Graphical abstract
Abstract
Objective
Fabrication of low shrinkage stress and strain dental resins containing highly available immobilized bactericidal moieties has been reported. The goal of this study is producing dental restorative materials with long-last antibacterial activity and reduced secondary caries.
It is anticipated that antibacterial properties of quaternary ammonium moieties chemically immobilized in the backbone of dental resins is directly depended on accessibility of these functions. In the present study the antibacterial effect of a series of antibacterial monomers polymerized in a ternary thiol–ene–methacrylate system were compared with corresponding classical methacrylate system against Streptococcus mutans (an oral bacteria Strain). Physical and mechanical properties of dental materials obtained from these two systems were also evaluated and compared.
Methods
The viscosities of the resin matrixes were measured on a MCR 300 rheometer. Degree of conversion (DC%) of monomers was measured using FTIR spectroscopy. The shrinkage-strain of photocured resins was measured using the bonded-disk technique. A universal testing machine combined with a stress measurement device was utilized to measure the polymerization-induced shrinkage stress. Viscoelastic properties of the samples were also determined by dynamic mechanical thermal analysis (DMTA). Assessment of antibacterial properties was performed through agar diffusion test (AD) to confirm non-release behavior of chemically anchored moieties. Quantitative assay of antibacterial activity was evaluated through direct contact test (DCT) against S. mutans . Direct contact cytotoxicity assay with fibroblast cell line L-929 was also performed to find more insight regarding cytotoxicity of the antibacterial matrixes. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level = 0.05).
Results
Neat methacrylate systems had significantly higher viscosity than thiol–ene–methacrylate analogous. The degree of conversion of methacrylate moieties in thiol–ene–methacrylate system was improved in comparison to conventional methacrylate system. Shrinkage stress and strain of thiol–ene–methacrylate system was lower than the neat methacrylate system. The thiol–ene-methacrylate systems show increased homogeneity and decreased glass transition temperature ( T g ) and crosslink density ( ν c ) in comparison to the neat methacrylate-based resins. The incorporated monofuctional quaternized monomer reduces degree of conversion, shrinkage stress and crosslink density of matrix. The results showed significant improvement in antibacterial activity and cytocompatibility of dental materials obtained from thiol–ene polymerization system.
Significance
It was shown that with proper control of monomers molar ratio, significant improvement in antibacterial activity and cytocompatibility as well as acceptable mechanical properties can be attained for dental resins prepared through the application of thiol–ene polymerization methodology.
1
Introduction
Secondary caries, which are the main reason of dental composites failure, have been mostly attributed to the plaque accumulation adjacent to the marginal, surface and bulk cracks of dental composites . Photopolymerization induced shrinkage strain and stress are the main reasons of crack propagation in dental matrixes . In fact, occurrence of shrinkage strain during photopolymerization of monomers is a result of conversion of intermolecular van der Waals distances of monomers to the covalent bond length . The shrinkage stress is another drawback occurs during photopolymerization of dental resins. This phenomenon depends on different factors in a complex manner. These factors may include monomer viscosity, shrinkage strain and photopolymerization rate . However, shrinkage stress has mainly arisen as a consequence of propagation of shrinkage strain which occurred under confinement after gelation . Many works have been carried out to reduce polymerization induced-shrinkage strain and stress .
Thiol–ene based dental restorative materials are currently attracted much interest as a versatile novel system for overcoming the limitation of conventional methacrylate resins . The step growth addition mechanism of thiol–ene system result in delayed gelation and consequently significant reduction in shrinkage stress as compared with conventional methacrylate analogous . Lower shrinkage stress and enhanced fracture toughness of the thiol–ene system causes significant reduction in crack development in dental composites .
On the other hand, the preparation of antibacterial restorative dental materials has attracted a great deal of attention in order to prevent secondary caries . Antibacterial activity in dental restorative materials can be provided by incorporation of biocides like silver and zinc metals as well as organic compounds such as chlorhexidine (CHX) and quaternary ammonium salts (QAS) . The gradual release of these antibacterial compounds may result in short-lasting antibacterial activity, reduction in mechanical properties of the dental composites and toxic side effects on the surrounding soft tissues. Therefore, preparations of dental materials with immobilized bactericidal moieties have currently attracted more attention for overcoming the deficiencies of systems based on releasing bactericides. While these compounds inhabit activity of contacted bacteria, the active agents chemically bonded to the matrix and will not leach out . For preparation of this category of materials, copolymerization of quaternary ammonium salt containing monomers (QASM) with proper multifunctional monomers have widely been considered. . The success of this approach directly depends on effective collision of negatively charged bacterial cell surface with the positive charge of QAS moieties; therefore, accessibility of these functions has prime importance.
In the present work, we have been trying to show that with the proper formulation of thiol–ene–methacrylate system, it is possible to tune accessibility of chemically embedded QAS moieties and reach to improved antibacterial effectiveness with reduced shrinkage strain and stress in cured matrix. We have shown that the thiol–ene polymerization method enable the preparation of matrixes with the higher possible loading of active monomer due to higher conversion of whole starting components. As well, higher degree of freedom for matrix components due to presence of flexible thiol–ether linkages will provide a higher chance for blooming and exposing of reactive groups on the surface of final networks. Different formulations based on ternary thiol–ene–methacrylate and neat methacrylate monomers containing QASM were prepared. Physical, mechanical and antibacterial activity as well as cytocompatibility of these systems was assessed and compared.
