In this review, the current status of the influence of added ions (i.e., SiO44-, CO32-, etc.) and surface states (i.e., hydrated and non-apatite layers) on the biocompatibility nature of hydroxyapatite (HA, Ca10(PO4)6(OH)2) is discussed. It is well known that HA is a type of calcium phosphate with high biocompatibility that is present in biological hard tissues such as bones and enamel. This biomedical material has been extensively studied due to its osteogenic properties. The chemical composition and crystalline structure of HA change depending on the synthetic method and the addition of other ions, thereby affecting the surface properties related to biocompatibility. This review illustrates the structural and surface properties of HA substituted with ions such as silicate, carbonate, and other elemental ions. The importance of the surface characteristics of HA and its components, the hydration layers, and the non-apatite layers for the effective control of biomedical function, as well as their relationship at the interface to improve biocompatibility, has been highlighted. Since the interfacial properties will affect protein adsorption and cell adhesion, the analysis of their properties may provide ideas for effective bone formation and regeneration mechanisms.
Keywords: biocompatibility; bone tissue formation; hydration layer; hydroxyapatite; mineralization; non-apatitic layer; surface properties.