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Flexible and wearable strain sensing fabrics
Chemical Engineering Journal 325 (2017) 396–403
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Flexible and wearable strain sensing fabrics Guangming Cai a, Mengyun Yang a, Zhenglin Xu a, Jiangang Liu c, Bin Tang a,b,⇑, Xungai Wang b,a a
Wuhan Textile University, Key Laboratory of Textile Fiber & Product, Ministry of Education, Wuhan 430073, China Deakin University, Institute for Frontier Materials, Geelong, Australia c Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Flexible strain sensor is fabricated
using elastic fabric and reduced graphene oxide. Reduced graphene oxide endows elastic fabric with electrical conductivity. Mechanical properties of fabrics change slightly after surface modification. Strain sensing fabric exhibits a large workable strain range and great stability. Real-time monitoring of human motions is achieved by the obtained strain sensor.
a r t i c l e
i n f o
Article history: Received 16 March 2017 Received in revised form 5 May 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Reduced graphene oxide Strain monitoring Flexible and wearable sensor Nylon/PU fabric Electromechanical performance
a b s t r a c t Flexible electronic devices have attracted considerable attention in recent years. Textile fabrics have been widely used to fabricate flexible strain sensors owing to their high flexibility. However, the elasticity of ordinary textile fabrics is low, which limits their strain sensing range. In this article, we used a simple method to fabricate flexible strain sensing fabrics (FSSFs) through the coating of graphene oxide (GO) nanosheets on elastic nylon/polyurethane (nylon/PU) fabric, followed by reduction of GO with sodium borohydride. The reduced graphene oxide (RGO) nanosheets were adsorbed on the elastic fabrics to impart electrical conductivity to the fabrics. The coated fabrics were characterized with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman scattering spectroscopy. The electromechanical performance and strain sensing properties of the FSSF were investigated. The fabricated strain sensor exhibited high sensitivity, a large workable strain range (0–30%), fast response and great stability. The mechanical property of fabrics did not change remarkably after the treatment with RGO. The surface resistance of the RGO/nylon/PU only increased from 112 KX/m2 to 154 KX/m2 after 8 washing cycles, exhibiting good washability. Furthermore, real-time monitoring of human motions, such as bending of finger and rotation of wrist, was achieved by the as-prepared FSSF. The RGO/nylon/PU fabrics as flexible strain sensors have potential applications in wearable electronic devices. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Corresponding author at: Wuhan Textile University, Key Laboratory of Textile Fiber & Product, Ministry of Education, Wuhan 430073, China. E-mail address: [email protected] (B. Tang). http://dx.doi.org/10.1016/j.cej.2017.05.091 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
Flexible and wearable electronic devices have attracted considerable attention because of their great potential applications, in areas such as wearable displays, smart clothing, human motion
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and health monitor [1–3]. Compared to traditional electronics, flexible electronic devices possess many particular features, including good flexibility, light weight, high sensitivity and large deformation ability [4–11]. Most of the flexible sensors are made of electrically conductive metal nanoparticles or nanowires, metal thin films, carbon nanotube and graphene [12–15]. Although these sensors are electrically conductive and have high sensitivity, the workable strain range of such sensors is still small, which limits their applications. It remains a challenge to develop strain sensors with a large workable strain range and high sensitivity. Textile materials, as flexible materials produced from fibers, possess many particular features, such as porous structure, high surface area, light weight, good flexibility and recoverable deformation [16–18]. Moreover, the textile materials exhibit high strength, good tear resistance, and excellent bending and stretching recovery. By virtue of these properties, textile materials have been utilized to prepare flexible strain and pressure sensors, recently [19–32]. It has been reported that the strain sensors were prepared by carbonization of textile fabrics [33–35]. The sensors have excellent sensitivity and large strain sensing range, but the carbonization process destroys the structure of fabrics, which lose their original mechanical properties. Therefore, the carbonized fabrics cannot be used alone as a strain sensor. Graphene, carbon nanotubes, silver nanowires and polyaniline have been coated on fabrics for preparation of flexible strain sensors. However, the sensors from ordinary textile fabrics have a relatively small strain range and low long-term stability due to the small deformation ability and poor elasticity recovery, which limit their applications. Polyurethane (PU) fiber is known for its high elasticity and elastic recovery, which has been widely used to manufacture elastic yarns and textiles. Recently, some researchers have proposed combining carbon materials such as carbon nanotubes (CNTs) onto PU/cotton yarns to develop lightweight, stretchable and flexible electronic devices [18,36,37]. However, the low strength, poor fatigue and chemical corrosion resistance of cotton fiber may limit their applications. Additionally, it is difficult to use a yarn alone as a wearable sensor, and combination of the yarn with other elastic materials is often needed. Nylon as a type of synthetic fiber, has high strength, elongation and good resistance to fatigue and chemical corrosion, and is widely used in textile industry [38]. Graphene has received tremendous attention owing to its outstanding mechanical, thermal, optical and electronic properties [39,40]. As one of carbon materials, graphene possesses high electrical conductivity. As such, modification of textile substrate with graphene can impart conductivity to the substrate materials [41–45]. In this study, we developed a simple method to fabricate flexible strain sensors by reducing graphene oxide in the presence of elastic nylon/PU fabrics, which serve as the skeleton for the reduced graphene oxide conductive layer. The reduction process of the GO on fabric was analyzed. The morphology and component of the RGO/nylon/PU fabric were characterized. The electromechanical performance and strain sensing properties of the FSSF were also investigated. These results show the RGO/nylon/PU fabrics have good application potential as strain sensors for wearable electronic devices.
