Caries Management with Non-Metallic Nanomaterials: A Systematic Review

Introduction

Dental caries is one of the most prevalent chronic diseases worldwide. Multiple pathological factors can trigger it, including cariogenic microbes, host or tooth surface, substrate and time.¹ The colonized cariogenic microbes on tooth surfaces can metabolize fermentable carbohydrates and generate organic acids. Although enamel and dentin are highly mineralized hard tissues, these acids over a period can dissolve and diffuse into enamel and dentin.^2,3 Continuous demineralization can destroy tooth structure and finally result in dental caries. Without treatment, dental caries can progress and lead to persistent pain and further infection.

The World Health Organization reported that dental caries is the fourth most expensive disease to treat, causing a significant global burden of disease.⁴ Clinicians largely use the conventional approach to manage dental caries with restoration.⁵ The current dental materials are principally silver amalgam, composite resin and glass ionomer cement. They have their own limitations that affect clinical life span of the restorations. The World Dental Federation published its policy statement that advocated evidence-based caries-control measures for caries management.⁶ Researchers have been developing various dental materials to inhibit demineralization by repressing the growth of cariogenic bacteria and/or to promote remineralization by enhancing the deposition of minerals.^3,7

Nanotechnology is a recent research trend that is particularly effective in developing various nanomaterials used in several application areas. Nanomaterials contain materials measured at the nanometer scale (1 to 100 nm). They exhibit unique mechanochemical and biological properties, such as huge surface-to-volume ratio, high strength and stability, strong solubility and chemical reactivity and promising antibacterial effects.^8,9 Therefore, various nanomaterials have been developed for caries management and have shown auspicious results.⁸

Metallic nanomaterials, such as silver nanoparticles, have been widely used in caries management. They generally exhibit antibacterial activity through oxidative stress induction.¹⁰ However, they demonstrate only antibacterial ability, with no remineralization ability. In addition, some metallic nanomaterials have high cell toxicity and staining effects, which limit their clinical application.

Due to these disadvantages of metallic nanomaterials, researchers are gradually studying non-metallic nanomaterials. Non-metallic nanomaterials are nanomaterials without any metal element. Some non-metallic nanomaterials enhance remineralization by promoting mineral deposition and inducing hydroxyapatite formation. They can be a biomimetic material to regulate and orient the growth of enamel-like apatite. Some non-metallic nanomaterials effectively inhibit the growth of cariogenic bacteria, owing to their nano-scale size, unique active constituents and loaded antibacterial agents.¹¹ They can protect teeth from demineralization without generating drug resistance. Figure 1 illustrates non-metallic nanomaterials’ remineralising and antibacterial properties in managing dental caries.

Figure 1 Effects of non-metallic cariostatic nanomaterials on carious lesion.

Moreover, non-metallic nanomaterials exhibit promising biocompatibility compared to metallic nanomaterials. The addition of non-metallic nanomaterials in dental materials neither significantly alters their color nor affects their application. However, a search revealed that reviews of non-metallic nanomaterials for caries management have not been conducted. The purpose of this study is to review and summarize systematically the non-metallic nanomaterials developed for managing dental caries. This outline could help researchers identify the proper research direction to develop new non-metallic nanomaterials for caries management.

Methods

Search Strategy

Two independent investigators conducted a literature search to identify publications in three common databases (PubMed, Scopus and Web of Science). The search was restricted to publications in English, with no limitations on the date of publication. The keywords were (nanoparticles OR nanocomposites OR nanomaterials) AND (caries OR tooth decay). The last search was conducted on 20 Jul 2022.

Study Selection and Data Extraction

This systematic review included original investigations of the non-metallic nanomaterials developed for caries management (Figure 2).

Figure 2 Flow chart of the literature search.

Two investigators independently checked and excluded duplicate publications from the three databases to generate a list of publications. They screened titles and abstracts of the publications to identify potentially eligible publications. They excluded literature reviews, studies on metal nanomaterials, studies not related to caries management and other irrelevant studies. Afterwards, the two investigators retrieved full texts of the remaining publications for review. They selected publications that non-metallic nanomaterials used as the active constituent for caries management. Then, they performed a manual screening to select eligible publications from the reference lists of the selected publications. They discussed with another investigator including the selected publications to determine the publications included in this review. They recorded the publications’ information, including authors, year, journal and issues; the nanomaterials studied; the study design; the anti-caries properties investigated and the potential uses of the nanomaterials.

