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ORIGINAL ARTICLE
Year : 2019  |  Volume : 11  |  Issue : 6  |  Page : 365-370  

Impact strength and dimensional accuracy of heat-cure denture base resin reinforced with ZrO2 nanoparticles: An in vitro study


1 Department of Prosthodontics and Crown and Bridge, Vinayaka Missions Sankarachariyar Dental College, Salem, Tamil Nadu, India
2 Department of Prosthodontics and Crown and Bridge, Vivekanandha Dental College for Women, Tiruchengode, Tamil Nadu, India
3 Private Practitioner, Salem, Tamil Nadu, India

Date of Web Publication28-May-2019

Correspondence Address:
Dr. R Ajay
Department of Prosthodontics and Crown and Bridge, Vivekanandha Dental College for Women, Elayampalayam, Tiruchengode, Namakkal, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JPBS.JPBS_36_19

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   Abstract 

Background: Polymerization shrinkage and fracture are the two common trouble shoots with denture base resins. Polymerization shrinkage affects the dimensional accuracy and fit of the prosthesis. The effect of zirconia (ZrO2) nanoparticles on polymerization shrinkage is not documented yet. Purpose: The aim and objective of this study were to evaluate the impact strength and dimensional accuracy of heat-cured poly methyl methacrylate (PMMA) on reinforcement with ZrO2 nanoparticles. Materials and Methods: Conventional heat-cure denture base resin (control) and the polymer reinforced with 3, 5, and 7 wt% of ZrO2 nanoparticles were prepared and used in this study. Forty bar-shaped specimens were prepared and tested for impact strength using Charpy’s type impact tester. Forty denture bases were fabricated and checked for dimensional accuracy by measuring the distance between the denture base and the cast in two different sections using the travelling microscope. Results: The impact strength decreased with increased concentration of ZrO2 and found to be least at 7 wt% concentration (2.01±0.26 J/mm2). The distance between the denture base and the cast significantly decreased both in the posterior palatal seal region (0.060±0.007cm) and mid-palatine section region (0.057±0.006cm) with ZrO2 nanoparticles reinforcement and was found to be least at 7 wt% concentration. Conclusion: Reinforcement of heat-cured PMMA with ZrO2 nanoparticles significantly increased the dimensional accuracy and decreased the impact strength.

Keywords: Dimensional accuracy, impact strength, PMMA, travelling microscope, ZrO2 nanoparticles


How to cite this article:
Begum S S, Ajay R, Devaki V, Divya K, Balu K, Kumar P A. Impact strength and dimensional accuracy of heat-cure denture base resin reinforced with ZrO2 nanoparticles: An in vitro study. J Pharm Bioall Sci 2019;11, Suppl S2:365-70

How to cite this URL:
Begum S S, Ajay R, Devaki V, Divya K, Balu K, Kumar P A. Impact strength and dimensional accuracy of heat-cure denture base resin reinforced with ZrO2 nanoparticles: An in vitro study. J Pharm Bioall Sci [serial online] 2019 [cited 2019 Jun 18];11, Suppl S2:365-70. Available from: http://www.jpbsonline.org/text.asp?2019/11/6/365/258868




   Introduction Top


The development of material science over time has led to a slow and steady increase in the quality of materials used for dental prostheses. Search for materials that are biocompatible, readily available, cost effective, easy to manipulate, less technique sensitive, functionally efficient, and esthetically pleasing is a persistent process.[1]Poly methyl methacrylate (PMMA) has been the most commonly used denture base material, because of its positive properties such as ease of working, processing, intraoral fit, stability, and aesthetics.[2] Regardless of these advantages, there are certain shortcomings pertaining to its strength properties and polymerization shrinkage. The denture base made of PMMA is exposed to different types of stresses such as compressive, tensile, shear, and impact stresses.

