self-assembled nano-layering at the adhesive interface
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http://jdr.sagepub.com/content/91/4/376The online version of this article can be found at:
DOI: 10.1177/0022034512437375
2012 91: 376 originally published online 1 February 2012J DENT RESY. Yoshida, K. Yoshihara, N. Nagaoka, S. Hayakawa, Y. Torii, T. Ogawa, A. Osaka and B.Van Meerbeek
Self-assembled Nano-layering at the Adhesive Interface
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- Feb 1, 2012OnlineFirst Version of Record
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376
RESEARCH REPORTSBiomaterials & Bioengineering
DOI: 10.1177/0022034512437375
Received November 15, 2011; Last revision January 1, 2012; Accepted January 7, 2012
A supplemental appendix to this article is published elec-tronically only at http://jdr.sagepub.com/supplemental.
© International & American Associations for Dental Research
Y. Yoshida1,2*#, K. Yoshihara3#, N. Nagaoka4, S. Hayakawa5, Y. Torii6, T. Ogawa7, A. Osaka5, and B.Van Meerbeek3,8
1Department of Biomaterials, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Okayama, 700-8525, Japan; 2Research Center for Biomedical Engineering, Okayama University, Okayama, Japan; 3KU Leuven BIOMAT, Department of Oral Health Sciences, Group of Biomedical Sciences, Faculty of Medicine, KU Leuven (University of Leuven), Leuven, Belgium; 4Laboratory for Electron Microscopy, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan; 5Biomaterials Laboratory Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan; 6Comprehensive Dental Clinic, Okayama University Hospital, Okayama, Japan; 7Cooperative Research Facilities, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan; and 8Department of Dentistry, University Hospitals Leuven, Belgium; #authors contributing equally as first author; *corresponding author, [email protected].
J Dent Res 91(4):376-381, 2012
AbstrActAccording to the ‘Adhesion–Decalcification’ concept, spe-cific functional monomers within dental adhesives can ioni-cally interact with hydroxyapatite (HAp). Such ionic bonding has been demonstrated for 10-methacryloy-loxydecyl dihydrogen phosphate (MDP) to manifest in the form of self-assembled ‘nano-layering’. However, it remained to be explored if such nano-layering also occurs on tooth tissue when commercial MDP-containing adhe-sives (Clearfil SE Bond, Kuraray; Scotchbond Universal, 3M ESPE) were applied following common clinical applica-tion protocols. We therefore characterized adhesive-dentin interfaces chemically, using x-ray diffraction (XRD) and energy-dispersive x-ray spectroscopy (EDS), and ultrastruc-turally, using (scanning) transmission electron microscopy (TEM/STEM). Both adhesives revealed nano-layering at the adhesive interface, not only within the hybrid layer but also, particularly for Clearfil SE Bond (Kuraray), extending into the adhesive layer. Since such self-assembled nano-layering of two 10-MDP molecules, joined by stable MDP-Ca salt formation, must make the adhesive interface more resistant to biodegradation, it may well explain the documented favorable clinical longevity of bonds produced by 10-MDP-based adhesives.
KEY WOrDs: adhesive, functional monomer, nano-layering, interface, XRD, TEM.
IntrODuctIOn
In nature, self-assembly of nano-layered structures, as occurs in the process of ‘biomineralization’, has often been observed to provide highly desirable
multiple functions (Stupp and Braun, 1997; Tai et al., 2007). In materials engi-neering, nano-layering has been shown to dramatically improve the perfor-mance of materials in general, and their resistance to damage or aging more specifically (Gandhi et al., 2007; Kagan et al., 2007; Sofos et al., 2009).
Recently, self-assembled nano-layering was found to result from interac-tion of the functional monomer 10-methacryloyloxydecyl dihydrogen phos-phate (MDP), first with synthetic hydroxyapatite (HAp) (Fukegawa et al., 2006; Yoshihara et al., 2010) and later also with enamel and dentin (Yoshihara et al., 2011a). We suggested that such nano-layering at the MDP/HAp inter-face may provide multi-functional properties to the interface with, in particu-lar, direct benefit to bond durability (Koshiro et al., 2006). Indeed, the strong hydrophobic nature of the nano-layered structure may help to protect the formed hybrid layer against biodegradation (Breschi et al., 2008). Besides direct protection of collagen against degradation, it may make residual HAp more resistant to acidic dissolution. It also forms a more gradual transition of an inorganic substrate (apatite in dentin/enamel) to an artificial biomaterial of organic nature (resin-based adhesive).
