ORIGINAL RESEARCH

https://doi.org/10.5005/jp-journals-10015-2568
World Journal of Dentistry
Volume 16 | Issue 1 | Year 2025

Comparative Evaluation of Shear Bond Strength between Pullulan-enhanced Bonding Agent and Conventional Bonding Agent: An In Vitro Study

Andlin Sahaya Sowmiya F¹, Sangeeta Chavan², Palanivel Pandian R³, Premkumar P⁴, Subhashini M⁵, Ramalakshmi G⁶

^1–6Department of Public Health Dentistry, Best Dental Science College, Madurai, Tamil Nadu, India

Corresponding Author: Andlin Sahaya Sowmiya F, Department of Public Health Dentistry, Best Dental Science College, Madurai, Tamil Nadu, India, Phone: +91 8637471937, e-mail: andlinsowmiya1998@gmail.com

Received: 12 January 2025; Accepted: 13 February 2025; Published on: 13 March 2025

ABSTRACT

Aim: To evaluate and compare the shear bond strength of pullulan (PL)-enhanced bonding agent and conventional bonding agent.

Materials and methods: A total of 45 intact human premolar teeth specimens were divided randomly into three groups, with 15 teeth specimens per group. These specimens were prepared and subjected to etching using 37% phosphoric acid. The group A specimens received conventional 5th-generation bonding agent (3M ESPE single bond), group B was treated with phosphorylated pullulan (PPL), and group C received PPL-enhanced 5th-generation bonding agent. Composite resin was then applied to all specimens, which were subsequently immersed in distilled water for 24 hours, followed by shear bond testing. ANOVA, followed by Tukey’s post hoc test, was used to analyze the test results.

Results: The shear force required to debond the PPL-enhanced commercially available 5th-generation bonding agent applied restoration [26.78 megapascals (MPa)] was significantly higher compared to commercially available 5th-generation bonding agent (24.39 MPa) and PPL solution (21.36 MPa) as bonding agent.

Conclusion: The dentin bonding agent enhanced with PL biopolymer exhibited higher bond strength at the tooth-restoration interface compared to the conventional bonding agent.

Clinical significance: Composite restorative failures are common nowadays. Improving the bonding agent constituent can provide better bonding, thereby preventing composite failures and enhancing durability, which in turn leads to longer-lasting restorations and decreased need for frequent replacements in clinical practice.

Keywords: Adhesive, Bonding agent, Dental composite, Phosphorylated pullulan, Shear bond strength

How to cite this article: Sowmiya ASFS, Chavan S, PR, et al. Comparative Evaluation of Shear Bond Strength between Pullulan-enhanced Bonding Agent and Conventional Bonding Agent: An In Vitro Study. World J Dent 2025;16(1):69–73.

Source of support: Nil

Conflict of interest: None

INTRODUCTION

The dental landscape underwent a transformative shift with the advent of composite resins. Prior to this era, metal fillings dominated the restorative scene, often compromising esthetic appeal and longevity. The introduction of composite materials marked a pivotal moment, ushering in an age of tooth-colored, durable, and versatile restorations. Dental composite resin consists primarily of organic polymers such as bis-glycidyl methacrylate (BisGMA), bis-glycidyl ethyl methacrylate (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA).¹ These materials are widely used in anterior and posterior tooth restorations. However, as the material gained popularity, so did the realization of its limitations. Despite advancements, composite restorations often exhibited issues such as wear, recurring dental caries, microleakage, fracture, and discoloration. These challenges not only compromised the esthetic appeal but also necessitated frequent replacements, increasing patient discomfort and dental costs.² It became evident that while composites had revolutionized dentistry, they had also introduced a new set of problems demanding innovative solutions. The need to address the shortcomings of composite restorations has spurred ongoing research and development, with the ultimate goal of creating materials that are not only esthetically pleasing but also exceptionally durable and long-lasting.

Some experts suggest that improving bonding interfaces or inhibiting cariogenic microbes could mitigate secondary caries and composite failures. Apart from composites, the development of novel adhesives is crucial, as composite restorations rely on adhesives to bond with tooth structures.³ Recent advancements in adhesives aim to reduce clinical application time, technique sensitivity, and offer various application options. However, there remains a lack of clinical data on their dentin bonding performance. The stability and durability of dentin adhesive interfaces are still debated, with microgaps at the interface contributing to nanoleakage.⁴ Additionally, the volumetric shrinkage of BIS-GMA monomers during polymerization weakens the dentin-adhesive bond, creating opportunities for cariogenic bacteria. Various factors influence shrinkage, including monomer types and quantities, filler loading, degree of polymerization, and polymerization rate. One approach to reduce shrinkage is through proprietary mixtures of monomers.⁵

