The container that holds a cryopreserved person for a century is, conceptually, the same object as the thermos that keeps your coffee hot. It is just much larger, and aimed at the opposite end of the temperature scale. That is not a cute simplification; it is the key to why long-term storage is so reliable. A freezer keeps things cold by working continuously and fails the instant the power goes out. A dewar keeps things cold by doing almost nothing, and that difference is the whole reason cryonics storage does not depend on the grid.
So it is worth understanding exactly how a vacuum flask defeats heat, and how that humble physics scales up to a three-meter steel vessel holding four people at -196°C.
A thermos beats heat three ways
Cryogenic storage dewars are vacuum containers for storing and transporting cryogens, in our case liquid nitrogen. They are named after Sir James Dewar, the British chemist and physicist who invented the vacuum flask, the "Dewar flask" or thermos, in 1892. Think about what a good thermos actually pulls off: it keeps a drink cold through hours of sun, or hot through a freezing day, with no power at all. The trick is that it attacks all three ways heat moves.
The design is not complicated. A container has two walls with the air between them pumped out to make a vacuum gap, a tight stopper, and a reflective layer on the surfaces facing that gap. Those three elements together shut down heat transfer. The vacuum gap blocks conduction: there are almost no molecules in the gap to carry heat across. The tight cap blocks convection, because air cannot circulate in and out. The reflective layer fights radiation, bouncing heat back rather than absorbing it. A cryogenic dewar is technically the same device, just specialized for liquids at extremely low temperatures and built much larger, especially when the contents are a person rather than a beverage. At that size the single reflective layer becomes many. Large cryogenic vessels line the vacuum gap with multi-layer insulation. That means dozens of alternating sheets of reflective foil and non-conducting spacer. Each sheet sends a little more of the radiant heat back where it came from.
From cell samples to whole people
Dewars come in many shapes and sizes, dictated by what they hold. Smaller ones store cryopreserved cell cultures, tissue, semen, and embryos, all of which can be warmed and used successfully later, a reminder that suspended-and-returned biology is routine, not speculative, the same foundation under biostasis itself. Suitably built dewars also hold cryopreserved patients, either the whole body or, for neuro patients, just the brain.
Providers like Tomorrow.bio typically use stainless-steel dewars that resist deterioration over long periods, stored and monitored in the long-term storage facility. The most common whole-body dewars stand about 3 meters tall and 1 meter wide and hold up to 4 full-body patients. Neuro patients are usually kept in a dedicated column in the center of the dewar.
Why the nitrogen has to be topped up
As anyone with a thermos knows, the coffee does eventually go cold. A high-quality flask delays it, but heat always wins in the end. A dewar is no different, even with advanced insulation, and the main reason is the lid. A perfectly sealed vessel would insulate better, but you would never be able to move patients in and out, so the lid is a deliberate trade of some insulation for access.
Liquid nitrogen exists in liquid form only at or below -196°C; warmer than that and it boils into gas and disperses. So a small amount of the nitrogen in a dewar slowly warms, boils, and turns to gas. The vessel must let that gas escape. Otherwise pressure would build until it ruptured, an outcome we very much want to avoid. The flip side of boil-off is refilling. Any dewar preserving biological material has to be topped up periodically. It is not frequent, but it is a real recurring cost of a process meant to last a very long time. In practice, facility operators refill cryogenic dewars roughly weekly, mostly as a safety margin rather than because the nitrogen runs that low. Even with this upkeep, liquid nitrogen remains the most practical, cheapest, and most environmentally friendly way to store patients today.
Why patients are stored head-down
One detail looks macabre until you see the logic: patients are stored upside down, head at the bottom. Some nitrogen will inevitably boil off over time, so the liquid level can fall. The part of the patient you least want exposed is the brain. Current understanding holds that everything making a person who they are is encoded in its structure. Storing the patient head-down keeps the brain at the bottom of the dewar, the last place to be affected by any loss of liquid nitrogen, even in the most extreme and unforeseeable circumstances. It is a simple geometric insurance policy on the most important tissue there is.
A cryonics dewar is a giant thermos that protects a person using no electricity at all, which is exactly why it keeps working when everything that needs electricity has already failed.