Need a skin graft? A new trachea? A heart patch? Turn on your printer, and let it spit one out.

A group of researchers hope printers' whirs and buzzes will soon be saving lives.

Led by University of Missouri-Columbia biological physics professor Gabor Forgacs and aided by a $5 million National Science Foundation grant, researchers at three universities have developed bio-ink and bio-paper that could make so-called organ printing a reality.

So far, they've made tubes similar to human blood vessels and sheets of heart muscle cells, printed in three dimensions on a special printer.

"I think this is going to be a biggie," said Glenn D. Prestwich, the University of Utah professor who developed the bio-paper. "A lot of things are going to be a pain in the butt to print, but I think we can do livers and kidneys as well."

Prestwich guessed initial human organ printing may be five or 10 years away.

The work started as a way to understand biological self-assembly -- such as how an embryo develops -- in the lab, Forgacs said.

While printed DNA and RNA chips have been around for a while, they have until now been printed in two dimensions, Forgacs said. Also, organ printing scientists have figured out how to print not just molecules, but clusters of cells, he said.

Here's how it works: A customized milling machine prints a small sheet of bio-paper. This "paper" is a variable gel composed of modified gelatin and hyaluronan, a sugar-rich material. Bio-ink blots -- each a little ball of cellular material a few hundred microns in diameter -- are then printed onto the paper. The process is repeated as many times as needed, the sheets stacked on top of each other.

Once the stack is the right size -- maybe two centimeters' worth of sheets, each containing a ring of blots, for a tube resembling a blood vessel -- printing stops. The stack is incubated in a bioreactor, where cells fuse with their neighbors in all directions. The bio-paper works as a scaffold to support and nurture cells, and should be eaten away by them or naturally degrade, researchers said.

Though it can take less than two minutes to print a sheet of bio-paper with bio-ink, it can take about a week for such a tube to fuse, Forgacs said.

It's currently feasible to print tubes, Prestwich explained, because the printers output bio-paper in a sort of ever-ascending spiral, like a Slinky.

Helen Lu, director of the Biomaterials and Interface Tissue Engineering Laboratory at Columbia University, thinks organ printing could eventually work. Still, she cautioned that scientists must determine additional details such as how blood vessels are formed in skin, because simply implanting them might not be optimal.

The researchers are aware of the difficulties they face; Forgacs didn't even want to guess at the technology's possibilities.

"There are so many questions at this point to tackle, even at the simpler level, that I really don't want to break my head over what kind of organ we would build," he said.