BME founder Shannon Larratt was interviewed by io9 about the grinder/biohack movement:
Making a wristwatch implant would actually be quite simple. The electronics need to be as small as possible of course. Even though implants can be quite large (a single double-D breast implant has more volume than many laptop computers at this point), if the implant is kept thin it will be inconspicuous, perhaps even undetectable without touching it. So the wristwatch would be built with surface mount components in a tight package. The LEDs would easily be visible through the skin — it’s quite possible that some small backlit panels could be visible through the skin but simple round or bar-shaped LEDs would be my choice for a watch.
One could do a numeric display, a geeky binary display, or even just use a single light and flash the time with morse code. You’re probably not going to leave the light on all the time in order to preserve the battery, but triggering could be accomplished in many ways. An accelerometer could be used to trigger it with a specific arm motion, a pressure switch could respond to touch, or in my case, or a magnetic switch could respond to me waving my finger over it — there are many options, but whatever is chosen would have to be versatile enough to also allow the time to be set.
Finally — and this is the biggest issue — there’s power. You could have yourself cut open have the battery replaced — but there’s no need for that. Inductive charging is easy to build, and wireless chargers are commonplace these days — personally I would include such a circuit.
Update: Here’s the full, uncut transcript of the interview