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CAM lithium disilicate-based glass ceramics on HF etching: An XPS study
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Physicochemical surface characterizations of four dental CAD/CAM lithium disilicate-based glass ceramics on HF etching: An XPS study Rodrigo Françaa,b,∗, Muna Bebshb, Asmaa Haimeura, Ana Carla Fernandesb, Edward Sacherc a
Dental Biomaterials Research Laboratory, Department of Restorative Dentistry, University of Manitoba, Winnipeg, Manitoba, Canada Department of Oral Biology, University of Manitoba, Winnipeg, Manitoba, Canada c Department of Engineering Physics, École Polytechnique, Montréal, Québec, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acid etching Glass ceramics Lithium disilicate SEM XPS
The surfaces of four lithium disilicate glass ceramics (LDGC) were characterized at the nanolevel. The goal was to detect the chemical alteration of the surface on etching with hydrofluoric acid (HF). The four LDGC tested were Celtra Duo, IPS e.max CAD, Straumann n!ce and Vita Suprinity. Four blocks of each LDGC were sectioned to ∼ 1 mm thicknesses. The requirement for firing, or not, was carried out following the manufacturer’s recommendations. The samples were then divided into two groups: a control and an etched group. Etching was carried out using a solution of 5% HF for 20 s, rinsed for 20 s and dried for 10 s, using an air jet. The atomic percentages of the first atomic layers were probed using X-ray photoelectron spectroscopy (XPS) in the survey mode (n = 12). The oxygen and silicon peaks (O1s and Si2p) were then analyzed in the high resolution mode. The samples were also characterized using scanning electronic microscopy at high magnification (60 k). XPS showed the amounts of the major elements, Si, O and Li, were changed on etching. For all samples, trace elements, such as P, Zn, Y, Na and Sr, disappeared on glassy phase dissolution. Zr and Al percentages varied, based on the LDGC analyzed. High resolution spectra of the O1s and Si2p peaks showed that the chemical environments were qualitatively different in all samples. Acid etching, using 5% HF for 20 s, modified not only the topographic structure, but also the chemical composition of the LDGC surface.
1. Introduction Monolithic lithium disilicate-based glass ceramics (LDGCs), used for computer-aided design and computer-aided manufacturing (CAD/ CAM), have become common in dentistry, due to their superior mechanical and satisfactory esthetic properties. Compared to feldspathicbased glass ceramics, LDGCs possesses improved toughness strength, thanks to a controlled crystallization process that results in a highly crystalline phase, and low void content. LDGCs present engineered microstructures, composed of a glassy matrix and a crystalline phase [1–10]. The matrix is typically formed by a vitreous silica glass, and the crystals are composed of a phyllosilicate, such as lithium disilicate (Li2Si2O5) [7,10–13]. A small number of other oxides are added to the structure to enhance supplementary features, such as color and fluorescence. In dentistry, the popularity of LDGC restorations is also due to their extended clinical longevities. A systematic review showed a 97.8% fiveyear cumulative survival rate for single crowns, and a 70.9% ten-year survival rate for multiple-unit prostheses [14]. In vitro studies reveal
∗
that, when etching the intaglio with hydrofluoric acid (HF), and using a silane coupling agent, the bond strength at the resin cement/LDGC interface can be between 20 and 32 MPa [15–19]. These values are related to an increase of the surface area by the dissolution of the glassy matrix, and the exposure of the residual crystalline phase. This surface treatment protocol produces superficial microporosities, increases the surface energy and permits good interlocking with the resin cement. The application of a silane coating promotes chemical reaction between the LDGC surface and the resin cement. While various silane chemical compositions have been proposed, only a few have obtained success [20]. One of the challenges is that the reaction stoichiometry between HF and the LDGC is not yet well understood. Thus, the development of an effective coupling agent becomes complicated when the chemical composition of the substrate surface is unknown. From their introduction in the 90’s, LDGCs have been in continuous evolution. Various chemical compositions are available on the dental market. The first product available, IPS e.max® CAD (see Table 1), was followed by products with high concentrations of either zirconium (Vita
Corresponding author. 780 Bannatyne Ave., room 227H, Winnipeg, R3E 0W2, MB, Canada. E-mail address: [email protected] (R. França).
https://doi.org/10.1016/j.ceramint.2019.09.105 Received 3 July 2019; Received in revised form 28 August 2019; Accepted 11 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rodrigo França, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.105
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Table 1 Chemical composition of four LDGC products provided by the manufacturer. Materials
Abbreviation
Manufacturer
Chemical composition
IPS e. max® CAD
e.max CAD
Ivoclar-Vivadent, Schaan, Liechtenstein,
Vita Suprinity®
Suprinity
Vita Zahnfabrik, Bad Sackingen, Germany
Celtra Duo®
Celtra
Straumann n!ce®
n!ce
Dentsply Sirona, Hanau-Wolfgang, Germany Straumann Basel Switzerland
57–80% SiO2; 11–19% Li2O; 0–13% K2O; 0–11% P2O5 0–8% ZrO2-ZnO, 0–5% Al2O3; 0–5% MgO - colouring oxides 56–64% SiO2; 5–21% Li2O; 1–4% K2O; 3–8% P2O5; 8–12% ZrO2; 0–4% CeO2 0–6% pigments 58% SiO2; 18.5% Li2O; 5% P2O5; 10.1% ZrO; 1.9% Al2O3; 2% CeO2; 1% Tb4O7 64–70% SiO2; 10.5 - 12.5% Li2O; 0–3% K2O; 0–0.5% P2O5; 10.1% ZrO; 10.5-11.5% Al2O3; 0–0.5% CaO; 0–9% pigments
[25,26] are provided as fully crystallized; no additional heat treatments were required. All samples were cleaned with alcohol and sonicated for 15 min, then stored in distilled water at room temperature. Specimens of each type of LDGC (n = 12) were randomly divided into two groups: a control and an etched group. Each sample of the etched group had one of the surfaces etched with a 5% HF solution for 20 s, rinsed for 20 s and dried for 10 s, following manufacturers’ recommendations.
