- Propylene glycol (PG), also known as 1,2‑propanediol, is a permeating cryoprotective agent widely used in cryobiology for the preservation of oocytes, embryos, and various mammalian cells. As a small, hydrophilic molecule, propylene glycol penetrates cells efficiently, modifies water behaviour during cooling, and reduces ice formation. Its favourable balance of permeability, glass‑forming ability, and moderate toxicity makes PG an important component of many cryopreservation and vitrification protocols.
- Propylene glycol acts by altering the thermodynamic and kinetic properties of water. It lowers the freezing point, increases solution viscosity, and interferes with ice nucleation and crystal growth. Because PG permeates cells relatively quickly, it helps maintain osmotic balance during CPA loading and removal, reducing the risk of excessive shrinkage or swelling. This rapid permeability is particularly advantageous for large, water‑rich cells such as oocytes and early embryos, which are highly susceptible to intracellular ice formation.
- The cryoprotective mechanism of propylene glycol involves several molecular interactions. PG forms hydrogen bonds with water molecules, disrupting the formation of ordered ice lattices. It also interacts with lipid bilayers, stabilizing membrane fluidity at low temperatures. In vitrification solutions, PG is often combined with other permeating CPAs—such as ethylene glycol or DMSO—and non‑permeating additives like sucrose or trehalose. These mixtures create synergistic effects that reduce the concentration of any single CPA while enhancing overall glass‑forming ability. More details on CPA combinations are discussed in Cryoprotective Agents.
- Propylene glycol exhibits moderate toxicity, which is lower than DMSO but higher than ethylene glycol. Toxicity arises from osmotic stress, membrane perturbation, and chemical interactions with intracellular components. PG can alter pH, disrupt protein structure, and interfere with metabolic processes if exposure times are too long. Temperature strongly influences toxicity: PG is significantly less harmful at low temperatures, which is why CPA loading is typically performed at 4–10 °C. In vitrification protocols, exposure times are kept extremely short—often under one minute—to minimize chemical injury. These toxicity mechanisms are explored further in Cryoprotectant Toxicity.
- Propylene glycol is widely used in reproductive biology. Historically, PG was one of the first CPAs used for oocyte and embryo freezing, and it remains a component of several slow‑freezing protocols. Although ethylene glycol has become more common in modern vitrification systems due to its lower toxicity, PG continues to play an important role in specific species, cell types, and experimental conditions. In some mammalian embryos, PG provides superior membrane stabilization and supports high post‑thaw developmental potential.
- Beyond reproductive applications, propylene glycol is used in the cryopreservation of certain tissues, stem cells, and small organisms. Its versatility makes it valuable in research settings where cell‑type‑specific optimization is required. PG’s moderate toxicity profile also makes it useful in CPA mixtures designed to reduce reliance on more harmful agents.
- Overall, propylene glycol remains an important cryoprotectant in modern cryobiology. Its permeability, ice‑suppression properties, and compatibility with vitrification mixtures make it a useful tool for preserving sensitive biological materials. Whether used in reproductive medicine, tissue preservation, or experimental cryobiology, propylene glycol continues to contribute to advances in low‑temperature biological storage.
