2022 Volume 22 Issue 2 Pages 21-28
Purpose: Rhodium-plated orthodontic wires are utilized for esthetic purposes. However, the decline in the corrosion resistance of rhodium (Rh)-plated nickel-titanium (Ni-Ti) wires has been a concern. In this study, Rh-plated nickel (Ni) wires and Ni-Ti orthodontic wires were prepared via electroplating, and the microstructures of the plated layers were investigated. Methods: The surface microstructure and thickness of the plated layers were analyzed using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). The corrosion resistance was tested by immersion into an acidic solution in addition to applying plated and non-plated wires to the oral cavity or subcutaneous tissues of animals. Ni dissolution and distribution into the contacted oral mucosa and implanted subcutaneous tissues were visualized using synchrotron radiated X-ray fluorescence (SR-XRF). Results: In Ni wires, the 1-2 µm plated rhodium layer was formed in contact with the substrate. This layer suppressed the corrosion of the Ni wire in acidic solutions. However, in Ni-Ti wires, defects in the plated layer were observed and subsequently, Ni and Ti dissolution in the acidic solution was enhanced because of galvanic corrosion. In Ni-Ti wires, Ni and Ti could not be detected in the tissues because of their low contents. Conclusion: The plated Rh layer was homogeneously formed in contact with the substrate in Ni wire compared to the plated layer on Ni-Ti wires. For the clinical use in orthodontic treatment, it might be suggested that Rh-plated Ni-Ti wires should be more stable in oral cavity without ion dissolution, however, the effect of dissolved Ni and Ti from Rh-plated Ni-Ti wires would not be a concern due to their extremely lower contents.
Nickel-titanium (Ni-Ti) orthodontic wires provide a low and constant force, providing a long treatment period because of their superelastic properties [ 1 ]. Thus, they play a crucial role in orthodontic treatment. However, the Ni-Ti superelastic alloy contains approximately 50 mol% of Ni, leading to concerns regarding corrosion and dissolution of Ni ions in the oral environment [ 2 ]. Ni poses toxicity and can cause carcinogenesis, and allergies [ 3 , 4 , 5 ], suggesting that the suppression of Ni ion dissolution in orthodontic appliances is vital.
Coatings with various polymers, e.g., epoxy resin, polyether ether ketone (PEEK), and polytetrafluoroethylene (PTFE), and rhodium (Rh) have been applied in recent years to improve the aesthetics of orthodontic appliances [ 6 , 7 ] and their corrosion resistance. Rh is a noble metal of the platinum group bearing high corrosion resistance and high visible-light reflection. Rh coating on orthodontic wires is achieved by the electroplating method, which provides a thin and homogeneous layer on the surface. Improvements in the mechanical and chemical properties are expected for Rh plating. The effects of Rh plating on the mechanical properties of orthodontic wires have been previously reported; Compared to the unplated wire, Rh-plated wire exhibited slightly higher friction force and was less affected by the flexural properties [ 8 , 9 ]. A reduction in corrosion resistance and enhancement of Ni dissolution by Rh plating for Ni-Ti wires have been reported by in vitro testing for artificial saliva [ 10 , 11 , 12 ], fluoride-containing artificial saliva [ 13 ], and fluoride-containing solutions or gels [ 14 , 15 ]. Katić et al. suggested that the galvanic reaction between Ni-Ti substrate and Rh caused by the defect of the plating layer leads to a reduction of corrosion resistance of Rh plating [ 10 ].
In this study, a thick Rh layer was electroplated on Ni-Ti orthodontic wires, and the corrosion behavior in an acidic solution was estimated. Ni wires were also plated, and the corrosion behavior was compared to that of Ni-Ti wires. In addition, Rh-plated and non-plated Ni and Ni-Ti wires were placed in the oral cavity and implanted in the subcutaneous layer to predict the in vivo corrosion behavior. Finally, the dissolved ion distribution in the surrounding tissues of the wires was measured using synchrotron radiation X-ray fluorescence (SR-XRF).
