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Gav comments on Dry Ice Cryonics- Preliminary Thoughts - Less Wrong
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I'm stuck at work for a while, so this is going to be painfully short, sorry.
The bit you're missing is getting below the glass transition temperature prevents both heterogeneous and homogenous nucleation. Dry ice is still well above the glass transition temperature.
Quickest online result I could find for the relevant graph is here: http://www.benbest.com/cryonics/vitrify.html, in section III. Axis labels are "Cryoprotectant concentration" and "Temperature (*C)"
(Although there's a nicer graph Fig3, p36 in Wolker's "Cryopreservation and Freeze Drying Protocols", which just came out this month. Probably not online as of yet)
At very high cryoprotectant concentration (right hand side of the graph) you can transition from 0C to below the Tg without getting in either danger regions (heterogenous nucleation, and homogeneous nucleation). At moderately high cryoprotectant concentration you can transition vertically from 0C to below Tg and only pass through the heterogeneous nucleation danger region, avoiding the homogeneous nucleation region. You typically do this as quickly as posssible, both CI and Alcor have computer controlled systems to accomplish this. With no cryoprotection, or poor perfusion, you pass through the homogeneous nucleation region and ice formation is impossible to prevent.
A typical cryopreservation of a person would have both well and poorly perfused areas, so getting through even the 'safer' danger region of heterogenous nucleation is something you want to do as quickly as possible to prevent ice crystals forming.
/I'm not a doctor, this is just what I've gathered from looking at the research. Hope this helps :-)
Also just another thing that might be interesting:
Check out 'intermediate temperature storage', the idea of storing at a slightly warmer than liquid nitrogen temps (-130'C as opposed to -196'C) is a good idea in order to avoid any fracturing*. This is right near the glass transition temp, so no nucleation can proceed.
Tricky part is there aren't any practical scalable chemicals that have a handy phase change near -130'C, (in the same way that liquid nitrogen does at -196'C) so any system to keep patients there would have to be engineered as a custom electrically controlled device, rather than a simple vat of liquid.
Not impossible, but adds a lot of compexity. They might end up doing it in a few years by putting a dewar in a dewar, and making a robust heater that will failsafe down to LN2 if there's any problem.
*Personally I'm not concerned with fracturing, it seems like a very information-preserving change compared to everything else.
Phase changes are also pressure dependent; it would be odd if 1 atm just happened to be optimal for cryonics. Presumably substances have different temperature/pressure curves and there might be a thermal/pressure path that avoids ice crystal formation but ends up below the glass transition temperature.
1 atm pressure has the advantage of costing nothing and requiring no equipment to maintain.