Diverse Strategies of Protein Sclerotization in Marine Invertebrates: Structure–Property Relationships in Natural Biomaterials

The conventional wisdom regarding sclerotization in arthropod exoskeletons is that tissue stiffness and polyphenolic content are tightly coupled. Recent progress on the biochemistry and mechanics of sclerotized structures from non-arthropod invertebrates such as mussels, polychaetes and squid suggests that this premise is too simple and needs to be more closely scrutinized. Emerging mechanistic insights about structure–property relationships in sclerotized extracellular composites include the following: (1) tissue stiffness can be significantly adjusted by the relative organization and density of rigid versus flexible domains in the protein components (mussel byssal thread); (2) structures can be rich in polyphenols without being sclerotized (mussel adhesive plaque); (3) stiffness and hardness can be adjusted by the relative abundance of interactions between protein-based imidazole and catecholate ligands and transition metals (polychaete jaws and mussel cuticle); (4) polyphenol-derived covalent cross-link density in a protein–chitin composite offers some stiffening, but gains from incrementally controlled dehydration are far greater (squid beak); and finally, (5) protein domains, ligand and metal densities, covalent cross-linking and dehydration are elaborately manipulated to create stiffness gradients between mechanically mismatched tissues.