Dielectric spectroscopy and dissolution studies of bioactive glasses

The first and rate-limiting step in the degradation of bioactive glasses is thought to be the ion exchange of hydrated protons in the external fluid with alkali metal cations in the glass. The activation energy (E-a) for alkali ion hopping can be followed by dielectric spectroscopy. The replacement of CaO by Na2O resulted in a reduction in the E-a for ion hopping. In contrast, increasing the glass network connectivity or reducing the nonbridging oxygen content of the glass resulted in an increase in E-a. Substitution of K2O for Na2O had little influence on E-a. Mixing alkali metals increased the E-a as expected on the basis of the mixed alkali effect. There was no correlation between the E-a for ion hopping and the dissolution behavior of the glass. Furthermore, the activation energy for Si, Ca Na, and K ion release was found to be approximately a factor of three lower than that for ion hopping suggesting that another rate-controlling mechanism is important in the degradation of bioactive glasses. The presence of a second relaxation process suggested that bioactive glasses undergo amorphous phase separation into silica-rich and orthophosphate-rich phases and the two relaxation processes are due to ion hopping in the two phases.