The “portable micro-grid” recovers energy from the human body to power electronic gadgets

The wearable micro-grid uses the energy of human sweat and movement to power an LCD wristwatch and electrochromic device. Credit: Lu Yin

Nanotechnology engineers at the University of California at San Diego have developed a “portable micro-grid” that captures and stores energy from the human body to power small electronic devices. It consists of three main parts: sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy storage supercapacitors. All parts are flexible, washable and can be screen printed on clothing.

The technology, reported in an article published today (March 9, 2021) in Nature Communication, is inspired by community microgrids.

“We apply the concept of the micro-grid to create portable systems powered in a sustainable, reliable and independent way,” said co-first author Lu Yin, a PhD in nanotechnology. student at UC San Diego Jacobs School of Engineering. “Just as an urban micro-grid integrates a variety of local and renewable energy sources such as wind and solar power, a portable micro-grid integrates devices that locally harvest energy from different parts of the body, such as sweat and movement, while containing energy storage.

This shirt collects and stores energy from the human body to power small electronic devices. Nanotechnology engineers at UC San Diego call it a “portable micro-grid” – it combines the energy of sweat and wearer movement to provide long-lasting power to portable devices. Credit: UC San Diego Jacobs School of Engineering

The portable microarray is constructed from a combination of flexible electronic parts that were developed by the nanobioelectronics team of UC San Diego nanotechnology professor Joseph Wang, director of the Center for Wearable Sensors at UC San Diego and corresponding author of this study. Each part is screen printed on a shirt and placed in such a way as to optimize the amount of energy collected.

The biofuel cells that recover energy from sweat are located inside the shirt at chest level. Devices that convert the energy of motion into electricity, called triboelectric generators, are placed on the outside of the shirt on the forearms and sides of the torso near the waist. They recover energy from the swinging motion of the arms against the torso while walking or running. Supercapacitors on the outside of the shirt on the chest temporarily store energy from both devices, then discharge it to power small electronic devices.

Portable micro-grid biofuel batteries

Biofuel cells recover energy from sweat. Credit: Lu Yin

Harnessing energy from movement and perspiration allows the portable micro-grid to power devices quickly and continuously. Triboelectric generators provide power as soon as the user starts to move, before they sweat. Once the user begins to sweat, the biofuel cells begin to provide energy and continue to do so after the user stops moving.

“When you add these two together, they make up for each other’s shortcomings,” Yin said. “They are complementary and synergistic to allow a quick start and a continuous supply. The whole system starts up twice as fast as biofuel fuel cells alone and lasts three times longer than triboelectric generators alone.

The portable microarray was tested on a subject for 30-minute sessions that consisted of 10 minutes of exercise on a bike or running machine, followed by 20 minutes of rest. The system was able to power either an LCD wristwatch or a small electrochromic display – a device that changes color in response to an applied voltage – throughout each 30-minute session.

Greater than the sum of its parts

Biofuel cells are equipped with enzymes that trigger an exchange of electrons between lactate and oxygen molecules in human sweat to generate electricity. Wang’s team first reported these portable sweat-harvesting devices in a 2013 article. Working with colleagues at UC San Diego’s Wearable Sensor Center, they then updated the technology. so that it is expandable and powerful enough to operate small electronic devices.

Triboelectric generators are made of a negatively charged material placed on the forearms and a positively charged material placed on the sides of the torso. When the arms swing against the torso while walking or running, the oppositely charged materials rub against each other and generate electricity.

Each laptop provides a different type of power. Biofuel cells provide low DC voltage, while triboelectric generators provide high voltage pulses. In order for the system to power the devices, these different voltages must be combined and regulated into a single stable voltage. This is where supercapacitors come in; they act as a reservoir that temporarily stores energy from both energy sources and can discharge it as needed.

Yin compared the installation to a water supply system.

“Imagine that biofuel cells are like a slow-flowing faucet and triboelectric generators are like a pipe that shoots jets of water,” he said. “Supercapacitors are the reservoir they both power into, and you can tap into that reservoir however you want.”

All parts are connected by flexible silver interconnects which are also printed on the shirt and insulated with a waterproof coating. The performance of each part is not affected by repeated bending, folding and creasing or washing with water, as long as no detergent is used.

The main innovation of this work is not the portable devices themselves, Yin said, but the systematic and efficient integration of all devices.

“We’re not just adding A and B and calling it a system. We’ve chosen parts that all have compatible form factors (everything here is printable, flexible, and expandable); performance matching; and complementary features, which means they’re all useful for the same scenario (in this case, a rigorous move), ”he said.

Other applications

This particular system is useful for athletics and other cases where the user is exercising. But this is just one example of how the portable micro-grid can be used. “We do not limit ourselves to this conception. We can tailor the system by selecting different types of energy harvesters for different scenarios, ”Yin said.

Researchers are working on other designs that can harvest energy while the user is sitting inside a desk, for example, or moving slowly outside.

Reference: “A Self-Sufficient Portable Multimodular Bioenergy Microarray System” by Lu Yin, Kyeong Nam Kim, Jian Lv, Farshad Tehrani, Muyang Lin, Zuzeng Lin, Jong-Min Moon, Jessica Ma, Jialu Yu, Sheng Xu and Joseph Wang, March 9, 2021, Nature Communication.
DOI: 10.1038 / s41467-021-21701-7

This work was supported by the UC San Diego Center for Wearable Sensors and the National Research Foundation of Korea.

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