Temperature-triggered smart milk-derived hydrogel with programmable adhesion for versatile skin-attached iontronics

Despite the current advances in adhesives, it is still a huge challenge to develop a programmable adhesion material for wearable and skin-attached devices. Herein, a skin temperature-triggered smart adhesive hydrogel is developed by one-sidedly infiltrating polyvalent metal ions into the UCST-typed thermosensitive copolymers comprising milk-derived sodium caseinate (SC). The resulting hydrogel presents asymmetric and tunable adhesion, which is capable of the repeated, on-demand detachments by a mild skin temperature trigger. Meanwhile, the hydrogel also shows the programmable turbidity-switching ability because of adjustable phase transformation, and outstanding antimicrobial properties (killing rate: >99.0%). Besides, the hydrogel can sensitively perceive external strain (GF: 37.59, response time: 169 ms) and temperature (TCR: -4.03%/celcius) variation by electrical signals. Thanks to these excellent features, the hydrogel can be applied to versatile wearable iontronics to precisely monitor various human motions, physiological signals and thermal distributions in real time. Therefore, our study establishes a novel and effective route for the rational design and fabrication of intelligent skin-attached materials.