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The Quest To Revive The Dead: Where Has Cryonics Succeeded?

Posted on 4 November 2022

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When discussing the science of life extension on this website, we dedicate most of our time to speculating about the future. What promising treatments are on the horizon, which mechanisms of the ageing process make the best targets, and what new drivers of ageing are being uncovered. Yet it’s also important to acknowledge our successes. When introducing the concept of human lifespan extension, I often try to emphasise the point that we have already succeeded in extending the lifespan of many animals, as this drives home the idea that the ageing process can be modified.

In a similar vein, perhaps it’s worth taking the time to acknowledge successes in the field of cryonics – the practice of preserving dead bodies in liquid nitrogen in the hope that they can one day be revived. Of course, a dead human has yet to be returned to life, but there are certainly examples of tissues, organs, and even some animals surviving cryopreservation – the preservation of biological material using extremely cold temperatures.


The story of successful cryopreservation begins in 1949 with Christopher Polge, an English biologist who discovered that rooster sperm could survive freezing in liquid nitrogen if the sperm sample was first mixed with glycerol. Glycerol prevented the formation of ice crystals, which usually resulted in the destruction of the sperm cells. By using glycerol, Polge found he could preserve the sperm samples and still use them to produce healthy chicks. Glycerol and other substances that can prevent ice crystal formation are known as cryoprotectants.

Scientists quickly figured out the optimal process for preserving various different cell types. Today, the cryopreservation of human cells is commonplace in fertility medicine, allowing human sperm and oocytes to be preserved in a viable state. It can also be used to store stem cells, blood, and gene therapy materials. So, at the very least, we can say that individual cells belonging to the human body have been successfully cryopreserved.

Tissues and organs:

Preservation of human organs is the next big step for cryopreservation. It would allow donated organs to be preserved for extended periods, greatly improving organ availability. Intact cryopreservation of human organs, in particular the brain, is also critical for the successful revival of a dead human for obvious reasons.

Preserving whole organs is a lot harder than preserving cells, because organs are made up of many different cell types, and these have different optimal freezing rates to minimise ice crystal formation. Because of this, the most promising approach to freezing whole organs is a process called vitrification. In vitrification, a cocktail of cryoprotectant chemicals is injected into the tissue in order to make its water solidify ‘like glass’. This water doesn’t form ice, in which water molecules are arranged into an orderly crystal structure. Instead, water molecules are immobilised in their liquid organisation. This completely eliminates ice crystal formation. Unfortunately, many chemicals used in vitrification are quite toxic.

Water freezes to form a crystalline solid. Ice crystals grow in size over time, causing havoc at the cellular level (left). In vitrification, water molecules are immobilised by cold temperatures, but are prevented form forming crystals by the addition of cryoprotectant chemicals (right).

So far, vitrification hasn’t been successfully reversed in a human organ. However, there have been some notable success stories in animals. Embryonic rabbit kidneys have been successfully vitrified, preserved for three months, and transplanted into rabbits to become functional organs. Other studies have shown that brain tissue can be vitrified with negligible damage to its microscopic structure. The additional difficulty of preserving human organs comes mainly from their size. It’s harder to cool larger objects rapidly and uniformly, and it’s also harder to get the cryoprotectant chemicals into every single cell of a larger organ.

Whole organisms:

As is often the case, nature has figured out how to cryopreserve whole organisms long before humans have. Some plants, insects, amphibians, reptiles and fish naturally concentrate substances in their body to protect their tissues against freezing damage during cold weather. The beetle Upis ceramboides can survive temperatures of down to -60 degrees Celcius, while North American wood frogs can survive even when about 65% of their body water freezes. One might argue that these temperatures are not low enough to adhere to the strict definition of cryopreservation. However, the principal here is mostly the same: tissues are able to survive freezing temperatures thanks to the presence of cryoprotectant chemicals.

To cryopreserve a corpse, we need much colder temperatures in order to prevent any of the chemical processes involved in decomposition. However, the fact that many animals can and regularly do survive temperatures below freezing should at least serve as encouragement that humans could one day do the same.


