Dental Materials Journal 24(2) : 251-256, 2005

Development of Self-etching Primer Adhesive in All-in-one Bonding System

Tohru HAYAKAWA1, Kazuyo KIKUTAKE-SUGIYAMA1, Tadao FUKUSHIMA2 and Kimiya NEMOTO1 1Department of Dental Biomaterials, Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo,

2-870-1 Sakaecho Nishi, Matsudo, Chiba 271-8587, Japan 2Department of Dental Engineering, Bioengineering Section, Fukuoka Dental College, 2-15-1 Tamura, Sawara-ku, Fukuoka 814-0193, Japan Corresponding author, E-mail : hayakawa@mascat.nihon-u.ac.jp

Received March 11, 2005/Accepted April 22, 2005

The self-etching primer adhesives examined in the present study were aqueous mixtures of 5 wt% MDP, 35 wt% HEMA, 3 wt% 2-hydroxyl-3- (3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy) -N,N,N-trimethyl-1-propanaminium chloride (QTX) , 0.5 wt% ethyl-p- (N,N-dimethylamino) benzoate (EDM) , and 30 wt% water-soluble dimethacrylate (i.e., 2,2-bis (4-methacryloxy

polyethoxypheny) propane (BPE) , Glycerol dimethacrylate (GMR) , or Glycerol methacrylate acrylate (GAM)) . Self-etching primer adhesive was directly applied to polished bovine teeth. Tensile bond strength of composite resin to bovine teeth was measured after one-day immersion in water at 37t. Differences in water-soluble dimethacrylate monomer did not produce any significant differences in tensile bond strength to dentin or enamel. The use of N-phenyliminodiacetic acid (PIDAA) in-stead of EDM showed significantly higher bond strength to dentin and enamel. In conclusion, GMR and GAM were useful as water-soluble dimethacrylates while the combination of PIDAA and QTX was effective in improving the bond strength of composite resin to teeth in an all-in-one bonding system.

Key words : Self-etching primer adhesive, Water-soluble photoinitiator, All-in-one bonding system

INTRODUCTION

Nowadays, most studies on the adhesion of dental resin to teeth have focused on the development of ad-hesion-promoting monomers or the improvement of

pre-treatment reagents, such as self-etching primers. As a result, the role of polymerization initiators in bonding has been grossly overlooked. It has been reported that interfacial polymerization is bene-ficial for strong adhesion between dentin and restora-tive resin1) . Further, Hirabayashi et a1.2) and Hirabayashi3) have elucidated the polymerization be-havior of PMMA/MMA resin when initiated by TBB, as compared with other initiators such as camphorquinone (CQ) /amine system.

For a new type of photopolymerization initiator, we introduced a unique water-soluble photopolymeriza-tion initiator, 2-hydroxy-3- (3,4-dimethyl-9-oxo-9H-thioxanthen - 2 -yloxy) - N, N, N - trimethyl - 1- propana-minium chloride (QTX) . QTX - which can polym-erize water-soluble monomers in an aqueous solution by photoirradiation4) - has been applied in the

printing industry5) . As for polymerization of dental resins, CQ is now widely used as the photo-

polymerization initiator. However, it is impossible for CQ to initiate water-soluble monomers, such as 2-hydroxyethyl methacrylate in an aqueous solution, and it is doubtful if CQ can sufficiently initiate the

polymerization of diffused monomers inside dentin because of the water content of dentin and the lower

partition coefficient of CQ in dentin than in resin4) We previously reported on the efficacy of QTX-

containing dentin primers or hydrophilic bonding agents for the adhesion of composite resin to dentin6-8) . It was also reported that self-etching

primers containing QTX could improve the bond strength of composite resin to dentin when used in combination with commercially available bonding agents9,10) . Nakanuma, on the other hand, reported on the effectiveness of a new type of hydrophilic bonding agent containing QTX4) . The usefulness of

QTX as a component of a primer for bonding resin-modified glass-ionomer cement was also reportedly

Based on favorable results mentioned above, Kikutake12) developed a one-step bonding system which employed a new type of self-etching primer ad-hesive using QTX. The self-etching primer adhesive consisted of 10-methacryloyloxydecamethylene phos-

phoric acid (MDP) , 2-hydroxyethyl methacrylate (HEMA) , QTX and tertiary amine, and water-soluble dimethacrylate. This original self-etching primer adhesive was applied directly to polished enamel and dentin without any pretreatment, and the result was tight bonding to enamel and dentin. For water-soluble dimethacrylate, 2,2-bis (4-methacryloxy

polyethoxypheny) propane (BPE) was used in the original self-etching primer adhesive. However, BPE has a high molecular weight of 1324 and a relatively high viscosity. The intrinsic diffusivity of molecule

is inversely related to its molecular weight or size13) It is speculated that dimethacrylate monomers with less molecular weight and less viscosity will infiltrate more efficiently into dentin, thereby improving the bond strength.