Suprinity® and Celtra Duo®), or aluminum (Straumann n!ce®). In addition, conventional LDGC blocks are furnished in an intermediary stage of crystallization, containing lithium metasilicate (Li2SiO3); these “soft” stage ingots can be easily milled by a CAD/CAM machine. The final, fully crystallized restoration is obtained after heat treatment (> 800 °C), where the Li2SiO3 phase becomes Li2Si2O5 crystals [7,11]. Recently, some manufacturers released LDGC blocks that are fully crystallized, advertising gains in completion time and improved mechanical strength. However, all these modifications and improvements brought more variables that could affect the reactions between both HF and silane, and the substrate. It has become clear that a knowledge of the morphologies and chemical compositions of the first atomic layers of the LDGC are fundamental to the understanding of the process of adhesion to resin cement. Manufacturers only provide the bulk composition of their products, and the amount is normally vague (Table 1). As a result of their thermodynamic instability, ceramic surfaces rarely present the same composition as that of the bulk [21]. Previous investigations provided good qualitative or semi-quantitative chemical analyses [12,15,22]. Unfortunately, techniques, such as FTIR, Raman, EDX or XRD, do not probe the nano-surface. To date, no information is available on the chemical composition of the outermost atomic layers of dental LDGC products. This study was designed to characterize the physicochemical compositions of the nano-surfaces of four LDGC products available on the dental market, using X-ray Photoelectron Spectroscopy (XPS),a powerful technique that can provide qualitative and quantitative (%) analyses of the first atomic layers. The morphologic effect of HF etching was also evaluated, using Scanning Electron Microscopy (SEM); to assess the effect of HF on the samples, the null hypothesis is that the acid etching protocol will not cause any difference to the nano-surface chemical compositions of the four LDGCs. The present study, limited to using XPS and SEM, is part of a program to evaluate new techniques to evaluate bonding. In particular, we explore their ease of use, feasibility and information gained. We intend to use the results to plan a larger, integrated study, using several additional techniques.
2.2. Sample characterization SEM: Topographic characterizations of samples, before and after acid etching, were performed using an FEI Nova NanoSEM 450 highresolution scanning electron microscope (ThermoFisher, Oregon, USA), with an SEI detector. The accelerating voltage for all materials was 20 kV, with a working distance around 10 mm. Magnifications were at the 20 μm and 500 nm scales. XPS: XPS was performed using a Kratos Axis, (Manchester, UK). An Al Kα monochromatic radiation source (1486.6 eV) was used, at a current of 15 mA and a voltage of 15 kV. Charge neutralization was used because the samples were not electrically conductive. Samples were placed into loading tray slots and introduced into the analysis chamber at a base pressure of < 1 × 10−9 Torr. Before analysis, the surfaces were cleaned with an argon (Ar) laser, for 75 s, to eliminate the adventitious carbon layer. Survey and high resolution (HR) photoelectron spectra were collected, at a takeoff angle of 15° from the perpendicular. Three regions of four slices of each LDGC were randomly analyzed, using a spot size of ∼ 2 mm2 (n = 12). The presence of residual Ar in the samples was negligible (< 0.1%). After Shirley background removal, elemental identification, atomic percentages, and peak deconvolutions were carried out using the CasaXPS software, version 2.3.1 (Casa Software Ltd, UK). Each Si2p spectral component was represented by two spin-orbit components (p3/2 and p1/2), separated by ∼ 0.63 eV, with a 2:1 area ratio. After the peaks were properly fitted, the spectral energies were calibrated by moving the highest intensity Si2p peak, attibuted Li2Si2O5 [27], to 102.5 eV; this was necesry because of the low intensity C1s spectrum. The full widths at half-maxima (FWHM), for peak deconvolution, were kept constant at 1.8 eV for O1s and 1.6 eV for Si2p. The peak attributions were established using the XPS database of the National Institute of Standards and Technology (NIST) [28]. The average and standard deviation for each chemical element were calculated using both survey and high resolution mode results.
2. Materials and methods 2.1. Samples tested Table 1 shows the LDGC samples used in this study, with their chemical compositions provided by the manufacturers. Four blocks of each LDGC product were sectioned (∼7 x 7 × 1 mm), using a lowspeed Isomet 1000 (Buehler Ltd, Lake Bluff, IL, USA) sectioning saw. IPS emax CAD and Vita Suprinity (both provided in a partially crystallized stage) [23,24] were then fully crystallized in a Programat P310 furnace (Ivoclar Vivadent, Schaan, Liechtenstein), following manufacturers’ recommendations. Celtra Duo and n!ce ceramic blocks
3. Results Figs. 1 to 4 are SEM images that show the surface morphologies of the LDGCs after being etched. The e.max CAD and n!ce samples presented needle-like crystals on dissolution of the glassy matrix. The sizes of the crystals were several nm wide and several μm long. Celtra and Suprinity samples were composed of equiaxial crystals of several nm. 2
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Fig. 4. SEM image of the surface microstructure of Straumann n!ce on etching. Fig. 1. SEM image of the surface microstructure of IPS e.Max CAD on etching.
and 4.9% Al, respectively, while the other LDGCs contained less than 1.8%. Except for Ce, trace elements (< 1%), such as P, Y, Sr, N and Na, were detected only in the control samples; Ce was found under both experimental conditions, except for etched Suprinity samples. Less than 1% F was detected in n!ce, for both conditions. Table 3 shows the results of peak deconvolutions obtained on HR XPS analyses, and are displayed in Figs. 5 to 8. The control groups for Celtra, e.max CAD and Suprinity showed three contributions for Si2p peak: A) ∼ 101.8 eV, attributed to an amorphous, partially oxidized Si, which is bonded to two O (Si2-Si-O2); B) ∼ 102.5 eV, attributed to Li2Si2O5, (LS2), with contributions from the silicates of the trace metals present [29,30]; C) ∼ 104 eV, attributed to SiO2, in which Si is bonded to four O [29,30]. The n!ce control group spectrum contained only contributions A and B. The O1s spectra, for both e.max and n!ce, contained two contributions: A) ∼ 531 eV, attributed to oxygen in lithium compounds such as LiOH, Li3PO4, LiPO3; B) ∼ 532 eV attributed to oxygen linked to silicon, as in silicates and Li2Si2O5. The Celtra and Suprinity spectra contained a third contribution: C) ∼ 534 eV, attributed to oxygen present in oxidized C [31]. The HR peak fitting showed that the peak intensities vary on etching. That is, the surface compositions were altered on etching, with some shifting of peak positions, well within experimental error. The amount of shift is in superscript, between parentheses, in Table 3. These shifts suggest compositional changes on etching.
