process [19] (Fig. 1c). Moreover, apatite (Ca-P based) formed onto the undissolved hydroxide layer because of the local alkalization and the saturation of Ca and P inside the body fluid. Also, cells adhere to the surface and proliferate to form tissues alongside the produced corrosion layer. At the same time, the eroded metal may disintegrate as irregular particles from the metal matrix and disperse in the surrounding media. Consequently, this process continues till the metal is degraded (Fig. 1d) [20] (Fig. 3). Moreover, the corrosion behavior is affected by the change in the microstructure. The change in the grain morphology and existing phases is directly related to the change in mechanical and corrosion properties. Furthermore, impurities and secondary phases (intermetallic compounds) affect the corrosion behavior of Mg and Mg alloys.

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Fig. 3 Schematic diagram of the biocorrosion at metal/medium interface. (Adapted with permission from Ref. [20] © 2014, Elsevier)

When the impurities concentration crosses a certain limit, the corrosion rate increases substantially because the Mg matrix works as the cathode of the micro galvanic cell and gets dissolved [19]. On the other hand, the intermetallic compounds along the grain boundaries cause segregation of the Mg grains. These segregated phases have a significant impact on the characteristics of Mg alloys, as shown in Fig. 4. Alloying is considered the normally used strategy for improving Mg0 s corrosion resistance and mechanical properties. Al, Zn, calcium (Ca), Zr, strontium (Sr), and REEs are the most used alloying elements to enhance the corrosion resistance and mechanical characteristics of Mg alloys. A brief review of each alloying element will be presented in this section.

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Fig. 4 A schematic illustration of the galvanic effect between the Mg matrix, secondary phase, and impurities

Mg – Al alloys Aluminum (Al) is one of the most used alloying elements to manipulate the mechanical properties and corrosion behavior of Mg alloys. The addition of Al causes grain refining, particularly with low content; therefore, a change in the mechanical and corrosion properties is expected. Alloying with Al affects the solid solution strengthening and precipitation strengthening, consequently increasing the strength [21]. The corrosion rate of Al-based Mg alloys is decreased by increasing the Al-content due to the formation of an insoluble Al2O3 layer, rather than Mg (OH)2, which is solvable in the chloride solution [22]. Moreover, the content of impurities or contaminants (Fe, Ni, and Cu) has a major play in the corrosion behavior of Mg-Al alloys. These impurities form efficient cathodes and lead to an effective galvanic cell if they are beyond a certain level. Therefore, localized corrosion occurs because of the formation of galvanic cells, which impairs the mechanical integrity of the implants [23].

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Many studies have revealed that Mg-Al alloys have higher corrosion resistance than pure Mg in different corrosive environments such as sodium chloride (NaCl) solutions [24], phosphate buffer saline (PBS) [25], and simulated body fluids (SBFs) [26]. Although Al is positively influencing Mg0 s mechanical and corrosion behavior, there are some major concerns regarding its biocompatibility, which makes Mg-Al alloys undesirable in the biomedical field [27]. Many medical reports have found that Al is unfavorable, particularly in higher concentrations. It was reported that the accretion of Al in the brain could result in neuropathological issues under certain conditions [28]. It is also a potential risk of developing Alzheimer’s disease. Also, Al substantially influences immunology, and Al-containing vaccinations may cause damage to lymphocyte and subcutaneous muscle fiber [29]. Other elements, including Ca, Zn, REEs, etc., can be added to improve one or more of Mg-Al alloys’ properties [30]. However, from the point of biocompatibility, it still requires deeper and detailed investigations. Table 2 summarizes the effect of main alloying elements on the properties of Mg alloys.

Mg – Zn alloys Zn is an essential and nutritional element in the human physique [27]. Using Zn as an alloying element with Mg improves the mechanical properties and corrosion behavior. Solid solution and precipitation strengthening were observed in Mg-Zn alloys [27]. However, due to the micro-galvanic effect, excessive Zn addition harms the corrosion resistance of Mg alloys. From the point of corrosion behavior, alloying Mg with Zn showed a considerable improvement compared to pure Mg [31], as Zn can turn impurities that affect the corrosion of Mg alloys like Fe and Ni into innocuous intermetallic compounds [32]. The biocompatibility of Mg – Zn alloys has been established by several studies [33–35]. However, many studies have been done on the adverse effects of a Zn overdose on growth, development, and health [36, 37]. Neurological problems have been linked to bivalent metal [38]. Zn buildup in the human physique may cause embryonic motor neuron death. Furthermore, it affects mature motor neurons because Zn cation may work as a mediator inhibitor of neurotrophins and sometimes lead to cell death. A normal Zn content keeps the body healthy. However, implanting a high Zn concentration Mg alloy into the body may lead to toxic effects that may compromise immune function [39]. As a result, the concentration of the Zn in such alloys is the most challenging aspect.

Mg – Ca alloys Calcium has a significant grain refining effect on Mg and Mg alloys. Therefore, it is expected that adding Ca to Mg alloys will result in significant improvement in the mechanical behavior, corrosion resistance, and biocompatibility of the Mg alloys [40]. Calcium is an essential nutrient and has the highest concentration of elements in the human body. It is the main component of the skeleton and has other important

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Table 2 Lists the effect of the main alloying elements on the mechanical behavior, corrosion resistance, and biocompatibility of Mg [17] Corrosion Element

Strength

Biocompatibility

Ductility resistance

Al

+

+

+

Neurotoxic

Zn

+

+

+

Yes

+ Zr

+

+

(Small

Yes

amounts) Ca

+

Sr

+

+ _

+

Yes

+

Yes Limited

Mn

+

_

+

(Excessive

amounts

can

produce

neurological disorder) + _

Ce

(Small

No

amounts) + Li

_

+

(Below

No

9% ) _ Limited REEs

+

(Binary (Accumulate in the liver and bone) alloys) Limited

Cu

+

_

(Excessive amounts can cause neurodegenerative

diseases

cellular cytotoxicity)

and

produce

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biological and functional roles in the body. Mg is very important for the incorporation of Ca into bone [41], which may be considered to be favorable to bone regeneration due to the co-release of Mg and Ca ions. Moreover, Mg – Ca alloys have an advantage due to their analogous density to bone [40]. This makes Ca one of the interesting alloying elements for orthopedic Mg alloys. Although Ca has a positive effect on the corrosion and mechanical characteristics of Mg alloys, it has some limitations from Ca concentration. It was reported that Mg-Ca alloys with a 0.6 wt % Ca concentration exhibited higher bending and compressive strength than pure Mg. However, both polarizations were notably dropped over this limit with increasing Ca concentration [2]. Moreover, the tensile strength was improved up to 4% Ca content [42]. The addition of Ca showed an improvement in corrosion resistance compared to pure Mg up to a specific Ca content limit [42]. The optimum Ca content is 0.6 wt % Ca, and any further increase in Ca content reduced the polarization resistance of Mg-Ca alloys in SBF [2]. The reason for the content limitation is that increasing the Ca content will increase the volume fraction of the Mg2Ca secondary phase, which increases the possibility of the galvanic cell and ultimately decreases the corrosion resistance and accelerates the degradation of Mg-Ca alloys [17]. Therefore, the concentration of Ca in Mg should be less than 1 wt % to avoid the localized corrosion phenomenon in the Mg – Ca alloys. The alloy of Ca concentration 0.6 wt % shows improved corrosion resistance compared to pure Mg [42, 43], and any further increase in Ca concentration reduces the corrosion resistance of Mg-Ca alloys in SBF solution [2]. The content limitation is that increasing the Ca content will increase the volume fraction of the Mg2Ca secondary phase, which may form galvanic cells and consequently decrease the corrosion resistance and accelerate the degradation of Mg – Ca alloys [17]. Therefore, the Ca concentration in Mg-Ca alloy should not exceed 1 wt % to prevent localized corrosion. Ca and Mg are essential elements inside the human body; therefore, it is expected that Mg-Ca alloys are intrinsically biocompatible. It has been confirmed that there are no toxic or harmful effects from the degradation of Mg-Ca implants within the body [43]. The in vivo Mg – 0.8 Ca study for multiple weeks [44]. In Vitro study on biomedical Mg-Ca alloy in albumin contained solution presented that the formation of threadlike corrosion triggered by chloride ions is successfully blocked owing to albumin molecule adsorption. Furthermore, the inhibitive impact increases with the increase in the albumin content [45]. However, the mechanical and corrosion properties of Mg – Ca alloys are inadequate to meet the criteria of bone implants. It shows rapid degradation inside the bone, showing gas bubbles over the total area of the Mg -Ca alloy pin after 1 month from surgery and an unhealed hole after the complete degradation of the implant 3 months after the surgery [46].

Mg – Zr alloys Zirconium (Zr) is well-known for improving corrosion resistance and grain refinement in Mg alloys [34]. Zr can hinder crystal grain growth as the undissolved Zr

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particles serve as nucleation sites during solidification, resulting in very refinedequiaxed grains [47]. Zr has high corrosion resistance in various environments such as alkaline, acidic, and salt water, and It is used as an alloying element with Mg in the presence of other corrosion protective elements [48]. When immersed in a borate buffer solution, Mg-Zr alloys exhibit high corrosion resistance due to the formation of Zr-Mg double oxyhydroxide enriched with Zr cations [17]. Moreover, it improves the corrosion resistance of Mg alloy by decreasing the harmful effect of iron impurity. [34]. Zr exhibits excellent biocompatibility and osseointegration in both vitro and in vivo, even excelling titanium [49]. The studies about Mg-Zn as binary alloys are scarce; however, the biocompatibility of ternary alloys containing Zr is reported in several studies. Zr addition up to 5% to Mg alloys showed superior biocompatibility, and no harmful impact was noted after implantation in rabbits [50]. Moreover, the results showed improvements in compressive strength, biocompatibility, and corrosion resistance at a concentration of 1% Zr and 1% Zr [51]. These results showed that Zr is promising in alloying biodegradable Mg alloys [52]. However, further investigations are still required as limited studies have been conducted on Mg-Zr alloys as a promising biodegradable material.

Mg – Sr alloys Strontium is also considered a grain refiner for Mg that can affect the mechanical and corrosion characteristics of Mg alloys [17]. Like Ca, Sr is a trace element essential to the human body [53]. A bone seeker element builds up in the skeleton as a Ca substitute in hydroxyapatite (HA), preferably in new trabecular bone [54]. Sr has been shown to play a biological role in bone, muscles, and the heart [55]. The alloying of Mg with Sr leads to the formation of a microstructure containing both α -Mg and Mg17Sr2 phases, increasing the Sr content up to 2% wt increases the amount of the secondary phases precipitate, which results in refined microstructure and enhanced strength, while elongation is decreased [53]. The corrosion resistance of Mg improves when alloyed with Sr. However, it is limited to 1.5–2% wt. Sr content [56]. A Further increase in the Sr content increases the volume percentage of the Mg17Sr2 secondary phase and, therefore, the formation of the galvanic cell, decreasing the corrosion resistance of Mg-Sr alloy. The biocompatibility of Sr is inevitable, with a great potential for bone formation. The biocompatibility and biodegradation of Mg-Sr alloys were reported in the literature. For instance, Zhao et al. [56] showed the lower cytotoxicity nature of Mg – Sr alloys. Their results proved no adverse or toxic effect on cells in Mg-0.5Sr. Gu et al. [53] revealed that up to 2 wt % Sr, there was an improvement of the corrosion resistance of Mg – Sr alloys in Hank’s solution at 37  C. Moreover, the in vivo experiments conducted on male mice confirmed the bone formation alongside the Mg-Sr implant degradation. The same trend was observed by Bornapour et al. [57], as they reported the lowest corrosion rate at 0.5 wt % Sr. in SBF at 37  C.

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Mg – REEs alloys The rare-earth elements (REEs) contain the 17 chemical elements in the periodic table; 15 lanthanides, Sc, and Y [17]. Recently, some REEs showed promising functions like enhanced mechanical properties, grain refining, and improvement in the corrosion resistance of Mg and Mg alloys [58]. REEs are often added to Mg alloys using master alloys or known as hardeners, which include mostly one or two REEs and nearly all other REEs in trace amounts [4]. There are two main groups of elements: those with high solubilities (Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and those with low solubilities (Nd, La, Ce, Pr, Sm), and Eu [59]. The most frequent REEs used in Mg –REE alloys are Ce, La, and Nd. The corrosion behavior of Mg-REE is related to the formation of the intermetallic compounds, which generally result in galvanic corrosion [57]. REEs with limited solubilities in Mg, such as La, Ce, and Nd, are more susceptible to precipitate the intermetallic compounds during casting and processing and thus induce galvanic corrosion. Therefore, the alloying elements with high solubility in Mg like Y, Dy, and Gd) were preferred in biomedical Mg alloys designs to avert the formation of intermetallic compounds [60, 61]. From biocompatibility, Mg – REEs alloys are not recommended [17]. However, few reports showed that some REEs are biocompatible [62]. The biocompatibility of magnesium alloy containing La, Nd, and Ce revealed no adverse effects on tissues. Mg – La and Mg – Nd had nearly the same cell vitality after 1, 3, and 5 days, whereas Mg – Ce had lower cell viability compared to the other two alloys at the same time [60]. The Mg alloys containing REEs demonstrated nearly the same biocompatibility as pure Mg, confirming that REEs are appropriate alloying elements for biodegradable Mg alloys [61]. The addition of REEs to Mg and Mg alloys significantly increases the mechanical properties. Nevertheless, the optimum concentrations of REEs and other alloying elements should be studied thoroughly to achieve the optimum combination between mechanical characteristics, corrosion behavior, and biocompatibility.

Surface Treatments of Biodegradable Mg Alloys The poor corrosion resistance is a major obstacle facing the widespread use of Mg alloys in many applications. Mg has high chemical activity compared to other structural metals like steel and Al alloys. Recently, several surface treatments techniques have been developed to protect Mg alloys. Figure 5 shows the wet processes, which are the most widely used commercially to protect Mg alloys against corrosion in film thickness and the film-formation rate. Using Mg alloys as biodegradable materials requires surface treatments to improve the anticorrosion property via the formation of the bioactive protective layer. This coating isolates the implant from the organism tissue and the physiological environment and thus controls the over-fast degradation of Mg alloys [63]. Some biodegradable Mg alloy surface treatments have an antibacterial property throughout

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Fig. 5 The wet processes most widely used commercially to protect Mg alloys against corrosion. (Adapted with permission from Ref. [11], © 2013, Woodhead Publishing)

Fig. 6 Scheme of coatings for biodegradable Mg alloys

the healing process [64]. There are several coating processes used for Mg and its related alloys. These include anodizing, micro-arc oxidation, PVD, chemical conversion, electrodeposition, CVD, bio-mimetic deposition, ion implantation, electrochemical deposition, etc. [65]. A Scheme of most techniques used for coatings of the biodegradable Mg alloys is shown in Fig. 6. The most important techniques will be covered in-depth in these sections. Several alteration methods may apply, bearing in mind the requirements of many clinical applications, particularly in orthopedics. This part concerns the development of biocompatible and bio-degradable coatings for Mg and Mg alloys to improve mechanical characteristics, corrosion resistance, and biocompatibility for orthopedic applications.

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Chemical Conversion Conversion coatings are cost-effective, simple operation, and provide useful features to extend the service life and enhance the treated Mg alloy parts [66]. The coating film is developed by a chemical reaction between an electrolytic solution and Mg substrate [67]. This technique is characterized by simplicity of operation, low cost, and no specific equipment or conditions are required, which make it an attractive surface treatment method in the industry. As shown in Fig. 7, the specimens are immersing in the conversion coating solution without applying an external current. The specimens may connect as a working electrode in an electrolytic cell to measure the open circuit potential (OCP). Furthermore, the process may need a heating source to examine the effect of temperature on the coating properties. Phosphate, fluoride, chromate, cerium, vanadate, and oxalate coatings are among the most used solutions to produce a barrier against corrosion, improve wear resistance, facilitate better paint adhesion, and enhance oil absorption form an electrically insulated surface [68]. However, fluoride [69], and phosphate conversion coatings [70] were used for orthopedic application as the toxicity of the other common processes is a major concern. Chemical conversion coating has been used for corrosion protection of metallic material due to several remarkable advantages over other existing coating methods. Chemical conversion coating is simple, easy to operate, low-cost with a short operation time. Moreover, it produces a layer that consistently covers the surface of components with irregular forms, such as porous, hollow, and screwed structures, which is impossible with some line-of-sight processes like plasma spray, RF sputtering, chemical/physical evaporation deposition, etc. Also, it produces more adhesive coatings because of the existence of chemical bonds and a transitional layer between the coating film and the metal substrate. However, the produced coating has low toughness/wear resistance and durability, which leads to defects and damage and

Fig. 7 A schematic representation of the chemical conversion coating

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a reduction in the protectiveness of the coating, which is strongly reliant on the coating integrity [71]. Fluoride conversion coating of Mg alloys can be proceeded by immersion in hydrofluoric acid solution [72]. The conversion coating film on pure Mg is composed of MgF2 coating and some Mg(OH)2. However, the conversion coating on Mg1.0Ca Mg alloy contains CaF2 and Ca(OH)2. The fluoride-coated Mg1.0Ca Mg alloy showed a lower corrosion rate (of 0.25 mm year
1) than fluoride-coated pure Mg (of 0.35 mm year
1), and both coated Mg, and Mg alloy are better than pure Mg. Moreover, the corrosion resistance of coatings is well related to the fluoride conversion coating composition [73]. A spontaneous reaction takes place between Mg alloy and HF, leading to the deposition of MgF2 on the surface (Eq. 1). Mg þ 2HF ! MgF2 þ H2 ", ΔG ¼
476:6 kJ mol
1

ð1Þ

The electrolyte solution contains a large amount of water; thus, the formation of Mg(OH)2 (Eq. 2) is also possible. Mg þ 2H2 O ! MgðOHÞ2 þ H2 ", ΔG ¼
359:3 kJ mol-1

ð2Þ

Therefore, the corrosion resistance in hank’s solution was improved with increasing the F/O ratio in the coating. Phosphate conversion coatings are carried out in a solution that contains phosphate ions only or with other metal ions such as Zn, Mn, Mg, Ca, molybdenum etc. Hydrogen evolution at the Mg substrate/solution interface increases the local pH of the bath and thus promotes the nucleation of nonmetallic phosphate crystals at the surface [68]. Recently, phosphate-based conversion coatings, in particular, Ca and zinc phosphate conversion coatings [70], have been considered a possible replacement to chromate conversion coating in biomedical applications because of their insolubility in water their outstanding biocompatibility, as well as high-temperature resistance, chemical stability. The most common phosphate-based conversion coatings were reported in the literature. For instance, Zheng-Yin et al. [74]. Figure 8 shows the most common combinations, including Ca, P, Z, Ce Sr, Mn, and Mo. From the several phosphate-based conversion coatings, calcium phosphate (Ca-P) coatings attract the most attention in biomedical applications, particularly bone substitution and orthopedic. Ca-P coatings have excellent biocompatibility and non-toxicity. The Ca and P are the main hydroxyapatite (HA) elements, which is the most important component in natural bone. However, the major challenge is controlling the phase and increasing the content as the Ca-P coatings were frequently amorphous with a small HA amount. According to chemical reaction thermodynamic calculations [55], HA is mainly formed over a specific range of Ca/P ratios in neutral and Semi-alkaline environments. Zai et al. [75] studied the phase structure, morphology, biocompatibility, and corrosion property of seven phosphate-based conversion coatings: Mg-P, Zn-P, Ca-P, ZnMg-P, CaMg-P, ZnCa-P, and ZnCaMg-P. They found that new beryite (MgHPO43H2O) is the main phase of MgP type coating (including Mg-P),

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Fig. 8 The common types of Phosphate-based conversion coatings on Mg alloys

dicalcium phosphate dehydrates (DCPD, CaHPO42H2O), the main phase of CaP coatings (including Ca-P and CaMg-P), and (Zn3(PO4)24H2O) is the main phase of ZnP coatings (including Zn-P, ZnMg-P, ZnCa-P, and ZnCaMg-P). Moreover, Ca-P coating consists of lath-like crystals and CaMg-P coating of flake-like crystals standing on the substrate, as shown in the SEM morphologies in Fig. 9c and e. The long-term immersion of Mg alloy substrate, MgP coating, and CaP conversion coatings in Hanks’ solution causes filiform and pitting corrosion; however, pitting corrosion is the main type of ZnP coatings (Zn-P, ZnMg-P, ZnCa-P, and ZnCaMg-P). The optimum anticorrosion performance is shown in ZnMg-P coatings since its most stable structure efficiently prevents filiform corrosion propagation. CaP type coatings have better biocompatibility than ZnP, MgP type coatings, and bare Mg alloy substrates, corresponding to cell viability test results.

Anodization The process of anodizing treatment is an electrolytic oxidation technique that converts the surface of a metal to a layer or film with the desired protective, functional, and even decorative features. The process is called “anodization” because the metal to be treated serves as the anode of an electrical circuit [15]. This process is one of the greatest commonly used surface treatment methods to enhance the corrosion resistance of Mg alloys [76]. Anodization uses the concept of classical electrochemical conversion when changing the metal’s surface chemistry through oxidation to generate a stable anodic oxide layer that can act as a barrier layer and thus improve the corrosion resistance and provide a decorative shape with functional properties [77].

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Fig. 9 SEM surface morphologies of coated samples: (a) Mg -P, (b) Zn -P, (c) Ca -P, (d) ZnMg -P, (e) CaMg -P, (f) ZnCa -P, (g) ZnCaMg -P and (h) bare substrate of AZ31 Mg alloy. (Adapted with permission from Ref. (121) © 2020, Elsevier)

Fig. 10 Schematic diagram of a primary electrochemical cell for anodizing using three electrodes (a), using two electrodes (b), and Pourbaix Diagram of Mg (c). (Adapted with permission from Ref. [11], © 2013, Woodhead Publishing)

The main content of the protective film formed on Mg alloy is Mg (OH)2. The formation of this film is governed by the thermodynamics described by Pourbaix (potential-pH) diagram given in Fig. 10c. Mg rapidly dissolves and spontaneously develops H2 gas in acidic or neutral solutions. Alternatively, in strong alkaline solutions, a thick, passive film of Mg(OH)2 was produced by the reaction with H2O. This passive film is thermodynamically preferred over a MgO, as explained by the standard enthalpy calculations

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MgðOHÞ2ðsÞ ! MgOðsÞ þ H2 O

ð3Þ

By using the sum of the three formation reactions, the above equation could be rewrite as: MgðOHÞ2 ðsÞ ! MgðsÞ þ O2ðgÞ þ H2ðgÞ

  ΔH1 ¼
ΔHf  MgðOHÞ2

ð4Þ

MgðsÞ þ 1=2O2ðgÞ ! MgOðsÞ

ΔH2 ¼ ΔHf  ½MgO

ð5Þ

H2ðgÞ þ 1=2O2ðgÞ ! H2 OðlÞ

ΔH3 ¼ ΔHf  ½H2 O

ð6Þ

Therefore, MgðOHÞ2 ðsÞ ! MgO ðsÞ þ H2 O ðlÞ, ΔH reaction ¼ ΔH1 þ ΔH2 þ ΔH3 Using Standard Enthalpy of Formation, we can calculate ΔH for the reaction as below ΔH reaction ¼ ð924:7 kJÞ þ ð
601:8 kJÞ þ ð
285:8 kJÞ ¼ 37:1 kJ The naturally produced Mg(OH)2 assumes losing protons during anodizing in a high electric field to form meta-stable MgO. Nevertheless, due to the lower solubility of Mg(OH)2 in comparison to MgO, the Mg(OH)2 film will reform again on the MgO film surface upon immersion in water. Perrault et al. [78] have studied the thermodynamic stability of MgH2 in aqueous solutions and noticed the stability of MgH2 in neutral and alkaline solutions, resulting in Mg surface passivity combined with Mg(OH)2 [79]. The general setting for anodization is consists of the working electrode as the metal substrate (anode), a counter electrode (cathode), an electrolyte, and a power supply, which is directly connected to anode and cathode as shown in Fig. 10b. Additionally, Potentiostatic with three electrodes cell, working counter, and reference electrodes may be used to measure the potential at the working electrode without passing a current through it, as shown in Fig. 10a [65]. The anodizing performance is generally depending on a range of main factors such as the electrolytic composition, solution temperature, anodizing time, applied voltage or current, substrate types, alloying element concentrations, and the quality of Mg substrate [80]. The anodizing treatment can also form a porous oxide film, with pores’ size and density being directly related to the processing parameters. Many of the applied anodizing processes on Mg alloys are toxic to the human body. Therefore, many researchers have aimed for the non-toxic, biocompatible surface treatment. Hiromoto et al. [81] have examined the effect of (Ca-P) deposition on pure Mg via anodizing and subsequent thermal treatment in an autoclave. The Ca-P coated substrates showed very little corrosion after immersion in Hanks’ solution [82]. Aaron et al. [83] have studied Mg0 s surface, degradation, and biological properties after anodizing in KOH. The electrochemical tests revealed that the anodizing at

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1.9 V followed with annealing has homogeneity of surface microstructure and chemical composition with improved corrosion resistance. Furthermore, instead of typical localized pitting and undermining, there was the continued growth of a surface film with high content of Ca and P and a higher rate of Mg2+ ion release when immersed in physiologically relevant solutions. Kalaiyarasan et al. [84] have studied the anodizing of AZ31 Mg alloy in alkaline silicate electrolytes. The results of hydrogen evolution studies showed that the realtime corrosion behavior of the anodized sample affects its degradation rate. The anodized sample’s current density was minimal and consistent over the entire surface. Moreover, Moreover, bone-like apatite was precipitated with a Ca/P ratio of 1.39 after immersion in Earle’s solution for 7 days. Zaffora [85] tried to provoke the growth of an Mg-P protective layer on AZ31 Mg alloy surface by anodizing in glycerol electrolyte containing K2HPO4 and K3PO4 at high temperature. Thermal treatment was conducted at 350  C for 24 to seal the outer porous layer and consequently achieve more corrosion resistance. The corrosion resistance improved significantly, and the Cytotoxicity tests also showed a physiological in vitro outgrowth in a 7-day interval indicating the biocompatibility of treated AZ31 samples.

Micro-Arc Oxidation (MAO) Micro-arc oxidation, also sometimes known as plasma electrolytic oxidation (PEO), is an electrochemical surface treatment for metals coating, and it is commonly used for the surface treatment of Mg and Mg alloys [86]. This technique is similar to anodizing; however, it carryout at higher potential; plasma discharge occurs, resulting in a partial fusing of an oxide layer and, as a result, the development of a very adhesive oxide coating on the substrate [87]. Zheng et al. [88] have studied the surface treatment of Mg–Zn–Y–Nd alloys with micro-arc oxidation/phytic acid (MAO/PA) composite coatings. In the in vitro experiments, the MAO/PA coatings significantly improved the corrosion resistance of the Mg alloy. They reduced the corrosion current by approximately one order of magnitude, indicating that the MAO/PA-coated groups had better corrosion properties than the uncoated samples in the rabbits at 3 weeks without significant tissue damage or signs of severe foreign body inflammation. Further, the cell survival rate satisfied the requirements of biomaterials and exhibited excellent cell adhesion. In another study, the effect of MAO coating on the degradation property of ZK60 alloy in SBF was examined thoroughly [89]. Silicate-based electrolyte composed of 10 g l
1 Na2SiO39H2O, 1 g l
1 KOH, and 8 g l
1 KF2H2O and the voltages of 230, 300, 370, and 450 V was chosen for the process. The coating is composed of Mg and MgO (periclase), Mg2SiO4 (forsterite). The corrosion current densities (Icorr) of all coated specimens at various four applied voltages were lower than the bare ZK60 specimen, implying that all the four MAO coatings have a good corrosion resistance compared to the bare ZK60 alloy. The MAO-coated ZK60 alloy showed a grade 0 cytotoxicity grade and no hemolytic potential. Suggesting the material has good cell and blood compatibilities.

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Physical Vapor Deposition The deposition of the coating using the physical vapor deposition (PVD) process is performed at substrate temperatures considerably lower than the melting temperature of the coating material. The low-temperature deposition is typically advantageous because it makes the coating relatively insensitive to thermodynamic processes phenomena [90]. In its most basic form, physical vapor deposition includes the evaporation of a condensable material and its consequent condensation on a substrate surface. The condensable material becomes vapor and moves in straight lines in a vacuum until it collides with a cold surface [91]. All kinds of inorganic substances and organic substances can be used with PVD. The process is environment-friendly compared to other processes. However, PVD has a drawback with coating complex shapes. The process also is complex with the high cost and low producibility [92]. According to Frey et al. [93], PVD processes include the types shown in Fig. 11. Bakhsheshi-Rad et al. [94] have studied the clinoenstatite (CLT)/tantalum nitride (TaN) coatings on Mg alloy via PVD followed by electrophoretic deposition (EDP) methods to improve the corrosion resistance and cytocompatibility of the substrate to develop orthopedic implants of Mg alloys.

Fig. 11 shows the summary of PVD processes

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Deposition of a CLT/TaN coating on the surface of an Mg-Ca-Zn alloy significantly increases Ecorr and decreases icorr of the substrate, suggesting a significant improvement in corrosion resistance. Compared to the thick CLT top layer, the TaN under-layer (950 nm) exhibited a highly compact structure with minor cracks. A larger number of cells were formed on TaN and CLT coating compared to Mg alloy substrate, which indicates the outstanding biocompatibility of the TaN and CLT coating.

Ion Implantation Ion implantation is also a highly effective method for improving the surface characteristics of Mg alloys [95]. This process maintains the specimen’s geometric dimensions while introducing a graded surface film without a sharp interface, reducing the probability of layer delamination. Therefore, Jin et al. [96] have studied the effects of ion implantation of Nd into WE43 Mg alloy on corrosion resistance and biocompatibility. The results indicate that formed a rather smooth and hydrophobic surface film consisting primarily of Nd2O3 and MgO. The corrosion resistance was enhanced due to the formation of a stable and protective Nd2O3 outer layer and a partly protecting MgO inner film. Cells attach and spread effectively on the Nd-implanted WE43 Mg alloy; cells nurtured with the removed medium of the Nd-implanted WE43 Mg alloy for 3 days display equivalent vitality to the whole cell culture medium, suggesting that the Nd-implanted WE43 Mg alloy has good biocompatibility. The microstructure, surface chemistry, and in vitro corrosion performance of the as-extruded Mg–1Ca (wt.%) after ion implantation with the biocompatible Ag, Fe, and Y. was investigated by Liu et al. [97]. The results suggest that Y ion implantation is promising for orthopedic applications. Y ion implantation can thicken the oxidized film either through Y&O implantation or oxidation treatment following Y-implantation. Figure 12 shows the cell viability of the ion-implanted coatings in 100% and 50% extract media. Compared to nontreated Mg–1Ca specimens, Ag and Fe-implanted Mg–1Ca exhibit lower cell viability. On the other hand, Y-implanted Mg–1Ca has similar viabilities to nontreated Mg–1Ca. After diluting the extracts to 50%, the cell viability of Ag and Fe-implanted Mg–1Ca was improved, and Y-implanted Mg–1Ca showed significant viability compared to nontreated specimens.

Electrochemical Deposition Electrochemical deposition is a promising technology, and it is one of the simple and inexpensive ways for the plating of metals and alloys. In this process, a solid metal film is formed by depositing an ion solution over an electrically conducting metal surface. The metal ions are reduced when sufficient current is passed through the solution.

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Mg-Ca matrix Agion implantation Fe ion implantation Y ion implantation

140 120

120 100 Viability / %

Viability / %

100 80 60 40

Mg-Ca matrix Agion implantation Fe ion implantation Y ion implantation

140

*

*

*

*

*

*

60 40

*

* *

20

*

* 80

*

0

20 0

1

2 Culture time / day

4

1

2 Culture time / day

4

Fig. 12 MG63 cell viability after 1, 2, 4 d incubation of sample extracts: (a) 100% extracts and (b) 50% extracts. (* indicates significant difference compared with untreated Mg–1Ca matrix ( p < 0.05).). (Adapted with permission from Ref. (146) © 2015, Elsevier) Fig. 13 SEM images of the coating obtained at
1.4 V before and after immersion in SBF solution for 1d and 7d. (Adapted from Ref. [89])

Song et al. [98] coated fluoridate hydroxyapatite (FHA) on Mg–Zn alloy and compared the in vitro degradation behavior with HA and DCPD coated samples. Although all the CaP coatings decreased the degradation rate of the alloy, they found that FHA and HA coatings promoted the nucleation of osteoconductive minerals. Noticeably, they found that FHA coating was more stable than HA coating under long-term exposure. Salman et al. [99] studied the electrodeposition method to produce HA coating on the substrate of AZ31 Mg alloy in calcium phosphate electrolyte to improve corrosion resistance and biocompatibility. The results revealed that HA was successfully electrodeposited on the surface of an Mg alloy after 30 min of immersion in a solution containing 3 mM Ca(H2PO4)2 + 7 mM CaCl2 solution at a temperature of 37  C. HA was deposited on the coating after immersion for 7 days in the SBF solution, confirming its bioactivity. The surface morphologies of the formed coatings at
1.4 V before immersion in SBF solution and after immersion for 1d and 7d are shown in Fig. 13. After immersion of 1 day in SBF solution, the growth of HA on the coating surface was confirmed. The particles sizes were finer at the beginning of immersion in SBF solution. With increasing the immersion time to 7 days, the rate of HA growth on the coating’s surface increases noticeably.

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Roncevi et al. [100] have studied the electrodeposition of HA coating on (Mg93Al6Zn1) alloy modified with self-assembled monolayers (SAM) using organic acids such as stearic acid (SA) or octadecyl phosphonic acid. In this study, the formation of HA coatings on well-defined surfaces was examined utilizing SA SAMs or octadecyl phosphonic acid (ODPA) on (Mg93Al6Zn1) alloy. The Ca/P coating was produced by electrodeposition from a solution containing Ca and P ions at pH 5, followed by an acid-base reaction to convert it to HA. The results show that the deposition of HA coatings on the Mg alloy surface treated with SA SAM improved the corrosion resistance. The SA SAM may promote CaP deposition by attracting the Ca2+ ions to incorporate in the surface. Consequently, – PO4H2 has more potential to initiate calcium phosphate deposition than –COOH. Furthermore, the coating exhibits a needle-like dendrite structure.

Conclusions Mg and Mg alloys are biodegradable with mechanical properties comparable to natural bone. This introduces the capability of using Mg alloys as biodegradable material for orthopedic applications. However, problems such as fast degradation and gas bubbles formation limit the widespread use of Mg and Mg alloys in such applications. The possible ways to overcome these limitations can be narrowed to alloying and surface treatment. Mg alloy with biocompatible elements like Al, Zn, Zr, Sr, Ca, and REEs is an excellent way to develop the mechanical and corrosion properties of Mg alloy. However, the biocompatibility of some of these elements still needs deeper and in detailed investigations. Al showed to cause some neuropathological issues such as Alzheimer’s disease and significantly impact immunology. Zn is proved to be a biocompatible element; however, Zn content should be limited to 4 wt.% of Zn. Otherwise, it will hurt growth, development, and health and cause some neurological disorders. Ca has a positive effect on the mechanical and corrosion properties of Mg alloys; however, the optimum Ca content is 0.6%, and any further addition will result in deterioration of the corrosion resistance and thus fast degradation. Zr is promising in alloying biodegradable Mg alloys and has limited studies as a biodegradable material. The addition of REEs to Mg and Mg alloys are significantly increases the mechanical properties. Nevertheless, the optimum concentrations of REEs and other alloying elements must be studied thoroughly to achieve the optimum combination between mechanical properties, corrosion resistance, and biocompatibility. Surface treatment can improve the anticorrosion property by forming the bioactive protective layer, which isolates the implant from the organism tissue and the physiological environment and thus controls the over-fast degradation of Mg alloys. Many coating techniques are available for Mg and related alloys, such as anodizing, micro-arc oxidation, chemical conversion, electrodeposition, bio-mimetic deposition, ion implantation, PVD, etc. FHA, HA, and DCPD contained coatings formed

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using these techniques, encouraging the nucleation of osteoconductive minerals and improving biocompatibility and corrosion resistance.

Future Prospective The biodegradable Mg alloys are promising implant material for orthopedic applications compared to the permanent implant to avoid a second surgery for implant removal. Choosing an appropriate biodegradable element during alloy design and biocompatible surface treatment is one of the major challenges and focuses of recent research in this field.

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93. H. F, Applications and developments of thin film technology, in Handbook of thin-film technology, KHR Frey H., Editor. 2015, Springer Berlin, Heidelberg. 94. Bakhsheshi-Rad H, Najafinezhad A, Hamzah E, Ismail A, Berto F, and Chen X (2020) Clinoenstatite/Tantalum coating for enhancement of biocompatibility and corrosion protection of Mg alloy. Journal of Functional Biomaterials 11(2) 95. Zhao Y, Jamesh M, Li W, Wu G, Wang C, Zheng Y, Yeung K, and Chu P (2014) Enhanced antimicrobial properties, cytocompatibility, and corrosion resistance of plasma-modified biodegradable magnesium alloys. Acta Biomaterialia 10(1):544–556 96. Jin W, Wu G, Feng H, Wang W, Zhang X, and Chu P (2015) Improvement of corrosion resistance and biocompatibility of rare-earth WE43 magnesium alloy by neodymium self-ion implantation. Corrosion Science 94:142–155 97. Liu Y, Bian D, Wu Y, Li N, Qiu K, Zheng Y, and Han Y (2015) Influence of biocompatible metal ions (Ag, Fe, Y) on the surface chemistry, corrosion behavior and cytocompatibility of Mg-1Ca alloy treated with MEVVA. Colloids and Surfaces B-Biointerfaces 133:99–107 98. Song Y, Zhang S, Li J, Zhao C, and Zhang X (2010) Electrodeposition of Ca-P coatings on biodegradable Mg alloy: In vitro biomineralization behavior. Acta Biomaterialia 6(5):1736– 1742 99. Salman S, Kuroda K, and Okido M (2013) Preparation and Characterization of Hydroxyapatite Coating on AZ31 Mg Alloy for Implant Applications. Bioinorganic Chemistry and Applications 2013 100. Škugor Rončević I, Vladislavić N, Buzuk M, and Buljac M (2019) Electrodeposition of hydroxyapatite coating on Mg alloy modified with organic acid self-assembled monolayers. Journal of Chemical Research 44(3–4):212–220

Part VII Foods and Agricultural Impacts of Biodegradation

Biochar and Chicken Manure Compost

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production of Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stability of Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fresh Chicken Manure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmaceutical Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Harmful Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inorganic and Organic Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chicken Manure Compost as Fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors of Aerobic Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surrounding Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon-to-Nitrogen Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bulking Agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxygen Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Texture of Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composting Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composting Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chicken Manure Composting Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pile Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sheet Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Biochar in Manure Compost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nutrient Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water-Holding Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Biological Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Nur Zulaikha Izzati binti Rosman Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia N. S. Jamaludin (*) Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_51

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Biochar Mitigates Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochar Mitigates Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Biochar is super charcoal, produced from heating organic materials or biomass in a high-temperature and low-oxygen process known as pyrolysis. Composting organic wastes is an aerobic process involving the decomposition of natural decaying of organic materials. It is being recognized for its effectiveness in improving soil quality which benefits the agricultural industry. Chicken manure compost is one example of organic waste compost, an excellent resource for soil amendment. However, fresh chicken manure contains a high amount of ammonia which can burn, damage, and kill the plants. Raw chicken manure may also contain harmful pathogens such as E. coli, Salmonella, and Cryptosporidium, which can stick on edible plants, thus causing serious diseases in humans and animals. Therefore, chicken manure will be composted first before being used as fertilizers to break down a high amount of nutrients into favorable conditions. Interestingly, biochar has been found to accelerate the composting process, mitigate pesticides, reduce odor, reduce the rate of greenhouse gas (GHG) emission, and improve the compost mixture’s homogeneity. Keywords

Biochar · Chicken manure · Compost · Biomass · Hydrochar · Pyrolysis Abbreviations

ATP BSF C/N ESBL GHG HA MCPA MSW O/C OM

Adenosine triphosphate Black soldier fly Carbon-to-nitrogen ratio Extended-spectrum β-lactamase Greenhouse gas Humic acid (4-Chloro-2-methylphenoxy) acetic acid Municipal solid waste Oxygen-to-carbon ratio Organic matter

Introduction Biochar is a heterogeneous, carbon-rich, porous solid obtained from thermally treated natural resources such as animal waste, rice husk, bean straw, and corn stalk. It can be produced from the lignocellulosic (agricultural waste and forestry

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residues) and non-lignocellulosic (animal manure, sewage sludge, and microalgae) groups on a large scale at a lower production cost [1]. Among all biomasses, agricultural waste is the key feedstock used for biochar production until today. The primary component in the biochar skeleton is carbon and ash, where the overall composition varies depending on the types of feedstock and the process conditions [2]. Biochar produced from animal manure and sewage sludge contains significantly higher nutrients and minerals than biochar produced from the lignin-rich biomass [3, 4]. The significant compositions of minerals in biochar influence the removal of heavy metals from wastewater [5]. In comparison, sewage sludge biochar contains about 161 g/kg of minerals, while corn biochar and poplar wood biochar comprised 28.6 g/kg and 19.5 g/kg of minerals, respectively. The considerably high mineral content in sewage sludge biochar promotes better heavy metal adsorption than the latter biochars. A sufficient amount of mineral content in biochar is essential for heavy metal removal and effective as a remover for pesticides, dyes, and phenolic compounds and as vapor or gas adsorber [1].

Production of Biochar Biochar is a viable nontoxic, environmentally friendly, economically feasible, and designable adsorbent for water treatment. Besides being utilized as membrane separation in aquatic, soil, and air pollution control, biochar has been found to possess catalytic activity adsorption efficiency and enhance anaerobic digestion and soil retention [6]. The potential of biochar is determined by the source of biomass, preparation method, reaction conditions, types of pollutants, and the mechanism of action [7]. Biochar can be produced via four thermochemical processes: pyrolysis, torrefaction, hydrothermal carbonization, and gasification.

Pyrolysis Pyrolysis is a thermal process used to produce solids (biochar), condensable liquid (bio-oil), and non-condensable gas (syngas) [8–14]. The process is conducted under an oxygen-deprived environment, and the product yield depends on the type of pyrolysis. There are three types of pyrolysis: slow, fast, and flash pyrolysis. Each type is conducted at a different heating rate, reaction temperature, and holding time. Slow pyrolysis, also known as conventional carbonization, is the ideal approach for biochar production. It is considered an effective treatment for high biochar and low bio-oil production [15]. Meanwhile, fast and flash pyrolysis targets bio-oil production [16]. Extremely high operating temperatures (800–1300 C) will lead to fast pyrolysis. When the temperature is high, the residence time in the reactor is shorter; thus, the biomass will undergo thermal cracking and prevent char formation. In contrast to fast pyrolysis, slow pyrolysis involves a slow rise in operating temperature. This will cause a very long residence time and maximum exposure to heat. The liquefied product is kept to a minimum, and solid char is preferentially produced [7].

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Biomass containing a higher calcium oxide content will result in highly stable biochar with low amounts of oxygenated functional groups upon heating above 450 C [17]. Biochar produced around 450 C shows a priming effect on organic matter mineralization, thus undergoing simultaneous degradation. The optimum temperature for the production of biochar is 500 C [15]. Its applications can be varied and expanded when the waste is heated with additives such as phosphorus and iron during pyrolysis. As a result of pyrolysis, the nature of biochar produced is determined by factors including the type of reactor, residence time in the reactor, reactor pressure and dimension, and the pre-treatment applied to the biomass. Pyrolysis can be used for a wide range of waste materials. The reactor types and designs have a great role in influencing pyrolysis. Generally, reactors such as auger, rotary, and drum kilns are common types used for pyrolysis [7]. A study on walnut shells confirmed that biochar produced from walnut shells through slow pyrolysis has effectively given a greater biochar yield than fast pyrolysis, disregarding the reaction temperature [18]. Additionally, the slow route is cost-effective too. The slow route mechanism involved three steps (Fig. 1). It begins with a pre-pyrolysis reaction where the heated raw biomass eliminates water, and an unreacted residue is formed in an inert atmosphere. In the primary reaction, the heating continues where the unreacted residue undergoes dehydration, decarboxylation, and dehydrogenation processes. All volatile components are released at this stage, leaving char as the product. Lastly, the secondary reaction is carried out at a high temperature to ensure all volatiles evaporate off the char. Heavy organic compounds undergo cracking and polymerization to produce a stable carbon-dense solid called biochar and syngas such as methylene, methane, carbon dioxide, and carbon monoxide [16]. Pyrolysis is a versatile and flexible process that can be operated at various reaction conditions with different types of feedstocks [19]. The properties of biochar are determined by the type of biomass and production temperature. Biochar derived from wood is highly stable and has up to 80% carbon content. Lignin, being one of

Fig. 1 Steps involved in pyrolysis

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the carbon sources, is highly stable to thermal degradation due to its high level of recovery from biomass [15]. Biochar produced from pyrolysis of wastewater sludge is useful for nutrient recyclability, heavy metal immobilization, alkalinity, and good pore structures. This makes pyrolysis an ideal method or process to form extremely valuable biochar for various important purposes [7].

Torrefaction Biochar can also be produced via a thermochemical process at 200–300 C, atmospheric pressure in an inert atmosphere. It is known as mild pyrolysis or torrefaction. The process is conducted at 50 C/min with 30–120 minutes of residence time. However, the biochar quality produced via this method is not as promising, although the production yield is undeniably high, approximately 70–80% [20]. Many volatile components are still found in the torrefied biomass, causing its physicochemical properties in between raw biomass and biochar. For example, the oxygen-carbon ratio in the torrefied biomass is above 0.4. In this regard, torrefaction is often employed in biomass densification, moisture removal, and improving biomass properties. A combination of torrefaction and pyrolysis will produce high-quality biochar with a higher yield and upgraded physicochemical characteristics [16]. Hydrothermal Carbonization Hydrothermal carbonization is referred to as wet pyrolysis or wet torrefaction since the process is carried out in the presence of water. It is used to produce “structure” charcoal and biochar from organic matter [7]. The main idea behind this process is to mimic natural carbon formation in a shorter time. It is done by heating a mixture of biomass and water in a pressure vessel for several hours at 180–250 C and at high pressures of 10 bars or more [7]. Hydrothermal carbonization utilizes water as a solvent, reactant, catalyst, and medium for mass and energy transfer. Water behaves like mild acid and mild base at 200–280 C, at the same time resulting in an acceleration of biomass decomposition [21]. Apart from that, it also facilitates hydrolysis, dehydration, decarboxylation, and depolymerization processes [16]. At high temperatures and pressure, the solution pH drops due to the elevation in the concentration of oxonium ions. This results in the release of a higher amount of organic matter into the water, which causes the formation of a sludge-like compound made of carbon. The sludge can be separated and dried, and the various products such as bio-oil, hydrochar, biochar, ketones, and aromatics can be used for many purposes. To obtain the desired products, the reaction can be halted at multiple stages [7]. The hydrothermal process has attracted interest for biochar production as it can avoid the preliminary energy-intensive drying process required for conventional pyrolysis, thus minimizing the operational costs. It is also convenient for biomass with over 50% moisture content. The technology offers the lowest reaction temperature compared to other thermochemical conversion techniques. Most often, hydrochar cannot be described as biochar since the reaction temperature is too low, carbon contents are low, and oxygen/carbon and hydrogen/carbon ratio is intolerable [22]. On the contrary, high-quality biochar can be produced if

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hydrothermal carbonization is integrated with pyrolysis. Experimental findings recorded that the preliminary hydrothermal treatment of brewery spent grains containing about 70–90% moisture before the pyrolysis process produces biochar with a greater product yield and carbon contents [23]. The carbon content of biochar is upgraded from 70% to 82% when the combined hydrothermal/pyrolysis process is applied, compared to the standalone hydrothermal process. Upgrading hydrochar is crucial since the hydrochar possesses a low surface area ( 50 C) conditions to obtain a stable material [89]. Chicken manure is biodegradable. Thus, it could favor microorganisms through aerobic digestion to degrade different organic solid wastes into compost due to its high nitrogen and low moisture content [90]. The microorganisms will secrete enzymes biochemically during composting process to convert organic matter into stable humus. Through this process, high-quality organic fertilizer from livestock manure can be produced, which can help in reducing environmental pollution [87]. Various methods are used for composting poultry manure; however, pile composting and sheet composting are standard methods employed by farmers and agricultural industries.

