SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... 445–452 INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON THE ELECTRICAL PARAMETERS OF NITINOL AND STAINLESS STEEL ARCHWIRES VPLIV UMETNE SLINE IN RAZTOPIN ETERI^NIH OLJ NA ELEKTRI^NE LASTNOSTI ORTODONTSKIH @I^K IZ NITINOLA IN NERJAVNEGA JEKLA Sanja Koji} 1* , Mitar Simi} 1 , Milica Aber 1 , Bojan Petrovi} 2 , Goran M. Stojanovi} 1 1 University of Novi Sad, Faculty of Technical Sciences, Trg Dositeja Obradovi}a 6, Novi Sad, Serbia 2 University of Novi Sad, Faculty of Medicine, Hajduk Veljkova 3, Novi Sad, Serbia Prejem rokopisa – received: 2022-02-19; sprejem za objavo – accepted for publication: 2022-06-09 doi:10.17222/mit.2022.415 This paper analyzes the variation in the electrical characteristics of NiTi and stainless steel archwires under exposure to different liquid media and temperatures. The archwires were enclosed in a microfluidic chip to obtain a controlled environment, mimick- ing oral cavity conditions. Five liquids were selected for testing: artificial saliva, Listerine mouthwash, cinnamon essential oil added to artificial saliva, eucalyptus essential oil added to artificial saliva and orange essential oil added to artificial saliva. Im- pedance spectroscopy was performed at three temperatures, (25, 37 and 50) °C, and in three different time intervals: at the start of the experiment, after three weeks and after six weeks of exposure. The impedance, its modulus and phase, and Nyquist dia- grams were plotted, as well as the root-mean-square (RMS) deviation parameter. The results showed that both materials exhib- ited comparable responses with small changes over time. Keywords: NiTi alloy, stainless steel, corrosion, essential oil, impedance spectroscopy Avtorji opisujejo analizo sprememb elektri~nih lastnosti ortodontskih pritrjevalnih `i~k iz zlitine na osnovi Ni in Ti (Nitinol) in nerjavnega jekla pod vplivom razli~nih raztopin in temperature. Preiskovane `i~ke so bile zaprte v folijo z mikrofluidom. S tem je bilo dose`eno kontrolirano okolje, podobno pogojem v ustni votlini. Za testiranje so uporabili pet razli~nih raztopin (umetno slino, ustno vodico Listerin, cimetovo, evkaliptusovo in pomaran~no eteri~no olje). Vsa tri eteri~na olja so bila raztopljena v umetni slini. Meritve z impedan~no spektroskopijo so izvajali pri treh razli~nih temperaturah (25, 37 in 50) °C, in treh razli~nih ~asovnih obdobjih izpostavitve. Meritve so opravili na za~etku preizkusa, po treh in po {estih tednih. Dolo~ili so impedanco, njen modul, fazo, izrisali Nyquistove diagrame in izra~unali povpre~ni kvadratni koren odstopanja. Rezultati analize so pokazali, da imajo vsi preiskovani materiali primerljive lastnosti z majhnimi spremembami glede na ~as trajanja preizkusa. Klju~ne besede: zlitina NiTi, nerjavno jeklo, korozija, eteri~na olja, impedan~na spektroskopija 1 INTRODUCTION Metals are ideal for biomedical applications because of their superior mechanical strength and corrosion resis- tance. Biomedical metallic devices are medical or dental devices that are designed to be used in various parts of the human body. A variety of chemical reactions occur on the surface of a material once it is placed intraorally. At the material-to-human-body-environment-system junction, all biomaterial surfaces form an interface. Bio- materials have two key characteristics: biofunctionality and biocompatibility. 1 Not all metals are suitable for bio- medical applications as these two stringent properties must be present. With the advancement in surgical in- struments and biomaterials, as well as the rising demand for biomaterials for the oral cavity, the use of biomaterials in the medical field is a growing area of in- terest. 2 Nitinol is a name for a group of titanium-based intermetallics with nearly equal amounts of nickel and ti- tanium. Because of the thermoelastic martensitic trans- formation, nitinol has shape-memory and superelastic properties. 3–5 The shape memory effect and super- elasticity in near-equiatomic NiTi alloys are caused by the thermoelastic martensitic transformation from the parent austenite phase with the B2 structure to the monoclinic (M) or rhombohedral (R) martensitic phase. 6–8 Fabrication of stainless steel is easier than that of Ti-alloys. The microstructures of stainless steel (SS) are austenite, martensite and ferrite. The properties of these structures reflect the fact that they are results of proper chemistry adjustments. The austenitic stainless steel used in orthodontics contains approximately 18 w/% Cr and 8 w/% Ni. The most important feature of stainless steel is its homogeneous corrosion resistance. On the other hand, stainless steel has a bad reputation regarding its use in body-fluid conditions due to pitting, intergranular, crev- ice and friction corrosion. 9,10 Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 445 UDK 620.193.16:616.314-089.23 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(5)445(2022) *Corresponding author's e-mail: sanjakojic@uns.ac.rs (Sanja Koji}) Corrosion of any kind releases a variety of harmful metal ions, such as nickel ions, that have negative effects on the human body. To resist corrosion, stainless steel and titanium alloys rely on the formation of a passive-surface oxide film. However, even if such a protective oxide film is present on a metal surface, metal ions can still be released. 