The cryoprotective properties of hydroxyethyl starch investigated by means of differential thermal analysis

Abstract

Phase transition temperatures of the water-rich part of the H 2O-NaCl-HES ternary system have been studied by means of differential thermal analysis. The experimental data indicate that the protective action of the macromolecular agent HES is different from that of DMSO and glycerol which seems essentially to be due to colligative properties. As expected, there is no evidence of a significant freezing-point depression caused by HES concentrated up to 40 wt%. The occurrence of a ternary eutectic point may therefore be excluded. Instead, an isothermal eutectic trough is observed. The extrapolated course of its projection onto the basal composition triangle indicates that a certain portion of water is absorbed by HES and kept from freezing, i.e., appears to be thermally inert within the range of temperatures studied. The protective action of HES against solution effects, therefore, is attributed to its water-absorptive capacity and kinetics instead of a postponement of lethal salt enrichment to lower temperatures as caused by DMSO or glycerol. Consequently, it is possible to determine a minimum initial HES concentration that completely prevents lethal salt enrichment during cooling. For the case of red blood cells, the derived algebraic expression yields an initial HES fraction of 11 wt% if the respective values are inserted.