A process that enables the scientists to store eggs or embryos. Vitrification is a fast/instant freezing process and has been proven to result in less damage to eggs or embryos compared to the old freezing method.

Vitrification is an embryo freezing technique that has been found to increase the chances of pregnancy through fertility treatment. For some specialists, the introduction of vitrification has meant that egg and embryo survival rates after freezing have almost increased to 100%, making it significantly more effective than previous methods. 

There are two commonly used methods for freezing eggs and embryos. The most well-established of these is the “slow cooling” or “slow freezing” method. In slow cooling, embryos and eggs are dehydrated by being exposed to one or more cryopreservation (freezing) solutions for 10-15 minutes. Once this is done, the eggs and embryos will then be loaded into plastic straws, which are sealed and placed into a freezing machine. This machine will then slowly drop the temperature to -30°C. 

The vitrification process, on the other hand, is considered rapid in comparison to the older process. It is a flash-freezing technique in which the cells of an egg or embryo will be cooled at rates close to 5,000 degrees per minute. Due to a much higher concentration of cryoprotectants (chemicals used to prevent cell or tissue damage), a much faster cooling method is used to avoid damage. 

The straws used to hold embryos are also much smaller in vitrification, compared to that of slow cooling. This means the amount of fluid surrounding the embryo will be around 100 to 200 times less than that of a slow-frozen embryo.

There are three stages to the vitrification process. As a first step, the eggs or embryos will be exposed to cryoprotectants that cause dehydration. They will then be put into / onto small storage straws that are designed to be able to cope with the ultra-rapid cooling method. The straws are cooled by plunging the sample / straw directly into liquid nitrogen (-196°C), with the temperature dropping thousands of degrees per minute. Overall, the process is usually complete in under 10 minutes, which makes it a much quicker process than slow freezing.

The fast rate of cooling carried out in the vitrification process allows for the contents of the straw to turn to a glass-like substance, rather than to ice. Avoiding the formation of ice crystals is essential in both the preservation and the cooling process, as these can damage eggs and embryos and result in a lower success rate. As vitrification has become more widely used, birth rates from frozen eggs and embryos have improved.

Once the eggs or embryos have been selected for use in a treatment, they will be thawed in a reversal of the vitrification process. They will be rehydrated at room or body temperature, before being placed back into incubators at 37°C. The process takes around 15 minutes, and the eggs or embryos will be ready for use as soon as this has happened, although it is common to allow the embryos to recover and re-expand following the vitrification-warming procedure prior to use (biopsy, Frozen Embryo Transfer, further culture to the blastocyst stage within the incubators, etc).