Red blood cell (RBC) deformability is important for tissue perfusion and a key determinant of blood rheology. Diseases such as diabetes, sickle cell anemia, and malaria, as well as prolonged storage, may affect the mechanical properties of RBCs altering their hemodynamic behavior 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 [Sherwood et al., "Viscosity and velocity distributions of aggregating and non-aggregating blood in a bifurcating microchannel," Biomech. Model. Mechanobiol. 13, 259-273 (2014); Sherwood et al., "Spatial distributions of red blood cells significantly alter local hemodynamics," PLoS One 9, e100473 (2014); and Kaliviotis et al., "Local viscosity distribution in bifurcating microfluidic blood flows," Phys. Fluids 30, 030706 (2018)] 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 flow rates were imaged, and velocity and hematocrit distributions were determined simultaneously using micro-Particle Image Velocimetry 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 velocity profiles, implying a reduction in shear thinning behavior. The hematocrit bluntness also decreased with hardening of the cells, implying an inversion of the correlation between velocity and hematocrit bluntness with loss of deformability. The study illustrates the complex interplay of various mechanisms affecting confined RBC suspension flows and the impact of both deformability and feed hematocrit on the resulting microstructure.

Microfluidic platforms have increasingly been explored for in vitro blood diagnostics and for studying complex microvascular processes. The perfusion of blood in such devices is typically achieved through pressure driven setups. Surface tension driven blood flow provides an alternative flow delivery option, and various studies in the literature have examined the behaviour of blood flow in such fluidic devices. In such flows, the influence of red blood cell (RBC) aggregation, the phenomenon majorly responsible for the non-Newtonian nature of blood, requires particular attention. In the present work, we examine differences in the surface tension driven flow of aggregating, non-aggregating RBC, and Newtonian suspensions, in a rectangular micro channel. The velocity fields were obtained using micro-PIV techniques. The analytical solution for blood velocity in the channel is developed utilising the power law model for blood viscosity. The results showed that RBC aggregation has an impact at the late stages of the flow, observed mainly in the bluntness of the velocity profiles. At the initial stages of the flow the shearing conditions are found moderately elevated, preventing intense RBC aggregate formation. As the flow decelerates in the channel RBC aggregation increases, affecting the flow characteristics.

Macroscopic hydrogels are commonly used as injectable scaffolds or fillers, however they may easily obstruct blood vessels, which poses risks and limits their clinical use. In the present study, three types of hyaluronic acid (HA)-based hydrogel micro-particles with non-covalent, covalent semi-interpenetrating and conventional 3D molecular networks, have been designed, fabricated and characterized. The micro-particles are spherical, biconcave or irregular in shape and their diameter ranged between 2.5 and 3.5 µm; their suspensions exhibit a tuneable viscosity, shear-thinning behaviour, dynamic stability and dispersity in microfluidic flow as a result of their specific particulate nature, providing thus a well-controlled injectable platform. Hydrogel particle suspensions also demonstrate an enhanced safety profile, in terms of the dispersity, cell safety, and hemocompatibility. In addition, Rhodamine 6G has successfully been loaded and released from the particles as a model for drug delivery. Functionalisation of hydrogel microparticles using synthetic polymers has been proven to be a cost-effective way to achieve desirable rheological properties and flow dynamic stability with improved physicochemical properties and biocompatibility in vitro, showing promise as a multifunctional biomedical material for various advanced surgical devices and therapies.

Blood oxygenators act as an extracorporeal artificial lung during certain types of cardiac surgery and intensive care therapies. Inside these devices, blood is forced to flow across an oxygenating bundle, encountering interstitial gaps comparable to those typical of the microvasculature. Despite the wellknown effects of such length scales on haemorheology and red blood cell (RBC) behaviour, these are generally overlooked in oxygenator modelling and design; it is persistently assumed that RBCs are homogeneously distributed throughout the oxygenating bundle, independently of their microstructure arrangement or main flow directions. The goal of this study is to provide preliminary experimental evidence of heterogeneous RBC distributions inside oxygenating fibre bundles. To this end, a number of microchannels were manufactured inspired by actual oxygenating devices, considering simplified versions of their microstructure. These comprise staggered arrays of micro pillars, which were perfused with RBC suspensions, with feed haematocrit (Ht) and velocities relevant for clinical use. The microchannels were imaged using a microscope and high-speed camera to accurately capture cell distribution. The imaged blood flows revealed the non-uniform nature of RBC distributions in the arrays, characterised by local Ht gradients particularly around the O2 sources inside the bundle. These heterogeneous distributions should be accounted for during oxygenator design, as RBC concentration plays a key role in O2 transport and, ultimately, overall device performance

Surface tension driven microfluidic flows offer low-cost solutions for blood diagnostics due to the pump-less flow handling. Κnowledge of the influence of the biomechanical properties of blood on such flows is key to design such devices, however a systematic examination of that influence is lacking in the literature. We report on the effects of specific haemorheological factors for flows in a superhydrophilic microchannel. Whole human blood and erythrocyte suspensions in phosphate buffer and Dextran solutions were tested. Heat-treated counterparts of the aforementioned samples were produced to alter the deformability of the cells. The flow of the samples was imaged and characterised using micro-Particle Image Velocimetry and tracking techniques to probe the effects of haematocrit, erythrocyte aggregation and deformability. Meniscus velocities, velocity profiles in the channel, and local and bulk shear rates were derived. The mean velocity of blood was affected by the increasing sample viscosity and the reduced erythrocyte deformability as expected. The increased erythrocyte aggregation appeared to affect more the shape of the velocity profiles in the normal, compared to the heat-treated samples. Very high shear rates are observed in the early stages of the flow, suggesting high erythrocyte disaggregation, persisting sufficiently strong until the flow reaches the end of the channel.