MIT researchers have used cells harvested from rats to develop tissue capable of beating like a heart. It grows to dime size and with electrical stimulation its capable of twitching like a beating heart. They are trying to graft this to an actual rat heart. Looking forward to the day that this can be used with a diseased human heart.
http://www.boston.com/news/science/articles/2004/12/14/mit_grows_beating_heart_tissue/
http://www.boston.com/news/science/articles/2004/12/14/mit_grows_beating_heart_tissue/