N95 masks can now be recharged and reused

By exposing N95 masks to 100 kV for 3 min after sterilisation in hot water or an autoclave, a researcher at The University of Tokyo has demonstrated how the masks can regain their static charge so that they can be reused. Published in the journal Soft Matter, the novel method can be rapidly applied to help meet the huge demand for protective equipment that can prevent the spread of SARS-CoV-2, the virus that causes COVID-19.

N95 masks have been indispensable for keeping healthcare workers and first responders safe during the current pandemic. The name comes from the fact that these masks can filter more than 95% of airborne particles, even though they have pore sizes 10 times larger than the small aerosol particles that can carry the virus. The trick is that the N95 masks are made of electrospun polypropylene fibres, which retain a static electric charge that can attract and trap the charged aerosols.

However, this static charge cannot withstand normal sterilisation procedures, such as washing in hot water or autoclaving. Even the moisture in the wearer’s breath can degrade the effectiveness of the electric attraction. For this reason, the masks are often discarded after a single use, which greatly increases the gap between the number of N95 masks needed and the number available.

Now, Tokyo researcher Kaori Sugihara has shown that sterilised masks can be returned to use after being recharged using a van de Graaff generator. These devices, familiar to many science museum visitors, use the friction from a spinning belt to generate very high voltages between two metal conductors.

“Making use of the high voltage provided by the van de Graaff generator, this method is much faster than alternative methods,” Sugihara said.

To regenerate a mask that had been previously sterilised, it was attached to the larger metallic sphere, while the smaller sphere was placed several centimetres away for 3 min. The regenerated masks were tested and shown to be comparable in filtering ability to unused masks.

Because van de Graaff generators are much cheaper and safer to use than other high-voltage sources, the method can be implemented easily in hospitals and other locations where N95 masks are most needed. Sugihara hopes it “will allow many more people to have access to N95 masks each day, which is our best line of defence against COVID transmission”.

Image credit: Institute of Industrial Science, The University of Tokyo.

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