Photomedicine and Laser Surgery Volume 33, Number 1, 2015 ª Mary Ann Liebert, Inc. Pp. 47–52 DOI: 10.1089/pho.2014.3813
Resin–Dentin Bonding Interface After Photochemical Surface Treatment Ken-ichi Tonami, DDS, PhD,1 Kazunobu Sano, DDS, PhD,2 Shizuko Ichinose, PhD, 3 and Kouji Araki, DDS, PhD 2
Abstract
Objective: The aim of this study is to elucidate the structure of the resin–dentin interface formed by photochemical dentin treatment using an argon fluoride (ArF) excimer laser. Background data: The ArF excimer laser processes material by photochemical reaction without generating heat, while also providing surface conditioning that enhances material adhesion. In the case of bonding between resin and dentin, we demonstrated in a previous study that laser etching using an ArF excimer laser produced bonding strength comparable to that of the traditional bonding process; however, conditions of the bonding interface have not been fully investigated. Methods: A dentin surface was irradiated in air with an ArF excimer laser followed by bonding treatment. Cross sections were observed under light microscope, transmission electron microscope (TEM), and scanning electron microscope, then analyzed using an energy dispersive X-ray spectroscope (EDS): EDS line profiles of the elements C, O, Si, Cl, P, and Ca at the resin–dentin interface were obtained. Results: The density of C in resin decreased as it approached the interface, reaching its lowest level within the dentin at a depth of 2 lm from the resin–dentin interface on EDS. There was no hybrid layer observed at the interface on TEM. Therefore, it was suggested that the resin monomer infiltrated into the microspaces produced on the dentin surface by laser abrasion. Conclusions: The monomer infiltration without hybrid layer is thought to be the adhesion mechanism after laser etching. Therefore, the photochemical processes at the bonding interface achieved using the ArF excimer laser has great potential to be developed into a new bonding system in dentistry.
strength.1,3 Although the hybrid layer contributes to bonding strength between resin and dentin, matrix metalloproteinase (MMP)-induced degradation of collagen within the layer was identified as a factor that shortens the clinical durability of the restoration.1,3–7 Therefore, other types of bonding interface should be investigated. The argon fluoride (ArF) excimer laser processes material by photochemical reaction and has an ultraviolet (UV) wavelength of 193 nm that can irradiate an area with sufficient energy to break covalent bonds such as C = C, O-O, C-H, C-C and C-N without generating heat.8 This nonthermal processing is useful for precision cutting without heat damage to the processed material. Practical applications include corneal surgery in ophthalmology.9 In industry, the ArF excimer laser is also available for surface modification to improve bonding properties of various materials. Many studies have reported that the increased surface energy of polymer following UV laser irradiation results in enhanced adhesion of metal or polymer films.10–15
Introduction
I
n recent years, adhesive restoration has prevailed in dentistry because of improvements in the bonding system. In current procedures to achieve bonding between resin and dentin, it is essential to form an interpenetrating network of polymer and dentinal collagen at the bonding interface; namely, a hybrid layer.1,2 This hybrid layer is formed by a three step chemical surface treatment composed of etching, priming, and bonding.1,2 In etching treatment, an acidic liquid removes minerals at the dentin surface and exposes the matrix collagen network. In priming treatment, a hydrophilic monomer is infused into the collagen network. In bonding treatment, water in the collagen network is replaced with a hydrophobic monomer followed by polymerizing. An acidic monomer, called ‘‘self-etching primer,’’ is frequently used, because it allows one step etching and priming treatment to avoid collagen network collapse caused by drying, thus improving bonding 1 2 3
Oral Diagnosis and General Dentistry, Dental Hospital, Tokyo Medical and Dental University, Tokyo, Japan. Educational System in Dentistry, Tokyo Medical and Dental University, Tokyo, Japan. Research Center for Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
47
48
In dentistry, the ArF excimer laser has been studied, but has not yet been used in clinical practice.16 We previously investigated the potential of this laser for clinical applications in dentistry, especially for laser etching in adhesive restoration. In a previous study by one of the authors of this article, increased surface energy of irradiated dentin was demonstrated by measuring surface wettability.17 Moreover, the authors of this study also reported that tensile bond strength between dentin and composite resin is identical to that of the conventional self-etching adhesive system when ArF excimer irradiation replaces etching and priming treatment, even though there is no hybrid layer.18 Therefore, we hypothesized that an effective adhesive structure at the resin–dentin interface is formed by ArF excimer laser treatment. To elucidate this hypothesis, the present study examined the resin–dentin interface formed by ArF excimer irradiation followed by bonding treatment using a light microscope (LM), a transmission electron microscope (TEM), a scanning electron microscope (SEM), and an energy dispersive X-ray spectroscope (EDS). Material and Methods Laser apparatus and bonding chemical
An EX5 Excimer Laser (GAM Laser, Orlando, FL) was used in this study. The beam size was 6 · 3 mm, the wavelength was 193 nm, and pulse length was 10 ns. Self-etching primer agent and bonding agent (Kuraray Noritake Dental, Clearfil Mega Bond Primer and Bond, Tokyo, Japan) were used as pretreatment chemicals. The manufacturer’s information indicates that the primer agent (pH 2.5) contains 2-hydroxylethyl methacrylate (HEMA) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP) as resin monomers along with water, a polymeric photoinitiator, and a coloring agent. The bonding agent contains the monomers bisphenol A-glycidyl methacrylate (BisGMA), MDP, and HEMA, along with a polymeric photoinitiator and a silica-based microfiller.
TONAMI ET AL.
