Freezing Stress, Osmotic Strain, and their Viscoelastic Coupling

Abstract

This paper attempts to explain why, against seemingly overwhelming odds,certain organisms can tolerate freezing to low temperatures, why someattempts to imitate this in nontolerant organisms have been partiallysuccessful, and why narrowly constrained rates of drying, cooling andheating are critical to that success. Below the ice point, temperaturelowering reduces the vapour pressure of water, and the semipermeabilityof membranes imposes a large mechanical or `osmotic' stress on cellswhich undergo a deformation, or strain, borne principally by thecytoskeleton. Nonetheless, even lethal stresses can have their effectsdeferred for minutes or even hours, providing a `window' forcryopreservation. We propose that because the connection between stressand strain is not elastic but viscoelastic, the strain is spread overtime and its intensity diluted. Beyond limits of time and intensity,relaxation in the cytoskeleton will become irreversible. We offer anAvrami model in which an Arrhenius expression models the temperatureeffect and elastic moduli supply an activation energy, to provide arational basis for the development of cryopreservation techniques.