Red blood cell deformation in a microfluidic sudden expansion at supraphysiological strain rates

Red blood cell (RBC) deformability is critical for in vivo tissue oxygenation and has been studied in ideal, canonical flows. In vitro frameworks are needed to study RBC deformation in more complex flows than those studied previously, as complex hemodynamics are common in blood-wetted medical devices and valvular pathologies. In this study, pseudo-three-dimensional RBC deformation and orientation angles are investigated in vitro in a microfluidic sudden expansion under two viscosity conditions (2.05 and 4.17 cP) and three flow rates (100, 200, and 400 μL/min). The velocity gradient in the sudden expansion is supraphysiological (3,000-200,000 s^-1) with coexisting shear, rotational, and extensional components to produce complex flows. An image post-processing workflow is developed to generate two-dimensional contour maps of RBC deformation and orientation throughout the sudden expansion. RBCs remain ellipsoidal throughout the channel except when transitioning between extended and compressed, where they exhibit a teardrop shape. Peak deformation index (DI) magnitudes in the complex flow regions reach 0.53 and 0.33 for 4.17 and 2.05 cP, respectively. DI magnitudes are 20%-27% higher in extension-dominant compared to compressive-dominant flows for 4.17 cP, while they are 11%-14% lower in extension-dominant compared to compressive-dominant flows for 2.05 cP. The pseudo-three-dimensional DI behavior between viscosity conditions also differ, with DI differences observed between 4.17 and 2.05 cP in the membrane cross section orthogonal to the principal straining direction. Two-dimensional contour maps of RBC deformation and orientation show high spatial variability, a result of the velocity gradient tensor's heterogeneity throughout the device. Contour map data also reveal higher RBC behavior variability in compressive and mixed flows compared to homogeneous extension. The study's novelty is capturing RBC deformation in off-centered regions of a sudden expansion across a suite of conditions, offering new insights into RBC behavior in complex flows.