What Is Failure? Nanoscaled Biomechanics of Collagen Disruption In Tissues & Semi-Synthetic Fibres

Collagen's evolutionary longevity is a tribute to the survival value of its complex, self-assembled structure. While both elasticity and overload damage have been studied extensively at the light microscope level in connective tissues, it is below that scale-really at the nanometer scale-where mechanical load is borne. It is there that collagen's strength is achieved-and perhaps more important for survival, where its toughness is determined. For the last 10 years or so, I have been interested in two fundamental questions: (i) What does mechanical damage in collagen look like at its most fundamental levels? (ii) Are there structural motifs for damage in collagen that activate physiologically appropriate cellular repair or replacement? These are questions of deep import for understanding the biomechanical evolution of the collagen fibril and for rational design of processed collagen products which can mimick native toughness and modulate inflammation and healing. We recently demonstrated that overloading of tendon collagen produces a characteristic, local, nano-scaled "kinking" of collagen fibrils (~5-200 nm dia.). Fibril-level damage leads to thermodynamic instability of the packed collagen molecules, consistent with local denaturation. Enzymolysis and very high magnification SEM (50-100kX) of overloaded tendons have shown that a sub-set (only) of the sub-fibrils at the kink zones are disrupted while others remain. Repeated plastic overload without rupture produces a linear densification of the kinks along individual damaged fibrils. The collagen fibril has thus been revealed to be unexpectedly heterogeneous: both across its diameter and along its length. We have a working theory that the local "kink" failure mechanism has 2-way evolutionary value: toughening tissues like tendons to prevent disabling injuries while providing the structural cues that guide resorption and/or repair of damaged fibrils. I believe this knowledge can be used. I propose to further explore the fundamental structuro-mechanical questions which have emerged from our work to date, applying additional high-mag tools (AFM, cryo-TEM), plus finer-scale samples to reduce heterogeneity of damage, and thereby visualize kink zone failures before elastic rebound. We will apply these methods to a range of animal tendons from across evolutionary timescales to examine the persistence of the mechanism. We will also produce laboratory-extruded collagen fibres to study the extent to which the heterogeneous fibril assembly necessary to the discrete plasticity mechanism is innate to collagen, or what interventions must be applied to produce it for technological value. Finally, I will look at processing of allograft tendons to determine under what conditions (decellularization, cryopreservation, sterilization) these products can lose/retain the discrete plasticity mechanism for optimal in vivo performance.