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Self-healing hydrogels can be made using either reversible covalent cross-links or coordination chemistry bonds. Here we present a multi-pH-responsive system inspired by the chemistry of blue mussel adhesive proteins. By attaching DOPA to an amine-functionalized polymer, a multiresponsive system is formed upon reaction with iron. The degree of polymer cross-linking is pH controlled through the pH-dependent DOPA/iron coordination chemistry. This leads to the formation of rapidly self-healing high-strength hydrogels when pH is raised from acidic toward basic values. Close to the pK(a) value, or more precisely the pI value, of the polymer, the gel collapses due to reduced repulsion between polymer chains. Thereby a bistable gel-system is obtained. The present polymer system more closely resembles mussel adhesive proteins than those previously reported and thus also serves as a model system for mussel adhesive chemistry.
Krogsgaard et al. (2013) studied this question.