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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 13  |  Issue : 2  |  Page : 83-88

Solvent-free self-etch adhesive as a breakthrough in bonding technology: Fact or fiction?


1 Department of Operative Dentistry, Faculty of Dentistry, Suez Canal University, Ismaileya, Egypt
2 Department of Operative Dentistry, Faculty of Dentistry, Minia University, Minia, Egypt

Date of Submission11-Mar-2016
Date of Acceptance04-Apr-2016
Date of Web Publication23-Aug-2016

Correspondence Address:
Mohamed M Zayed
Department of Operative Dentistry, Faculty of Dentistry, Minia University, Minia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1687-8574.188908

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  Abstract 


Objective
The aim of this study was to compare the microshear bond strength (μSBS) and adhesive dentin interfacial ultramorphology of a solvent-free self-etch adhesive with those of a solvent-containing adhesive.
Materials and methods
Twenty caries-free third molars were used to prepare specimens of dentin surfaces. Ten specimens were prepared for each material. Specimen surfaces were further divided according to the prepared dentin surface into superficial and deep. The adhesives were applied on dentin surfaces according to the manufacturer's instructions, after which Filtek Z250 XT composite resin was condensed through a polyethylene tube with a 0.75-mm internal diameter and 1-mm height attached firmly to the dentin surfaces and light cured. The bonded specimens were stored in distilled water at 37°C for 24 h before being tested. The μSBS values of the adhesives to dentin were evaluated and the collected data were analyzed statistically using one-way analysis of variance. In addition, in each experimental group, three specimens were prepared for analysis under scanning electron microscopy.
Results
There was a significant difference in μSBS between solvent-free self-etch adhesives and solvent-containing adhesives. Scanning electron microscopic findings confirmed the results.
Conclusion
Elimination of the solvent from a self-etch adhesive systems hindered the infiltration of adhesive components into dentin, which affects the bonding quality of resin composite to dentin.

Keywords: adhesive dentin interfacial ultramorphology, microshear bond strength, solvent-free adhesive


How to cite this article:
Abo-Alazm EA, Safy RK, Zayed MM. Solvent-free self-etch adhesive as a breakthrough in bonding technology: Fact or fiction?. Tanta Dent J 2016;13:83-8

How to cite this URL:
Abo-Alazm EA, Safy RK, Zayed MM. Solvent-free self-etch adhesive as a breakthrough in bonding technology: Fact or fiction?. Tanta Dent J [serial online] 2016 [cited 2017 Oct 22];13:83-8. Available from: http://www.tmj.eg.net/text.asp?2016/13/2/83/188908




  Introduction Top


The advent of composite resin materials resulted in many advantages to the field of operative dentistry. However, despite significant improvements over the years, the main reason cited for the failure of posterior composite restorations is secondary caries [1]. One of the challenges faced by dentists in this respect is achieving a durable bond and seal for the restoration. In light of this, a great deal of research has been focused on the adhesives used with composite resin fillings. The latest advancement in adhesion technology is the invention of one-step self-etch adhesive systems. These systems combine etching, priming, and bonding in one step. At a chemical level, they generally consist of ionic resin monomers with phosphate or carboxylic functional groups, hydrophilic monomers such as 2-hydroxyethyl methacrylate (HEMA), hydrophobic monomers such as bisphenol A diglycidyl methacrylate and urethane dimethacrylate, organic solvents (e.g. ethanol, acetone), fillers, and initiators. Water is also included, both as a solvent and as a source of the hydrogen ions required for effective dissolution and demineralization of tooth substrates. To keep water in solution with other resin monomers, hydrophilic and hydrophobic resin monomers of one-step self-etch adhesives were dissolved in a relatively high concentration of organic solvents [2],[3].

