Nanotechnology in textiles: theory and application 9780081026090, 0081026099

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Nanotechnology in Textiles

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The Textile Institute Book Series

Nanotechnology in Textiles Theory and Application

Rajesh Mishra Jiri Militky

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2019 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102609-0 (print) ISBN: 978-0-08-102627-4 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

List of contributors ix Acknowledgment xi 1

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Nature, nanoscience, and textile structures Rajesh Mishra, Jiri Militky 1.1 Introduction 1.2 Evolution of nanotechnology 1.3 New instruments 1.4 Natural nanosystems 1.5 Biomedical and industrial applications of nano science References Further reading Electrospun nanofibers Rajesh Mishra, Jiri Militky, Mohanapriya Venkataraman 2.1 Introduction 2.2 Posttreatment of nanofibrous membranes 2.3 Effect of electrospinning parameters and potential applications of nanofibers 2.4 Advanced centrifugal electrospinning 2.5 Blended polymer electrospinning 2.6 Three-dimensional nanofibrous macro-structures via electrospinning 2.7 Zirconium carbide nanofibers by electrospinning 2.8 Parameters on electrospinning process and characterization of electrospun nanofibers 2.9 Nano scaffolds prepared by disc-electrospinning 2.10 Electric field analysis of a multifunctional electrospinning platform 2.11 Free surface electrospinning for high throughput manufacturing of core-shell nanofibers 2.12 Electrospinning of high strength aqueous silk fibroin nanofibers 2.13 Coaxial electrospinning 2.14 Corona-electrospinning 2.15 Miscellaneous electrospinning methods 2.16 Recent developments in electrospinning 2.17 Applications of nanofiber membranes References Further reading

1 1 3 8 9 16 32 34 35 35 37 39 44 46 46 57 59 65 68 73 76 82 88 89 133 143 152 161

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Carbon-based nanomaterials Rajesh Mishra, Jiri Militky 3.1 Introduction 3.2 Carbon nanotubes yarns (CNY) 3.3 Electrochemical carbon based nanosensors 3.4 Fullerenes 3.5 Carbon nanotubes 3.6 Carbon nanofibers 3.7 Carbon nanotools as sorbents and sensors of nanosized objects 3.8 Carbon nanomaterials for nerve tissue stimulation and regeneration References

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Nanoparticles and textile technology Rajesh Mishra, Jiri Militky 4.1 Introduction 4.2 Selected features of nanoparticles 4.3 Nanoparticles preparation 4.4 Nanoparticles application in the textile industry 4.5 Carbon nanoparticles 4.6 Cellulosic nanoparticles 4.7 Conclusion References Further reading

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Characterization of nanomaterials in textiles Rajesh Mishra, Jiri Militky, Veerakumar Arumugam 5.1 Introduction 5.2 Environmental and health effects of nanomaterials in nanotextiles 5.3 Application of functionalized nano-fibres 5.4 Flame-retardant protective clothing 5.5 Tissue engineering 5.6 Affinity membranes 5.7 Super hydrophobicity 5.8 UV protection 5.9 Nanosensors 5.10 Protective clothing 5.11 Miscellaneous areas References

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6 Nanocomposites Rajesh Mishra, Jiri Militky 6.1 Introduction 6.2 Nanocomposite for photocatalytic degradation References

163 165 165 167 167 172 174 176 177

181 181 189 193 197 204 214 214 217

219 220 222 224 227 231 240 242 244 247 249 257 263 263 266 308

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Nanoporous materials Rajesh Mishra, Jiri Militky, Mohanapriya Venkataraman 7.1 Introduction 7.2 Nanoporous silica aerogel for thermal insulation 7.3 Nanoporous carbon materials 7.4 Nanoporous copper structures 7.5 Polypyrrole nanostructures 7.6 Suspended hydrophobic porous membrane for high-efficiency water desalination 7.7 Nanoporous chitosan materials 7.8 Bioinspired engineering of honeycomb structure 7.9 Bio-inspired superoleophobic and smart materials 7.10 Nanoporous graphene film 7.11 Decolorization of methylene blue by nanosheets 7.12 Hierarchically nanoporous nanofibers References

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Nanorisks and nanohazards Rajesh Mishra, Jiri Militky, Veerakumar Arumugam 8.1 Introduction 8.2 Toxicity of antibacterial nanoparticles 8.3 Toxicity of miscellaneous nanomaterials 8.4 Toxicity of carbon nanotubes References

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Future outlook in the context of nanoscale textiles as a technology for the twenty-first century Rajesh Mishra, Jiri Militky

311 312 328 332 334 336 336 339 341 344 346 346 351

355 359 364 378 384 387

Index 389

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List of contributors

Veerakumar Arumugam  Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic Jiri Militky  Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic Rajesh Mishra Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic Mohanapriya Venkataraman  Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

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Acknowledgment

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic and European Structural and Investment Funds. Part of the Operational Programme Research, Development and Education framework—project title Hybrid Materials for Hierarchical Structures (HyHi, Reg. no. CZ.02.1.01/0.0/0.0/16_019/00 00843).

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Nature, nanoscience, and textile structures

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Rajesh Mishra, Jiri Militky Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

1.1 Introduction Nanoscience and nanotechnology refer to the study, manipulation, engineering, and application of matter, particles, and structures on the nanometer (nm) scale (1 nm = 10−9 m). The “nano” in nanotechnology comes from the Greek word “nanos” that means dwarf. One nanometer is one-billionth of a meter, that is, 10−9 m. The fundamentals of nanotechnology lie in the fact that properties of substances dramatically change when their size is reduced to the nanometer range. Often, nanomaterials are defined by a size range limited by at least one of the dimensions. This range may be 1–100 nm (British Standards Institution 2007; ISO 2008), 0.1–100 nm, less than 100 nm, or less than 500 nm. The most common and accepted definition is probably the 1–100 nm range. In addition, it is sometimes suggested that to be counted as a nanomaterial, the material must have properties different from those of the bulk form of the same chemical substance. The nanorange is not characterizing the properties of matter. Usually, the low level of nano (approximately 1–10 nm) has very different behavior from moderate level of nano (approximately 10–30 nm) and far nanorange (over 30 nm). This is valid, for example, for influence of grain size on the yield strength of grained metals. This dependence for coarse-grained metals follows the Hall-Petch relationship where the yield strength is proportional to reciprocal value of grain size. Strength is therefore increasing with decreasing of grain size. Maximum yield strength is for grain size about 25 nm. For smaller grain sizes, yield strength is decreasing very seriously and at 20 nm reaches approximately the same value as for 30 nm. The European Commission released their suggested definition of nanomaterials: “‘nanomaterial’ means a natural, incidental, or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50 % or more of the particles in the number size distribution, one or more external dimensions is in the size range 1–100 nm” (European Commission: Commission recommendation of 18 October 2011 on the definition of nanomaterial. Brussels 2011). This definition is used in EU standard nanotechnologies—terminology and definitions for nanoobjects—nanoparticle, nanofiber, and nanoplate: ISO/TS 80004–2:2015(en). The big future of nano was launched by R. Feynman at an American Physical Society meeting at Caltech on 29 December 1959 in lecture “There’s plenty of room at the bottom.” Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00001-8 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Important properties of materials, such as the magnetic, electric, optical, thermal, and mechanical properties, are determined by the way molecules and atoms assemble at the nanoscale. Nanoscience and nanotechnology are at the forefront of modern research, and they are considered the new revolution for the 21st century. Nanoscience education is still emerging, and unlike other areas of science education, there are some gaps to fill and explore the crucial ideas of nanoscience and nanotechnology. The continuous advance of nanomaterials science and its unprecedented application in more and more nanotechnology-based consumer products indicate that nanomaterials are crucial to develop new applications: biological tagging, medical diagnostics and treatment, solar energy harvesting, catalysis, and electro-optical applications. Given the expected economic and social impact of nanotechnology products and the fact that many areas of application are still scarcely explored, it can foresee that industrial use of nanomaterials will continue to increase in the future. However, one of the “grand challenges” for nanotechnology is bottleneck for the development and implementation of the field. It is even the same situation where we may have the research results for new nanoapplications but without having skilled workers to translate them out of research centers. Nanoscience and nanotechnology scientific disciplines are situated at the interface between physics, chemistry, biochemistry, biotechnology, materials science, medicine, microelectronics, and computer science. Control of these disciplines therefore requires an academic and multidisciplinary scientific education. Then, it seems reasonable that a multidisciplinary scientific research is crucial to provide industry and research institutes with top quality experts. However, the physical infrastructure in nanoscale science is still in formation, being the multidisciplinary understanding one of the bottlenecks [1, 2]. In general, the researchers have difficulties to understand the underlying scientific principles that lead the unique properties at the nanoscale. And what are more important, researchers also have difficulties in implementing high-quality nanoscale material to produce a deep understanding of nanoscience concepts. Considering the previous facts, there is a need of both thinking minutely with efficiently scientific tools to assist in the knowledge transfer. Online resources are considered to be useful in areas of science where records of complex laboratory demonstrations or physical/chemical phenomena might be more effectively communicated than would prose. For instance, the use of images and documentaries and the ability to share them through the Internet have revolutionized scientific procedures, enhanced our ability to discover new things, and offered new opportunities for research. It is a valuable tool because it can be used to show researchers things that would be otherwise hard to transfer in a limited period of time. In fact, an increasing number of scientists use the Internet to present their results at scientific meetings, during lectures, or in their publications as online supplementary material. Then, it seems clear that the use of Internet to understand the concepts and phenomena occurring at a world where the scale is far beyond our dimensions could ease research in nanoscience. In nanoscience, the past advances and the future prospects in new topics ranging from properties of nanomaterials to their societal impacts are studied in much detail [3–6]. Nanoscience and nanotechnology (NST) are widely considered as one of the most promising areas of scientific and technological development for future  ­decades.

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As a consequence, almost every country in the world has chosen to invest significantly in this area. This choice, however, is only a first step in the investment decision process, given that almost any scientific discipline can be taken at the scale of a nanometer. In this chapter, it is argued that foresight studies to decide where to invest in the nanotechnology area should be designed in a different way from what is normally done. Nanotechnology, in fact, has specific characteristics that should be taken into account when evaluating its expected impacts and potentialities. Some nanomaterials (as nanofibrous assemblies) have some limitations for practical use because they are too weak and too sensitive to abrasion to be outer or inner part of structures in conditions of using and maintenance, they have some effect on nanolevel only, and they have serious limits for longer time durability in common conditions. Their effects are often only temporarily. Till now, there is unprecise information oriented to highlight “nano” from the point of view of scientific content and to suppress weakness in real conditions. Instead of seriousness, there are appeared “newspaper stories” oriented to dazzle customers (nanolayers with extremely thermal insulation, extreme surface area of nanolayers, nano is equivalent to stronger, etc.). Of course, there are some big advantages of nano, but there are serious limitations as well.

1.2 Evolution of nanotechnology In spite of the discovery of radioactivity at the end of the nineteenth century, although the general progress of physical, chemical, and biological sciences explicitly relies on the understanding of atomic nature of matter, real life could ignore the elementary bricks we are made of. However, at the end of the twentieth century, with the breakthrough of nanotechnologies, we realized that the nanometric size, close to that of atoms, is no longer beyond our perception and our range of action. Nanos are now guests of our everyday life. Let the history of nanosciences be briefly recalled. The discovery of the transistor in 1947 has deeply transformed electronics, as it was thus demonstrated that a small piece of germanium could do as well, if not better, as a complex and fragile vacuum tube. This piece of germanium, soon substituted by silicon, later became smaller and smaller; hence, the name “puce” (flea) designates a chip in French. Once the working mechanism of transistors has been well understood, it became possible to put several transistors on a single piece of semiconducting material and to connect them to obtain a complex electronic device, a microprocessor. As early as 1971, 2300 transistors could be assembled in the same processor. In the following years, the history of electronics became mainly a history of miniaturization, the problem to solve being: how to make the same circuits on smaller and smaller silicon pieces. Miniaturization had numerous advantages, the circuit becoming faster, cheaper, and more reliable. This miniaturization was made possible by breakthroughs achieved in several technical domains, including photographic masks, more and more controlled chemical etching, strongly directed ionic etching, the use of shorter and shorter wavelength radiation for the pattern transfer, and masks directly written with an electron beam. Size reduction proceeded steadily. The smallest elementary size achieved in a commercial circuit was 0.35 μm in 1995, 0.13 μm in 2001, 0.065 μm in

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2007, and 0.028 μm in 2010. An advantage of reduced size was the increased number of transistors per chip. From 2300 in 1971 as already said, it doubled every 2 years since that time and reaches more than 2 billion in 2010. The size reached at the end of the twentieth century is clearly in the nanometer range (1 nm = 10−9 m or 0.001 μm). A 22 nm wide motif that is presently available represents a silicon ribbon of only about 60 atoms. Microelectronics is thus becoming nanoelectronics. In other fields of sciences and technology, a similar nanorevolution took place, either spontaneously or stimulated by the progress of electronics, computer science, fabrication techniques (masks, etching, etc.), and observation methods (microscopes). These different fields will now be reviewed.

1.2.1 Nanomaterials While the small transistor is traditionally the remnant left after etching of a silicon crystal, another method appeared based on the relatively classical methods of chemical synthesis. Indeed, nanometric objects are naturally produced by chemistry, namely, more or less complex molecules. The trick is to orient the synthesis either toward the production of molecules suited for certain complex functions or toward the fabrication of “objects” that consist of thousands or more of properly assembled molecules that are dedicated to particular functions. This technique is called “bottom-up,” while the traditional etching approach is called “top-down.” A few examples will be given that demonstrate that sometimes, matter shows some good will for the creation of nanometric objects with remarkable properties [6]. Carbon nanotubes have been discovered accidentally in 1991 by the Japanese scientist Sumio Iijima as he observed with an electron microscope the soot generated by an electric arc. Carbon nanotubes are tubes made of a rolled graphene sheet. Graphene is a graphite layer of atomic thickness, in which all electrons of the external shell of the carbon atom are paired. A carbon nanotube has the periodic structure of a crystal. Its diameter is a few nanometers, while its length can reach hundreds or thousands of nanometers. Nanotubes can be either conductors or insulators. They exhibit remarkable mechanical properties since they can be bent drastically without breaking, and their mass is small since carbon is a light atom. They can now be synthesized at an industrial scale. They already have many applications: improvement of mechanical resistance (tennis rackets), an additive to resins to render them conducting, electron emitting cathodes, and high-efficiency gas absorption. How are nanotubes synthesized? They are generated from the surface of a nanometric metal droplet in the presence of carbon vapor. Metal absorbs carbon from the surrounding vapor, and the supersaturating carbon is eliminated as a ring at the surface of the drop. This process has some analogy with certain knitting techniques used to produce woolen tubes. Using similar principles, many other nanotubes or nanowires can be prepared, especially from semiconductors, for example, Si, ZnSe, ZnO, and MgO. Semiconductor nanowires can be doped during their growth, and the doping can be modulated along the tube so that a nanowire can be grown directly with the properties of an electronic diode. These new elements are appropriate for light emission or absorption, since nanowires, due to their shape, are not subject to some limitations

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encountered in planar components for incident or outgoing light. High-efficiency solar cells are being developed, as well as blue or UV light-emitting diodes (LEDs). The remarkable properties of carbon nanotubes, made of a rolled graphene sheet, have encouraged scientists to explore the properties of graphene itself. This material can be obtained directly by exfoliation of a graphite sample or by crystal growth on silicon carbide (SiC) as a substrate. Graphene has exceptional properties, such as a very high electronic mobility that can be modulated by a transverse electric field. Very-highfrequency field-effect-transistor (FET) prototypes have already been fabricated, as well as an element of logic circuit. The Nobel Prize in Physics was awarded in 2010 to Andre Geim and Konstantin Novoselov for their works on graphene. This material may be a good candidate for future microelectronics.

1.2.2 Nanoparticles Using recent synthesis techniques, nanoparticles can be obtained from various materials, for example, gold, silver, titanium oxide TiO2, carbon (fullerenes), and many types of semiconductors. Of particular interest are semiconductors, such as cadmium selenide that, in the bulk state, emit infrared light when irradiated by ultraviolet light. As nanoparticles, they turn out to emit visible light, of which color depends on particle size. These new fluorescent markers are efficient and stable. They are frequently used in biology. Nanoparticles can also receive complementary functions by grafting active molecules or specific coatings (drug, contrast agent, radiation absorber, etc.). This point will be addressed in more detail later when dealing with nanobiotechnologies [7].

1.2.3 Nanochemistry The fabrication of the above-described objects requires the contribution of various fields of chemistry that are now regarded as forming a single one, namely, nanochemistry. Chemistry has always been used for synthesizing molecules of nanometric or even subnanometric dimensions, but the term “nanochemistry” designates a branch of chemistry that aims at object synthesis by assembling elementary molecules. The overall size of such objects is in the nanometric range, so that they display specific properties. Among the many goals of nanochemistry, the following may be mentioned in addition to those cited above: ●

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Bulk materials with a nanometric structure obtained by self-organization Self-assembly of several molecules using supramolecular chemistry Nanostructuration by molecular printing that makes possible the synthesis of organic or inorganic materials incorporating cavities designed for specific molecular recognition. This wide range of materials leads to a variety of applications, a few of which are mentioned hereafter: TiO2 nanoparticles for solar protection (although the bulk material is white, nanoparticles become transparent to visible light but still absorb efficiently UV light). TiO2 nanoparticles that catalyze destruction of organic pollutants. For instance, their incorporation in concrete prevents its blackening. Self-cleaning glasses get dirt eliminated by rainwater. They are coated by a superhydrophobic film. Its properties are obtained by surface nanostructuration similar to that observed on lotus leaf.

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Silver nanoparticles have a bactericide power and may be incorporated into textiles. Tires can be reinforced by incorporation of silica particles to improve their mechanical properties, thermal properties, abrasion resistance, durability, etc. Thus, nanochemistry is an essential field of nanotechnologies. It creates the building bricks necessary for assembly of more complex structures. It will also play an essential part in the transfer of the discoveries of nanotechnologies to widely used products or materials where they were not necessarily expected (concrete, tires, and paints).

1.2.4 Nano-biotechnologies At the end of the last century, biology too has been affected by remarkable transformations. The genetic code was identified and deciphered, and it became clear that life itself was also a matter of atoms and molecules. Of course, living matter is mainly made of macromolecules, but its complexity is huge. Many contacts between biology and nanotechnology have progressively been established, and this led to define the field of nanobiotechnology. The main applications that follow on from these contacts will now be described.

1.2.5 Biochips A biochip is, as suggested by the terminology, something like a chip for biological use. In fact, they are not so small, but the word is commonly used. A first class of chips is constituted by DNA chips. The aim of these components is to simultaneously determine a large number of DNA sequences in a genome to be analyzed. Synthetic DNA probes, complementary of the researched sequences, are attached to a glass or a silicon slab. Each kind of sequence is deposited on a specific area of the chip. The slab is then brought into contact with a solution that contains the DNA to be analyzed, which has first been marked by fluorescent molecules. When there is complementarity between a sequence attached to the slab and one in the solution, the latter sticks to the slab and can be detected by fluorescence. In addition, it is identified through its location. Biochips are obtained by miniaturization techniques derived from microelectronics. A chip can contain tens of thousands of probes, so that a whole genome can be investigated with a single experiment. The identification of the whole DNA sequence in a cell is not identical with that of the subset that is active at a particular time in that cell. To obtain the latter information, protein chips have been devised. Their principle is similar to that of DNA chips. A set of receivers are prepared, each one with a strong affinity for a given protein that one wishes to identify and to quantify. The difficult point is to find for each investigated protein a peptide or another molecular component with a strong affinity and specificity. The receivers are attached to the chip that is then brought into contact with the solution to be analyzed, the proteins of which have been provided with a fluorescent or radioactive marker or whatever. Another variation of biochips is the cell chip. In that case, living cells are fixed on the chip. Each cell can be stimulated by an electrode or subjected to a specific treatment, for example, the introduction of a foreign gene (transfection) [7]. It is thus possible to simultaneously analyze a large number of identical cells subjected

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to different parameters. Although their dimensions are not really nanometric, these microsystems are considered to belong to the field of nanobiotechnology in an extended sense.

1.2.6 NEMS (nano electro mechanical systems) The progress of miniaturization now makes it possible to fabricate microsystems of smaller and smaller size that incorporate nanometric elements, hence their name NEMS. This research area is still in the stage of demonstrating the validity of the concept. According to the few teams working on this topic, the interest of such devices would be to allow working at the single-molecule level, for example, to detect or to analyze proteins one by one. Furthermore, one can design sensors working at a scale where quantum phenomena are predominant and thus take benefit of some useful properties (quantization of photons, phonons, or electronic transport). Moreover, the very fact of having tiny components allows manufacturing millions of them simultaneously. This may lead to the realization of systems with a high level of parallelism. A first example of significant achievement is the mass spectrometer dedicated to detection of a single molecule, realized by the team of Michael Roukes at Caltech. The sensitive element is a suspended silicon carbide wire only 100 nm wide for a length of 2 μm. It is incorporated in an electronic circuit controlled by the resonant vibration of the wire at very high frequency (450 MHz). The vibration frequency depends on the mechanical properties of the wire and on its mass. The sensor is associated with the first stage of a conventional mass spectrometer that generates ions in a vapor to be analyzed. When the wire captures a molecule, its mass changes, and the vibration frequency drifts abruptly. The frequency shift is used to identify the mass of incident molecules. The very small sensor size allows parallel connection of many of them, and thus, thousands or even millions of simultaneous measurements can be recorded. In another application, the nanowire can be treated to selectively adsorb a chemical species. The resonance frequency of the circuit associated with this sensor will be modified whenever a molecule of the targeted species will be captured. By selectively combining a number of dedicated sensors to different molecules, it is possible to determine the proportion of these molecules. This device called “electronic nose” is thus able to determine the profile of molecular species in a given sample. One objective of this project is the early detection of certain cancers by analyzing the exhalation of a patient (recall the current attempt to train dogs to perform this detection by their smell). One can also think of multiple applications in the food industry to control taste and fragrance products. Finally, the research on molecular motors can be mentioned. Such tiny motors are present in living systems as proteins responsible for transporting protons across the cell membrane (ATP synthase), for dragging the muscle fibers along one another (myosin), or for intracellular transport (kinesin). Some of these motors can be isolated, fixed, or adsorbed on a support, and their motion can then be detected. They could certainly provide new elements for an exciting “nanomeccano.” But now, we are still at the stage of speculation.

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1.3 New instruments The success of nanoscience and nanotechnology was made possible by the use of new, innovative instruments, which were powerful and relatively inexpensive, thus allowing their wide dissemination in laboratories and industrial plants.

1.3.1 The scanning tunneling microscope This was a real breakthrough in imaging techniques. While all microscopes operated according to the same principle as the eye (radiation illuminates an object, and the eye picks up the radiation and produces an image), the innovation was to make an instrument operating on the principle of the finger (as a blind reads a character in Braille by feeling the protrusions of the paper with his finger). The idea of Gerd Binnig and Heinrich Rohrer was exactly that. Approaching a metal tip close to a conducting surface, one measures the gap by measuring the electric current flowing between the tip and surface when a voltage is applied. This current is nonzero only if the tip is close enough, so that it flows thanks to tunnel effect. This current is about 1 nA for a gap of 0.5 nm. By moving the tip parallel to the surface while monitoring its distance to keep this current constant, it follows the roughness of the surface and allows one to reconstruct its topography. One can therefore reconstruct an “image” without ever having seen it. To obtain a good image resolution, the first condition to fulfill is to make a thin enough tip, so that its extremity consists of a single atom. Although very difficult, this problem is now commonly solved by experimentalists. A second condition is that the tip location must be very precisely tuned and measured in all directions of the space. This problem is solved by piezoelectric actuators. The tip sharpness and position accuracy are sufficient to reconstruct images of surfaces with a resolution of 0.01 nm. For this invention, Binnig and Rohrer received the Nobel Prize in 1986. Other types of microscopes have been invented according to analogous principles. The difference is usually the type of interaction between the tip and surface: Van der Waals force, electrostatic force, and magnetic force. These new microscopes produce multiple and complementary images of surfaces, highlighting various physical properties such as electron density, density of empty states, and localization of magnetic moments and electric charges [8].

1.3.2 Near-field optical microscope The use of light has not been abandoned, however. One tries to overcome the diffraction limit according to which, in conventional microscopy, it is not possible to see details whose size is below the half wavelength of the radiation used (i.e., 250 nm when using green light). This limit may be lowered if ultraviolet radiation is used, but this raises other problems (especially for biological samples that are degraded by UV light). In this way, the possibility of progress was limited. New instruments have now emerged, which overcome the limitation by other means. Instead of detecting light in the far field, subject to the diffraction limit, one considers the evanescent wave observable in the near field. Illumination and/or collection of

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light is made through a thinned and metalized optical fiber, which is located in the immediate vicinity of the diffracting object (a few nanometers). The fiber displacement is obtained with techniques similar to those used in the scanning tunneling microscope. It is possible to reconstruct an image of the surface with a resolution of up to 25 nm, that is, 1/20 of the wavelength of the illuminating light. All these new tools, called local probe microscopes, have greatly contributed to the development and dissemination of nanotechnologies in many laboratories. The above description of nanoscience and nanotechnology may look like a list of little correlated domains and multiple applications. Where is the unity of the field? What is the nature of the evolution (or revolution) proposed in the title? The domain is indeed very composite, it involves many fields of science and technology, and the objects of interest have dimensions that range from nanometer to micrometer. There are however a few unifying concepts: ●

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The genesis of all these innovations shows that one mover was the technology of microelectronics, which for 40 years has continuously provided new techniques to miniaturize circuits and reduce their cost. One result has been the considerable development of information technology and its dissemination in all fields. Another result was the introduction of miniaturization and new ways of computing in other technical areas: biology and biotechnology and mechanics. Nanochemistry and nanomaterials have also progressed but with less obvious connections. In all these areas, a number of innovations have been identified, but future projects are even more numerous. Another remarkable feature of this recent development is the combination of disciplines and different fields of knowledge. At the nanoscale, physics, chemistry, biology, and mechanics are linked. The collaboration of the knowledge and skills of different disciplines is mandatory. Basic science and technology are also inseparably linked. The manufacture of objects and the understanding of their properties have to move simultaneously. The term “nanorevolution” has been used more than once. Clearly, what we are witnessing is not really a revolution but rather a quite continuous progression of knowledge and techniques with sometimes remarkable qualitative leaps. However, the development of information technology, born in the last third of the twentieth century with the undeniable support of micro- and nanotechnology, has been a true revolution in many fields.

1.4 Natural nanosystems Biomineralization in nature produces diverse hierarchical structures of inorganic materials. (A) SEM images of cell walls from diatoms. Diatoms exhibit a wide range of multiscale structures containing biomineralized silica, such as the cell walls from Thalassiosira pseudonana (left) and Stephanopyxis turris (right). Fig. 1.1B SEM images of the microstructure of silica cell walls in Gyrosigma balticum (left) and Ditylum brightwellii (right). (C) SEM image of the nacreous layer in the mollusk shell of Atrina rigida. The nacreous layer forms the inner structure of the mollusk shell and consists of layered tablets of mineralized aragonite. (D) SEM image of the prismatic layer of the mollusk shell from A. rigida after partially dissolving inorganic minerals. The prismatic layer contains a fibrous matrix of proteins that direct mineralization of calcite.

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Fig. 1.1  Bionano systems [9]. (A) SEM images of cell walls from diatoms, (B) SEM images of the microstructure of silica cell walls in Gyrosigma balticum (left) and Ditylum brightwellii (right), (C) SEM image of the nacreous layer in the mollusk shell of Atrina rigida, and (D) SEM image of the prismatic layer of the mollusk shell.

Proteins in nature offer a multitude of compelling examples of their ability to direct the synthesis of complex, hierarchical structures through biomineralization. Moreover, outside of biomineralization, proteins, themselves, are well known for their ability to self-assemble into a variety of organizations not easily achieved by synthetic organic materials. However, with the development and increasing sophistication of recombinant techniques, proteins are no longer limited purely to those found in nature, and a vast design space exists for developing protein technologies with self-assembly capabilities not accessible synthetically or in nature. Consequently, inspiration from natural biomineralization scaffolds combined with opportunities through recombinant protein technology inspires researchers to pursue a variety of novel protein systems as templates in nanoparticle engineering. Some of the earliest evidence of the capacity of proteins to self-assemble nanostructures with properties exceeding those of synthetic systems arose from biomineralization studies on abalone shells. Abalone shells comprise a multilamellar hierarchy of highly oriented, interdigitating calcium carbonate crystals that are separated by thin organic lamellae. Their nano- and microstructure confer a tensile fracture toughness that is 3000 times that of synthetically produced calcium carbonate crystals. The unique properties and multiscale structure are intimately connected to the proteins that constitute the organic phase. Even in solution, polyanionic proteins isolated from either the calcite or aragonite phases of abalone shells were shown to direct mineralization of calcium carbonate crystals with distinct phases (calcite or aragonite) and morphologies, depending on the chemical identity of the protein templates, suggesting that protein expression provides a switchable mechanism for controlling phase and orientation of crystal growth in vivo. At a larger scale, atomic force microscopy (AFM)

Nature, nanoscience, and textile structures11

and scanning electron microscopy (SEM) studies of shell formation in mollusks suggest that the 2-D geometry and pore structure of the protein-based organic lamellae guide the growth of more complex 3-D mineral structures with mineral bridges that interconnect hierarchically stacked pearls. These studies highlight two promising features of protein-based inorganic templating: precise molecular recognition for control of crystal location, phase, and orientation and self-assembling higher-order structures that orchestrate the controlled formation of elaborate 3-D geometries.

1.4.1 Fibrous proteins Due to inspiration from biomineralization in nature, fibrous proteins and biopolymers have attracted interest in the materials science community as scaffolds for templated synthesis of inorganic materials. Collagen and chitin are two prominent examples of fiber-forming biopolymers that serve as organic templates for mineralization in vivo. Their tendency to form fibers at multiple length scales (nanofibers, microfibers, and macrofibers) results in hierarchically organized scaffolds that function both to guide the morphology of deposited minerals and to localize mineral forming precursors within the matrix, leading to unique morphologies and properties. Fibrous proteins have been demonstrated to provide similar functionality in vitro. Collagen scaffolds were shown to direct mineralization of alumina mesoporous materials, as well as titanium dioxide nanofibers with unusual catalytic properties for degradation of organic toxins. Similarly, native fish scales, which comprise a chitin scaffold that recruits a number of small proteins, proved to be a template for producing porous carbon materials with promising performance as electrochemical capacitors. On a smaller length scale, fibers formed from lysozyme were found to electrostatically direct the assembly of gold nanoparticles along the fibers into arrays with tunable particle spacing.

1.4.2 Cage proteins It is often desirable to control size and organization of inorganic nanoparticles on scales much smaller than the size scales of organization in fibrous proteins. Viral capsids are a simple and elegant example of the ability of proteins to self-assemble into precise, responsive nanostructures. In naturally occurring viruses, the viral capsid self-assembles from protein subunits encoded by the viral genome to form a protective coat that serves to package and protect nucleic acids and functions as a responsive delivery system for infecting a target host with the enclosed genetic cargo. The biological challenges encountered by viruses impose stringent demands on the ability of the capsid to robustly and stably self-assemble in its target host and to remodel in response to environmental cues. In addition, viral capsids typically possess a high degree of structural regularity and symmetry and are found to exist in a variety of self-assembled architectures spanning a range of length scales, including helical, icosahedral, and more complex geometries. In vitro studies reveal that even a particular protein coat composition can give way to a rich self-assembly phase diagram with diverse morphologies governed by ionic strength and pH. These appealing properties of naturally occurring capsids and the immense potential for engineering new protein capsids

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through recombinant approaches excite considerable interest in the nanoscience and nanotechnology communities to develop virus-like particles with new functional properties, either as delivery systems or templates for self-assembly. A number of groups have investigated the potential use of viral capsids as templates for engineering inorganic nanoparticle assemblies with constrained sizes and/or spatial organization. For example, multiple variants of cowpea chlorotic mottle virus (CCMV) capsids are capable of directing reduction of gold (III) precursors to form gold nanoparticle-decorated capsids through tyrosine residues, with high selectivity for the reduction of gold (III) over several other metal precursors. Alternatively, the endogenous histidine residues on the CCMV capsid can serve as highly specific nucleation sites for gold in the presence of a nonreducible precursor, yielding gold nanoparticle-patterned capsids with spatial organization dictated by the highly symmetrical, repeating organization of native histidine residues on the icosahedral capsid. Capitalizing on the cylindrical geometry of tobacco mosaic viral capsids, cylindrical nanoarrays of gold, platinum, and palladium nanoparticles were formed through the reduction of cationic precursors by acidic residues. It was further demonstrated that introducing mutations in the viral capsid to alter the density of negative charges produced distinct changes in the spatial organization of nucleated nanoparticles. In addition, the well-defined sizes of viral cages and their porous structures, which allow diffusion of reaction precursors, enable them to behave as size-constrained nanoreactors that direct the synthesis of inorganic nanoparticles with controlled size distributions within the interior of the capsid. These studies demonstrate the potential of viral capsids to produce nanoarrays of metal nanoparticles with geometry and dimensionality dictated by the underlying geometry of the self-assembled capsid and spatial distributions that can be tuned by engineering the presentation of nucleation-directing amino acid residues. Viral capsids thus provide a versatile platform for nanoparticle mineralization and assembly. Alternatively, inspired by the ability of many wild-type viral capsids to self-assemble around their electrostatically charged genetic cargo, other researchers explored inorganic-organic hybrid materials in which the viral protein cage encapsulates functionalized nanoparticle templates. The potential to enclose nanoparticles in protein coats similar to native viruses offers the possibility of producing nanoparticles equipped with proteins that are engineered for specific responsiveness or other functionality. For example, gold nanoparticles functionalized with carboxylate-terminated thiol-alkylated tetraethylene glycol chains can serve as templates for self-assembling protein cages from brome mosaic virus proteins. Under suitable conditions, a protein coat efficiently assembled around the gold nanoparticles to form caged structures with protein stoichiometry, symmetry, and low polydispersity reminiscent of the icosahedral capsids that form around the RNA template in the native brome mosaic virus. In order to develop a deeper understanding of the mechanisms that underlie the protein cage assembly around charged nanoparticles, a combinatorial study was designed to decouple geometric and electrostatic effects. Below a critical charge density, essentially, no protein cages are formed, irrespective of the total nanoparticle charge. This furnishes evidence of the possibility to rationally design protein cage assembly based on physical principles and independently tunable parameters. Selected nanosystems in proteins are shown in Fig. 1.2.

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Symmetry-directed self-assembly

Geometry-directed self-assembly

(A)

(B)

Size-constrained nanoparticle synthesis

(C)

Nanoparticle-templated virus-like-particle self-assembly

(D)

Fig. 1.2  Nanosystems in proteins [9]. (A) Symmetry-directed, (B) Geometry-directed, (C) Size-constrained, and (D) Nanoparticle-templated virus-like-particle self-assemblies.

1.4.3 Nanocrystalline chitin Chitin is the second most abundant naturally occurring polysaccharide, after cellulose, and shares many similarities to cellulose with respect to its physicochemical properties. However, while cellulose is a major structural component in plants, chitin is largely found in the exoskeleton of beetles and crustaceans and in the cell walls of some fungi. Chitin is commercially available and is typically produced by removing proteins and carbonate salts from seafood waste such as shrimp shells. Using acid hydrolysis, the amorphous regions in the chitin microfibrils are removed to give nanocrystalline chitin (NCh), very similar to the process for producing (cellulose nanocrystals) (CNCs). Dimensions and properties of NCh depend on the source of chitin and the chemical treatment with the length ranging from 5 nm to a few micrometers and a diameter from 4 to ca 70 nm. Although colloidal suspensions of chitin and NCh form LC phases, they have received much less attention than CNCs for templating, and only a few reports have made use of the chiral nematic phase. Much of the work on chitin templating has been inspired by the fascinating biomineralization of nacre, a biomaterial with remarkable physical properties. Although not all of the examples to be discussed below are based on true LC templating, these approaches illustrate the challenges that arise when using biomolecules as template. Researchers used chitin carbamate to prepare a nematic gel by soaking the LC solution (15 wt%) in methanol followed by manual stretching to further align the chitin backbone. The gels were dried and used as a template for the growth of CaCO3.

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Fig. 1.3  Nanocrystalline chitin [10]. (A) Gelation, (B) Gel in CaCl2, and (C) Mineralization with CaCO3.

Rod-like CaCO3 crystals aligned along the long axis of the chitin backbone were observed in the stretched gel but were not found when unstretched gels were used, suggesting that the inherent LC order plays a minor role in the growth of the crystals. Recognizing the importance of the ordered structure for directed crystal growth, the group investigated using the LC phase of NCh for templating. Gelation of the chiral nematic phase of chitin was induced by exposing an aqueous chitin suspension to ammonium carbonate vapor. The gel was then placed in CaCl2 and reexposed to the vapor, resulting in the mineralization of the gel with CaCO3 (Fig. 1.3). Images of the product, however, showed the composite material had layers but no chiral nematic structure, and no attempts to remove the template were made [10].

1.4.4 Viruses Viruses may be regarded as organic core/shell nanoparticles made from biopolymers. The core of these particles contains the genetic information (DNA or RNA) to replicate the virus and is surrounded by a protective shell of proteins. This so-called capsid typically self-assembles into either helical or icosahedral geometry and presents a versatile handle for materials scientists and chemists through both genetic engineering and chemical modification. Another distinct advantage over synthetic NPs is their monodispersity. Viruses have presented an ideal test system to study ordered phases of

Nature, nanoscience, and textile structures15

hard rod systems at the fundamental level. However, despite these intriguing properties, templating from LC phases has not yet seen a significant breakthrough.

1.4.5 Collagen Fibrous proteins, such as collagen, keratin, elastin, and silk, can be found throughout the animal kingdom. These proteins are typically composed of large hydrophobic regions and tend to form rod- or wirelike morphologies in in vivo aqueous environments. The well-defined anisotropic ordering of fibrous proteins makes them promising biotemplates for generating nanostructured materials. Collagen and silk are the most researched of the fibrous proteins, but only Type I collagen has been investigated in templating using its liquid crystalline properties. In humans, 28 different types of collagen comprise one-third of the total proteins. Type I collagen is a major structural protein found in modern vertebrates and provides the three-dimensional matrix for connective tissues like bone and cartilage. In vivo, collagen has a hierarchical structure resulting from the self-assembly of individual collagen monomers (~2 nm diameter and 300 nm long) into macromolecular fibers with diameters of N100 nm. Cholesteric twists between these larger macromolecular fibrils have been observed in biological tissues like bone [11].

1.4.6 Biomimetics The reasons for the need of the determination of composition and structure in nature are numerous. One example is a topic of growing importance, biomimetics—also called bionics or biognosis—derived from the Greek word “biomimesis” and with the meaning of mimicking biology or natural concepts. Nature has gone through natural evolution over several billion years and by trial and error evolved into objects with high performance. Hence, the way nature fabricates and uses nanostructures is considered an inspiration for taking ideas from nature and to exploit them in applications. One, often-quoted example of biomimetics consists in surface treatments to induce nonwetting, superhydrophobicity, and self-cleaning. Various natural surfaces including the leaves of several plants—for instance, the lotus plant—are superhydrophobic and do not wet. This is due to the presence of a wax coating on a high-surface-­ roughness surface structure as revealed by scanning electron microscopy. The effect has been called the “lotus effect” and may become of great biological and technological importance. When used in technological applications, this effect has various applications, for example, self-cleaning windows and solar cells, paints, utensils, roof tiles, and textiles. It can also reduce drag in fluid flow, for example, in microscopic nanochannels. “How it comes that geckos can climb walls?” is a question addressed by Andre Geim in his Nobel Lecture in 2010. Careful studies of composition, structure, and morphology have revealed that the answer of the question resides with the animal’s sticky feet due to submicron-size hairy toes. These can now be mimicked with polydimethylsiloxane (PDMS) structures of micron-sized dimensions at the surface.

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1.5 Biomedical and industrial applications of nano science Recently, nanoparticles have offered great possibilities for applications as antimicrobial agents. Metal and metal-oxide-based nanoparticles with antimicrobial action could find many applications in health-related and industrial products, like food preservation, cosmetics, home and personal care, water treatment, and crop protection. ZnONPs and colloidal size ZnO powders have numerous applications in pharmaceutical and cosmetic formulations, textile industry, electronics and electrotechnology industries, and photocatalysis due to their distinct properties such as large binding energy, wide bandgap, and chemical stability. Moreover, ZnONPs are used as antimicrobial agents for surface coatings on walls and wallpapers. Mg(OH)2NPs are approved as additives in a number of foods and drugs. Furthermore, the MgONPs can be utilized in medical treatments and in environmental preservation and food processing. TiO2NPs have already been utilized in cosmetics, wastewater treatment, and foods. AgNPs have also been used in textiles and other consumer goods for surface sterilization. The antifungal and antiviral activity of nanoparticles has not yet been studied extensively, but it is a very promising area with a huge potential. Silver nanoparticles were recently used as antiviral agents against HIV-1 strain at noncytotoxic levels. It showed good efficiency at the early stage of viral replication. Nanotechnology offers unconventional approaches for fighting microbes that do not rely on the existing pathways of antibiotic action [12–16]. This makes possible to address the challenge of antimicrobial resistance by using nanoparticles with engineered antimicrobial action designed to target specific pathogens. There is a lot of ongoing work on several classes of inorganic and organic colloid particles of added functionality that exhibit strong and universal antibacterial, antifungal, and antiviral action toward which microbes have not been able to develop resistance. We have discussed the mechanisms by which such nanoparticles attack microbial cells or inhibit their growth, which involve generation of reactive oxygen species (ROS) upon irradiation with UV light, cell membrane disruption due to the NP cationic surface, ROS scavenging, emission of heavy ions, as Ag+ and Cu2+ on the cell surface, etc. Various strategies have recently been pursued in search of antimicrobial agents based on natural and synthetic nanoparticles. The latter include nanoparticles synthesized from various metals, as copper, gold and silver, and metal oxides, for example, copper, zinc, titanium, aluminum, and magnesium, as well as low soluble metal hydroxides, as Mg(OH)2. These inorganic nanoparticles have very different mechanisms of antimicrobial activity and can retain their antimicrobial action in a range of adverse conditions. Smaller nanoparticles usually show greater antimicrobial activity due to larger surface-to-volume ratio in suspension and greater area of contact with targeted microbial cells. However, significant research effort is needed to carefully test their side effects, environmental impact, and potential nanotoxicity before nanoparticles can be safely and broadly used as efficient substitutes of conventional antimicrobials.

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1.5.1 Biomimetic multifunctional surfaces Over millions of years, animals have evolved to a higher intelligent level for their environment. A large number of diverse surface structures on their bodies have been formed to adapt to the extremely harsh environment. Just like the structural diversity existed in plants, the same also applies true in animals. Firstly, this article provides an overview and discussion of the most common functional surface structures inspired from animals, such as drag reduction, noise reduction, antiadhesion, anti-wear, antierosion, antifog, water capture, and optical surfaces. Then, some typical characteristics of morphologies, structures, and materials of the animal multifunctional surfaces were discussed. The adaptation of these surfaces to environmental conditions was also analyzed. It mainly focuses on the relationship between their surface functions and their surface structural characteristics. Afterward, the multifunctional mechanisms or principles of these surfaces were discussed. Models of these structures were provided for the development of structure materials and machinery surfaces. At last, fabrication techniques and existing or potential technical applications inspired from biomimetic multifunctional surfaces in animals were also discussed. The application prospects of the biomimetic functional surfaces are very broad, such as civil field of self-cleaning textile fabrics and nonstick pots, ocean field of oil-water separation, sports field of swimming suits, and space development field of lens arrays [17]. For the biology survived in desert, wear and tear of the sand wind on their body surface is the first big challenge. On the other hand, wear is also undesirable and can lead to catastrophic failure in most industrial applications. It limits the lifetime of components, and therefore, when parts fail, the problem of their recycling also arises. Fortunately, there exist numerous cases of soil-burrowing animals having peculiar surface geometries specifically evolved to resist against soil wear and prevent soil to adhere to the animals’ bodies. Animals that have recently been investigated include the dung beetle, the ground beetle, the earthworm, the Oniscidae, the Diplopoda, the centipede, the ant, and the mole cricket (Fig. 1.4).

Fig. 1.4  Natural nanosystems [17]. (A) Scale of insect, (B) Face, and (C) Scale of lizard.

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Fig. 1.5  Nonwetting nanostructures in nature [17]. (A) Pond scatter, (B) Barbs on foot, and (C) Single barb.

Fig. 1.6  Nanostructures on butterfly. (A) Butterfly, (B) Fins, and (C) Details of fins.

In nature, some animal surfaces with hierarchical structure and roughness can produce significant superhydrophobicity. Strider have the ability to stand and walk upon a water surface without getting wet (Fig. 1.5). On the other hand, the scales on the wing surfaces of some butterflies have regularly arranged edges that are overlapping like roof tiles, as shown in Fig. 1.6A–C. The lengths and widths of each individual scales are roughly ranged 50–150 and 35–70 μm, respectively, while the primary distance between the middle points on the long axis of two adjacent scales is within 100 μm. According to their analysis, the lower capacity of Parnassius butterfly wing surfaces to resist methanol wetting is due to the special microstructure of the scales (spindle-like shape) and ultrastructure (pinnule-like shape). The hierarchical architectures that included micro- and nanoscale layers on the upper surfaces of insect wings (Fig. 1.7) promote hydrophobicity, thereby enabling water droplets to roll off the wings and remove the dirt particles. A gecko is the largest animal that can produce high (dry) adhesion to support its weight with a high factor of safety. The ability of gecko (Fig. 1.8A) running up and down a vertical surface was observed in ancient time; however, only with the advent of the electron microscopy in the 1950s, it became possible to view a complex hierarchical morphology that covers the skin on the gecko’s foot (Fig.  1.8B) and toes (Fig. 1.8C). The skin comprises a complex fibrillar structure of lamellae, setae, branches, and spatula (Fig. 1.8D). This hierarchical structure allows a gecko to attach to and detach from the surfaces at will [18–22]. An explanation of the gecko’s ability to control adhesion is in its ability to adapt to the surface roughness and achieve very large real areas of contact between the gecko’s foot and the surface. The compliance and adaptability of setae are the primary sources of high adhesion.

Nature, nanoscience, and textile structures19

Fig. 1.7  Nanostructures on insect wings [17]. (A) Fly wings, (B) Scales of grasshopper, (C) Ordinary bees wings, and (D) Wings of honey bee.

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Nanotechnology in Textiles

Fig. 1.8  Nano enabled gecko effect [17]. (A) Gecko, (B) Gecko’s foot, (C) Toes, (D) Spatula, (E) Sensors on foot, and (F) Sensor functioning.

The UV light and most visible light energy (other than the blue light) are trapped into the nanostructures on butterfly wing scales through the multilayer interference of the shelf structure and the diffraction gratings of the quasi-honeycomb-like structure. In addition, in order to avoid predators, the color of butterfly wings would be changed in the wake of the variation of the incident light angle, resulting in the color and brightness of the butterfly wings being consistent with background. Researchers found that a certain frequency of light would be absorbed or diffracted by the nanostructures on butterfly wings, achieving the stealth effect (Fig. 1.9). Water sprayed on superhydrophobic patterns only forms small spherical droplets and is mainly collected on the hydrophilic patterns. Researchers demonstrated the fabrication of superhydrophobic/hydrophilic patterned surfaces to collect water. The water-collecting capabilities of different superhydrophobic/hydrophilic ratios on the surface are investigated in detail in their work. According to the above examples, one can anticipate the future application of superhydrophobic/hydrophilic patterned coatings in practical water collection apparatus [23,24]. Sonochemical methods utilize the energy of sonication to promote chemical reactions for material production. For the first time, the fine hierarchical structures of butterfly wing were successfully duplicated in manganese oxide using sonochemical reduction followed by calcination. The morphologies and surface structural details of the original butterfly wing and the calcined Mn2O3 butterfly wing are compared in Fig. 1.10. New functional materials with chosen hierarchical structures of biotemplates combined with the functionality of metal oxides could be synthesized in the future by the sonochemical method.

(E)

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Fig. 1.9  Optical effects of nanostructures in nature [17]. (A) Front of butterfly, (B) Back of butterfly, (C) Scales on wings, (D) Comparison of scales on front and back surface, and (E) Color change with time.

Nature, nanoscience, and textile structures21

(A)

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Fig. 1.10  Replication of butterfly wings by sonochemical technique [17]. (A) Butterfly wing, (B) In manganese oxide, (C) 1 μm resolution, (D) 2 μm resolution, (E) 500 nm resolution, (F) Detailed structures, (G) Barb an wing, and (H) Fixation of barb.

Till now, many new commercial products consisting of durable self-cleaning textile fabrics have been designed. Researchers developed a two-step procedure to obtain self-­ cleaning superamphiphobic (superhydrophobic and superoleophobic) cashmere textiles by low-temperature plasma treatment. Nanoscale roughness can be introduced on the texture surface after the plasma treatment. The further modification of fluorocarbons will largely decrease the surface energy of the textiles and thus makes the textiles water-/oil-repellent.

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Moreover, the treatments will not change the original properties of the textiles, such as color, permeability, soft, and flexibility. The oxidation and corrosion of metals in the humid atmosphere limit their application and bring huge waste and environmental contamination. Casting hydrophobic or superhydrophobic coatings on metal surfaces is a probable solution to these problems as the result of the intrinsic waterproof property of the coatings. Superhydrophobic coatings also have applications in eyeglasses, architectures, optical windows for electronic devices, and windows in automobiles. Superhydrophobic coatings have also shown the ability to minimize fluid drag for objects in water. Superhydrophobic surfaces are usually superoleophilic because the low-surface-energy chemicals on superhydrophobic surfaces usually have similar surface energies with the oil drops (hydrocarbon materials). The surface roughness will enhance the oleophilicity, leading to superoleophilicity. One intrinsic application for surfaces integrated with both superhydrophobicity and superoleophilicity is to be used in oil-water separation. Inspired by natural design, scientists are adding technological improvements to swimsuits by designing antimicrobial fabrics without the chemical treatments, especially in Olympic swimming competitions. Now, such important sport events are heading toward technological support since swimmers are using swimsuit designed on the hydrodynamics principles of a shark’s skin. These tightly fitting suits, covering rather a large area of the body, are made up of fabrics that are designed to mimic the properties of a shark’s skin by superimposing vertical resin stripes. This phenomenon is known as the riblet effect (Fig. 1.11). Swimsuits made with the new fibers

Patterns in shark skin help in reducing longitudinal and transverse vortices of water, limiting degree of moment transfer

Drag-reducing and nonadhesive surface to microbial cells can be designed in modern swim-suits

Fig. 1.11  Swimsuits inspired by shark skin [17].

Microanatomy of sharks skin exhibits unique pattern and arrangement of V-scales which provide special functional features in shark to compensate dense sea-water drag

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and weaving techniques mimicking shark scale microfeatures are produced to cling tightly to the swimmer’s body. It may give the wearer a 6 m equivalent head start in swimming competition by dampening turbulence in the immediate layer of water, next to the skin. Learning from natural creatures could simplify the development of advanced artificial functional surfaces. Over the past several decades, considerable efforts have been devoted to biomimetic fabrication of multiscale structures for multifunction integration. Mimicking natural masterpieces, the obtained artificial materials would exhibit both multiscale structures and superior properties similar to or even better than that of biomaterials. This makes the biomimetic fabrication a promising approach for the rational design and manufacture of various high-performance manmade devices [25–28]. Properties of animals’ functional surfaces result from a complex interplay between surface morphology and physical and chemical properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature to provide properties of interest. Animals have learned how to achieve most efficient multifunctional surfaces. The optimized biological solution should give us inspiration and design principles for the construction of multifunctional artificial surfaces. In the last few decades, inspired by the typical animals’ functional surfaces, a great number of multifunctional surfaces have been fabricated. Research is focused on the typical animals’ surfaces with drag reduction, noise reduction, antiadhesion, anti-wear, antierosion, antistealthy, antifog, low reflection of light, and others. The development of animal functional surfaces is important for basic research and various applications, including superamphiphobic textiles, self-cleaning nanotie, sticky bot, and applications requiring antifouling and a reduction in fluid flow. These surfaces can also be used in energy conversion and conservation [29–31]. In the future, the following research directions should be a growing and vigorous field: (i) to extend the new function of animals’ functional surface, (ii) clarification of structure-multifunction relationship, and (iii) the construction of animals’ functional surfaces. Although the biomimetic and bio-inspired research is in its infancy, it is a rapidly growing and enormously promising field, which will become the focus of international competition in the near future. Now, interdisciplinary cooperation is necessary for researchers in the area of science and engineering to further investigate the animals’ functional surfaces and discover the new function. The increasing collaboration work would also be useful for the improved understanding of structure-function relationship, extraction of useful engineering principles, and adaptation of models for practical applications [32,33].

1.5.2 Specific role of nanotechnology in textile structures The typical applications of nanoparticles, nanocoating, and nanofibers in textile structures are shown in Fig. 1.12 [34]. It is well known that many properties of matters depend on the size range. In nanoscale, in some cases, there are extra effects not following the bulk materials because

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Fig. 1.12  Some applications of nanostructures in textile branch.

the particle/wave nature of matter appears (quantum effects, tunneling, and self-­ assembling). Nanoobjects are generally divided into three categories (ISO/TS 27687, 2008(E), see Fig. 1.13): 1. Nanoparticles (three ext. dimensions in the nanoscale) 2. Nanofibers (two ext. dimensions in the nanoscale) 3. Nanoplates (one ext. dimension in the nanoscale)

Many characteristics are similar for all kinds of nanoobjects, but simplest are derivations of relations for the case of nanoparticles. Their benefit is not limitations according to the magnitude of dimensions, and therefore, the properties or characteristics can be expressed in relative units (usually

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V

l

SF

S 3

Nanoparticle No limitations of dimensions

r = m/(SF h)

r = m/(S l)

r = m/V

l/d ∼ 10 Nanofiber

d

h

Limitations due to restricted cross section area S

Nanoplate Limitations due to restricted thickness h

Fig. 1.13  Nanoobject geometry [34].

per unit of mass). Exclusive is relative mass expressed as density ρ where ρ = m/V. Density is the ratio between particle mass m and particle volume V. In the case of nanofibers, there are real limitations due to restricted cross section area, and relative mass is better expressed as fineness TT usually in [tex] units, that is, in grams per kilometer. It can be simply shown that ρ = TT/S. Nanofibers are usually in the form of assemblies, that is, nanoplates. In the case of nanoplates, there are real limitations due to restricted thickness h only, and it is better is to express relative mass as so-called planar mass W [kg m−2] usually called as gsm (gram per square meter). For this case, it is ρ = W/h. Textile structures are very unique materials with complicated hierarchical structure starting from nanodimensions. During construction, maintenance, and use is their structure dynamically changed starting from nanolevel. Second unique feature of textile materials and structure is the limited dimensions that are starting from narrow range of fiber diameter to cross section area, through limited thickness of yarns till limited thickness of fabrics. These geometric constraints are playing an important role in characterization of relative properties as relative surface area, mechanical properties, and sorption properties. Standard definition of these properties as properties per unit of mass, volume, or cross section area are due to geometric constrains not suitable. From definitions of density, it is visible that special textile characteristics, that is fineness TT and planar mass W, are more suitable for characterization of nanofibers and nanoplates instead of volume or mass. Basic elements of fibrous (textile) structures are fibers (see Fig. 1.14). Most of fibers are from the group of so-called semicrystalline polymers characterized by the presence of at least two phases, that is, amorphous and crystalline regions. The volume of crystallite unit for typical polymers is in the range from 0.16 to 1.2 nm3. The structural element of fiber is thin (diameter d ≈ 10 nm) long (l ≈ 1 μm) microfibril having regular arrangement of crystalline and amorphous parts. The length of amorphous part is about 1–6 nm, and the distance between two crystalline parts (long period) is about LA = 15 nm. The structure in microfibril is well arranged; the difference between densities of crystalline and amorphous parts is about 10% only. The microfibril thickness, for example, in polyethylene terephthalate fibers, is around 10 nm, and the length is comparable with that of macromolecular chains, that is, around 1 μm. Microfibrils are thin, long formations of nearly elliptical cross section. By nanotechnology, there are added some effects to the fibers (see Fig. 1.15).

Nature, nanoscience, and textile structures27

Fig. 1.14  Structural hierarchy in polymeric fibers [34].

Fig. 1.15  Nanoeffects added to fibers [34].

Due to limited range of fibers, surface area, and cross dimensions, there are natural limits for the size of particulate fillers, thickness of coatings, surface modifications, etc. Typical diameter of fibers around 10–20 μm limits the diameter of staple yarns (in the range 0.1–0.5 mm dependent on yarn fineness (see Fig.  1.16)) having hundred fibers in cross sections. Thickness of woven fabrics composed from two crossed yarns (dependent on the weave (see Fig. 1.17)) is typically in the range 0.3–0.9 mm. Nonwoven structures can be prepared from fibrous multilayers, but their standard thickness is usually around 1 cm as well.

28

Nanotechnology in Textiles 0.26 Combed ring

Yarn diameter [mm]

0.24

Carded ring

0.22 0.2

Rotor

0.18

Combed Novaspin

0.16 0.14

Carded Novaspin

0.12

Combed compact

0.1 0.08

Carded compact 3 23 Yarn fineness [tex]

Fig. 1.16  Influence of cotton yarn fineness and production technology on the yarn diameter [34].

Thickness [mm]

0.9

Plain weave

0.8

Twill

Satin

0.7 0.6 0.5 0.4 0.3 0.2 10

15

20

25 30 Yarn fineness [tex]

35

40

45

Fig. 1.17  Dependence between cotton yarn fineness and fabric thickness for various weaves [34].

Especially for nanofibrous assemblies prepared by electrospinning, the mean fibrous elements thickness (diameter) is some hundreds of nanometers. But in general, all these fibers with a diameter below 1 μm (1000 nm) are often accepted as nanofibers. Thickness h of nanofibrous assemblies depends critically on the production technology. In discontinuous (syringe-based) production of nanofibrous assemblies is thickness, usually around 20–60 μm, dependent on time of electrospinning. In case of continuous (needleless) electrospinning, it depends on thickness of the machine takeoff rate. Typical range of thickness is here 0.2–15 μm (planar mass 0.2–10 g/m2). The geometry of nano, micro, and macro textile layer is shown in Fig. 1.18. Precise thickness evaluation of textile layers is complicated because it depends on the selection of compression load used by thickness meters. Much simpler is to measure planar mass W.

Nature, nanoscience, and textile structures29

Fig. 1.18  Geometric characteristics of different typical textile structures [34].

Textile fabric (layer) thickness h [m] is not optional but functionally dependent on planar mass W, fibers density ρF and total porosity Po [–], or volume portion of fibers (packing density) v = 1 − Po: h=

W ρ F (1 − P )

The dependence of v on fabric planar mass for different fabric thickness is shown in Fig. 1.19. 1 0.9

Fabric thickness [cm]

Volume portion of fibers [–]

0.8

0.03 0.06 0.09

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 50

100

150

200 250 300 Fabric planar mass [g/m3]

350

400

Fig. 1.19  The dependence of packing density on fabric planar mass for different fabric thickness [34].

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It is visible that for planar mass 100 g/m2 is packing density for thick fabric about 0.2, that is, porosity is about 80%. For lower thickness is (at the same planar mass) the packing density decreasing, that is, the porosity is growing. In nanofibrous layers is porosity usually smaller because the very low thickness is not fully compensated by small planar mass. One of the main advantages of nanomaterials is very huge relative surface area (surface-area-to-volume ratio) [32]. This is in fact true for nanoparticles where there are no limitations according to their geometry (unit volume). It is widely published that by reducing fiber diameters down to the nanoscale, an enormous increase in relative surface area to the level of 1000 m2/g or much more is possible. Recognizing the potential nanoeffect that will be created when fibers are reduced to the nanoscale, there has been an explosive growth in research efforts around the world. Specifically, the role of fiber size has been recognized in significant increase in relative surface area, bioreactivity, electronic properties, and mechanical properties. The enhanced reactivity and efficiency of nanofibers are based on the claim that nanofibrous layers provide enormous availability of relative surface area (i.e., per unit mass) [33]. Especially for nanofibrous assemblies prepared typically by electrospinning, it is practically impossible to vary thickness in arbitrary range. Usually, the thickness of these nanolayers is up to few microns only. This is a serious limitation for volume of these objects because the unit of their macrosurface should be multiplied by real thickness and final total volume is then very low. It leads to low amount of nanofibers and their low total surface area per unit of macrosurface. Avoiding this limitation leads to the unbelievable huge relative surface area values that cannot be achieved in real products. The same situation appears when the properties as sorption capacity are calculated relative to mass. Relative surface area Sr [m2/g] is defined as surface area of fibrous phase divided by its mass. For the case of fibrous layer composed from cylindrical fibers having radius r, porosity Po, and geometry shown in Fig. 1.18 is macroscopic surface area SF = LF CF and relative surface area: Sr =

2 r ρF

This relation is not dependent on the total fibrous assembly porosity P (Fig. 1.20). Due to restricted thickness of nanolayers is their mass per unit macroscopic surface area SF (in fact, it is planar mass W) very low, and therefore, Sr is very huge. A much better characteristic of relative surface area is planar relative surface area (surface-­ area-to-macrosurface SF ratio) SSR [–]. Quantity of SSR is the ratio of total surface area

Fig. 1.20  Dimensions of fibrous layer [34].