252 DEVELOPMENT OF SELF-ETCHING PRIMER ADHESIVE

Kikutake12) evaluated the effectiveness of several kinds of amine in terms of tensile bond strength of composite resin to enamel or dentin, and concluded that N-phenylglycine (NPG) and Ethyl-p- (N ,N-dimethylamino) benzoate (EDM) were useful as amine compounds. But the self-etching primer adhesive containing NPG was not stable and posed a problem in handling property. Thus, EDM was selected as the amine compound of the original self-etching

primer adhesive. To further elucidate the efficacy of original self-

etching primer adhesive containing QTX in an all-in-one bonding system, we examined the efficacy of these water-soluble dimethacrylates and that of a new amine compound by measuring the tensile bond strength of composite resin to bovine dentin and enamel.

MATERIALS AND METHODS

Self-etching primer adhesives For phosphate methacrylate, MDP was used . Each self-etching primer adhesive then consisted of 5 wt% MDP, 35 wt % HEMA, 30 wt % water-soluble dimethacrylate, 3.0 wt % QTX and 0.5 wt% tertiary amine. For water-soluble dimethacrylates, BPE (Shin-

Nakamura Chemical Co., Wakayama, Japan) , Glyc-erol dimethacrylate (GMR, NOF Corp., Tokyo , Japan) , and Glycerol methacrylate acrylate (GAM, NOF Corp., Tokyo, Japan) were used. Molecular weight of BPE, GMR, and GAM was 1324, 229, and 214 respectively. Viscosity of each water-soluble dimethacrylate, as measured by viscometer (Model VM-10A, CBC Materials Co. Ltd., Tokyo, Japan) at 37C , was 574 mPa.s for BPE, 48.2 mPa.s for GMR , and 43.6 mPa.s for GAM.

For tertiary amine, EDM (Wako Pure Chemi-cal Industries Ltd., Osaka, Japan) or N-

phenyliminodiacetic acid (PIDAA, Wako Pure Chemi-cal Industries Ltd., Osaka, Japan) was used. The composition of each self-etching primer adhesive is listed in Table 1. Structural formulae of QTX and amine compounds are shown in Fig. 1, and monomers of the original self-etching primer adhesive are shown in Fig. 2.

Fig. 1 Structural formulae of QTX and tertiary amines of the self-etching primer adhesive.

Fig. 2 Structural formulae of the monomers of self-

etching primer adhesive.

Adhesion test Fig. 3 illustrates schematically the specimens for ten-

sile bond strength measurement. After extraction from mandibles, bovine anterior teeth were stored at

-70 °C to maintain freshness during storage . A flat enamel or dentin surface on the labial side was pre-

pared by wet grinding using # 800- and # 1000-grit stones as described in previous reports6-12). The ex-

posed enamel or dentin surface was then treated with one of the self-etching primer adhesives. After 30

Table 1 Composition of the self-etching primer adhesives .

HAYAKAWA et al. 253

Fig. 3 Schematic illustration of the specimen for tensile

bond strength measurement.

seconds, excess agent was removed using compressed air. A preliminary experiment demonstrated that bond strength decreased due to photoirradiation of applied self-etching primer adhesive, although the de-tailed mechanism is not still clear. Thus, without

photoirradiation of applied self-etching primer adhe-sive, a silicone ring of 3.2 mm diameter X 2.0 mm height was placed on the treated enamel or dentin surface to simulate a cavity. The ring was then di-rectly filled with photocurable composite resin (Clearfil AP-X, Batch No. 00989A, Kuraray Medical

Inc., Tokyo, Japan) . The filled composite was po-lymerized by photoirradiation for 40 seconds with a

photocuring unit (XL-3000, 3M Dental Products, St. Paul, MN, USA) . After curing, the silicone ring was removed and each specimen was stored in water at 37C for 24 hours. A brass attachment was fixed to the upper part of the cured resin, and tensile bond strength was measured using a universal testing ma-chine (TCM-500CR, Shinkoh, Tokyo, Japan) at a cross-head speed of 2 mm/min.

Ten specimens were measured for each test condi-tion. Significant differences in measurement were determined by two-way analysis of variance

(ANOVA) , and Scheffe's test for multiple compari-sons to determine statistical differences in bond strength between the enamel and dentin groups and among the self-etching primer adhesives at 5% sig-nificance level. In addition, bond strength was sta-

tistically compared among the self-etching primer ad-hesives in enamel and dentin groups, as well as be-tween enamel and dentin groups for each self-etching

primer adhesive using one-way ANOVA followed by Scheffe's multiple comparison tests at 5% significance level.