Pullulan (PL), a biopolymer derived from the fermentation of black yeast fungus Aureobasidium pullulans, has garnered attention for its biomedical applications, including drug and gene delivery and tissue engineering. Chemically modifiable, PL can be tailored to exhibit decreased water solubility or pH sensitivity, making it versatile for various applications.⁶ Phosphorylated pullulan (PPL) has the ability to form strong chemical bonds with hydroxyl groups, contributing to adhesion with hard tissues.⁷ To address various causes of composite failures, dentin adhesive enhanced with PPL is to be evaluated for its potential to enhance sealing properties. The aim of the study is to evaluate and compare the shear bond strength of PPL-enhanced 5th-generation bonding agent with commercially available 5th-generation bonding agent. The study proceeds with the hypothesized statement that ”PPL-enhanced conventional 5th-generation bonding agent is as effective as the conventional 5th-generation bonding agent.”

MATERIALS AND METHODS

This was an in vitro experimental study conducted in a private dental college in Madurai between June and July 2024. The study was performed after receiving approval from the Best Dental Science College Institutional Ethical Committee (Approval number: BDSC-IEC2022-STU-BrVII-ASA-26), and CRIS Guidelines for in vitro studies were followed. Using G*Power 3.1 software, a total sample size of 45 tooth samples (15/group) was estimated, with an effect size value of 0.52 obtained from the pilot study results. The study power was set at 80%, and the α error at 5%. An additional 10% for dropouts or loss to follow-up was added as a precautionary measure to account for any potential specimen errors during the procedure or testing phase. A total of 45 noncarious, intact human premolar teeth, designated for extraction as part of orthodontic treatment, were collected from dental clinics. These teeth were preserved in a saline solution. The inclusion criteria included orthodontically extracted sound human premolar teeth. Teeth with dental caries, white spot lesions, visible cracks, hypoplastic teeth, noncarious lesions such as attrition, abrasion, and erosion, and any other additional developmental anomalies were excluded. A simple random sampling method was used. All 45 tooth samples were sealed in separate nontransparent envelopes and sequentially numbered. The sealed samples were assigned into three groups—groups A, B, and C (n = 15/group)—by using the chit method numbered from 1 to 45. The allocation concealment and assignment of samples into three groups were implemented by two separate noninvestigators.

Synthesis of the Phosphorylated Pullulan

Initially, 300 gm of pharmacopeial PL (from Farmorganic Health and Beauty Pharmaceuticals Company, Maharashtra) was dissolved in 1360 gm of distilled water at room temperature. Following this, 6670 gm of a 1% aqueous phosphate solution, with its pH adjusted to 5.5 using NaOH, was gradually added while stirring. The resulting solution underwent stirring for 1 hour. Then, approximately 90% of the solution was evaporated by boiling at 100°C under reduced pressure. Subsequently, the solution was maintained at 170°C for a duration of 5 hours. Upon cooling to room temperature, the reactant was dissolved in distilled water, subjected to ultrafiltration, and finally freeze-dried, yielding a brown-colored powder. A 25% PPL solution was prepared by dissolving 25 mg of PPL powder in 100 mL distilled water.⁸

Preparation of Tooth Specimen

All the tooth samples underwent cleaning using an ultrasonic scaler, followed by polishing with a pumice and water slurry.

The root portions of the teeth in each group were removed, and only the coronal portions were embedded in cold-cure acrylic resin using custom-made molds sized 2 × 2 cm. These teeth were mounted horizontally. The labial surface of each tooth was reduced to expose a flat dentin surface using a high-speed handpiece with a #245 carbide bur, while ensuring a constant water spray. In all groups, the exposed tooth surfaces were etched with 37% phosphoric acid for 20 seconds, rinsed, and dried with gentle air blowing using a three-way syringe:

For group A, the commercially available 5th-generation bonding agent (3M ESPE single bond) was applied onto the surface using a microbrush and light-cured according to the manufacturer’s instructions for 20 seconds.
In group B, the PPL solution was applied onto the surface and allowed to dry, undergoing chemical curing.
In group C, a combination of PPL and commercially available 3M ESPE single bond was applied onto the surface and light-cured for 40 seconds.

Following the application of bonding agents, dental composite cylinders with a diameter of 2 mm (3M Espe Filtek Z350 XT) were placed on each tooth specimen and light-cured for 40 seconds. Additional curing was performed after removing the molds. Subsequently, all prepared specimens were stored in distilled water for 24 hours prior to shear bond testing.

