Fueled by sweat, a new bracelet can power a digital wristwatch

March 3, 2021

Sweat cools, but it can also power. (Unsplash/Nathan Dumlao)

Japanese scientists have unveiled a new biofuel cell bracelet that runs on the lactate in sweat, with its six interconnected cells generating up to 3.6 volts — enough power to run a digital wristwatch for 12 hours.

Described in a Feb. 20 paper in Biosensors and Bioelectronics, the research builds on an earlier patent. The team’s biofuel cell relied on a carbon-based bioanode fiber, which oxidizes the lactate found in sweat, and a carbon-based biocathode fiber, which reduces oxygen from the air. The bracelet powered a digital wristwatch at 80% power for 12 hours — and in fact, it still illuminated the watch’s LED screen after being stored in a refrigerator for a week.

The researchers wove the bracelet’s bioanode and biocathode fibers into a cotton textile for sweat storage, which had been coated with an ionic gel to lower resistance between the bioanode and biocathode. The resultant woven textile was itself then woven into two other textiles: a flax absorption textile, which gathered sweat from the skin, and a nylon evaporation textile, which dissipated water from the sweat in order to accumulate larger amounts of energy-rich lactate.

Three of these complexly woven structures, tied together by a narrow fiber, constituted a single biofuel cell. The researchers connected the cells by tying the cathode of one cell to the anode of the next.

The system’s bioanode, designed to receive energy in the form of lactate from sweat, was based on an enzyme known as lactate oxidase. The bioanode was multilayered, combining lactate oxidase with an osmium-based mediator and a carbon nanotube electrode — the mediator was used to boost electron transfer from lactate oxidase to the electrode. 

The biocathode was also multilayered. The team relied on another enzyme, bilirubin oxidase, for oxygen diffusion as well as another carbon nanotube bioelectrode.

Dr. Takeo Miyake, an associate professor at Waseda University in Tokyo and corresponding author of the paper, explained that bilirubin oxidase is much cheaper than platinum, the element typically used to reduce oxygen in biofuel cells of this sort.

The system can be reused indefinitely if the anode and cathode are replaced. This could make it a more eco-friendly and sustainable alternative to lithium batteries, which can be hazardous when discarded. “This is a big advantage,” said Miyake of the bracelet design.

Scalability is another big advantage, as the device can gain power through the attachment of additional biofuel cells. The bracelet is also less physically burdensome for users than some existing alternatives.

“Our bracelet was made by flexible and soft fibers, so people can wear it without losing comfort,” Miyake explained.

The device worked best when it was fueled by artificial sweat. When human sweat was used, the bracelet gradually lost power. However, when the team added chloride ions to the artificial sweat, making it more like human sweat, the cells performed much like they did on the real thing.

Miyake thinks that the chloride ions in human sweat interfere with oxygen reduction at the cathode. “We have several ways to improve the cathode performance,” he said. “One idea is to coat the mediator between enzyme and carbon materials.”

Miyake wants to capitalize on Japan’s economic strengths in applying his team’s innovation.

“There are many traditional companies for designing and weaving specific fabrics in Japan,” he noted, explaining that he is working with one firm to produce a power-generating bracelet product.

Miyake is also reaching out to a cosmetic company to explore the technology’s viability in medical patches and bandages. The sweat-powered bracelet could ultimately be used in many wearable electronics, which are still largely reliant on lithium batteries and other less sustainable power sources.

“The performance of wearable devices is determined by the performance of batteries,” Miyake pointed out.

The paper, “Fiber-crafted biofuel cell bracelet for wearable electronics,” published Feb. 20 in Biosensors and Biofuels, was authored by Sijie Yin and Xiaohan Liu, Waseda University; Takeo Miyake, Waseda University and PRESTO, Japan Science and Technology Agency; and Tatsuya Kaji and Yuta Nishina, Okayama University.

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