Pile Composting Pile composting involves microbes by maintaining sufficient moisture and oxygen level for the degradation process of animal manures, scraps, and food residues. It may take longer as the breakdown of raw materials depends on the microorganisms’ activities. This method can develop a strong ammonia fume if poultry manures make up more than other raw materials (Table 1). However, it can be solved by adjusting the carbon-nitrogen ratio and adding more carbon-rich waste (e.g., dried leaves, shredded paper, or dried grass) into the mixture. The pile also can be turned

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Table 1 Advantages, disadvantages, and ways to overcome disadvantages of manure composting methods Composting method Advantages

Disadvantages

Ways to overcome disadvantages

Pile composting 1. Produced high-quality compost 2. Compost has no offensive odor 3. No fumes were produced after composting process 1. Develop strong ammonia fumes before composting process 2. May take a long time 1. Adjusting carbon-to-nitrogen ratio 2. Constantly turning the pile to maximize the degradation process

Sheet composting 1. Easy (directly apply manure to soil) 2. Cheap (does not require a system like pile composting) 1. May produce low-quality products 2. Difficult to manage harmful microbes and unwanted weeds 1. Applying the manure after the garden or field has been harvested 2. Proper timing can eliminate the unwanted microbe and weed

constantly to allow good aeration and maximize the degradation of materials. As a result, the high-quality and stable compost can be easily produced and handled using this method [41, 91]. Signs of adequate aging of compost via this method can be indicated by their non-offensive odor and fume. Fully composted manure also will have non-recognizable particles of starting raw materials as all of them have already been degraded by microbes. Another good indicator for the composting process is the pile or windrow’s temperature. As microorganism activity starts to slow down, the temperature will drop from 48–60 C to normal air temperature (20–25 C) [41].

Sheet Composting Sheet composting is an ancient composting technique that remains practical until today. It is also known as lasagna composting, or sheet mulching is a method of layering or spreading raw (uncomposted) poultry manure directly to the soil as mulch. Sheet composting needs proper carbon, nitrogen, oxygen, and water to break down organic matters. The poultry manure is tilled lightly and left on the surface to decompose and decay. This method is easy and cheap compared to active pile composting. It can be done on a small or large scale. Additionally, sheet composting can improve the soil or add the compost to existing beds. Planting will start when the original materials are no longer recognizable and smell like fresh earth, indicating the bed is completely decomposed. Nevertheless, this composting method may produce a low-quality product. It is a slow process and may take at least 6 months to decompose properly before planting. It is also difficult to manage harmful microbes and unwanted weeds (Table 1). The poultry manure may introduce these contaminants to the soil as there is no heating process to kill and eliminate them from the manure. Therefore, proper timing is the key for this method to be safe by applying the manure after the garden or field has

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been harvested. Proper timing can help decompose manure and allow nutrients to be more desirable for plants, thus eliminating the unwanted salts [41, 91].

Effects of Biochar in Manure Compost Incorporating biochar into manure compost significantly enhanced and improved soil quality in terms of nutrient supply, water-holding capacity, soil pH, and soil biological process (Table 2).

Nutrient Supply Biochar and compost contain important macro- and micronutrients for plants depending on several conditions such as feedstock, composting, and pyrolysis conditions. Biochar helps restrain and fix nitrogen loss by reducing ammonia volatilization, adsorbing nitrous oxide emission, and reducing nitrogen leaching. Manure compost also can be an important source of plant nutrients, of which approximately 50% of nitrogen and 80% of phosphorus were observed in the soil after the addition of chicken manure. The high nitrogen and phosphorus content will rapidly lead to the decomposition of organic matter. Compost and biochar improve retention of calcium and potassium by enhancing the cation exchange capacity of the soils, which helps enhance soil fertility and crop yield. Both compost and biochar can also reduce the presence of toxic compounds in the soil, such as aluminum and manganese [43, 54, 92]. An appreciable amount of nitrogen, phosphorus, and potassium also helps the soil lose its fertility and avoid erosion. Due to the presence of humus, compost elevates the water capacity, improves soil structure, and aggregates the stability of the soil. The humus is a stable residue resulting from a high degree of organic matter decomposition that glues the soil constituents together [45]. Table 2 Effect of biochar and manure compost on soil Effect on soil Nutrient supply

Water-holding capacity Soil pH Soil biological process

Biochar Manure compost Restrains and fixes nitrogen Contains 50–80% of nitrogen and losses phosphorus • Improves retention of nutrients • Reduces toxic compounds Reduces surface runoff Contains organic matter to absorb and hold water Changing the volume and size of spaces between soil particles • Reduces acidity of the soil • Improves availability of nutrients Suppresses harmful Accelerates organic degradation pathogens Provides shelter for microbes

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Water-Holding Capacity Compost contains organic matter that can help to absorb and hold up to 90% of its weight in water and then release and supply it to the plants. Organic matter in compost indirectly improves the soil structure, resulting in increased pore volume and size. In contrast, biochar can reduce surface runoff that mainly causes water loss and some of the essential nutrients carried away from the soil. Biochar also can change the water-holding capacity by changing the volume and size of spaces between soil particles. The improvements in soil pore volume and size resulted in increased water movement and the available water capacity, thus enhancing the soil quality [92].

Soil pH It has been reported that biochar application on soil can help reduce the acidity level of soil because of their liming effect. The pH value of dry chicken manure was 7.9, while biochar can range from 4.6 to 9.3. Thus, both of them can be used as liming agents. The liming process can improve the availability and utilization of some nutrients such as nitrogen, calcium, magnesium, and phosphate. This liming process also improves the breakdown of organic matter by microbes, increases biological nitrogen fixation, and reduces the concentration of toxic elements such as Al3+ and Mn2+ [92, 93].

Soil Biological Process Compost acts as a biological remedy against plant diseases. Compost houses microorganisms that combat their pathogenic counterparts by competing for nutrients, becoming parasitic and predators, and producing antibiotics of which lytic, plant wilt, and damping-off disease can be controlled by Bacillus sp. Compost can also treat soil polluted with heavy metals in various ways, including precipitation, adsorption, complexation, and redox reactions. It is also useful in reducing soil toxicity by encouraging absorption or degradation of chlorinated and non-chlorinated hydrocarbons, wood-preserving chemicals, solvents, petroleum products, and explosives in soil [45]. The structure of compost and biochar provides a good shelter for soil microbes. Soil microorganisms play a key role in soil nutrients, as they accelerate the degradation of organic substances and mineralize nitrogen and phosphorus in composted manures into plant-available inorganic forms. On the other hand, the application of biochar on soil suppresses harmful pathogens by affecting the mobility of the pathogens and promoting antagonistic microbes to compete for nutrients [92, 94]. Composting is a safe way of managing organic wastes by transforming various organic wastes into bio-fertilizers. Recalcitrant materials such as synthetic polymer products cannot be composted. However, it is associated with odor

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production and the release of GHGs such as carbon dioxide, sulfur dioxide, and nitrous oxide. Through composting, soil quality and crop yield can be improved as compost contains plant-growth-promoting bacteria. A combined application of compost with synthetic fertilizer might increase the effectiveness in plant growth when used in an appropriate composition [45].

Biochar Mitigates Pesticides Biochar helps in mitigating pesticides in plants and improves the soil microbial activity by adsorbing pesticides and reducing the bioavailability of pesticides for soil microbes. Nutrient-rich biochar stimulates microbial activities in the biodegradation of pesticides [95]. The microbial-assisted degradation of pesticides is influenced by the physical and chemical properties and the types of biochar. For instance, the concentration of atrazine in the soil is reduced after 5% of biochar was applied. Similarly, adding biochar produced from cotton (Gossypium spp.) straw in the soil enhances microbial degradation, thus decreasing the concentrations of chlorpyrifos and fipronil. The same biochar lowers the bioavailability of pesticides in the soil in which Chinese chive (Allium tuberosum) is grown (Fig. 7) [95]. Small organic molecules undergo polymerization in the presence of biochar. Biochar increases organic carbon in soil and enhances soil productivity in the long term. Soil treated with biochar made from rice and wheat straw has shown an increase in adsorption of diuron. Biochar amendment of 0.5% and 5% also prevented leaching and suppressed biodegradation of simazine in loamy sand and sandy clay loam soil. This can be explained by the rapid and robust binding of pesticides that limit their availability for soil microbial and root uptake, which prevents the

Fig. 7 Types of biochar used to mitigate pesticides and microorganisms

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accumulation of pesticides in vegetables or plant tissues [95]. Besides biochar, the addition of hydrochar has also shown improvement in water retention capacity in the soil, enhancement in a microbial community, and increment in nutrient-holding capacity [7]. Another study has proven that the degradation and leaching of metalaxyl and tebuconazole are alleviated when biochar with high sorption capacities and specific surface area is employed. The type of feedstock, surface area, pore size, chemical composition, and polarity are biochar’s significant characteristics that attract pesticides in soil. Pesticides are attracted to the sorption capacity of biochar via electrostatic attraction, where the surface charges on biochar are electrically pulling polar organic molecules toward it. For example, biochar produced from pig manure attracted carbaryl and atrazine due to the increased sorption capacity. Soil amended with biochar also demonstrated no effect on plant growth, but an increase in sorption of (4-chloro-2-methylphenoxy) acetic acid (MCPA) and a decrease in leaching of this herbicide were observed. Additionally, the amount of ingested organochlorinebased pesticides by plants can significantly be minimized with the aid of biochar generated from soybean, rice straw, peanut shells, and sewage sludge (Fig. 7) [95].

Biochar Mitigates Microorganisms The macropores, mesopores, and micropores in biochar impart an additional valuable property of biochar and make it an ideal substance for developing bacteria, arbuscular mycorrhizal fungi, and ectomycorrhizal fungi. The highly porous structure of biochar makes it a favorable habitat for microorganisms’ growth and protects them against predators [96]. Biochar enhances better nutrient availability, provides organic matter, and increases water retention potential, which provides a conducive atmosphere for the soil microbiome. Soil treated with biochar shows an increment in the abundance of the Proteobacteria communities. Proteobacteria is a major phylum of Gram-negative bacteria divided into five classes: alpha-, beta-, gamma-, delta and epsilonproteobacteria [95]. This phylum encircles bacteria that are vital in recycling carbon, nitrogen, and sulfur, thus indirectly managing plant diseases [96]. Changes in pH and availability of nutrients in the soil expand the growth of Proteobacteria colonies. The presence of biochar affects the growth of Proteobacteria and expands the Actinobacteria communities, and this expansion is believed to result from the ability of Actinobacteria to decompose recalcitrant carbon compounds [96]. The pathogens that negatively affect the growth and quality of crops are usually found at the roots. Oomycetes, nematodes, fungi, bacteria, and viruses are the pathogens that cause diseases in plants by attacking the plants’ root system. The high carbon content in biochar promotes microbial activity in the soil [96]. Biochar suppresses diseases by accelerating the density and activities of useful microorganisms such as nitrogen-fixing bacteria, plant growth-promoting rhizobacteria, Trichoderma spp., and mycorrhizal fungi. Biochar increases nitrogen fixation and the Gram-positive, Gram-negative bacteria and actinomycetes. Azotobacter sp. and Azospirillum sp. are examples of nitrogen-fixers and Gram-negative and free-living

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bacteria used as inoculants in plants and soil. Biochar promotes fungi-toxic effect, induces resistance in plants that are a host to the pathogen, and adsorbs allelopathic phytotoxic compounds that directly harm the roots of plants. In biochar, microbes associated with the plant rhizosphere could utilize carbohydrates, amino acids, and phenolic compounds as carbon sources in soil [95]. Biochar suppresses the enzymatic reaction in soilborne pathogens that cause diseases in plants. Arylsulfatase and urease are examples of enzymes associated with the natural suppression of soil against cassava black root rot [96]. Soil enriched with wheat straw-based biochar and fomesafen herbicide promoted the growth of microbial communities’ alpha diversity, particularly some fungi such as Zygomycota, Ascomycota, Basidiomycota, Chytridiomycota, and Glomeromycota (Fig. 7) [95]. Applying 3 tons/hectare of rice straw-derived biochar helps to control wilt in tobacco caused by Ralstonia solanacearum bacteria. It was found that tobacco wilt can be reduced by 76.6%, while the severity of the disease is reduced by 73.9% [96].

Conclusion Composting is one of the simplest and most effective methods which produce a natural fertilizer for the growth of a plant. As a form of fertilizer, chicken manure compost will help reduce problems such as inorganic or organic contaminants and toxic gases released into the environment. The process of manure composting can be done by a variety of pile composting and sheet composting; both methods have different benefits and some drawbacks. Composting should be done by considering all the factors like surrounding temperature, carbon-to-nitrogen (C/N) ratio, oxygen level, pH of composted materials, amount of water in composted materials, particle size, and texture raw materials, which affect the process and the quality of compost. More studies in this field can generalize the use of chicken manure compost with the replacement of chemical fertilizer. The ability of compost to improve soil structure and improve nutrient availability by adding to the nutrient already available has been the major factor driving its use in crop production.

Future Perspectives Due to lacking technology and awareness, livestock manure was difficult to treat and recycle, thus becoming one source of nonpoint pollution. However, technology has been developed from time to time, followed by human’s awareness of environmental issues. Many researchers started to contribute and validate their ideas on reducing pollution caused by poultry manure and changing it into something beneficial such as compost fertilizer. Composting is one of the waste management methods that can replace common improper waste management intended to improve human health and environmental sustainability. The manure compost fertilizer made it possible for

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farmers to increase their crop yield without relying on chemical fertilizer, which can lead to environmental problems and affect agriculture products. Aerated static pile system was developed in the United States in the 1970s to control temperature change due to the changing rate of aeration [45]. The system generally blows or sucks air, which minimizes odors and controls temperature. However, globally, manure compost implementation still limits and requires development, especially in the agriculture industry. Therefore, composting can be improved in a few suggested ways for a sustainable environment and improved human health. Some insects and microorganisms can be utilized in composting for ingesting and degrading some organic wastes if the conditions are right [97]. This can help conserve biodiversity in the environment and increase productivity in agriculture industries. Furthermore, compost majorly consists of mixed nutrients. Therefore, the ability to extract single nutrients such as sodium, phosphorus, and nitrogen will tremendously contribute to the good health of the soil. These single nutrients can be used for nutrient addition for soil or plants. Besides, the general problem in composting is always odor. Thus, incorporating an odor trapping device in composting can benefit the industry. GHGs emitted during composting can also be a challenge for composting future improvement. That is why a proper composting management process is needed to effectively monitor and handle GHG emissions. By installing indicators and sensors, these facilities can aid proper control of composting operations and increase the opportunities for compost plant improvement. For future research, the researchers should grab the opportunity to understand composting-related studies to sustain the environmental condition and spread awareness. Greater use of manure with less waste could lead to increased circularity [97] and reduced fertilizer requirement.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composting Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In-Vessel Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Windrow Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vermicomposting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Static Pile Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors that Affect the Rate of Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Oxygen and pH Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composting Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major Chemical Elements in Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbes Used in Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composting and Biodegradation Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter examines and reviews the fundamental difference between biodegradable and compostable. Several composting techniques will be discussed, each with its own set of benefits and drawbacks. The composting rate can be affected by several factors, such as temperature, oxygen, pH, and moisture content, which will be discussed and analyzed. Anaerobic digestion involves B. H. Lee · S. M. Khor (*) Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_69

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the decomposition of organic matter without the consumption of oxygen. Although anaerobic digestion and composting processes are almost similar, there are some differences where a detailed explanation will be given regarding anaerobic digestion in this chapter. The major challenges of biodegradable and compostable materials are examined in greater depth, and process monitoring must be improved. In addition, the effects of composting on the environment, such as the release of volatile organic compounds and toxic odors, are highlighted, and some improvements could be made to mitigate the emission of toxic pollutants. Keywords

Biodegradation · Composting · Anaerobic digestion · Environmental impact · Biofiltration Abbreviations

Abbreviation BPA PAH VOC

Explanation Bisphenol A Polyaromatic Hydrocarbons Volatile Organic Compounds

Introduction With a rising population, urbanization, and industry, solid garbage has rapidly increased. Organic solid waste, which includes sewage sludge, kitchen trash, lignocellulosic waste, municipal or industrial wastewater, and manure waste, accounts for most solid waste [1–4]. The vast amount of organic solid waste and its various components make management a global challenge. Landfilling, incineration, and biological treatment methods such as composting and anaerobic digestion are currently used to manage organic solid waste [5–7]. The landfill competes with human living space and has the potential to damage the soil and groundwater and release a certain amount of greenhouse gases such as methane [8]. Incineration effectively reduces the volume of organic solid waste, but it may produce toxic pollutants such as nitrogen oxide, carbon monoxide, dioxins, furans, and polyaromatic hydrocarbon, especially during incomplete combustion of municipal solid waste [9]. Furthermore, organic solid waste with high moisture content makes the drying process longer, which is not ideal for incineration [10]. Biological treatment of organic solid waste, on the other hand, is an environmentally benign management strategy that necessitates a high moisture level for microbial growth. Anaerobic digestion and aerobic composting are the most common biological treatments for organic solid waste. In the absence of oxygen, organic solid waste is digested anaerobically to produce methane in the form of biogas and digestate [11]. Fig. 1 shows the fate and transport of plastic waste and its harmful impacts on the environment.

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Fig. 1 A schematic diagram shows the disposal of plastic waste and the adverse effects on the environment. (Adapted with permission from Ref. [12] (Copyright 2021, Elsevier))

Composting involves the decomposition of organic matter such as food and yard waste into humus, which is full of nutrients and has beneficial uses for fertilizing and improving soil health [13]. Composting simply accelerates the decomposition process by creating an optimal habitat for bacteria, fungi, and other decomposing organisms such as earthworms to work in [14]. Composting has lower technical complexity and capital expenditure [15]. Because pathogens are usually destroyed during the high-temperature phase of composting, it has a low environmental impact [15]. Organic solid waste is compostable because it contains heterogeneous organic matter such as sugars, lipids, and proteins, which are vital sources of energy for the microorganisms involved [16]. Microorganisms degrade organic solid waste into carbon dioxide and water and produce significant amounts of heat energy during the composting process. A portion of the created energy is utilized to keep microbial metabolism going, while the balance is usually lost as heat to the environment [17]. However, because the major goal of composting is the safe disposal of organic solid waste and the manufacture of soil improver, the generated heat is frequently overlooked [18]. With the tremendous rise in global energy consumption, the generated heat is gaining popularity as a viable alternative to fossil fuels and is one of the key options for mitigating global warming [19]. Composting has environmental benefits and is simple to undertake, but it raises health and safety concerns and the risk of nutrient imbalances in the soil [20]. Organic matter, moisture, oxygen, and bacteria are the four basic components of the composting process [13]. Plant materials and some animal manures are examples of organic matter. A mixture of brown organic material, such as dead leaves or

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manure, and green organic material, such as grass clippings, should be used to make compost. Brown compounds are carbon-rich, while green materials are nitrogenrich. The optimal ratio should be for brown and green organic materials [13]. Composting can be sped up by shredding, slicing, or mowing these items into smaller bits. To facilitate the breakdown of plant material by bacteria, oxygen is required. Compost piles need to be turned on to bring debris from the edges to the center to provide oxygen. Turning the compost pile is necessary for thorough composting and odor control [13]. The components have begun to decompose once the pile has cooled in the center. The composting process will speed up if the spinning is done frequently. Bacteria and other microbes are the major contributors to the composting process. In the presence of water, oxygen, and organic matter, the previously existent bacteria will break down the plant material into beneficial compost for the garden. Apart from bacteria, larger species such as insects and earthworms are active composters [13]. These critters break down large elements in the compost pile. In contrast, biodegradation is the degradation of organic materials into natural products such as water and carbon dioxide by using microorganisms or biological processes [21]. In other words, biodegradation can be defined as the biologically mediated reduction in the complexity of chemical compounds. For dangerous compounds, biodegradation is an important quality since a fast rate of biodegradation reduces the concentration and thus the toxic impact quickly, whereas nonbiodegradable substances maintain their poisonous effect for a long time [21]. Plant and animal matter and other compounds derived from living animals are examples of biodegradable matter, as are manufactured materials similar enough to plant and animal matter to be used by microorganisms. The availability of inorganic nutrients, the presence of numerous substrates, substrate concentration, temperature, and the effective response of the microbes are all known to influence biodegradation rates [22]. One of the most critical factors influencing xenobiotic biodegradation is the concentration of accessible substrate. Although large amounts of organic compounds are inhibiting or hazardous to soil microbial communities, the rate of biodegradation is directly related to substrate concentration [22]. There may be a concentration below which no degradation occurs under specific circumstances. Most importantly, biodegradable materials sometimes break down and produce a toxic waste metal residue. For example, polyethylene formed throughout the production cycle utilizing natural items may include high levels of manganese, which can prevent the items from breaking down [21]. In contrast, compostable materials break down into nutrient-rich humus and are released into the soil, improving plant growth. Because compostable materials are already organic, they do not produce a toxic residue. The difference between compostable and biodegradable products is that compostable products need specific conditions to break down. For example, composting rates are affected by temperature and the fact that all composting organisms require oxygen to achieve optimum aeration and moisture. Composting is a faster process, while biodegradable products break down naturally. Both compostable and biodegradable products are environmentally friendly. Biodegradable products can turn

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into organic matter, which eventually breaks down faster than synthetic products, which take hundreds of thousands of years to decompose. Table 1 summarizes the advantages, disadvantages, challenges, and limitations.

Composting Technique For biodegradable materials, composting is the most common method of disposal. Standard techniques have been devised for testing aerobic biodegradation in industrial compost. By tracking the evolution of carbon dioxide in a closed chamber, these techniques enable repeatable testing of plastics for biodegradation. Microbial biomass will absorb some carbon, while some carbon dioxide will be converted to carbonic acid [23]. While industrial composting has very standardized and regulated settings, home composting has a lot more variability, and temperatures rarely reach the same high levels as industrial composting. As a result, biodegradation in home composting is slower and less effective than industrial composting. Although the criteria have been determined to be adequate for industrial composting, compostable plastics may not biodegrade in residential areas where temperature and aeration are not controlled carefully [24]. Further instruction on composting processes and testing methodology is required in household composting [25]. However, the nonbiodegradable additives will remain in the compost. The following are composting techniques, such as in-vessel composting, windrow composting, vermicomposting, and static pile composting.

In-Vessel Composting Composting that takes place in an enclosed space, such as a container, structure, or vessel, is known as “in-vessel composting.” In-vessel composting can handle a large amount of any sort of organic waste without utilizing a large amount of area when compared with windrow composting [26]. These are commonly used in large food processing operations. This approach can be used all year due to careful climate management, often done electronically. Insulation or indoor use is available in exceptionally cold conditions. There is almost no odor or leachate created. This procedure is costly, and proper operation may necessitate technical knowledge [26]. When compared with windrow composting, in-vessel composting requires less land and labor. In-vessel technologies use a range of forced aeration and mechanical turning approaches [26]. Figure 2 shows a schematic diagram of an in-vessel composting system.

Windrow Composting Windrow composting involves putting raw materials such as yard waste, sewage sludge, and animal manure into long, narrow stacks called “windrows” that are

Biodegradable

Advantages The use of biodegradable materials can greatly reduce the amount of petroleum used and, as a result, the associated environmental risks. Microorganisms can break down organic matter and produce natural elements like carbon dioxide and water. Biodegradable materials degrade significantly more quickly than fossil-based materials, but this is dependent on various parameters such as temperature and aerobicanaerobic conditions

Disadvantages Biodegradable items can release toxic metals such as cadmium, lead, chromium, and manganese when they decompose. For example, dye fillers, plasticizers, and stabilizers act as additives that contain heavy metals to improve the physical properties of polymers used to produce biodegradable materials. When decomposed, heavy metals are nonbiodegradable and accumulate in the environment

Table 1 A comparison between biodegradable and compostable Challenges When biodegradable materials are thrown into landfills, they are frequently buried. Microorganisms buried beneath cannot survive in the absence or presence of oxygen. As a result, anaerobic biodegradation of biodegradable products produces methane, a greenhouse gas that contributes to global warming. Even though methane can generate electricity, most people do not do so. This is because methane can be released into the environment before it is burned to generate power; while methane has a shorter life span than carbon dioxide, it is more efficient at trapping heat in the atmosphere

Limitations Water is needed to facilitate the hydrolysis process and eventually increase the biodegradability rate

References [21]

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Compostable

It encourages the growth of beneficial bacteria and fungi that break down organic matter into humus, a nutrient-rich substance essential to plant growth. It takes a short time to break down completely in the specific condition. For example, all composting organisms require oxygen to achieve optimum aeration and moisture Ambient temperatures and meteorological conditions affect windrow and aerated static pile composting. In-vessel reactors are limited in adapting to changing conditions and are costly due to the maintenance often required Disease detection, low nutritional status, extended composting time, long mineralization time, and odor production, where most odors are toxic gases such as volatile organic compounds (VOC), e.g., formaldehyde, are all factors that need to be considered. Monitoring should be carried out to avoid the release of pungent odors. Composting in windrows and aerated static piles takes up much space, and odor control is regular Not all kinds of organic waste are suitable for the composting process. For example, organic waste containing pesticides or herbicides is not compostable [20]

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Water

Insulator

Condensation bottle

Composting materials Thermometer (upper)

Water Discharge

Thermometer (lower)

O2 analyzer

Fresh air Leachate overflow

Air flowmeter

Vacuum pump

Fig. 2 A schematic diagram of an In-Vessel Composting System. (Adapted with permission from Ref [27] (Copyright 2012, Elsevier))

turned regularly. The components are mixed, allowing for aeration of the setup. It is difficult and expensive to maintain, yet it is quick and efficient at retaining heat at 60  C. Large quantities, such as those created by entire towns and collected by municipal governments and high volume food-processing industries, are best served by aerated or turned windrow composting [28]. It will produce a large volume of compost, which may necessitate marketing the finished product. Residents may get compost for a low or no fee through local governments. This composting method entails separating the organic waste into rows of long piles known as “windrows“and aerating them regularly by turning the piles manually or automatically. This method can compost a wide range of wastes, including yard trimmings and animal by-products. Windrow composting often needs a large amount of land and a steady supply of workers to maintain and operate the facility. Windrows are occasionally placed under shelters in hot, arid regions to prevent water evaporation. During rainy seasons, the pile’s form can be altered so that water runs off the top instead of being absorbed by the pile. In frigid climates, windrow composting can be effective. The liquid that is released during the composting process is known as leachate. This has the potential to contaminate groundwater supplies and needs to be treated. Bacterial and heavy metal levels in compost should be evaluated in a laboratory because it is also important to keep odors under control. Figure 3 shows that a portable wind tunnel of compost windrows is essential to measure the bioaerosol emission flux that can cause respiratory and health problems, as well as odor problems [29].

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Emission surface 0.8 m Ae = 0.32 m2

Ac = 0.0084 m2

0.4 m

Mixing chamber

0.07 m 0.12 m Main section Am = 0.1

m2

0.25 m 0.4 m

Fig. 3 A schematic diagram of the wind tunnel and the dimensions of windrow composting. (Adapted with permission from Ref [29] (Copyright 2005, Elsevier))

Vermicomposting Vermicomposting is the process of degrading organic materials with earthworms [30]. Earthworms can eat almost any type of organic material and decay it. They can consume the equivalent of their body weight in food per day. The worms’ excrement is high in nitrate, which helps to improve soil fertility [30]. The presence of earthworms in the soil encourages the growth of bacteria and actinomycetes. Earthworms aid in decomposing complex organic waste by lowering the carbonto-nitrogen ratio and converting it to manure. They improve the microorganismexposed soil, making it more conducive to microbial activity and enhancing soil characteristics. Food scraps, yard trimmings, and other organic materials are fed to earthworms in bins to generate compost and decompose this material into castings [28], high-quality compost. Castings can also be used as soil for pots. Worm tea, another consequence of vermicomposting, is a natural, high-quality liquid fertilizer for house plants and gardens. Vermicomposting is ideal for those living in apartments or working in small offices. Vermiculture can be used in schools to teach youngsters about sustainable development and recycling. It is crucial to maintain the health of earthworms by providing them with the right circumstances and enough food. Worms are sensitive to climate change. High temperatures and direct sunlight harm the worms. The bin should be positioned in the shade in hot, arid climates. Many of these issues can be avoided by vermicomposting indoors. Figure 4 shows the diagrammatic representation of the vermicomposting system.

Static Pile Composting This is a conventional composting process in which wastes are decomposed aerobically with the help of passive aeration. This approach is time-consuming. However,

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Fig. 4 A schematic diagram showing the system boundary of the vermicomposting system. (Adapted with permission from Ref [31] (Copyright 2016, Elsevier))

this type of composting has cheap capital investment and operational costs compared to vermicomposting and other techniques [32]. Most types of feedstock, including green wastes, kitchen wastes, and all sorts of animal manure, are well suited to this method [32]. By introducing airflow into the compost piles, this composting technique reduces the need to turn the piles. The temperature will be managed by regulating the quantity of airflow, which will hasten the composting process while also assisting in producing high-quality compost. This technology can control noxious odors and insects [32]. The formation of a pile of raw materials is all that is required for this procedure, which requires little effort and equipment. The primary aeration method is the passive passage of air through the pile, which slowly degrades the organic waste [28]. Composting is an aerated static pile that normally takes place within 3–6 months. It works effectively for greater quantities of yard clippings and compostable municipal solid trash and is ideal for a somewhat homogeneous mix of organic waste. On the other hand, grease from food processing businesses does not function well with this procedure. The mound may need to be placed under a shelter to avoid water evaporation in a hot, arid region. In the winter, the pile’s core will retain its warm temperature. Aeration may be more challenging because passive airflow rather than active

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Fig. 5 A schematic diagram of the aerated static pile composting system. (Adapted with permission from Ref [33] (Copyright 2014, Elsevier))

rotation is used. It is also possible to bring the aerated static piles indoors with sufficient ventilation. Because there is no physical turning, this method necessitates close attention to guarantee that the outside of the pile heats up to the same degree as the center. A thick coating of finished compost applied over the pile may help to eliminate odors. Filtering the air through a biofilter built from completed compost will also eliminate any odors if the air blower sucks the air out of the pile. Large piles can be built with a controlled air supply, requiring less land than the windrow approach. Aerated static piles resemble windrows composting, but the materials are not turned or agitated once the pile is formed [33]. Figure 5 shows a schematic diagram of the aerated static pile composting system. Various composting methods are summarized in Table 2, each with its own set of benefits and drawbacks.

Anaerobic Digestion Anaerobic digestion is a biodegradable material end-of-life solution that is an alternative to composting [41]. In the absence of oxygen, bacteria break down organic materials such as animal dung and food wastes in anaerobic digestion, while composting involves the decomposition of organic matter in the presence of oxygen. The major output of composting is carbon dioxide and water, whereas anaerobic digestion produces methane gas, one of the greenhouse gases that can generate electricity. The fundamental reason that anaerobic digestion is regarded as more sustainable than composting is that composting necessitates energy to complete the process. The energy is used to turn the compost piles throughout the composting process. Meanwhile, anaerobic digestion can produce energy in the form of biogas. Anaerobic digestion for biogas generation occurs in a sealed vessel known as a reactor, which can be designed and built-in in various forms and sizes depending on the site and feedstock circumstances [42]. The waste is broken down in these

Static pile composting

Vermicomposting

Windrow composting

In-vessel composting

Advantages It improves the structure, workability, and rooting capacity of the soil. Moisture retention and soil drainage are improved. Furthermore, the amount of organic waste in landfills can be reduced. It helps to reduce odor and requires less land. Compost is a slow-release material that does not leach away A large volume of different waste can be composted. Product stabilization is excellent. It has a low initial capital outlay. It is simple to set up and use Vermicompost boosts plant development, increases soil porosity and microbial activity, and improves water retention and aeration. Vermicompost is also good for the environment because it reduces the demand for chemical fertilizers and the quantity of trash in landfills It has a low capital investment and low operational costs. Unpleasing odors and insects can be controlled It is time-consuming due to the slow degradation of organic waste

It is time-consuming because earthworms can take up to 6 months to decompose organic waste and transform it into a viable soil amendment

Composting necessitates a large amount of land

Disadvantages It requires a lot of capital investment

Careful monitoring is required to ensure that the outside of the pile heats up to the same degree as the center

During the rainy season, compost can turn anaerobic. Rainwater runoff may need to be processed It emits a foul odor if not done correctly. The stench comes from putting green plants in the compost bin, releasing ammonia

Challenges Personnel must undergo extensive training because the proper operation may necessitate technical knowledge

Bio-aerosols from composting can cause respiratory and health problems and odor problems For earthworms to be more effective, the temperature must be maintained. Pathogens and pest problems might thrive in vermicompost bins. As a result, the bin should be properly covered It requires a relatively large space to organic compost waste

Limitations High operational and maintenance costs

Table 2 A comparison between in-vessel composting, windrow composting, vermicomposting, and static pile composting

[32, 40]

[37–39]

[35, 36]

References [26, 34]

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reactors, producing two valuable outputs, biogas and digestate, which are expelled from the digester [43]. Biogas comprises the major components of natural gas, which is methane at a relatively high percentage, carbon dioxide, hydrogen sulfide, water vapor, and trace amounts of other gases [43]. Biogas can be utilized for various purposes, including providing heat, generating electricity, and powering cooling systems [43]. Biogas can be refined by removing inert or low-value elements to generate renewable natural gas. This can be converted into vehicle fuel or further processed to provide alternative transportation fuel, energy products, or advanced biochemicals and bioproducts [43]. The residue material left over from the digesting process is known as “digestate” consisting of both liquid and solid components [44]. Because each has a value that can be achieved with differing degrees of post-processing, they are frequently separated and handled separately. Both the solid and liquid portions of digestate can be used in various beneficial applications with proper treatment, including nutrient-rich fertilizer, which is a foundation material for bio-based products, and organic-rich compost as a soil amendment [44]. In short, digestive products can be an origin of revenue or cost savings and are frequently pursued to boost biogas projects’ financial and environmental benefits. Anaerobic digestion systems may also generate fewer greenhouse gas emissions than composting because they absorb methane from manure, which would otherwise be released as a strong greenhouse gas into the environment [45]. Anaerobic digestion is an alternative to composting, especially for biodegradable materials that do not decompose effectively, such as those found in home composting, which are not allowed for effective biodegradation [46]. Anaerobic digestion and composting can be combined to benefit both anaerobic and aerobic biodegradation [46]. Figure 6

Fig. 6 A schematic diagram of the anaerobic digestion system. (Adapted with permission from Ref. [47] (Copyright 2021, Elsevier))

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shows a schematic diagram of the anaerobic digestion system. Table 3 summarizes the advantages, disadvantages, challenges, and limitations of anaerobic digestion and composting.

Factors that Affect the Rate of Composting The factors that affect composting include the composting temperature, moisture content, oxygen content, and pH.

Temperature Temperature is important in composting because it speeds up the process and helps eradicate parasites and pathogens detrimental to soil organisms and plants. The temperature at which microorganisms are found during the composting process is classified. Microorganisms that thrive at low temperatures of around 30–40  C are known as mesophilic organisms, while bacteria that grow at moderately high temperatures of around 50  C to 60  C are known as thermophilic bacteria [49]. Mesophilic organisms initiate the composting process by breaking down the waste’s easily degradable components. As a result, the compost temperature rises quickly because of its metabolism. The temperature is occasionally affected by the volume of waste being processed. If the volume of waste is low, the high temperature may not be achieved [49].

The Oxygen and pH Levels During the composting process, oxygen is critical; especially when organisms oxidize carbon to make energy, the oxygen in the environment is depleted, and carbon dioxide is produced. The composting process will turn anaerobic if there is not enough oxygen. As a result, major components of gases such as methane and ammonia will be created, resulting in unpleasant aromas [28]. The composting rate is influenced by the pH of the materials being composted. Composting is said to be best when the pH is alkaline. Composting takes long when the pH is acidic because the microorganisms are destroyed [50].

Moisture Content The metabolic activities of microorganisms are supported by moisture. Compostable materials should have a 40–60% moisture content [50]. Compost moisture comes from either the water added at the beginning or the metabolic water created by microbial activity. Excess water limits oxygen diffusion, which reduces an organism’s metabolic activity. The metabolic activities of microbial cells are entirely

Composting

Anaerobic digestion

Advantages Low environmental impact because the released methane gas is used to generate energy in biogas, reducing methane emissions into the environment. As a result, less energy is required compared to composting Humus, a nutrient-rich substance essential to plant growth, enriches the soil and aids in the retention of moisture, is formed. Also, starting a composting facility requires a lower initial capital expenditure than starting an anaerobic digestion plant Composting necessitates a significant amount of energy to fuel and operate the equipment required to aerate and turn the compost piles. Apart from that, most composting techniques require a large space for the composting process

Disadvantages As methane-producing organisms develop slower than aerobic organisms, it takes longer to start the process

Table 3 A comparison between anaerobic digestion and composting

Odor production, where most of the odors are toxic gases like volatile organic compounds (VOC) such as formaldehyde, all these factors need to be considered. Monitoring should be carried out to avoid the release of pungent odors

Challenges Anaerobic digestion can produce a foul odor if it is not done properly

Not all kinds of organic waste are suitable for the composting process. For example, organic waste containing pesticides or herbicides is not compostable

Limitations The digestate output contains ammonia, handled carefully when dispersed on land to avoid ammonia gas pollution

[13, 20]

References [48]

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Fig. 7 A representative diagram shows factors that affect the rate of the composting process. (Adapted with permission from Ref. [51] (Copyright 2020, Elsevier))

dependent on water. Microorganisms can only use organic compounds that have been dissolved in water for metabolism. As the composting process progresses, the amount of moisture in the airdrops is reduced [49]. Figure 7 shows the several parameters that affect the composting process (open access, no permission required).

Composting Advantages Because of the recent campaign against synthetic fertilizers, the compound fertilizer form of compost is a welcome idea right now. Compost contributes to increased soil fertility and plant yield [52]. Another method of using compost for plant growth is to supplement it with synthetic fertilizer because research has shown that synthetic fertilizer is more productive than compost [53]. Composts also contain plant-growthpromoting bacteria that aid in soil fertility and plant growth. As a result, since erosion can deplete the soil’s fertility and consume a significant amount of nitrogen, phosphorus, and potassium, surface-applied organic amendments are very effective in preventing erosion. Compost improves the soil’s water retention capacity and the stability of the soil structure [54]. Furthermore, compost can be used to control plant diseases biologically [55]. To combat pathogenic microorganisms, compost microorganisms employ a variety of mechanisms. Compost can be used to remediate heavy metal-polluted soil. Compost is effective in degrading toxic pollutants such as chlorinated and non-chlorinated hydrocarbons and heavy metals in soil, reducing the toxicity of chemical pollutants [56]. Composting is a safe method of disposing of biodegradable organic waste. Composting waste that would otherwise be dumped into bodies of water, along

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roadsides, or burned is an option [57]. Composted waste products are used for a variety of beneficial purposes. Composting is an environmentally superior alternative to using organic material for landfills because it reduces methane production, which is a major source of greenhouse gases, and delivers a range of economic and environmental co-benefits [13]. Composting is an aerobic process that reduces or eliminates methane production when organic waste degrades. Bacteria break down organic matter in anaerobic circumstances without oxygen, releasing methane into the atmosphere. Anaerobic fermentation is common in landfills and open stockpiles like manure mounds. On the other hand, the aerobic composting process does not produce methane because methaneproducing microbes are not active in the presence of oxygen. As a result, composting methods that reduce anaerobic conditions while boosting aerobic conditions will greatly impact greenhouse gas emissions. Composting is one method of lowering methane emissions from organic waste currently held or disposed of in landfills.

Major Chemical Elements in Composting To be useful, compost must include particular elements in adequate proportions to offer the required nutrients to plants. Composting, for example, necessitates the inclusion of nitrogen, phosphorus, and potassium.

Nitrogen Plant growth and development are hampered by a lack of nitrogen, one of the most crucial components for plant growth. Nitrogen is an important component of chlorophyll, which gives plants their green hue. Compost is said to have the right amount of nitrogen for plant growth [58]. High nitrogen accumulation is uncommon because nutrients in compost fertilizer are gradually released due to mineralization. Excess nitrogen in plants from fertilizer overuse can induce quick growth and weaken the root system [59]. Excess nitrogen can cause leaf tissue to burn and the plant to die in extreme circumstances. Nitrogen deficiency results in a loss of leaf color and slowed development [59].

Phosphorus Plants have a sophisticated nucleic acid structure that regulates protein production, and phosphorus is a component of that structure [58]. Phosphorus is thus necessary for plant cell division, the development of new tissue, and complex energy conversions. Adding phosphate to low-phosphorus soil encourages root growth and hastens plant maturity. Phosphorus deficiency can cause growth, delayed maturity, poor seed, and fruit development. Compost has been found to promote plant growth by providing the optimum concentration of phosphorus [59].

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Potassium Potassium is an essential nutrient because it can boost plant growth, carotene levels, and chlorophyll levels [58]. It encourages plant vitality and color. The plant needs potassium to produce carbohydrates. It is also necessary since it aids in disease resistance and the plant’s ability to withstand harsh climatic conditions such as drought and cold [60]. A potassium shortage in plants can cause blistering and browning of the tips of older leaves, which eventually spreads to the entire leaf. A potassium deficit may also be linked to weak stalks. Composts are an excellent source of phosphorus, essential for plant growth.

Microbes Used in Composting The resident microbial community is primarily responsible for biodegradation during composting. Composting is accomplished through the action of a diverse microbial community. Bacteria and fungus have the highest number of microorganisms during composting [61]. Composting involves two types of aerobic microorganisms, such as mesophilic and thermophilic. Bacteria and actinomycetes are examples of organisms that dominate different stages of composting. The composting process could begin with a mesophilic stage, with temperatures ranging from 20  C to 40  C, while after the mesophilic stage, the thermophilic stage ranging from 40  C to 70  C takes over. In contrast to the mesophilic stage, active breakdown occurs in the thermophilic stage [49]. At this stage, mesophilic organisms are destroyed while the abundance and diversity of thermophilic bacteria, actinomycetes, and fungi grow. After the thermophilic stage, the compost is matured in the second mesophilic stage, also known as the curing phase [49]. Composting involves the use of lignocellulose-degrading bacteria. Polysaccharides such as cellulose, hemicellulose, and lignin are all components of lignocellulosic cellulose [62]. Organisms’ ability to decompose organic matter is determined by their ability to create necessary enzymes to break down the lignocellulosic cellulose. Hundreds of different fungi can degrade lignocellulose. Three major species of fungi, such as soft rot fungi, brown rot fungi, and white-rot fungi, are known to live in lignocellulose-rich dead forests [62]. These microbes degrade the wood components. Soft rot fungi can decompose cellulose but destroy lignin slowly and incompletely [62].

Composting and Biodegradation Challenges Separation procedures have been made easier thanks to technological advancements, which have resulted in significant changes in the methods and materials used in composting. For better waste management through composting, many primary separation tools have evolved. The inclusion of biochar as a co-compost element

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has improved the composting process [63, 64]. This cuts down on composting time, and it has been suggested that seed germination improves when seeds are placed directly in finished compost. The blockage of the pores reduces the surface area of biochar. Microbial communities are one of the activities impacted by the presence of biochar in composting [63]. Some microbes have become more prevalent because of it. It has an impact on and improves their ability to decompose quickly. Biochar is important in composting because of its high stability, porosity, and good waterholding capacity [65]. Biochar also helps to compost by balancing pH and acting as a catalyst [65]. Composting efficiency will be greatly improved if odor and bioaerosols generated during the process are controlled. Odors are an unavoidable by-product of the composting process, independent of the organic material used. Composting exhaust gases have high flow rates and, in most cases, low pollutant concentrations, with volatile organic compounds being one of the most common pollutants [2]. At a wastewater sludge composting facility, the most important odorous volatile organic compounds were dimethyl sulfide (Fig. 8a), dimethyl disulfide, limonene (Fig. 8b), and α-pinene (Fig. 8c) [66]. The most significant sources of volatile organic compounds are processes involving solvents, paints, or the use of chemicals, but waste and wastewater treatment facilities are also substantial volatile organic compound producers [67]. The presence of volatile organic compounds in gaseous streams causes an environmental problem for composting facilities and harms human health since many of them emit foul odors or are toxic, such as formaldehyde. Although the relative quantity of these pollutants varies, terpenes, aromatic hydrocarbons, and esters are the most typically released volatile organic compound families [21]. Figure 9 shows the emission of VOCs from the compost piles and the cycle of VOCs in the atmosphere. Ammonia is the other odor derived from the nitrogen compounds. The release of ammonia is greatly influenced by the pH and temperature of the composting pile, with high temperatures and alkaline conditions favoring the release of ammonia [21]. Trimethylamine (Fig. 8d), which is generally created in treatment plants, is Fig. 8 Chemical structures of (a) dimethyl sulfide; (b) limonene; (c) α-pinene; and (d) trimethylamine

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Precipitat ion VOC Gas Emissions

Condensation Rain Water

Compost Piles Ground Surface

Surface Water

Ground Water Contamination

Fig. 9 A representative diagram shows the composting process and the cycle in the atmosphere. (Adapted with permission from Ref. [68] (Copyright 2019, Elsevier))

another nitrogen-derived smell observed to be formed in composting. This molecule is significant because it has a low smell threshold, which contributes significantly to odor pollution [21]. The composting process must be optimized by maintaining the proper aeration rate and avoiding anaerobic conditions to reduce pollutant emissions. The use of gas treatment in composting facilities has been investigated. Because of its high pollutant destruction efficiency, biofiltration is a pollution control treatment used in composting facilities to help reduce the emission of toxic gaseous chemicals [69]. Biofiltration is also a biological technology that is both cost-effective and environmentally friendly. It is regarded as an appropriate technology in terms of trash recycling and filtering impact while also lowering construction and operational expenses and is currently being employed in composting facilities as one of the most cutting-edge technologies for odor control [70, 71]. A biofilter comprises filter materials such as compost or inert materials on which microorganisms are immobilized to form a biofilm. Target organic contaminants infiltrate into the biofilm as waste gases travel through the reactor due to adsorption and aerobic biodegradation carried out immediately [2]. Primary water, carbon dioxide, mineral salts, and microbial biomass are all by-products of microbial oxidation [72]. The effectiveness of biofilters is not consistent, and several crucial factors can influence it. Moisture content, pH, nutrient limitation, temperature, and the microbiology of the biofilter medium all influence microbial activity [69]. Natural

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Treated air

Mixing chamber

Calibration MeSH and VOC mixture

Biotrickling filter

Fresh MSM supply pH

Controller

Ambient air

Humidifying column Activated carbon Compressor filter

Mixing tank

NaOH storage

Fig. 10 A schematic diagram of the bio trickling filter setup. (Adapted with permission from Ref [73] (Copyright 2017, Elsevier))

filter materials provide sufficient nutrients for bacteria, so no additional nutrients are necessary [69]. Figure 10 shows the operating condition of a bio trickling filter where contaminated air, including acid gas, is allowed to travel through a densely packed bed of inert materials that are constantly supplied with a liquid containing the active bacteria’s required nutrients [68]. Figure 11 shows a biofiltration system set up by passing contaminated air or exhaust gases from compost piles through the biofilter. The odors could be decreased. Improved aeration in compost piles can also help to reduce odor emissions [74]. Bulking agents like rice straws and wheat straws will help remove moisture from the compost pile while also increasing air porosity [75]. The ammonia is trapped and recycled in the airbag bioreactors, resulting in higher nitrate content in the compost [76]. Agricultural wastes such as dead plants account for a substantial portion of the trash generated on the farm. Some have a high concentration of refractory chemicals and low nutrition levels, making composting challenging. When such wastes are put on a composting pile, they impede how other materials decompose. Due to the numbers and kinds of chemical components such as lignin and cellulose, the toughness of different plant taxa varies [62]. Tobacco leaves have a high nicotine content, which adds to their extended composting period [77]. Actinomycetes can degrade plant wastes with a high lignin concentration but take longer to degrade in a compost pile [65]. Due to its abrasive character, lignin is difficult to decompose.

