Muhammad Mustafa Hussain, a professor of electrical engineering at King Abdullah University of Science and Technology in Saudi Arabia, devotes nearly all of his time to building extremely tiny devices. “You make things very small, you get rapid results,” he says. So in 2010, when he set out to develop an abundant, renewable power source that could be used in extremely remote places for machines that might purify water or diagnose disease, it was inevitable that he would start small. A tiny microbial fuel cell, for example, would be a natural starting point. It was not inevitable, however, that he would choose to power that fuel cell with saliva.

The idea of using spit came from Hussain's colleague Justine E. Mink, then a Ph.D. candidate in his lab (now a researcher at Dow Chemical). At the time, Mink was trying to build glucose-monitoring devices for diabetics with power sources small enough to fit inside the body, near the pancreas. A microbial fuel cell—which generates power by feeding organic matter (which saliva has lots of) to bacteria, which, in turn, produce electrons—was a natural candidate for their projects. So the two took a highly conductive graphene electrode, loaded it with saliva-eating bacteria, and within weeks they were producing nearly one microwatt, a millionth of a watt of power.

A microwatt is a tiny amount of power, but it is enough for lab-on-a-chip devices, diagnostic tools and monitoring tools such as Mink's diabetes tracker. Hussain is working with companies that 3-D-print artificial organs to integrate his fuel cell into an artificial kidney, where a range of bodily fluids would provide fuel. He says this is simply the first step as he scales up: his long-term goal is to generate electricity from organic factory waste to power desalination plants in poor countries.