Revolutionizing dental restorations with nanoparticle-incorporated composites

Abstract

The field of dental composites encompasses a diverse array of materials utilized extensively in modern dentistry for both restorative and cosmetic procedures. These composites typically consist of an organic matrix, dispersed filler particles, and a coupling agent to integrate the two. One of the most significant advancements in composite resin development came in the late 1950s when Bowen’s successfully formulated a compromise between epoxy and methacrylate resins. Nanoparticles have gained significant attention in dentistry due to their unique properties, including small particle size, increased surface area, and enhanced mechanical and optical characteristics. Various nanomaterials, including nanorods, nanospheres, nanotubes, nanofibers, and dendritic structures, are being explored for their potential in dental composites. These materials offer advantages such as antibacterial properties, remineralization capabilities, and enhanced mechanical strength. Several techniques, including wet precipitation, sol-gel, multiple emulsion, biomimetic deposition, hydrothermal, and electrodeposition, are employed in the synthesis and processing of nanocomposites. Each method offers unique advantages in terms of scalability, cost-effectiveness, and control over material properties. In conclusion, nanoparticles in composites hold tremendous promise in revolutionizing dental restorative materials. By leveraging nanotechnology, researchers can develop materials with tailored properties to meet specific dental needs, ultimately improving patient outcomes and oral health. Ongoing research into antimicrobial mechanisms and fabrication techniques will further advance the field of dental nanocomposites.

INTRODUCTION

Dental composites represent a vital category of materials used extensively in modern dentistry for restorative and cosmetic dental procedures. Composite resins are formulated by amalgamating rigid, inorganic particles, encapsulated within a pliant resin matrix. The three main components of composites include organic matrix, dispersed filler particles, and interfacial coupling agent.[1]

Organic matrix

Amidst the quest to develop an effective matrix for dental composite, numerous challenges emerged, including the scarcity of apt curing agents and issues regarding color stability. In the late 1950s, to overcome these challenges, Bowen’s conducted experiments. His efforts resulted in a revolutionary compromise between methacrylate and epoxy resin. When paired with triethyleneglycol dimethacrylate (TEGDMA) and an appropriate catalyst system, he was able to determine in 1958 that the reaction product of bisphenol A and glycidylmethacrylate could cure in just 3 min at room temperature with only a negligible 5% polymerization shrinkage.[2,3] A composite incorporating this resin as its matrix displayed exceptional properties. Presently, numerous composites employ this monomer system, known as “Bis-GMA,” derived from bisphenol A-glycidyl methacrylate.

Bis-GMA, characterized by its high molecular weight, functions as a formable liquid for composite resins. Nevertheless, it is notable for its high viscosity, presenting a challenge in handling and application. To achieve an optimal consistency for ease of use, the Bis-GMA resin undergoes dilution by incorporating other methacrylate monomers characterized by low molecular weight and viscosity. Approximately 25% TEGDMA is commonly added by manufacturers to render the resin more manageable. While methylmethacrylate can serve as a thinner, its volatility and tendency to induce higher polymerization shrinkage render it unsatisfactory for this purpose.[3]

Additional components of the matrix include:

1. A polymerization inhibitor

2. A catalyst

3. A tertiary aromatic amine

4. A ultraviolet (UV)-activator

5. A UV absorber.

An alternative resin frequently employed is urethane dimethacrylate. Unlike Bis-GMA, urethane dimethacrylate lacks aromatic rings in its structure. Despite serving a similar purpose to Bis-GMA, it exhibits lower polymerization shrinkage and higher viscosity.[4]

Filler particles

To provide beneficial physical properties including stiffness, surface hardness, low shrinkage, and a lower coefficient of thermal expansion, inorganic fillers are added. Typically, a lower coefficient of thermal expansion is achieved using a larger percentage of stable filler in comparison to unstable resin.[3,5]

Initially, macrofill composites featured larger particles ranging from 10 to 50 μm, resulting in subpar polishability and aesthetics. Subsequently, there has been a prevailing trend toward particle size reduction. Over time, various types of dental resin composites (DRCs) have been developed with differing particle sizes and distributions. These include microfill (0.01–0.1 μm, late 1970s), hybrid (0.01–5 μm, early 1980s), microhybrid (0.01–1 μm, mid-1990s), nanofill (0.01–0.04 μm, around 2000), and nanohybrid (0.01–1 μm, around 2000) formulations. It is important to note that DRCs were originally called “microfill” composites, probably because the term “nano” was not widely used at the time, even though they are frequently filled with particles that are nanometers in size less than a micron.[6] Furthermore, it might be difficult to differentiate between microhybrid and nanohybrid composites because they both contain micro- and nanoparticles with comparable particle size distributions.[7]

Coupling agent

Coupling agents predominantly an organo-silane help in integrating the organic matrix and the filler particles.

