Template:SVA-NYC/Microfluidics

SVA-NYC

Experiments & Protocols

PCB Based Microfluidic Chip Fabrication

Description

Research recently published on the use of “Shrinky Dink” based microfluidic chip molds introduced the concept of in-lab fabrication of microfluidic chips at low cost. The material used to make the mold, however, is not as predictable as one would desire and the variability in the expanded polystyrene material leaves much to be desired. This protocol is for the fabrication of an inexpensive PDMS microfluidic chip using PCB etching technologies for rapid and reliable chip prototyping. By leveraging the tried and true methods of the electronics engineering industry, a positive master mold can be fashioned inexpensively that has near infinite reusability.

Materials

• 2oz Copper Clad PCB Boards (FR4 preferred)
• Dry Film Negative Photoresist (Forwardsell)
• Adobe Illustrator or 2D CAD program
• Laser Printer with at least 600dpi
• High Temperature Transparency Sheets
• Ferric Chloride Etchant (MG Chemicals)
• Sodium Carbonate 4g/L (Sigma Aldrich)
• Sodium Hydroxide 4g/L (Sigma Aldrich)
• CellGuard solar encapsulant and catalyst (ML Solar)

• 3x HDPE or chemical resistant plastic containers 500mL
• Fluorescent Light Box
• Band Saw
• Sander or Sand Paper
• Standard Laminator capable of 5+ mil thicknesses (Scotch TL-901)
• 2mm Biopsy Punch (Ted Pella)
• 6mm Biopsy Punch (Ted Pella)
• Barkeeper’s Friend Metal Cleaner and Polish
• 2mm OD Tygon tubing (McMaster-Carr)
• Blunt dispensing needle tip for luer lock syringe (right angle or straight)
• 20mm syringe

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1. In Adobe Illustrator or alternative 2D CAD program, design the desired circuit scaled to a 2” x 3” glass microscope slide. Channels should be no larger than 1mm and outlets should have a diameter of 6mm. Allow at least 1/8” clearance between the perimeter of the circuit and the edge of the glass slide so the mold may be cut and placed properly.

2. Print a 3 x 3 panel of circuit designs onto plain paper (8.5 x 11”) for visual inspection and fitting.

3. Cut out the circuits and overlay two of them together as to darken the negative space and ensure proper exposure. Ensure the designs are perfectly aligned. A visual inspection for blur is a good way of doing this. Place a small piece of tape on one of the ends to secure the two in place. This will act as the photomask for the circuit process.

5. Using a band saw, cut the raw 2oz copper clad FR4 fiber glass boards to the same dimensions as the glass slide. Use a belt sander to smooth the rough cut edges and bevel the corners for easy mold removal later. Ensure no raised corners are present on either side of the PCB as this will interfere with the molding process and later chip adhesion.

5. Scrub the copper boards with Barkeeper’s Friend (metal polish/cleaner) until completely free of oxidation, rinse with tap water and then wipe with isopropyl alcohol.

6. Cut a sheet of Dry Film Negative Photoresist into a shape at least ¼” wider than the copper board.

7. Using two pieces of tape placed at a 45 degree angle from any corner of the photoresist sheet, peel the clear protective layer from the film.

8. Place the remaining sheet (peeled side down) carefully onto the cut copper board such that no bubbles form. Reseat the film until no folds or bubbles are present. It is critical that the film is free of any imperfections as they will amplify during lamination and ruin the circuit. Using a paper towel, apply firm pressure evenly to the chip to secure the film to the board.

9. Using a preheat laminator, sandwich the board between two pieces of computer paper and pass it through several times so that the film tightly binds to the board. Two passes should suffice.

10. Lay the photomask onto the photoresist side of the copper board. Align the design so there is equal space on all sides. Tape the mask to the board to secure it in place.

11. Place the board design side down onto the lightbox and expose for 10-15 minutes. Place a weight on top of the board to keep the design flush to the surface of the board as to avoid blurred edges and circuit lines.

12. Place the exposed board (free of mask) into the first developing bath (Sodium Carbonate). Agitate the solution often until the copper cladding is exposed. Expedite the process by gentle rubbing with gloved hands. The light exposed areas will be preserved and appear dark blue in color. The exposed copper should NOT feel slippery to the touch. If so, continue developing until the film dissolves.

13. Rinse the board in tap water and then place into the etching bath (Ferric Chloride Etchant Solution). The following step is tricky and requires some trial and error.

