Researchers have developed electronic skin (e-skin) that is applied directly on top of real skin. Made from soft, flexible rubber, it can be embedded with sensors that monitor information like heart rate, body temperature, levels of blood sugar, and metabolic byproducts that are indicators of health as well as nerve signals that control muscles. It does so without the need for a battery, as it runs solely on biofuel cells powered by one of the body's own waste products.
Human sweat contains very high levels of the chemical lactate, a compound generated as a byproduct of normal metabolic processes, especially by muscles during exercise. The fuel cells built into the e-skin absorb that lactate and combine it with oxygen from the atmosphere, generating water and pyruvate, another byproduct of metabolism. As they operate, the biofuel cells generate enough electricity to power sensors and a Bluetooth device similar to the one that connects a phone to a car stereo, allowing the e-skin to transmit readings from its sensors wirelessly.
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. Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications.
Devising a power source that could run on sweat was not the only challenge in creating the e-skin. It also needed to last a long time with high power intensity with minimal degradation. The biofuel cells are made from carbon nano-tubes impregnated with a platinum/ cobalt catalyst and composite mesh holding an enzyme that breaks down lactate. They can generate continuous, stable power output (as high as several milliwatts per square centimeter) over multiple days in human sweat.
Next steps are to develop a variety of sensors that can be embedded in the e-skin so it can be used for multiple purposes. In addition to being a wearable biosensor, this can be a human-machine interface — the vital signs and molecular information collected using this platform could be used to design and optimize next-generation prosthetics.