Paper-Based Flow Fractionation System Applicable to

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Paper-based flow fractionation system applicable to preconcentration and field-flow separation Seokbin Hong, Rhokyun Kwak, and Wonjung Kim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03682 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 2, 2016

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Analytical Chemistry

Paper-based flow fractionation system applicable to preconcentration and field-flow separation Seokbin Hong,† Rhokyun Kwak,* ,‡ and Wonjung Kim*,† † ‡

Department of Mechanical Engineering, Sogang University, Seoul, 121-742, Republic of Korea Center for BioMicrosystems, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea

ABSTRACT: We present a novel paper-based flow fractionation system for preconcentration and field-flow separation. In this passive fluidic device, a straight channel is divided into multiple daughter channels, each of which is connected with an expanded region. The hydrodynamic resistance of the straight channel is predominant compared with those of expanded regions, so we can create steady flows through the straight and daughter channels. While the expanded regions absorb a great amount of water via capillarity, the steady flow continues for 10 min without external pumping devices. By controlling the relative hydrodynamic resistances of the daughter channels, we successfully divide the flow with the flow rate ratio of up to 30. Combining this bifurcation system with ion concentration polarization (ICP), we develop a continuous-flow preconcentrator on a paper platform, which can preconcentrate a fluorescent dye up to 33-fold. In addition, we construct a field-flow separation system to divide two different dyes depending on their electric polarities. Our flow fractionation systems on a paper-based platform would make a breakthrough for pointof-care diagnostics with specific functions including preconcentration and separation.

Microfluidic paper-based analytical devices (µPADs) provide unique advantages owing to the intrinsic properties of paper: low cost, feasible mass production, and pumpless operation.1-3 These characteristics are important requirements for point-of-care diagnostics, especially in resource-limited areas of the world, so that researchers have developed various microfluidic systems on a paper-based platform, including chemical and biological reactors, mixers, and detection devices.4-10 The concentration and separation of analytes are critical for diagnosis using µPADs, so substantial efforts have been made to the development of these techniques.2,3 Gong et al. presented the ICP preconcentrator for collecting targets in a wetted paper without internal flows,11 and Yang et al. developed the similar ICP preconcentrator in a straight channel with a converging area.12 Luo et al. suggested an electrophoretic separation device by stacking paper layers.13 However, these devices provide only transient performance while continuous-flow operations have been desired with conventional microfluidic devices14,15 because a steady operation is advantageous for reliable sensing by biological, chemical, or electrical reactions at a specific region.16 Difficulties in developing µPADs with steady performance stem from the transient characteristic of capillary flow through a porous medium. The hydrodynamic resistance in capillary flow increases with the imbibition length, which thus results in a decreasing flow speed with time.17 Recently, Mendez et al. suggested a design of paper channels with a steady flow,18 in which a fan-shaped channel with a negligible resistance is connected to a straight channel, so the hydrodynamic resistance is practically independent of the imbibition length. Here, we present a novel paper-based flow fractionation system utilizing paper channels with a steady flow. The system has a straight channel divided into multiple daughter channels, each of which is connected to an expanded region

with a negligible hydrodynamic resistance. By adjusting the relative hydrodynamic resistances of the daughter channels, the fluid flow is bifurcated with a fraction ratio of up to 30. Using this flow bifurcation system, we develop a paper-based preconcentrator and a field-flow separator working in a continuous manner.14,19,20

THEORY Capillary flow through a porous medium. Paper is a porous medium consisting of cellulose fibers. When paper comes into contact with liquid, capillary forces create fluid flow in paper. The relative magnitude of inertial to viscous effects is prescribed by Reynolds number, Re = ρud/µ, where ρ is the density of the liquid, u is the velocity of fluid flow, and d is the effective diameter of pores of paper. For the paper considered in the present study, d ~ 10 µm, ρ ~ 103 kg/m3, u ~ 10-4 m/s, and µ ~ 0.001 Pa·s, and Re is on the order of 10-3, indicating negligible inertial effects. The driving force thus balances with the viscous resistance, and the volumetric flow rate Q is given by Darcy’s law: = ∆ (1) where k is the permeability of the porous medium, A is the cross-sectional area, ∆P is the pressure difference over the length L, and µ is the dynamic viscosity of liquid.21 We define the hydrodynamic resistance R = (µL)/(kA), and eq 1 can be expressed as Q = ∆P/R. Since the pressure difference is generated by Laplace pressure ~ σ/d at the gas-liquid interface, the imbibition length L through a uniform channel is given by Washburn equation: = ′ (2)

