Microflow and crack formation patterns in drying sessile droplets of liposomes suspended in trehalose solutions

Anhydrobiotic preservation potentially provides a means of long-term storage of mammalian cells in carbohydrate glasses under ambient conditions. During desiccation, sessile droplets of glass-forming carbohydrate solutions exhibit complex phenomena, including fluid flow, droplet deformation, and crack formation, all of which may alter the cell preservation efficacy. Cell-sized liposomes were employed as a model system to explore these phenomena in diffusively dried sessile droplets of trehalose solutions. Two factors were identified that strongly influenced the features of the desiccated droplets: the underlying surface and the liposomes themselves. In particular, the surface altered the droplet shape as well as the microflow pattern and, in turn, the moisture conditions encountered by the liposomes during desiccation. A ring deposit formed when the droplets were dried on polystyrene, as would be expected owing to the capillary flow that generally occurs in pinned droplets. In contrast, when dried on the more hydrophilic glass slide, the resulting droplets were thinner, and the liposomes accumulated near their centers, which was an unexpected result likely owing to the glass-forming nature of trehalose solutions. As might be anticipated given the variations in liposome distribution, the choice of surface also influenced crack formation upon continued drying. In addition to providing a preferential path for drying, such cracks are relevant because they could inflict mechanical damage on cells. The liposomes themselves had an even more profound effect on crack formation; indeed, whereas cracks were found in all droplets containing liposomes, in their absence few of the droplets cracked at all, regardless of the surface type. These complex drying dynamics merit further investigation in the development of anhydrobiotic preservation protocols, particularly with regard to the role therein of surface hydrophobicity and the cells themselves.