There is abundant energy in the environment. Light, motion, body heat, and RF are examples of energy sources. The challenge is to convert that ambient energy into useable electrical energy to power an application. Technology is reaching the point where practical, useful systems down at the milliwatts or lower range can be made. At that range, it becomes possible to harvest some energy out of the environment. There are the obvious things like utilizing light, like users would get, for example, from the 100 W light bulb shown here. It's interesting to note that a human dissipates about 100 W as well. All that is needed to run a practical application like a body temperature sensor, pulse sensor or some other bio sensor, is to harvest 100 thousandth of the power from a light bulb or the human body.