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Origin of Interfacial Droplets with One-step Adhesives
K.L. Van Landuyt1,
J. Snauwaert2,
J. De Munck1,
E. Coutinho1,
A. Poitevin1,
Y. Yoshida3,
K. Suzuki3,
P. Lambrechts1 and
B. Van Meerbeek1,*
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 Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200G, B-3001 Heverlee, Belgium; and
3 Department of Biomaterials, Okayama University Graduate School of Medicine and Dentistry, 2-5-1 Shikata-cho, Okayama 700-8525, Japan
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Figure 1. Box-whisker plot (min-[lower quartile-median-upper quartile]-max) of the µTBS to enamel (top) and dentin (bottom) (mean ± standard deviation; n = total number of specimens; ptf = pre-testing failure). The diamonds represent the mean µTBS. NS = not statistically significant, p < 0.05 indicates statistical significance. Superscript letters indicate significant differences (Kruskal-Wallis non-parametric statistical analysis).
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Figure 2. Fracture analysis by LM and SEM (Appendix Table 2). SEM analysis confirmed the initial LM observations. Most frequently, the tested adhesives exhibited a mixed failure pattern. A striking shift was seen in the delay group of Clearfil S3Bond and G-Bond/HEMA, where failure occurred predominantly in the adhesive layer. Further SEM investigations showed that a large number of droplets within the adhesive layer of these HEMA-rich adhesives caused these fractures. SEM images of Clearfil S3Bond, G-Bond, and G-Bond/HEMA after µTBS are shown. a1 and a2 show a representative sample of the non-delay group of Clearfil S3Bond on dentin, with a1 the dentin side and a2 the composite side. Typical accumulations of small droplets (from 300 nm to 1.5 µm) can be seen in some areas. Notice the linear pattern parallel to the scratches of the smear layer (arrows). b1 and b2 show Clearfil S3Bond after delayed curing of the composite. In contrast to the non-delay group, almost the entire adhesive layer was affected by droplets, which had sometimes coalesced to larger droplets. No linear pattern was seen. c1 and c2, respectively, show G-Bond on dentin in the non-delay group and the delay group. c1 is a detail of a dentin side, showing the presence of droplets at different levels throughout the adhesive layer. In contrast to Clearfil S3Bond and G-Bond/HEMA, a mixture of both small and large droplets (from 0.5 to 10 µm) was observed. In the delay group (c2) (composite side of G-Bond on dentin), quantities of droplets comparable with those in the non-delay group were observed. d1 and d2 are images of G-Bond/HEMA on dentin in the non-delay and delay groups, respectively. d1 SEM revealed similar droplets in G-Bond/HEMA without delayed curing. After delayed curing (d2), similarly to Clearfil S3Bond, the adhesive layer was completely affected by droplets, which coalesced and had a disruptive effect. Abbreviations: Ar, Adhesive resin; Hy, Hybrid layer.
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Figure 3. TEM and SEM interface analysis. TEM was the most appropriate tool to determine the exact location of the droplets at the interface with reference to the adhesive layer. However, because a great many samples failed during diamond-knife ultra-microtomy for TEM, the TEM blocks themselves were also prepared for SEM, after a very smooth surface was obtained by ultra-microtomy. (a) An SEM image of Clearfil S3Bond on dentin with a flowable composite (Protect Liner, Kuraray) applied without delay. Yet, along the adhesive resin-composite interface, a distinct line of droplets can be seen (arrows). (b,c) TEM images of Clearfil S3Bond applied on dentin, showing samples of the delay group. Both samples failed before being embedded in epoxy resin, since epoxy resin is observed between the composite (AP-X) and the adhesive resin. (b) Detail of the adhesive resin on dentin. Notice how the dentin surface is irregular, due to the scratches of the smear layer. The adhesive resin, however, does not follow this pattern of "valleys" and "peaks" and has an even upper surface. As a consequence, at the "peaks" of the smear layer, the thickness of the adhesive layer was greatly reduced. (c) Detail of the composite, which was detached from the dentin side. In the delay group, most often a small zone of droplets (in contrast to a line of droplets in the non-delay group) was found. (d) Overview of the adhesive layer of G-Bond in the delay group. This sample was dyed with silver-nitrate. Notice the presence of a large droplet in the adhesive layer. Some small droplets are visible near the top of the hybrid layer, and were stained with silver (arrows). However, it must be noted that silver-staining was most inconsistent, with some droplets and areas heavily stained and other droplets and locations remaining unstained. Notice how this section was made perpendicular to the smear-layer pattern, showing that the thickness of the adhesive layer was variable. Also notable was the absence of an oxygen inhibition layer (also in samples of the delay group of Clearfil S3Bond and G-Bond/HEMA), which was easily detectable in non-delay samples (not shown). This may be due to the use, in the delay group, of a viscous composite (Clearfil AP-X) that was applied with pressure, in contrast to the flowable composite that was applied without pressure in the non-delay group. (e) SEM of G-Bond/HEMA on dentin after delayed curing of the composite (AP-X). A zone of droplets similar to that found with Clearfil S3Bond delay can be seen. (f) SEM of Clearfil S3Bond applied to dehydrated dentin (non-delay group). No droplets could be found. Abbreviations: Ar, Adhesive resin; C, composite; E, Epoxy resin; Hy, Hybrid layer; Ud, Unaffected dentin.
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Figure 4. Theoretical scheme illustrating the osmosis process in the non-delay and delay groups. A perpendicular section of dentin is shown. Due to the parallel scratches made by the diamond bur, the dentin surface is irregular and consists of "valleys" and "peaks". The upper part of the adhesive layer, however, does not follow this pattern and has an even upper surface. The uppermost part of the adhesive resin (1–3 µm) is not cured, due to oxygen inhibition. As a consequence, this "oxygen-inhibition layer" consisted mainly of uncured monomers. As such, this layer represents a zone with a "hypertonic" solution (high concentration of molecules and low concentration of water). The dentin and its intrinsic water (in the dentinal tubules and in smear crevices) represent a "hypotonic" area. Diffusion of water from the dentin through the cured adhesive layer will occur (osmosis), and the cured adhesive resin functions as a semi-permeable membrane. In HEMA-rich adhesives, uncured small HEMA-molecules must be important components of the oxygen-inhibition layer. The strong hydrophilic character and the small dimensions of this monomer explain why osmosis is fast and easy in HEMA-rich adhesives. In the non-delay group, water is able to reach the oxygen inhibition layer only in areas where the adhesive resin is minimal in thickness, which also explains the linear pattern of droplets according to the scratches of the smear layer. When the composite is cured after 20 min, water is able to diffuse, even in areas where the adhesive resin is maximal in thickness. The large accumulations of droplets have a severely weakening effect on the bond between the adhesive layer and the lining composite, resulting in low bond strengths. In HEMA-free adhesives, a similar osmotic process cannot be excluded. However, even if it occurs, it must be to a limited extent, since the bond strength is not reduced after delayed curing of the composite. HEMA-free adhesives do exhibit droplets, but, in contrast to HEMA-rich adhesives, these droplets are due to phase separation. Unlike the droplets in HEMA-rich adhesives, these droplets do not increase with time, but are usually larger in size and are located throughout the adhesive resin.
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Journal of Dental Research, Vol. 86, No. 8,
739-744 (2007)
DOI: 10.1177/154405910708600810
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