Human tooth enamel is a natural nanocomposite with a hierarchical structural architecture that spans from macroscale to nanoscale. Thus it offers the unique opportunity to study the physics of deformation at the nanoscale in a controlled manner using the novel nanoindentation technique. In spite of the wealth of literature, however, the information about the effect of loading rate on the nanoindentation behavior of human tooth enamel is far from being significant. Therefore, the major objective of the present work was to study the loading rate effect on nanoindentation behavior of enamel with a view to improve our understanding that could be used for development of better bioinspired synthetic structures for functional as well as biomedical utilities. The nanoindentation experiments were conducted at loading rates in the range of to ？ at peak load of ？ at room temperature with a Berkovich tip on the longitudinal section from a freshly extracted premolar tooth enamel surface from a 65-year-old Indian male. To the best of our knowledge here we report for the first time the experimental observation of the increase in intrinsic resistance against contact-induced deformation at the nanoscale with the loading rate applied to the enamel surface. The results were explained by considering the microstructural details and the shear stress underneath the nanoindenter. 1. Introduction The nanocomposite structure of the human tooth enamel provides a unique opportunity to study the physics of deformation during the nanoscale contact events because it consists of hydroxyapatite (HAP) nanocrystals embedded in an organic-protein matrix with hierarchical structures from macrostructure to microstructure to sub-microstructure to nanostructure to sub-nanostructure [1–3]. The tooth enamel can survive normally up to even a billion contacts between themselves. The tooth enamel has nanohardness ( ) in the range of 3 to 5？GPa [4–10]. The indenter shape [5], teeth type [6], location [7, 8], degree of biomineralization [9, 10], and depth of indentation [11] can affect the data. The HAP single crystals have nanohardness higher than that of enamel [12]. The tooth makes various mastications during all oral cavity movements. All these contact-induced events that determine the life of the enamel are the sum total of a multitude of nanoscale contact events under a wide variety of loading rates which have not been studied in significant details [1–12]. Neither the role of micro-pop-in nor micro-pop-out issues during nanoscale contact events happening under different loading rates in
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