Fig. 12.1 illustrates the hierarchy in bone. The basic building blocks (level 1) are the collagen fibrils and the extremely small plateshaped crystals of carbonate apatite, just hundreds of angstroms long and wide and some 20 to 30 Å thick.

These platelets are arranged in an ordered manner within the collagen framework (level 2) forming the mineralized collagen fibril. The mineralized fibrils are further arranged in parallel arrays (level 3). At the next hierarchical level, these arrays of mineral-filled collagen fibrils are organized into a three-dimensional structure (level 4). In the case of osteonal bone, the three-dimensional structure is readily seen in the bone lamellae. The lamellae can be further organized into cylindrical structures, the osteons (level 5). At any hierarchical level, the components show very different mechanical properties so that it is conceivable to consider bone an example of a hierarchical composite material.