The Glass Transition Behaviors of Hydroxyethyl Starch Solutions

Abstract

Despite the increasing use of hydroxyethyl starch (HES) as a cryo- and lyoprotectant, information about its glass transition behavior is scarce. The problem stems from the difficulty in detecting the glass transition of HES samples due to the polydispersity of HES and low sensitivity of calorimetric methods. Using an isothermal desorption (controlled air-drying) method and differential scanning calorimetry (DSC), the present study reports a complete glass state transition diagram of the HES-phosphate-buffered saline (HES-PBS) solution (average molecular weight, 262,600; substitution ratio, 0.46). This state diagram is described by the Gordon-Taylor equation. The glass transition temperature (T _g) of amorphous anhydrous solutes and the plasticization constant of water (k) as defined in the model are 406.3 K (133.1°C) and 4.75, respectively. T_g depression of the HES-PBS system is less sensitive to water plasticization than other carbohydrates and biopolymers. Heat capacity change (Δ C _p) associated with glass transition is also hydration-dependent. The relationship between water content and Δ C _p is defined by five hydration ranges, demonstrating complex hydration-dependent thermal behaviors. The thermal transition profile of the HES-PBS system depends on solute concentration. At solute concentrations >75% (wt/wt), no crystallization or devitrification was observed upon cooling or warming. Between 60% and 75%, the solution vitrified during slow cooling, but devitrified upon warming. Between 45% and 60%, the solution vitrified only at high cooling rates. Below <45%, complete vitrification was not achieved, even at a cooling rate about 2,200°C/min. Incomplete vitrification and devitrification resulted in the formation of heterogeneous domains that contribute to a complex transition profile upon warming. Devitrification temperatures of the HES-PBS solutions were observed at 23.3 ± 2.9°C (mean ± standard deviation) above T_g. Finally, the amount of unfrozen water in the freeze concentrate was calculated to be 0.328 g of water/g of solute in the HES-PBS system (i.e., 24.7%, wt/wt). The methodology described herein may be used to overcome the difficulty in glass transition determination of other cryo- and lyoprotectant systems.