Assessment of Internal porosities for different placement techniques of bulk-fill resin-based composites: A Micro-computed tomography study

doi:10.21203/rs.3.rs-3117442/v1

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Research Article

Assessment of Internal porosities for different placement techniques of bulk-fill resin-based composites: A Micro-computed tomography study

https://doi.org/10.21203/rs.3.rs-3117442/v1

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published 16 Nov, 2023

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Purpose: The aim was to compare the porosity of different bulk-fill resin-based composites (RBCs) placement techniques to the conventional incremental technique using microcomputed tomography (μ-CT).

Methods: Occlusal cavities were prepared on extracted human molars that were allocated into five placement technique groups (n=10): Monoblock-two-step technique: SureFil SDR flow+Ceram.X; Monoblock with sonic activation technique: SonicFill2; Monoblock-one-step technique: Tetric EvoCeram Bulk-Fill; Monoblock-two-step technique: Tetric EvoFlow Bulk-Fill+Tetric EvoCeram Bulk-Fill, and incremental technique: Filtek Z250. The number, volume of closed pores, and the total porosity was evaluated (μ-CT). Data were analyzed with analysis of variance on ranks (Student-Newman-Keuls method and Mann-Whitney rank-sum test) to verify the RBC viscosity and the socination placement technique significance. Porosity characteristics correlation was performed (Spearman correlation) (α= 0.05).

Results: The SonicFill2 group presented a higher number of closed pores than the other groups (p<0.05). The volume and total porosity characteristics were similar among all groups (p>0.05). The overall porosity within the restoration bulk seemed greater in the following order: Filtek Z250>SonicFill2>Tetric EvoFlow Bulk-Fill+Tetric EvoCeram Bulk-Fill>Tetric EvoCeram Bulk-Fill>SureFil SDR Flow+Ceram.X. Sonication tended to increase the number (p=0.005) and volume (p=0.036) of closed pores. Increased viscosity of the materials tended to increase the closed pores volume (p=0.036). There was a strong correlation between the number and volume of closed pores (R²=0.549, p<001).

Conclusions: The different placement techniques resulted in significantly less internal porosity than the monoblock with sonic activation technique (SonicFill2). Sonication during application contributed to the higher number and volume of closed pores than the passive bulk-fill RBCs application.

Voids

porosity

resin composite

bulk fill

placement technique

3D analysis

micro-CT

Dental resin-based composites (RBCs) are considered the materials of choice for tooth restoration due to the increased demand for a high-quality material that offers exemplary esthetic potential and acceptable longevity requirements [1]. Manufacturers introduced Bulk-fill RBCs to overcome the limited depth of cure of conventional RBCs by increasing the materials’ translucency, which is achieved by decreasing the filler loads or increasing the filler size, ultimately reducing light-scattering effect and thus increasing light transmission at thicker levels of the restoration [2]. Overall, bulk-fill RBCs can be classified into full-body or base bulk-filling materials, which depends on the material's viscosity level [3]. Low-viscosity bulk-fill RBCs possess low wear resistance. Thus, a layer of conventional RBC is required at the top surface to reinforce the occlusal surface [4]. However, placing multiple layers is always associated with the possibility of air entrapment and the creation of voids/defects between layers, which could lead to poor mechanical and esthetic properties over time [5–8]. Of note, two placing techniques of bulk-fill RBCs, such as sonic insertion or pre-heating, have been found to affect the internal and external homogeneity of the layered bulk-fill restorations [9, 10]. Additionally, viscosity and the filler size/content of RBCs have both contributed to the presence of voids within the bulk of restorations [11–13].

A recent study investigating the prevalence of interfacial gaps among different bulk-fill materials, layering techniques, and curing modes found that the RBC and floor (bottom) of the cavity interface has a higher probability of voids, leading to marginal gaps [14]. The voids' location could affect the restoration's success and longevity if they are found close to the occlusal surface or at the bonding interface. Previous work showed that aging by thermocycling and cyclic loading impacts the number, volume, and total percentage porosity within bulk-fill RBC restorations but not significantly, regardless of the light-curing unit used [15]. A previous study suggested three bulk-fill application techniques based on the manufacturers’ placement recommendation: the monoblock-two steps, the monoblock with sonic activation, or the monoblock-one step [16]. However, which placement technique yields fewer voids within the restoration is still unclear.

