TITLE: Microfluidic Digital Logic Chip Assembly
AUTHORS: Erik Werner, Elliot Hui [University Of California

[DESCRIPTION]
This protocol describes how to reversibly bond layers of poly(methyl methacrylate) (PMMA) and polydimethylsiloxane (PDMS) using thermo-compresison bonding. Usually, two layers of PMMA are sandwiched around a center layer of PDMS. While compressed, all materials are heated to the softening temperature of the PMMA. They are then allowed to slowly cool to room temperature before removing from compression. The result should produce a bubble-free lamination between both PMMA-PDMS interfaces without melting the PMMA or significantly distorting micro-scale features. The times and temperatures in the SOP may be adjusted to accommodate different thicknesses or types of materials where a similar lamination-effect is desired.

[GUIDELINES]
To ensure success:

The PMMA layers should be thicker than the middle PDMS layer (or at least substantially thicker than the thickest air pocket between the layers)
The thicker material (PMMA in this case) must have a lower softening temperature
The PMMA must be allowed to heat uniformly (see note)
The device must remain compressed until it is cooled completely (see note)
Uniform pressure should be applied to both sides of the device
Holes and pockets in the PMMA should be greater than 100 um


Note: Minimum hold times for heating and cooling will vary depending on the type of oven used. For a standard lab without forced air circulation, use 2 hours for 3mm of material thickness, and add 1 hour per additional mm. See the attached guidelines from plexiglass for more information.   


[STEPS]
SECTION: Clean
1. Rinse all parts to be assembled under a stream of DI water to remove charred material or debris from machining.
SECTION: Clean
3. Prepare a solution of ~1% Microsoap in DI water and fill each bag about half way
 

SECTION: Clean
2. Insert each component of the Microfluidic device into a 3" x 4" zip loc bag. 
 

SECTION: Clean
4. Sonicate the bags for 20 minutes

 

SECTION: Clean
5. Remove the parts from each bag, rinse thoroughly with DI water, and dry with compressed nitrogen. Transfer to a Petri dish to keep clean from dust.

SECTION: Assemble
6. In a clean work environment, insert four tapered dowel pins into a PMMA piece of the device. Let this side be the "bottom" layer. Ensure the small end of the pin faces the side with the device features that will be bonded.


 


 

SECTION: Assemble
7. Orient the chip so the features face up and inspect the surface of the device to ensure it is free from dust and debris. Remove dust with compressed air or lightly touching with Scotch MagicTape.
 

 


 

SECTION: Assemble
8. Orient the PDMS membrane over the prepared PMMA layer, and peel away the PET layer protecting the side that contacts the PMMA.
SECTION: Assemble
9. Clean the exposed PDMS with  MagicTape
 

SECTION: Assemble
10. Using the tapered dowel pins, lower the PDMS onto the PMMA chip. When the material get close, press in the middle of the PDMS to start the adhesion process, then allow the materials to adhere from the middle outwards. 
SECTION: Assemble
12. Remove one of the taper pins and crease the PDMS/PET at that corner. Peel off the top layer of PET.
 

 

SECTION: Assemble
11. If any bubbles remain under the PDMS, inspect for dust. Bubbles may be a result of the PDMS wrinkling over the PMMA or a spot of dust. Where there is dust, peel back the PDMS and blot with MagicTape to remove.
SECTION: Assemble
13. Clean the exposed PDMS with MagicTape
 

SECTION: Assemble
15. Reinsert the taper pins into the bottom PMMA /PDMS
SECTION: Assemble
14. Inspect clean the top layer of PMMA using compressed air and MagicTape
SECTION: Assemble
16. Lower the top layer, feature side down, over the taper pins onto the lower PMMA layer and PDMS.
SECTION: Assemble
17. When the materials are close, press firmly from the center outwards to adhere all layers together.
SECTION: Assemble
18. Remove the taper pins
SECTION: Assemble
19. Inspect the device to ensure features are aligned and there are no large air pockets. Small air pockets will smooth and disappear with annealing, wrinkles in the PDMS membrane will not.
SECTION: Compress
20. Clean the outside of the device with compressed air and MagicTape
 

SECTION: Compress
21. Prepare two more pieces of PET/PDMS/PET "bumper" slightly larger than the size of the device (at least 30mm extra length and width)
 

SECTION: Compress
22. Crease one corner and peel away one layer of the PET from one of the bumpers
 

SECTION: Compress
23. Place the completed chip onto the exposed PDMS

SECTION: Compress
24. Peel one layer off PET from the other PET/PDMS/PET bumper and repeat for the other side of the chip
SECTION: Compress
26. Sandwich the entire chip and bumpers between glass slides, ensuring the that entire device is between both slides. Parts that are not covered will not be compressed.
 

 


SECTION: Compress
27. Clamp with binder clips on all four sides.
SECTION: Compress
25. Trim away any excess PDMS/PET bumper material
 

SECTION: Anneal
28. Transfer to an oven, setting the device vertically in the middle of the oven.
SECTION: Anneal
29. Heat to 110ºC and maintain temperature for long enough to heat the entire device thoroughly.
 


SECTION: Anneal
30. Set the oven to room temperature and allow to cool completely (preferably over night).
SECTION: Anneal
31. Remove the binder clips, class slides, and peel away the outer PET/PDMS bumpers. The device is now ready to use.