sions or restorations. The tooth was sectioned perpendicular to its long axis using a diamond saw (Isomet 1000, Buehler, IL) to expose a flat dentin surface. Both surfaces were polished with #1000 wet abrasive papers and cleaned by ultrasonic washing for 30 sec. One dentin surface was conditioned by priming and bonding (PB) treatment (n = 1). The other dentin surface was irradiated in air with an ArF excimer laser followed by bonding treatment (LB) (n = 1). LM, TEM, and SEM observation and EDS analysis
After surface treatment, the specimens were prefixed in 2.5% glutaraldehyde, washed with 0.1M phosphate-buffered saline (PBS) for 2 h, postfixed with 1% OsO4, buffered with 0.1 M PBS for 2h, dehydrated with 50–100% alcohol, and embedded in epoxy resin (Epon 812, TAAB Laboratories, England, UK). Semithin (1 lm) sections were cut from the treated dentin using an ultracut S microtome (Reichert, Vienna, Austria), collected on glass slides and stained for 30 sec with toluidine blue, then observed by LM. The treated dentin surfaces were also cut in sections 80–90 nm thick using the ultracut S microtome, and double stained with uranyl acetate and lead citrate for TEM observation. Crosssections were observed by TEM (H-7100, Hitachi, Hitachinaka, Japan) with an acceleration voltage of 75 kV. Next, the TEM specimens were spatter-coated with osmium using NL-OPC80N (Filgen, Nagoya, Japan) and examined by SEM (S-4500, Hitachi, Hitachinaka, Japan) with an
Conditions of surface treatment
In the present study, the parameters of ArF laser irradiation were fixed at irradiation time 10 sec, pulse repetition rate 20 pps, and emission voltage 15 kV. The laser beam was focused on the dentin surface using a condenser lens (focal distance 30 cm, PCX-25-U-152.6, Lambda Research, Littleton, USA). The distance between lens and target was fixed at 30 cm. In the pilot study, the energy per pulse was measured, yielding an energy density per pulse of 217 mJ/cm2. The conditions were the same as those in our previous report.18 For etching and priming treatment, a self-etching primer agent was applied to the dentin surface for 20 sec. For bonding treatment, the bonding agent was applied on the dentin surface and polymerized by illumination with a halogen lamp (Tokuyama, Tokuso Power Lite Tokyo, Japan) for 10 sec. Preparation of dentin specimens
A single human molar that had been put in distilled water immediately after extraction and stored at 4C for < 1 year was used for the experiment. The tooth had no carious le-
FIG. 1. Transmission electron microscope (TEM) (x5000) and light microscope (LM) (x1000, left bottom inset) image of priming and bonding (PB). R, bonding resin; D, dentin. In the TEM image, filler particles were dispersed homogeneously in the resin. A 1–2 lm thick hybrid layer (H) was observed between the bonding resin and dentin and collagen bundle continuously ran from dentin to hybrid layer. In the LM image, the bonding resin was stained blue. The hybrid layer shows violet staining (h). Dentinal tubules were filled with bonding resin (t).
BONDING INTERFACE AFTER ArF EXCIMER LASER TREATMENT
49
acceleration voltage of 15 kV. Finally, the cross-sections were analyzed under an energy dispersive X-ray spectroscope (EMAX-7000, Horiba, Kyoto, Japan). Acquisition time of the EDS spectrum was 100 sec at 15 kV of acceleration voltage and 0.1 A of beam current. The EDS spectrum was quantitatively assessed according to the intensity of each spectrum. Overlapping lines were separated by the overlapping factor method, and quantitative correction was made after the line separation employing the standardless method.19 Then the density profile of the elements, C, O, Si, Cl, P, and Ca at the resin–dentin interface was obtained. Ethics
This study was approved by the Ethics Committee of Tokyo Medical and Dental University (No. 927). Results TEM and LM observation
TEM and LM image of PB is shown in Fig. 1. A 2 lm hybrid layer was observed between the bonding resin and dentin on both TEM and LM images. Bonding resin was observed in dentinal tubules on LM image. On TEM image, collagen bundles ran continuously from dentin to the hybrid
FIG. 2. Transmission electron microscope (TEM) (x5000) and light microscope (LM) (x1000, left bottom inset) image of laser followed by bonding treatment (LB). E, embedding epoxy resin; R, bonding resin; D, dentin. In the TEM image, no hybrid layer was observed. Instead, there was a filler sparse zone (fs) adjacent to the dentin surface in the bonding resin. Crack-like spaces (white arrowheads) were observed at subsurface of the dentin. In the bonding resin at the interface, multilocular phase (black arrowheads) were observed. The filler particles were dense in the epoxy resin close to the bonding resin (fd). In the LM image, a violetstained substance (v) was observed at area corresponding to the filler sparse zone on the TEM image, and was divided into small parts.
FIG. 3. Top: Scanning electron microscopic (SEM) image of priming and bonding (PB). E, embedding epoxy resin; R, bonding resin; D, dentin. Bottom: Linear analysis of the area indicated by the horizontal red line in the top photograph using energy dispersive X-ray spectroscope (EDS). The heights of C (blue line), O (red line), Si (green line), P (pink line), Cl (light blue line), and Ca (yellow line) indicate the distribution and relative quantity of each element at the interface between the bonding resin and dentin. In the SEM image, the bonding resin layer was observed as being 15 lm thick. In the EDS profile, the density of C in the bonding resin decreased as it approached the dentin and reached the baseline at the area corresponding to the hybrid layer (H).
50
TONAMI ET AL.
layer and filler particles were dispersed homogeneously in the bonding resin. TEM and LM image of LB is shown in Fig. 2. On TEM image, the filler particles were sparse at the resin–dentin interface in bonding resin, increased at the middle of the bonding resin layer, and were dense at the outer edge of the bonding resin. There was no detectable hybrid layer. There were crack-like spaces at a depth of 1 lm in the dentin subsurface but there were no collagen bundles observed in the subsurface dentin. Multilocular phases were observed on the surface of the dentin. On the LM image, a violet-stained granular substance was observed at the area corresponding to the filler-sparse zone on the TEM image. There was no bonding resin observed in dentinal tubules on the LM image. SEM observation and EDS analysis
Figures 3 and 4 show the SEM image and EDS profile of PB and LB, respectively. On SEM image, the bonding-resin layer was 15 lm thick for PB and 7 lm thick for LB, corresponding to elevated C on EDS. The layer of embedding epoxy resin on SEM corresponded to the Cl rise on EDS. For PB, the resin–dentin interface corresponded with the beginning of the simultaneous rise of P and Ca on the EDS profile. The O concentration rose at the same location as P and Ca. There was no hybrid layer observed on the SEM image. The density of C in the bonding resin decreased as it approached the dentin, and reached its lowest level before the resin–dentin interface. The location corresponded to the hybrid layer estimated on the TEM image. The Si profile on EDS showed a peak 2 lm from the resin–dentin interface, and then decreased, reaching its lowest level at the interface. For LB, the resin–dentin interface on the SEM image corresponded with the simultaneous rise of P and Ca on EDS, although the rate was less steep than that of PB. The profile of O rose at the same location as P and Ca. The density of C in resin decreased as it approached the dentin; however, it crossed below the rising P and Ca levels and reaching its lowest point at a 2 lm depth within the dentin. The Si density decreased as it approached dentin, reaching its lowest level at the resin–dentin interface and reaching its maximum density at the outer edge of the bonding resin, which corresponded to the bright layer in embedding epoxy resin on the SEM image. Discussion
FIG. 4. Top: Scanning electron microscopic (SEM) image of laser followed by bonding treatment (LB). E, embedding epoxy resin; R, bonding resin; D, dentin. Bottom: Linear analysis of the area indicated by the horizontal red line in the top photograph using energy dispersive X-ray spectroscope (EDS). In the SEM image, the bonding resin layer was observed as being 7 lm thick. In the EDS profile, the density of C in resin decreased as it approached the dentin, crossing below the rising P and Ca levels and reaching its lowest point at a 2 lm depth within the dentin, suggesting monomer infiltration into the dentin (I). Compared with priming and bonding (PB), the rise of P and Ca was less steep. Si indicated maximum density at the outer edge of the bonding resin, which corresponded to the bright layer in embedding epoxy resin on the SEM image.
