INTRODUCTION

One of the major concerns in adhesive dentistry is the durability of bonds to dentin substrate, because bonding is established on a complex hydrated biological composite structure.1–2 Adhesive systems are hydrophilic in nature, because hydrophilicity is desirable for bonding to a moist substrate such as dentin. However, subsequent water sorption within bonded interfaces is thought to contribute to degradation over time.3–4

Adhesive dentistry has been facing a trend towards simplification of bonding procedures. Different approaches, with different numbers of steps and degrees of sensitivity, have been used to bond resin-based materials to enamel and dentin.5–7 However, simplification of bonding procedures has resulted in increased hydrophilicity and, consequently, decreased long-term bonding effectiveness.8 One-bottle self-priming etch-and-rinse systems and single-step self-etching adhesives are more hydrophilic versions of their multiple-step precursors.7–8 The incorporation of increased concentrations of hydrophilic monomers into adhesive systems may compromise bond durability, as hydrophilicity and hydrolytic stability are antagonistic properties.9

It is well known that bond strength and quality of the seal produced by bonding agents decrease with time both in vitro and in vivo.4,10–12 Water sorption within resin-dentin interfaces has been quoted as one of the dominant factors involved in adhesion degradation.3 However, while numerous studies have examined the water sorption and solubility behavior of restorative resin composites, data on the water sorption and solubility characteristics of adhesive systems currently available in the market and their influence in bond longevity is scarce.13–14 This study evaluated the water sorption and solubility characteristics of five adhesive systems and two primer/adhesive mixtures over a 180-day water-storage period. The null hypotheses tested were: (1) different adhesive solutions present no significant difference in water sorption and solubility behavior and (2) increased water-storage time does not affect the water sorption and solubility behavior of the materials tested.

METHODS AND MATERIALS

Specimen Preparation

Five adhesive systems were used in this study. These systems comprised three categories: one single-step self-etching adhesive (One-up Bond F [OB], Tokuyama, Tokyo, Japan), two two-step self-etching primers (Clearfil SE Bond [CF] and an antibacterial fluoride-containing system, Clearfil Protect Bond [CP], Kuraray Medical Inc, Tokyo, Japan) and two two-step etch-and-rinse adhesives: a water/ethanol-based (Single Bod [SB], 3M ESPE, St Paul, MN, USA) and an acetone-based filled adhesive (Prime&Bond NT [PB], Dentsply/Caulk, Milford, DE, USA). In addition, two primer/adhesive mixtures were tested. The self-etching primers (P) were mixed with the respective hydrophobic bonding agents of Clearfil SE Bond (CF) and Clearfil Protect Bond (CP) in a 1:1 volume ratio (CF+P and CP+P). A 1:1 primer/adhesive volume ratio was used, because it can provide insight into how the water sorption and solubility behavior might be affected within the resin-tooth interface produced with the self-etching primer systems when primer and adhesive solutions are mixed. Composition, batch number and manufacturers of each adhesive system are listed in Table 1.

Table 1 Materials, Brand (Lot #), Composition, Application Technique and Manufacturers of Adhesive Systems Used in This Study

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All the adhesive systems tested present a certain amount of solvents and water in their composition, which could impair polymerization of specimens. Thus, the authors of this study tried to eliminate or at least reduce their content in the adhesive solutions of SB, PB, CP and CF primers. No attempt to reduce the presence of solvent in OB was done, because the manufacturer's instructions do not recommend air-drying after application. For solvent elimination, several drops of each adhesive or primer were dispensed in an adhesive dispenser, and the solution mass was recorded on an analytical balance (JEX-200, YMC Co Ltd, Kyoto, Japan) until reaching equilibrium at room temperature.15 The amount of time necessary for each adhesive system to reach a constant mass was determined. Solvent evaporation was done in a dark box to prevent early polymerization of the adhesives. Glass pipettes were then used to fill a hollow cylinder of approximately 12 mm that was cut from micro bore tygon tubing (TYG-030, Small Parts Inc, Miami Lakes, FL, USA) with an internal diameter of approximately 0.73 mm. A light curing unit (Astralis 5, Ivoclar Vivadent, Schaan, Liechtenstein) with an output of 650 mW/cm² was used to photo-activate specimens for 120 seconds. A razor blade was then used to section the cylinders into 2 mm high cylinders. The specimens were carefully removed from the tygon tubing and randomly assigned to each of the storage periods in water (n=5): 1, 7, 30, 90 and 180 days.

