Using auxetic structure for flexible strain sensor: simulation and experiment

Flexible strain sensors are pivotal in human motion monitoring and flexible robotics. Achieving large strain response, high sensitivity, and stability, while the simplifying preparation process and reducing costs, is still a challenge. In this paper, we demonstrate a novel flexible strain sensor based on auxetic structure, fabricated through layer-by-layer self-assembling polyetherimide (PEI) and multi-wall carbon nanotubes (MWCNTs). Simulation analysis is conducted to explore the effect of the auxetic structure on the film strain, and then the flexible strain sensors are designed, fabricated, and examined. We find that the flexible strain sensors based on auxetic structures exhibit interlaced microcracks in both the transverse and longitudinal directions under strain, significantly amplifying the resistance changes. Thus, the sensors achieve a maximum strain coefficient of 499 (13 times of those unpatterned strain sensors), a fast response with a response time of 37 ms and relaxation time of 38 ms, and excellent stability exceeding 15,000 cycles. The sensors perform well in detecting throat vocalization and human limb movements, proving their prospects in human physiological signal detection and human health care.