Shrinky Dink Microfluidics - Jennifer Boryczka
Shrinky Dink Microfluidics is a cutting-edge method to design and rapidly produce microfluidic devices with rounded channels. This method was first designed in 2008 by Michelle Khine’s group at the University of California Irvine . The method takes advantage of the shrinking capabilities of the toy called Shrinky Dinks to print larger designs and then shrink the design to allow for the microfluidic device size. It requires Shrinky Dink plastic papers, or other shrinking plastics, to take a printed design and with heat shrink down the width and increase the height of the design. This shrunk design is then used as a master for rapid casting of devices. This can be used for rapid prototyping or simple and rapid device development with the added benefit of having rounded channels. Subsequent reports have described the same approach using different plastics  or cutting the pattern out of the sheets rather than printing the designs .
Background and Motivation
Scientists have developed a method of rapidly developing rounded channels for microfluidic devices. This method utilizes a shrinking plastic sheet on the commercial market known as Shrinky Dinks. This product is designed to shrink the printed or colored design to a smaller size, commercially used for keychains or necklaces. A DIY version of this paper uses plastic #6, polystyrene, typically from to go boxes as a replacement for the Shrinky Dink plastic sheets but this has not been utilized for microfluidic devices. Shrinky Dinks are made of polystyrene, the same material as plastic #6. These polymer sheets are a bunch of polymer chains that have been stretched and flattened to an orderly configuration. This orderly configuration is not preferred energetically by the material. When the sheets are heated during the baking process then begin to bunch up thus shrinking the overall sheet .
The Shrinky Dinks’ shrinking properties can be utilized for microfluidic applications. Michelle Khine’s group at the University of California Irvine investigated utilizing the shrinking properties of this commercial product in 2008 and in 2011 other shrinking plastics. This group printed the device channels onto the Shrinky Dink sheets then shrunk them down to create a device master with raised areas that can then be used for to make the device out of PDMS. This method produces rounded channels in a short period of time. Where others have taken this method and spun off further methods, such as cutting out the device design to create the channels within the Shrinky Dink sheets. Further studies looked at other materials such as polyolefin sheets. The polyolefin sheets shrink by 95%  where the standard Shrinky Dink plastic shrinks by 63% .
The major advantages of this method are the rapid device development/prototyping, easy of manufacturing, simple equipment needs, and it creates a reusable master for PDMS casting or a useable device itself. The channels that are created are semicircular which is not an easy shape to produce using other fabrication methods. The major disadvantage of this method is that the channel may not be uniform through the entire length and once the master needs replacement there is a question of reproducing the exact master, due to the shrinking. The Shrinking is subject to change each time because the polymers within the sheets will bunch randomly. This random bunching can create narrower or wider sections of the channels. The resolution of the printer directly affects the minimum line width of the final microfluidic device .
Initial designs of the microfluidic channels are designed using AutoCad and then these designs were then printed onto Shrinky Dink plastic paper. The printing uses a LaserJet printer with both 600 dpi and 1200 dpi. With the 600 dpi there are smoother features but at the sacrifice of channel height, when compared with the 1200 dpi channels . Once the printing of the device was complete, the device sheet was placed in an oven for 5 minutes at 163 ℃. The heating can be done using a standard toaster oven or a laboratory-grade oven. Shrinky Dinks naturally curl during the heating process but if heated on with uniform heat and on a flat surface ensures that they will be flat at the completion of the shrinking. A post bake is used after the shrinking process to help maintain ink adhesion and smoothing of the features. The master that was created using the process described above and found in Figure 1 can be used for multiple device molding .
The rounded channels are formed during the shrinking process by compressing the area that the ink is printed on; this compresses the dots of ink to create an area where the ink is thicker. The thickness of the structure can be controlled based on the number of printed layers added and the exact printer that was used. The unshrunk ink height was ~10 um and when it was shrunk the height of the ink was 80 um .
In 2010, Khine’s group used polyolefin sheets in replacement of the Shrinky Dink plastic sheets to increase the shrinkage that can be obtained in comparison to the original design . The polyolefin sheets shrink by 95%  where the standard Shrinky Dink plastic shrinks by 63% .
