Cryopreservation involves cooling and storage of cells in liquid nitrogen at a temperature of -196° c where all metabolic processes are arrested. There are two methods of cryopreservation, slow rate freezing and vitrification.

Slow rate freezing

The basic objective of both these methods is the same, protect cells from cooling effects (chilling injury), intracellular ice formation, dehydration and toxic effects at both high and low temperatures. Slow rate freezing causes ice crystals to form within embryo's cells (blastomeres). Unsatisfactory survival rate. Only 50 % of embryo survives. All embryos do not survive the freeze-thaw, and those that do survive have less than half the likelihood of generating a pregnancy as do fresh embryos. Very time consuming. Embryos are slowly cooled using a programmable freezer before they are actually frozen. It takes approx. 2 hours to freeze embryos using this technique.


Vitrification is an advanced cell-freezing technique, which allows a higher survival rate after thawing due to the fact that high concentrations of cryoprotectants are used along with reduced volumes and timings. Thus, the formation of intracellular ice crystals is prevented, which are responsible, in most cases, for provoking irreparable cell damage. The vitrification process is used to freeze oocytes or embryos and it guarantees a survival rate of more than 80%. This technique enables the vitrified oocytes which have survived the thawing process to have similar attributes to fresh ones. They are then able to be fertilized by the spermatozoids. The generated embryos can then be implanted and develop into healthy children. Typical IVF treatment involves stimulating a woman's ovaries with hormones to produce eggs which are then collected and fertilized in the laboratory, with one or two embryos being transplanted into the womb two days later. The remaining embryos can be slow-frozen or vitrified, then stored, to be used later if the initial cycle fails.

New studies have provided further evidence that vitrified embryos may be better than fresh for IVF. Frozen embryos are more likely to produce successful, complication free IVF pregnancies than those that are fresh, research suggests. The studies indicate that using vitrified embryos rather than fresh embryos reduces the risk of stillbirth and premature delivery. The technique allows more embryos to survive the thawing process than the older and more widely used slow-freezing method.