Self-Repair of a Biological Fiber Guided by an Ordered Elastic Framework

Incorporating sacrificial cross-links into polymers represents an exciting new avenue for the development of self-healing materials, but it is unclear to what extent their spatial arrangement is important for this functionality. In this respect, self-healing biological materials, such as mussel byssal threads, can provide important chemical and structural insights. In this study, we employ in situ small-angle X-ray scattering (SAXS) measurements during mechanical deformation to show that byssal threads consist of a partially crystalline protein framework capable of large reversible deformations via unfolding of tightly folded protein domains. The long-range structural order is destroyed by stretching the fiber but reappears rapidly after removal of load. Full mechanical recovery, however, proceeds more slowly, suggesting the presence of strong and slowly reversible sacrificial cross-links. One likely role of the highly ordered elastic framework is to bring sacrificial binding sites back into register upon stress release, facilitating bond reformation and self-repair

Metal-Mediated Molecular Self-Healing in Histidine-Rich Mussel Peptides

Mussels withstand high-energy wave impacts in rocky seashore habitats by fastening tightly to surfaces with tough and self-healing proteinaceous fibers called byssal threads. Thread mechanical behavior is believed to arise from reversibly breakable metal coordination cross-links embedded in histidine-rich protein domains (HRDs) in the principle load-bearing proteins comprising the fibrous thread core. In order to investigate HRD behavior at the molecular level, we have synthesized a histidine-rich peptide derived from mussel proteins (His₅-bys) and studied its reversible adhesive self-interaction in the presence and absence of metal ions using PEG-based soft-colloidal probes (SCPs). Adhesion energies of greater than 0.3 mJ/m² were measured in the presence of metal ions, and the stiffness of the modified SCPs exhibited a 3-fold increase, whereas no adhesion was observed in the absence of metals. Raman spectroscopy confirmed the presence of metal-coordination via histidine residues by the peptide–supporting the role of His-metal complexes in the mechanical behavior of the byssus