In situ hydroxyapatite crystallization for the formation of hydroxyapatite/polymer composites

Abstract

In an attempt to synthesize hydroxyapatite (HAP)/polymer composites, HAP crystallization was investigated in solution in the presence of ionic synthetic polymers. The side groups of the polymers used include carboxylate, dihydrogen phosphate, sulfate, and primary amine. Spontaneous HAP precipitation and amorphous-crystalline transformation occurring in both the presence or absence of ionic polymers were studied by measuring the solution turbidity change and titrating the released protons, respectively. The rates of HAP nucleation and growth were determined from an induction period before onset of crystallization and the subsequent propagation of HAP crystallites. The added anionic and cationic polymers, were found to suppress the crystallization in a concentration-dependent manner. An exception was a concentrated poly(acrylic acid), which was precipitated by calcium ion binding to accelerate the nucleation and the growth of HAP crystallites. These results suggest that a molecular interaction is operative between the ionic polymer chains and the growing HAP crystallites. Infrared spectroscopy and X-ray diffraction analysis revealed that the HAP precipitated in the presence of ionic polymers incorporated the polymer chains in the HAP microcrystalline aggregates. Based on these findings, HAP/poly(acrylic acid) composites were prepared through in situ HAP crystallization in the presence of poly(acrylic acid). Tensile testing of the composites and electron microscopic observation of their fractured surfaces revealed that the composite prepared through this wet process was superior to that obtained by simple physical mixing with respect to the wet state mechanical properties and homogeneity in mixing.