in situ Nanomechanical Investigations of Bone

Abstract

Mineralized collagen fibrils(MCFs)are the fundamental building blocks that contribute to the extraordinary mechanical behaviour of bone. Despite its importance in defining bone mechanics, especially the high resistance to fracture recorded in bone tissue, MCFs have yet to be mechanically tested and and, thus, MCF contributions to the global mechanical properties of bone is unclear. In this thesis, a complete strategy for performing direct mechanical testing on nanosized fibrous samples including MCFs from bone using a novel in situ atomic force microscope (AFM) – scanning electron microscope (SEM) combination was scanning electron microscope (SEM) combination was established. This technique was used to mechanically test MCFs from antler bone tissue for the first time and resultant stress behaviour was recorded to highlight the inhomogeneous response of fibrils, which is associated with fibrillar compositional heterogeneity. Mechanical properties of MCFs and bone tissue were found to be controlled by biomineralization process using additional tensile testing of MCFs and bulk samples from mouse limb bones at different ages. Extrafibrillar mineralization was found to have effects on the Young’s modulus of bone tissue rather than fibrils, indicating the importance of fibrillar interfaces in controlling overall mechanical behaviour of bone tissue. Interfaces between fibrils in bone were examined by carrying out single fibril pullout tests. A weak but reformable interface, dominated by ionic bonds between fibrils, was recorded and the sacrificial bond reforming activity at the interface was found to be be dependent on pullout strain rate. Finally, considerations of bone as a fibrous composite was used to evaluate nanomechanical testing data, with approximately 50 % of the bone fracture energy accounted for in failure of fibril interfaces.