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 (Ea) for alkali ion hopping can be followed by dielectric spectroscopy. The replacement of CaO by Na2O resulted in reduction in the Ea for ion hopping. In contrast, increasing the glass network connectivity or reducing the non-bridging oxygen content of the glass resulted in an increase in Ea. Substitution of K2O for Na2O had little influence. Mixing alkali metals increased the Ea as expected on the basis of the mixed alkali effect. There was no correlation between the Ea for ion hopping and the dissolution behaviour 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.
