Printed Flexible Sensors: Fabrication, Characterization and Implementation [1st ed.] 978-3-030-13764-9, 978-3-030-13765-6

Table of contents :
Front Matter ....Pages i-ix
Introduction (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 1-15
Literature Review (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 17-81
Interdigitated Sensing and Electrochemical Impedance Spectroscopy (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 83-89
Carbon Nanotubes-Polydimethylsiloxane Sensor (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 91-114
Aluminium-Polyethylene Terephthalate Sensor (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 115-128
Graphite-Polyimide Sensor (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 129-168
Graphite-Polydimethylsiloxane Sensor (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 169-192
Conclusion, Challenges and Future Work (Anindya Nag, Subhas Chandra Mukhopadhyay, Jurgen Kosel)....Pages 193-198

Citation preview

Smart Sensors, Measurement and Instrumentation 33

Anindya Nag Subhas Chandra Mukhopadhyay Jurgen Kosel

Printed Flexible Sensors Fabrication, Characterization and Implementation

Smart Sensors, Measurement and Instrumentation Volume 33

Series editor Subhas Chandra Mukhopadhyay School of Engineering Macquarie University Sydney, NSW Australia e-mail: [email protected]

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.

More information about this series at http://www.springer.com/series/10617

Anindya Nag Subhas Chandra Mukhopadhyay Jurgen Kosel •

Printed Flexible Sensors Fabrication, Characterization and Implementation

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Anindya Nag School of Engineering Macquarie University Sydney, NSW, Australia

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

Jurgen Kosel King Abdullah University of Science and Technology Thuwal, Saudi Arabia

ISSN 2194-8402 ISSN 2194-8410 (electronic) Smart Sensors, Measurement and Instrumentation ISBN 978-3-030-13764-9 ISBN 978-3-030-13765-6 (eBook) https://doi.org/10.1007/978-3-030-13765-6 Library of Congress Control Number: 2019931841 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express 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 sensors have exhibited immense potential to be utilized for healthcare, environment and industrial applications. The full-blown exploitation of these types of sensors is yet to be carried out to cause an impact on the quality of life of people. The work shown here showcases a great dynamicity in the employment of some of the sensors in the application world. Among a range of techniques that can be used to fabricate the flexible sensing prototypes that differ with respect to size, cost and resolution, the use of printing technology had been considered to a large extent. The research done on printed flexible sensors has been constantly increased due to certain advantages like low cost, enhanced electrical and mechanical attributes. The work shown in this book explains the fabrication of novel flexible printed sensors using laser cutting and 3D printing techniques. Four types of printed flexible sensing prototypes were designed, fabricated and implemented for some of the healthcare, environmental and industrial applications. The main motive behind the development of each of the developed sensors can be ascribed to their low cost of fabrication, simple operating principle and multi-functional characteristics. The electrical nature of the sensor was based on the capacitive principle due to the interdigital design of their electrodes. Electrochemical impedance spectroscopy was used along with the sensor prototypes to analyse the change in their outputs with respect to different inputs. The differences among these prototypes were based on their characteristics as a result of the difference in raw materials used to fabricated them. Multi-walled carbon nanotubes, graphene, aluminium and graphite are some of the conductive materials that were considered to form the electrodes of the sensor prototypes for their lightweight, high electrical conductivity, robustness and high aspect ratio. Polydimethylsiloxane, polyethylene terephthalate and polyimide are some of the polymers that were considered to form the substrates for their low cost, biocompatibility, low Young’s modulus and capability to form flexible multi-layered structured devices. The sensor prototypes were considered for different fields like monitoring of physiological movements, respiration and taste sensing for healthcare, monitoring of salinity and nitrate concentrations in water bodies for environment, and tactile and low-force sensing for industrial applications. The futuristic v

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uses of the fabricated sensors could include their real-time applications for chemical and biological sensing of proteins and similar enzymes, different gases, temperature and humidity. Considering their small size and biocompatible nature, they can also be utilized as implantable sensors to analyse the anatomical changes taking place inside the body of a human being. They can also be employed for applications of national importance like military and defences, where they can be attached along with adaptive feedback systems on the wings of a plane to calculate their active flutter suppressions. The work elucidated in this book is obtained from a doctoral research done at Macquarie University, NSW, Australia. The objective of this work was to develop different flexible printed sensors to highlight and enhance the field of printing technology. The developed sensors were reliable with high durability, low response time and higher repeatability in their responses. The authors are highly obliged to their colleagues who had a significant contribution to this work: Nasrin Afsarimanesh, Md Eshrat E. Alahi, Van Nguyen Thi Phuoc, Ulrich Buttner, Dr. Ahmed Alfadhel, Niki Murray, Mario Codeniera, Walter Adendorff and Dr. Keith Imrie. A special thanks to Macquarie University, Australia, and Massey University, New Zealand, for providing research facilities to fabricate the sensors and utilize them for specific applications. We would also like to extend our gratitude to our families for their immense support, motivation and encouragement throughout the work. Sydney, Australia Sydney, Australia Thuwal, Saudi Arabia

Anindya Nag Subhas Chandra Mukhopadhyay Jurgen Kosel

Contents

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2 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Carbon Nanotubes and Their Sensor-Based Applications 2.2.1 Synthesis of Carbon Nanotubes . . . . . . . . . . . . . 2.2.2 Characterization and Properties . . . . . . . . . . . . . . 2.2.3 Electrochemical Sensors . . . . . . . . . . . . . . . . . . . 2.2.4 Strain Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Electrical Sensors . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Conclusion and Future Work . . . . . . . . . . . . . . . 2.3 Graphene and Its Sensor-Based Applications . . . . . . . . . 2.3.1 Synthesis of Graphene . . . . . . . . . . . . . . . . . . . . 2.3.2 Characterization and Properties . . . . . . . . . . . . . . 2.3.3 Electrochemical Sensors . . . . . . . . . . . . . . . . . . . 2.3.4 Strain Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Electrical Sensors . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Challenges with the Current Sensors . . . . . . . . . . 2.3.7 Conclusion and Future Work . . . . . . . . . . . . . . . 2.4 Wearable Flexible Sensors . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Materials for Wearable Flexible Sensors . . . . . . . 2.4.2 Sensor Networks for Wearable Flexible Sensors .

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1 Introduction . . . . . . . . . . . 1.1 Flexible Sensors . . . . . 1.2 Printed Electronics . . . 1.3 Conclusion . . . . . . . . . 1.4 The Aim of the Book . 1.5 Research Contributions References . . . . . . . . . . . . .

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2.4.3 Types of Activity Monitoring with Wearable Flexible Sensors . . . . . . . . . . . . . . . . . . . . . 2.4.4 Challenges and Future Opportunities . . . . . . . 2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Interdigitated Sensing and Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Planar Interdigital Sensors . . . . . . . . . . . . . . . . . . . . 3.3 Electrochemical Impedance Spectroscopy (EIS) . . . . 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Carbon Nanotubes-Polydimethylsiloxane Sensor . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Fabrication of the Sensor Patches . . . . . . . . . . . . . . 4.3 Frequency Response and Stress-Strain Measurements 4.4 Monitoring of Physiological Parameters . . . . . . . . . . 4.4.1 Experimental Setup . . . . . . . . . . . . . . . . . . . 4.4.2 Results and Discussion . . . . . . . . . . . . . . . . 4.4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Tactile Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Experimental Setup . . . . . . . . . . . . . . . . . . . 4.5.2 Results and Discussion . . . . . . . . . . . . . . . . 4.5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Aluminium-Polyethylene Terephthalate Sensor . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Fabrication of the Sensor Patches . . . . . . . . . . . . . . 5.3 Frequency Response and Stress-Strain Measurements 5.4 Tactile Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Graphite-Polyimide Sensor . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Fabrication of the Sensor Patches . . . . . . . . . . . 6.3 Complex Nonlinear Least Squares Curve Fitting 6.4 Salinity Sensing . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Experimental Setup . . . . . . . . . . . . . . . . 6.4.2 Results and Discussion . . . . . . . . . . . . .

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6.4.3 Microcontroller-Based Sensing System . . . . . . . 6.4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Taste Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . 6.5.2 Results and Discussion . . . . . . . . . . . . . . . . . . 6.5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Nitrate Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . 6.6.2 Comparative Analysis of Two Different Sensors 6.6.3 Temperature and Nitrate-N Measurement . . . . . 6.6.4 IoT-Enabled Smart Sensing System . . . . . . . . . 6.6.5 Results and Discussion . . . . . . . . . . . . . . . . . . 6.6.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Graphite-Polydimethylsiloxane Sensor . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . 7.2 Fabrication of the Sensor Patches . . . 7.3 Frequency Response and Stress-Strain 7.4 Strain Sensing . . . . . . . . . . . . . . . . . 7.4.1 Experimental Setup . . . . . . . . 7.4.2 Results and Discussion . . . . . 7.4.3 Conclusion . . . . . . . . . . . . . . 7.5 Force Sensing . . . . . . . . . . . . . . . . . . 7.5.1 Experimental Setup . . . . . . . . 7.5.2 Results and Discussion . . . . . 7.5.3 Conclusion . . . . . . . . . . . . . . 7.6 Chapter Summary . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

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8 Conclusion, Challenges and Future Work . 8.1 Conclusion . . . . . . . . . . . . . . . . . . . . . 8.2 Challenges of the Existing Work . . . . . 8.3 Future Work . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Introduction

Abstract This chapter showcases the significance of sensors, explaining the ideology behind the usage of printed flexible sensors, their fabrication techniques and some of their applications.

1.1 Flexible Sensors The utilization of sensors for daily activities has made a prominent impact on human being’s life. Sensors are being employed to monitor and detect the changes taking place in the surrounding environment. The use of sensors for different applications is increasing every day. They are installed or connected to different objects and operated via control engineering from distant locations. They have been used to collect, store and analyze data that are difficult to obtain from inaccessible and partially accessible locations without much human intervention and for security purposes. The use of sensors has been done for more than 2000 years (Soloman 2009). Technically, a sensor can be defined as a device that can detect and respond to the changes happening in its ambiance. Even though the first commercial sensor was a thermostat, there was a prominent rise in the usage of sensors around the early 19th century (Oberg et al. 2006). As the demand for commercial sensors gradually increased, work had been done to improve the quality of existing sensors (Muro 2013). The utilization of sensors has almost tripled in the last two decades (Wang 2001). Currently, almost all the applications are somewhat connected with sensing systems (Meixner et al. 2008). The types of any sensor are mainly dependent on the application for which they are to be used. The current sensors different among the current sensors lie in their operating principle, size, sensitivity, etc. Some of the popular types of sensors are magnetic, electrical, thermal, chemical, etc. The cost of a sensor depends on the price of the processed materials, the fabrication technique and post-processing costs. These characteristics reflect the properties of the sensor which in turn dictates the applications for which are to be used for. Initially, when the researchers started using sensors for measurement purposes, prototypes having rigid substrates were used for measuring different industrial (Nag et al. 2016b, c; Zia et al. 2011) and healthcare (Mainwaring et al. 2002; Otto et al. 2006; Szewczyk et al. 2004) applica© Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_1

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1 Introduction

tions ubiquitously. Until the end of the last decade, single-crystal silicon has been the dominant semiconductor material to develop the substrates for the sensors. Some of the distinct advantages of using rigid silicon sensors include their inexpensiveness, high sensitivity, and low leakage current, which is as a result of the high potential barrier in silicon. Even though these sensors are used largely to form sensors on a large-scale, there are certain disadvantages associated with them like the high cost of fabrication, high input power for operation, mechanical damage and stiffness, which opts for alternative options. Due to these limitations, flexible sensing prototypes are more and more preferred for different applications (Segev-Bar and Haick 2013). Some of the limitations mentioned above are rectified by flexible electronics. Thinfilm transistor circuits were developed around fifty years ago, which at that time, formed a new emerging segment for forming sensors. With time, the circuits were enhanced regarding their performances, cost of fabrication and efficiency. This was subsequently followed by microelectromechanical systems (MEMS), which were developed in the late 20th century. MEMS cover a wide range of systems with sizes varying from a few microns to a few millimeters. MEMS have significantly improved regarding size, efficiency, and power consumption compared to the earlier devices. After the advent of utilizing the sensors for continuous monitoring (Sze 1994) for different applications in daily life, there has been an ever rising demand for their commercial availability. They have revolutionized the quality of human life via their employment in a dynamic range of applications. Earlier, it took hours to study or monitor an event but can be tackled in minutes or even seconds via smart sensing systems. The dynamic applications of sensors have led to an ever-rising modification of the existing sensors. Nowadays, almost every industrial, domestic and environmental sector utilizes sensors for improving the quality of life (Nag and Mukhopadhyay 2014; Nag et al. 2016a, b; Rahman et al. 2013, 2014; Zia et al. 2014). They have been deployed for different sectors including gas-sensing (Nag et al. 2016b, c), environmental-monitoring (Suryadevara et al. 2012; Yunus and Mukhopadhyay 2011), determining the individual constituents in the food and other edible products (Mukhopadhyay and Gooneratne 2007; Zia et al. 2013, 2014) and physiological parameter monitoring (Nag et al. 2016a). The classification of sensors can be done in two categories depending on the type of materials used to fabricate them, flexible (Segev-Bar and Haick 2013) and non-flexible (Unno et al. 2011). The flexible prototypes are the ones which are fabricated from the materials which are malleable to a certain extent without changing their properties, whereas the latter ones are made from materials which are rigid and non-malleable. The non-flexible sensors were developed earlier among which the ones developed with silicon substrates are the most popular ones. Even though sensors of this kind are employed for a wide field of applications, there are certain limitations like their brittle nature, stiffness, which deters their usability. These disadvantages are more prominent, especially when the sensing system is deployed for monitoring physiological parameters of a person or any other uses which involves any strain applied on the sensor. This results into opt for an alternative approach where the electrical and mechanical properties of the sensor would be dynamically used, thus negating any inconvenience caused to the person or protecting the sensor from

1.1 Flexible Sensors

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Fig. 1.1 Schematic representation of the use of wireless sensors for physiological parameter monitoring (Patel et al. 2012)

damage for any specific application. Apart from this, lower fabrication cost, lighter weight, enhanced mechanical and thermal properties are some of the advantages of the flexible sensors in comparison to the non-flexible ones which make them a better option. Figure 1.1 shows a schematic of a monitoring system to sense physiological parameters like the heart rate and respiratory rate of a person and simultaneously transmit the data wirelessly to the cloud server via any information gateway (Patel et al. 2012). This is a quick and efficient real-time system as any abnormality of the person would be reflected in the transmitted data, which would give a notification to the healthcare provider or family members. The materials chosen for developing flexible sensors are processed with different techniques, depending on the dimensions of the final prototype. For example, for sensors with tiny dimensions having a few microns to a few millimetres, processes like photolithography (Revzin et al. 2001), screen printing (Chang et al. 2009), 3D printing (Muth et al. 2014), ink-jet printing (Wang et al. 2010), laser cutting (Schuettler et al. 2005) are commonly used. Photolithography is one of the frequently used techniques for microfabrication. Spin-coating of a photo-sensitive material called the photoresist is done on the substrate, to generate a geometrical pattern followed by the exposure of the substrate to the ultra-violet (UV) light. Two types of photolithographic processes occur, masked and maskless. The former one consists of a patterned mask through which the light passes on the substrate to imprint the design on the substrate. The maskless one does not involve any photomask but rather forms the patterns directly on the substrate, depending on the design uploaded on the system. Exposure of the photoresist to light polymerizes it on the patterned locations. Followed by the exposure to UV light to form the patterns, the residual photoresists are removed by an etching process. Two types of etching namely ionic or dry and chemical or wet etching, are performed for removing the photoresists. Screen print-

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1 Introduction

ing is one of the earliest techniques that has been employed to develop devices with finer dimensions. This process is similar to photolithography apart from the fact that smaller dimensions can be obtained with a photolithographic technique. In screen printing, a pattern is generated on a stencil through which the ink or any liquid is transferred to a substrate to obtain the developed pattern. The design is formed on a mesh where the ink is poured to develop the mesh design on the substrate. The design is formed on polymers where the mesh is fixed to a frame for support. Ink is dropped or squeezed by a squeegee to develop the pattern on the substrate. 3D printing is a relatively newer technique that has been used to generate three-dimensional objects by printing them. This method was developed in the early 1980s. The shapes or geometries formed by this approach are designed using a range of computer-aided modeling techniques. The designs are saved with a file extension that is compatible with the 3D model printer. The design or model is then subsequently divided into a series of thin layers which produces the G-code. These codes are specific to each instrument and represent the specific actions that are used to develop the designed object. This process has many industrial applications like metal wire processing and lamination. Inkjet printing is another process which was developed in the late 20th century to develop different types of small-scale printed devices. This technique has certain advantages like fine resolution, no warm-up time, low per-dye costs and improved picture quality in comparison to other printing technologies. Two types of drop-on-demand (DOD) processes are employed for this process: thermal and piezoelectric. The former one consists of a series of chambers with each one having a heater. When a pulse of current is passed through the heating element of the chamber, the ink vaporizes and subsequently forms a bubble. This increases pressure of the chamber, thus ejecting one droplet on the substrate. The piezoelectric sensors change their dimensions with pressure, with an input voltage, thus creating a rise in pressure which forces the bubble out of the nozzle. Laser cutting is a process developed in the early 1960s when this technique was mainly used to develop holes in the diamond. Two different types of laser cutters namely CO2 and Nd/Nd-YAG lasers, where the former one is more used than the latter one for research purposes. The laser material is stimulated which results in total internal reflection to the partial mirror as a result of which, a stream of monochrome coherent light generates out of the laser nozzle. The techniques used for laser cutting primarily include vaporization, melt-and-blow and reacting cutting. This approach has certain edges over other techniques like small sample preparation time, reduced contamination of the developed sample, final dimensions up to a few microns of the sample, low thermal influence, clean-cut edges, and smooth final product. Laser cutting devices are commercially available having a range of input powers which are decided by the sample to be sliced or ablated. A wide range of materials has been employed for developing substrates and electrodes for developing different flexible sensing prototypes. The materials that are selected for different prototypes depends upon their inherent properties. For example, commercially available tapes are cheaper than many of the organic polymers, but they would not form a very efficient substrate regarding the flexibility, due to their high Young’s modulus (E). Similarly, the electrical conductivity of gold is than

1.1 Flexible Sensors

5

aluminum but so is its brittle nature. Their breaking point threshold would be lower than that of aluminum electrodes. Thus, a material chosen for fabrication has to offer a balance between its cost of availability and extraction, its electrical, mechanical and thermal properties, and ultimately, its use as a sensing material. Some of the substrates commonly used to develop flexible sensors are Polydimethylsiloxane (PDMS) (Jo et al. 2000), Polyethylene Terephthalate (PET) (Mannsfeld et al. 2010), Polyethylene Naphthalate (PEN) (Cai et al. 2009), Polyimide (PI) (Engel et al. 2003), Poly(3,4-ethylene dioxythiophene): Polystyrene sulfonic acid (PEDOT:PSS) (Mainwaring et al. 2002), etc. PDMS is an organic polymer which is developed from the repetition of the siloxane monomers. It is most commonly used to develop flexible substrates for applications having rheological requirements. Some of its advantages are its transparency, non-toxicity, non-flammable and hydrophobic nature. Apart from this, one of the biggest advantages of using PDMS is its ability to form strong interfacial bonding with any nanomaterials during the formation of nanocomposites. PET is a carboxylate-group polymer that is mainly prepared for synthetic fibers. It is developed from ethylene glycol and dimethyl terephthalate. It is a semi-crystalline resin and is a more viscous polymer in comparison to PDMS. A few of the most common things made from PET are plastic bottles, packaging of foods and beverages. PEN is a polymer similar to PET but has higher dimensional and temperature stability. It is developed form carboxylate polymer and ethylene glycol. Its main uses include piezo-resistive tapes, for packaging, and solar-cell protection. PI is one comparatively of the oldest polymers and has been mass produced for its applications. The reaction between a diamine and dianhydride is commonly done to form these polymers. It mainly exists in two forms, a heterocyclic structure, and a linear structure. It is mostly used to create commercial tapes due to its high chemical resistance and excellent mechanical properties. PEDOT: PSS is one of the popular conductive polymers available in the market. It is developed from the two isomers of sulfonate and polystyrene groups. It is a transparent, flexible polymer, which makes it suitable for certain applications like printing and electrolytic capacitors. A range of flexible conductive materials has been used to develop flexible sensors. Some of them are Carbon Nanotubes (Karimov et al. 2015), silver (Yao and Zhu 2014), copper (Kim et al. 2009), gold (Gong et al. 2014), iron (Alfadhel et al. 2016), etc. The conductive materials chosen to develop the electrodes depends on the fabrication technique and application of the sensors. For example, conductive inks are used to develop the electrodes with the ink-jet printing technique. Nanotubes are cylindrical structures in nanoscale level with extraordinary electrical, thermal and mechanical properties. The tubes are bonded with sigma bonds, which makes them much stronger than their macro-counterparts. Nanowires have a high aspect ratio, where the ratio between its length to width is greater than 1000. These nanowires are synthesized by a range of techniques like vapor-liquid-solid method, solution-phase synthesis, and non-catalytic growth. But, the conductivity of nanotubes is greater than the nanowires as a result of their lower mean free electron path than that of the later. Also, the electrical conductivity of the nanotubes is higher than the nanowires due to the ballistic transport of electrons.

6

1 Introduction

1.2 Printed Electronics The recent progress in printing technology has brought a leap of advancements in the sensing world of electrical and electronics field. Printed technology has been adapted to develop sensitive electronic systems (Secor et al. 2014; Yin et al. 2010) as a result of certain advantages like their simplified processing steps, limited material wastage and overall low cost of fabrication. These features make it a very popular choice to use them for the development of large-area multifunctional electronic circuitries. The deployment of printed electronics has taken a major part for the fabrication of microelectronics using standard printing methodologies. Two of the major categories based on their fabrication techniques, namely contact and non-contact processes, are involved in printing technology. The contact process involves the physical contact of certain patterned inked surfaces with the substrates on which the electrode design has to be imprinted. Some of the common technologies of contact printing include R2R processing, flexographic and gravure printing. The second type of printing approach is the non-contact printing, which allows the solution to be distributed on the substrate through openings in a mask in a defined pattern to form the electrodes. The substrate below the mask is adjusted in a preprogrammed manner to define the pattern of the designed electronic device. Some of the commonly used printing technologies adapting this process are screen and ink-jet printing. The non-contact printing approach is more advantageous than that of the contact one as a result of their higher resolution of the patterns, speed, and reduced material wastage. Figure 1.2 shows various types of sensing materials developed with the contact and non-contact processes (Khan et al. 2015). Even though the non-contact approach has certain advantages over the contact ones, there are certain challenges that need to be addressed in both techniques. Table 1.1 provides a summary of some of the advantages and disadvantages of the contact and non-contact processes obtained from work done in this area to develop a range of flexible electrical and electronic systems. Apart from the approaches of the printing methodology, other classifications such as the choice of conductive inks to develop the colloidal solution have also been analyzed to determine the rheological properties of the solutions. Different types of materials including conducting (Leenen et al. 2009; Subramanian et al. 2008; Tobjörk and Österbacka 2011), semiconducting (Dahiya and Gennaro 2013; Nomura et al. 2003; Zhang et al. 2009) and dielectric (Choi et al. 2010; Pease and Chou 2008; Subramanian et al. 2008) materials have been opted for, to form pure and hybridized printed electronic systems along with organic and inorganic polymers. The choice of a material to develop printed sensing prototypes depends on their capability to obtain optimality for some of the parameters like surface tension, viscosity, flexible and malleable compatibility, electrical and thermal conductivity and affinity of the resultant colloidal solutions. There are three main categories of conductive materials that are used for printing technologies till date. In the first category, metallic elements like silver (Li et al. 2005), gold (Luechinger et al. 2008), and copper (Lee et al. 2008) inks are used to design the electrode part of the circuit. Apart from the high cost of

1.2 Printed Electronics

7

Table 1.1 Features and challenges of some of the commonly used printing techniques to develop flexible electronic systems (Khan et al. 2015) Printing type

Features

Challenges

Screen

• Conventional printing technique • Fast and controlled deposition of solutions • The open area of the mesh is defined with pre-structured patterns

• Resolution greater than 30 microns cannot be obtained • The occurrence of spreading and bleeding of printed solutions • Deteriorated patterns due to the spreading of the inks

Inkjet

• Lower viscosity of solutions compared to screen printing • Specific deposition of droplets • Lower material wastage compared to other techniques

• Unequal distribution of dried solute is causing a coffee-ring effect • High chances of clogging because of misfringing • Pixilation-related issues due to drop-on-demand

Gravure

• The substrate should be smooth, have compressible porosity and wettability • The ink should be high viscosity, solvent evaporation, and drying rate

• Expensive process • Detect and pick up related challenges due to contact printing • Proper ratio of cell spacing to cell width

Offset

• A popular technique for printing on hard surfaces • The goal is for 100% transference • Speed and pressure and the main process parameters

• Degree of solvent absorption effects the width of the printed line • Spreading of the line during the set process • High rolling resistance due to the fast rolling speed

Flexographic • Patterns are raised on the low-cost flexible plate using photolithography • Better pattern quality concerning the contact printing methodologies

• Layer cracks and non-uniform films • Tensile stress is occurring due to solvent evaporation or high temperature • Divergence from nominal specified values with speeding

Microcontact

• The stamp is inked and pressed against the substrate surface to transfer the design • They are used for biological sciences to develop micro and nano-structured surfaces

• Hydrophobic problems of some polymers like PDMS with the polar inks • Change in the pattern sizes due to swelling to the stamp during the inking process

Nanoimprinting

• The master material is pressed into a polymeric material cast at a specific temperature and pressure • Creating a thickness in contrast to the polymeric material

• Damage to the fragile nanostructures while removing the cast material • Time-consuming technique compared to other contact printing process • Difficult to replicate structures with resolution below 50 nm

Transfer

• Transfer and printing occur through solution casting • Combined techniques of photolithography and micro-contact printing

• Misalignment of neighboring strips during undercutting • Maintaining the surface quality of the backside of the flipped transferred structures is difficult

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1 Introduction

Fig. 1.2 Fabrication of flexible sensors using various printing techniques. a Direct printing using an ink-jet printer to coat gallium indium alloy on the paper. The wires developed from printed alloys are attached to the LED. b Optical images of the printed sensor for various components like Inductance coil and RFID antenna. c Schematic diagram of nano-printing done using a silicon mould. After the flowing of the molten precursor, demoulding is done followed by heating the sample at 250 °C. Three different types of nano-printing: hierarchical patterning, transfer stacking and polymer transfer are shown to demonstrate the differences in their techniques to develop sensors. d Schematic diagram of micro-contact printing. The pre-polymer is poured and cured on the pre-structured master to develop the elastomer stamp. The stamp is peeled off the master, cut into small pieces and used as a printing ink to stamp the replicated shape on a surface. e Transfer printing is done using the conformal stamp of a polymeric stamp with silicon wires. The wires are peeled off and transferred to the final substrate. e Screen printing of pressure sensors done on large flexible polymer films f with bumped structures on the top of the film. g Roll-to-roll production of flexible sensors for wearable sensing (Khan et al. 2015)

these materials, the oxidation in room temperature for some of the metallic pastes like copper and aluminum limits their range of applications. To tackle this limitation, some of the organic conductive polymers have been used (Blanchet et al. 2003; Zhou et al. 2012) along with these metals along with a range of printing technologies. These resultant conducting polymers are in different forms, which depends on the level of doping. Along with the use of the intrinsic ones (Kamyshny and Magdassi 2014), the doping level is varied with the n-type and p-type dopants, to have the resultant work function close to that of the semiconductors. Some of the common conducting polymers that are used for printing technology are polyacetylene (Sawhney et al. 2006), polyaniline (Crowley et al. 2008), polythiophene (Li et al. 2007), PEDOT: PSS (Sriprachuabwong et al. 2012), etc. Although these conducting polymers are employed on a large scale to fabricate printed electronic devices, the electrical con-

1.2 Printed Electronics

9

ductivities of these polymers are much lower than for the conductive metals, which curbs their applications. Another favorable choice of conductive materials for the printing technology is the formation of nanocomposite using nano-fillers of a range of metallic nanoparticles and organic elastomers at the varied ratio. The amount of nanofillers in the polymer matrix depends on the percolation threshold of these mixed nanofillers, which eventually defines their dispersion in the matrix. Although the electrical, thermal and mechanical properties of the resultant nanocomposites is dependent on the number of nano-fillers mixed in the polymer matrix, the agglomeration of the nanoparticles with the matrix affects the rheological properties of the subsequent nanocomposite. This is somewhat optimized using a range of surfactants and volatile additives along the nanoparticles. Among the semiconducting materials, the ability of the transduction of the free carriers of some of the materials like crystalline silicon (Dahiya et al. 2012) and certain oxides of transition metals (Nomura et al. 2003) makes them a very favorable choice for printing technology. Similar to that of the organic conducting polymers, the organic semiconducting polymers (Fortunato et al. 2008) are also employed to a certain extent to form colloidal solutions as a result of their solubility and capability to optimal dispersal solutions. For the utilization of the dielectric materials to synthesize the colloidal solutions of the printed devices, the organic materials are favored over the inorganic ones as a result of their low-cost and capability to diffuse in different solvents and solutions. The classification in a range of materials utilized to form the substrates of the flexible electronics is depended on their physical, chemical and optical properties that include their dimensional and thermal stabilities, bendability to a certain extent, transparencies and radiation properties. Thin glass, metallic foils and various plastics with different bendability are the three major types of substrates that are chosen to develop flexible systems. In accordance to the disadvantages of the intrinsic brittle property of the thin glass and the surface roughness to a certain degree of the metal foils, the plastics possess advantages over these two limitations in comparison to the other two types. Among the different plastics that are used as substrates for printing technologies, amorphous and semi-crystalline polymers are the most popular ones. A few examples of these types of plastics include Polycarbonate (PC), Polyethersulfone (PES) and Polyethylene Terephthalate (PET), Polyethylene Naphthalate (PEN), and Polyether ether ketone (PEEK) respectively. In a comparison between two types, the semi-crystalline ones are more advantageous that the amorphous ones as a result of their higher glass-transition (Tg ) temperature.

