Enamel of the human tooth is a highly mineralized hard tissue that exhibits a complex hierarchial structure and provides an interesting model for biomimetic design of new structural materials. Property gradients in this tissue have been recognized, but not studied in detail, particularly with regards to the potential changes that may arise with aging. Therefore, the purpose of this study was to examine property gradients in human enamel, and from patients of young (23±4) and old (73±15) age groups. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin Enamel Junction (DEJ) and within three different regions of the crown. Results showed that the elastic modulus and hardness increased with distance from the DEJ within each region examined, and regardless of patient age. Both the elastic modulus and hardness of the old enamel increased linearly with normalized distance from the DEJ. Although there were no age-dependent differences in properties near the DEJ, both the hardness and elastic modulus of old enamel were significantly greater at the tooth's surface. Changes in properties near the tooth's surface with age are expected to result from a gradual depletion of the interprismatic protein and replacement with apatitic mineral.
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