Hence, this study aimed to compare the internal porosity of different bulk-fill RBCs and application techniques compared to the conventional RBC layering technique. In addition, we aimed to correlate the number of porosities with the overall viscosity level of materials used. Two null hypotheses were tested: 1) there is no difference in the void numbers among the different bulk-fill layering techniques compared to the conventional technique, and 2) the number of porosities is not correlated with the viscosity of materials.

Cavity preparation

The Research Ethics at King Abdulaziz University Faculty of Dentistry Committee approved the project (proposal no.293-10-21). Fifty extracted, non-carious human molars were collected, cleaned from any connective tissue remanent, and stored in an aqueous 0.2% thymol solution for preservation purposes prior to the conduction of this study. Standardized occlusal class I, box-shaped cavities were prepared, providing a mesiodistal width of 3.0 mm, a buccolingual width of 3.0 mm, and a depth of 5.0 mm. Cavities were prepared under 3.5x magnification loupes with light-emitting diode (LED) light and at a working distance between 300–500 millimeters to ensure proper measurements. The tooth cavities were measured using a UNC-15 periodontal probe during cavity preparation. Preparations were made using a straight carbide fissure bur # F.G. 557L placed on a 1:5 ratio electric micromotor handpiece without compressed air (Ti-Max Z95L, NSK). The equipment was used at 200,000 revolutions per minute with a water-cooling system. The prepared teeth were randomly divided into five groups (n = 10) according to the restorative system and filling technique.

Restorative procedure

The universal adhesive system Prime and bond one Select (Dentsply, Germany) was applied following the manufacturer's instructions. Photoactivation was performed with a LED light-curing unit (Demi Ultra, Kerr Corporation, USA) with an 8 mm effective light diameter, with an irradiance of 850 mW/cm² as measured by a radiometer Bluephase Meter II (Ivoclar Vivadent, Schaan, Liechtenstein).

The different placement techniques were as follows and illustrated in Fig. 1:

Group 1: Incremental technique with conventional RBC (Filtek Z250)
Group 2: Monoblock technique-two steps using SureFil SDR Flow + Ceram.X
Group 3: Monoblock technique-two steps using Tetric Evoflow Bulk Fill + Tetric EvoCeram Bulk Fill
Group 4: Monoblock technique-one step using Tetric EvoCeram Bulk Fill
Group 5: Monoblock technique with sonic activation using SonicFill 2

The name, type, composition, and manufacturer of the RBCs used in the present study are listed in Table 1. Each group was filled with the desired RBC and light cured per the manufacturer’s instructions for the RBC used. Group 1: Filtek Z250 group was the control and placed using the incremental technique from cavity floor to the occlusal, placed in 2-mm, 2-mm, and 1-mm thickness, respectively; each layer was individually light-cured according to the manufacturer instructions. Group 2: SureFil SDR Flow was filled in one 4-mm increment followed by a 1-mm veneer layer of the conventional RBC Ceram.X. Group 3: Tetric EvoFlow Bulk Fill was placed in 1-mm followed by a 4-mm increment of Tetric EvoCeram Bulk Fill. Group 4: Tetric EvoCeram Bulk Fill was placed in two individual increments of 4-mm, followed by a 1-mm increment. Group 5: SonicFill 2 system was used to fill the 5-mm cavity.

