One of the most important calcium phosphates is hydroxyapatite (HA), the main mineral content of bones and teeth. It has long been used for coating titanium hip-joint stems for better integration between the implant and the femur bone, as well as numerous applications as bone void fillers. The chemical formula of HA is usually expressed as Ca10(PO4)6(OH)2, denoting the number of Ca, P, O, and H atoms that make up a crystallographic unit cell (Figure 7.2) (de Leeuw 2001). Notice that in this unit cell, the four positions

occupied by OH can be substituted by other anions such as F−, Cl−, and CO32−, and indeed, the mineral phase of bones and teeth consists of a mixture of HA, its substituted variants, and other forms of calcium phosphates. For this reason, even though the Ca:P ratio for pure HA is 1.67, in bone mineral the Ca:P ratio ranges from 1.37 to 1.87. HA and its substituted variants are extremely stable chemically in neutral or basic environments, but not in the presence of acid. This is the reason caries form in teeth as a result of HA erosion, which occurs when bacteria infection creates an acidic environment on the teeth. HA also has remarkable thermal stability: It can resist thermal decomposition at temperatures up to 1000°C, allowing sintering treatments that modify the crystallinity and porosity of the HA material; such treatments are routinely used to coat metal implants with HA. Currently, both natural and synthetic HA are available. Bones and corals are natural sources for HA, but for some applications, synthetic HA is preferred because the composition and crystallinity can be controlled. HA with lower crystallinity is found to be more bioactive.