Optical rheology for live cell membranes

Abstract

We present a novel optical methodology including both instrumentation and theory aimed at retrieving the full viscoelastic information of cell membrane material properties. Red blood cells (RBC) are chosen for this study because of their simple structure, which consists of a bi-layer cell membrane supported by a cytoskeleton enclosing a homogeneous fluid. The full complex modulus of RBC in terms of temporal frequency and spatial frequency is retrieved 'without contact, for the first time to our knowledge. Sub-nanometer sensitivity diffraction phase and fluorescence microscopy (DPF) quantifies non-invasively three dimensional morphological information of live cell with high speed. The fluctuation dissipation theory and generalized Stokes-Einstein relationship provide the complex modulus associated with the cell membrane, in a spatially-resolved manner. This information is used to retrieve the dynamic and spatial behavior of red blood cell membranes during the process of shape deterioration. The viscoelasticity results on RBC strongly correlate with cell morphology. Thus, we find that the cell evolution from a normal, doughnut shape to a spheroid can be interpreted from a viscoelastic point of view as a liquid-solid transition.