This week, researchers announced that infertile mice were able to give birth after being implanted with artificial 3D-printed ovaries, reports Ian Sample at The Guardian.

“Our hope is that one day this ovarian bioprosthesis is really the ovary of the future,” Teresa Woodruff at Northwestern University and an author of the study in the journal Nature Communications tells Sample. But, researchers caution, such procedures for humans are still a long way off.

As Katherine Kornei at Science reports, the researchers used a 3D printer to build the scaffolding of the organs, weaving layers of gelatin to create tiny (15 x 15 millimeter) ovaries on glass slides. They then tested the scaffolds by embedding a follicle—the tiny sacs composed of hormone-secreting cells that contain the maturing eggs.

This test suggested that the tightest weave supported the highest survival rates, reports Kornei. So the researchers punched tiny circles out of the tightly woven structures and stocked the ovaries with 40 to 50 follicles. Then they replaced seven mice's natural ovaries with the bioprosthetic version.

The follicles on the scaffolding were able to hook up with the blood supplies of the mice within a week, and the ovaries eventually released eggs, reports Sample, just like natural ovaries. Researchers allowed the mice to mate; out of the seven mice that received the ovaries, three gave birth, producing healthy offspring, Kornei reports. The mouse mothers also lactated normally, a sign that the follicles in their ovaries were producing the correct amount of hormones.

The system relies on a special gelatin or hydrogel, which was engineered to be strong enough to be handled during surgery, but also porous enough to allow the eggs to pass through.

“Most hydrogels are very weak, since they're made up of mostly water, and will often collapse on themselves,” says Ramille Shah, a material scientist and author of the study, in a press release. “But we found a gelatin temperature that allows it to be self-supporting, not collapse, and lead to building multiple layers. No one else has been able to print gelatin with such well-defined and self-supported geometry.”

Susan Scutti at CNN reports that the researchers were actually surprised that the ovaries worked the first time around. Now they are interested in building an ovary version 2.0 that has different size pores that can hold follicles at different stages of maturity.

“The goal of the project is to be able to restore fertility and endocrine health to young cancer patients who have been sterilized by their cancer treatment,” Woodruff tells Sample. Regaining naturally occurring hormones would help the patients enter puberty and also has benefits for the bones and cardiovascular health.

But getting to that point is still a long way off. Human ovaries are much larger than mouse ovaries and human follicles develop more rapidly. The scaffold will also need to host more and larger blood vessels—a problem that faces attempts to 3D print all types of artificial organs and tissues, Nicolas Sigaux, a surgeon who works with 3D-printed materials at Lyon-Sud Hospital Center in France tells Kornei.

The researchers now plan to implant artificial ovaries in mini-pigs, Nick Stockton reports for Wired, since they can handle larger bioprosthetics and have a menstrual cycle that is more similar to humans than mice.