(GO) nanosheets with a thickness of 0.8–1.2 nm and a twodimensional length of 0.5–5 lm were provided by Nanjing Xianfeng Nano Science and Technology Ltd, China. Sodium borohydride (NaBH4, >98%) was purchased from Aladdin Reagent Company (Shanghai, China). All chemicals were of analytical grade and used without further purification. 2.2. Preparation of sensing fabric The sensing fabric for strain monitoring was prepared by dipcoating of GO nanosheets and subsequent reduction of GO by NaBH4, which is illustrated in Fig. 1. A stable GO suspension was obtained through the dispersion of GO nanosheets in water and 30 min of sonication at room temperature. Nylon/PU fabrics were soaked in ethanol for 30 min and then washed thoroughly with deionized water. The fabric was dried for 24 h in vacuum. Fabric samples (4 4 cm) were dipped into GO suspensions with different concentrations (0.5, 1.0, 1.5 and 2.0 mgL 1) and kept in solution for 2 h at 40 °C under sonication. The fabrics were dried at room temperature. The fabrics changed to brownish yellow from white due to adsorption of GO nanosheets. Six cycles of ‘‘dip and dry” were performed to obtain the GO treated fabrics. And then the fabrics with GO were immersed in 200 mL of 0.5 mol L 1 NaBH4 aqueous solution. The mixtures were kept for 2 h at 40 °C under stirring. The color of fabrics turned to black from brownish yellow during treatment. The GO nanosheets on fabrics were reduced by NaBH4. Finally, the fabrics were rinsed with deionized water and dried in an oven at 40 °C. 2.3. Instruments Scanning electron microscopy (SEM) measurements were performed with a Supra 55 VP field emission SEM. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos XSAM800 XPS system with Ka source and a charge neutralizer. Raman analysis was performed on a Renishaw inVia Raman microscope system (Renishaw plc, Wotton-under-Edge, UK). A 50/N.A. 0.75 objective and a 785-nm near-IR diode laser excitation source (500 mW, 10%) were used in all measurements. Raman spectra were recorded using a mounted CCD camera with integration time of 10 s by single scan. The mechanical properties were measured using an Instron Model 5566 Materials Testing System. The changes in electric resistance of fabric samples at different strain levels were recorded by using a self-built fabric dynamic resistance tester. 2.4. Durability test to washing In this study, water washing durability test of the obtained RGO/nylon/PU fabric was carried out according to AATCC Test Method 61-2006. A standard color-fastness to washing laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China) was used in a washing procedure. The RGO/ nylon/PU fabric (5 10 cm) was washed in a rotating closed canister containing 200 mL of detergent aqueous solution (0.37 wt%)
2. Materials and methods GO solution
2.1. Materials White knitted nylon/PU fabrics (138 gm 2), 97% nylon and 3% PU, were used in this study. The nylon/PU core spun composite yarn were used to knit fabric, the liner density of composite yarn is 80 D. The wale density and course density of knitted nylon/PU fabric are 280/5 cm and 140/5 cm, respectively. Graphene oxide
Pristine fabric
NaBH4 solution
GO treated fabric
RGO treated fabric
1 cm
Fig. 1. Preparation process of strain sensing fabrics (FSSF).
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and 10 stainless steel balls. The electrical conductivity properties of RGO/nylon/PU fabric was evaluated after water washing.
3. Results and discussion 3.1. Fabrication and characterization of strain sensing fabrics (FSSF) Fig. 1 shows the images of pristine nylon/PU fabrics and strain sensing fabrics (FSSF). Conversion of fabric colors from white to brownish yellow implied that the GO nanosheets were coated on the surface of fabrics after the procedures of dipping and drying (Fig. 1). GO nanosheets on fabrics were reduced by NaBH4, which was demonstrated by darker color of fabrics after the fabrics with GO were immersed in NaBH4 aqueous solution. SEM characterization was performed to observe the morphologies of the samples (Fig. 2). The laminar structure of the GO was easily visible in the SEM image (Fig. 2a). SEM images of pristine fabrics exhibited a smooth surface without observable impurities (Fig. 2b). After the fabrics were treated with GO, laminar layers were seen clearly to be affixed to the surface of fibers from the SEM images (Fig. 2c and d), which indicates that GO nanosheets were successfully assembled on the fabrics. Reduction of GO by NaBH4 did not influence visibly the morphologies of nanosheets on the fabrics. The laminar structures of RGO nanosheets were kept and still observed on the surface the fabrics (Fig. 2e and f). XPS was used to analyze the components of the fabrics. Fig. 3 (a) and (b) show the XPS spectra of fabrics treated with GO and RGO, respectively. The main elements of nylon/PU fabrics, including carbon, oxygen and nitrogen, appeared in the both XPS spectra. The relative contents of C1s, O1s, N1s are shown in Table 1. The element summary indicates the oxygen content of the fabric decreased from 27.08% to 17.50% during reduction of GO. While the carbon content increased from 69.98% to 77.02%. The content ratios of O1s/C1s were calculated to be 38.70% and 12.72% corresponding to GO and RGO treated fabrics, respectively. Both the
reduction of oxygen content and the changes in the element ratios of the O1s/C1s testify that the graphene oxide on the fabric surface was reduced. Furthermore, peak fitting was applied to analyze the XPS spectra (Fig. 3). The peak areas of C1s spectra were shown in Table 2. According to previous reports [46], the carboncontaining groups on the graphene surface were ascribed to CAC (284.5 eV), CAO (285.6 eV), C@O (287.8 eV) and OAC@O (288.6 eV). As can be seen, the intensities of C@O and OAC@O decreased from 23.04% to 8.93% and 16.55% to 8.76% after reduction, respectively. Meanwhile, the peak intensities of CAO and CAC increased to 35.37% and 49.26%. The reduction of the graphene oxide under the NaBH4 aqueous solution treatment should be responsible for the decrease of C@O as well as the increase of CAC and CAO. Raman spectroscopy was also employed for characterization of samples. The Raman spectrum of the fabrics coated with GO showed two prominent peaks around 1310 and 1595 cm 1 (Fig. 4), assigned to the D and G band, respectively [47,48]. The intensity ratio of the D band to G band (ID/IG) is widely used as an indicator to investigate the relative disorder structures [49]. In this research, the ID/IG ratio of the fabric with GO decreased from 1.44 (GO treated fabric) to 0.99 (RGO treated fabric) after reduction with NaBH4 (Fig. 4), revealing an increase in sp2 crystallinity and elimination of oxygen species [50,51]. It is inferred that NaBH4 is effective to diminish defects and induce the formation of large aromatic domains, consistent with the previous report [49]. The Raman data indicates that the GO nanosheets adhered to nylon/ PU fabrics were reduced by NaBH4.
