Pig kidneys frozen for up to a year come back to life after rewarming

Pig kidneys cooled to -150°C, including one held in a freezer for 366 days, regained their pink colour and produced urine after rewarming and transplantation into pigs, University of Minnesota researchers reported.

The finding offers a potential answer to the tight timetable facing transplant teams. Human organs can currently be preserved for 24 to 36 hours, leaving limited time to pair a donor organ with a compatible recipient.

Erik Finger, a transplant surgeon who co-led the research, said a successful organ bank would change that calculation. “With a true organ bank… organs would wait for patients, instead of patients waiting for an organ.”

The study, led by Zonghu Han and titled ‘Functional recovery of vitrified and nanowarmed human-scale pig kidneys’, was posted on the preprint server bioRxiv on October 14. It has not yet been peer-reviewed.

The kidneys are similar in size to human kidneys. John Bischof, a bioengineer at the University of Minnesota, had previously worked with the team on rat kidneys, which were vitrified in research published in 2023 and stored for up to 100 days before transplantation into rats.

Vitrification leaves cells in a glass-like condition and is already used to preserve eggs, sperm and embryos. For the pig-kidney work, the researchers infused the organs with cryoprotectants, chemicals that act like antifreeze by preventing damaging ice crystals, before adding iron oxide nanoparticles.

An electromagnetic field using radiofrequency then activated the particles, generating heat across the organ during rewarming.

Eight rewarmed kidneys worked for two hours on a machine that circulated pig blood through them. They filtered blood at rates of 4.2 ml per minute per 100 grams.

Three more rewarmed kidneys were transplanted into pigs and monitored for two hours. They produced 181 ml per 100 grams of urine, compared with 73 after a day on ice and 411 for fresh kidneys.

The kidney stored for 366 days had blood flow matching the group average, though its urine output was one-third of that average. The results show that the organs recovered some function after storage, while their performance remained below that of fresh kidneys.

The work follows other advances in organ cryopreservation. In a 2025 study, a group including the cryobiologist Greg Fahy transplanted three vitrified rabbit kidneys weighing about 14 grams; one remained functional for 17 months.

Peter Kilbride is a cryobiologist and cofounder of Kyros Kairos. Korkut Uygun is a biomedical engineer and professor of surgery at Harvard Medical School.

The authors call the pig-kidney result an important milestone, while saying it remains unclear whether the approach will work in humans. Tens of thousands of organ transplants take place in the US alone each year, and transplant procedures are thought to have saved 2 million years of life in the last 25 years.