I would agree that there is alot of research going on in medical devices but I will warn you that the circuits that I have been seeing for implants are very simple. All of the research is really in the signals communicating with the body and there reaction. I would say MOST of the circuits will not be ground breaking, which I would think is not what you really want to showcase your PhD! I would go into converters, like pipeline and CT sigma delta ADCs. There is still alot going on in this field and after finishing your PhD you will know how to design alot of stuff! I guess it really depends on what you are looking for after you PhD, working in medical (do medical implants) , in semi's (converters), or being a professor ( you could do either).
Also you should do what your professor(supervisor) is into. If you join a department with the analog professor and do your PhD in digital, this is a very very bad idea!!!!
Hope this helps,
Jgk