Nanotechnology in Smart Textiles 9781773616490, 9781773615493

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NANOTECHNOLOGY IN SMART TEXTILES

NANOTECHNOLOGY IN SMART TEXTILES

Edited by: Dharani Sabba

ARCLER

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www.arclerpress.com

Nanotechnology in Smart Textiles Dharani Sabba

Arcler Press 2010 Winston Park Drive, 2nd Floor Oakville, ON L6H 5R7 Canada www.arclerpress.com Tel: 001-289-291-7705         001-905-616-2116 Fax: 001-289-291-7601 Email: [email protected] e-book Edition 2019 ISBN: 978-1-77361-649-0 (e-book) This book contains information obtained from highly regarded resources. Reprinted material sources are indicated. Copyright for individual articles remains with the authors as indicated and published under Creative Commons License. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data and views articulated in the chapters are those of the individual contributors, and not necessarily those of the editors or publishers. Editors or publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify. Notice: Registered trademark of products or corporate names are used only for explanation and identification without intent of infringement. © 2019 Arcler Press ISBN: 978-1-77361-549-3 (Hardcover) Arcler Press publishes wide variety of books and eBooks. For more information about Arcler Press and its products, visit our website at www.arclerpress.com

DECLARATION Some content or chapters in this book are open access copyright free published research work, which is published under Creative Commons License and are indicated with the citation. We are thankful to the publishers and authors of the content and chapters as without them this book wouldn’t have been possible.

ABOUT THE EDITOR

Dharani Sabba worked as a Research Fellow in Energy Research Institute of Nanyang Technological University (ERI@N), Singapore, on investigating novel semiconductor and photoactive materials for opto-electronic applications. She also obtained her Ph.D in Materials Science from Nanyang Technological University. Her research interests encompass development and synthesis of novel nanomaterials, investigation of their properties and their application in renewable energy sector.

TABLE OF CONTENTS



List of Contributors.......................................................................................xiii



List of Abbreviations..................................................................................... xix

Preface..................................................................................................... ....xxi Chapter 1

A Historical Review of the Development of Electronic Textiles................. 1 Abstract...................................................................................................... 1 Introduction................................................................................................ 2 Temperature Control Textiles....................................................................... 5 Materials Developments and Wearable Computing..................................... 6 Sensors....................................................................................................... 7 E-Textile Pressure Sensors and Textile Switches........................................... 9 Textile Energy Solutions............................................................................ 11 Communication Devices.......................................................................... 12 Illumination.............................................................................................. 12 Market Trends........................................................................................... 13 Potential Future Developments................................................................. 14 Conclusions.............................................................................................. 15 Author Contributionss............................................................................... 16 References................................................................................................ 17

Chapter 2

Smart Textiles and Nano-Technology: A General Overview.................... 31 Abstract.................................................................................................... 31 Introduction.............................................................................................. 32 History of Smart Textiles Development...................................................... 33 Classification of Smart Textiles.................................................................. 35 How Does a Smart Textile Work?.............................................................. 41 Incorporating Smartness Into Textiles........................................................ 42 Application of Smart Textiles..................................................................... 42

Latest Development In Smart Textiles and Nanotechnology...................... 47 Prospect of Smart Textiles.......................................................................... 48 Conclusions.............................................................................................. 50 References................................................................................................ 51 Chapter 3

Wearable E-Textile Technologies: A Review on Sensors, Actuators and Control Elements............................................................... 53 Abstract.................................................................................................... 53 Introduction.............................................................................................. 54 Materials, Connections and Fabrication Methods...................................... 55 Smart Fabrics Sensors................................................................................ 60 Wearable E-Textiles.................................................................................. 64 Future Perspectives................................................................................... 66 Conclusions.............................................................................................. 66 Acknowledgments.................................................................................... 67 Author Contributions................................................................................ 67 References................................................................................................ 68

Chapter 4

Textile-Based Flexible Coils for Wireless Inductive Power Transmission................................................................................. 75 Abstract.................................................................................................... 75 Introduction.............................................................................................. 76 Theory of Inductive Power Transmission.................................................... 77 Design...................................................................................................... 81 Fabrication................................................................................................ 83 Experimental Procedure............................................................................ 89 Electrical Characteristics of Printed Coils.................................................. 92 System Performance.................................................................................. 96 Safety Considerations................................................................................ 99 Conclusions............................................................................................ 100 Author Contributions.............................................................................. 101 Acknowledgments.................................................................................. 101 References.............................................................................................. 102

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Chapter 5

Printing Smart Designs of Light Emitting Devices with Maintained Textile Properties ............................................................... 105 Abstract.................................................................................................. 106 Introduction............................................................................................ 106 Results.................................................................................................... 107 Discussion And Conclusions................................................................... 115 Materials And Methods........................................................................... 116 Acknowledgments.................................................................................. 118 Author Contributions.............................................................................. 118 References.............................................................................................. 119

Chapter 6

Digital Inkjet Functionalization of Water-Repellent Textile For Smart Textile Application................................................................. 121 Abstract.................................................................................................. 121 Introduction............................................................................................ 122 Materials And Methods........................................................................... 124 Results And Discussion........................................................................... 127 Conclusion............................................................................................. 139 Acknowledgements................................................................................ 139 References.............................................................................................. 140

Chapter 7

Impact and Scope of Intelligent Textiles in Health Care........................ 145 Abstract.................................................................................................. 145 Introduction............................................................................................ 146 Materials and Methods........................................................................... 148 Results, Discussion And Conclusion....................................................... 164 References.............................................................................................. 166

Chapter 8

Antimicrobial Properties Of Diamond-Like Carbon/Silver Nanocomposite Thin Films Deposited On Textiles: Towards Smart Bandages...................................................................................... 171 Abstract.................................................................................................. 172 Introduction............................................................................................ 172 Results.................................................................................................... 175 Discussion.............................................................................................. 182 Materials And Methods........................................................................... 185 Conclusions............................................................................................ 189 xi

Acknowledgments.................................................................................. 189 Author Contributions.............................................................................. 190 References.............................................................................................. 191 Chapter 9

New Textile Sensors for In Situ Structural Health Monitoring of Textile Reinforced Thermoplastic Composites Based on the Conductive Poly(3,4-ethylenedioxy-thiophene)-poly (styrenesulfonate) Polymer Complex...................................................... 197 Abstract.................................................................................................. 198 Introduction............................................................................................ 198 Experimental........................................................................................... 202 Results And Discussion........................................................................... 213 Conclusions............................................................................................ 218 Acknowledgments.................................................................................. 218 Author Contributions.............................................................................. 219 Appendix A............................................................................................ 219 References.............................................................................................. 222

Chapter 10 Recent Advances In Soft E-Textiles......................................................... 229 Abstract.................................................................................................. 229 Introduction............................................................................................ 230 Materials For E-Textiles........................................................................... 231 Textiles Integrated With Electronics......................................................... 233 Smart Textiles In Energy Harvest And Storage.......................................... 235 Stretchable And Flexible Interconnects, And Conductive Textiles............ 238 Textiles In Advanced Health Care........................................................... 240 Conclusions And Future Scopes.............................................................. 241 Acknowledgments.................................................................................. 242 References.............................................................................................. 243 Index...................................................................................................... 253

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LIST OF CONTRIBUTORS Theodore Hughes-Riley  Nottingham Trent University, Advanced Textiles Research Group, School of Art & Design, Bonington Building, Dryden St, Nottingham NG1 4GG, UK Tilak Dias  Nottingham Trent University, Advanced Textiles Research Group, School of Art & Design, Bonington Building, Dryden St, Nottingham NG1 4GG, UK Colin Cork Nottingham Trent University, Advanced Textiles Research Group, School of Art & Design, Bonington Building, Dryden St, Nottingham NG1 4GG, UK Md. Syduzzaman Department of Textile Management and Business Studies, Bangladesh university of Textiles Sarif Ullah Patwary Department of Textile Engineering, National Institute of Textile Engineering and Research, Bangladesh Kaniz Farhana Department of Apparel Manufacturing Engineering, Bangladesh university of Textiles Sharif Ahmed Department of Yarn Manufacturing Engineering, Bangladesh university of Textiles Carlos Gonçalves Institute for Polymers and Composites IPC/I3N and MIT-Portugal Program, University of Minho, 4800-058 Guimarães, Portugal Center of Nanotechnology and Smart Materials (CeNTI), 4760-034 VN Famalicão, Portugal xiii

Alexandre Ferreira da Silva Center for Micro Electro Mechanical Systems (CMEMS-UMinho) and with the MIT Portugal Program, University of Minho, Campus of Azurem, 4804-533 Guimaraes, Portugal João Gomes Center of Nanotechnology and Smart Materials (CeNTI), 4760-034 VN Famalicão, Portugal Ricardo Simoes Institute for Polymers and Composites IPC/I3N and MIT-Portugal Program, University of Minho, 4800-058 Guimarães, Portugal Polytechnic Institute of Cávado and Ave (IPCA), 4750-810 Barcelos, Portugal Yi Li Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Neil Grabham Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Russel Torah Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK John Tudor Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Steve Beeby Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK Inge Verboven Institute for Materials Research (IMO-IMOMEC)—Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Interuniversity MicroElectronics Center (IMEC), IMOMEC, Universitaire Campus—Wetenschapspark 1, 3590 Diepenbeek, Belgium xiv

Jeroen Stryckers Institute for Materials Research (IMO-IMOMEC)—Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Interuniversity MicroElectronics Center (IMEC), IMOMEC, Universitaire Campus—Wetenschapspark 1, 3590 Diepenbeek, Belgium Viktorija Mecnika Institute of Textile Technology of RWTH Aachen, Otto Blumenthal Strasse 1, 52074 Aachen, Germany Institute for Design Technology, Riga Technical University, Kalku Street 1, LV1658 Riga, Latvia Glen Vandevenne Institute for Materials Research (IMO-IMOMEC)—Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Interuniversity MicroElectronics Center (IMEC), IMOMEC, Universitaire Campus—Wetenschapspark 1, 3590 Diepenbeek, Belgium Manoj Jose Institute for Materials Research (IMO-IMOMEC)—Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Interuniversity MicroElectronics Center (IMEC), IMOMEC, Universitaire Campus—Wetenschapspark 1, 3590 Diepenbeek, Belgium Wim Deferme Institute for Materials Research (IMO-IMOMEC)—Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Institute for Design Technology, Riga Technical University, Kalku Street 1, LV1658 Riga, Latvia Flanders Make vzw, Oude Diestersebaan 133, B-3920 Lommel, Belgium Junchun Yu Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, 501 90 Borås, Sweden

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Sina Seipel Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, 501 90 Borås, Sweden Vincent A. Nierstrasz Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, 501 90 Borås, Sweden Pravin Shende  Shobhaben Pratapbhai Patel School of Pharmacy and Technology Management, SVKM’s NMIMS, Vile Parle (W), Mumbai, India Anvi Desai Shobhaben Pratapbhai Patel School of Pharmacy and Technology Management, SVKM’s NMIMS, Vile Parle (W), Mumbai, India Tadas Juknius Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, 51423 Kaunas, Lithuania Modestas Ružauskas Veterinary Academy, Lithuanian University of Health Sciences, Tilžės St. 18, 47181 Kaunas, Lithuania Tomas Tamulevičius Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, 51423 Kaunas, Lithuania Department of Physics, Kaunas University of Technology, Studentų St. 50, 51368 Kaunas, Lithuania Rita Šiugždinienė Veterinary Academy, Lithuanian University of Health Sciences, Tilžės St. 18, 47181 Kaunas, Lithuania Indrė Juknienė Veterinary Academy, Lithuanian University of Health Sciences, Tilžės St. 18, 47181 Kaunas, Lithuania

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Andrius Vasiliauskas Veterinary Academy, Lithuanian University of Health Sciences, Tilžės St. 18, 47181 Kaunas, Lithuania Aušrinė Jurkevičiūtė Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, 51423 Kaunas, Lithuania Sigitas Tamulevičius Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, 51423 Kaunas, Lithuania Department of Physics, Kaunas University of Technology, Studentų St. 50, 51368 Kaunas, Lithuania Ivona Jerkovic Department of Textile Chemistry and Ecology, University of Zagreb Faculty of Textile Technology, 10000 Zagreb, Croatia Vladan Koncar Ecole Nationale Supérieure des Arts et Industries Textiles, GEMTEX Laboratory, 59056 Roubaix, France Ana Marija Grancaric Department of Textile Chemistry and Ecology, University of Zagreb Faculty of Textile Technology, 10000 Zagreb, Croatia Kunal Mondal Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA

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LIST OF ABBREVIATIONS ACPEL

Alternating current powder electroluminescence

AAS

Atomic absorption spectroscopy

BSA

Bovine serum albumin

CNT

Carbon nanotubes

CAD

Computer Assisted Design

DLC

Diamond-like carbon

ECG

Electrocardiogram

EMG

Electromyography

ESR

Equivalent Series Resistance

FBG

Fiber Bragg Grating

FSCs

Fiber supercapacitors

FES

Functional electrical stimulation

GPS

Global Positioning Systems

ICs

Integrated chips

ICNIRP International Commission on Non-Ionizing Radiation Protection ICPs

Intrinsically Conductive Polymers

LED

Light emission diode

MIT

Massachusetts Institute of Technology

MRSA

Methicillin-resistant Staphylococcus aureus

OECT

Organic electrochemical transistor

OLED

Organic light-emitting devices

OLED

Organic light emitting diodes

OTR

Oxygen transmission rates

PSS

Polystyrene sulfonic acid

RFIDs

Radio Frequency Identification

RGO

Reduced graphene oxide

SEM

Scanning electron microscope

SRF

Self-Resonant Frequency

SWNT

Single-walled carbon nanotube

SHM

Structural Health Monitoring

SIDS

Sudden Infant Death Syndrome

TEM

Transmission electron microscope

UI

User-interface

WCA

Water contact angle

WVTR

Water vapor transmission rates

WPT

Wireless power transmission

WHO

World Health Organization

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PREFACE

Extensive research is ongoing pertaining to the development of smart textiles, which incorporate technologies to provide the wearer with enhanced functionality, owing to their advanced and tailored properties. They are also referred to as electronic textiles (E-textiles) when electronically conductive fibers or components are incorporated into the textiles. These textiles have the technology to respond to external stimuli and then translate that into data in order to respond accordingly. Therefore they are being extensively researched for applications in health monitoring, protection and safety by integrating, validating and using smart clothing and other networked mobile devices as well as projects which target the full integration of sensors/ actuators, energy sources, processing and communication within the clothes. This technology in conjugation with nanotechnology is enabling further remarkable innovations. For example coating of a fabric with nano particles has resulted in the production of high durability fabrics as well as anti-bacterial, water repellent, oil repellent, stain repellent, flame retardant, wrinkle resistant, static resistant, UV-protected and self-cleaning fabrics. All these can be achieved without compromising the breathability and tactile properties of the fabrics to make them wearable. Chapters 1, 2 and 3 are review articles detailing the advent of smart textiles and their applications in various domains such as defense, sports, medicine, and health monitoring. Chapter 1 elucidates the history of these E-textiles by categorizing them as first generation, second generation and third generation textiles based on the pathway of integrating electronics into the textiles. It highlights the different advantages and disadvantages for each pathway as well as their applications. While in chapter 2 the origin and introduction of smart textiles for sport wear, industrial purpose, automotive and entertainment applications, healthcare and safety and military has been highlighted. It also features the working mechanism of these smart textiles as well as the latest developments in smart textiles and nanotechnology. Whereas the third chapter focuses on the production techniques to seamlessly embed electronic features into traditional wearable textiles to form wearable textile based sensors for daily use without a bionic stigma.

Chapters 4, 5 and 6 are research articles showcasing examples of smart textiles. In chapter 4 a wireless inductive power transmission system obtained by screenprinting flexible coils has been reported. It eliminates power cables or batteries and makes these wearable devices less obtrusive to users. These novel screenprinted flexible coils on textiles offer a low-cost and comfortable pathway. This chapter includes the theory, design, fabrication and characterization of these flexible coils. Whereas chapter 5 demonstrates the fabrication of light emitting devices on textile substrates using different designs and printing and coating techniques. One of the approaches was to fully screen-print alternating current powder electroluminescence (ACPEL) device whereas a second approach is to incorporate organic light emitting devices. These approaches which employ very thin layers of nanometer range thickness preserve the flexibility and drapability of the substrate and their low working voltage, makes these devices the possible future in light-emitting wearables. In the sixth chapter digital inkjet printing has been reported to yield water-repellent sports and work wear without compromising the functionalized textile’s softness and breathability. This chapter also includes the characterization of the physicochemical properties of the ink such as its rheology, particle size and surface tension. Use of smart textiles in health care domain has been detailed in chapters 7, 8 and 9. While chapter 7 is a review article on the progress made in the healthcare industry using chitosan, β-cyclodextrin, fullerene and alginate based smart textiles and their application in effective wound care management, medical implantable devices and surgical products. The chapters 8 and 9 are specific applications of these smart textiles such as antibacterial bandage and sensor for in situ structural health monitoring respectively. The bandage was fabricated using diamondlike carbon with silver nanoparticle (DLC:Ag) coated synthetic silk tissue as a building block. This chapter also encompasses extensive characterization of the nanoparticles and has reported that a silver ion concentration in the protective layer below the toxic level for organism cells was able to kill more than 99.9% of all strains of bacteria after 320 min. Whereas in chapter 9 the development and electromechanical properties of a new generation textile fibrous sensors based on the conductive polymer complex poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) has been featured which can monitor in situ structural health in real time. The final chapter 10 of this book not only underpins the recent advances in the field of e-textiles but also highlights the materials and their functionalities and the role of nanotechnology in this domain. As these smart textiles can be manufactured from a plethora of materials via different fabrication routes, it is a multidisciplinary research field, incorporating expertise from several fields - textile, materials, electronics, mechanics, and computer engineering. So this book will provide the material scientists, xxii

computational researchers, mechanical engineers, electrical engineers, chemists and device engineers with information and strategies from recent research to conceptualize novel approaches that would enhance the efficacy of the current technologies and promote further innovations.

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CHAPTER

1

A HISTORICAL REVIEW OF THE DEVELOPMENT OF ELECTRONIC TEXTILES Theodore Hughes-Riley , Tilak Dias and Colin Cork Nottingham Trent University, Advanced Textiles Research Group, School of Art & Design, Bonington Building, Dryden St, Nottingham NG1 4GG, UK

ABSTRACT Textiles have been at the heart of human technological progress for thousands of years, with textile developments closely tied to key inventions that have shaped societies. The relatively recent invention of electronic textiles is set to push boundaries again and has already opened up the potential for garments relevant to defense, sports, medicine, and health monitoring. The aim of this review is to provide an overview of the key innovative pathways in the development of electronic textiles to date using sources available in the public domain regarding electronic textiles (E-textiles);

Citation: Theodore Hughes-Riley, Tilak Dias and Colin Cork, A Historical Review of the Development of Electronic Textiles, doi:10.3390/fib6020034 Copyright: © This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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this includes academic literature, commercialized products, and published patents. The literature shows that electronics can be integrated into textiles, where integration is achieved by either attaching the electronics onto the surface of a textile, electronics are added at the textile manufacturing stage, or electronics are incorporated at the yarn stage. Methods of integration can have an influence on the textiles properties such as the drapability of the textile. Keywords: electronic textiles, E-textiles, smart textiles, intelligent textiles

INTRODUCTION This historical review will provide the reader with an insight into the development and employment of electronic textiles. While some academic sources have been used, a strong focus has been placed upon patented technology and commercialized products, which are often neglected in reviews of the literature. The innovation of textiles 27,000 years ago could be contested as humanity’s invention of the first material [1]. The passing of the millennia has consolidated humanity’s need of textiles either to be protected from the environment, or desire to outwardly convey a message about themselves; whether that be artistic, stylistic, or wealth-related. The creation of textiles has therefore been coupled closely with key inventions that have shaped society; the knitting frame by William Lee in 1589 [2], the flying shuttle by John Kay in 1733 and the spinning jenny by James Hargreaves around 1765 [3], and set the foundation for the first industrial revolution. A new revolution is now underway where the most widely used material by humankind has gained new functionality with the incorporation of electronic components. The first examples of electronic textiles date back to the use of illuminated headbands in the ballet La Farandole in 1883 [4]. More recently advances have appeared due to the reducing size and cost of electronic components, as well as an increased complexity of smallscale electronics, and have begun to show the true scope of possibilities for integrating electronics with clothing. The growth of E-textiles in the later part of the 20th Century was due to a series of developments in material science and electronics further expanding the potential scope for embedding electronics within clothing.

A Historical Review of the Development of Electronic Textiles

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The conductive polymer was a key innovation; invented by Heeger et al. in 1977 [5] this led to a Nobel Prize thirty-three years later [6]. A patent for this type of technology for use with textiles was granted shortly after its creation [7]. Another critical development were advancements in transistor technology, with the creation of the first MOS (metal–oxide–semiconductor field-effect transistor) in 1960 [8]. The use of transistor-based electronics were outlined in a patent describing illuminated clothing from 1979 [9]. For a greater adoption of E-textiles a better level of integration of the electronic components was required. Key patents from 2005, 2016 and 2017 described the encapsulation of semi-conductor devices within the fibers of yarns [10,11,12]. This represented the start of the work on electronically functional yarns. Three different pathways have been used to integrate electronics into textiles. These three distinct generations of electronic textiles are adding electronics or circuitry to a garment (first generation), functional fabrics such as sensors and switches (second generation), and functional yarns (third generation). Prior to the creation of E-textiles there are also many examples of the use of conductive fibers in textile fabrication, going back as far as the second century [13]. A timeline showing the evolution of E-textiles is given by Figure 1.

Figure 1:  A timeline of the different generations of electronic textiles. This timeline shows when significant interest in the technology began, and not earlier, isolated instances.

Each method of integration will have an influence on the textile properties such as the shear properties of the textile, or its flexibility, both of which effect the drapability. Figure 2 shows examples of each generation of electronic textiles.

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Figure 2: Photographs showing contemporary examples of each generation of electronic textile. (Left) An Adafruit coin cell battery holder. The first generation saw devices affixed to textiles. (Middle) A knitted electrode. The second generation of electronic textiles describes functional fabrics where conductive elements are integrated into a textile. (Right) An example of functional yarns (in this case LED yarns). The third generation of electronic textiles describe electronics embedded into textiles at a yarn level.

