Face masks can be 'recharged'

Scientists from the Institute of Industrial Science at The University of Tokyo have developed a compact system that can restore N95 respirators and surgical masks that have been exposed to moisture to 97% efficiency. By using a special circuit and a conductive plate, a large and uniform voltage distribution recharges the mask in about one minute. Described in the journal Heliyon, this machine can help address the need for high-performance masks while reducing plastic medical waste.

During the COVID-19 pandemic, surgical masks and N95 respirators were in high demand, because these were some of the best tools available for reducing the risk of catching or transmitting the SARS-CoV-2 virus. Although the pore size of these masks (~10 µm) is much larger than the size of virus-carrying aerosol droplets (~1 µm), the masks are good at capturing droplets because the polypropylene mesh inside them carries a static electric charge, which can attract the droplets via electrostatic force. However, over time or as a mask becomes wet, this effect becomes significantly reduced, hindering effectiveness.

“Humidity, including exposure to exhaled air, or washing in water removes a mask’s charge and compromises its filtering efficiency,” said Taisuke Ikawa, lead author on the new study. As a result, most used masks were discarded, instead of being sterilised and reused.

The device built by the research team is based on an electrical circuit called a Cockcroft-Walton’s voltage multiplier. This allows electrical potentials of up to 40,000 V to be obtained in a compact device. The researchers used a conductive aluminium plate as an electrode to ensure a homogenous voltage distribution and covered it with an insulating plastic PET board to suppress sparks.

After masks that had been washed in water were briefly exposed to the electric field generated by the device, their efficiency was found to be significantly restored. Future research may scale up the size of the device to help increase throughput.

“Our work raises the possibility that masks can be sterilised and recharged again and again, significantly increasing supply,” said senior author Kaori Sugihara.