Ni-Ti orthodontic wires and pure Ni wires ( n = 4, each) were used in this study. Ni-Ti orthodontic wires with cross-sectional dimensions of 0.406 mm × 0.559 mm (0.016 × 0.022 inches, Tomy International, Tokyo, Japan) and pure Ni wires with a round cross-section (99.9%, 1.0 mm diameter, Nilaco Corp., Tokyo, Japan) were cut into a constant length and divided into two groups. The first group was applied for the following experiments without plating, while the other was utilized after Rh plating. For Rh plating, first, a thin gold plating was applied to enhance the adhesion of the plated layer. Then, Rh plating was performed using an Rh-containing sulfuric acid solution. Rh content was between 1.5 to 2.5 g/L, plating current was 1 to 3 A/dm2, and the temperature was in the range of 40 to 50˚C. The surface structures of the wires before and after Rh plating were observed by scanning electron microscope (SEM, TM4000Plus, Hitachi High-Technologies Corp., Tokyo, Japan). In addition, the elemental distribution of the wire surface was analyzed using energy-dispersive X-ray spectroscopy (EDX, Quantax 75, Bruker Corp., Billerica, MA, USA) combined with SEM.
Ion dissolution testA 3% lactic acid (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) and 0.1 mol/L hydrochloric acid (HCl) solutions (Kanto Chemical Co., Inc., Tokyo, Japan) were prepared for the ion dissolution tests. Ni wires (15 mm in length), with and without Rh plating, were immersed in 10 mL of 3% lactic acid. Ni-Ti wires (15 mm in length) with and without Rh plating were also immersed in 0.1 mol/L hydrochloric acid solution (10 mL). Immersion was carried out in sealed plastic containers and kept at 37˚C for 1-2 weeks. The wires were carefully removed after immersion for certain periods and then subjected to Ni and Ti content analyses using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Spectro Arcos, Hitachi High-technologies, Tokyo, Japan). A 10 ppm multi-element solution (XSTC-622, Seishin Trading, Kobe, Japan) was employed as the standard. The ion dissolution measurements were statistically analyzed using GraphPad Prism Version 9.3.0 (GraphPad Software) and unpaired t -test with Welch’s correction. The P -value less than 0.05 considered to be statistically significant.
In vivo measurement of ion dissolution in rat oral mucosae and subcutaneous tissuesTwelve-week-old female Wistar rats were studied in this investigation. All animals were fed a powdered diet (CE-2; Clea Japan, Tokyo, Japan) and water ad libitum . All animal experimental procedures were approved by the Institutional Animal Care and Use Committee (authorization number: 0150174A) and performed in accordance with the Animal Care Standards of Tokyo Medical and Dental University. All experiments were performed under anesthesia using an intraperitoneal injection of chloral hydrate (300 mg/kg). The wires, half of them being Rh-plated, were cut into 10-mm long and washed through ultrasonication in 70% ethanol, followed by air seasoning. After anesthetizing rats, each wire was sutured on the buccal oral mucosa of both sides of the oral cavity using a silk thread to provide close contact with the mucous membrane. Rh-coated wires were set to the right side, while pure wires were sutured to the left side. In addition, each wire was implanted into loose subcutaneous connective tissue in the dorsal region. After two weeks, following deep anesthetizing and sacrificing of animals, the bilateral buccal mucosa and tissue of the dorsal area (2 × 12 mm) were extracted. Frozen 20-µm-thick sections were prepared from the extracted tissue specimens and mounted on 12.5-µm-thick polyimide films (Kapton, DuPont-Toray Co., Ltd., Tokyo, Japan) for elemental distribution analysis.
SR-XRF analysis for the elemental distributionSR-XRF analysis was conducted at BL-4A, Photon Factory, High-Energy Accelerator Research Organization (KEK-PF, Tsukuba, Japan) with 12.9 keV incident X-rays by capillary focusing into a beam diameter of 20 µm and scanning sample points with 40-µm steps in the X and Y directions, analyzing them as pixels of an image. The detailed conditions of the SR-XRF measurements and data processing have been described in a previous report [ 16 ].
