1 The effect of deformability on the microscale behaviour of red blood cell suspensions Andreas Passos 1 , Joseph M. Sherwood 2 Efstathios Kaliviotis 3 , Rupesh Agrawal 4,5 , Carlos Pavesio 5,6 , Stavroula Balabani 1, * 1 Department of Mechanical Engineering, University College London, UK 2 Department of Bioengineering, Imperial College London, UK 3 Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Cyprus 4 National Healthcare Group Eye Institute, Tan Tock Seng Hospital, Singapore 5 Moorfields Eye Hospital, UK 6 Biomedical Research Centre, Institute of Ophthalmology, University College London, UK *s.balabani@ucl.ac.uk Abstract Red blood cell (RBC) deformability is important for tissue perfusion and a key determinant of blood rheology. Diseases such as diabetes, sickle cell anaemia and malaria, as well as prolonged storage may affect the mechanical properties of RBCs altering their hemodynamic behaviour and leading to microvascular complications. However, the exact role of RBC deformability on microscale blood flow is not fully understood. In the present study we extend our previous work on healthy RBC flows in bifurcating microchannels 1–3 to quantify the effects of impaired RBC deformability on the velocity and hematocrit distributions in microscale blood flows. Suspensions of healthy and glutaraldehyde hardened RBCs perfused through straight microchannels at various hematocrits and flowrates were imaged and velocity and hematocrit distributions were determined simultaneously using micro Particle Image Velocimetry (PIV) and light transmission methods, respectively. At low feed hematocrits, hardened RBCs were more dispersed compared to healthy ones, consistent with decreased migration of stiffer cells. At high hematocrit, the loss of deformability was found to decrease the bluntness of the velocity profiles, implying a reduction in shear thinning behaviour. The hematocrit bluntness also decreased with hardening of the cells implying an inversion of the correlation between velocity - hematocrit bluntness with loss of deformability. The study illustrates the complex interplay of the various mechanisms affecting confined RBC suspension flows and the impact of both deformability and feed hematocrit on the resulting microstructure. 1. Introduction Microvascular flow plays a vital role in regulating physiological functions, such as vascular resistance, and maintaining organ health. A large number of pathologies alter these flows leading to complications and posing a risk to human health. Understanding the hemodynamics and rheology of blood flow in the microvasculature is thus of great importance in health and disease. Red blood cells (RBCs) or erythrocytes are the most abundant cells in blood, with a volume fraction of 45% (known as hematocrit); hence blood can be considered as a soft particle suspension, i.e. comprising deformable cells suspended in a continuous phase, the plasma (which is typically assumed Newtonian). As such, blood exhibits shear-thinning behaviour, determined to a great extent by the deformability and aggregation of RBCs. The ability of RBCs to deform is key to their function of nutrient and waste transport as it allows them to flow through microvessels with dimensions comparable to or smaller than their size. Pathologies such as sickle cell anaemia, malaria or diabetes are associated with both less deformable RBCs and microangiopathies, such as ischemia and vaso-occlusion. 4 Decreased RBC deformability has been reported in malaria, 5 sickle cell anaemia, 6 and other metabolic inflammatory or vascular diseases such as diabetes, Alzheimer and stroke. 7–10 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/1.5111189