The reduction of blood damage is still a big challenge in blood-carrying medical devices. In vitro experiments are performed to investigate the damage-causing effects, but due to the opaqueness of blood cells, only near-wall flows can be observed. Thus, several transparent blood models to visualize the rheologic behavior of blood have been proposed and examined. Nevertheless, two-phase blood models with added particles still represent the properties of blood inadequately or are very expensive and complex to produce. In this in vitro study, the viscosity, the flow behavior and the cell deformation of human red blood cells have been compared to a novel, easy-to-produce, two-phase blood model fluid with deformable alginate microspheres. The comparison has been performed in a cone-plate rheometer, a straight and a hyperbolic converging microchannel. The viscosity of the blood model fluid with a particle fraction of 30% showed a shear-thinning behavior, comparable to that of blood at room and human body temperature within shear rates from 7 to $$2000\,\hbox {s}^{-1}$$ 2000 s - 1 . The alginate microspheres were deformable in an extensional flow and formed a cell free layer comparable to that of blood in a straight microchannel. The experiments showed a good optical accessibility of the two-phase flow with traceable movements of individual microspheres in the center of the microchannel. It could be shown that our proposed blood model fluid is a promising tool for the analysis of two-phase flows in complex flow geometries. Graphical Abstract

The reduction of blood damage is still a big challenge in blood-carrying medical devices. In-vitro experiments are performed to investigate the damage-causing effects, but due to the opaqueness of blood cells, only near-wall flows can be observed. Thus, several transparent blood models to visualize the rheologic behavior of blood have been proposed and examined. Nevertheless, two-phase blood models with added particles still represent the properties of blood inadequately or are very expensive and complex to produce. In this in-vitro study, the viscosity, the flow behavior and the cell deformation of human red blood cells (RBCs) have been compared to a novel, easy-to-produce, two-phase blood model fluid with deformable alginate microspheres. The comparison has been performed in a cone-plate rheometer, a straight and a hyperbolic converging microchannel. The viscosity of the blood model fluid with a particle fraction of 30 % showed a shear-thinning behavior, comparable to that of blood at room and human body temperature within shear rates from 7 s-1 to 2000 s-1. The alginate microspheres were deformable in an extensional flow and formed a cell free layer (CFL) comparable to that of blood in a straight microchannel. The experiments showed a good optical accessibility of the two-phase flow with traceable movements of individual microspheres in the center of the microchannel. It could be shown that our proposed blood model fluid is a promising tool for the analysis of two-phase flows in complex flow geometries.