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

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The present work elucidates about the structure of bioactive glasses having chemical compositions expressed as (mol %) (50.0 – x )SiO 2 – x B 2 O 3 –9.3Na 2 O–37CaO–3.7P 2 O 5, 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 B 2 O 3 for SiO 2 are assessed by Raman and 29 Si, 31 P, 11 B, and 23 Na magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopies. The structural studies reveal the presence of Q Si 2 and Q Si 3 structural units in both silicate and borosilicate glasses. However, Q Si 4 (3B) units additionally form upon incorporating B 2 O 3 in B0 glass. B-containing silicate glasses exhibit both three-coordinated boron (B III ) and four-coordinated boron (B IV ) units. The 31 P MAS-NMR studies reveal that the majority of phosphate species exist as isolated orthophosphate (Q P 0 ) units. The incorporation of B 2 O 3 in B0 glass increases the cross-linking between the SiO 4 and BO 4 structural units. However, incorporation of B 2 O 3 lowers the glass thermal stability (Δ T ), 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 (BO 3 ) 3– ions from boron-modified bioactive glasses improves both the cell proliferation and the antibacterial properties, making them promising for hard tissue engineering applications.