On TEM and LM images of PB, the hybrid layer was observed at the resin–dentin bonding interface, which is congruent with previous findings.2–7,20 However, there was no hybrid layer detected on the SEM image of LB. For examination of the hybrid layer using SEM and TEM, pretreatment such as etching or decalcification is often employed to make the microstructure more prominent by changing the morphology2,6,7,21,22. In the present study, such pretreatment was not performed because a flat surface is necessary to obtain precise EDS results. Using the TEM image, the location of the hybrid layer on the SEM image and on the EDS profile of PB was estimated as the area 2 lm wide in the bonding resin at the resin–dentin boundary. In the hybrid layer, carbon in the EDS profile decreased as it approached the resin–dentin interface and reached its lowest level at the interface in the hybrid layer. Silicon
BONDING INTERFACE AFTER ArF EXCIMER LASER TREATMENT
demonstrated a peak at 2 lm from the resin–dentin interface and decreased in the hybrid layer as it approached the dentin. Carbon and silicon were, respectively, derived from cured resin monomer and filler in the bonding resin. Therefore, the result indicated that the bonding resin diffused into the decalcified dentin and diminished with depth to form the hybrid layer. On the TEM image of LB, there was no apparent hybrid layer in the laser-treated dentin, and the result coincides with our previous report.18 Instead, the EDS profile demonstrated increased carbon at the irradiated dentin surface extending to a 2 lm depth from the bonding interface. The ArF excimer laser breaks covalent bonds on an irradiated surface.8,23,24 Therefore, there is a possibility that resin monomer infiltrated into the molecular order dentinal defects produced by laser irradiation, which increased the relative amount of carbon at the dentin subsurface. According to Lambert–Beer’s law, the intensity of laser energy in the irradiated material decreases linearly with the length of the path. Therefore, the dentinal spaces produced by laser irradiation decreased with depth from the surface, corresponding to the linear change in carbon on the EDS profile of LB. Silicon did not infiltrate into dentin because the defects were smaller than the filler size. A violet-stained granular substance was observed in bonding resin close to the dentin surface on the LM image of LB. Under atmospheric pressure, abraded species easily form clusters and become reattached as debris on the irradiated surface after photochemical laser abrasion.23 Therefore, the origin of the violet-stained substance may be decomposed dentin removed by ArF excimer laser irradiation. Corresponding areas on the TEM image appeared transparent, and a high carbon concentration was detected on the EDS profile. According to the result, the main component of the debris was considered to be organic material. Therefore, there is a possibility that ArF excimer laser irradiation preferably decomposed the organic phase in dentin. The bonding resin layer in LB was thinner than that in PB. One reason is that the increased surface wettability of the irradiated dentin surface17 would allow the bonding resin to spread more easily on the surface, resulting in a thinner layer. Another reason is that resin infiltration into the irradiated dentin would decrease the volume of the bonding resin remaining on the surface. Prominent dense filler particles at the outer edge of the bonding resin were observed on the TEM image of LB, and were considered to be the result of a decrease in bonding resin surrounding the filler particles. Additionally, debris on the irradiated surface would push filler particles out of the bonding resin. Studies in polymers such as polyether ether ketone, polycarbonate, and nylon reported that the increase in surface wettability following ArF excimer laser irradiation was caused by surface oxidation.11,25 However, the present study did not detect changes in oxygen concentration on the surface of the irradiated dentin. Compared with the change in phosphorus or calcium, the change in oxygen at the resinbonding interface might have been too limited to be detected by EDS in this study. In a previous study, we demonstrated that the bonding strength and standard deviation between dentin and composite resin after dentin surface treatment in LB were 11.6 and 4.4 MPa, whereas those in PB were 13.0 and 3.3 MPa,
51
and the difference was not statistically significant18; therefore, ArF excimer laser etching is as effective as conventional acid etching. From the results of the present study, we considered that the bonding strength of the lased condition depended upon the 2 lm depth of monomer infiltration into dentin at the bonding interface. The structure of infiltration was similar to that of the hybrid layer from the point that resin and dentin were combined together, and the proportions changed gradually. On the other hand, under the lased condition, the amount of hydroxyapatite indicated by phosphorus and calcium in the monomer infiltration was greater than that in the conventional hybrid layer. The reason for the difference would be that laser etching removes mainly the organic phase in dentin, whereas acid etching removes hydroxyapatite. It is suggested that the greater amount of hydroxyl apatite remaining at the bonding boundary prevents MMP-induced degradation of the collagen. Therefore, the laser etching would be intrinsically more advantageous to clinical durability than acid etching. However, the debris on the irradiated surface observed in the present study may be a disadvantage for bonding, because it inhibits resin tag formation by obstructing the opening of dentinal tubules on the dentin surface. Additionally, the organic debris might also be a reason for nanoleakage in the same manner that resin-impregnated smear layer formed in conventional self-etching procedure would cause nanoleakage.26–31 Because it is known that such debris can be controlled by modifying atmospheric conditions at the irradiation field,23 the quality of bonding strength following ArF excimer laser treatment might be further improved under decompression. In order to clarify the correspondence among findings, the findings on TEM, SEM, and EDS in this study were obtained from the same section of one specimen. As a result, the analyses were mostly qualitative rather than quantitative, which is a limitation of the study. A quantitative study of monomer infiltration into the dentin after ArF excimer laser irradiation should be performed in the near future. Conclusions
Within the limitation of qualitative analysis, the results of this in vitro study showed that increased carbon on the dentin surface after ArF excimer laser irradiation extended to a 2 lm depth from the bonding interface. Therefore, it was suggested that the resin monomer infiltrated into the microspaces produced on the dentin surface by laser abrasion. This infiltration is thought to be the adhesion mechanism after laser etching. Therefore, the photochemical processes at the bonding interface achieved using the ArF excimer laser have great potential to be developed into a new bonding system in dentistry. Acknowledgments
This research was funded by the Japan Society for the Promotion of Science ( JSPS) and Grants-in-Aid for Scientific Research (KAKENHI, A; 25253100). Author Disclosure Statement
No competing financial interests exist.
