Water activity and glass transition temperatures of disaccharide based buffers for desiccation preservation of biologics

Studying the thermophysical properties of disaccharide based ternary solutions are gaining increasing importance because of their role as excepients in preservation protocols for biologics in general and mammalian cells in particular. Preservation strategies involve not only cryopreservation, but novel approaches like room temperature vitrification and lyophilization. In this study we investigate the water activity and glass transition temperature of citrate and tris buffers (widely used in the gamete preservation industry) with trehalose or sucrose after partial desiccation. After obtaining the water activity (aw) through equilibration at different relative humidity environments, we measured the glass transition temperature (Tg) of these partially desiccated solutions using a differential scanning calorimetry (DSC). The experimental data was used in conjunction with the Gordon-Taylor equation to obtain 3-D contours of Tg as a function of water content and relative salt/sugar concentration. Results indicate that the glass transition behavior is a strong function of the excepient combination. Overall, that trehalose solutions yielded larger values for Tg than sucrose counterparts at low moisture contents in combination with the same buffer. We also saw that citrate solutions yielded larger glass transitions than their tris counterparts. Based on these results, a trehalose-citrate mixture can be picked as the preferred composition for storage applications. The 3-D contours which show a wide variation in slope depending on the salt-sugar concentration constitute important information for the desiccation preservation of biologics.