3.2. The amount of deposited GO on the nylon/PU fabrics The amount of deposited GO as function of the GO concentration is shown in Fig. 5a, displaying a nearly linear increase of GO amount as increasing the GO concentration from 0.5 to 1.5 mg/ mL. When the GO concentration was higher than 1.5 mg/mL, the increase of GO deposition amount is not obvious. The amount of deposited GO on the nylon/PU fabrics would directly affect the electrical conductivity of fabric. In order to verify the effect of the GO concentration on the electrical conductivity, the surface electrical resistance of different RGO/nylon/PU fabrics were measured. Fig. 5b shows the surface resistance significantly decreased when the GO concentration increased to 2 from 0.5 mg/mL. The pristine nylon/PU fabrics exhibited extremely high surface electrical resistance (109 X/m2). When the GO concentration is 1.5 mg/ mL, the average value of surface electrical resistance for RGO/ nylon/PU fabric was 112 KX/m2. When the GO concentration is higher than 1.5 mg/mL, the surface resistance of RGO/nylon/PU fabrics tended to be stable, which may be due to the fact that the nylon/PU fabric cannot adsorb more GO. Therefore, we chose the RGO/nylon/PU fabrics obtained from 1.5 mg/mL of GO concentration to investigate the electromechanical property of FSSF.
3.3. Durability of the FSSF to washing
Fig. 2. SEM images of GO nanosheets (a), pristine fabrics (b), GO treated fabrics (c, d) and RGO treated fabrics (e, f).
Durability is an essential property for textile products. Fastness to washing was measured to test the bonding strength of the RGO nanosheets with nylon/PU fabric. The surface electrical resistance of RGO/nylon/PU fabrics after repeated washing were recorded (Fig. 6). As can be seen, the surface resistance increased slightly with after the first washing cycle (from 112 to 116 KX/m2). The surface resistance of the RGO/nylon/PU fabrics even after 8 washing cycles was less than 160 KX, which reveals that FSSF has good fastness to washing. The conductivity of FSSF would not be influenced notably by washing procedure.
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Fitted Original
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Fig. 3. C1s XPS analysis of (a) GO treated fabric and (b) RGO treated fabric.
Table 1 Relative surface chemical composition of different fabrics. Sample
Chemical composition of surface (%)
GO/fabric RGO/fabric
C 1s
O1s
N1s
O1s/C1s
69.98 77.02
27.08 17.5
2.54 5.49
38.70 12.72
Table 2 The peak areas of C1s spectra of different fabrics. Relative area corresponding to different chemical bonds (%)
GO/fabric RGO/fabric
CAC
CAO
C@O
OAC@O
46.24 49.26
11.15 35.37
23.04 8.93
16.55 8.76
3.4. Mechanical properties of the flexible strain sensor fabrics
GO treated fabric RGO treated fabric
Intensity (a.u.)
The mechanical properties of materials are important to the application of materials. The tensile strength of pristine and treated fabrics was tested to confirm whether the treatment with RGO deteriorated the mechanical properties of the nylon/PU fabrics. In this experiment, the wale direction strength of specimens with a width of 20 mm at a gauge length of 100 mm were tested. Typical strength–strain curves during stretching of pristine fabric and RGO treated fabric (FSSF) are depicted in Fig. 7. The breaking strength of the pristine fabric slightly decreased from 51.9 N to 47.9 N when the fabrics were treated with RGO. The RGO coating did not contribute to enhancement of the strength of fabric as the RGO nanosheets on the surface of the fabric did not form a thick continuous layer. The decrease in strength from the treat-
1200
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Fig. 4. Raman spectra of fabrics treated with GO and RGO.
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Fig. 5. (a) A plot of depositing amount of GO as a function of initial GO concentration. (b) Surface electrical resistance of different fabrics from different concentrations of GO.
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Suface electrical resistance (K /m2)
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ment with RGO may be due to the degradation of the fiber in the treatment process. However, the strength retention rate of fabric was more than 90%. Meanwhile, the strain retention rate of fabric was close to 93%. The tensile test results suggest that the fabrication process of FSSF did not significantly affect the mechanical properties of the fabric, which paves the way for practical applications of FSSF.
value of real-time resistance (R) when being stretched subtracted by initial resistance (R0). The relative resistance change increased monotonously as the loading strain increased in the range of 0– 33%. The results reveal that the sensing fabric has high elasticity and wide strain sensing range. It can be found that the change in resistance increases linearly when the strain is less than 10%, with a gauge factor of 18.5, which demonstrates the high sensitivity of FSSF. The gauge factor of elastic knitted fabric is higher than that of the previously reported wearable sensor which was fabricated by coating RGO on nonwoven fabrics [52]. Although the gauge factor decreased when the strain was more than 10%, the value of gauge factor still kept high level (12.1) in the strain range of 10– 18%. The FSSF exhibited significant strain sensing properties in the range of 0–18%. It should be noted that the resistance decreased when the strain of fabric was over 33%. Regarding the sensing mechanism of the FSSF in the present research, it is suggested that the changes in the contacting of the conductive RGO nanosheets, resulting from the deformation of the weaving structures of fabrics, give rise to the variation of the resistance of fabrics. The overlapping extent of RGO nanosheets on fabrics decreased as the fabrics were stretched to a certain degree. Fig. 9 shows the optical microscopic images of fabrics corresponding to different strain values and the structures of fabrics were seen clearly. The initial loop height of fabrics was 170 lm for the unstretched fabrics (Fig. 9a). The loop height increased to 224 and 247 lm when the strains of fabrics were 30% and 40%, respectively (Fig. 9b and c). The stretching led to reversible increase of loop height throughout the structure, which may result in the formation of discontinuous RGO layer on the surface of fabric. Nevertheless, the narrowing of fabrics in course direction under much strain would bring about the contacting of RGO layers on adjacent yarns or fibers, which may result in the decrease of electric resistance when the strain was more than 33%. Fig. 10a shows the resistance change of FSSF for different strains (3%, 6%, 12% and 15%) under cyclic stretching-releasing. The resistance change increased with increasing of the strain, which was consistent with the results shown in Fig. 9. The response of resistance change to strain under the cyclic stretching and releasing demonstrates the reliability of FSSF. In addition, the effect of tensile speed (strain rate) on the strain sensing properties of FSSF
3.5. Electromechanical performance of the FSSF
(a) A knitted structure of fabric is illustrated in Fig. 8a. In the knitted fabric, a ‘‘course” of knit is a predominantly horizontal row of needle loops and a ‘‘wale” of knit is a predominantly vertical column of interlaced needle loops (Fig. 8a). The FSSF composed of nylon/PU and RGO showed high sensitivity and good reliability to the loading of strain. Fig. 8b displays a plot of relative resistance change (DR/R0) of FSSF as a function of strain in wale direction, where R0 represents the initial resistance, and DR denotes the
(a)
30% (c)
(b)
0%
Fig. 9. Photographs of a flexible strain sensor fabric during (a) 0% strain, (b) 30% strain and 40% strain.