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where k' is the proportional constant, σ is the surface tension, and t is time.17 The proportional constant k' depends on the properties of the porous medium such as the pore size and contact angle between the liquid and porous medium. Eq 2 represents the transient characteristic of capillary flow because k', σ, and µ are constant with respect to time. Steady flow through a paper channel. We proceed by the design of paper channels with a steady flow rate. As shown in Figure 1a, Mendez et al. designed the paper channel that is connected with an expansion zone. The volumetric flow rate Q in this channel configuration is given by 2 1 + ln =

∆ (3)

where b is the paper thickness, Lr and W are the length and width of the channel, respectively, ω is the vertical angle of the expansion zone, r is the radial distance from the intersection between the channel and the expansion zone to the advancing front of liquid, and ∆PL is the Laplace pressure.18,22 Although eq 3 may aptly describe the flow rate through relatively wide channels with a thickness > 3 mm, it should be modified for a narrow channel constructed by wax printing. Because the surface tension at the hydrophobic wax boundaries is at work in an inverse direction to the flow, liquid imbibition along the wax-bounded channel becomes slower than that along with cut boundaries.23 We introduce the reduction coefficient α, which is defined as the flow rate ratio through a channel with wax boundaries to that through a channel with cut boundaries, and eq 3 is modified to =

1 2 " + ln % # #$

∆ (4)

where αr and αs are the reduction ratios corresponding to the straight channel and expansion zone, respectively, which can be experimentally determined. By noting that the reduction by wax boundaries becomes significant only for narrow channels, αs is presumed as 1 because the effective channel width in the expansion zone is much greater than 3 mm. In eq 4, two terms in the parenthesis are associated with the resistances of the straight channel and the expansion zone, respectively. The resistance in the expansion zone depends on r, which varies with the imbibition length and thus with t, so the flow rate would be unsteady if this term is dominant. However, when the expansion zone has a negligible hydrodynamic resistance compared with that of the straight channel, the hydrodynamic resistance becomes independent of time. Therefore, in the limit of (1/(αsω))ln(2r/W)/((Lr/(αrW)) « 1, we can create a steady flow in the straight channel. Flow bifurcation. We present a flow fractionation system by linking these steady flow channels. Figure 1b shows the flow bifurcation system with two split channels connected with the expansion regions. It can be assumed that the pressure differences between the bifurcation point and the advancing liquid-gas interface are the same in both channels. Therefore, the ratio of the volume flow rates through the split channels Q2/Q1 is governed by their hydrodynamic resistances: 1 2 ' ( # + ln ) * = = (5) ' 1 2 (' + ln ) ' * #' ' ' ' where the subscripts 1 and 2 indicate the upper and bottom channels, respectively (see Figure 1b).

