Liquid nitrogen dewar storage is the process of maintaining biological samples – sperm, embryos, oocytes, tissue, or cells – in a liquid nitrogen storage vessel (dewar) at -195.8 degrees C for a defined period. The dewar uses vacuum insulation between its inner and outer walls to minimise heat transfer from the environment, allowing liquid nitrogen to remain in a liquid state for days to weeks without external cooling. At -195.8 degrees C, biological samples enter a glassy amorphous state below the critical threshold of approximately -130 degrees C, at which biological processes effectively stop.
What is stored inside a liquid nitrogen dewar?
Inside a liquid nitrogen dewar, samples are organised in a layered system: individual samples in straws or cryovials are placed in goblets, goblets are attached to cryocanes, cryocanes are loaded into canisters, and canisters sit within the liquid nitrogen in the vessel. The position of every sample within this hierarchy must be documented in the laboratory’s storage management record for HFEA traceability compliance.
What are the two modes of liquid nitrogen dewar storage?
Samples can be stored in liquid phase (immersed in liquid nitrogen at -195.8 degrees C) or in vapour phase (suspended above the liquid surface, at temperatures typically between -140 and -190 degrees C depending on height within the vessel). Both modes maintain samples below the -130 degrees C critical threshold. Vapour phase storage eliminates the theoretical risk of cross-contamination via liquid nitrogen but requires closed carrier systems to maintain cryogenic temperature during the brief period samples are above the liquid surface.
How is liquid nitrogen replenished in dewar storage?
Liquid nitrogen is replenished by top-up from a dispensing dewar or bulk supply. In a licensed IVF unit, liquid nitrogen levels are checked and recorded daily, and top-up is performed before the level drops to a point that could compromise sample temperature. An automated liquid nitrogen level alarm provides early warning of abnormal evaporation.
Liquid nitrogen dewar storage has not fundamentally changed since the 1970s – but what has changed is the understanding that the quality of every component in the storage system, from the vessel to the cryosleeve on each cane, determines whether samples stored for 10 or 20 years are still viable at the point of use.
Cryolab supplies liquid nitrogen storage vessels and all supporting consumables. Browse at cryolab.co.uk/product-category/storage-vessels/.
