Effect of Photo-Initiators on the In Vitro Performance of a Dentin Adhesive Exposed to Simulated Oral Environment

Our previous study showed poor mechanical durability and nano-sized heterogeneities in cross-linked dentin adhesives cured in the presence of water. To further explore the relationship between nano-scale heterogeneities and the long-term mechanical properties of dentin adhesives, the properties of model dentin adhesives polymerized using hydrophilic photoinitiators were compared with those of adhesives polymerized using hydrophobic camphorquinone-based photoinitiators. There was a continuous decline of mechanical properties for the specimens cured in the presence of water during 3 months aqueous storage, especially for the specimens that contained hydrophobic photoinitiators. The multi-component systems containing hydrophilic photoinitiators were shown to produce superior model dental adhesives when these materials are cured in the presence of water

Diffusion Coefficients of Water and Leachables in Methacrylate-based Crosslinked Polymers using Absorption Experiments

The diffusion of water into dentin adhesive polymers and leaching of unpolymerized monomer from the adhesive are linked to their mechanical softening and hydrolytic degradation. Therefore, diffusion coefficient data are critical for the mechanical design of these polymeric adhesives. In this study, diffusion coefficients of water and leachables were obtained for sixteen methacrylate-based crosslinked polymers using absorption experiments. The experimental mass change data was interpreted using numerical solution of the two-dimensional diffusion equations. The calculated diffusion coefficients varied from 1.05 × 10−8 cm2/sec (co-monomer TMTMA) to 3.15 × 10−8 cm2/sec (co-monomer T4EGDMA). Correlation of the diffusion coefficients with crosslink density and hydrophilicity showed an inverse trend (R2 = 0.41). The correlation of diffusion coefficient with crosslink density and hydrophilicity are closer for molecules differing by simple repeat units (R2 = 0.95). These differences in the trends reveal mechanisms of interaction of the diffusing water with the polymer structure

The influence of chemical structure on the properties in methacrylate-based dentin adhesives

Objectives The objective of this study was to investigate the influence of the chemical structure of methacrylate monomers used in dentin adhesives on degree of conversion (DC), water sorption, and dynamic mechanical properties. Materials and methods Experimental adhesives containing 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane (BisGMA), 2-hydroxyethyl methacrylate (HEMA), and co-monomer, 30/45/25 w/w were photo-polymerized. Ethyleneglycol dimethacrylate (EGDM), diethyleneglycol dimethacrylate (DEGDM), triethyleneglycol dimethacrylate (TEGDMA), 1,3-glycerol dimethacrylate (GDM), and glycerol trimethacrylate (GTM) were used as a co-monomer. The adhesives were characterized with regard to DC, water sorption, and dynamic mechanical analysis and compared to control adhesive [HEMA/BisGMA, 45/55 w/w]. Results DC and water sorption increased with an increase in the number of ethylene glycol units in the monomer. Experimental adhesive containing GDM showed significantly higher storage moduli (p < 0.05) in both dry and wet samples than experimental adhesives containing EGDM or DEGDM. The rubbery moduli of adhesives containing GDM and GTM were found to be significantly greater (p < 0.05) than that of the control. Adhesives containing GTM exhibited the widest tanδ curves, indicating the greatest structural heterogeneity. Significance The hydrophilicity, functionality and size of monomers in dentin adhesives affected the water sorption, solubility, crosslink density and heterogeneity of the polymer network. The experimental adhesives containing GDM and GTM showed higher rubbery moduli, indicating higher crosslink density accompanied by a decrease in the homogeneity of the polymer network structure

Determination of neutralization capacity and stability of a basic methacrylate monomer using NMR