1.3 Conclusion The chapter showcases a brief introduction to flexible sensors and the associated printing technologies used to develop them. Initially, the raw materials used to fabricate the flexible sensors was explained along with their background, followed by the elucidation of the individual electrode and the substrate parts. In continuation of this, some of the commonly used fabrication techniques used to develop the flexible

10

1 Introduction

sensors are explained in this chapter. The commonly used printing techniques are explained in the second section, which is utilized to develop flexible electronic and electrical systems. The importance of printed electronics has also been highlighted regarding the different printing technology available to develop flexible sensors along with the features and challenges while operating them. Among the range of fabrication techniques available in printing technologies, laser and 3D printing are some of the commonly used techniques as a result of to their low fabrication cost, easy sample preparation and the capability to form smooth and flexible cuts of the final prototypes. The sensor patches developed with the laser cutting and 3D printing techniques are eventually utilized for varied applications like monitoring of the environment, industrial and health parameters. The utilization of different sensor prototypes for their subsequent applications as a result of their physiochemical suitability for that particular use. The advancement in the field of microfabrication of sensors has been focused along with their potential to be deployed for practical work.

1.4 The Aim of the Book Flexible sensors have showcased enormous potential to be deployed for monitoring purposes in the field of healthcare, environment, and industrial applications. The fullblown use of this category of sensors is yet to be done to generate an influence on the quality of life of people. The presented work shows great potential in the utilization of sensors in the real world. Among the various types of fabrication techniques that are utilized to generate flexible sensors differing regarding dimension, cost and resolution, the use of printing technology have been done on an enormous scale. The research work on printed flexible sensors has been constantly expanding as a result of their advantages of low-cost, enhanced electrical, mechanical and thermal properties. In this book, the explanation of the novel flexible printed sensors was done which were formulated using laser cutting and 3D printing techniques. Four different types of printed flexible sensor prototypes were developed, characterized and utilized for different applications. The purpose behind the formation of each of these sensor prototypes can be attributed to highlight their low cost of fabrication, simple operating principle, and multifunctional capabilities. The electrical behavior of the electrodes was based on a parallel-plate capacitor as a result of their interdigitated shapes. Electrochemical Impedance Spectroscopy was used in association with the sensor prototypes to determine the output concerning the corresponding changes in the input signals. The distinctions among these prototypes were based on their individualistic characteristics as a result of the different raw materials that have been processed to fabricate them. Multi-Walled Carbon Nanotubes, graphene, aluminum, and graphite are some of the conductive materials that were processed to form the electrodes of the sensor prototypes because of their light weight, high electrical conductivity, durability and high aspect ratio. Polydimethylsiloxane, polyethylene terephthalate, and polyimide are some of the polymeric materials that were processed to form the substrates of the sensor prototypes because of their low-cost, biocompatibility,

1.4 The Aim of the Book

11

low Young’s modulus and capability to form flexible, bi-layer structured devices. The sensor prototypes were utilized for different applications like monitoring of movements of different body parts, respiration and taste sensing in the point of view for healthcare, salinity and nitrate sensing for the environment, and low-force and tactile sensing for industrial applications.

1.5 Research Contributions The important contributions of this work lie in the fabrication, characterization, and utilization of four types of flexible sensing prototypes. The novelty in this work can be defined to be the usage of the processed materials and the fabrication techniques. The working principle of the flexible sensor prototypes was studied and presented along with the electrical and mechanical changes taking place during their experimentation. The sensor prototypes were used in different sectors including healthcare, environmental and industrial applications. The fundamental purpose of these sensors is for the advancement of the micro and nano-electronics for monitoring multiple applications ubiquitously. The major contributions of this research can be summarised as follows: 1. A brief introduction is given regarding the fabrication and implementation of wearable, flexible systems. It showcases the use of some common materials along with different techniques used to process them to form prototypes that are significant for the electronic world. 2. A detailed background study of the work done on the development of sensing prototypes with conductive materials like CNTs and graphene is done. The sensors fabricated using CNTs and graphene are categorized into different applications, based on their electrochemical, strain and electrical nature. The challenges faced by the current sensors along with some of the possible remedial solutions are also explained in the paper. 3. The detailed explanation of the working principle of all the fabricated sensor prototypes is also done in the book. The interdigitated structure of all the sensors prototypes worked on planar capacitive principle. Usage of electrochemical impedance spectroscopy (EIS) in conjugation with the impedance analyzers at different experimental setups is also elucidated in the paper. The sensors were connected to the analyzers via probes to determine the response concerning different inputs at specified frequencies. 4. The design, fabrication, and implementation of the first sensor prototype were done in the succeeding chapter. Carboxylic acid functionalized Multi-Walled Carbon Nanotubes (MWCNTs) and Polydimethylsiloxane (PDMS) were used as the conductive and substrate materials respectively. Laser cutting technique was used to curve the electrodes from a nanocomposite (NC) layer formed with the mixing of MWCNTs and PDMS at definite proportions. The developed were used for monitoring physiological movements like limb movements and respiration.

12

5.

6.

7.

8.

1 Introduction

They were also employed for measuring low-pressure tactile sensing to determine their capability to be used for prosthetic applications. The succeeding chapter describes the formation and implementation of the second sensor prototypes formed with metalized polymer films. Polyethylene terephthalate (PET) films being metalized with aluminum on one side was used as the singular processing material. The electrodes were carved via laser inducting from the aluminum side of the metallized polymer films to form the sensor prototypes. The developed sensors were used for tactile sensing via applying different pressure ranges through the index finger, thumb and palm. Highlight on the fabrication and employment of laser-induced graphene sensors was given in the succeeding chapter of the book. Commercial polymer films were laser-induced for the photo-thermal formation of graphene. This conductive material was transferred to the Kapton tapes to form the electrodes of a sensor. These sensors were used for monitoring the amount of salinity and nitrate content in different water bodies. They were also used as taste sensors where five different chemicals about individual tastes were experimentally tested to determine the differences in the responses. The fourth and final sensor prototype, developed from Graphite and PDMS was underscored to highlight their fabrication and application. 3D printed molds were used as templates for casting and curing of Graphite and PDMS at defined heights to form the electrodes and substrates respectively. The formed sensors were used for strain-induced applications like monitoring of limb movements, by connecting them to different bendable locations of the body like knee, elbow, neck, and finger. They were also used for low-pressure sensing by applying low forces on the sensing area of the prototypes. The conclusion of the explained work is drawn in the final chapter of the book which summarizes the work explained in the preceding chapters. It also highlights some of the future applications that can be done with the fabricated sensors, along with the possibility of forming new sensing prototypes with a range of raw materials.

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Chapter 2

Literature Review

Abstract This chapter elucidates on some of the work done by different researchers on sensors developed from Carbon Nanotubes (CNTs) and graphene. Work done on the preparation and properties of CNTs and graphene are explained in addition to their employment as electrochemical, strain and electrical sensors. It also explains the work done on a range of wearable, flexible sensors, some of the network protocols used to operate them, the current challenges availing in the present scenario and some of the future opportunities in terms of market survey and betterment of the existing sensors.

2.1 Introduction Due to the majority of the work showcased in this book is based on CNTs, and graphene and other flexible sensing prototypes, a lot of literature review was done on these topics. The research work done on the fabrication and characteristics of some of the flexible sensors based on these conductive elements are explained in the succeeding sections. The elucidation on the different types of flexible sensors that are developed with CNTs and graphene as electrodes operated as electrochemical, strain and electrical sensors. The literature review also includes some of the limitations and challenges of the current CNTs and graphene-based sensors with some of the remedial ideas for addressing these problems. This is followed by the work done on different kinds of wearable, flexible sensing prototypes that includes some of the flexible sensors, their operating principle and the fabrication techniques used to operate them. It also includes the explanation on the network protocols used to operate them. The chapter finally concludes on wearable, flexible sensors.

© Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_2

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2.2 Carbon Nanotubes and Their Sensor-Based Applications Carbon Nanotubes can be defined as the allotropes of carbon that are cylindrically shaped and has a crystalline nature. These elements have an extremely high aspect ratio with exceptional electrical, mechanical, and thermal properties. The history of CNTs dates to early 1980s where a fullerene ball consisting of six carbon atoms (C60 ) was discovered. These fullerene balls were explained being made up of elements formed from pure carbon atoms. The structural form of CNTs can be explained as the allotrope of carbon shaped in an sp2 hybridization. The sp2 hybridized form of carbon atoms in CNTs has a resemblance with graphite (alkynes), which makes them stronger than the sp3 hybridized molecules like alkanes. The sp2 hybridized carbon atoms have a strength of around 33%, which is higher than that of sp3 hybridized carbon atoms (Sinnott and Andrews 2001). The formation of CNTs involves the rolling of graphene sheet (Odom et al. 2002). The CNTs walls being graphene sheets are stacked on top of each other at definite angles, called as ‘chiral angles’. Chirality can be defined as the asymmetric nature of an element to its mirror image. The stacking of the nanotubes is known as π-stacking, taking place due to the Van der Waal’s forces of attraction for individual nanotubes against each other. The surface area of the atoms of the CNTs determines the amount of these Van der Waal forces. The consequence of the exposure of the atoms to both the interior and exterior sides of the tubes where the resultant surface area of CNT is quite high results in higher Van der Waal’s forces (Carraher 2016). These CNTs are having a high strength to weight ratio compared to other contemporary materials makes them a preferable choice to coagulate with other polymers to form nanocomposites.

2.2.1 Synthesis of Carbon Nanotubes CNTs are fabricated via different techniques like arc-discharge evaporation, laser vaporization, and chemical-vapor deposition (CVD). Arc-discharge evaporation is the oldest method among these where the CNTs are fabricated from an arc-discharge operation with two carbon electrodes in the presence of an inert gas (Ando and Iijima 1993). During the discharge process, one electrode was fixed, and the other one was adjustable. With the absence of a catalyst, the arc current used to very high in the range of 200–225 A, where one of the CNTs would be deposited in one of the carbon electrodes. As a result, the diameter of the electrodes would be altered after the process, thus change the number of fabricated nanotubes. The catalytic synthesis of CNTs was performed out with an arc current of around 70–80 A in an atmosphere containing argon and helium mixed at a certain ratio (Hutchison et al. 2001). The catalysts used for in technique were placed by evaporating them along with the carbon feedstock. The highly magnifying (100 nm) instruments like Scanning Electron Microscopic (SEM) and Transmission Electron Microscopic (TEM) were used take

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19

Fig. 2.1 a SEM image of vertically aligned CNTs (Liu et al. 2014b). b Highly magnified TEM image of CNTs grown with a catalyst. Reprinted with permission from (Chai et al. 2007)

images of vertically aligned CNTs grown with the presence of a catalyst are depicted in Figs. 2.1a (Liu et al. 2014b) and 2.1b (Chai et al. 2007). A stepping motor that is controlled with a computer (Ando et al. 2002) was utilized to develop the CNTs in the arc discharge method. This assisted in maintaining a fixed distance between the two electrodes. Hydrogen can be considered as an alternative option to helium to be used as a gas in the vacuum chamber during the arc-discharge fabrication process. Another alternative technique that can be considered for the arc-discharge process is the containing the positive electrode with a mixture of graphite powder and a metallic catalyst (Journet et al. 1997). This will assist to deposit a homogenous mixture of carbon filaments. Laser vaporization is another popular process that has been used a lot to develop CNTs. In this process, a laser beam contained with a controlled laser pulse speed was focused on a material developed with a composite of a metal and graphite (Guo et al. 1995). Carbon spots are formed on the target surface when this computer-controlled beam scans through it. This was followed by the deposition of the carbon in the collector in the presence of an inert gas. Another popular technique is the quartz tube, which is used to fabricate CNTs by the laser vaporization method (Lebedkin et al. 2002). The positioning of the tube on the oven is done during the evaporation process at a defined temperature and pressure. The fabricated CNTs were obtained inside the tube that is deposited on a filter. Near-infrared photoluminescence is another favorable option that is connected the laser vaporization method to fabricate CNTs (Lebedkin et al. 2003). Miniscule structures having a range of 1 nm can be developed via this approach. The acid-treated CNTs instead of raw CNTs reacted towards photoluminescence. One of the disadvantages of this method includes the low yield of CNTs in comparison to the arc-discharge and CVD techniques. The development of CNTs with CVD in the presence of catalysts is the most favorable and recently developed technique. The elements considered as catalysts differ depending on the configuration of the fabricated CNTs. Research has been done on magnesium-cobalt-molybdenumoxygen to consider it as a catalyst for double and triple-walled CNTs (Flahaut et al. 2005). The individual elements in this compound were mixed at defined proportions

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and dissolved in deionized water considering either urea or critic acid as the fuel. The iron-molybdenum mixture prepared from thermal decomposition process can also be used to fabricate SWCNTs (Li et al. 2001). Pyrolytic decomposition of certain hydrocarbon gasses include methane; benzene is another technique that is imposed in CVD (Qin 1997). The lower range of the operating temperature can be compared to other techniques like arc-discharge and laser-vaporization methods.

2.2.2 Characterization and Properties CNTs are classified based on structural different into three categories, namely singlewalled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). There is a high resemblance in the properties of DWCNTs with SWCNTs. The difference in the structure dictates the properties of these types of CNTs. SWCNTs can be observed as one strip of graphene sheet obtained from an infinite rolled-up sheet (Odom et al. 2002). The initial structures were first suggested after the proposal of fullerene balls in the early 1990s. The fabrication of CNTs was carried out by the arc-discharge process, having iron or cobalt as the catalyst in the reacting chamber. Subsequently, mixtures contained with nickel and cobalt or complexes developed from iron-carbon monoxide-acetylene were considered as catalysts to fabricate SWCNTs in certain processes like laser vaporization and CVD respectively (Chen et al. 2001). Hydroxyl groups are also considered for catalysts to fabricate high-purity SWCNTs (Maruyama et al. 2002). The suppression of the developed products other than SWCNTs was done using hydroxyl ion for its etching effect. The structure of MWCNTs is defined as containing multiple rolled sheets of carbon atoms. Figure 2.2a, b showcases the schematic diagram (Li and Chou 2003) along with its HR-TEM image (Andrews et al. 1999), for a single layer of an MWCNT. The behavior of SWCNTs comprises of having a higher probability of semiconductors due to their higher band gap between the conducting and insulation layers in SWCNTs in comparison MWCNTs. Due to the weaker assembly or accumulation of SWCNTs, their poor solubility in liquids is addressed by fabricating them in an aligned way (Zhang et al. 2001) via the application of an electric field with a range between 5 and 10 V applied DC bias voltage. The MWCNTs are defined to be consisting of multiple sheets of graphene that rerolled to form concentric cylindrical tubes. MWCNTs are preferred over SWCNTs for certain applications as a result of their formation of better interfacial bonds with the polymer matrix. MWCNTs form stronger covalent bonds with the matrix due to their kinetic stability in polar solvents. When insoluble, certain surfactants like sodium dodecyl sulfate (SDS) and chloroform (Wang 2009) are used to dissolve the CNTs to dissolve in solvents. Another advantage of MWCNTs over SWCNTs lies in the variation of their dispersity in the polar solvents with respect to variation in the length of the tubes (Li et al. 2005; Saito et al. 2002). Functionalisation of CNTs is another significant criterion with decides the final attributes of CNTs. Based on the functionalization group

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Fig. 2.2 a Schematic diagram of an MWCNT (Li and Chou 2003). b The HRTEM image of the multi-layered structure of a single MWCNT (Andrews et al. 1999)

like –OH, –COOH, the conductive and dispersive properties of the CNTs varies. The functionalization is done by conducting several steps, for example, treat in concentrated acid, ozone treatment, etc. (Kuzmany et al. 2004). Some of the significant works done on CNTs in terms of different sensing applications are showcased in the succeeding sections.

2.2.3 Electrochemical Sensors One of the primary applications for CNT-based sensors is for electrochemical sensing due to certain advantages of chemical, electronic and thermal properties in comparison to other conductive elements (Ahammad et al. 2009; Barsan et al. 2015). Due to their high hydrophobic nature, high electrical conductivity, high aspect ratio and resistance of change in its response towards oxygen and light, they have been largely used to developed ion-selective electrodes (ISE) in potentiometric sensors. One of the electrochemical sensors (Jin et al. 2016) developed using CNTs includes the coating of SWCNTs using a conductive poly(3,4-ethylene dioxythiophene) (PEDOT) layer, leading to the increase in conductivity and electrochemical properties of the composite layer. The thin conductive layer also protects the SWCNTs junctions from getting separated.

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Figure 2.3 depicts the change of the transmittance spectra, the resistance and LEDs of the composite films with respect to bending radius, strain and stretching cycles under 0 and 50% strains. It is seen that the sheet resistance and optical transmittance reduced in the 350–700 nm wavelength regions when there was an increase in the thickness of the composite film. The sheet resistance and transmittance of 82 /square and 81.5% respectively were observed for a thickness of 250 nm of the composite film. The films also showed excellent flexibility when they were wrapped around cylinders having curvature diameters between 1 and 26 mm. A strain of 50% could not change the intensity of light in LED. The electrochemical properties of CNTs have also been exploited for developing glucose (Hwa and Subramani 2014; Kaempgen and Roth 2006; Lee and Cui 2010; Lim et al. 2005; Periasamy et al. 2011; Pham et al. 2010; Qiu et al. 2009) and pH (Chien et al. 2012; Jung et al. 2014; Münzer et al. 2013) sensors. The hydrophobic nature and curved sidewall of the CNTs with its π-conjugative structure also creates a strong interaction with aromatic compounds via π-bonding and hydrophobic interactions, making them a popular choice to be mixed with polymers to develop nanocomposites. The mixture of SWCNTs with poly (diallyl dimethyl ammonium chloride) (PDDA) by layer-by-layer structured manner between two PET coated electrodes in one of the research works created sensors with p-type semiconductor materials. These sensors were functionalized with carboxylic acid (–COOH) groups, followed by employing them for monitoring glucose having a sensitivity and linear range of 18–45 μA/mN and 2–10 nM respectively and a pH with a range of 5–9 (Lee and Cui 2010). Figure 2.4 depicts the fabricated wafer and comparison of sensors between a coin and a single dye.

2.2.4 Strain Sensors The use of SWCNT papers dispersed in polydimethylsiloxane (PDMS) matrix had been done in one of the significant research work (Zhou et al. 2017b) to fabricate strain sensors. The sensors exhibited gauge factors of 2 × 106 and 107 at 10 and 50% strain respectively. The sensors had low electrical resistance from 5 to 28 , which varied up to 106  as a result of the cracks in the percolated SWCNT papers. The change in the SWCNT paper-based sensor having a thickness of 90-μm displayed no change in the resistance values for tests over 10,000 cycles at 20% strain. The employment of CNTs for pressure sensing has also been done include the fabrication of piezoresistive sensors (Hwang et al. 2011) via the homogenous mixture of MWCNTs within PDMS wrapped in poly (3-hexylthiophene) (P3HT). The change in conductivity of the nanocomposite was dependent on the concentration of P3HT. The ratio between the polymer and MWCNTs decided the distance between the MWCNTs electrodes. The responses of the sensors were optimal for the pressure ranging between 0 and 0.12 MPa. The use of SWCNTs to develop piezoresistive sensors has also been done (Chang et al. 2008a) where the CNTs grown on a silicon substrate was transferred into flexible substrates to employ them as pressure sensors. The sensors obtained a strain resolution and a gauge factor of 0.004% and

2.2 Carbon Nanotubes and Their Sensor-Based Applications

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Fig. 2.3 a Transmittance spectra of SWCNT/PEDOT film in the 350–700 nm region. b The resistance changes of SWCNTs@PEDOT/PDMS film as a function of the bending radius. c LED integrated circuits under 0% and 50% tensile strains. d A comparison of the resistance changes of SWCNTs (red line), SWCNTs@PEDOT/PDMS (black line), and SWCNTs@PEDOT/PDMS films with 30% pre-strain (blue line) as a function of tensile strains. e, f The variation of resistance for the SWCNTs@PEDOT/PDMS stretchable films in the first and second stretching cycles (e), and fourth and fiftieth (f) stretching cycles (Jin et al. 2016)

Fig. 2.4 Glucose/pH sensors developed from SWCNTs and PDDA. a The bending of standard 4-inch wafer-level devices. b The comparison of the sensor with a coin (Lee and Cui 2010)

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269 respectively. The pressure sensors used for tactile sensing has been developed from a nanocomposite formed with MWCNTs and photocurable monomer (Vatani et al. 2013). Followed by the uniform dispersion of MWCNTs and polymer via sonication and magnetic stirring, they were dispensed onto a polyurethane substrate. The conductive stripes had a spatial resolution of 2 mm for the cross-section of the developed device. Fast Fourier transform was used to determine the responses of the sensor with respect to different forces. A piezoelectric thin-film sensor had been developed with poly (vinylidene fluoride) (PVDF) being coated with acid-treated SWCNTs (Yu et al. 2006) to use them for pressure and strain sensing. –COOH functionalized SWCNTs were considered to develop sensors to assist their dispersion in water. Other thin-film pressure sensors were developed with iso-tropically oriented SWCNTs being deposited on a UV-activated PDMS layer (Lipomi et al. 2011). These pressure sensors displayed a change in conductivity up to 2200 cm−1 for maximum pressure and strain of 50 kPa and 150% from its reference position. The pressure sensors developed from flexible-CNT-array double helices (CNTADH) were also used for motion capturing application (Li et al. 2015a). Two CNTs strands were grown with Lamellar-layered double hydroxides (LDHs) were used as catalysts to grow two CNTs strands via sonication with a surfactant in water. The solution was poured and coated with a thermoplastic elastomer prior to its experimentation as a strain sensor. The sensors depicted small hysteresis with a maximum measured pressure of 410% from its reference position. (Amjadi and Park 2015; Amjadi et al. 2015; Choi et al. 2016; Dai et al. 2015; Ding et al. 2016; Kanoun et al. 2014; Li et al. 2015a; Lim et al. 2016; Loh et al. 2007; Michelis et al. 2014; Nakamoto et al. 2015; Park et al. 2008, 2016b; Roh et al. 2015; Sanli et al. 2017; Souri et al. 2015; Tadakaluru et al. 2014; Wang et al. 2016; Yan et al. 2014; Zhang et al. 2015a; Zhao et al. 2014, 2016a; Zhou et al. 2017a) are some of significant research work done in the past few years on pressure and strain sensors.

2.2.5 Electrical Sensors For the use of CNTs in electrical sensors, there has been prominent work done on certain plastic substrates like PI, PAA to develop devices like PMOS inverter. A sub-monolayer of SWCNTs that was developed by CVD on silicon substrates was etched into strips via soft lithography and used to develop the source, gate, and drain (Cao et al. 2008). The encapsulation of the source and drain electrodes were done using a layer of polyamic acid for transferring the liquid-polyurethane-coated PI. Figure 2.5a–e depict steps of fabrication and working principle of the PMOS inverter. The architecture of the circuit and its SEM image are shown in Fig. 2.5a, b. The zoomed view of SWCNTs bundles and its current distribution are shown in Fig. 2.5c, d. The final transistor and with its associated circuit is shown in Fig. 2.5e. The normalized on-state current and threshold voltage had standard deviations of around 20% and 0.05 V.

