Natural Selection on Hydroxyapatite Fiber Orientations for Resisting Damage of Enamel

Teeth have excellent mechanical properties, with high wear resistance and excellent fracture resistance. This is due to their well-organized multilevel hierarchical structure. While a number of studies in the last decades have revealed the relationship between tooth structure and mechanical properties, there is still no general agreement on how different orientations of hydroxyapatite (HAp) fibers affect the mechanical properties of enamel. With a scanning electron microscope and nanoindenter, the orientations of HAp fibers and their properties were investigated. HAp fibers have two different orientations: parallel and perpendicular to the surface. Fibers oriented parallel to the surface exhibited higher hardness, elastic modulus and wear resistance. Under applied force, fibers oriented perpendicular to the surface suffered deeper shearing in the protein along the long axis, resulting in lower mechanical properties. Teeth resist damaging fractures by combining hard and soft structures. This study may lead to new insights into how nature selects for tooth structure and provide a theoretical basis for the bioinspired design.