Known as the pizzicato knee-joint energy harvester, the circular device fits onto the outside of the knee and consists of an outer ring fitted with 72 plectra and a central hub with four protruding arms. With every bend of the knee the outer ring rotates, causing the plectra to pluck the arms.

This results in the arms vibrating, much like a guitar string, and generating electrical energy, which can then be used to charge devices. The researchers from Cranfield University, the University of Liverpool and University of Salford said the device can currently harvest around two milliwatts of power but could exceed 30 milliwatts with a few realistic improvements.

This could enable a new generation of GPS tracking, more advanced signal processing and more frequent and longer wireless transmission, the researchers said. The device could be particularly useful for soldiers who currently have to carry up to 10kg of power equipment when on foot patrol, or to power body-monitoring devices such as heart rate monitors, pedometers and accelerometers.

The knee is thought to be a suitable mechanism for energy generation, as it has a large change in angle during walking and does so at significant speeds. A device attached to the joint could therefore generate large amounts of power, the researchers claim.

"There is an on-going project looking at manufacturing a more compact and truly wearable harvester," said Dr Michele Pozzi, lead author of the study.