Abstract

Controlled exchange of solution around soft biological samples (e.g., cells, lipid vesicles, protein particles…) is a challenging task: hydrodynamic flows associated with the solution exchange can irreversibly deform the sample or simply flush it away. We have therefore developed a microfluidic diffusion chamber where the solution exchange is accomplished solely on the basis of diffusion. The sample is transferred into the chamber by optical tweezers and analysed under a standard optical microscope. In this talk I will describe the design of the chamber and show that the flow velocity in the chamber decays exponentially with the distance from its entrance, leaving its back part effectively flow-free. Also, we find that above a critical height/width ratio of the chamber, a vortex emerges at the chamber entrance. I will then show two applications of the diffusion chamber for soft biological samples. First, I will present reversible intercalation of LPS (lipopolysaccharide) into the membrane of flaccid giant lipid vesicles and subsequent changes of vesicle shapes. Second, I will show the reversible response of large protein aggregates (protein particles) to changes in pH, ionic strength, urea or sugar concentration, which shed light on the nature of interactions within the aggregate.