Nature, nanoscience, and textile structures31

of fibrous phase in layer SFT and layer macroscopic surface area SF. The total surface area of fibrous phase is equal to SFT =

2 (1 − Po ) h SF r

and then, the planar relative surface area is equal to SSR =

SFT 2 (1 − Po ) h . = SF r

The SSR is connected with SR by relation: SSR = SR (1 − Po ) h ρ F = SR W The dimensionless quantity planar relative surface area SSR is taking into account the porosity and thickness influence on the relative surface area of fibers in nanolayers. It is simple to derive that SSR = W SR, and it is then simple to calculate SSR from known SR by multiplying planar mass W. For comparison of both relative planar mass SR and SSR, there were compared PA 6 nanofibrous membrane (MN) with areal density of 1.3 g/m2 purchased from ELMARCO s. r. o Liberec and Spunbond PA 6 nonwoven fabrics (MM) with areal density of 100 g/m2 provided by Asahi KASEI Fibers Corporation. There were measured fiber diameters from scanning electron microscopy (SEM) images. Thickness was evaluated from the cross section images taken by SEM. Results are given in Table 1.1. The calculated relative surface areas are given in Table 1.1. The ratio SSR micro/ nano = 7.0850 indicates that real relative area of micromembrane is much higher in comparison with nanomembrane. On the other side, the ratio SR micro/nano = 0.0921, that is, relative area per mass is for micromembrane that is very low in comparison with nanomembrane due to differences in thickness mainly. It is interesting that for nanomembrane, the mass 100 g corresponds to huge real surface area 77 m2 but for micromembrane the mass 100 g corresponds to much smaller real surface area. Very interesting is comparison of sorption properties [5]. Under the same conditions, sorption of Acid Blue 41 by nanomembrane was expressed as concentration 138 mg/g and by micromembrane 15 mg/g only. This leads to (wrong) decision about better sorption properties of nanomembranes. By using real sample sizes Table 1.1  Characteristics of membranes Characteristic

Micromembrane

Nanomembrane

r (nm) h (mm) gsm (g/m2) PO (–) SR (m2/kg) SSR (–)

1520 0.53 100 0.83 1196.2 119.62

140 0.00185 1.3 0.36 12,987.1 16.88

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(membrane thickness), the concentration for nanomembrane was calculated 0.017 mg/ m2 but for micromembrane much higher value 0.15 mg/m2. From these differences, better sorption properties of micromembranes for real membrane thicknesses are visible. Similarly as for relative surface area, it is simple to obtain concentration in mg/ m2 by multiplying commonly used concentration in mg/g and multiplying it by planar mass W. These calculations clearly demonstrate that the real surface area of nanolayers prepared by standard techniques is very low due to very low thickness (planar mass).

References [1] A. Rios, M. Zougagh, M. Bouri, Magnetic (nano)materials as an useful tool for sample preparation in analytical methods: a review, Anal. Methods 5 (2013) 4558–4573. [2] X.L.O.  Barajas, T.  Hüffer, P.  Mettig, B.  Schilling, M.A.  Jochmann, T.C.  Schmidt, Investigation of carbon-based nanomaterials as sorbents for headspace in-tube extraction of polycyclic aromatic hydrocarbons, Anal. Bioanal. Chem. 409 (2017) 3861–3870. [3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature 359 (1992) 710–712. [4] E. Borsella, S. Botti, R. Giorgi, Laser-driven synthesis of nanocrystalline alumina powders from gas-phase precursors, Appl. Phys. Lett. 63 (1993) 1345–1347. [5] V. Moreno, M. Zougagh, A. Ríos, Hybrid nanoparticles based on magnetic multiwalled carbon nanotube nanoC18SiO2 composites for solid phase extraction of mycotoxins prior to their determination by LC-MS, Microchim. Acta 183 (2016) 871–880. [6] O.M.  Yaghi, H.  Li, Hydrothermal synthesis of a metal-organic framework containing large rectangular channels, J. Am. Chem. Soc. 117 (41) (1995) 10401–10402. [7] S.T.  Goodman, D.  Fanelli, J.P.A.  Ioannidis, What does research reproducibility mean? Science 8 (2016). 341ps12. [8] G.M. Whitesides, Reinventing chemistry, Angew. Chem. Int. 54 (2015) 3196–3209. [9] B.A. Krajina, A.C. Proctorc, A.P. Schoen, A.J. Spakowitz, S.C. Heilshorn, Biotemplated synthesis of inorganic materials: an emerging paradigm for nanomaterial synthesis inspired by nature, Prog. Mater. Sci. 91 (2018) 1–23. [10] M.  Wysokowski, M.  Motylenko, et  al., Extreme biomimetic approach for synthesis of nanocrystalline chitin-(Ti,Zr)O2 multiphase composites, Mater. Chem. Phys. 188 (15) (2017) 115–124. [11] M.  Valcárcel, R.  Lucena, Social responsibility of analytical chemistry, Trends Anal. Chem. 31 (2012) 1–7. [12] J.P.A. Ioannidis, Why most clinic research is not useful? PLoS Med. 13 (6) (2016). [13] G. Oberdórster, E. Oberdórster, Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles, Environ. Health Perspect. 113 (7) (2005) 823–839. [14] E. Caballero-Díaz, M. Valcárcel, Analytical methodologies for nanotoxicity assessment, Trends Anal. Chem. 84 (A) (2016) 160–171. [15] S.  Lanone, F.  Rogerieux, J.  Geys, A.  Dupont, E.  Maillot-Marechal, J.  Boczkowski, G.  Lacroix, P.  Hoet, Comparative toxicity of 24 manufactured nanoparticles in human alveolar epithelial and macrophage cell lines, Part. Fibre Toxicol. 6 (14) (2009) 1–12. [16] E. Caballero-Díaz, C. Pfeiffer, L. Kastl, P. Rivera-Gil, B. Simonet, J. Jiménez-Lamana, F. Laborda, W.J. Parak, The toxicity of silver nanoparticles depends on their uptake by cells and thus on their surface chemistry, Part. Part. Syst. Charact. 30 (2013) 1079–1085.

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[17] Z. Han, M. Zhengzhi, W. Yin, et al., Biomimetic multifunctional surfaces inspired from animals, Adv. Colloid Interf. Sci. 234 (2016) 27–50. [18] C.J. Murphy, A.M. Vartanian, Biological responses to engineered nanomaterials: needs for the next decade, ACS Cent. Sci. 1 (2015) 117–123. [19] E. Caballero-Díaz, R. Guzmán-Ruiz, M.M. Malagón, B. Simonet, M. Valcárcel, Effects of the interaction of single-walled carbon nanotubes with 4-nonylphenol on their in vitro toxicity, J. Hazard. Mater. 275 (2014) 107–115. [20] I.L. Bergin, L.A. Wilding, M. Morishita, K. Walacavage, A.P. Ault, J.L. Axson, D.I. Stark, S.A. Hashway, S.S. Capracotta, P.R. Leroueil, A.D. Maynard, M.A. Philbert, Effects of particle size and coating on toxicologic parameters, fecal elimination kinetics and tissue distribution of acutely ingested silver nanoparticles in a mouse model, Nanotoxicology 10 (3) (2016) 352–360. [21] J.L.  Axson, D.I.  Stark, A.L.  Bondy, S.S.  Capracotta, A.D.  Maynard, C.J.  Murphy, A.M. Gole, J.W. Stone, P.N. Sisco, A.M. Alkilany, E.C. Goldsmith, et al., Gold nanoparticles in biology: beyond toxicity to cellular imaging, Acc. Chem. Res. (2008) 41. [22] B. Fadeel, A.E. Garcia-Bennett, Better safe than sorry: understanding the toxicological properties of inorganic nanoparticles manufactured for biomedical applications, Adv. Drug Deliv. Rev. 62 (2010) 362–374. [23] Z. Nie, A. Petukhova, E. Kumacheva, Properties and emerging applications of self-assembled structures made from inorganic nanoparticles, Nat. Nanotechnol. 5 (2010) 15–25. [24] X. Fang, Y. Bando, U.K. Gautam, C. Ye, D. Golberg, Inorganic semiconductor nanostructures and their field-emission applications, J. Mater. Chem. 18 (2008) 509–522. [25] T.  Zhai, L.  Li, Y.  Ma, M.  Liao, X.  Wang, X.  Fang, et  al., One-dimensional inorganic nanostructures: synthesis, field-emission and photodetection, Chem. Soc. Rev. 40 (2011) 2986–3004. [26] S.R. Whaley, D.S. English, E.L. Hu, P.F. Barbara, A.M. Belcher, Selection of peptides with semiconductor binding specificity for directed nanocrystal assembly, Nature 405 (2000) 665–668. [27] S.  Hrapovic, Y.  Liu, K.B.  Male, J.H.T.  Luong, Electrochemical biosensing platforms using platinum nanoparticles and carbon nanotubes, Anal. Chem. 76 (2004) 1083–1088. [28] X.  Hu, S.  Dong, Metal nanomaterials and carbon nanotubes—synthesis, functionalization and potential applications towards electrochemistry, J. Mater. Chem. 18 (2008) 1279–1295. [29] M.  Mauter, M.  Elimelech, Environmental applications of carbon-based nanomaterials, Environ. Sci. Technol. 42 (2008) 5843–5859. [30] K.R.  Gopidas, J.K.  Whitesell, M.A.  Fox, Synthesis, characterization, and catalytic applications of a palladium-nanoparticle-cored dendrimer, Nano Lett. 3 (2003) 1757–1760. [31] H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda, Colloidal gold nanoparticles as catalyst for carbon-carbon bond formation: application to aerobic homocoupling of phenylboronic acid in water, Langmuir 20 (2004) 11293–11296. [32] F.K. Ko, Y. Wan, Introduction to Nanofiber Materials, Cambridge University Press, New York, 2014. [33] F.K.  Ko, Nanofiber technology: bridging the gap between nano and macro world, in: S.  Guceri, et  al. (Eds.), Nanoengineered Nanofibrous Materials, Kluwer Academic Publishers, Dordrecht, 2004. [34] Militký J., Wang Y. Mishra R.: Comparison of membranes functionality, n.d. Unpublished report.

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Further reading [35] Y. Benmassaoud, M.J. Villaseñor, R. Salghi, S. Jodeh, M. Algarra, M. Zougagh, Á. Ríos, Magnetic/non-magnetic argan press cake nanocellulose for the selective extraction of Sudan dyes in food samples prior to the determination by capillary liquid chromatograpy, Talanta 166 (2017) 63–69.

Electrospun nanofibers Rajesh Mishra, Jiri Militky, Mohanapriya Venkataraman Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

2

2.1 Introduction Fibrous structures with nanoscale diameters offer a multitude of fascinating features, such as excellent mechanical behavior and large surface-area-to-volume ratio, making them attractive for many applications. Their large surface area also gives them high functionalization ability. Among the many techniques available for generating nanofibers, electrospinning is rapidly emerging as a simple process in which careful control of operating conditions and polymer solution properties enables the production of highly porous structures of smooth nonwoven nanofibers. Compared with traditional phase inversion techniques for membrane fabrication, electrospinning allows the formation of interconnected pores with uniform pore size and porosities exceeding 90%. As a result, electrospun membranes are increasingly being applied to many water purification applications such as membrane distillation and pretreatment of feed prior to reverse osmosis or nanofiltration processes by the removal of divalent metal ions, grease, and other contaminants [1, 2]. Although the use of electrospinning for membrane fabrication has previously been reviewed, the rapid increase in developments over recent years has necessitated detailed study on the preparation and application of electrospun nanofiber membranes as the barrier layer for water treatment, with emphasis on the reinforcement and posttreatment of electrospun polymer membranes. A schematic of needle electrospinning setup is given in Fig. 2.1. Nanofibers are a unique class of nanomaterials with many interesting properties owing to their nanoscale diameters and large aspect ratio. They possess excellent mechanical properties, and their surface can be readily modified due to their high ­surface-area-to-volume ratio. Nanofibers can be produced with different techniques such as drawing, template synthesis, phase separation, self-assembly, and electrospinning. Among these, electrospinning is rapidly emerging as a simple and reliable technique for the preparation of smooth nanofibers with controllable morphology from a variety of polymers. Electrospinning involves the application of a high electric field to generate nanofibers from a charged polymer solution or melt. By varying electrospinning parameters and polymer solution properties, electrospinning can be used to produce different morphologies. Researchers controlled the branching and bending of the charged polymer jet by varying the electrospinning voltage to form “garlands” or columnar networks of polymer nanofibers forming closed loops over each other [3–5]. Other researchers have also demonstrated the use of electrospinning to form flat ribbons instead of round fibers. Careful control of operating conditions and solution parameters can lead to the production of highly porous structures of smooth, defect-free Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00002-X Copyright © 2019 Elsevier Ltd. All rights reserved.

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Nanotechnology in Textiles

Polymeric solution Metallic needle Fiber formation

High voltage supply

Fiber collection drum

Fig. 2.1  Schematic of needle electrospinning [2].

nonwoven nanofibrous membranes. Fiber formation in the electrospinning process is driven by repulsive electrostatic forces. Coulomb interactions in the charged fluid jet result in jet instabilities that dictate the end architecture of the fibers. The microscopic image of a nanofibrous membrane is shown in Fig. 2.2. The antibacterial property of polymer nanofibers can be improved through the use of biocides. Researchers studied the effect of incorporating different biocides on the pathogen removal efficiency of polyamide (PA) nanofibers. Due to its positive surface charge, WSCP (wet saturated chemical polyquat) was found to be the most effective functionalizing agent for killing bacteria and microorganisms. PA nanofibers with 5 wt% WSCP showed higher removal of bacteria compared with pure PA nanofibers. Researchers prepared electrospun poly(lactic acid) (PLA) membranes loaded with sepiolite fibrillar particles and studied the effect of functionalizing the sepiolite with silver or copper nanoparticles on the biofouling tendency of the membranes. The group tested the membranes for biofouling using suspensions of the yeast Saccharomyces cerevisiae and the bacterium Pseudomonas putida. They found that Ag-functionalized PLA/sepiolite nanofiber membranes had the least accumulation of active biomass on the surface. In addition, the water permeability through used membranes was greater for PLA/sepiolite nanofibers than for neat PLA. This is due to the repulsive electrostatic forces between negatively charged sepiolite and negatively charged regions on cell walls of microorganisms. This allows PLA/sepiolite nanofibers to be less prone to biofouling than PLA nanofibers alone [6]. The distribution of fibrillar sepiolite particles in electrospun PLA can be seen in the transmission electron microscope (TEM) image. In another study, researchers infused very fine cellulose nanofibers into electrospun polyacrylonitrile (PAN) on a microfibrous polyethylene

Electrospun nanofibers37

Fig. 2.2  Microscopic image of nanofibrous membrane [2]. (A) Nanofibers with respect to microfibers, (B) Nanoparticles on microfibers, (C) Interfiber friction, and (D) Nanoparticle aggregation on microfiber.

terephthalate (PET) support. Cellulose nanofibers with an average diameter of about 5 nm were prepared through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical oxidation in water. TEMPO-mediated oxidation was initiated by adding the TEMPO agent and NaClO solution to the cellulose dispersed in water. Upon washing away extra salts after the reaction, carboxylated cellulose nanofibers were obtained. These cellulose nanofibers were then modified with amine, such as polyvinylamine (PVAm), polyethylenimine (PEI), or ethylenediamine (EA). Modified and unmodified cellulose nanofibers were then loaded onto the electrospun membrane. The infusion of cellulose nanofibers resulted in a unique network structure, as shown in Fig. 2.3.

2.2 Posttreatment of nanofibrous membranes The posttreatment of the electrospun membranes is often carried out to add functionality or to improve intrinsic membrane properties such as pore size distribution and mechanical or thermal properties. Thermal treatment has shown to alter the pore size and improve the hydrophilicity of electrospun fibers. Researchers prepared the electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes. Polymer concentration was varied from 10 to 15 wt%. The effects of hot-pressing on

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Nanotechnology in Textiles

Fig. 2.3  Cellulose nanofiber network [2]. (A) Cellulose nanofiber and (B) Network of cellulose nanofiber.

the porosity, pore size, contact angle, and liquid entry pressure (LEP) were studied. A rapid contact method to hot-press the membranes with a household iron at a surface temperature of 200°C was developed. Two layers of the electrospun membranes were also hot-pressed together in order to study the effect of thickness on membrane properties. Hot-pressing caused the fibers to fuse at intersections and thus contributed to the mechanical stability of the membranes. These membranes were also tested for the filtration of a TiO2 nanosuspension with a feed pressure ranging from 1 to 2 bars. The treated membranes were more mechanically stable, but they demonstrated lower permeation and lower water flux than untreated membranes at the same feed pressure. Researchers prepared composite membranes from electrospun polyethersulfone (PES) nanofibers on a PET nonwoven support layer for prefiltration of wastewater. They found that increasing the operating pressure caused the pore structure of the membrane to deform, causing significant flux decline. This behavior is typical of electrospun mats under hydraulic flow as pressure-induced compression causes porosity and hence permeability of the membrane to decrease [7–9]. To overcome this issue and to improve interfacial stability between the PES and PET layers, the samples were subjected to continuous heating for 6 h at 190°C in air. Heat-treatment temperature was chosen above the boiling point of the solvent and below the glass transition of PES. After 24 h of filtration, they found that heat-treated membranes underwent a smaller decrease in flux compared with membranes without heat treatment. Heating resulted in better adhesion between the two layers and prevented the deformation of the membrane structure under pressure. Hot-pressing also resulted in lower contact angle and smaller pores as visible in Figs. 2.4 and 2.5. Electrospun membranes can also be posttreated, thermally or chemically, with the aim of modifying important features such as pore size, hydrophobicity, electric conductivity, and/or mechanical integrity. Thermal treatment has been used extensively on electrospun membranes in an attempt to allow greater control over pore size and thickness, in order to achieve higher permeation fluxes and separation efficiencies. Other chemically induced posttreatment methods such as polymer grafting are also

Electrospun nanofibers39

Fig. 2.4  Compaction of nanofibrous membranes [2]. (A) Before and (B) After compaction.

Fig. 2.5  Pores in hot-compacted nanofibrous membranes [2]. (A) Before and (B) After compaction.

useful for modifying the surface of electrospun fibers and making them more suitable for specific separation processes. While most existing setups are restricted to lab-scale production, electrospinning is slowly moving toward larger production volumes [10]. Free surface electrospinning, multinozzle electrospinning, and other modified electrospinning techniques have already been developed, effectively paving the way for industrial upscaling of electrospinning. A drum/rotating cylinder electrospinning setup is shown in Fig. 2.6.

2.3 Effect of electrospinning parameters and potential applications of nanofibers Nanotechnology is a budding technology that has been identified as a vital scientific and commercial venture with global economic benefits. With the increasing knowledge of nanomaterial manufacturing techniques, research groups around the globe are focusing more on the preparation of nanomaterials for various applications. Among the various techniques reported in the literature, electrospinning has gathered ­significant

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Grounded collector Supporting material

Air output

Rotating cylinder Nanofibers

Polymer solution

Input of conditioned air

80 kV

High-voltage supplier

Fig. 2.6  Rotating cylinder electrospinning [2].

interest because of its ability to fabricate nanostructures with unique properties such as a high surface area and inter-/intrafibrous porosity. Electrospinning has been the most widely used technique in the late 20th (1990) and early 21st (2000) centuries. Since its first use in the early 20th (1900) century, significant improvements have been made in the instrument design, material used, and nanomaterials produced. The production of nanomaterials (nanofibers) via electrospinning is affected by many operating parameters. Applied electric field, distance between the needle and the collector, flow rate, needle diameter, solution parameters (polymer concentration, viscosity, solvent, and solution conductivity), and environmental parameters (relativity humidity and temperature) affect the nanofiber fabrication. A wide application of nanofibers in tissue engineering, drug delivery systems, wound dressings, antibacterial study, filtration, desalination, protective clothing fabrication, and biosensors is proposed [11]. Until now, electrospun nanofibers have been prepared from approximately 100 different polymers with both synthetic and natural origins. All of these nanofibers have been prepared using either solvent or melt spinning. However, even with the widespread use of the electrospinning technique, the understanding of this method is still very limited. There are several factors that affect the electrospinning process. These factors are classified as electrospinning parameters, solution parameters, and environmental parameters. The electrospinning parameters include the applied electric field, distance between the needle and the collector, flow rate, and needle diameter. The solution parameters include the solvent, polymer concentration, viscosity, and solution conductivity. The environmental parameters include relativity humidity and temperature.

Electrospun nanofibers41

All of these parameters directly affect the generation of smooth and bead-free electrospun fibers. Therefore, to gain a better understanding of the electrospinning technique and fabrication of polymeric nanofibers, it is essential to thoroughly understand the effects of all of these governing parameters. Generally, it is a known fact that the flow of current from a high-voltage power supply into a solution via a metallic needle will cause a spherical droplet to deform into a Taylor cone and form ultrafine nanofibers at a critical voltage. This critical value of applied voltage varies from polymer to polymer. The formation of smaller-diameter nanofibers with an increase in the applied voltage is attributed to the stretching of the polymer solution in correlation with the charge repulsion within the polymer jet. An increase in the applied voltage beyond the critical value will result in the formation of beads or beaded nanofibers. The increases in the diameter and formation of beads or beaded nanofibers with an increase in the applied voltage are attributed to the decrease in the size of the Taylor cone and increase in the jet velocity for the same flow rate as shown in Fig. 2.7.

Fig. 2.7  Charge distribution and shape of Taylor cone [11]. (A) Speherical, (B) Tappered, (C) Taylor cone, (D) Charge on spherical, (E) Charge on Tappered and (F) Charge on Taylor cone.

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The flow of the polymeric solution through the metallic needle tip determines the morphology of the electrospun nanofibers. Uniform beadless electrospun nanofibers could be prepared via a critical flow rate for a polymeric solution. This critical value varies with the polymer system. Increasing the flow rate above the critical value could lead to the formation of beads. For example, in the case of polystyrene, when the flow rate was increased to 0.10 mL/min, bead formation was observed. However, when the flow rate was reduced to 0.07 mL/min, bead-free nanofibers were formed. Increasing the flow rate beyond a critical value leads not only to an increase in the pore size and fiber diameter but also to bead formation (due to incomplete drying of the nanofiber jet during the flight between the needle tip and the metallic collector) as shown in Fig. 2.8. The electrospinning process relies on the phenomenon of the uniaxial stretching of a charged jet. The stretching of the charged jet is significantly affected by changing the concentration and viscosity of the polymeric solution shown in Figs. 2.9 and 2.10. For example when the concentration of the polymeric solution is low, the applied electric field and surface tension cause the entangled polymer chains to break into fragments before reaching the collector. The selection of the solvent is one of the key factors for the formation of smooth and beadless electrospun nanofiber. Usually, two things need to be kept in mind before selecting the solvent. First, the preferred solvents for electrospinning process have polymers that are completely soluble. Second, the solvent should have a moderate boiling point. Its boiling point gives an idea about the volatility of a solvent. Generally, volatile solvents are fancied as their high evaporation rates encourage the easy evaporation (B)

(A)

Cone jet

(F)

(C)

(D)

Receded jet Semi-spherical droplet

(G)

(E)

Aggregated fluid

Cone jet and unspun droplets

(H)

Fig. 2.8  Different shapes of Taylor cone [11]. (A) Cone jet, (B) Receded jet, (C) Semispherical droplet, (D) Aggregated fluid, (E) Cone jet and unspun droplet, (F) Nanofibers, (G) Membrane, and (H) Porous membrane.

Electrospun nanofibers43

(A)

(B)

Droplet shape

(C)

Elongated droplet

(D)

Stretched droplet

Nanofibers

Increasing viscosity

(E)

(G)

(F)

13 cp

74 cp

(H)

289 cp

1250 cp

Fig. 2.9  Effect of viscosity on bead formation [11]. (A) Spherical droplet, (B) Elongated, (C) Stretched, (D) Naofibers, (E) Droplets at 13 cp viscosity, (F) Droplets at 74 cp viscosity, (G) At 289 cp, and (H) Fibers at 1250 cp viscosity.

Fig. 2.10  Effect of concentration on bead formation [11]. (A) Low concentration, (B) Medium concentration, (C) High concentration, (D) Smaller beads, (E) Medium beads, and (F) Fibers without beads.

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of the solvent from the nanofibers during their flight from the needle tip to collector. However, highly volatile solvents are mostly avoided because their low boiling points and high evaporation rates cause the drying of the jet at the needle tip.

2.4 Advanced centrifugal electrospinning An advanced centrifugal electrospinning setup is proposed for the fabrication of nonwoven textiles from polymer solutions using a pivotal feed unit and stationary collectors of large diameters and different designs: cylindrical collectors or collectors consisting of circularly arranged metal strips. The use of a collector composed of strips enables the production of aligned fibers. The feed reservoir is supplied with easy-tohandle interchangeable nozzles with the possibility of varying the nozzle-to-collector gap distance. The simultaneous use of four nozzles results in the enhancement of the production rate and shortens the time for the fabrication of a denser mat with a large surface area and enhanced exploitation properties. The special construction of the reservoir provides for its negligible dead volume [12]. The feeding unit consists of a reservoir for the polymer solution supplied with one, two, three, or four nozzles. The rotary motion of the reservoir is achieved by a belt-coupled electric motor (up to 3000 rpm). When higher rotational speed is required, the reservoir axis can be connected through a special gear to a high-speed electric motor. The axis of the reservoir is attached through a bearing to the console mounted on the stand of the electrospinning setup. The power supply is connected to the metal console, which ensures that high voltage is transmitted to the reservoir containing the polymer solution. A setup is shown in Fig. 2.11. The increase in the tip-to-collector distance allows mats with enhanced strength to be obtained. Mats collected at a distance of 17 cm have smaller elongation at break compared with those at 13 cm. An increase in the number of nozzles reduces the

Fig. 2.11  Centrifugal electrospinning [12].

Electrospun nanofibers45

deformation at break of the mats, at 40, 35, 15, and 12% for one-, two-, three-, or four-nozzle mats, respectively. The strength of the mats increases upon increasing the number of the nozzles. Mats obtained using four nozzles demonstrate the greatest strength (Young's modulus 250,718.3 MPa and breaking stress 3.7 MPa) compared with Young's modulus 230,717.2, 9477.1, and 7075.1 MPa and a maximal elongation at break of 3.5, 2.7, and 1.5 MPa, for three-, two-, and one-nozzle mats, respectively. It is known that under the combined action of an electric and a centrifugal field, the jet stability increases and significant stretching and orientation of the polymer macromolecules along the axis of the fibers takes place. Fibers with such orientation are likely to display better tensile properties even in cases when their diameters are smaller. The influence on the fiber orientation is shown in Fig. 2.12. Two stationary collectors of large-diameter design are proposed in order to obtain mats of random or aligned fibers. The multinozzle electrospinning feed unit leads to a significant increase in fiber productivity for shorter time. The designed reservoir

Fig. 2.12  Change of fiber orientation in centrifugal electrospinning [12]. (A) Centrifugal electrospinning without charged plate, (B) With charged plates, (C) Fiber orientation without plates, and (D) Better orientation with charged plates.

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allows maximum utilization of the spinning solution within eligible dead volume. The use of three or four nozzles or the increase in the tip-to-collector distance enhances the strength of the mats.

2.5 Blended polymer electrospinning Bulk chemical properties of electrospun polymers such as hydrophilicity, crystallinity, density, and degradation rate are important to the bioactivity of the scaffold, but often, it is the surface chemistry that dominates the cell-scaffold interface. In this regard, noncell-adhesive polymers are limited in their ability to interact directly with cells. Therefore, surface modification has become a common technique to increase cell interactions with noncell-adhesive scaffolds. The simplest method is nonspecific surface adsorption of cell-adhesive proteins like collagen, fibronectin, or laminin. While simple, the method is not mechanically robust since relatively weak, noncovalent interactions govern the interactions between the adsorbed proteins and the scaffold. An attractive alternative is to blend the polymer of interest with a cell-adhesive protein in the initial formulation, thus entrapping the protein within the entire fibrous architecture [13]. The different principles of blending polymers for electrospinning are shown in Fig. 2.13.

2.6 Three-dimensional nanofibrous macro-structures via electrospinning Spatial organization of electrospun fibers is another structural feature that has been actively researched. Fibers from electrospinning are often randomly aligned due to the inherent whipping of the fiber jet as the polymer ejects from the spinneret. To align fibers, researchers have developed different grounded collectors, the most common being a high-velocity, rotating mandrel that can align fibers parallel to the direction of rotation. Using this method of alignment, researchers have shown that cell morphology and cytoskeletal organization closely parallel fiber alignment as shown in Fig. 2.14. Compared with other nanofiber fabrication processes, electrospinning is versatile and superior in the production and construction of ordered or more complex nanofibrous assemblies. Besides traditional two-dimensional (2-D) nanofibrous structures, electrospinning is powerful in the fabrication of three-dimensional (3-D) fibrous macrostructures, especially for tissue engineering applications. Recent advances are made in various promising and cutting-edge electrospinning techniques, including multilayering electrospinning, postprocessing after electrospinning, liquid-assisted collection, template-assisted collection, porogen-added electrospinning, and self-­ assembly [14–17]. Furthermore, these 3-D nanofibrous macrostructures have been demonstrated to have potential applications in tissue engineering, energy harvesting and storage, and filtration.

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Fig. 2.13  Different principles of blending polymers for electrospinning [13]. (A) Fiber immersion in second polymer, (B) Mixed polymer spinning, (C) Coaxial spinning of two polymers, and (D) Chemical treatment of fibers before immersion in second polymer.

(I)

(H)

(A)

Syringe

(C)

48

(B)

(G)

Solution High voltage DC power supply

Needle

Fibers Aluminium foil

(D)

(E)

(F)

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Fig. 2.14  Spatial (3-D) structures produced by electrospinning [13]. (A) Spinning setup, (B) Multilayered, (C) Aligned multilayered, (D) Crossed multilayered, (E) Collected on crossed grids, (F) Collected as a bunch, (G) Collected in cross laying, (H) Spiral laying, and (I) Twisted.

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With a subsequent postprocess after electrospinning such as folding/rolling up, the aforementioned 3-D multilayer fibrous structures, even the as-spun 2-D layer-on-layer mats, can turn into a desired morphology for further application. For example, 3-D macrostructures based on double-layered nano-/microfibrous chitosan were prepared by a three-step process: First, chitosan microfibers were fabricated to form sheets with proper pores in advance; then, nanofibrous chitosan was electrospun to the upper side of microfiber sheet on the collector; finally, the double-layered sheets were rolled up, thus forming the 3-D structures as shown in Fig.  2.15. The proposed structures as scaffolds had dual porous architecture containing nanofibrous walls and microsized pores formed by microfibrous sheet. The collection of 3-D macrofibrous structures using a mechanical collector was first introduced, using 3-D collecting templates based on the manipulation of electric field and electric forces to fabricate micro and macro single tubes with multiple micro patterns; multiple interconnected tubes; and tubes with the same or different sizes, shapes, structures, and patterns. Fig. 2.16A and B shows the process for the fabrication of tubes with a multiple interconnected tubular structure and the as-prepared crossing fibrous tube. It is found that the distance between the individual 3-D collectors is a key parameter for the batch fabrication of tubes with different sizes, shapes, wall structures, and patterns. Namely, excessively small distance may cause fiber suspension between collectors. However, manipulation of the electric field is the basic principle of this technique [18,19].

(A)

(B)

(C)

Fig. 2.15  Three-dimensional rolled nanofibrous structures [13]. (A) Rolled sheet, (B) Bunched fibers, and (C) Multilayered sheets.

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Fig. 2.16  Interconnected tubular structure with electrospinning [13]. (A) Cross shaped, (B) fiber deposition on cross shape, (C) round cross section, and (D) fiber layer from round collector.

Apart from mechanical collectors, it is also possible to obtain 3-D fibrous macrostructures using a matrix. The matrix usually is microfibers that are fabricated by other methods in advance or at the same time with electrospinning. Namely, 3-D structures are usually fabricated via a hybrid technique that combines traditional electrospinning with other methods such as prototyping, polymer/fiber deposition, and melt electrospinning. For example, by combining direct polymer melt deposition (DPMD) with electrospinning, highly functionalized 3-D scaffolds were fabricated. As shown in Fig. 2.17, a microfiber layer was built by using the DPMD process firstly. Then, the microfiber layer was covered by a polymeric nanofiber matrix. Three-dimensional hybrid structures were fabricated by electrospinning polycaprolactone (PCL)/collagen biocomposite nanofiber matrices into the microfibrous structures [20]. Self-assembly, as an important “bottom-up” approach, is intensively studied in chemistry, physics, biology, and materials engineering. It can easily arrange small components into ordered systems (e.g., patterns) or aggregate structures without human intervention, which otherwise will be expensive, slow, and complex. Selfassembling processes are common throughout nature and technology, especially in the field of nanoscale materials. It is one of the few practical strategies for making ensembles of nanostructures and therefore an essential part of nanotechnology. For example, there are many reports on small components from molecules to nanoscale particles to be self-assembled into ordered aggregates with desired structures as is shown in Fig. 2.18.

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(A)

Repetitive process Nanofiber Hybrid structure

Electrospinning (B)

(C)

Microfiber

Direct Polymer Melt Deposition

(D)

(E)

Fig. 2.17  Three-dimensional fibrous microstructure from a matrix [13]. (A) Hybrid structures, (B) Multilayered, (C) Cross layered, (D) Dimensions, and (E) Nanofibers deposited over microfibers.

Honeycomb structures shown in Fig. 2.19 have a mechanical stability for intersecting angle of 120 degrees among the three branches. In nanoscience and technologies, 2-D or 3-D honeycomb structures are usually fabricated via breath-figure templating method, patterning on unusual substrates (e.g., nonplanar substrates and air/water interface), etc. Nanofiber yarns can be regarded as special 3-D fibrous assemblies. Single nanofibers with a small diameter have low mechanical strength and are difficult to tailor into a fibrous structure, restricting their further applications in some areas, such as high-performance clothing, filters, and composites. After precise handling of linear

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(A) Salt mixing Electrospinning

Grounded plate

High voltage power supply

(B)

(C)

Fig. 2.18  Self-assembled nanofibrous structures [13]. (A) Self assembly, (B) 3-D structure, and (C) Hollow 3-D structure.

Fig. 2.19  Honeycomb structures [13]. (A) Honeycomb and (B) Multilayered walls.

nanofiber assemblies, nanofiber yarns can be used directly in weaving, knitting, and embroidery. As special 3-D fibrous assemblies, fibers and yarns have a long history of thousands of years [21–23]. With the development of nanoscience and technology, nanofibrous yarns based on various materials such as polymers and composites have created an increasing use in high-value-added applications like filtration media, gas separation, sensors, and biomedical engineering. The fabrication of electrospun

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nanofiber yarns is usually based on a modified collector such as a water collector and a rotating drum, and then, the as-spun nanofibers were stretched manually or winded around the rotating grounded drum. Besides the rapid solidification of as-spun long fibers during the electrospinning process, electrostatic induction is believed to be the main factor for the formation of self-assembled 3-D fibrous stacks: Firstly, the polymer jets ejected from the metal needle are positively charged due to the applied high DC voltages. After the process of fluid extension, whipping and splitting, and the evaporation of solvent, the spun fibers are deposited on the grounded collector, and the surface charges are conducted away through the Al foil. Then, due to electrostatic induction and polarization under the influence of strong static electric field, the as-spun fibers on the collector are negatively charged. Hence, during the electrospinning process, with the rapid increase in the amount of spun fibers, a piece of thin membrane appears on the collector. The observation that some regions of the fibrous membranes are not smooth and thicker at microscopic scale suggests that the polymer fibers may be polarized under a strong electric field. Negative charges generated by static induction and polarization on the top fibers will attract the coming jets/fibers with positive charges. Namely, the stack acts as a new “collector” in this case; the coming fibers are deposited on top of the stack and make the stack grow fast within a short time. In addition, the formation of 3-D fibrous stacks also has relationship with the relative humidity. By changing the humidity of the electrospinning conditions, the concentration of dissociated ions in the fibers has been altered, which affects the like-charge repulsions between fibers and drives the 3-D mat formation. Accordingly, conversion between 3-D fiber stacks and 2-D thin film can be realized [24,25]. With an insulating Lucite plate placed over the Al foil collector during electrospinning, the fibers would fully spread on the plate, forming a 2-D nonwoven mat since the insulating Lucite plate blocked the dissipation of the positive charge on the fibers conducting through the ground wire. The fibers with positive charges on the Lucite plate are no longer attractive to the spinning needle, on the contrary, repelling to the coming fibers with positive charges. Furthermore, when an additional electrostatic generator was introduced in the subsequent experiment, the conversion from 2-D thin film to 3-D fiber stack on the Lucite plate can be achieved because the electrostatic generator generates enough negative charges. And the new fiber stack is almost the same as that without Lucite plate and electrostatic generator. The 3-D structure is shown in Fig. 2.20. Apart from the conventional electrospinning, melt electrospinning can also lead to a self-assembled 3-D fibrous macrostructure. Fig. 2.21A demonstrates an electrospun boxlike structure depending on a similar amount of fibrous layers stacked directly on top of each other with precise control with a thickness of approximately 1 mm, where electrostatic effects cause a loss of control over fiber deposition. Fig. 2.21B shows a combination of interweaving and fusion between the fibers in Fig. 2.21A, which may improve the structural integrity of melt electrospun scaffolds. However, because material parameters such as polymer molecular weight and molecular chain conformation are important in all polymer fiber spinning methods, it can be seen that the fibers' diameter in Fig. 2.21 is much larger than that from conventional solution electrospinning because of the stronger chain entanglement, higher viscosity, and poor crystallization of the melt.

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Fig. 2.20  Assemblies of nanofibrous layers [13]. (A) Nanofibrous assemblies, (B) Growth of assembly, and (C) Fiber arrangement.

Fig. 2.21  Self-assembly of melt electrospun nanofibers [13]. (A) Box shaped and (B) Microstructure.

A novel method was developed to fabricate 3-D nanofibrous scaffolds by using an array of focused halogen light bulbs, to localize the heat in the path of electrospun jet near the collector. It can be seen that the mat thickness from this method is 10 times that from the conventional electrospinning and the as-spun nanofibers are much more porous than those fabricated by conventional electrospinning [26–29]. It is potential to provide a suitable environment for viable cells to proliferate and migrate and may be appropriate for tissue engineering applications while enabling researchers to fabricate 3-D artificial tissues containing biodegradable polymers and functional cells.

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Compared with the conventional electrospun 2-D mats, 3-D fibrous macrostructures have larger inner surface area and pore size and thus improved cellular infiltration. It has been reported that cells can migrate up to approximately 4 mm and showed a spatial cell distribution. These physical and spatial architectural geometries and the excellent biocompatibility of electrospun 3-D scaffolds are important to applications in tissue engineering, such as nerve regeneration, vascular grafts, and bone regeneration. Besides tissue engineering applications, 3-D nanofibrous structures made of inorganic compound or composites may be also used as energy harvesting and storage materials such as cathode materials for solar cells and Li-ion batteries [30–34]. The small-diameter bilayered or multilayered 3-D nanofibrous tubular scaffolds especially with controllable nanofiber orientations have potential application in vascular grafts, because they not only have directional mechanical properties but also can facilitate the orientation of the cell attachment on the fibers. Another interesting application of 3-D fibrous scaffolds is bone tissue engineering, which has been reported by several research groups in recent years. Considering that the two layers of the skin, epidermis and dermis, have different ability to regenerate, the epidermis has less capacity to heal; however, while the dermis has an enormous capacity to regenerate, skin tissue engineering should be not only to close the skin wound but also to stimulate the regeneration of the dermis. Nanofibers reveal fascinating potential in this area due to their structural similarity to the extracellular matrix (ECM), large surface-area-to-volume ratio, superior mechanical properties, and high porosity [35,36]. Electrospun-nanofiber-based filters for industry, household, and defense applications have a history of more than 30 years, because electrospun nanofiber mats provide a dramatic increase in filtration efficiency at a relatively small decrease in permeability. Compared with other nanofiber fabrication processes, electrospinning is versatile and superior in the production and construction of ordered or more complex nanofibrous assemblies. Although a lot of progress has been achieved in the past three decades in the development of various designs and modification to the electrospinning process and wide potential applications of electrospun fibers, there are still many areas that require further improvement and refinement, for example, consistent production of nanofibers, controlling the fiber diameter and size distribution, reproducible placement of nanofibers in specific positions and orientations, and mass production [37,38]. To fully explore the extraordinary number of application opportunities of electrospun nanofibers, reliable consistent production of the nanofibers especially at the industrial level is essential. Since the morphology and diameter of the electrospun nanofibers are dependent on material and processing parameters, through controlling these interrelated variables such as solution conductivity, spinneret design, electric field intensity, auxiliary electric/magnetic field, applied voltage, flow rate, collection distance, and solution concentration, the morphology, average diameter, diameter distribution, and orientation of the electrospun nanofibers can be controlled and adjustable. The effect of the conductivity of poly(vinyl alcohol) (PVA)/water solution on the diameters of electrospun fibers was studied. By adding increasing concentrations of NaCl (ranging from 0.05% to 0.2%) to the PVA/water solution, the corresponding solution conductivity increased from 1.53 to 10.5 mS cm−1, the average fiber diameter decreased from

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214 to 159 nm, and the diameter distribution broadened. In another study, researchers examined the effects of the spinneret shape on the needleless electrospinning process and resultant fiber morphology [39]. It is found that compared with cylinder, the disk and coil produced thinner fibers with narrower diameter distribution, which could be ascribed to concentrated electric field formed on disk edge and coil wire surface, since the random orientation of fibrous mats fabricated by the conventional electrospinning may limit the potential applications of electrospun fibers, especially in the fields of electronics, photonics, photovoltaics, actuators, and tissue engineering that need direct, fast charge transfer or regular, uniform structures. To solve this problem, a variety of strategies have been proposed by many research groups, such as electrode pair collection, rotating drum or disk collection, auxiliary electric or magnetic electrospinning, double spinning, near-field electrospinning, and direct-writing electrospinning. Some approaches such as rotating drum collection have been used in the commercial electrospinning setup. However, a drum collector is not able to develop highly aligned fiber mesh with a large thickness due to electrostatic repulsion between the deposited fibers and the coming fibers. The ability to consistently fabricate highly aligned fibers in large quantity over a large area is still a challenge. In particular, recent advances in near-field electrospinning (precision electrospinning or direct-writing electrospinning) have demonstrated the ability to precisely control fiber deposition to form patterns. By fixing the collector or spinneret on an X-Y motion stage, the movement of the collector/spinneret can be controlled in the preprogrammed track via a host computer, and thus, precise fiber deposition and various complex patterns of polymer fibers with controlled orientation and spacing have been achieved, which is vital for device or biomedical applications. However, the assembly efficiency and the pattern area need to be improved for practical use [40–45]. There are a variety of approaches for building 3-D porous structures. It is interesting that the fibers in the 3-D fibrous macrostructures are similar to those of 2-D mats fabricated by conventional electrospinning, ranging from several micrometers down to tens of nanometers. For the large surface area and pore size and the existence of the interpore channels compared with the 2-D mats, these 3-D structures have potential applications in some fields such as tissue engineering, filtration, and energy materials. And other potential applications may lie in catalyst supports, microcontainers, sensors, drug delivery, sound absorption, and so on. Nevertheless, challenges in this area have also been met, and many practical problems have to be solved. For example, several 3-D fibrous materials such as self-assembled stacks and vertically aligned fibrous structures cannot be put in tissue engineering use directly due to their structure instability. These 3-D nanofibrous macrostructures require postprocessing for practical use. In order to improve the performance of 3-D fibrous scaffolds in tissue engineering applications, besides optimizing the porosity, size scale, and orientation of nanofibers, some other approaches such as surface functionalization of nanofibers with biological molecules and electrospinning of polymers with bioactive inorganic nanoparticles or nanotubes/wires may be effective. For example, surface modification of nanofibrous scaffolds with biological molecules such as gelatin, short peptide sequence, acrylic acid, collagen, polydopamine, and protein can be used to further enhance the interactions between cells and the scaffold material and improve

Electrospun nanofibers57

their in vivo usage [46,47]. Many studies on composite fibrous scaffolds containing bioactive inorganic nanoparticles such as hydroxyapatite (HA) and calcium phosphate have demonstrated enhanced in vitro attachment, differentiation, and proliferation of bone-forming cells and in vivo bone regeneration. In addition, since the engineering of complex tissues requires graded scaffolds that can mimic the complex spatial distributions of composition, structure, and functionality of native tissues, it is still a challenge to fabricate biodegradable 3-D scaffolds functionally graded in terms of porosity and composition distributions.

2.6.1 Three-dimensional porous nanofibrous scaffolds by the dual electrode electrospinning Application of traditional electrospun scaffolds in tissue engineering is limited due to the sheetlike nanofiber layers hindering cell infiltration. Three-dimensional (3-D) thick nanofiber stack with pore size gradient was fabricated via a novel electrospinning setup. Instead of a traditional flat-plate collector, a small copper plate that covered a hole of poly(methyl methacrylate) (PMMA) sheet was connected to negative voltage, resulting in focused collection of nanofibers and quickly enhanced scaffold thickness due to insulating PMMA preventing fibers from spreading elsewhere. The average pore size can reach more than 25 μm that benefit for cell infiltration. In the process of electrospinning, the mat with increased thickness may influence the electric field and has effect on fiber diameter and pore size [48].

2.7 Zirconium carbide nanofibers by electrospinning Zirconium carbide (ZrC) is one of the most attractive ultrahigh-temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid (Fig. 2.22). The polymer PZSA was prepared from the chemical reaction between polyzirconoxane (PZO) and salicyl alcohol.

Fig. 2.22  Electrospun nanofibers from ZrC [49]. (A) Nanofibrous layer and (B) Microstructure.

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The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermic reduction at 1300°C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles are dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Debye-Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by X-ray diffraction (XRD), SEM, and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase-change material support in thermal management systems [49]. The electrospun precursors need to be pyrolyzed to produce ZrC nanofibers. The crystalline structure of ZrC nanofibers heat-treated at 1300°C was further investigated by XRD. It is noted that all of the diffraction peaks in the XRD pattern match well with the cubic ZrC phase. The sharp diffraction peaks imply the better crystallinity of ZrC grains. The PVP in composite fibers had been completely carbonized after heat treatment at 1300°C for 2 h in argon. TEM investigation in Fig.  2.23 shows that ZrC fiber structure is well kept and nanosized hexagonal ZrC crystals are dispersed in the fibers. ZrC nanocrystals were

Intensity (a.u.)

ZrC

20

40 60 2theta (degree)

80

Fig. 2.23  TEM images of zirconium carbide nanofibers [49]. (A) Nanofibrous layer, (B) Microstructure, (C) Intensity, and (D) Single fiber.

Electrospun nanofibers59

embedded or engulfed by disordered carbon coming from salicyl alcohol; the average diameter of the fibers is ~200 nm, which is accordance with the SEM observation. These results confirm that the crystalline phases dispersed in the ceramic fibers uniformly and the ZrC crystalline is in the range of 30–50 nm. A quantitative analysis by energy-dispersive spectroscopy (EDS) shows that ZrC nanofibers consisted of Zr, C, and O elements. It is known that in the preparation of UHTCs, it is very difficult to remove the carbon and oxygen residues entirely. Continuous ZrC nanofibers are successfully fabricated. SEM (Fig. 2.23A and B), by combination of polymeric precursor chemistry and electrospinning technique. PZSA and PVP are used as zirconium source and spinning aid, respectively. ZrC nanofibers are obtained by pyrolyzing the as-spun green fibers at 1300°C for 2 h under argon. The obtained ZrC nanofibers via carbothermic reduction treatment at 1300°C exhibit an average diameter of 200 nm while preserving the fibrous morphology. TEM results show that ZrC nanoparticles disperse in the fiber uniformly containing free carbon and the average size is about 42 nm. The electrospun ZrC nanofibers in the form of freestanding nonwoven textile may serve as an ideal precursor for the synthesis of highly porous carbide-derived carbon materials, which would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phasechange material support in thermal management systems. Additionally, this reported method could be utilized to prepare other metal carbide/boride nanofibers [50].

2.8 Parameters on electrospinning process and characterization of electrospun nanofibers Electrospinning has been one of the simple, versatile, and promising processes to produce continuous nanofibers. Gelatin has been used widely at bulk state in foods for thickening and stabilizing purposes mostly. At nanoscale, electrospun gelatin nanofibers may be used in foods for the same purposes at smaller amounts giving more efficient results. In order to tailor properties of electrospun nanofibers in foods, the influence of affecting parameters on the functions of nanofibers should be known. The aim is to investigate the influences of the affecting parameters during electrospinning on properties of electrospun gelatin. The zeta potential and the diffusion coefficients of dispersions containing gelatin or electrospun gelatin were determined. Both values were higher for dispersions containing electrospun gelatin than for dispersions with gelatin at the same concentration. The zeta potential and diffusion coefficient values of dispersions containing electrospun gelatin decreased as the applied voltage during electrospinning increased. Lower applied voltage resulted in higher zeta potential and diffusion coefficient values for dispersions containing electrospun gelatin nanofibers, which may indicate that these nanofibers can be used for stabilizing food emulsions, whereas smooth nanofiber morphology without bead formation obtained at the highest voltage [51–54]. Different parameters such as gelatin concentration, electric conductivity, surface tension, rheological properties, applied voltage, and feed rate are selected to see their

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effects on the morphology of electrospun samples. The gelatin solution at 7% (w/v) did not produce nanofibers, whereas regular nanofiber formations occurred at 20% (w/v) gelatin concentration. The increasing voltage led to larger fiber diameters with lower zeta potential and lower diffusion coefficient values of dispersions containing nanofibers. The increasing feed rate leads to larger fiber diameters and bead formations. Smooth nanofiber morphology without bead formation was obtained at 35 kV applied voltage, 0.1 mL/h feeding rate, and 10 cm plate distance, from 20% (w/v) gelatin solution. Because of the complexity of the electrospinning process and effect of other process conditions, evaluating factors affecting nanofiber morphology is a challenge. To better interpretation especially for the effect of viscosity on electrospinning, the apparent viscosity at the ejection point can be calculated, and electrorheological ER measurements may be conducted. The zeta potential and diffusion coefficient of dispersions with electrospun samples can be measured or monitored for evaluating their functions in liquid media. Further investigations must be conducted to understand affecting parameters on the electrospinning process to tailor the properties of nanofibers for specific applications [55].

2.8.1 Electrospinning of corn cellulose with alcohols to fabricate ultrafine fibers As an important biomass, cellulose is recognized as the most promising substitute for the petroleum polymers in the wide range of areas, such as filtration, biomedical applications, and protective clothing, because of its renewability, biodegradability, and abundance in nature. Water and four small molecular alcohols are respectively used to activate corn cellulose (CN cellulose) with the aim to improve the dissolvability in dimethylacetamide/lithium chloride (DMAc/LiCl). Among all these activated agents, monohydric alcohols are found to produce the optimal effect of activation in the whole process including the activating, dissolving, and electrospinning of CN cellulose. Meanwhile, well-distributed fibers with the diameter of 500 nm to 2 μm are fabricated in electrospinning shown in Fig. 2.24. Understanding the activation effect of monohydric alcohols with water and polyhydric alcohols, the most effective activated agent is ascertained with the characteristics of small molecular size, low viscosity, and single functionality. This work is definitely initiated to understand the critical principle of CN cellulose in dissolving. Accordingly, a feasible methodology is also established to prepare ultrafine cellulose fibers with good morphology in electrospinning [56]. The coil size of the activated CN cellulose in dilute solution is also detected by dynamic light scattering. Water-activated CN cellulose shows the largest average diameter of random coil at 1575 nm. And the average diameters of the random coils of the methanol- and ethanol-activated cellulose are 597 and 993 nm, respectively, which are much smaller than that of the water-activated CN cellulose. This further expresses that the molecular entanglement of the water-activated CN cellulose in solution is stronger than that of monohydric alcohol-activated CN cellulose. Because of the rather small size and the particular chemical structure of water molecule, it probably induces a bridging effect between two cellulose molecules. Hence, the molecular entanglement is increased in solution. By using molecular simulation, it is proved that water is much easier to

Electrospun nanofibers61

Fig. 2.24  Electrospinning of corn cellulose [56]. (A) Corn cellulose fibers, (B) Microstructure of corn cellulose fibers, (C) Diameter distribution, (D) Surface of fiber, and (E) Surface roughness.

e­ nhance the interaction between two cellulose molecules than ethanol. In the meantime, small amount of water can induce the cellulose to precipitate from solution. Through these experimental facts, it can be concluded that water shows the dual functions to produce the best effect of activation and relatively enhance the molecular entanglement. By utilizing water and alcohols as the activation agents, it is successful to dissolve CN cellulose in DMAc/LiCl. Water can produce the best effect of activation. But it is evitable to result in a high molecular entanglement with low electrospinnability. The optimal activation agent of CN cellulose is recognized as the monohydric alcohols. Because of the suitable molecular size, low viscosity, and single functionality, monohydric alcohols provide the most effective promotion in the whole process including the activating, dissolving, and electrospinning of the CN cellulose. Well-distributed ultrafine cellulose fibers with the diameter of 500 nm to 2 μm are also fabricated. Finally, the establishment of an important principle has been initiated to choose suitable activation agent for CN cellulose in dissolving and processing [56].

2.8.2 Electrospinning of SAN/MWCNTS reinforced composite membranes Electrospinning is a fascinating fiber formation method that creates nanofibers through an electrically charged jet of polymer solution or polymer melt. The advantages of this fiber formation method are high speed of process, low cost, easy application in production, continuous fiber, easy-to-achieve nanoscale diameter, and flexibility in process parameters. The electrospinning process is governed by many solution, process, and ambient parameters. It was found that each of the solution parameters, which include viscosity, conductivity, molecular weight, and surface tension, has a significant impact on the morphology of the fibers and on the properties of the material obtained [57]. By proper manipulation of these parameters, membranes of desired structure and properties can be electrospun as is visible in Fig. 2.25.

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Fig. 2.25  Styrene acrylonitrile/multiwalled carbon nanotubes (SAN/MWCNTs) reinforced composite membranes [57]. (A) 1%, (B) 2%, (C) 3%, (D) 4%, (E) 5%, (F) 6%, (G) 7%, (H) 8%, (I) 9%, (J) 10%, (K) 15%, and (L) 20% MWCNTs.

Electrospun nanofibers63

The electric conductivity of all membranes without the ionic liquid is zero. MWCNTs do not form any conductive network in pure SAN fibers. The addition of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) increases the electric conductivity only up to 0.244 mS cm−1. But the addition of MWCNTs into the ionic liquid containing solution increases the electric conductivity of the membrane up to 5.95 mS cm−1. Thus, [Bmim]Cl and MWCNTs create a conductive network inside the SAN fibers and yield conductive membranes. It was found that MWCNTs have a certain effect on the tensile stress of the membrane only in [Bmim]Cl-containing membranes. The highest tensile stress was achieved at the MWCNT concentration 1.0% by weight of SAN. SEM investigation showed that both types of membranes, with and without [Bmim]Cl, contain beads at MWCNT concentrations higher than 1.5% by weight of SAN. But ionic liquid content has a certain effect on the average diameter of the fibers: Below 1.0% of MWCNTs by weight of SAN, it reduces, and above 1.0%, it increases again (till 3.0%). The solutions without ionic liquid showed no dependence of fiber diameter on the concentration of MWCNTs [57].

2.8.3 AC and DC electrospinning of hydroxy propyl methyl cellulose with polyethylene oxides as secondary polymer Alternating current electrospinning (ACES) capable to reach multiple times higher specific productivities than widely used direct current electrospinning (DCES) was investigated and compared with DCES to prepare drug-loaded formulations based on one of the most widespread polymeric matrices used for commercialized pharmaceutical solid dispersions, hydroxypropyl methylcellulose 2910 (HPMC). In order to improve the insufficient spinnability of HPMC (both with ACES and DCES) polyethylene oxide (PEO) as secondary polymer with intense ACES activity was introduced into the electrospinning solution. Different grades of this polymer used at as low concentrations in the fibers as 0.1% or less enabled the production of high-quality HPMC-based fibrous mats without altering its physicochemical properties remarkably. Increasing concentrations of higher-molecular-weight PEOs led to the thickening of fibers from submicronic diameters to several microns of thickness. ACES fibers loaded with the poorly water-soluble model drug spironolactone were several times thinner than drugloaded fibers prepared with DCES in spite of the higher feeding rates applied. The amorphous HPMC-based fibers with large surface area enhanced the dissolution of spironolactone significantly; the presence of small amounts of PEO did not affect the dissolution rate. The presented results confirm the diverse applicability of ACES, a novel technique to prepare fibrous drug delivery systems. The first electrospinning experiments revealed that HPMC alone cannot be considered as a good fiber-forming polymer as both AC and DC electrospinning produced large HPMC particles with a minor thin (around 0.5 mm) fibrous fraction shown in Fig. 2.26 [58]. Being one of the mostly used polymers for commercialized amorphous solid dispersion products, HPMC 2910 was processed with AC and DC electrospinning to improve the dissolution of a poorly water-soluble model drug, spironolactone. The insufficient morphology of electrospun HPMC was resolved by incorporating small amounts of PEO as secondary polymer overly sensitive to ACES. The use of lower

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Fig. 2.26  AC and DC electrospinning of hydroxypropyl methyl cellulose [58].

Electrospun nanofibers65

grades of PEO such as 100 kDa and 1 MDa yielded good-quality HPMC-based fibers. When PEO with molecular weight of 4 MDa was applied, the fiber formation was more sensitive to small variations in secondary polymer concentration. No traces of crystallinity could be detected in the prepared fibers when conducting differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) measurements; presumably, SPIR and PEO turned into an amorphous form as a result of processing. Accordingly, the dissolution of SPIR could be greatly enhanced regardless of the use of PEOs due to the large surface area, the hydrophilic HPMC carrier, and the amorphous SPIR content of the fibers [58]. The main benefit of alternating current electrospinning, that is, the multiple times higher feeding rates compared with regular direct current electrospinning, was also attainable during the production of HPMC-based fibers without remarkable deviations in morphology and physical state of the incorporated drug. Besides elevated feeding rates, an unexpected effect of alternating current electrospinning was observed: increasing fraction of SPIR resulted in thinner fibers as opposed to DCES of the same solutions. The exploration of alternating current electrospinning may be an important milestone on the way toward the industrial production of electrospun amorphous pharmaceutical solids; however, more detailed studies are still required to elucidate the mechanisms affecting ACES fiber formation.

2.9 Nano scaffolds prepared by disc-electrospinning Electrospinning is a versatile and convenient technology to generate nanofibers suitable for tissue engineering. However, the low production rate of traditional needle electrospinning hinders its applications. Needleless electrospinning is a potential strategy to promote the application of electrospun nanofiber in various fields. Disk electrospinning (one kind of needleless electrospinning) was conducted to produce polycaprolactone/gelatin (PCL/GT) scaffolds of different structures, namely, the nanoscale structure constructed by nanofiber and multiscale structure consisting of nanofiber and microfiber (Fig. 2.27). It was found that, due to the inhomogeneity of PCL/GT solution, disk-electrospun PCL-GT scaffold presented multiscale structure with larger pores than that of the acid-assisted one (PCL-GT-A). Scanning electron microscopy images indicated the PCL-GT scaffold was constructed by nanofibers and microfibers. Mouse fibroblasts and rat bone marrow stromal cells both showed higher proliferation rates on multiscale scaffolds than nanoscale scaffolds. It was proposed that the nanofibers bridged between the microfibers enhanced cell adhesion and spreading, while the large pores on the three-dimensional (3-D) PCL-GT scaffold provide more effective space for cells to proliferate and migrate. However, the uniform nanofibers and densely packed structure in PCL-GT-A scaffold limited the cells on the surface. The potential of disk-electrospun PCL-GT scaffold containing nanofiber and microfiber for 3-D tissue regeneration is explored [59]. The structure of scaffolds was treated as a priority for cellular colonization in tissue regeneration, which would affect the interaction of cells with the scaffolds. The morphology of cells on two scaffolds was observed by SEM as seen in Fig. 2.28. L929 on

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Fig. 2.27  Disk electrospinning of nanofibers [59].

(A)

(B)

(C)

(A)

(B)

(C)

(D)

(E)

(F)

(D)

(E)

(F)

Fig. 2.28  SEM images of nanofibrous scaffolds [59]. PCL-GT scaffolds after (A, A′) 1 day, (B, B′) 3 days, (C, C′) 7 days. L929 cultured on PCL-GT-A scaffolds after (D, D′) 1 day, (E, E′) 3 days, and (F, F′) 7 days. A′, B′, C′, D′, E′, and F′ are with higher magnification and 7 days (C), (C′). L929 cultured on PCL-GT-A scaffolds after 1 day (D), (D′), 3 days (E), (E′), and 7 days (F), (F′) (Bar: 100 m).

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both scaffolds showed a significant increase over time from day 1 to day 7. On the first day after cell seeding, cells adhered to the fibers and formed a typical spindle shape. On the PCL-GT scaffold, cells were thinner than that on PCL-GT-A scaffold. L929 cells were found not only on the surface but also inside the scaffold. Three days later, the number of cells in the scaffolds increased. However, for the PCL-GT-A scaffolds, the pore size between fibers was much smaller than cell, restricting the cells on the surface [60]. Electrospinning has been widely used in the fabrication of scaffolds with nanoscale structure, while the low production rate of traditional needle electrospinning limits the application. Generally, the production rate for the conventional electrospinning varies from 0.1 to 1.0 g/h. The development of needleless electrospinning raised a valid solution to scale up the production of nanofiber. Disk electrospinning was used to fabricate PCL/gelatin scaffold with homogeneous and inhomogeneous PCL/gelatin blends. The properties of both scaffolds were characterized, and in vitro experiment was conducted to study the biocompatibility [59,60]. Electrospinning PCL/gelatin blends into nanofiber has been extensively studied in previous literatures. Nanofiber constructed by PCL and gelatin possessed the merits of both synthetic and natural materials. Gelatin could improve the biocompatibility, while the PCL strengthen the mechanical properties. However, the obtained PCL/­gelatin scaffold was constructed by fibers of nanoscale, which reduced the pore size to several micrometers between fibers in the scaffold, inhibiting the cells on the very surface. Thus, the cells could only form a thin layer without migration into the scaffold. On the other hand, to generate the nanofiber into a 3-D scaffold allowing cell infiltration, several methods were invented, including salt leaching, freeze-­drying, and water bath collecting. All these methods generated large pore nonuniformly distributed in the scaffold. Some parts formed voids, while the rest was still densely packed nanofibers. Recently, the combination of nanofiber and microfiber provides a promising method to solve the existing problems. Microfiber formed the backbone of the scaffold, while the nanofibers bridged between microfibers. Experiments have verified that the multiscale scaffold could boost cell motility, survival, and proliferation. However, the existing methods were based on needle electrospinning, which always need a dual-needle system (Fig. 2.29). Nanofibers and microfibers were combined to form 3-D structure with large pores and efficient nanoscale binding sites. Bilayered constructs consisting of microfiber scaffold with various thicknesses of nanofibers were developed. The presence of nanofiber could enhance cell spreading, while increasing the thickness of nanofiber layer reduced cell infiltration. The multiscale 3-D scaffolds containing fibers with average diameter of 3.3 and 0.6 μm were produced via multimodal electrospinning. Results indicated that nanofibers properly inserted in the microfiber scaffold could crucially enhance the interaction between cells and scaffold. Due to the inhomogeneity of the PCL-GT in TFE solution, microfibers and nanofibers were generated simultaneously and formed multiscale scaffolds with large pores. Compared with the acetic acid-assisted PCL-GT-A scaffold constructed by uniform nanofibers, PCL-GT scaffold possesses larger pores with a diameter of 13.39 μm. In vitro experiments indicated that the L929 and BMSCs cultured on PCL-GT ­scaffold

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Fig. 2.29  Electrospinning PCL/gelatin blends into nanofiber [59]. (A) Gelatin on PCL individual fiber, (B) Gelation on fibrous web, (C) Gelatine in pores of fibrous web, (D) Macro clusters of gelatin, and (E) Microclusters.

show higher proliferation rate. PCL-GT also enhances the cell infiltration into the scaffold, while the nanofibrous structure of PCL-GT-A scaffold inhibited the cell on the surface. In the PCL-GT scaffold, cells seemed to settle and align on the microfibers but bridge between microfibers through the nanoweb constructed by the nanoscale fibers, distributing throughout the whole scaffold in a 3-D manner. Results demonstrate the potential of disk-electrospun PCL-GT scaffold with multiscale structure in the application of tissue engineering [59,60].

