- Email: [email protected]
Addition of nanoparticles for development of radiopaque dental adhesives
International Journal of Adhesion and Adhesives 80 (2018) 122–127
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Addition of nanoparticles for development of radiopaque dental adhesives a
a
a
b
MARK
c
Alexandra R. Cocco , Giana S. Lima , Fernanda B. Leal , Eliseu A. Munchow , Fabrício A. Ogliari , ⁎ Evandro Pivaa, a b c
School of Dentistry, Department of Restorative Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil Department of Dentistry, Health Science Institute, Federal University of Juiz de Fora, Governador Valadares, MG, Brazil Materials Engineering School, Federal University of Pelotas, Pelotas, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticles Etch-and-rinse Radiopacity Mechanical properties
This study evaluate the effect of nanoparticles (Bi2O3 - bismuth oxide; YbF3 - ytterbium trifluoride; SiO2 - silicon dioxide) on radiopacity (R), translucency parameter (TP), degree of conversion (DC), bond strength to dentin (µTBS), and quality of the adhesive interface of experimental two-step, etch-and-rinse adhesive systems. The R was evaluated using an x-ray equipment. TP was verified in five replications and DC was performed in triplicate. The µTBS was evaluated in specimens stored in distilled water after 24 h, 6, 12 and 24 months. The bonded interfaces were qualitatively examined by scanning electron microscopy (SEM), and using x-ray energy dispersive spectroscopy (EDS) for elemental analysis. Data were statistically analyzed at α = 0.05. YbF3 resulted in statistically greater R when compared with all the other groups (p = 0.024), which did not differ from each other. Bi2O3 was the adhesive with lowest TP. For DC, the YbF3 and C showed similar results (p = 0.627). Similar µTBS was obtained between groups. The addition of nanoparticles affected the chemical-mechanical properties. The incorporation of YbF3 could be considered a promising approach for the development of radiopaque dental adhesives.
1. Introduction With the purpose of improving the chemical and mechanical properties of dental resin-based materials, particles have been incorporated to reinforce the mechanisms in crystalline, semi-crystalline and amorphous materials [1]. Among these strategies, reducing the particle size down to the nanoscale level has been widely used [2–4]. Nanoparticles closer in size to those of the polymer chain have led to good interaction between the chain/polymer due to the increased surface to volume ratio of the fillers [5]. In adhesive systems, nanoparticles may increase mechanical properties such as strength, and viscosity [5,6]. Moreover, the incorporation of particles containing chemical elements with high atomic number may provide adhesive systems with radiopacity, which would be interesting, because the adhesive layer would typically be radiolucent. These radiolucent radiographic images may be similar to those of secondary caries or defective restorations. Consequently, clinical misdiagnosis and/or unnecessary replacement of restorations may occur [7], with additional costs, chair time, and discomfort to the patient. To avoid this problem, the incorporation of the oxides such as silicon dioxide (SiO2), barium oxide, barium sulfate, titanium dioxide, strontium oxide, zirconia dioxide have been used as radiopacifiers
⁎
[2,8]. In addition, ytterbium trifluoride (YbF3) has been shown to be a satisfactory source of radiopacity [9,10]; while the use of SiO2 nanoparticles was more interesting to increase the cohesive strength of adhesive resins [2]. Unfortunately, all the additives mentioned above also may bring some unwanted problems. For example, incorporation of great quantity fillers into dental resin may have some adverse effects on the mechanical properties of materials because this leads to significantly reduced inter-particle spacing, increasing the number of particle collisions and suspension viscosity. Moreover, excessive amounts of fillers could cause loss of dimensional stability of the composite materials or deterioration in bond strength, decrease in flexural strength and elastic modulus, diametral tensile strength, and fracture toughness. One study showed that increase in nanoparticle radiopacifier concentration was associated with exponential growth in viscosity and exponential decay in the translucency parameter [10,11]. Furthermore, excessive amounts of fillers could cause loss of dimensional stability of the composite materials or deterioration in bond strength [12–14]. Moreover, only few studies have evaluated the radiological, chemical and mechanical properties of experimental adhesives containing nanoparticles. In other words, there are other particles that may contribute to increasing the performance of these modified adhesives, but
Correspondence to: School of Dentistry, Federal University of Pelotas, Rua Gonçalves Chaves, 457/504, 96015-560 Pelotas, RS, Brazil. E-mail address: [email protected] (E. Piva).
http://dx.doi.org/10.1016/j.ijadhadh.2017.10.009 Accepted 17 October 2017 Available online 07 November 2017 0143-7496/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
2. Materials and methods
silicone molds, polyester strips were placed on the top and bottom surfaces, which were then photoactivated for 60 s. The specimens were stored in distilled water at 37 °C. The CIEL*a*b* color coordinates were measured after 24 h using a spectrophotometer (SP60; X-Rite, Grand Rapids, MI, USA). Color readouts were taken against white (L* = 93.07, a* = 1.28, b* = 5.25) and black (L* = 27.94, a* = 0.01, b* = 0.03) Munsell-like neutral value scale sheet backgrounds (AG-5330; BYK-Chemie, Wesel, Germany). The TP for each specimen was calculated using the formula: TP = [(L*W-L*B)2 + (a*W-a*B)2 + (b*Wb*B)2]1/2, where W and B referred to the color coordinates measured against the white and black backgrounds. Five specimens were tested for each material. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
2.1. Adhesive resin formulations
2.4. Degree of conversion (DC)
A stock methacrylate-based resin was formulated by mixing 35 wt% of 2-hydroxyethyl methacrylate (HEMA, Sigma-Aldrich, St. Louis, MO, USA), 15 wt% of urethane dimethacrylate (UDMA, Esstech, Essington, PA, USA), 10 wt% of triethyleneglycol dimethacrylate (TEGDMA, Esstech), 10 wt% of bisphenol-A glycidyl dimethacrylate (Bis-GMA, Esstech, and 10 wt% of glycerol dimethacrylate (GDMA, Esstech). A 20 wt% fraction of ethanol (Labsynth Ltda., Diadema, SP, Brazil) was used as solvent. A ternary photoinitiator system consisting of 0.4 M% of camphorquinone (CQ, Esstech), 1 M% of ethyl 4-dimethylaminobenzoate (EDAB, Fluka, Milwalkee, WI, USA), and 1 M% of diphenyliodonium hexafluorophosphate (DPI, Sigma-Aldrich) was used. YbF3 (40–80 nm average particle size, Nanostructured & Amorphous Materials, TX, USA) and Bi2O3 nanoparticles (90–210 nm average particle size, Nanostructured & Amorphous Materials) were superficially treated with a 10 wt% phosphate monomer/ethanol solution, whereas SiO2 nanoparticles (7 nm average particle size, Aerosil 380, Degussa, Germany) were silanized with a 10 wt% solution of organosilane (γmethacryloxypropyltrimethoxysilane, Sigma-Aldrich) in acetone (Labsynth Ltda.) The slurry was stored at 37 °C for 24 h to assure complete solvent removal (acetone). The SiO2 particles were then sonicated and filtered using sieves with 150 µm openings to avoid agglomeration. Next, the stock resin was divided into four groups, according to the type of nanoparticle incorporated: 10 wt%): Bi2O3, YbF3, SiO2, control (without particles). The nanoparticles were mechanically mixed with the resin, first with the help of a spatula and subsequently by using a motorized mixer (stirring process). After this, the resin adhesives were ultrasonicated for 1 h.
