Coacervate-Based Underwater Adhesives in Physiological Conditions

Soft underwater adhesives that can function in physiological environments are in high demand for biomedical applications. This study establishes a clear link between the composition and mechanical properties of complex coacervates from poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) and poly(N,N-[(dimethylamino) propyl]methacrylamide) (PMADAP) with degrees of polymerization (DP) close to 100. Choosing such low DP offers several advantages including low water contents corresponding to strong mechanical properties while remaining in the unentangled regime to allow injectability at high salt concentrations. Most importantly, this strategy favors the occurrence of the salt-induced sol–gel transition near physiological concentrations, where these materials form sticky hydrogels because of their viscoelastic dissipative nature. The fluidlike coacervate prepared at 0.75 M NaCl behaves as a soft adhesive when injected in physiological conditions. This adhesive satisfies a nontrivial trade-off between injectability and final mechanical properties. Alternatively, the gel-like coacervate prepared at 0.1 M NaCl offers an instant-stick solution in physiological conditions with a remarkable underwater adhesion energy reaching 65 J m–2 at 2 s−1 without the need for a trigger or any form of postreinforcement. These coacervates mimic the behavior of soft adhesives in air and may be useful as biomedical adhesives.