The presence of solvents, although traditionally necessary, presents a problem: all solvents should ideally be removed from the dentin surface before polymerization as residual solvent content is a major detrimental factor in bond integrity and longevity [4],[5],[6],[7],[8],[9],[10]. The solvents cause localized impediment of the conversion of monomer into polymer [11], leading to porosities, which enable the inward diffusion of oral fluids [9],[12],[13],[14] and increase water sorption. As a result, the resin and collagen fibrils undergo hydrolysis [15],[16] and the mechanical properties of the adhesive decrease [17], eventually leading to adhesive interface failure, marginal leakage, and secondary caries. Removal of the solvents from the dentin surface in clinical situations is generally carried out by drying with warm air [3],[18], the effectiveness of which depends on the properties of the solvents [19]. As the solvents evaporate, the concentration of nonvolatile monomers increases, which in turn decreases the vapor pressure of the remaining solvents, making it impossible to evaporate the entirety of the solvents under clinically relevant conditions [11],[20].

Recently, a one-step adhesive system, Bond-1 SF, was developed that contains neither water nor organic solvents to eliminate the issue of evaporation. The application of this 'solvent-free' adhesive system does not require air drying and therefore also requires fewer steps. However, as there is little information on its bonding performance, the aim of this study was to evaluate the microshear bond strength (µSBS) of this solvent-free one-step adhesive system to superficial and deep dentin. The null hypothesis tested was that the µSBS of a solvent-free adhesive to both superficial and deep dentin is similar to that of a solvent-containing one-step adhesive.


  Materials and Methods Top


Materials that have been used in this study are shown in [Table 1].
Table 1: Materials used, description, composition, manufacturers, and lot numbers

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Specimen preparation for the microshear bond strength test

Twenty human impacted third molars freshly extracted from patients aged 20–30 years old were collected, cleaned, and stored in distilled water containing a 0.2% thymol antiseptic solution for not more than 1 month at 4°C until testing [21]. Each tooth was embedded in cold-cured acrylic resin up to 1.5 mm apical to the cemento enamel junction, with its long axis parallel to that of the mold. The mounted specimens were assigned to two groups of 10 specimens according to the tested adhesive system used: group 1 using Bond-1 SF (Pentron Clinical, Wallingford, USA) adhesive and group 2 using Single Bond Universal (3M ESPE, St. Paul, USA).

Preparation of occlusal dentin surfaces

Our aim was to prepare the occlusal dentin surface of each molar tooth at two levels: superficial level [below the dentinoenamel junction (DEJ) by 0.5 mm] and deep level (below the DEJ by 1.5 mm).

To detect the level of the DEJ, two guiding grooves were placed on the mesial and distal surfaces using a cylindrical flat-ended diamond stone (ISO #111/012; Mani Inc., Tochigi, Japan) mounted on a high-speed hand piece with copious air–water spray and then the two surfaces were ground flat. With the use of a lead black pencil, a line was drawn on the proximal surfaces parallel to the DEJ, but below it by 0.5 mm. The two lines on the proximal surfaces were then connected carefully through the buccal and lingual surfaces to form a circle around the tooth representing the superficial level. The enamel and dentin tooth structure above the line were cut and the occlusal surface was ground flat to expose the superficial dentin level using a microsaw (IsoMet 4000; Buehler, Lake Bluff, Illinois, USA) under running water. The superficial dentin level was parallel to the occlusal surface, perpendicular on the longitudinal axis of the tooth, and 0.5 mm below DEJ.

A line was drawn on the middle of the flat occlusal surface, dividing it into two equal halves (mesial and distal). An IsoMet saw caliper was adjusted to prepare deep dentin on one half 1-mm deeper than that of the superficial half, and then the prepared surfaces were polished with carbide sandpaper #1000, #1500, and #2000 [22].

Preparation of composite specimens

The adhesive systems were applied on prepared dentin surfaces according to the manufacturer's instructions and before irradiation of the adhesive on each specimen, a hollow cylinder 1 mm in height and 0.75-mm internal diameter microbore tygon tubing (Norton Performance Plastic, Norton, Johannesburg, South Africa) was applied to the dentin surface and then cured for 20 s using a Bluephase C5 LED light-curing unit (Ivoclar Vivadent AG, Schaan, Liechtenstein) at an intensity of 500 mW/cm 2 at a fixed distance [23]. Composite resin was introduced and packed within each tube using a small condenser, care was taken to avoid air inclusion, and then light cured.

