Hydroxyapatite, (Ca10(PO4)6(OH)2 is chemically similar to the mineral component of bones and teeth and so it is used widely for biomedical application due to the high bioactivity and biocompatibility. It is used for the coating of the implants because of the bioactive and biodegradable properties and for reducing the failure of the implants. Due to the chemical similarity between HA and mineralized bone of human tissue, synthetic HA exhibits strong affinity to host hard tissues.
In order to achieve a good biocompatibility, high range bioactivity polyethylene, collagen, and chitosan (CTS) are used to modify the Hap as the polyamide has a great biocompatibility with human structure.
Synthetic nano-HA materials
HAp can be produced from biogenic materials like coral, seashell, eggshell, body fluids and by some chemical synthetic methods.
Common chemical methods used to produce synthetic nanocrystalline HA include precipitation, hydrothermal, hydrolysis, mechanochemical and solgel. These techniques can generate nano to micrometric size HA crystals. However, solgel method offers a distinct advantage of a molecular-level mixing of the calcium and phosphorus precursors, which are capable of improving chemical homogeneity of the resulting HA to a significant extent, in comparison with above conventional techniques.
Pure nanosize HA powder can be synthesized  via solgel process using calcium nitrate tetrahydrate, phosphoric acid (H3PO4) and ammonia as starting precursors and the nano powders can be obtained by calcination.
HAp can be produced from biogenic materials like coral, seashell and eggshell also. Seashell and eggshell are washed with detergent, and then calcined in air at 900degC for 3 h. During the first 30 minutes most of the organic materials get burnt out and the shells are converted to calcium oxide. Calcined shells are crushed and milled in a ball mill equipped with alumina balls and bowls. The crushed shell is reacted with phosphoric acid in an exothermic reaction. The mixtures are milled for 10 h at 350 rpm to achieve homogenous mixtures and to prevent agglomeration. After milling HAp powder is sintered at 900 deg C for 2h in air.
Uses
apatites are one of the most promising materials used as adsorbents or reductants for removing hexavalent uranium from groundwater. apatite is an ideal material for long-term sequestration of metals due to its high affinity for actinides and heavy metals and widely used for immobilizing heavy metals.
Nano hydroxyapatite can act as scaffolds for tissue engineering and chromatographic packing because of its high bioactivity and particular absorbability for various ions and organic molecules. It can support bone ingrowths and Osseo integration when used in orthopedic, dental and maxillofacial applications. Hydroxyapatite coatings are often applied to metallic implants, especially stainless steels and titanium alloys, to improve the surface properties. It may be employed in powders, porous blocks and hybrid composites to fill bone defects or voids when large sections of bone have had to be removed, or when bone augmentations are required (e.g. dental applications). HA has been used widely in bone implants.