Physics of Face Masks*

Central to the rapid spread of COVID-19 and other respiratory infections is the transmission by airborne viral particles. Indeed, ultrafine particles below 0.3 microns, such as majority of the pollutant particles found in smog and wild fires, can penetrate deep into the respiratory system, posing severe health risks. There is an urgent need for the development of efficient, durable, reusable and recyclable filters for the deep submicron size range. Currently, the gold standard N95 face masks are difficult to decontaminate and reuse. Their polymeric fibers rely on electrostatics, which discharge over time, limiting their efficiency. The sheer volume of the waste face masks generated during COVID-19 also poses significant environmental challenges, due to the long time it takes for polymer-based materials to decompose.

We have recently demonstrated a new type of deep submicron particulate filtration media using metallic nanowire foams with tunable porosity and density [1,2]. Their specially tailored nanostructures lead to excellent efficiencies and breathability. Analysis of the foam microstructures, including the overall surface areas and characteristic feature sizes, reveals correlations with the foam filtration performance [3]. The foams are light weight yet robust, can be easily cleaned and reused, and eventually reclaimed and recycled. They can accommodate additional air cleaning mechanisms, such as electrostatics, catalytic reactions, and antimicrobial characteristics, with the potential for also neutralizing viral pathogens or toxic gases. The foams are made by scalable processes, thus are economically viable. They can be readily integrated with other mask designs or household / vehicle air cleaners, as the filtration media or as inserts / cartridges. These results demonstrate a new type of smart filtration platform against multi-hazards carried by airborne pollutants and pathogens. Our mask design based on such foams has been selected as a Phase 1 Winner and a Phase 2 Finalist of the recent BARDA-NIOSH-NIST Mask Innovation Challenge [4-7].