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Article

The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires

by
Kento Numazaki
1,
Masatoshi Takahashi
2,*,
Arata Ito
1,
Yukyo Takada
2 and
Itaru Mizoguchi
1
1
Division of Orthodontics and Dentofacial Orthopedics, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
2
Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
*
Author to whom correspondence should be addressed.
Metals 2024, 14(2), 213; https://doi.org/10.3390/met14020213
Submission received: 15 January 2024 / Revised: 5 February 2024 / Accepted: 7 February 2024 / Published: 9 February 2024
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Versions Notes

Abstract

:
In orthodontic treatment, to achieve efficient tooth movement, it is important to reduce the frictional force between the wire and the bracket, especially the binding friction that occurs when the angle between the wire and the bracket is large. Electroless nickel–polytetrafluoroethylene (Ni-PTFE) coating is a coating technology used to deposit PTFE particles with a low coefficient of friction on the coating surface to provide a low-friction surface for metallic materials. The purpose of this study was to investigate the effect of Ni-PTFE-coated orthodontic wires on the frictional force between brackets. The surface morphology, surface roughness, and frictional properties of Ni-PTFE-coated stainless steel wires and Ni-Ti wires were evaluated. The results demonstrate that the Ni-PTFE coating reduced the frictional force between the orthodontic wires and brackets, despite the increased surface roughness. Even when the angle between the wire and bracket was increased, assuming binding friction, the frictional force was reduced by the Ni-PTFE coating. This suggests that the friction between the wire and the bracket was suppressed by the PTFE particles deposited on the wire surface in contact with the bracket.

1. Introduction

Multibracket appliances are most commonly used for orthodontic treatment. These appliances consist of brackets fixed to the tooth surface, archwires inserted into the bracket slot, and ligature wires, which are engaged by steel or elastic ligatures. Stainless steel (SS) alloys and nickel-titanium (Ni-Ti) alloys are commonly used as archwire materials. Nickel-titanium alloys, which have shape memory and superelasticity, are used in the levelling step, the initial phase of treatment, to apply weak and sustained forces to malposed teeth with large displacements. On the other hand, stainless steel alloys, with excellent formability, stiffness, and stability, are used in the sliding and final detailing step of the tooth, allowing for fine tooth positioning. Although the multibracket appliance enables precise three-dimensional tooth movement, frictional forces between bracket slots and archwires inhibit efficient tooth movement [1]. This frictional force is determined by two factors, i.e., the friction when the wire contacts the wall or bottom of the bracket and the binding friction when the wire contacts the corners of the bracket slot [1,2].
In self-ligating brackets, developed to achieve more efficient tooth movement by reducing the frictional force, the wires are not fixed by ligature wires or elastic modules [3]. Self-ligating brackets display lower frictional force than conventional brackets when the angle with the wire is 0° [4,5], but, as the angle between the bracket and the wire increases, so too does the binding friction, and the frictional force is comparable to that of conventional brackets [6]. Since there is no significant difference in treatment time between self-ligating and conventional brackets [2], it is assumed that most of the friction that occurs clinically is binding friction, and reducing the binding friction is therefore important to reduce the frictional force.
Controlling binding friction not only increases the speed of tooth movement and shortens treatment time, but also reduces the stress on the teeth. The orthodontic force used to move teeth tends to be excessive because, in addition to the force needed to move the teeth, the force necessary to overcome the frictional force must be added [1,2]. Since the application of excessive orthodontic force causes the side effect of tooth root resorption and tooth shortening, suppression of the frictional force is highly desirable.
Orthodontic appliance studies investigating frictional forces have examined various coatings, such as titanium nitride (TiN) plating [7], diamond-like carbon (DLC) [8], and polytetrafluoroethylene (PTFE) [9]. Frictional forces between the wire and bracket are reduced by TiN and DLC coatings, albeit only slightly compared with PTFE coatings [7,8,9]. PTFE is chemically stable, harmless to humans, and heat resistant, and it has an extremely low coefficient of friction; thus, it is used as a coating for various products such as frying pans and knives [10]. Attempts have been made to coat PTFE on temporary crowns and dental prosthetics, etc., and, as a result, PTFE has demonstrated excellent impact absorption, high wear resistance, and ease of cleaning. However, PTFE also has low surface energy and does not adhere easily to metallic materials [11]. Therefore, baking at 400 °C is required to obtain strong adhesion between PTFE and metals. When a PTFE coating is applied to Ni-Ti wire, superelasticity is lost due to this high-temperature baking.
Electroless Ni-PTFE (Ni-PTFE) coatings consist of PTFE particles co-deposited on an electroless Ni coating. Electroless Ni plating has enabled composite coatings due to its excellent compatibility with various materials such as steel, ceramics, carbon, and hydrophobic polymers. In particular, electroless composite plating is dominated by the deposition of highly advanced, lubricious, and ductile Ni-P alloys, which commonly contain phosphorus as a reducing agent derivative; for Ni-PTFE coatings, this technology has been used for many years and provides low friction, non-adhesive, and anti-bacterial properties to metals as well as in valves, train rails, molds, food production, and in a wide range of industries such as medical equipment [12,13]. Ni-PTFE coating layers show excellent performance with increasing PTFE content, such as a low friction coefficient, excellent wear resistance, high contact angle, and low surface energy, etc. It has been noted that PTFE particles tend to aggregate easily, and high concentrations hinder uniform plating; therefore, a concentration of around 30 g/L is appropriate. Particles are deposited in an aqueous solution; therefore, unlike PTFE coatings, high-temperature baking is not required, and the superelasticity of the Ni-Ti wire can be maintained.
Previous coatings on wires have not solved the problems encountered in clinical applications; although TiN and DLC coatings on wires reduce the frictional force with the bracket, the frictional force at an angle of 10° between the bracket and the wire is higher than at an angle of 0°, and this has not solved the binding friction that occurs in clinical practice [14]. However, Ni-PTFE coatings have the potential to realize clinically viable low-friction wires, as they have even lower friction than TiN or DLC and better adhesion to metal than pure PTFE coatings. In this study, we evaluated the frictional properties of stainless steel (SS) and Ni-Ti wires, which are the most commonly used orthodontic wires, when coated with Ni-PTFE.