Sorption and Solubility Analysis

A micro-balance (Sartorius 142P, Goettingen, Germany) with a precision of 0.001 mg was used for weighing the specimens. The water sorption/solubility test was performed according to ISO 4049 (1988) for resin-based filling materials, with the exception that the specimens were weighed shortly after preparation (m₁ was recorded. The specimens were then stored for 24 hours) and the dimensions of the specimens were reduced in relation to the original standard. The specimens were immersed individually in an eppendorf container with 1.5 ml of distilled water at 37°C for each storage period. After completing the storage periods (1, 7, 30, 90 or 180 days), the specimens were removed from water, any visible moisture was removed with a paper towel and 1 minute after removal from water mass m₂ was recorded. The specimens were then stored for 24 hours in a desiccator containing freshly dried silica gel and a constant mass (m₃) was obtained. The diameter and thickness of the specimens were measured at three different points using a digital caliper with a precision of 0.01 mm (Mitutoyo, Tokyo, Japan) and the volume (V) was calculated in cubic millimeters. The values of water sorption (W_sp) and solubility (W_sl) were measured and calculated using the following formula:

where:

m₁

is the mass of the specimen in micrograms before immersion in water;

m₂

is the mass of the specimen in micrograms after immersion in water;

m₃

is the mass of the specimen in micrograms after desiccation and

V

is the specimen volume in cubic millimeters.

RESULTS

Results for the water sorption and solubility tests are presented in Tables 2 and 3, respectively. Figure 1 graphically demonstrates water sorption and solubility behavior for the adhesive systems and primer/adhesive mixtures after storage in water. Two-way ANOVA revealed that there were statistically significant differences for the factor “adhesive” (p≤0.0001) and “storage time” (p≤0.0001), and it identified a significant interaction between factors (p≤0.0001). The above information is valid for both analyses (sorption and solubility). The Tukey post-hoc test showed significant differences among adhesive systems at different storage times (p<0.05).

Table 2 Mean Values and Standard Deviation for Water Sorption (μg/mm³) of the Adhesive Materials Tested After 1,7,30,90 and 180 Days of Water Storage

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Table 3 Mean Values and Standard Deviation for Water Solubility (μg/mm³) of the Adhesive Materials Tested After 1,7,30,90 and 180 Days of Water Storage

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Water sorption analysis revealed that tested groups present significantly different water sorption patterns and a different behavior after storage in water (Table 2). PB presented the lowest sorption values, followed respectively by CF, CP, SB and CF+P and CP+P and OB. Except for CP+P and OB, significant differences were observed among adhesives at all periods tested. Similar values were also observed between CF+P and CP+P after 180 days of storage in water. Except for SB, all groups presented a significant increase in water sorption values with increased water-storage time. Stability in water sorption values was only observed after 90 days of immersion in water for the other groups. The primer/adhesive mixtures (CF+P and CP+P) and the single-step self-etching adhesive OB absorbed considerably more water than the other groups.

A tendency towards increased solubility was observed with increased storage time for all groups (Table 3, Figure 1). The primer-adhesive mixture CF+P and the single-step adhesive OB presented the highest solubility values. On the other hand, CP presented the lowest values over the course of the experiment. Solubility increased significantly for all materials tested. After 180 days, the solubility values of all materials were at least two times higher than values observed after one day storage in water.

DISCUSSION

Bonding to a vital, wet substrate such as dentin has been proven to be a difficult task. The mineral phase of the substrate needs to be totally or partially removed and substituted with an adhesive solution that will permeate this collagen-rich layer and polymerize in situ, forming the hybrid layer.16 Adhesive systems are largely exposed to dentinal fluids and, to a lesser extent, to salivary fluids in the oral environment. The water sorption and solubility characteristics of adhesive materials are important in determining the longevity and marginal quality of a restoration. High water sorption values might contribute to marginal staining around composite restorations.13 Water plays an important role in the chemical degradation process of polymer materials.17–18 Thus, more hydrophobic materials tend to take up lower quantities of water and, consequently, present a lower hydrolytic degradation velocity.

Two different theories, which are believed to occur simultaneously, have been proposed related to the diffusion of water molecules into polymer matrices. In the “free volume theory,” water molecules diffuse through nanopores or micromorphological defects of the material without mutual relationship to the polar sites of the material. In the “interaction theory,” water molecules diffuse through the material, binding successively to the hydrophilic groups.19 There are several factors involved in polymer water sorption and solubility characteristics, such as pH of the storage media;20–21 degree of conversion,22 polarity of the molecular structure, presence of pendant hydroxyl groups capable of forming hydrogen bonds with water, degree of crosslinking,23 presence of residual water and presence and type of filler particles.14,24–25 After entering the polymer matrix, water triggers chemical degradation, resulting in the formation of oligomers and monomers.17 The microstructure might be changed due to progressive degradation through the formation of pores. Residual monomers, oligomers and degradation products might be released via these pores.17,26–27 In addition to the degradation process within the polymer, debonding between the polymer and filler particles might occur, resulting in leakage of the filler particles and ions. The degradation and erosion process leads to a mass loss of the adhesive material measured as solubility.

