An equilibrium study of phosphate precipitation on bioactive glass

The solubility of hydroxyapatite (HAP) depends on the concentration of calcium, phosphate and hydroxide ions. The paper discusses precipitation of HAP from a simulated body fluid (SBF). Thermodynamic calculations show that if equilibrium is attained, solid hydroxyapatite will precipitate from SBF. Metastable precursors, however, are reported to precede the apatite formation and equilibrium is attained only after several days. When in contact with a bioactive glass, alkali and alkaline earth ions are dissolved and a layer of hydrated silica is formed on the surface. In this gel pH drops from around 9.5 close to the glass to 7.2 close to the solution. The solubility of HAP decreases with increasing pH and as there further is an almost infinite number of nucleation sites in the hydrated gel, HAP precipitation starts earlier inside the gel than at its outer surface. It is suggested that the pH gradient and the heterogeneous nucleation sites explain the observed effect of silicate ions on HAP formation. Parallels are also drawn to formation of stem cells in blood vesicles.