Controlled-rate freezing

A more recent issue of debate among cryopreservation experts involves controlled-rate freezing vs. vitrification.  Controlled-rate freezing involves taking samples prepared for cryopreservation and slowly reducing the temperature to a sub-freezing temperature (often -80°C, but not always) before preserving at cryogenic temperatures for storage.  Vitrification, on the other hand, takes prepared samples at room temperature and plunges them directly into liquid nitrogen for extremely rapid freezing.

The two methods have similar goals: in both cases the idea is to minimize damage caused by ice.  Vitrification attempts to do this through turning the water into a glass phase rather than into solid ice.  Slow, controlled-rate freezing aims to do this by letting out just enough water but not too much, and minimizing the size of the ice crystals.

Both methods can be successful, and some scientists are quick to note that vitrification seems to be an “up-and-coming” technique for cryopreservation.  However, it is also a much younger technique, and not all studies have found that vitrification works better than slow, controlled freezing.

Here’s where my skepticism arises…  Very few labs (or journals, for that matter) publish negative data.  In some situations there may already be an optimized protocol for cryopreservation using controlled-rate freezing.  When that is the case, if a lab tries a vitrification protocol and it does not work as well, it will probably not be published.  However, if it does work and a better-performing protocol is established, it will get published.  Therefore we are made aware of those times when vitrification works better more frequently than those times that it does not.  Based on a topical analysis of the published literature, this may lead to a bit of bias.  Researchers may wrongfully start to believe that vitrification is much more favorable compared to controlled-rate freezing than it actually is based on published literature.

In the end, optimized protocols need to be developed and publicly displayed in a way that promotes best practice in cryopreservation for all labs.  Unfortunately, this would be beyond the scope of something such as “current protocols” as the volume of data would be far too large.