Biobanks have two main options for ultra-low temperature storage: 1) liquid nitrogen (LN2), which can be used in the liquid or vapor phase, and 2) mechanical cryopreservation.
In an earlier post, we discussed the advantages of using LN2 for long-term biobanking. Now, it’s time to examine the other side of the coin. While both cryopreservation methods come with both pros and cons, and both are widely used, here are the top five reasons that some biobanks choose mechanical cryopreservation:
Mechanical freezers provide a generally uniform top-to-bottom temperature throughout the storage chamber. By contrast, LN2 freezer chambers are subject to temperature gradients. In LN2 systems, LN2 is usually piped into the lower part of the freezer and the LN2 vapor is used to cool the upper racks. As a result, a heterogeneous chamber environment forms, and if the temperature in certain sections of the chamber rises above the glass transition point of water (approximately 135°C), biosample degradation can occur and the effectiveness of long-term cryopreservation can be compromised.
Mechanical freezers are less likely to present contamination issues. Under certain circumstances, LN2 can transmit contaminants. For instance, in a well-documented case, biospecimens were contaminated with Hepatitis B virus while stored in an LN2 freezer (Tedder RS, Zuckerman MA, Goldstone AH et al. (1995) Hepatitis B transmission from contaminated cryopreservation tank. Lancet 15, 137–140). Mycoplasma and other contaminants have been found in LN2 systems as well.
Mechanical freezers pose less risk to workers. LN2 is not poisonous to humans, but it does have the potential to displace breathable oxygen in a closed room (called Oxygen Deficiency (or Displacement) Hazard (ODH)). Therefore, it’s critical to vent LN2 systems properly. If a biobank encounters cost or logistics issues with ventilation, a mechanical freezer tends to be the better option.
Mechanical freezers can be less expensive in the long-run. Mechanical freezers may cost more initially and, in some cases, they may require more maintenance than LN2 systems. Over time, however, mechanical cryopreservation may be the more economical option because of the long-term cost of LN2, coolant tank storage, etc. Of course, this all depends on the price of electricity, which LN2 freezers use in lower quantities.
Mechanical freezers can be backed-up with LN2. In the event of an extended power outage or natural disaster, stable chamber temperatures can be maintained with a built-in LN2 backup system. When a biobank may be storing the only aliquot of a given sample in the world, contingency plans are absolutely vital.
Has you biobank opted for mechanical cryopreservation, rather than LN2 cryopreservation? If so, please tell us why. What features are most important to you when weighing options for long-term storage of biosamples?