Physical structure of tooth enamel at the nano- and micro-scales, revealed by x-ray linear dichroism, and displayed by polarization-dependent imaging contrast mapping
Abstract: Tooth enamel is an extremely tough and wear resistant material, withstanding hundreds of Newtons of force every day during mastication. Enamel’s superior mechanical performance is especially important in animals that have particularly high biting forces or use their teeth to break hard materials like wood, nuts, bone, and shells. Enamel is hierarchical, meaning that it has different structures at different scales, from centimeter to Ångstrom. This hierarchy confers toughness and longevity to enamel, especially the micro- and nanoscale structure and crystal orientations. We developed Polarization-dependent Imaging Contrast (PIC) mapping of hydroxyapatite and fluorapatite (Ca10(PO4)6(OH)2) and Ca10(PO4)6F2), based on a physical effect we discovered: x-ray linear dichroism in all apatite crystals. With PIC mapping, we reveal for the first time the complex and diverse crystal orientations in enamel from modern and fossil animals, with nanoscale resolution. Crystal misorientation of adjacent pixels in PIC maps is converted to toughness, producing the first ever toughness maps. Surprisingly, T. rex, a dinosaur that had an extremely high biting force (15,000 N), has the least tough enamel of the 30 animals we measured. The toughest enamel is the saltwater crocodile, with the greatest biting force of all living animals (16,000 N).