Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications (SpringerBriefs in Materials) 9819957796, 9789819957798

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SpringerBriefs in Materials Mariatti Jaafar · Ye Zar Ni Htwe

Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications

SpringerBriefs in Materials Series Editors Sujata K. Bhatia, University of Delaware, Newark, DE, USA Alain Diebold, Schenectady, NY, USA Juejun Hu, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA Kannan M. Krishnan, University of Washington, Seattle, WA, USA Dario Narducci, Department of Materials Science, University of Milano Bicocca, Milano, Italy Suprakas Sinha Ray , Centre for Nanostructures Materials, Council for Scientific and Industrial Research, Brummeria, Pretoria, South Africa Gerhard Wilde, Altenberge, Nordrhein-Westfalen, Germany

The SpringerBriefs Series in Materials presents highly relevant, concise monographs on a wide range of topics covering fundamental advances and new applications in the field. Areas of interest include topical information on innovative, structural and functional materials and composites as well as fundamental principles, physical properties, materials theory and design. Indexed in Scopus (2022). SpringerBriefs present succinct summaries of cutting-edge research and practical applications across a wide spectrum of fields. Featuring compact volumes of 50 to 125 pages, the series covers a range of content from professional to academic. Typical topics might include . A timely report of state-of-the art analytical techniques . A bridge between new research results, as published in journal articles, and a contextual literature review . A snapshot of a hot or emerging topic . An in-depth case study or clinical example . A presentation of core concepts that students must understand in order to make independent contributions Briefs are characterized by fast, global electronic dissemination, standard publishing contracts, standardized manuscript preparation and formatting guidelines, and expedited production schedules.

Mariatti Jaafar · Ye Zar Ni Htwe

Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications

Mariatti Jaafar School of Materials and Mineral Resources Engineering Universiti Sains Malaysia Engineering Campus Nibong Tebal, Penang, Malaysia

Ye Zar Ni Htwe School of Materials and Mineral Resources Engineering Universiti Sains Malaysia Engineering Campus Nibong Tebal, Penang, Malaysia

ISSN 2192-1091 ISSN 2192-1105 (electronic) SpringerBriefs in Materials ISBN 978-981-99-5779-8 ISBN 978-981-99-5780-4 (eBook) https://doi.org/10.1007/978-981-99-5780-4 © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Flexible and wearable printed strain sensors have received considerable interest and achieved rapid development in the past years due to their extensive and important applications in electronics skins, human–machine interaction, health monitoring, underwater detection, and other fields. Innovative materials, which vary from organic semiconductors to quantum dots and from carbon nanomaterials to conductive inks, are a crucial enabler component of many printed flexible electronic devices. The materials for printed flexible electronics must combine electronic and semiconductor functionality by being flexible, stretchable, wearable, and/or solution processable as well as being stable and straightforward to manufacture. This results in substantial technical challenge and encouraging broad innovation in the study. Nanomaterials-based printed strain sensor for wearable health monitoring applications serves as a systematic reference guide or textbook that aims to help students, researchers, and engineers of the present day understanding the state of printed flexible strain sensor and potential future trends. It also provides the reader with how to use materials and printed manufacturing process in the design of printed flexible strain sensor and how to fabricate a cross-functional strategy that integrates materials, printing, electronics, and procedure. This book provides a comprehensive introduction on basic principle of printed flexible electronics, printing technologies, formulations of conductive inks, characterization method, and fabrication process. Information on a wide range of nanomaterials used for printed flexible electronics for wearable printed electronics is introduced in the book. Finally, the state of advanced manufacturing of printed wearable strain sensor is discussed, along with its possibilities. We would like to express our gratitude to our family and friends for their support and encouragement. We would like to acknowledge the Universiti Sains Malaysia and Ministry of Higher Education, Malaysia for research fundings that supported most of our research works related to the nanomaterials based printed strain sensor.

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We are highly thankful to Springer Nature team for their generous cooperation at every stage of the book preparation and production. Nibong Tebal, Malaysia

Prof. Ir. Dr. Mariatti Jaafar Dr. Ye Zar Ni Htwe

Contents

1 Printed Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Conductive Ink Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Fabrication Methods of Printed Strain Sensor . . . . . . . . . . . . . . . . . . . . 1.4 Substrate for Printed Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 3 5 7 8 8

2 Carbon Based Printed Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Graphene Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 13 14 17 21 24 24

3 Silver Nanoparticles-Based Printed Strain Sensor . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Silver Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Silver Nanoparticles with Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Silver Nanoparticles with Conductive Polymer . . . . . . . . . . . . . . . . . . . 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29 29 29 34 37 39 39

4 Composites and Hybrid Based Printed Strain Sensor . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Carbon and Metal Based Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Carbon and Polymer Based Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . 4.4 Metal and Polymer Based Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 41 41 44 51 53 53

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5 Performance Evaluation of Strain Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Stretchability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 57 57 59 60 60 61 62 62

6 Printed Strain Sensor for Wearable Health Monitoring Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Body Motion Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Human–Machine Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Personal Healthcare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Printed Strain Sensor

Abstract Wearable printed strain sensor is attracting wide attention due to their extensive potential applications in various industries. In this chapter, the general information of printed strain sensor and their requirements are provided. Moreover, different types of conductive inks and substrate that commonly been used printed strain sensors are discussed. In addition, the printed fabrication processes used to fabricate the strain sensors are explained.

1.1 Introduction Wearable and flexible electronics for human or robot bodies are becoming increasingly popular for their potential uses in human healthcare monitoring, soft robotics, biomedical engineering, human–computer interaction, and electronic skin (Lin et al. 2021; Yan et al. 2021; Yoon et al. 2021). Figure 1.1 shows the requirement of wearable strain sensor and its applications. Among the various type of wearable sensor, strain sensors have gained popularity because of their easy interaction with the human body. Strain sensor is a device that can effectively convert the strain generated by external stimulation into electrical signal output (Song et al. 2021). It is mainly used to measure mechanical deformation on an object by varying its electrical properties. Traditional strain sensors were fabricated by metal or semiconducting piezoresistors (Ghumatkar et al. 2016). Generally, these sensors are fragile and rigid in nature which is not suitable for wearable printed electronics devices. These sensors exhibited low strain range (less than 5%) and low sensitivity. However, as in the case of wearable printed electronics applications, strain sensor must possess high stretchability (higher than 50%), high sensitivity, and high durability to accommodate multiscale and dynamic deformations induced by human activities (Souri et al. 2020; Lee et al. 2021; Yoon et al. 2021). The wearable strain sensors have been developed based on the various principles, including resistivity, capacitance, piezoelectricity, and inductance. Specially, the resistive-type strain sensors were widely studied due to the simple structures and the excellent comprehensive sensing performances (Jiao et al. 2021; Lin et al. 2021). The sensing performance of the printed strain sensor © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_1

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Fig. 1.1 Requirements of wearable strain sensor and their potential applications (Fu et al. 2023; Li et al. 2022a, b)

are classified from many aspects, such as linearity, strain range, hysteresis, stability, durability, sensitivity, and response time between strain and relative change in resistance (Torrisi and Carey 2018). Gauge factor (GF) is used to evaluate the sensitivity. GF value is calculated based on the equation; GF = (∆R/R0 )/ε, where ∆R is the resistance variation, R0 is the initial resistance, and ε is the mechanical strain (Fu et al. 2019). A typical wearable printed strains sensor is mainly composed of two essential components: conductive ink and flexible polymer substrate (Htwe and Mariatti 2022). Conductive inks have emerged and developed as a novel and interesting solution to the development of printed wearable strain sensor applications. Basically, conductive inks represent the dispersion of conductive fillers in an appropriate solvent or mixture of solvents, comprising stabilizers such as binder agent, surfactants, or polymer (Kamyshny and Magdassi 2019; Mehmood et al. 2020; Singh et al. 2021). The conductive material is considered as the most important component of conductive inks. Among the various nanomaterial-based conductive inks, carbon-based, metalbased, and their hybrid and composites based conductive inks have been observed to

1.2 Conductive Ink Materials

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have immerse potential for use in printed wearable strain sensor applications (Gao et al. 2017; Torrisi and Carey 2018). Besides the conductive ink, the flexible substrates are also important for the printed strain sensor. Among the flexible substrate for printed strain sensor, the most widely used are various polymeric films such as polyimide (PI), polyethylene terephthalate (PET), polyvinyl alcohol (PVA), poly(dimethysiloxane) (PDMS) and polyurethane (PU), which are excellent for large scale R2R production of strain sensor devices due to their high mechanical properties and easy processability (Li et al. 2022a; Lin et al. 2022; Wallbrink et al. 2022; Zhang et al. 2022). In addition, fabrication of the printed strain sensor is one of the foremost indentations for learn manufacturing. Various types of fabrication techniques have been developed for the printed strain sensor. The common methods currently used for the fabrication of strain sensors are vacuum deposition, photolithography, and spin-coating. However, these methods show some drawbacks, as they were mainly developed for the production of microelectronics, and involved multiple fabrication steps, high processing temperatures and produce toxic waste for a single-layer production (Yim et al. 2017; Miao et al. 2018; Gao et al. 2019). In recent years, different types of direct printing techniques such as inkjet printing, direct writing, screen printing have received increasing attention for the fabrication of printed strain sensor (Anderson et al. 2019; Türkmen and Acer Kalafat 2022; Wang et al. 2022).

1.2 Conductive Ink Materials Due to the latest advances in the field of printed strain sensor, nanomaterials-based conductive ink materials have emerged as very promising elements with high electrical conductivity and lustrous surfaces which may be explored for the development of different printed wearable electronics devices (Yan et al. 2018; Htwe et al. 2020). The conductive ink materials are one of the most important requirements to develop a good performance and efficient strain sensor. In the last few years some conductive ink materials were developed for the fabrication of strain sensors which are more sensitive and performed high flexibility (Zhang et al. 2020). These conductive ink materials including carbon based (graphene and carbon nanotubes (CNTs)), metal nanoparticles based (silver, gold, and copper), conductive polymer and in certain cases the hybrid or composites are used (refer to Fig. 1.2) (Huang et al. 2021; Htwe et al. 2022; Liu et al. 2018b). The choice of the conductive ink materials is depending highly on the desired physical characteristics of the printed strain sensor devices. These include conductivity, high patterning, stability, and durability of the printed strain sensor design to perform mechanical testing which is especially a requisite of wearable electronics applications (Robert 2015; Mau Dang et al. 2019; Franco et al. 2020). Comparison of typical conductive inks materials is shown in Table 1.1. Among the various types of conductive inks materials, carbon-based materials are popular due to the excellent mechanical properties, higher electrical and thermal conductivity, and environmental

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Fig. 1.2 Conductive ink materials for printed strain sensor (Huang et al. 2021; Htwe et al. 2022; Liu et al. 2018b)

stability. Graphene and CNTs are most widely used for printed strain sensor fabrication. Graphene is the single layer of carbon atoms in the 2-D planar structure. It has demonstrated superior mechanical and electrical properties is thus considered one of the most promising materials in printed wearable strain sensor (Varghese et al. 2015; Liu et al. 2022). Other than graphene, CNTs is a cylindrical or tubular form of the single or multi-layer concentric carbon or graphene sheets, named single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT), respectively. Due to their superior mechanical, electrical, and optical qualities, including their high strength, great flexibility, high current capacity, vast surface area, and metal/ semiconductor features, CNTs are also attracting more interest. However, compared to graphene, the dispersion of CNTs in liquids is an interactable issue due to van der Waals aggregation, which poses a significant obstacle to the manufacturing of printed strain sensor electrodes (Niu et al. 2021; Xue et al. 2022). Metal nanoparticles are widely used in the printed strain sensor for conductive electrode. Due to the high surface-to-volume ratio, their post-sintering temperature is much lower than the corresponding bulk materials (Benson et al. 2015; Tan et al. 2019; Yokoyama et al. 2020). For example, the sintering temperature of silver nanoparticles is 150–250 °C, much less than that of bulk silver materials (higher than 700 °C). In this reason, it effectively relieves the problem of high-temperature resistance of flexible substrates. Currently, silver (Ag), gold (Au), and copper (Cu) nanoparticles are the three most common metal nanoparticles used as conductive inks materials.

1.3 Fabrication Methods of Printed Strain Sensor

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Table 1.1 Comparison of typical conductive inks materials Conductive ink materials

Advantages

Limitations

Carbon based

Graphene

High mechanical properties, biocompatibility, chemical stability, high surface area

Still need to enhance electrical conductivity

CNTs

Higher immunity to oxidation, thermal stability, low resistivity and reasonably priced

Chiral problems of single-walled CNTs

AgNPs

Good oxidation resistance, Excellent electrical conductivity

Cost issue, easy to form a void defect, low mechanical properties

AuNPs

Superior stability and intertness

Expensive

CuNPs

Low cost

Oxidation issue

Metal based

AgNPs is frequently the materials of choice for the preparation of highly conductive materials on account of better stability and lower cost than AuNPs. Although, CuNPs is generally cheaper alternative to those of AgNPs and AuNPs, CuNPs have tendency to oxidize in ambient conditions (de Medeiros et al. 2021; Jiang et al. 2021; Niu et al. 2021).

1.3 Fabrication Methods of Printed Strain Sensor The printed wearable strain sensor have been applied in various applications such as in biomedical, environmental, and construction (Li et al. 2022b). The fabrication and improvement of the printed strain sensor have been done on specific factors such GF, flexibility, durability, and detective ranges, are some of the parameters that need to be considered before to the fabrication process (Huang and Zhu 2019). Several printing techniques for the fabrication of conductive material layers printed onto the flexible substrate are reported vis direct printing including inkjet printing, screen printing, gravure printing, direct writing (Khan et al. 2015; Liu et al. 2021). The schematic diagram shown in Fig. 1.3 and Table 1.2 show comparison and summarized of each printing technique in terms of resolution, key features, and limitations (Liang et al. 2016; Liu et al. 2018a; Zavanelli and Yeo 2021). These printing techniques are attractive strategies for processing large-scale electronics devices with less cost and high outcome, especially for large area printed electronics devices. As compared with the fabrication process of spin dip coating, spin coating, and drop casting, printing process would dramatically reduce materials waste and increase the production efficiency and enhance design capability. Screen-printing technique is a versatile and unique printing technique for printing conductive ink materials on the flexible substrate. Screen-printing approach is lesscost fabricating technique compared to conventional printing process. Therefore,

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Fig. 1.3 Schematic of printing techniques for fabricating printed strain sensor (Liang et al. 2016; Liu et al. 2018a; Zavanelli and Yeo 2021)

Table 1.2 Comparison and summary of different printing techniques (Gao et al. 2017; Kamyshny and Magdassi 2019; Wiklund et al. 2021) Printing techniques

Printing resolution

Key features

Limitations

Screen printing

40–150 μm

High aspect ratio of printed structure, less cost

Patterned stencils are needed for arch design

Inkjet printing

Tens of μm

High printing speed

Nozzle clogging, limited range of viscosity of the ink (0.005–0.02 Pa.s)

Gravure printing

2–50 μm

High printing speed, compatibility to the roll-to-roll printing process, low cost

Limited range of viscosity of the ink (0.1–1 Pa.s)

Direct writing

Several μm

Diverse printable

Nozzle clogging

screen-printing technique is a suitable process for the development of cost-effective printed strain sensor (Hyun et al. 2015; Grau et al. 2016). Inkjet printing is a noncontact printing technique without the need for any external tools and contact with the flexible substrate. It is also can directly produce pattern design without any masks and it can be considered as an alternative to lift-off process.

1.4 Substrate for Printed Strain Sensor

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By the inkjet printing method, the precise amounts of a wide variety of materials in the form of colloidal or chemical solutions are deposited through a micrometre-sized inkjet nozzle head forming conductive lines or single droplets on various substrates materials (Moonen et al. 2012; Grau et al. 2016; Liang et al. 2016; Liu et al. 2018a; Khosravani and Reinicke 2020; Zavanelli and Yeo 2021). The gravure printing is widely used printing technique based on transfer of the ink to the substrate via the small, engraved cavities in the engraving cylinder. The viscosity of diluted gravure inks is higher than inkjet inks, and it is between 0.01 and 0.2 Pa.s. The biggest obstacle encountered by the gravure printer is high-resolution print line, i.e., less than 20 μm. The inability to produce uniform structures with sharp edge pattern lines restricts the gravure printing to be able to manufacture top layers on electronic devices (Baeg et al. 2013; Feng et al. 2018). In the direct writing technique, air pressure is used to directly extrude materials onto substrates through a nozzle. Either inks with low viscosity or inks exhibiting reduced viscosity at the increasing shear rate (shear-thinning materials) can be printed by direct writing. The viscosity of the ink for direct writing ranges from 10 to 105 Pa.s at different shear rates (Roh et al. 2017; Senthil Kumar et al. 2019; Zhang et al. 2019).

