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Journal of Prosthodontic Research 56 (2012) 120–124 www.elsevier.com/locate/jpor
Original article
Effect of silver nano particles on flexural strength of acrylic resins§ Ahmad Sodagar DDS, MSca, Mohammad Zaman Kassaee PhDb, Azam Akhavan PhDc, Negar Javadi DDSd, Sepideh Arab DDSa,*, Mohammad Javad Kharazifard DDSe a
Department of Orthodontics, Faculty of Dentistry, Tehran University of Medical Sciences, Kargar Shomali Avenue, Tehran, Iran b Department of Chemistry, Tarbiat Modares University, Tehran, Iran c Radiation Applications Research School, Nuclear Science and Technology Research Institute, Tehran, Iran d Department of Endodontics, Faculty of Dentistry, Hamedan University of Medical Sciences, Hamedan, Iran e Dental Research Center, Faculty of Dentistry, Tehran University of Medical Sciences, Iran Received 5 November 2010; received in revised form 16 June 2011; accepted 23 June 2011 Available online 11 August 2011
Abstract Purpose: Poly(methyl methacrylate), PMMA, is widely used for fabrication of removable orthodontic appliances. Silver nano particles (AgNps) have been added to PMMA because of their antimicrobial properties. The aim of this study is to investigate the effect of AgNps on the flexural strength of PMMA. Methods: Acrylic liquid containing 0.05% and 0.2% AgNps was prepared for two kinds of acrylic resins: Rapid Repair & Selecta Plus. Two groups without AgNps were used as control groups. For each one, flexural strength was investigated via Three Point Bending method for the 15 acrylic blocks. Two-way ANOVA, one way ANOVA and Tukey tests were used for statistical analysis. Results: Rapid Repair without AgNps showed the highest flexural strength. Addition of 0.05% AgNps to Rapid Repair, significantly decreased its flexural strength while, continuing the addition up to 0.2% increased it nearly up to its primary level. In contrast, addition of AgNps to Selecta Plus increased its flexural strength but addition of 0.05% nano particles was more effective than 0.2%. Conclusions: The effect of AgNps on flexural strength of PMMA depends on several factors including the type of acrylics and the concentrations of nano particles. # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Silver; Nano particle; Flexural strength; Acrylic resin
1. Introduction Poly(methyl methacrylate), PMMA, based resins are widely used in dentistry for different purposes such as removable base plates, functional appliances and denture bases. The majority of prosthetic acrylic resins consist of PMMA, poly(ethyl methacrylate), PEMA, and additional copolymers [1–3]. Autopolymerizing acrylic resins have the advantage of rapid and easy handling. They have higher levels of residual monomers in comparison to heat-polymerized acrylic resins and that makes them appropriate for orthodontic applications § This research was presented in the 7th Congress of WFO (World Federation of Orthodontics, Sydney, 2010) as a poster entitled: ‘‘Effect of Silver Nano Particles on Flexural Strength of Two Types of Acrylic Resin: Rapid Repair and Selecta Plus’’. * Corresponding author. Tel.: +98 2188497407/9123545931; fax: +98 2166401132. E-mail address:
[email protected] (S. Arab).
[1,4]. Yet acrylic resins should have high proportional limits and high fatigue resistances to avoid plastic deformations and tolerate frequent masticatory loads [5,6]. Microorganism accumulations on acrylic appliances are one of the most discussable questions in using them. Indeed growth of some microorganisms, such as Candida albicans and bacteria like Streptococcus mutans on these appliances has been demonstrated [7,8]. Therefore, one of our goals in dental material studies should be to induce anti microbial capability in these appliances. Indeed induction of antimicrobial activity in dental materials has been widely a large concern in dentistry. Addition of t he quaternary-ammonium-compounds and chlorhexidine to alginate impression materials [9], development of MDPB to achieve resin-based restorative materials with antibacterial effects [10], mixing endodontic sealers with amoxicillin [11], and addition of silver ions to endodontic sealers [12] are a few examples of researches which have tried to enhance antimicrobial properties of dental materials.
