10 January 2020

What is the vitrification technique?

What is the vitrification technique?
Medical Director
Cesar Diaz-Garcia
MD PhD Assoc Prof
IVI London


A good embryo freezing technique is essential when trying to increase the chances of pregnancy with fertility treatment. The introduction of vitrification means that IVI can obtain embryo survival rates that reach almost 100% following freezing.

Due to the improvement of incubator conditions, IVI has been able to transfer and vitrify at the blastocyst stage in nearly all our treatments. The benefit of this is the increase in implantation and success rates, due to improving the embryo selection process. The term vitrification describes the process of cryopreservation in which an ultra-rapid cooling method is used to solidify the cell into a glass-like state avoiding the formation of ice. Over the past 30 years, two main preservation techniques have been used; slow freezing (SF) and vitrification. Due to better success rates, SF has been replaced by vitrification in nearly all cases when it comes to freezing embryos and oocytes.

What is slow freezing?

In slow freezing, the embryos and oocytes are dehydrated by being exposed to one or more cryopreservation solutions for 10-15 minutes. The oocytes or embryos are then loaded into plastic straws, which are heat-sealed and placed in the freezing machine which then slowly drops the temperature to -30‎°C.

What is vitrification?

Vitrification is an ultra-rapid process. A typical embryo vitrification process cools the cells in the embryo at rates close to 5,000 degrees per minute. However, due to a much higher concentration of cryoprotectants, a faster cooling method is used to avoid damage to the oocytes. Unlike SF, the straws that we use to hold embryos are much smaller and therefore the amount of fluid surrounding it is around 100-200 times less than that of a slow frozen embryo.

The process of vitrification consists of three stages. Firstly, the eggs or embryos are exposed to cryoprotectants causing dehydration. Then they are put into tiny storage straws that will cope with the ultra-rapid cooling method. Lastly, the straws are then cooled dropping the temperature thousands of degrees per minute. This procedure is normally completed in no more than 10 minutes. Therefore it is a much quicker process than that or slow freezing. The fast rate of cooling allows for the content of the straw to turn into a glass-like substance rather than ice. Avoiding the formation of ice crystals is essential when it comes to thawing the embryos at a later date and in fact, increases the survival rate to over 90%.

What about egg freezing?

The process for egg vitrification is very similar to that of embryos in that the loading of the straws and the same vitrification solution is used, albeit slightly slower to achieve the same level of dehydration without effecting the survival rate.

Whether it be eggs or embryos being frozen, once the procedure has taken place there is no ‘best before’ date and they can remain frozen for however long you wish. However, it is important to note that in the UK there is a law stating that eggs and embryos can only be frozen for a maximum of 10 years.

What next…?

To warm the eggs/embryos to be used for treatment the same process is used just in reverse. The oocytes and embryos are rehydrated at room temperature before being placed back into incubators at 37‎°C. The procedure to thaw the oocytes and embryos takes around 15 minutes. Once this is done the embryos can be put back into the uterus.

Since the introduction of vitrification, the number of frozen embryo cycles has increased significantly, increasing IVI’s cumulative success rates. IVI has one of the best success rates in the country when it comes to blastocyst survival rate after vitrification at 96.6%

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