Novel wearable device harvests energy from body movements

Scientists have developed a wearable device that can generate energy from the swing of an arm, paving the way for personal health monitoring systems that do not need traditional batteries. "The devices we make using our optimised materials run somewhere between five and 50 times better than anything else that's been reported," said Susan Trolier-McKinstry, from Pennsylvania State University in the US. Energy-harvesting devices are in high demand to power the millions of devices that make up the internet of things.

By providing continuous power to a rechargeable battery or supercapacitor, energy harvesters can reduce the labour cost of changing out batteries when they fail and keep dead batteries out of landfills. Certain crystals can produce an electric current when compressed or they can change shape when an electric charge is applied. This piezoelectric effect is used in ultrasound and sonar devices, as well as energy harvesting.

Researchers used a well-known piezoelectric material, PZT, and coated it on both sides of a flexible metal foil to a thickness four or five times greater than in previous devices. Greater volume of the active material equates to generation of more power. By orienting the film's crystal structure to optimise polarisation, the performance -- known as the figure of merit -- of 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. In future work, the team believes they can double the power output. They 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.