# Titanium Nitride Plating Reduces Nickel Ion Release from Orthodontic Wire
## 1 Introduction  
The number of patients with metal allergies has increased in recent years [1; 2]. The metals used in dentistry are considered to be among the factors responsible for metal allergies. Many orthodontic materials also contain such metals associated with allergic reactions, and therefore care is required in the orthodontic treatment of patients with metal allergies [3; 4; 5]. Some metals commonly used in dental practice, including nickel (Ni), cobalt (Co), and chromium (Cr), are known allergens for patients with metal-allergic disease [6; 7]. These metals are also present in various orthodontic materials [8]. Several studies have shown that Ni is the most common allergen associated with metal-allergic disease [2; 9; 10]. Previously, we evaluated the surface elements of orthodontic materials by X-ray fluorescence spectroscopy, and found that Ni was present in almost all the orthodontic metal materials [8]. Metal ions can be eluted from orthodontic materials due to the electrochemical corrosion caused by saliva, electrolytes in food debris, and acids produced by bacteria [11]. Therefore, orthodontic patients can be exposed to Ni from orthodontic appliances. In fact, [PERSON] et al. [12] reported significantly elevated Ni levels in the dental plaque of patients with orthodontic appliances than in that of non-orthodontic patients.  
Despite the development of many non-metal orthodontic appliances, including brackets, non-metal wires are still not available [5]. Therefore, it is necessary to improve orthodontic wires to achieve high corrosion resistance. Plating can be used to provide increased surface hardness, wear resistance, corrosion resistance, low friction, and improved esthetics, as well as to improve interactions with biological and material substrates [13]. Physicalvapor deposition (PVD) ion plating is a particularly suitable plating method to produce thin films on substrates. TiN layers fabricated by ion plating have demonstrated favorable characteristics in terms of corrosion resistance [14], wear resistance [15], hardness [16], and low friction [17], and may therefore be useful for the modification of the surfaces of orthodontic wires.  
We described the mechanical and frictional properties of TiN-coated wires previously [17], but the details of corrosion resistance have yet to be elucidated. We focused on two acids as potential sources of metal ion leaching from orthodontic wires in the oral cavity. First, as fixed orthodontic appliances induce the retention of bacterial plaque, leading to an increase in levels of caries-inducing bacteria in the oral cavity, such as _Lactobacillus_ spp. and _Streptococcus mutans_[18], and bacteria corrode orthodontic appliances [19], we focused on lactic acid, which is the main metabolite that causes low pH [20]. We also focused on gastric acid, mainly hydrochloric acid (HCl), which enters the oral cavity due to gastroesophageal reflux disease (GERD), which has a high incidence, of 10-20%, in adults [21]. In addition to acid erosion of teeth and dissolution of dental restorations [22], gastric acid has been reported to be involved in the elution of metal ions from orthodontic wires [23].  
In the present study, we evaluated the corrosion resistance of TiN-plated stainless steel (SS) and nickel titanium (NiTi) orthodontic arch wire immersed in acids by measuring the concentrations of released metal ions, by stereomicroscopic and scanning electron microscopic observation of the wire surface, and by surface roughness measurement.