Self-Powered Multidirectional Strain Sensor for Electronic Skin Based on Coiled Carbon Nanotube Yarns

Electronic skin (E-skin) is a wearable composite sensor that mimics human skin. It senses strain, pressure, and temperature and is applied in fields, such as health monitoring, motion recognition, and robotics. Traditional strain sensors can only sense the magnitude of the mechanical strain and not the direction of the strain and require an external power supply. Although piezoelectric and triboelectric strain sensors are self-powered, their output voltages are usually pulse signals, and it is difficult to output the signals accurately in synchronization with the action. Herein, we used two coiled carbon nanotube yarns arranged in an anisotropic structure to fabricate a multidirectional strain sensor, which can accurately measure the magnitude and direction of the strain. This sensor is self-powered and converts mechanical energy to electrical energy through an electrochemical system and works without an external power supply. This multidirectional strain sensor can sense strains up to 20% applied at 0°–90°, with a strain sensitivity of 1.28 mV/% for strains in the 0° direction. The angle sensitivity is 0.4 mV/° for the strains of 20%. We attached the multidirectional strain sensor as an E-skin to the wrist and tested its open-circuit voltage output with wrist movements. Tests show that this multidirectional strain sensor can sense the direction and magnitude of wrist motion. The sensor is expected to be used for energy harvesting and powering other components in the wearable device while enabling the miniaturization of the device.