What is Hydroxyapatite?

Hydroxyapatite, HAp is a basic calcium phosphate, and its chemical formula is Ca₁₀(PO₄)₆(OH)₂ existing as main material of bones, teeth and mineral naturally with high biaffinity. It is composited by below methods, and it is also used in various fields including biomaterials.

Manufacturing Method of HAp

Solution Technique (Wet Process）

Composition method by reacting calcium ion and phosphate ion in neutral or alkaline water solution at room temperature represented by neutralization reaction and reaction of salts.

Neutralization Reaction	10Ca(OH)2 + 6H3PO4 → Ca10(PO4)6(OH)2
Reaction of Salts	10Ca(NO3)2 + 6(NH4)3PO4 + H2O → Ca10(PO4)6(OH)2 + 18NH3 + 20 HNO3

Also, HAp has a characteristic of “bioaffinity and bone affinity,” “high sorbability with amino acid, protein, fat and sugar" and "ion-exchange performance.” It is known to be safe for biological bodies.
However, because HAp compounded by above method is amorphous, it is not enough stable depending on purpose of use, which needs to be calcined again for higher crystallinity.

Characteristics of HAp

Bioaffinity & Bone Affinity

HAp has excellent bioaffinity and it is researched widely as a biomaterial. Especially, it is known to have high bone affinity, and cells can penetrate into madreporic sintered bodies easily having excellent bone conduction capacity.※ By filling artificial bone of HAp into lost part of bone, its surface is covered by new bone, and autologous bone is displaced over time after successive absorption by osteoclast and rebirth by osteoblast. Then, the bone and HAp becomes stable enough to be bonded chemically due to the new bone. Therefore, it is effective for implant and artificial hip joint needs early immobilization and stabilization, which is already in practical use. HAp
※Bone Conduction Capacity：When biomedical materials are filled into autologous bone, new bone is formed on the surface of the material bonding the biomaterial and the bone.

High Sorbability

HAp has a high sorbability with amino acid, protein, fat and sugar. Therefore, it is used as a bulking agent of column chromatography used for disrupting biomolecules or used for cosmetics through its adsorption removal of extra fat and waste.

Functional Group Easy to Interact with HAp	Hydroxyl Group, Carboxyl Group and Amino Group etc.

Ion-Exchange Performance

HAp has an excellent ion-exchange performance, which exchanges calcium ion site with cation and phosphate group or hydroxide ion site with anion respectively. Indeed, in bones and teeth though it is fine, calcium ion is exchanged with iron ion, magnesium ion or strontium ion, and also hydroxide ion is exchanged with fluorine ion or carbonate ion. For example, cavity prevention using fluorine in dental field is a practical application by applying medicinal substance containing fluorine ion to teeth for the ion-exchange of fluorine ion and hydroxyl giving acid resistant to the teeth surface as a result.

Other Application Examples

Dentifrice	Effective for remineralization of primary dental caries and adsorptive removal of tartar.
Health Food	Composited as a food additive and effective for calcium refill.

REFERENCE

1. Hideki Aoki (1999) Extraordinary Biological Material Apatite』Ishiyaku Publishers,Inc. 193pp.
2. Junzo Tanaka, Toshiyuki Ikoma, Toshimasa Uemura, Kenichi Omori (2009) 『Bioceramic』CORONA PUBLISHING CO.,LTD. 183pp.
3. The Ceramic Society of Japan (2008) 『Biomaterial』（Environment-Conscious New Material Series）THE NIKKAN KOGYO SHIMBUN,LTD 295pp.