Indian Journal of Fibre & Texti le Research Vol. 31. M arch 2006. pp. 170- 176

Electronic textiles and their potential

Tushar Ghosh" & Anuj Dhawan

College of Te xti les. N C State University. Raleigh , North Caro lina 27695. USA

Tex tile matcr inl s nrc generall y li ghtweigh t. flex ible and unique in many ways compared to other mater ials. Most imponant ly, they are omnipresent in our li ves. Tex tiles are necessary next to our skin as well as in our envi ronment. They arc used for comfort nnd protection as 1vell as fashion. In the ncar future. almost all textile products including what we wear and walk on seem destined to be transformed f rom their present to mul tifunctional . adaptive and responsive systems. The functions may include communication. computation and cmcrtainmen t, as we ll as health care and threat detecti on. Tex ti les used in non-apparel appl icat ions may perform surve illance and detection funct ions. Some of the concepts bei ng explored currentl y may revo lutioni ze our undcrstnnding and ::1ppreciation of f'i ber- tex tile products. Th is paper reports the developments in the fi eld of' electronic text iles . focusing on the curren t state-of- the-art of electrotex tile products and the research being c;.~r r ied out in this field.

Keywords: Electronic textile. Electromagnetic induct ion shielding, Static charge di ss ipat ion, Sensors. Texti le ci rcuit

IPC Code: ln t. Cl 8 D02G3/00. G06K7/00. HOIR4/00

I lnti·oduction Electronic texti les or electrotextil es can be

described as textile products with 'i ntegrated' electronic capabilities. The level of the integrati on at thi s point may vary from simply using a textile product as a platform with plugged in passive electronic devices to higher level integration of electronic capabilities designed or fabricated into a fiber or texti le substrate. The ubiq uitous textile products havi ng large surface area and other desirable characteristics (l ightweight, etc.) arc obvious choices as platforms for deploying sensors and other electron ic devices over large areas in an economical manner. It has been an active area of research in recent times and by all indications seems to be gathering steam. Many commercial electrotextil e products are already being manufactured and marketed. In the last Tech texti l trade fair in Frankfurt, Germany, there were about ten companies displayi ng their products rangi ng from fabric-based heating systems to keyboards.

Currentl y, electron ic textiles are being developed for many applications. incl uding biomedical sensing, wearable computing. and large area sensors. Ongoing research and development activities in this area are truly multidisciplinary ; it is envisioned that the fundamental research being carried out in areas of materials/polymer sc ience and vari ous engineering

·'To whom all the correspondence should be addressed. E-mail : tghosh@ncsu.edu

discipl ines are likely to result in future unobtrusive electronic devices, sensors, batteries and solar cells. full y integrated at the fiber/polymer level or built into the text ile structures. The possi bili ties that this techno logy holds seem almost limitless.

In a world whi ch is becoming rapidly in terconnected by technology, the assimilation of these technologies to other systems in use today, as well as specifically targeted des igns for future products, wi 11 not only lead to enhanced functionalities in ex isting products but also wil l spawn new products. This paper reports the recent developments in electron ic textiles and highlights the ongoing research and developmen t in thi s fast­growi ng field.

2 Electronic Textiles Today Examination of the CUlTen t state-of-the-art of

electrotextile products being man ufactured as \Nell as being developed generally reveals two clearly del ineated areas of this technology in terms of the level of integration of electron ics in tex tile materi als. Textile product ancl/or fabr ic level in tegrati on of electronics seem to be more common today. This includes many examples of textile products that are used primarily as a carrier to plug in electronic devices, in some instances, into data buses integrated into the fabric. Examples of fiber/polymer level integration at thi s point are few. Most of these concepts are currently the subjects of research. The discussion that follows includes the applicati ons in

GHOSH & DHAWAN: ELECTRONIC TEXTILES & THEIR POTENTIAL 171

\vhat has been termed more recently as electrotextiles as well as some of the traditional uses of conductive fibers or other material s in textiles.

