What about a head?
For head (or other organ) artificial alive-keeping we still don't know enough about how they work - and in case of especially the head we don't have the adequate technology yet to properly reconnect the neurons.
It might be that we can do this sooner than keeping the full head alive though, given that there is way more investment in such research to help people with spinal injuries regain motor control.
Oxygenated, nutrition loaded blood mainly. But I'm sure that misses 8 unsolvable essentials...
Preserving sanity for the severed head is a whole other story.
But it's at least an understandable and defined problem.