Here’s an article about a researcher named John Fourkas who is using resin like your dentist uses to do transparent fillings and a very focused laser to build up really tiny structures. He can build up the letters spelling "hair" on top or a single human hair, for instance.
"Gee whiz," you’re thinking, "But, so what?" Here’s where the guy is going with this:
"So you start thinking, ‘Can I build microscopic devices that first of all might be able to monitor what’s going on in a biological tissue? Might I be able to look at somebody’s blood flow or test their glucose level with a really tiny sensor that’s embedded in their skin?’ In the long run you might even think about doing something like implanting things like this on a cancerous tumor and having it help to control the chemotherapy agents, and if it senses that the tumor is growing or not shrinking fast enough, adjust what’s happening to the tumor."