Temperature-responsive polyelectrolyte complexes for bio-inspired underwater adhesives

Adhesive proteins of marine organisms contain significant amounts of hydrophobic amino acids. Therefore, inter- and intramolecular hydrophobic interactions are expected to play an important role in both adhesion and cohesion. Here, we mimic the hydrophobicity of adhesive proteins by using temperature-responsive polyelectrolyte complexes (TERPOCs) with a high poly(N-isopropylacrylamide) (PNIPAM) content. Upon mixing aqueous solutions of PNIPAM-b-poly(acrylic acid)-b-PNIPAM and poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA), complexation between the oppositely charged polyelectrolytes occurs. At low temperatures, complex coacervate core micelles (C3Ms) with a PNIPAM corona are formed, and upon a temperature increase, the solution turns into a hydrogel by the formation of a network of hydrophobic PNIPAM domains. Consequently, an abrupt increase in viscosity is observed upon heating which facilitates injectability of the adhesive. The gelation temperature, Tgel, and (adhesive) strength of the TERPOC can be adjusted by altering the salt and polymer concentration, which changes the balance between the electrostatic and hydrophobic interactions. Despite the importance of hydrophobic groups in strong underwater adhesives, we conclude that TERPOCs with a high PNIPAM content (70 wt%) are unstable due to water release. Consequently, there is a limited amount of hydrophobic groups that can be inserted in this type of systems. Nevertheless, TERPOCs show promising and tunable properties for application as injectable underwater adhesives, for example in biomedical applications.