Raymond Kurzweil, the person who pioneered optical character recognition (OCR), text-to-speech synthesis, speech recognition technology, and electronic keyboard instruments (from Wikipedia.), and considered one of the most intelligent people in terms of predicting future technology is saying 2-3 decades nano-technology will be advanced enough to fix our bodies and play the roles of some of our cells.
This guy isn't a joke either: "Kurzweil isn't writing science fiction. In fact, Microsoft's Bill Gates, a robotics director at Carnegie Mellon University, an MIT professor, and a physicist have all endorsed his book. He has received the National Medal of Technology and the Lemelson-MIT prize."