Sweat-powered biofuel cell in e-skin drives Bluetooth
A biological fuel cell, or biofuel cell, powered by human sweat has enough power for a Bluetooth link in electronic skin and wearable designs. This is a key step for battery-less monitoring systems and takes advantage of the reducing power requirements and size of Bluetooth transceiver chips.
The researchers at Caltech in the US made the e-skin from soft, flexible rubber embedded with sensors that monitor information like heart rate, body temperature, levels of blood sugar and metabolic byproducts that are indicators of health, and even the nerve signals that control muscles.
The key is that it runs solely on a biofuel cell powered by sweat with an output sufficient to run a Bluetooth link rather than use near field communications to relay the sensor data via a mesh network. A previous design by researchers in France and the Uniersity of San Diego used a voltage booster to continuously power an LED light.
“One of the major challenges with these kinds of wearable devices is on the power side,” said Wei Gao, assistant professor in the Andrew and Peggy Cherng department of Medical Engineering. “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, a compound generated as a by-product of normal metabolic processes, especially by muscles during exercise. The biofuel cell built into the e-skin absorbs the lactate and combine it with oxygen from the atmosphere, generating water and pyruvate, another by-product of metabolism. The biofuel cell is made from carbon nanotubes impregnated with a platinum/cobalt catalyst and composite mesh holding an enzyme that breaks down lactate.
Next: Biofuel cell output
The output is 3.5mW/cm2 with stable performance during 60 hours of continuous operation. It selectively monitored key metabolic chemicals including urea, NH4+, glucose, and pH and the skin temperature during prolonged physical activities and wirelessly transmitted the data to the user interface using Bluetooth. The system was also able to monitor muscle contraction and work as a human-machine interface for human-prosthesis walking.
“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,” said Gao. “Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications.”
Gao says 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 optimize next-generation prosthetics. “
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