2.10 Electric field analysis of a multifunctional electrospinning platform Electric field is one of the most crucial parameters in electrospinning process. Moreover, it can be controlled by changing voltage, distance between the needle and the collector, type of the collector, and the characteristics of the needle. These parameters change the fiber morphology considerably. A minimum voltage of 6 kV, either positive or negative, is enough for the solution at the needle tip to turn into a Taylor cone throughout jet initiation. Higher voltages result in a more charge. Thus, this will accelerate the jet, and a greater amount of solution will come out from the needle tip. Changing the distance between the needle and the target will directly affect the flight time and electric field strength. The setup is shown in Fig. 2.30 [61]. Electrospinning is the most effective method for producing nanofibers. In this method, the polymer material that is melt or in the form of solution is fed to the needles. Then, high voltage is applied to the polymer that is sent to the needle tip through a pump.

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Fig. 2.30  Multifunctional electrospinning setup [61].

As soon as surface tension disappears, polymer material accelerates from the needles to the collector. At that moment, solvent evaporates rapidly, and fiber becomes longer and thinner due to the increase in speed. Thicknesses of these nanofibers are usually between 60 and 300 nm. The different types of electrodes are shown in Figs. 2.31 and 2.32.

Fig. 2.31  Electrodes in series [61]. (A) Flat electrode, (B) Disk, (C) Cylinder, and (D) Wire.

Fig. 2.32  Electrodes in parallel rows [61]. (A) Parallel nozzles and (B) Parallel cylinders.

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Electric field is the area where there is an electric force caused by the presence of electric charges. When there is a charge between two points, it is meaningful to consider Coulomb's law to account the interaction between them. But when there are more points included in the system, it is expedient to use electric field and potential together. To satisfy the increasing applications of the electrospun nanofibers, a novel method for the high-throughput production of nanofibers using upward cylinder-type electrospinning was studied. Previous experiments showed that a concentration of 10 wt% solution of polyurethane (PU) will be optimum for electrospinning and hence 10 wt% solution of PU is used as the model polymer for electrospinning. So, the performance test of cylindrical-type multinozzle system (Fig. 2.33) is carried out using PU 10 wt% solution, and nanofibers were collected [62]. In the conventional electrospinning setup, after the formation of tailor cone, there is a tendency to form droplets from the electrospinning nozzle tip. This droplet formation depends on many factors such as viscosity of the solution, applied voltage, and flow rate. In many cases, these droplets can seriously affect the morphology of the nanofiber mat. High-quality low-cost mass production of nanofiber mats is essential to support the fast-growing researches in the nanotechnology field. For enhancement in the applicability of nanofibers, various new revolutions in electrospinning were used. The lab-scale electrospinning machine that researchers currently used has a few numbers of nozzles. It takes quite long time to fabricate nanofiber mats. Moreover, the thickness of the mat depends on the spinning time. So, it is necessary to have a fast and convenient system for the production of nanofibers. The mass production of electrospun nanofibers by modified upward cylindrical-type multinozzle system method was explored. The results suggest that the upward cylindrical-type multinozzle electrospinning system can be applied for the mass production of nanofiber at laboratory and industrial site.

Solution A

−

−

+

+

Solution B

High voltage power supply Syringe pump

Fig. 2.33  Multinozzle electrospinning [62].

Grounded collection drum Syringe

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2.10.1 Steady state electrospinning of polycaprolactone PCL is a biocompatible aliphatic polyester with many possible applications in the medical field. PCL nanofibers, produced by electrospinning, could provide new characteristics that are of interest for these applications. However, a key prerequisite is the ability to obtain bead-free fibers with diameters in the nanoscale range. At present, the most commonly used solvent for electrospinning PCL is chloroform, but this only leads to fibers in the microscale range. Therefore, various solvent systems were examined in this study. The innovative solvent mixture formic acid/acetic acid was found to allow for nanofibers with a diameter 10 times smaller than the solvent chloroform. Moreover, steady-state conditions could be obtained that thus allow electrospinning in a stable and reproducible way. Further, it was noticed that the average fiber diameter decreased with decreasing polymer concentration while the diameter distribution decreased with increasing amount of formic acid. Also, the humidity, an often overlooked yet important parameter, was noted to affect both diameter characteristics. Generally, it can be concluded that the solvent system formic acid/acetic acid could fill the gap in electrospinning PCL since it is readily able to produce uniform fibers in the nanoscale range [63]. Based on the outcome of the solvent study, the binary solvent system formic acid/ acetic acid was chosen to study in more detail. As to refine the electrospinning, the steady-state conditions were studied. The polymer concentration was varied between 10 and 20 wt% and the acetic acid concentration between 0 and 90 v%. The applied voltage was adjusted as to obtain steady state, while all other electrospinning parameters were kept constant. These parameters were well chosen as to obtain the maximum steady-state area. This was possible based on the preliminary results during the solvent study. The SEM images of nanofibers are shown in Fig. 2.34.

Fig. 2.34  Nanofibers from polycaprolactone [63]. (A) Nanofibers, (B) Beads, (C) Membrane, (D) Microstructure with beads, and (E) Without beads.

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After the study of the steady-state conditions, the focus is given to the influence of the main electrospinning parameters on the fiber morphology of samples prepared under steady state. These parameters are generally divided in three groups being process, solution, and ambient parameters. The thermal conductivity detector (TCD) and flow rate are the most important process parameters, but the solvent study revealed that steady state was only possible between strict borders of TCD and flow rate. Therefore, it was chosen not to devote further attention to a variation of these parameters but to keep them constant at the already set values. The solution parameters solvent ratio and polymer concentration as well as the ambient parameter relative humidity can vary in a broader range and were thus of interest to study in more detail [63]. The effect of the solvent composition on the fiber morphology of the resulting nonwoven was investigated by varying the acetic acid concentration from 10 to 80 v%, at 14 wt% PCL and an RH of 10%. Although 80 v% of acetic acid is not part of the steady-state region at 10% RH, it does allow electrospinning in a stable way for a limited period of time and can thus be included. With this, it is important to realize that in order to allow stable electrospinning, the applied voltage was not constant during electrospinning this range. All produced nonwovens show uniform bead-free fibers as seen in the SEM images. The average fiber diameter shows only a minor increasing trend from 545 nm at 10 v% acetic acid to 662 nm at 80 v% acetic acid. However, particularly prominent is the increasing standard deviation with increasing amount of acetic acid [64]. The percent deviation rises from 17% at 10 v% acetic acid to 64% at 80 v% acetic acid. As the viscosity is not affected by the solvent composition, these effects must be attributed to the changes in the conductivity with varying acetic acid concentrations. It is known that the average fiber diameter increases with decreasing conductivity. However, the effect is small due to the varying voltages needed for electrospinning the whole range. Also, the broadening of the fiber diameter distribution with a decrease in conductivity is in line with previous investigations on other polymers and is likely caused by the more inhomogeneous charge distribution in solvents with low conductivity such as acetic acid (Fig. 2.35) [63].

Fig. 2.35  Uniform fiber diameter distribution [63]. (A) Variable diameters and (B) Uniform diameter.

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A solvent study with single and binary solvent systems revealed the major potential of the solvent mixture formic acid/acetic acid for electrospinning PCL. Fibers with diameters more than 10 times smaller than the ones obtained with the most commonly applied solvent chloroform could be produced. Moreover, the stability and reproducibility of the produced nonwovens were guaranteed by setting up a steady-state table. The steady-state conditions show that a minimum concentration of 30 v% formic acid and 10 v% acetic acid is necessary for steady-state electrospinning. Further, it was found that the polymer concentration is the dominant factor for the resulting average fiber diameter. The standard deviation of the nanofibers was lowered at high formic acid content and low RH. In conclusion, the solvent system formic acid/acetic acid has proved to be an excellent system for electrospinning PCL under steady-state conditions and can produce fibers in the nanoscale range with a small diameter distribution. This is a major breakthrough compared with the other solvent systems for PCL reported up till now [63,64].

2.10.2 Electro-rheological investigation of PVB for electrospinning The shear viscosity of some materials is subject to the presence or absence of an electric field. This dependence is significantly exhibited by so-called electrorheological materials, for which an increase in shear viscosity under the polyvinyl butyral (PVB) and two pairs of solvents (poor, methanol and ethanol, and good, isopropanol and butanol) are taken into account. The applicability and advantages of PVB in the process of electrospinning are investigated. Unlike good solvents, poor solvents have the potential to contribute to the good electrospinnability of the corresponding PVB solutions. In this case, a combination of the entanglements between chains results in the creation of a physical gel. Presumably, these junctions contribute to the stabilization of the viscoelastic jets. The electrospinning process is affected by a number of the entry parameters. The impact of an external electric field on the rheological properties of the polymer solutions is investigated. It was found that an increase in the complex viscosity ratio ɳE*/ɳ0* (where ɳE* and ɳ0* represent complex viscosities of a solution in the presence and absence of an external electric field, respectively) correlates with good electrospinnability of PVB solutions as visible in SEM images (Fig. 2.36) [65]. This increase is observed for poor solvents, while the complex viscosity ratio is constant for good solvents, for which the quality of nanofibrous web is unacceptable.

2.11 Free surface electrospinning for high throughput manufacturing of core-shell nanofibers While traditional single concentric spinneret coaxial electrospinning is convenient and widely used in research settings, typically low productivity inhibits the application of core-sheath nanofibers to commercial applications. Much attention has been paid to improve the electrospinning productivity, including the multineedle electrospinning and the needleless electrospinning. But all these methods can only produce single

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Fig. 2.36  SEM images of PVB nanofibers [65].

structure nanofibers in large scale. A modified free surface coaxial electrospinning setup was developed (Figs. 2.37 and 2.38) using one-stepped pyramid-shaped copper spinneret that is capable of producing core-sheath fibers in large scale [66]. Fig.  2.38A shows the schematic diagram of free surface coaxial electrospinning from two layers under the influence of electric field in two dimensions. When the electric field is high enough, the electric Maxwell stress on the free surface will overcome the stabilizing curvature surface tension to generate electrohydrodynamic instabilities, which lead to the generation of multiple jets. Under a given electric field, solutions with higher conductivity have higher surface charge density resulting in an increase in the electrostatic pressure. During this free surface coaxial electrospinning process, the lower solution is the driving liquid since the lower solution has higher conductivity and the viscous stress between the core and sheath solutions dragging the upper solution. As shown in Fig. 2.38B, a number of core-sheath Taylor cones were initiated around the curving part of free surface [66–69].

Fig. 2.37  Free surface electrospinning [66]. (A) Setup and (B) Fiber formation.

Fig. 2.38  Taylor cones on free surface electrospinning [66]. (A) Schematic and (B) Taylor cones.

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558 nm 268 nm

1 µm

Fig. 2.39  SEM images of core-sheath nanofibers [66]. (A) Fibrous layers and (B) Single fiber.

Fig. 2.39A shows the scanning electron micrograph. It is found that uniform nanofibers with average fiber diameter of 532 ± 44 nm are fabricated. Fig.  2.39B shows transmission electron micrograph of the core-shell structure. The diameters of the core and shell were approximately 268 and 558 nm, respectively. The core fiber showed a sharp interface with the shell fiber, and a relatively smooth core-shell interface was demonstrated. A novel and efficient free surface coaxial electrospinning setup for high-throughput core-shell nanofibers by utilizing one-stepped pyramid-shaped copper spinneret has been successfully developed. PAN/PU core-sheath fibers were successfully fabricated by this technique. This novel method provides a promising way toward the massive production of core-shell nanofibers.

2.12 Electrospinning of high strength aqueous silk fibroin nanofibers Silk fibroin (SF) derived from Bombyx mori (B. mori) silk cocoon has been used by the researchers worldwide as a promising biomaterial for various applications such as in drug delivery, in wound dressing, and very recently in the development of organ and tissue constructs. The unique properties such as biocompatibility, biodegradability, mechanical strength, and bioactivity made SF as an excellent biomaterial for developing tissue-engineered scaffolds. In tissue engineering, the fabrication of three-dimensional (3-D) porous matrices that mimic the structure and function of body extracellular matrix is a prime goal. In this context, SF nanofibers generated by electrospinning method are more advantageous than the other forms of SF such as film and 3-D porous structures because of their high surface area and high porosity and provide favorable microstructure for cell adhesion, proliferation, and new tissue regeneration. Images of such nanofibers are shown in Fig. 2.40 [67]. Electrospun nanofibers of high mechanical strength and low fiber diameter were produced by devising a novel and rapid technique of concentrating regenerated silk

Electrospun nanofibers77 PEO Nonextracted mats

PEO extracted mats

Native silk fibroin matrices

Day 01

Day 07

Day 14

Fig. 2.40  Nanofibers spun from silk fibroin [67].

fibroin (RSF) solution under mild shearing conditions without the formation of gel. The various parameters that influence the preparation of electrospinnable solution in aqueous solvent were investigated to establish an optimal electrospinning condition for producing aqueous RSF nanofibrous mats. The elucidation of mechanism to enhance the mechanical strength paves the way for further modulation in the sheet content and scaffold development for load-bearing applications. The posttreatment of the obtained nanofibrous mats with a cross-linking agent imparts significant improvement of mechanical and surface properties of nanofibrous mats, which can be further modified for various tissue engineering, drug delivery, and wound dressing applications. The upgradation of concentrating mechanism is expected to produce more heightened results that can offer a convenient solution toward the large-scale production of RSF nanofibers in less time [67].

2.12.1 Electrospinning of PVA/sericin nanofiber The extracellular matrix (ECM) affects the fate and activity of cells and tissues by both biochemical and biophysical factors. The interaction between resident cell and ECM is critical in determining resultant cell activity and fate, such as proliferation, differentiation, and migration. Surface topography of culturing material acts as the “first-step” factor affecting cell seeding and adhesion. Cells sense and transduce the physical and mechanical properties of their microenvironment through the direct interaction between the cell membrane and surface on ECM. With respect to physical

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properties, such as topography, elasticity, gradients, and geometry, cells can sense underlying topographic change and modified its physiological morphology. Thus, cell activity can be regulated as consequent of sensing different extracellular physical factors. Therefore, to understand the cascades of topographic sensing process and its influence on cell fate is critical in biomedical engineering design and health-care research. To investigate the interaction between the nanomaterial topology and cell fate, nanofabrication is the key step as controlling the topographic surface of culturing material as ECM, which regulates cell activity. Nanofabrication method such as soft lithography, electron-beam lithography, and photolithography are wildly used in surface etching to generate nanoscale patterns such as nanogroove, nanopillar, and nanoarray. Besides surface lithography technique as mentioned above, polymer by electrospinning is another conventional and efficient method to fabricate the designed nanostructure, especially for nanofibers. By controlling different parameters such as solution concentration, applied voltage, and spin distance, nanofibers can be equipped with unique properties such as high surface-area-to-volume ratio, high porosity, and designed alignment. The application of silk fibroin is a promising approach for designing biomaterials [68,69]. However, silk sericin (SS) protein has not attracted much attention in the field of biomaterials as a natural biopolymer due to its weak structural properties and high solubility. Research is focused on investigating the spinnability and biocompatibility for PVA/SS nanofibers. For electrospinning, the detrimental factors that control the surface morphology of the nanofibers are the properties of the spinning solution and other processing parameters, such as the applied voltage, distance, and flow rate. Among all the factors, the concentration of the polymer solution is the most important factor in determining the spinnability of nanofibrous mat. PVA and PVA/SS nanofibers as shown in Fig. 2.41 were fabricated with appropriate

Fig. 2.41  PVA-sericin nanofibers [69]. (A) Sericin, (B) Sericin over PVA, (C) Microstructure, (D) Membrane, (E) Microstructure of membrane, and (F) Microstructure of sericin.

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e­ lectrospinning parameters such as the properties of the spinning solution, applied voltage, spin distance, and flow rate. After cross-linking and detoxification, the nanofibers still maintain structural integrity [69]. By investigating the interaction between A549 cell and culturing nanofibrous mat with different topographic pattern and chemical composition, we found the culturing on nanofibrous mat increases A549 epithelial-mesenchymal transition rate with respect to the cell on the normal dish. And this indicates that the topology of nanofibers might increase the metastatic chance for the A549 cancer cells. Silk applications have been expanding beyond textile use. Especially, studies of silk-based materials intended for use as biomaterials used in medical applications have attracted a great deal of attention. This interest is attributable to the biocompatibility and mechanical advantages of silk, supported by its N2500 years of use for surgical sutures. Moreover, silk fibroin (SF) can be fabricated to various forms such as films, gels, resins, and sponges. Recently, electrospun nonwoven mats consisting of fine fibers from tens of nanometers to a few micrometers in diameter have been specifically examined for the development of cell scaffolds in tissue engineering. SF is also used as the spinning solution for electrospinning to produce nonwoven mats. Electrospinning of B. mori SF in a formic acid solvent indicates that concentration is the most important parameter when spinning uniform and cylindrical fibers of b100 nm in diameter by controlling various parameters. SF nanofibers of 50–300 nm diameter were prepared using an SF nanofilament solution in formic acid from b10 wt% concentration. The concentration of silk fibroins in hexafluoroacetone (HFA) affected the fiber diameter in a nonwoven mat formed by electrospinning. One expected application of the electrospun SF nonwoven mats is for cellular scaffolds in regenerative medicine. For medical use, the materials must be safe for cells and living human bodies. Silk's benefits for safety naturally derive from both the material itself and its fabrication processes. Regarding SF scaffolds, SF has been widely reported as safe and biocompatible. Therefore, the security of scaffold safety will depend on the solvents and chemical reagents used for fabrication. To prevent risks from residual solvents in the scaffold, solvents used for electrospinning must not be harmful to cells or the human body. The safest solvent for the human body is water. Therefore, several studies have examined the formation of nonwoven mats by electrospinning from an aqueous SF solution. Electrospun nanofibers were produced from an all-aqueous SF solution. It is reported that fibers of 400–800 nm diameter were electrospun with 28 w/v% solution, but fibers did not form at 17% solution. Such high concentrations are disadvantageous for industrial processing because high concentrated SF aqueous solution transforms into gel easily than low concentration by interactions including hydrophobic interactions and hydrogen bonds. SF fibers with diameters of around 800 nm were observed using this PEO blending technique. However, a complicated process to remove the PEO after spinning is required. It will be difficult to confirm the complete absence of the residual PEO in the nonwoven mat. Furthermore, the influence of the residual PEO on cell adhesion and proliferation was reported [70–73]. Silk fibroin (SF) nonwoven mats were fabricated by electrospinning from an all-aqueous solution at low concentration. SF extracted by degumming using boiling

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water without alkaline reagents maintained a higher range of molecular weight distribution than SF that was degummed with alkaline. The spinning solution pH is important for electrospinning of an SF aqueous solution at low concentration. Results show that pH 10.5 is appropriate. The electrospinnability of SF aqueous solution depends on the solution viscosity rather than the molecular weight of SF and the solution concentration. Mechanical properties of the SF nonwoven mat depend on the molecular weight of SF. The mean tensile stress and strain of the SF nonwoven mat electrospun from the higher-molecular-weight SF were 0.83 ± 0.05 MPa/(g/m2) and 12 ± 5.6% [73].

2.12.2 Selection of solvents for polymer electrospinning The selection of a desirable solvent or solvent system as the carrier of a particular polymer is fundamental for the optimization of electrospinning. Solvent selection is pivotal in determining the critical minimum solution concentration to allow the transition from electrospraying to electrospinning, thereby significantly affecting solution spinnability and the morphology of the electrospun fibers. Solvents are selected to produce binary solvent systems that have solvent parameters close to a good single solvent for electrospinning of the polymer solution. This work shows that solvents of high solubility do not necessarily produce solutions good for electrospinning. Polymethylsilsesquioxane solutions of the same concentration in solvents of partial solubility showed better spinnability than solutions in solvents of high solubility. A methanol-propanol binary solvent system produced electrospun fibers with high surface porosity, showing that high volatility and high vapor pressure difference among solvents mixed can induce phase separation in electrospinning. It is noteworthy that the binary solvent system mixing 2-nitropropane (high solubility) and dimethyl sulfoxide (nonsolvent), neither of which exhibited high volatility, also produced highly porous electrospun fibers. This demonstrates that phase separation can be induced by solubility difference in the electrospun polymer solution [74]. The characterization of the electrospun fiber morphology for single-solvent systems is explored. 2-Ethoxyethanol and cyclohexanone are good solvents for PMSQ. When their solutions at 60% w/w PMSQ concentration were electrospun, beads with “tapered tails” were produced, which demonstrate a transition state from electrospraying to electrospinning. Tetrahydrofuran (THF), acetone, methyl acetate, dimethyl carbonate (DCM), methanol, and ethanol showed partial solubility and produced white solutions of PMSQ. 60% w/w PMSQ solutions of methyl acetate, acetone, and THF produced bead-free smooth electrospun fibers of length 50–1000 mm and average diameters 3.3, 2.0, and 3.5 mm, respectively. Solutions in DCM showed high electrospinning productivity, but beading was present in the as-spun fibers. Average fiber diameter was 2.6 mm. Solutions in methanol and ethanol produced shorter fibers of length 1–500 mm and smaller average diameters of 0.40 and 0.83 mm, respectively. Electrospun fibers from binary solvent system mixing MeOH (high vapor pressure) and PrOH (moderate vapor pressure) showed high porosity. On close inspection, porous fibers spun from the MeOH/PrOH solution appeared to be solid, and porosity occurred only on the fiber surface (Fig. 2.42) [74]. It has been reported that solvents of high volatility and vapor pressure cause phase separation and surface ­porosity in

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Fig. 2.42  Porous nanofibers by electrospinning [74].

e­ lectrospun fibers, and the above result agrees with the literature. During electrospinning, the electrified solution jet accelerates toward the grounded substrate and elongates rapidly. The surface area of the jet is dramatically increased during this process, and this leads to an increased rate of solvent evaporation. The evaporative cooling during the loss of solvent leads to thermodynamic instability, which results in phase separation within the electrospun solution and the as-spun fiber phase into polymer-rich and solvent-rich phases. Electrospinning of the solution showed good spinnability, and the electrospun ­fibers showed high porosity with a ridged or a rough fiber surface (Fig. 2.43). This is noteworthy because neither 2NP nor DMSO exhibited high volatility, contrary to the highly volatile solvents often included in mixed solvent systems that have produced porous fibers. Furthermore, unlike the solid porous fibers with only surface porosity spun from the MeOH/PrOH solution, the fibers spun from the 2NP/DMSO solution showed large cavities inside the beads on the fibers (Fig. 2.43B). Electrospun fibers with no bead-on-string defects or very few beads were produced using solutions in partial solvents, whereas solvents with high solubility for PMSQ demonstrated electrospraying or transition state from electrospraying to electrospinning. It is suggested that lower solubility can be better suited for making good electrospinnable solutions than solvents of high solubility. High solvent dielectric constant showed a significant effect in reducing electrospun fiber diameter. Solution in binary solvent system mixing MeOH and PrOH produced electrospun fibers with surface porosity, supporting the theory that phase separation can be induced by high vapor pressure of at least one solvent component. Porous fibers were spun from solution

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Fig. 2.43  Surface morphology of porous electrospun nanofibers [74]. (A) Membrane and (B) Porous individual fiber.

in binary solvent system mixing 2NP, a solvent of high solubility, and nonsolvent DMSO, in which both solvent components have low volatility. This demonstrates that phase separation in electrospinning can occur even if none of the solvent components in the solvent system exhibits high vapor pressure. Solubility difference and high relative humidity can contribute to pore formation in electrospun fibers. A comparison between our empirical results and the existing extensional rheology and flow kinematics of the solution jet should be further explored.

2.13 Coaxial electrospinning Coaxial electrospinning is a robust technique for one-step encapsulation of fragile, water-soluble bioactive agents, including growth factors, DNA, and even living organisms, into core-shell nanofibers as shown in Fig. 2.44. The coaxial electrospinning process eliminates the damaging effects due to direct contact of the agents with organic solvents or harsh conditions during emulsification. The shell layer serves as a barrier to prevent the premature release of the water-soluble core contents. By varying

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Fig. 2.44  Coaxial electrospinning [77].

the structure and composition of the nanofibers, it is possible to precisely modulate the release of the encapsulated agents. Promising work has been done with coaxially electrospun nonwoven mats integrated with bioactive agents for use in tissue engineering, in local delivery, in wound healing, etc. The primary motivation to make use of coaxial electrospinning in controlled release is to circumvent the limitations of single-nozzle electrospinning in the encapsulation of fragile, water-soluble bioactive agents that play vital roles in regenerative medicine. Other advantages of coaxial electrospinning over single-nozzle electrospinning include more sustained release of the encapsulated agents and one-step coencapsulation of multiple drugs with different solubility characteristics [75]. The coaxial electrospinning process, including high voltage applied, shearing force imposed at the interface between core and shell fluids, and rapid protein dehydration, might still be harmful for the stability of fragile bioactive agents. Some reports did show the loss of bioactivity of the enzymes subjected to coaxial electrospinning. To avoid possible protein denaturation induced by the electrospinning process, a strategy was developed to immobilize growth factors onto fibrous nonwoven mats by combining coaxial electrospinning with heparin-affinity-based growth factor delivery system. The effect of coaxial electrospinning process on the viability of cells, using liquid poly(dimethylsiloxane) (PDMS) as shell fluid, was studied. Fig. 2.45 shows the compound jet just below Taylor cone, indicating successful encapsulation of the cells. No apparent loss of cell viability was found for two types of cells, primary porcine vascular and rabbit aorta smooth muscle cells, through long-term flow cytometry analysis [76].

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Fig. 2.45  Compound jet Taylor cone [81]. (A) Formation of Taylor cone and (B) Formation of nanofiber from Taylor cone.

Researchers encapsulated bifidobacteria into core-shell fibers with poly(vinyl alcohol) (PVA) as the shell material and bifidobacterium-containing milk constituting the core content and evaluated the protective effect of the core-shell fibers on the bacteria under different storage conditions. It was found that the encapsulation process had a beneficial effect on their storage stability instead of damaging the bacteria. Coaxial electrospinning was used to encapsulate three different species of bacteria into core-shell fibers for use in bioremediation of pollutants in water systems. In contrast with previous relevant studies where the cells were instantly deencapsulated as the membranes contacted with aqueous media so that cell viability could be accurately determined by plate counting, this report used PCL as the shell material to retain the bacteria within the fibers for a certain period of time. This raises practical applicability of such biosystem. PEG was added in the shell layer as a porogen to facilitate transport of small molecules through shell barrier. Efficacy of the bacteria-loaded system was assessed by analyzing the activity of cell membrane enzymes, while bacterial viability was indirectly evaluated by cell respiration and their ability to synthesize proteins. It was found that only partial bacteria survived from coaxial electrospinning process [77]. Coaxial electrospinning has been extensively explored for the encapsulation and controlled release of growth factors. The limited number of growth factors approved for clinical use, however, restricts the commercialization of growth-factor-loaded coaxial nonwoven mats. Furthermore, the combination of multiple growth factors in their biologically determined ratio is always necessary to mimic their in vivo environment where one growth factor only works as a part of growth factor network. In such context, natural source of growth factors, such as platelet-rich plasma (PRP) that contains several different growth factors and other cytokines, has gained popularity in academic and clinical settings over the past years. The integration of PRP with electrospun nanofibers has been achieved by either single-nozzle electrospinning of activated PRP or encapsulation of platelet α-granules into coaxial fibers. The resultant nonwoven mats could deliver multiple growth factors in a controlled manner over a long period of time and promote proliferation of the attached cells, showing great prospect for regenerative medicine [78–80].

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It is still not very clear how the solution pairs of inner/outer fluids affect electrospinning process and fiber structure. Contradictory results on this issue can be found in previous studies. The formation of core-shell fibers even when the same polymer solutions were used as core and shell fluids was reported; it was found that the immiscibility of inner/outer solutions was essential for preventing the mixing of the two fluids during electrospinning process. Clarification of this question is important because appropriate selection of solution pairs is critical not only for the formation of the expected core-shell structure but also for the stability of fragile bioactive agents. The mixing or partial mixing of inner/outer dopes could cause direct contacting of bioactive agents with organic solvents and the subsequent denaturation. In addition, solidification process of the compound jet, especially the dehydration of inner aqueous protein solution and effect of outer fluid and ambient conditions on water evaporation, remained unexplored. Since proteins are subjected to reversible/irreversible arrangement in secondary structure during the dehydration process, elucidation of the aforementioned question can help in the formulation of an aqueous protein solution with enhanced protein stability to be used as the core fluid of coaxial electrospinning [81]. The development of new techniques can promote the standardization of coaxial electrospinning setup. A prototype coaxial microfluidic device was constructed through soft lithography technique. Two layers of nonintersecting, stacked microchannels arranged in a branching-tree pattern are created within the device to provide constant flow of core/shell fluids to each of the eight outlet spinnerets. The coaxial spinneret that consisted of concentric stainless steel tubes were aligned and punched through the elastomer device. The advantages of the microfluidic device include simple molding methodology for the elastomeric device fabrication, flexible control over channel dimensions and geometry, and parallel coaxial electrospinning. In order to avoid the need for concentric spinneret, hydrodynamic focusing method was adopted to generate a coaxial stream of two immiscible fluids within a microfluidic device. Electrospinning of the coaxial stream resulted in the nanofibers with core-shell structure. Besides the controlled release of bioactive agents, this technique could be very useful for the encapsulation of living organisms into core-shell fibers to eliminate their possible agglomeration and evenly disperse the organisms in the fibers by taking advantages of the sorting effect of hydrodynamic focusing that has been successfully applied for flow cytometry. The introduction of the coaxial electrospinning to the field of controlled release provides a powerful tool for the encapsulation of fragile, water-soluble bioactive agents. This can include growth factors and DNA that play important roles in regenerative medicine. The method also provides fine control over the release rate of the agents. Coaxial electrospinning furnishes another tool that can be used to integrate the method of controlled release into tissue engineering and other biomedical applications. Although significant progress has been made, the true potential of coaxial electrospinning has yet to be realized. Recent applications of coaxial electrospinning for the encapsulation of cells, viruses, bacteria, and platelet α-granules show promise in biofabrication. However, a fundamental understanding of the coaxial electrospinning process, including the evolution of compound jets, solvent evaporation, and the discharge of the resultant core-shell fibers, has yet to be achieved. This is critical for the standardization of the setup and to achieve a stable operation of the

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process, which in turn is necessary for the formation of defect-free core-shell nanofibers and adequate stabilization of the agents. The introduction of new techniques and equipment, such as the microfluidic devices, also shows promise in the standardization of the technique [82].

2.13.1 Coaxial electrospinning multicomponent functional controlled-release vascular graft Fig. 2.46A shows the TEM micrographs of the core-shell structure of heparin-loaded collagen/chitosan/poly(ε-caprolactone-co-lactide) (PLCL) fiber (taking 40:10:50%– 15%, e.g., including TEM and SEM). The shell and the core in the image showing a clear interface indicated that heparin was encapsulated well into the fiber. As shown in Fig. 2.46B and C, the heparin-loaded collagen/chitosan/PLCL fibrous graft with a length of 7 cm, an inner diameter of 2.5 mm, and a wall thickness of 400 μm could be well fabricated, and these characters could match the human blood vessels very well. To further demonstrate the encapsulation of heparin into the tubular graft, a cross section of graft was made. After that, the graft was soaked into distilled water for 10 min and then dried in the fume hood for SEM test. Fig. 2.46D–G depicted the fiber cross

Fig. 2.46  Multifunctional vascular grafts by coaxial electrospinning [83]. (A) Coaxial electrospun fiber, (B) Dimensions, (C) Internal structure, (D) Core, (E) Magnified core, (F) After removal, and (G) Magnified core after removal.

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section, which obviously showed some fiber sections with holes, caused by the heparin diffused into water and formed the hollow fiber (showed in red arrow sections). In the meanwhile, the diameters of the fibers are different; the heparin diffusing time from the fiber inside to outside is different under normal condition; therefore, with larger fiber diameter, heparin diffused from the inside of the fiber to outer side needs more time and would get slower heparin releasing. As a result, it was speculated that the drug-release rate could be controlled via the different ratios of collagen, chitosan, and PLCL, which mainly lead to different fiber diameters [83]. SEM micrographs of cell morphology and the interaction between cells and grafts were shown in Fig. 2.47. After 3 days, porcine hip artery endothelial cells (PIECs) were more easily spread to develop an endothelial cell layer on the surface of 40:10:50%– 5%, 40:10:50%–15%, and 40:10:50%–30% grafts compared with others. Such results

Fig. 2.47  SEM micrographs of cell morphology [83]. (A) Cell growth, (B) Cell accumulation, (C) Functional tissues, (D) Coverage of scaffold, (E) Bulk absorption, and (F) Growth of tissues.

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further demonstrate that heparin in a proper concentration could enhance PIECs to grow, while very high heparin concentration could partially hinder cell proliferation. Fibrous grafts with heparin loading were successfully fabricated via coaxial electrospinning, and the heparin encapsulation efficiency was higher than 70% except 60:15:25%–15%. Moreover, the heparin could sustain release for more than 45 days. Especially for the 40:10:50%–15% graft, it got high drug-load amount (7.9 mg), high encapsulation efficiency (77%), and very stable sustained release rate and release amount, and it had low initial burst release (23%), and more importantly, it still had good performance of antiplatelet adhesion after heparin releasing for 3 weeks. In addition, it possessed excellent cell biocompatibility and suitable mechanical properties including tensile strength, suture retention strength, burst pressure, and compliance, which could match the native blood vessels. Thus, through controlling the parameter of fabrication process, this kind of graft might be a promising candidate for vascular tissue application.

2.14 Corona-electrospinning A novel spinneret and modified electrospinning method is introduced wherewith nanofibers can be produced with high throughput. The main conception of the system is to continuously supply the polymeric solution through a narrow but long gutter bounded by a metal electrode having sharp edge. As there is no high free liquid surface, volatile and low-boiling-point solvents can be applied that makes the method suitable for pharmaceutical and biomedical applications. In this study, the operation of the spinneret was tested with polyacrylonitrile/dimethylformamide and polyvinylpyrrolidone/­ ethanol solutions. The charge concentration—related from the construction—was investigated by finite element analysis. The highest electric charge density is formed along the sharp edge that results in many self-assembled Taylor cones that is also confirmed by the first operation experiences. The productivity of the technique can be two orders of magnitude higher than that of the single-capillary method [84]. The Taylor cone formation during the process can be seen in Fig. 2.48 for both the small, rapid prototyping made (d1 = 42 mm) and the bigger aluminum (d2 = 110 mm)

Fig. 2.48  Corona electrospinning [84]. (A) Setup and (B) Fiber formation.

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prototype spinnerets. Even the distribution of the forming nanofibers can be observed along the edge of the charged electrode. The corona electrospinning resulted in higher throughput compared with that of the single-needle setup. With the smaller, 42 mm-­ diameter electrode, the feed rate could reach 120 mL/h without overflowing, while at single-needle setup, only 8 mL/h could be reached. With the bigger (110 mm) setup, 300 mL/h could be set where intense ventilation was necessary to remove the evaporating solvent from the electrospinning space. Corona electrospinning requires higher voltage, as fiber formation began at around 30 kV, while with the single-needle setup, the initial voltage was around 15 kV. The average fiber diameter was 550 nm at corona electrospinning (spun at 55 kV) and 530 nm at single-needle electrospinning (at 25 kV). However, higher field strength is formed in the case of corona electrospinning; the small difference is formed that can be originated from the different solvent evaporation conditions (i.e., earlier solidification of the fibers) caused by ventilation. The novel approach makes possible to produce nanofibers by high throughput with a simple construction spinneret. The method is efficient as the applied electrode is a sharp edge that concentrates charges exactly at the location where Taylor cones are formed that was confirmed by FEA simulations. As there is no high free liquid surface, volatile and low-boiling-point solvents can be applied. The operation of the new spinneret and the method was demonstrated at PAN dissolved in N,N-dimethylformamide (DMF) and at PVP dissolved in ethanol. The diameter and morphology of the resulted nanofibers are close to those that were processed by the classical setup. The rotation of the spinneret made possible to avoid the overflow of the electrospinning solution and led to higher flow rates. A small-size prototype made possible a 20–50 times increase in productivity compared with the single-capillary method. The density of forming Taylor cones is in the magnitude of 100–200/m depending on the material, construction type, and spinneret size; however, detailed description needs further research. In the future, it is planned to determine the optimal distance between the round-shaped electrode and the lid and the optimal construction of the spinneret in order to further develop and exploit the promising preliminary results.

2.15 Miscellaneous electrospinning methods 2.15.1 Polycaprolactone mesh by emulsion electrospinning Use of PCL as 3-D porous scaffold, fibers, and matrices has proved importance of this polymer in applications for tissue engineering besides others. An approach to generate uneven surfaced meshes of PCL via emulsion electrospinning was explored with minimal use of organic solvent. Poly(vinyl alcohol) (PVA) was used as template polymer providing stability and alignment to PCL phase during electrospinning of oil-in-water emulsion of PCL. The emulsion properties including particle size, interparticle distance, and viscosity depended on the concentrations of PCL and PVA. Higher PVA content led to the formation of smaller oil phase particles resulting into higher viscosity of the emulsion, while a higher PCL content led to the formation of larger oil phase particles and correspondingly lower viscosity of the emulsion.

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A correlation between particle size of emulsion and diameter of the fibers obtained after electrospinning was found. The composite meshes of PCL-PVA obtained via emulsion electrospinning were washed with water to generate uneven surface on the meshes that was found to be highly favorable for cell growth in comparison with a uniform mesh of PCL made via solution electrospinning. Uneven surfaced mesh of PCL was generated via emulsion electrospinning with minimal use of organic solvents. A stable oil-in-water emulsion of PCL (dissolved in toluene forming the oil phase) was prepared with minimum amount of a water-soluble template polymer, PVA. PVA acted as a support to align PCL containing oil phase particles along the direction of electrospinning and also as stabilizer for the emulsion. The particle diameter in PCL-PVA emulsion depended upon concentrations of PVA and PCL. An increase in PVA concentration resulted into the formation of smaller oil particles, and thus, the viscosity of the emulsion increased due to higher interparticle interactions. On the other hand, an increase in PCL concentration in oil phase led to the formation of larger particles causing the viscosity of emulsion to decrease. The relative viscosity of the emulsion also decreased with increase in PCL concentration at shear rate varying from 0.001 to 1000 s−1. The composite mesh of PCL-PVA obtained via emulsion electrospinning was washed with water to remove PVA and to generate uneven surfaced fibers that turned out to be highly beneficial for cell attachment and growth. The added advantages of emulsion electrospinning and generation of conductive mesh of PCL can be extended for several applications in tissue engineering [85].

2.15.2 Conductive pani fibers Polyaniline doped with camphorsulfonic acid (CSA)/PEO conductive nanofibers was produced by electrospinning. The electrospinning window was determined by using a three-level, full factorial experimental design. The combined effects of the humidity, voltage, and flow rate on the fiber morphology and diameter were examined demonstrating that the ambient humidity is the critical factor affecting the electrospinning process and determining the electrospinning window for a conductive polymer. Low humidity favors the formation of defect-free fibers, while high humidity either hinders fiber formation or causes the formation of defects on the fibers either due to jet discharge or due to water absorption and phase separation. High level of doping with CSA led to the formation of crystalline structures. Data fitting was used to explore the behavior of conductive polymers in electrospinning, and very good agreement between experimental and theoretical predictions was obtained for only a limited range of experimental conditions, whereas deviation was observed for all other sets of conditions. In terms of flow rate, the field emission gun-scanning electron microscopy (FEG-SEM) images obtained (Fig.  2.49) show a trend that was expected, based on the literature. Higher flow rates generally resulted in higher productivity, and in some cases, the nanofibers were not completely dried when they reached the collector. This was observed for different voltages and different levels of relative humidity and could be attributed to the fact that longer time would be required for the solvent to fully evaporate at higher flow rates and constant tip-to-collector distance. However, when the highest value of voltage was applied, the process became more stable at high flow

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Fig. 2.49  Effect of flow rate on nanofibers [86]. (A) 3 mL/h, 13.5 kV, 18% RH, (B) 2 mL/h, 13.5 kV, 18% RH, (C) 1 mL/h, 13.5 kV, 18% RH, (D) 3 mL/h, 9.2 kV, 25% RH, (E) 2 mL/h, 9.2 kV, 25% RH, and (F) 1 mL/h, 9.2 kV, 25% RH.

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rates. Low flow rates resulted in the formation of some very fine fibers, as shown in Fig.  2.49C, probably due to the high surface charge density that may have caused splitting of the jet [86]. Humidity was found to play a crucial role in electrospinnability and mat morphology. As it is shown in the SEM images in Fig. 2.50, only low ambient humidity allows the formation of defect-free nanofibers throughout the whole range of applicable voltages. When the humidity increases to 25%, the fibers start to break, and irregular and uneven surfaces are formed, whereas electrospinning was not feasible at relative humidity higher than 40%. The roughness of the surface occurring at higher humidity (Fig. 2.50A) could be explained by phase separation that potentially occurred during the jet stretching caused by the absorption of water vapor by PEO. The electrospun solution consists of PEO that is a highly hydrophilic polymer, polyaniline that is insoluble in water, and chloroform that is immiscible with water; therefore, water absorption by the PEO could result in phase separation, precipitation of PANI, and hence uneven and rough nanofiber surface. As a general trend, it seems that higher voltages produce not only thinner nanofibers but also a bigger diversity in diameters and a broader diameter distribution that will be discussed in the next section. Especially at 13.5 kV, for all flow rates, there was a significant percentage of very thin nanofibers within the mat, as shown in Fig. 2.51, indicating splitting of the jet. It has been reported that the jet may undergo splitting into multiple subjets in a process known as splaying or branching. The solution containing PANI doped at a greater extent (100%) produced structures of very high crystallinity and orientation but not nanofibrous mats. The experimental design that was used covers low, medium, and high values of flow rate, voltage, and environmental humidity, so it can safely be concluded that the PANI (100% doped) of PEO solution 1.8% w/v is not electrospinnable. Typical samples of the mats produced are shown in Fig. 2.52, and they reveal a crystalline structure of the produced mats. The amount of CSA that was added in order to increase the doping level seemed to cause the formation of crystalline structures during spinning that had a certain orientation as shown in Fig. 2.52 [86]. Humidity was shown to be the most important parameter affecting electrospinning and defining the electrospinning window for PANI solutions. Only at very low humidity was electrospinning feasible, indicating that for conductive polymers, the effect of humidity may be significantly greater compared with that of nonconductive polymers. The importance of environmental humidity on electrospinning of a conductive polymer in an organic solvent is noteworthy. Higher ambient humidity disrupted the electrospinning process and resulted in irregular and rough fiber surfaces. Flow rate and strength of electric field were found to have an impact on the final nanofiber diameter [86]. Higher values of applied voltage resulted in thinner nanofibers at all flow rates. But when high voltage was combined with low flow rate, the phenomenon of branching of the jet was observed, resulting in broader diameter distribution. The impact of the flow rate was found to be dependent on the applied voltage. At high applied voltage, the nanofiber diameter decreases with the increase of flow rate, while at low voltage, the opposite trend is observed. At medium values of applied voltage, the effect of the

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Fig. 2.50  Effect of humidity on nanofibers [86]. (A) 1 mL/h, 5 kV, 32% RH, (B) 1 mL/h, 5 kV, 25% RH, (C) 1 mL/h, 5 kV, 18% RH, (D) 3 mL/h, 13.5 kV, 32% RH, (E) 3 mL/h, 13.5 kV, 25% RH, and (F) 3 mL/h, 13.5 kV, 18% RH.

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Fig. 2.51  Effect of voltage on nanofibers [86]. (A) 1 mL/h, 18% RH, 13.5 kV, (B) 1 mL/h, 18% RH, 9.2 kV, (C) 1 mL/h, 18% RH, 5 kV, (D) 2 mL/h, 18% RH, 13.5 kV, (E) 2 mL/h, 18% RH, 9.2 kV, and (F) 2 mL/h, 18% RH, 5 kV.

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Fig. 2.52  Effect of flow rate on the orientation of nanofibers [86]. (A) 1 mL/h, 9.2 kV, 18% RH, (B) 3 mL/h, 9.2 kV, 18% RH, and (C) 3 mL/h, 13.5 kV, 18% RH.

flow rate is counterbalanced by the increased voltage. Low doping level rendered the polyaniline insoluble in chloroform, and a high one caused the doping acid to crystallize and totally alter the morphology of the final mat.

2.15.3 Hierarchical structures by electrospinning or electrospraying To expand the application of electrospun fibers or electrosprayed beads, micro-/ nanohierarchical structures of polystyrene (PS) have been constructed through the adjustment of solvent, polymer concentration, environment humidity, electrospinning temperature, etc. Primary structures, such as fibers, beads, and bead-on-string structure, and secondary structures, such as nanopores, nanopapilla, and network structure, have been constructed. Solvent plays an important role in the construction of both primary structures and secondary structures. By using N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and mixed solvent of DMF/THF, the micro-/nanohierarchical structures (Fig. 2.53) can be controlled. Humidity is a key factor to the construction of secondary structures. The obtained fibers or beads have smooth surface at low humidity. While at high humidity, secondary structures tend to appear. For the PS/DMF

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Fig. 2.53  Micro-/nanohierarchical structures from DMF/THF [88]. (A) Microstructures by electrospraying, (B) Optimized microstructures, (C) Micro-nanocombined structures, (D) Micro-nano fibers, (E) Higher magnification of microstructures, (F) Microcapsules on nanofibers, (G) Uniform nanofibers, and (H) Uniform microfibers.

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s­ ystem, vapor-induced phase separation may be the most pertinent mechanism to explain the formation of secondary structures. While for the PS/THF system, breath-­ figure theory can explain the formation of uniform nanopores properly [87]. When solvent with low boiling point was applied, nanopores could be constructed at the surface of electrospun fibers or electrosprayed beads above certain humidity. The pore size distribution was in the range of tens to hundreds of nanometers. Several factors were found to be important to influence the pore size and distribution. Fig. 2.54 shows the SEM images of the enlarged fiber or bead surfaces prepared under different humidity, temperature, and polymer concentrations. The scale bar is the same and marked at the edge of Fig.  2.54I. Environment humidity played an important role in the formation of nanopores, as shown in Fig.  2.54A–C. The polymer concentration was fixed at 20% (w/v), and the electrospinning process was carried out at 25°C. Fig. 2.54A–C was the surface morphologies of samples produced under relative humidity of 10%, 30%, and 50%, respectively. The surface of the produced fibers or beads was smooth when the humidity was low, as shown in Fig.  2.54A. When the humidity increased to 30% and 50%, uniform nanopores were observed, as shown in Fig. 2.54B and C. An increase in humidity brought the appearance of nanopores at the surface of electrospun fibers or electrosprayed beads, which indicated that water vapor around the electrospun jets was the key factor for the construction of nanopores.

Fig. 2.54  Nanofiber surfaces under different humidities [88]. (A) 10% RH, (B) 30% RH, (C) 50% RH, (D) 25 degree centigrade, (E) 30 degree centigrade, (F) 35 degree centigrade, (G) 10% concentration, (H) 15% concentration, and (I) 20% concentration.

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Fig. 2.55  Beads from electrospraying [88].(A) DMF/THF=25:75, (B) DMF/THF=15:85, (C) DMF/THF=05:95, (D) DMF/THF=00:100, (E) Higher magnification of A, (F) Higher magnification of B, (G) Higher magnification of C, (H) Higher magnification of D, (I) Fibers, (J) Bowls, and (K) Porous membrane.

The surface morphologies of the electrospun fibers or electrosprayed beads were examined to study the influence of the solvent, as shown in Fig. 2.55. The samples prepared from PS solutions with the DMF/THF volume ratio of 25:75, 15:85, 5:95, and 0:100 correspond to Fig. 2.55A–C, D–F, G–I, J and K, respectively. Fig. 2.55B, E, H, and K was enlarged images of electrosprayed beads, and Fig. 2.55C, F, and I was enlarged images of electrospun fibers. When the DMF/THF volume ratio was varied from 25:75 to 0:100, fiber content decreased with the increase of THF content. Bead-on-string structure was produced when DMF/THF volume ratio was 25:75. Bowl-like beads were produced when DMF/THF volume ratio was 0:100. The surface morphology of electrospun fibers or electrosprayed beads was also influenced

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by the DMF/THF volume ratio. When DMF/THF was 25:75, the fibers and beads had a smooth surface. With the increase of THF content, pores occurred on the surface of the electrospun fibers or electrosprayed beads. When DMF/THF was 15:85, small amounts of pores were observed. When DMF/THF was 5:95, uniform pores on the surface of electrosprayed beads and stretched pores on the surface of electrospun fibers were observed. Bowl-like beads with densely arrayed pores were produced when pure solvent of THF was used [88]. The influence of humidity to the secondary structures was studied when DMF was used as the solvent in the electrospinning of PS. Keeping other conditions constant, 5% PS/DMF solution was electrospun at different humidities. When the humidity was 20%, the beads with the size of 0.5–2 mm had smooth surface. When the humidity increased to 40%, the size of the beads increased to about 5 mm. The surface of the beads became rough with nanopapilla of about 50 nm distributed on their surfaces. When using DMF as the solvent, smooth surface was observed when the environment humidity was low, while the bicontinuous pore structure occurred when the humidity reached a certain value. The vapor-induced phase separation may be the most pertinent mechanism to explain this formation of secondary structures. In this case, water vapor was the nonsolvent that had long time (due to the slow evaporation of DMF) to diffuse into the streaming jet and induced the phase separation process [88]. The initially homogeneous polymer solution became thermodynamically unstable as water vapor penetrating the fiber during the spinning process with the increase of environment humidity, which led to phase separation of the polymer solution. Besides the influence of humidity, polymer concentration was also considered in order to compare the morphology difference of secondary structure between electrospun fibers and electrosprayed beads. PS/DMF solutions with the concentration of 5% and 25% were used to produce PS beads and fibers, respectively, at the humidity of about 60%. Sample prepared from 5% PS/DMF solution was composed of beads and small amount of fibers. The beads had rough surface with nanopapilla of about 50–100 nm distributed on the surface. The polymer/solvent system phase is separated into a continuous polymer-rich phase and dispersed polymer-lean droplets. As DMF was a solvent with low evaporation rate, the fluid jet solidification rate was slow, and the phase separation process continued until all solvents were dried out and the fibers or beads were solidified. The polymer-lean droplets were developed and finally formed bicontinuous structure. For the bead case, structures formed during phase separation process were left behind after all solvents were dried out, and nanopapilla structure was observed on the bead surface. But for the fiber case, a polymer skin formed, and the porous structure is only observed inside the fibers. This was probably caused by the combined effects of the surface tension of the polymer solution and the elongation (or stretching) process of the charged liquid jets during electrospinning process [89]. Micro-/nanohierarchical structures of polystyrene have been obtained by electrospinning or electrospraying. Primary structures include fibers, beads, and bead-onstring structure; secondary structures include nanopores, nanopapilla, and network structure. Solvents and polymer concentration are the main factors that influence the primary structures. An increase of polymer concentration causes a morphology

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change from beads to fibers. Spherical beads and rounded fibers were produced when using DMF as the solvent, while bowl-like beads and ribbonlike fibers were formed when using THF solvent. Secondary structures such as nanopores or nanopapilla on the surface of beads, nanopores on surface of the fibers, and network structure inside the fibers have been obtained through the adjustment of solvents, environmental humidity, electrospinning temperature, polymer concentration, etc. The obtained fibers or beads have smooth surface at low humidity, while at high humidity, secondary structure tends to appear. Different solvents produced different secondary structures. When DMF was used, nanopapilla on the surface of electrosprayed beads and network structure inside electrospun fibers were formed. When THF was used, nanopores formed on the surface of both electrospun fibers and electrosprayed beads. For the PS/ DMF system, vapor-induced phase separation may be the most pertinent mechanism to explain the formation of secondary structures, while for PS/THF system, breath-­ figure theory can explain the formation of nanopores properly. By the construction of micro-/nanohierarchical structures, the application of electrospinning can be greatly expanded to the field of purification, superhydrophobic surfaces, catalyst carriers, drug delivery, sensors, etc. [89].

2.15.4 Mesoporous alumina nanofibers by electrospinning Mesoporous alumina nanofibers were obtained by a combined method including three steps, such as the sol-gel process, electrospinning, and calcination. Dendrimer polyamidoamine was employed as the structure-directing agent to form the mesoporous structures. The electrospinning process was applied to providing the alumina with a fibrous morphology and a flexible property, which were fixed during calcination. Products with different crystal structures and physicochemical properties were obtained at different calcination temperatures. The typical mesoporous alumina nanofibers showed a surface area of 417.7 m2/g according to the Brunauer-Emmett-Teller method and a total pore volume of 0.40 cm3/g on the basis of the Barrett-Joyner-Halenda model when the calcination temperature was set at 450°C. The possible formation mechanism of the mesoporous structures was analyzed. The typical product was applied to the adsorption of methyl orange from aqueous solutions. The influence of system pH on adsorption, the adsorption isotherm feature, the kinetics, and the reuse performance were investigated [90]. The PVP/alumina nanofibers and the typical product calcined at 450°C were characterized by SEM. The precursor nanofibers have a concentrated diameter distribution range 500–650 nm. The product shows favorable continuity and flexibility due to good spinnability of PVP in polar solvents. After calcined at 450°C for 6 h, the product has no obvious change on its continuity and flexibility, while the most probable fiber diameter distribution range decreases to 100–200 nm because of the removal of polymer coatings. Mesoporous alumina nanofibers have been successfully prepared via the combination of sol-gel, electrospinning, and calcination. The employment of dendrimer polyamidoamine generation 1 (PAMAM-G1) is crucial to the formation of products' mesoporous structures. The typical product calcined at 450°C is provided with a BET surface area of 417.7 m2/g and a total pore volume of 0.40 cm3/g, which

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lead to its desirable performance in methyl orange dye adsorption. Exhibiting a saturated adsorption capacity of 351.3 mg/g against MO, the typical product is regarded as a favorable adsorbent for dye separation. Weak acidic or neutral systems are favorable for the mesoporous alumina nanofibers to reach the highest efficiency in MO adsorption. The typical product also has a very stable structure, which leads to its good reuse performances. The adsorption processes fit the Langmuir isotherm well, and the adsorption kinetic model is identified as the pseudo-second-order equation. In general, the mesoporous alumina nanofibers templated by dendrimer PAMAM-G1 show great potential in such areas as dye separation and environmental purification. Nanofiber is one-dimensional material that has broad application. It can be formed by applying a high-voltage source to a polymer solution so that the polymer solution becomes charged. Using the high electric field, the charged polymer solution is formed as a Taylor cone and then drawn to the collector to form long, nanoscale fibers. This method is known as electrospinning. There are two types of electrospinning method; they are needle and needleless electrospinning. The latter is intended for mass production nanofibers because it can make tens to hundreds of jets at a time. Therefore, the high-voltage source required for the needleless electrospinning process must have a higher voltage and current compared with those for the needle one. Accordingly, the high-voltage power supply using a series-configuration Mazzilli zero voltage switching (ZVS) flyback converter was designed and developed. The Mazzilli flyback converter was able to generate a high voltage with relatively high power. Two converters were connected in series to achieve more output voltage. The output voltage was adjusted by changing the input voltage. The single converter could generate a high voltage up to 34 kV, whereas the series-configuration converter could increase the voltage by 98.41% to be 67 kV. The output voltage of converter was relatively stable and good enough to perform nanofiber synthesis using the needleless electrospinning. Visual observation confirmed that the nanofibers were formed well on the collector [91,92]. The designed HVPS was tested on the needleless electrospinning apparatus with polyvinyl acetate (PVAc) solution. The jet formation on the wire was recorded by a camera with 40 times magnification, and the collected electrospun fibers were captured by a microscope. The jet formation was initiated at 30 kV. The higher voltage applied to the apparatus would increase the number of jets that was formed along the wire. Furthermore, the fibers were formed well on the collector. Therefore, the series-connected Mazzilli ZVS flyback converter can be used as a HV source for the needleless electrospinning application. The high-voltage power supply (HVPS) using a series-connected Mazzilli ZVS flyback converter for needleless electrospinning application has been developed. The single Mazzilli converter could generate HV up to 34 kV, and the series-connected configuration could increase HV by 98.41% to be 67 kV. The performance test showed that the converter has good stability over time. The designed HVPS was tested to run needleless electrospinning process with a straight wire as the spinneret. The evaluation on the needleless electrospinning demonstrated that the HVPS is able to form polymer jets on the wire that was initiated at 30 kV. The microscope image of the electrospun fibers has showed that the nanofibers were formed well on the collector.

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2.15.5 Novel electrospinning setup for biomimetic scaffolds The tissue engineering field has provided great efforts toward the development of potential treatments for meniscal injuries. The success of these strategies is linked to the creation of scaffolds that are able to mimic the extracellular matrix architecture of the native meniscus. However, most conventional electrospinning setups can only produce either 2-D aligned fibrous structures or 3-D fibrous scaffolds with fibers randomly distributed. Herein, we designed a novel electrospinning setup, which consisted of two metallic devices as collectors: an external cylindrical hollow piece with a central pin and a mobile internal hollow cylinder. A feasible approach to create single-layer scaffolds with both circumferentially and radially aligned ultrathin fibers was developed. Then, this investigation demonstrated a great potential for the application of these scaffolds toward meniscus tissue engineering, once they are able to reproduce the orientation of the main collagen fibers present in the extracellular matrix of the knee meniscus [93].

2.15.6 Core-shell SF/PEO nanofibers via green electrospinning Silk fibroin has been widely investigated as a biomaterial for biomedical application such as tissue engineering and drug delivery carrier due to its perfect biodegradability, biocompatibility, and mechanical properties. In order to increase the viscosity of the solution and obtain a suitable surface tension to generate stable continuous spinning, PEOs are commonly added to SF solution; moreover, the PEO phase could be extracted from SF and then can enhance the porosity and surface roughness of nanofibers [94]. Silk fibroin (SF)/PEO nanofibers prepared by green electrospinning are safe, nontoxic, and environment-friendly; it is a potential drug delivery carrier for tissue engineering. Core-shell nanofibers named as Dex@SF/PEO were obtained by green electrospinning with SF/PEO as the shell and dexamethasone (Dex) in the core. The nanofiber morphology and core-shell structure were studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The Dex release behavior from the nanofibers was tested by high-performance liquid chromatography (HPLC) method. The protective effect of drug-loaded nanofiber mats on porcine hip artery endothelial cells (PIECs) against lipopolysaccharide (LPS)-induced inflammatory damage was determined by MTT assay. TEM result showed the distinct core-shell structure of nanofibers. In vitro drug-release studies demonstrated that dexamethasone can sustain release over 192 h and core-shell nanofibers showed more slow release of Dex compared with the blending electrospinning nanofibers. Antiinflammatory activity in vitro showed that released Dex can reduce the PIEC inflammatory damage and apoptosis that is induced by lipopolysaccharide (LPS). Dex@SF/PEO nanofibers are safe and nontoxic because of no harmful organic solvents used in the preparation; it is a promising environment-friendly drug carrier for tissue engineering. By emulsion electrospinning, core-shell structured SF/PEO nanofibers can be obtained, and Dex was successfully incorporated in the core part. In the process of electrospinning, organic solvent was discharged in consideration of its toxicity, so this nanofiber preparation method can be called “green electrospinning.”

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Dex loaded into the core of Dex@SF/PEO nanofibers prepared by emulsion electrospinning was released in a sustained and prolonged manner; in  vitro experiment showed the released Dex can protect the PIECs from damage induced by LPS. SF/PEO nanofiber mat is a potential drug carrier for blood tissue engineering and skin tissue engineering. Moreover, some other hydrophobic drugs may also be incorporated into the SF/PEO nanofibers and loaded into the core.

2.15.7 Direct electrospinning for producing 3D hybrid constructs Electrospinning has been widely used to fabricate micro-/nanosized fibers. However, use of three-dimensional (3-D) scaffolds consisting only of electrospun micro-/ nanofibers has several limitations, such as inadequate mechanical properties and low pore-structure controllability. To overcome these challenges, a new hybrid 3-D scaffold was developed, which was obtained by fusing two techniques: the conventional melt-plotting method and direct electrospinning writing (DE-writing). The macropores consisted of microsized struts in the hybrid scaffold were covered with size-controllable microfibrous mat obtained using direct electrospinning writing. By varying parameters of the direct electrospinning writing, such as the conical auxiliary electrode position, the width of the microfibrous mat on the melt-plotted structure could be controlled. A new combinatory method was developed for fabricating 3-D hybrid scaffolds with reasonable mechanical properties and outstanding cellular activities consisting of melt-plotted struts with interlayered microfibrous mat. The width of the electrospun fibrous mat in the hybrid scaffold (HS2) was reasonably controllable, which resulted in enhanced attachment, infiltration, proliferation, and mineralization of osteoblast-like cells. These results suggest that the newly designed hybrid scaffold has potential for hard tissue regeneration [95].