(b) 400 350
ΔR/R0 (%)
Loop height
Wale direcon
300
GF=12.1
250 200 150 100
GF=18.5
50 0 0
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40%
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Fig. 8. (a) Illustration of knitted structure of nylon/PU fabric. (b) Relative resistance change (DR/R0) as a function of tensile strain of FSSF.
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240
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Δ R/R0(%)
80 60 40 20 0 0
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Time (s) Fig. 10. Electromechanical performance of the flexible strain sensing fabric. (a) relative resistance variation (DR/R0) versus cyclic tensile strain of 3%, 6%, 12% and 15%; (b) resistance change under cyclic stretching–releasing with a strain of 15% at frequencies of 0.019 Hz; (c) resistance change of fabric in wale and course directions at a frequency of 0.047 Hz; (d) the durability test of FSSF under cyclic tensile strain of 3% for 120 cycles; (e) relative resistance variation (DR/R0) versus cyclic tensile strain of 3% corresponding to FSSF with different resistance values (1.5 KX and 5.5 MX).
90 80
R/R0 (%)
70 60 50 40 30 20 10 0 0
4
8
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36
Time (s) Fig. 11. Monitoring of the finger bending motions using FSSF.
was examined. As can be seen from Fig. 10b, the resistance change slightly increased as the tensile speed increased, which may be because that the immediate structure response of FSSF is limited at a high tensile speed. The textile fabric is anisotropic, leading to diversity of strain sensing properties of the fabric in different directions under stretching conditions. Fig. 10c shows the strain sensing properties of FSSF in wale and course direction at the same strain and tensile speed. It can be seen that amplitude and profile of the curves of FSSF in wale and course direction were noticeably different. The wale direction showed higher sensitivity, which could be due to higher elasticity of fabric in the wale direction. The anisotropic strain sensing features of FSSF inspired us to fabricate an integrated monitoring platform and develop sensing strategies of complicated motions based on special resistance changes in wale and course directions. While, isotropic fabrics such as nonwoven substrates cannot obtain distinguishing sensing properties in
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(a)
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Rotaon
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Fig. 12. Monitoring of the wrist motions using FSSF: (a) wrist flexion and (b) wrist rotation.
different directions. Furthermore, the remarkable cyclic stability and durability are very significant to practical applications of strain sensing fabrics. Fig. 10d shows the resistance change of FSSF sensor fabrics under 120 cyclic stretching-releasing of 3% strain at frequencies of 0.092 Hz. The electrical response of FSSF exhibited high stability to strain with low drift, during the repeated cyclic stretching-releasing process. No observable hysteresis was found in the plot of relative resistance changes versus strain (inset in Fig. 10d). The results indicate that the flexible strain sensing fabrics have long-term stability and excellent durability. Additionally, to investigate the influence of resistance on sensing features of the FSSF, the resistance change of FSSF with different resistance values (150 kX and 5.5 MX) under cyclic stretching-releasing with a strain of 3% were monitored (Fig. 10e). Comparing the curves of resistance changes for two FSSFs, tiny difference was found though the resistance difference between the two FSSFs was huge. The result implies that the resistance of fabrics in a certain range exhibited slight effect on sensing properties of FSSF.
3.6. Monitoring of human motions based on FSSF In order to demonstrate the potential applications of FSSF as wearable devices, the FSSF was used to monitor human motion in real-time. The FSSF was affixed on a finger for detecting a minor strain from bending a finger (inset in Fig. 11). The bending motions of the finger were precisely monitored by recording the resistance change of FSSF. When the finger bent to a certain angle, the resistance change of strain sensor sharply increased and then remained stable. Increasing bending angle brought about a further increase in the resistance of FSSF, forming a step signal, which demonstrates the fast response and high sensitivity of FSSF. As should be noted, FSSF exhibited a small overshoot of the resistance when fingers bent abruptly, which was attributed to the viscoelastic nature of the fabrics [22,49]. On account of the complicated deformation of human motion, it is meaningful to monitor the multi-directional motion characteristics by a single strain fabric sensor. In this study, we used FSSF to detect both the flexion and rotation of wrist. The intensity of curve of the relative resistance change increased as the wrist flexed and then returned to initial level after the wrist restored to its original status (Fig. 12a). The amplitude and shape of resistance curve varied with the angle and speed of wrist flexion. Significantly, repeatedly wrist rotation can be monitored by
recording the electrical resistance of FSSF (Fig. 12b). The resistance changes following the motion of wrist rotation consisted of electrical variations in wale and course directions. The characteristic of peaks in the curve of electrical resistance should be combining effects of anisotropic electromechanical properties of FSSF. The flexion and rotation of a wrist can be monitored easily by recording resistance changes of FSSF. The amplitude and frequency of resistance curves can detect the movement extent and speed of monitions, respectively. These results demonstrate the ability of FSSF as wearable devices to monitor the human motions.
4. Conclusions In summary, the flexible strain sensing fabrics have been successfully fabricated by coating and subsequently reducing graphene oxide (GO) on the surface of nylon/PU fabric. XPS and Raman spectroscopy revealed that the GO nanosheets on the surface of fabrics were reduced to RGO by NaBH4. SEM characterization showed that reduced graphene oxide (RGO) nanosheets were coated on the surface of nylon/PU fabric. Mechanical properties of fabrics changed slightly after modification with RGO. The investigation into electromechanical performance of fabrics showed that the flexible strain sensing fabrics (FSSFs) have great sensitivity, fast response, low drift and high stability. Furthermore, the FSSFs are able to monitor the human motions in real time, including bending of finger and the flexion and rotation of wrist. We envision that the FSSF as an effective flexible strain sensor will facilitate the development of novel wearable electronic devices.