Assessment of Risk of Bias

Two investigators independently assessed the risk of bias from individual studies. The assessment was adapted from a systematic review.^3,12 They evaluated the study’s quality with seven parameters, which were 1) presence of control, 2) sample size justification, 3) material characterization, 4) biocompatibility assessment, 5) amount of material for assessment, 6) assessment time and 7) blinding of observers. Publications that reported only one to two items were classified as high risk of bias. Publications reporting three to five items were classified as medium risk, and those reporting more five items were regarded as low risk.

Results

The initial literature search yielded 2497 potentially eligible publications (666 publications in PubMed; 1073 publications in Scopus; 758 publications in Web of Science). Duplicate records of publications were excluded. After the titles and abstracts were screened, and 1775 publications were removed because they were literature reviews, studies not regarding non-metallic nanomaterials, studies not related to dental caries or other irrelevant studies. Another 48 publications were excluded because the non-metallic nanomaterials in these papers were not the active constituent for caries management. A search of the references of the selected publications yielded 21 publications that met the inclusion criteria. Therefore, 75 publications were included in this review.

The 75 publications were assessed for the risk of bias (Table 1). Seventy-three publications (73/75, 97%) had a medium risk of bias, 1 publication (1/75, 1%) presented a high risk of bias, and 1 publication (1/75, 1%) presented a low risk of bias. Of the 75 publications included, most publications (67/75, 89%) were in vitro studies, 6 (6/75, 8%) were animal studies and 2 (2/75, 3%) were clinical studies. The non-metallic nanomaterials demonstrated in these publications could be incorporated in topical agent (29/75, 39%), dental adhesives (11/75, 15%), restorative filler (4/75, 5%), dental sealant (3/75, 4%), oral drugs (3/75, 4%), toothpaste (2/75, 3%) and functional candies (1/75, 1%). Other publications (22/75, 29%) did not mention specific applications. The non-metallic nanomaterials reported were categorized as biological organic nanomaterials (n=45), synthetic organic nanomaterials (n=15), carbon-based nanomaterials (n=13) and selenium nanomaterials (n=2). Figures 3 and Figure 4 show the four groups of non-metallic cariostatic nanomaterials and their mechanism, respectively.

Table 1 Risk of Bias of the Included Publications

Figure 3 Number of publications of the 4 non-metallic cariostatic nanomaterials.

Figure 4 Mechanism of the 4 non-metallic cariostatic nanomaterials.

Discussion

Biological Organic Nanomaterials

Biological organic nanomaterials are nano-scale materials derived from organisms. These materials have attracted widespread attention due to their great biocompatibility, biodegradability and bioavailability.

Amino Acid-Based Nanomaterials

Amino acid-based nanomaterials are amino acids and their derivatives, including peptides and proteins. Amino acid-based nanomaterials are novel materials used for caries management. Table 2 summarizes 28 publications on the amino acid-based nanomaterials for caries management. Free amino acids, such as D-cysteine, process antibacterial properties. D-cysteine can inhibit the metabolic activity and lactic acid production of Streptococcus mutans (S. mutans) and Streptococcus sanguinis (S. sanguinis) biofilms.¹³

Table 2 Studies on Amino Acid-Based Nanomaterials for Caries Management

Peptides are short chains of amino acids that the amide type of covalent chemical bonds link. The constitutions of peptides vary based on types and sequences of amino acids, which generate unique properties of each peptide. Antimicrobial peptides have potent, broad-spectrum antimicrobial properties against oral pathogens. Researchers obtain antimicrobial peptides from extracting immune responses to microbial infections or artificial synthesis. A casein peptide that the partial proteolysis of milk caseins produces inhibits biofilm formation independently. It can be combined and used with silver nanoparticles to improve significantly the antibacterial effect against S. mutans.¹⁴ Some antimicrobial peptides, including GH12 peptide, pHly-1 peptide, polyphemusin I peptide and diphosphoserine-polyphemusin I peptide, have shown an antibacterial effect on the biomass and cariogenic activity of S. mutans in vitro.^15–20 Notably, the GH-12 peptide exhibits good biocompatibility and a strong antibacterial effect against S. mutans on the molars of rats in vivo. GH-12 peptide is a potential topical agent to inhibit dental caries.¹⁶ Moreover, topical use of diphosphoserine-polyphemusin I peptide reduced the plaque on incisors of rabbits in vivo.²⁰ Besides a strong antimicrobial effect, some peptides have remineralising properties that can be used for caries management. Two studies demonstrated that adding 3 NSS peptide and RGDS peptide into bioactive glass can enhance bio-remineralization of enamel and dentin.^21,22 Casein phosphopeptide remineralizes the enamel when combined with stannous fluoride and amorphous calcium phosphate nanoparticles in toothpaste.^23–25 Oligopeptide amphiphile is a biomimetic remineralising material. It can self-assemble into nano-fibers in the presence of calcium ions and work with the calcium ions to foster the biomimetic mineralization on enamel surface.²⁶ Furthermore, GA-KR12 peptide is a dual-function peptide with an antibacterial and remineralising effect. It can inhibit the growth of multiple cariogenic microorganisms and enhance the remineralization on enamel and dentin.^27,28

