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Comparative Study on Adhesive Performance of Functional Monomers
1 Department of Biomaterials, Okayama University Graduate School of Medicine and Dentistry, 2-5-1 Shikata-cho, Okayama 700-8525, Japan; Correspondence: * corresponding author, bart.vanmeerbeek{at}med.kuleuven.ac.be
Mild self-etch adhesives demineralize dentin only partially, leaving hydroxyapatite around collagen within a submicron hybrid layer. We hypothesized that this residual hydroxyapatite may serve as a receptor for chemical interaction with the functional monomer and, subsequently, contribute to adhesive performance in addition to micro-mechanical hybridization. We therefore chemically characterized the adhesive interaction of 3 functional monomers with synthetic hydroxyapatite, using x-ray photoelectron spectroscopy and atomic absorption spectrophotometry. We further characterized their interaction with dentin ultra-morphologically, using transmission electron microscopy. The monomer 10-methacryloxydecyl dihydrogen phosphate (10-MDP) readily adhered to hydroxyapatite. This bond appeared very stable, as confirmed by the low dissolution rate of its calcium salt in water. The bonding potential of 4-methacryloxyethyl trimellitic acid (4-MET) was substantially lower. The monomer 2-methacryloxyethyl phenyl hydrogen phosphate (phenyl-P) and its bond to hydroxyapatite did not appear to be hydrolytically stable. Besides self-etching dentin, specific functional monomers have additional chemical bonding efficacy that is expected to contribute to their adhesive potential to tooth tissue.
Key Words: adhesion • functional monomer • XPS • TEM • dissolution rate
Self-etch adhesives are user-friendly because they do not require a rinse phase. This substantially reduces application time as well as technique sensitivity. In addition to their application after a two- vs. a single-step approach, a further distinction can be made between "mild" and "strong" current self-etch adhesives (Inoue et al., 2000; Tay and Pashley, 2001; Van Meerbeek et al., 2003). Like etch-and-rinse adhesives, strong self-etch adhesives completely remove hydroxyapatite from dentin, resulting in relatively deep dentin hybridization several micrometers thick. However, mild self-etch adhesives form only submicron-thick hybrid layers, in which hydroxyapatite partially remains around exposed collagen (Nakabayashi and Saimi, 1996; Inoue et al., 2000). Commonly, acidic monomers in self-etch primers/adhesives are esters originating from the reaction of a bivalent alcohol with methacrylic acid and phosphoric/carboxylic acid derivatives. Each self-etch adhesive contains its specific functional monomer that, to a large extent, determines its actual adhesive performance. Thus far, however, the interaction of functional monomers with dental tissues has seldom been characterized by chemical analytical techniques. We therefore comparatively characterized the adhesive interaction of 3 functional monomers with synthetic hydroxyapatite to test the hypothesis that the bonding mechanism of mild self-etch adhesives involves chemical interaction of functional monomers with residual hydroxyapatite in addition to micro-mechanical hybridization. The functional monomers investigated originated from 3 representative two-step self-etch adhesives, and their interaction with dentin was also ultra-morphologically characterized.
The functional monomer 4-MET (4-methacryloxyethyl trimellitic acid) and its calcium salt 4-METCa were provided by GC (Tokyo, Japan), whereas phenyl-P (2-methacryloxyethyl phenyl hydrogen phosphate) and 10-MDP (10-methacryloxydecyl dihydrogen phosphate), and their respective calcium salts (phenyl-PCa, 10-MDPCa), were provided by Kuraray (Osaka, Japan).
X-ray Photoelectron Spectroscopy (XPS)
Dissolution Rate of Ca Salts
Transmission Electron Microscopy (TEM) Following adhesive treatment, the resin-bonded dentin specimens were processed for TEM according to a protocol previously described (Van Meerbeek et al., 1998). Demineralized samples were also prepared by immersion in a 10% formaldehyde-formic acid solution (36 hrs). Sections (70–90 nm thick) were cut by means of a diamond knife (Diatome, Bienne, Switzerland) in an ultramicrotome (Ultracut UCT, Leica, Vienna, Austria), and were observed unstained and positively stained (5% uranyl acetate [UA] for 20 min/saturated lead citrate [LC] for 3 min) by TEM (Philips CM10, Eindhoven, The Netherlands).
