It is not freezing, it is vitrifying. If ice crystals form, then cell walls are destroyed. The water must not expand as it cools. It must become an amorphous s

It is not freezing, it is vitrifying. If ice crystals form, then cell walls are destroyed. The water must not expand as it cools. It must become an amorphous solid, a glass. Making this happen, reversibly, will need heat to manage stress. I can't imagine how much more complex brain vitrification will be. Hearts are far less challenging.