X-Message-Number: 5077
Date: Tue, 31 Oct 1995 11:45:30 -0500
Subject: hyperbaric freezing

Thanks to Wowk and Darwin for comments on mildly hyperbaric freezing.

First, as to Darwin's question about pulling a vacuum after cooling a
specimen to near zero centigrade; he asks if that will cause liquid to freeze
instantly at a higher temperature. No; reducing pressure from one atmosphere
doesn't affect the freezing point appreciably, only the boiling point. This
is freeze drying, or near-freeze drying. Of course, evaporation is a cooling
process, so pulling a vacuum will   tend to produce freezing  by this cooling
process (high school teachers like to demonstrate simultaneous boiling and
freezing), but you will not by any means get instant freezing or freezing at
a higher temperature.

Brian's point about the heat of fusion is one I remembered only after the
spur-of-the-moment post, and presumably the process wouldn't work (on large
specimens) for that reason unless the temperature before release of pressure
is below (roughly) negative 80 C, since the heat of fusion of water is about
80  cal/gm. 

If we have to go that low, we will be dealing with pretty high pressures, and
it is no longer so easy and cheap. However, it should still be much easier
and cheaper than the temperatures and pressures used by Fahy, Waitz and
others. Darwin says he gets around 2,000  atmospheres with a chamber large
enough for a brain, the chamber with accessories costing $75,000. Greg Fahy
has previously expressed the opinion that even a full-body capacity chamber
at 2,000--5,000 atmospheres would be affordable in the context of cryostasis,
after considering probable usage rates etc. 

Darwin says the small intracellular ice crystals formed by release of
overpressure will rather rapidly coalesce into larger crystals (because of
the dependence of vapor pressure on crystal size and geometry). How quickly?
At - 80 C the vapor pressure of ice is pretty low (again, I don't have my
references at hand) and hence the recrystallization should be pretty slow,
one would think. Mike says he has watched the recrystallization under the
microscope (at what temperature?), which suggests a matter of minutes or
hours, but maybe not. To avoid the crystal growth, presumably the cooling to
below around - 135 C (where the vapor pressure becomes negligible and
essentially independent of crystal size) would have to be rapid relative to
the rate of growth. This might be feasible. 

Further, one suspects that, since the tissues are mostly solid at this point,
migration of water molecules would have to be pretty slow and there would be
relatively little random churning, avoidance of which was the main idea in
the first place. 

Yes, small intracellular ice crystals can do a lot of damage, the apparent
damage being exaggerated by thawing effects and perhaps by artifacts of
preparation, staining etc; but once again, the idea was not so much to
minimize damage as to minimize degradation of information. 

Also, if we shift the damage somewhat from intercellular to  intracellular,
 this may save relevant information, because the intracellular information is
(I think) mostly generic and not unique to the individual, whereas
intercellular and intertissue connections and structures in the brain are
thought (at least by some) to represent our individuality and memories.

Robert Ettinger

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