The anomaly in bioactive sol–gel borate glasses

Abstract

Borate glasses differ from silicates as they do not always exhibit linear property trends with the addition of modifying elements. Rather, various property maxima, notably boron coordination, are observed at different modifier contents, a phenomenon termed as the borate anomaly. Furthermore, the higher dissolution rates of bioactive borate glasses have led to their consideration in not only hard tissue regeneration, but also in soft tissue repair. Yet, many of these borate compositions are based on traditional bioactive silicate glasses, which may not be optimized for their chemistries. Here, by exploiting the sol–gel process, we have for the first time extended the range of amorphous borate glasses to (x)CaO–(100−x)B₂O₃, where x = 20, 30, 40, 50, 60, 70 (mol%) to investigate the effect of alkaline-earth content on glass structure, texture, and bioactivity. Magic angle spinning nuclear magnetic resonance, infrared spectroscopy, and surface area and pore volume values confirmed that modifier content affected both the structural and textural properties of the glass in accordance with the borate anomaly. Modifier content also dictated the rate and conversion of glass to either calcite or hydroxycarbonated apatite in simulated body fluid, in vitro, according to X-ray diffraction, infrared spectroscopy, and scanning electron microscopy. We believe this is the first report demonstrating the borate anomaly in bioactive sol–gel glasses – over the widest compositional range for a binary alkaline-earth borate system – in terms of both structural and textural properties. The results aim to help provide a basis for designing borate glasses for targeted biomedical applications.