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Exhaust Biofilter

Colle

ction

tube

s (Su

ction )

Covered layer

Humidifier Air in

Motor Compost mass

Blower

Fig. 11 A schematic diagram of compost gas emissions and treatment through biofiltration. (Adapted with permission from Ref. [68] (Copyright 2019, Elsevier))

Compared to the amorphous component of the lignocellulose, the highly crystalline lignocellulose is more resistant to decomposition. Co-composting is a method for aerobically decomposing organic waste mixes to produce compost as a fertilizer or soil supplement [78]. The main difference between this and traditional composting is the use of multiple feedstocks to start and sustain the biodegradation process and the ability to combine various types of waste to create desired products with specific properties [78]. Co-composting can be done primarily to achieve an optimal carbon to nitrogen ratio to speed up the composting process and increase fertilizer quality [79]. A mixture of different materials may accelerate or inhibit the composting process for co-composting [75]. Microorganisms become more applicable when highly nutritious substrates are added to compost; the composting process gets accelerated. The composting process is impeded when materials with low nutrients or high lignocellulose content are added to a compost pile. Figure 12 shows a representative diagram of how the inclusion of biochar as a co-compost element has improved the composting process. Organic materials with a high carbon to nitrogen ratio are difficult to compost because they are inappropriate for microorganisms to use. To promote a fastercomposting process, activators must be used to reduce the carbon-to-nitrogen ratio of such substrates [81]. Activators are microorganisms supposed to break down the raw materials in the composting process. For instance, sewage, cow dung, pig dung, and goat dung are examples of activators. They have a poor carbon to nitrogen ratio, which makes composting them by itself challenging due to the odor issue [57]. As a result, mixing them with degradable materials containing a high carbon-to-nitrogen

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Fig. 12 A representative diagram of the inclusion of biochar for co-composting. (Adapted with permission from Ref. [80] (Copyright 2019, Elsevier))

ratio aids in achieving an appropriate carbon-to-nitrogen ratio. The population of microorganisms that may occur in the compost created is determined by the supply of degradable materials, animal manure [82]. To assure the environment’s safety, composts should be thoroughly examined for microbiological and chemical constituents. Furthermore, materials that take longer to compost can be decomposed separately so that the composting process of other wastes is not slowed. The agronomic evaluation of compost is used to determine the compost’s quality by observing the compost’s influence on specific plants’ growth and evaluating the outcome of plants when planted with compost [83]. Compost has been shown to boost crop yields due to its nutrients. In a similar vein, low nutritional status in compost has been reported as not affecting plant development. It is also critical to assess the nutritional content of composts and to supplement them with nutrient-rich materials to increase their value in terms of nutrition and agronomy [84]. Persistent organic pollutants and endocrine disruptors are dangerous substances that remain in soils and water, proven to be removed by using the composting method [85]. For instance, nonylphenols (Fig. 13a), polychlorinated biphenyl (Fig. 13b), bisphenol A (BPA) (Fig. 13c), and polyaromatic hydrocarbons (PAH) such as biphenyl are examples. Both toxic pollutants harm human health, so they must be taken seriously. Contact with contaminated air, water, and soil is the primary route of endocrine disruptor exposure [85]. Plants absorb these dangerous compounds from soils that come into contact with treated water. Ingesting these plants causes bioaccumulation of endocrine disruptors in humans, even though the amounts taken by plants may be minimal. It has not been easy to find a way to eliminate these dangerous chemical groupings. Although several traditional strategies have been

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Fig. 13 Chemical structures of (a) nonylphenol; (b) polychlorinated biphenyl; and (c) Bisphenol-A

used to eradicate them, no success has been established. On the other hand, composting techniques have proven to be capable of removing these hazards to human survival. With proper use, agricultural and environmental sustainability can be improved because bacteria in compost aid in absorbing persistent organic pollutants, and their bioavailability is crucial for their absorption [86]. The problem with polymer biodegradation is that the non-hydrolysable nature of the bonds linked between polymer monomers prevents them from breaking down into monomers. The rate of biodegradation and the presence and activity of polymerdegrading bacteria, are often affected by environmental factors such as temperature, aerobic-anaerobic conditions, pH, moisture content, and oxygen concentration. Although some biodegradable polymers decompose quickly in compost or an anaerobic digester, they may not degrade as quickly in the soil as well. As a result, the output aim for a biodegradable item must be precisely defined and followed. Even though some biodegradable polymers can be degraded in the marine environment, marine ecosystems should never be used as disposal sites because aquatic habitats do not have the optimum microbial activity and temperatures compared to compost and soil, making biodegradation a less attractive alternative in the aquatic environment. This can be problematic, particularly for micro or nano plastics transported to aquatic ecosystems before they have completely degraded. Biopolymers are made from renewable materials and can be destroyed by hydrolysis or enzymes. Even though biopolymers have been utilized for hundreds of years, their widespread applicability is limited due to their weaker functional qualities, such as poor mechanical properties compared to synthetic counterparts. Recent advancements have significantly enhanced their characteristics, allowing them to be used in various applications, including food packaging. Biopolymers can only be advantageous if they truly biodegrade. Hence, water-soluble biopolymers are preferable. Non-water-soluble biopolymers, on the other hand, require bioactive system infrastructure for disposal. Figure 14 shows recycling options for sustainable management of bioplastics, and their contributions to sustainable development are depicted schematically. Methane, a greenhouse gas that contributes to global warming, is frequently emitted during anaerobic biodegradation using methane-producing microbes because biodegradable materials often get buried in landfills in the absence of oxygen or low oxygen concentration. Methane can generate electricity, but most people do not do so. This is because methane can be released into the environment

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Fig. 14 A schematic diagram shows recycling options for sustainable bioplastics that contribute to sustainable development. (Adapted with permission from Ref. [87] (Copyright 2020, Elsevier))

before it is burned to generate power; while methane has a shorter life span than carbon dioxide, it is more efficient at trapping heat in the atmosphere. Due to the biodegradable product’s ability to be seen as easily decomposed in the environment by the public, it unavoidably increases the danger of littering. Littering risks must be reduced, and misconceptions must be dispelled by educating the public and offering funding and facilities to collect biodegradable items after usage properly. Furthermore, for instance, when biodegradable plastics decay, nonbiodegradable heavy metals such as cadmium, lead, chromium, and manganese present in the addictive-like plasticizers, stabilizers, or fillers used to improve the physicochemical characteristics of plastics are released and cause harm to the environment.

Conclusions Improper waste management is a widespread habit that is unsafe and can be changed with a more environmentally friendly waste management strategy like composting. Composting is an important waste management technique because of its durability

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and the prospect of producing a lucrative product with soil amendment potential. The globe is moving in the direction of bettering environmental and human health. The emphasis on composting will move away from chemical fertilizers in favor of compost. Furthermore, determining gaseous emissions is critical for maintaining the composting process’s long-term viability. This adjustment will undoubtedly benefit the environment and human health. Biodegradation is an important and necessary component of addressing global plastic waste pollution, which is undoubtedly a complex plan that includes reducing, reusing, and recycling. These are situations where plastics cannot be recycled or reused because they have been polluted by food residues or soil. Most of the biodegradable materials must be utilized and handled with composting or anaerobic digestion as the end-of-life option in mind because they are environmentally benign and can replace nonbiodegradable materials.

Future Perspectives There is still much awareness-building about its possibilities for composting technology to be fully accepted. In terms of improving technologies, several suggestions are made here to assist their advancement. Mixed nutrients make up most of the compost. As a result, the ability to extract nutrients such as nitrogen, potassium, and phosphate will significantly contribute to the soil’s overall health. Certain trace elements, for example, are contained within permissible limits in matured compost, but when the compost is used for soil remediation, instead of being reduced, certain trace elements can be added to the soil’s already existing trace elements. Compost’s capacity to increase soil structure and nutrient availability by supplementing nutrients already present has been a significant driving force behind its usage in crop cultivation. The focus on nutrients has traditionally been on nitrogen, but phosphate is a limited resource for plant production [13]. Therefore, combining effective phosphorous use in compost should greatly extend the life of our accessible mineral phosphate sources. The compost can be made more effective by adding plant-derived antimicrobials such as secondary metabolites present in plants, such as tannins, terpenoids, alkaloids, and flavonoids, which have been shown to have antibacterial activities [88]. Before applying compost to the field, it should always be tested for maturity and pathogens to avoid potential threats to the environment and other living creatures. Slowly decaying items such as lignocellulosic biomass, including corn stover, corn cobs, wheat straw, cotton residue, and others, should be composted separately from other materials so that the composting duration of other materials is not extended. Composting requires a significant amount of energy for forced aeration in piles and reactors. The introduction of solar energy could help lower energy costs. A solar composting pilot plant is employed to reduce the composting facility’s overall air emissions, a useful alternative to diesel generators and optimizing solar energy consumption to power aeration equipment.

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Insects can be used in composting in a controlled manner. For example, black soldier field larvae can digest certain chemical pollutants [89]. Composting with black soldier flies has been shown to lower greenhouse gas emissions when compared to other traditional composting processes. The black soldier fly could be used in composting resistant organic wastes because of its ability to break down a wide range of substrates. If the conditions are favorable, insects can be used in composting. The use of these insects in composting will aid in the conservation of these essential species and promote sustainable development. Instead of being thrown away, they might be captured and used in composting operations. Aside from that, the odor is always an issue when it comes to composting. The development of an odor trapping system can be employed in various composting techniques that arise in odor problems. Another key thing to consider for future improvement is compost process management. The probability of greenhouse gas formation during composting necessitates correct management methods, necessitating deeper knowledge of enzymes, microbial populations, and processing conditions to mitigate methane and nitrous oxide emissions successfully. It is vital to have sensors that can aid in comprehending the management process. For biodegradation, more research on the biodegradation process and variables is needed to examine the potential of various microbes to degrade the polymers. A greater mechanistic understanding of the degradation process, the duration of comprehensive microbial degradation, and the fate and movement of microplastics are required. The use of a custom microbial community could improve the efficiency of the plastic biodegradation process. Furthermore, molecular engineering approaches can enable modified microbes to integrate function-specific genes into their genomes. When it comes to finding the most effective polymer-degrading bacteria, molecular approaches are essential, such as proteomics and genomes, which, in our opinion, should be the emphasis of research [90]. The use of nanotechnology in the manufacture of bioplastics may help overcome biodegradation limitations and finetune the pace of biodegradation. By utilizing biodegradable materials, which is becoming a viable technique for fighting global plastic pollution, not only must end-of-life possibilities be considered, but also the accessibility of production resources such as raw materials and production equipment and the facility required to collect the biodegradable items for composting. It can be predicted that biodegradable materials will become more economically profitable when the use of conventional synthetic materials for certain items is outlawed, and disposal options become gradually limited. Although eliminating all plastics from society is neither practical nor desirable, prospective alternatives can play a key role in replacing traditional plastic-oriented items and reducing plastic waste. Alternatives must be part of a larger plan toward more sustainable manufacturing patterns, including the concepts of reuse, reduction, and facilitation of recycling. It is vital to strike a balance between lowering plastic packaging waste and increasing agricultural waste as a source of natural fabrics and a feedstock for bio-based manufacturing. Biopolymers derived from biomass, such as polylactic acid and thermoplastic starch, are promising as biochemically

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inert polymers. The use of recently commercialized polymers such as polyhydroxyalkanoates in industrial applications should be expanded under this scope. In the absence of competent industrial composting, its promotion as a sustainability brand is illogical. Acknowledgments This work was financially supported by the Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education of Malaysia (MOHE) (FRGS/1/2019/ STG01/UM/02/6).

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34. Pettitt T, D B, K K, M N, M F, Orthodoxou D, S S, P M, and Griffiths A, The challenges and benefits of in-vessel composting our food waste and catering waste to divert material from landfill and provide Eden Project with a valuable fertiliser. 2010. 35. Bhattacharjee N and Tollner E (2016) Improving management of windrow composting systems by modeling runoff water quality dynamics using recurrent neural network. Ecological Modelling 339:68–76 36. Zhu-Barker X, Bailey SK, Paw U KT, Burger M, and Horwath WR (2017) Greenhouse gas emissions from green waste composting windrow. Waste Management 59:70–79 37. Niedzialkoski RK, Marostica R, Damaceno FM, Costa LAM, and Costa M (2021) Combination of biological processes for agro-industrial poultry waste management: Effects on vermicomposting and anaerobic digestion. J Environ Manage 297:113127 38. Das D, Abbhishek K, Banik P, and Bhattacharya P (2021) A valorisation approach in recycling of organic wastes using low-grade rock minerals and microbial culture through vermicomposting. Environmental Challenges 5:100225 39. López R, Antelo J, Silva AC, Bento F, and Fiol S (2021) Factors that affect physicochemical and acid-base properties of compost and vermicompost and its potential use as a soil amendment. Journal of Environmental Management 300:113702 40. Isobaev P, Bouferguene A, Wichuk KM, and McCartney D (2014) An enhanced compost temperature sampling framework: case study of a covered aerated static pile. Waste Manag 34(7):1117–24 41. Piemonte V (2011) Bioplastic Wastes: The Best Final Disposition for Energy Saving. Journal of Polymers and the Environment 19(4):988–994 42. Shrestha A, van-Eerten Jansen MCAA, and Acharya B (2020) Biodegradation of Bioplastic Using Anaerobic Digestion at Retention Time as per Industrial Biogas Plant and International Norms. Sustainability 12(10) 43. Ileleji KE, Martin C, and Jones D, Chapter 17 - Basics of Energy Production through Anaerobic Digestion of Livestock Manure, in Bioenergy, A Dahiya, Editor. 2015, Academic Press: Boston. p. 287–295. 44. Teng Z, Hua J, Wang C, and Lu X, Chapter 4 - Design and optimization principles of biogas reactors in large scale applications, in Reactor and Process Design in Sustainable Energy Technology, F Shi, Editor. 2014, Elsevier: Amsterdam. p. 99–134. 45. Yagi H, Ninomiya F, Funabashi M, and Kunioka M (2009) Anaerobic biodegradation tests of poly(lactic acid) under mesophilic and thermophilic conditions using a new evaluation system for methane fermentation in anaerobic sludge. International journal of molecular sciences 10(9): 3824–3835 46. Hobbs SR, Parameswaran P, Astmann B, Devkota JP, and Landis AE (2019) Anaerobic Codigestion of Food Waste and Polylactic Acid: Effect of Pretreatment on Methane Yield and Solid Reduction. Advances in Materials Science and Engineering 2019:4715904 47. Chen B, Shao Y, Shi M, Ji L, He Q, and Yan S (2021) Anaerobic digestion of chicken manure coupled with ammonia recovery by vacuum-assisted gas-permeable membrane process. Biochemical Engineering Journal 175:108135 48. Xu F, Li Y, Ge X, Yang L, and Li Y (2018) Anaerobic digestion of food waste – Challenges and opportunities. Bioresource Technology 247:1047–1058 49. Chennaoui M, Chennaoui, Salama Y, Aouinty, Mountadar Mm, and Assobhei O (2018) Evolution of bacterial and fungal flora during in-vessel composting of organic household waste under air pressure. J Mat Environ Sci 9 50. Ameen A, Ahmad J, and Raza S (2016) Effect of pH and moisture content on composting of Municipal solid waste. International Journal of Scientific & research publication 6:35–2250 51. Rastogi M, Nandal M, and Khosla B (2020) Microbes as vital additives for solid waste composting. Heliyon 6(2):e03343 52. Majbar Z, Lahlou K, Ben Abbou M, Ammar E, Triki A, Abid W, Nawdali M, Bouka H, Taleb M, El Haji M, and Rais Z (2018) Co-composting of Olive Mill Waste and WineProcessing Waste: An Application of Compost as Soil Amendment. Journal of Chemistry 2018:7918583

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53. Pampuro N, Bertora C, Sacco D, Dinuccio E, Grignani C, Balsari P, Cavallo E, and Bernal MP (2017) Fertilizer value and greenhouse gas emissions from solid fraction pig slurry compost pellets. The Journal of Agricultural Science 155(10):1646–1658 54. Epelde L, Jauregi L, Urra J, Ibarretxe L, Romo J, Goikoetxea I, and Garbisu C (2018) Characterization of composted organic amendments for agricultural use. Frontiers in Sustainable Food Systems 2(44) 55. Olanrewaju OS, Glick BR, and Babalola OO (2017) Mechanisms of action of plant growth promoting bacteria. World J Microbiol Biotechnol 33(11):197 56. Zhang X, Wang H, He L, Lu K, Sarmah A, Li J, Bolan NS, Pei J, and Huang H (2013) Using biochar for remediation of soils contaminated with heavy metals and organic pollutants. Environ Sci Pollut Res Int 20(12):8472–83 57. Huang J, Yu Z, Gao H, Yan X, Chang J, Wang C, Hu J, and Zhang L (2017) Chemical structures and characteristics of animal manures and composts during composting and assessment of maturity indices. PLoS One 12(6):e0178110 58. Razaq M, Zhang P, Shen HL, and Salahuddin (2017) Influence of nitrogen and phosphorous on the growth and root morphology of Acer mono. PLoS One 12(2):e0171321 59. Loks NA, Manggoel W, Daar JW, Mamzing D, and Seltim BW (2014) The effects of fertilizer residues in soils and crop performance in northern Nigeria: a review. International Research Journal of Agricultural Science and soil Science 4:180–184 60. Kammoun M, Ghorbel I, Charfeddine S, Kamoun L, Gargouri-Bouzid R, and Nouri-Ellouz O (2017) The positive effect of phosphogypsum-supplemented composts on potato plant growth in the field and tuber yield. J Environ Manage 200:475–483 61. Galitskaya P, Biktasheva L, Saveliev A, Grigoryeva T, Boulygina E, and Selivanovskaya S (2017) Fungal and bacterial successions in the process of co-composting of organic wastes as revealed by 454 pyrosequencing. PloS one 12(10):e0186051–e0186051 62. Eriksson KEL and Bermek H, Lignin, Lignocellulose, Ligninase, in Encyclopedia of Microbiology (Third Edition), M Schaechter, Editor. 2009, Academic Press: Oxford. p. 373–384. 63. Jindo K, Suto K, Matsumoto K, García C, Sonoki T, and Sanchez-Monedero MA (2012) Chemical and biochemical characterisation of biochar-blended composts prepared from poultry manure. Bioresour Technol 110:396–404 64. Abd El-Rahim MGM, Sen D, Liu X, Shuai X, Sharaf A, Moussa AA, Eissa MA, Mustafa A-RA, Ali GAM, and Hamed MH (2021) Effect of biochar addition method on ammonia volatilization and quality of chicken manure compost. Zemdirbyste 108(4):331–338 65. Ahmed M, Ahmad S, Fayyaz ul H, Qadir G, Hayat R, Shaheen FA, and Raza MA (2019) Innovative processes and technologies for nutrient recovery from wastes: a comprehensive review. Sustainability 11(18) 66. Smet E and Langenhove HV (1998) Abatement of volatile organic sulfur compounds in odorous emissions from the bio-industry. Biodegradation 9(3):273–284 67. Anand SS, Philip BK, and Mehendale HM, Volatile Organic Compounds, in Encyclopedia of Toxicology (Third Edition), P Wexler, Editor. 2014, Academic Press: Oxford. p. 967–970. 68. Dhamodharan K, Varma VS, Veluchamy C, Pugazhendhi A, and Rajendran K (2019) Emission of volatile organic compounds from composting: A review on assessment, treatment and perspectives. Science of The Total Environment 695:133725 69. Tang H-M, Hwang S-J, and Hwang S-C (1996) Waste Gas Treatment in Biofilters. Journal of the Air & Waste Management Association 46(4):349–354 70. Sironi S and Botta D (2001) Biofilter Efficiency In Odor Abatement At Composting Plants. Compost Science & Utilization 9(2):149–155 71. Park KJ, Choi M, and Hong J (2013) Control of Composting Odor Using Biofiltration. Compost Science & Utilization 10:356–362 72. Nicolai RE and Janni KA (2001) Biofilter media mixture ratio of wood chips and compost treating swine odors. Water Sci Technol 44(9):261–7 73. Wypych G, 11 - METHODS OF ODOR REMOVAL, in Handbook of Odors in Plastic Materials (Second Edition), G Wypych, Editor. 2017, Chem Tec Publishing. p. 171–200

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74. Wei Y, Li J, Liu G, Zhao Y, and Shimaoka T (2017) Environmental challenges impeding the composting of biodegradable municipal solid waste: A critical review. Resources, Conservation and Recycling 122:51–65 75. Awasthi MK, Pandey AK, Bundela PS, and Khan J (2015) Co-composting of organic fraction of municipal solid waste mixed with different bulking waste: characterization of physicochemical parameters and microbial enzymatic dynamic. Bioresour Technol 182:200–207 76. Jiang Y, Liu J, Huang Z, Li P, Ju M, Zhan S, and Wang P (2019) Air bag bioreactor to improve biowaste composting and application. Journal of Cleaner Production 237:117797 77. Zhao GH, Yu YL, Zhou XT, Lu BY, Li ZM, and Feng YJ (2017) Effects of drying pretreatment and particle size adjustment on the composting process of discarded flue-cured tobacco leaves. Waste Manag Res 35(5):534–540 78. Giagnoni L, Martellini T, Scodellini R, Cincinelli A, and Renella G, Co-composting: An Opportunity to Produce Compost with Designated Tailor-Made Properties, in Organic Waste Composting through Nexus Thinking: Practices, Policies, and Trends, H Hettiarachchi, S Caucci, K Schwärzel, Editors. 2020, Springer International Publishing: Cham. p. 185–211. 79. Paul J, Cofie O, Josiane N, Impraim R, Adamtey N, and Koné D, Co-composting of Solid Waste and Fecal Sludge for Nutrient and Organic Matter Recovery. 2016. 80. Wang Y, Villamil MB, Davidson PC, and Akdeniz N (2019) A quantitative understanding of the role of co-composted biochar in plant growth using meta-analysis. Science of The Total Environment 685:741–752 81. Iewkittayakorn J, Chungsiriporn J, and Rakmak N (2018) Utilization of waste from concentrated rubber latex industry for composting with addition of natural activators. Songklanakarin Journal of Science and Technology 40:114–120 82. Alvarenga P, Mourinha C, Farto M, Santos T, Palma P, Sengo J, Morais MC, and Cunha-Queda C (2015) Sewage sludge, compost and other representative organic wastes as agricultural soil amendments: Benefits versus limiting factors. Waste Manag 40:44–52 83. Sharma A, Saha TN, Arora A, Shah R, and Nain L (2017) Efficient Microorganism Compost Benefits Plant Growth and Improves Soil Health in Calendula and Marigold. Horticultural Plant Journal 3(2):67–72 84. Lawal TE and Babalola OO (2014) Assessing the Associated Challenges in the Use of Animal Manure in Plant Growth. Journal of Human Ecology 48(2):285–297 85. Brambilla G, Abate V, Battacone G, De Filippis SP, Esposito M, Esposito V, and Miniero R (2016) Potential impact on food safety and food security from persistent organic pollutants in top soil improvers on Mediterranean pasture. Sci Total Environ 543(Pt A):581–590 86. Luo G, Li L, Friman V-P, Guo J, Guo S, Shen Q, and Ling N (2018) Organic amendments increase crop yields by improving microbe-mediated soil functioning of agroecosystems: A meta-analysis. Soil Biology and Biochemistry 124:105–115 87. RameshKumar S, Shaiju P, O'Connor KE, and P RB (2020) Bio-based and biodegradable polymers - State-of-the-art, challenges and emerging trends. Current Opinion in Green and Sustainable Chemistry 21:75–81 88. Cowan MM (1999) Plant products as antimicrobial agents. Clinical microbiology reviews 12(4):564–582 89. Mertenat A, Diener S, and Zurbrügg C (2019) Black Soldier Fly biowaste treatment – Assessment of global warming potential. Waste Management 84:173–181 90. Ali SS, Elsamahy T, Al-Tohamy R, Zhu D, Mahmoud YAG, Koutra E, Metwally MA, Kornaros M, and Sun J (2021) Plastic wastes biodegradation: Mechanisms, challenges and future prospects. Science of The Total Environment 780:146590

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polysaccharides-Based Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Protein-Based Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyesters/PHAs-Based Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemically Synthesized Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of Biofilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals of Food Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation Methodologies for Biofilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties Associated with the Biofilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The nonbiodegradability of synthetic plastics has emerged as one of the main challenges causing concerns among scientific researchers in recent years. Their excess usage as the food packaging material is especially discouraging as these materials have noxious impacts on the environment and human beings. Biodegradable polymers (natural materials capable of undergoing complete biodegradation) have now been considered an efficient alternative to these plastics as the biodegradable polymers efficiently mimic/improve the properties of synthetic polymers required for food packaging applications. Furthermore, discarding these materials in the environment is not a problem as the normal degradation pathways will be more than enough to assimilate these degradable polymers. This chapter aims to organize the literature associated with these specifics into J. Najeeb (*) · S. Naeem Department of Chemistry, University of Gujrat, Gujrat, Pakistan e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_56

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different categories depending on the origin of the biopolymers. The case studies were divided into natural polymers, biopolymers extracted from renewable resources, and synthetic polymers containing monomers extracted from fossil reservoirs. Moreover, the fundamentals associated with the food packaging applications, including preparative methodologies for biodegradable film, properties of the synthesized films, quality check experiments, etc., are discussed in detail for presenting an overview of the said topic. This chapter will act as a guideline for the new researchers having an intention of exploring this field and will present a summary of the recent progress in this field for the currently associated scientific community. Keywords

Food packaging · Biodegradable polymers · Plastics · Food industry · Biopolymer · Biodegradation · Biofilms Abbreviations 13

CNMR HNMR AFM Ag ASTM CEO CMC CNCs CNFs CO2 CO2PC CS CuO DCNP DSC EM FDA FTIR H2O HSPI LAE LCNFs MgO NMR NMs NPs OPC OTR 1

Carbon-13 nuclear magnetic resonance spectroscopy Proton nuclear magnetic resonance spectroscopy Atomic Force Microscopy Silver American Society of Testing and Materials Clove essential oil Carboxymethyl cellulose Cellulose nanocrystals Cellulose nanofibrils Carbon dioxide Carbon dioxide permeability coefficient Chitosan Copper oxide Dye-clay hybrid nanopigment Differential scanning calorimetry Elastic modulus Food and Drug Administration Fourier transform infrared spectroscopy Water Hybrid sorubium protein isolate Lauroyl arginate ethyl Lignocellulose nanofibrils Magnesium oxide Nuclear magnetic resonance Nanomaterials Nanoparticles Oxygen permeability coefficient Oxygen transmission rate

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PBAT PBS PCL PGA PHAs PHB PHBV PLA PVA SEM SM SPNCC SPS TEGO TEM Tg TGA TiO2 TPS TS US UV-VIS WPC WVPC WVTR XRD ZnO

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Polybutylene adipate terephthalate Polybutylene succinate Polycaprolactone Polyglycolic acid Polyhydroxyalkanoates Poly (3-hydroxybutyrate) Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) Polylactic acid Polyvinyl alcohol Scanning electron microscopy Sodium metabisulfite Sugar palm nanocrystalline cellulose Sugar palm starch Thermally exfoliated graphene oxide Transmission electron microscopy Glass transition temperature Thermal gravimetric analysis Titanium dioxide Thermoplastic starch Tensile strength United States Ultraviolet-visible spectroscopy Whey protein isolate Water vapor permeability coefficient Water vapor transmission rate X-ray diffraction analysis Zinc oxide

Introduction The accumulation of synthetic plastics in the natural environment (because of its improper waste management and disposal issues) is causing immense health and environmental concerns [1, 2]. Generally, nonbiodegradable materials, including ethylene, polyethylene, polystyrene [3, 4], poly (vinyl chloride) [5], polypropylene [6], and polyurethane [7], etc., are utilized in food packaging applications, which are known for their exceptionally high durability and enhanced mechanical properties [8]. These materials are further reinforced with the binding/strengthening agents (mostly aluminum foil) at their processing, contributing to their non-recyclability [9]. These improperly disposed of synthetic plastics constitute a significant portion of municipal solid waste. In terms of disposal methodologies associated with these wastes, the low-density plastic wastes are either incinerated (causing air pollution and noxious residue) or are preferably dumped into landfills [10]. According to an estimate, the concentration of these plastics in the landfills is continuously increasing, and if the strict regulations against these synthetic plastics are not implemented

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at a global scale, this amount is projected to increase exponentially [11]. Few studies have even called this enhanced speed of increase in plastic pollution “a serious threat to the sustainability of planet Earth” [12]. Although methodologies such as controlled bioreactors-based landfilling, controlled combusting, controlled composting, or recycling can reduce this issue [2, 13–16], the best-proposed methodology is to eliminate this issue at the starting level as indicated in Fig. 1. Instead of nonbiodegradable materials, natural biodegradable materials are presented as alternative food packaging materials [17]. Understanding and studying the materials that can effectively carry out plastics’ functions and exhibit biodegradability requires an hour [18]. The enhanced interest of the scientific community in this research domain can be validated by studying the following recent reviews published on a similar topic [1, 9, 19–23]. Biopolymers possess tremendous potential to be an efficient alternative to these harmful synthetic materials [3]. Biopolymers are the natural biodegradable polymeric materials either directly acquired from living organisms/biomass or indirectly synthesized by performing chemical processes (such as fermentation, polymerization, hydrolysis, etc.) on the biomass extracted from renewable resources [24]. The main property associated with these biopolymers is biodegradation as defined by the presence of an end-of-life breakdown route/pathway carried out via microorganisms (including algae, fungi, and bacteria) for the decomposition of these biopolymers [25]. These metabolic degradation processes of microorganisms either partially or completely degrade the large biopolymers into smaller environment-friendly constituents or molecules with a significant amount of time [26]. The nature and composition of the disposal environment are crucial factors that influence biodegradation as the disposal environment (such as landfill, agriculture soil, marine reservoir, industrial soil, etc.) relates directly with the number of microbes available for the decomposition. The ideal biodegradable food packaging biopolymer should be the one that completely dissociates into carbon dioxide (CO2) and water (H2O) molecules at the end of its degradation pathway [27].

Fig. 1 Numerous waste management strategies are utilized to remove organic pollutants from the environment. (Adapted with permission from Ref. [28], Copyright 2018, Elsevier)

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This chapter discusses the research domain of biodegradable food packaging material in this chapter. The introductory section provides a brief overview of the topic, while the second section provides the academic literature survey associated with biodegradable food packaging materials. The characterization section highlights the numerous analytical techniques associated with studying various characteristics/properties of these biopolymers. The fundamentals (including formation strategies, properties, and biodegradation tests of biopolymers) and the commercial aspects of the biodegradable plastics are also discussed in the fourth section. The research gaps and the future perspectives are also summarized in the last section. This chapter will act as a guideline for the scholars hoping to explore this research domain.

Biopolymers The biopolymers are broadly classified into two categories (i.e., natural and synthetic biopolymers), including biodegradable and nonbiodegradable materials [29]. The biopolymers utilized for food packaging applications (containing only biodegradable materials) are classified into three main groups. Group 1 includes the biopolymers that are extracted directly from the biomass. This class is subcategorized into polysaccharides (starch, starch derivatives, chitin, chitosan (CS), alginates, pectin, cellulose, etc.), proteins/polypeptides (animal proteins and plant proteins), and lipids-based biopolymers. All these biopolymers are hydrophilic and crystalline materials. Their crystalline nature makes processing these biopolymers quite difficult for food packaging applications. However, these biopolymers’ excellent gas barrier characteristics make them suitable in the form of blends for food packaging applications [20]. The studies documented by Lamareerat et al. [30], Marichelvam et al. [31], Bangar et al. [32], and Woggum et al. [33] report such Group 1-based biodegradable materials utilized for food packaging applications. Group 2 includes the biopolymers synthesized by microorganisms or genetically modified microbes. Mainly, the microbial polyesters (polyhydroxyalkanoates (PHAs), poly (3-hydroxybutyrate) (PHB), etc.), and bacterial cellulose constitute this category. For the processing of this class, blending the biopolymers with other biopolymers was found to be a preferable approach [8]. The studies reported by Cherpinski et al. [3], Anjum et al. [34], Manikandan et al. [35], Tripathi et al. [36], and Lin et al. [37] document the Group 2-based biodegradable materials for food packaging applications. Group 3 includes the biopolymers that are chemically synthesized by the conventional chemical processing (such as fermentation, hydrolysis, polymerization, etc.) of renewable monomers. The main constituents of this group include aliphatic polyesters, i.e., polycaprolactone (PCL), polyglycolic acid (PGA), polylactic acid (PLA), etc. [38]. The classic example of the biopolymers associated with this group is PLA, synthesized by polymerizing bio-based monomeric units of lactic acid. The monomeric units might be produced from the fermentation of numerous

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polysaccharides or carbohydrate food stocks. The enhanced barrier properties and high mechanical strength are why this is the first novel biopolymer commercialized at an industrial scale [23]. The case studies documented by Mahmoodi et al. [39], Villegas et al. [40], Yang et al. [41], Siakeng et al. [42], and Asadi et al. [43] represent Group 3-based biodegradable materials for food packaging applications. The subheadings provided in this section detail the numerous biodegradable biopolymers utilized individually or in the form of blends for food packaging applications. From group 1, emphasis has been given to polysaccharides and proteins-based biopolymers, while the subcategory of lipids is ignored. This is ascribed to the fact that synthesizing films from oily/greasy lipid materials is quite a difficult task, and lipids are not considered on the commercial scale for food packaging applications [23]. Figure 2 presents the structural analysis of some commonly utilized biodegradable materials for food packaging applications.

Polysaccharides-Based Biopolymers Starch Starch, regarded as the primary energy source, is a low-cost, abundant polysaccharide that usually constitutes almost 60–75% of the molecular weight of the natural grains and is responsible for approximately 80% of the calories consumed by humans. Starch is also known as hydrocolloid homopolymer of D-Glucopyranose.

Fig. 2 Chemical representation of the most utilized biodegradable materials, including (a) PLA, (b) PHAs, (c) PHB, (d) cellulose, (e) starch, and (f) PCL. (Adapted with permission from Ref. [44], Copyright 2020, Elsevier)

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The monomeric units of D-Glucopyranose can attach with its other subunits in two ways (i.e., alpha (1!4) or alpha (1!6) glycosidic linkages), leading to the formation of amylose (linear crystalline biopolymer) and amylopectin (branched amorphous biopolymer), respectively. Depending upon the source of extraction, the content of amylopectin and amylose in the overall content of the starch usually ranges from 80% to 90% and 10% to 20%, respectively [45]. Starch can be extracted from various sources such as rice, corn, potato, cassava, corn, etc. These are then utilized to prepare biopolymers. However, starch preference as an individual material is not recommended for food packaging applications because starch is thermoplastic in nature (i.e., variation in temperature changes its properties drastically) [46]. The starch is highly sensitive to the content of moisture/humidity and undergoes destruction quite rapidly via hydrolytic reactions. This reduces its practical viability as a food packaging material. Consequently, starch is utilized in blends for food packaging applications. Conventionally, the blend is prepared by intermixing the starch with other biopolymers in specific ratios, but in a new advanced approach nanomaterials (NMs, substances having one of its dimensions in the nano range) are also included during the formation of blends [47]. Guaras et al. [48] reported significant observation in the case of the blending approach usually utilized for forming blended films for food packaging applications. The authors observed that the blending components’ nature (polar/nonpolar) should match to form a blend. Otherwise, the blend will not form as the opposing components will not adhere properly to one another. Consequently, the phase separation will be observed in the medium. The authors observed this scenario in the case of blending thermoplastic starch (TPS) with PCL and indicated that adding an adhesion agent of maleic anhydride with PCL is essential for blending with the TPS. They documented that the films synthesized by 15 wt% addition of PCL into the TPS matrix exhibited the desirable properties (enhanced mechanical stability and relatively low water absorption potential) for food packaging applications. Apart from PCL, adding the green extracts (containing polyphenols, proteins, carbohydrates, chlorophyll, carotenoids, etc.) has also been documented to improve the mechanical properties of TPS acquired from cassava starch [49]. The addition of these extracts introduced an additional coating to the TPS film and enhanced the mechanical strength of the films for food packaging application. The synthesized film was able to withstand temperatures up to 240  C without altering any of the textural characteristics of the film. Peighambardoust et al. [50] also utilized a similar approach and validated that the blend of the active starch with the respective silver (Ag), zinc oxide (ZnO), copper oxide (CuO) nanoparticles (NPs) is an effective way of developing the efficient, biodegradable films for food packaging applications. The scanning electron microscopy (SEM) micrographs of these films are presented in Fig. 3. The morphological analysis indicated that the pure starch has a smooth and homogenous surface, as indicated in Fig. 3a. The introduction of the NPs in the starch imparted roughness to the starch matrix as indicated in Fig. 3b–e. Silva-Pereria et al. [45] took the blending mechanism one step further and prepared such smart biopolymeric film that could indicate the food condition by utilizing the variation in the color of the synthesized

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Fig. 3 SEM micrographs of (a) pure starch biofilm, (b) ZnO/starch blend film, (c) CuO/starch blend film, (d) Ag/starch blend film, and (e) ZnO/CuO/Ag/starch blend film. (Adapted with permission from Ref. [50] Copyright 2019, Elsevier)

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film. The corn starch/CS/red cabbages extract-based film contained pH-responsive sites capable of detecting the pH change in the stored food. The authors tested the spoilage of fish and observed that the initial spoilage of fish (16 h of storage in the prepared film) was indicated by the color change of transparent film into blue. The complete spoilage of fish after 72 h was indicated by the color change of film from blue to yellow. Thus, the synthesized films could be utilized as food packaging material and can also be used as a visual indicator to access the condition of the stored food in the film. Dash et al. [51] utilized NMs as the third component of the blend and synthesized starch/chitin/titanium dioxide (TiO2) NPs biofilms for food packaging applications. Apart from these blended films’ high mechanical strength and low water absorption capacity, the films containing TiO2 NPs also exhibited ultraviolet (UV) prevention capacity. The addition of the TiO2 NPs in the starch matrix also enhanced the thermal stability of the blends as the glass transition temperature (Tg) of the blended film increased compared to its pure counterpart. Castillo et al. [52] synthesized the blend of TPS (from corn)/CS oligomer by utilizing the process of melt-mixing and hot pressing. The acquired biofilm is presented in Fig. 4.

Cellulose Cellulose is another well-known polysaccharide, besides starch, that is utilized to develop biodegradable films for food packaging applications. This tasteless and odorless crystalline powder is characterized by the lack of the Tg, which indicates it is a hundred percent crystalline material. It is represented by the chemical formula (C6H10O5)n and possesses a linear structure with high molecular weight values (i.e., almost 100–1000) repeating monomeric units combined by beta (1 ! 4) linkages. Excellent compressive strength, tensile strength, and cross-bonding structure make cellulose an excellent reinforcing material for biodegradable films for food packaging applications [53]. Cellulose, particularly nanocellulose, has been documented extensively in the academic literature due to its enhanced

Fig. 4 Preparation and utilization of TPS/CS oligomer blend biofilm for food packaging applications. (Adapted with permission from Ref. [52], Copyright 2017, Elsevier)

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morphological features. Nanocellulose possesses various morphologies, including spheres, needle-shaped, rod-shaped, fibrous, or ribbon-like morphology, etc. [54]. The main characteristics of nanocellulose are its exceptionally large surface area values (for fibrous nanocellulose: several hundreds of m2 per gram), lower density (approximately 1.566 g/cm3), and high elastic modulus values of approximately 150 GPa [55]. These characteristics made them excellent materials for food packaging applications as not only is the formation of films easy with such high elastic modulus, but the fibrous nature provides the much-needed mechanical strength for these applications. El Achaby et al. [56] utilized cellulose nanocrystals (CNCs) as a reinforcing tool for the blend of polyvinyl alcohol (PVA)/carboxymethyl cellulose (CMC) to develop biodegradable films by the casting methodology. Remarkable results were acquired by incorporating 5 wt% CNCs in the medium. The water vapor permeability coefficient (WVPC) was observed to be reduced to 87% with the addition of CNC, while the properties of elastic modulus (EM) and tensile strength (TS) were found to be increased by 141% and 83%, respectively. Additionally, the incorporation of the CNCs in the films does not affect the transparency level of the pure PVA/CMC films. Authors attributed these enhanced results to the capability to develop nanoscale linkages of CNCs with the PVA/CMC matrix. Mostafa et al. [57] reported novel cellulose acetate biofiber acquired for flax fibers and cotton linters in the respective yields of 81% and 54%. The acquired biofiber was quite resistant to acidic and basic environments. The comparative tests performed by the authors revealed that this novel bioplastic could be applied in both the food industry and the medical field as an alternative to synthetic plastics. Huq et al. [55] synthesized CNCs reinforced alginate films by utilizing the casting technique. The CNCs concentration was varied from 1 wt% to 8 wt%, and the best results were acquired with the addition of 5 wt% of CNCs in the alginate films. The reduction of 31% in WVPC values and the increment of 37% in the tensile strength of the films indicated that CNCs reinforced films were better adapted to handle the extreme external conditions than non-reinforced alginate films. Azeredo et al. [58] presented an extensive study highlighting that bacterial cellulose can be utilized to replace synthetic plastics. However, the authors also acknowledged that modulating the process variables associated with the extraction of bacterial cellulose and processing cellulose into films is still a difficult task requiring further research before implementation at the commercial scale. He et al. [59] took the blending approach and reinforced the CMC/CNCs films with the Ag NPs to enhance its durability. The synthesized materials were primarily characterized by the techniques of ultraviolet-visible spectroscopy (UV-VIS), transmission electron microscopy (TEM), and X-ray diffraction analysis (XRD). The addition of Ag NPs resulted in a 45.4% decrease in WVPC, 1.26 times increment in TS, and a 93.3% reduction in air permeability properties. While investigating its food packaging applications, it was observed that novel films increased the shelf-life of stored strawberries by 7 days. The enhancement in properties of the textile strength and elongation at break with the addition of Ag NPs in the CMC/CNCs matrix is presented in Fig. 5.

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Fig. 5 Effect of adding different amounts of Ag NPs in the CMC/CNCs matrix on (a) thickness of biofilms and (b) elongation at break values. (Adapted with permission from Ref. [59], Copyright 2020, Elsevier)

Protein-Based Biopolymers Proteins such as gelatin, keratin, and casein possess unique properties (including moderate tensile modulus, moderate shear strength, high toughness/strength, and high elasticity) that make these materials significant for food packaging applications. Numerous biodegradable films based on protein polymers such as milk proteins, gelatin, corn protein, soy protein, wheat gluten, egg white protein, etc., have been documented in the academic literature. Apart from the food packaging applications, the biodegradable polymeric films generated by these proteins-based biopolymers are also edible [60]. Among the edible proteins, the plant proteins (particularly wheat, corn, and soybean) are the most commercially viable materials for food packaging applications owing to their enhanced mechanical and barrier properties [61]. Liu et al. [62] utilized soybean polysaccharide/gelatin blends to prepare edible and heat sealable food packaging material. The films were synthesized by using casting methodology in the presence of a plasticizer (glycerol). The acquired results indicated that the stretch-ability, fracture resistance properties, and heat sealability were greatly improved via the formation of blend in comparison to the individual films. The packaging test also reveals that these materials possess excellent potential for developing edible food packaging applications. Galus et al. [63] performed a detailed study on the whey proteins and investigated the moister sorption optical and mechanical properties under different conditions for food packaging applications. The findings of this study can be useful for the development of edible food packaging materials from whey proteins. Silva et al. [64] engineered hybrid sorubium (Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum) protein isolate (HSPI) biofilm incorporated with the clove essential oil (CEO) and the plasticizer of glycerol. Different properties were observed in the case of the numerous films acquired using different combinations of these three components. Best permeation and mechanical results desirable for

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food packaging applications were recorded in the case of the following combination: 2.5 g HSPI, 0.5 g CEO, and 25% glycerol solution. A blend of whey protein concentrate (WPC), corn oil, and TiO2 NPs were also documented in the academic literature to develop edible food packaging material [65]. The acquired films were investigated for WVPC, color, mechanical/tensile characteristics, and structure. It was found that the biofilm containing 60 nm TiO2 NMs with the highest concentration (7.5 wt%) of WPC exhibited the highest elastic modulus (19.2 MPa), Young modulus (19.4 MPa), and elongation at break (119%) values. All the biofilms synthesized exhibited a close correlation of the film morphology/structure with the mechanical/tensile characteristics. Figure 6 presents the CS/gelatin biofilms modified with the Ag NPs synthesized by Kumar et al. [66] for food packaging applications. The shelf-life of the red grapes stored in these biofilms was enhanced up to 2 weeks due to these synthesized films’ excellent permeability properties.

Polyesters/PHAs-Based Biopolymers For the biopolymers acquired directly from the microbes, the family of PHAs is the largest and most well-known family of bacterial polyesters utilized for food packaging applications. Naturally, the PHAs are generated by the bacterial fermentation of lipids and sugars carried out under extreme/unbalanced growth environments/ conditions [67]. Structurally, PHAs are composed of 3-hydroxy fatty acids monomeric units combined to generate large polymeric structures. According to an estimate, more than 100 copolymers or monomeric units can be developed from the conventional PHAs. The most common example of biopolymer of PHAs is the PHB, which is naturally occurring liner beta-hydroxyacid polyester. In terms of commercial significance, the properties of the PHAs are rated quite high for biodegradable food packaging applications. For instance, a US registered company “Metabolix” have released a commercial product of “Metabolix PHA.” which is a blend of PHB and poly (3-hydroxyoctanoate) and is Food and Drug Administration

Fig. 6 The CS/gelatin biofilms modified with the Ag NPs. (Adapted with permission from Ref. [66], Copyright 2018, Elsevier)

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(FDA) approved as the biodegradable packaging material [23]. Consequently, the class of PHAs is usually recommended to develop biodegradable food packaging applications. Cherpinski et al. [69] utilized the family of PHAs, including PHB and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), synthesized by using an electrospun methodology, as a reinforcing material for strengthening the properties of cellulose nanofibrils (CNFs) and lignocellulose nanofibrils (LCNFs) biofilms. The reinforced biofilms exhibited enhanced water contact resistance, higher permeation barriers against water vapors, and exceptionally high mechanical properties. However, one such disadvantage of these PHAs coated biofilms was a lower aroma barrier compared to non-reinforced biofilms owing to the intrinsic affinity of the PHB and PHBV polymers for the uptake of limonene. This area should be improved to utilize PHAs for food packaging applications further. Fabra et al. [70] added adhesive materials including zein, zein blends, pullulan, and whey protein isolate (WPI) into the PHAs biofilms, particularly PHBV, for improving its mechanical and permeability values. The WVPC and oxygen permeability coefficient (OPC) were improved in 38–48% and 28–35%, respectively, with these materials in the PHAs matrix. Furthermore, it was observed that the morphology of the PHAs matrix was influenced by the addition of adhesive materials as well, i.e., pullulan and zein addition resulted in the appearance of fibrous morphology in the PHAs matrix while WPI addition resulted in the formation of beaded or spherical morphology. Figure 7 represents the digital photographs of PHBV/natural rubber (NR) blend biofilm used for food packaging applications.