Table 1

The name, manufacturer, type, and composition of the composites used in this study.
Name	Manufacturer	LOT	Type	Composition
Prime and bond one Select	Dentsply, Charlotte, NC, USA	1605000143	Universal adhesive	Acetone, Urethane Dimethacrylate Resin, Dipentaerythritol pentacrylate phosphate, Polymerizeable dimethacrylate resin Polymerizeable trimethacrylate resin
Filtek Z250	3M, Saint Paul, MN, USA	1370A2	Conventional, high viscosity	Bis-GMA, Bis-EMA, TEGDMA, UDMA, zirconia, silica
SureFil SDR flow	Dentsply, Charlotte, NC, USA	1703001055	Bulk-fill, flowable	SDR patented UDMA, TEGDMA, EBPDMA, barium, and strontium alumino-fluoro-silicate glass
Ceram.X	Dentsply, Charlotte, NC, USA	1711001134	Conventional, high viscosity	Filler: barium-aluminum-borosilicate glass methacrylate functionalized silicone dioxide nanofiller Matrix: methacrylate modified polysiloxane, dimethacrylate resin, fluorescent pigment, U.V. stabilizer, stabilizer, camphorquinone, ethyl-4 (dimethylamino) benzoate, titanium oxide pigments, aluminum silicate pigments
Tetric EvoFlow Bulk Fill	Ivoclar Vivadent, Schaan, Liechtenstein	V29008	Bulk-fill, flowable	Bis-GMA, EBPADMA, TEGDMA, ytterbium trifluoride
Tetric EvoCeram Bulk Fill	Ivoclar Vivadent, Schaan, Liechtenstein	V34861	Bulk-fill, high viscosity	Bis-GMA, UDMA, Barium glass, ytterbium trifluoride, oxides, and prepolymers
SonicFill 2	Kerr Corporation, Brea, CA USA	5125884	Bulk-fill, high viscosity	Bis-GMA, TEGDMA, EBPDMA, silica, glass, oxide

Abbreviations: Bis-EMA, bisphenol-A, polyethylene glycol diether dimethacrylate; Bis-GMA, bisphenol-A diglycidyl ether dimethacrylate; EBPDMA, ethoxylated bisphenol-A-dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate.

Micro-CT (µ-CT) specifications

The µ-CT scanning was performed for all samples using an 1172 µ-CT scanner (SkyScan, Bruker, Belgium). The scanning settings were applied: 100 kV accelerating voltage, a 100 uA beam current, Al and Cu filters, pixel size of 6.7 µm, and flat field correction. The scans were reconstructed in 3D using NRecon software (Bruker), then reoriented in Data-Viewer (Bruker) to obtain 3D images of the restorations with a standardized Region of Interest (ROI) of 1460.6, 1460.6, and 3819 µm in XYZ planes. The generated standard-sized 3D images of the restorations were analyzed in CTAn software (SkyScan, Belgium) to measure the percent total porosity and the number and volume of closed pores. The analyzed 3D images were viewed on CTVol software (SkyScan, Belgium), after which 3D models were generated.

Statistical analysis

The data on the number and volume of closed pores and those concerning the total porosity of restorations were statistically analyzed using SigmaPlot (version 12.0; SPSS Inc., Chicago, IL, USA). The data were not normally distributed, so they were analyzed using Kruskal-Wallis One Way Analysis of Variance on Ranks and with the Student-Newman-Keuls Method when appropriate. For the latter analyses, the obtained data were allocated as having the type of RBC (restoration group) as the main variable factor. Additional analyses were performed using SPSS Statistics (version 22; IBM) and having two variable factors: the viscosity of the major restorative material used to fabricate the restorations (high vs. low) and the sonication of the restorative material during its application into the tooth cavities (no vs. yes). The data of the foregoing analyses were analyzed using the Mann-Whitney rank-sum test. The volume/porous characteristics of the restorations fabricated in the study were also correlated to each other using the Spearman Correlation test. The significance level of all analyses was set at α = 0.05.

The results of the characteristics investigated in this study for each group are shown in Table 2. Regarding the number of closed pores, restorations fabricated with SonicFill 2 (Group 5) presented more pores than all the other restorations (p < 0.05), which did not differ from each other (p > 0.05). However, the volume of closed pores was similar for all groups (p > 0.05). The total porosity of the restorations ranged from 0.2 ± 0.3% in Group 2 to 1.3 ± 2.4% in Group 1; however, no significant differences were detected among the groups (p > 0.05). 2D representative images and 3D models for each restoration group are given in Fig. 2. From the 3D representative models, the overall porosity within the bulk of restorations seemed to be greater in the following order: Filtek Z250 (Group 1, control) > SonicFill 2 (Group 5) > Tetric EvoFlow Bulk Fill + Tetric EvoCeram Bulk Fill (Group 3) > Tetric EvoCeram Bulf Fill (Group 4) > SureFil SDR Flow + Ceram.X (Group 2).

