Author name used in this publication: Hing Yee J. TsangAuthor name used in this publication: C. W. M. YuenVersion of RecordPublishe

Cheng, X

Leung, MY

Li, Y

Tao, X

Tsang, HYJ

Xue, P

Yuen, CWM

PolyU Institutional Repository

Proceedings of the 1st IEEE InternationalConference on Nano/Micro Engineered and Molecular SystemsJanuary 18 - 21, 2006, Zhuhai, ChinaPolypyrrole-coated Fabric Strain Sensorwith High Sensitivity and Good StabilityXiaoyin Cheng 1*, E, Yang Li 2, Xiaoming Tao, Member, IEEEPu Xue 1, Xiaoxiang Cheng', and C. W. M. ', Hing Yee J. Tsang ', Mei Yi Leung 1,V-i I ,n I'Nanotechnology Center, ITC, The Hong Kong Polytechnic University, China2Department ofPolymer Science and Engineering, Zhejiang University, ChinaAbstract-The sensitivity and stability are mainly factors to holdback the practical applications of Polypyrrole coated fabrics. Inthis paper, a flexible fabric strain sensor with high sensitivity,good stability and large deformation is reported. It is fabricatedby depositing a nano-layer (200nm to 300nm) of polypyrrole onthe fabric substrate at low temperature. Thickness andmorphology of the conducting thin film on the surface of thefibers were examined by scanning probe microscopy (SPM) andscanning electron microscopy (SEM). The measurement of theconductivity change with strain shows the fabrics so preparedexhibits a high strain sensitivity of 160 for a deformation aslarge as 50%, while its good stability is indicated by a small lossof conductivity after the thermal and humidity aging tests, andsupported by the slight change in conductivity and sensitivityover a storage of eighteen months. The flexible strain sensor isexpected to be a promising "soft" smart material with goodsensing properties in the preparation of smart garment, wearablehardware and biomedical applications.Keywords-fabric strain sensor; polypyrrole; stabilityI. INTRODUCTIONPolypyrrole (PPy) has been widely investigated asactuators, chemical sensors, biomaterials, batteries, gasseparation films, etc. [1]. When it forms composites withfabrics, the obtained conductive fabrics can also be used inelectromagnetic insulation, microwave absorbance, etc. due tothe combination of both the mechanical properties of theflexible fabrics and the electrical, microwave properties, andbiocompatibility of the PPy coatings [2, 3]. In recent years, itwas found the conductivity of the PPy-coated fabrics is alsosensitive to strain, thus the conductive fabrics can be used tomeasure and control the movement of human body andconstruct the wearable devices [4].However, these conductive fabric strain sensors exhibit lowsensitivity and unsatisfying stability. D. Rossi et al. developeda sensorized glove based on the PPy-coated Lycra/cotton, butthe sensor was aged severely in air and the conductivitydecreased continuously. Furthermore, the saturation of thesensor occurred at a small strain of about 6% in [4]. K. W. Ohet al. reported the PPy-coated Nylon-spandex was sensitive tostrain change until a deformation of 50%, but the sensitivity isas small as no more than 2 in [5]. X. P. Jiang et al. alsoproposed the PPy-coated PET/Sapndex can be used as a strainsenor for a large deformation of up to 50%, while thesensitivity is only 3 in [2].We report a PPy-coated flexible fabric strain sensor whichwas prepared by depositing a nano-layer (200nm to 300nm) ofpolypyrrole on the fabric substrate at low temperature. Thesensor so fabricated shows both high strain sensitivity for alarge deformation and a good environmental stability.II. EXPERIMENTA. Preparation ofthe Fabric Strain SensorPPy-coated fabric strain sensor was prepared by chemicalvapor deposition (CVD) at the low temperature. The typicalprocedure is as follows: Fabrics composed of 83% Tactel and17% lycra (195g/m2, from Sunikorn Knitters Limited) was firstimmersed into the solution containing sodium dodecyl benzenesulfonate (0.011 mol/L) and FeCl3 (0.1 mol/L), and the wettake-up was controlled by using a padding machine (PA-0,Rapid Labortex Co. Ltd). The fabrics and a beaker containing10 mL pyrrole monomer