Wearable devices are all the rage. But the fly in the technological ointment is battery life. Limited by size, rechargeables run out too fast, button batteries are not sustainable and newer tech brings heat and potential burn dangers. But scientists are working on another wearable power source – sweat. By Jeremy Torr.

Tokyo, 7 May 2021. Google recently bought wearables maker Fitbit for $2.1billion. Sales of Apple watches have peaked over 100 million in what has been the most rapid uptake of a new technology introduction, ever. Wearable tech is on a roll. But there is a downside.

Many users bemoan the poor battery life of the built in rechargeables. Users who opt for conventionally powered watches must put up with bulky casings and fiddly battery changeovers. And then there is the sustainability worry – where do all those batteries and potentially dangerous metals go when they have worn out? It’s a headache, for sure.

Already Apple is making the right noises. Recent Apple Watch models are made from recycled aluminium and other metals. Some manufacturers like Waltic make sports watches from recycled plastics. But batteries remain a problem – but not for long if Japanese researchers have their way.

"We managed to drive a commercially available activity meter for 1.5 hours using a single drop of artificial sweat,” explains power source researcher Dr. Professor Isao Shitanda from Tokyo University of Science. “And we expect (these sweat-powered biofuel cells) should be capable of powering all sorts of devices in future, such as smart watches and other commonplace portable gadgets."

The astonishing new power supplies developed by Shitanda and his team are miniature fuel cells – just like those that use hydrogen to power cars – but instead of hydrogen they use lactate, the key element in human sweat, to generate a small electric current. Add a stack of these tiny fuel cells together and stick them onto a permeable armband you can wrap around your wrist, and any watch will run on your own energy. No batteries required.

"In our experiments, our … biofuel cells generated a voltage of 3.66 V and an output power of 4.3 mW,” explained Shitanda. “To the best of our knowledge, this power is significantly higher than that of previously reported biofuel cells of this type." 

To show this was a real world output, and not just a research tick in the box, the Tokyo research team fabricated a self-driven lactate (sweat) biosensor. The device successfully powered itself using their novel sweat-power generator; then it sent real time feedback on various elements of the wearer’s skin condition to a smartphone, using Bluetooth. It worked!

The team’s new power source looks much like a bandage that can be wrapped around the arm or forearm. It consists of a water-repellent paper base which carries  multiple biofuel cells (electrodes) laid out and connected together in an array. The number of cells used depends on the voltage and power required.

“In each cell, electrochemical reactions between the user’s lactate (sweat) and an enzyme in each electrode produces an electric current, which flows to a current collector made from a special conducting carbon paste,” say the researchers.

Another key development is the use of screen printing to apply the cell materials to the paper base. As well as being light and inexpensive, this helps make the cells highly suitable for cost-effective mass production.

The research, carried out in collaboration with Dr. Seiya Tsujimura from University of Tsukuba, Dr. Tsutomu Mikawa from RIKEN, and Dr. Hiroyuki Matsui from Yamagata University, looks set to revolutionise the sustainability credentials of those sweaty runners that fill parks and laneways says Shitanda.

 “Although wearable electronic devices and biosensors are great tools for health monitoring, it has been difficult to find convenient power sources for them. This research opens doors to electronic health monitoring powered by nothing but bodily fluids,” say the boffins.

By-bye battery blues; roll on the perspiration proposition.