Now, researchers in ACS Applied Materials & Interfaces report a new hydrogel treatment that breaks apart cavity-forming biofilms and whitens teeth without damaging them.
Daily toothbrushing and flossing are good ways to prevent cavities from forming, according to the American Dental Association. However, these methods don’t effectively whiten teeth. For better whitening, consumers often turn to over-the-counter or professional treatments that combine hydrogen peroxide-containing gels and blue light, producing a chemical reaction that removes stains.
This method still required high-intensity blue light, which can damage nearby skin and eyes. So, the team wanted to find a material that would be activated by green light – a safer alternative – to both whiten teeth and prevent cavities.
The researchers combined bismuth oxychloride nanoparticles, copper oxide nanoparticles and sodium alginate into a thick mixture.
Then, they evenly coated the mixture onto the surface of teeth stuck to a slide and sprayed the concoction with a calcium chloride solution, forming a strongly adhering hydrogel. Next, the team tested the material on teeth that were stained with coffee, tea, blueberry juice and soy sauce and placed in a lab dish.
The green-light activated hydrogel effectively prevented moderate and deep cavities from forming on the surface of the animals’ teeth. The researchers say their safe, brush-free treatment both effectively prevents cavities and whitens teeth.
Dental caries (tooth decay) represents one of the most common and costly chronic diseases, causing unbearable suffering in 2.3 billion people around the world.[1-4] The development of dental caries is a long and complex process, which depends on the formation of microbial biofilms on tooth surfaces (dental plaque).[4-6] The formation of biofilms on tooth can increase the risk of enamel decalcification and periodontal diseases. To prevent the development of gingivitis, caries, and periodontal diseases, cutting off biofilm formation is an effective method. Scientists have devoted lots of research effort to interfering with the growth and metabolism of bacteria in the plaque biofilms, and preventing the formation or promotion of the dissociation of microbial biofilms, in hope of preventing dental caries.
Streptococcus mutans (S. mutans) is a predominant etiological agent of dental caries with the exceptional capability of acid production and biofilm formation.[7-9] There are multiple mechanisms of S. mutans contributing to the formation and development of dental plaque biofilm, including extracellular polymeric substance (EPS) synthesis, carbon catabolite repression, and quorum sensing.[10, 11] The EPS matrix of S. mutans is a water-insoluble, 3D natural physical barrier, which protects S. mutans from its host’s innate immune cells and prevents the penetration of antibacterial agents.[12, 13] Burying bacteria cells in biofilms is an ecological strategy that bacteria adopt to get rid of the host immune system and antimicrobial drugs, and the biofilms serve as a reservoir for chronic infections with high severity.[14-16] Besides, bacteria inside biofilm are usually metabolically changed with low growth rates and increased stress resistance, which invalidates antimicrobials and increases the difficulties of eradicating these bacteria.[15-17]
To inhibit the formation of biofilm, most commercially available oral care products contain ingredients for dental caries prevention, such as chlorhexidine (CHX) and fluoride. However, both of them cannot modulate biofilm composition or its virulence, and may lead to tooth staining.[18] Even with regular use of fluoride, carious lesions could still develop when exposed to over six dietary sugar per day, while high dose of fluoride could be associated with fluorosis, bone weakening, and developmental neurotoxicity. CHX has been reported to be toxic to host cells and may induce allergic reactions. More importantly, microorganisms have gained increasing resistance to these antimicrobials, which necessitates novel tooth biofilm eradication and tooth discoloration strategies to which microorganisms are difficult to generate resistance.
Besides biofilm eradication, there is an increasing need for good tooth appearance. The color of teeth can be significantly altered by stains from various sources, such as smoking, consumption of tannin-rich beverages (e.g., tea), and abuse of antibacterial agents such as CHX. Tooth whitening has thus emerged as one of the most demanded dental treatments by the general public. Compared with irreversible therapies such as veneers, tooth whitening represents a conservative and convenient treatment for colored teeth.[19] During regular tooth whitening, hydrogen peroxide (H2O2) and carbamide peroxide are widely used for the removal of tooth stains in the form of whitening strips (a tooth discoloration product). Most whitening strips contain over 10% of H2O2, whose bleaching effect is similar to that of household tray bleaching agents.[20] Such caustic treatment can cause burns to gingival and mucosal tissues,[21] and may also lead to dentine hypersensitivity.[22] It has been reported that H2O2 not only destroys the morphology and reduces the hardness of enamel,[23] but also increases surface porosities, which may lead to tooth re-coloration and adherence of certain cariogenic microorganisms on teeth.[24]
As aforementioned, both tooth biofilm inhibition and tooth whitening are thorny issues, and it will be great if we can tackle these two issues in one combined treatment. Photodynamic therapy (PDT) has been recently proposed for applications in dental therapies and tooth whitening, and it has been considered as a promising strategy to eradicate oral pathogenic bacteria, which may cause endodontic diseases, periodontitis, and caries.[25-30] During the course of PDT, photosensitizers (PSs) and light irradiation were employed to sensitize the generation of reactive oxygen species (ROS), which can cause oxidative damage to nucleic acids, proteins, and lipids.[31] Consequently, it induces irreversible microbial death with the superiority of minimal invasiveness, limited antibiotic resistance, low systemic toxicity, and minimal side effects. Recently, Zhang et al. reported a bifunctional photodynamic dental therapy strategy for tooth whitening and biofilm eradication by employing a zwitterion-modified porphyrin as the PS.[32] However, high PS concentrations, long incubation time, and long irradiation time were needed to achieve a good tooth-whitening effect and bactericidal effect in biofilm, which may increase the discomfort of patients and limit its clinical applications. Besides, conventional PSs usually suffer from decreased ROS sensitizing efficiency at high concentrations and in aggregated state, and thus further increasing their working concentrations may not improve PDT performance.
Materials with aggregation-induced emission (AIE) characteristics are gaining increasing attention, and they have been employed as PSs for microorganisms and tumor treatments due to their superior advantages of good photostability and enhanced ROS production in the aggregated state.[33-39] Previously, we reported an AIE-active PS, DTTPB (Figure 1a), for efficient inactivation of human coronavirus with PDT.[40] Encouraged by these exciting results, we further explored its potential for simultaneous photodynamic eradication of biofilm and tooth whitening (Figure 1b). DTTPB can sensitize the production of ROS, effectively inactivate S. mutans both in planktonic solution and biofilm, inhibit the formation of biofilm, and disintegrate biofilm by decomposing exopolysaccharides and glycoproteins in the EPS matrix. Besides, it can also whiten colored saliva-coated hydroxyapatite (sHA) and clinical teeth at low working concentrations and within a short irradiation time without inducing tooth erosion. Such highly efficient simultaneous tooth biofilm eradication and tooth whitening of DTTPB-mediated PDT is expected to open up new avenues for clinical oral treatment practices.
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More information: Qun Li et al, Fast Cross-Linked Hydrogel as a Green Light-Activated Photocatalyst for Localized Biofilm Disruption and Brush-Free Tooth Whitening, ACS Applied Materials & Interfaces (2022). DOI: 10.1021/acsami.2c00887
