Researchers Put Cloth Face Masks Under a Microscope. The Images Are Out of This World

Cloth face masks have become part of the daily routine for many of us. The first thing I did at the beginning of the pandemic last year was buy some cloth face masks for my mom, who is 68 years old and has delicate health. Nearly a year after that purchase, my mom is vaccinated, but she still wears her cloth face mask religiously every time she goes out in public. It’s her shield.

After seeing the destruction covid-19 has wreaked around the world, it can seem incredible that something as simple as a cloth face mask could slow the spread of the virus. (PSA: They do. Please wear a mask). However, you probably won’t feel the same way once you see the spectacular images of cloth face masks under a scanning electron microscope captured by researchers at the National Institute of Standards and Technology.

Edward Vicenzi, the microscopist at NIST who created the photomicrographs, said that when he first saw the images, he was impacted by the difference between woven and non-woven materials under the microscope.

“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 said. “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.”

Zangmeister and Vicenzi were part of a team at NIST that analyzed how well 32 natural and synthetic fabrics, including cotton, wool, synthetic, synthetic blends and synthetic/cotton blends, filter particles of a similar size to the coronavirus. Their findings, which were published in the journal ACS Nano last June, determined that three of the five most effective materials at blocking materials were 100% cotton with visible raised fibers or nap, such as that found in flannels. Meanwhile, four of the five lowest performers were synthetics.

To analyze the effectiveness of the fabrics, the team tested swatches (not complete masks) and flowed a stream of particles through it. They then counted the number of particles in the air before and after it passed through the fabric. They did not use real samples of the coronavirus in their experiment, as they are dangerous, and instead used salt as their “stand-in” particles, which is the substance recommended in these situations by the National Institute for Occupational Safety and Health.

Zangmeister said that when the pandemic started, there were only a handful of studies that had analyzed cloth masks. This meant that “everyone was kind of flying blind.” He stated that this is what motivated them to start studying cloth face masks.

“We wanted to understand what materials work best and why,” Zangmeister said. “And we hoped that with that knowledge we could help people choose the right materials to make masks that are effective at slowing the spread of the disease. The measurements we make in the lab allow us to answer these questions.”

Recently, they’ve also found that the humidity in our breath makes cotton face masks more effective. This is because cotton is hydrophilic, which means it likes water. When cotton fibers absorb some of the moisture in our breath, they grow larger, making it more likely that microscopic particles will get trapped.

So, what exactly happens when you’re out and about wearing a cloth face mask? For this example, let’s pretend we’re wearing a cotton flannel face mask, which the NIST researchers believe is a particularly good filter. Part of this is due to the chaotic arrangement of the fabric’s fibers.

As explained by NIST, when you breathe through a cotton flannel face mask, shown above at the individual fiber level, the air flows around these fibers and the aerosols “are pulled along for the ride.” Although the aerosols are very small, they do have some mass. Remember the chaotic arrangement I mentioned before? Well, the air you breathe in has to twist and turn around the fibers in the masks. Some aerosols can’t turn fast enough, which means they smash and stick to the fibers. Additionally, since the cotton fibers have kinks and bends, they also have greater surface area, which also helps trap aerosols.

As you can see from these photomicrographs, not all fabrics have the same structure or properties. Polyester, for instance, is made by extrusion. In the manufacturing process, the polymer raw material is squeezed out through an opening, like spaghetti coming out of a pasta maker. As a result, the fibers have a consistent cross-sectional profile.

If you’re interested learning about the differences of more fabrics, check out this great explainer by NIST that goes through the particularities of some of the fabrics analyzed by Zangmeister, Vincenzi, and the rest of their team.

The researchers’ work contributed to the development of the first standard for cloth face masks, called F3502, by the global standards organization ASTM. Zangmeister said he thinks the standard will help ease the confusion the public has over how well a cloth mask performs, how well it filters, and how breathable it is. As for their next steps, Zangmeister said that they’re trying to figure out how to best layer off-the-shelf materials that are both breathable and offer good particle filtration. There is a balance there, he affirmed.

“Like some things that seem simple on the surface, after you really study it, you find there is a lot of complexity. These masks definitely fit that bill,” Zangmeister said.