Auxetic Wearable Sensors Based on Flexible Triboelectric Polymers for Movement Monitoring

The methods to enhance the output voltage of triboelectric nanogenerators based on polymeric dielectric materials, such as building micro-nano structure and chemical modification, probably lower the structural uniformity of their devices and then decrease the performance stability because of stress aggregation. Here, we proposed a strategy based on auxetic structures for improving the triboelectric performance, which could concurrently uniformize the stress transfer due to their excellent friction properties. Using the negative Poisson's ratio (NPR) structure of the "I"-shaped structure and rotating polygon structure, we prepared the dielectric layers of polyvinylidene fluoride (PVDF) and copper with controllable Poisson's ratio from 0.84 to -1.51. Then, the finite element analysis proved that the auxetic NPR structure with tri-axial and tetra-axial symmetricity offered more uniform friction-force distribution and thus less fluctuation in a contact area than the "I"-shaped structure. We further designed triboelectric devices with polydimethylsiloxane (PDMS) deformation and triboelectric properties, and they showed that the triboelectric output voltage reached the maximum when their Poisson's ratio was close to 0, which was related to the minimal change in the contact area between the NPR layer and the PDMS layer. The devices were then inserted into wearable sports sensors, which could monitor the movement at different places of bodies and also have great potential in sensorium smart socks and brake sensors.