“Release-Occupy” strategy to fabricate an integrally formed Janus adhesive hydrogel for wearable sensor

The adhesive hydrogel is beneficial for achieving secure adhesion to the skin, promising stable signal output when used as a wearable sensor. However, traditional adhesive hydrogels are isotropic, thus susceptible to contamination or detached by other substrates. This study proposed a “Release-Occupy” strategy for fabricating an integrally formed Janus adhesive hydrogel. Adhesion is facilitated by inhibiting polymer crystallization through the in-situ generation of nanoparticles using ionic liquid (IL) and metal ions, thereby releasing hydroxyl groups. Non-adhesiveness top surface is further achieved by occupying the released active hydroxyl groups with ethylene glycol (EG) through coating EG on the adhesive hydrogel top surface. The effects of different types of ILs and metal ions on the tensile strength, adhesiveness, and ionic conductivity of the hydrogel are systematically investigated. It is found that [Emim]DEP and Ca 2+ can achieve the highest adhesiveness. The addition of EG significantly reduced the adhesion force while imparting higher strength and electrical conductivity to the hydrogel. Besides, these hydrogels show excellent anti-freezing, cytocompatibility and antibacterial properties. Furthermore, the integrally formed Janus hydrogel with adhesive bottom surface and non-adhesive top surface can adhere stably to the skin and resistant to contaminants, allowing for stable monitoring of human activities and information transmission without the need for a top protective film. This study opens a new avenue for designing stable and adhesive conductive hydrogels, offering valuable guidance for the development of wearable sensors.