Notes The authors declare no competing financial interest.
Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC 51503164 and 51403162), the MOE Innovation Team Project in Biological Fibers Advanced Textile Processing and Clean Production (No. IRT13086), the Natural Science Foundation of Hubei Province, China (No. 2014CFB760).
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Flexible and wearable strain sensing fabrics Guangming Cai a, Mengyun Yang a, Zhenglin Xu a, Jiangang Liu c, Bin Tang a,b,⇑, Xungai Wang b,a a
Wuhan Textile University, Key Laboratory of Textile Fiber & Product, Ministry of Education, Wuhan 430073, China Deakin University, Institute for Frontier Materials, Geelong, Australia c Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Flexible strain sensor is fabricated
using elastic fabric and reduced graphene oxide. Reduced graphene oxide endows elastic fabric with electrical conductivity. Mechanical properties of fabrics change slightly after surface modification. Strain sensing fabric exhibits a large workable strain range and great stability. Real-time monitoring of human motions is achieved by the obtained strain sensor.
a r t i c l e
i n f o
Article history: Received 16 March 2017 Received in revised form 5 May 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Reduced graphene oxide Strain monitoring Flexible and wearable sensor Nylon/PU fabric Electromechanical performance
a b s t r a c t Flexible electronic devices have attracted considerable attention in recent years. Textile fabrics have been widely used to fabricate flexible strain sensors owing to their high flexibility. However, the elasticity of ordinary textile fabrics is low, which limits their strain sensing range. In this article, we used a simple method to fabricate flexible strain sensing fabrics (FSSFs) through the coating of graphene oxide (GO) nanosheets on elastic nylon/polyurethane (nylon/PU) fabric, followed by reduction of GO with sodium borohydride. The reduced graphene oxide (RGO) nanosheets were adsorbed on the elastic fabrics to impart electrical conductivity to the fabrics. The coated fabrics were characterized with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman scattering spectroscopy. The electromechanical performance and strain sensing properties of the FSSF were investigated. The fabricated strain sensor exhibited high sensitivity, a large workable strain range (0–30%), fast response and great stability. The mechanical property of fabrics did not change remarkably after the treatment with RGO. The surface resistance of the RGO/nylon/PU only increased from 112 KX/m2 to 154 KX/m2 after 8 washing cycles, exhibiting good washability. Furthermore, real-time monitoring of human motions, such as bending of finger and rotation of wrist, was achieved by the as-prepared FSSF. The RGO/nylon/PU fabrics as flexible strain sensors have potential applications in wearable electronic devices. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Corresponding author at: Wuhan Textile University, Key Laboratory of Textile Fiber & Product, Ministry of Education, Wuhan 430073, China. E-mail address: [email protected] (B. Tang). http://dx.doi.org/10.1016/j.cej.2017.05.091 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
Flexible and wearable electronic devices have attracted considerable attention because of their great potential applications, in areas such as wearable displays, smart clothing, human motion
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and health monitor [1–3]. Compared to traditional electronics, flexible electronic devices possess many particular features, including good flexibility, light weight, high sensitivity and large deformation ability [4–11]. Most of the flexible sensors are made of electrically conductive metal nanoparticles or nanowires, metal thin films, carbon nanotube and graphene [12–15]. Although these sensors are electrically conductive and have high sensitivity, the workable strain range of such sensors is still small, which limits their applications. It remains a challenge to develop strain sensors with a large workable strain range and high sensitivity. Textile materials, as flexible materials produced from fibers, possess many particular features, such as porous structure, high surface area, light weight, good flexibility and recoverable deformation [16–18]. Moreover, the textile materials exhibit high strength, good tear resistance, and excellent bending and stretching recovery. By virtue of these properties, textile materials have been utilized to prepare flexible strain and pressure sensors, recently [19–32]. It has been reported that the strain sensors were prepared by carbonization of textile fabrics [33–35]. The sensors have excellent sensitivity and large strain sensing range, but the carbonization process destroys the structure of fabrics, which lose their original mechanical properties. Therefore, the carbonized fabrics cannot be used alone as a strain sensor. Graphene, carbon nanotubes, silver nanowires and polyaniline have been coated on fabrics for preparation of flexible strain sensors. However, the sensors from ordinary textile fabrics have a relatively small strain range and low long-term stability due to the small deformation ability and poor elasticity recovery, which limit their applications. Polyurethane (PU) fiber is known for its high elasticity and elastic recovery, which has been widely used to manufacture elastic yarns and textiles. Recently, some researchers have proposed combining carbon materials such as carbon nanotubes (CNTs) onto PU/cotton yarns to develop lightweight, stretchable and flexible electronic devices [18,36,37]. However, the low strength, poor fatigue and chemical corrosion resistance of cotton fiber may limit their applications. Additionally, it is difficult to use a yarn alone as a wearable sensor, and combination of the yarn with other elastic materials is often needed. Nylon as a type of synthetic fiber, has high strength, elongation and good resistance to fatigue and chemical corrosion, and is widely used in textile industry [38]. Graphene has received tremendous attention owing to its outstanding mechanical, thermal, optical and electronic properties [39,40]. As one of carbon materials, graphene possesses high electrical conductivity. As such, modification of textile substrate with graphene can impart conductivity to the substrate materials [41–45]. In this study, we developed a simple method to fabricate flexible strain sensors by reducing graphene oxide in the presence of elastic nylon/PU fabrics, which serve as the skeleton for the reduced graphene oxide conductive layer. The reduction process of the GO on fabric was analyzed. The morphology and component of the RGO/nylon/PU fabric were characterized. The electromechanical performance and strain sensing properties of the FSSF were also investigated. These results show the RGO/nylon/PU fabrics have good application potential as strain sensors for wearable electronic devices.
