Stretchable Solar Batteries May Power the Next Generation of Wearable Devices

Engineers at the University of California, Berkeley have developed flexible, rechargeable batteries that can be charged by the sun and threaded into clothes or wearable technologies.

In recent years, electronic devices have migrated into all sorts of strange areas — smart underwear, for instance, that can measure your heart rate, calorie consumption, and posture. As such, batteries are changing too.

Researchers at the University of California, Berkeley have developed a new kind of stretchy battery specifically designed for wearable devices and clothing that require more flexibility than old-school electronics. In fact, the engineering team has come up with two new stretchy battery designs that can expand, contract, and bend while preserving the cost-efficiency and safety of traditional batteries.

Originally developed by Alla Zamarayeva, a Ph.D candidate at the university, the batteries use helical springs in a stretchable S-shape or flexible linear wire that can be incorporated into clothing, bracelets, and other wearable objects. The batteries can also be used for certain medical devices.

“We developed stretchable batteries to power stretchable wearable electronics, including devices for health monitoring and sensing,” Zamarayeva said. “Having electronics in a stretchable format provides comfort to the wearer and often improves signal to noise ratio of the sensors, if the electronics is worn directly on the skin.”

The flexible silver-zinc batteries — a common type of high-performance battery used in electronics — can withstand repeated bending without loss of structural integrity or performance. The researchers used a custom-built flexing apparatus to approximate repeated use: 17,000 flex cycles at 0.5-centimeter radius, if you're keeping score at home.

“The market for flexible and stretchable batteries is still in its infancy, but there are no products with similar mechanical properties, as we know of,” Zamarayeva said.

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The flexible batteries can be plugged in and charged up like other rechargeable units, but the Berkeley team is looking farther ahead by incorporating solar power too. Fellow researcher Aminy Ostfeld specializes in the intersection of solar cells and batteries, and said that such options will be critical for real-world applications.

“For the applications we're targeting in wearables and the internet of things, it would be highly impractical to plug the batteries into the wall whenever they need charging,” she said. “Although there have been many efforts to develop high-performing batteries and high-performing solar cells, relatively few of them have focused on designing batteries and solar cells that perform well together, integrated into a single system for a given application.”

To demonstrate their system, Zamarayev and Ostfeld assembled a bracelet that harvests solar energy and stores it in a wire battery built into the wristband. The bracelet, in turn, can be used to power medical sensors or devices within the bracelet itself or elsewhere on the body.

“Stretchable batteries are especially well suited to applications where the system needs to be in close contact with the skin, for example to measure vital signs like temperature and heart rate,” Ostfeld said.

Research on the new designs was funded by the National Science Foundation and detailed in the journal Science Advances, which is published by the American Association for the Advancement of Science.

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