Controlling the ion release from mixed alkali bioactive glasses by varying modifier ionic radii and molar volume

Partially substituting one alkali oxide for another reduces the crystallisation tendency and improves the processing of bioactive glasses. Here, we investigate how we can use alkali ions of varying ionic radii to control glass degradation and ion release from Bioglass 45S5. Partially replacing sodium by lithium reduced ion release in static and dynamic dissolution studies in Tris buffer, while ion release increased with increasing potassium for sodium substitution. While the mixed alkali effect is known to reduce ion release from conventional silicate glasses (compared to compositions containing one alkali oxide only), in the glasses studied here ion release was controlled by the packing of the silicate network, described by glass molar volume and oxygen density. Incorporating an alkali ion of smaller ionic radius (Li for Na or Na for K) resulted in a more compact network of higher oxygen density, which reduced ion release. On the other hand, an alkali ion of larger ionic radius (K for Na or Na for Li) expanded the silicate network, allowing for faster ion release. This can be explained by water molecules penetrating an expanded silicate network more easily than a more compact one, thereby directly influencing the ion exchange between modifier ions and protons from the dissolution medium. This shows that the use of modifier ions of varying ionic radii allows for tailoring bioactive glass ion release and degradation while maintaining silicate network polymerisation and network connectivity. And, indeed, recent literature suggests that this concept can be extended to other modifiers besides alkali metal ions, making it possible to design bioactive glasses of tailored solubility.