Multiple role playing games make use of sub-dermal or dermal armor. This includes fusing plates of material directly to the skin and implanting material below the derma.
I see huge problems with all of this. Placing plates under the skin would likely end in migration. That is, the material would move out of place as skin, facia, other tissues refresh themselves. Anchoring this stuff in place would be a nightmare unless it is screwed into or simply coats bone -- coating the bone would be a massive problem too the least of the reasons being disruption to calcium storage. I can't figure out how it would it would lay between layers without disrupting the nerves and vasculature that inundadate the skin layers.
So let's assume that this is 300-400 years in the future. Therapeutic cloning exists -- specific organs can replace complex who organs. Organ rejection is a thing of the past. Nanites far exceed our red blood cells in their ability to deliver oxygen, they can deliver drugs directly the sites of action, and can even speed up thought. Genetic engineering has wiped out cancer and it is possible to increase the density of bone significantly as well as let you run for miles without exhaustion.
Is there a way to make implanted armor concept work?
When I say armor I don't mean something that could stop a 7.62 or 5.56 mm round. I mean small handgun (32-caliber) at point blank. The wearer would need to retain 90 - 90% of movement. However the aromor would not need to be concealable.