1.4 Substrate for Printed Strain Sensor Substrates are important role in the design of printing experiments because their physical, thermal, mechanical, and electrical properties are determined by the target application. Polymer substrates are the most common used substrate because they are lightweight, conformable, less cost, and ideal for printer strain sensor applications (Roh et al. 2017; Senthil Kumar et al. 2019; Zhang et al. 2019). The main polymeric substrates for the printed strain sensors are polyethylene terephthalate (PET), polyimide (PI), polycarbonate (PC), polyurethane (PU) and poly (vinyl alcohol) (PVA). According to previous works, the polymeric substrates are excellent substrates for printed electronics devices due to their high mechanical properties, easy processing, and high resistance to oxygen and water penetration (Zarek et al. 2016; Shin and Park 2018; Yang et al. 2018). Furthermore, poly (dimethylsiloxane) (PDMS) is also used for high stretchability printed strain sensor. The important issues in utilization of polymer as substrates for printed electronics is their low thermal stability (Harris et al. 2016; Kamyshny and Magdassi 2019). The glass transition temperature (Tg ) of the most common polymer films is less than 150 °C, (Example. 60–80 °C for PET, 120–125 °C for PEN, 70–80 °C for PU, 80– 85 °C for PVA, and 140–150 °C for PC. An important exception is PI with Tg in the range of 310–365 °C (360–410 °C for Kapton film, which is a commercial product of DuPont). Moreover, sintering polymer substrate above these temperatures for an extended time results in its irreversible deformation and even damage that leads to deterioration of printed device performance (Ghoshal 2017; Gonzalez et al. 2017). Nevertheless, obtaining high electrical conductivity of printed electrode, especially in the case of metal nano particles as conductive electrode materials, usually requires

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their sintering at elevated temperatures to remove the insulting organic dispersants, which prevent the formation of a continuous conductive structure upon drying of the solvent in the ink. The reduction in the sintering temperature can be achieved by a decrease in the nanoparticles size, selection of an appropriate stabilizing agent and its content in the ink (Niu et al. 2021; Zhou et al. 2022).

1.5 Summary In this chapter, an overview regarding the development of printed strain sensor for wearable electronics applications is reported. Conductive ink materials have emerged as a simple, cost-effective, and effective alternative in the manufacture of printed strain sensor. The different types of printing techniques and polymer substrates in the fabrication of printed strain sensor are discussed.

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Li S, Xu R, Wang J, Yang Y, Fu Q, Pan C (2022a) Ultra-stretchable, super-hydrophobic and highconductive composite for wearable strain sensors with high sensitivity. J Colloid Interface Sci 617:372–382. https://doi.org/10.1016/j.jcis.2022.03.020 Li Y, Liu Y, Bhuiyan SRA, Zhu Y, Yao S (2022b) Printed strain sensors for on-skin electronics. Small Struct 3:2100131. https://doi.org/10.1002/sstr.202100131 Liang J, Tong K, Pei Q (2016) A water-based silver-nanowire screen-print ink for the fabrication of stretchable conductors and wearable thin-film transistors. Adv Mater 28:5986–5996. https:// doi.org/10.1002/adma.201600772 Lin L, Choi Y, Chen T, Kim H, Lee KS, Kang J, Lyu L, Gao J, Piao Y (2021) Superhydrophobic and wearable TPU based nanofiber strain sensor with outstanding sensitivity for high-quality body motion monitoring. Chem Eng J 419:129513. https://doi.org/10.1016/j.cej.2021.129513 Lin L, Park S, Kim Y, Bae M, Lee J, Zhang W, Gao J, Paek SH, Piao Y (2022) Wearable and stretchable conductive polymer composites for strain sensors: How to design a superior one? Nano Mater Sci. https://doi.org/10.1016/j.nanoms.2022.08.003 Liu C, Huang N, Xu F, Tong J, Chen Z, Gui X, Fu Y, Lao C (2018a) 3D printing technologies for flexible tactile sensors toward wearable electronics and electronic skin. Polymers (basel) 10:1–31. https://doi.org/10.3390/polym10060629 Liu H, Li Q, Zhang S, Yin R, Liu X, He Y, Dai K, Shan C, Guo J, Liu C, Shen C, Wang X, Wang N, Wang Z, Wei R, Guo Z (2018b) Electrically conductive polymer composites for smart flexible strain sensors: a critical review. J Mater Chem C 6:12121–12141. https://doi.org/10.1039/C8T C04079F Liu H, Zhang H, Han W, Lin H, Li R, Zhu J, Huang W (2021) 3D printed flexible strain sensors: from printing to devices and signals. Adv Mater 33:1–19. https://doi.org/10.1002/adma.202004782 Liu T, Zhao J, Luo D, Xu Z, Liu X, Ning H, Chen J, Zhong J, Yao R, Peng J (2022) Inkjet printing high performance flexible electrodes via a graphene decorated Ag ink. Surf Interfaces 28:101609. https://doi.org/10.1016/j.surfin.2021.101609 Mau Dang C, Kim Huynh K, My Thi Dang D (2019) Influence of solvents on characteristics of inkjet printing conductive ink based on nano silver particles. Sci Stay True Here Biol Chem Res 6:126–134 Mehmood A, Mubarak NM, Khalid M, Walvekar R, Abdullah EC, Siddiqui MTH, Baloch HA, Nizamuddin S, Mazari S (2020) Graphene based nanomaterials for strain sensor application—a review. J Environ Chem Eng 8:103743. https://doi.org/10.1016/j.jece.2020.103743 Miao J, Liu H, Li Y, Zhang X (2018) Biodegradable transparent substrate based on edible starch-chitosan embedded with nature-inspired three-dimensionally interconnected conductive nanocomposites for wearable green electronics. ACS Appl Mater Interfaces 10:23037–23047. https://doi.org/10.1021/acsami.8b04291 Moonen PF, Yakimets I, Huskens J (2012) Fabrication of transistors on flexible substrates: from mass-printing to high-resolution alternative lithography strategies. Adv Mater 24:5526–5541. https://doi.org/10.1002/adma.201202949 Niu B, Yang S, Tian X, Hua T (2021) Highly sensitive and stretchable fiber strain sensors empowered by synergetic conductive network of silver nanoparticles and carbon nanotubes. Appl Mater Today 25:101221. https://doi.org/10.1016/j.apmt.2021.101221 Robert T (2015) “green ink in all colors”—printing ink from renewable resources. Prog Org Coatings 78:287–292. https://doi.org/10.1016/j.porgcoat.2014.08.007 Roh S, Parekh DP, Bharti B, Stoyanov SD, Velev OD (2017) 3D printing by multiphase silicone/ water capillary inks. Adv Mater 29:1–7. https://doi.org/10.1002/adma.201701554 Senthil Kumar K, Chen P-Y, Ren H (2019) A review of printable flexible and stretchable tactile sensors. Research 2019:1–32. https://doi.org/10.34133/2019/3018568 Shin IJ, Park MS (2018) Direct conductive patterning on 3d printed structure using laser. Phys Status Solidi Appl Mater Sci 215:1–9. https://doi.org/10.1002/pssa.201700597 Singh K, Sharma S, Shriwastava S, Singla P, Gupta M, Tripathi CC (2021) Significance of nanomaterials, designs consideration and fabrication techniques on performances of strain sensors—a review. Mater Sci Semicond Process 123. https://doi.org/10.1016/j.mssp.2020.105581

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Song P, Wang G, Zhang Y (2021) Preparation and performance of graphene/carbon black silicone rubber composites used for highly sensitive and flexible strain sensors. Sens Act A Phys 323. https://doi.org/10.1016/j.sna.2021.112659 Souri H, Banerjee H, Jusufi A, Radacsi N, Stokes AA, Park I, Sitti M, Amjadi M (2020) Wearable and stretchable strain sensors: materials, sensing mechanisms, and applications. Adv Intell Syst 2:2000039. https://doi.org/10.1002/aisy.202000039 Tan HW, An J, Chua CK, Tran T (2019) Metallic nanoparticle inks for 3d printing of electronics. Adv Electron Mater 5. https://doi.org/10.1002/aelm.201800831 Torrisi F, Carey T (2018) Graphene, related two-dimensional crystals and hybrid systems for printed and wearable electronics. Nano Today 23:73–96. https://doi.org/10.1016/j.nantod.2018.10.009 Türkmen D, Acer Kalafat M (2022) Improving inkjet printed flexible angle sensor performance by compression–tension sensor pair compensation. Sens Act A Phys 345:113752. https://doi.org/ 10.1016/j.sna.2022.113752 Varghese SS, Lonkar S, Singh KK, Swaminathan S, Abdala A (2015) Recent advances in graphene based gas sensors. Sens Act B Chem 218:160–183. https://doi.org/10.1016/j.snb.2015.04.062 Wallbrink C, Hughes JM, Kotousov A (2022) Application of an advanced piezoelectric strain sensor for crack closure measurement. Int J Fatigue 167:107286. https://doi.org/10.1016/j.ijfati gue.2022.107286 Wang X, Zhang M, Zhang L, Xu J, Xiao X, Zhang X (2022) Inkjet-printed flexible sensors: from function materials, manufacture process, and applications perspective. Mater Today Commun 31:103263. https://doi.org/10.1016/j.mtcomm.2022.103263 Wiklund J, Karakoç A, Palko T, Yigitler H, Ruttik K, Jäntti R, Paltakari J (2021) A review on printed electronics: fabrication methods, inks, substrates, applications and environmental impacts. J Manuf Mater Process 5. https://doi.org/10.3390/jmmp5030089 Xue SS, Tang ZH, Bin ZW, Li YQ, Huang P, Fu SY (2022) Stretchable and ultrasensitive strain sensor from carbon nanotube-based composite with significantly enhanced electrical and sensing properties by tailoring segregated conductive networks. Compos Commun 29:100987. https:// doi.org/10.1016/j.coco.2021.100987 Yan T, Wang Z, Pan ZJ (2018) Flexible strain sensors fabricated using carbon-based nanomaterials: a review. Curr Opin Solid State Mater Sci 22:213–228. https://doi.org/10.1016/j.cossms.2018. 11.001 Yan T, Wu Y, Yi W, Pan Z (2021) Recent progress on fabrication of carbon nanotube-based flexible conductive networks for resistive-type strain sensors. Sens Act A Phys 327:112755. https://doi. org/10.1016/j.sna.2021.112755 Yang H, Leow WR, Chen X (2018) 3D printing of flexible electronic devices. Small Methods 2:1–7. https://doi.org/10.1002/smtd.201700259 Yim C, Kockerbeck ZA, Jo SB, Park SS (2017) Hybrid copper-silver-graphene nanoplatelet conductive inks on PDMS for oxidation resistance under intensive pulsed light. ACS Appl Mater Interfaces 9:37160–37165. https://doi.org/10.1021/acsami.7b10748 Yokoyama S, Nozaki J, Motomiya K, Tsukahara N, Takahashi H (2020) Strong adhesion of polyvinylpyrrolidone-coated copper nanoparticles on various substrates fabricated from welldispersed copper nanoparticle inks. Coll Surf A Physicochem Eng Asp 591:124567. https://doi. org/10.1016/j.colsurfa.2020.124567 Yoon S, Kim YJ, Lee YR, Lee NE, Won Y, Gandla S, Kim S, Kim HK (2021) Highly stretchable metal-polymer hybrid conductors for wearable and self-cleaning sensors. NPG Asia Mater 13.https://doi.org/10.1038/s41427-020-00277-6 Zarek M, Layani M, Cooperstein I, Sachyani E, Cohn D, Magdassi S (2016) 3D printing of shape memory polymers for flexible electronic devices. Adv Mater 28:4449–4454. https://doi.org/10. 1002/adma.201503132 Zavanelli N, Yeo WH (2021) Advances in screen printing of conductive nanomaterials for stretchable electronics. ACS Omega 6:9344–9351. https://doi.org/10.1021/acsomega.1c00638

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Zhang Y, Shi G, Qin J, Lowe SE, Zhang S, Zhao H, Zhong YL (2019) Recent progress of direct ink writing of electronic components for advanced wearable devices. ACS Appl Electron Mater 1:1718–1734. https://doi.org/10.1021/acsaelm.9b00428 Zhang P, Chen Y, Li Y, Zhang Y, Zhang J, Huang L (2020) Erratum: a flexible strain sensor based on the porous structure of a carbon black/carbon nanotube conducting network for human motion detection Sensors 20, 1154. https://doi.org/10.3390/s20041154. Sensors (Switzerland) 20. https://doi.org/10.3390/s20102901 Zhang Y, Lin H, Zhang L, Peng S, Weng Z, Wang J, Wu L, Zheng L (2022) Mechanical exfoliation assisted with carbon nanospheres to prepare a few-layer graphene for flexible strain sensor. Appl Surf Sci 611:155649. https://doi.org/10.1016/j.apsusc.2022.155649 Zhou J, Men D, Zhang XE (2022) Progress in wearable sweat sensors and their applications. Chinese J Anal Chem 50:87–96. https://doi.org/10.1016/j.cjac.2021.11.004

Chapter 2

Carbon Based Printed Strain Sensor

Abstract Wearable printed strain sensor is attracting wide attention due to their extensive potential applications. In this chapter, general information of carbon-based materials such as graphene, carbon nanotubes (CNTs) and graphene oxide (GO) is provided. Factors that govern the electrical properties of carbon-based conductive inks such as the dispersion and stability are discussed. In addition, the fabrication process and substrates used for printed strain sensor are explained.

2.1 Introduction Carbon-based nanomaterials based conductive inks cover a various nanoscale dimension, including two-dimensional graphene, one-dimensional carbon nanotubes (CNTs). Carbon-based conductive inks are popular due to their high mechanical, electrical, electrical, and environmental stability (Fu et al. 2023). Graphene and CNTs are commonly studied for printed conductive inks-based strain sensor applications. Graphene is the single layer of carbon atoms in the two-dimensional planar structure. It is one of the most promising materials for flexible electronics since it has shown exceptional electrical and mechanical properties (Mehmood et al. 2020). However, a shortage of functional groups makes the development of graphene dispersions challenging, and oxygen-containing graphene oxide (GO) is utilised for the preparation of conductive inks. Because GO is not electrically conductive, it cannot be used to make conductive electrodes because the dispersibility was acquired at the expense of conductivity. The performance of the conductive electrodes is dependent on how much of the GO is reduced to create reduced graphene oxide (rGO) (Iqra et al. 2022). The single-walled (SWCNT) and multi-walled (MWCNT) carbon nanotubes are the cylindrical or tubular forms of the single or multi-layer concentric carbon atoms or graphene sheets, respectively. The qualities of the CNTs, such as whether they are metallic or semiconducting, are determined by chirality along the graphene sheets. The CNTs’ electrical properties is comparable to the metal based conductive inks. However, the defects occurred during the synthesis increase the resistivity of the CNTs (Yan et al. 2018). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_2

13

14

2 Carbon Based Printed Strain Sensor

2.2 Graphene In the past few years, the carbon-based conductive inks materials have been the source of excitement in the scientific community, due to its peculiar structural properties in one-, two-, and three-dimensional arrangements and high figures of merit (Afsarimanesh et al. 2020; Singh et al. 2021). Graphene is one of the widely used 2D layered materials and it is an allotrope of carbon. It is a single layer of carbon atoms arranged in strongly bound hexagon rings having sp2 hybrid C–C bonding (Mehmood et al. 2020). In the previous year, graphene has gained attention regarding the development of printed strain sensor due to its superior properties mainly due to high flexibility, tuneable 2D structure, excellent electromechanical properties and lightweight (Hasan and Hossain 2021). Nowadays, researchers from all around the world are working in the development of printed electronics devices using graphene and its composites. Research scientists are using different forms of graphene for fabricating strain sensor with different substrates (Fu et al. 2023). Therefore, the use of high-quality, defect-free graphene as a conductive material is widely preferred, particularly as a conductive ink for flexible printed strain sensor. According to previous studies, conductive inks-based graphene or its derivates often face several difficulties that need to be properly addressed. First, graphene inks and its derivatives are resistant to precipitation to produce constant output and uniform conductive patterns. Second, given the proper viscosity and surface tension, the produced ink should have the required fluid characteristics (Lv et al. 2021). There are many types of solvents that can be used with conductive inks-based graphene or its derivates, including liquid-based solvents like N,N-dimethylformamide (DMF), Nmethyl-2-pyrrolidone (NMP), terpineol, ethanol, isopropyl alcohol, ethylene glycol, glycerol, ethylene acetate (Franco et al. 2021; Zhang et al. 2021). Through liquid phase graphite exfoliation in NMP, Torrisi et al. (2012) developed a highly stable graphene ink and then printed thin film using the conductive inks. According to reports, they prepared conductive pattern with 80% transparency, 30 kΩm−1 sheet resistance. Another investigation into the printing of graphene inkjet on flexible substrates used conductive inks based on various solvent types (ethanol, DMF, and NMP). The prepared conductive traces, which had transparent graphene conductive circuits with a relatively low sheet resistance of 260 Ω/sq. at 160 nm thickness, was annealed at 350 °C for 150 min. Previously, Torrisi et al. (2012) developed high conductive, less-cost, and highly flexible conductive traces by using graphene conductive inks with screen-printing process. The high electrical properties were found from the prepared graphene conductive traces. Typically, graphene conductive inks based on different solvent types and printing techniques have been taken into consideration in previous research. However, there are some solvents such NMP and DMF that are harmful, have a high boiling point, and are not eco-friendly. On the other hand, the surface tension is typically inappropriate when graphene is dispersed in low-boiling-point solvents such acetone, isopropanol, and ethanol (Tran et al. 2018). Therefore, the research trend is shifting towards reducing the usage of organic solvents and looking into the