52 References
1. Rawls HR, Teixeira EC. Bonding and bonding agent. In: Phillips’ Dental Materials, 12th ed. KJ Anusavice, C Shen, HR Rawls (eds.). Philadelphia: Elsevier Sanders, 2013; pp. 257–274. 2. Van Meerbeek B, Dhem A, Goret–Nicaise M, Braem M, et al. Comparative SEM and TEM examination of the ultrastructure of the resin–dentin interdiffusion zone. J Dent Res 1993;72:495–501. 3. Van Meerbeek B, Yoshihara K, Yoshida Y, Mine A, et al. State of the art of self-etch adhesives. Dent Mater 2011;27: 17–28. 4. Liu Y, Tja¨derhane L, Breschi L, Mazzoni A, Li N, et al. Limitations in bonding to dentin and experimental strategies to prevent bond degradation. J Dent Res 2011;90: 953–968. 5. Breschi L, Mazzoni M, Ruggeri A, Cadenaro M, et al. Dental adhesion review: aging and stability of the bonded interface. Dent Mater 2008;24:90–101. 6. Hebling J, Pashley DH, Tjaderhane L, Tay FR. Chlorhexidine arrests subclinical degradationof dentin hybrid layers in vivo. J Dent Res 2005;84:741–746. 7. Koshiro K, Inoue S, Sano H, De Munck J, et al. In vivo degradation of resin–dentin bonds produced by a self-etch and an etch-and-rinse adhesive. Eur Oral Sci 2005;113, 341–348. 8. Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science 1986;234:559–565. 9. Reynolds A, Moore JE, Naroo SA, Moore CT, et al. Excimer laser surface ablation – a review. Clin Experiment Ophthalmol 2010;38:168–182. 10. Bagheri–Khoulenjani S, Mirzadeh H. Polystyrene surface modification using excimer laser and radio-frequency plasma: blood compatibility evaluations. Progress in Biomaterials 201;1:4. Available at: http://www.progressbiomaterials .com/content/1/1/4 (Last accessed January 28, 2014). 11. Laurens P, Bouali MO, Meducin F, Sadras B. Characterization of modifications of polymer surfaces after excimer laser treatments below the ablation threshold. Appl Surf Sci 2000;154/155:211–216. 12. Re´ve´sz K, Hopp B, Bor Z. Excimer laser induced surface chemical modification of polytetrafluoroethylene. Appl Surf Sci 1997;109/110:222–226. 13. Niino H, Okano H, Inui K, Yabe A. Surface modification of poly (tetrafluoroethylene) by excimer laser processing: enhancement of adhesion. Appl Surf Sci 1997;109/110: 259–263. 14. Weichenhain R, Wesner DA, Pfleging W, Horn H, et al. KrF-excimer laser pretreatment and metallization of polymers. Appl Surf Sci 1997;109/110:264–269. 15. Heitz J, Arenholz E, Kefer T, Baurle D, et al. Enhanced adhesion of metal films on PET after UV-laser treatment. Appl Phys A 1992;55:391–392. 16. Frentzen M, Koort HJ, Thiensiri I. Excimer lasers in dentistry: future possibilities with advanced technology. Quintessence Int 1992;23:117–133. 17. Ishida T, Tonam IK, Arak, IK, Kurosaki N. Properties of human dentin surface after ArF excimer laser irradiation. J Med Dent Sci 2008;55:155–161. 18. Sano K, Tonami K, Ichinose S, Araki K. Effects of ArF excimer laser irradiation of dentin on the tensile bonding strength to composite resin. Photomed Laser Surg 2012;30:71–76.
TONAMI ET AL.
19. Ichinose S, Muneta T, Sekiya I, Itoh S, et al. The study of metal ion release and cytotoxicity in Co-Cr-Mo and Ti-Al-V alloy in total knee prosthesis – scanning electron microscopic observation. J Mater Sci Mater Med 2003;14: 79–86. 20. Thanatvarakorn O, Nakajima M, Prasansuttiporn T, Ichinose S, et al. Effect of smear layer deproteinizing on resin– dentine interface with self-etch adhesive. J Dent 2014;42: 298–304. 21. Moretto SG, Azambuja N. Jr, Arana–Chavez VE, Reis AF, et al. Effects of ultramorphological changes on adhesion to lased dentin — Scanning electron microscopy and transmission electron microscopy analysis. Microsc Res Tech 2011;74:720–726. 22. Nikaido T, Weerasinghe DDS, Waidyasekera K, Inoue G, et al. Assessment of the nanostructure of acid-base resistant zone by the application of all-in-one adhesive systems: Super dentin formation. Biomed Mater Eng 2009;19: 163–171. 23. Ba¨uerle D. Nanosecond-laser ablation. In: Laser Processing and Chemistry. 4th ed. New York: Springer-Verlag, 2011; pp. 237–278. 24. Gillen D. Enhancing UV laser machining of medical grade polymers. Laser User 2009;54:27–29. 25. Yip J, Chan K, Sin KM, Lau KS. Study on the surface chemical properties of UV excimer laser irradiated polyamide by XPS, ToF-SIMS and CFM. Appl Surf Sci 2003; 205:151–159. 26. Mine A, De Munck J, Cardoso MV, Van Landuyt KL, et al. Dentin-smear remains at self-etch adhesive interface. Dent Mater 2014;30:1147–1153. 27. Pinzon LM, Watanabe LG, Reis AF, Power JM, et al. Analysis of interfacial structure and bond strength of selfetch adhesives. Am J Dent 2013;26:335–340. 28. Reis AF, Giannini M, Pereira PNR. Long-term TEM analysis of the nanoleakage patterns in resin–dentin interfaces produced by different bonding strategies. Dent Mater 200;23:1164–1172. 29. Suppa P, Breschi L, Ruggeri A, Mazzotti G, et al. Nanoleakage within the hybrid layer: A correlative FEISEM/ TEM investigation. J Biomed Mater Res Part B: Appl Biomater 2005;73:7–14. 30. Carvalho RM, Chersoni S, Frankenberger R, Pashley DH, et al. A challenge to the conventional wisdom that simultaneous etching and resin infiltration always occurs in selfetch adhesives. Biomaterials 2005;26:1035–1042. 31. Inoue S, Koshiro K, Yoshida Y, De Munck J, et al. Hydrolytic stability of self-etch adhesives bonded to dentin. J Dent Res 2005;84:1160–1164.
