# Sintering Behavior of a Six-Oxide Silicate Bioactive Glass for Scaffold Manufacturing
## 1 Introduction
The final result is a highly disrupted silicate network with high reactivity in physiological environment, as well as low glass transition temperature and enhanced devitrification tendency upon heating due to the low chemical stability of the network [29]. When a glass devitrifies, the crystalline phases which nucleate and grow within the amorphous matrix not only improve the strength of the material, but also make it chemically more stable [29; 30; 31]. In this regard, one of the major concerns related to the sinter-crystallization of BGs is in fact the subsequent decrease in bioactive potential, as the ion-release phenomena involved in the mechanism of bioactivity are mainly related to the amorphous phase. As a result, the optimal sintering window for BG processing is conventionally defined between the glass transition temperature and the crystallization onset of the material. Within this range, the temperature is not high enough to induce devitrification of the system but sufficient to allow the formation of sintering necks between adjacent glass particles through viscous flow and diffusive mechanisms, thus ideally preserving the glass bioactivity. However, the nucleation of crystalline phases is not the only factor affecting the reactivity of BGs and an optimal design of glass composition could be effective in conferring high bioactive potential to the material despite devitrification phenomena. Since 2009, the multicomponent 47.5B BG, with composition 47.5 SiO\({}_{2}\)-2.5F\({}_{2}\)O\({}_{5}\)-20 CaO-10 MgO-10 Na\({}_{2}\)O-10K\({}_{2}\)O (mol. %), has been highly appreciated for its wide workability window, thus allowing the production of highly densified structures while preserving its amorphous nature and excellent apatite-forming ability [32]. In addition, more recent studies proved that its bioactive potential can be preserved also at higher sintering temperatures, despite the development of crystalline phases [16; 20]. In a recent previous study, the effects of the sintering temperature on the overall microstructural properties of 47.5B-based scaffolds produced by the foam replication technique were determined as a function of the sintering temperature by combining mathematical modeling and experimental measurements of intrinsic permeability supported by tomographic characterization [33]. With the present study, the authors aim at completing the characterization of 47.5B-based scaffolds. In particular, the behavior of 47.5B BG upon thermal treatment was investigated in detail by means of isothermal differential thermal analysis (DTA) and hot-stage microscopy (HSM) measurements. The sintering process was simulated at six different temperatures, defined between 600 \({}^{\circ}\)C and 850 \({}^{\circ}\)C and corresponding respectively to the beginning of viscous flow phenomena and the end of crystallization of the glass, as previously reported [34]. Development of crystalline phases was assessed by XRD analyses and scaffolds produced by foam replica method underwent compressive tests in order to investigate the effect of the different thermal treatment on scaffold mechanical strength.