3D-Printed Auxetic Ionic Hydrogels with Moisture Retention and High Sensitivity for Sustainable Wearable Sensing

Conductive hydrogels face challenges in maintaining environmental and mechanical stability for practical sensor applications. In this study, a long-term, stable, and highly sensitive ionically conductive hydrogel was developed via a synergistic dual-humectant strategy: glycerol suppressed ice nucleation through hydrogen-bond competition, while LiCl provided dynamic water sorption. This synergy enables unprecedented stability─remaining unfrozen at -60 °C and retaining 70% moisture over 35 days at 25 °C. The hydrogel exhibits exceptional stretchability (1270% strain) and adhesion (60 kPa) through combined physical/covalent interactions. A three-dimensional (3D)-printed porous architecture enhances sensitivity, achieving a gauge factor of 32 (3 × higher than nonporous hydrogel). In particular, the auxetic-structured conductive hydrogel─when used as a wearable device─demonstrated an accurate recognition ability in detecting limb and subtle movements (including speech). These properties position the hydrogels as promising candidates for fabricating flexible wearable sensors with enhanced sensitivity and environmental sustainability.