In a microbial fuel cell, bacteria can release electrons by breaking down organic materials. The fuel cell can turn these electrons into a usable current running from an anode (the part that contributes electrons) to a cathode (which accepts them). But unlike a battery, a fuel cell runs on an external fuel supply—saliva, in this case.
The whole package is smaller than a dime and with effort could be shrunk even more, the scientists said. As an implant in the mouth, the device would pick up an individual's saliva.
An anode made of a carbon material called graphene that was seeded with bacteria would break down the saliva into electrons, protons and carbon dioxide. For the cathode needed to establish the current, the scientists simply relied on oxygen in the air; no chemicals were needed.
The setup managed to produce not quite one microwatt of power, or a mere 1/1,000,000th of a watt. But that's enough to run some low-power devices. Other scientists have demonstrated, for example, a low-power electroencephalograph on a chip that could warn of an approaching epileptic attack.
Tiny microbial fuel cells, the scientists wrote, "are an interesting and practical energy-harvesting option for health and environmental monitoring system-on-chips."
One potential use suggested by the researchers is a device, small enough to be a kind of dental appliance, that could predict a woman's ovulation from changes in her saliva's ability to conduct electricity, which is known to plummet five days before ovulation.