Synthesis of percolative hyperelastic conducting composite and demonstrations of application in wearable strain sensors
Introduction
Recently, flexible sensors are playing more and more important role in electronic skin, robotics, medical monitoring and diagnostic devices [1], [2], [3], [4], [5], [6], [7], [8], [9]. Especially, strain sensors are an important member of flexible sensor family, which highly require both good stretchability (up to strain of 60% for human skin deformation [10]), good reversibility and high gauge factor [6], [7], [8], [9]. Liu and coauthors developed strain sensors based on graphene woven fabric (GWF) embedded into PDMS matrix [5]. The sensor shows high gauge factor ((ΔR/R0)/ε: where ε represents the strain [6]) of 223 at strain of 3%. However, the maximum strain is less than 5% due to the interconnected graphene mesh structure and complicated preparation process of GWF. Li and co-workers decorated TPU fiber yarns with MWCNTs and SWCNTs and integrated it into strain sensors [7]. Despite of the high workable strain range of 100%, the average gauge factor is only about 1.67. Hao’s team constructed strain sensors based on CNT foam/PDMS [8]. Although the sensors are both compressible and stretchable, and the compressive gauge factor is as high as 104.8, the tensile gauge factor is only 8.8 and maximum strain is 60%. In this case, solutions from viewpoint of material to achieve low cost strain sensors that can sustain large deformation and show high performance are still highly needed. Hyperelastic materials could endure very large strains and are fully reversible [11]. Hyperelastic conductive composites are generally one of the preferred candidates for strain sensors because of their tunability in electric conductivity and elasticity [12], [13], [14]. When external force is applied to stretch the composite, the distance between internal conductive fillers is extended, consequently the probability of both conductive contact and tunneling current, which causes the global electric resistance to increase. While after releasing the strain, the conductive network becomes dense again, the electric resistance decreases. The conductivity of the conductive composite has been proven to follow the percolation theory [15], [16]. However, only in a certain range of volume ratio of filler, the composite remains both hyperelastic and conductive due to the strengthening effect of filler [17] for certain fillers.
In this work, a hyperelastic conductive polymeric composite AB-Ecoflex for strain sensors and wearable electronics is proposed. AB shows good electric conductivity and can chemically (by carboxyl, quinone, phenol and lactone groups) and physically (physisorption) bond with the polymeric matrix. The filler-matrix interaction enhances the internal continuity of composite [18] so that the mechanical performance of the composite could be maintained better than traditional metallic particles filler. Ecoflex®00-30 from Smooth-on Inc serves as the polymer matrix for its superior biocompatibility and high elasticity with a Young’s modulus low as 125 kPa [19]. It has been used in soft actuator for edible robotics [20] and wearable devices [21]. By combining filler of AB and matrix of Ecoflex, high performance of strain sensor could be assured.
Section snippets
Preparation of pure Ecoflex film, AB-Ecoflex and strain sensors
Same amount of part A and part B of Ecoflex were mixed by mechanical stirring for 5 min to form a uniform mixture and then kept in vacuum for 15 min to eliminate air bubbles. Revolution speed was set at 200 rpm for 30 s. The wet film was thermally cured at 60 °C for 15 mins for further use. Certain amount of AB was mixed with Ecoflex consisting of equal mass of part A and part B to form AB-Ecoflex composite. 50 mm × 5 mm × 2 mm AB-Ecoflex strips sandwiched between two half-cured Ecoflex films
Results and discussions
The schematic of AB-Ecoflex composite is illustrated in Fig. 1(a). The dependence of conductivity and Young’s modulus of the composite with the volume ratio of AB filler is shown in Fig. 1(b). It displays that conductivity increases dramatically by several orders when volume ratio of AB increases from 5% to 8%, which agrees well with the percolation theory [15], [16] and the threshold could be estimated to be around 5%. For Young’s modulus of composite with different volume ratio of AB, it
Conclusions
In this work, a percolative composite with AB as filler and Ecoflex as matrix has been developed. The composite with 14.5% volume ratio of AB was used to build a strain sensor with excellent sensitivity, stability and reversibility. The sensor is sensitive to large scale finger bending and even minor facial expression, which shows the promising application in flexible electronics as an integrated sensor to control motions of an artificial arm or leg.
Acknowledgments
The authors would like to thank the Open Projects Foundation (No. SKLD1708) of State Key Laboratory of Optical Fiber and Cable Manufacture Technology (Yangtze Optical Fiber and Cable Joint Stock Limited Company). The authors also want to thank the support of National Natural Science Foundation of China (Grant No. 51702285), Zhejiang Provincial Natural Science Foundation (Grant No. LY17F040003). All authors want to thank Prof. Dennis Chi Wah Leung of The Hong Kong Polytechnic University to
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Lei Zhang and Kaifeng Chen contribute equally to the work.