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The practice of preserving legally dead people at cryogenic temperatures in the hope that future medicine can repair and revive them.
Cryonics is the preservation of a legally dead human body or brain at cryogenic temperatures, on the premise that the damage of dying and of preservation itself may one day be reversible by medicine that does not yet exist. It is not a treatment and makes no claim to work now. It is a wager that the information constituting a person survives the process, and that some future technology will be able to read or restore it. The practice has been closely associated with Transhumanism since both emerged in the 1960s and 1970s, and remains marginal to mainstream medicine.
The procedure begins at legal death, which in most jurisdictions means cardiopulmonary arrest and a physician's pronouncement. Nothing may be done before that point. This constraint shapes everything: the ideal case is a patient who dies expectedly, with a standby team present, and the worst case is an unwitnessed death discovered days later.
Immediately after pronouncement, the team restores circulation mechanically and begins external cooling with ice, sometimes with drugs intended to limit reperfusion injury and blood clotting. The aim is to reduce the interval of warm ischemia during which cells continue metabolizing without oxygen. Ischemic injury before perfusion is generally regarded, including by cryonics organizations themselves, as the largest source of damage in real cases.
Blood is washed out and replaced by a cryoprotectant solution perfused through the carotid and vertebral arteries, or through the whole vasculature for whole-body cases. Concentrations are ramped upward over hours. The chemistry is the crux of the field: at high enough concentrations, the solution does not freeze as the temperature falls but becomes progressively more viscous until it forms a glass.
Vitrification avoids ice entirely. Ice crystals grow between cells, tear membranes, and concentrate solutes to damaging levels; a vitrified solution has no crystalline structure and no phase change. The cost is toxicity, because the agents that suppress ice — dimethyl sulfoxide, ethylene glycol, formamide and their mixtures — are themselves harmful to cells at the concentrations required. Modern solutions such as those developed at 21st Century Medicine are engineered to minimize toxicity at vitrifying concentrations, and their formulation is the main technical advance of the past twenty-five years.
Below roughly −124 °C the solution passes its glass transition and becomes mechanically brittle. Continued cooling to liquid nitrogen temperature produces thermal stress and, in whole organs and bodies, fracturing. Some organizations have proposed intermediate-temperature storage just below the glass transition to avoid this, at the cost of a more complex and failure-prone storage system.
Why temperature is not the hard partStorage at −196 °C is essentially free of chemical change: at that temperature, reactions that would degrade tissue take longer than the age of the universe. The unsolved problems are all in the transitions — getting cryoprotectant into a dying brain, and, in any revival scenario, getting it out again without killing what remains.
The case for cryonics does not require that current methods preserve a person perfectly. It requires a weaker claim, formalized by Ralph Merkle as information-theoretic death:1 a person is dead in the final sense only when the structures encoding their memory and personality have been degraded past any possible inference, not merely past the reach of present medicine. On this view the relevant question about a cryopreserved brain is whether the information is still there in some recoverable arrangement, not whether the tissue is alive.
Two revival routes follow, and they make very different demands. The biological route requires rewarming without ice damage, removing cryoprotectant, and repairing accumulated injury cell by cell — a task usually attributed to hypothetical Medical nanorobots and thus dependent on the molecular manufacturing programme that Eric Drexler set out and that has not materialized. The informational route abandons the body: the preserved brain is sectioned and scanned, its connectome and molecular state reconstructed, and the person instantiated as a whole brain emulation. That route is the explicit goal of Brain preservation as a separate field, and it inherits every objection to Mind uploading, including the Substrate independence premise and the question of whether the result would be the same person at all — the subject of Personal identity and continuity.
What the disagreement is really aboutAlmost no cryobiologist disputes that vitrification suppresses ice, or that −196 °C halts decay. The dispute is about whether the ischemia, toxicity and fracturing that occur in real cases leave enough structure behind to matter, and whether "enough structure" is even a well-defined criterion in the absence of a theory of how memory is physically encoded.
The relevant cryobiology has advanced, though not in the direction of human revival.
Small organisms survive vitrification routinely. Human embryos and oocytes are vitrified and thawed as standard IVF practice with high survival rates, which is the basis of egg banking discussed under Reproductive longevity; vitrified C. elegans have been reported to retain a learned association after revival, which is at least a demonstration that a memory trace can survive the process in a 302-neuron animal.2
Whole mammalian organs are the frontier. Fahy's group vitrified rabbit kidneys with the M22 solution, rewarmed them, and transplanted one that supported the animal's life.3 Rewarming was the bottleneck: heating a large mass fast enough and uniformly enough to avoid devitrification and cracking is harder than cooling it. That problem yielded partially in 2023, when a group reported vitrified rat kidneys rewarmed by inductively heating infused magnetic nanoparticles and then transplanted with restored function.4 These results matter for organ banking and the Organ shortage problem regardless of what one thinks of cryonics.
No brain has been recovered. No mammal has been revived from cryogenic temperature. The gap between a rat kidney and an adult human brain is several orders of magnitude in mass, and the brain is the one organ for which "restored function" cannot be assessed by perfusion pressure and urine output.
