Full Length ArticlePolysilsesquioxane with potent resistance to intraoral stress: Functional coating material for the advanced dental materials
Graphical abstract
Introduction
Developing dental materials is an exceptionally complicated and difficult process because the intraoral is a harsh environment in which various mechanical, chemical and biological stresses coexist. Clear overlay appliances (COAs) are popular dental transparent aligners that act as a retainer for dynamic orthodontic treatment or maintaining intercanine and molar width [1], [2], [3]. A COA is imprinted into a model suitable for the patient's tooth arrangement by heat-pressure processing of thermosetting polymers such as polyethylene and glycol-modified polyethylene terephthalate (PETG). However, due to the poor resistance of COAs to intraoral stress, not only does the corrosion and cracking of COAs occur frequently but also the accumulation of bacteria on the surface of COAs leads to the formation of plaque. Consequently, patients face the hassle and financial burden of having to change their COA every two weeks [4], [5].
Most researchers in dental materials have developed durable raw materials for COAs by blending or copolymerizing polymers with excellent mechanical properties [6], [7], [8]. However, these materials were opaque and heavy and had low rebound resilience. Recently, a strategy for imprinting COA without heat and pressure using 3D printing was reported [9], [10]. These COAs show better physical properties than the conventional COA, but it requires expensive equipment, and its mass production is difficult. Furthermore, the deterioration of the COAs by bacterial accumulation cannot be prevented since the previous studies above have not focused on the antibacterial properties of COAs. Although recent studies on dental antibacterial resins or adhesives have been steadily reported [11], [12], studies on antibacterial properties of dental retainers are rare. In our recent study, a Zwitterionic polymer-based coating was synthesized and applied to a dental retainer. The excellent hydration and anti-polyelectrolyte effects of Zwitterionic polymer exhibited a bacterial reduction of 85% and 80% in ex and in vivo biofilm formation, respectively [13]. Several conventional research have adopted methods of adding antibacterial silver nanoparticles or quaternary ammonium salts to the process of manufacturing raw materials [14], [15]. COAs prepared by these approaches are easily cracked, need to be changed frequently, and sometimes cause cytotoxicity due to the leakage of antibacterial agents. Ultimately, the mechanical strength and antibacterial properties are considered to exist in a tradeoff relationship in the development of orthodontic devices. Accordingly, more effective coatings and sophisticated evaluation systems are required for commercialization.
Polysilsesquioxane (PSQ) with an organic–inorganic hybrid structure is an optimal coating material for developing advanced COA that can exert excellent durability and antibacterial properties simultaneously. PSQ is a well-organized block containing organic functional groups in a durable and flexible siloxane matrix. PSQ is synthesized by the sol–gel reaction and has a random, cage-like, or ladder-like structure depending on the reaction conditions [16], [17], [18]. Moreover, various functions can be imparted to PSQ by using a precursor having a desired functional group [19], [20]. Recent strategies for functionalizing silica nanoparticles with PSQ containing quaternary ammonium salts (QAS) have been reported. Polymeric substrates or cotton fabrics coated with functionalized silica particles exhibited excellent antibacterial and antifouling properties [21], [22]. Another research group synthesized a Phenyl/vinyl polysilsesquioxane based ladder polymer as a novel additive for vinyl ester resin. The addition of the synthesized PSQ significantly improved the mechanical and flame retardant properties of the vinyl ester resin [23]. Ultimately, we can synthesize PSQ with both siloxane-based durability and functional chain-based antibacterial effects by using an appropriate silicate as a precursor.
In this study, we synthesized a ladder-like PSQ containing quaternary ammonium cations (QACs) and long alkyl chains (LACs) and then coated it on COAs to investigate the behavior of the coated COAs under intraoral conditions. The ladder-like PSQs indicate a linear configuration of double-strained siloxanes and organic functional groups stretching outward [16], [24]. These compounds exhibit improved film properties, as they have a higher molecular weight than PSQ with other structures and have excellent thermal/mechanical stability [24], [25], [26]. QACs and LACs act as antibacterial groups involved in bacterial contact and killing [27], [28]. Unlike conventional antibacterial agents that are associated with leakage concerns, the antibacterial chains in PSQ are less likely to leak due to their stable network. COAs are coated in a synthesized PSQ solution by a simple immersion method, and the thickness of the coating can be controlled according to the duration of reaction. The purpose of this study is to develop PSQ-coated COA with excellent mechanical strength and antibacterial properties in a harsh oral environment based on the ladder-like PSQ synthesis technology (Fig. 1). We established an experimental design that simulates a dynamic intraoral environment to evaluate the chemical stability and mechanical properties of PSQ-coated COAs. In addition, we discussed the possibility of the commercialization of PSQ-coated COAs based on the results of in-vivo antibacterial experiments. This study proposes a progressive strategy for using organic/inorganic hybrid coatings to develop advanced dental materials.
Section snippets
Synthesis of ladder-like PSQ and preparation of coatings
PSQ synthesis was performed via the sol–gel reaction. Tetraethyl orthosilicate (TEOS, Mw = 208.33, Sigma-Aldrich, St. Louis, USA) and 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride (TPDA, 42 wt% in methanol Mw = 496.28, Sigma-Aldrich) as a precursor, methanol (MtOH, Mw = 32.04, Sigma-Aldrich) as a solvent, and 1 M HCl as a catalyst were used. Precursors, deionized (DI) water, and acid catalyst were prepared in the amount calculated according to the feed molar ratio in Table 1.
Ladder-like structured PSQ coatings for COA modification
PSQs with the chemical formula [RSiO3/2] n are organic–inorganic hybrid materials embracing functional organic groups in the siloxane backbone. PSQs can implement various functions according to their synthesis conditions and precursor types and typically have random, cage-like, or ladder-like structures according to the arrangement of siloxane bonds and organic chains [16], [32], [33]. Since the physical properties of PSQs are different for each structure, the type of PSQ to be used is
Conclusion
In this study, we reported the effect of a ladder-like PSQ coating on the improvement of the mechanical properties and antibacterial properties of COAs. The ladder-like PSQ containing QACs and LACs in a durable siloxane matrix was synthesized via the sol–gel reaction under elaborately designed conditions. The PSQ-coated COA was prepared by immersing the COA substrate in the PSQ solution to induce the assembly of PSQ chains on the substrate surface. We controlled the duration of assembly to
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017R1E1A1A01074343). This work was supported by the Korea Medical Device Development Fund grant funded by the Korea government (the Ministry of Science and ICT, the Ministry of Trade, Industry and Energy, the Ministry of Health & Welfare, Republic of Korea, the Ministry of Food and Drug Safety) (202011D04). This work was supported
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