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A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 1 1 SCIENTISTS STUDY HUMAN ENAMEL In a new study of human enamel, materials scientists at Northwestern University, Evanston, Ill., identified a small number of impurity atoms that may contribute to strength, but also make enamel more soluble. The team is also the first to determine the spa- tial distribution of the impurities with atomic-scale resolution. The discovery about the building blocks of enamel, with detail down to the nanoscale, could lead to a better understanding of human tooth decay. One obstacle hindering enamel research is its complex structure, which consists of a 3Dweave of rods. Each rod, approximately 5 microns wide, is made up of thousands of individual hydroxyl- apatite crystallites. These nanoscale crystallites are the fundamental buil- ding blocks of enamel. Perhaps unique to human enamel, the center of the crystallite seems to be more soluble. The researchers set out to test if the composition of minor enamel constitu- ents varies in single crystallites. Using quantitative atomic-scale techniques, the team discovered that human enamel crystallites have a core- shell structure. Each crystallite has a continuous crystal structure with cal- cium, phosphate, and hydroxyl ions arranged periodically (the shell). How- ever, at the center, more of these ions are replaced with magnesium, sodium, carbonate, and fluoride (the core). Within the core, two magnesium-rich layers flank a mix of sodium, fluoride, and carbonate ions. Detecting and visu- alizing the sandwich structure required scanning transmission electron micros- copy at cryogenic temperatures (cryo- STEM) and atom probe tomography (APT). The team found strong evidence that the core-shell architecture and resulting residual stresses impact the dissolution behavior of human enamel crystallites. northwestern.edu. An atomic resolution STEM image of an enamel crystallite in which dark areas showmagnesium ions forming two layers on either side of the core. Courtesy of Northwestern University.