An ultra-flexible curvature sensor without relying on elongation *

In recent years, flexible sensors have rapidly become an emerging research topic. Nevertheless, most existing flexible sensors focus primarily on strain and pressure or force. Curvature, as a crucial measurement parameter, especially for evaluating the posture of soft & rigid robots or humans, however, has been rarely studied relatively. Here, we propose an ultra-flexible liquid metal curvature sensor based on a double-layer multi-microchannel structural design. The sensor can measure the bending by electric resistance variations, instead of relying on elongations like most flexible curvature sensors used now. When the sensor is bent, the embedded microchannel filled with liquid metal becomes narrow under compressive stress, and then due to the linear increase in electric resistance of the liquid metal, the curvature can be effectively measured. Subsequently, we introduce an easy and low-cost manufacturing method for curvature sensor. Then a theoretical model is established to further verify the rationality and functionality of the sensor. Finally, we present experimental results to estimate the sensor characterization. It proves that the ultra-flexible liquid metal curvature sensor has a higher sensitivity within a wide range degree, very low hysteresis and is still robust in withstanding >800 repeated loading and unloading cycles.