HK Mask is a reusable cloth mask with a disposable middle layer filter. It was invented by Dr. K. Kwong during the outbreak of COVID-19 in early 2020.
The design of HK Mask is referred by that of 3-layer surgical mask in the market. The HK Mask is composited with a disposable middle layer filter which is sandwiched by a cloth mask as the outer and inner layers. There are a number of material chosen for the middle layer filter ranging from Kitchen Towel Paper to Nanofilter.
The filtration efficiency of a piece of kitchen towel paper is close to that of N70. The mask can be safely used after replacing a new middle layer filter to a cleaning and sterilizing the inner and outer layer cloth.
Link: HKMask Manual | Reusable DIY Cloth Mask with Disposable Middle Layer Filter (vía kottke.org)