## 3 Results
### Glass-Ceramic Derivatives
#### 3.2.3 In Vitro Bioactivity Assessment  
Figure 4 shows the modifications that occurred on the surface of TT-CEL2 after soaking for seven days in SBF. A compact layer of hydroxyapatite (HA), which exhibits a typical \"cauliflower\" morphology constituted by globular agglomerates, is clearly visible in Figure 4a. Figure 4b reveals that the HA agglomerates are made up of nano-sized crystals, which were reported to typically characterize the HA layer formed on the surface of bioactive glasses [35]. The compositional analysis (Figure 4c) showed a Ca-to-P molar ratio of 1.66, which closely approaches the stoichiometric value (1.67) of HA. The presence of silver (Ag) peaks in the EDS pattern is due to the metal coating necessary for the analysis. A weak peak corresponding to silicon (Si) can be attributed to the presence of residual silica gel, which formed during the early stages of the bioactive process as described by [PERSON] and co-workers [36].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \(\sigma_{b}\) **(MPa)** & \(E\) **(GPa)** & \(HV\) **(GPa)** & \(K_{IC}\) **(MPa\(\cdot\)m\({}^{1/2}\))** \\ \hline TT-CEL2 & 65.0 \(\pm\) 21.0 & 85.0 \(\pm\) 2.0 & 7.4 \(\pm\) 0.8 & 2.40 \(\pm\) 0.25 \\ TT-FaGC & 70.0 \(\pm\) 26.0 & 55.0 \(\pm\) 2.0 & 8.8 \(\pm\) 1.3 & 2.19 \(\pm\) 0.20 \\ TT-SCNA & 125.0 \(\pm\) 24.0 & 98.0 \(\pm\) 3.0 & 11.6 \(\pm\) 1.2 & 2.98 \(\pm\) 0.40 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mechanical properties of the sintered samples.  
Figure 3: XRD patterns of the sintered materials: (**a**) TT-CEL2; (**b**) TT-FaGC; and (**c**) TT-SCNA.  
XRD investigations carried out on the sample surface further confirmed the presence of HA, as reported in Figure 5. The major peaks of HA were identified and indexed in the picture, whereas the peaks corresponding to the crystalline phases of TT-CEL2 lying underneath are no longer visible.  
Figure 6a shows the surface of TT-FaGC after soaking for seven days in SBF; the corresponding EDS pattern is reported in Figure 6b. The sample surface is still coated by a silica-gel layer: its presence is revealed by the compositional analysis (an intense peak for Si is visible in Figure 6b) and its typical \"clump-like\" morphology is due to the condensation of Si-OH groups (stage 3 of the bioactivity mechanism [36; 37]). Small globular agglomerates, rich in Ca and P, are visible on the top of the silica-rich layer: their presence demonstrates that the CaO-P\({}_{2}\)O\({}_{5}\)-rich film formed on the top of the gel layer begins to crystallize (stages 4 and 5 of the bioactive process [38]). It has to be noticed that the first stage of the bioactive process, i.e., the ion-exchange of cationic species from the glass and H\({}^{+}\) from the solution, is the rate-determining step for the whole mechanism of bioactivity. On the basis of this consideration, it is possible to give a qualitative explanation of the different bioactive behaviour of TT-CEL2 and TT-FaGC. The amount of residual glassy phase in TT-FaGC is expected to be low, as its parent material (FaGC) was a glass-ceramic and during the thermal treatment, the development of canasite and additional FA occurred (see the Section 3.2.1). Therefore, the limited amount of amorphous phase in TT-FaGC slows down the rate of the bioactive process and, thus, the formation of HA in comparison with TT-CEL2.  
TT-SCNA exhibited no apatite-forming ability because of its high content of SiO\({}_{2}\) which makes it, as expected, a bioinert material [37]. This behaviour was already studied in a previous work in which null mass loss was detected after prolonged immersion (one month) of thermally-treated SCNA-derived porous implants in SBF [39].  
Figure 4: In vitro bioactivity tests carried out on TT-CEL2 samples: (**a,b**) sample surface after soaking for seven days in simulated body fluid (SBF) and (**c**) corresponding energy dispersive spectroscopy (EDS) pattern.  
Figure 5: XRD pattern of TT-CEL2 after soaking for seven days in SBF.