Shear Bond Testing of Tooth Specimen

The specimens were affixed to the universal testing machine (INSTROM), with force applied by the machine at a crosshead speed of 1 mm/minute in a compression mode, utilizing a blade parallel to the adhesive-dentin interface. The bonded composite cylinder was oriented horizontally, ensuring that the shearing blade was perpendicular to the composite-dentin interface. Each specimen underwent loading until failure, and the shear force necessary to debond the specimen was documented. Subsequently, the debonding stress in megapascals (MPa) was calculated by dividing the maximum load in newtons by the surface area of the prepared resin cylinder (MPa = N/mm²).⁹^,¹¹ All the specimens were labeled A, B, C as of their respective study groups, and the procedure and the outcome, that is, the shear bond strength values, were measured by a noninvestigator blinded from information about the test groups.

Statistical Analysis

The study data were analyzed using IBM SPSS 20.0 statistical software. The shear bond strength values were estimated in MPa. The shear bond strength values between the three groups were compared using the ANOVA test, followed by Tukey’s post hoc test to identify the superior material among the three groups.

RESULTS

Table 1 represents the intergroup comparison of shear bond strength values (in MPa) among three groups. The mean shear bond strength values of commercially available 5th-generation bonding agent, PPL, and PPL-enhanced commercially available 5th-generation bonding agent were 24.39, 21.36, and 26.78 MPa, respectively, and there was a statistically significant difference with a p-value of <0.05. The mean shear bond strength value of PPL-enhanced commercially available 5th-generation bonding agent was comparatively higher, followed by commercially available 5th-generation bonding agent and PPL solution as bonding agent.

**Table 1:** Intergroup comparison of shear bond strength values (in Mpa) of three groups
	N	Mean	SD	SE	95% CI		p-value
	N	Mean	SD	SE	Lower bound	Upper bound	p-value
Group A	15	24.39	2.07	0.53	23.24	25.54	0.000^a
Group B	15	21.36	1.67	0.43	20.44	22.29
Group C	15	26.78	1.68	0.43	25.85	27.71

SD, standard deviation; SE, standard error; ^astatistically significant at p-value of <0.05

Table 2 demonstrates the multiple pairwise intergroup comparison of shear bond strength values (in MPa) among three groups. All three groups show a statistically significant mean difference at the 0.05 level. The results revealed that the shear force required to debond the PPL-enhanced commercially available 5th-generation bonding agent-applied restoration was significantly higher compared to commercially available 5th-generation bonding agent and PPL solution as bonding agent.

**Table 2:** Multiple pairwise comparison of shear bond strength among three groups
Groups	Groups	Mean diff.	SE	Sig.	95% CI
Groups	Groups	Mean diff.	SE	Sig.	Lower bound	Upper bound
Group A	Group B	3.024^b	0.66	0.000	1.407	4.641
	Group C	–2.390^b	0.66	0.002	–4.006	–0.773
Group B	Group A	–3.024^b	0.66	0.000	–4.641	–1.407
	Group C	–5.414^b	0.66	0.000	–7.031	–3.797
Group C	Group A	2.390^b	0.66	0.002	0.773	4.006
	Group B	5.414^b	0.66	0.000	3.797	7.031

SE, standard error; ^bthe mean difference is significant at 0.05 level

DISCUSSION

Adhesive dentistry is key to minimally invasive, esthetic, and tooth-preserving dental restoration.¹² For the past 35 years, secondary caries has remained one of the main reasons for replacement, with a predicted lifespan of 6–7.2 years for composite restorations. Secondary caries is strongly associated with a weak adhesive interface; in fact, all adhesive systems have been shown to be unstable and susceptible to degradation, which may be responsible for partial bond failure to the tooth, resulting in marginal leakage and subsequent restoration retention loss.¹³ The scientific area of dental adhesion is vast. Each component serves a unique purpose: adhesive systems are composed of hydrophilic and hydrophobic resin monomers, solvents, photoinitiators, and, in some circumstances, filler particles to boost strength and reduce polymerization shrinkage.¹⁴ Other additional molecules may be introduced, particularly to avoid degradation of the adhesive contact and extend its durability. Ideally, the adhesive dentin interface should be steady over time to ensure acceptable bond strength, marginal seal, and clinical durability.¹⁵ As a result, the quest for suitable biocompatible materials capable of overcoming the disadvantages of adhesive failure is critical.

