Improved Orthodontic NiTi Wires Coated With EP and PEO in Various pH saliva Fachmy F. Andriansyah

Department of Materials and Metallurgical Engineering, ITS, Surabaya 60111, Indonesia

Abstract Biomaterials in orthodontic field needs special material which has good corrosive resistance and good mechanical properties. This research has used one of them, Nickel Titanium Alloy (NiTi). NiTi is metal alloy that has ability to memorize its former shape called shape memory alloy. But some proven facts and research has found that this NiTi wire usage is harm and causes allergical damage in human body. Therefore, it needs special treatment through 2 methods, Electropolishing Pretreatment (EP) and Photoelectrocatalytic Oxidation (PEO).

This Massel’s NiTi wire has undergone several testings such as SEM, EDX and XRD. Its results show that it was successfully formed a number of TiO2 layers, flat on the surface. This causes reducing nickel release number from 1,627 ppm for uncoated speciments and 0,0359 ppm for EP-PEO coating speciments, which these speciments was sunk in pH 3 of saliva. In pH 6,25, it has nickel release number from 0,341 ppm for uncoated speciments and 0,272 ppm for EP-PEO coating. speciments Keywords: NiTi, Electropolishing Preatment, Photo Electro catalytic Oxidation, TiO2, Nickel Release. 1. Introduction Nickel-titanium shape memory alloys (SMA) with near equiatomic nickel and titanium have attracted considerable attention in biomedical applications due to their unique properties, in particular the shape memory effect, super-elasticity and corrosion resistance [1,2]. The NiTi SMA has been used as bone plates for internal fixation in bone fracture surgery, as staples for foot surgery, and orthodontic dental arch wires. Despite these advantages, the high Ni content (50 at.%) in NiTi SMA is of great health concern [3–6] and its toxicity, carcinogenicity and allergic hazard behaviors are associated with Ni have been reported [7,8].

To solve the problem of nickel ions release from the structure of NiTi, special treatment is required, so that nickel ions release can be reduced. One alternative to reduce Ni ions release is by using a combination methods of Electro polishing Pretreatment and Photoelectrocatalytic Oxidation Electro polishing (EP and PEO). Those steps are aimed to produce a TiO2

layer on the surface of NiTi. The advantages of thi methods are to combine two different methods that can be mutually reinforce the prevention of nickel ions release. [9].

2. Materials and Methods 2.1. Materials Material used in this experiment was NiTi wire MASEL 4832-204 with diameter 0,014 inch. 2.2. Methods 2.2.1. Experimental Details NiTi wires were chemically polished for several minutes using a solution of H2O, HF and HNO3 at a ratio of 5:1:4, then electro polished at a constant voltage of 10 V for 6 min at room temperature in an electrolytic cell with a magnetic stirrer and graphite cathode. The electrolyte consisted of 21% perchloric acid (HCLO4, 60%) and 79% acetic acid (CH3COOH, 99.5%) in volume. The samples were then washed with ultrasonic cleaner in acetone for 10 min and in deionized water for 10 min.

Fig 1. Schematic of Electro polishing Pretreatment (EP) method.

For the next step, the sample were

treated by Photo Electro catalytic oxidation to form a surface of TiO2 film at constant current of 0.3 A for 60 min in an electrolytic cell consisting of a graphite cathode and NiTi samples as the anode. The electrolyte was a 0.02 mol/ltr Na2SO4 aqueous solution with 0.0025% amount of Fe2+ ions at pH of 3.0. During photoelectrocatalytic oxidation, oxygen (O2) was purged continuously into the Na2SO4 electrolyte as near as possible to the graphite cathode, so H2O2 could be produced continuously under rather mild conditions by reduction of oxygen.

Fig 2. Schematic of Electropolishing Pretreatment (EP) method.

The electrolyte was irradiated vertically with 254 nm ultraviolet (UV) light for photo catalytic decompositing of the electrochemically generated H2O2 into hydroxyl radicals (OH-). Afterwards, the samples were cleaned ultrasonically in acetone for 10 min and in deionized water for 10 min.

3. Result and Discussion 3.1. Visual Observation Result The result of visual observation was presented in Fig. 2.

(a) (b)

(c) (d) Fig. 3. Result of visual observation for (a) non coated specimen (b) EP-PEO coated specimen (c) EP-PEO

coated specimen immersed in pH 3 saliva (d) EP-PEO coated specimen immersed in pH 6,25 saliva

Visual observation of the wire Niti

wire can be seen in figure 4.1 (b), thin layer of white coating is visible on EP-PEO coated specimens. In NiTi coated specimens which are immersed in pH 3 and pH 6.25salive, there are a layer of white deposits around the wire, as shown by the yellow arrows in Figure 4.1 (c) and (d). 3.2. SEM Result

SEM observation on normal specimen has a relatively smooth surface and a small amount of deposit passivation layer [10] in some parts there are extending streaks, as the result of the drawing process during manufacture of the NiTi product [11], the direction of the strokes are indicated by yellow arrows (1).

1

3

2

2

3

4

(a) (b) (c) (d) (e) (f)

Fig. 4. NiTi SEM result: (a) normal specimen (40 µm) (b) normal specimen immersed in pH 3 saliva (40 µm) (c) normal specimen immersed in pH 6,25 saliva (40 µm) (d) EP-PEO coated specimen (40 µm) (e) EP-PEO coated specimen immersed in pH 3 saliva (40 µm) (f) EP-PEO coated specimen immersed in pH 6,25 saliva

(100 µm).

