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A simple and cost-effective method for improving the sensitivity of flexible strain sensors based on conductive polymer composites
Sensors and Actuators A 298 (2019) 111608
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A simple and cost-effective method for improving the sensitivity of flexible strain sensors based on conductive polymer composites Dengji Guo, Xudong Pan, Hu He ∗ State Key Laboratory of High Performance Complex Manufacturing, College of Mechanical and Electrical Engineering, Central South University, Changsha, 410083, China
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Article history: Received 17 June 2019 Received in revised form 20 August 2019 Accepted 6 September 2019 Available online 9 September 2019 Keywords: Flexible strain sensors Conductive polymer composites Silicone fluid Carbon nanotubes PDMS
a b s t r a c t Conductive polymer composites (CPCs) based flexible strain sensors requires high performance in terms of sensitivity and stretchability towards potential applications in health monitoring, motion detection and human-machine interaction. In this work, a simple and cost-effective method was proposed to improve the sensitivity yet maintain the stretchability of CPCs based strain sensor. Compared with conventional multi-wall carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) flexible strain sensors, silicone fluid was introduced to configure the conductive networks as well as the mechanical properties. It was found that silicone fluid can increase the sensitivity of strain sensor by 10–30 times without reducing the stretchability, which was attributed to the non-covalently functionalized surface of the carbon nanotubes leading to higher ratio of the tunneling resistance to overall resistance of CPCs based flexible strain sensor. Furthermore, the synergetic effect between silicone fluid and CNTs was investigated through conductive network reconfiguration and electromechanical test. The proposed efficient and effective flexible strain sensor preparation approach provides useful insights for high-performance flexible strain sensor design and application. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Flexible strain sensors have recently attracted widespread attention due to their potential applications such as human motion detection [1,2], electronic skin [3] and human-machine interaction [4,5]. Generally, the preparation of flexible strain sensor could adopt either electrical conductive sensing films coupled with flexible substrates [6–9] or a conductive polymer composites (CPCs) [10,11]. In particular, CPCs are considered to be ideal for flexible strain sensors due to the ease of preparation, controllable electromechanical properties, and diverse substrate selection [12,13]. In order to adjust the electromechanical properties of CPCs, carbonbased conductive nanostructured fillers, such as carbon blacks (CBs) and carbon nanotubes (CNTs), have been widely employed [14]. Compared with CBs, CNTs are more attractable for CPCs based flexible strain sensors due to their excellent characteristics, such as exceptional mechanical, electrical properties and high aspect ratio [15]. Stretchability and sensitivity are the primary performance indicators of flexible strain sensors. Reducing the density of the
∗ Corresponding author. E-mail address: [email protected] (H. He). https://doi.org/10.1016/j.sna.2019.111608 0924-4247/© 2019 Elsevier B.V. All rights reserved.
conductive network generally improves the sensitivity of CPCs based flexible strain sensor with single conductive filler, while degrades the stretchability range [16,17]. Additionally, the density of conductive networks mainly depends on the content of conductive fillers in CPCs. Therefore, plenty of work improving the sensitivity and stretchability of CPCs based flexible strain sensors by adjusting the conductive fillers have been conducted. Zheng et al. showed the sensitivity and stretchability of CNTs/PDMS composites flexible strain sensors with different CNTs contents [2]. Although the sensitivity of the 0.48 vol.% CNTs was about 200 times than that of 2.45 vol.%, the effective strain range of the sensor reduced to be only a half [18]. Hu et al. proposed a CPCs based strain sensor using metal-coated MWCNTs, whose sensitivity was 4.3 times as large as that of untreated materials [12]. The metal-coated MWCNTs could significantly improve the sensitivity of the strain sensor due to a sparse electrical conductive network and a higher ratio of the tunneling resistance to overall resistance of CPCs. However, the preparation of metal-coated MWCNTs required complicated and costly nano-electrolytic plating process. Li et al. presented a CNTs/PDMS composites foam based strain sensor to improve the sensitivity with the degradation of effective strain range due to the porosity of the composites reduce the elongation [13]. Furthermore, a flexible strain sensor based on CNTs and CBs hybrid fillers was proposed to achieve 300% strain range yet keep the rea-
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Fig. 1. (a) Schematic of the preparation process the MWCNTs/SF/PDMS composites. (b) Injection molding die. (c) Size of the strain sensors. (d) Electronic universal testing machine and resistance tester.
sonable sensitivity, which is attribute to the synergetic effect of hybrid fillers in conductive path reconfiguration [2]. Nevertheless, the sensitivity of presented strain sensor was less than 1 with the strain range below 100%. Thus, an effective and low-cost method on the preparation of CPCs based flexible strain sensors using CNTs towards high sensitivity and stretchability is still required in the field. In this paper, silicone fluid (SF) was mixed into MWCNTs/PDMS to prepare a CPCs based strain sensor. It was found that SF could increase the sensitivity of strain sensor by 10–30 times without sacrificing the stretchability. Particularly, the sensitivity improved significantly at low strain range, which is applicable in common scenarios for wearable flexible sensors. Additionally, the synergetic effect between silicone fluid and CNTs was investigated in terms of conductive network forming and electromechanical performance evaluation. Furthermore, the improvement mechanism of silicone fluid for the performance of MWCNTs/PDMS strain sensor was discussed. It was believed the presented simple and cost-effective method for high-performance flexible strain sensor preparation could pave a way for wearable sensor applications in the future.
structural and physical drawings of the injection molding tool. The size of the test sample size was 30 mm × 6 mm × 1 mm (Fig. 1(c)). The microscopic morphology of the composites were characterized by a scanning electron microscope (SEM, Tescan Vega3). Fig. 1(d) showed the testing equipment for the mechanicalelectrical characteristics of the strain sensors. Electronic universal testing machine (Shenzhen Suns Technology Stock Co. Ltd) was employed to control the strain of the sample and record the change in stress. Precision resistance tester AT512(Applent Instruments Co. Ltd)was synchronized with the electronic universal testing machine to acquire the resistance change of testing samples. The gauge factor (GF) of the strain sensor was calculated by the following equation GF =
R/R0 (R − R0 ) /ε = ε ε
(1)
where ε is the mechanical strain applied to the strain sensor; R is the change in resistance; R0 and R are the initial resistance of the strain sensor and resistance at an applied strain, respectively. 3. Results and discussion
2. Materials and methods
3.1. Effect of SF on electrical properties
The PDMS (SYLARD 184) and PMX-200 silicone fluid 100cst were purchased from Dow Coring Corporation. The multi-walled nanotube (MWCNT) was purchased from Shengzhen Nanotech Port Co. Ltd (diameter:>5 um, length: 20–40 nm). Isopropanol alcohol (IPA) and other chemicals were purchased from aladdin Co. Ltd. All these material were employed as ordered. The fabrication process of the MWCNTs/Silicone fluid/PDMS nanocomposites strain sensors is illustrated in Fig. 1(a). The strain sensors was fabricated by liquid phase mixing to obtain a good dispersion of MWCNTs in the PDMS matrix [19]. Specifically, MWCNTs was first dispersed in IPA by ultrasonic process for 2 h. In the following, PDMS and SF were added into the MWCNTs/IPA solution and stirred for 2 h in a 60 ◦ water bath. The IPA in the mixed solution was completely volatilized by stirring in a water bath at 80 ◦ C. Finally, the curing agent was added into the mixed solution with the mass ratio of 10:1 of base polymer to curing agent. The mixture composites were stirred thoroughly and then vacuumed for use. The strain sensor test sample was prepared by injection molding in order to obtain a dense, non-porous sample. Fig. 1(b) showed the
As shown in Fig. 2a, the electrical property of the MWCNTs/PDMS and MWCNTs/SF/PDMS (PDMS:SF = 4:1) composites with different MWCNTs contents was presented. The initial resistance of the composites decreased as the MWCNTs content increased, which was in accordance with the percolation theory [20]. Moreover, the resistance reduced from disconnection to 30 k when the MWCNT content of the MWCNT/PDMS composites increased from 4 wt% to 5 wt%. The resistance varied significantly between 4 wt% and 5 wt%, which indicated a percolation threshold between 4 wt% and 5 wt%. However, the resistance of MWCNTs/SF/PDMS composites at 4 wt% was 1570 k, indicating that the percolation threshold was less than 4 wt%. The percolation threshold of MWCNTs/PDMS composites with SF was lower than that without SF. Thus, SF could be introduced to adjust electrical properties and reduce percolation threshold, which was attributed to that SF could effectively promote uniform dispersion of MWCNTs in PDMS matrix [21]. SF was coated on the surface of the MWCNTs by non-covalent bonding and could be attached to the PDMS matrix, which avoided re-aggregation of the MWCNTs during the
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Fig. 2. (a) The initial electrical resistance of MWCNTs/PDMS and MWCNTs/SF/PDMS at different MWCNT contents (4 wt%, 5 wt% and 6 wt%, respectively). (b) SEM images of the microstructure of the MWCNTs/SF/PDMS composites. (c) SEM images of the microstructure of the MWCNTs/PDMS composites.
