Water-Activated Soft TPE/PVA/CB-TPU Piezoresistive Strain Sensor for Wearable Knee Kinematics Monitoring

Additive manufacturing (AM), and thermoplastic material extrusion in particular, is increasingly enabling the fabrication of customized soft sensing platforms for wearable applications, owing to its accessibility, low cost, and compatibility with multifunctional polymeric materials. Among emerging substrates processable by material extrusion, water-activated thermoplastic elastomer/polyvinyl alcohol (TPE/PVA) composite filaments are particularly attractive: selective dissolution of the PVA phase upon water immersion yields a microporous elastomeric structure with enhanced compliance. This work reports, to the best of our knowledge, the first wearable strain sensor based on a hydrated TPE/PVA substrate, fabricated through a single-step dual-material extrusion AM process that directly embeds a carbon-black-filled TPU (CB-TPU) piezoresistive trace within the soft matrix, without post-printing assembly or bonding. The device was characterized under quasi-static tensile deformation up to 20% strain, showing a monotonic increase in relative resistance variation, a Gauge Factor of 0.27, and a maximum load of approximately 6 N. Cyclic tensile tests at 6, 12, and 20 cycles/min confirmed the ability of the sensor to follow repeated deformations, with average hysteresis errors of approximately 17% in all cases . Preliminary on-body validation during controlled knee flexion-extension and level walking demonstrated good agreement with a commercial inertial reference system in estimating movement timing, confirming the feasibility of TPE/PVA-based sensors produced by material extrusion for large-deformation wearable joint monitoring. These findings establish water-activated TPE/PVA composites as a viable and largely unexplored substrate class for fully 3D-Printed soft strain sensors, opening new directions in the design of ultra-compliant wearable devices.