Nanocomposites

By providing novel answers to enduring problems, nanotechnology has transformed a number of industries, including dentistry. Because of their unique qualities, nanocomposites – a byproduct of nanotechnology – have attracted a lot of interest in the dental sector. Small particle size, greater surface area, and improved optical qualities are some of the attractive characteristics of composite resins including nanoparticles and nanoclusters. When compared to conventional composites, nanocomposites exhibit better flexural strength, hardness, and finishing, polishing, and shade-matching capabilities.[8]

Nanomaterials and their approaches

Natural dental hard tissues, including dentin, enamel, and cementum, are comprised of natural nanoscale units. To replicate the properties of these natural tissues in artificial, nanomaterials will require the incorporation of fundamental components such as cells, biomolecules, tissue engineering scaffolds, and signals.[9]

The creation of nanomaterials with ordered or random nanotopographies is made possible by a variety of top-down and bottom-up nanofabrication techniques, such as electrospinning, phase separation, self-assembly, thin film deposition, chemical vapor deposition, chemical etching, nanoimprinting, photolithography, and lithographies such as electron beam or nanosphere lithographies.[10] Nanomaterials encompass diverse forms. Among them, the following nanomaterials are used widely in composite restorations.[9]

Nanoparticles

Presently, attention is directed toward refining the interfacial silanes responsible for bonding the inorganic fillers found in nanocomposites.

Nanorods

Chen et al.[11] have utilized nanorods to craft hydroxyapatite structures resembling enamel prisms, possessing self-assembly properties, to mimic artificial enamel.

Nanospheres

An amelogenin nanochain and nanosphere assembly can be used to evaluate calcium phosphate (CaP) deposition in a restorative framework to investigate normal tooth development.

Nanotubes

To increase the flexural strength of resin-based composites, carbon and titanium oxide nanotubes have been used in combination with silicon dioxide-treated organosilane bonding agents.

Nanofibers

Nanofibers employed in the production of ceramics containing hydroxyapatite and fluor-hydroxyapatite resulted in enhanced physical properties. Furthermore, silicate crystals serve to reinforce these nanocomposites, particularly through the combination of TEGDMA, acting as a thinning agent, and 2,2-bis glycol methacrylate.

Dendritic copolymers and dendrimers

Because of their potential to improve the performance of nanocomposites when mixed with other nanostructures, researchers are looking at dendrimers and dendritic structures in the context of dental composites.

The purpose of this review is to investigate the characteristics, advantages, and modes of action of different nanoparticles combined with dental composites.

MATERIALS AND METHODS

The review study carried out a thorough analysis of the body of research on different nanoparticles in restorative composites. To gather pertinent publications, scientific databases such as PubMed and Google Scholar were consulted. Terms like “Nanoparticles,” “nanomaterials,” “nanoparticles and restorative composites,” have been used to retrieve articles from the databases. We included studies that explained about the nanoparticles incorporated in restorative composites, their mechanism of action, and their properties. Studies with inadequate data have been excluded from the study. Thus, this review included 50 articles concerning nanoparticles in restorative dentistry.

LITERATURE REVIEW

Dental composites or adhesives now incorporate various nanoparticles to enhance their properties [Table 1].

Table 1.

Nanoparticles incorporated in restorative composites

Silver nanoparticles

Zinc oxide nanoparticles

Quarternary ammonium polyethylenimine nanoparticles

Calcium phosphate nanoparticles

Calcium fluoride nanoparticles

Titanium oxide nanoparticles

Hydroxyapatite nanoparticles

Chitosan nanoparticles

Silica nanoparticles

Zirconia nanoparticles

Silver nanoparticles

Silica particles that contain silver have been utilized as fillers in dental composites to provide them antibacterial qualities. Strong antibacterial effects can be achieved with a minimal amount of silver thanks to the high surface area-to-mass ratio of silver nanoparticles (Ag-NPs). Crucially, the color, appearance, and mechanical qualities of the composite are unaffected by this small quantity of silver.[11]

The precise method of operation through which Ag-NPs exhibit antimicrobial activity remains a topic of debate and ongoing research. Several theories have been proposed regarding their action on microbes. Ag-NPs interact with bacterial cell walls, penetrating them and increasing membrane permeability, ultimately cell death. In addition, these nanoparticles can generate free radicals that damage bacterial cell membranes, causing them to become porous and leading to cell death[12] [Figure 1a].

