‘Wearable microgrid’ harvests energy from sweat and movement

Human sweat and movement can power small electronic devices thanks to a new ‘wearable microgrid’ that harvests and stores energy from the body.

Inspired by small-scale community energy projects, the system was developed by nanoengineers at the University of California San Diego.

The wearable microgrid consists of three main parts – sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy-storing supercapacitors. All parts are flexible, washable and can be screen printed onto clothing.

“We're applying the concept of the microgrid to create wearable systems that are powered sustainably, reliably and independently,” said co-first author Lu Yin. “Just like a city microgrid integrates a variety of local, renewable power sources like wind and solar, a wearable microgrid integrates devices that locally harvest energy from different parts of the body, like sweat and movement, while containing energy storage.”

The system was built from a combination of flexible electronic parts developed by corresponding author Professor Joseph Wang’s nanobioelectronics team.

Biofuel cells that harvest energy from sweat were placed inside the shirt, on the chest. The triboelectric generators are on the outside of the top, on the forearms and sides of the torso near the waist. They harvest energy from the swinging movement of the arms against the torso while walking or running. Supercapacitors on the chest temporarily store energy from both devices and then discharge it to power small electronics.

Harvesting energy from both movement and sweat enables the wearable microgrid to power devices quickly and continuously, the researchers said. The triboelectric generators provide power as soon as the user starts moving, before breaking a sweat. Once the user starts sweating, the biofuel cells start providing power and continue to do so after the user stops moving.

“When you add these two together, they make up for each other's shortcomings,” said Yin. “They are complementary and synergistic, to enable fast start-up and continuous power.”

The entire system starts work two-times faster than having just the biofuel cells, and lasts three-times longer than the triboelectric generators alone.

The microgrid was tested on a subject during sessions consisting of 10 minutes of cycling or running 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 colour in response to an applied voltage – throughout each 30-minute session.

Each wearable provides a different type of power – the biofuel cells provide continuous low voltage, while the triboelectric generators provide pulses of high voltage. In order for the system to power devices, the different voltages were combined and regulated into one stable voltage by the supercapacitors.

All of the parts were connected with flexible silver interconnections, which were printed on the shirt and insulated by waterproof coating. The performance of each part was reportedly not affected by repeated bending, folding and crumpling, or washing in water, as long as no detergent was used.

The main innovation of the work was not the cells and generators themselves, Yin said, but the ‘systematic and efficient integration’ of all the devices.

“We're not just adding A and B together and calling it a system. We chose parts that all have compatible form factors (everything here is printable, flexible and stretchable), matching performance, and complementary functionality, meaning they are all useful for the same scenario,” he said.

The system could be used by athletes or during exercise. The researchers are developing other designs to harvest energy while the user is sitting down, or moving slowly.

The research was published in Nature Communications.