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A flexible strain sensor from polypyrrole-coated fabrics
Synthetic Metals 155 (2005) 89–94
A flexible strain sensor from polypyrrole-coated fabrics Y. Li a,∗ , X.Y. Cheng b , M.Y. Leung b , J. Tsang b , X.M. Tao b,∗,1 , M.C.W. Yuen b a b
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Institute of Textiles and Clothing, Hong Kong Polytechinc University, Hong Hum, Hongkong, China Received 16 May 2005; accepted 8 June 2005 Available online 10 October 2005
Abstract Conductive-polymer coated fabrics have been investigated as intelligent materials in the past years. In this paper, a flexible strain sensor from polypyrrole-coated fabrics which is featured with high sensitivity, good stability is reported. The strategies used for enhancing the sensitivity and stability of the sensor include: (a) the formation of thin coatings of polypyrrole (PPy) on the surface of fabrics using the chemical vapor deposition (CVD) method, (b) low temperature polymerization of pyrrole, (c) introduction of large docecyl benzene sulfonate anion in PPy film and (d) annealing of the PPy-coated conductive fabrics. The conductivity–strain tests reveal that the developed sensor exhibits a high strain sensitivity of ∼80 for a deformation as large as 50%, while its good stability is supported by the small changes in conductivity and sensitivity over a storage time of 9 months. The effect of the temperature and humidity on the conductivity of the strain sensor is investigated. © 2005 Elsevier B.V. All rights reserved. Keywords: Strain sensor; Polypyrrole; Conductive polymer; Intelligent materials
1. Introduction In the past decades, conductive polymers have been intensively investigated in the applications, including chemical and biosensors, actuators, photovoltaics, rechargeable batteries, separation films, etc. [1]. As one of the most important conductive polymers, polypyrrole (PPy) received much attention because of its ease of preparation, high conductivity, good environmental stability, and non-toxicity. PPy filament was used as chemical sensors for detection of gases and vapors [2] and PPy films can be employed to construct artificial muscles with tactile sensitivity [3] and up to 12% linear strain [4]. Due to the good adhesion with different substrates, PPy can also form composites with a number of non-conductive fibers or fabrics to prepare the electrical conductive textiles. Owing to the advantages of both the mechanical properties of the flexible and elastic substrate, and the electrical, microwave properties and biocompatibility of the PPy-coating, the composites can be used as flexible smart materials for applications in electromagnetic interference ∗
Corresponding authors. Tel.: +86 571 87952444. E-mail addresses: [email protected] (Y. Li), [email protected] (X.M. Tao). 1 Tel.: 852 2766 6490. 0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.06.008
(EMI) shielding, broadband microwave absorbing, static charge dissipation, biomedical and tissue engineering, etc. [1,5–11]. Some authors also reported the application of the PPy-coated fabrics as the sensing fabrics for detection of strain to enable the measurement and control of various movement of human body, which may be used as an intelligent material for preparation of wearable devices for training, fitting, rehabilitation, etc. [5,6,12,13]. However, the flexible strain sensors exhibit low sensitivity and unsatisfying stability. Rossi et al. developed a sensorized glove based on the sensing fabrics of polypyrrole-coated Lycra/cotton, but the sensor was aged severely in air and the conductivity decreased continuously. Furthermore, the saturation of the sensor occurred at a small strain of about 6% [13]. Oh et al. reported that the PPy-coated Nylon–Spandex was sensitive to strain change until a deformation of 50%, but the strain sensitivity is as small as not more than 2 [6]. Kim et al. also proposed the PPy-coated PET/Spandex can be used as a strain sensor for a large deformation up to 50%, while the strain sensitivity is only 3 [5]. In this paper, a flexible strain sensor from PPy-coated fabrics prepared by a chemical vapor deposition process instead of the traditional solution polymerization method is reported. The strain sensitivity, environmental stability and the effect of
