Research Article www.acsami.org
Design Rules for Fluorocarbon-Free Omniphobic Solvent Barriers in Paper-Based Devices Sana Jahanshahi-Anbuhi,†,‡ Kevin Pennings,‡ Vincent Leung, Balamurali Kannan,†,§ John D. Brennan,†,§ Carlos D. M Filipe,†,‡ and Robert H. Pelton*,†,‡ †
Biointerfaces Institute, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L8, Canada Department of Chemical Engineering, McMaster University, 1280 Main St W, Hamilton, Ontario L8S 4L7, Canada § Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4L7, Canada ‡
S Supporting Information *
ABSTRACT: The utility of hydrophobic wax barriers in paper-based lateral flow and multiwell devices for containment of aqueous solvents was extended to organic solvents and challenging aqueous surfactant solutions by preparation of a three layer barrier, consisting of internal pullulan impregnated paper barriers surrounded by external wax barriers. When paper impregnated with pullulan solution dries, the polymer forms solvent blocking lenses in the paper structure. Lens formation was illustrated by forming pullulan lenses in glass capillaries. The lens shapes were less curved compared to the predictions of a model based upon minimizing surface area. For barriers on Whatman # 1 filter paper, the pullulan molecular weight must be greater than ∼70 kDa, the mass fraction of pullulan in the barrier zone must be at least 32%, and there are restrictions on the minimum width of the pullulan impregnated zone. KEYWORDS: solvent barriers, lateral flow devices, porous media, paper devices, microfluidics, pullulan
■
INTRODUCTION Paper-based microfluidic devices are an attractive technology for point-of-care diagnostics, particularly for resource limited situations.1−6 Most paper-based sensors are formatted as spot sensors or as lateral flow devices. Spot sensors are typically circular zones in paper, where an array of spots can function like a multiwell plate.7−10 Lateral flow devices direct flow along the device and enable sequential chemical reactions or sample splitting into multiple sensor zones. Both spot and lateral flow sensors require liquid impermeable barriers to demarcate sensor zones on sheets of filter paper. Early paper-based devices were prepared by leaching channels in polystyrene impregnated paper9,11 or by photolithography.12−15 However, the most promising methods involve printing millimeter scale channel patterns on/into filter paper surfaces with the ink functioning as barriers that restrict capillary driven flow to a pathway described by the barriers. Reported barrier inks include wax, 16,17 alkyl ketene dimer,10,18,19 a hydrophobization agent used in papermaking,2,18 and methylsilsesquioxane.20 Of these, wax printing employing office printers is the easiest to implement. Unfortunately, none of these one-step inks provide barriers for the complete range of potential fluids used in bioassays. For example, B-PER, a cell lysing solution, breaches wax barriers. More complex barriers based on Teflon 8 and other fluorochemicals21 offer more robust barriers at a higher material cost and fabrication complexity and require hazardous materials to allow containment of organic solvents or surfactant solutions. © 2015 American Chemical Society
In this work, we describe a new barrier composite that blocks the flow of a wide range of liquids including aqueous surfactant solutions and alkanes, greatly expanding the range of solutions that can be used in printed, paper-based devices. The barrier consists of a polysaccharide (pullulan) zone which is applied throughout the paper thickness, bounded on both sides by printed wax layers. The wax layers block aqueous flow, whereas the pullulan acts as a barrier for organic solvents because pullulan is insoluble in methanol and less polar solvents.22−24 We present key design rules for liquid barrier fabrication in paper devices. Finally, we show that drying pullulan solutions in glass capillary tubes produces solvent blocking lenses; we propose that the same mechanism is operative in paper pores.
