Preparation and characterization of Bis-GMA-free dental composites with dimethacrylate monomer derived from 9,9-Bis[4-(2-hydroxyethoxy)phenyl]fluorene
Introduction
Bisphenol A (BPA) has been reported as one of many endocrine disrupting compounds [1] which may cause male reproductive abnormalities [2], [3], and spermatogenesis impairment [4], [5]. Indeed, a waste range of adverse effects have been associated with BPA in animal studies, such as diabetes, obesity, behavioral changes and infertility. A recent report by EFSA (European Food Safety Authority) evaluated the scientific bases for adverse health effects from BPA and concluded that an effect was likely on liver, kidney and breast of rat and mouse [6].
BPA exposure from dental materials has been documented in several studies [7], [8], [9], [10], [11], [12], [13], [14], even though BPA itself is not a component in methacrylate-based dental materials. However, these dental materials are manufactured using monomers derived from BPA, such as 2,2-di(4-methacryloxyphenyl)propane (Bis-DMA), 2,2-bis[4-(2′-hydroxy-3′-methacryloyloxy-propoxy)phenyl]propane (Bis-GMA), and ethoxylated Bisphenol-A-dimethacrylate (Bis-EMA). Leaching of BPA may then occur due to BPA-contamination or degradation processes, though some studies have shown that only Bis-DMA can be hydrolyzed into BPA [15], [16], [17]. Therefore, monomers that are BPA derivatives, and which are commonly used in methacrylate-based dental composite materials, have become a concern for patients, dentists and manufacturers.
The European committee SCENIHR (Scientific Committee on Emerging and Newly Identified Health Risks) recently presented an opinion on the safety of use of BPA in medical devices [18]. The report concluded that the long-term exposure from dental materials was below the temporary value for tolerable daily intake (t-TDI) set by EFSA. Still, the report recommended that medical devices, including dental materials, which do not leach BPA should be preferred when possible, and that the replacement of BPA should be considered. Thus, as a health precaution in order to minimize human exposure to BPA from dental materials, using monomers which are not derived from BPA to prepare dental materials may be effective.
In order to prepare Bis-GMA free dental materials, several new (meth)acrylate monomers such as urethane dimethacrylates [19], [20], [21], cycloaliphatic dimethacrylates [22], [23], hyperbranched methacrylates [24], [25], silicon containing dimehtacrylates [26], and fluorinated (meth)acrylates [27], [28], [29] have been used as Bis-GMA substituents. Some of these monomers showed significant improvements over Bis-GMA, but others had at least one disadvantage such as lower mechanical properties, or higher water sorption and solubility, when compared with Bis-GMA. In addition to new (meth)acrylate monomers, several new resin systems also have been introduced into dental application, such as oxiranes [30], [31], [32], thiol-enes [33], [34], [35], styrenic-methacrylate [36], thiol-Michael [37], vinylcyclopropanes [38], [39]. However, as many new methacrylate monomers or resin systems have been developed, as a principle, a new Bis-GMA-free dental material should not be prepared at the expense of sufficient material properties.
In this study, a new dimethacrylate monomer, abbreviated Bis-EFMA and derived from 9,9-Bis[4-(2-hydroxy-ethoxy)phenyl]fluorene(Bis-HEPF), was synthesized with the aim to prepare Bis-GMA-free dental resin and composite. Bis-HEPF was chosen because the rigid molecular structure could act similar to the BPA-core of the structure in Bis-GMA and reinforce the polymer structure. As well, this structure could endow the polymer with high refractive index and low birefringence [40], which would be beneficial for the light transmission in dental composites. The hypothesis of this study was that Bis-EFMA-based dental resin and composite will have similar or better properties when compared with Bis-GMA-based resin and composite. Bis-GMA/Triethylene glycol dimethacrylate (TEGDMA) dental resin was used as a control for the resin material. Bis-GMA/TEGDMA based composite and a commercial dental composite were used as control for the Bis-EFMA-based composite.
Section snippets
Materials
9,9-Bis[4-(2-hydroxyethoxy)phenyl]fluorene (Bis-HEPF), 2-(Methacryloyloxy)ethyl isocyanate (MEI), camphorquinone (CQ), and dibutyltin dilaurate (DBTL) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Bis-GMA, TEGDMA, and dimethyl aminoethyl methacrylate (DMAEMA) were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). Silanated dental glass fillers (SCHOTT® UltraFine, GM 27884, 9.4% silane, d50 = 0.4 μm) were purchased from SCHOTT AG (Mainz, Germany). Reference material 3M
Results
The dimethhacrylate monomer Bis-EFMA was confirmed as the product from the synthesis by FT-IR (Fig. 2) and 1H-NMR.
The curves of DC versus time of the experimental resins and composites and the controls are shown in Fig. 3, Fig. 4, respectively. The values of DC of resin and composites at 60 s and 24 h are listed separately in Table 1, Table 2. For all groups, DCs measured 24 h after the beginning of irradiation were higher than DCs measured 60 s after the beginning of irradiation (p < 0.05).
Discussion
The, dental resin matrix is usually a combination of different methacrylate monomers which influences the performance of dental composites. The selection of the monomers will influence the reactivity and the shrinkage of the composite when cured, as well as the mechanical properties, the water sorption and solubility, the cytotoxicity of the cured composite, to mention some effects [46]. The DC’s presented are those measured at 2 mm depths of the materials and it is acknowledged that higher
Conclusion
New dimethacrylate monomer Bis-EFMA was synthesized and used to prepare Bis-GMA-free dental resin and composite materials. Studies of the experimental materials showed that Bis-EFMA-based resin matrix and composite materials had improved properties compared to Bis-GMA-based resin matrix and composite materials, respectively. Compared with the commercial dental composite material Z250, Bis-EFMA-based composite showed similar or improved properties except for a lower FS after water immersion.
Acknowledgement
Mrs. Anne Wesmann, NIOM, is acknowledged for her assistance with the MTT-method.
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