by Ben Best
[The report below was written in 2005. To see information that is current as of 2010 see Computer-Controlled Cooling Boxes at CI (from 2010)]
The Cryonics Institute now has two computer-controlled cooling boxes, a large one for human patients and a small one for pets & testing. These cooling boxes are designed to bring the temperature of patients or pets from above water-ice temperature after perfusion to liquid nitrogen temperature for long-term storage. The use of vitrification solution for perfusion mandates rapid cooling before solidification temperature (Tg, near minus 120 degrees Celcius) to prevent ice formation. There should also be slow cooling below solidification temperature to liquid nitrogen temperature to prevent or minimize cracking. Other circumstances or testing may call for other cooling profiles. Our computer-controlled cooling boxes give us the ability to control cooling rates with some precision in any temperature range (with minimal manual supervision), and to log the temperature changes that actually occur so that cooling curves can be generated and studied.
We have previously done cooling with the patient above liquid nitrogen being cooled by gradual lowering toward the vapor -- after initial cooling in dry ice. With the vitrification protocol initial cooling needs to be faster, so we had placed a fan on the liquid nitrogen box. Computer control allows for more precise cooling rate and the automation provides relief from the demands of manual cooling.
The computer-controlled cooling box systems have been
created by the joint efforts of Marc McMaken (a cryogenics
engineer who owns a local cryogenics processing company,
HR & D, LLC),
of Mike R (a LabVIEW programmer who works for
Wineman Technology, Inc),
and of Andy Zawacki & Ben Best (me) of the Cryonics Institute.
Andy built the large cooling box, initially for shipping
and now customized for cooling. I provided the initial
specifications for the boxes and controlling software.
Marc McMaken built the small cooling box and provided the
controlling hardware for both boxes.
|Marc McMaken||Mike R|
The large cooling box is made of wood and extruded polystyrene foam insulation. The small cooling box is made of stainless steel and a type of foam insulation. Both cooling boxes have a long bar on the inside which is perforated with small holes that can shoot-out liquid nitrogen. Although the liquid nitrogen is quite cold, much of the cooling comes from heat absorbed when the liquid nitrogen vaporizes into gas -- ie, the liquid nitrogen vaporization is an endothermic process. The heat of vaporization for liquid nitrogen is 5.57 kiloJoules/mole.
(For an endothermic process the gain in entropy is greater than the loss of enthalpy, which is a way of saying that the molecules prefer the freedom of being in a gas to the contraints of being in a liquid -- and will take-up heat for the opportunity to be a gas. Burning firewood or gasoline in a car are exothermic processes because heat is released rather than absorbed.)
Injection of liquid nitrogen into the cooling boxes is
controlled by a
Magnatrol Type-M normally-closed valve (10M61Z)
which is rated for liquid nitrogen. A pressure regulator
keeps the injection pressure at 45 PSI (Pounds per Square
Inch). A 75 PSI pressure safety valve on the liquid nitrogen
tank blows-off whenever pressure reaches 75 PSI in the tank.
Too much pressure in the tank would hamper efficient
operation of the 45 PSI pressure regulator.
|Small Cooling Box||Relay box left, pressure regulator & Magnatrol valve right|
The Magnatrol valve is controlled by an electrical
relay switch located in a power/relay box mounted
on the cooling boxes, but only when a toggle switch on
the power/relay box is in the up position. A +24 volt
DC signal from the computer controller allows AC power
from a wall socket to open the Magnatrol valve. No voltage
(0 volt signal) comes from the computer controller when
the Magnatrol valve is to be closed. An orange indicator
light on the power/relay box is lit when the AC power
is activating the Magnatrol valve.
|Control Box, top view||Control Box, side view|
The signals received from the computer controller are based on temperature readings received from a thermocouple in the cooling box that has been placed in the patient or on the test object. We use T-type thermocouple beaded probes which have an operating range of −200ºC to +350ºC.
The "computer controller" is actually a combination of a control box and a laptop computer. The control box contains the power supply that converts 120 volt AC current to 24 volt DC and supplies a 0.5 amp current to the other components on the backplane. Fuses would blow if current reached 1.5 amps. A green ground wire (common) connects all components. Ducts run along the sides of the backplane to keep the wiring orderly.
The backplane in the control box consists of the actual
controller and 4 slots for plug-ins. Two of the slots are
empty. The controller itself is a Compact FieldPoint real-time
computer (cFP-2010) which runs a DOS-like operating system.
