  In recent years, tailoring the properties of bioactive glasses through
compositional design have become the subject of widespread interest for their
use in medical application, e.g., tissue regeneration. Understanding the mixed
alkali effect (MAE) in oxide glasses is of fundamental importance for tailoring
the glass compositions to control the mobility of ions and, therefore, the
glass properties that depend on it, such as ion release, glass transition
temperature, and ionic conductivity. However, most of the previously designed
bioactive glasses were based on trial-and-error, which is due to the complex
glass structure that is non-trivial to analyze and, thus, the lack of a clear
picture of the glass structure at short- and medium-range order. Accordingly,
we use molecular dynamics simulations to study whether using the MAE can
control the bioactivity and properties of 45S5 glass and its structural
origins. We showed that the network connectivity, a structural parameter often
used to access the bioactivity of silicate glasses, does not change with Na
substitution with Li or K. On the contrary, the elastic moduli showed a strong
dependence on the type of the modifier, as they increased with increasing mean
field strength. Similarly, the mobility of the glass elements was significantly
affected by the type of modifier used to substitute Na. The change of the
properties is further discussed and explained using changes at the short- and
medium-range structure by giving evidence of previous experimental findings.
Finally, we highlight the origin of the non-existence of the MAE, the effect of
the modifier on the bioactivity of the glasses, the importance of dynamical
descriptors in predicting the bioactivity of oxide glasses, and we provide the
necessary insights, at the atomic scale, needed for further development of
bioactive glasses.
