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.

Now Vacanti and his partners in the Center for Innovative Minimally Invasive Therapy-a Boston-area research consortium-may have solved that problem with a primitive circulatory system grown on a silicon wafer. The team first used photolithography to etch a template of interconnected veins, arteries and capillaries onto the wafer. Then they seeded the template with vascular cells from a rat lung, which spontaneously grew to coat the grooves, breeding a network of blood vessels. Vacanti's team then showed fluids could pass through the vessels.