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"As fascinating as it is, there are problems," he says. With the heat, electricity and spraying, "an inkjet printer isn't a very friendly environment." And, in fact, a lot of the bacteria Clemson researchers used died in the printing process. Much of the published research about using inkjets has been to demonstrate that cells can survive the printing, Forgacs says, "that in principle it is possible."

The method also is very slow. Inkjets spit out single cells, but organs consist of millions of cells. So Forgacs uses a bioprinter that delivers millions of cells at a time. To make blood vessels, he cultures the three primary types of cells that make up the cylindrical organs: connective tissue cells called fibroblasts that comprise the outer layer, muscle cells that form the middle layer, and the endothelial cells that line the vessels. Then Forgacs combines the cells in the right proportions and forms them into tiny spheres, each about half a millimeter or less in diameter and containing 10,000 to 30,000 cells.

These spheres, Forgacs' "bio-ink," are packed into micropipettes and loaded into the bioprinter's printer head. Before printing starts, the printer lays down a gel film, the "bio-paper" that will accept the bio-ink. Once the gel sets, the printer pumps out individual spheres, making a circle about three millimeters in diameter. Then it lays down another sheet of bio-paper on top and prints another circle of spheres.

The whole process takes about 15 minutes. "We've managed to print six or seven layers at a time," Forgacs says. "But there is really no limit to it."

The stack of printed sheets matures in a "bioreactor," an incubator set to the right temperature and humidity and providing fluids pumped at the right pressure to mimic the environment of a blood vessel. In the bioreactor, nature begins to work its magic. Over the course of about a week, the individual spheres of cells slowly fuse together to form a solid tube.

But do these cells form something comparable to an organ? Forgacs found that indeed they do. When he printed the chicken heart cells, the beating of the cells was unsynchronized at first. But as the spheres melded, the cells began to beat in unison. This experiment demonstrated that, once these spheres of cells merge, they behave just like the tissue they came from.

Just as extraordinary, the cocktail of cell types in the spheres turns into the distinct tissues of a blood vessel. The fibroblasts migrate to the exterior of the cylinder, the endothelial cells travel to the interior walls, and the muscle cells find their way in between. "The cells know what they are supposed to do," Forgacs says. "This is nature. We just use what nature can do."

To build longer vessels, Forgacs has begun printing cylinders lengthwise, laying down parallel lines of cell spheres that widen and narrow as the printer goes up from one sheet to the next. Ever the physicist, Forgacs is engineering his blood vessels so that their physical properties are close to those of the real thing.

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