Transition-metal coordinate bonds for bioinspired macromolecules with tunable mechanical properties

Transition-metal coordination complexes are emerging as a broad class of supramolecular crosslinks that can be used to engineer the mechanical properties of advanced structural materials. Unlike conventional covalent bonds, metal-coordination bonds have the capacity to reform after rupture, thereby enabling dynamic, tunable and reversible (self-healing) mechanical properties. Several biological organisms, such as marine mussels, have been found to take advantage of these unique properties of metal-coordinate complexes in the assembly of load-bearing materials for complex extraorganismal functions. Accordingly, efforts to integrate metal-coordinate crosslinking in bioinspired synthetic protein and polymer hydrogels are an increasingly active area of research. However, a deeper understanding of how metal-coordination bonds affect bulk mechanical properties is still missing, rendering predicting the mechanical properties of metal-coordinated materials challenging. In this Review, we survey recent advances and open questions in our understanding of how chemical properties of metal-coordinate complexes influence multiscale mechanical behaviour, with the aim of presenting metal-coordination bonding as a rich, inorganic crosslinking chemistry tool. We also review applications of metal-coordinate crosslinking in the design of novel materials with tunable mechanical properties, ranging from tough gels to soft robots. These applications highlight the opportunities arising from the integration of this class of load-bearing crosslinks in structural materials design. The reversibility of transition-metal coordination bonds affords broad control over the structural dynamics of materials. This Review surveys the design principles underlying the utilization of this dynamic crosslink chemistry to engineer tunable mechanical properties in biological materials and protein and polymer hydrogels.