To improve care for extremely premature newborns, researchers have been experimenting with technology that could act as an artificial placenta and mimic the womb environment to give delicate lungs more time to develop.

In the latest effort, doctors at Children’s Hospital of Philadelphia reported Tuesday that they kept premature lambs alive in a bag of fluid for longer and with better health outcomes than in previous artificial-womb experiments.

Advancements in medical care mean human babies born as early as 22 to 23 weeks of gestation have a chance of survival, but the lungs of babies delivered at this stage aren’t developed enough to breathe air and can be damaged by current methods of ventilation. The doctors envision their invention could one day support infants born at 23-25 weeks until 28 weeks when their organs are further developed and survival odds are better.

“We’ve developed a system that as closely as possible reproduces the environment of the womb and replaces the function of the placenta,” said Alan Flake, director of the Center for Fetal Research at Children’s Hospital of Philadelphia, who led the research reported in Nature Communications.

The system uses a translucent polyethylene bag the researchers dubbed the “Biobag,” filled with liquid similar to amniotic fluid. The premature lambs were sealed in the bags and connected via their umbilical cord vessels to a blood-oxygenation system, which acted as an artificial placenta.