Fig. 3. The schematic cross sectional diagram of the spatial relationship and resistance type of MWCNTs in the initial status. (a) MWCNTs/PDMS composites. (b) MWCNTs/SF/PDMS composites.
IPA high temperature evaporation and curing process [21]. It was also verified by SEM images that silicone fluid could significantly increase the dispersion of MWCNTs in PDMS matrix (Fig. 2(b) and (c)). The coating effect of SF achieved non-covalent functionalization of the surface of the carbon nanotubes to increase the degree of dispersion of the MWCNTs and reduce the percolation threshold. Uniformly dispersed MWCNTs is helpful to increase the number of conductive pathways in the MWCNTs/PDMS composites and reduced the initial resistance according to the percolation theory [22]. However, the initial resistance of 5 wt% and 6 wt% MWCNTs with SF were higher than that without SF, which was attributed to the coating effect of SF on MWCNTs (Fig. 2). SF functionalized MWCNTs surfaces changed the resistance composition of the MWCNTs/PDMS composites. The resistance of MWCNTs/PDMS composites consists of three type resistance, the intrinsic resistance of the MWCNT, the contact resistance between the MWCNT tubes and the tunneling resistance between the MWCNT tubes [23]. The intrinsic resistance depended on the nature of the carbon nanotubes. Both the tunneling resistance and the contact resistance are
related to the spatial position and the distance between the MWCNTs tubes [12]. The contact resistance was formed by the outer walls of the MWCNTs being in contact with each other. However, SF separated the originally contacted MWCNTs and created a certain distance, which was attributed to the non-covalently functionalized carbon nanotube surface of SF (Fig. 3(a)) [21]. The separation distance resulted from the functionalization of the SF satisfied the tunneling resistance condition, resulting in the conversion of the contact resistance into the tunneling resistance. Moreover, the original tunneling resistance was also turned into a disconnection due to the functionalization of the SF (Fig. 3(b)). Therefore, SF functionalized MWCNTs surfaces increased the proportion of tunneling resistance and reduced the proportion of contact resistance. In addition, the tunneling resistance was much larger than the contact resistance, resulting in an increase in the initial resistance of MWCNTs/PDMS composites with SF. Fig. 4(a) represents the initial resistance of MWCNTs/SF/PDMS composites with 5 wt% MWCNTs at different ratios of PDMS to SF. As the increase of PDMS matrix, the initial resistance firstly increased
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Fig. 4. (a) The initial electrical resistance for MWCNTs/SF/PDMS composites at different ratios of PDMS to SF. (b) The R/R0 as a function of the applied tensile strain during stretching test for MWCNTs/SF/PDMS composites at different ratio of PDMS to SF. (c) Stress-strain curve for MWCNTs/SF/PDMS composites at different ratio of PDMS to SF. The weight ratio of MWCNTs is 5 wt% and the tensile rate is 0.5 mm/s.
Fig. 5. The stretching test of the MWCNTs/PDMS and MWCNTs/SF/PDMS (containing 5 wt% and 6 wt% MWCNTs). (a) The R/R0 as a function of the applied tensile strain (tensile rate = 0.5 mm/s). (b) Stress-strain curve of the composites.
and then decreased. The lower resistance can be achieved with the ratio of PDMS to SF as 4:1 in comparing with that of 2:1 and 1:1. The non-covalent functionalization effect of SF on the surface of carbon nanotubes would increase as the content of SF increases, thus, the electrical resistance would increase. In addition, the standard deviation of the resistance increases significantly as the SF content increased, which might be related to the decrease in the degree of cross-linking of PDMS. Therefore, the stability of the conductive network was weakened with the degradation of the degree of cross-linking of PDMS when the SF content increased [24,25]. As shown in Fig. 4(b) and (c), the effective strain sensing interval of the strain sensor increased as the SF content decreased. Comparing with the sample with the ratio of PDMS to SF as 4:1, the
effective sensing range degrades to be almost half for that of 2:1 and 1:1. Moreover, the electrical response of the strain sensor fluctuates sharply within the strain range of 0.5–0.8 when the ratio of PDMS to SF was 2:1 and 1:1, which introduces difficulty for potential applications. Thus, the ratio of PDMS to SF as 4:1 was selected to prepare the sample in this work based on the mechanical and electrical performance. 3.2. Effect of SF on strain sensors performance Slope of the relative change of the resistance versus applied strain reflects Gauge factor (GF) of strain sensors and the GF value represents the sensitivity of the strain sensors. Following the com-
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Fig. 7. The relationship between R/R0 and stress of 4 wt% MWCNTs/SF/PDMS composites (tensile rate = 0.5 mm/s). Particularly, the dashed line represents our results while the solid line denotes literature results.
Fig. 6. The schematic cross sectional diagram of the change in the spatial relationship of the MWCNTs and the resistance during the stretching process. (a) MWCNTs/PDMS composites. (b) MWCNTs/SF/PDMS composites.
