Slow Freezing

Slow freezing of embryos is a procedure that has been mostly phased out in the modern IVF lab. The practicality and low cost of vitrification, as well as high proven success rates, have caused most laboratories to switch over to vitrification during the past 10–15 years. There is, however, a large remaining collection of slow-frozen embryos in existence, which are still periodically used for embryo transfer, embryo donation, and, in some cases, research. In addition, mouse embryos that are often used for quality control in the clinical lab are typically slow cooled instead of vitrified. Therefore, it is imperative that the modern IVF lab is able to handle these specimens and understand the techniques of slow freezing and the subsequent warming. In this chapter, we will discuss the history and fundamental principles of the slow-freezing and thawing technique, take a dive into the mechanism of cryodamage and how it can be prevented, look at ancillary procedures to improve success rate, then discuss the nuances of embryo storage, and finally discuss the future of slow freezing in IVF.