Cryonics is the practice of cooling a legally dead person to very low temperatures, usually in liquid nitrogen at about −196 °C (−321 °F), in the hope that future medicine might one day be able to repair them and restore them to life. It is not the same as freezing food, and it is not a way to "pause" a living person. Here is what really happens, in order, and where the hard scientific problems are.
Step 1: Legal death comes first
In the United States and most other countries, cryonics can only begin after a person has been declared legally dead. Doctors declare death in the normal way, usually when the heart and breathing have stopped and cannot be restarted. Performing cryonics on a living person would be treated as killing them, so it is not an option even for people with terminal illnesses who would like to start early.
This creates the central difficulty of cryonics: by the time the procedure is allowed to start, the body has already begun to suffer damage from lack of oxygen.
Step 2: Standby and stabilization
If death is expected, a cryonics organization may send a standby team to wait nearby. Once death is pronounced, they act quickly:
- The body is placed in an ice-water bath to start cooling.
- A mechanical device may restore circulation of the blood (without restoring life) so that cooling and medications reach the tissues.
- Medications are given to reduce blood clotting and slow cell damage.
The aim is to slow down the chemical breakdown that begins within minutes when blood stops flowing, especially in the brain. When no standby team is present, for example after a sudden death, this step may be delayed by hours or days, and the damage is greater.
Step 3: Transport
The cooled body is then taken to the cryonics facility, or in some arrangements, a surgical team does early steps closer to where the person died. Speed matters, but legal paperwork, funeral directors and airlines all take time.
Step 4: Replacing water with cryoprotectant
If you simply froze a human body, water inside and between cells would turn into ice crystals. Ice expands and tears delicate structures apart, which is one reason frozen strawberries go mushy when they thaw.
To reduce this, cryonics teams pump a cryoprotectant solution through the blood vessels. It works like a medical-grade antifreeze, replacing much of the water in the tissues. Common ingredients are related to chemicals used to preserve human embryos and eggs in fertility clinics.
Step 5: Vitrification, not freezing
With enough cryoprotectant, the tissue does not form ice as it cools. Instead it becomes a glass-like solid, a process called vitrification. Modern cryonics providers aim for vitrification, at least in the brain, because it avoids most ice damage.
Vitrification is real, well-established science for small things. Fertility clinics vitrify embryos and eggs every day and later warm them successfully. In 2023, researchers at the University of Minnesota vitrified rat kidneys, stored them for up to 100 days, rewarmed them and transplanted them into rats that then lived on the transplanted kidney. That was a major step for organ banking, but a kidney is far simpler than a whole human being.
Step 6: Cooling to liquid nitrogen temperature
The body or brain is cooled slowly over several days to around −130 °C and then to liquid nitrogen temperature. At that point, chemical activity essentially stops, so the patient can in principle be stored for a very long time without further change. Patients are kept in large insulated containers called dewars, which are topped up with liquid nitrogen regularly. Liquid nitrogen is cheap and the containers need no electricity, which helps with long-term reliability.
Whole body or brain only?
People can choose whole-body preservation or neuropreservation, where only the head is preserved. The idea behind neuropreservation is that memories and personality are stored in the brain, and that a future technology capable of repairing a brain might also be able to regrow or replace a body. It is much cheaper, which we cover in how much does cryonics cost. Whole-body preservation appeals to people who feel the body is part of who they are, or who doubt that future medicine could replace one.
The unsolved problems
Supporters and critics agree that several serious problems remain unsolved:
- Damage before preservation. Minutes without oxygen injure brain cells. Hours or days do much more harm.
- Cryoprotectant toxicity. The chemicals that prevent ice are themselves toxic at the high concentrations needed.
- Incomplete perfusion. Blood vessels may be blocked by clots, so some areas may not receive enough cryoprotectant and may form ice.
- Cracking. Large vitrified objects can fracture from thermal stress as they cool. Rewarming without cracks or ice is a further challenge, which is why the kidney study above used a special "nanowarming" method.
- No proof of revival. Nobody has ever revived a cryopreserved human or a whole cryopreserved mammal. Any revival would depend on technologies that do not exist yet, perhaps including tools that can repair tissue at the level of individual cells and molecules.
Because of these problems, mainstream cryobiology, the science of how living things respond to low temperatures, has long kept its distance from cryonics. Cryonics organizations themselves are careful to call it an experiment, not a treatment.
Why do people still choose it?
Most people who sign up see cryonics as a long shot that is better than the alternative. Their reasoning goes like this: burial or cremation leaves a chance of zero, while preservation leaves a small, unknown chance. Others think the money and effort are better spent elsewhere, or feel strongly that life should end once. Both views are reasonable. We look at the evidence in more depth in will science ever bring people back after death? and at the brain side of the question in brain preservation explained.
Your wishes matter either way
Whatever you think of cryonics, there is a simpler question everyone can answer: if revival ever did become possible, would you want it? Many families have never talked about it. Writing your answer down, with any conditions, means nobody has to guess on your behalf. See how to leave instructions for future revival for a simple way to do that.