(GO) nanosheets with a thickness of 0.8–1.2 nm and a twodimensional length of 0.5–5 lm were provided by Nanjing Xianfeng Nano Science and Technology Ltd, China. Sodium borohydride (NaBH4, >98%) was purchased from Aladdin Reagent Company (Shanghai, China). All chemicals were of analytical grade and used without further purification. 2.2. Preparation of sensing fabric The sensing fabric for strain monitoring was prepared by dipcoating of GO nanosheets and subsequent reduction of GO by NaBH4, which is illustrated in Fig. 1. A stable GO suspension was obtained through the dispersion of GO nanosheets in water and 30 min of sonication at room temperature. Nylon/PU fabrics were soaked in ethanol for 30 min and then washed thoroughly with deionized water. The fabric was dried for 24 h in vacuum. Fabric samples (4 4 cm) were dipped into GO suspensions with different concentrations (0.5, 1.0, 1.5 and 2.0 mgL 1) and kept in solution for 2 h at 40 °C under sonication. The fabrics were dried at room temperature. The fabrics changed to brownish yellow from white due to adsorption of GO nanosheets. Six cycles of ‘‘dip and dry” were performed to obtain the GO treated fabrics. And then the fabrics with GO were immersed in 200 mL of 0.5 mol L 1 NaBH4 aqueous solution. The mixtures were kept for 2 h at 40 °C under stirring. The color of fabrics turned to black from brownish yellow during treatment. The GO nanosheets on fabrics were reduced by NaBH4. Finally, the fabrics were rinsed with deionized water and dried in an oven at 40 °C. 2.3. Instruments Scanning electron microscopy (SEM) measurements were performed with a Supra 55 VP field emission SEM. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos XSAM800 XPS system with Ka source and a charge neutralizer. Raman analysis was performed on a Renishaw inVia Raman microscope system (Renishaw plc, Wotton-under-Edge, UK). A 50/N.A. 0.75 objective and a 785-nm near-IR diode laser excitation source (500 mW, 10%) were used in all measurements. Raman spectra were recorded using a mounted CCD camera with integration time of 10 s by single scan. The mechanical properties were measured using an Instron Model 5566 Materials Testing System. The changes in electric resistance of fabric samples at different strain levels were recorded by using a self-built fabric dynamic resistance tester. 2.4. Durability test to washing In this study, water washing durability test of the obtained RGO/nylon/PU fabric was carried out according to AATCC Test Method 61-2006. A standard color-fastness to washing laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China) was used in a washing procedure. The RGO/ nylon/PU fabric (5 10 cm) was washed in a rotating closed canister containing 200 mL of detergent aqueous solution (0.37 wt%)
2. Materials and methods GO solution
2.1. Materials White knitted nylon/PU fabrics (138 gm 2), 97% nylon and 3% PU, were used in this study. The nylon/PU core spun composite yarn were used to knit fabric, the liner density of composite yarn is 80 D. The wale density and course density of knitted nylon/PU fabric are 280/5 cm and 140/5 cm, respectively. Graphene oxide
Pristine fabric
NaBH4 solution
GO treated fabric
RGO treated fabric
1 cm
Fig. 1. Preparation process of strain sensing fabrics (FSSF).
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and 10 stainless steel balls. The electrical conductivity properties of RGO/nylon/PU fabric was evaluated after water washing.
3. Results and discussion 3.1. Fabrication and characterization of strain sensing fabrics (FSSF) Fig. 1 shows the images of pristine nylon/PU fabrics and strain sensing fabrics (FSSF). Conversion of fabric colors from white to brownish yellow implied that the GO nanosheets were coated on the surface of fabrics after the procedures of dipping and drying (Fig. 1). GO nanosheets on fabrics were reduced by NaBH4, which was demonstrated by darker color of fabrics after the fabrics with GO were immersed in NaBH4 aqueous solution. SEM characterization was performed to observe the morphologies of the samples (Fig. 2). The laminar structure of the GO was easily visible in the SEM image (Fig. 2a). SEM images of pristine fabrics exhibited a smooth surface without observable impurities (Fig. 2b). After the fabrics were treated with GO, laminar layers were seen clearly to be affixed to the surface of fibers from the SEM images (Fig. 2c and d), which indicates that GO nanosheets were successfully assembled on the fabrics. Reduction of GO by NaBH4 did not influence visibly the morphologies of nanosheets on the fabrics. The laminar structures of RGO nanosheets were kept and still observed on the surface the fabrics (Fig. 2e and f). XPS was used to analyze the components of the fabrics. Fig. 3 (a) and (b) show the XPS spectra of fabrics treated with GO and RGO, respectively. The main elements of nylon/PU fabrics, including carbon, oxygen and nitrogen, appeared in the both XPS spectra. The relative contents of C1s, O1s, N1s are shown in Table 1. The element summary indicates the oxygen content of the fabric decreased from 27.08% to 17.50% during reduction of GO. While the carbon content increased from 69.98% to 77.02%. The content ratios of O1s/C1s were calculated to be 38.70% and 12.72% corresponding to GO and RGO treated fabrics, respectively. Both the
reduction of oxygen content and the changes in the element ratios of the O1s/C1s testify that the graphene oxide on the fabric surface was reduced. Furthermore, peak fitting was applied to analyze the XPS spectra (Fig. 3). The peak areas of C1s spectra were shown in Table 2. According to previous reports [46], the carboncontaining groups on the graphene surface were ascribed to CAC (284.5 eV), CAO (285.6 eV), C@O (287.8 eV) and OAC@O (288.6 eV). As can be seen, the intensities of C@O and OAC@O decreased from 23.04% to 8.93% and 16.55% to 8.76% after reduction, respectively. Meanwhile, the peak intensities of CAO and CAC increased to 35.37% and 49.26%. The reduction of the graphene oxide under the NaBH4 aqueous solution treatment should be responsible for the decrease of C@O as well as the increase of CAC and CAO. Raman spectroscopy was also employed for characterization of samples. The Raman spectrum of the fabrics coated with GO showed two prominent peaks around 1310 and 1595 cm 1 (Fig. 4), assigned to the D and G band, respectively [47,48]. The intensity ratio of the D band to G band (ID/IG) is widely used as an indicator to investigate the relative disorder structures [49]. In this research, the ID/IG ratio of the fabric with GO decreased from 1.44 (GO treated fabric) to 0.99 (RGO treated fabric) after reduction with NaBH4 (Fig. 4), revealing an increase in sp2 crystallinity and elimination of oxygen species [50,51]. It is inferred that NaBH4 is effective to diminish defects and induce the formation of large aromatic domains, consistent with the previous report [49]. The Raman data indicates that the GO nanosheets adhered to nylon/ PU fabrics were reduced by NaBH4.
