Successful cryopreservation of cells requires a controlled cooling rate. Cooling too rapidly or too slowly could lead to a poorer outcome.
A controlled cooling rate is needed because of the physical environment experienced by the cells during cryopreservation. As ice forms in the extracellular space, pure water is locked away as ice. This results in cells being suspended in an increasingly concentrated solution, which dehydrates cells. However, cooling too slowly makes this solution toxic. And cooling too rapidly prevents cells from dehydrating sufficiently; intracellular ice forms.
Read how controlled-rate cooling is achieved using equipment such as VIA Freeze controlled-rate freezers.