Compositional Variations in the Calcium Phosphate Layer Growth on Gel Glasses Soaked in a Simulated Body Fluid

The growth of calcium phosphate layers on the surface of SiO 2 −CaO−(P 2 O 5 ) bioactive glasses immersed in a simulated body fluid (SBF) under static and dynamic conditions has been studied by XRD, SEM, EDS, TEM, and XPS. It has been found that the chemical composition of the layer is very heterogeneous and that there is evidence for the presence of a composition gradient across the layer. The Ca/P molar ratio in the layer decreases from a value of 1.6, corresponding to hydroxyapatite in the interior, to a value close to 1 in the outermost surface, where a high population of HPO 4 2 - groups is found. The chemical composition at the interphase is influenced by the presence of P in the starting glass substrate as well as by the conditions, static or dynamic, used to carry out the bioactivity tests. A continuous recrystallization of the calcium phosphate layer is proposed to account for these results.