In their search for eco-friendly energy sources, scientists have learned how to harness power from ever smaller living things: first corn, then algae, now bacteria. By figuring out how to generate electricity using the M13 bacteriophage, a virus that infects bacteria, engineers at the University of California, Berkeley, have gone smaller still. Although the virus-powered device produces only a tiny bit of energy, it may one day pave the way for cell phones that can be charged while you walk.

The device relies on a property known as piezoelectricity, which can translate mechanical energy, say, a finger tap, into electrical energy. Most cell-phone microphones are piezoelectric and convert the energy from sound waves into electrical output that is transmitted and translated back into sound waves in the recipient's phone. The problem with these piezoelectric devices, Berkeley bioengineer Seung-Wuk Lee says, is that they are made out of heavy metals such as lead and cadmium. Many biomolecules such as proteins and nucleic acids are also piezoelectric—they generate electricity when compressed—but lack the toxicity of traditional devices.

Lee and his colleagues found that the pencil-shaped M13 phage fits all their requirements. Because the virus infects only bacteria, it is safe for humans. And it is cheap and easy to create: scientists can get trillions of viruses from a single flask of infected bacteria. The shape of the virus is also important because M13 can easily self-assemble into thin sheets. To improve the electricity-generating power of M13, Lee's team tweaked the amino acid content of the virus's outer protein coat by adding four negatively charged glutamate molecules. The researchers stacked sheets of viruses on top of one another to amplify the piezoelectric effect.

When the scientists attached the one-square-centimeter virus film to a pair of gold electrodes and pressed firmly on one of those electrodes, the film produced enough electricity to light up a liquid-crystal display of the numeral 1. Although it generated only a small amount of power—400 millivolts, or about one quarter of the energy of a AAA battery—the study shows that biomaterial piezoelectrics are feasible, Lee says.

“This will bring a lot of excitement to the field,” says Zhong Lin Wang, an engineer at the Georgia Institute of Technology who was not involved in the study. “By utilizing the properties of these biomaterials, we can find unique applications in the future,” such as a pacemaker powered by the beating of one's heart.