Bioapatite

Bioapatite is the natural calcium phosphate mineral that forms the mineral phase of vertebrate hard tissues, notably bone, dentin, and tooth enamel. It is typically a carbonated calcium phosphate apatite that is chemically similar to hydroxyapatite but in biological systems is poorly crystalline and carries various substitutions. The general mineral framework is close to Ca10(PO4)6(OH)2, but biological apatites commonly replace carbonate for phosphate or hydroxide, and may incorporate fluoride or chloride, resulting in non-stoichiometric, nanocrystalline minerals with increased solubility compared with pure hydroxyapatite.

Substitutions and crystallinity: Carbonate is the most prevalent substituent in bioapatite, contributing to lower crystallinity and greater solubility. Other substitutions, such as fluoride or chloride, can modify lattice parameters and acid resistance. The mineral may exhibit a range of Ca/P ratios and is often described as carbonated apatite rather than a single chemical formula.

Biomineralization and structure: Bioapatite forms through biomineralization within an organic extracellular matrix produced by osteoblasts in bone, odontoblasts in dentin, and ameloblasts in developing enamel. The resulting mineral is typically nanocrystalline, with crystal sizes influenced by proteins and macromolecules that regulate nucleation and growth. The mineral content provides rigidity while allowing gradual remodeling in bone and mineralization in teeth.

Applications and relevance: In biology, bioapatite underpins mechanical properties and metabolic calcium storage. In materials science and dentistry, synthetic versions of carbonated apatites and hydroxyapatite are used as bone grafts, coatings for implants, and remineralization agents, reflecting the close link between bioapatite biology and biomedical applications.