Successful vitrification requires a cooling rate greater than 10,000 degrees C per minute, as established in ASRM best practice guidance. This rate is fast enough to prevent water molecules forming crystalline ice structures, instead driving them directly into a glass-like amorphous state – from which the term vitrification (from the Latin vitreum, meaning glass) derives.
The clinical implication is that vitrification success is highly sensitive to carrier design. The device that holds the cell during plunging determines how quickly that threshold cooling rate is achieved, and whether it is achieved consistently across different operators and procedure sessions.
Why does the cooling rate matter so much?
Below the glass transition temperature of approximately -130 degrees C, biological time effectively stops and ice crystal formation risk becomes negligible. The critical window is the rapid passage through intermediate temperatures during cooling – typically from 37 degrees C to below -130 degrees C. The speed through this window determines whether the cell vitrifies cleanly or whether ice crystals form and cause cellular damage.
How is the cooling rate achieved in practice?
The rate greater than 10,000 degrees C per minute is achieved by direct or indirect contact with liquid nitrogen, which boils at -195.8 degrees C at atmospheric pressure. Open carrier systems achieve the highest cooling rates. Closed carrier systems encase the cell within a sealed straw before liquid nitrogen contact, eliminating direct contact between the cell and potentially contaminated liquid nitrogen. CBS High Security Vitrification Kits from Cryolab (cryolab.co.uk/product-category/vitrification-kits/) use a closed carrier system validated to achieve the required cooling rate whilst providing containment integrity consistent with HFEA guidance.
The debate between open and closed vitrification is not simply about cooling rate performance – it is about risk balance. Open systems achieve marginally higher cooling rates. Closed systems eliminate the theoretical cross-contamination risk associated with direct liquid nitrogen contact. Most UK fertility clinics have moved to closed systems not because open systems produce worse outcomes in a clean laboratory environment, but because the risk management case for closed systems is stronger under current regulatory scrutiny.
Does the warming rate matter as much as the cooling rate?
Yes – and in many clinical settings it matters more, because warming is where the protocol deviates. Rapid warming through the intermediate temperature zone prevents devitrification – the formation of ice crystals during the transition back to physiological temperature. The warming protocol specified in the vitrification kit instructions is not interchangeable between kit manufacturers and should be followed precisely.