2.2 Graphene

15

possibilities of replacing them with eco-friendly alternatives in the formulations of conductive inks, based on the significance of environmentally friendly conductive ink for both humans and the environment. The use of proper, ecologically friendly solvent is important for the fabrication of conductive inks. Since it is non-toxic and has a low boiling point, water is the most chosen solvent, despite having a high surface tension of 72.8 mJm−2 (Franco et al. 2020). Nonetheless, due to the hydrophobic nature of graphite carbon, dispersing graphene in water is particularly challenging. Therefore, surfactants are widely used to improve graphene dispersion in conductive inks through van der Waals force, hydrogen bonding, electrostatic activity, and interactions between π–π. Recently, Htwe and Mariatti (2021) reported surfactant assisted water-based graphene conductive inks by using different types of surfactants such as (sodium dodecyl sulfate (SDS), polyvinylpyrrolodone (PVP), and Gum Arabic (GA). Figure 2.1 shows the fabrication process of surfactant assisted water-based graphene conductive inks. The study showed that PVP surfactant is better that SDS and GA in terms of stability, wettability, and electrical conductivity. Furthermore, the fabrication of flexible printed strain sensor is highly important for the preparation of conductive inks. Casiraghi et al. (2018) investigated waterbased graphene inks strain sensor by inkjet printing process with different printing parameters (i.e., drop-spacing, number of printing passes, etc.). The inkjet printed strain sensor showed good gauge factor close to 150 with large sensitivity (> 20%) even when small strains (0.3%) are applied. The schematic sensor configuration of

Fig. 2.1 Fabrication process of surfactant assisted water-based graphene conductive inks (Htwe and Mariatti 2021)

16

2 Carbon Based Printed Strain Sensor

printed sensor is shown in Fig. 2.2a. The printed graphene strain sensors showed a fast, significant change in electrical resistance under tensile and compressive strains as they were presented. The printed sensor was also used by the authors as a variable resistance in an electrical circuit to control an LED’s luminance when it was subjected to compressive or tensile loads (Fig. 2.2b–d). Tseng et al. (2021) developed sensing glass deformation by graphene-based screen-printed strain sensor. Ultrafast laser direct writing process were used to fabricate graphene strain sensor with different grid lengths (6, 8, 10 mm). The strain sensor with a grid length of 6 mm showed the highest gauge factor (550.14) and performed the highest sensitivity. Marra et al. (2021) reported the effect of water as the cosolvent with NMP in LPE for improving the yield and quality of graphene demonstrating how highquality graphene can lead to higher performance paper-based strain sensors. The gauge factor of the printed sensor is 1.41 in low strain range less than 0.1%. Lynch et al. (2020) fabricated graphene-based screen-printed conductor for cyclable strain sensors on elastomeric substrates. The printed tracks with 50% weight graphene show significantly higher gauge factors of 117. In another work, the strain sensor is fabricated from water-based graphene ink by screen printed on synthetic fabric (smart textile). The electromechanical tests, aimed at assessing the piezoresistive response of the coated fabrics, have shown an increasing sensitivity with strain: the results showed a GF of 30 at maximum strain (5%) and good repeatability throughout work cycles, even after the washing process (Lynch et al. 2020). In 2023, Zhang et al. (2022) prepared screen printed flexible graphene strain sensor from mechanical exfoliation assisted process. The relative

Fig. 2.2 Printed strain sensor a Response of inkjet printed sensor, b circuit of the sensor, c tensile, and d strain (Casiraghi et al. 2018)

2.3 Graphene Oxide

17

resistance of the sensor increased with the strain increases started from (1 to 6%) applied. The GF of the printed sensor is 20.02 under 1% strain applied. The sensor performed the excellent flexibility, which can maintain a stable working state after 100,000 times of flexing test or 1000 times of abrasion test and could be used as wearable printed strain sensor to monitor the relative change of resistance caused by the movement of the limbs. Table 2.1 shows the previous studies of graphene-based strain sensor.

2.3 Graphene Oxide The nanoscale carbon materials can be used as sensing materials to fabricate wearable strain sensors with or without combination with soft elastomeric. Graphene oxide (GO), as one of the most popular carbon-based nanomaterials, has been widely developed as a promising sensing element in strain sensors, thanks to its excellent mechanical properties, nanoscale flexibility, and chemical stability (Lin et al. 2022b). Reduces GO (rGO) can be easily prepared using thermal reduction methods from GO, obtained using Hummer’s method. Furthermore, GO has excellent processability for coating on porous structures owing to the abundant chemical functional groups on the surface dissolving into water (Tjong 2013). As a graphene derivative with benefits like water dispersibility and inexpensive bulk manufacturing, graphene oxide is the perfect choice to be employed as a filler in ink formulation. However, as GO is an insulator by nature, its conductivity must be restored by reducing it further to create rGO. The further reduction step is optional and should only be used in situations when conductivity is important. Because the oxygen functional groups become negatively ionized and release protons, there is a significant water dispersibility (Khan and Mariatti 2022). This causes electrostatic forces to develop between the GO flakes and stabilises them in the aqueous medium. To prepare in-situ reductions and restore electrical conductivity, Kim et al. (2014) demonstrated a fast and effective technique that involves first printing GO ink on a substrate, followed by printing reducing chemicals on top of the GO traces, as shown in Fig. 2.3. A similar study was applied to print GO ink, which included reduction on a textile substrate. They proposed that when reducing agent ratio and printing layer increased, the sheet resistance decreased. GO was applied on PI and reduced to make a four degree of freedom (4-DOF) strain sensor yielding a GF of 54.2 at a sensing range of 0.003%. To achieve six degrees of freedom (6-DOF), GO was applied to PDMS polymer material. Despite having lots of literature on 3D materials like graphene foam on PDMS, the effect of strain on 2D rGO/PDMS is hardly studied due to the hydrophobic property of PDMS which prohibits the GO adhesion with the PDMS layer (Xu et al. 2014). Therefore, Iqra et al. (2022) developed less-cost rGO based flexible strain sensor on PDMS substrate. GF of the sensor with different strain range such as 12.1, 3.5, and 90.3 for 140%, 130%, and 11.12% for stretching, bending, and torsion, respectively. Moreover, the sensitivity was improved by using a specific fabrication process

–

2.73 S/cm

–

–

2.2–15.4 S/m

Compression/ Coating

Tension/Spin coating

Compression/ Inkjet printing

Compression/ Coating

Tension

Tension/ Casting

Compression/ 3D printing

Compression/ Casting

Tension/ Coating

Tension/Inkjet printing

Epidermis microstructure inspired graphene

Graphene foam/ PDMS

Polyvinyl alcohol nanowires/ Graphene film

3D graphene film

Graphene foam/ PDMS

Pattern graphene

3D printed graphene aerogel

Graphene paper

Graphene/Porous PDMS

Graphene/Paper

266.78 kΩ

–

–

16 S/cm

–

Conductivity

Sensing type/ Method

Materials

20%

40%

0–20 kPa

0–20%

30%

100%

75 kPa

11%

3%

40 kPa

Sensing range

Table 2.1 Sensing performance of graphene-based strain sensor

Nonlinear

110 kPa−1 (0–0.2 kPa)

150

7–173

17.2 kPa−1 (8 layers, 0–2 kPa) 0.1 kPa−1 (1 layer, 2–20 kPa)

1.34

301–29,631 (10–25%)

Linear

Nonlinear

Nonlinear

Linear

Nonlinear

Nonlinear

Nonlinear

28.34 k Pa−1

100 (80% strain) 80 (100% strain)

Nonlinear

Nonlinear

25.1 kPa−1 (0–2–6 kPa)

223

Response type

GF

–

5000

–

–

10

11,000 bending cycles

10,000

6000

–

–

Durability (cycles)

(continued)

Casiraghi et al. (2018)

Yun et al. (2017)

Tao et al. (2017)

Zhang et al. (2016)

Lee et al. (2017)

Li et al. (2017)

Xia et al. (2018)

Liu et al. (2018a, b)

Liu et al. (2017)

Pang et al. (2018)

Ref

18 2 Carbon Based Printed Strain Sensor

Screen printing 34.832 kΩ

Screen printing 3.73 × 104 Sm−1

Graphene/Glass

Graphene/Fabric

Conductivity

Sensing type/ Method

Materials

Table 2.1 (continued)

20.02

550.14

200 μm (bending) 4% stain

GF

Sensing range

Linear

Linear

Response type

1000

–

Durability (cycles)

Zhang et al. (2023)

Tseng et al. (2021)

Ref

2.3 Graphene Oxide 19

20

2 Carbon Based Printed Strain Sensor

Fig. 2.3 Schematic of inkjet printing process and prepared GO inks (Kim et al. 2014)

whereby liquid PDMS was poured on rGO and GO and subsequently encapsulated with another PDMS layer. In earlier studies, strain sensor strain sensor have an extensive application range including smart clothing made by rGO coated elastic fibres which show GF of 8.8 below 5% sensing range (Shayan et al. 2018). Wearable strain sensors for the monitoring of different physical tasks such as hand folding, wrist-twisting, finger bending, and knee movement were tested by rGO/silicone strain sensor. The fabrication process of the rGO strain sensor is shown in Fig. 2.4a. From the results, rGO/silicone based strain sensor achieved the most promising stretchable stain sensor with an elasticity of 116%, GF of 4100, and durability of 4550 cycles. Figure 2.4b shows the response of the sensor corresponding to walking, running, head movement, disturbance to containers (Verma et al. 2022). In recent years, conductive elastomeric with rGO which were combined of electrically conductive filler and insulating elastomer matrices has been used as one promising candidate for the fabrication of strain sensor (Zhao et al. 2018; Song et al. 2021). On the other hand, elastomers enhance the stretchability of the strain sensor and conductive fillers can render electrically conductive that is sensitive to a strain stimulus. For the conductive elastomer, it generally needs high number of fillers to perform a conductive pathway. However, the higher amount loading of fillers normally issue from aggregation of fillers during the dispersion. For this reason, the dense conductive flow throughout the polymer matrix might perform in low GF and detrimental stretchability (Ma et al. 2020). Further, 3D network conductive filler in a polymer matrix has introduced as a promising strategy. Wang et al. (2022a; b) reported 3D rGO/elastomeric conductive network for strain sensor. Figure 2.4c, d shows the relative resistance change of the rGO/PDMS and the schematic of the changes of the filler network of rGO/ PDMS strain sensor. The performance of the sensor showed high GF (44.01) and cyclability (2500 cycles) coupled with high stretchability (0–300%), which exhibited very competitive for practical applications in the large-scale body motion detection. Based on the results, the rGO based strain sensor showed better performance in strain

2.4 Carbon Nanotubes

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Fig. 2.4 a The schematic diagram of rGO based strain sensor fabrication, b response of the sensor, c relative resistance change of the sensor, and d the diagram of the strain sensor during stretching (Verma et al. 2022, Wang et al. 2022a, b)

sensing and GF if compared with that of GO. The flexible strain sensor requires high electrical conductivity. However, the electrical performance of GO is very low. Therefore, the rGO based strain sensor is suitable to be used for wearable health monitoring applications.

2.4 Carbon Nanotubes Nowadays, the use of carbon nanotubes (CNT) based printed strain sensor monitoring is common among researchers. This is because CNT is well-known for electromechanical properties which is suitable for sensing applications, and its alignment has proven to facilitate some fabrication processes of CNT based sensor (e.g., CNT forest embedded into polymeric matrices), and increased its sensitivity to strain (Souri et al. 2020; Yan et al. 2021). Additionally, CNTs’ exceptional conductivity and physical characteristics are exhibited by their one-dimensional, slender tubular structure, which was created by crimping a single layer of several layers of graphene nanoplates (Jang et al. 2020). CNTs are more expensive than other conventional conductive materials, mostly because of the high cost of their fabrication and purifying process and their tiny production scale (Wu et al. 2019). With the development of batch production technology, the price of CNTs will drop significantly. However,

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due of their exceptional electrical conductivity, CNTs’ smaller level of reduction can compensate for their disadvantage of being expensive. CNT can be assembled by various techniques to form different shapes with elastic polymers. The assembly methods reported in the previous works are the uniform mixing, polymers coated by CNT and CNT coated by polymers. Recent results indicated the performance of these strain sensors are closely related to the assembly methods and the microstructures of conductive networks (Wang et al. 2017; Lu et al. 2018; Ren et al. 2019). The use of MWCNTs as a conductive material has allowed flexible printed strain sensor system to achieve excellent sensitivity and fast detection (Fiyadh et al. 2019). However, there are remain issues to be solved to fully increase their benefits. The main issue is that MWCNTs have a high surface energy and difficult to disperse well in aqueous solutions, which can lead to uncontrolled agglomeration (Arrigo et al. 2018). Several techniques, including (a) mechanical dispersion, (b) surface chemical functionalization, (c) physical adsorption of surfactants can increase the dispersion of MWCNTs conductive ink (Arrigo et al. 2018; Sigolaeva et al. 2019). The two first methods are regarded as difficult due to the possibility of them altering the nanotubes’ electronic structure because the electrical performance of MWCNTs is strongly dependent on their π-π conjugated configurations. Surfactant assisted dispersion has received a lot of attention in this area due to its simple process and ability to preserve the distinctive structure of MWCNTs. This process is typically used in conjunction with mechanical techniques like sonication, which could minimize the damaging process by lowering the time and energy needed to achieve a good dispersion (Atif and Inam 2016; Bricha and El Mabrouk 2019). White et al. (2007) reported that the dispersion and stability of CNT was greatly improved with the increase in SDS concentration alkyl chain length reduction of anionic surfactants. This resulted in enhanced zeta potential of the solution, thereby improving the dispersion and stability. In another work, MWCNTs conductive inks were prepared by using an aqueous mixture of polyethylene glycol and polyethylene glycol monomethyl ether. The ball milling techniques were used to mix the conductive inks. The prepared inks showed good dispersion, optimum viscosity, sufficient surface tension and desirable inks properties and suitable for normal printing. Moreover, there was no significant microchannel clogging up to about 1 week. Besides that, water-based CNT conductive inks with SDBS surfactant which was stable even after 30 days were reported by Han et al. (2014). In recent year, the performance, and properties of synthetic surfactants (pluronic F (PL)-127) and green surfactants (GA and alkali lignin (AL)) for water based MWCNTs conductive inks were reported by Kamarudin et al. (2021). From the study, it is reported that GA performed better dispersion and stability than those compared with other surfactants. In the same year, Akindoyo et al. (2021) investigated effect of functionalized and non-functionalized MWCNTs water-based conductive inks by using GA as a surfactant. Conductive ink with 0.75 wt% of functionalized MWCNTs showed the highest stability and electrical conductivity. Moreover, mix surfactant assisted dispersion of CNT are also effective to improve the dispersion quality for CNTs. Radman et al. (2019) reported dispersion of CNT in mixed surfactant SDS

2.4 Carbon Nanotubes

23

and GA in DI water at different concentrations ratio of SDS/GA. The CNT inks were found to be more stable with higher SDS content compared to GA, which was associated with the difference in their dispersion mechanisms. Specifically, whereas the non-polar head of SDS is adsorbed by nanotube bonds, its polar head is adsorbed by water thereby forming a stable solution. On the other hand, GA causes the nanotubes to float by circling in spiral around them. It is evident that surfactants help to achieve good dispersion of conductive fillers in water. Notably, the better dispersion and stability of CNTs inks were observed in the SDS and GA based surfactants. In the fabrication of CNTs based strain sensor, epoxy resin is a widely used elastic material to perform high stretchability sensor. For example, Bouhamed et al. (2017) fabricated screen printed MCNTs based strain sensor combined with epoxy resin. From the results, the sensitivity of the sensor decreased with increasing amount MWCNT wt% and thickness of substrate, and the highest values is 14.19 at 0.3 wt% of MWCNT. Similar study was reported by Rahman et al. (2016) and Tong et al. (2017). Additionally, because the concentration of water in the sensor changes with temperature and relative humidity, the influence of temperature was reduced as CNT concentrations increased (Gong et al. 2018). Due to the fast disappearance of the conductive channels under the greater stretching strain, the sensors’ strain range in the aforementioned tests is less than 2%. These findings showed that it is challenging to employ epoxy resin as a substrate for sensing substantial deformations (Cao et al. 2017). Fu et al. (2019) developed MWCNT based strain sensor on PDMS substrate by using casting process. The result reported that the sensitivity value was only 9 under 40% strain. These performances were significantly enhanced when the sensor was divided into MWCNT/PDMS fragmented particles. The GF valued reached 200 and the strain range was up to 80%. However, the linearity decreased obviously. Conducive networks were created using the functionalized CNTs to enhance the sensing capabilities of the CNT/PDMS film (Ahuja et al. 2020; Hu et al. 2020). The previous results demonstrate that employing homogenous CNT composites with intricate conducting networks, it is challenging to generate sensors with high sensitivity and a wide strain range at the same time. The sensitivity of the strain sensor is improved by the CNT composite with low concentration. However, the strain range is poor due to the weak conductive paths. Additionally, MWCNT film-based strain sensors have higher sensitivities than those made using homogeneous mixes and (ultrasonic mixing) (Choi et al. 2020). In the CNT film, more microcracks developed and spread as the tensile tension increased. The CNTs must be evenly scattered in order to create an excellent conductive network that will guarantee stable sensing performance. The homogenous CNT solution is frequently made using the ultrasonic technique. To enhance the aggregation, functionalization treatments and certain surfactants are required. These processes, however, will introduce certain impurities and cause flaws in CNTs. Giffney et al. (2017) developed a method to print highly stretchable large strain sensor using CNT/Ecoflex. Notably, this extrusion printing enabled pseudoalignment of the random CNT in the Ecoflex matrix during extrusion. It has been demonstrated that the sensor GF could be turned by controlling the mass ratio of CNT

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and silicon-based materials in the sensor. The obtained sensors possessed desirable properties for applications in wearable electronics and soft robots with high stretchability and low hysteresis. Kim et al. (2014) studied inkjet printed SWCNT/ poly(styrenesulfonic) (PEDOT:PSS) thin film strain sensor. The results showed that high GF with 9 (at 50% strain) and an excellent working range of 70% even after 1000 cycle test at 50% tensile strain. In another work, PDMS was used as a substrate for inkjet printed SWCNT stretchable strain sensor. Figure 2.3a shows fabrication process of SWCNT/PDMS printed strain sensor. The printed sensor showed high durability during repeated cycling (1000 cycles) and good sensitivity with GF of 2.75 (Park et al. 2019).

