###### Abstract  
Blood viscosity is influenced by several factors, including red blood cell (RBC) deformability, hematocrit (Hct), and plasma protein levels. To effectively isolate the individual contributions of several factors, it is necessary to simultaneously measure the viscosities of the blood and plasma. In this study, the viscosities of suspended blood and plasma were obtained sequentially by adopting an ultrasonic transducer for plasma separation and a co-flowing microfluidic channel for viscosity measurement. To improve the measurement accuracy of viscosity, the correction factor was obtained through experiments and numerical simulations, which was then inserted into the analytical expression for viscosity. To stabilize the pulsatile blood flow resulting from a micropump, the frequency (\(f\)) and voltage (\(v\)) were set to \(f=300\) Hz and \(v=140\) au, respectively. Flexible polyethylene tubing (i.d. = 500 \(\mu\)m, length = 40 mm) was connected to the microfluidic device as an air damper. Consequently, the coefficient of variance of the blood velocity decreased by up to 1%. As a demonstration, suspended blood (Hct = 20%, 30%, and 40%) was prepared by adding normal RBCs to autologous plasma. Compared with the previous method, the present method overestimates the viscosity values of both the fluids (i.e., suspended blood: 14-25% and plasma: 7-21%). The present method has the ability to sequentially measure the viscosities of suspended blood and plasma. The integrated system contributes to reducing blood-handling procedures (i.e., blood collection, blood loading into the syringe, and syringe installation into the syringe pump). bloch viscosity; plasma separation; microfluidic device; ultrasonic transducer; co-flowing method +
Footnote †: journal: Medical Engineering  
**Citation:** [PERSON]. Simultaneous Viscosity Measurement of Suspended Blood and Plasma Separated by an Ultrasonic Transducer. _Appl. Sci._**2023**, 13, 3574. [[https://doi.org/10.3390/app13063574](https://doi.org/10.3390/app13063574)]([https://doi.org/10.3390/app13063574](https://doi.org/10.3390/app13063574))
resistance \(\times\) flow rate) [12], fluidic resistance can be obtained by measuring either the pressure drop or flow rate. Fluidic resistance is proportional to the blood viscosity in a rectangular microfluidic channel. In principle, the blood viscosity can be obtained by measuring the fluidic resistance quantitatively, especially under a specific flow rate or pressure drop. According to previous studies, several methodologies for measuring fluidic resistance (i.e., interface location in the co-flowing channel [13; 14], digital flow compartment [15; 16; 17], and droplet length [8; 9]) or pressure drop (i.e., liquid-air interface movement [18], and membrane displacement [19; 20; 21]) have been suggested for obtaining blood viscosity. By supplying blood (or plasma) into a microfluidic channel, these methods are employed to measure the viscosity of blood or plasma. As the viscosity of whole (or suspended) blood is influenced by several factors, including RBC deformability, Hct, and plasma proteins, the contributions of several factors need to be understood. Therefore, it is necessary to isolate the individual effects of these factors. However, most of the previous methods focused on the accurate measurement of blood viscosity in microfluidic devices; they have a limitation in the simultaneous measurement of viscosities of whole blood (or suspended) blood and plasma. To resolve this issue, it is essential to separate the plasma from blood [22; 23; 24] and sequentially supply both fluids into a microfluidic device. Subsequently, the viscosities of both fluids were measured by adopting a coflowing method reported in previous studies [13; 25; 26; 27; 28]. Here, when blood and reference fluid are supplied at a specific flow rate ratio, interface between two fluids is determined by viscosity ratio of two fluids.