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Feed-back Controlled Thawing Experimental
Data
To
illustrate the benefits of controlled thawing with the TCU,
an experiment was conducted on a single well with DMSO, fitted with an
instrumented TCU; a thermocouple was also frozen in
place in the center of the well, as previously described. The
TCU was instrumented with a resistance heater and a
thermocouple for computer feed-back control, including a PID
controller algorithm.
The frozen well with DMSO was removed from
the freezer and allowed about a minute to equilibrate to ~ +6°C with
the room-temperature TCU. Then, at time zero, the
command was given to thaw the well, yielding the results shown in the
figure to the left.  The black line is the so called set-point
temperature, and represents the desire for the TCU to
instantly attain the maximum allowable temperature of 30°C. Since the
TCU is only at ~ +6°C, the feedback control calls for
heat input from the heater, thus causing the temperature of the
TCU to increase, as shown by the red curve in the
figure above. Because of the thermal mass of the system, the
TCU slightly lags behind the command, and slightly overshoots
before attaining 30°C in about one minute. For the center, frozen-in
thermocouple shown in blue, there is only a slight increase in
temperature, due to the onset of thawing at the well surface.
Thawing progressively carries on in this
fashion until the thaw-front reaches the center of the well; since
there is now at 3½ minutes liquid all around the center thermocouple,
its temperature increases rapidly, as seen in the figure above. That
is to say, the well was 100% thawed in
only 3½ minutes from start of command. A phenomenal
contrast to the 3½ hours needed on the bench! Of course, starting
from a lower freezer temperature the TCU thawing would
take longer, too, but minutes longer, not hours. After the thawing,
the liquid solution temperature approaches that of the controlled
TCU, as seen, and will remain at this temperature
indefinitely for sampling, or until the heater is turned off.
Feed-back Controlled Thawing Experimental
Data Conclusions
The thawing behavior of compounds frozen
in microplates was investigated in two types of experiments: (1)
conventional thawing on the bench, and (2) rapid thawing with a TCU.
The clearest observation from the bench
tests (1) is that samples in wells remain largely frozen for hours
after removal from freezer. Thus, for a center well, even 3 hours was
not enough time to cause 100% thawing on the bench.
This dramatically
illustrates the need for the Rapid Thaw System.
The rapid thaw experiment (2) was
conducted using computer feedback control of the TCU, which was
set to 30°C. For DMSO in a standard one-ml well frozen to -5°C, but
starting at ~+6°C equilibration, this resulted in 100% thawing in only
3½ minutes, without excessive temperature of the solution.
A standard RC-model was fitted to the
frozen-state data, which yielded the RC time constants for the initial
rate of heat transfer, in good agreement with heat transfer theory.
With the model and time constants, the time for onset of thawing was
estimated for various initial (freezer) temperatures.
Overall, it is concluded that there can be
great enhancement to laboratory efficiency with the RTS. In
addition, important savings in compound preservation can be realized
by thawing only those wells in the microplates that are selected for
sampling.
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