Skinner stated two major disadvantages of PMMA resins. They are large curing shrinkage during processing and its property of high water sorption. The heat produced during polymerization that is exothermic in turn produces an internal heat especially in the thickest portion of the denture, which is the posterior palatal seal (PPS) region in the maxillary denture and hence polymerization shrinkage is high in this region.[3]To overcome fracture susceptibility and polymerization shrinkage of PMMA, numerous modifiers have been used to reinforce the resin polymer. The various materials that have been used for the reinforcement include woven glass fiber, polystyrene fibers, silver, aluminum, and copper and titanium oxide powders.[4] These particles are reinforced in the form of nanoparticles due to better handling characteristics and even distribution.[4],[5],[6] They tend to increase the impact strength (IS) considerably. Zirconia (ZrO2) is a noncytotoxic metal oxide that is insoluble in water and lacks the bacterial adhesion property.[7] Recently, extensive research has been conducted on reinforcement of PMMA with ZrO2 to modify the mechanical properties such as transverse strength, flexural strength, and IS.[8] Despite the reinforcements of PMMA, the success of the complete denture in terms of retention and stability is closely related to its accurate fit, which in turn, depends on a series of factors, which include the clinical expertise of the dentist, accuracy of all the laboratory procedures of denture preparation, the type of materials used, and the dimensional stability of the mucosal tissues.[9] Evaluation of dimensional accuracy with ZrO2 reinforcements has not been yet documented in the literature. Hence, the IS and dimensional accuracy of heat-cure acrylic resins have been investigated with the focus on incorporation of ZrO2 nanoparticles at varying concentrations.


   Materials and Methods Top


Zirconia Powder (30–50nm; Nano Research Lab, Jamshedpur, Jharkhand, India) of 99.5% purity was selected as filler. The ZrO2 filler and PMMA (DPI, Bombay Burmah Trading Corp Ltd, Mumbai, India) were pre-weighed using an electronic balance (Avery India Ltd, Ballabgarh, India) in order to ensure a filler concentration of 3%, 5%, and 7% by weight.[10],[11],[12],[13],[14] ZrO2 particles were treated with 1 wt% of silane coupling agent before the mixing of filler particles and heat-cure PMMA resin polymer.[15] Mixing and blending was carried out thoroughly using ceramic ball milling, which rotates at a rate of 850rpm to obtain a uniform mix.

A total of 40 bar-shaped specimens of dimensions 80×7 × 4mm (ISO specification No: 1567) were prepared out of vacuum-formed thermoplastic sheet dies of 4-mm thickness.[15],[16] For fabricating the specimens of group A, the control (subgroup: A1), pre-weighed ZrO2-reinforced acrylic resins of concentrations 3% (subgroup: A2), 5% (subgroup: A3), and 7% (subgroup: A4) were mixed with monomer in a ratio of 3:1 and packed into mold space in the dough stage.[17] Trial closure was conducted and compressed with hydraulic press for 1 hour at 1200 psi.[18] The flasks were bench cured for 20 minutes and heat cured at 74°C for 2 hours and 100°C for 1 hour.[17] After curing, the flasks were bench cooled to room temperature. The specimens of each subgroup were finished and polished after retrieval. The specimens with internal or external porosities, warpage, broken edges, and surface defects were excluded from the study.

A total of 40 heat-cure denture bases were fabricated on dental stone casts. Thermoplastic sheets of 2-mm thickness were used as die. For fabricating the specimens of group B, the control (subgroup: B1), pre-weighed ZrO2-reinforced acrylic resins of concentrations 3% (subgroup: B2], 5% (subgroup: B3), and 7% (subgroup: B4) were mixed with monomer in a ratio of 3:1 and packed into mold space in the dough stage. The trial closure and processing regimens were same as that of group A. The specimens of each subgroup were finished and polished. All the specimens were immersed in distilled water at 37°C for 7 days.[13]

The IS of the 40 bar-shaped specimens were tested using Charpy’s impact tester (Modern Metallurgical and Scientific Services, Chennai, Tamil Nadu). The specimens were prepared by marking three lines. Two lines were drawn at a distance of 10mm from the borders of the specimen. The third midline was marked at 30mm away from the two lines. These two lines correspond to the location of supporting arm in the testing machine and conform to the span length of 60mm. At the midline, a V-shaped notch of 1.2mm was prepared with a notch cutter (Hounsfield notching machine, Tensometer Ltd., Croydon, UK).[19] The pendulum of the testing machine, which has an impact capacity of 164 J and a striking velocity of 5.6 m/s, would come and impact the specimen from the other side. The pendulum hit the specimen to fracture and this maximum load before fracture (F) was displayed in the machine. This value was recorded as the IS of the specimens in joule per square millimeter.

