Is there any work on long-term implantable technology? Not necessarily cranial, just in general as a precursor. As far as I'm aware, the body eventually reject

Is there any work on long-term implantable technology? Not necessarily cranial, just in general as a precursor. As far as I'm aware, the body eventually rejects or forms limiting scar tissue around pretty much all foreign bodies, with only a few exceptions. Because while I am not going to be signing up for sub-cranial implants any time soon, I could possibly be willing to try sub-dermal if there were promising results.