It should be noted that the inorganic composition of bone is not hydroxyapatite but carbonate apatite that contains 6-9 mass％carbonate. Although carbonate apatite cannot be fabricated by sintering due to thermal decomposition, it was found that the fabrication of carbonate apatite was possible through a dissolution–precipitation reaction using a precursor, such as calcium carbonate. Carbonate apatite artificial bone exhibits much higher osteoconductivity when compared with sintered hydroxyapatite, and it replaces new bone based on the bone remodeling process. Carbonate apatite cement and carbonate apatite coating on titanium also exhibit excellent properties.

Among metals, titanium （Ti） and the majority of its alloys exhibit excellent biocompatibility. Although their high corrosion resistance is a factor in their biocompatibility, it is clear that other factors exist. In this review, the corrosion resistance and passive film of Ti are compared with those of other metallic biomaterials, and their band gap energies （Egs） are also compared to discuss the role of Eg in reactivity with living tissues. From the perspective of the material's surface, it is possible to explain the excellent biocompatibility of Ti by considering the following factors: passive film showing high corrosion resistance; strong property as n-type semiconductor; good balance of positive and negative charges by dissociation of surface hydroxyl groups on passive film; low electrostatic force of passive film inducing natural adsorption of proteins maintaining their natural conformation; lower band gap energy of passive film on Ti generating optimal reactivity; and calcium phosphate formation caused by this reactivity. The optimal balance between high corrosion resistance and appropriate reactivity of Ti is the main factor determining the excellent biocompatibility of Ti.