2.2 Carbon Nanotubes and Their Sensor-Based Applications

Fig. 2.5 PMOS inverter developed from SWCNT/PI (Cao et al. 2008)

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Research related to the fabrication of RF analog devices, optoelectronics and photovoltaic devices based on CNTs has been done in the last few years (Arnold and Hersam 2013; Jariwala et al. 2013; Yin and Talapin 2013). One of them involves the printing of the transistors containing SWCNTs, had been done with inversegravure technique on flexible substrates (Lau et al. 2013). The sensor displayed excellent performance with high mobility and on/off current ratio of ~9 cm2 /(V s) and 105 respectively. The sensors also exhibited high bendability with the change in resistance up to 1 mm of the radius of curvature.

2.2.6 Conclusion and Future Work The work done on some of the CNT-based flexible sensors infers the success of the use of CNTs as electrodes. Functionalisation has been done on them with different groups enhance their properties as per the application. The applications of CNTs, since its discovery, has been increasing exponentially due to their excellent electrical, mechanical and thermal properties. Some of the primary areas they have been utilized are for gas sensing, electronic sensing, physiological parameter sensing. Continuous research work done on CNTs increases their chances to be used for applications with enhanced characteristics. The optimization of the properties of CNTs can affect the quality of human life by developing more biomedical devices by increasing the functional groups associated with them.

2.3 Graphene and Its Sensor-Based Applications Graphene has been a preferable choice since its commercialization because of its excellent electrical, mechanical and thermal properties. Graphene can be described as a single layer of carbon atoms that are compactly packed to form a 2D honeycomb crystal-lattice structure (Geim and Novoselov 2007). It is ascribed to be the basic component of all carbon allotropes, which can be simultaneously modified into other forms like 0D fullerenes, 1D CNTs and 3D graphite as depicted in Fig. 2.6. Even though the research done on graphene has been an ongoing process for the last sixty years, the free-standing 2D model of graphene has been experimentally proved recently (Novoselov et al. 2005; Wallace 1947). One of the widespread uses of graphene has been its implementation in batteries and cells as anodes, and also in supercapacitors as a result of its high strength-to-weight ratio, low charging time and large surface area. Its uses have also been explored in certain areas like sensors, biomedical engineering, nanotechnology, flexible electronics and catalysis due to certain attributes like distinctive nanopore structure, enhanced electrical, mechanical and thermal properties. Functionalisation has been done on graphene in order to reduce the cohesive forces between the graphene molecules which increases its potential applications due to precise changes in its physicochemical properties (Capasso

2.3 Graphene and Its Sensor-Based Applications

27

Fig. 2.6 Different forms (0D, 1D, and 3D) of modified graphene (Geim and Novoselov 2007)

et al. 2015; Ferreira et al. 2016; Liu et al. 2014a; Machado and Serp 2012; Novoselov et al. 2012; Wang et al. 2012c; Zhao et al. 2016b). Covalent and non-covalent forms of forms of functionalization are obtained for graphene molecules when the elements are chemically treated via different techniques like spin-coating, filtration, layer-bylayer (LBL) assembly to exert surface modification while maintaining its intrinsic properties (Kuila et al. 2012). Although a significant amount of research has been done on the preparation of graphene and its utilization in the form of sensors, a thorough background study combining all these aspects is yet to be done. Some of the advantages of graphene-like the very high surface-to-volume ratio, unique optical properties, high charge carrier mobility and exceptional electrical and thermal properties compared to the other allotropes of carbon has led to its inclusion as electrodes in sensors—these properties as seen to be constant for double and multi-layered graphene physical structures. With respect to the differences in the structure and working conditions, the employment of graphene sensing technology is decided by its application. For example, certain properties strain sensors like the detection limit, maximum sensing range, sensitivity, signal response and reproducibility of their outputs holds a key hold to ascertain the quality of that sensor. These attributes of the sensor are decided by the electrical

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and mechanical characteristics of graphene. For electrochemical sensing, the large surface of graphene assists in the loading of the desired biomolecules, causing a reaction between the analyte molecule and electrode surface because of the high ballistic transport capability and the very small band gap. Another major advantage of graphene can be ascribed to its low effects on the environment, pertaining it to be more popular for sensing applications (Graphene sensors: introduction and market status; Pumera 2011). Tables 2.1 and 2.2 showcases a comparative study between the performances of some of the electrochemical and strain sensors fabricated with graphene along with that of Carbon Nanotubes (CNTs) and silver. It is seen from Table 2.1 that the certain parameters significant for strain sensing like Gauge Factors (G.F.) and maximum attainable strain are mostly in the higher range for the graphenebased sensors. One demerit of these sensors lies in its the variation in the linearity in their response. For sensors developed with CNTs and silver, the GFs are much less than the graphene ones, even though most of them can withstand a high amount of detectable strain. It is seen from Table 2.2 that the graphene sensors can achieve high sensitivity with a wider linear range, having similar detection limits with the other types of sensors.

2.3.1 Synthesis of Graphene Optimization on the synthesis of graphene has been carried out since its invention and commercialization. Some of the common methods that are wide to fabricate graphene on a large scale are explained below.

2.3.1.1

Chemical Vapour Deposition (CVD)

An experimental chamber consisting of a heated up quartz-tube furnace (Zhu et al. 2016) with an inert gas like N2 , the deposition temperature carbonaceous gases of a mixture of Ar-H2 -CH4 (Kireev et al. 2017; Lavin-Lopez et al. 2017; Marchena et al. 2017) are flown through the screens of varied dimensions like one sq. an inch of Ni–Cu–Co–Pt–Ir–Nu metal that has been positioned inside the furnace. The gaseous deposition of carbon on the metal at higher temperature forms a single long atomthick monolayer/multilayer of graphene for a long period. The manufacturing process temperature gets reduced via the used of plasma in the absence of a catalyst in the CVD process (Li et al. 2016; Woehrl et al. 2014). Synthesis of Graphene via CVD via using the waste products is proceeded by the placement of certain sustainable materials like butter, tea tree (Melaleuca alternifoliate) extraction, waste plastic (solid form), camphor (C10 H16 O) (with/without iodine), Ni–Cu metal foils, polycrystalline Ni in an atmosphere containing Ar + H2 , H2 and Ar at normal or ambient pressure, where the temperature is increased with the presence of RF power for a certain period. Table 2.3 showcases a summary of some of the specific conditions for the preparation of graphene using CVD, mechanical exfoliation and Hummer’s method.

2.3 Graphene and Its Sensor-Based Applications

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Table 2.1 Comparison of the different characteristics of strain sensors where the electrodes are developed with graphene, CNTs, and silver Electrode material

Gauge factor

Max. attainable strain (%)

Linearity in the response

References

Graphene

~106

Carbon nanotubes (CNTs)

Silver

• • • • • •

120

Linear above 1%

Li et al. (2012b)

103

106.2

Linear up to 6%

Liu et al. (2015)

29

70

Linear up to 77%

Jeong et al. (2015)

300

500 GHz has also been achieved. It can be used to fabricate photodetectors that are CMOS compatible and can perform their operation over all the fiber-optic communication bands (Graphene photodetector enhanced by fractal golden ‘snowflake’; Pospischil et al. 2013; Xia et al. 2009). Table 2.7 showcases a summary of some of the graphene-based transistors with their corresponding strain and electron mobility (Jang et al. 2016). Work has also been done on the use of graphene-based electrical sensors for biomolecular, physical and chemical sensing (Zhan et al. 2014) due to the simplicity in the design, ease of mass production and the capability to capture and amplify the output signals. The physical sensors were

2.3 Graphene and Its Sensor-Based Applications

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Fig. 2.14 Graphene transistors on flexible substrate shown in 3-D (a), Optical (b), cross-sectional (c) and AFM (d) images (Hu et al. 2013b)

also developed with graphene electrodes in phototransistors and thermal transistors. The unimpeded transmission of the carriers in GFETs due to the very small band gap makes them a favorable choice for phototransistors (Nair et al. 2008; Wang et al. 2008). Some of the advantages like the high mobility of the charge carriers, ultrafast photodetectors have also been fabricated with graphene with a different number of layers. The highest photo-responsivity has not been very high for the graphene photodetectors has been of ~1 × 107 AW−1 with the response rate values and optical data links being higher than 20 GHz and 12 GBits/s respectively. The thermal transistors fabricated with graphene sometimes has an additive layer to enhance its stability during its protection from oxidation and water. Some researchers worked on the amalgamation of the certain attributes of graphene to develop Field-Effect Transistors (FETs) having some of the mechanical features like high strain-sensing capabilities (Konstantatos et al. 2012; Trung et al. 2012, 2014). The electrical sensing fields using GFETs has been employed for studying different metallic ions in different solutions. The detection of some of the heavy metal ions like cadmium, lead, mercury has been done at low concentrations of a few nanomolar ranges using GFETs. The detection of some of the commonly used ions like calcium, potassium, hydrogen has also been done via non-covalent functionalization of the electrodes with the analyte ions. The other applications of GFETs in chemical sensing involve the sensing of pH and a range of gasses like ammonia, nitrogen dioxide, and other inorganic gases. Graphene has been synthesized on different substrates like Silicon Carbide (SiC), poly (ethylene 2, 6-naphthalene dicarboxylate),

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Table 2.7 Summary of graphene-based electrical sensors based on the materials and technique used to compromise between the maximum strain and electron mobility (Jang et al. 2016) Material (with graphene)

Technique

Electron mobility (cm2 V−1 s−1 )

Maximum strain (%)

References

Al2 O3

CVD

190

0.62

Lu et al. (2012)

Ion gel

Aerosol jet printing

422

5

Lee et al. (2011)

Graphene oxide

Langmuir–Blodgett

4.1

3.5

Lee et al. (2012)

PMMA

Electron beam lithography

8

1.2

Meng et al. (2015)

Boron nitride

Mechanical exfoliation

60,000

–

Dean et al. (2010)

and shifted using the mechanical transfer to flexible substrates. Some of the research for gas sensing as depicted below shows very low detectable concentrations as ppb at different gate voltages. Table 2.8 showcases an overview for some of the selected work done on chemical sensing using GFETs. It displays the type of substrate that was used to grow graphene, the measurand, the range of gate voltages and the lowest detectable concentration.

2.3.6 Challenges with the Current Sensors Even though there has a significant amount of research work done on graphenebased sensors, there are still some existing challenges that need to be rectified at the basic level. The synthesis of graphene is a complicated and expensive process which requires a significant amount of time to generate a high-quality product. Some of the laboratory-based techniques to synthesize low-cost graphene are yet to be commercialized. The use of some of the catalysts during the synthesis of graphene increases its toxicity after its production (Akhavan and Ghaderi 2010), which eventually reduces its potential to be used for biomedical applications. This can also be rectified during its synthesis process. The nanocomposites formed with graphene to form the conductive part of the sensors degrade the intrinsic properties of the material. For example, there is a drastic reduction in the electrical conductivity of the composite (in terms of 103 ) in comparison to pure graphene. This is pivotal for applications demanding high conductivity of the electrodes. Secondly, the thermal stability also reduced for nanocomposites as a result of the weaker interaction between the graphene and matrix in comparison to the pure form of graphene. This problem can be handled by treating the mixed nanocomposites with an extra step involving chemical or thermal reduction (Papageorgiou et al. 2015; Prolongo et al. 2014), which would demand an

2.3 Graphene and Its Sensor-Based Applications

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Table 2.8 Comparison between the different parameters for the chemical sensing done by GFETs Type of substrate

Detected material

Gate voltage (V)

Detectable concentration/Range of detection

References

Cu

NH3 , CO2

>1

30, 4000 ppm

Inaba et al. (2013)

N-type silicon wafer

NO2 , NH3

−5 to 5

10, 50 ppm

Kim et al. (2012b)

Cu

NH3

15

50 ppm

Gautam and Jayatissa (2012)

SiO2 /Si

H2

0

1 ppm

Zhang et al. (2015b)

SiO2 layer

NO2 , Cl2

0

550 ppb

Rumyantsev et al. (2012)

SiO2, poly (ethylene 2,6-naphthalenedicarboxylate)

pH

>0.5

Neutral to acidic

MaillyGiacchetti et al. (2013)

SiC/silicon

pH

−1 to 1

4.3–7

Ang et al. (2008)

SiO2 layer (Degenerately doped silicon wafer)

pH

−0.1 to 0.1

4–9.3

Ohno et al. (2009)

SiO2 /Si

pH

−0.8 to 0.8

6–9

Sohn et al. (2013)

Scotch tape, SiO2 layer

pH

Gate free chemiresistor

4–10

Lei et al. (2011)

Silicone rubber/Glass

Pb2+

0–1

0.02 g/L

Wen et al. (2013)

Glass

pH

−0.2 to 0.4

3–10

Fu et al. (2013)

SiO2 /Adhesive tape

K+

−0.3 to 0.3

10 nM–1.0 mM

Maehashi et al. (2013)

SiO2

Hg2+

−20 to 20

1 nM

Zhang et al. (2010)

PDMS

Ca2+

−0.6 to 0.6

1 μM

Sudibya et al. (2011)

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extra step, and as a result, would generate a convoluted process. It is very difficult to achieve homogeneity of graphene in oxide forms in the composites because of its poor dispersion, which results in an additional step like the ball-mill mixing process to enhance the rate of dispersion (Tang et al. 2013). It is also very difficult to use pure graphene in oxidative environments due to its vulnerability of the formation of oxides. Another disadvantage is the loss of material while obtained rGO from graphite oxide (GO) via chemical reduction (Haag and Kung 2014). The conversion of graphene into different forms enhances the dynamicity of its characteristics, leading to an increase in its applications. Some of the research works reported on complete graphene-based sensing systems (HyungáCheong et al. 2016; Koester 2011; Mannoor et al. 2012) include mostly electrochemical sensing techniques. The data collected and transmitted by the sensors to the monitoring unit is done using different wireless protocols. Some of the electrochemical sensings include bacteria detection on tooth enamel where graphene was printed on water-soluble silk to form a sensing prototype. This system was utilized for the bio-detection of selective bacterial cells. The wireless conditioning circuit embedded a single-layer LC resonant circuit having a parallel resistive graphene monolayer. Another electrochemical application done with graphene-based wireless sensors is for developing transparent gas sensors using amalgamated graphene and silver nanowires. The wireless system consisted of an antenna-embedded Bluetooth system. Electrical sensors fabricated with graphene include graphene varactors that were developed with a combination of a metal-insulator-graphene structure with the capacitance altering with the charge concentration because of the quantum capacitance effect. These devices embedded with an LC oscillator circuit are very handy for wireless readout purposes. The quantum capacitance of graphene has also been exploited for wireless sensing systems (Deen et al. 2014), for applications like detection of ambient humidity. The results obtained with quantum capacitance have been verified with capacitance-voltage measurements done to validate their significance as sensing systems. Wireless Integrated Sensing Platform (Le et al. 2012) has also been used with graphene-based sensors where both the analog and digital wireless remote transmission principles were used to eliminate the use of any wire or battery-based operations. This system along with graphene-based gas sensors was used to detect NH3 and CO gases. Radio-Frequency Identification Tag (RFID) has also been used with graphene-based sensors consisting of a platinum-decorated reduced graphene oxide attached to an RFID sensor tag and an RFID reader antenna attached to a network analyzer. The sensing system was employed for detecting hydrogen gas at low concentrations (Lee et al. 2015a). The ZigBee standard wireless protocol was also used with graphene-based sensing systems which were used to measure different pH and glucose concentrations. The transmission of the sensed data was done using the XBee router to the XBee coordinator, would be subsequently interpreted using LABVIEW to extract the significant information. With a perspective to these sensing systems, the usability of the graphene-based sensors can be enhanced based on their strengths and limitations. Some of the advantages of graphene are based on their physicochemical structure. The sp2 hybridized carbon atoms of graphene make it highly electrically conductive due to the absence

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of an electron in the outermost shell. The conductivity of graphene is around 60 times more than that of CNTs, which makes it a very popular choice to develop the nanocomposite-based electrodes of the sensors. Graphene can also be used for developing supercapacitors due to its very low band gap between the valence and the conduction bands (Brownson and Banks 2012; Polat and Kocabas 2013; Wu et al. 2015; Yoo et al. 2011). Some of the other advantages of graphene includes high surface area, the availability of oxygen-containing groups in its structure, presence of the oxygen groups, opportunity of electrochemical modification in covalent and non-covalent ways to functionalised the electrodes of the sensors with additional operations, absence of heterogeneous materials during its production unlike CNTs (Pumera 2009). The advantages of graphene over CNTs and other allotropes of carbon lies their enhance characteristics during uses in the purest form to develop the electrodes, cost of production of pure graphene is lesser (Pumera et al. 2010), energystorage ability to produce fuel cells, lithium-ion batteries, and ultra-capacitors. Different lithium-ion batteries and fuel cells have been developed with graphene in the pure or composite to increase the storage capacity. Another advantage of graphene is its capability to exhibit the half-integer quantum Hall effect at the speed of light at room temperature. An ambipolar electric field effect with excellent electrode kinetics because of the high mobility of the carriers can be obtained because of the control of the charge density of graphene by the gate electrode. It has the ability to carry super current due to the continuous charge carrier exerting high crystal quality, causing light to travel thousands of inter-atomic distances without any scattering (Brownson and Banks 2010). Other advantage of graphene-based sensors for biosensing applications include having a high-density edge plane and can act as a nano-connector between the analyte and the electrodes and availability of the 2D electronic states on the surface which is done by tunnelling technique, which results in the mediator-less direct transfer of electrons between the enzymes and the surface of the electrodes. The flexibility of graphene sheets is higher than other allotropes of carbon, which increases their potential to be used for developing flexible electronic devices. Strain sensors with very high GF can be fabricated using graphene, whose performances remains constant even after many bending cycles. The limitations associated with graphene-based sensors if addressed and rectified effectively can increase the utilization of these sensors on a commercial basis. Some of the limitations related to current graphene-based sensors are described in Table 2.9. It is seen that a lot of work is yet to be done to enhance the characteristics of this material to fabricate and implement more efficient graphene-based sensors.

2.3.7 Conclusion and Future Work Even though there are some drawbacks of graphene-based sensors as mentioned in the previous section, it can still be considered one of the most promising materials that can be synthesized in the laboratory and employed for different applications. The potential applications of graphene-based sensors can be enhanced by using

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Table 2.9 Limitations of graphene as showcased in various research Sl. No.

Limitations

References

1.

The maximum photo-responsivity of graphene photodetectors is low. This is due to the small detection area of the graphene sheets and the very short photo-generated carrier lifetime

Liu et al. (2017)

2.

It is susceptible to oxidative environments. Thus, it cannot be used as a catalyst in redox reactions

(Advantages and disadvantages of graphene)

3.

Point defects are present in graphene due to its sp2 hybridizing property which results in the formation of various non-hexagonal structures. This alters the electro-mechanical properties of the resultant electrodes of the sensors

Lee et al. (2015b)

4.

Even though a lot of graphene-based strain sensors have been developed in the laboratory environment, the stretchability is still insufficient due to the defect density in its structure

5.

The presence of multilayers in graphene sheets results in interlayer sliding, leading to a difference in crack densities

6.

Defects occur in graphene’s structure during its interaction with metallic substrates

Karoui et al. (2010)

7.

There are oxides in the surface of graphene which affect the electronic and chemical properties

Dreyer et al. (2010)

8.

There are unknown cytotoxic limitations in graphene sensors which limit their usage in bio-sensing applications

9.

A lot of parasitic effects are present in graphene which influences the response of the graphene-based sensors

Wu et al. (2011a)

10.

The formation of different compounds with graphene changes its structural composition when an external load is applied, producing fracture lines, and increasing the number of dangling bonds in its structure

Schniepp et al. (2006)

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the graphene-based sensors to form real-time sensing systems. The sensors can be used to develop wearable sensing systems for ubiquitous monitoring of physiological parameters and chronic diseases. The robustness of the sensors attached to the body should be increased to sustain wear and tear for a long time. One of the ways to do this is by modifying the physical and chemical characteristics of the electrodes and substrates that are used to formulate the sensors. Many daily applications can be addressed by enhancing the selectivity and specificity of graphene sensors. The responses of the graphene sensors to target analytes and molecules should be enhanced while minimizing their responses to other interfering molecules. Functionalisation of graphene should be done more with captive agents that are used for specified target molecules. More focus should be pertained to obtain non-invasive, label-free detection of different biomolecules. The fabrication of graphene sensing systems should be improved to develop low-cost, reusable sensors. A few ways to ameliorate the quality of existing fabrication techniques is to employ devices with the simple operating principle, low input power, develop sensors with constant performance in terms of efficiency. The consistency can be obtained by addressing the performance and the signal conditioning circuit embedded in the sensor. An alternative approach to minimize the production cost is to develop multifunctional sensors which can be utilized with a single sensing system. The signal-to-noise ratio (SNR) of graphene-based systems should also be work upon to minimize the power loss during the transfer of the sensed data. The structure of graphene can also be modified during its synthesis to alter changes in its electrical, mechanical and thermal properties. Research can also be done on the diffusion process and surfactants that are associated with graphene to achieve uniform dispersion in the different polymer matrixes. The biocompatibility of graphene sensors should be ameliorated by working on the existing non-biocompatible commercial sensors that are currently available in the market. The global market for graphene is estimated to be above 250 million USD by 2020, which would consequently increase its uses for developing sensors for different applications. Figure 2.15 gives an overview of a few possible applications including energy conservation, electronics industry, and wearable devices, where graphene can be devised as a candidate (Graphene Market Trends). The utilization of graphene to develop sensors, supercapacitors and composites are forecasted to be the dominant trend for different sectors like aerospace, automobiles, defense, and biomedical science for the upcoming years (Graphene Market Overview; Graphene Market Reports). Topologically, the Asia-Pacific market region is shown to have the largest marketing industry with the use of most of the graphene in the academic and corporate sectors. The usage of graphene for potential applications is increasing with time and is expected to have an enormous impact on the quality of human life.

2.4 Wearable Flexible Sensors The last two decades have seen a growth of wearable sensors for where ubiquitous monitoring purposes. The sensors are attached to different places like arm, leg or

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Fig. 2.15 An overview of the potential applications and marketing areas of graphene in the upcoming years (Graphene Market Trends)

organ under consideration. These sensors have been proven very advantageous for of elderly people as the response time for the older people is very short in the case of an emergency due to their lack of immunity and low body strength and immunity towards diseases. These wearable sensing systems have become popular as they can be worn for a short duration or in a continuous manner depending on the problems faced and obtain a proper assessment. Some of the advantages of using wearable sensing systems are scrutinizing the minor changes in the patient via regular monitoring, exclusion of vision sensors that violates the privacy of a person, faster response in comparison to the non-wearable ones, and compact system which makes it easier to be used in comparison to the work-bench systems, which requires the person to go to a special place for examination purposes. These advantages have popularised the use of wearable technology (Rahman et al. 2014; Gong et al. 2014) for different applications. Although the use of wearable sensors obtained from non-flexible sensors did become popular in the biomedical field, there were certain disadvantages associated with them which forced to look for alternate options for fabricating wearable sensors. Some of these disadvantages are the high cost of production, high input power leading to wastage of energy, discomfort for the patient due to the brittle nature of the sensors, heavier in comparison to flexible sensors and risk of thermal injuries. The durability of the materials used to develop rigid sensing systems is lower due to their brittleness and rigidity. The power consumption, dynamicity, and sensitivity of the sensing system are some of the other characteristics where the flexible sensing systems are enhanced in comparison to their rigid counterparts. So, the researchers have started

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Fig. 2.16 Pictorial representation of the different prospects of wearable, flexible devices using organic electronics (Liao et al. 2015b)

working on the development of wearable, flexible systems on a large scale to integrate the advantages of flexible sensing prototypes with wearable devices. Some of the advantages of flexible sensors are their high mechanical flexibility and bendability, higher impact resistance, low cost of fabrication and reduced chances of thermal injuries.

2.4.1 Materials for Wearable Flexible Sensors The raw material used for developing the wearable, flexible sensors is calculated from several factors like the application of the sensor, its availability and total fabrication cost. Organic electronics is one prime area in the material side which has been considerably used to develop flexible wearable devices (Liao et al. 2015b). Some of the possibility in the utilization of organic devices for flexible wearable devices are depicted Fig. 2.16. These sensors have been considered for employment in the manufacturing of thin-film transistors, ionic pumps, and polymer electrodes. Organic and large-area electronics (OLAE) (van den Brand et al. 2015) is a phenomenon used to fabricate electronic devices that are printed in thin layers using functional inks. Certain attributes of PET and PEN-like transparency and lower cost makes them a favorable choice to be used for OLAE in comparison to other organic polymers. The OLAE process has been used largely to develop wearable health and medical devices. The utilization of PDMS (Chen et al. 2013a; Moon et al. 2010),

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PEN (Someya and Sekitani 2014), PI (Qin et al. 2015), P(VDF-TrFE) (Fujita et al. 2012), Parylene (Ha et al. 2012) and Polypyrrole (Tjahyono et al. 2013) has been preferably done to develop electrochemical, strain and pressure sensors (Alahi et al. 2018; Nag et al. 2016a, b, c, d, 2017a, b, c, 2018a, b, c; Nag and Mukhopadhyay 2018; Zang et al. 2015) for different applications. The electrodes of the sensor have been fabricated from a range of conducting materials like carbon-based and metallic nanoparticles. The carbon-based elements include graphene (Bae et al. 2013; Nag et al. 2017a; Sadasivuni et al. 2015; Tian et al. 2014), carbon nanotubes (CNTs) (Cohen et al. 2012; Shim et al. 2008) and carbon fibers (Jost et al. 2013). Silver (Amjadi et al. 2014; Wang et al. 2015a), gold (Gong et al. 2014; Hasegawa et al. 2008) and nickel (Tang 2007) are some of the metallic nanoparticles that are commonly used to develop flexible wearable sensors. Biomedical sensing is one of the significant areas which has been worked upon by the wearable, flexible electronic devices (Vilela et al. 2016). Single-Walled Carbon Nanotubes (SWCNTs) and Multi-Walled Carbon Nanotubes (MWCNTs) were used to develop sensors of different attributes for biomedical applications. SWCNTs were used to develop ion-electron potentiometric transducers for monitoring of metabolites on the skin (Zelada-Guillén et al. 2012). Multi-layered films of opposite polarity were developed with MWCNTs to fabricate chemo-resistive sensors (Saetia et al. 2014). Sensor formed with Cu/PI flexible electronic layer was used for the detection of sodium (Na+ ) and potassium (K+ ) ions. An antenna was attached to the sensor for wireless transmission of its data to an Android smartphone (Rose et al. 2015). Detection of bacterial infection on tooth enamel via saliva has been done using graphene-based nano-sensors. The sensors were attached to an inductive coil antenna patterned with interdigitated electrodes (Mannoor et al. 2012). Flexible Organic electrochemical transistors (OECTs) are another type of sensor considered for testing of saliva by converting biochemical signals to electrical signals. The sensors were fabricated with a PANI/Nafion—graphene bilayer film (Liao et al. 2015a). Another way of fabrication included the lamination of polypropylene films and amorphous silicon thin-film transistors on plasma-enhanced PI substrates. These sensors were employed for pressure sensing and large area electronic sensing skins (Graz et al. 2006). Magnetic-field sensors (Melzer et al. 2015) is one sector that is fabricated with inorganic functional nanomembranes with polymeric foils. A linear array of 8 sensors was developed to work on Hall Effect to obtain high bulk sensitivity. Another significant work involves the development of a wearable electronic nose (Lorwongtragool et al. 2014) using a sensor array fabricated from a nanocomposite of CNTs and PEN. Hydrogel systems and electrophysiological sensors (Jang et al. 2015) were also developed via spin-coating and thermally cured layer of PI placed above a layer of Poly (methyl methacrylate) (PMMA). The electrodes were fabricated with a bilayer developed with electron-beam-evaporated Cr and Au. These developed devices were employed for ECG, stress-strain measurements and other biomedical devices (Yeo and Lim 2016). Alloys were also used in WFS to fabricate biometric sensors (Francioso et al. 2010) where thin-film thermocouples like Sb2 Te3 and Bi2 Te3 along with a Kapton substrate were pertained to develop a low-power, flexible micro-thermoelectric generator, which was used for Ambient Assistant Living (AAL) applications.