Fig. 7 The PBHV/NR-based blend biofilms contain tomato sauce. (Adapted with permission from Ref. [68], Copyright 2019, Elsevier)

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Chemically Synthesized Biopolymers Polylactic Acid PLA is the polymer belonging to the class of aliphatic polyesters synthesized by co-polymerization of poly (L-lactic acid) and poly (D-lactic acid). The chemical fermentation process is performed on renewable sources (starch or sugar feedstock) to form PLA. The PLA is a highly transparent polymer, and changing the enantiomers ratio of poly (L-lactic acid) and poly (D-lactic acid) will vary its properties as well [71]. PLA is a highly brittle polymer and is not utilized individually to prepare biofilms for food packaging applications. Blends of PLA with materials having strong mechanical properties are made for the proper utilization of PLA in the packaging industry [39]. Trifol et al. [72] reinforced the pure PLA matrix with the CNC, CNFs, and cloisite30B (nanoclay) to improve the properties of PLA. The addition of the nanocellulose and nanoclay caused a 90% reduction in the oxygen transmission rate (OTR), while the water vapor transmission rate (WVTR) was reduced to 76%. Moreover, transparency of the PLA biofilms was still maintained with the addition of these reinforcing materials, while crystallization kinetics and thermomechanical properties were also greatly improved by this blending approach. Mahmoodi et al. [39] first synthesized a multifunctional filler composed of dyeclay hybrid nanopigment (DNCP) and then incorporated this filler into the PLA matrix to fabricate colored biodegradable film for food packaging applications. The incorporation of DNCP into the PLA matrix was achieved by solution casting methodology. Compared to the pure PLA matrix, the modified matrix exhibited an improvement of 12  C in Tg values while the storage modulus improved up to 20%. Similarly, a significant reduction from 54% and 36% was observed in the case of gas and water vapor permeability values with the addition of DNCP in the PLA matrix. The comparison was also drawn between the DNCP reinforced PLA matrix and nanoclay reinforced PLA matrix for the food packaging applications, where DNCP reinforced PLA matrix exhibited better thermomechanical and optical properties than other counterparts. Swaroop et al. [73] engineered the PLA biopolymer films by utilizing solvent casting methodology and further reinforced the PLA biofilms with the magnesium oxide (MgO) NPs for food packaging applications, as presented in Fig. 8. The PLA biofilm reinforced with 2 wt% MgO NPs exhibited the best results with a 29% improvement in tensile strength values and 25% improvement in oxygen barrier characteristics, respectively. The presence of MgO NPs further improves the

Fig. 8 The digital photographs of the (P0) pure PLA biofilm, (P1) 1 wt% MgO NPs/PLA biofilm, (P2) 2 wt% MgO NPs/PLA biofilm, and (P4) 4 wt% MgO NPs/PLA biofilms. (Adapted with permission from Ref. [73], Copyright 2018, Elsevier)

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antibacterial potential of the film and 46% lysis was observed in the case of the Escherichia coli culture after the interaction time of 12 h. Moreover, the synthesized biofilms were capable of performing UV screening, making these films extremely useful for improving the shelf-life of stored food.

Polycaprolactone PCL is another biodegradable polymer synthesized by utilizing chemical processes implemented on the biodegradable biomass. Generally, PCL is prepared by the ringopening polymerization methodology of caprolactone performed in stannous octoate acting as a catalyst. The physical properties, including high density (1.145 g/cm3) and high Tg (60  C), make biodegrading difficult to biodegrade, and mostly, extreme conditions are required to biodegrade this material into useful smaller compounds completely. Jeong et al. [74] incorporated 10–40 wt% sodium metabisulfite (SM) as an active ingredient for improving the properties of PCL matrix. Statistical analysis performed that significant (p90%) [92]. Soy flour, soy concentrate, and soy isolate are common soy proteins. The polymer made from soy protein has been reported to have good biodegradability but poor tensile strength and flexibility. As a result, this technique frequently employs plasticizers and reinforcing fillers. To improve the overall characteristics of the protein matrix, fillers such as nanoclay, chitin whisker, and lignin have been used. Chen et al., for example, made SPI/hydroxypropyl alkaline lignin (HPL) nanocomposites by combining the two materials in an aqueous solution with a minor quantity of glutaraldehyde as a compatibilizer [93]. HPL was well disseminated in the SPI matrix, according to the morphological examination. HPL domains appeared in SPI/HPL composites with diameters of around 50 nm when the HPL concentration was less than 6%. Furthermore, a significant increase in tensile strength was attained. Guo et al. conducted another research in which they used montmorillonite (MMT) as a layered silicate clay combined with soy protein isolate (SPI)/polyvinyl alcohol (PVA) to improve mechanical and water barrier qualities [94]. Melt processing created the SPI/PVA/ MMT composite films. Depending on the MMT content, the characteristics of the produced composite changed. The composites’ tensile strength and Young’s modulus rose as the MMT was increased. Due to the extremely disordered MMT in the matrix, the composites’ water sensitivity decreased, while their thermal stability improved. As a result, the nanocomposite films could be used as biodegradable materials in green packaging films. Soy protein isolate can be used as a cancer treatment following surgery, radiotherapy, chemotherapy, and biological therapy due to its eco-friendliness and effectiveness. Jiang et al. used a green process to make soy protein isolate/rGO (SPI/rGO) nanocomposites [95]. The SPI/rGO nanocomposites dispersed well in water and had acceptable biocompatibility and a great photothermal capability, killing HeLa cells efficiently with near-infrared irradiation, indicating that they might be used in photothermal therapy. For a flexible solidstate supercapacitor, Xun et al. produced a gel polymer electrolyte based on SPI and hydroxyethyl cellulose (HEC) with 1.0 mol/L Li2SO4 for energy storage devices [96]. With a capacitance of 91.79 F/g and an energy density of 7.17 W/kg at a current density of 5.0 A/g, the constructed solid-state supercapacitor demonstrated good electrochemical performance. Furthermore, it demonstrated superior flexibility under bending conditions to liquid supercapacitors, as well as similar electrochemical performance at various bending angles with high stability, as evidenced by the retention of about 100% cycling and a coulombic efficiency for over 5000 chargedischarge cycles. Jin et al. did similar work, incorporating GO into hyperbranched frameworks for the production of flexible protein-based films using Fe(III) ion-triggered simultaneous polymerization of dopamine and pyrrole [97]. The findings validated the network’s strong sacrificial metal-ligand linkages and covalent

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bonding, with toughness and strength increasing by 428.5% and 324.1%, respectively. Furthermore, as compared to pure materials, the produced film showed a significant improvement in electrical and thermal conductivities. Furthermore, the films developed demonstrated exceptional water resistance and UV-barrier efficacy.

Conclusions The growing desire for biodegradable materials in nearly every part of our lives has been the driving force behind the development of green, eco-friendly, and sustainable final goods with promising features compared to routinely used materials in recent decades. This is mostly due to their improved performance and cheaper cost when compared to conventional polymers, which remains a major obstacle for bio-based polymers. However, due to their poor mechanical qualities and excessive water solubility, pristine biodegradable polymers cannot replace synthetic polymers in real-world applications. Inorganic nanomaterials such as carbon nanotubes, graphene, nanoclays, and 2D layered materials were mixed with bio-based polymers and exhibited promising mechanical, electrical, and thermal properties. Our chapter presents the impact of introducing such nanoparticles as additives into biodegradable polymers for various applications, including food backing, energy conversion devices, and tissue engineering.

Further Perspectives The primary goal of this chapter is to present an overview of present research on the physical, thermal, rheological, and mechanical properties of biodegradableinorganic composites. The presented data showed promising performance in the overall properties of the prepared composites by incorporating various nanoparticles, thus being used to replace petroleum-based polymers in various applications. However, several disadvantages hinder their widely commercialized applications. Some of the new bio-based polymers have the drawback of being incompatible with conventional processing equipment. Moreover, the additive market for bio-based polymers is limited, making large development efforts difficult to justify. Therefore, much work should be devoted to overcoming this limitation to help the world become more sustainable.

Cross-References ▶ Biodegradable Materials: Fundamentals, Importance, and Impacts ▶ Biodegradation of Industrial Materials ▶ Biodegradation Process: Basics, Factors Affecting, and Industrial Applications ▶ Fundamentals of Biodegradation Process

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91. Karthika V, AlSalhi MS, Devanesan S, Gopinath K, Arumugam A, and Govindarajan M (2020) Chitosan overlaid Fe3O4/rGO nanocomposite for targeted drug delivery, imaging, and biomedical applications. Sci Rep 10(1):18912 92. Friedman M and Brandon DL (2001) Nutritional and Health Benefits of Soy Proteins. Journal of Agricultural and Food Chemistry 49(3):1069–1086 93. Chen P, Zhang L, Peng S, and Liao B (2006) Effects of nanoscale hydroxypropyl lignin on properties of soy protein plastics. Journal of Applied Polymer Science 101(1):334–341 94. Guo G, Tian H, and Wu Q (2019) Nanoclay incorporation into soy protein/polyvinyl alcohol blends to enhance the mechanical and barrier properties. Polymer Composites 40(9):3768–3776 95. Xuejiao J, Zhao L, Jinrong Y, Zhengzhong S, and Xin C (2016) One-step synthesis of soy protein/graphene nanocomposites and their application in photothermal therapy. Materials Science and Engineering: C 68:798–804 96. Xun Z, Ni S, Gao Z, Zhang Y, Gu J, and Huo P (2019) Construction of Polymer Electrolyte Based on Soybean Protein Isolate and Hydroxyethyl Cellulose for a Flexible Solid-State Supercapacitor. Polymers 11(11):1895 97. Shicun J, Kuang L, Qiang G, Wei Z, Hui C, Sheldon QS, and Jianzhang L (2020) Assembly of graphene oxide into the hyperbranched frameworks for the fabrication of flexible protein-based films with enhanced conductivities. Composites Part B: Engineering 196:108110

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact of Surfactants on the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Surfactants in the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Surfactant Biodegradations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anionic Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cationic Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-ionic Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amphoteric Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosurfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Surfactant Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ω-Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . β-Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzene Ring Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Influencing Surfactant Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixture Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Assessment of the Biodegradability of Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Although surfactants are being used extensively in detergents, food, pharmaceuticals, and cosmetics and in industries and various other sectors due to their diverse functions, they also play a critical role in environmental contamination. W. H. Danial (*) Department of Chemistry, Kulliyyah of Science, International Islamic University Malaysia, Kuantan, Pahang, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_26

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For this reason, efforts must be made to address the biodegradation of surfactants in the environment. This chapter presents an overview of the degradation of surfactants and discusses in detail the impact of surfactants on the environment and the types and mechanisms of surfactant biodegradation. The factors influencing the biodegradation of surfactants are briefly discussed, while an assessment of the biodegradability of surfactants is also presented. Keywords

Surfactant · Biodegradation · Environment · Mechanism · Biodegradability Abbreviations

AE AEO AES AOPs APEO AS BOD CO2 COD DO DOC H2O2 LAS NPEOn O3 OPEOn QACs SDS SPC SPE

Fatty acid polyoxyethylene ester Aliphatic alcohol polyoxyethylene ether Alcohol ether sulphate Advanced oxidation processes Alkyl phenol polyoxyethylene ether Alkyl sulfonate Biological oxygen demand Carbon dioxide Chemical oxygen demand Dissolved oxygen Dissolved Organic Carbon Hydrogen peroxide Linear alkylbenzene sulfonate Nonylphenol polyoxyethylene ether Ozone Octylphenol polyoxyethylene ether Quaternary ammonium compounds Sodium dodecyl sulphate Sulfophenyl carboxylic acids Solid-phase extraction

Introduction Surfactants, also known as surface-active agents, are chemical compounds that reduce the surface tension (also known as interfacial tension) between two liquids, a gas and a liquid or a solid. They also exhibit solubilization, dispersion, and emulsification properties. Surfactants are a structurally varied collection of chemical substances that are extensively employed in daily life and can be found in cleaning agents, fabric softeners, household detergents, and sanitary items, including toothpaste, shower gels, and shampoos [1]. They are also widely used as emulsifiers, dispersants, wetting agents, and foaming agents in various industries, such as in the food, pharmaceutical, paint, polymer, textile, and agricultural industries [2]. Surfactants are divided into four

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categories based on their molecular structure: anionic surfactants, cationic surfactants, non-ionic surfactants, and amphoteric surfactants. However, with the introduction of numerous adaptable and unique molecular designs of the aforementioned primary types, modern surfactant molecules have been further sub-classified into other varieties, such as Gemini and polymeric surfactants [3]. Aside from the main types stated above, there are certain particular categories of surfactants, such as stimuli-responsive surfactants and hybrid surfactants, that have unique and different physicochemical features compared to conventional surfactants [4]. Petrochemical feedstock or a mix of renewable and petrochemical feedstock is often used to make conventional surfactants. However, the present trend indicates that consumers are becoming more aware of and enthusiastic about surfactants made from renewable raw sources [5]. As one of the most commonly used chemical products, surfactants must also be produced sustainably to decrease their carbon footprint and reliance on petrochemicals. The growing need for different kinds of surfactants in a wide range of applications demands that new sustainable surfactant molecules be developed based on renewable building blocks [6]. Figure 1 depicts an ideal sustainable surfactant production cycle in which chemicals obtained from natural sources are used to produce new surfactant molecules. The worldwide market for surfactants is now valued at around 42 billion US dollars and is expected to grow significantly over the coming years due to the global rise in the manufacture and usage of hand sanitizers in response to the COVID-19 pandemic [7]. In addition, with their different compositions, surfactants have a

Fig. 1 Ideal cycle for the development of sustainable surfactants. (Figure adapted with permission from Ref. [6] (2020, Elsevier))

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variety of auxiliary functions, including anticorrosion, antimicrobial, lubricating, and tackifying properties. Non-ionic surfactants are now the most common type, accounting for over 40% of the entire market share, with demand steadily increasing in recent years [8]. Anionic surfactants are the next most extensively used surfactants due to their varied uses and low manufacturing cost. Cationic surfactants are more costly and, as a result, have a lesser market share. Surfactants of biological origin, or biosurfactants, have gained popularity in recent decades owing to their environmental friendliness [9]. However, their physicochemical behavior is comparable to that of their synthetic counterparts [10]. Due to their widespread use, high concentrations of surfactants in largely urban, industrial, and household wastewater might end up in municipal wastewater treatment facilities or are immediately discharged into the environment [11]. Surfactant removal has been critical in wastewater treatment for many sectors, including cosmetic, homecare, pharmaceutical, and paper industries [2]. Thus, the degradation of surfactants in the environment cannot be neglected, in addition to rigorous controls on the utilization of surfactants and regulations concerning the treatment of discharged surfactants. The purpose of surfactant degradation is to alter the structure in response to exterior influences and transform the detrimental structural components of the surfactant into environmentally benign components. Some surfactants are not biodegradable, resulting in relatively substantial residues and partly decomposed compounds in biologically treated effluents. Surfactants may be degraded via oxidation, sonication, photolysis, electrolysis, and biodegradation. During the oxidation process, surfactant-containing wastes are treated with strong oxides, such as ozone (O3), hydrogen peroxide (H2O2), and ferric salt, which cause the surfactants to undergo an oxidation-reduction reaction and decompose into non-toxic and non-harmful compounds [12]. The use of advanced oxidation processes (AOPs), such as photocatalytic degradations and Fenton and Fenton-like reaction processes, to effectively remove surfactants and their degradation intermediates from wastewater has also been investigated [13]. Mineralization routes of organic pollutants to CO2, H2O, and inorganic ions may be provided by AOPs that generate highly reactive oxygen species with poor selectivity, including hydroxyl radicals (OH), superoxide radicals, and O3 [14]. In ultrasonic degradation, surfactants are removed from a waste liquid by ultrasonic cavitation under a strong sound pressure, followed by high-pressure oxidation and high temperature [15]. Ultrasonic degradation may be used to remove surfactants from various waste liquids. As an alternative to the classic photolysis of UV-excited H2O2 oxidation surfactants, the current photolysis method frequently uses the non-toxic and effective semiconductor, TiO2, to perform the transition of electrons under UV light to enable the degradation of surfactants [16]. In the electrolysis approach, the strong oxidizing chemicals created on the surface of the electrode during the electrolysis process are used to mineralize and break down the surfactants in the waste liquid [17]. Given the extensive spread of surfactants in natural water systems, the breakdown of surfactants in the environment is mostly dependent on microorganisms, especially bacteria and algae [18]. Biodegradation is a process that converts surfactants into water and carbon dioxide (CO2), which serves as a source of carbon for the

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metabolism of microbial life. This occurs due to the disintegration of organic molecules in the setting by microbes. The oxidation technique employs chemical substances to carry out the oxidation process, and the resulting product is uncontrolled and prone to consequential contamination. However, the electrolysis and sonication methods have a high degrading effect but use great energy. Although the conditions for the photolysis reaction are moderate and do not result in secondary pollution, the degradation rate is rather slow. Surfactant biodegradation may be classified into three phases, based on the resultant degradation products [19], namely, (1) primary degradation: microbial degradation changes the parental structure of the surfactant, resulting in the loss of its initial surface activation properties; (2) secondary degradation: the degradation products may be thoroughly and quickly decomposed by microbes and thereby prevented from polluting the environment; and (3) end degradation: the surfactant is destroyed, and the end products are water, CO2, mineral salts, and other inorganic compounds.

Impact of Surfactants on the Environment The relevance and importance of surfactants in modern life have resulted in an exponential worldwide production of soaps, detergents, and other cleaning products in recent decades. Because of the widespread usage of surfactants, dangerous or hazardous chemicals have accumulated in the environment, posing major threats to human health and the environment [20]. According to the US Environmental Protection Agency, surfactants exhibit endocrine-disrupting properties and thus pose a health risk to animals and humans [21]. Surfactant-rich effluents include important nutrients like nitrogen and phosphorus, which are necessary for the development of organisms, and these components have the potential to disrupt the ecological relationships of aquatic bodies, hence impairing the healthy growth of aquatic biota [22]. These issues are associated with the trophic level of aquatic bodies, pH, dissolved oxygen level, and water surface tension fluctuations. Fish may be asphyxiated and killed by the build-up of water surfactants in their viscera and gills [23]. Surfactants also increase the water solubility of persistent organic pollutants, and the resulting aerosol and surfactant products thus have a substantial influence on the environment and climate [24]. The physiological and metabolic processes of aquatic species are affected by surfactants, which retard metabolism and development, damage the cell membrane, and disrupt the chlorophyll protein complex [25]. Among the most serious adverse effects of surfactants is their impact on aquatic gas exchange processes, resulting in a decrease in the development rate of unicellular photosynthetic organisms and the suppression of the chemoreceptor organs larvae and other aquatic life [26]. Surfactants in the soil are harmful to microorganisms, cause root damage, and hinder photosynthesis in plants [27]. Surfactants are also toxic to humans and may lead to skin and mouth ulcers and other ailments [28]. Consuming or drinking food contaminated with certain surfactants might pose serious health risks to humans. The liver may react to the surfactants, leading to long-term metabolic consequences and disturbances in the human endocrine

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system [29]. Besides, several surfactants have been linked to burning or irritation of the skin and eye and respiratory issues in humans [30]. As a result of these far-reaching consequences, high concentrations of surfactants have given rise to concerns about public health and the environment. Therefore, the environmental degradation of surfactants, the preservation of the ecological balance, and the safeguarding of human health have all become critical issues in the manufacture and use of surfactants. Despite surfactants being the major components in municipal and industrial effluents [31], they are also deliberately released into the environment in large quantities to aid in the clean-up of oil spills [32]. For instance, nearly 7 million liters of surfactants were used during the Deepwater Horizon oil leak in 2010 [33]. Therefore, surfactants are among the most significant environmental contaminants due to their widespread release into the environment [34]. However, they often demonstrate strong biological activity, such as the interaction of cellular membranes and the disruption of biochemical processes, and thus, they have a deleterious impact on living creatures [35]. Surfactants may inhibit microbial development and promote microbial mutation and mortality [36]. Nonylphenol ethoxylates, for example, may decouple energy generation, thereby impairing microbial growth and nitrification processes. In addition, surfactants also affect microbial processes, such as environmental resistance, competition, and reproduction [7]. Microorganisms may depolarize their membranes by ingesting surfactants. This will reduce their ability to collect nutrients, oxygen uptake, and the emission of harmful metabolites [7]. Due to these consequences, it is necessary to continuously monitor the concentration of surfactants and reduce the quantity of surfactants remaining in the environment to the lowest possible level by inventing effective removal methods and employing biodegradable initiatives [8, 37]. Surfactants, which are the most commonly used chemical commodity, must not only be produced using a renewable and sustainable strategy but also be intelligently constructed so that they may be quickly destroyed and degraded in the environment after usage. The usage of biodegradable and environmentally friendly surfactants must be considered. Utilizing renewable building blocks for the synthesis of surfactants also aids in lowering CO2 emissions since after degradation, renewable surfactants only return the quantity of carbon to the environment that plants previously used for making renewable building blocks. However, the use of petrochemical feedstocks for the manufacture of surfactants ensures the emission of carbon, which contributes to the upsurge in greenhouse gases [6]. While the C12 to C18 hydrocarbons found in tropical oils, such as palm kernel and coconut oil, are often touted as renewable building blocks for the production of surfactants, these tropical plants are grown at the expense of natural rain forests and their wild populations [7]. Several effects of surfactants on the environment are summarized in Fig. 2.

Analysis of Surfactants in the Environment Human health and the safety of the environment are at risk owing to the massive release of surfactants into the environment. Due to the environmental hazards presented by surfactants and their degradation products, it is essential that they be

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Fig. 2 The effects of surfactants on the environment

analyzed and quantified in the environment. Thus, a variety of methods can be used to separate, identify, and measure surfactants in an environmental matrix. However, the analysis of surfactants in the laboratory may prove to be a challenging process because surfactants are often found in mixes, including a variety of components, such as homologs, isomers, and other contaminants [38]. Surfactants are also often present in trace levels that are generally below the detection limits of analytical equipment. The analysis of surfactants is further complicated and hampered by the complexity and diversity of the matrices in which they naturally exist. Owing to this, the analysis of surfactants in an aqueous system might require modern advanced technology or instrumentations [39]. One strategy for overcoming the problems above concerning a practical analysis of surfactants is to conduct an extraction before the analysis, while sample purification and preconcentration are also critical procedures. Comprehensive research and evaluations have been carried out in surfactant extraction, isolation, and preconcentration [40]. According to the literature review, solid-phase extraction (SPE) is considered the most adaptable pretreatment procedure due to its multiple benefits [41]. Surfactants are often detected using various methods, including chromatography, spectrophotometry, titrimetry, and potentiometry [42]. The integrity and effectiveness of assessment methodologies

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are mostly determined by the amount of environmental pollution and the efficiency of remediation technology [7]. The spectrophotometric technique might be sufficient when the surfactant is present as a single component. When the surfactant is a component of an environmental matrix, a precipitating (such as titrimetry) or separating approach (such as chromatography) would be needed instead [7]. Analysis by separation or chromatography is the most commonly used technique to examine various surfactants due to their ability to distinguish between particular types of surfactants. Surfactant blends in a wide range of environmental matrices can also be detected using the chromatography technique to identify each isomer or homolog in the surfactant mixture. Supercritical fluid chromatography is one of the most costeffective, time-saving, and effective techniques available for assessing a wide variety of surfactants. When combined with high-performance liquid chromatography or a mass spectrometry detector, this application can further enhance the qualitative and quantitative analysis of a wide range of surfactants in the environment [43].

Types of Surfactant Biodegradations Surfactants have a distinct molecular structure, rendering them amphiphilic, with polar groups (hydrophilic) on one end and non-polar (hydrophobic) on the other. Based on the dissociation characteristics of polar groups, surfactants may be divided into anionic, cationic, non-ionic, and amphoteric surfactants. The biodegradability of surfactants varies according to the type of surfactant, although the biodegradability of the surfactant is primarily governed by its hydrophobic group.

Anionic Surfactants Anionic surfactants are the first and most developed among all the surfactants, and they constitute the most prominent surfactants. Anionic surfactants or negatively charged hydrophilic polar groups or ions are the most widely used surfactants in the food, detergent, cosmetics, and pharmaceutical industries, to name a few. The hydrophilic groups can be divided into five types: phosphate ester, fatty acid salt, sulfate salt, sulfonate, and carboxylic acid salt [50]. Alkyl sulfonate (AS), linear alkylbenzene sulfonate (LAS) , and alcohol ether sulfate (AES) are among the common examples of anionic surfactants [44]. Alkyl sulfonate (AS) is rapidly biodegradable, whereby the rate of primary breakdown may reach over 95% in a relatively short period [45]. The biodegradation of alkyl sulfonate can be achieved via the oxidation of aliphatic alcohols and de-vulcanization of sulfate esterase [46]. One of the most prevalent alkyl sulfonate anionic surfactants is sodium dodecyl sulfate (SDS), which has a strong foaming capacity and the capability to be fully degraded by microorganisms. SDS has also been used as a material for electrolytes in electrochemical processing [47, 48]. Figure 3 shows the molecular structure of SDS. The biodegradation of SDS may be accomplished by organisms such Pseudomonas putida R1, and Acinetobacter calcoaceticus [49]. Another type of common

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Fig. 3 Molecular structure of sodium dodecyl sulfate

anionic surfactant is linear alkylbenzene sulfonate (LAS), which accounts for 25–30% of synthetic surfactants and is composed of various isomers or homologous components [50]. The primary laundry surfactant used globally is LAS, a biodegradable yet inhibitory chemical compound. Under anaerobic circumstances, this anionic surfactant has been observed to be recalcitrant. Depending on the individual substrates and microbial species involved, the addition of readily degradable carbon sources may improve the biodegradation of persistent chemical compounds through co-metabolism. Khleifat [51] and Abboud [52] evaluated the effects of several carbon sources (glucose, mannitol, sucrose, maltose, and succinate) on surfactant biodegradation by facultative anaerobic cultures and discovered that glucose and sucrose increase the elimination of LAS. Another option is to use simpler co-substrates, such as short-chain alcohols like ethanol and methanol, which are low cost and easily degradable carbon sources [53]. However, the relevance of organic co-substrates in the degradation of LAS under anaerobic circumstances has yet to be consistently confirmed in the literature. LAS has low toxicity and is prone to biodegradation, with a final degradation rate of 80% [54]. In well-operating activated sludge wastewater treatment systems, LAS is rapidly biodegradable and can be substantially eliminated with total removal of 95–99.9% [55]. The molecular structure of LAS influences its degradation rate, depending on the position of the benzene ring, which becomes more unstable and vulnerable to degradation the further it is from the core carbon atom [45]. The primary breakdown of LAS begins with the ω-oxidation of the alkyl chain, followed by the β-oxidation that produces sulfophenyl carboxylic (SPC) acids as intermediates, and the benzene ring is finally cleaved by microorganisms to generate CO2, H2O, and SO42 [55]. Although the degradation of benzene by microorganism can be challenging, it is important for controlling the speed of biodegradation. Pseudomonas nitroreducens, Pseudomonas aeruginosa, Pantoea agglomerans, and Serratia odorifera are the strains that are capable of degrading LAS [51]. Since various strains may impart different degradation rates, the conditions, time, and selection of the best strain or its optimum combination need to be identified to achieve the best degradation method.

Cationic Surfactants Cationic surfactants, which can dissolve positively charged ions in solutions, are mostly utilized as industrial softeners, fungicides, and antistatic agents. In comparison to anionic surfactants, cationic surfactants are required in lower doses. Alkyl groups form the hydrophobic group of cationic surfactants, whereas groups

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containing nitrogen, phosphorus, and sulfur elements form the hydrophilic group. Several cationic surfactants in use today are made of nitrogenous compounds, such as amines, heterocyclic salts, and quaternary ammonium compounds (QACs). QACs consist of positively charged nitrogen atoms covalently bonded to four functional groups of long and short hydrophobic alkyl chains [56]. Due to their solubility in both acids and bases, QACs are commonly utilized as cationic surfactants. Besides, QACs account for around 10% of the global surfactant industry and are extensively employed in drug delivery, disinfection, fungicides, softeners, and preservatives, among other applications [57]. Cationic surfactants like QACs are also utilized in more specialized applications, including in antimicrobials and, more recently, ion-exchange membranes. After being used, QACs are inevitably dumped into wastewater and then released into the environment, where they might affect aquatic animals, such as fish, daphnia, algae, microbes, and coexisting contaminants [58]. Furthermore, QACs have been shown to have a deleterious influence on removing biological nitrogen and the digestion of sewage sludge [59]. Thus, it is critical to develop an efficient method for removing QACs before their release into surface waters, including surfactant biodegradation. Biodegradation techniques are generally regarded as effective, ecologically acceptable, and safe decomposing organic contaminants. However, the biodegradation of QACs is rather difficult as it is highly germicidal, positively charged in water, and easily adsorbed in sediment or minerals [60]. Even after biodegradation, treated wastewater may still include chemicals resistant to degradation, and the biodegradation process may take a long time to complete before the desired results are obtained. Therefore, an additional treatment process (e.g., Fenton oxidation) may be needed to achieve a highly efficient removal [61]. The biodegradation of QACs is primarily influenced by the complexity of their molecular structure and composition, concentration, and resistance to microorganisms [56]. Some of the strains responsible for the degradation of QACs are Thalassiosira sp., Xanthomonas sp., Aeromonas sp., and Pseudomonas sp. [60]. The initial step in the biodegradation of QACs is the hydroxylation of the alkyl chain, followed by ω-oxidation and β-oxidation to generate acetyl CoA, which enters the tricarboxylic acid cycle. The second step involves the partial hydroxylation of the alkyl chain, followed by the enzyme-assisted breaking of the C-N bonds, and then β-oxidation to complete the degradation. The third process involves the demethylation of methyl carbon and the hydroxylation of the long-chain carbon. Due to the structural variations between strains and the components of QACs, the three processes may exist independently or coexist in numerous forms [62]. Nonetheless, the data on the relevant, achievable kinetic degradation of QACs is still fairly limited [63]. Figure 4 shows the molecular structure of a quaternary ammonium compound.

Non-ionic Surfactants Non-ionic surfactants have excellent stability and do not dissolve in solutions to form ions, which are the second most widely used surfactants after anionic surfactants. The hydrophilic groups of non-ionic surfactants mostly comprise oxygen-containing

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Fig. 4 Quaternary ammonium compound

Fig. 5 Molecular structure of octylphenol polyethoxylene ether (OPEOn)

groups, such as hydroxyl and ether linkages. Due to the indissociable nature of non-ionic surfactants in solutions, they have high stability. They are unaffected by acids and alkalis, making them ideal for washing, dispersion, foaming, and solubilization, whereby they are being extensively used in the food, medical, textile, and paint industries. Due to their usage in household products, the major disposal option for these surfactants is down the drain. Non-ionic surfactants that are often used include ethoxylates, alkoxylates, cocamides, fatty acid polyoxyethylene ester (AE), fatty alcohol polyoxyethylene ether (AEO), and alkyl phenol polyoxyethylene ether (APEO). Extensive investigations into the environmental impact of these surfactants have shown that they are readily biodegradable in screening tests and quickly mineralize in highertiered activated sludge and river water experiments [64]. APEO is mostly constituted of octylphenol polyoxyethylene ether (OPEOn) and nonylphenol polyoxyethylene ether (NPEOn), and it is frequently employed as an insecticide, emulsifier, and solvent enhancer [65]. Figure 5 shows the molecular structure of OPEOn. The length and intricacy of the alkyl chain, as well as the number of components in the benzene ring and polyoxyethylene chain, are all important factors in the biodegradation of APEO, where the branched-chain structure is more difficult to degrade than the straight-chain structure, as is the number of polyoxyethylene units in the chain. Microorganisms may break down APEO to generate APEO with short polyoxyethylene chains, nonylphenols, octylphenols, and other metabolites. The most dangerous is octylphenols nonylphenols [66]. Endocrine disruptors like APEO metabolites, which are found in the environment, have been linked to a lower sperm count in men, and breast cancer and testicular cancer, among other disorders, in animals [45]. Therefore, complete biodegradation of APEO is required to hinder the impact of surfactants on the environment and microorganisms. Some strains responsible for the degradation of APEO include Moraxella osloensis, Brevibacterium sp., and Pseudomonas sp. [65]. The biodegradation of APEO begins with the breakdown of a single unit in the polyoxyethylene chain, followed by the

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oxidation of the terminal hydroxyl group to carboxylic acid and the removal of a single unit from the polyoxyethylene chain [67]. The degradation mechanism varies, depending on the type of microorganism. The degradation processes may occur through hydroxylation, resulting in the breakage of the alkyl chains and phenols, which are then degraded by microbes [68]. Another mechanism is converting alkylphenol to alkyl catechol by phenol hydroxylase, whereby, subsequently, the aromatic rings are broken by dioxygenase and then biodegraded by the microorganism [66]. However, AEO combines several congeners composed of an alkyl chain comprising 12–18 carbon atoms attached to a polyoxyethylene chain with a variable unit count. It is extensively used in cleaning, decontamination, and care, as well as in other sectors. Due to its low biodegradability and toxicity, APEO has been progressively prohibited in certain locations, thereby boosting the demand for AEO [69]. Pseudomonas stutzeri and Flavobacterium sp. are some of the strains responsible for the biodegradation of AEO [70], which begins with the breakdown of the ether link between the alkyl and polyoxyethylene chains, resulting in the formation of fatty acids and polyethylene glycols, respectively [71]. The metabolites are subsequently biodegraded. Moreover, non-ionic surfactants, such as Tween 20 and Tween-80, may be used as carbon and energy sources by microorganisms, such as Arthrobacter sp. (phylum Actinobacteria). However, these microorganisms might be inhibited in the presence of some anionic or non-ionic surfactants, such as sodium dodecyl sulfate and Tweeen-100, respectively [72].

Amphoteric Surfactants Surfactants with internal independent positive and negative charge centers are known as amphoteric surfactants, while those with amphoteric capabilities independent of pH are known as amphoteric ionic surfactants [73]. Amphoteric surfactants have a milder impact on the skin and eyes than the other three types of surfactants used in everyday items and cosmetics. Amphoteric surfactants are rarely produced wide due to their high cost and complicated manufacturing process [74]. The hydrophilic portion of these surfactants has unique qualities that are difficult to duplicate using traditional non-ionic, anionic, or cationic surfactants [75]. Amino acid and betaine-type amphoteric surfactants and imidazoline surfactants are the most common amphoteric surfactants. Amine-oxide-based surfactants (Fig. 6) offer a variety of useful qualities, including increasing and stabilizing foam in mixtures with other amphoteric or anionic surfactants, thickening, oxidation resistance, and

Fig. 6 Molecular structure of amine-oxide-based surfactant

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skin and hair compatibility [76]. These characteristics enable them to be used in a wide range of industrial, domestic, and cosmetic goods. Detergents, dishwashing solutions, antistatic preparations, shampoos, hair conditioners, and shaving foams include amine-oxide-based surfactants, which have previously been reported as components of specific formulations. Surfactants based on amine oxides are easily biodegradable, with ultimate biodegradation of 70% after 28 days. Furthermore, in most circumstances, the lag phase is minor. Changes in their molecular structure explain the discrepancies in biodegradability of amine-oxidebased surfactants. However, the cationic component of a betaine-type amphoteric surfactant comprises quaternary ammonium salts, which may be classified as carboxylate betaine, sulfobetaine, or phosphate betaine, based on the anions. The structure of betaine-type surfactants is determined by replacing various substituents for the methyl or carboxyl groups of trimethylglycine [67]. Alkyl betaine, cocoamidopropyl betaine, and hydroxysulfobetaine are examples of betaines with a high degree of primary biodegradability [77].

Biosurfactants Biosurfactants are generated by various microorganisms and have structures with varying chemical and surface characteristics [78]. Pseudomonas, Acinetobacter, Bacillus, Brevibacterium, Clostridium, Rhodococcus, Thiobacillus, Leuconostoc, Citrobacter, Candida, Corynebacterium, Penicillium, Ustilago, Aspergillus, Saccharomyces, Enterobacter, and Lactobacillus are all capable of producing biosurfactants ranging in molecular weight from low to high [78]. One of the primary benefits of biosurfactants over chemical detergents is their biodegradability. Biosurfactants are amphiphilic compounds made by living things, mostly on the surfaces of microbial cells or in the extracellular hydrophobic and hydrophilic moieties [79]. This enables them to accumulate between the fluid phases, lowering the surface and interfacial tension at the surface and interface. They have the same mechanism as chemical surfactants for lowering surface and interfacial tension [80]. Biosurfactants are categorized based on their microbiological origin and chemical structure. Glycolipids, phospholipids, polymeric biosurfactants, and lipopeptides are the major biosurfactants (surfactin) classes. Bioemulsifiers are another group that has often employed substitutivity with biosurfactants to represent surface-active biomolecules [81]. Bioemulsifiers have a large molecular weight because they are complex mixtures of protein, heteropolysaccharides, lipopolysaccharides, and lipoproteins [82]. They are also known as exopolysaccharides or high molecular weight biopolymers. Biosurfactants are the most sought-after biotechnological products in economics, profitability, and challenges to chemical surfactants in many industrial settings [83, 84]. Other biosurfactants are sophorolipids produced mostly by yeasts, such as Candida apicola, Candida batistae, Candida bombicola, Pichia anomala, Rhodotorula bogoriensis, Starmerella bombicola, and Wickerhamiella domercqiae [85]. Biosurfactants are biologically produced from various materials, including hydrocarbons, hydrophobic mixes, chemicals, solvents, vegetable oils, waste products, oil wastes, dairy products, and many more [86]. Several

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Fig. 7 Biosurfactants’ strengths, prospects, weaknesses, and threats in different industrial settings. (Adapted with permission from Ref. [84] (2019, Elsevier))

low-cost ingredients produced from industrial setups used in the manufacture of biosurfactants include starch waste, molasses, steep maize liquor, soap stock, and animal fat [86]. Many waste products are discharged into the environment as coproducts, enabling natural populations of bacteria to consume them. Industrial waste products may be used as affordable raw materials or substrates for the large-scale commercial manufacture of biosurfactant compounds, thus making them a viable alternative. The Pseudomonas aeruginosa strain was shown to produce rhamnolipid surfactants from soybean oil waste, as well as cassava flour, molasses, and whey [87]. Researchers have identified various agricultural products, such as rice straw, sugar cane straw, wheat, molasses, bran, rice, sugarcane bagasse, soy hulls, corn, and cassava flour, and their wastewater as candidates for agro-industrial wastes that are excellent substrates for the production of biosurfactants [86]. The strengths, prospects, weaknesses, and threats of biosurfactants in different industrial settings are summarized in Fig. 7.

Mechanism of Surfactant Biodegradation The biodegradation mechanism involves the breakdown of the surfactants through the degradation of the alkyl chain, hydrophilic head groups, and benzene ring by microorganisms [45]. The surfactants are used as a carbon source by microorganisms to

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Fig. 8 Degradation reactions of (a) ω-oxidation of alkyl chain, (b) β-oxidation pathway, and (c) benzene ring oxidation

catalyze the following degradation reactions: ω-oxidation, β-oxidation, and benzene ring oxidation [88]. Figure 8 depicts the degradation reactions for each oxidation.

v-Oxidation The ω-oxidation of the terminal methyl group initiates the degradation of the alkyl chain. Following the formation of terminal alcohol by alkane monooxygenase, the alkyl chain proceeds to the terminal aldehyde and carboxylic acid by the action of the two types of dehydrogenases, as seen in Fig. 8a.

β-Oxidation The production of carboxyl groups by the ω-oxidation of the alkyl chain ensures that the degradation of the alkyl chain proceeds to β-oxidation. Figure 8b illustrates that carboxyl and coenzyme A are esterified first in β-oxidation. Dehydrogenase catalyzes the formation of alpha and beta carbon-carbon double bonds, while hydrolytic enzyme activity results in the formation of a beta-hydroxy group. Further dehydrogenation results in the formation of beta-ketone and the interaction of coenzyme A, leading to the formation of acetyl coenzyme A and fatty acid coenzyme A ester with fewer carbons.

Benzene Ring Oxidation Benzene ring oxidation is a speed-determining step in surfactant biodegradation [67]. As illustrated in Fig. 8c, the benzene ring initially produces catechol under

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oxygenase action, followed by ortho- or meta-cleavage under the action of oxygenase. The cleavage of the adjacent position produces hexadiene diacid, while meta-cleavage is responsible for the final breakdown of acetic acid and succinic acid, whereby 2-hydroxyhexadiene semialdehyde acid is produced as a result of this process [67]. Finally, pyruvic acid, formic acid, and acetaldehyde are produced [88].

Factors Influencing Surfactant Biodegradation Many chemical and environmental variables influence the biodegradation of surfactants in the environment, including the media’s chemical structure and physicochemical conditions. As previously stated, the structure and characteristics of surfactants, including the length and degree of branching of the alkyl chains, the quantity of the polymer units, and the position and presence of benzene rings, all have a significant impact on the biodegradability of a surfactant. Biodegradation is further affected by the type and quantity of the microorganism, the degradation reaction circumstances, the type of surfactant in the waste, and the availability of these substances [19].

Reaction Conditions Efficient biodegradation can be achieved via optimum reaction conditions, such as pH, temperature, and oxygen presence or absence. Microorganisms thrive and proliferate most easily and efficiently at appropriate temperatures and pH levels. It can be seen from Table 1 that, in general, microorganisms grow normally at pH 7 and temperatures ranging from 25 to 37  C. Surfactant biodegradation is classified as either anaerobic or aerobic, and the amount of oxygen required varies according to the type of surfactant. For instance, most cationic surfactants degrade aerobically [60], whereas non-ionic surfactants degrade anaerobically [69]. Additionally, Cheng et al. [89] reported that the biodegradation of surfactants could also be enhanced with an anaerobic membrane bioreactor by introducing a micro-aeration technique. Although the micro-aeration technique is sparsely used, Cheng et al. [89] proved that the technique is an effective approach that results in a significant decrease of surfactant and soluble chemical oxygen demand concentrations. The group also revealed that micro-aeration activates the microorganisms, thus enhancing biodegradation. Furthermore, the addition of extra carbon and nitrogen sources alters the degree to which microorganisms degrade the surfactant. When sucrose was used as a carbon source in the breakdown of linear alkylbenzene sulfonate, for example, the degradation rate was more than twice as fast as when glucose was used. However, the inclusion of succinate as a carbon source hindered the breakdown of linear alkylbenzene sulfonate.

Pantoea agglomerans Serratia odorifera Pseudomonas sp. Brevibacterium sp. Pseudomonas stutzeri Aquamicrobium, Flaviflexus, Pseudomonas, Thiopseudomonas Pseudomonas putida A Alcaligenes faecalis, Enterobacter cloacae, Serratia marcescens

LAS

SLES

Quaternary ammonium compounds SDS, sodium dodecyl benzene sulfonate (SDBS), sodium lauryl ether sulphate (SLES) SLES

Aeromonas hydrophila strain S7, Pseudomonas stutzeri strain S8, Pseudomonas nitroreducens strain S11 Soil microbial, GammaProteobacteria

Pseudomonas nitroreducens Pseudomonas aeruginosa

LAS

Quaternary ammonium compounds Octylphenol polyethoxylates Polyethylene glycols SDS

Microorganism Pseudomonas aeruginosa

Surfactant SDS

Table 1 Various surfactant biodegradations

Neutral

7.3

7.4 7

7 7 7 n.a.

8.5

pH 7.0–7.5

20

30

30 30

30 30 30 33–38

32

25

Reaction conditions Temperature pH ( C) pH 7.2 30

Pressured liquid extraction + methylene blue active substances (MBAS) assay

Methylene blue active substances (MBAS) assay

[93]

[92]

[90] [91]

[60] [65] [70] [89]

[51]

[54]

Ref. [49]

Surfactant Biodegradation (continued)

Analytical method Methylene blue active substances (MBAS) assay Methylene blue active substances (MBAS) assay and by reverse-phase high-performance liquid chromatography (HPLC) analysis Methylene blue active substances (MBAS) assay HPLC-UV HPLC-MS Chromatography Methylene blue active substances (MBAS) assay Colorimetric method Methylene blue active substances (MBAS) assay

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Cetylpyridinium chloride Cocamidopropyl betaine

Surfactant LAS LAS

Table 1 (continued)

Microorganism Aerobic seed sludge Soil microbial community in commercial laundry wastewater and domestic sewage Rhodobacter, Asticcacaulis Pseudomonas sp., Rhizobium sp. n.a. n.a.

22–23 25

Reaction conditions Temperature pH ( C) 7–8 n.a. 7–8 n.a.

HPLC Dissolved organic carbon (DOC)

Analytical method HPLC n.a.

[96] [97]

Ref. [94] [95]

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Microorganisms Several strains of different genera can biodegrade the same organic molecule, although their degradability varies, as can be seen in Table 1. When it comes to the biodegradation of certain substances or surfactants, it is critical to select strains that have high adaptability and rapid breakdown rates. Although combining two or more strains may improve the ultimate degradation rate of organic materials, controlling the appropriate degradation conditions can be challenging. For instance, it was reported that linear alkylbenzene sulfonate degrades 70% faster using a combination of two strains than when using a single strain [51]. The concentration of various microorganisms has a significant impact on the degradation outcome. For example, at low concentrations, the microbes take a long time to adapt to the substrate, grow and reproduce slowly, and have a limited capacity to degrade. However, at high concentrations, chemicals readily adsorb on the microorganisms, thus affecting the degradation outcome. Consequently, it is vital to adjust the microbial concentration to correspond to the biodegradation of the chemicals being processed.

Mixture Components The surfactants in waste or wastewater do not simply exist as a single component but as a combination or mixture of many different compounds. Apart from the matrix structure of surfactants, the compatibility, concentration, and availability of chemicals all impact the degradation of these substances. For example, the biodegradation rate of linear alkylbenzene sulfonate decreases when its concentration increases [52]. A high surfactant concentration slows the adaptation of microbes to the environment and inhibits microbial activity. Besides, surfactant biodegradation might probably be hindered by the toxic effects on the heterogeneous bacterial mixture [76]. Therefore, it is also critical to investigate the inhibitory effects and biodegradation kinetics to understand the behavior of surfactant biodegradation in a mixture of components, such as in liquid waste or wastewater [76]. However, the availability of microorganisms also has a significant impact on biodegradation [19]. Some chemicals are poorly soluble in water and are readily absorbed by solid particles, while some substances can easily chelate with calcium and magnesium ions in water to form precipitates, which slow down the biodegradation rate. Even though the waste may contain the same type of surfactant, the combination of various components may alter the degradation rate, whereby some chemicals might inhibit the biodegradation and, thus, slow down the process. Furthermore, Tayag et al. reported that various structures formed by the interaction of the mixed surfactants might reduce the biodegradation of surfactant systems [98]. These structures may affect the normal degradation of microorganisms, thereby leading to a slower biodegradation rate or partial breakdown of the surfactants. However, it is widely known that the polarity of solvents has a significant impact on the production rate. It is predicted that the degradation process will also be

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sensitive to the solvent utilized and the polarity of the solvent since they might function as shields for particular reactive sites throughout the reaction. For example, in quaternary ammonium compounds (QACs), increased polarity is predicted to protect the vulnerable hydrogen site from possible removal. It is well established that some reactive sites may be shielded by polar (protic) solvents, such as water, methanol, and isopropyl alcohol. Furthermore, Nabeoka et al. reported on the effects of adsorbent carriers, such as silica gel, activated carbon, sea sand, and quartz sand on the biodegradation of QACs [99]. It was revealed that the QACs satisfied the biodegradability criterion using silica gel, except one, which had a branched alkyl chain and did not biodegrade [99]. As for the activated carbon, although the concentration of the QACs was reduced to around 0.1 mg/L, the QACs did not undergo biodegradation, but, instead, were strongly adsorbed on the activated carbon. No adsorption or biodegradation of the QACs was detected in the studies that employed sea sand and quartz sand [99].

