Using Adhesive Patterning to Construct 3D Paper Microfluidic Devices

Summary

We demonstrate the use of patterned aerosol adhesives to construct 3D paper microfluidic devices. This method of adhesive application forms semi-permanent bonds between layers, enabling single-use devices to be non-destructively disassembled after use and to ease folding complex nonplanar structures.

Abstract

We demonstrate the use of patterned aerosol adhesives to construct both planar and nonplanar 3D paper microfluidic devices. By spraying an aerosol adhesive through a metal stencil, the overall amount of adhesive used in assembling paper microfluidic devices can be significantly reduced. We show on a simple 4-layer planar paper microfluidic device that the optimal adhesive application technique and device construction style depends heavily on desired performance characteristics. By moderately increasing the overall area of a device, it is possible to dramatically decrease the wicking time and increase device success rates while also reducing the amount of adhesive required to keep the device together. Such adhesive application also causes the adhesive to form semi-permanent bonds instead of permanent bonds between paper layers, enabling single-use devices to be non-destructively disassembled after use. Nonplanar 3D origami devices also benefit from the semi-permanent bonds during folding, as it reduces the likelihood that unrelated faces may accidently stick together. Like planar devices, nonplanar structures see reduced wicking times with patterned adhesive application vs uniformly applied adhesive.

Introduction

In recent years, paper microfluidics has garnered considerable popularity for its potential to provide low-cost point of care (POC) diagnostic devices.^1-3 POC devices offer functionality similar to those of lab-based tests in a format that allows results to be obtained relatively quickly. POC devices made from paper are low-cost, lightweight, and easy-to-use alternatives to expensive microfluidic chips and miniaturized laboratories, making them ideal for use in resource-limited settings. The most common paper microfluidic devices are one-dimensional lateral flow devices, but planar three-dimensional (3D) paper microfluidic devices hold promise to provide multiplexed diagnostic devices⁴ that take up a much smaller footprint than would be required by a 2D device⁵ and correspondingly use a smaller sample volume.

Initially, planar 3D paper microfluidic devices were assembled individually, layer-by-layer with patterned paper layers alternating with laser-cut double-sided tape. Carefully aligned holes cut in the tape layer were filled with cellulose powder to ensure inter-layer fluid transport.⁴ A number of alternate methods were subsequently developed,^6-9 each improving different aspects of the devices. In particular, by eschewing adhesives, devices could be folded via origami techniques with layers held together by an external clamp.⁸ This eliminates any potential adhesive interference in a diagnostic test and allows the device to be unfolded post-use, potentially allowing even smaller sample volumes by displaying results internally. Alternatively, by using an aerosol adhesive applied between each paper layer, sheets of devices could be assembled simultaneously, without time-consuming patterning and alignment of tape.⁹

However, by applying an aerosol adhesive through a stencil, it is possible to gain the benefit of both of these techniques. By spraying the adhesive through a stencil, only a fraction of the adhesive is applied to the device, minimizing any potential interference with interlayer fluid transfer. Additionally, with careful stencil selection, a pattern of adhesive can be applied that results in semi-permanent adhesive bonding, allowing devices to be unfolded after use, while still providing sufficient interlayer contact to allow fluid to wick between layers.

Finally, applying aerosol adhesives through a stencil eases the construction of nonplanar 3D paper microfluidic devices, by minimizing the amount of adhesive applied to adjacent faces that may require frequent folding and unfolding during construction.¹⁰ Additionally, the use of patterned adhesive enables device to be unfolded after use for more convenient storage. Nonplanar 3D paper microfluidic devices are expected to be used for tasks that would otherwise be impossible in a planar 3D device. Figure 1 depicts the general process flow used to construct both planar and nonplanar 3D devices.

Protocol

1. Planar 4-layer Device (Stacked Layers) Construction

1. Print arrays of each layer of the device⁹ onto each piece of filter paper using a solid ink printer.^11,12 Place each filter paper on a hotplate at 170 °C for 2 min. This will melt the wax-based ink and allow it to fully penetrate the thickness of the paper, forming hydrophobic barriers.
   NOTE: The exact designs used are available as supplemental files.
2. Remove filter paper from hotplate and allow it to cool to RT.
3. Deposit 4 µl of 5 mM dye (red: Allura Red; yellow: tartrazine; blue: erioglaucine disodium salt; green: 10:1 mix of tartrazine:erioglaucine disodium salt) in each branch (one color per branch) of layer 3 (third layer from the top of the completed device) using a micropipette.
4. Begin with the bottom-most layer. Clamp the filter paper between the stencil and a stiff backing, such as a piece of plate glass, using binder clips, or another similar temporary method. Ensure that the stencil is flat against the paper. This will minimize any spray shadows cast by the stencil onto the paper.
5. Apply adhesive (see list of materials and equipment) with an approximately 1.33 sec (a four-count at 180 bpm) spray from about 24 cm.^9,10 During this time, move the can of adhesive across the stencil at a medium pace. Too slow of travel across the stencil will cause adhesive to accumulate on the stencil itself, clogging it. Too rapid of travel will fail to deposit sufficient adhesive on the paper. Four passes during this time (up-down-up-down) are sufficient in preventing spray shadows.
6. Remove stencil and place the next layer of the device (numbered layers are available as supplemental files) atop the freshly sprayed layer, aligning the edges of the paper. Firmly press the two layers together.
7. Replace stencil and repeat the spraying process for each layer of the device. Remove the stack of devices and place packing tape across the bottom layer. This prevents any fluid leakage from the device. Cut individual devices from the sheet using scissors, following the edge of the printed region.