2
Material and methods
2.1
Materials
Camphorquinone (CQ), N , N -dimethylaminoethyl methacrylate (DMAEMA) and N , N -diethylaminoethyl methacrylate (DEAEMA) were purchased from Merck (Germany). Tetrahydrofuran (THF) was purchased from Merck (Germany) and dried via distillation over sodium wire. 2,2-Bis-(2-hydroxy-3-methacryloxypropoxy)phenyl propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were kindly donated by Evonik (Germany) and used as received. Pentaerythritol tetra (3-mercaptopropionate) (PETMP) was supplied by Aldrich (Germany). Benzyl chloride and octyl bromide were supplied by Merck (Germany). Urethane tetra allyl ether monomer (UTAE), as ene monomer was synthesized by condensation reaction of isophorone diisocyanate (IPDI) and trimethylolpropane diallyl ether (DAE) according to the procedure reported in our previous article . The chemical structures of monomers are shown in Scheme 1 .
2.2
Synthesis of quaternary ammonium salt monomer (QASM)
Benzyl chloride quaternized dimethylaminoethyl methacrylate (DMAEMA-BC) and Benzyl chloride quaternized diethylaminoethyl methacrylate (DEAEMA-BC) were synthesized according to the procedure reported in . Briefly, DMAEMA or DEAEMA (5.35 ml, 0.032 mol), benzyl chloride (4 ml,0.035 mol), a small amount of hydroquinone and dichloromethane (15 ml) as solvent were charged into a 100 ml three-necked round-bottomed flask equipped with a condenser, oil bath and magnetic stirrer. The mixture was stirred at 40–45 °C for 4–5 h and then the resulting white crystals were filtered and washed several times with dry diethyl ether. The hygroscopic product was dried under vacuum at ambient temperature.
Octyl bromide quaternized dimethylaminoethyl methacrylate (DMAEMA-OB) was synthesized according to the procedure reported in . DMAEMA (10.7 ml, 0.064 mol) was added to octyl bromide (5.53 ml, 0.032 mol) in the presence of a small amount of hydroquinone as an inhibitor. The mixture was stirred over night at 50 °C. The white powder was collected after several washings with dry diethyl ether. The product was dried under vacuum at room temperature.
Synthetic routes for the preparation of different QASM monomers are depicted in Scheme 2 .
2.3
Preparation of the matrix resin of dental restorative materials (methacrylate and thiol–ene–methacrylate formulations)
All formulations of materials used in this study are shown in Table 1 . Combination of CQ (0.5 wt.%) and DMAEMA (0.5 wt.%) were used as visible light activating photoinitiator system in all formulations. The weight ratio of methacrylate part of formulations (BisGMA and TEGDMA) was kept constant at 70 to 30, respectively. The mole ratio of thiol to ene functional groups was also kept constant at 1:1. The photopolymerization reactions were carried out by irradiation of the resin mixture with a visible light source (450–500 nm, 550 mW/cm 2 , Optilux 501, Kerr, USA).
| Sample code | Thiol/ene (phr) | BisGMA/TEGDMA (phr) | DMAEMA–BC (phr) | DEAEMA–BC (phr) | DMAEMA–OB (phr) |
|---|---|---|---|---|---|
| TM55-BC | 50 | 50 | 20 | – | – |
| TM37-BC | 30 | 70 | 20 | – | – |
| M-BC | – | 100 | 20 | – | – |
| TM55 | 50 | 50 | – | – | – |
| TM37 | 30 | 70 | – | – | – |
| M | – | 100 | – | – | – |
| TM55-OB | 50 | 50 | – | – | 20 |
| TM37-OB | 30 | 70 | – | – | 20 |
| M-OB | – | 100 | – | – | 20 |
| TM55-EBC | 50 | 50 | – | 20 | – |
| TM37-EBC | 30 | 70 | – | 20 | – |
| M-EBC | – | 100 | – | 20 | – |
The structure of networks formed through thiol–ene–methacrylate photopolymerization is depicted in Scheme 3 .
2.4
Viscosity
The viscosity of the resin matrixes was measured on a MCR 300 rheometer (Anton Paar GmbH, Austria) with a cone and plate geometry (25 mm diameter) and a separation of 0.5 mm between the plates. The measurements were performed at 27 °C over the shear rate range of 0.1 to 1000 s −1 .
2.5
Degree of conversion
The degree of conversion of methacrylate functions was followed using FTIR spectroscopy (EQUINOX 55, Bruker, Germany). The thin resin specimens were placed between two polyethylene films to prevent oxygen inhibition during photopolymerization and photopolymerized using the light curing unit. DC was evaluated by comparing the absorbance spectrum of uncured methacrylate double bond (peak at 1638 cm −1 ) before and after 100 s curing of the specimen . The spectrum of aromatic carbon–carbon double bond (peak at 1608 cm −1 ) was used as internal reference. The degree of conversion was then calculated as follows:
DC % = 1 − 163 6/1 608 cm − 1 peak area after curing 163 6/1 608 cm − 1 peak area before curing × 100 %
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