What Is the Difference Between Cryopreservation and Vitrification?

Cryopreservation is the broad term for any process that preserves biological material through cooling to cryogenic temperatures. Vitrification is a specific cryopreservation method that uses very high concentrations of cryoprotectant and extremely rapid cooling – greater than 10,000 degrees C per minute – to convert the sample into a glass-like amorphous state without ice crystal formation. In an IVF context, vitrification has largely replaced slow-freeze protocols for embryo and oocyte cryopreservation because of its superior clinical outcomes.

Why did vitrification replace slow-freeze for embryos?

Slow-freeze protocols cool embryos at a controlled rate of approximately -0.3 to -2 degrees C per minute, using lower concentrations of cryoprotectant. Ice crystal formation during slow cooling causes physical damage to the embryo. Vitrification, by cooling at a rate greater than 10,000 degrees C per minute, bypasses the ice crystal formation phase entirely. Clinical outcomes including post-thaw survival rates and implantation rates are consistently higher with vitrification than with slow-freeze for embryos and oocytes.

Is slow-freeze still used in IVF?

Slow-freeze protocols remain in use for sperm cryopreservation, where the cell structure is more resistant to ice crystal damage than oocytes or embryos. Controlled rate freezers are still a standard piece of equipment in IVF and andrology laboratories specifically for this purpose. For embryos and oocytes, vitrification is now the standard of care in the UK and most of Europe.

What consumables does vitrification require compared to slow-freeze?

Vitrification requires specialist carriers – such as the CBS High Security Straw system – that are designed for the ultra-rapid plunge into liquid nitrogen that the technique demands. Slow-freeze protocols use standard straws or cryovials and require a controlled rate freezer. The downstream storage requirements are the same: both result in samples stored in liquid nitrogen at -195.8 degrees C in a compatible cryogenic storage system.

The shift from slow-freeze to vitrification for embryos and oocytes has been one of the most significant improvements in clinical IVF outcomes of the past 20 years. The equipment and consumable requirements are different, but the storage requirements are identical.

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