Assessment of the Biodegradability of Surfactants Due to the environmental concerns caused by surfactants and their degradation products, it is necessary to measure their quantities in environmental matrices. In general, the biodegradability of a surfactant can be assessed by determining the quantity of the surfactant before and after degradation, such as by using chemical quantitative, oxygen consumption, and dissolved organic carbon (DOC) methods [67]. Because certain surfactants may react with chemicals, the amount of surfactant before and after the interaction can be quantified to estimate the degree of degradation, for example, using methylene blue in a quantitative chemical method for anionic surfactants. The negatively charged anionic surfactant may react with the cationic dye and methylene blue, and the products obtained can be extracted using chloroform to determine the surfactant’s concentration. Given that surfactant biodegradation may potentially entail aerobic processes, the oxygen consumption method may be utilized to determine biodegradability. Methods for measuring oxygen consumption include the biological oxygen demand (BOD) and chemical oxygen demand (COD) techniques. The BOD is the quantity of dissolved oxygen (DO) required by aerobic biological organisms to degrade organic matter in a given water sample at a certain temperature and period. At the same time, the COD is defined as the oxygen equivalent spent during the chemical oxidation of organic materials using a strong oxidant. However, the DOC technique refers to the decomposition of organic materials into carbon dioxide during surfactant degradation. The quantity of CO2 generated and converted into organic carbon may determine the degradability. As technology advances, more precise tools, such as high-performance liquid chromatography, gas chromatography, and mass spectrometry, may be used to directly and quantitatively examine the number of substances in samples. However, an examination of the analytical techniques used to determine surfactant biodegradability in various studies revealed that, in many instances, it is challenging to ascertain whether the decrease in surfactant concentration is caused

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by biodegradation or sorption [8]. This is particularly precise in trials where the surfactant concentration is determined purely by a reduction in the analytical signal of the native compound. Besides, most of the reports deal with the use of activated sludge, which provides a significant sorption capacity for surfactants. Based on the findings in the literature, recommended guidelines have been developed to ensure the accuracy and reliability of the biodegradability of a surfactant. Cierniak et al. [8] ranked the accuracy in assessing biodegradability with increasing information by considering the environmental effects and reduced bias, as summarized in Fig. 9. The approach for the evaluation can be divided into several tiers. Within Tier 1, the primary biodegradation is essentially restricted to assessing residual concentrations of the primary compound. The structural alteration of a surfactant by microorganisms resulting in the loss of its surface-active qualities due to the parent substance’s degradation and subsequent loss of the surface-active feature is referred to as primary biodegradation. To ensure that the findings are consistent, it is essential to consider surfactant sorption in the analytical methodology. When the determination of sorption is added, the informational value of the approach is enhanced to the level of the bottom of Tier 2, which is the ultimate biodegradation. The demand for research into the ultimate biodegradability of surfactants has prompted a new wave of studies on the manufacture of biodegradable surfactants [100]. Ultimate biodegradation refers to the

Fig. 9 Results of accuracy and reliability guidelines in assessing biodegradability. (Figure adapted with permission from Ref. [8] (2019, Springer Nature))

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level of biodegradation attained when the surfactant is completely utilized by microorganisms, resulting in its breakdown to inorganic end-products, such as carbon dioxide, water, and mineral salts of any other elements present (mineralization), as well as new microbial cellular constituents (biomass) [19]. Biodegraded primary compounds and residues may be estimated with reasonably high accuracy at this level of analysis. An investigation and determination of the metabolites generated throughout the process and the identification of any stable and persistent structures are required to better understand the fate of the biodegraded surfactant, which will consequently be elevated to the bottom of Tier 3. Surfactant residual concentration, sorption behavior, stable metabolite production, and surfactant dissipation should all be determined at this level. The quantity of surfactant utilized by microorganisms as a carbon source during biodegradation is a crucial piece of information required to attain a high level of accuracy in determining biodegradability. Therefore, it is suggested that biomass growth be controlled to ensure that the reduction in surfactant concentration coincides with cell proliferation. This yields a very accurate and reliable assessment comparable to Tier 4. Additional analyses, such as identifying possible metabolites and monitoring microbial development, should also be included to appropriately address the actual environmental effect of the tested substances. Methods based on isotope-labeled compounds provide the best degree of compliance since such methods also reflect the quantity of the examined component that has been absorbed into the biomass. However, owing to the high cost and the need for specialized analytical equipment, this approach is not always viable. Mass balancing using unlabeled compounds is also possible, given that the analytical process includes surfactant recovery, metabolite measurement, and carbon dioxide assessment. In short, the biodegradability assessment should not be focused solely on surfactant determination since the analytical accuracy can be influenced by various factors, including selectivity, duration, quantification, and improper handling of the biomass, which may result in errors in distinguishing between surfactant depletion that is due to biodegradation or sorption [8]. The correctness of the produced data may be questioned if the experimental system is not well prepared, which involves identifying potential sources of error and developing relevant countermeasures. Therefore, depending on the experimental setting, additional procedures, including sorption to different parts of the test system, metabolite measurement, and biomass growth control, may be necessary to validate that the depletion in surfactant concentration is mainly due to biodegradation.

Conclusions Although surfactants are being extensively used in manufacturing, food, agriculture, medicine, and other sectors, which provide consumers with convenience, they are also a major cause of environmental pollution and are even hazardous to human health. Some surfactants and their converted products have severe environmental repercussions due to their non-degradability, persistence, and toxicity. Surfactants released into the water may asphyxiate aquatic life, while the presence of surfactants in the soil can be harmful to microorganisms and cause root damage or hinder

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photosynthesis in plants. Besides, they can be toxic to humans and cause diseases. Thus, efforts to promote surfactant biodegradation in the environment are urgently required and must be pursued. In surfactant biodegradation, various practical factors need to be considered to achieve an optimum environmentally benign output, such as selecting a suitable strain or a combination of strains for each type of surfactant, favorable reaction conditions, and taking account of the presence of other matrix components. In addition, a proper evaluation of the biodegradability of surfactants with improved accuracy and data reliability will assist in providing a better understanding and more innovative development of surfactant biodegradation in the future.

Future Prospects The degradation of surfactants in the environment is a pressing issue addressed globally. The microbial degradation of surfactants is an efficient, safe, ecologically beneficial, and broadly applicable degradation process. Besides, the development of more efficient biodegradation technologies and the improvement of biodegradability assessment methodologies are critical. The accuracy and reliability of biodegradability assessments are questionable since it is difficult to distinguish between the depletion of surfactant concentrations due to biodegradation or sorption since many studies in the literature used analytical methods, which focused only on the determination of the surfactant [8]. Thus, certain guidelines and proper analytical methods must be established in the future while considering various practical factors for obtaining highly accurate data in terms of surfactant biodegradation. In addition, the relationship between structure and degradability should be exploited to guide the synthesis of novel degradable green surfactants and decrease environmental pollution at the source. Further research should be explored to determine the mechanism by which the target surfactant is destroyed and the kinetics of the surfactant degradation. Policymakers and regulatory bodies are highly urged to develop recommendations and standards for the various classes of surfactants and their degradation products.

Cross-References ▶ Biodegradable Materials: Fundamentals, Importance, and Impacts ▶ Biodegradation of Crude Oil and Biodegradation of Surfactants ▶ Biodegradation of Industrial Materials ▶ Environmental Impact of Biodegradation Acknowledgments This work was supported by the Fundamental Research Grant Scheme (FRGS/1/2018/STG01/UIAM/03/2) (FRGS19-015-0623), Ministry of Higher Education (MOHE), Malaysia, and Department of Chemistry, Kulliyyah of Science, International Islamic University, Malaysia.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sources of Oil Industry Waste Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polycyclic Aromatic Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oil Spills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Impact of Oil Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oil-Polluted Systems Treatment Using Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Degradation of Petroleum Hydrocarbon Contaminants . . . . . . . . . . . . . . . . . . . . . . . . Degradation of Hydrocarbons by Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uptake of Hydrocarbons by Biosurfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Utilization of Petroleum Industry Wastes as Sustainable Building Materials . . . . . . . . . . . . . . . . . Drilling Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oily Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Tahany Mahmoud and Walaa S. Gado contributed equally with all other contributors. T. Mahmoud (*) Special Application laboratory, Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo, Egypt e-mail: [email protected]; [email protected] W. S. Gado (*) · A. H. Mady Petrochemical Technology Laboratory, Petrochemicals Department, Egyptian Petroleum Research Institute, Cairo, Egypt e-mail: [email protected] K. I. Kabel Additives laboratory, Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo, Egypt © Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2_35

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Abstract

The oil industry’s activities impact the environment due to the large amounts of oily waste generated. The technology management to treat oily waste is critical to promoting good environmental management and providing alternative waste reduction, reuse, and recycling options. This chapter discusses solid waste effluents, which are wastes generated by industries primarily engaged in refining crude oil and producing fuels, lubricants, and petrochemical intermediates, which are considered a significant cause of aquatic environmental pollution. It also discusses oily sludge, one of the essential semisolid wastes produced by petrochemical industries. Oily sludge, a mixture of several hydrocarbons (e.g., polycyclic aromatic hydrocarbons [PAHs]), has been identified as a hazardous waste that contributes significantly to environmental pollution. The most efficient method of removing these pollutants from the environment is biodegradation by microorganisms, discussed in this chapter. It is attaining significance as a more efficient and potentially cost-effective cleaning technique. Its potential contribution as countermeasure biotechnology for decontaminating oil-polluted systems might be enormous since it leads to the mineralization of complex organic molecules into simple ones, thus enhancing soil organic content. The chapter will provide an overview of leaks and accidental spills that occur regularly during the exploration, production, refining, transportation, and storage of petroleum and petroleum products and their treatment using microorganisms in different ecosystems. Keywords

Biodegradation · Waste materials · Oil industry · Refinery industry · Petroleum hydrocarbons Abbreviations

GEM PAHs PCBs TPH

Genetically modified microorganisms Polyaromatic hydrocarbons Polychlorinated biphenyls Total petroleum hydrocarbon

Introduction Massive amounts of different waste materials that need perfect valorization and elimination have increasingly grown in the petroleum industry. Waste materials are the unavoidable by-products of human activity: unusable, worthless, defective, and unwanted materials discarded after their main use. Waste materials appear in solid or liquid forms. Like all wash water, the liquid type contributed to household uses, as did waste solvents and liquids from factories. On the other hand, solid waste, such as food waste, wood, and plastic bottles, management involves medical, municipal, agriculture, vehicles, construction, electronics, demolition, food, households, radioactive waste,

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water, etc. Several waste materials are subjected to the environment daily. With no exceptions, waste materials cause a dangerous disaster [1]. Biodegradation is how living microbiological organisms break down organic molecules into smaller ones. The process of biodegradation is called “mineralization” when it is completed. In all cases, the term “biodegradation” refers to nearly any biologically driven alteration of a substrate. As a result, recognizing the biodegradation process necessitates knowing the microorganisms that make it possible. Microbial organisms transform the material via metabolic or enzymatic mechanisms. Growth and co-metabolism are the two processes on which it is founded. An organic contaminant is employed as the only carbon and energy source during growth. Organic contaminants are completely degraded (mineralized) as a consequence. The metabolism process of any organic substance in the presence of a growth material that serves as the principal energy and carbon source is known as co-metabolism. The biodegradation process involves a variety of microorganisms, such as fungus, bacteria, and yeasts. Biodegradation processes vary significantly, but carbon dioxide is produced as a by-product. Organic waste can be decomposed either aerobically (with oxygen) or anaerobically (without oxygen), as shown in Fig. 1.

Depolymerases

Polymer

Enzymes attach to the polymer and split the polymer chains

2

Oligomer

Dimer

Monomer

3

1

Excretion of extracellular enzymes from microorganism

Microbial degradation

Under anaerobic condition microorganisms use

3.2

CH4, CO2, H2O, residues

3.1

Under aerobic condition microorganisms use

CO2, H2O, Other metabolic products

Fig. 1 Schematic diagram illustrates the biodegradation pathway. (Adapted with permission from Ref. [1]; Copyright Springer, 2018)

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Plant and animal material and other compounds derived from living animals are examples of biodegradable substances, as are manufactured substances close enough to animals and plants to be used by microorganisms. Some hydrocarbons such as polyaromatic hydrocarbons (PAHs), oil, polychlorinated biphenyls (PCBs), and radionuclides are just a few chemicals that some bacteria can break down, alter, or retain. The expression “biodegradation” is frequently used in ecology, waste treatment, and environmental monitoring (bioremediation). The process of bioremediation can be divided into three main parts. First, pollutants are decreased by native microorganisms through natural attenuation, which does not require any human intervention. Second, biostimulation boosts efficacy and speeds up biodegradation by injecting nutrients and oxygen into the systems. Finally, the microorganisms are conducted through the process of bioaugmentation. The additional organisms must be more effective at degrading the target pollutant than native flora. Microorganisms capable of fast and efficient polluting matters, focusing on a specific situation with a fair amount of time, are required for a viable remedial method. Numerous factors affect microorganisms’ ability to subject pollutants and act as co-metabolizes or substrates, including environmental aspects like pH, temperature, nitrogen, phosphorus sources, and genetic potential. The intensity and frequency of degradation appear to be determined by these factors. As a result, genetically modified microorganisms (GEMs) in bioremediation have gotten much press. These GEMs have a greater degradative ability and have been effectively used to degrade various contaminants under controlled settings. However, ecological and environmental considerations and legislative limits are key roadblocks to field testing GEM, as shown in Fig. 2. This chapter discusses biodegradation about environmental bioremediation. In the microbiological sense, biodegradation refers to the destruction of any organic matter by a diverse group of life forms that include primarily fungi, bacteria, yeast, and maybe other creatures. Consequently, biodegradation is nature’s means of waste recycling or converting organic material into nutrients that may be spent and reclaimed by other organisms. Over the last few decades, hazardous chemical substances have been produced and reintroduced into the environment for long-term indirect or direct use. These substances include PCBs, PAHs, fuels, insecticides, and dyes. Other manufactured Fig. 2 The pathway of genetically modified microorganisms (GEM) in bioremediation

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chemicals, such as radionuclides and metals, are very resistant to biodegradation by native flora compared to naturally occurring organic molecules, which disintegrate rapidly after release into the environment. These substances could be categorized as follows: Hydrocarbons are organic molecules with hydrogen and carbon atoms in their structures. Hydrocarbons are molecules that are linearly connected, branched, or cyclic. Aromatic or aliphatic hydrocarbons are found in them. The first has the structure of benzene (C6H6), but the aliphatic has three types: the first one is alkanes, the second one is alkenes, and the last one is alkynes. PAHs are organic hydrophobic contaminants commonly detected in air, soil, and deposits. Industrial output is the primary cause of PAH pollution. PAHs can bind to organic-rich areas and sediments, aggregate in the aquatic matter, and pass on to humans via seafood. Both the removal of human pollutants from the environment and the natural process of the carbon cycle result from the biodegradation of PAHs. Polychlorinated biphenyls (PCBs) are synthetic organic chemical combinations. PCBs were used in thousands of industrial and commercial applications due to their nonflammability, chemical stability, high boiling point, and electrical insulating properties, along with electrical, heat transfer, and hydraulic devices; as plasticizers in paints, plastics, and rubber products; in pigments, dyes, and carbonless copy paper; and much other manufacturing products. Pesticides are compounds or blends of substances used to prevent, eradicate, repel, or mitigate pests. As a result, PCBs are hazardous substances that can alter the endocrine system and cause cancer. As a result, PCBs pollution is becoming a growing concern. Pesticides that dissolve quickly are referred to as nonpersistent. The other types that resist degradation are referred to as persistent. The popular degradation method occurs by microorganisms in the soil, particularly bacteria and fungi that feed on pesticides. Dyes are extensively utilized in textiles, rubber, paper, printing, color photography, medicines, cosmetics, and other sectors. Azo dyes are considered the largest category of synthetic dyes that are applied for commercial use. They are aromatic compounds having one or more (N¼N–) groups. Because of their architectures, many dyes are rarely biodegradable. The dye-containing wastewater treatment typically preserves chemical or physical procedures like coagulation-flocculation, adsorption, filtering, oxidation, and electrochemical approaches. Using microorganisms that successfully decolorize synthetic dyes of various chemical structures is critical to a biological method’s ability to remove color. Radionuclides are atoms with an unstable nucleus with extra energy that can be transferred to a newly produced radiation particle inside the nucleus or converted internally. The radionuclide is said to undergo radioactive decay during this operation, which leads to the production of alpha and beta particles, or gamma rays. Unlike biological pollutants, heavy metals cannot be eliminated, instead they must be transformed into a stable form or removed. Biotransformation is used to achieve metal bioremediation. Microorganisms act on heavy metals (redox reactions) through a variety of mechanisms, including biosorption (physicochemical metal sorption into the cell surface), biomineralization (heavy metal immobilization via the formation of polymeric complexes or insoluble sulfides), bioleaching (heavy metal mobilization via the excretion of methylation reactions or organic acids),

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Fig. 3 The microbial processes for bioremediation technologies modified by Lloyd and Lovely. (Adapted with permission from Ref. [2], Copyright Elsevier, 2013)

intracellular deposition, and enzyme-catalyzed biomineralization. The most important microbial activities that affect the bioremediation of heavy metals are shown in Fig. 3.

Sources of Oil Industry Waste Materials Common environmental pollutants worldwide are related to the petroleum industry and crude oil hydrocarbons. The nature of petroleum products’ processing leads to the release of hazardous organic compounds such as phenolic substances and PAHs, which are naturally degradable, cresol toxins, and chlorophenols [3–5]. Furthermore, crude oil spills are responsible for most oil pollution problems during transportation and storage. An effective cleanup treatment for a crude oil spill in saltwater is required. To remove crude oil spill pollution from saltwater, many biological and physicochemical treatment procedures were used worldwide. Because of its efficiency and low cost, a biological treatment procedure for biodegradation of crude oil employing fungi, bacteria, and algae has recently attracted focused interest. As part of the treatment, researchers employed bioaugmentation, which includes singlestrain and consortia of microorganisms, to break down the majority of the spilled crude oil. There is a movement to enhance and reintroduce high-potential agroindustrial waste microorganisms. Paneer whey, soybean oil, solid wastes, molasses, corn steep, and other low-cost substrates are available. Agro-industrial wastes are an

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impacted source of potential microorganisms that produce biosurfactants and nutrients for growth [6–12]. Research has been focused on other methods, such as applying ratios of nutrients [13, 14], and food wastage [15–19], and on questions like when we should use bioremediation, what kinds of bioremediation agents should be used, the method of applying and the evaluation process of outcomes, and what are the practical guidelines to apply this technology to seawater oil spills? [20]. The microorganism bioremediation of petroleum hydrocarbon pollutants is a priority [17–21]. The main challenge for the oil bioremediation application in marine is when and how to use this new technology to apply it where water movement is less in an encircled area considered the only beneficial factor that can occur in floating oil containment booms.

Polycyclic Aromatic Hydrocarbons PAHs are omnipresent environmental pollutants that appear when organic substances combustion is incomplete, such as coal, petroleum, oil, and wood, natural sources (natural losses and petroleum or coal deposits seepage, open burning, and volcano activities), and anthropogenic sources (coal gasification, residential heating, plant liquefaction, carbon black, asphalt production, coke and aluminum production, and catalytic crac towers). PAHs also exist in the gas phase as a sorbet for aerosols. The movement of PAH compounds in the atmosphere and how they enter the human body is heavily influenced by their atmospheric partitioning between particulate and gaseous phases. PAHs in soil are mostly derived from atmospheric deposition. PAHs’ gas/particle partitioning significantly impacts their removal from the atmosphere via dry and wet deposition mechanisms. Many PAHs have mutagenic, toxic, and carcinogenic properties. Due to their high lipid solubility, PAHs are absorbed from the gastrointestinal tract of mammals. They are rapidly distributed in different tissues, with a solid proclivity for localizing in body fat. The cytochrome P450-mediated mixed-function oxidase system is responsible for PAH metabolism, with oxidation or hydroxylation as the main step. Many different remediation technologies have been used to eliminate these toxins from the environment. Bioremediation, in particular, is showing the green light as a costeffective and more safe option. Gram-positive and gram-negative bacteria, algae, and fungi have been isolated and characterized due to their capability to use PAHs despite their xenobiotic properties (as shown in Fig. 4).

Oil Spills Total petroleum hydrocarbons (TPH) are released into the environment due to oil spills, industrial releases, or private or commercial by-products [21]. Ship operations, oil exploration, and production tanker accidents are the main causes of crude oil spills in coastal waters. The main causes of the oil spills are illustrated in Fig. 5. Although statistics for the occurrence of oil spills reflected a clear descending

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NH2-GRAFTED LACCASE

PAHs

Naphthalene Si-R-NH-

Phenanthrene

Anthracene

BIODEGRADATION SBA-15 silica

Fig. 4 The biodegradation pathway of polycyclic aromatic hydrocarbons Fig. 5 Major causes of all crude oil spills. (Adapted with permission from Ref. [26], Copyright 2007, Elsevier)

direction over the last half-century, the volume of oil spills continues to be a source of concern for the environment. As a result of over 140 major spills, more than 7 million tons of hydrocarbon oil have been released into the environment [22]. In 2020, tanker discharges alone were estimated to have lost around 1000 t of petroleum hydrocarbon oil [23]. This is the same figure as in 2012 and 2019. In the last 50 years, the list of global oil spills and recent spills in offshore and inland waters reveals more than 200 incidences [24]. Millions of tonnes of petroleum penetrate the marine environment every year through the natural seepages, besides the anthropogenic oil spills [25].

Environmental Impact of Oil Industry The oil spill disaster involving petroleum hydrocarbons has impacted the marine environment and ecosystem [27, 28]. As a result, TPH is discharged directly into water bodies due to oil spills (as illustrated in Fig. 6). Direct toxicity is achieved, and death by suffocation occurs in cases where the organism’s body is filled with petroleum hydrocarbon oil exposure [29]. Petroleum hydrocarbon oil spills have

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Fig. 6 Effect of the oil spill on the marine environment

harmed the shallow coral reefs, an important ecosystem. Extensive research has shown coral loss and damage from petroleum hydrocarbon oil exposure [4]. Because mammals and birds are covered with petroleum oil, their resilience to other environmental stresses, including temperature changes, infectious illnesses, and other pollutants, has declined [29]. Oil contact impairs seabirds’ ability to fly, making them particularly vulnerable. Ingestion of infected food, inhalation, and frequent contact with the oil-water interface result in severe personal poisoning and significant mortality rates [4]. Oil ingested or dissolved in the body through membranes, such as gill surfaces, causes direct lethal toxicity, sublethal effects, and reproductive failure in marine organisms [29]. Turtles caught in oil spills extend their physical contact with the petroleum-saturated breathing air and the floating oils and their ingestion of oil-polluted food. The esophagus of old and young tortoises had been blocked by petroleum hydrocarbons, causing them to starve to death [4]. It also has human consequences. Foremost, it is necessary to prevent or minimize human life loss and the side health effects and any close human communities and people affected by any petroleum oil spills in the marine environment [4, 29]. TPH ejected from the soil enters the groundwater via the surface. Evaporation into the atmosphere is after some of these volatile substances. A small percentage dissolves in the groundwater and disappears from the spill area. Most chemicals bond to soil particles and stay in the soil for an extended period, whereas soil bacteria break down certain hydrocarbons. Second, contact may occur via cutaneous continuous contact, inhalation, and ingestion depending on the grades and qualities of the chemical and media such as food, air, water, and soil, where the chemical will affect the human activities in and around each substance [4]. Contact with petroleum hydrocarbons can induce cancer, be transient, or be permanently noncancerous [30].

Oil-Polluted Systems Treatment Using Microorganisms Microbial Degradation of Petroleum Hydrocarbon Contaminants Petroleum hydrocarbon biodegradation is a complicated process influenced by the nature and amount of hydrocarbons present. Saturates, aromatics, asphaltenes (phenols, fatty acids, ketones, esters, and porphyrins), and resins (pyridines, quinolines,

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carbazoles, sulfoxides, and amides) are the four types of petroleum hydrocarbons [31]. Cooney et al. have identified several factors that influence hydrocarbon decomposition [32]. The limited availability of oil pollutants to microorganisms is one of the major factors limiting their biodegradation in the environment. Petroleum hydrocarbon molecules are difficult to remove or decompose because they adhere to soil components [33]. The vulnerability of hydrocarbons to microbial assault varies. The following is a general ranking of hydrocarbon susceptibility to microbial degradation: branched alkanes > tiny aromatics > cyclic alkanes > linear alkanes [34]. Some molecules, such as polycyclic aromatic hydrocarbons (PAHs) with a large molecular weight, may not be degraded [35]. Microbiological degradation is the most important and most effective natural mechanism for removing petroleum hydrocarbon contaminants from the environment [36–38]. Jones et al. reported the discovery of biodegraded oil-derived aromatic hydrocarbons in sediment samples [39]. They investigated the vast biodegradation of alkyl aromatics in marine sediments that previously belonged to detectable biodegradation of the crude oil’s n-alkane profile. They discovered that microorganisms like Burkholderia, Arthrobacter, Mycobacterium, Sphingomonas, Pseudomonas, and Rhodococcus were involved in alkyl-aromatic degradation. Adebusoye et al. [40] documented microbial decomposition of petroleum hydrocarbons in a polluted tropical stream in Lagos, Nigeria. As shown in Fig. 7, Bacillus subtilis, Bacillus sp., Alcaligenes sp., Acinetobacter lwoffi, Flavobacterium sp.,

Fig. 7 Microbial degradation of petroleum hydrocarbon contaminants

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Micrococcus roseus, and Corynebacterium sp. were among the nine bacterial strains identified from the contaminated stream that could breakdown crude oil. Bacteria, yeast, and fungi are the primary biodegraders of hydrocarbons in the environment. Biodegradation efficiency for soil fungus has been reported to range from 6–82%, 0.13% [41] to 50% [42], and 0.003% [43] to 100% [44] for marine bacteria. Several scientists have found that to digest complex combinations of hydrocarbons like crude oil in soil [45], freshwater [46], and aquatic systems [47, 48], mixed congregations with overall broad enzymatic abilities are necessary. Bacteria are by far the most effective agents in the degradation of petroleum, and they are the principal degraders of oil spilled in the environment [49, 50]. Several bacteria have been discovered to consume only hydrocarbons [51]. Floodgate [48] identified 25 genera of hydrocarbon-degrading bacteria and 25 species of hydrocarbon-degrading fungi from the aquatic environment. Bartha and Bossert [45] compiled a similar list that contained 22 bacterium genera and 31 fungus genera. The number of petroleum hydrocarbons biodegraded by yeast, bacteria, and filamentous fungi was previously unknown, but it appeared to be a function of local environmental circumstances and the ecosystem. Das and Mukherjee [52] found crude petroleum oil from petroleum-contaminated soil in Northeastern India. Acinetobacter sp. was discovered to use n-alkanes with chain lengths of C10–C40 as a single carbon source [53]. Brevibacterium, Dietzia, Burkholderia, Gordonia, Mycobacterium, and Aeromicrobium, obtained from petroleumcontaminated soil, were promising hydrocarbon-degrading microbes [54]. Sphingomonas has been shown to degrade polyaromatic hydrocarbons by McCracken and Daugulis [55]. From petroleum-contaminated soil, fungal genera Amorphoteca, Graphium, Talaromyces, and Yarrowia, as well as yeast genera Candida and Pichia were collected and found to be prospective hydrocarbon degradation species [54]. Singh [56] further mentioned a list of terrestrial fungi named Cephalosporium, Aspergillus, and Pencillium that were discovered to be promising crude oil hydrocarbon degraders. After being isolated from polluted water, Candida lipolytica, Rhodotorula mucilaginosa, Geotrichum sp., and Trichosporon mucoides were found to break down petroleum compounds after being isolated from it [57]. Although algae and protozoa are major constituents of the microbial population in terrestrial and aquatic ecosystems, research on their role in hydrocarbon biodegradation is limited. Walker et al. [58] identified Prototheca zopfi as an alga that could use crude oil and a mixed hydrocarbon feed and degrade n-alkanes and iso-alkanes and aromatic hydrocarbons extensively. Cerniglia et al. [59] discovered that naphthalene could be oxidized by one brown alga, one red alga, two diatoms, five green algae, and nine cyanobacteria. On the other hand, protozoa have never been proven to use hydrocarbons.

Mechanism of Petroleum Hydrocarbon Degradation and Pathways Under aerobic circumstances, the bulk of organic pollutants degrades at the fastest and most complete rate. The essential mechanism of aerobic hydrocarbon breakdown is depicted in Fig. 7 [60]. The earliest intracellular attack of organic

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contaminants is an oxidative process, with oxygenates and peroxidases catalyzing the activation and incorporation of oxygen as the primary enzymatic response. The tricarboxylic acid cycle, for example, is an example of a peripheral degradation route that converts organic contaminants into intermediates of the core intermediary metabolism. Gluconeogenesis produces the sugars needed for several biosynthetic processes and growth. The major precursor metabolites, such as acetyl-CoA, succinate, and pyruvate, are used to synthesize cell biomass. A particular enzyme system can mediate the breakdown of petroleum hydrocarbons. Figure 3 depicts oxygenates’ early attack on xenobiotic [60]. Other processes include microbial cell adhesion to substrates and biosurfactant synthesis [61]. Although the process of cell attachment to oil droplets is unknown, biosurfactant synthesis has been extensively investigated. Petroleum molecules are biodegraded in general, and then three steps take place. Petroleum compounds are absorbed on the microbial surface, then transported to the microbial cell membrane, and finally destroyed within the microbial cell. Finally, microbes convert these compounds into a variety of tiny molecules. Many studies have shown that all types of petroleum components, such as PAHs and alkanes, degrade via an oxidizing process [62–65]. Because of the varied structures of the petroleum components, such as resin, saturated, asphaltene, and fractional aromatic hydrocarbons, the degradation processes of these compounds differ.

Degrading Process of Alkane and Cycloalkane The structure of an alkane is representative of a saturated hydrocarbon with all single bonds. Another saturated hydrocarbon with multiple carbon rings in its structure is cycloalkane. Figure 8 depicts the alkane degradation process by microorganisms, which can be divided into alkyl hydroperoxides, subterminal oxidation, and cyclohexane degradation [65]. Essentially, alkane is converted to fatty acids by microbes’ enzymes like oxidase and then gradually metabolized to acetyl coA and the Krebs cycle of microbial metabolism, with the formation of CO2 and H2O as a result [66]. Enzymes such as fatty alcohol dehydrogenase, alkane monooxygenase, and fatty aldehyde dehydrogenase are vital because they catalyze the degradation process. Subterminal oxidation is the biological degradation basis of cycloalkane, much like alkanes. The cycloalkane is initially reduced to alcohol by several oxidizing enzymes. The alcohols are then converted to a ketone using dehydroge. The ketone is then converted to esterase or fatty acid. As illustrated in Fig. 8, cyclohexane is transformed into the equivalent molecules, cyclohexanol, cyclohexanone, dehydroge, and fatty acid. Finally, the molecules degrade into CO2 and H2O by biodegradation. Oxidation of cycloalkanes with different substituents is also possible. The characteristics of microbial organisms and any other considerations will affect the initial reaction position. The alicyclic compounds substituted with alkyl could be in two positions in the alicyclic compounds and side-chain oxidation. The initial position of the reaction will be affected by many factors such as microbial species and properties.

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Fig. 8 The main n-alkanes degradation pathways (terminal and subterminal oxidation). (Adapted with permission from Ref. [68], Copyright AIMS Press; 2017)

Fig. 9 Degradation pathway of aromatic hydrocarbons. (Adapted with permission from Ref. [67], Copyright Hindawi; 2011)

Degrading Process of Aromatic Hydrocarbon The following demonstrates the process of the degradation of aromatic hydrocarbons: aromatic hydrocarbons are first oxidized to dihydrodiol by oxidase. Dihydrodiol is then converted to o-dihydroxybenzene. The ring-opening reaction in both the o-position and the m-position are the two reactions that degrade o-dihydroxybenzene. These molecules are then oxidized to long-chain compounds, and then converted to acetyl coA over time. Figure 9 depicts the degradation process.

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Although some fungi and bacteria can degrade aromatic hydrocarbons, the processes are not the same. When two oxygen atoms in a bacterium oxidize an aromatic hydrocarbon, it is transformed into cis-dihydrodiol. On the other hand, fungus oxidizes aromatic hydrocarbons and converts them to trans-dihydrodiol.

Degrading Process of Polycyclic Aromatic Hydrocarbons PAHs are highly carcinogenic, mutagenic, and teratogenic, which has prompted researchers to investigate their breakdown mechanisms. It can be broken down into the catechol or glycol groups by enzymes, then subsequently broken down into the succinic acid or acetyl coA. By yeast monooxygenase, PAHs are gradually reduced into trans-diol, epoxide, trans-dihydro 2 phenol, and phenol, according to the general degradation process. Also, dioxygenase slowly metabolizes PAHs into cis-diol, epoxide, cis-dihydro 2 phenol, and other forms. Carbon dioxide and water are the end metabolites in both routes, and the total processes of PAH biodegradation are depicted in Fig. 10 [69]. PAHs degrade in diverse ways, and the amount of degradation should be graded according to the benzene rings number, solubility, the substituent organisms number and type, the features of heterocyclic atoms, and so on. Furthermore, the asphalt has the most complex structure, making biodegradation extremely difficult [70]. Nevertheless, many studies have shown that PAHs can be destroyed in aerobic environments. Other studies have shown that PAHs can be destroyed in anaerobic conditions through denitrification, sulfate reduction, or methanogenic fermentation. The rate of PAHs anaerobic breakdown is considerably down compared to the aerobic degradation rate. Furthermore, the anaerobic breakdown mechanism of PAHs is still unknown [71].

Specificity of Biodegradation Alcanivorax, Cycloclasticus, Marinobacter, Neptunomonas, Oleiphilus, and other marine oil–degrading bacteria strains could digest alkanes or aromatic hydrocarbons [72]. Cycloclasticus can thrive on aromatic hydrocarbons, but Alcanivorax strains favor n-alkanes and branched alkanes [72]. The bacteria strains (Bacillus, Pseudomonas, Corynebacterium, and Corynebacterium) isolated from the Liaohe oil field residual could break down petroleum components: bitumen 53%, aromatic hydrocarbons 80%, nonhydrocarbon 30%, and aliphatic hydrocarbon 37% [73]. Tropicibacter naphthalenivorans gen. nov. sp. can degrade the phenanthrene, naphthalene, C2-alkyl naphthalene, and C1-alkyl naphthalene, except for two alkyl benzothiophene, alkyl

Fig. 10 Degradation pathways of polycyclic aromatic hydrocarbons (PAHs). (Adapted with permission from Ref. [67], Copyright Hindawi, 2011)

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Fig. 11 The main principle of aerobic degradation of hydrocarbons by microorganisms. (Adapted with permission from Ref. [67], Copyright Hindawi, 2011)

phenanthrene, and alkyl fluorine, as shown in Fig. 11. Table 1 lists many typical oil-degrading microorganisms and their corresponding after-degradation compounds.

Degradation of Hydrocarbons by Enzymes Cytochrome P450 alkane hydroxylases are a ubiquitous family of heme-thiolate monooxygenases that play an important role in the microbial decomposition of oil, fuel additives, and chlorinated hydrocarbons, and a wide range of other chemicals [85]. To begin biodegradation, enzyme systems must incorporate oxygen into the substrate based on the chain length (Table 2 and Fig. 12). Higher eukaryotes have a variety of P450 families, each of which has a large number of distinct P450 forms that can work together to contribute to the metabolic conversion of a particular substrate. P450 multiplicity is only seen in a few microorganisms [86]. The biodegradation of petroleum hydrocarbons is assisted by cytochrome P450 enzyme systems (Table 2). Multiple microsomal cytochrome P450 forms are required for certain yeast species to use the n-alkanes and aliphatic hydrocarbons as their energy source and sole carbon. Such cytochrome P450 enzymes were isolated from Candidamaltosa, Candida tropicalis, and Candida apicola yeast species [87]. Van Beilen and Funhoff [88]

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Table 1 The oil-degrading microorganisms and their related and corresponding substances of degrading Substances of metabolic Cyclohexane, alkyl benzene, aliphatic hydrocarbon, and dicycloalkane Hydrocarbons with long carbon chain component of oil, benzo pyrene, and other compounds Naphthalene, 2,6-dimethylnaphthalene, 1-methylnaphthalene, 2-methylnaphtha-lene, and phenanthrene Pyrene, naphthalene, phenanthrene, and anthracene Chrysene and phenanthrene Naphthalene, n-Alkanes aromatic hydrocarbon, anthracene, and phenanthrene Branched alkanes, alkylbenzenes, and n-alkanes Alkanoles, aliphatic hydrocarbons, and alkanoates Alkanes PAHs Pyrene

Species Bacillus, Pseudomonas, and Corynebacterium Aspergillius sp. Penicillium sp. Vibrio cyclotrophicus sp. nov.

References [73]

Aeram onas punctata TII Pseudoalteromonas, Vibrio, and Marinomonas Oligotrophus Cycloclasticus Alcanivorax sp. Oleispira, Oleiphilusand. Desulfococcus Cycloclasticus Mycobacterium

[76, 77] [72]

[74] [75]

[78, 79] [80] [81, 82] [83] [79] [84]

Table 2 Enzymes which assisted in petroleum hydrocarbons biodegradation Name Soluble methane Monooxygenases

Substrates C1–C8 alkanes alkenes and cycloalkanes

Particulate Methane Monooxygenases

C1–C5 (halogenated) alkanes and cycloalkanes

AlkB-related alkane Hydroxylases

C5–C16 alkanes, fatty acids, alkyl benzenes, cycloalkanes, and so forth

Eukaryotic P450

C10–C16 alkanes, fatty acids

Bacterial P450 oxygenase system

C5–C16 alkanes, cycloalkanes

Dioxygenases

C10–C30 alkanes

Microorganisms Methylococcus Methylosinus Methylocystis Methylomonas Methylocella Methylobacter Methylococcus, Methylocystis Pseudomonas Burkholderia Rhodococcus, Mycobacterium Candida maltosa Candida tropicalis Yarrowia lipolytica Acinetobacter Caulobacter Mycobacterium Acinetobacter sp.

References [89]

[89]

[90]

[91]

[92]

[93]

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Monooxygenase reaction O2 H3C

OH

nCH3

n-alkane

O2

H3C

Rubredoxin Fe2+ H

H2O

O NADH

NAD+

benzene

H Arene oxide

NADH

Rubredoxin Fe3+ H2O

n Primary alcohal

H OH H NAD+ OH Trans-dihydrodiol

OH NADH

OH Catechol

Dioxygenase reaction H

O2

Benzene

H2O

NAD+

OH OH H Cis-dihydrodiol

OH

NAD+

NADH

Catechol

OH

Fig. 12 Enzymatic reactions involved in the processes of hydrocarbons degradation. (Adapted with permission from Ref. [67], Copyright Hindawi, 2011)

investigated the diversity of alkaneoxygenase systems in eukaryotes and prokaryotes that aid in alkane degradation under aerobic conditions, such as integral membrane di-iron alkane hydroxylases, cytochrome P450 enzymes, soluble di-iron methane monooxygenases, and membrane-bound copper that contains methane monooxygenases.

Uptake of Hydrocarbons by Biosurfactants Biosurfactants are diverse surface-active chemical compounds generated by microorganisms. Surfactants aid in the solubilization and removal of pollutants. Surfactants also aid in biodegradation by increasing the bioavailability of contaminants. Cameotra and Singh [94] described bioremediation of oil sludge utilizing biosurfactants. This study used a microbial consortium of two Pseudomonas aeruginosa isolates and one Rhodococcus erythropolis isolate from soil polluted with oily sludge. In liquid culture, in 6 weeks, the collaboration was able to destroy 90% of the hydrocarbons. Two distinct field trials assessed the consortium’s potential to decompose sludge hydrocarbons. In addition, the efficiency of the procedure was evaluated using two additives: crude biosurfactant preparation and nutrient mixture. A collaborative member created the biosurfactant, identified as a combination of 11 rhamnolipid congeners. The collaboration destroyed 91% of the crude oil

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Fig. 13 Involvement of biosurfactant (rhamnolipid) produced by Pseudomonas sp. in the uptake of hydrocarbons. (Adapted with permission from Ref. [67], Copyright Hindawi, 2011)

sludge–polluted soil (v/v). Separate use of anyone in addition with the collaboration resulted in a 91–95% hydrocarbon concentration depletion after 4 weeks, with the crude biosurfactant formulation being a more effective degradation facilitator. However, more than 98% hydrocarbon depletion was reached and confirmed when the consortium was combined with both additions. The findings backed up the use of a crude biosurfactant for hydrocarbon cleanup. Pseudomonads are the most wellknown bacteria that can use hydrocarbons as a carbon and energy source while also creating biosurfactants. Pseudomonas aeruginosa is one of the most studied Pseudomonads for generating glycolipid-type biosurfactants. Additional species, including P. putida and P. chlororaphis, have produced glycolipid-type biosurfactants. Biosurfactants enhance oil surface area, which means more oil is accessible for bacteria [95]. Biosurfactants can operate as emulsifying agents by lowering surface tension and generating micelles. The microdroplets trapped in the hydrophobic microbial cell surface are sucked into the cell and destroyed. The role of biosurfactants (rhamnolipids) produced by Pseudomonas sp. and the method of micelle production in the uptake of hydrocarbons is shown in Fig. 13.

Utilization of Petroleum Industry Wastes as Sustainable Building Materials Daily and industrial operations would be unimaginable without oil and gas, yet these valuable commodities generate many waste materials. Petroleum exploration, extraction, development, and production create enormous amounts of waste

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materials. They consist of various gases, high- and low-boiling constituents, wastewater, wasted caustic, filter clay, and solid waste. The Environmental Protection Agency classifies the majority of these wastes as hazardous due to their ignitability, flammability, corrosiveness, reactivity, and toxicity, and improper storage, transport, disposal, and treatment of these waste materials can have a serious impact on the environment and the health of living creatures. Depending on the disposal and treatment, these wastes include oil and other hydrocarbons, the primary source of CO2 emissions. Stabilization/solidification techniques, for example, create new goods with economic advantages and save resources and the environment. However, the ideal strategy is determined by environmental and economic performance data and life cycle assessment data.

Drilling Wastes Drilling generates a significant quantity of recalcitrant waste. According to previous research [96], oil and gas drilling wastes account for the second greatest waste after exploration and production wastes. In oil well drilling, drilling fluid circulates to keep the fluid pressure within the well constant and transport drill rock cuttings to the surface. This helps to keep the drilling bit cool and prevents it from overheating. Drilling waste is mainly composed of cuttings and drilling mud. It is a dark grey, high-viscosity paste. Drilling cuttings are composed of various kinds of rock particles, ranging in size from sand to gravel. The composition of drilling cuttings is intimately related to the rock formation’s composition and the chemistry of the drilling mud. Drilling fluid (sometimes termed drilling mud) is a synthetic chemical compound that improves drilling performance. It comprises a base fluid and various additives (mainly clay and organic stabilizers). Different fluid phases (mostly water, synthetic or natural oils, and gas or air) are utilized in industrial drilling fluids. Their formulation and quality are optimized for the physicochemical conditions in underground geological formations. As a result, drilling waste has a high concentration of inorganic and organic compounds, including heavy metals (such as Cd+2, Pb+2, Cr+3, Mn+2, and Cu+2) and petroleum hydrocarbons with a carbon chain length of 6–44, making them environmentally dangerous [97]. At the moment, waste drilling fluid is disposed of directly in landfills or through complex treatment processes such as chemical treatment, thermal treatment, stabilization/solidification, and bioremediation. Petroleum drilling wastes have been added to the list of wastes that may be processed and used productively. Recently, potential uses for these wastes in construction and building materials have been suggested, including road construction, asphalt pavement, as an aggregate in cold-mix and hot-mix asphalt, cement manufacturing, concrete, sand-crete blocks, bricks, and building ceramics. Drilling cuttings as solid waste include reactive calcium, silicon, aluminum, and iron oxides that may be used instead of limestone and clay as fine natural aggregate in the cement industry. They are either employed in trace amounts as fillers and components of the finished product or as an active component to enhance the technical properties of

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cement. Additionally, processed drill cuttings (i.e., cuttings that have been thermally treated to remove the hydrocarbon percentage) and screened or filtered cuttings with less liquid mud may be used as aggregates or fillers in the manufactured concrete, brick, or block as building materials. Cuttings that have not been treated are generally difficult to utilize in construction.

Oily Sludge The petroleum sector produces a variety of sludge wastes, including effluent treatment plants and bottom tank sludge. Sludge from oil refineries comprises sediments from separation processes, storage tank bottoms, cleaning of process equipment waste, biological sludge from wastewater treatment, and soil oil spills. For instance, a refinery producing 105,000 barrels per day generates around 50 t of oily sludge each year [98]. Petroleum sludge is formed primarily due to cooling below the cloud point; light terminates evaporation, combining with incompatible components and adding water for emulsion development. Oily sludge is classed as hazardous waste due to significant concentrations of various hydrocarbons; solid waste comprises heavy metals, metals, and metal salts. Meanwhile, several disposal and treatment methods for oily sludge remain the primary CO2 and CH4 emissions sources. According to Hu and his colleagues [99], conventional procedures such as landfilling or incineration are more hazardous to the environment than pyrolysis and solvent extraction. Petroleum sludge produced by refineries takes on a variety of forms and compositions. Sludge composition varies according to its origin, processing, and hydrocarbon recovery but generally ranges between 10% and 20% hydrocarbons and 5–20% solids and water. Vdovenko and colleagues [100] identified five distinct types of sludge: top layer sludge (PS1), fresh sludge (PS2), emulsive sludge (PS3), and suspension sludge (PS4), and bituminous sludge (PS5). Each kind of sludge has a unique chemical composition and physical qualities. Improper disposal of this hazardous material may significantly negatively impact the environment. Petroleum sludge may be processed and disposed of using a variety of processes, including thermal, mechanical, biological, and chemical approaches. Numerous techniques have been used to recover oil from petroleum sludge, including solvent extraction, freezing and thawing treatment, as well as electrokinetic, microwave, and ultrasonic irradiation. Incineration, oxidation, solidification/stabilization, and biodegradation are among the disposal techniques available after oil recovery. However, these methods are often expensive. The untreated reuse of oily sludge as a fuel or material resource in the building sector is a cost-effective and sustainable option. Due to the similar composition of various wastes to natural raw materials, they may be used in place of basic resource materials across multiple manufacturing processes, resulting in environmental and economic benefits. In conclusion, oily sludge production is unavoidable in the petroleum sector. Oily sludge requires good treatment due to its toxicity and negative environmental impact. The technology used to treat oily sludge should be determined by the

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sludge’s properties, treatment capacity, cost, disposal regulatory requirements, and time constraints. Several oil recovery and sludge disposal systems have been developed, some of them being used on a field scale. In general, one technique will not meet all of the reuse and disposal needs for various oily sludge wastes. Certain treatments may seem very promising in fuel recovery and/or decontamination of unrecoverable wastes, but their capital and/or operational costs may be prohibitively expensive, or their adoption on a large scale may be impossible. Other treatments, such as land farming and composting, may offer many applications and low operating costs for large-scale treatment, although the microbial degradation process may be lengthy. Certain techniques, such as centrifugation, enhanced oil recovery using surfactants, freeze/thaw, froth flotation, and bio-slurry treatment, may be more appropriate for treating oily sludge with high moisture content. However, other processes, such as incineration, pyrolysis, and stabilization/solidification, need the sludge to lower its moisture content.

Conclusion Hydrocarbon contamination caused by petrochemical sector activities is currently one of the most serious environmental issues. Accidental discharges of oil products are a major environmental problem. Carcinogens and neurotoxic organic pollutants have long been associated with hydrocarbon components. When considerable volumes of pollutants are present, current disposal methods such as cremation or burial in unsafe landfills can become prohibitively expensive. Mechanical and chemical procedures commonly remove hydrocarbons from contaminated locations, but they are ineffective and costly. Because it is cost-effective and leads to complete degradation, bioremediation is a potential approach for treating these contaminated sites. Bioremediation is based on biodegradation, which refers to the complete conversion of organic pollutants into carbon dioxide, water, inorganic compounds, and cell protein, and the transformation complex organic pollutants into simpler organic compounds by biological agents such as microorganisms. Hydrocarbon pollutants can be degraded by various indigenous microbes in water and soil. This publication provides an updated summary of microorganisms that degrade petroleum hydrocarbons in various habitats. Also, it discusses the utilization of petroleum industry wastes as sustainable building materials; these wastes include drilling wastes and oily sludge.