2.1 Electromagnetic Induction (EIVII) Shielding

Conducting fibers have been used for a long time 1 n producing fa brics used for e lec tromagnetic shi eld ing and fo r elect rostatic charge dissipation. Fabrics containing conducting fibers or coated with conducting polymers, like polyaniline and polyacetylene, provide effect ive sh ielding from rad iated electromagnetic interference. 1-3 Kim er a/.4

have developed polypyrrole coated polyester fabrics with relatively low surface resisti vities fo r providing shielding against electromagnetic in te rference. Kuhn et u/. 5 have discu ssed the EMl shielding properties of polypyrrole coated polyester fabrics. A patent by Ebneth and Fitzkl describes a layered arrangement of metallized texti le sheets (separated by insulating layers), \vh ich is used for screening electromagnetic waves in the broad range from 1 0 M Hz to 1000 G Hz. These textile-based metalli zed sheets are fl exible and conformable, have clas tic properties. and attenuate th e electromagnetic rad iation in transmiss ion by 30 - 40 dB. Another patent by Dordevic7 describes the development of a fabric from conducti ve yarns (cotlon and steel fiber blends) that guarantees a shielding of 20-40 dB aga inst electromagnetic rad iation at a frequency of I 0 GHz.

2.2 Static Chm·ge Dissipation

In the past few years, research has been carried out to investigate the use of conducting fabr ics fo r static charge dissipation. Tappura and Nurmi8 have discussed the di ss ipation of electrostatic charge from fabrics conta111 111g conducti ve fibers and have developed a computer model to explain the process. In a paper by Lowell and Mcintyre'1, the mechani sms for discharge of stat ic charge on fabri cs and carpets containing conducting fibers have been discussed. The effect of inco rporating conducting fibers (carbon­coated fibers and fibers with a conducting core) into the fab rics and carpets, on the elec trostati c discharge properties, has bee n reported in thi s paper. Dhawan et of. 1 al so can·ied ou t static charge measurements on blank polyester fabrics and polyaniline-coated polyester (conducti ng) fabrics and found that there was sign ifican t dissipation of static charge by the conducting fabrics. Kacprzyk and Domagalai(1

developed a model to ex plain the electrostat ic di scharge of carpets containing conducting fibers.

2.3 Thermal Applications

Conductive fibers are being used for many thermal applications like flexible heating pads, electrical heating blankets and jackets. Blankets and jackets that use stain less steel microfibers and yarns woven into a fab ri c have been developed by Polartech. 11 Based on the principle of res isti ve heating of the conductive elements, these conduct ive yarns act as a heatin g source and provide uni form distri bution of heat throughout the entire fabric. A heating jacket was also developed by Norrhfacc 12 wi th electrical wires integrated into the jacket.

2.4 Circuits and Circuit Boards

Electrical circuit built into a textile substrate is one of the necessary building blocks of electroni c text ile products . Fl ex ible circuit boards on fabric su bstra tes

. . . h . h b j il - l(> us1ng pnnt1ng tee 111ques ave een reportec. · Patents by DeA ngel is ef al. 1

', Pi ttma n and Kuhn 1" .

Gregory el a/. 15, and Kuhn and Kimbre11 16 describe

conductive text iles in \Vhi ch conductive polymer films are patterned on certain selected areas of woven or knitted fabrics according to given designs 01: patten1s. Post et a/.17 have described the use of electronic embroidery as a means of patterning conductive textiles by numerically controlled sewing processes to produce electronic textil e products. They have described applications of textile-based circuitry in fabri cating fabric keyboards, pressure se nsors , and others. One of the most sign ifi cant recent developments in the application of conducting threads to electron ics has been the attempt to fo rm networks of conducting threads in a fabric by weaving or knitting. Electrically or optically conducting fibers are arranged and woven in a particular manner so as to transmit elec tri cal or optical signals from one part of the fabric to the other for communi cation between different dev ices placed on a fabric or for wireless coupling with a remote electron ic device. 18-:!

4 In a patent by Lebby et al.21

, a communicative wristwatch whi.ch consists of a wristband made up of woven co nductive yarns, an electronic unit and a power source integrated into the woven wris tband, has been ·. described. The wristband has conductive and non­conducting fibers woven orthogonal to one another to define a simple grid . The conducti ve fibers/yarn s provide . wired coupling between the electroni c unit and the power source, and with all other co mponents attached to the wristband. The vvoven co nducti ve fab ric can also act as an antenna for transmission and receipt of signals.