2.5 Summary Findings based on previous studies on carbon based conductive inks and flexible conductive network for resistance type strain sensor based on graphene, graphene oxide, and carbon nanotubes are reported. The effects of preparation methods and conductive network on sensing performance are analysed and compared, this includes sensitivity, strain range, and response time.

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Chapter 3

Silver Nanoparticles-Based Printed Strain Sensor

Abstract Piezoresistive silver nanoparticles (AgNP)-based strain sensor are popular due to their high piezo resistivity and high electrical conductivity, which can readily be attached to clothes or human body. In this chapter, the overview of AgNPs based conductive inks and strain sensor is presented. Flexible strain sensors based on combination of AgNPs and polymer and conductive polymer are discussed. The materials structures, and fabrication process of representative strain sensor in relation to the electrical, mechanical and the sensing properties are also reported in this chapter.

3.1 Introduction Metal-based conductive inks have been widely explored for printed strain sensor for the last decades. Metal-based nanomaterials are the one-dimensional (1D) or zero-dimensional (0D) nanowires (NPs) forms of bulk metals that have low dimensional nanostructures. Silver nanoparticles (AgNPs), which are used to print highly conductive materials are thought to be the most durable nanoparticles among other metal-based nanoparticles. Furthermore, AgNPs have unique physical, chemical, electrical, and significant antioxidant properties. But the primary issue with AgNPs is the requirement for high-temperature sintering of the printed film to achieve exceptional conductivity. Additionally, metal nanowires like silver nanowires (AgNWs) have generated a lot of interest for conductive inks. AgNWs have showed promise in low sheet resistance and great visual transparency, both of which are necessary for printed flexible strain sensor because of the strong conductivity of Ag.

3.2 Silver Nanoparticles Silver nanoparticles (AgNPs) based wearable/stretchable strain sensors have become one of the fastest growing cutting-edge fields for human–machine interfacing and personalized healthcare-performance monitoring. The development and fabrication © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_3

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of the AgNPs based flexible/stretchable, and wearable strain sensor have been done on several factors (Mohammed Ali et al. 2018). Flexibility, durability, detective ranges and GF are a some of the factors that must be considered before for printed strain sensor. Substrate with desired mechanical flexibility acts as a support component of the AgNPs strain sensor. Suitable flexible substrate materials should have better adhesion to the AgNPs conductive ink, and subsequently exhibit high mechanical, chemical, and thermal properties (Segev-Bar and Haick 2013). The preparation of AgNPs conductive inks very important for the performance of the printed flexible strain sensor applications. Shen et al. (2014) developed stable dispersion of AgNPs inks by using deionized water and ethylene glycol as a solvent. The prepared conductive inks were printed on paper and PET substrates via inkjet printing process. Low resistivity (37 µΩ/cm) of the inkjet printed AgNPs conductive film were observed when the sintering temperature is 180 °C. Shahariar et al. (2019) prepared particle-free Ag ink by combining a silver salt mixture with an amine solution, which can transform into AgNPs once heated to a temperature were silver ion can be reduced. The conductive film is designed by inkjet printing on uncoated polyester textile knit, woven, and nonwoven fabrics without changing the feel, texture, durability, and mechanical behaviour of the material. Nevertheless, the sheet resistance and the resolution of the printed conductive film are significantly impacted by the packing and tightness of the fabric’s structures and fabric fibre sizes. To create a solid silver complex to produce silver ink, Zope et al. (2018) used ethylenediamine as a complexing agent and silver oxalate as a precursor. Longer shelf-life stability is provided by this solid silver complex. Additionally, a hybrid thermal-photonic curing technique that improved electrical properties and substrate adherence was demonstrated. The printed silver film has a 2.7 times higher electrical resistance than bulk silver. Ding et al. (2016) developed a one-step polyol process to scale up the AgNPs synthesis in the ethylene glycol in the addition of polyvinylpyrrolidone (PVP). The change in the mass ratio of AgNO3 and PVP determined the particle size (from 52 to 120 nm). To achieve conductivities that were close to those of bulk silver, the AgNPs were re-dispersed in ethanol at a 70-weight percent concentration and printed in various conductive shapes on PET substrate as shown in Fig. 3.1. Ramli et al. (2020) reported the development screen-printed stretchable strain sensor for human facial expression detection by using Ag ink (Fig. 3.2). The prepared

Fig. 3.1 Screen illustration of screen-printed silver conductive traces (Ding et al. 2016)

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strain sensor has several advantages, including high sensitivity, simple manufacture, and flexibility to human skin deformation. The GF of the sensor is around 84, and its peak stretchability is up to 20%. In another screen-printed strain sensor reported by Yoon and Kim (2020), cost effective stretchable AgNPs electrode printed on PU substrate (Fig. 3.3) was produced. It can be used for wearable electronics for humans and has a high stretchability compared to rigid substrate-based traditional strain sensors. It was stable up to 20% strain. In comparison to other stretchable materials, the 15–20% stretchability is low. Additionally, the strain sensor demonstrated a strong responsiveness to strain variations that depend on crack opening and closing. As a result, a screen-printed Ag electrode on a PU substrate might be employed as a permanent strain sensor. Furthermore, high performance screen-printed strain sensor based on a brittlestretchable conductive network that consists of both brittle and stretchable conductive layers by using AgNPs is shown in Fig. 3.4. This conductive network performs strain sensors, which have high sensitivity (GF > 870) for the whole strain range (100%) results in a higher electromechanical performance. Moreover, the prepared sensor showed the super-low detection limit of 0.05%, high reproducibility, low hysteresis, and good cyclic durability (over 5000 cycles) (Wang et al. 2020). This screen-printed strain sensor has advantages compared to other screen-printed strain sensors due to

Fig. 3.2 Fabrication process of screen-printed Ag strain sensor (Ramli et al. 2020)

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Fig. 3.3 Fabrication process of printed AgNPs ink on PU substrate (Yoon and Kim 2020)

both brittle (PEN) and stretchable (PDMS) substrates. Other substrates such as PET and PI substrate based AgNPs strain sensor were reported by Zlebic et al. (2016). In this research, different concentration of AgNPs and different types of substrates on the properties of printed flexible strain sensor was investigated. According to the results for sensor, the second series of tested resistive strain gauges have the highest sensitivity, with an average GF of 2.03, the first series of corresponding GF value was around 0.07 and the third series of GF were 1.94 for one layer and 1.58 for 2 layers. The findings of this study show that strain gauges with good GF can be made even with inexpensive equipment, like a desktop printer like the EPSON C88+ and a PET-based substrate. A polyurethane (PU) film was coated with a silver conductive layer using an e-beam to create a stretchable conductor. By cutting, the object can be moulded into any shape. Tolvanen et al. (2018) designed and fabricated Ag ink pattern on silicon elastomer-based strain sensor with Ag plated nylon structure (Ag-DS/CF). To achieve an excellent electrical performance, the distinctive construction combines precisely structured flexible conductive textiles with wrinkled Ag-ink pattern. By achieving great stretchability up to 75%, ultrahigh sensitivity (gauge factor > 104 – 106 ), and flexible sensing ranges (from 7 to 75%), the Ag-DS/CF might be used to detect both significant and subtle human motions and activities, pressure changes, and physical vibrations. On the other hand, the fabrication process of AgNPs based stretchable strain sensor also important to achieve the required performance of stretchability, sensitivity, and morphology. The fabrication process should have low temperature fabrication without the need for etching technique that cab be deform polymer-based substrates such as PET or PDMS. With these benefits, printing liquid materials containing functional particles, including dielectrics, semiconductors, and conductors, can be used to fabricate flexible and stretchable electronics. In this regard, the ‘direct’ printing or patterning process has been a different technique since it connects the deposition and drawing arbitrary pattern procedures together. Serhan et al. (2019) fabricated AgNWs strain sensor with arbitrary micro-pattern electrodes using dispensing nozzle printing on PDMS substrate. Further, this study highlights the adhesion problem of conductive filler and polymer substrate. The prepared strain

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Fig. 3.4 Schematic diagram of the structure printed strain sensor and testing (Wang et al. 2020)

sensor achieved highly stretchable strain sensor (up to 60% strain) with a proper electrode design. Based on experimental findings, it is believed that in the near future multifunctional sensors would be added to directly drawn electronic skin (E-skin) developed through the printing process. Besides that, flexible and stretchable strain sensor was designed by Agarwala et al. (2019) using thickness gradient films with a high gauge factor, while another research team was working on carbonization of plain-weave silk fabric that could be stretched up to 500%. In addition, flexible, bendable, and stretchable substrates such as PET, PI, PEN, and PDMS were used to fabricate AgNPs based strain sensor. However, these substrates are not suitable with postprocessing techniques like thermal sintering. Thermal sintering is an essential procedure to get the appropriate characteristics in the electronic mate by degrading organic additives, coalescing particles for a continuous electrical path, enhancing adhesion, and so on. Alternative methods for thermal sintering, such as electric current, plasma, photonic, and flashlight processes, have been studied in some detail. Additionally, increased conductivity values from laser sintering (20–40% of bulk silver) have showed promise and are suitable for printed electronic applications. Laser sintering can precisely control the material’s temperature and nanostructure. AgNPs on glass and silicon substrates were subjected to laser sintering in the research by Choi et al. (2015). For the purpose of developing

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Fig. 3.5 Schematic diagram depicting the process of printing (Agarwala et al. 2019)

a conducting silver mesh on a polymer substrate, (Hong et al. 2013) studied the use of laser sintering. They deposited a layer of spin-coated Ag before using a laser sintering machine to sinter the film in a specific pattern and remove the non-sintered material to form a grid. It has also been investigated how silver films’ structural and optical characteristics are affected by laser irradiation. Agarwala et al. (2019) developed wearable bandage-based strain sensor by using 3D aerosol jet printing and laser sintering for home healthcare applications. The fabrication process of 3D printed strain sensor was shown in Fig. 3.5. This study highlights the laser sintering of aerosol jet printed perform without damage on the substrate. The prepared sensor showed that the sensor is stretchable and has good sensitivity and stability for 700 cycles of repeated bending. Figure 3.5 showed the advantage of aerosol jet printing with the strain sensor on the commercial bandage. From the figure, the printed sensor can be stretched and bent to a certain degree without losing functionality.

3.3 Silver Nanoparticles with Polymer The high sensitivity and large stretchable performance of polymer composite-based flexible strain sensor gained high attention and interest among researchers in various engineering fields. AgNPs with polymer-based strain sensor offers the smooth conductive channels of metal sensing part, and stretchability performance, however investigation on suitable formulation and processing methods is required to produce homogeneous surface of conductive electrode with good mechanical performance

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(Afsarimanesh et al. 2020). The critical factor of conductive materials is allowing high electrical conductivity and mechanical compliance simultaneously. It is well known that generally AgNPs conductive filler contains good electrical conductivity and stability, but poor mechanical strength due to stiff and brittle nature. The soft polymer such as PDMS and polyurethane provide good flexibility and stretchability as mechanical properties (Khalid and Chang 2022). Therefore, the combination of conductive AgNPs with polymer is useful as it produces conductive composite with critical factor (high electrical conductivity and good mechanical strength) simultaneously. Based on the research observations, the mechanical compliance of AgNPs is very useful to produce conductive properties for stretchable electronics. A soft piezoresistive sensors have numerous potentials uses in skin-attachable electronics, human–machine interfaces, and electronic skins due to their special qualities, such as human skin, light weight, and multiple functions. Wang et al. (2019) fabricated stretchable piezoresistive sensor (SPS) from AgNPs/TPU composite and PDMS by using 3D printing process. The prepared sensor exhibits high sensitivity (5.54 kPa−1 ), large measurement range (from 10 Pa to 800 kPa), limited cross-correlation, and excellent durability. Another stretchable strain sensors were fabricated using AgNPs and PDMS with different types of fabrication processes such as drop-casting, screen-printing, and spin-coating. From the study, better performance of the sensor produced by screen-printing and drop-casting (Soe et al. 2020) was reported. The sensitivity of the prepared sensor showed (GF = 10.08) with 70% strain range in drop-casting. Similar studies were reported by using drop-casting method stretchable strain sensor with AgNPs/PDMS composites. From this work, the combination of the AgNPs and PDMS strain sensor with 0.25% filler loadings exhibited good sensitivity, and hysteresis behaviour at 70% strain range (Soe et al. 2021). Furthermore, Liu et al. (2017) reported highly sensitive strain sensor based on AgNPs and PDMS to develop a synergic conductive network and a sandwichstructure. The prepared sensor showed strong piezo resistivity with a high GF of 547.8 and strain range from 0.81 to 7.26%. The experimental procedure for recognising one finger’s motion is shown in Fig. 3.6. In the figure, a finger-shaped the stretchable strain sensor based on AgNPs/PDMS has been used to track the motion of a human finger. Besides human motion applications, the AgNPs and polymer-based strain sensor has been used in the biomedical and civil infrastructural industries. Yoon et al. (2021) prepared semi-transparent and stretchable Ag and polytetrafluoroethylene (PTFE) conductors on a PU substrate for use in high-performance wearable and self-cleaning sensors (Fig. 3.7). The prepared sensor performed high stretchability of 0–40%, a low sheet resistance of 3.09–17.23 Ω/sq., and a semi transmittance of 25.27–38.49%, which are suitable values for wearables, stretchable and self-cleaning sensors. The prepared sensor was also applied in electromyography (EMG) sensors.

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Fig. 3.6 The experimental process used for one finger motion (Liu et al. 2017)

Fig. 3.7 Fabrication and application of printed sensor (Yoon et al. 2021)

3.4 Silver Nanoparticles with Conductive Polymer

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3.4 Silver Nanoparticles with Conductive Polymer Conductive polymer such as PEDOT: PSS, polypyrrole (PPy), and polyaniline (PANI) have also been widely used in strain sensor preparation owing to simply of apply, excellent flexibility, low Young’s Modulus and good adhesivity to an elastomeric matrix. However, strain sensor based on conductive polymer have higher sheet resistance those compared with carbon and metalbased nanomaterials. A polymer based strain sensor with a sandwich structure of poly(3,4-ethylenedioxythiophene), poly(styrenesulfonate), doped with polyvinylalcohol (PVA-PEDOT: PSS)/highly conductive PEDOT: PSS/polydimethylsiloxane elastomer were reported by Fan et al. (2018). The stable structural integration and recoverability in conductance of the strain sensors resulted in high sensitivity and good durability at large high strain range. Additionally, the proposed plastic strain sensors have been successfully demonstrated for monitoring human activities such joint and muscle motions, arterial pulsation, voice vibration, and differentiating certain complex and different bending motions. Pani et al. (2016) developed a simple process to prepare a novel type of textile electrodes based on woven fabrics treated with PEDOT: PSS. The raw fabric was soaked in PEDOT: PSS squeezed, annealed, and used a second dopant. Human volunteers were used to test the electrodes for skin contact impedance and the quality of the ECG signals captured during and after physical activity. The findings demonstrated that the electrodes could function in both wet and dry conditions. Due to the irregular contact between the dry electrode and the skin, dry electrodes are more prone to noise artefacts, especially during physical activity. The strain sensor based on PEDOT: PSS is adopted to measure the bending angle of knee flexion and wrist rotation. According to the findings, bending sensors made by inkjet printing using PEDOT: PSS or silver nanoparticles have numerous advantages over other systems for tracking human movement (both inside and outside the body), including small size, low cost, and versatility. Teo et al. (2017) have demonstrated how 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB) affects PEDOT: PSS performance. At higher tensile strains, a considerable increase in electrical conductivity was observed. Many further studies have been carried out to enhance the properties of polymers, and the improved findings were placed to use in a variety of flexible electronics applications. Zein, Huppé, and Cochrane (2017) used a combination of PEDOT: PSS and silicone elastomer to develop flexible and light-weight strain gauge sensors. To increase the mechanical and adhesion properties, the elastomer was utilised as a flexible substrate in the form of a matrix and the PEDOT: PSS provided a conductive path for the measurement of deformation. It was discovered that by improving several sensor-related factors (such as the design, dimensions, geometry, conductive polymer concentration, etc.), low deformation measurements and negative temperature measurements could be enhanced. Nevertheless, to effectively connect sensor response and movement with substrate deformation considering the sensor’s mechanical and electrical response, thorough characterization of these sensors’ mechanical and electrical response is required.

