Collagen Denaturation as a Toughening Mechanism in Cortical Bone

Abstract

Bone is a highly versatile tissue. It is used for protecting internal organs as ribs, supporting locomotion as long bone, as a weapon in the form of antler, and many other uses. Depending on its use or function the bone may experience repeated cyclic loads, as in leg bones, or resist sudden impact as antler. To be physiologically useful as a biomaterial, bone must be stiff and resist deformation, but also be capable of dissipating large amounts of energy while resisting failure. All bone meets these biological requirements as a composite of hydroxyapatite mineral, protein (mainly type-I collagen) and water. Together these materials form a highly complex multi-scale structure that gives rise to varied and powerful toughening mechanisms. One putative - but yet unproven - mechanism is the mechanical denaturation (unravelling) of collagen. The native form of collagen is a triple helix with internal hydrogen bonds maintaining the molecular structure. In silico experiments have suggested that collagen does denature under mechanical stresses. If the collagen does mechanically denature during fracture, then some quantity of energy is dissipated disrupting the internal hydrogen bonding. The primary objective of this work is to test the hypothesis that “collagen denatures as a toughening mechanism during stable fracture of cortical bone”. A new biotechnology, fluorescently labelled collagen hybridizing peptides (F-CHP), has seen recent successes in identifying denatured collagen in a variety of tissues. These probes are specific for denatured collagen and not native, triple helical collagen. As such they provide a unique opportunity to probe the behavior of bone collagen during fracture. A notching and staining system was devised to reproducibly image denatured collagen on bovine cortical bone fracture surfaces. This imaging showed consistent increases in staining on surfaces produced by stable crack extension during fracture. This increase in staining correlated strongly with the energy per unit area dissipated by the sample. Furthermore, the staining was confined to a visibly rough region on the fracture surface produced by stable fracture extension. This result supports the hypothesis, suggesting that the denaturation of collagen is a crucial element of how bone resists fracture during stable fracture extension.