| The clinical application of the orthodontic archwire mostly follow the size andstiffness ascending order, which usually starts from the first use of nickel-titaniumarch wire, and then gradually transforms to the use of stainless steel arch wire. Thereare many shortcomings of using nickel-titanium or stainless steel arch wire alone.The use of one simple kind of arch wire could not meet clinical needs, especiallyunder the circumstances that different parts of the dental arch require the placementof different stiffness and performance of arch wires. Composite archwire (CAW) isformed by solder connection of nickel-titanium and stainless steel arch wire.Nickel-titanium part could be used in the part of dental arch which containsmalposed teeth, and stainless steel part could be used in the part which need toprovide anchorge. CAW combines the advantages of the individual material tocorrect malposed teeth whilst maintain the stability of anchored teeth. Li Hongmeiand Sun Daqian from the department of Materials process in Jilin Universitydeveloped the new CAW which could satisfy clinical needs in2011by using laserwelding and copper interlayer, and the design was patented in2011. CAW withcopper interlayer helds good mechanical properties and the application of CAW inclinical treatment will lead orthodontic technology into a new stage of development.The corrosion of nickel-titanium part and stainless steel part in CAW would result inthe precipitation of metal ion, in addition, the interlayer of CAW contains purecopper metal. Whether the precipitation of copper ions in oral after welding is withinthe limit of biosafety is urgently needed to be tested. Similar as other alloy materials,the corrosion resistance of CAW is also dependent on the passivation film formed onthe surface, the repairation ability of which to is limited. The damage of passivationfilm may cause further etch of deep material, leading to harmful ion precipitation.The effects of corrosion on CAW contain mainly three aspects: First, the reduction of mechanical properties like tensile and elastic strength, or even breakage in clinic.Secondly, the damage of oxide film on the wire surface. The corrosion concave couldincrease friction and prolong the clinical course of orthodontic treatment. Thirdly,the local oral tissue and systemic adverse reactions caused by the release of metalions after corrosion.The oral cavity is an extremely complex environment with a variety of factors whichcould effect the corrosion of material, such as the pH value, chloride, fluoride andvarious proteins. Before the clinical application of a new type of biological material,it is necessary to carefully study and explore the environment in which it mustfunction and also it effect. It is also important to discuss the stress corrosion of CAW,since it is not simple soaking in a static condition when used in clinic, but isconstantly subjected to the twisting force of correcting misaligned teeth, as well aschewing food pushing force. Laser welding was not a simple autologous metalremelting recrystallization process, the elements in the melting zone are changedwhere brings the involvement of copper. The primary cell anode corrosion of copperwould occur, therefore it is necessary to study the related electrochemical corrosionof CAW. In summary, the corrosion resistance of laser welding CAW with excellentphysical and mechanical properties in its clinical application is extremely important,but the related research has not yet been studied.In this study, the corrosion resistance of laser welding CAW with copper interlayer insimulated oral cavity environment was studied. A preliminary evaluation of the metalion release and micro-appearance surface corrosion of CAW was explored throughstatic immersion test and electrochemical assay in different pH value, fluoride ionconcentration, protein concentration and load values. The corrosion resist anceparameters in various aggressive environments were obtained by combining theelectrochemical results with weight loss and ion precipitation results. The corrosionresistance and bio-security of CAW was predicted not only to determine and lay thefoundation the clinical application of CAW in the process of clinical application, butalso to provide experimental and theoretical basis for its further improvement. In this study, the comprehensive materials science and biology evaluation of CAW by acombination of clinical condition and simulated oral environment, could provide abasis for its clinical use and promotion, and provide experimental route and referenceto the corrosion resistance studies with other new oral metal materials.The corrosion condition of CAW in chlorine solution and artificial saliva at differentpH: the copper ions precipitate and weight loss were significantly greater in chlorinesolution than artificial saliva group, and the weight loss and copper ions precipitatein acidic saliva group were slightly above the neutral group. The protective potentialof three medium is higher than the corrosion potential in cyclic polarization processindicates the limited capacity of re-passivation. Pitting potential was higher and andcorrosion current densities