For the purposes of this review it is important to disentangle the various terms used loosely in the field of advanced textiles. Electronic textiles will be discussed; the strict definition of electronic textiles are where electronically conductive fibers or components are incorporated into a textile (electronic textiles will be referred to as ‘E-textiles’ in the subsequent text). Here, the term smart textiles, often anachronistically used as a synonym for electronic or E-textiles, will not be employed unless the textile has some kind of intelligence. Critically, this historical review of the literature has a strong focus on innovation including patented technology and commercialized products, making heavy use of patents and internet sources. Non-internet sources have been referenced wherever possible; however, in many cases, particularly for products, only websites exist. These are areas that are typically neglected and this review serves as a complimentary piece to other more traditional reviews with the academic literature as the focus. As a historic review this work will principally focus on research prior to 2010. Interest in electronic textiles has increased significantly since 2010. A search of the literature using the term ‘electronic textiles’ yields 310,000 results of which 254,000 are between 2010 and 2018. For completeness some more contemporary resources will be examined, for example in the area of textile energy solutions, which has largely evolved since 2010. There are several excellent reviews covering more recent developments in electronic textiles [14,15,16] and recent books on this subject [17,18]. The aim of this review is to overview the key pathways to the development of electronic textiles. The review is structured over ten sections, with the

A Historical Review of the Development of Electronic Textiles

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seven sections covering examples from a specific types of E-textile or application, such as sensors or illumination. Each section covers a major area of research in the literature, for example, temperature control textiles. In some cases two areas of interest have been grouped together such as E-textile pressure sensors and textile switches as one of these technologies led to the other. Three final sections discuss market trends, possible future developments for E-textiles, and a brief conclusion.

TEMPERATURE CONTROL TEXTILES It is unsurprising that the use of embedded conductive fibers was employed in one of the first true E-textiles, the electrically heated glove patented in 1910 [19]. This invention used ohmic heating (Joule heating, or resistive heating [20]), where an electric current was run through the conductive fibers and the electric resistance in the wires led to a heating effect. This type of heating was the core application of many early E-textiles. Other patents refining the heated gloves were filed [21,22], and variations on the theme appeared such as the heated boot [23], and full heated garments including a jacket element [24,25]. The knitted heater was patented in 1910 [26]. Heated gloves and jackets based on these core concepts are still available today showing the relevance of these early innovations. Other devices utilizing heating elements were created between the 1930s and 1970s included the heated blanket [27], a heated baby carriage blanket [28], and electrically heated socks [29]. The year 1968 is often seen as the birth of modern E-textiles, when the Museum of Contemporary Craft in New York City held an exhibition, Body Covering, exploring a series of electric garments with functions such as heating and cooling [30]. Interest in developing new heated textiles remained well into the 21st Century with a patent from 2008 describing the incorporation of inductive heating elements into footwear and apparel [31]. Commercially, EXO Technologies have produced heated gloves that can be used by militaries or for a variety of outdoor activities such as skiing or motorcycling [32]. Marktek also make conductive textiles [33] that include heating products. WarmX produce heated clothing [34]. In addition to heating, a paper in 2012 described a wearable textile-based cooling system using thermoelectric modules and refrigerant channels [35].

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MATERIALS DEVELOPMENTS AND WEARABLE COMPUTING Wearable computing is a type of wearable that contains information technology, and is able to store, manipulate, or transmit data. The inclusion of wearable computing is what makes an E-textile a smart textile. The 1990s and early 2000s saw patents for devices either integrated onto the surface of garments or contained within pockets beginning to emerge [36,37,38,39,40,41,42]. These are generally regarded as the first generation E-textiles, where an electrical circuit or electronic components were attached to a garment. The initial commercialized ‘first generation E-textile’ garments began to appear on the market with the Industrial Clothing Division plusjacket in 2000 [43]. This jacket attached electronic devices into pockets. An important component towards the successful development of wearable computing included methods of creating electronic circuits within textiles. Post et al. filed a patent in the late 1990s describing how to integrate devices and circuits into textiles [44]. Another patent described using electrically insulating and electrically conductive yarns woven into a garment to create an electronic circuit [45]. A later patent further built on the use of knitting techniques to create electrical circuits and pathways [46]. In 2006 Ghosh et al. described an in-depth means of forming electrical circuits within textile structures [47]. Textilma were granted a patent describing an elastic compound thread with electrical conductivity [48]. A patent two years later from Seoul National University described a system for power transmission using conductive sewing thread [49]. An E-textile patents published in 2006 discussed the inclusion of devices such as Bluetooth and the electrical connections required between components [50]. With the concept of wearable computing existing for a number of years, in 2006 Frost and Sullivan provided an excellent review of wearable computing at that time [51]. A variety of other developments linked to wearable computing appeared in the 2000s. WearIT@work [52] was a project funded by the European Commission (€14.6 million) to investigate wearable computing. A particular focus was user acceptance. The European Space Agency (ESA) iGarment project was developed to create an Integrated System for Management of Civil Protection Units. The project was targeted to make a full-body smart garment. This would have incorporated sensors for monitoring vital signs and position. The garment was also to have included a communications unit

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and GPS [53]. Some recent developments in wearable computing have veered away from textiles including the Google Glass [54]. Despite these devices being wearables and not textiles, textile computing has still made significant strides in recent years. Graphene technology may be a key contributor to the further development of wearable computing. The US Army Research Laboratory discussed the potential of graphene-based nano-electronics for applications in wearable electronics in a report produced in 2012 [55]. Interfacing with E-textiles is also of importance for future developments: A recent Microsoft patent described an interface system for using muscle movements to control a computer or other device [56]. The use of organic conductive polymers may also improve the potential for wearable computing. Hamedi et al. have developed a fiber-level electrochemical transistor, opening up the potential to create larger circuits using a weave of these fibers [57].

SENSORS The development of E-textiles in an academic setting first gained traction in the late 1990s with a series of publications from the Massachusetts Institute of Technology (MIT) and the Georgia Institute of Technology [58]. The ‘The Wearable MotherboardTM’ proposed a smart-shirt capable of unobstructively monitoring human life signs [59]. Work by Farringdon et al. described a functional fabric in the form of a woven stretch sensors later 1990s. They produced a fabric stretch sensors for the monitoring of body movement [60]. A master’s thesis from the US Naval Postgraduate School in 2006 identified a further potential use of wearable sensors, suggesting that they may prove beneficial to locate sniper fire in combat situations [61]. Health and wellbeing applications also gained interest in the 2000s, with Cooseman et al. [62] describing a garment with an embedded patient monitoring system, which included wireless communication and inductive powering. In this instance, the garment was designed for infants. A paper in 2008 described a process for transforming cotton thread for use in E-textiles by applying carbon nanotubes [63]. It was claimed that the technology

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could be used for bio-sensing with a key result of the paper being that the carbon nano-tube cotton threads could be used to detect albumin, which is an important protein in the blood. Work has investigated the use of textile electrodes for heart rate monitoring or ECG (Electrocardiogram). MyHeart was an EU (FP 6) funded project to develop smart fabrics for both ECG and respiration [64], with the project concluding in 2009. Another EU FP 6 project, CONTEXT, also investigated the use of textile electrodes, in this case to measure muscle and heart electric signals [65]. A paper from 2009 described an integrated temperature sensor [66]. Also in 2009, a report described the development of a knitted biomedical sensor for the monitoring body temperature [67]. A patent was granted for a linear electronic transducer for strain measurement in 2011 [68]. A proliferation of sensor embedded garments began to emerge on the market with the company Clothing+ producing both the Sensor Belt [69] and the Pure Lime sensor bra. Both devices focused on the active-wear market. At the time of writing it is unclear whether the Pure Lime sensor bra is still in production. Other active-ware devices have included the Adidas MiCoach heartrate monitoring bra. Despite interest by a number of media outlets [70], at the time of writing it appears that this product is no longer sold by Adidas. The incorporation of sensors into garments has also continued to generate interest in the literature and from companies. A 2011 paper from described the development of a socks with integrated strain sensors for monitoring foot movement. Such systems could have had applications in stroke rehabilitation [71]. There have also been other example of incorporating temperature-sensing elements into textiles [72,73,74,75]. A variety of textile-based sensors and sensing garments have also appeared on the market. Nypro (formerly Clothing+) produces textilebased sensor systems [76]. Ohmatex [77] has created textile conductors and sensors. Polar [78] sell wearable monitoring equipment, including heartrate monitors. SmartLife [79] produce knitted physiological measurement devices for healthcare, sports and military applications. The Zephyr BioHarness [80] takes physiological meassurements from the wearer. The recorded data can then be transmitted. Under Armour have produced a shirt that monitors biometric data [81].

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E-TEXTILE PRESSURE SENSORS AND TEXTILE SWITCHES Many early functional fabrics took the form of textile switches [82] and this area of research was expanded upon heavily by academia. The work of Post et al. included the development of a textile-based keyboard using embroidered electrodes with a silk and twisted gold spacer fabric [83]. This technology was the first step forwards pressure sensitive fabrics. Further developments into textile transducers came in a paper from 2004 which described knitted transducers for motion and gesture capture together with ECG (electrocardiogram) measurement [84], which was also patented [85]. A related paper from 2005 described knitted capacitive transducers for touch and proximity sensing [86]. Sergio et al. initially proposed a textile based capacitive pressure sensor where a three-layer structure was implemented with a layer of rows of conductive fibers, an elastic spacer foam, and then another layer of conductive fibers orientated perpendicular to the top layer [87]. Their work described four methods for producing the conductive tracks; weaving in a mix of conductive and insulating fibers, embroidering the circuit using conductive thread, textile electrodes separated by conducting strips (called bundle routing in the paper), and the use of conductive paint. A follow-up paper described a measurement system in combination with their E-textile that was capable of producing pressure images [88]. Other researchers also developed this type of technology. Mannsfeld et al. developed a highly flexible and inexpensive capacitive pressure sensor using a micro-structured thin film as the capacitors dielectric layer [89]. Takamatsu et al. fabricated a pressure sensitive textile using a perfluoropolymer spacer, and rows of woven, die-coated yarns to make conductive rows [90]. Meyer et al. have contributed to the development of textile pressure sensors. Their general pressure sensor design consisted of three parts, an embroidered set of 2 cm × 2 cm electrodes, a 3D-knitted spacer fabric, and a woven back electrode [91,92]. Hoffmann et al. used a similar principle for a system to measure respiratory rate, where two conductive fabrics were placed on either side of a 3D-spacer textile [93]. A different proposal to monitor respiratory rate has used the change in induction of two knitted coils as the coils moved [94]. Holleczek et al. created a sensor using a pair of textile electrodes and a proprietary resin spacer material, integrating the sensor into socks [95]. Other capacitive textile designs have included a capacitive fiber by Gu et al.

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[96]. The capacitive fiber consisted of a conducting copper wire (0.12 mm diameter) embedded within a fiber. A similar design was employed by Lee et al. where a conductive coating was applied to a Kevlar fiber, which was then coated in a Polydimethylsiloxane (dielectric) layer. Capacitive junctions seen where the two fibers intersected [97]. Other copper wires were wrapped around the fibers surface acting as the second electrode. Pressure sensing textiles have not been limited to a research environment. Commercially, Novel sell a number of pressure sensing products including insoles, seating covers, and gloves [98]. The XSENSOR® Technology Corporation have focused on seating sensors (in particular for the automotive industry), health care monitoring, and sleep solutions using pressure sensitive mats to aid in mattresses selection [99]. Their technology has been implemented in a variety of medical studies, with a particular focus of managing pressure ulcers [100,101,102,103]. Pressure Profile Systems continue to sell capacitive pressure sensors as of mid-2016 for a variety of applications, including gloves, robotics, pressure mats, and medical applications [104]. LG have recently announced the release of a flexible textile sensor based on capacitive technology [105]. It is difficult to ascertain the exact make-up of many of these sensors and they may not be true E-textiles. Certainly, Peterson et al. stated that the XSENSOR® devices use proprietary capacitive technology, and described the sensor as a flexible thin pad [100]. The simpler textile switch also found its way into commercial products. In 2006 an intelligent push-button system was described in a patent [106]. Beyond this a patent in 2006 [107], followed by an article [108] described a fully integrated textile switch. Other patents by France Telecom (Now Orange S.A., Paris, France)) [109], Daimler Chrysler (Auburn Hills, MI, USA) [110] and Sentrix (New York, NY, USA) [111] also describe alternative textile switch technologies. The Burton Amp snowboarding jacket saw integrated textile switches on the arm of the jacket used to control an Apple iPod in 2002 [112]. Nike + iPod Sports Kit was another example of Apple engaging with sports apparel companies to create an E-textile device. In this case a sensor was incorporated into the shoe which communicated with an Apple product (such as an iPhone) or other Nike wearables (such as the Nike + Sportband) to track activity [113]. The recent Google and Levi’s Jacquard jacket also included textile switches [114].

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TEXTILE ENERGY SOLUTIONS By the early 2000s the first patent for textile-based energy harvesting appeared in the form of a mechanical generator scavenging energy through motion [115]. A paper by Qin et al. in 2007 described a technique for energy scavenging using piezoelectric zinc oxide nanowires grown radially around textile fibers [116]; this being one of the earlier examples of energy scavenging within textiles. The further development of energy storage and scavenging within textiles became a key area of interest in the 2010s as the viability of many of E-textile products and concepts are dependent on a suitable power supply. Existing textile energy scavenging (or energy harvesting) devices exploit either thermoelectrics, kinetics, or photovoltaics. Each system possesses different advantages or disadvantages, with many modern systems incorporated into textiles providing flexibility but lacking other textile properties such as bend or shear. In all cases the harvested power was minimal, with the most promising generators capable of producing sustained power on the order of milliwatts. Both thermoelectric and kinetic systems draw energy from the wearer while photovoltaic energy harvesting draws energy from light sources, such as the sun. Photovoltaics have also shown significant promise concerning the power that can be generated (~30 mW/m2) [117]. The use of carbon-nano tube based systems have gained significant popularity for both energy harvesting and storage [117,118,119,120,121]. For an energy storage device the textile can be used as a substrate for flexible films, or a carbon nano-tube infused film can be used to produce yarns [120]. Storage technology is either capacitive (normally supercapacitors) or chemical in nature. The use of carbon nano-tubes has caused some worry due to safety concerns [122], which would possibly impede future commercialization. Another popular storage method has seen flexible, solid electrolytebased batteries [123] woven into a garment as thin strips, however at present these batteries are still large relative to a normal yarn (width = 10 mm). Triboelectric nano-generators are also viewed as a potential source of energy for wearables given their very small size, high peak power densities, and good energy conversion efficiencies [124]. Triboelectric generators convert mechanical energy and therefore could be powered by human motion or vibration. Despite high energy densities, the converted energy has a small current and the generators energy output over time is unpredictable, requiring complex supporting power management electronics. Cui et al.

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have demonstrated a cloth-based triboelectric generator where frictional forces between the forearm of the wearer and their body was used to generate energy [125]. Comprehensive reviews of energy harvesting and storage in textiles are available elsewhere [14,120].

COMMUNICATION DEVICES A textile-based antenna fabricated from polymer (polypyrrole) strips has been described in the literature [126]. Two conference papers from 2010 have also describe conformable antennas for space suits [127,128]. The production of flexible embroidered antennas has been reported as suitable for megahertz frequency communications [129,130]. This type of antenna is highly sensitive and has been employed for unilateral magnetic resonance measurements [131]. Incorporating radio frequency identification (commonly known as RFID) tags into textiles has also been investigated by a number of entities. Textilma were granted a patent describing an RFID module textile tag [132]. Another patent described a method of attaching RFID chips to a textile substrate [133] however, full integration within yarns was not proposed. A patent from 2007 also described a RFID device, this time focusing on clothing [134]. In 2005 a patent described the incorporation of RFID devices within epoxy resin for use in laundries [135]. A patent from 2010 described an RFID tag with integrated antenna [136] whilst another patent from the same year described a method of incorporating a RFID device into a textile tag [137].

ILLUMINATION Interest in illuminated textiles continued into the mid-2000s with a patent from Daimler Chrysler in 2005 describing a textile-based lighting system for automotive applications [138]. A patent described creating a flat-panel video display by weaving electronically conductive fibers, such as dielectrics [139]. This led to patents being filed for other textile-based flexible displays in later years [140,141]. Other patents granted described a lighting system that used light leakage from optical fibers in specific locations [142] to create illumination. A patent from 2011 described a method of producing an illuminated pattern using light conducting fibres [143]. Another illuminated textile

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using optical fibres was described in a patent from the same year [144]. The company Sensing Tex Sl have used optical fibers to illuminate textiles [145]. In contrast, Philips have produced illuminated textiles using LED technology [146]. Philips have previously patented a flexible electro-optic filament [147]. There are a variety of companies that produce illuminated clothing for fashion applications, as well as use in performance (i.e. theatre), these include Cutecircuit [148] and LUcentury [149] which create garments by sewing LEDs onto existing clothing. Given advances in LED technology, reducing their size, LEDs can easily be incorporated into yarns using functional electronic yarns technology, and subsequently into garments [75]. Electroluminescent yarn was also developed on 2010 [150] with the technology fully described in a 2012 paper [151]. Another form of lighting used for E-textiles was by attaching lasers to a garment, as shown by Bono in 2009 [152]. This technology is not practical for a mass produced garment for a variety of reasons including cost and the weight of the final garment.

MARKET TRENDS According to IMS Research (Wellingborough, UK), 14 million wearable devices were shipped in 2011 [153]. This clear market coupled with the significant scientific advances in E-textiles led to a proliferation of devices entering the market in the early 2010’s, and an increased interest in the technology from academia. More recently the Fung Global Retail and Technology report on wearables in 2016 [154] identified a significant increase in the wearable market between 2015 and 2016, a 18.4% climb to $28.7 billion. This is far higher than the predictions of the IMS, expecting that the revenue for wearable technology would be $6 billion in 2016. Forbes currently predict that the wearable market will reach $34 billion by 2020 [155]. It is of interest to note that the Fung report also states that the top wearable brands of 2016 were Fitbit, Xiaomi, and Apple based on the number of units shipped: These company’s products are not textile based. The principal wearable device sold by each company are smart watches. It is unclear whether the growing market for wearables will become more focused on textile-based devices in the coming years, but the advantages offered, such as comfort to the wearer, will likely be an influential factor. The current market for E-textiles generates sales of around $100 million per year, with some sources predicting a $5 billion market by 2027 [156]. Current commercially available E-textiles include soft textile switches produced by International Fashion Machines (Seattle, WA, USA) [157] and

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systems for incorporating wires into clothing by the Technology Enabled Clothing [158]. Additionally, Fibretronic [159] have created a varied range of products including (but not limited to) flexible switches, textile cables for signal or power transport, and textile sensors, however as of 2018 it is unclear whether they are still trading. As discussed earlier some sportswear E-textile products are no longer available, presumably due to poor market demand. An important thing to note is that a significant number of website sources have been using in this review, particularly regarding commercial products, as information was not available from other sources. This is another possible indicator of poor uptake of certain products.

POTENTIAL FUTURE DEVELOPMENTS While this review is focused on the history and evolution of E-textiles, it is of interest to consider the direction that E-textile research will take in upcoming years. Originally, the vision of those working in the field of E-textiles was to incorporate all of the required electronic systems within the textile. More recently however some claim that the best approach is to use mobile phones as an interface [160]; which has been aided in part by the substantial advancement in mobile phone technology in recent years. Others claim that this approach is a temporary diversion and there are many advantages for fully embedded systems, and that as developments progress, the mobile phone itself will be integrated into textiles. A complete integration of the electronics may also be unfavorable for sustainability reasons as it makes the electronics more difficult to remove at the end of the life of a product [161]. It is expected that there will be a far greater uptake of wearable electronics when battery technology is improved or alternative energy sources, such as energy scavenging, become more viable. This is of particular importance to E-textiles over wearables more generally, as most conventional power sources are not well suited to textile integration due to size, inflexibility, and lack of washability. The reductions in size and cost of components will promote further development and uptake. A review paper from 2012 discussed the subject [162] and a BBC news item outlined on-going UK research in the area [163]. Ultimately, the adoption of E-textiles will depend on the cost. This will reduce with material costs and improvements to the manufacturing processes. It is also possible that developments in graphene technology will

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be improve the potential of what can be achieved with electronic textiles. Many major companies including Samsung, Nokia, and IBM have made significant investments into graphene technology [164]. There is the potential that graphene’s physical properties, including its strength and electrical conductivity, will allow it to replace silicon in many devices, possibly by the late 2020s after the technology has matured. In addition, work on carbon nanotubes is beginning to show some promise, especially for energy-based applications (such as energy harvesting and scavenging) [117,119,120,121]. Both technologies offer potential for further miniaturization of embedded electronics. With an enhancement of how much can be fit within a textile, and suitable energy solutions, E-textiles could move towards true wearable computing, with the textile managing and processing data on its own depending upon requirements. The decreasing size of microprocessors makes embedding this kind of intelligence within a textile likely in the immediate future.

CONCLUSIONS This review of the literature has clearly shown that the three pathways of integrating electronics into textiles have been applied in different ways. The methods of integrating electronics offer different advantages and disadvantages. The first generation E-textiles will always interfere with the textile properties of a garment, even thin film devices (while flexible), will not possess the shear properties of a normal textile. The second generation textiles may retain a textile feel but are limited in their applications; such as the creation of electronic pathways, and electrode-based sensing. The third generation of E-textiles, where electronics are contained within the yarn structure, do not interfere with the textile properties of a fabric. As this technology is principally limited by the size of the incorporated electronics (i.e., the electronic chip dimensions) the potential of this area will grow as smaller electronic chips become available. While the history of E-textiles has shown the development of new techniques to integrate electronics within a textile it is likely that the existing three methods will remain in use into the future. The attachment of electronics onto a garment is still common, in particular for illuminated textiles, despite this technology first being demonstrated in 1883.

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AUTHOR CONTRIBUTIONSS T.H.-R., C.C. and T.D. located the reference material used in the review. T.H.-R. and T.D. reviewed all of the reference material used in this review. T.H.-R. prepared the final manuscript.

