Microfluidics
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May 8, 2020Microfluidics
Objective(s)
After completing the activity, students will be able to:
1. Design and create microfluidic devices that can successfully mix two fluids and create concentration gradient to solve a problem.
2. Keep a record of the experimental procedures
3. Write a lab report that includes
a. Theory of microfluidics
b. Materials & methods
c. Results section 1 observe & record of performance of the three microfluidic designs provided in this handout
d. Results section 2 Interpret similarities and differences observed between the three microfluidic designs provided in this handout. Explain why & how these results influenced your design of a microfluidic device that is capable of mixing two dyes to produce a gradation of five different colors
e. Conclusion discussion of microfluidic design process, further work or extension of this lab.
Standards Addressed: Critical Thinking, Scientific Method
1. 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. 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.
Problem to be Solved
Design a microfluidic device that is capable of mixing two dyes to produce a gradation of five different colors in the microfluidic device.
Materials
¢ Computer and access to Microsoft Powerpoint or another graphic design program
¢ Clear Shrinky Dink film (Grafix, KSF50-C, amazon.com)
¢ Laser printer (we used a Hewlett Packard LaserJet 4100N)
¢ Scissors
¢ Mineral or vegetable oil
¢ Crystallization dish (125 x 65 mm) or 500 mL glass beaker
¢ Hot plate
¢ Thermometer
¢ Tweezers or forceps
¢ Glass plates
¢ Soap
¢ Glass microscope slides (75 x 50 mm and 25 x 75 mm)
¢ PDMS chemicals (Sylgard 184 Silicone Elastomer Kit, amazon.com)
¢ Plastic cup
¢ Wood stick/stir rod or plastic fork
¢ Vacuum desiccator or bell glass jar with pump
¢ Oven (60°C)
¢ Razor blade or scalpel
¢ Biopsy punch (2-mm diameter, sold for piercing, available on amazon.com for <$10)
¢ Scotch tape to clean the PDMS
¢ Double sided Scotch tape (wider tape is ideal)
¢ Plastic Petri dish (150-mm diameter, amazon.com)
¢ Transfer pipettes
¢ Food coloring or other colorimetric indicator (provide very concentrated solution)
¢ Small syringes (tapered tip or oral irrigator syringes, amazon.com)
¢ Tubing (Tygon PVC tubing OD = 3/32 and ID = 1/32 or similar, amazon.com)
Microfluidics 101
Student Instructions for the laboratory experiment
Safety Note: Observe safe laboratory procedures at all times. Wear goggles and gloves when handling the hot oil and PMDS polymer chemicals. CAUTION: hot oil will burn skin.
1. Fill a 250mL beaker about 1/3 full of oil, and heat the oil as directed by your instructor on a hot plate. Use a thermometer to ensure that the temperature of the oil maintains 150°C.
2. Insert one cut out Shrinky Dink film at a time 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. CAUTION: hot oil will burn skin.
3. Once the film has completely shrunk (~ 30 sec), gently remove the shrunken template from the oil with forceps or tweezers and quickly place it between two large 4 by 4 glass slides. Press the glass slides firmly together to flatten the template. Do not touch the printed channels with your hands. Maintain firm pressure on the plates until the template has cooled and hardened (~20 30 sec).
4. Remove the shrunken film from between the large glass slides and gently wash it with soapy water to remove the oil. You may touch the printed channels very gently with your hands; minimizing the touching of channels reduces the amount of oil on the channels and will help the fabrication process.
5. Using double-sided tape, secure the shrink dink templates (ink side up) to the inside of a plastic Petri dish. Fit as many templates as you can, side-by-side with no overlap, into your dish (usually three or four will fit), using the best ones.
6. 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).
7. Place the PDMS cup into a vacuum chamber for 15 min to remove large bubbles (optional, refer to teacher’s instructions) and then pour over the Shrinky Dink templates in the Petri dish. Apply a vacuum to the Petri dish and its contents to remove gas bubbles (10 min 1 h).
8. Using a wooden stick, gently pop any remaining bubbles on the PDMS surface. Bake the filled Petri dishes in a 60°C oven for 2-3 h or overnight to polymerize the PDMS.
9. Wear gloves for the remainder of the activity so that the oils on your hands are not transferred to the PDMS microfluidic device.
