A team of university researchers has developed a wearable device that generates energy when the user swings their arm.
Harvesting energy while the wearer is either walking or jogging, the device is about the size of a wristwatch and can, apparently, produce enough power to run a personal health monitoring system.
The researchers behind the device come from Pennsylvania State University's Materials Research Institute and the University of Utah. They said that such energy harvesting devices are in high demand as they could power the millions of devices that make up the internet of things .
For example, by providing continuous power to a rechargeable battery or supercapacitor, energy harvesters could reduce the labour cost of changing batteries when they fail, and keep dead batteries out of landfills.
"The devices we make using our optimized materials run somewhere between five and 50 times better than anything else that's been reported," said Susan Trolier-McKinstry, the Steward S. Flaschen Professor of Materials Science and Engineering and Electrical Engineering, Penn State.
The prototype watch is apparently made from a crystal material, which can produce an electric current when compressed or change shape when an electric charge is applied. This is called the "piezoelectric effect" and is used in ultrasound and sonar devices, as well as energy harvesting.
In development of the watch, Trolier-McKinstry and her team used a well-known piezoelectric material called PZT, and coated it on both sides of a flexible metal foil to a thickness four or five times greater than in previous devices.
In this case greater volume of the active material equates to generation of more power, so orienting the film's crystal structure, the figure of merit for energy harvesting was increased. The compressive stresses that are created in the film as it is grown on the flexible metal foils also means that the PZT films can sustain high strains without cracking, making for more robust devices.
"There were some good materials science challenges," Trolier-McKinstry added.
"The first was how to get the film thickness high on a flexible metal foil. Then we needed to get the proper crystal orientation in order to get the strongest piezoelectric effect."
In future work, the team believes they can double the power output using the cold sintering process, a low-temperature synthesis technology developed at Penn State. The researchers are also working on adding a magnetic component to the current mechanical harvester to scavenge energy over a larger portion of the day when there is no physical activity.
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