In Doris Taylor's cell- and molecular-­biology lab at the University of ­Minnesota, a small pink heart beats in a glass chamber amid a complex of tubes. With each twitch, the heart's bottom tip traces a small curve in space, and pink nutrient solution flows out through the aorta. Remarkably, this living heart was grown in the lab.

Taylor directs the university's ­Center for Cardiovascular Repair, where her team has created bio­artificial hearts using a novel approach in which animal hearts act as scaffolds. The researchers begin with a rat or pig heart and chemically wash away its cells. What remains is the extracellular matrix, a complex of carbohydrates and proteins that preserves the intricate structure of chambers, valves, and blood vessels. The researchers add heart cells harvested from a newborn animal and incubate the organ in a bioreactor, which provides physio­logical cues like pressure and electrical stimu­lation. Soon, the heart begins to beat weakly on its own.