2. Materials and Methods

2.1. Ni-PTFE Coating

SS wire (0.016 × 0.022 in2; Shofu, Kyoto, Japan) and Ni-Ti wire (0.016 × 0.022 in2, American Orthodontics, Sheboygan, WI, USA) were coated with Ni-PTFE (KEDC, Miyagi, Japan). Specifically, the wire surface was first degreased with an alkaline degreaser (50 °C, 10 min), rinsed with water, and then the surface was activated with acid (room temperature, 2 min). The wire was then plated with Ni-PTFE by immersion in an electroless nickel-plating solution containing PTFE particles (90 °C, 50 min). The concentration of PTFE in the plating solution was 30 g/L, and the expected thickness of the Ni-PTFE coating was 7.5 μm.

2.2. Evaluation of Surface Properties

The surface of the wire specimens was observed using a stereomicroscope (SZX7; Olympus Co., Ltd., Tokyo, Japan). An optical interferometer (TalySurf CCI HD-XL; Taylor Hobson, Leicester, UK) equipped with a 50× objective lens was used to analyze the surface topography and surface roughness of the wire samples. The resolution and measurement area were set to 2048 × 2048 pixels2 and 330 × 330 µm2, respectively. The acquired images were levelled and processed using a Gaussian filter with a cut-off value of 80 μm to obtain Sa (arithmetic mean height), Sq (root-mean-square height), Sp (maximum peak height), Sv (maximum valley depth), and Sz (distance from peak to valley). TalyMap Platinum (ver 7.4) software (proprietary TalySurf topography software) was used to analyze surface roughness. This method of measuring surface roughness is in accordance with ISO 25178 [15].

