The hidden structure of human enamel

E Beniash and CA Stifler and CY Sun and GS Jung and Z Qin and MJ Buehler and PUPA Gilbert, NATURE COMMUNICATIONS, 10, 4383 (2019).

DOI: 10.1038/s41467-019-12185-7

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, similar to 50 nm wide, microns-long nanocrystals, bundled either into 5-mu m-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1 degrees-30 degrees, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30 degrees-90 degrees within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges.

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