NY Times article discussing one approach to harvesting human waste energy for powering personal devices

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Move Your Body, Power Your Cellphone

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By HENRY FOUNTAIN

Published: February 26, 2010

It may not seem like it, but even the laziest of couch potatoes is a human dynamo. The act of breathing — of moving the ribs to draw air into the lungs and expel it — can generate about a watt of power. And if the potato actually gets up off the couch and walks briskly across the room, each heel strike can produce even more power, about 70 watts’ worth.

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Frank Wojciechowski

A flexible electricity-producing sheet of rubber that can be implanted in the body.

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That energy could be put to work, charging a cellphone, say, or a medical sensor in the body. The problem is how to harvest it.
Michael C. McAlpine of Princeton and colleagues have developed a promising approach for converting body movements into electricity: printing piezoelectric crystals onto flexible, biocompatible rubberlike material.
Piezoelectric crystals produce an electric current when bent and have many uses — the igniter on a gas barbecue grill being one of them. But highly efficient crystals of the kind that might be useful in the body are made at high temperatures that would destroy most plastics or rubbers.
The solution developed by Dr. McAlpine and colleagues, which is described in the journal Nano Letters, is to first make the crystals, in a series of narrow ribbons, on a rigid substrate of magnesium oxide. Then, after the substrate is etched away from the crystals, they are transfer-printed on a flexible biocompatible polymer, called PDMS.
Dr. McAlpine said his team had started building prototypes, in which tiny wires are deposited on the crystals so that the electricity can be harvested. The crystals are also covered with another layer of PDMS to protect them, and to safeguard the body since the crystals contain lead.
A first application might be in shoes, to produce enough power to keep a music player or phone charged. But the eventual goal would be to make a flexible power generator that could be implanted in the chest or elsewhere.