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MATERIALS AND METHODS Paper channel with wax boundaries. We used Whatman grade 1 filter paper with a pore diameter of 11 µm and a thickness of 180 µm. Using a commercial wax printer (ColorQube 8870, Xerox), we produced the channels with hydrophobic wax boundaries.24 The printed channels were then baked at 130 °C for 1 min on a hot plate. To avoid spreading of drop over the paper surface near the reservoirs, adhesive tape was attached on both sides of the paper channel.25 Paper channels for flow fractionation. We printed channels on paper. A mother channel with a length of 10 mm and a width of 4 mm bifurcated into two daughter channels, each of which was connected to the expanded regions with a vertical angle of 90°, so that ω1 = ω2 = π/2 in eq 5 (see Figure 2). In the case of even bifurcation (Q2/Q1 = 1), two daughter channels have the same geometry (1.5 mm width and 1 mm length). To achieve higher flow rate ratios, we fixed the geometry of the wider daughter channel as 3 mm width and 1 mm length and controlled the flow rate ratio of two daughter channels by adjusting the length of the narrower daughter channel, ranging from 1 mm to 10 mm while the width was maintained as 0.4 mm. We measured the reduction ratio α in eq 5 by comparing the imbibition speed through the channel with air boundaries with that through the channel with wax boundaries, and we obtained α1 = 0.45 and α2 = 0.9. Based on eq 5, we estimated that the ratios of flow rates through the daughter channels. For the lengths of upper channel of 1, 3, 7, and 10 mm, the flow rate ratios were calculated to be 1, 10, 20, and 30, respectively. A droplet with a volume of 10 µL was released on the inlet reservoir by a syringe. After the wetting fronts in both daughter channels entered the expansion zones, the imbibition flow was measured for 600 sec using a video camera (HDR-PJ340, SONY). Paper-based ICP preconcentrator. We fabricated the continuous-flow ICP preconcentration by integrating a cation exchange membrane (i.e. Nafion) on the flow bifurcation system. Nafion perfluorinated resin (20 wt%, Sigma-Aldrich) was dropped on the upper and lower sides of the mother channel adjacent to the bifurcation point. After the resin infiltrated through paper, the solvent was evaporated in room temperature. We tested the preconcentration of a negatively charged fluorescent dye (Alexa Fluor 488, Invitrogen, Carlsbad) with a concentration of 1.55 µM (Figure 3) and of a negatively charged protein (Alexa Fluor 488-conjugated bovine serum albumin, Invitrogen, Carlsbad) with a concentration of 15.15 µM (Figure 4) in a 1 mM NaCl solution. For the test of the Alexa Fluor 488-conjugated bovine serum albumin, we added 0.01% Tween 20 to a 1 mM NaCl solution to prevent the binding of the protein with the cellulose matrix of paper.26 A droplet of the solution was placed on the inlet reservoir. On the Nafion-infiltrated areas, 1 mM NaCl solution was also dropped to apply a voltage with Ag/AgCl electrodes. A DC voltage was applied across the mother channel using a sourcemeasurement unit (Keithley, 2635A). The dye concentration was quantified by measuring the fluorescent intensity on the inverted microscope (Olympus, IX-81) combined with a thermoelectrically cooled charge-coupled device (CCD) camera (Hamamatsu Co., Japan). Paper-based field-flow separator. We designed the fieldflow separation system consisting of three inlets and three outlets connected with expansion zones. The flows coming from the inlets were thus merged in the mother channel with a


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length of 7 mm and a width of 6 mm as shown in Figure 5. Power lines were connected to two reservoirs adjacent to the mother channel in order to apply an electric voltage of 20 V across the channel. The reservoirs and channel were separated by the wax patterns with a thickness of 1 mm, which could transfer ions while blocking a bulk liquid flow. We supplied a 100 mM NaCl solution in the reservoirs and 10 mM NaCl solution as buffer flows. The field-flow separation was visualized using positively charged molecules, Ru(bpy)32+, and negatively charged molecules, Alexa Flour 488 in a 1 mM NaCl solution.