10. Using a razor blade or scalpel, cut out your PDMS devices by following the edge of the Shrinky Dink template. Use tweezers and your hands to carefully remove the PDMS from the petri dish and Shrinky Dinks. Gently peel back the edges of the PDMS before trying to remove the entire device from the templates. CAUTION: razors and scalpels are sharp and may cut the skin if not used carefully.
11. Using the metal tip of a core punch, create holes (2 mm diameter) through the PDMS for the inlets and outlet. CAUTION: biopsy punches are sharp and may puncture skin if not used carefully.
12. Choose the best device that you have made. Use Scotch tape (by gently pressing and peeling) to remove dirt or dust particles from the PDMS mold prior to assembling the device.
13. 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. If double sided tape is not wide enough, use the double sided tape to secure a wider piece of regular tape, sticky side up. Ensure that the tape lies flat against the slide by rolling a clean, dust-free syringe barrel (or some other sturdy cylinder) over the tape. Make sure there are no bubbles between the glass slide and the tape.
14. Form the final device by placing the PDMS device, imprint side down, onto the double-sided tape on the microscope slide. Press gently and evenly 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.
15. Connect the tubing to the syringe and insert the tubing into the 2 mm outlet hole.
16. With plastic transfer pipettes, place the desired chemicals into the 2 mm inlet holes. For the chemistry design challenge, use yellow food coloring and blue food coloring.
17. 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 to the 0.1 mL mark.
18. Observe and record results.
19. You many clean your device to re-use it by running water through the device with the syringe.
20. Brainstorm changes to your design based on your observations. Use Powerpoint to design a microfluidic device that will mix two dyes to produce a gradation of five colors.
21. Using Powerpoint, create a drawing of your microfluidic device. The size of the final design should be no bigger than 6 cm by 8 cm to allow the resulting shrunken design to fit on a standard microscope slide. Make sure there is ~5 cm of white space around all areas of the device. The image (i.e., microfluidic channels) must appear black. There should only be one outlet in your initial device. Use the template as a guide. It is wise to make at least four copies of your device on one page. You will make all four and choose the best one for final testing.
22. Print the device on clear Shrinky Dink film. Cut out the four or more devices leaving ~5 cm of clear space around the device. Round the edges with scissors to reduce rippling during the shrinking process. CAUTION: there is a small risk that the Shrinky Dink film may damage a laser printer. An older, discarded laser printer is ideal for this activity.
23. Repeat step 1-19 above. Brainstorm changes to your design based on your observations. Use Powerpoint to design a microfluidic device that improves on your initial design. Make the device, observe and record your results.
Method of Assessment
You will be assessed on
1. your performance in the lab; i.e. did you turn up; your individual contribution to discussion any group or individual design decisions.
2. your lab book i.e. your record of the experimental procedures. Please note, your records should be complete and detailed such that another student with no knowledge of this procedure could take your lab book and repeat your work.
3. your lab report that includes
a. Theory of microfluidics
b. Materials & methods
c. Results section 1 observe & record of performance of the three microfluidic designs provided in this handout
d. Results section 2 Interpret similarities and differences observed between the three microfluidic designs provided in this handout. Explain why & how these results influenced your design of a microfluidic device that is capable of mixing two dyes to produce a gradation of five different colors
e. Conclusion discussion of microfluidic design process, further work or extension of this lab.
Microfluidic Design Templates
Microfluidics Name:
Microfluidics deals with the precise control of fluids on the microscale. Often at the microscale, smaller amounts of reagents are used so experiments are faster and cheaper. You will make and use three simple microfluidic devices to discover how microfluidic devices work.
Microfluidics Device Exploration
Prediction:
In the chart below predict what you think will happen if a drop of blue food coloring and a drop of yellow food coloring are placed in the two inlet holes of the device. Use colored pencils to draw your prediction and explain your reasoning. Repeat for all three devices.
Lab Observation:
1. Prepare your microfluidic devices.
2. Using pipettes, place a drop of blue food coloring and a drop of yellow food coloring in the inlet holes of the microfluidics device.
3. Insert the tip of the syringe (or tubing connected to the syringe) into the single outlet hole in the device.
4. Pull back very slowly on the syringe, until the food coloring is pulled through the device. Pulling too fast or too much will affect your observations.