Fig. 2. SEM image of the surface microstructure of Vita Suprinity on etching.
4. Discussion Our results show that, at the surface level, the concentrations of the major elements, O, Si and Li, vary significatively on etching for all LDGCs. The null hypothesis was therefore rejected. Our atomic percentages differ from those reported in the literature [15,22,25,32–35]: the manufacturers report different percentages of components, and some investigators did not note any changes. One reason for this discrepancy is that these investigations employed energy dispersive x-ray spectroscopy (EDS or EDX) [15,22,32]. EDX has a low surface sensitivity and, thus, a limited ability to characterize the surface composition [36]. The probe beam penetrates 1–2 μm in depth, assessing bulk, rather than surface, composition. Further, EDX cannot detect lithium, which affects the accuracy of the LDGC data. On the other hand, XPS can quantitatively detect even small concentrations (0.01%) of all elements, except for H and He, and is sensitive to a limited number of atomic layers in depth (≤4 nm), making it a perfect tool for surface analysis. The reaction between the glass and HF is described by equation (1):
Fig. 3. SEM image of the surface microstructure of Celtra Duo on etching.
The presence of the glassy matrix is noticeable for both e.max CAD and n!ce. Table 2 displays the XPS survey results of the samples before and after etching; O, Si and Li are the major constituents of all LDGCs. Although the amount of adventitious carbon present was greatly reduced in all samples by Ar cleaning, n!ce retained more than 10% C, and Zr was detected in all the samples. In the control group, Celtra and Suprinity had 2.4% and 4.7% Zr, respectively; these amounts changed to 2.9% and 2.2% after etching; n!ce samples displayed Zr only prior to etching (1%), and e.max CAD showed very small amounts under both experimental conditions. Control and etched samples of n!ce contained approximately 5.9%
SiO2 + 6 HF → H2SiF6 + 2 H2O
(1)
Among dental researchers, there is a predominantly held opinion 3
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Table 2 XPS survey analyses of four LDGC, with control and 5% HF etched groups showing the change of atomic percentage at the surface. Elements %
O 1s Si 2p Li 1s C 1s Al 2p P 2p K 2p Zn 2p Y 3p Zr 3p Sr 3p Ce 3d N 1s Na 1s F 1s
Celtra
e.max CAD
n!ce
Suprinity
Control
Etched
Control
Etched
Control
Etched
Control
Etched
49.2 22.9 19.7 2.1 1.5 0.8 0.5 – 0.2 2.4 0.2 0.1 0.2 0.2 –
45.5 21.4 21.2 6.6 1.5 – 0.5 – – 2.9 – 0.3 – – –
50 26.9 11.8 5.5 1.0 2.1 1.1 0.2 0.3 0.2 0.3 0.3 0.3 – –
46.6 25.6 12.1 9.5 1.1 – 2.2 0.2 – 0.2 0.3 0.3 – – –
44.7 25.2 8.4 12.8 5.9 – – – 0.3 1 – 0.1 1.4 – 0.2
45.6 22.1 14.2 11.5 4.9 – – – 0.2 – – 0.3 0.7 – 0.5
45.1 24.8 16.9 2.1 1.5 1.1 0.4 – 0.6 4.7 – 0.2 0.1 0.6 –
43.1 22.2 19.6 9.4 1.8 – 0.5 – – 2.2 – – – – –
of this residual quartz. The Si2p specrum in Fig. 5 informs us that the surface of Suprinity is principally composed of LS2 crystals, since that is the only peak present. Further, the O1s peak ratio reveals that the Li:Si ratio of this compound is 1:2.41, within a small error of the expected ratio of 1:2.39 for [Li2O]·2[SiO2], proposed by Holand et al. [7,43] However, for Celtra (referred to, by the manufacturers, as “zirconia-reinforced lithium silicate glass ceramic”, as is Suprinity) the atomic composition of the surface is quite different (Fig. 6). On etching, Celtra presented the same three chemical environments as those prior to etching. The amount of glassy phase was reduced from 28.7% to 9.1%, the LS2 phase remained the same, and the quartz phase increased from 8.8% to 27.5%. A possible explanation for this is that, as the glassy phase dissolves, the quartz phase increases in concentration. The O1s ratio presents a Li:Si ratio equal to 1:1.84, indicating that the Celtra surface has a higher content of lithium than that of Suprinity. This may indicate that the residual glassy matrix is rich in Li2SiO3 and/or Li3PO4. Of the four LDGC products that were assessed, e.max CAD appears to be the least affected by etching. The small change that was observed was due to the dissolution of the glassy phase Prior to etching, e.max CAD had the highest percentage of LS2 phase (Fig. 7), which contributes positively to its mechanical performance. The O1s spectrum shows a Li:Si ratio of 1:5.75, indicating a very small amount of lithium in the remaining glassy matrix. This was also be confirmed by the XPS survey results, which show that, on etching, e.max CAD had the greatest amount of Si and lowest amount of Li (Table 2). As shown in Fig. 8, the n!ce surface manifested distinctive features. First, n!ce is the only LDGC that, in the control group, does not have a third Si2p contribution. An explanation for this is that n!ce is furnished as a fully crystallized glass ceramic, for which the manufacturer does not recommend additional firing. As discussed earlier, should the e.max CAD ingot reach more the 900 °C during the fabrication phase, no crystalline SiO2 material (e.g., quartz) remains in the glassy phase.
that only the glassy matrix is dissolved [17,22,37]. Actually, LS2 crystals seem to be more resistant to dissolution than SiO2. As shown in equation (2), Li2Si2O5 + 14 HF → 2 LiF + 2 H2SiF6 + 5 H2O.
