The overall goal of this technique is to create planar and nonplanar three-dimensional paper microfluidic devices that are able to be unfolded during construction and after use through the patterning of aerosol adhesive. This method opens up a brand new design space for paper microfluidics, by removing constraints which previously kept researchers from fabricating out of plain channel networks. The primary advantages of this technique are that it significantly reduces the amount of adhesive applied during the fabrication of paper microfluidic devices, and that it enables the construction of nonplanar three-dimensional paper microfluidic circuits.
First, print an array of each layer for the device onto filter paper using a solid ink printer. Place each filter paper on a hot plate at 170 degrees Celsius for two minutes, to melt the wax-based ink, and allow it to fully penetrate into the paper, forming hydrophobic barriers. Once the paper has cooled, put a different dye color in each of the branches of layer three using a micropipette.
Four microliter aliquots of five millimolar dyes will suffice. Now, begin the construction. First, clamp the bottommost layer between the stencil and a stiff backing, such as a piece of plate glass.
Ensure that the stencil is flat against the paper to minimize spray shadows. Then, set a metronome to 180 beats per minute and spray adhesive from approximately 24 centimeters away for four beats. Move the can across the stencil in four even motions.
If the can is moved too slowly, the adhesive will accumulate on the stencil itself, clogging the stencil. If the can is moved too rapidly, not enough adhesive will be applied. Next, remove the stencil and position the next layer of the device over the freshly-sprayed layer, carefully aligning them at the edges.
Firmly press the layers together. Then, spray the adhesive again. Continue this process until all the layers are tightly adhered.
Onto the four-layer stack, stick a strip of packing tape across the bottom layer to prevent fluid from leaking out. Then, individual devices can be cut from the stack following the printed pattern. Like the previous procedure, print the device onto filter paper using a solid ink printer and melt the ink on a hot plate at 170 degrees Celsius for two minutes.
For this procedure, also print the crease pattern in the same manner, but on regular printer paper. Once the prints have cooled, align the lines of the crease pattern to the edges of the channel patterns. Then, secure them using tape.
Next, trace the crease pattern with a blunt stylus. Apply enough force to mark the device sheet but do not tear the paper. If a tear occurs, start over.
This pre-creasing technique increases the accuracy and precision of the folding. Now, begin folding the device using mountain and valley folds according to the crease pattern. Folding before application of the adhesive helps speed the device assembly.
Once folded, unfold it to expose the portions that require adhesive. Then, with a blade, cut out masks to limit where adhesive will get applied. Now, clamp the device flatly between the stencil with the mask and a stiff backing.
Use a metronome to time out 1.3 seconds and apply the adhesive as before. If the ambient humidity is low, apply the adhesive in a humidity-controlled area so the adhesive doesn't dry too quickly. Next, remove the stencil and mask and turn the sheet over.
Then, spray the back side of the paper in the same manner. Immediately remove the device from the stencil and begin folding the device. Once the device is completely folded, apply continuous pressure to the adhesive-containing portion until it is dried.
To perform a wicking test on the four-layer devices, randomly select 20 devices. Place the devices where they are shielded from air currents to minimize evaporation. Then, deposit 40 microliters of 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. For the origami devices, compare two origami peacocks, one made as previously described, and the other made without using the stencil while applying the adhesive. Then, insert one end of a small paper lead into the body of the peacock.
Under controlled relative humidity, above 90%place each leg and the paper lead of each peacock into a container filled with five millimolar dye. Average wicking times and success rates for four-layer devices constructed with different amounts of applied adhesive were compared. Uniform adhesive coverage resulted in relatively high success rates that decreased with increasing quantities of adhesive.
Success rates were also much higher with faster wicking times when the patterned adhesive was applied to both sides, than only one side. Failures occurred more frequently when adhesive was only applied to one side. Typical stacked device failure was characterized by outlets that failed to completely fill with dye or took longer than five minutes to fill.
In origami folded devices, device failure was characterized by outlets that failed to fill with any amount of dye. These outlets were exclusively located along the two edges of the device that contained the creases. By doubling the size of the border around the channels, the success rates for both single and dual-sided adhesive applications increased.
Both methods of adhesive application resulted in devices that successfully routed liquid the length of their channels and without mixing. However, the device with uniformly-applied adhesive was noticeably slower. While attempting this procedure, it's important to ensure even and consistent adhesive application.
Do note that aerosol adhesive should only be applied in well-ventilated areas. After watching this video, you should have a good understanding of how to apply and use patterned adhesives to construct planar and nonplanar 3D paper microfluidic devices. This technique will allow researchers to explore the use of nonplanar structures to achieve functionality not previously found in planar devices, such as integrated actuation and sensing.
