Compositional and structural gradients in dental enamel: from nano- to microscale*

Dental enamel has evolved to bear large masticatory forces, resist mechanical fatigue, and withstand wear over decades of use. Functional impairment or loss, as a consequence of developmental defects or tooth decay, has a dramatic impact on health and quality of life. While the last decade has seen great progress in our understanding of enamel formation and the functional properties of mature enamel, attempts to repair enamel lesions or synthesize enamel in vitro have had limited success. This is partly due to the highly hierarchical structure of enamel and the additional complexities arising from chemical gradients that we are only beginning to understand. Herein we show, using atomic-scale quantitative imaging and correlative spectroscopies, that human enamel is comprised of crystalline apatite and a Mg-rich amorphous intergranular phase. Individual crystallites have core-shell structure. The core is comprised of two thin layers enriched in Mg flanking a region that is poor in Mg and enriched in Na. Fluoride is often also present in layers. The sandwich core is surrounded by a shell largely free of substitutional defects. A mechanical model of coherent crystallites based on DFT calculations predicts that significant residual stresses, with important implications for enamel dissolution, crystallite and tissue mechanical properties, and crystal growth processes during amelogenesis. In addition to these gradients at length scales from single digit to 10s of nanometers, we will report on systematic changes in average lattice parameters and coherence length across single enamel rods, i.e. length scales on the order of 1-10 µm.