Biomineralization of sea urchin teeth

The sea urchin tooth, which is composed almost entirely of Mg-enriched CaCO₃, is of particular interest as a model for the study of biomineralization process due to its amazing mechanical toughness and hardness. Our recent work on the formation process, the crystal composition and orientation, and the mechanical properties of sea urchin tooth are summarized in this paper. First, transmission electron microscopy images and electron diffraction patterns, as well as crystal overgrowth experiments, show that the highly convoluted primary plate-lamellar needle complex grows into a single crystal of calcite from a transient amorphous precursor phase in the sea urchin tooth. Amorphous calcium carbonate exists in the center of both the primary plates and the needles, even though the surfaces are already well crystallized. Second, X-ray photoelectron emission spectromicroscopy demonstrates that the needles, primary plates, and polycrystalline matrix crystals are all aligned. And there are two alternating crystal orientations in the stone part of the sea urchin tooth. Microbeam X-ray diffraction patterns further prove the existence of the two crystal orientations in sea urchin tooth. The c axes of calcite in the two oriented crystals are only a few degrees from each other. Third, the mechanical properties of sea urchin tooth grinding tip were studied by nanoindentation. The polycrystalline matrix has a higher elastic modulus and hardness than single crystalline needles and plates. It is proposed that the grinding capability of the tooth can be attributed to the small and uniform sizes of the polycrystalline crystals, their high Mg contents, and the two co-orientations of single crystals and polycrystalline structure. The improved understanding of the biomineralization process of sea urchin tooth and the relations between their structures and mechanical properties may shed light on the design of mechanical grinding and cutting tools with tunable properties.