Chamber probe position effects on controlled rate freezing

Abstract

Background & Aim Background: Controlled Rate Freezers (CRF) enable long-term cryopreservation of cellular therapy products. Freezing curves record that cryopreservation temperatures are following optimized protocols. Two abnormal freezing curves were investigated. Methods, Results & Conclusion Method Retrospective case reviews. Results In the first case, the standard freezing protocol was followed with minor cyclic oscillations. Product integrity was not compromised. The CRF has a wire chamber temperature probe that makes a 90 degree bend and descends through a six inch metal straw which prevents liquid nitrogen vapor from directly contacting the probe. The insulated probe is uncoated for 3 mm on the distal protruding end allowing for accurate measurements. The straw and probe end are not visible in the CRF due to a metal plate. Upon inspection by the vendor after first abnormal curve, the chamber probe was not protruding far enough out of the straw. Repositioning counteracted the observed oscillations. Several months later, a second abnormal freezing curve was observed. Ten minutes into the run, it was noted that the chamber temperature was having drastic, wide-ranging swings. The sample temperature did not show the irregular swings, but indicated warming. Upon inspection, the uncoated tip of the probe was making contact with the metal straw. Both types of oscillation were reproducible by varying the position of the chamber probe. The exposed temperature probe allows the CRF to measure the temperature of the chamber and inject liquid nitrogen vapor into the system to allow for cooling. When the probe is not in the proper position it is either insulated by the straw (probe within the straw) or the probe is grounded and produces erroneous temperature swings (when the probe touches the metal straw). In the latter case, erroneously cold chamber temperatures occurred, producing large-scale temperature oscillations on the chamber probe which caused the CRF to respond with rapid warming. Product integrity was compromised by the second, more severe event. The vendor was called to discuss the findings and they reported other clients had similar experiences with the grounding of the probe. Currently, we ensure the chamber probe is pulled at least 1 cm through the straw and bent at a 90 degree angle. Conclusion: Probe position is essential for proper function of the CRF which may affect cryopreservation of products. Any oscillations in the freezing curve necessitate evaluation of the chamber probe position.