Back in around 2008 at the University of Utah I participated in a study someone was doing about detecting touches on any surface using a camera. When you touch a surface, the blood under your fingernails moves, and that movement can be detected with a camera and turned into a usable signal. I think they were even able to get force and direction as well. It was very interesting, but I haven't seen or heard anything about that method since.

I did, and still do, think it's a great idea.