WFU Physics Colloquium

Connective tissues and extracellular matrix predominantly consist of a support framework of fibrous biopolymers formed from members of the collagen superfamily of proteins, various elastic fibrils, and proteoglycans. Together these molecules provide a scaffold for cellular housing and tissue support. The primary structural proteins are the fibril forming collagens, which lend rigidity to the matrix. Efforts to characterize the mechanobiology of these systems have been impeded by the fact that, while both the macroscopic mechanics of the composite system and the nanoscopic mechanics of the collagen molecule have been studied extensively, the native mechanical properties of the fundamental fibril from which the composite system is built remain largely unknown. It is precisely the native form of collagen that needs to be fully understood in order to rationally design biomimetic systems for tendon or corneal replacements or for development of scaffolds for use in engineered soft tissue constructs.

We are able to extract collagen fibrils from Cucumaria Frondosa, a species of echinoderm whose dermis exhibits a neurally controllable rigidity. This source of collagen allows for the study of the mechanics of individual native, intact fibrils whose ultrastructure closely mimics that of collagen I. We have used atomic force microscopy to image and manipulate these fibrils and the proteoglycans of extracellular matrix. Through the use of nanoscale indentation and three-point bending, we have demonstrated that the elastic modulus of the collagen fibril is anisotropic, having an axial modulus that is some 150 fold stiffer than the radial. The presence of the fibril associated proteoglycans in the system under study has the added benefit that the interactions of these molecules can be investigated. We have made use of these interactions to induce the assembly of orthogonally stacked sheets of aligned collagen fibrils . an artificial corneal stroma.