How to go out if you can’t buy N95? Try DIY a cotton mask

Flannel, the N95 of the textile industry.

Written by | Xiaoye

As domestic epidemic prevention policies are opened up, some areas are once again in short supply of medicines and epidemic prevention materials, which reminds us of the early days of the epidemic, when there was a period of time when masks were in short supply around the world. At that time, there was a group of scientists who figured out how to make their own masks and also found the best fabric that could effectively resist the new coronavirus particles.

The COVID-19 pandemic has forced many people to start working remotely from home, and scientists are no exception. Edward Vicenzi is a scientist researcher at the Smithsonian Museum Conservation Institute (MCI). Before the pandemic, his main job was to use various precision equipment to examine rare ancient relics. Jamie Weaver is a chemist at the National Institute of Standards and Technology (NIST), investigating the chemical properties of Swedish glassware from the pre-Viking era.

Both scientists are proficient in materials research. At the beginning of the epidemic, they formed a team with two other earth atmospheric aerosol scientists, James Radney and Chris Zangmeister, who also worked at NIST. They responded to the call from academia and health departments, used their professional knowledge and skills, DIYed masks, and studied mask fabrics that could effectively resist viral particles.

In 2020, the Centers for Disease Control and Prevention (CDC) demonstrated and taught three ways to make simple cloth masks on its official website. To enhance the protective effect, it is also recommended to wear a disposable surgical mask inside the homemade mask.

The steps for making masks were once posted on the official website of the US CDC. However, everyone has different hands-on abilities, and the effects of personal DIY masks vary. Later, masks were in sufficient supply on the market. Now the CDC no longer recommends making masks by yourself, and has cancelled the DIY mask page.

Weaver uses the method shown in the picture above. You need to use a sewing machine to sew, but the production process is quite simple:

1. Cut two rectangular pieces of cotton cloth (bedsheets or old T-shirts are also OK), about 25 cm long and 15 cm wide;

2. Stack them together, fold the top, bottom, left, and right sides inward, and then sew them together with a sewing machine;

3. Then, take two long rubber bands, pass them through the two short sides of the mask and tie a knot, and stuff the knot into the side seam;

4. After adjusting the position of the mask and the rubber band, fit the mask to your face and fix the shape of the mask;

5. Finally, sew the elastic band to the side seam firmly to prevent it from sliding. In this way, a simple cloth mask is made.

When making her own masks, Weaver's family background played a big role. Her last name, Weaver, means "weaver." Indeed, five generations of her family were masters of sewing and collected all kinds of textiles. This time, Jamie herself used her grandmother's sewing machine to make masks with various fabrics, from her collection at home to fabrics found in craft stores. For added safety, Weaver sewed three layers of fabric and made a patch near the nose to prevent the mask from not fitting the face.

The four scientists wearing DIY masks in the picture were the first to publish research on mask materials in the journal. From left to right: Jamie Weaver, James Radney, Edward Vicenzi and Christopher Zangmeister | Source: NIST

The process of making cloth masks is simple, but it is not easy to find mask fabrics with excellent protective effects. This is also the focus of the four-member team's research. They read a lot of articles related to masks during the 1918 influenza pandemic. In fact, it was the outbreak of the 1918 influenza pandemic that made masks go from surgical medical instruments to public life.

A 1919 document describes homemade medical masks worn by medical staff at the time, as well as how to make them, and the CDC's method is similar. Source: George H. Weaver, Droplet Infection and its Prevention by the Face Mask, The Journal of Infectious Diseases, Volume 24, Issue 3, March 1919, Pages 218–230, https://doi.org/10.1093/infdis/24.3.218

The stay-at-home order kept Vicenzi and Weaver away from the advanced laboratory equipment. However, since DIY masks are not difficult for them, DIY laboratories are naturally no problem. They bought a batch of simple microscopes from Walmart supermarket for less than $30 each. Vicenzi also found a high-resolution microscope on the Internet, which can clearly see the detailed features of the fabric fibers with an accuracy of up to 2μm. Later, many high-definition and exquisite microscopic images of fiber structures in the articles published by the team were created by Vicenzi. In addition, Vicenzi also transformed a beer cooler into a simple humidifier and fixed the fabric on it with tape. All these simple experimental equipment helped the two scientists observe and test the properties of various fabric samples: porosity, fabric warp and weft density, fabric thread thickness and composition, etc., thereby revealing how the fabric resists tiny virus particles.