Some proteins can inhibit bacterial growth. Researchers used immunoglobulin Y as an oral drug and found that it can inhibit the growth of S. mutans through passive immunity in rats.^29,30 Loading immunoglobulin Y on hydroxyapatite also improved the antibacterial property of hydroxyapatite.³¹ Chimeric lysin can cause high lytic activity in cariogenic microorganisms. The S. mutans and Streptococcus sobrinus (S. sobrinus) biofilm viability of teeth treated with chimeric lysin significantly decreased on the premolars of rats.³² The chimeric lysin can be combined with amorphous calcium phosphate in carboxymethyl chitosan nanogel to inhibit biofilm formation and promote remineralization.³³

Apart from their antimicrobial properties, some proteins can act as biomimetic materials to regulate precipitation of calcium phosphate and protect teeth from demineralization. They can induce remineralization and form an enamel-like apatite under a physiological condition to prevent caries. Therefore, proteins are bioavailable alternatives to repair defective enamel. Amelogenin is a critical protein for controlling the organized growth of apatite crystals, which play a direct role in nucleation, crystal growth and the stretch spacing of hydroxyapatite crystallites.³⁴ Amelogenin-containing chitosan hydrogels can induce the apatite crystals to form on the damaged human enamel.^35–37 Amelogenin derivative is a small domain derived from native amelogenin. It can construct a mineral layer on the demineralized enamel.³⁸ Amelogenin derivative also generates biomimetical remineralization on silver diamine fluoride-treated dentin.³⁹ Besides, amelogenin mimics can bind to collagen fibrils and mimic the bio-mineralization process on enamel and dentin.^40,41

Natural Extractive-Based Nanomaterials

Although nanoparticles can be obtained using various physicochemical methods, green synthesis of nanoparticles using natural extracts minimizes the usage of toxic chemicals.⁴² Table 3 summarizes the 17 publications on the natural extractive-based nanomaterials for managing dental caries.

Table 3 Studies on Natural Extractive-Based Nanomaterials for Caries Management

Chitosan is a natural polymer obtained by the alkaline hydrolysis of chitin. Chitosan is widely used in biomedicine materials because it has promising remineralising, antibacterial and biocompatible properties without toxicity.^43,44 Chitosan nanoparticles have shown a remineralization ability on demineralized enamel.^45,46 Due to their nanometric dimensions, chitosan nanomaterials exhibit a greater ability to penetrate the oral biofilm than ordinary chitosan does. Chitosan nanomaterials can be used as antimicrobial agents for caries management because they inhibit the formation of S. mutans and S. sanguinis biofilm.⁴⁷ Moreover, chitosan nanoparticles can be applied as the matrix loaded with propolis, miswak, rutin and emodin to develop the anti-caries agent and dental varnish.^48–50 The chitosan nanoparticles have also been used as the capping agent combined with copper, copper oxide, zinc oxide and titanium dioxide nanoparticles to enhance the antibacterial property of dentin adhesive.^51–54 A clinical study demonstrated that orthodontic adhesive containing chitosan-titanium dioxide nanoparticles could prevent white spot lesions. The orthodontic adhesive containing chitosan-titanium dioxide nanoparticles significantly inhibits the S. mutans compared to the commercial orthodontic adhesive.⁵⁴