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Fig. 2.17 Schematic diagram of the transmission of data from the sensor to the monitoring unit Table 2.10 Network protocols standardized by IEEE (Song and Lee 2010) Standard

ZigBee (IEEE 802.15.4)

Bluetooth (IEEE 802.15.1 WPAN)

Wi-Fi (IEEE 802.11 WLAN)

Wi-Max (IEEE 802.11 WWAN)

Range (m)

100

10

5000

15,000

Data rate (kbps)

250–500

1000–3000

1000–45,000

75,000

Bandwidth (GHz)

2.4

2.4

2.4, 3.7 and 5

2.3, 3.5 and 3.5

Network Topology

Star, Mesh and Cluster trees

Star

Star, Tree, P2P

Star, Tree, and P2P

Applications

Wireless sensors (Monitoring and control)

Wireless sensors (Monitoring and control)

PC-based Data acquisition, Mobile internet

Mobile internet

2.4.2 Sensor Networks for Wearable Flexible Sensors Real-time applications including the monitoring of different physiological parameters are primarily dependent on the sensor network that is used to transfer the sensed data. After processing the data in the embedded circuit, it is trans-received between the sensor nodes and the monitoring unit via a router for further analysis. A schematic diagram depicting the transmission of data from the sensor to the monitoring is shown in Fig. 2.17. The choice of a communication network is dependent on the setup cost, power consumption, number of sensor node and range of trans-reception. Table 2.10 gives a comparative study for some network protocols standardized by IEEE (Song and Lee 2010). Comparatively, Bluetooth is the most popular one because of its advantages like cheaper installation, less hardware, and high compatibility. Substantial research has been done on forming Bluetooth integrated health-care systems (Haartsen 1998; Ohmura et al. 2006; Strauss et al. 2005) for different ubiquitous applications. Net-

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Fig. 2.18 Schematic diagram of the hardware architecture for the sensor node for WPMS (Hao and Foster 2008)

works other than the ones mentioned in Table 2.10 are available which is used for data transmission for different biomedical flexible systems. For example, SHIMMER uses a Chipcon radio transceiver with a 2.4 GHz Rufa™ antenna (Burns et al. 2010). Some of the other network remote technologies include Sun SPOT, IRIS, Mica2/MicaZ, Telos (Johnson et al. 2009). Among these, Telos, which is developed by UC Barkley, uses an IEEE 802.15.4 compliant radio that claims to be using one-tenth of the power of previous mote platforms (Polastre et al. 2005). Radio-frequency (RF) is another significant network protocol that is utilized by different flexible acoustic resonators for data transmission (Zhou et al. 2015). For example, ECG monitoring systems have employed the Tmote Sky platform which has an 802.15.4 radio interface at 250 kbps (Park et al. 2006). A wireless physiological management system (WPMS) (Hao and Foster 2008) that could transfer the real-time physiological measurement data wirelessly from the medical sensors to the processing unit had found potential applications in drug delivery systems like chemotherapy, diabetic insulin therapy, AIDS therapy (Jones et al. 2006). Figure 2.18 shows the schematic diagram of the hardware architecture for a specific sensor node for WPMS (Hao and Foster 2008). Wearable Based Sensor Networks (WBSNs), based on IEEE 802.15.4, is another protocol that was introduced for potential applications like ECG, a wearable platform for light, audio, motion and temperature sensing (Maurer et al. 2006).

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Toumaz Technologies, UK devised a wireless system-on-chip integrated system where the operation of the transceiver takes placed in 862–870 MHz and 902–928 MHz ISM bands for European and North American countries respectively (Wong et al. 2008). Research work on antennas and RF systems that have been integrated into clothing have also been done under Body Area Networks (BAN), where low-powered devices had been surface mounted on clothing in a fixed position (Jovanov and Milenkovic 2011). BAN can be broadly categorized into three categories, namely off-body, on-body, and in-body (Hall and Hao 2006; Hao and Foster 2008). Battery-operated systems is another option that has been worked upon to power the system with a battery integrated into the system (Malzahn et al. 2011; Pu et al. 2015). The merits of using self-powered systems (Fan et al. 2012; Ha et al. 2015; Xiao et al. 2012) is defined by the avoidance of the replacement of the battery or the power unit of the wireless system after the completion of charging and discharging cycles.

2.4.3 Types of Activity Monitoring with Wearable Flexible Sensors A range of flexible wearable sensors has been developed and utilized based on the specific parameter being monitored. These parameters dictated the fabrication technique of the sensor prototypes. For example, monitoring of physiological movements (Zeng et al. 2014) like limb motions (Nag et al. 2016b), walking and running (Yao and Zhu 2014) and gait analysis (Tao et al. 2012) would require the patches to be larger with more flexibility. But certain parameters like respiration (Jiang et al. 2010b), heart rate (Patterson et al. 2009) and cardiorespiratory signals (Choi and Jiang 2006) would require the sensors to have a smaller dimension with higher sensitivity. Another application of WFS is as sensing of glucose via secretion mediums like a tear (Iguchi et al. 2007) and immobilization of glucose oxidase (Kudo et al. 2006; Kwak et al. 2012). Electronic skins or e-skins (Katragadda and Xu 2008; Wang et al. 2014a) are another significant areas which was worked upon to mimic the functions of a natural skin in determining the changes in temperature, pressure or health conditions of a person at a specific time. These sensors (Bauer 2013) are embedded in thermal actuators and organic displays. Figure 2.19 depicts the schematic diagram of one type of electronic skin that was developed using elastomeric substrates. A few examples of e-skins include the development of a wearable-on-the-skin sensing systems for physiological sensing (Son et al. 2014), non-volatile memory and for drug release (Choi et al. 2015; Di et al. 2015; Minev et al. 2015) and therapeutic actuators (Mayer et al. 2006; Petrofsky et al. 2013). Figure 2.20 shows individual components of the device and the zoomed version of the finished product. Flexible sensors with high mechanical sensitivity, flexibility and durability were fabrication for monitoring of speech and physiological signals in the geometry of a spider’s sensory system (Kang et al. 2014). Biomedical signal monitoring is another area which was worked upon to

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Fig. 2.19 Schematic diagram of electronic skins with a sensory perception on a human arm (Bauer 2013)

monitor the hydration state and electrophysiological activity using optical, electrical and radio-frequency sensors (Jang et al. 2014). PDMS and PI were spin-coated and used as substrates with electrodes formed with bi-layered sputtered Chromium (Cr) and Gold (Au). Monitoring of skin hydration via thermal conductivity, blood oxygenation, electrocardiogram (ECG), electromyogram (EMG), electrooculogram (EOG) are few of the parameters that could be sensed using the developed prototypes. Figure 2.21 shows an optical diagram of a rugged and stretchable electronic sensor. Strain sensors (Chang et al. 2010; Wang et al. 2011b; Xiao et al. 2011) are one of the most significant categories of WFS and have been utilized for multiple disciplinary applications like human-motion detection (Park et al. 2015; Yamada et al. 2011), forces and acoustic vibrations (Gong et al. 2014) and artificial skins (Pang et al. 2012) to name a few. They have also been used for pressure sensing (Gong et al. 2014; Schwartz et al. 2013; Zang et al. 2015) due to certain attributes like high flexibility and bendability, which depended on the raw material used for the fabrication process. Some of their potential applications include in the field of robotics, aviation, etc. Another prominent application for WFSs is the monitoring of biological fluids like sweat and saliva (Matzeu et al. 2015), that is done using skin-tattooed nano-sensors connected on the wrist and within the mouth respectively. These sensors were also employed to monitoring glucose via electrochemically sensing it from the tears of a person by fixing the sensor with a contact lens. Tattoo-based sensors have been widely appreciated (Bandodkar et al. 2015) and have been used for applications like potentiometric (Bakker et al. 2000) and amperometric (Jia et al. 2013) sensor-based systems. These devices have potential applications for utilizing them in the skin-worn silver (Ag)-zinc (Zn) alkaline batteries (Berchmans et al. 2014), detecting the change

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Fig. 2.20 a Schematic diagram for the materials used to develop the sensor. b Finished product (Son et al. 2014)

in pH (Bandodkar et al. 2013) and the detection of ions like sodium and ammonium (Bandodkar et al. 2014; Guinovart et al. 2013). Chemical and biological sensing with WFS also includes pH measurements (Coyle et al. 2010) done via strapping the embedded system around the waist holding the sensor connected with the microcontroller and LED. Figure 2.22 shows the diagram of the sensing system and its attachment to a person. WFSs have also been fabricated and implemented for the detection of certain gases like Carbon monoxide (CO) and carbon dioxide (CO2 ) (Coyle et al. 2010), where the sensors were fitted in the garments or boots of people

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Fig. 2.21 Schematic diagram of the rugged and stretchable electronic sensor for electrophysiological activities (Jang et al. 2014)

like firefighters for safety measures. Other gas sensing systems include measuring Oxygen (O2 ) (Kudo et al. 2007; Tian et al. 2010) via fitting the system on the wrist of a person to analyze the continuous changes in the oxygen level occurring in hemoglobin during respiration. Microelectromechanical systems (MEMS)-based fabrication techniques have been largely used to develop WFSs for certain biological applications like blood-cell-counting sensing prototype with micro silicon chips (Satake et al. 2002). This technique was employed to aid patients with hearing ailment via fabricating micro-acoustic sensors for sound-source localization (Lisiewski et al. 2011) and hearing purposes (Ko et al. 2006). MEMS-based WFS systems have also been used to develop biomedical sensors to monitor the changes in temperature inside the brain during mental activities and to analyze circadian rhythms (Dittmar et al. 2006). Textile-based systems were also fabricated and implemented for monitoring purposes. One of the biggest advantages of these systems is the comfort level that the patient perceives while being monitored in the absence of wearing a separate sensing system. The monitoring of different parameters using textile-based systems has made it a popular choice (Hasegawa et al. 2008; Korhonen et al. 2003), thus involving make projects like VTAMN (France), Life Shirt (USA) and Wearable Health Care Systems (WEALTHY) (Europe), etc., to develop fibre-based sensor systems for medicine, home healthcare and prevention of diseases (Axisa et al. 2005). These fiber-based sensors also opted from piezo-resistive fibers, elastic and regular polyester fibers, which are used for conducting experiments for different applications like respiration (Huang et al. 2008) and cardiovascular diseases (Pacelli et al. 2006). Plastic optical fibers are also developed for pressure sensing (Rothmaier et al. 2008) where raw flexible silicone fibers were treated with acetone and weaved to form pressure sensors having a thickness of around 0.51 mm. The fiber-based generator (Zhong et al. 2014) is one of the applications where the electrostatic charge produced on the fibers during biomechanical vibrations can be converted into electricity. These Nano-generators operate on a non-contact mode basis, thus relying on air pressure (Li and Wang 2011), which can be used for ultrasensitive sensing during their uses

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Fig. 2.22 a Schematic diagram showing the pH-sensitive chip along with the LED and photodiode. b Place of the sensor on a person (Coyle et al. 2010)

in medical diagnostics and as measurement tools. The fiber can also be integrated with a computer (Kim et al. 2009) to form Planar Fashionable Circuit boards (PFCB) and used for monitoring of sweat using RFIP tag antennas. PFCBs were also used for monitoring of ECG (Yoo et al. 2009), physiological signals (Chang et al. 2008b) and general health (Kim et al. 2008; Yoo et al. 2010). Other applications for fiberbased WFS include a motion sensor (Colombo et al. 2014) and temperature sensor (Sibinski et al. 2010). The design of the Flexible printed-circuit boards (FPCBs) employed for in situ perspiration analysis (Gao et al. 2016) is shown in Fig. 2.23. Drug delivery pump (DDP) (Takei et al. 2015) is another interesting idea that the researchers have worked upon to develop the prototypes with PDMS and a negative photoresist via standard photolithographic technique. These sensors were used for pressure sensing, where the drug was ejected based on the applied pressure. One of the ways to implement the concept of DDP is to use it with a smart bandage and a temperature sensor that can detect the minute changes in the body temperature while performing physical activities.

2.4.4 Challenges and Future Opportunities Although a significant amount of work has been done with WFSs, there are still some glitches that need to be addressed. Researchers are trying to fabricate flexible sensing systems having better performance in terms of sensitivity and sustainability, in comparison to the existing ones. The enormous data generated by the WFS makes

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Fig. 2.23 Flexible Printed Circuit Board developed for in situ perspiration analysis (Gao et al. 2016)

it difficult to handle, store and operate on them to filter out the significant data them. There is also a question of a proper security system that needs to curb any mishandling and misuse of the received data. Time-varying traffic is another problem that the sensing systems face during data transmission in real-time topological systems. This leads to a delay in the data reception in the monitoring unit, thus leading to an overall decrease in the efficiency of the system. Also, some of the significant data might get lost as a result of the high traffic generated by AAL applications. The data transmission for a central coordinator system in Wireless Sensor Networks (WSNs) should be handled efficiently to reduce the traffic and data loss. The connectivity and interoperability of the embedded system should be enhanced, to reduce the power loss. From a patient’s point of view, any kind of discomfort should not be faced by the person when he is wearing the sensing system. The breach of privacy should also be avoided during and after monitoring duration. The attached system should not be loosely connected to the body or clothes worn by the person, as a result of which, the data could be altered based on movements and the surrounding environment. Thermal injuries from the WFS should also be avoided to protect the tissues of the patient. Some of the factors decide these thermal effects (Bagade et al. 2015) are the number, location, and positioning of the sensors. The operating frequency of the sensing system and the communication protocol should be kept low. Power consumption by WFSs is another pivotal issue that needs to be addressed in the current systems. Certain sensors like SHIMMER and Telos that uses low power should be considered for monitoring purposes to minimize the overall power consumed by the system. The continuous input power is another challenge that needs to be worked upon. The effects of motion artifact and distributed interference should be minimized via designing the system for on-node processing. Based on the market survey done on WFS, there is a prominent future (Flexible Smart Sensors and the Future of Health) in terms of printed and flexible electronics. The growth of flexible electronics is predicted to be over 75 billion USD by 2025

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(The State of Flexible and Printed Electronics) with a substantial increase in the usage of these flexible systems for monitoring health parameters. The predicted cost of WFSs is more than 3 billion USD by 2020 (2016–2026: Market Forecasts) with more than 240 million annual unit shipments by 2025 (The Wearable Technology Ecosystem: 2016–2030—Opportunities, Challenges, Strategies, Industry Verticals, And Forecasts). One of the challenges faced by the companies is designing the systems to reduce the overall fabrication cost. One of the ways to achieve this is via opting for cheap, safe and biocompatible materials to make the sensors. flexible, one of the UK-based companies, has estimated an increase in the usage of organic electronics among WFSs (Organic Electronics Will Play a Key Role in Increasing the Utility of Wearables). The companies should opt for designing the systems which would serve the people, based on their application purposes and economic condition. The systems should also be made cost-effective to address a wider section in the society.

2.5 Conclusions Some of the prominent research works done on WFS has been elucidated in the preceding sections of the chapter. Different sensors based on a range of materials along with different communication networks that are considered used for monitoring purposes. The scope of research done on this topic is increasing every day along with the increase in the demand for WFS. The predicted figures for the use of WFS for the next decade have been explained along with some of the challenges currently faced by WFS. The usage of MEMS and Nanoelectromechanical (NEMS) technology is expected to reduce in the upcoming years due to the reduction in the cost of fabrication of the flexible sensing systems. The utilization of the existing manufacturing techniques along with new ones will help in developing enhanced sensing systems to monitor a wider range of applications in order to have a better quality of life in the near future.

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Chapter 3

Interdigitated Sensing and Electrochemical Impedance Spectroscopy

Abstract This chapter elucidates the working principle of the different sensor prototypes that have been explained in the subsequent chapters. The electrodes of all the sensors were shaped in an interdigitated manner, working on the capacitive principle. Due to the flexible and interdigital nature of the sensors, the developed prototypes displayed dual nature of functionality. They were operated as electrochemical sensors with different solutions and as strain strains as a result of a change in dimensions with induced pressure. The sensors were conjugated with electrochemical impedance spectroscopy to determine the changes in impedance values with respect to different inputs provided as per specific applications.

3.1 Introduction The working principle in accordance with the structure of the sensors is explained in this chapter. It describes the changes in their electrical and mechanical behavior with respect to different inputs. The sensor prototypes were fabricated to serve a dual purpose in terms of the mechanical changes with respect to the applied force, as well as the electrical changes when they were used in an electrochemical cell. It also explains the phenomenon of impedance spectroscopy that was used to characterize the sensor patches and subsequently determine the changes take place during different operations. The sensor prototypes were flexible in nature contained with interdigitated electrodes. The electrical conductivity of the sensors was dependent on the processed material used to develop electrodes, while their mechanical diversion was decided by the fabrication process used to form them. The electrodes of all the sensor prototypes were developed to be the same due to the distinct advantages mentioned in the followed section of this chapter. Electrochemical impedance spectroscopy (EIS) was used as the computational tool in accordance with the fabricated prototypes to determine the changes for each application. The response of each of the systems was analyzed as a function of frequency to determine the changes occurring in linear and non-linear systems. EIS served as an excellent tool for measurement, as the sensors functioned at dynamic interfaces where specific system parameters were examined. © Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_3

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3.2 Planar Interdigital Sensors Flexible sensor prototypes with an interdigital pattern were chosen for designing the electrodes. The electrodes were formed in an interdigitated manner due to certain advantages like their non-destructive and non-invasive manner of operation, generating rapid responses to the input changes (Ngo et al. 2016), high sensitivity of the sensor is as a result of the high change in the net electric field due to the different dielectric solutions and applied stress (Matsuzaki and Todoroki 2007) and minimized self-resonant frequency of the sensors as a result of the planar shape of the electrodes. The sensor patches operated on capacitive sensing, where one electrode which is provided the voltage is named as excitation electrode, and the other one is termed as a reference electrode. The planar structure of the developed electrodes formed prototypes for single-sided and non-invasive measurements. Figure 3.1 shows the working principle of the designed electrodes (Khan and Kang 2015). When a frequencydepended voltage signal is applied to the excitation electrode, a resultant electric field is generated between the two oppositely charged electrodes. The electric field bulges from one electrode to another due to the planar nature of the electrodes. In order to determine The presence of the input material above the sensing area was determined via opting a one-directional measurement condition through the insulation of the other side of the substrate. When a material is considered for experimental purposes, it is positioned in contact with or in propinquity to the sensing area of the sensors. The field while bulging from one electrode to another of opposite polarity, penetrates through this Material Under Test (MUT). The properties of the electric field are changed as a result of penetration, which is subsequently studied using any of the response analyzers to determine the dynamics of the system (Mukhopadhyay and Gooneratne 2007). In accordance with the changes in the stimulus to the system or device, the corresponding changes in the characteristics of the electric field are analyzed to determine the changes in the MUT. The penetration depth of the electric field is altered by altering the spatial wavelength (distance between the electrodes of the same polarity), which makes it a favorable option for domestic (Nag et al. 2016), industrial (Zia et al. 2011, 2014, 2015) and scientific (Afsarimanesh et al. 2016, 2017; Alahi et al. 2018) applications. Due to the flexibility of the sensor prototypes, there is a subsequent change in their responses as a result of the applied stress. The working principle for the developed sensor prototypes is shown in Fig. 3.2. The interdigitated sensor replicates a parallel-plate capacitive device where the capacitance of the sensor depends on certain parameters shown in Eq. 3.1,  Cα (∈o ∗ ∈r ∗A) d

(3.1)

where C represents the capacitance, ∈o = 8.85 × 10−12 F m−1 represents the permittivity of vacuum, ∈r represents the relative permittivity, A represents the effective area (A  d), and d represents the effective spacing between electrodes of different polarity.

3.2 Planar Interdigital Sensors

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Fig. 3.1 Working principle of the sensor patch. a The sensor works on the idea of a parallel-plate capacitor. b Due to the planar structure of the electrodes, the electric field bulges from one electrode to another of opposite polarity (Khan and Kang 2015)

Fig. 3.2 Operating principle of the developed sensor patches was based on the capacitive principle (Nag et al. 2018)

The change of d or A causes a change of the effective capacitance of the sensor. This can be utilized to analyze a physiological event through the change in resultant capacitance based on the cyclic deformation and reformation of the sensor patch. The exertion of a strain on the patch via a physiological event changes the value of the capacitance (Cai et al. 2013; Matsuzaki and Todoroki 2007). Figure 3.2 depicts the notion of the working principle of the sensors in terms of their response as a result of mechanical changes. L and W represent for the length and width of the sensor patches, respectively. L, W, and d are the effective changes in the length, width and interdigital distance of the sensor patch respectively when any deformation is caused on them. Using Eq. 3.1, the change in capacitance can be determined as a function of the changes in length (L), width (W ) and interdigital distance (d) as depicted in Eq. 3.2,   W d L + − (3.2) C = C ∗ L W d where C represents the change in capacitance; L, W and d are the changes in length, width and interdigital distance of the sensor patches respectively.

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The effective reactive and impedance changes as a function of the changes in the resultant capacitance of the sensor patches: X = f (C)

(3.3)

The strain exerted on the sensor patches leads to the reorientation of the nanofillers in the polymer matrix. This subsequently changes the resistance and as a result, the conductivity of the electrodes,   A σ l − − (3.4) R = R ∗ l A σ So, the overall changes in the impedance can be expressed as Z = f (R, X )

(3.5)

where l represents the effective length of the electrodes, σ represents the effective conductivity, σ represents the change in conductivity, R, X and Z are the changes in the effective resistance, reactance and impedance respectively. The change in the responses of the sensors can also be correlated to the change in the complex conductivity. The advantages of measuring the output in terms of conductivity are due to a couple of reasons. Firstly, for the high electrical conductivity of the electrodes, even a small change in dimension of the sensor prototype because of the applied strain can be analyzed in terms of conductivity. Secondly, the employment of the sensor prototypes for a range of applications that would exert different ranges of strain on them, as a result of which, the change in the conductivity of the electrodes can be studied easily. Since the sensor prototypes are not ideal capacitors, the change in the conductivity values with respect to the change in frequency can be determined from the complex conductivity as shown from Eq. 3.6. σcomplex = σ + j2π f ε

(3.6)

where σcomplex is the complex conductivity, σ is the effective conductivity, ε is the permittivity, f is the operating frequency. The electric-field density distribution for an applied strain between two consecutive electrode fingers of opposite polarity for one of the fabricated sensor prototypes is shown in Fig. 3.3. The simulation was performed was done using COMSOL 3.2b. The simulation environment was taken to be vacuum while assigning graphite and PDMS as the electrode fingers and substrates of the sensor, respectively. The electric field density was analyzed in terms of the displacement field in the same direction (z-direction) as that of the applied stress. The maximum electric field is being concentrated between the fingers, thus causing a maximum change around that region. Due to the capacitive nature of the sensor patches, the charge density on the electrode fingers varies with the resultant electric displacement field. So, the net

3.3 Electrochemical Impedance Spectroscopy (EIS)

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Fig. 3.3 Three-dimensional simulation done using COMSOL 3.2b to determine the change in the electric field distribution on the sensor patch for the applied stress

charge density on the electrode fingers varies with the applied strain, thus causing a variation in their responses.

3.3 Electrochemical Impedance Spectroscopy (EIS) Electrochemical Impedance Spectroscopy (EIS) is one of the most powerful mechanisms used for sensor investigations as a result of its robust and non-invasive nature (Lasia 2002). Although it is mostly used for characterizing the electrical double layer at the electrode-electrolyte interfaces (Pajkossy and Jurczakowski 2017), the response of the sensors with respect to the change in frequency is also very useful in non-linear processes. EIS is a prominent technique with high resultant sensitivity towards the interfacial phenomenon that is used to analyze the changes in impedance of a cell in the presence of an external source. As a result, the frequency response analysis is performed in the presence of small amplitude of an AC signal on top of a controlled DC polarisation potential. The inner dynamics of the system are calculated from its responses towards the changes with respect to frequency. The resistive and reactive parts of the impedance values are generally considered to represent the changes happening inside a system. When an excitation voltage of small-amplitude is applied to a system, there is a corresponding change in the phase angle () between the input voltage and the output current of the system. This response is assumed to be pseudo-linear considering its change with respect to a low potential. In contrary, the linear system will generate a sinusoidal output current with respect to the input sinusoidal voltage with a shifted phase angle () as shown in Fig. 3.4.

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Fig. 3.4 Phase shift in the output current with respect to the input voltage

The excitation signal to an electrochemical cell can be represented as: E t = E 0 sin t

(3.7)

where E t represents the output voltage at time t, E 0 represents the amplitude of the input signal, ω represents the angular frequency (ω = 2πf ) expressed in terms of radians/second, f represents the frequency in Hertz. The output current of a linear circuit with a phase shift of  can be expressed as, It = I0 sin(t + Φ)

(3.8)

Therefore, the total impedance can be expressed as, Et E 0 sin ωt = It I0 sin(ωt + θ) sin(ωt) Z = Z0 sin(ωt + θ)

Z=

(3.9) (3.10)

The responses from the sensor prototypes developed and deployed for different applications were monitored using different impedance analyzers. Physiological movements, tactile sensing, salinity, and nitrate sensing, force and strain measurements and taste senses are some of the applications the developed prototypes were employed. The response analyzers were tuned in accordance with a specific application, operating over a fixed or a specific frequency range. The connection of the analyzers to the system has been elucidated in the experimental section for each application.

3.4 Conclusions

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3.4 Conclusions This chapter showcases the working principle of all the fabricated sensor prototypes. The flexible sensor patches were developed having interdigitated electrodes which aided as in dual purposes in terms of their electrical and mechanical behavior. The electrical changes in the sensors occurred due to the interdigitated nature of the electrodes, while the mechanical modifications occurred as a result of their flexible nature. EIS was conjugated with the sensor prototypes to monitor their responses for each of the application. The response analyzers were chosen to perform the measurements for the linear and non-linear systems in the succeeding chapters.

