Manipulation and Assessment of Human Red Blood Cells with Tunable "tug-of-War" Optical Tweezers

Understanding the biomechanical properties of red blood cells (RBCs) is crucial for many pathological analyses and diagnoses of human diseases. Here, we construct the so-called "tug-of-war" (TOW) optical tweezers, consisting of a pair of elongated diverging beams, to study the deformability of human RBCs. Such an optical tweezers system gives rise to object-adapted optical potentials that can stably trap, squeeze, and stretch single RBCs under different osmotic conditions without tethering or mechanical movement. Even at low trapping power, the TOW tweezers can exert a force of 18 pN, which is at least two times stronger than that of dual-trap optical tweezers based on conventional Gaussian beams, leading to more than 15% deformation of the cell shape. From a direct comparison of the trapping forces and shear modulus of the RBCs under different osmotic conditions, we find that the cell deformability follows a trend: Hypotonic > isotonic > hypertonic. This work exemplifies another photonic tool with advanced beam-shaping techniques for biomechanical studies of living cells that is promising for applications such as distinguishing healthy and diseased cells and intracellular delivery.