Electrical properties of resin monomers used in restorative dentistry

Abstract

Objectives

The application of an electric field has been shown to positively influence the impregnation of the resin monomers currently used in dentin bonding systems during hybrid layer formation. This study presents an experimental characterization of the electrical properties of these monomers with the aim of both correlating them to their chemical structures and seeking an insight into the mechanisms of the monomer migration under an applied electric field.

Methods

Some common monomers examined were TEGDMA (triethyleneglycoldimethacrylate), HEMA (2-hydroxyethyl methacrylate), UDMA (urethane dimethacrylate), 2-MP (bis[2-(methacryloyloxy)ethyl] phosphate, TCDM di(hydroxyethyl methacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexenyl-1,2-dicarboxylic anhydride) and Bis-GMA [2,2-bis(4-2-hydroxy-3-methacryloyloxypropoxyphenyl)propane]. A customized cell produced for the measurement of the electrical properties of monomers was manufactured and electrical conductivity and permittivity of resin monomers were measured.

Results

The permittivity of the tested monomers is largely affected by electrical frequency. The large values of permittivity and dielectric losses observed as frequency decreased, indicate a dominant effect of ionic polarization, particularly evident in materials showing the highest conductivity. Permittivity and conductivity of the tested monomers showed a similar behavior, i.e. materials with the lowest permittivity also show small values of conductivity and vice versa.

Significance

The results of the present study revealed a good correlation between electrical properties and Hoy solubility parameters and, in particular, the higher the polar contribution (polar forces plus hydrogen bonding) the higher the permittivity and conductivity. The most relevant outcome of this study is that the electrophoretic mechanism prevails on the electroendoosmotic effect in determining the monomer migration under the application of electric fields.

Introduction

Dentin bonding systems (DBS) allow restorative materials to bond to enamel and dentin . Dentin bonding is achieved by infiltrating the substrate via passive diffusion of solvent and DBS monomers into the insoluble demineralized dentin collagen fibril network that is saturated with residual interfibrillar water . The result is the formation of the hybrid layer, a structure composed by DBS monomers, residual solvent, dentin collagen fibrils and hydroxyapatite at different degrees in relation to the type of DBS used . In the case of self-etch DBS, residual smear layer particles are embedded within the hybrid layer .

Silver nitrate-impregnated resin-bonded specimens analyzed under electron microscopy revealed different degree of nanoleakage (i.e. interfacial nanoporosities within the hybrid layer that are claimed to represent the weakest area of the adhesive interface) due to poor or incomplete resin infiltration of both etch-and-rinse and self-etch (or etch-and-dry) DBS leaving residual interfibrillar water . High porosity hybrid layers lead to higher silver uptake along the adhesive interface, and result in reduced immediate bond strength and accelerated degradation .

The use of electricity has been proposed to facilitate resin monomers impregnation of the dentin . Preliminary in vitro studies performed with different prototypes (ElectroBond, Seti, Italy) delivering current intensities ranging from 25 to 175 μA, confirmed that the application of DBS under the influence of an electric field increases bond strengths and reduces interfacial nanoleakage . Initially iontophoresis was suggested to increase monomer impregnation, favoring the substitution of water by adhesive monomers . However a recent study revealed that DBS monomer migration within an electric field can be affected by several parameters related either to the dentin substrate (buffer pH and ionic strength), or to the electric field (applied voltage), strongly affecting the prevalence of electromigration forces or electroendoosmotic flux (EEO) within the matrix . Electromigration causes the migration of the negatively charged molecules toward the anode and the concurrent migration of mobile cations to the cathode (negative electrode) , thus determining monomers migration due to the pH-dependent ionization of molecules of weak acids . The second effect, EEO, which is the water movement within organic matrices during electrophoresis as a result of the fixed negative charges and electromigration of positive counterions on the matrix, causes the migration of water and all dissolved substances toward the cathode, irrespective of charge . If electromigration is higher than EEO monomers migrate toward the anode, while if electromigration is lower than EEO monomers migrate toward the cathode . Thus monomer migration toward the anode or cathode can be achieved as desired by selective choice of pH and ionic strength of the substrate and the applied electric field .

However, despite these findings the interaction between the dentin substrate and electricity, the mechanism of how an electrical field can facilitate impregnation of monomers remains unclear since little is known about the electrical properties of monomers currently used in restorative dentistry (i.e. their electrical conductivity and electrical permittivity). The electrical conductivity describes the ability of a material to transport current when subjected to an electric field. In liquid dielectrics, conductivity is closely correlated to the ionization status of the material and to the electrical mobility of ions and electrons .

