There was an idea kicking round at the turn of this century :), that an American company was trying to develop a digital “cartridge” incorporating sensor, memory and batteries, which would fit inside the back of any 35mm slr by taking up the space usually occupied by the film cassette and the spool of exposed film.

This would have converted the 35mm slr to a digital camera at a stroke.

Interesting idea which was touted round between 1997 (ish) and 2000, but never appeared because, i believe, of ongoing technical difficulties and the falling price of digital cameras making it progressively unfeasable to produce at an affordable price.

As you can now get an eos 400 body for £375, i dont think we will ever see such a device come to market.