IVF: what is the difference between slow and flash freezing?

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IVF: what is the difference between slow and flash freezing?

IVF: what is the difference between slow and flash freezing?

In the process of assisted reproduction, we tend to freeze eggs, sperm, embryos and blastocysts (embryos that have grown in the lab for 5 or 6 days).

Freezing is a stressful process and not all eggs, sperm, embryos and blastocysts survive. Cryoprotective agents are used to protect cells from damage during the freezing process. Liquid nitrogen is usually used for the cooling agent.

There are two freezing techniques:

  • Slow freezing
  • Flash freezing (vitrification)

Slow Freezing

During slow freezing, eggs, sperm, embryos or blastocysts are cooled very slowly, at a decrease of 0.3º–2º Celsius per minute, until it reaches the final storing temperature of -196º Celsius. In total, the entire process takes a couple of hours, which is why this technique is called “slow freezing”.

When freezing embryos, the water inside the embryo needs to removed to prevent the formation of ice crystals which would be deadly for the embryo. As the water is removed, it is replaced with an “antifreeze” solution which is a cryoprotective agent.

Freezing sperm is easier and more reliable, and only needs glycerol and a cryoprotective agent and sucrose.

Egg freezing is the most challenging process because the egg is very fragile when removed from the ovaries. It’s becoming a more reliable process with flash freezing.

Flash freezing (vitrification)

This is a more recent form of freezing that goes quickly to -196ºC leaving cells “glass-like” or “vitrified.” The process is so quick that it doesn’t allow for intracellular ice to form, avoiding trauma to the egg, embryo or blastocyst.

First water is removed from the eggs or embryos and replaced with the “antifreeze”.  Then, the eggs or embryos are flash frozen, with their temperature brought down to storage temperature.  By removing the water and quickly freezing eggs and embryos, they are protected against ice crystal formation.

And as it turns out, speed is really important in this process. Vitrified embyos have a better than 95% freeze-thaw survival rate (compared to 50% survival with slow freezing). This is also a better technique for egg freezing which have an over 83% freeze-thaw survival rate (compared to the approximately 66% survival rate of eggs during slow freezing).

Andreia Trigo

Fertility NLP Coach and Nurse Specialist

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