An Ultrafast Silicon Filter

Continued from page 1

By Prachi Patel

Thursday, February 15, 2007

McGrath says that the membrane would make a good substrate to culture neurological stem cells. Certain "helper" cells nurture stem cells and coax them into turning into neurons. To get a pure culture of the neurons, researchers are looking for ways to physically separate the helper cells from the stem cells while allowing them to exchange chemicals. "[With the new membrane,] the distance they'll be separated by will roughly be the same size as their own plasma membrane," McGrath says. "The pores will allow a signaling molecule to diffuse very quickly."

The researchers believe that because of a narrower range of pore diameters, the silicon membranes could separate proteins that are much closer in size than is possible with current sponge-like filters. There are thousands of different proteins serving crucial functions in the human body, and separating an individual protein is key to understanding its structure and function. Fauchet says that by engineering a narrower range of pore diameters, the researchers could get 100 percent separation of proteins--even those that are close in size.

In laboratory tests, one-nanometer-wide dye molecules in a solution pass through the nanoporous membrane 10 times faster than through a commercial blood-dialysis membrane. The researchers plan to make the membrane stronger--it can sustain pressures of 15 pounds per square inch--so that they can push more molecules through, potentially improving dialysis speed by a factor of 100 over commercial membranes.

Some experts, however, feel that it is too early to say whether the membrane will be useful for large-scale applications such as protein purification and blood dialysis. The drawback of the ultrathin membrane is that it is difficult to make large-area membranes using the technique, says Andrew Zydney, a chemical-engineering professor at Penn State University. Current protein-purification systems in the biotechnology industry effectively use 100 square meters of membrane, he says. Even if the new membrane filters 10 times faster, which means it can filter the same amount of fluid with a 10-times-smaller area, "you're still talking about 10 square meters of silicon membranes," Zydney says. "I'm not convinced that that can be done in a cost-effective way."