(2)
In order to dissolve one molecule of LS2, ons needs more than twice the HF concentration used in Equation (1). Further, the literature shows that the higher the degree of crystallization, the lower the solubility of the LDGC [38]. Our XPS results are consistent with this position, showing a reduction or elimination of the peaks attributed to the glassy phase, on etching. No fluorine was detected in the etched samples. Were there Si-F compounds (e.g., hexafluorosilicate) present, they would manifest a Si2p peak at ∼ 104.5 eV [39]; no sample possessed such a peak. A direct clinical implication of this finding validates that rinsing for 20 s is enough to wash off all by-products. An analysis of the Si2p and O1s peaks leads to a better understanding of what happened in the outermost atomic layers of the LDGCs on etching. As previously noted, prior to etching, Celtra, Suprinity and e.max CAD manifest three Si chemical environments: a glassy phase, the main phase (LS2) and a third, SiO2 phase, at ∼ 104 eV (Table 3) [29,30]. Tashiro and Wada reported that, in systems using ZrO2 as nucleating agents, partial crystallization of the base glass, as an αquartz solid solution precursor, can result [40]. Huang and coworkers also note the formation of quartz, cristobalite and tridymite during the crystallization of LS2, even in systems with low Zr content [41]. The same authors observed that the quartz normally disappears at 980 °C [42]. Certainly, the presence of this residual quartz (or another form of crystalline SiO2) would be beneficial in increasing mechanical properties. The amounts of SiO2 in our samples were 8.8% in Celtra, 13% in Suprinity and 7.5% in e.max. Despite the fact that n!ce contained 1% Zr, SiO2 was not detected. Further investigations, to occur in our projected integrated study, will be needed to better understand the nature
Table 3 The peak positions for silicone and oxygen of four LDCG, for control and 5% HF etched groups. Chemical Element
Peak deconvolution
Peak position (eV)
FWHM (eV)
Attribution
Celtra (%)
e.max (%)
n!ce (%)
Suprinity (%)
Control
Etched
Control
Etched
Control
Etched
Control
Etched
Silicon
Si2p A Si2p B Si2p C
∼ 101.8 ∼ 102.5 ∼ 104
1.6 1.6 1.6
SiO2 Glass Li2Si2O5 SiO2 Crystal
28.7 62.5 8.8
9.1 63.4 28.4
12.5 80 7.5
18.3 81.7 0
32.9 67.1 0
0 100 0
21.3 65.7 13
0 100 0
Oxygen
O1s A O1s B O1s C
∼ 531 ∼ 532 ∼ 534
1.8 1.8 1.8
O-Li O-Si O-P
30.5 57.5 11.9
35.2 64.8 0
11.1 88.9 0
14.8 85.2 0
13.5 85.4 0
15.4 84.6 0
32.5 61.1 6.4
22.9 77.1 0
4
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Fig. 5. Si2p and O1s XPS high resolution peaks for Suprinity, for control and etched samples. The deconvolution details are displayed in Table 3.
at ∼ 102.7 eV. Finally, the elevated amount of carbon in the n!ce samples indicates the continued presence of carbon, even after Ar laser pre-treatment. The clinical significance of this study rests on the fact that a 5% HF acid etching surface treatment has become the standard clinical protocol to obtain satisfactory bond strength for LDGC restorations. SEM images have shown the effective dissolution of the glassy phase and the exposure of the crystal phase in all samples. This dissolution increases both surface area and surface energy, and these two features together play a major role in aiding the adhesion process [18,37,47,48].
Second, the amount of aluminum in n!ce is about four times greater than in the other LDGCs. The manufacturer describes n!ce as a lithium aluminosilicate ceramic that is reinforced with lithium disilicate. Our study detected Al only in the glassy phase. This finding was confirmed by the deconvolution of the Al2p peak (Fig. 9). In the control group, the Al-Li compounds were found, at ∼ 75.8 eV [44]. After etching, the concentration of Al-Li compounds seems to decrease drastically, due to the Al peak shifting to 74.6 eV. This new peak position may indicate the presence of silicates, such as Al2SiO5 and/or Kaolinite ( Al4Si4O10(OH)8) [45,46], an assignment supported by the Si2p peak
Fig. 6. Si2p and O1s XPS high resolution peaks for Celtra, for control and etched samples. The deconvolution details are displayed in Table 3. 5
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Fig. 7. Si2p and O1s XPS high resolution peaks for e.Max CAD, for control and etched samples. The deconvolution details are displayed in Table 3.
etching that leads to strong adhesion.
However, mechanical interlocking is only one part of the bonding process, the chemical adhesion is achieved by using silane coupling agents. The efficiency of the silane treatment is basically dependent on the chemical composition of the etched surface. For the first time, we have described the physicochemical composition of the first atomic layers and the effect of HF etching on dental LDGCs. This information will help to understand the challenges of adhesive processes, particularly in selecting coupling agents for specific surface chemical compositions. By these methodologies, future studies, such as the one we propose, will be able to assess the ideal ratio/concentration/time for HF
5. Conclusions Within the limitations of this investigation, confined to XPS and SEM, it is possible to conclude that the surfaces of all LDGCs have distinctly different chemical compositions on acid etching. At the nanolevel probed, Celtra, e.max CAD and Suprinity displayed SiO2 crystalline phases. After etching, these phases disappear, except for Celtra. LS2 crystal concentrations vary from 62.5% to 80% prior to etching.
Fig. 8. Si2p and O1s XPS high resolution peaks for n!ce, for control and etched samples. The deconvolution details are displayed in Table 3. 6
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[13]
[14]
[15]
[16] [17]
[18]
[19]
[20]
Fig. 9. Al2p XPS high resolution peak for n!ce, for control and etched samples; dotted lines show the peak shift after HF etching.
[21]
Suprinity and n!ce are the only ones that present no glassy phase on etching.
[22]
[23]
Conflicts of interest:
[24]
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
[25]
Declaration: [26]
We hereby declare that this manuscript is original, and is not currently being considered for publication elsewhere.
[27]
Acknowledgments [28] [29]
The authors thank the University of Manitoba for financial support through a College of Dentistry Research Grant, as well as the manufacturers for furnishing the samples.
