The influence of various adhesive applicators on the bond strength to dentin: an in vitro experimental study
Article information
Abstract
Objectives
The aim of this in vitro study was to evaluate the influence of various adhesive applicators on the microshear bond strength (µSBS) of flowable resin composite to dentin.
Methods
Forty-two extracted molars were prepared as disks and divided into two groups. In the first group, GLUMA Bond Universal (Heraeus Kulzer GmbH) was applied to the tooth surface for 20 seconds using a bristle brush, while in the second group, the same adhesive was applied by the same operator using a microbrush. Photopolymerization was performed for 10 seconds using a Valo light-curing unit (Ultradent Products Inc.). Microcylindrical silicone tubes (2 mm long, 0.8 mm inner diameter) were used as molds on the bonded dentin surfaces, and Charisma Bulk Flow ONE resin composite (Heraeus Kulzer GmbH) was injected and cured for 20 seconds. Both groups underwent a µSBS test, and the failure modes were examined microscopically. Student t-test compared the failure stress between the groups (p < 0.05).
Results
The bristle brush group had the highest mean stress at failure (49.09 ± 8.76 MPa) compared to the microbrush group (37.20 ± 6.77 MPa), which also had the highest percentage of adhesive failure (95.24%).
Conclusions
The choice of adhesive applicator plays a decisive role in determining the µSBS of flowable resin composite to dentin. It is advisable to use a bristle brush applicator when applying adhesive.
INTRODUCTION
In recent years, adhesive dentistry has made considerable progress, particularly in improving the mechanical properties and bond strength of resin-based adhesive systems [1,2]. Nevertheless, these procedures remain highly technique-dependent due to the many steps involved, making them susceptible to operator-induced variations. Consequently, the success of an adhesive system depends not only on its inherent material properties but also on the clinician’s technique and the tools used during application [3,4].
As dental adhesives are increasingly used in restorative treatments, the application method has become a crucial factor influencing bond strength and clinical results. There are several techniques for applying adhesives, each of which can impact the final result. These include: (a) direct application followed by immediate light-curing; (b) application with removal of excess adhesive with a gentle air stream; (c) removal of excess adhesive with a dry microapplicator; and (d) removal with both a dry microapplicator and a gentle air stream. A thin and uniform layer of adhesive is considered essential for achieving optimal bond strength [5,6].
To minimize the technique’s sensitivity, newer adhesive formulations have been developed that are easier to apply and require fewer steps, making them more suitable for routine clinical use. [1]. Universal adhesives, for example, contain water as a key component that enables ionization of the acidic monomers and promotes adequate bonding with both dentin and enamel surfaces [7,8].
Accordingly, solvent evaporation is a crucial step during application. It has been shown that extending the air-drying time to 10–15 seconds, as recommended by the manufacturer, significantly improves the bond strength to dentin after 24 hours [9,10]. Other approaches to improve adhesion include applying multiple layers of universal adhesive, which can enhance hybrid-layer formation and resin penetration [11–13]. During application, clinicians also need to control critical parameters, such as active rubbing and air-drying, which together increase the adhesive’s contact time and the pressure exerted on the tooth surface, thereby promoting a stronger micromechanical bond to both enamel and dentin [6,14,15].
Initially, dental adhesives were applied using simple tools, such as cotton pellets, tissue, sponge tips, or disposable brushes with elongated bristles [16]. The introduction of the microbrush in the early 2000s represented a significant advance in the application of adhesives [17]. This tool, a flexible, hand-held, disposable applicator with a fine nylon tip, allows for more precise placement and handling of adhesive materials. It is commonly used to apply, rub, or clean various substances, such as etchants, disclosant dyes, and resins [15], and is usually made from fibers such as nylon, cotton, or synthetic polymers [18]. Comparative studies have shown that the use of microbrushes can produce a more uniform, micromechanically stable bond than conventional brushes, especially when placing endodontic posts [19,20].
Scanning electron microscopy (SEM) is often used in such investigations to identify defects and assess the integrity of the adhesive layer. In one study, Alshawi et al. [15] used SEM to examine the residual contaminants on adhesive surfaces of different applicators. Their results showed that microbrushes consistently left fiber residues on the bonded surfaces, while bristle brush applicators left no visible residues. Although Alshawi et al. [15] did not explicitly investigate the mechanical effects of these residues, the presence of microfibers could weaken the bond between the restorative material and dentin, leading to microleakage, secondary caries, or restoration failure.
A review of the literature shows that previous studies have found differences in bond strength between different applicator types when used on root dentin [4]. In contrast, the effects of the different applicator types on bond strength to crown dentin during adhesive application in the prepared cavity remain uninvestigated.
