**1. Introduction**

Many orthodontic materials are formed from metals, which typically have superior mechanical properties compared with other materials. However, there are aesthetic issues with metal orthodontic materials [1]. More aesthetically attractive orthodontic materials are desirable, especially for adult patients. Aesthetic brackets made from ceramics and plastics have been widely used in clinical orthodontics [2,3]. Unfortunately, ceramic brackets have shortcomings stemming from their brittle nature, e.g., occasional fracture when tying the ligature and fracture from archwire forces, along with tooth wear during treatment and enamel fracture at debonding [1,4]. Plastic brackets also have deficiencies, such as a tendency to discolor, wear and creep due to their poor mechanical properties [1,5]. To overcome these issues, glass fiber-reinforced polymer wires have been investigated [6–9] but have yet to be used widely because of their brittleness and inability to withstand sufficient force [6–8]. Recently, coated archwires, including metal wires coated with polymers and rhodium-plated wires, have been developed [10–15]. These are preferred by many patients and orthodontists because of their

improved aesthetic qualities. However, polymer-coated wire loses a significant amount of its coating layer when used in the areas of archwire engagement [11,12], which affects frictional properties and bacterial adhesion [13,15].

Acid-etching of enamel surfaces for bracket bonding procedures has been accepted in modern clinical orthodontics since the direct bonding of orthodontic brackets to enamel was introduced in the mid-1960s [16,17]. The enamel surface around bonded brackets etched with phosphoric acid is more susceptible to demineralization because the areas stagnate with plaque, making tooth-cleaning more difficult and limiting the efficacy of natural self-cleaning mechanisms. Additionally, the mechanical properties of the enamel surface region decreased after bracket bonding with the etch-and-rinse adhesive system [18], and irreversible alteration of the enamel might increase the risk of enamel micro-cracks forming during debonding procedures. Therefore, further demineralization of the enamel after bracket bonding should be prevented and, ideally, remineralization should be enhanced.

One reasonable way to enhance the remineralization of tooth surfaces is to increase the calcium or fluoride concentrations of oral fluids [19,20]. Various bioactive glass (BG) have been investigated since the first ones were reported by Hench et al. (1971) [21]. These studies have included their osteo-inductive behavior, ability to bond to both soft and hard tissues, the capacity of the glass to release ions (Ca, Na, Si), and the ability to form a hydroxyapatite layer [22–25]. More recently, attention has focused on their modification to further enhance osteogenic behavior, or on further compositional changes to introduce additional multifunctional properties such as antimicrobial activity [26]. If the surface of the metallic orthodontic materials can be modified with a BG, it may help to prevent the demineralization of tooth surfaces surrounding brackets and enhance remineralization after bracket debonding; these features are attractive in the clinical orthodontic setting.

Electrophoretic deposition (EPD) is a simple, rapid, and versatile coating technique, whereby colloidal particles suspended in a liquid medium migrate under the influence of an appropriate electric field and are deposited onto an electrode, leading to film formation and coatings with high microstructural homogeneity and tailored thickness [27,28]. Among the different techniques used for surface modification in the biomedical field, EPD is particularly attractive because it is does not require expensive equipment and can be used with colloidal BG particles to form complex-shaped orthodontic materials.

In this article, BG particles were deposited onto orthodontic stainless steel disks by an EPD process under various conditions, and the BG coating was characterized esthetically, morphologically, and compositionally using various methods. Additionally, the effects of the BG coating on the remineralization ability of etched dental enamel and frictional properties were investigated.

#### **2. Materials and Methods**

#### *2.1. Materials*

Mechanically polished stainless steel (SUS316) disk specimens (diameter: 14 mm; thickness: 2 mm; Nogata Denki Kogyo, Tokyo, Japan) were purchased and cleaned ultrasonically and subjected to the BG coating process. Non-coated specimens served as a control.

The BG (45.0% SiO2 + 24.5% Na2O + 24.5% CaO + 6.0% P2O5) was prepared by melting the raw materials in a platinum crucible at 1550 ◦C for 90 min using an electrically heated furnace (model SSFT-1520; Yamada Denki, Tokyo, Japan). The molten glass was rapidly quenched by malleating (rolling) between two stainless steel plates of 10-mm thickness. After cooling overnight, the glass was ground for 2 min in a vibrational rod mill (model TI-200; CMT Co., Fukushima, Japan) to yield a particle diameter of ca. 100 μm. The powders were further milled using a high-pressure gas-milling apparatus (Nano Jetmizer; Aishin Nano Technologies, Saitama, Japan) under a grinding pressure of 1.4 MPa to provide particles with a median diameter (D50) of 1.98 μm. Analysis of the BG by X-ray diffraction (XRD) confirmed its amorphous structure.