2.15.8 Hybrid nanofibres composed of nanospheres via electrospinning The direct fabrication of hybrid nanofibers composed of poly(methyl methacrylate)/ SiO2 (SiO2-PMMA) nanospheres via electrospinning (Fig. 2.56) was investigated in detail. SiO2-PMMA nanospheres were successfully prepared, with the SiO2 nanospheres synthesized via the Stober method, followed by in situ surface-initiated atom transfer radical polymerization of methyl methacrylate (MMA). Electrospinning was carried out with N,N-dimethylformamide (DMF) as the solvent to disperse SiO2PMMA nanospheres. The size of the SiO2 core, the molecular weight of the PMMA shell, and the concentration of the SiO2-PMMA/DMF solution all had substantial effects on the morphology and structure of electrospun nanofibers composed of SiO2PMMA nanospheres. When these determining factors were well tailored, it was found that one-dimensional necklace-like nanofibers were obtained, with SiO2-PMMA nanospheres aligned one by one along the fiber. The successful fabrication of nanofibers by directly electrospinning the SiO2-PMMA/DMF solution verified that p­ olymer-grafted particles possess polymerlike characteristics, which endowed them with the ability to be processed into desirable shapes and structures.

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(A)

(B)

1 mm

1 mm

Fig. 2.56  Hybrid nanofibers composed of nanospheres [96]. (A) SiO2 nanospheres by Stober method and (B) In situ surface-initiated atom transfer radical polymerization of methyl methacrylate (MMA).

SiO2-PMMA nanospheres with controllable SiO2 core size and PMMA shell thickness were readily prepared by the Stober method and ATRP-initiated “graft-from” polymerization of MMA. As the molecular weight of the grafted PMMA chains exceeded the critical concentration of SiO2-PMMA nanospheres in solution, nanofibers composed of SiO2-PMMA nanospheres could be obtained by directly electrospinning the SiO2-PMMA/solvent system without the addition of extra polymer. The morphology of the resulting hybrid nanofibers, however, depended strongly on various factors, including the size of the SiO2 core, the concentration of the electrospinning solution, and the molecular weight of the PMMA shell. In achieving desirable one-dimensional necklace-like structures of SiO2-PMMA nanofibers composed of nanospheres, the key was to achieve a balance between the electrostatic force and the adhesive force among nanospheres in stretching, which could also be regulated by features of the SiO2-PMMA nanospheres and electrospinning solution. The successful preparation of one-dimensional necklace-like nanofibers via directly electrospinning of SiO2PMMA nanosphere solutions not only confirmed the polymerlike characteristics of polymer-grafted inorganic particles but also suggested a new way to prepare functional organic/inorganic hybrid nanofibers designed for various applications. The approach used was an advanced study of the preparation of hybrid nanofibers containing homogeneously distributed nonspherical nanoparticles, such as hybrid nanofibers composed of hydroxyapatite nanorods, intended for bone repair [96]. Microfluidics is a rapidly emerging technology that provides platforms for realizing biological, chemical, and medical applications in a miniaturized and integrated lab-on-a-chip format. In general, the fluidic structure is essential for most microfluidic systems, particularly to manipulate, trap, and analyze cell types, such as circulating tumor cells in the blood samples of cancer patients. Although using microfluidic devices

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is feasible for these cases, the demonstrated trapping efficiency of the microfluidic devices remains low. Hence, the study of the ability to generate functional patterns for lab-on-a-chip microfluidic devices as an alternative solution has become crucial. The design fabrication of microfluidic devices by using standard photolithography and etching techniques always depends on the material used, such as glass, silicon, and polymer substrates. These methods experience process restrictions in terms of both material choices and patterned structures. In addition, the complex procedures require a considerable amount of time. For unique structures, the cell trapping process must be developed for surface nanostructures in the manufactured microfluidic device, which remains a challenge. A direct fabrication process of nanofiber scaffolds within a pillar-based microfluidic device was proposed using electrospinning and picosecond laser pulses. The picosecond laser irradiation source is a type of ultrafast laser, which facilitates adequately controlled processing in a glass microfluidic device at a wavelength of 355 nm. The required nanofiber scaffolds can be formed through electrospinning with an optimal maximum pore area of 8.4 μm2 at a concentration of 11 wt%. Subsequently, three types of pillar structures with nanofiber scaffolds in the microfluidic device were fabricated. Self-organization of nanofibers can be achieved within the microfluidic device because of the inclined structures. The nanofiber scaffolds were observed to fill the space between the pillars, including the side wall and bottom layer of the pillars and microfluidic device. This micromachining technique can be employed to fabricate downscaling patterned device structures for microfluidic applications. To reduce the laser processing time, pillar-based microfluidic devices can be fabricated using a laser fluence of 43.01 J/cm2 and scanning speed of 1200 mm/s. The patterned device utilizing the direct writing process was scanned by 10 scanning times to form on the glass substrate where the depth of its formed device was 355 μm. It indicated the edge of microfluidic device where thermal damage should be concerned. The width of the microfluidic device was 38 mm, and an array of pillars was formed. The diameter of the pillar structures was 577 μm. Laser ablation with multiple pulses can cause ablation craters when a high laser influence and scanning speed are used. The picosecond laser process, based on the ablation threshold flounce, energy fluence, and scanning speed, can facilitate adequately controlled processing in a glass substrate at a wavelength of 355 nm. The required nanofiber scaffolds were fabricated with the lowest maximum pore area of 8.4 μm2 at a concentration of 11 wt%. In addition, three types of pillar structures are made with nanofiber scaffolds in the microfluidic device. The nanofiber scaffold deposition between the pillars depended on the angle of the pillar shape and the gap between pillars to yield the required high pore volume (i.e., high porosity). Thus, the required nanofiber scaffolds can be obtained in a pillar-based microfluidic device. This approach can be used in microfluidic applications such as cell trapping and on-demand biosensing [97,98]. Electrospinning is a very simple yet efficient process to produce nonwoven fiber mats from solutions of materials. Unlike fibers generated by common dry spinning, wet spinning, and dry-jet wet spinning process with diameters of micrometer, the fibers generated by electrospinning process usually are much smaller in 10–300 nm level. Because of the small diameter of the fibers, the fiber mats have a large surface area

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with reported BET surface area ranging from 7 to 110 m2/g, useful for applications including membrane for fuel cell, electrodes in solar cell, filters, sensors, and drug delivery. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the intrinsic conductive polymers that is commercially available in a range of properties usually in the form of poly(styrene sulfonate) (PSS)-doped PEDOT (PEDOT/PSS). Intrinsic conductive polymer materials, like PEDOT/PSS, offer unique material design pathways for a range of emerging flexible electronics applications, including flexible transparent electrodes, LEDs, and photovoltaics. In these applications, conductivity and interfacial area of the intrinsic conductive polymer strongly affect the efficiency of the final assembled device. High conductivity increases the efficiency of the device by reducing the resistance. Large interfacial area provides more location for electron–hole generation or recombination. It provides a simple and easy way to generate highly conductive nonwoven nanomat of commercially available intrinsic conductive polymers. Spinnability and conductivity are achieved by using a very small amount of very-high-molecular-weight PEO that provides stability in electrospinning process without interfering the percolation path of PEDOT/PSS within nanofibers. High-speed video observations revealed a unique spinning pattern of fiber standing at the collecting plate in electrospinning. This was solved by introducing an airstream flowing along the direction deposition. Effect of humidity, viscosity, and electrospinning voltage on electrospun fiber diameters was also investigated.

2.15.9 Honeycomb-like structures by electrospinning Honeycomb-like 3-D polymeric structures are useful to grow cells inside pores or in between nanofibers in cellular containment, but their generation and morphology control involves different processing and forming procedures. Approximately 35 mm-­ diameter deposits were generated under several conditions of humidity, while all other process parameters were kept constant. The morphologies of the structures, that is, their porosity and nanofibers, were studied and are related to the jet behavior characteristics to explain why 3-D honeycomb structures are obtained under selected optimum humidity conditions. Three-dimensional honeycomb structures were obtained only at an optimized 73% RH. All other parameters were kept constant; they included the concentration of PEO solution, solution flow rate, applied voltage, fiber collection distance, ambient temperature, and fiber formation time. The formation of nanofibers and the honeycomb-like structure was observed using a high-speed camera (Phantom v654) that is capable of capturing 15,000 frames per second. All the processing devices were calibrated before operation. The effect of relative humidity on the morphology both of the nanofibers generated by electrospinning and the honeycomb-like structures produced from them was investigated systematically using electrospinning under constant process parameters. Process evolution was also studied using high-speed camera videos and images. The diameter of the macrostructures produced was found to be similar throughout the range of relative humidity varied from 53% to 93%. A well-defined 3-D ­honeycomb-like structure was observed at 73% relative humidity. At humidities below 73%, although patterns were generated, nothing as well defined as a honeycomb-like cocoon

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emerged. Increasing the humidity to 83% caused the formation of large beads in the central region. Pores were generated in the middle region, and the resulting wall cannot be aligned high enough to form a well-defined structure. Increasing the humidity to 93% made the nanofibers remain in a semidissolved state on the substrate. The diameter of both the pores and nanofibers was observed to decrease sharply from the central region to the edge region of the macrostructure at 73% relative humidity. The number of beaded nanofibers was found to increase when the relative humidity was increased. Videos produced using the high-speed camera revealed that the generation of honeycomb-like structure takes place after a conical envelope of fibers occur, post jetting. The formation of structures was influenced by the arcing of the nanofibers when reaching the substrate [99].

2.15.10 Growth of nanostructured fibers One-dimensional (1-D) metal oxide nanostructures (1D-MONS) shown in Fig. 2.57 play a key role in the development of functional devices including energy conversion, energy storage, and environmental devices. They are also used for some important biomedical products like wound dressings, filter media, drug delivery, and tissue engineering. The electrospinning (ES) is the versatile technique for making of 1-D growth of nanostructured nanofibers, an experimental approach and its applications. This study is focused on the 1-D growth of nanostructured nanofibers in different applications like dye-sensitized solar cells, perovskite solar cells, fuel cells, lithium-ion batteries, redox-flow batteries, supercapacitors, photocatalytic sensors, and gas sensors based on ZnO, TiO2, MnO2, WO3, V2O5, NiO, SnO2, Fe2O3, and other metal oxides; their composites; and carbon. The various technical details such as preparative parameters, postdeposition methods, applied electric field, solution feed rate, and a distance between a tip to the collector are the key factors in order to obtain exotic 1-D nanostructured materials [100,101]. In the occurrence of a metal catalyst, the chemical energy that is converted into electric energy is known as a fuel cell. In fuel cells, the hydrocarbons such as ethanol and methanol were used as a fuel source. The direct methanol fuel cell (DMFC) was established to be cost-effective, simple, low-temperature, and inexpensive. Proton exchange membrane fuel cell (PEMFC) and DMFC were being established for transportable applications like personal digital assistants, cellular phones, and laptops. In the PEMFC, the treatment of hydrogen as a fuel is critical. In order to get efficient proton conduction, the water organization of proton exchange membrane (PEM) is very vital. Although different mixtures were planned for this problem, DMFC was an option to PEMFC in moveable applications. Presently, 40% efficiency has been achieved to DMFC. Hence, scientists are focusing on the improvement of an efficient membrane that will exclude the methanol crossover. Different contributing factors include methanol concentration, the thickness of PEM, operating current density, PEM materials, temperature, and pressure. The 1-D nanostructure can propose increase rate capability, improved cycle stability, and high capacity in Li-ion batteries. This is due to the make possible Li-ion transport and better electron resulting from a high interfacial contact area with the accommodation strain, electrolyte, and volume changes without

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(A)

500nm

(B)

100nm

(C)

500nm

(D)

100nm

(F)

100nm

(E)

500nm

Fig. 2.57  One-dimensional metal oxide nanostructures [101]. (A) ZnO, (B) TiO2, (C) MnO2, (D) NiO, (E) SnO2, and (F) Fe2O3.

any crumbling or cracking. Supercapacitors (SCs) are very promising energy storage devices in different areas such as defense, communication, transportation, consumer electronics, and electricity applications owing to their long cycle life, good safety, simple mechanism, short charging time, and high power density. SCs have two types: electric double-layer capacitors (EDLCs) and pseudocapacitors. At the electrode and electrolyte, the EDLCs store energy using ion adsorption and desorption, whereas PCs store energy based on fast reversible surface redox reactions. Recently, carbon-based materials have been investigated for high-performance EDLCs. The electrospun carbon nanofibers (CNFs) can be employed as an electrode for EDLCs after undergoing the process of carbonization, activation, and stabilization, in which the porosity and surface area of the NFs can be improved.

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The bismuth ferrite (BiFeO3) NFs and NTs were prepared from bismuth nitrate and iron nitrate precursors. The distilled water and PVP polymer were used to maintain an appropriate viscosity and surface tension. The average diameter of the BiFeO3 NFs and NTs was around 100 nm. The rhombohedral perovskite structure is confirmed via XRD study. The bandgap energy is 2.38 eV of BiFeO3 NFs and NTs that was obtained by UV-vis absorption and diffuse reflectance spectrometry [101]. The ES technique has been widely adopted to produce a wide variety of 1-D nanostructured NFs owing to their high surface-area-to-volume ratio. The utility of the ES technique to grow high-quality device-grade material details its versatility. It is one of the potential techniques to produce nanomaterials with greater reproducibility and subsequently has the potential to scaling-up. Electrospun NFs have potential application in the field of energy conversion (DSSCs, perovskite solar cell, and fuel cell), energy storage (lithium-ion, redox-flow, and lithium‑sulfur batteries and supercapacitors), environmental (photocatalytic and gas sensor)- and biomedical (wound dressing, filter media, tissue engineering, and drug delivery)-related applications, and device fabrication. Most of the inorganic metal oxide materials in 1-D NFs structure exhibit better performance when used in intended devices. TiO2 NF-based DSSCs exhibited maximum 9.63% power conversion efficiency (PCE), and perovskite showed maximum 14.37% PCE and 17.89% PCE for Zn2SnO4 NFs. Similarly, other devices also showed superior characteristics when ES 1-D NF materials are used. However, ES has become a vital technique for making 1-D nanostructure; the research investigation is unmoving young but promising in energy applications. Though, it is also noted that the size of the 1-D NFs varies approximately over the range 100–600 nm. Overall, it seems to be the limitation of ES to grow 1-D NFs with size less than 100 nm. Also, it has relatively low making rate. In the future, it is expected that research labors will be paying attention to engineering the ES process, with an eventual goal of producing NFs with diameter below 100 nm and at a more rapidly making rate. In the long time, it is predictable that vertically aligned NFs, perfect morphology via ES, should be potential to make in order to accomplish highest electron transport in energy and electronic devices and prescribed pore sizes for environmental filtration. Further attempts on modifications of chemical strategies and device geometries are necessary to shrink the size of the NFs to obtain higher effective surface area, more functionality, and higher chemical reactivity. This will need dedication and research teamwork around the world. Nanofibrous membrane processes signify for technologies that deliver an answer to pollution in water and air environments. It is estimated that the future of membrane technology lies in the progress of more efficient methods. Moreover, the improvement of clean energy and environmental technology will deliver huge opportunities for the formation of high-value additional products and linked business improvement. Therefore, not only attention in ES technology is controlled to research laboratory, but also world famous industries have been involved in making nanofibrous products for enhanced performance. Recently, ES processes involve the exploitation of organic solvents, which are harmful, poisonous, and corrosive to the environment [101].

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2.15.11 Electrospinning-based (bio)sensors Sensors and biosensors for monitoring food traceability, quality, safety, and nutritional value are of outmost importance nowadays. Electrospinning, a simple, straightforward, and versatile technique to fabricate 1-D micro- and nanomaterials, is among the most potential strategies to further advance the development of chemical (bio)sensors. Electrospun nanofibers are capable of improving several attributes of chemical (bio) sensors due to the high specific surface area, high porosity, and 1-D confinement characteristics. Furthermore, the possibility to build up multifunctional nanostructures by functionalizing the nanofiber surface with a wide range of distinct nanomaterials (such as carbon nanotubes, graphene, nanoparticles, and conjugated polymers) enhances the (bio)sensing capabilities through additional properties and synergistic effects. Coaxial spinneret system, which consists of a syringe-like apparatus with an inner capillary, coaxially placed inside an outer one, is a versatile method to prepare coreshell nanofibers. Recently, the synthesis of core-shell NFs based on In2O3 and SnO2 via coaxial electrospinning approach for gas was investigated in sensing applications, which exhibited an outstanding selectivity and rapid response/recovery in comparison with the sensors based on single nanofibers. These phenomena are closely associated with the electron flow caused by the work function difference between the metal oxide semiconductors of the core and the shell. Apart from multiple and coaxial electrospinning, some needleless electrospinning setups have been developed for mass nanofiber production, including bubble and disk electrospinning. Specifically, the high surface area of nanofibers enhances the reactivity of the material, speeds up adsorption or release mechanisms, and increases the number of sites for the interaction or loading of catalysts or other reactive materials in the development of (bio)sensors. In addition, the morphology of ESNFs is also consisted of empty volume such as pores and channels. As the porosity of nanofibers increases, the mass-­transport resistance encountered by the fluid passing through the empty channels of the membrane is reduced. Therefore, the possibility to fill this 3-D “empty” nanostructure with gas, solvents, or reactive chemical and biochemical species opens to a myriad of potential applications. As a consequence, electrodes coated with nanofiber membranes offer, in principle, a negligible barrier to the analyte diffusion toward the electrode surface. Moreover, the opportunity to customize and functionalize the nanofibers on large scale enables the electrospinning technique to match a wide range of requirements for specific sensing applications, giving it a benefit over other methods commonly used for the production of micro-/nanostructures. For instance, electrospinning has been applied toward functional fiber formation based not only on polymers but also on metals; ceramics; and organic/organic, organic/inorganic, and inorganic/inorganic composite systems. All these features make electrospinning as an outstanding technology in the fabrication of (bio)sensors suitable for the detection of different analytes. In practical terms, electrospinning versatility has become well studied and started to gain industrial market. Despite the positive prospects, more intense efforts are still required for making electrospinning-based (bio)sensors for agri-food commercially applicable at an industrial scale. For reaching such aim, some challenges must be overcome, which include increasing the relatively low output production rate of

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c­ ommercial electrospinning setups, establishing strategies to improve the dispersion of nanomaterials into the bulk or onto the surface of the functional nanofibers, and improving the control of processing parameters for fabricating reproducible and standardized hybrid functional nanofibers. In addition, there is a great demand to develop flexible platforms based on NFs using lab-on-a-chip technologies that demand less assay steps, less solution consumption, and in situ applications at affordable costs [102].

2.15.12 Electrospinning complexly shaped vascular grafts The use of vascular grafts is indicated in a wide range of medical treatments. While autologous tissue is the graft of choice in most surgical bypass procedures, the next best option is the use of synthetic vascular grafts. While significant advances have been reported in the use of electrospinning for vascular grafts both at in  vitro and in  vivo level, most of the work is limited to straight, tubular shapes with uniform diameters. In order to generate resorbable scaffolds with curving and bifurcated tubular shapes with nonuniform diameters, the combination of directed electric field and dynamic positioning of electrospun fibers aimed at a custom, 3-D printed mandrel is possible. The proposed approach produced a woven membrane of electrospun fibers shown in Fig. 2.58 [103]. In order to generate resorbable scaffolds with bifurcated tubular shapes, the combination of directed electric field and dynamic positioning of the mandrel was proposed.

2-DOF mechanism Electrospinning

Flexible mechanism

Scaffold

20 µm

EHT = 10.00 kV WD = 12.5 mm

Signal A = VPSE G3 Mag = 500 X

Date : 22 Feb 2017 ZEISS Vacuum Mode = Variable Pres

Fig. 2.58  Complex-shaped vascular grafts [103].

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The proposed approach produced a mat of electrospun fibers on top of a bifurcated tubular mandrel. Preliminary mechanical testing of the bifurcated scaffolds is reported, with maximum indentation force between 0.7 and 2.3 N. In tension tests, the scaffolds showed an average maximum strength of 0.60 (no indexing condition) and 1.37 MPa (indexing in the B direction). The positioning of the electrospun fiber source and/or the mandrel requires further refinement in order to improve the precision of continuous motion in A and B directions and specific indexing angles at particular positions to control the weave. Further experimentation is required to establish process capability in terms of tolerances for the key dimensions of the bifurcated scaffold (such as diameter and thickness) and mechanical properties. Additional testing is required in order to assess the burst pressure strength of this kind of bifurcated scaffolds.

2.15.13 Electrospinning composite nanofibers Nonwovens of polymer/clay composite nanofibers (viz., polyacrylonitrile/ Na-montmorillonite, PAN/Na-MMT) are produced by electrospinning a solution of PAN in dimethylformamide containing synthetic Na-MMT. The influence of both NaMMT amount and applied voltage on the properties of electrospun composite nonwovens was studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis-differential thermal analysis (TGA-DTA) were used to evaluate the morphology, structure, and thermal properties of composite nanofibers. SEM observations revealed that increasing the amount of Na-MMT in the solution or the applied voltage increases the average diameter of electrospun composite nanofibers. The prepared composite showed a higher thermal stability than the pristine PAN nanofibers. It was proved that the ion exchange properties of Na-MMT were maintained in the obtained composite [104,105]. Nonwovens of PAN/Na-MMT hybrid nanofibers containing 5, 10, or 19 wt% synthetic Na-MMT were produced by electrospinning PAN/Na-MMT/DMF solution at three different applied voltages: 11.5, 13, and 14.5 kV. Incorporating Na-MMT into PAN composite nanofibers induces an increase in the thermal stability of the hybrid nonwoven. This is evidenced by the shifting of exothermic peak of PAN toward higher temperature. A difference between the measured loading percent of Na-MMT within the composite nanofibers and those expected is marked indicating that PAN/Na-MMT suspensions are not stable and some Na-MMT particles have precipitated before electrospinning. A method to stabilize the electrospinning suspensions is envisaged. SEM observations reveal that increasing both Na-MMT and applied voltage, nanofiber diameter increases with a dominant influence of loading percent over that of applied voltage. X-ray patterns of PAN/Na-MMT nanofibers indicate that Na-MMT incorporated within the nanofibers is intercalated either by PAN macromolecules, by solvent molecules, or by water molecules. This was confirmed by the shifting of the characteristic 0 0 1 peak of Na-MMT into smaller 2θ values that correspond to an increase in the interlayer space from 12.7 Å for powder Na-MMT to 14.8 Å for incorporated Na-MMT. It was found that neither Na-MMT loading nor the applied voltage has a significant effect on the extent of silicate layer intercalation. However, the change in 0 0 1 peak skewness as a function of applied voltage confirms that the higher the applied

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voltage is, the more the layers parallel to each other. No information can be confirmed concerning the alignment of Na-MMT layers to the axis of nanofiber. Observation by transmission electron microscopy will be envisaged for this purpose. One interesting feature is that the interlayer space of Na-MMT is still accessible in the composite; applications in the field of environmental protection (wastewater treatment and entrapping of organic pollutant) can therefore be expected. Studies are underway in order to tailor the Na-MMT dispersion and distribution in the nanofibers. Further dispersing of Na-MMT silicate layers in the composite nanofibers is envisaged. Quantification of accessible galleries within the composite nanofibers will also be investigated [105].

2.15.14 Electrospinning cross-linking hydrogelators Many hydrogel materials of interest are homogeneous on the micrometer scale. Electrospinning, the formation of submicrometer to micrometer diameter fibers by a jet of fluid formed under an electric field, is one process being explored to create rich microstructures. However, electrospinning a hydrogel system as it reacts requires an understanding of the gelation kinetics and corresponding rheology near the ­liquid-solid transition. In this study, we correlate the structure of electrospun fibers of a covalently cross-linked hydrogelator with the corresponding gelation transition and kinetics. Polyethylene oxide (PEO) is used as a carrier polymer in a chemically cross-linking poly(ethylene glycol)/high-molecular-weight heparin (PEG-HMWH) hydrogel. Using measurements of gelation kinetics from multiple-particle tracking (MPT) microrheology, we correlate the material rheology with the formation of stable fibers. An equilibrated, cross-linked hydrogel is also spun, and the PEO is dissolved [106]. In both cases, microstructural features of the electrospun fibers are retained, confirming the covalent nature of the network. The ability to spin fibers of a cross-linking hydrogel system ultimately enables the engineering of materials and microstructural length scales suitable for biological applications. First, hydrogel spinning experiments study the time a hydrogel is able to spin during the gelation reaction and the associated rheological properties. Second, batch experiments are performed that test the spinning of an equilibrated, cross-linked hydrogel with rheological properties chosen from the results of the reacting gel. It can be noted that the concentrations and functionalities of the materials selected for equilibrated and kinetically evolving electrospinning are different. This is necessary, as the aim of kinetically evolving electrospinning is to correlate the change in spinnability with the material’s rheological properties as the material transitions from a sol to a gel [107]. The fiber diameter decreases as the separation distance increases. This corresponds to a diminishing number of elliptical beads, shown in Fig.  2.59A. After dissolving out the PEO, a porous microstructured material is recovered for all separations. The corresponding SEM images are shown in Fig. 2.59B. To electrospin a hydrogel, we must first identify the material properties that are suitable for this process. The extensional viscosity of the (gelling) hydrogel material must be such that it can flow and form stable filaments during spinning, but is not too stiff or viscous that the material only extrudes from the charging needle. Using electrospinning,

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Fig. 2.59  Electrospun cross-linked hydrogels [107]. (A) Diminishing number of elliptical beads and (B) Porous microstructured material after dissolving out the PEO.

a porous network of fibers is created with diameters ranging from a few hundred nanometers to micrometers and pore sizes on the order of tens of micrometers, a length scale similar to the ECM. The fibers fabricated of PEG-HMWH hydrogel have the physically relevant length scales to mimic the extracellular matrix. This recapitulates biophysical cues necessary for applications such as tissue regeneration, wound healing, and stem cell culture.

2.15.15 Electrospun poly(l-lactide)/zinc oxide nonwoven textile New hybrid fibrous materials from the biocompatible and biodegradable aliphatic polyester poly(l-lactide) (PLA) and pristine or surface-functionalized nanosized zinc

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oxide were prepared. The application of the techniques—(i) electrospinning of a suspension of ZnO in PLA solution or (ii) simultaneous electrospinning of PLA solution and electrospraying of a ZnO suspension in PLA solution (at low PLA concentration)—enabled the fabrication of hybrid materials of diverse design: nonwoven textile consisting of fibers in which ZnO was deposited on the fibers’ surface (designated as type “on”) or was mainly in the fibers' bulk (designated as type “in”). The photocatalytic activity of the new fibrous materials was estimated in respect to methylene blue (MB) and reactive red (RR) dyes. Type “on” hybrid materials had higher photocatalytic activity as compared with type “in” materials. It was shown that type “on” materials preserved their photocatalytic activity in respect to MB even after three repeated uses, while for the RR dye, the same held true for ZnO-on-PLA mats only. The type “on” materials exhibited antimicrobial activity against the pathogenic microorganism Staphylococcus aureus as evidenced by the performed microbiological tests [108–110]. It was found that the use of electrospinning/electrospraying (type “on” mats) is more efficient for the preparation of hybrid materials. This is due to the fact that for the type “on” mats, the zinc oxide is deposited only on the fibers’ surface, while for the type “in” mats, it is mainly in the fibers’ bulk. The new type “on” hybrid materials may find application for biomedical applications (as antibacterial scaffolds) and for heterogeneous degradation of organic pollutants such as the used MB and RR model dyes. ZnWO4 fibers are fabricated by an electrospinning process. The phase, size, and morphology, as well as the photocatalytic properties, are studied carefully. The XRD, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) results confirm the formation of ZnWO4 with a monoclinic phase. The SEM and TEM images indicate that ZnWO4 fibers are composed of lots of nanoparticles with diameters about 20 nm. The ZnWO4 fibers display high photocatalytic activities in the degradation of RhB and photocurrent response under simulated sunlight irradiation. The recycling tests suggest the photocatalytic stability of ZnWO4 fibers. All of the results obtained by the work tell us that ZnWO4 fibers are excellent catalysts for the photocatalytic decomposition of pollutants. The phase, size, and morphology, as well as the photocatalytic properties, are studied carefully. The excellent photocatalytic activity for the RhB degradation is obtained on ZnWO4 fiber photocatalysts. The hierarchical structure of ZnWO4 fiber increases the amount of photogenerated electrons and holes and hence enhances the catalytic rate. The recycling tests show that the photocatalytic activity is stable for ZnWO4 fibers. This work tells us that ZnWO4 fibers are excellent catalysts for the photocatalytic decomposition of pollutants [111].

2.15.16 Electrospinning of agar/PVA aqueous solutions The successful fabrication of agar-based nanofibers by electrospinning technique was explored, using water as solvent media. A tubeless spinneret was attached inside the electrospinning chamber, operating at 50°C, to avoid agar gelation. Agar pure solution (1 wt%) showed inadequate spinnability regardless of the used electrospinning conditions. The addition of a coblending polymer such as PVA (10 wt% starting solution)

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improved the solution viscoelasticity and hence the solution spinnability. Agar/PVA solutions were prepared with different mass ratios (100:0, 50:50, 40:60, 30:70, 20:80, and 0:100) and electrospun at various sets of electrospinning conditions. Best nanofibers were obtained with 30:70 and 20:80 agar/PVA blends, while samples with higher agar contents (50,50 and 40:60 agar/PVA) were harder to process and led to discontinuous fibrous mats. This first set of encouraging results can open a new window of opportunities for agar-based biomaterials in the form of nanofibers. The nature of the polymer jets seemed to indicate some incompatibility between both polymers in aqueous media, particularly at higher agar contents. However, this is a suggestion that needs to be confirmed. Currently, there are ongoing researches aiming at improving the temperature control inside the electrospinning chamber and exploring other components to improve the solution spinnability. Agar-containing PVA nanofibers were successfully obtained by electrospinning technique using a tubeless spinneret attached inside the electrospinning chamber, with the temperature set at 50°C. The PVA addition was crucial to improve the solution spinnability as seen by the rheological profiles of the tested agar/PVA blends. Best nanofibers were obtained at higher PVA concentrations (i.e., 30:70 and 20:80 agar/PVA), while blends with higher agar contents (i.e., 40:60 and 50:50 agar/PVA) were harder to process. This can open a new window of opportunities for the fabrication of agar-based biomaterials in the form of nanofibers [112].

2.15.17 Electrospinning of alginate/soy protein isolated nanofibers Natural polymer-based nanofibers with functions of loading and releasing bioactive cues or drugs have recently gained interest for biomedical applications. Nanotopography and large surface-area-to-volume ratio of hydrophilic polymer fibers promote their use as carriers of hydrophilic drugs. Sodium alginate (SA) and soy protein isolated (SPI) blended fibers encapsulated with vancomycin were fabricated via electrospinning with the assistance of polyethylene oxide (PEO). Morphological results showed submicron-sized, smooth, and uniform as-spun SA/PEO/SPI fibers with an average diameter of 200 nm. Beads on the fiber mats were formed with increasing SPI content in the blending system. The optimal polymer composition of the electrospinning solution was determined as 5.6:2.4:2 SA/PEO/SPI. Polymer blends were maintained after ionic “cross-linking,” as indicated by the FTIR result. Investigation of release characteristic of vancomycin-loaded SA/PEO/SPI electrospun fibers exhibited initial burst release followed by a controlled release after 2 days of immersion in a phosphate-buffered saline. The release rate of SA/PEO/SPI fibers was significantly slower than that of SA/PEO fibers, and drug-loaded fibers inhibited bacterial growth against S. aureus after 24 h of incubation. Nontoxicity and biocompatibility of the fibers were confirmed by an indirect cytotoxicity test using human dermal fibroblasts. These results suggest that the vancomycin-loaded SA/PEO/SPI blended fibers are a promising nanomaterial for use in biomedical fields such as scaffolds for tissue engineering and drug delivery systems [113].

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Electrospinning was used to fabricate nanosized fibers based on alginate, soy protein isolated, and poly(ethylene oxide) blended aqueous solutions with vancomycin antibacterial drug loading. Aqueous solutions of vancomycin-loaded SA/PEO/SPI (5.6:2.4:2) were successfully electrospun to produce uniform fibers with diameter range of 60–600 nm. The composition of polymer blends strongly affected the fiber morphology and consequently the drug-release behavior. The SA/PEO/SPI fibers provided a slower release of vancomycin in the initial stage, followed by a constant release over a longer time compared with the SA/PEO fibers. According to the Ritger and Peppas model, the SA/PEO/SPI fibers followed the release mechanisms of polymer chain relaxation. In addition, the fibers provided antibacterial activity against gram-positive S. aureus related to the released vancomycin dose. Finally, the result of in  vitro cytotoxicity of the fibers tested with HDFs confirmed their noncytotoxicity and biocompatibility. As a result, the electrospun SA/PEO/SPI fibers could act as a biomedical device offering several advantages including drug encapsulation and controlled release, antibacterial activity, and compatibility with cells suitable for biomedical applications.

2.15.18 Electrospinning of calcium carbonate fibers Various electrospun fiber materials have been studied for bone scaffolds. There are several publications about the electrospinning of biocomposite fibers with inorganic nano- or microparticles of bioglass, hydroxyapatite (HA), or calcium carbonate (CaCO3) embedded in the polymer matrix of the fibers. Also, CaCO3 has been mineralized as calcite on electrospun cellulose acetate (CA) fibers by a gas-liquid diffusion technique in CaCl2 solution with polyacrylic acid (PAA) as the crystal modifier. The process was quite slow with an incubation period of 10 days required to obtain a continuous calcite coating around the CA fibers, but it also enabled the preparation of CaCO3 microtubes by dissolving the CA core of the fibers with acetone post mineralization. Calcium carbonate (CaCO3) fibers were prepared by electrospinning followed by annealing. Solutions consisting of calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and polyvinylpyrrolidone (PVP) dissolved in ethanol or 2-methoxyethanol were used for the fiber preparation. By varying the precursor concentrations in the electrospinning solutions, CaCO3 fibers with average diameters from 140 to 290 nm were obtained. After calcination, the fibers were identified as calcite by X-ray diffraction (XRD). The calcination process was studied in detail with high-temperature X-ray diffraction (HTXRD) and thermogravimetric analysis (TGA). The initially weak fiber-to-­substrate adhesion was improved by adding a strengthening CaCO3 layer by spin or dip coating Ca(NO3)2/PVP precursor solution on the CaCO3 fibers followed by annealing of the gel formed inside the fiber layer. The CaCO3 fibers were converted to nanocrystalline hydroxyapatite (HA) fibers by treatment in a dilute phosphate solution. The resulting hydroxyapatite had a platelike crystal structure with resemblance to bone mineral. The calcium carbonate and hydroxyapatite fibers are interesting materials for bone scaffolds and bioactive coatings [114,115].

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Fig. 2.60  Electrospun shape memory fibers [116].

2.15.19 Electrospinning of shape memory fibers A mat of fibers as shown in Fig.  2.60 with shape memory effect is obtained using commercially available components and a simple electrospinning process. For obtaining this goal, a blend of diglycidyl ether of bisphenol A (DGEBA) and PCL is electrospun, and the obtained mats are cured by UV radiation, avoiding the melting of the PCL component. The cationic photoinitiator based on iodonium salts, used for the first time in the electrospinning process, increases the solution conductivity and consequently the spinnability of the blend. The obtained mats show shape memory properties through several cycles, with shape fixity ratios that range from 95% to 99% and shape recovery ratios of between 88% and 100% [116]. The results obtained for the shape memory effect are displayed in Fig. 2.6A and B. The shape fixity ratio ranges from 95 to 99% and shape recovery ratio from 88% to 100%, maintaining the shape memory properties after five consecutive cycles. Shape memory polymer (SMP) composites of electrospun PCL imbibed with epoxy resin were analyzed, and similar values were obtained. This indicates that the method employed in this study makes it possible to obtain a great shape memory effect maintaining the fibrous structure that is electrospun. There is no significant difference in the shape memory effect among the mats obtained from different solvents. Electrospun mats from PCL/DGEBA mixtures were obtained adding an iodonium salt to the solutions. This UV initiator not only was necessary for curing the DGEBA after electrospinning but also improved the spinnability allowing bead-free fibers to be obtained. The epoxy polymerization characterized by FTIR and photo-DSC allowed the integrity of the material and the fiber morphology to be maintained when the mats were heated up to the melting point of the PCL. These mats showed great shape memory effect during several cycles.

2.15.20 Electrospinning of continuous poly (l-lactide) yarns Electrospinning poly(l-lactic) acid (PLLA) solutions from two oppositely charged nozzles gives a triangle of fibers, also called E-triangle, that assemble into yarns at the convergence point. The formed yarn at the E-triangle was taken up by a unit comprising a take-up roller and coupled twister plate, as shown in Fig. 2.61. At all twist rates, uniform and smooth fibers without any beads were formed. The apex angle of the deposited fibers at the E-triangle was larger at higher twist rates. By increasing the

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Take-up roller

High voltage supply

Twister

E-triangle

Grounded cylinder

Syringe pump

Fig. 2.61  Electrospinning PLA yarns [117].

twist rate from 80 to 320 rpm, the orientation angle of fibers in the yarn changes from 18.8 to 41.5 degrees. Increasing the twist rate revealed a higher polymer crystallinity likely due to the polymer orientation by the applied tension to the fibers. The ultimate strength and modulus of electrospun yarns were higher when prepared at higher twist rates. However, at the highest twist rates, the strength and modulus of electrospun yarns leveled off and even decreased slightly. The results revealed that the mechanical properties depend not only on the polymer crystallinity but also on the alignment of the fibers in the yarn and the angle at which they were deposited. These biodegradable materials are promising materials to be used in a wide range of applications where environmentally friendly products are required [117]. The formed E-triangle was approximately symmetrical at all twist rates. In addition, when keeping the electrospinning process parameters constant, an increase in the twist rate places the fiber convergence point closer to the neutral cylinder. At the convergence point, the twist angle α increased with twist rate, while the height and the base of the triangle decreased. The considerable reduction of E-triangle dimensions at twist rates of 240 and 320 rpm leads to a limited space for fiber deposition at the convergence point and thus for yarn formation. As a result, at high twist rates and due to the high amount of produced fibers, electrospun “out-of-control” fibers are depositing and wrapping on the already formed yarn (Fig. 2.62B). This phenomenon leads to an increase in yarn diameter at twist rates of 240 and 320 rpm [117]. Higher magnifications show that the fibers are oriented and arranged in an angle to the yarn axis called twist angle. The results showed that by increasing the twist rate from 80 to 320 rpm, the twist angle increased from 18.8 to 41.5 degrees. Compared with fibers in the E-triangle zone, the fiber diameter in the yarn is somewhat smaller caused by the rolling and twisting. Despite the tension exposed to the fibers upon

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Fig. 2.62  E-triangle during nanoyarn formation [117]. (A) Fiber triangle and (B) Deposition on already formed yarn.

electrospinning and subsequent twisting and rolling, the yarn diameter increased by increasing the twist rate. As mentioned above, by increasing the twist rate, the dimensions of the E-triangle zone become smaller, and parts of the electrospun fibers wrap the formed yarns. This phenomenon leads to form bulky yarns at high twist rates. Actually, the electrospun yarns produced at high twist rates of 240 and 320 rpm can be considered to be composed of an inner layer with deposited fibers assembled on it. The difference between these two layers is related to the inner layer of the yarn that is formed from twisting the fibers in the E-triangle zone. In this part of the yarn, the fibers are densely packed with high orientation. The fibers in this outer layer have a lower orientation and are less densely packed. The E-triangle of fibers formed in electrospinning PLLA solutions from two oppositely charged nozzles has a large influence on the assembly of fibers and consequent properties of twisted yarns. It was shown that by changing the twist rate, the average fiber diameter decreased as the twist rate increased. At higher twist rates, despite the compression of the fibers in the inner structure of the yarn, the yarn diameter increased due to the formation of an outer layer of out-of-control depositing fibers. Increasing the twist rate gave an increased tension on the fibers, resulting in a crystallinity. The mechanical properties of the yarns showed that the strength at break and modulus of electrospun yarns increased up to a twist rate of 160 rpm and leveled off and even decreased slightly at the higher twist rates. Increasing the twist amount caused an increase in the elongation at break of electrospun yarns.

2.15.21 Electrospinning of complex fast-dissolving nanofibrous The volatility and limited water solubility of linalool is a critical issue to be solved. Here, we demonstrated the electrospinning of polymer-free nanofibrous webs of cyclodextrin/linalool-inclusion complex (CD/linalool-IC-NFs). Three types of modified cyclodextrin (HPbCD, (2-hydroxypropyl)-beta-cyclodextrin; MbCD, methyl-β-­cyclodextrin; and HPcCD, (2-hydroxypropyl)-chloride-cyclodextrin) were

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used to electrospin CD/linalool-IC-NFs. Freestanding CD/linalool-IC-NFs ­facilitate maximum loading of linalool up to 12% (w/w). A significant amount of linalool (45%–89%) was preserved in CD/linalool-IC-NFs, due to enhancement in the thermal stability of linalool by cyclodextrin inclusion complexation. Remarkably, CD/linaloolIC-NFs have shown fast-dissolving characteristics in which these nanofibrous webs dissolved in water within 2 s. Furthermore, linalool release from CD/linalool-IC-NFs inhibited the growth of model gram-negative (Escherichia coli) and gram-positive (S. aureus) bacteria to a great extent. Briefly, characteristics of liquid linalool have been preserved in a solid nanofiber form, and designed CD/linalool-IC-NFs confer high loading capacity, enhanced shelf life, and strong antibacterial activity of linalool. Freestanding nanofibers were produced from nonpolymeric systems of cyclodextrin/linalool-inclusion complexes (CD/linalool-ICs) via electrospinning. High amount of linalool (45%–89%) was preserved in CD/linalool-IC-NFs owing to cyclodextrin complexation. Short-term temperature release (3 h at 37, 50, and 75°C), long-term open-air release (50 days, at RT), and humidity-dependent release (60 ± 2% RH at RT) tests were carried out for CD/linalool-IC-NFs. MbCD/linalool-IC-NF released less linalool compared with HPbCD/linalool-IC-NF in short-term temperature release and long-term open-air release tests, due to its higher thermal stability and stability constant. CD/linalool-IC-NFs were shown to have quite high antibacterial activity against model gram-negative (E. coli) and gram-positive (S. aureus) bacteria. CD/ linalool-IC-NFs are shown to dissolve completely in water within 2 s. In brief, high preservation of linalool along with antibacterial activity and slow release was achieved by the electrospinning of CD/linalool-IC nanofibrous webs, which may be used as fast-dissolving supplement material in food and pharmaceutical products [118,119].

2.15.22 Electrospinning of doped and undoped blends Electrospun mats have been prepared by blending emeraldine base (EB) or dodecylbenzenesulfonic acid (DBSA)-doped polyaniline (PANI.DBSA) with poly(vinylidene fluoride) (PVDF). In order to understand the effect of doped and undoped PANI on the structure and properties of PVDF/PANI mats, the electric conductivity, doping degree, and morphology of the electrospun mats were investigated. The effect of PANI.DBSA and EB content on the properties of PVDF solution and fiber morphology was investigated. The addition of either doped or undoped PANI increases the viscosity of the PVDF solution, but the ionic conductivity was changed significantly by adding doped PANI. The PVDF/EB fibers manifested a core-sheath structure, while PANI agglomerates were homogeneously distributed in the PVDF/PANI.DBSA. This morphology associated with the high porosity of mat resulted in insulating mats, even when 23 wt% of PANI salt was used and high doping levels were achieved. The PVDF/EB mats obtained from electrospinning displayed a considerable doping level, indicating that the electric field could induce the protonation of PANI. Nonwoven mats were obtained through electrospinning of PVDF/EB and PVDF/ PANI.DBSA blends. Comparing the systems, it can be noted that both salt or base PANI affect the solution properties and have an influence on the process, morphology, and fiber diameter. The electrospinning of solution with PANI base is easy to be performed

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when compared with solution containing PANI salt, since both viscosity and ionic conductivity of solution are greatly modified by the addition of doped PANI. Consequently, in order to obtain uniform fibers, the voltage applied to neat PVDF needs to be adjusted when PANI salt is used, which result in thinner fibers. The electrospun mats from solution containing EB displayed a grayish green color, typical of PANI salt, similar to the color of PVDF/PANI.DBSA mats. In fact, the doping level of PVDF/EB obtained from XPS analysis was in the range from 19% to 25%, indicating that the electrospinning could be able to induce some protonation on the PANI. The possibility to protonate polyaniline through a free-acid process can be interesting for a wide range of applications. The PVDF/EB mats behave as an insulating material due to its porosity and the structure, like core-sheath, which result in high values of electric resistivity. This same insulating behavior was observed to PVDF/PANI.DBSA mats due to the inability to form continuous pathways through the fibers. For PVDF/PANI.DBSA, a doping level higher than 50% was not enough to reach electric conductivity similar to that of neat PANI. The results reported in this study showed that the introduction of doped and undoped PANI into fiber mats by mixing with PVDF does not provide a good level of electric conductivity [120].

2.15.23 Electrospinning of ethyl cellulose fibers A novel device to produce ethyl cellulose fibers, an important biomaterial in modern food processing, using a glass needle in a modified electrospinning setup has been investigated. The effect of applied voltage on the fiber aspect ratio was analyzed during electrospinning with a metallic (stainless steel) needle and then compared with that obtained with a glass-steel needle combination. A distinct difference in fiber diameter was observed between the two needle setups for the same processing conditions. A detailed quantitative study of the fiber length and diameter with respect to applied voltage was also carried out in order to determine any relationship between the needle material and the resulting electrospun fibers. There was an increase in fiber diameter in the case of steel-steel needle electrospinning with increasing applied voltage, while a decrease in fiber diameter was observed with glass-steel needle electrospinning for the same voltage [121]. Electrospinning using a combination of a glass and a steel needle was successfully carried out for the first time using ethyl cellulose as the processed material. A thorough comparison was made with a steel-steel needle electrospinning setup with regard to fiber aspect ratio and the influence of applied voltage thereon. Introduction of the glass needle to the electrospinning setup resulted in thinner fibers with increasing applied voltage, whereas the reverse was true when only steel needles were employed to form the fibers. The analysis of the electric field generation during electrospinning illustrated the need to take into consideration many other factors (such as needle geometry) associated with the entire fiber-forming process in order to fully understand the electric field and its effects on fiber generation and to move away from ­computer-simulated models. All the results indicate that glass-steel needle electrospinning setup can generate fibers with smaller diameters compared with steel-steel needle electrospinning setup [121,122].

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2.15.24 Electrospinning of gelatin The electrospinning of gelatin with tunable fiber morphology from round to flat/ribbon was shown, and the detailed studies were conducted to correlate the fiber morphology with electrospinning process parameters and gelatin concentration in electrospinning solution. Particularly, variations in the applied voltage and the concentration of gelatin led to the transition of fiber shape from round to flat/ribbon. The formation of flatshaped fibers was attributed to rapid evaporation of the solvent (formic acid) from the fiber matrix with increasing the applied voltage and gelatin concentration. On the other hand, round fibers were due to the steady evaporation of formic acid throughout the cross section of fibers. Wide-angle X-ray scattering (WAXS) analysis revealed the loss of triple-helical crystalline structure in gelatin after the electrospinning process. The gelatin fibers were cross-linked through treatment with toluene-2,4-diisocyanate (TDI) in a mixed solution of acetone and pyridine, and XPS confirmed the cross-­ linking of the fibers over an increased carbon content on the elemental composition of the fiber surface due to the incorporated TDI moieties. Overall, this study focuses on morphological tuning of gelatin electrospun fibers toward a flat/ribbonlike structure by the variation of electrospinning parameters and polymer concentration, and thus, the proposed concept can be adapted toward flat/ribbonlike fibers of other p­ rotein-based systems by electrospinning. Electrospinning is a process that involves electric forces to form fibers from a wide range of molecules, including numerous synthetic and natural polymers, and small molecules like cyclodextrins and cyclodextrin inclusion complexes. Thus, it is expected that the strength of electric field should lead to structural variations in electrospun fibers where higher applied voltage leads to thinner fibers due to rapid electrospinning of polymer solution. In most cases, the electrospinning produces fibers with circular cross sections, but in some cases, deviations from circular fibers can be observed [123]. For better understanding the effect of electric field on fiber morphology, gelatin concentration was increased from 20% to 25% (w/v) and the applied voltage from 10 to 22 kV. Intriguingly, with an increase of the applied voltage, a combination of flatand round-shaped fibers was obtained. This high-voltage-driven shape transition is generic for gelatin solutions (in formic acid) as observed for 20% (w/v) gelatin. The influence of electric field on the flat-/ribbon-shaped fiber formation was obvious and could be attributed to rapid electrospinning of gelatin solution. Likewise, flatshaped Nylon11 fibers were previously reported at the applied voltage of 20 kV in formic acid. The formation of flat/ribbon fiber structure was attributed to the applied voltage and gelatin concentration for the electrospinning. For instance, by increasing the applied voltage from 10 to 25 kV in the electrospinning process, the formation of flat/ ribbon-type fibers was clearly observed. During the electrospinning process, rapid release of formic acid at high voltages and gelatin concentrations might lead to flat/ ribbonlike electrospun gelatin fibers, whereas the formation of fibers with circular cross sections can be attributed to the steady evaporation of formic acid (i.e., homogenous shrinkage of the jet) from the fiber matrix. WAXS analysis revealed that the

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electrospinning of gelatin led to structural changes in gelatin polypeptides. The fibers were cross-linked by diisocyanate linkers (TDI) in a mixed solution of acetone and pyridine. This cross-linking route was highly efficient and led to highly cross-linked fiber mats. However, the fiber morphology was greatly influenced by the used pyridine content. XPS analysis demonstrated the cross-linking of the fibers with an increased carbon content of the fibers.

2.15.25 Electrospinning of graphene-oxide The graphene oxide (GO) was dispersed in poly(vinyl alcohol) (PVA) diluted aqueous solution, and the mixture was electrospun into GO/PVA composite nanofibers. The morphology of GO and surface-plasmon-coupled emission (SPCE) modified with electrospun nanofiber was studied using SEM micrographs and confirmed the uniform distribution of GO sheets in nanofibers. The alkaline phosphatase (ALP) was immobilized onto nanofibers grown on the SPCE, and the activity was checked using 1-­naphthyl phosphate (1-NP) by applying differential pulse voltammetry (DPV). The ALP activity was inhibited in the presence of HMs (heavy metals), and a difference in activity was recorded after inhibition. The inhibition was calculated based on the data of preand postinhibition current values. The inhibition of ALP was observed with mercury (Hg2+), lead (Pb2+), and cadmium (Cd2+) with a detection limit of 0.0075, 0.015, and 0.0312 ppb, respectively. The developed biosensor was also tested to evaluate the matrix suitability through recovery studies. Excellent HM recoveries were obtained in the range 94.6–99.75 with percent relative standard deviation (%RSD) value of 3.77. For the application of biosensors, it should be being not only sensitive but also specific. In order to determine the specificity of the biosensor, it was noticed from the obtained results that ALP is more specific to Hg2+ as against Pb2+ and Cd2+ in terms of inhibition. The Hg2+ was always predominant over other tested metals, and the discrimination of these three metal ions is easier based on the I% data. Considering the fact that SPCEs are cost-effective, it will be easier to use a sensor as a single-use device. However, for this purpose, there must be a good sensor-to-sensor reproducibility. The reproducibility of the developed biosensor was also investigated with interassay precision. The precision test was evaluated with the same Hg2+ concentrations (ppb) using three biosensors independently prepared in the same experimental conditions. A %RSD was calculated to 3.25, indicating the acceptable precision and reproducibility of electrodes. The long-term stability of the developed biosensor is further studied since it is an important parameter for the practical implementation of biosensor. The long-term stability may be expected due to the covalent interaction between 4-­chlorophenol (4-CP) modified electrodes and primary amine group in biomolecules that could prevent the leaking out of the ALP. The developed assay is rapid in terms of detecting metal ions in particular matrix [124–127].

2.15.26 Electrospinning of hyaluronic acid nanofibers For several reasons, the electrospinning of nanofibrous mats composed purely of biopolymers, such as hyaluronic acid (HA), has been difficult to achieve. Most notably,

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due to its polyelectrolytic nature, very low polymer concentrations exhibit very high solution viscosities. Thus, it is challenging to obtain the critical chain entanglement concentration necessary for biopolymer electrospinning to ensue. While the successful electrospinning of HA fibers from a sodium hydroxide/dimethylformamide (NaOH/ DMF) system has been reported, the diameter of these fibers was well above 100 nm. Moreover, questions regarding the degradation of HA within the solvent system arose. These factors support research into determining an improved solvent system. In this study, the use of a less basic (pH 11) aqueous ammonium hydroxide (NH4OH) solvent system, NH4OH/DMF, allowed for the fabrication of HA mats having an average fiber diameter of 39 ± 12 nm. Importantly, while using this solvent system, no degradation effects were observed, and the continuous electrospinning of pure HA fibers was possible [128–130]. Pure HA mats with an average fiber diameter of 39 ± 12 nm have been successfully electrospun using a new solvent system that features aqueous NH4OH. This solution disrupts the rigid structure of HA and permits the critical chain entanglement to be reached. Thus, the electrospinning of continuous, cylindrical, and randomly oriented HA fibrous mats devoid of remnant solvent was demonstrated. Working with solvents at either end of the pH spectra can pose compatibility issues for the bulk biopolymer and biocompatibility of the mats. Thus, our fabricating of nanofibrous HA mats with an unaltered electrospinning system and a less basic solvent system is encouraging for their potential use in biomedical applications.

2.15.27 Electrospinning of chitosan nanofibers Specifically, there is a wide range of spinnable polymers; however, only a few of them have the necessary parameters to make them suitable for use in such critical applications. One polymer that does exhibit the necessary characteristics is chitin, along with its deacetylated derivative, chitosan. Chitin is the second most abundant polysaccharide after cellulose and has a similar structure. Chitosan is insoluble in water, alkali, and most mineral acidic systems. However, though its solubility in inorganic acids is quite limited, chitosan is in fact soluble in organic acids, such as dilute aqueous acetic, formic, and lactic acids. In the presence of a limited amount of acid, chitosan is soluble in water-methanol, water-ethanol, and water-acetone mixtures. Chitosan also has free amino groups that make it a positively charged polyelectrolyte in pH below 2–6 and that contribute to its higher solubility in comparison with chitin. However, this property makes chitosan solutions highly viscous and complicates its electrospinning. Furthermore, the formation of strong hydrogen bonds in a 3-D network prevents the movement of polymeric chains exposed to the electric field. The problem of chitosan high viscosity, which limits its spinnability, is resolved through the application of an alkali treatment that hydrolyzes chitosan chains and so decreases its molecular weight. Solutions of the treated chitosan in aqueous 70%–90% acetic acid produce nanofibers with appropriate quality and processing stability. Decreasing the acetic acid concentration in the solvent increases the mean diameter of the nanofibers. Optimum nanofibers are achieved with chitosan that is hydrolyzed for 48 h. Such nanofibers result in a moisture regain that is 74% greater than that of treated and untreated chitosan powder.

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The diameter of this nanofiber, 140 nm, is strongly affected by the electrospinning conditions and by the concentration of the solvent. FTIR investigations prove that neither the alkali treatment nor the electrospinning process changes the chemical nature of the polymer [131]. The electrospinning of chitosan was investigated using different molecular weights of chitosan and different concentrations of AcOH as a solvent, and the optimum conditions leading to high-quality chitosan nanofibers in a steady electrospinning process were introduced. The diameter of the optimum product is 140 nm, which corresponds to chitosan hydrolyzed for 48 h with the molecular weight of 2.94 × 105 g/mol. These nanofibers present 13.95% moisture uptake, which is 74% higher than that of the polymer powder. FTIR spectroscopy also revealed that alkali treatment has no effect on the chemical nature of chitosan and its nanofibers, though it may affect secondary bonds. Chitosan hydrolyzed for 48 h is spinnable in lower concentrations of AcOH, and the solution of this polymer at 7 and 7.5 wt% in AcOH 80 and 70% leads to nanofibers with mean diameters of 250 and 284 nm, respectively.

2.15.28 Electrospinning of immiscible systems: The wool keratin/ polyamide-6

Keratin

PA6

g

stin

a pc

o

Dr

ng

r yi

d low

S

Segregation

Blends of wool keratin and polyamide 6 (PA6) have shown interesting adsorbent properties in filtration. The miscibility of keratin and PA6 was studied through rheological measurements of diluted blend solutions. In particular, the immiscibility between the two polymers in different blend proportions was observed by the segregation of phases in the cast films. Nevertheless, notwithstanding the immiscibility, homogeneous keratin/PA6 blend nanofibers can be obtained by electrospinning as a result of rapid solvent evaporation, where kinetic effects prevail on thermodynamic effects, thereby avoiding phase segregation. The obtained nanofibers have diameters ranging from 100 to 250 nm, depending on the experimental conditions as shown in Fig. 2.63 [132].

ctr os pin Fa nin st dr y g ing

Fig. 2.63  Electrospinning of wool keratin/PA6 blends [132].

No segregation

Ele

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The miscibility degree of wool keratin/PA6 blend solutions and the related electrospinning process were studied. Notwithstanding similar chemical structures, that is, both polymers are characterized by amide bonds, viscometric measurements of blend diluted solutions suggest immiscibility between two polymers in all blending ratios. The immiscibility was confirmed through observations of phase segregations in the blend films obtained by casting. However, differently to inhomogeneous mixtures obtained by the slower solvent evaporation of the casting process, we obtained blend nanofibers with good homogeneity and no phase segregations through the electrospinning process. In this case, the formation of nanofibers occurs through a rapid solvent evaporation that is dominated by kinetic effects preventing polymer separation. The effects of the electrospinning process parameters on the morphology of the nanofibers were investigated by principal component analysis revealing that the percentage of keratin is negatively correlated to increasing diameters while viscosity and conductivity are positively correlated to increasing diameters. On the other hand, voltage and flux appear not to be significantly important for diameters of fibers. Moreover, spectroscopic and thermal analysis pointed out that keratin and PA6 interfere each other in the supramolecular arrangements: in particular, the keratin presence seems to hinder the formation of α crystallites of PA6. Finally, an intriguing kinetic effect was observed in the supramolecular organization of keratin and PA6, since both keratin and PA6 took on an unusual crystalline configuration in the nanofibers [132].

2.15.29 Electrospinning of ion jelly fibers Gelatin is a widely available, inexpensive, and well-studied gelling agent. It is prepared through the thermal denaturation of collagen, after acid or alkaline pretreatment. The viscoelastic properties of gelatin allow for a wide range of applications, such as water-soluble, gelatin-based electrolytes. Ionic liquids (room-temperature ionic liquids, ILs) are salts composed of an organic cation and an organic or inorganic anion, with a melting point at or close to room temperature. Ionic liquids (ILs) have been pointed out as a very promising solution to electrolytes for different electrochemical devices. This is motivated by the fact that these compounds exhibit high conductivity (10−4–10−2 S cm−1), high electrochemical stability (4–5.7 V), and high thermostability (up to 300°C) but especially because most ILs are nonvolatile and nonflammable, which are essential requisites for electrochemical devices. In addition, IL properties (e.g., polarity and ionic conductivity) can be tuned through changes in cation and anion, which has led to the designation “designer solvents.” The introduction of ILs in gelatin polymers could extend the applications of gelatin to different fields of research. Conductive biomaterials can provide unique electroactive surfaces by electric or electrochemical stimulations to be useful for various cell- and tissue-culture applications, such as biosensors, medical devices, or tissue repair and regeneration [133,134]. The conductivities of Ion Jelly fibers are of the same order of magnitude as the conductivities of Ion Jelly dense films (~10−4 S cm−1). This was an expected result since the conductivity of Ion Jelly materials is imparted by the IL. On the other hand, the conductivities of Ion Jelly materials are about one order of magnitude lower when compared with the IL alone. This decrease in conductivity is related with the higher

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ion mobility in the liquid when compared with the gel. Nevertheless, these conductivity values are quite acceptable for solid/gel/polymer electrolytes.

2.15.30 Electrospinning of nonpolymeric systems Electrospinning of nanofibers from nonpolymeric systems is rather challenging; however, it was carried out from two of the native cyclodextrins (CDs): a-CD and b-CD. Electrospinning was carried out for highly concentrated solutions of a-CD (120%– 160%, w/v) and b-CD (120%–150%, w/v) in basic aqueous system. At optimal concentration level, the electrospinning of CD solutions yielded bead-free uniform CD nanofibers without using carrier polymeric matrix. Similar to polymeric systems, the electrospinning of CD solutions resulted in different morphologies and average fiber diameters depending on the CD type and CD concentration. The dynamic light scattering (DLS) and rheology measurements were performed in order to examine the electrospinnability of CD solutions. The existence of CD aggregates via hydrogen bonding and very high solution viscosity and viscoelastic solid-like behavior of CD solutions were found to be the key factors for obtaining bead-free nanofibers from CDs. The addition of urea disrupted CD aggregates and lowered the viscosity significantly, and therefore, the urea-added CD solutions yielded beaded fibers and/or beads. Although the as-received CDs in powder form are crystalline, the structural analyses by XRD and high-resolution transmission electron microscopy (HR-TEM) indicated that electrospun CD nanofibers have amorphous characteristic without showing any particular orientation or crystalline aggregation of CD molecules [135–139]. Nanofibers were formed from native CDs of a-CD and b-CD via electrospinning technique without using any carrier polymeric matrix. At lower CD concentrations, beaded nanofibers were obtained, but as the CD concentrations were increased, the transformation from beaded to bead-free nanofibers was observed. The optimal concentrations for producing bead-free nanofibers were 160% and 150% (w/v) for a-CD and b-CD, respectively. The DLS and rheology measurements indicated the presence of self-associated CD aggregates in the solutions. It was found that the high solution viscosity and viscoelastic solid-like behavior of CD solutions played a key role for the electrospinning of bead-free nanofibers from these two native CD types. The size of CD aggregates got smaller, and the viscosity of the CD solutions decreased significantly with the addition of urea. This situation affected the electrospinnability of CD solutions, and beaded fibers and/or beads were obtained. The XRD and HRTEM studies revealed that electrospun CD nanofibers were in amorphous state. CDs are naturally occurring nontoxic cyclic oligosaccharides having host-guest inclusion complexation capability with other molecules. So, electrospinning of CD nanofibers would have unique properties by combining the very large surface area of nanofibers with specific functionality of the CD. For instance, native CDs have different cavity sizes that can allow selective inclusion complexation with various molecules of different sizes. In addition, CDs are already being used in various fields including pharmaceutical, food, textiles, biotechnology, and filtration/separation systems. Hence, CDs in the form of nanofibrous web may extend the use of CDs in the aforementioned areas [140].

