Brain only required

The human brain was the only thing they couldn't replace, or hadn't replaced yet.

The first line in my new story, or the story I haven't written yet but resides in my head. This story is about some mid-future (as opposed to far-future) timeframe in which humans have, through the process of time, replaced all body parts including head, with mechanics, all but the brain. Also evolved is the physical body in which the brain resides. I'm sure this idea has been done before. I may have even read about it somewhere and this 'idea' is only a regurgitation of the original, long dormant and out of context. If someone does know of a source where this idea was written about, do let me know. And I don't mean CS Lewis' version, where a disembodied head is kept alive by scientists.

I'm thinking of piggybacking off my wife's site and putting the full text there. Once it's written of course.