The pKa effects of the carboxylic acid in N-methacryloyl-omega-amino acid on the demineralization and bond strengths to the teeth

Light-curable urushiol enhanced bisphenol A glycidyl dimethacrylate dentin bonding agent

OBJECTIVES

The low durability of composite resin restorations can be attributed to the degradation of the resin dentin bonding interface. Owing to the presence of hydrophilic components in the adhesive, the integrity of the resin dentin bonding interface is easily compromised, which, in turn, leads to a reduction in bond strength. The hydrophilic nature of the adhesive leads to water sorption, phase separation, and leaching of the resin component. Therefore, hydrophobic adhesives could effectively be used to stabilize the integrity and durability of the resin dentin bonding interface.

METHODS

We synthesized a novel hydrophobic dentin adhesive by partially replacing bisphenol A glycidyl dimethacrylate (Bis-GMA) with a light-curable urushiol monomer. The properties of the produced adhesive, including the degree of conversion, viscosity, contact angle, water sorption/solubility, and mechanical strength, were comprehensively examined and compared to those of the commercially adhesive Adper Single Bond2 as a positive control. The adhesive properties were determined using microtensile bond strength measurements, laser confocal microscopy, scanning electron microscopy observations, and nanoleakage tests. Finally, the novel adhesive was subjected to biocompatibility testing to determine its potential cytotoxicity.

RESULTS

At a light-curable urushiol content of 20 %, the synthesized adhesive exhibited high degrees of conversion and hydrophobicity, low cytotoxicity, good mechanical properties, and outstanding adhesive strength.

CONCLUSIONS

The introduction of the light-curable urushiol into dentin adhesives can significantly enhance their hydrophobic, mechanical, and bonding properties, demonstrating potential to significantly improve restoration longevity.

CLINICAL SIGNIFICANCE

The integration of light-curable urushiol has endowed the experimental adhesives with several enhanced functionalities. These notable benefits underscore the suitability of this monomer for expanded applications in clinical practice.

Chemical Interaction and Interface Analysis of Self-Etch Adhesives Containing 10-MDP and Methacrylamide With the Dentin in Noncarious Cervical Lesions

Objectives:

To characterize the chemical interactions and analyze the interface of adhesive systems containing 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) and N-methacryloyl glycine (methacrylamide) functional monomers with the dentin in noncarious cervical lesions (NCCLs) compared with artificial defects (ADs).

Methods and Materials:

Twenty human teeth with natural NCCLs on the buccal surface were used. Class V cavities, similar to NCCLs, were created on the lingual surface to serve as controls. Teeth were randomly allocated to two groups according to the functional monomer in the adhesive (N=10): G1, 10-MDP; and G2, methacrylamide. NCCLs and ADs were characterized by their mineral composition (MC) and degree of demineralization (DD) using micro-Raman spectroscopy, adhesive/dentin chemical interactions (CIs) were assessed with infrared photoacoustic spectroscopy, and interface morphology was evaluated with scanning electron and light microscopy. MC, CI, and DD data were submitted to Shapiro-Wilk and Student t-tests (p<0.05).

Results:

Compared with ADs, dentin in NCCLs was hypermineralized (p<0.05). In G1, CI, and DD in the first 2 μm, and adhesive projections in NCCLs and ADs interfaces were similar. Additionally, a thin layer of dentin collagen was observed in ADs, while it was hardly present in NCCLs. In G2, although CI could not be identified, changes in the mineral components were observed. The DD in the ADs and NCCLs were statistically similar, while SEM showed a lack of adhesion at NCCLs interface. DD and collagen exposure in the ADs and NCCLs were more pronounced than in G1.

Conclusions:

Results suggest that the G1 adhesive could be applied directly on the superficial sclerotic layer in NCCLs. In contrast, previous cavity preparation should be conducted to improve the micromechanical interaction of G2 with the dentin.

