I only spent 2 years studying biology in college before I found my true love IT. I'm an insanely pragmatic person and I Have dabbled into a little bit of backyard engineering I have drawn up a bunch of things on my cad programs, mostly mechanical. But Im still dumbfounded as to why artificial organs are not a bigger thing. I understand the artificial heart is basically on the market and just waiting for a rich industrialist to mass produce the things. But kidneys are essentialy filters are they not? I know some people have done some research into this but It just seems to me that it shouldn't be an issue to have chemical filtration sheets that act in the same way as the human kidney. Samething with livers(although the liver is a much more diverse machine so I can kind of understand that one) But lungs are really simple as well you really just need a membrane that blood can flow across and allow for diffusion of oxygen (maybe even from a portable container) If CO2 is the big problem then you could even look at a carbon scrubber and reprocess the O2
Two hypothesis come to mind immediately
the first being the problem of an autoimmune rejection. I don't consider Immunosuppressant's an answer here. But I could be wrong but I think its completely possible to insert large foreign bodies into the human body without excessive buildup of scar tissue.
The second one being the big problem of microsurgery. From my understanding one of the reason nerve injury surgery is as primitive as it is because theyre are incredibly small blood vessels nerves muscle fibers and a lot of them that all have to be connected with an insane degree of precision to get anything functioning well.