NHPA

Stunning

IN DECEMBER 2007, a Japanese aquarium hooked up the lights on a Christmas tree to a tank containing an electric eel. Metal plates at the ends of the tank collected the electricity that the fish generated, which was used to power the bulbs. It was certainly effective as a publicity stunt. Now some researchers in America have developed a battery that generates electricity in a similar way.

Many creatures use differences in the concentration of ions (electically charged atoms) within the body to do work. Human brains, for example, rely on electrical impulses to release calcium ions that bind to neurotransmitters that, in turn, communicate with the rest of the nervous system. The mechanism that allows Electrophorus electricus to produce a shock as strong as a wall socket employs differences in the concentration of sodium ions in some 6,000 specialised cells called electrocytes. These cells are normally electrically isolated from one another. When the eel locates its prey, it opens a series of cellular gates through which the ions can flow. This movement of charged ions, when the eel is in a conductive solution like water, creates an electric current.

David LaVan of the National Institute of Standards and Technology in Maryland and his colleagues wanted to study the operation of living cell membranes and their proteins. They began by experimenting on artificial “protocells”. These, like real cells, were surrounded by membranes made of fatty molecules. Proteins “floating” in the membranes would let only certain ions pass. Using this system, the researchers realised that they might be able to copy the eel's electricity-generation mechanism.

The team fused two protocells together, so that they shared part of their respective membranes. They then added a dilute concentration of potassium chloride to one protocell and a more concentrated solution to the other. The difference in concentrations of potassium and chloride ions would normally cause ions to move from the less concentrated protocell to the more concentrated one. In this case, however, the membrane between the protocells was too thick to permit much of this kind of movement.

Next, Dr LaVan and his colleagues installed a protein called alpha-hemolysin into the protocell membrane. This functioned as a selective bridge, permitting the passage of positively charged sodium ions, but not negatively charged chloride ions. As the selected ions moved in one direction, electrons (which are negatively charged) flowed in the opposite direction. To make use of this electrical current, the team connected tiny electrodes to the protocells.

They report, in Advanced Materials, that by using electrodes made from a material with the same electrical resistance as that experienced by the ions when they cross from one protocell to the other, they were able to sustain a usable current for a useful period of time. Adding more protocells would increase the battery life. Dr LaVan reckons that two of his protocells, measuring several centimetres across, could run a portable MP3 player for about 10 hours. A novel Christmas present, perhaps?