In the story linked below an engineering professor discusses creating electronics small enough to travel through the blood stream. The prototype is not really very small as far as electronic things go. It looks to be a single IC on a 3mm x 4mm PCB, so the story is a bit on the vaporware side of things. But the idea of powering small electronic devices with electromagnetic fields is interesting. This is how some early RFID tag (radio frequency identification) systems were designed to work. You move the RFID through an energized loop antenna (for example something around the perimeter of a warehouse door) and the field creates electron flow in a small antenna on the RFID. This powers the RFID and initiates an exchange of information again using the antennae.
The idea always struck me as elegant, in part because no battery was on the RFID tag. You can imagine the RFID connected to products or pallets moving through a factory, and the location or assembly state being reported to a central database whenever the tag moves through a portal. Trucks, boats, and cargo aircraft could be configured to read the RFID tags. I’m sure technology has changed quite a bit, and I’m also sure companies like UPS and FedEx have figured out better ways to do this (bar codes ). But I still fancy the idea.
Anyway here’s the story, if you are also intrigued..