## 4 Discussion  
Orthodontic treatment seeks to enhance occlusal function and improve aesthetic appearance by improving the arrangement of teeth. However, the fixed orthodontic appliances used to shift teeth lead to plaque deposits that cause bacteria to grow on the enamel surface, thereby creating WSLs or caries that contradict the aesthetic purpose of orthodontic treatment [7]. While there are conservative and prosthodonic treatments that can treat WSLs, these methods can damage natural teeth; thus, it is important to prevent WSLs before they occur [4]. Previous studies have developed preventive methods by adding biomaterials to the dental resins used to attach fixed orthodontic appliances [8; 9; 10; 11]. Figure 12: Remineralization length of GaMBN-containing resins, measured using ImageJ. The same alphabet letter indicates no statistically significant difference between the groups (p < 0.05) by [PERSON]’s multiple comparison test. Error bars represent the standard deviation (n = 10). Figure 13: Remineralization points (red arrows) of the GaMBN-containing resins determined by micro-CT: (**a**) GaMBN0, (**b**) GaMB1, (**c**) GaMBN3, and (**d**) GaMBN5. This study added GaMBN to dental resin to investigate whether the antibacterial properties of Ga and remineralization effect of Ca\({}^{2+}\) and PO\({}_{4}\)[3]- can prevent WSLs. Characterization of the GaMBN-containing resins confirmed that the Ga\({}^{3+}\), Ca\({}^{2+}\), and PO\({}_{4}\)[3]- ions formed the same heterogeneous structure as the previous study [18]. Moreover, the N\({}_{2}\) adsorption-desorption isotherm results determined that the resins have uniform pore structures (type IV isotherms) as shown in the previous study [17]. Ion release tests demonstrated that ions were barely released by GaMBN0, and that ion release increased with the GaMBN concentration. In particular, the release of Ca\({}^{2+}\) ions by GaMBN5 continued to increase after the first 7 days, reaching a solution concentration of 27.2 ppm after 14 days. This result implies that the GaMBN resin can supply Ca ions for remineralization. Although the amount of Ca and PO\({}_{4}\)[3]- ions released GaMBN1 and GaMBN3 did not change after 7 days, GaMBN5 exhibited the potential to form the ion layer necessary for hydroxyapatite reformation by continuously releasing PO\({}_{4}\)[3]- ions [19]. In terms of Ga, the amount of ions released by GaMBN5 was initially 0.5 ppm in solution and increased over the first 7 days to 6.7 ppm. The microhardness of the resins increased with the GaMBN concentration, which is in agreement with a previous study [11]. This increase in microhardness occurs because GaMBN acts as a filler in the resin and its strength increases with its concentration. Moreover, there were no statistically significant differences between the bracket retention force and ARI of the GaMBN-containing resins, which has also been reported by previous studies [8; 9; 10; 11]; therefore, GaMBN-containing resins are not expected to cause clinical problems when maintaining a fixed orthodontic appliance. Human dental pulp stem cells were used to evaluate the cell viability of the resins, which resulted in stricter cell viability compared to previous studies because stem cells have higher sensitivity than other cells. The cell viability tests also demonstrated that there were no statistical differences between the GaMBN-containing resins and GaMBN0, a commercial product. The antibacterial test showed that bacterial activity on the GaMBN5 resin decreased relative to GaMBN0 after 24 and 48 h; however, the change was not statistically significant. This reduction in bacterial activity is attributed to the small amount of Ga (0.5 ppm in solution) released after 24 h. Another study has tested the antimicrobial activity of Ga released from Ga-doped MBN, and reported approximately 100 ppm Ga ion elution [13]. Thus, the amount of Ga ions released in this study was too low to result in significant antibacterial activity.