Future Perspectives To maximize the renewal of the environment and maintain our global carbon cycle, microbial applications are vital solutions that can be translated into biodegradation. Many synthetic chemicals and materials have great ecotoxicological effects, such as heavy metals and hydrocarbons, which can be transformed and degraded by microorganisms. Moreover, using selected cultures under optimum growth conditions,

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common numbers of these materials were estimated in the laboratory, which reflected the potential degradability. Under natural conditions, biodegradation is considered limited due to several factors, such as the insufficient domain with the necessary substrates, the microorganism’s competition, undesirable external conditions like moisture, aeration, pH, and small pollutants bioavailability, and temperature. As a result, environmental biotechnology aims to solve and assist with the limitations above to allow microorganisms in bioremediation systems. So, biostimulation is important to support the functions of the indigenous microorganisms in contaminated biotopes and enhance their degradative activities. Applying such techniques as bioremediation to pollutants should make a visible difference in our capability to minimize waste and eliminate pollution for a more sustainable future. On the other hand, genetic engineering plays a vital role in the biodegradation abilities of microorganisms. However, there are several problems related and linked to the application of genetic engineering in the real world. Another point of view for the future is the degradation of refinery oily waste, which may be a cost-effective fermentation technique for biomass production, similar to the conversion of other hydrophobic compounds. Depending on this view, recent studies have established the culture’s value as a de-emulsifier for oil-field emulsions, a liquid volatile organic hydrocarbon, and an inoculant for bioaugmentation of TPH-contaminated soil, which can be more promising for an investigation into the mixed culture’s potential to serve other environmental pollutants as major components of crude oil. We are looking for novel and low-cost transformation applications for this culture without previous acclimation or exposure to the target substrate of interest.

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Index

A AATCC Soil Burial Method 30-1993, 1475 Abaca fibers, 1462, 1463 Abiotic biodegradation, 1412–1413 Abiotic biodeterioration, 583, 585 Abiotic bioplastic biodegradation, 586 Abiotic degradation, 1177 and chemical degradation, 26 definition, 24 factors influencing, 27 importance of, 24 role of, 24 Abiotic deterioration, 585 Abiotic hydrolysis, 825 Absorption of inorganic substances, 181 mechanism, 181 Acetate, 1546 Acetoacetyl-CoA, 822 Acetochlor, 850 Acetoclastic/acetotrophic methanogenesis, 1491 Acetogenesis, 90, 92–93, 1491 Acetone biodegradation, 743 Acetone-removing efficiency, 744 2-Acetylalkanoic acids, 1501 Acetyl-CoA, 822 Acetyl groups, 1471 Acidobacteria, 1108 Acidogenesis, 90–92, 1416, 1491 Acids, 231 Acinetobacter sp., 848 Acrylamide, 237 Acrylic acid, 745 Acrylic monomers, 1376 Actinomycetes, 1292, 1295 Activated partial thromboplastin time (APTT), 1079 Activated sewage sludge test, 1464

Activated sludge simulation test, 1119 Activated sludge treatment (AST), 1107, 1108 Activators, 1296 Active packaging material, 559 Actuators, 401 Adaptation, 121, 122 Adipose-derived MSCs (ADSCs), 1068 Adsorbent-based nanocellulose-composites, 768 Adsorbents, 766, 772, 773, 775, 776 Adsorption, 772 Adult stem cells (ASC), 1137, 1139, 1167 Advanced eco-friendly alkyds coatings, 1378 Advanced hyperbranched alkyd nanocomposites, 1380 Advanced oxidation processes (AOPs), 792, 1624 Aerobic-anaerobic conditions, 1183, 1298 Aerobic bacteria, 588 Aerobic bacterial granules (ABGs), 226 Aerobic biodegradation, 64, 66, 184, 185, 243, 551–554, 1117, 1414–1415, 1454, 1455, 1488–1489 Aerobic biological analysis, 751 Aerobic circumstances, 1114 Aerobic composting, 740, 1276 Aerobic composting test, 1119 Aerobic conditions, 1111 Aerobic degradation, 209, 911–914 of hydrocarbons, 1493–1497 Aerobic deterioration, 1422 Aerobic microorganisms, 1292 Aerobic respiration, 1454 Aerobic transformation, 1108 Aerosol particles in air, 1192 Affordability, 1478 Agar bioplastics chemical structure, 509 plasticizers, 509 red algae, 508

© Springer Nature Switzerland AG 2023 G. A. M. Ali, A. S. H. Makhlouf (eds.), Handbook of Biodegradable Materials, https://doi.org/10.1007/978-3-031-09710-2

1679

1680 Agar bioplastics (cont.) structural polysaccharide, 508 thermo-reversible gelation, 508 transparency and retraction ratio, 509 water evaporation, 509 Agaropectin, 508 Agarose, 508 Agave sisalana, 1461 Ag NPs in the CMC/CNCs matrix, 1316–1318 Agricultural crop residues, 736 Agricultural cycle, 827 Agricultural polymers, 490 Agricultural products, 1634 Agricultural soils, 1107 Agricultural waste, 13, 1295 Ag/starch blend film, 1314 Air bioremediation, 183 Airborne particles, 1201 Air filtration, 1201 Air permeability, 1200 Air pollution, 868 γ-Al2O3 nanorods, 1383 Albeit normal polymers, 557 Alcaligenes eutrophus, 483 Alcohol dehydrogenase enzyme, 226 Alcohol ether sulfate (AES), 1628 Alcohol ethoxy sulfates, 1507, 1511 Algae, 117, 159–160, 238, 239, 802, 803 bioplastics, 494, 593 classified, 503 oil, 1385 and protozoa degradation, 919–920 Algae-derived polymers food packaging and coatings, 515–518 Mulching films, 521 pharmaceutical and biomedical applications, 518–520 water purification and desalination, 520–521 Algal bioremediation, 186 Algal cellulose, 514–515 Algal decoloration, 803 Algal-derived polymers, 517 Algal enzymes, 117 Algal-mediated biodegradation, 117 Algal proteins, 513–514 Algal starch, 515 Alginate(s), 431–432, 988, 989, 992, 1069–1071 bioplastic films, 504 bioplastics, 492, 506 biopolymers, 1023 brown algae, 503

Index chemical structure, 504 electrostatic interaction, 505 external gelation, 505 guluronic acid blocks, 503 internal gelation method, 504 ionic-crosslinking, 505 ionic crosslinking process, 504 M/G ratio, 503, 504 Na-alginate solution, 504 sodium salt, 503 structural polysaccharides, 503 Alicyclic compounds, 73 Aliphatic-aromatic co-polyesters, 369 Aliphatic hydrocarbons, 73 Aliphatic polyester(s), 369, 491, 1311 based polymeric structures, 1597 biotic and abiotic degradation, 381 Aliphatic polymers, 580 Alkaloids, 1300 Alkanes, 73 paraffin, 226 Alkoxylates, 1631 Alkyd resins classification according to chain length, 1374 fatty acid composition, 1374, 1375 fatty acid procedure, 1374 monoglyceride procedure, 1374 Alkyl alkoxy methylammonium chlorides, 1511 Alkylbenzene sulfonate, 1636, 1639 Alkyl dimethyl imidazolium chlorides, 1511 Alkyl phenol polyoxyethylene ether (APEO), 1631 Alkyl polyglucosides, 1514 Alkyl sulfonate (AS), 1628 Alkynes, 227 biodegradation, 227 Allotropes of carbon, 614 Alloying, 1216 Alpha-cellulose, 1460 Alzheimer’s disease, 1218 American Society for Testing and Materials (ASTM), 368 Amine-oxide-based surfactants, 1632 Amines, 1630 Amino acid(s), 1457, 1472, 1473 glycine, 1540 Aminopolycarboxylates, 1537–1540, 1551, 1552 capacity, 1540 ethylenediamine-N, N'-disuccinic acid (EDDS), 1543–1545, 1549

Index iminodisuccinic acid, 1541, 1542, 1549 metal complex, 1541 methylglycinediacetic acid, 1542, 1543, 1549 nitrilotriacetic acid, 1544–1547, 1549 tetrasodium glutamate diacetate, 1547, 1548 Ammonia, 101–102, 870, 1293, 1295, 1551 Ammonium, 1187 Amphoteric surfactants, 1517–1518, 1522 biodegradability, 1633 characteristics, 1633 classified, 1633 cost, 1632 manufacturing process, 1632 methyl or carboxyl groups, 1633 positive and negative charge centers, 1632 Amylopectin, 542, 1185, 1197, 1344 Amylose, 542, 1185, 1197, 1344 Anacardic acid (AA), 880 Anaerobic biodegradation, 64, 66, 88–96, 103–106, 185, 209, 551–554, 915–916, 1415–1416, 1455, 1489 acetogenesis, 1491 vs. aerobic biodegradation, 1493 chemical reaction, 1489 fermentation/acidogenesis, 1491 hydrolysis, 1491 methanogenesis, 1492 stages in, 1492 test, 1119 Anaerobic degradation, 487 carboxylation, 1501 fumarate addition, 1499 of hydrocarbons, 1499–1502 hydroxylation, 1500 reverse methanogenesis, 1501 Anaerobic digestion, 870, 874, 1276, 1285, 1287–1289, 1300 acetogenesis, 92–93 acidogenesis, 91–92 ammonia, 101–102 application fields, 89 assessment techniques, 96–99 biochemical methane potential, 96–97 carbon-to-nitrogen ratio, 103 design of the digesters, 105 energy and economic recovery of biogas, 94–96 factors affecting, 99–105 hydrogen potential, 99 hydrolysis, 91 industrial applications of, 43–49

1681 kinetics of biogas and methane production, 98–99 load and organic composition, 104 methanogenesis, 93–94 microbiology and metabolic pathways of, 91–94 pretreatment, 104–105 sulfide, 103 temperature, 99–100 Anaerobic environment, 1473 Anaerobic fermentation, 1291, 1490 Anaerobic sewage, 824 Anaerobic transformation, 1108 Angiogenesis, 937, 1056 Angiogenic factor drug delivery systems, 965 Aniline pentamer (AP), 397 Aniline pentamer crosslinking chitosan (AP-cs-CS), 398 Aniline tetramer (AT), 399 Animal-based biofibers, 327 Animal fibers, 1457 Anionic surfactants, 1506–1511, 1521, 1624 alkyl sulfonate, 1628 biodegradation, 1628 degradation of benzene, 1629 examples of, 1507 LAS, 1507 microbial species, 1629 negatively-charged hydrophilic polar groups, 1628 short-chain alcohols, 1629 Anodization, 1226–1229 Anoxic conditions, 1112 Anthraquinone dyes, 225 Anthropogenic activity, 1534 Anthropogenic pollutants, 224 Anti-aging cosmetics, 706 Anti-bacterial finishing of fabrics, 1468 Antibacterial properties, 705 Antibiotic(s), 126, 234, 775, 890 chloramphenicol, 519 production, 234 Antibiotic fluoroquinolone (QF), 239 Antibiotic-resistant bacteria, 1100 Anti-corrosive polyurethane coatings, 1383 Anti-inflammatory medications, 1107 Antimicrobial activity, 433, 887 activity of biodegradable materials, 624 agent, 623, 1189 antiviral agents, 1199 applications, 885

1682 Antimicrobial (cont.) and biodegradable food packaging materials, 885 chitosan nanocomposite, 888 polyurethane coatings, 1383 property, 885 Anti-thrombogenic materials[, 705 Antitumor activity, 707 Antiviral agents, 1201 Applications of CNTs, 654 biomedical applications, 654, 655 membranes filtration and adsorption, 655, 656 nanoelectronics, 654–656 Aquatic environment, 1096 Aquatic organisms, 1535, 1536 Aquatic wildlife, 1178 Arabidopsis thaliana phytochelatin synthase, 77 Arbuscular endomycorrhiza, 236 Arbuscular mycorrhiza, 236 Arbuscular mycorrhizal fungi (AMF), 916 Arginine-glycine-asparagine (Arg-Gly-Asn), 989 Aromatic compounds, 848, 1108–1110 Aromatic hydrocarbons, 227, 228 Aromatics compounds, 74 Arsenic, 249 Arteriovenous loop (AVL), 1054 Artificial biodegradable materials, 901 Artificial fibers, 1456 Aspergillus A. flavus, 142 A. niger, 688 A. tubingensis, 142 Asphyxiate aquatic life, 1642 Assimilation, 1326, 1424–1425 ASTM D 5209-92, 1327 ASTM D 5210-92, 1327 ASTM D 5247-92, 1327 ASTM D 5512-94, 1327 ASTM D 5988-12, 1467 Atomic force microscopy (AFM), 367, 1321 Auto-graft approach, 932 Automated determination of biodegradability, 1118–1119 Auxins, 233 Azadirachta indica, 1193 Azo dyes (ADs), 251, 1410 algal remediation, 1434 biodegradation using algae, 802, 803 biodegradation using bacteria, 798, 799 biodegradation using fungi, 800, 801 biodegradation using yeast, 801, 802

Index biological processes, 783 classification of, 796 enzymes, 1434–1438 fungal degradation, 1433–1434 role, 782 Azoreductase-mediated dye degradation, 251

B Bacillus, 147, 1102 B. cereus, 845 B. laterosporus DSP, 842 B. subtilis, 1120 B. thuringiensis B1, 1123 Bacteria, 114, 122, 147–156, 730, 798, 799, 1199–1202, 1278, 1292, 1428–1429 Bacterial biodegradation, 225, 226, 911–916 ABGs, 226 accumulation of plastic, 229 aerobic biodegradation, 184, 185 alkynes biodegradation, 227 anaerobic biodegradation, 185 anthraquinone dyes, 225, 226 aromatic hydrocarbons, 227, 228 cyclohexane, 227 processes, 229 Bacterial cellulose (BC), 278, 443, 620, 1575 Bacterial degradations, anthraquinone dyes, 226 Bacterial-mediated biodegradation, 113–115 Bacterial nanotube gene transfer method, 248 Bacterial strains, 226, 241 Barium titanate (BT), 958 Basic fibroblast growth factor (bFGF), 1065 Bast fibers, 1457 Batch tests, 1111 Batteries, 522–523 Bengal Delta, 1460 Bentonite (BT), 1606 Benzene-ring molecules, 1109 Benzene ring oxidation, 1635 17b-estradiol (E2) and (E1) hormones, 1102 Betaine-type surfactants, 1633 Bifunctional nanostructured materials, 1380 Bioaccumulation, 117, 1535 Bioactive material, 616 Bioactive pharmaceutical compounds, 185 Bioactivity, 937 Bioaerosols, 1200 Bioassimilation, 27, 488 Bioaugmentation, 76, 187, 245, 902 Bio-based biodegradable materials, 1183

Index Bio-based biodegradable plastics, 1426–1427 Bio-based fillers, 279 Bio-based materials, 866 Biobased packaging materials, 554 Bio-based plastics, 489, 574, 813–815 Bio-based polyester polyols, 486 Bio-based polymers, 419 Bio-based starch, 486 Bio-bioplastic degradation, 729 Bioceramics, 953 calcium sulfates, 948 di-calcium phosphate, 948 hydroxyapatite, 948 TCP, 948 Bio-ceramization, 963 Biochar, 1292, 1293, 1296 application on soil, 1265 carbon content in, 1248, 1267 characteristics, 1244 compost and, 1264 gasification, 1248 half-life of, 1249 vs. hydrochar, 1248 hydrothermal carbonization, 1247 hydrothermal process, 1247 life span of, 1249 in manure compost, 1264 mineral content in, 1245 mitigates microorganisms, 1267 mitigates pesticides, 1266–1267 nutrients supply, 1264 potential of, 1245 presence of, 1258 production of, 1245, 1246 properties of, 1246 pyrolysis, 1245 rice straw-derived, 1268 sorption capacity of, 1267 stability of, 1249 stable, 1246 surface area of, 1258 torrefaction, 1247 types of, 1266 via electrostatic attraction, 1267 wheat straw-based, 1268 Biochemical conversion, 874 Biochemical methane potential, 96–97 Biocompatibility, 4, 14, 368, 936, 984, 987, 995, 998, 1006, 1007 biopolymers, 328 of HA, 334 Biocompatible polymers, 393, 396, 397, 400 Biocompatible technologies, 1586

1683 Biocomposite polymers, durability of, 340 Biocorrosion, 1216 Biodegradability, 113, 937, 998, 1058 accuracy and reliability, 1641 analytical techniques, 1640 biodegraded primary compounds, 1642 biomass growth, 1642 DOC technique, 1640 fibers (see Fibers) high-performance liquid chromatography, 1640 isotope-labeled compounds, 1642 mass balancing, 1642 negatively-charged anionic surfactant, 1640 quantitative chemical method, 1640 residual concentrations, 1641 surfactant, 1622, 1640, 1641 surfactant concentration, 1641 surfactant determination, 1642 surfactant residual concentration, 1642 Biodegradability of plastic, factors affecting, 576 chemical properties of polymer, 579 physical properties of polymer, 578 polymer additives, 579–580 Biodegradability of PPCPs automated determination of biodegradability, 1118–1119 bacteria/fungi, 1110 behavior in aquatic ecosystems, 1115 bioavailability, 1110 categories, 1111–1113 3,5-dichloro-ethylparaben, 1114 3,5-dichloro-4-hydroxybenzoate’s, 1114 DOC, 1114 environmental samples, 1113 inherent biodegradability (OECD 302), 1117–1118 inherent degradability, 1113 mineralization, 1110 molecular structure, 1113 NP, 1114 OP, 1114 pathways, 1096, 1098, 1099, 1102, 1120, 1121 physicochemical properties, 1113 polycyclic aliphatic compounds, 1113 ready biodegradability (OECD 301), 1115–1117 retention time, 1113, 1114 WWTP, 1111, 1115 Biodegradable agricultural plastics, 728 Biodegradable bioplastics, 593

1684 Biodegradable composites, with nanosized fillers bacterial cellulose, 443 cellulose, 443 lignocellulosic fibers, 440–442 Biodegradable conducting polymers biomedical applications of, 406 block polymer, 397 electrochromic applications, 402 electronic devices, sensors, and actuators, 400–401 energy conservation and storage, 402 graft polymer, 398 polymer hydrogels, 399, 400 polymeric composites, 398 properties and applications, 403 synthesis, 396 tissue regeneration, 403–405 water and wastewater treatment, 402 Biodegradable contaminants, 908–909 Biodegradable electrodes (BGE), 1571, 1572, 1577, 1582–1585 bacterial cellulose, 1575 biomass-derived electrodes, 1574 biomass-derived materials/compositing, 1575 biomass-derived precursors, 1576 biopolymer-containing biomass, 1574 biopolymer resources, 1573 carbon nanotubes, 1575 cellulose, chitin/chitosan, 1577 characteristics, 1571 composites, 1577, 1578 electrochemical activity, 1577 electrochemical investigations, 1577 electrochemical performance, 1580 ionic adsorption behavior, 1580 molybdenum-made electrodes, 1574 nitrogen-containing groups, 1579 pinecone-derived carbon, 1579 single-stage carbonization method, 1572 for supercapacitors, 1572 synthesized carbon, 1578 Biodegradable face masks, 1199–1202 Biodegradable face shields, 1199–1202 Biodegradable fibers abaca fibers, 1462, 1463 applications, 1469, 1470 cotton, 1457, 1458 and eco-friendly, 1459 enzymatic hydrolysis, 1471–1474 flax fibers, 1458, 1459 in geotextiles, 1470, 1471

Index hemp fibers, 1459, 1460 jute fibers, 1460 kenaf fibers, 1461 lyocell fibers, 1463, 1464 nature fibers, 1455 non-wovens, 1469, 1470 ramie fibers, 1461 sisal fibers, 1461, 1462 and sustainable fibers, 1464–1466 synthetic fibers, 1455 wool, 1457 Biodegradable films and coatings biological synthesis, 885 biopolymer film and coating steps, 884 CS-nanocomposites, 887 electrospinning technique, 883 food packaging-based materials, 883 metal and metal oxides, 887 physical properties, 883 polymer-polymer interaction, 883 solvent casting, 883 Biodegradable hand gloves, 1199–1202 Biodegradable industrial materials’ potential azo dyes, 1433–1438 bacterial biodegradation and bioconversion of industrial lignocellulosic streams, 1428–1429 biodegradable film-forming materials, 1432 cellulose and derivatives, 1430–1431 chitin and chitosan, 1431 composite materials, 1439–1440 ecological isolation of wastewater, 1433 edible films and coatings, 1430 lignocellulose-degrading bacteria, 1435 lignocellulose-degrading enzymes, 1435 lignocellulose-degrading fungi, 1435 lipid compounds, 1431–1432 nanocomposite packaging materials, 1432–1433 packaging materials based on biodegradable polymers and nanocomposite, 1429–1433 plastics, 1422–1428 proteins, 1432 starch, 1431 Biodegradable inorganic nanocomposites, 605 antibacterial activity, 623–624 approaches to produce, 611 biomedical applications, 628–635 carbon nanostructures, 614 cellulose nanocrystals, 608 classification of, 610–621 dry process, 627–629

Index future research, 636–637 mechanical properties, 624–625 nanocellulose fibres, 619–621 nano-fillers particles, 612–614 nano-hydroxyapatite, 615–619 scaffold material for bone, 629–631 stem cells, 631–632 thermal properties, 625–626 tunable biodegradability, 622–623 type of composites, 607 wet process, 626–627 Biodegradable materials, 9–11, 966, 1301, 1571 advantages and disadvantages, 1584–1585 face masks, 1191–1199 plastics biodegradation, 7–10 polymers biodegradation, 6–8 utilization, 1183 Biodegradable nanocellulose (BNC), 762 beneficial effects, 767 drugs, 774, 775 fertilizers, 773, 774 organic compounds, 770 pesticide, 771, 772 properties (see Properties of BNCs) removal of dye pollutants, 767–769 synthesis (see Synthesis of BNCs) types, 763 uses, 767 water/wastewater treatment processes, 768 Biodegradable nanocomposite(s), 963 bone scaffold, 952 calcium-phosphate based composites, 953 inorganic phase, 953 organic phase, 953 supercapacitor electrodes, 1582–1584 Biodegradable non-woven fibers, 1469 Biodegradable packaging materials bioplastic industry, 875 biopolymers, 876 economic model, 875 hydrophobic substances, 875 Biodegradable plastics, 123, 574, 575, 577, 594, 1183, 1452, 1453 advantages by bacteria, 830 compostability, 830 petroleum, 830 recycling, 829 traditional products, 830 waste produced, 829 aerobic or non-aerobic, 482 applications, 492–493 bio-based plastics, 815

1685 biodegradable, 490 biosynthetic pathway, 822 cellulose-based plastics, 818 chemical/biotechnological processes, 815 chemical formula representation, 817 classification, 814 cyanobacteria, 822–823 disadvantages cost, 831 metals, 830 methane, 830 ocean pollution problems, 831 processing and recycling, 831 fermentation, 815 in food packaging, 816 fossil-based, 818 fossil-based raw materials, 815 fossil resources, 815 hydrolysis, 816 natural polymers, 481 PBAT, 819 PBS, 819 PHAs, 816–818, 821 PHB components, 821 photodegradable, 490 PLA, 815–816 polymers, 815 properties, 482–483 protein-based plastics, 818 PVA, 820 renewable raw materials, 820–821 scategories, 490 semibiodegrdable, 490 smicrobial fermentation, 490 source of raw material, 491 synthesis, 483–487 transgenic plants, 822 types, 489–492 Biodegradable polymeric materials (BPMs), 1059 Biodegradable polymers, 321, 368–370, 582, 813, 981, 984, 1183, 1298, 1341 banning of problematic conventional plastics, 309–310 bio-composites, 37 biodegradation mechanism for, 548–550 biopolymers (see Biopolymers) capacity of, 37 carbon dioxide (CO2) and water (H2O) molecules, 1310 cellulose, 543 challenges and mitigation, 308–311 challenges and opportunities, 296–300

1686 Biodegradable polymers (cont.) characteristics of, 1146–1147 chemical and physical properties, 1184 chemical structure, complexity, and compositions of, 281 classification of, 33–39 classified, 1184 completely, 37 vs. conventional plastics, 546–547 definition, 23 degradation evaluation, 266 degradation in soil environments, 550 deposit refund schemes policy, 310–311 derived from petroleum resources, 266–274 dissolution and degradation, 1183 emerging and advanced technologies, 554–559 extended producer responsibility, 310 face shields and face masks, 1184–1191 factors affecting biodegradation, 281–284 future of, 285 future research, 560–563 global production of, 301, 302 green economy, 561–562 home products, 557 market size of, 264 materials, 1310 in medicine and medical approaches, 556 modification in synthetic strategies for, 309 nanotechnology, 557–560 natural, 35, 301 from natural resources, 274–280 occurence, 26 packaging industry, 555–556 partially, 37 polybutylene succinate, 304–305 polycarbonates, 307–308 polyhydroxyalkanoates, 303–304 polylactic acid, 305–307, 545 polylactide, 305–307 soybeans, 544 starch plastics, 542 starting materials for, 264, 265 sustainable design for product development, 562–563 synthesized, 1321 types of, 37 Biodegradable polymers in cardiac tissue engineering alginate, 988, 989, 992 chitosan, 987, 988, 992 collagen, 986, 987 fibrin, 984, 989, 990, 992

Index gelatin, 992 hyaluronic acid, 990–992, 994 matrigel, 990–992 natural polymers, 984, 992 silk fibroin, 985, 986, 992 Biodegradable polysaccharide, 492 Biodegradable scaffolds, 982, 997, 1018 Biodegradable sensors, 6 Biodegradation, 113, 138, 180, 582, 584, 1278, 1301, 1408, 1409, 1411, 1412 abiotic, 1412–1413 abiotic degradation, 24 adsorption and absorption, 1419 aerobic, 1414–1415, 1454, 1455, 1488–1489 aerobic biodegradation, 64, 66 on air purification, 743–745 algae and protozoa role, 238–241 alicyclics compounds, 73 aliphatic hydrocarbons, 73 alkanes, 73 among aromatic compounds, 42 anaerobic, 41, 1415–1416, 1455, 1489–1492 anaerobic biodegradation, 64, 66 animals and plants, 813 in aquatic environments, 487 aromatics compounds, 74 bioassimilation, 27, 488 bioavailability, 1420 bioceramics, 947–948 biodegradable polymers and microorganisms, 124 biofragmentation, 488 bioglass, 949 biological factors, 71, 72, 1359, 1417–1419 of biomass, 1258 of bioplastics, 488 bioremediation of contaminants, 69 biotic, 1413–1414 bisphenol, 225 by body fluids, 374 bone scaffolds, 945 BTE, 946 CEN in, 23 classification of enzymes in, 161 cometabolism, 67, 68 contaminant, 42 contaminant migration in groundwater, 1420 cycle of discarded biodegradable polymer, 296 cyclohexane, 227

Index definition, 23, 59, 901–902, 1278, 1453, 1454, 1653 dioxins, 74 ecosystem balancing, 751 efficiency, 31 end-products formed from, 23 environmental conditions, 22, 487 environmental factors, 1419–1420 environmental impacts, 11–12 extent of contaminant degradation, 1418 extracellular enzymes, 813 factors affecting, 28, 34, 299 fermentation, 66 fiber production, 1451 foods and agricultural impacts, 13–14 fundamentals of, 5 future prospective, 755 gaseous end products, 813 genetically modified microbes, 76, 77 green, 813 halogenated aliphatics, 73 health impacts, 12–13 heterocyclic compounds, 74 of human life, 753–754 of hydrocarbons, 41 hydrolysable plastics, 489 importance of, 5–6 improvement of productivity of plants and animals, 750–751 industrial and technological impacts, 12 industrial applications of, 37–41, 48, 49 industrial composting, 488 in laboratory and environmental conditions, 115 LD and HD, 489 lignin-based polymer, 489 mechanism, 24, 1326, 1327, 1412–1416 mechanism for biodegradable plastics, 300 medical waste (see Medical waste) metal extraction (see Metal extraction) metals, 944–947 metals biotransformation, 62, 63 microbial, 41 (see also Microbial biodegradation) microbial interaction with inorganic pollutants, 61 microbiological biofilms, 813 microorganisms, role of, 22 microorganisms participating in, 139 misconception, 813 moisture, 1418–1419 molecule changes, 60 national standards in, 23

1687 natural and pollution-free, 487 natural mechanism, 813 nitrogen, 71 organic compounds, 1454 organic matter, 70 of organic pollutants, 60 PASHs, 234 pathway, 1653 pathways of di-n-butyl phthalate, 157 PCBs, 74 PCL degradation, 29 pesticides, 75 petroleum hydrocarbon (see Petroleum hydrocarbons) pH, 72, 1419 physical changes in materials, 22 plastic polyethylene bags, 489 plastics, 813 polycyclic aromatic hydrocarbons, 1658 polymeric plastics, 488 of polymers, 26, 30 principles of, 24 process, 488 protozoa, 139 rate, 49 rates of contaminant degradation, 1417 3R (diminish, recycle, reuse) philosophies, 1451 recycling wastes and impurities, 1451 redox conditions, 71 redox potential, 1421 requirement for, 1416–1417 roles of microorganisms in, 141–145 salinity, 72 soil matric potential, 1420–1421 specificity of, 1664–1665 surfactants, 1667–1668 sustainability, 1451 temperature, 72, 1418 tests, 1327, 1328 textiles (see Textiles) using algae, 159–160 using fungi, 156–158 waste reduction, 1451 on water purification, 746–748 white-rot fungi IZU-154, 489 of xenobiotics, 145 Biodegradation by soil microorganism, 371 assimilation, 373 bio-fragmentation process, 373 Biodegradation challenges and composting actinomycetes, 1295 activators, 1296

1688 Biodegradation challenges and composting (cont.) agricultural wastes, 1295 agronomic evaluation, 1297 ammonia, 1293 biochar, 1293 biofilters, 1294 biofiltration, 1294–1296 bio trickling filter, 1295 bisphenol A (BPA), 1297, 1298 bulking agents, 1295 co-composting, 1296, 1297 gas treatment, 1294 methane, 1298 microbial communities, 1293 microorganisms, 1296 natural filter materials, 1295 nonylphenols, 1297, 1298 odors, 1293 organic materials, 1296 organic pollutants and endocrine disruptors, 1297 polyaromatic hydrocarbons (PAH), 1297 polychlorinated biphenyl, 1297, 1298 polymer biodegradation, 1298 recycling for sustainable management of bioplastics, 1298, 1299 trimethylamine, 1293 volatile organic compounds, 1293 waste management, 1292 Biodegradation of azo dye algae, 802 bacteria, 798 fungi, 800 yeast, 801 Biodegradation of CNTs, 661–663 economic cost, 668 enzymatic degradation, 667, 668 microbial degradation, 662–666 thermal degradation, 661 Biodegradation of plastics biodegradable plastic, 575 compostable plastic, 575 degradable plastic, 575 Biodegradation of polymers abiotic biodeterioration, 583, 585 assimilation and mineralization processes, 587 biodeterioration, 583 biofragmentation, 587 biotic degradation, 585–586 phases, 582

Index Biodegradation on soil fertility agricultural crop residues, 736–740 herbicides, pesticides, and insecticides, 733–736 oil, 740–743 of plastics/bioplastics, 724–733 Biodegradation polymers (BPs), 1048 angiogenesis, 1060 biodegradable polymeric materials, 1060 formulations, 1061 natural biodegradation polymers, 1061–1068 synthetic BPs, 1071–1078 Biodegradation reactions, 62 Biodegradation technology, 129 Biodegraded contaminants, 60 Bio-deterioration, 198, 210, 211, 229, 370, 583, 1326, 1423 biodeterioration, 372 chemical deterioration process, 372 enzymes, 372 physical deterioration process, 371 Biodiversity, 576 Bioemulsifiers, 1633 Bioenergy production, 872 Bioengineering process, 970 Bio-fabrication methods, 1162 Biofertilizers, 233 Biofibers animal-based biofibers, 327–328 definition of, 321 plant-based, 323–326 Biofilms, 141 casting methodology, 1323, 1324 characterization, 1322, 1323 CS/gelatin, 1318 development, 143 extrusion methodology, 1324 film blowing methodology, 1324 HSPI, 1317 (KGM)/PCL/Ag NPs, 1321, 1322 LCNFs, 1319 organo nanoclay/CS, 1321 PBHV/NR based blend, 1319 PHAs, 1319 PHBV/natural rubber (NR) blend, 1319 PLA, 1320 properties, 1324–1326 pure starch, 1314 TEGO/PCL, 1321 TiO2 NPs, 1315 TPS/CS oligomer blend, 1315 Biofilters, 1294

Index Biofiltration, 183, 1294–1296 Bio-fragmentation, 211, 229, 283, 373, 549, 587, 1326, 1358, 1423–1424 Biogas, 94, 740, 874, 1287 Bioglass, 622 bioceramics nanocomposite scaffolds, 953 bone regeneration, 949 degradation rate, 949 elastic modulus, 949 metal nano-composite scaffolds, 953 polymers nanocomposite scaffolds, 954 porosity, 949 silicate, 949 Bioinks, 943 Bioleaching, 61, 181 Biolistic transformation technique, 246 Biological cell assembly, 1054–1055 Biological enzymes, 72 Biologically modified montmorillonite (BMMT), 1606 Biological method, 764 Biological oxygen demand (BOD), 73, 792, 1118, 1640 Biological reactors for air purification, 752 Biological sewage treatment, 1537 Biological therapy, 1114 Biological transformation of PPCPs, 1101–1103, 1114 Biological waste treatment, 829 Biomarkers, 245 Biomass, 724, 730, 731, 740, 750, 751, 1556 acceleration of, 1247 biodegradation of, 1258 gasification, 1248 lignin-rich, 1245 pre-treatment, 1246 source of, 1245 torrefied, 1247 type of, 1246 Biomass-derived carbon, 1573 Biomaterials, 393, 396, 403, 980, 1050 applications, 354 classification, 355 Biomedical, 1185, 1186 applications, 518–520 Biomedicine, 471 and medication delivery, 466 Biomineralization, 61 Biomodification, see Biotransformation Biomolecules, 1049 Biomorphic transformation, 963

1689 Bionanocomposites advantages in biomedical applications, 628 interaction with biological entities, 633–635 layered-particle-reinforced, 612 types based on morphology, 612, 613 Bionanomaterials, 611 Bio-oil, 873 Biopaper, 880 Bioplastic biodegradation abiotic, 586 biomass residues, 578 carbon-bound utilization in polymer, 578 Bioplastics, 574, 576, 582, 590 advantages, 591–592 aerobic and anaerobic biodegradation, 551–554 algal cellulose, 514–515 algal starch, 515 based on algal proteins, 513–514 batteries, 522–523 biodegradation of water-insoluble polymers, 732 biofragmentation, assimilation and mineralization, 589 capacities of global production, 726 catering products manufacturing process, 492 classes, 537 classification and examples, 575 classification of, 489, 537 commodity, 502 containers and bottles, 493 degradability, 593 disadvantages, 546, 591–593 electricity conduction, 522 electromagnetic interference shielding, 522 enzymatic degradation of bioplastics, 730 EPS, 515 estimated land use for production, 727 examples, 606 future perspectives, 494 global production of, 725, 726 macro- and microorganisms, 502 Mirel bioplastic, 557 packaging business, 493 vs. petroleum plastics, 548 plant-based substances, 735 production, 301 production capacities, 732 production capacities in the world continents, 728 reuse via photosynthesis, 731

1690 Bioplastics (cont.) sustainable manufacturing strategy, 554 toxicological indicator of plant-based substance, 734 types of, 606 types of degradable plastics, 729 waste management, 540, 594 BiopolTM, 484 Bio-polyethylene (bio-PE), 814 Biopolymer chitosan, 331 natural, 334 and wound healing, 329 Biopolymer-containing biomass, 1574 Biopolymeric substances, decomposition rate, 374 Biopolymers, 517, 578, 724, 875, 1298, 1326 aliphatic polyesters, 1311 biodegradable bio polyesters, 878 1-4 butanediol, 877 challenges, 879 chemically synthesized biopolymers, 1319–1322 chitosan sources, 878 classifications, 877 classified, 1311 crystalline nature, 880 decomposition, 1310 hydrophilic and crystalline materials, 1311 living organisms/biomass or indirectly synthesized, 1310 methods, 878 microorganisms/genetically modified microbes, 1311 natural biomass-extracted, 877 natural monomer sources, 876 PGA, 879 PHA production, 879 PHBV purification processes, 881 plasticizing agents, 878 polycondensation of succinic acid, 877 polyesters/PHAs based biopolymers, 1318–1319 polysaccharides-based biopolymers, 1312–1317 property, 1310 protein-based biopolymers, 1317–1318 scaffold tissue, 983 semirigid recyclable packaging, 880 structural analysis of biodegradable materials, 1312 synthesis and resources of PHA, 879 valuable products, 876

Index Bioremediation, 117, 181, 187, 244, 250, 1409, 1654, 1671 algal, 186 classification, 245 ex situ bioremediation, 904 GEMs (see Genetically engineered microorganisms (GEMs) in bioremediation) genetically modified microorganisms in, 1654 historical and ecological context, 903 in situ bioremediation, 903–904 limitations, 245 and metagenomics, 851 microbial processes for, 1656 microorganism remediation, 906–908 oil sludge, biosurfactants, 1667 phytoremediation, 904–906 proteomics in, 853 transcriptomics in, 852 yeast, 186, 187 Bioremediation processes air bioremediation, 183 soil bioremediation, 183 water bioremediation, 183, 184 Biosensor definition, 455 eyeball, 455 fluorescent, 470 glucose, 470 Biosorption, 61, 181, 1556, 1558–1562 Biostimulation, 76, 183, 187, 244, 245 Biosurfactants, 580, 1518–1520, 1523 biodegradability, 1633 categorized, 1633 chemical and surface characteristics, 1633 exopolysaccharides, 1633 industrial settings, 1634 low-cost ingredients, 1634 in molecular weight, 1633 rhamnolipid surfactants, 1634 solubilization, 241 strengths, 1634 Biosynthesis of collagens, 1019–1020 Biotic biodegradation, 1413–1414 Biotic degradation, 585 definition, 24 factors affecting, 29–30 mechanisms of, 24, 1117 steps in, 26–28 Biotransformation, 61, 181 of metals, 62 Bio trickling filter, 1295

Index Biowaste, 870 biodegradable food packing, 875 pomegranate fruit, 881 precursor, 1574 treatment, 871 into valuable products, 872 Bisphenol, 1107 Bisphenol A (BPA), 1191, 1192, 1297, 1298 Black soldier fly, 1301 Bleaching agents, 1543 Blending, 517 Block-copolymer nanoparticles (BPN), 706 Block polymer, 397, 398 Bond cleavage, 145 Bone architecture, 933–934 cells, 933–935 defects and healing mechanism, 935–936 defects repair, 932 fixing, 405 grafting, 615 implants, 1035–1038 inter-scale, 934 multicellular units, 934 scaffolds (see Bone scaffolds) and vascular network formation analysis, 1036 Bone graft replacements (BGSs), 1158 Bone marrow-derived mononuclear cells (BMMNCs), 1077 Bone marrow stem cells (BMSCs), 1075, 1142, 1143, 1167 Bone morphogenetic protein 2 (BMP-2), 1158 Bone regeneration, 932, 950, 1031–1036 Bone remodeling, 615 Bone repair and bone regeneration strategies, biomaterials and nanomedicine growth factors, role of, 1160 nanoparticle-based strategies, 1157–1159 scaffold-based strategies, 1158–1161 scaffolds for GF delivery, 1161–1162 Bone scaffolds, 330, 332 bioactivity, 937 biocompatibility, 936 biodegradability, 937 4D printing technique, 943 fabrication, 938–944 graft materials, 936 mechanical properties, 937 micro-architecture, 937 requirements, 937 self-healing tissue, 936 Bone sialoproteins, 933

1691 Bone Tissue Engineering (BTE), 384, 931 biodegradable materials, 946 biomaterials/scaffolds, 932 bone regeneration, 932 gratitude, 932 nanocomposites, 932 scaffolds (see Bone scaffolds) triad of cells, 936 Boron nitride (BN), 958 Botanical filtration, 743 Breakdown mechanism, 182 Brevibacillus laterosporus, 1120 British Textile Technology Group (BTTG), 704 2-Bromoethanesulfonate (BES), 128 Brown adipose-derived SCs (BADSCs), 1071 Brown compounds, 1278 Brownian diffusion, 1201 Brown rot fungi, 1292 BTEX, 848, 1109 Bulk erosion, 372 Bulking agents, 1257, 1295 1,4-Butanediol diglycidyl ether, 519 β-Butyrolactone (BL), 821

C Calcium caseinate coatings, 1351 Calcium-cross-linked alginate hydrogel, 1070 Calcium-phosphate based composites, 953 Calcium-phosphate-based materials, 950 Calcium phosphate-based scaffolds, 1035 Calcium sulfates, 948 Cancer, 936 treatment, 405 Candida curvet, 801 Carbamates, 124 Carbamazepine, 1113, 1114, 1124–1125 degradation pathway, 1124, 1125 stracture, 1124 Carbazole degradation, 129 Carbofuran, 738 Carbohydrate-active enzymes (CAZymes), 213 Carbohydrate polymer, 483 Carbon, 1574, 1581 Carbon-based materials, 1585 Carbon-based nanoparticles, 419 Carbon dioxide (CO2), 1199 emissions, 1175 Carbon dioxide-free air, 1116 Carbon monoxide, 1276 Carbon nanofibers (CNFs), 1074 Carbon nanofiller, 1380, 1389

1692 Carbon nanohorns (CNHs), 647, 660 Carbon nanospheres, 1575 Carbon nanostructures, 614 Carbon nanotubes (CNTs), 400, 418, 614, 1083, 1385, 1389 applications, 645 biodegradation, 645 degradation, 659–660 environmental impact, 656–659 MWCNTs, 646, 647 MWNTs, 645 SWCNTs, 646 SWNTs, 645 Carbon quantum dots (CQDs), 400 Carbon-to-nitrogen ratio, 1297 Carboxylation, 1501 Carboxymethyl cellulose (CMC), 514, 1147–1150, 1343, 1344 Carboxymethyl chitin (CMC), 702 Carcinogen, 1099 Cardiac failure, 404 Cardiac grafts, 404 Cardiac matrix, 1055–1056 Cardiac progenitor cells (CPCs), 1049 Cardiac SCs (CSCs), 1049 Cardiac tissue engineering (CTE), 982–983, 1152 biological cell assembly, 1054–1055 BPs (see Biodegradation polymers (BPs)) categories, 1049 decellularization of the cardiac matrix, 1055–1056 heart tissue engineering, 1052–1053 natural/synthetic hybrid biodegradation polymers, 1080–1081 scaffold-less cell sheet/cell patch technology, 1053–1054 scaffolds and cells, 1049–1050 strategies, 1048–1050 Cardiomyocyte (CM), 982, 1049, 1152 Cardiovascular disease (CVD), 982, 1048 tissue engineering applications, 1151–1157 Cardiovascular progenitor cells (CPCs), 1071 Carrageenan, 492 ι-carrageenan membranes, 508 cations, 507 characteristics, 507 chemical structure, 507 Eucheuma, 506 Kappaphycus, 506 mechanism, 508 refined, 506 semi-refined, 507

Index sorbitol-plasticized films, 508 structure, 506 sulfated polysaccharides, 506 sulfate groups, 506 Casein, 1317 Casting methodology, 1323, 1324 Castor oil-derived alkyd, 1382 Catalytic or enzymatic pathways, 398 Catalyzed enzyme hydrolysis, 225 Catechol, 1121 Catechol 2,3-dioxygenase, 848 Catechol dioxygenases, 254 Cationic cellulose nanofibrillated (CCNF) paper, 774 Cationic surfactants, 1511–1514, 1521, 1624 biodegradation of QACs, 1630 biodegradation techniques, 1630 in lower doses, 1629 nitrogenous compounds, 1630 partial hydroxylation, 1630 QACs, 1630 quaternary ammonium compound, 1631 CBX-IBF, 1122 Ceftazidime, 1098 Cellobiohydrolases, 765 Cell patch technology, 1053–1054 Cell seeding technique, 942 Cellulases enzymes, 207 Cellulose, 207, 255, 277, 324–326, 401, 433–436, 443, 462–463, 491, 543, 620, 820, 1147, 1184–1186, 1190, 1315–1317, 1343–1344, 1455, 1457–1459, 1462, 1607 Cellulose acetate (CA), 815 Cellulose-based BGE, 1582 Cellulose-based plastics, 818 Cellulose ethers (CE), 426–427 Cellulose fibers, 1466–1468 mechanisms of enzymatic reactions, 1471–1473 Cellulose microfibrils, 764 Cellulose micro (nano) fibrillated structures, 427–428 Cellulose nanocrystallites (CNs), 326–327 Cellulose nanocrystals (CNC), 514, 515, 608, 1185, 1186, 1316 Cellulose nanofibrils (CNFs), 772, 774, 1185, 1186, 1319 Cellulose reinforcement, 885 Cellulosic fabrics, 1465 Cellulosic fibers, 1463, 1464 Central Pollution Control Board (CPCB), 794 Ceramic 1D nanomaterials, 1391

Index Ceramic nanocomposite, 1383 Cereplast, 557 C6-fluorocarbon spray-coated cotton, 1193, 1194 Chain cleavage, 607 Chelants, 1554 Chelating agents, 1538, 1548 aminopolycarboxylate, 1538 biodegradability, 1539 and chelates, 1538 concentration, 1554 vs. EDTA, 1553 eliminate/chelate key metals, 1538 extraction efficiency, 1538 soil washing, 1538, 1539 vs. ethylenediaminetetraacetic acid (EDTA), 1539, 1540 Chelonibia patulaisis, 687 Chemical biodegradation, 1413 Chemical commodity, 1626 Chemical contaminants, 901 Chemical conversion, 1224–1226 Chemical deterioration process, 372, 1358 Chemical elements in composting nitrogen, 1291 phosphorus, 1291 potassium, 1292 Chemical extraction of chitosan, 1021 Chemical fertilizers, 225, 230, 232 Chemically synthesized biopolymers polycaprolactone (PCL), 1321–1322 polylactic acid (PLA), 1320, 1321 Chemically synthetic biopolymers, 876 Chemical method, 394 Chemical mobility, 1107 Chemical oxidative technique, 394 Chemical oxygen demand (COD), 104, 239, 792, 1117 Chemical polymers, 490 Chemical recycling, 1204 Chemical vapor deposition (CVD) approach, 653, 654, 1386 Chemical waste degradation, 225 Chemoorganotrophic microorganisms, 144 Chicken manure bird’s carcasses in, 1253 compost as fertilizers, 1254–1255 composting method, 1262 direct application of, 1250 fresh, 1249 macronutrients, 1255 pathogens in, 1253

1693 Chitin, 878, 1068–1069, 1345, 1431 acetylated derivative, 683 anti-aging cosmetics, 706 anti-allergic effect, 706 antibacterial properties, 705 anti-thrombogenic materials, 705 antitumor activity, 707 biomedical application, 690, 691 biosynthesis, 685, 686 cancer diagnosis, 702 chemical structure, 683, 685 chitin-based dressings, 704 cuticle, 710 deacetylation of, 684 decomposition, 709 degradation, 688, 689 enzymatic pathways, 685 fibers, 988 future prospects, 711 hemostatic materials, 705 hydrogen bonding, 684 isolation at industrial level, 687 major forms, 681 nanofibrils, 707 N-deacetylation, 684 in ophthalmology, 704 synthetase, 686 tissue engineering, 690, 700 vaccine adjuvant, 708 in wound healing, 690, 701 β-chitin, 684 γ-Chitin, 684 Chitin/chitosan, 453–459 biopolymer sources, 1585 α-Chitin/nano-silver composite, 701 Chitin hydrolysis (GlcNAc), 688 Chitosan, 235, 278, 431, 434–437, 443, 505, 951, 987, 988, 992, 1020–1021, 1068–1069, 1185, 1191, 1195, 1196, 1198, 1201, 1345, 1431 anti-aging cosmetics, 706 anti-allergic effect, 706 antibacterial properties, 705 anti-thrombogenic materials, 705 antitumor activity, 707 applications, 681 biomedical application, 691 cancer diagnosis, 702 chitosan-based dressings, 704 cuticle, 710 deacetylation, 684 decomposition, 709 FE-SEM images, 886

