Metals & polymers in the mix: fine-tuning the mechanical properties & color of self-healing mussel-inspired hydrogels

Reversible sacrificial bonds play a crucial role in various biological materials where they serve as load-bearing bonds, facilitating extensibility and/or impart self-healing properties to the biological materials. Recently, the coordination bonds found in blue mussel byssal threads have been mimicked in the design of self-healing hydrogels. Herein we show how the mechanical moduli of mussel-inspired hydrogels based on DOPA-polyallylamine (DOPA-PAA) can be straight-forwardly adjusted by systematically varying the coordinating metal from Al^III, Ga^III to In^III. These gels are transparent and only slightly tanned opposite to the black hydrogels obtained using Fe^III. Additionally, dark Fe^III:DOPA-chitosan gels were synthesized and showed remarkably high storage modulus. The strongest hydrogels were formed around pH 8, which is closer to physiological pH than what was observed in the Fe^III:DOPA-PAA system (pH_max∼ 9.5). This finding supports the hypothesis that the maximum in the storage modulus distribution can be adjusted to match a given application by selecting the cationic polymer based on its pK_a value.