Materials Science and Engineering C 55 (2015) 267–271

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Susceptibility to corrosion of laser welding composite arch wire in artificial saliva of salivary amylase and pancreatic amylase Chao Zhang a,⁎, Jiming Liu b, Wenwen Yu b, Daqian Sun c, Xinhua Sun b a b c

Guangdong Provincial Stomatological Hospital, No. 366 South of Jiangnan Road, Guangzhou 510280, PR China Department of Orthodontics, Jilin University, No. 1500 Qinghua Road, Changchun 130021, PR China Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China

a r t i c l e

i n f o

Article history: Received 10 September 2014 Received in revised form 5 March 2015 Accepted 7 May 2015 Available online 11 May 2015 Keywords: Copper Corrosion Interface Protein adsorption

a b s t r a c t In this study, laser-welded composite arch wire (CAW) with a copper interlayer was exposed to artificial saliva containing salivary amylase or pancreatic amylase, and the resultant corrosion behavior was studied. The purpose was to determine the mechanisms by which salivary amylase and pancreatic amylase contribute to corrosion. The effects of amylase on the electrochemical resistance of CAW were tested by potentiodynamic polarization measurements. The dissolved corrosion products were determined by ICP-OES, and the surfaces were analyzed by SEM, AFM and EDS. The results showed that both exposure to salivary amylase and pancreatic amylase significantly improved the corrosion resistance of CAW. Even isozyme could have different influences on the alloy surface. When performing in vitro research of materials to be used in oral cavity, the effect of α-amylase should be taken into account since a simple saline solution does not entirely simulate the physiological situation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Composite arch wire (CAW) is a new type of orthodontic arch wire, which is formed by laser solder connection of nickel-titanium (TiNiSMA), stainless steel (SS) arch wire and copper (Cu) interlayer [1]. CAW combines the advantages of base metals and holds great potential for more efficient clinical applications. Systematic research results show that CAW reveals sufficient corrosion resistance in neutral, acidic and fluoric artificial saliva [2–4]. More importantly, CAW has demonstrated good biocompatibility in ion release and in vitro cytotoxicity [4]. Inadequate knowledge often exists about advanced materials, so clinicians rely on in vitro test procedures to make appropriate material selections for use in vivo [5]. In clinical orthodontic arch wire applications, one issue that doctors highly care about is the corrosion resistance of arch wire after placement in oral environment. The relatively fragile copper interlayer of CAW is also facing this problem. There are two important aspects on the corrosion effect related to the orthodontic application of CAW: one is the breakage of arch wire, and the other is the release of toxic ions. To better understand the corrosion resistance of a new type of biomedical material, it is essential to simulate and study the environment in which it must function. For dental materials, saliva in oral cavity is the most basic corrosion environment. Saliva secreted by salivary glands is composed of mainly salt solutions with a variety of mucus, glycoproteins, lipids, enzymes, ⁎ Corresponding author. E-mail address: [email protected] (C. Zhang).

http://dx.doi.org/10.1016/j.msec.2015.05.022 0928-4931/© 2015 Elsevier B.V. All rights reserved.

and antibacterial compounds [6]. The effect of inorganic constituents in saliva on arch wire has been extensively studied, but relatively fewer studies have dealt with the effect of proteins on the corrosion behavior of metals. Williams and Brown have demonstrated that SS is subjected to pitting corrosion in the presence of proteins [7]. Yang and Black determined the percentage of chromium, cobalt, and nickel that bind to murine serum proteins [8]. Clark and Williams indicate that proteins significantly increase the dissolution of cobalt and copper [9]. It is possible that the pretreatment of material into powders rendered them especially corrosion-susceptible, and it is difficult to extrapolate from their work directly to alloy systems. In oral cavity, solid materials are covered by proteinaceous films, formed by selective adsorption of proteins [10]. The chemical composition and properties of the biofilm are reported to be specific to the nature of surface on which it is formed and will influence subsequent surface reactions to corrosion [10,11]. Furthermore, salivary proteins, including mucin, α-amylase, histatins, gustin and lactoferrin, have metal-binding capacity with Ca2+, Cu2+, Zn2+, Ni2+ and Fe3+, whereas the effect of binding with metallic ion on corrosion resistance is still unknown [12–16]. Svare showed that specific amino acids could influence alloy passivation: for cysteine in Ringer's the passivation of Cu was improved, but alanine and albumin had little or no effect [17]. Therefore, interaction of metallic materials and protein composition, especially the enzymes in saliva, and their consecutive influences on corrosion need more research. The enzymes found in saliva are essential in beginning the process of dietary starch and fat digestion. These enzymes also play a role in breaking down food particles entrapped within dental crevices, and