Effect of the concentration of water in an MDP-based all-in-one adhesive on the efficacy of smear layer removal and on dentin bonding performance

The effects of the water concentration in an experimental 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based all-in-one (EX) adhesive were examined on the ability of MDP to remove the smear layer from the ground dentin surface and on the dentin bonding performance. Four types of EX adhesives were prepared by varying the amount of water (46.6, 93.2, 149.8 and 208.1 mg/g), but the MDP concentration was kept a constant at 49.9 mg/g. Scanning electron microscopy and bond strength measurements were performed on the dentin surface demineralized by each EX adhesive. Increased amount of water in the EX adhesive increased the ability of MDP to remove the smear layer. However, the solubilization of the smear layer into the EX adhesive decreased the dentin bond strength. The water concentration in the EX adhesive affected the efficacies of smear layer removal and dentin bonding performance more strongly than the pH value of the EX adhesive.

Effect of calcium salt of 10-methacryloyloxydecyl dihydrogen phosphate produced on the bond durability of one-step self-etch adhesive

Five experimental 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based one-step self-etch adhesives were designed by varying amounts of MDP. The aim of this study was to examine the effect of the quantity of calcium salt of MDP (MDP-Ca) salt produced by demineralization on the bond durability between experimental one-step adhesives and enamel or dentin. Bond strengths of experimental adhesives to the enamel and dentin were measured, before and after 30,000×thermocycling. The fractured enamel and dentin samples as well as the fractured adhesive surfaces obtained during adhesion test were analyzed by a scanning electron microscope and energy-dispersive X-ray microscope. An increase in the amount of MDP-Ca salt to above 37.2 mg/g drastically decreased the dentin bond strength and changed the fracture type during the thermocycling process. In contrast, the enamel bond strength remained unchanged, although the fracture type changed to an interfacial failure with increasing the amount of MDP-Ca salt to 78.3 mg/g.

Determination of molecular species of calcium salts of MDP produced through decalcification of enamel and dentin by MDP-based one-step adhesive

Enamel and dentin particles were added to an experimental 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based one-step adhesive to react for 30 s. After enamel and dentin reactants were analyzed using X-ray diffraction (XRD) and phosphorus-31 nuclear magnetic resonance ((31)P NMR) techniques, curve-fitting analysis was performed on the (31)P NMR spectra of enamel and dentin reactants. By varying the molar ratio of calcium chloride to MDP, a series of three types of MDP-Ca salts were synthesized. The molecular species of calcium salts of MDP (MDP-Ca salts) produced by decalcification of enamel or dentin were determined based on the XRD and (31)P NMR analysis results of these three types of synthesized MDP-Ca salts. Curve-fitting analysis showed that enamel and dentin developed several types of MDP-Ca salts and amorphous dicalcium phosphate dihydrate (DCPD) during decalcification. The molecular species of MDP-Ca salts produced by enamel and dentin were mono-calcium salts of MDP monomer and MDP dimer. In addition, dentin produced a di-calcium of MDP dimer.

Development of MDP-based one-step self-etch adhesive--effect of additional 4-META on bonding performance

We designed three experimental 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based one-step (EX) adhesives consisting of MDP, urethane dimethacrylate, and triethylene glycol dimethacrylate adhesives with different water contents (98.4, 196.8, and 294.4 mg/g), and 4-methacryloyloxyethyl trimellitic anhydride (4-META) or 2-hydroxyethyl methacrylate (HEMA)-containing onestep adhesive. The effect of the amount of MDP-calcium (MDP-Ca) salt produced through demineralization of enamel and dentin on the bonding performance was examined. The efficacy of 4-META and HEMA was then discussed. When the amount of water in EX adhesive was increased, the production amount of MDP-Ca salt of enamel increased, but not the dentin. The enamel bond strength slightly increased with increasing the production amount of MDP-Ca salt, in contrast to the dentin. However, addition of 4-META in the EX adhesive (water content=98.4 mg/g) increased both bond strengths, although the production amounts of MDP-Ca salt significantly decreased. The 4-META enhances both bond strengths more effectively than the HEMA.

Role of 2-hydroxyethyl methacrylate in the interaction of dental monomers with collagen studied by saturation transfer difference NMR

OBJECTIONS

Functional adhesive monomers are formulated with solvents and hydrophilic resin monomers, such as 2-hydroxyethyl methacrylate (HEMA). In theory, exposed collagen fibrils should be covered and protected by the resin matrix. We examined if the atomic- and molecular-level interaction of monomers with collagen would be affected when the monomers are blended with HEMA.

