Freezing whole organs without ice: where organ cryopreservation stands

What organ cryopreservation means

Organ cryopreservation means cooling a whole organ to a very low temperature so it can be stored for a long time and warmed later for transplant. Donor organs are normally kept cold, and some are kept alive on machines, but neither method allows storage for weeks, months or years. A stored organ could give surgeons more time to match donors and recipients, and it could allow organs to be banked in the way blood is.

The central technical problem is ice. When water freezes into crystals, the crystals can tear cells and tissue apart. Researchers therefore aim for vitrification, a process in which the organ solidifies into a glass-like state without crystals forming.

Why ice is the problem

Fast cooling helps avoid crystals, but a whole organ is large, and heat takes time to leave its centre. To reach a glass-like state, researchers load the organ with cryoprotectants, chemicals that stop ice from forming. The trade-off is that cryoprotectants can harm cells.

Cryoprotectants and their toxicity

A 2015 peer-reviewed review in Rejuvenation Research describes cryoprotectant toxicity as the major impediment to vitrification, and notes that these chemicals become more toxic as their concentration rises (Best, 2015). The author is affiliated with the Life Extension Foundation in Florida. The review lists harms reported for several common agents in laboratory and animal work, including damage to proteins and DNA and effects on heart rate. It also says that, without exact assaying of molecular damage, explanations of cryoprotectant toxicity can only be speculation.

Larger organs need higher concentrations, so the problem grows with size. A 2025 bioRxiv preprint by Ahmadkhani and colleagues, which is not peer reviewed in the version reviewed here (the page notes that a peer-reviewed version has since appeared in Cryobiology, which we have not reviewed), notes that a major obstacle in translating these results to human organs is their larger size (Ahmadkhani et al., 2025). Its laboratory screening found that toxicity generally rose with both concentration and exposure time, and that some mixtures of agents were less toxic than their single components.

Cooling and warming problems

Two failure modes matter for larger samples. If warming is too slow, ice can form and crystallise. If warming is uneven, the temperature difference within the tissue can cause cracking. A 2024 review chapter in the Annual Review of Heat Transfer names ice recrystallisation and physical cracking as the two main challenges, and links them to slow rates and thermal non-uniformity respectively. It describes both as inherent to traditional boundary rewarming (Gangwar et al., 2024).

The same chapter says nanowarming, which is radiofrequency rewarming with magnetic nanoparticles, has shown scalability to about 1,500 degrees Celsius per minute and volumes as large as 2 litres. These figures describe heating performance. The chapter's abstract-level record does not report transplant outcomes at that size.

The evidence, sorted by strength

Established

A peer-reviewed review identifies cryoprotectant toxicity as the major impediment to vitrification (Best, 2015). This is the only item in this group drawn from a journal review.

Demonstrated

In a 2023 study in Nature Communications, Han and colleagues vitrified rat kidneys, stored them for between 1 and 100 days at minus 150 degrees Celsius, and warmed them with nanowarming (Han et al., 2023). The kidneys produced urine straight away during perfusion outside the body. After transplant, they made urine within 40 to 45 minutes of reperfusion, compared with a few minutes for fresh controls. At day 30, creatinine, a marker of kidney function, did not differ significantly from controls. Group sizes were small, at 4 to 8 animals per group across experiments, and all animals in the vitrified and fresh control groups survived the 30-day study period. All experiments used rat kidneys. The study did not test human organs.

Reported or speculative

The Gangwar et al. (2024) chapter is a review chapter, not a journal article. Its heating figures are reported above and have not been tested in transplant studies.

A 2023 news article from the Lifespan Research Institute, a non-profit organisation, describes cryobanking of human organs as a future goal rather than an achievement. It says nanowarming would need to work in larger human organs, which has not yet been tested. It describes whole-body cryopreservation only as a conditional possibility (Lifespan Research Institute, 2023). This is press coverage, not a primary study or a neutral report.

Limits and open problems

• Size. Rat kidneys are small. The authors say the work was not done in human or human-sized kidneys, such as porcine ones, and list scaling up to them as future work (Han et al., 2023).
• Duration. Storage was tested only up to 100 days, and recipients were followed for 30 days. Longer survival is not known.
• Early function. The nanowarmed kidneys showed two to three weeks of initial graft dysfunction (Han et al., 2023).
• Toxicity. The 2025 preprint reports that some mixtures were less toxic, but its results come from cell-based screening rather than whole-organ transplant outcomes (Ahmadkhani et al., 2025).
• Uniform rewarming at scale. Heating a large volume quickly is not the same as keeping every part of a whole organ intact and working afterwards.
• Human use. No source reviewed for this article reports a human organ that has been cryopreserved and transplanted.

What to watch next

• Transplant studies in human-sized kidneys, which the authors list as future work.
• Follow-up beyond 30 days, and storage beyond 100 days.
• Peer-reviewed evidence on cryoprotectant mixtures at organ scale.
• Any regulatory route to human use. This article did not review current rules.