Figure 1.

(a) Mechanism of action of silver nanoparticles (NAg) on microbial cells, (b) Mechanism of action of zinc nanoparticles (NZn) and Quaternary ammonium polyethylenimine nanoparticles (QA-PEI-NPs) on microbial cells, (c) Mechanism of action of Calcium phosphate nanoparticles (CaP-NPs) on microbial cells

Silver ions released by the nanoparticles bind with bacterial DNA and prevent replication by interacting with electron transport chain enzymes, which is another potential mechanism.[13]

However, there have been concerns regarding the adverse effects of the same. To approach this concern more constructively, it’s crucial to comprehensively assess the impact of Ag NPs, especially in high doses, on human health and the environment. This necessitates a systematic evaluation of their toxicity across different doses, considering various factors such as size, shape, and other morphological traits.[14] Notably, Ag NPs below 10 nm may exhibit enhanced efficacy in targeting bacteria due to their potential for deeper penetration. Hence, a thorough understanding of these dynamics is essential for informed decision-making regarding their usage, ensuring both safety and efficacy in diverse applications.[15]

Zinc oxide nanoparticles

The incorporation of Zinc oxide nanoparticles in flowable resin composite inhibited the growth of Streptococcus mutans without significantly affecting the mechanical properties of the composite. This antibacterial property is attributed to the generation of reactive oxygen species[16] [Figure 1b]. By efficiently targeting and breaking down bacterial cell constituents such as proteins, lipids, and DNA, these organisms prevent the growth of bacteria. Furthermore, the release of zinc ions disrupts enzyme systems within dental biofilms by dislodging necessary magnesium ions, which in turn actively prevents the active transport and digestion of carbohydrates. This is another possible antibacterial mechanism.[17,18] However, it was found that these antimicrobial effects did not last long and were only short term.[19,20] While short-term evaluations of zinc nanoparticle-infused composites against bacteria offer insights, they may not fully reflect their performance in the oral environment over an extended period. With approximately 60% of composite resin restorations expected to endure for over a decade with proper placement techniques, it’s evident that long-term studies are imperative. Specifically, assessing the efficacy of ZnO-NP dental composites in vivo over an extended duration is crucial for determining their antibacterial effectiveness in oral restorations. However, before initiating such studies, it’s essential to develop ZnO-NP composites that exhibit sustained antimicrobial activity for longer durations (>1 week) in in vitro tests, a criterion that remains to be fulfilled. This sequential approach ensures a more comprehensive understanding of the potential of ZnO-NP as antibacterial agents in dental restorations, promoting informed decision-making in clinical applications.[21]

Quaternary ammonium polyethylenimine nanoparticles

Cross-linked quaternary ammonium polyethylenimine nanoparticles (QA-PEI-NPs) are being explored as antimicrobial agents in resin-based composites. These nanoparticles possess surface quaternary ammonium groups that confer antibacterial properties by disrupting cell membrane function [Figure 1b]. They have proven effective against a variety of pathogenic strains of bacteria, including both Gram-positive and Gram-negative bacteria. Furthermore, in vitro, QA-PEI-NPs have demonstrated efficacy against cariogenic bacteria such as S. mutans, and in in vivo investigations, they were discovered to inhibit intraoral biofilm.[22] The antibacterial efficacy of these nanoparticles also depends on the crosslinker character as well as its density. The enhanced hydrophilicity of the crosslinker correlates positively with its antibacterial effectiveness.[23] Furthermore, as the concentration of the crosslinking agent rises, there’s a notable decrease in antibacterial activity. This phenomenon is attributed to the heightened stiffness of the nanoparticle core.[24,25,26]

Calcium phosphate nanoparticles

When compared to conventional composites, nanocomposites that contain nanoparticles of amorphous calcium and phosphate ions have double the flexural strength and elastic modulus, which makes them appropriate for load-bearing restorations. In addition, these composites effectively counteract lactic acid challenges, unlike conventional restoratives. This feature makes them particularly promising for high-caries-risk patients, individuals with xerostomia, and in cases where absolute removal of caries tissues is not feasible.[27]