METHODS

We performed saturation transfer difference (STD) NMR spectroscopy to investigate the binding interaction of two functional monomers, 4-methacryloyloxyethyl trimellitic acid (4-MET) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP), with atelocollagen as a triple-helical peptide model. The STD NMR measurement was performed by adding 4-MET or MDP to the atelocollagen solution.

RESULTS

When the atelocollagen was saturated, the STD signals were detected in the MDP spectrum for the protons in the aliphatic chain when MDP was dissolved in DMSO. However, the STD signals disappeared when MDP was mixed with HEMA. No STD signal was visible for the 4-MET ligand samples in either DMSO or for the HEMA blend sample.

DISCUSSION

The interaction of MDP with atelocollagen is hydrophobic; however, the MDP-HEMA blend may form an aggregate in the atelocollagen solution, which would suppress the hydrophobicity of MDP. The formation of the MDP-HEMA aggregate may compromise the MDP-collagen interaction, and leave the collagen fibrils unprotected by MDP and HEMA. Unstable chemical interaction of the monomers with the exposed collagen may deteriorate hybrid layer integrity and strong dentine bonding.

Novel fluoro-carbon functional monomer for dental bonding

Among several functional monomers, 10-methacryloxydecyl dihydrogen phosphate (10-MDP) bonded most effectively to hydroxyapatite (HAp). However, more hydrolysis-resistant functional monomers are needed to improve bond durability. Here, we investigated the adhesive potential of the novel fluoro-carbon functional monomer 6-methacryloxy-2,2,3,3,4,4,5,5-octafluorohexyl dihydrogen phosphate (MF8P; Kuraray Noritake Dental Inc., Tokyo, Japan) by studying its molecular interaction with powder HAp using solid-state nuclear magnetic resonance ((1)H MAS NMR) and with dentin using x-ray diffraction (XRD) and by characterizing its interface ultrastructure at dentin using transmission electron microscopy (TEM). We further determined the dissolution rate of the MF8P_Ca salt, the hydrophobicity of MF8P, and the bond strength of an experimental MF8P-based adhesive to dentin. NMR confirmed chemical adsorption of MF8P onto HAp. XRD and TEM revealed MF8P_Ca salt formation and nano-layering at dentin. The MF8P_Ca salt was as stable as that of 10-MDP; MF8P was as hydrophobic as 10-MDP; a significantly higher bond strength was recorded for MF8P than for 10-MDP. In conclusion, MF8P chemically bonded to HAp. Despite its shorter size, MF8P possesses characteristics similar to those of 10-MDP, most likely to be associated with the strong chemical bond between fluorine and carbon. Since favorable bond strength to dentin was recorded, MF8P can be considered a good candidate functional monomer for bonding.

Effect of calcium salt of functional monomer on bonding performance

To determine the amount of 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-calcium (MDP-Ca) salt produced through the demineralization of enamel or dentin by MDP, we designed experimental MDP-based one-step adhesives with different amounts of MDP. The null hypotheses were that (1) the amount of MDP-Ca salt produced through the demineralization of enamel was the same as that for dentin, and (2) the amounts of MDP-Ca salt have no effect on bonding performance. Increases in the amount of MDP resulted in increased amounts of MDP-Ca salt. The production amount of MDP-Ca salt of the dentin was 1.3 times higher than that of the enamel. The predominant species of the MDP and enamel reactants was a calcium hydrogen phosphate of MDP. In contrast, the dentin yielded both calcium phosphate and calcium hydrogen phosphate of MDP. Increases in the amount of MDP-Ca salt decreased both enamel and dentin bond strengths. An optimal concentration of MDP exists in one-step self-etch adhesives.