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temperature and humidity on the conductivity of the sensor are investigated. 2. Experimental 2.1. Preparation of the polypyrrole-coated fabrics A typical procedure for preparation of PPy-coated fabrics by CVD is as follows: plain knitted fabric of 83% Tactel blended with 17% (40 denier) Lycra in weight of 195 g/m2 (Sunikorn Knitters Limited, HK) was first immersed into the aqueous solution containing sodium dodecyl benzene sulfonate (0.011 mol/L) and the ethanol solution of FeCl3 (0.1 mol/L) in sequence, and the wet take-up after each immersion was controlled to be ∼100% by using a padding machine (PA-0, Rapid Labortex Co. Ltd.). The fabrics and a beaker containing 10 mL pyrrole monomer were transferred into a desiccator, which was put in a fridge where the temperature is kept at −26 ◦ C. The vapour phase polymerization of polypyrrole proceeded on the surface of the fabrics under vacuum for 50 h. The black fabrics so obtained were then washed with de-ionized water and ethanol, respectively, and dried at 40 ◦ C under vacuum. The annealing was carried out by heating the dried fabrics at 60 ◦ C for 40 h under vacuum. A typical procedure for preparation of PPy-coated fabrics by solution polymerization is as follows: fabrics were immersed in a bottle containing 100 mL aqueous solution of FeCl3 (0.2 mol/L) and stirred for 1 h, then 100 mL of the aqueous solution of sodium dodecylbenzenesulfonate (0.05 mol/L) and 0.8 mL of pyrrole was added into the bottle slowly with stirring. The polymerization proceeded at room temperature for another 1 h with continuous shaking. The black fabrics so obtained were washed with water and ethanol, respectively, dried in vacuum at 40 ◦ C for 16 h. 2.2. Measurement The strain–stress properties of the PPy-coated fabrics were obtained using an Instron testing Instrument (Model 4466) under the standard testing conditions (T = 25 ◦ C and RH = 65%). The fabrics were repeatedly stretched and relaxed for 10 cycles with the maximum extension up to 12.5 mm (50% deformation) in each cycle. The conductivity change of the sensing fabrics in both stretched and relaxed states was recorded using a digital multimeter (Keithley Model 2010) to investigate its strain sensitivity. SEM images of the pristine and PPy-coated fabrics were
obtained using a scanning electron microscope (Leica stereoscan 440). ATR FT-IR spectra were measured on a Bio-Rad FTS6000 spectrometer. The effect of temperature and humidity on the conductivity of the sensor was investigated by recording the conductivity change of the PPy-coated fabrics put in a climatic chamber (Hotpack Series 922), where both the temperature and humidity can be controlled. For temperature test, the humidity is always kept at 65% RH. And the humidity test was carried out at 30 ◦ C. 3. Results and discussions The strategy to prepare a flexible strain sensor with both high sensitivity and good stability from PPy-coated fabrics is shown in Scheme 1. It was reported that the conductive fabrics prepared by solution polymerization of pyrrole on the surface of the fabrics usually show a low strain sensitivity of not more than 3 even though the deformation is as large as 50% [5,6]. It is proposed that the low strain sensitivity is related to the thick coatings composed of multi-layers of PPy deposited on the fabric surface during the solution polymerization. When the coated fabric is elongated, the multi-layers of PPy may not change in the same way as the substrate. Unless under a very large deformation, the relative position and the contact of the conductive PPy-coatings may not change much with the elongation of the fabrics, this leads to a small change in conductivity and thus low strain sensitivity resulted. For this reason, we employed the method of chemical vapor deposition (CVD) to obtain a very thin coating of PPy on the surface of a Tactal (83%)/Lycra (17%) fabrics. Due to the existence of elastic Lycra, the as-prepared composites show good reversibility under large-strain deformation of up to 100%. The coated fabrics show almost the same ATR FT-IR spectra as the pristine fabrics, which suggests that the PPy-coatings formed by CVD are very thin [9]. The typical curves of conductivity change with strain of the PPy-coated fabrics prepared by CVD and solution polymerization during 10 cycles of elongation and relaxation measurements are illustrated in Fig. 1a and b, respectively. The resistance of the sensor prepared by CVD increased sharply with the extension up to a large deformation of 50%. The strain sensitivity is defined as R/εR0 , where ε is the deformation, R are the resistance change of the fabric under extension and R0 is the original resistance, i.e. the resistance of the fabric in relaxed state, respectively. It can be calculated from Fig. 1 that the strain sensitivity of the sensor is as high as 80 for a deformation of 50% (ε = 0.5). As to our best knowledge, this is the highest sensitivity for the
Scheme 1. Strategy for preparing a flexible strain sensor from PPy-coated fabrics with both high sensitivity and good stability.