■
EXPERIMENTAL SECTION
Materials and Reagents. Allura Red, Dowex MB mixed ionexchange resin, Gum Arabic (250 kDa), Whatman #1 filter paper, reagent grade pullulan (100 kDa), methyl cellulose (17 kDa), carboxymethyl cellulose (250 kDa), dextran (148 kDa), and sodium dodecyl sulfate (SDS) were obtained from Sigma-Aldrich and used without further purification. Pullulan (PI20, 200 kDa) was obtained from Hayashibara Co, Ltd., Okayama, Japan. Poly vinyl alcohol (125 kDa) was obtained from BDH Chemicals Ltd., Poole, England. Polyvinylamine (125 kDa, with an 88% degree of hydrolysis) was a gift from BASF. B-PER Bacterial Protein Extraction Reagent was purchased from Thermo Scientific. Distilled deionized water was Received: September 4, 2015 Accepted: October 23, 2015 Published: October 23, 2015 25434
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces obtained from a Milli-Q Synthesis A10 water purification system. Latex was a 20 wt %, 254 nm diameter dispersion of copolymer beads based on styrene and n-butyl acrylate, 71 mol % with a glass transition temperature of −2 °C.25 Adelphi tubes (1 mL) were open ampules from Schott Verrerie Médicale made of DIN standard neutral type I Fiolax glass. Glass capillary tubes (Microhematocrit capillary tubes; length, 75 mm ± 0.02 mm; inner diameter, 1.15 mm ± 0.05 mm) were supplied by Fisher Scientific. Most experiments, unless otherwise stated, were performed with P120 pullulan solution (9.1 wt % in water). The literature contains only limited density information for pullulan solutions. We measured the density of dried pullulan films to be 1.2 g/mL, and from this we estimated the density of the 9.1% solution to be 1.09 g/mL at 25 °C based on Nishinari’s26 dilute solution data. Details are given in the Supporting Information (SI) file. Pullulan Hydrolysis-Customized Molecular Weight. A total of 20 g of pullulan (200 kDa) was dissolved in 200 mL of water and placed in a water bath at a temperature of 50 °C. The pH was then lowered to 1.0 using HCl (1.0 N), and 10 mL samples of the partially hydrolyzed pullulan solution were taken over time. The isolated samples were immediately neutralized with NaOH (1.0 N) and chilled in ice to reach room temperature. A total of 1 mL of these hydrolyzed pullulan solutions (10% wt/vol) was mixed with 0.25 g of mixed bed DOWEX resin and was filtered after 20 min. The solutions were further filtered through a 0.45 μm membrane syringe filter. MW Characterization. Molecular weight distributions of the pullulan samples were determined by gel permeation chromatography (GPC) based on column calibration with polyethylene glycol standards in 500 mM NaNO3 and 25 mM CHES buffer at a pH of 10. Composite Barrier Fabrication and Evaluation. Concentric black wax squares with inner diameters of 2.1 and 3.2 cm and thicknesses of 0.44 mm each (prior to melting) were printed on Whatman #1 filter paper using a Xerox Color Qube 8570DN PS printer. The printed lines were heated at 120 °C for 3 min, causing the wax to penetrate the thickness of the filter paper. The total superficial area of the paper between the two rectangles after the wax melting was approximately 400 mm2. Pullulan solutions were manually pipetted on top of the channels defined by the printed wax rectangles and were left to dry overnight at room temperaturesee Figure 1. In most cases, the 200 kDa pullulan
melting) with a center-to-center separation of 4 mm were printed and melted into the paper. A total of 200 μL of pullulan or other test solutions was manually pipetted between the wax strips. The strips were lowered vertically into the test liquid until the liquid level was approximately 10 mm below the barriers. Eight strips were tested for each solvent, and visual observation was used to assess barrier performance. Results were recorded with a Canon Powershot SX40HS camera. Pullulan Lens Formation in Capillary Tubes. Pullulan solutions (200 kDa, 9.1% w/v in water) containing 20 mg/mL Allura Red were pipetted into 1.15 i.d. Fisherbrand glass capillaries. The volume of the added solutions ranged from 0.25 to 10 μL. The capillaries were supported either vertically or horizontally and allowed to dry under ambient conditions, leading to pullulan lens formation. The integrity of the lenses was determined by visual inspection and by an air leakage test in which the lens was pressurized with a syringesee Figure S1. The minimum quantity of pullulan required to form effective lenses was determined by immersing the lens-containing capillary tubes into methanol. Intact lenses prevented the capillary rise of methanol.
