## 3 Results and Discussion
The error bars indicate the variation (\(\pm\)one standard deviation) of the repeatedly measured values by the particle detector. Only representative bars are displayed and the error for all data is the same. Without the operation of the air cleaning system, the PM density remained at a hazardous level (above the detection limit of 500 \(\upmu\)g/m\({}^{3}\)) for 400 min, as expected. On the other hand, when the simple air cleaning system was installed and operational, it took only \(\sim\)23 min for the PM density in the closed chamber to be reduced to a safe level (below 50 \(\upmu\)g/m\({}^{3}\)). It should be noted that the dimensions of the Kapton filter were only 40 \(\times\) 40 \(\times\) 0.025 mm\({}^{3}\), which are much smaller than the filters in commercial air purifiers. The Kapton air filter used was a single layer and thus, a very low-power fan can be utilized. The advantages of Kapton and polymer films include their water resistance, corrosion resistance, and high thermal stability. In order to exploit these advantages, we investigated the possibility of reusing the filter by washing it with tap water. Figure 2e,f show the surface morphology of the Kapton air filter after washing it with tap water. As mentioned for the single-pass filtering experiment, very fine particles were captured, which is shown in Figure 2f. It can also be seen that the surface of the Kapton filter has been effectively cleaned after being washed with tap water, compared to the SEM images in Figure 2a-d. No special cleaning agents or processes were needed as previously reported [14]. Thus, the filter can be recycled very easily and is very economical. To investigate the durability and reliability of the filter, multiple washing processes were performed. Figure 3b shows the PM removal performance test after each cleaning cycle. Each filtration test was performed in the same conditions, which is shown in Figure 2. The PM2.5 density in all the experiments dropped to 40 \(\upmu\)g/m\({}^{3}\) within 30 min. In this study, the proposed Kapton air filter  
Figure 2: SEM images of a Kapton filter operated for 7 days. Micro dust is captured at the surface and around the pores. The magnifications are (**a**) 200\(\times\), (**b**) 900\(\times\), (**c**) 1500\(\times\), and (**d**) 2900\(\times\). Representative dimensions of the PM are shown in (**d**). (**e**) Optical microscope and (**f**) SEM images of a Kapton filter washed with tap water after filtration. demonstrates good PM removal performance and excellent durability and reliability for multiple uses after being cleaned with tap water. Lastly, in order to verify the endurance of the Kapton films under high temperature conditions, such as gas outlets in factories and laboratories, the Kapton air filter was heated at 350 \({}^{\circ}\)C for 10 min. Figure 4 shows a SEM image of a Kapton film after heating. As expected, there has been no significant deformation or introduction of defects during the heating process. The decomposition temperatures (on-set temperature and 10% mass loss temperature) of the Kapton film determined by thermogravimetric analysis are 558.97 \({}^{\circ}\)C and 577.21 \({}^{\circ}\)C, respectively, and the glass transition temperature was determined to be 401.67 \({}^{\circ}\)C by differential scanning calorimetry analysis. To the best of our knowledge, no other commercial air filters can withstand such high temperatures. These results also indicate that the filter can easily be sterilized by being heated in home appliances, such as conventional and microwave ovens, preventing any micro-organisms from surviving. This is another notable advantage of the proposed air filter. Figure 4: SEM images of a Kapton filter after heating at 350 \({}^{\circ}\)C for 10 min. The magnifications are (**a**) 200\(\times\) and (**b**) 1500\(\times\). Figure 3: (**a**) Evolution of PM2.5 density as a function of time with and without the Kapton air filter. The detection limit was 500 \(\mathrm{\SIUnitSymbolMicro g}\mathrm{/}\mathrm{m}^{3}\).