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2.15.31 Electrospinning of nickel oxide nanofibers Nickel oxide (NiO) nanofibers (NFs) are of great practical interest in several engineering applications such as sensing electrochemical capacitance, photocatalysis, and other chemical catalysis. Moreover, their morphology has gained significant attention due to strong dependence of properties on the morphology. Whereas several physical techniques have been successfully employed for their synthesis, electrospinning has turned out to be a better choice because of its simplicity, economy, scaling capability, and control over the NF morphology. A variety of metal oxide NFs have been synthesized through this technique that have shown promising potential in different engineering domains such as sensing, catalysis, and solar energy conversion. It has been demonstrated that electrospinning can also be employed for synthesizing pure NiO NFs using an appropriate precursor composed of NiAc salt and any suitable polymer such as PVP, PAN, or PVA. However, no report exists regarding the morphology control of these electrospun NiO NFs. It is found that using different proportions of NiAc in the solution containing PVA as polymeric component, the diameter and roughness of NiO NFs can be easily controlled, and an optimum proportion of NiAc is mandatory for obtaining smooth and continuous fibers. The diameter of these NFs can be further reduced by increasing the electrospinning voltage. These electrospun NiO NFs with varying morphologies and microstructures that are producible in an economical and scalable fashion have potential applications in sensing, catalytic, magnetic, and photovoltaic domains [141,142]. The rough and discontinuous NiO NFs are of great practical interest in applications such as fuel cell electrodes and several sensing and catalytic applications, where high specific surface area is of key importance. On the other hand, smooth and continuous NiO NFs are highly desirable in electric, thermal, and magnetic applications where minimum electron scattering from irregular nanofiber boundaries is required, especially when these NiO NFs are further reduced to obtain pure Ni NFs. Moreover, since the diameter of NFs can be easily tuned via electrospinning parameters such as voltage and flow rate, a systematic study is possible on these NFs to determine the characteristic diameter where the quantum confinement effects become more pronounced. Thus, electrospinning provides a simple and economic way of fabricating NiO NFs with varying morphologies that can be applied in plenty of engineering applications.

2.15.32 Electrospinning of Nylon11 Nylon11 (N11) is a high-performance polymer exhibiting piezoelectric and pyroelectric properties, good chemical resistance, low water absorption, and high impact strength. Nylon11 forms a crystalline phase in which the aliphatic chains are aligned in parallel, while the amide group dipoles are oriented perpendicular to the chain direction. The crystalline structure and polymorphism of Nylon11 have been studied by several researchers who described the influence of electrospinning parameters on the morphology of Nylon11 electrospun mats and the microstructure of the fibers constituting the mats. Specifically, the effects of solution concentration, applied voltage, and distance between the electrodes on the diameter and shape of the nanofibers

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are investigated. Various techniques were used to characterize the microstructure of the Nylon11 electrospun nanofibers, and the same was compared with the structure of melt-pressed and solution-cast films of Nylon11. It is shown that rapid formation of electrospun nanofibers causes the Nylon11 chains to crystallize in the metastable gamma phase and show lower degree of crystallinity. Further, the fibers show higher glass transition temperature and higher activation energy for the glass-to-rubber transition suggesting the possibility of a rigid amorphous phase [143]. The morphology of the mats and the microstructure of the nanofibers of the electrospun mats were characterized using multiple techniques. These were compared with melt-pressed and solution-cast films of Nylon11. The polymer concentration affected the solution viscosity and conductivity, with the effect of viscosity being more dominant. Increasing Nylon11 concentration increased the fiber diameter in the electrospun mats. Other process variables did not have much effect on the fiber diameter. X-ray diffraction data showed that while the melt-pressed film exhibits alpha crystalline structure, the solution-cast film and electrospun mats showed the presence of gamma crystalline structure. The formation of gamma crystals in electrospun mats is attributed to the rapid solvent evaporation combined with stretching experienced by the polymer during electrospinning. The thermal properties of electrospun nanofibers showed reduced crystallinity than solution-crystallized samples. Viscoelastic measurements of the electrospun mats showed higher ɑ-relaxation temperature and higher activation energy for ɑ-relaxation compared with melt-pressed film and solution-cast film.

2.15.33 Electrospinning of PLGA/gum tragacanth nanofibers Periodontitis is a major chronic inflammatory disorder that can lead to the loss of periodontal support for the periodontal ligament that leads to the formation of an abnormal gap between the tooth and gum. If the process continues, the tooth can eventually get lost. For chronic periodontitis, local antimicrobial agents are used as an adjunct to scaling, root planning, and restoring the periodontal health. Multiple investigations have been conducted to incorporate antibiotics into the polymeric carriers, in order to develop a DDS for the treatment of periodontal diseases. Tetracycline hydrochloride (TCH)-loaded blend and core-shell nanofibers with smooth and beadless morphology were successfully fabricated from poly(lactic-co-glycolic acid) (PLGA) and GT for application as new and controlled drug delivery systems. Drug delivery systems are engineered technologies for the controlled release of therapeutic agents to achieve therapeutic purposes in humans or animals. Controlled drug-release systems have shown benefits over conventional drugs, such as improved adequacy, reduced side effects, improved patient compliance, and reduced toxicity. Electrospinning is one of the developed techniques, which enables the design and production of nanostructured drug carriers with high loading capacity, encapsulation efficiency, multidrug delivery with ease of operation, and cost-effectiveness. In most cases, drugs are blended with the polymeric solution to produce drug-incorporated nanofibers, which might result in low delivery efficiency and burst release, while other electrospinning techniques such as emulsion or coaxial electrospinning showed capabilities to overcome some of these problems. Use of nanofibrous scaffolds loaded with

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antibiotic drugs for biomedical applications especially for the treatment of infections after tissue damages such as burn, ulcers, surgery, or periodontal disease has evoked considerable interest. Mefoxin incorporated PLGA membrane displayed a controlled release of the drug for over 6 days. However, controlled release of antibiotics is required for a longer period of time for the treatment of some of the chronic infections such as periodontal diseases. Nevertheless, long-term release of hydrophilic drugs (such as TCH) from nanofibrous scaffolds is still challenging due to high solubility of the drug molecule in aqueous mediums. It was previously demonstrated that the compatibility between hygroscopic properties of drug and polymer is essential to obtain a sustained drug release from nanofibrous delivery system [144,145]. The cytocompatibility studies showed that GT could successfully support cell growth on nanofibrous membranes in both blending and core-shell architecture. This might be attributed to the more hydrophilic properties of GT-contained membranes, which improves protein adsorption and subsequent cell attachment and proliferation. On the other hand, incorporation of TCH showed to decrease cell growth in some of the formulations due to the inhibitory effect of this drug on mitochondrial protein synthesis. However, none of the GT-contained scaffolds showed significant changes in cell viability compared with the control, which demonstrates good cytocompatibility of the composite membranes. Antibacterial assessment of drug-loaded PG (PLGA-GT, poly lactic-co-glycolic acid-gum tragacanth) and PG(cs) (PLGA-GT, core shell) nanofibers showed that these scaffolds are strong enough against of S. aureus bacteria.

2.15.34 Electrospinning of polyaniline for anticorrosion For metallic materials, corrosion reactions have been unevitable natural processes. Great achievements have been obtained to prevent or at least weaken the corrosion processes. One of the promising strategies is the utilization of barrier coatings. Recently, an arising technique of employing conducting polymers has attracted ­extensive attentions in corrosion inhibitors for metals. Among known conducting polymers, polyaniline (PANI) has a high place because of its high electric conductivity, good environmental stability, relatively low cost, and reversible doping-dedopingredoping chemistry. More importantly, PANI played not only physical barrier but also electrochemical protection effects, allowing for potential anticorrosive coatings on metallic substrates. These promising performances are attributed to the elevation of the corrosion potential along with a protective passivation layer from redox properties of PANI. Researchers have launched studies of utilizing PANI nanofibers as anticorrosion coatings for carbon steels in NaCl solution (3.5 wt%). One of the crucial issues for drop-casting fabricated PANI nanofiber coatings is that the uniformity and poor adhesiveness of the PANI nanofiber coatings limit the further enhancement of anticorrosion properties. In this fashion, a prerequisite of synthesizing highly efficient anticorrosion coatings is to homogeneously disperse PANI nano-/microstructures in coatings. Electrospinning has been recognized as a considerable technique in the fabrication of micro- or nanoscaled conductive polymer fibers. For example, PANI electrospun fibers were prepared by using the mixture of PANI-H2SO4 and commonly used polymers such as poly(ethylene oxide), polystyrene, polycaprolactone, or

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polyacrylonitrile. A remaining but important issue is the inherently poor solubility of PANI in the used solvents, making it difficult for PANI to disperse uniformly in the polymeric matrix. PANI/PMMA microfibers have been successfully prepared by an electrospinning strategy and used as anticorrosion coatings for carbon steel. By optimizing PANI dosage, the 25 wt% PANI/PMMA coating displays promising anticorrosion properties in 0.1 M H2SO4 solution. The maximum corrosion protection efficiency of electrospun PANI/PMMA coating is as high as 99.99%, which is nearly 500 times higher than that from traditional drop-casting PANI/PMMA coating. Equally importantly, the corrosion protection efficiency remains as high as 99.96% even suffering 20-day immersion in 0.1 M H2SO4. These results pave the way for further advances in efficient and cost-effective anticorrosion coatings [146].

2.15.35 Electrospinning of TiO2 nanofibers Nanotechnology can provide ultrahigh porosity and surface area. As a result, the photocatalytic activity of TiO2 and the efficiency of DSSCs using TiO2 films can be significantly improved by fabricating it in nanostructured form. Electrospinning has been successfully applied in the fabrication of many ceramics, such as SnO2, BaTiO3, In2O3, and complex cobaltite. The structural properties of the synthesized fibers can be controlled by changing the composition of the solution, the strength of the applied electric field, and the temperature for heat treatment after electrospinning. Dopants in TiO2 can improve its properties. Li-doped TiO2 is considered to be a promising material for Li-ion batteries and supercapacitors. Ca-doped TiO2 has been shown to increase the energy conversion efficiency in DSSCs. In general, the anatase phase of TiO2 has the highest activity as a photocatalyst, which means that increasing the anatase content of TiO2 can potentially improve the efficiency of any catalytic processes. However, few investigations have been conducted on the influence of vacuum processing on this phase transformation [147]. A simple method of sol-gel electrospinning was employed to fabricate pure, Lidoped, and Ca-doped TiO2 nanofibers. Electrospinning is able to produce TiO2 nanostructures with high surface area and porosity, suggesting that it is an appropriate technique for applications of TiO2 that require high surface area, such as photocatalysis and DSSCs. SEM and XRD analyses show that Li doping can reduce the diameters of TiO2 nanofibers, increase the grain size, and lower the ratio of anatase to rutile phase, while Ca doping has the opposite effects. Compared with the normal heating process, calcining in a vacuum furnace gives decreased fiber diameters and increased formation of the rutile phase. Hence, Li doping and vacuum heating may be beneficial to applications requiring high surface area, while Ca doping may be beneficial for applications where performance is enhanced by the presence of the anatase phase.

2.15.36 Bubble electrospinning Bubble electrospinning was invented in 2007. In contrast to the classical electrospinning, of which the electrospinnability mainly depends on solution properties, bubble

Electrospun nanofibers133 Collecter

Solution reservoir

Jets

Gas pump

Power supply

Gas tube Metal electrode Tube support

Fig. 2.64  Principle of bubble electrospinning [148].

electrospinning depends geometrically on sizes of produced bubbles. When no voltage is applied, the surface tension depends geometrically upon the size of the bubble. When an electric field is present, it induces charges onto the surface of bubbles in clusters and solution surface. The surface charge coupling and the external electric field create a tangential stress, leading to the deformation of the bubble into a p­ rotuberance-induced upward-directed reentrant jet, as demonstrated in Fig.  2.64. Once the electric field exceeds the critical value needed to overcome the surface tension, one fluid jet ejects from the apex of the conical bubble. When the bubble is broken, the surface charges will be redistributed, and the bubble surface was pulled upward by electric force again; thus, multiple jets are formed in a very short period [148].

2.16 Recent developments in electrospinning Electrospinning (e-spinning) has been extensively explored as a simple, versatile, and cost-effective method in preparing ultrathin fibers from a wide variety of materials. Electrospun (e-spun) ultrathin fibers are now widely used in tissue scaffold, wound dressing, energy harvesting and storage, environment engineering, catalyst, and textile. However, compared with conventional fiber industry, one major challenge associated with e-spinning technology is its production rate. Over the last decade, compared

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with conventional needle e-spinning, needleless e-spinning has emerged as the most efficient strategy for large-scale production of ultrathin fibers. For example, rolling cylinder and stationary wire as spinnerets have been commercialized successfully for significantly improving throughput of e-spun fibers. The significant advancements in needleless e-spinning approaches, including spinneret structures, productivity, and fiber quality, are reviewed. In addition, some striking examples of innovative device designs toward higher throughput, as well as available industrial-scale equipment and commercial applications in the market, are highlighted [149]. The strong electrostatic force has been regarded as the driving force for initiating the e-spinning process. In the role of strong electrostatic field, the charged solution jet at the spinneret tip changes its size to maintain the force balance. With the increasing electrostatic field intensity, the induction charges on the surface repel each other and produce shear stresses. These repulsive forces act in the opposite direction to the surface tension, which leads to the extension of the solution drop into conical shape (generally called Taylor cone) and plays a role of initiating the surface. When the electrostatic field achieves the critical voltage Vc, the balance of repulsive forces is broken, and thus, a charged jet ejects from the tip of the conical drop. The critical voltage Vc for e-spinning is given by the following expression based on Taylor’s calculation. Fig. 2.65 shows a wide range of electrospun nanofibrous structures.

2.16.1 Upward e-spinning from stationary spinnerets Fig. 2.66 shows the schematic summary of the various stationary spinnerets for upward needleless e-spinning. What they have in common is that the fiber generators are right below the fiber collectors. In this method, bubbles produced by compressed air or nitrogen are blown into the solution. As the bubbles burst on the surface, multiple temporary jets are created, and e-spinning is initiated. This technique has a higher fiber throughput than needle e-spinning; however, strip-like and sphere-like morphologies occurred easily because of the broken bubble. Compressed gas through a porous surface was injected into polymer solutions to form charged multiple jets. The key design, processing, and solution parameters for producing uniform fibers were also identified. Above all, with more established needleless high-throughput e-spinning process, it remains to see whether bubble e-spinning can present some new features [150]. A 1-D electrohydrodynamic theory was developed to explain the e-spinning process of conductive liquids from an open plane surface. During the free liquid surface e-spinning process, the amplitude of characteristic wavelength grew faster because of the electric force. As reported, the fastest-growing stationary wave marked the onset of e-spinning from a free liquid surface. This theory not only predicated the critical values of the e-spinning process but also explained the fundamental of upward needleless e-spinning. Though many ways have potential in scaling up the e-spun fibers, very few can make actual application. Elmarco introduced production lines with stationary wire spinneret in 2010. During the e-spinning process, the high voltage is applied on the stationary wires, and polymer solution is loaded on the surface of the wires by a reciprocating movement polymer solution container. Then, numerous jets

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Fig. 2.65  Genesis of electrospun nanofibers [149]. (A) Fibrous web, (B) Spiral, (C) Twisted, (D) Woven, (E) Intermeshed, (F) Bundles, (G) Zigzag, (H) Bilobal, (I) Coaxial, (J) Concentric, (K) Multichannel, (L) Hollow, (M) Beaded strand, (N) Beads, (O) Fibers with beads, (P) Globules, (Q) Porous fibers, (R) Multilayered porous, (S) Fiber net, and (T) Multiscale fibrous web.

are generated from the wires. Figs. 2.67 and 2.68 show the newest design of Elmarco company’s commercialized Nanospider, and it has a better performance in producing e-spun fibers in industrial level than its first generation of Nanospider that uses rotary cylinder as spinneret [149–154].

2.16.2 Sideward e-spinning from stationary spinnerets Besides the upward and downward e-spinning, some kinds of spinnerets, such as flat electrode edge, porous tube, and bowl edge, are much more appropriate to put the

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(A)

(B)

Collector Jets

Bubble

High voltage power supply

(C)

Gas supply Grounded collection plate

Power supply Polymer foam column

Copper electrode + Pressure gauge P

Plate

Holes

Compressed gas tank Rotameter

Fig. 2.66  Upward e-spinning [149]. (A) Electrospinning from single bubble, (B) Bubble image, and (C) Foam.

collector sideward (Fig.  2.69), which is called “sideward e-spinning.” E-spinning utilizing edge-plate configuration functions in a remarkably similar manner to traditional needle e-spinning. However, this method is much easily implemented, without the possibility of clogging, and has high scale-up potential. During e-spinning, polymer solution flows down to the edge along the plate surface under the gravity. Subsequently, e-spinning jet will be generated near the plate edge. Unlike hollow porous polytetrafluoroethylene tube drilled with linear holes, porous polyethylene tube was used as a spinneret (Fig. 2.69) with a surrounded circle collector. The polymer filled in the porous walled cylindrical tube was pushed through the pores to form drops on the outer surface of the tube. When the solution was charged, jets issued from the drops and formed many e-spun fibers [155].

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Fig. 2.67  Elmarco commercial e-spinning [149]. (A) Device, (B) Drum, and (C) Various diameters.

Fig. 2.68  Principle of Elmarco e-spinning [149]. (A) Schematic and (B) Actual image.

2.16.3 Needleless e-spinning from rotary spinnerets Besides the stationary spinneret e-spinning techniques, rotary spinneret e-spinning (Fig. 2.70) has also attracted much attention. In the rotary spinneret e-spinning equipment, the spinnerets are all connected with a high-voltage power supply and driven by a motor. Polymer solution is loaded on the surface of spinnerets and e-spun at the nearside. These needleless rotary spinnerets are featured as simplicity of design and high production rate of e-spun fibers. The comparison of different modifications of rotary

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Fig. 2.69  Sideward e-spinning [149]. (A) Porous drum and (B) Twisted wire.

Uncharged

Charged

Fig. 2.70  Needleless e-spinning from rotary spinnerets [149].

spinnerets for needleless e-spinning is shown. Mostly, the needleless rotary spinnerets are partially immersed into the polymer solution, and the ultrathin fibers are e-spun upward, which effectively prevents the polymer liquid from dropping onto the fiber collector. Numerous jets will be generated from the upper part of the spinneret that is closer to the collector when high voltage is supplied.

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Solution

Edge

Stationary spinnerts --Sideward ES--

Porous tube

Bowl

Fig. 2.71  Other principles of needleless e-spinning [149].

Fig. 2.71 represents three different methods of needleless e-spinning. The rim radius of cylinder spinneret can reduce the discrepancy of electric field intensity and influence the fiber productivity. Thinner disk spinnerets increased the electric field intensity, leading to finer nanofibers and higher throughput. Ball spinnerets generated evenly distributed electric field but failed to electrospun fibers when the diameters are below 60 mm [149,156]. Maximum of 40 jets from a bowl as shown in Fig. 2.72 are obtained at the same time, and the total number of jets tends to decrease with the increase of the viscosity of the solution. Besides, the influence of surface tension and conductivity on the productivity was also studied. Experimental observations, including time required for the initial jet formation, total number of jets, feed rate per jet, and resultant fiber diameter, are reported and compared with theoretical predictions. Although the e-spinning setups with rotary e-spinning spinnerets that are partially immersed into the polymer

(A)

Collector

(B)

(C)

Bowl

Working distance

HVPS

5 cm

Fig. 2.72  Bowl electrospinning [149]. (A) Schematic, (B) Actual picture, and (C) Fibrous web.

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solution have simpler structures, the flow rate of polymer solution cannot be controlled precisely. Moreover, the polymer concentration will increase gradually because the open design of the solution container cannot protect the solvent of polymer solution from evaporation [157]. Based on the above, the needleless e-spinning has great potentials in large-scale production of ultrathin fibers for both industrial production and laboratory research. Nevertheless, there has been significantly less industrial transposition of this technique compared with melt blowing and melt spinning. On one hand, the inherent challenge of achieving sufficient economies of scale has been a key factor in the commodity fiber industry. On the other hand, for example, even the most commercialized Nanospider has drawbacks such as unstable jets and electric field intensity around the spinnerets, which leads to the limitation of this technology in industry [149,157]. As a particular form of electrospraying or electrostatic atomization, e-spinning shares distinguishing features of both electrospraying and conventional solution dry spinning of fibers. From a translational research point of view, the market for e-spinning equipment and e-spun materials with high surface areas and porosities is expected to grow significantly in the 21st century. For both laboratory research and industrial production, e-spun ultrathin fibers have already given increasing competition among e-spinning equipment suppliers. A large number of companies develop different spinning designs, devices, and accessories to struggle for market shares. Due to the unique features and properties that distinguish from other ultrathin fiber production techniques, e-spun ultrathin fibers have found extensive applications in a wide range of industrial fields (as shown in Fig. 2.73). In addition, plenty of productions fabricated by industrial e-spinning equipment have been fruitfully transferred into commercial applications. Currently, with higher filtration efficiency and lower pressure drop, filtration and purification of fibrous materials show great competitive force in environment engineering [158]. As one of the remarkably simple and powerful techniques for generating ultrathin fibers, e-spinning has explored a broad range of applications like drug delivery, tissue engineering, energy conversion and storage, and environmental engineering. Thanks to the efforts of many research groups, the morphology and structure of e-spun ultrathin fibers have been managed by various designs and modifications. Furthermore, the range of e-spun materials has been developed into a rich variety of organic polymers, ceramics, and composite materials. Compared with other 1-D fiber fabrication processes, the production rate of conventional e-spinning method does not meet the demand for practical application. Needleless e-spinning has now emerged as a technique capable of generating ultrathin fibers in the absence of needlelike spinnerets, providing benefits including eliminating the problem of clogging and increasing the fiber productivity sharply. At the current stage of development, even though the cylinder and stationary wire approaches have been commercialized for high throughput, limited available designs are suitable for practical application in industry level. On the one hand, the productivity and throughput of some functional ultrathin fibers are far below what is needed by practical demand. On the other hand, the quality control and cost of production are also expected to handle [159,160].

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e on s s ati on Ga r filtr trati Ai d fil ui Liq

rgy

Con

Ene

er y batt ium ll Lith l ce Fue ll r ce Sola

sum er

t en m n ro vi n E

Health

tect ive clot Foo hing d pa ckin Cos g met ic

E Ph lectr on o t Pi ez ovo ic d oe lta e lec ic d vice tri e c d vic El ev e ec ice tro ni c

rs

Pro

o ns

Wound healing Tissue engineering Drug delivery

Fig. 2.73  Nanofiber applications in industrial scale [149].

The architecture of e-spun nonwoven should be defined by a series of standard characterization for practical application. Although e-spun fibers have been explored for the broadest potential applications, the standard and norms of e-spinning have not yet been established. Ultimately, further development of e-spinning technique will need extensive collaborations and combines with other well-established techniques to significantly improve their performances. With the continuous development of e-­ spinning technology driven by the academic and industrial researches, a high-impact practical application is expected to achieve in the near future.

2.16.4 Liquid shear-driven fabrication of polymer nanofibers The shear precipitation process takes place during direct injection of polymer solutions in the bulk of a viscous medium under shear. The polymer solvent is miscible with the shearing medium. A critical fourth component of the systems is a polymer antisolvent, mixed within the shear medium, which induces the precipitation of the injected dissolved polymer. The typical media contained glycerin and 20%–90% of antisolvent, such as water or ethanol, inducing polymer precipitation in the injected sheared solution. The initial characterization of the role of process parameters on the

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Fig. 2.74  Shear-driven electrospinning [161].

resulting micro- and nanomaterials was performed by using a simple benchtop batch device with concentric cylinder (Taylor-Couette) geometry, assuring controlled and easily variable uniform shear rate. A small amount of polymer solution is injected directly in the viscous fluid between the rotating cylinders, and the polymer structures formed in the liquid are examined microscopically as shown in Fig. 2.74 [161]. This continuous flow device is presently being scaled up to the commercial fabrication of nanofibers at rates that could exceed tens or hundreds of kilograms per hour. The process has a few specific features that can make it of high value to commercial nanotechnology, largely stemming from its ability to make nanofibers dispersed and carried in a flux of liquid as opposed to methods such as electrospinning and melt blowing, where dry fibers are carried and deposited by gas phase. Streams of liquidborne staple nanofibers can be easily integrated with industrial wet-laying lines and spraying devices for large-volume production of filters, coatings, mats, and nonwovens. Before wet laying or spraying, the suspended nanofibers can be mixed with commercial microfibers, resulting in intermixed micro-/nanofiber materials with very highly developed area and high particle capturing capability. Such liquid-­deposited nanofiber/microfiber composites could also serve as excellent cell scaffolding and growth substrates, which are being presently investigated. The ability to control morphologies, together with the multitude of functionalization possibilities by particle additives, could in the future open the way to the large-scale fabrication of diverse classes of other nanomaterials. The liquid-shear fabrication of nanofibers and nanoribbons at unprecedented rates and volumes could enable their use as bulk components of products ranging from filters to bioscaffolds and could have a transformative impact on the emerging area of nanomanufacturing [162].

2.16.5 Vertical rod method for electrospinning polymer fibers Electrospinning is often used to produce submicron-sized fibers. Most electrospinning applications employ charged needles or nozzles but have low production rates. For higher production rates, several needleless systems have been developed. The vertical rod system (shown in Fig. 2.75) on a simple unique needleless method employs charged vertically oriented threaded rods for holding multiple drops to launch many simultaneous jets. Experiments were conducted with a single rod or multiple rods

Electrospun nanofibers143 0.5 cm

(A)

Rubber stopper to hold the rod in the PVC pipe

Copper wire

(C)

Resorvoir Solution flow down through the grooves

+ 35 kV Pover supply

Multiple jets

Pipe wall

Multiple jets

Grooves for solution flow

(B) 20 cm

Circular collector

Threaded rod covered with solution

Solution flows down through the grooves

Fig. 2.75  Vertical rod electrospinning [162]. (A) Schematic, (B) Rod surrounded by fibers, and (C) Displacement of solution.

arranged in a linear array. The direction of the launched jets was controlled using a secondary electrode to direct the jets toward the grounded collector. Results show a single rod of about 50 cm in length can produce PVP fibers at a rate of 4.5 g/h and 200–400 nm size range [162]. A needleless vertical rod electrospinning method was successfully applied to fabricate polymer electrospun fibers. Multiple polymer jets were launched from the surface of the vertical rod electrode. The fiber morphologies and average fiber sizes were unaffected by the method (syringe or rod), but the total production rate of fibers was significantly greater with the vertical rods. The single vertical rod with coaxial cylindrical collector had the greatest total fiber production rate of about 4.5 g/h, and the single needle had the least production rate of 0.17 g/h. The vertical rods in arrays with secondary electrodes to direct the fibers toward a smaller planar collector surface had total production rates greater than the single needle, and the five-rod production rate per area of collector surface was higher than that of the single rod with coaxial cylindrical collector.

2.17 Applications of nanofiber membranes In the biomedical field, it is now an established fact that almost all tissues and organs such as skin, collagen, dentin, cartilage, and bone, in one way or another, have some sort of resemblance to highly organized, hierarchical, nanosized fibrous structures. Therefore, research on biomedical applications has focused on (i) the generation of fibrous scaffolds for tissue engineering, (ii) wound dressing, (iii) drug delivery mechanisms, and (iv) enzyme immobilization to achieve faster reaction rates in biological reactions.

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There are many articles in the literature that highlight the importance of the biomedical applications of electrospinning. Because of their unique properties such as their morphology (dimensions), high surface-area-to-volume ratio, and inter-/intrafibrous porosity, electrospun nanofibers are regarded as promising scaffold materials. They have shown the ability to initiate/evoke (stimulate) special biological responses in cells when cells are cultured on them. Furthermore, nanofibrous scaffolds have shown enhanced cell adhesion, stimulated cell growth, and protein adsorption and assisted in cell differentiation. In addition to biomedical applications, nanofibers have been widely studied as a potential filter material in the environmental protection field. Based on the design and construction of the membrane and the size of the contaminants, filters are of two main types: nanofilters and microfilters. To achieve the easy removal of a targeted contaminant, the filter membrane should have pores or passage channels. These channels allow liquid and particles with the appropriate dimension to pass while arresting the particles or contaminants with a larger particle size. For instance, one of the most commonly used filters in daily life is a paper coffee filter, which has the ability to prevent the movement of large and undissolved particles through its pores while allowing dissolved particles with smaller diameters to pass. In addition to normal fibrous filter membranes, researchers have developed a new type of fibrous membrane known as an affinity membrane. These membranes have selective sites that assist in the selective immobilization of targets and removal of the target contaminant. This membrane has shown an extensive range of applications in both the environmental and biomedical engineering fields [163].

2.17.1 Tissue engineering A range of methods have been reported in the literature for the fabrication of tissue engineering scaffolds. However, in the past decade, nanofiber systems have been targeted for the preparation of scaffolds for tissue engineering. For the regeneration of tissue, biocompatible and biodegradable fibrous scaffolds are generally preferred over conventional scaffolds because of their unique nature and ability to provide the target cells/ tissues with a native environment by mimicking the extracellular matrix. Therefore, the use of electrospun nanofibers in tissue engineering is increasing with every passing day. The literature published on tissue engineering utilizing electrospun nanofibers has so far surpassed the literature published on the conventional materials. Fibrous scaffolds not only have shown an impact on the cell-to-cell interaction but also have increased the interaction between the cells and matrix. Because of the aforementioned properties and similarities between the hierarchical structure of electrospun nanofiber scaffolds and the natural extracellular matrix, electrospun nanofiber scaffolds have exhibited an excellent cell-growing capability. Furthermore, until recently, researchers have mainly focused on biopolymers/natural polymers (hyaluronic acid, alginate, collagen, silk protein, fibrinogen, chitosan, starch, and poly(3-­ hydroxybutyrateco-3-hydroxyvalerate) (PHBV)) for tissue engineering, because these polymers showed excellent biocompatibility and biodegradability. However, more recently, attempts have been made to utilize a wide range of natural and synthetic polymers for the regeneration of new tissues, specifically cartilage tissue dermal tissue. Among the

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synthetic polymers, PLGA is considered to be the ideal material for tissue regeneration because of its tunable and biodegradable nature, easy spinnability, and the presence of multiple focal adhesion points. Silk fibroin is another polymer fiber that in a blended form with bone morphogenetic protein 2 (BMP-2) and hydroxyapatite nanoparticles (nHAP) has exhibited excellent bone tissue regeneration. Researchers have also explored the potential application of PCL in bone tissue regeneration. The results obtained revealed that PCL electrospun nanofiber scaffolds enhanced the MC3T3-E1 preosteoblast cell adhesion and proliferation and assisted in the differentiation of the cell. A huge amount of literature is available on the tissue engineering applications of electrospun nanofibers. However, there are some limitations in the use of electrospun nanofiber scaffolds in tissue engineering. One such hurdle is the infiltration of the cells inside the scaffolds because of the smaller intrafiber pore size. In order to overcome this hurdle, various attempts have been made to fabricate scaffolds with a larger intrafiber pore size to allow the scaffolds to present a 3-D environment instead of a 2-D environment. As compared with conventional 2-D electrospun scaffold, 3-D scaffolds have more exposed inner surface area and pore size and therefore show enhanced infiltration of cell. Literature shows that cells migrated approximately up to 4 mm and exhibited a spatial cell distribution. Therefore, excellent biocompatibility and physical and spatial geometries of 3-D electrospun scaffolds are important in tissue engineering applications such as nerve regeneration, vascular grafts, and bone regeneration [163]. Repair and regeneration of human tissues and organs using biomaterials, cells, and/ or growth factors is a great challenge for tissue engineers and surgeons. The convergence of advanced materials science, nanotechnology, stem cell science, and developmental biology, which we define as regenerative engineering, represents the next multidisciplinary paradigm to engineer complex tissues. One of the grand challenges in this field is to mimic closely the hierarchical architecture and properties of the extracellular matrices (ECM) of the native tissues. A bioinspired approach to creating biomaterials with nanoscale topographical features, micro- and macroscale gradient structures, and biological domains to interact with target growth factors and cells is key to overcoming this challenge for successful tissue regeneration. Furthermore, the healing and repair of diseased musculoskeletal tissues rely on many signaling pathways, involving numerous growth factors and their receptors. Thus, pharmacological manipulation of the signaling pathways with bioactive molecules is an important component of tissue regeneration. The tissue-building scaffold is shown in Fig. 2.64 [163]. One of the most common surgical procedures performed is regeneration or replacement of ruptured/torn ligament or tendon tissues. For example, anterior cruciate ligament (ACL) rupture is one of the most common knee injuries related to disease or trauma. In addition, tendon damage is the most common injury in the young and physically active population. For the past several decades, the use of autografts has been considered the gold standard for ACL reconstruction and tendon regeneration, while allograft tissue has also been commonly used. The use of tissue grafts has a high success rate; however, it is associated with limitations such as donor site pain and morbidity with autografts and potential disease transmission with allografts. Consequently, tissue engineering has become a promising alternative approach for the regeneration of ligament and tendon tissues by using specifically

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designed scaffolds, cells, and/or growth factors. Electrospun nanofibrous scaffolds have been investigated to repair ligament and tendon by providing an artificial ECM that mimics the collagen fiber bundles present in the natural tissues. Electrospun nanofibrous scaffolds are intrinsically advantageous as artificial matrices for muscle regeneration. A large number of electrospinnable polymers allow for the screening and selection of a graft material with suitable biological and mechanical properties that match those of skeletal muscle. In addition, the electrospinning process permits the formation of unidirectional nanofibers, which guide the growth of different types of cells. Aligned nanofibrous scaffolds fabricated from both synthetic and natural polymers have been evaluated for skeletal muscle regeneration. Aligned PLGA nanofibrous scaffolds were synthesized and evaluated with respect to growth and functions of C2C12 murine myoblasts. The myoblasts not only aligned themselves on the oriented nanofibrous scaffolds but also produced significantly higher expression of the differentiation marker, fast myosin heavy chain, than those on the random nanofibrous scaffolds [164]. Electrospinning is a convenient and versatile technique to fabricate nanofibers and nanofibrous scaffolds for tissue engineering applications. By altering the polymer solution properties and a number of processing parameters such as voltage, flow rate, distance between the needle tip and the collector, and the type of spinneret and collector, one can fabricate a variety of nanofiber assemblies. Nanofibrous structures are highly porous, possess extremely high specific surface area, and mimic the native microenvironments that cells face in vivo. These attributes make nanofiber scaffolds highly advantageous in tissue engineering and drug delivery applications. Nevertheless, challenges remain when applying the electrospinning process and electrospun nanofibrous scaffolds to musculoskeletal tissue regeneration. The main challenge encountered during fabrication is the inability to control fiber size and arrangement for optimized cell infiltration. Aligned nanofibers are especially ideal for tissue regeneration applications as research has shown that cells tend to elongate, proliferate, and differentiate well along the aligned fibers. However, methods to obtain a large area of consistent arrangement need to be refined. A rotating drum is the common collector used to create aligned nanofibers, but the rotation speed needs to be finely adjusted for differences in fiber diameter. Increased fiber diameter or introduction of a sacrificial polymer component is often utilized to increase pore size in order to enhance cell infiltration; however, increased pore size is also associated with decreased mechanical integrity. Another challenge faced when creating nanofibrous scaffolds for musculoskeletal tissue regeneration is the development of suitable mechanical properties. For example, mineral gradients are often introduced to the surfaces of fibers in order to mimic mineralized bone tissue. Unfortunately, the resulting mechanical strength is often not comparable with that of bone. Carefully modifying the ion concentrations of the mineral coating solution can enhance modulus and decrease toughness, providing mechanical properties similar to those of bone tissue. Since polymeric nanofiber scaffolds act as temporary templates at sites of tissue regeneration, it is important that the scaffolds maintain this structural integrity for the time needed for cells to infiltrate and secrete their own ECM. The issue of scaffold shrinkage during degradation has prompted researchers to fabricate novel patterns of electrospun polymers, such as

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nest-like-patterned PLGA matrices, to maintain scaffold morphology and strength. Although PLGA is the most widely used polymer for electrospinning nanofibrous scaffolds for tissue regeneration, researchers are exploring other polymer options to provide unique features to suit their desired applications. Since many polymer solutions are not easily spun due to characteristics such as low conductivity and high surface tension, special methods that utilize an electrospinnable carrying solution can be explored to fabricate double-layered fibers. The carrier layer can be subsequently removed, leaving the desired polymer nanofiber intact. Overall, the development of three-dimensional nanofibrous scaffolds that exhibit all of the properties necessary to mimic musculoskeletal tissue and facilitate cell infiltration still needs to be optimized for successful application in the clinic. The emergence of regenerative engineering provides tissue engineers with invaluable tools to achieve the repair and regeneration of musculoskeletal tissues using biomaterials, cells, and growth factors/signaling molecules. In the realm of regenerative engineering, we believe that future research should focus on advanced materials science, stem cells, developmental biology, and strategies to integrate these components into a functional biological system. Creating nanoscale scaffolds with nanotopographical features has been suggested as a key to successfully regenerating musculoskeletal tissues by mimicking the natural microenvironments that cells face in the body [164]. Polysaccharides are the homopolymers or copolymers of monosaccharides. In nature, polysaccharides can be found in many organisms, including polysaccharides of algal origin (e.g., alginate), plant origin (e.g., cellulose and starch), microbial origin (e.g., dextran), and animal origin (e.g., chitosan and hyaluronic acid). Polysaccharides are also diverse in their chemical structure, chemical composition, molecular weight, and ionic character, all of which contribute to their functionality and biological activity. Several fabrication methods for nanofibers, including drawing, template synthesis, phase separation, self-assembly, and electrospinning, have been developed. Among these, the electrospinning process has become the most attractive because it is cost-effective, highly productive, and applicable to a variety of polysaccharides. Many studies have been conducted to date using polysaccharides and their derivatives for the fabrication of electrospun nanofibers that could be potentially useful in regenerative medicine [164]. Electrospun nanofibers from natural and synthetic polymers have been widely used in regenerative medicine, including tissue engineering applications. Tissue engineering aims to provide man-made tissues or organs to patients who suffer the loss or failure of a tissue or organ. Tissues and organs are typically engineered using a combination of a patient’s own cells and polymer scaffolds. Electrospun polysaccharide nanofibers have shown great potential in many biomedical applications, including regenerative medicine. A critical future challenge of electrospun polysaccharide nanofibers may include proper selection of polysaccharides, use of mixed solvents, synthesis of various derivatives, hybrid of natural and/or synthetic polymers, fabrication of core-shell structures, blowing-assisted electrospinning, and fabrication of micro-/nanofiber composites. Typically used polysaccharides are alginate, cellulose, chitin, chitosan, hyaluronic acid, starch, dextran, and heparin. Although most of these polysaccharides are of fundamental interest for electrospinning and have been found to be useful in many biomedical applications, there are still limitations to

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be overcome for electrospinning. In particular, one of the most difficult barriers to overcome may be the limited solubility of several polysaccharides, such as cellulose and chitin. A variety of approaches have been reported to improve the solubility, including the synthesis of derivatives and the use of mixed solvent systems. A high viscosity caused by inherently high molecular weights and electric charges also produces poor electrospinnability of some polysaccharides, such as alginate, chitosan, and hyaluronic acid. These issues can be overcome by varying the blend ratio with other polymers and by varying the solvent composition. Naturally occurring polysaccharides have been known to be biocompatible and safe for many biomedical applications. However, improving the surface functionality of electrospun nanofibers with bioactive molecules could be very important for specific biomedical applications. The surface chemistry, microstructure, and architecture of nanofibrous matrices significantly influence cellular adhesion, proliferation, and differentiation. Aligned nanofibers significantly induced neurite outgrowth and enhanced skin cell migration during wound healing compared with randomly oriented nanofibers. Furthermore, immobilized biochemical factors (e.g., soluble factors) significantly promoted neurite outgrowth. Electrospun nanofiber matrices were chemically modified by oxygen or ammonia plasma treatment, and the adhesion and proliferation of fibroblasts seeded onto the plasma-treated matrices were significantly improved compared with nontreated ones. It would also be challenging to fabricate polysaccharide-based electrospun micro−/nanofiber composites for various applications in regenerative medicine, as nanofiber matrices provide neither sufficient space for cell migration within the matrices nor effective points of cell attachment. Thus, composites composed of microfibers and nanofibers electrospun from synthetic polymers in the same construct have been studied. In addition, the critical issue of electrospun polysaccharide nanofibers in regenerative medicine is that more animal studies may be required and rapid progress is certainly expected by collaboration between materials scientists and clinicians [165].

2.17.2 Drug delivery Delivering drugs in the most feasible physiological manner is of prime importance in the medical field. Providing a drug with a smaller size and suitable coating material enhances its ability to be digested or absorbed by the targeted site. Targeted drug delivery using electrospun nanofibers banks on the idea that the drug dissolution rate increases with an increase in the surface area of the carrier and the drug itself. Numerous reports have been published highlighting the benefits of using electrospun nanofibers as a drug delivery carrier. Until now, many kinds of drugs, including anticancer agents, proteins, antibiotics, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA), have been loaded on electrospun nanofibers. The diversity offered by the electrospinning technique, simply by tuning its parameters according to the target study, has made the use of electrospinning in drug delivery and tissue engineering highly attractive. Various methods such as incorporating the drug into the electrospun nanofibers and coating the drug on the surface of the electrospun nanofibers have been employed for the preparation of electrospun nanofiber scaffolds, which can act as a nanocargo carrier. All of these methods can be helpful in providing a controlled and sustained release of a drug at the target site by simply tailoring the drug-release kinetics [166].

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2.17.3 Wound dressing Wound healing is a dynamic process that follows an intricate sequence of events, including homeostasis, inflammation, proliferation, and remodeling. This sequence is controlled by various factors, signaling molecules, and cells. The roles of these factors are still not completely known. Much work is needed to identify and understand the roles of these factors. Therefore, wound dressing plays a pivotal role in the protection of a wound site, elimination of exudates, appearance, and inhibition of microorganisms. From the analysis of the obtained data, they concluded that electrospun nanofiber scaffolds can be a better substitute than scaffolds prepared by the freeze-drying method. The better performance of the electrospun nanofiber scaffolds was attributed to the better cellular organization on the nanofibers compared with the conventional freeze-dried scaffolds. Furthermore, another research group revealed that electrospun collagen nanofiber scaffolds treated with type 1 collagen and laminin exhibited better cytocompatibility as compared with the untreated collagen nanofiber scaffolds. Similar attempts have been made to fabricate blended electrospun nanofiber scaffolds using various biocompatible polymers such as chitosan and PEO [167]. This electrospun composite nanofiber scaffold was subjected to various cytocompatibility experiments. The results obtained from those cytocompatibility experiments suggested that the electrospun composite nanofiber scaffolds were noncytotoxic. Because chitin and chitosan have structural similarities with glycosaminoglycans (GAGs, the main component of proteoglycans), their antibacterial activity makes this electrospun nanofiber scaffold one of the candidates to be used in the regeneration of skin tissues. Scientists also reported the effectiveness of using chitin, in either a pure or blended form with other polymeric materials. Researchers prepared chitosan-based nanofiber scaffolds containing PEO, chitosan, and Triton X-100. Based on the biocompatibility data analysis, they came to the conclusion that the electrospun composite nanofiber scaffolds facilitated the adhesion of human osteoblastic cells. They fabricated chitosan/PLA blend micro-/nanofibers by using electrospinning technique.

2.17.4 Filtration Various heavy metals are used in the manufacturing processes of various industries. The ions released in effluent can cause severe damage to human health and the environment. Heavy metal ions can easily be mixed into the water reservoir (that acts as a carrier), which distributes metal ions to the surroundings. The separation of metal ions from reservoir water is a serious problem [168].

2.17.5 Desalination To meet the increasing demands for pure drinking water, various techniques have been implemented for the purification of water with a high salt content. These techniques include membrane distillation (MD), electrodialysis (ED), freeze desalination (FD), and reverse osmosis (RO). Because of their flux and cost-effectiveness, electrospun membranes are considered to be the most effective method for ­purifying

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saline  ­water. Such electrospun nanofiber scaffolds/membranes are used as self-­ supporting membranes for the purpose of desalination. The application of electrospun nanofiber scaffolds/membranes in the purification of water was explored extensively. It is highlighted that electrospun nanofiber scaffolds/membranes can remain stable for up to 4 weeks. Therefore, these electrospun nanofiber scaffolds/membranes can be used as an alternative to conventional distillation membranes. The blending of clay nanoparticles with PVDF followed by electrospinning was carried out for a direct contact membrane distillation (DCMD) process, and up to 99.95% salt rejection was achieved [169]. With the increasing knowledge in the field of nanotechnology, many techniques are being employed for the synthesis of materials at the nanometer level. Electrospinning is considered to be one of the most efficient techniques used for the synthesis of nanomaterials. Although this technique was discovered way back in the nineteenth century, the bulk of the work has been done in the late 1990s and early part of the 21st century. The work in the field of electrospinning has intensified more recently. Many polymer and high-molecular-weight compounds with sufficient viscosity have been electrospun. At present, not only can the morphology and inter- and intraporosities be controlled, but also the dimension and direction of the nanofiber deposition can be controlled. All of these factors have led to the extensive utilization of nanofibers in almost every field, including filtration, enzyme immobilization, sensing membranes, cosmetics, protective clothing, affinity membranes, tissue engineering scaffolds, drug delivery, and wound healing applications. In biomedical applications and particularly in tissue engineering, it is very important for artificial scaffolds to mimic the original biological structure and exhibit similar biological properties. Therefore, more work is needed to provide a natural environment for cells and avoid toxicity, which would lead to greater cell proliferation. This could be done by the immobilization of spacers (functional groups) onto scaffolds. Such immobilizing species should also be biocompatible. A similar approach is also finding much interest in environmental and sensor applications. However, environmental applications with surface-­functionalized nanofibers are facing few challenges that need to be tackled. These include a capacity reduction and kinetic slowness after surface modifications. Similarly, the adsorption and removal capacities of nanofibers are dramatically reduced after regeneration. The first effect is related to the porosity of the membrane, which changes after surface modification, whereas the latter is related to the occupation of the adsorption sites by water molecules. Hence, it is suggested that surface functionalization strategies should be designed that not only avoid pore changes but also prevent a decrease in the adsorption capacities after desorption. In sensor applications, the surface group should have an affinity for the material that needs to be determined. Electrospinning is a simple, unique, versatile, and cost-effective technique that is widely used for the fabrication of nonwoven fibers with a high and tunable porosity and high surface area. The morphology of the electrospun nanofibers is significantly affected by various parameters such as the polymer concentration, viscosity, molecular weight, applied voltage, tip-to-collector distance, and solvent. By controlling these parameters, it is possible to easily fabricate electrospun nanofiber scaffolds for the desired function. Electrospun nanofiber scaffolds/membranes have found numerous potential

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applications in almost every field, including filtration, enzyme immobilization, sensing membranes, cosmetics, protective clothing, affinity membranes, tissue engineering scaffolds, drug delivery, and wound healing applications [169]. Because of the aforementioned properties of electrospun nanofibers, the electrospinning technique is considered to play a vital role in different biomedical fields and, more specifically, in the area of tissue engineering. Despite having unique properties, electrospun nanofibers have a few limitations. One hurdle is the poor infiltration of cells into electrospun nanofiber scaffolds. However, progress is being made to fabricate electrospun nanofiber scaffolds with enhanced cell infiltration ability, which will actually allow them to act as 3-D scaffolds [159]. In general, the electrospinning technique has exhibited excellent potential to be used in various fields, specifically in the field of tissue engineering.

2.17.6 Biomedical applications Increased understanding of the nanoscale structural features of the extracellular matrix (ECM) in directing cell and tissue function has led to the development of materials with nanofibrous architectures to mimic the biophysical cues of natural ECM. Although electrospinning originates from the early twentieth century, increased understanding of the importance of structural features on cell-matrix interactions within the past two decades has renewed interest in the technique. The electrospinning process involves a polymer solution that is extruded from an electrically conductive spinneret (often a metal needle). Concurrently, voltage is applied between the spinneret and a grounded target that is placed at a fixed distance (often 5–25 cm) from the tip of the spinneret. As the electric potential within the polymer solution overcomes the surface tension of the formed droplet, the polymer ejects from the spinneret, the solvent is evaporated, and the polymer fibers collect onto the grounded target. The result is a nonwoven fibrous polymer mesh with typical fibers ranging from a hundred nanometers to a few microns in diameter depending on the particular polymer solution and electrospinning parameters [170].

2.17.7 Concluding remarks The fundamentals of nanotechnology lie in the fact that properties of substances dramatically change when their size is reduced to the nanometer range. When a bulk material is divided into small-size particles with one or more dimensions (length, width, or thickness) in the nanometer range, the individual particles exhibit unexpected properties, different from those of the bulk material. The nanometer range is characterized by the transition of a material’s behavior to “quantum-like” behavior of atoms and molecules from the “continuum-like” behavior of bulk materials. Often, nanomaterials are defined by a size range limited by at least one of the dimensions. This range should be 1–100 nm according to EU standard. Especially for nanofibrous assemblies prepared by electrospinning, the mean fibrous element thickness (diameter) is some hundreds of nanometers. But in general, all these fibers with a diameter below 1 μm (1000 nm) are often accepted as nanofibers.

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One of the main advantages of nanomaterials is very huge relative surface area (surface-area-to-volume ratio). This is in fact true for nanoparticles where there are no limitations according to the macrogeometry (unit volume). It is widely published that by reducing fiber diameters down to the nanoscale, an enormous increase in specific surface area to the level of 1000 m2/g or much more is possible. By reducing the fiber diameter from 10 μm to 10 nm, a million times increase in flexibility is expected. Recognizing the potential nanoeffect that will be created when fibers are reduced to the nanoscale, there has been an explosive growth in research efforts around the world. Specifically, the role of fiber size has been recognized in significant increase in surface area, bioreactivity, electronic properties, and mechanical properties. The enhanced reactivity and efficiency of nanofibers are based on the claim that nanofibrous membranes provide enormous availability of surface area per unit mass. Especially for nanofibrous assemblies prepared typically by electrospinning, it is practically impossible to vary thickness in arbitrary range. Usually, the thickness of these nanolayers is up to few microns only. This is serious limitation for the volume of these objects' calculation because the unit of their macro surface should be multiplied by real thickness and final macro volume is then very low. It leads to low amount of nanofibers and their low total surface area per unit of macro surface. Avoiding this limitation leads to the unbelievable huge relative surface area values that cannot be achieved in real products. The same situation appears when the properties such as sorption capacity are calculated relative to mass. Unlike the recent overwhelming trend of nanofibers assumed to provide enormous relative surface area, the reality is quite different. The nanofibrous assemblies (membranes) are extremely thin in the order of a few microns only. They have often relatively smaller porosity compared with microfibrous membranes. Thus, by considering their porosity and thickness, the nanofibrous membranes do not offer so huge real surface area as is evaluated from standard approach (surface-area-to-volume or surface-area-to-mass ratio) because the volume or mass for nanofibrous materials is too small.

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Carbon-based nanomaterials Rajesh Mishra, Jiri Militky Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

3

3.1 Introduction Carbon is very unique material with many allotropes having quite different behavior from amorphous (charcoal and carbon black) to crystalline (diamond and graphite) structures. The properties of carbon structures vary uniquely with allotrope type. For example, there are huge differences between diamond and graphite. Diamond is highly transparent and very hard (microhardness of 100 GPa), with highest thermal conductivity but extremely low electric conductivity. Graphite is opaque and black, relatively soft, and a very good electric conductor. Nanocarbon materials play a critical role in the development of new or improved technologies and devices for sustainable production and use of renewable energy. This perspective paper defines some of the trends and outlooks in this exciting area, with the effort of evidencing some of the possibilities offered from the growing level of knowledge, as testified from the exponentially rising number of publications and putting bases for a more rational design of these nanomaterials. The basic members of the new carbon family are fullerene, graphene, graphene oxide, and carbon nanotube. Derived from them are carbon quantum dots, nanoplates, nanohorn, nanofiber, nanoribbon, nanocapsulate, nanocage, and other nanomorphologies. Second-generation nanocarbons are those that have been modified by surface functionalization or doping with heteroatoms to create specific tailored properties. The third generation of nanocarbons is the nanoarchitectured supramolecular hybrids or composites of the first- and second-generation nanocarbons or with organic or inorganic species. The major advantages of nanocarbon are the following: ●

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Nanodiamond ≥ hard, extraordinary thermal conductivity Graphite ≥ soft, clean industrial lubricant Graphene ≥ electrically conductive Fullerenes ≥ very good electric conductivity Nanotubes ≥ stiff, strong, variable electric conductivity

Nanodiamonds are more thermodynamically stable than graphite when the particle size is less than 5–10 nm. In macroscale, diamond is metastable. Nanodiamond stability is restricted by the smallest sizes of ~1.9 nm, below which fullerene-related structures are more stable. The advantages of the new carbon materials, relating to the field of sustainable energy, are discussed, evidencing the unique properties that they offer for developing next-generation solar devices and energy storage solutions [1, 2] (Fig. 3.1). Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00003-1 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Fig. 3.1  Porous carbon materials for future energy application [2].

Fig. 3.2  TiO2 nanoparticles over graphene [2].

Fig. 3.2 shows an example of TiO2 nanoparticles over graphene acting as an electron acceptor to enhance charge separation and improve the performance of TiO2 in water photoelectrolysis. Nanocarbons can also act as electron donors, that is, photosensitizers (carbon quantum dots), as will be discussed later. In addition, the intrinsic properties of TiO2 are changed by this interaction with nanocarbons, creating hybrid materials. Supercapacitors and pseudocapacitors are another area discussed in detail, where introduction of the new third-generation nanocarbon materials with a specific design is essential to exploit the massive use of nanocarbons in these devices. Some of the strategies to enhance the performances, from developing high surface nanocarbons with tailored pore sizes to modification of the surface characteristics and nanoarchitecture design, are explored. In the field of energy conversion, there is also an urgent need to develop novel nanocarbon-based materials. Fuel and solar cells are two of the specific areas of

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d­ evelopment and need some critical attention. There is great potential of new materials/approaches such as carbon quantum dots and graphene quantum dot solar cells. In terms of application, nanocarbons for polymer solar cells appear to be one of the main driving forces for research in a short/medium term, while solar fuel cells for developing PEC and artificial leaf-type devices are a priority in a longer-term vision. There is a bright future for nanocarbons in the field of sustainable energy (use and storage), but more complex architectures such as those in third-generation nanocarbon materials are necessary to address the very challenging and demanding requirements for next-generation solar devices and energy storage solutions [2].

3.2 Carbon nanotubes yarns (CNY) An innovative solid-state dye-sensitized photovoltaic carbon nanotube yarn (DSPCNY) has been developed using thermally stable and highly conductive carbon nanotube (CNT) yarns (CNYs). These CNYs are highly interaligned, ultrastrong, and flexible with excellent electric conductivity, mechanical integrity, and catalytic properties. The CNYs are coated with a dye-incorporated TiO2 microfilm and intertwined with a second set of CNYs as a counter electrode (CE). The DSPCNYs were developed without using any metal wires or any expensive transparent conductive oxides (TCOs), liquid electrolytes, or glass or plastic cladding. The maximum photon to current conversion efficiency (ηAM 1.5) achieved with prolonged-time stability was 2.57%. The yarn-shaped flexible cells were able to transport photocurrent over a significant distance using a simple cell configuration with a wide range of structural flexibilities (30–3301). These cells are capable of efficiently harvesting incident photons regardless of direction and generating photo currents with high efficiency and long-term stability. Micrographic images can be seen in Fig. 3.3 [3].

3.3 Electrochemical carbon based nanosensors Nanotechnology has become very popular in the sensor fields in recent times. It is thought that the utilization of such technologies and the use of nanosized materials could well have beneficial effects for the performance of sensors. Nanosized materials have been shown to have a number of novel and interesting physical and chemical properties. Low-dimensional nanometer-sized materials and systems have defined a new research area in condensed-matter physics within past decades. Apart from the aforesaid categories of materials, there exist various materials of different types for fabricating nanosensors. Carbon is called a unique element, due to its magnificent applications in many areas. Carbon is an astonishing element that can be found in many forms including graphite, diamond, fullerenes, and graphene. This review provides an overview of some of the important and recent

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(A) (C)

4 nm

20 µm

(B)

500 nm

(D)

2 µm

Fig. 3.3  SEM of carbon nanotube ropes [3]. (A) Single nanotube, (B) Surface morphology, (C) Web of nanotubes, and (D) Web generation.

developments brought about by the application of carbon-based nanostructures to nanotechnology for both chemical and biological sensor development and their application in pharmaceutical and biomedical area. A nanosensor is a class of sensor devices or systems in which a nanoscale interaction is exploited as the basis of detecting the presence or level of a known analyte. A size of several nanometers in two dimensions (wires, rods, etc.) or all three (clusters, particles, etc.) is a simplistic criterion for a system to be considered “nano.” The target analyte may be a pharmaceutically active compound, any other electroactive compounds, or even sometimes nonelectroactive compounds. Nanosensors are composed of two main parts: a nanomaterial recognizing component (nanometals, nanotubes, nanowires, nanoparticles, etc.) directly connected to a physical transducer (voltammetric, amperometric, conductometric, spectrophotometric, etc.). The broad variety of nanomaterial systems that can be used as recognizing agents allows the development of specific nanosensors for a very large pool of analytes. Furthermore, electrochemical transducers confer high sensitivity for these devices [4]. Carbon can form many different chains with different length and electronic configuration. This hybridization property gives the possibility for carbon to form more than hundred million compounds with different properties. Carbon-based nanomaterials are common forms with hollow spheres, ellipsoids, or tubes. Spherical and ellipsoidal carbon nanomaterials are referred to as fullerenes, and the cylindrical ones are called nanotubes.

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3.4 Fullerenes Fullerenes (originally buckminster fullerenes) are a new class of carbon-only molecules where 60 carbon atoms (C60) are arranged in a soccer-ball structure. The first example of fullerenes was discovered in 1985. Fullerenes are a large class of allotropes of carbon and have attracted considerable attention in different fields of science since their discovery. Fullerene molecules are made of carbon atoms, and their shapes are as hollow sphere, ellipsoid, or tube. Spherical fullerenes are also referred to as bucky balls. They are carbon clusters, whose surface is formed by 12 pentagons and any number of hexagons as shown in Fig. 3.4 [5].

3.5 Carbon nanotubes Carbon nanotubes (CNTs) represent an increasingly important group of nanomaterials with unique geometric, mechanical, electronic, and chemical properties. A fullerene nanotube has tensile strength about 20 times that of high-strength steel alloys and a density half that of aluminum. Comparing fullerenes with nanotubes, it is obvious that the first are curved with regard to all three directions in space, whereas no curvature at all is observed in the axial direction of nanotubes. CNTs can be viewed as a hollow cylinder formed by rolling graphite sheets. Since CNTs are derived from fullerenes, they are referred to as tubular fullerenes or bucky tubes. Their length can be very relatively high (up to 3 mm) in comparison with their diameter (minimum of about 0.4 nm for single-walled CNT and maximum of 30 nm for outer shell of multiwalled CNT). The discovery of carbon nanotubes (CNTs) in 1991 opened up a new area in materials science. CNT technology is rapidly growing to become an integral part of our lives due to the unique mechanical and electric properties of nanotubes. CNTs have novel properties that make them potentially useful in a wide variety of applications in nanotechnology, electronics, optics, and other fields of materials science. Nanotubes can be envisioned as one-atom

(A)

(B)

Fig. 3.4  Fullerenes [5]. (A) Spherical and (B) Hexagonal.

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thick sheets of carbon that have been rolled into tubes. CNTs are composed of carbon atoms linked in hexagonal shapes that have diameters as small as 1 nm and lengths up to several centimeters, with each carbon atom covalently bonded to three other carbon atoms. Another type of nanotube has closed ends, formed by some of the carbon atoms combining into pentagons on the end of the nanotube (Fig. 3.5) [5]. The structure of CNT depends upon the orientation of the hexagons created from carbon atoms in the CNT cylinders with respect of their axis. These different CNT structures have different behavior. The symmetrical structures are for armchair and zigzag forms, and chiral structure (screw axis) is for chiral forms of CNT. Depending on their crystallographic structure, CNTs may have the conductivity of copper (“metallic” armchair single-wall nanotubes) or silicon (“semiconducting” zigzag or chiral ­single-wall nanotubes). In metallic CNT, the mean-free paths for phonon scattering have been found to be about 1 μm at low field and in the range of 10–100 nm at high field. For semiconducting CNT, values close to 300–500 and 10–100 nm have been obtained at low and high field, respectively. The high electron mobility's (104– 105 cm2 V−1 s−1) in CNT indicate that semiconducting nanotubes should be an excellent material for a number of semiconductor applications. The thermal conductivity at room temperature was found ~3000 W m−1 K−1 for the SWCNT and MWCNT. This conductivity is growing with increasing temperature [6]. Physical or chemical modification of carbon nanotubes gives them new and different properties. Furthermore, using chemically modified carbon nanotubes as electrode materials, a field in its infancy, would make the electrodes more sensitive and selective materials. Surface modifications with carbon nanotubes have been performed in both oriented and nonoriented ways. Their reactivity, however, is less pronounced due to the curvature in just one direction. There are some types of functionalization that can be described as covalent or noncovalent, respectively, on the outer wall and a filling of the central cavity. The modification of carbon nanotubes has recently been in the focus of much research, primarily to improve their solubility in various solvents. A first covalent functionalization of the tubular structure is already achieved upon purification of the nanotubes obtained from different methods of production. Carboxyl derivatives obtained from the oxidative opening of nanotubes can be further modified with the classical methods of organic chemistry. The reaction with hot, concentrated

Fig. 3.5  Carbon nanotubes [5].

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oxidizing mineral acids (like nitric or sulfuric acid) introduces carboxyl groups to the ends of the tubes and to defects of the side wall. For MWCNT, the extent intensity of the reaction may as well be adjusted to decide whether or not outer walls of the tubes are oxidatively removed. Recent research has taken a surprisingly low-tech approach to improve carbon nanotube technology. Researchers from the National Institute of Advanced Industrial Science and Technology in Japan have found that adding a small amount of water vapor during standard chemical vapor deposition (CVD) dramatically improves the efficiency of the growth of single-walled carbon nanotubes (SWNTs). Water acts as a weak oxidizer, selectively removing amorphous carbon without damaging the nanotubes, thus enhancing the activity and lifetime of the catalyst used, such as Fe nanoparticles or sputtered metal thin films. Dense, vertically aligned forests of 1–3 nm diameter SWNTs, which are free from amorphous carbon or metal nanoparticles, can be grown at rates up to 2.5 mm in 10 min [6]. The SWNT forests can be grown on lithographically patterned substrates into large-scale arrays (as shown, top). The researchers believe that the technique could be used with various growth systems and could pave the way for scalable, cost-effective mass manufacture. In a second innovation, researchers from the University of Texas at Dallas and CSIRO Textile and Fiber Technology in Australia have applied the Stone Age technique of spinning to carbon nanotubes. Multiwalled nanotubes (MWNTs) ~10 nm in diameter are simultaneously drawn from an MWNT forest and twisted. The MWNT yarns remain twisted even when the ends are released and can be knotted (as shown at the bottom) without degrading their strength. Two- and four-ply yarns can be fabricated in this way with strengths up to 460 MPa and high electric conductivity. Yarns up to 1 m in length were fabricated by hand, but the researchers believe the process could be automated to produce yarns of unlimited length (Fig. 3.6) [6].

