Flexible Sensors for Energy-Harvesting Applications 3030995992, 9783030995997

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Smart Sensors, Measurement and Instrumentation 42

Anindya Nag Subhas Chandra Mukhopadhyay   Editors

Flexible Sensors for Energy-Harvesting Applications

Smart Sensors, Measurement and Instrumentation Volume 42

Series Editor Subhas Chandra Mukhopadhyay, School of Engineering, Macquarie University, Sydney, NSW, Australia

The Smart Sensors, Measurement and Instrumentation series (SSMI) publishes new developments and advancements in the fields of Sensors, Instrumentation and Measurement technologies. The series focuses on all aspects of design, development, implementation, operation and applications of intelligent and smart sensors, sensor network, instrumentation and measurement methodologies. The intent is to cover all the technical contents, applications, and multidisciplinary aspects of the field, embedded in the areas of Electrical and Electronic Engineering, Robotics, Control, Mechatronics, Mechanical Engineering, Computer Science, and Life Sciences, as well as the methodologies behind them. Within the scope of the series are monographs, lecture notes, selected contributions from specialized conferences and workshops, special contribution from international experts, as well as selected PhD theses. Indexed by SCOPUS and Google Scholar.

More information about this series at https://link.springer.com/bookseries/10617

Anindya Nag · Subhas Chandra Mukhopadhyay Editors

Flexible Sensors for Energy-Harvesting Applications

Editors Anindya Nag Faculty of Electrical and Computer Engineering Technische Universität Dresden Dresden, Germany

Subhas Chandra Mukhopadhyay School of Engineering Macquarie University Sydney, NSW, Australia

Centre for Tactile Internet with Human-in-the-Loop (CeTI) Technische Universität Dresden Dresden, Germany

ISSN 2194-8402 ISSN 2194-8410 (electronic) Smart Sensors, Measurement and Instrumentation ISBN 978-3-030-99599-7 ISBN 978-3-030-99600-0 (eBook) https://doi.org/10.1007/978-3-030-99600-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Flexible sensing prototypes have proven to be the cornerstone of the sensing industry. A vast spectrum of flexible sensors has been developed, which varies in fabrication techniques and processed materials. Diversified polymers and nanomaterials have been utilized to form the substrates and electrodes of these flexible sensors. The nanomaterials considered to form the flexible sensors can be classified into two forms, including carbon-based allotropes and metallic nanomaterials. The elements belonging to each of these categories have been processed to form pure or composite flexible electrodes. As a result of the variation in physical, mechanical and thermal characteristics of the flexible sensors, their deployment in the chosen applications achieves enhanced performances. One of the recent pivotal uses of flexible sensors is harvesting energy. The need to harvest energy has been a state of the art in the current electronics world. The enormity of the sensing devices used for different applications demands a constant supply of energy. Although the researchers have been able to cope up until date, it has become paramount to generate and harvest energy as a replacement for the conventional techniques. The book highlights some of the significant technologies in which flexible sensors have been exploited to harvest energy by converting tactile, vibrational and other forms of energy. The last decade has seen exponential growth in the design and development of energy harvesting flexible devices. These prototypes have operated with different mechanisms, including piezoelectric, triboelectric, pyroelectric and self-powered sensors. Among them, the first two (piezoelectric and triboelectric) types are the more popular due to their high signal-to-noise ratio, simple structure and reliable operation. The self-powered sensors generally include a singular form or a combination of the other three types of nanogenerators. The high electrical conductivity, mechanical flexibility and thermal stability of these four flexible sensors have allowed the researchers to deploy them in a wide range of ambiances to harvest energy. The verdict on the capability of the sensors to harvest energy is done using the short-circuit voltage, open-circuit current and power density. This work aims to show the fabrication of low-cost sensors and their ubiquitous implementation for energy-harvesting applications. The examples cited in the book showcase the sensors’ potential to harvest energy based on their fabrication technique, v

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Preface

raw materials, and working mechanism. The developed sensors were reliable with high sensitivity, high durability and robustness, low response time and higher stability and repeatability in their responses. This book would help to study the synergy between the opted nanomaterials and the corresponding energy harvested in the sensors. It would assist in evaluating the quality of different types of flexible sensors that have been developed with various printing techniques. The editors, Anindya Nag and Subhas Chandra Mukhopadhyay are thankful to the authors who significantly contributed to this work. We would also like to thank the production team for their quick response and publishing the book on time. We would also like to extend our gratitude to our families for their immense support, motivation and encouragement throughout the work. Dresden, Germany Sydney, Australia

Anindya Nag Subhas Chandra Mukhopadhyay

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anindya Nag and Subhas Chandra Mukhopadhyay

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Need of Flexible Sensors in the Sensing World . . . . . . . . . . . . . . . . . . . . . . . . Anindya Nag and Subhas Chandra Mukhopadhyay

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Impact of Nanotechnology on the Quality of the Flexible Sensors . . . . . . . Anindya Nag, Subhas Chandra Mukhopadhyay, and Joyanta Kumar Roy

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Fabrication and Implementation of Nanomaterials-Assisted Flexible Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mariana Arpini Vieira

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Necessity and Available Technologies for Energy Harvesting . . . . . . . . . . . 109 Anindya Nag and Subhas Chandra Mukhopadhyay Flexible Piezoelectric and Triboelectric Sensors for Energy Harvesting Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Nasrin Afsarimanesh, Anindya Nag, and Ghobad Shafiei Sabet Flexible Pyroelectric Sensors for Energy Harvesting Applications . . . . . . 153 Anindya Nag, Nasrin Afsarimanesh, and Subhas Chandra Mukhopadhyay Self-Powered Implantable Energy Harvesters for Medical Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Md. Eshrat E. Alahi, Anindya Nag, and S. C. Mukhopadhyay Energy Harvesting in IoT-Enabled Flexible Sensors: Smart Sensing and Secure Access Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Shantanu Pal and Anindya Nag

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Challenges of Existing Flexible Sensors for Energy Harvesting . . . . . . . . . 211 Pham Thi Quynh Trang and Nguyen Thi Phuoc Van Conclusion and Future Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Anindya Nag and Subhas Chandra Mukhopadhyay

Introduction Anindya Nag and Subhas Chandra Mukhopadhyay

Abstract The impact of sensors can be felt in every step of human life. Different kinds of sensors are being employed every day by academic researchers and industries for electrochemical and strain sensing applications. This chapter elucidates the significance of sensors and the type of sensors designed, developed and implemented to date. The categorization of sensors has been primarily done in two groups, including microelectrochemical-based silicon sensors and flexible sensors. The focus has been given towards flexible sensors, in which the raw materials and fabrication processes used to form the electrodes and substrates of the prototypes have been explained. Finally, some of the significant applications for flexible sensors have been shown in the chapter.

1 Introduction The advancement of science and technology has driven the field of microelectronics to upgrade its existing conditions. In earlier times, when the concept of microelectronics did not get popularized, very large-scale integrated (VLSI) circuits were mainly used to device the electrical and electronic systems. These circuits consisted of bipolar junction transistors (BJTs) [1, 2] and field-effect transistors (FETs) [3, 4] that primarily operated on the flow of current based on the direction of the voltage. The need for sensors gradually grew to keep up with the ever-growing pace of life. Back in the days when everything was operated manually, a single sensorial job was A. Nag (B) Faculty of Electrical and Computer Engineering, Technische Universität Dresden, 01062 Dresden, Germany e-mail: [email protected] Centre for Tactile Internet With Human-in-the-Loop (CeTI), Technische Universität Dresden, 01069 Dresden, Germany S. C. Mukhopadhyay School of Engineering, Macquarie University, Sydney 2109, Australia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Nag and S. C. Mukhopadhyay (eds.), Flexible Sensors for Energy-Harvesting Applications, Smart Sensors, Measurement and Instrumentation 42, https://doi.org/10.1007/978-3-030-99600-0_1

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completed over days. This was a wastage of time, money and energy. The intervention of sensors not only assisted in reducing these three factors but also helped people gather knowledge in a much efficient manner. The size of computers and other electronic gadgets has decreased a lot due to the presence of miniaturized sensors, which have made them more efficient in terms of portability and access. The introduction of semiconducting sensors in the late 1970s [5, 6] has revolutionized the electronics industry in multiple ways. Along with miniaturizing the size of the resultant devices, their effective performances in terms of sensitivity, longevity and robustness have increased to a great extent. Fairchild [7] was the first company to commercially develop semiconductor sensors that were based on their charge-coupled operation. The invention of these sensors was done at Bell Labs. These prototypes showed enhanced performances for which they are still used to date. Similar to Fairchild, companies like The Silicon Engine [8] also grew up around that time to develop semiconducting devices. Their work was primarily based on the fabrication of memory devices that were able to compete against the dominant magnetic core technology that was available at that time. The window for customization of sensors was very small at that time as the target was to fabricate the sensors on a large-scale basis. With time, different companies have established themselves to fabricate and implement semiconducting sensors on an academic and industrial basis. The fabrication techniques of these semiconducting techniques have also been optimized in terms of cost, requirement of expertise and laboratory facilities, which has simultaneously enhanced their electrical, mechanical and thermal characteristics [9–11]. Continuous use of these sensors has compelled researchers to come up with ideas where they can be used for multiple applications. Sensors primarily hold importance in day-to-day activities, for example, using passive infrared sensors for opening an automated door or switching a light on. The cost of each sensor depends on its simplicity, which in turn depends on its operating mechanism. While dealing with widening the working area of the semiconducting sensors, there were certain limitations that needed to be dealt with. Even though these sensors served to a great extent for sensing different kinds of applications [12–18], their deployment for biomedical and healthcare-related uses was of limited use. This is mainly due to their brittle nature, intransigency and degradation of performance over time. This has led to the consideration of sensors with alternative processed materials. Scientists started using flexible sensors that had a certain degree of mechanical flexibility associated with most of the raw materials. In addition to their bendability, the raw materials also had enhanced electrical and thermal characteristics. These flexible sensors had the capability to perform with better performances of stretchability, sensitivity, working range, linearity and consistency in performances over a longer period, which was not possible with the conventional silicon-based sensors [19–23]. The flexible sensors have been considered and popularized primarily for the last fifteen years [24–29]. The application spectrum has constantly been widening with the corresponding increase in the conjugation of materials to form the electrodes and substrates of the sensors. While using them for various kinds of biomedical [30– 33], industrial [34–37] and environmental [38–41] uses, these sensors are fabrication using different kinds of printing techniques.

Introduction

3

Fig. 1 Representation of the increase in the use of flexible sensors in the global scenario [42]

The structural dimension and choice of the materials used to make the sensors primarily depend on the chosen application. Figure 1 shows a constant increase in the use of flexible sensors in the upcoming years [42]. This comparison has been made in a global scenario where there is almost an equal distribution in the increase of usage in all the continents. The entire chapter has been organized as follows. Followed by the brief introduction on the need for sensors in the real world, along with the alteration in their types, the classification in the major types of sensors has been shown in the succeeding section. This section elucidates the types of MEMS-based and flexible sensors that are fabricated and deployed for a range of applications. The characteristics of these sensors have been varied with respect to the materials used to form their electrodes and substrates.

2 Types of Sensors The types of sensors can be varied based on their structural variation, processed materials and working mechanism. Each of these categories consists of certain attributes that enhance their performance towards the chosen application. These three factors can be divided on the basis of two major categories.

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2.1 MEMS-Based Sensors Semiconducting sensors, which have been mainly formed using silicon substrates, have been devised through the microelectromechanical (MEMS) technique [43–45]. This technique consists of a number of sub-processes that cover the entire fabrication spectrum of the silicon-based sensors. For forming the substrates of semiconducting sensors, silicon and germanium have been the two common choices due to their semiconducting nature. Among them, silicon has been preferred over the other one due to its lower cost, ability to work over a wide temperature range can be used to form prototypes that have the ability for ubiquitous performance. Pure silicon obtained from the earth is used to make single-crystal silicon wafers with a thickness ranging between 200 and 600 microns. These silicon wafers are taken for the thermal oxidation process to form a thin oxidized film to obtain an insulative nature. The electrodes of these semiconducting sensors are being formed using metallic nanoparticles. Some of the common metals used to form the electrodes of the MEMS-based sensors are gold, chromium and platinum. Some of the major steps carried out using the MEMS technique are spin-coating, baking process, photolithography, wet chemical etching, sputtering and sonication. These processes are carried out inside cleanroom facilities with minimized particle size. The classification of cleanrooms is done on the basis of the maximum size of the particles that are present in the room [47, 48]. After the sensors are formed, the entire silicon wafer is cut into individual prototypes to use for the chosen application. This division of individual dyes is done using a laser cutting machine. The number of dyes used for characterization and experimentation purposes is lesser than that of the fabricated ones. This is because the ones that are present near the edges of the wafer do not have uniform spin-coated photoresists or sputtered metals. Normally, this problem is addressed using beads to maximize the uniformity of the spin-coated layers. Figure 2 [46] shows the representation of the major steps that are involved in formulating the silicon sensors. It is important to note that each of these processes is significant for the fabrication of high-quality silicon sensors. If any of the processes are omitted or not followed in a sequential manner, the error present in the silicon sensors can have a disastrous effect on their performances. These MEMS-based sensors have been deployed for a wide range of applications [49–51] for the last three decades. Although their industrial and environmental uses have been very efficient due to their high tolerance, their healthcare-related applications have been pretty limited. This has led to the formulation of flexible sensors that could tackle the loopholes present in the silicon sensors.

