Why is cryopreservation at -196 degrees Celsius?

Here is a question worth taking literally: what are we actually trying to do when we preserve a person? The answer is not make them cold. Cold is the method, never the goal. The goal is to stop time for a body, to halt the biology so completely that the gap between one year and ten thousand years stops mattering. Almost everything else, including the oddly specific number in the title, falls straight out of that single requirement.

So the useful question is not why so cold. It is: how cold do you have to get before chemistry stops? That threshold is the glass transition of the cryoprotectant solution, around -130°C. Below it, molecular motion is arrested and metabolism has stopped completely, so there is no chemistry left to run. -196°C is not that threshold. It is the boiling point of liquid nitrogen, which sits well below the line that matters and holds itself there without help.

Decay is just chemistry that has not stopped yet

When we say a body decays, we are describing chemistry: enzymes cutting molecules apart, reactions running, microbes doing what microbes do. Every one of those processes needs molecules to move and collide. Slow the motion and you slow the chemistry. Slow it far enough and, for all practical purposes, you stop the clock.

There is a handy rule of thumb here. As a rough approximation, dropping the temperature by about 10°C roughly halves the rate of a typical reaction. That does not sound dramatic until you stack enough of those halvings on top of each other. Going from body temperature down into deep cold does not slow decay by a factor of two or ten, it slows it by factors with a great many zeros after them. The race against cellular decay that starts the moment the heart stops is, in the end, a race to get the temperature low enough that the race itself no longer matters.

Ice is the villain. Glass is the hero.

You might assume the trick is simply to freeze someone. It is not, and water is the reason. Water, that most dependable of household substances, betrays you the instant it freezes: it expands, and the growing ice crystals shred cell membranes and tear apart delicate structures. The crystals are not the only problem. As pure water crystallizes out, the salts and other solutes left behind concentrate in the shrinking pocket of liquid that remains. Cells sitting in that increasingly concentrated solution lose water and are damaged osmotically. No crystal ever touches them. Freezing a person the way you freeze a steak would be an efficient way to destroy exactly the things you are trying to keep.

The way around this is vitrification. Most of the body's water is replaced with cryoprotective agents, a kind of medical antifreeze, and the tissue is cooled fast enough that it never crystallizes. Instead it sets into a glass-like state: a solid with no ice, no sharp crystal edges, nothing expanding and rupturing. Vitrified tissue is frozen in the everyday sense of very cold and solid, but it is emphatically not frozen in the destructive, crystal-forming sense. That distinction is the entire game.

The number that really matters is about -130°C

Here is the part that surprises people: the temperature that truly matters is not -196°C at all. It is the glass transition temperature, somewhere around -130°C, the point below which the vitrified state locks rigidly into place. Above it, the glass-like state can slowly relax. Worse, it can begin to recrystallize. Recrystallization is just ice showing up late to the party. Below it, molecular motion has dropped so far that the structure simply holds.

So if -130°C is where the glass-like state becomes stable, why go all the way down to -196°C? For the same reason you do not set your freezer to exactly 0°C and cross your fingers: you want margin. Storing tissue more than 60 degrees below the glass transition keeps it deep inside the safe zone. That is far from any temperature where the glass-like state might soften or ice might creep back in. It turns stable-if-nothing-goes-wrong into stable-with-a-large-buffer-against-things-going-wrong, which is the only kind of stable worth betting a life on.

Why -196 exactly? Nature hands us a free thermostat.

The specific value of -196°C is not chosen by committee. It is the temperature at which liquid nitrogen boils, and that one fact makes it almost unfairly convenient.

A boiling liquid holds its temperature. As long as there is liquid nitrogen in the container, the contents sit at -196°C, no warmer and no colder, no matter what the room is doing. That is a self-regulating thermostat with no moving parts, no compressor, and crucially no dependence on electricity. Patients and samples rest inside vacuum-insulated dewars, essentially very serious thermos flasks, which slow the heat leaking in to a crawl. The only routine maintenance is topping up the nitrogen that slowly boils away. Compare that to a mechanical freezer, which fails the moment the power does. The laws of thermodynamics, unlike the local grid, never call in sick.

Nitrogen also happens to be cheap, abundant (it is most of the air you are breathing right now), inert, and non-flammable. If you set out to design an ideal long-term storage coolant from scratch, you would struggle to beat the stuff we can pull straight out of the atmosphere.

The cost of going that cold

Liquid nitrogen has no moving parts, and that convenience has a price. Between the glass transition and -196°C there are another sixty-six degrees of cooling, and the vitrified solid contracts the whole way down. By then it is brittle, so the contraction cannot relax. That builds mechanical stress, and past a threshold the material relieves the stress by cracking.

The effect is reproducible in the laboratory. Cracking in vitrified cryoprotectant solutions tracks thermal contraction and the temperature gradients that fast cooling creates across a large object. Solutions with a higher glass transition temperature crack less, because less cooling remains once the material has turned solid.

A whole body is the hard case. Fracturing usually starts below the glass transition. If cryoprotectant perfusion went poorly it can start as warm as -90°C. The cracks run from ones you could see down to microscopic separations. A fracture parts tissue along a plane rather than scrambling it, so the structure on either side survives, but every crack is damage a future repair has to undo.

The current answer is patience. The whole cooldown takes close to a week. The descent through the glass transition and below it is the slowest part, run at roughly one degree per hour under computer control. Slower cooling means smaller gradients, and smaller gradients mean less stress.

Intermediate Temperature Storage

The better answer is to stop cooling earlier. Intermediate Temperature Storage holds the patient near -140°C instead of -196°C. That is still ten degrees below the glass transition rather than sixty-six, so molecular motion stays arrested and nothing decays, with far less contraction to survive.

The engineering is passive. A reservoir of liquid nitrogen sits at the bottom of the dewar and the patient is held in the cold vapour above it, with the vapour space regulated to the target temperature. No compressor, and no dependence on the power grid, which is the same reason liquid nitrogen is trusted for storage in the first place.

The first human-size ITS dewar arrives at the European Biostasis Foundation facility in Switzerland in August 2026. The first job is measuring how much liquid nitrogen it actually consumes, because that number sets the cost of storing a patient for a century. ITS is not offered to members until that cost is known.

What actually survives down there

At -196°C, biological time effectively stops. Enzyme activity halts, microbes cannot grow, and the spontaneous reactions that would otherwise dismantle tissue have nowhere near enough energy to proceed. The molecular architecture of the body, and above all the brain, stays exactly where it was.

That last point is the one that matters most. The bet behind biostasis is that what makes you you is encoded in physical structure: the connections and patterns inside the brain, which is the subject of memory, identity and the brain. Hold that structure still and you preserve the information, even if the technology to read it back and restore function does not exist yet. The cold is not magic. It is a pause button, pressed hard enough that the information has time to wait for the future to catch up.

None of this requires a flawless machine or an unbroken supply of electricity. It requires a glass-like state instead of ice, and a temperature comfortably below the glass transition. It also requires a boiling liquid that pins itself to exactly the right value. That is the quiet elegance of -196°C: it is the temperature at which a preserved person can, in the most literal sense available to us, wait.

Further reading

• Why can't we cryopreserve a person alive?
• Preservation without freezing
• Why is revival currently not possible?
• Critical glossary