Capillary flow-driven microfluidic device with wettability gradient and sedimentation effects for blood plasma separation

Abstract

We report a capillary flow-driven microfluidic device for blood-plasma separation that comprises a cylindrical well between a pair of bottom and top channels. Exposure of the well to oxygen-plasma creates wettability gradient on its inner surface with its ends hydrophilic and middle portion hydrophobic. Due to capillary action, sample blood self-infuses into bottom channel and rises up the well. Separation of plasma occurs at the hydrophobic patch due to formation of a 'self-built-in filter' and sedimentation. Capillary velocity is predicted using a model and validated using experimental data. Sedimentation of RBCs is explained using modified Steinour's model and correlation between settling velocity and liquid concentration is found. Variation of contact angle on inner surface of the well is characterized and effects of well diameter and height and dilution ratio on plasma separation rate are investigated. With a well of 1.0 mm diameter and 4.0 mm height, 2.0 μl of plasma was obtained (from <10 μl whole blood) in 15 min with a purification efficiency of 99.9%. Detection of glucose was demonstrated with the plasma obtained. Wetting property of channels was maintained by storing in DI water under vacuum and performance of the device was found to be unaffected over three weeks.

Figure 1. Schematic of the device structure, plasma filtration due to moving self-built in filter and sedimentation shown, variation of contact angle inside the cylindrical channel (well) due to oxygen plasma exposure is also shown.

Figure 2

Photograph of the (a) proposed device (b) schematic showing the arrangement of PDMS blocks and (right) positions of the blood drops on the inner face of the block (right) for the contact angle studies.

Figure 3

(a) Capillary meniscus of whole blood in bottom microchannel (b) Distance travelled by blood meniscus x(t) at different time instants t: comparison of theoretical model predictions (Supplementary Information) and experimental data (n = 6, error bars represent standard error of the mean).

Figure 4

Sedimentation of RBCs in a cylindrical well, diameter 1.5 mm, height 6.0 mm (a) whole blood at 30 min and (b) whole blood at 60 min and (c) 1:10 diluted blood, 30 min (d) 1:10 diluted blood, 60 min (e) Location of interface h_i at different time instants for different dilution (n = 6, error bars represent standard error of the mean).

Figure 5. Variation of sedimentation velocity with liquid concentration ε (n = 6, error bars represent standard error of the mean).

Figure 6

Images showing the contact angle of a blood droplet, 4 μl at (a) directly exposed channel edge, y = 0 (b) midpoint between centre and edge, y = 0.25 h_c (c) centre of the inner surface, y = 0.5h_c, gap d_c = 1.5 mm, height h_c = 6 mm, Variation of contact angle with (d) different gaps between PDMS blocks, h_c = 8 mm (e) different height, gap d_c = 2.0 mm (n = 6, error bars in the plots represent standard error of the mean).

Figure 7. Variation of contact angle at three locations on the side wall (y/hc = 0, 0.25, 0.5) with time after storing in water under vacuum (n = 6, error bars represent standard error of the mean).

Figure 8

Images showing the separation and rise of plasma in the well, 1.5 mm diameter and 6.0 mm height at (a) 3 min, (b) 10 min (c) 20 min and (d) 30 min, plasma meniscus h_m and cell-plasma interface h_i are shown.

Figure 9

(a) Variation of the location of plasma meniscus h_m and cell-plasma interface h_i at different instants of time, d_c = 1.5 mm (b) variation of location of plasma meniscus h_m in wells of different diameters d_c, h_c = 6.0 mm (n = 6, error bars represent standard error of the mean).

Figure 10

(a) Location of the plasma meniscus h_m at different instants of time t for different dilutions (c) Location of the plasma meniscus at different instants of time t for different channel heights h_c (n = 6, error bars represent standard error of the mean).

Figure 11

(a–b) Optical images showing the flow of separated plasma in the top channel at two different locations x = 1.0 mm and 10 mm at time t = 10 min and 12 min, respectively (c–d) whole blood and plasma on the haemocytometer grid.

Figure 12

(a) ATR-FTIR analysis and comparison of plasma samples from proposed device and centrifugation (b) Table shows the component groups corresponding to the characteristic bands.

Figure 13. Comparison of difference in the gray scale intensities ΔI_g of glucose detection strip on reaction with plasma of non-diabetic and diabetic patients (n = 3, error bars represent standard error of the mean.

For statistical significance, one-way analysis of variance (ANOVA) was performed on these values followed by Dunnett’s Multiple Comparison Test for multiple comparison of D-1, D-2, D-3 with control sample (C) with statistical significance threshold set at 0.05 (p < 0.05)).