2.2 Flexible Sensors The flexible sensing prototypes have been primarily developed using materials that have optimized characteristics as compared to the ones using for MEMS-based

Introduction

5

Fig. 2 Schematic diagram of the fabrication process of the MEMS-based sensors [46]

sensors. A wide range of polymers and nanomaterials are being used to form the substrates and electrodes of the sensors. These materials are used in both pure and composite forms to maximize the mechanical strength and performance of the prototypes. Some of the common types of polymers that are used to fabricate flexible sensors include synthetic, organic and inorganic polymers [52, 53]. Among each of these categories, some of the common ones are polydimethylsiloxane (PDMS) [54–56], polyethylene terephthalate (PET) [57–59], polyimide (PI) [40, 60, 61], polystyrene (PP) [62–64], thermoplastic polyurethane (TPU) [65–67] and polyvinyl chloride (PVC) [68–70]. Apart from these ones, there are certain conductive polymers like poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) [71–73] that recently got very popular due to their game-changing nature. These polymers can be integrated with nanomaterials to form nanocomposites that can avoid the reduction of electrical conductivity, as it normally happens with other kinds of polymers. The nanocomposites are generally formed to devise the electrodes of the sensors. These composites can include more than one type of nanomaterial in order to induce specific characteristics on the resultant product. The nanocomposites have also been very effective in developing strain sensors [74–76], as the inclusion of polymers improves the total integrity of the nanomaterials. Different kinds of nanomaterials have been considered to form flexible sensors to date [77–79]. These nanomaterials

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have differed in shapes, which has varied their mechanical attributes. Some of the common shape of the nanomaterials are nanobeads [80, 81], nanosheets [82, 83], nanoparticles [84, 85], nanotubes [86, 87], nanowires [88, 89] and quantum dots [90, 91]. Each of these shapes is different in tensile strength, elasticity, elongation, stiffness and fatigue limit. Conductive materials like graphene, being available in different forms like nanosheets and powdered form, can be used to form a wide range of sensors on the basis of these individual structures. The processing techniques used for each type of nanomaterial are different. This is not only because the precursor materials required to synthesize these nanomaterials are different, but each of their sizes also varies. Among the conductive elements, these shapes of nanomaterials can be broadly classified into two types, namely carbon-based allotropes [92–94] and metallic counterparts [95–97]. The first category includes the ones containing aromatic carbon rings like Carbon Nanotubes (CNTs) [98–100], graphene [101–103] and graphite [104–106]. Figure 3 [107] shows the schematic diagram of three types of carbon-based allotropes. Here, carbon atoms are bonded in an sp2 hybridized manner, giving them low-density values. CNTs are available primarily in two different types, including Single-Walled Carbon Nanotubes (SWCNTs) [108–110] and Multi-Walled Carbon Nanotubes

Fig. 3 a–c Schematic diagram of the three types of carbon-based allotropes [107]

Introduction

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Fig. 4 Schematic diagram of the three types of CNTs, namely a SWCNTs, b DWCNTs and c MWCNTs [107]

(MWCNTs) [111–113]. Apart from these two types, researchers have sparsely used Double-Walled Carbon Nanotubes (DWCNTs) [114–116] and Few-Walled Carbon Nanotubes (FWCNTs) [117–119]. Figure 4 [107] shows the schematic diagram of the SWCNTs, DWCNs and MWCNTs. But these two types have not been used as much as SWCNTs and MWCNTs due to the superior properties of the latter ones. Graphite is the unmodified form of graphene that has recently gained popularity [120, 121] due their high electrical conductivity, high durability and high surface area. The structural diversity of CNTs can be exemplified into three categories, namely armchair, zigzag and chiral [122, 123]. Each of these types is defined based on the way the graphite is rolled up during the synthesis process. The nature of graphene can be said to be the pure form of carbon allotrope, having exceptional characteristics. Some of the advantages like high charge carrier mobility, high tensile strength, high surface-to-volume ratio have led to their large-scale exploitation for electrochemical [124, 125] and strain-sensing [126, 127] applications. In the pure form, the bandgap of graphene is almost nil. Thus, it is impossible to use them for form switches. Also, the use of graphene as catalysts for reactions is not popular as it is susceptible to oxidative environments. Other than the carbon-based allotropes, the metallic counterparts mostly include nanowires that are formed using elements like iron [128, 129], gold [130, 131], copper [132, 133] and silver [111, 134]. Some of the advantages of these metallic nanowires include high mechanical flexibility, high optical transparency, high electrical conductivity and excellent solution-processing

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capability [134]. These nanowires are deposited in polymers to form electrodes for the prototypes that are capable of operating as chemical sensors [135]. These raw materials are processed using different kinds of fabrication techniques that include a certain number of steps. These techniques are different from the ones used for developing semiconducting sensors, as processing is mostly done in laboratory environments. The printing techniques [136, 137] used to develop each of the prototypes primarily depend on the nature of the processed raw materials. The difference in the nature of the fabrication techniques for the flexible sensors as compared to the MEMS-based sensors leads to the development of prototypes that had numerous advantages like cost-efficient mass production, quick response to stimuli, development of smaller and flexible devices and sensing capabilities over large areas [138, 139]. Some of the common contact and non-contact printing techniques used to develop the flexible sensors are screen printing [141, 142], inkjet printing [143, 144], laser ablation [145, 146], gravure printing [147, 148], transfer printing [149, 150], rollto-roll printing [151, 152], patterned printing [153, 154], 3D printing [155, 156] and micro-contact printing [157, 158]. Figure 5 [140] shows a pie-chart of some of the common printing fabrication techniques. Each of these fabrication processes is capable of generating thin-film sensors that are subsequently characterized and used for the experimental process. Along with the prototypes, researchers are nowadays to use these mentioned fabrication techniques for signal-conditioning circuits on flexible printed circuit boards (FPCBs) [159, 160]. The FPCBs are able to contain the entire electronic circuitry containing for detecting, processing and transmitting the sensed data.

Fig. 5 Representation of different printing techniques used for forming flexible sensors [140]

Introduction

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3 Application of Sensors 3.1 Electrochemical Sensors The applications of sensors can be broadly into three categories, namely electrochemical and strain sensing. The validity in the use of each of the above-mentioned sensors is done by proper characterization using a range of techniques. Some of those techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-Ray diffraction (XRD), Fouriertransform infrared spectroscopy (FTIR) and ultra-violet visible spectroscopy (UV– vis). The detection methods comprise electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The major difference between these two detections lies in their experimental setup and monitored parameters. EIS is a two-electrode detection system, where the change in the responses is determined in terms of impedance with respect to the corresponding change in concentrations. CV is a three-electrode system that operates in the change in current with respect to voltage for different concentrations. The significance of MEMS-based sensors is higher as compared to the flexible ones, as the change in the responses of the sensors should take place only due to the molecular ions in the solution, not due to any kind of deflection. For the electrochemical sensors, the profiling of the prototypes is done in the control solution. Once the change in impedance or current is determined for the control solution, the sensors are tested for the electrochemical ions specific to the solution. The analytical performances of the sensors are tested to different static and dynamic characteristics like sensitivity, reproducibility, stability, linearity, working range, limit of detection, response time, recovery time and others. The flexible sensors that are currently used as electrochemical sensing prototypes have been able to detect concentrations at ppm and ppb levels with high sensitivity. The electrochemical sensing activities include the detection of biomolecules available in food [161, 162]. The presence of these biomolecules above the threshold levels leads to different kinds of anomalies related to the heart, kidney, liver and other body organs. Table 1 [163] shows the use of carbon-based flexible sensors for the detection of biomolecules available in food. By determining the specific concentrations, they assist in determining the quality of the food in which these biomolecules are present. It is seen that the carbon-based nanomaterials have been used in conjugation with different kinds of conducting and semiconducting materials to increase the sensitivity and linearity of the chosen molecules. Apart from the biomedical applications, the sensors have also been used for electrochemical sensing of ions related to environmental applications. Some of the essential ions like sulfur [164, 165], nitrate [16, 166, 167] and phosphate [168, 169] have been detected using flexible sensors at low concentrations. One of the primary advantages of using the sensors for the detection of environmental ions is the ability to operate in harsh ambiances which are partially accessible. Although the replacement of the sensors in such ambiances is an issue, they are still advantageous as compared to other non-portable technologies that are expensive and require expertise on the operation spot. The industrial uses of electrochemical sensors consist of

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Table 1 Comparison between the work on the utilization of carbon-based materials for determining biomolecules in food [163] Material

Linear range

Limit of detection Target product

References

Sulphur, nitrogen, mesoporous carbon

0.001–1000 nM

0.00045 nM

Mercury ion

[170]

Multi-Walled Carbon Nanotubes (MWCNTs), Molybdenum disulfide nanosheets (MoS2 )

0.08–1392 µM

0.015 µM

Chloramphenicol [171]

Multi-Walled Carbon Nanotubes (MWCNTs), Salen, Cobalt (III)

0.5–6.0 mg L−1

0.048 mg L−1

Methimazole

[172]

Reduced graphene oxide (RGO)

1 pM–20 pM

1 pM

Kanamycin

[173]

Modified glassy carbon electrode, graphene quantum dots, riboflavin

0.001 µM– 1.0 µM

0.2 µM

Persulfate

[174]

Multi-Walled Carbon Nanotubes (MWCNTs)

0.75–20 mg L−1

0.5 mg L−1

Quinoline yellow [175]

Molybdenum disulphide (MoS2 )

300 nM–30 mM

300 nM

Glucose

[176]

Graphene sheet, Nafion, thionine, platinum nanoparticles

0.01–12.0 ng/mL

0.00574 ng/mL

Kanamycin

[177]

Graphene nanosheets, platinum-catalysed hydrogen

0.05–100 ng mL−1

0.006 ng mL−1

Tetracycline

[178]

0.007 ng mL−1

Streptomycin

[179]

Glucose oxidase, 0.01–10 ng mL−1 aniline, o-phenylenediamine

the detection of the ions in the chemicals that are discharged in the water bodies. The presence of these ions in the water above a certain threshold level leads to disastrous effects for the flora and fauna residing in the water bodies. The chemicals being detected by the sensors are tested initially in the laboratory environments and then in real-time situations. The laboratory testing is done by preparing aqueous solutions of different concentrations. The concentration range is dictated by the amount in which these chemicals are deposited in the water bodies.

Introduction

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3.2 Strain Sensors The second categorization of sensors where mostly flexible sensors are employed is strain-induced sensing applications. The strain sensing is done by applying pressure/force on the sensing area to obtain a corresponding change in the structure of the prototypes. The amount of compression caused on the sensors depends on the amount of stress-induced on them. The characterization of the sensors is performed slightly in a different manner as compared to the electrochemical ones, as the mechanical performances of these sensors are tested with respect to the applied stress. The Young’s Modulus (E) is an important parameter for the strain sensors, which decides their strength and eventually their capability for the chosen application. The characterization part also includes determining their responses in terms of bending radii of the prototypes, reproducibility and stability in the results. With the standardization of the mechanical properties of the nanomaterials and polymers mentioned in the previous section, nanocomposites have also been formed where nanomaterials and polymers are mixed at defined percolation thresholds to devise the conductive electrodes [180, 181]. Figure 6 [182] shows the illustration of the use of 3D printing techniques for forming graphene-based strain sensors for automobile industries. Similar to 3D printing, other printing techniques have also been used to develop thin-film sensors for strain sensing. One of the essentialities that come with biomedical strain sensors is their wearable nature. The wearability of the sensors is particularly important for biomedical uses, where a particular body organ like limbs or fingers or neck would induce a certain amount of periodic strain on the prototypes. Fig. 6 Illustration of the use of 3D printed graphene-based strain sensors for automobile industries [182]

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The wearable, flexible strain sensing systems consist of a signal-processing unit, including a wireless communication protocol, that would be able to transmit the data at defined intervals. Wearable sensors nowadays are in higher demand as they assist in ubiquitous monitoring of different kinds of acute and chronic diseases. Tactile sensing has been another significant phenomenon carried out using flexible strains sensors, where stress with very low intensity is applied to the sensing area. Many tactile sensors are developed with the motivation from the cilia present on the bodies of the organisms. These cilia act as a sensory organ and help the animal for determining the location, food hunting and other factors. The tactile sensors are used for applications like robotics and hardware electronics like touch-screen mobiles [183, 184]. Researchers and industries are constantly working on improving the sensitivity of these tactile sensors so that the future generation would be able to operate devices like computers and mobile through the integration of 3D imaging and tactile sensing. The tactile sensors mainly operate on two mechanisms, including piezo-resistivity [185, 186] and piezo-electricity [187, 188]. The former category includes the change in the resistance with the corresponding change in the applied pressure, whereas the latter one includes the induction of voltage on applied pressure. The piezoelectric sensors have been used for energy-harvesting applications, where the piezoelectric materials have been used to develop prototypes for generating and harvesting energy [189, 190]. Some of the common piezoelectric polymers include polyvinylidene fluoride (PVDF) [191, 192], polyvinylidene fluoride-trifluoro ethylene (P(VDF-TrFE)) [193, 194], cellulose and its derivatives [195, 196], whereas oxides like zinc oxide (ZnO) [197, 198] has also been used for forming energy-harvesting devices.

4 Conclusion The chapter elucidates the work done on the significance of sensors in the current world. Sensors with varied processed materials and fabrication techniques have been designed, developed and implemented over time. The need for each type of sensor has been significant for a specific application that it is employed for. The MEMSbased silicon sensors were the initial prototypes, followed by flexible sensors formed using materials with a certain degree of mechanical flexibility. Both these types of sensors have been deployed for electrochemical sensing, where different kinds of ions have been detected over a wide range of concentrations. The electrochemical sensors have operated on the basis of the change in current or impedance monitored against voltage or frequency, respectively. These responses are monitored for a range of concentrations specific to each tested ion. The flexible prototypes have been additionally used as strain sensors for determining the intensity of force/pressure exerted on them. The stress has been induced on these sensors in a wide spectrum of applications, which compromise both industrial and biomedical sectors. The next step taken by the researchers is the integration of these sensors with the internet of things (IoT) for increasing the efficiency of transmission of the sensed data [199,

Introduction

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200]. This would help to achieve a fast and compact operation system for analyzing the data at different levels. Funding This study was funded by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) as part of Germany’s Excellence Strategy—EXC 2050/1—Project ID 390696704— Cluster of Excellence “Centre for Tactile Internet with Human-in-the-Loop” (CeTI) of Technische Universität Dresden.

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Need of Flexible Sensors in the Sensing World Anindya Nag and Subhas Chandra Mukhopadhyay

Abstract The need for flexible sensors arises from the fact their working performances are much better than their rigid counterparts. A range of flexible sensors has been developed by scientists in research labs and industries with varied fabrication techniques and processing materials. The nanomaterials and polymers used to form these sensors have enhanced electrical, mechanical and thermal characteristics. The biocompatible nature of carbon allotropes has also been popularized to devise prototypes for medical purposes. Among the fabrication techniques, the printing methods have been stressed upon due to their low cost and capability for high roll-to-roll production. These flexible differ in their physiochemical forms and operates on a specific or a combination of working mechanisms for the chosen application. This chapter explains the significance of flexible sensors by showing the techniques used to develop them, the associated raw materials and the application of these sensors. It shows the variance in some of the significant fabrication techniques used to develop the flexible sensors.