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protecting teeth from bacterial decay [18]. Amylase is the enzyme that catalyzes the breakdown of starch into sugars, serves as one of the specific receptors for bacterial adherence and mediates further interaction between bacteria and dental materials [19,20]. In human physiology, both the salivary and pancreatic amylases are α-amylases, and the optimum pH is 6.7–7.0 [21]. There are two metal binding sites in α-amylase, one exclusive for Ca2+ and the other for Cu2+ [22]. It is assumed that Cu2+ could bind with proteins through electrostatic interaction following a mechanism similar to that of Ca2 + [23]. Therefore, there is an urgent need to study the exact interaction of salivary amylase and the copper interlayer of CAW. From a dental application point of view, facing a corrosive oral environment for a greater span of time would induce damage to the oxide film on the appliance surface, and then the loss of protection could allow active metal to react with the surroundings and cause a toxicity reaction [24]. Overall, the aim of this study is to evaluate the corrosion resistance of CAW in the presence of salivary amylase and pancreatic amylase in artificial saliva, and study the effect of adding different proteins on corrosion. The dissolved corrosion products were determined by ICP-OES, and the surfaces were analyzed by SEM, AFM and EDS. 2. Materials and methods 2.1. Materials and sample preparation Ti–44.73 wt.% NiTiSMA wire (Ms = 33.l °C, Mf = 27.4 °C, As = 63.8 °C, Af = 76.8 °C; Smart Co., Beijing, China), Fe–18Cr–8Ni SS, and pure Cu were used as base metals in this investigation. The dimensions of the wires were 30 mm (length) × 0.64 mm (width) × 0.48 mm (thickness). The thickness of the pure Cu interlayer was 0.2 mm. The base metal was ground using silicon carbide (SiC) papers of 800, 1200, and 2000 grit to remove the oxide layer and then ultrasonically degreased in acetone. The NiTiSMA and SS wires were fixed end-toend in a self-constructed apparatus with a pure Cu interlayer. A Nd:YAG laser welding system (JHM-1GY 300B) with a wavelength of 1064 nm was used for welding (Fig. 1c). The optimized laser parameters used in the study were laser power of 5.23 J, welding time of 6 ms, and Φ = 0.5 mm [25,26]. 2.2. Test solution preparation and immersion tests The composition of artificial saliva (AS) was KCl (0.4 g/L), NaCl (0.4 g/L), CaCl2·2H2O (0.906 g/L), NaH2PO4·2H2O (0.690 g/L), Na2S·9H2O (0.005 g/L), and urea (1 g/L) with the pH adjusted to 6.75