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To improve the above issue, Borghetti et al. (2016) reported inkjet printed strain sensor based on the AgNPs and PEDOT: PSS inks printed on polymer substrate. The sensitivity of the Ag inks sensor is about 3.7 and PEDOT: PSS ink sensor the sensitivity is less than 1. In another work, Han et al. (2022) fabricated PEDOT: PSS with AgNWs to increase the stretchability of AgNWs and conductivity of PEDOT: PSS. Figure 3.8 shows the experimental setup of the strain sensor. The prepared sensor electrode endured over 40% stretch strain rate and 20% stretching repetition test for more than 350 times. Comparing these results to bare Ag NW electrodes revealed improved stretching endurance. Other mechanical tests, like bending, folding, twisting, and rolling, demonstrated outstanding results. Eom et al. (2017) demonstrated highly sensitive textile-based strain sensor using PEDOT: PSS and AgNWs coated nylon threads as a strain sensing component for electronic textile applications. Figure 3.8 showed the fabrications process of strain sensor and SEM images of AgNWs coated nylon thread with PLL surface modification. The strain GF of 1.69–3.31 (strain range of 5–20%) and stable operation up to ca. 1000 stretch-release cycles were observed from this study. Therefore, a textile strain sensor with a high gauge factor and good operating stability may be made employing the PEDOT:PSS/Ag NW/nylon threads. As a discussed above, the Ag and conductive polymer are in demand for flexible strain sensor due to their lightweight, chemically diverse, adjustable in terms of structure and morphology, environmental stable and easy to fabricate. However, they struggle with long-term stability. The covalent bonds that are present in polymer structures have an unstable character. Due to the complex material structures, detecting the sensing behaviour of conducting polymers is a difficult process. Although conductive polymer and Ag have demonstrated flexible electronic characteristics, these materials conductivity still needs to be improved.

Fig. 3.8 Schematic representation of the experiment setup of the sensor

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3.5 Summary Findings reported in the previous works on the dispersion and stability of AgNPs conductive inks are reported. Flexible and stretchable/wearable strain sensors produced using AgNPs, AgNPs/polymer, and AgNPs/conductive polymer are discussed. The parametric evaluations indicated the performance improvement of these wearable strain sensor through single AgNPs and with polymer structuring. However, for particular and demanding applications including prosthetic devices, implantable systems, and body-worn robotic stimulation devices, design and performance enhancements through advanced composites are still required.

References Afsarimanesh N et al (2020) A review on fabrication, characterization and implementation of wearable strain sensors. Sens Act A: Phys 315 Agarwala S et al (2019) wearable bandage-based strain sensor for home healthcare: combining 3D aerosol jet printing and laser sintering. ACS Sens 4(1):218–226 Borghetti M, Serpelloni M, Sardini E, Pandini S (2016) Mechanical behavior of strain sensors based on PEDOT:PSS and silver nanoparticles inks deposited on polymer substrate by inkjet printing. Sens Act, A 243:71–80. https://doi.org/10.1016/j.sna.2016.03.021 Choi JH, Ryu K, Park K, Moon SJ (2015) Thermal conductivity estimation of inkjet-printed silver nanoparticle ink during continuous wave laser sintering. Int J Heat Mass Transf 85:904–909. https://doi.org/10.1016/j.ijheatmasstransfer.2015.01.056 Ding J et al (2016) Preparing of highly conductive patterns on flexible substrates by screen printing of silver nanoparticles with different size distribution. Nanoscale Res Lett 11(1):1–8. https:// doi.org/10.1186/s11671-016-1640-1 Eom J et al (2017) Highly sensitive textile-based strain sensors using poly(3,4ethylenedioxythiophene): polystyrene sulfonate/silver nanowire-coated nylon threads with poly-l-lysine surface modification. RSC Adv 7(84):53373–53378 Fan X et al (2018) Highly sensitive, durable and stretchable plastic strain sensors using sandwich structures of PEDOT:PSS and an elastomer. Mater Chem Front 2(2):355–361 Han JW et al (2022) Highly stretchable, robust, and conductive lab-synthesized PEDOT:PSS conductive polymer/hydroxyethyl cellulose films for on-skin health-monitoring devices. Org Electron 105(January):106499. https://doi.org/10.1016/j.orgel.2022.106499 Hong S et al (2013) Nonvacuum, maskless fabrication of a flexible metal grid transparent conductor by low-temperature selective laser sintering of nanoparticle ink. ACS Nano 7(6):5024–5031 Khalid MAU, Chang SH (2022) Flexible strain sensors for wearable applications fabricated using novel functional nanocomposites: a review. Compos Struct 284(January):115214. https://doi. org/10.1016/j.compstruct.2022.115214 Liu L et al (2017) Preparation and property research of strain sensor based on PDMS and silver nanomaterials. J Sens Mohammed Ali M et al (2018) Printed strain sensor based on silver nanowire/silver flake composite on flexible and stretchable TPU substrate. Sens Act A: Phys 274(2010):109–115.https://doi.org/ 10.1016/j.sna.2018.03.003 Pani D et al (2016) Fully textile, PEDOT:PSS based electrodes for wearable ECG monitoring systems. IEEE Trans Biomed Eng 63(3):540–549

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Ramli NA, Nordin AN, Azlan NZ (2020) Development of low cost screen-printed piezoresistive strain sensor for facial expressions recognition systems. Microelectron Eng 234(September):111440. https://doi.org/10.1016/j.mee.2020.111440 Segev-Bar M, Haick H (2013) Flexible sensors based on nanoparticles. ACS Nano 7(10):8366–8378 Serhan M et al (2019) Total iron measurement in human serum with a smartphone. In AIChE annual meeting, conference proceedings 2019-November Shahariar H, Kim I, Soewardiman H, Jur JS (2019) Inkjet printing of reactive silver ink on textiles. ACS Appl Mater Interfaces Shen W et al (2014) Preparation of solid silver nanoparticles for inkjet printed flexible electronics with high conductivity. Nanoscale 6(3):1622–1628 Soe HM, Manaf AA, Matsuda A, Jaafar M (2020) Development and fabrication of highly flexible, stretchable, and sensitive strain sensor for long durability based on silver nanoparticlespolydimethylsiloxane composite. J Mater Sci: Mater Electron 31(14):11897–11910. https://doi. org/10.1007/s10854-020-03744-6 Soe HM, Manaf AA, Matsuda A, Jaafar M (2021) Performance of a silver nanoparticles-based polydimethylsiloxane composite strain sensor produced using different fabrication methods. Sens Actuators, A 329:112793. https://doi.org/10.1016/j.sna.2021.112793 Teo MY et al (2017) Highly stretchable and highly conductive PEDOT:PSS/ionic liquid composite transparent electrodes for solution-processed stretchable electronics. ACS Appl Mater Interfaces 9(1):819–826 Tolvanen J, Hannu J, Jantunen H (2018) Stretchable and washable strain sensor based on cracking structure for human motion monitoring. Scien Rep (August):1–10. https://doi.org/10.1038/s41 598-018-31628-7 Wang Z et al (2019) Full 3D printing of stretchable piezoresistive sensor with hierarchical porosity and multimodulus architecture. Adv Func Mater 29(11):1–8 Wang YF et al (2020) Printed strain sensor with high sensitivity and wide working range using a novel brittle-stretchable conductive network. ACS Appl Mater Interfaces 12(31):35282–35290 Yoon S, Kim HK (2020) Cost-effective stretchable ag nanoparticles electrodes fabrication by screen printing for wearable strain sensors. Surf Coat Technol 384(June 2019):125308. https://doi.org/ 10.1016/j.surfcoat.2019.125308 Yoon S et al (2021) Highly stretchable metal-polymer hybrid conductors for wearable and selfcleaning sensors. NPG Asia Mater 13(1). https://doi.org/10.1038/s41427-020-00277-6 El Zein A, Huppé C, Cochrane C (2017) Development of a flexible strain sensor based on PEDOT: PSS for thin film structures. Sensors (switzerland) 17(6):1–14 Zlebic C et al (2016) Inkjet printed resistive strain gages on flexible substrates. Facta Universitatis— Series: Electron Energetics 29(1):89–100 Zope KR, Cormier D, Williams SA (2018) Reactive silver oxalate ink composition with enhanced curing conditions for flexible substrates. ACS Appl Mater Interfaces 10(4):3830–3837

Chapter 4

Composites and Hybrid Based Printed Strain Sensor

Abstract Composites and hybrid based flexible strain sensor have better performance in sensitivity, and stretchability. In this chapter, the development of composite and hybrid such as carbon/metal, carbon/polymer, and metal/polymer based flexible/stretchable strain sensor are reported. Furthermore, the fabrication process and their performance in sensitivity, stretchability, stability of the strain sensor are also discussed in this chapter.

4.1 Introduction Flexible conductive hybrid and composites consist of a conductive filler dispersed in elastomer and which have a much greater mechanical stretchability than rigid metal or carbon conductive materials (Yun et al. 2021). These composite or hybrid materials for flexible strain sensor applications are fabricated using a variety of synthesis and treatment process, including mixing and printing, implantation, spray deposition, layered structuring, surface modifications. These processes are used to determine if the distribution of nano-reinforcements inside the polymer matrix is biased or uniform. The ultimate objective of each process scenario is to produce a flexible functional nanocomposite and hybrid that can adapt its electrical properties in response to mechanical inputs (Khalid and Chang 2022).

4.2 Carbon and Metal Based Strain Sensor Unique devices for specialised applications in health monitoring, general motion detection, sports activity tracking, smart textiles, soft robotics, and other fields have been made possible by the development of advanced composite and hybrid materials and novel production methods (Senthil Kumar et al. 2019). Limited detection range and complex fabrication process of traditional metal- or semiconductor-based strain sensors are becoming outdated for the strain sensor applications. Flexible © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_4

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and stretchable strain sensor based on carbon–metal nanocomposite have therefore emerged as the most suitable replacement candidates (Wang et al. 2014; Sun et al. 2020). Typically, flexible strain sensor is fabricated from three parts: a transducing electrode, a functional composite structure, and connection wires. The functional stretchable composite and hybrid structures are fabricated from these parts primarily by utilising conductive nanofillers in an insulating or conducting polymer matrix, taking into account particular characteristics like electrical conductivity, mechanical strength, and structural interface (Fang et al. 2015; Yan et al. 2018a; Soe et al. 2021). Carbon-based composites and hybrid structures, including graphene, carbon nanotubes (CNTs), carbon black (CB), graphene oxide have been studied extensively for the development of flexible strain sensor (Zhang et al. 2018; Aziz et al. 2019). Besides carbon based, metal conductive filler such as silver nanoparticles (AgNPs), silver nanowire (AgNWs), copper nanoparticles (CuNPs), gold nanoparticles (AuNPs) are also studied for flexible composites and hybrid structures (Wang et al. 2016; Luan et al. 2019). According to their geometrical interfaces, electrical conduction, percolation networks, crack propagation, or tunnelling effects, strainresponsive functional composites’ operating mechanisms can be well-described (Zhang et al. 2020a). To increase the electrical properties of pure carbon-based conductive filler, previous studies has concentrated more on hybrid carbon and metal-based composite. Among the various types of metal based inks, AgNPs conductive inks is the most popular choice for printed flexible hybrid strain sensor due to its excellent oxidation resistance, electrical conductivity, and other suitable physical properties that give it good substrate adhesion. Wang et al. (2015) developed an Ag organic complex ink using annealed graphene nanosheets and AgNPs and inkjet printed on a PI substrate. The resistivity of the prepared conductive traces is 4.62 × 104 Ω/m with 15 printing cycles and annealing at 300 °C for 40 min. The conducive ink was developed through the hybridization of graphene with AgNPs, which is ideal for flexible electronics. Another study used an aerosol-jet approach to create and print hybrid conductive graphene/AgNPs inks. The printed graphene/AgNPs conductive trcaes exhibited electrical conductivity of 1.07 × 10–4 Ω/cm after being annealed at 250 °C, which is higher than that of pure Ag and graphene inks (Jabari and Toyserkani 2016). Lee et al. (2021) reported pintable electrodes strain sensor fabricated from AgNPs embedded with SWCNTs. The fabricated screen-printed strain sensor showed good electrical conductivity with 4907 S cm−1 and a highly stretchable stability of over 10,000 cycles under a 20% strain. Moreover, the printed sensor showed low strain range, under 20% with gauge factor (GF) 76 and 5% with GF 5.4. In another work, CNTs/AgNWs/TPU composites strain sensor were fabricated by soaking in PDMS solution (Lin et al. 2021). The higher electrical conductivity (3506.8 Sm−1 ) of the composite film were found in this study. AgNWs self-assembled within the pores of the TPU/CNT nanofiber composites, resulting in high sensitivity and stability. By both the effects of capillary action, surface tension, and hydrogen bonding, the TPU/ ACNT acts as a template that draws Ag NWs into the pores. PDMS also acts an interface and binding the AgNWs. When exposed to high strain, this combined interaction keeps the contacts between AgNWs from breaking. As a result, the sensor’s

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Fig. 4.1 Schematic illustration of graphene, AgNPs and grapehen/AgNPs hybrid conductive film (Htwe et al. 2020)

working range was broad, and its sensitivity was high (Lin et al. 2021). In 2020, Htwe et al. (2020) reported inkjet printed strain sensor based on graphene/AgNPs hybrid on PVA substrate. From this study, the electrical conductivity was notably increased by the number of printing cycle, as well as the annealing time. The sensitivity performance of the printed hybrid sensor is higher than that of single graphene and AgNPs printed sensors under the strain range up to 20%. The hybrid of graphene and AgNPs demonstrated additional advantages in comparison to the ink based on single nanoparticles, as illustrated in the schematic picture in Fig. 4.1. Feng et al. (2021) fabricated carbon nanofibers (CNF) and AgNWs composite strain sensor by using drop-casting techniques on silicon dioxide wafer. The laser irradiation helps to enhance the electrical properties of the composite structure. It can be revealed that the CNF and AgNWs are consequently electrically connected, and the interfacial resistance at the junction is drastically reduced because of the laser radiation. Qi et al. (2020) fabricated screen-printed strain sensor based on carbon black/Ag composite with PET substrate. Figure 4.1 shows the schematic diagram of printed strain sensor. The printed composited based strain sensor performed a high gauge factor of 444.5 for an applied strain of 0.6–1.4% with a durability of 1000 cycles and linearity of R2 is 0.9974. Aerodynamically focused nanomaterials (AFN) printing technology was utilised to create flexible strain sensors that have a high degree of sensitivity and durability. Min et al. (2019) investigated directed printed strain sensor fabricated from AgNPs/MWCNTs on PDMS substrate. Figure 4.2a depicts the method by which AgNPs/MWCNTs nanocomposites are deposited onto a soft and flexible polymer substrate, and Fig. 4.2b–e show pictures taken using a scanning electron microscope (SEM) at various stages of printing. From this work, the printed nanocomposites strain sensor has good mechanical stability, a broad measured range, and high sensitivity. The sensitivity of the printed strain sensor showed 58.7 under 74% strain range and 1000 times life cycle evaluation test. Although the fact that the market for flexible strain sensors for wearable technology is increasing, it is still difficult to attain good sensitivity at 100% stretchability of human motions. Niu et al. (2021) reported CNTs/AgNPs composite based

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Fig. 4.2 a Schematic diagram of aerodynamically focused nanomaterials (AFN) printing mechanism for AgNPs/MWCNTs nanocomposites onto soft substrates, b–e SEM image of printed AgNPs/ MWCNTs nanocomposites onto flexible substrates on each printing step (Min et al. 2019)

strain sensor PU stretchable substrate. In terms of high sensitivity (gauge factor of up to 1004.5), wide usable stretching range (150%), good stability (3000 cycles @ 150%), and fast response (53 ms) and recovery, the as-prepared PU/CNTs/PD/AgNPs sensor outperforms the standard PU/CNTs sensor by a significant margin (80 ms). Besides that, highly stretchable and sensitive strain sensor using AgNWs/MWCNTs on hair band is fabricated by Cao et al. (2021). By dipping a hair band into a suspension of Ag NWs/MWCNTs composites, the fabrication technique of these sensors allowed for easy, cost-effective, and scaleable fabrication. The best performance was demonstrated by the hair band-based strain sensor with an AgNWs/MWCNT mass ratio of 30, which had a high sensitivity of 416.0 and high stretchability up to 70%. Table 4.1 shows previous studies of carbon and metal-based composites/hybrid strain sensor applications. Based on the table, high electrical properties of composite were observed in AgNWs based composite strain sensors. The percentage of the strain range are also higher in AgNWs based composite sensor. The substrate performs as the flexible support part of the sensor, which provide desirable mechanical flexibility and stretchability. TPU based strain sensors showed higher strain range and sensing performance. Moreover, the fabrication process of the sensors is also important for stability/durability of the sensor. From the table, screen-printed strain sensor showed higher durability performance those compared with other strain sensor.

