Nano-scale thermo-mechanical structure-property relationships in human dental tissues studied by nanoindentation and synchrotron X-ray scattering

Human teeth are hierarchical mineralized structures consisting of collagen fibres decorated with hydroxyapatite (HAp) crystallites at the nano-scale. Teeth are composed of two major calcified tissues: harder outer enamel layer and softer dentine core. These tissues are joined by an interface known as the dentine-enamel junction (DEJ). The knowledge of the mechanical properties related to the hierarchical structure of dentine, enamel and the DEJ is essential for understanding and predicting the effects of microstructural alterations due to disease, treatment, or environmental or thermal exposure on the performance of dental tissues and their artificial replacements. So far, few studies have focused on the nano-scale mechanical properties of human teeth, in particular the response to thermal treatment that is thought to be strongly affected by the evolution of the mineral crystallite size, their spatial arrangement and orientation. In this study, small and wide angle synchrotron X-ray scattering (SAXS/WAXS) was used to investigate the structural alterations at the nano-scale, while nanoindentation was introduced to map the nanomechanical properties variation in thermally treated dental tissues. The thermo-mechanical structure-property relationships in human dental tissues were analysed. This property alteration is likely to occur due to changes in the size and orientation of HAp crystallites, but is also strongly affected by the presence of the organic phase (collagen). The results are likely to be broadly useful in understanding the mechanical property correlations in hierarchical biomaterials in the context of forensic and archaeological investigations.