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79. Smartlife. Available online: https://www.smartlifeinc.com/ (accessed on 22 August 2017). 80. Zephyr™ Performance Systems. Performance Monitoring Technology. Available online: https://www.zephyranywhere.com/ (accessed on 22 August 2017). 81. Under Armour-Sportswear, Sport Shoes, & Accessories. Available online: http://www.underarmour.co.uk/ (accessed on 22 August 2017). 82. Schwabe, D. Electrical Switch for Automotive or Clothing Use is Integrated into a Flexible Textile Material and is Actuated by Applied Force. DE102005038988 (A1), 22 February 2007. 83. Post, E.R.; Orth, M.; Russo, P.R.; Gershenfeld, N. E-broidery: Design and fabrication of textile-based computing. IBM Syst. J. 2000, 39, 840– 860. 84. Wijesiriwardana, R.; Mitcham, K.; Dias, T. Fibre-meshed transducers based real time wearable physiological information monitoring system. In Proceedings of the ISWC 2004. Eighth International Symposium on Wearable Computers, Arlington, VA, USA, 31 October–3 November 2004; Volume 1, pp. 40–47. 85. Tilak, D.; Beatty, P.C.W.; Cooke, W.; Wijesiriwardana, R.; Mitcham, K.; Mukhopadhyay, S.; Hurley, W. Knitted Transducer Devices. U.S. Patent Application 10/557,074, 19 May 2004. 86. Wijesiriwardana, R.; Mitcham, K.; Hurley, W.; Dias, T. Capacitive fibermeshed transducers for touch and proximity-sensing applications. IEEE Sens. J. 2005, 5, 989–994. 87. Sergio, M.; Manaresi, N.; Tartagni, M.; Guerrieri, R.; Canegallo, R. A textile based capacitive pressure sensor. In Proceedings of the IEEE Sensors, Orlando, FL, USA, 12–14 June 2002; Volume 2, pp. 1625– 1630. 88. Sergio, M.; Manaresi, N.; Campi, F.; Canegallo, R.; Tartagni, M.; Guerrieri, R. A dynamically reconfigurable monolithic CMOS pressure sensor for smart fabric. IEEE J. Solid-State Circuits 2003, 38, 966–975. 89. Mannsfeld, S.C.; Tee, B.C.; Stoltenberg, R.M.; Chen, C.V.H.; Barman, S.; Muir, B.V.; Sokolov, A.N.; Reese, C.; Bao, Z. Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nat. Mater. 2010, 9, 859–864. 90. Takamatsu, S.; Kobayashi, T.; Shibayama, N.; Miyake, K.; Itoh, T. Fabric pressure sensor array fabricated with die-coating and weaving

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techniques. Sens. Actuators A Phys. 2012, 184, 57–63. 91. Meyer, J.; Lukowicz, P.; Troster, G. Textile pressure sensor for muscle activity and motion detection. In Proceedings of the 10th IEEE International Symposium on Wearable Computers, Montreux, Switzerland, 11–14 October 2006; pp. 69–72. 92. Meyer, J.; Arnrich, B.; Schumm, J.; Troster, G. Design and modeling of a textile pressure sensor for sitting posture classification. IEEE Sens. J. 2010, 10, 1391–1398. 93. Hoffmann, T.; Eilebrecht, B.; Leonhardt, S. Respiratory monitoring system on the basis of capacitive textile force sensors. IEEE Sens. J. 2011, 11, 1112–1119. 94. Wijesiriwardana, R. Inductive fiber-meshed strain and displacement transducers for respiratory measuring systems and motion capturing systems. IEEE Sens. J. 2006, 6, 571–579. 95. Holleczek, T.; Rüegg, A.; Harms, H.; Tröster, G. Textile pressure sensors for sports applications. In Proceedings of the IEEE Sensors, Kona, HI, USA, 1–4 November 2010; pp. 732–737. 96. Gu, J.F.; Gorgutsa, S.; Skorobogatiy, M. A Fully Woven Touchpad Sensor Based on Soft Capacitor Fibers. Unpublished. 2011. Available online:  https://www.researchgate.net/profile/Maksim_Skorobogatiy/ publication/51912313_A_fully_woven_touchpad_sensor_based_ on_soft_capacitor_fibers/links/00b4951cad5dfa7912000000. pdf (accessed on 4 May 2016). 97. Lee, J.; Kwon, H.; Seo, J.; Shin, S.; Koo, J.H.; Pang, C.; Son, S.; Kim, J.H.; Jang, Y.H.; Kim, D.E.; et al. Conductive Fiber-Based Ultrasensitive Textile Pressure Sensor for Wearable Electronics. Adv. Mater. 2015, 27, 2433–2439. 98. Sensors. Available online: http://www.novel.de/novelcontent/ sensors (accessed on 21 August 2017). 99. Xsensor Technology. Available online: http://www.xsensor. com (accessed on 21 August 2017). 100. Peterson, M.J.; Gravenstein, N.; Schwab, W.K.; van Oostrom, J.H.; Caruso, L.J. Patient repositioning and pressure ulcer risk-monitoring interface pressures of at-risk patients. J. Rehabil. Res. Dev. 2013, 50, 477. 101. Kahn, J.A.; Kerrigan, M.V.; Gutmann, J.M.; Harrow, J.J. Pressure ulcer risk of patient handling sling use. J. Rehabil. Res. Dev. 2015, 52, 291.

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102. Wong, H.; Kaufman, J.; Baylis, B.; Conly, J.M.; Hogan, D.B.; Stelfox, H.T.; Southern, D.A.; Ghali, W.A.; Ho, C.H. Efficacy of a pressuresensing mattress cover system for reducing interface pressure: Study protocol for a randomized controlled trial. Trials 2015, 16, 1. 103. Higer, S.; James, T. Interface pressure mapping pilot study to select surfaces that effectively redistribute pediatric occipital pressure.  J. Tissue Viability 2016, 25, 41–49. 104. Capacitive Tactile Pressure Sensors. Available online: http://www. pressureprofile.com/capacitive-sensors (accessed on 21 August 2017). 105. LG Innotek Unveils Flexible Textile Pressure Sensors. Available online:  http://m.phys.org/news/2016-07-lg-innotek-unveils-flexibletextile.html?utm_source=nwletter&utm_medium=email&utm_ campaign=daily-nwletter (accessed on 21 August 2017). 106. Schedukat, N.; Gries, T. Intelligent Push-Button System for Use in Smart Textile, Has Upper and Lower Push-Button Halves with Two Electric Contacts Connected with One Another Electro-Conductively for Data, Signal and Power Transmission, While Closing Connection. DE102004026554, 16 March 2006. 107. Dias, T.; Hurley, W.; Wijesiriwardana, R. Switches in Textile Structures. WO2006045988, 4 May 2006. 108. Dias, T.; Hurley, W.; Monaragala, R.; Wijeyesiriwardana, R. Development of Electrically Active Textiles. In Advances in Science and Technology; Trans Tech Publications: Zürich, Switzerland, 2008; Volume 60. 109. Deflin, E.; Weill, A.; Bonfiglio, J.; Athimon-Pillard, B. Flexible Textile Structure for Producing Electric Switches. WO03050832, 19 June 2003. 110. Kuebler, S.; Seidel, F.-P. Textile with Built-in Electrical Switches is Used as Internal Lining or Seat Covering in Vehicles. DE102004009189, 15 September 2005. 111. Leftly, S.A. Switches and Devices for Textile Articles. WO2006030230, 23 March 2006. 112. Greenfield, A.  Readings from Everyware: The dawning age of Ubiquitous Computing; New Rider: San Francisco, CA, USA, 2006. 113. Nike + iPod Sensor. Available online: https://manuals.info.apple. com/MANUALS/1000/MA1139/en_US/nike_plus_ipod_sensor. pdf (accessed on 8 August 2017).

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114. Jacquard by Google. Available online: https://atap.google.com/ jacquard/(accessed on 17 November 2017). 115. Muglia, H.A.; Refeld, J.; Eiselt, H. Generator Device for Converting Motion Energy of Person’s Respiration into Electrical Energy is Integrated into Clothing Item Normally Arranged at One or More Positions on Person that Undergoes Change in Dimensions during Respiration. DE10340873, 28 April 2005. 116. Qin, Y.; Wang, X.; Wang, Z.L. Microfibre-nanowire hybrid structure for energy scavenging. Nature 2008, 451, 809–813. 117. Velten, J.; Kuanyshbekova, Z.; Göktepe, Ö.; Göktepe, F.; Zakhidov, A. Weavable dye sensitized solar cells exploiting carbon nanotube yarns. Appl. Phys. Lett. 2013, 102, 203902. 118. Uddin, M.J.; Davies, B.; Dickens, T.J.; Okoli, O.I. Self-aligned carbon nanotubes yarns (CNY) with efficient optoelectronic interface for microyarn shaped 3D photovoltaic cells. Solar Energy Mater. Solar Cells 2013, 115, 166–171. 119. Meng, Y.; Zhao, Y.; Hu, C.; Cheng, H.; Hu, Y.; Zhang, Z.; Shi, G.; Qu, L. All-graphene core-sheath microfibers for all-solid-state, stretchable fibriform supercapacitors and wearable electronic textiles.  Adv. Mater. 2013, 25, 2326–2331. 120. Jost, K.; Dion, G.; Gogotsi, Y. Textile energy storage in perspective. J. Mater. Chem. A 2014, 2, 10776–10787. 121. Zhang, D.; Miao, M.; Niu, H.; Wie, Z. Core-spun carbon nanotube yarn supercapacitors for wearable electronic textiles.  Acs Nano  2014,  8, 4571–4579. 122. Greenemeier, L. Study says carbon nanotubes as dangerous as asbestos. Sci. Am.  2008,  20. Available online: https://www. scientificamerican.com/article/carbon-nanotube-danger/  (accessed on 8 August 2017). 123. Liu, Y.; Gorgutsa, S.; Santato, C.; Skorobogatiy, M. Flexible, solid electrolyte-based lithium battery composed of LiFePO4 cathode and Li4Ti5O12 anode for applications in smart textiles. J. Electrochem. Soc. 2012, 159, A349–A356. 124. Fan, F.R.; Tian, Z.Q.; Wang, Z.L. Flexible triboelectric generator. Nano Energy2012, 1, 328–334. 125. Cui, N.; Liu, J.; Gu, L.; Bai, S.; Chen, X.; Qin, Y. Wearable triboelectric generator for powering the portable electronic devices. ACS Appl.

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Mater. Interfaces 2015, 7, 18225–18230. 126. Pillai, P.; Paster, E.; Montemayor, L.; Benson, C.; Hunter, I.W. Development of Soldier Conformable Antennae Using Conducting Polymers; Massachusetts Inst of Tech Cambridge Institute for Soldier Nanotechnologies (ISN): Cambridge, MA, USA, 2010. 127. Campbell, T.G.; Hearn, C.W.; Reddy, C.J.; Boyd, R.C.; Yang, T.; Davis, W.A.; Persans, A.; Scarborough, S. Development of Conformal Space Suit Antennas for Enhanced EVA Communications and Wearable Computer Applications. In Proceedings of the 2010 Antenna Applications Symposium Volume II of II, Tangshan, China, 15–18 October 2010. 128. Yang, T.; Davis, W.A.; Campbell, T.G.; Reddy, C.J. A LowProfile Antenna Design Approach for Conformal Space Suit and Other Wearable Applications. In Proceedings of the 2010 Antenna Applications Symposium Volume II of II, Monticello, IL, USA, 21–23 September 2010. 129. Acti, T.; Zhang, S.; Chauraya, A.; Whittow, W.; Seager, R.; Dias, T.; Vardaxoglou, Y. High performance flexible fabric electronics for megahertz frequency communications. In Proceedings of the Antennas and Propagation Conference (LAPC), 2011 Loughborough, Loughborough, UK, 14–15 November 2011; pp. 1–4. 130. Chauraya, A.; Zhang, S.; Whittow, W.; Acti, T.; Seager, R.; Dias, T.; Vardaxoglou, Y.C. Addressing the challenges of fabricating microwave antennas using conductive threads. In Proceedings of the 6th European Conference on Antennas and Propagation (EUCAP), Prague, Czech Republic, 26–30 March 2012; pp. 1365–1367. 131. Morris, R.H.; McHale, G.; Dias, T.; Newton, M.I. Embroidered coils for magnetic resonance sensors. Electronics 2013, 2, 168–177. 132. Speich, F. RFID Transponder Chip Module with Connecting Means for an Antenna, Textile Tag with an RFID Transponder Chip Module, and Use of an RFID Transponder Chip Module. TW200905574, 1 February 2009. 133. Muehlbauer, A.G. Method for Attaching and Contacting RFID Chip Modules to Produce Transponders Comprising a Textile Substrate, and Transponder for Fabrics. WO2007104634, 20 September 2007. 134. Corbett, B.G. Textile Identification System with RFID Tracking. US2005183990, 25 August 2005.

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135. Gravina, D. Method Using RFID Technology for Surveillance of Textile Goods in Laundries. EP1528504, 4 May 2005. 136. Shpajkh, F. Textile RFID Label. RU2009114415, 27 October 2010. 137. Speich, F. Method for the Production of a Textile Label Having an RFID Transponder Chip and Interlaced Information Carrier, and System for Carrying out the Method. US2010085166, 8 April 2010. 138. Boll, W. Illumination System for Automobile Passenger Compartment e.g., for Cabriolet Automobile, Using Flexible Light Conductors or Electrical Lighting Devices Incorporated in Textile Material Forming Automobile Roof. DE10345002, 21 April 2005. 139. Christensen, A.O. Woven Polymer Fiber Video Displays with Improved Efficiency and Economy of Manufacture. U.S. Patent US 6,229,259, 8 May 2001. 140. Murasko, M.; Kinlen, P.J. Illuminated Display System and Process. U.S. Patent US 6,811,895, 2 November 2004. 141. De-Flin, E.; Mourot, E.; Remy, M. Textile Display. WO 2004/100111 A2, 18 November 2004. 142. DO UK HO. Self-Lighting Textile Using Optical Fiber. KR20080040815, 9 May 2008. 143. Peng, C.-T.; Wang, C.-T. Textile with Pattern-Lighting Effect. US2011309768, 22 December 2011. 144. Yu, Z. Lighting Textile Fabric. CN201873891, 22 June 2011. 145. Ridao, M. Self Illuminating Spaces. In Proceedings of the Smart Fabrics Conference, Miami, FL, USA, 17–19 April 2012. 146. Van De Pas, L. Bring Spaces Alive. In Proceedings of the Smart Fabrics Conference, Miami, FL, USA, 17–19 April 2012. 147. Eves, D.A.; Chapman, J.A.; Bechtel, H.-H.; Wagner, P.C.; Martynov, Y. Electro-Optic Filament or Fibre. WO/2004/055576, 1 July 2004. 148. Cutecircuit. Available online: http://cutecircuit.com/ (accessed on 22 August 2017). 149. Lucentury. Available online: http://www.lucentury.com/ (accessed on 22 August 2017). 150. Dias, T.; Monaragala, R.M. Electro-luminant Fabric Structures. US2010003496, 7 January 2010. 151. Dias, T.; Monaragala, R. Development and analysis of novel electroluminescent yarns and fabrics for localized automotive interior

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illumination. Text. Res. J. 2012, 82, 1164–1176. 152. Bono’s Laser Stage Suit by Moritz Waldemeyer. Available online: https://www.dezeen.com/2010/02/28/bonos-laser-stage-suitby-moritz-waldemeyer/(accessed on 14 November 2017). 153. World Market for Wearable Technology—A Quantitative Market Assessment—2012; IMS Research of Wellingborough: Wellingborough, UK, 2012. 154. The Wearables Report 2016: Reviewing a Fast-Changing Market. Available online:  https://www.fbicgroup.com/sites/default/files/ The%20Wearables%20Report%202016%20by%20FBIC%20 Global%20Retail%20and%20Technology%20June%2021%202016. pdf (accessed on 22 August 2017). 155. Wearable Tech Market to be Worth $34 Billion By 2020. Available online: https://www.forbes.com/sites/paullamkin/2016/02/17/ wearable-tech-market-to-be-worth-34-billion-by2020/#5f690ca83cb5 (accessed on 22 August 2017). 156. Hayward, J. E-Textiles 2017–2027: Technologies, Markets, Players. Available online: https://www.idtechex.com/research/ reports/e-textiles-2017-2027-technologies-markets-players-000522. asp (accessed on 17 November 2017). 157. International Fashion Machines. Available online: http://www. ifmachines.com/(accessed on 21 August 2017). 158. ScotteVest. Available online: https://www.scottevest.com/ (accessed on 21 August 2017). 159. Fibretronic. Wearable Tech. CrunchWear. Available online: http:// crunchwear.com/category/companies/fibretronic/  (accessed on 21 August 2017). 160. Russell, B. Smart Fabrics for Consumer Health. In Proceedings of the Smart Fabrics Conference, Miami, FL, USA, 17–19 April 2012. 161. Köhler, A.R.; Hilty, L.M.; Bakker, C. Prospective impacts of electronic textiles on recycling and disposal. J. Ind. Ecol. 2011, 15, 496–511. 162. Sun, C.H.; Shang, G.Q.; Tao, Y.Y.; Li, Z.R. A review on application of piezoelectric energy harvesting technology. Adv. Mater. Res. 2012, 516, 1481–1484. 163. Can Electricity from the Human Body Replace Batteries? BBC News. Available online: http://www.bbc.co.uk/news/scienceenvironment-19470850 (accessed on 22 August 2017).

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164. Burns, M.L. Medical Trauma Assessment through the Use of Smart Textiles; Final Technical Report 7/14/94–2/28/95; Science, Math & Engineering, Inc.: Billerica, MA, USA, 1995.

CHAPTER

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SMART TEXTILES AND NANO-TECHNOLOGY: A GENERAL OVERVIEW Md. Syduzzaman1, Sarif Ullah Patwary2, Kaniz Farhana3, Sharif Ahmed4 Department of Textile Management and Business Studies, Bangladesh university of Textiles

1

Department of Textile Engineering, National Institute of Textile Engineering and Research, Bangladesh 2

Department of Apparel Manufacturing Engineering, Bangladesh university of Textiles

3

Department of Yarn Manufacturing Engineering, Bangladesh university of Textiles

4

ABSTRACT Smart textiles are fabrics that have been designed and manufactured to include technologies that provide the wearer with increased functionality. These textiles have numerous potential applications, such as the ability to communicate with other devices, conduct energy, transform into other materials and protect the wearer from environmental hazards. Research and

Citation: Md. Syduzzaman, Sarif Ullah Patwary, Kaniz Farhana, Sharif Ahmed, Smart Textiles and Nano-Technology: A General Overview, http://dx.doi.org/10.4172/21658064.1000181. Copyright: © 2015 Syduzzaman, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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development towards wearable textile-based personal systems allowing e.g. health monitoring, protection and safety, and healthy lifestyle gained strong interest during the last few years. Smart fabrics and interactive textiles’ activities include personal health management through integration, validation, and use of smart clothing and other networked mobile devices as well as projects targeting the full integration of sensors/ actuators, energy sources, processing and communication within the clothes to enable personal applications such as protection/safety, emergency and healthcare. This writing includes the origin and introduction of smart textile and integrated wearable electronics for sport wear, industrial purpose, automotive and entertainment applications, healthcare & safety, military, public sectors and new developments in smart textiles. Keywords: Health care and safety, Interactive textiles, Nanotechnology, Sportswear, Smart textiles

INTRODUCTION SMART TEXTILES are defined as textiles that can sense and react to environmental conditions or stimuli, from mechanical, thermal, magnetic, chemical, electrical, or other sources. They are able to sense and respond to external conditions (stimuli) in a predetermined way. Textile products which can act in a different manner than an average fabric and are mostly able to perform a special function certainly count as smart textiles [1]. Other examples of smart textiles include fabrics capable of releasing medication or moisturizer in to the skin, fabrics that help control the vibration of muscles during athletic activities and materials that regulate body temperature. There are also simpler, aesthetic applications for smart textiles, including those that can change color, light up in patterns or potentially display pictures and video [2]. The original function of textiles was to shield man from cold and rain. Later on in history aesthetic aspects also came to play a role in clothing. Much more recently a new generation of textiles has arisen; smart and interactive textiles. Interactive textiles are a relatively new discipline in the textile sector. They are active materials that have sensing and actuation properties. Their potential is enormous. one could think of smart clothing that makes us feel comfortable at all times, during any activity and in any environmental conditions, a suit that protects and monitors, that warns in case of danger and even helps to treat diseases and injuries. Such clothing could be used

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from the moment we are born till the end of our life. Some of the more important efforts include applications that Aid in patient health monitoring through sensor embedded garments that track and record biometric data, helps to improve athletic performance both by analyzing sensor data and adapting to changing conditions [3]. So as to improve performance over the time. Provides environmental sensing and communication technologies for military defense and other security personals. Present new structural and decorative solutions for fashion design. The smart textile can sense and react to environmental conditions or stimuli from mechanical, thermal, chemical, electrical, magnetic or other sources. Three components must be present in smart textiles. i.e. sensors, actuators and controlling units [4]. Modified textile material and miniaturized electronic devices create smart cloths. These cloths are like ordinary cloth providing special function in various situations according to the design and application.

HISTORY OF SMART TEXTILES DEVELOPMENT The basic materials needed to construct e-textiles, conductive threads and fabrics have been around for over 1000 years. In particular, artisans have been wrapping fine metal foils, most often gold and silver, around fabric threads for centuries [5]. Many of Queen Elizabeth I’s gowns, for example, are embroidered with gold-wrapped threads. (See the entry on Goldwork for more information) At the end of the 19th century, as people developed and grew accustomed to electric appliances, designers and engineers began to combine electricity with clothing and jewelry—developing a series of illuminated and motorized necklaces, hats, broaches and costumes [6,7]. For example, in the late 1800s, a person could hire young women adorned in light-studded evening gowns from the Electric Girl Lighting Company to provide cocktail party entertainment [8]. In 1968, the Museum of Contemporary Craft in New York City held a groundbreaking exhibition called Body Covering that focused on the relationship between technology and apparel. The show featured astronauts’ space suits along with clothing that could inflate and deflate light up, and heat and cool itself [9]. Particularly noteworthy in this collection was the work of Diana Dew, a designer who created a line of electronic fashion, including electroluminescent party dresses and belts that could sound alarm sirens [10].

