Transforming non-adhesive hydrogels to reversible tough adhesives via mixed-solvent-induced phase separation

Realizing tough adhesion between hydrogels and solid surfaces is crucial for developing emerging soft-rigid hybrid devices with a high level of complexity. However, this is extremely challenging for numerous non-adhesive hydrogels due to their weak interfacial interaction with solid surfaces and negligible mechanical dissipation. Here, we report a phase-separation strategy to transform traditionally non-adhesive hydrogels to tough glues for diverse solid surfaces, without the need for chemical treatment. Equilibrating a hydrogel in a mixture of both good and poor solvents induces phase separation, resulting in a significant increment of polymer volume fraction at the surface and in the bulk of the gel. The high-density polymer chains at the gel surface facilitate the formation of dense arrays of noncovalent bonds with the solid surfaces, improving the intrinsic work of adhesion and favoring the force transmission from the crack front to the bulk gel. Meanwhile, the phase-separated structure in the bulk gel allows significant mechanical dissipation upon interfacial separation. Such a synergy contributes to a high interfacial toughness. The tough adhesion, reaching over 1000 J m(-2), is instant and repeatable, with inappreciable loss in the interfacial toughness after over 100 attach/detach cycles. This facile, repeatable phase-separation approach is also universal, and can be induced by various mixed solvents and applies to multiple types of common non-adhesive hydrogels for tough yet detachable gel-solid adhesion.