using lactic acid. The concentration ratio of salivary amylase (Sigma, US) was 50 mg/L according to the natural saliva composition, and concentration of pancreatic amylase (Sigma, US) was set the same in order to contrast. Each group contained 5 specimens and all the wires were mechanically grinded using SiC papers up to 2000# grit and ultrasonically cleaned in 95% alcohol and then rinsed with double-distilled water. The samples were immersed and incubated at 37 °C for periods up to 28 days. After 14 days of immersion, the medium was removed from each sample and new medium added. After 28 days, each sample was cleaned and weighed (Precision electronic balance, M2-P, Sartorius, Gottingen, Germany) to calculate weight loss incurred during immersion. The collected solutions were individually analyzed for Cu by inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 3300DV, PerkinElmer, Boston, MA, USA). The detection limit of ICP-OES was 0.01 ppm for Cu ion, and an internal standard solution was used for the ion release test. 2.3. Electrochemical measurements All electrochemical measurements were performed using a CHI 920C electrochemical workstation (CH Instruments, Shanghai, China). The counter electrode was a rectangular platinum plate, and the reference electrode was a saturated calomel electrode (SCE). After grinding with SiC papers up to 2000 grit to guarantee consistent surface roughness, all samples were embedded in cold-curing epoxy resin, with an exposed sample surface area of 20 × 0.64 mm2. After an initial delay of 60 min to achieve the steady state, the scanning rate was 1 mV/s, starting from −1 V/SCE. The potentiodynamic polarization measurements were taken between −1000 and +1000 mV. 2.4. SEM observation of surface morphology The surface morphology of the interlayer in CAW samples was observed using environmental scanning electron microscopy (SEM, ZEISS EVO18, Jena, Germany) with an energy dispersive spectrometer (EDS) analyzer (INCA-X-Max, Oxford, England). The salivary and pancreatic amylase specimens were tested for topographical characterization with Atomic Force Microscopy (AFM) in tapping mode, using a SPA300HV with a SPI3800N controller (Seiko Instruments, Inc., Japan). A silicon microcantilever (spring constant 2 N/m and resonance frequency ~ 70 kHz, Olympus Co., Japan) with an etched conical tip was used for the scan. Each measurement was 10 × 10 μm in size. The scan format was 256 × 256, which resulted in a distance of 39 nm of each pixel value in the horizontal direction (X- and Y-axis). 3. Results 3.1. Microstructures of the CAW components The TiNi and SS components of CAW were joined together via a pure Cu interlayer. Fig. 1a shows SEM images of the microstructures of the dissimilar materials at the welding zone. The surface of the welding zone was smooth, complete, and free of any apparent pores or other defects. According to EDS analysis, the welding zone showed a heterogeneous composition. From the SS to the TiNi side, the concentrations of Fe and Cr decreased and the concentrations of Ti and Ni tended to increase. Cu was distributed homogeneously within the welding zone (Fig. 1b). 3.2. Electrochemical measurements

Fig. 1. SEM surface morphologies and EDS analysis of the laser-welded composite arch wire (a, b) and schematic diagram of laser welding (c).

The effects of the experimental solutions on the typical polarization behavior of CAW in each solution are shown in Fig. 2, and the detailed electrochemical parameters are listed in Table 1. The slope of curve indicated that oxygen consumption had a vertical stage in the cathodic

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Fig. 2. Polarization curves for composite arch wires in different solutions.

section. Ecorr in both two amylase groups were almost the same, but both of that were higher than that in simple AS group. In the anodic polarization section, CAW samples exhibited a typical passive region up to the pitting potential. After the cathodic section, the anodic current densities were very stable up to a high potential. Both of the icorr of protein groups were smaller than simple AS, especially the pancreatic amylase group. 3.3. Surface morphology, weight loss, and Cu release of CAW after immersion tests After soaking in the different solutions, the surface morphology of specimens was examined by SEM, and representative images were shown in Fig. 3. No obvious degradation was observed on the interlayer surface of any samples after 28 days of immersion. There was significant protein deposit formation on the surface of protein group specimen. The corrosion pits generated regardless of the protein adding, but the pits were small and shallow in protein group, especially the pancreatic amylase group. The microscopic morphologies scanned by AFM are shown in Fig. 4. Both kinds of amylase could form hump-shaped deposited film on the surface of specimens, but the morphologies were different. Pancreatic amylase particles seemed easier to reunite and the undulating shape were more obvious. The weight loss of samples after immersion was calculated and expressed relative to the specimen surface area (Table 2). The release of Cu in both amylase groups was almost the same as simple AS, but the weight loss was smaller than simple AS group. 4. Discussion Both of the biocompatibility and in vivo corrosion resistance of biomaterials are closely dependent on the protective oxide film forming onto the surface thereof. The densification and fracture resistance of the oxide film will significantly influence the results of corrosion and harmful ion release of biomaterials [27]. For a new material such as

Table 1 The pitting potential (Epit), corrosion potential (Ecorr), and corrosion current density (icorr) values calculated from potentiodynamic polarization curves. Solution type

Epit (mV/SCE)

Ecorr (mV/SCE)

icorr (μA/cm2)

AS Salivary amylase Pancreatic amylase

102 (±9) 89 (±10) 114 (±12)

−572 (±20) −258 (±11) −246 (±19)

1.02 (±0.09) 0.91 (±0.03) 0.05 (±0.009)

Data are mean ± standard deviation.

