The glass transition temperature is -130 degrees Celsius, the threshold below which all molecular motion effectively stops. It is one of the most important concepts in cryopreservation and is the scientific basis for the entire field of biological sample storage at cryogenic temperatures.
Below -130 degrees Celsius, water and biological fluids transition from a supercooled liquid into a glass-like amorphous solid. In this state, molecules have no kinetic energy to drive chemical reactions, enzymatic processes, or cellular degradation. A biological sample stored below the glass transition temperature is in a state of complete biological stasis. Time, in a practical sense, stops.
Above -130 degrees Celsius, molecular motion resumes. Biological processes restart. Ice crystals can form, grow, and recrystallise. Oxidative damage begins to accumulate. This is why a temperature excursion above -130 degrees Celsius during storage, even a brief one, can cause irreversible damage to cryopreserved samples. The embryo or sperm sample that looked intact when it went into the liquid nitrogen tank may be compromised by a storage incident that was never detected.
This is also why vapour phase storage at -140 to -190 degrees Celsius is considered safe for biological samples, even though it is warmer than liquid phase storage at -196 degrees Celsius. Any temperature below -130 degrees Celsius provides the same fundamental protection, provided it is maintained consistently. The CryoBank Series maintains temperatures as low as -190 degrees Celsius in vapour phase mode at the highest storage points within the vessel, well below the glass transition threshold at every level of the tank.
Understanding the glass transition temperature also explains the urgency of the warming step in vitrification. When a sample is warmed, it must pass through the -130 degree threshold as rapidly as possible to prevent ice recrystallisation, the formation of ice crystals during warming that can destroy a sample that survived the freeze intact.