Address correspondence to: Ken-ichi Tonami Oral Diagnosis and General Dentistry Dental Hospital Tokyo Medical and Dental University 1-5-45 Yushima Bunkyo-ku Tokyo, 113-8549 Japan E-mail: [email protected]
Resin–Dentin Bonding Interface After Photochemical Surface Treatment Ken-ichi Tonami, DDS, PhD,1 Kazunobu Sano, DDS, PhD,2 Shizuko Ichinose, PhD, 3 and Kouji Araki, DDS, PhD 2
Abstract
Objective: The aim of this study is to elucidate the structure of the resin–dentin interface formed by photochemical dentin treatment using an argon fluoride (ArF) excimer laser. Background data: The ArF excimer laser processes material by photochemical reaction without generating heat, while also providing surface conditioning that enhances material adhesion. In the case of bonding between resin and dentin, we demonstrated in a previous study that laser etching using an ArF excimer laser produced bonding strength comparable to that of the traditional bonding process; however, conditions of the bonding interface have not been fully investigated. Methods: A dentin surface was irradiated in air with an ArF excimer laser followed by bonding treatment. Cross sections were observed under light microscope, transmission electron microscope (TEM), and scanning electron microscope, then analyzed using an energy dispersive X-ray spectroscope (EDS): EDS line profiles of the elements C, O, Si, Cl, P, and Ca at the resin–dentin interface were obtained. Results: The density of C in resin decreased as it approached the interface, reaching its lowest level within the dentin at a depth of 2 lm from the resin–dentin interface on EDS. There was no hybrid layer observed at the interface on TEM. Therefore, it was suggested that the resin monomer infiltrated into the microspaces produced on the dentin surface by laser abrasion. Conclusions: The monomer infiltration without hybrid layer is thought to be the adhesion mechanism after laser etching. Therefore, the photochemical processes at the bonding interface achieved using the ArF excimer laser has great potential to be developed into a new bonding system in dentistry.
strength.1,3 Although the hybrid layer contributes to bonding strength between resin and dentin, matrix metalloproteinase (MMP)-induced degradation of collagen within the layer was identified as a factor that shortens the clinical durability of the restoration.1,3–7 Therefore, other types of bonding interface should be investigated. The argon fluoride (ArF) excimer laser processes material by photochemical reaction and has an ultraviolet (UV) wavelength of 193 nm that can irradiate an area with sufficient energy to break covalent bonds such as C = C, O-O, C-H, C-C and C-N without generating heat.8 This nonthermal processing is useful for precision cutting without heat damage to the processed material. Practical applications include corneal surgery in ophthalmology.9 In industry, the ArF excimer laser is also available for surface modification to improve bonding properties of various materials. Many studies have reported that the increased surface energy of polymer following UV laser irradiation results in enhanced adhesion of metal or polymer films.10–15
Introduction
I
n recent years, adhesive restoration has prevailed in dentistry because of improvements in the bonding system. In current procedures to achieve bonding between resin and dentin, it is essential to form an interpenetrating network of polymer and dentinal collagen at the bonding interface; namely, a hybrid layer.1,2 This hybrid layer is formed by a three step chemical surface treatment composed of etching, priming, and bonding.1,2 In etching treatment, an acidic liquid removes minerals at the dentin surface and exposes the matrix collagen network. In priming treatment, a hydrophilic monomer is infused into the collagen network. In bonding treatment, water in the collagen network is replaced with a hydrophobic monomer followed by polymerizing. An acidic monomer, called ‘‘self-etching primer,’’ is frequently used, because it allows one step etching and priming treatment to avoid collagen network collapse caused by drying, thus improving bonding 1 2 3
Oral Diagnosis and General Dentistry, Dental Hospital, Tokyo Medical and Dental University, Tokyo, Japan. Educational System in Dentistry, Tokyo Medical and Dental University, Tokyo, Japan. Research Center for Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
47
48
In dentistry, the ArF excimer laser has been studied, but has not yet been used in clinical practice.16 We previously investigated the potential of this laser for clinical applications in dentistry, especially for laser etching in adhesive restoration. In a previous study by one of the authors of this article, increased surface energy of irradiated dentin was demonstrated by measuring surface wettability.17 Moreover, the authors of this study also reported that tensile bond strength between dentin and composite resin is identical to that of the conventional self-etching adhesive system when ArF excimer irradiation replaces etching and priming treatment, even though there is no hybrid layer.18 Therefore, we hypothesized that an effective adhesive structure at the resin–dentin interface is formed by ArF excimer laser treatment. To elucidate this hypothesis, the present study examined the resin–dentin interface formed by ArF excimer irradiation followed by bonding treatment using a light microscope (LM), a transmission electron microscope (TEM), a scanning electron microscope (SEM), and an energy dispersive X-ray spectroscope (EDS). Material and Methods Laser apparatus and bonding chemical
An EX5 Excimer Laser (GAM Laser, Orlando, FL) was used in this study. The beam size was 6 · 3 mm, the wavelength was 193 nm, and pulse length was 10 ns. Self-etching primer agent and bonding agent (Kuraray Noritake Dental, Clearfil Mega Bond Primer and Bond, Tokyo, Japan) were used as pretreatment chemicals. The manufacturer’s information indicates that the primer agent (pH 2.5) contains 2-hydroxylethyl methacrylate (HEMA) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP) as resin monomers along with water, a polymeric photoinitiator, and a coloring agent. The bonding agent contains the monomers bisphenol A-glycidyl methacrylate (BisGMA), MDP, and HEMA, along with a polymeric photoinitiator and a silica-based microfiller.
TONAMI ET AL.
