This derives from a blog post I made on a discussions about cryonics and I was asked to write it up fully.

Background, cryonics is the general name for the theory that at the point of death you can be stored non frozen in liquid nitrogen and a future civilisation who has the technology to revive you will do so. This is available at a cost of $165,000 (roughly £110,000) to UK citizens. The aim is to preserve the brain such that your memories and personality remain in tact at point of restoration.

The current state of the art is a 50% survival rate of rabbits when you cool their kidney to -130C, reheat it and transplant it back. From there we extrapolate that the same technique will work for human brains stored long term at -196C.

This requires a number of leaps of faith in that the future society is able to regrow a suitable vessel for you to be reanimated into, has solve the problem of eternal life, can cure whatever was about to kill you and can do perfect restoration of cryonically stored brains.

We can't say anything directly about that, so that's just philosophy. However, the part of the process that's implemented today we can look at.

Is the current cryonic freezing processes likely capture the full information of the brain?

I thought I'd attempt to estimate this. A very simplistic model has the brain made up of a large number of cells. Clearly if all the cells have been destroyed you are dead, and if only one cell dies it doesn't matter. So a reasonable first question is

How many brain cells can die in one go without it being a serious issue?

According to The Stroke Association, a stroke is a brain injury with effects which may include difficulty thinking, learning, concentrating, remembering, making decisions, reasoning and planning. Rehabilitation consists of relearning skills, not having your brain recover naturally.

So a reasonable position is that the cryonic chilling process should cause less damage to the brain than a stroke. Jeffrey Saver reports that we usually lose 31 million neurons a year during aging, and 1.9 million neurons per minute during a stroke, with a total of about 1.2 billion during a severe stroke.

The total number of neurons in the average brain is about 130 billion. That means that the loss of 1% of the cells is a reaonable upper bound for the freezing process, and 0.03% would be a sensible target which is consistent with aging one year.

So the next question is,

What fraction of cells die during the current cryonic freezing process?

Alcor currently uses M22 solution to cryopreserve patients, being the same solution they used to cryo-preserve a rabbit kidney and thaw it out leaving the rabbit happily alive. Personal experience within my family suggests that you can happily survive despite losing 90% of your kidney cells.

This protocol for cryopreservation of cell lines suggests that following the protocol will increase the cell survival rate from ~ 1% to > 90% Cryopreservation of cell lines.

Now this doesn't directly answer the question, although freezing individual cells is clearly a much easier problem than freezing the entire brain immediately after legal death. Still a 10% cell failure rate is obviously far too high being way above our upper bound of 1% and our target of 0.03%.

Now some of the cell death occurs during the current rewarming procedure, although again reheating an individual cell is clearly much easier than reheating the entire brain.


Aschwin de Wolf writes at Depressed Metabolism that my analysis is completely incorrect, in particular.

Securing viability of brain cells is a sufficient but not a necessary condition for resuscitation of cryonics patients. Future cell repair technologies are assumed to infer the original viable state of the cells from their morphological properties.

Which I think can be paraphrased as 'nanobots will fix it'.

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