4.3 Carbon and Polymer Based Strain Sensor In the few decades, extensive research on carbon nanomaterials and their interconnections with polymer matrices has been conducted. It is challenging to evenly distribute these carbon materials throughout the polymer composite complex due

4.3 Carbon and Polymer Based Strain Sensor

45

Table 4.1 Previous studies of carbon and metal-based composites/hybrid strain sensor applications Materials

Fabrication Conductivity Sensing process range

GF

AgNPs/ SWCNTs PDMS

Screen printing

4907 S cm−1

20% and 5%

76 and Linear 5.4

10,000

Lee et al. (2021)

CNF/ AgNWs Silicon wafer

Drop casting

105 Ω

76%

660

–

Feng et al. (2021)

CNT/ AgNWs/ TPU PDMS

Soaking

3506.8 Sm−1

38–100% 1.36 × Linear 105

1200

Lin et al. (2021)

carbon black/Ag PET

Screen printing

–

0.6–1.4% 444.5

Linear

1000

Qi et al. (2020)

AgNPs/ MWCNTs PDMS

Direct printing

–

74%

Linear

1000

Min et al. (2019)

CNTs/ AgNPs Nylon/PU

Dip coating

–

80–130% 125.4

Linear

1000

Zhao et al. (2021)

10,000

10%

Linear

3000

Han et al. (2017)

Ag/ Syringe Nanocarbon pump

58.7

60

Response Durability References type (cycles)

Linear

to their nanoscale characteristics. To obtain the desired sensor performance, many types of treatments and processes are carried out to ensure an ordered collection of nanomaterial interconnects and their dispersion (Afsarimanesh et al. 2020). The flexibility of the sensor was taken into consideration during the design and development of the flexible strain sensors. Flexibility, durability, detective ranges, and GF are a some of the factors that must be taken into consideration before manufacturing process. The desired properties can be achieved with the right carbon/polymer composite selection (Khalid and Chang 2022). Among all the carbon nanomaterials, 1-D CNTs have been widely used to fabricate flexible polymer composite for strain sensor (Hao et al. 2018; Ren et al. 2019). The fabrication process of aligned CNT with PDMS composite for the multidimensional sensing of strains of up to 260% with high durability (Fig. 4.3a) (Ma et al. 2018). The stable response of the sensor in the non-aligned direction while maintaining a high GF of 437 at the highest strain in the aligned direction due to damage, slippage, and cracks in the CNT networks can be used to explain the mechanisms of multidimensional sensing. As can be seen in Fig. 4.3b, a strain sensor made of an aligned CNT/PDMS composite has been constructed by Akhtar and Chang (2021). The flexible strain sensor was fabricated and analysed with different wt% of chemically functionalised CNTs, and an electric field was then used to align the CNTs in PDMS matrix to

46

4 Composites and Hybrid Based Printed Strain Sensor

establish the ideal concentration beyond the percolation threshold (Sang et al. 2019). The substrate and encapsulating PDMS layers were applied using an house applicator. High GF and good repeatability were displayed by the highly aligned CNT/PDMS strain sensor (approximately 780). The sensing performance was further enhanced (GF = 1533) in an extension of this study using a super radially aligned CNT/PDMS composite, which also offered radial strain sensing capabilities (Akhtar and Chang 2021). In addition to robotic hand control, it was employed for hand gesture detection. To fabricate a flexible strain sensor, Chen et al. (2020) developed a simple spray deposition and transfer method employing a sandwich-like PDMS/carbon nanotubes (CNTs)/PDMS composite comprising with an even and ultrathin conducting layer. The stretchability and sensing characteristics of the prepared strain sensors were excellent. With applications in human motion detection, wearable electronics, and e-skin, it demonstrated a good optical transmittance of 53.1% at 550 nm, a wide sensing range of over 130%, and the capacity to feel both the small motions of facial expressions and the massive motions of human joints. In another work, screen printed MWCNTs/PDMS composite ink was prepared for flexible strain sensor (Fig. 4.4) (Yang et al. 2022). The printed flexible sensors demonstrated an excellent linearity up to 100% deformation and a high gauge factor of 1.55. More than 4000 strain cycles without deterioration served as evidence of the good sensor stability, repeatability, and quick dynamic response. Further to develop a highly flexible strain sensor based on the nano composites mixing PDMS with the hybrid CNTs and CB conductive fillers, Zheng et al. (2018) used a solution mixing-casting technique. The developed strain sensor based on CNTs/CB/PDMS composite showed good stretchability (330%), sensitivity, and linearity in a monotonic stretching test. The strain sensor showed excellent repeatability, stability, and durability through stretching-releasing cycles (2500 cycles at 200% strain). In another study, a strain sensor was recently fabricated by Zhang et al. (2020b) based on CNTs/CB/PDMS. The fabrication procedure of the flexible strain sensor is showed in Fig. 4.5. A porous resistance strain sensor fabricated of CB and MWCNTs was developed by this group. The surface of a porous PDMS substrate was sprayed with a layer of synergized conductive networks created by CB and multi-walled CNTs

Fig. 4.3 Fabrication process of strain sensor based on carbon-based composite a Aligned carbon nanotube/PDMS (Ma et al 2018) and b Aligned CNTs/PDMS composite-based strain sensor (Akhtar and Chang 2021)

4.3 Carbon and Polymer Based Strain Sensor

47

using an easy and less cost spraying approach. By combining the advantages of the synergistic effects of mixed CB and CNTs with their porous PDMS structure, the performance of the sensor was enhanced. The indicated porous flexible strain sensor displayed strong linearity, high stability, a high GF (up to 61.82) and wide strain range (0–130%). Another interesting carbon-based two-dimensional nanomaterial graphene has been employed to develop high-performance flexible strain sensor based on nanocomposite materials. Graphene has high electrical conductivity, which can be mixed to PDMS to form graphene/PDMS conductive film. Zhang et al. (2019) developed flexible strain sensor based graphene/PDMS was fabricated by using a conventional spin-coating technique to prepare graphene/PDMS conductive film. The prepared sensor shown high stability and it showed high sensitivity. A onestep laser patterning process was used by Tao et al. (2017) to fabricate graphene/ Ecoflex strain sensor with self-adapted and tunable performance. The prepared sensor displayed GF of up to 457 with a 35% strain range or 268 with 100% strain range. By electrospinning carbon/graphene composite nanofiber yarn (CNY) and thermoplastic polyurethane (TPU), Yan et al. (2018b) fabricated flexible strain sensor. Due to linearity, brittleness, and high conductivity of the CNYs, the prepared flexible strain sensor demonstrated outstanding sensitivity and stability. They investigated that the

Fig. 4.4 Illustration of fabrication process and screen-printed flexible strain sensor (Yang et al. 2022)

48

4 Composites and Hybrid Based Printed Strain Sensor

Fig. 4.5 Fabrication process of the porous flexible strain sensor (Zhang et al. 2020b)

average GF value was observed around 1700 under 2% strain when the quantity of yarns and substrate thickness were 4 and 129 m, respectively. Conversely, after 300 stretching-releasing cycles, the developed sensor showed a high level of stability. The flexible strain sensor was also capable of correctly detecting minute deformations. The fabrication process and testing such as bending and twisting were shown in Fig. 4.6. Recently, polyurethane and silicone elastomers were added to highly porous and associated graphene gels to create two different types of stretchable and conductive networks in Fig. 4.7 (Song et al. 2020). In the low percentage filler of 0.8 wt%, the polymer-infiltered graphene network displayed a high electrical conductivity of about 100 S/m. The graphene composites showed different electrical characteristics under stretching and bending situations due to its outstanding structural integrity. For example, under 50% tensile strain, a maximum conductivity of 47 S/m was reached and after 500 cycles of bending, the resistance variation was less than 5%. Figure 4.8 illustrates the creation of a skin-like strain sensor made of a closed-pore porous structured laser-induced graphene (LIG)/PDMS/photosensitive PI (PSPI) composite (Jeong et al. 2021). A skin-like PDMS and PSPI substrate was produced by mixing and spin coating it, and then graphene was written directly onto the closedpore porous structures using a laser. This provided the sensor the additional ability to flex the conductive networks in response to the applied strain. The skin-like sensor displayed quick reaction and recovery times, great stability, and a 120% strain range. It served as a useful electronic skin for the purpose of detecting different human motions. The fabrication of a flexible and bendable strain sensor using a nanocarbon black (nCB)/PDMS composite is shown in Fig. 4.9 (Zhang et al. 2020c). After

4.3 Carbon and Polymer Based Strain Sensor

49

Fig. 4.6 Fabrication and characterization process of composite nanofiber yarn strain sensor (Yan et al. 2018b)

Fig. 4.7 Schematic illustration of the fabrication processes of graphene-based nanocomposites films (Song et al. 2020)

50

4 Composites and Hybrid Based Printed Strain Sensor

Fig. 4.8 Fabrication processes of graphene (LIG)/PDMS/photosensitive PI (PSPI) compositebased skin-like strain sensor (Jeong et al. 2021)

being prepared with a toluene bath, a swollen PDMS substrate was placed into a nCB suspension for a diffusion-based filling mechanism to create a gradient-filled nanocomposite structure in the PDMS. By performing numerous morphological and electromechanical characterisations, this study demonstrated the diffusion-based gradient filling of nCB reinforcements in considerable detail. The produced strain sensor had a large strain range and a high GF, which put it in a difficult parametric evaluation region. Due to its compatibility with conventional UV photolithography, an SU-8/carbon composite-based cantilever strain sensor was created by Gammelgaard et al. (2006), who took inspiration from the micro-machined cantilever design. The SU-8 photoresist and carbon particles were ultrasonically mixed to produce the polymer composite. Based on deflections, the cantilever sensor identified variations in surface stress, which changed the piezo resistance.

Fig. 4.9 a PDMS/nCB nanoposites b photographs of the samples c sample under stretching d wrapping e folding, and f twisting (Zhang et al. 2020c)

4.4 Metal and Polymer Based Strain Sensor

51

4.4 Metal and Polymer Based Strain Sensor Metal and polymer composites are also popular for synthesising functional nanocomposites for wearable strain sensor applications. In fact, they are better for composite structure because of their higher electrical conductivities. A composite composed of silver nanowires, polyacrylic acid, phytic acid, and aniline is shown in Fig. 4.10a (Soe et al. 2021). It was created by mixing and agitating the ingredients. Ammonium persulfate was also included in the final composite solution, which was degassed before being cast into a polytetrafluoroethylene mould. The resulting composite membrane was cut into strips of the required sizes for electromechanical characterization after the cast had time to sinter in a sintering furnace. The sensor showed a high stretchability of above 500%, but the GF is low. According to the explanation, the functioning mechanism depended on bubble-like formations that tended to expand as a result of the applied strain, breaking the associated Ag NWs network. The sensor was utilised to record electrocardiograms (ECGs), analyse speech signals, and detect human motion. Similar to this, Soe et al. (2021) developed AgNW, AgNP, and styrene–butadiene– styrene (SBS) hybrid composite fibres using wet spinning techniques. This method benefited from the reinforced electrical conductivities of the AgNWs that bridged the AgNPs conductive networks in the polymer matrix, even when stretched. Due of this, the composite was able to have high initial conductivities and high elongation at break of 900%. In some cases, metallic NPs have also been investigated for the fabrication of flexible strain sensors’ nanocomposite materials. A composite made of AgNP and PDMS was produced as seen in Fig. 4.10b, using a variety of fast

Fig. 4.10 Fabrication of metal nanowire based composites a Polyaniline with polyacrylic acid and phytic acid are mixed with AgNWs and b AgNPs/PDMS strain sensor (Soe et al. 2021)

52

4 Composites and Hybrid Based Printed Strain Sensor

fabrication techniques as drop casting, screen printing, and spin coating (Soe et al. 2021). The sensor’s responses were compared when the three construction procedures were used, and it was constructed as a sandwich composite of PDMS, Ag NPs, and PDMS. For both drop casting and screen printing, the sensor recorded the highest GF of 10.08. The propagation of the microcracks in the conductive networks of AgNPs was a major factor in the strain sensing process. The drop-casting method produced the most advantageous performance characteristics. Zhang et al. (2018) reported the development of an AuNP/PDMS nanocomposite structure in the shape of a conductive fibre; they demonstrated the continuation of the conductive pathways after releasing the strain on the deposited gold films, which was a significant issue in conventional gold-film-based strain sensors. The crack length in the fibre-structured composites was decreased. Even though sensitivity went through, the strain range was increased to 100%. Sun et al. used photolithography to fabricate a micropatterned Au/singlewalled CNT (SWCNT) composite on a PDMS substrate that was patterned after a spider’s sensory slit organ. The most responsive and sensitive to small and light stresses were the highly conductive Au routes, although the less conductive SWCNTs increased the sensing range by up to 100%, with the GF reaching 3.4 × 106 . Metal nanowires (NWs) have comparable optical transparency to other nanomaterials. Based on crack-induced AgNW networks, Lee et al. (2017) developed a highly transparent (more than 90% at 550 nm wavelength) and flexible strain sensor. The PDMS layer on the glass substrate was spin-coated with AgNW, and the AgNW/PDMS film was then covered with a synthetic encapsulant. The device’s sensing capabilities can be improved by working with gold nanowires in addition to Ag nanowires. A flexible and wearable strain sensor was developed by Yin et al. (2019) using an AgNW network structure as a conductive layer on PDMS. This sensor demonstrated superior transmittance, stability, and ultra-high sensitivity with a huge gauge factor of 84.6 at a strain of 30%. Additionally, ultra-transparency was achieved by controlling the concentration of droplets during the pulling process to control the AgNW film’s thickness. As illustrated in Fig. 4.11, the sensor’s performance in detecting human muscle and touching motions showed great repeatability and reproducibility, a rapid reaction, and high stability. Although metal nanowires have demonstrated remarkable compatibility with various solvents (Wang et al. 2014), their main drawback is their inability to adhere to flexible polymer substrates. A change in the conductive path results from the segregation of nanowires on polymer substrate that occurs with the application of strain. This behaviour was magnified over time, and a nonlinear reaction was created. The cost and limited sensitivity of nanowires are other issues that need to be improved.

References

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Fig. 4.11 Fabrication process of the sensitive and transparent flexible sensor (Yin et al. 2019)

4.5 Summary In this chapter, the previous research trends in the development of flexible and stretchable strain sensors, from the perspective of nano-composite and hybrid strain sensor for wearable electronics applications were discussed. Flexible and stretchable strain sensors based on carbon, metal and polymer-based composites and hybrid were considered. Carbon based composite were found to be the most studied nanocomposites for stretchable strain sensor applications, due to their high-performance characteristics. However, there is still a need for unique and demanding applications, such prosthetic devices, implantable systems, and body-worn robotic stimulation devices, to increase design and performance through advanced composites and structures.

References Afsarimanesh N, Nag A, Sarkar S et al (2020) A review on fabrication, characterization and implementation of wearable strain sensors. Sens Act, A Phys 315:112355. https://doi.org/10.1016/j. sna.2020.112355 Akhtar I, Chang SH (2021) Radial alignment of carbon nanotubes for directional sensing application. Compos Part B Eng 222:109038. https://doi.org/10.1016/j.compositesb.2021.109038

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Aziz S, Jung KC, Chang SH (2019) Stretchable strain sensor based on a nanocomposite of zinc stannate nanocubes and silver nanowires. Compos Struct 224:111005. https://doi.org/10.1016/ j.compstruct.2019.111005 Cao Y, Lai T, Teng F et al (2021) Highly stretchable and sensitive strain sensor based on silver nanowires/carbon nanotubes on hair band for human motion detection. Prog Nat Sci Mater Int 31:379–386. https://doi.org/10.1016/j.pnsc.2021.05.005 Chen J, Zhu Y, Jiang W (2020) A stretchable and transparent strain sensor based on sandwichlike PDMS/CNTs/PDMS composite containing an ultrathin conductive CNT layer. Compos Sci Technol 186:107938. https://doi.org/10.1016/j.compscitech.2019.107938 Fang W, Jang HW, Leung SN (2015) Evaluation and modelling of electrically conductive polymer nanocomposites with carbon nanotube networks. Compos Part B Eng 83:184–193. https://doi. org/10.1016/j.compositesb.2015.08.047 Feng J, Tian Y, Wang S et al (2021) Femtosecond laser irradiation induced heterojunctions between carbon nanofibers and silver nanowires for a flexible strain sensor. J Mater Sci Technol 84:139– 146. https://doi.org/10.1016/j.jmst.2020.12.060 Gammelgaard L, Rasmussen PA, Calleja M et al (2006) Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor. Appl Phys Lett 88:7–9. https://doi.org/10.1063/1.2186396 Han JT, Jang JI, Cho JY et al (2017) Synthesis of nanobelt-like 1-dimensional silver/nanocarbon hybrid materials for flexible and wearable electroncs. Sci Rep 7:1–9. https://doi.org/10.1038/ s41598-017-05347-4 Hao B, Mu L, Ma Q et al (2018) Stretchable and compressible strain sensor based on carbon nanotube foam/polymer nanocomposites with three-dimensional networks. Compos Sci Technol 163:162–170. https://doi.org/10.1016/j.compscitech.2018.05.017 Htwe YZN, Hidayah IN, Mariatti M (2020) Performance of inkjet-printed strain sensor based on graphene/silver nanoparticles hybrid conductive inks on polyvinyl alcohol substrate. J Mater Sci Mater Electron 31:15361–15371. https://doi.org/10.1007/s10854-020-04100-4 Jabari E, Toyserkani E (2016) Aerosol-Jet printing of highly flexible and conductive graphene/silver patterns. Mater Lett 174:40–43. https://doi.org/10.1016/j.matlet.2016.03.082 Jeong SY, Lee JU, Hong SM et al (2021) Highly skin-conformal laser-induced graphenebased human motion monitoring sensor. Nanomaterials 11:1–15. https://doi.org/10.3390/nan o11040951 Khalid MAU, Chang SH (2022) Flexible strain sensors for wearable applications fabricated using novel functional nanocomposites: a review. Compos Struct 284:115214. https://doi.org/10.1016/ j.compstruct.2022.115214 Lee CJ, Park KH, Han CJ et al (2017) Crack-induced Ag nanowire networks for transparent, stretchable, and highly sensitive strain sensors. Sci Rep 7:1–8. https://doi.org/10.1038/s41598017-08484-y Lee JW, Cho JY, Kim MJ et al (2021) Synthesis of silver nanoparticles embedded with singlewalled carbon nanotubes for printable elastic electrodes and sensors with high stability. Sci Rep 11:1–10. https://doi.org/10.1038/s41598-021-84386-4 Lin L, Choi Y, Chen T et al (2021) Superhydrophobic and wearable TPU based nanofiber strain sensor with outstanding sensitivity for high-quality body motion monitoring. Chem Eng J 419:129513. https://doi.org/10.1016/j.cej.2021.129513 Luan J, Wang Q, Zheng X et al (2019) Flexible metal/polymer composite films embedded with silver nanowires as a stretchable and conductive strain sensor for human motion monitoring. Micromachines 10.https://doi.org/10.3390/mi10060372 Ma L, Yang W, Wang Y et al (2018) Multi-dimensional strain sensor based on carbon nanotube film with aligned conductive networks. Compos Sci Technol 165:190–197. https://doi.org/10. 1016/j.compscitech.2018.06.030 Min SH, Lee GY, Ahn SH (2019) Direct printing of highly sensitive, stretchable, and durable strain sensor based on silver nanoparticles/multi-walled carbon nanotubes composites. Compos Part B Eng 161:395–401. https://doi.org/10.1016/j.compositesb.2018.12.107