This study was conducted to evaluate the bonding efficiency in terms of shear bond strength of PPL-enhanced conventional 5th-generation bonding agent with PPL alone and conventional 5th-generation bonding agent. Since PL is an adhesive material with the potential to resist temperature and pH changes, and its modified form, that is, PPL, has been reported to achieve an adequate bond with hard tissues in recent times, it was considered in this study for creating durable adhesion. Conventional 5th-generation bonding agent was used as a gold standard control in this study, which is of the etch-and-rinse type, so that separate etching steps could be carried out for etching the tooth surface in all three groups to avoid any confounding effects. Also, it is a single-bottle system where both the primer and the bonding agent are in the same bottle and were easier to use. According to studies conducted by Bouillaguet et al.,¹⁶ Chuang et al.,¹⁷ and Kerby et al.,¹⁸ it was stated that self-etching adhesives (6th and 7th-generation) have lower bond strength compared to total etch bonding systems. According to Hashimoto et al., self-etch adhesives produce thinner and shorter resin tags than those produced by phosphoric acid etching, resulting in lower bond strength compared to total etch adhesive systems. Self-etching adhesive techniques use acidic monomers to both demineralize and penetrate enamel and dentin. The mineral component of the tooth structure must neutralize this acidity for the adhesive layer to polymerize completely. During the rinse stage, the complete etch adhesive removes the smear layer and dissolved minerals. Because of some concerns about residual acidity and the fact that the smear layer is not eliminated, the hydrolytic instability of self-etching adhesive systems, 5th-generation bonding agents were considered for modification with PPL biomaterial.¹⁹ Shear bond tests are used to evaluate the efficiency of adhesive materials at the tooth-restoration interface. Shear bond strength testing is done with a universal testing machine, Instron, which is conventionally popular for evaluating the adhesive ability of adhesive/restorative materials. The findings highlighted that the shear bond strength of the conventional 5th-generation bonding agent containing PPL was higher compared to the conventional one. Takahata et al. conducted an in vitro study in which they found that titanium implant surfaces treated with PPL positively influence osteogenesis in a rabbit model, and the mechanical properties of PPL demonstrated that both PPL and PPL/β-TCP (phosphorylated pullulan with β tricalcium phosphate) composites have higher shear bond strength than materials in current clinical use, including polymethylmethacrylate (PMMA) cement, α-TCP cement, and Biopex-R.⁷ This biomaterial has excellent adhesion properties to the deep dentinal surface of the tooth, which was scientifically proven by research conducted by Pedano et al., in an ex vivo human tooth-culture model and an in vivo minipig animal study using an injectable phosphopullulan-based calcium-silicate cement (GC, Tokyo, Japan) as pulp capping material. This study showed no pulpal inflammatory reaction and excellent reparative dentin formation capacity by adhering to the dentinal interface.²⁰ A similar effect was identified upon using a novel mineral trioxide aggregate containing phosphorylated pullulan-based material (MTAPPL) as a pulp capping material by Toida et al.²¹

The mechanism behind its high bonding efficiency seen in this study could be explained by the fact that the PPL mixes well and forms efficient cross-linking with the polymers present in the conventional bonding agents, resulting in a stable, strong compound, as seen in a study by Kamoun et al., where hydroxyethyl methacrylate (HEMA) polymer was cross-linked with PL in the presence of various initiators to form a stable hydrogel used for drug delivery.²² Yet, this needs further confirmation by examining the degree of cross-linking using spectrometric analysis, which was not confirmed in this study. Also, the hydroxyl groups within the pyranose rings of PL are available for substitution with phosphate groups. The pharmacopeial polysaccharide PL was chemically functionalized with dihydrogen phosphate groups to chemically bridge the biomaterial with the hard tissue via ionic binding of the phosphate functional groups to calcium of apatite that was present in the tooth. PL’s structure has a unique characteristic, that is, the coexistence of –(1 → 4) and –(1 → 6) linkages. PL nanoparticles consist of both hydrophobic and hydrophilic characteristics, which are due to its unique structure,²³ making it an ideal characteristic for an efficient adhesive.

Strengths of this study include its controlled environment and the reasonable and comparable bond strength achieved with PPL solution alone. Additional samples were included in all three groups as compensation for any specimen errors. This new polymer additive is safe and biocompatible with oral tissues, environmentally friendly, and cost-comparable to other resin monomers. Since PL is a food-grade material and was used for the production of cosmetic products, as a binder in the cooking industry, as a coating on apples to prevent spoiling, and also as a pulp capping agent in animal studies by Toida et al., Pedano et al., and Takahata et al. for bone implants in rats.⁷^,²⁰^,²¹ However, this needs to be further confirmed in future studies.

However, limitations exist, including the in vitro nature of the study, which may not fully replicate clinical conditions, and the sensitivity of testing procedures, and therefore, it lacks generalizability. Future research should focus on surface texture analysis at the resin-dentin interface, material characterization at varying proportions, and clinical trials to evaluate long-term efficiency in oral environments. Additionally, efforts should be made to enhance material shelf life and storage stability through further experimentation.

CONCLUSION

The study revealed that the shear bond strength was notably higher for the combination of PPL with a commercially available 5th-generation bonding agent compared to either the conventional 5th-generation bonding agent alone or PPL solution used independently. This suggests that PPL holds promise as a polymer additive, offering substantial adhesion at the interface between the restoration and the tooth. Its potential to provide stability and resist degradation could help prevent adhesive failures, ultimately contributing to the longevity of restorations.

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