[30] [31]
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Physicochemical surface characterizations of four dental CAD/CAM lithium disilicate-based glass ceramics on HF etching: An XPS study Rodrigo Françaa,b,∗, Muna Bebshb, Asmaa Haimeura, Ana Carla Fernandesb, Edward Sacherc a
Dental Biomaterials Research Laboratory, Department of Restorative Dentistry, University of Manitoba, Winnipeg, Manitoba, Canada Department of Oral Biology, University of Manitoba, Winnipeg, Manitoba, Canada c Department of Engineering Physics, École Polytechnique, Montréal, Québec, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acid etching Glass ceramics Lithium disilicate SEM XPS
The surfaces of four lithium disilicate glass ceramics (LDGC) were characterized at the nanolevel. The goal was to detect the chemical alteration of the surface on etching with hydrofluoric acid (HF). The four LDGC tested were Celtra Duo, IPS e.max CAD, Straumann n!ce and Vita Suprinity. Four blocks of each LDGC were sectioned to ∼ 1 mm thicknesses. The requirement for firing, or not, was carried out following the manufacturer’s recommendations. The samples were then divided into two groups: a control and an etched group. Etching was carried out using a solution of 5% HF for 20 s, rinsed for 20 s and dried for 10 s, using an air jet. The atomic percentages of the first atomic layers were probed using X-ray photoelectron spectroscopy (XPS) in the survey mode (n = 12). The oxygen and silicon peaks (O1s and Si2p) were then analyzed in the high resolution mode. The samples were also characterized using scanning electronic microscopy at high magnification (60 k). XPS showed the amounts of the major elements, Si, O and Li, were changed on etching. For all samples, trace elements, such as P, Zn, Y, Na and Sr, disappeared on glassy phase dissolution. Zr and Al percentages varied, based on the LDGC analyzed. High resolution spectra of the O1s and Si2p peaks showed that the chemical environments were qualitatively different in all samples. Acid etching, using 5% HF for 20 s, modified not only the topographic structure, but also the chemical composition of the LDGC surface.
1. Introduction Monolithic lithium disilicate-based glass ceramics (LDGCs), used for computer-aided design and computer-aided manufacturing (CAD/ CAM), have become common in dentistry, due to their superior mechanical and satisfactory esthetic properties. Compared to feldspathicbased glass ceramics, LDGCs possesses improved toughness strength, thanks to a controlled crystallization process that results in a highly crystalline phase, and low void content. LDGCs present engineered microstructures, composed of a glassy matrix and a crystalline phase [1–10]. The matrix is typically formed by a vitreous silica glass, and the crystals are composed of a phyllosilicate, such as lithium disilicate (Li2Si2O5) [7,10–13]. A small number of other oxides are added to the structure to enhance supplementary features, such as color and fluorescence. In dentistry, the popularity of LDGC restorations is also due to their extended clinical longevities. A systematic review showed a 97.8% fiveyear cumulative survival rate for single crowns, and a 70.9% ten-year survival rate for multiple-unit prostheses [14]. In vitro studies reveal
∗
that, when etching the intaglio with hydrofluoric acid (HF), and using a silane coupling agent, the bond strength at the resin cement/LDGC interface can be between 20 and 32 MPa [15–19]. These values are related to an increase of the surface area by the dissolution of the glassy matrix, and the exposure of the residual crystalline phase. This surface treatment protocol produces superficial microporosities, increases the surface energy and permits good interlocking with the resin cement. The application of a silane coating promotes chemical reaction between the LDGC surface and the resin cement. While various silane chemical compositions have been proposed, only a few have obtained success [20]. One of the challenges is that the reaction stoichiometry between HF and the LDGC is not yet well understood. Thus, the development of an effective coupling agent becomes complicated when the chemical composition of the substrate surface is unknown. From their introduction in the 90’s, LDGCs have been in continuous evolution. Various chemical compositions are available on the dental market. The first product available, IPS e.max® CAD (see Table 1), was followed by products with high concentrations of either zirconium (Vita
Corresponding author. 780 Bannatyne Ave., room 227H, Winnipeg, R3E 0W2, MB, Canada. E-mail address: [email protected] (R. França).
https://doi.org/10.1016/j.ceramint.2019.09.105 Received 3 July 2019; Received in revised form 28 August 2019; Accepted 11 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rodrigo França, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.105
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Table 1 Chemical composition of four LDGC products provided by the manufacturer. Materials
Abbreviation
Manufacturer
Chemical composition
IPS e. max® CAD
e.max CAD
Ivoclar-Vivadent, Schaan, Liechtenstein,
Vita Suprinity®
Suprinity
Vita Zahnfabrik, Bad Sackingen, Germany
Celtra Duo®
Celtra
Straumann n!ce®
n!ce
Dentsply Sirona, Hanau-Wolfgang, Germany Straumann Basel Switzerland
57–80% SiO2; 11–19% Li2O; 0–13% K2O; 0–11% P2O5 0–8% ZrO2-ZnO, 0–5% Al2O3; 0–5% MgO - colouring oxides 56–64% SiO2; 5–21% Li2O; 1–4% K2O; 3–8% P2O5; 8–12% ZrO2; 0–4% CeO2 0–6% pigments 58% SiO2; 18.5% Li2O; 5% P2O5; 10.1% ZrO; 1.9% Al2O3; 2% CeO2; 1% Tb4O7 64–70% SiO2; 10.5 - 12.5% Li2O; 0–3% K2O; 0–0.5% P2O5; 10.1% ZrO; 10.5-11.5% Al2O3; 0–0.5% CaO; 0–9% pigments
[25,26] are provided as fully crystallized; no additional heat treatments were required. All samples were cleaned with alcohol and sonicated for 15 min, then stored in distilled water at room temperature. Specimens of each type of LDGC (n = 12) were randomly divided into two groups: a control and an etched group. Each sample of the etched group had one of the surfaces etched with a 5% HF solution for 20 s, rinsed for 20 s and dried for 10 s, following manufacturers’ recommendations.
