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RESEARCH REPORTS |
1 Leuven BIOMAT Research Cluster, Department of Conservative Dentistry, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium;
2 Laboratory of Solid-State Physics and Magnetism, Department of Physics, Catholic University of Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium;
3 Department of Biomaterials, Okayama University Graduate School of Medicine and Dentistry, 2-5-1 Shikata-cho, Okayama 700-8525, Japan; and
4 Division for General Dentistry, Hokkaido University Dental Hospital, Kita 13 Nishi 7, Kita-ku, Sapporo 060-8586, Japan;
* corresponding author, bart.vanmeerbeek{at}med.kuleuven.ac.be
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: adhesion • monomer • solvent • phase separation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The objective of this study was to investigate whether the lower bonding effectiveness of one-step self-etch adhesives should be attributed in part to phase separation between adhesive ingredients. Therefore, we compared the adhesive interaction of 5 experimental and 3 commercial self-etch adhesives with dentin using transmission electron microscopy (TEM). All adhesives were examined for homogeneity by light microscopy (LM). Bonding effectiveness to dentin was determined with the use of a micro-tensile bond-strength (µTBS) protocol. The actual hypothesis tested was that phase separation may occur upon evaporation of primer solvents and may account for the lower bonding effectiveness of one-step adhesives.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). Three experimental adhesives differed in composition for the solvent used, being either ethanol/water (Exp-Eth), acetone/water (Exp-Ac), or 2-hydroxyethyl methacrylate/water (Exp-HEMA). Two experimental adhesives were applied differently, with Exp-Ac/SA representing the acetone-based adhesive strongly air-dried before samples were light-cured. Exp-Eth/UB transformed the master one-step adhesive (Exp-Eth) into a two-step self-etch adhesive when Exp-Eth (that served as a self-etching primer) was not light-cured, and a HEMA-containing bonding agent was additionally applied (UB; from Unifil Bond, GC, Tokyo, Japan). One commercial one-step self-etch adhesive (iBond, Hereaus-Kulzer, Hanau, Germany) and 2 commercial two-step self-etch adhesives (Clearfil SE Bond, Kuraray, Osaka, Japan; Unifil Bond, Tokyo, Japan, GC) served as controls.
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). The specimens were processed for TEM according to the procedure previously described in detail (Van Meerbeek et al., 1998). Non-demineralized and lab-demineralized (10% formaldehyde-formic acid for 36 hrs) ultrathin sections were cut (Ultracut UCT, Leica, Vienna, Austria) and examined unstained and positively stained (5% uranyl acetate for 20 min/saturated lead citrate for 3 min) by TEM (Philips CM10, Eindhoven, The Netherlands).
LM of Adhesive Homogeneity
All adhesive solutions were examined (uncured) by LM for homogeneity (Olympus BH2, Hamburg, Germany). A drop of each self-etching solution was dispensed onto a glass plate, and imaged real-time at different magnifications (140-280x) by means of a digital camera (JVC TK-870E, Yokohama, Japan).
µTBS Testing
Human third molars (gathered from patients following informed consent obtained according to a protocol approved by the Commission for Medical Ethics of KU Leuven) were used within 1 mo of extraction. They were stored in 0.5% chloramine/water (4°C) until used. The occlusal crown third was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA), thereby exposing a flat mid-coronal dentin surface. A bur-cut smear layer was produced by removal of a thin layer of the surface by means of a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA), equipped with a high-speed regular-grit (100 µm) diamond (842, Komet, Lemgo, Germany). After application of the experimental and control adhesives according to the manufacturers’ instructions (Table), dentin was immediately built up with Gradia Direct Anterior (GC).
After samples were stored overnight in distilled water (37°C), rectangular sticks (2x2 mm wide; 8–9 mm long) were sectioned perpendicular to the adhesive-tooth interface by means of the Isomet saw. Only the 4 central sticks were used, to eliminate substrate regional variability (Yoshiyama et al., 1998). The sticks were trimmed at the interface into an hourglass shape (diameter of ± 1.1 mm) by means of the MicroSpecimen Former, equipped with a fine-grit (30 µm) diamond (5835KREF, Komet) in a high-speed handpiece under air/water coolant. The specimens were fixed to a Ciucchi’s jig with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Ohtawara, Japan) and stressed in tension at a crosshead speed of 1 mm/min in a universal testing device (LRX, Lloyd, Hampshire, UK). We derived the µTBS by dividing the imposed force at the time of fracture by the bond area (mm2). When a specimen failed during processing (pre-testing failure), the µTBS was set at 0 MPa (De Munck et al., 2004; Nikolaenko et al., 2004). Statistical differences were examined by Kruskal-Wallis non-parametric statistics (
= 0.05). The mode of failure was determined with a stereomicroscope at 50x magnification.
