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 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.
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.
(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. 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.
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 -250ºC 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.