Nanolayering of phosphoric acid ester monomer on enamel and dentin

Following the "adhesion-decalcification" concept, specific functional monomers possess the capacity to primary chemically interact with hydroxyapatite (HAp). Such ionic bonding with synthetic HAp has been demonstrated for 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), manifest as self-assembled "nanolayering". In continuation of that basic research this study aimed to explore whether nanolayering also occurs on enamel and dentin when a 10-MDP primer is applied following a common clinical application protocol. Therefore, the interaction of an experimental 10-MDP primer and a control, commercially available, 10-MDP-based primer (Clearfil SE Bond primer (C-SE), Kuraray) with enamel and dentin was characterized by X-ray diffraction (XRD), complemented with transmission electron microscopy interfacial ultrastructural data upon their reaction with enamel and dentin. In addition, XRD was used to study the effect of the concentration of 10-MDP on nanolayering on dentin. Finally, the stability of the nanolayers was determined by measuring the bond strength to enamel and dentin when a photoinitiator was added to the experimental primer or when interfacial polymerization depended solely on the photoinitiator supplied with the subsequently applied adhesive resin. XRD confirmed nanolayering on enamel and dentin, which was significantly greater on dentin than on enamel, and also when the surface was actively rubbed with the primer. Nanolayering was also proportional to the concentration of 10-MDP in the primer. Finally, the experimental primer needed the photoinitiator to obtain a tensile bond strength to dentin comparable with that of the control C-SE primer (which also contains a photoinitiator), but not when bonded to enamel. It is concluded that self-assembled nanolayering occurs on enamel and dentin, even when following a clinically used application protocol. The lower bonding effectiveness of mild self-etch adhesives to enamel should be ascribed in part to a lower chemical reactivity (nanolayering) with enamel HAp.

Bond strength and morphology of enamel using self-etching adhesive systems with different acidities

Objectives:

To assess the bond strength and the morphology of enamel after application of self-etching adhesive systems with different acidities. The tested hypothesis was that the performance of the self-etching adhesive systems does not vary for the studied parameters.

Material and methods:

Composite resin (Filtek Z250) buildups were bonded to untreated (prophylaxis) and treated (burcut or SiC-paper) enamel surfaces of third molars after application of four self-etching and two etch-and-rinse adhesive systems (n=6/condition): Clearfil SE Bond (CSE); OptiBond Solo Plus Self-Etch (OP); AdheSe (AD); Tyrian Self Priming Etching (TY), Adper Scotchbond Multi-Purpose Plus (SBMP) and Adper Single Bond (SB). After storage in water (24 h/37°C), the bonded specimens were sectioned into sticks with 0.8 mm² cross-sectional area and the microtensile bond strength was tested at a crosshead speed of 0.5 mm/min. The mean bond strength values (MPa) were subjected to two-way ANOVA and Tukey's test (α=0.05). The etching patterns of the adhesive systems were also observed with a scanning electron microscope.

Results:

The main factor adhesive system was statistically significant (p<0.05). The mean bond strength values (MPa) and standard deviations were: CSE (20.5±3.5), OP (11.3±2.3), AD (11.2±2.8), TY (11.1±3.0), SBMP (21.9±4.0) and SB (24.9±3.0). Different etching patterns were observed for the self-etching primers depending on the enamel treatment and the pH of the adhesive system.

Conclusion:

Although there is a tendency towards using adhesive systems with simplified application procedures, this may compromise the bonding performance of some systems to enamel, even when the prismless enamel is removed.

Degradation-stage effect of self-etching primer on dentin bond durability

It is well-known that self-etching primers can be altered. However, the effects from altered primers on the dentin bond durability have yet to be thoroughly identified. In this study, we examined the effects from 5 altered Liquid A primers in different stages of degradation-where 2-hydroxyethylmethacrylate (HEMA) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP), used in Liquid A primers, were altered by the hydrolysis of the methacryloxy ester portion in the HEMA and MDP-on the hybrid layer's quality and dentin bond durability. The hypothesis was that degradation stages of altered Liquid A primers have no effect on the hybrid layer's quality and on dentin bond durability. Bond strengths, obtained after thermo-cycling, were strongly dependent on the degradation stage of the altered Liquid A primer. Alterations of self-etching primers reduced dentin bond durability and decreased the created hybrid layer's quality.

