Hydroxyapatite based ceramic materials

Hydroxyapatite is the major mineral component of human bones. From the point of view of its chemistry, it is calcium orthophosphate which has in its structure hydroxyl groups and in its stoichiometric is described with formula – Ca10(PO4)6(OH)2. Biological properties of hydroxyapatite are very favorable; it is not only biocompatible (which means that after implantation it causes no side effects) but also bioactive (this means that implants made with this material bond with the surrounding bone tissue through formation of a chemical bond) [Hench 1993]. These properties made hydroxyapatite an excellent material for production of ceramic materials intended for use as implants in dentistry and treatment of orthopedic injuries. Hydroxyapatite which is used to treat bone defects can originate from a wide variety of sources. The gold standard is obtained during preparation of the so called autografts, which are small parts of patient’s own bone tissue harvested from a donor site and then transplanted to the damaged region. Unfortunately, autologous bone can be harvested in limited amount only and may be accompanied by a damage to the donor site. A solution to this problems based on preparation of allografts (i.e. extraction of bone tissue from cadavers of another patient) has also a number of drawbacks, as the use of allografts may be accompanied by disease transmission and requires application of immunosuppressant drugs. Moreover, additional ethical and religious concerns appear [Baino, 2011]. As hydroxyapatite derived via various synthesis routes or prepared through transformation of animal bones is readily available, such materials are almost exclusively used in the therapy of damaged bone tissues. A number of synthesis routes to produce hydroxyapatite has been developed in recent years and they can be classified into precipitation [Jarcho, 1976], hydrolysis [Yoshimura, 1994], solid state [Yeong, 2001] and combined [Luo, 1995] methods. Synthetic hydroxyapatite most often is a nano-powder of specific surface area of a few dozen m/g and high thermal stability. It can be synthesized either in stoichiometric or non-stoichiometric Mg, Si, F, CO3 doped form. Methods utilizing thermal treatment of bones [Murugan, 2003], supercritical fluids [Barakat, 2008] and leaching of organic matter with lye [Haberko, 2003] are used to extract hydroxyapatite from bones (i.a. bovine and porcine). Fig. 1 shows cortical parts of porcine bones obtained after leaching out of organic matter with sodium hydroxide solution, this material will be ground to obtain hydroxyapatite powder at a later preparation stage.