The aim of this in vitro study is to evaluate the influence of different applicator types (specifically, bristle brush vs microbrush) on the microshear bond strength (µSBS) of a universal adhesive applied to crown dentin when used with a flowable resin composite. Previous microscopic investigations have indicated that microbrush applicators consistently leave fiber residues on bonded surfaces, whereas bristle brush applicators do not leave visible contaminants [15]. Based on the premise that residual contaminants at the adhesive interface can compromise bonding integrity, the research hypothesis is that the use of bristle brush applicators will result in significantly higher µSBS values to crown dentin compared to microbrush applicators.
METHODS
Determination of the specimen size
The specimen size was determined based on a pilot study with both groups (n = 21), which resulted in a large effect size (f = 0.89) based on the calculated means and standard deviations. At least 30 specimens (15 per group) were required to achieve 96% significance at a significance level of 0.05. To improve reliability, the total specimen size was increased to 42 (n = 21 per group).
Preparation of the specimens
The study protocol was approved by the Clinical Research Ethics Committee of the Faculty of Dentistry, Istanbul University (project number: 2024/10).
A total of 42 sound human molars that had been extracted for periodontal reasons and showed neither caries nor structural damage were used for this study. Informed consent was obtained from all patients prior to extraction, and they were informed that their teeth would be used for research purposes. After extraction, the teeth were stored in sterile saline solution (0.9% NaCl, w/v) at 4°C for up to 14 days. All visible calculus, discoloration, and biological debris were removed with manual curettes. Each tooth was then embedded in a 16 × 16 × 28 mm self-curing acrylic resin block (Varidur; Buehler, Lake Bluff, IL, USA).
The occlusal enamel of each molar was removed with a slow-speed diamond saw (Isomet, Buehler) under constant water cooling to a depth of 3 mm from the highest cusp, exposing a flat dentin surface to produce a disk-shaped specimen. All specimens were prepared by a single examiner (AA) and examined at ×3.5 magnification to ensure smooth margins and no sharp edges. To simulate an artificial smear layer, the exposed dentin was polished with 150- and 320-μm grit silicon carbide abrasive papers [20]. The surfaces were then rinsed with sterile deionized water for 10 seconds and carefully air-dried.
Adhesive procedure
Specimens were randomly assigned to two experimental groups using a computer-generated randomization table created by an independent researcher not involved in the laboratory procedures. All bonding procedures were performed by a single calibrated operator (AA) according to the manufacturer’s instructions for GLUMA Bond Universal (Heraeus Kulzer GmbH, Hanau, Germany).
To ensure standardization of the adhesive volume, the specific ‘drop control’ nozzle design of the adhesive bottle GLUMA Bond Universal was used. According to the manufacturer, this system ensures a consistent, precise, and clean dosage for every application. Only one drop of the adhesive was dispensed and applied strictly in accordance with these manufacturer’s instructions for each applicator [21]. Following application, the adhesive was air-dried carefully with a gentle stream of oil-free air until the adhesive film was immobile. This step was performed strictly according to the manufacturer’s instructions to ensure solvent evaporation and prevent the formation of excessive puddles or variations in film thickness [21].
Group 1 (n = 21): bristle brush applicator
In this group, GLUMA Bond Universal was actively applied to the prepared dentin surfaces in self-etch mode using a bristle brush applicator (Ultrabrush; Young Innovations, Algonquin, IL, USA) (Figure 1).
Photograph of the disposable bristle brush applicator (Ultrabrush fine ± 100 fibers; Young Innovations, Algonquin, IL, USA) moistened with GLUMA Bond Universal (Heraeus Kulzer GmbH, Hanau, Germany) for application to dentin specimens.
The adhesive was rubbed over the surface for 20 seconds with a manual pressure of approximately 0.35 N (35 gf) [19,22,23]. This was followed by air drying with an oil-free air stream of increasing intensity, directed from the periphery to the center, until the adhesive layer stopped moving. A new disposable applicator was used for each specimen. The adhesive was light-cured for 10 seconds using a multi-LED light-curing unit (Valo LED Cordless; Ultradent Products Inc., South Jordan, UT, USA) at a standard mode of 1,000 mW/cm².
For the microshear strength test, silicone tubes with an inner diameter of 0.8 mm were cut into 2 mm long pieces that served as molds [24]. A tube was placed on the bonded dentin surface, and Charisma Bulk Flow ONE (Heraeus Kulzer GmbH) flowable resin composite was injected into the tube from bottom to top to prevent bubble entrapment. The filled tube was covered with a transparent Mylar strip and light-cured for 20 seconds. The silicone molds were then carefully removed by cutting vertically with a sharp lancet to avoid failure before testing.
