Some viruses cause illness, pandemics, and death. But scientists have found a new way to put at least one type of virus to good use. A team of researchers has harnessed bacteria-infecting viruses to generate power by converting mechanical energy into electricity. The virus power pack isn't yet powerful enough to run your cell phone or iPod. But because the microbes are harmless to humans, they may one day prove useful for powering medical sensors inside our bodies.
Devices that convert mechanical energy into electricity, or vice versa, aren't anything new. They take advantage of the "piezoelectric effect," which was first discovered in 1880 and is a property of certain crystals, proteins, and even DNA. Piezoelectric materials consist of molecules that have more positive electrical charges on one end of the molecule than on the other. These molecules lock together in a repeating array, with their positive ends all facing one way and their negative ends facing the opposite way. Compressing the material increases this polarization and generates an electric voltage that can be used to do work. Alternatively, by adding electricity, you can change the shape of a piezoelectric material. Today, piezoelectrics are used in everything from electric lighters to scanning tunneling microscopes.
Most piezoelectric generators in use today are made with crystals of the ceramic lead zirconate titanate (PZT). PZT is toxic, so in recent years researchers have been developing nontoxic alternatives, such as zinc oxide. But some of these alternatives are expensive and challenging to manufacture. So Seung-Wuk Lee, a bioengineer at the University of California, Berkeley, and his colleagues there and at neighboring Lawrence Berkeley National Laboratory opted to see if viruses could help them out.
The idea isn't as wacky as it seems. While a graduate student at the University of Texas, Austin, Lee had developed bacteria-infecting viruses called phages that bind to specific types of inorganic semiconductor nanoparticles. He also knew that DNA and certain proteins—the building blocks of the phages—are piezoelectric. So he and his colleagues went looking for piezoelectric phages. They found one called M13 bacteriophage, whose narrow, tube-shaped outer coat consists of about 2700 copies of a rod-shaped protein with positive charges on one end and negative charges on the other. The proteins in the phage assemble with their positive ends leaning into the hollow core, which allows them to hold onto the negatively charged DNA that the phages inject into bacteria during an infection.
To test whether the phages could produce power, Lee and his colleagues first genetically engineered the virus's proteins to harbor additional copies of a negatively charged amino acid called glutamate. They added glutamates to the negatively charged end of the protein to increase its negative charge and thus its piezoelectric properties. To make a generator, the researchers laid down a film of millions of these phages atop one electrode. The phages naturally assemble themselves lying flat, side by side, all pointing in the same direction.
The Berkeley team layered several of these viral films atop one another to enhance the piezoelectric effect and then capped the stack with a second electrode. As the researchers report online this week in Nature Nanotechnology, pressing a finger to the upper electrode compressed the phages in the film enough to generate an electric current that could light up the number one on a small liquid crystal display.
The new generator produces far less power than conventional piezoelectric devices. Nevertheless, Zhong Lin Wang, a materials scientist at the Georgia Institute of Technology in Atlanta says, "It shows the possibility of expanding the nanogenerator into biostructures, which can be important for medical and biological applications," such as implantable sensors for diagnosing blood sugar levels for diabetics. In an effort to make that possible, Lee and his colleagues are now working to direct the evolution of the viruses to make them better power producers.