Filtration Properties of Silk
Mechanical vs Electrostatic Filtration
Mechanical filtration is simply the fabric physically catching the particles. This type of filter stops particles because of the small size of their pores. In fabrics such as cotton, high thread count works best. Basically, the smaller the pores in the fabric, the fewer particles can escape.
Thread count is measured as threads per inch or TPI. If a material is 100 threads per inch across the warp (vertically) and 100 threads per inch across the weft (horizontally), that fabric is 200 threads per inch, or 200 TPI. See more about cotton filtration here.
Silk is Electrostatic
Electrostatic-based filtration is a little different. Think of a static material such as polyester. These filters are electrostatically charged, and thus attract and capture charged particles. Silk has electrostatic properties, meaning it’s positively charged. When a face mask has a silk layer, small particles stick to it due to the positive charge and those particles consequently don’t go through the fabric. An electrostatic filter keeps the aerosols on either side of the static environment. N95 and KN95 masks have an electrostatic layer as well.
Silk Repels Water
Patrick Guerra is a biological sciences professor at the University of Cincinnati, who’s wife is a doctor on the front lines during the COVID19 pandemic. Guerra studies the complex architecture of silk moth cocoons. He wondered if a silk mask might serve as a better protective barrier over the N95 because he’d observed that the cocoons are naturally water repellent. Guerra’s lab found that “silk face coverings repelled droplets in spray tests as well as disposable single-use surgical masks. Due to its unique hydrophobicity (or ability to repel water) silk successfully helps prevent more droplets from being soaked into a face covering than other materials.
This hydrophobicity is retained even after washing. Although I would recommend hand washing, or machine washing in a pillow case or mesh bag. Washing this way is gentler on the fabric. Silk should be hung to dry in order to protect it’s water repellent nature.
Antimicrobial and Antiviral Properties of Silk
Perhaps best of all, silk contains natural antimicrobial, antibacterial and antiviral properties that could help ward off the virus. Studies have shown that copper, in particular, can kill bacteria and viruses on contact. And that’s where the little caterpillars have their own superpower, Patrick Guerra has said. “Silk has copper in it. Domesticated silk moths eat mulberry leaves. They incorporate copper from their diet into the silk.” This property, like the hydrophobic quality, seems to be retained even after washing.
Our Chosen Silk Filter Material
There are numerous fabrics which would provide electrostatic-based filtration. Because both polyester and polypropylene can be difficult to breathe through, we use one layer of silk organza between an inner and outer layer of cotton. We chose silk organza for our face covering filter material, as it is thin and lightweight enough to be breathable, and yet has a fairly tight weave. The fibres in organza are twisted very tightly before it is woven. It is crisper and has more body than most other silk as well, and therefore allows our face coverings to hold their sculptural shape.
Chart Showing Filtration Efficiency of Various Fabrics
If you are viewing the chart below on your mobile device, turn it horizontally to see the full image.
See the full study here ACS Publications: Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
For further reading:
- A Laboratory Based Study Examining Filtration Properties of Silk
- “We are recommending that you consider a three-layer non-medical mask,” Dr Tam (CBC)
- New York Times: What is the Best Material for a Mask? (has helpful info on filters as well)
- The Lancet: Stability of SARS-CoV-2 in Different Environmental Conditions
- ACS Publications: Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
- Why Silk is One of the Best Materials for Face Masks
See face covering use and care here:
Use & Care
See more info on cotton and mechanical filtration here:
Info on eyeglasses & fogging here:
Eyeglasses & Masks