Doctors have for the first time successfully used artificial blood to treat patients.
Blood supplies are stretched
The product is a powder which can be stored for years, say scientists at Stockholm's Karolinska Hospital.
It is made from donated supplies of real blood, which normally has a shelf-life of just 42 days.
The powder can then be mixed into liquid form when needed, and used immediately regardless of the patient's blood type.
Dr Pierre LaFolie, chief physician at the Karolinska Hospital, said that if artificial blood were approved for use it could lead to dramatic changes in health care.
He said: "If this really works all the way, then mankind will have taken a big step forward. This is like landing on the moon."
Dr LaFolie said synthetic blood could save time after an accident - particularly as there is no need test a patient's blood type before administering a transfusion.
The synthetic blood has also been shown to transport oxygen through the body better than real blood, which can help limit damage to the body, for example during a heart attack.
Dr LaFolie said: "In acute situations time is of the essence, within an hour all
these things have to be done.
"That's why I think this artificial blood is so important for people."
The blood would be used to complement a patient's own blood, not
replace it, as real blood has qualities that the artificial blood lacks.
The synthetic blood has been developed by researchers in the US - the exact process for developing it has been kept secret - and it has been tested for the first time on eight patients at Karolinska Hospital.
Lead researcher Professor Bengt Fagrell said: "There has been no sign that the blood would be rejected.
"This is a molecule that the body's immune system gladly welcomes."
Professor Fagrell said the patients had been administered artificial blood
made out of human red blood cells.
However, he said red blood cells from any mammal could be used.
"We have chosen to use human blood for ethical reasons. But we could use blood within the entire mammal chain, such as cow blood."
Belinda Linden, head of medical information at the British Heart Foundation said: "There is limited availability of fresh blood for transfusion.
"This is due to the short-fall of donors and an increased number of surgical procedures which require blood.
" It is becoming more important to consider safe and effective alternatives.
"There have been previous studies examining the potential of red blood cell substitutes, but this process would need to be evaluated with large groups of people before it is considered for approval."