Representative dentin and composite µTBS-fracture planes, exhibiting the most frequently observed failure mode and a µTBS close to the mean, were processed for field-emission-gun scanning electron microscopy (Feg-SEM; Philips XL30, Eindhoven), by a common specimen-processing procedure described previously (Perdigão et al., 1995).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). The presence of droplets was confirmed by Feg-SEM (Fig. 1). Exp-Ac showed more droplets than Exp-Eth. These droplets were round/oval, with various sizes (0.5–10 µm), and localized mostly at the bottom of the adhesive layer. Smaller droplets were seen to coalesce into larger ones. Only at the occurrence of defects in the TEM section were the droplets filled with embedding resin. No droplets were entrapped in the adhesive layer when ethanol/acetone was replaced by HEMA (Exp-HEMA), and when a HEMA-containing bonding agent was applied to the non-cured Exp-Eth primer according to a two-step application procedure (Exp-Eth/UB) (Fig. 2). Strong air-drying of the experimental adhesive (Exp-Ac/SA) substantially reduced the number of droplets, though it never completely eliminated them (Fig. 2). Among the commercial adhesives, only iBond revealed droplets with features/localization similar to those observed with the experimental adhesives (Fig. 2). All adhesives created a hybrid layer of 0.5–1 µm thickness, without much resin tag formation. Residual hydroxyapatite was clearly present throughout the hybrid layer.
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No significant difference was found among the µTBSs of the 5 experimental adhesives (Fig. 3). They bonded significantly better than the one-step self-etch adhesive iBond, significantly worse than the two-step self-etch adhesive Clearfil SE Bond, and equally as effective as the two-step self-etch adhesive Unifil Bond. All adhesives proved to fail mainly according to a ‘mixed’ failure pattern, which was confirmed by Feg-SEM (Figs. 1, 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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TEM clearly revealed the entrapment of droplets throughout the adhesive, in particular at the bottom of the adhesive layer. LM of adhesive drops dispensed on a glass plate showed that the droplets were caused by turbulent mixing reactions after exposure to surrounding air. Due to the formation of droplets, the solution promptly lost its transparency and turned opaque. Since this happened a few seconds after the drop was dispensed, solvent evaporation must have triggered this reaction. While turbulence streams, though varying in intensity, continued in larger globules for a few minutes, the single droplets disappeared only very slowly with time, suggesting that they represent water. Upon further solvent evaporation, the drop of adhesive became transparent again, beginning gradually from the outside, where the solution film was thinnest, toward the thickest film at the center of the drop. We accepted our hypothesis, because the following evidence strongly suggests that the droplets are due to phase separation: (1) the particular dynamic behavior of the adhesive observed microscopically; the prompt disappearance of droplets (2) upon the addition of pure HEMA and (3) upon application of a HEMA-containing UB bonding agent; (4) the entrapment of droplets within the adhesive layer of the HEMA-free Exp-Eth, Exp-Ac, and iBond, as observed by TEM; and (5) the droplet-free adhesive layer of the HEMA-containing adhesives Exp-HEMA, Exp-Eth/UB, Clearfil SE Bond, and Unifil Bond. Once ethanol/acetone starts to evaporate, the solvent-monomer balance is broken, with water separating from the other adhesive ingredients. This phase separation was also observed with another commercial one-step self-etch adhesive, AQ-Bond (Sun Medical, Shiga, Japan; unpublished LM observation). Droplet entrapment within the adhesive layer was confirmed by Tay et al.(2002a), who also mentioned phase separation as a possible explanation. Droplets were also identified at the fracture planes of fatigued iBond (Hereaus-Kulzer) specimens, and were considered to be responsible for its low fatigue resistance as compared with that of a two-step self-etch adhesive and three-step etch & rinse adhesive (De Munck, 2004).
