Influence of light intensity on surface-free energy and dentin bond strength of single-step self-etch adhesives

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1 Dental Materials Journal 2015; 34(5): Influence of ligt intensity on surface-free energy and dentin bond strengt of single-step self-etc adesives Kie NOJIRI, Akimasa TSUJIMOTO, Takayuki SUZUKI, Syo SHIBASAKI, Saki MATSUYOSHI, Tosiki TAKAMIZAWA and Masasi MIYAZAKI Department of Operative Dentistry, Nion University Scool of Dentistry, Kandasurugadai, Ciyoda-Ku, Tokyo , Japan Corresponding autor, Akimasa TSUJIMOTO; In tis study, we investigated te influence of ligt intensity on te surface-free energy and dentin bond strengt of single-step selfetc adesives. Te adesives were applied to te dentin surfaces of bovine mandibular incisors and cured wit ligt intensities of 0 (no irradiation), 200, 400, and 600 mw/cm 2. Surface-free energies were determined by measuring te contact angles of tree test liquids placed on te cured adesives. Dentin bond strengts of te specimens were also measured. Polymerization wit a iger ligt intensity resulted in a lower surface-free energy of te cured adesives. Te greatest bond strengt was acieved wen a ligt intensity of 400 mw/cm 2 or greater was used. Our data suggest tat te surface-free energy and dentin bond strengt of single-step self-etc adesives are affected by ligt intensity of te curing unit. Keywords: Ligt intensity, Surface-free energy, Dentin bond strengt, Single-step self-etc adesive INTRODUCTION Current dental adesive systems feature simplified application metods to acieve optimum clinical results 1). Single-step self-etc adesives tat combine te functions of a self-etcing primer and a bonding agent ave been developed 2). Tis kind of adesive system combines acidic functional monomers, ydropilic monomers, ydropobic monomers, fillers, water, and organic solvent in a single bottle 3). Te use of water as a solvent is indispensable for single-step self-etc adesives to ensure te ionization of te acidic functional monomers, and te organic solvents are added to facilitate mixing of te ydropilic and ydropobic components 4). However, it as been suggested tat water and te solvents mixed in te adesive resin lead to reduced mecanical properties and accordingly poorer bonding performance 5). Terefore, removing tese components from single-step self-etc adesives is important for optimal bonding. Single-step self-etc adesives reportedly ave a ig ydropilicity even after polymerization, wic may increase teir solubility and water uptake compared wit tose of conventional multi-step adesives 6). Longterm leacing of water into te material may furter compromise its mecanical properties, leading to poor bonding performance of single-step self-etc adesives to dentin 7). Tese penomena may be dependent upon te degree of conversion of te adesive, wic is strongly influenced by ligt intensity during curing. Ligt intensity during curing is dependent upon te clinical circumstances, suc as cavity preparation, toot position, or te existence of adjacent teet 8). Wen te distance between te curing ligt tip and cavity wall increases, ligt intensity reacing te adesive surface decreases 9). It as been demonstrated tat reduced ligt intensity causes lower bond strengts between self-etc adesives and dentin 10). In addition, after te ligt irradiation of adesives at a reduced ligt intensity, te top surface of te adesive is covered wit a relatively tick, uncured adesive layer due to te oxygen inibition 11). In te case of single-step selfetc adesives, residual water and acidic functional monomers may be present witin tis oxygen-inibited layer 12). Te presence of water and acidic functional monomers may compromise te bonding durability of single-step self-etc adesives. Terefore, te bonding performance of single-step self-etc adesives may be influenced by ligt intensity during curing to a greater degree tan te conventional multi-step adesive systems wit solvent-free bonding agents. However, tere is little information on te mecanism by wic te ligt irradiation of single-step self-etc adesives affects dentin bond strengt; terefore, it is necessary to determine te influence of ligt intensity on te bonding performance of tese adesives. Previous studies 13,14) ave sown tat te interfacial caracteristics of self-etc adesives can be analyzed on te basis of te surface-free energy measurement. Generally, te surface-free energy caracteristics of polymers make a