The degree of conversion of the adhesives was evaluated using realtime Fourier Transform infrared spectroscopy (FTIR, Prestige21, Shimadzu, Tokyo, Japan) with an attenuated total reflectance device. A micropipette was used to drop a controlled amount of each material onto the total reflectance accessory (10 µl), and a preliminary readout for the uncured material (monomer) was taken using 24 co-added scans and 4-cm-1 resolutions. Data was taken from readouts in triplicate (n = 3). The adhesive was photoactivated for 20 s using a light-emitting diode (LED) curing unit (Radii, SDI, Bayswater, Victoria, Australia) with 1400 mW/cm2 irradiance, and readouts were taken again for the polymer. DC was calculated (%) as previously described [17]. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
they have not yet been evaluated. For example, bismuth oxide (Bi2O3) is an agent commonly used for radiopacity purposes in dental materials, including but not limited to mineral trioxide aggregate (MTA) materials [15]. Therefore, the aim of this study was to investigate the effect of nanoparticles (Bi2O3, YbF3, and SiO2) on radiopacity, translucency parameter degree of conversion, immediate and long-term bond strength to dentin, and on the quality of the bonded interfaces. The null hypothesis to be tested was that the incorporation of nanoparticles would provide radiopacity without compromising the properties and bonding performance of the adhesive systems.
2.5. Microtensile bond strength (µTBS) test Forty bovine incisors were used in this study. The middle dentin layer was exposed and wet-polished with 600-grit SiC papers for 60 s under water-cooling. The dentin was etched with 37% phosphoric acid gel for 15 s and cleansed with air-water spray for 15 s. Two coats of the experimental adhesives were applied; the solvent was evaporated for 10 s using an air stream; and the samples were photoactivated for 20 s using the LED unit. Composite resin restorations (Filtek Z250, 3 M ESPE, St Paul, MN, USA) were built up on the surfaces in 2 mm increments. The samples were stored in distilled water at 37 °C. After 24 h, the bonded samples were sectioned into beam-shaped specimens with an area of ~ 0.7 mm2. The specimens were randomly divided into four time intervals (24 h, and 6, 12, or 24 months). The specimens were stored in distilled water at 37 °C (n = 20). After this, µTBS test was performed with a universal testing machine (DL500, EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 1 mm/ min. µTBS values were recorded in MPa, and the data were subjected to two-way ANOVA (adhesive × storage period as factors) and the Tukey test (α = 5%). The fractured specimens were observed at 500X magnification, and the failure modes were classified as adhesive, cohesive within resin, cohesive within dentin, or mixed.
2.2. Radiopacity The radiographic test was performed in accordance with ISO 4049 [16]. Radiographic images of five cylindrical specimens (5 mm in diameter, 1 mm thick) per adhesive were distributed on periapical film (Insight; Kodak, Rochester, NY, USA) and the images were captured by means of an X-ray device (Spectro 70X Seletronic; Dabi Atlante, Ribeirão Preto, SP, Brazil) using 70 kV, 8 mA; a 40 cm focus-film distance; and exposure to irradiation for 0.4 s. An aluminum step-wedge (aluminum purity range > 99.0%) with thickness ranging from 0.5 to 5 mm; 0.5 mm for every increasing step was exposed simultaneously to the radiation (control). The gray levels (pixel density) of the digital images were analyzed by image software, and the aluminum equivalence values (mm) of each specimen were recorded. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
2.6. SEM and EDS analyses The different adhesives with nanoparticles were analyzed by scanning electron microscopy at 15 kV (Jeol, JSM - 6610LV, USA). Six bovine teeth were obtained and the adhesive system was applied as described earlier (n = 2). The dentin discs were bonded to each other using a thin layer of photo-activated composite resin, generating a dentin-composite resin-dentin sandwich specimen. The specimens were embedded cross-sectionally in epoxy resin so that the dentin-resin composite-interface was visible. After 24 h, the specimens were wet polished with 600, 1200, 1500 and 2000-grit SiC papers and polished with 3-, 1- and 0.5-µm diamond suspensions. The surfaces were etched with 50% phosphoric acid solution for 5 s and deproteinized by
2.3. Translucency parameter (TP) The TP of cylindrical specimens of each adhesive (7 mm in diameter, 1 mm thick) was measured. The adhesives were placed in 123
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
Fig. 2. Values microtensile bond strength to dentin. MPa. In 12 months, there was an increase in bond strength of the adhesive containing SiO2 nanoparticles compared with the control material.
Fig. 1. Evaluation of radiopacity (pixel density) of the experimental adhesive resins. Distinct letters indicate statistically significant differences.
immersion in 2.5% NaOCl solution for 10 min. The specimens were ultrasonically cleaned with distilled water for 1 h and stored in a receptacle with silica gel for 24 h, at room temperature. Thereafter, the specimens were coated with carbon, and the bonded interfaces were examined by SEM and by x-ray energy dispersive spectroscopy (EDS), for elemental analysis.
3.4. Microtensile bond strength (µTBS) to dentin The bond strength results are shown in Table 1. The factors “adhesive” (p = 0.026) and “storage period” (p < 0.001) were both statistically significant, whereas the interaction between the factors was not significant (p = 0.619). No significant differences in bond strength were observed between the materials for any storage time (p = 0.084) except for the significantly higher bond strength of the adhesive containing SiO2 nanoparticles at 12 months compared with the control material (p = 0.015) (Fig. 2). For the control adhesive and the adhesive containing Bi2O3 nanoparticles, the bond strength values at 24 h were significantly higher than those of all the other storage time intervals (p = 0.007). The material with SiO2 nanoparticles showed an early bond strength that was significantly higher than that found at 6 months and 24 months (p = 0.002), whereas no significant differences in bond strength for any storage period were observed for the adhesive with YbF3 nanoparticles. Mixed failures were predominant for the time intervals of 6 months and 12 months of storage, while adhesive and mixed failures were predominant at 24 h and 24 months (Fig. 3). In general, the failure modes of the different adhesives were similar.
3. Results 3.1. Radiopacity The radiopacity results are shown in Fig. 1. The radiopacity value for the adhesive containing YbF3 nanoparticles was significantly higher compared with all the other groups (p = 0.024). All the other groups had radiopacity similar to that of the control group (p = 0.672). When using 10 wt% of fillers only the formulation with YbF3 nanoparticles reached an average radiopacity level above 1.0 mm Al; whereas, the other materials had an average radiopacity below 0.5 mm Al. 3.2. Translucency parameter (TP) The TP results are shown in Table 1. The adhesive containing Bi2O3 showed lower TP, and differed from that of control. YbF3 nanoparticles also showed lower TP and differed significantly from that of control (p = 0.129).
3.5. SEM and EDS SEM micrographs of the bonded interfaces (adhesives and dentin) are shown in Fig. 4. Long resin tags were obtained for the YbF3-based adhesive (Fig. 4a); within the hybrid layer of the Bi2O3-based group, agglomerations of the Bi2O3 could be observed (Fig. 4b); and silica deposition was noted along the tubules for the SiO2 group (Fig. 4c). EDS analysis identified the presence of ytterbium, bismuth and silicon, among others, within the experimental adhesives analyzed (Fig. 5), showing characteristic peaks in the emission energy spectra of these elements.
3.3. Degree of conversion The adhesive containing YbF3 nanoparticles and the control showed similar DC, however a higher DC value than the other adhesives (p = 0.022), as shown in Table 1. The material with SiO2 nanoparticles also showed a significantly higher degree of conversion than the adhesive containing Bi2O3 (p = 0.007).
Table 1 Means (standard deviations) for degree of C = C conversion (DC), microtensile bond strength to dentin, MPa, and translucency parameter (TP). Nanoparticle*
YbF3 Bi2O3 SiO2 None (control)
%DC (n = 5)
91.9 45.3 69.4 88.6
( ± 2.8)a ( ± 7.6)c ( ± 10.8)b ( ± 9.3)a
µTBS, MPa (n = 20)**
TP (n = 5)
24 h
6 months
37.8 ( ± 15.7)A,a 40.6 ( ± 10.5)A,a 40.9 ( ± 12.4)A,a 39.1( ± 14.4)A,a
24.4 26.0 27.8 24.0
( ± 11.5)A,a ( ± 9.2) B,a ( ± 9.1) B,a ( ± 9.0)B,a
12 months 29.3 29.2 32.7 23.8
( ± 10.0)A,ab ( ± 8.7)B,ab ( ± 6.3)AB,a ( ± 8.9)B,b
24 months 29.6 31.1 27.2 24.1
( ± 13.2)A,a ( ± 13.3)B,a ( ± 11.0)B,a ( ± 6.1)B,a
Different lowercase letters in the same column indicate significant differences between groups (p < 0.05) YbF3: ytterbium trifluoride; Bi2O: bismuth oxide; SiO2 (silicon dioxide); control (without nanoparticles) ** Different capital letters in the same row and lowercase letters in the same column indicate significant differences between groups (p < 0.05).
