New air filter may help city residents breathe easy

Stanford scientists have developed a new low-cost, highly efficient air filter for polluted cities such as Beijing that could be used to improve facemasks and window screens.
Yi Cui, an associate professor of materials science and engineering at Stanford University, and his graduate students set to work designing an inexpensive, efficient air filter that could ease the breathing for people in polluted cities.
"We think we could use this material for personal masks, window shades and maybe automobiles and industrial waste. It works really well, and it might be a game-changer," said Cui.
The team looked for polymers that would have a strong attraction to the main components of smog, particularly particle matters that are smaller than 2.5 microns, known as PM2.5.

These pose the greatest risk to the human respiratory system and overall health; current filtration systems that can remove them from the air are very energy-intensive.
It turned out that polyacrylonitrile (PAN), a material commonly used to make surgical gloves, met these requirements.

"It was mostly by luck, but we found that PAN had the characteristics we were looking for, and it is breathtakingly strong," said Po-Chun Hsu, co-author on the study and a graduate student in Cui's lab.
Using a technique called electrospinning, the researchers converted liquid PAN into spider-web-like fibres that are just a thousandth the diameter of a human hair.

In the study, the researchers approximated Beijing's smog by flowing smoke from burning incense over different densities of the fibre, and later performed a field test in Beijing.
The final material allows about 70 per cent transparency and yet collects 99 per cent of the particles.
"The fibre just keeps accumulating particles, and can collect 10 times its own weight," said Chong Liu, lead author on the paper and a graduate student in Cui's lab.
"The lifespan of its effectiveness depends on application, but in its current form, our tests suggest it collects particles for probably a week," said Liu.

The first two immediate applications, Cui said, would probably be simple passive systems, such as personal masks and window screens, or possibly hospital air filtration systems.
"The transparency and distance between the fibres 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," Cui added.
The material might also have a place in filtering exhaust from cars, or from the smoke stacks of power plants and industrial complexes.

The research was published in the journal Nature Communications.