We synthesized a new polymerizable monomer.
The new monomer can endow dental resin with antibacterial and radio-opaque properties.
Increase new monomer will decrease polymerization shrinkage of dental resin.
Polymers with 20 wt.% and 30 wt.% new monomer had higher fracture energies.
A new polymerizable quaternary ammonium monomer (IPhene) with iodine anion was synthesized and incorporated into Bis-GMA/TEGDMA (50/50, wt/wt) to prepare antibacterial and radio-opaque dental resin.
IPhene was synthesized through a 2-steps reaction route, and its structure was confirmed by FT-IR and 1 H-NMR spectra. IPhene was incorporated into Bis-GMA/TEGDMA (50/50, wt/wt) with a series of mass fraction (from 10 wt.% to 40 wt.%). Degree of monomer conversion (DC) was determined by FT-IR analysis. Polymerization shrinkage was determined according to the variation of density before and after polymerization. The flexural strength, modulus of elasticity, and fracture energy were measured using a three-point bending set up. Radiograph was taken to evaluate the radio-opacity of the polymer. A single-species biofilm model with Streptococcus mutans ( S. mutans ) as the tests organism was used to evaluate the antibacterial activity of the polymer. Bis-GMA/TEGDMA resin system without IPhene was used as a control group.
FT-IR and 1 H-NMR spectra of IPhene revealed that IPhene was the same as the designed structure. ANOVA analysis showed that when mass fraction of IPhene was more than 10 wt.%, the obtained resin formulation had lower DC, polymerization shrinkage, FS, and FM than control resin ( p < 0.05). Polymers with 20 wt.% and 30 wt.% IPhene had higher fracture energies than control polymer ( p < 0.05). IPhene containing samples had higher radio-opacity than control group ( p < 0.05), and radio-opacity of IPhene containing sample increased with the increasing of IPhene mass fraction ( p < 0.05). Only polymers with 30 wt.% and 40 wt.% of IPhene showed antibacterial activity ( p < 0.05).
IPhene could endow dental resin with both antibacterial and radio-opaque activity when IPhene reached 30 wt.% or more. Though sample with 30 wt.% of IPhene had lower FS and FM than control group, its lower volumetric shrinkage, higher fracture energy, higher radio-opacity, and antibacterial activity still made it having potential to be used in dentistry.
Dental composites, which consist of resin matrix and inorganic filler, are popular restorative materials because of their natural tooth color and direct-filling capabilities . Dental composites have been reported to be used in more than 95% of all anterior tooth direct restorations and in about 50% of all posterior tooth direct restorations . However, dental composite restorative materials have been reported to accumulate more bacteria or plaque than other restorative materials in vitro or in vivo because of their lack of antibacterial activity, and bacteria or plaque accumulation adjacent to the restoration margins may lead to secondary caries in vivo and shorten the life of composite restoration.
Quaternary ammonium compounds, which are active against a broad spectrum of micro-organisms such as Gram-positive and Gram-negative bacteria, fungi, and certain types of viruses , are well known antibacterial agents and have already been used in fields like water treatment, medicine, food applications, and textile products . Quaternary ammonium compounds with polymerizable groups can immobilize the antibacterial quaternary ammonium group into the polymer backbone and give long-term antibacterial activity to the polymer . In dentistry, Imazato firstly applied a polymerizable quaternary ammonium named methacryloyloxydodecyl-pridinium bromide (MDPB) as an antibacterial agent to prepare long-term antibacterial dental restorative materials .
Besides the antibacterial activity, quaternary ammonium compounds with iodine anion (I − ) also have radio-opacity , which is an important property for dental materials, because of the high electronic density of iodine . Though radio-opacity of dental materials can be implemented by adding radio-opaque inorganic fillers, there still exist some dental materials like E-glass fiber reinforced composites and flowable resin composites that have insufficient radio-opacity . Therefore, radio-opaque dental resin can be used to solve this problem.
In our previous study, a polymerizable quaternary ammonium named 2-dimethyl-2-dodecyl-1-methacryloxyethyl ammonium iodine (DDMAI) was synthesized and used to prepare antibacterial and radio-opaque dental resin system . Unfortunately, DDMAI had miscible problem with hydrophobic methacrylate monomer tri-ethyleneglycol dimethacrylate (TEGDMA), and only 5 wt.% of DDMAI could be added into 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl]propane (Bis-GMA)/TEGDMA resin system. Thus, the antibacterial activity and radio-opacity of DDAMI containing polymer were not obvious.
In this work, a new polymerizable quaternary ammonium monomer (IPhene) with iodine anion was synthesized and incorporated into Bis-GMA/TEGDMA dental resin with different mass ratio. The hypotheses are (I): IPhene could be mixed well with Bis-GMA/TEGDMA at high mass ratio; (II): IPhene could endow Bis-GMA/TEGDMA with both antibacterial and radio-opaque activity. Double bond conversion, polymerization shrinkage, flexural strength and modulus, antibacterial activity, radio-opacity of IPhene containing resin formulations were investigated and compared with resin formulation without IPhene.
