Squishing a stack of virus sheets generates enough electricity to power a small liquid crystal display. With increased power output, these virus films might one day use the beating of your heart to power a pacemaker, the researchers behind them say.

Piezoelectric materials build up charge when pushed or squeezed. These materials may be familiar to you: they generate the spark in a gas lighter, and motors powered by such materials vibrate some cell phones. Piezoelectric materials made of metals or polymers require large inputs of energy to build up a charge. Bone, DNA, and protein fibers are weakly piezoelectric, but it’s hard to efficiently organize these materials on a large scale to yield electricity.

To handle this organizational issue, Seung-Wuk Lee, of the University of California in Berkeley and the Lawrence Berkeley National Laboratory, and his colleagues looked for a biomaterial that had intrinsic order and was easy to make. They settled on the M13 bacteriophage, a rod-shaped virus that only infects bacteria. One bacterium can produce one million copies of the virus within four hours, so starting material isn't a problem. And the virus neatly arranges itself in stacked rows when spread on a surface.

The researchers first tested the virus to see if it was piezoelectric. Instead of pushing on the virus and measuring a current, they looked for the opposite effect. They electrified a film made with the virus and watched for mechanical motion. The scientists saw the helical proteins covering the virus twist.

To understand why the virus is piezoelectric, we need to look at its structure. About 2700 copies of a helical protein stretch along the length of the virus, tipping out from that central axis about 20°. Each helix has a positively charged end and a negatively charged end. The amount of this charge difference and the distance between the two charged areas sets up an electric dipole, which runs along each helix.

Normally these dipoles cancel each other out because the proteins are symmetrically arranged around the outside of the virus—the amount of negative charge around the virus surface balances out the amount of positive charge. But when the virus is squished from above, its rod shape elongates into an oval, and the dipole moments become uneven. One area of the virus coat can now hold negative charges while another builds positive charge. Establishing that charge difference causes current to flow along the virus.

Since the structure of the coat proteins is well known, the researchers engineered the virus to increase its piezoelectric properties. They added four extra negatively charged amino acids, specifically a string of glutamates, to one end of the helical surface protein. That increased the charge difference between the positive and negative ends of the helix, thus raising the amount of electrical energy it produced when squished.

Next, the scientists sandwiched sheets of engineered virus between two gold electrodes about the size of a postage stamp. When pushed with a thumb, the virus stack produces 6 nA of current with 400 mV of potential. That’s about one-quarter the voltage of an AAA battery. Combining two of these stacks provides enough energy to bring up a “1” on a small liquid crystal display.

Lee is working to increase the amount of current that these viral particles can produce by tweaking the viral coat proteins and playing with their arrangement on the electrode surface. In five to ten years, he estimates, viral piezoelectric films in your shoes could be personal electricity generators to power your iPod as you run. Or they could use the thumping of your heart to power a pacemaker, Lee says.

Though the current produced now is small because only a thin layer of the virus deforms, virus-based devices could still be useful for small scale applications, writes S. Michael Yu, of Johns Hopkins University, in the News and Views article accompanying the paper. This flexible film has a “self-assembling capability that no other piezoelectric materials can even dream about,” he writes. That reliable self-organization forms tidy structures gives the material its piezoelectric activity, Yu writes.

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.69 (About DOIs).