Implanted active RFID to come?....

Also, as a general rule, the more complex a device is, the more likely it is to break down. A passive tag has no battery and no interaction with the body... it just sits there. That means it can stay operational for 100 years or more, while a chemical device (like the biocell or a catalytic converter in your car) or mechanical device (like a generator) will likely break down and need replacing (just like battery powered devices) eventually. The difference in the working lifespan between a battery powered device and a power cell/generator may be several years... maybe even a decade or more, but eventually it will have to come out and be replaced.

Still, in terms of medical implants, there is a definite need for something like this. I really hope they can get it working reliably, and with minimal danger to the implantee. Odd though to think it will be acting like a parasite... pulling energy and oxygen from your own blood. Patients with poor lung function or diabetes probably could not use such a device.