The specimens were stored in distilled water at 37°C for 24 h and then the tygon tube around the composite cylinders was removed by gently cutting the tube into two hemicylinders using a feather-edge blade. This procedure was carried out under a stereomicroscope (Nikon, Belmont, United State) and special caution was exercised to avoid application of any stress to the bonded composite cylinders. The cylinders that showed apparent interfacial gap formation, bubble inclusion, or any other relevant defects were excluded from the study ([Figure 1]).
Figure 1: Diagram summarizing the steps of tooth preparation from (left to right). (a) Sound tooth mounted in acrylic block. (b) Prepared superficial dentin surface. (c) Prepared superficial and deep dentin surfaces. (d) Buccal view of composite rods on dentin. (e) Top view of composite rods on dentin. (f) Specimen secured to the testing machine lower head.

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Microshear bond strength test

Each acrylic-embedded tooth with the bonded composite microcylinder was secured with tightening screws to the lower fixed compartment of the materials testing machine (model 3345; Instron) with a load cell of 500 N. Data were recorded using computer software (Bluehill 3; Instron). A loop prepared from an orthodontic wire (0.14 mm in diameter) was wrapped around the bonded microcylinder assembly as close as possible to the base of the microcylinder and aligned with the loading axis of the upper movable compartment of the testing machine. A shearing load with tensile mode of force was applied through the materials testing machine at a crosshead speed of 0.5 mm/min. The maximum load of failure was recorded and the bond strength (expressed in MPa) was calculated according to the following equation:



where δ is the bond strength (MPa), δ is the load of failure (N), and r is the radius of a composite microcylinder (mm). Values of µSBS data were calculated and analyzed statistically using a one-way analysis of variance test.

Resin–dentin interface analysis

An additional 12 extracted human third molars were selected and dentin surfaces were prepared for bonding, as for the µSBS test, three teeth per each subgroup. The adhesive systems and composite build-ups for all specimens were then applied on the dentin surface of each tooth according to the manufacturer's instructions. The specimens were sectioned parallel to the long axis using a water-cooled diamond saw wheel (IsoMet 4000; Buehler) at the mid-coronal part at a low speed.

The sections were then polished with 1200-grit paper discs and immersed in 6 mol/l hydrochloric acid for 30 s. This was followed by immersion in 1 wt% sodium hypochlorite for 10 min. Specimens were then rinsed with distilled water and stored in a desiccator overnight to dry. The specimens were then examined under a scanning electron microscope (SEM) (Philips 505; Philips, Eindoven, The Netherlands).


  Results Top


µSBS test results are shown in [Table 2]. Statistically significant differences were found between the two materials. The solvent-free adhesive system (Bond-1 SF) showed the lowest significant difference compared with solvent-containing (Single Bond Universal) at P less than or equal to 0.001 for superficial dentin. In addition, the solvent-free adhesive system also showed the lowest significant difference compared with the solvent-containing system at P lees than or equal to 0.001 for deep dentin.
Table 2: Mean and SD for different tested adhesives and substrates

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SEM images of the dentin–adhesive interfaces formed by Single Bond Universal adhesive system showed a uniform hybrid layer and excellent and extensive penetration into dentinal tubules, forming numerous and prominent tags for both superficial and deep dentin. The resin tag numbers and lengths seemed to be much higher for deep dentin than superficial dentin in the Single Bond Universal adhesive. The junction between the adhesive and dentin appeared tight and continuous ([Figure 2] and [Figure 3]). For both superficial and deep dentin, the resin tags numbers and lengths seemed to be less numerous and shorter with Bond-1 SF. The hybrid layer formed by Bond-1 SF was less successful than that with the Single Bond Universal ([Figure 4] and [Figure 5]).
Figure 2: Superficial dentin–adhesive interface using a Single Bond Universal bonding agent.

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Figure 3: Deep dentin–adhesive interface using a Single Bond Universal bonding agent.

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Figure 4: Superficial dentin–adhesive interface using a Bond-1 SF bonding agent.

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Figure 5: Deep dentin–adhesive interface using a Bond-1 SF bonding agent.

