Researchers from Saint Louis University, who earlier this year developed a fuel cell using enzymes to generate electricity from ethanol, have built a microchip-based version of the device.

The trick to constructing the biofuel cell was creating a sheltered environment for the enzymes, which are fairly sensitive. The researchers coated the carbon anode, or positive electrode, of the fuel cell with polymethylene green, an electocatalyst, then added a nafion membrane containing the immobilized enzyme alcohol dehydrogenase.

The biofuel cell showed an electrical potential of 0.34 volts and current density of 53 microamps per square centimeter, according to the researchers. Multiple cells can be stacked, and the device can be integrated into a computer chip.

The microchip biofuel cell could eventually be used in place of rechargeable batteries. Instead of recharging by plugging into a wall outlet like batteries, the biofuel would be recharged by adding a few milliliters, or thousandths of a liter, of alcohol. The micro fuel cell could also be used to power sensors and labs-on-a-chip.

The researchers' prototype consists of a 200-micron-wide, three-centimeter-long channel in a plastic chip. The bottom of the channel is lined with the carbon anode, which is covered by the electric catalyst and membrane. The researchers tested the fuel cell by measuring the electricity generated as one microliter, or millionths of a liter, per minute of ethanol flowed through the channel. A drop of water contains about 50 microliters.

The biofuel cell's enzyme catalysts are renewable. Most fuel cells catalyze reactions with metals, which are relatively expensive and not renewable.

The work is scheduled to appear in an upcoming issue of Lab-on-a-Chip.