- The glass transition temperature (Tg) is one of the most critical concepts in cryobiology and cryopreservation. It refers to the temperature at which an aqueous or cryoprotectant‑containing solution transitions from a viscous, super‑cooled liquid into a rigid, glass‑like solid. Unlike freezing, which involves the formation of crystalline ice, the glass transition produces an amorphous solid with no ice crystals. This distinction is essential because ice crystals can damage cells, whereas the glassy state preserves biological structures by halting molecular motion.
- In biological systems, Tg typically occurs around −120 °C to −140 °C, depending on the composition of the solution. Pure water has a Tg near −137 °C, but intracellular and extracellular solutions containing salts, proteins, and cryoprotectants exhibit slightly different values. The widely used benchmark of −135 °C represents the approximate Tg of many cryoprotectant mixtures used in cryobiology. Below this temperature, molecular mobility becomes negligible, chemical reactions cease, and ice recrystallization stops. This is why cryopreservation requires storage below Tg to ensure long‑term stability.
- The glass transition is not a phase change like melting or freezing; instead, it is a kinetic event. As temperature decreases, molecules lose mobility until they can no longer rearrange themselves. The solution becomes increasingly viscous until it solidifies into a glass. In this state, the arrangement of molecules is disordered, but the structure is stable because molecular motion is essentially frozen. This stability is what allows biological samples to be stored for decades without degradation when kept below Tg.
- In cryobiology, Tg is central to understanding the difference between slow freezing and vitrification. Slow freezing allows ice crystals to form, requiring careful control of cooling rates and cryoprotectant concentrations to minimize damage. Vitrification, on the other hand, aims to bypass ice formation entirely by using high concentrations of cryoprotectants and extremely rapid cooling. When successful, vitrification locks the sample into a glassy state, eliminating the risk of ice‑crystal injury. This approach is widely used for oocytes, embryos, and other highly sensitive biological materials. More details are available in Vitrification.
- The glass transition temperature also explains why storage at −20 °C or −80 °C is insufficient for long‑term cryopreservation. At these temperatures, ice crystals can still reorganize and grow—a process known as ice recrystallization—and chemical reactions, though slow, still occur. Only temperatures below Tg ensure complete molecular arrest. Liquid nitrogen systems, which operate at −150 °C (vapour phase) or −196 °C (liquid phase), maintain samples well below Tg, guaranteeing long‑term stability.
- Understanding Tg is essential for designing effective cryopreservation protocols. It influences cryoprotectant selection, cooling rates, thawing procedures, and storage conditions. Whether preserving stem cells, reproductive materials, engineered tissues, or biological samples for research, Tg defines the boundary between unstable frozen states and true cryogenic preservation. It is the scientific foundation behind the concept of “stopping biological time.”
