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A team of researchers at NIST — the U.S.’s National Institute Standards and Technology — put the cloth face masks people use to curb the spread of COVID-19 under a microscope to understand how different materials perform in the terms of protecting the wearer. 

Cloth face masks block some of the virus-filled droplets and smaller particles called aerosols that an infected person exhales or coughs, according to a report by NIST. They also protect the wearer by filtering the incoming air.

However, ever since the start of the pandemic, it was debated whether cloth masks perform as well as surgical masks. To understand how they work, one of the researchers, Edward Vicenzi, decided to take a look up close by using a scanning electron microscope.

Cotton flannel (left) vs. polyester (right). Source: E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

The images he captured provided many insights into the particle-filtering qualities of different fabrics. In fact, the images resulting from the study don’t seem like a cloth face mask at all at first glance. In fact, they rather look like something out of an art gallery.

How do the fabrics differ? 

The team analyzed 32 natural and synthetic fabrics, including cotton, wool, synthetic, synthetic blends, and synthetic/cotton blends. Vicenzi told Gizmodo in an email that he was particularly taken aback by the differences between woven and non-woven materials.

Cloth Face Masks Under a Microscope Reveal Spectacular Views
Polyester. Source: E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

“I was instantly drawn into the beautiful interlocking patterns made by woven materials. Despite the simplicity of the patterns, each thread, which is made up of a bundle of fibers, has its own complex shape,” he stated. “On the other hand, the non-woven materials like N95 and surgical masks were like viewing a wildly chaotic scene filled with fibers of all sizes going in every direction. The contrast between the two types of textures hit me right in the face.”

Cloth Face Masks Under a Microscope Reveal Spectacular Views
Individual fibers in cotton flannel. Source: E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

In order to see what makes some fabrics have better filters than others, the researchers looked at details such as the diameter of the fibers, how much open space there is, the fabric’s thickness, and how randomly constructed is. The images were used to measure the thickness of the fabrics, for example. All this enabled researchers to understand how these variables affect filtration. 

Cloth Face Masks Under a Microscope Reveal Spectacular Views
The cross-section of cotton flannel. Source: E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

The team’s findings, which were published in the journal ACS Nano last June, showed that cotton fabrics tend to perform better than synthetics. In fact, cotton flannels were particularly effective. Even after they were exposed to the moisture of breathing, cotton fabrics still performed better than the others.

The reason why cotton flannel masks are good at filtering the aerosols is hidden in their tightly clustered individual fibers. NIST explains that when the wearer breathes through the mask, the arrangement of the fibers makes it hard for aerosols to escape through and helps trap them. NIST scientists are still trying to better understand the filtration process of cloth masks.

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