Ray Optics Model for Optical Trapping of Biconcave Red Blood Cells.

Red blood cells (RBCs) or erythrocytes are essential for oxygenating the peripherical tissue in the human body. Impairment of their physical properties may lead to severe diseases. Optical tweezers have in experiments been shown to be a powerful tool for assessing the biochemical and biophysical properties of RBCs. Despite this success there has been little theoretical work investigating of the stability of erythrocytes in optical tweezers. In this paper we report a numerical study of the trapping of RBCs in the healthy, native biconcave disk conformation in optical tweezers using the ray optics approximation. We study trapping using both single- and dual-beam optical tweezers and show that the complex biconcave shape of the RBC is a significant factor in determining the optical forces and torques on the cell, and ultimately the equilibrium configuration of the RBC within the trap. We also numerically demonstrate how the addition of a third or even fourth trapping laser beam can be used to control the cell orientation in the optical trap. The present investigation sheds light on the trapping mechanism of healthy erythrocytes and can be exploited by experimentalist to envisage new experiments.