Hydroxylapatite

Hydroxylapatite





Hydroxylapatite, also called hydroxyapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxylapatite 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 hydroxylapatite 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 hydroxylapatite, known as bone mineral. Carbonated calcium-deficient hydroxylapatite is the main mineral of which dental enamel and dentin are composed. Hydroxylapatite crystals are also found in the small calcifications, within the pineal gland and other structures, known as corpora arenacea or 'brain sand'






Chemical synthesis of hydroxyapatite


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 wet-chemical route can be improved by power ultrasound. The ultrasonically assisted synthesis (sono-synthesis) of hydroxyapatite is a successful technique to produce nanostructured hydroxyapatite at high quality standards. The ultrasonic route allows to produce nano-crystalline hydroxyapatite as well as modified particles, e.g. core-shell nanospheres, and composites.






Calcium deficient hydroxyapatite


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.

9.6 Ca(NO3)2 + 6 (NH4)2HPO4 → Ca9.6(PO4)5.6(HPO4)0.4(OH)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.

Ca10−x(PO4)6−x(HPO4)x(OH)2−x → (1−x) Ca10(PO4)6(OH)2 + 3x Ca3(PO4)2



Medical uses


Hydroxylapatite can be found in teeth and bones within the human body. Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. Although many other phases exist with similar or even identical chemical makeup, the body responds to them very differently. Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as proteins, preventing an immune response and rejection.

Many modern implants, e.g. hip replacements, dental implants and bone conduction implants, are coated with hydroxylapatite. It has been suggested that this may promote osseointegration.[citation needed] Porous hydroxylapatite implants are used for local drug delivery in bone. It is also being used to repair early lesions in tooth enamel.

In spite of attractive biological properties, hydroxylapatite, and materials based thereon, have some drawbacks, such as low bioresorption rate in vivo, poor stimulating effect on the growth of new bone tissues, low crack resistance and small fatigue durability in the physiological environment. The application of modified hydroxylapatite opens up the opportunities for the preparation of artificial bone substances for implants and a large variety of drugs for curing different lesions of bone, soft and mucous tissues of the individual. A promising method of modification is the introduction of fluorine or silicon into the primary structure with the formation of fluorine- or silicon-substituted hydroxylapatite. The introduction of fluorine increases the resistance to biodegradation and improves the adsorption of proteins and adhesion of the coating to the metal substrate