Single crystal
Consisting of rod and interrod regions within the microstructure of enamel, the mechanical properties of enamel vary depending on the location within the microstructure. The rod and interrod structure causes anisotropy in enamel as both components have different mechanical properties. The interrod enamel has around 53% and 74% decreased hardness and elastic modulus when compared to the rod structures. This leads to a composite like hierarchical structure of enamel. The hardness and stiffness parallel to the rod axis results in high hardness and modulus, shown to have modulus values of 85-90 GPa and hardness values of 3.4-3.9 GPa. The hardness and modulus in the direction perpendicular to the rod directions are lower values, shown to have an elastic modulus between 70-77 GPa and a hardness value of 3.0-3.5 GPa. The anisotropy between the two directions can be as high as 30%. This is due partially to the structure of the material and the directionality of the rods in the c-direction. The structure of enamel is also composite in nature between the interrod and rods which also leads to anisotropy. Single crystal hydroxyapatite, the crystal enamel is based off, is also anisotropic. Single crystal hydroxyapatite has a higher hardness and young’s modulus, which may be due to the defects present in enamel, such as substitutional ions as well as the presence of organic materials.