Y. Li et al. / Synthetic Metals 155 (2005) 89–94
Fig. 1. The conductivity changes with strain of PPy-coated fabrics prepared by (a) CVD and (b) solution polymerization (deformation = 50%).
large deformation reported up to date. In contrast, the sensor prepared by solution polymerization shows the strain sensitivity as low as about 6. The difference of the highest sensitivity of each cycle of extension and recovery is termed RD, which is used to evaluate the sensing reproducibility. It was found that RD is less than 1 for the sensor prepared by the CVD method, indicating that the sensor has very good reproducibility. In addition to high sensitivity, good environmental stability is especially important for its possible application in smart garment, rehabilitation and biomedical fields. Thin PPy-coatings were prepared on the surface of the fabrics by using the CVD method to improve the strain sensitivity. However, the thickness of PPy can influence its air stability as reported by Wang et al. [14] and the thin coatings of PPy are unbeneficial for its stability. It was reported that a smooth and dense layer of PPy film is more resistant to the attack of oxygen and water molecules, thus enhances its stability [7,15,16]. Kanynak et al. [17] proposed that the low temperature polymerization of pyrrole on surface of poly(ethylene terephthalate) fabrics was helpful for obtaining thinner coatings, resulting in more adherent film and more ordered structure of PPy and thus higher conductivity. Therefore, in the present study, polymerization was carried out at a
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low temperature of −26 ◦ C in the preparation of the PPy-coated fabrics. Fig. 2a–c shows scanning microscopy (SEM) photography of the film of pristine fabrics, PPy-coated fabrics prepared by CVD at low temperature and by solution polymerization, respectively. It reveals that the fabrics coated with PPy formed by CVD exhibits a very smooth surface as the pristine one, while coated fabric prepared by the solution polymerization is featured with a rough surface. Annealing is another method used for improving the stability of the conductive fabrics. Singh et.al. investigated the effect of the heat annealing on the conductivity and surface structure of PPy, and found that annealing will lead to the removal of residue solvent, the restructuring and reordering of PPy chain, thus stabilizing the conductivity [18]. Therefore, the conductive fabrics developed in the project were annealed by heat treatment under vacuum. In order to avoid destroying the structure and stability of the fabrics, the annealing was carried out by heating at only 60 ◦ C under vacuum for ∼40 h. Fig. 3 shows the curves of conductivity change of a strain sensor under cyclic extension before and after the annealing. It can be seen that the annealing process increases the strain sensitivity. In addition, the values of RD of the sensor decreased from 16 to 6 after the annealing, indicating improved reproducibility and stability. It was known that the dopant can effectively affect the stability and also the conductivity of PPy [1,19–24]. PPy film doped with larger anionic species, such as dodecylsulfate and dodecylbenzenesulfate displayed smother morphology than the cauliflower-like appearance of PPy/Cl− or PPy/ClO4 − film [25]. Kudoh found that the introduction of large-sized anions in the PPy film resulted in the enhancement of the thermal and humidity stability, since the large size dopants are difficult to de-dope even at high temperature and humidity [19]. Kuhn et al. also proposed that dopant anions can affect the stability of PPy film, which may partly be ascribed to its effect on the film morphology [7]. Also, the PPy/Cl− system was reported to have worse stability than those doped with organic acid. In our work, it was also found that the PPy-coated fabrics prepared with only Cl− as the counterion increase the resistance by more than 10-folds after storage of several months. The result agrees with the results from Scilingo et al. that the conductive fabrics prepared by solution polymerization with inorganic ClO4 − as the counterion faced the problem of rapid decrease in conductivity [13]. Considering the fact that the doping agent should be of low toxicity for practical application, sodium dodecyl benzene sulfonate, a bulky aromatic sulfonate was used as the additional doping agent for the preparation of conductive fabrics so as to provide good stability. This bulky anion has long aliphatic chain and its large dopant ion is more stable than the small Cl− ion. In addition, its inclusion in the PPy film is expected to result in a smoother morphology and more compact coatings. It may protect the PPy from the penetration and attack of air and moisture, and thus improve the stability of the sensor from PPy-coated fabrics. The stability of the PPy-coated fabrics was supported by the small variation of the conductivity and strain sensing properties of the sensor after storage of a long time. Table 1 presents the resistance of the sensor prepared at both low and room temperatures before and after the storage of about 9 months. Their strain
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Fig. 2. SEM images of (a) pristine fabrics and PPy-coated fabrics prepared (b) by CVD at low temperature and (c) by solution polymerisation.
