Highly Linear and Sensitive Hydrogel Strain Sensor Enabled by PVA/CS–HEDP Dynamic Networks for Wearable Motion Monitoring

Flexible wearable strain sensors that accurately capture joint motion are essential for rehabilitation assessment, motion monitoring, and human–machine interaction. However, conventional hydrogel sensors often suffer from limited mechanical robustness, poor durability, insufficient conductivity, or biocompatibility concerns caused by conductive fillers and chemical crosslinkers. Here, we report a multifunctional biocompatible hydrogel strain sensor, PCH, constructed from a polyvinyl alcohol/chitosan (PVA/CS) double-network matrix, hydroxyl ethylidene diphosphonic acid (HEDP)-mediated dynamic coordination crosslinking, and potassium chloride (KCl)-based ion conduction. FTIR analysis confirms the formation of a hybrid PVA/CS–HEDP network stabilized by hydrogen bonding and phosphonate-associated interactions, contributing to enhanced mechanical integrity. The PCH hydrogel exhibits favorable tensile properties, stable cyclic loading–unloading behavior, and reliable durability. Electromechanical tests show a linear and sensitive strain response with a gauge factor of 0.84 and high linearity (R2 = 0.9976). In joint mobility monitoring, PCH generates precise angle-dependent signals during ±90° wrist bending and maintains stable output under continuous bidirectional motion. It also shows repeatable finger-joint sensing performance and favorable cytocompatibility, hemocompatibility, and skin compatibility, demonstrating strong potential for real-time wearable joint monitoring.