X-Message-Number: 3644
From: Brian Wowk <>
Date: Sun, 8 Jan 95 20:37:57 CST
Subject: SCI.CRYONICS Reply to Ben Best


	I now admit, Ben, that I may have dismissed your molecular
mobility concerns too casually the last time we spoke about it.
However I think it is premature for you to say we need lower temperature
storage without some real numbers to talk about.  Right now this
is an argument in a vacuum.  Just what is the rate of molecular
diffusion in vitified water/cryoprotectant solutions at -130'C or
-196'C ?  More to the point, what is the rate of diffusion of *large
chunks* of material (weighing millions of daltons) that are probably
most important for inferring the undamaged state?

	Remember also that we are not talking about completely
vitreous media.  (When and if true brain vitrification is developed,
there will be no damaged material to diffuse around, and none of
this will be a concern anyway.)  When tissue freezes (as it does
with today's cryopreservation methods) we are left with pockets
of unfrozen (vitreous) water/cryoprotectant *in between* ice
crystals of pure water.  The very ice crystals that cause the
damage in the first place later serve as framework to help hold
things in place.  Although I cannot rigorously prove it, I am
extremely doubtful that diffusion of debris in vitrified spaces
below the glass transition temperature (-130'C) is of concern
in cryonics.

	As a starting point for further discussion, it might be
worthwhile to look once again at Peter Mazur's class review paper,
"Mechanisms of freezing injury, and implications for living tissue"
(title paraphrased from memory).  ("class" should be "classic" in
line above.)  You know the one I mean.  There is a brief mention
there of the *quantitative* change in viscosity that occurs below
Tg in tissue, and perhaps a further reference.  Perhaps the
physical chemists in the audience can tell us if there is a
simple way to compute rates of molecular diffusion from viscosity
data.

--- Brian Wowk

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