Dimensional accuracy was measured in terms of the distance between the denture base and the cast at the PPS and mid-palatine section (MPS) regions. At MPS, the distance between the casts and the denture bases was measured after sectioning the cast–denture base assembly anteroposteriorly using a diamond disk. The distance was measured with the help of travelling microscope (INCO, Ambala, India) with an accuracy of 0.001cm. The distance between the denture base and cast was measured in three regions of the PPS namely hamular notches on either side of cast and in the midline using the travelling microscope and the average was calculated in centimeter.[3],[20-24] The distance between the cast and the denture base were measured in three regions in MPS using the travelling microscope. The regions selected were incisive papilla, palatal vault, and posterior border of the denture base.[22],[23] The average of the three readings was calculated in centimeter. The obtained values of both group A and B were subjected to statistical analysis using one-way analysis of variance and Bonferroni multiple comparison tests.


   Results Top


The mean and standard deviation IS of subgroups A1, A2, A3, and A4 were 3.93±0.17, 3.73±0.19, 3.24±0.35, and 2.01±0.26 J/mm2, respectively [Table 1] and [Figure 1]. The IS had decreased considerably from A1 to A4 and the least IS was with subgroup A4. While comparing the mean IS of the subgroups, a statistically significant difference (P < 0.001) existed. In [Table 2], the Bonferroni multiple comparison tests were conducted at 95% confidence interval to compare the mean IS within the subgroups. The difference in the mean IS between A1 and A2 was statistically insignificant. However, statistically significant difference in mean IS existed when comparing between other sub-groups.
Table 1: One-way analysis of variance to compare mean IS values between sub-groups

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Figure 1: Mean impact strength

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Table 2: Bonferroni multiple comparisons of mean IS values

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The mean distance and standard deviation between the denture base and the cast of subgroups B1, B2, B3, and B4 were 0.148±0.031, 0.116±0.017, 0.090±0.016, and 0.060±0.007cm, respectively [Table 3] and [Figure 2]. The mean had decreased considerably from B1 to B4 and the least distance was at subgroup B4. While comparing the mean dimensional accuracy in relation to the distance between the denture base and the cast at PPS of the subgroups, a statistically significant difference (P < 0.001) existed. In [Table 4], the Bonferroni multiple comparison tests were conducted to compare the mean dimensional accuracy in terms of distance between the denture base and the cast at PPS section within the subgroups. The difference in the mean values on comparing between the subgroups was statistically significant.
Table 3: One-way analysis of variance to compare mean distance at PPS

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Figure 2: Mean distance between denture base and cast at posterior palatal seal

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Table 4: Bonferroni multiple comparisons of mean dimensional accuracy in relation to the distance between the denture base and the cast at PPS

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The mean distance and standard deviation between the denture base and the cast at MPS of subgroups B1, B2, B3, and B4 were 0.128±0.025, 0.097±0.008, 0.076±0.010, and 0.057±0.006cm, respectively [Table 5] and [Figure 3]. The mean had decreased considerably from B1 to B4 and the least distance was at subgroup B4. While comparing the mean dimensional accuracy in relation to the distance between the denture base and the cast at MPS of the subgroups, a statistically significant difference (P < 0.001) existed. In [Table 6], the Bonferroni multiple comparison tests were conducted to compare the mean dimensional accuracy in terms of distance between the denture base and the cast at MPS within the subgroups. The difference in the mean values on comparing between subgroups was statistically significant.
Table 5: One-way analysis of variance to compare the mean distance at MPS

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,
Figure 3: Mean distance between denture base and cast at mid-palatine section