Fewer than a thousand people are in cryopreservation worldwide, held by a small number of organizations: Alcor in Arizona, the Cryonics Institute in Michigan, Tomorrow Bio in Europe, and smaller operations elsewhere including Australia's first case in 2024. Several thousand more are signed up as members. Growth has been slow and roughly linear for decades, not exponential.
Alcor Life Extension Foundation, which was led from 2011 to 2020 by the philosopher Max More, sets minimum funding at $200,000 for whole-body preservation and $80,000 for neuropreservation — the head alone, on the reasoning that a future capable of reviving anyone can also grow a body, an assumption that leans on Lab-grown organs; the Cryonics Institute charges several times less than Alcor for whole-body. Most members fund it with a life insurance policy naming the organization as beneficiary, which converts a large capital cost into a modest premium. Whether that makes the practice broadly available or merely cheap for the already insured is one instance of the Access and inequality problem that runs through life-extension technology.
The membership has always been small and unrepresentative, skewing technical, male, and American. Among the better-known patients is the futurist FM-2030, preserved at Alcor in 2000.
The structural risk is organizational rather than technical. Patients must be maintained for an unknown period, possibly centuries, by institutions with no revenue from them after the initial payment. Alcor's response is a patient care trust holding invested funds separately from operating accounts, sized so that liquid nitrogen and maintenance can be paid from returns. Whether any private institution can survive on that basis for two hundred years is untested, and the Chatsworth failure is the field's own worked example of what happens when one does not.
Cryonics is legal in most countries but exists in a category the law does not otherwise recognize. Preservation may begin only after death is pronounced, which guarantees a period of ischemia and rules out preserving a patient whose brain is being destroyed by a progressive disease while they are still alive. A challenge to that rule in California in the early 1990s, brought by a man with a brain tumor who sought pre-mortem cryopreservation, was rejected by the courts — a decision that sits awkwardly beside both assisted-dying law and the Morphological freedom arguments made elsewhere in this field.
Patients are usually handled under anatomical gift or body-donation law, giving them the legal status of remains rather than persons. Estate law offers no mechanism for a preserved person to retain assets, which is why some members establish personal revival trusts of uncertain enforceability. An English High Court ruling in 2016 permitted a terminally ill 14-year-old's wish to be cryopreserved to be carried out, in a judgment that dealt with parental dispute over disposal of a body rather than with cryonics itself.
The Society for Cryobiology has distanced itself from cryonics for decades, at one point barring practitioners from membership, and its position is that the practice is not supported by science and should not be described as medicine. The core objections are specific rather than general.
Cryoprotectant toxicity is not eliminated, only reduced, and the doses reaching brain tissue in a real case are unknown and unmeasurable. Perfusion is uneven; ischemic tissue swells and blocks capillaries, so parts of the brain may receive little cryoprotectant and freeze rather than vitrify. Fracturing at cryogenic temperature is documented and unresolved for whole organs. And the field's central premise, that structure preserves the person, is an assumption about the physical basis of memory that neuroscience has not established — synaptic connectivity is clearly necessary, but whether it is sufficient, or whether molecular states that fixation and cooling do not preserve also matter, is open.
A separate criticism concerns epistemics rather than biology. Cryonics is unfalsifiable in practice: no experiment run today can show that a preserved patient will not be revived in 2300, which makes the claim immune to disconfirmation and therefore, critics argue, more like a belief than a hypothesis. Proponents reply that the relevant decision is a bet under uncertainty with an asymmetric payoff, a framing that critics in turn regard as a rebranding of Pascal's wager.
What is and is not demonstratedVitrification of small tissue volumes is routine laboratory practice. Long-term storage without chemical change is established physics. Revival of a cryopreserved mammal, recovery of a cryopreserved brain, and repair of ischemic and toxic injury at cellular scale are all undemonstrated, and the last requires technology that does not exist.
The field's near-term progress is more likely to come from organ banking than from cryonics itself. If vitrification and nanowarming scale from rat kidneys to human hearts and livers, transplant medicine acquires a supply chain it currently lacks, and cryonics acquires evidence that a large vitrified mammalian organ can be brought back. That would not demonstrate revival of a person, but it would move one of the field's assumptions from argument to data.
The deeper question is not technical. Cryonics asks whether a structure that is not alive, not conscious, and not repairable by any known means should be treated as a patient rather than a corpse. Nothing in cryobiology answers that, and nothing will until either a mammal is recovered or a preserved brain is scanned and shown to contain what it was supposed to contain. Until then the honest statement of the odds is that they are unknown and that the people making the bet are not in a position to collect on it themselves.
paperMerkle, R.C. "The Molecular Repair of the Brain." Cryonics, 1994.↩Published in a cryonics movement magazine rather than a peer-reviewed journal; it proposes a criterion for death and reports no experiment.
paperVita-More, N. and Barranco, D. "Persistence of Long-Term Memory in Vitrified and Revived Caenorhabditis elegans." Rejuvenation Research, 2015.↩The animal has 302 neurons and the assay is a single learned odour association, which is a long way from a mammalian memory.
paperFahy, G.M., Wowk, B., Pagotan, R. et al. "Physical and biological aspects of renal vitrification." Organogenesis, 2009. ↩
paperHan, Z. et al. "Vitrification and nanowarming enable long-term organ cryopreservation and life-sustaining kidney transplantation in a rat model." Nature Communications, 2023. ↩