sensing properties before and after the storage is illustrated in Fig. 4. It can be seen that the sensor prepared at low temperature exhibits a conductivity loss of only ∼20% after storage for 9 months, and the change in strain sensitivity is only about one
for a deformation of 50%. In contrast, the one prepared at room temperature shows a conductivity loss of ∼80% and its strain sensitivity also decreases by a half. It can thus be concluded that the flexible strain sensor from the PPy-coated fabrics prepared at low temperature show good environmental stability and the low temperature preparation is beneficial for improving the stability of the sensor. The conductivity of PPy is usually affected by temperature. The effect of temperature on the conductivity of the PPy-coated fabrics is presented in Fig. 5. It can be seen that the conductivity of the sensor increases with the increase in temperature, and the temperature coefficient is calculated to be −0.6%/◦ C. Humidity can also exert an influence on the conductivity of PPy. The adsorbed moisture can promote the free motion of Table 1 The resistance changes during the storage of the sensors prepared at different temperatures
Fig. 3. The conductivity changes with strain of PPy-coated fabrics (a) before and (b) after annealing (deformation = 50%).
Preparation temperature
Initial resistance (k)
Resistance after the storage (k)
Low temperature Room temperature
9.2 69.0
11.5 351
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Fig. 6. The effect of humidity on the conductivity of the strain sensor.
doping ions in the PPy film, thus increase its conductivity [20]. It was reported that PPy can even be used for detecting humidity [26]. The conductivity change of the sensor with humidity was plotted in Fig. 6. It can be seen that the humidity increase can slightly decrease the resistance of the sensor. The conductivity of the sensor changes only 0.66% when the humidity increased from 40 to 90%RH. 4. Conclusions
Fig. 4. The conductivity changes with strain of the sensors prepared at (a) low temperature and (b) room temperature before and after the storage (deformation = 50%).
In summary, a flexible strain sensor from polypyrrole-coated fabrics that is featured with both high strain sensitivity and good stability has been developed. It is proposed that the chemical vapor deposition of thin coatings of polypyrrole on fabric surfaces results in high strain sensitivity of the sensor, while the low temperature polymerization of pyrrole, annealing of the PPycoated fabrics and the introduction of bulky dodecyl benzene sulfonate anion into the polypyrrole backbone helps to improve the stability of the PPy-coated sensors. The PPy-coated fabrics are expected to find applications in sensing garment, wearable hardware and rehabilitation, etc. Acknowledgements The work is financially supported by the Innovative Technology Fund, The SAR Government of Hong Kong, and the Internal Research Fund, The Hong Kong Polytechnic University. References
Fig. 5. The effect of temperature on the conductivity of the strain sensor.
[1] G.G. Wallace, G.M. Spinks, L.A.P. Kane-Maguire, P.R. Teasdale, Conductive Electroactive Polymers: Intelligent Materials Systems, 2nd ed., CRC Press, Boca Raton, FL, 2003. [2] G. Jin, J. Norrish, C. Too, G. Wallace, Curr. Appl. Phys. 4 (2004) 366. [3] T.F. Otero, M.T. Cort˙es, Adv. Mater. 15 (2003) 279. [4] L. Bay, K. West, P.S. Larsen, S. Skaarup, M. Benslimane, Adv. Mater. 15 (2003) 310. [5] H.K. Kim, M.S. Kim, S.Y. Chun, Y.H. Park, B.S. Jeon, J.Y. Lee, Y.K. Hong, J.S. Joo, S.H. Kim, Mol. Cryst. Liq. Cryst. 405 (2003) 161. [6] K.W. Oh, H.J. Park, S.H. Kim, J. App. Polym. Sci. 88 (2003) 1225. [7] H.H. Kuhn, A.D. Child, W.C. Kimbrell, Synth. Met. 71 (1995) 2139.
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[8] S.H. Kim, S.H. Jang, S.W. Byun, J.Y. Lee, J.S. Joo, S.H. Jeong, M.J. Park, J. Appl. Polym. Sci. 87 (2003) 1969. [9] D. Tessier, L.H. Dao, Z. Zhang, M.W. King, R. Guidoin, J. Biomater. Sci. Polym. Edn. 11 (2000) 87. [10] C.Y. Lee, D.E. Lee, C.K. Jeong, Y.K. Hong, J.H. Shim, J. Joo, M.S. Kim, J.Y. Lee, S.H. Jeong, S.W. Byun, D.S. Zang, H.G. Yang, Polym. Adv. Technol. 13 (2002) 577. [11] X.P. Jiang, D. Tessier, L. Dao, Z. Zhang, J. Biomed. Mater. Res. 62 (2002) 507. [12] D.D. Rossi, A.D. Santa, A. Mazzoldi, Mater. Sci. Eng. C 7 (1999) 31. [13] E.P. Scilingo, F. Lorussi, A. Mazzoldi, D.D. Rossi, IEEE Sens. J. 3 (2003) 460. [14] Y.D. Wang, M.F. Rubner, L.J. Buckley, Synth. Met. 41–43 (1991) 1103. [15] Y.C. Liu, B.J. Hwang, J. Electroanal. Chem. 501 (2001) 100. [16] V.T. Truong, B.C. Ennis, T.G. Turner, C.M. Jenden, Polym. Int. 27 (1992) 187.