You may be surprised to learn that humans have been successfully cryopreserved for 14 years and survived the process. You may be disappointed to learn that I’m talking about human embryos for in vitro fertilisation (IVF). The largest human organism that can be reversibly cryopreserved is typically a blastocyst of around 100-120 cells. No more developed humans have successfully been revived after cryopreservation, but cold temperatures have certainly saved the lives of people who otherwise would have been beyond help.

People who suffer cardiac arrests in cold conditions can sometimes survive for hours.
Photo by Fabrizio Conti on Unsplash

One of the most extreme recent examples of such an occurrence is the case of Audrey Mash, who suffered a cardiac arrest while hiking in the Pyrenees during a snowstorm. She survived for 6 hours with a stopped heart until Spanish doctors resuscitated her. Audrey’s body temperature dropped to around 17 degrees Celsius, which slowed the chemical reactions in her cells sufficiently to prevent her death. She also appeared to avoid any brain damage. There are also numerous examples of people ‘drowning’ in very cold waters, only to be revived hours later.

Why are these examples relevant to cryonics? What occurred by accident in these cases is essentially what cryonicists are attempting to do, but over a much larger timescale and with conditions that are currently considered irreversible. When Audrey Mash’s cold body arrived at the hospital, doctors had the medical knowledge and equipment necessary to revive her. Would Audrey have survived the same ordeal 500 years ago? It’s unlikely. Given another 500 years, we might be able to revive many people who would currently be considered dead. These examples show that cooling the body can extend the ‘revival window’ until that body can be brought to a hospital. Cryonicists want to extend the revival window until the body can be brought to the hospital of the year 2500 (though preferably sooner than that).

Cryopreservation before death:

The process of decomposition begins the moment the heart stops beating. In cells throughout the body, chemical reactions begin to expend the remains of the now limited oxygen supply, causing a build-up of carbon dioxide. This quickly kills neurons in the brain, and creates acidic conditions that destroy cellular membranes, releasing digestive enzymes that cause further damage.

All of the examples of cryopreservation given above involve the preservation of living tissue. Cryonics faces an additional challenge: it must not only preserve the entire organism with minimal damage, but must also reverse whatever killed the organism, as well as repairing any damage that was sustained between the organism’s death and its cryopreservation. Given the above, would getting cryopreserved before death increase one’s chances of being revived? Suppose someone is on their deathbed, and has already decided to be cryopreserved. Why not begin cooling them immediately to slow the chemical reactions in their cells, and then set about replacing their blood with vitrification chemicals? This way, scientists of the future need only figure out how to reverse the vitrification, following which the patient can be given a new body, have their mind uploaded to the cloud, or whatever form of immortality is available at the time.

Currently, a person needs to be declared legally dead before they can be cryopreserved, which usually involves irreversible cessation of heart rate, breathing and brain activity. Since hypothermia can suppress brain activity, patients usually need to be warm in order to be declared dead. Of course, there’s also the fact that a doctor deliberately inducing hypothermia and then draining someone of blood would in most legal systems be considered to have murdered their patient. For the above reasons, no one has ever been cryopreserved alive, at least to the best of our knowledge.

There is one path that could allow people to be cryopreserved alive, and that’s under the guise of euthanasia. The idea is that you would sign up for cryopreservation, then travel to a country where assisted dying is legal and get yourself ‘euthanised’ at a cryopreservation lab. Russian cryonics company KrioRus (which we discussed in our article summarising the main cryonics companies) previously stated its ambition to set up such a centre in Switzerland, though I wasn’t able to find any information on whether any progress has been made towards that goal, or what legal hurdles would need to be overcome.

People with little knowledge of the science behind cryonics are often quick to dismiss it as hopelessly unrealistic or even as a scam. I hope that this article has convinced you that even if the ability to revive the dead is still a very long way off, there is plenty of real science behind the cryonics industry, some of which will have important medical applications in the not-too-distant future.

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