The surface textures of the non-plated and Rh-plated Ni and Ni-Ti wires were observed by SEM ( Fig. 1 ). Scratch marks caused by the drawing process were observed for both non-plated and Rh-plated Ni wires. The surface of the Ni wire was homogeneously covered. However, defects in the plating were found at the surface of the Ni-Ti wires, as indicated by the black arrows. SEM and elemental distribution images of the Rh-plated Ni-Ti wires were obtained by SEM/EDS ( Fig. 2 ), revealing that Rh mostly covered the surface. However, Ni and Ti derived from the substrate appeared at defects, suggesting that the Ni-Ti wire surface was not entirely coated by Rh plating.
Cross-sectional SEM and elemental distribution images of Rh-plated Ni and Ni-Ti wires were obtained (Figs. 3 and 4). Close contact between the undercoat layer of Au, the upper coat layer of Rh, and the Ni substrate can be observed ( Fig. 3 ). The thicknesses of the Au-and Rh-plated layers were estimated to be 1-2 µm. In contrast, a thick intermediate layer was found between the plated Rh (and Au) and Ni-Ti substrate ( Fig. 4 ). Oxygen was abundant in the intermediate layer, suggesting the formation of this layer by oxidation during plating. The existence of an oxide layer inhibits the adhesion of the metal substrate and the plated metal layer. In addition, Rh-plated Ni-Ti wires showed signs of defects in the plated layer.
SEM images of non-plated and Rh-plated Ni and Ni-Ti wires
SEM image and elemental distribution images of Rh, Ni, and Ti of Rh-plated Ni-Ti wire surface
SEM image and elemental distribution images of Ni, Au, and Ti of Rh-plated Ni wire cross-section
SEM image and elemental distribution images of Ni, Ti, Au, Rh, and O of Rh-plated Ni-Ti wire cross-section
Ion dissolution
Ni dissolution in 3% lactic acid from the non-plated and Rh-plated Ni wires was measured ( Table 1 ). The concentration of the dissolved Ni increased during the immersion period. Ni dissolution was significantly suppressed by approximately half (in a week, P = 0.0051) to one-third (in 2 weeks, P = 0.0001) after Rh plating. In the case of Ni-Ti wires, ion dissolution was not detected during lactic acid immersion.
Next, the Ni and Ti dissolution of non-plated and Rh-plated Ni-Ti wires was measured after exposure to 0.1 mol/L HCl ( Table 2 ). In contrast to the Ni wire, the Ni-Ti wire showed no significant difference for Ni and Ti dissolution after 1w immersion between non-coated and Rh-plated Ni-Ti wires. However, Ni and Ti dissolution of Rh-plated Ni-Ti wire showed significant increase after 2-week immersion. It is suggested that instead of acting as a barrier to suppress dissolution, Rh plating enhanced the corrosion of Ni-Ti wires.
Table 1 Released Ni from the non-plated and Rh-plated Ni wires
| Immersion period* | non-plated Ni (µmol/L) | Rh-plated Ni (µmol/L) | P -value |
|---|---|---|---|
| 1 week | 1,249 ± 44.2 | 505 ± 216 | 0.0051** |
| 2 weeks | 1,797 ± 80.6 | 599 ± 185 | 0.0001*** |
*3% lactic acid aq.
** P < 0.01
*** P < 0.001
Table 2 Released Ni and Ti from the non-plated and Rh-plated Ni-Ti wires
| Immersion period* | non-plated Ni-Ti (µmol/L) | Rh-plated Ni-Ti (µmol/L) | ||
|---|---|---|---|---|
| Ni | Ti | Ni | Ti | |
| 1 week | 6.7 ± 0.8 | 7.9 ± 0.9 | 58.0 ± 37.8 | 54.7 ± 36.6 |
| 2 weeks | 13.8 ± 1.8 | 13.4 ± 1.8 | 228.1 ± 43.3** | 207.2 ± 43.0*** |
*0.1 mol/L HCl aq.
