Skin-printable, anti-freezing, and recyclable ionic hydrogel bioelectrode for continuous electrophysiological monitoring

• Reversible Thermal Phase Transition : The hydrogel shows reversible yet fast sol-gel transition enabling precise in-situ printing on human skin. • Outstanding Anti-Freezing and Transparency : The hydrogel retains optical clarity and mechanical flexibility even in liquid nitrogen (−196 °C). • Multifunctional Performance : The hydrogel exhibits robust skin adhesion, anti-freezing, self-healing, water retention, and recyclability. • High-Fidelity Signal Acquisition: The hydrogel skin electrode enables real-time and continuous biopotential monitoring. The rapid advancement of wearable electrophysiological monitoring has driven a growing demand for soft, conductive materials, particularly hydrogels, due to their exceptional mechanical properties, high conductivity, and biocompatibility. However, hydrogel materials are prone to freezing at subzero temperatures, which leads to material failure. Additionally, prefabricated hydrogel electrodes often lack sufficient conformability and long-term stability, thereby limiting their practical applications. We report a skin-printable hydrogel-based epidermal bioelectrode designed for continuous, high-precision wireless electrophysiological monitoring. The hydrogel, synthesized via an effective one-pot process, comprises gelatin, glycerol, ammonium chloride, and water. Its reversible thermal phase transition enables precise printing onto glabrous and hairy human skin. The in-situ formed hydrogel electrode exhibits excellent adhesion at both the hydrogel/skin and hydrogel/electrode interfaces, with adhesion force reaching 0.9 N/cm and 2.6 N/cm, respectively. The incorporation of glycerol and ammonium chloride imparts remarkable anti-freezing properties, allowing the hydrogel to maintain outstanding mechanical flexibility across a broad temperature range, from ultra-low temperatures of −80 °C to the phase transition temperature of 43.9 °C. Remarkably, the hydrogel retains its optical transparency even when immersed in liquid nitrogen. Additionally, the hydrogel demonstrates superior self-healing, high water retention, and recyclable properties. We successfully validated the hydrogel’s potential for personalized healthcare through high-fidelity electrocardiogram recordings over 80 min during various daily activities, demonstrating its practical applicability. This work highlights the versatility and functionality of skin-printable hydrogel as a promising material for next-generation epidermal electronics.