## 3 Results and Discussion
### Blood Flows Controlled with Peristaltic Pump
Without the flow stabilizer, blood did not enter the coflowing channel for a certain duration (i.e., \(\beta\) = 0). Blood flow exhibited peristaltic instability over time. To invade blood into the  
Figure 8: Contribution of flow stabilizer to four parameters (i.e., \(P_{r}\) [\(L_{c}\)], \(\tau_{r}\), \(w_{P}\), and \(w_{\tau}\)) of blood flowing under in vitro circulation. (**A**) Schematic diagram of the experimental setup for monitoring blood flow in vitro fluidic circuit. (a) The experimental setup was composed of a microfluidic device equipped with a flow stabilizer, peristaltic pump for delivering blood, and syringe pump for supplying glycerin solution (20%). The cavity of a flow stabilizer was secured by adjusting tubing length (\(L\) = 20, 100, 200, and 300 mm), and closing one-side end with a drawing pin. A tube (\(\sim\)5 mL) was filled with blood (Hct = 50%, and blood volume = 4 mL). The peristaltic pump set to \(\Omega\) = 0.5 rpm. The syringe pump is set to 0.5 mL/h. (b) Microscopic image of microfluidic channels filled with blood and glycerin solution (20%). (**B**) Quantifications of four parameters of blood with respect to tubing length (\(L\)). (a) Temporal variations of \(\beta\) with respect to \(L\). (b) Variations of \(P_{r}\) (\(L_{c}\)) and \(\tau_{r}\) with respect to \(\beta\) and \(L\). (c) Variations of \(w_{P}\) and \(w_{\tau}\) with respect to \(L\). right upper channel (i.e., coflowing channel) continuously, it was necessary to minimize unstable blood flow by adjusting \(L\). By installing a flow stabilizer into the microfluidic device, blood was filled in the coflowing channels and flowed continuously over time. At \(L\) = 20 mm, blood did not invade the right upper channel for a certain duration. When \(L\) was increased from 100 mm to 300 mm, the alternating amplitude of \(\beta\) (i.e., the maximum value of \(\beta\)--minimum value of \(\beta\)) tended to decrease over time. As air compliance increased substantially with respect to \(L\), it contributed to stabilizing the unstable blood flow considerably. Figure 8B(b) showed variations in \(P_{r}\) (\(L_{c}\)) and \(\tau_{r}\) with respect to \(\beta\) and \(L\). When \(L\) was increased from 20 mm to 300 mm, the variation range of \(P_{r}\) and \(\tau_{r}\) decreased significantly. The variation range of \(\beta\) also tended to decrease with respect to \(L\). Figure 8B(c) showed variations of \(w_{p}\) and \(w_{\tau}\) with respect to \(L\). As the area enclosed between \(P_{r}\) (or \(\tau_{r}\)) and \(\beta\) decreased with increasing \(L\), as shown in Figure 8B(b), \(w_{p}\) and \(w_{\tau}\) tended to decrease substantially with respect to \(L\). Two properties related to the work (i.e., \(w_{p}\), \(w_{\tau}\)) tended to decrease owing to the flow stabilizer. Thus, four properties estimated by analyzing blood flow could be considered as quantitative factors for analyzing blood flow. \(P_{r}\) and \(\tau_{r}\) exhibited similar trends with respect to \(L\). In addition, \(w_{p}\) and \(w_{\tau}\) exhibited similar variations with increasing \(L\). Among the four properties, two properties (i.e., \(P_{r}\), \(w_{p}\)) could be used to monitor blood flow. During the demonstration, aggregation-enhanced blood was used to change the hematocrit inside the blood flow. RBC sedimentation in the reservoir caused variations in the hematocrit of blood over time [30]. With respect to the degree of RBC aggregation, the image intensity (\(l_{b}\)) and interface (\(\beta\)) were obtained. Thereafter, two properties of blood flow (\(P_{r}\), \(w_{p}\)) were obtained by analyzing the temporal variations in \(\beta\). To enhance RBC aggregation or RBC sedimentation in a tube, aggregation-enhanced blood (Hct = 50%) was prepared by adding normal RBCs into a dextran solution at several concentrations (\(C_{dex}\) = 10, 20, and 30 mg/mL). Control blood (\(C_{dex}\) = 0) was prepared by adding normal RBCs to the plasma. The tube was filled with aggregation-enhanced blood (\(\sim\)4 mL). The tube length (\(L\)) of the flow stabilizer was fixed at 200 mm.