Viscosities encountered during the cryopreservation of dimethyl sulphoxide systems.

This study determined the viscous conditions experienced by cells in the unfrozen freeze concentrated channels between ice crystals in slow cooling protocols. This was examined for both the binary Me ₂ SO-water and the ternary Me ₂ SO-NaCl-water systems. Viscosity increases from 6.9 ± 0.1 mPa s at -14.4 ± 0.3 °C to 958 ± 27 mPa s at -64.3 ± 0.4 °C in the binary system, and up to 55387 ± 1068 mPa s at -75 ± 0.5 °C in the ternary (10% Me ₂ SO, 0.9% NaCl by weight) solution were seen. This increase in viscosity limits molecular diffusion, reducing adsorption onto the crystal plane. These viscosities are significantly lower than observed in glycerol based systems and so cells in freeze concentrated channels cooled to between -60 °C and -75 °C will reside in a thick fluid not a near-solid state as is often assumed. In addition, the viscosities experienced during cooling of various Me ₂ SO based vitrification solutions is determined to below -70 °C, as is the impact which additional solutes exert on viscosity. These data show that additional solutes in a cryopreservation system cause disproportionate increases in viscosity. This in turn impacts diffusion rates and mixing abilities of high concentrations of cryoprotectants, and have applications to understanding the fundamental cooling responses of cells to Me ₂ SO based cryopreservation solutions.
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