Ever been shocked by an electric eel? Probably not; if you had you’d know it. The ‘eel,’ which is actually a knifefish, can discharge an electric current of about 500 volts at 1 Ampere (500 watts). That’s not far from the shock you’d get from making contact with a wall circuit. It’s impressive enough that scientists have wondered for a long time if an artificial means couldn’t be developed from the eel’s form of bioelectrogenesis.
Four-fifths of an electric eel’s body is, in a sense, a battery. This ‘battery’ is composed of cells, called electrocytes. The cells contain ions (electrically charged particles) that build up from metabolic chemical processes. A fully-charged cell has only about 0.15 volt, but the cells are arranged in rows. When the eel decides to use is electric charge for hunting or defense, it opens a flow of positively charged sodium ions over the electrocytes, which attracts and conducts the electric charge within the cells to discharge points. This process is what a research team in Maryland (USA) set out to reproduce.
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.
[Source: The Economist]
The lab results were encouraging, producing a useable and sustainable current. Of course, this is lab work – a long way from practical testing, and even longer from commercial application. It is indicative, however, of the many approaches being taken to one of the most stubborn and important requirements for new technology – the better battery.