Comparative Analysis of Acetylated Hydroxyethyl Methacrylate (Ac-HEMA) as a Novel Direct Restorative Material in Dentistry: An In vitro Study on Mechanical Properties and Microleakage Performance

In modern dentistry, restorative materials play a crucial role in maintaining oral health by preserving tooth structure, enhancing functionality and improving aesthetics. An ideal restorative material should be biocompatible, durable and capable of mimicking the natural appearance of teeth while ensuring minimal risk of secondary caries [1,2].

Dental amalgam has been widely used for many years due to  its  exceptional  durability,  reliability  and  affordability for tooth restoration. Its ability  to  withstand  strong  occlusal forces makes it suitable for use in posterior teeth. However, amalgam has several drawbacks, including poor aesthetics, mercury toxicity, thermal conductivity, delayed expansion and susceptibility to microleakage, which limit its appeal in contemporary restorative dentistry [3,4].

In recent years, composite resins have emerged as a preferred choice for restorative procedures among both patients and dentists. Their primary advantage lies in their ability to closely match the natural appearance of teeth, providing superior aesthetics, particularly in anterior restorations [5]. Furthermore, composite resins often require less removal of healthy tooth structure, aligning well with the principles of minimally invasive dentistry. However, composite materials are prone to polymerization shrinkage during curing, which can compromise marginal integrity and increase the risk of microleakage unless incremental layering techniques are employed [6].

Glass Ionomer Cement (GIC) is another commonly used restorative material due to its unique chemical bonding properties, which make it suitable as a liner, bonding agent, or cement. While GIC offers strong adhesion to dental tissues, it exhibits lower wear resistance compared to composite resins, especially in areas with high occlusal forces, limiting its suitability for high-stress applications [7,8].

2-Hydroxyethyl methacrylate (HEMA) is a monomer derived from methacrylic acid (MAA) and is known for its excellent biocompatibility and adhesive properties. HEMA, a colorless and transparent liquid with the chemical formula C6H10O3, is widely utilized in the synthesis of various polymers and resins. Its ability to form strong chemical bonds with the tooth structure is a key factor contributing to the success and durability of dental restorations [9,10]. HEMA’s biocompatibility, low toxicity and chemical stability ensure its safety and longevity in dental applications [11,12]. Additionally, HEMA’s compatibility with various dental materials, such as composites, bonding agents and sealants, makes it a versatile component in restorative dentistry. By mitigating polymerization shrinkage in dental composites, HEMA helps minimize marginal gaps and reduces the risk of secondary caries [13].

When HEMA-based materials are applied to tooth surfaces, they adhere effectively to dentin and enamel, forming stable and long-lasting restorations. HEMA undergoes polymerization through light-curing (photo polymerization) or self-curing methods, forming strong polymers or copolymers that reinforce marginal integrity [14]. Despite  its  advantages,  polymerization  shrinkage  remains a concern in HEMA-based restorations, potentially compromising marginal adaptation [15].

To address these limitations, Acetylated Hydroxyethyl Methacrylate (Ac-HEMA) was developed as a novel restorative material designed to improve mechanical strength, enhance  marginal  integrity  and  minimize  microleakage. Ac-HEMA combines the chemical bonding properties of HEMA with improved hydrophilic characteristics,  enhancing its adhesion to tooth structures. By forming a well-characterized matrix, Ac-HEMA is designed to improve structural durability and reduce microleakage [16].

This in-vitro study was undertaken to evaluate Ac-HEMA as a direct restorative material and compare its mechanical and chemical properties with established materials such as composite resins and GIC. The study aims to determine whether Ac-HEMA can provide superior mechanical strength, reduced microleakage and improved marginal integrity, thereby advancing restorative techniques in modern dentistry.

Synthesis  of  Acetylated  Hydroxyethyl  Methacrylate (Ac-HEMA)

Ac-HEMA was synthesized through a controlled acetylation process. Hydroxyethyl methacrylate (HEMA), dichloromethane and ethylene triamine were reacted to form chloroacetic acid. The mixture was maintained at -20̊C for 15 minutes, followed by incubation at 20̊C for 2 hours. Subsequently, 30 ml of distilled water was added and the solution was stirred overnight to ensure complete acetylation, resulting in the formation of Acetylated Hydroxyethyl Methacrylate (Table 1). This controlled acetylation process was chosen to enhance the chemical stability and structural integrity of the material.

Fourier Infrared Spectroscopy (FTIR)

To confirm the acetylation of HEMA, Fourier Transform Infrared Spectroscopy (FTIR) analysis was conducted. FTIR spectroscopy utilizes infrared light to analyze the chemical properties of the sample. The spectra  of  both  HEMA and Ac-HEMA were recorded, revealing a distinct shift in peaks and an additional peak at 1632 cm‾¹ and 1453 cm‾¹, confirming the successful acetylation of HEMA. The FTIR  spectra of Ac-HEMA were acquired three times using a Perkin Elmer Spectrum Two spectrometer equipped with a Universal Diamond attenuated total reflectance attachment (Perkin Elmer, London, UK). Spectra were collected with 16 scans accumulated over a range of 4000 cm‾¹ to 450 cm‾¹ with a resolution of 4 cm‾¹ (Figure 1). This rigorous confirmation ensured the precise identification of Ac-HEMA and its  cetylated structure.

