Simple, Commercially Viable Insole Sensor for Simultaneous Plantar Pressure and Shear Stress Measurement Using Tilted Piezoresistive Sensors

Abstract Accurate measurement of plantar stress is essential for biomechanical monitoring, injury prevention, and clinical diagnosis. While most insole sensors are limited to detecting normal pressure, shear stress, which is critical for assessing postural imbalance and movement dynamics, remains largely unmeasured. This study presents a simple and scalable method to simultaneously measure plantar pressure and shear stress using conventional piezoresistive sensors. Each sensor module consists of five sensors: one centrally placed and oriented parallel to the insole surface and four tilted at 45° in the forward, backward, left, and right directions. The sensors are fabricated on a flexible polyimide substrate and mounted using polyethylene terephthalate film fixtures to maintain their tilted orientations; they are then encapsulated with an ultraviolet-curable (UV-curable) resin for durability. Vertical and shear compression tests were conducted to characterize the orientation-dependent responses of the sensors. Additionally, finite element analysis was performed to verify the positional and orientational changes of the sensors during compression, with results correlated to the experimental data. By analyzing the differential signals from the five sensors, shear stress magnitude and direction were inferred based on characteristic variations in signal output under combined loading. Four sensor modules were embedded into an insole system and tested during treadmill walking and running. The results demonstrated effective, real-time detection of both plantar pressure and shear stress. The system offers a promising solution for gait analysis, athletic-performance monitoring, injury risk assessment, and rehabilitation in both sports and clinical settings.