So far, this nano-layering was proven to be formed upon direct interaction of MDP, dissolved in ethanol-water, with synthetic HAp powder and enamel/dentin fragments. However, today’s adhesives are complex mixtures of not only functional but also cross-linking monomers, curing initiators, inhibitors or stabilizers, solvents, and often filler. In continuation of our previous research, we used chemical [x-ray diffraction (XRD) and energy-dispersive x-ray spectroscopy (EDS)] and ultrastructural techniques [(scanning) trans-mission electron microscopy (TEM/STEM)] to investigate whether two of
self-assembled nano-layering at the Adhesive Interface
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J Dent Res 91(4) 2012 Interfacial Nano-layering 377
today’s commercial MDP-containing adhesives would pro-duce nano-layering at their interface with dentin.
MAtErIAls & MEthODs
XrD
Two dentin samples (10 × 8 × 1 mm) were cut from non-cari-ous human third molars, gath-ered following informed consent approved by the Commission for Medical Ethics of Okayama University, and stored in 0.5% chloramine at 4°C for a maximum of 1 mo. The exposed surface was ground with 600-grit SiC paper. All dentin surfaces were carefully verified, by stereomicroscopy, for the absence of enamel/pulp tissue (Wild M5A, Wild, Heerbrugg, Switzerland). The MDP-containing primer ‘Clearfil SE primer’ (C-SE; Kuraray, Tokyo, Japan), as the first step of the two-step self-etch adhesive ‘Clearfil SE Bond’ (Kuraray; for composition, see the Appendix Table), was applied according to the manufacturer’s instructions by light rubbing of the dentin with a micro-brush (Centrix Benda Brush, Centrix, CT, USA). After 20 sec, the samples were vigorously air-dried prior to being examined by thin-film XRD (CuKα XRD; RINT2500, Rigaku, Tokyo, Japan) under 40 kV acceleration and 200 mA current, with the angle of the incident x-ray beam fixed at 1.0° and a scanning time of 0.02°/sec for 2θ scan (referred to as ‘C-SE_dentin’). As reference, XRD patterns of untreated dentin (‘dentin’) and of dentin rubbed for 20 sec with an experimental MDP:EtOH:H2O solution at a 15:45:40wt% ratio (‘MDP_dentin’) were also acquired. Likewise, the recently marketed MDP-contain-ing adhesive Scotchbond Universal (3M ESPE, Seefeld, Germany; see the Appendix Table) was applied to dentin in a ‘self-etch’ mode and analyzed by XRD (‘S-U_dentin’).
ultrastructural and chemical Interfacial Analysis by tEM, stEM, and EDs
Dentin specimens from two additional third molars were flattened by means of a medium-grit (100 μm) diamond bur (842, Komet, Lemgo, Germany) in a water-cooled high-speed contra-angle hand-piece mounted in a MicroSpecimen Former (University of Iowa, Iowa City, IA, USA). The bur-cut dentin surface was further ground with 600-grit SiC paper. The two-step self-etch adhesive Clearfil SE Bond (Kuraray) and the one-step adhesive Scotchbond Universal (3M ESPE) were applied according to the manufacturers’ instructions, followed by a thin layer of flowable composite
(Clearfil Protect Liner F, Kuraray). Light-curing of the adhesive and flowable composite, respectively, was performed separately in an Optilux 500 (Demetron/Kerr, Danbury, CT, USA) device with a light output not less than 600 mW/cm2. After bonding, specimens were stored for 1 day in tap water at 37°C and further processed for TEM by common EM-lab procedures. Non-demineralized sections were cut (Ultracut UCT, Leica, Vienna, Austria), prior to TEM (200 kV TEM, JEM-2100, JEOL, Tokyo, Japan) and STEM (200 kV STEM, JEM-2100F, JEOL), the lat-ter being equipped with a spherical aberration corrector (Ceos, Heidelberg, Germany). This provided a minimum electron-probe diameter of about 0.09 nm. Elemental analysis was per-formed by EDS, with a Si(Li) semiconductor EDS detector [JEM-2300T, JEOL: ultra-thin window detector with a size of 30 mm2, a solid angle of 0.19 sr (URP objective lens), and an energy resolution of 133 eV at Mn Kα]. High-angle annular dark-field (HAADF) images were taken with a 167–228 mrad detector.