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In 1985, an inventor by the name of Harry Wainwright (www. hleewainwright.com) created the first fully animated sweatshirt consisting of fiber optics, leds, and a microprocessor to control individual frames of animation resulting in a full color cartoon on the surface of apparel. Wainwright went on to invent the first machine in 1995 enabling fiber optics to be machined into fabrics, the process needed for manufacturing enough for mass markets and hired a German machine designer, Herbert Selbach, from Selbach Machinery to produce the world’s first CNC machine able to automatically implant fiber optics into any flexible material (www.usneedle. com) in 1997. Receiving the first of a dozen patents based on LED/Optic displays and machinery in 1989, the first CNC machines went into production in 1998 beginning with the production of animated coats for Disney Parks in 1998. The first ECG Bio-Physical display jackets employing LED/ Optic displays were created by Wainwright and David Bychkov, the CEO of Exmovere at the time in 2005 using GSR sensors in a watch connected via Bluetooth to the embedded machine washable display in a denim jacket and were demonstrated at the Smart Fabrics Conference held in Washington D.C. May 7th, 2007. Additional Smart Fabric technologies were unveiled by Wainwright at two Flextech Flexible Display conferences held in Phoenix, AZ, showing Infra-Red digital displays machine embedded into fabrics for IFF (Identification of Friend or Foe) which were submitted to BAE Systems for evaluation in 2006 and won an “Honorable Mention” award from NASA in 2010 on their Tech Briefs, “Design the Future” contest. MIT personnel purchased several fully animated coats for their researchers to wear at their demonstrations in 1999 to bring attention to their “Wearable Computer” research. Wainwright was commissioned to speak at the Textile and Colorists Conference in Melbourne, Australia on June 5th, 2012 where he was requested to demonstrate his fabric creations that change color using any smart phone, indicate callers on mobile phones without a digital display, and contain WIFI security features that protect purses and personal items from theft. In the mid 1990s a team of MIT researchers led by Steve Mann, Thad Starner, and Sandy Pentland began to develop what they termed wearable computers. These devices consisted of traditional computer hardware attached to and carried on the body. In response to technical, social, and design challenges faced by these researchers, another group at MIT, that included Maggie Orth and Rehmi Post, began to explore how such devices might be more gracefully integrated into clothing and other soft substrates. Among other developments, this team explored integrating digital electronics

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with conductive fabrics and developed a method for embroidering electronic circuits [11,12].

CLASSIFICATION OF SMART TEXTILES Passive Smart Textiles The first generations of smart textiles, which provide additional feature in a passive mode i.e. irrespective of the change in the environment [13]. For example, a highly insulating coat would remain insulating to the same degree irrespective of the outside temperature. Wide range of capabilities, including anti-microbial, antiodour, anti-static, bullet proof are the other examples.

Active Smart Textiles The second generation has both actuators and sensors. Textiles which adapt their functionality to changing environment automatically are active smart textiles. Active smart textiles are shape memory, chameleonic, waterresistant and vapor permeable (hydrophilic/ nonporous), heat storage, thermo regulated, vapor absorbing, and heat evolving fabric and electrically heated suits.

Ultra Smart Textiles Very smart textiles are the third generation of smart textiles, which can sense, react and adopt themselves to environmental conditions or stimuli. A very smart or intelligent textile essentially consists of a unit, which works like the brain, with cognition, reasoning and activating capacities. The production of very smart textiles is now a reality after a successful marriage of traditional textiles and clothing technology with other branches of science like material science, structural mechanics, sensor and actuator technology, advance processing technology, communication, artificial intelligence, biology etc. New fibre and textile materials, and miniaturized electronic components make the preparation of smart textiles possible, in order to create truly usable smart clothes. These intelligent clothes are worn like ordinary clothing, providing help in various situations according to the designed applications.

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Smart Materials and Fibers in Smart Textiles ‘Smart’ or ‘Functional’ materials usually form part of a ‘Smart System’ that has the capability to sense its environment and the effects thereof and, if truly smart, to respond to that external stimulus via an active control mechanism. Smart materials and systems occupy a ‘Technology space’, which also includes the areas of sensors and actuators [14].

Materials The materials of our surroundings are being “intellectualized”. These materials can interact, communicate and sense. Polymeric or carbon coated threads Conductive yarn, conductive rubber, and conductive ink have been developed into sensors or used as an interconnection substrate. Conductive yarns and fibers are made by mixing pure metallic or natural fibers with conductive materials. Pure metallic yarns can be made of composite stainless steel or fine continuous conductive metal-alloy combination of fibers with conductive materials can be completed by the methods namely: Fibers filled with conductive material (e.g., carbon -or metallic particles); Fibers coated with conductive polymers or metal and Fibers spun with thin metallic or plastic conductive threads. Metallic silk, organza, stainless steel filament, metal clad aramid fiber, conductive polymer fiber, conductive polymer coating and special carbon fiber have been applied to the manufacture of fabric sensors. Materials such as metallic, optical fibers and conductive polymers may be integrated into the textile structure, thus supplying electrical conductivity, sensing capabilities and data transmission. Organic polymers may provide a solution to overcome the stiffness of inorganic crystals such as silicon. These materials are light, elastic, resilient, mechanically flexible, inexpensive and easy to process.

Metal Fibers Metal threads are made up of metal fibers which are very thin metal. The fibers are produced either through a bundle-drawing process or else shaved off the edge of thin metal sheeting. Metallic threads and yarns may be knitted or woven into a textile and used to form interconnects between components (Figure 1). They may also be used as electrodes for monitoring electrical physiological activity such as electrocardiogram (ECG) signals.

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Conductive Inks A layout can be screen-printed using conductive inks to add conductivity to specific areas of a garment. Carbon, copper, silver, nickel and gold may be added to conventional printing inks to make them conductive (Figure 2). Printed areas can be subsequently used as switches or pressure pads for the activation of circuits.

Figure 1: Metal fiber.

Figure 2: Conductive Inks.

Figure 3: Electrically conductive textiles.

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Inherently Conductive Polymers Inherently conductive polymers have both sensing and actuation properties. Some commonly had known ICPs include polyacetylene, polypyrrole, polyaniline. Polypyrrole (PPy) is most suitable as it has high mechanical strength with high elasticity, is relatively stable in air and electro. The major advantage of this approach is that the sensors retain the natural texture of the material. The problem with these devices is a variation in resistance over time and high response time.

Electrically Conductive Textiles Electrically conductive textiles are already used for years in various industrial application fields for the purpose of controlling static and electromagnetic interference shielding. Nowadays, textiles are modified to offer a good electrical conductivity to be applied in smart textiles (Figure 3). Here electrically conductive textiles are used as electrodes or as interconnection between the different components.

Optical Fibers Plastic optical fibers may be easily integrated into a textile. They have the advantage of not generating heat and are insensitive to EM radiation. Optical fibers may serve a number of functions in a smart garment-transmit data signals, transmit light for optical sensing, detect deformations in fabrics due to stress and strain and perform chemical sensing. Commercially available Luminex ®fabric is a textile with woven optical fibers capable of emitting its own light (Figure 4). While this has aesthetic appeal for the fashion industry it is also used in safety vests and potential to be used for data transmission.

Coating with Nano-particles Coating a fabric with nano particles is being widely applied within the textile industry to improve the performance and functionality of textiles. Nanotechnology can add permanent effects and provide high durability fabrics. Coating with Nano-particles can enhance the textiles with properties such as anti-bacterial, water-repellence, UV-protection and self-cleaning, while still maintaining breath-ability and tactile properties of the textile. Nano-tex has a range of products using such coatings to resist spills, repel and release stains, and resist static.

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Shape Memory Materials Shape memory alloys, such as nickel-titanium, have been developed to provide increased protection against sources of heat. A shape memory alloy possesses different properties below and above the temperature at which it is activated. At the activation temperature, the alloy exerts a force to return to a previously adopted shape and becomes much stiffer. The temperature of activation can be chosen by altering the ratio of nickel to titanium in the alloy (Figure 5). Cuprous-zinc alloys are capable of producing the reversible variation needed for protection from changeable weather conditions. Shape Memory Polymers have the same effect as the Ni-Ti alloys but, being polymers, they will potentially be more compatible with textiles. Electro active polymers EAPs are generally made up of high functionalized polymer. One of the most famous EAPs is the “Gel robots” made up of poly 2-acrylamido-2methylpropane sulfonic acid that is fully researched for applications in the replacement of muscles and tendons.

Figure 4: Optical fibers.

Figure 5: Shape memory materials.

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Nanotechnology in Smart Textiles

Chromic Materials Chromic materials can change their color according to external conditions. These materials have mostly used in fashion, to create funny color changing designs [15]. According to the stimuli type, chromic materials can be categorized as Photo chromic: External stimulus is light. Thermo chromic: External stimulus is heat. Electro chromic: External stimulus is electricity. Piezoro chromic: External stimulus is pressure. Solvate chromic: External stimulus is liquid or gas.

Phase Change Materials Now days, phase change materials are highly applied in the field of textiles for different kinds of products such as apparel, underwear, socks, shoes, bedding accessories and sleeping bags. For multifunctional products also are applicable in the specialty items like anti - ballistic vests, automotive, medical or for other industrial applications. Application of PCMs in textiles: For a Suitable application of PCMs in textiles the temperature must be within a temperature range of human skin. This exciting property of PCMs would be useful for the application of producing protective garments in all- kinds of weathers from the strongest winter to the hottest summer. Textile materials treated with PCMs can store the heat if it is excess and release it back when the heat is needed. The PCMs can be applied either in the fibre spinning or during chemical finishing processes like Coating, lamination and others.

Incorporating Smartness into Apparels The key steps in the integration of electronic devices with apparel during the manufacturing process interface can be seen in Figure 6. If the sensors and micro processors are integrated into the yarn itself they will not interfere with the normal manufacturing process of the garment. The technology is based on the encapsulated area. The vision is the development of novel technology for fabricating electronically active and sensor fibers. In the future a shirt could monitor your ECG, be your iPod, and talk to you! It is possible to incorporate electronic chips, optical and thermal devices, into yarn (Figure 6). Yarns were created with LEDs in them so that

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the functionality could be easily demonstrated. Hitachi is the first company to produce a chip small enough to be embedded with textile fibers, called the Mu-chip. It is 0.4 mm x 0.4 mm x 0.15 mm.

Figure 6: Steps to incorporating smartness in apparel.

Work is now being carried out on light emitting yarns, ones that will measure stresses and strains on fabrics and that will sense fluids and liquids. The fabric of the future won’t be just plain chiffon, silk or cotton. Instead electroluminescent material, microprocessors and LEDs may be woven together with clothing fibers to create smart textiles.

HOW DOES A SMART TEXTILE WORK? Smart textiles can be made by incorporating smart materials, conductive polymers, encapsulated phase change materials, shape memory polymers and materials and other electronic sensors and communication equipments. These materials interact – according to their designed feature with the stimuli in their environment. All smart materials involve an energy transfer from the stimuli to response given out by the material. They are integrated and complex materials. They have the ability do some sort of processing, analyzing and responding. Even they can adapt to the environment. They got full ability to change themselves depending on — temperature, pressure, density, or internal energy—change [16]. The amount of energy transferred to make this change is determined by the properties of the material. This relationship between the amount of energy required and the degree of the specific change governs the behaviour of all materials, including smart ones. If they get energy or any stimuli from the outer environment they do not do any change on it .They just resist it or absorb it. For example, a material‘s

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Nanotechnology in Smart Textiles

specific heat (property) will determine how much heat (energy) is needed in order to change its temperature by a specified amount.

INCORPORATING SMARTNESS INTO TEXTILES Textile to behave smartly it must have a sensor, an actuator (for active smart textiles) and a controlling unit (for very smart textiles). These components may be fiber optics, phase change materials, shape memory materials, thermo chromic dyes, miniaturized electronic items etc. These components form an integrated part of the textile structure and can be incorporated into the substrate at any of the level namely: fiber spinning level; yarn/fabric formation level; finishing level. The active (smart) material can be incorporated into the spinning dope or polymer chips prior to spinning e.g. lyocell fiber can be modified by admixtures of electrically conductive components during production to make an electrically conductive cellulosic fiber. Sensors and activators can also be embedded into the textile structure during fabric formation e.g. during weaving. Many active finishes have been developed which are imparted to the fabric during finishing. The electronic control units can be synchronized with each other during finishing. Techniques such as microencapsulation are generally preferred for incorporation of smartness imparting material in the textile substrate.

APPLICATION OF SMART TEXTILES Health The development of wearable monitoring systems is already having an effect on healthcare in the form of “Telemedicine”. Wearable devices allow physiological signals to be continuously monitored during normal daily activities [17]. This can overcome the problem of infrequent clinical visits that can only provide a brief window into the physiological status of the patient. Representative examples are e.g.: Wireless-enabled garment with embedded textile sensors for simultaneous acquisition and continuous Monitoring of ECG, respiration, EMG, and physical activity. The “smart cloth” embeds a Figure 6: strain fabric sensor based on piezo resistive yarns and fabric electrodes realized with metal based yarns. -Sensitized vest including fully woven textile sensors for ECG and respiratory frequency detection and a Portable electronic board for motion assessment, signal pre-processing, and bluetooth connection for data transmission. Wearable sensitized garment that measures human heart rhythm and respiration using a three lead ECG

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shirt. The conductive fibre grid and sensors are fully integrated (knitted) in the garment (Smart Shirt) (Figure 7).

Life Belt Life belt is a trans-abdominal wearable device for long-term health monitoring that facilitates the parental monitoring procedures for both the mother and the fetus. This life belt is very useful in case of pregnant women. Pregnant women living in remote areas work during pregnancy and face certain health problems (e.g. high blood pressure, kinetic problems requiring immobilization, kidney or heart diseases, multiple pregnancies). Life belt is a support tool for the obstetrician, who is enabled to monitor patients remotely, evaluate automated preliminary diagnosis of their condition based on collected and analyzed vital signs, access patients’ medical data at any time and most importantly be alerted.

Life Jacket Life jacket is a medical device worn by the patient that consequently reads their blood pressure or monitors the heart rate; the information is transferred to a computer and read by medical staff. A specialized camera in the form of headwear has been developed to be worn by paramedics. Visual information captured by the camera can be transferred directly to medical staff at the hospital enabling them to advise instantly on appropriate treatment. Cuff-less BP can be measured from the radial pulse waveform by arterial tonometry by using this life jacket.

Military/Defense Around the world military forces are exploring how smart clothing can be used to increase the safety and effectiveness of military forces. In extreme environmental conditions and hazardous situations there is a need for real time information technology to increase the protection and survivability of the people working in those conditions. Improvements in performance and additional capabilities would be of immense assistance within professions such as the defense forces and emergency response services. The requirements for such situations are to monitor vital signs and ease injuries while also monitoring environment hazards such as toxic gases (Figure 8). Wireless communication to a central unit allows medics to conduct remote triage of casualties to help them respond more rapidly and safely [18].

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Fashion and Entertainment As the technology is becoming more flexible various electronic devices and components clothes becoming truly portable devices.

Figure 7: Wearable devices in health.

Figure 8: Smart textiles in Military/Defense

Figure 9: Light emitting smart textiles.

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Figure 10: Smart sportswear.

Already there are textile switches integrated into clothing for the control of such devices. While technology may be hidden through invisible coatings and advanced fibers, it can also be used to dramatically change the appearance of the textile, giving new and dazzling effects (Figure 9). Light emitting textiles are finding their way onto the haute couture catwalks, suggesting a future trend in technical garments.

Sportswear Sports are area of important smart clothing developments. In general a number of important functions can be implemented using smart devices or clothing (Figure 10). These include: Monitoring heart rate, breathing, body temperature and other physiological parameters; Measuring activity, for example determining the number of steps taken, the total distance travelled; Acting to actively stimulate muscles e.g. using electrical muscle stimulation; Work against activity to provide ‘smart’ resistance training; Record aspects of performance, such a foot pressure or specific joint movements; Protect against injury.

Smart Sports Shoe Global Positioning Systems (GPS) incorporated into walking shoes which allow the user to be tracked by mountain rescues services. They can also used to monitor the where about of young children. Gloves that contain heaters, or built in LED’s emitting light so that a cyclist can be seen in the dark.

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Nanotechnology in Smart Textiles

The Sensory Baby Vest The sensory baby vest is equipped with sensors that enable the constant monitoring of vital functions such as heart, lungs, skin and body temperature which can be used in the early detection and monitoring of heart and circulatory illness.

Figure 11: Smart Bra.

Figure 12: Ultrasonic bonding.

It is hoped to use this vest to prevent cot death and other life-threatening situations in babies. The sensors are attached in a way that they do not pinch or disturb the baby when it is sleeping.

The Smart Bra One of the best examples for improving comfort is an Australian invention: the Smart Bra. They have developed a bra that will change its properties in response to breast movement. This bra will provide better support to active women when they are in action. The Smart Bra will tighten and loosen its straps, or stiffen and relax its cups to restrict breast motion, preventing

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breast pain and sag (Figure 11). The conductive polymer coated fabrics will be used in the manufacture of the Smart Bra. The fabrics can alter their elasticity in response to information about how much strain they are under. The Smart Bra will be capable of instantly tightening and loosening its straps or stiffening cups at excessive movement.

LATEST DEVELOPMENT IN SMART TEXTILES AND NANOTECHNOLOGY Ultrasonic Assembly Ultrasonic bonding occurs when high frequency electrical energy– converted to acoustical, mechanical vibrations and channeled through a horn–creates a rapid heat build up at the material contact point, causing the fabric between the horn and anvil–or the rotating pattern wheel in the case of the Seam Master–to soften and fuse (Figure 12). According to the company, in one pass, the machine seals and trims without thread, glue or other consumables, as much as four times faster than conventional sewing machines and ten times faster than adhesive methods. The Seam Master is also said to be easy to operate with minimal training required.

Life on Earth “Radiation is a danger faced by the military, health care workers, and the first-responder in many different scenarios. Our anatomically correct applications addressing protection as well as comfort, will reach the people who need it much faster as extreme career wear, in military, medical, energy, transportation, safety, protection and first responders.

Life on Mars Fab-designs President and textile engineer, Connie Huffa presents several soft and stretchable 3-dimensional fabric swatches with different textured sides that she calls a spacer. A spacer is a fabric with two face sides and a yarn constructed hollow gap between the faces (Figure 13). “The initial focus for our fabrication efforts is to meet NASA’s challenge of a potential space mission to Mars, since astronauts are exposed to many different situations that have the potential to cause irreversible harm to the body. This extremely hostile environment requires a sophisticated fabrication that will protect the wearer from hazards emanating from outer space, the

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Nanotechnology in Smart Textiles

space craft itself, and health problems from the human body’s reaction to antigravity conditions,” she says.

Fiber in Space Elevator Challenge A space elevator is a proposed type of space transportation system, whose main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space. It is designed to permit vehicle transport along the cable from a planetary surface, such as the Earth’s, directly into space or orbit, without the use of large rockets by mechanical means to orbit, and descended to return to the surface from orbit. Battery-powered robots, called climbers, made by participating teams will race up and down the Technora belt and rope to see which can travel the fastest and farthest.

Innovative Sportswear New product developments in sportswear not only make garments look and fit better, they also help athletes perform better. Many of these require uses of new or specialist technology within the manufacture of the garments, not just the materials they were made from. The market leaders present these specialist technologies at Reprocess. Smart textiles are an example. The Adidas miCoach Elite System has been introduced to football to help with coaching and game monitoring. For the Olympics, Speedo introduced its Fastskin Racing System which combines the swimsuit, cap and goggles into a unified system, which Speedo claim enhances both comfort and hydrodynamic efficiency. Threedimensional CAD software is used to help develop the design for sportswear (Figure 14). It is used to create custom fit models, build life-like digital clothing samples, and adjust these based on virtual fit [19].

PROSPECT OF SMART TEXTILES Rising demand for smart and interactive textiles from the transportation industry is likely to be one of the major factors for the growth of the market. Furthermore, growth in medical and healthcare industries is anticipated to lead to higher demand for smart textiles during the forecast period.

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Figure 13: 3-dimensional fabric swatches.

Figure 14: Innovative Sportswear.

Figure 15: Innovative Sportswear.

However, market growth is likely to be sluggish due to high prices of finished products of smart textiles as compared to conventional textiles. Rising number of research and development activities for product and

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technological innovation is expected to provide new opportunities for market growth.  With over 38% share, the transportation segment accounted for the largest demand for smart textiles in 2013. New applications such as measuring heart rate, heating of seats, smart seat belts and functioning of steering wheels hold immense potential in the automotive industry. The industrial application segment is anticipated to lead the market in the near future (Figure 15). It is projected to grow at a CAGR of 17.3% between 2014 and 2020 due to a rise in the number of industrial activities such as logistics and supply chain management in emerging regions such as Asia Pacific. Other prominent applications of smart textiles in industrial applications include various protection devices such as personal protective equipment, which is significantly used in industrial plants, manufacturing facilities, etc. Extensive research and development has resulted in commercialization of smart textiles for fire fighters; however, a gap still exists between commercialization and prototype development.

CONCLUSIONS Previously smart textiles were presented as imaginary products and used in very limited areas. After scientific efforts and development phases, nowadays smart textiles are an implanted customer interest and are presented as the future of the textile industry. Now many commercial products are available and, as it have been presented in this article. A lot of scientists are developing new solutions, ideas and concrete products with the emerging demand of smart textiles in various phases of life. The global markets of smart textiles are expected to reach USD 1500 million according to new study of Grant View Research, Inc.

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REFERENCES 1. 2. 3.

4.

5.

6. 7. 8. 9. 10.

11. 12. 13.