2.3. Friction Test

In order to measure the frictional force between the bracket and the wire in actual clinical conditions, the wire must be inserted into the bracket slot and ligated with a ligating tool such as an elastic module. The typical method of measuring frictional force in orthodontic appliances is to bond the bracket to the table and measure the load on the sliding wire using a creep meter [7,8]. In this study, we referred to the method of Sugisawa et al. [7], who prepared two types of bracket positions considering binding friction. Specifically, a creep meter (Re2-33005S; Yamaden, Tokyo, Japan) equipped with a 19.6 N load cell was used to measure the frictional force on the wire (Figure 1A). Brackets for maxillary canine teeth (Crystabrace7; Dentsply Sirona, Tokyo, Japan) were used with slots made of SS. The slot size was 0.018 × 0.025 in2, and no torque or tip was applied. Brackets were bonded to acrylic plates fixed to a creep meter with 4-META/MMA-TBB resin (Superbond; Sun Medical, Shiga, Japan) (Figure 1A). A 60 mm long wire was ligated to the bracket with an elastic module (TP Orthodontics, La Porte, IN, USA). The upper end of the wire was fixed to a grip attached to a load cell, while the lower end of the wire was free. The distance between the center of the grip and the bracket was standardized to 20 mm (Figure 1A). An acrylic plate with the bracket attached was fixed to the stage and moved 5.0 mm downward at a rate of 0.1 mm/s to measure the frictional force. Binding friction is the friction that occurs when the angle between the bracket and wire is large and accounts for most of the friction that occurs in clinical practice [16]. In this study, the binding friction was taken into account, and the conditions were set for an angle of 10° between the wire and bracket in addition to 0° (Figure 1B,C). The frictional force was calculated as the average value from 0.5 s after the start of the measurement to the end of the measurement.

2.4. Nickel Dissolution Test

Wire samples (15 mm) were immersed in artificial saliva (Fusayama/Meyer Artificial Saliva; Sigma–Aldrich, St. Louis, MO, USA) in a polypropylene tube (15 mL). The period of immersion was set to 7 days because peak Ni release from SS and Ni-Ti wires immersed in artificial saliva occurs on day 7 [17]. Artificial saliva had a pH of 4.9 and the following composition (g/L): 0.906 CaCl2·2H2O, 0.690 NaH2PO4·2H2O, 0.4 KCl, 0.4 NaCl, 0.005 Na2S·9H2O, and 1 urea. All experiments were performed at 37.0 ± 0.1 °C. The concentration of Ni released from the wire samples into the artificial saliva was measured using triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS) (ICP-QQQQ-Agilent 8800; Agilent Technologies, Santa Clara, CA, USA).

2.5. Statistical Analyses

Student’s t-test was used to calculate the mean and standard deviation of surface roughness (Sa, Sq, Sp, Sv, and Sz) from five different measurements, the frictional force from five measurements for each wire–bracket combination, and the Ni elution from four measurements for each wire. Statistical significance was set at p < 0.05.

3. Results

3.1. Surface Texture

First, the appearance of the SS and Ni-Ti wires was observed by stereomicroscopy (Figure 2). The SS wire changed to a brighter color, and the Ni-Ti wire changed to a darker color after Ni-PTFE plating. No pitting corrosion or wire delamination was observed on the wire surface for the Ni-PTFE-coated wires. Next, the surface morphology of the Ni-PTFE-coated wires was observed via optical interferometry (Figure 3). Clumps of PTFE particles were homogeneously distributed on the surface of the Ni-PTFE-coated SS wires. On the other hand, the number of clumps of PTFE particles was smaller on the Ni-PTFE-coated Ni-Ti wires than on the Ni-PTFE-coated SS wires, and they were sparsely distributed.

3.2. Surface Roughness

The surface roughness of a wire was measured using an optical interferometer (Figure 4). The values of Sa, Sq, Sp, Sv, and Sz on the SS wire surface were significantly increased by Ni-PTFE coating by 1.4, 1.6, 6.8, 1.6, and 3.7 times, respectively. On the other hand, the values of Sa, Sq, and Sp for Ni-Ti wires were significantly increased by 3.4, 2.9, and 2.5 times, respectively, while Sv and Sz were not significantly different.

3.3. Frictional Force

The frictional force between the wire and the bracket was measured using a creep meter (Figure 5). The frictional force of the Ni-PTFE-coated SS wire was significantly smaller than that of the as-received wire by approximately 67% at an angle of 0° and by approximately 42% at 10°. For the Ni-PTFE-coated Ni-Ti wire, the frictional force was significantly reduced to approximately 62% at 0° and approximately 69% at 10°. For the as-received SS wire, the frictional force was 1.5 times higher at 10° compared with 0°, while for the Ni-PTFE-coated SS wire, there was no significant difference between 0° and 10°. The frictional force of the Ni-Ti wire was reduced to 90% at 10° compared to 0°, but there was no significant difference between 0° and 10° for the Ni-PTFE-coated Ni-Ti wire. There was no visual difference before and after the friction test, and no coating loss was observed.

