Scheme 1.

Proposed pH dependence of covalent and coordination bond formation in catechol polymer hydrogels containing Fe³⁺. Reaction of catechol-terminated branched PEG with Fe³⁺ at acidic pH results in covalently cross-linked hydrogels. Subsequent equilibration of these gels at pH 5, 7, or 9 introduces varying amounts of Fe³⁺-catechol coordination bonds that mechanically enhance the covalent network. Under the influence of a mechanical force, these coordination bonds reversibly rupture and re-form, acting as a mechanism for energy dissipation. Both oligomeric and monomeric (unreacted) catechols are believed to participate in the coordination network.