Cui isn't sure just how the filter works, but he knows the two materials are better together. Silver has long been known to have antimicrobial properties, and carbon nanotubes are highly conductive. One guess is that very strong local electrical fields are formed at the tip of the silver nanowires, piercing the cell walls. When the electricity is off, the silver prevents bacteria from fouling the surface, a common problem with filters.

There have been no definitive studies of the effects of water-borne carbon nanotubes and silver nanowires on people and lower organisms; experiments with airborne carbon nanotubes have shown that their effect on mice lungs is similar to the effect of asbestos. But early tests on thousands of gallons of water suggest that the nanomaterials are not leaching into the water. The researchers will perform further tests to determine whether the nanomaterials remain enmeshed in the filter or are dislodged into the water over time.

"I believe there is tremendous potential for technological breakthroughs such as this to dramatically improve the options for low-cost water treatment," says Kara Nelson, professor of civil and environmental engineering at the University of California, Berkeley. Now it's important to take this proof-of-concept device to the next step, says Nelson, by improving the filter's efficacy and demonstrating that it can work with a broad range of water-borne pathogens, including viruses and protozoa.

Chad Vecitis, professor of environmental engineering at Harvard University, says the most impressive aspect of the filter is its speed. Many university researchers are addressing the clean-water problem, but other low-power solutions take too long or are too complex. For example, some systems use a light-activated catalyst to kill bacteria in clear containers of water that are set out in the sun. This takes several hours, and it's not easy to tell when the sterilization is done.