Dynamics of Optically Trapped Red Blood Cells by Phase Contrast Microscopy

We report red blood cell (RBC) stretching using a Zeiss Axioplan microscope, modified for phase contrast and optical trapping using a 808 nm diode laser bar, as a tool to characterize RBC dynamics along a linear optical trap. Phase contrast offers a convenient method of converting small variations of refractive index into corresponding amplitude changes, differentially enhancing the contrast near cell edges. We have investigated the behavior of RBCs within both static and dynamic microfluidic environments with a linear optical stretcher. Studies within static systems allow characterization of cell interactions with the line optical force field without the complicating forces associated with hydrodynamics. In flowing, dynamic systems, cells stretch along the optical trap down microfluidic channels and are eventually released to recover their original shape. We record the dynamic cell response with a CMOS camera at 250 fps and extract cell contours with sub-pixel accuracy using derivative operators. To quantify cell deformability, we measure the major and minor axes of individual cells both within and outside of the trap, which also allows measurement of cell relaxation. In these studies, we observe that cell rotation, stretching, and bending along the linear optical trap, are tightly coupled to the modulation of optical power and cell speed inside our microfluidic systems.