The adhesive materials tested in this experiment present different amounts of hydrophilic and hydrophobic monomers in their composition. Water and solvents are also present in the adhesive blend. In order to bond to the intrinsically wet dentin surface, adhesive systems incorporate resin molecules with both hydrophilic and hydrophobic moieties, or hydrophilic resins, such as 2-hydroxyethylmethacrylate (HEMA). The self-etching ability of contemporary adhesives is commonly achieved by incorporating polymerizable, methacrylate-based resin monomers that contain carboxylic/phosphoric acid moieties or their esters or by incorporating mineral or organic acids as additives to non-acidic hydrophilic resin monomers.6,28 The presence of water is also an essential component, in order to enable ionization of acidic monomers and demineralization of underlying enamel and/or dentin.6 Besides the presence of water in their composition, the ionizable moieties of acidic monomers are hydrophilic. These observations might explain the highest water sorption rates recorded for the self-etching adhesive OB and the self-etching primer/adhesive blends (CF+P and CP+P) tested in this study. High solubility values were also observed for these groups. The primer/adhesive volume ratio (1:1) was certainly higher than it would be in actual clinical conditions. During application of the self-etching primer adhesive systems to the tooth surface, primer and adhesive solutions mix to some extent. The actual ratio of each component (primer and adhesive) of the mixture within the adhesive layer cannot be easily determined, as it varies along the resin-tooth interface.12 Concentration of the self-etching primer tends to be higher closer to the hybrid layer and concentration of the hydrophobic adhesive solution increases as it gets closer to the resin composite.12 It has been demonstrated that mixtures containing higher amounts of primer present a lower degree of conversion and inferior mechanical properties when compared with mixtures containing a lower proportion of primer.29 Both null hypotheses were rejected, because different water sorption and solubility rates were observed for the groups tested in this study, and they tended to increase with increased water-storage time.

Except for OB, all filled adhesives (PB, CF and CP) presented lower water sorption rates than unfilled adhesives. The presence of filler might provide the adhesives with improved mechanical properties and decreased water sorption.24,30 The bonding agents of the two-step self-etching systems proved to be more hydrophobic resins. Low water sorption rates and decreased solubility in water were observed for CP and CF. Even though CF presented lower water sorption means than CP, CP presented the lowest solubility rates over the experiment. The same trend was observed for the primer/adhesive mixtures. CP+P presented higher water sorption rates but lower solubility in water than did CF+P. Moreover, CP also showed a mass increase after one and seven days of storage in water. A possible explanation for the increased mass of CP and the lower water solubility of CP and CP+P is chemical reactions with water within the adhesive and the production of reaction products.31 An important feature of two-step self-etching systems that might contribute to the better results when compared to one-step systems is the hydrophobic resin layer that is placed over the primer, which might reduce water sorption.12

The solvent depleted UDMA/PENTA filled solution of PB presented the lowest water sorption values. However, solubility values were higher than those observed for SB, CF and CP and were comparable to those recorded for CP+P. The low water sorption values are probably due to the presence of nanofillers within the bonding solution. However, even though the fillers, themselves, are relatively inert inorganic materials, the coupling agents associated with them are prone to hydrolysis via ester linkages within the molecules or siloxane links that are formed with the filler particle.32 The elution of filler particles might have occurred for PB after storage in water.

The materials' compositions are inherently prone to hydrolysis due to the presence of unprotected ester linkages in the monomers.17 The presence of monomers with different properties, such as molecular weight, hydrophilicity (hydrophobicity) and degradation behavior, might induce preferential degradation of one phase in a polymer system.33 This could lead to the formation of pores within the polymer matrix. Yiu and others34 recently reported that the increased hydrophilicity of resin blends, such as those employed in dentin adhesives, resulted in decreased mechanical strength after long-term water storage. Increased water-storage time resulted in increased solubility values for all adhesives tested in this investigation. For SB, stabilization in solubility values was observed after seven days; 30 days for CP+P and 90 days for CF+P, CF and PB. Increased solubility was still observed for CP and OB after 180 days of storage in water. Elution of degradation products might have occurred over the water-storage period. The leaching of monomers has a potential impact on the structural stability and biocompatibility of the material.35 The reduction of some mechanical properties of resin composites (such as modulus of elasticity, strength and hardness) has been attributed to the plasticizing effect of water.23 The same plasticization effect can be expected to occur in adhesive systems.36

The water sorption and solubility values of bonding agents have been reported to be much higher than composite filling materials.14 High water sorption and solubility values could lead not only to marginal discoloration, but to decreased mechanical properties of the resin-dentin interface and possibly to compromised restoration longevity. Simplification of clinical application procedures has resulted in a loss of bonding effectiveness.8,37 High water sorption and solubility rates were observed for the 1-step self-etching adhesive OB and the primer/adhesive blends CP+P and CF+P. Further research is necessary for the promotion of simplified adhesives able to bond to a moist substrate such as dentin, but with more hydrophobic characteristics after polymerization and less susceptibility to water solubility.