Calcium hydroxylapatite

Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis. Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis. Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. Hydroxyapatite crystals are also found in the small calcifications, within the pineal gland and other structures, known as corpora arenacea or 'brain sand'. Hydroxyapatite can be synthesized via several methods, such as wet chemical deposition, biomimetic deposition, sol-gel route (wet-chemical precipitation) or electrodeposition. Yagai and Aoki proposed the hydroxyapatite nanocrystal suspension can be prepared by a wet chemical precipitation reaction following the reaction equation below: 10 Ca(OH)2 + 6 H3PO4 → Ca10(PO4)6(OH)2 + 18 H2O Several studies have shown that hydroxyapatite synthesis via the wet-chemical route can be improved by high-power ultrasound. The ultrasonically assisted synthesis (sono-synthesis) of hydroxyapatite is a successful technique for the production of nanostructured hydroxyapatite to high quality standards. The ultrasonic route allows the production of nano-crystalline hydroxyapatite as well as modified particles, e.g. core-shell nanospheres and composites. Calcium deficient (non-stochiometric) hydroxyapatite, Ca10−x(PO4)6−x(HPO4)x(OH)2−x (where x is between 0 and 1) has a Ca/P ratio between 1.67 and 1.5. The Ca/P ratio is often used in the discussion of calcium phosphate phases. Stoichiometric apatite Ca10(PO4)6(OH)2 has a Ca/P ratio of 10:6 normally expressed as 1.67. The non-stoichiometric phases have the hydroxyapatite structure with cation vacancies (Ca2+) and anion (OH–) vacancies. The sites occupied solely by phosphate anions in stochiometric hydroxyapatite, are occupied by phosphate or hydrogen phosphate, HPO42–, anions. Preparation of these calcium deficient phases can be prepared by precipitation from a mixture of calcium nitrate and diammonium phosphate with the desired Ca/P ratio, for example to make a sample with a Ca/P ratio of 1.6: Sintering these non-stoichiometric phases forms a solid phase which is an intimate mixture of tricalcium phosphate and hydroxyapatite, termed biphasic calcium phosphate: The clubbing appendages of the Odontodactylus scyllarus (peacock mantis shrimp) are made of an extremely dense form of the mineral which has a higher specific strength and toughness than any synthetic composite material; these properties have led to its investigation for potential synthesis and engineering use. Their dactyl appendages have excellent impact resistance due to the impact region being composed of mainly crystalline hydroxyapatite, which offers significant hardness. A periodic layer underneath the impact layer composed of hydroxyapatite with lower calcium and phosphorus content (thus resulting in a much lower modulus) inhibits crack growth by forcing new cracks to change directions. This periodic layer also reduces the energy transferred across both layers due to the large difference in modulus, even reflecting some of the incident energy. Hydroxyapatite is present in bone and teeth; bone is made primarily of HA crystals interspersed in a collagen matrix—65 to 70% of the mass of bone is HA. Similarly HA is 70 to 80% of the mass of dentin and enamel in teeth. In enamel, the matrix for HA is formed by amelogenins and enamelins instead of collagen.