Professor Ionnis Ieropoulos of the University of the West Of England, Bristol put 24 flexible microbial fuel cells (MFCs) in socks with tubes that feed urine in and take the processed liquid out. The good news is, provided the MFCs and tubes don't break, your socks and feet stay urine-free, although we're still not sure there will be a rush of demand.
The fuel cells draw on electrons released by microbes as they process food in an oxygen-free environment. Any organic waste can serve as their source of nutrients, so options even less pleasant than urine could be running through your socks, but urine is widely availalble and flows easily. The system comes with a container that can store up to 648 millelitres, (1.37 pints) of urine, which the wearer can top up separately; you can't add to the system while walking along.
The walker's energy pumps the urine, rich in the nitrogen and phosphorus microbes need, into the cells and then takes the used liquid back out again afterward. The design is based on the cardiovascular system of fish, the simplest closed circulatory system in nature. The electrical energy produced by Ieropoulos' prototype was used to power a wireless transmitter that every two minutes sent a message to a receiver module.
"Having already powered a mobile phone with MFCs using urine as fuel, we wanted to see if we could replicate this success in wearable technology,” Ieropoulos said in a statement. “We also wanted the system to be entirely self-sufficient, running only on human power – using urine as fuel and the action of the foot as the pump."
As a method of transforming mechanical energy into electrical it's not very efficient, but Ieropoulos added: "This work opens up possibilities of using waste for powering portable and wearable electronics. For example, recent research shows it should be possible to develop a system based on wearable MFC technology to transmit a person's coordinates in an emergency situation. At the same time this would indicate proof of life since the device will only work if the operator's urine fuels the MFCs."
The work has been published in Bioinspiration and Biomimetics where the authors report: “The maximum achievable power is about 110 μW [microwatts].” This is a tiny fraction of even the smallest commercial solar cell's production, but of course it works at night, in a cave, or in terrible weather. Further work might increase the power to the point where small electronic devices can be charged, although the paper acknowledges obstacles to stepping up the scale.
Even though the system could save the life of a hiker lost in the wilderness, many might prefer to carry a solar panel or extra batteries. However, the technology is part of wider exploration of ways to extract energy from human waste in ways that could bring light to refugee camps. The MFCs cost just $1.50 (£1), and Oxfam is exploring their use in places where light is unavailable, with waste disposal as an added benefit. If the wearable version becomes popular, even as a gimmick, mass production could bring costs down further.