Red blood cell in the field of the laser beam of optical tweezers
All optical methods for studying the blood cells usually measure their characteristic parameters on a large ensemble of cells (ektacytometry, diffuse light scattering, etc.) or on an individual cell level (laser tweezers, atomic force microscopy, etc.) [1,2]. Laser tweezers (LT) are devices that allow for trapping and manipulation of living cells without direct mechanical contact with them using highly focused laser beam . LT are a powerful tool for the study of properties of single red blood cells (RBC) and mechanisms of their interaction [4,5]. The effect of optical trapping on the RBC elastic and other properties were studied by many authors [4,6]. However, there is no full understanding of the shape changes of RBC in the field of the laser beam as well as the influence of laser exposure on RBC membrane.
The aim of this work was to study the effect of the strongly focused laser beam with the wavelength 1064 nm on the trapped RBC using LT given the trapping of a single RBC is performed in aqueous environment at room temperature. The dependences of the trapped cell’s transverse linear dimensions and shape on the radiation power and the time of laser exposure were studied.
The process of trapping a singe RBC is presented in the Fig.1. It was found that the single RBC in the optical trap changes its orientation and acquires an asymmetric shape. The linear dimensions of different parts of the cell depend on the radiation power at the focal point as it is shown in the Fig.2. In our earlier work, it was shown  that at the given wavelength and trapping time the cell heating effects are negligible: the temperature of the trapped cell increases only by ∼1 °C per each 10 mW of laser beam power during the trapping time of about 5-10 min. In our study, RBC does not undergo significant changes during 5 min at the laser beam power less than 60 mW (Fig.2). At the laser beam power higher than 80 mW the RBC starts to fold and at that higher than 130 mW the RBC’s membrane breaks after 5 min of trapping. Additionally, it was found that the amplitude of the change in linear dimensions decreases with repeated short-term trapping of the cell by the laser beam of fixed power (Fig.3), which indicates a change in the rigidity of the RBC membrane.
The obtained results are important for understanding the mechanisms of the laser beam interaction with trapped RBC and for optimizing the technique when conducting optical experiments with them, especially for performing measurements of membrane mechanical properties.
Acknowledgment: The authors acknowledge the financial support provided to this study by Russian Foundation for
Basic Research grant № 19-52-51015.
 O. Baskurt et al., “Red Blood Cell Aggregation,” CRC Press, Boca Raton, United States, 2012.
 A.V. Priezzhev et al., “Optical Study of RBC Aggregation in Whole Blood Samples and Single Cells,” Chapter 1 in “Handbook on Optical Biomedical Diagnostics”, V. V. Tuchin – editor, 2nd Edition, SPIE Press Bellingham, WA, United States, 2016.
 A. Ashkin et al., “Observation of a single-beam gradient force optical trap for dielectric particles,” Optics Letters 11(5), 288-290, 1986.
 A. Ashkin, “Optical Trapping and Manipulation of Neutral Particles Using Lasers: a reprint volume with commentaries”, World Scientific Publishing Co. Pte. Ltd., Singapore, 2006.
 P. B. Ermolinskiy et al., “Interaction of erythrocytes in the process of pair aggregation in blood samples from patients with arterial hypertension and healthy donors: measurements with laser tweezers”, J of Biomedical Photonics & Eng 4(3), 030303-1 – 030303-8, 2018.
 A. Ghosh et al. “Euler buckling-induced folding and rotation of red blood cells in an optical trap” Phys. Biol. 3, 67-73 (2006).
 A. Y. Maklygin et al., “Measurement of interaction forces between red blood cells in aggregates by optical tweezers,” Quantum
Electronics 42(6), 500–504, 2012.
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Physics Department, Lomonosov Moscow State University, Russia
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