1883-1958/$ – see front matter # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. doi:10.1016/j.jpor.2011.06.002
A. Sodagar et al. / Journal of Prosthodontic Research 56 (2012) 120–124
To achieve this aim, one solution is application of silver nano particles (AgNps) which are one of the important products of nano technology [13,14]. The AgNps are one of the most commonly used nano particles because of their ductility, electrical and thermal conductivity, and antimicrobial activity [15–18]. They have shown anti microbial effects on many microorganisms such as E. coli, Staphylococcus eureus, Staphylococcus epidermidis, Candida albicans and Streptococcus mutans [19–22]. Therefore it seems that using AgNps in acrylic resins, induces antimicrobial property in them [23]. Utilization of elemental silver benefits the long-term silver release in comparison to utilization of silver ions [24,25]. In addition silver release of polymers containing silver nano particles is more effective than polymers using silver in micrometer dimensions [26]. As nano particles have the potential of imparting some dental materials ‘mechanical properties [27], one might assume that addition of AgNps to acrylic resins affects their mechanical characteristics. Therefore, although addition of AgNps has antimicrobial advantage, we should also be concern for its effects on the PMMA flexural strength. The aim of this study was to investigate the effects of AgNps on the flexural strength of two types of self-cure poly(methyl methacrylate) consisting of Selecta Plus and Rapid Repair.
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Fig. 1. TEM electron microscopy micrograph shows formation of nano particles.
2. Materials and methods Two commonly used brands of auto polymerizing acrylic resins, Rapid Repair and Selecta Plus (Dentsply, Weybridge, UK) are used in this study. The AgNps are synthesized in two concentrations of 0.05% and 0.2% through addition of silver nitrate and isopropyl alcohol to the monomer. After 30 min the gray color of solution indicates formation of AgNps. Transmission electron microscopy (TEM) micrographs and scanning electron microscopy (SEM) images ensure formation of AgNps (Figs. 1 and 2). These images confirm the formation of nano particles with an average diameter of 38 nanometers. The method used in this study follows ISO1567:2000 [28] standard for denture base resins. Dimension of specimens are considered as 50 10(0.2) 3.3(0.2) mm for flexural strength. For precise fabrication of samples regarding shrinkage of acrylic during setting, a larger metallic model (54 14 7.3) is provided primarily and a silicone impression is taken from it. Number of necessary specimens in each experimental group is determined through a pilot study in which flexural strength test is performed on four groups: Rapid Repair with AgNps, Rapid Repair without AgNps, Selecta Plus with AgNps and Selecta Plus without AgNps. After probing the pilot study, a more comprehensive test is carried out which consists of the following six groups each of which contains 15 specimens with different concentrations of the AgNps. 1. Rapid Repair containing 0.00% AgNps (the first control group). 2. Rapid Repair containing 0.05% AgNps. 3. Rapid Repair containing 0.2% AgNps. 4. Selecta Plus containing 0.00% AgNps (the second control group).
Fig. 2. SEM electron microscopy image illustrates formation of nano particles.
5. Selecta Plus containing 0.05% AgNps. 6. Selecta Plus containing 0.02% AgNps. Powder and monomer containing 0%, 0.05%, and 0.2% AgNps are mixed in the proportion of 5 g powder to 3.5 ml monomer, at 25 8C (in the same place and by the same examiner for all groups). The mixture is placed in the silicon mold at the doughy stage of polymerization. When setting of acrylic specimens is completed, desired dimensions are obtained through grinding in the turnery. Then specimens are kept in 37 8C water for 50 2 h, and then placed in the machine and the force is increased with the speed of 5 1 mm/min, until the models break. The amounts of flexural strengths in MPa are calculated according to the following formula: I s ¼ 3 F b h2 2
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Table 1 The results of the flexural strength test in six experimental groups.
