parasitic sensors

The term 'parasitic' in this context refers to the device's ability to obtain the necessary power from something else in its environment.

A parasitic sensor in a building could have, for example, a battery which is charged by proximity to wiring, charging itself off electrical fields produced by ac current.

A parasitic sensor that is worn or implanted on your body could run on piezoelectric charges generated by your movements.

Vibration is another source of power for small aps like sensors.

Try a search for 'energy scavenging technology'.