What is the difference between slow-freezing and the newer vitrification process?

When embryos are cryopreserved, they are taken from the incubator in our Austin IVF lab, where they have been growing in an atmosphere and temperature that mimic conditions within the reproductive tract of the woman. They are then transferred into a medium containing an alcohol product, such as glycerol, and sucrose. This cryoprotectant medium is important because it replaces the water that is in the cells of the embryo. It is introduced into the cells of the embryos in precisely timed stages according to exact formulas that have been determined through the work of researchers over several years.

During slow-freezing, embryos are placed in the different concentrations of cryoprotectant medium for 5 to 10 minutes each. This results in a slow diffusion of the cryoprotectant into the cells of the embryos. The embryos are then loaded individually into a vessel, such as a straw-like device. They are then placed into a cryopreservation freezing machine which lowers their temperature in two different stages over the course of two hours. This gradual lowering of temperature results in a gradual increase in the concentration of the cryoprotectant media and a gradual increase in the amount of cryoprotectant that enters the cells of the embryos. At the end of the two hours, they are removed and placed into labeled holders and immediately plunged into the liquid nitrogen storage tanks in our Austin IVF lab.

During vitrification, embryos are placed into much higher concentrations of cryoprotectant media for very short periods of time. This results in a very fast movement of water out of the cells of the embryos. As the water leaves the cells, the high concentration of cryoprotectant enters the cells very quickly. The embryos are moved through the drops of cryoprotectant rapidly, loaded into a labeled vessel in a micro droplet and immediately plunged into liquid nitrogen. They are placed into a labeled holder and transferred into the liquid nitrogen storage tanks.

The slow-freezing method has been the standard in IVF for twenty-five years, but vitrification is now the predominant freezing process in the US.