172 INDIAN 1. FIBR E TEXT. RES .. M ARCH 2006

Recently, Dhawan et a/. ~c. - 24 reported fabrication of electrical circuits in woven fabrics . They explored vari ous potential methods to form interconnects and disconnects at crossover points in woven circuits, necessary to route signals. They fou nd res istance vvelding: as an effecti ve means of fo rming the interconnects. They al so discussed two kinds of resistance welding processes. i.e. top-bottom probe weldi ng and para llel gap weld ing. Interestingly, they reported formation of di sconnects - necessary to route

. . ..,.., signals usmg parallel gap res tstance welc!mg.--Scanning electron microscope (SEM) images of resistance-welded and unwelded fabric samples of copper and steel conductive threads reported by Dhawan era/ . ~2 -24 is shown in Fig. I.

In design ing fabric-based circuits, signal integrity issues (crosstalk noise) are also of importance. Ohm-van

~ . . et at.- observed that the magnttucle of cross talk nmse increased as the spacing between the conducting lines decreased (Fig. 2). They reported significant reduction in crosstalk noise when twis ted-pair and coaxial yarns were used to form these circuits.

2.5 Sensing Devices and Systems

The area of sensi ng may be the most ac ti ve research area in electrotex tiles. Unobtrusive sensing or human physiological signs to sensing of ex ternal envi ronmental parameters includi ng threat are major areas of interes t. Woven fabric networks with integrated sensing elements have already been described. lnaba et a/.25 described a conducting fabric-based tacti le sensor system in which a garment made from a mu ltilayer sensing fabric covers the entire body of a robot. This fabric helps the robot to sense wheneve r any object or human being touches it. Fu lly integrated fabric-based tactile sensors in thi s sensor suit arc di stributed on the entire body of the

(a) (b)

Fig. l -SEM minograph:-; of w ppcr (l ight) and polyes ter (cla rk) lhro:: ads. Plan view of a welded interconnect using (a) top-bollol!l probe res istive welding, <l nd (b) par;d lel -gap res isti ve we lding~~

iii ~

robot and a network of conducting fibers conveys the sensed signals to the sensor processoL De Rossi et ol.26

-30 at the University of Pisa have done some of the

pioneeri ng work in the area of strai n sensing ustng textile substrates. Their work is relevant fo r many appl ications, including motion sensi ng ;:mel physiolor, ical monitori ng. De Rossi et a/. Hi

demonstrated remarkable strain and te mperature sensin g capabili ty of conventional clastorneric fa bri cs coated with a thin layer of conducting polymer polypyrrole. They demonstrated the piezores istive properties of these fabrics by fabr icating a sensing glove. The change in resistance caused by fabric strain cl ue to finger movement was measured. Lorussi et a/. 2~ applied the same concept to instrLiment a sleeve that is capable of sensing body posture . They used printed carbon fi lled rubber layers on fab ric and created an interconnected array of sensors to determine the surface strain fi elds induced by movemen t. The composite materi al used in the sensor fab rication has appropriate level of conduc tive particl es embedded in an elastomeric bincleL Resi sti vity of the of the compos ite is altered due to application of press ure.

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Spaci ng between parallel l ines (i nches)

Fig.2-Crossralk noise between woven interconnect li nes for a I vol t signal ( l M l-lz frequency) sent into an aggressor li ne: (a )

signal on the quiet l ine for 0.125 inch spacing between paralkl twisted pair li nes. and (b) crosstalk noi se l 's . spac ing between tw is1ed pair and ba re copper lin es~3

GHOSH & DHAWAN: ELECTRONIC TEXTrLES & THEIR POTENTIAL 173

Various sensing devices and systems compatible with textiles (o r sometimes textile based) for continuous monitoring of physiological and behavioral parameters for health care have been proposed? 9

-32 ln all cases, the use of functionalized

materials enabled design and fabrication of sensors and electrodes. Sci!ingo et al.29 reported the use of knitted fabrics made of stainless steel wrapped viscose yarn as electrodes to monitior signals of electrocardiogram and electromyogram. Additiona!!y, they also reponed the use of e lastic fabrics coated with carbon loaded rubber to monitor respiratory changes. Similar efforts to design electrodes and strain sensors for health monitoring have also been reported by Paradiso et a/. 31

