Pig kidneys frozen for a year work again, in animal tests

On a pig kidney, the warming is the moment of truth. For the first time, a team led by transplant surgeon Erik Finger and bioengineer John Bischof has frozen whole pig kidneys into a glass-like state, kept them for months at about -150°C and brought them back. After rewarming, the organs regained their color and made urine, the key signs of function. One kidney had stayed frozen for just over a year.

The trick is to avoid ice, which tears tissue apart. The team first loads each kidney with chemical cryoprotectants, which work like antifreeze, then infuses iron oxide nanoparticles. Cold nitrogen vapor cools the organ to about -120°C, where it turns vitrified, meaning glass-like, before settling at -150°C. Rewarming is the hard part. An electromagnetic field makes the nanoparticles release heat throughout the organ, and adjusting the field's power controls how quickly it warms. Pig kidneys are roughly 100 times heavier than the rat kidneys the team vitrified in 2023, and Bischof said the cryoprotectant mix had to be modified, a change that took three years.

In the tests, rewarmed kidneys were run on perfusion machines, which pump fluid through an organ outside the body, and transplanted into pigs for two hours. In both cases, "they pink right up," Finger said. They made urine within about a minute, though only about two-thirds of the normal amount. The year-old kidney, tested on a perfusion machine, produced urine immediately and looked better than many of the others.

The result follows a 2025 milestone. Researchers at Massachusetts General Hospital, working within an engineering research center called ATP-Bio, stored a pig kidney for 10 days at about -15°C, then transplanted it into a living pig and restored its function. The University of Minnesota, which cites National Institutes of Health data, notes that about 20% of donated kidneys cannot be used because they cannot be kept cold long enough to reach recipients.

So who gains, and when? Banking organs for long periods would give doctors time to match donors to recipients, and possibly to prepare patients' immune systems, which might reduce the need for lifelong anti-rejection drugs. Kevin Myer, who leads the organ procurement organization LifeGift, pictures banks of organs ready for emergencies such as drug overdoses and gunshot wounds. Those are horizons, not dates: the sources give no timeline for use in people.

The kidneys are not perfect. Beyond the low urine output, the researchers attribute some injury to the cryoprotectants. Greg Fahy, a cryobiology pioneer who was not involved, said the experiments cannot show whether the kidneys could have supported life. Peter Kilbride, another cryobiologist not involved, called the work "one big breakthrough that solves half the problem." The next steps are longer tests in pigs, less toxic cryoprotectants and experiments with human kidneys.