Nanoengineers at the University of California San Diego have developed a “wearable microgrid” that harvests and stores energy from the human body to power small electronics. There are three main components: sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy-storing supercapacitors. All parts are flexible, washable and can be printed on clothing.
The technology, reported in a paper published March 9 in Nature Communication, drawing inspiration from community microgrids.
“We are applying the concept of the microgrid to create accessible systems that are sustainably, reliably and independently powered,” said co-author Lu Yin, an engineer Ph.D. student at UC San Diego Jacobs School of Engineering. ”Just as the city’s microgrid integrates several local renewable energy sources such as wind and solar, an affordable microgrid weaves its includes devices that extract energy locally from various parts of the body, such as sweat and movement, and have energy storage. ”
The wearable microgrid is built from a combination of flexible electronic components developed by the Nanobioelectronics team of UC San Diego nanoengineering professor Joseph Wang, who is the director of the Center for Wearable Sensors at UC San Diego and a co-author of the current study. Each part is printed on a shirt and placed in a way that maximizes the energy collected.
Biofuel cells that extract energy from sweat are located inside the shirt at the chest. Devices that convert energy from transmission to electricity, called triboelectric generators, are located on the outside of the shirt on the forearms and sides of the torso near the middle. They extract energy from the moving movement of the arms against the torso while walking or running. Supercapacitors on the outside of the chest on the chest store energy from both devices for a short time and then release it to power small electronics.
By extracting energy from both movement and perspiration the accessible microgrid will allow power devices quickly and continuously. The triboelectric generators deliver power immediately as soon as the user begins to move, before breaking a sweat. As soon as the user starts taking a shower, the hybrid cells begin to supply power and continue to do so after the user stops moving.
“When you put those two together, they make up for each other’s shortcomings,” Yin said. “They are supportive and consensual to enable quick start-ups and sustained power.” The whole system boils twice as fast as just the dual-fuel cells, and lasts three times longer than the triboelectric generators alone.
The wearable microgrid was tested on a subject in 30-minute sessions consisting of 10 minutes of exercise on a cycling or running machine, followed by 20 minutes of breathing. The system was capable of powering an LCD watch or a small electrochromic display – a device that changes color in response to activated voltage – throughout each 30-minute session.
Greater than the sum of its parts
The biofuel cells are equipped with an enzyme that stimulates the exchange of electrons between lactate molecules and oxygen in human sweat to create electricity. Wang’s team first described these sweat-harvesting wearables in a paper published in 2013. Working with colleagues at the UC San Diego Center for Wearable Sensors, they later updated the technology to become easy to stretch and powerful enough to run small electronics.
The triboelectric generators are made of a well-charged material, placed on the forearms, and a well-charged material, placed on the sides of the torso. As the arms move against the torso while walking or running, the opposite materials rub against each other and generate electricity.
Each wear provides a different type of power. The dual-fuel cells provide continuous low voltage, while the triboelectric generators deliver high voltage pulses. In order for the system to power machines, these different voltages must be combined and regulated into a single constant voltage. That’s where the supercapacitors come in; they become a reservoir that stores the energy from both power sources for a short time and can release it as needed.
Yin compared the establishment to a water supply system.
“Imagine that the dual-fuel cells are like a slow-flowing faucet and the triboelectric generators are like a hose that burns out water jets,” he said. “The supercapacitors are the tank they both feed into, and you can draw from that tank however you need.”
All parts are attached with flexible silver interlayers which are also printed on the shirt and covered with a waterproof cover. The performance of each part is not affected by re-bending, folding and crimping, or water washing – as long as a vacuum cleaner is not used.
The main innovation of this work is not the accessible devices themselves, Yin said, but a systematic and efficient integration of all the devices.
“We don’t just combine A and B and name it as a system. We chose parts that have corresponding shape factors (everything here is printable, flexible and accessible); matching performance; and complementary functionality, meaning they are all useful for the same situation (in this case, a hard move), “he said.
Other applications
This particular system is useful for gymnastics and other cases where the user is exercising. But this is just one example of how the wearable microgrid can be used. “We are not limiting ourselves to this design. We can change the system by choosing different types of energy harvesters for different situations,” Yin said.
The researchers are working on other designs that can harness energy while the user is sitting inside an office, for example, or moving slowly outside.
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Paper: “Self-sustaining multi-mode wearable biomass microgrid system.” Co-authors include Kyeong Nam Kim *, Jian Lv *, Farshad Tehrani, Muyang Lin, Zuzeng Lin, Jong-Min Moon, Jessica Ma, Jialu Yu and Sheng Xu.
* These authors contributed equally to this work.
This work was supported by the UC San Diego Center for Wearable Sensors and the Korea National Research Foundation.
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