Doctors need blood plasma to measure the viral load of a person living with HIV. Separating this yellowish liquid from whole blood is usually done with a centrifuge, in which a tube of blood is rapidly spun so that its red blood cells collect at the bottom and its plasma can be poured off the top. But owning and maintaining even such a basic piece of lab equipment—or finding a source of electricity to run it—can present a challenge in the parts of the world hit hardest by HIV/AIDS.

A team from the School of Engineering and Applied Science and the Perelman School of Medicine, led by research assistant professor Changchun Liu and professor Haim Bau of the Department of Mechanical Engineering and Applied Mechanics, has recently designed a plasma-separating device to address this problem. It is elegant in its simplicity: made out of inexpensive, disposable plastic with a footprint not much bigger than a dime, it uses no electricity and has no moving parts. Gravity and filtration do all of the work.

The device consists of two vertical chambers separated by a sieve-like membrane. When whole blood is placed in the first chamber, cells drift toward the bottom, but plasma can move sideways through the membrane into the second chamber.   

Other membrane-based plasma separators exist, but something as simple as changing its orientation gives the Penn team’s device an edge.

“In existing devices, the membrane is placed horizontally at the bottom of the sample introduction chamber,” Liu says. “This results in limited capacity and can lead to blood cells blocking the membrane’s pores.”

The team successfully tested their device on HIV-positive blood, filtering out enough plasma to run viral load tests in about seven minutes. 

“Two key needs in the developing world are to be able to do this type of testing in remote locations without a need for electrical power and to do it with on-the-spot, rapid turn-around results, so that doctors and patients can make decisions about treatment then and there,” says Liu. “That’s what our plasma separator does.”