References Afsarimanesh N, Mukhopadhyay SC, Kruger M (2017) Molecularly imprinted polymer-based electrochemical biosensor for bone loss detection. IEEE Trans Biomed Eng Afsarimanesh N, Mukhopadhyay SC, Kruger M, Yu P-L, Kosel J (2016) Sensors and instrumentation towards early detection of osteoporosis. In: Instrumentation and measurement technology conference proceedings (I2MTC), 2016 IEEE International. IEEE, pp 1–6 Alahi MEE, Nag A, Mukhopadhyay SC, Burkitt L (2018) A temperature-compensated graphene sensor for nitrate monitoring in real-time application. Sens Actuators, A 269:79–90 Cai L et al (2013) Super-stretchable, transparent carbon nanotube-based capacitive strain sensors for human motion detection. Sci Rep 3 Khan MRR, Kang S-W (2015) Highly sensitive multi-channel IDC sensor array for low concentration taste detection. Sensors 15:13201–13221 Lasia A (2002) Electrochemical impedance spectroscopy and its applications. In: Modern aspects of electrochemistry, Springer, pp 143–248 Matsuzaki R, Todoroki A (2007) Wireless flexible capacitive sensor based on ultra-flexible epoxy resin for strain measurement of automobile tires. Sens Actuators, A 140:32–42 Mukhopadhyay SC, Gooneratne CP (2007) A novel planar-type biosensor for noninvasive meat inspection. Sens J IEEE 7:1340–1346 Nag A, Afasrimanesh N, Feng S, Mukhopadhyay SC (2018) Strain induced graphite/PDMS sensors for biomedical applications. Sens Actuators A 271:257–269 Nag A, Zia AI, Li X, Mukhopadhyay SC, Kosel J (2016) Novel sensing approach for LPG leakage detection—part ii: effects of particle size, composition, and coating layer thickness. IEEE Sens J 16:1088–1094 Ngo T-T, Bourjilat A, Claudel J, Kourtiche D, Nadi M (2016) Design and realization of a planar interdigital microsensor for biological medium characterization. In: Next generation sensors and systems, Springer, pp 23–54 Pajkossy T, Jurczakowski R (2017) Electrochemical impedance spectroscopy in interfacial studies. Curr Opin Electrochem 1:53–58 Zia AI, Mohd Syaifudin A, Mukhopadhyay S, Al-Bahadly I, Yu P, Gooneratne C, Kosel J Development of Electrochemical Impedance Spectroscopy based sensing system for DEHP detection. In: 2011 fifth international conference on sensing technology (ICST). IEEE, pp 666–674 Zia AI, Mukhopadhyay S, Al-Bahadly I, Yu P, Gooneratne CP, Kosel J (2014) Introducing molecular selectivity in rapid impedimetric sensing of phthalates. In: Instrumentation and measurement technology conference (I2MTC) proceedings, 2014 IEEE International, pp 838–843 Zia AI, Mukhopadhyay SC, Yu P-L, Al-Bahadly IH, Gooneratne CP, Kosel J (2015) Rapid and molecular selective electrochemical sensing of phthalates in aqueous solution. Biosens Bioelectron 67:342–349

Chapter 4

Carbon Nanotubes-Polydimethylsiloxane Sensor

Abstract This chapter depicts the design, fabrication, and employment of the first novel sensor prototype formed from Carboxylic acid functionalized Multi-Walled Carbon Nanotubes (MWCNTs) and Polydimethylsiloxane (PDMS). Casting and laser cutting techniques were used to develop the patches where the electrodes were curved out off a nanocomposite layer that was formed by mixing MWCNTs and PDMS. The sensors were then employed for monitoring limb movements and respiration by attaching them to the joints of the limbs and lower part of the diaphragm. They were also deployed for low-pressure tactile sensing purposes.

4.1 Introduction The design, fabrication, and execution of novel flexible sensor prototypes were done using PDMS and MWCNTs are elucidated in this chapter. The utilization of polydimethylsiloxane (PDMS) was done as the substrate and a nanocomposite formed from PDMS and Carbon Nanotubes (CNTs) as electrodes. PDMS is one of the materials that has been widely used (Armani et al. 1999; Fujii 2002; Jo et al. 2000) for fabricating flexible sensors due to specific advantages like low cost, non-toxicity, inertness and hydrophobic nature. CNTs were preferred as the conducting material over other nanoparticles due to certain advantages like their biocompatibility, high flexibility, high resistance towards concerning the changes in temperature, low stiffness, and high tensile strength. Carboxylic group (–COOH) functionalized multiwalled carbon nanotubes (MWCNTs) were considered for developing these sensor patches due to their enhanced dispersing capability within a polymer in comparison to non-functionalised or single-walled carbon nanotubes (SWCNTs). An enhanced interfacial bonding is formed between the nanotubes and the polymer, thus resulting in increased electrical conductivity of the resultant nanocomposites. Interdigital electrodes were patterned on the conductive layer to form sensors for non-invasive and single-sided measurements. The interdigitated patterning was done using laser ablation (Gower 2000; Snakenborg et al. 2004) using a CO2 laser cutter. In comparison to other fabrication techniques like 3D printing (Lam et al. 2002), photolithography (Herzer et al. 2010) or inkjet printing (De Gans et al. 2004), laser © Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_4

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cutting has certain distinct advantages like ease of sample preparation without the necessity of the requirement of any template to develop the electrodes. It also forms very thin and flexible final products while forming smooth edges that are perpendicular to the surface. The employment of these formed sensors was done by attaching them to the skin to measure physiological movements and respiration of different people. They were also employed for tactile sensing by exerting low pressure on the sensing area of the patches via attaching them on the finger.

4.2 Fabrication of the Sensor Patches Figure 4.1 shows the schematic diagram of the fabrication process. PDMS (SYLGARD® 184, Silicon Elastomer Base) was formed at a ratio of 10:1 for the base elastomer (pre-polymer) and curing agent (cross-linker) and cast on a Poly (methyl methacrylate) (PMMA) template. PMMA was used as the main template for casting the PDMS due to its impassiveness towards PDMS, and the ease of peeling off PDMS from the template without the requirement of any additional step. A casting knife (SHEEN, 1117/1000 mm) was used to adjust the thickness of the PDMS cast on the PMMA to 1 mm. Then the desiccation of the sample was done for 2 h to remove the air bubbles trapped in the sample. Then the curing of the sample was done 80 °C for 8 h to solidify the substrate of the sensor patch. This was followed by forming a nanocomposite layer on top of the cured PDMS by mixing functionalized MWCNTs (Aldrich, 773840-100G) and PDMS. 4 wt% of CNTs was considered as the optimal value by obtaining a trade-off between the electrical conductivity and mechanical flexibility of resultant nanocomposite. An SEM image of the developed nanocomposite is shown in Fig. 4.2. The white and black regions in the image represent the mixed PDMS and MWCNTs respectively. Similar to the cured PDMS, the height of the nanocomposite layer was adjusted using the casting knife to around 600 µ, which was then subsequently desiccated for 2 h and cured at 80 °C for 8 h. Laser cutting (Universal Laser Systems) was then conducted on the cured nanocomposite layer to form the defined electrode patterns. Figure 4.3 shows the individual steps of fabrication that were followed to form the sensor patches. The electrodes of the formed patches were tested with a

Fig. 4.1 Schematic diagram of the fabrication steps. PDMS: polydimethylsiloxane. NC: nanocomposite

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Fig. 4.2 SEM top-view image of the nanocomposite consisting of CNTs (4 wt%) and PDMS

Fig. 4.3 Fabrication steps: (1) Casting of PDMS, (2) Desiccation of PDMS, (3) Curing of PDMS, (4) Casting of nanocomposite (NC), (5) Desiccation of NC, (6) Curing of NC, (7) Laser patterning of electrodes

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Fig. 4.4 Front and rear-view images of the sensor patch Table 4.1 Power and speed combinations of the laser cutting tool to obtain different electrode thicknesses Power (W)

Speed (m/min)

z-axis (mm)

Thickness of the electrodes (µ)

1.2

4.2

2

137

2.4

7

2

140

12

42

2

150

24

70

2

175

42

126

2

180

series of short-circuit connection tests to decide the combination of the power and speed parameters of the laser system. A combination of 24 W and 70 m/min was eventually turned out to be comparatively the most viable one that was set for forming rest of the sensor patches. Figure 4.4 shows the front and rear views of the developed sensor patches. The black spots present in the two views of the sensor patches are unscanned agglomerations of CNTs that lie within the PDMS layer. Table 4.1 shows a combination of power and speed settings that were tested to achieve the optimal combinations for the sensor patches. Power (W) defined the intensity of how energetically the laser fired on the material. Speed (m/min) defined the rate of movement of the laser nozzle over the substrate in the X-Y directions. Z-axis was altered to determine the focal point of the laser beam on the substrate material. A profilometer (XP-200) was used to determine the thicknesses of the formed electrodes. When the sensors were developed with very low power and speed settings, a hardening effect of the patches was observed which caused the nanocomposite layer to come off upon application of stress on the patches.

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4.3 Frequency Response and Stress-Strain Measurements Impedance measurements of the sensor patches were taken using a Precision Impedance Analyser (Agilent 4294A). Open and short circuit calibrations were done before the measurements to minimize the effects of stray capacitances. A frequency sweep was done between 10 kHz and 10 MHz to obtain the corresponding impedances (Z) and phase angles () values as depicted in Figs. 4.5 and 4.6 respectively.

Fig. 4.5 Impedance behavior of the sensor patch as a function of frequency

Fig. 4.6 Phase angle of the sensor patch as a function of frequency

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Fig. 4.7 Directions of the applied stresses for the stress/strain characterization of the sensor patch

Fig. 4.8 Stress-strain relationship of the sensor patch

The capacitive nature of the sensor patches caused to have a peak of the phase angle at a frequency value of 150 kHz. This frequency of 150 kHz was fixed to perform further characterization and experiments. The stress-strain experiments of the sensor patches were performed using an INSTRON extensometer tensile/compressive force testing system (VS02477052 R: F). Figure 4.7 shows the directions of the stresses that were being applied on the sensor patches. It is seen from Fig. 4.8 that an anticipated stress-strain relationship (Frankland et al. 2003) was obtained after the stress was applied in the horizontal direction but not in the vertical direction. This was due to the anisotropic geometry of the electrode design. The fracture points acquired from the tensile stress were (1420 µm, 2060 mN) and (−1680 µm, −840 mN). The lower limit of the negative strain was a result of the excessive bending of the sensor patches. The relationship between capacitance and strain of the sensor patches was obtained by exerting strain at three different frequencies to obtain the respective capacitances. It is seen from Fig. 4.9 that there was a prominent change in capacitance values concerning applied strain, the highest was being obtained at the operating frequency (150 kHz). The sensitivity was calculated from the curve at the optimum frequency of 150 kHz from the given formula,

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Fig. 4.9 Relation between capacitance and strain of the sensor patch in the horizontal direction

Sensitivity =

179.2 − 122.3 C = = 0.124 pF/µm. Strain 1187.5 − 728

The linear portion of the frequency line was obtained between 610 and 1120 µm. The saturation value at 150 kHz was obtained at 2160 µm.

4.4 Monitoring of Physiological Parameters The first group of experiments performed with the CNT-PDMS sensor patches was to determine some of the physiological parameters of human beings. The sensor patches were attached to specific locations of the human body to determine the movement of limbs and respiration. The idea behind choosing these two applications is to highlight the applications of CNT-PDMS sensor patches in the biomedical sector. Due to the distinct advantage of mechanical flexibility of these sensor patches, they would certainly be useful to detect the voluntary physical changes taking place on a person’s body. Some of the potential applications lie in their deployment to determine the post-rehabilitative movements of the patients who just suffered from a stroke or muscle spasms. The concept of the use of sensors to monitor people’s health and lifestyle has been capitalized for the past two decades (Altun and Barshan 2010; Najafi et al. 2003) where different types of sensors have been deployed to monitor certain activities and physiological parameters of individuals to analyze and generate a pattern of the human behavior (Kelly et al. 2013; Malhi et al. 2012; Suryadevara and Mukhopadhyay 2012). Flexible sensors are one area where prominent research work (Arena et al. 2010; Ashruf 2002; Briand et al. 2011; Charton et al. 2006) has been largely done in recent times. Lightweight, low cost of fabrication and high longevity

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are some of the ground reasons for their increased preferences over rigid substrates. The sensors fabricated for smart home usage are mainly used for specific monitoring purposes like PIR sensors (Hayes et al. 2009), pressure sensors and other ones. Multiple parameter monitoring is of great interest as a result of the disadvantage caused by a large number of sensors used for individual applications. This is due to the reduced cost of a single sensor that can be used for multiple functionalities. The fabrication of the sensor patches explained here is much simpler regarding the quick and cheaper fabrication process in comparison to previously developed sensors, which were developed for multiple functions containing a coil (Kaniusas et al. 2006; Mehnen et al. 2004; Pfützner et al. 2006) operating on a magnetic principle. Noteworthy research work has also been done on the determination of joint and limb movements. But the majority of them is related to fixed sensors (Crabtree et al. 2003) or the study involving an artificial robot (Aminian and Najafi 2004) to determine the human behavior. Wearable sensors (Nguyen et al. 2011), accelerometers (Yang and Hsu 2010) are other similar sensors have been used to monitor human movement. Shoe sensors (Sazonov et al. 2011) and braces (Bell et al. 2015) are some of the types of wearable sensors that were used for monitoring of physical activities that involves limb movements. The existing sensors have certain disadvantages like constant wearing for ubiquitous measurements, complicated circuitry of the sensing system which require specific expertise to operate them. So, it is state-of-the-art to develop a simple, non-invasive, sensing device that can monitor the movements when they are attached to certain regions. Research work to monitor the rate of respiration has also been done earlier using rigid and flexible sensors. The photoplethysmographic technique (Leonard et al. 2006; Nilsson et al. 2000) is widely preferred for the detection of the rate of respiratory. But due to the complexity, time consumption and requirement of technicians of this technique, researchers have worked on piezoresistive (Reinvuo et al. 2006), fabric-based (Jung et al. 2006; Merritt et al. 2009) and optical sensors (Warkentin et al. 2007) as an alternative to monitor the rate of respiratory. The technical complexity, cost and specific location of the subject during the experimental procedure are some of the demerits of these techniques. Monitoring of respiration and other physiological parameters has also been considered using types of sensors like PVDF-based piezoelectric sensors (Bu et al. 2007; Chiu et al. 2013). But the demits of using PDVF-based sensors are their strong dependency on temperature and high hysteresis. Apart from this, there are different types of force sensors available in the market that are used for these applications. Table 4.2 classifies some of them based on price, size and certain applications related to the monitoring of physiological parameters. It is seen that either the price of the sensors is very high, or the size of the sensor size is reasonably large. In the experiments shown in the subsequent section, we showed the change in the capacitance values of the developed sensor patches by simply attaching them to the joints of the limbs and lower part of the diaphragm of an individual. The inhalation and exhalation rates for the rate of respiration were monitored based on the induced strain on the sensor patch. This idea is useful for certain applications like the anomaly caused in the rate of respiration as a result of hypoxemia and hyperemia, which can be subsequently

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Table 4.2 Comparison of different force-sensing resistors available in the market Sensor

Size (mm)

Force sensing capacity (lbs)

Price (USD)

Application

RB-Phi-121

25 * 11

11.24

45.00

Pressuresensitive touch user interface

Flexiforce A101 Sensor

15.6 * 7.6

10

34.00

Bed monitoring systems, force-sensitive video games

SKU: SEN 09376

3.5 * 3.5

4.49

14.28

Tactile sensor for robotic appendages

SEN-09375

2.375 * 0.75 * 0.5

2.24

6.75

Bicycle handlebar glove, human symbiotic robot

analyzed by determining the differences between the capacitance values between a healthy person and a patient.

4.4.1 Experimental Setup To verify the functionality of the sensor for biophysical parameter monitoring, the patch was fixed to the skin using biocompatible tapes (VHB 3 M RP) as shown in Fig. 4.10. The sensor was fixed only after the skin was completely dried to reduce the effect of sweat or water due to the attachment of the tapes. The existence of sweat on the skin would lead to an additional capacitive layer between the sensor and skin, causing erroneous results. The experimental results regarding the change in capacitance of the sensor were taken by a Precision LCR meter (E4980A) at 150 kHz. BNC-to-alligator clips were connected to the LCR meter from one end and the sensor patch from the other which was attached to the body. Respiratory measurements were taken by attaching the sensor patches at the lower end of the diaphragm of a person. The readings were taken at two different conditions. The connection of the sensor patches was made at the trochlea of the elbow and the patella of the knee to detect the limb movement. The terms ‘flexed’ and ‘extended’ mentioned in the figures refer to states of the limb positions. The movement of the arms was done from a fully extended position, i.e., resting on the table to a fully flexed position via bending the elbow at different angles. The movement of the leg was done similarly by causing an of bending of the knee from an extended to a flexed position. The position of the sensor on the body is crucial regarding the life span and reproducibility of the results. For example, if the

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Fig. 4.10 Sensor attachment for the biophysical parameters: a monitoring of respiration with the sensor attached on the lower part of the diaphragm, b sensor attached on the elbow to monitor arm movement, c Sensor attached on the knee to monitor leg movement

sensor is positioned in a tilted position, the stress exerted on the sensor patch would not be distributed equally. This would lead to a non-uniform stretching of the patch, thus generating an unequal interdigital distance (d) between the electrodes and as a result, producing erroneous results.

4.4.2 Results and Discussion People of different age groups were considered for testing for monitoring of movement of limbs and respiration to affirm the functionality of the sensor patch. Figures 4.11, 4.12, 4.13 and 4.14 show the response of the sensor when the subject was at rest and an oscillatory movement was done for the limbs. The results show that the detection of the movement of the limbs can be clearly done by the connected sensor patch. The limbs were flexed up to an angle of 130°, considering a reference of zero degrees for the extended limb. Regarding the results, there are a few issues that can be addressed to optimize the performance of these sensors for this application. For example, in Figs. 4.11 and 4.12, minute fluctuations are observed during the flexed

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101

Fig. 4.11 Detection of left-arm movement when the subject is at rest

Fig. 4.12 Detection of right-arm movement when the subject is at rest

state. This can lead to contradictory assumptions of the condition of the limb. One of the reasons for this could be a movement of the cables during the movement of the limbs. This issue could be rectified by the wireless operation. Another reason could be the loosening of the sensor patch during the movements of the limbs which lead to the observed artifacts. The signals in the flexed position showed some extent of contradictory values. The reason for this could be that the movement of the limbs was not completely identical for two different cycles. Figures 4.15 and 4.16 show the sensor response when the person is at motion. The changes in capacitance values was monitored when the person moved his limbs while walking.

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Fig. 4.13 Detection of left-leg movement when the subject is at rest

Fig. 4.14 Detection of right-leg movement when the subject is at rest

Not much difference is found for this case in comparison to the output when the subject is at rest. The differences between the two conditions for the flexed and extended situations can still easily be distinguished. The different changes in the angle concerning a reference state of a limb of a subject are shown in Fig. 4.17. The evaluations were done using a Winkeltronic angle finder (450 mm). This experiment was performed to quantify a relationship between the changes in the capacitance values on the change in each degree of the limb. It is seen from Fig. 4.18 that the changes of capacitance values are linear concerning the change in the degree of the

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Fig. 4.15 Detection of right-arm movement when the subject is in motion

Fig. 4.16 Detection of right-leg movement when the subject is in motion

limbs. The maximum angular variation concerning the reference was considered up to 130° due to restricted bending of the limbs. Figure 4.19 showed the sensor response when it was used for the detection of respiratory activity. The readings shown in two colors represent two different individuals. Figure 4.20 shows the three respiration signals taken at different rates. The contraction and expansion occurring on the sensor patch are due to the movement of the diaphragm, which results in the change in the interdigital distance (d) and total area (A) of the sensing surface of the sensor patch. Hence, the sensor patch can precisely distinguish the different rates of inhalation and exhalation lead by a

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Fig. 4.17 Different angular measurements of the limbs of a subject

prominent change in the capacitance values. The inhale and exhale rates were also varied in a controlled fashion to further determine the sensitivity of the sensor patch as depicted in Fig. 4.21.

4.4.3 Conclusion Using carboxylic acid functionalized MWCNTs as a conductive material to develop the electrodes and PDMS as substrates, flexible sensors had been designed and fabricated. The MWCNTs were chosen as a filler to form the nanocomposite due to

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105

Fig. 4.18 Change of capacitance as a function of the right-arm limb movement

Fig. 4.19 Monitoring of respiration for two different individuals at rest

their advantages like high electrical conductivity, high mechanical flexibility, and high aspect ratio. The functionalization groups like –COOH, –C=O and other oxygen carboxyl groups assist in increasing electrical conductivity and allows better dispersion of the nanotubes in the polymer matrix (Lau et al. 2008). PDMS has a low Young’s modulus that has a high-performance patch for its robustness and durability. It is cheaper in comparison to other similar polymers like Polyethylene naphthalate (PAN), Polyethylene terephthalate (PET), that are commonly used commonly to develop flexible sensors. Due to its hydrophobicity, PDMS as a sensor substrate reduces the effects of sweat or other liquids on the response of the sensor output and its attachment to its skin. Another advantage of these sensor patches is their

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Fig. 4.20 Measurement of respiration activity of an individual who was simulating respiration rates of 10/s for the first 80 s, 5/s for the next 45 s and 1/s for the next 20 s

Fig. 4.21 Measurement of respiration activity of an individual who was simulating inhalation and exhalation at different speeds

smaller sizes, which is around 50 mm2 , thus making them convenient for monitoring physiological parameters of elderly people or infants while reducing discomfort to a large extent.

4.5 Tactile Sensing The prosthesis has been considered for a long time as a replacement for a missing part of an amputee’s body with an artificially developed one. The most common

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107

category of prosthesis done in the medical field is the prosthesis of limbs (Erik Scheme and Kevin Englehart 2011; Fougner et al. 2012; Muzumdar 2004), where different amputee parts of the upper or lower portions of the body are placed or replaced to help in restoring a normal life to the affected person. The common materials used for developing the prosthetic organs are fiberglass, Nylon, Dacron, Carbon, and Kevlar (Fortino et al. 2013). Controlling the stiffness and strength of the resultant organs are some of the basic advantages of these materials. But certain disadvantages like their brittle nature and the inability to remold the device once formed deters their applications. For example, the use of Dacron in prosthetic devices can lead to the inability of the original amputated region to repair and grow due to the stiffness of the prosthetic organ. This adversely effects on the growth of tissue of the affected portion which can eventually lead to multiple replacements. In the case of Kevlar, it has been difficult to shape the prosthetic material into the form of the amputated organ unless it is being operated with special tools and equipment. This increases the cost of the overall treatment (Odame and Du 2013). Kevlar also corrodes when they are in contact with chlorine. Thus, it is state-of-the-art to develop sensors that can be considered as a replacement for the commercially used prosthetic devices. Touch-sensitive prosthetic limbs are an interesting development in the prosthetic sector where substantial work (Boland 2010; Carrozza et al. 2003) has been done over the past decade. There is a constant effort from the researchers develops touchsensitive somatosensory organs which would feedback the sensory signals to the amputee’s brain. The devices that have been developed tested are so far on robots (Cannata et al. 2008; Dargahi 2000) and artificially fabricated organs (Trampuz et al. 2003; Zimmerli 2006). An attempt was made (Touch-Sensitive Prosthetic Limbs Take Step Forward in Monkey Study 2013) to fabricate a brain-machine interfaced prosthetic limb. The high cost of fabrication, complex testing systems, and weak sensor response are some of the demerits of such systems. In the succeeding sections, the use of the fabricated CNT-PDMS sensor patches for tactile sensing has been described.

4.5.1 Experimental Setup The performances of the sensors were evaluated by deploying it as a tactile sensor via attaching it at the tip of the forefinger. The patch was connected to the LCR meter via Kelvin probes as shown in Fig. 4.22. An alternating voltage of 1 V peak-to-peak was provided as an input signal to the connected sensor patch. The change in response was determined to analyse the changes in capacitance in pF concerning time.

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Fig. 4.22 The sensor patch attached to the fingertip and connected to the LCR meter for tactile sensing

4.5.2 Results and Discussion Figures 4.23, 4.24, 4.25, 4.26 and 4.27 show the sensor responses for two conditions; touch and no touch as shown by ‘touched’ and ‘non-touched’ conditions. The results are shown in the change of absolute capacitance values against time. Touched is considered for the situation when the sensor attached to the finger is pressed against a plane at different pressures. Not touched is considered for the situation when no stress is applied to the sensor. The results differed in the range of capacitance values during the touched condition as a result of the application of different pressures on the finger on the object. It is seen that the sensor patch responded well to the two defined conditions where the touched condition obtained higher values, thus distinguishable from the no-touch condition. The change in capacitance values with different weights is shown in Fig. 4.28. Different weights were positioned on the sensing area of the patch to determine the changes in capacitance with different weights. It is seen from the Figure that the capacitance values change linearly concerning the change in weights. There were some glitches that were observed during the touched condition, which could be due to the aberrations related to the experimental setup, where the pressure

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Fig. 4.23 Response of the sensor patch for a pressure of 42.2 µPa

Fig. 4.24 Response of the sensor patch for a pressure of 54.6 µPa

applied on the finger was not constant. Also, since the connection of the sensor patches were done using tapes at the tip of the finger, these tapes would have loosened during the bending of the patch, which resulted in the sudden change in capacitance. The capacitance values in the no-touch condition have some non-zero values. This is due to the minimum strain caused on the sensor patch as a result of the bending curvature of the finger.

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Fig. 4.25 Response of the sensor patch for a pressure of 68.2 µPa

Fig. 4.26 Response of the sensor patch for a pressure of 62.4 µPa

4.5 Tactile Sensing

Fig. 4.27 Response of the sensor patch for a pressure of 84.6 µPa

Fig. 4.28 Variation of capacitance for different weight values

111

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4.5.3 Conclusion The application of the CNT-PDMS sensor patches for tactile sensing has been described in the above section. Manual forces were exerted on the sensing area of the patches to analyze their respective responses towards low pressure. The experimental results obtained are very promising that could be based upon to extend the work behind this proposal. This idea could reduce the cost of fabrication cost and equip the existing prosthetic organs with efficient and high-sensitive tactile sensors. The next step would be to deploy these sensor prototypes in real-time applications to validate their functionality via embedding them to a signal-conditioning circuit to develop a complete prosthetic system for human uses.

