## 4 Discussion  
This study was an in vitro study that used a plasma source instead of the existing light source and performed teeth bleaching with DW instead of an oxidizing bleaching agent. A significant advantage of the plasma bleaching method is that it does not require the use of a high-temperature light source. Moreover, it exhibits a substantial bleaching effect within a short duration even when a bleaching gel containing a low concentration of HP is used. Furthermore, it does not induce any significant side effects on the dental hard tissues and the soft tissues surrounding the teeth at this level [20]. These findings  
Figure 4: The mean VHN for the enamel treated with NCP + DW or DW alone. Figure 5: The mean of the quantitative mineral elements present in the enamel after tooth bleaching. indicate that the plasma tooth bleaching method may be a good alternative to the existing tooth bleaching methods. \(\bullet\)OH released from HP-containing bleaching gels plays a crucial role in the tooth whitening mechanism. The electrons in the plasma facilitate the effective breakdown of the hydrogen bonds of HP, thereby producing a large amount of \(\bullet\)OH [21]. Thus, a better whitening effect can be expected in a shorter duration if a plasma device more powerful than existing devices is developed. Ordinary DW may be a suitable agent for tooth whitening if bleaching gels can be replaced with DW without HP, and \(\bullet\)OH can be produced in DW at a level similar to that of a bleaching gel containing HP at low concentrations. The present study aimed to identify alternatives to peroxide-based bleaching agents, which are associated with several drawbacks and limitations. Bleaching with NCP and DW has been used to address this problem. The efficacy and safety of NCP and DW, and whether this method can be employed efficiently and safely, with no adverse effects on dental hard tissues, was evaluated in the present study. Performing tooth bleaching using NCP with DW, a non-peroxide method, for 30 min led to a significant improvement in the appearance of the tooth. Furthermore, this method also yielded higher brightness values than the other methods for up to 6 months. Thus, Figure 2 demonstrates that the combination of DW and NCP may be an effective and accelerative method for tooth bleaching, compared with existing procedures. The findings of the present study are consistent with those of earlier experimental studies on tooth bleaching using non-peroxide materials. [PERSON] et al. [22] reported that the tooth whitening effect achieved after 20 min via direct-current atmospheric-pressure cold-air plasma was superior to that of the effect achieved with the existing HP gel treatment. However, these results did not reveal the effect of the chemical composition of saline on the mechanism of tooth bleaching. Nevertheless, the use of pure DW in the present study ensured that uncontrolled chemical components had no effect on the results. A high concentration of the HP bleaching agent yields an excellent effect; however, high concentrations of HP are known to have adverse effects on the tooth and surrounding tissue. [PERSON] et al. [23] reported that treatment with 30% HP for 10 days yielded significantly higher efficacy than treatment with a low concentration of HP. However, several treatment sessions were required to achieve the desired results despite the use of a high concentration of the HP bleaching agent. The present study proposed the use of a new method, whose efficacy surpasses that of all previous methods, without harming dental tissue. In the case of DW and NCP treatments, the number of treatments has increased, but this represents a great advantage in that it can avoid various serious effects of HP. Several instruments and devices developed for the color matching of teeth are commercially available for use in the field of esthetic dentistry. The CIELAB color system is used most widely for matching the color of the teeth [20; 24; 25; 26].