Thanks to advances in tissue engineering, doctors can now replace burned skin or worn out cartilage with lab-grown replacements. But engineering larger body parts, such as desperately needed livers, remains out of reach-in part because there's no way to keep these tissues fed with blood. Although a patient's own blood vessels can penetrate and sustain a skin graft, they can't grow fast enough to keep a mass of liver cells alive. "You get cell death before the vessels arrive," says pediatric surgeon and tissue engineer Joseph Vacanti of Massachusetts General Hospital.

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