311 7 Synthesis, Properties and Applications of Hydroxyapatite Lech Pawlowski 7.1 Introduction Hydroxyapatite (HA), with the formula Ca 10 (PO 4 ) 6 (OH) 2 , is one of the calcium phosphates or to be precise calcium orthophosphates. 1 Te calcium phosphates occur naturally in the human body, but also in nature as mineral rocks. Te can also be synthesized. Te compounds may be categorized using the Ca/P molar ratio. Te ratio may vary from 0.5 to 2, but the calcium phosphates that are useful as biomaterials have a ratio equal to or greater than 1. Te calcium phosphates with a ratio lower than 1 have solubility and speed of hydrolysis too high to be used as biomaterials [7.2]. Te existence of calcium phosphates in bones was discovered as early as 1769. Te beginning of their application as biomaterials began in 1920, with the surgical repair of a defect in a rabbit bone using tricalcium phosphate (TCP) by US surgeon F.H. Albee [7.3]. Table 7.1 shows the major calcium phosphate compounds. HA is a main crystalline component of bone and this compound has been intensively studied over the last 30 years. Its main application is related to implants of various kinds. However, these applications are largely limited to the non-major-load-bearing parts of the skeleton because of its poor mechanical properties [7.2]. To extend the possible applications HA has been used as a coating on metallic implant substrates: the metallic substrate improves the mechanical strength and the HA contribution is reduced to its biocompatibility with bone. 7.2 Phase Diagram HA monocrystal has a complex hexagonal structure, with the cell constants a = b = 0.942 nm and c = 0.688 nm [7.2]. Te primitive cell includes 44 atoms, as represented in Figure 7.1. Te positions of the ions and groups are as follows: • Ca + are placed in columnar (four ions) and hexagonal positions (six ions). • Six groups of PO 4 3- tetrahedra are placed on planes parallel to the basal plane. • Two OH - groups are placed in columns parallel to the c-axis. 1 Orthophosphates are the inorganic forms of phosphates such as PO 4 3- , HPO 4 -2 , and H 2 PO 4 -1 [7.1]. Industrial Chemistry of Oxides for Emerging Applications, First Edition. Lech Pawłowski and Philippe Blanchart. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.