124
50.6 ( ± 1.1)b 35.4 ( ± 3.4)c 54.9 ( ± 0.7)ab 57.7a ( ± 0.1)a
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
YbF3-modified adhesive improved radiopacity and maintained the bond strength to dentin stable over time; in addition, the degree of conversion was not altered. Consequently, the hypothesis of the study was partially accepted. The degree of radiopacity of a given particle depends on the atomic number of its components, as well as their density and size [18,19]. Elements with high atomic numbers can absorb or reflect light, thus influencing light scattering within the material, and consequently, making the material radiopaque [20]. Particles containing elements of high atomic number have been used in dentistry as radiopacifying agents. For example, YbF3 nanoparticles were shown to improve radiopacity in resin-based cements [18,21], composite resins, and glassionomer cements [22]. Indeed, the inclusion of 40% by weight of YbF3 nanoparticles in methacrylate-based cements resulted in radiopacity values similar to 3 mm of Al [21]. In this study, the ytterbium-based nanoparticle, which has a high atomic number (Yb = 70) [21], produced higher radiopacity values than the other nanoparticles and higher than 1 mm of aluminum (Fig. 1), thereby meeting the ISO requirements for resin-based materials [16]. On the other hand, the Bi2O3 or SiO2 nanoparticles added to adhesives not contribute to improving radiopacity. This may be explained because provide radiopacity, the material needs to allow the passage/transmission of light, and considering that both Bi2O3- and SiO2-modified adhesives resulted in lower degree of conversion than the control, it can be inferred that these
Fig. 3. Frequency distribution of failure modes for all materials and storage periods.
4. Discussion For nano-based materials, it is of utmost importance that nanoparticles do not jeopardize the chemo-mechanical stability of the modified material; in the present case, the experimental adhesive systems. According to the present findings, only one type of nanoparticle, i.e., YbF3, did not compromise the performance of the material. Differently from the nanoparticle-free adhesive used as control, the
Fig. 4. SEM micrographs of the interface bonded with adhesive system experimental and demineralized dentin (D). In (a) ytterbium particles. In (b) bismuth particles. In (c), silica deposition was noted long tubule. T, resin tags formed by adhesive filling into dentinal tubules. HL, hybrid layer between the adhesive and the underlying mineralized dentin. The circle show particle agglomeration.
125
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
adhesive might have negative effects on the esthetic qualities of dental restorations. This should not be neglected, and should be analyzed in further studies. A recent study showed that adhesives play an important role in altering the color of composite restorations [30]. Although the immediate bond strength was not negatively affected by the incorporation of nanoparticles into the adhesive systems, the long-term bond strength of adhesives containing Bi2O3 and SiO2 was significantly reduced. According to some studies, the lower degree of conversion exhibited by these aforementioned adhesives may have increased the permeability and water sorption phenomena within the adhesive layer, thus resulting in reduced bond strength durability [31,32]. Worth mentioning, the bond strength between restorative material and dental substrates is expected to decrease over time, as demonstrated by the nanoparticle-free adhesive tested; indeed, polymer-based materials such as dental adhesives and restoratives are very prone to suffer hydrolysis, breaking the intermolecular bonds formed during the bonding procedure [33]. The nanoparticles-modified adhesives synthesized here also underwent hydrolysis, similarly to the control material, except for the adhesive system containing YbF3. A similar result was observed in the study of Macedo et al. [11], in which the adhesive containing YbF3 was more resistant to strong hydrolytic degradation, maintaining the bond strength to dentin stable after 6 months of water storage. The authors suggested that due to the electronegative nature of the YbF3 nanoparticle (i.e., the fluorine ion may be considered an excellent hydrogen bond acceptor), strong intermolecular hydrogen bonds could be formed between the nanoparticles and the monomers present within the resin matrix, thus contributing to protecting the network system from fast hydrolysis. Moreover, the YbF3-modified adhesive presented a higher degree of conversion when compared with the other nanoparticle-based adhesives (Table 1), thereby contributing to keep the bond statistically stable, even after 24 months of water storage. However, all the adhesives containing nanoparticles, except YbF3modified adhesive, showed bond strength decreased by about 40%, after 6 months aging. And after 12 months, all adhesive showed a small increase, but SiO2-modified adhesive was the only one that showed statistical difference (Fig. 2), suggesting that 10% silanized silica nanoparticles increased the water resistance of the adhesive [34]. Many authors have found that the silanes are resistant to water leaching [35–37]. Moreover, one study showed that an experimental resin with silanized fillers had higher diametral tensile strength and compressive strength than that without silanization [38]. According to the SEM micrographs (Fig. 5) obtained in the present study, all the nanoparticle-modified adhesives produced a uniform hybrid layer with a similar degree of demineralization. This may explain the similar immediate bond strength results. Although SiO2 presented better particle dispersion, all the adhesive systems had nanoparticles successfully incorporated into the matrix. This fact can be justified by the dispersion method adopted for the study, since it has previously been shown that mixing followed by ultrasonication formed stable suspensions and decreased sedimentation during storage. Furthermore, the SiO2-modified adhesive produced a better particle distribution/dispersion, probably due to the small size of these particles (7 nm) compared with the other nanoparticles, which were around 90–210 nm. The smaller the particle size, the greater the flow into the small dentinal tubules during bonding [39]. Considering the different results found for the nanoparticle-modified adhesives evaluated in this study, it seems that the gain in radiopacity without jeopardizing important characteristics of dental bonding agents depended on the type of radiopacifiers incorporated into the material. Furthermore, YbF3 nanoparticle could be considered a feasible alternative for the development of novel radiopaque dental adhesives.
Fig. 5. EDS analysis of the (a) ytterbium (b) bismuth and (c) silica groups.
