An onion dewar is a traditional style of liquid nitrogen storage vessel characterised by its distinctive rounded, layered outer casing – a shape that earned the name in early cryogenic laboratory practice. In IVF, onion dewars were widely used from the 1980s onwards to store sperm, embryos, and oocytes in liquid nitrogen at -195.8 degrees C. Whilst the onion dewar design has largely been superseded in modern IVF laboratories by purpose-built cryogenic storage vessels with higher canister capacity, better neck insulation, and improved hold times, they remain in use in some older clinical settings and in veterinary fertility applications.
What are the limitations of an onion dewar compared to modern storage vessels?
The primary limitations are canister capacity and liquid nitrogen evaporation rate. Onion dewars typically accommodate fewer canisters than modern wide-neck storage vessels, and their static evaporation rate – the volume of liquid nitrogen lost per day without any access to the vessel – is higher than current designs. For a laboratory expanding its sample inventory, this creates operational pressure: more frequent top-ups, less working space per vessel, and reduced resilience during supply disruption.
When would an IVF laboratory still use an onion dewar?
Where an onion dewar is already in service and in good condition, some laboratories retain it as secondary or overflow storage. The cryogenic principle is identical to any liquid nitrogen vessel: samples stored below -130 degrees C are in a glassy state and do not deteriorate. The vessel type does not affect sample viability, provided the liquid nitrogen level is maintained and the vessel is in serviceable condition.
How does Cryolab support laboratories transitioning away from onion dewars?
Cryolab advises laboratories on vessel replacement and capacity planning, including decanting protocols for moving samples from legacy onion dewars into modern storage vessels without temperature excursion. The CryoNest and CryoCan ranges offer a range of capacities suited to laboratories at all stages of sample inventory growth.
An onion dewar that was adequate for a clinic doing 200 cycles a year in 1995 is almost certainly under-specified for the same clinic doing 800 cycles today. Capacity planning should start with where the inventory will be in five years, not where it is now.
Browse Cryolab’s storage vessel range at cryolab.co.uk/product-category/storage-vessels/.