mon way to calculate the GF in the literature [13,18,26], the regions with good linearity are commonly employed to calculate the GF. As shown in Fig. 5(a), the GF of the MWCNTs/PDMS composites strain sensor of 5 wt% of MWCNTs was higher than 6 wt%. For MWCNTsbased strain sensors, the sensitivity of the strain sensor decreased as the MWCNT content increasing, which was attributed to the density of the conductive network composed of MWCNTs. When the strain sensor was subjected to strain, the low-density conductive network was prone to damage easier, resulting in a more significant resistance change. Moreover, the sensitivity of the 5 wt% MWCNTs/PDMS composites with SF was 8.68 at the strain range of 0–0.8 and the sensitivity was 23 at the strain range of 0.8–1.4. The sensitivity of the strain sensor with SF was 10–30 times of that without SF, which implicated MWCNTs surface functionalized with SF would alter the process of resistance changes during strain. As shown in Fig. 5(b), the modulus of elasticity was represented by the slope of the stress-strain curve of the strain sensor. The elastic modulus of the MWCNTs/PDMS composites strain sensors of 6 wt% of MWCNTs was higher than 5 wt%. MWCNTs with higher content increased the elastic modulus of composites [23]. Moreover, the elastic modulus of the MWCNTs/PDMS composites with SF was lower than that without SF while the strain range of strain sensors was sustained, which indicated SF changed the interfacial relationship between MWCNTs and PDMS matrix and weakened the reinforcing effect of MWCNTs as well as the degree of crosslinking of the PDMS. Therefore, SF improved the flexibility of the MWCNTs/PDMS composites strain sensor without sacrificing stretchability, which improved the adaptability of the flexible strain sensor to complex surfaces. In addition, SF could affect the sensing mechanism. Fig. 6 showed the change in the spatial relationship of the MWCNTs and the resistance change during the stretching process. The distance between the carbon nanotubes in the MWCNTs/PDMS composites would
gradually increase in the direction of stretching and the resistance changed from the contact resistance to tunneling resistance until it was broken (Fig. 6(a)). However, SF functionalized MWCNTs only underwent the tunneling resistance to the disconnection process (Fig. 6(b)). Since the resistance change from contact resistance to tunneling resistance was smaller than that from tunneling resistance to disconnection, the resistance change of the composites with SF was more significant than that without SF. In other word, SF functionalized carbon nanotubes surface not only increased the ratio of tunneling resistance to the overall resistance, but also changed the process of resistance change under strain loading. 3.3. Effect of MWCNTs aspect ratio on strain sensors performance As shown in Fig. 7, the sensitivity of strain sensor composed of MWCNTs with the low aspect ratio was higher than that of the high aspect ratio. The sensitivity of the strain sensors with the aspect ratio of 300 was 26.8 at the strain range of 0–17% and the sensitivity was 7.5 at the strain range of 17–40%. It was found the sensitivity of strain sensor with the aspect ratio of 300 was 17–62 times of that with the aspect ratio of 1000. For the MWCNTs/PDMS composites strain sensors, the sensitivity decreased with the increase of the aspect ratio of MWCNTs, which was attributed to the robustness of the conductive network constructed by MWCNTs. In addition, the elastic modulus of the MWCNTs/PDMS composites with low aspect ratio was lower than that with high aspect ratio, which was in agreement with that the elastic modulus of MWCNT-based composite increased as the aspect ratio of the carbon nanotubes increased [23,27]. Thus, MWCNT with low aspect ratio might be more appropriate for strain sensor in terms of sensitivity and flexibility. In Fig. 8, the contact points between MWCNTs with high aspect ratio was more than that with low aspect ratio in the conductive network of the strain sensors. In particular, the number of contact points for MWCNTs with different aspect ratio depended on the excluded volumes which refers to one part of a long chain molecule cannot occupy space that is already occupied by another
Fig. 8. Schematic diagram of the conductive paths of carbon nanotubes with different aspect ratios. (a) High aspect ratio MWCNT. (b) Low aspect ratio MWCNT.
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Fig. 9. (a) The dynamic durability test of the MWCNTs/SF/PDMS composites based strain sensors. (b) Electrical response of strain sensor in the first five cycles. (c) Electrical response of strain sensor in the 51–55 cycles. (d) Electrical response of strain sensor in the 96–100 cycles. The strain range was configured as 0.8.
Fig. 10. (a) The presented CPCs based strain sensor for finger joint bending monitoring application. (b) The relative resistance change of the strain sensor during finger bending.
part of the same molecule [28]. For MWCNTs/PDMS composites, the excluded volume decreases with the increase of the aspect ratio of MWCNTs [29]. Therefore, the overlapping area between adjacent MWCNTs with higher aspect ratio increased against the excluded volume decreasing, which leads to the increase of the contact points of adjacent MWCNTs with higher aspect ratio in the conductive network. Consequently, the robustness of conductive networks for composites with MWCNTs with higher aspect ratio was better than that with low aspect ratio against strain loading. In other word,
the variation of contact points between MWCNTs with low aspect ratio is more significant during strain test, which contributes higher sensitivity achieved with respect to that with high aspect ratio. 3.4. The dynamic durability of the MWCNTs/SF/PDMS composites stain sensor and application In order to demonstrate the resistance response and stability of the strain sensor, a mechanical stretching cycle test was performed
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100 times for the MWCNTs/SF/PDMS composites based strain sensor. As shown in Fig. 9(a)–(d), the resistance response and stress of the strain sensor exhibited good stability and repeatability, which implicated the presented strain sensor could be applied for the continuous monitoring application. However, it is worth to note that the R/R0 change of the strain sensor becomes more stable as the number of cycles increased, which is commonly referred to be R/R0 drift that is ubiquitous for the CPCs based strain sensors [15,18,30]. In addition, the shoulder peak phenomenon in the continuous cycle test was also observed (Fig. 9(b)), which was generally considered to be the competition mechanism of the destruction and reconstruction of the conductive network and the dependence of the mechanical properties of the polymer [18,31]. In order to demonstrate the potential application of prepared strain sensor in this work, the presented strain sensor was wrapped around the joint of fingers and the dynamical electrical signal was measured to reflect the movement of finger joints. As shown in Fig. 10, the relative resistance change could respond with respect to different bending angle of finger joint. The larger bending angle is, the larger change of relative resistance obtains, which leads to finger movement distinguishable. Therefore, the proposed MWCNTs/SF/PDMS composites strain sensor could effectively measure the joint movement of human body. 4. Conclusions In summary, a simple and efficient method to improve the sensitivity and flexibility of MWCNT-based strain sensors was proposed. It was found that SF improved the dispersion of MWCNTs in the matrix and increased the sensitivity of strain sensor by 10–30 times without reducing the stretchability. Additionally, the sensitivity of MWCNTs/PDMS composites strain sensors decreased with the aspect ratio of the carbon nanotubes increasing, which was attributed to the robustness of conductive network constructed by MWCNTs with high aspect ratio. It was believed that the presented results could promote the development of flexible strain sensors. Acknowledgments This work was supported by National Natural Science Foundation of China (51605497), State Key Laboratory of High Performance Complex Manufacturing (ZZYJKT2019-05), and Postgraduate Research Innovation of Central South University (502211823). References [1] Y.L. Wang, J. Hao, Z.Q. Huang, G.Q. Zheng, K. Dai, C.T. Liu, C.Y. Shen, Flexible electrically resistive-type strain sensors based on reduced graphene oxide-decorated electrospun polymer fibrous mats for human motion monitoring, Carbon 126 (2018) 360–371. [2] Y.J. Zheng, Y.L. Li, K. Dai, Y. Wang, G.Q. Zheng, C.T. Liu, C.Y. Shen, A highly stretchable and stable strain sensor based on hybrid carbon nanofillers/polydimethylsiloxane conductive composites for large human motions monitoring, Compos. Sci. Technol. 156 (2018) 276–286. [3] D. Kang, P.V. Pikhitsa, Y.W. Choi, C. Lee, S.S. Shin, L.F. Piao, B. Park, K.Y. Suh, T.I. Kim, M. Choi, Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system, Nature 516 (7530) (2014) 222–226. [4] T. Lee, W. Lee, S.W. Kim, J.J. Kim, B.S. Kim, Flexible textile strain wireless sensor functionalized with hybrid carbon nanomaterials supported ZNO nanowires with controlled aspect ratio, Adv. Funct. Mater. 26 (34) (2016) 6206–6214. [5] J.W. Jeong, W.H. Yeo, A. Akhtar, J.J.S. Norton, Y.J. Kwack, S. Li, S.Y. Jung, Y.W. Su, W. Lee, J. Xia, H.Y. Cheng, Y.G. Huang, W.S. Choi, T. Bretl, J.A. Rogers, Materials and optimized designs for human-machine interfaces via epidermal electronics, Adv. Mater. 25 (47) (2013) 6839–6846. [6] M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu, I. Park, Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite, ACS Nano 8 (5) (2014) 5154–5163.