adhesives suffered from light scattering, thus interfering with light penetration into the bulk of the material. As a consequence, both the conversion of monomers and the possible gain in radiopacity were affected. This effect was also stressed if the size of the particles approached the output wave length of the light-curing unit [23]. Moreover, it should be considered that distinct particle agglomeration of the different nanoparticles will also play a role in light transmission within the material; the agglomeration of particles could cause phase separation and loss of homogeneity in the resultant material, impairing the degree of conversion and mechanical properties [24,25]. For example, the agglomeration of particles may also create clusters around the openings of the dentin tubules, hampering proper monomer infiltration within the interfibrillar spaces, thereby creating voids within the hybrid layer (shown in Fig. 3). This effect may reduce the adhesion/bond strength, especially to dentin [2,22]. However, the present findings demonstrated that the addition of nanoparticles did not affect the immediate bond strength to dentin when compared with the control group (Table 1). This corroborates the findings of a previous study that showed that an increase in the concentration of zirconia nanoparticles in both the primer and/or the adhesive formulations gradually increased the microtensile bond strength to dentin [5]. The authors supposed that the increase in bond strength was due to the reinforcement promoted by the adhesive resin with filler incorporation, because nanofillers have larger surfaces in contact with the organic matrix [5,26]. In the same way, the Bi2O3-modified adhesive resulted in lower TP than that of the control, which is known to cause detrimental effects on optical properties of translucent materials due to its grayish discoloration [27,28]. Change in TP is a problem because it may affect the light polymerization behavior of the adhesives [29]. Despite the fact that adhesives are applied clinically in very thin films, the color of the 126
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
[17] Collares FM, Ogliari FA, Zanchi CH, Petzhold CL, Piva E, Samuel SM. Influence of 2hydroxyethyl methacrylate concentration on polymer network of adhesive resin. J Adhes Dent 2011;13:125–9. [18] Michl, R.J., Rheinberger, V.M., Ott, G., U.S. Patent and Trademark Office, U.S. Patent No. 4,629,746., Washington, DC; 1986. [19] Schulz H, Schimmoeller B, Pratsinis SE, Salz U, Bock T. Radiopaque dental adhesives: dispersion of flame-made Ta2O5/SiO2 nanoparticles in methacrylic matrices. J Dent 2008;36:579–87. [20] Aoyagi Y, Takahashi H, Iwasaki N, Honda E, Kurabayashi T. Radiopacity of experimental composite resins containing radiopaque materials. Dent Mater J 2005;24:315–20. [21] Collares FM, Ogliari FA, Lima GS, Fontanella VR, Piva E, Samuel SM. Ytterbium trifluoride as a radiopaque agent for dental cements. Int Endod J 2010;43:792–7. [22] Prentice LH, Tyas MJ, Burrow MF. The effect of ytterbium fluoride and barium sulphate nanoparticles on the reactivity and strength of a glass-ionomer cement. Dent Mater 2006;22:746–51. [23] Turssi CP, Ferracane JL, Vogel K. Filler features and their effects on wear and degree of conversion of particulate dental resin composites. Biomaterials 2005;26:4932–7. [24] Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater 1999;15:128–37. [25] Nunes TG, Pereira SG, Kalachandra S. Effect of treated filler loading on the photopolymerization inhibition and shrinkage of a dimethacrylate matrix. J Mater Sci: Mater Med 2008;19:1881–9. [26] Conde MC, Zanchi CH, Rodrigues-Junior SA, Carren NL, Ogliari FA, Piva E. Nanofiller loading level: influence on selected properties of an adhesive resin. J Dent 2009;37:331–5. [27] Sanz O, Haro-Poniatowski E, Gonzalo J, Navarro JM Fernández. Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption. J Non-Cryst Solids 2006;352:761–8. [28] Marciano MA, Costa RM, Camilleri J, Mondelli RF, Guimaraes BM, Duarte MA. Assessment of color stability of white mineral trioxide aggregate angelus and bismuth oxide in contact with tooth structure. J Endod 2014;40:1235–40. [29] Moraes RR, Garcia JW, Wilson ND, Lewis SH, Barros MD, Yang B, et al. Improved dental adhesive formulations based on reactive nanogel additives. J Dent Res 2012;91:179–84. [30] Alabdulwahhab BM, AlShethry MA, AlMoneef MA, AlManie MA, AlMaziad MM, AlOkla MS. The effect of dental adhesive on final color match of direct laminate veneer (DLV): in vitro study. J Esthet Restor Dent 2015;27:307–13. [31] Cadenaro M, Antoniolli F, Sauro S, Tay FR, Di Lenarda R, Prati C, et al. Degree of conversion and permeability of dental adhesives. Eur J Oral Sci 2005;113:525–30. [32] Armstrong SR, Vargas MA, Fang Q, Laffoon JE. Microtensile bond strength of a total-etch 3-step, total-etch 2-step, self-etch 2-step, and a self-etch 1-step dentin bonding system through 15-month water storage. J Adhes Dent 2003;5:47–56. [33] De Munck J, Poitevin A, Luhrs AK, Pongprueksa P, Van Ende A, Van Landuyt KL, et al. Interfacial fracture toughness of aged adhesive-dentin interfaces. Dent Mater 2015;31:462–72. [34] Wang Z, Gu Z, Hong Y, Cheng L, Li Z. Bonding strength and water resistance of starch-based wood adhesive improved by silica nanoparticles. Carbohydr Polym 2011;86:72–7. [35] Donath SMH, Mai C. Wood modification with alkoxysilanes. Wood Sci Technol 2004;38:555–66. [36] Hill CAS, Farahani MRM, Hale MDC. The use of organo alkoxysilane coupling agents for wood preservation. Holzforschung 2004;58:316–25. [37] Abdelmouleh MBS, Ben Salah A, Belgacem MN, Gandini A. Interaction of silane coupling agents with cellulose. Langmuir 2002;18:3203–8. [38] Lin CT, Lee SY, Keh ES, Dong DR, Huang HM, Shih YH. Influence of silanization and filler fraction on aged dental composites. J Oral Rehabil 2000;27:919–26. [39] Melo MA, Cheng L, Zhang K, Weir MD, Rodrigues LK, Xu HH. Novel dental adhesives containing nanoparticles of silver and amorphous calcium phosphate. Dent Mater 2013;29:199–210.
5. Conclusion Within the limitations of the study, the addition of nanoparticles affected the radiopacity, translucency parameter, degree of conversion and long-term bond strength to dentin. The formulation of YbF3-modified adhesive tested showed to be a promising approach for the development of radiopaque dental adhesives. Acknowledgments We thank CEME-Sul – Centro de Microscopia Eletrônica da Zona Sul at Federal University of Rio Grande – Brazil for SEM equipment support. References [1] Gersappe D. Molecular mechanisms of failure in polymer nanocomposites. Phys Rev Lett 2002;89:058301. [2] Conde MC, Zanchi CH, Rodrigues-Junior SA, Carreno NL, Ogliari FA, Piva E. Nanofiller loading level: influence on selected properties of an adhesive resin. J Dent 2009;37:331–5. [3] Kim S, Cho JK, Shin S, Kim BJ. Photo-curing behaviors of bio-based isosorbide dimethacrylate by irradiation of light-emitting diodes and the physical properties of its photo-cured materials. J Appl Polym Sci 2015:132. [4] Sadat-Shojai MAM, Nodehi A, Khanlar LN. Hydroxyapatite nanorods as novel fillers for improving the properties of dental adhesives: synthesis and application. Dent Mater 2010;26:471–82. [5] Lohbauer U, Wagner A, Belli R, Stoetzel C, Hilpert A, Kurland HD, et al. Zirconia nanoparticles prepared by laser vaporization as fillers for dental adhesives. Acta Biomater 2010;6:4539–46. [6] Klapdohr S, Moszner N. New inorganic components for dental filling composites. Monatshefte Chem 2005;136:21–45. [7] Mjor IA. Clinical diagnosis of recurrent caries. J Am Dent Assoc 2005;136:1426–33. [8] Van Meerbeek B, Braem M, Lambrechts P, Vanherle G. Adhesion of composite to dentin. Mechanical and clinical results. Ned Tijdschr Tandheelkd 1993;100:489–94. [9] Collares FM, Klein M, Santos PD, Portella FF, Ogliari F, Leitune VC, et al. Influence of radiopaque fillers on physicochemical properties of a model epoxy resin-based root canal sealer. J Appl Oral Sci 2013;21:533–9. [10] Carreño Neftali LV, Oliveira Thiago CS, Piva Evandro, Leal Fernanda B, Lima Giana S, Moncks Marcelo D, Raubach Cristiane W, Ogliari Fabrício A. YbF3/SiO2 fillers as radiopacifiers in a dental adhesive resin. Nano-micro Lett 2012;4:3. [11] Macedo Cármen Lúcia Rodrigues, Münchow Eliseu Aldrighi, da Silveira Lima Giana, Zanchi Cesar Henrique, Ogliari Fabrício Aulo, Piva Evandro. Incorporation of inorganic fillers into experimental resin adhesives: effects on physical properties and bond strength to dentin. Int J Adhes Adhes 2015;62:78–84. [12] Miyazaki T, Sugawara-Narutaki A, Ohtsuki C. Organic-inorganic composites toward biomaterial application. Front Oral Biol 2015;17:33–8. [13] Miyazaki M, Ando S, Hinoura K, Onose H, Moore BK. Influence of filler addition to bonding agents on shear bond strength to bovine dentin. Dent Mater 1995;11:234–8. [14] Miyazaki M, Hinoura K, Onose H, Moore BK. Effect of filler content of light-cured composites on bond strength to bovine dentine. J Dent 1991;19:301–3. [15] Antonijevic D, Medigovic I, Zrilic M, Jokic B, Vukovic Z, Todorovic L. The influence of different radiopacifying agents on the radiopacity, compressive strength, setting time, and porosity of Portland cement. Clin Oral Investig 2014;18:1597–604. [16] ISO 4049. Dentistry Polymer Based Filling Restorative and Luting Materials. Geneva, Switzerland: International Organization for Standardization; 2000. p. 1–27.
