The purpose of this study was to prepare antibacterial and radiopaque dental resin through adding one polymerizable quaternary ammonium compound with iodine as counter anion into Bis-GMA/TEGDMA (50/50, wt/wt) dental resin system, and evaluate the degree of monomer conversion, mechanical properties, radiopacity, and antibacterial effectiveness of the polymer.
2-Dimethyl-2-dodecyl-1-methacryloxyethyl ammonium iodine (DDMAI) (3 wt.% and 5 wt.%) was added into a Bis-GMA/TEGDMA (50/50, wt/wt) resin system with CQ (0.7 wt.%) and DMAEMA (0.7 wt.%) as photoinitiation system. Degree of monomer conversion (DC) was determined by FT-IR analysis. The flexural strength (FS) and flexural modulus (FM) of the polymer were measured using a three-point bending set up. Radiographs were taken to determine the radiopacity of the polymer, and aluminum step-wedge (0.5–4 mm) was used as calibration standard. A single-species biofilm model with Streptococcus mutans as the tests organism was used to evaluate the antibacterial property of the polymer. Bis-GMA/TEGDMA without DDMAI was used as control material in all of the tests.
ANOVA analysis revealed that there was no significant difference in DC between polymer with and without DDMAI ( p > 0.05). Polymer with 3 wt.% DDMAI had higher FS than the control material ( p < 0.05), while polymer with 5 wt.% DDMAI had comparable FS with the control material ( p > 0.05). Though average FM of control material was lower than that of 3 wt.% DDMAI containing polymer and higher than 5 wt.% DDMAI containing polymer, but the differences were statistically insignificant. By increasing the quantity of DDMAI, the radiopacity of the polymer increased. Of the three formulations, the polymer with 5 wt.% DDMAI inhibited biofilm the formation of the clinically relevant young biofilm.
Incorporation of DDMAI into resin system could endow it with radiopacity and antibacterial effectiveness, and these two properties seem to be improved with increasing the quantities of DDMAI.
Since methyl methacrylate was firstly used in tooth restoration in 1937, methacrylate monomers having good biocompatibility and adhesive property have been extensively used as dental resin-based materials, e.g., denture base materials , endodontic sealers , direct-filling restorative materials , and dental bonding agents . The most commonly used methacrylate monomers in commercial dental resin-based materials are methyl methacrylate (MMA), 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl]propane (Bis-GMA), 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (UDMA), and tri-ethyleneglycol dimethacrylate (TEGDMA) . By free radical polymerization of these monomers, a three dimensional network is formed, and the selection of the monomers influences the properties of dental resin-based materials directly.
In order to improve properties of dental resin-based materials, several new methacrylate monomers were introduced into dental resin with the aim of reducing polymerization shrinkage , decreasing water uptake and solubility , and improving mechanical properties .
Besides these properties, dental resin-based materials also need to be radiopaque and antibacterial. However, acrylic polymers formed by commonly methacrylate monomers do not have radiopacity and antibacterial activity intrinsically, so some radiopacifying fillers, e.g., BaO, BaSO 4 , TiO 2 , SrO, or ZrO 2 , and antibacterial agents, e.g., chlorhexidine, cetylpyridinium chloride, or titania nanoparticles , are incorporated into dental resin by physical blending.
Unfortunately, all the additives mentioned above also bring some unwanted problems. For example, incorporation of radiopaque fillers into dental resin has some adverse effects on the physical and chemical properties of composites materials and excessive fillers could cause loss of dimensional stability of the composites materials . The addition of antibacterial agent into dental materials could reduce its mechanical properties even at a small concentration , and the releasing agent may exert toxic effects as well as cause short-term antibacterial effectiveness of the material .
It was reported that addition of methacrylate compounds containing radioopaque agent or antibacterial agent could also render dental materials relevant function. Davy et al.  synthesized iodinate methacrylate and copolymerized it with MMA for the application as X-ray opaque denture base material. Imazato et al. synthesized methacryloyloxydodecyl pyrimidinium bromide (MDPB) and used it as an antibacterial monomer in several kinds of dental materials to give long-term antibacterial effects . Though many methacrylate compounds have been taken for the aim of radiopacity or antibacterial activity, there is no report on one methacrylate compound with both of these two functions so far.
In this study, a methacrylate monomer containing quaternary ammonium structure, 2-dimethyl-2-dodecyl-1-methacryloxyethyl ammonium iodine (DDMAI, structure is shown in Fig. 1 ), whose minimum inhibition concentration against Streptococcus mutans is 6.25 μg/mL , was used in dental resin as antibacterial agent, because the counter anion of this reactive quaternary ammonium is iodine, so it may also has radiocontrast property. Therefore, radiopacity and antibacterial activity of dental resin with this monomer were studied, and influences of this monomer on monomer conversion and mechanical properties were also examined in this work.
Materials and methods
2-Dimethyl-2-dodecyl-1-methacryloxyethyl ammonium iodine (DDMAI) was synthesized in our lab. 2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl]propane (Bis-GMA) was purchased from Esstech Inc. (Essington, PA, USA), tri-ethyleneglycol dimethacrylate (TEGDMA), camphoroquinone (CQ), and N,N′-dimethyl aminoethyl methacrylate (DMAEMA) were purchased from Sigma–Aldrich Co. (St Luois, MO, USA), all of the reagents were used without purification.
The formulation of every dental resin used in this experiment is shown in Table 1 . All of them were well blended to obtain a homogenous mixture, and stored at darkness before used.
|System||Content of every monomer (g)|
Monomer (double bond) conversion
The degree of monomer conversion (DC) was determined by using an FTIR spectrometer (Spectrum One, Perkin-Elmer, Waltham, MA, USA) with an attenuated total reflectance (ATR) accessory. The FTIR spectra were recorded with one scan at a resolution of 4 cm −1 . All the samples were analyzed in a mold that was 2 mm thick and 6 mm in diameter. First, the spectrum of the unpolymerized sample was measured. Then, the sample was irradiated for 60 s with a visible light-curing unit (Curing Light 2500, λ = 400–520 nm, I ≈ 550 mW cm −2 , 3 M Co., St Paul, MN, USA). The sample was scanned for its FTIR spectrum every 5 s until 15 min after the beginning of irradiation.
To determined the percentage of reacted double bonds, the absorbance intensities of the methacrylate C C absorbance peak at 1636 cm −1 , which were decreased after being irradiated (as shown in Fig. 2 ), and an internal phenyl ring standard peak at 1608 cm −1 , were calculated using a baseline method. The ratios of absorbance intensities were calculated and compared before and after polymerization. The DC at every irradiation time was calculated by using the equation
DC ( t ) = 1 − ( A c c / A ph ) t ( A c c / A ph ) 0 × 100 %