significant contribution to teir interfacial interactions wit liquids, wit wic tey can form ydrogen bonds. Wetting properties and te ydropilicity of adesives may be expressed in terms of te surface-free energy and its components. Tus, analysis of te surface-free energy of single-step self-etc adesives may provide an insigt in te development of teir bonding performance. In tis study, we examined te influence of ligt intensity during curing on te surface-free energy of adesive-treated dentin and bond strengt of single-step self-etc adesives to bovine dentin. Our null ypotesis was tat tere would be no significant differences in surface-free energy or bond strengt at different ligt intensities. Received Feb 3, 2015: Accepted Apr 15, 2015 doi: /dmj JOI JST.JSTAGE/dmj/

2 612 Dent Mater J 2015; 34(5): MATERIALS AND METHODS Adesives used As presented in Table 1, te single-step self-etc adesive systems used were: Scotcbond Universal Adesive (3M ESPE, St. Paul, MN, USA); Clearfil Tri-S Bond (Kuraray Noritake Dental, Tokyo, Japan); and G-Bond Plus (GC, Tokyo, Japan). A visible-ligt curing unit (Optilux 501, Demetron Kerr, Danbury, CT, USA) was connected to a variablevoltage transformer (Type S , Yamabisi Electric, Tokyo, Japan). Te ligt intensities used were 100, 200, 400, and 600 mw/cm 2 ; tese values were determined using a dental curing radiometer (Model 100, Demetron Kerr). Te curing unit was placed in a jig to maintain te distance between te ligt tip end and specimen surface (2 mm). Surface-free energy measurement Mandibular incisors were extracted from 2- to 3-yearold cattle and frozen until use. After removing te roots of eac toot wit a slow-speed saw equipped wit a diamond-impregnated disk (IsoMet, Bueler, Lake Bluff, IL, USA), te pulp was removed, and te pulp camber was filled wit cotton to avoid penetration of te embedding medium. After ultrasonic cleaning for 1 min in distilled water to remove debris, te surface of eac toot was wased and ten dried wit oil-free compressed air. Te labial surface was ground wit wet 240-grit silicon carbide (SiC) paper to create a flat dentin surface. Eac toot was ten mounted in selfcuring acrylic resin (Tray Resin II, Sofu, Kyoto, Japan) to expose te flattened area, and placed under tap water to reduce te temperature increase caused by te exotermic polymerization reaction of te acrylic resin. Te final finis was created by grinding wit wet 320- grit SiC paper. Finally, te dentin surface of eac toot was dried wit oil-free compressed air. Eac adesive was applied to te dentin surface according to te manufacturers instructions, followed by ligt irradiation for 10 s. Ligt irradiation was performed at intensities of 0 (no irradiation), 200, 400, and 600 mw/cm 2. Surface-free energy was determined by measuring te contact angle wit te surface of tree test liquids, 1-bromonaptalene, diiodometane, and distilled water, eac of wic as known surface-free energy parameters (Table 2). Te surface-free energy of five specimens per ligt intensity group for eac adesive-treated dentin surface was determined by measuring teir contact angle (Drop Master DM500, Kyowa Interface Science, Saitama, Japan). Te device was fitted wit a carge-coupled device camera, wic allowed automatic measurement of te contact angle (Fig. 1). For eac test liquid, te equilibrium contact angle (θ) was measured, using te sessile-drop metod at 23±1 C, for five specimens of eac adesive 15). Te surface free-energy parameters of te solids were ten determined based on te fundamental concepts of wetting. Te Young-Dupré equation describes te work of adesion (W) between a solid (S) and a liquid (L) in contact as follows: Table 1 Single-step self-etc adesives used in tis study Adesive Code Lot No. Main components Manufacturer Scotc Bond Universal SU MDP, Bis-GMA, HEMA, Vitrebond copolymer, polyetelene glycol, water, initiator, colloidal silica, aluminiam oxide 3M ESPE, St. Paul, MN, USA Clearfil Tri-S Bond TS 00038B MDP, Bis-GMA, HEMA, etanol,water, initiator, silanated colloidal silica Kuraray Noritake Dental, Tokyo, Japan G-Bond Plus GB MDP, 4-MET, metacrylate monomer, acetone, water, initiator, silica filler GC, Tokyo, Japan MDP: 10-metacryloyloxydecyl di-ydrogen pospate, Bis-GMA: 2,2-bis[4-(2-ydroxyl-3-metacryloyloxypropoxy)penyl] propane, HEMA: 2-ydroxyetyl metacrylate, 4-MET: 4-metacryloyloxyetyl trimellitate Table 2 Surface free energy and teir components values of test Liquid Lot No. Manufacturer γ L γ L d γ L p γ L 1-Bromonaptalene ALH4513 Wako Pure Cemical Industries Diiodometane ALL2310 Wako Pure Cemical Industries Distilled water Unit: mn/m. γ L: surface free energy, γ Ld : dispersion force, γ Lp : polarity force, γ L : ydrogen bonding force

