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 some attempts to imitate this in nontolerant organisms have been partially successful, and why narrowly constrained rates of drying, cooling and heating are critical to that success. Below the ice point, temperature lowering reduces the vapour pressure of water, and the semipermeability of membranes imposes a large mechanical or ‘osmotic’ stress on cells which undergo a deformation, or strain, borne principally by the cytoskeleton. Nonetheless, even lethal stresses can have their effects deferred for minutes or even hours, providing a ‘window’ for cryopreservation. We propose that because the connection between stress and strain is not elastic but viscoelastic, the strain is spread over time and its intensity diluted. Beyond limits of time and intensity, relaxation in the cytoskeleton will become irreversible. We offer an Avrami model in which an Arrhenius expression models the temperature effect and elastic moduli supply an activation energy, to provide a rational basis for the development of cryopreservation techniques.