127
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Addition of nanoparticles for development of radiopaque dental adhesives a
a
a
b
MARK
c
Alexandra R. Cocco , Giana S. Lima , Fernanda B. Leal , Eliseu A. Munchow , Fabrício A. Ogliari , ⁎ Evandro Pivaa, a b c
School of Dentistry, Department of Restorative Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil Department of Dentistry, Health Science Institute, Federal University of Juiz de Fora, Governador Valadares, MG, Brazil Materials Engineering School, Federal University of Pelotas, Pelotas, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticles Etch-and-rinse Radiopacity Mechanical properties
This study evaluate the effect of nanoparticles (Bi2O3 - bismuth oxide; YbF3 - ytterbium trifluoride; SiO2 - silicon dioxide) on radiopacity (R), translucency parameter (TP), degree of conversion (DC), bond strength to dentin (µTBS), and quality of the adhesive interface of experimental two-step, etch-and-rinse adhesive systems. The R was evaluated using an x-ray equipment. TP was verified in five replications and DC was performed in triplicate. The µTBS was evaluated in specimens stored in distilled water after 24 h, 6, 12 and 24 months. The bonded interfaces were qualitatively examined by scanning electron microscopy (SEM), and using x-ray energy dispersive spectroscopy (EDS) for elemental analysis. Data were statistically analyzed at α = 0.05. YbF3 resulted in statistically greater R when compared with all the other groups (p = 0.024), which did not differ from each other. Bi2O3 was the adhesive with lowest TP. For DC, the YbF3 and C showed similar results (p = 0.627). Similar µTBS was obtained between groups. The addition of nanoparticles affected the chemical-mechanical properties. The incorporation of YbF3 could be considered a promising approach for the development of radiopaque dental adhesives.
1. Introduction With the purpose of improving the chemical and mechanical properties of dental resin-based materials, particles have been incorporated to reinforce the mechanisms in crystalline, semi-crystalline and amorphous materials [1]. Among these strategies, reducing the particle size down to the nanoscale level has been widely used [2–4]. Nanoparticles closer in size to those of the polymer chain have led to good interaction between the chain/polymer due to the increased surface to volume ratio of the fillers [5]. In adhesive systems, nanoparticles may increase mechanical properties such as strength, and viscosity [5,6]. Moreover, the incorporation of particles containing chemical elements with high atomic number may provide adhesive systems with radiopacity, which would be interesting, because the adhesive layer would typically be radiolucent. These radiolucent radiographic images may be similar to those of secondary caries or defective restorations. Consequently, clinical misdiagnosis and/or unnecessary replacement of restorations may occur [7], with additional costs, chair time, and discomfort to the patient. To avoid this problem, the incorporation of the oxides such as silicon dioxide (SiO2), barium oxide, barium sulfate, titanium dioxide, strontium oxide, zirconia dioxide have been used as radiopacifiers
⁎
[2,8]. In addition, ytterbium trifluoride (YbF3) has been shown to be a satisfactory source of radiopacity [9,10]; while the use of SiO2 nanoparticles was more interesting to increase the cohesive strength of adhesive resins [2]. Unfortunately, all the additives mentioned above also may bring some unwanted problems. For example, incorporation of great quantity fillers into dental resin may have some adverse effects on the mechanical properties of materials because this leads to significantly reduced inter-particle spacing, increasing the number of particle collisions and suspension viscosity. Moreover, excessive amounts of fillers could cause loss of dimensional stability of the composite materials or deterioration in bond strength, decrease in flexural strength and elastic modulus, diametral tensile strength, and fracture toughness. One study showed that increase in nanoparticle radiopacifier concentration was associated with exponential growth in viscosity and exponential decay in the translucency parameter [10,11]. Furthermore, excessive amounts of fillers could cause loss of dimensional stability of the composite materials or deterioration in bond strength [12–14]. Moreover, only few studies have evaluated the radiological, chemical and mechanical properties of experimental adhesives containing nanoparticles. In other words, there are other particles that may contribute to increasing the performance of these modified adhesives, but
Correspondence to: School of Dentistry, Federal University of Pelotas, Rua Gonçalves Chaves, 457/504, 96015-560 Pelotas, RS, Brazil. E-mail address: [email protected] (E. Piva).
http://dx.doi.org/10.1016/j.ijadhadh.2017.10.009 Accepted 17 October 2017 Available online 07 November 2017 0143-7496/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
2. Materials and methods
silicone molds, polyester strips were placed on the top and bottom surfaces, which were then photoactivated for 60 s. The specimens were stored in distilled water at 37 °C. The CIEL*a*b* color coordinates were measured after 24 h using a spectrophotometer (SP60; X-Rite, Grand Rapids, MI, USA). Color readouts were taken against white (L* = 93.07, a* = 1.28, b* = 5.25) and black (L* = 27.94, a* = 0.01, b* = 0.03) Munsell-like neutral value scale sheet backgrounds (AG-5330; BYK-Chemie, Wesel, Germany). The TP for each specimen was calculated using the formula: TP = [(L*W-L*B)2 + (a*W-a*B)2 + (b*Wb*B)2]1/2, where W and B referred to the color coordinates measured against the white and black backgrounds. Five specimens were tested for each material. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
2.1. Adhesive resin formulations
2.4. Degree of conversion (DC)
A stock methacrylate-based resin was formulated by mixing 35 wt% of 2-hydroxyethyl methacrylate (HEMA, Sigma-Aldrich, St. Louis, MO, USA), 15 wt% of urethane dimethacrylate (UDMA, Esstech, Essington, PA, USA), 10 wt% of triethyleneglycol dimethacrylate (TEGDMA, Esstech), 10 wt% of bisphenol-A glycidyl dimethacrylate (Bis-GMA, Esstech, and 10 wt% of glycerol dimethacrylate (GDMA, Esstech). A 20 wt% fraction of ethanol (Labsynth Ltda., Diadema, SP, Brazil) was used as solvent. A ternary photoinitiator system consisting of 0.4 M% of camphorquinone (CQ, Esstech), 1 M% of ethyl 4-dimethylaminobenzoate (EDAB, Fluka, Milwalkee, WI, USA), and 1 M% of diphenyliodonium hexafluorophosphate (DPI, Sigma-Aldrich) was used. YbF3 (40–80 nm average particle size, Nanostructured & Amorphous Materials, TX, USA) and Bi2O3 nanoparticles (90–210 nm average particle size, Nanostructured & Amorphous Materials) were superficially treated with a 10 wt% phosphate monomer/ethanol solution, whereas SiO2 nanoparticles (7 nm average particle size, Aerosil 380, Degussa, Germany) were silanized with a 10 wt% solution of organosilane (γmethacryloxypropyltrimethoxysilane, Sigma-Aldrich) in acetone (Labsynth Ltda.) The slurry was stored at 37 °C for 24 h to assure complete solvent removal (acetone). The SiO2 particles were then sonicated and filtered using sieves with 150 µm openings to avoid agglomeration. Next, the stock resin was divided into four groups, according to the type of nanoparticle incorporated: 10 wt%): Bi2O3, YbF3, SiO2, control (without particles). The nanoparticles were mechanically mixed with the resin, first with the help of a spatula and subsequently by using a motorized mixer (stirring process). After this, the resin adhesives were ultrasonicated for 1 h.
The degree of conversion of the adhesives was evaluated using realtime Fourier Transform infrared spectroscopy (FTIR, Prestige21, Shimadzu, Tokyo, Japan) with an attenuated total reflectance device. A micropipette was used to drop a controlled amount of each material onto the total reflectance accessory (10 µl), and a preliminary readout for the uncured material (monomer) was taken using 24 co-added scans and 4-cm-1 resolutions. Data was taken from readouts in triplicate (n = 3). The adhesive was photoactivated for 20 s using a light-emitting diode (LED) curing unit (Radii, SDI, Bayswater, Victoria, Australia) with 1400 mW/cm2 irradiance, and readouts were taken again for the polymer. DC was calculated (%) as previously described [17]. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
they have not yet been evaluated. For example, bismuth oxide (Bi2O3) is an agent commonly used for radiopacity purposes in dental materials, including but not limited to mineral trioxide aggregate (MTA) materials [15]. Therefore, the aim of this study was to investigate the effect of nanoparticles (Bi2O3, YbF3, and SiO2) on radiopacity, translucency parameter degree of conversion, immediate and long-term bond strength to dentin, and on the quality of the bonded interfaces. The null hypothesis to be tested was that the incorporation of nanoparticles would provide radiopacity without compromising the properties and bonding performance of the adhesive systems.
