Stretchable, sweat-powered battery developed for wearables

Scientists from Nanyang Technological University, Singapore (NTU Singapore) have developed a soft and stretchable battery, measuring 2 x 2 cm and as flat as a small paper bandage, that is powered by human perspiration.

Described in the journal Science Advances, the battery was created by printing ink containing silver flakes and hydrophilic poly(urethane-acrylate) (HPUA), which function as the battery electrodes, onto a stretchable textile that can be attached to wearable devices, like watches, wrist bands or arm straps. When the silver flakes come into contact with sweat, its chloride ions and acidity cause the flakes to clump together, increasing their ability to conduct electricity. This chemical reaction also causes an electric current to flow between the electrodes.

When the battery material is stretched, its resistance is further lowered, meaning it can be used when it is exposed to strain, such as when its user is exercising. Furthermore, as the stretchable textile is very absorbent, it retains a lot of sweat, so that the battery remains powered even when the rate of sweating is inconsistent. This is important for consistent functioning as the amount of human sweat secreted is variable and depends on the area of the body it is in, the environmental conditions and the time of day.

Another advantage is that, unlike conventional batteries, the battery does not contain heavy metals or toxic chemicals. As noted by Dr Lyu Jian, who is the co-first author of the study, “Conventional batteries are cheaper and more common than ever, but they are often built using unsustainable materials which are harmful to the environment. They are also potentially harmful in wearable devices, where a broken battery could spill toxic fluids onto human skin. Our device could provide a real opportunity to do away with those toxic materials entirely.”

To demonstrate its potential use when it becomes incorporated in wearable biosensors and other electronic devices, the team of scientists tested their device with artificial human sweat. In a separate trial, the team reported that an individual wearing the battery around their wrist and cycling on a stationary bicycle for 30 min was able to generate a voltage of 4.2 V and output power of 3.9 mW that was sufficient to power a commercial temperature sensor device and send the data continuously to a smartphone via Bluetooth.

“Our device could be more durable than current technology, as we showed it could withstand strain from a wearer’s daily activities and repeated exposure to stress or sweat,” said study leader Professor Lee Pooi See, Dean of NTU Graduate College.

“The slim size of our battery also solves two problems in wearable tech: traditional button batteries are a problem for achieving the sort of sleek aesthetics that are attractive to consumers, while thinner batteries reduce the item’s ability to carry enough charge to last throughout the day.”

A patent application for the sweat-powered battery has been filed through NTU’s enterprise and innovation company, NTUitive. The researchers now plan to further explore the effects of other components of human sweat and how factors such as body heat may affect the performance of the battery.

“Our technology heralds a previously unreachable milestone in the design of wearable devices,” Prof Lee said. “By capitalising on a ubiquitous product — perspiration — we could be looking at a more environmentally friendly way of powering wearable devices that does not rely on conventional batteries. It is a near-guaranteed source of energy produced by our bodies. We expect the battery to be capable of powering all sorts of wearable devices.”

Image caption: NTU School of Materials Science & Engineering (MSE) Senior Research Fellow Dr Gurunathan Thangavel, materials scientist and Dean of NTU Graduate College Professor Lee Pooi See, and MSE Research Fellow Dr Lyu Jian. Image credit: NTU Singapore.

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