Materials scientists in the US say they have cracked one of the secrets of tooth decay by identifying a small number of atoms that contribute to the strength of tooth enamel but also may make it more soluble.
Writing in the journal Nature, the team from Northwestern University says it is the first to determine the spatial distribution of the impurities with atomic-scale resolution.
Enamel can be several millimetres thick and covers the entire crown of a tooth, but it is difficult to study because of its complex structure. It is a 3D weave of rods, each about five microns wide, and these rods are made of thousands of hydroxylapatite crystallites, each just tens of nanometres thick.
Perhaps unique to human enamel, the centre of each crystallite seems to be more soluble than the rest, and Derk Joester and his colleagues wanted to know why.
Using quantitative atomic-scale techniques, they discovered that crystallites have a continuous crystal structure with a core-shell architecture. Calcium, phosphate and hydroxyl ions are arranged periodically to form the shell, but at the core a greater number of these ions are replaced with magnesium, sodium, carbonate and fluoride.
Surprisingly, within the core the magnesium ions form two layers on either side of the core, “like the world’s tiniest sandwich, just six billionths of a metre across”, says co-author Karen DeRocher.
Working at the nanoscale and at cryogenic temperatures, the researchers were able to determine the chemical nature and position of small numbers of impurity atoms with sub-nanometre resolution.
They suspected that the irregularities introduced by magnesium layers give rise to areas of strain in the crystallite and computer modelling supported this, predicting higher stresses in the core than in the shell.
“Stress may sound bad, but in material science it can be useful, and we think it may make enamel stronger overall,” says DeRocher. “On the other hand, those stresses are predicted to make the core more soluble.”
When the researchers exposed crystallites to acid – similar to what happens in the mouth – the cores did more erosion than the shell. More modelling and experiments are planned.
In the image above, the left panel shows the magnesium (magenta) sandwich at the crystallite’s core from data acquired by atom probe tomography. The right panel shows an atomic resolution scanning transmission electron microscopy image of an enamel crystallite looking down the long axis of the crystal.
The dark areas are distortions in the crystal lattice due to the presence of impurities such as magnesium and sodium, identified by atom probe tomography (left panel).
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