[17] A. Kaynak, R. Beltran, Polym. Int. 52 (2003) 1021. [18] R. Singh, A.K. Narula, R.P. Tandon, S.U.M. Rao, V.S. Panwar, A. Mansingh, S. Chandra, Synth. Met. 79 (1996) 1. [19] Y. Kudoh, Synth. Met. 79 (1996) 17. [20] C. Cassignol, P. Olivier, A. Richard, J. Appl. Polym. Sci. 70 (1998) 1567. [21] L.X. Wang, X.G. Li, Y.L. Yang, React. Funct. Polym. 47 (2001) 125. [22] R.V. Gregory, W.C. Kimbrell, H.H. Kuhn, J. Coat. Fabrics 20 (1991) 167. [23] J.C. Thi˙eblemont, A. Brun, J. Marty, M.F. Planche, P. Calo, Polymer 36 (1995) 1605. [24] M. Omastov´a, J. Pionteck, M. Trchov´a, Synth. Met. 135–136 (2003) 437. [25] K. Crowley, J. Cassidy, J. Electroanal. Chem. 547 (2003) 75. [26] L.S. Hwang, J.M. Ko, H.W. Rhee, C.Y. Kim, Synth. Met. 55–57 (1993) 3671.
A flexible strain sensor from polypyrrole-coated fabrics Y. Li a,∗ , X.Y. Cheng b , M.Y. Leung b , J. Tsang b , X.M. Tao b,∗,1 , M.C.W. Yuen b a b
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Institute of Textiles and Clothing, Hong Kong Polytechinc University, Hong Hum, Hongkong, China Received 16 May 2005; accepted 8 June 2005 Available online 10 October 2005
Abstract Conductive-polymer coated fabrics have been investigated as intelligent materials in the past years. In this paper, a flexible strain sensor from polypyrrole-coated fabrics which is featured with high sensitivity, good stability is reported. The strategies used for enhancing the sensitivity and stability of the sensor include: (a) the formation of thin coatings of polypyrrole (PPy) on the surface of fabrics using the chemical vapor deposition (CVD) method, (b) low temperature polymerization of pyrrole, (c) introduction of large docecyl benzene sulfonate anion in PPy film and (d) annealing of the PPy-coated conductive fabrics. The conductivity–strain tests reveal that the developed sensor exhibits a high strain sensitivity of ∼80 for a deformation as large as 50%, while its good stability is supported by the small changes in conductivity and sensitivity over a storage time of 9 months. The effect of the temperature and humidity on the conductivity of the strain sensor is investigated. © 2005 Elsevier B.V. All rights reserved. Keywords: Strain sensor; Polypyrrole; Conductive polymer; Intelligent materials
1. Introduction In the past decades, conductive polymers have been intensively investigated in the applications, including chemical and biosensors, actuators, photovoltaics, rechargeable batteries, separation films, etc. [1]. As one of the most important conductive polymers, polypyrrole (PPy) received much attention because of its ease of preparation, high conductivity, good environmental stability, and non-toxicity. PPy filament was used as chemical sensors for detection of gases and vapors [2] and PPy films can be employed to construct artificial muscles with tactile sensitivity [3] and up to 12% linear strain [4]. Due to the good adhesion with different substrates, PPy can also form composites with a number of non-conductive fibers or fabrics to prepare the electrical conductive textiles. Owing to the advantages of both the mechanical properties of the flexible and elastic substrate, and the electrical, microwave properties and biocompatibility of the PPy-coating, the composites can be used as flexible smart materials for applications in electromagnetic interference ∗
Corresponding authors. Tel.: +86 571 87952444. E-mail addresses: [email protected] (Y. Li), [email protected] (X.M. Tao). 1 Tel.: 852 2766 6490. 0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.06.008
(EMI) shielding, broadband microwave absorbing, static charge dissipation, biomedical and tissue engineering, etc. [1,5–11]. Some authors also reported the application of the PPy-coated fabrics as the sensing fabrics for detection of strain to enable the measurement and control of various movement of human body, which may be used as an intelligent material for preparation of wearable devices for training, fitting, rehabilitation, etc. [5,6,12,13]. However, the flexible strain sensors exhibit low sensitivity and unsatisfying stability. Rossi et al. developed a sensorized glove based on the sensing fabrics of polypyrrole-coated Lycra/cotton, but the sensor was aged severely in air and the conductivity decreased continuously. Furthermore, the saturation of the sensor occurred at a small strain of about 6% [13]. Oh et al. reported that the PPy-coated Nylon–Spandex was sensitive to strain change until a deformation of 50%, but the strain sensitivity is as small as not more than 2 [6]. Kim et al. also proposed the PPy-coated PET/Spandex can be used as a strain sensor for a large deformation up to 50%, while the strain sensitivity is only 3 [5]. In this paper, a flexible strain sensor from PPy-coated fabrics prepared by a chemical vapor deposition process instead of the traditional solution polymerization method is reported. The strain sensitivity, environmental stability and the effect of
