Basics of hydroxyapatite—structure, synthesis, properties, and clinical applications

This chapter introduces hydroxyapatite (HA) as a key bioactive material primarily used in regeneration of bone. HA is a synthetic ceramic material similar to biological apatite (osseous mineral), which is the mineral component of bone. Biological apatite varies in composition (it contains additional ions that play a key biological role in the development and mineralization of bone) and crystallinity. Synthetic HA can be synthesized using a range of synthesis methods that include wet-chemical and solid-state methods. Wet-chemical synthesis methods include sol–gel methodology, coprecipitation, emulsion technology, and batch and continuous hydrothermal synthesis. HA has been the focus of significant research and development due to its similarity to biological apatite. It is bioactive, bonds to surrounding tissue and facilitates bone regeneration. As a result of this key property and similarity, it has also been used as a coating on metallic implants to offer bioactive fixation (as compared with mechanical fixation of traditional metal surfaces). Calcium phosphate (including HA) coatings have been developed primarily by thermal processes. Electrophoretic deposition, physical vapor deposition, sputtering, and precipitation are some of the methods used for developing HA coatings. Furthermore, this chapter also highlights the crystal structure of HA and its key physical, thermal, and mechanical properties. Biological performance including in vitro and in vivo studies and clinical trials have been introduced. Other biological applications of HA such as drug delivery and bioimaging have also been highlighted.