Dual-Functional Ion-Conductive γ-PGA-GMA Hydrogels for Wearable Strain Sensing and Thermal Energy Harvesting

Ion-conductive hydrogels have emerged as promising materials for wearable healthcare monitoring devices owing to their tissue-like softness, stretchability, and biocompatibility. This study describes a photo-cross-linkable, ion-conductive, and biocompatible hydrogel obtained by functionalizing poly(γ-glutamic acid) (γ-PGA) with glycidyl methacrylate (GMA). The degree of γ-PGA substitution and polymer concentration were optimized to achieve a mechanically stretchable hydrogel that reached up to 160% elongation at break. The optimized γ-PGA-GMA hydrogel exhibited a Young’s modulus of approximately 250 kPa, comparable to that of human skin, which suggested its potential for conformal integration without mechanical mismatch in wearable applications. The incorporation of sodium chloride (NaCl) into the γ-PGA-GMA hydrogel yielded an ion-conductive γ-PGA-GMA hydrogel (IPGH). The IPGH-based strain sensor exhibited a gauge factor of 1.84, rapid response (<500 ms), and stable operation over 800 strain cycles. Furthermore, the IPGH exhibited ionic thermoelectric behavior under a temperature gradient based on the Soret effect. Thermodiffusion of Na+ generated an output voltage, and the ionic Seebeck coefficient reached a maximum of 1.82 mV K–1 when 0.4 M NaCl was incorporated in the IPGH. The findings of the study highlight the dual strain sensing and thermal energy harvesting functionalities of the IPGH, demonstrating its potential for integration into self-powered wearable systems.