XPS wide-scan spectra of untreated hydroxyapatite and spectra of hydroxyapatite treated with 4-MET, phenyl-P, and 10-MDP, respectively, are alike, except for the C 1s peak at a binding energy of approximately 285 eV that appeared when hydroxyapatite was exposed to functional monomers (Fig. 1  ). The intensity of the C 1s peak of untreated hydroxyapatite (Fig. 1a) increased slightly when hydroxyapatite was treated for 30 sec with 4-MET (Fig. 1b), while it increased considerably when hydroxyapatite was exposed to 10-MDP for 30 sec (Fig. 1c). Application of 4-MET to hydroxyapatite for 30 min further increased the intensity of the C 1s peak in comparison with intensity from the 30-second application (Fig. 1d). A slightly increased C 1s peak appeared when hydroxyapatite was exposed to phenyl-P for 30 min (Fig. 1e). No increase in intensity of the C 1s peak was recorded when hydroxyapatite was exposed to 10-MDP for 30 min (Fig. 1f), as compared with the short 30-second application (Fig. 1c).
Application of 4-MET to hydroxyapatite (Fig. 2a ) resulted in a significant shift of the peak representing carboxyl groups and esters at 288.6 eV, to a lower binding energy and an increase of its FWHM (full width at half-maximum). Deconvolution of the shifted peak disclosed two components, representing esters and unreacted carboxyl groups at 288.6 eV, and carboxyl groups reacted with Ca of hydroxyapatite at 288.2 eV.
Application of phenyl-P to hydroxyapatite resulted in a two-fold C 1s peak with a shape totally different from that of phenyl-P itself (Fig. 2b  ). Application of 10-MDP to hydroxyapatite resulted in a single C 1s peak, resembling that of 10-MDP itself (Fig. 2c). Deconvolution disclosed: an intense sub-peak at 284.6 eV, representing C-C/C-H/C=C bindings; a second, much smaller peak at 286.0 eV, representing C-O bindings; and the smallest peak at 288.6 eV, representing esters. With regard to chemical bonding efficacy, quantitative analysis revealed that the carbon concentration detected on hydroxyapatite exposed to 15% (w/w) 4-MET for 30 min (17.2 ± 0.6%) was not significantly different (p > 0.05) from that of hydroxyapatite exposed to 15% (w/w) 10-MDP for 30 min (17.3 ± 0.6%). However, 4-MET and 10-MDP have 15 and 14 carbon atoms, respectively, in a single molecule. Thus, for comparison of bonding potentials, the atom% should be adjusted by the number of carbon atoms in a single molecule. The atom%/14 value for hydroxyapatite exposed to 10-MDP (1.23 ± 0.04%) was significantly (p < 0.05) higher than the atom%/15 value of hydroxyapatite exposed to 4-MET (1.15 ± 0.04%). AAS (n = 5) revealed that phenyl-PCa was significantly more soluble ([Ca2+] in water = 1.91 ± 0.14 g/L) than 4-METCa ([Ca2+] in water = 1.36 ± 0.27 g/L). The calcium salt of 10-MDP (10-MDPCa) was the least soluble in water ([Ca2+] in water = 6.79 ± 0.43 mg/L).
TEM revealed that all 3 self-etch adhesives formed a shallow (0.5–1 µm) hybrid layer (Fig. 3
High-resolution microscopic analysis showed that all 3 self-etch adhesives hybridized dentin, indicating that their bonding mechanism depended, at least in part, on micro-mechanical interlocking of resin into a micro-porous surface (Nakayabashi et al., 1982; Nakabayashi and Saimi, 1996). In contrast to etch-and-rinse adhesives that involve phosphoric-acid etching, the self-etch adhesives demineralized dentin only partially, leaving hydroxyapatite partially attached to collagen. Since hydroxyapatite remains available for interaction, the main objective of this study was to characterize any potential chemical interaction between the functional monomers and this residual hydroxyapatite. Therefore, we used synthetic hydroxyapatite to exclude any influence from other organic tooth components that would have made unambiguous interpretation of the XPS spectra impossible. We selected 1 carboxyl- and 2 phosphate-based monomers that constitute the functional monomers of 3 representative two-step self-etch adhesives (Van Meerbeek et al., 2003). We used XPS to investigate interfacial interaction of adhesive molecules with hydroxyapatite (Yoshida et al., 2000, 2001); it showed that all 3 functional monomers interacted differently with hydroxyapatite. As can be derived theoretically from the chemical formula of 4-MET, the ratio of C-C/C-H/C=C bindings to C-O bindings and to -COO- bindings is 9 to 2 to 4. Peak area analysis (n = 13) of the peak that appeared when 4-MET interacted with hydroxyapatite revealed areas of, respectively, 9.1 ± 0.2 for the C-C/C-H/C=C peak and 2.0 ± 0.0 for the C-O peak, when the area of the -COO- bindings was taken as 4. The theoretical and measured peak ratios are alike, thus indicating that the recorded C 1s peak represented 4-MET that remained attached to hydroxyapatite. This C 1s peak cannot represent any carbon contamination, since it would have resulted in totally different peak area ratios. Previously, we demonstrated—for polyalkenoic acids (Yoshida et al., 2000) as well as for some mono-, di-, and tri-carboxylic acids (Yoshida et al., 2001)—that the shift of the carboxyl peak to a lower binding energy could be explained by the formation of an ionic bond between the carboxyl groups and the Ca of hydroxyapatite. Likewise, the peak shift resulting from the interaction of 4-MET with hydroxyapatite indicates that the carboxyl groups formed an ionic bond with Ca of hydroxyapatite. While a short 30-second exposure resulted in rather weak interaction intensity, 4-MET interacted more intensely with hydroxyapatite after a 30-minute exposure.