11 Not only is the protective oxide layer susceptible to a me- chanical and chemical breakdown, but it can also dis- solve over time. While a metal disintegrates due to the presence of oxygen in the environment, some metals and alloys suffer from corrosion and deterioration due to the acidic environment of the oral cavity, the presence of flu- oride ions in toothpastes and mouthwashes, and other biomedical solutions. 12 Nickel compounds have been classified as human carcinogens by the International Agency for Research on Cancer (IARC). 3 Nickel has been shown to have toxic, carcinogenic and immune-sensitizing properties. 4,5 The release of nickel from medical devices may contribute significantly to an oral nickel intake of up to 1 mg per day. 6–8 Nickel has the potential to cause genotoxicity, neurotoxicity, gastrointestinal, cardiovascular, muscular, dermal, metabolic, immunological and carcinogenic ef- fects. 13 Acidic foods and beverages, such as soft drinks, are constantly attacking metals in oral appliances, promoting the cathodic corrosion reaction and, as a result, also the anodic corrosion reaction (metal dissolution). The effect of pH on nitinol corrosion has been extensively studied. In 2003, Huang et al. measured the amount of ions re- leased from NiTi wires immersed in artificial saliva with various pH values as a function of time. 14 They discov- ered that, in all conditions, the amount of metal ions re- leased increased with the immersion time, and that their amount was higher when larger amounts of acidic solu- tions were used. Fluoride-containing mouthwashes are recommended for the patients with stainless steel crowns, orthodontic appliances and immobilization appliances. Most of these patients are children and adolescents who do not always maintain good oral hygiene and are at high risk of devel- oping caries. To reduce the prevalence of caries, dentists frequently prescribe fluoride-containing mouthwashes on a daily or weekly basis. Fluoride mouthwashes are typi- cally available in concentrations of 0.05 % and 0.2 % of fluoride ions. Due to its ionic, thermal, microbiological and enzymatic properties, the oral environment aids the biological degradation of a metal. As a result, patients are likely to be exposed to corrosion processes to some extent. 15–17 Fluoride promotes the formation of calcium fluoride spheres that adhere to the teeth and stimulate remineralization while also protecting against acid at- tacks. Mouthwashes with fluoride help to prevent cavi- ties and protect tooth enamel. Essential oils, which con- tain high-concentration terpenic hydrocarbon mixtures, have a lot of potential for naturally treating oral infec- tions. Lavender, eucalyptus, peppermint, clove and cinnamon oils are the most important essential oils for maintaining oral health. The antimicrobial properties of these essential oils have been well documented. Cinnamon oil (CO), derived from cinnamon plants in the Lauraceae family, has been extensively studied for its antibacterial properties and has been shown to be effec- tive against oral bacteria. Its antibacterial properties against Streptococcus mutans make it ideal for prevent- ing dental caries. Because of their powerful anti- microbial properties, natural mouthwashes have been shown to be effective in treating gingivitis and plaque. Herbal extracts of medicinal plants have been used in dentistry as plaque antimicrobial agents, antioxidants, analgesics and antiviral agents to prevent histamine re- lease due to fewer side effects and low toxicity. The bio- logically active compound eucalyptol is found in Euca- lyptus globulus essential oil, also known as eucalyptus oil, which is used as an endodontic solvent in dentistry for mouthwash and dental preparations. 18–22 With the advancement of modern wireless technolo- gies and wearable electronic devices, the oral cavity is becoming a place for diagnostics as well as therapy. Smart orthodontic brackets, tooth-mounted sensors and biosensors for detecting various parameters in the oral cavity have been reported recently. However, there is a lack of research data on how to determine the electrical performance of metallic dental appliances, and how they perform within a microfluidic chip. 22–24 The aim of this work was to investigate electrical properties of the most often used metallic dental materi- als made from NiTi and SS. These characteristics were studied in different fluidic environments, such as artifi- cial saliva, a commercial essential oil based mouthwash and two experimental essential oils added to artificial sa- liva. To mimic the oral cavity environment, we designed a microfluidic chip as a system for analyzing the varia- tion in the electrical parameters of the NiTi and SS archwires through their exposure to different fluids and temperatures. 