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human motion detection. ACS Appl Mater Interfaces 12:44360–44370. https://doi.org/10.1021/ acsami.0c13442 Zhang P, Chen Y, Li Y et al (2020b) Erratum: a flexible strain sensor based on the porous structure of a carbon black/carbon nanotube conducting network for human motion detection. Sensors 20:1154. https://doi.org/10.3390/s20041154. Sensors (Switzerland) 20. https://doi.org/10.3390/ s20102901 Zhang R, Li S, Ying C et al (2020c) Bioinspired design of flexible strain sensor with high performance based on gradient filler distributions. Compos Sci Technol 200. https://doi.org/10.1016/ j.compscitech.2020.108319 Zhao SQ, Zheng PX, Cong HL, Wan AL (2021) Facile fabrication of flexible strain sensors with AgNPs-decorated CNTs based on nylon/PU fabrics through polydopamine templates. Appl Surf Sci 558:149931. https://doi.org/10.1016/j.apsusc.2021.149931 Zheng Y, Li Y, Dai K et al (2018) A highly stretchable and stable strain sensor based on hybrid carbon nanofillers/polydimethylsiloxane conductive composites for large human motions monitoring. Compos Sci Technol 156:276–286. https://doi.org/10.1016/j.compscitech.2018.01.019

Chapter 5

Performance Evaluation of Strain Sensor

Abstract The performance of flexible strain sensor is very important for the application for health monitoring devices. In this chapter, the performance evaluations of flexible strain sensors such as stretchability, sensitivity, linearity are discussed.

5.1 Introduction Flexible strain sensor can be converted physical deformations into electrical signals. The sensing technique has a significant impact on how well flexible and stretchable strain sensors function. When a material is under tensile stress, strain is defined as the ratio of the changes in length to the initial length; when a material is under compression, strain can be either positive or negative. Different measurements of performance, such as stretchability, sensitivity, linearity, hysteresis, and durability, could be used to assess the effectiveness of flexible/stretchable strain sensors in human motion monitoring and other applications. These factors are important for flexible/stretchable and wearable strain sensor representation because in these applications, expensive, long-lasting, and continuous strains may be attached to the strain sensors.

5.2 Stretchability Stretchability is a measure of how much strain a strain sensor can withstand while still maintaining its structural integrity and response stability. Stretchability refers to a material’s capacity to tolerate extension to a specific length without suffering permanent distortion. This parameter mainly depends on the type of conductive materials or substrate, fabrication techniques, and the aspect ratio of the material used as sensing elements. Due to the rigidity of the material, conventional strain sensors (metal or semiconductor) typically have low stretchability. Numerous studies have described their efforts to develop high stretchability and sensitivity in a strain sensor © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_5

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5 Performance Evaluation of Strain Sensor

in order to overcome this obstacle. The movement of human motion can cause a large strain range more than 50%, and pulse and respiration cause a subtle strain less than 1% (Jeong et al. 2015; Yamada et al. 2011). As suggested by Roh et al. (2015) two methodologies are often employed to obtain a high stretchability, namely, “materials that stretch” and “structures that stretch”. For the materials, it is known that the strain sensors are mainly composed with the sensing materials, such as CNTs, graphene, metal nanoparticles, and metal nanowires, the selection of a flexible and stretchable polymer could be a facile and preferred option, and the commonly used polymers include PDMS, PU, PET, PI, natural rubber, and gel. Furthermore, it is thought that 1D materials are advantageous for improving stretchability since the development of conductive networks can be facilitated by their high aspect ratio and the networks can remain stable even under high strain levels. In contrast, the created networks can break under light strain and additional conductive networks are easily generated due to the significantly lower aspect ratio. On the other hand, good structure design might result in a better stretchability, for instance by creating distinctive open-mesh geometries (Fig. 5.1a), wherein the strain sensor made of aligned SWCNTs can break into gaps and islands but bridged by bundles. The SWCNTs strain sensor now has a substantially wider work range of 280% thanks to this method (Yamada et al. 2011). Another example is the wrinkled CNT film strain sensor in Fig. 5.1b, the wrinkled CNT networks can measure an ultrahigh strain of 750% with high sensitivity (Park et al. 2016). Other structures include wavy geometry helical structure (Feng et al. 2011; Wang et al. 2011; Xu et al. 2011).

Fig. 5.1 (a) Scheme showing the fabrication procedures of SWCNT stain sensors with open-mesh geometries

5.3 Sensitivity

59

5.3 Sensitivity The level of responsiveness to both internal and external changes is referred to as sensitivity.. Sensitivity of a strain sensor is represented by GF, where the ratio of the changes in relative electrical resistance to the applied tensile strain. Figure 5.2 shows the illustration of the strain measurement in printed materials. It can be calculated by the following Eq. (5.1): GF =

∆R Ro

ε

(5.1)

where Ro refers to the initial resistance, ∆R represents the change between Ro and the final resistance (Rf ) when applied with a certain strain (ε). The equation states that the more sensitive the substrate is to the same applied strain, the higher the change in resistance and lower the starting resistance. The GF maintains at 2–5, which is insufficient to be used for a wearable and practical strain sensor for the conventional metal-foil-based strain sensor. In contrast, a flexible strain sensor’s sensitivity can be adjusted to an extremely high degree. The mechanisms, components, and structural elements of the strain sensor influence a wide variety of sensitivity variations. In general, due to theoretical limitations, the capacitive strain sensors display a very small GF ( ≤1) (Cai et al. 2013; Cooper et al. 2017). As a result, resistive strain sensors are far superior to capacitive ones. The sensitivity of the flexible resistive strain sensor is mostly determined by the conductive materials, including the manufacturing processes, concentrations, and assembly structure (Frutiger et al. 2015). For instance, it has been discovered that a yarn-based graphene/PU strain sensor’s sensitivity can be greatly influenced by the concentration of graphene and the number of coatings or printings applied to the sensors; a higher graphene concentration and less coating lead to a higher sensitivity (FIG). Additionally, the hybrid structure with two distinct shaped conductive materials can significantly increase sensitivity in comparison to the single conductive material. Fig. 5.2 Illustration of the strain measurement in materials

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5.4 Linearity A relationship’s ability to be represented as a straight line is known as linearity. When evaluating whether a sensor’s transfer function can be roughly approximated by a straight line, linearity is used (Souri et al. 2020). The coefficient of determination (R2) produced from a linear regression is used to quantify the linearity of the strain sensor. A linear strain sensor’s R2 value is higher and its output signal barely deviates from a straight line. In contrast, if R2 is low, a strain sensor is nonlinear. Stretchable strain sensors’ linear behaviour across a wide strain range is highly desired since nonlinearity complicates calibration and data processing of the output signal (Mao et al. 2021). The nonlinear response of resistive-type strain sensors is attributed to the non-homogeneous microstructural and morphological changes in the sensing films upon stretching (Cao et al. 2021). Overall, a significant barrier still exists in the creation of extremely stretchy, sensitive, and linear electrochemical response strain sensors. Most stretchable strain sensors have been found to exhibit a trade-off between “high sensitivity and linearity” and “high stretchability” (Yang et al. 2017). High nonlinearity (or linearity in many locations) and limited stretchability are common characteristics of strain sensors that are highly sensitive. For instance, flexible strain sensors with high nonlinearity that rely on the development of microcracks and a disconnection mechanism have shown better sensitivities than those with other sensing techniques (Lu et al. 2018).

5.5 Hysteresis Hysteresis is a retardation of the effect when the forces acting upon a body are changed. Since extreme hysteresis prevents the detecting qualities from returning to their former condition and causes uncertainty, weakening, and a longer response time for the strain sensor, a strong hysteresis performance becomes crucial, especially under long-term operation. The hysteresis can be evaluated by hysteresis error (δh) as following Eq. (5.2) (Li et al. 2017): δh =

∆Rmax di f f er ence × 100% ∆R

(5.2)

where ∆Rmax difference , ∆R refers to the maximum difference between resistances as decided by the stretching and releasing stages, and the whole resistance change, respectively. In fact, it is commonly accepted that the viscoelastic properties of polymers play a major role in causing hysteresis (Amjadi et al. 2016), as internal of viscoelastic material tends to decrease over time and it cannot be completely recovered after releasing it from strain. One way to improve the hysteresis is the structuring design of flexible polymer substrate (Wang et al. 2014), for it is proven that the sensor will have enhanced mechanical properties, followed by higher sensitivity and faster

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response time than unstructured one. When applied with external strain, the microstructured polymer can elastically deform, which will store and release the energy reversibly to minimize the inherent visco-elastic behaviour (Mannsfeld et al. 2010). Moreover, the interaction between conductive filler and polymers can also cause hysteresis (Claypole et al. 2021). With a few exceptions, such as AgNWs-PDMS nanocomposite-based sensors for greater strains, where hysteresis is brought on by the PDMS elastomer itself, the hysteresis for strain sensors is typically low (40%) (Singh et al. 2021). Flexible nanomaterial conductive filler and polymers are strongly interfacially bonded by CNT and graphene, which improves performance (Mehmood et al. 2020). In the case of weak elastic nanomaterials, they can slide inside the polymer grids when exposed to high stretching. They cannot return to their previous locations right away following the release of tension because of the weakness in their elasticity, which causes excessive hysteresis (Gu et al. 2019). On the other hand, rigid nanomaterials and polymers have very weak interfacial adhesion when it comes to metallic nanowires. (Jiang et al. 2019). They can fully recover to their initial positions upon releasing the stress. While releasing cycle bucking and fracture of nanomaterials, the interfacial binding is strong enough. Research shows that the hysteresis behaviour of the CNT-polymer nanocomposite-based strain sensors can also be explained (Kanoun et al. 2021).

5.6 Durability Durability is especially important for the practical application of flexible strain sensor. In daily using, for human motion detection, the strain sensors will undergo large, complex, and varying strains. In addition, the environment, which includes temperature, moisture, sweat, and external friction, has a significant impact on the strain sensor’s sensing abilities (Jiang et al. 2019). The two most common ways that a strain sensor can malfunction are fractured sensing materials and plastic deformation and fatigue of the soft polymer substrate. The most preferred choice for encapsulating sensing materials in a flexible polymer layer that protects them from external influences is flexible PDMS and PVA due to their outstanding elasticity, transparency, forming, and biocompatibility (Wang et al. 2019). The durability of the strain sensor for dynamic stretching/releasing tests can be significantly increased with the outside protective covering (Zheng et al. 2020). However, in the most reported results, the durability for continuous testing stays in the level of 102 –103 times, although some strain sensors can reach a relatively high level of 104 –106 times (Wang et al. 2016).

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5.7 Summary In this chapter, we have reported the important evaluation of flexible strain sensor. To improve the performance characteristics for practical applications, it is necessary to further investigate issues such greater percolation thresholds, non-linearity, variable strain gauges, multidimensional sensing, breathability, biocompatibility, self-healing, and wearable system integration.

References Amjadi M, Kyung KU, Park I, Sitti M (2016) Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review. Adv Func Mater 26(11):1678–1698. https:// doi.org/10.1002/adfm.201504755 Cai L, Song L, Luan P, Zhang Q, Zhang N, Gao Q, Zhao D, Zhang X, Min T, Yang F, Zhou W, Fan Q, Luo J, Zhou W, Ajayan PM, Xie S (2013) Super-stretchable, transparent carbon nanotube-based capacitive strain sensors for human motion detection. Sci Rep 3:1–9. https://doi.org/10.1038/ srep03048 Cao Y, Lai T, Teng F, Liu C, Li A (2021) Highly stretchable and sensitive strain sensor based on silver nanowires/carbon nanotubes on hair band for human motion detection. Progr Nat Sci: Mater Int 31(3):379–386. https://doi.org/10.1016/j.pnsc.2021.05.005 Claypole A, Claypole J, Kilduff L, Gethin D, Claypole T (2021) Stretchable carbon and silver inks for wearable applications. Nanomaterials 11(5):1200. https://doi.org/10.3390/nano11051200 Cooper CB, Arutselvan K, Liu Y, Armstrong D, Lin Y, Khan MR, Genzer J, Dickey MD (2017) Stretchable capacitive sensors of torsion, strain, and touch using double helix liquid metal fibers. Adv Func Mater 27(20). https://doi.org/10.1002/adfm.201605630 Feng X, Yang BD, Liu Y, Wang Y, Dagdeviren C, Liu Z, Carlson A, Li J, Huang Y, Rogers JA (2011) Stretchable ferroelectric nanoribbons with wavy configurations on elastomeric substrates. ACS Nano 5(4):3326–3332. https://doi.org/10.1021/nn200477q Frutiger A, Muth JT, Vogt DM, Mengüç Y, Campo A, Valentine AD, Walsh CJ, Lewis JA (2015) Capacitive soft strain sensors via multicore-shell fiber printing. Adv Mater 27(15):2440–2446. https://doi.org/10.1002/adma.201500072 Gu Y, Zhang T, Chen H, Wang F, Yueming P, Gao C, Li S (2019) Mini review on flexible and wearable electronics for monitoring human health information. Nanoscale Res Lett 14(1):1–15. https://doi.org/10.1186/s11671-019-3084-x Jeong YR, Park H, Jin SW, Hong SY, Lee SS, Ha JS (2015) Highly stretchable and sensitive strain sensors using fragmentized graphene foam. Adv Func Mater 25(27):4228–4236. https://doi.org/ 10.1002/adfm.201501000 Jiang Z, Nayeem MOG, Kenjiro Fukuda S, Ding HJ, Yokota T, Inoue D, Hashizume D, Someya T (2019) Highly stretchable metallic nanowire networks reinforced by the underlying randomly distributed elastic polymer nanofibers via interfacial adhesion improvement. Adv Mater 31(37):1–9. https://doi.org/10.1002/adma.201903446 Kanoun O, Bouhamed A, Ramalingame R, Bautista-Quijano JR, Rajendran D, Al-Hamry A (2021) Review on conductive polymer/CNTs nanocomposites based flexible and stretchable strain and pressure sensors. Sensors 21(2):1–29. https://doi.org/10.3390/s21020341 Li X, Hua T, Bingang X (2017) Electromechanical properties of a yarn strain sensor with graphenesheath/polyurethane-core. Carbon 118:686–698. https://doi.org/10.1016/j.carbon.2017.04.002 Lu H, Chen J, Tian Q (2018) Wearable high-performance supercapacitors based on Ni-coated cotton textile with low-crystalline Ni-Al layered double hydroxide nanoparticles. J Colloid Interface Sci 513:342–348. https://doi.org/10.1016/j.jcis.2017.11.046

