hydroxyapatite

Hydroxyapatite is a calcium phosphate mineral with the chemical formula Ca10(PO4)6(OH)2. It crystallizes in a hexagonal lattice and is the principal inorganic component of bone mineral and tooth enamel. In biological systems, the mineral phase often deviates from the ideal formula through carbonate and other substitutions, forming non-stoichiometric, substituted versions known as biological apatites.

In nature, hydroxyapatite occurs in bones and teeth. Bone mineral is a poorly crystalline, carbonate-containing hydroxyapatite that provides stiffness and a reservoir for calcium and phosphate. Enamel is largely crystalline hydroxyapatite, sometimes with fluoride incorporated, which can influence solubility and acid resistance.

Because of its biocompatibility and chemical similarity to natural bone mineral, hydroxyapatite is widely used in medicine and dentistry. It serves as a bone graft substitute, a coating on metallic implants to promote osseointegration, and a scaffold material in bone tissue engineering. In dentistry, hydroxyapatite is used in remineralization therapies and as a filler or coating in some restorative materials.

Synthetic hydroxyapatite can be prepared by aqueous precipitation, hydrothermal synthesis, or sol-gel methods. It is often doped with trace ions such as carbonate, fluoride, silicon, strontium, or zinc to alter its solubility, bioactivity, or mechanical properties. Its relatively low solubility at neutral pH and surface bioactivity contribute to its suitability for biomedical applications.