Methacrylate saccharide-based monomers for dental adhesive systems

Abstract

The aim of this in vitro study was to synthesize three new methacrylate monomers based on the modification of saccharides structures (glucose-Gluc, sucrose-Sucr and chitosan-Chit) with glycidyl methacrylate, and to use them in the composition of dental adhesives. Three methacrylate saccharide monomers were synthesized and characterized by mid-IR, ¹H and ¹³C NMR, antioxidant activity and cytotoxic effect. Monomers included: one monosaccharide – Gluc-MA; one disaccharide – Sucr-MA; and one polysaccharide – Chit-MA. Primers containing HEMA, methacrylate saccharide monomers at concentrations of 0 (control), 1, 2 or 4 wt%, 60 wt% ethanol aqueous solution (pH3.0) and initiator system were formulated. Primers were used in conjunction with a bond step and composite paste to restore caries-free third molars, and dentin bond strength (24 h and 6 month of storage in water), and antimicrobial activity (Alamar Blue test) were tested. Degree of conversion (DC) and maximum rate of polymerization (Rp_max) of the primers themselves were also analyzed. The mid-IR, ¹H and ¹³C spectrum confirmed the presence of vinyl group on the structure of saccharides. Chit-MA showed low antioxidant activity and did not present a cytotoxic effect. Gluc-MA and Sucr-MA possess antioxidant and cytotoxic activity, concentration dependent. In the presence of methacrylate saccharide monomers, the primers showed DC comparable to the control group, except Gluc-MA4%, Sucr-MA4% and Chit-MA1%, which showed a range of 64.6 from 58.5%DC. Rp_max was not statistically different for all the groups (p = 0.01). The bond strength of Sucr-MA1% increased from 25.7 (± 2.8) to 40.6 (± 5.3) MPa after 6 months of storage. All the synthesized monomers showed some antimicrobial activity after polymerization. Gluc-MA and Chit-MA 4% and Sucr-MA 1, 2 and 4% led to decrease bacterial metabolism. Sucr-MA 1% showed better results regarding the decrease in bacterial metabolism and increasing the bond strength after 6 months of storage.

Introduction

The development of adhesive systems revolutionized esthetic restorative procedures, by modifying the cavity preparation concepts and allowing for conservation of the remaining healthy tooth structure [1], [2], [3]. The total etching technique proposed by Fusayama et al. allowed for the hybridization of the demineralized dentin and has become the most common bonding mechanism in dental practice [4], [5]. Dentin adhesion with three-step etch-and-rinse strategy was first used with clinical success around 1990s [6]. However, adhesion failure between restorative materials and dental structures continues to be one of the biggest practical problems in clinical Dentistry, leading to marginal leakage, discoloration, marginal fractures, secondary caries, post-operative sensitivity and pulpal reactions [7], [8], [9].

The success of restorations with margins in enamel has been described since the introduction of the etching technique, which is credited to the high inorganic content (~ 90%) of enamel. On this substrate, the mechanical imbrication by tag formation within the demineralized tissue is very efficient, and leads to very stable bonding [10], [11]. Dentin, however, still poses the major challenge for adhesive procedures, due to its tubular structure and organic and aqueous content [11], [12], [13]. The adhesive resin must infiltrate the wet network of exposed collagen fibrils, and polymerize in situ, forming what is known as the hybrid layer [5], [14].

The monomer composition of dental adhesives is one of the determining factors in adhesion performance. Hydrophilic monomers are essential in adhering to a wet substrate such as dentin. 2-hydroxyethyl methacrylate (HEMA) is the hydrophilic monomer most commonly used in adhesive systems and is present in the composition of the primer (hydrophilic monomers with organic solvents with or without water in it formulation) [15]. This monomer has a hydrophilic moiety (hydroxyl) with affinity with the wet substrate and, a hydrophobic tail (methacrylate functionality) which promotes the polymerization with other monomers. HEMA has a low molar mass allowing the infiltration of the resin adhesive in the dental substrate. However, HEMA monomers do not form very strong polymer networks, due to the linear nature of its chains, and potential phase separation during polymerization in a highly solvated state, in the presence of tubular water [15], [16].

Despite the advances achieved in dental adhesive technology, studies point to the degradation of the material over time in the presence of water [17]. This degradation may be a result of hydrolysis of the material and/or the collagen, thereby weakening the physical properties of resin-dentin bonding. The dentin-adhesive interface is porous and permeable, allowing the leaching of unreacted monomers, water sorption, swelling the polymer, and also be susceptible to the enzyme activity by metalloproteinases (MMPs), which degrade mainly type I collagen, hence exposing the fibrils at the bottom of the hybrid layer [18], [19].

The biodegradation of the interface can also increase the bacterial infiltration through a gap between the restorative material and dentin (interproximal areas), which may lead to secondary caries formation [20], [21]. Therefore, one additional feature of interest for adhesives is direct antimicrobial activity. Antimicrobial agents can potentially limit the infiltration, growth and the formation of a cariogenic biofilm, particularly by decreasing the viability of Streptococcus mutans (S. mutans), considered the main pathogen of tooth decay [22], [23], [24], [25]. Using this concept, antibacterial agents with broad antimicrobial spectra have been added with no concern regarding the promotion of bacterial resistance and the production of undesirable outcomes on oral health [26], [27]. Much attention has been given to antimicrobial compounds in natural products, as an alternative to synthetic compounds [28], [29], [30]. Thus, the search for new therapies to stabilize the resin-dentin interface is the key to improving the biomechanical and biochemical properties of hard dental tissues in restorative therapy.

A series of multi-functional monomers based on bile acids, colic acids and saccharides has shown high biocompatibility and low cytotoxicity compared to conventional polymers and monomers found in literature [30], [31], [32], [33], [34], [35], [36], [37], [38]. These bi- or multi-functional monomers are being used to provide resistance in the crosslinked monomer formed from the monomeric matrix [39]. Therefore, the aim of this in vitro study was to synthesize and characterize methacrylate monomers based on saccharides (mono-, di- and poly-saccharides) and analyze their behavior when incorporated into dental adhesive systems, including antimicrobial properties, hydrolytic stability and quality of the bonded interface with dentin. Our hypotheses were: (1) the synthesis route proposed here will successfully modify the saccharides structure to incorporate photopolymerizable groups; (2) the adhesives formulated with methacrylate saccharides monomers will demonstrate: a. greater antimicrobial activity against Streptococcus mutans; (3) greater/longer-lasting bonding to the dentin substrate when compared to control.