A natural functionally graded biocomposite coating--human enamel.

Human enamel has been found to be a coating with excellent mechanical performance, and has undergone extensive investigation and discussion. However, most of the reported studies consider the enamel as a homogeneous anisotropic biocomposite. The current study illustrated the graded properties of the biocomposite from its functional load-bearing direction. Within the thickness of the enamel, from the outer surface towards the enamel-dentin junction (EDJ), the elastic modulus (E(x)) and hardness (H(x)) of enamel exist in an exponential relationship with normalized thickness (x) as E(x)=111.64x(0.18) (R(2)=0.94) and H(x)=4.41x(0.16) (R(2)=0.87) GPa, respectively. Moreover, the creep ability of enamel increases towards the EDJ. The graded properties of the biocomposite can be explained by both microstructural and compositional changes along the thickness of the material towards the EDJ. Finite element analysis indicates that the graded properties of enamel have important roles in reducing the enamel-dentin interface stresses and maintaining the integrity of the multilayer tooth structure. The results provide a new angle to understand the excellent mechanical behaviour of the multilayer tooth structure and may inspire the development of new functionally graded materials and coating structures.