Coordination Geometry and Mineralization in Self-Healing Mussel-Inspired Hydrogels

Mussel-inspired polymeric materials have found a broad range of applications not only in adhesives, coatings, and tough hydrogels but thanks to self-healing and stimuli responsiveness in drug delivery, tissue engineering, and soft robotics. These unique properties stem from the reversible dissociation of transient bonds made by protic ligands, specifically catechol, and their fascinating sensitivity to a wide range of stimuli like pH. Nevertheless, their predictability is undermined not only by the chemical side reactions and possible physical phase separation but also by the complex dynamics of the polymer backbone and its interplay with the dynamics of transient bonds. To address this gap, herein, we synthesize side-chain supramolecular polymers by incorporating nitrocatechol (nCAT) groups along poly(dimethyl acrylate) chains. We form hydrogels upon the addition of Fe 2+/3+ metal ions and raising the pH value, thereby changing the coordination geometry at varying hydroxyl and metal ion concentrations. Most of the hydrogels follow a single relaxation process, whose plateau modulus and lifetime can be explained by the sticky Rouse relaxation mechanism. Surprisingly, a low-frequency relaxation mode appears upon the progressive formation of mono complexes either at high metal ion concentration or low pH values, which according to SAXS results is associated with mineralization of metal ions. Density functional theory simulations demonstrate a higher affinity of Fe 3+ to tris complexes, in contrast to the bis selectivity of Fe 2+, which explains the highest plateau modulus that could be obtained with the former. Our results suggest possible control over the mineralization and dual dynamicity of mussel-inspired hydrogels, which is useful in the development of novel self-healing tough materials.