Fig. 3.6  Carbon nanoyarns [6]. (A) Yarn view, (B) Surface morphology, (C) Web structure, (D) CNT yarn wrapped over TiO2 wire, and (E) Surface rupture.

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3.5.1 Carbon nanotube yarn and 3-D braid composites Macroscopic textile preforms were produced with a multilevel hierarchical carbon nanotube (CNT) structure: nanotubes, bundles, spun single yarns, plied yarns, and 3-D braids. The 3-D braided preform was the first of its kind produced by textile processing technique and used as a composite reinforcement consisting solely of carbon nanotubes. Four different epoxy systems that possessed a wide range of mechanical properties (owed to an added modifier) were infused into the CNT yarns and 3-D braids. Mechanical characterization of the resulting composites was conducted through the use of tensile testing. It was found that the tensile strength, stiffness, and especially strain-to-failure values for each preform type were close regardless of the properties of the matrix whose strain-to-failure values ranged from 3.6% to 89%. This is hypothetically attributed to the nanoscale interaction between individual nanotubes and polymeric macromolecules in the composites. Therefore, one can expect that very complex interactions between individual nanotubes (or nanotube conglomerates) on one side and resin macromolecules (or their aggregates) on the other would take place during composite processing. One or another form of their mutual interaction would be evident from the very start of the resin penetration into the nanotube reinforcement, reaching the deepest possible level. Such interaction could severely affect all steps of the resin cross-linking and solidification into final composite. Typical braided CNT yarns are shown in Fig. 3.7. In principle, any continuous CNT yarn (single or multi-ply) and 3-D nanotube braids described above can be used as the reinforcement for composite preforms. However, currently available single CNT yarns made by draw-twist spinning CNTs from their forests have diameter slightly larger than typical diameter of a single carbon filament; besides, they are characteristic of very low breaking force (in the range of few grams) and coiling (due to inherent twist) when left free. According to the number of plies, they have much higher breaking force than the single CNT yarns and thus can be processed on special textile machinery to make preforms and then composites. This was demonstrated by a number of experimental composite samples fabricated

Fig. 3.7  Carbon nanotube braided structure [7].

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with the use of 5- and 25-ply nanotube CNT yarns and 3-D nanotube braids as the reinforcements. Composites were produced using a soaking technique. The resin was heated, and the preform was allowed to soak in epoxy resin system for 30 min under vacuum to help remove trapped air. The viscosity of the resin at this temperature was below 100 cp. The composites were cured under slight tension at room temperature to ensure their straightness for testing and to force out extra resin to maximize their volume fraction [7]. Carbon nanotubes can, hypothetically, make significant effect on the chemical cross-linking process, by generating additional covalent bonds between the macromolecules in the epoxy networks and, consequently, by reducing the ability of cured epoxy to deform. However, no chemical substantiation has been found up to this point to validate this hypothesis. One argument against it is that chemically inert (nonfunctionalized) CNTs cannot cause so significant additional cross-linking of epoxy resins that the failure strain of the neat matrix could decrease, say, from 89.3% to 1.61% at the same temperature. Even if assuming some possible random impurities associated with the CNTs (those might act as additional catalysts), it is hard to imagine that so substantial increase in the cross-linking density of the epoxy can be achieved. The other argument is that no experimental evidence is available to support the suggestion that by artificially increasing cross-linking density in some epoxy resin to the highest possible level above the regular one, the normal strain-to-failure value can be reduced by 10, 30, or even 50 times at the same temperature. It is well known that cross-linking density strongly affects the Tg values, and through Tg, it also affects the mechanical properties when the resin transitions from the rubbery state to the glassy state take place. However, the strain-to-failure reduction effect discussed here was observed for the neat epoxies and respective composites in their glassy state at room temperature. Based on the above two arguments, we ruled out the hypothesis of a chemical cross-linking causing the investigated effect [7]. Novel composites based on 3-D braided preforms utilizing CNT yarns have unique mechanical properties not seen in any traditional composite or in any dispersed CNT composite. When long continuous CNT yarns or 3-D braids are infused with low-­ viscosity, low-molecular-weight epoxy having long pot life, the resin is able to penetrate into very small (several micron scale) spaces within the reinforcement. This initiates very complex interactions between the CNTs on one side and their surrounding polymer macromolecules on the other. Such an interaction ultimately determines tensile properties of the nanocomposite. This is a new physical phenomenon that can be only found in nanocomposites and has never been observed in traditional microcomposites, where the fiber properties mainly determine mechanical properties of the composite in tension. In the case of our nanocomposites, all five hierarchical levels, mentioned in the beginning of the paper, make their own (more or less important) contribution to the deformation and failure mechanisms under tensile loading [7, 8].

3.5.2 Recent advances in inkjet printing of CNT inks Typically, the composition of an ink consists of filler, binder, solvent, and additives. Depending on the significance of particular applications, a suspension of the m ­ etallic

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nanoparticle, carbon materials, and conductive polymer are included in filler. Dispersion of these compositions in the solution media and turning them into ink are among the most important issues. Several steps should be carried out to make CNT ink after determination of the required concentrations of the desired material. At first, the CNTs must be well dispersed in the ink material through mixing. High intertube interaction energy (500 eV/m) of van der Waals forces in carbon nanotubes and the nature of nanomaterials challenge the dispersion of CNTs in liquids medium. Also, the high aspect ratio makes them prone to be entangled and bundled. In the perspective of dispersion, high aspect ratio of CNTs can create an issue where it is required to limit CNT length based on the diameter of the print head orifice to prevent clogging. All of these factors have made it challenging to achieve a suitable dispersion of carbon nanotubes in nanoscale device applications. Physical (mechanical dispersion) and chemical functionalizations have been employed by researchers to solve this issue [9, 10]. There are two general mechanical approaches for dispersing CNTs including ultrasonication and high-shear mixing. The most common one is ultrasonication that uses high-frequency vibrations to separate carbon nanotubes within a liquid. Despite of higher dispersion level it can achieve, it can also cause defects in tubular structure and even may shorten CNTs. The intrinsic conductivity of CNTs can get reduced as the defect density rises, while the conductivity of the printed film can be decreased by the reduced probability of formation of electron pathways with shortened CNTs. Sonication can even affect the solvents and dispersants used in a carbon nanotube ink both physically and chemically. Researchers have reported that the ultrasonication can result in fragmentation of CNTs and decrement in their aspect ratio. Moreover, the dispersion was unstable [11–13].

3.6 Carbon nanofibers Carbon fibers are classified as microelectrodes because of their small size. The growing interest in microelectrodes and carbon-reinforced structural materials has led to a widespread use of carbon fibers in electroanalytical chemistry. It has been described as a fiber containing at least 90% carbon obtained by the controlled pyrolysis of appropriate fibers. Carbon fibers are a new breed of high-strength materials. Carbon fiber materials are produced mainly for the preparation of high-strength composites by high-temperature pyrolysis of polymer textiles or via catalytic chemical vapor deposition. The heat treatment process is similar to the glassy carbon process. Like all forms of conductive carbon, carbon fibers are made by heating a carbon-based precursor to several hundred or thousand degrees Celsius. A wide variety of synthesis and heat treatment procedures exist for the preparation and posttreatment of the fibers. For this reason, fibers are available with different exposed microstructures and surface chemistries. They are often stronger than steel but much lighter. Typical dimensions of the carbon fiber tip range from 5 to 20 μm in diameter and 5–15 mm in length. They can be classified into three broad categories: low-, medium-, and high-modulus types. Carbon fibers have a wide variety of structures and properties. The finished fiber has a cross section of the “onion,” “radial,” or “random” type, and the fiber end generally exhibits a high fraction of edge plane. Because of their smaller surface areas, reduced

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capacitance and fast time constants can be obtained with carbon fibers. These specifications make them ideal for use with fast voltammetric techniques such as fast-scan cyclic voltammetry. Because of this advantage, carbon fibers are very attractive for anodic measurements in various micromeasurements. However, precautions should be taken to avoid contamination of the carbon surface with the epoxy during its preparation steps. Carbon fibers have been used extensively for microelectrodes because they are durable, have small ohmic drops, and exhibit good electrochemistry. The carbon fiber is cut to a length of 0.5 mm prior to use. Carbon fibers are typically mounted at the tip of a pulled glass capillary with epoxy adhesive [14, 15]. CNFs have become increasingly attractive in creating interfaces between electrodes and local neural tissues in electric stimulation applications, such as deep brain stimulation (DBS). The main problem with application of aligned CNFs as electrodes for nerve stimulation is their tendency for agglomeration creating bundles with bigger sizes and micron-sized fibers. The advantages of CNFs are their small sizes (10 nm diameters) that enable the probing 3-D neural networks, extracting and modulating neural signals more precisely with less damage to the tissue than micron electrode arrays (whose electrode diameters are 100 μm or larger). Researchers developed a CNF-based neural chip and proved its in  vitro capability of both stimulating and recording electrophysiological signals from brain tissues. In this study, long-term potentiation was induced and detected through CNF arrays. Good biocompatibility combined with excellent electric and mechanical properties makes such a system useful for use as nerve prostheses (electrodes) and in the electric stimulation of nerves in the CNS and PNS. Currently, most traditional microelectrodes are fabricated with rigid metals and semiconductors. CNFs have two superior characteristics to be employed in the neural electric interfaces compared with metal-based microelectrode arrays. One of them is the high resolution that is difficult to obtain for conventional metal-based electrodes since decreasing their size increases the electrode impedance and thermal noise; this reduces the sensitivity of the electrode in the detection of electric signals in the nervous system. Secondly, CNFs not only can work at the extracellular level but also may penetrate into neurons and then work at the intracellular level (Fig.  3.8). Thus, the carbon-based electrodes may be potentially superior to conventional metal electrodes [16].

Fig. 3.8  Carbon nanofibers [16]. (A) Web and (B) Single fiber.

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Besides CNTs and CNFs, other types of carbon nanoforms such as graphene are investigated as materials for neuronal stimulation and monitoring. Unique electric properties of graphene offer a significant advantage for a variety of clinical diagnostics and treatments in the nervous system. Graphene, like carbon nanotubes, was also cultured in contact with hippocampal neurons chosen to be the model of investigation due to their well-known plasticity and regeneration properties. The hippocampus is a major component of the human brain and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of several forms of learning and memory and especially during the formation of declarative memories. Humans and other mammals have two hippocampi, one in each side of the brain. The authors indicate the noncytotoxic effects of graphene in contact with nerve cells derived from the hippocampus in comparison with tissue culture polystyrene (TCPS). Moreover, the length and number of neurites during the developing period (7 days) on graphene were increased as compared with control samples. They also observed that the greatest statistical differences of average neurite number and length between graphene and TCPS were observed on the second day when compared with a period from 3 to 7 days [17]. The high electric conductivity of carbon nanomaterials is an important property for the functional recovery in the central nervous system of learning processes, neuronal plasticity, and adaptation. These forms of plasticity might also depend upon the presence of proper chronic electric instructions. For this reason, many important questions should be taken into consideration. Many questions appear in terms of biocompatibility of carbon nanoparticles, and in particular, what happens to them when they are released into the body? How to interact with the surrounding cells and tissues? Whether and how are they released from the body? What are the factors associated with carbon nanomaterials that affect cellular responses? To answer these questions, in particular referring to the biocompatibility of carbon nanoparticles, further research is needed. This is mainly due to a number of factors that may affect this response. Carbon nanomaterials have gained a special importance in recent years, when it was observed that their properties (particularly their electric and mechanical characteristics, their high specific surface area, and their appropriate dimensions, close to the dimensions of single axons) may constitute a kind of matrix, scaffold, and a factor that stimulates their growth and the regeneration of synaptic connections. In spite of the existence of many positive reports concerning the possibility of applying carbon nanomaterials for the treatment of the central and peripheral nervous systems, one should not forget about a number of issues related to the toxicity of nanomaterials and the incompletely understood mechanisms that have an influence on the stimulation of nerve tissue cells to grow. Therefore, for better understanding of this extremely interesting and complex area of medicine, further research needs to be explored [18, 19].

3.7 Carbon nanotools as sorbents and sensors of nanosized objects Engineering nanoparticles hold promise as novel materials in a wide range of applications owing to their exceptional properties by virtue of their reduced size. In the

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last years, the extensively production of engineering nanomaterials and their recently incorporation into a variety of industrial processes, in commercial products, and even in medicine have experienced an exponential growth, entailing potential risks and great concerns about their impact on the environment and human health. Although many toxicological investigations on nanoparticles have been carried out using different cell lines and living organisms, their potential harmful effect is still unclear and sometimes even contradictory by the lack of standardized toxicological procedures for nanomaterials. Nanoparticles are naturally or intentionally be present in the environment and afterward entered in living organisms through direct routes. In biological systems, the unexpected toxicity of nanomaterials related to the cellular uptake, biodistribution, possible transformations over time, or biomolecules is influenced by a variety of factors such as the nanoparticle nature and size, ­surface-to-volume ratio, colloidal stability, and surface reactivity (Fig.  3.9). Taking into account that some of these nanomaterials are considered as human life-­ threatening, emerging analytic methodologies are recently reported to provide innovative detection strategies toward nanoparticles in a wide variety of scenarios (simple and complex matrices). In this context, a substantial progress in the use of nanoparticles as nanotools for analytic applications was accomplished in the last decades as a consequence of their outstanding adsorption capacities and their optical and magnetic properties by virtue of their large surface areas, chemical reactivity and composition, and quantum effects. Such fascinating properties, which are different from other materials, take the lead to improve and simplify available analytic methods in terms of sensitivity and specificity and help us to gain a better understanding and controlling of the nanoworld [20]. Interestingly, carbon nanostructures have recently been paid great attention in analytic applications since they allow obtaining more sensitive methods to determine a wide variety of analytes, being involved in any of the steps of the analytic process.

So na rbe nt not o ols

Standard nanoanalyte

Sample nanoanalyte Samp treatm le ent

Ana l

Sepa ration

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Senso Nano r too ls

Fig. 3.9  Carbon nanomaterials as sorbents [20].

Detec

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Results Nanoanalytes Characterization Determination

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Furthermore, many methods have been described by combining different carbon nanostructures or even by mixing of nanoparticles of different nature. Carbon-based nanoparticles, mainly carbon nanotubes (SWCNTs and MWCNTs) and, most recently, graphene and its derivate graphene oxide (GO), are the most commonly nanomaterials employed as sorbent in conventional extraction techniques. A great affinity between such sorbent and the analyte can be achieved by virtue of their large specific surface area and their aromatic character. Their unique morphology determines the extraction efficiency and selectivity and also confers them the ability to achieve a quick adsorption equilibrium and analyte elution. Some research works were reported until now using fluorescent carbon-based nanodots as sensor of other CNPs because of their toxicity, for instance, the similarity of CNTs to asbestos as well as the generation of reactive oxygen species and high aggregation tendency of CNTs, GO, and fullerene. Their extensive uses in consumer products meant human damages or even their accumulation, and thus, the development of analytic methods to determine them is crucial [21].

3.8 Carbon nanomaterials for nerve tissue stimulation and regeneration Nanotechnology offers new perspectives in the field of innovative medicine, especially for reparation and regeneration of irreversibly damaged or diseased nerve tissues due to the lack of effective self-repair mechanisms in the peripheral and central nervous systems (PNS and CNS, respectively) of the human body. Carbon nanomaterials, due to their unique physical, chemical, and biological properties, are currently considered as promising candidates for applications in regenerative medicine. The applications of various carbon nanomaterials including carbon nanotubes, nanofibers, and graphene for regeneration and stimulation of nerve tissue and in drug delivery systems for nerve disease therapy are reported [22, 23]. There are high hopes for the progress of research on regeneration and stimulation of the nervous system associated with nanotechnology, in particular nanomaterials. It is expected that nanomaterials will be able, on the one hand, to prevent the activity of astrocytes (in CNS) and, on the other, to stimulate axon growth and the restoration of synaptic connections. Carbon nanomaterials are proposed as promising candidates for nerve stimulation and regeneration. Currently, the best known are the three types of carbon nanostructures: carbon nanotubes (CNTs), carbon nanofibers (CNFs), and graphene. Carbon nanostructures have unique mechanical, electric, and physicochemical properties, and their shape (CNTs and CNFs) is similar to neurites. Biostable CNTs are attempted to be used as implants where long-term extracellular molecular cues for neurite outgrowth are necessary, for example, in regeneration after spinal cord or brain injury. Moreover, these materials can be fictionalized and modified chemically using biomolecules stimulating neurite growth. The chemical and biological modification of carbon nanomaterials produces various surface charges affecting the

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Fig. 3.10  Carbon nanomaterials for nerve tissue regeneration [23]. (A) Nerve tissues, (B) Tissue formation, (C) Tissue growth, (D) Enlarged view, (E) Tissue linear growth, and (F) Connecting tissues.

nerve response. Moreover, the surface charge can influence the length of neurite outgrowth, their number, branching, and the number of synaptic connections as shown in Fig. 3.10 [24].

References [1] S.R. Bakshi, V. Singh, K. Balani, D.G. Mccartney, S. Seal, A. Agarwal, Carbon nanotube reinforced aluminum composite coating via cold spraying, Surf. Coat. Technol. 202 (2008) 5162–5169. [2] D.S.  Su, G.  Centi, A perspective on carbon materials for future energy application, J. Energy Chem. 22 (2) (2013) 151–173. [3] L.K.  Randeniya, Chapter  5 – Alloy hybrid carbon nanotube yarn for multifunctionality, in: Nanotube Superfiber Materials: Changing Engineering Design, William Andrew, Oxford; Waltham, MA, 2014, pp. 137–165.

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[4] K. Balani, A. Agarwal, Process map for plasma sprayed aluminum oxide carbon nanotube nanocomposite coatings, Surf. Coat. Technol. 202 (2008) 4270–4277. [5] S.  Kurbanoglu, S.A.  Ozkan, Electrochemical carbon based nanosensors: a promising tool in pharmaceutical and biomedical analysis, J. Pharm. Biomed. Anal. 147 (5) (2018) 439–457. [6] J. Yan, M.J. Uddin, et al., Carbon nanotubes (CNTs) enrich the solar cells, Sol. Energy 96 (2013) 239–252. [7] A.E. Bogdanovich, P.D. Bradford, Carbon nanotube yarn and 3-D braid composites. Part I: Tensile testing and mechanical properties analysis, Compos. A: Appl. Sci. Manuf. 41 (2) (2010) 230–237. [8] K. Balani, T. Zhang, A. Karakoti, W.Z. Li, S. Seal, A. Agarwal, In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating, Acta Mater. 56 (2008) 571–579. [9] K.  Balani, R.  Anderson, T.  Laha, M.  Andara, J.  Tercero, E.  Crumpler, et  al., Plasmasprayed carbon nanotube reinforced hydroxyapatite coatings and their interaction with human osteoblasts in vitro, Biomaterials 28 (2007) 618–624. [10] K. Balani, Y. Chen, S.P. Harimkar, N.B. Dahotre, A. Agarwal, Tribological behavior of plasma-sprayed carbon nanotube-reinforced hydroxyapatite coating in physiological solution, Acta Biomater. 3 (2007) 944–951. [11] K. Hentour, A. Marsal, V. Turq, A. Weibel, F. Ansart, J.M. Sobrino, et al., Carbon nanotube/alumina and graphite/alumina composite coatings on stainless steel for tribological applications, Mater. Today Commun. 8 (2016) 118–126. [12] C.K. Lee, Wear and corrosion behavior of electrodeposited nickel-carbon nanotube composite coatings on Ti-6Al-4V alloy in hanks’ solution, Tribol. Int. 55 (2012) 7–14. [13] M.A.  Samad, S.K.  Sinha, Mechanical, thermal and tribological characterization of a UHMWPE film reinforced with carbon nanotubes coated on steel, Tribol. Int. 44 (2011) 1932–1941. [14] X.  Li, Y.  Zhou, J.  Sun, H.  Zhang, W.  Wu, Tribological behavior of the electroless Ni-P-CNTs-SiC (nanometer) composite coating, Rare Metal Mater. Eng. 36 (2007) 712–714. [15] L. Esposito, J. Ramos, G. Kortaberria, Dispersion of carbon nanotubes in nanostructured epoxy systems for coating application, Prog. Org. Coat. 77 (2014) 1452–1458. [16] D.E.  Kshirsagar, V.  Puri, H.  Dubey, M.  Sharon, Giga hertz frequency absorber carbon nano fibers synthesized using linseed oil, Mater. Today Commun. 13 (2017) 23–25. [17] W.  Guo, W.  Zhong, Y.  Dai, S.  Li, Coupled defect-size effects on interlayer friction in multiwalled carbon nanotubes, Phys. Rev. B Condens. Matter 72 (2005) 075409. [18] A.K. Keshri, J. Huang, V. Singh, W. Choi, S. Seal, A. Agarwal, Synthesis of aluminum oxide coating with carbon nanotube reinforcement produced by chemical vapor deposition for improved fracture and wear resistance, Carbon 48 (2010) 431–442. [19] P.  Nie, C.  Min, H.J.  Song, X.  Chen, Z.  Zhang, K.  Zhao, Preparation and tribological properties of polyimide/carboxyl-functionalized multi-walled carbon nanotube nanocomposite films under seawater lubrication, Tribol. Lett. 58 (2015) 1–12. [20] A. Cayuela, S. Benítez-Martínez, M. Laura Soriano, Carbon nanotools as sorbents and sensors of nanosized objects: the third way of analytical nanoscience and nanotechnology, TrAC Trends Anal. Chem. 84 (Part A) (2016) 172–180. [21] E.E.  Anand, S.  Natarajan, Influence of carbon nanotube addition on sliding wear behaviour of pulse electrodeposited cobalt (Co)-phosphorus (P) coatings, Appl. Phys. A Mater. Sci. Process. 120 (2015) 1–6.

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[22] N.W.  Khun, B.C.R.  Troconis, G.S.  Frankel, Effects of carbon nanotube content on adhesion strength and wear and corrosion resistance of epoxy composite coatings on AA2024-T3, Prog. Org. Coat. 77 (2014) 72–80. [23] R. Rafiee (Ed.), Carbon Nanotube-Reinforced Polymers: From Nanoscale to Macroscale, Elsevier Inc., Amsterdam, 2018. Chapter 1. [24] A.  Fraczek-Szczypta, Carbon nanomaterials for nerve tissue stimulation and regeneration, Mater. Sci. Eng C 34 (1) (2014) 35–49.

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Nanoparticles and textile technology

4

Rajesh Mishra, Jiri Militky Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

4.1 Introduction Despite the current interest, nanoparticles are not a new phenomenon, with scientists being aware of colloids and sols for more than 100 years. The scientific investigation of colloids and their properties was reported by Faraday [1] in his experiments with gold. He used the term “divided metals” to describe the material that he produced. Zsigmondy [2] describes the formation of a red gold sol that is now understood to comprise particles in the 10 nm size range. Throughout the last century, the field of colloid science has developed enormously and has been used to produce many materials including metals, oxides, organics, and pharmaceutical products. When a bulk material is divided into small-size particles with one or more dimensions (length, width, or thickness) in the nanometer range, the individual particles exhibit unexpected properties, different from those of the bulk material. The nanometer range is characterized by the transition of a material’s behavior from “quantum-like” behavior of atoms and molecules to the “continuum-like” behavior of bulk materials. Nanomaterials when engineered at the atomic and molecular level and are integrated into fabrics can exhibit certain properties that alter the physical properties of textiles. Nanotechnology also has real commercial potential for the textile industry. This is mainly due to the fact that conventional methods used to impart different properties to fabrics often do not lead to permanent effects and will lose their functions after laundering or wearing. This technology provides high durability for fabrics because nanoparticles have a large surface-area-to-volume ratio and high surface energy, thus presenting better attraction to fabrics and increase in durability of the function [2]. One of the promising ways is to use textile-based wastes for preparation of nanoparticles. Classical waste pyrolysis for preparation of carbon nanoparticles is still very useful. Relatively new is preparation of cellulose-based nanofibrils by proper treatment of jute textile wastes. This process is described here as well.

4.2 Selected features of nanoparticles Nanoparticles (less than 100 nm) [3, 4] contain 1 million atoms or less (1 nm radius has approx. 25 atoms), and the majority of atoms are on the surface. It is well known that many properties of matters depend on the size range. In nanoscale, there are in Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00004-3 Copyright © 2019 Elsevier Ltd. All rights reserved.

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some cases extra effects not following the bulk materials because the particle/wave nature of matter appears (quantum effects, tunneling, and self-assembling). ●

●

Nanoparticles are particles with at least one dimension smaller than 1 μm and potentially as small as atomic and molecular length scales ~0.2 nm. Nanoparticles can have amorphous or crystalline form, and their surfaces can act as carriers for liquid droplets or gases.

Typical nanoparticle properties are the following: ●

●

●

●

Extreme surface area, the majority of atoms are on the surface. Dimensional similarity with visible and UV radiation. Color and scattering are dependent on the particle size. Critical length (mean free length) for diffusion and conductivity is comparable or higher than nanoparticle dimension. Particle toxicity is growing due to their size decreasing.

One carbon microparticle with a diameter of 60 μm has a mass of 0.3 g and a surface area of 0.01 mm2. The same mass of carbon in nanoparticulate form, with each particle having a diameter of 60 nm, has a surface area of 11.3 mm2 and consists of 109 nanoparticles. As the material in nanoparticulate form presents a much larger surface area for chemical reactions, reactivity is enhanced roughly 1000-fold. It is very simple to compute the geometric changes due to use of particles with smaller dimension. Simple calculations are based on the ideal spherical particles. The specific surface area of spherical particle of diameter D and density ρ is defined by relation Sa =

A π D2 6 = = 3 ρ Vo ρ π D / 6 ρ D

(4.1)

The specific surface area is growing dramatically below particles of diameter 10 nm (see Fig. 4.1). Qualitatively similar trends are valid for related properties such as the ratio of surface/bulk atoms and the fraction of particle volume comprised by a surface layer of finite thickness. It is illustrative to calculate some geometric characteristics of smaller particles as a result of cutting of big particle. Let the spherical particle of diameter D with the surface area of S0 = π D2 be divided into n identical spherical particles of diameter d with total surface area S = n π d2. Because the volume (and mass) of bigger particles V0 = (4/3) π (D/2)3 is the same as the sum of volumes of smaller particles n V = (4/3) π (d/2)3, the number of smaller particles is simply 3

D n=  d Relative gain of surface area SR while maintaining the volumes is D  S R = ( S − S0 ) / S0 = n π d 2 − π D 2 / S0 =  − 1  d 

(

)

(4.2)

(4.3)

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104 Specific surface area of sphere r = 1000 r = 5000 r = 7000

3

Surface area [ m2/g]

10

102

101

100 0

20

40

60

80

100

120

140

160

180

200

Sphere diameter [nm]

Fig. 4.1  Influence of spherical particle diameter D on the specific surface area for various material densities ρ.

For example, if the particles of diameter 10 nm are produced from the particle of diameter 10 μm, the number of particles is n = 109, and SR = 999. Increase of surface area Sn is then equal to = Sn S= / S0 D / d = 3 n

(4.4)

Dispersion F of a particle is defined as the fraction of atoms at a surface relative to the total number of atoms in the particle: F⊕

surface area volume

(4.5)

For sphere of radius r is F = 3/r; for tubular rod is F = 2/r. Cubic crystals, containing total number of N atoms, have F ⊕2 / 3 N . Very interesting is the computation of surface layer volume portion for particle with diameter D and total volume Vc. Let the volume of the surface layer having thickness t be Vs = VC − Vo. Then, relative volume of surface layer Vp is Vp = (Vc − Vo ) / Vc =

( 4 / 3) π ( D / 2 + t ) − ( 4 / 3) π ( D / 2 ) 3 ( 4 / 3) π ( D / 2 + t ) 3

3

= 1−

( D / 2)

3

(D / 2 + t)

(4.6) 3

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0.9

Relative volume change [–]

0.8

VT

0.7 0.6 0.5 0.4

Vp

0.3 0.2 0.1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Particle diameter [nm] D t = 2 nm Fig. 4.2  Changes of relative volumes of particle surface layer Vp and core VT.

and relative volume of VT of core sphere of diameter D is VT = 1 − Vp. For the case of D = 100 nm and t = 2 nm is Vp = 0.11. Relative portion of volume in the surface layer of thickness 2 nm is then around 11%. For thicker particles with dimensions about 10 μm is Vp-negligible. The majority of volume is then under surface layer not at disposal for surface or interface phenomena. In smaller particles, the passive volume out of surface layer is quickly decreased (Fig. 4.2). One of main benefits of nanoparticles is more even covering of space and smaller interparticle distance h. This distance should be sufficiently small for overcoming of threshold, for example, for transition from nonconductive and conductive materials. Let the particles be arranged in the space in cubic order with the same interparticle distances h (see Fig. 4.3). The portion of each particle volume V0 = (4/3) π (D/2)3 in the elementary cubic cell of volume Va = (D + h)3 is Vf = Vo/8. The volume ratio is defined as

φ=

particles volume V0 = total volume Va

(4.7)

Volume concentration c (%) is in fact the volume ratio expressed as percentage, that is, c = 100 φ

(4.8)

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Fig. 4.3  Ideal arrangement of particles in the space. 400 350 5 nm

Distance [nm]

300

100 nm 50 nm

250 200 150 100 50 0

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Particle volume fraction

Fig. 4.4  Relation between particle volume fraction and interparticle distance for selected particle diameters.

The ratio between the actual volume ratio ϕ and limited volume ratio ϕm for h = 0 is then

φ D3 = φm ( D + h ) 3

and φm =

π = 0.524 6

(4.9)

Ideal interparticle distance h between particles is then defined by relation   φ h = D  3 m − 1 (4.10)  φ    The dependence of interparticle distance h on the volume fraction of particles is shown in Fig. 4.4.

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For nanoparticles with typical diameter D = 6 nm is interparticle distance for volume fraction of 1% equal to the h (1%) = 16.4 nm, and for volume fraction of 5% is interparticle distance equal to the h (5%) = 7.13 nm. For bigger (colloidal) particles with typical diameter D = 600 nm is interparticle distance for volume fraction of 10% equal to h (10%) = 441.9 nm, and for volume fraction of 50% is interparticle distance equal to the h (50%) = 9.3 nm. It is visible that for bigger particles is necessary to use much higher concentration to obtain prescribed interparticle distance. Degree of space covering by particles can be characterized by the total number of particles N in the defined volume

φ=

particles total volume = N V0 system total volume Va

N=

6 φ Va π D3

(4.11)

where V0 is one particle volume, Va is total volume of system, and N is total number of particles in volume Va. The dependence of total number of particles in volume Va = 50,000 μm3 on the particles diameter is shown in Fig. 4.5. The degree of space covering by particles of different diameter D is schematically shown in Fig. 4.6. Typical length ratio L = D/d for d equal to the atom diameter can be simply used for the computation of the following: ●

●

●

Number of atoms in particle N = L3 Particle mass Mc = Mh L3/6.022 1023, where Mh is molecular mass of material Particle volume Vc = π d2 L3/4

Influence of particle size on selected geometric characteristics is shown in the Table 4.1.

4

x 106

Number of particles in volume Va

Particles volume concentration

3.5

5% 10% 30%

3 2.5 2

Va = 50,000 mm3

1.5 1 0.5 0 0.2

0.25

0.3 0.35 0.4 0.45 0.5 Particle diameter [micrometers]

0.55

Fig. 4.5  Influence of particles diameter on the total number of particles in volume Va = 50,000 μm3.

0.6

Nanoparticles and textile technology187

50 mm

100 mm

(A)

(B)

9 x Magnification

(C) Fig. 4.6  Degree of space covering by particles of different diameter D. (A) 10 μm, (B) 1 μm, and (C) 100 nm. Table 4.1  Selected geometric characteristics of fine nanoparticles Particle size (nm)

Crosssectional area (10−18 m2)

Mass (10−25 kg)

Number of molecules

% of molecules at the surface

0.5 1.0 2.0 5.0 10.0 20.0

0.2 0.8 3.2 20 80 320

0.65 5.2 42 650 5.2 × 103 4.2 × 104

1 8 64 1 × 103 8 × 103 6.4 × 104

– 100 90 50 25 12

Very common mistake is the assumption that nanoparticles are stronger in comparison with more voluminous particles. In fact, cohesion energy per atom (diameter d) is dependent on the diameter of particles (D) by relation d   1 E = Eb  1 −  = Eb  1 −  D    L where Eb is cohesive energy for bulk material. For nanoparticles, it is ratio d/D from 0.1 till 0.01, and cohesive energy is increasing with particle diameter. It is interesting that starting from diameter of nanoparticles around 30 nm is E practically constant. It was found [78] that the size-dependent thermodynamic properties of nanoparticles (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature, and specific heat capacity) vary approximately linearly with reciprocal value of their diameter 1/D. Therefore, the size-dependent thermodynamic properties Gn follow the relation Gn = Gb(1 − K/D), where Gb is the property bulk value and K is the material constant. This is in fact a scaling law for most of the size-dependent thermodynamic properties for different materials.

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The size-dependent surface free energy of nanoparticles and nanocavities can be expressed as  1.45 d   1.45  γ = γ b 1 ±  = γ b 1 ± L  D     Here, minus sign corresponds to nanoparticles, while plus sign corresponds to nanocavities. This relation is valid for free nanoparticles. A nanoparticle embedded in a matrix can be regarded as being composed of a free nanoparticle and a cavity with the same size [78]. The melting point of embedded nanoparticle Tcm is then P P   Tcm = Tbm  1 − + 2  D D   where P and Q are constant and Tbm is melting temperature of bulk material corresponding to the nanoparticles. The mass fraction of the nanoparticles embedded in the polymer matrix can remain very low (on the order of 0.5%–5%) due to very high covering of space and small interparticle distances and number density of nanoparticles [7–20]. It was shown that many particle and molecular cluster properties (e.g., ionization energy, electron affinity, melting temperature, and cohesive energy) have a smooth variation with size in the large-particle regime. The following scaling laws can be applied for a general property (G) [5]. G ( N ) = G (∞) + b N −β where N is size (the number of atoms) in particle or cluster. Usually, β = 1/3. Large deviations (oscillations about the smooth trend) are observed for many properties in the medium- and (especially) the small-particle-size regimes. Deviations arise due to quantum size effects (electronic shell closings) and surface effects (geometric shell closings); see Fig. 4.7.

Large

Medium N

Small 1

G(N)

G(1)

Liquid drop behavior G(¥)

Quantum size and surface effects

N–b

Fig. 4.7  Scaling law (influence of particle size on the selected properties) [6].

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Between particles generally occur gravitational and electromagnetic forces. Gravitational force is a function of mass and distance and is extraordinary weak between (low-mass) nanosized particles. Electromagnetic force on the other hand is a function of charge and distance. It is not affected by mass, so it can be very strong even when we have nanosized particles. The electromagnetic force between, for example, two protons is 1036 times stronger than the gravitational force. The stability of liquid in continuous phase depends on the settling velocity vs, which is proportional to their diameter square vs ~ D2. For nanoparticles, D is very small, and settling velocity is very small (sedimentation occurs after long time). For these particle diameters, the diffusion is due to molecular collision, that is, Brownian motion.

4.3 Nanoparticles preparation Specific mechanical, physical, and chemical processes are employed to produce the various nanoparticles, coatings, dispersions, or composites. Particle characteristics can be controlled by temperature, pH value, concentration, chemical composition, surface modifications, and process of preparation [21]. Two basic strategies are used to produce nanoparticles: “top-down” and “bottom-up.” ●

●

Top-down approach (from bigger to smaller objects). Etching by laser, electron or ion beam, milling, etc. Bottom-up approach (molecular machines—molecular biology and molecular chemistry). Self-organization—the spontaneous transition from chaos to order

The term “top-down” refers here to the mechanical crushing of source material using usually a milling process. In the “bottom-up” strategy, structures are built up by chemical processes. The selection of the respective process depends on the chemical composition and the desired features specified for the nanoparticles. The various techniques of nanoparticles preparation are summarized in the work [22]. Here, the milling process only is discussed. In mechanical milling, mixtures of elemental or prealloyed powders are subjected to grinding (often under protective atmosphere) in equipment capable of high-energy compressive impact forces. A variety of ball mills have been developed for different purposes including tumbler mills, attrition mills, shaker mills, vibratory mills, and planetary mills. Powders with typical particle diameters of about 50 μm are placed together with a number of hardened steel- or tungsten carbide (WC)-coated balls in a sealed container that is shaken or violently agitated. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials are preferable to ceramic balls. During the continuous severe plastic deformation associated with high-energy mechanical attrition, a continuous refinement of the internal structure of the powder particles to nanometer scales occurs. Results of milling are dependent on the diameter of milling balls. Fig. 4.8 depicts the size distribution of polypyrrole (PPy) milled particles on the Planetary Ball Milling FRITSCH PULVERISETTE-7 [23]. Milling with 3 mm diameter balls gives a narrow distribution, whereas 5 mm diameter balls showed a bimodal distribution, and milling with 10 mm diameter balls gives the extremely wide distribution results with large particle size.

190

Nanotechnology in Textiles 16 91.28

14

3 mm diameter ball 5 mm diameter ball 10 mm diameter ball

295.3

12 220.2 Intensity %

10 8

396.1

6 4 2 0

0

100

200

300

400

500

600

700

800

900

1000

1100

Particle size (nm)

Fig. 4.8  Particle size distributions of the PPy particles after 300 min of milling at 800 rpm with different ball sizes [24].

Fig. 4.9 represents the SEM images of unmilled and some milled particles of PPy. Nonspherical particles with nearly uniform sizes can be seen in Fig. 4.9B and C only which are the results of milling PPy with 3 and 5 mm dia balls for 300 min at 800 rpm. Small size of nanoparticles leads to particle-particle aggregation, hereby making physical handling of nanoparticles difficult in liquid and dry powder forms. Due to their high reactivity, nanoparticles have a high tendency to build aggregates or agglomerates, which could lead to a loss of their advantages and to creation of bigger structures. In the limiting case, the bigger particles are created (see Fig. 4.10). Therefore, it is generally necessary to stabilize the nanoparticles with additional treatments. The commercial success or failure of nanoparticles in a particular application usually depends upon the ability to prepare stable dispersions in water or organic fluids. The ability to prepare stable nanoparticle dispersions with controlled rheology is enabled by tailoring nanoparticle coatings. On the other hand, coating nanoparticles with another material of nanoscale thickness is a simple way to alter the surface properties of nanoparticles. Core-shell structured nanoparticles have, for example, advanced optical, mechanical, and magnetic properties. One common method to stabilize or modify the reactivity of the nanoparticles is the encapsulation with a molecular or polymeric layer. A thin polymeric shell enables compatibility of the particles with a wide variety of fluids, resins, and polymers [6]. In this way, the nanoparticles retain their original chemical and physical properties, but the coating can be tailored for a wide variety of applications.

Nanoparticles and textile technology191

(B)

(A) SEMMAG:10.00Kx HV: 30.0KV VAC: HIVac

DET: BE Detector DATE: 03/25/11 Device: TS5130

5 um

Vega ©Tescan TU Liberec

SEMMAG:10.00Kx HV: 30.0KV VAC: HIVac

DET: BE Detector DATE: 03/25/11 Device: TS5130

5 um

Vega ©Tescan TU Liberec

SEMMAG:10.00Kx HV: 30.0KV VAC: HIVac

DET: BE Detector DATE: 03/25/11 Device: TS5130

5 um

Vega ©Tesc TU Liber

(C) SEMMAG:10.00Kx HV: 30.0KV

DET: BE Detector DATE: 03/25/11

VAC: HIVac

Device: TS5130

5 um

Vega ©Tescan TU Liberec

(D)

Fig. 4.9  SEM images of PPy (A) unmilled; (B), (C), and (D) are milled samples with 3, 5, and 10 mm diameter balls at 800 rpm for 300 min [25].

Another way to ensure the stability of the collected nanoparticle powders against agglomeration, sintering, and compositional changes is to collect the nanoparticles in a liquid suspension. For semiconducting particles, stabilization of the liquid suspension has been demonstrated by the addition of polar solvent surfactant molecules that have been used to stabilize the liquid suspension of metallic nanoparticles [7]. Alternatively, nanometer-thick silica-shell encapsulation of nanoparticles by a gasphase reaction and by oxidation in colloidal solution has been shown to be effective for metallic nanoparticles [8]. For carbon nanotubes, it is often necessary to disperse them in fluid suspensions to obtain a regular orientation in the composite material resulting in unique mechanical or electric characteristics. Milling, ultrasonication, high

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Sintering

Ripening

Fig. 4.10  Nanoparticle enlargements.

shear flow, elongational flow, functionalization, and surfactant and dispersant systems are used to affect the morphologies of carbon nanotubes and their interactions in the fluid phase [9]. Nanoparticles dispersed in aqueous solutions also tend to build aggregates due to attractive van der Waals forces. By altering the dispersing conditions, repulsive forces can be introduced between the particles to prevent the aggregation. There are two general ways of stabilizing nanoparticles in aqueous solutions. Firstly, by adjusting the pH of the system, the nanoparticle surface charge can be manipulated in such a way that an electric double layer is generated around the particle. Overlap of two double layers on different nanoparticles causes repulsion and hence stabilization. The magnitude of this repulsive force can be measured via the zeta potential. Electrostatic stabilization involves the coulomb repulsion between the particles caused by the electric double layer formed by ions adsorbed at the particle surface (e.g., sodium citrate) and the corresponding counterions (see Fig. 4.11). As an example, gold sols are prepared by the reduction of [AuCl4−] with sodium citrate. The second method involves the adsorption of polymers onto the nanoparticles in such a way that the particles are physically prevented from coming close enough for the van der Waals attractive force to dominate. This is termed steric stabilization. Steric stabilization is achieved by coordinating sterically demanding organic molecules that act as protective shields on the surface. In this way, nanoparticle cores are separated from each other, and agglomeration is prevented (see Fig. 4.12). The main classes of protective groups are polymers and copolymers, phosphines, amines, thioethers, solvents, long-chain alcohols, surfactants, and organometallics. A combination of these two mechanisms is called electrostatic stabilization and occurs when polyelectrolytes are adsorbed on the nanoparticles surface [10].

Nanoparticles and textile technology193

E Electrostatic repulsion

Van der Walls attraction

Fig. 4.11  Electrostatic repulsion.

Fig. 4.12  Steric stabilization.

4.4 Nanoparticles application in the textile industry The selected nanomaterials used frequently in textile branch are shown in Fig. 4.13. Especially, nanoparticles have versatile applications in textile industry, composites, buildings, and other industries. They can beneficially replace the classical micron particles used in finishing for obtaining soil release; stain resistance; flame retardation; wrinkle resistance; moisture management; antibacteria, antistatic, and UV protection; improvement of dyeability; and so on. Incorporating nanoparticles into a textile can affect a host of properties, including shrinkage, strength, and electric and thermal conductivity. [34]. The formation of a barrier layer is one of the fundamental mechanisms of fire retardancy due to the presence of nanoparticles in a polymer [35]. Various other kinds

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Fig. 4.13  Some nanostructured materials.

of mechanisms can be involved, some of which are connected to the nature of the nanoparticles, such as ●

●

●

●

●

●

●

●

catalytic effects promoting the formation of charred structures and reinforced by the nanoparticles, purely physical barrier effects due to the particles because of their specific aspect ratio, formation of an insulating char structure that is able to dissipate incident heat by radiative emission, modification of heat diffusivity through the material, restriction of macromolecular mobility and an increase in viscosity, modification of the degradation pathway of the polymers, trapping of radicals formed during combustion, formation of new chemical species by reaction with flame retardants or additives.

Nanoparticles such as metal oxides and ceramics are also used in textile finishing, altering surface properties, and imparting textile functions. Nanosized particles have a larger surface area and hence higher efficiency than larger-size particles. Besides, nanosized particles are transparent, and do not blur the color and brightness of the textile substrates. The particle size is therefore one of main factors influencing efficiency of their use. As an example, the fabric treated with nanoparticles TiO2 and MgO replaces fabrics with active carbon, previously used as chemical and biological protective materials [36]. The photocatalytic activity of TiO2 and MgO nanoparticles can break harmful and toxic chemicals and biological agents. These nanoparticles can be pre-engineered to adhere to textile substrates by using spray coating or electrostatic methods. Finishing with nanoparticles can convert fabrics into sensor-based materials. If nanocrystalline piezoceramic particles are incorporated into fabrics,

Nanoparticles and textile technology195

the finished fabric can convert exerted mechanical forces into electric signals enabling the monitoring of bodily functions such as heart rhythm and pulse if they are worn next to the skin [37]. In the textile branch, the following nanomaterials are used: nanofibers (electrospinning), surface nanoroughening, nanoparticles (powders), nanoporous materials, nanocomposites, quantum dots, and carbon nanotubes. The application of nanotechnology in the textile branch is described, for example, in the review [38]. The most important principles of textile functionalization by nanoparticles are the following [79]: 1. Incorporation of functional nanoadditives (organic or biological compounds, inorganic particles, and polymers) into the polymer melt or polymer solution before spinning. The advantage is the high permanence, but drawbacks are low flexibility, the high nontextile portion, and also the procedure not practicable for natural fibers. 2. Chemical grafting of nanoadditives on the fiber surface directly or by means of linkers. This effective method yields excellent permanent effects but is restricted to fibers containing reactive sites and additive structures. 3. Postequipping textiles (fiber and fabrics) with functional coatings. This universal method is very flexible with regard to coating technology and productive capacity and is largely independent from fabric type. It requires low quantities of additive and enables the combination of different functionalities in a simple way.

It seems that the last principle is more economical and flexible also for future commercial success of nanotechnology applications on textiles. There are various techniques to carry out the coating on textile surface like sol–gel process, layer-by-layer deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), or simply pad-dry-cure process. More details are provided on coating the textile substrate via sol-gel process and dip-paddry-cure process as they are most economical and flexible compared with other techniques. One example of use of silver nanoparticles created in situ on the cotton fabric for antimicrobial finishing was published in the work [39]. Deposition of these particles on cotton fiber surface is visible from Fig. 4.14.

Fig. 4.14  Cotton fibers with deposited silver nanoparticles [20].

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Table 4.2  Nanoparticles in continuous phase Dispersed phase (nanoparticle)

Continuous phase (medium completely surrounding the nanoparticle)

Solid

Liquid

Gas

Solid

Liquid

Gas

Solid suspension (solid sol) certain ceramics (Corel) and alloys, ruby glass Suspension (sol, in H2O hydrosol) muddy water, paint, ink Smoke (aerosol) smoke, dust

Gel jello, jelly, cheese, certain rubbers

Solid foam foam rubber, Styrofoam

Emulsion mayonnaise, milk

Foam shaving cream, whipped cream

Fog (liquid aerosol) fog, clouds

Does not occur (all gases are miscible)

Coating is a common technique used to apply nanoparticles onto textiles. The coating compositions that can modify the surface of textiles are usually composed of nanoparticles, a surfactant, ingredients, and a carrier medium [40]. Several methods can apply coating onto fabrics, including spraying, transfer printing, washing, rinsing, and padding. Of these methods, padding is the most commonly used [41–44]. The nanoparticles are attached to the fabrics with the use of a padder adjusted to suitable pressure and speed, followed by drying and curing. The application of nanoparticles is dependent on their dispersion medium (see Table 4.2) as well. The suspension of nanoparticles is commonly used. It is attractive to use nanoparticles as a form of fillers into polymeric materials used as coatings or composites. A definition of nanotechnology for the textile industry was elaborated, which is also the basis for the “Hohenstein Quality Label for nanotechnology” [24]: “Nanotechnology refers to the systematically arranged functional structures which consist of particles with size-dependent properties.” A textile product therefore does not qualify for the Hohenstein Quality Label merely on the basis that it has nanoparticles incorporated within the fibers or that the fibers are enclosed in a nanoscale coating [24]. Rather, the nanoparticles or nanolayers in or on the textile must be systematically arranged (Fig. 4.15) and thus demonstrably result in a new function. At the same time, there must not be any negligible negative effect on the textile properties [24]. The definition of textile nanotechnology according to Hohenstein Institute should follow these requirements: ●

●

●

●

Application of functional systems in the submicron range based on the use of “subunits” (nanoparticles/molecules) systematically arranged Specific size-dependent properties Products with all new or far improved functions Textile properties not being negatively affected

Nanoparticles and textile technology197

NANOTECHNOLOGY

NANOTECHNOLOGY

Fig. 4.15  Definition of nanotechnology according to the Hohenstein Institute [24].

4.5 Carbon nanoparticles Activated carbon is prepared by controlled pyrolysis of acrylic fibrous waste under the layer of charcoal using physical activation in high-temperature furnace. For the preparation of carbon particles from acrylic wastes with required properties, the process parameters, that is, final pyrolysis temperature, holding time at final temperature, heating rate per hour, and the number of steps should be optimized. The Box-Behnken design and response surface modeling is for these cases effective for experimental design-based optimization. Later on, the carbonized acrylic fibrous waste was pulverized in dry conditions by high-energy planetary ball milling to get activated carbon nanoparticles. In addition to refinement of size, the specific surface area and electric conductivity of pulverized carbon particles were found to increase with an increase in milling time. The activated carbon particles obtained after 3 h of dry milling revealed the particle size of 521 nm, the electric conductivity of 21.78 s/m for 0.5 wt% concentration of aqueous dispersion, and the specific surface area of 432 m2/g. In recent years, the quality of fresh air surrounding the living place has gained an importance as it affects health, comfort, satisfaction, and productivity of people. In fact, the indoor quality of air is more important than the outdoor air since most people spend an average of 90% of their time in enclosed buildings [45–47]. The volatile organic compounds are regarded as a major source of indoor air contaminants. The volatile organic compounds are a highly diverse class of chemical contaminants having 50–300 compounds of boiling point in the range of 50–260°C [47,48]. The few examples of volatile organic compounds are formaldehyde, benzene, toluene, acrolein, radon, ozone, and fine particles, which can be found in cooking or tobacco smoke [49]. A long-term exposure to these compounds causes headaches, dizziness, nausea, or allergic reaction. Due to diverse nature of the presence of volatile organic compounds in indoor air, there is no single method for their removal from the atmosphere. The commonly used indoor air purification methods are adsorption, photocatalytic oxidation, negative air ions, and nonthermal plasma [50]. However, the adsorption-based techniques are most attractive than others since they do not generate harmful intermediates [51,52]. The removal of volatile organic compounds by adsorption on activated carbon, activated

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carbon fibers, zeolites, porous clay ore, activated alumina, silica gel, etc. have been studied previously [53]. However, the activated carbon-based adsorbents are effective at removing a broad spectrum of indoor volatile organic compounds due to their high adsorption capacity and considerable surface area [54]. Activated carbon is a highly porous product, usually derived from carbon sources such as bituminous coal, lignite, wood, or coconut shell [55]. Activated carbon has a very high porous structure with a large internal surface area around 500–2000 m2/g [56]. In recent years, research on exploring alternative inexpensive sources together with methods for preparation of activated carbon materials has attracted attention. The idea of converting acrylic fibrous wastes generated in textile industries into activated carbon is considered as one of the favorable approaches. The short acrylic fiber wastes are suitable for porous activated carbon because of their excellent natural structure and low ash content [57]. The activated carbon derived from acrylic fiber wastes are advantageous over carbon made from other materials because of its low cost, high density, better purity, and virtually dust-free nature [58]. The acrylic fibrous waste can be converted into activated carbon by physical activation in the presence of air via controlled thermal treatment under the layer of charcoal using high-temperature furnace (Fig. 4.16). The carbonization process was studied under the four variables, that is, final pyrolysis temperature (FPT), holding time at final temperature (HTFT), heating rate per hour (HRPH), and the number of steps (NOS) using Box-Behnken design and response surface modeling. These four variables were optimized to get higher specific surface area and higher electric conductivity. The details about carbonization of acrylic fibrous wastes are collected in article [59]. Using the Box-Behnken experimental design, 27 runs with appropriate combinations of FPT, HTFT, HRPH, and NOS were conducted. The results for specific surface area and surface resistivity are given in Table 4.3. During the initial pyrolysis stage of stabilization at 250°C, the acrylic fibrous waste was softened with the evolution of volatile matter causing subsequent bubbling, hardening, and shrinkage of the char [59]. The subsequent pyrolysis cycles of the char at increased temperature found to result in micropore formation followed by pore enlargement as the atmospheric air reacts with the carbonized acrylic fibrous wastes. This phenomenon indicated the increase in specific surface area with increase in pyrolysis temperature. The experimental results were fitted to a full quadratic second-order model by applying multiple regression analysis using SYSTAT software. The model equations representing specific surface area and surface resistivity are given in [59]. With the help of canonical analysis in SYSTAT software, a set of optimized parameters for both specific surface area and surface resistivity are calculated and given in Table 4.3. The development of porous morphology having higher surface area is found to increase with an increase in pyrolysis temperature, an increase in the number of steps, a decrease in holding time, and a decrease in heating rate till some optimum value. This behavior is attributed to gradual reaction of atmospheric oxygen with carbonized acrylic fibrous waste, which resulted in the opening of previously inaccessible pores through the removal of tars and disorganized carbon [60,61]. Moreover, these four

Acrylic fibrous wastes CH2

CH2 CH

CH

C N

H

N

N

H

H

N

N

H

H

N

400–600°C

Dehydrogenation

CH2 HC

C

H

C N

N

N

N

N

N

N

N

N

N

N

N

N

N

600–1200°C

Denitrogenation

N N N N N N

Fig. 4.16  Preparation of activated carbon from acrylic wastes.

Nanoparticles and textile technology199

N

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Table 4.3  Optimum values of pyrolysis parameters Pyrolysis factor

Specific surface area (m2/g)

Surface resistivity (Ω mm)

FPT (°C) HTFT (min) HRPH (°C/h) NOS

769.63 17.19 382.27 1.47

970.05 58.40 337.52 1.72

factors also found to have significant effect on the development of electric conductivity than surface area of activated carbon. It is clear from Fig. 4.16 that the effect of heating rate was more pronounced for increased specific surface area than the effect of holding time. The slower heating rate below 300°C/h was found more advantageous for gradual increase in reactivity between atmospheric oxygen and carbon. However, with an increase in holding time, the chances of sudden increase in reactivity between atmospheric oxygen and carbon are higher. As a result, the reduction in values of specific surface area with increase in holding time can be observed. Fig. 4.17A–C show the particle size distribution results of dry-milled activated carbon particles for different time of milling from 1 h to 3 h. It can be seen from Fig.  4.17A that the rate of particle size reduction is higher during the initial 1 h of milling during which the characteristic particle diameter Z-average reduced to 1563 nm. However, the particle size was gradually reduced later and reached to 521 nm after 3 h of dry milling as shown in Fig. 4.17C. When milling was performed for longer time, particle size distribution changed from multimodal distribution to near unimodal distribution. The reason behind multimodal distribution of particles is due to an increase in temperature within the mill because of continuous impact of balls [62,63]. The increased temperature of mill rendered the activated carbon particles to undergo cold welding and deposited a layer on the surface of milling media. The growth of deposited layer changed the impact force of balls on the material with least impact on particles at the bottom of the layer. The morphology of activated carbon particles after 3 h of dry milling was further investigated with the help of SEM images shown in Fig. 4.18A–C. The shape of activated carbon particles was observed in the form of mixture of both nanoparticles and nanosegments. The few carbon particles with higher aspect ratio were found due to inability of milling process as a result of increased temperature. From Fig. 4.19, the specific surface area of activated carbon particles was found to increase with an increase in milling time. This behavior was found more significant during initial 1 h of milling during which surface area of activated carbon particles changed from 278 to 346 m2/g for 30 min of milling interval. Afterward, there was steady improvement in surface area up to 432 m2/g for 3 h of milling. Fig. 4.20 shows the electric conductivity of aqueous dispersion of activated carbon particles measured under different concentrations from 0.5 to 4.0 wt%. The influence of dry milling time on electric conductivity is clearly observed under lower concentration of carbon particles below 1 wt%, where electric conductivity of aqueous dispersion of carbon particles increased significantly with an increase in dry milling time. This behavior was attributed to an increase in surface area and

% Volume

Width (nm)

Peak 1:

2534

100.0

943.9

43.12

Pdl: 0.230

Peak 2:

0.000

0.0

0.000

662.3

Intercept: 0.854

Peak 3:

0.000

0.0

0.000

% Volume

Width (nm)

Peak 1:

1996

35.8

573.2

Pdl: 0.800

Peak 2:

199.2

10.0

Intercept: 0.876

Peak 3:

5431

54.2

Result quality Good

Result quality Refer to quality report

15

60

10

40

5

20 1

10

0 10000

1000

100

Volume (%)

80

16 14 12 10 8 6 4 2 0

(B)

Size (d.nm)

100 80 60 40 20 1

10

100

1000

0 10000

Size (d.nm) Diam. (nm) % Intensity

Width (nm)

Z-Average (d.nm): 521.6

Peak 1:

878.1

100.0

705.2

Pdl: 0.352

Peak 2:

0.000

0.0

0.000

Intercept: 0.850

Peak 3:

0.000

0.0

0.000

Result quality Good Size distribution by volume 100

7

80

5 4

60

3

40

2

20

1 0

(C)

Undersize

Intensity (%)

6

1

10

100

1000

0 10000

Size (d.nm)

Fig. 4.17  Particle size distribution. (A) One hour of dry milling. (B) Two hour of dry milling. (C) Three hour of dry milling.

Undersize

20

0

(A)

Size distribution by volume 100

Undersize

Volume (%)

Size distribution by volume 25

Nanoparticles and textile technology201

Diam. (nm) Z-Average (d.nm): 1640

Diam. (nm) Z-Average (d.nm): 1563

Fig. 4.18  SEM image of activated carbon particles. (A) One hour of dry milling. (B) Two hour of dry milling. (C) Three hour of dry milling.

Fig. 4.19  Effect of milling time on specific surface area of activated carbon particles.

Nanoparticles and textile technology203

Fig. 4.20  Effect of milling time on electric conductivity of activated carbon particles.

reduction in size of activated carbon particles at extended milling time. For higher concentration of carbon particles (i.e., 2 and 4 wt%), there is a gradual increase in electric conductivity with an increase in dry milling time. This behavior is attributed to early achievement of percolated network of carbon particles due to their higher loading in aqueous dispersion. As is shown in [59], the acrylic fibrous waste can be successfully converted into activated carbon by physical activation in the presence of air using controlled thermal treatment in high-temperature furnace. The multistage pyrolysis with 1200°C of final pyrolysis temperature resulted into activated carbon having higher specific surface area and higher electric conductivity. The lower heating rate and shorter holding time are found to have significant effect on the development of porous morphology with higher surface area. This behavior is attributed to gradual reaction of atmospheric oxygen with carbonized acrylic fibrous waste. For the analysis of variance of results in Box-Behnken design, the coefficient of determination is found to be 94.0% for specific surface area model and 86.0% for surface resistivity model. After getting the optimum pyrolysis parameters, dry pulverization of carbonized acrylic fibrous waste was carried out using high-energy planetary ball milling. The activated carbon particles obtained after 3 h of dry milling revealed the particle size of 521 nm, the electric conductivity of 21.78 s/m for 0.5 wt% concentration of aqueous dispersion, and the specific surface area of 432 m2/g. In this way, ball milling process was found to mechanically activate the surface of carbon nanoparticles with an increase in electric conductivity and surface area for better compatibility with printing pastes or resins.

