The tattoo that charges your phone

We all feel like we need to recharge our batteries after a tough workout. But what if you could use the power from your workout to charge batteries like the ones in your phone? Researchers at Joseph Wang laboratory at the University of California, San Diego, have developed a solution that does just that: a temporary tattoo that converts sweat into tiny amounts of electricity.The transfer was originally used to monitor the health of athletes. During exercise, the body produces energy by converting glucose into lactic acid, or lactate. “This is a very strong indicator of your fitness during exercise,” explains Wenzhao Jia, who coordinated the research. However, the system required blood to be taken, and this was inconvenient, to say the least. Then came the idea of measuring the lactate present in sweat instead of blood – and the printed sensor in tattoo form came into being.The transfer was originally used to monitor the health of athletesA mini bio-batteryThe tattoo also turns into an electric micro-generator: an enzyme acts as a catalyst to extract the electrons from the lactate molecules. The electrons are then transferred to another molecule, generating electricity on the way. The basic principle is the same as that of a standard battery – the exceptions being that it is tiny and uses organic elements.Working up good-sized drops of sweat on exercise bikes for half an hour, volunteers were able to generate a maximum of 70 microwatts per square centimetre of skin. But the technology is not yet up to speed with the concept. “Our electrodes generate around four microwatts. That isn’t enough to power a watch, which needs at least 10 microwatts,” explains Jia.Sustainable and non-toxicBefore we can charge our phones or fitness monitors by working up a good sweat, the system must be further developed – for example, using bracelets or headbands with larger electrodes – and linked to a storage battery. But Jia is confident the invention has potential: “Bio-batteries charge more quickly, use renewable energy sources, and are safer because they don’t leak toxic products. They could have a wide range of uses in the future.”To go furtherGenerating power through music
Piezoelectrics, the future-proof human dynamo
Epidermal Biofuel Cells: Energy Harvesting from Human Perspiration (Wiley Online Library)

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