Researchers like the University of Missouri's Gabor Forgacs who have worked in bioprinting see these findings as an interesting though preliminary application. "What is important about this work is they show in vitro that by changing the pattern or the concentration of this [growth factor], cells respond differently and choose their lineages according to the concentration," says Forgacs. "Controlling stem-cell specification is very important, and we're still at the beginning of this endeavor."

Ink-jet printing may ultimately open up a whole new way of studying stem cells. Lee Weiss, a research professor at Carnegie Mellon's Robotics Institute who helped design the custom printer, says the technology is there; the question is how to better use it.

"I can print fairly complex things," says Weiss. "But probably one of the biggest limiting factors is understanding the biology in order to know what to print." Weiss hopes to eventually print specific patterns of growth factors that can then either be combined with stem cells in vitro or implanted directly into damaged regions to create new tissue.

To that end, researchers are now exploring even more complex patterns and printing with other growth factor-based "inks," all with a view toward tissue therapy. Campbell adds that to take this to the next level, they are also working on 3-D patterning, which may be used to direct stem-cell transplantation. For now, Campbell is looking to experiment with other types of stem cells, including those found in human adults.