Hydroxyapatites are naturally occurring mineral forms of calcium apatite, with the general formula Ca5(PO4)3(OH). Hydroxyapatite is the hydroxyl end-member of the complex apatite group, where the OH- ion can be replaced by fluoride, chloride or carbonate ions.
It crystallises in the hexagonal crystal system, has a specific gravity of 3.08 and occupies the 5th position in the Mohs hardness scale. Pure hydroxyapatite powder is white. However, naturally occurring apatites can also be brown, yellow or green. Fifty per cent of human bone is made from the inorganic mineral hydroxyl-apatite.1 Carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are composed of.
Hydroxyl-apatite can be found in teeth and bones in the human body. Scientific studies report its medical use as a filler in amputated bones or as a coating to promote bone ingrowth into prosthetic implants. It has been suggested that this material may promote ossification2–4 and many modern implants are coated with hydroxyl-apatite. Recently, hydroxyl-apatite has been used as a semi-permanent filler in non-surgical options for treating wrinkles and textural changes in skin rejuvenation. Moreover, as the main component of dental enamel, it is reported to protect it from acid erosion5–7 and to exhibit enamel restoring effects, as well as an anti-plaque and anti-stain activity.
It is possible to calibrate the physical and chemical properties of synthetic HAP for specific biomedical applications. By developing new routes or modifications of pre-existing methods to produce HAP crystallites with reduced particle size and increased surface area of controlled morphology, or both. In particular, in order to have these biomedical applications, HAP must be produced in small size, similar to the biogenic one, it must have the same morphology, not too crystalline, the presence of carbonate ions in the crystal lattice, a comb-like morphology, and, finally, must have a fixed ratio Ca/P. As far as the cosmetic field is concerned, its characteristic as a slow-release source of phosphate and calcium ions to the skin cells suggests its use as an anti-ageing ingredient for aged skin. HAP is highly hydrophilic, but insoluble in water at pH >5. The particles have a very large surface, full of irregular protrusions and cavities. Moreover, it has been possible to extend and fine-tune the bioactivity of this compound by functionalisation of watersoluble biomolecules. In this regard, amino acids are ideal candidates for the production of bio-inorganic HAP due to their intrinsic biocompatibility and ability to interact with HAP surfaces.8
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