1 Introduction The widespread use of sensors in the late’90s of the previous century has compelled researchers and scientists all over the world to form efficient prototypes. The diversified nature of the sensors has allowed their uses in areas that are partially accessible [1, 2]. These devices have helped in monitoring data at a faster rate, which in turn has assisted in taking required measures for the particular application [3, 4]. The use A. Nag (B) Faculty of Electrical and Computer Engineering, Technische Universität Dresden, 01062 Dresden, Germany e-mail: [email protected] Centre for Tactile Internet With Human-in-the-Loop (CeTI), Technische Universität Dresden, 01069 Dresden, Germany S. C. Mukhopadhyay School of Engineering, Macquarie University, Sydney 2109, Australia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Nag and S. C. Mukhopadhyay (eds.), Flexible Sensors for Energy-Harvesting Applications, Smart Sensors, Measurement and Instrumentation 42, https://doi.org/10.1007/978-3-030-99600-0_2

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of flexible sensors mainly started in the early 2000s at a time when silicon-based sensors were highly popular [5–8]. Although the silicon sensors served most of the purposes at that time, there were some limitations that urged the researchers to go for alternative options. Some of the common limitations were low signal-to-noise ratio (SNR), degradation of performance over time, low life and unreliable connections at harsh conditions [9, 10]. These issues were addressed by sensors that were developed by flexible materials. There has been a constant rise in the use of flexible sensors as estimated in different market surveys [11–15]. The use of these sensors includes applications like biosensing, image sensing, optical sensing and others. Dynamic industrial applications, including automotive, medical and consumer electronics, have been covered using these sensors. The rise in the expenditure on flexible sensors is estimated to rise from around 8 billion USD in 2019 to around 13 billion USD by 2027, with a compound annual growth rate of 6.8%. Some of the advantages of these sensors include their lightweight, portable nature and ruggedness. When these sensors are integrated with wearable technology, the spectrum of their application increases exponentially [16– 18]. Acute and chronic diseases are being monitored and analysed by these wearable sensors. The concept of this wearability has led the research groups and companies to design the prototypes in such a way that aids ubiquitous sensing [19–21]. In addition to the flexibility of the prototypes, the wearable systems also consist of the signalconditioning circuit that processes the raw data and transmits them to the monitoring unit. In order to make the wearable systems more comfortable and compatible for the patients, the recent trend is the fabrication of flexible electronics for embedding with the sensors [22, 23]. This will reduce the brittleness of the entire system that might cause difficulty for the patients while wearing them for a long time. This led to the development of the concept of flexible printed circuit boards (FPCBs) [24, 25] consisting of flexible passive electronic elements. Some advantages of FPCBs are reduced weight and space, increased reliability and repeatability of the production of these devices and elimination of connectors. Figure 1 [26] shows an example of the use of FPCBs with flexible electronics. As the connections of the flexible sensors with the other electronic elements in the FPCBs needs to be done carefully to avoid short circuiting, printing techniques are used to develop them [27, 28]. Normally, in the

Fig. 1 Schematic diagram of the a sensors printed on form b FPCBs [26]

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researchers working the academic labs tend to obtain the FPCBs from the commercial companies that are fabricating them. The significance of these companies has recently increased as the research groups and industries have opted to develop entire flexible sensing systems. The raw materials and fabricating techniques that have been considered to form the flexible years have greatly varied over the years. In earlier years, when the formation of flexible sensing was just getting popularized, Carbon Nanotubes (CNTs) [29–31] as conductive elements were largely favored. The design and development of flexible transistors and transducers have been developed in the initial days [32]. These devices were formed on the thin-film substrates that formed a podium for devising flexible sensing systems. CNTs are available in two different forms, namely Single-Walled Carbon Nanotubes (SWCNTs) [33, 34] and Multi-Walled Carbon Nanotubes (MWCNTs) [35, 36], Although, apart from these forms, Few-Walled Carbon Nanotubes (FWCNTs) [37, 38] and Double-Walled Carbon Nanotubes (DWCNTs) [39, 40] have also been used for researchers to develop high-quality flexible sensors. These CNTs have been widely used to develop sensors for biomedical [41, 42], industrial [43, 44] and environmental [45, 46] applications. While the CNTs were being used in the early 2000s for fabricating electrochemical and strain sensors, Geim et al. [47] in 2010 introduced graphene as a conducting element with enhanced electrical, mechanical and thermal properties. Along both CNTs and graphene are carbon-based allotropes that are biocompatible and biodegradable in nature, high electron mobility, ultra-low band gap and high aspect ratio of graphene have brought about a general in the paradigm from using CNTs to graphene for fabricating flexible sensors. Graphene, to date, has been considered as one of the most effective materials for the formation and deploying flexible sensors for different uses [48, 49]. During all these years, when graphene and CNTs were used in pure and composite forms to form flexible sensors [50, 51], researchers have also started working on the development of flexible metallic counterparts that were more efficient than metallic nanoparticles. While metallic nanoparticles are mostly preferred for MEMS-based silicon sensors [52, 53], the flexible prototypes used a variety of metallic nanomaterials with different kinds of structures. Some of the types of nanomaterials are nano-beads [54, 55], nano-sheets [56, 57], nanowires [58, 59] and nano-rods [60, 61]. Figure 2 [62] shows the schematic diagram of some of these types of nanomaterials. Some of the common metals that are considered to develop these structural variations are iron [63, 64], silver [65, 66], gold [67, 68] and copper [69, 70]. While each of these

Fig. 2 Illustration of the range of nanomaterials used to develop flexible sensors [62]

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conductive elements is conjugated with polymeric matrixes to form the thin-film sensors, their individualistic attributes play a major role in the resultant prototypes. That is why some of the sensors prefer the doping of metallic elements to form the resultant sensing area. One of the issues with these metallic conductive elements is the amount of toxicity they impart in the formed sensors. While the effect of toxicity of the sensors is critical in every application [71, 72], their repercussions are mostly judged when the sensors are used for biomedical applications. The in vitro and in vivo uses of the flexible sensors has the maximum effect of the toxicity of the metallic nanoparticles. The increased toxicity of the elements leads to inflammation, damage of proteins, cell membranes and DNA [73]. Continuous work has been going on to reduce the effect of toxicity of the nanoparticles by replacing these elements with the ones with similar properties better less toxicity [74]. The minimization of the sensing surface, dissolution of the metallic elements and reacting them with other associated elements are some of the steps that assist in decreasing the toxicity. Due to the increased probability of toxicity of smaller nanoparticles as compared to the larger ones, a trade-off is done to balance the toxicity and reduction in the size of the nanoparticles. The biocompatibility of these metallic counterparts is another issue that scientists are currently trying to circumvent. Since it is not possible to use carbon-based allotropes to develop all the flexible sensors, the biocompatibility issue of the metallic parts has become a serious issue for the devices that are used for healthcare applications. Some of the uses like therapeutic efficacy, prosthesis, tissue repair and replacement technologies, drug delivery systems and diagnostics use [75]. Silica nanoparticles are one of the materials that have recently shown a good deal of biocompatibility. The target is to develop materials that can show high resilience to detrimental substances that affect the body by conjugating with them. The optimization in the biocompatibility of nanoparticles can be done by customizing certain physical attributes like surface properties, shape, size, charge and hydrophobicity, as shown in Fig. 3 [76]. It is seen that apart from metallic

Fig. 3 Schematic diagram of the physical attributes related to the biocompatibility of nanoparticles [76]

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nanoparticles, other nanomaterials like quantum dots, lipid-based nanoparticles and others have been studied, characterized and implemented. The polymer matrixes considered to integrate with the conductive elements have also been varied as per the requirement of the application. Some of the types of polymers commonly used to form flexible sensors are thermoplastics [77, 78], organic [79, 80] and synthetic fibers [81, 82]. Each of these kinds forms efficient prototypes depending on the degree of binding and interfacial bonding with the conductive elements. The percolation threshold is one of the concepts popularized that deals with the minimum amount of nanofillers present in the polymer matrix to form nanocomposites, after which there would be no significant changes in the electrical and mechanical attributes. The nanocomposites are formed with polymers like polydimethylsiloxane (PDMS) [83, 84], polyvinyl alcohol (PVA) [85, 86] and polycarbonate (PC) [87, 88]. Researchers recently have been working on altering certain properties of the polymers, like the length of the polymer chains, creating different branches in the polymers and cross-linking the polymer chains in order to enhance their mechanical properties. The fabrication techniques used to amalgamate the conductive nanomaterials and polymers greatly vary and have evolved over the years. These techniques mostly fall under the category of printing methods [89, 90], some of which are explained in the next section. Some of the significant examples have been highlighted there with respect to each technique. The applications of flexible sensors include a wide spectrum that can be divided into three main categories, namely strain [91, 92], electrochemical [93, 94] and electrical [95, 96] uses. Figure 4 [97] shows an illustration of the use of different kinds of nanostructures to develop devices of gas sensors operating on the electrochemical sensing principle. The variation in the type of nanostructures to form the sensing area of these types of sensors results in the corresponding differences in the analytical performances of the sensors. Fig. 4 Representation of the use of different kinds of nanomaterials used to form prototypes for gas-sensing applications [97]

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Some of the common dynamic parameters used to determine the performance of the sensors are linearity, sensitivity, working range, limit of detection, response time, reproducibility and recovery time. One of the criteria recently used to develop flexible sensors for biomedical and industrial applications are the biomimetic sensors. The sensing area of these prototypes are formed in the shapes which have been inspired from different kinds of animals. The inspiration comes from the notion that these flexible organs of the animals have assisted them to operate physiological functions and perform basic day-to-day functions. Figure 5 [98] represents some of the research work done on the fabrication of tactile sensors through the inspiration taken from flexible organs of the domestic animals. The operational imitation of these sensors helps in multiple ways. Along with efficient operation, they also assist in studying the animal from whom the organs have been mimicked. They also offer high structural advantages with a high chance of recyclability [99]. The fabrication process of these biomimetic devices has been more complex as compared to the other flexible sensors, as these prototypes require Fig. 5 Some of the biomimetic structures formed with inspiration from animal organs [98]

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cilia-type structures that have a dimension of a few microns. The high aspect ratio provided by these sensors provides excellent results while operated for electrochemical [100, 101] and strain [102, 103] sensing applications. These sensors have also been beneficial to develop new materials as the study of the organs of real living systems inspires to design and develop the next generation of materials. With the ultimate outcome of developing low-cost multifunctional sensors for a few chosen applications, biomimetic sensors help in laying a podium to come up with devices for such purposes.

2 Fabrication Techniques of Flexible Sensors The fabrication techniques chosen to develop the flexible prototypes can be collectively called printing methodologies. This is because each of the processes involves some kind of printing method that consists of forming different designs using conductive ink and substrates. Apart from the printing techniques that are mentioned below, there are some other ones that can be categorized under contact and non-contact printing techniques [28]. The advantages of all these techniques include low fabrication cost, quick roll-to-roll production, generation of highly efficient sensors in terms of electromechanical properties and smaller products with high customization and flexibility.

2.1 Screen Printing Screen printing has been one of the oldest techniques that have been used to fabricate samples with printed patterns. Some of the advantages of this technique include easy customization on different types of substrates, easy operational nature, high-quality printing, high durability of the patterns and high dynamicity in terms of printable inks [104]. One of the works that can be exemplified with this work can be shown in [105], where graphene nanoflakes were printed on paper substrates to form dipole antenna for low-cost radio frequency identification and sensing applications. Some of the advantages of these devices were their small size and simple structure. The graphene nanoflakes were mixed in dispersants to keep the ink in solution. Prior to characterization, the samples were dried at a temperature of 100 ˚C for 10 min. As a result of the irregularly-shaped nanoflakes stacked on the paper surface, the electrical conductivity was limited. The prototypes were compressed to decrease the overall thickness from 31.6 microns to 6 microns. These meandered-line devices showed an electrical conductivity of 0.43 × 105 S/m. The range and gain of the sensors were −4 dBi and 4 m, respectively. The maximum gain was obtained at a frequency between 835 and 900 MHz. Another work employing screen printing techniques to develop strain sensors can be showcased in Moorthi et al. [106]. The sensors were printed with silver ink on polyethylene terephthalate (PET) substrates. Figure 6

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Fig. 6 Schematic diagram of the screen printing process of silver ink/PET-based strain sensors [106]

[106] shows a schematic diagram of the screen printing of the silver ink/PET-based strain sensors. The sensors were heated at a temperature of 90 ˚C for 20 min. The overall structural dimension of the sensors was 2 cm × 1.5 cm. The sensors were capable of showing excellent strain sensing results when they were characterized and experimented for analysing the dynamic characteristics. The sensors responded with high stability to over 10,000 cycles at a frequency of 0.5 Hz. The prototypes showed an average increase in resistance of 9% when the calculations were done for an interval of 0.004 mm. A total change in resistance was around 115.5% for a corresponding maximum strain of 400%. One of the recent works highlighting the use of screen printing techniques to develop flexible sensors can be shown in [107]. The sensors were formed by using silver nanoparticles and polyurethane (PU) as electrodes and substrates, respectively. The electrodes were shaped in wavy and horseshoe patterns with silver nanoparticles having a sheet resistance of 1.64–2.85 /sq. at room temperature. The experiments were conducted at a temperature and humidity of 23 ˚C and 60%, respectively. The thickness of the PU substrates was around 60 microns, while the silver paste had 75% silver by weight. The mask chosen to print the patterns contained 420 meshes made of polyester. The printing process was followed by curing the sensors at a temperature of 130 ˚C for 10 min. The line width for the wavy and horseshoe patterns ranges between 1 and 3 mm. The stretchability of the sensors had an upper limit of 20%, with an outer bending radius of 1 mm. The changes in resistance values occurred due to the consecutive cracking and healing cycles happening as a result of the stretching and releasing mechanism of the silver nanoparticles-based electrodes. The sensors were used as stretchable interconnections for LEDs and as strain sensors.

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These prototypes also had high potential as wearable sensors for detecting human motions. Finger and wrist bending actions were monitored with high efficiency using these printed sensors. The bending angles for the wrist and finger were 40˚ and 90˚, respectively. Another work on the use of graphene in conjugation with screen printing technique can be seen in [108], where graphene nanoplatelets (GNPs) were used to develop smart textile-based strain sensors. Water-based dispersions were formed and were subsequently used as conductive inks on synthetic fabrics. The entire fabrication process was carried out at a temperature of 23 ˚C and humidity of 40%. The GNPs were synthesized via thermal expansion of graphite intercalated compounds inside a muffle furnace at a temperature of 1150 ˚C. The GNPs were dried and mixed with deionized water to form dispersions. Then, the screen printing technique was carried out to form devices on fabrics consisting of 96% polyester and 4% elastane yarns. The printing process was carried performed with an aluminium mesh consisted of 55 weaved polyester threads. The angle of the mesh and diameter of the individual wires were 90˚ and 80 microns, respectively. Weight values of 3% and 3.8% were used for forming the sensors, after which characterization was done using quasi-static tensile tests. The prototypes were heated in the oven at 150 ˚C for 3 min. A gauge factor (G.F.) of 30 was obtained at a strain value of 5%. The prototypes were capable of monitoring physiological parameters like heartbeat and respiration. The association of the screen-printing technique has also been done with other types of nanomaterials and polymers. For example, MWCNTs and PDMS have been used to form nanocomposites to form flexible piezoresistive sensors [109]. The composites were printed on PET substrates to obtain sensor arrays with parallel plate structures. The nanocomposites were formed by mixing the optimized value of MWCNTs in the polymer matrix. The silver paste was used as the top and bottom electrodes that had a viscosity range of 15–30 Pa. sec. An array containing 4 modules was formed, each of which had an active area of 1 × 1 mm2 . The interconnected lines had a width of 100 microns and a distance of 5.6 mm between two consecutive sensors. The printed top layers were sintered at a temperature of 120 ˚C for 60 min. The bottom electrodes were printed and aligned by using solutions formed with MWCNTs/PDMS composites. The deposited bottom layers were again sintered at a temperature of 80 ˚C for 5 h. Finally, the counter electrodes were formed on top of the previously patterned composites with an oriental difference of 90 ˚C. After fabrication, the characterization was done to check the alignment and short-circuiting of the devices. The nanocomposites were sandwiched between printed silver plates that had dimensions of 1 × 1 mm2 . Finally, force concentrators were formed on separate substrates and consequently laminated to the bottom substrates. The printing of these force concentrators was done using UV-curable dielectric ink. Figure 7 [109] shows the side view of the printed piezoresistive sensors. The changes in the responses were determined in terms of resistance for applications like tactile sensing for electronic skins. Khan et al. [110] showed a similar work with the use of MWCNTs/PDMS nanocomposites to form flexible tactile sensors. These sensors were also formed using the screen printing technique to form pressure sensor arrays. The parallel plate structures were formed by sandwiching piezoelectric polymer polyvinylidene fluoride

