Polysilsesquioxane with potent resistance to intraoral stress: Functional coating material for the advanced dental materials

Highlights

• Polysilsesquioxanes are introduced as a coating material for surface modification of dental retainers.

• Polysilsesquioxanes have a ladder-like structure with antibacterial groups in a siloxane matrix.

• The surfaces of the dental retainers are functionalized by the self-assembly of the polysilsesquioxane.

• The coated surfaces have excellent antibacterial activity and mechanical resistance to intraoral stress.

• The clinical application potential of this polymeric nano-coating was demonstrated in in vivo experiments.

Abstract

Since the oral cavity is a harsh environment where various germs and chemical and mechanical stress exist simultaneously, well-organized hybrid technology should be considered in the design of cutting-edge dental materials. Although the Clear Overlay Appliance (COA) is widely used as a transparent orthodontic device, it still requires specific modifications to overcome the poor durability and frequent contamination that cause the patient's therapeutic burden. Herein, robust polysilsesquioxane (PSQ) containing quaternary ammonium cations (QACs) and long alkyl chains (LACs) in a durable siloxane matrix are reported as coating materials for the advanced COAs. The PSQ coatings are engineered to have a ladder-like structure that exhibits excellent stability under various pH and enzymatic conditions. Experiments simulating the dynamic intraoral are established to investigate the effects of PSQ coatings for the functional improvement of COA. The QACs and LACs firmly fixed to the siloxane matrix cause bacterial contact killing to prevent the contamination of COAs in the environment exposed to bacteria. The ladder-like siloxane matrix of the coating protects the COAs from intraoral abrasion forces and shearing movements with its flexible adaptability to external stresses. This study clarifies the clinical application potential of the coated COAs via in-vivo experiments using the beagle model.

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.