3.4. Nickel Release

The amount of Ni released from Ni-PTFE-coated wires immersed in artificial saliva for 7 days was measured (Table 1). The amount of Ni ions released from the as-received SS and Ni-Ti wires was low. On the other hand, the Ni-PTFE-coated SS wires released approximately 3000 times more Ni than the as-received SS wires, and the Ni-PTFE-coated Ni-Ti wires released approximately 6000 times more Ni than the as-received Ni-Ti wires. After immersion in artificial saliva, the surfaces of both Ni-PTFE-coated SS wires and Ni-Ti wires became rough.

4. Discussion

In orthodontic treatment, the frictional force between the wire and bracket is one factor that reduces the efficiency of tooth movement, and most of the frictional force has been attributed to binding friction [1,2,16]. In this work, when the angle between the bracket and wire was increased from 0° to 10°, the frictional force for the SS wires increased by approximately 1.5 times (Figure 5), consistent with previous studies. On the other hand, the frictional force of the Ni-Ti wires was reduced to 90%, which is attributed to the difference in stiffness between SS and Ni-Ti. The Young’s modulus of Ni-Ti is 17% lower than that of SS, and the binding friction is reduced because the deflection of the wire corresponds to the angle [18]. For both types of wire, the frictional force of the Ni-PTFE-coated wires was significantly lower than that of the as-received wires at both 0° and 10°. Furthermore, there was no significant difference between 0° and 10° for the Ni-PTFE-coated wires. This may be due to low friction between the PTFE particles and the bracket, which reduced the friction between the wire and bracket and contributed to the suppression of binding friction. In the friction test, changing the friction environment, such as temperature and humidity, may produce different test results [19]. The oral cavity has a moist environment due to saliva, and the temperature in the mouth is approximately 37 °C. As a future issue, it is necessary to study the friction test under a reproduction of the oral environment, e.g., immersion in artificial saliva at 37 °C. Frictional force is also affected by wear on the material surface, which is said to basically increase friction [19,20]. PTFE particles maintain low friction even when the surface wears down due to friction, as fresh surfaces appear [21]. In orthodontic treatment, the number of times a bracket slides over the same spot on the wire surface is considered to be one or two times at most, and since the wire is replaced within a short period of time, wear is not considered a concern. Although the debris produced by wear would normally be of toxic concern, PTFE debris from abrasion is also not considered a problem, as PTFE is bio secure [10].
The Ni-PTFE-coated Ni-Ti wires exhibited the same reduction in friction as the Ni-PTFE-coated SS wires (Figure 5), even though the number of clumps of PTFE particles was sparse (Figure 3). In Ni-PTFE coatings, PTFE particles aggregate and exist as clumps about 5 μm in diameter, some of which are deposited on the coating surface [21]. This suggests that even a small number of clumps of PTFE particles in contact with the bracket might reduce the frictional force. It has also been reported that the frictional force is reduced even when the amount of PTFE particles on the material surface is small [22], which is consistent with the frictional force reduction results for the Ni-PTFE-coated Ni-Ti wires.
The surface roughness parameters (Sa, Sq, Sp, Sv, and Sz) of the SS wire were significantly increased by Ni-PTFE coating (Figure 4). This is attributed to the increase in height caused by the deposition of PTFE particles. For the Ni-Ti wires, Sa, Sq, and Sp were increased by coating with Ni-PTFE, but Sv and Sz were not significantly different (Figure 4). This may be because the Sv and Sz values were larger than those of the SS wires due to grooves created by the wire-drawing process during the manufacture of the Ni-Ti wires. Generally, frictional force increases with increasing surface roughness [23,24], but our results indicated that the Ni-PTFE-coated wires exhibited lower frictional force with the bracket despite their greater surface roughness. These results confirm that the addition of PTFE particles to the coating layer is effective in suppressing frictional force with the bracket.