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Fig. 7 Schematic diagram of the printed piezoresistive structure formed with MWCNTs/PDMS on PET substrates [109]

trifluoroethylene [P(VDF-TrFE)] between two metallic layers. One of the metallic layers was formed using silver and the other one was formed using MWCNTs/PDMS nanocomposites. The [P(VDF-TrFE)] was mixed with dimethylformamide (DMF) to form solutions for screen printing. The design parameters for the stencil mask were optimized, along with pallets of 70/30 wt.% of [P(VDF-TrFE)] in methyl ethyl ketone to form compatible solutions. Then, the mixture was cured at 90 ˚C for six hours. The MWCNTs/CNTs nanocomposites consisted of three different weights of 1, 3 and 5%. A total of 64 sensors were formed using the screen printing technique to determine the piezoelectric responses towards static forces. The arrays consisted of 4 × 4 sensors, with each prototype having a sensing area of 1 mm2 . The sensors were printed on plastic substrates and experimented with to determine their responses with respect to contact forces. Another work showing the use of CNTs/PDMS-based nanocomposites can be seen in [111]. Fabrication and implementation of flexible three-axis tactile sensors were done using the screen printing process. The nanocomposites formed using the solvent evaporation method assisted in obtaining a homogeneous matrix. The screenprinted sensors were employed as force sensors with four sensing cells. The changes in electrical resistance values were determined as a result of the applied force. These piezoresistive sensors obtained a linear relationship for the applied force. The normal and shear force values were 6.67%/N and 86.7%/N for force ranges up to 2 N and 0.5 N, respectively. High repeatability was obtained along with a maximum deviation of 2% for the tested force range. Other than nanoflakes or nanoplatelets, nanowires have also been used to form conductive inks for the screen printing process. Ke et al. [112] showed the formation of flexible strain sensors using silver nanowires (Ag NWs). The Ag NWs were formed using a simple polyol reduction process. After formation, Ag NWs were mixed with polyvinyl pyrrolidone (PVP), ethanol and water

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to form the resultant dispersions. The water-based nanowires were printed on PET substrates to form the thin-film substrates. The widths of the electrode lines were 0.5, 1, 2, 4 and 8 mm. The sensors were then heated in the oven at 120˚ for 10 min, cooled at room temperature and then cut into individual prototypes. The length and width of each sensor were 70 mm and 20 mm, respectively. The characterization was done on the basis of the change in resistance values with respect to bending angles. At an angle of 45˚, the G.F. values ranged between 16.7 and 23.4. The angles of 90˚ and 135˚ obtained G.F. ranges of 8.9–17.5 and 7.14–24.5, respectively. The sensors also showed excellent mechanical stability as there was almost nil decay in the response even after 1000 cycles. The sensors were used for intelligent and tamperevident packaging applications. After powering the printed patterns with a DC power supply, the LED lamps were lit for both flexed and extended situations of the patterns. One of the interesting works showing the use of the screen-printing technique for developing piezoelectric devices can be seen in the work done by Emamian et al. [113]. The sensors were formed with silver ink on PET and paper substrates. The entire sensing system consisted of 4 × 4 arrays, where each of the prototypes of a piezoelectric layer sandwiched between two metallic layers. Polyvinylidene fluoride (PVDF) was used as the piezoelectric layer, while the top and bottom layers were formed using printed silver. The thicknesses of the PET and paper substrates were 541 microns and 17,712 microns, respectively. The force experiments were conducted by performing a sweep from 0.2 N to 1.4 N with a step of 0.2 N. The sensors obtained a linear response in terms of voltage with respect to the applied force. The sensitivities for the PET and paper-based sensors were 1.2 V/N and 0.3 V/N, respectively. The correlation coefficient for PET-based printed sensors was 0.9954 and that of paper-based sensors was 0.9859.

2.2 Inkjet Printing The second type of printing process that has been extensively used to develop highly efficient flexible sensors is inkjet printing. Some of the advantages of this process are low fabrication process, easy customization, minimal requirement of expertise, no warm-up time and high quality printed patterns. One of the examples can be shown in [114], where graphene-based field-effect transistors were developed using the inkjet printing process. The fabrication process was carried out on flexible Kapton films and graphene films as substrates and electrodes, respectively. Figure 8 [114] represents the fabrication process and signal out of these graphene/Kapton-based biosensors. An ion-gel film was printed on top of the gate electrodes after the first electrodes were formed. The ionic liquid was formed by mixing poly(styrene-methyl methacrylate– styrene) and 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) amide at a weight ratio of 15 k-81 k-15 k. After the printing process was done on 3 M films, the sensors were baked at 400 °C for 20 min. Finally, the sensors were cut to form microfluidic structures, after which, functionalization process was carried out on the printed graphene films. These prototypes with a sheet resistance of 110 /sq. were

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Fig. 8 Illustration of the fabrication process of the graphene-based FET biosensors [114]

used as biosensors to detect infectious organisms. These responses of the sensors changed with respect to the concentration of the biological agents. The linear response of the sensors at a frequency of 10 GHz for S12 is between 0.07 dB and 3.70 dB for a corresponding increase in concentrations of Norovirus protein between 0.1 and 100 µg/ml. The sensors showed a R2 value of 0.99, along with three orders of magnitude for the detection of the biological targets. The inkjet printing process was also used to form gas sensors, one of which is exemplified in work done by Alshammari et al. [115]. CNTs-based gas sensors were formed on flexible sensors that had the capability to detect ethanol gas at different concentrations. The fabrication process of the sensors was carried out in two steps, first printing the electrodes and then developing the active CNTs layer. The sensitivity of the sensors was increased by functionalizing the prototypes with composites formed using carboxylic acid and poly (3,4-ethylenedioxythiophene) poly (styrene sulfonate) (PEDOT: PSS). Interdigitated electrodes were formed using silver nanoparticles ink on flexible PET substrates. Some of the specifications of the inkjet printing process included temperature, drop spacing and voltage values 60 ˚C,

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25 microns, 13–16 V, respectively. The testing was done using ethanol vapors for concentrations from 200 to 1000 ppm. The resultant conductive polymer-wrapped nanotubes showed an increase in sensitivity of 2.5 times as compared to the bare sensors. The change in response was calculated in terms of resistive values ranges with respect to the adsorption and desorption gas cycles. The calculated response and recovery times at 95% of the total time change were 8–13 s and 40–135 s for a concentration of 1000 ppm. The LOD of the sensors was 13 ppm. Dankoco et al. [116] showed the fabrication of flexible sensors using inkjet printing and subsequently used them for temperature sensing applications. These thermistors were formed by depositing silver nanoparticles on polyimide substrates. The Kapton films with a thickness of 125 microns were pasted on the glass substrates that were used as a reference. Drop-on-demand was done to form the meandering patterns with silver ink having a viscosity of 9–15 cps. Prior to the printing process, the ink was sonicated for 10 min provide certain attributes like minimal nozzle clogging, increased stability of the suspension and removing larger particles in ink. The printing process was carried out at a frequency and droplet velocity of 400 Hz and 4.5 m/sec, respectively. The thickness of the two silver printer layers was 365 nm. The thermal curing steps of the fabrication process consisted of two sub-steps, including drying of the heated samples and sintering them at 150 ˚C for 30 min. The sheet resistance and resistivity values were 0.163 /sq. and 5.9 µ cm, respectively. The sensors were used for measuring temperature ranging between 20˚C and 60 ˚C with an input voltage between 0 and 1 V. The total sensing area was 6.2 cm2 with a width and gap of 300 microns and 60 microns, respectively. The mean sensitivity was 2.23 × 10−3 ˚C−1 . The sensors also showed high linearity and a hysteresis lesser than 5%. Another work on the fabrication and implementation of gas sensors can be shown in the research done by Lv et al. [117]. Ammonia gas sensors were formed on PEDOT: PSS-based thin films have ferric chloride (FeCl3 ) as additives. Polyimide films were used as substrates on which PEDOT: PSS and FeCl3 aqueous solutions were mixed. Some of the advantages of these sensors included low cost, quick fabrication process, easy customization of the film thickness and additive levels, high selectivity and low response and recovery times. Figure 9 [117] represents the fabrication process of these thin-film sensors. Polyimide films with a thickness of 50 microns were cleaned

Fig. 9 Schematic diagram of the fabrication process of PEDOT: PSS/FeCl3 -based ammonia gas sensors [117]

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and screen printed to form interdigitated electrodes using silver paste. The samples were subsequently heated at 100 °C, followed by performing inkjet printing process on the electrodes. The effective sensing area of the electrodes was 70 mm2 , while the interdigital spacing and width of the fingers were 0.5 mm. The ink used for inkjet printing was prepared by mixing precursor aqueous solutions of PEDOT: PSS, DI water, sodium lauryl sulfate and FeCl3 at a fixed ratio. After the printing process, the samples were heated for a temperature and duration of 60 °C and two hours, respectively. The increase in the concentration of FeCl3 in the PEDOT: PSS thin films correspondingly improved the sensitivity of these gas sensors. With the increase in concentration, the reduction in the response time was around 30 times to a final value of 20 s. The sensitivity also increased tenfold to around 44% for the experimentation done at a concentration of 50 ppm of NH3 gas. When the experiments were done at room temperature, the LOD and response of these sensors were 0.5 ppm and 7.6%, respectively. Another type of application where the flexible sensors formed using inkjet printing technique has been employed can be shown in Delekta et al. [118]. Transparent and flexible graphene micro-supercapacitors were fabricated using a low-cost and scalable inkjet printing process. The solid-state devices consisted of graphene flakes printed to form interdigital electrodes. A combination of inkjet printing dry etching processes was used to form the thin-film sensors. Initially, a large area having coffee stains was printed on glass substrates. This was followed by printing the interdigitated electrodes using silver ink on the inner regions of the sensing area. The samples were finally plasma etched to pattern the graphene flakes and removing the coffee stains from the substrates. The samples were then subsequently treated with nitric acid solution and gel electrolyte to remove the mask and clean the devices, respectively. The sensors also consisted of vertical sidewalls that had high repeatability and high uniformity in their thickness and transparency. The number of printed graphene layers were fabricated between 5 and 20, with the thickness ranging from 15 and 27 nm. The graphene flakes with the thickness layer had a transmittance of 90% at 550 nm and sheet resistance of around 80 k/sq. The sensors showed high transparency with transmittance values of 90% and 71% at corresponding single-electrode areal capacitance values of 16 µF/cm2 and 99 µF/cm2 , respectively. The change in capacitive values was negligible when the sensors were bent from a radius of 9 cm to 2.75 cm. Touch sensors have also been developed using the inkjet printing technique, as shown in work done by Chen et al. [119]. Some of the attributes of these sensors are their green nature, high transparency, high flexibility and recyclable nature. The sensors were formed using PEDOT: PSS and cellulose nanofibrils (CNF) as electrodes and substrates, respectively. The transparent nanopapers that consisted of CNF were developed using a tape casting process. With a surface roughness of 15.3 nm, doped-PEDOT: PSS was transferred on the CNF to form the microelectrodes of the devices. The doping was done using p-toluenesulfonic acid with an optimized value of 0.75 wt.% of PTSA and 30% of DI water, followed by post-treatment with and dimethyl sulfoxide. An eco-friendly, water-based ink formulation technique was used to form the sensors that had a transparency of 80% at 400 nm and electrical conductivity of 1360 S/cm. The sensors had a cost, dimension and density of $

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0.060, 3.5 cm × 2.5 cm × 0.05 cm and 52.11 g/m2 , respectively. The sensors had Young’s modulus of 6.44 GPa and tensile strength of 104.23 MPa. The nanopaperbased touch sensors consisting of four-keypad were integrated with a microcontroller Arduino UNO development board for characterization and experimental purposes. The sensors were able to completely degrade in the natural soil in around 3–4 weeks. The sensors showed a change in capacitive values with respect to the folding, bending and multiple fingers touches when exerted on the prototypes.

2.3 Laser Ablation The third most popular fabrication technique used to develop flexible sensors is laser ablation or laser induction that includes the processing of precursor polymer materials using different types of lasers. The type of lasers chosen to process the graphitic material depends on the characteristics of electrodes needed for the sensing applications. The types of lasers vary based on the wavelength, mode and subsequent applications the sensors are used for. Table 1 [120] shows a summary of some of the common lasers used to develop a wide range of flexible sensors. The preciseness and resolution of the flexible sensors depend on the type of cuts induced by these lasers. The choice of wavelength is one of the crucial parameters as the intensity of the laser varies the amount of precursor material curved out off, thus varying the characteristics of the induced conductive material. The use of laser ablation for one of the interesting works can be shown in work done by Jiang et al. [121]. Multifunctional energy-harvesting sensors were developed based on LIG, PDMS and MXene-enabled porous films. These triboelectric nanogenerators (TENG) with enhanced tribo-electronegativity showed a sevenfold increase in the output performance as compared to the pure PDMS-based TENG. Some of the advantages of these sensors were high flexibility, excellent performance and high adhesion. The composites were formed by mixing stable MXene nanosheets, PDMS, lithium fluoride and HCl solutions. Due to the insoluble nature of the mixture of MXene aqueous solutions and PDMS, the MXene solutions were stirred to break the bubbles into numerous tiny bubbles. The MXene was mixed with PDMS at Table 1 Comparison of the parameters of some of the significant types of lasers [120]

Laser type

Wavelength (nm)

Mode

CO2

10,600

Pulse or continuous-wave

Nd: YAG

1064

Pulse

Er: YAG

2940

Pulse

Er, Cr: YSGG

2780

Pulse

Argon

572

Pulse or continuous

Diode

810 or 980

Pulse or continuous wave

HO: YAG

2100

Pulse

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different concentrations of 4,8, 10, 15 and 31 mg/ml to form the porous composites. The laser-induced graphene was formed from PI substrates having a thickness of 0.06 mm. A wavelength of 5.2 W and power of 470 nm was used as the optimized values to carry out a single-step ablation process. These composite sensors showed a high open-circuit voltage of −301 ± 8 V and a square resistance of 17.48 ± 0.81 /cm. The sensors showed high potential for harvesting energy from the agricultural field and human activities. Nag et al. [122] also showed the use of laser ablation techniques to develop LIG-based sensors. These sensors were used to detect the saline concentrations in different natural water bodies. One of the major advantages of this paper includes a quick fabrication process, which will assist in the large-scale production of these types of sensors. Figure 10 [122] shows the fabrication process of the LIG-based salinity sensors. LIG was initially formed from commercial polyimide films by laser ablating the substrates. Interdigital electrodes were formed, having a length of 500 microns and a width of 100 microns. Photothermal induced graphene was generated with optimized values of laser parameters. The final values of power, energy and zaxis were 9 W, 70 m/min and 1 mm, respectively. Then, the conductive material and was manually transferred to Kapton tapes to form the electrodes of the sensors. The thickness of the Kapton tapes was around 1000 microns. Sticky Kapton tapes having the same mechanical characteristics as polyimide films were used as substrates. The manual transfer was done carefully, starting from the bonding pads and then slowly moving towards the sensing area. The sensors were used for testing concentrations between 4 ppm and 40,000 ppm to cover the entire range of saline concentrations ranging from rivers to oceanic bodies. After obtaining a resolution of 1 ppm, the sensors were embedded into a microcontroller-based system for real-time operations. Yoon et al. [123] show the use of laser ablation technique to develop graphene electrodes to detect glucose molecules. The sensors were chemically modified to increase their sensitivity as electrochemical sensing devices for testing glucose at very low concentrations. The surface modification of the electrodes was done using a facile and practicable dip-coating technique. With a simple acidic acid treatment, there was an increase in the ratio of carbon–carbon bonds leading to the increase in electrical conductivity and decrease in sheet resistance. Figure 11 [123] represents

