A Thermoswitchable, Hydrophobically-Crosslinked Hydrogel as an Injectable Underwater Adhesive

The progress in the field of underwater adhesion is indebted to insights and inspirations from marine organisms such as mussels and sandcastle worms. Both of these organisms deliver their glue components via injection which is considered a major asset for potential biomedical applications. They cure their adhesives through a rich variety of molecular interactions including “hydrophobic interactions”.[۱] Most of the manmade underwater adhesives rely on strong interactions for adhesion. This common strategy has several disadvantages: it relies on the dynamic chemistry of tissues while raising concerns regarding the toxicity of the reaction and the removability of the adhesive.Developing generic adhesives which can stick onto different surfaces in the presence of water remains a major challenge in the field. Such generic adhesives could be inspired from hydrophobic pressure-sensitive-adhesives (PSAs) that rely on a fine balance of viscoelasticity to make rapid and intimate contact with substrates and dissipate large amounts of energy upon detachment.[۲] Yet, typical PSAs lose their performance on wet substrates.In order to target an injectable polymer solution able to switch into a viscoelastic material, we synthesized a graft copolymer with a thermoresponsive backbone carrying short hydrophilic sidechains. With this topology, the collapse of the backbones upon the thermal transition leads to the formation of a percolating network of strong, hydrophobic domains. Similar to PSAs, this hydrogel goes through fibrillation and extensive energy dissipation in large deformations. This capability comes from the hydrophobic nano-scaffold which resists large deformations to minimize its contact with water. Since hydrophobic interactions are insensitive to water, the behavior of the hydrogel is maintained in aqueous medium. Chemistry-independent adhesion of this model hydrogel offers a major advantage over current tissue adhesives.