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CAW, the surface oxide film formation in various environments urgently needs to be studied, particularly in the simulated oral environment of clinical application. In an aqueous milieu, a solid surface is instantaneously covered by an organized mono- or bilayer of water molecules, the arrangement of which is the basis of protective film and influences subsequent interactions between the surface and environment [28]. When in contact with organic environment, e.g. saliva, blood or interstitial fluids, the composition, chemical character and function of bio-film could be influenced by time, material and surroundings. Adsorption of protein from an aqueous solution is the result of various types of interactions that simultaneously occur, no matter the solid or the solubilized proteins. The polarity of protein has great impact on the adsorption process, and the acid–base interactions or electron interactions are regarded as the predominant non-covalent forces on adsorption and conformation after adsorption [29–31]. Both of the isoenzymes, salivary and pancreatic amylase, have the same molecular size (approximately 2.9 nm), but they carry different electric charge and have different specific spatial conformation. Isoelectric point of salivary amylase is 5.9 to 6.4, and pancreatic amylase is 7.0. Salivary amylase carries more negative charge than pancreatic amylase, and the differences on electron interactions could lead to different extents of adsorption. In natural oral cavity, the binding of organic molecules to enamel is thought to occur via divalent cations (Ca+2 etc.) as a bridge between the negatively charged molecules and the negative phosphate groups on the surface of hydroxyapatite [32]. In the case of metal, there exists a similar mechanism; furthermore CAW, Cu and Ca also play an important role. Cu is an element of IB, and is generally considered as one of the transition elements. The “d” electron subgrade of Cu contains an empty electron orbit, and is apt to form a stable complex compound with N, C and O elements, which are able to provide lone pair electrons. Cu could perform a variety of oxidation states during the interaction of oxygen and other corrosive elements. For simple AS without protein, Cu of CAW interlayer combined with O to form a passivation film under the action of current to block further corrosion. Clark et al. believed that protein could rob metal elements from oxide to form complex compound [9]. In the presence of amylase, the protein molecules could compete with O to bind with Cu by electrostatic attraction, and form a metal–protein complex film. The deposition of amylase and formation of a protein complex are likely responsible for corrosion rate decrease as well as the dissolution of metal ion increase. Corrosion rate decrease may be due to the protection of surface from aggressive ions (Clˉ etc.) in the solution which is the so called blocking effect. The electrostatic properties of α-amylase agglomerates in a suspension are decided by the electrostatic potential generated by the accumulation of ions on the surface of CAW [33]. The ions are arranged in an electrical double layer consisting of a Stern layer and the diffuse layer [34]. Instead of only having to diffuse through the simple electrical double layer, anions must now diffuse through the adsorbed Cu/Ca-amylase films. Even if aggressive anions became available at the metal–film interface, many of the active surface sites on metal would bound to protein, and reduce the combination of Cu to aggressive anions. Other released Cu would become bound to proteins in the succeeding deposited layers. The electrochemical properties of arch wire are important for its performance as a biomedical orthopedic material, owing to its ability to spontaneously passivate and maintain passivity under physiological conditions [35]. In polarization curves of CAW in different solutions, the higher Ecorr in amylase AS than simple AS indicates that the corrosion resistance of CAW was improved by adding these two proteins. The much smaller icorr of protein groups further testified that amylase could gradually deposit on the surface of specimens and block the appearance of pitting corrosion. The process of protein adsorption onto material surface is multifactorial, as isoelectric point, repulsion, structural stability (orientations and conformations), heterogeneity of the surface and the adsorbed

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Fig. 3. SEM interface surface morphologies of composite arch wires immersed in different artificial saliva solutions: (a) pancreatic amylase; (b) salivary amylase and (c) simple artificial saliva.

