X-Message-Number: 18925
From: 
Date: Sun, 14 Apr 2002 20:28:05 EDT
Subject: Magnetic Fluid Heating

In a message dated 4/14/02 2:00:59 AM Pacific Daylight Time, 
 writes:

> "Magnetic fluid"? A changing magnetic field will produce electric potential 
>  differentials (voltages), fluid or no fluid (Maxwell's equations), and if 
>  there is a material there that conducts electricity there will be an 
induced 
> 
>  current and Joule heating. As far as I can see, introducing actual 
magnetic 
>  dipoles would not be especially helpful.

In theory what is said above is true. However, in practice there are a number 
of considerations that make intravascular "magnetic fluid" warming 
interesting for near term applications. Two such application which come to 
mind are recovery of vitrified organs, or revival of human patients who have 
been cryopreserved with little or no injury from the preservation or dying 
process and are young enough to benefit from revival when the proximate cause 
of their deanimation is treatable.

There are several practical problems associated with radio frequency (RF) or 
microwave rewarming in real world applications:

1) Coupling of the radiation to the material being re-warmed is usually poor. 
Adjusting the frequency or wavelength can help, but it is still a practical 
problem of some magnitude.

2) It is difficult in practice to re-warm even very homogenous and uniformly 
shaped objects (such as a sphere or cylinder of pure cryoprotective solution) 
using either of these techniques, and the larger the object the harder it 
becomes. In biological systems the problems are much greater because the size 
is irregular, the solids in the material are very in-homogenous, and the 
water and cryoprotective agent distribution may be in-homogenous as well. 
Consider a single organ such as a kidney: it has a thin outer cortex, a 
medulla which is composed of structures such as the renal pyramids, and a 
calyx in the center which is where urine collects and is which "empty" space 
(actually space filled with perfusate ultrafiltrate or urine in a living 
animal). All are very different materials in terms of composition (lipids, 
water content, connective tissue, and so on).

So, you have a very in-homogenous object, and trying to re-warm such an 
object without local hot spots or thermal runaway using RF or microwaves is 
challenging. It becomes incredibly more difficult when something as big, 
in-homogeneous, and geometrically complex as a human body is considered. Even 
a human head has bone surrounding the brain and also skin, muscle and 
connective tissue. In short, lots of different tissues which will tend to 
couple with radiation differentially. Further, the water content and CPA 
content of these tissues will be different at the end of perfusion. Bone is 
relatively "dry" compared to brain...

3) Some materials like bone are mineral rich or otherwise alter penetration 
or create reflection of RF and microwaves. This also contributes to localized 
thermal runaway.

Perfusate that is doped with materials to facilitate RF or microwave coupling 
are not a magic solution. However, they do improve coupling as a whole and 
they offer the potential to selectively or regionally increase coupling and 
thus the heating in areas that might normally experience a lag in rewarming 
or re-warm slowly enough to allow for ice growth and freezing. The use of 
such intravascular or intra-viscus media may also allow for more rapid 
overall rates of rewarming decreasing the concentration needed to vitrify 
(CNV) of CPA and thus decreasing its toxicity.

Also not to be despised is the potential for reducing the raw amount of power 
it takes to re-warm, and thus the size of the power source. In the near term, 
power source size can be a serious consideration, especially in the research 
setting. At very high rates of rewarming (300-400 degrees C/min) an RF power 
source the same size and cost of one used by an FM radio station is required 
to re-warm a rabbit kidney or a human kidney! The cost is very high and it is 
complicated by the need to get an FCC license for the transmitter; a process 
which has taken months in the past!

With newer, less toxic vitrification solutions the critical warming rate has 
been reduced drastically so that such rapid rates of rewarming are not likely 
to be required for something as small as a rabbit kidney. However, for large 
organs or whole people some kind of electromagnetic assisted rewarming may 
still be necessary given the current stability of vitrification solutions. 
Even if you could re-warm at 0.5 C/min without freezing this would not be 
possible using conduction for a whole body. Gas perfusion of the vasculature 
or other strategies might achieve this rate of rewarming, but even then it 
might not be sufficient.

In dealing with a whole human body or head it is probably reasonable to 
assume that there will be in-homogenous distribution of cryoprotectant. In 
fact, this was verified for glycerol in the late 1980s in work conducted by 
Leaf and Hixon using a whole body feline model. The less well equilibrated 
areas will require higher warming rates. 

A simple (nonvascular) application of this technology might be to fill areas 
such as the renal calyx, the gut, or other areas which typically end up with 
lower concentrations of CPA with just the right amount of doping material to 
facilitate selective rewarming of the region. In such a situation 
nanoparticles would probably not even be required; micron scale particles 
would probably work since the space is nonvascular.

At any rate, thanks to Brent Thomas for posting this interesting idea. It may 
turn out to be of use after all.

Mike Darwin

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