5. Draw your observation in the chart below using colored pencils. Explain why you think this happened and answer the questions. Repeat for all three devices.
Laminar Flow
Laminar Flow Definition: Non-turbulent fluids flow in parallel layers.
Very little lateral mixing occurs between adjacent layers.
Fluids flow smoothly.
Reynolds number (Re) = (density)(velocity)(channel diameter)
viscosity
Re < 2000 ? laminar flow
Re > 4000 ? turbulent flow
Apply the Concept:
1. Circle all the areas in the three microfluidic designs where different fluids meet but do not mix.
2. What features of a microfluidic device promote laminar flow? How do you know?
3. If a scientist would like to have two solutions completely mix in a microfluidic device, what would you tell them to include in their microfluidic design? Why?
4a. Calculate your Reynolds number in water versus molasses (use the values provided below).
Density (g/m3) Viscosity (g/(m?s))
Water 1 x 106 1
Molasses 1.4 x 106 1 x 104
Assume your velocity = 0.2 m/s, and your diameter’ is roughly = 0.25 m.
4b. For comparison, the Re of a blue whale in water is roughly 1 x 108. And the Re of a bacterium in water is about 1 x 10-5. How would your motion in water versus molasses compare to the motion of these organisms in nature?
5. Use the above results to design, make and record two rounds of microfluidic devices that answer the problem to be solved on page 1 of this handout.
Overall procedure
We had to create microfluidic samples, which are basically plastic shrinky-dink material which has channels etched into one site, in a shape. The idea is to drop two colors of ink, one into each entry point (dot) on the sample, and use a vacuum device to pull it through and out of the end point (ending dot). We study how the fluid flows and mixes as it goes through the channels.
The prof gave us 5 initial samples, pre-printed on shrink-dink film. We used these samples, to go through step 1-19, which turned these films into molds, from which we create the fluidic devices to run the ink through. After that, steps 20-22 tell us to now use software (Powerpoint) to create our own design(s), print them on shrinky-dink paper, and run through the process (1-19) again to create new molds from our own designs, and test with ink.
Steps
Here are our notes on the steps, to be incorporated into the lab report, along with the photos. The numbered item is the instructional step(s) for the lab, and the red text is our comments from performing the step(s). This is what goes into the report.
1. Fill a 250mL beaker about 1/3 full of oil, and heat the oil as directed by your instructoron a hot plate. Use a thermometer to ensure that the temperature of the oil maintains150°C.
2. Insert one cut out Shrinky Dink film at a time 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. CAUTION: hot oil will burn skin.
Steps 1,2 went smoothly.
3. Once the film has completely shrunk (~ 30 sec), gently remove the shrunken templatefrom the oil with forceps or tweezers and quickly place it between two large 4 by 4 glass slides. Press the glass slides firmly together to flatten the template. Do not touch the printed channels with your hands. Maintain firm pressure on the plates until the template has cooled and hardened (~20 30 sec).
This step went smoothly, and the template very rapidly cooled, and properly flattened.
4. Remove the shrunken film from between the large glass slides and gently wash it with soapy water to remove the oil. You may touch the printed channels very gently with your hands; minimizing the touching of channels reduces the amount of oil on the channels and will help the fabrication process.
This step went smoothly.
5. Using double-sided tape, secure the shrink dink templates (ink side up) to the inside ofa plastic Petri dish. Fit as many templates as you can, side-by-side with no overlap, into your dish (usually three or four will fit), using the best ones.
This step when smoothly and we were able to fit 6 samples into the dish.
6. Prepare the PDMS by mixing the base and cross-linker at a ~10:1 (w/w) ratio (thisis 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).
This step went smoothly, and we did not have many bubbles at all.
7. Place the PDMS cup into a vacuum chamber for 15 min to remove large bubbles(optional, refer to teacher’s instructions) and then pour over the Shrinky Dink templates in the Petri dish. Apply abubbles (10 min 1 h).
8. Using a wooden stick, gently pop any remaining bubbles on the PDMS surface. Bake the filled Petri dishes in a 60°C oven for 2-3 h or overnight to polymerize the PDMS.
9. Wear gloves for the remainder of the activity so that the oils on your hands are nottransferred to the PDMS microfluidic device.
