Preparation of nanosized hydroxyapatite particles at low temperatures

Hydroxyapatite [Ca10(PO4)6(OH)2, HAp] has been used to manufacture biomaterials for hard tissue repair and replacement, because of its excellent biocompatibility with the human organism as well as its close similarity to the main inorganic constituent of bone in both composition and structure [1, 2]. Numerous processes have been developed for the preparation of HAp powders, which include chemical co-precipitation [3], sol–gel process [4], spray-pyrolysis [5], hydrothermal syntheses [6] and emulsion techniques [7]. The preparation of HAp powders via the above methods is usually carried out at the condition of heating or with the help of after-heat-treating, which excludes the application of these methods to preparing hydroxyapatite/collagen (HAp/Col) composites, because collagen will lose its bioactivity at the temperature higher than 40 ◦C. Collagen is the major organic component of bone tissue. It has an improved bioactivity and osteoconduction compared with HAp. The combination of collagen with HAp particles can enhance the mechanical properties and bioactivity of the composite formed. So, many researchers tried to synthesize the HAp/Col composite at low temperatures [8, 9]. The key step in HAp/Col composite synthesis is the growth of HAp on a collagen matrix in aqueous medium. Up to now, most of researchers have synthesized crystallite HAp particles using the reaction of calcium hydroxide (Ca(OH)2) and phosphoric acid (H3PO4) at temperatures ≤40 ◦C [8, 10]: 10Ca(OH)2 + 6H3PO4 = Ca10(PO4)6(OH)2 + 18H2O