The development of garments with capabilities to continuously monitor the· wearer's biomedical information and body vital signs, environmental conditions, and garment penetration by a ballistic material have also been reported.:13

.34 In a patent by

Jayaraman et al 3\ a complete process for the

development of a woven garment with intelligence and sensing capabilities has been described. One of the intelligence capabilities of the garment (called Sensate liner) described in the patent is the ability to monitor garment penetration by a ballistic material and this intelligence capability can be achieved by including optical fiber sensing components in the weave of the garment. Another such sensor system developed is the life-shirt system34 which is used to collect physiological data of patients without affecting their normal routines. Sensors and conductive elemen ts are woven into the life-shirt at optimal locations (near the chest and abdomen) to get data on the patient's respiratory system.

15 . . Jones and Leftly· at SOFTSw1tch Ltd have

developed a technology that enables fabric-based materials to be used for pressure sensing. The mechanism of sensing is similar to what has been described earlier. Soft switch fabrics show a very broad resistance change when compressed. The soft switches can be used for occupancy sensing based on seat pressure mapping for fabric control surfaces in automotives, airlines , home furnishings and tloors, for wearable computing, and for developing keypads (Fig . 3). In a patent by Gilbert and Boyce36

, a fabric­based weight sensor for the seat of a motor vehicle has been described . Thi s sensor includes a compressible, preferably foam layer disposed between two conductive fabric ·layers. In another patent by

1 -~~ l·i rr=i f~r~1u:

.;, ~~-.~1•1"il ...., __ · ·: - ·- - ' '- -- . ~ ..

(a)

(b)

Fi~r. 3-(a) Some appli ca ti ons of Soflswitch technology" , and (b) fabric-based ci rcuits with e lectronics imegratecl inro the l"ab ric 17

Sandbach et a/. 37, a laminate conducting fabric sensor

has been described. It consists of two conducting planes (with strips of conductive lines) separated by a mesh of insulating fibers. On application of pressure, the outer conducting planes are brought into mechanical contact through the insul ating mesh. The amount of current tlowing as a result of this contact will vary in dependence upon the actual position of the plane where the mechanical interaction takes pl ace.

Application of context awareness to wearable I . h b d. d . I 18. 19 e ectron1cs as een 1scusse 111 severa papers.- -

Context awareness can be defined as making the electronic devices CaiTied by people in everyday life more aware of the environment and activity of the users so that the functionality of the devices could be modified accordingly to suit the activities and situation of their wearers. This context awareness can be achieved by using sensors that can feed information into the electronic devices CaiTied by the user. In a paper by Randell and Muller38

, a method of determination of context awareness using Global Posi tioning Systems (GPS) has been described.

El-Sherif et a/. 40 have integrated optical fiber-based chemical and biological sensors into uniforms or soldiers. A miniature chemical sensor has been developed for the detection of the presence of organophosphate dimethylmethylphosphonate (DM­MP) gas. Development of the sensor includes replacing the cladding material of the optical fibers with active

174 INDIAN J. FI BRE TEXT. RES .. MARCH 2006

materi als like polypyrrole and polyaniline. Exposure to DMMP changes the refracti ve index of fiber cladding material (polypyrrole) and thereby reduces the transmitted in tensity of light signal transmitted through the optical fiber. These optical fiber sensors have been integrated into woven (with twill , satin and plain weaves) and knitted structures. Influence of the different fabric structures and constructions on the loss of intensity of signals transmitted through the \.\10ven or knitted opti cal fibers has been studied. El-Sherif et a/.41

have also reported the incorporati on of fiber optic strain sensors into textile structures.

2.6 Energy Storage and Harvesting Devices

Power supply is an essential component of electron ic texti le products. A number of publications reported developments in this area. Reppelinger and Coates42 disclosed development of a nonwoven fabri c bat tery electrode in a patent. It uses conductive fibers to form a porous mat, whi ch is used as a negative electrode fo r a hydrogen storage ce ll. fn another paten t by lkkanzaka et a!. -n , the development of nonwoven fab ri cs for a storage battery separator has been described. This sheet has a property of holding electrolytes and is produced from conjugate fibers that have a specific cross-sectional form. These conjugate or bi -component fibers are formed using the melt ex trusion process with one of the components of the fiber being a random copolymer of ethylene and polyacrylic acid and th e other being polypropylene. These developments have paved the way for the development of batteries based enti rely on textiles. Inni s er al.H have reported the development of complete textil e-based batteries. Inherently conducting polymers are also being developed for wearable energy storage systems. These fabric batteries and photovoltaics have been developed utili zing ionic liquid electrolytes that could be incorporated into textile membranes . Fabric-based electrodes have been used to form a tlexible and conformable battery system. These textile-based electrodes were fo rmed by coating conducting polymeric materials on fabrics or by in situ or vapor phase polymeri zation of the monomers of conductive polymers on fabric substrates.