This is an Accepted Manuscript of an article published by Taylor & Francis in International Journal of Polymeric Materials and Polymeric Biomaterials in 2012, available online: http://www.tandfonline.com/10.1080/00914037.2011.574660.The durability of dental resin depends on the stability of the polymer. The neutralizing capacity of a basic methacrylate monomer and its chemical stability were measured using nuclear magnetic resonance (NMR) spectroscopy. Lactic acid solution was titrated with 2-(dimethylamino)ethylmethacrylate (DMAEMA) or 2-hydroxyethylmethacrylate (HEMA) and its chemical shifts monitored. Addition of DMAEMA alters the chemical shift proportionally to pH neutralization, whereas HEMA has no impact. Chemical shifts were used to quantify both the change in pH and monomer stability. The results demonstrate that neutralization by basic monomer can be achieved and that this can be measured using an NMR assay

Water sorption and dynamic mechanical properties of dentin adhesives with a urethane-based multifunctional methacrylate monomer

Objectives Our previous study showed the synthesis and characterization of a novel urethane-linked trimethacrylate monomer for use as a co-monomer in dentin adhesives. The objective of this work was to further investigate the performance of dentin adhesives containing a new monomer, with particular emphasis on the water sorption and visco-elastic behavior of the crosslinked networks. Materials and methods Dentin adhesives contained 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane (BisGMA), 2-hydroxyethyl methacrylate (HEMA), and a new multifunctional methacrylate with urethane-linked groups-1,1,1-tri-[4-(methacryloxyethylaminocarbonyloxy)-phenyl]ethane (MPE) and were photo-polymerized in the presence or absence of water. Adhesives were characterized with regard to degree of conversion (DC), viscosity, water sorption/solubility, and via dynamic mechanical analysis (DMA) and compared with BisGMA/HEMA controls. Results The experimental adhesives exhibited DC and solubility comparable to that of the control, regardless of the presence or absence of water. All the experimental adhesives tested showed less water sorption, lower tan δ peak heights, and higher rubbery modulus than the control. Significance Dentin adhesives containing a new multifunctional methacrylate showed better dynamic thermomechanical properties and water sorption relative to controls, without compromising DC and solubility. Thus, MPE, when included as a component of methacrylate dentin adhesives, may provide enhanced durability in the moist environment of the mouth

Mechanical properties of methacrylate-based model dentin adhesives: Effect of loading rate and moisture exposure

The aim of this study is to investigate the mechanical behavior of model methacrylate-based dentin adhesives under conditions that simulate the wet oral environment. A series of monotonic and creep experiments were performed on rectangular beam samples of dentin adhesive in three-point bending configuration under different moisture conditions. The monotonic test results show a significant effect of loading rate on the failure strength and the linear limit (yield point) of the stress-strain response. In addition, these tests show that the failure strength is low, and the failure occurs at a smaller deformation when the test is performed under continuously changing moisture conditions. The creep test results show that under constant moisture conditions, the model dentin adhesives can have a viscoelastic response under certain low loading levels. However, when the moisture conditions vary under the same low loading levels, the dentin adhesives have an anomalous creep response accompanied by large secondary creep and high strain accumulation

Characterization of Acid-neutralizing Basic Monomers in Co-solvent Systems by NMR

This is an Accepted Manuscript of an article published by Taylor & Francis in International Journal of Polymeric Materials and Polymeric Biomaterials in 2010, available online: http://www.tandfonline.com/10.1080/00914037.2013.845192Metabolic activity of the oral microbiota leads to acidification of the microenvironment and promotes demineralization of tooth structure at the margin of composite restorations. The pathogenic impact of the biofilm at the margin of the composite restoration could be reduced by engineering novel dentin adhesives that neutralize the acidic micro-environment. Integrating basic moieties into methacrylate derivatives has the potential to buffer against acid-induced degradation, and we are investigating basic monomers for this purpose. These monomers must be compatible with existing formulations, which are hydrophobic and marginally miscible with water. As such, cosolvent systems may be required to enable analysis of monomer function and chemical properties. Here we present an approach for examining the neutralizing capacity of basic methacrylate monomers in a water/ethanol co-solvent system using NMR spectroscopy. NMR is an excellent tool for monitoring the impact of co-solvent effects on pKa and buffering capacity of basic monomers because chemical shift is extremely sensitive to small changes that most other methods cannot detect. Because lactic acid (LA) is produced by oral bacteria and is prevalent in this microenvironment, LA was used to analyze the effectiveness of basic monomers to neutralize acid. The 13C chemical shift of the carbonyl in lactic acid was monitored as a function of ethanol and monomer concentration and each was correlated with pH to determine the functional buffering range. This study shows that the buffering capacity of even very poorly water-soluble monomers can be analyzed using NMR

