Hydroxyapatite gels and nanocrystals prepared through a sol-gel process

We have investigated the effect of the Ca/P molar ratio on the structural and morphological properties of hydroxyapatite (HA) gels and nanocrystals. The sol-gel process was carried out in aqueous, and alternatively in alcoholic medium (50% water-50% ethanol), at 37degreesC. Gel samples were obtained by drying the sols at 37degreesC or at 80degreesC, whereas powder samples were obtained by filtering the sols. Heat treatment at temperatures as low as 300degreesC is enough to obtain pure HA from the gels with a Ca/P molar ratio of 1.00 and 1.67. At variance heat treatment of the gels with a Ca/P of 2.55 always produces secondary phases. The degree of crystallinity of HA increases with the Ca/P molar ratio of the sols, and it is slightly affected by the presence of ethanol in the precipitation medium. Filtering of the sols provides powders constituted of nanocrystalline HA that exhibit degree of crystallinity, crystal morphology and thermal stability closely related to the sols composition. (C) 2004 Elsevier Inc. All rights reserved.