2.5. Microtensile bond strength (µTBS) test Forty bovine incisors were used in this study. The middle dentin layer was exposed and wet-polished with 600-grit SiC papers for 60 s under water-cooling. The dentin was etched with 37% phosphoric acid gel for 15 s and cleansed with air-water spray for 15 s. Two coats of the experimental adhesives were applied; the solvent was evaporated for 10 s using an air stream; and the samples were photoactivated for 20 s using the LED unit. Composite resin restorations (Filtek Z250, 3 M ESPE, St Paul, MN, USA) were built up on the surfaces in 2 mm increments. The samples were stored in distilled water at 37 °C. After 24 h, the bonded samples were sectioned into beam-shaped specimens with an area of ~ 0.7 mm2. The specimens were randomly divided into four time intervals (24 h, and 6, 12, or 24 months). The specimens were stored in distilled water at 37 °C (n = 20). After this, µTBS test was performed with a universal testing machine (DL500, EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 1 mm/ min. µTBS values were recorded in MPa, and the data were subjected to two-way ANOVA (adhesive × storage period as factors) and the Tukey test (α = 5%). The fractured specimens were observed at 500X magnification, and the failure modes were classified as adhesive, cohesive within resin, cohesive within dentin, or mixed.
2.2. Radiopacity The radiographic test was performed in accordance with ISO 4049 [16]. Radiographic images of five cylindrical specimens (5 mm in diameter, 1 mm thick) per adhesive were distributed on periapical film (Insight; Kodak, Rochester, NY, USA) and the images were captured by means of an X-ray device (Spectro 70X Seletronic; Dabi Atlante, Ribeirão Preto, SP, Brazil) using 70 kV, 8 mA; a 40 cm focus-film distance; and exposure to irradiation for 0.4 s. An aluminum step-wedge (aluminum purity range > 99.0%) with thickness ranging from 0.5 to 5 mm; 0.5 mm for every increasing step was exposed simultaneously to the radiation (control). The gray levels (pixel density) of the digital images were analyzed by image software, and the aluminum equivalence values (mm) of each specimen were recorded. The data were statistically analyzed using one-way ANOVA and the Tukey test (α = 5%).
2.6. SEM and EDS analyses The different adhesives with nanoparticles were analyzed by scanning electron microscopy at 15 kV (Jeol, JSM - 6610LV, USA). Six bovine teeth were obtained and the adhesive system was applied as described earlier (n = 2). The dentin discs were bonded to each other using a thin layer of photo-activated composite resin, generating a dentin-composite resin-dentin sandwich specimen. The specimens were embedded cross-sectionally in epoxy resin so that the dentin-resin composite-interface was visible. After 24 h, the specimens were wet polished with 600, 1200, 1500 and 2000-grit SiC papers and polished with 3-, 1- and 0.5-µm diamond suspensions. The surfaces were etched with 50% phosphoric acid solution for 5 s and deproteinized by
2.3. Translucency parameter (TP) The TP of cylindrical specimens of each adhesive (7 mm in diameter, 1 mm thick) was measured. The adhesives were placed in 123
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
Fig. 2. Values microtensile bond strength to dentin. MPa. In 12 months, there was an increase in bond strength of the adhesive containing SiO2 nanoparticles compared with the control material.
Fig. 1. Evaluation of radiopacity (pixel density) of the experimental adhesive resins. Distinct letters indicate statistically significant differences.
immersion in 2.5% NaOCl solution for 10 min. The specimens were ultrasonically cleaned with distilled water for 1 h and stored in a receptacle with silica gel for 24 h, at room temperature. Thereafter, the specimens were coated with carbon, and the bonded interfaces were examined by SEM and by x-ray energy dispersive spectroscopy (EDS), for elemental analysis.
3.4. Microtensile bond strength (µTBS) to dentin The bond strength results are shown in Table 1. The factors “adhesive” (p = 0.026) and “storage period” (p < 0.001) were both statistically significant, whereas the interaction between the factors was not significant (p = 0.619). No significant differences in bond strength were observed between the materials for any storage time (p = 0.084) except for the significantly higher bond strength of the adhesive containing SiO2 nanoparticles at 12 months compared with the control material (p = 0.015) (Fig. 2). For the control adhesive and the adhesive containing Bi2O3 nanoparticles, the bond strength values at 24 h were significantly higher than those of all the other storage time intervals (p = 0.007). The material with SiO2 nanoparticles showed an early bond strength that was significantly higher than that found at 6 months and 24 months (p = 0.002), whereas no significant differences in bond strength for any storage period were observed for the adhesive with YbF3 nanoparticles. Mixed failures were predominant for the time intervals of 6 months and 12 months of storage, while adhesive and mixed failures were predominant at 24 h and 24 months (Fig. 3). In general, the failure modes of the different adhesives were similar.
3. Results 3.1. Radiopacity The radiopacity results are shown in Fig. 1. The radiopacity value for the adhesive containing YbF3 nanoparticles was significantly higher compared with all the other groups (p = 0.024). All the other groups had radiopacity similar to that of the control group (p = 0.672). When using 10 wt% of fillers only the formulation with YbF3 nanoparticles reached an average radiopacity level above 1.0 mm Al; whereas, the other materials had an average radiopacity below 0.5 mm Al. 3.2. Translucency parameter (TP) The TP results are shown in Table 1. The adhesive containing Bi2O3 showed lower TP, and differed from that of control. YbF3 nanoparticles also showed lower TP and differed significantly from that of control (p = 0.129).
3.5. SEM and EDS SEM micrographs of the bonded interfaces (adhesives and dentin) are shown in Fig. 4. Long resin tags were obtained for the YbF3-based adhesive (Fig. 4a); within the hybrid layer of the Bi2O3-based group, agglomerations of the Bi2O3 could be observed (Fig. 4b); and silica deposition was noted along the tubules for the SiO2 group (Fig. 4c). EDS analysis identified the presence of ytterbium, bismuth and silicon, among others, within the experimental adhesives analyzed (Fig. 5), showing characteristic peaks in the emission energy spectra of these elements.
3.3. Degree of conversion The adhesive containing YbF3 nanoparticles and the control showed similar DC, however a higher DC value than the other adhesives (p = 0.022), as shown in Table 1. The material with SiO2 nanoparticles also showed a significantly higher degree of conversion than the adhesive containing Bi2O3 (p = 0.007).
Table 1 Means (standard deviations) for degree of C = C conversion (DC), microtensile bond strength to dentin, MPa, and translucency parameter (TP). Nanoparticle*
YbF3 Bi2O3 SiO2 None (control)
%DC (n = 5)
91.9 45.3 69.4 88.6
( ± 2.8)a ( ± 7.6)c ( ± 10.8)b ( ± 9.3)a
µTBS, MPa (n = 20)**
TP (n = 5)
24 h
6 months
37.8 ( ± 15.7)A,a 40.6 ( ± 10.5)A,a 40.9 ( ± 12.4)A,a 39.1( ± 14.4)A,a
24.4 26.0 27.8 24.0
( ± 11.5)A,a ( ± 9.2) B,a ( ± 9.1) B,a ( ± 9.0)B,a
12 months 29.3 29.2 32.7 23.8
( ± 10.0)A,ab ( ± 8.7)B,ab ( ± 6.3)AB,a ( ± 8.9)B,b
24 months 29.6 31.1 27.2 24.1
( ± 13.2)A,a ( ± 13.3)B,a ( ± 11.0)B,a ( ± 6.1)B,a
Different lowercase letters in the same column indicate significant differences between groups (p < 0.05) YbF3: ytterbium trifluoride; Bi2O: bismuth oxide; SiO2 (silicon dioxide); control (without nanoparticles) ** Different capital letters in the same row and lowercase letters in the same column indicate significant differences between groups (p < 0.05).
