The behavior of Fe3+ coordination in alginate-catechol networks

Abstract

Mussel byssal threads allow mussels to remain steadfast on ocean rocks despite ocean turbulence, facilitated by the simultaneous elasticity and hardness of the byssus coating. Inspired by the metal-coordination chemistry found in byssus coating, scientists have synthesized an array of self-healing hydrogels with catechol-modified, 4-branched PEG (4cPEG) and various metal ions, primarily Fe3+. While considerable testing has been conducted with 4cPEG, the effects of changing the polymer backbone have not been investigated extensively. Here, alginate was chemically modified with catechol attachments (Alg-C), and metal-coordinated with Fe3+ to yield a self-healing network with similar qualities to 4cPEG gels. Rheological measurements indicated that metal-coordination played a dominant role in the bulk mechanics of the network, although the ionic crosslinking caused the gel to act as a solid across all frequencies, in contrast to 4cPEG. In addition, the stiff alginate backbone caused the metal-coordinate bond in itself to act on a longer time scale. Color changes in the Alg-C gel indicated that excess catechol on the backbone was oxidizing. While rheology confirmed the metal-coordination in the Alg-C network, UV-vis absorption measurements provided less certain data. Nonetheless, this study shows that metal-coordination is highly dependent on the polymer backbone, but may still be used in a variety of polymer networks.