## 3 Results and Discussion
### Blood Flows Controlled with Peristaltic Pump
Figure 9A showed RBC sedimentation in the tube as well as blood flow in microfluidic channels for highly aggregated blood (\(C_{dex}\) = 30 mg/mL). As shown in Figure 9A(a), the dextran solution accelerated RBC sedimentation in the tube. The end of the suction tube was fixed inside the tube. Blood was sectioned through the tube, and RBCs were sedimented continuously. Blood was separated into two layers (i.e., diluent in the upper layer and RBCs in the lower layer) after a certain time. As shown in Figure 9A(b), microscopic images of blood flow were obtained at specific times (\(t\) = 0, 320, 640, and 960 s). Initially (\(t\) = 0), blood did not invade the microfluidic channel. At \(t\) = 320 s, blood flowed into the microfluidic channel. The right upper channel (i.e., the coflowing channel) was partially filled with a blood and glycerin solution. However, due to RBC sedimentation in the tube, the hematocrit of blood flows (i.e., RBCs distributed in the channel) decreased significantly from \(t\) = 640 s to \(t\) = 960 s. Figure 9B showed temporal variations in \(\beta\) and \(l_{b}\) with respect to \(C_{dex}\). The control blood showed consistent variation in \(l_{b}\) and \(\beta\) over time. However, when the concentration of the dextran solution increased from \(C_{dex}\) = 10 mg/mL to \(C_{dex}\) = 30 mg/mL, \(l_{b}\) varied largely over time due to RBC sedimentation in the tube. RBC sedimentation causes a decrease in \(\beta\) over time. In other words, \(l_{b}\) and \(\beta\) varied largely from the specific time when the hematocrit of blood flow began to decrease. Figure 9C showed the temporal variations of \(\Delta l_{b}\) with respect to \(C_{dex}\). \(\Delta l_{b}\) was calculated as \(\Delta l_{b}\) = \(l_{b}\) (\(t\) = 0)--\(l_{b}\) (\(t\)). When the hematocrit of blood flow began to decrease, it tended to decrease at higher concentrations of dextran solution. Figure 9D showed the temporal variations of the averaged pressure (\(P_{r}\) [\(L_{c}\)]) calculated for each period with respect to \(C_{dex}\). Control blood (\(C_{dex}\) = 0) and aggregation-enhanced blood (\(C_{dex}\) = 10 mg/mL) exhibited remarkably similar trends in \(P_{r}\) (\(L_{c}\)) over time. Above \(C_{dex}\) = 10 mg/mL, the dextran solution contributed significantly to increasing \(P_{r}\) (\(L_{c}\)). Furthermore, it tended to decrease gradually after the hematocrit of the blood flow began to decrease. Figure 9E showed the temporal variations of \(w_{p}\) with respect to \(C_{dex}\). As the pressure-unit volume work (\(w_{P}\)) was calculated using \(P_{r}\) (\(L_{c}\)), \(w_{P}\) exhibited similar variations in \(P_{r}\). The \(w_{P}\) of the control blood remained constant over time. As shown in Figure 9B, the alternating amplitude of \(\beta\) tended to decrease at higher concentrations of the dextran solution. Thus, aggregation-enhanced blood contributed to decreasing the pressure-unit volume work (\(w_{P}\)) substantially from the specific time when the hematocrit of blood flows began to decrease. From the demonstration, the present method was used effectively to quantify blood flow when the hematocrit of blood varied continuously over time. For the conventional micro-PIV technique, to obtain consistent velocity fields of blood flow, it is necessary to maintain the uniform distribution of RBCs within blood flows. However, as hematocrit varies greatly over time, it might be difficult to get consistent monitoring of blood flow with the micro-PIV technique. Throughout the demonstration, the present method is better than the micro-PIV technique. As a limitation, the present method was demonstrated using a syringe pump that supplied the reference fluid. To extend the application of the present method, it will be necessary to devise a miniaturized delivery system for supplying the reference fluid at a constant flow rate. The delivery system should be equipped with two vital functions, such as fluid delivery with a micropump [60; 61; 62] and flow stabilization with a compliance mechanism [58; 63; 64].