Crystallization mechanism of B12.5 bioactive borosilicate glasses and its impact on in vitro degradation

Understanding the thermal properties and crystallization mechanisms are crucial upon sintering of bioactive glasses. In this study, crystallization mechanism and sintering ability of S53P4-based B12.5 borosilicate glass series, containing varying amounts of magnesium and/or strontium, was assessed. Additionally, the effect of crystallization onto these glasses bioactive properties was investigated. Glasses were composed of 47.12 SiO₂ - 6.73 B₂O₃ - 21.77-x-y CaO - 22.65 Na₂O - 1.72 P₂O₅ - x MgO - y SrO, where x,y = 0, 5 or 10 (mol%). Thermal properties were analysed with DTA, and glass transition temperatures and onset of crystallization were determined to gain an overview of temperature range suitable for heat treatments, and for calculation of activation energies related to viscous flow and crystallization. Further, sintered bodies were formed by heat-treating coarse glass particles in large temperature range; their porosity was assessed, cross section were analysed by SEM and crystallinity was studied with XRD. To evaluate the impact of crystallization on the in vitro reactivity, dissolution studies were executed in SBF-solution up to one week, with pH and ion content of solution measured at the end of immersion. Immersed particles were studied with FTIR to observe changes in the glasses structure. The main crystallization mechanism of B12.5-based glasses was determined to be surface crystallization. While the crystallization interfered with viscous flow sintering of the pure borosilicate glass, Mg and Sr addition enabled sintering of amorphous bodies more easily and with wider temperature range. Mg in the composition especially enabled densification. In vitro studies presented that surprisingly, partially crystallized specimen were initially more reactive than the amorphous specimen.