124
50.6 ( ± 1.1)b 35.4 ( ± 3.4)c 54.9 ( ± 0.7)ab 57.7a ( ± 0.1)a
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
YbF3-modified adhesive improved radiopacity and maintained the bond strength to dentin stable over time; in addition, the degree of conversion was not altered. Consequently, the hypothesis of the study was partially accepted. The degree of radiopacity of a given particle depends on the atomic number of its components, as well as their density and size [18,19]. Elements with high atomic numbers can absorb or reflect light, thus influencing light scattering within the material, and consequently, making the material radiopaque [20]. Particles containing elements of high atomic number have been used in dentistry as radiopacifying agents. For example, YbF3 nanoparticles were shown to improve radiopacity in resin-based cements [18,21], composite resins, and glassionomer cements [22]. Indeed, the inclusion of 40% by weight of YbF3 nanoparticles in methacrylate-based cements resulted in radiopacity values similar to 3 mm of Al [21]. In this study, the ytterbium-based nanoparticle, which has a high atomic number (Yb = 70) [21], produced higher radiopacity values than the other nanoparticles and higher than 1 mm of aluminum (Fig. 1), thereby meeting the ISO requirements for resin-based materials [16]. On the other hand, the Bi2O3 or SiO2 nanoparticles added to adhesives not contribute to improving radiopacity. This may be explained because provide radiopacity, the material needs to allow the passage/transmission of light, and considering that both Bi2O3- and SiO2-modified adhesives resulted in lower degree of conversion than the control, it can be inferred that these
Fig. 3. Frequency distribution of failure modes for all materials and storage periods.
4. Discussion For nano-based materials, it is of utmost importance that nanoparticles do not jeopardize the chemo-mechanical stability of the modified material; in the present case, the experimental adhesive systems. According to the present findings, only one type of nanoparticle, i.e., YbF3, did not compromise the performance of the material. Differently from the nanoparticle-free adhesive used as control, the
Fig. 4. SEM micrographs of the interface bonded with adhesive system experimental and demineralized dentin (D). In (a) ytterbium particles. In (b) bismuth particles. In (c), silica deposition was noted long tubule. T, resin tags formed by adhesive filling into dentinal tubules. HL, hybrid layer between the adhesive and the underlying mineralized dentin. The circle show particle agglomeration.
125
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
adhesive might have negative effects on the esthetic qualities of dental restorations. This should not be neglected, and should be analyzed in further studies. A recent study showed that adhesives play an important role in altering the color of composite restorations [30]. Although the immediate bond strength was not negatively affected by the incorporation of nanoparticles into the adhesive systems, the long-term bond strength of adhesives containing Bi2O3 and SiO2 was significantly reduced. According to some studies, the lower degree of conversion exhibited by these aforementioned adhesives may have increased the permeability and water sorption phenomena within the adhesive layer, thus resulting in reduced bond strength durability [31,32]. Worth mentioning, the bond strength between restorative material and dental substrates is expected to decrease over time, as demonstrated by the nanoparticle-free adhesive tested; indeed, polymer-based materials such as dental adhesives and restoratives are very prone to suffer hydrolysis, breaking the intermolecular bonds formed during the bonding procedure [33]. The nanoparticles-modified adhesives synthesized here also underwent hydrolysis, similarly to the control material, except for the adhesive system containing YbF3. A similar result was observed in the study of Macedo et al. [11], in which the adhesive containing YbF3 was more resistant to strong hydrolytic degradation, maintaining the bond strength to dentin stable after 6 months of water storage. The authors suggested that due to the electronegative nature of the YbF3 nanoparticle (i.e., the fluorine ion may be considered an excellent hydrogen bond acceptor), strong intermolecular hydrogen bonds could be formed between the nanoparticles and the monomers present within the resin matrix, thus contributing to protecting the network system from fast hydrolysis. Moreover, the YbF3-modified adhesive presented a higher degree of conversion when compared with the other nanoparticle-based adhesives (Table 1), thereby contributing to keep the bond statistically stable, even after 24 months of water storage. However, all the adhesives containing nanoparticles, except YbF3modified adhesive, showed bond strength decreased by about 40%, after 6 months aging. And after 12 months, all adhesive showed a small increase, but SiO2-modified adhesive was the only one that showed statistical difference (Fig. 2), suggesting that 10% silanized silica nanoparticles increased the water resistance of the adhesive [34]. Many authors have found that the silanes are resistant to water leaching [35–37]. Moreover, one study showed that an experimental resin with silanized fillers had higher diametral tensile strength and compressive strength than that without silanization [38]. According to the SEM micrographs (Fig. 5) obtained in the present study, all the nanoparticle-modified adhesives produced a uniform hybrid layer with a similar degree of demineralization. This may explain the similar immediate bond strength results. Although SiO2 presented better particle dispersion, all the adhesive systems had nanoparticles successfully incorporated into the matrix. This fact can be justified by the dispersion method adopted for the study, since it has previously been shown that mixing followed by ultrasonication formed stable suspensions and decreased sedimentation during storage. Furthermore, the SiO2-modified adhesive produced a better particle distribution/dispersion, probably due to the small size of these particles (7 nm) compared with the other nanoparticles, which were around 90–210 nm. The smaller the particle size, the greater the flow into the small dentinal tubules during bonding [39]. Considering the different results found for the nanoparticle-modified adhesives evaluated in this study, it seems that the gain in radiopacity without jeopardizing important characteristics of dental bonding agents depended on the type of radiopacifiers incorporated into the material. Furthermore, YbF3 nanoparticle could be considered a feasible alternative for the development of novel radiopaque dental adhesives.
Fig. 5. EDS analysis of the (a) ytterbium (b) bismuth and (c) silica groups.
adhesives suffered from light scattering, thus interfering with light penetration into the bulk of the material. As a consequence, both the conversion of monomers and the possible gain in radiopacity were affected. This effect was also stressed if the size of the particles approached the output wave length of the light-curing unit [23]. Moreover, it should be considered that distinct particle agglomeration of the different nanoparticles will also play a role in light transmission within the material; the agglomeration of particles could cause phase separation and loss of homogeneity in the resultant material, impairing the degree of conversion and mechanical properties [24,25]. For example, the agglomeration of particles may also create clusters around the openings of the dentin tubules, hampering proper monomer infiltration within the interfibrillar spaces, thereby creating voids within the hybrid layer (shown in Fig. 3). This effect may reduce the adhesion/bond strength, especially to dentin [2,22]. However, the present findings demonstrated that the addition of nanoparticles did not affect the immediate bond strength to dentin when compared with the control group (Table 1). This corroborates the findings of a previous study that showed that an increase in the concentration of zirconia nanoparticles in both the primer and/or the adhesive formulations gradually increased the microtensile bond strength to dentin [5]. The authors supposed that the increase in bond strength was due to the reinforcement promoted by the adhesive resin with filler incorporation, because nanofillers have larger surfaces in contact with the organic matrix [5,26]. In the same way, the Bi2O3-modified adhesive resulted in lower TP than that of the control, which is known to cause detrimental effects on optical properties of translucent materials due to its grayish discoloration [27,28]. Change in TP is a problem because it may affect the light polymerization behavior of the adhesives [29]. Despite the fact that adhesives are applied clinically in very thin films, the color of the 126
International Journal of Adhesion and Adhesives 80 (2018) 122–127
A.R. Cocco et al.