3 Dent Mater J 2015; 34(5): (a) (b) Fig. 1 Drop Matser DM500 apparatus fitted wit a carge-coupled device camera (a) allowing automatic measurements of te contact angles to be made (b). W SL=γ L+γ S γ SL=γ L (1+cosθ) Here, γ SL is te interfacial free energy between te solid and liquid, γ L is te surface-free energy of te liquid, and γ S is te surface-free energy of te solid. By extending te Fowkes equation, te γ SL can be expressed as follows: γ SL=γ L+γ S 2(γ Ld γ Sd ) 1/2 2(γ Lp γ Sp ) 1/2 2(γ L γ S ) 1/2 γ L=γ Ld +γ Lp +γ L, γ S=γ Sd +γ Sp +γ S were γ Ld, γ Lp, and γ L are components of te surface-free energy (γ) arising from te dispersion force, te polar (permanent and induced) force, and te ydrogenbonding force, respectively. Surface-free energy (γ) values were determined for te tree test liquids, and te surface energy parameters of te treated dentin surfaces were calculated based on tese equations using add-on software and an interface measurement and analysis system (FAMAS, Kyowa Interface Science). Bond strengt measurement Te sear-bond strengt between te single-step selfetc adesives and te dentin surface was measured by a notced-edge sear-bond strengt test, as described by ISO ). A total of 120 mandibular incisors from cattle were prepared as described in te preceding section. Te adesives were applied according to te manufacturers instructions, followed by ligt irradiation for 10 s. Ligt irradiation was performed at intensities of 0 (no irradiation), 200, 400, and 600 mw/cm 2. Te specimens were clamped in a bonding jig (Ultradent Bonding Jig, Ultradent Products, Sout Jordan, UT, USA), and plastic molds (2.4 mm in internal diameter, 2.5 mm in eigt) were used to form and old te resin composite on te adesive-treated dentin surface. Te resin composite (Clearfil AP-X, Kuraray Noritake Dental) was condensed into te mold and ligt cured for 30 s at a ligt intensity of 600 mw/cm 2. Te finised specimens were transferred to distilled water, and stored at 37 C for 24. Ten specimens per ligt intensity were tested in sear mode using bond-testing notced-blade semicircular apparatus (Ultradent Products) in a universal testing macine (Type 5500R, Instron, Norwood, MA, USA) at a crossead speed of 1 mm/min. Bond strengt values (MPa) were calculated from te peak load at failure divided by te specimen surface area. After testing, te specimens were examined under an optical microscope (SZH-131, Olympus, Tokyo, Japan) at a magnification of 10 to determine te location of bond failure. Te type of failure was classified based on te percentage of substrate-free material as follows: adesive failure; coesive failure in te dentin; or coesive failure in te composite. Statistical analysis Te dentin bond strengt data obtained were analyzed using a commercial statistical software package (SigmaStat Version 3.1, SPSS, Cicago, IL, USA). Because te data were normally distributed (Kolmogorov- Smirnov test), two-way analysis of variance (ANOVA) was used to analyze te effects of ligt intensities and te adesive systems tat were used. Multiple comparisons were ten conducted using te Tukey- Kramer test, wit a significance level of Scanning electron microscopy Te resin-dentin interface was ultrastructurally observed by scanning electron microscopy (SEM). Bonded specimens from eac group (n=5) were stored in distilled water maintained at 37ºC for 24, embedded in self-curing epoxy resin (Epon812, Nissin EM, Tokyo, Japan), and stored at 37ºC for an additional 12. Te embedded specimens were sectioned troug te diameter of te composite resin post, and te surfaces of te cut alves were polised wit an Ecomet 4/Automet 2 (Bueler) using SiC papers wit a grit size of 600, 1,200, and 4,000 in succession. Te surface was finally polised by a soft clot using diamond paste (Bueler) wit a grit size of 1.0 lm. All SEM specimens were deydrated in ascending concentrations of tert-butanol

