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

At first glance, the images don’t seem like a cloth face mask at all. They look like something out of an art gallery. Some materials, like polyester, look like graceful braids. (I must admit, I was hungry when I first saw these images, so to me, they also looked a little bit like pasta). Others like cotton flannel look like dried grass spread haphazardly on the ground. When you stop and realize that you actually wear these intricately beautiful fibers on your face, it’s quite mind blowing.

Christopher Zangmeister, the lead NIST scientist on a study that analyzed the effectiveness of cloth face masks, told Gizmodo via email that the images really bring the materials to life. Not only that, but they help scientists understand why one material works better than another. Details like the diameter of the fibers, how much open space there is, the fabric’s thickness, and how randomly constructed it is all impact how well a material captures particles, he said.

According to the Centers for Diseases Control and Prevention, covid-19 most commonly spreads through close contact via exposure to the respiratory droplets of an infected person. Droplets are produced when people cough, sneeze, sing, talk, or breathe. Cloth masks block some of the virus-filled droplets and smaller particles, called aerosols, that an infected person exhales. They also offer some protection to the wearer by filtering incoming air, per NIST.

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.