## 1 Introduction  
Recently, emerging environmental pollution issues--especially, air pollution produced by emissions from residential energy use (heating, cooking), traffic, and power generation--have become serious problems worldwide. Such air pollution has serious long-term effects on the global atmosphere and public health. In recent years, there has been growing concern about the harmful effect of very fine particulate matter (PM) in the air as the PM can be absorbed into and eventually settle in the human body [1; 2; 3; 4; 5; 6; 7]. For example, PM with a diameter that is smaller than 10 \(\upmu\)m (PM10) can pass through human tissue and cause lung disease [8]. Furthermore, PM with a diameter that is smaller than 2.5 \(\upmu\)m (PM2.5) leads to increased plaque deposition in arteries, causing vascular inflammation and atherosclerosis, which can lead to heart attacks and other cardiovascular problems [9]. Conventionally, PM10 is filtered using general thread filters, and the smaller PM2.5 is filtered using high-efficiency particulate arresting (HEPA) filters. In modern air-conditioning systems, HEPA filters are generally made from a fabric of tangled, warped fibers and they work by pulling or pushing the air through a filter screen that catches the PM. The fibers are randomly arranged and these randomly oriented individual fibers force the air through an indirect, non-linear path. As the particlespass through the filter, they are caught in the fabric maze by four mechanisms: impaction, sieving, interception, and diffusion. However, the conventional PM filter systems have many inherent problems. First, the bulky structure of the HEPA filter requires a large working space and causes a large pressure difference between the inlet and outlet of the filter, limiting the efficiency of air circulation [10; 11]. Second, HEPA filters cannot be cleaned, allowing potentially hazardous micro-organisms to accumulate and prosper in the filters. Lastly, the HEPA filters are not recyclable and must be discarded after use, causing environmental pollution. In order to remove these limitations, various PM filter systems based on new materials and production technologies have been developed. Polar, polymeric nanofiber networks based on surface chemistry, using polyacrylamide, nylon, and polyimide, have been proposed as potential transparent PM filters with high efficiency [12; 13; 14; 15]. However, such filters are single-use and not reusable. [PERSON] et al. examined hot-gas filters composed of a polyimide nanofiber membrane sandwiched between woven-carbon fabrics that can be recycled by back-air flushing [16]. Recently, [PERSON] et al. employed metal nanowire percolation networks to develop a transparent, reusable, and active PM filter, with a high efficiency and small volume, using a hierarchical Ag nanowire percolation network on a nylon mesh [17]. Their filter can be reused multiple times only after cleaning with certain agents, such as ethylene glycol. Furthermore, the use of clay nanoparticles [18] and biopolymers [19] has been suggested for adsorption of pollutants. In this research, our aims in developing a new air filter can be summarized as follows. First, we aimed to minimize the pressure difference between the inlet and outlet while maintaining the performance of the filter in order to allow a less powerful fan to be used, thus lowering the power consumption. Second, the new filter should be easily cleanable to prevent the accumulation and growth of molds of fungus of micro-organisms in the filter, and be reusable. Third, the large-area filter should be made from only one material and its fabrication process should be as simple as possible. Lastly, the filter should be flexible and heat-resistant. The filter is not required to be rectangular, allowing innovative designs and applications. The filter should have adequate thermal stability. To achieve these goals, we present a demonstration of the application of a Kapton film--with micro-sized, patterned thru-holes--to air filtration.