Filtration & fluid control
was provided by cloth masks. However, cloth facemasks are better at minimising the spread of Covid-19 than no mask at all. They have also been the only option for people who couldn’t source other types of masks.
Researchers at Georgia Institute of Technology recognised this early in the pandemic and decided to use their expertise to investigate the filtration properties of different fabrics. Ryan Lively, associate professor at the School of Chemical and Biomolecular Engineering, says: “Around April last year, we realised there was not a great understanding of home-made masks, but everyone was wearing them.”
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As a filtration expert, Lively teamed up with colleagues – including aerosol scientist associate professor Nga Lee Ng – to design a study testing 33 different commercially available materials. They included single-layer woven fabrics (like cotton and polyester), mixed fabrics, non-woven materials, and materials used in hospitals – such as sterilisation wrap used to package surgical instruments. The researchers used an atomiser to generate submicron particles, which have a diameter of <1µm. By comparison, a human hair has a diameter of around 70µm. “In terms of filtration, submicron particles are the most difficult to be filtered out,” explains Ng. “In the study, we focused on these submicron particles because those larger than 1µm are not that hard for materials to filter out.” The particles were passed through two separate channels, one of which had the fabric sample attached to it, while the other did not. “We measured the aerosol concentration downstream of those two channels, and from the difference we were able to calculate a filtration efficiency,” she adds. Using submicron particles of different sizes – some as small as 50nm (0.05µm) – enabled the researchers to measure the filtration rate for each size particle and calculate a total filtration rate for the entire distribution of particles. However, testing multiple layers of even the poorer-performing cotton fabrics yielded improved results. “If you are going to use a cotton mask, it’s important to double or triple up. We have tested two and three layers of materials, and the filtration efficiency of those materials is about 50%, which is not too bad in terms of a homemade mask,” Ng comments. It’s important to note that the filtration efficiency of a mask – say, 95% for an N95 respirator – often refers to the percentage of particles of the smallest size that are rejected. “But we humans expel a broad distribution of particles and what matters in terms of limiting exposure [to Covid-19] is the sum or the total rejection of all of those particles,” Lively says.
“So, even the cloth masks, which have poor rejection of 0.3µm particles, still have good overall rejections from the total distribution of particles that we expel. That’s important, because one message we don’t want is that cloth masks are ineffective. A cloth mask is significantly better than no mask and it makes a big difference.”
Medical Device Developments /
www.nsmedicaldevices.com
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