Frozen pig kidneys stored for over a year have been successfully rewarmed and transplanted by researchers at the NSF-funded ATP-Bio research center, headquartered at the University of Minnesota and Massachusetts General Hospital. The pig kidneys were cooled to −150 °C in a vitrified state — a glass-like, crystal-free solid — and, after nanowarming with iron oxide nanoparticles activated by an electromagnetic field, regained normal coloration and began producing urine when connected to perfusion machines and subsequently transplanted into live pigs.
The work, led by transplant surgeon Erik Finger and biomedical engineer John C. Bischof, was shared as a preprint on bioRxiv in October 2026 and has not yet undergone peer review. The team presented the results at the American Transplant Congress, where they drew significant attention from the transplant community.
The cryopreservation procedure described by the authors combines three steps. First, the kidney is loaded with cryoprotective agents that allow ice to form only in the extracellular space and within the vasculature while cells remain unfrozen. This approach drastically reduces metabolic demand during storage. Second, the organ is infused with iron oxide nanoparticles distributed throughout the vascular system. Third, the kidney is cooled with nitrogen vapor to approximately −120 °C, where it assumes a glassy consistency, and then transferred to storage at −150 °C.
For rewarming, a high-frequency electromagnetic field activates the nanoparticles, heating the tissue volumetrically and uniformly. The achieved warming rate was sufficient to prevent ice recrystallization during passage through the glass transition — a barrier that had eluded organ cryopreservation for decades.
The historical context for this advance is extensive. In 2023, the same research line published in the journal Nature Communications the first successful functional transplant of a cryopreserved rat kidney stored for up to one hundred days. Five kidneys were transplanted successfully, and all recipient animals survived the thirty-day observation period. The step to pig kidneys, however, required substantial adaptations: rodent kidneys weigh only a few grams, while mature pig kidneys weigh approximately 150 grams — volume comparable to a human kidney. The larger scale introduces more pronounced thermal gradients and an additional challenge of mechanical stress known as the cracking problem, where microfractures form in glassy tissue during warming if heat is not sufficiently uniform.
In September 2025, researchers at Texas A&M University, led by Matthew Powell-Palm and Guillermo Aguilar, published in Scientific Reports a solution to the cracking problem. By altering the chemical composition of the vitrification solution, they raised the glass transition temperature, reducing the thermal differential between the organ surface and interior during warming. In another study, also published in 2025 in Nature Communications, the Minnesota team scaled nanowarming technology to volumes of two liters, achieving warming rates of 88 degrees Celsius per minute.
These two advances — crystallization-free vitrification and crack-free warming — formed the foundation for the current pig kidney experiment. Three years were needed to adapt the cryopreservation chemical cocktail from the rodent model to the porcine model, as the team reported.
The observed results are promising but still incomplete. After rewarming, the kidneys recovered their natural pink coloration and produced functional urine in ex vivo perfusion. When transplanted into live pigs, the organs maintained urinary output and biochemical function for a limited period. The authors report that the kidneys produced approximately two-thirds of the urine expected compared to non-frozen kidneys and showed signs of cellular injury attributed to the toxicity of the cryoprotectant chemicals themselves.
This point — cryoprotectant toxicity — remains the principal outstanding barrier. The chemicals that prevent ice formation are equally aggressive toward cells when administered at the elevated concentrations required for whole organs. Reducing this toxicity without compromising crystallization protection requires a more precise combination of cryoprotectants or entirely new approaches, such as biocompatible materials being researched at the University of California, Riverside's Materials Laboratory.
Additionally, long-term survival of the transplanted animals has not yet been demonstrated. The experiment showed acute and subacute renal function but has not yet sustained life over weeks or months. This is the next milestone the team intends to reach.
What to watch for in the coming months is the peer-reviewed publication of the work. The preprint is already available, but reviewers in the scientific community will likely question methodological details and the actual duration of renal function post-transplant. Finger and Bischof's team has already received 2.7 million dollars from the NIH to apply the same technology to pig livers, indicating that cryopreservation of larger organs will be tested in parallel.
If the next results confirm the method's viability, the impact on transplant medicine would be profound. Currently, about 20 percent of donated kidneys in the United States cannot be used because they cannot be transported on ice for more than a few hours. With the ability to store organs for months or years, it would be possible to create a global bank of compatible kidneys, schedule surgeries at elective times, perform extended compatibility testing, and drastically reduce waiting lists. In the United States, more than 100,000 people wait for a kidney transplant, and dozens die daily while waiting.
The research also sits within the broader context of xenotransplantation. In 2024 and 2025, several human patients received genetically modified pig kidneys through FDA expanded access programs. One kidney transplanted in 2026 functioned for 271 days in an elderly patient. Cryopreservation adds a logistical dimension to these efforts: beyond genetic and immunological compatibility, organs could be stored indefinitely before surgery.
The field remains far from routine clinical application in humans, but the milestones accumulated — from the vitrification of embryos in the 1980s, to the rat kidney transplant in 2023, and now the transplant of pig kidneys cryopreserved for more than a year — indicate a consistent technological progression. What began as a theoretical possibility for microscopic tissue cryopreservation is approaching the reality of whole, functional, and indefinitely storable organs.
Sources: MIT Technology Review, Talk With Tech, University of Minnesota
✓ Independent sources cross-checked and verified before publishing