RESULTS AND DISCUSSION Flow fractionation system. Figure 2 presents the experimental results of the flow bifurcation. In Figure 2a, dashed lines indicate the advancing water fronts in the expansion regions. Based on the position of the advancing water fronts with respect to time, we calculate flow rates through the upper (Q1) and lower (Q2) daughter channels. The sum of two flow rates is equal to the total flow rate through the mother channel (Qtotal). As can be seen in Figure 2b, the total flow rate is almost steady for 600 sec for all four cases owing to the negligible resistance in the expansion regions. The flow rate is independent of the length of the upper channel, through which flow rate is much lower than those through the lower channel. Therefore, our measurement data are collapsed on the single line in Figure 2b, which corresponds to the prediction by eq 4 for the case of R2/R1 = 1. In Figure 2c, we estimate the flow rate ratio based on the wetting area in the expansion regions. The experimental measurements indicate Q2/Q1 is determined by R1/R2, as predicted by eq 5. The ratios of flow rates remain approximately steady only with a decrease by 10% of the initial value after 10 min. Paper-based ICP preconcentrator. Figure 3 shows the paper-based ICP preconcentrator utilizing the flow bifurcation system. Although the arrangement of the channels is similar to the ICP preconcentrator developed on a polydimethylsiloxane (PDMS) platform,14 our system has two Nafion regions patterned on the upper and lower sides of the mother channel. Under an electric field, the Nafion membrane allows only cation to pass through it.27 As cations are drawn toward the cathode, anions should be moved away to satisfy electroneutrality, which creates an ion depletion zone on the anodic side of the Nafion. With a sufficiently high voltage, this depletion zone evolves far from the bifurcation point (red line in Figure 3a). When liquid with the charged particles flows toward the depletion zone, the charged particles aggregate at the ion depletion zone boundary because this zone is impermeable to charged particles.14,27 Therefore, the flow through the filter channel does not carry the charged targets, which are exclusively carried to the concentration channel (Figure 3a,b). Figure 3b shows the fluorescence image of the ICP concentrator with the highest bifurcation ratio of 30 (see Video S1). A voltage of 100 V is applied to the Nafion patterns when the advancing front enters the expansion zones, which takes 3 min after we put the droplet at reservoir. The fluorescent dyes are evidently condensed in the concentration channel and depleted in the filter channel. The concentration factor, the ratio of the target concentration in the concentration channel to that in the mother channel, can be controlled with the flow rate ratio of the bifurcation

system. In Figure 3c,d, we present the fluorescent images and the intensity profiles measured along the dotted lines near the bifurcation point. Given that the charged targets in the mother channel flow only to the concentration channel, the concentration factor is given by Qtotal/Q1, = R1/R2 + 1 from eq 5. In Figure 3e, we compare our model prediction and experimental measurements of the concentration factors, which shows a good agreement over a concentration ratio range from 1 to 33fold. The concentration efficiency, the ratio of the measured volumetric fractionation ratio to that predicted by the model, is 91%, which is comparable with other recently developed paper-based devices.11,28 In addition, we successfully applied the ICP preconcentrator to concentrate a protein, Alexa Fluor 488-conjugated bovine serum albumin, up to 20-fold, as shown in Figure 4. Because the ion depletion zone was smaller than that in the case of the Alexa Fluor 488, owing to the difference in the surface charges, we reduced the width for the mother channel to 2 mm, which resulted in the limit of the concentration factor up to 20fold. We discuss the limit of the concentration factor of our paper-based ICP preconcentrator. In the current production process with wax printing and thermal reflow, the minimum feature width is approximately 400 µm. Owing to the limit of the minimum width, a higher concentration factor may be pursued by designing the width of the mother and filter channels much greater than that of concentration channel. However, such a channel configuration requires a large volume of the test fluid, which may be inappropriate for point-of-care diagnostics. Alternatively, one may increase the hydrodynamic resistance in the concentration channel with the length the channel. However, a channel with a long length requires an extension of time before the advancing front of fluids enters the expansion zone, which may cause the appreciable effects of the evaporation. For the further study, 3-D paper platforms with a controllable thickness would be useful to overcome this issue. Paper-based field-flow separator. Combined the technique of electrophoretic separation, our flow fractionation system allows us to develop a paper-based field-flow separator. As illustrated in Figure 5, the flow of the sample solution with mixed fluorescent dyes (Alexa Fluor 488 and Ru(bpy)32+) is sandwiched by two sheath flows. When the sample flow is subjected to the electric field orthogonal to the flow direction, the electrophoretic force arrays the charged fluorescent dyes transversely depending on their surface charges. As a result, we can divide negatively charged Alexa Fluor 488 (green fluorescence) and positively charged Ru(bpy)32+ (red fluorescence), and collect them in the different outlets (Figure 5b,c).