■
RESULTS The ability of potential barrier materials to withstand a wide range of solvents was evaluated with both spot test and lateral flow formats. Spot Tests. Figure 1 illustrates the two-step fabrication of wax−pullulan−wax barriers. In the first step, concentric wax squares were printed and melted into Whatman filter paper. Most results were with Whatman #1 (W1), although we compare other filter papers below. In a second step, pullulan solution (9.1 wt %/wt) was added along the thin paper band between the wax layers. The pullulan solution penetrated the thickness of the filter paper and was allowed to air dry. Note that the pullulan content in our spot test devices was high. The superficial pullulan solution coverage was normally 375 mL/m2, giving a dry pullulan superficial coverage of 37 g/m2. Lower coverages were evaluated as barriers for absolute methanol, acetone, and 5% SDS solutions. The minimum required dry pullulan coverage was 275 ± 25 mL/m2 (see Table S1). The pore volume fraction of W1 filter paper was about 0.69, which means the pores should be filled by application of only 125 mL/m2. Normally we added 3 times this coverage. Therefore, about 1/3 of the added pullulan solution fills the pores and 2/3 of the polymer solution initially remains on the surface. We do not know why excess volume of pullulan solution was required. After drying the paper impregnated with pullulan solution, the pore volume fraction inside the paper will be decreased. A pore filled with 9% pullulan solution will decrease in pore volume by ∼9% after drying because of the volume occupied by the pullulan. SEM images confirm that the pores are not fully filled with solid pullulan, Figure S2. Therefore, total pore filling is not the mechanism by which pullulan acts as a solvent barrier. The wax-pullulan barriers were evaluated by adding test fluids to the central paper square, followed by the visual assessment of breakthrough. Figure 2 compares results with and without the pullulan using a few solvents. All six solvents broke though the wax-only barrier, while only heptane breached the wax-pullulan combination barriers. We were initially puzzled by the heptane breach, because pullulan is completely insoluble in heptane. Close observation revealed that the heptane did not penetrate the pullulan but, instead, crossed over the top of the pullulan film. Large sessile drops of test fluid were used in our spot tests. A successful barrier for a spot test must not only prevent transport through the paper structure but also prevent spreading across the top of the barrier.
Figure 1. (A) Schematic representation of the process used to form the pullulan barrier. (B) Photograph of a pullulan barrier (9.1 wt % pullulan solution with minimum coverage of 375 mL/m2 on filter paper) constructed using this method. concentration was 9.1 wt %, and the volume of the polymer solution was 0.15 mL, giving a superficial pullulan solution coverage of 375 mL/m2, corresponding to a dry pullulan superficial coverage of 37 g/ m2. The composite barriers were evaluated by manually spotting 0.1 mL of test liquid in the uncoated center paper square. Lateral Flow Barrier Tests. Vertical lateral flow tests were performed with 0.75-cm-wide strips of filter paper with barriers across the center of the strip. Parallel wax lines (0.44 mm width before 25435
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces
Figure 2. Example barriers. From the photographs, it is evident that the pullulan allows for retention of several organic solvents and surfactants, which are not retained by the wax barriers alone. In these cases, pullulan solution (9.1 wt %) with 375 mL/m2 coverage on filter paper was used.
Figure 3. Comparing polymeric barriers for methanol in lateral flow. (A) Schematic of the experimental setup. The barrier zone was 4-mmwide, and the coverage of polymer solution in the barrier zone was 375 mL/m2. (B) Success rate of the barrier when alternative polymers are used for the barrier creation.
Similar tests were performed to evaluate the influence of pullulan barrier width with a pullulan coverage of 375 mL/m2 of 9.1 wt %, 200 kDa pullulan. Barrier widths of 2.4 mm (after melting the wax borders) and greater were effective, whereas 2 mm and thinner barriers failedsee data in Table S2. Comparing Filter Papers. Methanol blocking experiments were performed using Whatman # 2, 3, and 4 papers, and the results are summarized in Table 1. Five tests were performed
effective. We focused on pullulan in the remainder of this work because PVA was difficult to dissolve and formed very viscous solutions which made barrier formation challenging. Pullulan Lenses in Capillary Tubes. We argue in the Discussion section that pullulan barrier properties result from the formation of pullulan lenses that block pores in paper. To support this explanation, plugs of pullulan solution were introduced into glass capillary tubes and allowed to dry to form lensessee Figure 4. A series of experiments was conducted, spanning a range of pullulan solution (9.1 wt %, 200 kDa) volumes to determine the minimum volume required to form a lens. The integrity of the lenses was measured by visual inspection, by air leak tests, and by immersion in methanol (see Figure 4), and the results are summarized in Table 2. The minimum volume of 9.1% pullulan solution for a methanol-blocking lens was between 2 and 2.5 μL. This value is compared to a model prediction in the Discussion section. Finally, we also confirmed that PVA also formed lenses in glass capillaries. Pullulan Molecular Weight. A series of lower molecular weight pullulan polymers was prepared by acid hydrolysis GPC traces and molecular weight assignments are given in Figure S5 and Table S3. The capillary lens test with methanol, described above, was used to evaluate the polymers. Lenses prepared from the lowest molecular weight pullulan samples did not form methanol resistant lenses. The molecular weight transition from poor to good lenses was between 68 and 79 kDa.
