Osmocapillary adhesion: Reversible and strong adhesion between any hydrogel

Reversible and strong adhesion can enable reversible part exchanges in soft machines. This paper reports a novel mechanism for reversible hydrogel adhesion based on the recently discovered osmocapillary phase separation. A hydrogel surface is generally biphasic due to osmocapillary phase separation: liquid solvent in equilibrium with the bulk hydrogel can be pulled out by surface tension. When two hydrogel surfaces contact, the surface liquid fills the interfacial gap. The interfacial liquid is under tension since the hydrogel tries to absorb the liquid. The tension pulls the two interfaces together and generates adhesion. Since osmocapillary phase separation is a result of thermodynamic equilibrium, it is intrinsically reversible and universally applicable to any hydrogel. We have demonstrated that osmocapillary adhesion can reach 0.1 MPa level strength, be reversible over 50 cycles, and be stable over 24 h of constant loading.