Haematocrit heterogeneity in blood flows past microfluidic models of oxygenating fibre bundles

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 well-known effects of such length scales on haemorheology and red blood cell (RBC) behavior, these are generally overlooked in oxygenator modeling 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, characterized 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.