Improvement of the control strategy of a Pulse Tube refrigerator for the cells cryopreservation

Cells cryopreservation is crucial for the treatment of several diseases and the research, both in basic as well as in medical applications, is going in the direction of the investigation of new processes and technologies. Currently, programmable freezers based on liquid nitrogen technology are usually adopted for the freezing of the cells. These solutions require liquid nitrogen distribution systems and cannot be transportable. The freezing of cells must be carried out at a specific cooling velocity, normally established by experimental results, in order to reduce the death of the cells due to undesired crystallization, membrane damage or osmotic shock. In the recent years, Pulse Tube (PT) refrigerator has proved to be a technically and commercially viable solution as an alternative to the liquid nitrogen technology. However, the PT characteristic curve shows a cooling speed always faster that the one required by the cell to preserve its vitality Therefore, a Joule effect passive resistance is used to counterbalance the freezing curve of the PT and minimize the difference between the two cooling curves. Temperature control indeed is of paramount importance to assure the right cryopreservation of the cells. The research carried out at Sapienza University of Rome has been aiming at improving the control strategy for cryocooling applications of a commercial PT refrigerator tested in its laboratories. In the past, some of the authors designed and tested an own control system for the cells cryopreservation of biological material with varying test tubes configurations. Results have shown that the control temperature, i.e. the cooling temperature inside the test tubes, diverges in a range ± 5°C compared to the requested ideal temperature. Therefore a new control logic and system has been developed by the authors of the present paper and experimental tests in the university premises are going to be carried out to prove the improvement of the control logic and better evaluate the inertia of the system.