Y. Li et al. / Synthetic Metals 155 (2005) 89–94
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temperature and humidity on the conductivity of the sensor are investigated. 2. Experimental 2.1. Preparation of the polypyrrole-coated fabrics A typical procedure for preparation of PPy-coated fabrics by CVD is as follows: plain knitted fabric of 83% Tactel blended with 17% (40 denier) Lycra in weight of 195 g/m2 (Sunikorn Knitters Limited, HK) was first immersed into the aqueous solution containing sodium dodecyl benzene sulfonate (0.011 mol/L) and the ethanol solution of FeCl3 (0.1 mol/L) in sequence, and the wet take-up after each immersion was controlled to be ∼100% by using a padding machine (PA-0, Rapid Labortex Co. Ltd.). The fabrics and a beaker containing 10 mL pyrrole monomer were transferred into a desiccator, which was put in a fridge where the temperature is kept at −26 ◦ C. The vapour phase polymerization of polypyrrole proceeded on the surface of the fabrics under vacuum for 50 h. The black fabrics so obtained were then washed with de-ionized water and ethanol, respectively, and dried at 40 ◦ C under vacuum. The annealing was carried out by heating the dried fabrics at 60 ◦ C for 40 h under vacuum. A typical procedure for preparation of PPy-coated fabrics by solution polymerization is as follows: fabrics were immersed in a bottle containing 100 mL aqueous solution of FeCl3 (0.2 mol/L) and stirred for 1 h, then 100 mL of the aqueous solution of sodium dodecylbenzenesulfonate (0.05 mol/L) and 0.8 mL of pyrrole was added into the bottle slowly with stirring. The polymerization proceeded at room temperature for another 1 h with continuous shaking. The black fabrics so obtained were washed with water and ethanol, respectively, dried in vacuum at 40 ◦ C for 16 h. 2.2. Measurement The strain–stress properties of the PPy-coated fabrics were obtained using an Instron testing Instrument (Model 4466) under the standard testing conditions (T = 25 ◦ C and RH = 65%). The fabrics were repeatedly stretched and relaxed for 10 cycles with the maximum extension up to 12.5 mm (50% deformation) in each cycle. The conductivity change of the sensing fabrics in both stretched and relaxed states was recorded using a digital multimeter (Keithley Model 2010) to investigate its strain sensitivity. SEM images of the pristine and PPy-coated fabrics were
obtained using a scanning electron microscope (Leica stereoscan 440). ATR FT-IR spectra were measured on a Bio-Rad FTS6000 spectrometer. The effect of temperature and humidity on the conductivity of the sensor was investigated by recording the conductivity change of the PPy-coated fabrics put in a climatic chamber (Hotpack Series 922), where both the temperature and humidity can be controlled. For temperature test, the humidity is always kept at 65% RH. And the humidity test was carried out at 30 ◦ C. 3. Results and discussions The strategy to prepare a flexible strain sensor with both high sensitivity and good stability from PPy-coated fabrics is shown in Scheme 1. It was reported that the conductive fabrics prepared by solution polymerization of pyrrole on the surface of the fabrics usually show a low strain sensitivity of not more than 3 even though the deformation is as large as 50% [5,6]. It is proposed that the low strain sensitivity is related to the thick coatings composed of multi-layers of PPy deposited on the fabric surface during the solution polymerization. When the coated fabric is elongated, the multi-layers of PPy may not change in the same way as the substrate. Unless under a very large deformation, the relative position and the contact of the conductive PPy-coatings may not change much with the elongation of the fabrics, this leads to a small change in conductivity and thus low strain sensitivity resulted. For this reason, we employed the method of chemical vapor deposition (CVD) to obtain a very thin coating of PPy on the surface of a Tactal (83%)/Lycra (17%) fabrics. Due to the existence of elastic Lycra, the as-prepared composites show good reversibility under large-strain deformation of up to 100%. The coated fabrics show almost the same ATR FT-IR spectra as the pristine fabrics, which suggests that the PPy-coatings formed by CVD are very thin [9]. The typical curves of conductivity change with strain of the PPy-coated fabrics prepared by CVD and solution polymerization during 10 cycles of elongation and relaxation measurements are illustrated in Fig. 1a and b, respectively. The resistance of the sensor prepared by CVD increased sharply with the extension up to a large deformation of 50%. The strain sensitivity is defined as R/εR0 , where ε is the deformation, R are the resistance change of the fabric under extension and R0 is the original resistance, i.e. the resistance of the fabric in relaxed state, respectively. It can be calculated from Fig. 1 that the strain sensitivity of the sensor is as high as 80 for a deformation of 50% (ε = 0.5). As to our best knowledge, this is the highest sensitivity for the
