For the first time in an organ transplant, frozen rat kidneys can be used after 100 days.

A team of cryo-engineers and surgeons at the University of Minnesota has found a way to cryogenically freeze and thaw rat organs in a way that preserves their energy. In their study, it was written in a journal Nature Communicationsthe team overcame previous problems with frozen parts with their new system.

Medical scientists have been working on freezing and thawing human organs since they were successfully transplanted from one person to another—freezing would greatly expand the pool of donated organs. The problem has always been the same—freezing the organs leads to the formation of crystals that destroy the tissue and thawing them in different ways leads to further damage. In this new effort, a research team in Minnesota has found a solution to all of these problems.

When muscles freeze, ice cubes form between the cells, causing damage. To overcome this problem, the researchers used a technique called snap-freezing, where a cryoprotective drug was injected into the tissue that was frozen very quickly. Instead of crystals, a glass-like substance was created that did not damage. Then the group turned to the melting task.

Most methods of shrinking tissue involve placing the sample at room temperature until it melts, sometimes using heat or using microwaves. Unfortunately, each of these methods has the same problem: uneven melting, where the muscles heat differently in the same organ. This results in cracks or tears, rendering the organ useless. To overcome this problem, the team injected iron oxide particles into the organ before it dried. Then they used a magnetic field to heat the tissue so that it didn’t get too hot—the nanoparticles acted like a tiny heater. The result was a similar thawed muscle.

To test their ability, the research team froze several rats’ kidneys and kept them frozen for 100 days. They then thawed the organs, removed the drug and nanoparticles and implanted five of them into living rats. Tests showed that all the rats survived and that all of them regained kidney function within 30 days of transplantation. More testing is needed before similar trials can be carried out in humans, but the researchers suggest that their work could improve the chances of patients who need an organ transplant before their end of life.