Abstract
Objective
To evaluate the effect of photo-initiator on the degree of conversion (DC) and elution of Bis-GMA and HEMA for 8 one-step adhesive formulations.
Methods
We used Scotchbond Universal (‘SBU-CQ/amine_4.0’, 3M ESPE), containing about 2 wt% camphorquinone (CQ) and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), an experimental ‘SBU-TPO_2.1’ version, containing 2.1 wt% diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and 6 experimental LUB adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, respectively containing 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO. DC was measured using micro-Raman spectroscopy. Additional specimens were immersed in ethanol for 24 h to determine the elution of Bis-GMA and HEMA using HPLC.
Results
DC of the respective SBU and LUB adhesives was alike at high photo-initiator concentration. At low concentration, TPO was significantly more efficient than CQ/amine (LUB adhesives only). A statistically significant positive photo-initiator concentration effect on DC was noted for both CQ/amine and TPO (LUB adhesives only). A statistically significant inverse photo-initiator concentration effect on HEMA elution was noted for both the CQ/amine- and TPO-containing LUB adhesives. A significantly strong correlation was found between DC and Bis-GMA elution ( R 2 = 0.744, p = 0.026), and between DC and HEMA elution ( R 2 = 0.913, p = 0.002) for the LUB adhesives.
Significance
The photo-initiator kind and concentration affect DC and the Bis-GMA/HEMA elution. TPO can be used as an alternative photo-initiator for CQ/amine.
1
Introduction
Camphorquinone (CQ) combined with tertiary amine as co-initiator is the most generally used photo-initiator system in dental adhesives and composites. Although the tertiary amine co-initiator boosts polymerization , the low pH of especially one-step self-etch adhesives may neutralize the tertiary amine by acid-based reaction . Also water, present in self-etch adhesives to ionize acidic monomers, and the hydrophilic nature of one-step self-etch adhesives, thereby also attracting water through osmosis from the host dentin , may make the rather hydrophobic CQ less efficient . Therefore, as an alternative photo-initiator to CQ/amine, the polymerization efficiency of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (commonly being referred to as TPO) has been studied ; TPO has already been used in some commercially available products. TPO was used alone without any co-initiator , or added as second initiator . Studies showed the advantage of TPO over CQ regarding polymerization efficiency, water compatibility and color stability . A possible drawback may be the different light-absorption spectrum of TPO (380–425 nm) versus that of CQ/amine (470 nm), necessitating the use of a polywave LED-curing unit .
Previous studies revealed that the photo-initiator concentration influences DC, the latter obviously of importance to many material properties, such as hardness, water sorption, bond strength, and degradation resistance . A resin-based material should be optimized for the photo-initiator concentration so that maximum physical properties are obtained with a minimum amount of photo-initiator that otherwise may leach out and be potentially toxic .
Bisphenol A glycidyl dimethacrylate (Bis-GMA) and 2-hydroxyethyl methacrylate (HEMA) are two common monomers used in dental adhesives. Both monomers can elute when they get insufficiently immobilized in the 3D resin matrix upon polymerization; leached monomers may then come into contact with the adjacent ‘soft’ gingival, mucosal and/or pulpal tissues, and so have been associated with biocompatibility issues . Several studies have indeed shown that the monomers themselves or their degradation products may induce cytotoxic, genotoxic, mutagenic, apoptosis, and estrogenic effects .
The purpose of this study was to evaluate the effect of two photo-initiator systems on DC and elution of Bis-GMA and HEMA for 1 commercial one-step adhesive and 7 experimental one-step adhesive formulations. The null hypothesis tested was that the kind and concentration of the photo-initiator systems CQ/amine and TPO do not affect DC, nor have an effect on Bis-GMA/HEMA elution.