2 EXPERIMENTAL PART 2.1 Archwires and microfluidic chip design Two as-received commercial archwires (from Astar Orthodontic Inc., China) with dimensions of (0.4826 × 0.635) mm were used in this study (Fig- ure 1a). The wires were randomly chosen from a packet containing ten archwires. Unlike some other studies, where a cut from the end of an archwire was used, we performed an experiment on the whole archwire, with the intention to imitate the real application of a complete archwire, being a part of an orthodontic appliance in the oral cavity. For the fabrication of the microfluidic chips, 120-μm-thick PVC foils were used in this study. Each SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... 446 Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 chip was made of five layers of foil. The drawing of indi- vidual layers of a microfluidic chip was performed in the AutoCad program, Figure 2. The first layer is full foil, the last layer has two small round holes for the inlets, through which the solutions are injected into the chip, and the three middle layers are cut in the shape of a wire so that the solution can easily coat the entire wire during the injection. Figure 1b depicts the fabricated micro- fluidic chips with a NiTi wire (left) and an SS wire (right). A Graphtec CE6000-60 Cutter Plotter was used for cutting the foil. Before the cutting, the setting was made in the Graphtec Pro Studio device software. After load- ing the design made in the AutoCad program, it was nec- essary to adjust the position of the cutter plotter knife. When the knife was positioned, the speed and force of cutting the foil were determined. The force and speed had to be carefully determined to avoid damaging the foil (high force and speed) or the skipping of the knife, not cutting all the parts of the foil properly (low force and speed). The final step was to stack the layers of foil onto each other and insert a certain wire into the middle layer of the chip (the microfluidic channel). After every- thing was put together, lamination was done. An MBL laminator was set to speed 3 and a temperature of 160 °C. The ends of each archwire were protruded from the microfluidic chip by approximately 3 mm, enabling the connections with alligator clips for measuring electri- cal characteristics. 2.2 Preparation of the liquids and injections into the chips Five types of liquids were used in the study: 1) artificial saliva, 2) Listerine mouthwash, 3) cinnamon essential oil added to artificial saliva (dilu- tion 1 : 100), 4) eucalyptus essential oil added to artificial saliva (di- lution 1 : 100), 5) orange essential oil added to artificial saliva (dilution 1 : 100). Narrow pipette-shaped syringe tips were used to in- ject the above liquids into the inlets of the microfluidic chips. 2.3 Instruments The devices used for the testing included: 1) PalmSens4 galvanostat/potentiostat, 2) PC with installed PSTrace 5.8 software, 3) adjustable hot-air source, 4) thermal infrared camera (ULIRVision, China). Electrochemical impedance spectroscopy was applied using the PalmSens4 instrument. To start the measure- ment, the PalmSens4 device was connected to the PC with the PSTrace software via Bluetooth. The first mea- SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 447 Figure 1: Archwires: a) NiTi and SS wires, b) wire integration into a PVC microfluidic chip Figure 2: Layer design of a microfluidic chip Figure 3: Thermal-camera view of the archwires heated to: a) 37 °C and b) 50 °C surement was performed on a sample of the chip with the NiTi wire and sample of the chip with the SS wire with- out injecting the solution. We performed three consecu- tive measurements for each chip at different tempera- tures, at room temperature (25 °C), 37 °C and 50 °C. We used the hot-air source to change the temperature in the chip, and we monitored the exact temperature with the thermal camera that can be seen in Figure 3. All the measurement data was stored as excel files and plotted via the GraphPad Prism software. 3 RESULTS The main purpose of the study was to evaluate the in- fluence of the mouthwash and different essential oils on the archwires made of two different materials. In order to achieve the main goal, the impedance of the archwires enclosed in the microfluidic chips was obtained. An EIS analysis was chosen over a physical-optical cross-exami- nation as it provides several important advantages: 1) EIS is non-invasive and it does not require any spe- cific pre-treatment of the sample; 2) EIS can be performed in-situ with low-cost readout devices, reducing the overall costs and increasing ac- cessibility to real-life in-vivo applications; 3) EIS provides the response of a complete sample, rather than just one small part which is very common in the case of a physical-optical cross-examination. The impedance of the NiTi and SS archwires was measured firstly in a microfluidic chip without any liquid inside. These measurements are marked as Empty in the subsequent graphs. Next, the liquids were injected into the chips and measurements were made and repeated af- ter 3 weeks and 6 weeks. The impedance as the modulus of all samples is depicted in Figure 4. The graphs indi- cate the types of archwire (SS – left, or NiTi – right) and the temperatures: 25 °C – first row, 37 °C – second row and 50 °C – third row. Moreover, the liquids are marked with colors according to the legend below the graphs (the same for each graph) and