Hierarchical Structure and Fabrication of Functionally Graded Biointerfaces in the Mussel Byssus

ABSTRACT Mussels produce a protein‐based byssus to anchor in rocky seashore habitats. The byssus is glued at one end to the substrate and is anchored into the living tissue at the other end. Moving from substrate to tissue, the byssus is comprised of defined functional regions that are compositionally, structurally and mechanically distinct—yet, these different regions are fully integrated and function seamlessly to prevent dislodgement of the mussel from the surface. Currently, very little is understood about the hierarchical structure and the fabrication of the interfaces between these different functional regions—especially the living/nonliving interface. Yet, this information can be valuable for bio‐inspired design. Here, tissue histology, confocal Raman spectroscopic imaging, and electron microscopy are combined to elucidate the structure and composition of several key interfaces in the byssus, identifying the exquisite control of fine‐scale structure and composition ranging from the nanometer to millimeter scale. Further investigation of the assembly process reveals that this remarkable hierarchical structure is achieved through a highly orchestrated and intricate fabrication process involving stepwise secretion and extrusion of fluid condensates of numerous protein precursors. Insights are highly relevant for inspiring new avenues for fabricating complex (bio)polymeric structures for technical and biomedical applications.