Ultrathin GPU/CNTs@Ag electrospun fibers for use as strain/piezo-resistive flexible wearable sensors

Carbon-based nano-materials are excellent candidates for constructing conductive sensing networks in flexible polymer matrix, which is widely used. However, it remains a challenge to improve the flexibility and sensitivity of multifunctional sensor by synergistically combining several conductive materials in the sensor fabrication processes. In this study, graphene (GR) dopped thermoplastic polyurethane (GPU) was adopted as the matrix material with surface grafting of carbon nano-tubes (CNTs) and silver nano-particles (Ag) for fabricating fibrous membranes by electrospinning and ultrasonic adsorption. Then GPU/CNTs@Ag-based sensors were made and tested for their piezo-resistive and strain sensing properties. The results showed that GR and Ag nano-particles increased the initial piezo-resistance response, and the piezo-resistive sensor had the sensitivity of 0.08 kPa-1 within 0-6 kPa with the response/recovery time of 15 ms/35 ms. The strain sensor performed well up to 400% deformation with gauge factor of 349.8, and response/recovery time of 68 ms/199 ms. The sensors had stability for more than 5,000 cycles. The sensors showed excellent detection of human body movements including limb motion, respiration, and muscle rhythm. A 4×4 piezo-resistive array was fabricated and the corresponding data acquisition system have been developed to monitor the real-time signals of external stress distribution. Therefore, the GPU/CNTs@Ag sensors fabricated with the novel design strategy demonstrated considerable prospective applications such as human motion detection, smart skin, and machine haptics.