Permittivity is a physical quantity that describes how an electric field affects a dielectric medium . It is determined by the ability of a material to polarize in response to electric fields, and thus reduce the total electric field inside the material. In general, the response of a solid medium to external fields depends on the frequency of the field. This frequency dependence reflects the fact that material polarization does not respond instantaneously to an applied field due to material inertia . This leads to power losses generated by the polarization process (dielectric losses) and to a phase delay between polarization and electric field. When the applied electric field exhibits a sinusoidal behavior, this delay can be mathematically expressed by considering a complex quantity for the electrical permittivity, <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='ε¯’>ε¯ε¯
ε ¯
:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='ε¯=ε′−jε″’>ε¯=εjε′′ε¯=ε′−jε″
ε ¯ = ε ′ − j ε ″

The real part ( ε ′) and the imaginary part ( ε ″) of the permittivity are associated with the extent of polarization and (1) by the vacuum permittivity constant ε 0 = 8.85 × 10 −12 F/m.

The aim of the present study was to investigate the electrical conductivity and the electrical permittivity of resin monomers currently used in DBS blends, since such information is not currently available in the literature. Additionally these electrical properties will be correlated to the chemical properties of these monomers, such as their structure and Hoy’s solubility parameters . The tested null hypothesis was that no correlation exists between electrical conductivity, electrical permittivity and Hoy’s solubility parameters.

Materials and methods

Tested monomers

TEGDMA (triethylene glycol dimethacrylate), HEMA (2-hydroxyethyl methacrylate), UDMA (urethane dimethacrylate), 2-MP (bis[2-(methacryloyloxy)ethyl] phosphate, TCDM di(hydroxyethyl methacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexenyl-1,2-dicarboxylic anhydride) and Bis-GMA [2,2-bis(4-2-hydroxy-3-methacryloyloxypropoxyphenyl)propane] were purchased from Sigma Chemical (St Louis, MO, USA) and used without further purification ( Table 1 ).

Cell fixture

A customized cell fixture for the measurement of the electrical properties of monomers was manufactured, as no device was commercially available . In fact, due to the high viscosity of some of these monomers, these liquids are not compatible with commercial measurement cells for liquid dielectrics . Moreover, the cell has been specifically designed for applied voltages much lower than those used in standard high voltage dielectric measurements. This is a key feature of the experimental setup, as it allows experiments to be performed at electric field levels similar to those applied in a clinical environment.

The material adopted for the cell is poly(methylmethacrylate) (PMMA) in order to ensure chemical compatibility with the monomers tested in the present study . An image of the customized cell is shown in Fig. 1 . The cell is composed of a hollow cylinder closed at both ends with two stainless steel circular electrodes having a diameter of 44 mm and separated by a 3 mm gap. The area of the electrodes is therefore equal to 1520 mm 2 . Two holes on the top part of the cylinder allow the test monomers to be poured in between the electrodes and for the trapped air to be expelled.

Fig. 1
Cell fixture used to measure the complex permittivity and electric conductivity of the monomers under test.

With this fixture, two-terminal impedance measurements were carried out by means of an Alpha dielectric analyzer (Novocontrol, Hundsangen, Germany) . The complex permittivity and the electric conductivity were then measured for liquid monomers in the frequency range from 10 −2 Hz to 6 MHz and for a supply voltage of 1 V rms.

Materials and methods

Tested monomers

TEGDMA (triethylene glycol dimethacrylate), HEMA (2-hydroxyethyl methacrylate), UDMA (urethane dimethacrylate), 2-MP (bis[2-(methacryloyloxy)ethyl] phosphate, TCDM di(hydroxyethyl methacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexenyl-1,2-dicarboxylic anhydride) and Bis-GMA [2,2-bis(4-2-hydroxy-3-methacryloyloxypropoxyphenyl)propane] were purchased from Sigma Chemical (St Louis, MO, USA) and used without further purification ( Table 1 ).

Cell fixture

A customized cell fixture for the measurement of the electrical properties of monomers was manufactured, as no device was commercially available . In fact, due to the high viscosity of some of these monomers, these liquids are not compatible with commercial measurement cells for liquid dielectrics . Moreover, the cell has been specifically designed for applied voltages much lower than those used in standard high voltage dielectric measurements. This is a key feature of the experimental setup, as it allows experiments to be performed at electric field levels similar to those applied in a clinical environment.

The material adopted for the cell is poly(methylmethacrylate) (PMMA) in order to ensure chemical compatibility with the monomers tested in the present study . An image of the customized cell is shown in Fig. 1 . The cell is composed of a hollow cylinder closed at both ends with two stainless steel circular electrodes having a diameter of 44 mm and separated by a 3 mm gap. The area of the electrodes is therefore equal to 1520 mm 2 . Two holes on the top part of the cylinder allow the test monomers to be poured in between the electrodes and for the trapped air to be expelled.

Fig. 1
Cell fixture used to measure the complex permittivity and electric conductivity of the monomers under test.