Scheme 1. Strategy for preparing a flexible strain sensor from PPy-coated fabrics with both high sensitivity and good stability.
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Fig. 1. The conductivity changes with strain of PPy-coated fabrics prepared by (a) CVD and (b) solution polymerization (deformation = 50%).
large deformation reported up to date. In contrast, the sensor prepared by solution polymerization shows the strain sensitivity as low as about 6. The difference of the highest sensitivity of each cycle of extension and recovery is termed RD, which is used to evaluate the sensing reproducibility. It was found that RD is less than 1 for the sensor prepared by the CVD method, indicating that the sensor has very good reproducibility. In addition to high sensitivity, good environmental stability is especially important for its possible application in smart garment, rehabilitation and biomedical fields. Thin PPy-coatings were prepared on the surface of the fabrics by using the CVD method to improve the strain sensitivity. However, the thickness of PPy can influence its air stability as reported by Wang et al. [14] and the thin coatings of PPy are unbeneficial for its stability. It was reported that a smooth and dense layer of PPy film is more resistant to the attack of oxygen and water molecules, thus enhances its stability [7,15,16]. Kanynak et al. [17] proposed that the low temperature polymerization of pyrrole on surface of poly(ethylene terephthalate) fabrics was helpful for obtaining thinner coatings, resulting in more adherent film and more ordered structure of PPy and thus higher conductivity. Therefore, in the present study, polymerization was carried out at a
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low temperature of −26 ◦ C in the preparation of the PPy-coated fabrics. Fig. 2a–c shows scanning microscopy (SEM) photography of the film of pristine fabrics, PPy-coated fabrics prepared by CVD at low temperature and by solution polymerization, respectively. It reveals that the fabrics coated with PPy formed by CVD exhibits a very smooth surface as the pristine one, while coated fabric prepared by the solution polymerization is featured with a rough surface. Annealing is another method used for improving the stability of the conductive fabrics. Singh et.al. investigated the effect of the heat annealing on the conductivity and surface structure of PPy, and found that annealing will lead to the removal of residue solvent, the restructuring and reordering of PPy chain, thus stabilizing the conductivity [18]. Therefore, the conductive fabrics developed in the project were annealed by heat treatment under vacuum. In order to avoid destroying the structure and stability of the fabrics, the annealing was carried out by heating at only 60 ◦ C under vacuum for ∼40 h. Fig. 3 shows the curves of conductivity change of a strain sensor under cyclic extension before and after the annealing. It can be seen that the annealing process increases the strain sensitivity. In addition, the values of RD of the sensor decreased from 16 to 6 after the annealing, indicating improved reproducibility and stability. It was known that the dopant can effectively affect the stability and also the conductivity of PPy [1,19–24]. PPy film doped with larger anionic species, such as dodecylsulfate and dodecylbenzenesulfate displayed smother morphology than the cauliflower-like appearance of PPy/Cl− or PPy/ClO4 − film [25]. Kudoh found that the introduction of large-sized anions in the PPy film resulted in the enhancement of the thermal and humidity stability, since the large size dopants are difficult to de-dope even at high temperature and humidity [19]. Kuhn et al. also proposed that dopant anions can affect the stability of PPy film, which may partly be ascribed to its effect on the film morphology [7]. Also, the PPy/Cl− system was reported to have worse stability than those doped with organic acid. In our work, it was also found that