Molecular Engineering of a Spider Silk and Mussel Foot Hybrid Protein Gives a Strong and Tough Biomimetic Adhesive

High performance bio-based materials are an important part of future sustainable technology, and engineered proteins provide excellent possibilities as functional polymers. Adhesives are widely needed for composite materials and biomimetic structures. In biological adhesives, two features have emerged as especially interesting—the role of coacervation and the presence of 3,4-dihydroxyphenylalanine (DOPA). To study these, protein engineering is used to construct a hybrid silk-mussel foot protein (mfp) adhesive. Tyr residues in the purified mfp are oxidized to DOPA and an encoded SpyCatcher-Tag system allowed easy click-chemistry to couple silk and mfp and to study the parts separately. The combined silk-mfp protein have a strong tendency to coacervate. DOPA affected the properties of coacervates and increased adhesion by several ways of measuring. In lap shear testing, the combined mfp-silk protein is superior to any of the components studied separately. Coacervation is suggested to contribute to the adhesion of silk-mfp, and shows several features suggested to lead to the strength and toughness of natural adhesives. In the lap shear system, coacervation have a stronger overall effect on adhesion than the presence of DOPA. The results show that protein design provides a route toward high performance biosynthetic polymers and future sustainable materials.