Significant research is being carried out in the development of flex ible and conformable solar cells. Samuelson et a/.45 have reported development of solar cells on polymeric films and woven fabrics by coating these substrates with organic dye molecules and titanium dioxide. The mi xture of organic dye

molecules and the titani um dioxide nano-particles is sanclviichecl between two conductive electrodes. There is a layer of electrolyte prese nt between the organic dye and titanium dioxide mix ture and the lower electrode. The top conductive electrode is transparent and allows li ght to pass through it and react with the organic dye molecules. There is charge generati on clue to the absorpti on of light by the dye molecules. The charge generated is transferred to the titanium di oxide nano-particles and collected by the electrodes, thereby producing electrical energy fro m solar energy.

2.7 Transistors on Thin Films/Thn!ads

In the area of fl ex ible electron ics and e:ectronic textiles, one of the most signifi cant research developments is the fabrication of trans istors on thin films that cou ld be sl it into narrow film s and woven into a fabric (Fi g. 4). Research in this area is being carried out at Princeton, North Carolina State, and North Texas Universities.~6-48 Fabrication of these transistors has been done on amorphous sili con. Layers of n-cloped and intrinsic amorphous silicon are deposited on silicon nitride, which is deposited at a low temperature on fl ex ible Kapton films and the transistors are formed on the amorphous si licon. Pl as ma enhanced chemical vapor deposition process was used for the deposition of amorphous sili con and . si licon nitride. The gate, source and the drain contacts of the transistor are depos ited by usmg thermal

Source contact <:late conl.act TFT Drain contact

(a)

I (b)

Fig. 4-(a) Source, gate, and dra in comacts of the transistor fabricated on u thin Kapton film, and (b) thin fi lm transistors woven in to a fabri c and interconnected to form a desired logic c ircuitr/ 6

GHOSH & DHAWAN: ELECTRONIC TEXTILES & THEIR POTENTIAL 175

deposi tion. All the patterning IS done us in g conventional photolithography . The film s with transistors formed on it are cut into thin strips which are then woven into a fabric-like structure. Interconnecti ons between the different trans istors are made using conductive st rips to demon strate digi tal logic fun ctional ity. Development of these transistors is significant because it proves that if good quality transistors could be fabr icated on thin fil ms and these thin films could be woven into a fabric , an all fab ric­based integrated circuit could be formed o n fabri c substrates at the speed of large-scale fabr ic production . Lee and Subramanian49 reported fabrication of transisto rs directly on fibers without us ing conven ti onal lithography process . They achi eved patte rning v ia shadowing from fiber to fiber crossover.

3 Future of Electronic Textiles Based o n the current state of e lectronic textiles

research, it can be assumed that in the short term, the fi e ld of electronic textiles would involve attachment of electronic devices , sensors, etc. to conductive e lemen ts integrated in to a textile to form fl ex ible electron ic products. In the lo ng run , inco nspicuou s devices , inc lud ing power supp ly and some level of information process ing will be built into the structures of the fibe rs, yarns and fab rics. Many other issues such as redundancy, fault to lerance, packaging, e tc. have to be addressed . Additionally, a ll-polymer or po lymer-metal conductive ' tex tile ' yarns have to be developed to rep lace curre ntly used co nnecting medium copper or s teel. The f uture electrotextil e products not o nly inc lude wearables to address indi vidual needs but also sensor arrays useful for c ivil ian and military applicatio ns. Additi onall y, in the future, the e lectroac ti ve materials are like ly to be incorporated into text ile struc tures to make it adapti ve and respo nsive to a llow for shape, surface and structure modification desirab le for many applications. It is envis ioned that future electronic textil es would have both sensing and actuation e lements built into it.

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