Adhesion of a self-etching system to dental substrate prepared by Er:YAG laser or air abrasion

The purpose of this study was to assess the microtensile bond strength of a self-etching adhesive system to enamel and dentin prepared by Er:YAG laser irradiation or air abrasion, as well as to evaluate the adhesive interfaces by scanning electron microscopy (SEM). For microtensile bond strength test, 80 third molars were randomly assigned to five groups: Group I, carbide bur, control (CB); II, air abrasion with standard tip (ST); III, air abrasion with supersonic tip (SP); IV, Er:YAG laser 250 mJ/4 Hz (L250); V, Er:YAG laser 300 mJ/4 Hz (L300). Each group was divided into two subgroups (n = 8) (enamel, E and dentin, D). E and D surfaces were treated with the self-etching system Adper Prompt L-Pop and composite buildups were done with Filtek Z-250. Sticks with a cross-sectional area of 0.8 mm(2) (+/-0.2 mm(2)) were obtained and the bond strength tests were performed. Data were submitted to ANOVA and Tukey's test. For morphological analysis, disks of 30 third molars were restored, sectioned and prepared for SEM. Dentin presented the highest values of adhesion, differing from enamel. Laser and air-abrasion preparations were similar to enamel. Dentin air-abrasion with standard tip group showed higher bond strength results than Er:YAG-laser groups, however, air-abrasion and Er:YAG laser groups were similar to control group. SEM micrographs revealed that, for both enamel and dentin, the air-abrasion and laser preparations presented irregular adhesive interfaces, different from the ones prepared by rotary instrument. It was concluded that cavity preparations accomplished by both Er:YAG laser energies and air abrasion tips did not positively influence the adhesion to enamel and dentin.

Evaluation of intermolecular interactions of self-etch dentin adhesive primer molecules with type 1 collagen: computer modeling and in vitro binding analysis

The objective of this investigation was to study adhesion of self-etch primer systems to dentin by computer-modeled docking simulations and in vitro binding assay methods. Computer modeling employed analysis of docking simulations of a self-etch primer molecule 10-methacryloxydecamethylene phosphoric acid (MDP) and its calcium salt (MDPCa) as ligands. Typical type 1 collagen segments were selected as targets to reflect potential differences in the amino acid residues in dentinal type 1 collagen triple helix motif. The binding assay involved immunochemical analysis of the modification of anti-collagen binding to collagen by prior exposure of the demineralized dentin to MDP. The estimated mean docking energy values ranged between -4.5 and -8.9kcalmol(-1). The results revealed significant differences in the docking energy estimates as a function of ligand and target structures (p<0.01). Van der Waals and electrostatic contributions were also significantly influenced by ligand selection and collagen structure. Both MDP and MDPCa appear to be important in the overall interactions. Binding assay studies also lend evidence of collagen-ligand intermolecular interactions. It is suggested that the ability of self-etch dentin primer systems to bond effectively to dentin is not limited to the interaction of the primer with the hydroxyapatite of dentin, but also due to the ability to prime dentin efficiently through intermolecular interactions between the primer and its calcium salt with the collagen matrix. Virtual screening methods may be very valuable to select primer molecules for dentin bonding.

Systematic review of the chemical composition of contemporary dental adhesives

Dental adhesives are designed to bond composite resins to enamel and dentin. Their chemical formulation determines to a large extent their adhesive performance in clinic. Irrespective of the number of bottles, an adhesive system typically contains resin monomers, curing initiators, inhibitors or stabilizers, solvents and sometimes inorganic filler. Each one of these components has a specific function. The aim of this article is to systematically review the ingredients commonly used in current dental adhesives as well as the properties of these ingredients. This paper includes an extensive table with the chemical formulation of contemporary dental adhesives.

NMR Study on the Adhesion Efficacy of Experimental Phosphonic Acid Monomers

Three experimental self-etching primers - consisting of N-methacryloyl-omega-aminoalkyl phosphonic acid (NMomegaP) with different methylene chain lengths and N-methacryloyl glycine (NMGly) - were formulated. The influence of methylene chain length in NMomegaP derivatives on the chemical nature of calcium salts was examined following their application to tooth components. Bond strengths of experimental self-etching primers created with these monomers to enamel and dentin were also investigated. Nuclear magnetic resonance spectroscopy showed that NMomegaPs decalcified tooth components with formation of calcium salts, which changed from calcium hydrogen phosphonate to calcium phosphonate with increase in methylene chain length within the NMomegaP structure. Disparity in calcium salt formation was related to increases in bond strength to enamel from 18 to 24 MPa. However, bond strength to dentin remained unchanged (22 MPa). The relative dependency of bond strength on monomer methylene chain length was probably attributable to the sites where these NMomegaP calcium salts had deposited on the bonding substrates.

