Hierarchical structure of ivory: from nanometer to centimeter

Hierarchical structure of ivory has been investigated in terms of the organization of collagen fibrils in three dimensions, the spatial relationship of apatite crystals with collagen fibrils as well as the morphology and sizes of the crystals. Ivory can be characterized as two sets of radially-distributed layers of collagen fibril bundles interweaving to form a network. Within one layer, the collagen fibril bundles with diameter of about 2 μm lie nearly parallel to one another. They rotate by about 90° from one layer to its neighbor. The collagen fibril bundles are composed of densely-packed mineralized collagen fibrils with a diameter of 60–200 nm. Plate-shaped apatite crystals deposit mainly within the gap zone of the collagen fibrils. The average sizes of the crystals are measured to be 31 nm in length, 20 nm in width and 3 nm in thickness.