Suprinity® and Celtra Duo®), or aluminum (Straumann n!ce®). In addition, conventional LDGC blocks are furnished in an intermediary stage of crystallization, containing lithium metasilicate (Li2SiO3); these “soft” stage ingots can be easily milled by a CAD/CAM machine. The final, fully crystallized restoration is obtained after heat treatment (> 800 °C), where the Li2SiO3 phase becomes Li2Si2O5 crystals [7,11]. Recently, some manufacturers released LDGC blocks that are fully crystallized, advertising gains in completion time and improved mechanical strength. However, all these modifications and improvements brought more variables that could affect the reactions between both HF and silane, and the substrate. It has become clear that a knowledge of the morphologies and chemical compositions of the first atomic layers of the LDGC are fundamental to the understanding of the process of adhesion to resin cement. Manufacturers only provide the bulk composition of their products, and the amount is normally vague (Table 1). As a result of their thermodynamic instability, ceramic surfaces rarely present the same composition as that of the bulk [21]. Previous investigations provided good qualitative or semi-quantitative chemical analyses [12,15,22]. Unfortunately, techniques, such as FTIR, Raman, EDX or XRD, do not probe the nano-surface. To date, no information is available on the chemical composition of the outermost atomic layers of dental LDGC products. This study was designed to characterize the physicochemical compositions of the nano-surfaces of four LDGC products available on the dental market, using X-ray Photoelectron Spectroscopy (XPS),a powerful technique that can provide qualitative and quantitative (%) analyses of the first atomic layers. The morphologic effect of HF etching was also evaluated, using Scanning Electron Microscopy (SEM); to assess the effect of HF on the samples, the null hypothesis is that the acid etching protocol will not cause any difference to the nano-surface chemical compositions of the four LDGCs. The present study, limited to using XPS and SEM, is part of a program to evaluate new techniques to evaluate bonding. In particular, we explore their ease of use, feasibility and information gained. We intend to use the results to plan a larger, integrated study, using several additional techniques.
2.2. Sample characterization SEM: Topographic characterizations of samples, before and after acid etching, were performed using an FEI Nova NanoSEM 450 highresolution scanning electron microscope (ThermoFisher, Oregon, USA), with an SEI detector. The accelerating voltage for all materials was 20 kV, with a working distance around 10 mm. Magnifications were at the 20 μm and 500 nm scales. XPS: XPS was performed using a Kratos Axis, (Manchester, UK). An Al Kα monochromatic radiation source (1486.6 eV) was used, at a current of 15 mA and a voltage of 15 kV. Charge neutralization was used because the samples were not electrically conductive. Samples were placed into loading tray slots and introduced into the analysis chamber at a base pressure of < 1 × 10−9 Torr. Before analysis, the surfaces were cleaned with an argon (Ar) laser, for 75 s, to eliminate the adventitious carbon layer. Survey and high resolution (HR) photoelectron spectra were collected, at a takeoff angle of 15° from the perpendicular. Three regions of four slices of each LDGC were randomly analyzed, using a spot size of ∼ 2 mm2 (n = 12). The presence of residual Ar in the samples was negligible (< 0.1%). After Shirley background removal, elemental identification, atomic percentages, and peak deconvolutions were carried out using the CasaXPS software, version 2.3.1 (Casa Software Ltd, UK). Each Si2p spectral component was represented by two spin-orbit components (p3/2 and p1/2), separated by ∼ 0.63 eV, with a 2:1 area ratio. After the peaks were properly fitted, the spectral energies were calibrated by moving the highest intensity Si2p peak, attibuted Li2Si2O5 [27], to 102.5 eV; this was necesry because of the low intensity C1s spectrum. The full widths at half-maxima (FWHM), for peak deconvolution, were kept constant at 1.8 eV for O1s and 1.6 eV for Si2p. The peak attributions were established using the XPS database of the National Institute of Standards and Technology (NIST) [28]. The average and standard deviation for each chemical element were calculated using both survey and high resolution mode results.
2. Materials and methods 2.1. Samples tested Table 1 shows the LDGC samples used in this study, with their chemical compositions provided by the manufacturers. Four blocks of each LDGC product were sectioned (∼7 x 7 × 1 mm), using a lowspeed Isomet 1000 (Buehler Ltd, Lake Bluff, IL, USA) sectioning saw. IPS emax CAD and Vita Suprinity (both provided in a partially crystallized stage) [23,24] were then fully crystallized in a Programat P310 furnace (Ivoclar Vivadent, Schaan, Liechtenstein), following manufacturers’ recommendations. Celtra Duo and n!ce ceramic blocks
3. Results Figs. 1 to 4 are SEM images that show the surface morphologies of the LDGCs after being etched. The e.max CAD and n!ce samples presented needle-like crystals on dissolution of the glassy matrix. The sizes of the crystals were several nm wide and several μm long. Celtra and Suprinity samples were composed of equiaxial crystals of several nm. 2
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Fig. 4. SEM image of the surface microstructure of Straumann n!ce on etching. Fig. 1. SEM image of the surface microstructure of IPS e.Max CAD on etching.
and 4.9% Al, respectively, while the other LDGCs contained less than 1.8%. Except for Ce, trace elements (< 1%), such as P, Y, Sr, N and Na, were detected only in the control samples; Ce was found under both experimental conditions, except for etched Suprinity samples. Less than 1% F was detected in n!ce, for both conditions. Table 3 shows the results of peak deconvolutions obtained on HR XPS analyses, and are displayed in Figs. 5 to 8. The control groups for Celtra, e.max CAD and Suprinity showed three contributions for Si2p peak: A) ∼ 101.8 eV, attributed to an amorphous, partially oxidized Si, which is bonded to two O (Si2-Si-O2); B) ∼ 102.5 eV, attributed to Li2Si2O5, (LS2), with contributions from the silicates of the trace metals present [29,30]; C) ∼ 104 eV, attributed to SiO2, in which Si is bonded to four O [29,30]. The n!ce control group spectrum contained only contributions A and B. The O1s spectra, for both e.max and n!ce, contained two contributions: A) ∼ 531 eV, attributed to oxygen in lithium compounds such as LiOH, Li3PO4, LiPO3; B) ∼ 532 eV attributed to oxygen linked to silicon, as in silicates and Li2Si2O5. The Celtra and Suprinity spectra contained a third contribution: C) ∼ 534 eV, attributed to oxygen present in oxidized C [31]. The HR peak fitting showed that the peak intensities vary on etching. That is, the surface compositions were altered on etching, with some shifting of peak positions, well within experimental error. The amount of shift is in superscript, between parentheses, in Table 3. These shifts suggest compositional changes on etching.