10. Using a razor blade or scalpel, cut out your PDMS devices by following the edge of the Shrinky Dink template. Use tweezers and your hands to carefully remove the PDMS from the petri dish and Shrinky Dinks. Gently peel back the edges of the PDMS before trying to remove the entire device from the templates. CAUTION: razors and scalpels are sharp and may cut the skin if not used carefully.
Steps 7,8,9,10all went smoothly.We polymerized the samples by baking for 3 hours.
We separated the devices from the templates using a scalpel and tweezers.
11. Using the metal tip of a core punch, create holes (2 mm diameter) through the PDMS forthe inlets and outlet. CAUTION: biopsy punches are sharp and may puncture skin if not used carefully
12. Choose the best device that you have made. Use Scotch tape (by gently pressing and peeling) to remove dirt or dust particles from the PDMS mold prior to assembling the device.
Steps 11,12went smoothly, and we selected the best 3 devices from the batch.
13. 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. If double sided tape is not wide enough, use the double sided tape to secure a wider piece of regular tape, sticky side up. Ensure that the tape lies flat against the slide by rolling a clean, dust-free syringe barrel (or some other sturdy cylinder) over the tape. Make sure there are no bubbles between the glass slide and the tape.
14. Form the final device by placing the PDMS device, imprint side down, onto the double-sided tape on the microscope slide. Press gently and evenly 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.
15. Connect the tubing to the syringe and insert the tubing into the 2 mm outlet hole.
16. With plastic transfer pipettes, place the desired chemicals into the 2 mm inlet holes. For the chemistry design challenge, use yellow food coloring and blue food coloring.
17. 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 to the 0.1 mL mark.
18. Observe and record results.
19. You many clean your device to re-use it by running water through the device with thesyringe.
For step 18, per the photos below, the ink flowed well through the first sample. You can see the ink mixed and gave us an intermediate color. On sample two, the ink flowed through the channel well, but did not mix well. The blue color overtook and filled the vast majority of the channel. On the final sample, the ink did not flow through well, and some obstructions and blockages can be seen.
20. Brainstorm changes to your design based on your observations. Use PowerPoint todesign a microfluidic device that will mix two dyes to produce a gradation of five colors.
21. Using PowerPoint, create a drawing of your microfluidic device. The size of the final design should be no bigger than 6 cm by 8 cm to allow the resulting shrunken design to fit on a standard microscope slide. Make sure there is ~5 cm of white space around all areas of the device. The image (i.e., microfluidic channels) must appear black. There should only be one outlet in your initial device. Use the template as a guide. It is wise to make at least four copies of your device on one page. You will make all four and choose the best one for final testing.
The samples were initially created using paint program. We attempted to use the best measuring and guideline features of the program to ensure the designs were symmetrical. One of the best methods to make the designs accurate was zooming in to a very high magnification when aligning the points and channels. 4 of them were selected to test. We had 2 with different channel thickness.
Here are the samples we created in PowerPoint.
22. Print the device on clear Shrinky Dink film. Cut out the four or more devices leaving ~5 cm of clear space around the device. Round the edges with scissors to reduce rippling during the shrinking process. CAUTION: there is a small risk that the Shrinky Dink film may damage a laser printer. An older, discarded laser printer is ideal for this activity.
The printing procedure was handled by Dr. Sutton. The resulting samples where cut into individual products, and holes were punched in each to allow them to be suspended in oil. (no picture)
23. Repeat step 1-19 above. Brainstorm changes to your design based on your observations. Use PowerPoint to design a microfluidic device that improves on your initial design. Make the device, observe and record your results. The following are steps 1-19:
1. Fill a 250mL beaker about 1/3 full of oil, and heat the oil as directed by your instructor on a hot plate. Use a thermometer to ensure that the temperature of the oil maintains 150°C.
2. Insert one cut out Shrinky Dink film at a time into the hot oil. Wait for the Shrinky Dinkfilm to curl and then uncurl. There may be a slight curve to the final device in the oil. CAUTION: hot oil will burn skin.
Steps 1, 2: Although the instructions state to heat to 150, the professor suggested 130-140. At these the ink would cooking off of the sample in strands, into the oil. We then proceeded to go down to 120, and the same thing happened. We then went back up to 150 and the same thing happened again. So we went to the printer to check that the printing process had no changed in any way. We found that it had not changed in any way. We noticed that the oil looked burnt, so we hypothesized that it had become oxidized, so we used new oil.
