# Titanium Nitride Plating Reduces Nickel Ion Release from Orthodontic Wire
## 4 Discussion
Such uniform corrosion is known to occur when metal is immersed in HCl [29], and this was confirmed in the present study. Pitting corrosion tends to occur in TiN with a coating thickness \(<\)6 \(\upmu\)m [30]. In the present study, the TiN coating was 0.3 \(\upmu\)m thick, which was intended to be used for orthodontic treatment, so the pitting corrosion may have occurred easily. Preliminary experiments showed that the peeling of TiN coating gradually became noticeable under the stereomicroscope when immersed in HCl for more than 30 min, so the immersion time was set to 30 min in this study. The surface roughness of both the uncoated and the TiN-coated wire immersed in HCl was increased significantly compared to that of the non-immersed samples, suggesting that the wire surface was corroded by HCl, confirming the results of the microscopic analyses. Furthermore, the surface roughness of the TiN-coated wire was greater than that of the uncoated wire despite the reduced amount of metal ion elution. This may have been due to the difference in corrosion patterns between the TiN-coated and uncoated wires, as revealed by the stereomicroscopy and SEM analyses. The uniform corrosion of the uncoated wire was seen over its entire surface, but the pitting corrosion of TiN-coated wire induced by HCl in this study was randomly scattered, and the SEM images showed small fragments of coating material that seemed to have been peeled off around the pores, which may have caused an increase in the surface roughness. The surface roughness (Ra) measured in this study is a two-dimensional measurement, so a three-dimensional measurement of surface roughness (Sa) may be useful for further detailed analyses [31]. Microscopic examination indicated that the surface condition of the TiN-coated SS and NiTi wire was not altered by immersion in lactic acid and there was no change in the surface roughness, suggesting no corrosion by lactic acid. On the other hand, for the uncoated SS and NiTi wire, although microscopy revealed no changes in the surface condition, the surface roughness was increased by immersion in lactic acid, suggesting the possibility of corrosion by lactic acid. Taken together, the results outlined above indicated that pitting corrosion occurred on the surface of TiN-coated SS and NiTi wire immersed in HCl, but that the corrosion did not spread over the entire surface of the wire, so the leaching of metal ions was significantly suppressed compared to the uncoated wire. The leaching of Ni ions from the TiN-coated SS and NiTi wire immersed in lactic acid was also shown to be significantly suppressed compared to the uncoated wire. These results indicated that TiN coating inhibits acid-induced corrosion and the elution of metal ions. GERD is associated with the regurgitation of gastric acid, mainly HCl, into the oral cavity. Thus, the results presented above suggest that the use of TiN-coated wires may be effective in preventing the leaching of metal ions from orthodontic appliances by HCl in gastric acid and reduce the rates of induction and exacerbation of metal allergy in patients with GERD. In addition, _Lactobacillus acidophilus_ present in the oral cavity has been suggested to be related to the corrosion of metals mediated by lactic acid production [11]. The TiN-coated wire used in this study was resistant to corrosion by lactic acid, and is expected to be applicable to orthodontic treatment of patients with metal allergies. We analyzed the acid corrosion resistance of TiN-coated wires in vitro. The following issues should be considered in future experiments. First, it is necessary to examine how long the TiN coating can be maintained in good condition in the oral cavity during orthodontic treatment. Second, the TiN coating is expensive, so cost reduction must be considered for clinical use. We plan to reduce the cost by coating as many wires as possible at once.