Microfluidics

Microfluidics

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.

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