X-Message-Number: 14646
Date: Mon, 9 Oct 2000 11:37:46 -0700
From: Hugh Hixon <>
Subject: coacervats ~= micelles or microcapsules

Yvan Bozzonetti Wrote:

>Now, the question of the day:
>It seems, many good cryoprotectants are very viscous. Has anybody tried to 
>load them in coacervats (sorry, this is the French name, I don't know the
>English translation). They are small vesicles similar to cell wall and
>exploited in some health care products.

Probably *micelles*.  Another, more general term is *microencapsulation*.
Keith R. Dugue suggests *liposomes*, which I think would be a subclass of
micelles.

On microencapsulating viscous cryoprotectants, a major flaw is that at the
endpoint for cryoprotection for, say, glycerol, the volume/volume % is
around 50%.  The concentration of red blood cells in the blood (a good
example of microencapsulation, micelles, and liposomes, but you almost never
see these terms used for RBCs) is only about 40% of blood volume, and blood
exhibits a *non-Newtonian rheology*, which is to say the viscosity effects
get noticably non-linear.  I'm not sure, but I think Einstein examined the
problem of the viscosity of  a particle-filled fluid at one time.  Think of
it roughly as pumping marbles.  The microcapsules have a Reynolds number of
their own.  There is probably some coverage of these problems in a
biophysics text.

So microencapsulation  is good for drug delivery, where the volume is
usually quite small, but would fail to relieve the viscosity problems at the
concentrations needed for useful cryoprotection.  Would probably add to them.

The new Fahy-Wowk cryoprotectants are reportedly substantially less viscous
at vitrification concentrations than is glycerol, but I don't think anyone
has made the effort to make comparable viscosity-temperature-concentration
maps.  That would take lots of points, and would probably have to be an
automated experiment to do it in reasonable time at reasonable cost.  Since
we already know we can get glycerol in at 50% v/v under equilibrium
conditions, it's unlikely that such an experiment will be done, unless we
run into some new problems.

Hugh Hixon

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