Effect of calcination temperature on the bioactivity of a sol-gel-derived 40CaO-60B₂O₃ glass composition

Sol-gel-derived borate glasses are highly bioactive where they demonstrate rapid production of hydroxycarbonated apatite (HCA), in vitro; making them promising candidates for biomedical applications. Calcination, a critical step in sol-gel processing, significantly influences product properties. This study investigated the effect of calcination temperature on the texture, structure, as well as the reactivity and bioactive properties of sol-gel-derived 40CaO-60B₂O₃ (mol%) samples synthesized from calcium lactate pentahydrate. X-ray diffraction (XRD) confirmed that samples calcined between 400 and 600 °C were amorphous, whereas crystallization occurred when samples were calcined at 700 °C. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy revealed a decrease in BO₄ units with increasing calcination temperature. There was a decrease in specific surface area, attributable to reduced network connectivity and densification, which correlated with a decrease in vapor reactivity, as indicated by dynamic vapor sorption analysis. Bioactivity was confirmed by HCA formation within 7 days in simulated body fluid in all calcined samples, as characterized by ATR-FTIR, XRD, and scanning electron microscopy. In particular, samples calcined at 400 and 500 °C exhibited HCA formation within 1 day. Thermogravimetric analysis revealed that the sample calcined at 400 °C contained the highest amount of organic residues from sol-gel processing. In contrast, the sample calcined at 500 °C combined high bioactivity with optimal thermal decomposition, indicating that this calcination temperature may be suitable for this glass composition. In summary, the successful use of calcium lactate pentahydrate as a low-cost precursor highlights its scalability and potential for producing high-performance sol-gel-derived bioactive borate glasses.