Rapid freezing using vitrification
During vitrification, embryos are exposed in special cryoprotective solutions, then loaded onto special vessels and plunged directly in liquid nitrogen. In this way the formation of glass is induced in the inside of the cells, avoiding the formation of ice crystals that are detrimental for embryo viability.
Vitrification is a more recent method of freezing, and has been successfully applied on all embryonic stages, as well as oocytes. Especially for freezing of oocytes, vitrification is now the method of choice as it is related with higher survival and pregnancy rates compared to slow freezing. Also, vitrification does not require the use of a special machine, and is much faster than slow freezing, which last for 2-3 hours.
However, vitrification requires experienced embryologists as it is a matter of seconds during freezing and thawing if the embryos will retain their viability or not.