the time is marked as follows: a full line for the first measurement, a dashed line for the measurements after 21 d (3 weeks) and a dotted line for the measurements after 42 d (6 weeks). The impedance phase of all the samples following the same pattern and labelling is presented in Figure 5. In these figures the only clear difference in the impedance is noted between the empty chips and filled ones. The differences in the impedance caused by the ap- plication of selected liquids and the time are not clearly visible. Because of that, Nyquist plots were also included in Figure 6, with the same notation and color scheme as used for the impedance modulus and phase angle. The Nyquist plots present the real part of impedance (Z’) ver- sus the inverse-signing imaginary part of impedance (Z"). If a Nyquist plot shows a semi-circle capacitive loop, its diameter is directly proportional to the corrosion SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... 448 Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 Figure 5: Measured phase angle for all studied samples Figure 4: Modulus of the impedance for all studied samples resistance of the sample for the corroding system under kinetic control. Thus, by comparing the diameters with- out fitting, one can compare the behaviors of two sam- ples in the same medium, or the effects of any tested pa- rameters on the behavior of the system. However, the differences between the Nyquist plots caused by differ- ent liquids and times are not clearly visible. Because of that, no equivalent circuit modelling was performed as it was reasonable to expect that very similar results would be obtained for all 108 configurations, not providing new useful data. As it can be seen from Figure 6, the plot of Z’/Z’’ for various solutions varies more with the increas- ing temperature for SS. This can be attributed to the higher temperature resistivity of SS, when compared to NiTi, causing higher temperature resistivity of the SS wire without any solution (red curves in Figure 6). 26,27 Despite the fact that a visual observation of impedance spectra or Nyquist diagrams can provide initial clues re- garding the corrosion level, more precise metrics are needed to quantify the level for different materials ex- posed to corrosion. One such parameter is the root-mean-square deviation (RMSD) of the real part of impedance. 28 The RMSD for the real part of impedance is defined with Equation (1): () () RMSD YY Y ii i i N = − = ∑ (Re( ) (Re( ) Re( ) ,, , 12 2 1 2 1 (1) SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 449 Figure 7: RMSD values for all measured samples Figure 6: Nyquist plots for all analyzed samples where the real part of impedance of the first measure- ment (no solution) is Re(Y i,1 ), while the real part of im- pedance for different solutions is Re(Y i,2 ) and N is the number of measurement points in a frequency sweep. The result is a scalar value, which can easily be used to compare corrosion levels of different materials when compared to the baseline (empty). The RMSD values were calculated and presented in Figure 7, with the aim to clarify the influence of selected liquids on the archwire corrosion. As can be seen, both materials in all five solutions demonstrated very similar responses, with small changes over time (from the initial measurement to six weeks af- terwards). 4 DISCUSSION The oral cavity is one of the most hostile environ- ments in the human body, with temperature and pH vari- ations that are greater than in most other parts of the body. The corrosion caused by the graded degradation of materials due to an electrochemical attack is a cause for concern, especially when materials are exposed to the aggressive electrolytic environment of the human mouth. Corrosion is unavoidable and its consequences can jeop- ardize the material mechanical and biocompatibility properties. 29 Immersion tests revealed that at a pH of 4.2, the highest release of Ni ions occurred, while the surface morphology of orthodontic appliances changed as they became inhomogeneous and rough. 27 Various synthetic physiological fluids are used to simulate bodily, oral, dentinal or bony conditions. The most reliable and similar medium for testing the proper- ties of biometals used in dentistry is artificial saliva. With this in mind, during this experiment, artificial saliva was used as a solution for analysis, but also as a solution to which three experimental essential oils were added, whose corrosion potential was also tested. Mouthwashes are becoming increasingly popular as a means of preventing tooth decay, periodontal disease, odors, and for implant maintenance, even for children with metal restorations or metallic appliances. The role and impact of prophylactic agents on corrosion, mechan- ical properties, surface characterization and friction caused by the sliding motion have all been studied. Mouthwashes, commonly used in orthodontic appli- ances, have the potential to cause ion leaching from these alloys. Furthermore, metals such as chromium and nickel can produce atom species from molecular oxygen, resulting in superoxide, a highly toxic gas. A nitinol wire behaves passively in a simulated saliva solution, whereas an SS wire exhibits pitting corrosion. Fluoride ions raise the passivity current density in both wires, lowering nitinol breakdown potential while inverting the effect on SS. 28 Application of green inhibitors based on plant ex- tracts and essential oils (EOs) are very important in den- tal medicine. EOs have antibacterial, antifungal effects and can be used against biofilm forming on Ti and SS surfaces. 