Sweat could power battery-free health trackers of tomorrow
Many of us now keep tabs on how we're doing by wearing health and fitness trackers like those from Fitbit, which run on batteries that need charging. Researchers from Caltech have developed an electronic skin that can be packed with sensors, and that's powered by the sweat of its wearer.
"One of the major challenges with these kinds of wearable devices is on the power side," said the project's Wei Gao. "Many people are using batteries, but that's not very sustainable. Some people have tried using solar cells or harvesting the power of human motion, but we wanted to know, 'Can we get sufficient energy from sweat to power the wearables?' and the answer is yes."
Our sweat contains concentrations of lactate. This by-product of normal metabolic processes is absorbed by the electronic skin's fuel cells – which are made from carbon nanotubes that host a platinum/cobalt catalyst and an enzyme that breaks down the lactate and is held in composite mesh. Here, the body's waste product is mixed with oxygen from the air around us to generate water and pyruvate.
The Caltech team says that these biofuel cells can generate enough continuous, stable electricity (as much as "several milliwatts per square centimeter") to power sensors for monitoring such things as heart rate, body temperature and blood sugar levels, as well a Bluetooth radio.
"While near-field communication is a common approach for many battery-free e-skin systems, it could be only used for power transfer and data readout over a very short distance," explained Gao. "Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications."
The fuel cells, sensors and wireless comms system are all contained within a soft, flexible rubber patch that's applied directly to the wearer's own skin. And the researchers say that the sweat-powered electronic skin can run "over multiple days."
Going forward, the team sees the development as a platform for a number of different battery-free biosensors, as well as having potential for use as a human/machine interface to help operate future prosthetics.
A paper on the research was published in the journal Science Robotics.