Moreover, plant extractives attract researchers’ interest due to their antibacterial properties for caries management. Cinnamon essential oil nanoemulsion exhibited a strongly inhibited the growth of an oral microcosm.⁵⁵ Dodonaea viscosa var. angustifolia flavone nanoparticles significantly reduced the formation and acid production of S. mutans biofilm.⁵⁶ Epigallocatechin gallate is an extracted polymer from green tea that shows remarkable anti-cariogenic bioactivity with poor stability.⁵⁷ To increase the stability and efficacy of epigallocatechin gallate, researchers entrapped it in the oral nanovesicle to inhibit the production of glucan and formation of S. mutans biofilm.⁵⁷ Farnesol is present in essential oils from many plants. Adding Farnesol halloysite nanotubes into composite resin creates antibacterial activity against S. mutans without compromising mechanical properties.⁵⁸ Thymol-cardamom-Lactobacillus plantarum nanoparticles are prepared as functional candies to inhibit the growth of S. mutans for managing dental caries.⁵⁹ In addition, propolis nanoparticles reduce the virulence of S. mutans and promote remineralization on enamel.^60,61

Synthetic Organic Nanomaterials

Apart from natural extractive organic compounds, many synthetic organic nanomaterials have been investigated as anti-caries agents. They are often used as active components with carriers to construct hybrid nanomaterials. Table 4 summarizes 15 publications on the synthetic organic nanomaterials for managing dental caries. All were in vitro studies, and the systematic search yielded no clinical or animal studies on synthetic organic nanomaterials for caries management.

Table 4 In vitro Studies* on Synthetic Organic Nanomaterials for Caries Management

Chlorhexidine is an antibacterial agent commonly used in dental practice as an antiseptic and disinfectant. Chlorhexidine nanoparticles inhibit S. mutans and Lactobacillus casei (L. casei) from growing and from forming their biofilms. They can be used as the filler in dentin adhesive and dental sealant for caries management.^62,63 Nano-carriers loaded with chlorhexidine can build a drug delivery system for advanced antibacterial properties.⁶⁴ Researchers have used various types of nano-carriers to synthesize a desirable anti-caries agent, including nanotubes, mesoporous nanoparticles and polymers. Chlorhexidine-loaded halloysite nanotubes effectively kill S. mutans, which can be used in a pit-and-fissure sealant. Additionally, some researchers have utilized poly (N,N-dimethylaminoethyl methacrylate-co-2-hydroxyethyl methacrylate), a bio reagent for cell culture, as the pH-sensitive nano-carrier and chlorhexidine as the active ingredients. The system could control the release rate of chlorhexidine according to the pH value and it could exhibit lower cytotoxicity against human oral keratinocytes than free chlorhexidine does. The chlorhexidine in these delivery systems showed the same antibacterial effects on S. mutans biofilms as free chlorhexidine.⁶⁵ A research group synthesized the chlorhexidine encapsulated with mesoporous silica and found it could improve the anti-microbial performance of glass ionomer cement without affecting its mechanical properties.⁶⁶

Quaternary ammonium compound nanoparticles are cationic surfactants used to prevent caries. They are broad-spectrum antimicrobials with low toxicity.⁶⁷ Quaternary ammonium polyethylenimine nanoparticles inhibit oral pathogens, such as S. mutans and L. casei. They can be incorporated into glass ionomer cements and resin composite.^68,69 Adding quaternary ammonium polyethylenimine nanoparticles did not compromize the composite materials’ flexural modulus or the flexural strength.⁷⁰ Cetylpyridinium chloride is another quaternary ammonium compound used in mouthwashes and toothpastes. Cetylpyridinium chloride nanoemulsions can inhibit S. mutans, L. casei, Actinomyces viscosus (A. viscosus) and Candida albicans (C. albicans) when applied to tooth surfaces.^71–73 Furthermore, quaternary ammonium compound nanoparticles can cooperate with silver to synthesis a core-shell silver bromide–cationic polymer nanocomposite. This composite showed long-term antimicrobial properties against S. mutans. The cationic-polymer shell provides a sustained antibacterial effect in dental resins and sealants.^74,75

