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Researchers from India's Tata Institute of Fundamental Research and Israel's Technion-IIT developed the method to restore the efficiency of N95 masks to ‘out-of-box’ levels, as long as they have not been structurally compromised.
N95 masks are a critical part of the personal protective equipment (PPE) used by frontline healthcare workers. They achieve 95% efficiency at filtering out tiny 0.3 micron particles while maintaining reasonable breathability, thanks to a layer of fine melt-blown polypropylene fibres with electrical charges to attract particles.
Extended use and decontamination seen in response to severe global shortages during the Covid-19 pandemic can easily remove the charges and degrade filtration efficiency, the researchers said.
“Today, N95 masks are being worn by healthcare workers for extended periods,” said research co-author Shankar Ghosh. “This gives rise to very humid conditions, and humidity is detrimental to electrostatics.”
During use, all electrostatics-based masks slowly lose their efficiency due to humidity.
“It's much worse in a place like Mumbai during the Indian monsoon, where ambient humidity levels can reach more than 90%,” Ghosh said.
Under high electric fields, the polypropylene conductivity is high, which makes introducing excess charges into the material possible by connecting it to a battery. When the charge source is switched off, the applied electrical field becomes zero, and the conductivity of the polypropylene drops effectively to zero. As a result, the added charge carriers immobilise, and the material remains charged.
The researchers cleaned an N95 mask in a standard washing machine, which significantly reduced its filtration efficiency. They discovered they could then recharge it by ‘sandwiching’ it between two electrodes at high voltage, to recover its 95% efficiency.
“We've also shown a proof-of-concept construction of a battery-operated ‘smart mask’, where the lost charge gets replenished periodically by plugging the mask into a charging station – akin to how you would charge your smartphone,” Ghosh said.
The group believes its method to keep masks charged will lead to highly energy-efficient smart masks.
“Currents are in microamps, and the power requirement is extremely low, on the order of a milliwatt, so a compact and practical solution may soon be feasible,” he added.
The method could also be useful for air filtration applications, such as HVAC or industrial filters.
The research was published in Physics of Fluids.
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