2. Planar 4-layer Device (Origami Folded Layers) Construction

1. Print sheets containing all layers of the device onto filter paper using a solid ink printer. Place filter paper on a hotplate at 170 °C for 2 min. Remove filter paper from hotplate and allow it to cool to RT.
   NOTE: The exact designs used are available as supplemental files.
2. Deposit 4 µl of 5 mM dye (red: Allura Red; yellow: tartrazine; blue: erioglaucine disodium salt; green: 10:1 mix of tartrazine:erioglaucine disodium salt) in each branch (one color per branch) of layer 3 (third layer from the top of the completed device) via micropipette.
3. Clamp the sheet of devices between the stencil and a stiff backing, such as a piece of plate glass, using binderclips, or another similar temporary method. Ensure that the stencil is flat against the paper.
4. Apply adhesive (see list of materials and equipment) with an approximately 1.33 sec (a four-count at 180 bpm) spray from about 24 cm. Four passes during this time (up-down-up-down) are sufficient in preventing spray shadows.
5. Remove stencil and turn the sheet over. Replace stencil and spray back side of the paper. Remove the sheet of devices and begin folding in an accordion pleat, as depicted in Figure 1. Cut each device out from sheet using scissors, following the edge of the printed region. Place packing tape across the bottom layer.

3. Nonplanar (Origami) Device Construction

1. Print device (Figure 2A) onto filter paper using a solid ink printer and place the filter paper on a hot plate at 170 °C for 2 min. Remove device from hotplate and allow it to cool to RT.
   NOTE: The exact designs used are available as supplemental files.
2. Print crease pattern (Figure 2C) onto printer paper using a solid ink printer and cut to the size of the filter paper. Place crease pattern on a hotplate at 170 °C for 2 min, to melt the wax, causing the pattern to be visible from both sides of the paper. Remove crease pattern from hotplate and allow it to cool to RT.
3. Align the edges of the crease pattern to the edges of the paper containing the channel patterns and attach the two pieces of paper using binder clips, or another similar temporary method.
4. Trace the crease pattern with a blunt stylus, applying enough force that marks appear on the device sheet, but not so hard that the crease pattern paper rips. If that occurs, the device risks being damaged. Precreasing causes the paper to fold much more easily and allows for greater accuracy and precision in folding.
5. Begin folding the device with mountain and valley folds according to the crease pattern. Once the adhesive has been applied, the entire device must be assembled very quickly, so folding the device as much as possible before adhesive application is very helpful.
6. Once the device is folded, unfold the device to expose the portions of the device that require adhesive. Cut out masks (Figure 2D) that limit where on the device adhesive may be applied, using a razor blade.
7. Clamp the device between the stencil and mask and a stiff backing, such as a piece of plate glass. Ensure that the stencil is flat against the device. Apply adhesive (see list materials and equipment) with an approximately 1.33 sec (a four-count at 180 bpm) spray from about 24 cm. Four passes during this time (up-down-up-down) are sufficient in preventing spray shadows. Remove stencil and turn the sheet over. Replace stencil and mask and spray back side of the paper.
8. Immediately remove device from stencil and begin folding the device. Once the device is completely folded, apply pressure to the adhesive containing portion until the adhesive has dried.
   NOTE: The drying time of the adhesive is very sensitive to ambient humidity, so in locations with low humidity, folding in a humidity controlled chamber allows more time to fold the device.

4. Wicking Test for 4-layer Devices

1. Randomly select 20 devices, previously assembled according to the above protocols. Place devices in a location shielded from any wind or breezes to minimize evaporation. Deposit 40 µl water at the inlet of each device. Record the time it takes for each device to have all of its outlets completely filled with dye.

5. Origami Wicking Comparison

1. Construct two origami peacocks — one according to the above protocol (Section 3), and the other without the use of a stencil during adhesive application.
2. Insert one end of a small paper lead (approximately 5 mm wide by 5 cm long) into the body of each peacock.
3. Place both peacocks in a chamber kept at a high relative humidity (>90%) to minimize evaporation. Place each leg and lead of each peacock into a container filled with 5 mM dye (red: Allura Red, yellow: tartrazine, blue: erioglaucine disodium salt). Record wicking process with a digital camera.

Results

The 4-layer device tests were performed in a sealed chamber, shielding them from any wind or breezes that might cause excessive evaporation of the limited deposited fluid volume. The majority of the wicking in the 4-layer devices is in the middle layers of the device, so differences in wicking speeds due to evaporation were expected to be minimal. Additionally, there is minimal lateral wicking, with only 13 mm between the inlet and any individual outlet, suggesting that variations in wick...

Discussion

The above protocols use perforated metal sheets as stencils for applying aerosol adhesives to construct planar and nonplanar 3D paper microfluidic devices. In planar devices, this has the advantage of allowing devices to be completely unfolded after the adhesive has dried without destroying the device. In other adhesive based construction techniques, this is almost impossible, although, some designs allow for partial destructive disassembly by unpeeling two halves held together with a removable adhesive.¹⁴ Adh...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Camera	Nikon	D5100
Solid-ink printer	Xerox	ColorQube 8880
Hotplate	Torrey Pines	HS60
Humidity chamber	Electro-Tech Systems	5503-E
Spray adhesive	3M	62497749309	Super 77 (16.75 oz can)
Filter paper	Whatman	Grade 4
Perforated steel sheet	MetalsDepot	PS16116
Tartrazine	Sigma-Aldritch	T0388
Allura Red	Sigma-Aldritch	458848
Erioglaucine disodium salt	Sigma-Aldritch	861146