From: kqb@cbnewsl.cb.att.com (kevin.q.brown)
Subject: Re: Cryoprotectant Issues
Organization: AT&T Bell Laboratories
Date: Wed, 23 Dec 1992 17:10:35 GMT
X-Crossposted-To: cryonics mailing list

    ------------------------ Forwarded Message ------------------------

> Message-Subject: CRYONICS - Re: Gas perfusion
> From: edgar@spectrx.Saigon.COM (Edgar W. Swank)
> Date: Tue, 22 Dec 92 16:51:00 PST

I'd like to thank Mike Darwin for publicly commenting on mine and
Rich Schropel's ideas in CRYOMSG 1465.
 
His very informative commentary prompts some further questions.
 
I'm glad Mike has determined some candidates for "safe, nontoxic
cooling fluids", Dow Corning polydimethylsiloxane 200 and FC-75.
 
Are both these fluids miscible with water?  If not, is there a
problem using them to "wash out" water-based cryoprotective fluids?
That is, are "pockets" of water-based fluids left inside the
circulatory system?
 
Are these fluids more volatile than water?  Higher vapor pressures and
lower boiling points at given ambient pressures?  If so, then it seems
that a good answer to the "short-circuit" problem when transiting to a
gas would be to surround the subject with a (partial) vacuum.  (note:
keep circulatory system at same or only slightly higher pressure).  At
some pressure and temperature Y (below 0xC) organic fluid will
vaporize and will easily mix with/be washed out by circulating helium.
If organic fluid is more volatile than water, then this procedure
should not result in excessive removel of (solid) water through
sublimation (freeze drying).  Once organic fluid is washed out, vacuum
may be gradually released.  Circulating He may then be used to cool
all the way to LN2 temperature.  Hopefully, since cooling is even, and
strains can be absorbed by flexure of open circulatory system, gross
fractures will be avoided.?
 
Since circulatory system is clear, this process can be reversed in an
attempt at revival.  i.e., warm with circulating He to temp Y, apply
vacuum, washout He with organic fluid vapor, release vacuum, vapor
condenses (no bubbles), continue to warm via circulating organic fluid
to above 0xC, washout organic fluid with water-based cryoprotectant,
washout that with blood or blood substitute, etc, warm to 98.6xF and
try rescucitation.
 
The object is to try to use a clear circulatory system to duplicate
what already works for single cells and early zygotes, which are
routinely preserved in LN2 with good survival rates.
 
I don't completely understand Mike's comment:
 
    As to going down to liquid nitrogen temperature, as far as I know
    there is no reason to do this.  All the interesting events are
    over by - 80xC.  Once you are down to -80xC, and certainly by the
    time you are at the glass transition point (TG) of the water
    cryoprotectant mixture you are using you can cool at as leisurely
    a rate as you like.  Indeed, you may not want to cool much below
    TG unless you want the organ to fracture into pieces.
 
The interest in LN2 is for long-term storage. Referring to Hugh
Hixon's article from CRYOMSG 15, -80xC is clearly too warm for
long-term storage (1 second of body temp decay takes 17 minutes).
 
I would guess -164xC (L Methane) is the warmest useable (1 sec at body
temp = 42 years) long-term storage temp.  LN2 is overkill (24 Million
yrs.),  but is used because it's economical.
 
This is not to say that a "round trip" suspension/revival to
-80xC would not be a monumental accomplishment.
 
I would guess that what you need that you don't already have to
perform the above experiment is a vacuum chamber suitable for
a cannulated small animal.  Any idea what such a tool would cost?
 
You didn't comment on use of trehalose as a water-based
cryoprotectant. Seems to me this is worthy of investigation as
it's likely to be less toxic than either glycerol or DMSO.

--
edgar@spectrx.saigon.com (Edgar W. Swank)
SPECTROX SYSTEMS +1.408.252.1005  Silicon Valley, Ca
Newsgroups: sci.cryonics