Scientists have developed a novel wearable device that could generate energy from the swing of an arm while walking or jogging. The device, about the size of a wristwatch, produces enough power to run a personal health monitoring system, according to the research published in the journal Advanced Functional Materials. "The devices we make using our optimised materials run somewhere between 5 and 50 times better than anything else that's been reported," said Susan Trolier-McKinstry, a professor at 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, researchers said. 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, they said. 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. Trolier-McKinstry and her former doctoral student, Hong Goo Yeo, 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. Researchers at the University of Utah and in Penn State designed the novel wristwatch-like device that incorporates the PZT\/metal foil materials. The device uses a freely rotating, eccentric brass rotor with a magnet embedded, and multiple PZT beams with a magnet on each beam. When the magnet on the rotor approaches one of the beams, the magnets repel each other and deflect the beam, plucking the beam in a process that is referred to as frequency up-conversion. The slow frequency of a rotating wrist is converted into a higher frequency oscillation. The design of this device is more efficient than a standard electromagnetic harvester - like those used in self-powered watches - according to Trolier-McKinstry.