Titanium dioxide nanotubes with triazine-methacrylate monomer to improve physicochemical and biological properties of adhesives
Introduction
The replacement of dental tissues with resin composites is a common procedure performed when patients are affected by caries [1]. However, the lifespan of direct technique with resin composites remains about five years [2], leading to materials replacement and a spiral sequence of restorative treatments [3,4]. A greater extent of dental tissues may be lost over time due to the progressive substitution of unsuccessful composite restorations, jeopardizing patients’ oral health conditions [4]. Materials fracture and recurrent caries around dental-resin interfaces are the major reasons for the failure of composite restorations [5].
The development of caries seems to be the most important factor related to the survival of composite restorations after the first five years since their placement [6]. While the annual failure rate (AFR) of direct composite restorations was reported as 4.6% for high-caries-risk patients, this value was reported as 1.6% for low-caries-risk patients. This difference led to cumulative survival values of around 35% for those with high risk and 60% for low risk after 20 years [6]. Furthermore, knowledge about caries prevention is an essential factor that positively affects the survival of direct restorations [7]. Therefore, the quality of restorations depends on individual factors, and clinical patient-centered approaches must always be in focus [7]. However, restorative materials with antibacterial properties could be adjuvants in this process, may assisting in decreasing the AFR in those patients who present greater difficulty in controlling biofilm.
Another essential factor for restoration survival is the quality of the bonding to the hard-dental structures, which is influenced by the mechanical and chemical properties of the adhesive and the hybrid layers formed [8]. The hybrid layer is composed of a collagen network embedded by adhesive resin, and it represents the weakest site of the restoration set [9]. Mechanical forces application, patient-related variables, and a deficit of appropriate marginal sealing make the restoration prone to loss its mechanical stability and increase its degradation [10]. Marginal gaps caused by polymer degradation and polymerization shrinkage, together with increased roughness of composites over time [[10], [11], [12]], influence the development of recurrent caries adjacent to the bonding interface. Within this framework, efforts should be addressed to enhance the quality of the hybrid layer formed, improving the mechanical properties of dental adhesives, and providing antibacterial activity for this class of material.
Inorganic filler addition has been a strategy investigated to improve the physicochemical and biological properties of adhesive resins [[13], [14], [15], [16], [17]]. Researches on dental bonding have been progressively demanding inorganic nanoparticles due to their small size and high surface area, which may be an advantage to provide biointeractivity with dental tissues [18]. Titanium dioxide (TiO2) nanostructures were incorporated into dental materials to improve their physicochemical and biological properties. Hardness, flexural strength, degree of conversion, elastic modulus, shear bond strength, and antibacterial activity are among the superior features that TiO2 can confer [[19], [20], [21], [22], [23], [24], [25], [26], [27]]. Furthermore, TiO2 presents bioactivity, which could provide remineralization ability for resin-based dental materials [28]. Due to these properties, materials derived from TiO2 have been highlighted as reasonable fillers to be added to dental materials. In this context, titanium dioxide nanotubes (nt-TiO2) are nanocarriers that could provide TiO2 properties to adhesives besides being a suitable method to decrease the release rate of therapeutic agents such as antibacterial molecules [29]. nt-TiO2 are biocompatible nanocarriers with elongated shape, tubular structure, and significant loading ability. These features make them attractive for drug delivery systems [[30], [31], [32]].
Dental adhesives have been modified with antibacterial agents to prevent bacterial colonization at restoration margins [33,34]. Methacrylate monomers with antibacterial activity that are able to copolymerize with the resin may offer a long-term antibacterial effect because of their covalent bonding within the resin network [[34], [35], [36], [37]]. 1,3,5-triacryloylhexahydro-1,3,5-triazine (TAT) is a methacrylate monomer with three functional groups of carbon-carbon double bonds. This positively charged monomer was previously tested in dental formulations for orthodontic [38,39] and restorative [40] purposes. TAT increased the resins’ stability against solvents and the bond strength to enamel, besides had demonstrated antibacterial effect against Streptococcus mutans [38,40,41].
Based on the characteristics above mentioned of nt-TiO2 and TAT, the incorporation of nt-TiO2 into dental adhesives could be a strategy to confer superior mechanical properties besides providing antibacterial activity and bioactivity for the polymer. Moreover, nt-TiO2 could serve as a drug carrier of other agents, such as TAT. Considering that TAT was already tested purely in methacrylate dental resins, inducing antibacterial effect, and improving the physical and chemical properties, it could be mixed with nt-TiO2. These properties led us to investigate nt-TiO2 or nt-TiO2 with a triazine-methacrylate monomer (nt-TiO2:TAT) as fillers. This study aimed to formulate experimental adhesive resins containing nt-TiO2 or nt-TiO2:TAT and evaluate the effect of these fillers on resins' physical, chemical, and biological properties. The null hypothesis to be tested is that the addition of nt-TiO2 or nt-TiO2:TAT does not affect resins' physical, chemical, or biological properties.
Section snippets
Study design
In this in vitro study, five experimental adhesive resins were formulated and tested with different concentrations (2.5 or 5 wt.%) of two separated fillers: titanium dioxide nanotubes (nt-TiO2) or titanium dioxide nanotubes mixed with a triazine-methacrylate monomer (nt-TiO2:TAT). The following dependent variables were evaluated: antibacterial activity against biofilm formation and planktonic bacteria, cytotoxicity against human pulp cells, polymerization kinetics, degree of conversion, Knoop
Results
The nt-TiO2 applied as filler for the adhesives was synthesized according to a previously reported method [49], and the nt-TiO2:TAT was characterized via TEM, showing mean size of 45.8 (± 7.4) nm, which is similar to the size already described in the literature [49]. The nt-TiO2:TAT presented a typical morphology of nanotubes (Fig. 1 B–C). The particles were arranged in aggregates, and they presented clearer areas suggestive of the lumen presence.
The nt-TiO2, TAT, and nt-TiO2:TAT were evaluated
Discussion
This study used two fillers for an adhesive resin: nt-TiO2 and nt-TiO2 mixed with a triazine-methacrylate monomer (TAT), to improve adhesives physicochemical and biological properties. Overall, the fillers composed of nt-TiO2 or nt-TiO2:TAT led to a set of superior properties for the adhesive resins. Higher hydrolytic stability and μ-TBS at long-term, along with antibacterial activity and no adverse effects on biocompatibility, were also achieved when nt-TiO2 were mixed with the
Conclusion
In this work, we described the synthesis of nt-TiO2 combined with a triazine-methacrylate monomer and its incorporation into an adhesive resin. The nt-TiO2:TAT material was characterized by TEM, FTIR, UV–vis, and micro-Raman. From the combination of these techniques, it was possible to confirm the nanotubular morphology of TiO2 as well as the presence of TAT at the surface of nt-TiO2. The addition of 5 wt.% of nt-TiO2 in the adhesive resin increased the μ-TBS in comparison to the unfilled
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgments
The authors gratefully acknowledge Microscopy and Microanalysis Center (Federal University of Rio Grande do Sul) for the transmission electron microscopy analysis and the National Council for Scientific and Technological Development - Brazil (CNPq) for the scholarship of M. Stürmer. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 – scholarship.
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