Vicnezi started building a home laboratory in April last year. Source: Ed Vicnezi

Compared with them, Radney and Zangmeister were much luckier. They received special permission to work in the laboratory during the epidemic. During that time, the two kept going back and forth between Weaver and Vicenzi's homes, bringing bags of fabric samples to scientists at home. In the unit laboratory, they used sodium chloride to simulate the new coronavirus, and used a "magic spray bottle" to launch small particles or aerosols of different sizes, ranging from 50 nanometers to 825 nanometers in diameter, to observe the penetration of these particles into the fabric in the closed test tube, and to measure the ratio of the number of particles on both sides of the fabric, so as to judge the filtering effect of each fabric.

Soon in June 2020, the four-member team’s results were quickly published in the journal ACS Nano[1], sparking a wave of attention. According to statistics, the article has been viewed more than 64,000 times since it was published, becoming the journal’s top article in 2020.

Magnified images of several fabrics with the best protective effect under a microscope: A. 100% cotton, towel, B1 and B2 are the outer and inner surfaces of 100% cotton, lightweight flannel, C. 100% polyester, garment fabric, D. 100% cotton, pillowcase, E. 100% polyester staple fiber | Source: ACS Nano 2020, 14, 7, 9188-9200

According to the paper, the four scientists selected 32 kinds of fabrics and measured their filtration efficiency, differential pressure, quality factor and construction parameters. They also compared them with 7 kinds of polypropylene fiber filter materials used in surgical masks and N95 masks. The results showed that 100% cotton flannel performed best in resisting virus particles, and was no less effective than N95 masks, followed by woven synthetic fibers. If one or two more layers of filtration are added to the flannel, such as HEPA (high efficiency particulate air filter) filters, coffee filters or other materials that can capture tiny aerosol particles, the mask's defense capabilities will be even better.

So, how are N95 masks made? The prototype of the N95 mask comes from the design idea of ​​the female underwear cup of Sara Little Turnbull, an American fashion editor in 1958. After continuous modification, the final mask can block 95% of aerosols of the same size as the new coronavirus particles. Generally speaking, a single virus particle is about 110 nanometers in diameter, but according to Zangmeister's records, the virus clusters in the exhaled breath of patients with new coronary pneumonia are wrapped in proteins and salts, with a diameter of up to one or two microns. When N95 masks are made industrially, melt-blowing extrusion is used to deform the plastic fibers of N95. In this way, fiber bundles of different thicknesses are mixed together to form different shapes and textures. This messy mixed weave prevents aerosols from passing through the mask.

The plastic fiber structure inside the N95 mask is in a mess. Source: Ed Vicenzi

Vicenzi's team's structural imaging reveals the mechanism by which cloth masks protect against viral particles just as well as N95 masks. Common fabrics like flannel have an internal structure somewhere between N95 and polyester. Although its fibers are woven in a certain pattern, they are still very irregular. This is because during the weaving process of flannel, the fiber bundles protrude from the surface, forming what is known as "fluff." The researchers speculate that this fuzzy surface not only gives the fabric a soft touch, but also captures more aerosol particles like an N95 mask. "Flannel is the N95 of the textile industry," Vicenzi said.

The microstructure of flannel fabric, which is second only to N95 masks in terms of effectiveness. Source: Ed Vicenzi

On March 8, 2021, the team’s second article was published in the journal ACS Applied Nano Materials [2], further revealing another advantage of cotton masks: humidity enhances the protective effectiveness of masks.