Synthesis and evaluation of novel dental monomer with branched aromatic carboxylic acid group

A new glycerol-based dimethacrylate monomer with an aromatic carboxylic acid, 2-((1,3-bis(methacryloyloxy)propan-2-yloxy)carbonyl)benzoic acid (BMPB), was synthesized, characterized, and proposed as a possible dental co-monomer for dentin adhesives. Dentin adhesives containing 2-hydroxyethyl methacrylate (HEMA) and 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]propane (BisGMA) in addition to BMPB were formulated with water at 0, 5, 10, and 15 wt % to simulate wet, oral conditions, and photo-polymerized. Adhesives were characterized with regard to viscosity, real-time photopolymerization behavior, dynamic mechanical analysis, and microscale 3D internal morphologies and compared with HEMA/BisGMA controls. When formulated under wet conditions, the experimental adhesives showed lower viscosities (0.04–0.07 Pa s) as compared to the control (0.09–0.12 Pa s). The experimental adhesives showed higher glass transition temperature (146–157°C), degree of conversion (78–89%), and rubbery moduli (33–36 MPa), and improved water miscibility (no voids) as compared to the controls (123–135°C, 67–71%, 15–26 MPa, and voids, respectively). The enhanced properties of these adhesives suggest that BMPB with simple, straight-forward synthesis is a promising photocurable co-monomer for dental restorative materials

Viscoelastic and fatigue properties of model methacrylate-based dentin adhesives

The objective of the current study is to characterize the viscoelastic and fatigue properties of model methacrylate-based dentin adhesives under dry and wet conditions. Static, creep, and fatigue tests were performed on cylindrical samples in a 3-point bending clamp. Static results showed that the apparent elastic modulus of the model adhesive varied from 2.56 to 3.53 GPa in the dry condition, and from 1.04 to 1.62 GPa in the wet condition, depending upon the rate of loading. Significant differences were also found for the creep behavior of the model adhesive under dry and wet conditions. A linear viscoelastic model was developed by fitting the adhesive creep behavior. The developed model with 5 Kelvin Voigt elements predicted the apparent elastic moduli measured in the static tests. The model was then utilized to interpret the fatigue test results. It was found that the failure under cyclic loading can be due to creep or fatigue, which has implications for the failure criterion that are applied for these types of tests. Finally, it was found that the adhesive samples tested under dry conditions were more durable than those tested under wet conditions

Fatigue life prediction of dentin-adhesive interface using micromechanical stress analysis

Objectives The objective of this work was to develop a methodology for the prediction of fatigue life of the dentin-adhesive (d-a) interface. Methods At the micro-scale, the d-a interface is composed of dissimilar material components. Under global loading, these components experience different local stress amplitudes. The overall fatigue life of the d-a interface is, therefore, determined by the material component that has the shortest fatigue life under local stresses. Multiple 3d finite element (FE) models were developed to determine the stress distribution within the d-a interface by considering variations in micro-scale geometry, material composition and boundary conditions. The results from these models were analyzed to obtain the local stress concentrations within each d-a interface component. By combining the local stress concentrations and experimentally determined stress versus number of cycle to failure (S-N) curves for the different material components, the overall fatigue life of the d-a interface was predicted. Results The fatigue life was found to be a function of the applied loading amplitude, boundary conditions, microstructure and the mechanical properties of the material components of the d-a interface. In addition, it was found that the overall fatigue life of the d-a interface is not determined by the weakest material component. In many cases, the overall fatigue life was determined by the adhesive although exposed collagen was the weakest material component. Comparison of the predicted results with experimental data from the literature showed both qualitative and quantitative agreement. Significance The methodology developed for fatigue life prediction can provide insight into the mechanisms that control degradation of the bond formed at the d-a interface