Fig. 10 Schematic illustration of the fabrication process of the LIG-based salinity sensors [122]

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Fig. 11 a Schematic diagram of the surface modification of the LIG electrodes prior and subsequent to the treatment with acetic acid. b Polyimide substrates having a thickness of 125 microns were used to form three-electrode sensing systems to detect glucose molecules

the surface of the LIG electrodes before and after their treatment with acetic acid. The presence of a uniform and stable layer of highly catalytic platinum nanoparticles on the laser-induced graphene (LIG) led to the avoidance of the agglomeration of the nanoparticles during the electrodeposition technique. Three-electrode sensors were formed with the electrodeposited platinum nanoparticles. Prior to the sensing of glucose molecules, composites consisting of chitosanglucose oxidase were immobilized on the Pt-decorated LIG for forming sweat glucose biosensors. Electrochemical impedance spectroscopy (EIS) was used to detect the target molecules at an amplitude of 5 mV and frequency range of 1–1 MHz. Cyclic voltammetry (CV) process was used as the detection technique where the input values consisted of the voltage range of –0.2–0.8 V and a scan rate of 50 mV/s. The sensors had a R2 value of was 0.99 and the standard deviation was from 0.36 µA to 0.66 µA. The LOD of the sensors was less than 300 nM, with sensitivities of 4.622 µA/mM and 65.6 µA/mM cm2 . The signal-to-noise ratio of the sensors was 3, while the sensing range was between 0.0003 and 2.1 mM. Das et al. [124] depicted the use of laser ablation technique to develop pressure sensors to monitor human gestures and finger pulse. These sensors operating on TENG were fabricated based on a novel, eco-friendly and low-cost process. The sensors consisted of three layers, including a bottom electrode layer of microstructure

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PDMS layer have LIG and another layer of PET/ITO having opposite polarity. The third layer consisted of two acrylic sheets. The sensors were formed on a silicon wafer attached with tapes to a piece of sandpaper. The PDMS was spin-coated on the sandpaper for 35 s, followed by performing a laser ablation technique on precursor polyimide material. CO2 lasers were used for this purpose with an optimized power value of 15 W. Finally, the PET/ITO films having a thickness of 127 microns were attached to the surface of the microstructure PDMS layer. The sensors had a sample size of 3 cm × 3 cm with a total sensing area was 12 cm2 . These wearable prototypes operated in a self-powered manner to determine human gesture-like motions. The sensors showed a high sensitivity of 7.697 kPa−1 , a LOD of 1 Pa and a fast response time of 9.9 ms when tested over 4000 compression-releasing cycles. These sensors showed a high potential for e-skin and healthcare applications. Another example of the design and development of flexible sensors using the laser ablation process can be seen in the work done by Rahimi et al. [125]. Highly stretchable and sensitive unidirectional strain sensors were formed that showed a high G.F. up to 20,000. Some of the advantages consisted of high robustness and high stretchability. Figure 12 [125] shows the schematic diagram of the fabrication process of the laser carbonized strain sensors. The process was carried out using the selective layer pyrolysis process of polyimide films, subsequently followed by transfer and embedment of the carbonized patterns. The polyimide tapes were initially attached to PET sheets, after which CO2 lasers were used for inscribing highly porous carbon patterns. A local pyrolysis process was carried out on the surface of the polymer, followed by treating the patterns with n-heptane for 20s to improve the resultant adhesion and penetration of the elastomers in the laser-ablated carbonaceous materials. This is followed by pouring uncured PDMS to improve the impregnation of the carbon patterns. Finally, the PDMS was peeled off the polyimide substrates for characterization and experimental purposes. The carbonized particles are comprised of partially aligned graphene and CNTs. The patterns having a sheet resistance of 60 /cm were able to measure the real-time finger motions. Another work on the development of composite sensors using laser ablation technique can be seen in [126], where ultrasensitive MWCNTs/PDMS-based sensors

Fig. 12 (i)–(v) Representation of the fabrication process of the stretchable carbon nanocomposites using laser ablation technique [125]

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were fabricated and implemented for strain-sensing applications. Initially, the composites were formed using the stirring process and a self-made coating machine was used to adjust the thickness to around 0.3 mm. After the composites were formed using optimized MWCNTs, content, CO2 laser ablation technique was carried out on their surfaces. After the laser ablation process, the resultant electrical conductivity and sensing G.F. were improved by adding a 1 wt.% of MWCNTs. However, when the amount of MWCNTs increased to 1.2 wt.%, the electrical conductivity of the composites increased to 0.18 S/m as a result of the denser MWCNTs network. High sensitivity was obtained due to the rearrangement of the MWCNTs after the surface of the composites was laser ablated to form a novel boscage-like structure. After the laser ablation process, two wires were attached to the conductive layer with a silver layer and then a PDMS layer was encapsulated over the entire structure for maximum protection during strain-sensing applications. The thickness of the encapsulated PDMS layer was around 0.6 mm. The mechanical characterization of the sensors obtained the tensile strength, Young’s modulus and elongation limit values of 6.418 MPa, 1.964 MPa and 148%, respectively. A strain of 5% exerted on the sensors obtained a G.F. of 513.

2.4 3D Printing The final fabrication technique popularized to develop high-quality flexible sensors is 3D printing and 3D printed mold-based techniques. As compared to the abovementioned techniques, this process is relatively new and has been particularly considered due to its distinctive advantages like high customization, minimal electronic waste, easy access and lightweight parts. The work done by Maurya et al. [127] shows the use of 3D printing techniques to develop graphene-based self-powered strains sensors for operating in autonomous vehicles. These smart tires combined direct maskless 3D printed strain gauges, operating as a flexible piezoelectric energy harvester. Some of the advantages of these prototypes were easy to design and fabrication process, cost-effective smart sensing and wireless operation of the sensing systems. The 3D printing process was used to print various architectures having a range of line widths of 10–100 microns. Two types of conductive inks, namely graphene and silver nanoparticles, were used to develop the electrodes of the strain sensors. Graphite powder was reduced to form graphene oxide (GO) using the Hummers’ method. The obtained GO was mixed with DI water to form suspensions and reduced to form reduced graphene oxide (rGO) using HI acid. Finally, rGO-based sensors were formed on Kapton films and were used for measuring the change in resistance values with respect to the different radius of curvature. A large number of deformations were resisted by the sensors due to the wrinkled microstructures present in the graphene sheets. The change in resistance values was measured under tensile and compressive strains. These devices functioned for powering the sensors, transferring the data wirelessly and machine learning for predictive data analysis. Another example of the use of the 3D printing technique in developing

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flexible sensors can be shown in work done by Kim et al. [128]. Fused deposition modeling 3D printing was used from the multiaxial force sensors that had CNTs and thermoplastic polyurethane (TPU) filaments as the processing materials. The sensors consisted of two individual elements, namely a structural and a sensing part. The structural part consisted of TPU, while the sensing part consisted of composites made of CNTs and TPU filaments. The TPU filaments had lower stiffness as compared to the ABS and PLA filaments. CNTs with different amounts of 1,2 and 4 wt.% were mixed with TPU filaments at a temperature of 220 °C. The prototypes formed with CNTs amount more than 3 wt.% displayed a higher electrical conductivity as compared to the ones developed with lower CNTs contents. The filaments were produced by setting the nozzle temperature at 190 °C. The average diameter resistivity of the CNTs/TPU filaments was 1.64 mm and 0.143 .m. These piezoresistive sensors were characterized in three axial directions, where the forces were applied in each direction to measure the changes in resistive values. These sensors were able to detect the sub-millimeter scale deflection, where the change in resistance values for y and z directions were measured when the force has induced the x-direction. One of the interesting works highlighting the use of 3D printing techniques for developing electrochemical sensors can be seen in the research done by He et al. [129]. Flexible composite sensors were formed to develop free-standing graphenesupported graphene-CNTs-ionic liquid (IL). Figure 13 [129] shows the fabrication process of these composite-based sensors. The IL was used as a binder that assisted in the adhesion between the 3D graphene-CNTs nanohybrids to the 2D graphene substrates. The co-assembly between the graphene nanosheets and CNTs occurred with the 3D porous graphene-CNTs cylinder aerogels. Some of the attributes of these sensors include high mechanical robustness and high electrical conductivity as a result of the densely packed layered structures. The free-standing composites were prepared by following certain steps, including coating GO dispersions on the surface of the printing paper, drying them at room temperature and reduced the GO in the presence of HI solutions. Finally, pintable rGO-CNTs-IL gel was dropped on the graphene surface. The rods were then rolled back and forth to increase their uniformity on the entire surface. These composite-based sensors were loaded with an alloy of platinum and gold nanoparticles to increase the effective surface area and

Fig. 13 Fabrication process of the electrochemical sensors using graphene-CNTs-IL compositebased sensors [129]

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electromechanical properties of the sensors. These prototypes showed high sensitivity, selectivity and reproducibility of the responses towards the tested glucose molecules. The sensors showed a rapid response time of 3 s and a linear range from 0.1 to 11.6 mM. The sensitivity and LOD of the sensors are 0.19 mA cm−2 · mM−1 and 8 µM, respectively. The sensors had an SNR of 3 and a relative standard deviation of less than 4.2%. The use of the 3D printing technique has also been done to develop flexible sensors that have been used as supercapacitors and temperature sensors [130]. 3D printed fiber electrodes were formed using a simple, low-cost technique that combined the dual functions of printed fiber-shaped temperature sensors and fiber-shaped asymmetric supercapacitors. Two of the major advantages of the sensors included high manufacturing efficiency and high scalability. After fabricating the CNTs-based electrode fibers into a variety of patterns, the positive and negative polarities were assigned by molding them into Archimedean spiral and six-angle spiral forms, respectively. An extrusion process was followed to orient SWCNTs and active materials to increase the overall mechanical strength of the fibers. The positive electrodes of the sensors consisted of V2 O5 /SWCNTs and the negative electrodes consisted of VN/SWCNTs. The negative electrodes were then uniformly wrapped with gel electrolyte that had a porous structure and a thickness of 8– 16 microns. The diameter of the positive fiber V2 O5 was around 250 microns. The size of the nozzle of the electrode ink was 410 microns and that of the GO ink was 500 microns. Each of the nozzles was immersed inside the coagulation bath for 300s. The GO fibers were then reduced to rGO fibers by chemically reducing them with HI solution at 80 ˚C for 8 h. The temperature sensors displayed a responsivity of 1.95% ˚C−1 , while the supercapacitors showcased a stable output power. One of the recent interesting works by Mousavi et al. [131] showed the direct 3D printing of highly anisotropic and flexible sensors and subsequently using them for soft robotic applications. CNTs-reinforced PLA printing filament was used to develop the sensing element and conductive interconnects of these resistive constriction sensors. After synthesizing the CNTs/PLA composites with concentrations ranging between 5 and 14 wt.%, an optimized CNTs content of 12 wt.% was chosen to form the final sensors. After the optimized value, the composites became brittle and unsuitable for the continuous fuse deposition modeling printing process. The resultant composites had an electrical conductivity of 149 S/m for a failure strain of 3.2%. Certain parameters like the air gap between the printed adjacent tracks, infill density and build orientation of the main loading directions were adjusted to optimize and sensitivity and the anisotropic nature of the sensors. Figure 14 [131] represents the step-wise fabrication process of these printed anisotropic tactile (PAT) sensors. After these PAT sensors were integrated into the interior of the soft robots, G.F. of 1342 and 1 in the directions perpendicular and parallel to the raster orientation, respectively. The directional sensitivity of these sensors was 31.4. These sensors showed a high potential for the development of highly customizable and highly multifunctional 3D printed sensors for soft robotic systems.

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Fig. 14 Schematic diagram of the step-wise fabrication process of the PAT sensors [131]

3 Conclusion This chapter explains the significance of flexible sensors in terms of the prototypes formed using some of the major types of fabrication techniques. Four major printing processes, including screen printing, inkjet printing, laser ablation and 3D printing, were elucidated by highlighting the design, development and implementation of some of the efficient, flexible sensors. These sensors have been used for a wide range of applications that include electrochemical and strain-sensing operating principles. The analytical performances of these flexible sensors validated the necessity of integrating flexible sensors with day-to-day activities. The presence of these sensors not only protects us from different ailments but also improves the quality of life as a whole. While some of these flexible sensors are integrated with communication protocols to operate them in controlled and real-time environments, the existence of the flexible sensors can be used as a podium to further contribute to enhancing the quality of the sensing world in the microelectronics industry. Funding This study was funded by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) as part of Germany’s Excellence Strategy—EXC 2050/1—Project ID 390696704— Cluster of Excellence “Centre for Tactile Internet with Human-in-the-Loop” (CeTI) of Technische Universität Dresden.

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Impact of Nanotechnology on the Quality of the Flexible Sensors Anindya Nag, Subhas Chandra Mukhopadhyay, and Joyanta Kumar Roy

Abstract The introduction of nanotechnology to the sensing world has opened up a wide spectrum of applications. The flexible sensors formed using a range of nanomaterials have shown high diversity in terms of their physicochemical nature. The micro and nano-sized sensors formed using these nanomaterials showed enhanced attributes in terms of analytical parameters when used in different electrochemical and strain-sensing applications. The mechanical and chemical flexible sensors were formed using nanomaterials having varied sizes, shapes, and structural dimensions. This chapter highlights the deployment of some of the primary types of nanomaterials to form flexible sensors. A classification has been done on the use of some of the nanomaterials like nanoparticles, nanotubes, nanowires and nanosheets, and nanowires, where the synthesis process and application of the flexible sensors have been elucidated. Finally, some of the challenges existing with the current nanotechnology field associated with flexible sensors have been showcased, along with certain possible remedies.

