Cells and Materials


This investigation determined the effects of reaction temperature and bulk composition on the microstructural features of hydroxyapatites (HAp) synthesized from reaction between particulate CaHPO4·2H2O and Ca4(PO4)2O. These data were used in combination with previous work to further elucidate the mechanistic reaction path taken by these calcium phosphate cements. HAps having two different compositions (Ca/P = 1.50 and 1.67) were synthesized between 15.0 and 70.0°C. All reactions reached completion as indicated by X-ray diffraction. Single point nitrogen absorption was performed on all samples to determine specific surface areas. The HAp which is approximately stoichiometric (Ca/P = 1.67) had a surface area equal or greater than that of calcium-deficient HAp (Ca/P = 1.50) synthesized at the same temperature. All surface areas were equal to or greater than those typically reported for bone and were in accord with average crystallite sizes as determined by dark field transmission electron microscopy (TEM) for all but one sample. Observations using scanning electron microscopy support previous X-ray diffraction studies and indicate the reactivity of Ca4(PO4)2O limited the rate of HAp formation. Electron diffraction by TEM verified the presence of very small proportions of an electron amorphous phase. The proportion of this phase increased with a decrease in reaction temperature, and it was more prevalent in the calcium-deficient HAp. Two distinct morphologies, plates and nodules, were observed by TEM. The nodules contained more amorphous material, while the plates were determined to have the c-axis approximately perpendicular to the plate face.