Multi-purpose Bonding Performance of Newly Synthesized Phosphonic Acid Monomers

Multi-purpose bonding performance of three kinds of newly synthesized phosphonic acid monomers was investigated. Methacryloxyalkyl or acryloxyalkyl phosphonoacetates of 6-MHPA, 6-AHPA, 10-MDPA were synthesized in 42.8-51.9% yields with a light yellow viscous liquid, and identified as new compounds by 1H NMR, IR, and elemental analysis. Conventional adhesive monomers, namely VBPA, 4-META, and 4-AETA, and CEBA-p-TSMo-t-BPMA initiator were also used. Seven experimental composite-type adhesive resins comprising these six kinds of adhesive monomers and None (control) with the initiator were prepared. Tensile bond strengths of adhesive resins to unetched ground enamel and dentin, ground porcelain, and sandblasted Ni-Cr alloy were measured at a crosshead speed of 1.0 mm/min. Results showed that except with VBPA, there were no significant differences among 6-MHPA, 6-AHPA, 10-MDPA, 4-AETA, and 4-META in bonding performance to the adherends (p<0.01). It was found that the new phosphonic acid monomers provided good multipurpose adhesion to all adherends tested.

Development of Bonding System for Resin Core Construction by M.OMEGA.A Adhesive

We designed self-etching primers consisting of a series of four N-methacryloyl-omega-amino acids, MomegaA, of different methylene chain numbers for resin core construction. The interacted amount of MomegaA's carboxylic acid with root or crown dentin apatite was determined, and its effects on dentin bond durability examined. The addition of both dentin particles to the MomegaA solutions caused the carbonyl carbon peak of carboxylic acid in MomegaA to shift to a lower field, chiefly because of an acid-base interaction between carboxylic acid and calcium. Then, as the pKa value of MomegaA's carboxylic acid increased, the amount of carboxylic acid that interacted with calcium decreased. In terms of dentin bonding durability, the four tested MomegaA adhesives provided noticeably higher bond strengths of resin to root or crown dentin than ED Primer II. Therefore, from the perspective of restoring pulpless teeth with minimal intervention, carboxylated MomegaA adhesives seemed to be very useful as functional monomers for self-etching primers.

Water concentration in self-etching primers affects their aggressiveness and bonding efficacy to dentin

Water is required to ionize acid resin monomers for demineralization of tooth substrates. We tested the null hypothesis that altering the water concentration in two-step self-etching primers has no effect on their aggressiveness and bonding efficacy to dentin. Five experimental self-etching primers were prepared with resin-water-ethanol volume ratios of 9-0-1, 8-1-1, 7-2-1, 5-4-1, and 3-6-1. They were applied to smear-layer-covered dentin, followed by a bonding resin and composite build-ups for microtensile bond testing and TEM examination of tracer penetration. Increasing water concentration from 0-60 vol% improved acidic monomer ionization that was manifested as increasing hybrid layer thickness. However, significantly higher bond strength was observed in the 7-2-1 group, with minimal nanoleakage in the corresponding hybrid layer. When self-etching primers are formulated, a balance must be achieved to provide sufficient water for adequate ionization of the acidic monomers, without lowering the resin concentration too much, to optimize their bonding efficacy to dentin.

Efficacy of varying the NMEP concentrations in the NMGly–NMEP self-etching primer on the resin-tooth bonding

It is well understood that the application of a self-etching primer enhances the bonding at the resin-teeth interface. In this study, we designed a self-etching primer consisting of N-methacryloyl glycine (NMGly) and N-methacryloyl-2-aminoethyl phosphonic acid (NMEP). The demineralization effects on the hydroxyapatite or dentin by the carboxylic acid in the NMGly and by the phosphonic acid in the NMEP and their effects on the bond strength of the resin to the tooth were examined. The application of the NMGly-NMEP solution to the enamel resulted in an increase in the bond strength when additional amounts of NMEP were added to the NMGly aqueous solution. This increase was due to the phosphonic acid in the NMEP demineralizing the enamel. Conversely, the addition of the NMEP to the NMGly solution resulted in a decrease in the bond strength to the dentin. The optimal concentration of the NMEP in the NMGly-NMEP solution resulted in bond strengths of over 20 MPa for both the enamel and dentin.