Table 2

The mean, standard deviation (S.D.), median of the closed pores, the volume of closed pores (mm³), and total porosity (%) for each placement technique group.
Characteristic evaluated	Resin-based composite	Mean (S.D.)	Median (minimum - maximum)*
Number of closed pores	Group 1: Incremental technique: Filtek Z250	44.1 (79.0)	13.5 (9.0–238.0)^B
	Group 2: Monoblock-two-step technique: SDR + Ceram.X	53.8 (83.5)	9.0 (0–237.0)^B
	Group 3: Monoblock-two-step technique: Tetric EvoFlow Bulk Fill + Tetric EvoCeram Bulk Fill	9.6 (8.7)	7.0 (2.0–28.0)^B
	Group 4: Monoblock-one-step technique: Tetric EvoCeram Bulk Fill	6.9 (6.5)	5.5 (1.0–21.0)^B
	Group 5: Monoblock with sonic activation technique: SonicFill 2	33.0 (18.3)	28.5 (14.0–59.0)^A
Volume of closed pores (mm³)	Group 1: Incremental technique: Filtek Z250	0.008 (0.015)	0.001 (0–0.043)^A
	Group 2: Monoblock-two-step technique: SDR + Ceram.X	0.002 (0.003)	0.001 (0–0.006)^A
	Group 3: Monoblock-two-step technique: Tetric EvoFlow Bulk Fill + Tetric EvoCeram Bulk Fill	0.004 (0.005)	0.003 (0–0.015)^A
	Group 4: Monoblock-one-step technique: Tetric EvoCeram Bulk Fill	0.004 (0.008)	0.001 (0–0.022)^A
	Group 5: Monoblock with sonic activation technique: SonicFill 2	0.019 (0.024)	0.007 (0–0.061)^A
Total porosity (%)	Group 1: Incremental technique: Filtek Z250	1.3 (2.4)	0.5(0–7.1)^A
	Group 2: Monoblock-two-step technique: SDR + Ceram.X	0.2 (0.3)	0.1(0–0.6)^A
	Group 3: Monoblock-two-step technique: Tetric EvoFlow Bulk Fill + Tetric EvoCeram Bulk Fill	0.7 (0.9)	0.2(0–2.4)^A
	Group 4: Monoblock-one-step technique: Tetric EvoCeram Bulk Fill	0.7 (1.8)	0.1(0–5.3)^A
	Group 5: Monoblock with sonic activation technique: SonicFill 2	1.2 (2.6)	0.3 (0.1–7.6)^A
*Different letters of median (minimum and maximum) values indicate statistically significant differences among the groups for each characteristic (p < 0.05); Kruskal-Wallis One Way Analysis of Variance with Ranks and Student-Newman-Keuls Method.

Figure 3 shows the results for the volume/porous characteristics investigated in the study and having as variable factors the viscosity and the sonication of the RBCs during the fabrication of the restorations. The viscosity of the major RBC was a significant factor for the volume of closed pores (p = 0.036), with more viscous materials contributing to more voluminous pores within the restorations (nearly 19-fold increase). The sonication of the RBC during its application into the tooth cavity was a significant factor for the number (p = 0.005) and volume (p = 0.036) of closed pores, with the sonic application of materials contributing to the occurrence of a higher number of closed pores (3-fold increase), and greater volume of closed pores (7.3-fold increase) as compared to the passive application of materials without using sonication. The volume/porous characteristics of the restorations were correlated to each other in this study, with the strongest relationship occurring between the number of closed pores and the volume of closed pores (R² = 0.549; p < 0.001), followed by the relationship between the volume of closed pores and the total porosity (R² = 0.385; p = 0.014) and between the number of closed pores and the total porosity of restorations (R² = 0.315; p = 0.048).

