Гидроксиапатит: Табиғи кальций фосфатының минералдық түрі

Naturally occurring mineral form of calcium apatite

Hydroxyapatite (IMA name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring mineral form of calcium apatite with the formula |Ca5(PO4)3(OH), often written |Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The |OH− ion can be replaced by fluoride or chloride, 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 pathological calcifications such as those found in breast tumors, as well as calcifications within the pineal gland (and other structures of the brain) known as corpora arenacea or "brain sand".

Hydroxylapatite is present in bones 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. Hydroxylapatite deposits in tendons around joints results in the medical condition calcific tendinitis. Hydroxylapatite is a constituent of calcium phosphate kidney stones.

Remineralisation of tooth enamel involves the reintroduction of mineral ions into demineralised enamel. Hydroxyapatite is the main mineral component of enamel in teeth. During demineralisation, calcium and phosphorus ions are drawn out from the hydroxyapatite. The mineral ions introduced during remineralisation restore the structure of the hydroxyapatite crystals.

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; this has 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.

, the use of hydroxyapatite, or its synthetically manufactured form, nano hydroxyapatite, is not yet common practice. Some studies suggest it is useful in counteracting dentine hypersensitivity, preventing sensitivity after teeth bleaching procedures and caries prevention. Avian eggshell hydroxyapatite can be a viable filler material in bone regeneration procedures in oral surgery.

Nano hydroxyapatite possesses bioactive components which can prompt the mineralisation process of teeth, remedying hypersensitivity. Hypersensitivity of teeth is thought to be regulated by fluid within dentinal tubules. Hydroxylapatite has shown significant medium and long term desensitizing effects on dentine hypersensitivity using evaporative stimuli and the visual analogue scale (alongside potassium nitrate, arginine, glutaraldehyde with hydroxyethyl methacrylate, hydroxyapatite, adhesive systems, glass ionomer cements and laser).

Teeth bleaching agents release reactive oxygen species which can degrade enamel. In addition to remineralisation, in vitro studies have shown that toothpastes containing nano hydroxyapatite have the potential to reduce biofilm formation on both tooth enamel and resin based composite surfaces.

Hydroxyapatite is widely used within dentistry and oral and maxillofacial surgery, due to its chemical similarity to hard tissue. In the future, there are possibilities for using nano hydroxyapatite for tissue engineering and repair. The main and most advantageous feature of nano hydroxyapatite is its biocompatibility. It is chemically similar to naturally occurring hydroxyapatite and can mimic the structure and biological function of the structures found in the resident extracellular matrix. Therefore, it can be used as a scaffold for engineering tissues such as bone and cementum. The European Commission's Scientific Committee on Consumer Safety (SCCS) reissued an updated opinion in 2023, where it cleared rod shaped nano hydroxyapatite of concerns regarding genotoxicity, allowing consumer products to contain concentrations of nano hydroxyapatite as high as 10% for toothpastes and 0.465% for mouthwashes. However it warns of needle shaped nano hydroxyapatite and of inhalation in spray products. It stated:

Along with its medical applications, hydroxyapatite is also used in downstream applications under mixed mode chromatography in polishing step. The ions present on the surface of hydroxyapatite make it an ideal candidate with unique selectivity, separation and purification of biomolecule mixtures. In mixed mode chromatography, hydroxyapatite is used as the stationary phase in chromatography columns. The combined presence of calcium ions (C sites) and phosphate sites (P sites) provide metal affinity and ion exchange properties respectively. The C sites on the surface of the resin undergo metal affinity interactions with phosphate or carboxyl groups present on the biomolecules. Concurrently, these positively charged C sites tend to repel positively charged functional groups (e. g., amino groups) on biomolecules. P sites undergo cationic exchange with positively charged functional groups on biomolecules. They exhibit electrostatic repulsion with negatively charged functional groups on biomolecules. For the elution of molecules buffer with high concentration of phosphate and sodium chloride is used. The nature of different charged ions on the surface of hydroxyapatite provides the framework for unique selectivity and binding of biomolecules, facilitating robust separation of biomolecules. Hydroxyapatite is available in different forms and in different sizes for the purpose of protein purification. The advantages of hydroxyapatite media are its high product stability and uniformity in various lots during its production. Generally, hydroxyapatite was used in the polishing step of monoclonal antibodies, isolation of endotoxin free plasmids, purification of enzymes and viral particles.

In archaeology, hydroxyapatite from human and animal remains can be analysed to reconstruct ancient diets, migrations and paleoclimate. The mineral fractions of bone and teeth act as a reservoir of trace elements, including carbon, oxygen and strontium. Stable isotope analysis of human and faunal hydroxyapatite can be used to indicate whether a diet was predominantly terrestrial or marine in nature (carbon, strontium); the geographical origin and migratory habits of an animal or human (oxygen, strontium) and to reconstruct past temperatures and climate shifts (oxygen). Post depositional alteration of bone can contribute to the degradation of bone collagen, the protein required for stable isotope analysis.

Hydroxylapatite is a potential adsorbent for the defluoridation of drinking water, as it forms fluorapatite in a three step process. Hydroxylapatite removes |F− from the water to replace |OH− forming fluorapatite. However, during the defluoridation process the hydroxyapatite dissolves, and increases the pH and phosphate ion concentration which makes the defluoridated water unfit for drinking. Recently, a ″calcium amended hydroxyapatite″ defluoridation technique was suggested to overcome the phosphate leaching from hydroxyapatite. This technique can also affect fluorosis reversal by providing calcium enriched alkaline drinking water to fluorosis affected areas.