In normal specimen with pH 3 immersion, there is thin coarse deposit layer attached to its surface. This layer is damaged by the influence of acidic pH [12], indicated by the yellow arrows (2). Normal specimens with pH 3 and pH 6.25 immersion seems to have small clumps deposit and thin fibers on their surface. These fibers are thin and brighter than their surrounding areas, which showed that they have greater molecular weight than their surrounding areas [11], it is indicated by the yellow arrows (3).

EP-PEO coated specimens without immersion, are having a layer of crust on their surface, these layers serve the protection of Niti wire from corrosion and prevent the release of Nickel [9]. EP-PEO coated specimens with pH 3 immersion, also have the thin layer but it is peeled off as a result of the acidic pH [12] as shown by the yellow arrows (2). When compared with normal specimens which is immersed in pH3 saliva , this coated specimen which

immersed in pH 3 saliva has a thicker protective layer.

EP-PEO coated NiTi specimen which is immersed in pH 6.25 saliva has a visible layer that covers the entire surface of the wire, but there is excess residu of saliva on the surface of the coatings, that is indicated by the yellow arrows (4). Saliva deposits can still be left on the specimens, despite being cleaned [13]. 3.3. EDX Result

By EDX observations, normal specimens consisted of Ni and Ti elements, which are 43.04% Wt and 48.08% At of Ti, whereas the Ni contents are 56.96% Wt and 51.92% At. In EP-PEO coated specimens are consisted of Ni, Ti, and O with the Ti contents are 40.56% Wt and 40.68% At, Ni contents are 54.55% Wt and 44.63% At, and contents of O are 4.89% Wt and 14.68% At.

(a) (b) (c) (d)

Fig. 5. EDX result for: (a). normal NiTi specimen and (b). unsure content, (c). EP-PEO coated NiTi specimen and (d). unsure content

As the results of EP-PEO coatings, Ti

elements will bind to O elements forming the TiO2 films, but It is not capable to produce nickel-free zone on the surface of the Niti wire yet. 3.4. XRD Result

From the observation and analysis of XRD data, TiO2 compounds is obtained at 38.47o with an intensity of 785 and Niti compounds at 78.22o with an intensity of 747. The TiO2 compund is EP-PEO coating product, while NiTi compound is NiTi wire base metal. This result is similar to previous study that observed NiTi wire with XRD [14]. Peak with Fe sign indicated an unsure in specimen holder, it can be neglected.

Fig 6. X-ray diffraction pattern of the specimens with Coating Wire NiTi EP-PEO

Element Wt% At% TiK 43.04 48.08 NiK 56.96 51.92 Matrix Correction ZAF

Element Wt% At% OK 04.89 14.68 TiK 40.56 40.68 NiK 54.55 44.63 Matrix Correction ZAF

3.5. ICP Result

Fig. 7. NiTi wire Ni release immersed in pH 3 saliva

Fig. 8. NiTi wire Ni release immersed in pH 6,25 saliva

Fig 7. and 8. shows the leached Ni

concentrations in artificial saliva after immersion tests at 37 ± 0.5 _C as determined by ICP. It can be observed that surface modification by EP and PEO dramatically reduces Ni release from the NiTi substrate compared to the control (NiTi non coating). [9].

On pH 3, nickel release on normal specimen are 1.627 ppm while on EP-PEO coated specimen the release are 0.0359 ppm. In pH 6.25 Nickel release decreased into 0.241 ppm for normal specimen and 0.0272 ppm for EP-PEO coated specimens.

From the ICP results, we will search the relationship between Nickel loss vs Immersion time by the formula:

m = v.c/s

Where m is the lost mass of Nickel (µgr/cm2), v is the mass of nickel wire (0.14 g for normal specimens and 0.13 for coated specimens), and s is the specimen surface area (1.957 cm2 for wire diameter of 0, 03556 cm and 17.5 cm long, surface area was calculated using the formula L = 2πr (r + t)). Graph Fitting is conducted using the equation log m = a + bu + Cu 2, with u = log h (hours) [15].

On the Nickel loss vs Immersion times it is found that the most mass loss is occured on pH3 normal specimen, while the least mass lost is occurred on pH 6.25 EP-PEO coated specimen.

Fig. 9. Mass loss vs immersion times at NiTi

specimen

Fig. 10. Mass loss vs immersion times at NiTi

specimen coated EP-PEO.

On the Nickel loss vs Immersion times it is found that the most mass loss is occured on pH3 normal specimen, while the least mass lost is occurred on pH 6.25 EP-PEO coated specimen. i cor = zF dm A dt = (b+2cu) zFm A h 3600 with z is the valence of Nickel (2), A is the atomic weight (58.70), m is the nickel lost mass (µgr/cm2), t is the immersion time (seconds), F is the Faraday constant (96,500 C / mol), and h is the time in hours (hours).

Fig. 11. iCorr vs immersion times at NiTi specimen

Fig. 12. iCorr vs immersion times at NiTi specimen

coated EP-PEO

On pH 3 normal specimen, icorr tends to increase, while the pH 6.25 normal specimen tends to decrease. on pH 3 and pH 6.25 EP-PEO coated specimens, both icorr are increased but they are decreased when

the immersion time reaches 1000 hours. This is happened due to the occurrence of passivation process on NiTi wire, so that the icorr is decreased [16]. 4. Conclusions

A surface modification technique encompassing EP and PEO has been developed to improve the surface properties of biomedical nickel titanium wire orthodontic. The method gives rise to a sturdy TiO2 layer. Although this film has a porous structure on the nanometer scale, it is very effective in suppressing Nickel ion release from the NiTi substrate, as shown by immersion tests for 8 weeks in different pH saliva. References [1] S.A. Shabalovskaya, Bio-Med. Mater. Eng. 6 (1996)

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