1 2 3 4 5 6 7
Group description
Mean flexural strength (MPa)
S.D.
S.E.
Minimum
Maximum
Rapid Repair Rapid Repair (0.05% np) Rapid Repair (0.2% np) Selecta Plus Selecta Plus (0.05% np) Selecta Plus (0.2% np) Total
67.6660 55.2000 66.4700 56.0280 60.0760 59.0180 60.7430
4.21 3.90 7.65 3.75 4.97 3.51 6.75
1.08 1.00 1.97 0.96 1.28 0.90 0.71
62.79 46.23 53.13 50.37 48.30 53.82 46.23
73.14 58.65 80.04 64.17 66.24 64.86 80.04
where F is the maximum applied force, in Newton; I distance between the supporter arms of the machine in mm 0.01; b the width of the specimens in mm, measured prior to water storage; and h is the height of the specimens in mm, measured prior to water storage. As it is mentioned, the amount of I, b, and h are 50 mm, 10 mm, and 3.3 mm, respectively. One-way and two-way analysis of variance (ANOVA) and Tukey tests are performed for statistical analysis.
Table 2 Statistical comparison of groups using ANOVA test. (I) group
(J) group
Mean difference (I J)
Sig.
Rapid Repair
Rapid Repair (0.05%) Rapid Repair (0.2%) Selecta Plus Selecta Plus (0.05%) Selecta Plus (0.2%) Rapid Repair (0.2%) Selecta Plus Selecta Plus (0.05%) Selecta Plus (0.2%) Selecta Plus Selecta Plus (0.05%) Selecta Plus (0.2%) Selecta Plus (0.05%) Selecta Plus (0.2%) Selecta Plus (0.2%)
12.466 1.196 11.638 7.590 8.648 11.270 0.828 4.828 3.818 10.442 6.394 7.452 4.048 2.990 1.058
0.000 0.985 0.000 0.001 0.000 0.000 0.997 0.079 0.276 0.000 0.007 0.001 0.217 0.550 0.991
Rapid Repair (0.2%)
3. Results The highest flexural strength among the six groups was found in Rapid Repair without AgNps (group 1). The lowest was found in Rapid Repair with 0.05% nano particles (group 2). For each group, the standard deviations and the means were calculated for the flexural strength (Table 1). Addition of 0.05% AgNps in Rapid Repair, significantly decreased the flexural strength ( p < 0.05), while further addition to 0.2% increased the strength nearly up to its primary level. The difference between group 1 and 3 did not appear statistically significant. In Selecta Plus, with both 0.05% and 0.2% AgNps concentrations, no significant increase in the flexural strength was observed (Table 2). 4. Discussion Recently we reported various synthetic methods for preparation of stable AgNps [29–31] and explored their antibacterial effects on PMMA [21]. Our main objective in this work is to investigate the effects of AgNps concentrations on the flexural strength of two types of self-cure poly(methyl methacrylate) consisting of Selecta Plus and Rapid Repair. Results of this study-in brief-indicates that addition of AgNps alters flexural strength of PMMA, somewhat depending on the brand of acrylic resin and the amount of AgNps employed. Explicitly, due to very high surface area of the nano particles in our PMMA/AgNps, the applied stress is expected to be easily transformed from the matrix onto the AgNps, resulting in an enhancement of the mechanical properties. Therefore, the degree of the particles dispersion in the matrix is an important factor. In addition, polar interactions are more favorable to be formed between C O groups of the PMMA chains and AgNps which improve the compatibility between the polymeric matrix and the nano particles [31], possibly increasing the mechanical strength.