** P < 0.01
*** P < 0.001
S and Ni distribution images of oral mucosa which were in contact with non-plated and Rh-plated Ni wires The number at the side of the color bar is the maximum characteristic X-ray counts (counts per second, CPS) of the corresponding elements. White bar = 1 mm
S and Ni distribution images of subcutaneous tissues which were implanted with non-plated and Rh-plated Ni wires The number at the side of the color bar is the maximum characteristic X-ray counts (counts per second, CPS) of the corresponding elements. White bar = 1 mm
Ion diffusion into the mucosa and subcutaneous tissues
Diffusion of dissolved Ni from Ni wires with and without Rh plating into the mucosa and subcutaneous tissues was visualized by SR-XRF. In this method, the characteristic X-rays of each element are measured at various spots. At low concentrations, the concentration of the element is roughly proportional to the intensity of its characteristic X-ray counts. Therefore, the elemental distribution can be visualized by mapping the characteristic X-ray intensities of the elements. The elemental distribution was visualized as a pseudo-color image, in which red shows a high concentration of the element while blue exhibits its low concentration. Sulfur (S) and Ni distribution images of the mucosa in contact with Ni wires, with and without Rh plating, are presented in Fig. 5 a and 5b.
The line profile of the intensity of characteristic X-ray of Ni along the red line shown in Fig. 6
The entire shape of the specimen can be observed in the S-distribution image. A clear Ni distribution originated by leaching of Ni from the wire was observed in the non-plated Ni wire contacting the mucosa. The Ni ion penetration into the contacted mucosa was measured approximately 1 mm deep from the mucosal surface. However, Ni distribution was not detected in the Rh-plated Ni wire contacting mucosa. In the case of subcutaneous implantation, the dissolved Ni ions accumulated in the surrounding tissue, leading to a clear concentric distribution ( Fig. 6 a). Furthermore, a slight Ni distribution was also observed for the Rh-plated Ni wire. The line profile of the Ni concentration along the red line, that was shown in Fig. 6 , was estimated ( Fig. 7 ). The X-axis is the lateral position along the red line in Fig. 6 , while the Y-axis is the intensity of the Ni characteristic X-ray at each spot. The Ni content of the Rh-plated Ni wire in implanted tissue was more than 10 times lower than that of the non-plated Ni wire. In the case of Ni-Ti wires, no indication of Ni and Ti distribution could be found by SR-XRF because of the low concentration of dissolved ions.
Ni-Ti superelastic alloys have unique mechanical properties, enabling them as crucial materials for orthodontic wires production. However, as the visible-light reflection of Ni-Ti is relatively low, Rh plating has been utilized to achieve sufficient reflection. Rhodium shows high reflection for visible light and high corrosion resistance, the electroplated surface of Rh has been jewelry, mirror, and others [ 17 ]. Therefore, Rh plating on Ni-Ti wire exhibits bright visible-light reflection, enhancing the esthetics of wires. Moreover, compared to the epoxy-coated wires, a slighter color change with a coffee-stained solution was reported for the Rh-plated wire [ 18 ]. Therefore, Rh plating is a promising coating method for improving the esthetics of orthodontic wires.
As shown in Table 1 , Rh plating effectively suppressed Ni dissolution of the Ni wires. The plated Rh layer was uniformly coated on the wire surface, as shown in Fig. 1 , with the plated layer being in close contact with the Ni substrate, as demonstrated in Fig. 3 . Therefore, Rh plating can provide a physical barrier to corrosion and suppress Ni dissolution. The thickness of the Rh-plated commercial Ni-Ti wire was too small to be observed using SEM [ 19 ]. Hence, compared to commercial sources, thicker (1-2 µm) and more uniform Rh plating was produced for this study. Defects in the Rh-plated layer have been reported for Rh-plated Ni-Ti wires [ 10 ]. As shown in Fig. 2 , similar defects were observed in the Rh-plated Ni-Ti wire in this study. Strong adhesion between the plated layer and metal substrate is required to obtain a stable coating layer. Figure 3 exhibits close contact between the Rh/Au layers and the Ni substrate for Rh-plated Ni wires, suggesting that Rh plating on such wires can be carried homogeneously without defect formation. In the case of Ni-Ti wires, a thick oxide layer was noticed between the Rh/Au layers and the Ni-Ti substrate, as shown in Fig. 4 . The oxide layer on the Ni-Ti wire is believed to be formed when the electroplating was immersed in the sulfate solution, an oxidizing acid used as the electrolyte for Rh plating. This oxide inhibits stable contact between the Rh/Au plating and the Ni-Ti substrate, resulting in defect formation and exposed Ni-Ti substrate. As Rh is nobler than Ni and Ti, local galvanic cells are formed in this system, and galvanic corrosion enhances the dissolution of Ni and Ti ions [ 14 ]. As shown in Table 2 , similar concentrations of Ni and Ti are released into the immersed HCl solution from the non-plated and Rh-plated Ni-Ti wires. The Ni-Ti super-elastic alloy contains approximately 54% Ni, 44% Ti, and 2% or less Co, suggesting approximately a 1:1 atomic ratio of Ni and Ti. Therefore, Ni and Ti dissolutions originated from the general corrosion of the Ni-Ti wire and are not a consequence of selective dissolution.