1694 Chitosan (cont.) future prospects, 711 hemostatic materials, 705 mucosal immunity against allergic reactions, 707 nanocomposite, 888 in ophthalmology, 704 tissue engineering, 699 vaccine adjuvant, 708, 709 in wound healing, 701 Chitosan-based biomaterials, 1150–1151 Chitosan-based nanohydroxyapatite composite, 385 Chitosan-coated PBS filter, 1188 Chitosan glutamate (CSN), 708 Chitosan nanocomposites, 1609, 1610 Chitosan-PANI electrode, 402 Chitosomes, 686, 687 Chlorella, 159 C. vulgaris, 159 Chlorinated, 1290 chemicals, 185 pollutants, 66 solvents, 73 Chlorine-free bleaching processes, 1538 2-Chloro-N-(2-methyl-6-ethyl phenyl) acetamide (CMEPA), 850 Chlorpyrifos, 772, 773 Cholesterol esterase, 166 Chromatography, 1627 Chromium, 846, 847 Ciprofloxacin, 1098 biodegradation, 128 Circular economy, 1339, 1362, 1363 cis-dichloroethene (cDCE), 852 Citrate acid, 1548 Citric acid, 1548, 1551 Clay particles, 1536 Clean air delivery rate (CADR), 744 Climate change mitigation, 722 Clinoenstatite/tantalum nitride coatings, 1230 Closed bottle test (CBT) (ISO 10707), 1116 Clove essential oil (CEO), 1317 Clustered regularly interspaced short palindromic repeats (CRISPR-Cas) systems, 843 CMC/CNCs films, 1316 CMC/CNCs matrix, 1316, 1317 CO2/DOC test, 1117 CO2 evolution test (ISO 9439), 1116 Coated-based technology, 883 Coatings, 515–518, 878 Cocamides, 1631

Index Cocamidopropylamine oxide, 1517 Coconut husk fiber-reinforced biocomposites, 10 Coir fiber-reinforced biocomposites, 10 Collagen, 332, 949, 986, 987, 1061–1065, 1349, 1350 AuNPs, 1083 CNTs, 1083 fibrin, 1084 Comamonas testosteroni, 843 Combustion method, 358 Co-metabolic biodegradation, 67, 68 Co-metabolism, 211, 1653 Cometabolism, 67, 68, 145, 901 Compatibility, 1478 Compatibilizers, 514 Compatible electrolytes, for biodegradable electrodes supercapacitors, 1581–1582 Complexones, 1540 Composite materials, 1439–1440 Composite nanostructured delivery systems, 963–964 Compost ability of, 1268 aerobic, 1256 agents, 1261 aging of, 1263 chicken manure, 1254, 1262 duration of, 1261 fertilizer, 1268 immature, 1254 manure, 1263 maturity of, 1254, 1259 moisture content, 1260 municipal solid waste, 1259 odor trapping device in, 1269 organic matter in, 1265 pH for, 1259 pile, 1262 poultry wastes, 1254 quality of, 1256 sheet, 1263 structure of, 1265 unstable, 1254 Compostability, 830 Compostable plastic, 575 Composting, 539, 551, 553, 561 advantages, 1279, 1290, 1291 aerobic, 1276 vs. anaerobic digestion, 1276, 1285, 1287–1289 bacteria, 1277, 1278 vs. biodegradable, 1278–1281

Index and biodegradation challenges, 1292–1299 black soldier fly, 1301 brown compounds, 1278 capacity, 1300 challenges, 1279 chemical elements, 1291–1292 decomposition process, 1277 disadvantages, 1279 disposal, 1279 environmental benefits, 1277 high-temperature phase, 1277 inorganic nutrients, 1278 in-vessel composting, 1279, 1282, 1286 limitations, 1279 materials, 1278 microbes, 1292 microbial biomass, 1279 microbial metabolism, 1277 mixed nutrients, 1300 moisture content, 1288, 1290 organic matter, 1277 organic solid waste, 1277 oxygen, 1277 oxygen and pH levels, 1288 process management, 1301 static pile composting, 1283–1286 technology, 1300 temperature, 1288 vermicomposting, 1283, 1284, 1286 windrow composting, 1279, 1282, 1283, 1286 Computed tomography (CT), 940 Computer-aided design (CAD), 940 Concentrated emulsion method, 395 Condensation polymerization processes, 1176, 1353–1354 Conducting-polymer containers (CPCs), 403 Conducting polymers biodegradable (see Biodegradable conducting polymers) chemical methods, 394 concentrated emulsion method, 395 doping, 394 electrochemical methods, 394 metathesis methods, 395 photochemical polymerisation, 394 plasma polymerization, 395 pyrolysis method, 395 solid state polymerization, 395 Conductive fillers, 959 Conductive polymers, 1029 Consumption of fossil, 864 Contact lenses, 704

1695 Contaminated water treatment, 785 Continuous air purification process, 743 Controlled bioreactors-based landfilling, 1310 Controlled combusting, 1310 Controlled composting, 1310 Conventional activated sludge treatment plant (CAS), 1112–1114 Conventional carbonization, 1245 Conventional plastics, 480, 536 vs. biodegradable plastics, 546–547 degradation in soil environments, 550 waste management options, 538 Conventional pollutants, 785 Conventional polymers, 866 Copolymer’s lactide, 1060 Copolymerization, 491 Copper nanoparticles, 419 Corchorus capsularis, 1460 Corn oil, 1318 Corn protein, 1317 Corn zein, 1346–1347 Corynebacterium diphtheria, 708 Cosmetic wastewater, 791, 792 Cotton, 1457, 1458, 1471 fabrics, 1467 and fibers, 1464 Covalent immobilization, 255 COVID-19, 1199, 1203 disaster, 1202 epidemic, 1175, 1191 pandemic, 1175, 1176, 1203, 1623 Cprofloxacin biodegradation, 126 CRISPR-Cas system, 843 Crop-based bioplastics, 592 Crop residues, 736–740 Crop starch, 437 Cross-linked glucomannan, 430 Crosslinker, 519 Crude agro waste, 557 Crude oil, 847 aerobic degradation of hydrocarbons, 1493–1497 anaerobic degradation of hydrocarbons, 1499–1502 economic benefits, 1493 spills, 1658 Crustaceans, 679, 682, 687, 688, 711 Crystalline cellulose, 278 Crystalline hydroxyapatite, 630 CS/gelatin biofilms, 1318 CS-selenium nanoparticles (SeNP), 1069 Ca2+ signal transduction pathway, 957 Cu2O nanocubes, 1381

1696 Cu2O nanofillers, 1381 Cunninghamella elegans, 688 CuO/starch blend film, 1314 Cupriavidus metallidurans, 845 Cupriavidus necator AEO106, 852 Cupriavidus sp., 1109 Curing phase, 1292 Cuticle, 710 Cutinase enzymes, 489 Cyanobacteria, 147, 229, 822–823 Cycloalkane biodegradation pathway, 227 Cyclohexane, 73 Cyclohexatriene, 74 Cyclopentane, 73 Cypermethrin, 125 Cytochrome P450 enzymes, 254, 1667 Cytocompatibility, 953

D Daidzein, 544 3D bioprinting technique, 1143–1144 extrusion, 943 inkjet, 942–943 laser-assisted, 942 micro-valve, 943 4D bioprinting, 1144 3D chitosan/hydroxyapatite scaffold, 617 Decellularization, 1055–1056 Decellularized cartilage matrix (DCM), 967 Decellularized extracellular matrix (dECM), 1055 Decellularized matrices, 1050, 1051 Dechloromonas sp., 1108 Decolorization techniques, 800 Decomposition, 709, 710 Defatted soybean meal (DSM), 1349 Degradable compostable plastic, 729 Degradable conducting block polymer, 397 Degradable plastic, 575 Degradation of microorganisms, 1639 natural biodegradation polymers, 1057–1059 synthetic biodegradable polymers, 1052–1059 Degree of substitution (DS), 814 Dehalogenation reactions, 67 Dehydrogenase, 750 3D engineered cardiac tissue models, 1144–1146 Depolymerization, 123, 145, 588 Deposit refund schemes policy, 310–311

Index Desalination, 520–521 Desulfobulbaceae, 853 Desulfonation reaction, 1511, 1513 Detergent, 793 D-gal-inducing aging, 707 D-Glucopyranose, 1312 D-glucose, 1184 Diallyl 4-phenylphosphonicbenzoic (DAPB), 1371, 1372 Diaphragmatic myoblasts (DM), 1072 Diazene, 802 Diazinon, 842 Dibenzofurans, 74 Di-calcium phosphate, 948 2,4 dichlorophenoxyacetic acid, 842 Diclofenac, 1111, 1114 Bacillus subtilis, 1120 bacterial strains, 1119 Brevibacillus laterosporus, 1120 classified, 1119 Enterobacter hormaechei D15, 1120 Labrys portucalensis F11, 1120 pathway, 1122 Rhodococcus ruber IEGM 346, 1120, 1121 strucre, 1120 Diesel fuel, 234 Diesel hydrocarbons, 226 Diethylenetriaminepentaacetic acid (DTPA), 1538 Differential scanning calorimetry (DSC) measurement, 368, 625 Digestate, 1287 Dihydrodiol, 1663 Dimethylolpropionic acid (DMPA), 399 Dimethyl sulfide, 1293 Di-n-butyl phthalate, 156, 157 Dioxins, 74, 1276 Dioxygenases catalyze, 254 Direct contact test, 634 Direct mixing of polymer and nanofillers, 362, 363 Direct phytoremediation enhancement, 908 Disposable non-woven surgical face masks, 1175 Disposal methodologies, 1309 Dissolved organic carbon (DOC), 1115 Dissolved oxygen (DO), 1640 Dissolved oxygen concentration (DOC), 1114 Diterminal oxidation, 73 D-Lactic acid, 545 DNA transfer, 246 DOC die-away test (ISO 7827), 1116, 1117 Domestic sewage, 1097

Index Dopant-free conductive polyurethane elastomer (DCPU), 399 Doping, 394 Doubling time, 1114 3-D porous scaffolds, 1152 3D printing techniques FDM, 941 selective laser sintering, 941 stereolithography, 940 4D printing technique, 943 Dried soy polymers, 545 Drilling wastes, 1669–1670 Drinking water, 1097 Drug(s), 681, 703–705 delivery, 383 nanocarriers, 963 Drying oils, 1371 Dry process, 627–629 3D scaffold manufacturing processes, 1163 Durability, 1478 Dye-clay hybrid nanopigment (DNCP), 1320 Dye-decolorizing peroxidase, 164 Dyes Ads (see Azo dyes (ADs)) classification of, 795 degradation processes, 251 environment’s sources and pathways, 794 molecules, 795 sources and extractions, 795 textile dyes, 793 Dye-sensitized solar cell (DSSC), 1602 Dynamic air-vegetation-soil model, 752 Dynamic mechanical analysis (DMA), 625 Dynamic modulus analysis (DMA), 368 E Earthworms, 1283 Ecoflex, 268 Eco-friendly alkyds coatings, 1377 Eco-friendly alternatives/solutions, 1452 Eco-friendly biodegradable, 1537 Eco-friendly ceramic alkyd resin, 1381 Eco-friendly method, 234 Ecological contamination, 245 Ecologically friendly, 1543 Ecosystem, 722–724, 728, 738, 747, 751, 752, 754, 869 Ecosystem service (ES), 752 Ecotoxicity, 826–827 Edible-biodegradable films, in food industry, 1356–1357 Edible films, 518 Edible food packaging materials, 1317

1697 Edible proteins, 1317 Education, 1182, 1183 Egg-box structure, 503 Egg white protein, 1317 Elastin, 334 Electrical conduction, 518, 522 Electric conductive nanocomposites bone regeneration, 959 non-load-bearing healing, 959 Electricity conduction, 522 Electroactive surface, 635 Electrochemical deposition, 1231–1233 Electrochemical double-layer capacitors (EDLC), 1583 Electrochemical sensors, 400 Electroconductive polymers (ECPs), 959, 960 Electroconductive scaffolds, 959 Electrode, 1571 Electrolytes, 1582 Electromagnetic interference shielding, 522 Electronic(s), 1571 devices, 400 waste, 181 Electroporation, 246 Electrospinning, 511, 880, 939, 1003–1005, 1007, 1192, 1197 method, 1189 technique, 1192 Electrospraying, 511 Electrospun, 700 encapsulated polylactic acid, 1198 fibers, 883 methodology, 1319 microfiber, 1187 polyimide/metal-organic framework, 1200 Electrospun encapsulated polylactic acid-based nanomembrane, 1192–1194 Electrostatic layer-by-layer deposition, 878 Electro-transformation, 246 Embedded emulsion polymerization, 421 Embryonic stem cells (ESCs), 1049, 1071, 1137 Encapsulation, 694, 702, 706, 708 Endocrine-disrupting compounds, 1514 Endocuticle, 710 Endoglucanases, 765 Energy consumption, 829, 870 Energy conversion fuel cells, 465 solar technique, 458 Energy storage devices, 1571, 1585 mechanism, 1583 polymer-based supercapacitors in, 460

1698 Engineered cardiac tissue, 982 Engineered heart tissue (EHT), 1052–1053 Enterobacter hormaechei D15, 1120 Environment, 181, 185, 187, 902, 903, 905–909, 911, 915, 917–919 analysis of surfactants, 1626 hazards, 1452, 1453 legislation, 1451, 1469 pH, 120 pollutants, 224 pollution, 864, 910, 1451 problems, 224 quality, 1182 rehabilitation, 5 remediation, 1409 safety, 1297 surfactants impact, 1625 Environmental impact, 1277, 1291, 1293, 1294, 1298 abiotic and biotic degradation mechanisms, 1177 of biodegradation, 11–12 bisphenol A, 1177 of carbon nanotubes, toxicity mechanisms, 657 components of plastics, 1176 COVID-19 pandemic, 1176 greenhouse gas emissions, 1176 mask waste, 1178 microplastics, 1177, 1178 non-biodegradability, 1176 personal protection equipment, 1176 phthalates, 1177 surgical mask, 1176 UV light, 1178 Environmental impact of CNTs biodegradation, 657 and non-polar aromatic compounds, 659 SWNTs, 656 toxicity of MWNTs, 656 transformation and breakdown, 658 Environmentally friendly materials, 574 Environmental Protection Agency (EPA), 902, 1191, 1192 Environmental Quality Act, 1182 Enzymatic degradation of bioplastics, 730 Enzymatic degradation of CNTs, 662 Enzymatic hydrolysis, 374, 375, 1475 bacteriological progressions, 1471 cellulose fibers, 1471–1473 microbial analysis, 1471 proteinic fibers, 1473, 1474

Index Enzyme(s), 117, 118, 161, 182, 251, 586, 987, 1359 in bioremediation, 254 cellulase, 1471 characteristics, 580 classification in biodegradation, 161 endo and exo activity, 1473 hydrogenases, 162 hydrolysis, 1464, 1475 hydrolytic, 1472 immobilization technique, 256 laccases, 163 lyases, 167 microorganisms, 1466 microorganism-secreted, 1460 oxidoreductases, 161 oxygenases, 161 proteolytic, 1472 proteolytic and keratinolytic, 1473 reductases, 165 sources, 1471 Enzyme-catalyzed transformation, 61 EPCBZ, 1124 Epipremnum aureum (golden pothos), 743 Epistylis sp., 140 Epoxidized soybean oil (ESO), 482 Epoxy resin (EP), 1391 Equilibrium modified atmosphere packaging (EMAP), 880 Escherichia coli, 844 Escherichia coli O157:H7 (EcO157), 240 Essential metals, 1535 Esterase enzymes, 577 Estrogens, 1111 17a-Ethinylestradiol (EE2), 1097 Ethoxylated surfactants, 1513 Ethoxylates, 1631 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), 1084 Ethylenediaminedisuccinic acid, 1543 Ethylenediamine-N, N'-disuccinic acid (EDDS), 1537, 1540, 1543–1545, 1549 Ethylenediaminetetraacetic acid (EDTA), 1536–1540, 1553, 1561 Ethylene glycol dimethacrylate (EGDMA), 399 Ethylene hormone, 234 Ethylene-vinyl alcohol (EVOH), 418 2-Ethyl hexanoate (EHT), 819 e-waste, 1571 Exfoliated nanocomposites, 560

Index Exfoliation adsorption, 421 Exo-chitinas, 689 Exocuticle, 710 Exoenzymes, 370, 587, 588 Exogenous versus indigenous, 121 Exoglycanases, 1472 Exophiala pisciphila, 852 Exothermic processes, 188 Exposure conditions, 580 Ex situ bioremediation, 904 Extended producer responsibility (EPR) policy, 310 Extracellular enzymes, 61, 197, 212 Extracellular matrix (ECM), 982, 1000, 1138, 1143, 1144, 1149, 1152–1154, 1156, 1161, 1163, 1165, 1167 biomimetic platforms, 1050 CTE, 1048 tissue-engineered cardiac patches, 1050 Extracellular polymeric matrix (EPS), 211 Extracellular polysaccharides (EPS), 515 Extraction efficiency, 1536, 1538, 1548, 1561 concentration of chelating agents, 1554 pH condition, 1551, 1554 Extrusion, 878 bioprinters, 943 methodology, 1324 Ex-vivo tissue engineering, 1152

F Fabrication, 971 Fabrication methods, biodegradable polymers, 1001 electrospinning, 1003–1005 freeze-drying method, 1002 gas foaming technique, 1005 hydrogen scaffold, 1000 leached scaffold, 1000 melting-based technique, 1001 rapid prototyping technique, 1006 scaffold, 1000 solvent-based technique, 1001 solvent casting/particle leaching, 1002 thermal-induced phase separation, 1002–1003 Face masks, 1175, 1181, 1202 biodegradation, 1193 bisphenol A, 1191, 1192 chitosan, 1195, 1196, 1198 electrospun encapsulated polylactic acid, 1198

1699 electrospun encapsulated polylactic acid-based nanomembrane, 1192–1194 and face shields, 1184–1191 gluten, 1193–1195, 1198 N95, 1191 petrochemical-based non-degradable polymers, 1191 phthalates, 1191 polyethylene, 1191 polylactic acid, 1193 polypropylene-based, 1191, 1192 respiratory masks, 1191 starch, 1197–1199 surgical masks, 1191 World Health Organization, 1191 Face shields, 1175, 1203 and face masks, 1184–1191 Fashion brands, 1478 Faster-moving consumer goods organizations, 555 Fatty acid-containing stains, 1542 Fatty acids composition of oils, 1375 double bond positions, 1376 Fatty alcohol ethoxylates, 1515 Fe3O4@SiO2 nanofillers, 1382 Feeding zone, 1324 Fenoprofen, 1111 Fenton-like reaction processes, 1624 Fermentation, 66, 491 Fertilizers, 69, 773, 774 Ferulic acid, 506 Fetal Stem Cells (FSC), 1137 Fiber(s) biodegradable (see Biodegradable fibers) cell walls, 1458 surface, 1467 Fibrils, 681, 691–701, 704 Fibrin, 951, 992, 1066–1067, 1084 gel, 989, 990 Fibrinogen, 989, 1022 Filamentous fungi, 158, 236, 238 Fillers, 884 Film blowing methodology, 1324 Film packaging, 1185, 1186 Fire-retardant bioplastics, 523–524 Fish, 1625 Fixed bed reactor (FBR), 1114 Flame-retardant finished fabric, 1468 Flavin mononucleotide (FMN), 846 Flavonoids, 1300

1700 Flax fabrics, 1459, 1468 Flax fibers, 1458, 1459, 1464 Flax stem, 1458 Flexible thin-film transistors (TFTs), 401 Fluid barrier qualities, 1200 5-Fluorouracil, 681, 702 Flux values, 521 Foamed polylactide, 816 Food and Drug Administration (FDA), 1192, 1319 Food industry, 1316 Food packaging, 493, 515–518, 876 applications, 831–832, 1298, 1309, 1311, 1312 biodegradation, 1324–1328 biopolymers (see Biopolymers) casting methodology, 1323, 1324 extrusion methodology, 1324 film blowing methodology, 1324 materials, 1310, 1328 PLA, 555 properties, biofilms, 1324–1326 thermoplastic starch, 543 TPS/CS oligomer blend biofilm, 1315 Food packaging materials, 865, 1360–1362 biodegradable polymers, 1341 cellulose, 1343–1344 chitin and chitosan, 1345 collagen and gelatine, 1349–1350 corn zein, 1346–1347 microorganisms’ polymers, 1355–1356 milk protein, 1351–1352 polymers from biomonomers, 1352–1355 polysaccharides, 1343–1345 proteins, 1345–1352 soy protein, 1347–1349 starch, 1344–1345 sustainable polymers from renewable resources, 1340 wheat gluten, 1347 Food processing waste, 790 Formaldehyde, 1293 Fossil-based biodegradable plastics, 818, 1426–1428 Fossil-based biodegradable polymers, 492 Fossil-based bioplastics, 489 Fossil resources, 814 Fourier transform infrared spectroscopy (FTIR), 486, 487, 1321, 1477 Fragmentation, 488 Free cell biodegradation, 129 Freeze casting, 700 Freeze-drying technique, 938, 1070

Index Freeze gelation technique, 699, 700 Fucoidan, 519 chemical structure, 512 F residues, 512 fucose-rich-sulphated polysaccharides, 511 gel-forming polymers, 512 heteropolysaccharides, 511 laminariales, 512 Fuel cells fire-retardant bioplastics, 523–524 other applications, 524 5-FU (Fluorouracil), 702 Fumarate addition, 1499, 1500 Fungal biodegradation, 185 of carbofuran, 738 Fungal-mediated biodegradation, 114, 116 Fungal strains, 124 Fungi, 114, 156–158, 236, 730, 800, 801, 1371, 1428 degradation, 917–918 Fungi-based biodegradation, 236 Furans, 1276 Fused deposition modeling (FDM), 941, 1006

G Gas chromatography/Mass spectrometry (GC/MS), 747 Gas foaming technique, 699, 938, 1005 Gasification, 1248 Gas-phase glutaraldehyde, 1197 G-code, 940 Gelatin, 992, 1025–1026, 1065–1066, 1084, 1317 nanofilms, 885 Gelatin/polycaprolactone/graphene (Gt/PCL/ graphene), 1079–1082 Gelation, 505 Gemcitabine (GEM), 405 Gemfibrozil, 1111 GenBank database, 1110 Genetically engineered microorganisms (GEMs), 76, 246, 902, 1654 advantages, 249 applications, 253 in biodegradation of dyes pollutants, 251 in HMs removal, 252 in industrial food enzyme production, 251 killer and anti-killer genes, 248 Genetically engineered microorganisms (GEMs), in bioremediation chromium, 846, 847 future perspectives, 854

Index genome editing tools, 843 mercury, 845, 846 nickel, 844 pesticides, 849, 850 petroleum hydrocarbons, 847 recombinant DNA techniques, 841 Genetic engineering, 1562 Gene transfer agents (GTAs), 246, 247 Gene transfer method, 246 Genome editing tools, 843 Genome shuffling, 842 Geobacter sp., 1109 Geotextiles, 1470, 1471 Gliadins, 1347 Global non-biodegradable plastic wastes, 864 Global plastics industry, 813 Global plastic waste production, 865 Global warming, 1179 Glucomannan hemicellulose, 430 Gluconeogenesis, 1662 Glucose molecules, 1471 Glucuronic acid/sulfate, 1110 Glucuronidation, 1100 Gluten, 1191, 1193–1195, 1198 coatings, 1347 Glycerin, 486 Glycerol, 13, 704 Glycerol-plasticized soy-based bio-plastic (GSBP), 482 Glycidyl methacrylate (GMA), 399 Glycine ethyl ester (GEE) block polymers, 397 Glycine max (L.) Merr., 746 Glycosaminoglycans (GAGs), 333 Glycosyl hydrolases (GH), 681 GO-γ-Al2O, 1394 Gold nanoparticles (AuNPs), 1083 GO-NanoTi composite, 1392 Gordonia alkanivorans strain Sh6, 145 GO-SiC nanowires, 1394 Graft polymers, 398 Gram-negative bacteria, 114 Gram-positive bacteria, 114, 485, 1110 Graphene, 400, 615 PLA composite, 1601 Graphene-based alkyds graphene-alkyd nanocomposite coatings, 1386 graphene-based coatings, 1385 Graphene-based materials, 890 Graphene-based nanocoatings technology, 1386 Graphene oxide (GO), 487

1701 Graphene oxide-bio-chitosan nanocomposite, 380 Graphene oxide nanosheets (GONSs), 885 Green economy, 561 Green energy, 87 Greenhouse gases (GHG), 868, 1179, 1255, 1276 emissions, 1176 Green solution electrospinning, 1192 Guanidine hydrochloride (GuHCL), 482 Guide Ribonucleic acid (gRNA), 843 Gymnopus luxurians, 158 H Hand gloves, 1175, 1203 Hardeners, 1222 Hard fibers, 1461 Hardwood lignin, 608 Hawroth projection, 689 Hazard assessment, 1453 Hazardous pollutants, 785, 786 Healing mechanism, 935–936 Health impacts, of biodegradation, 12–13 Heart failure, 1050, 1152 Heart function, 1068 Heart tissue engineering biomaterials, 1052 3D matrix, 1052 polymeric systems, 1053 tissue-engineered materials, 1052 Heating zone, 1324 Heavy metals, 234, 236, 250, 724, 842, 844, 852, 1290 in agriculture, 1534 bioaccumulation and biomagnification, 1535, 1536 classified, 1535 contamination, 238 environmental rules, 1535 extraction, 1538 fate and transport of mercury, 1535 microbial biosorption, 1556, 1558–1560 non-biodegradable properties, 1556 phytoremediation, 1555–1560 pollution, 1534, 1535, 1554–1561 recycling, 1559, 1561 sludge, 1535 sources and migratory pathways, 1537 in toxicity, 1535 toxicity, 1555 Hematopoietic SCs, 1049 Hemicelluloses, 429–430, 1460, 1462 Hemodynamics, 1068

1702 Hemostatic materials, 705 Hemp fibers, 1459, 1460 Hemp stalks, 1460 Hepatocyte growth factor (HGF), 1070 Herbal ingredients, 1203 Herbicides, 76, 209, 733 Heterocyclic compounds, 74, 75 Heterocyclic salts, 1630 Hexachlorobenzene, 848 High density (HD), 489 High-density polyethylene (HDPE), 202, 205, 418, 1175 High-performance liquid chromatographytandem mass spectroscopy (HPLCMS/MS), 1100 High-purity microcrystalline cellulose (HPMC), 426, 427 ® HOB cells, 1033 Horizontal gene transfer (HGT), 246, 247, 249 Hormones, 233, 234 Horseradish peroxidase, 164 Human activity, 551 Human bone marrow mesenchymal stem cells (hBMSCs), 947 Human cardiac progenitor cells (hCPCs), 1077 Human cardiomyocyte progenitor cells (hCMPCs), 1066 Human elastin-like polypeptide, 518 Human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM), 990 Human interactions, and PPCPs, 1099–1101 Human osteoblast-like cells, 961 Human periosteum-derived cells (hPDCs), 968 Human scaffold-free cell system, 1141 Human umbilical cord matrix stem cells, 635 Human umbilical vein endothelial cells (HUVECs), 1078 Humic acids (HAs), 1254 Humus, 1254 Hyaluronan and chitin nanofibrils (PHHYCN), 706 Hyaluronic acid (HA), 333, 951, 990–992, 994, 1022–1024, 1084 Hybrid polymers, 1079, 1085 Hydraulic retention time (HRT), 1113, 1114 Hydro-biodegradation, 1058 Hydrocarbon degraders, 741 Hydrocarbon-oxidizing microbes, 741 Hydrocarbon toxicity, 226 Hydrochar addition of, 1267 vs. biochar, 1248 characteristics, 1248 Hydrocolloid(s), 558 homopolymer, 1312

Index Hydrogels, 399, 944, 1054 bioactivity and biocompatibility, 954 biocompatible materials, 954 BTE, 955 categorized, 954 Hydrogen bonding, 684 Hydrogenophaga sp., 1109 Hydrogen sulfide, 1179 Hydrolases, 168, 255, 283 Hydrolysis, 91, 588, 689, 1416, 1491 reaction, 551 Hydrophilic phycocolloids, 516 Hydrophobic cells, 30 Hydrophobicity, 770 Hydrophobins, 158 Hydroquinone monooxygenase, 1123 Hydrothermal carbonization, 1247 3-Hydroxyalkanoic acids, 483 Hydroxyapatite, 948 lattice, 631 SiO2 composite surface, 618 Hydroxyapatite-bacterial cellulose composite, 621 Hydroxyapatite–graphene nanocomposites, 960 4-Hydroxybutyric acid (HB), 483 Hydroxyethyl methacrylate (HEMA), 399 Hydroxylapatite (HA), 933 Hydroxyl-group oxidation, 201 2-(4-hydroxyphenyl) propionic acid, 1123 Hydroxypropylated starch, 609 5-Hydroxyvaleric acid (HV), 483 Hyperaccumulator plants, 1562 Hyperaccumulators, 1555 Hyperbranched alkyd polymer, 1382 Hyperbranched alkyds, 1379, 1380 Hyperbranched polymers, 1379 Hypholoma fasciculare, 158

I IBU biodegradation, 1109 Ibuprofen (IBU), 1100, 1107, 1111, 1113 Bacillus thuringiensis B1, 1123 metabolic mechanism, 1122 metabolites, 1121 nonsteroidal anti-inflammatory drug, 1121 pathway, 1123 Sphinomonas sp. Ibu2, 1121, 1122 structre, 1122 variovorax Ibu-1, 1122 Ideal cycle, 1623 Ideonella sakaiensis, 205, 842 Iminodisuccinic acid, 1537, 1540–1543, 1549 Immobilization, 907 Immobilized oxidative enzymes, 256

Index Incineration, 481, 533, 539, 561, 1179, 1180, 1183 Indigenous microbial communities, 126 Indirect phytoremediation enhancement, 908 Indomethacin, 1111 Induced pluripotent stem cells (iPSC), 1139, 1142, 1144, 1145, 1157 Inherent biodegradability (OECD 302) mineral medium composition, 1117 modified Zahn-Wellens test, 1118 SCAS, 1117 Zahn-Wellens/EMPA, 1117, 1118 Inkjet bioprinting, 942–943 Inorganic acids, 231, 1548 Inorganic cellulose esters, 427 Inorganic nanocomposite, 1613 Inorganic nanofillers, 1380 Inorganic NPs, 1391 Inorganic nutrients, 1278 Inorganic pollutants, 61, 787, 788 Inorganic strong acids, 1548 Insects, 1301 In situ bioremediation, 122, 903–904 In situ polymerization method, 360, 361, 423 Intercalated nanocomposites, 421 Intercalation methods, 359, 360 International Society for Cellular Therapy (ISCT), 1138 Intracellular accumulation, 61 Intrinsic bioremediation, 904 Intrinsic tissue regeneration, 1052 In-vessel composting, 1279, 1282, 1286 In vitro vascularization, 1055–1056 In vivo vascularization, 1056–1057 Iodine values, 1375 Ion implantation, 1231–1232 Iron, 235 composite scaffolds, 946 Isotopic tracking, 1425 IsPETase, 842

J Joint Committee on Powder Diffraction Standards (JCPDS) data, 367 Jute fibers, 1460

K Kenaf fibers, 1461 Keratin, 1317, 1457, 1473 Kneading zone, 1324 Kraft lignin, 206 Krebs cycle, 1494

1703 L Laboratory conditions, 581, 582 Labrys portucalensis F11, 1120 Laccases, 117, 164, 166, 206 Lactic acid, 335, 545 Lactide monomer, 1598 Lactoperoxidase, 667 Lamellae link fiber cells, 1462 Lamivudine, 681, 702 Landfilling, 539, 1179, 1180, 1183 Langmuir isotherm, 1607 Laser ablation, 652 Laser-assisted bioprinting, 942 Lateral gene transfer (LGT), 246, 249 Layer-by-layer assembly, 627 Layered double hydroxide (LDH), 819 L-cysteine, 1387 Leachate, 1282 Leached scaffold, 1000 Left ventricular (LV) myocardium, 1053 Levofloxacin biodegradation, 240 Levofloxacin hydrochloride (Levo-HCl), 775 Life cycle assessments (LCAs), 826 in product development, 562 Lifecycle management, 563 Light, 188 deterioration, 585 Lignin, 206, 608, 1246, 1460 Ligninolytic enzyme secretion, 207 Ligninolytic fungi, 918 Lignin peroxidase (LiP), 164, 801 Lignocellulose, 324–325, 1186 biodegradation, 1428 Lignocellulose-degrading bacteria, 1435 Lignocellulose-degrading enzymes, 1435 Lignocellulose-degrading fungi, 1435 Lignocellulose nanofibrils (LCNFs) biofilms, 1319 Lignocellulosic biodegradation, 1429 Lignocellulosic biomass, 1300 Lignocellulosic fibers, 440–442 Limonene, 1293 Linear alkylbenzene sulfonate (LAS), 73, 1507, 1628, 1629 Linseed oil-based hyperbranched alkyd matrix, 1385 Linseed oil-derived hyperbranched alkyd, 1388 Lipid regulators, 1107 Lipoprotein, 1474 Liquid hydrocarbons, 847 Livestock feeding techniques, 750 Living cells’ metabolism, 1057 L-Lactic acid, 545 Long-chain polymeric plastic degradation, 481 Long-term biodegradation, 728

1704 Long-term manure storage, 750 Low density (LD), 489 Low-density polyethylene (LDPE), 202–205, 238, 418, 728, 1175 LV end-diastolic diameter (LVEDD), 1075 Lyases, 168 Lyocell fibers, 1463, 1464, 1479 Lysozyme, 1196

M Macroalgae, 117 Macroalgal polysaccharides, 503 Macrofibrils, 679 Macrolide phosphotransferases (MPH), 849 Macrophage colony-stimulating factor (M-CSF/CSF-1), 934 Macroporosity, 937 Macroscopical birds, 1453 Magnesium, 1212 atomic, physical, and mechanical properties of, 1215 biodegradability, 946 characteristics of, 1213 matrix, secondary phase, and impurities, 1217 mechanical behavior, corrosion resistance, and biocompatibility of, 1218–1219 Magnesium alloys advantages and disadvantages of, 1214 anodization, 1226–1229 chemical conversion, 1224–1226 coatings for biodegradable, 1223 electrochemical deposition, 1231–1233 future research, 1234 ion implantation, 1231–1232 Mg–Al alloys, 1217–1218 Mg–Ca alloys, 1218–1220 Mg–REEs alloys, 1222 Mg–Sr alloys, 1221 Mg–Zn alloys, 1218 micro-arc oxidation, 1229 for orthopedic implant materials, 1222–1233 physical vapor deposition, 1230–1231 Magnetically responsive composites advantages and disadvantages, 962 chitosan-based scaffolds, 961 Fe3O4 MNPs, 961 in vitro studies, 961 stimulation therapy, 960 Magnetic nanoparticles (MNPs), 961 Magnetic resonance imaging (MRI), 940, 969

Index MAGNUM, 1063 Malaysia’s clinical/biomedical waste management, 1182 Malaysian Department of Environment, 1182 Malignancy, 405 Manganese peroxidase (MnP), 801 Manganese peroxidases, 164, 165, 255 Manila hemp, 1462, 1463 Manometric respirometry test (ISO 9408), 1117 Manure compost on soil and biochar, 1264 Marine biomass polysaccharides alginates, 431–432 cellulose, 433–436 chitosan and chitosan derivatives, 431 semolina with embedded nanokaolin, 432–433 Marine environment, 1178 Marine oil spill, 743 Mask waste, 1178 Mater-Bi™, 268 Matrigel, 990–992, 1067 Matrix metalloproteinases (MMPs), 404, 1067 Medical waste disposable non-woven surgical face masks, 1175 education, 1182, 1183 environmental impact, 1176–1178 face mask, 1175 incineration, 1179, 1180 landfilling, 1179, 1180 mismanagement, 1175 non-biodegradable petrochemical-based materials face masks, 1175 personal protective equipment, 1175 plastic trash, 1175 pollution, 1204 respiratory protection, 1175 3R principle (reduce, reuse, and recycle), 1181, 1182 synthetic plastic, 1175 utilization of biodegradable materials, 1183 Melt compounding, 363 Melt electrospinning writing (MEW), 1075 Melting temperature, 579 Melt intercalation method, 360 Melt mixing, 422–423 Membrane bioreactor (MBR), 1112, 1113 Mercury, 845 Mercury-resistant (HgR) bacteria, 842 Mesenchymal stem cells (MSCs), 947, 1049, 1137, 1138, 1140, 1143 Mesophilic organisms, 1288 Meso-PLA, 1071

Index Messenger ribonucleic acid (mRNA) transcript levels, 156 Metabolic biodegradation, 64, 67 Metabolism carbamazepine, 1124–1125 diclofenac, 1119–1121 ibuprofen (IBU), 1121–1124 of organic compounds, 67 Metabolites, 813, 1100 Metabolix PHA, 1318 Meta-cleavage metabolic pathway, 1122 Metagenomic(s), 851 analysis, 159 Metal-binding properties, 1544 Metal extraction aminopolycarboxylate, 1540–1548, 1551, 1552 chelants (see Chelating agents) efficiency, 1551–1554 ethylenediaminetetraacetic acid (EDTA), 1536, 1537 organic acid, 1548, 1551, 1552 soils, 1536 Metallic implants, 1214 Metallothionines, 844 Metals biotransformation, 62 iron composite scaffolds, 946 magnesium’s biodegradability, 946 plating, 1543 strontium, 947 zinc, 947 Metaproteomics, 853 Metathesis method, 395 Methane, 87, 104, 105, 870, 1179, 1298 biochemical methane potential, 96–97 biogas, 95 fermentation, 90 gas, 487 methanogenesis, 93 production, 98–100 Methanogenesis, 90, 93–94, 1416, 1492 Methanogenic bacteria, 1491 Methanogens, 139 Methanosarcina acetivorans, 1502 Methotrexate, 1097 Methylation, 63 Methylcellulose (MC), 1344 Methylglycinediacetic acid, 1537, 1540, 1542, 1543, 1549 Methylmercury, 1535 Methylotrophic methanogenesis, 1491 Methyl parathion hydrolase, 842 Methyl tert-butyl ether (MTBE), 1109

1705 Meyerozyma caribbica, 165 MFC A/O system’s biodegradation, 1108–1110 Mg–Al alloys, 1217–1218 Mg–Ca alloys, 1218–1220 MgO nanoparticles, 1201, 1320 Mg–REEs alloys, 1222 Mg–Sr alloys, 1221 Mg–Zn alloys, 1218 Mg–Zr alloys, 1220–1221 Micro-aeration technique, 1636 Microalgae, 229, 492 Microalgae-based wastewater treatment plants, 159 Microalgae culture, 186 Micro-architecture, 937 Micro-arc oxidation, 1229 Microbe-killing mechanism, 1189 Microbes, 113, 114, 121, 180 composting, 1292 consortium vs.individual (pure) microbe, 119, 121 exogenous versus indigenous, 121 Microbial adaptation, 122 Microbial aggregate, 1102 Microbial biodegradation, 184, 908 absorption mechanism, 181 algal-mediated biodegradation, 117 bacterial-mediated biodegradation, 113–115 breakdown mechanism, 182 enzymes, 117, 118 fungal-mediated biodegradation, 114, 116 mechanism of microbial decomposition, 180, 181 Microbial biofilm formation, 211 Microbial biomass, 1279 Microbial biosensors, 245 Microbial biosorption, 1556, 1558–1562 Microbial cells, 1472 Microbial cellulases, 255 Microbial communities, 181, 737, 1293 Microbial consortia, 130 Microbial consortium, 145 anaerobic biodegradation, 146 bacterial consortium, 145 studies on, 147 xenobiotic biodegradation, 145 Microbial degradation, 114, 180, 199, 208, 502 antibiotics, 126 ciprofloxacin, 127 pesticides, 124, 125 petroleum hydrocarbons, 1659–1664 plastics, 122, 123

1706 Microbial degradation, factors affecting, 187, 189 biological factors, 241–242 environmental factors, 243–244 light, 188 moisture, 119 oxygen, 187, 188 pH, 120 temperature, 120, 188 water, 187 Microbial degraders, 577 Microbial enzymes, 253, 370 advantages and disadvantages in bioremediation, 253, 254 hydrolases, 255 oxidoreductases, 253–255 Microbial exopolysaccharides, 231 Microbial fuel cells (MFCs), 875 Microbial growth, 243 Microbial infallibility hypothesis, 140–141 Microbial metabolism, 908 Microbial methylation, 63 Microbial modes of action, on bioavailability processes, 242 Microbial oxidoreductases, 253 Microbial pollutants, 786 Microbial polymers, 490 Microbial proteases, 255 Microbial remediation, 1554 Microbiological plastic degradation mechanism, 1422–1425 Microbiology, 180 Microfungi, 235 Micro-organisms, 181, 824, 909–911, 1108, 1119, 1296, 1359, 1408, 1409, 1411, 1413, 1414, 1417, 1419–1423, 1426, 1435, 1439, 1441, 1471, 1473, 1639 assimilation, 229 biodeterioration, 210 biofragmentation, 211 as biosensor devices, 224 environmental factors, 214 as innovative biotechnology, 238 microbial biofilm formation, 211 microbial enzymes, in biodegradation process, 212 microbial species and metabolic activities, 213 mineralization, 211 pesticide-degrading microorganisms, 207 polymer-degrading microorganisms, 197 remediation, 907–908 substrate characteristics, 214 types of polymers, 577

Index Microplastics, 225, 536, 573, 868, 1175, 1177, 1178, 1204 Microporous annealed particle (MAP), 1054 microRNAs, 972 Micro-valve bioprinting, 943 Migration, 444–445 Milk protein, 1317, 1351–1352 Milkweed, 1469, 1470 Minamata disease, 1535 Mineralization, 196, 204, 205, 211, 229, 607, 1110, 1425, 1488, 1506 Mini-transposon, 248 Mirel bioplastic, 557 MITI, 1116 Mixed bacterial culture technology, 880 Mixed plasticized soy-based bio-plastic (MSBP), 482 Mixture components, 1639 β–MnO2 nanorods, 1387 Mobilization factors, 1050 Mobilizing metals, 1537 Modified OECD screening, 1117 Modified sturm test, 1116 Modified Zahn-Wellens test, 1118 Mohair, 1457 Moisture content, 1260, 1288, 1290 Moisture-rich environments, 120 Molecular engineering approaches, 1301 Mo-nitrogenase, 230 Monochloramine co-metabolism pathways, 146 Mono crop cultivation, 739 Monooxygenase analysis, 1123 Monooxygenases, 1100 Montmorillonite (MMT), 1606 MS/MS formulation, 1124 Mulches biodegradable plastic, 826 ecotoxicity, 826–827 in the environment, 828 plastic films, 826 plastic pollution, 826 waste management options, 828–829 Mulch film, 750 Mulching, 739 films, 521 Multialdehyde sodium alginate (MASA), 398 Multiblock copolymer (PLA-co-AP), 397 Multifunctional nanofiber scaffolds, 964, 965 Multiple-walled carbon nanotubes (MWCNTs), 645, 646, 954, 1076, 1380, 1391 adsorbents, 656 biotransformed MWCNTs, 664 chirality, 649 thickness, 649

Index Multipolymers, 880 MXenes, 364, 365 Mycelium, 800 Mycobacterium austroafricanum, 67 Mycorrhizae, 235 Mycorrhizal fungi, 235 Myocardial infarction (MI), 404, 1054, 1151–1153 N N95, 1191 n-alkanes, 73 Nano γ-Al2O3 rods, 1381 Nanobiodegradation for immobilization of microorganisms, 129 nanoparticles enhance microbial growth, 127, 128 Nano-bioglass (nBG)-titania (nTiO2), 954 Nanocellulose, 619, 1184–1186, 1577 cellulase enzymes, 620 and cellulose, 620 composite, 1585 composite with inorganic materials, 620 uses, 620 works on biodegradable, 621 Nanoclays, 613, 1599 Nanocomposites, 355, 1049, 1075, 1076, 1082, 1085 biodegradable, 932 in bone tissue engineering, 932 drug delivery systems, 964 materials, 885 packaging materials, 1432–1433 Nano-delivery systems barriers, 968 ethical issues, 971 intelligent materials and modular fabrication, 965–968 multifunctional nanofiber scaffolds, 965 scientific and technological challenges, 970 surface alterations, 965 translational challenges, 971 Nanoelectronics, 654–656 NanoFe3O4@SiO2 hybrid fillers, 1381 Nanofibers, 944, 967 Nanofillers, 1378, 1380, 1381, 1387, 1389, 1395, 1396 lignin, 609 Nano-fillers particles definition, 612 silver nanoparticles, 613 TEM and FESEM, 613 use in composites, 613

1707 Nano-hydroxyapatite, 615–619 conjugation, 952 Nanomaterials, biodegradation processes, 128 Nanometric scale, 933 Nanoparticle(s), 127, 702–704, 706, 960, 1378, 1379, 1381, 1391, 1395 as biodegradation enhancers, 127 delivery, 703 enhance microbial growth, 127, 128 for immobilization of microorganisms, 129 synthesis, 355 Nanoparticle-based strategies, 1158–1159 Nanopiezoceramics, 958 Nanopiezoelectric ceramic materials, 958 Nanoplastics, 557 Nano-scale membrane, 1192 Nanosilica, 483 Nanostructured delivery systems, 965–967 Nanostructured drug carriers, 965 Nanostructured materials, 962 Nanotechnology, 129, 557–560, 766, 887, 891, 1479 scaffold bioactivity, 932 Nanovibrational bioreactor, 972 Nano-whiskers, 625 2-Naphthoic acid, 1501 Naproxen-contaminated wastewater, 1110 Naproxen degradation, 1110 Natural attenuation, 244, 902 Natural biodegradable materials, 1310 Natural biodegradable polymers, 301 biomedical applications, 1026–1038 bone implants, 1035–1038 bone regeneration, 1031–1036 chitosan, 1020–1021 collagens, 1019–1020 fibrin, 1022 gelatin, 1025–1026 hyaluronic acid, 1022–1024 PHAs, 276 PLA, 275 polysaccharides, 277 skin regeneration and wound healing, 1027–1031 starch, 1023–1025 synthesis of, 1019–1026 Natural biodegradation, 750 Natural biodegradation polymers, 1057–1059 bioactive characteristics, 1061 chemical structure, 1063 polysaccharides, 1067–1068 proteins, 1061–1068 Natural biomass sources, 880

1708 Natural biopolymers polysaccharides, 1343–1345 proteins, 1345–1352 Natural cellulose, 278 Natural environment, 1192 Natural fibers, 1464, 1469 classification, 1456 classifications of, 322 Natural microbial communities, 751 Natural polymeric plastic, 491 Natural polymers, 984, 992, 995, 1183, 1184, 1597 biodegradable, 453 cellulose, 462 chitosan, 951 collagen, 949 fibrin, 951 hyaluronic acid, 951 natural biodegradable polymers in, 471 silk, 951 sodium alginate, 459 Natural polymers based biocomposites, 1146 characteristics of biodegradable polymers, 1146–1147 chitosan-based biomaterials, 1150–1151 CMC, 1147–1150 Natural surfactants, 1518 Natural textiles, 1480 Nature fibers, 1455 n-doping, 394 Neosartorya fischeri, 852 Neo-vascularization strategy angiogenesis, 1056 in vitro vascularization, 1055–1056 in vivo vascularization, 1056–1057 oxygenation and nutrition, 1056 Nerve cell, 404 Neurons, 404 Neutral electrolytes, 1582 New product development, 564 Nif-genes activation, 230 Nitrilotriacetic acid, 1537, 1540, 1544–1547, 1549 Nitrogen, 71, 740, 750, 1291 oxide, 1276 Nitrogen-containing groups, 1579 Nitrogen-doped graphene (NG), 1610 Nitrogen-fixing bacteria, 229 Nitrosomonas europaea strain, 146 Nitrous oxide (N2O), 870 NixA gene, 844 N-(3-maleimidopropionyloxy) succinimide, 405