protein layer all have an impact [36]. For metal base material, the most important factor on protein adsorption is isoelectric point. The isoelectric point (pI) is the pH at which a protein carries no net electric charge. The pH value of this study, the same as natural pH of oral cavity, is higher than the isoelectric point of salivary amylase, so amylase could carry more negative charge than pancreatic amylase, which leads to more Cu combination [37]. That should be the reason for the higher Ecorr and smaller icorr of pancreatic amylase than salivary amylase. In the polarization process, the action of metallic ions is balanced electrochemically by the consumption of electrons on cathodic sites. For Cu cations, either binding to proteins or converting to products with Cl−1, S− 2, O2 or OH−1 will have occurred. For the reduction process, electrons are most likely to have been consumed by dissolved O2. In the event of amylase, O2 is restricted from the metal–film interface, and electrons transfer to acceptor groups in amylase molecule, such as by the splitting of disulfide bridges [38,39]. For the immersion test, the surface morphology of amylase groups has show to have a distinct deposition morphology in the results of SEM. The surface oxide layer from the AFM chart (Fig. 4) was a layer of deposited membranous structure showing a continuous mountainlike shape. The oxide film was formed by protein aggregation, but the deposition morphologies of the two isoenzymes are not exactly the same. This indicates that there are still different patterns of deposition formation on the metal surface in the same concentrations of isoenzyme. This would lead to different corrosion resistance of the surface, though the specific combination mechanisms still need more research. In the environment of amylase, Cu and Ca could respond to the protein binding sites of amylase. This macromolecular metal-combination effect could increase the quantity of harmful dissociative metal ions, but this effect could be offset by the protein layer blocking effect and lead to equilibration [40]. The data in Table 2 also supports this conclusion. In our previous study, adding bovine serum albumin could also cause a reduction of Cu ion elution [2]. The effect was not exactly the same as amylase, since amylase contains the specific spatial structure

for Ca and Cu binding. This concentration of salivary amylase set in this study is based on the natural state in the oral cavity, but the effect of salivary and pancreatic amylase may have a certain relationship with their concentrations. High concentrations of protein may form a more dense protective film on the metal surface to prevent further corrosion occurs. However this is only a speculation, the actual results need to be confirmed by later study. Protein adsorption is characterized by a reorientation and structural rearrangement after first adhesion to the surface which result in conformational changes. Increased protein retention after corrosion was probably due to corrosion-induced surface changes. In an aqueous environment, metallic materials will corrode mainly through ion-exchange processes, leaving behind hydrated surface layers of varying depth and composition [41]. α-Amylase formed clusters or aggregates when adsorbed onto the surface of interlayer like IgG and fibrinogen, resulting in differences in layer thickness and morphology [42] (Figs. 3 and 4). Whether the morphological appearance of the protein film is characteristic for the specific adsorbed protein irrespective of the substrate, or it is a substrate-modified morphology of the protein, is not known. The introduction of metal alloys in vivo is not completely innocuous, since they would suffer corrosion. Corrosion is therefore responsible for the release of toxic and potentially carcinogenic metallic species, like nickel and copper, from the arch wire surface. Released species would either accumulate in the surrounding tissues, or be transferred by body fluids and then accumulate in liver, spleen, etc. [43]. Since elemental release correlates with the cytotoxicity of alloys, conditions of in vitro tests which influence elemental release could profoundly affect the results. The dental industry relies on the accuracy of these in vitro tests for alloy evaluation and development. Under quiescent conditions, proteins reach the surface by Brownian molecular motion in a stochastic manner. However, in oral cavity, saliva is oversaturated with proteins which could adsorb from a flowing solution. Therefore, the actual effects in oral between salivary amylase and CAW still need further in vivo studies.

Fig. 4. The microscopic morphologies scanned by AFM of composite arch wires immersed in different artificial saliva solutions: (a) pancreatic amylase; (b) salivary amylase; (c) simple artificial saliva.

C. Zhang et al. / Materials Science and Engineering C 55 (2015) 267–271 Table 2 Copper element release and weight loss after 28 days immersion. Solution type

Copper element release (μg)

Weight loss (%)

AS Salivary amylase Pancreatic amylase

0.03 (±0.007) 0.07 (±0.006) 0.06 (±0.009)

0.075 (±0.003) 0.052 (±0.007) 0.049 (±0.005)

Data are mean ± standard deviation.

