Metal Coordination-Mediated Functional Grading and Self-Healing in Mussel Byssus Cuticle.

Quan Xu,Yida Zhang,Meng Xu,Xiaoxiao Dong,Shouceng Tian,Yu Tian,Qiang Zhao,Rui Zhang,Zhenhai Xia,Chun‐Yu Lin

doi:10.1002/advs.201902043

Abstract

Metal‐containing polymer networks are ubiquitous in biological systems, and their unique structures enable a variety of fascinating biological behaviors. Cuticle of mussel byssal threads, containing Fe‐catecholate complexes, shows remarkably high hardness, high extensibility, and self‐healing capability. Understanding strengthening and self‐healing mechanisms is essential for elucidating animal behaviors and rationally designing mussel‐inspired materials. Here, direct evidence of Fe3+ and Fe2+ gradient distribution across the cuticle thickness is demonstrated, which shows more Fe2+ inside the inner cuticle, to support the hypothesis that the cuticle is a functionally graded material with high stiffness, extensibility, and self‐healing capacity. The mechanical tests of the mussel threads show that both strength and extensibility of the threads decrease with increasing oxygen contents, but this property degradation can be restored upon removing the oxygen. The first‐principles calculations explain the change in iron coordination, which plays a key role in strengthening, degradation, and self‐healing of the polymer networks. The oxygen absorbs on metal ions, weakening the iron‐catecholate bonds in the cuticle and collagen core, but this process can be reversed by sea water. These findings can have important implications in the design of next‐generation bioinspired robust, highly extensible materials, and catalysis.

Highlights

Metal-containing polymer networks are ubiquitous in biological systems, and energy conversion, brilliant structural colors, intelligence, and so on, which protheir unique structures enable a variety of fascinating biological behaviors
Waves, the byssus must be strong, tough, The first-principles calculations explain the change in iron coordination, which plays a key role in strengthening, degradation, and self-healing of the polymer networks
Our experiment combined with the first-principles and finite element simulations demonstrated that oxygen might weaken and even break the Fe-DOPA bonds, which led to the degradation of mussel threads, but the weakened and broken bonds in Fe-DOPA complexes could be restored spontaneously in the presence of sea water. These findings provide a theoretical base of designing next-generation mussel-inspired coating surfaces which have high stiffness, extensibility, and self-healing capacity

Summary

The extraordinary properties of the byssal threads stem from their unique

To understand how the cuticle and collagen core degrade at the molecular scale in the oxygen-containing environments, we further performed the first-principles calculations, using the density function theory (DFT) methods, and determined the electronic structures and chemical reaction pathways under different environments (Note S3, Supporting Information). The damaged area of cuticle could be healed through oxygen evolution reaction (Reactions (c) and (d) in Figure 5A), the oxygen molecule would desorb from the iron ions in the iron-DOPA complex to form Fe2+DOPA2 since the change in Gibbs free energy of the oxygen dissociation is −0.14 eV in sea water (pH = 8.1) (Table S2, Supporting Information). These findings provide insight into the strengthening and self-healing mechanisms of the mussel cuticle and may have implication in designing/fabricating bioinspired materials with high strength, high toughness, and self-healing capacity

Experimental Section

Findings

Conflict of Interest