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Table 6: Bonferroni multiple comparisons of mean dimensional accuracy in relation to the distance between the denture base and the cast at MPS

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   Discussion Top


Several studies have been carried out to improve the properties of PMMA, which include addition of reinforcing material as fibers, fillers, hybrid reinforcement, and recently, nanoparticles. However, the most effective reinforcement is not apparent, and research scholars are confused about designing such reinforcements. Reinforcement has two important purposes on prosthesis. The initial purpose is to improve the strength and prevent fracture, and the second purpose is to improve the dimensional accuracy in order to prevent residual ridge resorption of the associated structures.[25] Currently, reinforcements are carried out at the nanoparticles level.[4-6],[10-14],[25-36] The properties of resin reinforced by nanofillers depend highly on the factors that include size, shape, type, and concentration of the reinforced material.[35] In this study, ZrO2 nanoparticles were selected to evaluate the effect of reinforcement on the IS and dimensional accuracy of heat-cure denture base acrylic resin.

Studies about the ZrO2 reinforcement on the IS of the acrylic resin are very few in the dental literature. Charpy’s impact test was chosen for this study in which V-shaped notches were made in the specimens to act as areas of stress concentration.[13] In this in vitro study with respect to IS, the mean IS value decreased with increasing concentration of ZrO2 nanoparticles. However, the difference in the mean IS values between A1 (control) and A2 (3%) was not statistically significant (P = 0.541). This result was in agreement with the previous experiments.[13],[15],[37] Silanation of ZrO2 nanoparticles further enhanced and improved the IS. However, in this in vitro study, despite the silanation of ZrO2 nanoparticles, the IS decreased significantly with increase in ratio of reinforcement.

Accurate fit of the dentures is very important for maintaining healthy and stable tissues and helps in reducing the degree of tissue changes.[21] There are many studies conducted by various authors, which have evaluated the dimensional accuracy of the denture bases, and have concluded that processing technique and water sorption does have an influence on dimensional accuracy.[3] There are no studies in the dental literature evaluating the dimensional accuracy of heat-cure denture base acrylic resin reinforced with ZrO2 nanoparticles.

Hence, in this study, the fit was measured in PPS[3] and MPS.[21] Polymerization shrinkage tends to draw the denture flanges inwards and as a result the denture gets slightly elevated in the MPS. The lesser the distance, the better is the dimensional accuracy of the denture base. The distances between the denture base and the cast at PPS and MPS were significantly lesser with 7% ZrO2 nanoparticles reinforcement than the control. Thus, the dimensional accuracy or fit of the dentures improved significantly with increase in ratio of reinforcement of ZrO2 nanoparticles.

This is an in vitro study and hence, an exact clinical implication of the test results is questionable. The effect of water sorption, which has an influence on the polymerization shrinkage, is not taken into consideration. This study used only simulations of the oral environment. However, it could not accurately reproduce all the oral factors such as thermal fluctuations, masticatory load, masticatory cycles, salivary pH, its buffering capacity, and flow rate.


   Conclusion Top


Within the limitations of this in vitro study, the following conclusions were deduced:

  1. The reinforcement of ZrO2 nanoparticles with heat-cure denture base resin decreased the IS of the resin.


  2. The reinforcement of ZrO2 nanoparticles with heat-cure denture base resin increased the dimensional accuracy and fit at both PPS and MPS.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

1.
Phoenix RD. Denture base materials. Dent Clin North Am 1996;40:113-20.  Back to cited text no. 1
    
2.
Smith DC. The acrylic denture. Mechanical evaluation, mid-line fracture. Br Dent J 1961;110:257-67.  Back to cited text no. 2
    
3.
Firtell DN, Green AJ, Elahi JM. Posterior peripheral seal distortion related to processing temperature. J Prosthet Dent 1981;45:598-601.  Back to cited text no. 3
    
4.
Hamedi-Rad F, Ghaffari T, Rezaii F, Ramazani A. Effect of nanosilver on thermal and mechanical properties of acrylic base complete dentures. J Dent (Tehran) 2014;11: 495-505.  Back to cited text no. 4
    
