Slow freezing and vitrification are both methods of cryopreservation designed to preserve embryos at very low temperatures for extended periods. They differ fundamentally in how they achieve this and in their outcomes.
Slow freezing, also called controlled-rate freezing, cools the embryo gradually at a rate of around one degree Celsius per minute. Despite careful use of cryoprotectant chemicals, some ice crystal formation is almost inevitable, particularly in extracellular spaces. This causes physical damage to cells during both the freezing and thawing processes.
Vitrification takes the opposite approach. Rather than accepting some ice formation, vitrification eliminates it entirely by cooling so rapidly that the fluid transitions directly to a glass-like solid. The embryo is plunged directly into liquid nitrogen within seconds of loading onto the carrier device. The cooling rate, often exceeding 20,000 degrees Celsius per minute, prevents ice nucleation entirely.
The clinical outcomes reflect this difference clearly. Slow-frozen embryos typically survive at rates of 70 to 80 percent. Vitrified embryos in competent UK laboratories survive at 90 to 95 percent or higher. The difference in subsequent implantation rates is also measurable.
Slow freezing is still sometimes used for sperm. For embryos and eggs, vitrification is now the clear standard of care across UK fertility practice.
Cryolab supplies vitrification workstations and carrier systems for UK IVF laboratories. Visit cryolab.co.uk“>Cryolab