The current study investigated the volume of voids and overall porosity associated with some bulk-fill RBCs placed using varying techniques, which were based on different clinical utilization of bulk-filling materials.[16] Considering the three characteristics evaluated, i.e., number of closed pores, the volume of closed pores, and total percentage porosity of restorations, our findings revealed a statistical difference only in terms of the number of closed pores (Table 2). Of note, the restorations prepared with SonicFill 2 presented a higher quantity of pores than all other groups, indicating that the sonic delivery of the material may create more voids compared to the passive delivery method. It is possible to suggest that sonication changed the rheological behavior of the RBC, reducing viscosity and perhaps allowing the incorporation of air within the RBC mass, thereby increasing the total number of closed pores. One may attempt to the fact that despite the broader range of pores shown by Groups 1 and 2 (i.e., from 0 to 238), it was the sonicated Group 5 that presented with the highest minimum number of closed pores, ranging from 14 to 59 pores, so all restorations in Group 5 displayed a porous structure (Fig. 2). Conversely, Groups 3 and 4 showed the lowest and narrowest range of closed pores of the study, i.e., from 1 to 28, suggesting that the respective placing techniques tend to produce a more uniform and densely packed structure for direct RBC restoration.

A similar µ-CT study compared the void formation volume between two placement techniques based on conventional (two-layers) and sonication methods [13]. The study concluded that there was no difference in the void volume of SonicFill 2 regardless of placement techniques. However, the results showed that the volume of voids was significantly increased in the sonication group for other bulk-fill RBCs [13]. Another study that used the µ-CT to compare the effects of conventional, sonic, or pre-heating placement techniques on the internal void formation within bulk-fill RBCs has also concluded that other materials rather than SonicFill 2 were negatively affected by the sonic insertion technique [9]. In contrast, the latter was not influenced by the insertion method. In our study, the sonic device was used according to the manufacturer's instructions, but it still resulted in more voids than the conventional incremental technique.

It is worth mentioning that Groups 3 and 4 were prepared with bulk-fill RBCs placed using the two-step and one-step monoblock techniques, respectively. While the former group combined a flowable RBC as the first 1-mm increment with a high-viscosity RBC as the second 4-mm increment, the latter group was based solely on a high-viscosity RBC. Here, both groups were fabricated using a high-viscosity RBC as the major restorative material, differing from Group 2, which was mainly prepared using a flowable material. Despite the high viscous behavior of SonicFill 2, it was probably applied into the tooth cavity at a more fluid state due to the sonication delivery method applied in Group 5, as discussed earlier. Thus, considering the minimum and maximum values of the porous characteristics displayed by each group, one may consider that the viscosity of the major RBC may play a role in the structural quality of direct restorations.

According to our findings, high viscous RBCs were associated with an increased volume of closed pores compared to the use of low viscous material (Fig. 3). The number of closed pores and total porosity were not affected, so it can be suggested that the higher viscosity makes the material stickier, favoring the occurrence of larger voids/defects in case a pore is formed within the restoration. In theory, the flowing ability of low viscous RBCs is expected to facilitate internal adaptation with the tooth margins, improving the fit between the restorative material and tooth substrate. This was indeed observed in Group 2, in which maximum values for the volume of closed pores (0.006 mm³) and total porosity (0.6%) were the lowest in the study.

The current study used the µ-CT to detect and compare the internal porosity among different bulk-fill RBCs. This imaging modality offers a non-destructive, precise, and reproducible image of the tooth structure instead of using the non-precise dye penetration and sectional scanning of specimens [17]. Micro-CT imaging produces a 3D reconstruction and is reliable for viewing internal and external tooth structures [18]. Furthermore, we standardized our investigation by focusing on a consistent rectangular area in the middle of the restoration. The specimen cavity preparation and bonding agent were also standardized to minimize confounding factors.

From the 3D images shown in Fig. 2, it is clear that the incremental technique using conventional RBC (Group 1; control) and the monoblock technique with bulk-fill material and using sonic activation (Group 5) resulted in more apparent porous restorations. Concerning the incremental technique, the handling of small-sized increments may induce air entrapment and the creation of voids/defects in the bulk and surface of RBC restorations [19, 20]. Remarkably, the use of modeler resins has been suggested to aid in the sculpting of direct RBCs, reducing the stickiness of composite increments and thus contributing to more uniform, less-defective restorations [19, 21]. Nonetheless, there is still no consensus on the effects of modeler resins on the overall quality of RBC restorations over time [22, 23], so the use of the monoblock technique with bulk-fill materials should be preferred. Regarding the monoblock technique using sonic activation, the results shown in Fig. 3 demonstrated that this restorative method was significantly associated with a greater number of closed pores and volume of closed pores as compared to the non-sonicated counterparts, which corroborate the 3D images presented in Fig. 2. From the different placement techniques tested in this study, the incrementally- and the sonicated-driven methods resulted in the poorest structure for RBC restorations, at least in terms of the volume and internal porosity features investigated. Therefore, the first null hypothesis of the study was partially rejected since the number of voids (closed pores) depended on the type of bulk-fill layering technique, with some bulk-fill materials performing similarly or worse than the conventional technique. In contrast, the second null hypothesis was entirely rejected since the number of porosities was significantly associated with tested RBCs' viscosity.