4.6 Chapter Summary The design, fabrication of the CNT-PDMS sensor patches and their implementation of two different applications were described in this chapter. Due to the mechanical flexibility of the sensor patches, they were being employed for different types of applications like monitoring of physiological parameters and tactile sensing. Some of the major advantages of these sensor patches lie in their low cost of fabrication, simple operating principle, and multiple functionalities. The potentiality of these developed sensor patches regarding the applications can be extended for ubiquitous monitoring of elderly-care purposes to improve the quality of human life. Apart from limb movements and respiration, the detection of other physiological parameters like heart monitoring, fall detection, etc. can also be considered with these sensor patches. The embedding of these patches can be done to make them portable and point-ofcare devices, which would be applicable to the ubiquitous monitoring of the daily activities of a person. The above-mentioned explanation of these sensor patches and their future opportunities does make them a viable option to be considered for using them in multifunctional sensing systems.

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Jung S, Ji T, Varadan VK (2006) Point-of-care temperature and respiration monitoring sensors for smart fabric applications. Smart Mater Struct 15:1872 Kaniusas E et al (2006) Method for continuous nondisturbing monitoring of blood pressure by magnetoelastic skin curvature sensor and ECG. Sens J 6:819–828 Kelly SDT, Suryadevara NK, Mukhopadhyay SC (2013) Towards the implementation of IoT for environmental condition monitoring in homes. Sens J 13:3846–3853 Lam CXF, Mo X, Teoh S-H, Hutmacher D (2002) Scaffold development using 3D printing with a starch-based polymer. Mater Sci Eng C 20:49–56 Lau CH et al (2008) The effect of functionalization on structure and electrical conductivity of multi-walled carbon nanotubes. J Nanopart Res 10:77–88 Leonard PA, Douglas JG, Grubb NR, Clifton D, Addison PS, Watson JN (2006) A fully automated algorithm for the determination of respiratory rate from the photoplethysmogram. J Clin Monit Comput 20:33–36 Malhi K, Mukhopadhyay SC, Schnepper J, Haefke M, Ewald H (2012) A Zigbee-based wearable physiological parameters monitoring system. IEEE Sens J 12:423–430 Mehnen L et al (2004) Magnetostrictive bilayer sensors—a survey. J Alloys Compounds 369:202–204 Merritt CR, Nagle HT, Grant E (2009) Textile-based capacitive sensors for respiration monitoring. IEEE Sens J 9:71–78 Muzumdar A (2004) Powered upper limb prostheses: control, implementation and clinical application. Springer Science & Business Media, Berlin Najafi B, Aminian K, Paraschiv-Ionescu A, Loew F, Büla CJ, Robert P (2003) Ambulatory system for human motion analysis using a kinematic sensor: monitoring of daily physical activity in the elderly. IEEE Trans Biomed Eng 50:711–723 Nguyen KD, Chen I, Luo Z, Yeo SH, Duh HB-L (2011) A wearable sensing system for tracking and monitoring of functional arm movement. IEEE/ASME Trans Mechatronics 16:213–220 Nilsson L, Johansson A, Kalman S (2000) Monitoring of respiratory rate in postoperative care using a new photoplethysmographic technique. J Clin Monit Comput 16:309–315 Odame K, Du D (2013) Towards a smart sensor interface for wearable cough monitoring, IEEE Global Conference on Signal and Information Processing. December 3–5, Austin, Texas, USA, pp 654–657 Pfützner H et al (2006) Magnetostrictive bilayers for multi-functional sensor families. Sens Actuators A 129:154–158 Reinvuo T, Hannula M, Sorvoja H, Alasaarela E, Myllylä R (2006) Measurement of respiratory rate with high-resolution accelerometer and EMFit pressure sensor. In: Sensors applications symposium. IEEE, pp 192–195 Sazonov ES, Fulk G, Hill J, Schutz Y, Browning R (2011) Monitoring of posture allocations and activities by a shoe-based wearable sensor. IEEE Trans Biomed Eng 58:983–990 Snakenborg D, Klank H, Kutter JP (2004) Microstructure fabrication with a CO2 laser system. J Micromech Microeng 14:182 Suryadevara NK, Mukhopadhyay SC (2012) Wireless sensor network based home monitoring system for wellness determination of elderly. Sens J 12:1965–1972 Touch-Sensitive Prosthetic Limbs Take Step Forward in Monkey Study (2013). http://www. livescience.com/40405-touch-sensitive-prosthetic-limbs-monkey-study.html Trampuz A, Steckelberg JM, Osmon DR, Cockerill Iii FR, Hanssen AD, Patel R (2003) Advances in the laboratory diagnosis of prosthetic joint infection. Rev Med Microbiol 14:1–14 Warkentin M, Freese HM, Karsten U, Schumann R (2007) New and fast method to quantify respiration rates of bacterial and plankton communities in freshwater ecosystems by using optical oxygen sensor spots. Appl Environ Microbiol 73:6722–6729 Yang C-C, Hsu Y-L (2010) A review of accelerometry-based wearable motion detectors for physical activity monitoring. Sensors 10:7772–7788 Zimmerli W (2006) Prosthetic-joint-associated infections. Best Practice Res Clin Rheumatol 20:1045–1063

Chapter 5

Aluminium-Polyethylene Terephthalate Sensor

Abstract This chapter explains the fabrication and implementation of the second type of sensor prototype developed from Aluminium and Polyethylene Terephthalate (PET). Metallized PET films were laser ablated in a single-step process to develop the sensor patches. These sensors were then employed for tactile sensing purposes.

5.1 Introduction This chapter explains the fabrication and implementation of the second type of novel sensor patches that were formed from Aluminium (Al) and Polyethylene Terephthalate (PET). One distinct advantage of these sensor patches is their fabrication from a single raw material. Due to this reason, these sensor patches behave differently in terms of performances in comparison to other sensors. This also helped in curbing the complexity during the fabrication process of the sensor prototypes and thus, decreasing the overall cost of fabrication. PET is one of the commonly preferred polymers that is used to develop substrates of flexible sensors for different applications like strain (Lee et al. 2012; Singh et al. 2012) and pressure (Akiyama et al. 2006; Wang et al. 2015) sensing. Some of the advantages of using the metalized version of this polymer are the absence of any post-processing steps, high mechanical flexibility and smooth cut edges (Cutting PET with CO2 laser; Laser cutting and engraving of PET).

5.2 Fabrication of the Sensor Patches The use of aluminum as the conductive material has certain advantages which makes it a viable option for the electrode material (De La Escosura-Muñiz et al. 2015). Some of them are high resistance to corrosion, high electrical and thermal conductivity, and mechanical flexibility (Aluminium—Advantages and Properties of Aluminium). Similar to the first type (CNT-PDMS) of sensors, laser cutting technique was used to develop the electrodes of these sensor patches. Followed by the fabrication and © Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_5

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characterization of the sensor patches, they were deployed for tactile sensing for low pressure. The reasons for considering PET as a substrate material are its high clarity, high recyclability, good chemical resistance, and high impact resistance, and it has a good resistance towards moisture. This gives PET an edge over other polymer materials for developing devices that can be employed for sensing physiological parameters. CO2 laser cutting (Radovanovic and Madic 2011) is a popular technique for developing flexible sensing prototypes on a large scale. Some of its advantages are easy to sample preparation and formation of very thin and flexible products. It also omits the requirement of any mask or template for designing the electrodes, unlike photolithography (Sundaramurthy et al. 2006) or screen printing (Ito et al. 2007). Laser cutting of polymers (Lippert 2004) (Wagner 2000) has been done previously as a result of its reduced cost of fabrication and complicacy in comparison to other techniques. CorelDraw X7 software was considered for designing the schematic of the interdigital electrodes as shown in Fig. 5.1. The interdigital distance obtained for this sensor patch was optimized to 150 µ. The laser ablation process done on the PET film that was attached to the glass substrate is shown in Fig. 5.2. The glass substrate was fixed to the laser platform with tapes (3MTM VHBTM ) on the sides to restrict its movement during the ablation process. A schematic of the individual steps of fabrication of the sensor patches is shown in Fig. 5.3. The processing material was attached to a glass substrate for its support before laser ablation. Metallized PET films (HO-107) were used for fabrication purposes, with Al present on one side of the film. The differences between the electrode and the substrate side were based on the higher electrical conductivity and higher smoothness of Al than the PET side. The sample was then placed inside the laser system (Universal Laser Systems) to develop the interdigital electrodes on the Al side. After the laser ablation process,

Fig. 5.1 Schematic of the electrode design

5.2 Fabrication of the Sensor Patches

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the sensor patch was peeled off the glass substrate. A total of 24 fingers were present in each sensor patch with each one having a length of 41 µ and a width of 1.2 mm. The pitch between two electrodes of the same polarity was 300 µ. The thicknesses of the PET substrate were around 500 µ, and Al electrodes were around 300 µ. The sensing area of the patch was fixed at 44 mm2 . The properties of the laser cutting system like power, speed, frequency, and zaxis were optimized in order to develop the electrodes of the sensor patches. The optimization was done via a series of experiments where different values were considered for the laser properties as shown in Eq. (5.1). Each parameter was altered as a function of the other three parameters until all the parameters were optimized. The values considered for the three parameters are given below.

Fig. 5.2 Laser ablation to form electrodes

Fig. 5.3 Fabrication steps of the sensor patch: (1) Raw material used for fabrication. (2) The raw material is attached to a glass substrate for support. (3) The laser beam is shone on the sample. (4) Interdigitated Al electrodes are formed on PET. (5) Patch is detached from the glass substrate

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f ( p) → f (s) → f (z−axis)

(5.1)

p = 18, 21, 24, 27, 30; s = 20, 30, 40, 50, 60; z-axis = 1, 1.1, 1.2, 1.3, 1.4. Where p defines the power in watts, s defines the speed in mm/min, z-axis defines the distance of the laser nozzle above the sample material in mm. Power—21%, speed—30%, and z-axis—1.20 mm were the final values for which the best electrode line was obtained. These values were fixed during the entire fabrication process of the sensor patches. The width of each electrode line cut by the laser beam was fixed to 41 µ as shown in Fig. 5.4. The image was taken using a Zeiss confocal microscope. The front and rear views of the developed sensor patches are shown in Fig. 5.5. The top-view SEM images of the electrode lines and their edges of the sensor patches are shown in Fig. 5.6. The smooth surfaces shown in the figure are the substrate PET film, and the corresponding rough surfaces are the Al electrodes. The uneven cavities shown in the electrode lines were caused due to the simultaneous molding and demoulding process of the aluminum part of the thin film due to the heat generated by the laser. It is seen from the images that the electrode lines are symmetrical and mostly perpendicular to the substrate whereas the edges at the cross-section are quite smooth. Therefore, no post-processing steps were required to remove any sharp or uneven edges for using the patches for biosensing purposes.

Fig. 5.4 Microscopic image of a single line of the electrode

5.3 Frequency Response and Stress-Strain Measurements

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Fig. 5.5 Front and rear views of the sensor patch

5.3 Frequency Response and Stress-Strain Measurements The sensor patches were then characterized to determine the optimal frequency for further experiments. A HIOKI IM 3536 LCR High TESTER was used to test the sensor patches. A frequency-dependent peak-to-peak voltage (Vin ) of 1 V was applied as the input signal to the sensor. The frequency was swept between 1 kHz and 100 kHz to determine the operating frequency. Figures 5.7 and 5.8 depict the response of the sensor patches in terms of impedance and phase angle with respect to the defined frequency range. The operating frequency for the test sensor patch was fixed as 305 kHz. Figure 5.9 depicts the equivalent circuit for the sensor operation. Vsense is the voltage supplied across the series resistor to determine the current through the sensor. Rsense and Csense are the real and imaginary parts of the sensor patches respectively. Figure 5.10 shows the experimental setup used for analyzing the relationship between the forces applied to the sensor patch with respect to the displacement caused on the patch from its reference position. Elongative and compressive stresses were exerted in the horizontal direction of the sensor patch which was perpendicular to the electrode fingers whose responses are shown in Fig. 5.11. The sensor patches were clamped and moved in the vertical direction depending on the direction of the force. The elongated stress was applied by fixing one end of the sensor and pulling it upwards from the other direction to the highest point (breaking point).

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Fig. 5.6 Zoomed-in SEM image of the top view of the a edges, b electrode lines of the fabricated sensor

Fig. 5.7 Impedance behavior of the sensor patch as a function of frequency

5.3 Frequency Response and Stress-Strain Measurements

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Fig. 5.8 Phase angle of the sensor patch as a function of frequency

Fig. 5.9 Equivalent circuit to determine the response from an interdigital sensor

The compressive stress was applied by referencing the highest point (breaking point) as the reference and moving downwards until the patch reached its normal position (reference point for the elongative force). The breaking points obtained for the tensile and compressive stresses are (388.7 µm, 420 mN) and (−510 µm, −250 mN) respectively.

5.4 Tactile Sensing Tactile sensing (Capek et al. 1988) is one of the recently growing sensing fields (Cutkosky et al. 2008; Dargahi and Najarian 2005; Yang et al. 2008), spanning different applications in everyday life like elevators, automobiles, strain gauges, etc. The current sensing technologies available for tactile measurements have certain disadvantages like high cost (Engel et al. 2005; Ramuz et al. 2012; Takei et al. 2010;

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Fig. 5.10 Experimental set-up for characterization of the sensor

Fig. 5.11 Force-displacement relation for the sensor

5.4 Tactile Sensing

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Fig. 5.12 Schematic for the experimental setup for tactile sensing

Ying et al. 2012), complicated operation (Li et al. 2008; Schwartz et al. 2013; Yeo et al. 2013) or have been tested only in artificial intelligence (Dahiya et al. 2010, 2011). Some of the sensors have high threshold pressure (kPa) (Dargahi and Najarian 2005; Jeong et al. 2015) causing it unavailable for low-pressure measurements. Thus, it is a state-of-the-art to develop novel tactile sensors which have a low cost of fabrication, a simple operating principle with high-performance efficiency and sensitivity. The testing was done manually to determine the responses of the developed sensor patches for low-pressure applications. High sensitivity and repeatability were two of the merits observed in the developed patches. These patches hold high potential to be used as cheap sensing devices on a large scale which can replace the currently used commercial devices for tactile sensing. Figures 5.13, 5.14, 5.15, 5.16, 5.17 and 5.18 show the sensor responses for the different forces applied to them. An oscillatory response was obtained to validate the repeatability of the results upon exertion of manual pressure. In order to test the sensor patches for tactile sensing, forces of different magnitudes were applied manually over the sensing area of the patch to determine their sensitivity. The sensor signals were studied using a HIOKI 3532-50 LCR High TESTER that was considered to study its characterization. The LCR device was interfaced with a computer using an RS-232C interface device to obtain the experimental data. Figure 5.12 depicts the schematic diagram of the experimental setup. The frequency of operation for the following experiments was fixed at 305 kHz. The index finger, thumb, and palm were used to exert manual pressure on the sensing area. These two particular fingers and the palm were considered for experimentation due to the reliability of these portions of the hand for sensing purposes on commercial tactile sensors (Kawasaki et al. 2002; Shimojo et al. 2004). Phase angle was the measured parameter to determine the change in sensor responses against time. Initially, the values of the phase angle of the patch at no pressure condition were around −90°. When pressure was applied at the sensing area, the sensor patches responded with an increase in the phase angle. Two different forces of 2.25 and 2.94 mN were exerted by the index finger (Figs. 5.13 and 5.14) and thumb (Figs. 5.15 and 5.16). These forces were measured by using a digital force gauges (DFX-II) to the patch. The pressure exerted on the patch was calculated by dividing the force applied to the sensing area of the patch (0.36 cm2 ). Different pressures were exerted on the sensor patch to determine its corresponding responses. The pressures applied to the sensor patches as shown in the figures (Figs. 5.13, 5.14,

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Fig. 5.13 Response of the sensor for a pressure of 51.2 Pa of the index finger

Fig. 5.14 Response of the sensor for a pressure of 66.8 Pa of the index finger

Fig. 5.15 Response of the sensor for a pressure of 51.2 Pa of the thumb

5.15, 5.16, 5.17 and 5.18) was considerably lesser in comparison to a normal finger pressure ranging between 540 Pa to 400 kPa (Park et al. 2015; Saccomandi et al. 2014). This can be considered as a soft touch on the patch by the finger. Figures 5.17 and 5.18 show the responses of the sensor patches upon exertion of pressure from the palm. Table 5.1 shows the different pressure values exerted on the sensor patch along with their corresponding repeatability on the sensor output. It is seen that the repeatability of the sensor output ceases when the applied pressure exceeds beyond a limit. The reason behind this phenomenon could be attributed to

5.4 Tactile Sensing

Fig. 5.16 Response of the sensor for a pressure of 66.8 Pa of the thumb Fig. 5.17 Response of the sensor for a pressure of 245 Pa of the palm

Fig. 5.18 Response of the sensor for a pressure of 289.54 Pa of the palm

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Table 5.1 Different pressure values exerted on the sensor along with the repeatability of the output Pressure (Pa)

Finger used for pressure

Repeatability

51.2

Index

Yes

66.8

Index

Yes

51.2

Thumb

Yes

66.8

Thumb

Yes

245

Palm

No

289.54

Palm

No

Fig. 5.19 Sensitivity values for different pressures for the index finger, thumb and palm

the breaking point of the sensor. The sensitivity of the sensors for the defined range of pressure can be calculated from Eq. (5.2). Sensitivity =

Change o f phase angle Change in pr essur e  Sp = p

(5.2) (5.3)

where S p defines the pressure sensitivity, ΔΦ defines the change in phase angle, Δp defines the change in pressure applied on the sensor. The sensitivity curves for the exerted pressure with the two fingers and the palm are shown in Fig. 5.19. There is not much difference in the readings with respect to the applied pressure by the palm from that with the fingers. This shows the consistency in their responses irrespective of the pressure is exerted on it.

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5.5 Chapter Summary This chapter described the fabrication and implementation of a novel sensor prototype developed from metallized polymeric films. Laser ablation was done on the aluminum side of the commercial PET films to develop interdigitated electrodes. Experimental results for tactile sensing were done manually to analyze the responses of the sensor patch to human touch. The response to the pressures from two different fingers and the palm were tested was determined in terms of phase angle with time. It is seen from the results that the patches were capable of responding well to very faint pressures (in Pa). The utilization of these low cost, easily fabricated patches would help to replace the existing tactile sensors for those used in prosthetic limbs, robotic grippers, and pressure sensors. The repeatability of the sensor responses was observed for the applied pressures. The next step will be to employ these sensors for specific applications to validate their functionality for real-time applications.

References Akiyama M, Morofuji Y, Kamohara T, Nishikubo K, Tsubai M, Fukuda O, Ueno N (2006) Flexible piezoelectric pressure sensors using oriented aluminum nitride thin films prepared on polyethylene terephthalate films. J Appl Phys 100:114318 Aluminium—Advantages and Properties of Aluminium. http://www.azom.com/article.aspx? ArticleID=1446 Capek J, Nevesely M, Lansky M (1988) Tactile sensor. Google Patents Cutkosky MR, Howe RD, Provancher WR (2008) Force and tactile sensors. In: Springer handbook of robotics. Springer, Berlin, pp 455–476 Cutting PET with CO2 laser. http://www.gccworld.com/showcase.php?act=view&no=138 Dahiya RS et al (2011) Towards tactile sensing system on chip for robotic applications. Sens J 11:3216–3226 Dahiya RS, Metta G, Valle M, Sandini G (2010) Tactile sensing—from humans to humanoids. IEEE Trans Robot 26:1–20 Dargahi J, Najarian S (2005) Advances in tactile sensors design/manufacturing and its impact on robotics applications—a review. Ind Robot Int J 32:268–281 De La Escosura-Muñiz A, Espinoza-Castañeda M, Hasegawa M, Philippe L, Merkoçi A (2015) Nanoparticles-based nanochannels assembled on a plastic flexible substrate for label-free immunosensing. Nano Res 8:1180–1188 Engel J, Chen J, Fan Z, Liu C (2005) Polymer micromachined multimodal tactile sensors. Sens Actuators A Phys 117:50–61 Ito S, Chen P, Comte P, Nazeeruddin MK, Liska P, Pechy P, Grätzel M (2007) Fabrication of screenprinting pastes from TiO2 powders for dye-sensitised solar cells. Progress Photovoltaics Res Appl 15:603–612 Jeong Y et al (2015) Psychological tactile sensor structure based on piezoelectric nanowire cell arrays. RSC Adv 5:40363–40368 Kawasaki H, Komatsu T, Uchiyama K (2002) Dexterous anthropomorphic robot hand with distributed tactile sensor: Gifu hand II. IEEE/ASME Trans Mechatronics 7:296–303 Laser cutting and engraving of PET. https://www.eurolaser.com/materials/pet-petg/ Lee C, Ahn J, Lee KB, Kim D, Kim J (2012) Graphene-based flexible NO2 Chem Sens Thin Solid Films 520:5459–5462

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Li C, Wu P-M, Lee S, Gorton A, Schulz MJ, Ahn CH (2008) Flexible dome and bump shape piezoelectric tactile sensors using PVDF-TrFE copolymer. J Microelectromech Syst 17:334–341 Lippert T (2004) Laser application of polymers. In: Polymers and Light. Springer, Berlin, pp 51–246 Park J, Kim M, Lee Y, Lee HS, Ko H (2015) Fingertip skin–inspired microstructured ferroelectric skins discriminate static/dynamic pressure and temperature stimuli. Sci Adv 1:e1500661 Radovanovic M, Madic M (2011) Experimental investigations of CO2 laser cut quality: a review. Revista de Tehnologii Neconventionale 15:35 Ramuz M, Tee BCK, Tok JBH, Bao Z (2012) Transparent, optical, pressure-sensitive artificial skin for large-area stretchable electronics. Adv Mater 24:3223–3227 Saccomandi P, Schena E, Oddo CM, Zollo L, Silvestri S, Guglielmelli E (2014) Microfabricated tactile sensors for biomedical applications: a review. Biosensors 4:422–448 Schwartz G, Tee BC-K, Mei J, Appleton AL, Kim DH, Wang H, Bao Z (2013) Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nat Commun 4:1859 Shimojo M, Namiki A, Ishikawa M, Makino R, Mabuchi K (2004) A tactile sensor sheet using pressure conductive rubber with electrical-wires stitched method. Sens J 4:589–596 Singh J, Chu H, Abell J, Tripp RA, Zhao Y (2012) Flexible and mechanical strain resistant large area SERS active substrates. Nanoscale 4:3410–3414 Sundaramurthy A, Schuck PJ, Conley NR, Fromm DP, Kino GS, Moerner W (2006) Toward nanometer-scale optical photolithography: utilizing the near-field of bowtie optical nanoantennas. Nano Lett 6:355–360 Takei K et al (2010) Nanowire active-matrix circuitry for low-voltage macroscale artificial skin. Nat Mater 9:821 Wagner FR (2000) Scanning excimer laser ablation of poly(ethylene terephthalate) (PET) and its application to rapid prototyping of channels for microfluidics, PhD Thesis, Ecole Polytechnique Fédérale de Lausanne (EPFL) Wang X et al (2015) Dynamic pressure mapping of personalized handwriting by a flexible sensor matrix based on the mechanoluminescence process. Adv Mater 27:2324–2331 Yang Y-J et al (2008) An integrated flexible temperature and tactile sensing array using PI-copper films. Sens Actuators A 143:143–153 Yeo WH et al (2013) Multifunctional epidermal electronics printed directly onto the skin. Adv Mater 25:2773–2778 Ying M et al (2012) Silicon nanomembranes for fingertip electronics. Nanotechnology 23:344004

Chapter 6

Graphite-Polyimide Sensor

Abstract This chapter explains the fabrication and implementation of sensor patches developed from laser-induced polymer films. Photo-thermal induction of commercial films was done to form graphene that was subsequently used as electrodes in sensor patches via transferring them on sticky tapes. The sensor patches were used for different environmental and industrial applications like salinity sensing, taste sensing, and nitrate sensing.

6.1 Introduction A novel sensor prototype fabricated from the laser induction of commercial polymer films is showcased in this chapter. This type of sensor patch is different from most of the sensors that are developed for different applications. The fabrication technique used for this sensor can generate conductive material from low cost processing material. The manufacturing cost for each sensor is one of the issues that are currently faced by researchers. One of the reasons for this is due to the high value of the equipment or the raw materials that are used to fabricate the sensors. For example, techniques like photolithography (Herzer et al. 2010) is one of the standard methods that is used for fabricating flexible sensors, has a high equipment cost and requires many steps to prepare the sample and form the electrodes on the substrate. It requires expertise to handle the processing materials to operate them. Thus, it is a state-of-theart to develop a technique for developing quick and cheap sensors. Recently, the idea of developing conductive materials from polymers (Lin et al. 2014) has been proposed by researchers. This is interesting as it helps to fabricate sensors at a low cost on a larger scale. Graphene is one of the popular conductive materials that is used for fabricating sensors due to its certain advantages over other elements like high electrical conductivity, corrosion resistance (Graphene paints a corrosion-free future), which would reduce the oxidation effect on the electrodes. This chapter showcases the design and development of the laser-induced sensor patches that simultaneously uses the formed graphene as electrodes on commercial tapes to develop the sensor patches. The fabricated sensor patches were then operated on solutions of different saline concentrations to determine their ability to be used salinity sensors in a range © Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_6

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of large and small water bodies. Analysis was also done with these sensor patches for taste sensing by experimenting them with different concentrations formed with five chemicals. The sensors were also tested for determining the nitrate concentration in water bodies. Some of the advantages of these sensor patches lie in their easy fabrication process, and excellent electrical and mechanical properties.

