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New e-Skin Embedded with Sensors can Monitor Health

Of the many ways humans experience the world around them, one is through their skin. The numerous nerve endings in the skin give people a great deal of information.

Sweat-powered electronic skin.Image Credit: Caltech.

At the California Institute of Technology (Caltech), Wei Gao, an assistant professor in the Andrew and Peggy Cherng Department of Medical Engineering, intends to study much more about humans from their skin. For this purpose, he has designed an electronic skin, or e-skin, that can be directly applied to the real skin of humans.

The e-skin has been prepared from soft and flexible rubber. Sensors for tracking data like body temperature, heart rate, levels of metabolic by-products, and blood sugar that are signs of health, and even the nerve signals that control the muscles, can be fitted into the e-skin.

The e-skin can function without a battery, as it works only on biofuel cells driven by one of the body’s waste products.

One of the major challenges with these kinds of wearable devices is on the power side. 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.

Wei Gao, Assistant Professor, Andrew and Peggy Cherng Department of Medical Engineering, Caltech

According to Gao, human sweat includes extremely high levels of the chemical lactate. It is a compound produced as a by-product of regular metabolic processes, particularly by muscles at the time of exercise.

Lactate is absorbed by the fuel cells integrated into the e-skin and blends it with oxygen from the air, thus producing water and pyruvate, which is another by-product of metabolism.

As these by-products function, a sufficient amount of electricity is produced by the biofuel cells to power a Bluetooth device and sensors, similar to the one that connects a phone to a car stereo, thereby enabling the e-skin to wirelessly transmit readings from its sensors.

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.

Wei Gao, Assistant Professor, Andrew and Peggy Cherng Department of Medical Engineering, Caltech

Gao stated that while developing the e-skin, creating a power source that has the ability to run on sweat was not the only difficulty faced, but it also required to last for a long period with minimum degradation and with high power intensity.

The biofuel cells have been produced from carbon nanotubes impregnated with a catalyst of cobalt or platinum and composite mesh containing an enzyme that disintegrates lactate. These biofuel cells can produce steady and constant power output (as high as several milliwatts per square centimeter) over many days from human sweat.

The plan is to design a range of sensors that can be incorporated into the e-skin so that it can be utilized for multiple purposes, stated Gao.

We want this system to be a platform. 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.

Wei Gao, Assistant Professor, Andrew and Peggy Cherng Department of Medical Engineering, Caltech

The paper explaining the study titled “Biofuel-powered soft electronic skin for multiplexed and wireless sensing” was published in the April 22nd issue of Science Robotics.

The study was financially supported by a California Institute of Technology startup grant, the National Institutes of Health, the Carver Mead New Adventures Fund, and the Rothenberg Innovation Initiative (RI2) program.

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