Lately, I've heard about a fascinating new technology called DNA Origami, where incredibly tiny machines are folded out of DNA. Using CAD software they are able to build all sorts of interesting things, not the least of which are floating robots small enough to deliver drugs to specific cells, upon receiving a radio signal.
There is a talk about these robots up on youtube.
That got me to wonder, what is the size limitation on building radio transmitters? What factors constrain their size? And could one be built that was the size of a cell? Would it be able to communicate outside the body?
I'd really like to learn more about this!