2
Materials and methods
Eight different adhesive formulations were used in this study. The commercially available adhesive Scotchbond Universal (referred further in this manuscript to as ‘SBU-CQ/amine_4.0’; 3M ESPE), used in a self-etch mode and containing according to the manufacturer about 2 wt% CQ and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), and an experimental derivative, referred to as ‘SBU-TPO_2.1’ and containing 2.1 wt% TPO, were, respectively, provided and experimentally prepared by the manufacturer. We additionally used 6 experimental ‘LUB’ adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, with the photo-initiator respectively consisting of 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO ( Table 1 ). The experimental LUB adhesive was provided by the manufacturer without photo-initiator. The photo-initiators CQ and TPO, and the co-initiator EDMAB were purchased from Sigma–Aldrich (Sigma–Aldrich Chemie, Steinheim, Germany). The respective amounts of CQ, TPO and EDMAB were measured on an analytical balance with 0.01 mg accuracy (AB304-S’ analytic balance, Mettler-Toledo, Greifensee, Switzerland) and added to the LUB adhesive in amber vials wrapped with aluminum foil to protect them from light. The photo-initiators were dissolved in the adhesive using a closed container immersed in an ultrasonic bath for 1 min.
| Adhesive | Composition a | Initiators |
|---|---|---|
| SBU-CQ/amine_4.0 b | Bis-GMA 15–25 wt%, HEMA 15–25 wt%, DMDMA 5–15 wt%, MDP, ethanol 10–15 wt%, water 10–15 wt%, silica filler 8–12 wt%, copolymer of acrylic and itaconic acid 1–5 wt%, toluene < 0.02 wt% | CQ ∼2 wt%, EDMAB <2 wt% |
| SBU-TPO_2.1 | TPO 2.1 wt% | |
| LUB-CQ/amine_0.7 | Bis-GMA 15 wt%, HEMA 30 wt%, MDP 10 wt%, TEGDMA 15 wt%, ethanol 15 wt%, water 15 wt%, colloidal silica 10 wt%, stabilizers (minute amount) | CQ 0.35 wt%, EDMAB 0.35 wt% |
| LUB-CQ/amine_1.4 | CQ 0.7 wt%, EDMAB 0.7 wt% | |
| LUB-CQ/amine_4.0 | CQ 2 wt%, EDMAB 2 wt% | |
| LUB-TPO_0.35 | TPO 0.35 wt% | |
| LUB-TPO_0.7 | TPO 7 wt% | |
| LUB-TPO_2.0 | TPO 2 wt% |
a Abbreviations : Bis-GMA, bisphenol A glycidyl dimethacrylate; CQ, camphorquinone; DMDMA, decamethylene dimethacrylate; EDMAB, ethyl 4-(dimethylamino)benzoate; HEMA, 2-hydroxyethyl methacrylate; MDP, 10-methacryloyloxydecyl dihydrogenphosphate; TEGDMA, triethylene dimethacrylate; TPO, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.
b Commercialized as Scotchbond Universal (Lot: 468651, 3M ESPE).
Non-caries human third molars (collected following informed consent approved by the Commission for Medical Ethics of KU Leuven) were stored in 0.5% chloramine/water at 4 °C and used within 3 months after extraction. Each adhesive (5 μl) was applied to 12 mid-coronal human dentin discs, covered by a glass slide and light-cured for 10 sec using a Bluephase 20i LED light-curing unit (Ivoclar-Vivadent, Schaan, Liechtenstein) with an output of around 1100 mW/cm 2 . The curing parameters, the light-intensity output and emission spectrum, were monitored using a MARC Patient Simulator (Bluelight Analytics, Halifax, NS, Canada). The specimen-preparation procedure is graphically presented in Fig. 1 . DC of 4 specimens was immediately measured using micro-Raman spectroscopy (Senterra, Bruker, Ettlingen, Germany), using a near-infrared (785 nm) laser with a power output of 100 mW, a 50× microscope objective and 50-μm pin-hole aperture. The collected spectra ranged from 50 to 3500 cm −1 with a resolution of around 9–15 cm −1 . The integration time was set to 20 s with 2 co-additions. The CCD detector possessed a 1024 × 256 pixel resolution, and was cooled down thermo-electrically to a temperature of −65 °C. The results were processed using OPUS 7.0 software (Bruker, Ettlingen, Germany), while the laser was calibrated using Sure-cal software (Bruker). DC was calculated as the ratio of peak intensities of the aliphatic 1639 cm −1 and aromatic 1609 cm −1 peaks in cured and uncured materials.
