# Repetitive Bacterial Disinfection of Respiratory by Polydopamine Coating
## 1 Introduction  
Wearing medical-grade masks (e.g., N95) is presumably the easiest and most effective method of preventing the spread of pandemic and epidemic diseases, including the current COVID-19 virus. External pathogens can be efficiently captured in the filtering material (melt-blown polypropylene) of a mask [1]. However, repetitive usage of a single mask may result in unexpected infection from random pathogens, as the inner environment of masks is ideal for pathogen growth. For example, the humidity and temperature of the filtering N95 respirator can increase rapidly to 90% RH (relative humidity) and 30-35 \({}^{\circ}\)C after 10 min of usage [2]. The rapid growth of random pathogens may result in pathogenic consequences. However, repetitive usage of medical-grade masks is occasionally inevitable owing to the unexpected shortage of masks [3]. Several methods have been developed to overcome this issue. Thermal disinfection [4; 5], ultraviolet germicidal irradiation [6; 7], and H\({}_{2}\)O\({}_{2}\)-based decontamination [8] are representative examples. However, none of these methods have successfully solved the mask shortage problem for the following reasons: (1) The polymeric filtering material in masks undergoes structural deformation during the continuous thermal decontaminationprocess; thus, only a few cycles of decontamination are possible in the thermal disinfection method [9; 10]. (2) In the case of ultraviolet germicidal irradiation, the installation of a UV source is essential, and the effectiveness of decontamination varies greatly depending on the distance between the mask and the UV source [11]. (3) Decontamination using vaporized H\({}_{2}\)O\({}_{2}\) may be the most reasonable method for disinfection; however, a large amount of H\({}_{2}\)O\({}_{2}\) may also result in structural deformation of the filtering materials in masks [9]. It may be ideal to locally produce a small amount of H\({}_{2}\)O\({}_{2}\) in masks for disinfection purposes (removal of pathogens). For this purpose, H\({}_{2}\)O\({}_{2}\)-generating ability should be conferred to the filtering material, melt-blown polypropylene. However, methods of adding new functions (functionalization) to polypropylene are extremely limited owing to the inherent hydrophobicity of polypropylene [12; 13]. Polydopamine, inspired by the adhesion mechanism of marine mussels, is a thin multifunctional coating material that can be introduced into any kind of substrate [14; 15; 16; 17; 18; 19], including polypropylene [20]. The inherent catechol functionality of polydopamine allows various chemical conjugations, such as hydrogen bonding, \(\pi\)-\(\pi\) stacking, metal coordination, and other covalent bonds [21], and is the origin of material-independent coating ability. Moreover, the catechol group has a high potential for transformation into H\({}_{2}\)O\({}_{2}\)[22; 23], which then generates hydroxyl radicals (OH-) [24] via a chemical oxidation process. Reactive oxygen species (ROS) oxidize the crucial components of pathogens, resulting in disinfection [25]. Thus, we postulate that successive polydopamine coatings may be a way of permanently producing H\({}_{2}\)O\({}_{2}\). This is beneficial for avoiding the spontaneous growth of pathogens in the mask, enabling the reuse of masks without structural deformation of the filtering materials. Herein, we present a method for repetitively imparting antibacterial properties to the conventional N95 mask through polydopamine (PDA) coating. For this purpose, melt-blown polypropylene, which can capture external pathogens when used as a mask filter, is coated with PDA to introduce a catechol functional group. Catechol acts as a raw material for generating H\({}_{2}\)O\({}_{2}\), and the generated H\({}_{2}\)O\({}_{2}\) exhibits an antibacterial effect owing to the continuous conversion to the hydroxyl radical (OH-). Successful coating is confirmed through X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and other techniques.