3.2. The amount of deposited GO on the nylon/PU fabrics The amount of deposited GO as function of the GO concentration is shown in Fig. 5a, displaying a nearly linear increase of GO amount as increasing the GO concentration from 0.5 to 1.5 mg/ mL. When the GO concentration was higher than 1.5 mg/mL, the increase of GO deposition amount is not obvious. The amount of deposited GO on the nylon/PU fabrics would directly affect the electrical conductivity of fabric. In order to verify the effect of the GO concentration on the electrical conductivity, the surface electrical resistance of different RGO/nylon/PU fabrics were measured. Fig. 5b shows the surface resistance significantly decreased when the GO concentration increased to 2 from 0.5 mg/mL. The pristine nylon/PU fabrics exhibited extremely high surface electrical resistance (109 X/m2). When the GO concentration is 1.5 mg/ mL, the average value of surface electrical resistance for RGO/ nylon/PU fabric was 112 KX/m2. When the GO concentration is higher than 1.5 mg/mL, the surface resistance of RGO/nylon/PU fabrics tended to be stable, which may be due to the fact that the nylon/PU fabric cannot adsorb more GO. Therefore, we chose the RGO/nylon/PU fabrics obtained from 1.5 mg/mL of GO concentration to investigate the electromechanical property of FSSF.
3.3. Durability of the FSSF to washing
Fig. 2. SEM images of GO nanosheets (a), pristine fabrics (b), GO treated fabrics (c, d) and RGO treated fabrics (e, f).
Durability is an essential property for textile products. Fastness to washing was measured to test the bonding strength of the RGO nanosheets with nylon/PU fabric. The surface electrical resistance of RGO/nylon/PU fabrics after repeated washing were recorded (Fig. 6). As can be seen, the surface resistance increased slightly with after the first washing cycle (from 112 to 116 KX/m2). The surface resistance of the RGO/nylon/PU fabrics even after 8 washing cycles was less than 160 KX, which reveals that FSSF has good fastness to washing. The conductivity of FSSF would not be influenced notably by washing procedure.
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Fitted Original
(a)
(b)
C-C Fitted
C-C
Original C=O
C-O C=O O-C=O
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290
C-O O-C=O
288
286
284
282
280
278
292
290
Binding Energy (eV)
288
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284
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Binding Energy (eV)
Fig. 3. C1s XPS analysis of (a) GO treated fabric and (b) RGO treated fabric.
Table 1 Relative surface chemical composition of different fabrics. Sample
Chemical composition of surface (%)
GO/fabric RGO/fabric
C 1s
O1s
N1s
O1s/C1s
69.98 77.02
27.08 17.5
2.54 5.49
38.70 12.72
Table 2 The peak areas of C1s spectra of different fabrics. Relative area corresponding to different chemical bonds (%)
GO/fabric RGO/fabric
CAC
CAO
C@O
OAC@O
46.24 49.26
11.15 35.37
23.04 8.93
16.55 8.76
3.4. Mechanical properties of the flexible strain sensor fabrics
GO treated fabric RGO treated fabric
Intensity (a.u.)
The mechanical properties of materials are important to the application of materials. The tensile strength of pristine and treated fabrics was tested to confirm whether the treatment with RGO deteriorated the mechanical properties of the nylon/PU fabrics. In this experiment, the wale direction strength of specimens with a width of 20 mm at a gauge length of 100 mm were tested. Typical strength–strain curves during stretching of pristine fabric and RGO treated fabric (FSSF) are depicted in Fig. 7. The breaking strength of the pristine fabric slightly decreased from 51.9 N to 47.9 N when the fabrics were treated with RGO. The RGO coating did not contribute to enhancement of the strength of fabric as the RGO nanosheets on the surface of the fabric did not form a thick continuous layer. The decrease in strength from the treat-
1200
1300
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GO adsorption amount (g/m 2)
Fig. 4. Raman spectra of fabrics treated with GO and RGO.
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C (mg/mL)
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Fig. 5. (a) A plot of depositing amount of GO as a function of initial GO concentration. (b) Surface electrical resistance of different fabrics from different concentrations of GO.
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ment with RGO may be due to the degradation of the fiber in the treatment process. However, the strength retention rate of fabric was more than 90%. Meanwhile, the strain retention rate of fabric was close to 93%. The tensile test results suggest that the fabrication process of FSSF did not significantly affect the mechanical properties of the fabric, which paves the way for practical applications of FSSF.