Table 1. Mass Fraction of Pullulan Required to Form a Methanol Barrier, χ, in the Spot Testsa Whatman paper grade
retained particle size (μm)
basis weight (g/m2)
thickness (μm)
pore volume fraction
χ (wt %)
Lmin eq 5 (μm)
1 2 3 4
11 8 6 20−25
88 103 187 96
180 190 390 205
0.687 0.668 0.692 0.709
24 26 25 32
23 13 14 30
a
The pore volume fraction was calculated assuming a cellulose density of 1.54 g/mL. The other paper properties were specified by Whatman.
under each set of conditions. The mass fractions in Table 1 are the lowest values needed to form a good barrier in every test. The complete data set is plotted in Figures S3 and S4. The pullulan mass fractions in Table 1 fell within the narrow range of 0.24−0.32; we were unable to correlate these values with the paper properties. Comparing Polymers in Lateral Flow Devices. Pullulan is a linear nonionic polysaccharide; thus it seems reasonable to assume that other polymers could be used for barrier formation. We compared a number of polymer candidates by evaluating the ability to block capillary driven vertical flow of ethanol, and the results are summarized in Figure 3. Poly(vinyl alcohol) (PVA) performed as well as pullulan, and methylcellulose (MEC) showed some promise. The other materials were not
■
DISCUSSION In the following discussion, we recast our results as barrier design rules that have applicability beyond our specific barriers. We then discuss the barrier mechanism, and we present a simple model that predicts pullulan lens properties in filter paper. 25436
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces
requires that the liquid wets the barrier material and that the liquid surface is above the barrier surface, giving a hydrostatic driving force. This form of breach is most problematic for paper-based multiwell plates, where large volume liquid drops can be initially present on top of the well. Whether or not a liquid will spread on a solid surface can be predicted by the critical surface tension of the solid, which is an experimental quantity that approximately corresponds to the maximum surface tension of a liquid that will spontaneously spread over the surface. Dried pullulan has a critical surface tension of 15− 20 mJ/m2,27 explaining why alkanes with a lower surface tension, but not methanol with a higher surface tension, can spread across the top of a pullulan barrier. 2. The surface tension of blocked fluids must be greater than the critical surface tension of the barrier material to prevent breaching over the top of the barrier. This rule is most important for paper-based multiwell plates. Rules for Pullulan Barriers in Whatman #1 Filter Paper. We have focused on pullulan as a barrier material because it is a good film-forming polymer. However, most commercial pullulans are high molecular weight, giving viscous aqueous solutions that are difficult to force into the paper structure. Low molecular weight pullulan samples have a low viscosity and are easier to introduce into the paper structure. However, if the molecular weight is too low, the dried pullulan lenses are too weak. 3. Pullulan molecular weight must be greater than ∼70 kDa for effective lens formation. There must be enough impregnated pullulan to block capillary paths through the paper. 4. A pullulan mass fraction of 24−32% is required to block solvent flow in Whatman #1−4 filter papers. 5. The minimum width of the pullulan zone between wax barriers is 2.4 mm in Whatman #1 paper. Pore Blocking Mechanisms. We now discuss how pullulan blocks solvent flow. The physical blocking of pores by dried pullulan is the obvious mechanism. However, it is impossible for pullulan solutions to fill completely all the pores after dryinga 9.1% aqueous pullulan solution will fill about 30% of the pore volumes after drying. The presence of pullulan in the pores is not sufficientthe polymer must be partially located as flow-blocking lenses in sufficient quantities to block all pathways through the paper barrier. Support for the presence of pore-blocking pullulan lenses in paper comes from our glass capillary experiments (see Figure 4) showing pullulan lens formation when a plug of pullulan solution is dried. The surface tension of pullulan solutions is close to that of pure water,28 maximizing capillary forces that tend to drive lens formation with drying. Pullulan films are flexible and with few defects,29 ideal properties for solvent blocking lenses. In the next section, we present a model predicting the amount of polymer solution required to form a lens in a glass capillary tube. Lens Formation in Capillary Tubes. The lenses are assumed to have circular curvature. The volume of dry polymer in a lens can be estimated as follows. The model parameters are illustrated in Figure 5. There are two characteristic radii: r, the capillary radius, and Rc, the radius of curvature of the spherical lens. These radii are related by the contact angle that pullulan forms with the glass wall.
