Nanoengineers have developed a wearable microgrid that harvests and stores energy from the human body to power small electronics. It consists of three main parts: sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy-storing superca-pacitors. All parts are flexible, washable, and can be screen-printed onto clothing.

The wearable microgrid is built from a combination of flexible electronic parts. Each part is screen-printed onto a shirt and placed in a way that optimizes the amount of energy collected. Biofuel cells that harvest energy from sweat are located inside the shirt at the chest. Devices that convert energy from movement into electricity, called triboelectric generators, are positioned outside the shirt 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 outside the shirt 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 triboelectric generators provide power right away 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. The entire system boots two times faster than having just the biofuel cells alone and lasts three times longer than the triboelectric generators alone.

The wearable microgrid was tested on a subject during 30-minute sessions that consisted of 10 minutes of either exercising on a cycling machine or running, followed by 20 minutes of resting. 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.

The biofuel cells are equipped with enzymes that trigger a swapping of electrons between lactate and oxygen molecules in human sweat to generate electricity. The 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. As the arms swing against the torso while walking or running, the oppositely charged materials rub against each and generate electricity.

The wearable microgrid uses energy from human sweat and movement to power an LCD wrist-watch and electrochromic device. (Photo: Lu Yin)

Each wearable provides a different type of power. The biofuel cells provide continuous low voltage while the triboelectric generators provide pulses of high voltage. For the system to power devices, these different voltages need to be combined and regulated into one stable voltage. That's where the supercapacitors come in, acting as a reservoir that temporarily stores the energy from both power sources and can discharge it as needed.

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

This particular system is useful for athletics and other cases where the user is exercising. But this is just one example of how the wearable microgrid can be used. The system can be adapted by selecting different types of energy harvesters for different scenarios. The researchers are working on other designs that can harvest energy while the user is sitting inside an office, for example, or moving slowly outside.

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