Precipitation of hydroxyapatite from dilute solutions upon seeding.

Abstract

The kinetics of precipitation of hydroxyapatite, HA, was studied by seeding dilute supersaturated solutions with well characterized HA crystals. In solutions having initial degrees of supersaturation comparable to those present in human serum, the precipitation rates were related to the thermodynamic driving force (degree of supersaturation with respect to HA) and not to the solution composition. The following relationshipR 0=KA(DS) n , whereR 0=initial precipitation rate, A=amount of seeds, DS=degree of supersaturation, and K andn are parameters obtained from the experimental data, was found to apply over a DS range of 6.6×1010 to 3.3×106. These observations are consistent with the occurrence of a simple growth process on the HA seeds. No evidence for the formation of discrete calcium phosphates other than HA was detected. The Ca/P molar ratio of the precipitating phase, calculated from solution compositions, was invariably higher than that expected for HA; this result is shown to be consistent with an initial adsorption phenomenon. Anomalous kinetic behavior was observed at low seeding levels and may relate to the surface phenomenon described. It is concluded that, most probably, under physiological conditions, formation and remineralization of hard tissues occur through the reported crystal growth process.