PARIS (AFP) - A new "supercooling" technique keeps rat livers alive three times longer than before, boosting hopes for easing shortages of human transplant organs, scientists said on Sunday.

The method involves cooling the livers while flushing them with oxygen and nutrients and preserving them in a solution containing a form of antifreeze.

The livers can be conserved at temperatures below zero degrees Celsius yet not freeze and thus suffer cell damage.

All rats given livers "supercooled" for three days (72 hours) were healthy after three months, a benchmark for survival.

Of those who received livers stored for 96 hours, 58 per cent survived to the three-month mark, said study results published in the journal Nature Medicine.

Rats that received transplant livers preserved with current methods survived only for hours or days.

"To our knowledge, this is the longest preservation time with subsequent successful transplantation achieved to date," said study co-author Korkut Uygun of the Massachusetts General Hospital's Center for Engineering in Medicine.

"If we can do this with human organs, we could share organs globally, helping to alleviate the worldwide organ shortage." Existing technology can preserve human livers well for up to about 12 hours outside the body.

Since the 1980s, donor organs have been preserved at temperatures at or just above freezing in a solution that reduces metabolism and organ deterioration.

The new method saw the addition of protective, anti-freeze ingredients to the preservation solution.

In a three-step technique, a perfusion machine was used to flood the livers with oxygen, nutrients and protective compounds before they were cooled and submerged in the preservation solution, whose temperature was then lowered to -6 deg C at which the tissue was stored for either three or four days.

After supercooling, the temperature was gradually increased back to 4 deg C and the livers reflushed with oxygen and nutrients before being transplanted, the study paper said.

"The next step will be to conduct similar studies in larger animals," said Rosemarie Hunziker of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), a body of the US National Institutes of Health (NIH) which supported the work.

The method will have to be thoroughly tested and refined before it can be considered for use in humans.

Some 120,000 people are waiting on donor organs in the United States alone, said the study authors.

"Extending even further the time a liver can survive outside the body would provide many benefits," according to a NIBIB statement.

"It would allow more time to prepare the patient and ease logistics at the donor hospital site, reduce the urgency of rushing the organ to its destination, and expand the donation area to allow for transcontinental and intercontinental transplantations."

This would boost the chances of patients finding better donor organ matches, and reduce costs.