Amongst the many problems we've had with the spread of COVID-19 is the coronavirus's ability to survive on surfaces for hours on end. While we can effectively wipe down hard materials or sterilize them with alcohol, what about more delicate surfaces like cardboard?
Even in the atmosphere, SARS-CoV-2 can survive up to a few hours; on cardboard it can last for up to 24 hours, and viable particles have been detected on plastic up to three days after it was contaminated.
Scientists across many disciplines are throwing their vast talents into tackling the pandemic. Now, a team led by engineer Zhitong Chen from the University of California in Los Angeles may have found a solution. They just demonstrated cold plasma has the ability to destroy the virus on a wide range of surfaces without damaging the material.
"Everything we use comes from the air," explains aerospace engineer Richard Wirz. "Air and electricity: It's a very healthy treatment with no side effects."
Plasma, the least well known of the four main states of matter (the other three being solid, liquid and gas), occurs naturally in our upper atmosphere. It forms when electrons become separated from their atoms (making the atoms positively charged), and together create a soup of charged particles that are unstable and so more reactive than in their equivalent gas state.
Chen, Wirz and colleagues designed and 3D-printed an atmospheric plasma jet device fuelled by argon gas - an inert and stable element that's one of the most abundant gases in our air. The device sends speeding electrons through the gas, stripping the gas atoms of outer electrons as they collide; it requires just 12 W of continuous power to work.
The team directed a near-room-temperature stream of reactive particles onto contaminated surfaces, exposing them to an electric current, charged atoms and molecules (ions), and UV radiation.
They tested the plasma's effect on six surfaces, including cardboard, football leather, plastic and metal and found that on each of these, most of the virus particles were inactivated after only 30 seconds. Three minutes of contact with the plasma destroyed all of the virus.
The team believes it's the reactive oxygen and nitrogen ions, formed as the plasma interacts with air, that are destroying the viral particles; when they tested a helium-fed plasma, which produces less of these species of atoms, it was not effective even after five minutes of application.
They explain that as charged particles gather on the virion's surface, they can damage its envelope through electrostatic forces leading to its rupture. The ions can also break structurally important bonds such as those between two carbon atoms, carbon and oxygen, and carbon and nitrogen atoms.
"These results also suggest that cold plasma should be investigated for the inactivation of aerosol-borne SARS-CoV-2," the Wirz and colleagues wrote in their paper.
Last year another team created a plasma filter that could sterilise the air from 99 percent of viruses. In their device, as air moves through gaps in a bed of borosilicate glass beads, it's oxidised the unstable atoms that form the plasma. This damages viral particles, leaving them with a greatly diminished ability to infect us.
Of course, there is still a way to go from proof of concept to a device we can all use. But Wirz and team are now working on building such a device.
"This is only the beginning," Wirz said. "We are very confident and have very high expectations for plasma in future work."
This research was published in Physics of Fluids.