14. Agitate the etchant by slowly lifting and lowering the board onto its side with a PLASTIC spatula or tool. DO NOT USE METAL. The etching speed may vary depending on temperature and freshness of the Ferric Chloride Solution. The material is reusable for several etchings but decreases in efficiency with subsequent runs. Allow the board to etch until the edges of the copper side begin to show signs of the fiberglass board beneath. This means that there is only a very thin layer of copper left on the board. This step is required for casting a chip that is free of any texture for optimal glass adhesion in later steps. Do not allow the board to be etched completely.

15. Submerge the board into a small plastic container with 25-50mL of tap water to rinse off excess Ferric Chloride. DO NOT POUR FERRIC CHLORIDE DOWN THE DRAIN. Store waste liquid in a plastic reagent bottle labeled Dilute Ferric Chloride Etchant.

16. Place the board into the second development bath (Sodium Hydroxide 4g/L) and let sit until the photoresist film dissolves or dislodges from the surface of the board.

17. Rise board with tap water and dry with a paper towel.

18. Clean and polish the board with Barkeeper’s Friend Metal Polish to a brilliant copper shine.

19. Place the copper board (etched side up) into a custom acrylic casting tray (1” tall, encloses the perimeter of the board with a small gap for removal).

20. Preheat an incubator to 60 degree Celsius.

21. Mix 10:1 (by weight) CellGuard solar encapsulant and catalyst in a small flexible container or 50mL centrifuge tube until cloudy with bubbles. 20g of encapsulant and 2g of the catalyst was the common mixture.

22. Pour the mixed encapsulant onto the surface of the copper board. Using a spatula or spoon, ensure entire contents (or as much as possible) of the mix is dispensed into the casting tray.

23. Degas the mold via partial vacuum and rapid decompression several times in a desiccation/vacuum chamber. A single stage vacuum pump will suffice. Ensure the contents of the casting tray do not bubble over. Repeat the process several times or until little or no bubbles form.

24. Incubate mold at 60C for at least one hour.

25. Allow mold to cool to room temperature.

26. Cut out the chip from the mold with a #11 scalpel (triangular pointed blade) with smooth even pressure as to not rip or tear the chip.

27. Carefully remove the mold from the casting tray and peel off the casted chip from the copper board. Place the chip in a dust free environment (chemical or laminar flow hood ideal) feature side up.

28. Thoroughly clean a 2”x3” microscope slide with dish detergent (NO LOTION OR OILS) and wipe with alcohol spray.

29. Place the molded chip and slide side by side (but not touching) onto a non-conductive surface. We use a plastic cutting board.

30. Using a biopsy punch (2mm for tubing outlet/inlets and 6mm for reservoirs) punch out the required holes into the chip. Ensure any punched holes are free of obstruction.

31. Ensuring gloved hands are not wet and immediate vicinity of the cutting board is free of conductive sources, turn on the corona wand with wide wire attachment to medium strength. DO NOT TOUCH THE CONDUCTOR OR PLASMA WHILE WAND IS ACTIVE.

32. Slowly but deliberately pass the wand across the width of the chip at an elevation close to but not actually touching. Ensure at least 30 seconds is spent on both the chip and the slide. Cover every corner of the chip and angle the wand so the edges of the wide wire adapter is directly above any holes. Allow the oxygen plasma to arc down through the holes so the surface area of the holes are also treated. DO NOT TOUCH THE CONDUCTOR OR PLASMA WHILE WAND IS ACTIVE. DO NOT BREATHE IN THE RESULTING OZONE AS ANY AIRBORNE DUST PARTICLES CHARGED BY THE OZONE MAY STICK TO LUNGS. Changing the hydrophobictiy of the PDMS chip and glass substrate via oxygen plasma treatment.

33. Once oxygen plasma treatment is complete, carefully sandwich the feature side of the PDMS chip and the plasma treated glass slide together and gently apply pressure to remove any air pockets while not collapsing any channels. A collapsed channel looks clearer than the surroundings. If a channel collapses, simply reseat the chip and try again. Do not touch any of the surfaces with or without gloves if possible as to not interfere with the covalent bonding of the glass and PDMS.

34. If time allows, place the chip for one hour in the 60C incubator to complete the bonding.

35. Using the corona wand, treat the surface of the bonded chip paying extra attention to the holes. Using the edges of the wide wire attachment, angle the plasma arc down into the holes such that the plasma flows into the channel and walls of the holes. This ensures extra hydrophilicity at the junction point between holes and tubing. Do not treat holes that are to be reservoirs else the liquid may already begin to flow into the channels from the reservoir.

36. Load any reservoirs on the chip with their respective reagent and then seal the opening with a strip of clear tape. Do not apply pressure to the reservoir once filled so it does not dispense reagent into the channels prematurely.

37. Connect the appropriate tubing and pump accessories to the chip as per the experiment in question.

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