Deformation of biconcave Red Blood Cell in the Dual-beam Optical Tweezers

Red Blood Cell (RBC) suspended in aquatic buffer is trapped and deformed by the dual-beam optical tweezers in order to measure the elasticity of the cell’s membrane. In this experiment, the RBCs were in the natural form of biconcave disc shape. We modeled a highly focused trapping beam of numerical aperture NA=1.25 as a spherical wave with Gaussian intensity distribution in a stationary study mode, and built the background optical field by adding two such waves shifted in a lateral direction. The perfect matched layers (PML) were selected to define the computation domain. The 3D radiation stress distribution on the biconcave RBC surface was computed by Comsol multi-physics Radio Frequency Module via the Maxwell stress tensor, and the 3D deformation of the biconcave RBC was then computed with the elastic membrane theory with Comsol Structure Mechanics Module. The linear elastic material model with nearly incompressible material and geometric nonlinearity were set up to study large deformation of the cell. The two modules were fully coupled and used the moving mash to compute in a recursive process the electromagnetic field and the radiation stress distribution on the deformed RBC of any geometric shape and its consequent deformation until the final equilibrium state. The computed deformations in the background field with different beam separations and laser powers can fit to the experimental data in order to determine the Young’s elasticity module of the cell membrane.