As I understand, one of the reasons we haven’t figured out something like cryogenic sleep is that cells are irreparably damaged during the freezing and thawing process. How do we manage to avoid those problems when freezing and storing embryos for the purposes of IVF?
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The challenge when freezing cells is that, when you freeze them slowly, the freezing water inside them has time to form large ice crystals. These crystals poke holes in the cells and thereby destroy them.
However, if you freeze cells very quickly (and apply some other tricks), you can make it so the ice crystals that form are very tiny and do not cause any damage (at least to most of the cells). This makes it possible to freeze living cells that are still viable when they are thawed.
So why can’t we do this to a whole human body? Or even just, like, a head? Well, the problem is that you cannot freeze them quickly enough. See, even if you apply really cold temperatures to the outside of a human body, that cold will seep into the body from the outside, in a gradual way. So, even if you can flash-freeze the outside few millimeters, the inside lags behind and still freezes slowly, meaning you get those big harmful ice crystals again.
With something as small as an embryo, this is not a problem (as long as you freeze them early enough in development).
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