rEsults
XrD
Untreated dentin (Fig. 1a, ‘dentin’) revealed XRD peaks at 2θ = 26.0°, 28.8°, 31.8°, 32.2°, and 33.0°, which must be ascribed to
Figure 1. TF-XRD patterns of untreated dentin (‘dentin’), of dentin exposed to an experimental MDP:EtOH:H2O solution at a ratio of 15:45:40wt% for 20 sec (‘MDP_dentin’), and of dentin with Clearfil SE Bond primer (Kuraray; ‘C-SE_dentin’) and Scotchbond Universal (3M-ESPE; ‘S-U_dentin’) at a wide angle in (a) and at a low angle in (b) for the latter three experimental conditions. The principle of thin-film XRD is briefly explained in (c), with, according to Bragg’s law, ‘n’ being an integer, λ being the wavelength of the incident wave (a CuKα target with a wavelength of 1.54 Å was used), ‘d’ being the spacing between the planes in the atomic lattice, and 2θ being the angle between the incident X-ray and the scattered planes.
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Figure 2. TEM photomicrographs illustrating the interface of Clearfil SE Bond (Kuraray) bonded to dentin. The photomicrographs in (a-d) were taken by TEM (JEM-2100, JEOL), those in (e-h) by STEM (JEM-2100F, JEOL) equipped with a Cs corrector, and, finally, those in (i-m) by energy-dispersive x-ray spectroscopy (EDS; JED-2300T, JEOL).
Figure 3. TEM (JEM-2100, JEOL) photomicrographs illustrating the interface of Scotchbond Universal (3M ESPE) bonded to dentin at two different locations in (a-c) and (d-f).
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HAp. When dentin was exposed to the MDP:EtOH:H2O solu-tion (Figs. 1a, 1b, ‘MDP_dentin’), three characteristic peaks in the range of 2θ = 2.52° (d = 3.50 nm), 4.84° (d = 1.82 nm), and 7.16° (d = 1.23 nm) appeared and must be assigned to the forma-tion of MDP_Ca salt (Yoshihara et al., 2011a). The three char-acteristic peaks have been previously identified as ‘nano-layering’ of MDP-Ca, more specifically, the structural self-assembly of two MDP molecules joined together by Ca (Fukegawa et al., 2006; Yoshihara et al., 2010, 2011a). The dentin sample treated with Clearfil SE primer (Kuraray) revealed three characteristic peaks in the range of 2θ = 2.53° (d = 3.49 nm), 4.96° (d = 1.78 nm), and 7.36° (d = 1.20 nm) (Figs. 1a, 1b, ‘C-SE_dentin’), which were not detected for untreated dentin (Fig. 1a, ‘dentin’). Three such peaks at 2θ = 2.56° (d = 3.45 nm), 5.04° (d = 1.75 nm), and 7.44° (d = 1.19 nm) were also detected when Scotchbond Universal (3M ESPE) was applied to dentin (Figs. 1a, 1b, ‘S-U_dentin’).
tEM/stEM/EDs
(S)TEM of the interface produced by Clearfil SE Bond (Kuraray) at dentin revealed a 0.5- to 0.7-μm-thick hybrid layer (Figs. 2a, 2e, 2f, 2i). At higher magnification, HAp could be seen to have remained within the submicron hybrid layer (Fig. 2b). In par-ticular, at the top of the hybrid layer, regular longitudinally layered structures that are often curved could be observed (Figs. 2c, 2f, 2g). Higher magnification confirmed nano-layering with a periodicity of about 3.5 nm, as was calculated from the elec-tron diffraction pattern in the inset of Fig. 2d, and as was most clearly detectable by STEM (Fig. 2h). EDS maps of Ca (Fig. 2j) and P (Fig. 2k) disclosed the distribution of HAp within dentin, but also identified the curved zones of nano-layering. The distri-bution of O (Fig. 2l) represented HAp, but also dentinal colla-gen, the zone of nano-layering, and the SiO2-microfiller within the adhesive. The latter filler was also identified by Si (Fig. 2m).
TEM (complementing EDS; data not shown) of the interface produced by Scotchbond Universal (3M ESPE) at dentin revealed a 0.2- to 0.5-μm-thick hybrid layer, in which HAp remained (Figs. 3a, 3b, 3d, 3e). Nano-layering was discovered particularly near the tubule orifices where the adhesive infil-trated residual smear (Figs. 3c, 3f).