Figure 4. Pullulan lens formation in capillary tubes. (A) Shows a dried pullulan lens in a capillary tube. (B) Shows a dyed pullulan lens preventing methanol penetration. Refraction at the air/methanol interface caused the distortion of the capillary images.
Table 2. Pullulan Lens Properties As a Function of the Volume of 9.1 wt % 200 kDa Pullulan Solution in 1.15 mm i.d. Glass Capillaries
Barrier Design RulesGeneral Requirements. Cellulose is insoluble in water and in any other solvent likely to be involved in a paper device. Therefore, liquid transport through paper involves flow through the porous network formed by the insoluble cellulose fibers. Although it is possible to prevent fluid flow by cellulose surface modification to give a high contact angle, this strategy is not robust for low surface energy alkanes or for surfactant solutions. This leads to our first rule: 1. An effective barrier must physically block the porous network in paper. An obvious corollary is that the blocking material must be insoluble in the fluid of interest. The heptane experiment in Figure 2 demonstrates that, under the right conditions, heptane can flow across the top of pullulan treated paper. Transport across the top of a barrier
Rc = 25437
r cos(θ)
(1) DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces
μm-thick, randomly arranged in the x−y plane. The voids between a single layer of randomly deposited, uniform ribbons are polygons, usually treated as circles.32 The pore radius distributions fit a gamma distribution for a single layer of fibers.34 Sampson has extended the analysis to give distribution functions and mean minimum radii for voids formed by multiple layers of ribbons.31 The effect of adding layers is to lower the minimum radius in channels through the paper. Much less has been reported on the lateral porosity of paper. Lateral porosity is lower than the z directional porosity, and the key lateral pore dimensions should be similar to the fiber thickness (∼5−10 μm). Finally, cellulose fibers are not ideal solid ribbons. Instead, fiber walls contain small pores which are due to the removal of lignin from the fiber walls during the pulping process. Alince et al. reported that bleached wood pulp fiber walls have pore sizes of typically 100 nm.35 These very small pores are unlikely to influence barrier properties. In the following section, to explain our results, we treat paper as a collection of short capillary tubes. However, it is important to recognize that the capillary driven wetting of paper is far more complex than filling capillary tubes. Senden and coworkers showed that a wetting liquid first travels along the triangular shaped grooves formed by fiber−fiber junctions, and this flow may or may not lead to complete filling of large pores.36,37 Considering the complexity of the porous network in filter paper, our capillary modeling can only offer hints to the most important phenomena in paperthese are now discussed. Pullulan Lens Formation in Paper. We propose that the solvent blocking mechanism results from pullulan lens formation across pores. If we take the minimum retained diameter (see Table 1) as a measure of the effective pore size of cylindrical pores in filter paper, eq 5 can be used to estimate the minimum length of these capillaries required to provide enough dilute pullulan solution to form a lens. The resulting Lmin values are shown as the last column in Table 1. The values range from 13 to 30 μm for the Whatman filter paper series for pullulan concentrations corresponding to the χ values, the experimentally determined minimum pullulan contents required to block methanol. Details of this calculation are given in the SI. This analysis is significant because it shows that Lmin values are of the dimensions of cellulose fiber widths, which seems reasonable for horizontal pores in the x−y paper plane. We showed that the minimum width of the pullulan zones between the parallel wax lines was 2.4 mm for effective barriers, nearly 2 orders of magnitude greater than the Lmin values. Capillary modeling does not explain the 2.4 mm thickness limitation. When fabricating our barriers, it is desirable to minimize the pullulan concentration in the impregnation solution for both cost and ease of impregnation during barrier manufacture. However, the lower the polymer solution concentration, the higher the volume of polymer solution required to form complete lenses. In the case of filter paper, we propose that reservoirs of pullulan solution in large pores near each lens site in the paper feed polymer to the lenses during the drying process. Furthermore, we speculate that the lower limit of pullulan concentration giving effective barriers occurs when the reservoir volumes are too low to deliver sufficient pullulan solution to give a sealing lens during drying.
Figure 5. Cross section of an ideal lens in a capillary.