Future directions of our study could assess the mechanical properties of bulk-fill materials with different void locations, comparing the internal porosity achieved at the surface versus the middle and marginal areas of the restoration; the comparison between heated and unheated bulk-fill materials is also encouraged since it may demonstrate essential differences in terms of the overall quality of the bulk of RBC restorations. Further investigations within these lines will improve the clinical practice and ensure the delivery of the best dental care to the patient in an efficient way.

Clinical Relevance

The bulk-fill RBC placement technique in different clinical situations should be selected based on the mechanical and physical properties of the materials. To reduce the number of voids, it is advisable to use bulk-fill materials when available in large and deep cavities instead of using the incremental technique of conventional RBC. Highly viscous bulk-fill RBCs could be used as monoblocks-one-step technique, whereas less viscous bulk-fill RBCs are suitable for monoblock-two-step placement techniques covered by a thin or thick layer of conventional RBCs to minimize the voids. The use of sonic activation was demonstrated to increase the internal porosity of restorations.

Based on the results, the following conclusions can be considered:

The number of closed pores was significantly less with the monoblock-two-step technique, monoblock-one-step technique, and the incremental technique compared with the monoblock with sonic activation technique when using SonicFill 2.

The viscosity of the major restorative material was significantly associated with the increase in the volume of closed pores.

Acknowledgments

The authors acknowledge with thanks to the facilities of The Advanced Technology Dental Research Laboratory, Faculty of Dentistry, King Abdulaziz University, for their technical support. This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R350), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Author contribution

Conceptualization [Afnan O. Al-Zain]; Methodology [Afnan O. Al-Zain]; Data curation [Lina Baeesa], [Elaf Jassoma], [Maryam A. Alghilan]; Formal analysis and investigation [Afnan O. Al-Zain], [Lina Baeesa], [Elaf Jassoma], [Maryam A. Alghilan], [Murooj Hariri], [Eman H. Ismail], [Eliseu A. Münchow]; Funding acquisition [Eman H. Ismail]; Project administration and supervision [Afnan O. Al-Zain]; Resources [Afnan O. Al-Zain], [Lina Baeesa], [Elaf Jassoma], [Eman H. Ismail], [Maryam A. Alghilan], [Eliseu A. Münchow]; Software [Maryam A. Alghilan], [Eliseu A. Münchow]; Validation and visualization [Afnan O. Al-Zain], [Lina Baeesa], [Elaf Jassoma], [Maryam A. Alghilan], [Murooj Hariri], [Eman H. Ismail], [Eliseu A. Münchow]; Writing – original draft, review & editing [Afnan O. Al-Zain], [Lina Baeesa], [Elaf Jassoma], [Maryam A. Alghilan], [Murooj Hariri], [Eman H. Ismail], [Eliseu A. Münchow].

Funding Statement: This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R350), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Conflict of Interest: The authors declare that there is no conflict of interest regarding the publication of this paper.

Ethical approval: The Research Ethics Committee at King Abdulaziz University Faculty of Dentistry approved the project (proposal no.293-10-21). The study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.

Informed consent: For this type of study, formal consent is not required.

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No competing interests reported.

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published 16 Nov, 2023

Read the published version in Clinical Oral Investigations →

Editorial decision: Major revision
03 Aug, 2023
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Reviewers agreed at journal
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Assessment of Internal porosities for different placement techniques of bulk-fill resin-based composites: A Micro-computed tomography study

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Abstract

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Introduction

Materials and Methods

Cavity preparation

Restorative procedure

Micro-CT (µ-CT) specifications

Statistical analysis

Results

Discussion

Clinical Relevance

Conclusions

Declarations

Acknowledgments

References

Additional Declarations

Status:

Journal Publication

Version 1