If you’ve ever worn a knee brace, then you may have noticed what a large change in angle your knee goes through with every step you take, and how quickly it does so. A team of scientists from the U.K.’s Cranfield University, University of Liverpool and University of Salford certainly noticed, and decided that all that movement should be put to use. The result is a wearable piezoelectric device that converts knee movement into electricity, which could in turn be used to power gadgets such as heart rate monitors, pedometers and accelerometers.

Known as the pizzicato knee-joint energy harvester, the device fits onto the outside of the knee. It is circular, and consists of a central hub equipped with four protruding arms, surrounded by an outer ring bearing 72 plectra (a plectrum is a plucking tool, such as a guitar pick). The ring rotates about a quarter of a turn with every bend of the knee, causing the plectra to pluck the arms. This causes the arms to vibrate (not unlike a guitar string), and it’s those vibrations that are used to generate electrical energy.

The device was tested on a knee motion simulator rig. In order to make sure that the rig accurately simulated a typical human gait, the researchers first used a motion capture system to analyze the walking patterns of a human subject, who was wearing strategically-placed reflective markers.

The gait of that person was also analyzed when wearing backpacks of three different load weights, in order to see how carrying extra weight might affect their knee movements. This was an important consideration, as the energy harvester might end up being used to ease the burden of soldiers, who are currently heavily-laden with electronic devices and their accompanying batteries.

So far, the device has been able to harvest about two milliwatts of power. The researchers, however, believe that it should be a relatively easy to improve its performance to the point that it is able to provide at least 30 milliwatts – this ought to be enough to power a GPS tracking system, and to allow for advanced signal processing electronics, plus more frequent and longer wireless transmissions.

Cranfield University’s Dr. Michele Pozzi, who is the lead author of a paper on the project, is now part of another team that is looking into the manufacturing of a more compact, cost-effective version of the energy harvester. He estimates that the production model may cost under £10 (US$15.57) per unit.

Pozzi’s paper was published today, in the journal Smart Materials and Structures.

This isn’t the first time that people have looked at harvesting power from the knees. A few years ago, industrial designer Kyle Toole built a prototype device that generated electricity not from the bending of the knee, but from the shock forces that traveled up to it from heel strikes against the ground.

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