DIscussIOn
Laboratory and clinical studies have proven that the MDP-based adhesive Clearfil SE Bond (Kuraray) bonds effectively and durably to dentin (Inoue et al., 2005; Waidyasekera et al., 2009; Peumans et al., 2010). The underlying mechanism of bonding was shown to be based upon submicron micro-mechanical inter-locking (Van Meerbeek et al., 2003), supplemented by primary chemical interaction of the functional monomer MDP with HAp that remained around the partially exposed collagen (Yoshida et al., 2004; Fu et al., 2005). When, according to the ‘Adhesion-Decalcification’ concept (Yoshida et al., 2001), MDP chemi-cally bonds to HAp, it thereby keeps HAp as the natural shelter around collagen. Among the different functional monomers so far tested for their chemical bonding potential to HAp, the func-tional monomer MDP outperformed other monomers, like
Phenyl-P and 4-MET (Yoshida et al., 2004; Yoshihara et al., 2010), and three different phosphonate monomers (Van Landuyt et al., 2008; Yoshihara et al., 2011b). This primary chemical interaction of MDP with HAp was recently shown to occur within a clinically manageable time of 20 sec (Yoshihara et al., 2011a). While such chemical interaction did not raise the ‘immediate’ bond strength, studies testing the biodegradation resistance of adhesive interfaces have shown that it improved the bond stability (Inoue et al., 2005; Erhardt et al., 2011).
This finding favors the use of a ‘mild’ self-etch rather than an ‘etch-and-rinse’ approach with dentin (Van Meerbeek et al., 2011). Increasingly, research indicates that etch-and-rinse adhe-sives suffer from poor adaptation to HAp-denuded collagen (Van Meerbeek et al., 2003; Brackett et al., 2011; Liu et al., 2011). The latter etch-and-rinse approach nevertheless remains preferred for enamel that requires sufficient etching (Erickson et al., 2009; Perdigão et al., 2012). Selectively etching enamel combined with a ‘mild’ self-etch adhesive can therefore today be recommended to achieve effective and durable bonding to tooth enamel and dentin (Van Meerbeek et al., 2011).
Further chemical analysis of the molecular interplay at the adhesive interface disclosed the process of self-assembled nano-layering, first documented upon interaction of MDP with HAp powder (Fukegawa et al., 2006; Yoshihara et al., 2010), and later also with dentin/enamel fragments (Yoshihara et al., 2011a). However, it had not been determined thus far if such nano-layering occurred at the interface of an MDP-containing adhesive bonded to dentin. This study confirmed, chemically and ultrastructurally, that MDP-Ca salts were produced by two MDP-containing commercial adhesives, a process that manifested as interfacial nano-layering. XRD revealed average data on MDP-Ca salt formation for the whole sample surface. TEM pro-vided ultrastructural evidence of nano-layering at the actually formed hybrid layer, albeit in different degrees, depending on the adhesive applied. Indeed, nano-layering appeared less promi-nent for Scotchbond Universal (3M ESPE), as both XRD and TEM revealed correlatively. Actual compositional differences and
Figure 4. Schematic explaining the formation of MDP-Ca salt and interfacial nano-layering. When the MDP-containing adhesive is rubbed onto dentin, the surface is partially demineralized up to a depth of 0.5-1 micrometer. Ca ions released upon partial dissolution of HAp diffuse within the hybrid layer and assemble MDP molecules into nano-layers, a process that is driven by MDP-Ca salt formation. The measured size of one nano-layer is about 3.5 nm.
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possibly different MDP concentrations may explain this vari-ance in nano-layering recorded for the two adhesives tested. Also, the polyalkenoic-acid co-polymer in Scotchbond Universal (3M ESPE) may have competed with MDP for the same Ca-bonding sites at HAp. Previous research actually proved that the nano-layering intensity, and thus also the XRD-peak inten-sity representing nano-layering, was directly proportional to the concentration of MDP (Yoshihara et al., 2011a). In another project (unpublished observations), more intense nano-layering was also found for a filler-free formulation of the commercial MDP-based one-step adhesive Clearfil S3 Bond (Kuraray). Moreover, Clearfil SE Bond (Kuraray) involves a two-step application procedure, with the much more fluid and thus prob-ably more chemically active primer applied first. Most likely, the primer itself may have produced the nano-layering. Scotchbond Universal (3M ESPE) is a one-step adhesive and thus contains proportionally less MDP. The reduced interaction intensity of Scotchbond Universal (3M ESPE) is also reflected in the thinner hybrid layer as compared with that produced by Clearfil SE Bond (Kuraray). Finally, nano-layering was ultra-structurally relatively easily observed at the transition of the hybrid layer to the adhesive resin for Clearfil SE Bond (Kuraray). For Scotchbond Universal (3M ESPE), nano-layering was found primarily near the dentin tubules, most likely having made use of Ca present within the remained smear plugs.