Characterization  of  Acetylated  Hydroxyethyl Methacrylate

Ac-HEMA was characterized for its adhesive properties, marginal  integrity  and  hydrophilic nature. Ac-HEMA bonds

Table 1: Composition of HEMA

Figure 1: Ac-HEMA confirmation by Fourier Infrared Spectroscopy (FTIR)

chemically to the tooth surface, enhancing adhesion and improving marginal integration. To assess its practical application, Ac-HEMA was evaluated only in small controlled amounts, as its adhesive efficiency diminishes when applied in excessive quantities [17].

For this study, ten extracted teeth (six premolars and four molars) were collected from the Oral Biology Department of Saveetha Dental College. The  teeth  were  manually debrided using scaling instruments, cleaned with pumice paste and stored in distilled water for a maximum of 14 days to maintain hydration and minimize bacterial growth.

The teeth were divided into three groups for comparative evaluation:

• Group I: The occlusal surface of one premolar was etched with 35% phosphoric acid gel, air-dried and treated with a bonding agent followed by composite resin restoration using light curing
• Group II: The occlusal surface of one premolar was restored with Glass Ionomer Cement (GIC)
• Group III: The occlusal surface of one molar was etched with 35% phosphoric acid gel, air-dried and treated with Ac-HEMA, which was cured using a 380 nm light source

The use of identical preparation protocols ensured consistency in restorative application across all groups, minimizing procedural variability.

Scanning Electron Microscopy (SEM) Analysis

To evaluate the structural morphology and fracture characteristics of Ac-HEMA, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analyses were performed. Ac-HEMA samples, cured in disc form, were crushed between steel plates using controlled low-impact force to generate fracture surfaces. For SEM observation, the fractured samples were affixed to carbon tabs and analyzed using a JEOL JSM5410 LV electron microscope. An Oxford Instruments X-MaxN EDX detector was employed for elemental analysis.

SEM imaging was conducted in low vacuum mode at an accelerating voltage of 1.0 kV with an 8.4 mm working distance to reduce charging artifacts and improve image clarity. This method allowed for high-resolution analysis of Ac-HEMA’s surface morphology and elemental composition, providing insights into its bonding characteristics and potential defects [18].

Microleakage Analysis

To evaluate microleakage, teeth restored with Ac-HEMA, GIC and composite resins were immersed in distilled water at 37̊C for 24 hours. Following this, the samples were subjected to 2500 thermal cycles between 5̊C and 55̊C, with a 10-second transfer time and a 30-second dwell time in each temperature phase. To isolate the restorative margins, a protective layer of nail varnish was applied to the samples, leaving a 2 mm uncovered area around the restoration. The roots were embedded in acrylic resin cylinders (Meliodent, Bayer Co., Leverkusen, Germany) for stability. Each sample underwent 5-minute immersion in 0.5% basic fuchsin dye to evaluate dye penetration patterns. Following dye immersion, the samples were sectioned in the bucco-lingual direction using a water-cooled diamond saw to obtain three precise slices for analysis [19].

Fracture resistance of Ac-HEMA

The fracture resistance of Acetylated Hydroxyethyl Methacrylate (Ac-HEMA) was evaluated through compressive strength testing. Ac-HEMA demonstrated a maximum force of 474.24 N, with a corresponding compressive stress of 8.98 MPa. The recorded compressive displacement was  3.84 mm and  the  compressive  strain  was 3.84%, with an additional recorded compressive strain value of  1.37%.  These   values   indicate   that   while   Ac-HEMA exhibits moderate mechanical strength, it falls short when compared to conventional restorative materials such as Glass Ionomer Cement (GIC) and composite resins (Figure 2, 3, Table 2).

In comparison, GIC demonstrated a maximum force of 1130.59 N with a compressive stress of 11.64 MPa, while composite resins exhibited the highest performance with a maximum force of 2198.49 N and a compressive stress of 21.85 MPa. These findings suggest that while Ac-HEMA offers moderate fracture resistance, its performance is inferior to that of GIC and composite resins, particularly in high-stress areas.

Microleakage

Microleakage assessment was performed using dye penetration   methods   to   evaluate   the   sealing   ability  of Ac-HEMA in comparison to GIC and composite resins. The results revealed that Ac-HEMA exhibited dye penetration extending beyond half of the fissure involvement, indicating increased microleakage compared to the other materials tested (Figure 3, Table 3).

Table 2: Fracture resistance of Ac-HEMA

Figure 2: Compressive stress and strain of Ac-HEMA