ONE IN SIX PEOPLE lacks access to clean water worldwide, making diarrheal illness—a direct result of poor sanitation—the leading cause of death globally. Water filters would do the trick, but they are generally too expensive to distribute in great enough quantities. By combining nanotechnology with cheap materials such as cotton and tea bags, however, researchers have recently developed mobile water filters that can be manufactured for less than a penny.

Most conventional water filters are equipped with small pores that “trap” bacteria, but the pores have a tendency to get clogged, which requires expensive maintenance. Yi Cui found a way to use silver and electricity to kill the bacteria instead. Cui, a materials scientist at Stanford University, dipped woven cotton that he purchased at Walmart into a mixture of electrically conducting carbon nanotubes and silver nanowires. Silver works as an effective bactericide in part because silver ions damage genetic material. Additional killing power comes from a light electric current (powered by two nine-volt batteries) that breaks the bacterial cell membranes. In the lab, Cui's filter killed more than 98 percent of E. coli bacteria in water. Because the pores in the cotton are large, the filter is 80,000 times faster than filters that trap microbes.

An even cheaper invention is a kind of a tea bag filled with carbon granules coated with a microbicide. Researchers at South Africa's Stellenbosch University figured out a way to encapsulate the chemicals within nanofibers to increase their surface area and help them trap toxic substances and bacteria. The filter fits in the neck of an ordinary bottle and costs about half a U.S. cent apiece. Each bag is good for cleaning one liter of polluted water. The technology is currently being tested by the South African Bureau of Standards, after which the researchers plan to dole it out to communities in need.