Elemental composition and structural characteristics of Bio-active™ orthodontic archwire

The Bio-active™ archwires are the latest generation multi-force orthodontic archwires made of a Ni-Ti alloy. It is of particular importance to orthodontists to know what their composition and structural characteristics are so that they can determine which one is suitable for a given stage of orthodontic treatment. The aim of this work is to characterize as-received Bio-active™ archwires, consisting of three segments (anterior, bicuspid and posterior), by determining their physicochemical properties. Laser-induced breakdown spectroscopy (LIBS) was used to determine the elemental composition in the three different segments of the archwires, along with X-ray diffraction analysis (XRD), scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and differential scanning calorimetry (DSC). A LIBS and EDX analysis of the elemental composition showed that nickel (55wt.%) and titanium (45wt.%) are the main elements, and in some segments Fe and Cr registered as trace elements. A XRD analysis, at room temperature, showed two similar peaks, characteristic of a Ni-Ti alloy, proof that the archwire is an austenite phase. The DSC data was obtained by measuring the Af temperatures for each segment (heated up to +80°C and cooled down to -80°C), showing that they can be classified as martensite-active wires (heat-activated). Based on that a recommendation can be made to cool down the unused, as-received archwires before clinical use to ensure that they will fit in the brackets easier. On the surface of the as-received archwires small grains can be seen from the SEM micrographs. The obtained results provide orthodontists important information regarding the physicochemical properties of the as-received Bio-active™ archwires. The results can also serve as a foundation for future research on the elemental composition and morphology of clinically applied Bio-active™ archwires.


Introduction
Orthodontic arhwires are an indispensible part of the Fixed technique in orthodontic treatment, which involves the use of brackets and the insertion of an archwire into said brackets to help move the patient's teeth.This treatment occurs in stages and the selection of a proper archwire at each stage is crucial to the treatment's overall success [1].
Due to its superior corrosion resistance, biocompatibility, and innate capacity for osseointegration, nickel-titanium (NiTi) alloy was originally utilized for medical purposes in the early 1970s, including orthodontic archwires.This alloy can be regarded as one of the major materials in orthodontics as it delivers a low and steady force throughout lengthy activations.Due to this, dental archwires have been produced more and more frequently using NiTi alloy [2].
Currently on the market there is a wide variety of multi-force NiTi dental archwires, which have been improved over the years with the addition of other elements or the application of high-quality heat treatment methods.The Bio-active™ multi-force archwires are one of the most widely used brands of that type.Looking at previous studies on the subject [3,4], there is insufficient information on their physicochemical properties available.When an orthodontist is choosing which archwire to use in a particular stage of a patient's treatment, it is of utmost importance for them to know the physicochemical properties of as-received Bio-active™ archwires.

Materials
The as-received Bio-active™ archwire (GC orthodontics, Alsip, Illionois, USA) was single packed in an airtight plastic bag.Its dimensions are 0.016×0.022 in [5].It was cut in three pieces (figure 1).

Methods
2.2.1.LIBS.The LIBSCAN25+ system (Applied Photonics Ltd, Skipton, UK) was used for qualitative determination of the Bio-active™ archwires' elemental composition.The system contains a Qswitched Nd:YAG pulsed laser and operated at 1064 nm with a 5 ns pulse duration.The laser beam was focused on the sample by a lens of 25 cm focal length.Four spectrometers are contained in the experimental console, which cover a spectral range of 185 to 785 nm with a resolution of 0.01 nm.A special LIBSoft V8.2.0 software (Applied Photonics Ltd, Skipton, UK) was used to analyze the registered data.

EDX and SEM. A scanning electron microscope ZEISS FE-SEM Ultra 55 (Zeiss, Oberkochen,
Germany), together with an Esprit 1.82 system (Bruker, Billerica, Massachusetts, USA) was used to obtain the elemental composition of the archwires by performing an energy dispersive X-ray spectroscopy.A 4 kV accelerating voltage (AV) and high-resolution in-lens secondary electra mode was used to perform the SEM and a 20 kV AV was used for EDX.An etching process was performed for both methods in order to get a clear image and exact results.The ImageJ software (developed by Wayne Rasband, USA) was used for the analysis.

