**1. Introduction**

The concept of mechanical circulatory support (MCS) was developed rapidly to resolve the shortage of donor hearts for heart transplantation and the sustained increase of heart failure (HF) mobility and mortality. MCSs with ventricular assist devices (VADs) were originally designed as a temporary therapy for bridge-to-transplant (BTT). Nevertheless, VAD implantation is an efficacious treatment as a destination therapy for patients with end-stage HF, which is refractory to current medical therapy and can effectively improve the long-time survival rate to approximately 81% and 70% at 1 year and 2 years, respectively [1,2]. The therapeutic benefits of VAD implantation are that it

also improves the functional capacity and quality of life. However, patients who undergo long-term application of VADs experience a series of VAD-related adverse events, including right heart failure, pump thrombosis, gastrointestinal bleeding, driveline infection, stroke, and aortic insufficiency [2–10].

Among these VAD-related complications, thrombosis is concerning as a multifactorial complication which is related to other adverse events, such as hemolysis, stroke and bleeding, and can induce rapid clinical deterioration [11]. The mechanism of thrombosis has been studied for decades, and the induced factors of thrombosis lead to a decreased or turbulent flow pattern in VADs and suboptimal anticoagulation. Furthermore, hemolysis and von Willebrand deficiency, which are related to mechanical damage caused by rotate impellers, can also induce thrombosis. It is difficult to visualize the thrombosis in vivo. Therefore, various in vitro test methods and devices have been applied to monitor and evaluate blood clotting and platelet function to avoid thrombosis, such as assays for bleeding time, activated clotting time (ACT), activated partial thromboplastin time (APTT), thromboelastography, and platelet aggregometry. However, all of these parameters are tested under static or irrelevant flow conditions, which fail to incorporate the true flow pattern and status in blood vessels. Furthermore, the test devices are also quite different from actual vessel structures, especially the structure of stenosis arteries segments. Therefore, there is an urgent demand for a new method to investigate the real-time thrombus formation process and evaluate the relationship with the assessment of the blood cell damage in the region of VAD application.

Microfluidic devices, on which the blood flow pattern in simulated small vessels can be observed directly, have recently been used to study erythrocyte movement and blood flow in small vessels in vitro [12]. Based on bionic microfluidic devices, atherosclerotic confinement of vessels and blood flow patterns (including flow rate and physiological and pathological shear stress) in microscale arterioles can be simulated for basic combined research on whole blood and extracellular matrix surface coating or treatment [13–15]. However, the flow patterns were controlled by injecting flow rate and pressure, but were too coarse to maintain consistent inner shear stress force and flow rate in a device with several microscale channels. Furthermore, traces of experiment blood were difficult to collect and measure for the evaluation of the blood cell damage. In our previous research, the relations among erythrocyte morphology, membrane damage, and the concentration of specific plasma proteins had been revealed and confirmed, and the changes of plasma free hemoglobin (PFH) and lactic dehydrogenase (LDH) concentrations in plasma can be measured and analyzed to evaluate the degree of erythrocyte rupture and the degree of erythrocyte membrane injury, respectively [16]. Here, we collected fresh whole blood from sheep and exposed it to a fixed shear stress environment generated by a rheometer for blood cell damage assessment. Then, the damaged blood was collected and injected into a bionic microfluidic device, which was designed to mimic arterioles with several continuous stenosis segments, to investigate and evaluate the effects of red cell damage at different degrees on the flow pattern inside the microfluidic channel.
