CHICAGO (Reuters) - U.S. researchers have taken a major step toward the use of frozen or cryopreserved tissues and organs for transplantation, an advance that may one day ease the shortage of available organs, experts said on Tuesday.

Although scientists have been able to successfully preserve organs cooled at ultra-low temperatures, they have had less success in thawing them out without causing cracks in the fragile tissues.

To address this issue, researchers at the University of Minnesota developed a new heating method using iron oxide nanoparticles, which surround the frozen tissues. They act like tiny heaters when activated by an external magnetic field.

Using this method, the researchers were able to uniformly warm frozen animal heart valves and tissues without causing harm to the tissues.

“This paper takes the first practical step towards making tissue banking a reality,” said Caitlin Czajka, an editor at Science Translational Medicine, which published the research on Tuesday.

Currently, more than 60 percent of donated hearts and lungs are thrown out each year because they can only be kept on ice for four hours before they deteriorate.

In prior work, teams have succeeded in reheating only about 1 milliliter of tissue and solution. In the new study, the team reheated arteries and heart valves in 50 milliliter vials, said study senior author John Bischof, a mechanical engineering professor at the University of Minnesota which holds two patents on the technology.

“What we found is we could bring these tissues back at very rapid rates and we were able to maintain the viability and functionality of the tissues,” Bischof told reporters on a telephone briefing.

After thawing, none of the tissues showed signs of damage and the nanoparticles were washed away.

Bischof said the team is working on a pilot study to thaw frozen rabbit kidneys in 80 milliliter vials, and he thinks it will be possible to scale up the technology to even bigger organs.

Bischof and colleagues said on the call it will probably take several more years of research before the technology can be cleared for use in human organ transplants, but he believes the new warming method could make that possible.