| Abstract
| - The growth of calcium phosphate layers on the surface of SiO2−CaO−(P2O5) bioactiveglasses immersed in a simulated body fluid (SBF) under static and dynamic conditions hasbeen studied by XRD, SEM, EDS, TEM, and XPS. It has been found that the chemicalcomposition of the layer is very heterogeneous and that there is evidence for the presenceof a composition gradient across the layer. The Ca/P molar ratio in the layer decreases froma value of 1.6, corresponding to hydroxyapatite in the interior, to a value close to 1 in theoutermost surface, where a high population of HPO42- groups is found. The chemicalcomposition at the interphase is influenced by the presence of P in the starting glass substrateas well as by the conditions, static or dynamic, used to carry out the bioactivity tests. Acontinuous recrystallization of the calcium phosphate layer is proposed to account for theseresults.
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