Human sweat-powered e-skin points to future robotics

Human sweat-powered e-skin embedded with sensors could be used as a biosensor to monitor physiological parameters and eventually help in the development of next generation robots and medical devices.

human sweat
Sweat-powered electronic skin (Image: CALTECH)

This is the claim of Caltech’s Wei Gao, assistant professor in the Andrew and Peggy Cherng department of Medical Engineering, who has developed the electronic ‘e-skin’ to be applied directly to human skin.

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Made from soft, flexible rubber, the e-skin can be embedded with sensors to monitor heart rate, body temperature, levels of blood sugar and metabolic by-products that are indicators of health. The e-skin can reportedly monitor nerve signals that control muscles and it does so by running on biofuel cells powered by human sweat. The research is detailed in Science Robotics.

“One of the major challenges with these kinds of wearable devices is on the power side,” Gao said in a statement. “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.”

Human sweat contains very high levels of the chemical lactate which is absorbed by the fuel cells and combined with atmospheric oxygen, generating water and pyruvate, another by-product of metabolism. As they operate, the biofuel cells generate enough electricity to power sensors and a Bluetooth, 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,” Gao said. “Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications.”

According to Gao, the e-skin also needed to last a long time with high power intensity and minimal degradation. The biofuel cells are made from carbon nanotubes 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 centimetre – over multiple days in human sweat.

Gao said the plan is to develop a variety of sensors that can be embedded in the e-skin so it can be used for multiple purposes.

“We want this system to be a platform,” he said. “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 optimise next-generation prosthetics.”