They simulated the high humidity environment when people breathe and found that when droplets containing the new coronavirus hit, the filtering effect of hydrophilic fabrics (such as pure cotton fabrics) would be enhanced. The principle is relatively easy to understand: the humidity of our exhaled breath is 100%, and cotton masks are water-absorbent. After absorbing water, the fibers expand and thicken greatly, making the tiny internal space even smaller, and it is increasingly difficult for aerosols to pass through the mask and enter the nasal cavity.

In contrast, the synthetic plastic fibers used in N95 masks are hydrophobic, and the water vapor we exhale condenses on the inner surface of the mask. Vicenzi uses an analogy: "... like a buzzing insect hitting a fly net, it is firmly trapped as soon as it touches the surface of the net." Similarly, in the extremely humid environment inside a cotton mask, aerosol particles grow larger due to water absorption, and eventually become trapped in a small space.

A simulation of the filtration and protection effects of cotton masks in comparison with dry and wet materials. It can be seen that more aerosol particles pass through dry fabric (a) than through wet fabric (b) | Source: Hydration of Hydrophilic Cloth Face Masks Enhances the Filtration of Nanoparticles

However, this does not mean that you need to soak the mask in water before wearing it, but it means that if you keep wearing a cotton mask, the moist filter layer inside the mask will enhance the virus protection effect over time.

As we all know, at the beginning of the COVID-19 pandemic, the issue of whether masks should be worn caused widespread controversy around the world. The research by Vicenzi's team revealed the structural mechanism of masks blocking virus particles, proving that China's practice is correct: during the epidemic, everyone should wear masks to protect the health of themselves and others.

In May 2021, a study published in the journal Science[3] once again theoretically proved the view that wearing masks in daily life can reduce the spread of the new coronavirus. In public environments, we cannot avoid breathing in the air exhaled by others, especially in hospitals with the highest virus concentration. Even the most effective medical masks cannot provide sufficient protection if you do not wear protective clothing and maintain ventilation. However, in the living environment of ordinary people, the concentration of the new coronavirus in the aerosols exhaled by people is not high, which belongs to the virus-limited regime. As long as everyone wears a mask, even the simplest disposable surgical mask, it can effectively reduce the possible exposure to the virus, and better filter the residual virus, further reducing the infection rate. In short, to achieve the best protection effect, both patients and healthy people should wear masks and maintain good ventilation and social distance.

In a virus-limited environment, where ordinary people live, the difference between wearing a mask and not wearing a mask, red dots represent aerosol particles containing the new coronavirus, while green dots represent aerosol particles that do not contain the virus. 丨Source: Tinu CA from www.freeicons.io, distributed under CC-BY 3.0.

The global fight against the epidemic is an extremely complex and systematic activity. Small changes in human behavior may greatly change the trajectory and effect of virus transmission. The strain Omicron that has evolved from the novel coronavirus is highly contagious, and we need to do a good job of epidemic prevention to protect the health of ourselves and our family and friends. When we cannot buy enough N95 masks or other masks, we might as well make cotton masks with suitable patterns and colors by ourselves to protect ourselves and the environment. After all, scientific research and life practice have proved the practical significance of wearing masks, which is indeed one of the most powerful weapons for ordinary people to fight the epidemic.

Original reference:

https://www.smithsonianmag.com/smithsonian-institution/using-store-bought-microscopes-and-eye-detail-heres-what-smithsonian-scientists-have-learned-about-mask-effectiveness-180977216/

References

[1] https://pubs.acs.org/doi/10.1021/acsnano.0c05025

[2] https://pubs.acs.org/doi/10.1021/acsanm.0c03319

[3] https://science.sciencemag.org/content/372/6549/1439

[4] https://doi.org/10.1093/infdis/24.3.218