Freezing is a method of preserving tissues and cells over extended periods in liquid nitrogen at a temperature of —196 degrees Celsius.

Sperm Freezing

Sperm is preserved in a special environment through rapid freezing. Once frozen, sperm can endure dozens of years without harm. When required, the spermatozoa are defrosted in a set manner and are then activated in special environment to be used in diverse assisted reproduction techniques.

Embryo Freezing

As distinct from sperm, embryos are very vulnerable during freezing and defrosting. A tenth to a fifth of them fail to survive freeing and defrosting. This means that only good quality (grades I and II) embryos may be frozen and preserved only where sufficient good quality embryos remain after embryo transfer.
There are two main methods for embryo freezing: slow and rapid (vitrification).
Slow freezing involves reducing temperature at the rate of 1/3 degree Celsius per minute under computer control. The environment is special and cryoprotectors cause liquids to leave the embryos preserving them from the ill effects of low temperatures.
Rapid freezing is used ever more in assisted reproduction in recent years. Vitrification is a very fast freezing technique which causes living cells to become glass-like, thus avoiding the formation of ice crystals during freezing and defrosting. The main benefits of Vitrification are the elimination of mechanical damage caused by ice crystals within and outside cells and the reduction of damage caused by prolonged exposure to gradually reducing temperatures.
The Sofia Hospital of Obstetrics, Gynaecology and Reproductive Medicine applies embryo vitrification at different developmental stages. Over 19 out of every 20 vitrified embryos survive defrosting and embryo transfer success rates are similar to those using fresh embryos.

Egg Cell Vitrification

This technique freezes egg cells amid cryopreservation. It offers a way to preserve or extend female fertility and is applied for a number of reasons. Egg cells contain significant amounts of water, making the slower freezing techniques used so far much more prone to forming ice crystals that may destroy egg cells. In egg vitrification, temperature drops exceptionally rapidly, at the rate of 23,000 degrees per minute, making egg cells assume a glass-like state instantly. The scientific community feels that vitrifying egg cells offers great prospects to egg donation and preservation of female fertility.