SEM observation The resin/dentin interface - after treatment with GMR or GMR-PIDAA self-etching primer adhesive and after bonding with resin composite restoration - was observed using scanning electron microscopy

(SEM, T-200, JEOL, Tokyo, Japan) . Polished bovine dentin surface was treated with

GMR or GMR-PIDAA self-etching primer adhesive for 30 seconds. A dentin specimen bonded with resin composite was prepared in the same manner as for the tensile test. The bonded specimen was cut verti-cally to obtain cross-sectional samples of the resin/ dentin interface. Cross-sectional samples were pol-ished with # 1000-grit stone, as well as with special soft polishing discs using diamond suspensions of 6

um, 3um, 1um, and 0.25um grit size (Struers, Co-

penhagen, Denmark) . The samples were ultrasoni-cally cleaned for three minutes to remove all rem-nants of the polishing debris and paste. Then, they were etched with an argon-ion beam for 60 seconds with an EIS-200R (Elionix, Tokyo, Japan) prior to SEM examination14) . Operating conditions for the ion source were kept at a constant anode voltage of 1 kV and ion current density of 15 mA/cm2, with the ion beam directed at 90 degrees to the specimen sur-face. Finally, specimens were ion-coated with gold and observed using SEM.

RESULTS

Table 2 shows the results of tensile bond strength of

composite resin to bovine dentin and enamel when BPE, GMR, and GMA were used as self-etching

primer adhesives. Two-way ANOVA showed signifi-cant differences in bond strength between dentin and enamel for three original self-etching primer adhe-

Table 2 Tensile bond strength of composite resin to bovine dentin and enamel with different self-

etching primer adhesives (BPE, GMR, and GAM) . (mean±SD, MPa)

254 DEVELOPMENT OF SELF-ETCHING PRIMER ADHESIVE

Table 3 Tensile bond strength of composite resin to bovine dentin and enamel with GMR-PIDAA self-etching primer adhesive (mean±SD, MPa).

Fig. 4 SEM views of the cross-sectional interface between resin and dentin. A hybrid layer is observed at the resin/dentin interface. (R: resin; D: dentin; Between arrows: hybrid layer.)

sives, BPE, GMR, and GAM (p<0.05). No signifi-cant differences existed among the monomers

(p>0.05). Two-way interaction was not found for teeth (dentin and enamel) and monomers (BPE, GMR, and GAM) (p<0.05).

There were no significant differences in tensile bond strength to dentin or enamel among the three different self-etching primer adhesives (p>0.05). When comparing tensile bond strength to dentin and enamel, BPE and GMR did not produce significantly different bond strengths between dentin and enamel

(p>0.05). GAM, on the other hand, showed signifi-cantly different bond strengths between dentin and enamel (p<0.05). After debonding, macroscopic in-spection revealed that failure was a mixture of cohe-sive failure inside the resin and adhesive failure at the resin/enamel or resin/dentin interface.

Table 3 shows the results of tensile bond strength of composite resin to bovine dentin and enamel when GMR-PIDAA was used as the self-etching primer adhesive. Among the dimethacrylate monomers, GMR produced the highest mean bond strength to bovine teeth although no significant dif-ferences existed among the three groups (Table 2) . Thus we employed GMR as the dimethacrylate mono-mer. There were no significant differences in tensile bond strength between dentin and enamel (p>0.05). Comparing the obtained values with those of GMR, the tensile bond strength of GMR-PIDAA to dentin

(19.8 ± 5.2 MPa) was significantly higher than that of GMR to dentin (13.2±3.3 MPa). For enamel ad-hesion, there were no significant differences in tensile bond strength between GMR-PIDAA to enamel (20.8 ±6.4 MPa) and GMR to enamel (17.2±6.1). After debonding, macroscopic inspection revealed that fail-ure was mainly a cohesive failure inside resin.

Fig. 4 shows the SEM views of the interface be-tween resin and dentin. In composite resin part, the inorganic fillers were clearly visible. Both pictures showed the presence of a hybrid layer (between ar-rows in Fig. 4) at the dentin/resin interface, and the thickness of the hybrid layer was approximately 1-1. 5um. Dentin resin tags were also observed.

DISCUSSION

This study demonstrated the effectiveness of an original self-etching primer adhesive for bonding composite resin to bovine dentin and enamel in an all-in-one step bonding system.