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4.6 Cellulosic nanoparticles The utilization of different types of cellulosic wastes has been studied in the past in order to obtain cellulose nanofibrils at reasonably lower cost [26]. Although the cellulose nanostructures have a great potential for reinforcement into biopolymers, the major challenge in order to use them is the extraction. The variety of techniques like acid hydrolysis [27], enzymatic hydrolysis [28], ultrasonication [29], and high pressure homogenization have been employed. However, most of these techniques used in the extraction are time-consuming, expensive in nature, and low in yields [30–32]. The commonly used strong acid hydrolysis method has a number of important drawbacks such as potential degradation of cellulose, corrosivity, and environmental incompatibility. In order to promote the commercialization of cellulose nanofibrils, the development of more flexible and industrially viable processing technique is needed [33]. The increased demands of textiles brought the challenges to dispose significant amount of wastes generated during the processing [64,65]. In the context of environment protection and current disposal of textile wastes, it becomes essential to recover useful products from them for economic reasons. Traditionally, textile wastes are converted to individual fiber stage through cutting, shredding, carding, and other mechanical processes. The fibers are then rearranged into products for applications in garment linings, household items, furniture upholstery, automotive carpeting, automobile sound absorption materials, carpet underlays, building materials for insulation and roofing felt, and low-end blankets [66]. However, due to a recent increase in competition and reduced profit margins in these industries, it has become important to search for new recycling techniques of waste textiles in order to utilize them for high-end applications. One such interesting way is to separate the nanofibrils or nanocrystals from the textile wastes and subsequently incorporate them as fillers into high-performance composite materials [67–71]. Cellulose fibers are popularly used in the textile industry due to their high aspect ratio, acceptable density, good tensile strength, and modulus [72]. These properties make them attractive class of textile materials traditionally used in manufacture of yarn by spinning process. But due to certain limitations of the spinning process, shorter fibers (i.e., less than 10 mm) generated during mechanical processing are not suitable to reuse in yarn manufacture and consequently result in waste [73]. In order to exploit the intrinsic mechanical properties of short cellulose fibers in textile industries, the idea of separating nanofibrils or nanocrystals of cellulose could provide interesting applications in other fields. The previous studies have reported the remarkable properties of cellulose materials at nanoscale dimensions [74–76]. The extreme improvement in mechanical properties, in the range of 130–160 GPa, of cellulose nanofibrils is attributed to their increased rigidity obtained from parallel arrangement of molecular chains present without folding [77]. As a result, cellulose nanofibrils are increasingly used in applications of reinforced biodegradable nanocomposites, foams, aerogels, optically transparent functional materials, and oxygen-barrier layers [77]. In a variety of agricultural wastes like coconut husk fibers, cassava bagasse, banana rachis, mulberry bark, soybean pods, wheat straw, and soy hulls and cornstalks, some researchers investigated woods for extraction of cellulose nanofibrils. However, there

Nanoparticles and textile technology205

is no information available in literature on utilization of cellulosic wastes in textile industries in spite of the fact that large amounts of short fibers are generated during the mechanical processing of yarn manufacture. Over the last two decades, reinforcement potentials of lignocellulose fibers have been investigated in numerous studies of biocomposites made from polylactic acid (PLA). However, the properties are found not consistent to meet demands of value-added applications. This pattern of inconsistent improvements in properties of lignocellulosic fiber composites are explained due to the variations in properties of lignocellulosic fibers derived from different resources. As the individual lignocellulosic fibers are made from the packing of several micro-/nanocellulose fibrils together, the number of defects present in the structure varies from source to source. One of the basic ideas to further improve fiber and composite properties is to eliminate the macroscopic flaws by disintegrating the fibers and separating the almost defect-free, highly crystalline nanofibrils. This can be achieved by exploiting the hierarchical structure of the natural fibers. In theory, the performance of reinforced materials relies on the efficiency with which mechanical stress is transferred from an external energy source to the reinforcing phase through the matrix. The efficiency of transfer is a function of the amount and quality of the interfacial area between the reinforcing agent and the matrix. For composite materials, the interphase region is usually thin but plays an important role in their overall mechanical properties. The interphase is the region between the surface of the reinforcement and the polymer matrix where the chemistry is different from that of the bulk matrix. The interphase has a heterogeneous nature, with different morphological features, chemical compositions, and mechanical properties from those of the reinforcing fiber or the matrix polymer. Nowadays, as the optical-electronic industry developed, polymer materials are gradually increasing their importance in the area of thin-film structures. For such kind of thin-film structures, conventional test techniques, for example, tensile, compression, and bending tests, are inapplicable due to the difficulties of meeting sample requirements for such tests. Therefore, there is a need to propose novel techniques to study the mechanical properties of thin films or small volumes of materials. Nanoindentation technique has been developed for this purpose. However, relatively few nanoindentation studies of polymeric film have been presented. Recently, Beake and researchers studied the nanomechanical behavior on uniaxially and biaxially drawn poly(ethylene terephthalate) films using nanoindentation and nanoscratch tests. Some researchers studied the viscoelastic effects during in-depth-sensing indentation. It is generally very interesting to evaluate the reinforcement potentials of garneted jute fibers of millimeter scale (GJF), dry-milled jute fibers of micrometer scale (DMJF), and wet-milled jute fibers of nanometer scale (WMJF) in PLA matrix using nanoindentation testing. Details about preparation and characterization of milli (GJF), micro (DMJF), and nano (WMJF) jute particles and nanocomposite film testing (nanoindentation, tensile, and dynamic mechanical analysis) are described in article [48]. Under 1 h of dry milling, jute fibers were pulverized to microparticles with average size of 1480 nm in wider particle size distribution as is shown in Table 4.4 and Fig. 4.21. The reason behind multimodal distribution of particles was due to an increase in temperature within the mill because of continuous impact of balls. The increased

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Table 4.4  Particle size from DLS measurement for different milling conditions Sample name

First peak (nm)

Second peak (nm)

Third peak (nm)

Z-average (nm)

Polydispersity index

One hour of dry mill One hour of wet mill

1821

4889

297.5

1480

0.407

448.8

2365

0

640.1

0.420

Fig. 4.21  Particle size distribution of jute particles.

t­emperature of mill rendered the jute particles to undergo cold welding and deposited a layer on the surface of container and balls as milling progressed. The growth of deposited layer on the milling media changed the impact force of balls on the material with least impact on particles at the bottom of the layer. In the case of wet milling, the increase in temperature was slowed down by deionized water that consequently resulted in narrow particle size distribution with significant reduction in average particle size to 640 nm after 1 h of wet milling as shown in Fig. 4.22A and B. This can be attributed to the uniformity in the impact action of balls on every individual particle in wet condition. To further refine the jute particles to smaller size, wet milling was performed for extended duration. The average particle size reached to 443 nm after 3 h of wet milling, and the particle size distribution changed slowly from multimodal nature to unimodal nature as shown in Fig. 4.23 and Table 4.5. This showed the consistency and homogeneity in milling action on every individual particle as milling continued for longer time. However, the rate of refinement became slower while grinding the smaller particles in addition to the severe damage of milling balls due to direct collision. This could have introduced some inorganic contaminations from mill to the material, so further pulverization was stopped and jute particles in 500 nm range were used as nano-/microfillers.

Nanoparticles and textile technology207

SEM MAG: 1.00 kx HV: 30.0 kV VAC: HiVac

DET: BE Detector DATE: 04/04/11 Device: TS5130

50 um

Vega ©Tescan TU Liberec

(A)

SEM MAG: 5.00 kx HV: 30.0 kV VAC: HiVac

DET: BE Detector DATE: 04/04/11 Device: TS5130

10 um

Vega ©Tescan TU Liberec

(B)

Fig. 4.22  SEM image of jute particles. (A) One hour dry milling. (B) One hour wet milling.

Fig. 4.23  Effect of extended wet milling time on particle size reduction. Table 4.5  Particle size from DLS measurement for different wet milling time Sample name

First peak (nm)

Second peak (nm)

Third peak (nm)

Z-average (nm)

Polydispersity index

One hour Two hours Three hours

448.8 605.1

2365 4781

0 0

640.1 508.8

0.420 0.333

449.9

4966

0

443.1

0.299

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Nanotechnology in Textiles

44.04 nm 64.30 26.01 nm

42.83 nm 43.30 nm 23.76 nm 62.28 nm

60.86 nm

67.50 nm

34.76 nm 64.29 nm 56.21 nm

28.35 nm 48.27 nm

30.03 nm 56.34 nm 64.30 nm 33.30 nm 57.38 nm 37.36 nm

51.87 nm

43.51 nm

26.09 nm

1 µm

EHT = 5.00 kV WD = 1.2 mm

44.04 nm

Signal A = SE2 Mag = 50.00 K X

51.91 nm

Date :4 Oct 2012

Fig. 4.24  SEM image of jute nanofibrils.

Density 0.030

Density estimate

0.020

0.010

0.000 10

Fibril diameter 20

30

40

50

60

70

80

90

Fig. 4.25  Probability density function of diameter distribution of jute nanofibrils.

The shape and size of jute fibers after 3 h of wet milling was precisely investigated with the help of FESEM image (Fig. 4.24) due to its better resolution at nanoscale. The shape of jute particles was seen in the form of nanofibrils with certain aspect ratio. The few jute particles without aspect ratio were considered as agglomerates of hundreds of individual jute nanofibrils. In order to measure the diameter of nanofibrils, NIS-Elements BR software was employed, and a total of 25 observations were made. The probability density function of diameter distribution of 25 readings is shown in Fig. 4.25. In this way, the mean diameter and standard deviation were calculated as 46.52 nm and 13.58 nm, respectively.

Nanoparticles and textile technology209

Table 4.6  Comparison of jute nanofibril size from different techniques Sample name One hour of dry milling One hour of wet milling Three hours of wet milling

DLS technique (spherical shape)

BET technique (spherical shape)

Image analysis (actual shape)

1480 nm

1120 nm

480 nm

619 nm

430 nm

85 nm

493 nm

190 nm

46 nm

The sizes of nanofibrils obtained after dry and wet pulverization of jute fibers are compared on the basis of different techniques of measurements as given in Table 4.6. The techniques based on DLS and BET measurements were found to overestimate the size of nanofibrils with regard to actual size. The size obtained after image analysis was found much lesser as compared with DLS and BET measurements. The significant difference in size was attributed to the principles of working of different techniques that were based on certain assumptions. The DLS and BET techniques assume the shape of particles as spherical while calculating the particle size, whereas image analysis measurements are based on actual projected image of particles.

4.6.1 Nano indentation In order to investigate the indentation behavior, neat PLA and composite PLA films were subjected to the nanoindentation measurement. Fig.  4.26 shows the load-­ penetration depth curves obtained from neat PLA and after reinforcing with 3 wt% of GJF, DMJF, and WMJF in PLA matrix at a maximum load of 0.5 mN. The load-­ penetration depth behavior was typical of that for soft materials with a very little signature of an elastic recovery indicating a permanent deformation of the surface beneath the indenter (Table 4.7). The neat PLA was soft in comparison with the composite films. There was an increase in the measured hardness after addition of fillers (Table  4.8). The WMJF/ PLA films showed maximum hardness of 0.70 GPa compared with 0.68 and 0.54 GPa hardness of DMJF and GJF reinforced composite films, respectively. It was also seen that the elastic recovery (area enclosed between the unloading portion of the ­load-penetration depth profiles and the depth at maximum load) was maximum in the case of WMJF/PLA composite films. This was attributed mainly due to the effect of higher interaction of nanofibrils and PLA that transmitted the elastic properties of jute nanofibrils to PLA matrix effectively. For the GJF/PLA and DMJF/PLA composite films, the elastic recovery was less as compared with the WMJF/PLA composite films. This might be due to their reduced area of interaction as a result of bigger size of fillers. The sharpness of indentation mark shown in Fig. 4.27 further explained the higher elastic recovery of WMJF/PLA composite films compared with neat PLA and other composite films. The indentation mark was less sharp in the case of WMJF/PLA composite films due to faster elastic recovery of deformation (Fig. 4.27D).

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Fig. 4.26  Load-penetration depth profile of neat and composite PLA films. Table 4.7  Nanoindentation properties of neat and composite PLA films Nanolevel properties Sample name

Elastic modulus (GPa)

Hardness (GPa)

Neat PLA 3% GJF + PLA 3% DMJF + PLA 3% WMJF + PLA

4.37 ± 0.45 4.94 ± 0.41 5.48 ± 0.47 7.22 ± 0.52

0.52 ± 0.06 0.54 ± 0.05 0.68 ± 0.08 0.70 ± 0.10

Table 4.8  Storage modulus of neat and composite PLA films at different temperature Sample name

Tα

E′ (Tα) (GPa)

E′ (35°C) (GPa)

E′ (60°C) (GPa)

Neat PLA 3% GJF + PLA 3% DMJF + PLA 3% WMJF + PLA

40 ± 1 35 ± 1.8 35 ± 1.4 50 ± 1.1

2.42 ± 0.15 1.01 ± 0.40 3.15 ± 0.33 3.27 ± 0.21

3.09 ± 0.20 1.01 ± 0.38 3.15 ± 0.30 5.92 ± 0.23

0.48 ± 0.02 0.01 ± 0.07 0.04 ± 0.03 0.52 ± 0.02

4.6.2 Dynamic mechanical analysis Fig.  4.28 and Table  4.8 showed that WMJF/PLA composites have higher load-­ bearing capacity than neat PLA due to the transfer of stress from matrix to stiff nanofibrils. The storage modulus of PLA composites at 35°C increased from 3.09 GPa to the level of 5.92 GPa after the addition of WMJF. The maximum increase in storage

Nanoparticles and textile technology211

Fig. 4.27  Indentation mark. (A) Neat PLA film. (B) GJF/PLA film. (C) DMJF/PLA film. (D) WMJF/PLA.

modulus for WMJF/PLA composites was attributed to the higher surface area of ­nanofibrils interacting with the matrix. The deterioration in load-bearing capacity of PLA after addition of GJF and DMJF was attributed to the poor bonding of these fillers with PLA due to their bigger particle size. Fig. 4.29 shows that the tan delta peak of PLA was positively shifted only after the addition of WMJF. The maximum shift of 14°C was reported in the case of WMJF/PLA composites due to the higher surface area of nanofibrils interacting with PLA matrix.

4.6.3 Tensile testing Fig. 4.30 and Table 4.9 showed that PLA composite films reinforced with WMJF have significantly higher initial modulus compared with neat PLA film and other composite films. The slight increase in initial modulus was observed in the case of GJF and DMJF composite films.

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Fig. 4.28  Storage modulus of neat and composite PLA films.

Fig. 4.29  Damping factor of neat and composite PLA film.

The significant increase in initial modulus from 1.04 to 3.14 GPa in the case of WMJF was attributed to the increased interaction of WMJF and PLA matrix due to higher surface area of fillers at nanoscale. The least improvements in composite films of GJF and DMJF were attributed to the bigger size of these fillers, which resulted in least interaction with the matrix.

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Fig. 4.30  Stress-strain curve of neat and composite PLA films. Table 4.9  Tensile properties of neat and composite PLA films Sample name

Initial modulus (GPa)

Tensile strength (MPa)

Elongation (%)

Neat PLA 3% GJF + PLA 3% DMJF + PLA 3% WMJF + PLA

1.04 ± 0.03 0.72 ± 0.09 1.30 ± 0.08 3.14 ± 0.05

25.98 ± 0.13 15.14 ± 0.30 18.41 ± 0.25 67.90 ± 0.46

4.84 ± 0.72 3.28 ± 0.48 2.40 ± 0.43 1.82 ± 0.32

The goal of present study was to utilize the waste jute fibers in textile industry as a source of cellulose nanofibrils for reinforcement of biodegradable packaging films. The jute nanofibrils were obtained by wet pulverization using high-energy planetary ball milling process instead of strong acid hydrolysis due to its simple, economical, and environment friendly approach. The extended wet milling for the duration of 3 h resulted into unimodal distribution of jute nanofibrils with diameter below 50 nm. In the subsequent step, composite films at 3 wt% filler content were prepared by mixing the calculated amount of GJF, DMJF, and WMJF with 5% PLA in chloroform solution. The mechanical properties of thin composite films of PLA were studied after reinforcement of nanofibrils, microfibers, and millifibers with the help of nanoindentation technique. The PLA composite films reinforced with jute nanofibrils showed maximum hardness of 0.70 GPa compared with 0.68 and 0.54 GPa hardness of microfiber- and millifiber-reinforced composite films, respectively. The enhancement in

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elastic recovery was observed in the case of nanofibril-reinforced composite films, which was attributed mainly to the higher surface area of interacting nanofibrils compared with the micro- and millifibers. As area of interaction between fibers and matrix increased, the elastic properties of jute fibers transmitted effectively to the PLA matrix. The sharpness of indentation mark also explained the higher elastic recovery of nanofibril-reinforced PLA composite films compared with neat PLA and other composite films. The indentation mark was found less sharp in the case of nanofibril-­ reinforced PLA composite films, which was attributed to the faster elastic recovery of deformation.

4.7 Conclusion Although a number of so-called nanotextiles are now available on the market, it is questionable whether these materials have been classified according to definition of textile nanotechnology. There are several manufacturing processes still in the research stage, and the production of nanotextiles is to a degree still cost-intensive. Usually, much nano is no nano, properties are often badly described, most are at proof of concept stage, processes are still small-scale, functionalities are not stable, and life-time behavior is not known. It can be assumed that some conventional textiles are only promoted using the term “nano.” However, it is certain that nanomaterials can be released by mechanical load, abrasion, and other external influences. It is likely that nanoparticles enter the environment when textiles are washed. Some in vivo and in vitro studies provide indications about the hazardous potential of certain nanoparticles.

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Further reading [80] British Standards Institution, Terminology for Nanomaterials, PAS Publicly Available Specification, London, 2007136. [81] ISO, Nanotechnologies—terminology and definitions for nano-objects — nanoparticle, nanofibre and nanoplate, 2008. ISO/TS 27687, 2008(E). [82] N. O’Brien, E. Cummins, Recent developments in nanotechnology and risk assessment strategies for addressing public and environmental health concerns, Hum. Ecol. Risk. Assess. 14 (3) (2008) 568–592. [83] R.  Handy, F.  von der Kammer, J.  Lead, M.  Hassellöv, R.  Owen, M.  Crane, The ecotoxicology and chemistry of manufactured nanoparticles, Ecotoxicology 17 (4) (2008) 287–314. [84] Royal Society, Nanoscience and Nanotechnologies: Opportunities and Uncertainties, Royal Society, London, 2004. [85] P. Satyamurthy, et al., Carbohydr. Polym. 83 (2011) 122. [86] W. Li, et al., Ultrason. Sonochem. 19 (2012) 479. [87] V. Baheti, R. Mishra, J. Militky, B.K. Behera, Fibers Polym. 15 (2014) 1500–1506. [88] G.S. Bhat, F.L. Cook, A.S. Abhiraman, L.H. Peebles, Carbon 28 (1990) 377.

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Characterization of nanomaterials in textiles

5

Rajesh Mishra, Jiri Militky, Veerakumar Arumugam Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

5.1 Introduction With nanotechnology moving from development to commercialization at a more rapid rate, so too are calls increasing for a more comprehensive understanding of the production costs, environmental and occupational health risks, and broader societal impacts associated with various nanomanufacturing processes. Commercialization of nanotechnology is proceeding quickly, with over 1000 products containing nanoparticles identified in commerce. Global spending in 2006 was cited at more than 12 billion US dollars. However, this enormous investment dwarfs the funding dedicated to the environment, health, and safety (EHS) implications of nanotechnology. There are indications that a range of engineered nanomaterials, including nanoparticles, agglomerates of nanoparticles, and particles of nanostructured materials, are likely to present potential risks to human health and the environment. Possible negative properties of these materials include their ability to penetrate dermal barriers, cross cell membranes, travel neuronal pathways, breach the gas exchange regions of the lung, travel from the lung throughout the body, and interact at the molecular level [1]. Although there are few examples of engineered nanoparticles (ENPs) now being used as substitutes for more hazardous chemicals in industrial settings, that situation is likely to change rapidly. The very large current investment in ENP research and development holds the promise for many more such possible substitutions reaching the commercial stage in the near future. The possible use of ENPs as substitutes for toxic materials in manufacturing holds not only great promise but also many risks. For every possible application, alternative assessment tools must be used to carefully analyze the risks and benefits. In the near future for most cases, such assessments will be made more difficult because the risks of the current process will be well known, but the risks associated with the ENP will not have been fully studied. In addition, effective measures for the control of exposure to nanomaterials are under development, as well as the end of life treatment for ­nanotechnology-enabled products. Such developments will help to overcome the challenges currently experienced when considering the use of nanomaterials and since well-controlled nanomaterials can successfully substitute for the current use of some toxic materials even with unanswered questions as to their toxicity. However, it is recommended at the present that a precautionary approach be taken and that such substitutions be made only in cases where the ENP clearly ­represents an improvement over the current materials being used and where nanoparticle releases to the environment can be controlled [2,3]. Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00005-5 Copyright © 2019 Elsevier Ltd. All rights reserved.

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5.2 Environmental and health effects of nanomaterials in nanotextiles Engineered nanomaterials (ENMs) are expected to hold considerable potential for products that offer improved or novel functionalities. For example, nanotechnologies could open the way for the use of textile products outside their traditional fields of applications, for example, in the construction, medical, automobile, and environmental and safety technology sectors. Consequently, nanotextiles could become ubiquitous in industrial and consumer products in the future. Another ubiquitous field of application for ENM is facade coatings. The environment and human health could be affected by unintended release of ENM from these products. The product life cycle and the product design determine the various environmental and health exposure situations. For example, ENM unintentionally released from geotextiles will probably end up in soils, whereas ENM unintentionally released from T-shirts may come into direct contact with humans and end up in wastewater. ENM is already applied or may be applied in future to textile products and facade coatings. These ENMs are mainly nanosilver (nano-Ag), nano-titanium dioxide (nano-TiO2), nanosilica (nano-SiO2), nano-zinc oxide (nano-ZnO), nanoalumina (nano-Al2O3), layered silica (e.g., montmorillonite, Al2[(OH)2/Si4O10]●nH2O), carbon black, and carbon nanotubes (CNT) [4,5]. Knowing full well that innovators have to take decisions today, we have presented some criteria that should be useful in systematically analyzing and interpreting the state of the art on the effects of ENM. For the environment, the following criteria are established: (1) the indication for hazardous effects, (2) dissolution in water that increases/decreases toxic effects, (3) tendency for agglomeration or sedimentation, (4) fate during wastewater treatment, and (5) stability during incineration. For human health, the following criteria were defined: (1) acute toxicity, (2) chronic toxicity, (3) impairment of DNA, (4) crossing and damaging of tissue barriers, and (5) brain damage and translocation and effects of ENM in the (6) skin and (7) gastrointestinal or (8) respiratory tract. Interestingly, some ENM might affect the environment less severely than they might affect human health, whereas the case for others is vice versa. This is especially true for CNT. The assessment of the environmental risks is highly dependent on the respective product life cycles and on the amounts of ENM produced globally. A product life cycle for nanoparticles is shown in Fig. 5.1 [5]. Sustainable consumption and sustainable production require both an increase in the efficiency of consumption and a change in consumption patterns and reductions in consumption levels. The first prerequisite is not sufficient on its own and can be named weak sustainable consumption. Therefore, technological improvements and ecoefficiency must support a necessary reduction in resource consumption. The unsustainable way in which we use resources has resulted in the challenges we face today: climate change, environmental pollution, ecosystem degradation, and raw materials exhaustion. This last point is particularly important, since industrial development should focus on materials and products that are more resource- and ­energy-efficient [6].

Characterization of nanomaterials in textiles221 Product life cycle Transport/Storage Use

Production Resources

Residues

Technosphere: Waste water treatment, waste incineration, recycling systems, landfill

Effect

Fate of ENP

Disposal

Exposure ENM released to

Environmental compartments: water, soil, air Biota

Humans

Fig. 5.1  Life cycle of engineered nano particles [5].

The main benefits of the self-cleaning textiles can be outlined as follows: ●

●

●

●

●

Ease of maintenance and environmental protection because of reduced cleaning efforts Time, material, energy reduction, and consequently cost efficiency during production Makes textiles longer lasting People need not to suffer from heavy laundry bills Improved aging behavior by extended surface purity effect

5.2.1 Nanotechnology and metagenomics Recent progress in metagenomics and nanotechnology has provided exciting new products for white and red biotechnology. However, the true potential of these approaches can only be realized if they are integrated along with protein and enzyme engineering in future research efforts. Novel and potentially more efficient enzymes obtained from metagenomics are likely to require further modification by protein or enzyme engineering to obtain increased efficiency and stability in large-scale fermenters. More importantly, formulations are needed that can stabilize the enzyme and protect them from adverse conditions, including those arising from washing, leaching, solvents, water, and protease attack. Emerging nanotechnological tools could provide such protection to novel enzymes. It is therefore important that these approaches are combined from the conceptual stage onward. Such an interdisciplinary approach, combining genomics (mainly functional metagenomics), protein engineering, and nanotechnology, might expedite the exploitation of metabolic capabilities of environmental microbes, which in turn will bring sustainable wealth creation and efficient resource management and other socioeconomic benefits [7,8]. Further exploitation of environmental microbes is anticipated to increase the production of renewable and efficient fuels and of high-value bioproducts used in industry and medical treatments. There is a need for a sustained metagenomic data archive comprising data collected by different research groups. The data inventory, which archives data from marine metagenomes, is a good example. However, it is desirable

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to create a global inventory database that contains exclusively metagenomic data obtained from various environments and that should be freely available to researchers and other interested groups. The best model of such an archive is the Human Genome Project, where generated sequence data are freely accessible [9]. The promise of metagenomics and nanotechnology cannot be realized without coordinated and well-funded research and development. For this to happen, a strong and sustained political support is essential. Policy guidance on public funding with an emphasis on the social impacts along with minimal disturbance to the environment will go a long way to help on this count. To accomplish these goals successfully, a strong legal framework is needed that would clearly define the freedom and limitation under which experiments can be carried out for creating new biological and/or ­nanotechnology-based products. Currently, several misconceptions regarding nanotechnology prevail among the general public and these need to be tackled urgently. A legal framework, along with the recognition that information generated by these emerging technologies is for common societal benefit, would enhance the public confidence in this technology. Thus, making research data freely available will go a long way in gaining approval and acceptance of stakeholders so that mistakes, such as those made by industries, are not repeated. The public opposition to GM crops was not driven by the lack of scientific data or risk of hazard but was due to the perception of the lack of institutional and cultural responsibilities associated with new innovations. Therefore, public involvement from the onset, together with openness and availability of research data with regard to environmental, social and health impacts of these emerging technologies and enforceable regulations, would be essential for public support and ultimately their successful application. If the global research and industrial communities are able to overcome the above challenges, then they are poised to harness the enormous potential of uncultivable environmental microbes for industrial applications [10].

5.3 Application of functionalized nano-fibres Active polymer nanofibers for opto- and nanoelectronics benefit from low-cost and versatile fabrication processes and exhibit an unequaled flexibility in terms of chemical composition, physical properties, and achievable functionality. For these reasons, they have rapidly emerged as powerful tool for nanotechnologies and as building blocks of a wide range of devices. Both bottom-up and top-down nanofabrication concepts were developed to produce nanofibers made of conjugated or other functional polymers and blends. This article summarizes and reviews the chemicophysical and functional requirements for polymer nanofibers to be used in opto- and nanoelectronics, as well as recent advances in various promising device architectures, such as light-emitting and photovoltaic devices, photodetectors, field-effect transistors, piezo- and thermoelectric generators, and actuators. The outlook of functional polymer nanofibers and of devices based on them is also outlined and discussed [11,12]. Synthetic and processing technologies to produce, functionalize, and assemble polymer nanofibers and materials and devices based on them have achieved enormous advances in the last decade. Indeed, the use of polymer nanofibers for an enormous

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variety of applications grounds on solid motivations due to their ultrahigh surface-to-­ volume ratio, morphological and optical anisotropy, unique mechanical characteristics, and other physicochemical properties, such as charge transport, thermal conductivity, and molecular adsorption capability, which can be significantly enhanced compared with bulk, film-forming, or macroscopic fiber-based materials. These features make polymer nanofibers interesting for all those technological fields where a very large surface is needed in order to allow nanostructures to interact effectively with external liquid or gas media. Applications include the realization of sensors and biosensors, the development of most efficient materials for catalysis, filtration, biotechnology, regenerative medicine, and tissue engineering. In all these fields, technologies to realize nanofiber-based materials have already reached commercialization, with a lot of dedicated start-up companies. As a matter of fact, some industrial, goods-producing segments, such as those addressing the commercialization of dedicated equipment and of products whose added value is directly provided by nanofibers, are gaining increasing visibility on the market. This is making the field one of the most vibrant in the broad framework of nanotechnologies, due to the actual perspectives of socioeconomic impact. In other areas of applications, polymer nanofibers are still relatively immature as newly developed material. However, a few of these fields, such as nanophotonics and nanoelectronics, are tremendously intriguing from a scientific and technological viewpoint. Here, future, novel materials and devices have being seeded over the last few years, by realizing nanofibers made of optically and electronically active molecules. The classes of materials utilizable to this aim are well known. On one side, one can consider conjugated polymers, that is, macromolecules presenting unpaired electrons. These polymers can exhibit electric conductivity, together with an either one-dimensional or two-dimensional topological structures and various other peculiar physical properties that make them excellent materials for many applications in optoelectronics, nanophotonics, and nanoelectronics. Such features comprise semiconducting behavior, wide tuneability of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and at least partially controllable supramolecular ordering and anisotropy of the resulting optical and electronic properties. Nowadays, it has become clear that active nanofibers can be realized with conjugated polymers, following a number of complementary fabrication approaches [13–15]. Nanotechnology has high technological potential for textile industry. An important application of this technology in this sector is a novel concept of textile, called “self-cleaning textiles,” that can be easily washed and maintained, capable to improve the process performances in terms of energy and resource consumption. The new textile is realized by depositing on the surface a nanocrystalline TiO2 photocatalytic layer, which is able to destroy organic material by solar irradiation. This finishing process is able to reduce the maintenance costs of textile products, including a reduction in the consumption of water and chemicals/detergents, and to significantly reduce the temperature required for the removal of persistent stains. Life-cycle assessment (LCA) was applied to a self-cleaning textile in order to quantify its environmental advantages. The calculations were performed with the SimaPro software version 7.3.3 and the main database used is Ecoinvent version 2.2. In particular, the ecological earnings

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were evaluated by comparison of the production and the use phases of the innovative and conventional materials in a variety of application scenarios. The results show that the innovative system has lower impact with all the chosen methods and reduced tap water consumption [16,17].

5.4 Flame-retardant protective clothing Textile fabric was coated using layer-by-layer assembly in an effort to enhance flame-retardant properties. Two systems were deposited using large (30 nm) and small (10 nm) negative colloidal silica alternated with positively charged alumina-coated silica (10 nm). SEM images (Fig. 5.2) show a change in surface morphology for treated fabrics with an increased presence of the coating as a function of bilayers deposited. Fabric thermal stability in oxidative conditions was found to be slightly improved by the presence of the coating, as assessed with TGA [18]. Flammability and combustion properties were found to be greatly influenced by the morphology of the coating and its physical stability during testing. The coatings improved fabric fire properties as long as they were able to maintain continuous coverage of the fibers, which leads to the formation of an effective inorganic barrier. Cone calorimetry results showed that the best system contained the smallest nanoparticles, which increased time to ignition by 99 s (ca. 45%) and reduced the flammability. The same system tested with vertical flame test was able to reduce the burning time by 95% and eliminate melt dripping, which is one of the most significant issues with PET. This ability to dramatically reduce flammability of PET fabric, using environmentally friendly process and relatively few layers (5–10), makes this a promising alternative to current antiflammable treatments. Indeed, the great advantage of this novel technique is represented by the use of water as the solvent and the possibility to recycle the dilute suspension bath after its use [18]. Among the surface engineering approaches, the layer-by-layer (LbL) technique has arisen as a new method for obtaining molecularly controlled nanostructured thin films and coatings. Nowadays, LbL processes can be employed for fabricating nanostructured assemblies on different substrates, aiming to (i) define and provide the surface functionality, through which every object/substrate can interact with the surrounding (A)

AL D12.1 ×250 300 µM

(B)

AL D12.4 ×250 300 µM

(C)

AL D12.7 ×250 300 µM

Fig. 5.2  Flame-retardant nanocoated fabrics [18]. (A) Nanocoated fabric, (B) Nanofiber coated textile, and (C) Nanoassemblies.

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environment, and (ii) design and fabricate surface-based devices (membrane reactors and photonic devices like LED or complex wave guides) [19,20]. Concerning fabrics flame retardancy, LbL has been addressed to exploit two possible different effects provided by the deposited assembly: thermal shielding and/or intumescence. The former is mainly based on the deposition of nanoparticles in completely inorganic or hybrid organic-inorganic structures, which, during combustion, favor the formation of a ceramic barrier on the surface that protects the underlying polymer from oxygen and reduces the heat transfer. As the nanoparticles are already nanostructured on the fabric surface, they can exert the protective effect from the very beginning of the fire stages [21]. Indeed, the high freedom in the design of the LbL nanocoatings allows finely tuning the final properties of the obtained assemblies in order to make them very effective in protecting the underlying material from a flame or a heat source. Therefore, this nanotechnology approach seems to represent a possible efficient alternative to conventional flame-retardant systems, some of which are encountering limitations in use, due to their suspected/assessed toxicity and low ecosustainability. Conversely, the major current limitations and challenging issues of this technique refer to (i) the possibility of its scale-up for an industrial exploitation (this could be fulfilled by spraying LbL assemblies on designed roll-to-roll industrial plants) and (ii) the durability of the most currently employed LbL assemblies, which are based on electrostatic interactions. Indeed, the waterborne character of the LbL architectures strongly limits their resistance to the washing treatments the fabrics usually have to undergo. To overcome this issue, the exploitation of covalent interactions taking place in between the assembled layers is currently being investigated: in particular, the use of thermally or UV-curable organic layers in the design of the final flame-retardant LbL assembly could represent a reliable solution to this problem [22].

5.4.1 Materials engineering for surface-confined flame retardancy Polymer materials flammability represents a major limitation to their use and hence to the development of most polymer-based advanced technologies. Environmental and safety concerns are leading to progressive phasing out of versatile and effective ­halogen-based fire retardants that, so far, ensured a satisfactory polymer fire hazard control. Among the intensive efforts that are being made to develop new, environmentally safe, polymer fire protection approaches, the recognition of the paramount role played by the polymer surface during combustion and the exploitation of the new nanotechnologies developed for polymer surface engineering offer a promising perspective for polymer fire retardance. Indeed, heat transfer to the polymer and diffusion to the gas phase of polymer degradation combustible volatiles, which both fuel the combustion, occur across the polymer surface, which characteristics regulate the polymer combustion process. It is shown that by engineering the polymer material surface by intumescent coatings or layer-by-layer nanodeposition or by oxidic nanostructure sol-gel synthesis, polymer combustion can be conveniently slowed down to extinguishment, complying fire safety rules of specific applications, through the creation of a surface barrier to heat and mass transfer across the polymer surface [23].

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Heat

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Fig. 5.3  Nanoenabled flame-retardant fabrics [26].

Intumescent coating technology has been widely applied since the 1970s in fire protection of different substrates, mainly on wood boards and steel structures, as an alternative to inorganic protections such as rigid mineral-based boards and fiber blankets. Despite that metals and woods exhibit obviously different reaction and resistance to fire, the coating with an organic intumescent layer aims in both cases to the thermal protection of the substrate, in particular to extend the time to reach its critical temperature, that is, the thermal decomposition temperature of wood or the temperature corresponding to an elastic modulus drop for steel. Paints and varnishes technology for these substrates is well established and is based on blowing of the coating layer to a thick insulating charred foam when heated above a certain temperature, as a consequence of fire exposure. The thermal resistance for a coating is the parameter quantifying the heat shield from a protective foam, and it depends on several factors, including foam thickness, bubble size, defects, and intrinsic heat transport characteristics of the foam material [24,25]. Up to now, most of the LbL coatings have been deposited on substrates characterized by high surface-to-bulk ratio; this means that these substrates possess a high surface available for the coating with respect to the bulk that needs fire protection. Among this kind of substrates, textiles certainly represent the most used ones, recently followed by foams and thin films; for this reason, the following paragraphs will aim to provide the reader with a detailed presentation on textile LbL protective coatings (thermal, intumescent, and hybrid) and, subsequently, an insight into the recently developed substrates (foams and thin films) shown in Fig. 5.3 [26]. Nanosol particles exhibit diameters in the range from a few nanometres up to 100 nm, while coatings formed by nanosols can reach a thickness of up to several hundred nanometres. Therefore, the length of a nanosol coating can cover a broad range of the structural elements, starting from molecules up to three-dimensional, large-scaled objects such as fibers forming a textile. In relation to the process parameters, the

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inorganic metal oxide forms mainly amorphous networks after moderate heat treatment (so-called xerogels); on the other hand, if a treatment at high temperatures (> 500°C) is carried out, the resulting networks give increasingly crystalline structures. The basic nanosols can be modified in a wide range, leading to numerous new functionalities that can be applied to various surfaces. Thus, this type of coating is a suitable tool for modifying a large number of materials, such as glass, paper, synthetic polymers, wood, metal, and textiles. In conclusion, sol-gel technology promises the possibility to tailor surface properties to a certain extent and to combine different functionalities in a single material [26]. Engineering the polymer surface is shown to provide a potential promising, environmentally friendly, and effective approach to polymer fire retardance, particularly when combined with nanostructurating technologies. Feasibility is demonstrated for textiles, films, and foams, while present efforts are directed toward composites with possible future extension to thick polymer materials. A major interest in this approach to surface polymer properties is the possibility to simultaneously confer multifunctional features that, besides fire retardance, may involve gas barrier, hydrophobicity, biocide activity, surface electric conductivity, etc.

5.5 Tissue engineering Tissue engineering is an interdisciplinary field that has attempted to utilize a variety of processing methods with synthetic and natural polymers to fabricate scaffolds for the regeneration of tissues and organs. The study of structure-function relationships in both normal and pathological tissues has been coupled with the development of biologically active substitutes or engineered materials. The fibrillar collagens, types I, II, and III, are the most abundant natural polymers in the body and are found throughout the interstitial spaces where they function to impart overall structural integrity and strength to tissues. The collagen structures, referred to as extracellular matrix (ECM), provide the cells with the appropriate biological environment for embryologic development, organogenesis, cell growth, and wound repair. In the native tissues, the structural ECM proteins range in diameter from 50 to 500 nm. In order to create scaffolds or ECM analogues, which are truly biomimicking at this scale, one must employ nanotechnology. Recent advances in nanotechnology have led to a variety of approaches for the development of engineered ECM analogues. To date, three processing techniques (self-assembly, phase separation, and electrospinning) have evolved to allow the fabrication of nanofibrous scaffolds. With these advances, the long-awaited and much anticipated construction of a truly “biomimicking” or “ideal” tissue engineered environment or scaffold, for a variety of tissues, is now highly feasible [27]. Since there is no consensus on the gold standard for creating analogues to the native ECM, all three and even combinations of these processing techniques have been utilized to produce nanofibrous structures. Researchers used both electrospinning and phase separation to create submicron diameter fibers with pores or pits on the order of 100 nm. The appeal of nanofibers in tissue engineering is their structural similarity to native ECM; however, ECM has not only a structural role but also a functional

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one. This network creates a dynamic, three-dimensional microenvironment in which cells are maintained. Signals are transmitted between the cell nucleus and the ECM enabling communication between both for cell adhesion, migration, growth, differentiation, programmed cell death, modulation of cytokine and growth factor activity, and activation of intracellular signaling. In addition, the interactions between cell receptors and ECM molecules are critical for wound healing and the development of nature's provisional matrix for cellular migration, proliferation, and subsequent tissue remodeling. The influence of the ECM on cellular activities occurs via binding of specific factors to specific ECM molecules and binding of ECM molecules to cell surface receptors known as integrins, which then influence local release of growth factors or separation of molecules (for cell attachment, spreading, and growth) [28,29]. Current nanolevel processing techniques have been developed with the goal of mimicking ECM geometry. All three of the aforementioned techniques are capable of producing fibers with submicron diameters. Efforts have also been made to go beyond geometry and attempt to truly mimic the ECM in terms of physiological ability. By engineering material properties of synthetic polymer structures and surfaces, they can be made more conducive to cell adhesion and function. On the other hand, natural polymers may already contain/present signaling capabilities required by cells. Thus, materials utilized as nanofibers in tissue engineering include a variety of synthetic and natural polymers that offer advantages and disadvantages. There have been investigations into the use of xenogeneic and allogeneic ECM as tissue engineering devices, but the problems associated with this approach include age-related factors, cell lysis, and calcification. The polymers electrospun should offer the advantages of being readily available and having known degradation times and mechanical strengths. However, these synthetic polymers lack an ultrastructure that mimics ECM; thus, we are also electrospinning natural polymers, that is, native ECM proteins, as previously mentioned, for tissue engineering applications. Yet, even natural polymers have some disadvantages, including immunogenicity and variations in mechanical properties, degradation, and reproducibility [29]. The mechanical properties of the scaffolds, including tangential modulus and stress and strain to failure, can be tailored by controlling the geometry and orientation of the fibers in the scaffolds. The authors have evaluated the mechanical properties of the different PGA electrospinning concentrations in both aligned and random fiber orientation scaffolds. The overall results exhibit a correlation between the fiber diameter and orientation and the elastic modulus and strain to failure. Additionally, it has been shown that the greater surface-area-to-volume ratio of smaller fibers results in a faster loss of strength during degradation. When an initially tough (high strength and elasticity) and fast degrading material is desired, PGA is a good choice. However, the higher rate of degradation may result in sharp increases in localized pH, which may cause unwanted tissue responses if the region does not have a high buffering capacity or sufficient mechanisms for the rapid removal of metabolic waste. Biocompatibility studies have been performed by the authors, in which PGA and acid-treated PGA were evaluated in cell culture and in an animal model. Both fiber diameter and acid pretreatment influenced the in vitro and in vivo cellular responses. The acid pretreatment improved biocompatibility via a hypothesized mechanism of surface hydrolysis of ester bonds,

Characterization of nanomaterials in textiles229

thereby exposing carboxylic acid and alcohol groups. This may improve binding that, in turn, may improve the ability of cells to adhere to the surface [30]. When a single polymer does not have the properties desired for a tissue engineering application, a copolymer or blend (simple mixture) of polymers may be employed to achieve the desired geometric, mechanical, and biodegradation properties. Electrospinning again allows the ability to produce nanofibers of such a copolymer or blend. The incorporation of glycosaminoglycans (GAGs) into an ECM analogue during electrospinning could potentially be an important aspect in truly mimicking the native ECM. GAGs serve a variety of functions including linking collagen structures and binding growth factors. The specific GAGs of physiological and tissue engineering scaffold significance are hyaluronic acid, dermatan sulfate, chondroitin sulfate (most abundant GAG in tissues), heparin, heparan sulfate, and keratan sulfate [31–33]. The mechanical properties of the electrospun collagen-blended vascular prosthetics (Fig. 5.4) fall within the ranges of the corresponding values for traditional vascular materials. Human dermal fibroblasts seeded onto the PDO/collagen type III scaffolds displayed favorable cellular interactions; the cells migrated into the thickness of the scaffolds containing collagen but merely migrated on the seeded surface of the 100% PDO scaffold [34].

Fig. 5.4  Vascular prosthetics from nanofibrous membranes [34].

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Electrospun materials composed of nanofibers have a high surface-area-to-volume ratio. This is potentially an important consideration when designing a hemostatic product. A high surface-area-to-volume ratio increases the surface area available for blood components to interact with to initiate clot formation. A fibrinogen scaffold with an average fiber diameter of 700 nm, dimensions of 60 × 60 × 0.7 mm, and weighing approximately 0.08 g has an estimated total surface area of approximately 3300 cm2. This translates into a relative surface-area-to-volume ratio of 1300 cm2/cm3 or a relative surface-area-to-weight ratio of 4.1 m2/g [34]. Development of functional tissue engineering products requires an appropriate scaffolding to mimic the native ECM. Current research in tissue engineering is approaching a major breakthrough in the treatment of injury and disease due to the ability to routinely create ECM analogous nanofibers. Since the inception of the field of tissue engineering, many methods have evolved from the simple concept of placing cells in a degradable scaffold to building native tissues either in vivo or in vitro. These advances come on the heels of advances in the life sciences that provide critical information about the nature and development of tissues and diseases. Ultimately, engineers must match applications, materials, and fabrication processes to best meet the needs of the tissue they wish to build. It is anticipated that nanotechnology will be a key component in the development of the next generation of scaffolds, particularly with respect to the fabrication component [35,36]. Recently, nanomaterials derived from natural renewable resources have drawn much attention in the nanotechnology research thrust. Lignocelluloses are composed of cellulosic nanofibrils that can be disintegrated by chemical, mechanical, and enzymatic methods in order to obtain nanocellulose. Further, nanocellulose can also be synthesized by bacterial method in a suitable culture. Nanocelluloses have many interesting properties (viz., nanodimension, renewability, low toxicity, biocompatibility, biodegradability, easy availability, and low cost) that make them ideal nanomaterials for diverse applications. Various methods of nanocellulose preparation and their properties, surface modifications of nanocellulose, and applications of nanocellulose in the diverse fields of tissue regeneration have been explored. Some typical nanocellulosic tissues are shown in Fig. 5.5 [36].

Fig. 5.5  Nanocellulose-based tissue engineering [36].

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Nanocellulosic materials have many interesting properties (such as nontoxicity, biodegradability, renewability, and biocompatibility) that make them ideal candidates for many potential applications. Nanocellulose can be extracted from the lignocellulosic biomass by using various methods that influences their properties. There are enormous potential for the nanocellulosic materials that includes nanocomposites membrane, hybrid film, hydrogel, and aerogel that are few of many to states. Cellulose nanomaterials could have wide applications in food packaging energy, water treatment, biomedical filed, etc. Further, nanocellulose derived from the bacterial synthesis has proved as a promising biomaterial for various biomedical applications such as tissue engineering, drug delivery, wound dressing, and cardiovascular applications. It is hoped that comprehensive further research for the utilization of huge renewable lingo cellulosic biomass for the preparation of nanomaterials could be useful in diverse applications [36].

5.6 Affinity membranes The skin is a marvelous natural system model, which maintains the integrity of the human body acting as a large barrier organ against environmental hazards such as cold, heat, chemical, and mechanical force. However, the skin is quite sensitive, and the use of an extra barrier is requested. This barrier is provided by a large variety of clothing solutions, along with esthetics and thermophysiological and sensorial comfort. The requirement for establishing a microclimate between the skin and fabric arises from the need to avoid irritations, allergies, and intolerance as well as the direct contact of the skin with harmful chemicals and physical agents. The microspace airstream between the skin and clothes is defined as a “microclimate.” Depending on some environmental parameters, including temperature and humidity, this microclimate can be seriously compromised, thereby causing discomfort to the person. Indeed, a water loss through the skin via perspiration needs to be regulated by heat exchange. When water evaporates due to secretion, the absorption of an amount of heat is requested to normalize the body temperature for comfort. Depending on external conditions, clothing makes possible the transport of moisture and heat exchange through the material, maintaining a regulated equilibrium between temperature and humidity inside the air gap immediately surrounding the body [37,38]. In this respect, natural fibers such as cotton and wool show a large “wicking,” that is, an ability to create natural ventilation, leading to less chance of skin rashes, itching, and allergies and, therefore, being suitable for garments, sheets, and pillow cases. Indeed, cotton is fresh to wear in summer, while air pockets entrapped in the wool texture act as insulators against cold and heat. The full awareness that traditional synthetic fibers cannot capture air and “breathe” like natural fibers and a desire to improve the performance of current fabrics, for clothing, construction, architectures, medical, marine, agricultural, environmental, and transport solutions, have solicited the textile industry to look for advanced engineered materials. Indeed, the use of nanotechnology is becoming a common practice in the textile industry for the creation of miniaturized structures and functions, which make the fabrics adaptable. In this respect, nature is a living school that can teach about materials, hierarchical architectures, and textures with associated functions and properties ­providing

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innovative responses to shared technical, economic, and ecological demands. In this respect, polymers and membranes can offer many concrete opportunities for the design of adaptive materials, thereby responding to microclimate needs through mechanisms that in part remain still unexplored. Bioinspired membrane materials, capable of working as heat exchangers, water condensers, and barriers against dirty and toxic agents, are just practical examples of how materials science and nanotechnologies can improve the quality of life, health, and lifestyle [39]. Membrane technology is consolidated in many fields of chemistry, medicine, biology, energy, and the environment. Each single process uses a semipermeable membrane with specific structural and chemical features through which certain substances can pass, while others are caught according to various mechanism types. In water desalination, for example, hydrophobic porous membranes with high permeability to water vapor and high resistance to liquid intrusion are successfully utilized for the production of fresh water free from salts, sand, oils, and metals. This type of membrane has recently been proposed in a membrane condenser configuration in order to recover 20% of drinkable water and dry flue gases. After compression, the flue gas is fed to the membrane, which works as a condenser to cool the gaseous stream until saturation in a process involving a membrane contactor (MC) unit. The hydrophobic nature of the porous membrane based on polyvinylidene fluoride (PVDF) prevents the liquid water from entering inside the pores, leaving the dehydrated gases to pass through the membrane [39,40]. In order to condense water vapor and cool flue gas, a certain amount of heat is obviously required. This principle of cooling process could be used in textile technology in order to regulate the exchange of vapor and heat between the body and fabric, thereby maintaining the desired microclimate conditions. Nanostructured membranes are, however, also used for purifying air of contaminants and pollutants. The recovery and reuse of value-added compounds are possible through separation processes such as microfiltration, ultra- and nanofiltration, and reverse osmosis, along with membrane contact or sand gas separation as shown in Fig. 5.6 [40].

Nanoparticles

Nanofibers CNT

Nanosheets

Fig. 5.6  Functional membrane [40].

Composite membrane

Mono-component nanomaterial membrane

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Characterization of nanomaterials in textiles233

Fig. 5.7  Membrane for filtration [40]. (A) Schematic of membrane and (B) Distillation mechanism.

In every case, the success of membrane technology depends on the multidisciplinary approach used to set up a membrane operation. Membrane design and process engineering along with study of surface phenomena, that is, fouling, bioadsorption, and sensing, and transport phenomena, including thermodynamics and kinetics, are various aspects merging in the membrane process. Membrane systems are of interest in various fields and could become successful also in textile technology. As an example, basic concepts of membrane distillation are common to textile technology as shown in Fig. 5.7 [40]. This means that well-established knowledge and expertise can be easily moved from the membrane to textile sectors, with exciting prospects. A revolutionary transformation in the area of manufacturing and processing of novel textiles can be achieved by using eco-friendly technologies based on membrane science. In porous membranes, the transfer of gases/vapors can be directed by convective flow as the pores are 0.1–10 μm. No separation occurs; in membranes with pores smaller than 0.1 μm, the gas diffusion occurs according to the Knudsen mechanism. In this case, the pore diameter has same or smaller size than the mean free path of the gas molecules. This promotes a temporary adsorption of the molecules on the pore walls during collision, while gas-gas collisions are rare. The diffusant moves independently and in random directions after desorption from the pore walls as in Fig. 5.8 [41]. The concentration of diffusant in membrane can be regarded as the contribution of different fractions, depending on the type of free volume distributed through the matrix. The free volume is intended as the sum of small gaps, sizes, and interspaces between the polymer segment chains, which stay in the amorphous region. In rubbery polymer, the free volume is fluctuating and is indicated as the equilibrium free volume, due to high mobility of the chains. In glassy polymer, wherein the mobility of the polymer is limited, the free volume consists of discrete and continuous microcavities and d-spaces frozen in the matrix and is assumed as nonequilibrium free volume [41].

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Fig. 5.8  Diffusion through membranes [41].

Deviations from the dual-mode sorption can be observed owing to clustering, swelling, and plasticization events. These phenomena are frequent when the diffusant is condensable and, therefore, capable of interacting with itself or polymer chains, thereby generating time and concentration dependence. Under unsteady-state conditions, the sorption rate can be classified as case II or anomalous case (non-Fickian) depending on the mobility of the diffusant with respect to the polymer chains. In case II, the molecules diffuse more quickly than the polymer segments mobility, while in anomalous sorption, a comparable mobility of both the penetrant and polymer chains is discerned. Sorption/desorption kinetics are powerful tools to classify the sorption rate as Fickian or non-Fickian type. Experimentally, the gravimetry is one of the most used techniques to measure the solubility coefficients of gas or vapor in polymer membrane [41,42]. Monolithic dense membranes are also used in textiles, being more advantageous than porous films of disallowed liquid intrusion. In this case, the limitation is a lower water vapor transmission rate (WVTR) due to the major resistance of the polymeric film. Hydrophilic elastomeric membranes are often preferred for outdoor shell clothing and protective apparel in order to solve this limitation. Forming a very thin dense layer on a thicker porous support reduces resistance to vapor diffusion. The hydrophilicity of the membrane is expected to facilitate the adsorption of water vapor into the matrix according to the solution-diffusion model. When materials with great affinity to polar species are used, water condensability is promoted, and solution selectivity-based permeation is induced. In addition, under a normal regime of temperature, the rubbery component is usually predominant in elastomers; this means that rapid fluctuations of the polymer chains can facilitate the diffusion of water vapor, thereby promoting higher breathability. In order to modulate the transport of water vapor, fillers and modifiers are often embedded in the elastomeric membranes. Studies that focused on the

Characterization of nanomaterials in textiles235

role of modifiers in the dissolution process of water into membranes have provided useful indication about the role of intermolecular interactions in perspiration [42]. One of the major concerns for textiles falling into the categories of sportswear, medical garments, and military clothing is dermal protection against liquid water, blood, bacteria, viruses, aerosol particles, warfare agents such as nerve agents, and many others. Initially, the barrier function did not allow for perspiration, causing discomfort. In the last few decades, there has been much research about the development of membranes, which work as barriers toward liquids and toxic agents, while striving to maintain significant breathability. Considering that hydrophilic materials tend to swell in the presence of hydration water, they must be used carefully to prevent a severe modification of the membrane structure under high relative humidity conditions. Indeed, significant swelling could destroy the barrier function facilitating the permeation of toxic chemicals [41,42]. Another important aspect of membranes for textiles is the simultaneous regulation of mechanical properties, permeability, and waterproofness. Engineered textiles often imply the creation of composite structures, which can affect the lightness, strength, toughness, and flexibility, along with integrity and damage tolerance of the fabrics. Shape memory membranes (SMMs) are another attractive category of material with high potential for textile applications. Their prerogative is to deform into a temporary shape and return to their original shape under the influence of external triggers such as temperature, pH, light, or chemicals. Thermally induced deformation is one of the commonest changes. The use of smart gels as sensing membranes in textile is having a great success for the attractive and concrete opportunity to develop autoadjusting body temperature clothing, temperature-regulating permeability, antibacterial coatings, and odor capture and nutrient/drug delivery fabrics. Before examining the potential of this class of material in the textile field, it would be interesting to give a definition of smart gels and related properties. The gel is between a solid and liquid system because of a 3-D cross-linked network, enabling a certain viscous consistency to be given. Low diffusion of the solvent is permitted, causing the polymer network to extend or shrink reversibly with a dynamic volume phase transition. The gels can be classified into various categories, depending on the solvent: aerogel or xerogel, whether the solvent is air, and hydrogel, whether it is water and organic gel and whether it is oil. Abrupt changes in volume can take place under external stimuli such as temperature, pH, solvent, light, ionic strength, magnetic or electric field, and pressure, yielding smart gels [43,44]. The intelligence of gel-based membranes (Fig. 5.9) and, more generally, gel materials is related to their ability to change the volume in a discontinuous manner up to more than 1000 times of its original size as a function of environmental conditions. This variation can further be isotropic and anisotropic during swelling/deswelling phenomena. A discontinuity of the volume phase transition of poly(acrylamide) has, for example, been demonstrated to depend on the solvent composition, resulting in a change from swelling to shrinking as the percentage of a solvent increased in the mixture [44]. Durable fabrics able to preserve their functions after laundering or wearing, while offering resistance to dirt and chemicals, are current needs for the textile industry.

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Fig. 5.9  Gel-based membranes [44].

In particular, dirty and fouling events cause the performance of membranes assembled in garments and footwear to decay in a short term. This is due to the fact that the pores are blocked, preventing normal perspiration and inducing alteration of the surface free tension. For this reason, there is a growing tendency to make self-cleaning coatings, enabling to remove inorganic and organic contaminates by two different mechanisms: (a) rolling water droplets and (b) photocatalysis. In the first case, microscopic roughness, that is, lotus or cauliflower-like surface, combined with low surface energy gets dirty particles onto the textile surface, allowing the water droplet to roll off and pick up dusts, soils, inorganic, and organic contaminants [44,45]. Membrane science may also promote the penetration of advanced and smart technologies into other advanced textile sectors such as the military and the space industry. The development of original clothing material solutions is imperative in the latter sectors, since the necessity for interaction with the body, the self-maintenance, and the adaptability to hazard environments and space workstations is quite essential for wearing comfort and life support. From the sustainability point of view, changes in lifestyle, technology, economy, and society are expected from the development of advanced functional textiles able to mimic the behavior of living systems, which are concrete prototypes of excellence in adaptive, self-healing, and renewal processes [45].

5.6.1 Biomimetic and bioinspired membranes By imitating the exceptional compositions, structures, formations, and functions of biological or natural materials, a myriad of biomimetic and bioinspired membranes have been designed and prepared using cell membrane, lotus, and mussel as representative prototypes and biomineralization, bioadhesion, and self-assembly as major tools (Fig. 5.10). These membranes have displayed fascinating properties and outstanding performances such as multiple interactions, hierarchical organizations, multiple selective transport mechanisms, superior stability/resistance, and distinct adaptability. Meanwhile, these membranes have made tremendous contributions in coping with energy and water stress and environment threats. Biomimetics focuses on the basic science by fundamentally exploring the principles of biological systems, while bioinspiration focuses on the applied engineering by technologically implementing the

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Fig. 5.10  Bio inspired membranes application [46].

principles from biological systems. Biomimetics and bioinspiration, as the complementary and interchangeable strategies for sustainable innovation and development of membrane technology, have great implications in exploring membrane materials and intensifying membrane processes. This review will present a brief overview on the prototypes, preparation, application, and perspective of biomimetic and bioinspired membranes [46]. Biomimetic and bioinspired membranes are those membranes that are fabricated with natural or natural-like (inorganic, organic, or hybrid) materials via biomimetic and bioinspired approaches (biomineralization, bioadhesion, self-assembly, etc.) to tailor specific properties (sophisticated structures, hierarchical organizations, controlled selectivity, antifouling, or self-cleaning properties, etc.). Research on biomimetic and bioinspired membrane has developed rapidly during the last decade, with enhanced knowledge on mechanisms, models, and functions from the contributions of many scientific disciplines. Ideally, biomimetic and bioinspired membranes should possess the following features: Membrane fabrication is often conducted through self-assembly under mild conditions close to natural environment, such as atmospheric pressure, room temperature, and aqueous environment.

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Membrane materials are usually common materials with excellent hydrodynamic, mechanical, wetting, and adhesive properties, primarily composed of the lightest ­elements—the first two rows of the periodic table. Membrane structures are of hierarchical organization, spanning from molecular scale to nanoscale, to microscale, to macroscale and bearing controlled configuration, mutable surface, and robust interface. Membrane properties are often highly dependent on the content and state of water in the structure, and membrane processes can be intensified by rationally manipulating the multiple selectivity mechanism in a facile way. Scientists are always amazed by the high degree of sophistication, miniaturization, and multifunctionalization found in cell membranes. According to the current knowledge, the delicate structures and potent function of cell membranes are mostly based on the fluid lipid bilayer and its embedded proteins (Fig. 5.11) [46]. Cell membranes are constructed with amphipathic lipids (phospholipids, glycolipids, cholesterols, and cholesterol esters), membrane proteins (integral proteins, lipid anchored proteins, and peripheral proteins), and carbohydrates (polysaccharide and oligosaccharides) [46]. Contaminating particles on lotus leaves can be picked up by the water droplets and removed with the droplets. Different plant surfaces always appear in very different surface structures. The unique structures in two scales are beneficial to trapping air, lowering surface energy, and forming the self-cleaning surfaces. The physical adhesion forces between contaminating particles and the structured surfaces can be largely reduced. In nature, the self-cleaning is not restricted to plant surfaces. A great variety of self-cleaning surfaces have also been found in the insect wings, water strider legs, insect eyes, fish scales, shark skin, gecko feet, spider silks, bird feathers, etc. [47]. Beautiful examples are butterfly wings. For the morpho butterfly wings, the specific multiscale and highly ordered photonic structures enhance superhydrophobicity and self-cleaning features. The directional easy-cleaning property of the

Fig. 5.11  Bio membrane structure [46].

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morpho butterfly wings is attributed to the direction-dependent arrangement of flexible nanotips on the lamella-stacked nanostripes and microscales overlapped on the wings. Gecko feet can keep self-cleaning while walking with sticky toes. The self-cleaning ability could be attributed to the microstructure (on overlapping lamellar pads in uniform arrays) and nanostructure (single seta with branched structure terminating in hundreds of spatular tips). Nonadhered lamellar surfaces appear to be highly nonwettable, and particles contacting the unloaded surface should wash away easily in the presence of water. Moreover, gecko feet contaminated with micro spheres could also recover their ability to cling within only a few steps on dry clean glass. Self-cleaning is derived from the energetic disequilibrium between the adhesive forces attracting a dirt particle to the substrate and those attracting the same particle to one or more spatula [47,48]. Many artificial superhydrophobic micro-/nanoporous fibrous membranes have been facilely fabricated by creating the second-level hierarchical surface geometric structure from nanohybrid systems. Nanomaterials, such as nanoparticles and graphene nanoflakes, assembled in the polymeric fibers could change the surface morphology and chemistry, leading to superhydrophobicity with self-cleaning properties. An artificial composite fibrous membrane from polyaniline (PANI) doped with azobenzenesulfonic acid blended with PS was developed via electrospinning. As an emerging area, biomimetics and bioinspiration have already won a foothold in the scientific and technical arena. “Learn from nature” and “innovation through imitation” have gradually evolved as the intriguing shortcut and pragmatic philosophy in the research and development of membranes and membrane processes. By imitating the exceptional compositions, structures, formations, and functions of biological or natural materials, a myriad of biomimetic membranes and bioinspired membranes has been designed and prepared using cell membrane, lotus, and mussel as representative prototypes and biomineralization, bioadhesion, and self-assembly as major tools [49,50]. Biomimetics and bioinspiration have provided potent tools for the design of innovative membranes and membrane processes. Biomimetic membranes are often limited to copying or imitating natural solutions in particular the structure and function of cell membranes. Bioinspired membranes, on the other hand, expand upon biomimetic membranes, not only copying the concepts of cell membranes but also borrowing the preparation principles of natural materials for the engineering and technological implementation. Obviously, the scope of bioinspired membranes is much broader and application oriented. From this perspective, the research and development of biomimetic and bioinspired membranes may undergo the following three transitions: (1) from biomimicry, which entails merely superficial imitation of the biological systems; (2) to biomimesis, which aims to copy and reconstruct the structure-function relationships observed in biological systems; (3) to finally bioinspiration, through which properties and performances are elevated to higher levels, even surpass biological systems. In comparison with biomimetic membranes, bioinspired membranes are less mature albeit they may become more important in the future membrane technology. Moving from biomimetics to bioinspiration will represent the translational nature of membrane technology development [50].

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5.7 Super hydrophobicity Inspired by lotus leaves, smart interfaces/surfaces with high water contact angle (WCA) (>150 degrees) and small sliding angle (SA) ( 1, and for circular platelet with thickness H and diameter D is A = H/D 90% in 210 min under visible light irradiation. The small size, high surface area, and synergistic effect in the ZnO/Ag/CdO nanocomposite are responsible for high photocatalytic activity. These results also showed that the Ag nanoparticles induced visible light activity and facilitated efficient charge separation in the ZnO/Ag/CdO nanocomposite, thereby improving the photocatalytic performance [3]. The as-synthesized ZnO/Ag/CdO nanocomposite showed better photocatalytic degradation of MB, MO, and industrial textile effluent under visible light irradiation in comparison with the binary ZnO/Ag and Zn/CdO nanocomposites. The small size, high surface area, and synergistic effect within the ZnO/Ag/CdO nanocomposite seem responsible for the high visible light-induced photocatalysis [3].