Figure: The two on the left did not come out well, we believe due to bad oil. The one on the right was done with good oil, and came out perfectly.
Figure: The darker oil on the left is the bad oil which produced bad results. We believe the oil had become oxidized. The lighter oil on the right side is the good oil being heated.
3. Once the film has completely shrunk (~ 30 sec), gently remove the shrunken templatefrom the oil with forceps or tweezers and quickly place it between two large 4 by 4 glass slides. Press the glass slides firmly together to flatten the template. Do not touch the printed channels with your hands. Maintain firm pressure on the plates until the template has cooled and hardened (~20 30 sec).
4. Remove the shrunken film from between the large glass slides and gently wash it with soapy water to remove the oil. You may touch the printed channels very gently with your hands; minimizing the touching of channels reduces the amount of oil on the channels and will help the fabrication process.
Steps 3, 4 went smoothly, however we had to go through many samples due to the problems with the oil. We had to borrow samples from peers as well.
This figure shows the samples in the petri dish. We were able to fit 6 samples in without overlap.
5. Using double-sided tape, secure the shrink dink templates (ink side up) to the inside ofa plastic Petri dish. Fit as many templates as you can, side-by-side with no overlap, intoyour dish (usually three or four will fit), using the best ones.
6. 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).
7. Place the PDMS cup into a vacuum chamber for 15 min to remove large bubbles (optional, refer to teacher’s instructions) and then pour over the Shrinky Dink templates in the Petri dish. Apply a vacuum to the Petri dish and its contents to remove gas bubbles (10 min 1 h).
8. Using a wooden stick, gently pop any remaining bubbles on the PDMS surface. Bake thefilled Petri dishes in a 60°C oven for 2-3 h or overnight to polymerize the PDMS.
Steps 5-8 went smoothly. When we retrieved the specimens we noticed many bubbles within the PDMS, and most of them were in the device channels.
These three figures show the bubbles in the PDMS.
9. Wear gloves for the remainder of the activity so that the oils on your hands are nottransferred to the PDMS microfluidic device.
10. Using a razor blade or scalpel, cut out your PDMS devices by following the edge of the Shrinky Dink template. Use tweezers and your hands to carefully remove the PDMSfrom the petri dish and Shrinky Dinks. Gently peel back the edges of the PDMS before trying to remove the entire device from the templates. CAUTION: razors and scalpels are sharp and may cut the skin if not used carefully.
11. Using the metal tip of a core punch, create holes (2 mm diameter) through the PDMS for the inlets and outlet. CAUTION: biopsy punches are sharp and may puncture skin if not used carefully.
12. Choose the best device that you have made. Use Scotch tape (by gently pressing and peeling) to remove dirt or dust particles from the PDMS mold prior to assembling the device.
13. On a clean 25 x 75 mm glass slide, put down a strip of double sided tape large enoughto seal the footprint of the microfluidic network. If double sided tape is not wide enough, use the double sided tape to secure a wider piece of regular tape, sticky side up. Ensure that the tape lies flat against the slide by rolling a clean, dust-free syringe barrel (or some other sturdy cylinder) over the tape. Make sure there are no bubbles between the glass slide and the tape.
14. Form the final device by placing the PDMS device, imprint side down, onto the double-sided tape on the microscope slide. Press gently and evenly 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.
Steps 9-14went smoothly.
15. Connect the tubing to the syringe and insert the tubing into the 2 mm outlet hole.
16. With plastic transfer pipettes, place the desired chemicals into the 2 mm inlet holes. Forthe chemistry design challenge, use yellow food coloring and blue food coloring.
17. 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 to the 0.1 mL mark.
Steps 15-17 went smoothly.
18. Observe and record results.
Step18 went smoothly. On the first image you can see that we were able to obtain a gradient of color despite the bubbling in the device. On the second image, you can see that the bubbles obstructed the way for the ink and it did not flow smoothly. The yellow color went more smoothly than the blue. In the last image, you can see that the ink did not make it through at all on one side, and in the center it is no balanced, mostly likely due to the complexity of the design as well as the many obstructions from air bubbles.
DONE LAB STEPS
Also answer questions at the end of the lab report instructions.
