Temperature-Mediated Phase Separation Enables Strong yet Reversible Mechanical and Adhesive Hydrogels.

Hydrogels with strong yet reversible mechanical and adhesive properties fabricated in a facile and friendly manner are important for engineering and intelligent electronics applications but are challenging to create and control. Existing approaches for preparing hydrogels involve complicated pretreatments and produce hydrogels that suffer from limited skin applicability. Copolymerized hydrogels are expected to present an intriguing target in this field by means of thermoresponsive features, while the perceived intrinsic flaws of brittleness, easy fracture, and weak adhesion enervate the development prospects. Herein, we report a hydrogel with strong yet reversible mechanical and adhesive properties using cellulose nanofibrils to simultaneously address multiple dilemmas inspired by a temperature-mediated phase separation strategy. This strategy applies temperature-driven formation and dissociation of hydrogen bonds between common copolymers and cellulose nanofibrils to trigger the onset and termination of phase separation for dynamically reversible on-demand properties. The resulting hydrogel exhibits up to 96.0% (117.2 J/m 2 vs 4.8 J/m 2 for interfacial toughness) and 85.7% (0.02 MPa vs 0.14 MPa for mechanical stiffness) adhesive and mechanical tunability when worked on skin, respectively. Our strategy offers a promising, simple, and efficient way to directly achieve robust adhesion performance in one step using common copolymers and biomass resources, with implications that could go beyond strong yet adhesive hydrogels.