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

Abstract

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.

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Fig. 1: Properties enabled by metal-coordinated bonds.
Fig. 2: Hierarchical organization and resulting mechanical properties of mussels and Nereis virens.
Fig. 3: Mechanical signatures of metal-coordination bonds.
Fig. 4: Engineering metal-coordinated polymers.
Fig. 5: Chemical factors influencing the relaxation time of the network.
Fig. 6: Multiscale modelling of metal-coordinated materials.
Fig. 7: Examples of ligands used for metal coordination with different metal ions.
Fig. 8: Future directions in the study of metal-coordination bonds.

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Acknowledgements

This work was supported by ONR (N00014-19-1-2375), U.S. Air Force Office of Scientific Research (FA9550-15-1-0514), NIH (U01 EB014976), ARO (W911NF1920098) and NSF Graduate Research Fellowship, as well as MIT CAST through a grant from the Mellon Foundation.

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Khare, E., Holten-Andersen, N. & Buehler, M.J. Transition-metal coordinate bonds for bioinspired macromolecules with tunable mechanical properties. Nat Rev Mater 6, 421–436 (2021). https://doi.org/10.1038/s41578-020-00270-z

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