were transferred into a desiccator,which was evacuated under vacuum and flushed with N2repeatedly and put in a fridge where the temperature is keptbelow -10 °C. The vapour phase polymerization of polypyrroleproceeded on the surface of the fabrics under vacuum. Theblack fabrics so obtained were then washed with deionizedwater and ethanol, respectively, and dried at 400C undervacuum. The annealing was carried out by heating the driedfabrics at 600C for 40 hours under vacuum.B. MeasurementThe strain-stress properties of the PPy-coated fabrics wereobtained using an Instron testing Instrument (Model 4466)under the standard testing conditions (T = 250C and RH =65%). The fabrics were repeatedly stretched and relaxed for 10cycles with the maximum extension up to 12.5 mm (50%deformation) in each cycle. The conductivity change of thesensing fabrics in both stretched and relaxed states wasrecorded using a digital multi-meter (Keithley Model 2010) toinvestigate its strain sensitivity. SEM images of the pristine andPPy-coated fabrics were obtained using a scanning electronmicroscope (Leica stereoscan 440). The morphology andthickness of PPy film on the surface of the fibers wereexamined by a scanning probe microscope system(SPI4000SPA300HV from Seiko Instruments) under ambientconditions. SPM images were obtained from cross-sections andthe surface of a composite fiber, respectively. Both thermal andhumidity aging tests were carried out by recording theconductivity change of the PPy-coated fabrics put in a climaticchamber (Hotpack Series 922), where both the temperature andThis project wasfunded by the Innovative Technology Fund, The SARGovernment ofHong Kong (Project code. (ZPOD (ITS/071/02)).*Contact author. tcxyc@inet.polyu.edu.hk).1-4244-0140-2/06/$20.00 C)2006 IEEE 1245humidity can be controlled. For the thermal aging, the humidityis always kept at 65%RH. The temperatures investigatedinclude 20, 35, 50 and 600C and the temperature changes everytwo hours. The humidity aging was carried out at 300C. Thehumidities investigated include 40, 55, 70 and 90%RH and thehumidity changes every two hours.III. RESULTS AND DISCUSSIONWith respect to low sensitivity of PPy-coated fabric sensorprepared by the solution polymerization in [2, 5], we proposethe low sensitivity is related to the thick coatings composed ofmultilayers of PPy deposited on fabric substrate. Themultilayers of PPy may not change in the same way as thesubstrate when the fabrics are elongated. To obtain a flexiblestrain sensor with both high sensitivity and good stability fromPPy coated fabrics, we employ the method of CVD at lowtemperature to obtain very thin and dense coatings of PPy onthe surface of a Tactal (83%)/Lycra (17%) fabrics.To understand the mechanism well, morphology andthickness of PPy film on the surface of the fibers wereexamined by SPM. In Fig. 1(a), the insulating region is shownin the darker region and the electrically conductive region inthe lighter region. PPy displayed a continuous pathway ofconductive domain with 200nm to 300nm in thickness onTactal fibre base. Fig. l(b), (c), and (d) gives the phase imagesof Tactal fibre and topographical views from differentviewpoints, respectively. Fig.1 (b) and (c) show that the PPycoating film is composed of well-connected PPy granules,covering up whole fibre surface with finer, denser, and moreuniform. Fig. 1 (d) shows the height of the rugged PPy coatinglayer on the Tactal fibre. As shown in Fig. 1, the PPy tended toform a connected layer on the substrate when polymerizationwas carried out. Before the well-connected first layer formed,PPy oligomer nucleated on the substrate in numerous places.Due to the strong interaction between the substrate and PPy,the nucleation rate is fast and the distance between nuclei isrelatively small. The nuclei grows quickly in the lateraldirections and spread throughout the surface of the fibres. Theinitially connected Layer comes from multiple nucleation siteswith either isotropic or anisotropic growth rates on thesubstrate. Further work is required to investigate the interactionbetween the PPy and substrate polymers.Fig. I (a) Conductive layer of PPy-coated fibre. The scanned area is 6 X6umFig. 