value of real-time resistance (R) when being stretched subtracted by initial resistance (R0). The relative resistance change increased monotonously as the loading strain increased in the range of 0– 33%. The results reveal that the sensing fabric has high elasticity and wide strain sensing range. It can be found that the change in resistance increases linearly when the strain is less than 10%, with a gauge factor of 18.5, which demonstrates the high sensitivity of FSSF. The gauge factor of elastic knitted fabric is higher than that of the previously reported wearable sensor which was fabricated by coating RGO on nonwoven fabrics [52]. Although the gauge factor decreased when the strain was more than 10%, the value of gauge factor still kept high level (12.1) in the strain range of 10– 18%. The FSSF exhibited significant strain sensing properties in the range of 0–18%. It should be noted that the resistance decreased when the strain of fabric was over 33%. Regarding the sensing mechanism of the FSSF in the present research, it is suggested that the changes in the contacting of the conductive RGO nanosheets, resulting from the deformation of the weaving structures of fabrics, give rise to the variation of the resistance of fabrics. The overlapping extent of RGO nanosheets on fabrics decreased as the fabrics were stretched to a certain degree. Fig. 9 shows the optical microscopic images of fabrics corresponding to different strain values and the structures of fabrics were seen clearly. The initial loop height of fabrics was 170 lm for the unstretched fabrics (Fig. 9a). The loop height increased to 224 and 247 lm when the strains of fabrics were 30% and 40%, respectively (Fig. 9b and c). The stretching led to reversible increase of loop height throughout the structure, which may result in the formation of discontinuous RGO layer on the surface of fabric. Nevertheless, the narrowing of fabrics in course direction under much strain would bring about the contacting of RGO layers on adjacent yarns or fibers, which may result in the decrease of electric resistance when the strain was more than 33%. Fig. 10a shows the resistance change of FSSF for different strains (3%, 6%, 12% and 15%) under cyclic stretching-releasing. The resistance change increased with increasing of the strain, which was consistent with the results shown in Fig. 9. The response of resistance change to strain under the cyclic stretching and releasing demonstrates the reliability of FSSF. In addition, the effect of tensile speed (strain rate) on the strain sensing properties of FSSF
3.5. Electromechanical performance of the FSSF
(a) A knitted structure of fabric is illustrated in Fig. 8a. In the knitted fabric, a ‘‘course” of knit is a predominantly horizontal row of needle loops and a ‘‘wale” of knit is a predominantly vertical column of interlaced needle loops (Fig. 8a). The FSSF composed of nylon/PU and RGO showed high sensitivity and good reliability to the loading of strain. Fig. 8b displays a plot of relative resistance change (DR/R0) of FSSF as a function of strain in wale direction, where R0 represents the initial resistance, and DR denotes the
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Time (s) Fig. 11. Monitoring of the finger bending motions using FSSF.
was examined. As can be seen from Fig. 10b, the resistance change slightly increased as the tensile speed increased, which may be because that the immediate structure response of FSSF is limited at a high tensile speed. The textile fabric is anisotropic, leading to diversity of strain sensing properties of the fabric in different directions under stretching conditions. Fig. 10c shows the strain sensing properties of FSSF in wale and course direction at the same strain and tensile speed. It can be seen that amplitude and profile of the curves of FSSF in wale and course direction were noticeably different. The wale direction showed higher sensitivity, which could be due to higher elasticity of fabric in the wale direction. The anisotropic strain sensing features of FSSF inspired us to fabricate an integrated monitoring platform and develop sensing strategies of complicated motions based on special resistance changes in wale and course directions. While, isotropic fabrics such as nonwoven substrates cannot obtain distinguishing sensing properties in
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different directions. Furthermore, the remarkable cyclic stability and durability are very significant to practical applications of strain sensing fabrics. Fig. 10d shows the resistance change of FSSF sensor fabrics under 120 cyclic stretching-releasing of 3% strain at frequencies of 0.092 Hz. The electrical response of FSSF exhibited high stability to strain with low drift, during the repeated cyclic stretching-releasing process. No observable hysteresis was found in the plot of relative resistance changes versus strain (inset in Fig. 10d). The results indicate that the flexible strain sensing fabrics have long-term stability and excellent durability. Additionally, to investigate the influence of resistance on sensing features of the FSSF, the resistance change of FSSF with different resistance values (150 kX and 5.5 MX) under cyclic stretching-releasing with a strain of 3% were monitored (Fig. 10e). Comparing the curves of resistance changes for two FSSFs, tiny difference was found though the resistance difference between the two FSSFs was huge. The result implies that the resistance of fabrics in a certain range exhibited slight effect on sensing properties of FSSF.
3.6. Monitoring of human motions based on FSSF In order to demonstrate the potential applications of FSSF as wearable devices, the FSSF was used to monitor human motion in real-time. The FSSF was affixed on a finger for detecting a minor strain from bending a finger (inset in Fig. 11). The bending motions of the finger were precisely monitored by recording the resistance change of FSSF. When the finger bent to a certain angle, the resistance change of strain sensor sharply increased and then remained stable. Increasing bending angle brought about a further increase in the resistance of FSSF, forming a step signal, which demonstrates the fast response and high sensitivity of FSSF. As should be noted, FSSF exhibited a small overshoot of the resistance when fingers bent abruptly, which was attributed to the viscoelastic nature of the fabrics [22,49]. On account of the complicated deformation of human motion, it is meaningful to monitor the multi-directional motion characteristics by a single strain fabric sensor. In this study, we used FSSF to detect both the flexion and rotation of wrist. The intensity of curve of the relative resistance change increased as the wrist flexed and then returned to initial level after the wrist restored to its original status (Fig. 12a). The amplitude and shape of resistance curve varied with the angle and speed of wrist flexion. Significantly, repeatedly wrist rotation can be monitored by
recording the electrical resistance of FSSF (Fig. 12b). The resistance changes following the motion of wrist rotation consisted of electrical variations in wale and course directions. The characteristic of peaks in the curve of electrical resistance should be combining effects of anisotropic electromechanical properties of FSSF. The flexion and rotation of a wrist can be monitored easily by recording resistance changes of FSSF. The amplitude and frequency of resistance curves can detect the movement extent and speed of monitions, respectively. These results demonstrate the ability of FSSF as wearable devices to monitor the human motions.
4. Conclusions In summary, the flexible strain sensing fabrics have been successfully fabricated by coating and subsequently reducing graphene oxide (GO) on the surface of nylon/PU fabric. XPS and Raman spectroscopy revealed that the GO nanosheets on the surface of fabrics were reduced to RGO by NaBH4. SEM characterization showed that reduced graphene oxide (RGO) nanosheets were coated on the surface of nylon/PU fabric. Mechanical properties of fabrics changed slightly after modification with RGO. The investigation into electromechanical performance of fabrics showed that the flexible strain sensing fabrics (FSSFs) have great sensitivity, fast response, low drift and high stability. Furthermore, the FSSFs are able to monitor the human motions in real time, including bending of finger and the flexion and rotation of wrist. We envision that the FSSF as an effective flexible strain sensor will facilitate the development of novel wearable electronic devices.
Notes The authors declare no competing financial interest.
Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC 51503164 and 51403162), the MOE Innovation Team Project in Biological Fibers Advanced Textile Processing and Clean Production (No. IRT13086), the Natural Science Foundation of Hubei Province, China (No. 2014CFB760).
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