Similarly, h (see Figure 5) is given by the following h = R c(1 − sin(θ ))
(2)
The volume of polymer in the lens is given by the following, where Vcap is the volume of the spherical capssee Figure 5 Vlens = πr 2(2h + δl) − 2Vcap
(3)
and Vcap =
1 3 πR c (2 − 3 sin(θ) + sin(θ )3 ) 3
(4)
Therefore, the lens volume is dependent upon three parameters: θ, the contact angle; r, the capillary radius; and δl > 0, the thinnest dimension of the lens. Finally, Lmin, the minimum length of the polymer solution plug (i.e., before drying) in the capillary that is required to give a lens is calculated as follows, where cp is the polymer concentration. Lmin increases linearly with capillary radius. This model is now applied to our capillary tube experiments Lmin =
Vlens 2 πR cap
×
1 cp
(5)
The contact angle of pullulan on glass influences the curvature, and thus the capillary pressure. Contact angle measurements were performed during the drying of pullulan solutions on smooth clean glass surfaces and on cellulose membranes; the results are shown in Figure S6. We chose a contact angle of 25° for the modeling. The results in Table 2 show that 2.25 ± 0.25 μL of 9.1% pullulan is required to form a blocking lens in a 1.15 mm i.d. glass capillary. Applying eqs 1−5 and assuming the minimum required film thickness, δl = 1 μm, and the contact angle, θ = 25°, the model predicts Vlens/0.09 = 4.1 μL of 9.1% solution is requirednearly twice the experimental value. The photograph in Figure 4 suggests that the lens is nearly flat, and not the curved surface suggested by the model. Dried lenses formed with dyed pullulan indicated a coating of pullulan on the glass surface beyond the lens. We propose that during the drying process, the three phase contact line was pinned, preventing the constant curvature surface predicted by the model. To summarize, the capillary lens model illustrates the essential features of lens formation; however, it overestimates the required amount of polymer to be nearly a factor of 2. To apply this model to paper, a simple model of the porous structure is required. Paper Structure. The “paper physics” community, scientists who study the properties of paper, have presented many theoretical and experimental descriptions of the pore structure of paper. Much of this work is found in refereed conference proceedings that can be difficult to find. Nevertheless, there are textbooks30 and recent reviews in the mainstream literature.31−33 A simplified view of filter paper structure is layered cellulose ribbons, ∼20-μm-wide and ∼525438
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces
■
Active Bacteria using Tungsten Trioxide Nanoprobes. Sci. Rep. 2015, 5, 1−7. (4) Whitesides, G. M. The Origins and the Future of Microfluidics. Nature 2006, 442, 368−373. (5) Jahanshahi-Anbuhi, S.; Henry, A.; Leung, V.; Sicard, C.; Pennings, K.; Pelton, R.; Brennan, J. D.; Filipe, C. D. M. Paper-Based Microfluidics With an Erodible Polymeric Bridge Giving Controlled Release and Timed Flow Shutoff. Lab Chip 2014, 14, 229−236. (6) Jahanshahi-Anbuhi, S.; Chavan, P.; Sicard, C.; Leung, V.; Hossain, S. M. Z.; Pelton, R.; Brennan, J. D.; Filipe, C. D. M. Creating Fast Flow Channels in Paper Fluidic Devices to Control Timing of Sequential Reactions. Lab Chip 2012, 12, 5079−5085. (7) Carrilho, E.; Phillips, S. T.; Vella, S. J.; Martinez, A. W.; Whitesides, G. M. Paper Microzone Plates. Anal. Chem. 2009, 81, 5990−5998. (8) Deiss, F.; Matochko, W. L.; Govindasamy, N.; Lin, E. Y.; Derda, R. Flow-Through Synthesis on Teflon-Patterned Paper To Produce Peptide Arrays for Cell-Based Assays. Angew. Chem., Int. Ed. 2014, 53, 6374−6377. (9) Sameenoi, Y.; Nongkai, P. N.; Nouanthavong, S.; Henry, C. S.; Nacapricha, D. One-Step Polymer Screen-Printing for Microfluidic Paper-Based Analytical Device (Mpad) Fabrication. Analyst 2014, 139, 6580−6588. (10) Wang, J.; Monton, M. R. N.; Zhang, X.; Filipe, C. D. M.; Pelton, R.; Brennan, J. D. Hydrophobic Sol−Gel Channel Patterning Strategies for Paper-Based Microfluidics. Lab Chip 2014, 14, 691−695. (11) Abe, K.; Suzuki, K.; Citterio, D. Inkjet-Printed Microfluidic Multianalyte Chemical Sensing Paper. Anal. Chem. 2008, 80, 6928− 6934. (12) Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Thomas, S. W., III; Sindi, H.; Whitesides, G. M. Simple Telemedicine for Developing Regions: Camera Phones and Paper-Based Microfluidic Devices for Real-Time, Off-Site Diagnosis. Anal. Chem. 2008, 80, 3699−3707. (13) Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Patterned Paper as a Platform for Inexpensive, Low-Volume, Portable Bioassays. Angew. Chem., Int. Ed. 2007, 46, 1318−1320. (14) Martinez, A. W.; Phillips, S. T.; Whitesides, G. M. ThreeDimensional Microfluidic Devices Fabricated in Layered Paper and Tape. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 19606−19611. (15) Martinez, A. W.; Phillips, S. T.; Wiley, B. J.; Gupta, M.; Whitesides, G. M. FLASH: A Rapid Method for Prototyping PaperBased Microfluidic Devices. Lab Chip 2008, 8, 2146−2150. (16) Lu, Y.; Shi, W.; Jiang, L.; Qin, J.; Lin, B. Rapid Prototyping of Paper-Based Microfluidics With Wax for Low-Cost, Portable Bioassay. Electrophoresis 2009, 30, 1497−1500. (17) Carrilho, E.; Martinez, A. W.; Whitesides, G. M. Understanding Wax Printing: A Simple Micropatterning Process for Paper-Based Microfluidics. Anal. Chem. 2009, 81, 7091−7095. (18) Li, X.; Tian, J.; Garnier, G.; Shen, W. Fabrication of Paper-Based Microfluidic Sensors by Printing. Colloids Surf., B 2010, 76, 564−570. (19) Li, X.; Tian, J.; Nguyen, T.; Shen, W. Paper-Based Microfluidic Devices by Plasma Treatment. Anal. Chem. 2008, 80, 9131−9134. (20) Wang, J.; Monton, M. R. N.; Zhang, X.; Filipe, C. D. M.; Pelton, R.; Brennan, J. D. Hydrophobic Sol-Gel Channel Patterning Strategies for Paper-Based Microfluidics. Lab Chip 2014, 14, 691−694. (21) Chen, B.; Kwong, P.; Gupta, M. Patterned Fluoropolymer Barriers for Containment of Organic Solvents Within Paper-Based Microfluidic Devices. ACS Appl. Mater. Interfaces 2013, 5, 12701− 12707. (22) Singh, R. S.; Saini, G. K.; Kennedy, J. F. Pullulan: Microbial Sources, Production and Applications. Carbohydr. Polym. 2008, 73, 515−531. (23) Farris, S.; Unalan, I. U.; Introzzi, L.; Fuentes-Alventosa, J. M.; Cozzolino, C. A. Pullulan-Based Films and Coatings for Food Packaging: Present Applications, Emerging Opportunities, and Future Challenges. J. Appl. Polym. Sci. 2014, 131, 40539 (1−12). (24) Jahanshahi-Anbuhi, S.; Pennings, K.; Leung, V.; Liu, M.; Carrasquilla, C.; Kannan, B.; Li, Y.; Pelton, R.; Brennan, J. D.; Filipe,
CONCLUSIONS Including a layer of pullulan impregnated paper sandwiched between wax barriers in filter paper extends the range of liquids beyond those blocked by wax alone, giving an omniphobic barrier. The wax restricts aqueous solution penetration, whereas the pullulan prevents organic liquid transport. The wax− pullulan−wax barrier blocked liquid flow within filter paper for every solvent evaluated from water to heptane. This has been an important development for our paper-supported assay development because we can now use BPER, an aggressive cell lysing solution, without breaching printed barriers. Our results are represented by five design rules for barriers in paper devices: (1) An effective barrier must physically block the porous network in the paper substrate. (2) The surface tension of blocked fluids must be greater than the critical surface tension of the barrier material to prevent breaching over the top of the barrier. (3) Pullulan molecular weight must be greater than ∼70 kDa for effective lens formation. (4) A pullulan mass fraction of at least 32% is required to block solvent flow in Whatman #1 filter paper, and (5) the minimum width of the pullulan zone between external wax barriers is 2.4 mm. We proposed that pullulan blocks flow through paper by forming lenses across paper pores, preventing the transport of liquids that cannot dissolve pullulan. In support of this explanation, we showed the conditions under which pullulan solution dries to form lenses in glass capillary tubing. It is the well-documented ability of pullulan to form defect-free, flexible films that makes it an ideal polymer for this application.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08301. Pullulan characterization after hydrolysis, SEM of a crosssection of pullulan treated paper, barrier performance statistics, and calculations (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada for funding through the SENTINEL Bioactive Paper Network. We also thank the Canadian Foundation for Innovation and the Ontario Ministry of Research and Innovation for Infra-structure funding to the Biointerfaces Institute. JDB holds the Canada Research Chair in Bioanalytical Chemistry and Biointerfaces. RP holds the Canada Research Chair in Interfacial Technologies.
