Small biofuel cells can harvest enough energy from the sweat on a person’s fingertips to power wearable medical sensors that track health and nutrition – and because our fingertips are one of the sweatiest parts of the body, the sensors could be powered all day.
Lu Yin at the University of California, San Diego, and his colleagues created a device that breaks down a dissolved compound in sweat called lactate. It comprises biofuel cells that fit into thin pads that are stuck to the fingertips. They soak up sweat into a thin layer of foam, where an enzyme oxidises lactate in the sweat to create an electrical charge.
Each finger pad can generate 20 to 40 microwatts of power and harvest 300 millijoules of energy per square centimetre during 10 hours of sleep. This isn’t enough to run power-hungry devices like smartwatches or mobile phones, but more than enough for lightweight sensors that detect a range of metrics such as heart rate, vitamin deficiencies and glucose levels.
Researchers have created devices that are powered by sweat before, but they needed large volumes of the liquid, such as when a subject was jogging. The fingertips have the highest concentration of sweat glands on the body and produce continuous charge even if the wearer isn’t exercising.
“Even with the minute amount of sweat compared to the sweat you got from a really intense workout, this power is still very sizeable,” says Yin. “No matter how clean your hand is, it’s very easy to leave your fingerprint everywhere. That’s basically the residue of your sweat, with a lot of metabolites. What we did is to take advantage of this.”
Currently, the enzyme that is key to the reaction begins to break down and become ineffective after two weeks. Yin says that further research is needed to create a stable enzyme that can be used in permanent sensors.
Journal reference: Joule, DOI: https://www.cell.com/joule/fulltext/S2542-4351(21)00292-0
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