By Sophie Lindqvist

Researchers at the University of Minnesota have rewarmed pig kidneys that were vitrified at -150 °C and stored in a freezer for up to a year — and the organs regained blood flow and produced urine, the first demonstration of its kind in whole organs of human-relevant size. The team posted its results on the preprint server bioRxiv on October 7, and the work has not yet passed peer review.

The study, co-led by transplant surgeon Erik Finger and bioengineer John Bischof, moves the field from rat kidneys weighing about one gram to pig kidneys around 100 times heavier and close in size and appearance to human organs. After rewarming, the kidneys were tested on perfusion devices and transplanted into pigs, where they "pink right up," Finger said. By the key functional measure — urine output — the organs produced roughly two-thirds of the expected volume within about a minute of reperfusion.

"Ten years ago … I wouldn't say I had high hopes it was actually going to work," Finger said. "After we did the last transplant, I walked out of the room and said, 'This is a big deal.'"

What did the team actually measure?

The researchers ran two sets of experiments. In one, they placed rewarmed kidneys on perfusion systems of the kind already used in transplant surgery to feed and assess organs outside the body. In the other, they transplanted rewarmed kidneys into pigs and monitored them for two hours.

In both settings, the kidneys recovered blood flow and made urine. One kidney, banked for just over a year, produced urine "immediately" on the perfusion machine and "actually looked better than many of the [other] kidneys," Finger said. "We're comfortable now saying we can store them for a year and rewarm them with excellent function."

The caveats are material. The rewarmed organs produced less urine than fresh kidneys would, and they showed signs of injury that the researchers attribute to the cryoprotectant chemicals. The team monitored transplants for two hours only; it has not yet shown the organs can sustain life over the long term.

"This is one big breakthrough that solves half the problem," said Peter Kilbride, a cryobiologist and cofounder of Kyros Kairos, a company focused on cryopreservation research funding, who was not involved in the work. "Thank God that someone's actually managed to do it in one organ and prove that it works."

Greg Fahy, a cryobiologist widely regarded as a pioneer in the field and chief scientific officer at Intervene Immune and 21st Century Medicine, noted the open question. "We can't tell from these experiments whether these kidneys might have supported life or not," he said.

How does the rewarming work?

Vitrification — cooling tissue into a glass-like state — is routine for eggs, sperm and embryos, but whole organs pose three problems: toxic cryoprotectants, uneven cooling and uneven rewarming. Ice crystals destroy tissue; glass does not, but getting a 100-gram organ to the glassy state at about -120 °C and back out without cracking it is an engineering problem the field has worked on for years.

The Minnesota solution loads the organ with a cryoprotectant cocktail over roughly two hours, then infuses iron oxide nanoparticles for seven to eight minutes. Cold nitrogen vapor cools the organ; the team pauses the process at the vitrification point to confirm the organ is glassy throughout, because otherwise it can crack, before taking it down to -150 °C.

Rewarming is where the nanoparticles matter. An electromagnetic field switching between two states flips the particles back and forth, releasing heat throughout the organ. Adjusting field power lets the team accelerate heating once the organ passes -120 °C, avoiding the thermal gradients that crack large tissues.

The group published the same approach in rat kidneys in 2023 in Nature Communications, rewarming organs banked for up to 100 days before transplantation. Scaling to pigs took three years, largely because the rat cryoprotectant formula performed poorly in the larger organs. "We had to rediscover and modify the cocktail," Bischof said. "That's why it's taken us three years."

Why organ banking matters for the transplant pipeline

Tens of thousands of organ transplants take place in the US each year, and transplants over the past 25 years are credited with saving an estimated 2 million years of life. Current preservation limits give clinicians 24 to 36 hours to get an organ from donor to recipient, constraining both matching and logistics.

Indefinite banking would invert that model. "With real organ banking … instead of the patients waiting for the organ, it's the organs that are waiting for the patients," Finger said. Longer matching windows could also reduce rejection risk, potentially allowing recipients' immune systems to be prepared in advance — relevant given that transplant patients take immunosuppressive drugs for life, with elevated infection and cancer risks.

Kevin Myer, president and CEO of LifeGift, a Texas organ procurement organization, called the one-year storage result "a very big deal" and pointed to emergency use cases — drug overdoses, gunshot wounds — where patients cannot wait for a matched organ.

Outside researchers framed the result as a field-level shift rather than a clinical one. "Their work is amazing and will enable us to truly, indefinitely bank organs," said Korkut Uygun, a biomedical engineer and professor of surgery at Harvard Medical School who was not involved in the study. "It is a revolutionary time for transplantation."

What comes next

The immediate agenda is durability: longer-term transplants in pigs to test whether rewarmed kidneys can sustain life, a continued search for less toxic cryoprotectants, and planned experiments with human kidneys. Finger also expects some repair capacity — prior work shows organ health can improve on perfusion machines, and damaged kidneys may self-repair in the body. Human kidneys, he suggested, may tolerate the process better than pig kidneys.

"We're going to see that there's injury, and it's not perfect … but now we've got to the point where it works," Finger said. The next dataset — pig survival beyond two hours, and eventually human-kidney vitrification — will determine whether that assessment holds.