Why, because you saw it in a shitty tom cruise movie from the 90s?
Liquid nitrogen at -196°C is not going to flash-freeze a head to the centre, any more than a deep fryer set to 200°C will flash-defrost something frozen dropped into it. It takes minutes, crystals will form, cells will break, proteins will denature.
Freezing organs recoverably. (We already do this routinely with embryos and there's good work, though little success as yet, on freezing and recovering organs. It attracts lots of cryobiology funding.)
Fixing the freezing damage to the original frozen brain. The dendrites (10,000 connections for each of the 100 billion neurons — that's 1015 dendrites to check) are cracked badly by the freezing process — "acoustic fracturing events," like when you drop an ice cube into a drink. What is the process for fixing a frozen brain that's cracked into several or hundreds of pieces, with dendrites shattered at a microscopic level? This is a problem even with vitrification.[21]
The damage may not be mappable, let alone repairable. Damaging energies are required to scan at 5 nm resolutions, where things start going quantum.[22]
Reattaching a severed head or transplanting the brain.
Alternately: reading the patterns from the original brain and writing those to the cloned brain (uploading and downloading minds). Cryonicists speak of mind-uploading as if it's a mere technical detail that's just around the corner, rather than something that we don't even know can be meaningfully discussed.
The cryopreservatives that prevent ice crystal damage are themselves toxic and would need to be removed from the tissues. (This is really a pretty minor problem compared to everything else listed herein.)
Cryonics needs to preserve a high proportion of the mind if it is to live up to its promise. But what is an acceptable threshold? A typical stroke patient loses ~5% of their brain (over 10% in some severe strokes).[23] A severe stroke can be associated with loss of large chunks of personality and memory and the sufferer is frequently severely disabled afterwards, although stroke victims are still considered to be the same person (occupying the same body and all that). For comparison, an adult naturally loses up to 0.5% of their brain volume every year.[24] For another comparison, hemispherectomy, removing half a person's brain (as pioneered by everyone's favourite neurosurgeon[25]) is survivable with, thanks to neuroplasticity, surprisingly tolerable effects on memory, personality and cognitive function, particularly in young children.[26] Of course, there is at present no evidence that cryonics preserves more than 0%.
Once you've fixed the body's cells and the brain paths, you have a recovered corpse. Your next task is to resurrect the dead.
Cryonics is the practice of freezing clinically-dead people in liquid nitrogen (N2) with the hope of future reanimation.
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