“The feasibility study of pulse tube application in molecular cryoconservation”

Cryopreservation is the set of practices applied to bring cells to cryogenic temperatures without producing injuries. In fact, the obsolescence of cells is caused by chemical reactions, and the rate at which these reactions occur can be drastically reduced by lowering the temperature. The lower the storage temperature, the longer cells can be kept. For instance, at temperatures of -80°C (dry ice) cells can be preserved for months, even if some chemical reactions can occur, while temperatures of -196 °C (liquid nitrogen) can guarantee indefinite storage. Nowadays the freezing of cells is carried out through programmable freezers: this kind of machines use liquid nitrogen to reach the storage temperature. However, the use of liquid nitrogen is a limiting factor in many situations of inefficient or absent nitrogen distribution network, such as small industries, isolated places and during transport of cells. Since the early 1950s, many studies have been carried out on the possibility to reduce injuries to cells subjected to freezing. No analytical approach has emerged, but an “unwritten rule” states the optimal range for cooling velocity between 0,1°C/min and 10°C/min [1]. The characteristic curve (temperature on the cold-head vs time) of a Pulse Tube refrigerator observed during experimental analysis has shown that the close cycle cryorefrigerator can be used for cell preservation, if equipped with a control system for the cooling rate. Almost all the machines employed in the field of freezing cells use liquid nitrogen. The interest in the Pulse Tube refrigerator is due to a reduced need of continuous maintenance for adding technical gas. Even the absence of magnetic field and the reduction of vibrations near the cold-head make this kind of refrigerator suitable for the cryopreservation of cells. The mathematical approach theorized aims at regulating the cold-head temperature through the prevision of electric power to be dissipated in a passive component, allowing to obtain a freezing curve induced by the PT, coincident with the one established by the molecular theory. A resistor linked with a power amp was used to produce heat for Joule effect to control the cooling rate. The dissipated power was handled by a software realized with LabView™ that monitors temperature both of the cold-head and of the test tube containing the cells. The results obtained theoretically have then been compared with experimental data.