CONCLUSIONS We have developed a paper-based flow fractionation system with a designated flow rate ratio. On the basis of a combined theoretical and experimental analysis on the flow in paper channels, the flow rate ratio in the fractionation system can be precisely controlled. Utilizing the fractionation system, we have created a paper-based ICP preconcentrator using a Nafion membrane, which successfully concentrates charged fluorescent dyes up to 33-fold. We have also suggested a paperbased field-flow separator that can separate two different charged dyes. Our paper-based fractionation system is suitable for handling generic samples, and thus would make a break-



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through for reproducing numerous continuous-flow microfluidic systems in paper-based platform.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. supplementary video (avi)

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(25) Starov, V. M.; Kostvintsev, S. R.; Sobolev, V. D.; Velarde, M. G.; Zhdanov, S. A. J. Colloid Interface Sci. 2002, 252, 397-408. (26) Batteiger, B.; Newhall, W. J.; Jones, R. B. J. Immunol. Methods 1982, 55, 297-307. (27) Kim, S. J.; Song, Y.-A.; Han, J. Chem. Soc. Rev. 2010, 39, 912-922. (28) Wong, S. Y.; Cabodi, M.; Rolland, J.; Klapperich, C. M. Anal. Chem. 2014, 86, 11981-11985.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: +82-2-958-6910. *E-mail: [email protected]. Tel: +82-2-705-8824.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (NRF-2015R1A2A2A04006181). R.K. was supported by the KIST institutional program 2E25590 and 2N40800.

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Figure 1. The schematic illustrations of the paper channels with a steady flow rate. (a) After the water front enters the expansion zone, the flow rate becomes steady because the viscous resistance in the expansion region is much smaller than that in the rectangular channel. (b) The flow bifurcation system consists of a mother channel and two daughter channels, each of which is connected with an expanded region.


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Figure 2. Paper channels for bifurcating flows. (a) Flow bifurcation platforms with various flow rate ratios. The dashed lines indicate the advancing water front in the expansion regions at specific time points with a 60 sec interval. (b) The total flow rate through the mother channel is calculated based on the wetting area in the expansion regions. The solid line indicates the model anticipation by eq 4 with αr ~ 0.9, µ ~ 0.001 Pa·s, k ~ 6.5 , 10-13 m2, and Pc ~ 3200 Pa. The flow rate remains approximately steady for 10 min. (c) Experimental measurements of the flow rate ratio through the two daughter channels Q2/Q1 for 600 sec. The solid lines are calculated by eq 5.


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Figure 3. Paper-based ICP preconcentrator. (a) The schematic illustration of the preconcentrator. A NaCl solution with a concentration of 1 mM is put on the Nafion regions (yellow zones). The inset image shows the channel design for a flow rate ratio of 30. (b) Fluorescence image of the 30-fold preconcentration with a fluorescent dye (Alexa Fluor 488). (c) Fluorescence images and (d) fluorescent intensity profiles with various flow rate ratios (1, 10, 20, and 30). The yellow dotted lines indicate the channel boundaries. The fluorescence intensity is measured along the dashed lines A-A'. The dashed lines are the reference fluorescent intensities corresponding to 1, 10 ,20 and 30-fold. (e) The comparison of the concentration factor measured from the fluorescent images with those expected by our model given by eq 5.


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Figure 4. (a) Fluorescence image of the 20-fold preconcentration with Alexa Fluor 488-conjugated bovine serum albumin. (b) Fluorescent intensity profiles with flow rate ratios of 1 and 20.


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Figure 5. Paper-based field-flow separator. (a) Schematic illustration and (b) fluorescence images with two mixed fluorescent dyes: negative dyes (green, Alex Fluor 488) and positive dyes (red, Ru(bpy)32+). The yellow dashed lines indicate the channel boundaries. The relative flow rates through Region 1, Region 2, and Region 3 are 1, 1.5, and 1, respectively. (c) Fluorescence intensity profile along the dotted line (A-A'). The two dyes flow together in a straight without applied voltage, but they bend toward the opposite directions and flow into the separate outlets when a voltage of 20 V is applied.


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Paper-Based Flow Fractionation System Applicable to

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