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.

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.

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

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.