Rotational behaviour of erythrocytes in optical trap: Revisited by confocal fluorescence microscopy

There has been considerable current interest in rotational behavior of red blood cells (RBC) in optical tweezers. However, the mechanism of rotation in polarized tweezers is still not well understood and there exists conflicts in the understanding of this phenomenon. Therefore, we re-examined the underlying phenomenon by use of confocal fluorescence microscopy. Under different osmolarities of the buffer, the three dimensionally reconstructed images showed that the trapped RBC maintains its discotic shape and is oriented in vertical direction. Using dual optical tweezers, the RBC could also be oriented three-dimensionally in a controlled manner. Since, no folding of the RBC was observed under optical trapping beam, the rotational mechanism based on optical birefringence caused by folding of RBC can be ruled out. The alignment of RBC with polarization of the tweezers beam can be attributed to its form-birefringence. We also present the mechanism for possible rotational behavior of RBC in circularly polarized beam.