7
[7] M. Amjadi, Y.J. Yoon, I. Park, Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes-Ecoflex nanocomposites, Nanotechnology 26 (37) (2015). [8] J. Lee, S. Kim, J. Lee, D. Yang, B.C. Park, S. Ryu, I. Park, A stretchable strain sensor based on a metal nanoparticle thin film for human motion detection, Nanoscale 6 (20) (2014) 11932–11939. [9] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi, D.N. Futaba, K. Hata, A stretchable carbon nanotube strain sensor for human-motion detection, Nat. Nanotechnol. 6 (5) (2011) 296–301. [10] L. Xie, Y.T. Zhu, Tune the phase morphology to design conductive polymer composites: a review, Polym. Compos. 39 (9) (2018) 2985–2996. [11] H. Liu, M.Y. Dong, W.J. Huang, J.C. Gao, K. Dai, J. Guo, G.Q. Zheng, C.T. Liu, C.Y. Shen, Z.H. Guo, Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing, J. Mater. Chem. C 5 (1) (2017) 73–83. [12] N. Hu, T. Itoi, T. Akagi, T. Kojima, J.M. Xue, C. Yan, S. Atobe, H. Fukunaga, W.F. Yuan, H.M. Ning, Y.L. Surina, Alamusi Liu, Ultrasensitive strain sensors made from metal-coated carbon nanofiller/epoxy composites, Carbon 51 (2013) 202–212. [13] Q. Li, J. Li, D.Q. Tran, C.Q. Luo, Y. Gao, C.J. Yu, F.Z. Xuan, Engineering of carbon nanotube/polydimethylsiloxane nanocomposites with enhanced sensitivity for wearable motion sensors, J. Mater. Chem. C 5 (42) (2017) 11092–11099. [14] M.C. Qu, F. Nilsson, Y.J. Qin, G.D. Yang, Y.M. Pan, X.H. Liu, G.H. Rodriguez, J.F. Chen, C.H. Zhang, D.W. Schubert, Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black, Compos. Sci. Technol. 150 (2017) 24–31. [15] R. Zhang, H. Deng, R. Valenca, J.H. Jin, Q. Fu, E. Bilotti, T. Peijs, Strain sensing behaviour of elastomeric composite films containing carbon nanotubes under cyclic loading, Compos. Sci. Technol. 74 (2013) 1–5. [16] L.Y. Duan, S.R. Fu, H. Deng, Q. Zhang, K. Wang, F. Chen, Q. Fu, The resistivity-strain behavior of conductive polymer composites: stability and sensitivity, J. Mater. Chem. A 2 (40) (2014) 17085–17098. [17] O. Kanoun, C. Muller, A. Benchirouf, A. Sanli, T.N. Dinh, A. Al-Hamry, L. Bu, C. Gerlach, A. Bouhamed, Flexible carbon nanotube films for high performance strain sensors, Sensors 14 (6) (2014) 10042–10071. [18] Y.J. Zheng, Y.L. Li, Z.Y. Li, Y.L. Wang, K. Dai, G.Q. Zheng, C.T. Liu, C.Y. Shen, The effect of filler dimensionality on the electromechanical performance of polydimethylsiloxane based conductive nanocomposites for flexible strain sensors, Compos. Sci. Technol. 139 (2017) 64–73. [19] M. Amjadi, K.-U. Kyung, I. Park, M. Sitti, Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review, Adv. Funct. Mater. 26 (11) (2016) 1678–1698. [20] W. Bauhofer, J.Z. Kovacs, A review and analysis of electrical percolation in carbon nanotube polymer composites, Compos. Sci. Technol. 69 (10) (2009) 1486–1498. [21] J.H. Kim, J.Y. Hwang, H.R. Hwang, H.S. Kim, J.H. Lee, J.W. Seo, U.S. Shin, S.H. Lee, Simple and cost-effective method of highly conductive and elastic carbon nanotube/polydimethylsiloxane composite for wearable electronics, Sci. Rep. 8 (1) (2018) 1375. [22] J. Li, P.C. Ma, W.S. Chow, C.K. To, B.Z. Tang, J.K. Kim, Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes, Adv. Funct. Mater. 17 (16) (2007) 3207–3215. [23] J.W. Zha, K. Shehzad, W.K. Li, Z.M. Dang, The effect of aspect ratio on the piezoresistive behavior of the multiwalled carbon nanotubes/thermoplastic elastomer nanocomposites, J. Appl. Phys. 113 (1) (2013). [24] L. Gan, M. Dong, Y. Han, Y.F. Xiao, L. Yang, J. Huang, Connection-improved conductive network of carbon nanotubes in a rubber cross-link network, ACS Appl. Mater. Interfaces 10 (21) (2018) 18213–18219. [25] L. Chen, G.H. Chen, L. Lu, Piezoresistive behavior study on finger-sensing silicone rubber/graphite nanosheet nanocomposites, Adv. Funct. Mater. 17 (6) (2007) 898–904. [26] Y. Gao, X.L. Fang, J.P. Tan, T. Lu, L.K. Pan, F.Z. Xuan, Highly sensitive strain sensors based on fragmentized carbon nanotube/polydimethylsiloxane composites, Nanotechnology 29 (23) (2018) 11. [27] A. Haque, A. Ramasetty, Theoretical study of stress transfer in carbon nanotube reinforced polymer matrix composites, Compos. Struct. 71 (1) (2005) 68–77. [28] I. Balberg, C.H. Anderson, S. Alexander, N. Wagner, Excluded volume and its relation to the onset of percolation, Phys. Rev. B 30 (7) (1984) 3933–3943. [29] M.R. Ayatollahi, S. Shadlou, M.M. Shokrieh, M. Chitsazzadeh, Effect of multi-walled carbon nanotube aspect ratio on mechanical and electrical properties of epoxy-based nanocomposites, Polym. Test. 30 (5) (2011) 548–556. [30] Y.J. Zheng, Y.L. Li, K. Dai, M.R. Liu, K.K. Zhou, G.Q. Zheng, C.T. Liu, C.Y. Shen, Conductive thermoplastic polyurethane composites with tunable piezoresistivity by modulating the filler dimensionality for flexible strain sensors, Compos. Part A: Appl. Sci. Manuf. 101 (2017) 41–49. [31] H. Yang, X.F. Yao, L. Yuan, L.H. Gong, Y.H. Liu, Strain-sensitive electrical conductivity of carbon nanotube-graphene-filled rubber composites under cyclic loading, Nanoscale 11 (2) (2019) 578–586.
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Biographies Dengji Guo received the B.Eng. degree in microelectronics manufacturing engineering from Central South University in 2017, and currently pursues Master degree on mechanical engineering in Central South University. His research topic is focusing on the flexible strain sensor based on carbon nanotubes. Xudong Pan received the B.Eng. degree in mechanical engineering from Central South University in 2018, and currently pursues Master degree on mechanical engineering in Central South University. His research topic is focusing on the flexible strain sensor based on carbon nanotubes and liquid metal.