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Mannsfeld SCB, Tee BCK, Stoltenberg RM, Chen CVHH, Barman S, Muir BVO, Sokolov AN, Reese C, Bao Z (2010) Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nat Mater 9(10):859–864. https://doi.org/10.1038/nmat2834 Mao J, Zhao C, Liu L, Li Y, Xiang D, Yuanpeng W, Li H (2021) Adhesive, transparent, stretchable, and strain-sensitive hydrogel as flexible strain sensor. Compos Commun 25(April):100733. https://doi.org/10.1016/j.coco.2021.100733 Mehmood A, Mubarak NM, Khalid M, Rashmi Walvekar EC, Abdullah MTH, Siddiqui HA, Baloch SN, Mazari S (2020) Graphene based nanomaterials for strain sensor application—a review. J Environ Chem Eng 8(3):103743. https://doi.org/10.1016/j.jece.2020.103743 Park SJ, Kim J, Chu M, Khine M (2016) Highly flexible wrinkled carbon nanotube thin film strain sensor to monitor human movement. Adv Mater Technol 1(5). https://doi.org/10.1002/admt.201 600053 Roh E, Hwang BU, Kim D, Kim BY, Lee NE (2015) Stretchable, transparent, ultrasensitive, and patchable strain sensor for human-machine interfaces comprising a nanohybrid of carbon nanotubes and conductive elastomers. ACS Nano 9(6):6252–6261. https://doi.org/10.1021/acs nano.5b01613 Singh K, Sharma S, Shriwastava S, Singla P, Gupta M, Tripathi CC (2021) Significance of nanomaterials, designs consideration and fabrication techniques on performances of strain sensors—a review. Mater Sci Semiconductor Process 123(October 2020):105581. https://doi.org/10.1016/ j.mssp.2020.105581 Souri H, Banerjee H, Jusu A, Radacsi N, Stokes AA, Park I, Sitti M, Amjadi M (2020) Wearable and stretchable strain sensors: materials, sensing mechanisms, and applications, 2000039. https:// doi.org/10.1002/aisy.202000039 Wang X, Hong H, Shen Y, Zhou X, Zheng Z (2011) Stretchable conductors with ultrahigh tensile strain and stable metallic conductance enabled by prestrained polyelectrolyte nanoplatforms. Adv Mater 23(27):3090–3094. https://doi.org/10.1002/adma.201101120 Wang X, Yang G, Zuoping X, Zheng C, Ting Z (2014) Silk-molded flexible , ultrasensitive , and highly stable electronic skin for monitoring human physiological signals, 1336–1342. https:// doi.org/10.1002/adma.201304248 Wang Z, Huang Y, Sun J, Huang Y, Hong H, Jiang R, Gai W, Li G, Zhi C (2016) Polyurethane/ cotton/carbon nanotubes core-spun yarn as high reliability stretchable strain sensor for human motion detection. ACS Appl Mater Interfaces 8(37):24837–24843. https://doi.org/10.1021/acs ami.6b08207 Wang L, Chen Y, Lin L, Wang H, Huang X, Xue H, Gao J (2019) Highly stretchable, anti-corrosive and wearable strain sensors based on the pdms/cnts decorated elastomer nanofiber composite. Chem Eng J 362(October 2018):89–98. https://doi.org/10.1016/j.cej.2019.01.014 Xu F, Wei L, Zhu Y (2011) Controlled 3D buckling of silicon nanowires for stretchable electronics. ACS Nano 5(1):672–678. https://doi.org/10.1021/nn103189z Yamada T, Hayamizu Y, Yamamoto Y, Yomogida Y, Izadi-Najafabadi A, Futaba DN, Hata K (2011) A stretchable carbon nanotube strain sensor for human-motion detection. Nat Nanotechnol 6(5):296–301. https://doi.org/10.1038/nnano.2011.36 Yang T, Jiang X, Zhong Y, Zhao X, Lin S, Li J, Li X, Jianlong X, Li Z, Zhu H (2017) A wearable and highly sensitive graphene strain sensor for precise home-based pulse wave monitoring. ACS Sensors 2(7):967–974. https://doi.org/10.1021/acssensors.7b00230 Zheng Y, Li Y, Zhou Y, Dai K, Zheng G, Zhang B, Liu C, Shen C (2020) High-performance wearable strain sensor based on graphene/cotton fabric with high durability and low detection limit. ACS Appl Mater Interfaces 12(1):1474–1485. https://doi.org/10.1021/acsami.9b17173

Chapter 6

Printed Strain Sensor for Wearable Health Monitoring Applications

Abstract Wearable electronics have received a lot attention since they are revolutionize several areas of healthcare, motion tracking, rehabilitation, robotics, human– machine interaction, among others. Discussion in this chapter is focused on the potential applications of printed wearable strain sensors focusing in three areas: body motion, human–machine interface, and health care.

6.1 Introduction Printed flexible strain sensor can be easily mounted to the skin surface for the purpose of identifying skin deformations linked to bodily functions and medical conditions. High sensitivity is preferred for monitoring subtle deformations like blood pressure, breathing, phonation, and facial movements (Li et al. 2022). On the other hand, to detect large deformations such as finger flexing, wrist bending, and knee motions, a large sensing range and stretchability are required. Flexible strain sensor is certain to becomes standard in the area of medical care in the future, as demonstrated by the successful fabrication of flexible sensing devices with high sensitivity, low-cost, portability, and long-term stability (Gu et al. 2019).

6.2 Body Motion Detection The full-range strain sensor for body motion can also monitored using flexible printed strain sensors, including facial expressions and joint motions (Sun et al. 2020b). The strains sensor with high stretchability can be operated for the body motion detection such as finger motion, keen joints bending etc. Figure 6.1 shows a flexible strain sensor was mounted to the forehead and skin near the mouth to analyze expression (Roh et al. 2015), such as laughing, crying, blinking and eyeball movement, indicating the strains sensor is a potential tool to assess the disease like Alzheimer.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jaafar and Y. Z. Ni Htwe, Nanomaterials Based Printed Strain Sensor for Wearable Health Monitoring Applications, SpringerBriefs in Materials, https://doi.org/10.1007/978-981-99-5780-4_6

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Fig. 6.1 Time-dependent ∆R/Ro response for the sensor attached to the forehead, skin near the mouth, and the eye (Roh et al 2015)

The tension and relaxing of a muscle were also monitored to provide programs for athletes (Yan et al. 2018). By placing strain sensors on the joints in the neck, fingers, arm, wrist, and knee, it was possible to identify the significant strains brought on by human joint movements (Chen et al. 2018). Figure 6.2a, b shows strain sensor fixed to the knee and the relative change in resistance of leg movement during jumping (He et al. 2019). After a single leap by a volunteer, a steady signal might be recorded. It was possible to discern the various feedback signals produced by jogging, walking, jumping, and squatting. The bending angles of joints could also be detected according to the resistance of the sensor (Lu et al. 2019a). Lin et al. (2021) reported on wearable and conductive nanofiber composite (WCNC) strain sensor with handy preparation method. The strain sensor performed high sensitivity and large working strain (gauge factor is nearly 1.36 × 105 with the working strain ranging from 38 to 100%), which illustrates that the WCNC has a quite large work strain under extremely high GF. WCNC is adhered to different positions

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Fig. 6.2 a Photograph of the sensors attached to the human knee and b the resistance response in detecting leg movement during jumping (He et al. 2019)

of the human skin to monitor its corresponding movements, as shown in Fig. 6.3a. Further, Fig. 6.3b–d illustrates the R/R0 of the WCNC strain sensor behavior when monitoring the movements of the elbow. Figure 6.3e shows the detection signal R1 reflected by WCNC is 2.8 when a nodding movement is performed. Moreover, as shown in Fig. 6.3f–h, other large body movements are also detected, such as leg movements (walking and knee bending) and finger bending. In addition, vocal cord movement and pulse were shown in Fig. 6.3i, j. In another study, Kaidarova et al. (2019) reported wearable resistive bending sensors by LIG on polyimide. The LIG sensors possess an outstanding range for strain measurements reaching > 10%. Different measurements including joint-bendingrelated motions, finger bending, knee-related motion monitoring: walking, jogging, and squatting, microsleep detection by monitoring head nodding and waking up were observed as shown in Fig. 6.4. These results suggest that these sensors are very useful for monitoring the body’s health condition and evaluating athletes’ sport performance.

6.3 Human–Machine Interfaces To create human–machine interfaces for controlling smart devices, printed wearable strain sensors are also being investigated (e.g., computers, robots, and prosthetics). Interactions between humans and machines enable peaceful coexistence and productive collaboration in the digital age (Fu et al. 2023). Key enabling technologies for human machine interfaces include gesture and motion recognition, which can

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Fig. 6.3 a Schematic diagram illustrating the WCNC in monitoring mode for body motions. Monitoring of numerous body motions by transient resistance change (R/R0 using the WCNC as WSS: b–d) Elbow movements with different bending degrees (45°, 90°, and 135°). e Neck movement. f, g show the motions of walking and knee squatting. h Finger movement with water dropping. i Vocal cord movement (speaking “SNU”). j Pulse rate. (The insets reflect the various positions of human body where the WCNC is attached to detect the body motions) (Lin et al. 2021)

Fig. 6.4 a Schematic of wearable LIG bending sensors attached to different positions of a human body to monitor joint-bending-related motions. b Monitoring the response of finger bending. c Kneerelated motion monitoring: walking, jogging, and squatting. d Microsleep detection by monitoring head nodding and waking up (Kaidorava et al. 2019)

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be accomplished by either directly installing strain sensors on the skin surface or incorporating strain sensors into hand-worn data gloves (Zhou et al. 2020). Zhou et al. (2018) designed screen-printed pressure sensor. AgNPs-based screenprinted forked-finger textile electrodes and AgNWs cotton fabric made up this sensor. A microcontroller unit (MCU) also included a sensor array. The sensor’s response times can be as quick as 6 ms. Data exchange between the PC and sensor is made possible by amplifying and translating the sensor’s current response into digital form. It is possible to print this integrated array on garments. The researcher plays computer games and the piano on the wearable MCU. Hand movements are currently the most often used measurement data. For instance, Zhu et al. (2020) presented a tactile feedback smart glove with programming. This glove can reflect the movement of human fingers onto Unity’s virtual hand for flexible control since it has a piezoelectric mechanical stimulator, a sliding sensor for the palm, and a frictional electric based finger bending sensor. It can substitute the standard mouse control to start the cursor’s virtual pointer. Smart gloves have a lower cost than multi-controller systems. People’s lives, entertainment, healthcare, and other social lifestyles are projected to alter because of wearable piezoresistive strain sensors. The PCC-based systems need to be improved but have great potential (Lu et al. 2019c). The piezoresistive sensor, for instance, necessitates an external power source, which limits the portability of wearable issues. It can be improved by integrating a self-powered system or flexible battery. In order to detect minor deformations like blood pressure and the rapid stretch rate, strain sensors must also be developed with a broad detection range and high sensitivity. To solve the issues with motion artefacts and signal quality, it is also required to increase the sensor’s adherence to the skin (Zhang et al. 2018). Biocompatibility is the primary challenge for an implantable sensor device. It should be able to be absorbed and broken down by the body into a chemical that is safe and non-hazardous. Choosing materials that are non-toxic, biocompatible, and environmentally friendly can therefore offer solutions to this issue. Additionally, the challenges of data processing and algorithm optimisation in human–machine interactions necessitate continual advances (Amjadi et al. 2016; Fu et al. 2023; Lu et al. 2019c).

6.4 Personal Healthcare Since the printed wearable strain sensors can permanently adhere to the skin’s surface and can be mounted in various locations on the body of a person to monitor physiological functions like respiration, heartbeat or pulse rate, and phonation, they are crucial healthcare monitoring devices. The conductive network’s resistance will alter because of the tiny distortion caused by human activity. Because of the slight strains brought on by the blood flow pressure, the sensor’s resistance rose throughout each cardiac cycle. In order to collect data on health, strain sensors are currently mounted mostly in the wrist, gauze mask, neck, and abdomen (Souri et al. 2020; Yan et al. 2021).

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Compared with the hospital-based healthcare, wearable health monitoring at home setting offers a comfortable and cost-effective way to collect key information from daily activities (Lee et al. 2019). Figure 6.5a shows a stretchable strain sensor attached to the radial of the wrist for pulse rate monitoring. Due to the tiny strains brought on by the blood flow pressure, the sensor’s resistance increased with each cardiac cycle. Furthermore, the sensor was able to sufficiently identify the pulse features, such as percussion, tidal, and diastolic waves (You et al. 2016). Figure 6.5b shows the e-skin apexcardiogram (ACG) strain sensor for tracking the temporal volume and pressure changes in the heart. Wearable strain sensor could be utilized to recognize respiration and vocal cords vibration. For example. When the strain was attached to a mask, it could accurately monitor the respiration signals with different frequencies (Fig. 6.5c). Figure 6.5d shows a wearable strain sensor attached to the artery of the wrist to monitor the pulse rate (Wang et al. 2014). The slight strain that the blood pressure caused increased the relative amplitude of resistance. By assessing the pulse rate and intensity of data containing percussion wave, tidal wave, and diastolic wave, the device could tell a pregnant lady apart from a healthy individual. The pulse rates before and after exercise could be also detected in the real timeThe curves of relative resistance change with typical distinctive spectra can be obtained for swallowing and different words (e.g., “a”, “sensor”, “textile”) when the sensor was positioned on the neck to detect the tiny movement during speaking. They can be used to diagnose throat conditions and correct language learners’ speech. These investigations show that flexible strain sensors are useful for physiological applications (Lu et al. 2019b; Qin et al. 2020; Sun et al. 2019). Real-time tracking of phonation, chewing, and facial strain can assist in disease diagnosis and rehabilitation. Contrary to large deformations in human joints, the strain involved in these skin movements is much smaller. A screen-printed strain sensor based on the cracking mechanism was proposed for tracking such smaller deformations (Tolvanen et al. 2018). The strain sensor can be used to detect facial movements and phonation due to its excellent sensitivity (GF > 104 –106 ), low hysteresis (< 20%), low overshoot (< 2.5%), and good durability (> 3000 repeated cycles). When the strain sensor attached to the throat, the resistance changed corresponding to the different behaviours such as swallowing, coughing, sniffing, and phonation (Fig. 6.6a, b). Likewise, when the sensor was attached to the cheek and frontalis, various facial movements such as chewing, uplifting of the eyebrows, and looking down could be detected (Fig. 6.6c). Decoding of facial kinetics (e.g., facial strains and phonation vibrations) with printed strain sensors also offers a novel strategy to develop nonverbal communication interfaces. Such interfaces can improve and voice disorders and serve as promising way for controlling smart machines through facial expressions and speech (Sun et al. 2020a).

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Fig. 6.5 Low strain applications of printed strain sensor a The resistance changes of the sensor with respect to the blood flow pressure in the wrist, b schematic illustration of the locations of the mounted ACG sensor and photograph of the metal composite-based sensor, c respiration monitoring, and d pulse monitoring (You et al. 2016, Wang et al. 2014)

Fig. 6.6 Relative resistance changes of the printed stain sensor a Swallowing, coughing, sniffing, b the pronunciation of different words, and c chewing and different facial expressions (Sun et al. 2020a)

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6.5 Summary Numerous uses of the printed wearable strain sensors have been investigated, including personal healthcare, sports performance tracking, and human–machine interfaces. Additionally, strain sensor form factors and aesthetics will advance the transition from proof-of-concept prototypes to commercial goods. The goal of the research should change from the creation of simple strain sensors to integrated, high-performance, and user-friendly strain sensors. The goal will be achieved through continuous innovations in materials section, structural design, and advanced manufacturing and integration.

References Amjadi M, Kyung KU, Park I, Sitti M (2016) Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review. Adv Func Mater 26(11):1678–1698. https:// doi.org/10.1002/adfm.201504755 Chen Q, Xiang D, Wang L, Tang Y, Harkin-Jones E, Zhao C, Li Y (2018) Facile fabrication and performance of robust polymer/carbon nanotube coated spandex fibers for strain sensing. Compos Part a: Appl Sci Manuf 112(June):186–196. https://doi.org/10.1016/j.compositesa. 2018.06.009 Fu R, Zhao X, Zhang X, Zhiqiang S (2023) Design strategies and applications of wearable piezoresistive strain sensors with dimensionality-based conductive network structures. Chem Eng J 454(P3):140467. https://doi.org/10.1016/j.cej.2022.140467 Gu Y, Zhang T, Chen H, Wang F, Yueming P, Gao C, Li S (2019) Mini review on flexible and wearable electronics for monitoring human health information. Nanoscale Res Lett 14(1):1–15. https://doi.org/10.1186/s11671-019-3084-x He Z, Zhou G, Byun JH, Lee SK, Um MK, Park B, Kim T, Lee SB, Chou TW (2019) Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite fibers for ultrasensitive, wearable strain sensors. Nanoscale 11(13):5884–5890. https://doi.org/10.1039/C9N R01005J Kaidarova A, Khan MA, Marengo M, Swanepoel L, Przybysz A, Muller C, Fahlman A, Buttner U, Geraldi NR, Wilson RP, Duarte CM, Kosel J (2019) Wearable multifunctional printed graphene sensors. NPJ Flexible Electron 3(1):1–10. https://doi.org/10.1038/s41528-019-0061-5 Lee J, Pyo S, Kwon D, Jo E, Kim W, Kim J (2019) Ultrasensitive strain sensor based on separation of overlapped carbon nanotubes, 1805120:1–7. https://doi.org/10.1002/smll.201805120 Li Y, Liu Y, Bhuiyan SRA, Zhu Y, Yao S (2022) Printed strain sensors for on-skin electronics. Small Structures 3(2):2100131. https://doi.org/10.1002/sstr.202100131 Lin L, Choi Y, Chen T, Kim H, Lee KS, Kang J, Lyu L, Gao J, Piao Y (2021) Superhydrophobic and wearable TPU based nanofiber strain sensor with outstanding sensitivity for high-quality body motion monitoring. Chem Eng J 419(March):129513. https://doi.org/10.1016/j.cej.2021. 129513 Lu L, Zhou Y, Pan J, Chen T, Yajie H, Zheng G, Dai K, Liu C, Shen C, Sun X, Peng H (2019a) Design of helically double-leveled gaps for stretchable fiber strain sensor with ultralow detection limit, broad sensing range, and high repeatability. ACS Appl Mater Interfaces 11(4):4345–4352. https://doi.org/10.1021/acsami.8b17666 Lu L, Zhou Y, Pan J, Chen T, Hu Y, Zheng G, Dai K, Liu C, Shen C, Sun X, Peng H (2019b) Design of helically double-leveled gaps for stretchable fiber strain sensor with ultralow detection limit, broad sensing range, and high repeatability. https://doi.org/10.1021/acsami.8b17666

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