Effect of cryoprotectant on optimal cooling rate during cryopreservation.

The effect of initial Me2SO concentration (cgi) inside the cell lines on the optimal cooling rate is studied using a well established water transport model. A correlation formula is proposed for the determination of optimal cooling rate of freezing biological systems which depends on the cell activation energy, reference membrane permeability, initial Me2SO concentration, and the cell geometrical parameters. Here, the optimal cooling rate is defined as the highest cooling rate for which amount of trapped water inside the cell is equal to 5% of the initial cell water content at an end temperature of -40 °C. It is found that the optimal cooling rate varies linearly with the reference membrane permeability and the ratio of surface area for water transport to the initial volume of intracellular water. The developed correlation is valid for cell activation energy between 20 and 80 kcal/mole and initial Me2SO concentration between 0.1 and 1.3M. It has been observed that the optimal cooling rate does not follow a single trend for the studied initial concentration of Me2SO. However, three regions are identified within which, the variation is almost similar; the three regions are: 0.1 M ≤ cgi ≤ 0.7, 0.7 M ≤ cgi ≤ 0.9, and 0.9 M ≤ cgi ≤ 1.3M. It has been shown that the predicted optimal cooling rate is in a very good agreement with the published experimental/numerical prediction.