Effect of strontium substitution on the structure, ionic diffusion and dynamic properties of 45S5 Bioactive glasses

Strontium ions can promote bone growth and enhance bioactivity in bioactive glasses that find critical biomedical applications. In this paper, the effect of SrO/CaO substitution on the structure, ionic diffusion, and dynamic properties of 45S5 bioactive glasses has been studied using constant pressure molecular dynamics simulations with a set of effective partial charge potentials. The simulated structure models were validated by comparing with experimental neutron diffraction results. It was found that the SrO/CaO substitution leads to an increase of glass density and decrease of oxygen density, a measure of compactness of the glass, in excellent agreement with available experimental data. On the other hand, the substitution does not significantly change of the medium range glass structures as characterized by silicon and phosphorous Qn distributions, network connectivity, and ring size distributions. The diffusion and dynamic behavior of these glasses and their melts were also determined by calculating the mean square displacements and velocity autocorrelation functions. It was found that the diffusion energy barriers of sodium, calcium and strontium ions remain nearly constant with respect to the level of substitution. However, strontium ions do influence the diffusion behaviors of calcium and sodium ions at high temperature, as evidenced from their velocity autocorrelation functions. Like calcium and sodium, strontium ions only contribute to the lower frequency (around 100 cm− 1) of the vibrational spectra the substitution has little effect on high frequency features and the general shape of the vibrational density of states. These results suggest that the increase of the dissolution rate in strontium containing glasses are mainly due to the increase of free volume and the non-local effect that weakens the silicon-oxygen network due to strontium ions.

► Short and medium range structure of SrO/CaO substituted 45S5 bioglass studied using MD simulations.
► Sr2+ has similar local environment as Ca2+ but with slightly higher coord. number and longer bond length to oxygen.
► Modifier ions diffusion energy barriers change little with the level of SrO substitution.
► Phosphorus Qn species was found to be sensitive the cooling rate during glass formation.