Piezo-ionic Materials and Structures for Complex Shear Field Monitoring

Capturing shearing stresses is crucial for accurately monitoring multidirectional mechanical deformation, enabling advanced motion analysis and enhancing functionality in wearable and robotic applications. Here, we present self-powered porous piezo-ionic shear-sensing materials that simultaneously resolve normal and tangential stresses with high sensitivity, a broad dynamic range, and high linearity. A composite of thermoplastic urethane and ionic liquids, reinforced with silica nanoparticles, was exploited to establish and sustain a porous framework. This configuration amplifies piezo-ionic responsiveness and broadens the response range. The materials design uses a stacked two-layered architecture with one reversed trapezoidal layer interlocked with a second macrodome layer. Such an arrangement of elastomeric structural elements converts macroscopic shearing deformation into localized directional compression while providing a four-electrode multidirectional signal readout. This hierarchical macro-to-nano structure achieves a sensitivity of 0.23 mV kPa^-1 and offers reliable and linear performance over a broad pressure range (up to 634 kPa), a critical advantage for continuous monitoring applications, compared to conventional piezo-ionic materials which often exhibit saturation at high loads. Integrated into a thin single-stack film, these sensors can map complex gait patterns, including walking, pivoting, and tiptoeing, with low power consumption and scalable fabrication, establishing a universal platform for high-fidelity shear monitoring in wearable robotics and industrial friction sensing.