Investigation of the deformation and fracture behavior of hierarchical dental enamel

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In the previous funding period of this DFG project we experimentally investigated the influence of the different hierarchical levels on the mechanical properties of dental enamel, in particular by using micromechanical experiments under tension and compression. We could reveal for the first time, that there exists a pronounced mechanical tension compression asymmetry especially in regard to the influence of hierarchy. This asymmetry has not been included in the existing theoretical micromechanical models yet. Notably our investigations showed that it is not clear, whether the hierarchical structure or the hard and soft combination of hydroxyapatite with its soft protein sheets is dominant for the exceptional mechanical properties of enamel. A major reason for this gap in understanding is, that there does not exist a 3 dimensional structural imaging of the structure of the hydroxyapatite nanofibers and the interfaces between the enamel prisms. The objective of this renewal proposal is therefore to understand, whether the presence of soft proteins is necessary for the damage tolerant behavior of dental enamel, or whether its complex hierarchical structure is more essential. In order to reveal that we want to compare mechanical experiments with enamel samples with and without proteins. In parallel we want to use TEM tomography to image the 3D structure of the hydroxyapatite nanofibers, their interfaces and FIB-tomography to image the interfaces of the enamel prisms. We expect, that the combination of the micro- and macromechanical experiments on different hierarchical levels together with high resolution imaging will allow answering the central scientific question of this project. A secondary objective of this renewal proposal is to understand, whether dental enamel as a non living tissue has the ability of crack healing, as indicated in one publication. It is related to the first objective of this proposal and will elucidate the role of proteins in enamel. Finally we think that the findings of this project will contribute in developing better and novel dental materials and non-metallic engineering materials.