sions or restorations. The tooth was sectioned perpendicular to its long axis using a diamond saw (Isomet 1000, Buehler, IL) to expose a flat dentin surface. Both surfaces were polished with #1000 wet abrasive papers and cleaned by ultrasonic washing for 30 sec. One dentin surface was conditioned by priming and bonding (PB) treatment (n = 1). The other dentin surface was irradiated in air with an ArF excimer laser followed by bonding treatment (LB) (n = 1). LM, TEM, and SEM observation and EDS analysis
After surface treatment, the specimens were prefixed in 2.5% glutaraldehyde, washed with 0.1M phosphate-buffered saline (PBS) for 2 h, postfixed with 1% OsO4, buffered with 0.1 M PBS for 2h, dehydrated with 50–100% alcohol, and embedded in epoxy resin (Epon 812, TAAB Laboratories, England, UK). Semithin (1 lm) sections were cut from the treated dentin using an ultracut S microtome (Reichert, Vienna, Austria), collected on glass slides and stained for 30 sec with toluidine blue, then observed by LM. The treated dentin surfaces were also cut in sections 80–90 nm thick using the ultracut S microtome, and double stained with uranyl acetate and lead citrate for TEM observation. Crosssections were observed by TEM (H-7100, Hitachi, Hitachinaka, Japan) with an acceleration voltage of 75 kV. Next, the TEM specimens were spatter-coated with osmium using NL-OPC80N (Filgen, Nagoya, Japan) and examined by SEM (S-4500, Hitachi, Hitachinaka, Japan) with an
Conditions of surface treatment
In the present study, the parameters of ArF laser irradiation were fixed at irradiation time 10 sec, pulse repetition rate 20 pps, and emission voltage 15 kV. The laser beam was focused on the dentin surface using a condenser lens (focal distance 30 cm, PCX-25-U-152.6, Lambda Research, Littleton, USA). The distance between lens and target was fixed at 30 cm. In the pilot study, the energy per pulse was measured, yielding an energy density per pulse of 217 mJ/cm2. The conditions were the same as those in our previous report.18 For etching and priming treatment, a self-etching primer agent was applied to the dentin surface for 20 sec. For bonding treatment, the bonding agent was applied on the dentin surface and polymerized by illumination with a halogen lamp (Tokuyama, Tokuso Power Lite Tokyo, Japan) for 10 sec. Preparation of dentin specimens
A single human molar that had been put in distilled water immediately after extraction and stored at 4C for < 1 year was used for the experiment. The tooth had no carious le-
FIG. 1. Transmission electron microscope (TEM) (x5000) and light microscope (LM) (x1000, left bottom inset) image of priming and bonding (PB). R, bonding resin; D, dentin. In the TEM image, filler particles were dispersed homogeneously in the resin. A 1–2 lm thick hybrid layer (H) was observed between the bonding resin and dentin and collagen bundle continuously ran from dentin to hybrid layer. In the LM image, the bonding resin was stained blue. The hybrid layer shows violet staining (h). Dentinal tubules were filled with bonding resin (t).
BONDING INTERFACE AFTER ArF EXCIMER LASER TREATMENT
49
acceleration voltage of 15 kV. Finally, the cross-sections were analyzed under an energy dispersive X-ray spectroscope (EMAX-7000, Horiba, Kyoto, Japan). Acquisition time of the EDS spectrum was 100 sec at 15 kV of acceleration voltage and 0.1 A of beam current. The EDS spectrum was quantitatively assessed according to the intensity of each spectrum. Overlapping lines were separated by the overlapping factor method, and quantitative correction was made after the line separation employing the standardless method.19 Then the density profile of the elements, C, O, Si, Cl, P, and Ca at the resin–dentin interface was obtained. Ethics
This study was approved by the Ethics Committee of Tokyo Medical and Dental University (No. 927). Results TEM and LM observation
TEM and LM image of PB is shown in Fig. 1. A 2 lm hybrid layer was observed between the bonding resin and dentin on both TEM and LM images. Bonding resin was observed in dentinal tubules on LM image. On TEM image, collagen bundles ran continuously from dentin to the hybrid
FIG. 2. Transmission electron microscope (TEM) (x5000) and light microscope (LM) (x1000, left bottom inset) image of laser followed by bonding treatment (LB). E, embedding epoxy resin; R, bonding resin; D, dentin. In the TEM image, no hybrid layer was observed. Instead, there was a filler sparse zone (fs) adjacent to the dentin surface in the bonding resin. Crack-like spaces (white arrowheads) were observed at subsurface of the dentin. In the bonding resin at the interface, multilocular phase (black arrowheads) were observed. The filler particles were dense in the epoxy resin close to the bonding resin (fd). In the LM image, a violetstained substance (v) was observed at area corresponding to the filler sparse zone on the TEM image, and was divided into small parts.
FIG. 3. Top: Scanning electron microscopic (SEM) image of priming and bonding (PB). E, embedding epoxy resin; R, bonding resin; D, dentin. Bottom: Linear analysis of the area indicated by the horizontal red line in the top photograph using energy dispersive X-ray spectroscope (EDS). The heights of C (blue line), O (red line), Si (green line), P (pink line), Cl (light blue line), and Ca (yellow line) indicate the distribution and relative quantity of each element at the interface between the bonding resin and dentin. In the SEM image, the bonding resin layer was observed as being 15 lm thick. In the EDS profile, the density of C in the bonding resin decreased as it approached the dentin and reached the baseline at the area corresponding to the hybrid layer (H).
50
TONAMI ET AL.
layer and filler particles were dispersed homogeneously in the bonding resin. TEM and LM image of LB is shown in Fig. 2. On TEM image, the filler particles were sparse at the resin–dentin interface in bonding resin, increased at the middle of the bonding resin layer, and were dense at the outer edge of the bonding resin. There was no detectable hybrid layer. There were crack-like spaces at a depth of 1 lm in the dentin subsurface but there were no collagen bundles observed in the subsurface dentin. Multilocular phases were observed on the surface of the dentin. On the LM image, a violet-stained granular substance was observed at the area corresponding to the filler-sparse zone on the TEM image. There was no bonding resin observed in dentinal tubules on the LM image. SEM observation and EDS analysis
Figures 3 and 4 show the SEM image and EDS profile of PB and LB, respectively. On SEM image, the bonding-resin layer was 15 lm thick for PB and 7 lm thick for LB, corresponding to elevated C on EDS. The layer of embedding epoxy resin on SEM corresponded to the Cl rise on EDS. For PB, the resin–dentin interface corresponded with the beginning of the simultaneous rise of P and Ca on the EDS profile. The O concentration rose at the same location as P and Ca. There was no hybrid layer observed on the SEM image. The density of C in the bonding resin decreased as it approached the dentin, and reached its lowest level before the resin–dentin interface. The location corresponded to the hybrid layer estimated on the TEM image. The Si profile on EDS showed a peak 2 lm from the resin–dentin interface, and then decreased, reaching its lowest level at the interface. For LB, the resin–dentin interface on the SEM image corresponded with the simultaneous rise of P and Ca on EDS, although the rate was less steep than that of PB. The profile of O rose at the same location as P and Ca. The density of C in resin decreased as it approached the dentin; however, it crossed below the rising P and Ca levels and reaching its lowest point at a 2 lm depth within the dentin. The Si density decreased as it approached dentin, reaching its lowest level at the resin–dentin interface and reaching its maximum density at the outer edge of the bonding resin, which corresponded to the bright layer in embedding epoxy resin on the SEM image. Discussion
FIG. 4. Top: Scanning electron microscopic (SEM) image of laser followed by bonding treatment (LB). E, embedding epoxy resin; R, bonding resin; D, dentin. Bottom: Linear analysis of the area indicated by the horizontal red line in the top photograph using energy dispersive X-ray spectroscope (EDS). In the SEM image, the bonding resin layer was observed as being 7 lm thick. In the EDS profile, the density of C in resin decreased as it approached the dentin, crossing below the rising P and Ca levels and reaching its lowest point at a 2 lm depth within the dentin, suggesting monomer infiltration into the dentin (I). Compared with priming and bonding (PB), the rise of P and Ca was less steep. Si indicated maximum density at the outer edge of the bonding resin, which corresponded to the bright layer in embedding epoxy resin on the SEM image.
