Has anyone yet figured out how sensory signals for eyes and ears are encoded by our organ into electrical signals?
I think the encoding is quite well understood, and in fact there are artifical cochleas available for deaf people: https://en.wikipedia.org/wiki/Cochlear_implant
For retina replacements, I think the hurdle is establishing the connections from any device to the nerve.  There are roughly 120 million rods and 6 million cones in the human eye. (Per Wikipedia: https://en.wikipedia.org/wiki/Photoreceptor_cell )  These connect to about 1.2 million optic nerves, so making the proper connections would seem to be a non-trivial matter.