Effect of the substitution of Y2O3 for CaO on the bioactivity of 2.5CaO · 2SiO2 glass

Glasses were prepared whose composition is defined by the following general formula: (2.5 − x)CaO · x/3Y2O3 · 2SiO2 (0 less-than-or-equals, slant x less-than-or-equals, slant 1). Their behaviour when they were soaked in a simulated body fluid (SBF) and their thermal properties (glass transformation and softening temperatures, Tg and Ts respectively) were studied. Tg and Ts increase with the Y2O3 content. The trend can be explained on the basis of the increased structural rigidity when Ca2+ ions are substituted by Y3+ ions, because of the formation of stronger bonds to the oxygen. The bioactivity was studied by means of electron microscopy equipped with an energy dispersive system for elemental analysis and IR spectroscopy. All the glasses studied except the one with the greatest amount of Y2O3, x = 1.0, reacted with SBF by forming a calcium phosphate layer. The experimental results suggest that the bioactivity is negatively influenced by the Y2O3 content: the tendency to form a calcium phosphate layer is reduced the greater the amount of CaO substituted. A comparison with literature data indicates that the amount of Y2O3 that can be substituted depends on the CaO content of the base CaO-SiO2 glass. The experimental results are in good agreement with the mechanism reported in the literature. After 7 days soaking, crystalline hydroxyapatite is formed in the Y2O3-free glass and in the glasses of low Y2O3 content (x = 0.2).