Effects of collagen and gelatin on the formation of cementitious hydroxyapatite-protein composites at 38°C [hard tissue prosthetics application]

Composites comprised of microcrystalline calcium deficient hydroxyapatite and either gelatin or collagen were synthesized at 38°C. A calcium phosphate cement composed of CaHPO₄ and Ca₄(PO₄)₂O was used to form the mineral phase of the composites. Hydroxyapatite-to-protein weight ratios of 4.5-to-1, 11-to-1, and 22-to-1 along with the pure mineral were investigated. Complete reaction as indicated by isothermal calorimetry and X-ray diffraction was reached within 5 hr. Solution pH was monitored as a function of protein type and content. The presence of gelatin had a negligible affect on the formation kinetics, solution pH, and hydroxyapatite microstructure. Conversely, the presence of collagen increased the rate of hydroxyapatite formation and depressed the solution pH. Scanning electron microscopy indicates that the acceleration due to the collagen resulted from its provision of a nucleation template for hydroxyapatite precipitation