Designing Microfluidic Devices for the High School Classroom

Authors

• RET Fellow: Melissa Hemling
• RET Leadership Team: Kira Jacobson, Evelyn Montalvo, Scott Mullee, Elizabeth Waldinger, Angela Johnson, Ben Taylor, George Lisensky, Doug Weibel, Katie Brenner, John Crooks, Piercen Oliver, and Anne Lynn Gillian-Daniel.

Audience

High School Chemistry Students

Time Frame

• Set-up: 3 hours
• Activity: 5 Block Classes (90 min. each)
• Clean-up: 10 minutes

Objective(s)
After completing the activity, students will be able to:
Design and create microfluidic devices that can successfully mix and create concentration gradients to solve a problem.

Standards Addressed: Engineering Principles<

1. HS-ETS-ED-B Analyze input and output data and functioning of a human-built system to define opportunities to improve the system’s performance so it better meets the needs of end user while taking into account constraints.
2. HS-ETS-ED-D Plan and carry out a quantitative investigation with physical models or prototypes to develop evidence on the effectiveness of design solutions, leading to at least two rounds of testing and improvement.

Activity Materials

• Computer and MS Powerpoint access
• Clear Shrinky Dinks Film (Grafix, KSF50-C)
• LaserJet Printer (Hewlett Packard LaserJet 4100N)
• Scissors
• Mineral or Vegetable Oil
• Crystallization dish (125 x 65 mm)
• Hot Plate
• Thermometer
• Tweezers
• Glass plates
• Soap
• Glass microscope slides (75 x 50 mm and 25 x 75 mm)
• PDMS Chemicals (Sylgard 184 Silicone Elastomer Kit)
• Plastic cup
• Wood Splint/Stick or Stir Rod
• Vacuum desiccator or Bell Glass Jar with pump
•  Oven (60°C)
• Razor blade
• Punches (Harris Unicore 2mm)
• Scotch tape
• Double Sided Tape
• Plastic Petri dish (100 mm)
• Transfer Pipettes
• Food coloring or other colorimetric indicator (provide very concentrated solutions)
• Acid and Base solutions, weak (example: 100 mM NaH2PO4 and 100 mM Na2HPO4)
• Universal Indicator solution, concentrated
• Small syringe (1 mL, Luer taper)
• Tubing (Tygon microbore PVC tubing OD 0.03″ and OD 0.2″)

Activity Instructions

Set-up(3 hours day before)
Using the 2 premade powerpoint files and following the student directions for making a microfluidic device, make two different introductory microfluidic devices for student groups. Alternatively, you could have your students make these devices in class.

Introduction (10 min)
Conduct a pre-assessment by having students brainstorm an answer to the question: “What do you think microfluidics means?” Review with students the relative size of a micrometer. You may also want to review basics about acid and base neutralization and pH if you have not already for the design portion of the activity.

Introduction to Microfluidics 101 (5 90 min class periods total)
Handout the Microfluidics 101 worksheet. Students will follow the directions on this worksheet to interact with 3 different microfluidic devices. This worksheet is structured as a guided inquiry. The devices have been specially designed to show students unique features of microfludics: laminar flow, mixing, and gradients. After interacting with all 3 devices, the students will be asked questions on the worksheet to help guide them to an understanding of the concept of laminar flow. Before the class engages in the design part of the worksheet, a class discussion should occur about laminar flow and the unique features of each device. At the end of the worksheet, they will be presented a design challenge where students with design their own more complicated device using MS Powerpoint. The following day students will take their microfluidic design and create it using shrinky dink masks and then test it using double sided tape and suction. Once their first microfludic design is successfully designed, created, and tested, students will brainstorm changes to their first design to optimize performance. Students will then create and test a second device based on the data gained from the first.

Creating a Microfluidics Device in a High School Lab (2 90 min class periods or 3 hours)

Safety Note: Observe safe laboratory procedures as outlined by your instructor. Wear goggles at all times and gloves when handling the PMDS polymer chemicals. Be cautious of the hot oil as it will burn.

