Enamel--a functionally graded natural coating.

OBJECTIVE: The aim of this study is to illustrate the graded proper properties of enamel from the outer (near occlusal surface) to the inner region (near enamel-dentine junction) in a cross-sectioned surface and discuss how natural design achieve the graded functions.
METHODS: Nanoindentation, Raman spectroscopes, and SEM were employed to compare the inner and outer regions of the cross-sectioned enamel from different angles, namely mechanical properties such as elastic modulus and hardness, indentation energy absorption ability, indentation creep ability, indentation residual stress distribution pattern, compositional differences, and microstructural differences.
RESULTS: Inner enamel has lower elastic modulus and hardness but higher creep and stress redistribution abilities than outer counterpart, which is related to the gradual compositional change through the enamel. SIGNIFICANCE: Enamel can be regarded as a functionally graded natural biocomposite, which will require special attention using numerical analysis to fully appreciate the consequences of such a structure for the mechanical behaviour of teeth and restorations placed therein. Moreover, the smart design of nature can be a good model for us in functional graded materials/coatings design and development.