The earlier onset of the separation reaction and the faster return of transparency in Exp-Ac and iBond, as compared with Exp-Eth, must be explained by the differences in solvents. Since the vapor pressure (at 25°C) for acetone is 200 mm Hg, as compared with 54.1 mm Hg for ethanol, acetone is more volatile than ethanol. Although occurring in slightly different patterns, the fact that phase separation was apparent in all 3 one-step adhesives indicates that it is solvent-induced, but not solvent-specific.
The higher concentration of droplets at the bottom of the adhesive layer adjacent to the hybrid layer must be attributed to the upward movement of droplets toward the surface, where they emerge. The short 10-second application time is not enough to allow all droplets to move upward, and subsequent light-curing entrapped the droplets within the adhesive layer. LM showed that complete disappearance of droplets through evaporation was achieved only after 4–10 min, depending on the adhesive. Both the oval/circular shape of the droplets and the fact that they were not filled with embedding resin during TEM specimen-processing suggest that they contain fluid. The disappearance of droplets upon the application of a small drop of HEMA or the HEMA-containing UB bonding agent must be ascribed to HEMA acting as a solvent and bringing all adhesive ingredients back into solution. TEM confirmed the absence of droplets within the adhesive layer for Exp-HEMA and Exp-Eth/UB. In the HEMA-containing two-step self-etch adhesives, only convection streams representing rapid solvent evaporation were seen without any droplet formation or phase separation. The fact that droplets could be viewed by LM when the adhesive was dispensed on a glass plate (no water-containing dentin tissue underneath), and that TEM of the one-step self-etch adhesive Exp-HEMA did not reveal any droplet entrapment in the adhesive layer, exclude any other origin, such as water-uptake from the tooth or from the specimen storage medium, as has been demonstrated before under different circumstances (Tay and Pashley, 2003).
Conventional adhesives usually contain HEMA in a concentration between 35 and 55 vol% (Pashley et al., 1998). In etch & rinse adhesives, HEMA acts as a wetting agent and helps monomers to diffuse into the relatively deeply (3–5 µm) exposed collagen network within a clinically manageable time, thereby improving bond strength (Toledano et al., 2001). Besides the drawback of potential allergenic effects, HEMA may also retain water within the adhesive, thereby weakening the mechanical strength of the adhesive itself and potentially jeopardizing bond durability (Jacobsen and Söderholm, 1995; De Munck et al., 2003; Shirai et al., 2005). The submicron hybrid layer produced by mild self-etch adhesives should make diffusion of monomers easier, decreasing the need for HEMA. In this respect, the omission of HEMA in the adhesive formulation of the one-step adhesive, separating water from the other ingredients upon ethanol/acetone evaporation, may be advantageous by removing most of the water that would otherwise only weaken the bond. Very strong air-drying appeared sufficient to blow the droplets out, leaving only a transparent film of co-monomers behind, as observed by LM. Reduced droplet entrapment in such heavily dried adhesive layers was observed by TEM. Especially in the long term, a void-free adhesive layer should be beneficial to bond integrity. This obviously should be confirmed by durability testing. Nevertheless, AQ-Bond (Sun Medical), with a composition similar to that of the experimental one-step adhesive and proven to be sensitive to phase separation (see above), maintained the best marginal adaptation after 1 yr of water storage and 2 thermo-cycling sessions among all one-step self-etch adhesives studied (Blunck, personal communication), and performed as well as two-step self-etch adhesives and even the generally best-performing three-step etch & rinse adhesives. A prerequisite is ‘air-blowing the adhesive with full power’, which is a less ambiguous instruction than ‘gently air-drying’.
In conclusion, although one-step self-etch adhesives appear to be easy to use, some stringent problems remain. Since HEMA-free one-step adhesives are complex blends of hydrophilic/hydrophobic ingredients, water and solvents, they are prone to phase separation, which accounts partially for their lower bonding effectiveness. In contrast, strongly air-drying the phase-separated adhesive might be a clinical technique for removing substantial interfacial water, thereby improving bonding effectiveness. How successfully this can be done in complex cavity preparations in vivo remains to be determined.
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ACKNOWLEDGMENTS |
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Received June 15, 2004; Last revision November 1, 2004; Accepted November 3, 2004
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REFERENCES |
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