The airflow of a typical human breath travels at less than 2 meters per second. Instead of lamenting its weakness, engineers at the University of Wisconsin-Madison decided to try to make a material that could react to this airflow in such a way as to convert it to electrical energy.
So they turned to polyvinylidene fluoride (PVDF), a material in which an electrical charge can build up in response to applied mechanical stress. (There's even a name for this: the piezoelectric effect.) The trick, then, was to get this material thin enough to be sufficiently stressed by human breath.
"We calculated that if we could make this material thin enough, small vibrations could produce a microwatt of electrical energy that could be useful for sensors or other devices implanted in the face," says Xudong Wang, a materials science and engineering assistant professor who reports on these findings in the journal Energy & Environmental Science.
Wang's team had go about thinning this material very carefully, so as to preserve its piezoelectric properties. They used an ion-etching process that, with some improvements, might eventually enable them to control thickness to the submicron level.
The obvious benefits of using respiration to power biomedical devices (think blood glucose monitors or pacemakers) are that the source is local and it is consistent.