30 The effects of NaF and the extracts of differ- ent plants (Artemisia, clove and Celtis australis) on the corrosion resistance of NiTi and CuNiTi archwires were investigated by Fatene at al. 31 Peppermint essential oil was used as the coating for Ti alloys or SS and mechani- cal/chemical attributes were determined in the paper. 32 The clove EO and ultraviolet light (UV-C) were com- bined to inactivate the biofilms on SS. 33 Several studies used electrochemical impedance spectroscopy (EIS) to investigate the behavior of passive Ti films and some of their alloys in simulated physiologi- cal solutions. In most cases, impedance data was ana- lyzed using equivalent circuit models, but it was only found in a few of these studies. The importance of controlling the temperature during the experiments was already emphasized in the study conducted by Friedli et al. In their investigation, orth- odontic wires were stored at four different temperatures (5, 22, 36 and 60) °C for twenty-four hours before being mechanically tested. 34 The obtained results are in agree- ment with our current investigation. In our study, all the samples showed a uniform low corrosion potential at all testing temperatures and throughout the duration of the experiment. The Listerine contains four essential oils that are an- tibacterial antiplaque agents, shown to penetrate biofilms. The oils included 0.092 % of eucalyptol, 0.064 % of thymol, 0.060 % of methyl salicylate and 0.042 % of menthol. In the conducted research, we tested another three essential oils at much higher concentra- tions, at a dilution of 1 : 100. The following oils were tested: eucalyptol, cinnamon oil and orange oil. With re- gard to eucalyptus oil, its antimicrobial activity was dis- covered to be linked to the synergistic effects of the ma- jor and minor components rather than the concentration of a single component. 16 It was shown that eucalyptus has antimicrobial activity against both gram-negative and gram-positive bacteria. Trans-cinnamaldehyde, eugenol and linalool are three of the most important components of cinnamomum zeylanicum cinnamon oil, accounting for 82.5 % of the total composition. Cinnamaldehyde is the most active component of cinnamon and it is the main constituent. The antimicrobial effect is based on the growth inhibitory effect on a variety of bacteria, includ- ing gram-positive, gram-negative and fungal isolates. 35 The essential oils found in orange peels have a lot of promise as an antimicrobial agent against oral patho- gens. 36,37 All the tested samples, a commercial one, List- erine and three experimental ones, used in this study showed very similar corrosion potentials, which did not exceed the corrosion potential of artificial saliva. In this study, we considered a time period of 6 weeks, with one injection per sample, to provide information on the static response of archwires, which is not compre- hensively covered in the literature. The purpose of our SANJA KOJI] et al.: INFLUENCE OF ARTIFICIAL SALIVA AND ESSENTIAL OIL-BASED SOLUTIONS ON ... 450 Materiali in tehnologije / Materials and technology 56 (2022) 5, 445–452 study was to validate the assumption that three types of essential oils do not make a strong corrosion impact on any of the two types of wire, and that they are safe to use over a prolonged period of time in various environmental conditions (at three temperatures that were studied). 5 CONCLUSIONS For the majority of orthodontic patients, using fixed orthodontic appliances is part of the regular treatment. The usual materials applied as archwires are nitinol and stainless steel and their properties were analyzed in this study by exposing them to different solutions containing essential oils, and different temperatures, imitating the conditions in the oral cavity. The modulus of impedance as a function of frequency was measured and a range of 0.6–3.5 was obtained; the SS archwire demonstrated a slightly higher electrical resistance than the NiTi arch- wire. At low frequencies both archwires had a phase an- gle of around zero, indicating that they had a resistive character, whereas at high frequencies, the phase angle increased up to 50°, indicating an inductive character. The studied archwires did not demonstrate changes in the impedance as a function of temperature variation. Our findings can be potentially very useful when recom- mending the use of chemoprophylactic agents to patients with orthodontic appliances. In future, it will be impor- tant to analyze the variation in the electrical parameters of these archwires for a period longer than six weeks (presented in this paper) to be closer to real applications where fixed orthodontic appliances are used for up to six months, and to evaluate all the effects of treatment. 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