Shaping up: A polymer scaffold (purple) encourages heart cells grown on it (green) to align themselves in the same direction. Credit: Engelmayr et al., Nature Materials 2008

Heart patches grown from stem cells on biodegradable scaffolds could one day help treat heart attacks or congenital heart problems. But heart tissue is a mechanical marvel that's hard to mimic. Any substitute must be flexible enough to permit the contractions that drive blood through the body but strong enough to withstand them.

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