In 2011, Patel et al. from the Rust group used a cutting plotter to cut out the design of the microfluidic channels in the Shrinky Dink plastic paper with the inlet channels being cut into a separate layer using a 3-hole punch. A cutter plotter has a resolution of 10 um where a standard inkjet printer can get down to a resolution of 20 um. Fluid connections were made by gluing the tubing to the device. These cut sheets were aligned and adhered together with a full sheet to close the channel; the stack order outlined in Figure 2. The layers where then placed between two plates and then were heated to allow shrinking. Once cooled, these devices were ready to be tested with no further steps needed .
This method can be used for a variety of applications, some of which are listed below. The method rapidly creates rounded channels that can be cast in PDMS or be used to make a device directly, which means that this fabrication method can be used to quickly test prototype devices and adjust the device designs.
Rounded Channel Applications
Rounded channels are difficult to produce using standard lithography methods, it can and is done but it is time consuming. These rounded channels are used in a wide variety of applications, such as to hold cells or in a valve. These devices take time to build and understand. Simulations are used to either verify a device is working as it seem or to test a device before fabrication begins. The rapid ability of this Shrinky Dinks method to create a variety of rounded microfluidic channels can be utilized to verify the simulations of these channels. Simulations are essential to determine exactly the flow within the device. Knowing that the simulation is correct makes the further development of devices easier without test devices needing to be produced. Combining the rapid device development with adjustments to simulations, an accurate simulation and device development can be achieved . Simulations can be used to know the flow of a device or the device manufactured using this method can be used as a quick way to validate the simulations. Since rounded channels have endless possible uses, the ability to rapidly manufacture them allows for rapid protype testing.
Due to the printing of larger structures that can be shrunk down, complex devices can be made rapidly, such as gradient generators. An example of the gradient generator that was produced using this method is located in Figure 3.
3D Chip Fabrication
Complex 3-Dimensional microfluidic devices can be difficult to produce. The manufacturing of the 3-D devices involves making individual layers and then stacking them together to create the finished device. In a proof of concept, a 3-D vortex mixer was produced, Figure 4. This vortex can be created at a low Reynolds number with the use of the various layers of a 3-D system. This mixing is difficult in 2-D due to the laminar flow regime. Developing 3-D micromixers and other 3-D chips are difficult due to the alignment and bonding issues. These 3-D chips can allow for faster reaction times, allow for further device miniaturization, and increase processing power by these chips .
 Grimes A.; Breslauer, D.; Long, M.; Pegan, J.; Lee, L.; and Khine, M. Shrinky-Dink microfluidics: rapid generation of deep and rounded patterns. Lab on a Chip. 2008, 8, 170-172. http://dx.DOI.org/10.1039/b711622e.
 Nguyen, D.; Taylor, D.; Qian, K.; Norouzi, N.; Rasmussen, J.; Botzet, S.; Lehmann, M.; Halverson, K.; and Khine, M. Better shrinkage than Shrinky Dinks. Lab on a Chip. 2010, 10, 1623-1626. http://dx.DOI.org/10.1039/c001082k.
 Patel, T.; Tencza, L.; Daniel, L.; Criscuolo, J.; and Rust, M. A Rapid Prototyping Method for Microfluidic Devices Using a Cutting Plotter and Shrinky Dinks. 2011. IEEE 37th Annual Northeast Bioengineering Conference (NEBEC), Troy, NY, 2011, pp. 1-2. http://dx.DOI.org/10.1109/NEBC.2011.5778520.  Rhodes, J. (2010, October 14). The Science of Shrinky Dinks. Retrieved from https://www.smithsonianmag.com/science-nature/the-science-of-shrinky-dinks-36715644/
 Jović, A.; Janićijević, Ž.; Janković, M.; Janković, N.; Barjaktarović, M.; Čantrak, Đ.; and Gadjanski, I. Simulating fluid flow in “Shrinky Dink” microfluidic chips - potential for combination with low-cost DIY microPIV. 15th IEEE EAST-WEST DESIGN & TEST SYMPOSIUM (EWDTS), At Novi Sad, Serbia, 2017. http://dx.DOI.org/10.1109/EWDTS.2017.8110052.
 Chen, C.; Breslauer, D.; Luna, J.; Grimes, A.; Chin, W.; Lee, L.; and Khine, M. Shrinky Dink microfluidics: 3D polystyrene chips. Lab on a Chip. 2008,8, 622-624. http://dx.DOI.org/10.1039/b719029h.