14. 15.

http://www.fibre2fashion.com/industry-article/4/335/smart-textile3. asp http://www.ask.com/beauty-fashion/smart-textiles-d7ff8fb58a817a26 Shin Y, Yoo DI, Son K (2005) Development of thermoregulating textile materials with microencapsulated phase change materials (PCM). II. Preparation and application of PCM microcapsules. J Appl Polym Science 96: 2005-2010. Bendkowska W, Tysiak J, Grabowski L (2005) Determining temperature regulating factor for apparel fabrics containing phase change material. Int J Clothing Science and Technology 17: 209-214. Marvin C (1990) When Old Technologies Were New: Thinking About Electric Communication in the Late Nineteenth Century. Oxford University Press, USA. Gere C, Rudoe J (2010)] Jewellery in the Age of Queen Victoria: A Mirror to the World. http://query.nytimes.com/gst/abstract. html?res=FA0912FB3A5C15738DDDAF 0A94DC405B8484F0D3 Smith P (1968) Body Covering. Museum of Contemporary Crafts, the American Craft Council, New York. http://www.thecreatorsproject.com/blog/the-original-creators-dianadew Post R, Orth M, Russo P, Gershenfeld N (2000) E-broidery: design and fabrication of textile-based computing. IBM Systems Journal 39: 840-860. Electrically active textiles and articles made therefrom. Gregory RV, Samuel RJ, Hanks T (2001) National Textile Centre Annual Report, USA. Oakes J, Batchelor SN, Dixon S (2005) New method for obtaining proper initial clusters to perform FCM algorithm for colour image clustering. Coloration Technology 12: 237-244. Krasovitskii BM, Bolotin BM (2002) Organic Luminescent Materials, Weinheim NY. http://www.innovationintextiles.com/smart-textiles-nanotechnology/ nanofrontused-in-bicycle-and-motorcycle-gloves/#sthash. eG8cQmbM.dpuf

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16. h t t p : / / w w w. i n n o v a t i o n i n t e x t i l e s . c o m / s m a r t - t e x t i l e s nanotechnology/anaselects-performance-polyesters-for-inflightcomfort/#sthash.82x57GjJ.dpuf 17. http://textilelearner.blogspot.com/2013/04/applications-of-smartandinteractive.html 18. http://www.stitchprint.eu/news/new-technologies-for-innovativesportswear/ 19. http://www.transparencymarketresearch.com/pressrelease/smartfabrics-andinteractive-textiles-market.htm

CHAPTER

3

WEARABLE E-TEXTILE TECHNOLOGIES: A REVIEW ON SENSORS, ACTUATORS AND CONTROL ELEMENTS Carlos Gonçalves 1,2, Alexandre Ferreira da Silva 3, João Gomes 2 and Ricardo Simoes 1,4 Institute for Polymers and Composites IPC/I3N and MIT-Portugal Program, University of Minho, 4800-058 Guimarães, Portugal 1

Center of Nanotechnology and Smart Materials (CeNTI), 4760-034 VN Famalicão, Portugal

2

Center for Micro Electro Mechanical Systems (CMEMS-UMinho) and with the MIT Portugal Program, University of Minho, Campus of Azurem, 4804-533 Guimaraes, Portugal 3

Polytechnic Institute of Cávado and Ave (IPCA), 4750-810 Barcelos, Portugal

4

ABSTRACT Wearable e-textiles are able to perform electronic functions and are perceived as a way to add features into common wearable textiles, building Citation: Carlos Gonçalves , Alexandre Ferreira da Silva, João Gomes and Ricardo Simoes, Wearable E-Textile Technologies: A Review on Sensors, Actuators and Control Elements, doi:10.3390/inventions3010014 Copyright © This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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competitive market advantages. The e-textile production has become not only a research effort but also an industrial production challenge. It is important to know how to use existing industrial processes or to develop new ones that are able to scale up production, ensuring the behavior and performance of prototypes. Despite the technical challenges, there are already some examples of wearable e-textiles where sensors, actuators, and production techniques were used to seamlessly embed electronic features into traditional wearable textiles, which allow for daily use without a bionic stigma. Keywords: e-textiles, digital textiles, smart textiles, conductive yarns, heat regulating textiles

INTRODUCTION Nowadays, the wearable e-textile technologies are facing an exponential growth. Day by day new textiles come to the market with functionalities such as: heat regulation, luminescent, touch, and sensitivity. Those functionalities are useful for several applications in different fields such as: healthcare, sports, space exploration, and gaming. The gaming industry revenue using wearable e-textile technologies is growing, resulting in $66 billion in 2013 for the mobile games on smartphones and tablets, and the growth in 2017 was estimated at around $78 billion [1]. The increasing miniaturization of electric circuits enables seamless incorporation of functionalities, which helps to avoid a potential bionic stigma and to embrace market penetration of wearable e-textiles. The potential of electronic wearable textiles has been perceived by several companies, like Google, among others, which is developing capacitive touching textiles in its project, called Google Jacquard. This project enables a seamless and reliable wearable computing concept that can help costumers to perform daily tasks like answering phone calls without stopping an ongoing activity [2]. The main wearable e-textiles have embedded capacitive, resistive, and optical sensors allowing the textile to sense touch, strain, pressure, temperature, and humidity. The sensors are normally connected to control boards responsible to process information. Several review works have been published summarizing developments in wearable e-textile technologies [3,4,5,6]. A variety of sensing and output devices have been used into e-textiles using touch sensitive buttons

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[7], pressure sensors [5], Radio Frequency Identification (RFIDs), or electrocardiography (ECG) sensors in electronic socks and sports bras [8]. Moreover, electromyography (EMG) sensors are used to control active orthotics [9,10], prostheses, mobility assistive devices [11] and provide electrical stimulation [12]. Accelerometers have been used in e-textiles to access fall risks [13] and to monitor functional ability at home. Output devices used Light Emission Diode (LED) arrays, thermo chromic ink, vibration, and shape memory alloys [4]. The most common communication methods for interactive textiles are Wi-Fi and Bluetooth with a hybrid approach to power. All of the wearable e-textiles have battery requirements that must be fulfilled either by detachable batteries or by thin, flat, and flexible batteries that are able to survive washing, drying, ironing, and dry cleaning [14]. In this paper, it is presented a review of wearable e-textile technologies. The paper is structured as follows: • •

•

First materials, connections and fabrication methods are explained in the context of wearable e-textile technologies; and, Secondly, textile capacitive and resistive sensors are explained, highlighting measurement ranges and fabrication methods. Examples of commercial wearable e-textiles are presented and shortly described. Finally, a conclusion is done mentioning the importance of wearable e-textile technologies with a small future perspective.

MATERIALS, CONNECTIONS AND FABRICATION METHODS The wearable e-textiles can be made with several materials using different fabrication methods. The selected materials and fabrication methods are always interconnected with the final application. This makes e-textile a multidisciplinary research field, with the need of expertise in several fields, such as textile, materials, electronics, mechanics, and computer engineering [15].

Adapted Fabrics: E-Textiles Fabrication Methods Over the past decade, it has been proved that traditional fabrication methods that are used to produce conventional textiles could be used in e-textiles

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production too. The development of flexible conductive yarns with diameters that are similar to the conventional textile yarns enable the use of traditional fabrication methods to merge conductive threads with non-conductive threads. The conductive yarns incorporation processes into conventional textiles threads can be manually done by sewing conductive yarns [9] or automatically through embroidery [16], weaving [17], knitting [18], and breading machines [14]. Coating non-conductive yarns with metals, galvanic substances or metallic salts can also be used to make electrical conductive yarns from pure textile threads, which also enables an e-textile production. Common textile coating processes include electroless plating [19], chemical vapor deposition [20], sputtering [21], and with a conductive polymer coating [15]. Stamping conductive inks is also an alternative to embed conductive lines into textiles. There are several technologies that can print conductive material on textile substrates, but all of them use conductive inks with high conductive metals, such as silver (Ag), copper (Cu), and gold (Au). Table 1 shows a list of manufacturing techniques with a qualitative comparison of fabrication attributes. All of the manufacturing techniques can be used to produce e-textiles. Table 1: Qualitative comparison of e-textiles fabrication attributes E-Textile Manufacturing Technique

Machinery Costs

Material Costs

Process Complexity

Resistance to Wear

Embroidery

High

Low

High

High

Sewing

Low

Low

Low

High

Weaving

Low

High

High

High

Non-woven

Low

Low

Low

Low

Knitting

Low

High

High

Low

Spinning

Low

Low

Low

Low

Breading

Low

Low

Low

High

Coating

High

Low

Low

Low

Printing

High

High

Low

Low

Connections to data acquisition systems are achieved by either mechanical [15] or electrical mechanisms [22]. This way, textile structure platforms as woven, knitted, or nets can be used to produce e-textiles, avoiding attaching electronics to textile substrates.

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Electrical Components The wearable e-textiles would not be possible without electrical components, such as electrodes, connectors, and interconnectors. When wearable e-textiles are used for the acquisition of electrical biological signals such as electrocardiogram (ECG), the electrodes are the bridge between the body and the circuit. When there is no need of electrical signal acquisition, there is still the need of connectors and interconnectors in order to bridge the textile with the electronics. Copper wire can be used in applications without skin contact, and silver thread can be used in applications that requires direct contact with skin [23]. The energy needed to power e-textile circuits is normally provided from Lithium Polymer (LiPo) batteries. The LiPo batteries are selected accordingly to a tradeoff between power autonomy and battery size. The goal is to select the smallest LiPo battery that is able to supply the e-textile circuit power demands during a predefined amount of time. There are also research projects developing energy harvesting solutions that are embedded into e-textiles [24]. With energy harvesting solutions, it is possible to charge small LiPo batteries, keeping the e-textile energy demands during use. Figure 1 shows some examples of the connection techniques that are used in e-textile circuits and transducers.

Figure 1: Textile and electronic materials used in e-textiles. (a) Solder and polyester thread used into e-textiles; (b) E-textile capacitor; (c) Printed Circuit Board (PCB) for e-textiles; (d) Casing shell for e-textiles; (e) Vibration

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motor; (f) Elektrisola textile conductive wire; (g) Bekintex conductive thread; (h) Lithium-ion battery used to power e-textiles; and, (i) Slide switch used to switch On/Off e-textiles [25].

The two main bond categories for connectors and interconnects are mechanical and physical. Mechanical connections are made with snaps that are directly pressed into conduction lines, and are normally made when there is a need to detach any electrical module from the e-textile. Physical connections include microwelding, thermoplastic adhesion [26], mixed conductive polymer adhesion [27], joint soldering, and electroplating [28]. Physical connections are made when there is a need for a permanent connection. E-textile connectors remain an open research field due to the diversity of application environments where each solution is customized and is almost unique.

Textile Circuitry Textile circuits are electrical circuits built on textile substrates. Embroidery conductive thread into textile substrates is a widely used technique. This technique is used to stitch patterns that define circuit traces, component connection pads or sensing surfaces using Computer Assisted Design (CAD) tools [29]. The conductive patterns can also be done using inkjetprinted techniques of graphene-based conductive inks [30]. Normally a textile circuit is designed to have a low power consumption rate and high input impedance, which is opposite to the conventional requirement of low impedance for component interconnections. Many yarns available in the market can be used for connections and circuit elements. These include silverized yarns, stainless steel thread, titanium, gold, and tin. Another technique to fabricate textile circuits is to iron a welded circuit to the textile substrate [31]. Once the circuit is attached to the textile, it can be soldered like a traditional printed circuit board. There are also commercial printed control boards made to be wearable. Table 2shows a qualitative attribute comparison from a list of wearable control boards that are available in the market. According to the information presented at Table 2, Xadow is the best wearable control board that is available in the market due to the analog/digital pins and the wireless communication in board. The possibility to be washed is also an important

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advantage that enables a permanent connection with a textile and textile fibers. Table 2: Atribute comparisson of wearable control boards [32] Control Boards

Washable

Analog Pins

Digital Pins

Wireless Communication

Lilypad

Yes

Yes

Yes

No

Intel Edison

Yes

Yes

Yes

Yes

Flora

Yes

No

Yes

No

Xadow

Yes

Yes

Yes

Yes

SquareWear

Yes

Yes

Yes

No

Printoo

Yes

Yes

Yes

No

BITalino

Yes

Yes

Yes

No

Igloo

Yes

Yes

Yes

No

WaRP7

No

Yes

Yes

Yes

nRF52832

No

Yes

Yes

Yes

Flexible conduction lines could also be made of any conductive ink and conductive polymer. Thick and thin printing processes are two production techniques that are used to print conductive inks. An example of a thick film process is silk screening, where an adhesive conductive ink is applied to the open areas of a textile mesh allowing for the ink to penetrate into the fabric [33]. A sputtering process can also be used to produce highresolution circuits on textile substrates. The textile substrate, kept at 150 °C, needs to be placed in a vacuum chamber with an inert gas like argon and a shadow mask to make the circuit patterns. There are also research projects reporting the use of nanosoldering methods to produce e-textiles with carbon nanotubes (CNT) conductive lines. The CNTs are soldered onto the fiber surface of non-woven fabric by ultrasonication, which brings a strong adhesion between the carbon nanotubes and the textile fibers. The CNTs do not detach when the e-textile is under vigorous mechanical stirring, or even after being washed [34].

Textile Circuit Elements Textile circuit elements can be built to be adapted to the textile substrates. Small electric components can be sewn into the conductive lines on fabrics [35] either directly or using sockets attached to the fabric with connection resistivity that is lower than 1 Ω [36]. Gripper snaps and textile switches can also be used in order to ensure connectivity, allowing strong connections [37].

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Electronic elements can be made out of conductive thread by sewing thread fibers in patterns, with multiple crossings, to achieve desired electrical properties. Conductive properties can be given to threads by several techniques before and after the thread manufacturing process [38]. Another very common technique entails the application of metal or conductive polymer coatings to the textile substrate. Laminating techniques are also used, including those that are adapted from conventional and flexible electronics [39]. With those techniques, passive elements can be formed with conductive inks and polymers. Resistors (i.e., 2–8 Ω/mm), capacitors (i.e., 1 pF to 1 nF), and inductors (i.e., 500 nH to 1 µH at 10 MHz) can be made by planar printing techniques, such as screen printing or sputtering metal inks onto fabric substrates, such as cotton, polyester, silk, wool, polyacrylonite, and fiberglass fabrics [33]. It has also been shown that resistive elements can be made by adjusting the dimension of an already coated conductive polymer fabric [40]. In the case of transistors, the core of a metalized yarn can be used as gate, while source and drain contacts can be made by depositing metals or polymers using evaporation or soft lithography processes [41]. Transistors can also be fabricated on strips of Kapton and be later interlaced into a textile substrate [42]. Figure 2 shows a textile electrode suitable for acquiring biological signals, such as ECG.

Figure 2: Textile electrode used to sense biological signals such as electrocardiography [43].

SMART FABRICS SENSORS Capacitive Pressure Sensors Usually, capacitive pressure sensors are made on textiles that can be sewn, snapped, or glued to a fabric substrate and welded to other electronics or

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wires. Textile capacitors can also be made from compliant conductive materials that are acting as conductive plates separated by dielectrics. The conductive plates can be woven [28], sewn [44], and embroidered with conductive thread/fabrics, or they can be painted, printed, sputtered, or screened with conductive inks [45], or conductive polymers [46]. The dielectrics used are typically synthetic foams, fabric spacers, and/or soft non-conductive polymers. Capacitive fibers can also be manufactured using techniques that are similar to those found in flexible electronics, such as a silicon fiber sputtered with metals [47]. The capacitance of a capacitive pressure sensor depends on the area of two conductive parallel plans, the conductive material and the distance between each other. Keeping the same area for the conductive plates the capacitance will change with the distance between them. When the distance between the conductive plates decreases, the capacitance increases, and when the distance between the conductive plates increases, the capacitance decreases. Table 3 presents a list of production techniques used to produce capacitive pressure sensors. From Table 3, it is possible to see that the conductive element and production technique influence not only the pressure range measurement but also measurement sensitivity. Embroidery of conductive thread into textile substrates produces capacitor pressure sensors with low resolution that are good to make seamless e-textile press buttons. The CrossliteTM capacitor production technique is able to produce capacitive pressure sensors with higher resolutions that can be used sense pressures over time. Table 3: Fabric capacitor pressure sensor production techniques [48] Production Technique

Elements

Measured Vari- Sensitivity able

Pressure

Embroidery

Conductive thread

Electrical contact

Contact sens- mm2–cm2 range ing

Switching threshold

Size

Patterned electrodes Conductive ink

Thickness com- 0.214 V/pF pression

0–13 kPa

32 mm2

Surface touch

Capacitance fingers/surface

0.02 pf/mm

0–2 Pa

Diameter = 5 cm

Laminated electrodes Thin film deposited metals

Capacitance at intersecting points

0.01 ΔC/mN

0–50 N/cm2

Diameter = 250 µm

3D textile capacitor Conductive fabric 3D textile

Thickness com- 2 pF/N/cm2 pression

PEDOT Nylon

0–0.75 N/cm2 9 cm2

CrossliteTM capacitor Silver-coated Thickness com- 0.05 pF/N/cm2 0–30 N/cm2 textile PCCR pression

100 mm2

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Resistive Pressure Sensors The resistive pressure sensors have a correlation between pressure and electrical resistance. These sensors can be made of different conductive materials in different structures using different production techniques. The variable resistive materials can be sewn, embroidered or glued to the textile substrate to measure pressure. The working principle of a resistive pressure sensor is based on an electric resistance that increases when the resistive material is stretched or compressed. According to Ohm’s Law (V = R*I), for the same electric current, a higher resistance makes the output voltage increase. This way, the stretch or compression can be correlated to the sensed voltage [49]. Table 4 shows a list of textile production techniques that are used to produce textile pressure sensors. The conductive material and production technique influence the sensitivity and sensed pressure range. Table 4: Textile pressure sensors based on resistive mechanism [48] Production Technique

Elements

Sensitivity

Pressure range

Size

Characteristics

Switch tactile Plated fabric sensor Cu, Ni

Threshold at 70–500 g/ 500 g/mm2 mm2

8 mm2

Active sensing cells

Tooth structured

2.98 × 10−3 kPa−1

760 mm3

Strain in under pressure fabric

Conductive fabric

–2000 kPa

Polyurethane PPyPolyurefoam thane

0:0007 mS/N 1–7 kN/m2

4 cm3

Conductance increases with compression

Conductive Carbon polyRubber based mer

250 Ω/MPa

9 mm2

Resistance changes with applied load

0–0.2 MPa

QTC-Ni based Pressure com- ~106 Ω/1% 25% composite compression pression

Diameter = Switching behavior 5.5 mm

Optical Textile Sensors The working principle of optical textile sensors is based on the variation of the light intensity or the amplitude that can be sensed by a fiber Bragg grating (FBG) sensor. These type of sensors were first developed in 1978 by Hill et al. when the photosensitivity in optical fibers were found [50]. Since then, several configurations were developed and incorporated into fabrics [51,52]. The small glass optical fibers diameters (in the microns range) make these materials suitable for seamless textile integration with industrial processes. The optical fiber light source can be a small light emission diode (LED), and the light amplitude at the end of the optical fiber can be sensed with a small photodetector. Depending on the textile movements,

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the light amplitude will change allowing to sense textile displacements and pressures. The optical textile sensors can be used to sense textile displacements and pressures in applications where the electrical currents cannot cross textile substrates. Figure 3shows a schematic of a fabric with optical fibers incorporated. When the elastic fabric is stretched, the light amplitude passing thought the fiber increases, which increases the output voltage coming out the photodetector.

Figure 3: Schematic of a fabric with optical fibers incorporated.

Temperature and Humidity Sensitive Textile There are several possibilities to build an e-textile to sense temperature and humidity changes. Resistance and capacitance are the main principles to build humidity textile sensors. The resistive humidity textile sensors answer to moisture variation by changing its conductivity, while the capacitive humidity textile sensors answer to water vapor by varying its dielectric constant [53]. Combinations of polymer/substrate; PEDOT–PSS/ PAN nanofibers [48], PEDOT–PSS/polyimide, PEDOT [54]—PSS/lycra tactel and Polypyrrole, are sensitive to humidity changes by changing their electrical conductivity [44]. These sensitized substrates can also be woven into textiles. Polymers that are suitable for capacitive humidity sensors include polyethersulfone (PES), polysulfone (PSF), and divinyl siloxane benzocyclobutene (BCB). Other humidity sensing devices have flexible transistors that changes conductivity with the humidity levels [55]. Coated sensors on fabrics typically react to humidity if they are organic or carbon based. Temperature sensors compatible with fabrics can be made on flexible substrates, such as plastics and polyimide sheets. These sensors can be later attached to fabrics or integrated into their structure. Resistance temperature detectors (RTDs) have elements, such as platinum/nichrome (NiCr) and

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related materials that can be coated on flexible surfaces. Kapton based plastic stripes of platinum RTDs can be woven into fabrics to manufacture a temperature sensitive textile [56]. A gold RTD has been manufactured on a flexible polyimide substrate [57]; its resistance changes linearly with temperature. Thermoelectric generators can also be attached to fabrics using molding techniques and fabric connection technologies [58]. All of the conductive polymers and carbon based conductive particle polymers have a temperature dependent response. For instance, PEDOT–PSS coated fibers experience a decrease in resistance when under high temperatures [59]. Fiber optic sensors can also be used to sense temperature changes as well as temperature sensitive inks [60].

WEARABLE E-TEXTILES Everyday, new wearable e-textile products come to the market with different useful functionalities. Figure 4 shows three examples of commercially available textile based wearable e-textiles from three different brands. Figure 4a shows a hat commercialized by Zeroi with bone conduction technology. With this hat, the user can listen music or answer phone calls. The bone conduction technology is seamless embedded into the hat, which avoids a bionic stigma and makes this e-textile suitable for outdoor activities. The sound waves bypass the eardrums, going directly to the cochlea where the sounds waves are decoded. Figure 4b shows a smart sock with a foot pressure measurement technology and walking distance measurement that can be used to measure sports performance. The electronic components of this product are detachable in order to wash the textile part of the socks. The collected data is sent wirelessly to a mobile application running into a smartphone. With this e-textile, runners can see the pressure profile of the foot sole and then practice their gait cycle to achieve a better performance. Figure 4c shows an airbag jacket that can be used to prevent serious injuries when a motorcycle accident occurs. The jacket has an accelerometer that detects falls and triggers the airbag before the road impact. The airbag is inflated with a small bottle of gas that needs to be replaced when the airbag is triggered. Tests performed with the airbag jacket show that, in the case of a fall, the impact forces with the ground do not exceed 2 kN. The traditional motorcycle protecting jackets have impact forces that range from 20 to 35 kN.

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Figure 4: Wearable e-textile transducers. (a) Zeroi—Bone conduction hat [61]; (b) Sensoria—Intelligent sock with heart rate measurement system [62]; (c) Dair—Airbag jacket (motorcycling protection) [63].

Besides the commercial wearable e-textile products, there are also research projects in the medical field that aim to develop new e-textiles to improve diagnostic and treatment of several diseases. Wearable e-textiles with embedded textile electrodes are being developed to detect a variety of biological signals, such as electrocardiogram (ECG) [64] and electromyogram (EMG) [65], as well as to measure body impedance and skin conductance [66]. With the ECG detection, it is possible to do an early diagnostic of heart diseases, which can prevent sudden deaths. The EMG signal measurement is useful to evaluate physiotherapy treatment of gait cycle disorders or stroke rehabilitation [67]. The body impedance and skin conductance measurements are useful to evaluate the body hydration levels, thus helping to prevent dehydration [68]. Wearable e-textiles with functional electrical stimulation (FES) are also under development to be used in the treatment of gait disorders such as foot drop [69]. New wearable e-textiles are under development to build functional soft orthotic devices useful in the treatment of ankle-knee injuries [70]. Insomnia disorders can also be treated with wearable e-textiles able to control body temperature during sleep [71].