Hu He received the B.Eng. degree in microelectronics manufacturing engineering from Central South University in 2008, and received the Ph.D. degree in computer science and electrical engineering from the Queensland University of Technology in 2014. Currently he is an associate professor of mechanical engineering at Central South University. His current research interests include micro/nano structure fabrication, advanced packaging, reliability and failure analysis, image processing.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A simple and cost-effective method for improving the sensitivity of flexible strain sensors based on conductive polymer composites Dengji Guo, Xudong Pan, Hu He ∗ State Key Laboratory of High Performance Complex Manufacturing, College of Mechanical and Electrical Engineering, Central South University, Changsha, 410083, China
a r t i c l e
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Article history: Received 17 June 2019 Received in revised form 20 August 2019 Accepted 6 September 2019 Available online 9 September 2019 Keywords: Flexible strain sensors Conductive polymer composites Silicone fluid Carbon nanotubes PDMS
a b s t r a c t Conductive polymer composites (CPCs) based flexible strain sensors requires high performance in terms of sensitivity and stretchability towards potential applications in health monitoring, motion detection and human-machine interaction. In this work, a simple and cost-effective method was proposed to improve the sensitivity yet maintain the stretchability of CPCs based strain sensor. Compared with conventional multi-wall carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) flexible strain sensors, silicone fluid was introduced to configure the conductive networks as well as the mechanical properties. It was found that silicone fluid can increase the sensitivity of strain sensor by 10–30 times without reducing the stretchability, which was attributed to the non-covalently functionalized surface of the carbon nanotubes leading to higher ratio of the tunneling resistance to overall resistance of CPCs based flexible strain sensor. Furthermore, the synergetic effect between silicone fluid and CNTs was investigated through conductive network reconfiguration and electromechanical test. The proposed efficient and effective flexible strain sensor preparation approach provides useful insights for high-performance flexible strain sensor design and application. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Flexible strain sensors have recently attracted widespread attention due to their potential applications such as human motion detection [1,2], electronic skin [3] and human-machine interaction [4,5]. Generally, the preparation of flexible strain sensor could adopt either electrical conductive sensing films coupled with flexible substrates [6–9] or a conductive polymer composites (CPCs) [10,11]. In particular, CPCs are considered to be ideal for flexible strain sensors due to the ease of preparation, controllable electromechanical properties, and diverse substrate selection [12,13]. In order to adjust the electromechanical properties of CPCs, carbonbased conductive nanostructured fillers, such as carbon blacks (CBs) and carbon nanotubes (CNTs), have been widely employed [14]. Compared with CBs, CNTs are more attractable for CPCs based flexible strain sensors due to their excellent characteristics, such as exceptional mechanical, electrical properties and high aspect ratio [15]. Stretchability and sensitivity are the primary performance indicators of flexible strain sensors. Reducing the density of the
∗ Corresponding author. E-mail address: [email protected] (H. He). https://doi.org/10.1016/j.sna.2019.111608 0924-4247/© 2019 Elsevier B.V. All rights reserved.
conductive network generally improves the sensitivity of CPCs based flexible strain sensor with single conductive filler, while degrades the stretchability range [16,17]. Additionally, the density of conductive networks mainly depends on the content of conductive fillers in CPCs. Therefore, plenty of work improving the sensitivity and stretchability of CPCs based flexible strain sensors by adjusting the conductive fillers have been conducted. Zheng et al. showed the sensitivity and stretchability of CNTs/PDMS composites flexible strain sensors with different CNTs contents [2]. Although the sensitivity of the 0.48 vol.% CNTs was about 200 times than that of 2.45 vol.%, the effective strain range of the sensor reduced to be only a half [18]. Hu et al. proposed a CPCs based strain sensor using metal-coated MWCNTs, whose sensitivity was 4.3 times as large as that of untreated materials [12]. The metal-coated MWCNTs could significantly improve the sensitivity of the strain sensor due to a sparse electrical conductive network and a higher ratio of the tunneling resistance to overall resistance of CPCs. However, the preparation of metal-coated MWCNTs required complicated and costly nano-electrolytic plating process. Li et al. presented a CNTs/PDMS composites foam based strain sensor to improve the sensitivity with the degradation of effective strain range due to the porosity of the composites reduce the elongation [13]. Furthermore, a flexible strain sensor based on CNTs and CBs hybrid fillers was proposed to achieve 300% strain range yet keep the rea-
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Fig. 1. (a) Schematic of the preparation process the MWCNTs/SF/PDMS composites. (b) Injection molding die. (c) Size of the strain sensors. (d) Electronic universal testing machine and resistance tester.
sonable sensitivity, which is attribute to the synergetic effect of hybrid fillers in conductive path reconfiguration [2]. Nevertheless, the sensitivity of presented strain sensor was less than 1 with the strain range below 100%. Thus, an effective and low-cost method on the preparation of CPCs based flexible strain sensors using CNTs towards high sensitivity and stretchability is still required in the field. In this paper, silicone fluid (SF) was mixed into MWCNTs/PDMS to prepare a CPCs based strain sensor. It was found that SF could increase the sensitivity of strain sensor by 10–30 times without sacrificing the stretchability. Particularly, the sensitivity improved significantly at low strain range, which is applicable in common scenarios for wearable flexible sensors. Additionally, the synergetic effect between silicone fluid and CNTs was investigated in terms of conductive network forming and electromechanical performance evaluation. Furthermore, the improvement mechanism of silicone fluid for the performance of MWCNTs/PDMS strain sensor was discussed. It was believed the presented simple and cost-effective method for high-performance flexible strain sensor preparation could pave a way for wearable sensor applications in the future.
structural and physical drawings of the injection molding tool. The size of the test sample size was 30 mm × 6 mm × 1 mm (Fig. 1(c)). The microscopic morphology of the composites were characterized by a scanning electron microscope (SEM, Tescan Vega3). Fig. 1(d) showed the testing equipment for the mechanicalelectrical characteristics of the strain sensors. Electronic universal testing machine (Shenzhen Suns Technology Stock Co. Ltd) was employed to control the strain of the sample and record the change in stress. Precision resistance tester AT512(Applent Instruments Co. Ltd)was synchronized with the electronic universal testing machine to acquire the resistance change of testing samples. The gauge factor (GF) of the strain sensor was calculated by the following equation GF =
R/R0 (R − R0 ) /ε = ε ε
(1)
where ε is the mechanical strain applied to the strain sensor; R is the change in resistance; R0 and R are the initial resistance of the strain sensor and resistance at an applied strain, respectively. 3. Results and discussion
2. Materials and methods
3.1. Effect of SF on electrical properties
The PDMS (SYLARD 184) and PMX-200 silicone fluid 100cst were purchased from Dow Coring Corporation. The multi-walled nanotube (MWCNT) was purchased from Shengzhen Nanotech Port Co. Ltd (diameter:>5 um, length: 20–40 nm). Isopropanol alcohol (IPA) and other chemicals were purchased from aladdin Co. Ltd. All these material were employed as ordered. The fabrication process of the MWCNTs/Silicone fluid/PDMS nanocomposites strain sensors is illustrated in Fig. 1(a). The strain sensors was fabricated by liquid phase mixing to obtain a good dispersion of MWCNTs in the PDMS matrix [19]. Specifically, MWCNTs was first dispersed in IPA by ultrasonic process for 2 h. In the following, PDMS and SF were added into the MWCNTs/IPA solution and stirred for 2 h in a 60 ◦ water bath. The IPA in the mixed solution was completely volatilized by stirring in a water bath at 80 ◦ C. Finally, the curing agent was added into the mixed solution with the mass ratio of 10:1 of base polymer to curing agent. The mixture composites were stirred thoroughly and then vacuumed for use. The strain sensor test sample was prepared by injection molding in order to obtain a dense, non-porous sample. Fig. 1(b) showed the
As shown in Fig. 2a, the electrical property of the MWCNTs/PDMS and MWCNTs/SF/PDMS (PDMS:SF = 4:1) composites with different MWCNTs contents was presented. The initial resistance of the composites decreased as the MWCNTs content increased, which was in accordance with the percolation theory [20]. Moreover, the resistance reduced from disconnection to 30 k when the MWCNT content of the MWCNT/PDMS composites increased from 4 wt% to 5 wt%. The resistance varied significantly between 4 wt% and 5 wt%, which indicated a percolation threshold between 4 wt% and 5 wt%. However, the resistance of MWCNTs/SF/PDMS composites at 4 wt% was 1570 k, indicating that the percolation threshold was less than 4 wt%. The percolation threshold of MWCNTs/PDMS composites with SF was lower than that without SF. Thus, SF could be introduced to adjust electrical properties and reduce percolation threshold, which was attributed to that SF could effectively promote uniform dispersion of MWCNTs in PDMS matrix [21]. SF was coated on the surface of the MWCNTs by non-covalent bonding and could be attached to the PDMS matrix, which avoided re-aggregation of the MWCNTs during the
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Fig. 2. (a) The initial electrical resistance of MWCNTs/PDMS and MWCNTs/SF/PDMS at different MWCNT contents (4 wt%, 5 wt% and 6 wt%, respectively). (b) SEM images of the microstructure of the MWCNTs/SF/PDMS composites. (c) SEM images of the microstructure of the MWCNTs/PDMS composites.
