The plastic "biobag" womb is filled with filled with an electrolyte solution which acts like amniotic fluid in the uterus.
Dr Alan Flake, director of the Center for Fetal Research at the Children's Hospital of Philadelphia, US, said: "Our system could prevent the severe morbidity (illness) suffered by extremely premature infants by potentially offering a medical technology that does not now exist".
Premature human babies can suffer irreparable lung damage if they breathe air before the 25th week, not to mention the trauma their bodies can sustain just by being outside the womb too early. They all appeared to grow normally, without any significant changes in their blood pressure and other important health measures.
It's something that wouldn't be out of place in a sci-fi movie - a lamb inside a plastic bag with tubes and fluids helping it grow.
Experts who have carried out the experiment at The Children's Hospital of Philadelphia hope that this success may provide a better chance of survival to premature human babies in a few years. The environment of each artificial womb allowed all of the lamb fetuses to survive and thrive.
The researchers say they hope the technology might prove a breakthrough in improving the prospects for critically premature babies, who face a fight for survival even with heavy medical intervention, and often face serious health complications if they make it.
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In this photo provided by the Children's Hospital of Philadelphia, fetal physiologist Marcus G. Davey of the Children's Hospital of Philadelphia, who helped design the artificial womb system.
"And there are some unpublished reports that these animals have been progressing normally after they've so-called "delivered" them at the end of their experience". It's going to take time before the external womb can be used on humans.
Based on the study, the key to making premature babies survive is to treat them like fetuses inside the womb and extend the gestation period, contrary to caring for them in an incubator.
If the animal results translate into clinical care, Flake said he envisions that a decade from now, extremely premature infants would continue to develop in chambers filled with amniotic fluid, rather than lying in incubators, attached to ventilators.
The latest prototype was tested on five lambs that were surgically removed from their mothers after 105 to 108 days of gestation, when they were similar in development to a 23-week-old human fetus. They are now using the bag on human-sized lambs and tracking the progress of those that survived after being taken off the ventilator, "I think it's realistic to think about three years for first-in-human trials", Flake said. "This could establish a new standard of care for this subset of extremely premature infants".