Living cells shift electrons in the process of converting fuels such as sugars. Unfortunately, they usually do it within the membrane of the cell and leave no net external current. The trick is to get in the way of the electron flow so that it can be ported to an external circuit. One approach is to extract the enzymes from Escherichia coli bacteria which will convert ethanol to current flow in a circuit.
In another approach, two enzyme coated microfibers laid side by side capture electrons from glucose on one and pass it to oxygen on the other. A 1/4 inch long microfiber pair can generate 600 nanoWatts. It is small enough to reside in the blood stream and might be able to sense blood-sugar levels.
A technique which leaves the cell of rhodoferax ferrireducens intact harvests electrons with earth minerals such as iron oxide. Up to 80% of the electrons are passed to the electrode. The hope is to generate electricity from sewage while breaking down the waste.
Discover, January 2004, pp. 15-16
Singapore’s Institute of Bioengineering and Nanotechnology develop a novel way to generate electric current from urine. The battery unit is made from a layer of paper that is steeped in copper chloride and sandwiched between strips of magnesium and copper. This “sandwich” is then held in place by being laminated, which involves passing the battery unit between a pair of transparent plastic films through a heating roller at 1200C. The final product has dimensions of 60 mm x 30 mm, and a thickness of just 1 mm (a little bit smaller than a credit card).
Writing in the Journal of Micromechanics and Microengineering, Lee describes how the battery was created and quantifies its performance. Using 0.2 ml of urine, the scientists generated a Voltage of approximately 1.5 Volts with a corresponding maximum power of 1.5mWatts. The scientists also found that the battery performance (such as Voltage, power or duration) may be designed or adjusted by changing the geometry or materials used.
“Our urine-activated battery would be integrated into biochip systems for healthcare diagnostic applications,” says Lee. He envisions a world where people will easily be able to monitor their health at home, seeking medical attention only when necessary. “These fully-integrated biochip systems have a huge market potential,” adds Lee.