Cryoprotectant equilibration in tissues

The first step in the cryopreservation of cells or tissues is often the movement of a permeating cryoprotectant into the cells or tissues from the solution into which they have been placed. The cryoprotectant enters the cells or tissues by thermodynamic equilibration with the surroundings. In the reverse case, thermodynamic equilibration also drives the removal of permeating cryoprotectants by a dilution solution at the end of the preservation process when the cells or tissues are being readied for use. There have been reports of tissues having equilibrium cryoprotectant concentrations lower than that of the surrounding carrier solution. For various tissues, the equilibrium concentration of cryoprotectant inside the tissue is either equal to, or lower than the cryoprotectant concentration of the surrounding solution. A simple thermodynamic treatment of the solution-tissue equilibrium shows that an equilibrium concentration difference can exist between a tissue and the surrounding solution if a pressure difference can be maintained.