I once watched a special on Dr. Roger Leir, a doctor made famous for removing supposed ET implants. Whether they were as such or not isn't important for this question. What is important is that he documented the removal of several devices that were at least partially made of material that tissue would somewhat grow into so that the body wouldn't reject it.
The question is: what, if any, such materials exist? Of course what the body won't reject and what it will grow into are two different things. I'm interested in both. It would also be helpful to know where people tend to discuss this and what companies are working on it. This is a question of biology and material science.
When I think of materials that won't be rejected, the only thing that comes to mind are materials made out of tissues themselves (like cat guts). Of course, we have things like pacemakers and all manner of implants but I don't know to what degree the body accepts them. Keep in mind that devices wouldn't have to be made entirely of such materials, just the outer shells of them.