As shown in Figs. 5 and 6, the distribution of released Ni from the Ni wires in the contacted mucosa or surrounding tissues was visualized by SR-XRF, a highly sensitive elemental analysis method. These images show the distribution of the characteristic X-rays of Ni, which is linearly correlated with the Ni content. The maximum count of the characteristic X-rays of Ni in the mucosa that has been in contact with non-plated Ni wire ( Fig. 5 a) was approximately 1,000 cps. In contrast, that in subcutaneous tissue ( Fig. 6 a) was nearly 2,000 cps or more, suggesting lower Ni content in the mucosa than in the wire-implanted surrounding tissue. The lower Ni concentration in the contacted mucosa is believed to be due to the dilution of Ni ions by saliva in the oral cavity. In both cases, the Rh-plated Ni wire showed a lower Ni content than the non-plated Ni wire, revealing that the in vivo Ni dissolution could be efficiently suppressed by Rh plating.
Rh-plated Ni-Ti wires showed enhanced Ni dissolution after immersion in acidic solution. However, the Ni contents were lower than those of the non-plated and Rh-plated Ni wires, even though Ni-Ti wires were exposed to more acidic solutions than Ni wires. In the case of Ni-Ti wires, Ni dissolution in the oral cavity of the subcutaneous tissues was too minimal to be detected by SR-XRF, although this characterization method is highly sensitive. It is noteworthy that, in contrast to the procedure used in this study in which the wires were sutured to the oral mucosa for close contact, the actual orthodontic wires are not typically designed to provide close contact with the oral mucosa. Therefore, our results suggest that the effect of dissolved Ni from Rh-plated Ni-Ti wires would not be a concern in clinical applications.
In this study, thick Rh plating was applied to pure Ni and Ni-Ti orthodontic wires. The plated Rh layer was homogeneously formed and was in close contact with the substrate in the case of the Ni wire, whereas the plated layer on the Ni-Ti wires had defects, leading to the exposure of the wire substrate. Subsequently, Ni and Ti dissolution in the acidic solution was enhanced because of galvanic corrosion between the plated Rh and Ni-Ti substrates. Ni dissolution and distribution into the contacted oral mucosa and implanted subcutaneous tissues were visualized using synchrotron radiated X-ray fluorescence. The leaching of the Ni content to the tissues was suppressed by the Rh plating of the Ni wires. In the case of Ni-Ti wires, Ni and Ti were not detected in the tissues using these methods because of their extremely low quantity.
Acknowledgments
We wish to thank Dr. Toshihiro Imamura for his helpful support for animal study.
Supported in part from the Grants-in-Aid for Scientific Research in Japan (KAKEN) (grant number 18K09853).
Conflicts of Interest
The authors declare no conflicts of interest.
Author contributions
EC contributed to the design of the study, data acquisition, data interpretation, and drafted the manuscript. ZK contributed to the design of the study and animal handling. IY contributed to draft the manuscript. HK contributed to resources and methodology. NO contributed to resources and methodology. TO conceived the study. MU participated in manuscript design and formatting. All authors read and approved the final manuscript.
