system
Given the following post, respond as if you're commenting on Reddit:
human
Why can human embryos be frozen and thawed but fully grown humans cannot survive the thawing process? (or can they?)

Why can human embryos be frozen and thawed but fully grown humans cannot survive the thawing process? (or can they?)
gpt
As you know, a chunk of ice occupies more volume than the water that froze to create it. If you fill a bottle with water and put it in a freezer, the bottle will burst when the water freezes.

Human cells are mostly water, and if you freeze one, the ice will crystallize, increasing in volume, damaging components of the cell and potentially rupturing it. If you want to freeze a cell, or group of cells like an embryo, without damaging it, you need to prevent the formation of ice crystals. This can be achieved by either adding a cryoprotectant, a chemical that prevents the formation of ice crystals, or flash freezing. Flash freezing requires that the water be frozen so quickly that there is no time for crystallization to occur. Instead, the water simply "stops" in place, forming an amorphous solid, like glass, without increasing in volume the way ice crystallization does. 

Modern cryopreservation of embryos uses a combination of cryoprotectants and flash freezing, or vitrification. In order for flash freezing to be effective, the entire cell needs to be cooled very quickly. This means it only works on a small volume of water, such as the tiny amounts present in an embryo. If you tried to flash freeze a large quantity of water, say a gallon jug by submerging it in a vat of liquid nitrogen, you'd find that only the outer layers would cool quickly enough to vitrify, and the water at the centre of the jug would cool more gradually, forming ice crystals. This is part of why the process can't work on a fully-grown human. We're too big to be cooled fast enough.

Another way of preserving embryos is to cool them so slowly that water is able to escape the cell as ice forms. This prevents bursting of the cell by preserving the volume as it freezes. Again, this would be impossible to achieve in a human body, since our body regulates its own temperature. Were you able to cool an entire human uniformly, it would still be impossible for water to escape from every cell in the body.