4 614 Dent Mater J 2015; 34(5): (50% for 20 min, 75% for 20 min, 95% for 20 min, and 100% for 2 ) and transferred to a critical-point dryer for 30 min. Tese surfaces were subjected to Argon ion-beam etcing (Type EIS-200ER, Elionix, Tokyo, Japan) for 30 s, wit te ion beam (accelerating voltage, 1.0 kv, ion current density, 0.4 ma/cm 2 ) directed perpendicularly to te polised surface. Surfaces were coated in a vacuum evaporator (Quick Coater Type SC-701, Sanyu Densi, Tokyo, Japan) wit a tin film of gold and observed by SEM (ERA 8800FE, Elionix) at an accelerating voltage of 10 kv. All of te specimens were observed under SEM. RESULTS Te surface free energies and teir components, for eac cured adesive, are sown in Table 3. Total surfacefree energy (γ S=γ Sd +γ Sp +γ S ) values decreased wit increasing ligt intensity, and were significantly lower wen adesives were irradiated wit a ligt intensity of 400 mw/cm 2. For all surfaces, te estimated γ S d values remained relatively constant, in te range of mn/m. Decreased γ S p and γ S values were observed for specimens irradiated wit a greater ligt intensity. Te influence of ligt intensity during curing on sear bond strengt is sown in Table 4. Two-way ANOVA revealed tat bot adesive type (p<0.001) and ligt intensity (p<0.001) significantly influenced bond strengt, altoug tere was no significant interaction between te two factors (p=0.601). For eac adesive, greater bond strengt was acieved by irradiation wit a ligt intensity of 400 mw/cm 2 or greater. Statistically significant differences were evident between te values obtained, except for tose obtained at ligt intensities of 400 and 600 mw/cm 2 (p<0.005). Failure type was not associated wit bond strengt: te predominant failure type was adesive failure. SEM observations of te resin-dentin interface are sown in Fig. 2. Te dentin-resin interface of eac groups sowed excellent adaptation, wit te formation of a transitional layer between te adesive resin and Table 3 Influence of ligt intensity on surface free energies and teir components for single-step self-etc adesives γ S γ S d γ S p γ S SU (3.2) 68.0 (3.1) 55.1 (3.4) 54.8 (2.5) 38.5 (0.8) 39.1 (0.7) 39.6 (1.0) 39.8 (0.8) 10.6 (2.0) 8.3 (2.0) 5.3 (2.7) 4.9 (2.0) 24.4 (2.3) 20.6 (2.2) 10.2 (1.8) 10.1 (1.2) TS (3.1) 67.7 (3.6) 57.2 (3.5) 57.1 (3.0) 38.9 (0.6) 38.7 (1.2) 38.9 (1.1) 39.1 (0.9) 9.8 (2.2) 9.6 (2.8) 7.7 (2.7) 6.8 (2.2) 23.5 (2.1) 19.4 (1.9) 10.6 (1.9) 11.2 (1.8) GB (3.3) 64.3 (3.2) 57.2 (2.4) 57.6 (2.0) 38.5 (1.2) 38.3 (0.8) 39.8 (0.9) 39.9 (1.2) 10.2 (2.2) 9.3 (2.2) 5.4 (1.5) 5.1 (1.9) 20.3 (2.2) 16.7 (2.2) 12.0 (1.7) 12.6 (2.0) Unit: mn/m, values in parentesis are standard deviations (n=5). Values connected by vertical lines indicate no significant difference (p>0.05). Table 4 Influence of ligt intensity on dentin bond strengt of single-step self-etc adesives Adesive Ligt intensity SU 18.8 (3.2) a,a 24.8 (3.7) a,b 30.3 (3.7) a,c 29.2 (3.2) a,c TS 15.5 (2.8) b,a 19.2 (4.4) b,b 27.9 (3.8) b,c 27.3 (3.2) b,c GB 15.3 (3.5) b,a 19.0 (3.4) b,b 25.3 (3.9) b,c 26.0 (3.0) b,c Unit: MPa, values in parentesis are standard deviations (n=15). Values wit same letter indicate no significant difference (p>0.05). [ ] : failure mode [adesive failure/ coesive failure/ mixed failure]

5 615 Dent Mater J 2015; 34(5): Fig. 2 (a) (c) (e) (g) (b) (d) (f) () Representative SEM potomicrograps of te dentin-resin interface (Original magnification, 5,000). Tickness of adesive was indicated by wite arrows in eac SEM. (a) SU wit 0 mw/cm2, (b) TS wit 0 mw/cm2, (c) SU wit 200 mw/cm2, (d) TS wit 400 mw/cm2, (e) SU wit 400 mw/cm2, (f) TS wit 400 mw/cm2, (g) SU wit 600 mw/cm2, () TS wit 600 mw/cm2. toot structure. Te tickness of te adesives was lesser for te specimens irradiated wit a weaker ligt intensity of <200 mw/cm2. DISCUSSION Generally, te surface-free energy of organic substances (γs) as tree component forces: te dispersion (γsd), polarity (γsp), and ydrogen-bonding (γs) forces17). Te dispersion force represents London interactions between apolar molecules, wereas te polar (nondispersion) force represents electrostatic, metallic, and dipolar interactions. Furter to tese two parameters of te γs, we also calculated te ydrogen-bonding force, wic is related to water and ydroxyl components. Because te ydration of te aderend is of great importance to wettability wic is, in turn, key to dentin bonding polar interactions and ydrogen-bonding caracteristics must be accurately determined to clarify te interactions between te adesives and water18). An estimation of dipolar interactions separately from ydrogen-bonding interactions may provide a novel insigt into te mecanisms contributing to wettability and te bonding caracteristics of te adesives. Te γsd values of te dentin surfaces remained relatively constant ( mn/m) regardless of te adesive