Here is a genuinely awkward fact about how we store patients today: the temperature that makes storage cheap and reliable, -196°C, is colder than the temperature that would best protect the tissue. We do not cool patients that far because the physics demands it. We do it because that is simply how cold liquid nitrogen happens to be, and liquid nitrogen is what we have. This article is about the gap between what is convenient and what is optimal, and the experimental storage method that tries to close it.
Tomorrow.bio continuously researches ways to improve preservation. The early stages are already refined: SST teams deliver the only professional whole-body field cryoprotection procedure available, state-of-the-art cryoprotectants limit cell damage during cooling, and liquid nitrogen gives a very stable storage environment. But further refinement is possible, and one open question is the storage temperature itself.
The problem is not storage, it is the trip to storage
Most cryopreserved people are stored in liquid nitrogen at -196°C. The method is inexpensive, sustainable, uses no electricity, and is easy to maintain, with refills needed only about weekly. So what is the issue?
During cryopreservation, patients enter a glass-like vitrified state at around -130°C. After that, they are cooled further to -196°C, the natural temperature of liquid nitrogen, where their temperature then holds steady. The trouble is not the resting at -196°C. It is the journey between -130°C and -196°C.
Bluntly, -196°C does not preserve meaningfully better than -130°C. Liquid nitrogen just happens to sit at a temperature cryopreservation can work with; if anything, it would be more convenient if its temperature were closer to, but still below, the glass transition point. Uniform cooling minimizes thermal stress as tissue passes below the vitrification temperature, and holding the temperature near the glass transition for a while just after vitrification lets stress relax before cooling continues. Driving all the way down to -196°C, by contrast, causes tissue to fracture. These fractures do not necessarily destroy the crucial neuroanatomical information, but they do complicate future recovery, which makes them one of the real technical challenges for high-quality preservation.
Why cooling cracks the glass
The mechanism is ordinary physics. Molecules vibrate, and that vibration sets a characteristic volume at any given temperature; cool an object and it contracts. This is thermal contraction. When a vitrified body cools, its warmer interior cools and shrinks slightly faster than its outer shell. [1] Inside and outside are bonded together, so the mismatch in shrinkage builds stress. Past a point the glass-like solid relieves that stress the only way a brittle solid can, by fracturing. Scale sets how severe this is. A few milliliters of tissue or a single embryo equilibrates fast enough that no meaningful gradient forms, so such samples reach -196°C intact. A human body holds a gradient across tens of centimeters for hours, which is why fracturing shows up at our scale and not in the lab. The colder you push below the glass transition, the more contraction mismatch you accumulate.
Intermediate Temperature Storage: stop the cooling sooner
The fix follows directly from the diagnosis. If most fracturing happens on the way from -130°C down to -196°C, then cool slowly through the glass transition and stop sooner, holding the patient nearer the vitrification point. That is exactly what Intermediate Temperature Storage (ITS) does, named for a storage temperature sitting somewhere between vitrification and liquid nitrogen, around -140°C. Thermal stress is lower when cooling beyond the glass transition is halted earlier, and past data shows damage is more likely the faster the tissue was cooled. [2] On paper, ITS is the better preservation.
So why is it not standard? An honest cost-benefit
Because better preservation is not the only variable, and this is where calibration beats enthusiasm. ITS is not commonly used outside research, for reasons that are practical rather than theoretical, and they compound:
- Far less nitrogen buffer. A traditional -196°C dewar is kept nearly full, over 1000 liters, enough to maintain preservation for months before it would run dry. The weekly refill is a routine, not a deadline. A typical ITS dewar holds only about 120 liters at the bottom, at most enough for roughly 5 days at -140°C. Less buffer means less margin for error.
- Higher consumption, smaller capacity. ITS dewars burn through about twice the liquid nitrogen of their colder counterparts. They also offer only about a third of the holding capacity. Much of the interior has to stay empty, because that is the vapor space the patient sits in. Like standard dewars they can refill automatically, but they demand more resources and more frequent checks to hold a stable temperature.
- More ways to drift. Holding a precise intermediate temperature is inherently harder than letting a vessel sit at the fixed boiling point of nitrogen. Potentially damaging temperature fluctuations are more likely, which makes ITS, as currently built, the less reliable and less safe option.
These drawbacks are solvable, but solving them drives storage upkeep costs up sharply. So the field sits with a real trade-off. The method reduces fracturing in principle. In practice it currently raises the risk of a temperature excursion. An excursion threatens the whole patient, not one fracture plane. For now, classic dewars at -196°C win. They are stable and they carry an enormous nitrogen buffer. Over a century the metric that matters most is not the lowest theoretical damage. It is the lowest chance of a catastrophic failure.
Intermediate Temperature Storage trades less tissue fracturing for less margin against catastrophe, and until that margin closes, the safest place for a patient is still the deep cold that needs nothing but topping up.