[17] Collares FM, Ogliari FA, Zanchi CH, Petzhold CL, Piva E, Samuel SM. Influence of 2hydroxyethyl methacrylate concentration on polymer network of adhesive resin. J Adhes Dent 2011;13:125–9. [18] Michl, R.J., Rheinberger, V.M., Ott, G., U.S. Patent and Trademark Office, U.S. Patent No. 4,629,746., Washington, DC; 1986. [19] Schulz H, Schimmoeller B, Pratsinis SE, Salz U, Bock T. Radiopaque dental adhesives: dispersion of flame-made Ta2O5/SiO2 nanoparticles in methacrylic matrices. J Dent 2008;36:579–87. [20] Aoyagi Y, Takahashi H, Iwasaki N, Honda E, Kurabayashi T. Radiopacity of experimental composite resins containing radiopaque materials. Dent Mater J 2005;24:315–20. [21] Collares FM, Ogliari FA, Lima GS, Fontanella VR, Piva E, Samuel SM. Ytterbium trifluoride as a radiopaque agent for dental cements. Int Endod J 2010;43:792–7. [22] Prentice LH, Tyas MJ, Burrow MF. The effect of ytterbium fluoride and barium sulphate nanoparticles on the reactivity and strength of a glass-ionomer cement. Dent Mater 2006;22:746–51. [23] Turssi CP, Ferracane JL, Vogel K. Filler features and their effects on wear and degree of conversion of particulate dental resin composites. Biomaterials 2005;26:4932–7. [24] Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater 1999;15:128–37. [25] Nunes TG, Pereira SG, Kalachandra S. Effect of treated filler loading on the photopolymerization inhibition and shrinkage of a dimethacrylate matrix. J Mater Sci: Mater Med 2008;19:1881–9. [26] Conde MC, Zanchi CH, Rodrigues-Junior SA, Carren NL, Ogliari FA, Piva E. Nanofiller loading level: influence on selected properties of an adhesive resin. J Dent 2009;37:331–5. [27] Sanz O, Haro-Poniatowski E, Gonzalo J, Navarro JM Fernández. Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption. J Non-Cryst Solids 2006;352:761–8. [28] Marciano MA, Costa RM, Camilleri J, Mondelli RF, Guimaraes BM, Duarte MA. Assessment of color stability of white mineral trioxide aggregate angelus and bismuth oxide in contact with tooth structure. J Endod 2014;40:1235–40. [29] Moraes RR, Garcia JW, Wilson ND, Lewis SH, Barros MD, Yang B, et al. Improved dental adhesive formulations based on reactive nanogel additives. J Dent Res 2012;91:179–84. [30] Alabdulwahhab BM, AlShethry MA, AlMoneef MA, AlManie MA, AlMaziad MM, AlOkla MS. The effect of dental adhesive on final color match of direct laminate veneer (DLV): in vitro study. J Esthet Restor Dent 2015;27:307–13. [31] Cadenaro M, Antoniolli F, Sauro S, Tay FR, Di Lenarda R, Prati C, et al. Degree of conversion and permeability of dental adhesives. Eur J Oral Sci 2005;113:525–30. [32] Armstrong SR, Vargas MA, Fang Q, Laffoon JE. Microtensile bond strength of a total-etch 3-step, total-etch 2-step, self-etch 2-step, and a self-etch 1-step dentin bonding system through 15-month water storage. J Adhes Dent 2003;5:47–56. [33] De Munck J, Poitevin A, Luhrs AK, Pongprueksa P, Van Ende A, Van Landuyt KL, et al. Interfacial fracture toughness of aged adhesive-dentin interfaces. Dent Mater 2015;31:462–72. [34] Wang Z, Gu Z, Hong Y, Cheng L, Li Z. Bonding strength and water resistance of starch-based wood adhesive improved by silica nanoparticles. Carbohydr Polym 2011;86:72–7. [35] Donath SMH, Mai C. Wood modification with alkoxysilanes. Wood Sci Technol 2004;38:555–66. [36] Hill CAS, Farahani MRM, Hale MDC. The use of organo alkoxysilane coupling agents for wood preservation. Holzforschung 2004;58:316–25. [37] Abdelmouleh MBS, Ben Salah A, Belgacem MN, Gandini A. Interaction of silane coupling agents with cellulose. Langmuir 2002;18:3203–8. [38] Lin CT, Lee SY, Keh ES, Dong DR, Huang HM, Shih YH. Influence of silanization and filler fraction on aged dental composites. J Oral Rehabil 2000;27:919–26. [39] Melo MA, Cheng L, Zhang K, Weir MD, Rodrigues LK, Xu HH. Novel dental adhesives containing nanoparticles of silver and amorphous calcium phosphate. Dent Mater 2013;29:199–210.
5. Conclusion Within the limitations of the study, the addition of nanoparticles affected the radiopacity, translucency parameter, degree of conversion and long-term bond strength to dentin. The formulation of YbF3-modified adhesive tested showed to be a promising approach for the development of radiopaque dental adhesives. Acknowledgments We thank CEME-Sul – Centro de Microscopia Eletrônica da Zona Sul at Federal University of Rio Grande – Brazil for SEM equipment support. References [1] Gersappe D. Molecular mechanisms of failure in polymer nanocomposites. Phys Rev Lett 2002;89:058301. [2] Conde MC, Zanchi CH, Rodrigues-Junior SA, Carreno NL, Ogliari FA, Piva E. Nanofiller loading level: influence on selected properties of an adhesive resin. J Dent 2009;37:331–5. [3] Kim S, Cho JK, Shin S, Kim BJ. Photo-curing behaviors of bio-based isosorbide dimethacrylate by irradiation of light-emitting diodes and the physical properties of its photo-cured materials. J Appl Polym Sci 2015:132. [4] Sadat-Shojai MAM, Nodehi A, Khanlar LN. Hydroxyapatite nanorods as novel fillers for improving the properties of dental adhesives: synthesis and application. Dent Mater 2010;26:471–82. [5] Lohbauer U, Wagner A, Belli R, Stoetzel C, Hilpert A, Kurland HD, et al. Zirconia nanoparticles prepared by laser vaporization as fillers for dental adhesives. Acta Biomater 2010;6:4539–46. [6] Klapdohr S, Moszner N. New inorganic components for dental filling composites. Monatshefte Chem 2005;136:21–45. [7] Mjor IA. Clinical diagnosis of recurrent caries. J Am Dent Assoc 2005;136:1426–33. [8] Van Meerbeek B, Braem M, Lambrechts P, Vanherle G. Adhesion of composite to dentin. Mechanical and clinical results. Ned Tijdschr Tandheelkd 1993;100:489–94. [9] Collares FM, Klein M, Santos PD, Portella FF, Ogliari F, Leitune VC, et al. Influence of radiopaque fillers on physicochemical properties of a model epoxy resin-based root canal sealer. J Appl Oral Sci 2013;21:533–9. [10] Carreño Neftali LV, Oliveira Thiago CS, Piva Evandro, Leal Fernanda B, Lima Giana S, Moncks Marcelo D, Raubach Cristiane W, Ogliari Fabrício A. YbF3/SiO2 fillers as radiopacifiers in a dental adhesive resin. Nano-micro Lett 2012;4:3. [11] Macedo Cármen Lúcia Rodrigues, Münchow Eliseu Aldrighi, da Silveira Lima Giana, Zanchi Cesar Henrique, Ogliari Fabrício Aulo, Piva Evandro. Incorporation of inorganic fillers into experimental resin adhesives: effects on physical properties and bond strength to dentin. Int J Adhes Adhes 2015;62:78–84. [12] Miyazaki T, Sugawara-Narutaki A, Ohtsuki C. Organic-inorganic composites toward biomaterial application. Front Oral Biol 2015;17:33–8. [13] Miyazaki M, Ando S, Hinoura K, Onose H, Moore BK. Influence of filler addition to bonding agents on shear bond strength to bovine dentin. Dent Mater 1995;11:234–8. [14] Miyazaki M, Hinoura K, Onose H, Moore BK. Effect of filler content of light-cured composites on bond strength to bovine dentine. J Dent 1991;19:301–3. [15] Antonijevic D, Medigovic I, Zrilic M, Jokic B, Vukovic Z, Todorovic L. The influence of different radiopacifying agents on the radiopacity, compressive strength, setting time, and porosity of Portland cement. Clin Oral Investig 2014;18:1597–604. [16] ISO 4049. Dentistry Polymer Based Filling Restorative and Luting Materials. Geneva, Switzerland: International Organization for Standardization; 2000. p. 1–27.
127