used, and tere were no significant differences in γsd between ligt intensity groups. Reportedly, te γsd values of many oxidized surfaces measured using tis metod are approximately 40 mn/m12,15,19), wereas tose of surfaces coated wit polytetrafluoroetylene are around 20 mn/m. Te γsp values of all specimens were relatively low (from 4.9 to 10.6 mn/m), and decreased wit increasing ligt intensity. Te γs values ranged from 16.7 to 24.4 mn/m at ligt intensities of 0 and 200 mw/cm2, and from 10.1 to 12.6 mn/m at ligt intensities of 400 mw/cm2 or greater. Te γsp value, wic reflects polar interactions, and te γs value, wic relates to water and ydroxyl components, togeter measure ydropilic interactions. Te functional monomers of te adesives used ave tree distinct structural components: functional, spacer, and polymerizable groups20). Te functional group exibits ydropilic properties tat enance te wettability and demineralization of te toot surface21). Te spacer group influences te properties of te monomer, including water solubility, viscosity, wettability, and penetration22). However, after ligt irradiation at sufficient ligt intensities, te polymerizable groups of te functional monomers become polymerized, increasing te ydropobicity of te cured adesive23). Terefore, sufficient ligt energy may enance te polymerization reaction and encourage te double-bond conversion of resin monomers in te adesives. Conversely, incomplete polymerization may lead to residual water witin te adesive24). Irradiation of te adesive at a lower ligt intensity may impair cross-linking witin te adesive as a result of insufficient cross-linked functional monomers and excess water witin te adesive; tis may lead to iger γsp and γs values in adesives irradiated at a lower ligt intensity. Wen te adesives were cured at a greater ligt intensity, te resultant dentin bond strengt values

6 616 Dent Mater J 2015; 34(5): were iger tan tose obtained wen te adesives were polymerized wit a ligt intensity of 200 mw/ cm 2 or less. From SEM observation of te dentin-resin interface, te tickness of te adesive was muc less in te group irradiated wit a weaker ligt intensity. Tese data suggest tat te adesives acquire inferior mecanical properties as a result of insufficient polymerization by inadequate ligt energy, in turn reducing bond strengt. Furtermore, irradiation of single-step self-etc adesives at a lower ligt intensity retards te polymerization process, leading to te creation of a ticker, uncured adesive layer tat includes an oxygen-inibited layer 25). Presumably, residual acidic functional monomers, water, and solvent are present in tis oxygen-inibited layer 12). Tese remnants may impair te secondary polymerization of te adesive and te polymerization of te filler resin, because tertiary amines in te adesives and filler resin composite may be neutralized by te acidic functional monomers in te uncured adesive 26). Acidic conditions affect polymerization driven by dl-camporquinone/ amine catalysts, resulting in poor polymerization and attenuating interactions wit te filler resin composite 27). In tis study, wen te surface-free energy of single-step adesives decreased following irradiation at a greater ligt intensity, dentin bond strengt values increased. Optimal wettability is essential to enable te materials to spread across te entire surface and establis adesion 28). Altoug in general, te surfacefree energy of te aderend surface must be maximized, te maximum bond strengt was evident in properly polymerized adesives, wic ave a lower surface-free energy 14). Terefore, full polymerization of te adesive as a greater influence on sear bond strengt tan te surface-free energy of te adesive. Tis suggests tat te degree of polymerization of adesives wit a lower surface-free energy significantly influenced dentin-bond strengt. Our null ypotesis tat tere would be no significant differences in surface-free energy and bond strengt at different ligt intensities, can terefore be rejected. CONCLUSION Regarding te single-step self-etc adesives evaluated in tis study, polymerization wit a greater ligt intensity reduced te surface-free energy of adesivetreated dentin surfaces, because conversion of te polymerizable group of functional monomers was enanced. Sufficient ligt energy may enance te polymerization reaction and perpetuate te doublebond conversion of resin monomers in te adesives, leading to iger dentin bond strengt. 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