Vitrification and slow freezing are the two established methods for cryopreserving embryos in UK IVF clinics. Both result in embryos stored in liquid nitrogen at minus 196 degrees Celsius but they get there in fundamentally different ways, with meaningfully different outcomes.
Slow freezing, the older method, uses a controlled rate freezer to reduce the embryo’s temperature gradually over several hours. Lower concentrations of cryoprotectant are used. The process allows time for water to leave the cells in a controlled manner but the slower cooling rate means there is greater risk of ice crystal formation within and around the embryo, which can cause cellular damage.
Vitrification is a rapid flash freezing technique. The embryo is exposed to higher concentrations of cryoprotectant and then plunged into liquid nitrogen almost instantaneously. The speed of cooling is so fast that water molecules do not have time to form ice crystals — instead the cellular fluid vitrifies into a glass-like state. Survival rates with vitrification are consistently higher than slow freezing, and the technique has become the dominant method in UK fertility clinics over the past decade.
From an equipment standpoint, vitrification requires a specifically designed workstation, appropriate carriers (such as Cryotop or Cryolock), warming media and trained embryologists. Slow freezing requires a controlled rate freezer which is a more significant capital investment. Both methods require the same downstream cryogenic storage infrastructure — liquid nitrogen dewars, monitoring systems and reliable cryogenic sample identification. Cryolab.co.uk supplies vitrification consumables and cryogenic storage equipment to UK fertility clinics.