Fig. 3. The schematic cross sectional diagram of the spatial relationship and resistance type of MWCNTs in the initial status. (a) MWCNTs/PDMS composites. (b) MWCNTs/SF/PDMS composites.
IPA high temperature evaporation and curing process [21]. It was also verified by SEM images that silicone fluid could significantly increase the dispersion of MWCNTs in PDMS matrix (Fig. 2(b) and (c)). The coating effect of SF achieved non-covalent functionalization of the surface of the carbon nanotubes to increase the degree of dispersion of the MWCNTs and reduce the percolation threshold. Uniformly dispersed MWCNTs is helpful to increase the number of conductive pathways in the MWCNTs/PDMS composites and reduced the initial resistance according to the percolation theory [22]. However, the initial resistance of 5 wt% and 6 wt% MWCNTs with SF were higher than that without SF, which was attributed to the coating effect of SF on MWCNTs (Fig. 2). SF functionalized MWCNTs surfaces changed the resistance composition of the MWCNTs/PDMS composites. The resistance of MWCNTs/PDMS composites consists of three type resistance, the intrinsic resistance of the MWCNT, the contact resistance between the MWCNT tubes and the tunneling resistance between the MWCNT tubes [23]. The intrinsic resistance depended on the nature of the carbon nanotubes. Both the tunneling resistance and the contact resistance are
related to the spatial position and the distance between the MWCNTs tubes [12]. The contact resistance was formed by the outer walls of the MWCNTs being in contact with each other. However, SF separated the originally contacted MWCNTs and created a certain distance, which was attributed to the non-covalently functionalized carbon nanotube surface of SF (Fig. 3(a)) [21]. The separation distance resulted from the functionalization of the SF satisfied the tunneling resistance condition, resulting in the conversion of the contact resistance into the tunneling resistance. Moreover, the original tunneling resistance was also turned into a disconnection due to the functionalization of the SF (Fig. 3(b)). Therefore, SF functionalized MWCNTs surfaces increased the proportion of tunneling resistance and reduced the proportion of contact resistance. In addition, the tunneling resistance was much larger than the contact resistance, resulting in an increase in the initial resistance of MWCNTs/PDMS composites with SF. Fig. 4(a) represents the initial resistance of MWCNTs/SF/PDMS composites with 5 wt% MWCNTs at different ratios of PDMS to SF. As the increase of PDMS matrix, the initial resistance firstly increased
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Fig. 4. (a) The initial electrical resistance for MWCNTs/SF/PDMS composites at different ratios of PDMS to SF. (b) The R/R0 as a function of the applied tensile strain during stretching test for MWCNTs/SF/PDMS composites at different ratio of PDMS to SF. (c) Stress-strain curve for MWCNTs/SF/PDMS composites at different ratio of PDMS to SF. The weight ratio of MWCNTs is 5 wt% and the tensile rate is 0.5 mm/s.
Fig. 5. The stretching test of the MWCNTs/PDMS and MWCNTs/SF/PDMS (containing 5 wt% and 6 wt% MWCNTs). (a) The R/R0 as a function of the applied tensile strain (tensile rate = 0.5 mm/s). (b) Stress-strain curve of the composites.
and then decreased. The lower resistance can be achieved with the ratio of PDMS to SF as 4:1 in comparing with that of 2:1 and 1:1. The non-covalent functionalization effect of SF on the surface of carbon nanotubes would increase as the content of SF increases, thus, the electrical resistance would increase. In addition, the standard deviation of the resistance increases significantly as the SF content increased, which might be related to the decrease in the degree of cross-linking of PDMS. Therefore, the stability of the conductive network was weakened with the degradation of the degree of cross-linking of PDMS when the SF content increased [24,25]. As shown in Fig. 4(b) and (c), the effective strain sensing interval of the strain sensor increased as the SF content decreased. Comparing with the sample with the ratio of PDMS to SF as 4:1, the
effective sensing range degrades to be almost half for that of 2:1 and 1:1. Moreover, the electrical response of the strain sensor fluctuates sharply within the strain range of 0.5–0.8 when the ratio of PDMS to SF was 2:1 and 1:1, which introduces difficulty for potential applications. Thus, the ratio of PDMS to SF as 4:1 was selected to prepare the sample in this work based on the mechanical and electrical performance. 3.2. Effect of SF on strain sensors performance Slope of the relative change of the resistance versus applied strain reflects Gauge factor (GF) of strain sensors and the GF value represents the sensitivity of the strain sensors. Following the com-
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Fig. 7. The relationship between R/R0 and stress of 4 wt% MWCNTs/SF/PDMS composites (tensile rate = 0.5 mm/s). Particularly, the dashed line represents our results while the solid line denotes literature results.
Fig. 6. The schematic cross sectional diagram of the change in the spatial relationship of the MWCNTs and the resistance during the stretching process. (a) MWCNTs/PDMS composites. (b) MWCNTs/SF/PDMS composites.
