*3.6. Enamel Remineralization Ability in Artificial Saliva and Mechanical Property Changes Determined by Nanoindentation*

Figures 7 and 8 show the mean hardness and elastic modulus values of the enamel surface before and after etching and during the 3 months of immersion. There was no significant difference in the hardness or elastic modulus between the groups before and immediately after etching. Phosphoric acid etching markedly decreased the hardness and elastic modulus of the enamel surfaces. The hardness and elastic modulus of the enamels of BG-coated specimens increased gradually with increasing immersion time. The recovery of the mechanical properties of a specimen immersed with a non-coated specimen was unremarkable. However, the hardness and elastic modulus of etched enamel specimens immersed with most BG-coated specimens recovered significantly compared with a non-coated specimen. Similar behavior was observed for both load conditions (10 and 100 mN).

**Figure 7.** Mean hardness values of the enamel surface before and after etching, and during the 3-month immersion period. BE, before etching; AE, after etching; 1D, 1-day immersion; 1W, 1-week immersion; 1M, 1-month immersion; 3M, 3-month immersion.

**Figure 8.** Mean elastic modulus values of the enamel surface before and after etching, and during the 3-month immersion period. BE, before etching; AE, after etching; 1D, 1-day immersion; 1W, 1-week immersion; 1M, 1-month immersion; 3M, 3-month immersion.

#### *3.7. Cytocompatibility Assays*

Figure 9 shows the absorbance at 450 nm as a function of the L929 fibroblast cell number at 36 h. No significant difference was noted in the mean fibroblast cell growth for 36 h, which indicated that the BG coatings were not cytotoxic.

**Figure 9.** Absorbance at 450 nrn as a function of the L929 fibroblast cell number at 36 h.

#### **4. Discussion**

In the present study, thin BG coating layers with a milky-white appearance were formed on mechanically polished stainless steel specimens using an EPD process with a BG suspension and this is the first study that has investigate the esthetic performance of the BG coating. Quantitative color measurements showed that the EPD process using AC at 15 V produced higher values for both the reflectance (%) in the range of 350–800 nm and *L\** (mean value: 70.42). The range of *L\** values measured for the BG coating layers in the present study (54.88 to 70.42) was greater than that (36.2 to 50.3) reported for ceramic and plastic brackets [34], although the color values (*a*\*, −0.50 to −0.41; *b*\*, 6.12 to 8.12) for the BG coating layers were similar with published values (*a*\*, −1.3 to 3.8; *b*\*, −2.9 to 11.2) [34]. The color for orthodontic appliances, such as brackets and archwires, should ideally match that of natural teeth, although natural tooth color varies according to race, gender, and age [35]. A previous study measured the CIE *L\*a\*b\** color values for the Vita Lumin Vacuum shade guide (A3.5, B1, B3, C4), which is the color selection scale most widely used in dentistry; the values ranged from 43.2 to 61.4 for *L\**, from −1.6 to 6.8 for *a*\*, and from 13.2 to 28.8 for *b*\*. The *L\** and *a*\* values from the present

study are fairly similar, although our mean *b*\* value was smaller than that of the Vita Lumin Vacuum shade guide. Thus, the BG coatings formed by the EPD process in the present study likely produced a clinically acceptable color appearance, but further research is warranted to fine-tune the color.

SEM observations showed that the thickness of the BG coating layers were similar in both specimens coated with AC and DC. However, surface-sensitive parallel-beam XRD analysis revealed that the peaks at 2θ = 43.5◦ and 51.0◦ associated with the austenite (γ-Fe) phase of the bulk substrate were observed for the BG-coated specimens when DC (DC 15 V) was used. This suggested that the BG coating layer that formed on the specimen using DC had a too-low density. On the other hand, the XRD pattern obtained for thin BG coating layer formed on the specimen using AC (AC 15 V) indicated a completely amorphous phase, even for thinner BG coating layers, which suggested that thin BG coating layers formed using AC had better quality with high density. This difference in crystallinity is because undesired electrolysis of water at the electrodes occurred with the DC, which entrapped the coating and degraded the coating quality [36]. This was partly confirmed by the nano-indentation test results, which found that the elastic modulus of the BG layers formed by AC were significantly higher than those of the BG layers formed using DC.

The frictional force between the bracket and the archwire during tooth movement is a primary issue in orthodontics. If the frictional force can be decreased, then the efficiency of tooth movement can be improved [37,38]. The frictional properties are attributed to multiple factors, such as surface roughness, hardness, elastic modulus, and the cross-sectional dimensions of the orthodontic wires and brackets [37,38]. A recent study reported that commercially available esthetic coating wire influences the frictional properties [15] and the esthetic polymer coating may increase the frictional resistance due to increased wire-binding at the edge of the bracket. The progressive load scratch test used in the present study showed that the BG-coated specimens (DC 15 V) displayed significantly higher frictional forces than the other BG-coated specimens (DC 10 V; AC 10 and 15 V), although the other BG-coated specimens produced slightly higher frictional forces compared with the non-coated stainless steel specimen. The stainless steel wire alloy generally generates lower levels of frictional resistance than coated wires [15], and the BG-coated specimens, in some conditions, displayed frictional performance that was similar to that of the non-coated stainless steel specimens. Thus, the BG coating can likely provide acceptable clinical frictional characteristics. Evaluation of specimens with thin, high elastic modulus coatings are required to fully explore this aspect.

The results of this study showed that BG-coated specimens had significant acid-neutralizing capability due to their ability to release various ions. This suggests that the BG layer may be able to mitigate enamel demineralization [39,40]. Additionally, this in vitro study demonstrated that remineralization of etched enamel was accelerated for the BG-coated specimens, which is the most important finding of the present study. Nano-indentation testing with a 100-mN load showed that the hardness recovered by 49%–60%, and elastic modulus by 77%–84%, after 3 months of immersion. In comparison, the recovery of the mechanical properties of the etched enamel surface of non-coated specimens was unremarkable: The hardness recovered by 13% and the elastic modulus by 15%, although artificial saliva contains the inorganic ions necessary for remineralization [41]. A similar trend was observed for the mechanical properties of the top surface regions measured by nano-indentation testing with a 10-mN load (ca. 1000-nm depth analysis).

BG with an amorphous structure can release multiple ions into the oral environment [22–25], which may help to prevent the demineralization of tooth surfaces surrounding brackets and enhance remineralization after bracket debonding. In the present study, XRD analysis confirmed that the EPD coating process had little influence on the crystal structure of the BG coatings, and these enhanced enamel remineralization. However, the BG coatings slightly increased the frictional force. The BG coating formed using AC had acceptable quality and a completely amorphous structure, favorable esthetic character and mechanical properties. Thus, AC may be more suitable for the EPD coating process, although there was no significant difference between the BG-coated specimens formed by DC and AC in terms of their enamel remineralization ability. EPD as a new BG coating process in the

present study can possibly produce acid-neutralizing and remineralizing capabilities of the enamel surfaces around orthodontic appliances. Further research to optimize the EPD coating conditions is warranted.
