Preprint: one human-sized pig kidney was stored for 366 days at −150 °C and rewarmed using nanoparticles

Preprint: one human-sized pig kidney was stored for 366 days at −150 °C and rewarmed using nanoparticles

On 14 October, bioengineer John Bischof, transplant surgeon Erik Finger and their colleagues at the University of Minnesota posted a preprint, a manuscript that has yet to undergo peer review. They infused pig kidneys weighing about 100 grams with a cryoprotectant, a substance that acts as antifreeze for cells, cooled them into a glassy state and rewarmed them using nanoparticles.

A donor kidney is usually stored on ice for only 24–36 hours, leaving that much time to find a recipient. Vitrification puts an organ into a glassy state without ice crystals, but heating a large organ quickly and evenly is difficult. Nanoparticles in the blood vessels generate heat throughout the organ when exposed to an alternating magnetic field. In 2023, Bischof’s group used this approach to restore function in rat kidneys; after up to 100 days of storage, the kidneys kept rats alive after their own kidneys had been removed.

Moving from rat to pig kidneys changed the cooling conditions because of the difference in size. A rat kidney cooled at about 20 °C per minute, whereas the centre of a pig kidney cooled at about 3 °C per minute. The cryoprotectant reaches the inner kidney tissue less readily, so the previous formulation required a cooling rate that could not be achieved. The authors adjusted the formulation to allow it to form and remain in a glassy state during slow cooling. In the initial experiments, kidneys lost function because of severe toxicity after receiving the full dose of cryoprotectant at 4 °C. The researchers therefore introduced it gradually while lowering the temperature below −15 °C. They held the kidney at −114 °C until its temperature was uniform throughout, reducing the risk of cracks.

Eight rewarmed kidneys functioned for two hours on a machine that pumped pig blood through them. They were compared with fresh kidneys, kidneys stored on ice for a day and kidneys exposed only to the cryoprotectant. This last control separates chemical damage from damage caused by storage and rewarming. Blood flowed through the rewarmed kidneys, and they produced urine, but they cleared creatinine, a metabolic waste product, from the blood at a rate of 4,2 ml per minute per 100 grams, compared with 12,6 after a day on ice and 18,4 in fresh kidneys. Their blood flow, urine output and blood clearance measurements fell within the same range as those of kidneys exposed only to the cryoprotectant. The authors attribute most of the remaining damage to cryoprotectant toxicity and the way it reaches the tissue.

One of the eight kidneys spent 366 days in a glassy state; most of the others were stored for one to seven days. The kidney stored for a year was tested only on the machine. Its blood flow matched the group average, but its urine output was one-third of that average. Separately, three rewarmed kidneys were transplanted into pigs and monitored for two hours. They began producing urine immediately, yielding 181 ml per 100 grams, compared with 73 after a day on ice and 411 for fresh kidneys. Creatinine clearance remained low, and the layer of cells lining the blood vessels sustained more damage than in the other transplant groups.

In a 2025 study, a group that included cryobiologist Greg Fahy transplanted three vitrified rabbit kidneys weighing about 14 grams. One recovered function and remained functional for 17 months. Fahy told MIT Technology Review: “These experiments cannot tell us whether these kidneys would sustain life.” Erik Finger described a possible organ bank this way: “With a true organ bank… organs would wait for patients, instead of patients waiting for an organ.”