Innovative strategy for in-office tooth bleaching using violet LED and biopolymers as H₂O₂ catalysts

Highlights

• Higher production of OH^• improves the esthetic outcome of the tooth bleaching.

• Violet LED causes a discrete chromatic change in dental tissues.

• Catalysis applied to a bleaching gel with 10% H₂O₂ optimizes the esthetic efficacy.

Abstract

Objective

To assess the influence of coating the enamel with a nanofiber scaffold (NS) and a polymeric catalyst primer (PCP) on the esthetic efficacy, degradation kinetics of hydrogen peroxide (H₂O₂), and trans-amelodentinal cytotoxicity of bleaching gels subjected or not to violet-LED irradiation.

Methodology

The following groups were established (n = 8): G1- No treatment (negative control); G2- NS+PCP; G3- LED; G4- NS+PCP+LED; G5- 35% H₂O₂ (positive control); G6- NS+PCP+35% H₂O₂+LED; G7- 20% H₂O₂; G8- NS+PCP+20% H₂O₂+LED; G9- 10% H₂O₂; G10- NS+PCP+10% H₂O₂+LED. For esthetic efficacy analysis, enamel/dentin discs were stained and exposed for 45 min to the bleaching protocols. To assess the cytotoxicity, the stained enamel/dentin discs were adapted to artificial pulp chambers, and the extracts (culture medium + components diffused through the discs) were collected and applied to MDPC-23 cells, which had their viability, oxidative stress, and morphology (SEM) evaluated. The amount of H₂O₂ diffused and hydroxyl radical (OH^•) production were also determined (two-way ANOVA/Tukey/paired Student t-test; p < 0.05).

Results

G6 had the highest esthetic efficacy compared to the other groups (p < 0.05). Besides the esthetic efficacy similar to conventional in-office bleaching (G5; p > 0.05), G10 also showed the lowest toxic effect and oxidative stress to MDPC-23 cells compared to all bleached groups (p < 0.05).

Conclusion

Coating the enamel with a nanofiber scaffold and a polymeric catalyst primer, followed by the application of 10%, 20%, or 35% H₂O₂ bleaching gels irradiated with a violet LED, stimulates H₂O₂ degradation, increasing esthetic efficacy and reducing the trans-amelodentinal toxicity of the treatment.

Introduction

Conventional in-office tooth bleaching (CIOTB) is considered a non-invasive procedure that causes a rapid chromatic change in hard dental tissues and has been extensively used in contemporary Esthetic Dentistry [1]. However, most patients subjected to this professional therapy report some type of tooth sensitivity [2], [3], [4]. Such adverse effect has been related to the intense trans-amelodentinal diffusion of unreacted (H₂O₂-free) hydrogen peroxide (H₂O₂) that causes irreversible damage to pulp cells [5], [6], [7], [8]. This occurs because of the high concentration of this low-oxidation-potential reactive molecule in bleaching gels, which main objective is to break the double links of colored pigments (chromophores), favoring the chromatic change of dental tissues [9], [10], [11], [12].

Several studies show that cell toxicity and tooth bleaching effectiveness depend directly on the H₂O₂ concentration in gels and application time to the tooth enamel [5,6,13,14]. However, reducing the H₂O₂ concentration in gels and/or application time requires various bleaching sessions, limiting the use of this strategy in the current clinical practice [7,13]. Thus, the search for faster and biocompatible bleaching protocols has driven researchers to study the kinetic behavior of H₂O₂ [15], [16], [17]. Proposing to catalyze H₂O₂ and increase the production of molecules of higher oxidation potential, catalyzing agents stood out because this strategy favored esthetic efficacy and decreased the cytotoxicity caused by CIOTB [17,16]. In this context, the heme peroxidase (HPR) enzyme, original from the root of Amoracia Rusticana, which has a high potential for catalyzing H₂O₂, started to be incorporated and tested successfully for professional bleaching protocols [17,18].

Several polymers, such as hydroxypropyl methylcellulose (HPMC), may be used to develop biomaterials with the potential to carry catalyzing agents [19]. The HPMC is a biodegradable, biocompatible, and nontoxic semi-synthetic polymer. Considering that HPMC can form a hydrophilic gel, it has been extensively used in the pharmaceutical industry, particularly for the controlled release of drugs [20,21]. Polycaprolactone (PCL) is a biodegradable and nontoxic biopolymer [22], [23], [24] that has been extensively used to produce electrospun scaffolds [24]. Jin et al. (2002) [25] reported that the reduced pore diameter of electrospun scaffolds prepared with PLC may limit the passage of microorganisms and toxic particles, and the desired permeability of this biopolymer may be achieved to perform specific functions in the application site.

The photocatalysis of H₂O₂ has been comprehensively studied in Dentistry [26]. Although light increases the temperature of the bleaching gel, thus favoring H₂O₂ degradation to produce two mols of the hydroxyl radical [27], the efficacy and safety of photocatalysis remain controversial [28,26]. Among the different light sources used in Dentistry, the violet LED associated or not with the use of a bleaching gel seems to have the highest potential to produce color changes in dental tissues [29], [30], [31], [32].

Considering the limited current knowledge on the safety and efficacy of using violet LED for tooth bleaching, as well as the role of catalyzing biopolymers in this esthetic therapy, this in vitro study aimed to assess the effect of associating a nanofiber scaffold (NS) and a polymeric catalyst primer (PCP) on esthetic efficacy, degradation kinetics, and cytotoxicity of bleaching gels, irradiated or not with a violet LED. The null hypothesis was that using violet LED to irradiate bleaching gels applied to the enamel previously coated with biopolymers (NS+PCP) does not interfere with the chromatic change and cytotoxicity commonly caused by CIOTB.