The controller has a TCP/IP line which makes an Ethernet
connection to the laptop through an orange crossover wire
connecting the laptop to the control box.
|Illustration of power supply & backplane components||Photo of power supply & backplane components|
In one slot is the Digital Output module (cFP-DO-401) which sets the output line to 24 volts upon receiving signals from the controller. In another slot is the ThermoCouple module (cFP-TC-120) which receives input from up to six thermocouples and transmits that information to the controller. The wires in this module are specifically matched with the metals in the T-type thermocouples. Both modules have adjoining Connector Blocks (cFP-CB-x) to handle the actual wiring. The cFP-CB-1 is used for the Digital Output module, whereas the cFP-CB-3 is more suited for the ThermoCouple module because it has more accurate cold-junction compensation -- compensating for the thermoelectric voltages generated at the junctures.
The laptop computer contains configuration screens for programming input, graphical display screens for monitoring cooling curves and a report generatioin screen. Cooling curves can be stored as tabular data files or as PDF files containing graphical plots. Operator screens control system log-in and user authorization.
The configuration screen on the laptop allows the operator to program a sequence of cooling steps, where each step specifies a target temperature. The rate at which the Magnatrol valve opens and the duration of the valve opening is determined by calculations in the computer based on the specifications in a cooling step and the temperature of the controlling thermocouple. The operator can manually override the preprogrammed step at any time -- opening and closing the Magnatrol valve by clicking on an icon. A cooling curve will be displayed on the computer screen, which can be saved for future examination or for printing.
An example of a sequence for cooling a patient from water-ice temperature to liquid nitrogen temperature might be the following:
(1) cool to −130ºC in 8 hours
(2) hold at −130ºC for 4 hours
(3) cool to −196ºC in 200 hours
The computer will calculate a cooling rate (slope of a
line on a cooling curve) that is necessary to achieve the
desired target temperature (eg, −130ºC) in the desired time
(eg, 8 hours) for each step. A dotted line on the computer
graph indicates the three-step profile desired. When the
program is started, the computer begins opening the Magnatrol
valve at a rate required to achieve the desired cooling rate
based on feedback from the controlling thermocouple (which
would be the thermocouple in the throat used to approximate
brain temperature). Plots of the controlling thermocouple
and thermocouples placed elsewhere on the patient appear in
different colors on the same plot showing the desired cooling
profile. The computer should make the readings from the
controlling thermocouple match the plot of the desired
cooling profile by opening the Magnatrol valve at the
desired rate. Theory and practice can be well-illustrated by the
vitrification cooling done for the
72nd patient of the Cryonics
|First Six Hours||First 130 Hours|
With each cryonics patient we treat, we learn from the experience and are thereby improving our equipment and methods. The cooling of the 72nd patient is the best we have done so far, but what we have learned from this cooling will allow our next vitrification patient to be treated with a better protocol. The above representation shows a somewhat idealized cooling curve, although this too will be improved-upon.
The controlling thermocouple should be the thermocouple just under the skin -- or at the surface of the brain. The brain surface will cool more rapidly than the center of the brain. We want to cool as rapidly as possible to glass transition temperature (eg, −130ºC), but the entire brain does not cool at the same rate. So the controlling thermocouple should cool the surface of the brain to around −130ºC and then wait at −130ºC until the center of the brain also comes to −130ºC. When the surface of the brain and the center of the brain are at −130ºC, very slow cooling to −196ºC can then begin. Attaining uniform temperature throughout the brain before cooling the vitrified solid to liquid nitrogen temperature will minimize the thermal stress that will develop in the glass.
Holding the brain surface temperature just above glass transition temperature while waiting for the center of the brain to reach that temperature should not result in devitrification (freezing) because although the nucleation rate is highest just above glass transition temperature, the viscosity is so great that devitrification is unlikely to occur. An alternate approach might be to cool the surface of the brain to just below glass transition temperature -- thereby achieving a glassy solid that will not freeze -- and then raising the brain surface temperature to just above glass transition temperature when the brain core temperature reaches that temperature. With temperature uniformity achieved just above glass transition temperature, slow cooling to liquid nitrogen temperature can then proceed with minimal thermal stress.
To ensure good patient care as experience is being gained
with the system and required parameters, close monitoring
and manual overriding of the preprogrammed sequence will be
|Marc and Mike survey the problems||Marc and Mike get to work|
|Mike, Andy and Marc discuss the big cooling box||Marc and Mike hard at work while Andy shoots the breeze|
|The Test Patient is a Meathead||Mike works on software while Curtis Henderson watches|
|Big Cooling Box, side view||Big Cooling Box, inside view|
|Big Cooling Box cools 69th patient||Monitoring cooling of CI patient 69|
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