Investigation of the use of Pulse Tube in cell cryopreservation systems

In order to obtain an acceptable rate of survival of the frozen cells, they must be first cooled in programmable freezers, while controlling the cooling rate, and then stored in maintenance freezers. Different solutions are already used to preserve cells at cryogenic temperatures without liquid nitrogen, while this is not true for controlling the cooling rate. A Pulse Tube (PT) cryorefrigeration system type, can be used in the automatic freezing if combined with a system that monitors and "drives" the temperature generated on the cold part (cold head). To make the Pulse Tube system a suitable one for freezing processes, the cooling curve must be "corrected" in a linear one with a slope given by requested cooling rate. The temperature regulation is obtained with the use of a power dissipator based on Joule effect. In this study, the power to be dissipated is calculated individualizing the trend of temperature on the Pulse Tube cold head and inside the cell test tubes. The control system is not based on an historical series of data so it can be used in different operative conditions. The obtained temperature curve is in good agreement with the theoretical values, with errors within those accepted by commercial systems. The Pulse Tube cryorefrigerator may represent a valid alternative solution to programmable liquid nitrogen freezer, especially where nitrogen's supply is difficult or extremely expensive.