Investigation of the deformation and fracture behavior of hierarchical dental enamel

在本DFG项目前期的资助期内，我们通过实验研究了不同层次对牙釉质力学性能的影响，特别是通过拉伸和压缩的微观力学实验。我们首次揭示了存在明显的机械张力压缩不对称性，特别是考虑到等级的影响。这种不对称性尚未被包括在现有的理论微观力学模型中。值得注意的是，我们的研究表明，羟基磷灰石及其软蛋白片的层次结构或软硬结合是否对牙釉质的特殊力学性能起主导作用尚不清楚。造成这种理解上的差距的一个主要原因是，羟基磷灰石纳米纤维的结构和牙釉质棱柱之间的界面没有三维结构成像。因此，这项更新建议的目的是了解软蛋白的存在是否对牙釉质的损伤耐受行为是必要的，或者其复杂的层次结构是否更为必要。为了揭示这一点，我们想比较含和不含蛋白质的牙釉质样品的力学实验。同时，我们希望利用TEM断层成像技术对羟基磷灰石纳米纤维的三维结构及其界面进行成像，并利用fib断层成像技术对牙釉质棱镜的界面进行成像。我们期望，不同层次的微观和宏观力学实验与高分辨率成像的结合将允许回答这个项目的核心科学问题。这个更新建议的第二个目标是了解牙釉质作为一种非活组织是否具有裂缝愈合的能力，正如一篇出版物所指出的那样。这与本提案的第一个目标有关，并将阐明蛋白质在牙釉质中的作用。本课题的研究成果将有助于开发出更好的新型牙科材料和非金属工程材料。

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.