# Titanium Nitride Plating Reduces Nickel Ion Release from Orthodontic Wire
## 4 Discussion  
As the release of metal ions from orthodontic materials in the oral environment is closely related to their corrosion, metal surfaces are coated to improve corrosion resistance [13]. A number of plating techniques and materials are available for coating such metal components, and TiN ion plating was used in this study. The Ti of TiN components forms a passivation film, which improves the surface properties of the substrate material [25; 26]. TiN ion plating results in little degradation of the mechanical properties of the substrate material because it is performed at low temperatures. Previously, we reported  
Figure 5: Comparison of surface roughness of orthodontic arch wires after immersion in HCl or lactic acid. After immersion in HCl or lactic acid for 30 min, the surface roughness (Ra) of orthodontic arch wires was assessed using a surface roughness tester: (**a**) SS wire immersed in HCl; (**b**) NiTi wire immersed in HCl; (**c**) SS wire immersed in lactic acid; (**d**) NiTi immersed in lactic acid. The statistical of differences was determined by the Tukey-Kramer HSD test. (* \(p<0.05\), **\(p<0.01\)). that TiN-coated orthodontic wire has a reduced friction coefficient, and the mechanical properties of the base material are maintained [17]. This study was performed to investigate whether the elution of metal ions can be suppressed by coating orthodontic wires with TiN film. The ICP-MS data showed that the elution of metal ions was significantly suppressed in the TiN-coated wire compared to the uncoated wire after immersion in HCl for 30 min. Similarly, the DMG test confirmed that that the amount of Ni leached from both the SS and the NiTi wire after immersion in HCl were significantly lower for the TiN-coated than for the uncoated wire. The leaching of metal ions by HCl was suppressed by TiN coating. On the other hand, with immersion in lactic acid, Ni ions were leached from the uncoated but not the TiN-coated SS and NiTi wire, suggesting that the TiN-coated SS and NiTi wire was resistant to corrosion by lactic acid. Wire immersed in lactic acid showed no positive reaction on the DMG test, which may have been because the concentration of Ni was below the limit of detection of the test (10 \(\upmu\)g/mL, 10 ppm) [27]. ICP-MS also showed that the amount of Ni ions eluted from the wire immersed in lactic acid was \(<\) 10 \(\upmu\)g/mL, suggesting that it would be difficult to detect Ni elution associated with immersion in lactic acid on the DMG test. The amount of metal ions eluted from the TiN-coated SS and NiTi wire and the variations in the data were considered to be greater because of the high concentration of HCl used in this experiment. As the HCl derived from GERD has a lower concentration than that used in this study [23] and the immersion time in GERD-derived HCl is likely to be \(<\) 30 min in the oral cavity, the amount of elution is expected to be much smaller than in this experiment. However, as the threshold for Ni allergies is 1.5 \(\upmu\)g [28], and there is a risk of onset of metal allergy when exposed to very small amounts of the metal, further experiments are required to examine the use of HCl at a concentration closer to that likely to be present in the oral cavity. Furthermore, the amount of Ni ion elution from uncoated SS wire immersed in lactic acid showed a high standard deviation. The immersion time of 30 min in lactic acid is considered to be the starting point of erosion for the SS wire used in this experiment. In some samples, the amount of Ni ion elution was small because dissolution had only just begun, and this may have resulted in the high degree of variation in the data. The stereomicroscopy and SEM analyses showed the corrosion of all the wires following immersion in HCl, but the types of corrosion differed between the TiN-coated and uncoated wires. That is, pitting corrosion occurred in the TiN-coated SS and NiTi wire, while the uncoated SS and NiTi wire showed uniform corrosion.