Coupling piezoelectric and piezoresistive effects in flexible pressure sensors for human motion detection from zero to high frequency

Flexible wearable sensors have received considerable popularity due to their potential application in monitoring human activities and health conditions. However, traditional pressure sensors always rely on a single mechanism (such as piezoelectric or piezoresistive effect), which shows great limitation in the cooperative monitoring of static and highly dynamic pressure detection and severely restricts their further practical applications. Herein, a new strategy of combining piezoelectric and piezoresistive mechanisms in one single flexible sensor is reported for the realization of dual function detection. In detail, a piezoelectric unit (non-array and array) based on a flexible poly(vinylidene fluoride) (PVDF) polymer is deployed for highly dynamic detection, while the composite of graphite sheet (GS)-decorated PVDF electrospun mat composite serves as a piezoresistive unit for static detection. The device exhibits excellent piezoresistive durability (over 600 cycles) and higher piezoelectric sensitivity of 0.5 N V-1 at the applied frequency of 5 Hz due to the fact that the introduction of a soft piezoresistive layer increases the transverse strain on the piezoelectric layer originating from large deformation of the low stiffness surface. On the basis of the collaborative and complementary advantages, accurate and reliable detection of human motions such as elbow and heel activities are realized, indicating great potential in wearable electronics applications from zero to high frequency. This study provides general guidance for the development of flexible pressure sensors that require dynamic and static cooperative detection capabilities.