Structure-solubility relationships in fluoride-containing phosphate based bioactive glasses

The dissolution of fluoride-containing bioactive glasses critically affects their biomedical applications. Most commercial fluoride-releasing bioactive glasses have been designed in the soda-lime-silica system. However, their relatively slow chemical dissolution and the adverse effect of fluoride on their bioactivity are stimulating the study of alternative biodegradable materials with higher biodegradability, such as biodegradable phosphate-based bioactive glasses, which can be a good candidate for applications where a fast release of active ions is sought. In order to design new biomaterials with controlled degradability and high bioactivity, it is essential to understand the connection between chemical composition, molecular structure, and solubility in physiological fluids. Accordingly, in this work we have combined the strengths of various experimental techniques with Molecular Dynamics (MD) simulations, to elucidate the impact of fluoride ions on the structure and chemical dissolution of bioactive phosphate glasses in the system: 10Na₂O-(45 -x)CaO-45P₂O₅-xCaF₂, where x varies between 0-10 mol%. NMR and MD data reveal that the medium-range atomic-scale structure of these glasses is dominated by Q² phosphate units followed by Q¹ units, and the MD simulations further show that fluoride tends to associate with network modifier cations to form alkali/alkaline-earth rich ionic aggregates. The impact of fluoride on chemical dissolution of glasses has been studied in deionized water, acidic (pH = 3.0), neutral (pH = 7.4) and basic (pH = 9.0) buffer solutions, while the bioactivity and cytotoxicity of glasses has been studied in vitro through their apatite-forming ability in simulated body fluid (SBF) and cell culture tests on mesenchymal stem cells (MSCs), respectively. The macroscopic trends observed from various chemical dissolution and bioactivity studies are discussed on the basis of the effect of fluoride on the atomistic structure of glasses, such as F-induced phosphate network re-polymerization, in an attempt to establish composition-structure-property relationships for these biomaterials.