A. Nag (B) Faculty of Electrical and Computer Engineering, Technische Universität Dresden, 01062 Dresden, Germany e-mail: [email protected] Centre for Tactile Internet With Human-in-the-Loop (CeTI), Technische Universität Dresden, 01069 Dresden, Germany S. C. Mukhopadhyay School of Engineering, Macquarie University, Sydney 2109, Australia J. K. Roy Eureka Scientech Research Foundation, Kolkata, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Nag and S. C. Mukhopadhyay (eds.), Flexible Sensors for Energy-Harvesting Applications, Smart Sensors, Measurement and Instrumentation 42, https://doi.org/10.1007/978-3-030-99600-0_3

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1 Introduction The beginning of the twenty-first century has faced an exponential increase in the design and development of the sensing world. While earlier the sensors mostly consisted of very-large-scale integrated circuits [1, 2], some of the disadvantages like low accuracy, large size, and low repeatability force the researchers to develop sensors with miniaturized versions. After popularizing silicon-based sensors for ubiquitous applications [3–5], researchers realized the need for prototypes with mechanical flexibility to increase their robustness and longevity. Although they have been used for different kinds of industrial [6–8] and environmental applications [9, 10], the flexible sensors could also provide enhanced electrical, mechanical, and thermal attributes compared to the rigid silicon sensors. These sensors consisted of different kinds of processing materials that have been chosen based on their electromechanical properties. The size of the processing materials chosen to form the flexible sensors have been between a few microns and hundreds of nanometres. Among them, the coexistence of the nanotechnology field with the sensors has helped researchers to fabricate high-quality flexible sensors. This nanotechnology field has dramatically impacted the real world as the flexible sensors that have been developed with the nanomaterials have been used in multiple disciplines included in biomedical [11, 12], environmental [10, 13], and industrial [14, 15] applications. Some of the daily activities on which nanotechnology-based sensors had a major impact are electronics, energy, chemical manufacturing, defense, and aerospace. The future trend also shows an increase in the use of nanotechnology to form enhanced flexible sensors [16, 17]. It is estimated that the total revenue worth $536.6 million is projected to increase to more than $1300 million by the next five years, with a compound annual growth rate of 11%. Multidisciplinary fields like chemical, biomedical, mechanics, and material science are some of the sectors where nanomaterials-based sensors have been used. Nanotechnology consists of different types of materials that have been used to form sensors. The differentiation of these materials lies in their shape, size and structural dimensions. The metals used to form these nanomaterials also decide the electrical conductivity, mechanical flexibility and thermal conductivity. The selectivity and sensitivity of the electrodes of the flexible sensors increased as a result of certain attributes of the nanomaterials like high carrier capability, high stability, improved biocompatibility, high porosity, improved mechanical properties and large surface area. The nanomaterials are available in different forms, the primary ones being nanoparticles [18, 19], nanotubes [20, 21], nanowires [22, 23], and nanosheets [24, 25]. These three types have been formed using different types of pure and conjugated metals. The fabrication techniques used to form these nanomaterials have also been fixed as per the requirement of the application. For example, while choosing the phase of the polyvinylidene difluoride (PVDF) [26, 27], different research groups have tried certain processes like annealing, poling, mechanical rolling for the conversion between α and β phases. Similar to this, conductive metals and polymers have been developed in the form of the three types of nanomaterials. For the conductive

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elements, gold [28, 29], platinum [30, 31], palladium [32, 33], silver [34, 35], iron [36, 37] and copper [38, 39] are some of the metals used to form the nanoparticles. Carbon has been mostly used to form Carbon Nanotubes (CNTs) [40, 41] and carbon dots [42, 43]. Apart from the advantages imparted by both nanoparticles and nanotubes, the nanotubes provide an additional advantage of biocompatibility. This attribute increases their field of application in different biomedical fields when the sensing prototypes have functioned as biosensors [44, 45], brain activity electrical sensors [46, 47], and implantable sensors [48, 49]. Similar to CNTs, other carbonbased allotropes like graphite [50, 51] and graphene [52, 53] have also been used as nanomaterials for forming biocompatible sensors. These two elements have mostly been employed in the form of nanosheets, which is the third most type of nanomaterial used to form flexible sensors. Graphene and graphite have also been used in other forms like reduced oxides [54, 55], nanoplatelets [56, 57], nanoribbons [58, 59], and nanorods [60, 61]. Apart from these forms, the last but one of the most significant types is the nanowires that have been widely used to develop high-quality electrochemical and strain sensors [62–65]. These metallic nanowires’ advantages are high electrical and thermal conductivities, excellent mechanical flexibility, high aspect ratio, and good optical transparency [22, 23]. Figure 1 [66] shows the representation

Fig. 1 Illustration of the types of nanomaterials used to develop flexible sensors [66]

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of the different types of nanomaterials used to develop flexible sensors. However, other nanomaterials that exist with different nanostructures are not discussed above, have been considered to increase the sensitivity of the prototypes. With the presence of each of these nanomaterials on the sensors, the binding affinity of the electrodes towards the target analyte for the electrochemical sensing applications. In strainsensing applications, the nanomaterials increase the overall aspect ratio, offering an increased contact area and material strength for determining the change in responses to the applied pressure/force. The conductive nanomaterials mentioned above are integrated with different kinds of polymers that differ in mechanical attributes like Young’s Modulus (E), biocompatibility, chemical resistance, and thermal stability. The types of polymers considered for fabricating flexible sensors can be differentiated based on the application of the sensors. Some of the organic and synthetic types of polymers that are commonly used are polydimethylsiloxane (PDMS) [67, 68], polyethylene terephthalate (PET) [69, 70], polyimide (PI) [71, 72], poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) [73, 74] and Polyethylene naphthalate (PEN) [75, 76]. In order to form the electrodes and substrates of the flexible sensors, each of these polymers has been used individually or integrated with the nanomaterials to form composites. This chapter highlights the significance of the use of nanomaterials to form flexible sensors in terms of their analytical performances. Tables 1 and 2 display a comparison between some of the significant physical properties of mentioned nanomaterials that have been used to form the flexible sensors. While Table 1 showcases the performances of the sensors for electrochemical sensing, Table 2 highlights the performances for strain-sensing applications. It is observed from each of the tables, the presence of nanomaterials has increased the selectivity and sensitivity of the sensors to a great extent. The subsequent sections elucidate some of the work from different research groups, where the use of each type of nanomaterial to design, develop and implement flexible sensors have been shown.

2 Nanotechnology-based Flexible Sensors The use of nanotechnology in the field of flexible sensors has benefited the research groups to a great extent. It has not only increased the application spectrum but has also simultaneously increased the chances of developing multifunctional flexible sensors [91, 92]. This multifunctional nature has been exploited mainly by conjugating different types of metallic elements to form single and multi-layered structures. The nature of the nanomaterials is vital for flexible sensors, which decide the overall characteristics and application of the sensors. Two different types of nanomaterials have also been mixed [93] to induce additional advantages like reinforced mechanical strength, thermal stability, and multiple physiochemical properties. The quality of the flexible sensors is determined by optimizing the amount of each material used to form the prototypes. The fabrication techniques used to integrate the nanomaterials in the flexible sensors can be summarized within printing methods [94, 95]. Some of

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Table 1 Comparison between the performances of different nanomaterials-based flexible sensors for electrochemical-sensing applications Materials

Nature of nanomaterial

Sensitivity

Limit of detection

Linear range

Ref.

Silver nanoparticles, PET

Nanoparticle

–

10 μg/ml

10–1000 μg/ml

[34]

Platinum NPs, MWCNTs, PEDOT: PSS

Nanoparticle

2.60 ± 0.15 nA

0.2 μM

10–600 μM

[30]

MWCNTs, silk fabric, platinum microspheres

Nanotube

288.86 μA mM−1 cm−2

1 mM

0–5 mM

[77]

CNTs, Gold nanotubes, PDMS

Nanotube

–

0.1 mM

1 − 100 mM

[78]

Graphene, MnO2

Nanosheet

422.10 μA mM−1 cm−2

0.19 μM

0.5–350 μM

[79]

Graphene nanosheets, polypyrrole

Nanosheet

–

200 ppb

–

[80]

Copper nanowires, graphene oxide, PET

Nanowire

1100 μA/mM.cm2 1.6 μM

0.005–6 mM

[81]

59 μA cm−2 mM−1

0.1–45.4 mM

[82]

Manganese oxide Nanowire nanowires, graphene

10 M

Table 2 Comparison between the performances of different nanomaterials-based flexible sensors for strain–sensing applications Materials

Nature of nanomaterial

Gauge factor

Detection limit

Ref.

Reduced graphene oxide, polystyrene,

Nanoparticle

250

1.05% (lower)

[83]

ITO NPs

Nanoparticle

18–157

1.5% (lower)

[84]

MWCNTs, hydrogel

Nanotube

3.39

700% (higher)

[85]

CNTs/ethylene vinyl acetate Nanotube

33.29

190% (higher)

[86]

Graphene nanosheets, reduced graphene oxide

Nanosheet

0.7–1.5

25% (lower)

[87]

Graphene nanosheets, gelatine, epoxidized natural rubber

Nanosheet

45,573.1

0.2% (lower)

[88]

Silver nanowires, PDMS

Nanowire

1.5

60% (higher)

[89]

Silicon nanowires, PDMS

Nanowire

0.15

60% (higher)

[90]

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the standard printing techniques used to fabricate the nanotechnology-based flexible sensors are screen printing [96, 97], inkjet printing [98, 99], gravure printing [100, 101], laser cutting [102, 103], and 3D printing [104, 105]. Each of these contact and non-contact printing methods is responsible for forming and finishing the sensors. Some of the advantages provided by these sensors include fabrication of smaller and flexible prototypes, large-scale production of the sensors leading to high-cost efficiency, reduced material wastage, and generation of sensors with high mechanical flexibility and integrity.

2.1 Types of Flexible Sensors Nanoparticles are the most common type of nanomaterials that are used to develop flexible, high-quality flexible sensors. Rosati et al. [34] show the use of it inkjet printing techniques to process silver nanoparticles (Ag NPs) to develop flexible biosensors. Four external ink reservoirs with cyan, magenta, yellow and black colors namely were used to form the sensors. Photographic glossy commercial papers were used as substrates, whereas PET was used as an absorbing layer to increase the penetration power and stability of the Ag NPs ink. The electrodes had a width of 50 microns, with each printed sheet consisting of three sets of horizontal and vertical lines. The interdigital distance was 200 microns and reproducibility of 90%. The functionalization of the sensors was done on both printed electrodes and PET substrates via thiol covalent binding. These sensors were employed for electrochemical sensing, where rapid label-free detection of antibiotics was done in milk. While the aptamerbased sensing process was carried the printed electrodes, the sensors showed high stability in their responses during the entire experimental process. The sensors having interdigital electrodes were tested using the electrochemical impedance spectroscopy (EIS) technique. In the experimentation, the excitation frequency of 10 Hz was used. Each round of EIS experiments was continued ten times. The prototypes had a LOD of 10 μg/ml and a response time of 6 min. With the application of certain antibiotics in nanomaterial, such as ampicillin, amoxicillin, neomycin, and kanamycin, the coefficient of variation in the experimental result was found 18% lower. Another interesting research regarding the fabrication of nanoparticles-based flexible sensors can be shown in work done by Nguyen et al. [30]. The sensors were fabricated using a direct writing process, where platinum nanoparticles (Pt NPs)based composite ink were used to form the sensors’ electrodes. Figure 2 [30] shows the schematic diagram of the fabrication process of these MWCNTs/Pt NPs-based sensors. The nanocomposites were formed by mixing the individual conductive components at fixed ratios. Then, PEDOT: PSS used as the conductive polymer was mixed in the nanocomposites, followed by adding Ecoflex and rotating the samples at a speed of 10,000 RPM. Then, PEDOT: PSS was modified using dimethyl sulfoxide to increase the resultant electrical conductivity of the nanocomposites. The printing process was done to form the working and counter electrodes using Pt NPs. The conductive traces were

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Fig. 2 Schematic diagram of the fabrication process of Pt NPs-based sensors on PDMS substrates [30]

then insulated using PDMS to leave open the electrodes and contact pads. This was followed by micromachining the implantable biosensors using femtosecond lasers to form lines on PDMS films on Parylene C-coated glass slides. These prototypes were used as amperometric biosensors for in vivo monitoring of glutamate molecules. The prototypes having nanocomposite-based electrodes consisted of multi-walled carbon nanotubes (MWCNTs), platinum nanoparticles, and a conductive polymer. These prototypes were immobilized with glutamate oxidase to measure the extracellular dynamics of glutamate and other potential biomarkers. Some of the advantages of the sensors were excellent linear range, high repeatability, high stability, and high mechanical flexibility. The sensors showed high potentiality for events like traumatic spinal cord injury. The sensors showed a LOD of 0.2 μM and a linear range from 10–600 μM. The response time of the sensors was between 15 and 20 s. The significance of nanoparticles in the performance of strains sensors has also been very prominent in recent years. The work done by Gong et al. [83] can be highlighted as one of the interesting works done on this area. Polystyrene nanoparticles (PS NPs) were mixed using reduced graphene oxide (rGO) to form highly responsive, flexible piezoresistive strain sensors. Initially, after forming PDMS substrates, the doping process was carried out on the GO. A chemical exfoliation process was used to form the GO, which was later used to form the GO dispersions using modified Hummers’ method. It was followed by mixing PS NPs and performing an ultrasonication process to form homogeneous solutions. The sensors were formed by drop-casting the homogeneous solution on the PDMS-coated CDs. The samples were cured for a couple of hours to obtain a dry film with a 1.8 cm × 0.6 cm size. Then, a laser induction process was used to pattern the GO-based sensing film to form rGO film. Then, the films were torn off the CD and treated with polypropylene

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flakes. Finally, the ends of the samples were sputtered with silver nanoparticles with a size of 80 nm to form the electrodes. The laser thermally treated rGO was mixed with the PS NPs to increase the conducting channels, thus increasing the subsequent sensitivity towards the applied strain. The Gauge Factor (G.F.) increased up to 250 under a small linear deformation of 1.05%. The sensors were employed for real-time monitoring of different human activities like the swallowing process, movement of lower back posture, and neck pulse. The resistance values of the prototypes formed with 250 nm-sized rGO were 240%, 113%, and 34% for doping ratios of 12.5 wt.%, 2.5 wt.% and 1.56 wt.%, respectively. Another example regarding the use of nanoparticles to form high-quality strain sensors can be highlighted in work done by Lee et al. [84]. The sensors were formed by doping tin with indium oxide nanoparticles (ITO NPs). The diameter of the ITO NPs was chosen to be less than 50 nm. The pulling process was carried out using a syringe pump and vertically standing centrifugation. The substrates were immersed in the homogeneous suspensions formed with 10 wt.% ITO NPs and deionized (DI) water. The patterns on the prototypes were formed using a shadow mask, followed by overlaying them with gold and titanium nanoparticles. The thermal evaporation process was used to form the electrodes for gold and titanium metals with 30 nm and 5 nm thicknesses, respectively. The electrode width and gap between the electrodes were around 1 cm and 100 microns, respectively. Around 330 parallel lines were formed on the ITO NPs-based film with a width of 20 microns and a gap of 10 microns between two consecutive lines. The line patterns formed on the strain sensors had additional advantages of adjustable G.F. and optical transparency. The G.F. varied between 18 and 157. The nanoparticles assembly speed was customized to control the dimensional and electrical properties of the prototypes. To minimized crosstalk between the transverse and longitudinal bending directions, a high optical transmittance up to 93% at a wavelength of 500 nm was obtained with these sensors. The crosstalk was around 7% and 2.5% for the transverse and longitudinal directions, respectively.

