ABSTRACT
Pure crystalline hydroxyapatite (HA) has the composition of Ca10(PO4)6(OH)2 with the calcium to phosphor mole ratio Ca/P = 1.67. The main composition of the biological bone is nano-grained hydroxyapatite (HA) with the grain size of about 5 to 50 nm (see Figure 24.1). In a physiological environment, bone is a nonstoichemical HA of Ca/P mole ratio = 1.5 to 1.67, dependent on the age and bone site. Some ions such as HPO2−4 , CO
2− 3 , and F
−1 replace partial PO3−4 and OH−1. Some other earth elements such as Mg2+ and Sr2+ can replace Ca2+, too. The common formula should be Ca10−x+y (PO4)6−x (OH)2−x−2y , where 0 < x < 2 and 0 < y < x/2. For example, the bone composition can be represented by the formula of
Ca8.3(PO4)4.3(HPO4, CO3)1.7(OH, CO3)0.3
Bone is a living tissue and undergoes a constant change in composition by either dissolving or deposition of bone minerals through osteoclast and osteobalst cells, respectively. The nano-HA has a nano-crystalline feature similar to the bone, thus being sued as the bone substitute material [1-3].