Group 2 (n = 21): microbrush applicator
In the second group, the same adhesive system was applied with a microbrush applicator (Microbrush, Young Innovations) (Figure 2).
Photo of the disposable Microbrush applicator (Microbrush Plus, regular 2.0 mm, green; Young Innovations, Algonquin, IL, USA) moistened with Gluma Bond Universal (Heraeus Kulzer GmbH, Hanau, Germany) for application to dentin specimens.
The operator (AA) used the same technique, time, and pressure parameters as in group 1. Similarly, four resin composite microcylinders (0.8 mm diameter and 2 mm height) were fabricated on each dentin surface using the same materials and procedures as described in group 1.
After placement of the adhesive and resin composite, all specimens were stored in distilled water at 37°C in an incubator for 24 hours prior to mechanical testing.
Bond strength testing
The µSBS testing was performed using a universal testing machine (Instron 3365; Instron Corp., Norwood, MA, USA) at a crosshead speed of 1 mm/min. A chisel-shaped loading device applied force until fracture occurred. The fracture stress was calculated from the measured specimen diameter.
Failure mode analysis
After the test, both the fractured resin composite cylinders and the corresponding dentin substrates were collected for fractographic analysis. Each specimen was examined under an optical microscope at ×50 magnification (Olympus BX51 SM Analysis System; Olympus Corporation, Tokyo, Japan).
Scanning electron microscopy surface analysis
Representative specimens (n = 11 per group) were randomly selected for SEM analysis. Specimens were dehydrated in an ascending series of acetone concentrations (70%, 80%, 90%, and 100%) and then stored in 100% acetone. Specimens were dried using a critical point dryer (Leica EM CPD300; Leica Microsystems, Wetzlar, Germany). After drying, a gold-palladium coating was applied using a sputter coater (Leica EM SCD500) in diffuse mode for 10 seconds. The SEM images were taken with a GeminiSEM 500 (Carl Zeiss, Jena, Germany) equipped with an Inlens electron detector and operated at 10 kV with different magnifications.
All the steps of specimen preparation, along with the materials, instruments, and devices employed in this study, are illustrated in Figure 3.
Schematic representation of the specimen preparation, materials, and equipment used in the individual phases of this study.
Statistical analysis
Data normality was assessed using the Shapiro-Wilk test. As the µSBS data followed a parametric distribution, comparisons between groups were performed using Student t-test. Statistical significance was set at p < 0.05. All analyses were performed using IBM SPSS Statistics for Windows, version 26.0 (IBM Corp., Armonk, NY, USA).
RESULTS
Microshear bond strength
The comparison of the µSBS between the two groups showed statistically significant differences (Table 1 and Figure 4). The group in which the universal adhesive was applied with the bristle brush applicator (group 1) had a significantly higher mean shear stress at failure (49.10 ± 5.39 MPa) than the group with the microbrush applicator (group 2), which had a mean value of 37.20 ± 3.50 MPa. The difference in mean bond strength between the two groups was statistically significant (p < 0.001, Student t-test).
Comparison of micro-shear bond strength between the two groups
Comparison of microshear bond strength between the two groups. Box plots are schematic representations based on mean and standard deviation, as raw data were not provided. Different lowercase letters indicate statistically significant differences (p < 0.05), with p < 0.001 between groups.
These results indicate that the bristle brush applicator resulted in a stronger adhesive bond to dentin under the same experimental conditions, supporting the hypothesis that the applicator type may influence adhesive performance. The lower standard deviation in the microbrush group (± 3.50) compared to the bristle brush group (± 5.39) also indicates a more consistent but weaker adhesive performance when using the microbrush.
Failure mode analysis
Analyzing the distribution of failure modes provides additional support for the mechanical data. In the bristle brush group, 71.40% of the specimens showed adhesive failure, while 28.60% exhibited cohesive failure within the resin composite substrate. In contrast, the microbrush group showed a higher proportion of adhesive failures (95.24%) and a lower proportion of cohesive failures (4.76%) (Table 2 and Figure 5).
Comparison of failure modes between the two groups
A graphic compares the adhesive failure modes observed in both groups.
The higher rate of adhesive failures in the microbrush group indicates a weaker adhesive interface, which is consistent with the lower µSBS values in this group. Conversely, the higher frequency of cohesive failures in the bristle brush group suggests that the bond between the adhesive and the dentin was strong enough that failures occurred more frequently within the restorative material rather than at the adhesive interface.
