(Intro)

It's an idea that's been in science fiction for decades. Could you take a dying person, freeze them, and then, in the future, thaw them, cure them, and then introduce them to their robot butler? It's the promise of cryonics: the process of preserving people at very low temperatures so they can be revived in the future. But no one's really sure if it'll ever be possible.

Cryonics as we know it today began in 1962, when college physics teacher Robert Ettinger suggested in his book, The Prospect of Immortality, that freezing people could preserve them so that future, more advanced medicine could revive them.

Cryonics researchers work under the assumption that a person isn't truly dead until the information in the brain becomes lost. The idea is that if you preserve the mind, you can preserve the person - the body itself can be fixed in the future, once we develop the technology for it. But for cryonics to work, there are a lot of problems that need to be solved.

To successfully cure a person who's been preserved, you have to cure the effects of being preserved, and then you'd have to cure what killed them, and then you'd have to bring them back to life. Sounds easy enough, right?

The first step - preserving people in a way that doesn't do too much damage - is the one where we've made the most progress. But even it hasn't been perfected. To preserve someone, cryonics experts bring their temperature down to -196 degrees Celsius, usually with liquid nitrogen, in a process called vitrification, from vitreum, the Latin word for glass. Vitrification is used today to preserve human embryos for people undergoing IVF therapy, and those preserved embryos can be revived and implanted after as long as 12 years. But it's a lot easier to preserve and then revive 8 cells than an entire human body. For one thing, the cooling process can take a while, which means more time for cells to degenerate.

There are companies that have the option to just preserve the brain, because it's a faster process than preserving the whole body. They figure that someday medicine will get to the point where brains can be attached to something else, or even uploaded to a computer.

But just trying to freeze someone forms ice crystals, which damages blood vessels and dehydrates cells. Experts deal with this by totally replacing the blood with chemical cryopreservants - antifreeze, basically. When it freezes, the antifreeze doesn't crystallize the way plain old blood would - instead, it forms a glass-like, crystal-free solid.

Even with cryopreservants, though, bigger things like organs have a nasty habit of fracturing when cooled - which is bad, if you want to use them later. In 2009, researchers from the California-based group called 21st Century Medicine announced that they'd used similar techniques to preserve and then thaw a rabbit kidney. They put the kidney back into a rabbit, where it worked fine for 48 days. At that point, the group decided that the experiment was a success and they euthanized the rabbit so they could study it.

The results are promising for the future of organ donations, because it would be much easier to find a match if organs and tissues from organ donors could be preserved. The fact that the kidney still worked could also mean that we could effectively preserve at least some organs. But the human brain is much more complicated than a rabbit kidney. Even if current techniques are enough to keep a kidney in working order, they might damage structures in the brain. And anyway, we don't really know how much of a person's consciousness is still stored in their brain structure, once they're dead.

There is some evidence supporting the idea that memory sticks around after cryopreservation. In 2015, Nematode worms that were trained to respond to certain smells still responded to these smells after they were cryopreserved and revived. That's encouraging, but still: worms are a lot simpler than people.

Then there's the matter of curing whatever killed the cryopreserved person in the first place. There are always new treatments being developed, though, so it does seem likely that in the future, we'll be able to cure at least some of the things that we consider fatal now. But reviving people so they can be cured and then go on living their lives? That's where cryonics runs into a lot of problems, because no one has any idea how to do it. The brain would definitely be at least somewhat damaged from the cryopreservation process. And the chemicals used in the cryopreservant that replaces the blood are toxic, which means more damage.

So how do we fix the damage and get everything up and running again? Some cryonics experts say that nanobots are the answer: they could go in and fix the damage, maybe even on the cellular level. But on both the technical and medical sides of things, nanobots that could do that are a very long way off.

So when it comes to preserving people for the future, there are still a lot of unknowns. Cryonics may have worked for Captain America, but in the real world, there's still a lot more research that needs to be done before we even know if it's possible.

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