# Sintering Behavior of a Six-Oxide Silicate Bioactive Glass for Scaffold Manufacturing
## 3 Results and Discussion
The sample shrinkage was quantified in terms of height percentage as a function of both time and temperature, as depicted in Figure 3; the values are summarized in Table 4.  
As can be seen, it was not possible to observe a clear trend depending on the temperature increase since comparable values were registered for all the samples, regardless of the sintering conditions. Interestingly, the specimens sintered above 700 \({}^{\circ}\)C seemed to shrink less than all the others, with a progressive height reduction from 27 to 21%, with a minimum value of 18% registered at 750 \({}^{\circ}\)C in correspondence of the crystallization temperature of the glass. This could be likely attributed to microstructural adjustments resulting from the devitrification of the amorphous matrix, which becomes progressively stiffer due to the development of crystalline species, thus inhibiting densification phenomena. This is consistent with previous results reported by [PERSON] et al. about the sinter-crystallization of 45S5 Bioglass(r)-derived glass-ceramic scaffolds [41]. A similar trend was also observed by [PERSON] et al. [42] in borosilicate, borophosphate and phosphate glasses for bone regeneration. The study, indeed, reported that the density of the samples decreased as a result of the increase in sintering temperature due to the inhibition of viscous flow of the particles caused by crystallization phenomena at the material surface, thus reducing the densification ability of the material. This was thought to have a clear effect on the mechanical performance of foam-replicated 47.5B-based scaffolds sintered at the same temperatures; the physical and structural parameters of such scaffolds are also collected in Table 4.  
As can be seen, all the scaffolds examined in the present work exhibited compressive strength within the typical physiological ranges of human trabecular bone, defined between 0.1 and 16.0 MPa [37]. The total porosity of all samples is also within the typical range recommended for bone tissue engineering scaffolds (\(>\)0.50 [35]). \begin{table}
\begin{tabular}{c c c c} \hline
**Ref. Code** & **Compound Name** & **Chemical Formula** & **Crystal System** \\ \hline
01-075-1686 & Combeite & Na\({}_{2}\)Ca\({}_{2}\)(Si\({}_{3}\)O\({}_{9}\)) & Rombohedral \\
00-002-0455 & Tremolite & CaMg\({}_{3}\)(SiO\({}_{4}\))\({}_{3}\) & Monoclinic \\
01-075-1332 & Sodium-calcium silicate & Na\({}_{15.6}\)Ca\({}_{3.84}\)(Si\({}_{12}\)O\({}_{36}\)) & Cubic \\ \hline \end{tabular}
\end{table} 
Table 3: Crystalline phases detected in 47.5B samples sintered at 700, 750, 800 and 850 \({}^{\circ}\)C.  
In particular, while in Figure 4a, corresponding to the scaffold sintered at 600\({}^{\circ}\)C, adjacent glass particles are clearly delimited by visible borders, by increasing the sintering temperature, contact points between adjacent particles increased in number and sintering necks became gradually thicker, reducing progressively the interstitial voids (Figure 4b-d). As a result, the surface of scaffolds sintered between 650 and 750 \({}^{\circ}\)C appeared almost smooth and continuous, while the roughness observed in samples treated at 800 and 850 \({}^{\circ}\)C (Figure 4a,e,f) was related to the growing of crystalline phases on the scaffold's surface, as a result of the devitrification process of the glass. As expected, an increase in the compressive strength was observed as the sintering temperature increases, which was attributable both to viscous flow sintering and to the concomitant development of crystalline species within the amorphous matrix. Interestingly, fully amorphous 3D structures treated at 600\({}^{\circ}\)C revealed a compressive strength value comparable to that of 4555 glass-ceramic scaffolds produced by the same technique (\(\sigma_{\rm max}\) = 0.3-0.4 MPa) [14]. The increase in compressive strength observed in the temperature range of 600-750 \({}^{\circ}\)C followed a highly linear trend with R\({}^{2}\) coefficient equal to 0.9986.