Stretchable anisotropic piezoelectric sensors with modulus engineering for monitoring multiaxial joint motion

Real-time tracking of human joint motion is crucial for gesture and posture monitoring, requiring wearable sensors capable of detecting anisotropic signals through form-changing capabilities. Herein, we introduce an anisotropic piezoelectric sensor (APS) developed via modulus engineering of stretchable heterogeneous materials. The APS integrates high-modulus active-line patterns with a low-modulus inactive matrix, utilizing Poisson's effect to generate directional anisotropic outputs based on strain variations in active regions. Single-electrode APS devices exhibited varied signal polarities in response to bending directions induced by joint movements, thereby enhancing sensing diversity. Remarkably, these devices demonstrated up to a 21-fold variation in output for identical strain across different tensile axis, indicating excellent anisotropic sensitivity. Additionally, an orthogonally stacked APS (s-APS) enabled the detection of eight distinct motion types with a single device, providing consistent feedback during joint deformation and highlighting its potential as a versatile wearable sensor for gesture and posture monitoring across diverse applications.