Wu et al. developed a novel dental composite with three advantages: antimicrobial, remineralization, and self-healing properties. The original characteristics, including flexural strength, elastic modulus, and fracture toughness, were unaffected by the addition of microcapsules up to 7.5%. Self-healing was accomplished with a 65%–81% recovery in virgin fracture toughness. Furthermore, when applied to dental plaque microcosm biofilms, the composite demonstrated potent antibacterial properties that markedly decreased biofilm survival, lactic acid generation, and colony-forming unit count [Figure 1c]. Thus, the self-healing nanoparticles of calcium phosphate (NACP) and dimethylaminohexadecyl methacrylate composite promise for tooth cavity restorations, with potential applications in various restorative and preventive dental materials.[28]

Remineralization and caries prevention depend on the sustained long-term release of calcium and phosphorus ions, which Zhang et al. demonstrated in their groundbreaking production of rechargeable CaP composites. The capacity to recharge and re-release Ca and P ions was demonstrated by adding an acidic monomer to the composite. Interestingly, the effectiveness of various acidic monomers in recharge/re-release using the NACP nanocomposite varied. Sustained re-release was maintained without decline over the course of six cycles of recharging and re-release. Furthermore, for 42 days, the composites showed constant Ca and P ion release without the need for further recharge. In comparison to commercial control composites, the NACP nanocomposite in the pyromellitic glycerol dimethacrylate and ethoxylated bisphenol A dimethacrylate resin showed flexural strength that was three times greater than that of rechargeable fluoride-releasing resin-modified glass ionomer materials.[29]

Calcium fluoride nanoparticles

Dental nanocomposites incorporating calcium fluoride (CaF₂) were identified as proficient, boasting superior mechanical strength and fluoride release capabilities. The ion release property of the CaF₂ can be attributed to the smaller size and thus high surface area. Fluoride ions at low pH form hydrofluoric acid, which penetrates bacteria and inhibits enzymes, leading to bacterial cell lysis at high concentrations. In addition, fluoride ions impede bacterial metabolism and adhesion. The presence of CaF₂ nanoparticles reduces microbial adhesion to tooth surfaces, subsequently decreasing biofilm formation and mitigating caries development.[30]

Kulshrestha et al. demonstrated that CaF₂ nanoparticles possess potent antibacterial properties against S. mutans, resulting in a 90% reduction in biofilm formation, decreased acid production, and extracellular polysaccharide formation.[31]

The first rechargeable nCaF2 composites were created by Mitwalli et al., and they showed remarkable promise for recharging and maintaining long-term release of F and Ca ions, which are essential for remineralization [Figure 2a]. Different resin monomers demonstrated distinct recharge/re-release performance. Interestingly, the nanocomposites that included nCaF2 in the BisGMA-TEGDMA resin showed the best balance between initial mechanical characteristics and the ability to recharge and re-release.[32]

Figure 2.

(a) Mechanism of action of Calcium fluoride nanoparticles (CaF2-NPs) on microbial cells, (b) Mechanism of action of Titanium oxide nanoparticles (TiO2-NPs) and Chitosan nanoparticles on microbial cells, (c) Mechanism of action of nanohydroxyapatite (nHAP) on microbial cells, (d) Mechanism of action of Zirconia Nanoparticles (ZrO2-NPs) and Silica Nanoparticles (Si-NPs) on microbial cells

Titanium oxide nanoparticles

Titanium oxide nanoparticles are effective antimicrobial agents because of their ability to generate free radicals and inhibit the growth of various microorganisms [Figure 2b].[33] Thus, they can be effectively used in composites as a potent caries-reducing agent.

Nanohydroxyapatite

Numerous techniques have been employed in the synthesis and processing of hydroxyapatite, such as wet precipitation, sol-gel, multiple emulsion, biomimetic deposition, hydrothermal, and electrodeposition.[34,35,36,37,38,39]

The hydroxyapatite composite is composed of a polymeric matrix (Bis-GMA, TEGDMA, HEMA) and hydroxyapatite nanoparticles as a filler. The hydroxyapatite particles’ surface was altered using either acrylic or methacrylic acid as a coupling agent to improve adhesion.[40] Hydroxyapatite nanoparticles incorporated in composites can be used in the remineralizing of early carious lesions.[41] A 10% suspension of hydroxyapatite nanoparticles (10–20 nm diameter; 60–80 nm length) facilitated the remineralization of the surface layer of initial caries lesions [Figure 2c], reaching depths of 20–40 μm with limited remineralization was observed in the core of the lesion.[8]

