Sequential quantification of blood and diluent using red cell sedimentation-based separation and pressure-induced work in a microfluidic channel

The erythrocyte sedimentation method has been widely used to detect inflammatory diseases. However, this conventional method still has several drawbacks, such as a large blood volume (similar to 1 mL) and difficulty in continuous monitoring. Most importantly, image-based methods cannot quantify RBC-rich blood (blood) and RBC-free blood (diluent) simultaneously. In this study, instead of visualizing interface movement in the blood syringe, a simple method is proposed to quantify blood and diluent in microfluidic channels sequentially. The hematocrit was set to 25% to enhance RBC sedimentation and form two layers (blood and diluent) in the blood syringe. An air cavity (similar to 300 mu L) inside the blood syringe was secured to completely remove dead volumes (similar to 200 mu L) in fluidic paths (syringe needle and tubing). Thus, a small blood volume (V-b = 50 mu L) suctioned into the blood syringe is sufficient for supplying blood and diluent in the blood channel sequentially. The relative ratio of blood resident time (RBC-to-diluent separation) was quantified using lambda(b), which was obtained by quantifying the image intensity of blood flow. After the junction pressure (P-j) and blood volume (V) were obtained by analyzing the interface in the coflowing channel, the averaged work (W-p [Pa mm(3)]) was calculated and adopted to detect blood and diluent, respectively. The proposed method was then applied with various concentrations of dextran solution to detect aggregation-elevated blood. The W-p of blood and diluent exhibited substantial differences with respect to dextran solutions ranging from C-dex = 10 to C-dex = 40 mg mL(-1). Moreover, lambda(b) did not exhibit substantial differences in blood with C-dex > 10 mg mL(-1). The variations in lambda(b) were comparable to those of the previous method based on interface movement in the blood syringe. In conclusion, the W-P could detect blood as well as diluents more effectively than lambda(b). Furthermore, the proposed method substantially reduced the blood volume from 1 mL to 50 mu L.