Put it this way: one day everything in our body we’ll be able to replicate and improve. For now though we can only make simple things like semi-autonomous prosthetics or neural-prosthetics (controlled by your thoughts).
I’m not really sure – new things are getting developed every day. What do you consider to be robotic body parts? There are now artificial heart pumps and things like nerve controlled prostheses – as Neil says.
Perhaps it’s an area you can help to develop!
I’m not sure on the actual answer to this, but as well as artificial limbs researchers are now working on artificial organs and even artificial blood that could be classified as robotic. There’s a Channel 4 programme that talks all about this:
It’s important to note that only 2 of these are able to be used by people as replacements for damaged parts – the bionic eyes and exoskeleton, numbers 6 and 8 – but it’s interesting to see what researchers in robotics are developing!