Microfluidic viscometer using capillary pressure sensing

Blood viscosity is considered as a vital determinant of the efficiency of blood flow in blood-vessel networks. The coflowing method is considered as a promising technique for measuring blood viscosity. However, it requires two precise syringe pumps to supply two fluids (i.e., the reference fluid and blood), calibration in advance, and long waiting time for securing steady blood flow. To solve these problems, a single syringe pump is adopted to supply blood into a microfluidic device without requiring a reference fluid. Two key parameters—fluidic resistance and compliance coefficient—are suggested and obtained by analyzing the fluid velocities in a microfluidic channel and calculating the air pressure in the air compliance unit. Using a discrete fluidic circuit model, the pressure difference is analytically derived and utilized as the nonlinear regression formula. The two key parameters are then obtained through nonlinear regression analysis. According to experimental results, the air cavity and flow rate contribute to increasing the compliance coefficient. The fluidic resistance increases significantly at higher concentrations of glycerin solution ranging from 20% to 50%. The proposed method underestimates the values by approximately 27.5% compared with the previous method. Finally, the proposed method is adopted to detect the effects of hematocrit and red blood cell sedimentation in the driving syringe based on two vital parameters. Regarding the fluidic resistance, the normalized difference between the proposed and previous methods is less than 10%. Therefore, two key parameters can be considered as effective for quantitatively monitoring the hematocrit variation in blood flow. In conclusion, from a biomechanical perspective, the proposed method is highly promising for quantifying blood flow in a microfluidic channel.