The interaction of phenyl-P with hydroxyapatite cannot be explained by simple bonding to hydroxyapatite. As can theoretically be derived from the chemical formula of phenyl-P, the ratio of C-C/C-H/C=C bindings to C-O bindings and to -COO- bindings is 8 to 3 to 1. Unreacted phenyl-P (Fig. 2b
As can theoretically be derived from the chemical formula of 10-MDP, the ratio of C-C/C-H/C=C bindings to C-O bindings and to -COO- bindings is 11 to 2 to 1. Peak area analysis (n = 11) of the peak that appeared upon interaction of 10-MDP with hydroxyapatite (Fig. 2c When we compared the chemical bonding efficacy of the 3 functional monomers investigated, quantitative determination of carbon concentration on hydroxyapatite revealed that the bonding potential of 10-MDP to hydroxyapatite is significantly stronger than that of 4-MET, even after a 30-minute exposure. Moreover, the fact that even a short 30-second exposure of hydroxyapatite to 10-MDP produced a significant C 1s peak, and that the peak intensity did not increase further when the exposure time was extended, strongly suggests that 10-MDP has a high chemical bonding potential to hydroxyapatite within a clinically reasonable application time. The chemical bonding capacity of 4-MET is weaker, and it is doubtful that 4-MET, even within a short application time, is capable of chemically bonding to hydroxyapatite. This confirms the rather weak adsorption of 4-META from ethanol and dichloromethane onto synthetic hydroxyapatite reported in a study by Misra (1989). The chemical bonding efficacy of phenyl-P is clearly the lowest among the 3 functional monomers investigated. However, chemical bonding potential on its own is insufficient to contribute to bonding performance. The ionic bindings formed should be stable in an aqueous environment. XPS showed that chemical bonding induced, in particular, by 10-MDP, even after a short 30-second application, resisted ultrasonic cleaning. In addition, AAS revealed that the calcium salt of 10-MDP was hardly soluble. The calcium salt of 4-MET could be dissolved more easily, whereas phenyl-P was very soluble in water. These data conform to the adhesion-decalcification (AD) concept previously presented (Yoshida et al., 2001; Yoshioka et al., 2001). According to the AD-concept, the less soluble the calcium salt of the acidic molecule, the more intense and stable the molecular adhesion to a hydroxyapatite-based substrate. The higher and more stable bonding performance of 10-MDP vs. that of 4-MET is also reflected in the higher micro-tensile strength to dentin of the 10-MDP-based Clearfil SE (Kuraray) compared with that of the 4-MET-based Unifil Bond (GC) (Inoue et al., 2001). In addition to increased bond strength, improved monomer-dentinal tissue interaction may also enhance sealing potential for the prevention of nano-leakage (Sano et al., 1995), and thus extend bonding longevity (Sano et al., 1999). In conclusion, besides self-etching dentin, some functional monomers may, within a clinically reasonable time, interact chemically with hydroxyapatite. Hydroxyapatite, however, needs to remain available at the partially demineralized dentin surface. The specific molecular formula of the functional monomer and the subsequent dissolution rate of its calcium salt determine actual bonding efficacy and stability.
This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan and by funds from the Uehara Memorial Foundation, The Nakatomi Foundation, and the Toshio Nakao Chair for Adhesive Dentistry, inaugurated at the Catholic University of Leuven (B. Van Meerbeek and P. Lambrechts, Chairholders). We thank GC and Kuraray for providing the monomers and calcium salts. We also thank J. Yearn (GC Europe) for critically reviewing the manuscript. Received for publication January 21, 2003. Revision received April 7, 2004. Accepted for publication April 14, 2004.
Journal of Dental Research, Vol. 83, No. 6,
454-458 (2004) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
monochromatic x-ray with a source power of 150 W. Wide and narrow scans were measured at a pass energy of, respectively, 80 and 40 eV. Quantitative data were obtained from peak areas, and identification of chemical states was made from detailed measurement of peak positions and separations. Significant differences in adhesive performance among the functional monomers were analyzed by Student’s t test (
