## 4 Discussion  
In this work, three non-commercial multicomponent silicate glasses (CEL2, FaGC, SCNA) were prepared by a melting-quenching process. The corresponding glass-ceramic derivatives (TT-CEL2, TT-FaGC, TT-SCNA) were obtained via thermal treatment of the parent glasses in order to finally produce massive cylindrical or bar-shaped samples. Figure 8: Cross-sections of a bilayered implant after soaking for seven days in SBF: (**a**) TT-SCNA region and (**b**) TT-CEL2 region. Figure 9: Cross-sections of a bilayered implant after soaking for one month in SBF at different magnifications: (**a,c**) TT-SCNA region and (**b,d**) TT-CEL2 region. Due to the thermal treatment, the development of different crystalline phases occurred in the materials, as shown in Figure 3. As regards TT-CEL2 and TT-SCNA, XRD results are clearly consistent with the data from thermal analysis (Table 3). In fact, the crystalline phases assessed by XRD investigations (Figure 3a,c) have a direct correspondence with the crystallization temperatures (T\({}_{\rm x}\)) found by DTA. With regard to TT-FaGC, the situation is more complex. In fact, it has to be noticed that as-poured FaGC is a glass-ceramic material containing FA crystals. The thermal treatment of sintering induced the development of a new crystalline phase (canasite), as shown in Figure 3b. The DTA analysis showed the presence of two crystallization temperatures, which means that the crystallization of FA, too, continues from the residual glassy phase upon sintering. In general, XRD results are in good agreement with previous findings about thermally-treated CEL2-, FaGC-, and SCNA-derived materials reported elsewhere [28; 40; 41]. All the three glass-ceramic materials are expected to be highly biocompatible with human tissues, especially bone. As regards the crystalline phases of TT-CEL2, Na\({}_{2}\)Ca\({}_{2}\)Si\({}_{3}\)O\({}_{9}\) is the same crystalline phase that develops in 45S5 Bioglass(r) when thermally-treated above 600\({}^{\circ}\)C [42; 43; 44]--This commercial bioactive glass has been implanted since the early 1990s in millions of patients to regenerate bone [45]--And Ca\({}_{2}\)Mg(Si\({}_{2}\)O\({}_{7}\)) was proved effective to promote new bone tissue formation in vivo, too [46; 47]. FA, found in TT-FaGC, is a natural component of tooth enamel [48]; canasite is biocompatible, although its use for bone repair requires caution due to its tendency to fast resorption in vivo [49]. This limitation could be overcome by properly adjusting the composition of the starting glass through the addition of SiO\({}_{2}\) and AlPO\({}_{4}\), as suggested by [PERSON] et al. [50]. The single crystalline phase identified in TT-SCNA, CaSiO\({}_{3}\), has been successfully used since the 1980s for osseous repair in load-bearing bone defect sites [51; 52]. Other favourable characteristics of the materials prepared in this work include the thermal expansion coefficients that are comparable to those of the tooth structures, which typically range from \(8\times 10^{-6}\) to \(11\times 10^{-6}\)\({}^{\circ}\)C\({}^{-1}\)[53]. It has to be noticed that the density of all the produced glass-ceramic samples, too, closely approaches that of tooth hard substances (\(\sim\)2.5 g\(\cdot\)cm\({}^{-3}\)[5]). One of the major challenges in the development of novel dental materials is to produce biomaterials exhibiting mechanical properties able to match those of the tooth hard structures, i.e., dentine and enamel. Table 5 summarizes the major mechanical features of a tooth [5; 54; 55; 56; 57; 58]. In general, the tooth mechanical properties are dependent on the patient's age [57; 59; 60]; therefore, the choice of one particular material for dental applications rather than another one should ideally be done by taking into account this parameter. Natural enamel and dentine exhibit unique characteristics and, at present, an ideal material able to match all their mechanical properties (strength, stiffness, toughness, hardness) does not exist.