■
REFERENCES
(1) Pelton, R. Bioactive Paper - a Low Cost Platform for Diagnostics. TrAC, Trends Anal. Chem. 2009, 28, 925−942. (2) Li, X.; Ballerini, D. R.; Shen, W. A Perspective on Paper-Based Microfluidics: Current Status and Future Trends. Biomicrofluidics 2012, 6, 011301−13. (3) Marques, A. C.; Santos, L.; Costa, M. N.; Dantas, J. M.; Duarte, P.; Gonçalves, A.; Martins, R.; Salgueiro, C. A.; Fortunato, E. Office Paper Platform for Bioelectrochromic Detection of Electrochemically 25439
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
Research Article
ACS Applied Materials & Interfaces C. D. M. Pullulan Encapsulation of Labile Biomolecules to Give Stable Bioassay Tablets. Angew. Chem. 2014, 126, 6269−6272. (25) Yang, S.; Razavizadeh, B. M.; Pelton, R.; Bruin, G. Soft Nanoparticle Flotation Collectors. ACS Appl. Mater. Interfaces 2013, 5, 4836−4842. (26) Nishinari, K.; Kohyama, K.; Williams, P. A.; Phillips, G. O.; Burchard, W.; Ogino, K. Solution Properties Of Pullulan. Macromolecules 1991, 24, 5590−5593. (27) Adachi, S.; Iwase, H.; Matsuno, R. Relationship Between the Amount of Lipid Exposed at the Surface of the Lipid Emulsions Encapsulated by a Polymer and the Critical Surface Tension of the Dehydrated Polymer Film. Nippon Shokuhin Kagaku Kogaku Kaishi 1995, 42, 137−139. (28) Whistler, R. E. Industrial Gums: Polysaccharides and Their Derivatives; Elsevier: 2012; p 309. (29) Shingel, K. I. Current Knowledge on Biosynthesis, Biological Activity and Chemical Modification of the Exopolysaccharide, Pullulan. Carbohydr. Res. 2004, 333, 447−460. (30) Niskanen, K. Paper Physics. Fapet Oy: Helsinki, 1998; Vol. 16, p 324. (31) Sampson, W. A Multiplanar Model for the Pore Radius Distribution in Isotropic Near-Planar Stochastic Fibre Networks. J. Mater. Sci. 2003, 38, 1617−1622. (32) Eichhorn, S. J.; Sampson, W. W. Statistical Geometry of Pores and Statistics of Porous Nanofibrous Assemblies. J. R. Soc., Interface 2005, 2, 309−318. (33) Alava, M.; Niskanen, K. The Physics of Paper. Rep. Prog. Phys. 2006, 69, 669−723. (34) Johnston, P. R. Revisiting the Most Probable Pore-Size Distribution in Filter Media: The Gamma Distribution. Filtr. Sep. 1998, 35, 287−292. (35) Alince, B. Porosity of Swollen Pulp Fibers Revisited. Nord. Pulp Pap. Res. J. 2002, 1, 71−73. (36) Senden, T. J.; Knackstedt, M. A.; Lyne, M. B. Droplet Penetration Into Porous Networks: Role of Pore Morphology. Nord. Pulp Pap. Res. J. 2000, 15, 554−563. (37) Roberts, R. J.; Senden, T. J.; Knackstedt, M. A.; Lyne, M. B. Spreading of Aqueous Liquids in Unsized Papers is by Film Flow. Journal of Pulp and Paper Science 2003, 29, 123−131.
25440
DOI: 10.1021/acsami.5b08301 ACS Appl. Mater. Interfaces 2015, 7, 25434−25440