The self-etching primer adhesive used in the pre-sent study consisted of MDP, HEMA, water-soluble dimethacrylate, QTX and amine compound. MDP is known as an adhesive monomer and is reported to be an effective component of self-etching primers in the adhesion of composite resin to dentin and enamel15-20). HEMA is used as a dentin primer com-

ponent, and it has been reported that HEMA

HAYAKAWA et al. 255

enhanced the permeability of dentinal substrates21,22) QTX is a unique photopolymerization initiator, which can polymerize water-soluble monomers in aqueous solutions4,5) . Once MDP, HEMA, and dimethacrylate monomer had penetrated and diffused inside dentin, they were polymerized by the QTX-amine compound combination to produce a tight bonding between com-

posite resin and dentin. Kikutake12) speculated that the addition of BPE

to original self-etching primer adhesive improved the latter's mechanical properties, thereby resulting in a high bond strength to tooth substrate. In fact, ten-sile bond strength was almost twice that of non-BPE adhesive. However, BPE has a high molecular weight and high viscosity. When monomers infil-trate into dentin, they diffuse across the spaces of dentin substrate and along collagen fibrils - and the rate of diffusion is proportional to the concentration, temperature, and viscosity of the solvent13) . Some studies reported that as the diffusion of monomers

progressed more deeply into dentin, a more excellent bonding between adhesive resin to dentin could be ob-tained thereby23-25) . Thus, it was expected that dimethacrylate monomer with less viscosity would in-filtrate into dentin more easily, and as a result pro-duced a higher bond strength to dentin than BPE. Among the dimethacrylate monomers, GMR and GAM are water-soluble and have a smaller molecular weight than BPE. On these grounds, we selected GMR and GAM as water-soluble dimethacrylate monomers for our self-etching primer adhesive.

Viscosities of GMA and GAM were less than 10% that of BPE. However, contrary to our expecta-tion, no significant differences existed in the tensile bond strength to dentin among BPE, GMA, and GAM. This was because the present self-etching

primer adhesive produced a decalcified layer of dentin that was too thin to make any difference in mono-mer penetration.

For enamel adhesion, no significant differences existed among BPE, GME, and GAM. However, GAM showed a significantly higher tensile bond strength to enamel than to dentin. It is well known that photopolymerization of acrylate monomer pro-

gresses better than methacrylate monomer26). It was thus speculated that the photopolymerization degree of GAM on enamel surface was more advanced than that on or inside dentin.

In the photopolymerization of dental resins, the structure of amine compound influences the polym-erization degree of dental resin or its adhesiveness to tooth substrate. Nikiaido27) investigated the effect of several kinds of amine compound on polymerization of bonding liners and their adhesion to dentin, and found that NPG and 4- (dimethylamino) benzoic acid were effective in improving bond strength to dentin. Kikutake13) also found that NPG was effective in im-

proving the bond strength of original self-etching

primer adhesive to dentin. But the addition of NPG creates a storage problem for self-etching primer ad-hesives. Schumacher et al.28) reported that PIDAA, of which the structure resembles NPG, was likewise effective as a component of self-etching primer. Thus, we employed PIDAA as the amine compound in our original self-etching primer adhesive. As a re-sult, significant improvement in tensile bond strength was obtained not only for dentin, but also for enamel (Table 3) . Nikaido27) insisted that amine compounds such as NPG aided in the diffusion of monomers into the dentin substrate although no linear correlation was obtained between polymerization degree of adhe-sive liners and their bond strength to dentin. On the other hand, Imai et al.29) insisted that moisture on the dentin surface could contribute to improving bond strength in dentin bonding systems using NPG by promoting polymerization at the dentin interface. Against this diverse multitude of speculations and suggestions with regard to bond strength improve-ment, it warrants further investigation into how PIDAA improved the bond strength to enamel and dentin. Bovine teeth were used as a substitute for

human teeth because of their morphological similari-ties. However, the results obtained using bovine teeth cannot always be extrapolated to human teeth. As such, the bonding effectiveness of the original self-etching primer adhesive to human dentin or enamel should be further examined.

In conclusion, GMR and GAM - which are water-soluble dimethacrylates - were useful as a component of our original self-etching primer adhe-sive, and the combination of PIDAA and QTX was effective in improving the bond strength of composite resin to dentin and enamel in an all-in-one bonding system. Based on the results of the present study, the original self-etching primer adhesive, GMR-PIDAA, proved to be a promising material for com-

posite resin restorations in dental clinics.

ACKNOWLEDGEMENTS

We are grateful Mr. Naoto Kidokoro, Dainippon Ink and Chemicals Inc., for his offer of QTX, and to Dr. Junichi Yamauchi, Kuraray Medical Inc., for his co-operation in the preparation of the self-etching

primer adhesive. Part of this study was supported by a Grant-in-aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan to

promote 2001 - Multidisciplinary Research Projects (2001-2005) .

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