# Sintering Behavior of a Six-Oxide Silicate Bioactive Glass for Scaffold Manufacturing
## 3 Results and Discussion  
BGs are commonly recognized to be promising materials for the production of 3D structures for bone tissue engineering applications [37]. However, the use of BG-based scaffolds is still limited in clinical practice due to the tendency of these materials to crystallize upon heating treatments, with undesired and sometimes unpredictable effects on their macroporous structure and bioactive properties in contact with body fluids. One of the main concerns related to the crystallization of BGs upon sintering is in fact the partial loss in the bioactive potential, as the ion-exchange mechanisms involved in the process of deposition of the hydroxyapatite reaction layer are mainly attributed to the amorphous glassy matrix, while crystalline species are usually characterized by lower reaction kinetics in aqueous environment. Despite this, producing dense glass-ceramic scaffolds is sometimes necessary to achieve better mechanical performances and obtain structures able to support the physiological healing process of bone tissue while maintaining mechanical integrity over the whole duration of the therapeutic treatment. As a result, when designing a BG-based scaffold for bone regeneration, achieving the right balance between mechanical, morphological and bioactive properties has to be considered as a primary need. The silicate 47.5B BG, based on the six oxides system SiO\({}_{2}\)-P\({}_{2}\)O\({}_{5}\)-CaO-MgO-Na\({}_{2}\)O-K\({}_{2}\)O, was greatly appreciated in the past for its wide workability window, which makes it possible to obtain highly densified structures upon sintering in a wide range of temperatures without inhibiting the reactivity of the system in contact with body fluids. In a previous study, it was demonstrated that the activation energy for viscous flow in 47.5B glass, assessed by the Kissinger and Matusita-Sakka equations, is lower than that required for crystallization, thus proving the potentiality of this material to achieve significant densification levels before the beginning of devitrification processes upon thermal treatments above \(T_{g}\)[34]. In addition, other studies showed that 47.5B composition is able to retain its exceptional apatite-forming ability even at temperatures much higher than the crystallization onset of the system [20]. This can be partly explained by the fact that 47.5B composition, designed by [PERSON] and coworkers in 2009 [32], exhibits a high Ca/P ratio (just like [PERSON]'s 4555 Bioglass(r)[4]), which ensures good bioactivity in physiological environment even in spite of devitrification phenomena upon thermal treatments. As a result, in recent years, 47.5B glass has been extensively employed in the production of highly densified and bioactive porous scaffolds for bone tissue engineering applications by different manufacturing processes [16; 20; 21; 36; 38]. In the present work, thermal analyses were performed to simulate the pressure-less sintering process of 47.5B BG particles at different temperatures, in order to investigate the evolution of the glassy system upon heating; moreover, the effect of sintering temperature on glass devitrification and scaffolds mechanical properties was investigated by XRD and compression mechanical test. Figure 1 shows the graphical output of DTA measurements, while Table 2 summarizes the characteristic temperatures of the six systems analyzed.