Here's a short description of the process from an NPR interview done by host Guy Raz with Dr. Gabor Forgacs, a scientist at the University of Missouri:
Dr. GABOR FORGACS (Scientist, University of Missouri): You scoop out cells from the patient.
RAZ: So if you want a new heart, some cardiac cells, if you want a new stomach, some stomach cells, and so on. And he takes this cluster of cells.
Dr. FORGACS: That may contain anything between 10 to 30,000 cells.
RAZ: And he mixes it into a liquid, something Gabor Forgacs calls bio-ink, and just like in the printer connected to your computer, this bio-ink shoots out of a cartridge. And it's printed, dot by dot, onto a gelatin-like sheet of paper, or what he calls bio-paper.
Dr. FORGACS: It is a material that mimics what we have in our body between the organs, that surrounds the organs. It's called the extracellular matrix. Cells love it.
RAZ: And when placed together on the bio-paper, the bio-ink, those cell clusters, starts to fuse and form shapes, but at this point, the printout is still two-dimensional. So another sheet of bio-paper is layered right on top with another cluster of bio-ink.
The principle is a little like building a skyscraper. You start with the bottom level, then build up.
Dr. FORGACS: Then imagine that comes the second story, and then comes the third story.
RAZ: And on and on until you have something that starts to look like an organ. Forgacs' printer is connected to a computer that then directs how those layers should be shaped, a predesigned scheme a little like paint-by-numbers.
Dr. FORGACS: And that scheme you can get by taking an X-ray or a CT image of the organ, and you try to repeat the outline of the organ. Of course, it's very complicated, but we have now the precision to place the cells according to this scheme and end up with a three-dimensional object.