## 3 Results and Discussion
### Simultaneous Viscosity Measurement of suspended Blood and Plasma with Respect to Hematocrit  
To date, the contributions of vital factors have been quantitatively evaluated in terms of blood velocity or interface. As a demonstration, the proposed method was applied to measure the viscosities of suspended blood and plasma. Suspended blood (Hct = 20, 30, and 40%) was prepared by adding normal RBCs to the same autologous plasma. This was intended to detect the contribution of Hct in blood viscosity as the suspended blood was prepared using the same diluent. Based on the operational protocol suggested in the present study, as shown in Figure 4A, temporal variations in \(\alpha_{b}\), \(I_{b}\), and \(U_{b}\) were obtained with respect to Hct ([i] Hct = 20%, [ii] Hct = 30%, and [iii] Hct = 40%). Considering the suspended blood, \(U_{b}\) tended to decrease at higher Hct values. The Hct contributed to the increase in \(\alpha_{b}\) and \(I_{b}\). The time elapsed for separating plasma from the suspended blood increased at higher Hct values. When the micropump was turned on, \(\alpha_{b}\) immediately reached a steady state. However, \(U_{b}\) took a considerable amount of time to arrive at a steady value. It was inferred that the air damper contributed to the increase in the time delay of \(U_{b}\) rather than that of \(\alpha_{b}\). Figure 3: Quantitative evaluation of blood flow delivered by micro pump and blood flow stabilization with air damper. (**A**) Microscopic images showing variations of interface with respect to operational frequency (\(f\)) ([i]\(f\) = 100 Hz, [ii] \(f\) = 300 Hz, [iii] \(f\) = 400 Hz, and [iv] \(f\) = 800 Hz). (**B**) Contributions of operational frequency to blood velocity and interface. (i) Temporal variations of \(U_{b}\) with respect to \(f\). (ii) Variations of \(U_{b}\) and COV with respect to \(f\). (iii) Temporal variations of \(\alpha_{b}\) with respect to \(f\). (iv) Variations of \(\alpha_{b}\) and COV with respect to \(f\). (**C**) Contribution of air damper to blood flow stabilization. (i) Schematic diagram showing air damper connected to a microfluidic device. (ii) Temporal variations of \(U_{b}\) with respect to \(L_{t}\) = 0, 20, 40, and 60 mm. (iii) Variations of CV with respect to \(L_{t}\). Based on the temporal variations of \(\alpha_{b}\) and \(U_{b}\), the individual viscosities of the suspended blood and plasma were obtained using Equation (3). As shown in Figure 4B, temporal variations in \(\mu_{b}\) were observed with respect to Hct. Owing to the dynamic delay resulting from the air damper, \(\mu_{b}\) of suspended blood reached a steady value after approximately 120 s. Blood viscosity tended to increase at higher values of Hct. The \(\mu_{b}\) of the plasma reached a steady value within 60 s. According to a previous study, the time constant (i.e., time delay) is influenced by fluidic resistance and compliance (i.e., time constant = fluidic resistance \(\times\) compliance) [30; 34]. The time delay of suspended blood was longer than that of plasma because the viscosity of blood was higher than that of plasma. There was no significant difference in the viscosities of the plasma separated from each suspended blood sample. Figure 4C shows a quantitative comparison between the proposed method (i.e., blood delivery with a micropump) and previous method (i.e., blood delivery with a syringe pump). In the present method, the individual viscosities of suspended blood and plasma were obtained as the mean \(\pm\) standard deviation after the viscosity reached a steady value. However, in the previous method, to measure viscosity of suspended blood, the suspended blood was loaded into disposable syringe and installed in the syringe pump; the blood flow rate was set at 1 mL/h. As the shear rate of the blood flow was estimated to be greater than 1000 s\({}^{-1}\), it was assumed that the blood behaved as a Newtonian fluid. Next, to measure the viscosity of the plasma, it was collected from the ultrasonic transducer, as shown in Figure 1(A-ii).