Japanese scientists have made a biofuel cell that produces enough power to run an mp3 player or a remote controlled car.

Inspired by power generation in living organisms, Tsuyonobu Hatazawa, from the Sony Corporation, Kanagawa, and colleagues developed a bio-battery that generates electricity from glucose using enzymes as catalysts.

Four biofuel cell units in series can power an mp3 player with speakers

A typical biofuel cell consists of an anode and a cathode separated by a proton-conducting membrane. A renewable fuel, such as a sugar, is oxidised by microorganisms at the anode, generating electrons and protons. The protons migrate through the membrane to the cathode while the electrons are transferred to the cathode by an external circuit. The electrons and protons combine with oxygen at the cathode to form water.

Until now, the energy output from biofuel cells has been too low for practical applications. Electron transfer in a biofuel cell can be slow so Hatazawa used a naphthoquinone derivative - known as an electron transfer mediator - to shuttle electrons between the electrodes and enzymes. This increased the current density - a measure of the rate of an electrochemical reaction - and increased the power output.

To increase the current density further, Hatazawa packed the mediator and enzymes on to a carbon-fibre anode. The large surface area and porosity of the electrode avoided disruption to glucose transport and maintained enzyme activity. They used a similar design to optimise the cathode so it supplied oxygen efficiently to the fuel cell.

When the researchers stacked four of the cells together, they achieved a power output of 100 milliwatts - enough to run an mp3 player with speakers or a small remote controlled car.

Adam Heller, an expert in bioelectrochemistry from The University of Texas at Austin, US, says the research 'will give much needed impetus to the development of useful biofuel cells, after years of studies aimed at unachievable goals'.

Nicola Burton