6.2 Fabrication of the Sensor Patches A schematic diagram for the steps of fabrication of the sensor patches is shown in Fig. 6.1. Commercial PI films (Zibo Zhongnan Plastics Co., Ltd.) were considered as the raw materials to fabricate the sensor patches. A CO2 laser system (Model: OLS 6.75 CO2 laser system, laser spot diameter: 150 μ) was used for the operation on the thermal induction of graphene on the PI films. The illustrations of the fabrication steps are shown in Fig. 6.2. Initially, the polymer film was attached to a glass substrate as a template for its support as shown in Fig. 6.2a. CorelDraw designing software was utilized for designing the electrodes in an interdigital pattern. Six pairs of electrode fingers were designed with each one having a length of 500 μ and a width of 100 μ. Figure 6.2b depicts the laser induction process done on the attached polymer film. The sp3 hybridized carbon atoms of the PI film was photo-thermally converted to sp2 hybridized atoms of graphene. Power, speed and z-axis were some of the laser parameters that were optimized for the laser-induction process. Power (W) is defined as the amount of energy that the laser nozzle operates on. Speed (m/min) defines the agility of the nozzle to determine the movement of the nozzle over the sample in the x-y direction. Z-axis (mm) defines the position of the focal point of the laser over the sample. This was achieved by operating on the height of the laser platform in the z-direction. The optimized values considered for fabricating the sensor patches were: Power: 9 W, speed: 70 m/min, z-axis: 1 mm. The photo-thermally induced graphene was

Fig. 6.1 Schematic diagram of the steps of fabrication of the graphene sensor. a The polyimide film, with a thickness of around 120 μ, is taken as the substrate material for the laser writing process. b The laser writing was done on the film. c The thermally induced material is shifted manually to the Kapton tape by compressing it against the generated graphene. d The final product

6.2 Fabrication of the Sensor Patches

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Fig. 6.2 Fabrication steps of the sensor

manually transferred to a commercial Kapton tape having a thickness of 1000 μ. Although both the processing material and Kapton tape are made of PI, the commercial polymer films were considered as the processing material instead of Kapton tapes for laser writing is due to two reasons. Firstly, due to the higher thickness of Kapton tapes, photo-thermal induction was not possible to disintegrate the sp3 hybridized carbon atoms of the PI films to sp2 hybridized carbon atoms of graphene. This could have been done only possible with thinner films. Secondly, the sticky nature of the Kapton tape would lead to the subsequent coagulation of the thermally-induced graphene, damaging the design of the electrodes. The positioning of the Kapton tape over the induced graphene is shown in Fig. 6.2c. The transfer of graphene was done via the application of manual pressure over the tape. The pressure was initially applied first in the region of the electrodes of the sensor and consequently moving towards the sensing area of the sensor patches to ensure proper adherence of the induced graphene on the Kapton tapes. The tape was then carefully pulled off from the PI film to avoid the damage on the design of the transferred graphene. Figure 6.2d depicts the final step of the process where the sensor patch consisted of transferred conductive material on the Kapton tape. A difference in electrical conductivities of less than 20 mS/m was obtained between the induced and transferred graphene. The front and rear views of the final product are shown in Fig. 6.3. The SEM images depicting the top and cross-sectional views are shown in

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Fig. 6.3 Front and rear views of the sensor patch

Fig. 6.4 SEM image of a top view of an electrode finger of the sensor patch, b zoomed top view showing the electrode lines

Figs. 6.4a, b. It is seen from the SEM images that the edges and electrode lines of the transferred induced material came off cleanly on the Kapton tapes.

6.3 Complex Nonlinear Least Squares Curve Fitting

133

6.3 Complex Nonlinear Least Squares Curve Fitting The characterization of the developed sensor patches was done to analyze their responses to the dielectric materials operating on different permittivity values. The equivalent circuit was determined to obtain the electrical parameters using the complex nonlinear least square curve fitting (CNLS) technique. It operates on the electrochemical spectrum analyzer algorithm by obtaining a comparative study between the experimental values with the predictive responses. This was done to obtain the system kinetics explaining the diffusion process occurring at the electrode-electrolyte interfaces. The Nyquist (Cole-Cole) plot for each chemical was analyzed by plotting a curve between the real part (resistance) and imaginary part (reactance) of the impedance as the x-axis and y-axis respectively. Statistical analysis was done using a specific algorithm to determine the amplitude of the residual mean-square value using Eq. 6.1 (Zia et al. 2015): 2 = ramplitude

N  (Z 

iobs

i=1

 2  − Z icalc )2 + (Z iobs − Z icalc )2 2 2 Z iobs + Z iobs

(6.1)

where 2 determines the quantitative values for the differences between the simulated ramplitude and experimentally obtained values.   and Z icalc determines the observed and calculated values for the real part of Z iobs the impedance.   and Z icalc determines the observed and calculated values for the imaginary part Z iobs of the impedance.

The characterization of the sensor patches was done by analyzing their profiles in the air to obtain their equivalent circuits as shown in Fig. 6.5. The change in the responses of the sensor patches was studied concerning the real (R) and imaginary (X) parts of the impedance (Z). An average of three values was considered for each sensor to ensure repeatability of their responses. These impedance values were set in the electrochemical spectrum analyzer to obtain the equivalent circuit concerning the responses of the sensor patch in the air. The least square curve was fitted in the Nyquist plot of the experimental values to obtain the equivalent circuit as shown in the inset of the figure. The impedimetric components of the equivalent circuit consisted of different components (Cint , Cool , and Rsol ). Each of these individual parameters influences the resultant kinetic response of the sensor patches. Csol and Rsol define the solution capacitance and resistance values respectively which relies on the properties of the experimental medium. When there is a change in the solution medium, the conductive properties related to the sensor and also the relative permittivity (εr ) changes. This subsequently changes the solution resistance and capacitance values. Cint defines the internal capacitance which occurs due to the electrode design. The internal capacitance also alters when the experimental medium changes, but the quantity of this change is negligible in comparison to the changes in the values

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Fig. 6.5 Least squares curve fitting plot regarding Nyquist plot for the profiling of the sensor done in air. The red dots indicate the experimental data with the green line being the fitted curve Table 6.1 Equivalent circuit parameters (as shown in the inset of Fig. 6.7b)

Equivalent circuit electrical parameters

Curve fitted value

Error%

Cint (pF)

71.22

1.95

Csol (pF)

8.99

0.82

Rsol (k)

343

4.05

of solution capacitance and resistance. The internal capacitance mainly depends on the surface area (A) and interdigital distance (d) of the electrodes. The electrode resistance can be considered very small in this case. The simulation was done with the electrochemical spectrum analyzer for the same range of frequency sweep done between 1 Hz and 10 kHz to obtain a comparison between the experimental results with the theoretical results. Table 6.1 shows the values of the electrical parameters that includes the corresponding errors obtained by determining the differences between the experimental values and curve fitted values for the passive electrical parameters in the equivalent circuit. It is seen that the error values obtained for each parameter were within 5%, which suggests that mean square values is within the range of 10−2 (EIS Spectrum Analyzer).

6.4 Salinity Sensing Water bodies contain different elements that sustain plant and animal life. Each of the constituents has a certain effect on the life-sustaining in the water bodies. The effect of salinity on marine flora and fauna is one of the main factors (Changes in Marine Salinity Levels; Natural environment). Salinity in the water bodies like sea

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and rivers can be defined as the quantity of salt present there. The unit of salinity is defined by the electrical conductivity per unit distance (μS/cm) at a temperature. There are ongoing research works to calculate the effects of changes in the salinity level in different water bodies. Some of the common factors affecting the levels are the rise in temperature, oil spills in the water bodies, discharging of waste materials and climatic changes. A rise of the saline levels in the water bodies is detrimental to the plants living in it. The growth of plants and seed germination is adversely affected by even slight increase or decrease in the salt concentrations. Higher levels of salinity can lead to difficulty for the plants in extracting water from the soil and can be toxic for their sustainability. A small increase in salinity level increases the density of certain water bodies like rivers, thus sinking it to the bottom and floating it across the river basins. A lot of research work (Rahman et al. 2012; Robinson and Nakkeeran 2012; Wang et al. 2012; Wu et al. 2011) is going on currently to determine the optimal concentrations of salt in the water bodies. The available techniques are expensive to remove the excess salt present in the water bodies as a result of the high-quality equipment required to extract the salt. The Murray-Darling Basin report 2015–16 (Murray-Darling Basin Authority Annual Report 2015–2016) showcased the removal of around 524,728 tons of salt water from the River Murray, Australia in one a single year. The presence of this much amount of salt water is alarming considering its presence in just one water body. The research work done (Bhat 2005; Gadani et al. 2012; Huber et al. 2000; Jonsson et al. 2013; Le Vine et al. 2010; Men et al. 2008; Rahman et al. 2011; Reul et al. 2014; Scudiero et al. 2012) based on the monitoring and evaluation of salinity involves certain demerits that deters them to be used on an industrial scale. Some sensors like that optical ones are used to measure the salinity (Huber et al. 2000) for a concentration from 200 mM to 2 M. Apart from the high cost of fabrication of the fabricated sensor formed with the chloride-quenchable fluorescent probe, the prototypes do not measure the small changes having small sensitivity. Other techniques involve the utilization of SMOS satellites developed by NASA (Reul et al. 2014) for measuring moisture in soil and salinity in the ocean. Even though this approach would be much more convenient than the previous one, the cost of the system is high and would be difficult to implement in smaller water bodies. Grating sensors used for salinity and temperature measurements are made up of acrylate and polyimide-coated fibers via determining Bragg’s wavelength (Men et al. 2008). The biggest demerit of employing a coating layer on the fibres is their difficulty to be reused. Thus, it is state of the art to fabricate a low-cost, efficient sensing system that can be used to determine the changes in the salinity level to apply immediate remedial measures. Some of the other impedance measurement systems that have been proposed earlier (Jonsson et al. 2013) have certain demerits like low sensitivity and a complicated embedded system that need time and expertise to be operated. The sensors that are being developed with MEMS technology have a high fabrication cost. Another problem is the operation of these sensors in the MHz range which would require high input power. The development of laser-induced graphene sensors shown in this paper was initially tested with a specific range of salt concentrations with the proposal of fabricating a fully functionalized sensing system. The validation of the

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Fig. 6.6 Block diagram for the proposed microcontroller-based sensing system for salinity measurement

initial sensing system was followed by fabricating a microcontroller-based sensing system. Figure 6.6 depicts the block diagram of the working principle of the proposed microcontroller-based sensing system. The operation of the sensor was controlled by the embedded signal-conditioning circuit, which operates on its output to generate an amplified resultant signal. The fabrication and implementation of this system are explained step by step in subsequent sections.

6.4.1 Experimental Setup Sodium Chloride (SA046-500G) and De-ionised MilliQ® water (Resistance: 18.2 M cm and pH: 6.71) were used as the solute and solvent respectively to develop the salt solutions. Serial dilution process was done to develop a series of solutions ranging between 40,000 and 4 ppm. Initially, a stock solution of 40,000 ppm was formed via the mixture of 4 g of solute to 100 ml of solvent. After the experimentation with this stock solution, the second experimental solution was formed by pipetting 10 ml of the previous solution to 90 ml of de-ionized MilliQ® water to prepare 4000 ppm of salt solution. Followed by the experimentation, the third solution was prepared by pipetting 10 ml of the previous solution into 90 ml of deionized MilliQ® water to form 400 ppm of salt solution. This procedure was continued till a 4-ppm salt solution was prepared and experimented. The experimental setup depicted in Fig. 6.7 shows the use of a HIOKI IM 3536 LCR Hi precision tester for data acquisition purposes. The LCR meter connected to the sensor patch via Kelvin probes from one end was connected to the computer via a USB–USB cable for data collection using an Excel file via an automated data-acquisition algorithm. The sensor patch was attached to a board using biocompatible tapes (3 M Ruban MagiqueMC ) to keep it stable in the water during experimentation. The sensing area of the patch was immersed carefully into the solution to avoid the connect of the bonding pads of the

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Fig. 6.7 Experimental setup is showing the attachment of the sensor patch to the LCR meter via Kelvin probes. The sensing area of the patch was immersed in the solution. The LCR meter was connected to the laptop to collect the data

sensor and the Kelvin probes with the solution. After each round of experiment, the sensor patch was washed properly with MilliQ® water and dried inside the oven for 10 min before reusing it for the next solution. A sinusoidal signal having a voltage of 1 V peak-to-peak was applied as an input from the LCR meter where the frequency was swept between 1 Hz and 10 kHz. The resistance (R) and reactance (X) values were taken from the LCR meter.

6.4.2 Results and Discussion The responses of the sensor patch to the tested samples are depicted in Figs. 6.8 and 6.9 for the resistance (R) and reactance (X) values respectively. It is seen that the sensor patch can clearly distinguish the different tested salt concentrations. Although it is seen from Fig. 6.8 that the changes in the resistance values are symmetric for the tested solutions, the change in the reactance values as seen from Fig. 6.9 is only prominent for a certain range. There is a decrease in the resistance concerning the increase in the salt concentration as a result of the increase in the ionic current passing through the circuit. The operating frequency of had an important role to play for the changes in reactance values in comparison to the changes in the resistance

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Fig. 6.8 Response of the sensor patch for different concentrations of salt regarding resistance and frequency

Fig. 6.9 Response of the sensor patch for different concentrations of salt regarding the reactance and frequency

values. For the changes in reactance values, the frequency ranged between 0.4093 and 7.152 kHz showed a strong change for the tested concentrations. The reactance values changed due to the capacitive part of the sensor was due to both the frequency and the concentration of the sample. The reactive changes of the sensor patches were due to the changes in the faradic currents going through the tested concentration. Figure 6.10 shows the Nyquist plotted between the real and imaginary parts of the impedance. The readings depict a show a clear distinction between the different sample concentrations. A frequency (4.205 kHz) was among the operating range was selected to form the characteristic curve as shown in Fig. 6.11. The selected frequency obtained the highest difference in the experimental values among the salt concentrations. The sensitivity of the sensor patches was 0.005 /ppm, as can be obtained from the slope of Fig. 6.11. A real sample of unknown concentration was taken from the river was taken for testing to validate the proposed salinity system. After sweeping the frequency between 1 Hz and 10 kHz, the response of the patches towards the selected frequency value was considered to form the standard curve. The experimental val-

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Fig. 6.10 Response of the sensor patch for different concentrations of salt regarding Nyquist plot

Fig. 6.11 Standard curve for different experimental concentrations

ues of the tested sample at operating frequency (4.205 kHz) were: Resistance (R): 1828.35 , reactance (X): 1651.7 . Placing the resistance value to the equation acquired from the standard curve, y = 0.005 ∗ (x) + 1651.7

(6.2)

1828.35 = 0.005 ∗ (x) + 1651.7

(6.3)

x=

1828.35 − 1651.7 0.005 x = 35,330

(6.4) (6.5)

The concentration of the sample obtained from the above calculated was 35,330 ppm. This was confirmed with a refractometer which is a standardized device used to calculate the salinity level of real samples in the marine and biological industries (Grosso et al. 2010; Malarde et al. 2008). The actual concentration of the unknown sample was 35,000 ppm, which ensures the experimental value to be very close to the actual value with an error % of 0.9. Figure 6.12 depicts the equivalent

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Fig. 6.12 Least-square curve is fitting for the Nyquist plot with the experimental data Table 6.2 Equivalent circuit parameters along with their limits, result and error % to determining the fitted curve on the experimental data

Equivalent circuit electrical parameters

Curve fitted value

Error%

Csol (μF)

1.171

2.46

Rsol (k)

18.6

0.49

Cint (μF)

6.8

0.95

Cad (μF)

113

2.53

circuit for the Nyquist plot of the values obtained from the tested samples. It is seen there ca change in the components of the two equivalents circuits in Figs. 6.5 and 6.12. A constant-phase element (CPE) has been added in the equivalent circuit in series with the internal capacitor of the sensor. This CPE represents the adsorption capacitance (Cad ) that arises as a result of the ionic diffusion process between the sensing surface of the electrodes and the bulk solution (Mamishev et al. 2004). The solution capacitance (C sol ) and resistance (Rsol ) are two pivotal parameters that determine the change in response of the patches. Table 6.2 represents the different parameters of the equivalent circuit after the experiments along with their values and error rate. The error rates obtained for each of the elements in the generated equivalent circuit is less than 5% (Rahman et al. 2014). Figure 6.13 depicts the response of the sensor patches towards the changes in temperature regarding resistance values for the measurements taken at five different concentrations at the chosen frequency (4.205 kHz) for five temperature values ranging between 10 and 50 °C. It is seen that the changes in resistance values for each sample is negligible concerning the changes in temperature. Figure 6.14 depicts the repeatability of the sensor responses where experiments with each tested were carried out six times with a gap of two hours between each experimental cycle. Similar to the temperature responses, it is seen from the repeatability of sensor responses for each of each concentration

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Fig. 6.13 Dependence of the sensor response towards the temperature changes for different salt concentrations

Fig. 6.14 Repeatability of the sensor response illustrated with six measurements with each salt concentration

is very high and the deviation from the mean value lies within 2%. To develop a real-time sensing system, the same frequency value (4.205 kHz) was used in the microcontroller-based system to determine if the changes in the concentration of the sample can be determined using the embedded system. A conditioning circuit was formed with the microcontroller before it was used for testing the samples.

6.4.3 Microcontroller-Based Sensing System Arduino Uno and the Arduino Integrated Development Environment (IDE) were considered as the microcontroller (μC) device and the associated software, respectively, to form the embedded system. Figure 6.15 shows a schematic diagram for the user to connect the sensor to the microcontroller-based system. The output from the sensor was passed via a buffer and a low-pass filter to the microcontroller. The voltage of 3.3 V with a sinusoidal signal was fed to the microcontroller, and the output was passed through an amplification circuit. This amplifier consisted of a buffer and an active low-pass filter to gain the maximum response from the sensor by reducing the noise and amplifying the signal. Figure 6.16 shows the connection of the sensor with

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Fig. 6.15 Block diagram depicting the connection of the sensor to the microcontroller. The buffer and low-pass filter were added to the circuitry to improve the output from the sensor

Fig. 6.16 Circuit design for the conditioning circuit used to process the sensor output before feeding it to the microcontroller

the amplifying circuit. The non-inverting gain of 23 was provided to the output signal of the sensor to have an amplified response of the output voltage from the sensor taking place due to the difference in salt concentrations. The output of the low-pass filter was fed as the analog input to the microcontroller’s analog-to-digital (ADC) converter. Figure 6.17 shows the voltage readings for the measured samples. It is seen that the response of the sensor is almost linearly dependent on the concentration. The aim of this proposed idea to obtain a system that gives a full-scale voltage reading during the testing with real-time seawater samples. The microcontroller-based system would be necessary in real-life situations where a selective part of the sensing system can be capsuled inside a waterproof case to determine the changes in saline concentrations of the water.

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Fig. 6.17 Response of the microcontroller for different salt concentrations

6.4.4 Conclusion This section describes the fabrication and implementation of a sensing system to detect different salt concentrations in water bodies. The main aim to develop this system was to analyze the current problems faced by flora and fauna existing inside water bodies. Even though the system can detect a range of saline concentrations, there are some issues that need to be addressed before implementing the developed system for real-time applications. Firstly, in addition to sodium chloride, there are other molecular constituents present in the water bodies which can affect the response of the sensor in real-time applications. This can be attained by including the selectivity towards the chosen element. Secondly, the response of the sensor for the saline concentrations can also be influenced by elements having ionic properties like sodium and chloride ions. The similarity in the properties as a result of the similarity in their structures, densities and electron affinities. The issues related to similarity occurs especially with salts having sodium as the cation. This would mislead the monitoring unit leading to erroneous results. These problems if addressed can lead to the development of an enhanced low cost, fully-functionalized salinity measurement system.

6.5 Taste Sensing Among the five different senses of sight, touch, smell, hearing, and taste, the work is done on the last one is the least. The significance of taste sensing holds its role to determine the condition of a person. A human tongue includes thousands of taste buds, each of which is made up of hundreds of taste cells (Ha et al. 2015). People with taste disorders (Taste disorders) are affected with one of the more significant problems in recent years. More than 200,000 people are effected every year from taste disorders, especially the number of children being one out of every ten (Taste disorder ‘prevalent in children’) in Australia. This is a very alarming, especially

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with the view of leading other disorders like obesity and high blood pressure from taste abnormalities. Although the loss of the taste buds for a normal person starts happening after the age of 50, some of the other causes like respiratory infections, radiation therapy and surgeries near the head or neck region can lead to the loss of taste buds. Although one of the common choices to deal with the people suffering from taste buds problems is otolaryngologists, researchers have been working to deal with it in a scientific manner (Chen et al. 2010; Riul et al. 2003). An electronic tongue (e-tongue) has recently been a popular choice for taste sensing (Krantz-Rülcker et al. 2001; Tahara and Toko 2013), where a complicated system had been proposed which replicates the functions of a real human tongue. Even though the e-tongue does function similar to a real tongue, there are certain demerits associated with them which makes it compelling for the researchers to find alternative options. Some of the demerits are the high cost of fabrication of the structure (Haddi et al. 2015), increased complexity (Toko et al. 2016) and difficulty of common people to afford it (Etoh et al. 2008). Another major disadvantage related to the e-tongue is the change in its responses based on temperature and humidity (Baldwin et al. 2011). The adsorption of the analyte on the selective sensing surface does limit the reusability of the sensor prototypes (Ciosek and Wróblewski 2011). So, it is desirable to opt for alternative options that could be considered for taste sense. Although some of the groups working in this area have developed sensing systems for taste assessment (Huang et al. 2006), certain limitations associated with their work reduces the potentiality of their uses. For example, the systems that were based on the recognition of just one (Medeiros et al. 2009) or two (Zhao et al. 2003) taste types can be considered for real-time applications. The motive behind our work is to have a sensor prototype which can clearly distinguish the five fundamental taste types. This could help us to fabricate a real-time system that can be used to determine the concentration of a taste in a food material. The succeeding sections report the implementation of the laser-induced graphene sensors for taste sense. The taste buds of a human being can be categorized into five categories namely sour, salty, bitter, sweet, and umami. Although there are lots of food materials that can be sub-divided into one of these sectors, researchers have classified (The Five Senses of Sensors—Taste) the following chemicals for experimental purposes that can be precisely replicated to these five tastes: • • • • •

Sour—Citric acid Salty—Sodium chloride Bitte—l-tryptophan Sweet—Sucrose Umami—Guanosine monophosphate (GMP)

After the sensor was fabricated, the chemicals mentioned above were tested at four fixed concentrations. The concentrations were based on the minimum amount (Concentration of chemicals in food samples; Medeiros et al. 2009) of that chemical present in food. Then a comparison was done on each of the four concentrations of the five taste types to verify the differences in their responses.

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6.5.1 Experimental Setup The experiments with the graphene-based sensor patches along with the different chemicals were performed inside the laboratory environment at a fixed temperature (25 °C) and humidity (RH 50%) conditions. Figure 6.18 shows the experimental setup used for testing and data collection. Biocompatible tape (3M 810D Ruban MagiqueMC ) was used to attach the sensor patch to a wiring board to keep it fixed inside the sample during experimentation. The sensing area of the patch was immersed carefully inside the sample in a way such that the bonding pads were not touching the solution. A HIOKI IM 3536 LCR High Precision Tester was attached to the sensor with Kelvin probes from one end during experimentation to analyze the changes occurring in the sensor. An input signal of 1Vrms was provided as the input from the tester to the sensor. The analyzer was in turn connected to a desktop via a USB-USB cable for collecting the data obtained from the sensor in Microsoft Office Suite using an automatic-data-acquisition algorithm. Three readings per solution were taken as an average having an interval of 5 s between each cycle to validate the repeatability of the responses of the sensor patch. The sensor was washed thoroughly after each cycle of experiment with deionized water followed by drying it in an oven for 10 min followed by using it for the next experiment. Five different chemicals namely Sodium Chloride (Chem-Supply, SA046-500G), Citric acid (Sigma-Aldrich, 791725-500G), Sucrose (Sigma-Aldrich, S7903-250G), l-Tryptophan (Sigma-Aldrich, T0254-25G) and GMP (Sigma-Aldrich, G8377-5G) were used to analyse the responses of the sensor patches to determine the tastes of salty, sweet, sour, bitter and umami respectively. The solutions were formed by adding the chemicals with deionized MilliQ water® (Resistance: 18.2 M cm and pH: 6.71) as solute and solvent respectively. Experiments with four different concentrations namely 1000, 100, 10 and 1 ppm for each chemical were carried out

Fig. 6.18 Experimental setup during the analysis of different chemicals

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using serial dilution mechanism. The first solution of 1000 ppm for each chemical was formed by adding 0.1 gm of the solute to 100 mL of deionized water. After the experiments with the first concentration, 10 mL from this solution was pipetted into 90 mL of deionized water to develop a solution of 100 ppm. Similar steps were performed to form solutions of 10 and 1 ppm for experimental purposes.

6.5.2 Results and Discussion Figures 6.19, 6.20, 6.21, 6.22, 6.23, 6.24, 6.25, 6.26, 6.27 and 6.28 depict the responses of the sensor patches for different chemicals regarding resistance and reactance for a defined frequency range. Figures 6.19 and 6.20 show a significant change in the responses for citric acid concerning frequency. The presence of the two distinct operating frequency ranges for the resistance values of citric acid can be ascribed to the presence of four single-bond hydroxyl ions (OH) in its structure (Electrical Conductivity of Aqueous Solutions). The increase in the resistive values at higher frequencies can be ascribed to the presence of solution resistance (Rs ) (Kim et al. 2015). The change in the reactance values for citric acid can be ascribed to the adsorption capacitance (Cad ) of the solution and the presence of the double-layer capacitance (Cdl ) at the electrode-electrolyte interface. This leads to an initial increase in reactance values concerning frequency, which after a certain point goes down due to the shorting of the double-layer capacitance. The electrochemical nature of sucrose as seen from Figs. 6.21 and 6.22 can be studied by noting the presence of two isomeric forms of this compound. The existence of the glycosidic linkage between the carbon atoms of the glucosyl and fructosyl subunits can be mainly attributed to the change in the impedance behavior of sucrose. The resistance values increase with an increase in frequency as a result of the increase in ion-solvent interactions at higher frequencies (Juansah and Yulianti 2016). The behavior of the reactance values for sucrose can be ascribed to the same mechanism as that of citric acid. The double-layer capacitance (Cdl ) plays an important

Fig. 6.19 Response of the sensor for citric acid (sourness) in terms of resistance versus frequency

6.5 Taste Sensing Fig. 6.20 Response of the sensor for citric acid (sourness) regarding reactance versus frequency

Fig. 6.21 Response of the sensor for sucrose (sweetness) regarding resistance versus frequency

Fig. 6.22 Response of the sensor for sucrose (sweetness) regarding reactance versus frequency

147

148 Fig. 6.23 Response of the sensor for sodium chloride (saltiness) regarding resistance versus frequency

Fig. 6.24 Response of the sensor for sodium chloride (saltiness) regarding reactance versus frequency

Fig. 6.25 Response of the sensor for l-tryptophan (bitterness) regarding resistance versus frequency

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6.5 Taste Sensing Fig. 6.26 Response of the sensor for l-tryptophan (bitterness) regarding reactance versus frequency

Fig. 6.27 Response of the sensor for GMP (umami) regarding resistance versus frequency

Fig. 6.28 Response of the sensor for GMP (umami) regarding reactance versus frequency

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role in the change like the reactance values at lower and higher frequencies. The nature of the response of the sensor towards sodium chloride as seen in Figs. 6.23 and 6.24 infers that the conductivity of sodium chloride is highest for the tested chemicals. This is as a result of the ionic properties of its structure. The resistance and reactance values for sodium chloride at lower concentrations can be ascribed to the polarisation of the ions. After a specific concentration, the nature of the resistive and reactive values remains constant for all frequencies, as the sensor output is only due to the solution resistance (Rs ) and adsorption capacitance (C ad ) respectively. The electrochemical behavior of l-tryptophan and GMP, as seen from Figs. 6.25, 6.26, 6.27 and 6.28, are very similar. This is due to their similar chemical structures. The change in the resistance values for both these chemicals can be solely attributed to the solution resistance (Rs ). The nature of the reactance for l-tryptophan can be ascribed to the presence of a carboxylic functional group and a secondary amino group in its chemical structure. The double bond in the oxygen ion of the carboxylic group leads to an increase in the current at higher frequencies (Zhao et al. 2014), eventually reducing the resultant reactance values. The electrochemical behavior of GMP can only be attributed to the presence of the phosphate group in its structure. At lower frequencies, the sp3 hybridized bonds between the phosphorus and oxygen atoms deter the change in response of GMP to the applied electric field. The higher frequencies create a polarisation of the oxygen atoms which increases the electronegativity (Properties of the Phosphate Group) and as a result of which, the reactive values slightly changes. A comparison has been done for the reactive responses of the sensor patches to the five tested chemicals to analyze the differences between their responses. The reactance was selected instead of resistive values as they indicated changes in the relative permittivity (εr ) between the electrode and electrolyte interfaces. Figures 6.29, 6.30, 6.31 and 6.32 shows the reactive values for the five chemicals, each of which has four tested concentrations. Prominent differences can be seen between all the values of the chemicals for each of the tested concentration. Even though for bitter and umami, the responses are in proximity to each other, the nature of their changes in the reactance values with the change in concentration is different. The comparative responses of the reactive values can validate that the sensor patches are capable of differentiating each chemical. One of the concentrations (100 ppm) was selected to determine the change in conductivity concerning frequency for these chemicals. The change in the electrical conductivity of the five solutions can be attributed to the relative permittivity of each of the samples. Even though the relative permittivity and electrical conductance are defined to be static and dynamic properties of the sample respectively, an increase in the electrical conductivity will gradually increase the relative permittivity of the sample (Malicki and Walczak 1999; Robinson et al. 2003). Figure 6.33 depicts the differences in conductivities expressed in μS/m among the five chemicals. As seen, sucrose has the least conductivity of the five chemicals which is evident from the comparative reactance values shown in Fig. 6.22. Similarly, sodium chloride depicted the highest conductivity, as evident from its least impedance shown in Fig. 6.24. l-tryptophan and GMP lie in the second and third

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Fig. 6.29 Comparison between the responses of the sensor patch to different chemicals for the concentrations of 1 ppm

Fig. 6.30 Comparison between the responses of the sensor patch to different chemicals for the concentrations of 10 ppm

Fig. 6.31 Comparison between the responses of the sensor patch to different chemicals for the concentrations of 100 ppm

least conductive chemicals due to the stability of their structures in comparison to sodium chloride and citric acid.

