Synergistic Physical Crosslinking and Ionic Hydration-Reinforced Flame-Retardant Zwitterionic Hydrogel for Multimodal Flexible Sensing

Abstract

Hydrogel-based flexible sensors are promising candidates for wearable biomechanical monitoring, soft robotics, and human–machine interfaces because of their compliance, stretchability, and intrinsic ionic conductivity. However, their practical implementation in demanding environments is constrained by subzero freezing, environmental dehydration, interfacial delamination, and high flammability. Herein, a filler-free PAM/WPU/GPC hydrogel is synthesized via one-pot, thermally initiated, free-radical polymerization. Polyacrylamide (PAM) establishes the structural skeleton, waterborne polyurethane (WPU) provides dynamic energy dissipation, and zwitterionic glycerophosphocholine (GPC) stabilizes aqueous ion channels through intense hydration. The resulting hydrogel demonstrates robust multisubstrate adhesion, delivering a lap-shear strength of 137 kPa on paper and maintaining 73.6–91.5% adhesion retention across all tested substrates after 24 h of ambient exposure. Simultaneously, the hydrogel exhibits high moisture retention (89.3% after 48 h) and remains mechanically flexible and ionically conductive over a wide temperature range of −30 to 100 °C. Cone calorimetry verifies enhanced flame-retardant behavior, characterized by marked reductions in total heat release and smoke production. As an electromechanical strain sensor, it operates across a 0–400% strain window with gauge factors of 1.47 (0–200% strain) and 2.01 (200–400% strain), prompt response/recovery kinetics (0.3 s/0.2 s), and stable output over 300 continuous cycles. The assembled device effectively tracks biomechanical joint motions and transmits Morse code signals, highlighting its potential for packaged wearable systems, soft robotics, and emergency communication under harsh conditions.