Salting-Out-Induced Fiber Reinforcement and Multiple Hydrogen Bonding Enable Robust, Freezing-Tolerant, and Dehydration-Resistant Hydrogel Strain Sensors

Developing hydrogel strain sensors that simultaneously possess mechanical robustness, stretchability, environmental stability, and reliable ionic signal transmission remains a major challenge for wearable electronics. Many existing reinforcement strategies often improve strength at the expense of deformability or rely on externally introduced fillers that may compromise network uniformity and ion transport. Herein, we report a salting-out-induced rigid–soft synergistic reinforcement strategy to construct an ionically conductive sodium alginate (SA)/NaCl/poly(vinyl alcohol) (PVA)/tannic acid (TA) hydrogel (SNPT hydrogel) containing dispersed SA-derived reinforcing domains through a simple solution-mixing and freeze–thaw process. In this system, NaCl plays a dual role: it promotes the pregelation association of sodium alginate (SA) chains in the precursor solution, leading to the formation of dispersed SA-derived reinforcing domains, while providing mobile ions for electrical conduction. Meanwhile, tannic acid (TA) establishes multiple dynamic hydrogen-bonding interactions with SA and poly(vinyl alcohol) (PVA), forming reversible physical cross-linking sites that stabilize the network and dissipate deformation energy. Benefiting from this hierarchical structure, the optimized SN2PT1.2% hydrogel exhibits a balanced combination of tensile strength (0.40 MPa), elongation at break (344%), and ionic conductivity (0.029 S/m). The incorporation of glycerol and NaCl further enables the hydrogel to retain flexibility and conductivity after exposure to −20 °C and to resist dehydration under accelerated heating conditions. When assembled into a flexible strain sensor, the SNPT hydrogel exhibited stable stepwise resistance responses under strains up to 90% and maintained a highly linear ΔR/R0–strain relationship over the tested strain range, with a moderate gauge factor of 0.29. These features enable reliable monitoring of finger, knee, elbow, and wrist motions.