Walking could soon power mobile devices: a group of British researchers has created an energy harvester that generates electricity thanks to the motion of your knees. The device could eliminate the need for carrying batteries, an advantage especially important for the military, where 22 pounds of a soldier's backpack is made up of batteries alone.

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The device fits on the outside of the knee. It consists of an outer ring and central hub. The ring rotates as the knee joint goes through a walking motion. The outer ring has 72 guitar-pick-like teeth that "pluck" four energy-generating arms attached to the inner hub. As each tooth deflects off one of the arms it causes it to vibrate, like a guitar string, generating the electrical energy.


The "strings" are made of a piezoelectric material, which generates current when it's under strain. Piezoelectric materials are common in many electronics, especially sonar and microphones. They are also part of the sparkers used to light gas stoves.

The energy harvester picks generate about two milliwatts. That isn't enough to be much use, but the researchers think that with a few improvements it could get to 30 milliwatts. That's enough for some types of GPS tracking, though it's only a small fraction of the watt or so an iPhone uses when idle.

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The harveseter was tested with a simulated knee that reproduced the way a human walks. A person was fitted with reflective markers and a motion capture system was used to record the patterns of movement both without a load and with one — a backpack, in this case. Three different weights were used to see what the knees motions were like and how much energy they generated.   

The device will be presented June 15 in the journal Smart Materials and Structures by researchers from Cranfield University, The University of Liverpool and University of Salford.

The lead author of the study, Dr Michele Pozzi, estimated the cost of each unit at about $15 each, once they are mass-produced.

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