1 (b) Morphology of the PPy coating layer from differentviewpoint.Fig. I (c) Morphology of the PPy coating layer from differentviewpoint.Fig. I (d) Height of PPy-coated layer. The scanned area is 6 X 6um1246Fig. 2 (a) SEM images of pristine fabrics.The low temperature polymerization of pyrrole on surfaceof fabrics was helpful for obtaining thinner coatings resultingin more adherent film and more ordered structure of PPy withhigher conductivity. Fig. 2(a) , (b) and (c) show SEMphotography of the film of pristine fabrics, PPy-coated fabricsprepared by CVD at low temperature and by solutionpolymerization, respectively. It reveals that the fabrics coatedwith PPy formed by CVD exhibits a very smooth surface as thepristine one, while that prepared by the solution polymerizationis featured with a rough surface. The thin PPy coatings arehelpful to improve strain sensitivity; however they areunbeneficial for environmental stability. A smooth and denselayer of PPy film is more resistant to the attack of oxygen andwater molecules, thus enhances its stability.Fig. 2 (b) SEM images of PPy-coated fabrics prepared by CVD at lowtemperatureFig. 2 (c) SEM images of PPy-coated fabrics prepared by solutionpolymerization.The typical conductivity change-strain curves of the PPy-coated fabrics prepared by CVD at low temperature during tencycles of elongation and relaxation measurements areillustrated in Fig. 3. The resistance of the sensor increasedsharply with the extension up to a large deformation of 50%,and the sensing curves are almost the same during ten cycles ofthe test, suggesting good repeatability of the sensor.Normalized resistance FAR] is about 80, and the strainL Ro jsensitivity AR ] is as high as about 160, where AR are theresistance change of the fabric under extension and Ro is theoriginal resistance, i.e. the resistance of the fabric in relaxedstate, £ is the extension, respectively. To our best knowledge,this is the highest sensitivity reported up to date.ruN 48 -UE200D so IC. ISO C0 2Tl 300Tkii e OCFig. 3. The conductivity changes with the elongation of PPy-coatedfabrics (deformation=50%).1247In addition to high sensitivity, good environmental stabilityis especially important for the practical application. Theconductivity degradation of PPy is closely related to itsreaction with oxygen, especially under elevated temperatures,and moisture. Therefore, the stability of the strain sensors wasfirst investigated by the thermal aging and humidity aging tests.The thermal aging test is composted of six cycles of heatingand cooling tests. The temperature ranged from 20 to 600C, andthe conductivity change with temperature of the sensors duringthe thermal aging process is presented in Fig. 4. It can be seenthat the difference in resistance between the heating andcooling cycle is quite small. In addition, only a slight increasein resistance is observed after each heating-cooling cycle,which may be related to the stability of the dopant ion and therearrangement of the PPy chain. It is found that theconductivity drop at 200C is as small as 16% after the thermalaging of six cycles, which is indicative of the good stability ofthe sensor. Furthermore, we find that the conductivity of thesensor shows only a slight dependence on the temperature, andthe temperature coefficient is calculated to be only -0.6%/°C.a)(ntY)151stm 2nd= 3rd4th-C>-- 5th1111,11V6th10 V---,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-------X-------t105. ,,,, I10 20 30 40 50 60Humidity can also exert an influence on the conductivity ofPPy. The stability of the sensor was further investigated by thehumidity aging. The conductivity change with humidity of thesensor during six cycles of humidifying and desiccating testswas plotted in Fig. 5. It can be seen that the difference inconductivity for the humidifying and desiccating process isvery small, suggesting little hysteresis. The largest conductivityloss was observed at 40%RH, which is only 15 % afterhumidity aging of six cycles, suggesting the good stability ofthe sensor. It was also found that the humidity increase canonly slightly decrease the resistance of the sensor, and theconductivity change when the humidity is increased from 40 to90%RH is only 0.66 %.The thermal and humidity aging tests show that theconductivity of the strain sensor from PPy-coated fabrics isonly slightly dependent on the humidity and temperature, whilethe sensor exhibits very high strain sensitivity. Therefore, it canbe concluded that the effect of the temperature and humidity onthe conductivity of the strain sensor can be ignored providedthey fall in the range already investigated during the aging tests,which is an advantage for its potential application.<V0cM70z70Temperature (°C)Fig. 4. Resistance change versus temperature during the thermal agingtests0a)a)cJ),cn30 40 50 60 70Humidity (%)80 90ThIe See.)Fig. 6 (a) Conductivity changes versus strain of the sensors prepared atlow temperature before and after the storage.0:100Fig. 5. Resistance change versus humidity during the humidity aging testsThe (tec.,Fig. 6 (b) Conductivity changes versus strain of the sensors prepared atroom temperature before and after the storage124814-1 St*-2nd3rd4th12- 5th6th.... ............... ......................10 610- * S ------- @~~~~~~~~*8 ==-TABLE I. THE RESISTANCE CHANGES DURING THE STORAGE OF THESENSORS PREPARED AT DIFFERENT TEMPERATURESPreparationtemperatureLow temperatureRoom temperatureInitial resistance(k Ohm)11.270.1Resistance afterstorage (k Ohm)13.2312A more direct proof of the stability of the PPy-coatedfabrics is the variation of the conductivity and sensingproperties of the sensor during storage of a long time. Table Ipresents the resistance of the sensor prepared at both low androom temperatures before and after the storage of abouteighteen months. And their strain sensing properties before andafter the storage is illustrated in Fig. 6. It can be seen that thesensor prepared at low temperature exhibits a conductivity lossof only -15% after eighteen months, and its strain sensingproperties is almost unchanged. In contrast, the one prepared atroom temperature shows a conductivity loss of- 80%. Inaddition, the strain sensitivity also decreased by a halfTherefore, it can be concluded that the flexible strain sensorfrom the PPy-coated fabrics so prepared shows goodenvironmental stability and the low temperature preparation isbeneficial for the improving the stability of the sensorIV. CONCLUSIONIn summary, a polypyrrole-coated fabric strain sensorfeatured with high strain sensitivity, good stability and largedeformation has been developed. It is proposed that thechemical vapor deposition of thin coatings of polypyrrole onfabrics surface leads the high strain sensitivity of the sensor,while the low temperature polymerization of pyrrole helpsimprove its stability. Morphology and thickness of PPy filmon the surface of the fibers examined by SPM and SEM helpunderstand the mechanism well. The PPy-coated fabric strainsensor so prepared is expected to find applications in sensinggarment, wearable hardware and rehabilitation, etc.REFERENCES[1] G. G. Wallace, G. M. Spinks, L. A. P. K. Maguire, and P. R. Teasdale,Conductive Electroactive Polymers. Intelligent Materials Systems, 2nded. CRC Press, Boca Raton, FL 2003.[2] X. P. Jiang, D. Tessier, L. Dao, and Z. Zhang, "Biostability ofelectrically conductive polyester fabrics: An in vitro study," J. Biomed.Mater. Res., vol. 62, pp. 507-513, 2002.[3] D. D. Rossi, A. D. Santa, and A. Mazzoldi, "Dressware: wearablehardware," Mater. Sci. Eng. C, vol. 7, pp. 31-35, 1999.[4] E. P. Scilingo, F. Lorussi, A. Mazzoldi, and D. D. Rossi, "Strain-sensingfabrics for wearable kinaesthetic-like systems," IEEE Sensors J.2003,vol. 3, pp. 460-467, 2003.[5] K. W. Oh, H. J. Park, S. H. Kim, "Stretchable conductive fabrics forelectrotherapy," J. App. Polym. Sci., vol. 88,1225-1229, 2003.1249

Polypyrrole-coated fabric strain sensor with high sensitivity and good stability

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