The monomer elution was measured using the 8 remaining specimens, which were immediately immersed in 1 ml absolute ethanol (99.99%; CAS: 64-17-5, VWR, Haasrode, Belgium) at 37 °C for 24 h, after which the Bis-GMA and HEMA concentration in ethanol were determined using high-performance liquid chromatography (HPLC-DAD, Agilent 1200); the instrument was equipped with an XDB-C18 column with a length of 250 mm, a diameter of 3.0 mm, and a particle size of 5 μm (Zorbax Eclipse, Agilent Technologies, Santa Clara, CA, USA). The mobile phase consisted of a mixture of water and acetonitrile (CH 3 CN), applied successively in a concentration of 10% CH 3 CN (0–7 min), 60% CH 3 CN (7–10 min), 100% CH 3 CN (10–17 min) and 10% CH 3 CN (17–20 min). The flow rate was 1 ml/min and the injection volume was 10 μl. The diode array detector was set to 210 nm for monitoring Bis-GMA and HEMA monomer. Both monomers were identified and quantified by comparison with reference compounds with known composition measured under the same HPLC conditions. Reference standards of Bis-GMA and HEMA (Sigma–Aldrich Chemie, Steinheim, Germany) were used as obtained to produce stock solutions of 1000 μg/ml each. These stock solutions were diluted with absolute ethanol to produce the calibration solutions: 3.125, 6.25, 12.5, 25, 50, 100 and 200 μg/ml. The peak area for each monomer was determined and plotted versus concentration using linear regression analysis for Bis-GMA ( R 2 = 0.9996) and HEMA ( R 2 = 0.9999), and used to quantify monomer concentration in the sample solutions.
2.1
Statistical analysis
The DC and monomer elution data were analyzed by one-way ANOVA and Tukey’s multiple comparison test ( α = 0.05). Any correlation between DC and monomer elution was also calculated (R 2.13.2, R Foundation for Statistical Computing, Vienna, Austria).
2
Materials and methods
Eight different adhesive formulations were used in this study. The commercially available adhesive Scotchbond Universal (referred further in this manuscript to as ‘SBU-CQ/amine_4.0’; 3M ESPE), used in a self-etch mode and containing according to the manufacturer about 2 wt% CQ and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), and an experimental derivative, referred to as ‘SBU-TPO_2.1’ and containing 2.1 wt% TPO, were, respectively, provided and experimentally prepared by the manufacturer. We additionally used 6 experimental ‘LUB’ adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, with the photo-initiator respectively consisting of 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO ( Table 1 ). The experimental LUB adhesive was provided by the manufacturer without photo-initiator. The photo-initiators CQ and TPO, and the co-initiator EDMAB were purchased from Sigma–Aldrich (Sigma–Aldrich Chemie, Steinheim, Germany). The respective amounts of CQ, TPO and EDMAB were measured on an analytical balance with 0.01 mg accuracy (AB304-S’ analytic balance, Mettler-Toledo, Greifensee, Switzerland) and added to the LUB adhesive in amber vials wrapped with aluminum foil to protect them from light. The photo-initiators were dissolved in the adhesive using a closed container immersed in an ultrasonic bath for 1 min.
| Adhesive | Composition a | Initiators |
|---|---|---|
| SBU-CQ/amine_4.0 b | Bis-GMA 15–25 wt%, HEMA 15–25 wt%, DMDMA 5–15 wt%, MDP, ethanol 10–15 wt%, water 10–15 wt%, silica filler 8–12 wt%, copolymer of acrylic and itaconic acid 1–5 wt%, toluene < 0.02 wt% | CQ ∼2 wt%, EDMAB <2 wt% |
| SBU-TPO_2.1 | TPO 2.1 wt% | |
| LUB-CQ/amine_0.7 | Bis-GMA 15 wt%, HEMA 30 wt%, MDP 10 wt%, TEGDMA 15 wt%, ethanol 15 wt%, water 15 wt%, colloidal silica 10 wt%, stabilizers (minute amount) | CQ 0.35 wt%, EDMAB 0.35 wt% |
| LUB-CQ/amine_1.4 | CQ 0.7 wt%, EDMAB 0.7 wt% | |
| LUB-CQ/amine_4.0 | CQ 2 wt%, EDMAB 2 wt% | |
| LUB-TPO_0.35 | TPO 0.35 wt% | |
| LUB-TPO_0.7 | TPO 7 wt% | |
| LUB-TPO_2.0 | TPO 2 wt% |
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