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FUTURE PERSPECTIVES The future of wearable e-textiles will be dictated by the ability to manufacture products with seamless embedded electronics useful to fulfill daily needs. Multinational business companies are endorsing the design and development of e-textiles, helping the market penetration. At the same time, traditional textile business companies envision e-textiles production as a competitive advantage. Private users, compelled by the advantages of e-textiles, buy these products to use them in sports and daily life activities (where they can have an important role, e.g., in healthcare), thus increasing the e-textile market. The interest of business companies in the production of e-textiles together with the interest by the market creates a new e-textile cluster. This e-textile cluster will help in the development of production standards setting higher quality levels. The production standards will help decrease the e-textile manufacturing time and final fabrication costs. However, many e-textile technologies are still in the research phase where requirements such as washability, nontoxicity, and resistance to tensile strength forces still need to be addressed. The development of new standard tests to control the sources of e-textile failure, such as cyclic loads and current flow, are crucial to ensure e-textile resistance overtime. Strategies for the encapsulation of components and extension of e-textile lifetime also need to be improved. The development of e-textiles with energy harvesting features is also an important challenge to overcome. These challenges increase with higher integration levels, due to the higher number of degrees of freedom. When all of these challenges are met, e-textile mass production will become a reality, thus achieving a major milestone for these materials. New wearable e-textiles will always be both an opportunity for new markets but also a challenge to interact with conventional electronics devices.

CONCLUSIONS The wearable e-textiles became one of the main research avenues in the textile field. The useful features that are incorporated into e-textiles bring market advantages in several areas, such as sports and healthcare. Common textile manufacturing machines and production techniques can be used to produce e-textiles. Electrical components that are used in conventional electrical circuits can also be used into e-textiles and new textile based electrical components, such as resistors, capacitors, and antennas are also being developed. Thus, the available materials and technologies provide

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sufficient range to be tuned and adapted for e-textiles. Wearable textile based sensors are developed in order to produce wearable seamless sensing solutions avoiding bionic stigma. Business companies already perceived the wearable e-textiles business potential and are developing e-textile products to incorporate them in their product portfolios. The wearable e-textiles are still a new field with opportunities to build innovative products that can revolutionize the way that persons interact with their garments.

ACKNOWLEDGMENTS The authors would like to acknowledge the support from CeNTI—Centre for Nanotechnology and Smart Materials, MIT MVL—Man Vehicle Laboratory, and UMN WTL—Wearable Technology Laboratory. This work is funded by is funded by National Funds throught FCT—Portuguese Foundation for Science and Technology, Reference UID/CTM/50025/2013 and Ph.D. grant SFRH/BD/52352/2013 (CG), MIT Portugal Program, and FEDER funds through the COMPETE 2020 Programme under the project number POCI01-0145-FEDER007688.

AUTHOR CONTRIBUTIONS Carlos Gonçalves, Alexandre Ferreira da Silva, João Gomes and Ricardo Simoes contributed to the analysis of wearable e-textile technologies and the paper writing process.

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TEXTILE-BASED FLEXIBLE COILS FOR WIRELESS INDUCTIVE POWER TRANSMISSION Yi Li, Neil Grabham, Russel Torah, John Tudor and Steve Beeby Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK

ABSTRACT Wireless inductive power transmission systems can potentially supply wearable devices. Power cables or batteries can be eliminated by implementing a wireless power transfer system, making the wearable devices less obtrusive to users. However, rigid coils can cause discomfort to users in wearable applications. The novel screen-printed flexible coils on textiles reported here are intended to be a low-cost and comfortable solution when integrated into clothing. A constant-width circular-spiral flat coil has been designed to Citation: Yi Li, Neil Grabham, Russel Torah, John Tudor and Steve Beeby, Textile-Based Flexible Coils for Wireless Inductive Power Transmission, doi:10.3390/ app8060912. Copyright © This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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minimize the detrimental effect of the low conductivity of the screen-printed flexible conductors on the efficiency of the wireless power transfer system. The coils are printed on 65/35 polyester/cotton textile with a screen-printed Fabink-UV-IF1 interface layer coating. The interface layer provides a relatively flat and smooth surface to prevent the permeation of the conductive paste into the textile and allows the printing of finer-profile coils. A 5 V 1.2 W DC output has been achieved by a wireless power transfer system using the printed flexible coils with Qi standard circuitry; a DC-DC efficiency of 37% has been measured. Keywords:  inductive power transmission, coil design, smart textile, screen printing, resonant coupling

INTRODUCTION Wireless power transmission (WPT) using inductive coupling has been employed in numerous applications [1], in particular where cable-free devices are desired. E-textile technology covers the integration of electronics into clothes to achieve a wearable flexible implementation that is easy to use. Furthermore, the watt-level power supplied by WPT makes it possible to improve the functionality of wearable devices and reduce their size and weight by eliminating power cables or batteries [2] and make the wearable devices less intrusive for users. Most current research involves the use of coils fabricated either by winding copper wire or by using a track fabricated onto a PCB [3,4]. These rigid inflexible coils are uncomfortable and so less suited to e-textile applications due to their impact on the user. In this paper, screen-printed flexible coils have been designed and fabricated for a WPT system. These flexible coils provide a convenient and comfortable option to integrate a WPT system into clothing which is less invasive and more comfortable for users, by having improved flexibility and breathability over the conventional wire-wound or PCB-based coil fabrication approaches. Screen printing is a well-established, low-cost, and textile-compatible technology for fabrication which can be readily applied to volume production [5,6]. In this paper, the textile used is 65/35 polyester/cotton which has widespread use in everyday clothing. Conventionally flexible printed electronics for e-textiles are fabricated by printing the conductive paste to form the electronic circuitry onto a substrate material, such as a polyimide

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film, which is then attached to the host textile by a mechanical fixing method such as sewing, as such the printed electronics are not directly integrated with the actual textile. Printing the conductive paste directly on the textile causes several problems in manufacture and application. Firstly, a textured or porous substrate causes a higher effective resistance as conductive paste sinks into the gaps between the textile fibers [7]. Secondly, conductive paste printed directly on textile without a supporting structural material is brittle, and when the textile is deformed the conductor can be fractured, increasing the resistance of conductive tracks, or even causing an open circuit [8]. Finally, the profile of a printed conductive paste is poorly defined when printed onto a rough substrate. These factors lead to unpredictability and variation of the electrical properties which can cause the coils of the WPT system to diverge from their tuned operational frequency or cause the driver circuit to fail. An interface layer is therefore required on the textile to create a smooth surface for the printing of subsequent conductive layers with uniform thickness. The flexible coils used in this work are printed onto interface-coated textile achieved by screen printing to produce novel printed coils that are directly integrated with the textile substrate. This paper is organized as follows: in Section 2, the theory of inductive power transmission is discussed to introduce the essential parameters used in determining the behavior of the flexible coils.  Section 3 describes the specific design for the flexible coils in this work, based on the essential parameters identified in  Section 2. The practical fabrication of the coils is described in detail in Section 4. Section 5describes the experimental performance evaluation of flexible coils and the WPT system. The essential parameters of coils are detailed in Section 6 and the performance of the WPT system using the printed flexible coils is given in Section 7. Considerations for the safe operation of the WPT system are identified in Section 8. Final conclusions are given in Section 9.

THEORY OF INDUCTIVE POWER TRANSMISSION Typical inductively coupled wireless power transfer systems are based on coupled-mode theory, the resonant components being the Transmitter (TX) and Receiver (RX) coils, which are coupled in an oscillating electromagnetic field. A system diagram showing a typical arrangement is shown in Figure 1.

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Figure 1: System diagram of wireless inductive power transmission system.

The inductances of the TX and RX coils allow the transfer of power wirelessly via inductive coupling. The TX and RX coils are represented by inductors LTX and LRXwith their equivalent series resistances (ESRs) represented by ReTX and ReRX, and their parasitical capacitances by CpTX and CpRX respectively [9]. Resonant tank capacitors CrTX and CrRX are connected in parallel with the TX and RX coils to tune them causing both driving and receiving circuits to resonate at same operating frequency. The load, represented by a resistance RLoad, is powered by the rectified DC voltage from the receiver circuit. The ESRs introduced by the coupled coils waste power when the alternating currents pass through the coil, and thereby reduce the efficiency of system. These losses due to the ESRs are especially prominent in a printed coil due to the high sheet resistance of printable conductive pastes compared to bulk conductive materials. Consequently, the ESRs of printed coils are should be as low as possible. The inductance of the coils is a key parameter required to tune the system correctly, the inductance of a flat spiral coil with an air core is determined by its shape. The circular coil used in this paper is shown in Figure 2. The engineering calculations for circular and square coils as derived in [10] are given in Equations (1) and (2) respectively: (1)

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(2)

where  μ0 is absolute magnetic permeability, N is the number of turns the spiral coil has, davg=(dext+dinner)/2 is the average diameter of the flat spiral coil, and the fill ratio Δ=(dext−dinner)/(dext+dinner) which are defined in [10] to simplify the effect of dimensions for a coil with external diameter dext and inner diameter dinner as shown in Figure 2. The ESRs of the coils are composed of their DC resistance, the skin effect, and the proximity effect [11]. The DC resistance of the coils can be calculated for a conductive track as: (3) where Rsheet is the sheet resistance for a given thickness, which is provided by the manufacturer; for other thicknesses the sheet resistance may be calculated as discussed in Section 4, and W is the width of the track. The ESR Re has been defined in [12] as a combination of RDC, the skin effect, and the proximity effect, given by: (4) where  A′=W×tc−(W−2δ)(tc−2δ)  is the reduced effective area due to skin effects for a track with thickness tc, and skin depth   [13] (where ρ is resistivity and μr is relative permeability), this reduced effective area is derived in [14] from the effective area, which is the cross section of the track A=W×tc,  ω  is the operating frequency, and  ωcrit=3.1(W+S)ρ/ (μ0μrW2tc) is the critical frequency [15] of a coil with track spacing S where the resistance begins to increase significantly relative to the operating frequency due to the skin and proximity effect [16]. The skin effect depth for the conductors in the printed flat coils is of the order of 1 mm and is therefore negligible in this case but is included in the calculations for completeness.

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Figure 2: Design of circular-spiral flat coil. The external diameter of the coil is designed based on the application, and other parameters are optimized for the maximized unloaded Q factor. The coils are printed on interface-coated 65/35 polyester/cotton textile.

The parallel combination of parasitic capacitance and inductance of the coil causes self-resonance of a coil at a particular frequency. The selfresonant frequency (SRF) is an essential parameter of an inductively coupled coil because the effective inductance is zero at the SRF which prevents an alternating electromagnetic field from being induced. A connection cannot be made without an alternating electromagnetic field. The parasitic capacitance is affected by the surrounding materials which are air, the insulator, and the substrate textile. This can be calculated as described in [12] as:

(5)

where εrCo, εrSu are the relative permittivity of the coating insulator material and the substrate textile material, ε0 is the vacuum permittivity, and lgap is the overall length of the gap between the tracks. The unloaded Q factor of inductors expresses the ratio of stored against lost energy which is used to evaluate the quality of coils, and is found as [17]:

(6)

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To employ the printed coils in a WPT system, the coils are attached with resonant tank capacitors which are calculated as [18]: (7) where ReTX, ReRX, QuTX, and QuRX are the ESRs and unloaded Q factors of the TX and RX coils, respectively, L is the inductance of the coil with the resonant tank capacitor attached. The coefficient is based on the unloaded Q factors of both coils and the coupling factor, k, which has an approximate formula for flat spiral coils of [19]:



(8)

where  D is the center separation distance between the TX and RX coils,  dextTX and dextRX are the external diameters of the TX and RX coils, respectively. For coils with a known size, the coupling factor k is affected by the distance between two coils. In this work, the coupling factor was also measured for each pair of printed coils to determine the resonant tank capacitor required for each individual coil at the desired operating frequency.

DESIGN The flat coil was designed as a circular spiral shape to provide the desired inductance as well as to retain flexibility for both TX and RX coils as shown in Figure 2. The external diameter of the coils was specified based on the target application. As the printed coils increase the thickness of a textile this reduces the flexibility and breathability of the textile. Therefore, the coil size is limited to ensure that the WPT system enabled textile is as comfortable as possible. An area less than 140 mm × 140 mm is an acceptable size for flexibility and fits the printable area of the semi-automatic screen printer used in this paper. The external diameter was 138 mm and the remaining space was for the connection pads and screen-printing alignment marks. Zierhofer et al. [20] presented experimental work on a geometric approach for the enhancement of the unloaded Q factor and the coupling coefficient of flat coils. They concluded that when dinner≈0.4×dext, the optimum

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unloaded Q factor is obtained for a given coupling coefficient. Therefore, the inner diameter of the coil was set as 53 mm to get an integral number of turns for the coil. Ng et al. [11] found that the optimal spacing depended on the size of coil and is essential to improve the unloaded Q factor. In this paper, the coils have a 2 mm spacing between conductive tracks, the spacing has been standardized at this value to reduce inter-turn proximity effects while leaving scope for the width of the conductive track to be varied within the overall coil-dimension limits. When increasing the number of turns, N, of a coil, both the numerator and denominator of its unloaded Q factor in Equation (6) are increased. The relationship between N and unloaded Q factor is simulated and illustrated in Figure 3 with parameters as dext = 138 mm, dinner = 53 mm, S = 2 mm, for both circular and square forms, and using a sheet resistance Rsheet = 24 mΩ/□ given from the manufacturer (specified as Ω per square area for their recommended printed thickness, denoted as Ω/□). The width of the track was varied in simulation to fit a given number of turns within the specified area, varying the width for a given printed thickness affects the effective conductor size as the resulting cross-sectional area changes. For both circular and square coils the 6-turn coil has the highest unloaded Q factor and the width of the track W is 4.5 mm with a pitch of 6.5 mm. By comparing coils with different shapes, it was found that the circular coils have a larger Q, a shorter length of track and a lower resistance than the square coil for the same number of turns. The resistance of the printed coils is approximately 50 times higher than that of a wound copper coil because of the limitation in respect of conductivity of the screen-printable conductive paste caused by the non-conductive polymer binder and gaps between the metal particles in the printed paste. This material-based limitation makes resistance the limiting factor of the unloaded Q factor of the printed coil. As a result, the circular coil has a higher unloaded Q factor than the square one due to the longer track length in the square coil.

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Figure 3: Unloaded Q factor versus the turns of spiral coil for a coil design that has 138 mm external diameter, 53 mm inner diameter, and is printed with a conductive paste which has 24 mΩ/□ sheet resistance.

FABRICATION Process of Fabrication The coils were fabricated by screen printing using a DEK 248 semiautomatic printer (ASM Assembly Systems, Munich, Germany), using a series of patterned screens to deposit the desired layers individually [7], prior to curing as appropriate. The 65/35 polyester/cotton textile substrate used is very flexible and it can withstand curing process at up to 130 °C for up to 60 min without significant damage or degradation. To keep it flat and to ease handling during the printing and curing processes it is glued on to an alumina tile with a demountable spray adhesive. Multiple wet prints prior to curing can be performed to give the desired printed thickness with the thickness of each print controlled by the screen emulsion thickness. In this work an emulsion thickness of 40 μm was used for both the interface and conductive layers. After the printing stages, ultraviolet (UV) curing for the interface paste was carried out using a UV light cabinet with a 400 W mercury bulb, and thermal curing for conductive paste was performed in a BTU belt furnace as appropriate for the material deposited. This print cure process was repeated until the desired value of thickness was obtained. Film

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thickness measurements were taken using a Nikon eclipse LV 100 microscope (Nikon Instruments Europe, Amsterdam, The Netherlands), with a Nikon LU Plan Fluor 100×/0.90 lens (Nikon Instruments Europe, Amsterdam, The Netherlands), to perform a non-contact measurement. This avoided damage to the surface of the printed layer, and possible contamination issues which may affect the adhesion of subsequent layers. Multiple print-cure cycles can be correctly aligned using the alignment marks included in the screen design. The use of multiple print-cure cycles allows thicker conductors to be realized than can be achieved in a single print-cure cycle of the printing process. The overall structure of the screen-printed coil and associated interface material is shown in the exploded view shown in Figure 2, consisting of conductive and interface layers printed onto the flexible textile substrate.

Interface Layer The selected standard 65/35 polyester/cotton textile has a rough surface with an arithmetic mean deviation of 140 μm [21] so it was necessary to print an interface layer on the textile to create a smooth surface for the printing of subsequent conductive layers with uniform thickness. An UV curing interface ink Fabink-IF-UV4 from Smart Fabric Inks Ltd. (Smart Fabric Inks Ltd., Southampton, UK) [22] was printed and cured to form the interface layer on the textile, to achieve a smoother surface. After each print, UV curing is performed for 60 s at an intensity of 50 mW/cm2. The thickness of each cured layer was then measured using the non-contact method. An acceptably smooth surface was achieved with six sub-layers of interface with a total thickness of 96 μm ± 10%. A comparison of surface roughness is shown in Figure 4which shows example conductive tracks printed on a smooth interface layer with six sub-layers on the left side compared with a rough surface of the interface layer with two sub-layers on the right side. Based on SEM measurements it can be concluded that the maximum diameter of surface defects is less than 5 μm with a 6 sub-layer interface having a thickness of 96 μm as shown in the SEM micrograph in Figure 5.

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Figure 4: Roughness of interface layer comparison. Demo tracks printed on a smooth surface of the interface with 6 sub-layers on the left side, and a rough surface of the interface with 2 sub-layers on the right side.

Figure 5: SEM image of the interface and conductive layer on textile. The interface layer provides a smooth surface for conductive layer over the textile.

Conductive Track Layer The conductive paste was printed onto the interface layer and cured to form the planar spiral coil; a low sheet resistivity is required to minimize resistive losses with the coil. Thermally cured silver polymer paste Fabink-TCAG4002 from Smart Fabric Inks Ltd. (Smart Fabric Inks Ltd., Southampton,

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UK) [23] was used to print the conductive layer; it has 20 mΩ/□ sheet resistance and high flexibility, the maximum permissible bend radius for the printed conductor is dependent on the number of layers printed and resulting geometry, the ink has been found to be stable once cured and to perform well with repeated bending cycles though is not machine washable at present due to the increased bending during the process. From the datasheet, the recommended curing temperature of Fabink-TC-AG4002 is 110 to 130 °C, and the recommended curing time is 2 to 10 min, this curing parameters are for the recommended deposit thickness of 25 μm and a printed area of up to 100 mm2. After fully curing, the 25 μm printed thickness of the silver layer shrinks to 16–18 μm. These recommended curing parameters from the manufacturer are intended for printed conductive layer in applications such as printed resistors. Since the textiles maximum curing temperature is 130 °C, curing at a higher temperature is not an option. It is investigated in following section that the designed coil, which has a printed area over 100 cm2, required an increased curing duration to provide a homogeneous cured layer. In the uncured liquid state, the conductive paste contains a solvent which has higher resistivity than the conductive silver particles so consequently the DC resistance of a conductive track decreases as the solvent evaporates throughout the curing process. Therefore, the DC resistance of a track is an indicator of the state of the curing process. The appropriate curing time was identified by identifying the relationship between curing state and DC resistance. The DC resistance of a conductor is used as a standard test to establish when the curing process has been completed and volatile components of the conductive paste have been removed, at which point the conductor’s properties should have reached their optimum. For this test a conductive sub-layer was printed with two deposits onto the interface layer or the previously cured conductive layers and then cured at 130 °C for 5 min. This was then followed by measurement of the DC resistance. The sample was then cured for a further 5 min and the DC resistance measured again. This curing and measuring cycle was repeated until the DC resistance of the coil stabilized indicating that it is fully cured. This process was completed for samples of conductive tracks composed of one, two, four and eight conductive sub-layers on an interface-coated textile for curing durations from 15 to 60 min. All the numbers of sub-layers tested show consistent decreases in the normalized DC resistance for curing times from 10 to 40 min. For curing durations over 40 min they show varying levels of increase in DC resistance. This occurs with all numbers of sub-layers, but the effect is reduced as the total thickness increases and the number of sub-

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layers increases. The cause of this effect is due to interactions between the interface layer and the conductive layer which occur once the conductive paste is over-cured. Comparative tests for the same conductive layer printed directly onto Kapton without an interface layer show no increase in DC resistance for curing durations over 40 min. The final selected curing process for a 140 mm × 140 mm printed coil conductive layer is 40 min at 130 °C, which minimizes the DC resistance and avoids increases due to over curing. The sheet resistance (Ω/□) of the realized conductive layer can be calculated from Equation (3) using the RDC measured and the length of the conductive track, ltrack, from the CAD software used to produce the printing screens is 2417 mm. The manufacturer of the conductive paste recommends a cured layer thickness, tcRX, of 18 μm to achieve optimal sheet resistance. The thickness of a printed conductive layer is greater than the recommended value in this paper because the minimum DC resistance was desired. Consequently, a mathematical theoretical sheet resistance is calculated as RsheetCal=RsheetRec×tcRec/tc, where tc is the average thickness of the conductive layer measured using the non-contact method. These theoretical sheet resistances, RsheetCal, can be compared with the practical sheet resistance Rsheet to verify that the sheet resistance on the samples was as expected. The thicknesses, theoretical sheet resistance, practical sheet resistance, and DC resistance of the conductive layer are listed in Table 1, each cured sub-layer has two printed deposits. The results are based on the measurements of five different samples. Table 1: Thickness and resistance characteristics of printed coils Symbol

Quantity

Value for 4 SubLayers

Value for 8 Sub-Layers

tc

Thickness of conductive layer

116 μm ± 13%

224 μm ± 11%

RDC

Measured DC resistance

2.10 Ω ± 25%

1.05 Ω ± 5%

RDCtheo

Theoretical DC resistance

2Ω

1Ω

Rsheet

Measured sheet resistance

3.91 mΩ/□

1.97 mΩ/□

RsheetCal

Calculated sheet resistance

3.10 mΩ/□

1.61 mΩ/□

In Table 1, the thicknesses of the overall layer, which are comprised of eight sub-layers, are approximately 5% less than twice the thickness of layers with four sub-layers. This means that the thickness of upper sub-layers is less than the lower sub-layers due to the reduced separation between substrate and screen mesh when the upper sub-layers are printed. As more sub-layers

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of conductive track are printed the thickness of the entire conductive layer is increases and the standard deviation is reduced, and consequently the surface roughness and deviation of the DC resistance are also reduced. By comparing the theoretical sheet resistance, it can be observed that the practical value was higher than expected on all samples, due to the mix of different resistivities between the conductive binder and silver particles. Measured values of DC resistances of printed coils with four different numbers of conductive sub-layers are shown in Figure 6. They are compared with the theoretical calculation of DC resistance the conductors with given thickness have, the values of thickness and resistances are calculated based on 10 measurements of three different samples of each number of sublayers. By adding up to eight conductive sub-layers, a conductive layer of approximately 224 μm thickness was printed to achieve a nominally 1 Ω DC resistance. The relationship between DC resistance and the thickness of the conductive layer is a continuous function from the minimal thickness to the maximal. The practical DC resistance is within −15% to +10% of the theoretical calculation because of the variation of the thickness of printed layer in different areas due to the process used.

