When one of Proteus’s pills is taken, stomach fluids activate the edible communications device it contains, which sends wireless signals through the body to another chip worn as a skin patch or embedded just under the skin. That, in turn, can upload data to a smart-phone or send it to a doctor via the internet. Thus it is easy to make sure a patient is taking his pills at the right time, to spot adverse reactions with other drugs and so on.

Found by Keith Gibson.