Index N,N0 -diacetylchitobiose [(GlcNAc)], 683 N, N- Diethyl-meta-toluamide (DEET), 1097 Non-biodegradability, 1176 Non-biodegradable additives, 1279 Non-biodegradable masks, 1203 Non-biodegradable materials, 295, 866, 867, 1309, 1310 Non-biodegradable packaging materials chemical additives, 868 conventional plastic materials, 866 GHGs emission, 868 life cycle, 868 non-biodegradable materials challenges, 867 plastic production, 868 Non-biodegradable petrochemical-based materials face masks, 1175 Non-biodegradable plastics, 729, 814, 865, 1469 Non-chlorinated hydrocarbons, 1290 Non-degradable plastics, 1204 Non-drying oils, 1371 Nonidet-P40 electrons, 1391 Non-ionic surfactants, 1513–1517, 1522, 1624 benzene ring, 1631 biodegradation of APEO, 1631 hydrophilic groups, 1630 indissociable nature, 1631 metabolites, 1632 molecular structure, 1631 polyoxyethylene chain, 1631, 1632 stability, 1630 Non-steroidal anti-inflammatory medications (NSAIDs), 1109 Non-toxicity, 368 Non-water-soluble biopolymers, 1298 Non-woven(s), 1469, 1470 face masks, 1175 polypropylene fiber, 1192 Nonylphenol (NP), 1100, 1114, 1297, 1298 isomers, 1515 Nonylphenol polyethoxylate surfactants (NPEOs), 1102, 1113 Nonylphenol polyoxyethylene ether (NPEOn), 1631 Novosphingobium aromaticivorans, 845 Nuclear magnetic resonance (NMR), 367 Nuclear transfer stem cells (NTSC), 1139 Nucleophilic substitution process, 397 Nutrient accessibility, 908 Nutrient availability, 242 Nutrient-rich environments, 1111 Nylon 4, 274

Index O Ocean conservancy, 1453 Octahedral coordination, 1539 Octylphenol (OP), 1100, 1114 Octylphenol polyoxyethylene ether (OPEOn), 1631 Octylphenols nonylphenols, 1631 Odors, 1293 OH-IBF isomers, 1122 Oil biodegradation, 740–743 Oil bioremediation industrial applications of, 50 strategies in, 39 Oil-degrading microorganisms, 1666 Oil industry environmental impact of, 1658–1659 oil spills, 1657–1658 polycyclic aromatic hydrocarbons, 1657–1658 sources of waste materials, 1656–1658 Oil-polluted soil, 742 Oil-suspended particulate matter aggregate (OSA), 742 Oily sludge, 1670–1671 Oily wastewater, 792, 793 Omics approaches, 156 Open circuit potential (OCP), 1224 Opercularia sp., 140 Ophthalmology, 704 Organic acid, 231, 1539, 1548, 1551, 1552 production, 231 Organic carbon, 1116, 1117 Organic chelators, 1551 Organic coatings, 1386, 1389 Organic light-emitting diodes (OLED), 401 Organic materials, 113, 116, 1107, 1296 Organic matter (OM), 70, 71, 1277 Organic pollutants, 60, 184, 901 cosmetics, 791, 792 detergents and surfactants, 793 food processing waste, 790 oils, 792, 793 pesticides, 789, 790 pharmaceuticals, 791 phenols, 789 textile dyes, 793 Organic polymer matrix, 422, 423 Organic solid waste, 1276, 1277 biological treatment, 1276 Organic solvents, 1192 Organic thin-film transistor (OTFT), 401 Organochlorine pesticides, 723

1709 Organophilic montmorillonite (OMMT) nanocomposite, 376 Organophosphates, 124, 733 Organophosphorus hydrolase (OPH), 849 Organophosphorus pesticides (OPs), 209 Osteocalcin, 933 Osteoclastogenesis, 934, 935 Osteoclasts, 616 Osteoconductivity, 948 Osteocytes, 933 Osteoinduction, 937, 948 Osteonectin, 933 Osteopetrosis, 935 Osteoporosis, 1158 Osteoprotegerin, 935 Osteosarcoma, 936 Oxalic acid, 1548 β-oxidation, 1635 ω-oxidation reactions, 1511, 1512, 1635 Oxidation reactions, 751 Oxidoreductases, 168, 283 OxiTop-C, 1118, 1119 Oxo-biodegradation, 489, 1058 Oxy-degradable plastics, 828 Oxygen and pH levels, 1288 Oxygen-independent hydroxylation, 1500 Oxygen penetration prevention, 885 Oxygen permeability coefficient (OPC), 1319 Oxygen to carbon (O/C) ratio, 1249 Oxygen transmission rate (OTR), 1320 Oxygen uptake, 1116

P PAH bioremediation, 1109 PANI-dextrin nanocomposite, 402 PANi-gelatin blend nanofibers, 398 Paraffin, 875 Paramagnetism, 960 Paramecium sp., 140 Parent compounds, PPCPs, 1102, 1107 Partially bio-based non-biodegradable polymers, 301 Particulate leaching, 938 Particulate leaching techniques, 699 Particulate matter (PM), 752 2.5 particles, 1199–1202 Passive absorption, 1558 Pathogens’ cellular components, 1201 PBHV/NR based blend biofilms, 1319 PBST, 272 pDOC, 842

1710 p-doping, 394 Pedobacter steynii DX4 transcriptomics analysis, 852 Perchlorate reduction, 746 Percolation threshold, 634 Peroxidase, 162, 206 Persistent organic pollutants (POPs), 480 Personal protection equipment, 1175, 1176, 1191, 1202–1204 Pervaporation (PV) films, 546 Pesticide(s), 69, 75, 113, 124, 125, 225, 723, 733, 736, 737, 753, 754, 771, 772, 789, 849, 850, 1655 degradation, 125 Pesticide-degrading microorganisms, 207 Petrochemical-based materials, 1176 Petrochemical-based non-degradable polymers, 1191 Petrochemical-based plastic, 1186, 1203 Petroleum, 186 contamination, 185 drilling wastes, 1669 Petroleum-based biodegradable polymers Nylon 4, 274 PBST, 272 poly-caprolactone, 268 poly(p-dioxanone), 274 polyvinyl alcohol, 271 Petroleum-based materials, 1370, 1371 Petroleum-based non-biodegradable materials, 1183 Petroleum-based plastics, 502 Petroleum-based techniques, 1186 Petroleum hydrocarbons, 139, 847, 848 degradation mechanism and pathway, 1661–1662 degrading process of alkane and cycloalkane, 1662–1663 degrading process of polycyclic aromatic hydrocarbons, 1664 enzymes assisted in biodegradation, 1665–1667 microbial degradation, 1659–1664 oil spill, 1658 process of degradation of aromatic hydrocarbons, 1663–1664 Petroleum industry wastes drilling wastes, 1669–1670 oily sludge, 1670–1671 as sustainable building materials, 1668–1671 Petroleum plastics, vs. bioplastics, 548 Petroleum-polluted aquifer, 1109

Index PGPR degradation growth hormones regulation by plant, 232–234 microbial role in nitrogen fixation, 229–230 microbial role in phosphorous solubilization, 230–232 microfungi and mycorrhiza biodegradation, 235–236 protection from phytopathogenic microorganisms, 234–235 PGPR strains, 229 pH, 72, 120 Phanerochaete chrysosporium (a white-rot fungus), 116, 207 Pharmaceuticals, 518–520, 791 biodegradability, 1112 Pharmaceuticals and personal care products (PPCPs) aromatic compounds, 1108–1110 AST, 1107, 1108 bioaccumulation, 1100 biodegradability (see Biodegradability of PPCPs) biological transformation, 1098, 1101–1103 biotransformation, 1096, 1101–1103 category, 1096, 1097 chemical structures, 1107 classifications, 1100 destiny, 1100 domestic sewage, 1097 drinking water, 1097 drinking water source, 1099 environmental pollution, 1096 and human interactions, 1099–1101 hygiene, 1097 industries, 1096 metabolism (see Metabolism) MFC A/O systems, 1108–1110 microbiological modification, 1098 micropollutants, 1111 parent compounds, 1102, 1107 pathways, 1098, 1099 production, 1097 protective products, 1097 removal efficiency, 1098 SAT, 1107, 1108 seawater, 1097 soil, 1107 sources, 1096, 1099 STPs, 1097 water contamination, 1096 WTPs, 1097 and WWTPs, 1097–1099

Index pH condition, 1551, 1554 Phenanthrene carboxylic acid, 1501 Phenols, 186, 789, 1468 Phlorotannins, 517 Phosphate-based conversion coatings, 1226 Phosphorus, 230, 233, 1291 Photocatalytic degradations, 1624 Photochemical polymerisation, 394 Photodegradable plastics, 491 Photodegradation, 188, 869, 1179, 1358, 1555 Photodynamic therapy, 703 Photolithography, 1140 Photoluminescence spectroscopy (PLS), 367 Photolysis, 1108 Photosynthetic organisms, 113 Phthalates, 1191 Phycoremediation, 159 Physical biodegradation, 1413 Physical deterioration process, 371 Physical properties of CNTs atomic structure, 649 bulk density, 650 electrical characteristics, 650 length, 649 optical and thermal properties, 650 specific surface area, 650 thickness, 649 Physical vapor deposition, 1230–1231 Phytoaccumulation, 904–905 Phytodegradation, 906, 1555 Phytoextraction, 1555, 1557 Phytofiltration, 905, 1555, 1557 Phytopathogenic microorganisms, 234 Phytoplankton, 1546 Phytoremediation, 117, 904–906, 1554–1560, 1562 Phytostabilization, 905–906, 1555, 1557 Phytovolatilization, 906, 1555, 1557 Piezoelectric ceramics, 958 Piezoelectric polymer-ceramic composites calcium-modulated protein, 957 direct and converse effects, 956 inorganic, 958 living bone, 956 mechanical stress, 956 Piezoelectric polymers bone regeneration applications, 958 cell adhesion and proliferation, 958 ceramic-polymer composite materials, 958 PHBV, 958 PLLA, 958 PVDF limits, 958

1711 α-pinene, 1293 pJP4 plasmid, 842 PLA/neem, 1193, 1194 Placental growth factor (PlGF), 1069 PLA fabrication, 1006 Plant and animal productivity, 750 Plant-based biofibers, 323–327 Plantbottle, 489 Plant cellulose, 543 Plant-derived antimicrobials, 1300 Plant growth hormones, 233 Plant growth-promoting rhizobacteria (PGPR), 232, 234 Plant hormones, 234 Plant proteins, 1317 Plasma electrolytic oxidation, 1229 Plasma polymerization, 395 Plasmids, 76 Plasticizers, 480, 506, 508, 586, 878, 883, 1317 Plastic material(s), 1452 biodegradation, 578 Plastics, 7–10, 113, 122, 123, 185, 238, 480, 573, 1176, 1309, 1311, 1316, 1328, 1422, 1452 applications, 555 are non-biodegradable, 481 bio-based biodegradable, 1426–1427 biodegradation, 197, 481, 578, 866 biodegradation rate, 583 classifications, 883 components, 1176 damaging effects, 1179 debris, 1453 decomposition, 1177 enzymatic breakdown, 588 ethylene manufacturing, 868 films, 826 fossil-based biodegradable, 1426–1428 garbage, 539, 1175–1177, 1179, 1181, 1203 global waste production, 865 with management options, 590 market, 1176 materials, 863 microbial decomposition mechanism, 576 microbiological plastic degradation mechanism, 1422–1425 pollution, 297, 480, 481, 1452, 1453 polymers, 576 production and consumption, 864 products, 573 recycling, 541, 564 as thermosetting/thermoplastic, 573 trash, 1175, 1204

1712 Plastics’ biodegradability, 574, 580–582 chemical structure and crystallinity, 825 kinetics, 825 molecular weight, 825 polymer’s glass temperature, 825 tests, 581 Plastic waste, 197, 200, 207, 215, 573, 588, 591, 594, 1276, 1277, 1301, 1452 management, 297, 310 recycling, 7 Platelet-derived growth factor-BB (PDGF-BB), 1067 Platinum Nanoparticles (NPts), 960 Please replace Fig. with the attached version., 646 Pleurotus ostreatus, 206 Plexiglass, 745 Polar amide groups, 1187 Pollutants bacterial biodegradation, 911–916 biodegradable contaminants, 908–909 biodegradation, 234, 241 chemical pollutants, 224 ex situ bioremediation, 904 microbial biodegradation, 908 microfungi and mycorrhiza degradation, 915–920 microorganism remediation, 906–908 microorganisms, role of, 909–911 phytoremediation, 904–906 in situ bioremediation, 903–904 Pollution, 480 classification, 224 heavy metal, 1534, 1535 microbial biosorption, 1556, 1558–1560 phytoremediation, 1555–1560 Poly (β-(1–4)-N-acetyl-D-glucosamine), 682 Poly(3,4-ethylenedioxythiophene) (PEDOT), 960, 1610 Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), 376, 880, 1319 Poly(3-hydroxybutyrate) (PHB), 303, 821, 880 biodegradability, 823–824 biological consideration, 823–824 biosynthesis, 821 cyanobacteria, 822–823 detection and analysis, 823 synthesis, 821–822 Poly (3-hydroxyoctanoate), 1318 Polyacrylonitrile, 1466 Polyalcohols, 1373 Polyamide, 418, 419

Index Polyamide, and acrylic (PP&A) fiber, 480 Polyanhydrides, 1060 Polyaniline (PANI), 394, 960, 1072 Polyaromatic hydrocarbons (PAH), 74, 1276, 1297 Poly-butylene adipate-co-terephthalate (PBAT), 267, 269, 271, 273 Polybutylene adipate terephthalate (PBAT), 490, 1183, 1185, 1187, 1190, 1324 Poly (butylene furandicarboxylate)-b-poly (glycolic acid), 880 Polybutylene succinate, 304–305, 490, 819, 1185–1188, 1190, 1604 Poly (butylene succinate-co-glycolate) (PBSGL), 880 Polybutylene terephthalate (PBT), 480, 865 Poly butyrate adipate terephthalate (PBAT), 819 Polycaprolactone, 482, 952, 1185, 1187, 1189, 1190, 1197 Poly(ε-caprolactone), 336 Polycaprolactone diol (PCL), 399, 699, 1313 Polycaprolactone fumarate (PCLF), 398, 399 Polycaprolactone nanocomposites, 1076 Poly(ε-caprolactone) (PCL), 29, 378, 1602 Poly-caprolactone (PCL), 124, 267–269, 393, 396–399, 403, 814, 817, 819, 960, 1075–1077, 1183, 1311, 1321–1322 biodegradation of films, 283 enzymatic degradation, 281 starch blend, 270 weight loss of, 282 ε-polycaprolactone polymer emulsion, 965 Polycarbonates, 307–308, 1060 Polycarboxylic acids, 1373 Polycarprolactone (PCL), 994 Polychlorinated biphenyls (PCBs), 1297, 1298, 1655 Polycyclic aliphatic compounds, 1113 Polycyclic aromatic hydrocarbons (PAHs), 207, 208, 224, 237, 255, 658, 1655 Polycyclic aromatic sulfur heterocycles (PASHs), 234 Poly (D, L-lactic acid) (PDLLA), 396, 398, 1071 Polydioxanone, 339 Poly(D-lactic acid) (PDLA), 1071, 1320, 1354 Polydopamine-capped reduced graphene oxide (PDG), 1603 Polyester, 480 amide, 482

Index Polyester-based PU, 273 Polyesterification process, 1381 Polyesterification reactions, 1381 Polyesters, 725, 877 PHAs based biopolymers, 1318–1319 Polyethene terephthalate (PET), 1440, 1441 Poly(ether-ether-ketone) (PEEK) polymer, 366 Polyether polyols, 485, 486 Polyethylene, 480, 1278, 1452 Poly(ethylene glycol) (PEG), 954, 1067, 1074–1075, 1084 Polyethylene oxide (PEO), 335 Polyethylene (PE), 201, 202, 418, 573, 865 Polyethylene succinate (PES), 124 Polyethylene terephthalate (PET), 205, 239, 418, 480, 573, 733, 865, 1468, 1469 Polyethylene-vinyl alcohol, 543 Poly furfuryl methacrylate (PFMA), 405 Poly(glycerol sebacate) (PGS), 995, 1053 Poly (glycolic acid-co-ε-caprolactone), 880 Polyglycolic acid (PGA), 335, 466–467, 879, 952, 994, 1059, 1072–1073, 1311 Poly-glycolide-co-caprolactone (PGCL), 1077 Polyheterocycle group, 960 Polyhydroxyalkanoates (PHAs), 124, 303–304, 481, 491, 815–818, 821, 995, 1302, 1355, 1356, 1426 anoxygenic phototrophic bacteria, 484 bioplastic films, 513 bioplastics, 513 biosynthetic, 483 classified, 513 hydroxy alkalotic acid residues, 512 3-hydroxyalkanoic acids, 485 microorganisms, 484 petroleum-based plastics, 512 5-phenylvaleric acid, 483 pseudomonas, 484 storage materials, 513 structure of, 484 Polyhydroxy butyrate (PHB), 205, 369, 481, 484, 490 Poly(hydroxybutyrate) and Poly(3hydroxybutyrate-co-3hydroxyvalerate), 381 Poly-β-hydroxybutyrate (PHB), 823 Polyhydroxybutyrate valerate (PHBV), 481 Poly(hydroxymethylglycolideco-ε-caprolactone) (pHMGCL), 1075 Polyhydroxy valerate (PHV), 369

1713 Poly(lactic acid) (PLA), 124, 265, 296, 305–307, 335, 377, 397, 467–469, 481, 490, 545, 724, 725, 729–731, 733, 735, 814–816, 952, 993, 994, 1059, 1071–1072, 1183, 1185, 1191, 1193, 1311, 1320, 1321, 1341, 1352–1355, 1426, 1465, 1466, 1598 electrospun fibers, 1193 Poly(lactic acid)/chitosan (PLA/CS), 1078–1079 Poly(lactic-co-glycolic acid) (PLGA), 700, 1053, 1059, 1073–1074 Poly (lactic-co-glycolide), 880 Polylactide (PLA), 275, 369, 397, 399, 400, 491, 815–816, 825 Poly (lactide-co-glycolide) (PLGA), 399, 469–470 Poly(L-lactic acid) (PLLA), 816, 952, 994, 1071, 1354 Polymer, 725, 821, 1058, 1191, 1597 biodegradable, 321 chains, 826 characteristics of, 30–33 extrusion, 628 fibrils, 699, 700 food packaging materials, 1339–1341 hydrogels, 399, 400 nanoparticle composites production, 610 natural, 949–952 properties, 578, 995–996 synthesis, 1543 synthetic, 949, 952 synthetic polymers, 1596 thermal properties, 981 Polymer-based nanoclays, 366 Polymer-based supercapacitors in, 460 Polymer biodegradation, 1298 by soil microorganism, 371 Polymer-bioglass, 954 Polymer-degrading bacteria, 1301 Polymer-degrading microorganisms, 197 biodiversity and occurrence, 576–577 Polymeric alkyds, 1386 Polymeric biomaterials, 354, 369 Polymeric composites, 398 Polymeric membranes, 520 Polymeric molecules, 1471 Polymerization, 394, 492 Polymer nanocomposites, 420 high barrier characteristics, 424–425 inorganic synthesis and in situ polymerization, 424

1714 Polymer nanocomposites (cont.) in situ polymerization, 423 in situ synthesis nanoparticle preparation, 423–424 interface’s role, 417–418 as matrices for biomolecules, 418–419 melt mixing, 422–423 of polysaccharides, 426–430 solution, 421 Polymer-nanocomposites biodegradation aliphatic polyesters biotic and abiotic degradation, 381 graphene oxide-bio-chitosan nanocomposite, 380 PHB and PHBV, 381 PHBV/OMMT, 376, 377 poly (ε-caprolactone) nanocomposites, 378, 379 polylactic acid, 377 products, 381 Polymer nanocomposites (PNCs), 356 AFM, 367 applicability and safety of, 386 bone tissue engineering, 384 catalytic properties, 366 characterization, 367 chitosan-based nanohydroxyapatite composite, 385 direct mixing of polymer and nanofillers, 362, 363 drug delivery, 383 EDAX, 367 electrical and dynamic mechanical properties, 364, 366 environmental applications, 385 formation, 357 future perspectives, 387 intercalation methods, 359, 360 interface, 364 luminescence, 366 melt compounding, 363 melt intercalation methods, 360 NMR, 367 optical clarity, 366 preparation methods, 359 in situ polymerization method, 360, 361 sol-gel method, 361, 362 solvent method, 363 synthesis and fabrication methods of, 357 TEM, 367 thermal stability, 366 tribological properties, 366 WAXD, 368

Index wound dressing, 382 XPS, 367 XRD, 367 Polymers biodegradability, 6–8, 1454 chitosan, 434–437 starch and thermoplastic starch, 437–441 Polymethyl methacrylate (PMMA) composites, 1389 Poly-nanocomposites, 607 Poly(N-isopropyl acrylamide (PNIPAAm), 1053 Polyols, 485, 1383 Poly(orthoesters), 337 Polyoxyethylene chain, 1632 Poly(p-dioxanone) nanocomposites, 274, 1604 Polyphosphazene, 338 Polypropylene (PP), 201, 418, 480, 483, 573, 865, 1175, 1191 Polypropylene-based face masks, 1192 Polypropylene fumarate (PPF), 960 Polypropylene oxide (PPO), 335 Poly(pyrrole-co-aniline)-coated titania/ nanocellulose, 620 Polypyrrole (PPy), 398, 960 Polysaccharides, 13, 277, 426, 491, 681, 682, 1183, 1201, 1343–1345, 1468 alginate, 1069–1071 cellulose esters, 427 cellulose ethers, 426–427 cellulose micro (nano) fibrillated structures, 428 chitin, 1068–1069 chitosan, 1068–1069 hemicelluloses, 428–430 Polysaccharides-based biopolymers cellulose, 1315–1317 starch, 1312–1315 Polystyrene (PS), 198, 340, 480, 573, 865, 1175 film, 144 Polythiophene (PTh), 960 Poly (trimethylene carbonate) (PTMC), 994 Polyurethane (PU), 238, 480, 485, 865, 994, 1077–1078 Polyurethane-alkyd copolymers, 1382 Polyvinyl alcohol (PVA), 201, 270, 271, 401, 434, 463, 820, 954, 995, 1183, 1197 carboxymethyl cellulose (CMC), 1316 films, 511 Polyvinyl chloride (PVC), 201, 418, 419, 480, 573, 865, 1441 Poly(vinylidene fluoride (PVDF), 521, 957, 1386 Polyvinylpyrrolidone, 401

Index Pomegranate rind powder, 883 Porogens, 938 Porosity, 521, 999 Porous carbon, 1572 Porphyran chemical structure, 511 composite bioplastics, 511 partial methylation, 510 and phycobiliprotein-enriched fractions, 511 sulfated polysaccharide, 510 water susceptibility, 511 WVP, 511 Post-consumer plastic trash, 1179 Potassium, 1292 Potentiometry, 1627 Pourbaix diagram, 1227 Pressure-assisted gelatin extraction, 1026 Primary recycling, 1181 Pristine, 1613 cotton fabric, 1193, 1194 Probiotic cellulose antibacterial activity, 433–436 Production of biodegradable plastic with microorganisms, 821–824 renewable resources, 820–821 Product lifecycle management, 563 Product-life extra time, 1451 Professional home-care services, 936 Programmable modular self-assembly, 1143 Properties of BNCs high aspect ratio, 765 high surface area, 765 mechanical strength and rigidity, 766 solubility, 765 surface energy, 765 surface modifications, 766 Properties of CNTs chemical properties, 648, 649 physical properties (see Physical properties, CNTs) Propionyl-coenzyme A, 485 Protective coatings, 1388, 1392 Protein-based biopolymers, 1317–1318 Protein-based plastics, 818 Proteinic fibers, 1473, 1474 Protein-rich microalgal biomass, 514 Proteins, 1345–1352 Proteoglycan, 957 Proteolytic enzymes, 1473 Proteomics, 853 Prothrombin time (PT), 1079 Protozoa, 139

1715 Protozoans, 239 Protozoan species, 140 Pseudo-first-order reaction rate constant, 1111 Pseudomonas, 124, 147, 842 P. aeruginosa, 842, 1109 P. oleovorans, 483 P. putida, 848, 850 P. putida strain ShA, 145 strain YJB6, 157 Pullulan, 1319 Pure starch biofilm, 1314 Purification, 1627 of air pollutants, 746 Puronic hydrogels, 954 PVA/SrTiO3 hybrid nanocomposites, 401 Pyrethrin, 235 Pyrethrin secretion, 235 Pyrethroids, 124 Pyrolysis, 874, 1572 conventional, 1247 definition, 1245 fast, 1245 flash, 1245 method, 395 mild, 1247 pre-pyrolysis reaction, 1246 slow, 1245 steps in, 1246 vs. torrefaction, 1247 of wastewater sludge, 1247 wet, 1247 Pyrrolizidine alkaloids (PAs), 723

Q Quantum dots, 702 Quaternary ammonium compounds (QACs), 1511, 1513, 1514, 1630, 1640

R RAD16-II peptide gels, 1055 Radioactive compounds, 251 Radioactive wastewater, 788 Ralstonia eutropha, 842 Ralstonia oxalatica A5 strain, 842 Ramie fibers, 1461 Randomly DNA fragments, 246 Rapid prototyping (RP), 1135, 1150, 1163 technology, 699 3R (diminish, recycle, reuse) philosophies, 1451 Reaction conditions, 1636

1716 Reactive blue 19, 225 Reactive embedded oxygen species (ROS), 887 Reactive orange 16, 796 Reactive oxygen species (ROS), 1201 Reactor, 1285 Ready biodegradability (OECD 301) in aerobic aqueous medium, 1115 CBT (ISO 10707, 1116 CO2/DOC test, 1117 CO2 evolution test (ISO 9439), 1116 DOC die-away test (ISO 7827), 1116 manometric respirometry test (ISO 9408), 1117 MITI, 1116 modified OECD screening, 1117 properties, 1116 Recalcitrance, 141 Recalcitrant petroleum hydrocarbons, 1519 Receptor activator of nuclear factor-kB (RANK), 935 Recombinant DNA technology, 76, 841 Recyclability, 1204 Recycling, 539, 1181, 1183, 1310, 1451, 1453, 1469, 1477, 1478, 1480 heavy metals, 1559, 1561 process, 481 productivity, 1204 sustainable management of bioplastics, 1298, 1299 Red-9 dye, 795 Redox conditions, 71 Redox potential, 1421 Redox potential profiles, in anaerobic respiration, 1490 Reduced graphene oxide (rGO), 1056 Reduced graphene oxide-silver (rGO-Ag), 1077 Reductases, 165 Remazol Brilliant Blue R, 225 Renewable feedstock, 727 Renewable raw materials, 820–821, 832 Representative oils, 1375, 1377 Resistance random access memory (RRAM), 401 Respiratory airways, 1189 Respiratory disorders, 1201 Respiratory masks, 1191 Respiratory protection, 1175 Retention time, 1113, 1114 Reverse methanogenesis, 1501 Rhamnolipids, 1519 Rhizodegradation, 1555 Rhizofiltration, 1556 Rhizopus arrhizus, 688

Index Rhizospheric microorganisms, 908 Rhodococcus erythropolis D310-1, 853 Rhodococcus ruber IEGM 346, 1120, 1121 Rhodococcus ruber TH, 843 Rhodopseudomonas palustri, 846 Ribosomal ribonucleic acid, 139 Rice husk flour (RHF) filler, 279 Ring-opening polymerization (ROP), 821, 1353 3R principle (reduce, reuse, and recycle), 1181, 1182 Runt-related transcription factor 2 (RUNX2), 933 S Saccharomyces cerevisiae, 186 Safe water biological functions, 783 importance of, 784 Salinity, 72 Sapromat, 1118, 1119 SARS-CoV2 illness, 709, 1175 Saturated mutagenesis, 842 Scaffold(s), 1018–1019 angiogenesis, 1063 biologic, 1055 biomorphic transformation, 963 and cells, 1049–1050 cell sheet, 1053–1054 3D hydrogel, 1066 3D printing systems, 961–962 myocardial reconstruction, 1066 PEG-based hydrogel-forming, 1075 PLLA, 1071 transplanted, 1057 Scaffold-based strategies, 1158–1161 Scaffold fabrication bone, 939 3D bioprinting, 942–944 3D printing techniques, 940–942 electrospinning, 939 freeze-drying, 938 gas foaming, 938 phase separation, 938 solvent casting/particulate leaching, 938 Scaffold-free CM cell patch, 1053–1054 Scaffolding polymers properties in cardiac tissue engineering bioactivity, 997 biocompatibility, 997, 998 biodegradability, 998 mechanical, 999, 1000 morphology, 999 porosity, 999

Index Scanning electron microscopy (SEM), 487, 1321 micrographs, 1313, 1314 of polyester, 1467 Scenedesmus, 159 Seawater, 1097 Secondary recycling, 1181 Selective laser sintering, 941 Self-degrading materials, 1480 Semi-continuous activated sludge test (SCAS), 1117 Semi-crystalline polymers, 419 Sensors, 400 Sequential biotransformation, 685 Serine hydrolase (SH), 198 Serratia marcescens, 1380 Sewage treatment, 747 bioreactors, 183 Sewage treatment plants (STPs), 774, 1097, 1121 Shape memory polymers (SMPs), 1602 Shellfish, 688 Short-chain alcohols, 1629 Shrimps, 679, 687, 688, 704 Shrimp shell, 689 Silica (SiO2), 964 aerogel, 618 Silicate-based bio-ceramics, 948 Silk, 951 cocoons, 985 fibroin, 985, 986, 992 Silver nanoparticles (AgNPs), 437–438, 519, 613, 885 Simulated body fluids, 616 Single-use masks, 1178 Single-use plastic cutlery, 492 Single-walled carbon nanotubes (SWCNTs), 645 authentic SWCNTs, 649 biodegradation, 667, 668 cylindrical tubes, 646 degradation, 659 degradation rate, 670 electrical properties, 650 in vitro transformation, 658 metal catalysts, 649 and MWCNTs, 646 optical and thermal properties, 650 ox-SWCNTs, 662 rate of degradation, 666 small geometric size, 654 specific surface area, 650 structure, 647

1717 Sisal fibers, 1461, 1462 Sisal plant, 1479 Skeletal muscles, 405 Skeletal progenitor cells, 1049 Skeleton-maintenance mechanism, 935 Skin, 403 regeneration, 1027–1031 tissue engineering, 1028, 1162–1163 SM/PCL, 1321 Small-angle X-ray scattering (SAXS), 368 Sodium alginate, 459–461, 505 extraction, 1038 Sodium caseinate films, 1351 Sodium dodecyl sulphate (SDS), 482, 1629 Sodium hypochlorite (NaClO), 880 Sodium metabisulfite (SM), 1321 Sodium silicate (SS), 1606 Soft rot fungi, 1292 Soil, 232 biological process, 1265 bioremediation, 183 burial test, 1327, 1328, 1464 contamination, 183 fertility, 1290 matric potential, 1420–1421 microorganisms, 1107 parameters, 1107 pH, 1259, 1265 PPCPs, 1107 sludge pH, 1554 structure, 1536 transport, 1102 washing, 1538, 1539 Soil aquifer treatment (SAT), 1107, 1108 Solar energy, 1300 Sol-gel method, 361, 362 Solid inorganic acids, 1548 Solid-phase extraction (SPE), 1100 Solid reinforcing action, 625 Solids retention time (SRT), 1113, 1114 Solid state polymerization, 395 Solid waste (SW) environmental impact, 866 food industry, 864 food packaging materials, 865 non-biodegradable food packaging, 865 packaging and construction, 864 plastic materials, 863 plastic production, 864 production, 863 waste sources, 864 Soluble chitin oligomers (GlcNAc), 688

1718 Solution casting, 627 approach, 1387 process, 1381 Solvent casting, 623, 938 Solvent method, 363 Sorbed state, 1538 Sorbitol, 704 Sorbitol plasticized soy-based bio-plastic (SSBP), 482 Soya-based bioplastics, 482 Soya protein-based plastics, 818 Soybean(s), 544 Soybean and linseed (soy-linseed) oils, 1389 Soybean oil (SO), 482 Soybean oil-based elastomers, 1381 Soy protein, 1317, 1349 Soy protein isolates (SPI), 1349 Spectrophotometry, 1627 Spherical nanocellulose forms (SCNFs), 1598 Sphingobacteriales bacterium, 1109 Sphingobium chlorophenolicum, 847, 848 Sphingomonadales, 853 Sphingomonas sp., 849, 1109 Sphingomonas sp. Strain RD1, 1106 Sphinomonas sp. Ibu2, 1121, 1122 Sponge-like hydroxyapatite-collagen, 963 Staphylococcus, 842, 1109 Staphylococcus aureus ATCC6538, 844 ® Starbucks , 264 Starch, 430–431, 491, 609, 877, 1023–1025, 1191, 1197–1199, 1312–1315, 1344–1345 foam-based cassava, 882 nanocomposites, 1605 plastics, 542 Starch-based bioplastics, 491 Starch-based films, 483 Starch-based polymers, 877 Starch-PANI nanocomposites, 402 Starch-rich tubers, 486 State-level biomedical waste management guidelines, 1182 Static pile composting, 1283–1286 Stem cells (SCs), 631–632, 1049, 1137 sources, 1138 Stem fibers, 1457 Stereolithography (SLA), 330, 940 Stoichiometric hydroxyapatite, 616 Streptomyces coelicolor, 848 Strontium, 947 Substrate-induced gene expression (SIGEX), 852

Index Succinate, 1499 Sulfamethoxazole, 126 Sulfide, 103 Sulfonamides, 1108 Sulfophenyl carboxylic (SPC), 1629 Sum parameter method, 740 Sunlight transmission, 885 Supercapacitors, 1571, 1584 Supercritical fluid chromatography, 1628 Superior wearing properties, 1200 Surface change, 1476 Surface erosion, 372 Surface functionalization, 966 Surface plasmon resonance, 614 Surfactant(s), 793, 1502, 1667–1668 aggregation and micellization of, 1505 amphiphilic structure of, 1505 amphoteric, 1517–1518 analysis, environment, 1626 anionic, 1506–1511, 1624 assessment, 1640 biodegradation, 1624 biodegradation mechanism, 1634 biodegradations, 1636 biosurfactants, 1518–1520 categories, 1623 cationic, 1511–1514, 1624 chemical substances, 1622 classified, 1625 classification of, 1507 classification, structure, and abbreviation of, 1508 efficient biodegradation, 1636 electrolysis approach, 1624 environmental degradation, 1626 factors, 1636 hand sanitizers, 1623 ideal cycle, 1623 impacts, on the environment, 1625 non-ionic, 1513–1517 oxidation process, 1624 petrochemical feedstock, 1623 photolysis reaction, 1625 physiological and metabolic processes, 1625 primary and ultimate biodegradation of, 1506 removal, 1624 role in biodegradation of oil droplets, 1505, 1506 skin and mouth ulcers, 1625 surface-active agents, 1622 types, 1507, 1628 ultrasonic degradation, 1624

Index Surfactant-rich effluents, 1625 Surgical face masks, 1201 Surgical masks, 1176, 1191 Sustainability, 233, 244, 1480 textile and clothing field, 1478–1480 Sustainable design approaches, 1451 Sustainable design for product development, 562–563 Sustainable development goals (SDGs), 832 Sustainable fibers cellulosic fibers, 1464 lyocell fiber, 1464 non-cellulose content and composition, 1464 polyacrylonitrile, 1466 poly (lactic acid), 1465, 1466 tencel, 1464, 1466 tencel fabrics, 1465 viscose fiber, 1464 viscose rayon, 1464 Sustainable polymers, 1340 Symbiotic mycorrhizae, 235 Symbiotic nitrogen fixation, 231 Syngas, 1246 Synthesis of BNCs characterization, 765, 766 chemical method, 764 enzymatic method, 764 mechanical methods, 763 pre-treatment methods, 763 process of extracting BNCs, 764 routes, 763 Synthesis of CNTs arc discharge, 650, 651 CVD, 653, 654 laser ablation, 652 Synthetic, 1597 biodegradable plastics, 491 biopolymers, 419 dyes, 237, 767 fibers, 1455, 1456 microbial plastics, 491 petrochemical-based face shields, 1175 plastic, 1175 plasticizers, 13 plastics, 197, 198, 728, 729, 1309 textiles, 1479, 1480 Synthetic biodegradation polymers (BPs), 1052–1059 biological fluids, 1071 chemical structure, 1072 PCL, 1075–1077 PEG, 1074–1075

1719 PGA, 1072–1073 PLA, 1071–1072 PLGA, 1073–1074 properties, 1071 PU, 1077–1078 Synthetic polymers, 6, 991, 994, 1147, 1183, 1184, 1452, 1587 biodegradable, 464 disadvantage, 1456 invention of, 453 materials, 1596 natural and, 453 PLGA, 469 PLLA/PGA, 952 polycaprolactone, 952 polylactic acid, 952 polyvinyl alcohol, 463 with hydrophilic properties, 465

T Talc nanoparticles, 438–441 TALENS, 843 Tannins, 1300 Teflon, 113 Temperature, 120, 188, 1059, 1298 Tencel, 1463, 1464, 1466 fabrics, 1465 Tensile properties (breaking load), 1477 Tensile strength, 419 Terminal electron acceptor (TEA), 63 Terpenoids, 1300 Tert-butyl alcohol (TBA), 1109 4-tert-octylphenol, 1516 Tetraaniline-graft-multialdehyde sodium alginate (MASA-AT) graft copolymer, 398 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 74 Tetracycline (TC, C22H24N2O8), 775 Tetrasodium glutamate diacetate, 1540, 1547, 1548 Tetrazolium, 634 Textile AATCC Soil Burial Method 30-1993, 1475 activated sludge relative biodegradability, 1475 biodegradability, 1454 biodegradable substrate, 1454 cellulose fibers, 1466–1468 changes of internal structure, 1476 classified, 1455

1720 Textile (cont.) dye wastewater, 794 enzymatic hydrolysis, 1475 and fabric manufacturing processes, 1455 fibers and fabrics recycling, 1477, 1478 fibers (see Fibers) and garments fiber, 1451, 1452 generations, 1456 ideal and practical environmental conditions, 1474 laboratory, simulation and field tests, 1474 manufacturing, 1543 observation of surface change, 1476 recycling, 1480 sustainability, 1478–1480 tensile properties (breaking load), 1477 weight loss, 1475, 1476 Textile wastewater, biodegradation of Ads, see Biodegradation of azo dye The American Society of Testing and Materials (ASTM), 1327, 1328 Theoretical Oxygen Demand (ThOD), 1115 Thermal degradation of CNTs, 660, 661 Thermal gravimetric analysis (TGA), 1321 Thermally exfoliated graphene oxide (TEGO)/PCL biofilms, 1321 Thermal processing, 439 Thermal properties (TGA), 486 Thermochemical conversion, 872 Thermochemical liquefaction technology, 873 Thermoforming process, 628 Thermomechanical analysis (TMA), 368 Thermophilic bacteria, 1288 Thermoplastic(s), 817 biodegradable polymers, 491 packaging, 1339 polymers, 981 Thermoplastic starch (TPS), 270, 437, 491, 543, 1313 with silver nanoparticles, 437–438 with talc nanoparticles, 438–441 Thermoplastic sugar (TPS), 207 Thermosetting plastics, 573 Thermostability, 521 Thick-walled cells, 1458 Time cycle of hydrocarbon fraction concentrations, 749 Time cycle of TPH, 749 TiO2 NMs, 1318 TiO2 NPs, 1315, 1318 Tissue-Engineered Vascular Grafts (TEVGs), 1154

Index Tissue engineering, 403, 629, 690, 1026, 1029, 1136, 1137 advantages, 984 applications, 463, 1048 biomaterial scaffolds and stem cell for skin tissue engineering, in wound healing, 1162–1163 biomaterial scaffolds for cardiac tissue engineering, 1154–1156 biopolymers for, 327–331 bioprinting technologies and cell sheet, 1138–1146 bio-resorbable stents and vascular grafts, 983 for bone, 405 bone repair and bone regeneration strategies, biomaterials and nanomedicine for, 1157–1162 BTE, 931 for cancer treatment, 405 cardiac regeneration, 1157 cardiac repairing and function, 1155 cardiac tissue engineering products, 1154 cardiovascular tissue engineering, 1152–1154 chitosan-based biomaterials, 1150–1151 CMC-based scaffolds, 1149–1150 combination therapy, 1163 composite porous polymeric structure for, 470 description, 931, 1048 drug delivery and, 469 engineered tissue, 983 for heart, 404 for nerve, 404 scaffolds, 467 for skeletal muscles, 405 for skin, 403 strategies, 1048 Tissue regeneration, 404 Titanium dioxide, 418 Titanium dioxide-polyethylene glycol/chitosan, 1082 Titanium oxide, 964 Titrimetry, 1627 Tobacco, 1295 Tone™, 268 Torrefaction definition, 1247 wet, 1247 Total petroleum hydrocarbons (TPHs), 747, 748 Toxicity of CNTs, 645, 654, 656, 669, 670 TPS/CS oligomer blend biofilm, 1315

Index Traditional scaffold fabrication, 1163 Transcription activator-like-effector nucleases (TALEN), 843 Transcriptomics, 852 Transduction, 247 Transformation transfer, 246 Transforming growth factor (TGF-), 934 Transmission electron microscopy (TEM), 367, 1316 Transparent polycarbonate sheets for greenhouses, 307 Transplantation technique, 629 Transposons, 842 Trash recycling, 1294 Triblock copolymer, 397 2,4,6-Tribromophenol (TBP), 166 Tri-calcium phosphate (TCP), 946, 948 Tricarboxylic acid (TCA) cycle, 1494, 1662 Triclosan (TCS), 1097, 1106–1108 Triglyceride chain, 1372 Triglycerides, 1371, 1375 Trimethylamine, 1293 Tripolyphosphate, 519 Tumor necrosis, 934 Type I interferons (IFNs), 708 TyRx, 556 U UDP-N-acetylglucosamine (UDP-GlcNAc), 687 Ultrasonic degradation, 1624 Ultraviolet-visible spectroscopy (UV-VIS), 1316 Ulvan chemical structure, 510 enzymatic extraction, 510 repeating disaccharides, 509 structural features, 510 sulfated heteropolysaccharide, 509 utilization, 510 Underground storage tanks (USTs), 69 United States Environmental Protection Agency (USEPA), 69, 786, 1099 Urban trees, 752 Urban wastewater, 785 UV light, 1108 UV-Vis-NIR spectroscopy (UV-Vis), 367 V Vaccines, 708 Variovorax Ibu-1, 1122 Vascular endothelial growth factor (VEGF), 1054

1721 Vegetable oil, 486 applications, 1371, 1373 benefits, 1371 Vegetable oil-based alkyd nanocomposite film advanced hyperbranched alkyd nanocomposites, 1380–1383 anti-corrosive and physico-mechanical properties, 1389 corrosion protection, 1393 GO-γ-Al2O3, 1394 GO-NanoTi composite, 1392 GO-SiC nanowires, 1394 linseed oil-derived hyperbranched alkyd, 1388 modifications of alkyd resins, 1389 polyesterification reaction, 1390 Vegetable oil-based polymeric alkyds eco-friendly alkyds coatings, 1377 hyperbranched alkyds, 1379 waterborne alkyd coatings, 1378 Vermicomposting, 1283, 1284, 1286 Vero cells, 701 Versatile peroxidases (VP), 254 Very small embryonic-like stem cells (VSEL), 1139 Very tiny embryonic-like SCs, 1137 Veterinary drugs, 1097 Virgin oils, 1371 Viruses, 1199–1202 Viscose fiber, 1464 Viscose rayon, 1464 Visual inspection, 581 Volatile fatty acid, 100 Volatile organic compounds (VOCs), 770, 870, 1293 Volatile organic content, 1371, 1373, 1377, 1380, 1381, 1383, 1386, 1395, 1396 Volatile pesticides, 224 Vorticella sp., 140

W Waste, 87, 89 accumulation, 5 hierarchy, 588 plastics landfill, 589 Waste management, 5, 1179, 1182, 1292 options, 828–829 strategies, 1310 technologies, 1178 Waste management, for bioplastic energy recovery by incineration, 589 landfill, 589

1722 Waste management, for bioplastic (cont.) recycling, 588 treatment for biological waste, 590 Wastewater, 766 treatment, 402, 785–787, 790–794, 797, 800 Wastewater treatment plants (WWTPs) biological activity, 1098 catabolism, 1098 cometabolism, 1098 effluent concentrations, 1099, 1100 and environment, 1102 homeostasis, 1107 micropollutant, 1121 ozonation and fenton oxidation, 1099 pH, 1099 PPCPs, 1114 removal effectiveness, 1098 TCS biotransformation, 1106 treating/dissolving pollutants, 1098 water quality and behavior, 1118 Water, 119 availability, 243 bioremediation, 183, 184 contamination, 1096 evaporation prevention, 885 loss, 1468 pollution, 762 purification, 520–521 Water-accommodated fractions (WAFs), 742 Waterborne alkyd coatings, 1378, 1379 Water contact angle (WCA), 1386 Water-insoluble polymers, 732 Water pollutants conventional pollutants, 785 forms, 785 hazardous pollutants, 785, 786 inorganic pollutants, 787, 788 microbial pollutants, 786, 787 organic pollutants (see Organic pollutants) Water-soluble biopolymers, 1298 Water-soluble polymer materials, 1192 Water treatment plants (WTPs), 1097 Water vapor permeability (WVP), 432, 504 Water vapor permeability coefficient (WVPC), 1316 Water vapor transmission rate (WVTR), 1320 Weight loss, 1468, 1475, 1476 Wet process, 626–627 Wet torrefaction, 1247 Wheat gluten, 1317, 1347 Whey protein concentrate (WPC), 1318

Index Whey protein isolate (WPI), 1319 White-rot fungi, 1292 Wide-angle X-ray diffraction (WAXD), 368 WikiPearls™, 264 Windrow(s), 1279, 1282 composting, 1279, 1282, 1283, 1286 Wool, 327, 1457 ropes burying, 1469 Wound dressing, 382 Wound healing, 701, 702, 1027–1031, 1162 GFs in, 1163–1164 potential applications of biomaterials for, 1166 Wound stitches, 493

X Xenobiotic(s), 66, 145 biodegradation, 1278 X-ray diffraction (XRD), 367, 1321 analysis, 1316 X-ray energy dispersive analysis (EDAX), 367 X-ray photoelectron spectroscopy (XPS), 367

Y Yeast(s), 238, 801, 802 biodegradation, 238 bioremediation, 186, 187 degradation, 917 Yellowish liquid, 1542

Z Zahn-Wellens/EMPA, 1117, 1118 Zahn-Wellens test, 1117 Zein, 1319, 1346–1347 blends, 1319 ZEK100, 946 Zeolitic imidazolate framework-8 (ZIF-8), 1200 Zero-dimensional materials, 614 Zero emissions, 722 Zero-waste strategy, 1451 Zinc, 947 Zinc-finger nucleases (ZFNs), 841, 843 Zinc oxide (ZnO), 958 nanoparticles, 401, 1321 starch blend film, 1314 Zirconia titanate, 957 ZnO/CuO/Ag/starch blend film, 1314 Zoogloea sp., 1109 Zwitterionic surfactants, 1517–1518, 1522