## 4 Discussion  
The atomic ratio of silica in all the samples decreased after 7 days of soaking in SBF, but the atomic ratio of titanium in all samples increased greatly after 7 days due to a large amount of silica dissolution [20] and hardly any dissolution of titanium oxide. The proposed materials were silica-based bioceramics, and the O-Si-O bonds were breaking off due to OH\({}^{-}\) groups, which were formed by the ionic exchange between Ca\({}^{2+}\) ions in the material and H\({}^{+}\) and H\({}_{3}\)O\({}^{+}\) ions coming from the surrounding environment. This caused the silica network to break and the soluble silica to be released from the material and directly into the SBF solution [21]. However, titanium oxide is not a water-soluble material, but its hydrophilicity can help the titanium oxide nanoparticle dispersion and suspension in the water solution [22]. The proposed materials were bioceramics composed of silica and titanium oxide with various concentrations of titanium oxide. After soaking in the SBF solution, the soluble silica was released from the material and made the composites decompose to release titanium oxide nanoparticles into the SBF solution. This might mean that these bioceramics can be controlled by tuning the Si/Ti ratio to modify the dissolution rate of samples. However, the titanium precursor (TBDT) was rapidly formed titanium oxide due to the hydrolysis of TBOT [23], so the reaction environment of the solution used in the sol-gel process was not stable. In this study, the solution used in the sol-gel process was controlled in an acid environment to fabricate nanoscale particles [24], but titanium oxide formed by hydrolysis of TBOT was slightly dissolved in the solution. As evident in Figure 1a, the added amount of titanium oxide caused the formation of coarse micro-structures due to the slightly basic solution. A large amount of titanium oxide due to the hydrolysis of TBOT not only changed the reaction environment but also formed a lot of TiO\({}_{2}\) precipitate in the sol-gel process. As in Table 1, the much lower Si/Ti ratio of samples at 0 days showed that only a little titanium oxide could be formed inside silica networks. After 7 days of soaking in SBF, the atomic ratio of calcium was an important factor for bioceramics as a bioactive bone graft material. The calcium atomic ratio of all samples was increased, besides BG_Ti100, based on Table 1. This means that BG_Ti100 became the main calcium supplier to the  
Figure 6: The cell toxicity test of MG63 cells incubating for 1 day in DMEM, DMSO, PBS, the extraction of bioglass, and the extractions of bioceramics composed of silica and titanium oxide with various concentrations of titanium oxide. (* means \(p\) value \(<\)0.05, ** means \(p\) value \(<\)0.01, and *** means \(p\) value \(<\) 0.001). environment but was not superior for calcium salt being deposited onto the surface. It also can be seen in Figure 4c that the calcium concentration of BG_Ti100 measured by ICP-MS suddenly rose high after 1 day. A lot of calcium ions were released into the environment in vitro, and this sample displayed a weak ability to deposit HA, as shown in Figure 2b. However, the samples of BG_Ti25, BG_Ti50, and BG_Ti75 have several advantages in the in vitro tests, such as the increase of both the atomic ratios of calcium and phosphate after 7 days soaking in SBF, increased deposition on the sample surfaces to make bigger particle sizes (shown in Figure 1b), increased formation of HA crystal structures (shown in Figure 2b), keeping a stable pH value of the soaking environment (shown in Figure 3), and released calcium ions under a stable state (shown in Figure 4c). The cell morphology of MG63, which incubated in the extraction of BG_Ti25 for 1 day, showed obvious cell deformation (shown in Figure 5e) and some precipitation on the bottom of culture plate.