2.2 Nanotubes-based Flexible Sensors The second most common type of nanomaterials is used to develop flexible sensors, which are nanotubes with varied physiochemical characteristics. The nanotubes for certain conductive elements are present in various forms, thus being used in a varied range of applications. Carbon Nanotubes (CNTs) are primarily present in two forms, single-walled carbon nanotubes (SWCNTs) [106, 107] and multi-walled carbon nanotubes (MWCNTs) [108, 109]. Due to their structural differences, the electrical, mechanical, and thermal characteristics also differ from each other. While both types are preferred for individualistic applications, MWCNTs are advantageous over SWCNTs in terms of higher electrical conductivity, higher purity, easier and cheaper production [110, 111]. Other types of CNTs like few-walled carbon nanotubes (FWCNTs) [112, 113] and double-walled carbon nanotubes (DWCNTs) [114, 115]

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Fig. 3 Schematic diagram of the fabrication process of the platinum microspheres-decorated MWCNTs/CSF sensors [77]

are also being used to form enhanced flexible sensors. One of the works done by Chen et al. [77] shows the fabrication and implementation of electrochemical sensors using CNTs. The prototypes were formed by coating MWCNTs on carbonized silk fabric (CSF) and decorating them with platinum microspheres. Figure 3 [77] illustrates the fabrication process of these prototypes. The MWCNTs-coated CSF composites were formed using a carbonization process at a temperature of 950 °C and under an inert atmosphere. The electrodeposition process was used to decorate the coated composites with platinum microspheres. Cyclic voltammetry (CV) process was deployed for this process with a scanning rate of 10 mV/s. After the sensors were immobilized with glucose oxidase, they showed a good response in terms of analytical performances. The sensitivity and linear range of the prototypes were 288.86 μA mM−1 cm−2 and 0–5 mM, respectively. The sensors also had high repeatability and reproducibility in the responses with a relative standard deviation (RSD) of 3.25%. It was seen that the selectivity of these sensors was also high when they were simultaneously tested with other molecules like ascorbic acid, uric acid, and acetaminophen. The prototypes also showed a good response towards the hydrogen peroxide (H2 O2 ) gas. Another work highlighting the electrochemical sensing performance of the nanotubes-based sensors can be shown in work done by Liu et al. [78]. The sensors were wearable in nature and were able to perform in a non-invasive manner. These electrochemical sensors were formed by an electropolymerization process of 3,4-ethylene dioxythiophene (EDOT) monomers. These monomers were formed on the hierarchical network of CNTs and gold nanotubes (Au NTs). Figure 4 [78] shows the schematic diagram of these flexible electrochemical sensors. After PDMS films were soaked in the dopamine hydrochloride solution, silver nanowires were spin-coated on the surface of the soaked PDMS films. Then, in situ galvanic displacement process was used to form the Au NTs/PDMS films at a temperature of 90 °C. Then, a membrane transfer process was employed to load the CNTs membrane on the Au NTs-loaded PDMS films. The last step included the connection of copper wires on the sensing films using conductive silver adhesives. Then, PDMS was used to insulate and fix the joints. The active area of the sensors

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Fig. 4 Representation of the fabrication steps of the flexible CNTs/Au NTs-based electrochemical sensors [78]

was 0.5 cm × 0.5 cm. The next step included the electro-polymerization process of the molecularly imprinted membranes on the CNTs/Au NTs/PDMS-based sensing films. Then, urea-PEDOT composites were deposited on these flexible membranes, followed by removing the templates by washing them with DI water for an hour. The sensors showed high selectivity and good linear response towards relevant urea levels. The prototypes deployed as urea sensors were able to detect human sweat excreted after aerobic exercise. The average change in current for three different urea concentrations of 14.41 mM, 12.46 mM, and 30.33 mM was 27.99%, 27.34%, and 33.26%. The recovery range of the sensors was between 99.97% and 100.57%. The nanotubes-based flexible sensors have also been efficient for strain-sensing applications. Sun et al. [85] showed the use of CNTs-reinforced hydrogel to form highly flexible strain sensors with high mechanical toughness and high stretchability. Nanocomposite hydrogels were formed using oxidized MWCNTs and polyacrylamide, where the CNTs were uniformly dispersed in the presence of gelatine via hydrogen bonding. These strain sensors showed high performance due to the physical interactions between the oxidized MWCNTs, gelatine, and polyacrylamide chains. A one-step method was used to prepare the nanocomposite hydrogels comprising polyacrylamide and oxidized MWCNTs. The Ultrasonication process was used to form the CNTs-based dispersions, followed by adding monomers to the solutions. The solutions were then stirred and transferred into molds formed using silicone

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rubber. The rubber was placed between two glass plates having an internal diameter of 9 mm. Then, a free-radical polymerization process was deployed on these samples at a temperature of 50 °C for four hours. The sensors showed excellent responses when used to detect large and subtle human movements like elbow rotation, wrist and knee bending, swallowing, and phonation. The sensors had a high stretchability and high tensile strength of 700% and 0.71 MPa, respectively. While the recovery efficiency of the prototypes was 90%, the G.F. was 3.39 when the sensors were tested for strain values between 250% and 700%. As a result of high conductive pathways formed by oxidized MWCNTs, other advantages of the sensors were a fast response time of 300 ms, excellent durability for over 300 cycles. The prototypes had high potential for applications like human–machine interactions and monitoring personal health. Another interesting work related to the use of CNTs to form high-quality strain sensors can be seen in [86]. Self-repairing strain sensors were formed that had certain attributes like shape memory and large linear working range. Other advantages of the sensors were high stretchability, good dynamic durability, and fast response. Figure 5 [86] represents the fabrication process of these flexible strain sensors. After EVA fibers were treated with xylene solution, carboxylic group-functionalized MWCNTs were dispersed in the EVA solutions. It was followed by rinsing the nanocomposite fibers with DI water, followed by bonding copper sheets on the two opposite ends of the CNTs/EVA fibers at a distance of 20 mm. The prototypes were formed by embedding CNTs on the surface of the ethylene–vinyl acetate fibers (EVA) using the ultrasonic method. The shape memory nature of the prototypes was instigated as a result of the viscoelasticity of the elastic EVA polymer. The electrical conductivity of the CNTs/EVA fibers was around 2.646 S/m after the CNTs/EVA fibers were heated in the oven at 70 °C and then cooled at room temperature. Strain values up to 190% and 88% were measured for stretchability and linear working range. The durability of the sensors was over 5000 cycles, along with a response time of 312 ms. The sensors were capable of accurate measurement of heartbeat and human pulse.

Fig. 5 Schematic diagram of the fabrication process of the CNTs/EVA fibers for strain-sensing applications [86]

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2.3 Nanosheets-based Flexible Sensors The use of nanosheets is primarily done via graphene, which is stacked in the form of sheets in an sp2 hybridized manner [116, 117]. The graphene nanosheets have been used to form flexible sensors that display high electrical conductivity, high charge mobility, excellent mechanical strength, and high chemical and structural stabilities [118]. One of the ways to use graphene nanosheets to develop flexible sensors has been through nanocomposites, where other types of nanomaterials and polymers have been mixed to alter the electrical conductivity and mechanical integrity. Guan et al. [79] showcased the fabrication of non-enzymatic electrochemical sensors to detect H2 O2 . Figure 6 [79] shows the schematic diagram of the fabrication process of the graphene nanosheets-MnO2 -based electrochemical sensors. After potassium permanganate was reduced using the hydrothermal process in the presence of graphene nanosheets, nanocomposites were formed using manganese oxide and graphene nanosheets using the one-step hydrothermal technique. The formed nanocomposites were collected on a polytetrafluoroethylene (PTFE) membrane having a pore size of 0.22 microns. Then, the samples were washed and dried at 80 °C for 12 h. These nanocomposites were immobilized on glassy carbon electrodes using the drop-casting method. The nanocomposite-based suspensions having a concentration of 2 ppm were dropped on polished glassy carbon electrodes at room temperature. The nanocomposites-modified glassy carbon electrode was used to detect H2 O2 using CV, EIS, and amperometry techniques. The potential range and scan rate of the CV technique were 0–0.6 V and 50 mV/s, respectively. The linear range and LOD of the sensors were from 0.5 μM to 350 μM and 0.19 μM, respectively. The sensors had a high sensitivity of 422.10 μA mM−1 cm−2 and a signal-to-noise ratio (SNR) of 3. The RSD of the sensors was between 1.48% and 4.47% and the range of recovery

Fig. 6 Representation of the fabrication process of the MnO2/graphene nanosheets/glass carbon electrode-based electrochemical sensors [79]

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rate was 96.50–101.22%. Additionally, the sensors showed excellent reproducibility and repeatability of the responses and good anti-interference. Qin et al. [80] depicted the fabrication of nanosheets-based flexible sensors that were used as micro-supercapacitors (MSC) and gas sensors. The sensors consisted of 2D hierarchical order dual-mesoporous polypyrrole and graphene nanosheets as bi-functional active materials. The patterning of the dual mesoporous polypyrrolegraphene nanosheets was done using a soft and hard dual-template process. These novel planar prototypes were used to detect ammonia gas and store energy as supercapacitors. The sensors showed enhanced performances as a result of the effective coupling effect of graphene nanosheets and pseudo-capacitive polypyrrole polymer. The mesoporous values of 7 nm and 18 nm obtained for the hierarchical mesoporous network and a large surface area of 112 m2 /g led to extraordinary performances in terms of selectivity and sensitivity. The LOD of the sensors for NH3 gas was 200 ppb, along with a stable response when they were charged for 100 s. The stability of the responses was seen between 10 and 40 ppm of NH3 gas. The prototypes also showed high mechanical flexibility of 82% and high sensitivity towards NH3 gas as compared to other volatile compounds. When operated as MSC, an excellent capacitive value of 376 F/g was obtained at 1 mV/s. Another major advantage of the sensors was high overall compatibility that allowed them to use as ultrathin, miniaturized, body-attachable, and portable devices. Among the efficient strain sensors formed using nanosheets, the work done by Manna et al. [87] highlights the fabrication and implementation of printed prototypes using graphene nanosheets. Graphite was used as the precursor material from which graphene nanosheets were produced using the liquid-phase exfoliation technique. Water and N-methyl pyrrolidinone were used as co-solvents in the synthesis process. The consideration of the water-based solution assisted in minimizing the negative effect on the mechanical properties of the paper substrates. These paper-based sensors were cut in dimensions of 14 × 1 cm2 , followed by forming electrodes with two copper stripes sandwiched between two layers of silver paste. The presence of water enhanced the total yield and higher stability of the graphene nanosheets-based dispersions. Thin-film strain sensors were formed using both graphene nanosheets dispersions and rGO and were experimented with for comparative purposes. Two processes, namely centrifugation and bath-sonication, were repeated a number of times to generate rGO from the precursor material. The ones formed using graphene nanosheets exhibited better electromechanical characteristics like excellent electrical conductivity and lower noise floor. While determining the GF for both annealed and non-annealed cases, the total range of GF varied between 0.7 and 1.5. The prototypes were cooled down to room temperature before the electrical measurements were taken for the annealed samples. Another example of the synthesis of highly efficient strain sensors using nanosheets can be seen in Tang et al. [88]. Scalable strain sensors were manufactured that were based on ion-intercalated mechanical milling and interfacial coordination methods. A simple, cost-efficient method was used to form the autonomously self-healing sensors that showed enhanced mechanical characteristics. Large-scale

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Fig. 7 Representation of the nanocomposite-based strain sensors formed using gelatine, epoxidized natural rubber, and graphene nanosheets [88]

production of these sensors was possible as the interfacial metal–ligand coordination was obtained between the well-organized graphene nanosheets network and a rubber matrix. Figure 7 [88] shows the schematic diagram of the fabrication process of these self-healing strain sensors. The exfoliation of the expanded graphite was done in aqueous solutions with the assistance of ferric ions and Kelvar fiber pulp as an intercalator. The process took place by co-milling in a grinder. The exfoliated graphene nanosheets had a thickness and later size of 2 nm and 200–400 nm, respectively. The prototypes showed high healing capability even in harsh conditions like underwater, at sub-zero temperature and at extreme acidic and alkaline conditions. With a gradual increase in the applied strain, the G.F. increased slowly up to a strain value of 73.7%. The G.F. of the sensors was around 45,573.1, thus exhibiting a potential in different wearable applications like artificial skins and smart robotics. The prototypes had a LOD of 0.2% strain.

2.4 Nanowires-based Flexible Sensors The last type of nanomaterials has gained high popularity in recent times due to the attributes they impart on the resultant prototypes. Apart from the high electromechanical characteristics imparted by different types of metallic nanowires, the easy customization in their structural dimensions has allowed the researchers to form nanocomposites of high caliber [119]. One of the examples of the use of nanowires can be seen in the use of copper nanowires to develop non-enzymatic glucose sensors [81]. The sensors were flexible and disposable in nature, formed by modified transparent graphene electrodes with copper nanowires. The sensors were formed on

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PET sheets, using four individual steps. The first step was preparing the GO films via filtration of GO colloids onto cellulose acetate microfiltration membrane. Then, the films were presented onto PET sheets manually by applying a 5 kg on top of it. Then, acetone was used to rise and dissolve the membranes, followed by converting the GO/PET films into graphene transparent electrodes. This conversion process was carried out by treating the GO/PET sheets with HI and CH3 COOH acids at a volume ratio of 2/5, respectively. The copper nanowires were modified by the Wiley method, where the precursor material copper nitrate was treated with sodium hydroxide at a temperature of 60 °C. The deposition of the copper nanowires on the hybrid electrodes was done using the spin-coating process. CV process was used to determine the change in the responses during the characterization and experimentation processes. The sensors showed a wide linear range of 0.005–6 mM and a LOD of 1.6 μM. The sensors showed an SNR of 3 and a sensitivity of 1100 μA/(mM cm2 ). Dong et al. [82] showed the fabrication of electrochemical sensors using manganese dioxide (MnO2 ) nanowires on graphene paper substrates. Figure 8 [82] illustrates the synthesis process of these MnO2 nanowires/graphene-based electrochemical sensors. After GO was synthesized from graphite powder using the modified Hummers’ method, the GO papers were formed using a simple, scalable mold-casting method. The fabrication process was carried out using a one-step electrochemical process which included the loading of MnO2 nanowires on effectively reduced graphene oxide (GO). The casting mold was formed using a PTFE membrane to

Fig. 8 Schematic diagram of the MnO2 nanowires/graphene-based electrochemical sensors [82]