were lower in acidic saliva group than neutral group. Thesurface of CAW after electrochemical corrosion is relatively smooth, but there arealso a small number of cracks generated, especially in chlorine solution with morerough, deeper, and etch pits. The corrosion morphology in acidic saliva was similarto neutral saliva, but not severe as chlorine solution group. The corrosion resistanceof CAW in artificial saliva was higher than in chlorine solution, and acidity changescould affect the corrosion resistance in artificial saliva. The protective oxide film ismore prone to rupture in acidic condition, leading to accelerate ion precipitation andcorrosion.In different values of fluorine-containing solution: the corrosion potential oflow-fluoride solution was similar as simple artificial saliva, but significantly lowercorrosion potential was found in high-fluoride solution with the current increasingrapidly after cathodic polarization. The cyclic retrace curve in fluorinated solutionhas the trend of coincide with the previous scan, whereas the polarization curve ofsimple artificial saliva group shows a typical hysteresis loop. The microscopemorphology of fluorinated group showed obviously visible etch pit, andhigh-fluoride group appear greater diameter and depth of corrosion pits thanlow-fluoride group, along with greater copper ion precipitation amount and weightloss. With the increase of the concentration of fluoride, the corrosion resistance of CAW decreased. CAW has strong corrosion resistance in low fluoride solution, andhigh-fluoride could reduce the corrosion resistance. However, even fluorideaccelerate the corrosion process of CAW, the overall amount of copper ionsprecipitation still within the specified limit of WHO.In different values of protein-containing solution: the corrosion potential oflow-protein group was significantly reduced. The corrosion current densities of threegroups were similar. The microstructure of low-protein group were rough with pits,but were not significantly increased as the values of protein concentration. Theprecipitation amount of copper ions in low-protein group was most, but the weightloss was not obvious. In general, CAW showed good corrosion resistance in solutionscontaining protein. Low protein concentration reduces the corrosion resistance ofCAW, while high concentrations of protein would reduce this effect. Protein couldform film on the surface as ion diffusion barrier, while the precipitation of copperions can be reduced as well as hinder calcium and phosphorus ions from artificialsaliva deposited. The reason for high protein concentrations did not significantlyaccelerate the corrosion process may be the increase of deposited protein layerthickness on the sample surface, which further hinder the intrusion of oxygen ions.However the specific mechanism remains to be further studied.The corrosion condition of CAW under stress conditions and different acidities ordifferent concentrations of fluoride: the surface of CAW was intact withoutsignificant degradation occurs after immersed in acidic and neutral artificial saliva,regardless of the applied stress; the tension side of three-point bending stress showedrougher surface with the increase of bending displacement. The corrosion p its oninterlayer also showed irregular morphology. The main elements composition ofinterlayer contain oxygen and phosphorus according to the energy spectrum analysis.Applied stress would cause increase of copper ions precipitation either in acidic orneutral saliva, and the amount of increase is proportional to the increase of appliedstress. Three-point bending stress as the applied load, could cause deformation ofCAW and the rupture of oxide film on the surface. Then the rupture subsequently extended to exacerbate the breakage of the metal protective film which made thebottom metal expose to corrosive media leading to further erosion. Three-pointbending stress can cause rupture of the passivation film of interlayer surface,followed by hydrogen ions penetrate in the point of displacement and spread alongthe stress. Then hydrogen and fluoride ions could combine to form corrosivehydrofluoric acid and accelerate the dissolution of copper interlayer. Under thesynergy action of high fluorine and large stress, the corrosion resistance of CAWinterlayer significantly reduced, which is an even rare condition in clinical treatment.Overall, CAW showed good corrosion resistance in simulated oral environment. Itstill exhibits corrosion resistance in inorganic ion, organic protein and applied stressthat could meet the needs of clinical orthodontic arch wire applications.Electrochemical experiment and static corrosion experiment are important means todetect oral metallic materials corrosive resistance, and combine the results of thesetwo methods could effective assess the influence of different simulated oralenvironment on the corrosion behavior of metallic materials and corrosion resistanceof the material. The test methods and results in this study could provide theappropriate theoretical basis for further development and improvement of dentalmetallic materials. |
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