Scanning electron microscopy observations (descriptive)
Qualitative SEM analysis of representative specimens from both groups revealed remarkable differences in surface characteristics after application. The bristle brush group exhibited a more uniform and homogeneous adhesive layer with minimal surface contamination (Figure 6).
(A–C) Scanning electron microscopy images of the examined specimens from the disposable bristle brush applicator group. They show a more consistent and uniform adhesive layer with no surface contamination (residue-free) on the adhesive surface.
In contrast, there is a widespread distribution of fiber residues within the microbrush group. Fiber residues were identified embedded at the adhesive interface in 100% of the examined specimens, with fiber diameters ranging between 14.67 and 19.66 μm (minimum–maximum) (Figure 7). The consistent presence of these inclusions confirms that fiber shedding is a systematic occurrence rather than a stochastic defect. These embedded fibers created visible interfacial discontinuities that appeared to act as stress concentrators, initiating adhesive failure.
(A–C) Scanning electron microscopy images of the examined specimens from the microbrush group, displaying the fiber residues embedded on the surface, and showing the diametric measurements of the embedded fiber residues. The analysis confirmed that fiber diameters ranged from 14.67 to 19.66 μm (minimum–maximum).
DISCUSSION
The present study showed that the type of applicator used to apply a universal adhesive significantly affected the µSBS to dentin. The bristle brush applicator resulted in a significantly higher mean bond strength than the microbrush, with the difference confirmed by both failure mode analysis and SEM observations. These results support the research hypothesis that the use of bristle brush applicators will yield significantly higher µSBS values to crown dentin than microbrush applicators. Our results are consistent with previous findings that application technique variables, such as the rubbing action and mechanical contact between the applicator and the substrate, directly influence the quality of the bonding surface [25]. In our study, the bristle brush group not only achieved higher µSBS values but also exhibited a greater proportion of cohesive failures, suggesting that the bond to dentin was strong enough to shift the failure site to the resin composite substrate rather than the adhesive interface. In contrast, the microbrush group showed mainly adhesive failures, indicating a weaker interfacial bond.
An important factor influencing these results could be the presence of fiber residues from the microbrush applicator. SEM examination revealed visible nylon fiber residues embedded in the adhesive layer of all examined microbrush specimens, while no such residues were found in the bristle brush specimen. This observation of fiber residues in 100% of the examined microbrush specimens is consistent with findings from previous studies, which similarly reported contamination of the adhesive interface when fibrous microbrushes were employed [15,16]. Embedded fibers may act as stress concentrators or barriers to resin infiltration, reducing the cohesive strength of the adhesive layer and resulting in lower µSBS values.
This is consistent with the findings of Alshawi et al. [15], who reported that microbrushes consistently left fiber residues on the bonding surfaces in Class I cavities, whereas this was not the case with bristle brushes. Similarly, Berton et al. [16] observed fiber residues in both Class I and Class II cavities when adhesives were applied with microbrushes.
A potential causal link between fiber embedding and reduced bond strength is supported by the fractographic data. The microbrush group demonstrated a distinct pattern of adhesive failures (95.24%) that corresponded directly to the interface, with SEM analysis confirming 100% fiber contamination. This suggests that the embedded fibers may have acted as stress concentrators or flaws at the interface, potentially preventing intimate contact and likely contributing to premature adhesive failure. The fiber residues appear to act as mechanical discontinuities, potentially interfering with polymerization and reducing bond strength by creating weak zones within the adhesive layer. Microbrushes are widely used, accounting for approximately 76% of all applicators sold in Italy in August 2022 [16]. Despite their popularity, their design leads to fiber shedding. To avoid this, non-absorbent, lint-free nylon fibers have been developed that allow for practical adhesive application [17]. However, these fibers are not always firmly attached to the plastic handle, leading to fiber shedding during heavy scrubbing [15]. Since most adhesive manufacturers recommend active application with firm rubbing and multilayer application can improve adhesive performance [25], mechanical stress during application probably increases the likelihood of fiber loss from the microbrush tips. Interestingly, a group of Italian researchers from the College of Siena published numerous papers on post-cementation in endodontic canals between 2001 and 2002. They reported better resin tag formation and higher bond strength when the adhesives were applied with microbrushes rather than small plastic bristle brushes [26–30]. These differences are probably due to the particular anatomical challenges of intracanal application, where microbrushes are better able to adapt to narrow, deep spaces compared to bristle brushes. However, for preparations in shallow cavities - as in the present study - the risk of fiber contamination by microbrushes might outweigh the ergonomic advantages. Another factor that could affect these results is the angle of the fiber tips of the bristle brushes, which may be at 90° to the dentin surface in the specimens used in this study. However, it is unlikely that the bristle tips were oriented at exactly 90° to the root dentin. This study hypothesizes that the active application protocol (scrubbing) employed on a flat surface exerts direct compressive and shear forces on the microbrush fibers against the hard dentin substrate. Unlike the lateral friction in a canal, this direct perpendicular scrubbing likely accelerates fiber detachment and embedment. Consequently, in the absence of anatomical constraints, the material stability of the applicator becomes the dominant factor, explaining why the fiber-free bristle brush performed significantly better.
