Body-fluid battery.

Kidnapped by armed terrorists, the diplomat surreptitiously licks a plastic card and slips it back into a pocket. Soaked by the captive's saliva, a battery on the card springs to life, powering a transmitter that beams out an SOS.

A prototype of such a battery has now been devised by a researcher in Singapore. Because the device may be activated by a mere droplet of a body fluid--saliva, urine, or blood, for example--it's likely to find its widest use in supplying power to plastic electronic components to be used in disposable home health-care kits, says Ki Bang Lee of the Institute of Bioengineering and Nanotechnology, a government-run research center.

For instance, Lee notes, a microchip-based diabetes-test device could measure the concentration of glucose in urine while using the urine as battery fluid.

To devise a battery that would be both inexpensive and compatible with plastics-processing techniques (SN: 3/19/05, p. 189), Lee resorted to low-tech methods and materials. He infused a piece of filter paper with crystals of copper chloride and then sandwiched the paper between electrodes hand cut from copper and magnesium foils. The Singapore engineer then laminated the sandwich with clear plastic pierced by slits that allow fluid to enter and air to escape.

When body fluid, or even water, enters the battery, the liquid soaks the crystal-infused paper, instigating a chemical reaction that generates free copper, magnesium chloride, and electricity. Lee unveils a 3-centimeter--by-6-cm prototype battery in the September Journal of Micromechanics and Microengineering.--P.W.