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  Discussion Top


In this study, impacted third molar teeth were used in this study for making sure of their virginity of caries. To decrease the number of teeth used and the natural structural variation between teeth, superficial and deep dentin surfaces assigned for bonding were prepared in the same tooth. The µSBS test was used in this study for testing bond strength to dentin because of its simple preparation procedures compared with the microtensile method, which is more complicated, with a high risk of stresses induction during preparation. Another advantage of the µSBS test is that it is less demanding in terms of specimen production, and bond test areas can be much better controlled with the use of known diameter microbore tubing. Because bonded surfaces for the microshear test were very small (∼0.5 mm 2), we could test many specimens on a single surface of prepared dentin, resulting in significant conservation of extracted teeth [24].

The durability of the adhesive bond is a major factor in the overall integrity of a restoration, and thus research into adhesive systems is of significant interest in the field of restorative dentistry [25]. Many factors can influence the bonding performance of adhesive systems to dentin, such as dentin substrate, dentin treatment, and chemical composition of the adhesive systems including the solvents [26],[27]. It has already been established that the presence of residual solvent exerts a detrimental effect on monomer infiltration and polymerization [28],[29]; therefore, air drying is used to facilitate the removal of the remaining solvent from the adhesive after it has been applied [30] and to reduce the thickness of the adhesive layer, which enables further solvent removal [31]. However, in clinical conditions, full removal of residual solvents is difficult or impossible to achieve. Various techniques have been suggested to improve monomer infiltration to decrease water sorption and degradation of collagen. One of these techniques is to manufacture adhesives that do not contain water, ethanol, or acetone as conventional solvents, referred to as solvent-free adhesives.

Although Bond-1 SF is labeled as a solvent-free self-etch adhesive, it is worth noting that it actually contains HEMA, a low-molecular-weight monomer, as a solvent and diluent in place of water and the two other conventional solvents, with the goal of reducing phase separation and droplet entrapment. HEMA is frequently used in adhesives because of its positive effect on bond strength because of prevention of phase separation. Its polarity and small dimensions improve the wetting properties of the adhesive solution and the penetration efficacy of the adhesive into demineralized dentin; however, HEMA has a limited H-bonding capacity [32]. It seems that HEMA plays the role of a solvent and a diluent in Bond-1 SF. Therefore, phase separation and droplet entrapment in this adhesive are decreased by the elimination of water and the two other conventional solvents. In contrast, the resin tags seem to be less numerous and shorter with Bond-1 SF for both superficial and deep dentin ([Figure 3]), which might be attributed to less opportunity of the adhesive to penetrate into the demineralized dentin. It is likely that the disadvantages of HEMA, especially limited H-bonding capacity and limitations in cross-linking, result in a significant reduction in bond strength to both superficial and deep dentin and less marginal integrity at dentin margins. Therefore, the results of this study indicate that solvents are beneficial for the dentin margin integrity of self-etch adhesives.

Bond-1 SF also contains the 4-MET monomer, which is proven to form ionic bonds with hydroxyapatite [33], and thus it was reasonable to expect a certain degree of chemical interaction and to observe a shallow resin–dentin interface under SEM. However, SEM samples showed a less homogenous and thinner hybrid layer than that of the Single Bond Universal adhesive. In addition, the pH value of Bond-1 SF (pH=3–4) is comparatively higher than that of the Single Bond Universal adhesive (pH=2.7), which may decrease the dentin demineralization depth.

The lack of a solvent in this adhesive system could be linked to with all the factors described above and may explain the poor hybrid layer formation and the relatively lower bond strength of the Bond-1 SF adhesive system. Keeping this in mind, the results of this study indicate that solvents are beneficial for the integrity of self-etch adhesives. However, further studies are needed to support these hypotheses.


  Conclusion Top


Under the limitations of the present study, it could be concluded that elimination of the solvent from a self-etch adhesive system hindered the infiltration of the adhesive components into the dentin and antagonized the penetration of resin tags into dentinal tubules and formation of a hybrid layer, which affected the bond quality of resin composite to dentin at the different depths tested.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]



 

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