## 2 Materials and Methods  
### Microfluidic Device and Experimental Setup  
The microfluidic device used for measuring blood pressure and equivalent compliance of blood samples consisted of a main channel (mc) (width = 250 \(\upmu\)m, length = 16.2 mm) and a pressure channel (pc) (width = 250 \(\upmu\)m, length = 7.5 mm), as shown in Figure 1(A-a). The main channel contained inlet and outlet (a). The pressure channel was bifurcated at 7.6 mm from outlet (a) of the main channel. The pressure channel contained outlet (b). The channel depth of a microfluidic device was fixed at \(h\) = 10 \(\upmu\)m. Conventional micro-electromechanical system techniques were used to fabricate a four inch silicon mold. Based on a soft lithography process with PDMS (Sylgard 184, Dow Corning, Midland, MI, USA) [15], a PDMS block was fabricated from the silicon mold. Using an oxygen plasma system, the PDMS block was bonded to the glass slide.  
Polyethylene tubing (\(L_{2}\)) (length = 200 mm, inner diameter = 500 \(\upmu\)m) was connected to outlet (a). Outlet (b) of the pressure channel was connected to polyethylene tubing (\(L_{3}\)) (length = 100 mm, inner diameter = 500 \(\upmu\)m). After a disposable syringe (\(\sim\)1 mL) was filled with the blood sample (\(\sim\)0.5 mL), the needle of the syringe was fitted into the tubing (\(L_{1}\)). The disposable syringe was  
Figure 1: Proposed method for measuring pressure and equivalent compliance. (**A**) Schematic of the experimental setup, including a microfluidic device, a syringe pump, and an image acquisition system. (**a**) A microfluidic device consisting of a main channel (mc) and pressure channel (pc). (**b**) A syringe pump for delivering the blood sample. (**c**) Optical image acquisition. (**B**) Two regions-of-interest (ROIs) selected for evaluating averaged blood velocity in the main channel (\(<\)\(U_{mc}\)\(>\)) and averaged blood velocity in the pressure channel (\(<\)\(U_{mc}\)\(>\)). (**C**) As a preliminary demonstration, the blood sample (normal RBCs in 1\(\times\) PBS, hematocrit (Hct) = 50%) was supplied into inlet with a syringe pump. (**a**) Microscopic images of the pressure channel at specific times (\(t\) = 0, 60, 120, 180, 240, 300, 360, and 420 s) during a single period. (**b**) Temporal variations of \(<\)\(U_{mc}\)\(>\) and \(<\)\(U_{pc}\)\(>\) during a single period.  
then installed into a syringe pump (neMESYS, Cetoni Gmbh, Germany). As shown in Figure 1(A-b), the syringe pump was positioned horizontally. The syringe pump was controlled in a periodic on-off fashion (i.e., period [\(T\)] = 480 s, duty ratio = 0.5, flow rate = \(Q_{0}\)). After filling both channels with the blood sample, the end of the tube (\(L_{3}\)) was clamped with a drawing pin. A microfluidic device was then positioned on an inverted microscope (BX51, Olympus, Tokyo, Japan). As shown in Figure 1(A-c), based on a high-speed camera (FASTCAM MINI, Photron, Tokyo, Japan), microscopic images were captured at a frame rate of 5000 frame per second (fps). Using an external trigger, two microscopic images were captured sequentially at intervals of 1 s. The camera offered a spatial resolution of \(1280\times 1000\) pixels. Each pixel corresponded to 10 \(\upmu\)m.