XRD.
The crystalline structure was assessed by powder diffraction method.A D8-Advance powder diffractometer (Bruker, Billerica, Massachusetts, USA), with a Cu-Kα target was used.The measurement was performed at room temperature, within the range from 5-80° 2 θ at a constant step of 0.02° 2 θ.

DSC method.
Apparatus DSC250 TA Instruments New Castle, DE, USA was applied to investigate all archwire segments.About 4 mg of the material were placed in aluminum pan and another empty aluminum pan is used as an inert reference.Argon purge gas with a flow rate of 30 ml/min was used in the heating/cooling chamber to avoid water condensation and oxidation of the material.The system was heated to + 80° C and cooled to -80° C with a rate of 10 degrees per minute.The apparatus was calibrated right before testing, using indium as standard.The experimental arrangement and processing procedure were provided by specialized software (TA TRIOS; TA Instruments, New Castle, DE, USA).

Results and discussion
By using LIBS analysis, at room temperature, in an environment of air, a qualitative examination of the studied archwires' elements composition was completed.The presence of Ni and Ti, discovered by the EDS analysis, was confirmed by the LIBS spectra of the Bio-active™ archwires (figures 2 and 3), which also show the presence of Fe and Cr.Fe is presented in the 290-310nm wavelength range, while Cr is presented in the 340-440nm wavelength range.The registered spectrums were contrasted with the pure element spectra found in the NIST database [6].In our circumstance, there was no LIBS analysis calibration, thus it was only utilized for qualitative aims.EDX analysis was used for quantitative elemental analysis.It has been demonstrated that the LIBS analysis may be utilized to qualitatively characterize such samples, and the findings from this study helped to explain the characteristics of the Bio-active™ orthodontic archwires in general.The SEM micrographs of the wire's surface shows it has a rough microstructure (figure 4).Small grains can be seen, without a specific shape, with a size of around 500nm.The properties of the archwire, such as mechanical characteristics, corrosion behavior, biocompatibility and aesthetic appearance, are influenced by the topography of its surface, since it is an essential property of the archwire [7].EDX analysis were performed on all segments of the as-received Bioactive™ archwire.The elemental composition of all three segments is shown on table 1.It shows that all segments contain Ni and Ti.The anterior segment contains 0,5 wt.% Fe, while the posterior segment contains 0,3 wt.% Fe and Cr.The XRD patterns were all identical in the three segments (figure 5).The diffractogram, obtained at room temperature, shows that all three segments of the archwire are made of NiTi alloy with an austenite crystal structure (2θ = 42.48 and 2θ = 77.73).The DSC curves of each segment (anterior, bicuspid, posterior) of Bio-active™ archwires are shown on figure 6.Three phases were observed on the DSC (visualized by three peaks), namely austenite at heating and both rhombohedral -R-) phase and martensite at cooling [8].Especially important for the purposes of our study is the finish temperature of Austenite (Af) because it shows full austenite transformation of each segment sample.This temperature was determined from lines tangent to the DSC curve, in which there was a deviation of the adjacent baseline.In this way the anterior segment of Bio-active archwires demonstrates an Af close to the oral temperature, whereas premolar and molar segments have an Af value close to the room temperature.The differences between the Af in the three regions, may lead to some changes in modulus of elasticity of each sector, resulting in a variable force release along archwire's length without changing the crosssection size.

Conclusions
As received Bio-active™ archwires were characterized by various physicochemical methods in order to provide important information for orthodontists prior to their clinical use, namely: (i) Both LIBS and EDX analyses showed that Ni and Ti are main elements, while Fe and Cr are registered as trace elements in some segments; (ii) The XRD analysis showed that austenite is the only archwire phase at room tempearature; (iii) The SEM data showed rough microstructure with small grains on the surface; (iv) The DSC data showed that this type of archwires can be classified as martensite-active wires (heat-activated) due to the fact that the Af to be above room temperature and close to the temperature of the oral cavity.Based on that a recommendation can be made to cool down the unused as-received archwires before clinical use to ensure that they will fit in the brackets easier.

Figure 2 .
Figure 2. LIBS spectra of the Bio-active™ archwires, showing the presence of Cr.

Figure 3 .
Figure 3. LIBS spectra of the Bio-active™ archwires, showing the presence of Fe.

Figure 4 .
Figure 4. Representative SEM micrographs of a) anterior, b) bicuspid and c) posterior segments of the Bio-active™ multi-force archwire.