Fig. 2. SEM image of the surface microstructure of Vita Suprinity on etching.
4. Discussion Our results show that, at the surface level, the concentrations of the major elements, O, Si and Li, vary significatively on etching for all LDGCs. The null hypothesis was therefore rejected. Our atomic percentages differ from those reported in the literature [15,22,25,32–35]: the manufacturers report different percentages of components, and some investigators did not note any changes. One reason for this discrepancy is that these investigations employed energy dispersive x-ray spectroscopy (EDS or EDX) [15,22,32]. EDX has a low surface sensitivity and, thus, a limited ability to characterize the surface composition [36]. The probe beam penetrates 1–2 μm in depth, assessing bulk, rather than surface, composition. Further, EDX cannot detect lithium, which affects the accuracy of the LDGC data. On the other hand, XPS can quantitatively detect even small concentrations (0.01%) of all elements, except for H and He, and is sensitive to a limited number of atomic layers in depth (≤4 nm), making it a perfect tool for surface analysis. The reaction between the glass and HF is described by equation (1):
Fig. 3. SEM image of the surface microstructure of Celtra Duo on etching.
The presence of the glassy matrix is noticeable for both e.max CAD and n!ce. Table 2 displays the XPS survey results of the samples before and after etching; O, Si and Li are the major constituents of all LDGCs. Although the amount of adventitious carbon present was greatly reduced in all samples by Ar cleaning, n!ce retained more than 10% C, and Zr was detected in all the samples. In the control group, Celtra and Suprinity had 2.4% and 4.7% Zr, respectively; these amounts changed to 2.9% and 2.2% after etching; n!ce samples displayed Zr only prior to etching (1%), and e.max CAD showed very small amounts under both experimental conditions. Control and etched samples of n!ce contained approximately 5.9%
SiO2 + 6 HF → H2SiF6 + 2 H2O
(1)
Among dental researchers, there is a predominantly held opinion 3
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Table 2 XPS survey analyses of four LDGC, with control and 5% HF etched groups showing the change of atomic percentage at the surface. Elements %
O 1s Si 2p Li 1s C 1s Al 2p P 2p K 2p Zn 2p Y 3p Zr 3p Sr 3p Ce 3d N 1s Na 1s F 1s
Celtra
e.max CAD
n!ce
Suprinity
Control
Etched
Control
Etched
Control
Etched
Control
Etched
49.2 22.9 19.7 2.1 1.5 0.8 0.5 – 0.2 2.4 0.2 0.1 0.2 0.2 –
45.5 21.4 21.2 6.6 1.5 – 0.5 – – 2.9 – 0.3 – – –
50 26.9 11.8 5.5 1.0 2.1 1.1 0.2 0.3 0.2 0.3 0.3 0.3 – –
46.6 25.6 12.1 9.5 1.1 – 2.2 0.2 – 0.2 0.3 0.3 – – –
44.7 25.2 8.4 12.8 5.9 – – – 0.3 1 – 0.1 1.4 – 0.2
45.6 22.1 14.2 11.5 4.9 – – – 0.2 – – 0.3 0.7 – 0.5
45.1 24.8 16.9 2.1 1.5 1.1 0.4 – 0.6 4.7 – 0.2 0.1 0.6 –
43.1 22.2 19.6 9.4 1.8 – 0.5 – – 2.2 – – – – –
of this residual quartz. The Si2p specrum in Fig. 5 informs us that the surface of Suprinity is principally composed of LS2 crystals, since that is the only peak present. Further, the O1s peak ratio reveals that the Li:Si ratio of this compound is 1:2.41, within a small error of the expected ratio of 1:2.39 for [Li2O]·2[SiO2], proposed by Holand et al. [7,43] However, for Celtra (referred to, by the manufacturers, as “zirconia-reinforced lithium silicate glass ceramic”, as is Suprinity) the atomic composition of the surface is quite different (Fig. 6). On etching, Celtra presented the same three chemical environments as those prior to etching. The amount of glassy phase was reduced from 28.7% to 9.1%, the LS2 phase remained the same, and the quartz phase increased from 8.8% to 27.5%. A possible explanation for this is that, as the glassy phase dissolves, the quartz phase increases in concentration. The O1s ratio presents a Li:Si ratio equal to 1:1.84, indicating that the Celtra surface has a higher content of lithium than that of Suprinity. This may indicate that the residual glassy matrix is rich in Li2SiO3 and/or Li3PO4. Of the four LDGC products that were assessed, e.max CAD appears to be the least affected by etching. The small change that was observed was due to the dissolution of the glassy phase Prior to etching, e.max CAD had the highest percentage of LS2 phase (Fig. 7), which contributes positively to its mechanical performance. The O1s spectrum shows a Li:Si ratio of 1:5.75, indicating a very small amount of lithium in the remaining glassy matrix. This was also be confirmed by the XPS survey results, which show that, on etching, e.max CAD had the greatest amount of Si and lowest amount of Li (Table 2). As shown in Fig. 8, the n!ce surface manifested distinctive features. First, n!ce is the only LDGC that, in the control group, does not have a third Si2p contribution. An explanation for this is that n!ce is furnished as a fully crystallized glass ceramic, for which the manufacturer does not recommend additional firing. As discussed earlier, should the e.max CAD ingot reach more the 900 °C during the fabrication phase, no crystalline SiO2 material (e.g., quartz) remains in the glassy phase.
that only the glassy matrix is dissolved [17,22,37]. Actually, LS2 crystals seem to be more resistant to dissolution than SiO2. As shown in equation (2), Li2Si2O5 + 14 HF → 2 LiF + 2 H2SiF6 + 5 H2O.
