Ordering in sol-gel-derived bioactive glasses and its influence on the dissolution/precipitation behavior in a complex culture medium

Sol-gel-derived bioactive glasses obtained via sol-gel can exhibit inherent structural ordering even below the glass-transition temperature (Tg) depending on the levels of modifier and intermediate oxides that are included in their composition. To answer whether this previous structural ordering can affect their surface reactivity, we synthesized two distinct systems that are known to present ordering below the Tg: a standard bioactive glass without MgO [Na2O-CaO-P2O5-SiO2] and another that contained different MgO proportions in substitution for CaO [Na2O-CaO(MgO)-P2O5-SiO2]. The bioactive glass without MgO suffered an ordering process below the Tg with the formation of nanocrystalline apatite domains. The insertion of MgO caused the ordering of nanocrystalline Mg2+-containing β-tricalcium phosphate domains. After immersion in a complex medium (McCoy's 5A modified culture medium), the MgO-containing bioactive glass had a higher apatite precipitation rate than the bioactive glass without MgO. Although the MgO reduced the Tg, the increased precipitation was not caused directly by the decrease in Tg but rather by the previous presence of the ordered phase. Additionally, the layer precipitated onto the MgO-containing bioactive glass presented a higher level of Mg2+ and CO32− ions, suggesting the formation of an Mg2+-containing carbonate apatite similar to that observed in bone tissue. Therefore, the inherent ordering of sol-gel-derived bioactive glasses below the Tg can define their bioactivity, which constitutes a consistent way of controlling their surface reactivity.