the PPy-coated fabrics prepared with only Cl− as the counterion increase the resistance by more than 10-folds after storage of several months. The result agrees with the results from Scilingo et al. that the conductive fabrics prepared by solution polymerization with inorganic ClO4 − as the counterion faced the problem of rapid decrease in conductivity [13]. Considering the fact that the doping agent should be of low toxicity for practical application, sodium dodecyl benzene sulfonate, a bulky aromatic sulfonate was used as the additional doping agent for the preparation of conductive fabrics so as to provide good stability. This bulky anion has long aliphatic chain and its large dopant ion is more stable than the small Cl− ion. In addition, its inclusion in the PPy film is expected to result in a smoother morphology and more compact coatings. It may protect the PPy from the penetration and attack of air and moisture, and thus improve the stability of the sensor from PPy-coated fabrics. The stability of the PPy-coated fabrics was supported by the small variation of the conductivity and strain sensing properties of the sensor after storage of a long time. Table 1 presents the resistance of the sensor prepared at both low and room temperatures before and after the storage of about 9 months. Their strain
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Fig. 2. SEM images of (a) pristine fabrics and PPy-coated fabrics prepared (b) by CVD at low temperature and (c) by solution polymerisation.
sensing properties before and after the storage is illustrated in Fig. 4. It can be seen that the sensor prepared at low temperature exhibits a conductivity loss of only ∼20% after storage for 9 months, and the change in strain sensitivity is only about one
for a deformation of 50%. In contrast, the one prepared at room temperature shows a conductivity loss of ∼80% and its strain sensitivity also decreases by a half. It can thus be concluded that the flexible strain sensor from the PPy-coated fabrics prepared at low temperature show good environmental stability and the low temperature preparation is beneficial for improving the stability of the sensor. The conductivity of PPy is usually affected by temperature. The effect of temperature on the conductivity of the PPy-coated fabrics is presented in Fig. 5. It can be seen that the conductivity of the sensor increases with the increase in temperature, and the temperature coefficient is calculated to be −0.6%/◦ C. Humidity can also exert an influence on the conductivity of PPy. The adsorbed moisture can promote the free motion of Table 1 The resistance changes during the storage of the sensors prepared at different temperatures
Fig. 3. The conductivity changes with strain of PPy-coated fabrics (a) before and (b) after annealing (deformation = 50%).
Preparation temperature
Initial resistance (k)
Resistance after the storage (k)
Low temperature Room temperature
9.2 69.0
11.5 351
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Fig. 6. The effect of humidity on the conductivity of the strain sensor.
doping ions in the PPy film, thus increase its conductivity [20]. It was reported that PPy can even be used for detecting humidity [26]. The conductivity change of the sensor with humidity was plotted in Fig. 6. It can be seen that the humidity increase can slightly decrease the resistance of the sensor. The conductivity of the sensor changes only 0.66% when the humidity increased from 40 to 90%RH. 4. Conclusions
Fig. 4. The conductivity changes with strain of the sensors prepared at (a) low temperature and (b) room temperature before and after the storage (deformation = 50%).
In summary, a flexible strain sensor from polypyrrole-coated fabrics that is featured with both high strain sensitivity and good stability has been developed. It is proposed that the chemical vapor deposition of thin coatings of polypyrrole on fabric surfaces results in high strain sensitivity of the sensor, while the low temperature polymerization of pyrrole, annealing of the PPycoated fabrics and the introduction of bulky dodecyl benzene sulfonate anion into the polypyrrole backbone helps to improve the stability of the PPy-coated sensors. The PPy-coated fabrics are expected to find applications in sensing garment, wearable hardware and rehabilitation, etc. Acknowledgements The work is financially supported by the Innovative Technology Fund, The SAR Government of Hong Kong, and the Internal Research Fund, The Hong Kong Polytechnic University. References
Fig. 5. The effect of temperature on the conductivity of the strain sensor.
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