Researchers also developed synthetic nanomaterials against cariogenic microorganisms. Doxycycline-nanoparticles-doped dental composite resins can reduce viability of oral biofilm.⁷⁶ Another study encapsulated doxycycline in the halloysite nanotube to enhance the antibacterial effect of dental adhesive against S. mutans.⁷⁷ Many fungicides contain imidazole and benzimidazole. Researchers synthesized imidazole and benzimidazole nanoparticles and incorporated them to inhibit bacteria. Studies have shown these antibacterial composite resins inhibit Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Enterococcus faecalis (E. faecalis).⁷⁸

Carbon-Based Nanomaterials

Researchers also developed carbon-based nanomaterials for caries management. Carbon-based nanomaterials exhibit high mechanical strength.^79,80 They have antibacterial and remineralising properties. Table 5 summarizes 13 publications on carbon-based nanomaterials for managing dental caries. All were in vitro studies, and the systematic search yielded no clinical or animal studies on carbon-based nanomaterials for caries management. The carbon-based nanomaterials in these publications can be categorized as carbon nanoparticles, graphene nanoplatelets, graphene oxide nanoplatelets and reduced graphene oxide nanoplatelets.

Table 5 In vitro Studies* on Carbon-Based and Selenium Nanomaterials for Caries Management

Carbon nanoparticles consist of pure carbon in various structures. They inhibit the growth of oral microcosms without toxicity to human cells.⁸¹ Graphene nanoplatelets are single layers of carbon allotrope arranged in a 2-dimensional honeycomb lattice nanoplatelet. Graphene nanoplatelets provide antibacterial protection against S. mutans.⁸² They can be used as the filler in dental adhesive to prevent caries without affecting the mechanical properties of adhesive.^83,84 Graphene nanoplatelets can be platform-doped with other nanoparticles, such as zinc oxide nanoparticles, to obtain the enhanced inhibitory effect of biofilm formation.⁸⁵ Graphene oxide nanoplatelets are oxidized graphene nanoplatelets.^86,87 They can be added into an orthodontic adhesive to prevent white spot lesions after orthodontic treatment because of their antibacterial effects against S. mutans.^88,89 Graphene oxide nanoplatelets can destroy the cell wall and membrane of S. mutans, Fusobacterium nucleatum (F. nucleatum) and Porphyromonas gingivalis (P. gingivalis) to inhibit biofilm formation.^90–92 In addition, graphene oxide nanoplatelets showed a remineralising ability to occlude the exposed dentinal tubules and to promote the growth of hydroxyapatite crystal on demineralized dentin.⁹³ Due to abundant oxygen groups on the surface of graphene oxide nanoplatelets, graphene oxide nanoplatelets can be functionalized with diverse antiseptics to give them enhanced antibacterial properties. Graphene oxide nanoplatelets combined with silver and copper nanoparticles have antimicrobial effects against S. mutans, with a superior aesthetic effect on dentin compared to silver diamine fluoride.^94,95 A report also showed that loading graphene oxide nanoplatelets with polyethylenimine complexes and with antisense vicR RNA can reduce virulent-associated gene expressions and reduce the S. mutans biofilm from forming.⁹⁶ Reduced graphene oxide nanoplatelets are another graphene derivative reduced from graphene oxide nanoplatelets. Reduced graphene oxide nanoplatelets have greater surface area and better mechanical strength than graphene oxide nanoplatelets do. Researchers found that the reduced graphene oxide nanoplatelets could link with silver nanoparticles to inhibit the growth of S. mutans biofilm.⁹⁷

Selenium Nanomaterials

Selenium is an indispensable trace element for the human body’s cellular functions. Selenium in antioxidant enzymes and functional protein molecules plays a crucial role in reducing oxidative stress.⁹⁸ Table 5 shows the 2 publications on selenium nanomaterials for managing dental caries. All were in vitro studies, and the systematic search yielded no clinical or animal studies on selenium nanomaterials for caries management. Selenium nanoparticles can inhibit growth of S. mutans, E. faecalis, L. casei, C. albicans and Staphylococcus aureus (S. aureus) and their biofilms.^98,99 However, these studies were not performed on tooth tissue. Further studies are needed to explore the application of selenium nanomaterials for management of dental caries.

Conclusion

The most common type of non-metallic nanomaterials for caries management is organic nanomaterials. The literature showed non-metallic nanomaterials have antibacterial and/or remineralising properties. Non-metallic nanomaterials can be incorporated into dental sealants, toothpaste, dental adhesives, topical agents and even candies and drugs. However, the majority of the publications are in vitro studies, and only 2 publications are clinical studies.