With this fixture, two-terminal impedance measurements were carried out by means of an Alpha dielectric analyzer (Novocontrol, Hundsangen, Germany) . The complex permittivity and the electric conductivity were then measured for liquid monomers in the frequency range from 10 −2 Hz to 6 MHz and for a supply voltage of 1 V rms.

Results

The measured electric properties of the tested liquid monomers are shown in Figs. 2 and 3 .

Fig. 2
Real part ε r ′ (a) and imaginary part ε r ″ (b) of the relative electrical permittivity ε r as a function of frequency for the tested monomers.

Fig. 3
Electrical conductivity σ as a function of frequency for the tested monomers.

The real part, ε r ′ (polarization, Fig. 2 a) and imaginary part, ε r ″ (polarization delay, Fig. 2 b) of the relative electrical permittivity as a function of frequency for the different monomers tested at room temperature are reported in Fig. 2 . Both real and imaginary permittivity are largely affected by frequency. In particular, ε r ′ is constant at high frequencies, i.e. from 5 MHz down to a frequency characteristic for each material, ranging from 5 kHz (max) to 1 Hz (min) for the two monomers 2-MP and UDMA, respectively.

Fig. 3 reports the frequency response of electrical conductivity, σ , for the same monomers as in Fig. 2 at room temperature. The electrical conductivity shown exhibits an opposite behavior compared to permittivity, being constant in the low-frequency range and variable at high frequencies ( Fig. 3 ). The frequency of transition between these two regimes depends on the material, ranging from 10 Hz to 100 kHz for UDMA and 2-MP, respectively.

Fig. 4 shows the comparison of real part of permittivity, ε r ′ ( Fig. 4 a) and conductivity, σ ( Fig. 4 b) for the tested monomers obtained from plots of Figs. 2a and 3 , at frequencies for which the permittivity and conductivity are frequency-independent, i.e. at high (10 MHz) and low (10 Hz) frequencies, respectively. Permittivity and conductivity showed a similar behavior, i.e. materials having the lowest permittivity also showed small values of conductivity and vice versa. It should be noted that among the different materials, conductivity varied in a wider range with respect to permittivity. Interestingly 2-MP showed the largest values of electrical permittivity and conductivity, TEGDMA had both the lowest conductivity and permittivity ( ε r ′ < 10) while UDMA and Bis-GMA exhibited similar values of conductivity and real permittivity.

Fig. 4
Comparison of the real part of the permittivity at 1 MHz (A) and conductivity at 10 Hz (B) for the tested monomers.

Discussion

The results of the present study revealed that electrical properties can be correlated with the chemical structure and with the Hoy solubility parameters since they are correlated with the polarity of the molecules. Thus the tested null hypothesis was rejected.

Additionally, this study revealed a linear behavior of electrical conductivity of the tested adhesive resins at high frequencies in log–log scale ( Fig. 3 ) in accordance with the fact that conductivity increases follow a power law of “universal dielectric response” developed by Jonscher :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='σ(f)=σ(0)+Afn’>σ(f)=σ(0)+Afnσ(f)=σ(0)+Afn
σ ( f ) = σ ( 0 ) + A f n

where f is the frequency, A and n are the model parameters.

It can also be seen that the large values of permittivity ( Fig. 2 a) and dielectric losses ( Fig. 2 b) observed as frequency decreases, indicate a dominant effect of ionic polarization, particularly evident in materials showing the highest conductivity ( Fig. 3 ). Polarization at the electrode/monomer interface could also play a role, increasing permittivity and losses in the low frequency range. This behavior is quite typical of liquid dielectrics having large polar groups .

Hoy’s solubility parameters shown in Table 1 ( δ d , δ p , δ h , δ t ) are indices of the intermolecular forces that occur between molecules. In particular, δ d is the dispersive contribution (i.e. all non-polar contribution), δ p the polar contribution, δ h the hydrogen bond contribution and δ t the total solubility parameter that is calculated according to the formula <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='δt2=δd2+δp2+δh2′>δ2t=δ2d+δ2p+δ2hδt2=δd2+δp2+δh2
δ t 2 = δ d 2 + δ p 2 + δ h 2
. Hoy’s parameters can be used to predict whether a given material is compatible with another one. Materials with similar Hoy’s parameters behave similarly and are compatible. Hoy’s solubility parameters have been calculated using the Computer Chemistry Consultancy software . The molecules that exhibit higher δ p (polar contribution) and δ h (contribution from hydrogen bonds) had higher electrical permittivity and conductivity. This is to be expected since the dielectric constant of liquids can be calculated from their chemical group contributions . Permittivity of the tested monomers showed a highly linear correlation with Hoy’s total solubility parameters ( δ t ) yielding an R 2 = 0.89, p < 0.005 ( Fig. 5 a ). Conductivity of the tested monomers correlated with Hoy’s δ p + h values, producing R 2 = 0.69, p < 0.05 ( Fig. 5 b).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Electrical properties of resin monomers used in restorative dentistry
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