(2)
In order to dissolve one molecule of LS2, ons needs more than twice the HF concentration used in Equation (1). Further, the literature shows that the higher the degree of crystallization, the lower the solubility of the LDGC [38]. Our XPS results are consistent with this position, showing a reduction or elimination of the peaks attributed to the glassy phase, on etching. No fluorine was detected in the etched samples. Were there Si-F compounds (e.g., hexafluorosilicate) present, they would manifest a Si2p peak at ∼ 104.5 eV [39]; no sample possessed such a peak. A direct clinical implication of this finding validates that rinsing for 20 s is enough to wash off all by-products. An analysis of the Si2p and O1s peaks leads to a better understanding of what happened in the outermost atomic layers of the LDGCs on etching. As previously noted, prior to etching, Celtra, Suprinity and e.max CAD manifest three Si chemical environments: a glassy phase, the main phase (LS2) and a third, SiO2 phase, at ∼ 104 eV (Table 3) [29,30]. Tashiro and Wada reported that, in systems using ZrO2 as nucleating agents, partial crystallization of the base glass, as an αquartz solid solution precursor, can result [40]. Huang and coworkers also note the formation of quartz, cristobalite and tridymite during the crystallization of LS2, even in systems with low Zr content [41]. The same authors observed that the quartz normally disappears at 980 °C [42]. Certainly, the presence of this residual quartz (or another form of crystalline SiO2) would be beneficial in increasing mechanical properties. The amounts of SiO2 in our samples were 8.8% in Celtra, 13% in Suprinity and 7.5% in e.max. Despite the fact that n!ce contained 1% Zr, SiO2 was not detected. Further investigations, to occur in our projected integrated study, will be needed to better understand the nature
Table 3 The peak positions for silicone and oxygen of four LDCG, for control and 5% HF etched groups. Chemical Element
Peak deconvolution
Peak position (eV)
FWHM (eV)
Attribution
Celtra (%)
e.max (%)
n!ce (%)
Suprinity (%)
Control
Etched
Control
Etched
Control
Etched
Control
Etched
Silicon
Si2p A Si2p B Si2p C
∼ 101.8 ∼ 102.5 ∼ 104
1.6 1.6 1.6
SiO2 Glass Li2Si2O5 SiO2 Crystal
28.7 62.5 8.8
9.1 63.4 28.4
12.5 80 7.5
18.3 81.7 0
32.9 67.1 0
0 100 0
21.3 65.7 13
0 100 0
Oxygen
O1s A O1s B O1s C
∼ 531 ∼ 532 ∼ 534
1.8 1.8 1.8
O-Li O-Si O-P
30.5 57.5 11.9
35.2 64.8 0
11.1 88.9 0
14.8 85.2 0
13.5 85.4 0
15.4 84.6 0
32.5 61.1 6.4
22.9 77.1 0
4
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Fig. 5. Si2p and O1s XPS high resolution peaks for Suprinity, for control and etched samples. The deconvolution details are displayed in Table 3.
at ∼ 102.7 eV. Finally, the elevated amount of carbon in the n!ce samples indicates the continued presence of carbon, even after Ar laser pre-treatment. The clinical significance of this study rests on the fact that a 5% HF acid etching surface treatment has become the standard clinical protocol to obtain satisfactory bond strength for LDGC restorations. SEM images have shown the effective dissolution of the glassy phase and the exposure of the crystal phase in all samples. This dissolution increases both surface area and surface energy, and these two features together play a major role in aiding the adhesion process [18,37,47,48].
Second, the amount of aluminum in n!ce is about four times greater than in the other LDGCs. The manufacturer describes n!ce as a lithium aluminosilicate ceramic that is reinforced with lithium disilicate. Our study detected Al only in the glassy phase. This finding was confirmed by the deconvolution of the Al2p peak (Fig. 9). In the control group, the Al-Li compounds were found, at ∼ 75.8 eV [44]. After etching, the concentration of Al-Li compounds seems to decrease drastically, due to the Al peak shifting to 74.6 eV. This new peak position may indicate the presence of silicates, such as Al2SiO5 and/or Kaolinite ( Al4Si4O10(OH)8) [45,46], an assignment supported by the Si2p peak
Fig. 6. Si2p and O1s XPS high resolution peaks for Celtra, for control and etched samples. The deconvolution details are displayed in Table 3. 5
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Fig. 7. Si2p and O1s XPS high resolution peaks for e.Max CAD, for control and etched samples. The deconvolution details are displayed in Table 3.
etching that leads to strong adhesion.
However, mechanical interlocking is only one part of the bonding process, the chemical adhesion is achieved by using silane coupling agents. The efficiency of the silane treatment is basically dependent on the chemical composition of the etched surface. For the first time, we have described the physicochemical composition of the first atomic layers and the effect of HF etching on dental LDGCs. This information will help to understand the challenges of adhesive processes, particularly in selecting coupling agents for specific surface chemical compositions. By these methodologies, future studies, such as the one we propose, will be able to assess the ideal ratio/concentration/time for HF
5. Conclusions Within the limitations of this investigation, confined to XPS and SEM, it is possible to conclude that the surfaces of all LDGCs have distinctly different chemical compositions on acid etching. At the nanolevel probed, Celtra, e.max CAD and Suprinity displayed SiO2 crystalline phases. After etching, these phases disappear, except for Celtra. LS2 crystal concentrations vary from 62.5% to 80% prior to etching.
Fig. 8. Si2p and O1s XPS high resolution peaks for n!ce, for control and etched samples. The deconvolution details are displayed in Table 3. 6
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[13]
[14]
[15]
[16] [17]
[18]
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Fig. 9. Al2p XPS high resolution peak for n!ce, for control and etched samples; dotted lines show the peak shift after HF etching.
[21]
Suprinity and n!ce are the only ones that present no glassy phase on etching.
[22]
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Conflicts of interest:
[24]
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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Declaration: [26]
We hereby declare that this manuscript is original, and is not currently being considered for publication elsewhere.
[27]
Acknowledgments [28] [29]
The authors thank the University of Manitoba for financial support through a College of Dentistry Research Grant, as well as the manufacturers for furnishing the samples.
[30] [31]
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