mon way to calculate the GF in the literature [13,18,26], the regions with good linearity are commonly employed to calculate the GF. As shown in Fig. 5(a), the GF of the MWCNTs/PDMS composites strain sensor of 5 wt% of MWCNTs was higher than 6 wt%. For MWCNTsbased strain sensors, the sensitivity of the strain sensor decreased as the MWCNT content increasing, which was attributed to the density of the conductive network composed of MWCNTs. When the strain sensor was subjected to strain, the low-density conductive network was prone to damage easier, resulting in a more significant resistance change. Moreover, the sensitivity of the 5 wt% MWCNTs/PDMS composites with SF was 8.68 at the strain range of 0–0.8 and the sensitivity was 23 at the strain range of 0.8–1.4. The sensitivity of the strain sensor with SF was 10–30 times of that without SF, which implicated MWCNTs surface functionalized with SF would alter the process of resistance changes during strain. As shown in Fig. 5(b), the modulus of elasticity was represented by the slope of the stress-strain curve of the strain sensor. The elastic modulus of the MWCNTs/PDMS composites strain sensors of 6 wt% of MWCNTs was higher than 5 wt%. MWCNTs with higher content increased the elastic modulus of composites [23]. Moreover, the elastic modulus of the MWCNTs/PDMS composites with SF was lower than that without SF while the strain range of strain sensors was sustained, which indicated SF changed the interfacial relationship between MWCNTs and PDMS matrix and weakened the reinforcing effect of MWCNTs as well as the degree of crosslinking of the PDMS. Therefore, SF improved the flexibility of the MWCNTs/PDMS composites strain sensor without sacrificing stretchability, which improved the adaptability of the flexible strain sensor to complex surfaces. In addition, SF could affect the sensing mechanism. Fig. 6 showed the change in the spatial relationship of the MWCNTs and the resistance change during the stretching process. The distance between the carbon nanotubes in the MWCNTs/PDMS composites would
gradually increase in the direction of stretching and the resistance changed from the contact resistance to tunneling resistance until it was broken (Fig. 6(a)). However, SF functionalized MWCNTs only underwent the tunneling resistance to the disconnection process (Fig. 6(b)). Since the resistance change from contact resistance to tunneling resistance was smaller than that from tunneling resistance to disconnection, the resistance change of the composites with SF was more significant than that without SF. In other word, SF functionalized carbon nanotubes surface not only increased the ratio of tunneling resistance to the overall resistance, but also changed the process of resistance change under strain loading. 3.3. Effect of MWCNTs aspect ratio on strain sensors performance As shown in Fig. 7, the sensitivity of strain sensor composed of MWCNTs with the low aspect ratio was higher than that of the high aspect ratio. The sensitivity of the strain sensors with the aspect ratio of 300 was 26.8 at the strain range of 0–17% and the sensitivity was 7.5 at the strain range of 17–40%. It was found the sensitivity of strain sensor with the aspect ratio of 300 was 17–62 times of that with the aspect ratio of 1000. For the MWCNTs/PDMS composites strain sensors, the sensitivity decreased with the increase of the aspect ratio of MWCNTs, which was attributed to the robustness of the conductive network constructed by MWCNTs. In addition, the elastic modulus of the MWCNTs/PDMS composites with low aspect ratio was lower than that with high aspect ratio, which was in agreement with that the elastic modulus of MWCNT-based composite increased as the aspect ratio of the carbon nanotubes increased [23,27]. Thus, MWCNT with low aspect ratio might be more appropriate for strain sensor in terms of sensitivity and flexibility. In Fig. 8, the contact points between MWCNTs with high aspect ratio was more than that with low aspect ratio in the conductive network of the strain sensors. In particular, the number of contact points for MWCNTs with different aspect ratio depended on the excluded volumes which refers to one part of a long chain molecule cannot occupy space that is already occupied by another
Fig. 8. Schematic diagram of the conductive paths of carbon nanotubes with different aspect ratios. (a) High aspect ratio MWCNT. (b) Low aspect ratio MWCNT.
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Fig. 9. (a) The dynamic durability test of the MWCNTs/SF/PDMS composites based strain sensors. (b) Electrical response of strain sensor in the first five cycles. (c) Electrical response of strain sensor in the 51–55 cycles. (d) Electrical response of strain sensor in the 96–100 cycles. The strain range was configured as 0.8.
Fig. 10. (a) The presented CPCs based strain sensor for finger joint bending monitoring application. (b) The relative resistance change of the strain sensor during finger bending.
part of the same molecule [28]. For MWCNTs/PDMS composites, the excluded volume decreases with the increase of the aspect ratio of MWCNTs [29]. Therefore, the overlapping area between adjacent MWCNTs with higher aspect ratio increased against the excluded volume decreasing, which leads to the increase of the contact points of adjacent MWCNTs with higher aspect ratio in the conductive network. Consequently, the robustness of conductive networks for composites with MWCNTs with higher aspect ratio was better than that with low aspect ratio against strain loading. In other word,
the variation of contact points between MWCNTs with low aspect ratio is more significant during strain test, which contributes higher sensitivity achieved with respect to that with high aspect ratio. 3.4. The dynamic durability of the MWCNTs/SF/PDMS composites stain sensor and application In order to demonstrate the resistance response and stability of the strain sensor, a mechanical stretching cycle test was performed
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100 times for the MWCNTs/SF/PDMS composites based strain sensor. As shown in Fig. 9(a)–(d), the resistance response and stress of the strain sensor exhibited good stability and repeatability, which implicated the presented strain sensor could be applied for the continuous monitoring application. However, it is worth to note that the R/R0 change of the strain sensor becomes more stable as the number of cycles increased, which is commonly referred to be R/R0 drift that is ubiquitous for the CPCs based strain sensors [15,18,30]. In addition, the shoulder peak phenomenon in the continuous cycle test was also observed (Fig. 9(b)), which was generally considered to be the competition mechanism of the destruction and reconstruction of the conductive network and the dependence of the mechanical properties of the polymer [18,31]. In order to demonstrate the potential application of prepared strain sensor in this work, the presented strain sensor was wrapped around the joint of fingers and the dynamical electrical signal was measured to reflect the movement of finger joints. As shown in Fig. 10, the relative resistance change could respond with respect to different bending angle of finger joint. The larger bending angle is, the larger change of relative resistance obtains, which leads to finger movement distinguishable. Therefore, the proposed MWCNTs/SF/PDMS composites strain sensor could effectively measure the joint movement of human body. 4. Conclusions In summary, a simple and efficient method to improve the sensitivity and flexibility of MWCNT-based strain sensors was proposed. It was found that SF improved the dispersion of MWCNTs in the matrix and increased the sensitivity of strain sensor by 10–30 times without reducing the stretchability. Additionally, the sensitivity of MWCNTs/PDMS composites strain sensors decreased with the aspect ratio of the carbon nanotubes increasing, which was attributed to the robustness of conductive network constructed by MWCNTs with high aspect ratio. It was believed that the presented results could promote the development of flexible strain sensors. Acknowledgments This work was supported by National Natural Science Foundation of China (51605497), State Key Laboratory of High Performance Complex Manufacturing (ZZYJKT2019-05), and Postgraduate Research Innovation of Central South University (502211823). References [1] Y.L. Wang, J. Hao, Z.Q. Huang, G.Q. Zheng, K. Dai, C.T. Liu, C.Y. Shen, Flexible electrically resistive-type strain sensors based on reduced graphene oxide-decorated electrospun polymer fibrous mats for human motion monitoring, Carbon 126 (2018) 360–371. [2] Y.J. Zheng, Y.L. Li, K. Dai, Y. Wang, G.Q. Zheng, C.T. Liu, C.Y. Shen, A highly stretchable and stable strain sensor based on hybrid carbon nanofillers/polydimethylsiloxane conductive composites for large human motions monitoring, Compos. Sci. Technol. 156 (2018) 276–286. [3] D. Kang, P.V. Pikhitsa, Y.W. Choi, C. Lee, S.S. Shin, L.F. Piao, B. Park, K.Y. Suh, T.I. Kim, M. Choi, Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system, Nature 516 (7530) (2014) 222–226. [4] T. Lee, W. Lee, S.W. Kim, J.J. Kim, B.S. Kim, Flexible textile strain wireless sensor functionalized with hybrid carbon nanomaterials supported ZNO nanowires with controlled aspect ratio, Adv. Funct. Mater. 26 (34) (2016) 6206–6214. [5] J.W. Jeong, W.H. Yeo, A. Akhtar, J.J.S. Norton, Y.J. Kwack, S. Li, S.Y. Jung, Y.W. Su, W. Lee, J. Xia, H.Y. Cheng, Y.G. Huang, W.S. Choi, T. Bretl, J.A. Rogers, Materials and optimized designs for human-machine interfaces via epidermal electronics, Adv. Mater. 25 (47) (2013) 6839–6846. [6] M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu, I. Park, Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite, ACS Nano 8 (5) (2014) 5154–5163.