While hydroxyapatite boasts favorable bioactive and osteoconductive properties, its mechanical strength and fracture toughness are relatively low. This limitation presents a challenge to its utilization in load-bearing regions.[42] Various enhancements, such as particles, platelets, whiskers, long fibers, partially stabilized zirconia, metal dispersoids, and polymers, have been employed in hydroxyapatite to bolster its reliability. To improve the mechanical properties, hybrid fillers that combine hydroxyapatite and silica were developed. The homogeneity and nontoxicity of these materials have been confirmed, making them suitable for potential dental restorations. Nano-sized silica cosynthesized with HA (n-SiO2-HA) demonstrated superior performance. Restorative dental composites filled with n-SiO2-HA exhibited enhanced depth of cure and compressive strength compared to those filled with HA alone while also retaining their remineralization potential, characterized by the release of calcium ions. The mechanism underlying this remineralization potential is linked to the dissolution of hydroxyapatite, which occurs more prominently in caries-affected environments, offering potential benefits for patient treatment.[40]

Chitosan nanoparticles

A naturally occurring polysaccharide, chitosan has become more and more popular in dentistry, especially for restorative uses. It has chelating activity, high biocompatibility, and antibacterial efficiency against a variety of microorganisms. In addition to improving resin infiltration and the creation of the hybrid layer beneath composite restorations, it encourages the biomimetic regeneration of enamel. When added to dentin bonding agents, chitosan nanoparticles (CSNs) strengthen their antibacterial and anti-inflammatory qualities, protecting the pulp. Compared to other antibacterial powders, chitosan nanoparticles (CSNPs) have potent antimicrobial capabilities. By interacting with the surface of negatively charged bacterial cells, they result in cellular death [Figure 2b].[43,44] In addition, it serves as a cavity liner and preconditions dentin to stop pulp inflammation.[45] According to studies, adding chitosan nanoparticles (CSNs) to dental adhesives or composite resins boosted their antibacterial qualities without weakening the bond[46] and shows good responses in various aspects, which may make it a suitable candidate for clinical use.[47]

Silica nanoparticles

The most prevalent substance on Earth, silicon dioxide, makes up silica nanoparticles (SiNPs), which are utilized extensively in a variety of applications because of their huge surface area, excellent biocompatibility, and tunable particle size.[48] When SiNPs up to 0.5 weight percent were added, the flexural strength and modulus of Fiber Reinforced Composites were significantly and favorably affected. A stronger connection between fibers and the resin matrix is ensured by SiO2 nanoparticles due to the high surface energy of nanoparticles [Figure 2d].[49]

Zirconia nanoparticles

The high dielectric constant, ion-exchange ability, high refractive index, high optical transparency, low thermal conductivity, low coefficient of thermal expansion, polymorphism, and remarkable chemical, and optical properties are just a few of the intriguing characteristics of Zirconia (ZrO2), an n-type semiconductor. ZrO2 NP coatings for teeth also improve their longevity and external strength. Since acid enters teeth, destroys enamel, and creates cavities, ZrO2 NPs would adhere to the surface of bacteria and inhibit their metabolic activity with food, preventing acid generation and enamel deterioration [Figure 2d].[50]

Limitations

1. The studies included in the research are mostly limited to in vitro studies

2. Further studies are to be carried out in vivo.

CONCLUSION

By incorporating nanofillers and nanostructures, composites offer a wide array of benefits, including enhanced mechanical properties, improved esthetics, and superior antibacterial capabilities. The diverse range of nanofabrication techniques available enables precise control over the composition and structure of nanocomposites, allowing for tailored properties to meet specific dental needs. Furthermore, ongoing research into the antimicrobial mechanisms of nanoparticles opens up new possibilities for combating dental pathogens and preventing biofilm formation. As nanocomposites continue to evolve, they hold tremendous potential for revolutionizing dental restorative materials, ultimately improving patient outcomes and oral health.

Clinical significance

The incorporation of nanoparticles into composite materials revolutionizes healthcare, particularly in dentistry, tissue engineering, and drug delivery. These nanoparticles significantly enhance the mechanical, biological, and functional properties of composites, leading to improved clinical outcomes. It enhances strength, toughness, and wear resistance, reduces polymerization shrinkage, and improved antibacterial properties. Thus, the incorporation of nanoparticles in composites could serve as an advantage in improving all the demerits of composites.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.