Materials and methods
N-methyl diethanol amine (MDEA), 1-iodododecane, dibutyltin dilaurate (DBTDL), TEGDMA, camphoroquinone (CQ), N,N′-dimethylaminoethylmethacrylate (DMAEMA), and 3-isopropenyl-α,α-dimethylbenzyl isocyanate (IDI) were purchased from Sigma–Aldrich Co. (St Luois, MO, USA). Bis-GMA was applied from Esstech Inc. (Essington, PA, USA). All of the compounds were used without further purification.
Synthesis of polymerizable quaternary ammonium monomer (IPhene)
As shown in Fig. 1 , IPhene was synthesized through a 2-steps route. FT-IR (Spectrum One, Perkin-Elmer, Waltham, MA, USA) and 1 H-NMR (AV 400 MHz, Bruker Co., Germany) spectra of intermediate products and IPhene were obtained to confirm their structures. The FT-IR spectra were recorded with 32 scans at a resolution of 4 cm −1 . The chemical shifts of 1 H-NMR spectra were reported in ppm on δ scale with tetramethylsilane as the internal reference and CDCl 3 as the solvent.
Synthesis of intermediate product N,N-bis(2-hydroxyethyl)-N-methyldodecyl ammonium iodide (HQAI-12)
A mixture of MDEA (0.05 mol), 1-iodododecane (0.051 mol), and 30 mL acetone were stirred at reflux. After 24 h reaction, the acetone was removed by distillation under vacuum. The obtained raw product was washed with ethyl ether and filtered for several times. Then the white intermediate product HQAI-12 was dried under vacuum at 35 °C for 48 h. The results of spectroscopic studies for HQAI-12 are as follows: IR (neat): ν (cm −1 ) 3294, 2956, 2927, 2855, 1381, 1090, 720. 1 H-NMR (CDCl 3 , 400 MHz): δ 4.22, 4.09, 3.77–3.89, 3.57–3.62, 3.38, 1.80, 1.31–1.42, 0.94–0.95.
Synthesis of final product IPhene
Acetone used here was dried over 4 Å molecular sieves for 2 weeks. A mixture of HQAI-12 (0.04 mol), IDI (0.08 mol), 20 mL acetone, and 2 droplets of DBTDL were stirred at 45 °C. The reaction was continued until the infrared absorbance peak of the –NCO group (2270 cm −1 ) disappeared in the FT-IR spectra of the samples taken from the reaction medium. After removing the acetone by distillation under vacuum, the product was washed with diethyl ether to remove DBTDL. Then the yellow viscose liquid was dried under vacuum at 35 °C for 48 h to obtain IPhene. The results of spectroscopic studies for IPhene are as follows: IR (neat): ν (cm −1 ) 3284, 3083, 2956, 2925, 2855, 1710, 1629, 1600, 1578, 1521, 1485, 1221, 1173, 1091, 724. 1 H-NMR (CDCl 3 , 400 MHz): δ 7.31–7.49, 6.15, 5.36, 5.10, 4.47, 3.38–3.82, 2.16, 1.66, 1.54, 1.28–1.38, 0.91.
Preparation of resin formulation
The formulation of each dental resin used in this work 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)|
Double bond conversion
The degree of double bond conversion (DC) was determined by using an FT-IR spectrometer (Spectrum One, Perkin-Elmer, Waltham, MA, USA) with an attenuated total reflectance (ATR) accessory. The FT-IR 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 180s 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. Each group was measured for 5 times.
To determined the percentage of reacted double bonds, the absorbance intensities of the C C absorbance peak at 1636 cm −1 , which were decreased after being irradiated, 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 each irradiation time was calculated by using the equation
DC ( t ) = 1 − ( A C = C / A ph ) t ( A C = C / A ph ) 0 × 100 %
where A C C and A ph are the absorbance intensity of C C at 1636 cm −1 and phenyl ring at 1608 cm −1 , respectively; ( A C C / A ph ) 0 and ( A C C / A ph ) t ate the normalized absorbance of functional group at the radiation time 0 and t , respectively; DC( t ) is the conversion of C C as a function of radiation time.
Volumetric polymerization shrinkage of dental resin was measured according to the method reported previously . The specimen’s densities ( n = 5 per group) were measured to determine polymerization shrinkage using Archimedes’ principle with a commercial Density Determination Kit of the analytical balance Mettler Toledo X (Mettler Instrument Co., Highstone, NJ, USA). The kit provides a special holder for the specimens, a special container to keep the water, a computer, and appropriate software. The specimens were weighed in air and in water, and the density was directly calculated in gram per cubic centimeter by the software of the Mettler Toledo XS balance according to the equation:
d = M a M a − M w × ( d w − d a ) + d a
where d density of sample, M a weight of sample in air, M w weight of sample in water, d w density of water at the exactly measured temperature in °C according to the density table of distilled water, d a air density (0.0012 g/cm 3 ). An internal balance correction factor (0.99985) of the Mettler Toledo XS balance software took the air buoyancy of the adjustment weight into account.
The volumetric polymerization shrinkage S was calculated from the densities according to the equation:
S = d after − d before d after × 100 %