5. Conclusion The results of the present study indicate that the presence of amylase does not result in any fracture or release of excessive copper ion. CAW has good corrosion resistance in the artificial saliva containing amylase. Even isozyme could have different influences on the alloy surface. Adding amylase lead to the increase of Ecorr and the decline of icorr, which is much obvious in pancreatic amylase than salivary amylase. Both exposure to salivary and pancreatic amylase significantly improved the corrosion resistance of CAW. Amylase could act as complexing agents for dissolved metal ions to affect the electrochemical behavior of a base metal. When performing in vitro research of materials to be used in oral cavity, the effect of α-amylase should be taken into account since a pure saline solution does not entirely simulate the physiological situation. References [1] Chao Zhang, Xinhua Sun, Susceptibility to stress corrosion of laser-welded composite arch wire in acid artificial saliva, Adv. Mater. Sci. Eng. 738954 (2013). [2] Chao Zhang, Xinhua Sun, Hongmei Li, Xu Hou, Daqian Sun, The corrosion resistance of composite arch wire laser-welded By NiTi shape memory alloy and stainless steel wires with Cu interlayer in artificial saliva with protein, Int. J. Med. Sci. 10 (2013) 1068–1072. [3] Chao Zhang, Shuang Zhao, Xiumei Sun, Xinhua Sun, Daqian Sun, Corrosion of laserwelded NiTi shape memory alloy and stainless steel composite wires with a copper interlayer upon exposure to fluoride and mechanical stress, Corros. Sci. 82 (2014) 404–409. [4] Chao Zhang, Xinhua Sun, Shuang Zhao, Yu. Wenwen, Daqian Sun, Susceptibility to corrosion and in vitro biocompatibility of a laser-welded composite orthodontic arch wire, Ann. Biomed. Eng. 42 (2014) 222–230. [5] S.K. Nelson, J.C. Wataha, A.M. Neme, R.M. Cibirka, P.E. Lockwood, Cytotoxicity of dental casting alloys pretreated with biologic solutions, J. Prosthet. Dent. 81 (1999) 591–596. [6] G.N. Jenkins, The Physiology and Biochemistry of the Mouth, Blackwell Scientific Publications, Oxford press, 1978. [7] R.L. Williams, S.A. Brown, K. Merritt, Electrochemical studies on the influence of proteins on the corrosion of implant alloys, Biomaterials 9 (1988) 181–186. [8] J. Yang, J. Black, Competitive binding of chromium, cobalt and nickel to serum proteins, Biomaterials 15 (1994) 262–268. [9] G.C.F. Clark, D.F. Williams, The effects of proteins on metallic corrosion, J. Biomed. Mater. Res. 16 (1982) 125–134. [10] H.J. Busscher, M. Rinastiti, W. Siswomihardjo, H.C. van der Mei, Biofilm formation on dental restorative and implant materials, J. Dent. Res. 89 (2010) 657–665. [11] C.E. Christersson, R.G. Dunford, Salivary film formation on defined solid surfaces in the absence and presence of microorganisms (special issue: microbial adhesion and its prevention in dentistry), Biofouling 3 (1991) 237–250. [12] G.D. Brayer, G. Sidhu, R. Maurus, E.H. Rydberg, C. Braun, Y. Wang, et al., Subsite mapping of the human pancreatic α-amylase active site through structural, kinetic, and mutagenesis techniques, Biochemistry 39 (2000) 4778–4791. [13] H. Gusman, U. Lendemann, J. Grogan, R.F. Troxler, F.G. Oppenheim, Is salivary histatin 5 a metallopeptide? Biochim. Biophys. Acta 1545 (2001) 86–95. [14] A.M. Wu, G. Csako, A. Herp, Structure, biosynthesis, and function of salivary mucins, Mol. Cell. Biochem. 137 (1994) 39–55. [15] S. Melino, M. Gallo, E. Trotta, F. Mondello, M. Paci, R. Petruzzelli, Metal-binding and nuclease activity of an antimicrobial peptide analogue of the salivary histatin 5, Biochemistry 45 (2006) 15373–15383.

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Susceptibility to corrosion of laser welding composite arch wire in artificial saliva of salivary amylase and pancreatic amylase.

In this study, laser-welded composite arch wire (CAW) with a copper interlayer was exposed to artificial saliva containing salivary amylase or pancrea...
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