An artificial womb resembling a liquid-filled plastic bag could one day keep premature newborns alive and healthy, medical researchers say.

Key points: The artificial womb mimics a uterus, allowing the foetus to continue breathing oxygen-filled liquid

The artificial womb mimics a uterus, allowing the foetus to continue breathing oxygen-filled liquid The mortality of infants born at 24 weeks is about 50 per cent

The mortality of infants born at 24 weeks is about 50 per cent One expert says the hopes for human trials in two years are "optimistic"

A team at the Centre for Foetal Research in Philadelphia successfully tested a prototype on two lambs and say they hope to have the technology available for humans within two years.

The artificial womb mimics a mother's uterus, allowing the foetus to continue breathing oxygen-filled liquid just as it would in the womb.

One of the leaders of the project, Australian foetal physiologist Marcus Davey, said the breakthrough could one day help improve outcomes for premature babies.

"So many research groups have been trying to develop a system like this since the 1950s, however they've met with limited success," Dr Davey said.

"If we take a look at mortality of infants born at 24 weeks, it's around about 50 per cent.

Medical advances mean premature babies can survive outside the womb to as early as 22 weeks' gestation. ( Flickr: Sarah Hopkins )

"The majority of those infants born at 24 weeks have some sort of illness, and most of the time that is due to lung immaturity."

In recent years medical advances have pushed back the point when premature babies can survive outside the womb to as early as 22 weeks' gestation.

But that has presented new challenges in moving toward the point where they can survive outside a humidicrib.

Human trials 'a long way off'

In Australia, experts have said that if the artificial womb could meet its promise, it could become a valuable new tool in the critical care arsenal.

Dr David Tingay, from the Murdoch Children's Research Institute, called it an exciting development.

"This is still pre-clinical or experimental data, but the research group in Philadelphia have managed to overcome many of the limitations others have had in supporting a baby or foetus with womb-like conditions," he said.

The development opens up potential to understand how to better support pre-term babies. ( Supplied: Children's Hospital of Philadelphia )

"They've managed to now keep some of these lambs alive for at least a month.

"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."

Dr Tingay said the development opened up potential to understand how to better support pre-term babies with a very high risk of lung disease after being born.

"But I would just caution this is pre-clinical and new and experimental data — we're still a long way off being able to use the findings from this work in the neo-natal intensive care unit," he said.

"I think [two years] is optimistic. It will take a long time to be able to demonstrate that this technology can be safely applied in humans.

"That being said, the techniques that they've used each independently are either not already being used in clinical care, or are not far from being able to be adapted to clinical care.

"I think the difficulty will be putting the whole package into a clinical package, and for that reason I think we're further off than the researchers would hope."