5.
Shirkavand S, Moslehifard E. Effect of TiO2 nanoparticles on tensile strength of dental acrylic resins. J Dent Res Dent Clin Dent Prospects 2014;8:197-203.  Back to cited text no. 5
    
6.
Mahross HZ, Baroudi K. Effect of silver nanoparticles incorporation on viscoelastic properties of acrylic resin denture base material. Eur J Dent 2015;9:207-12.  Back to cited text no. 6
[PUBMED]  [Full text]  
7.
Raigrodski AJ. Contemporary materials and technologies for all-ceramic fixed partial dentures: A review of the literature. J Prosthet Dent 2004;92:557-62.  Back to cited text no. 7
    
8.
Vagkopoulou T, Koutayas SO, Koidis P, Strub JR. Zirconia in dentistry: Part 1. Discovering the nature of an upcoming bioceramic. Eur J Esthet Dent 2009;4:130-51.  Back to cited text no. 8
    
9.
de Gee AJ, ten Harkel EC, Davidson CL. Measuring procedure for the determination of the three-dimensional shape of dentures. J Prosthet Dent 1979;42:149-53.  Back to cited text no. 9
    
10.
Ihab NS, Moudhaffar M. Evaluation the effect of modified nano-fillers addition on some properties of heat cured acrylic denture base material. J Baghdad Coll Dent 2011;23:23-9.  Back to cited text no. 10
    
11.
Ihab NS, Hassanen KA, Ali NA. Assessment of zirconium oxide nano-fillers incorporation and silanation on impact, tensile strength and color alteration of heat polymerized acrylic resin. J BaghColl Dent 2012;24:36-42.  Back to cited text no. 11
    
12.
Gad MM, Al-Thobity AM, Shahin SY, Alsaqer BT, Ali AA. Inhibitory effect of zirconium oxide nanoparticles on Candida albicans adhesion to repaired polymethyl methacrylate denture bases and interim removable prostheses: A new approach for denture stomatitis prevention. Int J Nanomedicine 2017;12:5409-19.  Back to cited text no. 12
    
13.
Gad MM, Rahoma A, Al-Thobity AM, ArRejaie AS. Influence of incorporation of ZrO2 nanoparticles on the repair strength of polymethyl methacrylate denture bases. Int J Nanomedicine 2016;11:5633-43.  Back to cited text no. 13
    
14.
Ahmed MA, Ebrahim MI. Effect of zirconium oxide nano-fillers addition on the flexural strength, fracture toughness, and hardness of heat-polymerized acrylic resin. World J Nano Sci Eng 2014;4:50-7.  Back to cited text no. 14
    
15.
Asopa V, Suresh S, Khandelwal M, Sharma V, Asopa SS, Kaira LS. A comparative evaluation of properties of zirconia reinforced high impact acrylic resin with that of high impact acrylic resin. Saudi J Dent Res 2015;6:146-51.  Back to cited text no. 15
    
16.
Uzun G, Hersek N, Tinçer T. Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. J Prosthet Dent 1999;81:616-20.  Back to cited text no. 16
    
17.
Koran A III. Prosthetic applications of polymers. In: CraigRG, PowersJM, editors. Restorative dental materials. 11th ed. St. Louis, Missouri, USA:Mosby Inc.; 2002. ISBN 0-323-01442-9.  Back to cited text no. 17
    
18.
Ranganath LM, Shet RG, Rajesh AG, Abraham S. The effect of fiber reinforcement on the dimensional changes of poly methyl methacrylate resin after processing and after immersion in water: An in vitro study. J Contemp Dent Pract 2011;12: 305-17.  Back to cited text no. 18
    
19.
Faot F, Panza LH, Garcia RC, Cury AA. Impact and flexural strength, and fracture morphology of acrylic resins with impact modifiers. Open Dent J 2009;3:137-43.  Back to cited text no. 19
    
20.
Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes in complete dentures on drying, wetting and heating in water. J Am Dent Assoc 1962;65:495-505.  Back to cited text no. 20
    