6.2.1 Nanocomposites for textile effluent degradation Nanomaterial-based biosensors and photocatalysts have gained tremendous importance in recent years, with respect to their application in the highly sensitive detection of various enzymes and the degradation of industrial pollutants. They have significant impact on human health and the ecosystem. Metal oxide nanosemiconductors have been extensively investigated in the past decade for properties that are suitable for photocatalytic and biosensing applications. Among all semiconductors, zinc oxide is one of the hardest polar inorganic materials with a large bandgap and is nonhazardous in nature. The low-cost and high thermal stability of ZnO enable its use in versatile applications [4]. ZnO/CdO nanocomposites as shown in Fig. 6.4 prepared by vapor to solid mechanisms are good catalysts. The photocatalytic degradation activity of the nanocomposites with greater efficiencies and the high degradation effect on the industrial effluents (dyes) enable their use in wastewater treatment. CdO-modified ZnO has also been proved to have higher enzyme sensing activity. Thus, the developed nanocomposites are among the most efficient catalysts in wastewater treatment and in the degradation of industrial pollutants. Low-cost synthesis and high efficiency and sensitivity make the catalysts a suitable material for the degradation of pollutants and enable us to move a step further toward a greener environment [4].

6.2.2 TiO2 nanocomposite based polymeric membranes Following the studies conducted during the last decade, considerable progress in the development of membrane materials has been gained that has made it possible for researchers to operate at higher temperatures while preserving acceptable and suitable conductivity. Consequently, membranes have found a significant position in chemical

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Fig. 6.4  ZnO/CdO nanocomposites [4]. (A) Rod like, (B) Cylindrical, (C) Triangular, (D) Grain like, (E) Thicker, and (F) Fractured.

technologies. More specifically, membrane separation technology (MST) has become increasingly important because of different advantages including high productivity and selectivity, compact and small size equipment, and lower cost and energy consumption, and consequently, MST has found extensive applications in various fields including environment protection, petrochemical industry, desalination and water

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treatment, and biorefinery. Recently, there have been considerable progresses in the development of membrane materials for various purposes, which in turn has increased the demand for new membranes with modified characteristics. One of the most versatile and effective modification approaches is the incorporation of metal oxide particles to enhance the performance of the membranes. Titanium oxide (TiO2) is one of the most practical nanoparticles that offer a promising platform for different applications, for example, high-performance catalysis, photocatalysis, and electrocatalysis systems. In fact, TiO2 plays an important role in performance development of membranes. This review comprehensively discusses TiO2 and its application in various aspects of membranes and membrane engineering processes [5]. A photocatalytic membrane reactor (PMR) coupled with photocatalysis and direct contact membrane distillation (DCMD) was applied. Researchers reported that due to the shorter exposure time of the TiO2 NPs to UV irradiation, the photodegradation of MB in the batch room was faster than in the PMR. A PVDF-TiO2 mixed-matrix UF membrane was developed. Despite changing some of the physical properties at higher concentration of MB, performance of the mixed-matrix membrane exceeds the neat, which is due to the extra adsorption sites provided by TiO2 NPs for MB dye. Multifunctional composite polyurethane (PU) membrane was developed by using a sol-gel system containing TiO2 and fly ash NPs. The influence of TiO2 and fly ash NPs on membrane performance was studied by removing the MB dye, adsorption of heavy metals (Pb and Hg), water flux, and antibacterial properties. The improvement in antibacterial capacities of the membrane was attributed to the antibacterial properties of TiO2 NPs and the adsorptive property of fly ash. TiO2 nanoparticle-filled membrane and fibers are shown in Figs. 6.5 and 6.6, respectively [5,6].

(A)

Membrane surface

(B)

TiO2 nanoparticles

Membrane pores

PAA chain

Fig. 6.5  TiO2 nanoparticle-filled membrane [6]. (A) Initial bacterial deposition and (B) Final stage.

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Fig. 6.6  TiO2 nanoparticle-containing fibers [6].

6.2.3 Barrier applications of polymer nanocomposites When food will not be consumed immediately after production, it must be contained in a package that serves numerous functions. In addition to protecting the food from dirt or dust, oxygen, light, pathogenic microorganisms, moisture, and a variety of other destructive or harmful substances, the packaging must also be safe under its intended conditions of use, inert, cheap to produce, lightweight, easy to dispose of or reuse, able to withstand extreme conditions during processing or filling, impervious to a host of environmental storage and transport conditions, and resistant to physical abuse. This is a tall order for any material to fill [7].

6.2.4 Permeability The permeability of polymeric materials to gasses is determined by the adsorption rate of gas molecules into the matrix at the atmosphere/polymer boundary and the diffusion rate of adsorbed gas molecules through the matrix. The adsorption rate is generally dependent on the rate of formation of free-volume holes in the polymer created by random (Brownian) or thermal motions of the polymer chains, and diffusion is caused by jumps of molecular gas molecules to neighboring (empty) holes. Thus, the permeability of polymer films is dependent on free-volume hole sizes, degree of polymer motion, and specific polymer-polymer and polymer-gas interactions, all of which can be affected by intrinsic polymer chemistry and external properties such as temperature and pressure. Of course, the overall rate of gas diffusion is also directly dependent on the film thickness. The dispersal of nanosized fillers into the polymer matrix affects the barrier properties of a homogeneous polymer film in two specific

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Water vapor, oxygen

Water vapor, oxygen

(A)

(B)

Fig. 6.7  Diffusion of gas molecules in nanoporous membranes [10]. (A) Straight path and (B) Tortuous path.

ways. The first way is by creating a tortuous path for gas diffusion. Because the filler materials are essentially impermeable inorganic crystals, gas molecules must diffuse around them rather than taking a (mean) straight-line path that lies perpendicular to the film surface. The result is a longer mean path for gas diffusion through the film in the presence of fillers, as illustrated in Fig. 6.7. Essentially, the tortuous path allows the manufacturer to attain larger effective film thicknesses while using smaller amounts of polymer [8–10].

6.2.5 Barrier properties By far the most promising nanoscale fillers for PNCs are nanoplatelets composed of clays or other silicate materials. The popularity of nanoclays in food contact applications derives from their low cost, effectiveness, high stability, and (alleged) benignity. The prototypical clay utilized in PNC applications is montmorillonite (MMT), a soft 2:1 layered phyllosilicate clay composed of highly anisotropic platelets separated by thin layers of water. The platelets have an average thickness of nm and average lateral dimensions ranging between a few tens of nm and several lm. Each platelet contains a layer of aluminum or magnesium hydroxide octahedra sandwiched between two layers of silicon oxide tetrahedra. The faces of each platelet have a net negative charge, which causes the interstitial water layer (known as the gallery) to attract cations (Ca2+, Mg2+, Na+, etc.) and allows for the construction of multilayer polymer assemblies under appropriate conditions. Individual MMT clay platelets possess surface areas in excess of 750 m2/g and aspect ratios on the order of 100–500 [11,12]. These structural characteristics contribute to MMT's excellent utility as a filler material for PNCs, typically giving rise to impressive increases in polymer strength and barrier properties with only a few wt% added to the polymer matrix. However, because they have such large surface energies, clay nanoplatelets tend to stick together, particularly when dispersed in nonpolar polymer environments. Agglomeration of clay platelets leads to

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tactoid structures (microcomposites) with reduced aspect ratios and, according to the Nielsen model, reduced barrier efficiencies. When polymer-clay interactions are more favorable or when steps are taken (e.g., sonication) to disaggregate the platelets, intercalated and fully exfoliated PNC structures are formed. Intercalated morphologies are characterized by moderate intrusion of polymer strands into the gallery volume, and the shape of the layered stack is preserved. In fully exfoliated structures, on the other hand, individual platelets are well separated and have extremely favorable interactions with the polymer matrix. These various nanoclay morphologies are depicted in Fig. 6.8 [10]. The first successful example of a polymer-clay nanocomposite (PCNC) was a nylon 6 MMT hybrid material developed by the Toyota Corporation in 1986. The initial interest in PCNC materials stemmed from gains in strength and fire retardancy, and it was not until over a decade after their first appearance in the literature that their impressive barrier properties were fully realized and PCNC-based food packaging material development commenced. Over the last 25 years, hundreds of clay-based PNC systems have been reported, and nanoscale clay materials have been successfully incorporated into virtually every important class of synthetic or natural polymer. This body of work is especially exciting in light of the evident success in using PCNCs to improve the mechanical and barrier properties of biocompatible polymers, which in their virgin states are either too brittle or water-sensitive to enjoy widespread commercial use in the food industry. The dispersion of nanoclay in polymer resin is depicted in Fig. 6.9 [10]. Most commercially available PCNC products are marketed toward a very specific application, including several in the food and beverage industry. PCNC packaging materials have, for example, become popular with beverage manufacturers, such as Miller Brewing Company, which has used them to manufacture plastic bottles possessing both high barriers to oxygen and carbon dioxide migration. Other interested parties in PCNC technology include the US Army Natick Soldier Research, Development and Engineering Center (NSRDEC) that has invested considerable time and money researching the potential use of PNC plastics to package meals ready to eat (MREs) for soldiers; in addition to currently creating an enormous amount of waste, MREs have incredibly stringent shelf life and robustness requirements that PCNC-based packages may be uniquely able to satisfy [13]. While the studies demonstrate that PCNCs may slow down the migration of potentially harmful additives into foods, the body of safety research is at this time fragmentary and incomplete. A theoretical treatment has predicted that montmorillonite particles with surface modification embedded in various polymer matrices are unlikely to migrate into foods from a polymer nanocomposite food contact material in any detectable quantities. Nevertheless, more comprehensive experimental studies need to be done in PCNCs made from common-use food contact polymers such as PET, especially since some food and beverage companies are already utilizing these materials in their products. More importantly, the availability of clay nanoparticle toxicology data is still lacking, as are developments in strategies to detect and categorize clay and other nanoparticles in complex food matrices. One study determined that exfoliated silicate nanoclays exhibited low cytotoxicity and genotoxicity, even when part of a diet fed to rats (measured acute oral toxicity, ­median

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Fig. 6.8  Nanoclay structures [10].

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Fig. 6.9  Nanoclay dispersion in polymer system [10]. (A) Intercallation, (B) Lamellar, and (C) Exfoliation.

lethal dose, and LD50 > 5700 mg/kg body weight under the conditions probed); however, the authors of this study only tested a single clay type and morphology, so it is unclear whether it can be applied in a general sense. Due to the unique electric and optical properties of nanomaterials and the ease with which bottom-up engineering can provide multifunctional nanoscale architectures, pathogen detection strategies are increasingly abandoning conventional microbiological analysis methods in preference of a reliance on nanomaterials themselves as the means of detection. The general idea is that not only can the nanoscale magnetic particles be used to bind and isolate analytes from the matrix but also they (or other nanomaterials that constitute parts of multicomponent systems) can be directly detected without the need for time-consuming biological assays. This is particularly easy with microbial detection because the small size of nanoparticles relative to those of the target organisms causes large, readily observable electric/optical property modulations before and after binding events. It is also worth pointing out that nanomaterials lend themselves well to multiplexing assays, as in the case of a barcode-style method that utilizes binding of selective antibodies to specific regions of magnetic (and nonmagnetic) multimetal nanowires for the simultaneous, multiplexed optical detection of bacteria, viruses, and protein-based toxins [14]. There are numerous examples that demonstrate the utility of nanomaterials and in particular magnetic nanomaterials, as vehicles for the simultaneous isolation and optical or magnetic detection of microorganisms. Magnetic nanoparticles can be used to isolate Mycobacterium avium spp. paratuberculosis from contaminated whole milk and determine the bacterial concentration by observing effects of conjugation-­ induced magnetic particle agglomeration on the spin-spin (T2) relaxation times of nearby water protons; importantly, this method is not susceptible to interference from other bacterial species that may be present in the matrix. A similar approach that measures changes in the magnetic susceptibility (correlated to changes in particle

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hydrodynamic volume) of bound and unbound iron oxide particles efficiently detects Brucella antibodies in the blood serum of infected cows [15,16]. One of the most significant advantages of highly sensitive nanotechnology-based techniques is the reduced incubation and measurement times required for accurate detection. For instance, one research group used sugar molecules attached to nanosized magnetic iron oxide particles to isolate up to 88% of Escherichia coli in a sample with only a 45 min incubation time; the E. coli were subsequently detected using fluorescent staining. Researchers improved upon this approach by using species- and strain-specific antibodies instead of sugar molecules to isolate and optically detect (benchtop FTIR/portable mid-IR) the target organisms in 2% milk and spinach extract. In a separate series of studies, they used magnetic nanoparticles in conjunction with gold nanorods (AuNRs) to separate and detect key foodborne bacteria; here, the magnetic particles facilitate separation, while the AuNRs are used for optical detection in the near-infrared. Notably, because AuNRs have length-dependent absorptive properties and efficient light-to-heat energy conversion, they offer the possibility of multiplexed detection (i.e., simultaneous detection of multiple organisms) and efficient photoactivated antimicrobial activity via thermal ablation, all from a single multicomponent entity. Optical colorimetric detection and thermal ablation of Salmonella using oval-shaped AuNPs have also been demonstrated. With this in mind, it is also worth pointing out that nanomaterials can be useful for the rapid detection of microorganisms after IMS, even IMS performed using conventionally sized magnetic particles: as an example, Su and Li used IMS to separate E. coli from test samples and semiconductor nanocrystals (quantum dots) as fluorescent tags. Their protocol offered a detection range of 103–107 CFU/ml and total detection time of only 2 h, compared with 18–24 h for traditional bacterial plating/incubation methods [17].

6.2.6 Hybrid nanocomposite particles Hybrid nanocomposites based on Ag and Au nanoparticles and chitosan (CS) in form of films with high antibacterial activity and retained cytotoxicity against eukaryotic cells have been recently investigated. In terms of the potential biomedical application, since Ag and Au nanoparticles are entrapped in the solid polymeric matrix, these bioactive materials prevent the availability of NPs for eukaryotic cells while preserving the pronounced antimicrobial activity against selected resistant biofilm-forming strains (i.e., Staphylococcus aureus, Pseudomonas aeruginosa, E. coli, or Candida albicans). Bacterial cell wall morphology studies upon incubation with medium molecular weight (MW) chitosan-based nanocomposites confirmed that for the tested bacterial strains, significant and progressive damage on the cell wall was observed, which resulted in a total cell lysis. The examples of demonstrated biocidal activity of Ag and AuNPbased composites are investigated in detail. It is suggested that the lack of significant cytotoxicity against mammalian cells is a consequence of biocompatible chitosan layer surrounding the surface of metal NPs. Due to the presence of chemical bonds between NPs and chitosan, the potential direct interactions of bare nanoparticles with cellular components are diminished. In the case of the resulting nanomaterials, the main bactericidal effect is mainly a result of the Ag and AuNP activity. Certainly, since the p­ ositive

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charge of the polymer is reduced during NP synthesis and further film formation, the antibacterial activity of chitosan films decreases in comparison with the polymeric dispersion; however, bacteriostatic activity of the pure polymer is still observed [18]. In recent years, infections caused by multidrug-resistant (MDR) microbes have become a global health challenge. To stop the spread of drug-resistant infections, several parallel actions seem to be urgent and imperative including development of new rapid diagnostic systems to cut the unfounded use of antibiotics, fundamental changes in prescriptions and consumption of existing antibiotics to preserve their usefulness, applying combination therapies, and the development of effective alternative approaches in antimicrobial drugs discovery. Recently, bioinorganic platforms like metal complexes, metal-modified macromolecules, and metal oxide nanoparticles have become attractive alternatives to combat microbes that are resistant to various classes of antibiotics. The vast array of physicochemical properties of these platforms enables them to act as antimicrobial agents through various mechanisms. They can also serve as carriers for drugs delivery. Moreover, combination of bioinorganic platforms and light offers very promising alternative antimicrobial strategies like photothermal therapy or photodynamic microorganism inactivation. Unique electronic, structural, spectroscopic properties and redox or acid-based thermal or photochemical reactivity make bioinorganic platforms appropriate to provide innovative antimicrobial strategies with simultaneous prevention of resistance development and protection of the natural host microbiome [19,20].

6.2.7 Nanocomposite fibers for antibacterial fabrics Cellulose-silver nanocomposite fibers (Fig. 6.10) with excellent antimicrobial properties were successfully developed from a green process where AgNO3 was reduced with natural carbohydrates, GA and GG. The composites were characterized by spectral, thermal, and electron microscopy techniques. The results showed that the silver nanoparticles were greatly dispersed in the cellulose matrix with strong interaction between the cellulose and polymer/silver particles. The CSNCFs exhibited good antibacterial activity against E. coli. Therefore, it was concluded that the AgNP composite cellulose fibers developed can be suggested for their utilization as effective tissue scaffolding for burn/wound treatments. The fibers alone can be used as higher durable antibacterial finishings in textile industries and also as potential surgical fabrics in the medical fields [21].

6.2.8 Preparation of nanocomposite films Nanocomposite films at 3 wt% filler content were prepared by mixing the calculated amount of jute fibrils with 5% PLA in chloroform solution using a magnetic stirrer. The stirring was performed at room temperature for 3 h. The composite mixture was further ultrasonicated for 10 min on Bandelin ultrasonic probe mixer with 50-horn power. The final mixtures then cast on a Teflon sheet. The films were kept at room temperature for 2 days until they were completely dried and then removed from the Teflon sheet. One neat PLA film was also prepared without addition of fillers for comparison purpose [22].

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Fig. 6.10  Cellulose-silver nanocomposite fibers [21]. (A) Cotton fiber, (B) Polymer coated cotton fibers, and (C) Polymer AgNPs coated cotton fibers.

6.2.9 Testing of nanocomposite films The nanoindentation experiments were performed using a CSM Instruments (NHTX S), Switzerland, with a diamond probe. A diamond Berkovich indenter with a tip radius of 50 nm and a maximum load of 0.50 mN was used for evaluating Young's modulus and hardness of the films. The loading and unloading rates were fixed at 1 mN/min, whereas the approach speed was kept at 2000 nm/min. Subsequently, the images of indentation marks were observed under optical microscope attached to the nanoindenter. Total of 10 samples were used to characterize each material. Tensile testing was carried out using a miniature material tester Rheometric Scientific MiniMat 2000 with a 1000 N load cell at a crosshead speed of 10 mm/min. The samples were prepared by cutting strips from the films with a width of 10 mm. The length between the grips was kept at 100 mm. A total of 10 samples were used to characterize each material. Dynamic mechanical properties of the nanocomposite films were tested on dynamic mechanical analysis (DMA) DX04T RMI instrument, Czech Republic, in tensile mode. The measurements were carried out at constant frequency of 1 Hz, strain amplitude of 0.05%, temperature range of 35–100°C, heating rate of 5°C/min, and gap

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distance of 30 mm. The samples were prepared by cutting strips from the films with a width of 10 mm. Four samples were used to characterize each material [23].

6.2.10 TiO2 nanoparticles in basalt/polysiloxane composites New structural materials are being sought for applications under hostile conditions at elevated temperatures and in an aggressive (oxidative, acidic, and alkaline) environment. Among them, composites reinforced with thermally stable fibers (e.g., R-glass fibers or fine ceramic fibers based on silicon carbide or oxides like alumina) are perhaps the most promising. The merit of expensive fibrous reinforcement consists in improving the toughness and strength of naturally brittle ceramic matrix, which also can be obtained by a relatively cheap polymer, for example, pyrolysis route from polymer precursors like polysiloxane resins or others. Utilization of cheap basalt fibers in combination with commercially available resins therefore offers a unique potential for developing inexpensive composite materials with remarkable performance at temperatures limited by the thermal stability of basalt [24]. The attractiveness of basalt fibers as an alternative reinforcement comes from its relatively high specific properties (strength, stiffness, and high temperature resistance) and its good eco-friendly performance when compared with traditional fibers such as glass. Glass fiber-reinforced composites, although used very successfully in many applications, are found to be hazardous to the people working with them. Therefore, in this study, basalt fabric has been used for the composite preparation. The resin used is polysiloxane, which is inert, nontoxic, and nonflammable, hence providing a good working environment. The nanostructure offers outstanding attributes as reinforcement to improve the mechanical properties of materials. The high surface areas of the nanoparticles enhance the organic-inorganic adhesion. Fabric composites have outstanding specific strength, load-carrying capacity, and wear-resistant properties. They are increasingly used in aerospace, automotive, naval, and other industries. There are a lot of reports on the tensile properties of fabric composites. Various kinds of fabric materials are used, various tensile directions are applied, and also many simulation methods are used to study the tensile mechanisms of the fabric composites. But these reports are mainly focused on the effects of the fabric materials and structures; the influence of the matrix modification has hardly been studied. Modification of matrix can enhance the fiber-matrix adhesion, improving the tensile and thermal properties of the fabric composites. In this work, TiO2 nanofillers have been used to modify the matrix properties in order to get improved thermal and mechanical properties of the fabric composite. TiO2 nanoparticles are among the smallest nanoparticles available commercially. This allows for huge areas of contact between the nanofillers and the polymeric matrix. The objective is to investigate the influence of the of TiO2 nanoparticle contents on the tensile and thermal properties of the fabric composites [24]. The yarn (roving) count was 100 per 100 mm in both warp and weft directions, and the yarn fineness was 908 dtex. The tenacity of the basalt fiber was about 1.71 N/ tex. The thickness of the basalt fabric was 0.25 mm. Polysiloxane (Lukosil M130) was used as the matrix. It is a low-viscosity resin. The TiO2 nanoparticles used in

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the study were obtained from Sigma Aldrich, the United States. The precalculated amount of TiO2 nanoparticles and resin was carefully weighed and mixed together in a beaker. The mixing was carried out through a high-intensity ultrasound irradiation by a type of sonicator probe (Bandelin Sonopuls sonicator, the United Kingdom) for 10 min with the maximum energy of 30 kJ. The fabric composites were prepared by hand layup process. The weight ratio between fabric and matrix (matrix comprising of both resin and nanoparticles) was maintained to be 55:45 for all samples. This higher proportion of the basalt fabric was chosen in order to have greater effect of reinforcement material on overall mechanical property of the composite. Basalt fabric was coated with the prepared TiO2/polysiloxane resin and left for room-temperature curing for 16 h. The fabric was again coated with the same resin, and the prepregs were then postcured at 200°C for 6 h on a hot press to produce composite sheets of 1 mm thickness. Specimens of various dimensions were then cut from the sheets for various testings [25].

6.2.11 Nanoparticle size distribution Nanoparticles were dispersed in distilled water by 10 min of rigorous ultrasonication. Particle size distribution of nanoparticles in the solution was then measured by Malvern Mastersizer. This instrument measures the particle size on dynamic light-scattering principle. Fig.  6.11 shows the size dispersion of TiO2 nanoparticles; average TiO2 nanoparticle size obtained was 238 nm [24].

6.2.12 Tensile testing Mechanical testing is limited to tensile testing as the mechanical parameters are very sensitive to particle identity and concentration. Tensile properties were measured on TIRAtest 2300 universal material test machine equipped with a 10 kN load cell at a constant speed of 6 mm/min. The specimen was 18 cm in length, 2 cm in width, and 1 mm in thickness. The grip separation was 10 cm. The loading was in the direction

100

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80

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60

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4

20

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10

100 Size (d.nm)

Fig. 6.11  TiO2 nanoparticle size distribution by intensity [24].

1000

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Size distribution by intensity 12

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of the warp. The load elongation behavior and Young's modulus were obtained. All tensile tests were performed at room temperature. The tensile stress-strain curves were obtained by determining the engineering stress and engineering strain as done by previous researchers. Experiments were carried out to observe the effect of TiO2 nanoparticles on tensile properties of the fabric composites. In the fabric composite formation, there is a mechanical interlocking between the fibers and the resin due to friction force. Friction force is highly dependent on the area of contact between the resin and the fibers. Nanoparticles undoubtedly have very high surface area. When these particles are dispersed inside the resin, the total surface area of contact between the particles and resin increases abruptly. This high contact area between the particles and the resin increases friction force between the two but not really between matrix and fibers. From Fig. 6.12, one can see that with inclusion of TiO2 nanoparticles in the resin, tensile strength somehow goes down. However, the tensile modulus increases for 1 and 1.5% particle content. This is because of a lower extension at break, resulting from frictional restraint due to particle and resin interaction. Tensile modulus again decreases with higher percentage of nanoparticles. This is due to agglomerate formation, which leads to stress concentration and further reduces the tensile properties of the fabric composite [25]. Fig.  6.12 shows the load elongation behavior of nanocomposites containing different wt% of TiO2 nanoparticles, and also shows variation in tensile modulus as a function of wt% of TiO2 nanoparticles in the composites. For 1.5 wt% of TiO2 nanoparticles, highest increment in modulus was found as compared with the other loading percentages, but >1.5 wt% again decreases the modulus, which is in good agreement with the scanning electron microscope (SEM) images of the nanocomposites. Reduction in tensile strength of nanocomposite at higher loading percentage is due to particle agglomeration, which starts acting as defect inside the composite.

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Fig. 6.12  Load elongation curve for nanocomposites containing different wt% of TiO2 nanoparticles [24].

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6.2.13 Dynamic mechanical analysis One of the basic ideas of modifying polymers with nanoparticles is to enhance the material's mechanical properties. DMA measurements are used to detect changes in the molecular mobility between polymer segments in the vicinity of nanoparticles and to evaluate the stiffness of the nanocomposites. DMA has been frequently used in nanocomposite characterization since it allows the measurement of two different moduli of the nanocomposites, a storage modulus (E″) that is related to the ability of the material to return or store mechanical energy and a loss modulus (E″) that is related to the ability of the material to dissipate energy as a function of temperature. Ratio of loss modulus to storage modulus is given by tan delta [26]. DMA measurements were performed with DMA DX04T instrument (dual cantilever beam test configuration) with frequency of 1 Hz. Specimen dimensions were 30 × 10 × 1 mm. Fig. 6.13 shows the storage modulus of the composite as a function of wt% of TiO2 nanoparticles for the composites. It is visible from Fig.  6.13 that, for the 1.5 wt% of TiO2 nanoparticles, maximum improvement of storage modulus is obtained in comparison with the neat resin composite system. Storage modulus of the matrix affects the mechanical properties of the composite material. Hence, improvement in storage modulus also gives improvement in mechanical properties of the composite system, specifically the tensile properties. The results for storage modulus explain the highest gain in tensile modulus of the nanocomposites for 1.5 wt% of nanoparticle content. Trends of graphs obtained for storage modulus and tensile modulus are similar with respect to the change in wt% of TiO2 nanoparticle content in the nanocomposites.

Storage modulus 1.80E+09 1.60E+09

Storage modulus

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1%

1.50%

2%

2.50%

3%

% TiO2 content

Fig. 6.13  Storage modulus of nanocomposites as a function of wt% of TiO2 nanoparticles [24].

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6.2.14 Thermal stability of nanocomposites Nanoparticles, when reinforced into the polymer matrix, create restrictive environment of the polymer chains surrounding the nanoparticles that greatly affects molecular relaxation, mobility, and also their crystallization. Hence, nanoparticle induces a crystalline phase transition. Polymer molecules are sensitive to the local environment because of the characteristic long-chain morphology. Therefore, the values of Tg of a polymer at glass transition and heat distortion temperature (HDT) are important parameters that provide information about the structural changes undergone by the polymer during the transition. Nanoparticle is known to induce phase transitions in some polymer systems (e.g., nylon 6) by providing nucleation sites. Generally, nanoparticles are grafted with matrix-compatible ligands that act as plasticizers as the nanoparticle is very rigid. Chemical bonding takes place between these ligands and the matrix molecules. This restricts the polymer chain mobility, which results in increasing Tg of the polymer matrix. Differential scanning calorimetry (DSC) is used to study the polymer chain relaxation behavior and mobility in the vicinity of the glass transition in view of these different nanocomposite morphologies [27]. Two different types of analyses were performed for determining thermal stability of nanocomposites. 1. Differential scanning calorimetry (DSC)—the instrument was used to measure the glass transition temperature of the composites. 2. Thermal gravimetric analysis (TGA)—heat distortion temperature was measured by TGA.

6.2.15 DSC analysis DSC measurements were performed with a Perkin Elmer Differential Scanning Calorimeter (Pyris 6 DSC) under nitrogen atmosphere. Appropriate amount of samples (~6 mg) were sealed in aluminum pans and heated from 25 to 400°C at a scanning rate of 10°C/min. The DSC was introduced to investigate the glass transition temperature (Tg) of the composites. Fig. 6.14 shows the DSC thermograms of nanocomposites containing TiO2 and Tg values, respectively. The Tg of the nanocomposites increased all the way from 252.6°C for pure polymethylsiloxane to 264.4°C for 3 wt% TiO2filled composites [28]. Increase in Tg of nanocomposites is attributed to nanoparticle dispersion and interaction with matrix.

6.2.16 TGA measurements The TGA was performed with thermogravimetric analyzer (Mettler Toledo TAC 7/ DX) under nitrogen atmosphere. Appropriate amount of samples (~7 mg) were sealed in the sample holder and heated from 25 to 1000°C. Fig. 6.15 shows the dependence of mass loss on temperature for nanocomposites with various amounts of TiO2 nanoparticles. It is noticed that the temperature at which the mass loss reaches the highest value (i.e., the inflection point or Tm) increases as the filler content in the composite increases or weight loss for the composite decreases as the wt% of TiO2 nanoparticles

Nanocomposites283 2.5

Tg

Heat flow (mW/mg)

2

0.5% TiO2 1% TiO2

1.5

1.5% TiO2 2% TiO2

1

2.5% TiO2 3% TiO2 Neat methylsiloxane

0.5

0 0.0

100.0

200.0

300.0

400.0

500.0

Temperature (°C)

0.5 mg

Fig. 6.14  DSC curves in the glass transition region of nanocomposites [24].

0.5% TiO2

1% TiO2

1.5% TiO2 2% TiO2 2.5% TiO2 3% TiO2

50

100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 °C

Fig. 6.15  TGA thermograms of nanocomposites loaded with various amounts of TiO2 (wt%) [24].

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in the composite increases. The TGA curves for all samples are falling in character due to weight loss. However, the slope is somehow lower with the sample having 1% TiO2, which can be attributed to lower rate of weight loss resulting from very good adhesion between nanoparticles and the resin molecules. From the TGA table, it is clear that at 400°C, wt% loss reduces with the increase in wt% of TiO2 nanoparticles in the matrix all the way from 0.5% TiO2 content to 3% TiO2 content. The addition of lower percentage of TiO2 nanoparticles can enhance the tensile properties of the polysiloxane matrix, which can be attributed to increase in friction force between the nanofillers and the matrix resulting in increased mechanical interlocking in the composite system. The increased frictional force facilitates load transfer from the polysiloxane resin to TiO2 nanoparticles; thus, the tensile properties of matrix are improved, thereby improving the tensile properties of the composite. 1.5% is the critical overall content of the fillers, exceeding which the fillers have a strong tendency to agglomerate, lowering the tensile strength. Hence, the mechanical strength enhancement of nanocomposite up to 1.5 wt% TiO2 can be attributed to the intrinsic characteristics of the nanoparticle based on the rule of mixture [29]. The results of DSC and TGA of nanocomposite show that the Tg and HDT of the nanocomposite increase with the increase of nanoparticle content in the polymer. These results reveal that the increase in Tg and HDT is attributed to a good adhesion between polymer and reinforcing particle, so that the nanometric size particle can restrict the segmental motion with a consequent increase in Tg and HDT.

6.2.17 3D woven composites and nanocomposites Woven fabrics in 2-D sheet form have many properties, such as being drapable, flexible, strong, and warmth keeping, and all of these make them suitable to be used as materials for clothing and other domestic end uses. But when high-performance fibers (such as glass, aramid, and carbon) are used for constructing such 2-D fabrics, the same woven fabrics find many technical applications. These include textile composite reinforcements for the aerospace industry and body armors for personal protection for the military and the police. These fabrics, when considered for body armors, can’t be used in single layer because certain amount of bulk is necessary, and ballistic resistance increases with overall areal density. This necessitates the use of many layers, typically between 5 and 20, to produce a ballistic pack, which will perform adequately. Many researchers reported that this multilayer packed armor fail to high-velocity bullets and is not suitable for higher levels of protection. The main cause behind the failure is delamination process of 2-D woven laminates whose interface strength is determined by the bond strength between the matrix and multiple layers of fabrics. Also importantly, crimp in the 2-D fabric as composite reinforcement significantly reduces in-plane stiffness and strength of composite. These drawbacks of 2-D fabrics as reinforcement material for composite demand a better integral structure with substantial thickness, possibly crimpless and having better strength in through-the-thickness direction [30]. The potential usage of 3-D woven fabrics in the ballistic protection applications is the main motivating factor of this research. Thus in its initial part, the project has

Nanocomposites285

been focused to investigate the ballistic properties of 3-D woven fabrics of various structures in comparison with 2-D fabrics. The gap in the research of 3-D woven composites embedded with nanoparticles has given the opportunity to carry out the research work further. By keeping in mind this wide scope, further exploratory work has been carried out to study the mechanical characteristics of the 2-D and 3-D woven nanocomposites. All composites were prepared by compression molding technique. LY556 epoxy resin was used as a matrix component for all the fabrics. The principal advantage of compression molding is its ability to produce parts of complex geometry in short period of time. SANTEC compression molding equipment is used to prepare composites. The parameters set on the machine while preparing composites are curing time, 900 s; breathing pressure, 6 N/m2; curing pressure, 12 N/m2; and curing temperature 120°C.

6.2.18 Preparation of nanocomposites 6.2.18.1 Preparation of nano fly-ash The fly ash was collected from source in Plzeň, Czech Republic. Mechanical activation of fly ash was carried out using a high-energy planetary ball mill of Fritsch Pulverisette 7 in a sintered corundum container of 80 ml capacity using zirconia balls of 3 mm diameter under wet condition in distilled water for 1, 2, 3, 4, and 5 h. The ball mill was loaded with ball-to-powder weight ratio of 10:1. The rotation speed of the planet carrier was 850 rpm. In this mechanical treatment, powder particles are subjected to a severe plastic deformation due to the repetitive compressive loads arising from the impacts between the balls and the powder. The milled sample powder was taken out at a regular interval of every 1 h of milling to test for particle size distribution on Malvern Zetasizer Nano based on dynamic light-scattering principle. The dispersion medium was deionized water. The dispersion was ultrasonicated for 5 min with Bandelin ultrasonic probe before characterization. The fly ash nanoparticles prepared by wet milling process are dried first in the oven at 120°C for 6 h. Then, the amount of epoxy needed was calculated by considering ­fabric-to-epoxy weight ratio is 60:40. Fly ash nanoparticle was taken 10% by wt to the matrix (epoxy + hardener). This calculated amount of nanoparticles was carefully weighted and dispersed in the hardener first with the help of sonicator. This n­ anoparticle-dispersed hardener was then added to the beaker already containing premeasured amount of epoxy with gentle stirring to avoid formation of air bubbles [31]. The quantitative elemental analysis of fly ash showed 53.80% of oxide content. Among all other elements, silica and aluminum particles contribute to over 75%. The variation in particle size of fly ash with milling time is depicted in Fig. 6.16. The average particle size of the fly ash procured was 3547 nm. Ball milling of fresh fly ash up to 5 h reduced its size by a magnitude of 10 times to 396 nm. The reduction of particle size after regular interval can be seen from Fig.  6.16. The particle size distribution before milling and after milling for 5 h is plotted in graphs as shown in Fig. 6.17A and B, respectively.

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Nanotechnology in Textiles 18 FA-Original FA-1Hr-Wet

16

FA-2Hr-Wet

14

FA-3Hr-Wet FA-4Hr-Wet FA-5Hr-Wet

Intensity %

12 10 8 6 4 2 0

1

10

100

1000

10,000

Size (d nm)

Fig. 6.16  Particle size distribution after regular intervals of milling [32].

The SEM fractographs of the nanocomposites are depicted in Fig.  6.17 at 60, 100, 500, and 1000 times magnification in alphabetical order of figures. As shown in Fig. 6.17A and B, it can be found that the dispersion of fly ash particulates in the matrix for the composite is uniform and the fly ash particulates disperse easily in the matrix. This means that the fly ash particulates could distribute uniformly in the matrix due to their high dispersibility. SEM photograph in Fig. 6.18D shows the broken and fractured surface of the glass fiber. Agglomeration of nanoparticles is also observed at few places in otherwise uniformly dispersed nanoparticles in the composites. The peak force of warp interlock fabric-reinforced composite is highest, while ­angle-interlock fabric performed well in terms of total energy absorption. This performance is a result of intimate binding of the layers of yarns in these two structures. A comparative account of breakdown strength and energy at break for regular composites and nanocomposites is shown in Figs. 6.19 and 6.20, respectively. The effect of fly ash nanoparticles can be observed from the figures. The addition of nanoparticles improved the breakdown strength and energy by approximately 30%–40%. The uniform distribution of the nanoparticles on the surface and between the layers of plain woven fabrics while preparing the composites gave the highest change in the breaking strength [32]. The flexural test is carried out for all composites prepared with and without the nanoparticles. It can be seen from Fig.  6.21A and B that the flexural modulus and flexural strength of the glass fabric-reinforced composites increased significantly with the addition of fly ash nanoparticles.

Nanocomposites287 Diam. (nm)

% Intensity

Width (nm)

Z-Average (d.nm): 3547

Peak 1:

2744

86.8

853.4

Pdl: 0.264

Peak 2:

5261

13.2

370.7

Intercept: 0.813

Peak 3:

0.000

0.0

0.000

Result quality Refer to quality report Size distribution by intensity 20

100

Intensity (%)

Undersize

80

15

60 10 40 5

20

0 1

10

1000

100

0 10,000

Size (d.nm)

(A)

Intensity (Record 4: original fa 2)

Undersize (Record 4: original fa 2)

Diam. (nm)

% Intensity

Width (nm)

Z-Average (d.nm): 396.8

Peak 1:

493.1

100.0

194.4

Pdl: 0.184

Peak 2:

0.000

0.0

0.000

Intercept: 0.890

Peak 3:

0.000

0.0

0.000

Result quality Good Size distribution by intensity 100

14

80

10 8

60

6

40

4

20

2 0

Undersize

Intensity (%)

12

1

10

100

1000

0 10,000

Size (d.nm)

(B)

Intensity (Record 2: fly ash 5hr 2)

Undersize (Record 2: fly ash 5hr 2)

Fig. 6.17  (A) Particle size distribution before milling [32]. (B) Particle size distributions after milling [32].

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Fig. 6.18  SEM fractographs of the nanocomposites [32]. Breakdown strength (N) 700 600 500 400 300

Regular composite

200

Nanocomposite

100 0 2D 3 layers

3D angle interlock

3D warp interlock

3D orthogonal

Fig. 6.19  Effect of nanoparticles on breakdown strength of composites [32].

Nanocomposites289

Energy at break (N.m)

25 20 15

Regular composite

10

Nanocomposite

5 0 2D 3 layer

3D angle interlock

3D 3D warp interlock orthogonal

Fig. 6.20  Effect of nanoparticles on energy at break of composites [32].

Flexural strength of regular composites 700 600 Stress (MPa)

500

3D warp interlock

400

3D orthogonal

300

3D angle interlock

200

2D 3 layers

100 0

0

0.005

(A)

0.01

0.015

Strain Flexural strength of nanocomposites

700

Stress (MPa)

600 500

3D warp interlock

400

3D orthogonal

300

3D angle interlock

200

2D 3 layers

100 0

(B)

0

0.005

0.01

0.015

Strain

Fig. 6.21  (A) Flexural behavior of regular composites [32]. (B) Flexural behavior of nanocomposites [32].

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Among the composites with different reinforcement structures, 3-D warp interlock composites proved to have maximum stiffness, while orthogonal structure composite has lowest stiffness. The linear (uncrimped) arrangement of warp yarns in orthogonal structures fails to support the composite in terms of flexural stiffness and modulus. In both the composites with and without nanoparticles, 3-D woven nanocomposites show higher flexural strength and flexural modulus compared with regular 3-D woven composites. The 3-D angle-interlock composite shows an improvement of 11%, while ­angle-interlock composite shows 33% improvement in the flexural modulus. This increase is attributed to excellent interphase between nanoparticles and the matrix molecules [33].

6.2.19 Functional properties of 3D woven glass nanocomposites Woven fabrics in 2-D sheet form have many properties, such as drapability, flexibility, and comfort, and all of these make them suitable to be used as materials for clothing and other domestic end uses. However in case of composite applications, the main cause behind the failure is delamination failure of 2-D woven laminates whose interface strength is determined by the bond strength between the matrix and multiple layers of fabrics. Thus in its initial part, the research has been focused to investigate the thermomechanical properties of 3-D woven fabrics of various structures in comparison with 2-D fabrics. The nanostructure offers outstanding attributes as reinforcement to improve the functional performance of composite materials. The high surface areas of the nanoparticles enhance the organic-inorganic interaction. Nanocomposites are expected to exhibit improved electric, thermal, and magnetic properties. By keeping in mind this wide scope, further exploratory work has been carried out to study the thermal, electric, and electromagnetic characteristics of the 2-D and 3-D woven nanocomposites [34].

6.2.20 Knife penetration test The breakdown strength and the work at breaking load are shown in Figs. 6.22 and 6.23, respectively. Three-dimensional woven fabric-reinforced composite shows higher breaking load as compared with the 2-D plain woven composite. The 3-D orthogonal fabric shows lowest resistance, which may be because of less number of interlacement points in the structure giving chance to easily propagate the crack between the parallel threads when knife penetrates in the composite. The higher deformation of the plain woven composite while during penetration of knife results into higher energy absorption, and the case is similar for 3-D warp interlock woven composite also. The effect of fly ash nanoparticles can be observed. The addition of nanoparticles improved the breakdown strength and energy by 30%–40%.

6.2.21 Thermo-mechanical properties (DMA test) Thermomechanical behavior of the epoxy-based composites with different reinforcement structures for regular composites and different loadings of nano-fly ash in case of nanocomposites has been investigated in this study. The effect of different ­reinforcement structures can be seen from Figs. 6.24 and 6.25. The composites with

Nanocomposites291

Breakdown strength (N)

700 600 500 400 300

Regular composite

200

Nanocomposite

100 0 2D 3 layers

3D angle interlock

3D warp interlock

3D orthogonal

Fig. 6.22  Breakdown strength of composites and nanocomposites [34]. Energy at break (N.m)

25 20 15

Regular composite

10

Nanocomposite

05 0

2D 3 layer

3D angle interlock

3D 3D warp interlock orthogonal

Fig. 6.23  Energy at break of composites and nanocomposites [34]. Storage modulus (GPa) 2D plain 3 layer composite

Storage modulus (GPa)

20 15

3D orthogonal composite

10 3D angle interlock composite

05 0 40

140

240

340

3D warp interlock composite

Temperature (°C)

Fig. 6.24  Storage modulus of composites with different reinforcement structures [34].

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Tan delta 0.35

2D plain 3 layer composite

Tan delta

0.30 0.25

3D orthogonal composite

0.20 0.15

3D angle interlock composite

0.10 0.05 0 40

140

240

340

3D warp interlock composite

Temperature (°C)

Fig. 6.25  Tan δ for composites with different reinforcement structures [34].

the 3-D woven fabrics as reinforcement show higher values of storage modulus. When compared with the three-layer plain woven fabric composite, 14% increase in storage modulus of angle-interlock fabric and 26% improvement in the orthogonal and warp interlock fabric-reinforced composites are observed. The integral structure of the 3-D orthogonal fabric with uncrimped yarns absorbs higher amount of energy [35,36]. The curves of the temperature dependence of tan δ in case of regular composites shown in Fig. 6.25 give peaks at 68°C that is closely related to the Tg of the matrix.

6.2.22 Thermo-mechanical behavior of nanocomposites Fig.  6.26 shows the effect of nanoparticle loading on thermomechanical behavior of the composites. In all these graphs, storage modulus is higher at initial low temperature up to 50–60°C, which indicate glassy region in the composite. This could be an indication of stronger matrix/filler interaction. But immediately, steep drop in storage modulus can be observed at higher temperature, which may be because of the increased thermal energy that easily exceeds the matrix/filler interaction forces Storage modulus (GPa) 2D plain 3 layer composite

Storage modulus (GPa)

30 25

3D orthogonal composite

20 15 10

3D angle interlock composite

05 0 40

140 240 Temperature (°C)

340

3D warp interlock composite

Fig. 6.26  Storage modulus of nanocomposites with different reinforcement structures [34].

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leading to a sharp decrease in storage modulus. It can be seen from the graph that the addition of nanoparticles led to increase in the storage modulus of the composites. The order of the ranking with respect to storage modulus of these nanocomposites is same as regular composites. The reason behind this can be explained as the nanoscale dimension of fly ash nanoparticles has got very large surface area that leads to interaction sites and hence efficient load transfer between reinforcing agent and matrix. Also functionalization of the fly ash nanoparticles influences the interfacial interaction between the epoxy matrix and nanoparticles. When compared among these composites, 3-D orthogonal fabric-reinforced composite with nanoparticles shows dramatic improvement in storage modulus. This may be because of relatively easier dispersion of matrix-filler in the interstices of the yarns in a 3-D orthogonal structure. Thus, the nanoparticles further enhance the load-bearing capacity of this composite by filling up the empty spaces in the structure [37]. The glass transition temperature (Tg) in Fig. 6.27 is defined by the peak value of tan delta in the curves. The glass transition temperature has not changed too much by the addition of nanoparticles. Also the values are almost similar for all composites except 3-D orthogonal woven composites. The tan δ values of composites filled with fly ash are higher than that of the unfilled epoxy matrix, which proves that the addition of fly ash enhances the damping capacity of epoxy resin. All fabric constructions showed improvement in damping capacity of composites after the addition of fly ash. This fact may be explained by the addition of fly ash that increases the contributions of the hollow structure in fly ash and frictional damping, thereby leading to the increment of the loss of energy, and thus, the damping loss factor increases [38].

6.2.23 Electrical properties As electric conductivity is the property of material, so, there is no significant effect of fabric structure on the electric properties. All values of surface resistivity of the composites lie in a narrow range of 4.2 × 109 to 4.8 × 109 Ω m. And the volume resistivity is also between 6.5 × 1010 and 7.2 × 1010 Ω m. As this value itself is so high, the

0.35

Tan delta 2D plain 3 layer composite

0.30 Tan delta

0.25

3D orthogonal composite

0.20 0.15

3D angle interlock composite

0.10

3D warp interlock composite

0.05 0 40

140 240 Temperature (°C)

340

Fig. 6.27  Tan δ for nanocomposites with different reinforcement structures [34].

294

Nanotechnology in Textiles Surface resistivity (Ohm.m) Regular composites

Surface resistivity (Ohm.m)

6 × 109

Nanocomposites

5 × 109 4 × 109 3× 109 2 × 109 1 × 109 0 2D plain 3 layer

(A)

3D warp interlock

3D angle 3D interlock orthogonal

Volume resistivity (Ohm.m)

Regular composites Nanocomposites

Volume resistivity (Ohm.m)

70 × 109 60 × 109 50 × 109 40 × 109 30 × 109 20 × 109 10 × 109 0

(B)

2D plain 3 layer

3D warp interlock

3D angle interlock

3D orthogonal

Fig. 6.28  (A) Surface resistivity of composites and nanocomposites [34]. (B) Volume resistivity of composites and nanocomposites [34].

change in the values of resistivity between the structures is not significant. Addition of particles improved the conductivity of the material in all cases. Results are shown in Fig. 6.28A and B.

6.2.24 Electromagnetic shielding As shielding effectiveness of the material is influenced by the conductivity of the material, slight improvement is observed in the SE value of the nanocomposites. Fig. 6.29 shows the shielding effectiveness in dB for both regular and corresponding nanocomposites. The addition of nanoparticles improving the conductivity of the matrix material enhanced the electromagnetic (EM) shielding of the nanocomposites [39,40].

Nanocomposites295 Shielding effectiveness (dB)

Regular composites Nanocomposites

1.0

(dB)

0.8 0.6 0.4 0.2 0 2D plain 3 layer

3D warp interlock

3D angle interlock

3D orthogonal

Fig. 6.29  Electromagnetic shielding of composites and nanocomposites [34].

The concept of adding nanoscale filler in the matrix material with fabric reinforcement in a new three-phase composite has been shown to be very successful. The fly ash nanoparticles needed for these composites were prepared by high-energy ball milling technique. This method of preparation of fly ash nanoparticle was found as universal and quick, giving mean particle size of 450 nm. Functional properties of the nanocomposites were determined and compared with those of regular composites. All nanocomposites show improved knife penetration resistance compared with regular composites. However, the addition of nanoparticle improved the breaking strength and energy of 2-D plain woven composite to the maximum. The uniform distribution of nanoparticle in between the layers of plain woven composite could be the reason for giving maximum change in this case. From DMA results, it is seen that storage modulus increased by significant amount, while glass transition temperature remained almost unchanged for different composite samples determined by the peak in tan delta. Therefore, the changes in thermomechanical properties were due to the physical presence of the nanoclay as opposed to changes in the polymeric network structure. Improvement in the conductivity of nanocomposite is observed due to the addition of fly ash nanoparticle. This enhancement caused slight improvement in the shielding effectiveness of the nanocomposites. But both these changes are not so significant to be taken into consideration in related applications. This is attributed to properties of fly ash particles. Conductive particles can be used for further enhancement of EM shielding [40].

6.2.25 Basalt nanoparticle reinforced hybrid woven composites The encouraging and surprising results in various fields with the use of nanosized objects have led to new horizons in terms of applications. Nanotechnology is playing a vital role in various fields of energy, electronics, materials science, aerospace, optics, and pharmaceuticals. As per definition in law for nanotechnology, a drastic change in

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properties of a particular substance takes place when it is reduced in size to nanometer scale. As surface area of nanomaterials is extremely high in comparison with bulk materials, it exhibits physical and chemical properties that are totally different from individual molecule and bulk material. The production of polymeric nanocomposites by the use of nanoscale reinforcement fillers has evolved with high level of interest. Nanocomposites are particle-filled polymers with at least one dimension of the dispersed particle in nanometer scale. A variety of nanoparticles such as silica, zinc, alumina, micro- and nanosized silicon carbide, carbon black nanoparticles, and calcium carbonate are commonly used as fillers to reinforce polymer nanocomposites for enhanced material properties. The polymer nanocomposite technology is a relatively new branch of materials science that has an inherent versatility in terms of new prospects. This century will see the domination of composite family in major applications for structure development. Some of many important features of nanocomposites include mechanical performance, thermal stability, dielectric behavior, excellent tribological properties, adhesion to most substrates, good corrosion resistance, and good scratch resistance. A variety of potential applications have led to development of wider interest in nanocomposites for their usage in paints, coating, and sealants. It is a common practice to use additional phase, that is, organic filler that will help in strengthening the properties of an epoxy resin, where the nanoparticles fill weak microareas of resin and boost the interacting force at resin filler interface. An increase in interfacial area between matrix and fillers can dramatically increase and improve properties of composite material. The reinforcement efficiency depends on volume fraction of nanoparticles in composite structure, dispersion of nanoparticles, and size of particles. Many techniques such as shearing, mixing, ultrasonic homogenizer, and sol-gel technique are used for better dispersion of nanoparticles, some of which are in situ techniques. Recent research suggests that for fabrication of polymer nanocomposites, ultrasonic homogenizer is an effective tool. Development of environment-friendly processes and innovative methodologies to use biodegradable or recyclable materials is a new challenge for engineers in order to “go green.” Technical, environmental, and economic reasons are key issues while developing natural material as a replacement for synthetic fiber composites. Some of the beneficial properties of natural fibers are reduced solidity in comparison with synthetics, low density, and lower cost, which qualifies them for commercial applications. Fibers such as flax, jute, wood, bamboo, cotton, hemp, and sisal are studied for usage as reinforcement of polymeric matrices. In the recent years, natural fibers such as jute and sisal are acting as potential replacements of carbon and glass fibers owing to their lower cost and easier availability. Among all the reinforcement materials that are obtained naturally, jute appears as a promising material as it is available on commercial scale in every required form and is inexpensive as compared with others. The applications of natural fiber composites other than car industry are sports, office products, construction, boats, aerospace, machinery, and partition boards. Due to their vulnerability to environmental attacks like ultraviolet (UV) lights, the natural fiber composites are limited to panels, door frames, and other indoor components in civil applications. Green buildings are desired as healthy places to live and ecologically mindful place to work and live. In current times, as a part of green materials, biocomposites are considered as major materials. The mechanical

Nanocomposites297

properties of natural fiber composites are somewhat inferior. The increasing economic and environmental demands on use of reinforcements in load-bearing polymer structural materials have encouraged researchers for development of new reinforcement structures and materials. The reinforcement of epoxy composites is possible by using inorganic fillers that include special fibers and nanoparticles [41,42]. The properties can be enhanced if the composites are produced by hybrid formation. In a hybrid composite that contains two or more types of fiber, disadvantages of one type of fiber can be complemented by the other having consequent advantages. A proper material design leads to achievement of a balance in terms of performance and design. The combinations of cheaper basalt fiber as compared with glass fiber along with other natural fibers, which are available commercially, offer a potential for hybrid materials, which are inexpensive and remarkable in terms of performance. In recent times, basalt fiber, which is extracted from basaltoic rocks, has gained attention and is widely used in the field of composite reinforcement. Basalt along with carbon, polyamide, glass, and aramid fibers is used for enhancement of performance. Basalt fiber exhibiting high modulus, excellent resistance to fire, and resistance to acid/alkali exposure is successfully used in composite industry. The prominent advantages of basalt fiber are advanced mechanical properties and excellent heat resistance. It is evident from experiments that basalt can withstand temperatures as high as 600°C without affecting its mechanical properties and with no/negligible weight loss. Glass fiber, among fibers that are used currently for FRP, shows high sensitivity on exposure to alkaline conditions and is susceptible to surface damage. The carbon fiber that is chemically inert and stiffer in nature has a disadvantage of anisotropy and high cost. Synthetic fibers, mainly polymeric fibers, normally have lower melting point, lower elastic modulus, and poor interfacial bonding. Basalt fibers were investigated as reinforcement in composites with epoxy, polypropylene, or concrete or phenol-­formaldehyde resin matrix. The interfacial properties between fiber and matrix were studied in various basalt fiber polymer matrix systems. Researchers mentioned that after seawater treatment, the interfacial property of basalt fiber-reinforced epoxy composite was still better than that of glass fiber-reinforced epoxy composites. It is observed that the compressive and tensile strength of basalt fiber-reinforced epoxy composites is higher in comparison with glass fiber composites. In the past few decades, a comprehensive investigation was done on the role of inorganic fillers for enhancement of mechanical properties of polymer composites. It has been found specifically that the size of the particles plays a vital role for improvement of toughness and stiffness at the same time. It is assumed that a significant efficiency is achieved as particle size is reduced to nanoscale. Polymer nanocomposites that are polymers filled with nanoparticles are promising materials for various applications. Some research was conducted on the use of other materials as nanoparticles on basalt fabrics. An investigative study was carried out to use TiO2 nanoparticles on basalt fabric composites. The basalt hybrid and nonhybrid composites were fabricated using single ply of the reinforcing fabric. Epoxy resin was used as matrix (matrix comprising of both resin and nanoparticles) with a resin/hardener ratio of 100:32 (by weight) according to manufacturer recommendations. The prepared resin mixture was poured on fabric layers and spread out by a roller. The gentle rolling action confirmed the wetting of

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Nanotechnology in Textiles

fabrics, and the excess resin was squeezed out of the panel layup by the roller. The composite layup along with Teflon sheets was sandwiched between a pair of steel plates and cured at 120°C for 1.0 h in mechanical convection oven with predetermined weight. The fiber volume fraction (Vf) of all composites was maintained around 0.45. Specimens of desired dimensions were then cut from the sheets for various testings.

6.2.26 Nanoparticle size distribution and zeta potential For many varieties of nanoparticles, whether produced dry or wet, final applications are often in wet form. Analyzing nanoparticles in wet, concentrated slurry, or diluted suspension therefore is the natural choice. The instrument used in this study was a Zetasizer Nano ZS (Malvern Instruments, the United Kingdom) that was capable of both particle size analysis (using dynamic light scattering as the basic principle of operation) and zeta-potential measurement (using Doppler electrophoresis as the basic principle of operation). Malvern Instruments Dispersion Technology software (version 4.0) was used to control and analyze all data from the instrument. Before the measurements, the instrument was warmed up for 30 min in order to make sure that the instrument is calibrated and gives consistent results. For both, zeta potential and particle size, 0.01 g/L w/v solution was prepared. Nanoparticles were dispersed in distilled water by 10 min of rigorous ultrasonication. Size distribution of nanoparticles in the solution was then measured. For this measurement, 1 mL of solution was taken. Each sample was treated three times for better results. This instrument measures the particle size on dynamic light-scattering (DLS) technology. Light scatters at an angle, which is directly proportional to the size of the particle: larger particles scatter at lower angles, whereas smaller particles scatter at higher angles [43]. Zeta potential is the charge on a particle at the shear plane. It describes the amount of energy needed to transfer an electric charge between two points. It measures the potential difference across the boundaries between liquid and solid phases. In other words, zeta potential is the measure of electric charge of the solid particles that are held suspended in the liquid phase. This value of surface charge is useful for understanding and predicting interactions between particles in suspension. The higher the zeta potential, the higher is the stability of the solid-liquid interface. The value of zeta potential between +30 and −30 mV shows adequate stability of the solid particles in liquids. A value less than −15 mV shows the agglomeration of the particles. The instrument measures the zeta potential by laser Doppler microelectrophoresis technique. The particle motion due to the applied electric field is measured by light scattering.

6.2.27 UV absorption spectra The UV-vis absorption spectrum of basalt nanoparticles had been carried out at room temperature on UV-1600PC spectrophotometer to measure the UV absorption intensities of the samples. The UV absorption was carried out in liquid form. To obtain good results, it is necessary to keep the concentration of the solute (basalt nanoparticles) to a minimum in the solution. The solution was prepared with 0.01 g/L concentration [43,44]. The tensile and three-point bending tests of the nanocomposites were carried out by TIRAtest universal tensile tester according to standards EN-ISO 527-5 and EN-ISO

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Fig. 6.30  Basalt nanoparticle (A) size distribution and (B) Zeta potential [44].

14125, respectively. All tested specimens were adhered with aluminum tape to prevent failure at the grips as shown in Fig. 6.30. Young's modulus and tensile strength were evaluated under standard room-temperature conditions. Five specimens were tested for each composite in both warp and weft directions. Impact test (Charpy test) was carried out on an impact tester EN-ISO 14125 rectangular type using Ceast Resil 5.5 with a force of 22 J at a velocity of 2.9 m/s. The width and thickness of the specimen were measured and recorded. The work of fracture/impact strength values was calculated. The average values of five specimens for each sample in warp and weft directions have been reported. Dynamic mechanical analysis was performed on a DMA 40XT RMI equipment. The samples were tested using three-point bending mode at a frequency of 1 Hz in temperature scan mode. The DMA test was executed in the temperature range of 27–200°C at a heating rate of 3°C/min. This test was performed according to ENISO 6721-1. For each sample, five measurements were done.

6.2.28 Microscopic analysis/scanning electron microscopy SEM fractographs of the fractured specimens were used to reveal the mode of failure like fiber pullout, fiber/matrix debonding, matrix cracking, and fiber breaking. The fiber-matrix adhesion at the fracture surface of the composites was examined by scanning electron microscope TS5130-Tescan SEM at 20 kV accelerated voltage.

300

SEM MAG: 1.00 kx DET: BE Det + SE Det 100 um HV: 20.0 kV DATE: 08/26/15 Device: TS5130 VAC: HIVac

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SEM MAG: 5.00 kx VegaTescan HV: 20.0 kV TU Liberec VAC: HIVac

DET: BE Det + SE Det DATE: 08/26/15 Device: TS5130

20 um

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Fig. 6.31  SEM images of nanoparticles [44].

The nonconducting surface of the composites was coated with gold in agar auto sputter coater before being subjected to scanning electron microscope (SEM). A conducting gold layer was always evaporated onto the surface, and the specimens were fixed onto the metal sample holder using an electrically conductive adhesive in order to avoid the electrostatic charging of the samples. For investigation of morphology (shape and size) of the prepared nanoparticles, SEM images, with different magnifications from range of 1000–5000, were taken as shown in Fig. 6.31. Coating of samples was done with gold using plasma sputtering apparatus before SEM investigation. It has to be noted, however, that due to this gold layer, the size of the nanoparticles seems to be larger as the thickness of the gold coating amounts to several tens of nanometers [43,44].

6.2.29 Ultraviolet (UV) absorption spectra The results in Fig. 6.32 show a very narrow absorbance peak visible at a wavelength of 227 nm, which means that the basalt fibers can be utilized as a UV protector. It is visible in the figure that the effect of UV light even at high intensity is almost near to zero, and it has minor effects in a very short span of wavelength, that is, 225–250 nm.

6.2.30 Scanning electron microscope images of nanocomposites The distribution of the nanoparticles in the sample was examined by scanning electron microscopy (SEM) as shown in Fig. 6.33. One of the major challenges, when preparing polymer matrix nanocomposites, is homogeneous dispersion of nanofillers in a polymer. Agglomeration of nanoparticles (usually in micrometer size clumps) often gives adverse effects on the thermal and mechanical properties of the epoxy as smaller

Nanocomposites301 A

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Fig. 6.32  UV-vis absorption spectrum [44].

Fig. 6.33  SEM images of (A) B/B and (B) B/J nanocomposites [44].

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number of reinforcing particles are present and aggregates may act as defect centers; in case of failure, they are sometimes crack initiators. Hence, this does not represent the true properties of a desired nanocomposite. In this study, a uniform distribution of nanobasalt particles in composites was achieved. There was no agglomeration of the nanoparticles as shown in Fig. 6.33. Experiments were conducted for investigation of the effect of nanoparticle reinforcement on tensile properties of the composites. During formation of hybrid nanocomposites, mechanical interlocking between fiber and resin, due to frictional force, takes place. Frictional force is directly related and dependent upon the area of contact between the resin and the fibers. Nanoparticles undoubtedly have very high specific surface area. When these particles are dispersed inside the resin, the total surface area of contact between the particles and resin increases abruptly. This high contact area between the particles and the resin increases friction force between the two, but not really between matrix and fibers. It was found that the presence of nanoparticles enhances the tensile stress-strain behavior of the epoxy polymer. Nanocomposites exhibited higher tensile modulus (as measured at the initial slope of the graph). The increase in modulus is expected because the modulus of basalt is high. In addition, the homogeneous dispersion of these nanofillers in the matrix enhances the fracture resistance. As the tensile load increases, the matrix tries to elongate in its usual way. However, the nanofillers resist deformation. This is because of a lower extension at break, resulting from frictional restraint due to particle and resin interaction. This results in a smaller resultant deformation. Therefore, nanocomposites sustain more loads compared with the pure systems and contribute to a higher tensile modulus. Overall, as compared with B/J, increase of tensile moduli in B/B composites is not significant with respect to pure composites. Increase is