On TEM and LM images of PB, the hybrid layer was observed at the resin–dentin bonding interface, which is congruent with previous findings.2–7,20 However, there was no hybrid layer detected on the SEM image of LB. For examination of the hybrid layer using SEM and TEM, pretreatment such as etching or decalcification is often employed to make the microstructure more prominent by changing the morphology2,6,7,21,22. In the present study, such pretreatment was not performed because a flat surface is necessary to obtain precise EDS results. Using the TEM image, the location of the hybrid layer on the SEM image and on the EDS profile of PB was estimated as the area 2 lm wide in the bonding resin at the resin–dentin boundary. In the hybrid layer, carbon in the EDS profile decreased as it approached the resin–dentin interface and reached its lowest level at the interface in the hybrid layer. Silicon
BONDING INTERFACE AFTER ArF EXCIMER LASER TREATMENT
demonstrated a peak at 2 lm from the resin–dentin interface and decreased in the hybrid layer as it approached the dentin. Carbon and silicon were, respectively, derived from cured resin monomer and filler in the bonding resin. Therefore, the result indicated that the bonding resin diffused into the decalcified dentin and diminished with depth to form the hybrid layer. On the TEM image of LB, there was no apparent hybrid layer in the laser-treated dentin, and the result coincides with our previous report.18 Instead, the EDS profile demonstrated increased carbon at the irradiated dentin surface extending to a 2 lm depth from the bonding interface. The ArF excimer laser breaks covalent bonds on an irradiated surface.8,23,24 Therefore, there is a possibility that resin monomer infiltrated into the molecular order dentinal defects produced by laser irradiation, which increased the relative amount of carbon at the dentin subsurface. According to Lambert–Beer’s law, the intensity of laser energy in the irradiated material decreases linearly with the length of the path. Therefore, the dentinal spaces produced by laser irradiation decreased with depth from the surface, corresponding to the linear change in carbon on the EDS profile of LB. Silicon did not infiltrate into dentin because the defects were smaller than the filler size. A violet-stained granular substance was observed in bonding resin close to the dentin surface on the LM image of LB. Under atmospheric pressure, abraded species easily form clusters and become reattached as debris on the irradiated surface after photochemical laser abrasion.23 Therefore, the origin of the violet-stained substance may be decomposed dentin removed by ArF excimer laser irradiation. Corresponding areas on the TEM image appeared transparent, and a high carbon concentration was detected on the EDS profile. According to the result, the main component of the debris was considered to be organic material. Therefore, there is a possibility that ArF excimer laser irradiation preferably decomposed the organic phase in dentin. The bonding resin layer in LB was thinner than that in PB. One reason is that the increased surface wettability of the irradiated dentin surface17 would allow the bonding resin to spread more easily on the surface, resulting in a thinner layer. Another reason is that resin infiltration into the irradiated dentin would decrease the volume of the bonding resin remaining on the surface. Prominent dense filler particles at the outer edge of the bonding resin were observed on the TEM image of LB, and were considered to be the result of a decrease in bonding resin surrounding the filler particles. Additionally, debris on the irradiated surface would push filler particles out of the bonding resin. Studies in polymers such as polyether ether ketone, polycarbonate, and nylon reported that the increase in surface wettability following ArF excimer laser irradiation was caused by surface oxidation.11,25 However, the present study did not detect changes in oxygen concentration on the surface of the irradiated dentin. Compared with the change in phosphorus or calcium, the change in oxygen at the resinbonding interface might have been too limited to be detected by EDS in this study. In a previous study, we demonstrated that the bonding strength and standard deviation between dentin and composite resin after dentin surface treatment in LB were 11.6 and 4.4 MPa, whereas those in PB were 13.0 and 3.3 MPa,
51
and the difference was not statistically significant18; therefore, ArF excimer laser etching is as effective as conventional acid etching. From the results of the present study, we considered that the bonding strength of the lased condition depended upon the 2 lm depth of monomer infiltration into dentin at the bonding interface. The structure of infiltration was similar to that of the hybrid layer from the point that resin and dentin were combined together, and the proportions changed gradually. On the other hand, under the lased condition, the amount of hydroxyapatite indicated by phosphorus and calcium in the monomer infiltration was greater than that in the conventional hybrid layer. The reason for the difference would be that laser etching removes mainly the organic phase in dentin, whereas acid etching removes hydroxyapatite. It is suggested that the greater amount of hydroxyl apatite remaining at the bonding boundary prevents MMP-induced degradation of the collagen. Therefore, the laser etching would be intrinsically more advantageous to clinical durability than acid etching. However, the debris on the irradiated surface observed in the present study may be a disadvantage for bonding, because it inhibits resin tag formation by obstructing the opening of dentinal tubules on the dentin surface. Additionally, the organic debris might also be a reason for nanoleakage in the same manner that resin-impregnated smear layer formed in conventional self-etching procedure would cause nanoleakage.26–31 Because it is known that such debris can be controlled by modifying atmospheric conditions at the irradiation field,23 the quality of bonding strength following ArF excimer laser treatment might be further improved under decompression. In order to clarify the correspondence among findings, the findings on TEM, SEM, and EDS in this study were obtained from the same section of one specimen. As a result, the analyses were mostly qualitative rather than quantitative, which is a limitation of the study. A quantitative study of monomer infiltration into the dentin after ArF excimer laser irradiation should be performed in the near future. Conclusions
Within the limitation of qualitative analysis, the results of this in vitro study showed that increased carbon on the dentin surface after ArF excimer laser irradiation extended to a 2 lm depth from the bonding interface. Therefore, it was suggested that the resin monomer infiltrated into the microspaces produced on the dentin surface by laser abrasion. This infiltration is thought to be the adhesion mechanism after laser etching. Therefore, the photochemical processes at the bonding interface achieved using the ArF excimer laser have great potential to be developed into a new bonding system in dentistry. Acknowledgments
This research was funded by the Japan Society for the Promotion of Science ( JSPS) and Grants-in-Aid for Scientific Research (KAKENHI, A; 25253100). Author Disclosure Statement
No competing financial interests exist.