From a clinical perspective, our results suggest that bristle brush applicators are a more reliable option for applying adhesives to accessible dentin surfaces, thereby reducing the likelihood of interface contamination and improving bond strength. Nevertheless, microbrushes may be valuable in endodontics, as their flexibility and reach are advantageous, provided that potential problems with residue are avoided through improved manufacturing or adapted application protocols [4,26,28,29].
This study employed GLUMA Bond Universal exclusively, a singular adhesive system. Variations among universal adhesives in solvent composition—such as acetone, ethanol, and water—and viscosity can influence their interaction with applicator fibers and shedding. Consequently, the findings may not be generalizable to all universal adhesives, particularly those with differing solvents or viscosities. Further research should quantify the clinical impact of microbrush fiber residues on the longevity of restorations and evaluate alternative applicator designs or materials. It would also be helpful to investigate whether preconditioning of microbrushes (e.g., pre-rinsing or fiber inspection) could reduce the transfer of residues. Until such findings are available, clinicians should be aware of the potential for fiber contamination and consider applicator choice as a variable that may impact adhesive performance and ultimately the success of bonded restorations.
CONCLUSIONS
Overall, the results indicate that the bristle brush applicator can achieve a higher and more uniform dentin bond strength than the disposable microbrush when used with the tested universal adhesive under the experimental conditions. The persistent presence of microbrush fiber residue, confirmed by SEM examination, appears to contribute to lower mechanical performance. These results emphasize the importance of applicator choice as a controllable variable in adhesive dentistry and highlight the need for greater awareness of potential applicator-induced contamination.
From a restorative perspective, the results suggest that bristle brush applicators may work better when bonding cavity walls, especially when the bonding surface is fully accessible. The absence of fiber residue reduces the risk of interface contamination, which could otherwise affect polymerization or cause microcracks. In contrast, the microbrush remains useful in narrow spaces, such as root canals, due to its geometry, which provides better access and adaptation, though fiber detachment may occur. Therefore, the applicator used should be tailored to the clinical situation, balancing ease of access with potential risks to the adhesive’s integrity.
This in vitro study was conducted on flat dentin surfaces under controlled conditions that may not fully reflect the clinical environment. Variables such as saliva contamination, cavity shape, and operator variability could influence residue deposition and bond strength. In addition, only one brand of each applicator type was tested, and differences in manufacturing could influence the tendency for residue detachment.
Furthermore, to definitively isolate the effect of fiber shedding from other applicator mechanics, future studies should include a fiber-free silicone microbrush as a control group. This would help clarify the specific contribution of fiber inclusion to the reduction in bond strength. Further investigations should explore the following aspects: the effect of fiber residues on polymerization kinetics and nano-leakage; the influence of various adhesive formulations on residue adhesion; and strategies to modify the design of microbrushes or fiber anchoring to mitigate contamination risks.
Notes
CONFLICT OF INTEREST
No potential conflict of interest relevant to this article was reported.
FUNDING/SUPPORT
The authors have no financial relationships relevant to this article to disclose.
AUTHOR CONTRIBUTIONS
Conceptualization, Funding acquisition, Resources: Alshawi A. Data curation, Validation: Erdemir U, Alshawi A, Lal F. Formal analysis: Ozel Yildiz S, Lal F, Alshawi A. Investigation: Dikmen B, Alshawi A, Lal F. Methodology: Erdemir U, Alshawi A. Project administration: Erdemir U. Software: Ozel Yildiz S, Lal F. Supervision: Erdemir U. Visualization: Dikmen B, Alshawi A. Writing - original draft: Alshawi A, Dikmen B, Ozel Yildiz S, Lal F. Writing - review & editing: Erdemir U, Alshawi A. All authors read and approved the final manuscript.
DATA SHARING STATEMENT
The datasets are available from the corresponding author upon reasonable request.
DISCLOSURE OF GENERATIVE AI IN SCIENTIFIC WRITING
Generative AI was utilized solely to improve the language, readability, and grammar of the manuscript. It was not used in data collection, data analysis, or the generation of scientific content or conclusions. The authors take full responsibility for the final content of the publication.