Biaxially oriented polystyrene thermoplastic sheets (Shrinky Dinks) will contract to about 50-percent of their original size when heated, becoming thicker and hard. Any printing on the sheets will become a raised pattern which can used as a mold. In this experiment the printing will be lines drawn where fluid should flow. Polydimethylsiloxane (PDMS) can be cured by an organometallic crosslinking reaction to give an optically transparent polymer with the ability to reproduce surface features. In this experiment the PDMS is cured in contact with the Shrinky Dink mold to create channels that correspond to the original printing. Peeling the PDMS from the mold and attaching it to a glass slide produces a microfluidic device.
In addition to important applications where very small quantities of reagents are used, the small dimensions of the microfluidic channels alter the behavior of fluids compared to the macroscale. Microfluidic devices exhibit laminar flow where parallel fluid streams in the same channel will flow independently of each other with little or no mixing at the interface.
Using Powerpoint, create a drawing of a microfluidic device. The size of indvidual patterns should be no bigger than 2" by 3" to allow the shrunken design to fit on a standard microscope slide.
Several devices can be drawn within an 8.4 inch circle. Leave space between them so the PDMS casts can be cut out individually and stay at least 1/2 inch from the edge of the circle.
Line widths of 2 or 3 points work well but may vary with individual laser printers (see the test patterns). There should only be one outlet. Use the sample file as a guide.
Print on a laser printer using manual feed and straight pass through to avoid heating up the plastic sheet. Feed the same sheet through again and print the same pattern a second time to increase the height of the printing.
Cut out the 8.4" circle so that it will fit in a petri dish after shrinking.
Heat corn oil to 160°C in a 9x9" pan. When the oil is hot, add one circle at a time. Wait 5-6 seconds for the Shrinky Dink film to curl and then uncurl. Grab with tongs in an area away from the patterns. There may be a slight curl remaining so press between two glass plates immediately after removing from the hot oil.
Wash the corn oil off the mold with soap and water.
Using double-sided tape, secure the mold (ink side up) inside the bottom of a petri dish. Without the tape the mold may float in the next step.
Dispensing the viscous PDMS reagents can be messy. Cover the work surface and the balance with aluminum foil. Wear gloves.
Pour 25 g or mL of PDMS base into a plastic cup and add 3 g or mL of PDMS cross-linker to the cup. Stir with a wooden stick 100 times.
Add enough PDMS to cover the mold by 3-4 millimeters. Place the top on the Petri dish.
Stack covered Petri dishes in a vacuum desiccator with a layer of foil on the bottom. To remove the bubbles from the PDMS, pump out the air then let the air back in several times.
Bubbles in the PDMS will seriously affect the results.
Let sit overnight or place the petri dish in an oven (70°) for 1 hour to polymerize the PDMS. (Higher temperatures would be quicker but the peri dish does not survive.)
You should get your petri dish in the oven by the end of the first lab period.
After the PDMS has hardened, break off a portion of the wall of the petri dish.
Remove the remainder of the petri dish.
Use a single-edge razor blade to trim off any PDMS that went under the Shrinky Dink. (There should not be much if the disk was taped to the bottom of the dish.)
Cut the wide end off a micropipet tip so that it will fit on a syringe.
Connect the syringe tip to 2 mm PVC tubing.
Wear gloves for the remainder of the activity so that the oils on your hands are not transferred to the PDMS microfluidic device.
Peal the PDMS from the mold. The mold can be used multiple times so do not discard.
Place the PDMS channel-side up on a cutting mat. Press straight down with a single-edge razor blade to cut the PDMS. (Dragging with the blade to cut like a knife often tears the PDMS.)
Place the PDMS channel-side up on a cutting mat. Punch holes for the outlet (2 mm) and the inlets (3 mm). Always use the punch on a cutting mat. Do not press on the plunger while cutting. (Press the plunger afterward to eject the core.)
Using larger reservoirs for the inlets allows flow for a longer time.
Roll out a strip of double-stick tape onto a clean 25 x 75 mm glass microscope slide. Use a polystyrene vial to roll over and press down the tape.
Make sure there are no bubbles between the glass slide and the tape.
Place the PDMS mold, channel side down, onto the double-sided tape on the microscope slide. Press gently to remove any air pockets between the double sided tape and the PDMS mold. Take care not to collapse the microfluidic channels by pressing too hard.
Insert the 2 mm tubing connected to the plastic syringe into the 2 mm outlet hole.
Use small droppers to add food coloring to the 3mm inlet holes. Use the syringe to slowly and gently pull the solution through the channel.
Do the streams of food coloring exhibit laminar flow as the solutions are pulled through the channels?
Use the syringe to slowly and gently pull the solution through the channel. Do your observations vary with flow rate?
Use the syringe to slowly and gently pull the solution through the channel.
If your device does not work, try assembling the same device again. You many clean your device to re-use it by running water through the device with the syringe or disassembling the device and rinsing with water.
Use tape (by pressing and peeling) to remove macro dirt or dust particles from the PDMS mold prior to assembling the device. Watch carefully as your press the device to the tape: you want all the flat parts and only the flat parts to connect to the tape.
Report your observations by taking a photograph or video or using colored pencils to make a sketch of your results for each tested pattern. Make use of a partner to record this information while the device is operating.
What happens at a Y-junction? What happens at a T-junction? What happens at square corners (90° turn)? What happens at zig-zag corners (<90° turn)? Sketch your answers on the figures below.
Do your observations vary with flow rate? How could you measure flow rate?
What features of a microfluidic device promote turbulent flow (mixing)? What features of a microfluidic device promote laminar flow? How do you know?
Clear Shrinky Dinks Film (Grafix KSF50-C 8-1/2-Inch by 11-Inch Shrink Film, Clear, 50-Pack)
A less expensive punch alternative can be fabricated from 1/8 x 0.014 inch round brass tube. A tubing cutter is used to cut the tubing into 3 inch lengths and a grinding wheel is used to sharpen one end. If the core gets stuck in the punch use a copper wire to eject the PDMS core.