Influence of pH values on preparation of hydroxyapatite/gelatin composites

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) biomaterials have been studied as the substitute materials for the hard tissue because of its biocompatibility, bioactivity, and osteoconductivity [1]. Unfortunately, HA biomaterials cannot be used for heavy load-bearing applications due to their low mechanical properties [2]. Therefore the mechanical properties of HA biomaterials have been frequently improved by including a series of organics, such as collagen, gelatin, and Polylactide (PLA) [3– –6]. It is well known that the hard tissue is composed of HA crystals and the organic matrix, of which the main constituent is type I collagen [7]. But the biggest practical problem with collagen is its cost and commercial availability. Gelatin is the metaprot of collagen and is commercially available. Same as collagen it also shows good biocompatibility and osteoconductvity [8, 9]. Therefore, gelatin has always been used instead of collagen to prepare composites with HA in many investigations. Chang et al. had developed a chemical co-precipitation process to synthesize HA/gelatin nanocomposites [10]. However, a detailed study of the effect of pH of reactive solution on the synthesis of HA/gelatin composites had not yet been studied till date. The pH values would affect both the HA formation and the gelatin incorporation. The aim of the present study was to investigate the effect of pH values on the structure, phase constitutes, and gelatin content of HA/gelatin composites. The staring materials were Ca(NO3)2 (analytic grade), (NH4)2HPO4 (analytic grade) and gelatin (MW250,000), and they were all dissolved in distilled water. NH3·H2O (analytic grade,) was added to above solutions to adjust the pH. Then the gelatin and (NH4)2HPO4 solutions were gradually added to the stirred Ca(NO3)2 solution, respectively. White precipitates of HA/gelatin was formed when PO3− 4 solution was dropped into the Ca 2+ solution according to the following equation.