Programming Fluid Transport in Paper-Based Microfluidic Devices

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Programming fluid transport in paper-based microfluidic devices using razor-crafted open channels Dimosthenis L. Giokas, George Z Tsogas, and Athanasios G. Vlessidis Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 10 Jun 2014 Downloaded from http://pubs.acs.org on June 17, 2014

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Analytical Chemistry

Programming fluid transport in paper-based microfluidic devices using razor-crafted open channels Dimosthenis L. Giokas∗, George Z. Tsogas, Athanasios G. Vlessidis Department of Chemistry, University of Ioannina, 45110, Ioannina, Greece

∗

Corresponding author. E-mail: [email protected], Tel: +30-26510-08402, Fax:

+30-26510-08781

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ABSTRACT Manipulating fluid transport in microfluidic, paper-based analytical devices (µPADs) is an essential pre-requisite to enable multiple timed analytical steps on the same device. Current methods to control fluid distribution mainly rely on controlling how slowly the fluid moves within a device or by activating an on/off switch to flow. In this article, we present an easy approach for programming fluid transport within paper-based devices that enables both acceleration as well as delay of fluid transport without active pumping. Both operations are programmed by carving open channels either longitudinally or perpendicularly to the flow path using a craft-cutting tool equipped with a knife blade. Channels are crafted after µPADs fabrication enabling the end user to generate patterns of open-channels on demand by carving the porous material of the paper without cutting or removing the paper substrate altogether. Parameters to control the acceleration or delay of flow include the orientation, length and number of open channels. Using this method accelerated as well as reduced fluid transport rates were achieved on the same device. This methodology was applied to µPADs for multiple and time-programmable assays for metal ion determination.


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INTRODUCTION Microfluidic paper-based analytical devices (µPADs) is a portable and cheap platform which can perform analytical assays in a simple manner, at low cost and minimal external resources. Following their introduction by the Whitesides group,1,2 two and three dimension templates have been successfully described for various applications ranging from single analyte determinations to multiplex assays.3-7 Critical variables to the design of µPADs are the fabrication process and the controlled transport of reagents within them. The fabrication processes is currently well developed and various techniques have been described that offer a wide range of capabilities such as photolithography, plotting, inkjet printing, plasma etching, flexographic printing, wax printing and xyrography (knife cutting).5,

8-10

Of these methods, wax-printing is

probably the simplest and cheapest micro-patterning method affording high flexibility to the design of µPADs with various technical specifications. The technique relies on printing patterns of solid wax on the paper surface which is melted under increased temperature to penetrate the full thickness of the paper.11 By this process it is feasible to create hydrophobic barriers that define hydrophilic channels, fluid reservoirs and reaction zones with good precision. After fabrication, fluid control is the next most crucial parameter in the operation of µPADs. Fluid transport on paper is an autonomous passive process governed by capillary forces and depends on the physicochemical properties, the homogeneity and uniformity of the paper, the dimensions of the channels, the viscosity of the fluid, and the environmental conditions.12 Therefore, paper itself offers limited control over fluid transport. However, to enable more complex applications of µPΑDs that require multi-step sequences instead of single step protocols, it is necessary to regulate and control the delivery of single or multiple 3 ACS Paragon Plus Environment


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fluids.13,14 Such control would provide µPADs with additional attributes such as sample processing, real-time mixing of reagents, improved sensitivity, sample incubation and the realization of sequence of reactions, much like the timed pipetting steps used in many laboratory protocols. A convenient method to manipulate fluid dynamics in µPADs, is to adjust the topologies and geometries of the flow channels (length and width).13 However, as the dimensions of the hydrophilic flow path increases the fluid wicking time increase.

8,15

In

addition, the sample volume requirements also increase due to increased sample retention in the porous cellulose matrix.16 Therefore, tuning of channel dimensions may be impractical when small amounts of sample are available or the distribution rates of the fluids must differ by several minutes on the same device.12 To accommodate more accurate timing and actuation of fluid transport without active pumping, several innovative fluidic tools have been developed that include disposable sugar barriers as programmable on-switches to flow,13,17 dissolvable bridges as off-switches,18 tunable-delay shunts that divert flow into an absorbent pad,19 modification of the wetting properties of the channels,12,20 diodes and valves employing surfactants,14 user-activated mechanical means21-23 and magnetically actuated valves.24 However, a common feature of these tools is that programmable delivery of flows is accomplished by delaying the fluid transport rate or as on/off switches to flow. On the contrary, fluidic tools that can accelerate fluid transport, such as crafting hollow channels,25 sandwiching paper channels between two flexible films,26 or fabricating channels of variable geometry27 are still at an early stage of development, although they could provide a basis for more complex manipulations and a reduction in analysis time. In this work we describe a new and simple approach for controlling fluid transport without active pumping, by adjusting capillary and laminar flow on paper-based substrates. 4 ACS Paragon Plus Environment

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The method relies on razor-crafting open channels on the paper longitudinally or perpendicularly to the direction of the flow. These open channels are made by direct carving the paper surface with the aid of a cutting blade, creating a thin micro-groove on the hydrophilic fluid path of the paper, instead of removing the cellulose paper matrix or craftcutting channels in two dimensional patterns.25,28-30 Based on this principle, channels crafted along the fluid flow path (longitudinally) were used to create a free flow regime of the liquid which accelerated fluid transport kinetics. Channels carved orthogonally to the direction of the flow (perpendicularly), acted as a barrier delaying fluid transportation and movement. Either pattern represent a new useful approach to the manipulation of fluid transport in µPADs that can accommodate various types of microfluidic manipulations to adjust the delivery time of liquids from several minutes to a few dozens of seconds.