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Fig. 6.32 Comparison between the responses of the sensor patch to different chemicals for the concentration of 1000 ppm

Fig. 6.33 Comparison of the conductivity detected by the sensor for different chemicals at a concentration of 100 ppm

6.5.3 Conclusion This portion of the chapter elucidated the utilization of the graphene-polyimide sensors as a taste sensor. These patches were tested to test different chemicals that resemble the constituents of a range of food products. The sensor patches were capable of individualizing the chemicals via different sensor responses to the tested chemicals. The sensor patches displayed significant repeatability in their responses to the experimental solutions. There was no hysteresis present in any of responses to the tested concentrations. The response time of these sensor patches was around two seconds that depended on the impedance analyzer, showing their capability to respond to dynamic conditions. The recovery time of the sensor patches obtained during the experimental process was around 10 min that included the thorough washing of the experimented sensor patch with deionized water and subsequently by drying it. The experimental results look promising for considering the sensor patches to be a replacement for the commercially available e-tongue for taste sense. These patches can be considered for any person who is suffering from a taste-disorder disability or can preferably be installed in any food-producing industry to analyze

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the concentration of the taste of a food product. The sensor patches can also be utilized by the pharmaceutical industries before the injection of drugs in a patient to determine the consequences (Bhattacharyya and Bandhopadhyay 2010). Other than the consumable products, the patches can also be used for testing certain chemicals like dyes (Tahara and Toko 2013), where the precise concentration of any particular synthesized material is pivotal to its performance. Another big merit of these sensor patches is related to the independence of their output on temperature or humidity, as a result of which, it can be used at any operating condition. The simple operating principle and low fabrication cost of the sensor patches are some of the other merits of these patches in comparison to other prototypes that are used in previous research work done on taste sense. However, there are some glitches that need to be rectified before the real-time application of these sensors. The reactive values of the sensor patches at low frequencies for l-tryptophan and GMP for the four tested concentrations do not differ much from each other. This can provide erroneous results to the monitoring unit about the exact value of a concentration. This makes it mandatory to operate the sensor patches at higher frequencies, thus having a high input power. Secondly, when the sensor patches are considered for real-time applications, the reuse of these patches regarding the optimal sensing area conditions holds a pivotal role in their appropriate responses. One of the proposed ways to achieve this is to install an automated robotic arm fitted with the sensor patches and a water-hose nozzle. The arm could be automated in such a way that the nozzle of the hose would wash the sensing area after each of the experimental rounds, and subsequently allowing it to dry for 10 min before they are used for next round. Thirdly, the responses shown above gives us with a platform for a novel sensing system that can be considered as a taste sensor, by the experimentation with individual chemicals individually. The analysis of amalgamation of these chemicals should be done to determine their combined response for as a real-time system. The sensor patches can be tested with a mixture of chemicals to determine the presence of a chemical with a specific concentration. One of the proposed technique would be the introduction of selectivity to identify a particular analyte. The sensing area can be coated with a layer containing the template of the chosen molecule. This would make the sensor selective to that chemical, as a result of which, the concentration can be determined. Another idea is to introduce the wireless operation of the sensor patches by including RFID tags along with the patches to upgrade their performances for taste-sensing purposes. RFID tags can be integrated on the sensor patches during their fabrication and operated on a specific frequency. Based on the application of taste sensing, low-frequency passive RFID tags can be preferred to be installed in the chips. This would be advantageous for industrial uses, as a reader could be used at the monitoring unit for avoiding any manual intervention during the experimental and data-collection stages. The amendment of the glitches mentioned above with their possible remedies would certainly assist to fabricate and implement a fully-functionalized taste sensing system.

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6.6 Nitrate Sensing Nitrogen is one of the essential nutrients occurring in the nitrogen cycle have to be one of the nitrate nutrient sources for the living beings of the Earth. Excessive nitrate leaching causes a loss from soil into water bodies, treating the aquatic life and human health (Cameron et al. 2013; Hester et al. 1996; Wild and Cameron 1980). The presence of nitrogen can be widely found in the environment as a result of their solubility in water. The surface water in New Zealand is contaminated by the presence of nitrate ions due to excessive agricultural usage of land and cattle farming, which consequently possess a serious threat to the quality of the surface water (Holland and Doole 2014; Monaghan et al. 2007). An excessive quantity of nitrate-N in rivers assist to grow periphyton and macrophytes to very high levels (Davies-Colley and Wilcock 2004). They cause a reduction in the oxygen levels in water, which results in hampering the aquatic life. Contaminated nitrate-N water may lead to serious illnesses like congenital disabilities, spontaneous abortions intrauterine growth restriction and increasing the potential of cancer (Brender et al. 2004; Daniel et al. 2009; Greer and Shannon 2005; Squillace et al. 2002). Also, long-term accumulation of nitrate-N increases the risk of affecting animal and human health. The blue-baby syndrome is one of the diseases that can be caused by drinking water with elevated nitrate concentrations (Organization 2017). There has been a considerable amount of work done to determine the concentration of nitrate ion in water. The spectrophotometric method is one of the popular techniques used to determine nitrate-N in water using chemical reagents (Narayana and Sunil 2009). The Griess reaction is considered for the reduction of nitrate ions (Miranda et al. 2001). Ion chromatography (Dudwadkar et al. 2013), optical fibre sensors (Ensafi and Amini 2012; Pellerin et al. 2013), planar electrode sensors (Wang et al. 2015), ion-selective electrodes (Schazmann and Diamond 2007), palladium nanostructures (Pham et al. 2014) are some of the techniques that are used to measure nitrate ions in water. Even though the techniques mentioned above have been successfully validated, most of them are laboratory-based and create a lot of chemical waste, which has detrimental effects on the environment. Some of the regional communities in countries like New Zealand monitor the water samples from different water bodies like rivers, lakes, and groundwater. The water management collects the samples at regular intervals, usually with an interval of a monthly basis. The laboratory-based methods such as spectrophotometry or ion chromatography were employed to determine nitrate-N concentrations. However, it is difficult to determine the accurate effects of leaching of nitrate-N into rivers or lakes as a result of the fluctuating dynamic water system. Therefore, the monthly sampling measurements would be ineffective to determine the actual nitrateN profile. The misplacement of information would influence the understanding of the seasonal effects, which would subsequently affect the total nitrate-N estimation. This causes trouble for the policymakers wanting to come to proper conclusions. The work described below employs an Internet of Things (IoT)-enabled smart sensing system to determine the nitrate-N concentration for real-time application. The aim

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of using the IoT-based system is to create a situation in which the basic information of the developed system is shared in real time. The proposed sensing system will measure the nitrate-N concentration in real time situation and transfer the data to the cloud server pertaining to the system to be considered as a distributed network. The temperature compensation of the proposed system is also included to increase the efficiency of the measured samples. The nitrate-N and temperature measurement along with their validation are explained in the subsequent sections.

6.6.1 Experimental Setup Similar to the previous applications explained in this chapter, Electrochemical Impedance Spectroscopy (EIS) has been employed for the measurement of nitrate concentration. Among the different methods are available for impedance measurement, Frequency Response Analyser (FRA) can be considered as the de facto standard for EIS measurement. A single input sinusoidal signal with an amplitude ranging between 5 and 15 mV is provided as an input in FRA. The frequency is swept in a certain range with a direct current bias voltage, where the signal is applied between the working and sensing electrode. The measurements were done at a certain frequency range to obtain a complete impedance profile. The equation given below can be used to determine the total impedance of an electrochemical system: Z = R + jX

(6.6)

where Z defines the impedance (); R defines the resistance () which is the real part of the impedance; X defines the reactance () which is the imaginary part of the impedance. The impedance profile data can be depicted graphically as a Bode plot and a Nyquist plot which is also known as Cole-Cole plot. The plot also shows the electrochemical changes taking place at the electrode-electrolyte interfaces. A clamp was used to connect the sensor with the HIOKI IM 3536 LCR meter. The frequency sweep was done between 10 Hz and 100 kHz to determine the profile the sensor in air. A standard temperature and humidity condition were maintained during measurement. Deionized water was considered to be the control solution whereas a standard nitrate-N solution was used to develop a working solution for different concentrated samples. The average pH of all the samples was 6.60. Followed by the characterization of the sensors in the air to determine the experimental reference curve, they were tested with deionized water and sample water for the experiments. Figure 6.34 depicts the laboratory setup used for EIS measurements.

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Fig. 6.34 Experimental setup for nitrate measurement

6.6.2 Comparative Analysis of Two Different Sensors 10-ppm Nitrate-N sample water was considered to determine the real part of the impedance of the sensors. The real part of the impedance of the deionized water as the control solution was also measured. The sensor’s response was determined from Eq. 6.7. Sensor ’s r esponse (%) =

R(milliq) − R(sample) × 100 R(milliq)

(6.7)

where R defines the real part of the impedance of the sensors.

6.6.3 Temperature and Nitrate-N Measurement The sensors were initially used to measure the change in temperature in water. The change in temperature changes the mobility of ions present in water which was determined by the graphene sensors. A GEX MS 7-H550 Digital Hotplate, mercury thermometer, LCR meter, and a desktop were used for data acquisition and collection. The thermometer was immersed inside the sample water to analyze its temperature. The sensing surface of the sensor was carefully immersed inside the water, and a frequency sweep was done between 10 Hz and 100 kHz to characterize the sensor at different temperatures. 1, 10, 30, 50 and 70-ppm standard nitrate-N solutions were prepared for experiments in five 100 ml beakers. The immersing of the sensor inside the solution was done until the completion of the measurements. Real and imaginary parts impedance were considered by the LCR meter to develop the graphs.

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6.6.4 IoT-Enabled Smart Sensing System Internet of Things (IoT- enabled smart sensing system was proposed to monitor and collect the data for nitrate-N measurements. AD5933 (Devices), being the impedance analyzer was used to measure the impedance and the phase shift of the sensor. Before the measurements, the impedance analyzer was be calibrated to obtain its gain. Arduino Uno Wi-Fi (Arduino 2017), consisting of an integrated Wi-Fi module, was considered as the master microcontroller to collect the impedance data from the impedance analyzer. Thingspeak (Thingspeak 2017) was used as the web server to gather the measured data and display it in real-time. The Arduino Ciao (Website 2017) library, which is capable of interfacing with system resources and communicating with some of the most common protocols such as (MQTT, XMPP, HTTP, SMTP), was used to transfer the data to the defined private channel in the Thingspeak server. HTTP POST was employed to transfer the measured concentration to the ThingSpeak cloud server. Security is an important issue in IoT research due to the heterogeneity of the employed sensors. An ADG849 was employed as a switch connected to a calibration resistor and the sensor, where the impedance was calculated using the calibrated gain. The phase shift was also calculated from the connected analyzer. The I2 C protocol that was considered to communicate to the impedance analyzer was able to extract the real part of the impedance from the analyzer and gather the information in the main microcontroller. A data processing algorithm was used to convert that resistive part into meaningful temperature and nitrate-N concentration values. The temperature and nitrate-N information were sent to the IoT-based cloud server followed by the measurement of the water sample that was collected from different locations. Figures 6.35 and 6.36 show the block diagram and software flow of the proposed sensing system respectively. Arduino Sketch and Circuit maker (Circuit maker) were used to writing the programming code and draw the circuit diagram of the sensing system respectively. The first step starts with the initialization of the microcontroller with the setup function that is established with the rest connector to provide the cloud-server API (Application Programming Interface) number that is subsequently important to send the measured data to the defined IoT server. The developed calibration curve used to measure the temperature of the deionized water and calculate the nitrate-N concentration along with the compensated temperature — the system communicated with the IoT cloud server to transfer the measured data. The last step is for the microcontroller to go into sleep mode for saving power. Figures 6.37 and 6.38 show a schematic diagram of the signal conditioning circuit and the prototype of the proposed sensing system respectively.

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Fig. 6.35 Block diagram of the smart sensing system

Fig. 6.36 Software flow of the individual steps of the operating of the IoT-based system to calculate and transmission of the nitrate concentration to the cloud server

6.6.5 Results and Discussion Nitrate Measurement Figure 6.39 depicts the change in the real part of the impedance concerning different concentrations for the swept frequency range. It is seen that there is a prominent change in the real part of the impedance can be distinguished clearly for the different concentrations of nitrate-N in sample water. The presence of nitrate-N ions in the water caused a change in the impedance profile for tested concentrations. It is seen that 1200–1700 Hz is the most sensitive frequency region for tested concentrations where the real part of the impedance showed a higher change in comparison to the imaginary part for the different nitrate-N samples. Temperature Measurement The dielectric properties of the deionized water were analyzed via this proposed

6.6 Nitrate Sensing

Fig. 6.37 Schematic diagram of the smart sensing system

Fig. 6.38 Prototype of the smart sensing system

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Fig. 6.39 The change of the real part of the impedance concerning frequency Fig. 6.40 Real part of the impedance as a function of temperature

method. The EIS measurement done on the sample showcased more change in the real part compared to imaginary impedance. The variation of temperature was done between 7 and 50 °C to obtain the real part of the impedance shown in Fig. 6.40. It is seen from the linear regression analysis that the temperature is correlated with the real impedance (R2 = 0.99), that can be theoretically calculated from Eq. 6.8: T (◦ C) =

(RT + 29,153) 404.87

(6.8)

where RT defines the measured real part of the impedance for a certain temperature and T defines the calculated temperature. The slope of the straight line dictates the change of real part of the impedance concerning the change in temperature, which is α = 404.87 /°C. The operating frequency of the sensor for the temperature measurement was obtained at 1650 Hz. Figure 6.41 depicts the comparison of the measured and actual temperature.

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Fig. 6.41 Comparison of actual and measured temperatures

Fig. 6.42 Calibration Standard of Nitrate-N concentration (ppm)

It is seen that the experimental and measured temperature are well correlated with each other and R2 = 0.99 which shows that the sensor can provide measurement of the temperature of sample water can be done quite accurately. 1650 Hz being considered as the operating frequency, was used to develop the calibration standard from a standard nitrate-N sample measurement. All the measured concentrations were taken as the x-axis, and the corresponding real part of the impedance values were taken as the y-axis. Figure 6.42 depicts the final calibration curve for nitrate-N measurements. Linear regression analysis can be done from Eq. 6.9: C=

Rcal − 9196.9 −667.97

(6.9)

where C (ppm) defines the actual concentration, and Rcal () defines real impedance measured by the graphene sensor. Equation 6.10 shown below, was used to determine any unknown nitrate-N concentration in water. The sensor is sensitive to temperature and change in the mobility of the ions with temperature, the measured resistive values were adjusted by a correction factor α. The Ractual defined the modified real part of the impedance due to the change in temperature and was calculated by:

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Table 6.3 Unknown sample measurement (in ppm) compared with laboratory standard method Sl. no.

Sampling number

1st run

2nd run

3rd run

4th run

5th run

Laboratory method

1.

River Water

21.69

21.56

21.54

21.63

21.35

21.5

2.

Tap Water

5.25

5.1

5.35

5.15

5.75

5.5

3.

Canal Water

56.75

56.7

56.6

56.65

56.55

56.5

4.

Stream Water

65

65.2

65.1

65.3

65.15

65

5.

River Water

32.35

32.65

32.55

32.45

32.62

32.5

Ractual = Rcal + α × (T − 25)

(6.10)

Therefore, the standard formula to calculate the actual concentration after applying a correction factor for the graphene sensor is represented from Eq. 6.11. Cactual =

Ractual − 9196.9 −667.97

(6.11)

where C actual is the final corrected concentration due to the temperature change. The final equation was used to determine the unknown nitrate-N concentration along with including temperature compensation. It is seen that the sensitivity of the sensor is changed to 667.97 /ppm, which can be used to measure nitrate-N concentrations accurately. Different water samples of unknown concentration were collected from different locations like river, lake, stream and tap water. Since the concentration of nitrateN level is not very high in the natural water bodies; some nitrate-N samples were spiked with those water samples to increase the level of nitrate-N concentration. The graphene sensors used to measure the temperature of the water samples was followed by the measurement of nitrate-N samples. Among all the other samples, river water was considered to depict the calculation process and measure the nitrate-N concentration. The resistive part of the impedance gave a value of RT = −19,750 . Therefore, inserting the value of resistance, the calculated temperature was found out to be T (°C) = 23.2 °C. The real part of the impedance was found, Rcal = − 4568 , whereas using Eq. 7.6, Ractual becomes −5288.67 . So, finally, the final concentration was 21.69 ppm that is measured from the water sample. This output was verified by a standard UV-spectrometric method. Other sample waters were measured and compared with the laboratory standard method as shown in Table 6.3. It is seen that the sensor showed a very reliable performance in comparison to the laboratory method. The error rate was found out to be less than 5%, which can be considered to be acceptable performance for the sensing system. The error can be assumed to be due to measurements and presence of other ions in water. But still, the accuracy of the sensor and sensing system was seen to be more than 95% and fairly consistent. The developed sensor is robust and maintains good repeatability.

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Fig. 6.43 Repeated unknown sample measurements by smart sensing system

Figure 6.43 displays the reusability performance of the sensor and the sensing system. The sensor can obtain an almost identical result with each experimental run. Graphene as electrodes is corrosion free (Cochlin 2014) and can protect the sensing area from oxidation during the experimentation, thus increasing its repeatability. Mechanically, it is very robust (Geim 2009) and can perform very well during measurements. Figure 6.44 depicts the transfer of data to the IoT-based cloud server showing the concentration of nitrate-N and temperature to be 25.5 ppm and 22 °C respectively. The measurement was done for nearly three hours to determine the actual data in real time. It was found out that the developed system was fairly consistent for monitoring continuous data at defined intervals. The transfer of data has been done simultaneously which helped to monitor the experimental data in real time. There was a delay of 30 s in ThingSpeak cloud server. The developed sensing system will also be useful to considering a distributed monitoring system to determine the temperature and nitrate-N concentration in real-time conditions.

6.6.6 Conclusion This section of the chapter describes the IoT-enabled sensing system for the detection of nitrate-N concentrations in water samples. Temperature compensation was done to enhance the performance of the developed sensing system. The sensor performance was analyzed regarding its capability to determine the different concentrations of the nitrate-N in the water samples. Due to the low fabrication cost and simple operating

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Fig. 6.44 Data transferred to the IoT based web server

principle of the sensor patches, it was easier to develop a low-cost and efficient sensing system to monitor the water samples resembling real-time situations. Another advantage of the sensor was its robustness to assist in their repeatability of the measurements. The developed smart sensing system can be considered for monitoring the real-time nitrate-N concentrations and fabricating a low-cost distributed network with effective performances.

6.7 Chapter Summary This chapter explains the fabrication and employment of laser-induced graphene sensors from commercial polymer films. The induced graphene was transferred to sticky tapes for using them as electrodes on sensor patches. A few of the major advantages of the developed patches were their quick fabrication process, low cost, simple operating principle, high conductivity and the corrosion-resistant nature of the conductive material. The sensor patches were employed for environmental monitoring purposes where laboratory-made samples to determine the capability of the developed patches to differentiate the nitrate and saline solutions at different concentrations. They were also utilized for industrial applications via experimenting with different solutions formed with five fundamental chemicals which are considered for taste-sensing purposes. The ability of the developed patches to distinguish the five chemicals depended on the differences of their responses. Apart from employing these patches for industrial and environmental purposes, they can also be used for health monitoring via embedding them with a conditioning circuit in conjugation with wearable sensors. The corrosion resistance of graphene makes them preferable to be used to develop the electrodes for sensor patches that can be used for ubiquitous monitoring of swear, urine (Nag et al. 2017a, b), etc. The inclusion of selectivity to the sensor patches would further increase their potentiality for different applications.

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Chapter 7

Graphite-Polydimethylsiloxane Sensor

Abstract This chapter represents the fabrication and implementation of a novel sensor prototype developed from 3D printed mold-based technique. 3D printing was done to develop molds of specific dimensions where the casting of Graphite and PDMS was done to develop the electrodes and substrates of the sensor patches respectively. The developed sensors were used for monitoring strain-induced physiological movements by attaching them to different joints of the body. They were also employed for low-force sensing where identical objects with different weights were positioned in the sensing area of the patches to determine their responses.

7.1 Introduction This chapter presents the development and implementation of novel Graphite-PDMS sensors for different strain-sensing applications. The fabrication of the sensor patches was done via 3D printed molds which were formed using acrylonitrile thermoplastic polymer as the printing filament. One of the enhanced techniques to fabricate sensors with flexible substrates has been the utilization of 3D printing technique where the master mold can be printed with specified dimensions quickly at a very low price, followed by their deployment to develop the final sensor patches. Another big advantage of using 3D printing for fabrication purposes is the usage of thermoplastic polymers to form the templates. Some of the advantages of these polymers lie in their high tensile strength, easy bendability, easy recyclability and high performance in terms of fatigue properties in comparison to metals (Advantages of thermoplastics), which makes these thermoplastic polymers (Kanoun et al. 2014) cheaper and muchpreferred choice for 3D printing purposes. Graphite and PDMS were considered to fabricate the electrodes and substrates of the sensor patches respectively. The merits of using PDMS over the other commonly used polymers are its low cost, high tensile strength, hydrophobicity, resistance towards temperature and ability to form excellent interfacial bonding with the polymer matrix (Nour et al. 2012). The advantages of using graphite lie in its high compressive strength, high electrical and thermal conductivity and corrosion resistance. One of the biggest merits of using graphite is its biocompatibility (Chang-bin et al. 2011), which makes it a preferred choice over © Springer Nature Switzerland AG 2019 A. Nag et al., Printed Flexible Sensors, Smart Sensors, Measurement and Instrumentation 33, https://doi.org/10.1007/978-3-030-13765-6_7

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other conductive materials to fabricate the devices for biomedical applications. The electrodes were formed in an interdigital manner via the casting of graphite powder on the trenches of 3D printed molds having specific dimensions. The working principle of the sensor patches is explained along with the COMSOL simulation result which depicts the electric field density distribution between the two electrode fingers of opposite polarity under applied stress. The characterization of the sensor patches was done to analyze their responses to the change of frequency and applied stress. The experimental results sensor patches were obtained by determining the changes in their complex conductivities and capacitances at different operating conditions. The sensor patches were then deployed for two applications. One was regarding straininduced physiological parameter monitoring by attaching them to different parts of the body like finger, elbow, neck, and knee. Strain sensing was done based on the bending of the sensor patches along with the bending of the different joints. The other application was related to low-force sensing measurements. The results depicted by the developed sensor patches were promising, thus increasing the chances of utilizing them in future for dynamic fields like biomedical field, microfluidic and tactile applications. The merits of the proposed sensor patches over the existing methods include its low-cost of fabrication, low-cost of processing materials, simple operating principle and a wide range of strain-induced applications. The proposed sensor patches also have lightweight and small dimension, which makes them easier to be replaced or fixed if damaged. The sensors had a single-sided measurement with the electrodes being capacitive in nature, thus making them easier to be adjusted and fabricated while achieving high sensitivity with little stress. The input power required is also low but obtaining a good resolution and response to tested frequency ranges. Apart from utilizing the flexible nature of these sensor patches, they can also be employed for monitoring non-metallic targets and dielectric materials.

7.2 Fabrication of the Sensor Patches The fabrication of the sensor patches was done inside the laboratory conditions. Figure 7.1(a) shows the 3D printing system that was used to fabricate the molds. Acrylonitrile Butadiene Styrene (Tymrak et al. 2014) was considered as the printed filament to form the moulds. This material was considered because of high flexural strength and ductility in comparison to other thermoplastics used for 3D printing purposes. The filament was attached to the printing device (3D PRINTING SYSTEM) followed by pre-heating the later for fifteen minutes before starting the process. This was done to let the device to achieve a fixed temperature required for the operation. This was followed by uploading the designs of the electrodes that are to be fabricated on the sensor patch using the designing software CREO Parametric 2.0, that was associated with the printing system. The height of the trenches to be developed on the molds was adjusted to 500 µ. The mold formed by the printing system is depicted in Fig. 7.1(b). After the molds were developed, they were thoroughly

7.2 Fabrication of the Sensor Patches

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Fig. 7.1 The 3D printing systems (a) were used to develop the molds (b) using the Acrylonitrile Butadiene Styrene as the printing filament. The height of the trenches on the mold was adjusted to 500 microns

washed using 2-isopropanol to remove the presence of any kind of snippets of the printing filament. Graphite powder (Sigma-Aldrich 282863-25G,