Coating with Ni-PTFE could also be applied to orthodontic brackets, not only to orthodontic wires. In a report of DLC coating on brackets by applying the technology of DLC coating on orthodontic wires, a reduction in the frictional force with the wire was confirmed [8]. Although PTFE coating of brackets was explored as a way to improve antimicrobial resistance, the poor adhesion of the PTFE resulted in coating loss, which is considered an issue [25]. Since a Ni-PTFE coating bonds not only to metal brackets but also to plastic and ceramic brackets, it could solve the coating loss issue. In addition, contact between PTFE particles and the wire and bracket would be expected to further reduce the frictional force.
PTFE coatings have a low coefficient of friction and low energy, which inhibit the adhesion of oral bacteria to the wire [9,25]. Since adhered bacteria inhibit sliding with the bracket, it is desirable for the wire to have antimicrobial properties. PTFE particles deposited on the wire surface might reduce the adhesion of oral bacteria because their low-friction and low-energy properties might be as beneficial as a PTFE coating. Further studies are warranted to examine the antimicrobial properties of PTFE particles in these coatings.
Electroless Ni plating was used in this study to adhere the PTFE particles to the wire surface. This resulted in wires with almost no coating loss after friction testing. Although facile coating loss has been an issue in PTFE coatings, this method suggests that the coating can withstand clinical use. On the other hand, Ni is the main component of the coating in this method. After immersion in artificial saliva, a large amount of Ni was released, and the surface of the Ni-PTFE coating was roughened (Table 1). Notably, the Ni-PTFE-plated Ni-Ti wire released 1.6 times more nickel than the SS wire. This may be due to the difference in the adhesion of Ni plating to metal; Ni plating is known to adhere well to SS, while adhesion to Ni-Ti is poor, and the manufacturer responsible for Ni-PTFE plating also reported poor adhesion to Ni-Ti. Therefore, although no significant degradation was observed on the specimen surface after immersion in artificial saliva compared to SS, it is possible that the crevice corrosion between Ni-Ti and Ni plating resulted in more nickel release from Ni-Ti. Orthodontic wires do not remain in the mouth permanently like dental prostheses, such as metal crowns or dental implants, and are replaced with new ones after each (short) period of time (less than a month). For this reason, the standard for orthodontic wires, ISO 15841:2014 [26], does not mention corrosion resistance. In addition, silver braze is used for orthodontic appliances such as lingual arches and retainers, but the ISO 9333:2022 [27] standard for brazing materials does not describe the corrosion resistance of silver braze. Under these circumstances, unlike metal materials for dental prostheses, for which corrosion resistance testing is specified in ISO 22674:2022 [28], there is little need to evaluate the corrosion resistance of orthodontic wires as rigorously as for dental prostheses.
However, even small amounts of nickel released from orthodontic wires can be allergens if they come into contact with the oral mucosa or if nickel is released into saliva [29,30,31]. Metal release from Ni-based oral appliances has been reported to cause burning mouth syndrome and oral lichenoid reactions, which are due to delayed allergic reactions (type IV) [32]. However, nickel is also inoculated through dietary intake at 300–500 µg per day, and the actual threshold at which nickel causes metal allergies is not yet clear, largely due to the individual’s constitution. Since these results indicate that composite plating by adding insoluble particles (PTFE) and codepositing the particles with the metal is effective in reducing frictional force, it is necessary to consider methods other than electroless Ni plating. In addition to Ni-PTFE coating, the method of adding PTFE particles to electrolytic rhodium (Rh) plating has been reported, but since Rh plating easily induces corrosion due to the effect of a galvanic electrode [33,34], it is necessary to select a plating method that is resistant to corrosion. Accordingly, it has been suggested that gold or silver plating, which would provide excellent corrosion resistance, should be considered as an alternative to electroless Ni plating [34]. In addition to composite plating, plasma treatment of metal surfaces has also been reported to enhance adhesion to PTFE [11], and research on improving the adhesion of PTFE is still ongoing. As low-friction properties of PTFE particles were demonstrated in this study, the development of methods to adhere PTFE particles to wire surfaces is important to perfect low-friction orthodontic wires.