Figure 6: Thickness and practical DC resistance of printed conductive layer as designed coil compared with theoretical DC resistance in different number of sub-layers and thickness.

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Flexibility of Coils A nominal DC resistance of 1 Ω has been achieved by increasing the thickness of the conductive layer. The DC resistances of the printed coils are measured after bending round a mandrel with a radius of 150 mm to demonstrate the functionality of the coils after deformation. The DC resistance is increased to 1.15 Ω ± 7% in 6 samples which is an increase of 15% over an un-deformed coil. This demonstrates an acceptable degree of flexibility.

EXPERIMENTAL PROCEDURE Measurement of Electrical Characteristics of Printed Coils After printing and curing, the following five essential electrical characteristics were measured and calculated, then they were compared with the theoretical model and calculations to characterize the performance of a coil for WPT: • Equivalent Series Resistance (ESR) • Inductance • Self-Resonant Frequency (SRF) • Parasitic capacitance • Unloaded Q factor ESR was directly measured using a Wayne Kerr 6500B (Wayne Kerr Electronics, London, UK) impedance analyzer at 200 kHz drive frequency. To find the SRF a Rohde & Schwarz ZVB 4 (Rohde & Schwarz, Munich, Germany) vector network analyzer was used to analyze the impedance phase angle in a range of frequencies from 300 kHz to 3 GHz. Using these instruments, the inductance, parasitic capacitance and SRF are found as follows: • •

• •

Locate the SRF fSR where the phase angle of the input impedance is zero, using the vector network analyzer. Read the inductance Lcoil from the impedance analyzer at the frequency 1/10 of fSR where the parasitic capacitance has negligible effect. Calculate the parasitic capacitance as Cp=1/(4π2fSR2Lcoil). Read the ESR Re from the impedance analyzer at the frequency 1/10 of SRF and calculate the unloaded Q factor Qu using Equation (6).

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To quantify the performance of the printed coil in a WPT application two other type of coils were also measured. One was a flat spiral air-core coil wound using 1.2 mm diameter copper wire with the same equivalent diameter davg and Δ as the printed coil. The second one was formed using copper adhesive tape on paper to give the same geometry as the printed coil. The resonant tank capacitors for each coil were calculated using Equation (7), based on the measured parameters. The tank capacitors were implemented using polypropylene film capacitors which have lowest dielectric absorption and are suitable for the compensation of inductive coils.

Measurement of Performance of Wireless Power Transfer System The printed coils with their resonant tank capacitors were connected to Qi standard compliant bqTESLA commercial driver bq500211EVM and receiver bq51013AEVM evaluation modules from Texas Instruments [24,25] to evaluate the performance of the coils. The transmitter module operates between 110 kHz and 205 kHz, with the operating frequency automatically varying as a function of the load. The receiver module has an internal output voltage regulator that provides 5 V DC at up to 1 A, subject to sufficient incoming energy from the transmitter module. In the bqTESLA wireless power transfer system, a pair of rigid wound coils are provided. In this experiment, these original coils and their resonant tank capacitors were replaced by the tuned copper tape coils and printed coils. Measurements of wireless power transfer were performed on a plastic support to avoid influencing the magnetic field as shown in Figure 7. The center separation distance, misalignment, and rate of deformation of coils mounted on these supports can be adjusted. The DC output power and DC to DC efficiency were measured as follows: • •

• •

Mount the paired coils on the supports with the driver and receiver circuit and adjust their position to align the centers. With the driver powered vary the load on the output of the receiver circuit and record the input DC current to the driver circuit and the output voltage from the receiver circuit and the corresponding load resistance. Use the load resistance and output voltage values calculate the output current and power transferred. Plot the output power of the receiver circuit versus its output

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•

• •

•

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current and locate the maximum output power point. Record the input DC voltage and current to the transmitter circuit to calculate the input power for wireless power transfer and overall DC to DC efficiency. Plot the overall DC to DC efficiency of the system versus the output current and locate the maximum efficiency point. Adjust the separation distance between the TX and RX coils, and then repeat steps 2 to 6, plot the maximum DC to DC efficiency for each separation distance. Adjust the curvature of the RX coil to simulate deformation of the wearable device while keeping the TX coil flat, and then repeat steps 2 to 7, plot the maximum DC to DC efficiency versus the deformation of flexible coil.

Figure 7: Measuring DC output power of Qi-standard wireless power transfer circuit with adjustable radius support for RX coil (highlighted blue) and planar support for TX coil (highlighted green).

To perform these measurements the receiver output was loaded using a resistance box to allow adjustment of the load current, load resistances between 1 kΩ and 10 Ω were used to give a nominal current load range of 5 mA to 500 mA at the nominal 5 V DC output level. The voltage across the load was measured using a 12-bit A/D convertor (NI USB-6008 in differential

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mode) to take into account any output voltage variations with load, and the output power under each different distance and rate of deformation was then calculated. The DC to DC efficiency can then be calculated as: (9) where PLoad is the power delivered to the load and PDC is the power supplied from the DC power supply. Varying the load resistance changes the output current of the receiver circuit and the input power. The output voltage of the receiver circuit is fixed at 5 V DC by its internal regulation process, providing there is sufficient energy available at its input to provide the required amount of output power. A series of system-level input and output power values can therefore be measured. The DC to DC efficiency can be calculated and graphs can then be plotted to show the output power and the DC to DC efficiency against the output current of wireless power transfer. The maximum output power and DC to DC efficiency points of the WPT can be found by locating the maximum values on the P/I and ηDC−DC/I curves. The center separation distance between the TX and RX coils can be measured between the TX and RX coil supports, DC-DC efficiency can be calculated and plotted against the separation distance. To achieve a controllable curvature of deformation of coils an assumption was made that the bent spring plate to which the flexible coil was attached has a constant curvature forming an arc of a circle. The curvature of the coil Kcoil can be changed by varying the vertical distance h as defined in Equation (10) and shown in Figure 7, in the figure the fixed TX coil is highlighted in green and the adjustable curvature RX coil is highlighted in blue. (10)

ELECTRICAL CHARACTERISTICS OF PRINTED COILS The self-resonant frequencies (SRFs) of coils are located as shown in Figure 8. The SRF of the wound copper coil was at 10.3 MHz, and the highest SRFs of both copper tape coil and the printed coil are 17.6 MHz for the un-deformed coils. A deformation of 16 m−1 gives a 1% increase in the observed SRFs. The inductance was then measured to calculate the

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parasitic capacitance at 1.03 MHz and 1.76 MHz for the wound copper and flexible coils, respectively. Equivalent series resistances (ESRs), and unloaded Q factors are measured and calculated as specified in Section 5. The coil parameters are listed in Table 2 and compared with the theoretical values obtained using Equations (1)—Lcircular, (4)—Re, (5)—Cp, and (6)— Qu. The values of practical inductance, ESR, and unloaded Q factor are calculated based on the measurements on five samples of each type of coil.

Figure 8: Impedance phase angle of different type of coils on frequency range from 300 kHz to 50 MHz. The self-resonant frequency can be located from this measurement as 10.3 MHz and 17.6 MHz for wound copper coil and flexible coils, respectively. Table 2: Electrical characteristics of coils Symbol

Wound Copper Coil

Copper Tape Coil

Printed Coil

Lcoil

5.24 μH ± 1%

4.40 μH ± 2%

3.88 μH ± 8%

Lcircular

5 μH

4.52 μH

4.52 μH

Cp-coil

8 pF ± 0.5%

10 pF ± 1%

24 pF ± 9%

Cp

5 pF

6 pF

20 pF

Re-coil

6.1 Ω ± 2%

5.9 Ω ± 4%

12.0 Ω ± 25%

Re

6Ω

6Ω

11.3 Ω

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Qu-coil

10.7 ± 2%

10.17 ± 5%

3.05 ± 25%

Qu

10.8

9.37

4.06

The practical inductances of all types of coils are within 14% of theoretical value for flat coils. The difference in inductance between the copper tape and the printed coil is 12%, which implies the inductance of a coil is mainly affected by the winding dimensions and geometry rather than the fabrication method used. The theoretical calculation of the parasitic capacitance of printed coils from the design has a discrepancy of 17% from the experimental value obtained from the printed coil based on the measured SRF and inductance. One reason for this difference is the varying relative permittivity of the surrounding materials (i.e., the interface material and textile). The difference between the measured and theoretical ESRs of all types of coil tested are within 6%, which is within an acceptable margin. The ESR of the printed coil has the largest deviation followed by the copper tape coil on paper, and then the wound copper coil. This is due to the variation in the conductor thickness which is greatest in the printed coils when compared with the other two types of coils. Compared with the wound copper and copper tape coils, the printed coils have a reduction of up to 55% in their unloaded Q factor, which is predominantly attributable to the higher ESR of the printed flexible conductive paste compared with copper. To measure the coupling factor the printed coils are tuned in pairs as TX and RX coils for use in the WPT system and then the coupling factor k is measured following the process used by Kazimierczuk and Czarkowski [26]. The variation in coupling factor as a function of spacing between two coils is illustrated in Figure 9 and compared with the theoretical calculation from Equation (8) for the different coil types. The experimental results of coupling factors versus separation distance for the tested coils follows the theoretical calculation, although the coupling factor limits to a value of approximately 0.9 where the separation distance was less than 6 mm. Values at separation distances of 5, 10, 15, and 20 mm between the two coils were used to calculate the resonant tank capacitors using Equation (7). The value of the capacitors used is given in Table 3. With the appropriate tank capacitors fitted, the maximum output currents and overall DC-DC efficiencies were found and are reported in Table 4.

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Figure 9: Coupling factor k of different type of coupled coils against distance between them compared with the theoretical calculations. Table 3: Tuning of printed coils Separation

Symbol

Wound Copper Coil

Copper Tape Coil

Printed Coil

5 mm

k

0.88

0.87

0.85

Cr_TX

68 nF

83 nF

96 nF

Cr_RX

68 nF

82 nF

97 nF

k

0.80

0.80

0.77

Cr_TX

74 nF

88 nF

104 nF

Cr_RX

74 nF

85 nF

105 nF

k

0.68

0.66

0.66

Cr_TX

82 nF

101 nF

115 nF

Cr_RX

82 nF

97 nF

116 nF

k

0.59

0.58

0.57

Cr_TX

90 nF

108 nF

122 nF

Cr_RX

90 nF

104 nF

124 nF

10 mm

15 mm

20 mm

Table 4: Maximum output current and DC to DC efficiency of wireless power transfer system deployed with different coils Separation

Symbol

Wound Copper Coils

Copper Tape Coils

Printed Coils

5 mm

ILoad

0.39 A

0.37 A

0.37 A

ηDC-DC

52%

51%

37%

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ILoad

0.37 A

0.36 A

0.36 A

ηDC-DC

46%

44%

30%

ILoad

0.28 A

0.28 A

0.27 A

ηDC-DC

39%

38%

22%

ILoad

0.16 A

0.16 A

0.14 A

ηDC-DC

24%

24%

10%

SYSTEM PERFORMANCE Output Power of WPT The plots of output power versus output current obtained by employing paired coils of the different types are shown in Figure 10. The given standard deviation of maximum output power is calculated based on 10 measurements with each pair of coils.

Figure 10: Output powers of wireless power transfer system deployed with different type of coupled coils against output DC current at 5 mm and 10 mm separation between transmitter and receiver coils. An approximate 1.51 W output power can be achieved for all types of coils at 5 mm separation, and 9% less output power, 1.37 W, can be achieved at 10 mm separation.

The maximum output DC power of the WPT system using the printed coils can be observed to be 1.51 W at 0.37 A output current. For all the coils tested, the errors of maximum output power are 1%, 1.5%, 1.7%, and 1.8% at 5, 10, 15, and 20 mm separation, respectively.

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DC to DC Efficiency The DC to DC efficiency of a WPT system given in Table 4 indicates the overall performance of the WPT system as a whole. The system with wound copper coils had an optimal 52% DC to DC efficiency under full load at 5 mm separation distance [24,25]. The DC to DC efficiency of the system equipped with copper tape and printed coils were measured at 5, 10, 15, and 20 mm separation distances with varying load. The highest efficiencies given in Table 4 occur at the maximum output power point, which was as expected since the Qi WPT system is designed to achieve optimal efficiency under full load. The WPT system equipped with the printed coils has up to a 60% reduction in the DC to DC efficiency compared to that of the wound copper coils, this reduction in efficiency is attributed to the losses associated with the ESRs of the printed coils. The DC to DC efficiency at optimal output current against varying separation distance for the different types of paired TX and RX coils is shown in  Figure 11. It can be seen that the efficiency reduces as the separation distance increases for all the types of coils tested.

Figure 11:  DC to DC efficiency at optimal output current against separation distance between different types of paired coils. The same trends of the effect of separation on DC to DC efficiency are shown on all types of coils.

Effect of Deformation on Coils A pair of axially aligned printed coils with varying curvature of RX coil was used to simulate the deformation of the coil in the wearable device. The

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maximum output power and DC to DC efficiency against the curvature of the deformed RX coil are shown in Figure 12 with a coil separation distance of 5 mm. The maximum output power is reduced by 33% and the efficiency reduces by 20% as the coil was deformed to vary its curvature from 8 m−1 to 16 m−1. This range of curvature matches the diameters of human legs for 90.1% people in UK [27].

Figure 12: Maximum output power and DC to DC efficiency of WPT employed flexible coils with varying curvature of receiver coil at 5 mm center separation distance from transmitter coil. The maximum output power drops 33% as a result of the deformation of the receiver coil.

When the separation distance between the TX and deformed RX coil was increased, the DC to DC efficiency reduces as shown in  Figure 13. It shows that the separation distance of center aligned printed coil pairs is the major influencing factor for the DC to DC efficiency of WPT systems utilizing flexible coils. In an application of a device worn on the body the separation distance between the TX and RX coils will typically be in the range between 0 and 20 mm; however, the range of anticipated deformation of the coil is in the range 0 to 16 m−1. Using these range values and the data presented in Figure 11 and Figure 13 the separation distance can be seen to cause a DC to DC efficiency drop of 27.7% when the curvature is kept constant, whereas varying the curvature from 0 to 16 m−1 while keeping the separation distance constant only causes the DC to DC efficiency to fall by 8.8%.

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Figure 13:  DC to DC efficiency of the WPT system employing a deformed receiver coil under different curvature against the separation distance from the flat transmitter coil.

SAFETY CONSIDERATIONS A WPT system is by nature going to be radiating electromagnetic energy and while the primary intention is that as much of this energy as possible is absorbed by the receive coil, it is inevitable that some may be absorbed by other materials. In the case of an on-body system, it is likely that some of the radiated energy will be absorbed by the wearer’s body. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) publish guidelines for acceptable field strengths at different frequencies [28]. These guidelines provide limits for electric field strength, magnetic field strength and magnetic flux density for both occupational (repeated) and general public (occasional) situations at a selected operating frequency. The field strengths and flux densities can be simulated using models of the human body as done by Jonah et al. [29], this approach uses ANSYS HFSS simulation software to calculate the magnetic field distribution within the body resulting from an external radiator. A theoretical calculation and experimental verification method has been presented by Clare et al. [30] to calculate the maximum permissible drive power for a WPT system operating at 100 kHz so as to comply with the ICNIRP guidelines. This approach is continued by Worgan et al. [31] to define a design space for the design of WPT devices to be used in on-body applications considering safety, power transfer and user comfort.

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For the purpose of this work on-body testing was not performed since the intention is to concentrate upon the underlying fabrication techniques and the verification of these for use in a WPT system. For compliance with the ICNIRP guidelines, the maximum output of the transmitter drive circuitry should be set so that the measured field strengths and flux densities at the closest possible separation distance are within the limits defined for the frequency of operation. As a further exposure limiting action, commercial examples such as the Qi standard used as the power driver and receiver in this work, operate a polling-type approach whereby the output is energized for a short while and the presence of an appropriate receiver system is queried using communication back from the energy sink. This communication can be achieved by pulsed disconnection of the load, causing the output of the transmitter to be modulated, which can in turn be detected by the driver system. In the case where a suitable energy sink is located power transmission continues, with periodic checks that the energy sink is still present. When an energy sink is no longer present the transmitter output is shut down and the transmitter waits a pre-determined time before repeating the polling process.

CONCLUSIONS A constant-width single-layer circular-spiral coil has been designed and fabricated using screen printing on an interface-coated flexible textile. This provides a novel means of producing a flexible coil directly integrated with a host textile as opposed to methods such as the addition of a flexible PCBbased coil to a separate textile by a method such as sewing, the integration of the coil directly with the textile can befit the end-user’s comfort. The functionality of this flexible coil for use in a WPT system is examined in this paper. The experimental results from the tests of the flexibility and electrical characteristics of the screen-printed coil have been described and show that the flexible screen-printed coils on 65/35 polyester/cotton textile achieve an acceptable flexibility. This has been achieved by using an interface layer (Fabink-UV-IF1) to facilitate the successful printing of the conductive paste with the desired geometry and resolution and to prevent the spreading and permeation of the conductive paste into the textile. The electrical characteristics of screen-printed coils have been measured and the results have been compared with theoretical calculations of the SERs, capacitance and inductance of the coils and found to agree within a reasonable range

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of 17%. A 1.51 W DC output has been achieved by a WPT system using Qi wireless power standard compliant driver and receiver circuitry in conjunction with the screen-printed flexible coils. The ESRs and parasitic capacitance of the printed coils lead to increased losses in the coils when compared to wound copper coils resulting in the loss of energy in the inductive coupling process and therefore reducing the overall efficiency of the WPT system. A comparison of the DC to DC transfer efficiency of the WPT system with different types of coils has shown the DC to DC efficiency of the printed coils to be limited to 37% compared to a 52% efficiency recorded for the wound copper coils. The separation distance of two coils was found to be the major influencing factor on the DC to DC efficiency compared with the effect of deformation of the flexible coils from a planar form.

AUTHOR CONTRIBUTIONS Y.L., N.G. and J.T. conceived and designed the experiments; Y.L. performed the experiments; Y.L., N.G. and J.T. analyzed the data; R.T. contributed materials and fabrication expertise; Y.L., N.G. and S.B. wrote the paper.

ACKNOWLEDGMENTS The authors wish to acknowledge the support of the UK Engineering and Physical Sciences Research Council (EPSRC). Part of this work was performed under the SPHERE IRC funded by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant EP/K031910/1. The authors also acknowledge funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 260034 for the project titled: ‘TIBUCON: Self Powered Wireless Sensor Network for HVAC System Energy Improvement Towards Integral Building Connectivity’, which is part of the ICT for Sustainable Growth Research Program. Beeby also gratefully acknowledges EPSRC support through his Fellowship ‘Energy Harvesting Materials for Smart Fabrics and Interactive Textiles’ (EP/I005323/1). The data for this paper can be found at doi. org/10.5258/SOTON/D0401.

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Hui, S.Y.R.; Wenxing, Z.; Lee, C.K. A Critical Review of Recent Progress in Mid-Range Wireless Power Transfer. IEEE Trans. Power Electron. 2014, 29, 4500–4511. 2. Young-Sik, S.; Minh Quoc, N.; Hughes, Z.; Rao, S.; Chiao, J.C. Wireless power transfer by inductive coupling for implantable batteryless stimulators. In Proceedings of the IEEE MTT-S International Conference on Microwave Symposium Digest (MTT), Montreal, QC, Canada, 17–22 June 2012; pp. 1–3. 3. Sun, X.; Peng, X.; Zheng, Y.; Li, X.; Zhang, H. A 3-D Stacked High-Q PI-Based MEMS Inductor for Wireless Power Transmission System in Bio-Implanted Applications. J. Microelectromech. Syst.  2014,  23, 888–898. 4. Islam, A.B.; Islam, S.K.; Tulip, F.S. Design and optimization of printed circuit board inductors for wireless power transfer system. Circuits Syst. 2013, 4, 237–244. 5. Kipphan, H. Handbook of Print Media: Technologies and Production Methods; Springer: Berlin, Germany, 2001. 6. Mondal, S. Phase change materials for smart textiles–An overview. Appl. Therm. Eng. 2008, 28, 1536–1550. 7. Paul, G.; Cao, F.; Torah, R.; Yang, K.; Beeby, S.; Tudor, J. A Smart Textile based Facial EMG and EOG Computer Interface. IEEE J. Sens. 2014, 14, 393–400. 8. Wagner, S.; Bonderover, E.; Jordan, W.B.; Sturm, J.C. Electrotextiles: Concepts and challenges. Int. J. High Speed Electron. Syst. 2002, 12, 391–399. 9. Younghwan, K.; Sungjoon, L. High efficient misaligned wireless power transfer using magnetic resonant coupling and additional capacitor. In Proceedings of the Microwave Conference Proceedings (APMC), Kaohsiung, Taiwan, 4–7 December 2012; pp. 1049–1051. 10. Zolog, M.; Pitica, D.; Pop, O. Characterization of Spiral Planar Inductors Built on Printed Circuit Boards. In Proceedings of the 30th International Spring Seminar on Electronics Technology, Cluj-Napoca, Romania, 9–13 May 2007; pp. 308–313. 11. Ng, D.C.; Boyd, C.; Shun, B.; Felic, G.; Halpern, M.; Skafidas, E. High-Q flexible spiral inductive coils. In Proceedings of the

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Electromagnetic Compatibility Symposium—Melbourne (EMC Melbourne), Melbourne, Australia, 8–10 September 2010; pp. 1–4. Ashoori, E.; Asgarian, F.; Sodagar, A.M.; Euisik, Y. Design of double layer printed spiral coils for wirelessly-powered biomedical implants. In Proceedings of the IEEE Annual International Conference on Engineering in Medicine and Biology Society, Boston, MA, USA, 30 August–3 September 2011; pp. 2882–2885. Song, Z.; Yahyaoui, W.; Duval, F.; Su, D.; Louis, A. Capturing skin effect with an effective nonuniform mesh and coupled R-L circuits. Electron. Lett. 2011, 47, 94–95. Jakubiuk, K.; Zimny, P. Skin effect in rectangular conductors. J. Phys. A Math. Gen. 1976, 9, 669. Lan, J.; Tang, H.; Gen, X. Frequency Splitting Analysis of Wireless Power Transfer System Based on T-type Transformer Model. Electron. Electr. Eng.2013, 19, 109–113. Greconici, M.; Madescu, G.; Mot, M. Skin effect analysis in a free space conductor. Facta Univ.-Ser. Electron. Energ. 2010, 23, 207–215. Kiani, M.; Uei-Ming, J.; Ghovanloo, M. Design and Optimization of a 3-Coil Inductive Link for Efficient Wireless Power Transmission. IEEE Trans. Biomed. Circuits Syst. 2011, 5, 579–591. Xue, R.F.; Cheng, K.W.; Je, M. High-Efficiency Wireless Power Transfer for Biomedical Implants by Optimal Resonant Load Transformation.  IEEE Trans. Circuits Syst. I Regul. Pap.  2013,  60, 867–874. Wei, W.; Qiang, F. Design and simulation of printed spiral coil used in wireless power transmission systems for implant medical devices. In Proceedings of the IEEE Annual International Conference on Engineering in Medicine and Biology Society, Boston, MA, USA, 30 August–3 September2011; pp. 4018–4021. Zierhofer, C.M.; Hochmair, E.S. Geometric approach for coupling enhancement of magnetically coupled coils. IEEE Trans. Biomed. Circuits Syst. 1996, 43, 708–714. Li, Y.; Torah, R.; Beeby, S.; Tudor, J. An all-inkjet printed flexible capacitor for wearable applications. In Proceedings of the IEEE Symposium on Design Test, Integration and Packaging of MEMS/ MOEMS (DTIP), Taipei, Taiwan, 28–31 October 2012; pp. 192–195.

