Embryos were traditionally frozen using a ‘slow-freezing’ method where the temperature is gradually reduced using a specially designed machine. However, the survival rates of frozen embryos have skyrocketed since the development of a new freezing technique called vitrification. Vitrifying embryos means that they are frozen at such a rapid rate that there is no time for ice crystals to form inside the cells which is the main cause of damage, often leading to embryo death. Instead, the water inside the cells freezes at -20,000°C per minute into a glass-like state. Because no ice is formed, technically vitrification isn’t freezing… it is referred to as ‘cooling’. This super-fast rate of cooling is achieved by holding the embryo in extremely small volumes of liquid and plunging it into liquid nitrogen which keeps the embryo perfectly preserved at -196°C.
Before embryos can be frozen, they need to be gradually dehydrated to remove the water in and around the cells. They are moved individually through different liquids which vary in their concentration of cryoprotectant (solutions that replace the water and protect the cells). Although cryoprotectants protect the embryos from ice damage, they can also be toxic. This means that the embryos can only be in contact with the media for very specific amounts of time to ensure the cells are sufficiently preserved, without excessive exposure which can do more harm than good. The water in the cells is gradually drawn out and the cryoprotectant is slowly transferred in.
It is normal for a blastocyst to ‘collapse’ during freezing because it contains a central cavity filled with water which needs to be removed before freezing. When it is ready, the embryo is picked up in a tiny amount of liquid and transferred onto a vitrification device which usually contains a microscopic well or trough that the embryo sits in. The device is quickly plunged into a canister of liquid nitrogen which instantly freezes it in its equilibrated state.
Cryopreserved eggs, sperm and embryos are stored in containers called dewars. They are essentially specially-designed reinforced freezers that can withstand extremely cold temperatures. They are fitted with individual canisters which hold specimens for many patients - often hundreds in one dewar. They are also carefully monitored for fluctuations in temperature as even the slightest change could have detrimental effects on anything stored in it.