7
[7] M. Amjadi, Y.J. Yoon, I. Park, Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes-Ecoflex nanocomposites, Nanotechnology 26 (37) (2015). [8] J. Lee, S. Kim, J. Lee, D. Yang, B.C. Park, S. Ryu, I. Park, A stretchable strain sensor based on a metal nanoparticle thin film for human motion detection, Nanoscale 6 (20) (2014) 11932–11939. [9] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi, D.N. Futaba, K. Hata, A stretchable carbon nanotube strain sensor for human-motion detection, Nat. Nanotechnol. 6 (5) (2011) 296–301. [10] L. Xie, Y.T. Zhu, Tune the phase morphology to design conductive polymer composites: a review, Polym. Compos. 39 (9) (2018) 2985–2996. [11] H. Liu, M.Y. Dong, W.J. Huang, J.C. Gao, K. Dai, J. Guo, G.Q. Zheng, C.T. Liu, C.Y. Shen, Z.H. Guo, Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing, J. Mater. Chem. C 5 (1) (2017) 73–83. [12] N. Hu, T. Itoi, T. Akagi, T. Kojima, J.M. Xue, C. Yan, S. Atobe, H. Fukunaga, W.F. Yuan, H.M. Ning, Y.L. Surina, Alamusi Liu, Ultrasensitive strain sensors made from metal-coated carbon nanofiller/epoxy composites, Carbon 51 (2013) 202–212. [13] Q. Li, J. Li, D.Q. Tran, C.Q. Luo, Y. Gao, C.J. Yu, F.Z. Xuan, Engineering of carbon nanotube/polydimethylsiloxane nanocomposites with enhanced sensitivity for wearable motion sensors, J. Mater. Chem. C 5 (42) (2017) 11092–11099. [14] M.C. Qu, F. Nilsson, Y.J. Qin, G.D. Yang, Y.M. Pan, X.H. Liu, G.H. Rodriguez, J.F. Chen, C.H. Zhang, D.W. Schubert, Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black, Compos. Sci. Technol. 150 (2017) 24–31. [15] R. Zhang, H. Deng, R. Valenca, J.H. Jin, Q. Fu, E. Bilotti, T. Peijs, Strain sensing behaviour of elastomeric composite films containing carbon nanotubes under cyclic loading, Compos. Sci. Technol. 74 (2013) 1–5. [16] L.Y. Duan, S.R. Fu, H. Deng, Q. Zhang, K. Wang, F. Chen, Q. Fu, The resistivity-strain behavior of conductive polymer composites: stability and sensitivity, J. Mater. Chem. A 2 (40) (2014) 17085–17098. [17] O. Kanoun, C. Muller, A. Benchirouf, A. Sanli, T.N. Dinh, A. Al-Hamry, L. Bu, C. Gerlach, A. Bouhamed, Flexible carbon nanotube films for high performance strain sensors, Sensors 14 (6) (2014) 10042–10071. [18] Y.J. Zheng, Y.L. Li, Z.Y. Li, Y.L. Wang, K. Dai, G.Q. Zheng, C.T. Liu, C.Y. Shen, The effect of filler dimensionality on the electromechanical performance of polydimethylsiloxane based conductive nanocomposites for flexible strain sensors, Compos. Sci. Technol. 139 (2017) 64–73. [19] M. Amjadi, K.-U. Kyung, I. Park, M. Sitti, Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review, Adv. Funct. Mater. 26 (11) (2016) 1678–1698. [20] W. Bauhofer, J.Z. Kovacs, A review and analysis of electrical percolation in carbon nanotube polymer composites, Compos. Sci. Technol. 69 (10) (2009) 1486–1498. [21] J.H. Kim, J.Y. Hwang, H.R. Hwang, H.S. Kim, J.H. Lee, J.W. Seo, U.S. Shin, S.H. Lee, Simple and cost-effective method of highly conductive and elastic carbon nanotube/polydimethylsiloxane composite for wearable electronics, Sci. Rep. 8 (1) (2018) 1375. [22] J. Li, P.C. Ma, W.S. Chow, C.K. To, B.Z. Tang, J.K. Kim, Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes, Adv. Funct. Mater. 17 (16) (2007) 3207–3215. [23] J.W. Zha, K. Shehzad, W.K. Li, Z.M. Dang, The effect of aspect ratio on the piezoresistive behavior of the multiwalled carbon nanotubes/thermoplastic elastomer nanocomposites, J. Appl. Phys. 113 (1) (2013). [24] L. Gan, M. Dong, Y. Han, Y.F. Xiao, L. Yang, J. Huang, Connection-improved conductive network of carbon nanotubes in a rubber cross-link network, ACS Appl. Mater. Interfaces 10 (21) (2018) 18213–18219. [25] L. Chen, G.H. Chen, L. Lu, Piezoresistive behavior study on finger-sensing silicone rubber/graphite nanosheet nanocomposites, Adv. Funct. Mater. 17 (6) (2007) 898–904. [26] Y. Gao, X.L. Fang, J.P. Tan, T. Lu, L.K. Pan, F.Z. Xuan, Highly sensitive strain sensors based on fragmentized carbon nanotube/polydimethylsiloxane composites, Nanotechnology 29 (23) (2018) 11. [27] A. Haque, A. Ramasetty, Theoretical study of stress transfer in carbon nanotube reinforced polymer matrix composites, Compos. Struct. 71 (1) (2005) 68–77. [28] I. Balberg, C.H. Anderson, S. Alexander, N. Wagner, Excluded volume and its relation to the onset of percolation, Phys. Rev. B 30 (7) (1984) 3933–3943. [29] M.R. Ayatollahi, S. Shadlou, M.M. Shokrieh, M. Chitsazzadeh, Effect of multi-walled carbon nanotube aspect ratio on mechanical and electrical properties of epoxy-based nanocomposites, Polym. Test. 30 (5) (2011) 548–556. [30] Y.J. Zheng, Y.L. Li, K. Dai, M.R. Liu, K.K. Zhou, G.Q. Zheng, C.T. Liu, C.Y. Shen, Conductive thermoplastic polyurethane composites with tunable piezoresistivity by modulating the filler dimensionality for flexible strain sensors, Compos. Part A: Appl. Sci. Manuf. 101 (2017) 41–49. [31] H. Yang, X.F. Yao, L. Yuan, L.H. Gong, Y.H. Liu, Strain-sensitive electrical conductivity of carbon nanotube-graphene-filled rubber composites under cyclic loading, Nanoscale 11 (2) (2019) 578–586.
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Biographies Dengji Guo received the B.Eng. degree in microelectronics manufacturing engineering from Central South University in 2017, and currently pursues Master degree on mechanical engineering in Central South University. His research topic is focusing on the flexible strain sensor based on carbon nanotubes. Xudong Pan received the B.Eng. degree in mechanical engineering from Central South University in 2018, and currently pursues Master degree on mechanical engineering in Central South University. His research topic is focusing on the flexible strain sensor based on carbon nanotubes and liquid metal.
Hu He received the B.Eng. degree in microelectronics manufacturing engineering from Central South University in 2008, and received the Ph.D. degree in computer science and electrical engineering from the Queensland University of Technology in 2014. Currently he is an associate professor of mechanical engineering at Central South University. His current research interests include micro/nano structure fabrication, advanced packaging, reliability and failure analysis, image processing.