Scientists at the US Massachusetts Institute of Technology (MIT) have figured out a way to use viruses to build ultra-thin lithium-ion batteries that pack three times the normal energy level for their weight and size, they said this week.

By manipulating genes inside the viruses, the scientists coaxed them into coating themselves with cobalt oxide molecules and gold particles and then lining themselves up to form tiny wires that serve as the anode electrode in a battery.

The eight-person team, led by MIT Professors Angela Belcher, Paula Hammond and Yet-Ming Chiang, describe their work in this week's issue of the journal Science.

Among other applications, the work could contribute to the development of more useful car batteries, which today are too heavy and weak to compete effectively with petrol, the scientists said.

Each wire measures six nanometres, or six billionths of a metre, in diameter and 880 nanometres in length. Once the genes have been altered, the viruses can be cloned millions of times to form batteries as small as a grain of rice or as large as a hearing-aid battery, the team said.

The nanowires can be made at room temperature and pressure, meaning expensive gear isn't needed to create an artificial environment. But the work is delicate, with just the right amount of cobalt oxide and gold needing to be formed exactly where it belongs.

In 2003 Belcher cofounded a company called Cambrios Technologies, in Mountain View, California, which aims to commercialise biological technologies. Cambrios holds "exclusive license rights to Dr. Belcher’s technology," according to its website.