"This is a tremendous leap forward in membrane technology," says materials researcher Christopher Striemer of the University of Rochester, part of a group that created the filters and has formed a start-up company to commercialize them.
Striemer says simple laboratory tools incorporating the filters may hit store shelves within a year. He says the new technology could also be used in specialized air and water filters for emergency and military personnel, and, in the long run, could help improve dialysis. "We expect this to be a disruptive technology in a number of application areas," he says.
Most filters are spongelike and relatively thick—about 50 microns, or half the width of a human hair. Made from tangles of polymer, they have trouble cleanly separating molecules of similar sizes, because their nooks and crannies tend to trap particles that are only modestly bigger than the ones they let through. Thinner or more porous membranes would be faster, which might aid dialysis, but they also could allow a greater number of unwanted molecules to slip through.
Striemer and his colleagues stumbled onto their silicon filters while trying to prepare thin samples to study under an electron microscope. They splattered silicon atoms onto a surface and then heated the 15-nanometer-thick layer to crystallize the atoms. "We noticed that under certain conditions this very thin material could be made porous," Striemer says. Why the pores form is unclear, but a likely explanation, he says, is that the material contracts as it rearranges itself into small crystals, leaving tiny spaces in-between.
The resulting films can separate one protein molecule from another onethat is only twice its weight, compared with a 10-fold difference needed for normal membranes, the group reports in this week's Nature. The silicon layers, which span nearly a millimeter in width, can stand up to a full atmosphere of pressure (15 pounds per square inch) without bending or breaking. They also work about 10 times faster than other nanoscale filters and conventional dialysis membranes, the researchers found.
The pores are simple to produce and can be made in different sizes, note nanotech researchers Albert van den Berg and Matthias Wessling of the University of Twente in the Netherlands in an accompanying editorial. Their greatest promise might be in microfluidics, or "lab on a chip" technologies, for medical testing or drug research, they say.