To further protect Cord Blood cells against the formation of ice crystals, it is important that the cells freeze slowly. This is done in a special controlled-rate freezer, and then the blood is transfered to long-term storage. The final storage temperature should remain stable at -196 C. Freezers and handling procedures are designed to avoid “Transient Warming Events (TWEs)”. In particular, the cells must stay below -130 C while they are in storage. If the temperature fluctuates above -130 C and then below again, the blood will suffer irreparable cell damage.
A few public Cord Blood banks with big budgets use “BioArchive” freezers from Thermogenesis (Nasdaq: KOOL). These freezers were originally developed at the New York Blood Center. This liquid nitrogen freezer uses a proprietary, computer-driven robotic system to cryopreserve and archive up to 3,626 bag units. The location and retrieval of each sample is computerized. The goal of using a robotic arm to circulate and retrieve samples is to avoid “Transient Warming Events (TWEs)” when samples are added or removed from storage.
Most of the public Cord Blood banks in the USA are using “dewars”, which are basically insulated tanks with lids. The disadvantage of these units is the need to open the lid when adding or removing samples. However, temperatures are monitored constantly and they have been proven effective for long-term storage.
All private cord blood banks in the USA are also using “dewars”, like most of the public banks.
Blood is stored in either bags or vials. When the racks in the freezer are loaded with blood containers, they should be spaced evenly to ensure temperature stability. A given freezer will have racks designed to hold either bags or vials, you cannot mix containers in the same freezer. Hence, most banks build their business around one storage container or the other.
The cryogenic temperatures are maintained with cold nitrogen. Some banks use the liquid phase of nitrogen and others use cold nitrogen vapor (gas). The pros and cons of the nitrogen options are given in the table below.