Structure and Stability of High CaO- and P₂O₅-Containing Silicate and Borosilicate Bioactive Glasses

The present work elucidates about the structure of bioactive glasses having chemical compositions expressed as (mol %) (50.0 - x)SiO₂-xB₂O₃-9.3Na₂O-37CaO-3.7P₂O₅, where x = 0.0, 12.5, 25, and 37.5, and establishes a correlation between the structure and thermal stability. The structural modifications in the parent boron-free glass (B0) with the gradual substitutions of B₂O₃ for SiO₂ are assessed by Raman and ²⁹Si, ³¹P, ¹¹B, and ²³Na magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopies. The structural studies reveal the presence of Q_Si² and Q_Si³ structural units in both silicate and borosilicate glasses. However, Q_Si⁴(3B) units additionally form upon incorporating B₂O₃ in B0 glass. B-containing silicate glasses exhibit both three-coordinated boron (B^III) and four-coordinated boron (B^IV) units. The ³¹P MAS-NMR studies reveal that the majority of phosphate species exist as isolated orthophosphate (Q_P⁰) units. The incorporation of B₂O₃ in B0 glass increases the cross-linking between the SiO₄ and BO₄ structural units. However, incorporation of B₂O₃ lowers the glass thermal stability (ΔT), as shown by differential scanning calorimetry. Although both silicate and borosilicate glasses exhibit good in vitro apatite-forming ability and cell compatibility, the bactericidal action against Escherichia coli bacteria is more evident in borosilicate glass in comparison to silicate base glass. The controlled release of (BO₃)^3- ions from boron-modified bioactive glasses improves both the cell proliferation and the antibacterial properties, making them promising for hard tissue engineering applications.