Bone and teeth as fibrous biological composites: in situ nano-mechanical investigations

Man-made composite materials are used extensively in a variety of structures where high strength and stiffness is required as well as low weight. These composites are almost exclusively constructed from a lightweight polymer reinforced with fibres. The fibres have advantages over other geometries as the mechanical properties are excellent in one particular direction, making the fibre anisotropic. The numerous structures existing in nature are optimized through evolutionary processes for a particular mechanical function. Common examples of biological materials with a mechanical role are bone, required for structural integrity in skeletal systems, and teeth, primarily used for chewing and tearing of food. These materials have striking resemblances to man-made composites as fibrous constituents are used as reinforcement in an organic matrix. However, two main differences are apparent in biological composites when compared to synthetic composites. The first is that the reinforcing fibres in biological composites are much smaller than typical fibres used in engineering composites. Decreasing fibre diameter is widely acknowledged to increase strength and utilization of scale effects in nature highlights the optimization processes used. Furthermore, many organizational levels exist in biological composites from the nano-scale level of the reinforcing fibre building blocks up to the large scale architectures. This structural hierarchy is currently far more complex than any synthetic composite.Understanding how the nano-scale fibre building blocks influence the overall mechanical properties of the biological composite is experimentally challenging due to the structural hierarchy and small size of the fibre reinforcements. Mechanical tests on large samples give results that are difficult to interpret because of the various different fibre organizations. Therefore, testing on the individual nano-scale fibre reinforcements in biological composites would give fundamental information and help to understand how materials like bone and teeth are optimized for their mechanical functions. In addition, the understanding of biological composites at the nano-scale could provide a pathway for developing new synthetic composites with nano-material reinforcements.The project will test samples of bovine bone femur and limpet teeth, which are representative of many different types of bone and teeth found in nature. The nano-scale fibres will be mechanical tested by pulling at the ends of these fibres. The pulling will be done by a scanning probe microscopy, which is ideal for measuring the very small forces needed to deform and break the nano-scale fibres. An electron microscope will also be used to visualize these tests and observe if the nano-scale fibres are fractured during the pulling process or slide out of the surrounding organic matrix. Mechanical models used in conventional composite theory will be applied to assess the mechanical properties of the reinforcing nano-fibres and the surrounding organic matrix. The results of this research will therefore provide unique insight into how natural materials have remarkable mechanical properties from using nano-scale building blocks.