Nature’s toolkit for killing viruses and bacteria

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They burst out of toilet bubbles, swim across drinking water, spread through coughs. Tiny infectious microbes—from the virus that causes COVID-19 to waterborne bacteria—kill millions of people around the world each year. Now engineers are studying how zinc oxide surfaces and natural hydrodynamic churning have the power to kill pathogens first.

“Bacterial contamination of common surfaces and of drinking water have been traditionally the main infection routes for transmission of serious diseases, often leading to mortality,” said Abinash Tripathy, a researcher in mechanical and process engineering at ETH Zurich. “Our goal was to design a surface that can address both issues.”

His group submerged clean zinc in hot water for 24 hours, which formed a zinc oxide surface covered in sharp nanoneedles. Then they introduced E. coli bacteria.

The surface kills almost all bacteria cultured on top of it very efficiently. And the biggest surprise? When sitting in contaminated water, the surface kills all waterborne E. coli within three hours—even bacteria it didn’t touch.

This water disinfection at a distance works because the process generates a reactive oxygen species, which damages the cell walls of bacteria. The group from ETH Zurich, IIT Ropar India, and Empa, Switzerland, presented their initial findings at the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics.

In Southeast Asian and African countries where clean drinking water is scarce, current solar water disinfection methods take up to 48 hours and require a minimum intensity of sunlight. The new zinc oxide surface speeds up the disinfection process and does not need light.

“This surface can be used to disinfect water in remote areas at a very low cost,” said Tripathy. “The fabrication technique is environmentally friendly, simple, and economical.”

Surface and waterborne pathogens aren’t the only killers. As the COVID-19 pandemic has reinforced, airborne viruses and bacteria pose a serious global challenge for disinfection.

The very droplets that carry pathogens through the air can play a role in destroying them. In the microseconds that droplets take to form, their fluids rearrange rapidly—stressing the microbes within.

“Think of a bucket with a fish in it. One imagines that if you start churning the fluid in the bucket too quickly, the fish won’t be very happy,” said Oliver McRae, a mechanical engineer. “It’s a similar kind of thing—albeit on a much, much smaller scale—when you have, say, a pathogen in a droplet. Eventually the fluid’s going to agitate too much for that bacteria or virus to survive.”

McRae and a team from Boston University and the Centers for Disease Control and Prevention were studying how hydrodynamic agitation works when environmental bubbles produce droplets. After the onset of the pandemic, they started modeling droplets similar to those produced by the lungs and respiratory tract.

Using computational fluid dynamics, the team predicted how agitation works during aerosol formation. They discovered that stressors are very sensitive to droplet size. If the droplet shrinks or grows by one order of magnitude, the stressors change by

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Modern aircraft ventilation systems aren’t spreading viruses, DoD study suggests

A new study released Thursday suggests that people don’t need to worry about circulating air spreading coronavirus on airplanes.

a large passenger jet flying through a cloudy blue sky: TOPSHOT - Fleecy clouds are seen in the sky as an airplane prepares to land at the airport in Frankfurt am Main, western Germany, on November 5, 2018. (Photo by Silas Stein / dpa / AFP) / Germany OUT    (Photo credit should read SILAS STEIN/DPA/AFP via Getty Images)

© SILAS STEIN/DPA/DPA/AFP via Getty Images
TOPSHOT – Fleecy clouds are seen in the sky as an airplane prepares to land at the airport in Frankfurt am Main, western Germany, on November 5, 2018. (Photo by Silas Stein / dpa / AFP) / Germany OUT (Photo credit should read SILAS STEIN/DPA/AFP via Getty Images)

The US Department of Defense study supports earlier research showing the ventilation systems on aircraft filter the air efficiently and take out particles that could transmit viruses.

The study, which was released without peer review, did not take into account other ways that people could catch the virus on aircraft — including from others coughing or breathing directly on them, from surfaces or from confined spaces such as restrooms.

The US Transportation Command, The Defense Advanced Research Project Agency (DARPA) and the Air Mobility Command used Boeing 777-200 and 767-300 aircraft loaded up with sensors meant to duplicate the effect of a fully loaded passenger flight.

A dummy wearing a surgical mask simulated a coughing passenger infected with a respiratory virus.

The team used fluorescent aerosol tracers to see where particles emitted from the coughing “passenger” went. They were sucked quickly into the ventilation system, the team concluded, and were unlikely to contaminate nearby surfaces or blow into the breathing zones of people seated nearby.

“Testing assumes that mask wearing is continuous, and that the number of infected personnel is low,” the research team wrote. “Contamination of surfaces via non-aerosol routes (large droplets or fecal contamination) is more likely in lavatories and other common areas and is not tested here,” they added.

“These alternative routes of exposure are more challenging to predict because of uncertainty in human behavior.”

Other reports have found people became infected with coronavirus on flights, perhaps when they took off masks to use restrooms.

“Testing did not include substantial movement throughout the plane or in the airport, lounge or jetway, where air change rates and human interactions will vary,” the researchers added.

“Similarly, the mannequin remained facing forward, uncertainty in human behavior with conversations and behavior may change the risk and directionality in the closest seats to an index patient, especially for large droplets.”

A lot left to learn

Much is still unknown about Covid-19 transmission aboard planes. Two previous studies documented real-life cases of suspected transmission aboard flights.

Both studies involved cases connected to long flights early in the pandemic, before airlines began requiring face masks.

Another study documenting a case of suspected coronavirus transmission aboard a flight involved a woman who wore an N95 mask throughout her flight except when she used the lavatory.

The lavatory was shared by an asymptomatic patient who was seated three rows away.

The US Centers for Disease Control and Prevention notes that “most viruses and other germs do not spread easily on flights because of how air circulates and is filtered on airplanes,” adding

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