Researchers at the University of Minnesota, co-led by transplant surgeon Erik Finger, have vitrified human-sized pig kidneys at -150°C and rewarmed them without fracture. One kidney spent 366 days frozen and later recovered blood flow and urine production on a perfusion machine, though less urine than most vitrified kidneys in the study. The work is a preprint and has not passed peer review. It is a real advance in scale, but not proof that such an organ can be banked and keep an animal alive.
- What worked: Human-scale pig kidneys were rewarmed after long storage, with perfusion and urine output restored in tests.
- What remains unproven: The 366-day kidney was never transplanted; transplants used other kidneys in a 120-minute acute model.
- Why it matters: Human kidneys last only 24-36 hours, so organ banking would transform transplant logistics.
One year frozen is not one year transplanted
The kidney that spent 366 days at -150°C was evaluated only on a perfusion machine, outside the body, for two hours. It was never implanted. The transplant experiments used other vitrified kidneys, three per group, in an acute 120-minute model after which the animals were sacrificed. The paper says most vitrified kidneys were stored for one to seven days and does not say how long the transplanted organs had been frozen. No one has transplanted a kidney stored for a year.
How nanowarming prevents the ice problem
Vitrification cools tissue quickly with cryoprotectants so water becomes a glass-like solid instead of ice crystals. Rewarming is harder: heating from outside warms the surface before the core, creating ice and cracks. The Minnesota team uses nanowarming: silica-coated iron oxide nanoparticles circulate through blood vessels, then an electromagnetic field heats them simultaneously from inside. Rewarming takes under six minutes.
The group developed a new cryoprotectant, PEZ42, because the rat-kidney solution did not vitrify reliably inside a 100-gram organ. Staged perfusion limited toxicity during longer loading and unloading. The size jump is the real headline: these kidneys weigh about 100 grams, roughly 100 times more than the rat kidneys the same team vitrified and transplanted in 2023. A 2025 pig-kidney study involved only 10 days of storage.
Urine output is not the same as life support
Urine output shows the organ is perfused, its tubules have not collapsed and its tissue is not necrotic. It does not prove the kidney can sustain life. In the transplant model, vitrified grafts produced urine while developing endothelial damage and hemorrhage. Creatinine clearance was 1.0 mL/min/100 g, about 2% of the 47.8 seen in fresh transplanted kidneys. Kidneys stored for 24 hours in cold conditions, today’s clinical standard, also scored low at 3.6, and the paper calls the vitrified result “similar.” With three kidneys per group, the comparisons are exploratory and underpowered; the authors warn that a lack of significant differences is not equivalence.
On the perfusion machine, clearance was 4.16, versus 12.56 for 24-hour cold storage and 18.35 for fresh kidneys. Damage was concentrated in tubules, while glomeruli and vessels were nearly intact. Cryobiologist Peter Kilbride, who was not involved, said the field now has proof the approach can work in a whole organ, but called it only “half the problem.” Greg Fahy, a pioneer in the field, was more direct.
We cannot know, from these experiments, whether these kidneys could have sustained life or not.
The bottleneck has moved from physics to chemistry
The practical stakes are large. Human kidneys last only 24 to 36 hours outside the body, forcing transplant networks into a race against time. Longer storage could enable organ banks, calmer matching and fewer discards; the US National Institutes of Health estimates that about 20% of donated kidneys are discarded. The study also contains a quieter clue. In the perfusion machine, vitrified kidneys did not perform worse than kidneys exposed only to cryoprotectant without freezing. That control group was not transplanted, so the endothelial damage seen in animals cannot yet be pinned on freezing and rewarming. The data instead point to the cryoprotectant as the main source of remaining injury, though the authors cannot rule out a smaller contribution from the cryogenic process.
The decisive experiment is still missing: a long-term survival transplant showing a human-sized vitrified kidney can support life. The researchers acknowledge that groups are very small, that the one-year storage was tested in a single kidney, and that human kidneys have not been tried. They give no clinical timeline. The engineering feat is fracture-free rewarming of an organ 100 times larger than earlier successes; the medical promise remains unconfirmed. Until a large animal survives with one of these kidneys inside it, the field has a stronger result in preservation technology than in transplantation.