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22. Smart Fabric Inks Ltd. UV Curable Pastes for Use on Textiles: FabinkUV-IF1 Datasheet. Available online: www.fabinks.com (accessed on 12 January 2018). 23. Smart Fabric Inks Ltd. Smooth Interface Layer Paste for Printing on Fabrics. Available online: www.fabinks.com (accessed on 12 January 2018). 24. Texas Instruments. bq500211 bqTESLA Wireless Power TX EVM. Available online: www.ti.com/lit/ug/slvu536a/slvu536a.pdf (accessed on 12 January 2018). 25. Texas Instruments. bq51013AEVM-765 Evaluation Module. Available online: www.ti.com/lit/ug/sluu911a/sluu911a.pdf (accessed on 12 January 2018). 26. Kazimierczuk, M.K.; Czarkowski, D. Resonant Power Converters; John Wiley & Sons: Hoboken, NJ, USA, 2012. 27. Bogin, B.; Varela-Silva, M.I. Leg length, body proportion, and health: A review with a note on beauty. Int. J. Environ. Res. Public Health 2010, 7, 1047–1075. 28. ICNIRP. ICNIRP Guidelines for Limiting Exposure to Time-Varying Electric and Magnetic Fields (1 Hz–100 kHz). Health Phys. 2010, 99, 818–836. 29. Jonah, O.; Georgakopoulos, S.V.; Tentzeris, M.M. Wireless power transfer to mobile wearable device via resonance magnetic. In Proceedings of the IEEE 14th Annual IEEE Conference on Wireless and Microwave Technology Conference (WAMICON), Orlando, FL, USA, 7–9 April 2013; pp. 1–3. 30. Clare, L.; Worgan, P.; Stark, B.H.; Adami, S.-E.; Coyle, D. Influence of Exposure Guidelines on the Design of On-Body Inductive Power Transfer. In Proceedings of the IEEE Wireless Power Transfer Conference (WPTC), Boulder, CO, USA, 13–15 May 2015; pp. 1–4. 31. Worgan, P.; Clare, L.; Proynov, P.; Stark, B.H.; Coyle, D. Inductive power transfer for on-body sensors defining a design space for safe, wirelessly powered on-body health sensors. In Proceedings of the IEEE 9th International Conference on Pervasive Computing Technologies for Healthcare (PervasiveHealth), Istanbul, Turkey, 20–23 May 2015; pp. 177–184.

CHAPTER

5

PRINTING SMART DESIGNS OF LIGHT EMITTING DEVICES WITH MAINTAINED TEXTILE PROPERTIES Inge Verboven 1,2, Jeroen Stryckers 1,2, Viktorija Mecnika 3,4, Glen Vandevenne 1,2, Manoj Jose 1,2, and Wim Deferme 1,4,5 Institute for Materials Research (IMO-IMOMEC)—Engineering Materials Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium 1

and

Interuniversity MicroElectronics Center (IMEC), IMOMEC, Universitaire Campus— Wetenschapspark 1, 3590 Diepenbeek, Belgium 2

Institute of Textile Technology of RWTH Aachen, Otto Blumenthal Strasse 1, 52074 Aachen, Germany 3

Institute for Design Technology, Riga Technical University, Kalku Street 1, LV-1658 Riga, Latvia 4

Flanders Make vzw, Oude Diestersebaan 133, B-3920 Lommel, Belgium

5

Citation: Inge Verboven, Jeroen Stryckers, Viktorija Mecnika, Glen Vandevenne, Manoj Jose, and Wim Deferme, Printing Smart Designs of Light Emitting Devices with Maintained Textile Properties, doi:10.3390/ma11020290 Copyright © This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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ABSTRACT To maintain typical textile properties, smart designs of light emitting devices are printed directly onto textile substrates. A first approach shows improved designs for alternating current powder electroluminescence (ACPEL) devices. A configuration with the following build-up, starting from the textile substrate, was applied using the screen printing technique: silver (10 µm)/barium titanate (10 µm)/zinc-oxide (10 µm) and poly(3,4ethylenedioxythiophene)poly(styrenesulfonate) (10 µm). Textile properties such as flexibility, drapability and air permeability are preserved by implementing a pixel-like design of the printed layers. Another route is the application of organic light emitting devices (OLEDs) fabricated out of following layers, also starting from the textile substrate: polyurethane or acrylate (10–20 µm) as smoothing layer/silver (200 nm)/poly(3,4ethylenedioxythiophene)poly(styrenesulfonate) (35 nm)/super yellow (80 nm)/calcium/aluminum (12/17 nm). Their very thin nm-range layer thickness, preserving the flexibility and drapability of the substrate, and their low working voltage, makes these devices the possible future in lightemitting wearables. Keywords:  Electroluminescence, OLED, printing, textiles

INTRODUCTION Smart luminous textiles are of great interest for applications such as clothing, interior design and visual merchandizing. Moreover, luminous textiles are beneficial for protective clothing and sportswear in order to improve safety by a higher visibility and interactive design for non-verbal communication. Additionally, luminous textiles have potentials for healthcare and medicine applications such as phototherapy. At present, such smart textiles are mostly limited to the integration of light emitting devices (LED) or optical fibres [1]. This approach however is limited to small-scale luminous textiles. An optional solution for the implementation of large-scale luminous surfaces on textiles is brought by applying printing technologies. There are a number of research projects that have investigated scenarios to incorporate light-emitting devices on textiles [2,3]. These address alternating current powder electroluminescence (ACPEL) and organic light emitting diodes (OLED) technology. Nevertheless, these are mostly limited to printing luminous structures on non-textile substrates and subsequently integrating them onto textile surfaces.

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This work addresses two different approaches implemented directly on textiles substrates: improved screen-printed designs of ACPEL devices and direct deposition of OLEDs. Diverse smart designs of the ACPEL devices were suggested to preserve textile properties such as flexibility, drapability and air permeability. The complete layer stack silver (Ag) (10 µm)/barium titanate (BaTiO3) (10 µm)/zinc-oxide (ZnO) (10 µm)/poly(3,4ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) (10 µm) was applied using the screen printing technique. A pixel-like design of the printed layers was selected and different geometries were implemented. In order to comprehend the interaction between the textile substrate, the applied functional layers and the selected design, the samples were tested on flexibility, air permeability and light output and their morphology after mechanical stress was investigated. Organic light-emitting devices (OLED) are more challenging to apply directly on textiles, but promise to be the future in light-emitting wearables. These devices were build out of the following layers: polyurethane (PU) or acrylate (10–20 µm) as the smoothing layer/Ag (200 nm)/PEDOT:PSS (35 nm)/super yellow (80 nm)/calcium/ aluminum (Ca/Al) (12/17 nm) ending up with a device stack of maximum 0.5 µm and therefore maintaining the flexibility and drapability of the textile substrate. Due to the roughness of the textile substrate a planarizing layer of polyurethane (PU) or acrylate (10–20 µm) had to be applied directly on the substrate before completing the rest of the stack. OLEDs have a high brightness and a low power consumption. To protect these devices from fast degradation from contact with oxygen or water vapour, an encapsulation layer is necessary.

RESULTS ACPEL Devices Literature shows that ACPEL devices can be printed on a variety of substrates [4]. The thickness of the complete device is about 40 µm and mostly it is applied as full area coverage. However, this will mask the benefits of the textile such as air permeability and drapability. Therefore, in this work, a special design, based on a hexagonal cell structure is proposed. The design of the stack can be seen in Figure 1a.

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Figure 1: (a) Design of the single layers of the ACPEL (alternating current powder electroluminescence) build-up; (b) a zoom on a hexagon cell, showing clearly the applied size and number of the pixels; (c) SEM (Scanning electron microscopy) micrographs of the different printed ACPEL devices; (d) images of the light-emitting area of the ACPEL stacks.

Both the bottom layer (Ag) and the top layer (PEDOT:PSS) are screen printed in this honeycomb structure. The line width is 0.5 mm and both layers are deposited in such a way that they are not touching, to prevent electrical shorts. The dielectric layer and the light emitting layer consist of 1.5 to 2.5 mm pixels. They can be printed on each crossing of the hexagon structure or on half of them. By changing the diameter and the number of pixels per hexagon, the light emission, but also the air permeability and the crease recovery, can be adapted. A schematic view of a hexagon cell structure is depicted in Figure 1b. Scanning electron microscopy (SEM) is applied to look at the final printed ACPEL device in detail. In Figure 1c, SEM images of the ACPEL devices printed on the polyester textile are shown. From left to right, the diameter of the pixels is changed from 1.5 mm over 2 mm up to 2.5 mm. This is the case for the first three images from the left, where only three pixels are printed in one hexagon cell. Also, for the last three SEM

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images, this change of diameter is applied but now six pixels are printed in one hexagon cell. It is clear from these SEM micrographs that the area of uncoated textile changes (dark grey area) when altering the diameter and the number of pixels per cell. In Figure 1d, the light emission can be noted. Due to the design of the ACPEL stack, only the pixels show light emission. Finally, Figure 2 indicates the influence of the design on the properties of the textile substrate after screen printing the ACPEL device.

Figure 2: The air permeability (a) and the crease recovery angle (b) of the screen printed ACPEL devices on polyester textile substrates.

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In Figure 2a, the air permeability, as measured by a FX3300 LabAir IV Air Permeability tester (Textest AG, Schwerzenbach, Switzerland), for the different designs is shown. It is clear from this graph that more air can pass if the diameter of the pixels is smaller. This is logical, of course, as less textile surface is covered. The difference between the single pixel structure, where only three pixels are printed per hexagon call, for the different diameters is however not as big as for the dual pixel structure, with six pixels per hexagon cell. It can also be noted that the difference between the dual pixel structure, with a diameter of 1.5 mm and the single pixel structure, with a pixel diameter of 2.5 mm and 2 mm, is comparable. This graph is in correspondence with the calculated area coverage of the textile substrate. In Figure 2b one can see the crease recovery measurements. In this experiment, the textile is folded and kept as such for 5 min and for 30 min by applying a weight of 1 kg on top of the double folded textile. After these 5 or 30 min, the weight is removed and it is recorded how far the textile will reverse back to its initial state. This is denoted as the crease recovery. From the figure it can be seen that, in comparison to an uncoated polyester substrate (last line of the graph), the crease recovery was smaller for all samples. However, in comparison with ACPEL devices printed as a full covering on the polyester textile, the crease recovery, especially for the single pixel structures, is very good. The light output performances were acquired using a Keithley 2401 (Keithley, Cleveland, OH, USA) source to measure the current and voltage characteristics and an absolute calibrated integrating sphere spectrometer from Avantes to determine the irradiance per wavelength [5]. The light output was obtained by comparing the electrical power to the coated area. In Figure 3 the light output of the different designs are compared to that of a fully covered ACPEL device. This demonstrates that the light output is halved when a dual pixel design is used instead of a fully covered surface. The light output of a device with the single pixel design is even less than one fourth of that of a fully covered device.

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Figure 3: Light output of the ACPEL devices.

Based on these experiments and on the light emission measurements, which can be found in [5], the single pixel structure with a diameter of 2 mm is seen as the most optimal, when reserving the textile properties has priority. When, however, the light output is of high importance, the best option can be found in the dual pixel design with a diameter of 1.5 mm.

OLED Devices Applying OLEDs to textiles is not that straightforward as is the case for the screen-printed ACPEL devices discussed above. First of all, the total thickness of the OLED stack is only 0.5 µm, which is even smaller than the roughness of the underlying textile substrate. Further, the deposition techniques to do so are not as standard as the screen printing technique from above. The advantages of using OLEDs, however, are numerous. Since they are made of very thin nm layers, the devices can be applied to flexible substrates. The emitted light has a high brightness, a uniform light output and a wide range of vision. OLEDs require a low power supply (3–5 V), have a low energy consumption and a good efficacy. Important disadvantages or challenges, however, have to be taken into account. The devices degrade very quickly due to water vapour and oxygen. Therefore water vapor transmission rates (WVTR) and oxygen transmission rates (OTR) must be lower than respectively 10−6 g·m−2 per day and 10−3 cm3·m−2 per day, indicating a

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very high barrier layer is necessary [6]. Some of the applied techniques to deposit the OLED layers are very expensive and not roll-to-roll compatible. However, more and more less expensive and roll-to-roll compatible printing techniques are emerging. These other deposition techniques and the OLED stack to be applied to textiles will be discussed in more detail in this part of the paper. As mentioned, the surface of the textile substrate is quite rough (µm-range) compared to the nm-range layer thickness of the OLEDs. This roughness can be ruled out by the deposition of a planarizing or covering layer. Printable PU or acrylate are therefore laminated on top of the textile substrates as is shown in Figure 4, with a thickness between 10 and 20 µm to bring the micrometer roughness of the textile substrate to a nanometer roughness.

Figure 4: The textile substrate covered with PU (polyurethane) (a) or acrylate (b) to smoothen the textile roughness towards nm-range.

As previously stated, OLEDs degrade immediately in ambient conditions, making good encapsulation indispensable. Therefore a transparent barrier layer is applied using plasma techniques. A first oxygen-free silicon nitride (SiN) base layer serves as a protection for later depositions. This layer is followed by an alternating system of high barrier inorganic materials (such as silicon oxide (SiOx)) and a softer, low barrier organic materials. This barrier system brings a halt to defect formation and subsequently increases the diffusion length and the barrier properties. More information on the topic of encapsulation can be found in [7]. The bottom electrode or anode is a thermally evaporated silver (Ag) layer of 200 nm. Subsequently the hole injection/transport layer PEDOT PSS, a polymer mixture, is spin coated to obtain a 35 nm film. As an active layer, the PPV polymer Super Yellow is used to spin coat a layer of 80 nm inside an inert atmosphere glovebox system. Both the lab-scaled spin coating and thermal evaporation technique

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can be replaced by inkjet printing and ultrasonic spray coating. Inkjet printing is a contactless printing process where a digital image is recreated by ejecting ink droplets onto a substrate. The large-area deposition technique ultrasonic spray coating forms layers by atomizing the ink at the nozzle of the spray head into a continuous flow of micro sized spherical droplets. Both techniques are less expensive and roll-to-roll compatible. It was shown in earlier work of the authors [8] that the active light-emitting layer can be ultrasonically spray coated without changing or damaging the polymer side-chain or backbone of the PPV polymer. As the textile substrate is not transparent, a top emitting polymer OLED (TEOLED) is prepared where the photons have to escape the device through the top transparent electrode or cathode. To obtain a transparent cathode, two different methods are tested in this work, i.e., applying printed metal grids or evaporating very thin metal layers. For comparison, inkjet-printed Ag grids and very thin thermally evaporated golden (Au) layers were assessed by their transparency and sheet resistance. Hexagonal and triangular shaped Ag grids were inkjet-printed on glass substrates with a thickness of 150–250 nm. They showed a low sheet resistance of 0.82–2.7 Ω/□ and a high transparency of 70–90%. Very thin and completely covering Au layers of 1–15 nm were thermally evaporated on glass substrates. Here a higher sheet resistance of 3.2–123.7 Ω/□ and a lower transparency between 25–70% was found. An overview of these results can be seen in Figure 5.

Figure 5: The transparent top electrode can be deposited by applying a full covering Au layer or by inkjet printing Ag grids.

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Considering only these two characteristics, the Ag grids score much better on both as can also be found back in earlier work of the authors [9]. However, the used commercially available Ag ink has to be sintered at a temperature of 200 °C, which will destroy all underlying layers. New Ag inks, based on precursors rather than on Ag nanoparticles, are now available with a considerable lower sintering temperature [10] and therefore, applying this grid structure is the most promising. A low work function material, such as calcium (Ca), has to be used in between the light emitting layer and the top Ag layer to align the energy levels for the proper functioning of the OLED. This material is usually thermally evaporated. Therefore, at this time, preference was given to a thermally-evaporated Ca/Ag cathode of respectively 12 and 17 nm. In this work, the complete OLED stack was deposited on glass, PET and textile substrates. The encapsulated glass OLED sample had some visual defects and pinholes, as can be seen in Figure 6. After applying the barrier layer, the OLED sample was taken out of the glovebox system to investigate the effects of ambient conditions on the encapsulation. After 19 h the OLED had already lost more than half of its light output and after 43 h only a few luminous pixels were visible. This shows that applying the barrier layer is a promising encapsulation strategy, but more research is needed to improve the OLED’s characteristics and lifetime.

Figure 6: The OLED (organic light emitting diodes) after before and after encapsulation showing a fast degradation with full fading after only 43 h.

Finally, the OLED stack has been deposited onto PET foil and textiles. The OLED on PET had a nice uniform light output and could be bent without any output loss or cracks in the layers, displaying the flexibility of the OLED device as shown in Figure 7. However, for the textile-based OLED, only a few luminous pixels could be distinguished. The reasoning behind this bad light emission for the textile-based OLED is that this device employed printable PU as planarizing layer. This PU layer was effected by the chlorobenzene used as solvent for the light emitting polymer Super

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Yellow. Consequently a lot of defects were introduced into the OLED stack, making an informal light output impossible.

Figure 7: From left to right: OLED on PET (polyethylene terephthalate) just after deposition; OLED on PET showing the stable light emission even during flexing and bending; OLED stack printed on textile.

DISCUSSION AND CONCLUSIONS It has been shown in this work that light emitting devices can be printed on textile substrates applying different designs and different printing and coating techniques. First of all, an ACPEL device is fully screen-printed in an adapted, smart design such that the breathability and the drapability of the textile substrate are enhanced. It was shown that adapting the design (diameter and number of pixels per hexagon cell) can influence the air permeability and the crease recovery. The application of OLEDs on textiles shows several advantages, being very thin and flexible devices with a low power supply, low energy consumption, a good efficacy, a bright and uniform light output and a wide range of vision. Nevertheless, the usage of a high barrier layer is necessary and some applied deposition techniques are not rollto-roll compatible and quite expensive. At the moment high barrier layers are still applied using a combination of printing and plasma techniques, but for the actual OLED stack layers alternative techniques have been pushed forward, such as inkjet printing and ultrasonic spray coating. Adequate research into a proper barrier layer, a planarizing layer, a transparent top electrode and roll-to-roll deposition techniques is ongoing and will bring the OLED technology from the class of PET foils towards textile substrates. The combination of both results presented in this paper can finally lead to a pixelated OLED structure on textile substrates for enhanced light emission without hampering the textile properties.

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MATERIALS AND METHODS As mentioned above two diverse technologies for lighting are examined on their printability on textile substrates. Figure 8 shows the typical layer build-up of an ACPEL device. All of the layers, with a thickness of 10 µm, are deposited on top of each other using the screen printing technique. The textile used in this work was a polyester woven fabric (100% PES—washed and fixated—kw11401 from Concordia Textiles (Valmontheim, Belgium) with a roughness average Ra of 6 µm. The first Ag layer (from Gwent) fulfils a dual purpose, as a bottom electrode and as a planarizing layer. This layer is followed by a dielectric layer (BaTiO3 from Gwent, Pontypool, United Kingdom) and a light emitting layer (Cu-doped ZnS from Gwent, Pontypool, United Kingdom). They are stacked in between two electrodes and therefore, a capacitor build-up is achieved. A transparent top-electrode (PEDOT:PSS EL-P 3145 ink from Orgacon, Mortsel, Belgium) completes the stack. After screen printing each layer, they are subsequently thermally annealed at 130 °C for 10 to 30 min. When a AC voltage of 80 V is applied with a frequency of 400 Hz, light is generated and coupled out through the transparent top-electrode.

Figure 8: Build-up of the ACPEL technology.

For the second approach, organic light-emitting diodes (OLED) are deposited. The TEOLED stack (Figure 9) is produced by implementing different deposition techniques to apply the layers on glass, polyethylene terephthalate (PET) and textile substrates. Due to the relatively high surface

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roughness of the textile substrates, an additional planarizing/covering layer is required. To equalize the surface polyurethane (PU) or acrylate is laminated onto the textile substrate with a thickness between 10–20 µm. By applying plasma techniques, a barrier layer, composed out of a stack of alternating organic and inorganic layers with a total thickness of 1 µm, was added on top of the substrate or planarizing layer. Afterwards, an Ag anode of 200 nm is thermally evaporated at a base pressure of 10−7 mbar. Under a fume hood a hole injection/transport layer PEDOT PSS (Clevios™ P AI 4083 from Heraeus, Hanau, Germany) of 35 nm is spin coated. As an active layer the PPV–polymer super yellow (PDY-132 from Merck, Darmstadt, Germany) (Figure 10) was dissolved in chlorobenzene with a mass concentration of 5 mg/mL and stirred overnight at 50 °C. A layer of 80 nm was spin-coated in an inert atmosphere glovebox system (O2/H2O ppm