New Nanofiber Filter Could Help Residents Breathe Easier on Smoggy Days

A new filter can capture many times its weight in hazardous air pollution while letting air and light pass through easily. The material is made of a polymer called polyacrylonitrile, the same ingredient used to make acrylic yarns for clothing, some boat sails and surgical gloves. Using a fiber-pulling process called electrospinning, the liquid polymer is converted into nanofibers each a thousand times thinner than a human hair.

The Stanford University engineers who made the filter say its surface chemistry and the positioning of the fibers lets it absorb more than 95 percent of the smallest particulate matter (PM) in air pollution while remaining 90 percent transparent.

“The fiber just keeps accumulating particles, and can collect 10 times its own weight,” says Chong Liu, a materials science and engineering graduate student and lead author of a paper published this week in the journal Nature Communications. “The lifespan of its effectiveness depends on application, but in its current form, our tests suggest it collects particles for probably a week.”

Their study, which subjected the nanofiber filter to high levels of particulate matter found when incense is burned, found that the chemistry of the strands actually pulled particles to it. They found particles stuck all the way around the fibers, indicating that they had been pulled out of passing air through the polymer’s adhesive qualities.

“We have demonstrated that electrospun [polyacrylonitrile] nanofibers can be highly effective transparent PM filters because of its small fibre diameter and surface chemistry,” the authors conclude in the study. “Such nanofibrous filters can shut off PM from entering the indoor environment, maintain natural ventilation and preserve the optical transparency when installed on windows.”



Yi Cui, an associate professor of materials science and engineering who came up with the idea and heads the lab where the filter was produced, said it performed very well during field trials in Beijing. They tested it on a common bad air quality day in the city when PM levels were classified as hazardous. One of their filters, which had been designed to allow 77 percent of light through, filtered 98.69 percent of PM particles less than 2.5 microns in diameter. These tiny particles can travel deep into the lungs and have been linked to increased cardiovascular disease, emergency room visits and death. Commercial filters and masks tested for comparison showed either significantly less scrubbing power or were not transparent to light.



Cui said the passive ventilation filter could be used for face masks or as window screens. With additional work, they might be useful in car exhausts or in smokestacks.

In addition, the authors write that the electrospinning technique they employed is scalable and the nanofibers can be directly spun onto a wire screen to make the filter mechanically strong. The common polymers used to make the filter are available at low cost.

“The transparency and distance between the fibers means that light and air can pass through very efficiently, which makes it a very good application for windows,” Cui said. “It might be the first time in years that people in Beijing can open their window and let in a fresh breeze.”

Top two gifs created from video courtesy of Stanford University. Bottom gif created from video courtesy Liu et al./Nature Communications.