Genetically manipulated viruses could replace standard lithium-ion batteries, packing two to three times more energy than other batteries, researchers say.The virus batteries could be thin, transparent, and lightweight, according to a US study published online recently in the journal Science by Professor Angela Belcher of the Massachusetts Institute of Technology and team.Because less material is devoted to packaging, more of the battery is used just for generating power."What we're trying to do is have all of the mass and volume be used for the purpose it is to be used for, which is to power the device," says Belcher.The researchers say such a battery should last as long as conventional batteries. And it could power anything from microelectronics, including chemical and biological sensors, 'lab on chip' devices, and security tags to larger items such as mobile phones, computer displays and even electric cars.Building batteries, like building anything, requires assembly. The smaller the battery, the more challenging that is.Current manufacturing techniques involve arranging nanoparticles, nanotubes, or nanowires on surfaces using expensive, high-temperature methods.Belcher and her team decided to capitalise on biology's inherent knack for organising microscopic structures and apply it to battery technology.To make the viruses work like conducting wires, the scientists genetically altered the organisms so that proteins on their surfaces would be attracted to metal particles, including cobalt and gold.Four different solutions went into the battery component: a negatively charged polymer, a positively charged polymer, negatively charged viruses, and charged particles, or ions, of cobalt.The scientists spread the negatively and positively charged polymer solutions onto a glass slide in alternating layers. Next, they dipped the slide into a solution containing millions of the altered viruses.The wire-like viruses automatically spread themselves evenly across the slide, as they have a natural tendency to slightly repel each other.When the slide was dipped into the ion solution, proteins on the surface of the viruses attracted the metal ions, causing the organisms to become, essentially, conducting wires.And because viruses naturally replicate, scientists say that growing more to make many batteries shouldn't be hard."All you do is grow them in a bigger fermenter and you're done. Once you do, there's no roadblock to scale up to industrial level production," says Brent Iverson, professor of chemistry and biochemistry at the University of Texas at Austin When the polymer solution dries, it becomes a transparent anode, the battery's positively charged terminal.A piece of film about 10 centimetres by 10 centimetres contains about a billion conducting viruses.Belcher and her team are working next to produce the negatively charged cathode with the viruses and believe they will have a working prototype in about two years.