In Situ Nanophase-Engineered, Filler-Free Dynamic Hydrogels Enable Cycle-Stable and Drift-Resistant Wearable Strain Sensors

Wearable ionic hydrogel strain sensors often suffer from limited cyclic reliability because mechanical fatigue and deformation-induced changes in ion-transport pathways can lead to structural damage and signal drift. Here, we report a one-pot, filler-free hydrogel design that addresses this challenge by combining reversible dynamic bonding with in situ nanophase formation. Poly(vinyl alcohol) (PVA) was acetal-grafted with 2,4-dimethyl-5-formyl-1H-pyrrole-3-carboxylic acid (C8H9O3N), followed by reversible Fe3+ coordination. Subsequent ethanol/water solvent exchange induced the formation of rigid nanohydrophobic domains, yielding a hierarchical triple dynamic network based on hydrogen bonding, Fe3+ coordination, and nanoscale phase reinforcement. This structural design is proposed to improve energy dissipation, facilitate network reconstruction, and help maintain ionic conduction during repeated deformation. The optimized hydrogel (5% grafting, 75% Fe3+) exhibited a tensile strength of 466 kPa, an elongation at break of 488.66%, and a dissipated energy density of 156 kJ m–3 at 250% strain. As a resistive strain sensor, it showed repeatable responses over the 10–60% strain range and retained more than 90% of its signal after 2600 cycles at 55% strain. The hydrogel also enabled monitoring of representative human motions and showed preliminary degradability in soil-burial tests, with 11.92% residual mass after 50 days. These results demonstrate a simple and potentially scalable strategy for developing mechanically robust and cycle-stable hydrogel sensors for wearable monitoring applications.