21.
Anthony DH, Peyton FA. Dimensional accuracy of various denture-base materials. J Prosthet Dent 1962;12:67-81.  Back to cited text no. 21
    
22.
Peyton FA, Anthony DH. Evaluation of dentures processed by different techniques. J Prosthet Dent 1963;13:269-82.  Back to cited text no. 22
    
23.
Kraut RA. A comparison of denture base accuracy. J Am Dent Assoc 1971;83:352-7.  Back to cited text no. 23
    
24.
Antomopoulos AN. Dimensional and occlusal changes in fluid resin dentures. J Prosthet Dent 1978;39:605-15.  Back to cited text no. 24
    
25.
Gad MM, Fouda SM, Al-Harbi FA, Näpänkangas R, Raustia A. PMMA denture base material enhancement: A review of fiber, filler, and nanofiller addition. Int J Nanomedicine 2017;12:3801-12.  Back to cited text no. 25
    
26.
Harini P, Mohamed K, Padmanabhan TV. Effect of titanium dioxide nanoparticles on the flexural strength of polymethylmethacrylate: An in vitro study. Indian J Dent Res 2014;25:459-63.  Back to cited text no. 26
[PUBMED]  [Full text]  
27.
Andreotti AM, Goiato MC, Moreno A, Nobrega AS, Pesqueira AA, dos Santos DM. Influence of nanoparticles on color stability, microhardness, and flexural strength of acrylic resins specific for ocular prosthesis. Int J Nanomedicine 2014;9:5779-87.  Back to cited text no. 27
    
28.
Gad M, ArRejaie AS, Abdel-Halim MS, Rahoma A. The reinforcement effect of nano-zirconia on the transverse strength of repaired acrylic denture base. Int J Dent 2016;2016:7094056.  Back to cited text no. 28
    
29.
Ghahremani L, Shirkavand S, Akbari F, Sabzikari N. Tensile strength and impact strength of color modified acrylic resin reinforced with titanium dioxide nanoparticles. J Clin Exp Dent 2017;9:e661-5.  Back to cited text no. 29
    
30.
Cevik P, Yildirim-Bicer AZ. The effect of silica and prepolymer nanoparticles on the mechanical properties of denture base acrylic resin. J Prosthodont 2018;27:763-70.  Back to cited text no. 30
    
31.
Hameed HK, Rahman HA. The effect of addition nano particle ZrO2 on some properties of autoclave processed heat cure acrylic denture base material. J Baghdad Coll Dent 2015;27:32-9.  Back to cited text no. 31
    
32.
Ibrahim RA. The effect of adding single walled carbon nanotube with different concentrations on mechanical properties of heat cure acrylic denture base material. J Baghdad Coll Dent 2015;27:28-32.  Back to cited text no. 32
    
33.
Ismail IJ, Jasim BS. The effect of silanized alumina nano-fillers addition on some physical and mechanical properties of heat cured polymethyl methacrylate denture base material. J Baghdad Coll Dent 2014;26:18-23.  Back to cited text no. 33
    
34.
Mahmood WS. The effect of incorporating carbon nanotubes on impact, transverse strength, hardness, and roughness to high impact denture base material. J Baghdad Coll Dent 2015;27:96-9.  Back to cited text no. 34
    
35.
Safi IN. Evaluation the effect of nano-fillers (TiO2, Al2O3, SiO2) addition on glass transition temperature, e-moudulus and coefficient of thermal expansion of acrylic denture base material. J Baghdad Coll Dent 2014;26:37-41.  Back to cited text no. 35
    
36.
Tahereh G, Fahimeh H, Baharak E. In vitro comparison of compressive and tensile strengths of acrylic resins reinforced by silver nanoparticles at 2% and 0.2%. Concentrations 2014;8:204-9.  Back to cited text no. 36
    
37.
Ayad NM, Badawi MF, Fatah AA. Effect of reinforcement of high-impact acrylic resin with zirconia on some physical and mechanical properties. Rev Clin Pesq Odontol 2008;4: 145-51.  Back to cited text no. 37
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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