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casting the aqueous solutions of GO nanosheets. The next step included the evaporation of water vapor and peeling off the GO papers from the PTFE molds. The working principle of these sensors was based on the high electro-catalytic activity shown towards the redox reaction of hydrogen peroxide (H2 O2 ) secreted from live cells macrophages. A higher number of electrochemically active sites were provided, which increased the overall utilization efficiency and catalytic performance. These flexible sensors had other advantages like high stability, selectivity, and reproducibility of the responses. The sensors had a linear range of between 0.1 mM and 45.4 mM, an SNR of 3, and a LOD of 10 μM. The sensitivity of the sensors was 59 μA cm−2 mM−1 . With certain attributes like intrinsic flexibility, tailorable shapes, and adjustable properties, the sensors had high potential as point-of-care (POC) testing devices and portable instruments. The flexible strain sensors that have been formed using nanowires can be showcased in work done by Jiang et al. [89]. The strain sensors formed in the sandwich structure contained silver nanowires and self-healing substrates. Super-flexible 3D architectures having sandwiched structures had silver nanowires-decorated selfhealing polymer between two layers of PDMS. The sandwiched structures were formed by initially drop-casting silver nanowires on the middle of the PTFE membrane, followed by curing them in the oven at 60 °C for an hour. Then, the self-healing polymer composites were used to cover the plate and subsequently heated at 90 °C for 30 s. Then, the samples were cut with dimensions having a length and width of 40 mm and 10 mm, respectively. Finally, two copper conductors were attached at the opposite ends of the prototypes with the help of silver adhesives. Some of the advantages of these sensors were excellent stability, high electrical conductivity, excellent reliability, large sensing area, high stretchability, and self-healable nature. The polymers were further reinforced with carbon fibers to improve the overall mechanical integrity of the prototypes. The self-healing nature also helped in repairing the electromechanical damages of the prototypes. Different types of human motions like bending, recovering the forearm and shank, palm, fist, and fingers have been exerted on the sensors. The sensors had a tensile stress limit of 10.3 MPa and an elongation breakpoint of 8%. Another illustration of the nanowires-based strain sensors can be seen in the work done by Huang et al. [90], where ultra-minimized stretchable strain sensors were formed using single silicon nanowires. Figure 9 [90] shows the schematic diagram of the fabrication process of stretchable silicon nanowires. The nanowires were formed using the temperaturegradient-assisted vapor–liquid-solid technique. The developed silicon nanowires had

Fig. 9 Illustration of the fabrication of the silicon nanowires-based flexible strain sensors [90]

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a length of 7–10 mm. Then, a single nanowire was placed on the silicon wafer, followed by transferring it to PDMS thin layer contact printing process. The PDMS films were then cured at 75 °C for an hour to solidify the substrates and release them from silicon wafers to form stretchable prototypes. When the strain applied on the stretchable PDMS increased from 10 to 50%, the resultant amplitude of the silicon nanowires increased from 40 μm to 60 μm. The change in relative resistance was around 4% with respect to the applied strain in the range of 30– 35%. The breaking point of the silicon nanowires was 60%. With single-centimeter nanowires as active materials, the prototypes exhibited a large strain sensing strain over 45% and high durability over 10,000 cycles. The sensors were used to detect human motions like joint motion, swallow and touch.

3 Challenges of the Current Nanotechnology-based Sensors Even though much work has been done to form flexible sensors with different types of nanomaterials, some issues still need to be addressed in the current scenario. The conjugation of the nanomaterials with flexible sensors using printing methods has greatly influenced in carrying out the research in developing miniaturized flexible sensors. Although there has been a significant paradigm shift from microelectromechanical (MEMS)-based silicon sensors to flexible sensors, proper exploitation of the nanomaterials has still not yet been done. The formation of nanocomposites with specific nanomaterials like nanotubes and nanosheets is still an issue. Even though the presence of surfactants does improve the percolation threshold, the mechanical integrity of the nanocomposites is compromised in the presence of the same. One of the disadvantages of graphene is its structural defect [120, 121], whose effect can be associated with sensors with semiconducting nature. The issue with nanowires can be related to their synthesis process, which is an expensive and complicated process. The association of nanowires has not yet been fully done with polymers to form prototypes that can be used as wearable and implantable sensors. The conjugation of different types of nanomaterials should be further encouraged to develop nanocomposite-based sensors with higher quality in terms of electrical conductivity and mechanical flexibility. The quality of the electrochemical sensors used to detect the common ions showed to be improved in terms of reusability and washable nature. The tuneable G.F. of the strain sensors also improved by inducing the tunneling effect to increase the overall conduction pathway in the sensors. The use of nanomaterials inside the human body for drug delivery in cells is another issue that needs to be further studied. The adverse effects of the nanomaterials inside the human body can be a major reason to cause long-term health-related problems. The embodiment of the flexible sensors with flexible printed circuit boards [122, 123] should be done for realtime operations in biomedical applications. This would increase the comfortability for the patients and increase the multifunctional nature of the prototypes.

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4 Conclusion The chapter elucidates the effect of some of the common types of nanomaterials on the properties of flexible sensors. Nanomaterials have been used to modify the electrodes of the sensing prototypes to enhance their quality in terms of electrical and mechanical attributes. Some common nanomaterials like nanoparticles, nanotubes, nanosheets, and nanowires have been employed with other types of conductive elements and polymeric matrices to form enhanced electrochemical and strain sensors. These electrochemical sensors have been successfully used to detect ions at micro levels with high selectivity and sensitivity, fast response and recovery times, high stability and reproducibility of the results, and a wide linear range. Similarly, the strain sensors had a high G.F. and wide working strain range. After validating the proof of concept of these nanotechnology-based flexible sensors in the laboratory and controlled environments, the next step would be to deploy these sensors for real-time applications. Different communication protocols need to be embedded with these flexible sensors to process the sensed signal and transfer them to the monitoring unit for further processing. With an ever-increasing demand for flexible sensors in the modern world, the presence of nanomaterials would greatly suffice their capability in improving the quality of life. Funding This study was funded by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) as part of Germany’s Excellence Strategy—EXC 2050/1—Project ID 390696704— Cluster of Excellence “Centre for Tactile Internet with Human-in-the-Loop” (CeTI) of Technische Universität Dresden.

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Fabrication and Implementation of Nanomaterials-Assisted Flexible Sensors Mariana Arpini Vieira

Abstract Research on flexible electronics has grown remarkably in the last decade due to the mechanical limitations of conventional rigid electronics. With the advent of nanotechnology, bulky, thick, and rigid electronic materials have been replaced by nanomaterials that exhibit intrinsic mechanical deformability as well as superior electrical properties. These nanoscale-based soft materials can be envisioned as great assets for the sensing field as conventional sensors usually fail to efficiently capture analytes and, therefore, suffer from poor-quality signal transduction. Herein, we review the literature on nanomaterials-enabled flexible sensors in the past decade, addressing the kinds of nanomaterials that have been used in their fabrication, as well as listing and describing them according to their dimensions (0D, 1D and 2D nanomaterials). Moreover, we discuss some of the different fabrication techniques that have been used in order to obtain flexible sensors, either by fabricating nanocomposites from the combination of nanomaterials and flexible substrates, or by integrating nanomaterials by depositing them on top or between layers of other flexible materials. Then we discuss the advantages of using nanomaterials-assisted flexible sensors, such as simple fabrication processes, or at least the vast number of different techniques that can be applied, the ability to vary the nanomaterials sizes and shapes, elevated sensitivity and selectivity and so on. At last, we bring some examples of applications of nanomaterials-based flexible sensors, more specifically in the fields of electrochemical, strain and electrical sensing, ranging from environmental to medical applications.

M. A. Vieira (B) Department of Physics, University of Alberta, 116 St & 85 Ave, Edmonton, AB T6G 2R3, Canada e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Nag and S. C. Mukhopadhyay (eds.), Flexible Sensors for Energy-Harvesting Applications, Smart Sensors, Measurement and Instrumentation 42, https://doi.org/10.1007/978-3-030-99600-0_4

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1 Introduction The advances on flexible technologies research over the years has enabled the fabrication of devices on large substrates with thin and ultra-flexible intuitive systems and, among these, the fabrication of slender, lightweight, stretchable, and foldable sensors. Due to their rigidity, conventional sensors usually fail to efficiently capture analytes and, therefore, suffer from poor-quality signal transduction. On the other hand, flexible sensors can be folded or rolled and applied onto substrates with complex interfaces (soft or curved shapes, for example) with no change of their functionality and still efficiently detect various types of stimulation [1–3]. Han et al. have already stated that it is expected that the possibilities of flexible electronics will expand remarkably due to the rapid developments in the design of ultrathin sensors, electronic and optoelectronic devices and soft biocompatible layers, giving rise to novel curved panels and foldable displays as well as soft systems with interfaces that offer curved surfaces and complex geometries [1]. The production of flexible sensors requires novel approaches in materials design, including active materials and conductors, as well as the selection or synthesis of flexible substrates, depending on the fabrication technique and application of the sensors. In light of the different components that make up a sensor, there is a wide range of promising materials that have already been used according to the most common role each individual material has been found to perform in flexible sensors (Fig. 1). That is, the different kinds of materials that have been used as conductors, semiconductors, insulators and substrates [4]. Special attention is given to the most relevant properties of materials for the development of flexible sensors, such as stability under bending, stretchability and fabrication options. In light of these needs, and as an alternative to the conventional materials that were once used for the fabrication of sensors, nanomaterials in the form of nanoparticles [5–8] or nanowires, [9, 10] for example, have been studied due to their relatively easy application in different materials and because they show a resistance to bending much smaller than that of its bulk equivalents [11, 12]. Carbon-based nanomaterials, such as graphene [13–15] and carbon nanotubes, [16, 17] have also been the subject of intensive research, given their unique characteristics, such as high conductivity, mechanical flexibility and low cost. Among all the materials and their applications in the production of flexible sensors, the use of nanomaterials can be easily found in almost any part of these devices. This can be explained by the fact that the nanoscale dimensions of the materials dramatically decrease flexural rigidity of devices [18]. In this chapter we will discuss the types of nanomaterials that have been used for the fabrication of flexible sensors, what are the fabrication processes that have been followed over the past decade as well as the implementation of nanomaterials-assisted flexible sensors and the advantages provided by nanomaterials for the sensing field.

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Fig. 1 Common materials and respective fabrication methods used for the fabrication of flexible sensors [4]. For the visually impaired: The image describes a flexible sensor schematically. In the center of the image there are four wavy thin rectangles, diagonally stacked, representing different layers of a flexible sensor. Each layer is described in four stickers around the central image. The first bottom layer represents the sensor substrates and the materials commonly used for this. Second bottom layer represents the dielectric layer and the materials used for this function. The third layer represents the layer in which the sensor contacts are attached. These contacts are represented by colorful squares of top of this layer. The fourth and last layer represents the sensing area with the materials that are usually applied on it. There are also colorful squares that represent the electronic components of the sensing layer

2 Nanomaterials That Have Been Used in Flexible Sensors Nanomaterials can be identified based on their structure, shape, size, chemical composition and chemical synthesis. In this way, there are four major categories into which nanomaterials can be classified: carbon nanomaterials, metal and metal oxide nanomaterials, organic nanomaterials and nanocomposites (when the nanomaterials either combine with other types of nanomaterials or combine with larger sized materials). However, most of these types of nanomaterials can usually be synthesized into different shapes, like tubes and sheets for example. Thus, nanomaterials can also be classified based on their shape or, basically, depending on the movement of electrons in the material. For instance, in zero-dimensional (0D) materials the movement of the electrons is very limited, being generally considered as fixed. For one-dimensional materials (1D), electrons can move freely along only one of the axes, which is usually less than 100 nm. Accordingly, for two-dimensional (2D) and

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three-dimensional (3D) nanomaterials, electrons move along two and three axes, respectively [19]. Here we will address the kinds of nanomaterials that have been used in the fabrication of flexible sensors, listing and describing them according to their shape classification. In this way, we will define and describe these nanomaterials, starting with 0D nanomaterials, followed by 1D, and so on.

2.1 Zero-Dimensional Nanomaterials 2.1.1

Nanoparticles

Nanoparticles (NPs), are a major class of nanomaterials, zero-dimensional and possessing nanometric dimensions in all three dimensions, with sizes ranging from 10–100 nm [20]. Because of their extremely reduced size, the electronic and atomic structures of nanoparticles provide them properties different from those of their bulk materials. However, the larger they become, the closer their properties become of their bulk counterparts. Some of the size-dependent properties of nanoparticles include electronic, magnetic, optical, and chemical. Besides that, nanoparticles can be either amorphous or crystalline. That is, a nanoparticle composed of atoms in either a single- or poly-crystalline arrangement is called a nanocrystal. In a flexible sensor, NPs can be applied in the sensing layer either by being deposited on flexible substrates or by being integrated into the composite of a flexible material. Integrating NPs and the polymer flexible substrate into a composite material make it possible to tune the properties of the final material by tailoring the NPs type, coating and size, since properties of the initial polymer would be modified due to the association with the NPs. As an example of the application of NPs in flexible sensors, Xie et al. presented in their work a resistive type H2 sensor using Pd nanoparticles as sensing active material. The nanoparticles were deposited on a polyethylene terephthalate (PET) substrate which enabled the fabrication of a flexible and optically transparent sensor with great tolerance towards repeated bending, as well as good electrical stability, mechanical robustness and the ability to detect a H2 concentration as low as 15 ppm [7] (Fig. 2). Mahesh et al. produced a dopamine (DA) flexible sensor by growing α-Fe2 O3 nanoparticles on acid treated carbon cloth (ACC) as an alternative to glassy carbon electrodes that have been previously used. The results showed that the ACC-α-Fe2O3 electrode exhibits impressive electrochemical sensitivity, stability and selectivity for the detection of DA, with excellent response even upon cyclic folding and bending [8].

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Fig. 2 A H2 flexible sensor device showing convex and concave bending. The corresponding lower diagrams show the device exposed to H2 molecules (red) under strain; the Pd NPs are represented by the white spheres while the interdigital electrodes are simplified as a pair of yellow plane electrodes Xie et al. [7]. For the visually impaired: there are four tiled images. The top left picture contains a transparent acrylic cylinder being held by a hand, and on the external top of the cylinder is attached the flexible stripe-like sensor array, on a convex configuration, connected to two electrodes. The top right image is similar to the first, but the sensor array is located in the inner bottom part of the cylinder, on a concave configuration. The two bottom images are diagrams of the two upper images representing how H2 molecules interact with the device in the convex and concave configurations. The stripe-like sensor array is represented by a gray rectangle, the Pd NPs are white small spheres attached to the rectangle, the interdigital electrodes are simplified as a pair of yellow stripes around the rectangle, and the H2 molecules as represented by red small spheres “floating” close to the sensor array

2.1.2

Quantum Dots

Within the class of nanoparticles, quantum dots (QDs), drag special attention due to their unique optical, magnetic and electronic properties, caused by the quantum confinement effect that occurs due to their extremely small diameter (in the range of a few nanometers). This effect happens when the particle dimensions are near to and below the Bohr exciton radius of the material, which also makes the materials’ properties size-dependent [21]. Being zero-dimensional also provides QDs a sharper density of states than structures of higher dimension, which explains their excellent optical and transport properties and make them a promising material for the use in amplifiers, diode lasers and biological sensors. Compared with other materials that are commonly used for the fabrication of sensors, QDs are believed to offer superior performance due to its luminescence lifetimes, resistance against photobleaching, narrow emission bands, and especially broad absorption bands that allow for a diverse

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selection of possible excitation wavelengths from the visible to the near infrared regions [22]. QDs have been traditionally used in the production of solar cells, transistors, LEDs, etc. However, their extremely large surface-to-volume ratio also makes them capable of active interaction with target gas molecules [23]. Hosseini et al. developed a highly sensitive flexible humidity sensors based on graphene quantum dots (GQDs), with potential to detect low humidity levels (