Rapid Repair (0.05%)
Selecta Plus Selecta Plus (0.05%)
In the Selecta Plus samples, the presence of AgNps improves the flexural strength, as it was expected. However, in the Rapid Repair, addition of AgNps changed the composite properties unexpectedly (Table 1). This is probably related to the composition or chemical formulation of the Rapid Repair acrylic resin (Table 3). As this acrylic is a commercial product, its composition is not really available in detail and the definite mechanism could not be explained. However, as it is obvious that the degree of nano particles’ dispersion in the PMMA matrix is affected by chemical composition of Rapid Repair acrylic resin. Probably, at low concentration, particle dispersion and chemical interactions between PMMA and AgNps are low which result in flexural strength reduction. We can say that here, at the 0.05% concentration, AgNps act as impurities which usually decrease mechanical strength in the composites [32]. Along with increase in concentration of the nano particles, the more chemical interaction between the C O groups and the silver overcome the aforementioned negative effects, leading to the flexural strength improvement. In our preliminary studies, we had used 0.5% concentration of AgNps and encountered no significant effect on the flexural strength of PMMA [21]. This concentration is more than that we have employed in this work (0.2% and 0.05%). Since the higher concentration of AgNps in our former studies could have led to a different dispersion of AgNps in the previous specimens, we anticipated a different effect in our current work. Consequently using 0.05% concentration of AgNps in
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Table 3 Comparison between chemical formulation of Rapid Repair and Selecta Plus. Acrylic resin
Company
Powder composition
Liquid composition
Curing method
Batch no.
Rapid Repair
Dentsply (Weybridge, UK)
Poly(methyl methacrylate)
Self cure
05MP01
Selecta Plus
Dentsply (Weybridge, UK)
Poly(methyl methacrylate)
Methyl methacrylate Ethylene dimethacrylate N,N-Dimethyl-p-toluidine Methyl methacrylate
Self cure
04AP03
Table 4 Number and percent of specimens in each group which had the minimum acceptable flexural strength. Group number
Group description
Number of specimens with acceptable flexural strength
Percent of specimens with acceptable flexural strength
1 2 3 4 5 6
Rapid Repair Rapid Repair (0.05% np) Rapid Repair (0.2% np) Selecta Plus Selecta Plus (0.05% np) Selecta Plus (0.2% np)
15 0 11 2 8 6
100 0 73.3 13 53 40
Rapid Repair decreased flexural strength significantly. Nevertheless, for 0.2% concentration of AgNps in Rapid Repair, as well as 0.2% and 0.05% in Selecta Plus, the effect on flexural strength immerged insignificant (Table 1). Hence, the type of acrylic is an important factor in the effect of AgNps on flexural strength of PMMA. These results appear more or less similar to those of WJ She whose addition of AgNps to denture base inhibited the growth of Streptococcus mutans and Candida albicans but did not have any significant effect on the mechanical properties of the denture base resin [20]. Interestingly, the method of addition of AgNps to the acrylic in that study was completely different with ours. We produced nano particles inside the monomer through chemical reduction, while She utilized prefabricated nano particles (FUMAT T200-4). This difference could have had remarkable effects on the results. Another important point is 2% concentration of AgNps in She’s study which was 10 times more that our current highest concentration (0.2%). Nevertheless, the results obtained by She appear similar with our results in Selecta Plus and 0.2% silver nano particle in Rapid Repair. Our data shows that only group 1 (Rapid Repair without AgNps) falls in accord with ISO 1567:2000 standard which requires at least 4 of 5 samples (i.e. 80%) of a self-cure acrylic to have flexural strength of 60 MPa or more (Table 4) [28]. Since our research is limited to two types of acrylics and two concentrations of AgNps, future studies to evaluate effects of different concentrations of AgNps on other types of acrylics seem to be advantageous. We should keep in mind that AgNps might affect flexural strength of some types of acrylics and therefore their advantage of antimicrobial properties should be evaluated against probable influence on flexural strength or some other physical and mechanical properties. Therefore it seems that addition of AgNps to PMMA for antimicrobial activity would be beneficial but it should be performed under particular care and after evaluation of its effect on the other physical and mechanical features of the acrylic.
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