The actual process of nano-layering is schematically explained in Fig. 4. When the MDP-containing adhesive is rubbed onto dentin, the surface is partially demineralized up to a submicron depth. Ca ions released upon partial dissolution of HAp diffuse within the hybrid layer and assemble MDP mole-cules into nano-layers, a process that is driven by MDP-Ca salt formation. The theoretical size of one MDP molecule is 1.95 nm, and thus the size of two MDP molecules is 3.90 nm. This dimension approximated the 4-nm-thick nano-layer formed by MDP at synthetic HAp, as has been measured previously (Fukegawa et al., 2006; Yoshihara et al., 2011a). Like a finger-print, the dimension of nano-layering can disclose which par-ticular functional monomer the adhesive contains. When the ca. 4-nm-thick nano-layer at synthetic HAp is compared with that detected upon interaction of the two MDP-containing adhesives with dentin, a slight XRD-peak shift was detected, thus referring to a slightly thinner nano-layering of about 3.5 nm. TEM con-firmed the ca. 3.5-nm-thick nano-layering for both adhesives tested. This thinner nano-layering is difficult to explain, although differences in the size of the Ca-layer connecting two nano-layers, but also some shrinkage upon polymerization, may be plausible explanations (Fig. 4b).
High-magnification TEM clearly revealed abundant residual HAp within the submicron hybrid layer produced by the two adhesives tested. This may have obscured the presence of nano-layering to a certain extent. In general, the nano-layering appeared to be formed parallel to the c-axis of HAp, which, in turn, is aligned with the longitudinal axis of the collagen fibrils (Figs. 2c, 2d, 2g, 2h, 3c, 3f, and 4a; Elliott, 2002; Tay and Pashley, 2009). In particular, at the interface of Clearfil SE Bond (Kuraray) bonded to dentin, nano-layering was observed by (S)TEM to extend above the hybrid layer. EDS elemental mapping confirmed the extension of Ca within the curved nano-layered
features that were embedded within the adhesive layer above. This particular nano-layering arrangement must be ascribed to the rubbing motion with which the primer was applied, and/or to the subsequent air-blowing. Adhesive ingredients from Clearfil SE Bond (Kuraray) along with dentin fragments (dis-solved hydroxyapatite) and water must have been mixed and have self-arranged into this peculiar form of nano-layering extending above the hybrid layer.
Although the actual relevance of nano-layering is currently not known, one could speculate that it may contribute to the better bond stability typically documented for mild self-etch adhesives that also chemically interact with dentin (Inoue et al., 2005). In gen-eral, a hybrid layer behaves in a sponge-like manner, absorbing and releasing water and ions via osmosis (Chersoni et al., 2004; Carrilho et al., 2009); as such, it is even ‘repairable’ by biomi-metic re-mineralization (Kim et al., 2010). Upon the interaction of MDP with HAp, highly hydrolytically stable MDP-Ca salts were formed. In a study by Yoshida et al. (2004), atomic absorp-tion spectrophotometry (AAS) revealed a low dissolution rate of 6.79 ± 0.43 mg/L for MDP-Ca, vs. 1.36 ± 0.27 g/L and 1.91 ± 0.14 g/L for 4-MET-Ca and PhenylP-Ca, respectively. According to Van Landuyt et al. (2008), the AAS dissolution rate was 6.8 mg/L for MDP-Ca, vs. 14 mg/L, 50 mg/L, and > 200 mg/L for the experimental phosphonate monomers MAEPA, EAEPA, and HAEPA, respectively. In a self-etch procedure, the Ca-salts are embedded within the hybrid layer (and not rinsed off as in the case of an etch-and-rinse procedure). Stably formed MDP-Ca salts will definitely contribute to the clinical longevity of the hybrid layer and thus the bond to dentin. In addition, the nano-layering itself forms a stronger phase at the adhesive interface, which must also increase the mechanical strength of the adhesive assembly, a hypothesis that naturally needs to be explored further.
It is concluded that MDP-containing adhesives do form nano-layering at the adhesive interface, albeit in different degrees, depending on the adhesive formulation. Stable MDP-Ca salt deposition along with nano-layering may explain the high stability of MDP-based bonding, as has been proven previously in laboratory as well as clinical research.
AcKnOWlEDgMEnts
The current research was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan, and by the G.0496.10 research grant of the Research Foundation-Flanders. We thank the respec-tive manufacturers for donating the adhesives. The authors declare no potential conflict of interest with respect to the authorship and/or publication of this article.
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