5. Conclusions

This is the first report showing that Ni-PTFE coating on SS and Ni-Ti wires, which are typical orthodontic wires, reduces the frictional force between wires and brackets. In addition, the frictional force at an angle of 10° between the wires and the brackets was also reduced, suggesting the possibility of reducing binding friction, which accounts for the majority of friction occurring in clinical practice and has remained unresolved to date. A reduction in friction with the bracket, despite the increase in surface roughness, was caused by the Ni-PTFE coating. In general, frictional resistance increases with increasing surface roughness. It is likely that the surface roughness was unaffected by the contact between the bracket and the PTFE particles with a low coefficient of friction attached to the surface of the Ni-PTFE-coated wire. Although a reduction in frictional force by the Ni-PTFE coating was clearly demonstrated, the amount of Ni released was significant. Orthodontic wires are devices that are replaced after a short period of time, and there are no ISO regulations regarding corrosion resistance as there are for permanently worn dental prosthetics. However, it has been reported that even a small amount of Ni can cause metal allergies, and it is necessary to consider methods other than electroless Ni plating for fixing PTFE particles to orthodontic wires.

Author Contributions

Conceptualization, K.N. and A.I.; investigation, K.N.; data curation, K.N. and A.I.; writing—original draft preparation, K.N.; formal analysis, K.N.; methodology, K.N., M.T., A.I. and Y.T.; validation, M.T., Y.T. and I.M.; writing—review and editing, M.T. and I.M.; supervision, Y.T. and I.M.; project administration, Y.T. and I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

The authors thank the Industrial Technology Institute (Miyagi Prefectural Government) for technical support with the creep meter and optical interferometer measurements and the Instrumental Analysis Group (Technical Division, School of Engineering, Tohoku University) for technical support with the ICP-MS measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Friction testing system. The creep meter used to measure the frictional forces. The distance between the creep meter grip and the center of the bracket was set at 20 mm (A). Bracket angulation of (B) 0° and (C) 10°.
Figure 1. Friction testing system. The creep meter used to measure the frictional forces. The distance between the creep meter grip and the center of the bracket was set at 20 mm (A). Bracket angulation of (B) 0° and (C) 10°.
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Figure 2. Stereomicroscope image of as-received and Ni-PTFE-coated wires (bar = 500 μm).
Figure 2. Stereomicroscope image of as-received and Ni-PTFE-coated wires (bar = 500 μm).
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Figure 3. Representative optical interferometer images of as-received and Ni-PTFE-coated wires.
Figure 3. Representative optical interferometer images of as-received and Ni-PTFE-coated wires.
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Figure 4. Surface roughness parameters of as-received and Ni-PTFE-coated wires (n = 5). (A) Sa, (B) Sq, (C) Sp, (D) Sv, and (E) Sz. ** p < 0.01 according to Student’s t-test.
Figure 4. Surface roughness parameters of as-received and Ni-PTFE-coated wires (n = 5). (A) Sa, (B) Sq, (C) Sp, (D) Sv, and (E) Sz. ** p < 0.01 according to Student’s t-test.
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Figure 5. Frictional forces for bracket angulations of 0° and 10° for as-received and Ni-PTFE-coated wires (n = 5). ** p < 0.01 according to Student’s t-test; ns indicates not significant.
Figure 5. Frictional forces for bracket angulations of 0° and 10° for as-received and Ni-PTFE-coated wires (n = 5). ** p < 0.01 according to Student’s t-test; ns indicates not significant.
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Table 1. Nickel release from as-received and Ni-PTFE-coated wires (μg/cm2/week).
Table 1. Nickel release from as-received and Ni-PTFE-coated wires (μg/cm2/week).
As-ReceivedNi-PTFE Coated
WireMeanSDMeanSD
SS0.0510.003152.34512.475
Ni-Ti0.0470.005239.8525.374
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MDPI and ACS Style

Numazaki, K.; Takahashi, M.; Ito, A.; Takada, Y.; Mizoguchi, I. The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires. Metals 2024, 14, 213. https://doi.org/10.3390/met14020213

AMA Style

Numazaki K, Takahashi M, Ito A, Takada Y, Mizoguchi I. The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires. Metals. 2024; 14(2):213. https://doi.org/10.3390/met14020213

Chicago/Turabian Style

Numazaki, Kento, Masatoshi Takahashi, Arata Ito, Yukyo Takada, and Itaru Mizoguchi. 2024. "The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires" Metals 14, no. 2: 213. https://doi.org/10.3390/met14020213

APA Style

Numazaki, K., Takahashi, M., Ito, A., Takada, Y., & Mizoguchi, I. (2024). The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires. Metals, 14(2), 213. https://doi.org/10.3390/met14020213

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