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EXPERIMENTAL SECTION Materials and Methods. We used a Xerox Phaser 8560N printer for depositing solid wax onto Whatman No. 1 chromatography paper in pre-defined patterns designed on a white background.11 To create hydrophobic barriers we heated the paper in an oven for 2.0 min at 138±3 oC in order to melt the wax and penetrate the paper (Figure S1). A Leica stereoscope (Model S8 APO) equipped with a Canon S50 digital camera was used to capture images of the channels. Paper thickness was determined with a digital Vernier Caliper. To investigate fluid transport, we created a template closely resembling a thermometer. After heating, the device consisted of a hydrophilic flow path of 15.2 mm length and 2.1 mm width and a circular sample deposition area of 3.8 mm diameter. Channel Crafting. To notch without cutting the paper surface, we simulated the operation of automated crafting tools by using a line plotter equipped with a knife blade. A Linear 1200 single channel recorder (Barnstead International) was employed for that purpose. This recorder has a removable print-head that can be equipped with simple pens to draw graphs on 200 mm width chart paper. The recorder has inputs of up to 5 volts and electronic as well as manual pen lift. In addition, it can run in reverse via external control. Initial efforts to use an engraving tip were unsuccessful because the tip would exert a significant downstream force on the paper surface causing it to tear. A cutting blade and a precision knife were more suitable because they could carve paper on a single pass and their contact angle and position could be adjusted to ensure that the paper surface would be carved but not cut. Therefore, a precision knife with a Chisel-type grind was used to engrave the channels on the paper surface. For the crafting process, the µPADs were placed on the plotter surface just below the blade and each µPAD was separately carved. To carve channels of different orientation with 6 ACS Paragon Plus Environment

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respect to the flow the paper orientation was adjusted as appropriate (vertical, parallel or at an angle). By manipulating the input voltage of the recorder the print-head hosting the blade could move upwards (along the x- or y-axis of the paper depending on its orientation) for a specified distance. In this manner, microfluidic channels (micro-grooves) of various patterns were fabricated on the hydrophilic area of the µPADs (Figure 1). Chromatography paper (Whatman No. 1) was selected for this process due to its high thickness and mass per area (0.18 mm, 87 g m-2), uniform composition (relative to other types of paper) and lack of additives that affect flow rate. Other types of paper such as filter paper (0.18 mm, 62 g m-2) could not be used because it could easily be torn due to its lower mass per area. Images of the carved channels are shown in Figure S2. The average channel width, calculated by averaging the channel width in various positions (n=7) along its length, was approximately 130±20 µm. For the evaluation experiments, a solution of Methyl red diluted in water was used to visualize timing. All experiments were carried out by depositing 3.5 µL of the aqueous dye solution with an aid of an automatic micro-pipette (Orange Scientific, TIPOR-VL, 0.5-10 µL). To ensure reproducibility and remove bias, all experiments were run in ten replicates (n=10) and 4.6% outlier cases (21 out of 455 individual measurements) were statistically excluded with the Grubbs test (p=0.05). Colorimetric detection of metal ions. As proof-of-concept, we designed two µPADs that enable the determination and identification of metal ions. The chemistry for the paper-based analysis of Ni2+ and total Fe was based on their well-established reactions with dimethylglyoxime, 1,10-phenanthroline, respectively,31,32 while Fe3+ was determined as its thiocyanate complex.



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RESULTS AND DISCUSSION

The influence of open channels on fluid delivery time was evaluated in comparison to unmodified paper by determining the % change in the fluid arrival times. To minimize uncertainty due to paper inhomogeneity and variation between each preparation cycle, the influence of each pattern was evaluated in comparison to control experiments (un-crafted paper) that were carried out on the same sheet of paper. The reproducibility of fluid transport time, expressed as the relative standard deviation of all data collected with un-crafted paper, ranged between 6.34-13.36% which was deemed as satisfactory. Typically, flow transport on a hydrophilic paper path (in the absence of any open channels or barriers) is governed by capillary forces and the distance traveled by the fluid (L) is related to the square root of time

according to the Washburn equation:

, where t is time, D is the average pore

diameter, γ is the effective surface tension and µ is viscosity.15,27 Therefore, in the absence of open channels or barriers, the length of the paper path determines the time of delivery of reagent. Accelerating fluid flow. To increase fluid transport rate, we razor crafted open channels along the direction of the flow (i.e. downstream horizontal grooves along the x-axis of the flow), right after the sample deposition area and towards the finish line (Figure 1, Design 1). Depending on channel length (2-15 mm), the fluid transport rate and consequently the time necessary for the fluid to reach the finish line decreased with increasing channel length. The results from this study are depicted in Figure 2A. As we can observe, fluid delivery time decreases with increasing channel length up to 6 mm (approx. 40% of the overall flow path), then rapidly accelerates up to 10 mm (approximately 67% of the overall flow path) and approaches a plateau at longer lengths which corresponds to almost 60% faster delivery of 8 ACS Paragon Plus Environment

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the fluid compared to that recorded in untreated devices. This profile can be explained on the basis of fluid transport by a laminar flow regime along the open channel and then by wicking on the remaining paper surface. From the results of Figure 2A it is inferred that fluid delivery time is mainly influenced by wicking for open channels shorter than 50% of the total flow path, while laminar flow is the predominant factor when the channels are longer than 50% of the total flow path. Therefore, capillary and laminar flow can be adjusted by varying channel length. In addition to channel length, we observed that fluid transport time also depends on the location of the channel. When open channels were crafted upstream with respect to the flow (i.e. longitudinally along the x-axis of the flow but close to the finish line) (Figure 1, Design 2), fluid delivery time slightly increased for channels shorter than 40% of the total flow path (

Programming Fluid Transport in Paper-Based Microfluidic Devices

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