1. Using Powerpoint, create a drawing of a microfluidic device. The size of the final design should be no bigger than 6 cm by 8 cm to allow the shrunken design to fit on a standard microscope slide. Make sure there is about 3cm of white space around all areas of the device to help with cutting it out later on. The image needs to be in black. There should only be one outlet in order to use the syringe and hose. Use the template as a guide.
2. Print the device out on clear Shrinky Dink film using the transparency setting on a laser printer. Even though this type of Shrinky Dink is not designed for a laser printer, the Shrinky Dink film will not wreck the standard computer printer. Depending on the exact printer used you may want to re-print the picture of the device on the same Shrinky Dink film so there are two coatings of ink on the film.
3. Cut out the devices leaving about 1″ of clear space around the device. Round the edges with scissors to reduce rippling during the shrinking process.
4. Heat a crystallization dish 1/3 full of vegetable oil on a hot plate. Use a thermometer to ensure that the temperature of the oil maintains 120°C.
5. Insert one cut-out Shrinky Dink film into the hot oil. Wait for the Shrinky Dink film to curl and then uncurl. There may be a slight curve to the final device in the oil.
6. Once the film has completely shrunk (~ 30 sec), gently remove the shrunken template from the oil and quickly place it between two large glass slides. Press the glass slides firmly together to flatten the template. Try not to press directly on the printed channels. Maintain pressure on the plates until the template has cooled and hardened (~20 – 30 sec).
7. Remove the device from the large glass slides and gently wash it in soapy water to remove the oil.
8. Using double-sided tape, secure the device (ink side up) to the inside of a plastic petri dish.
9. Prepare the PDMS by mixing the base and cross-linker at a 10:1 (w/w) ratio (this is easily done by slowly pouring the PDMS materials into a plastic cup that is placed directly on a balance). Pour 25 g of PDMS base into a plastic cup and add 3 g of PDMS cross-linker to the cup. A final weight between 25 and 30 g is ideal for filling a 100 mm diameter petri dish. Stir with a wooden stick until base and cross-linker are completely mixed (~100 times).
10. Place the PDMS cup into a vacuum chamber for 15 min to remove bubbles and then pour over the shrinky dink templates in the petri dish. Vacuum the petri dish for 1 hour to remove bubbles.
11. Using a wooden stick, gently pop any remaining bubbles. Bake the filled petri dishes in a 60°C oven for 1 hour to polymerize the PDMS.
12. Wear gloves for the remainder of the activity so that the oils on your hands are not transferred to the PDMS microfluidic device.
13. Using a razor blade, cut out your device by following the edge of the shrinky dink template. Use tweezers to carefully remove the PDMS mold from the petri dish and shrinky dinks. Gently peel back the edges of the mold before trying to remove the entire device.
14. Using a punch, create holes (2 mm diameter) through the PDMS mold for the inlets and outlet.
15. Use scotch tape (by pressing and peeling) to remove macro dirt or dust particles from the PDMS mold prior to assembling the device.
16. On a clean 25 x 75 mm glass slide, put down a strip of double sided tape large enough to seal the footprint of the microfluidic network. Ensure that the double sided tape lies flat against the slide by rolling a syringe barrel (or some other sturdy cylinder) over the tape. Make sure there are no bubbles between the glass slide and the tape.
17. Form the final device by placing the PDMS mold, imprint 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.
18. Connect the tubing to the syringe and insert the tubing into the 2 mm outlet hole.
19. With plastic transfer pipettes, place the desired chemicals into the 2 mm inlet holes. For the chemistry design challenge, use 100 mM NaH2PO4 and 100 mM Na2HPO4. Mix both solutions with concentrated Universal Indicator Solution. The red and violet colors of the acid and base respectively should be fairly dark as the colors will appear much lighter in the microfluidic device.
20. Slowly and gently pull back on the syringe plunger just enough to create suction and pull the chemicals through the device. You may only need to pull the plunger slightly, such as 0.1 mL.
21. Observe and record results.
22. You many clean your device to re-use it by running water through the device with the syringe.
23. Brainstorm changes to your design based on your observations. Make the desired changes in MS Powerpoint and repeat the procedure with your new microfluidic design.

Conclusion (20 min)
To wrap-up the lab experience, discuss changes that the students made to their microfluidics devices and why they made those changes. Lead a class discussion about how scientists problem-solve and learn from trials that do not work. Show students video clips of more complicated microfluidics devices and the TED talk on Lab of a Chip potential for microfluidics. Have students brainstorm ways microfluidics could help solve problems in the real-world. Discuss current research using microfluidics.

Background

• None

Supplemental Materials

• Microfluidics 101 Worksheet
• Powerpoint Template
• Handout of Procedure

References

• A. Grimes, D.N. Breslauer, M. Long, J. Pegan, L.P. Lee, and M. Khine, Lab Chip. 8, 170-172 (2008). “Shrinky-Dink microfluidics: rapid generation of deep and rounded patterns.” doi: 10.1039/B711622E
• M. Chia, C.M. Sweeney, and T.W. Odom, J. Chem. Educ. 88, 461 (2011). “Chemistry in Microfluidic Channels.” doi: 10.1021/ed1008624
• J. Greener, E. Tumarkin, M. Debono, A.P. Dicks, and E. Kumacheva, Lab Chip. 12, 696-701 (2012). “Education: a microfluidic platform for university-level analytical chemistry laboratories.” doi: 10.1039/C2LC20951A
• D.J. Beebe, G.A. Mensing, and G.M. Walker, Annual Review of Biomedical Engineering. 4, 261-286 (2002).”Physics and applications of microfluidics in biology.” doi: 10.1146/annurev.bioeng.4.112601.125916

Additional References