52 References
1. Rawls HR, Teixeira EC. Bonding and bonding agent. In: Phillips’ Dental Materials, 12th ed. KJ Anusavice, C Shen, HR Rawls (eds.). Philadelphia: Elsevier Sanders, 2013; pp. 257–274. 2. Van Meerbeek B, Dhem A, Goret–Nicaise M, Braem M, et al. Comparative SEM and TEM examination of the ultrastructure of the resin–dentin interdiffusion zone. J Dent Res 1993;72:495–501. 3. Van Meerbeek B, Yoshihara K, Yoshida Y, Mine A, et al. State of the art of self-etch adhesives. Dent Mater 2011;27: 17–28. 4. Liu Y, Tja¨derhane L, Breschi L, Mazzoni A, Li N, et al. Limitations in bonding to dentin and experimental strategies to prevent bond degradation. J Dent Res 2011;90: 953–968. 5. Breschi L, Mazzoni M, Ruggeri A, Cadenaro M, et al. Dental adhesion review: aging and stability of the bonded interface. Dent Mater 2008;24:90–101. 6. Hebling J, Pashley DH, Tjaderhane L, Tay FR. Chlorhexidine arrests subclinical degradationof dentin hybrid layers in vivo. J Dent Res 2005;84:741–746. 7. Koshiro K, Inoue S, Sano H, De Munck J, et al. In vivo degradation of resin–dentin bonds produced by a self-etch and an etch-and-rinse adhesive. Eur Oral Sci 2005;113, 341–348. 8. Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science 1986;234:559–565. 9. Reynolds A, Moore JE, Naroo SA, Moore CT, et al. Excimer laser surface ablation – a review. Clin Experiment Ophthalmol 2010;38:168–182. 10. Bagheri–Khoulenjani S, Mirzadeh H. Polystyrene surface modification using excimer laser and radio-frequency plasma: blood compatibility evaluations. Progress in Biomaterials 201;1:4. Available at: http://www.progressbiomaterials .com/content/1/1/4 (Last accessed January 28, 2014). 11. Laurens P, Bouali MO, Meducin F, Sadras B. Characterization of modifications of polymer surfaces after excimer laser treatments below the ablation threshold. Appl Surf Sci 2000;154/155:211–216. 12. Re´ve´sz K, Hopp B, Bor Z. Excimer laser induced surface chemical modification of polytetrafluoroethylene. Appl Surf Sci 1997;109/110:222–226. 13. Niino H, Okano H, Inui K, Yabe A. Surface modification of poly (tetrafluoroethylene) by excimer laser processing: enhancement of adhesion. Appl Surf Sci 1997;109/110: 259–263. 14. Weichenhain R, Wesner DA, Pfleging W, Horn H, et al. KrF-excimer laser pretreatment and metallization of polymers. Appl Surf Sci 1997;109/110:264–269. 15. Heitz J, Arenholz E, Kefer T, Baurle D, et al. Enhanced adhesion of metal films on PET after UV-laser treatment. Appl Phys A 1992;55:391–392. 16. Frentzen M, Koort HJ, Thiensiri I. Excimer lasers in dentistry: future possibilities with advanced technology. Quintessence Int 1992;23:117–133. 17. Ishida T, Tonam IK, Arak, IK, Kurosaki N. Properties of human dentin surface after ArF excimer laser irradiation. J Med Dent Sci 2008;55:155–161. 18. Sano K, Tonami K, Ichinose S, Araki K. Effects of ArF excimer laser irradiation of dentin on the tensile bonding strength to composite resin. Photomed Laser Surg 2012;30:71–76.
TONAMI ET AL.
19. Ichinose S, Muneta T, Sekiya I, Itoh S, et al. The study of metal ion release and cytotoxicity in Co-Cr-Mo and Ti-Al-V alloy in total knee prosthesis – scanning electron microscopic observation. J Mater Sci Mater Med 2003;14: 79–86. 20. Thanatvarakorn O, Nakajima M, Prasansuttiporn T, Ichinose S, et al. Effect of smear layer deproteinizing on resin– dentine interface with self-etch adhesive. J Dent 2014;42: 298–304. 21. Moretto SG, Azambuja N. Jr, Arana–Chavez VE, Reis AF, et al. Effects of ultramorphological changes on adhesion to lased dentin — Scanning electron microscopy and transmission electron microscopy analysis. Microsc Res Tech 2011;74:720–726. 22. Nikaido T, Weerasinghe DDS, Waidyasekera K, Inoue G, et al. Assessment of the nanostructure of acid-base resistant zone by the application of all-in-one adhesive systems: Super dentin formation. Biomed Mater Eng 2009;19: 163–171. 23. Ba¨uerle D. Nanosecond-laser ablation. In: Laser Processing and Chemistry. 4th ed. New York: Springer-Verlag, 2011; pp. 237–278. 24. Gillen D. Enhancing UV laser machining of medical grade polymers. Laser User 2009;54:27–29. 25. Yip J, Chan K, Sin KM, Lau KS. Study on the surface chemical properties of UV excimer laser irradiated polyamide by XPS, ToF-SIMS and CFM. Appl Surf Sci 2003; 205:151–159. 26. Mine A, De Munck J, Cardoso MV, Van Landuyt KL, et al. Dentin-smear remains at self-etch adhesive interface. Dent Mater 2014;30:1147–1153. 27. Pinzon LM, Watanabe LG, Reis AF, Power JM, et al. Analysis of interfacial structure and bond strength of selfetch adhesives. Am J Dent 2013;26:335–340. 28. Reis AF, Giannini M, Pereira PNR. Long-term TEM analysis of the nanoleakage patterns in resin–dentin interfaces produced by different bonding strategies. Dent Mater 200;23:1164–1172. 29. Suppa P, Breschi L, Ruggeri A, Mazzotti G, et al. Nanoleakage within the hybrid layer: A correlative FEISEM/ TEM investigation. J Biomed Mater Res Part B: Appl Biomater 2005;73:7–14. 30. Carvalho RM, Chersoni S, Frankenberger R, Pashley DH, et al. A challenge to the conventional wisdom that simultaneous etching and resin infiltration always occurs in selfetch adhesives. Biomaterials 2005;26:1035–1042. 31. Inoue S, Koshiro K, Yoshida Y, De Munck J, et al. Hydrolytic stability of self-etch adhesives bonded to dentin. J Dent Res 2005;84:1160–1164.
Address correspondence to: Ken-ichi Tonami Oral Diagnosis and General Dentistry Dental Hospital Tokyo Medical and Dental University 1-5-45 Yushima Bunkyo-ku Tokyo, 113-8549 Japan E-mail: [email protected]
