Making a PDMS Microfluidic Device With Maskless Molds

With Maskless molds, you can now make functional microfluidic devices with a few small pieces of lab equipment. This instructable is a demonstration of the steps for using Maskless molds. The mold used in the photos is for a simple mixer, but you can design any microfluidic device you would like from Maskless.co.

## Supplies:

1. safety glasses
2. gloves
3. paper towels
4. 1 mL syringes (optional)
5. flat-tipped tweezers
6. small plastic cup
7. plastic stirring spatula (or popsicle stick)
8. Dow Corning Sylgard 184 polydimethylsiloxane (PDMS)
9. 75 mm x 50 mm glass slide (optional)
10. plastic tubing correctly sized for your design
11. flat-tipped needle of appropriate gauge for tubing outer diameter (to punch holes)
12. Al foil or plastic Petri dish
13. flat weight (such as piece of metal or glass)
14. analytical balance
15. vacuum chamber (such as small vacuum pump and plastic dessicator)
16. warm surface (such as hot plate or some other heater plate)
17. plasma treater (such as hand-held corona treater)
18. for this demo: food coloring, syringes and flat tipped needles of appropriate gauge for tubing INNER diameter to pump liquid into device (or external pump)

Remove Maskless mold from packaging. Maskless molds are printed onto a glass slide. This glass can be easily removed or kept for convenient handling. If the mold falls off of the glass, it can be taped back on with double-sided tape or arranged and taped onto a different substrate, if desired. Place mold in a petri dish or aluminum foil dish to prevent spilling of PDMS throughout the process below.

Optionally, you can rinse the mold with water and/or isopropyl alcohol (IPA) and dry in an oven at 80 ºC for 30 to 60 minutes prior to use. Allow mold to return to room temperature.

Prepare PDMS precursor according to desired protocol. For example, Sylgard 184 may be used in a 10:1 ratio by weight and mixed thoroughly. It is essential to thoroughly mix the PDMS precursor: mix with a plastic spatula or popsicle stick for at least 2 minutes. Make sure that the container you have weighed the PDMS into has a flat bottom so that you don't miss any PDMS precursor while mixing.

Degass the PDMS precursor under vacuum until all visible bubbles disappear. It is advisable to put a piece of aluminum foil or other material beneath the PDMS in case of spills. Keep an eye on the PDMS in the vacuum chamber until the bubbles have maximized their expansion. If the PDMS starts bubbling over, turn the vacuum off and/or vent until the bubbles collapse.

For Maskless molds with walls: pour or inject the PDMS precursor into the mold. The volume of PDMS precursor may be measured in order to keep the thickness of the device consistent: use up to 1 mL for the small sized molds and up to 4 mL for the medium sized molds. For Maskless molds without walls, place mold in a plastic petri dish and pour PDMS on top of the mold.

Degass the PDMS for a second time in the mold.

Cure the PDMS in the mold. Oven curing at 80 ºC for 15 – 40 minutes is recommended. A low cost alternative is placing the mold on a heating plate until the PDMS is cured (it will no longer be liquid or tacky to touch). Remove the mold from heat and allow to cool to room temperature.

Use a flat tweezer or other small, flat edge to scrape PDMS from mold on one end of the mold. Use the tweezers to pull the PDMS out of the mold.

The Maskless mold can be reused without additional treatment. If needed, the mold can be rinsed with water or isopropyl alcohol and dried at 80 ºC, taking care not to scratch the fine features in the mold.

Prepare holes for inlets to the device. A dedicated microfluidics puncher may be used. A low-cost alternative is punching holes through the PDMS replica at the inlet and outlet locations using a blunt, flat-tipped needle and discarding the punched out PDMS. Select the needle gauge for the appropriate tubing you plan to insert. For instance, a 17 gauge needle has an outer diameter of 1.473 mm, which should fit with 1/16" or 1.58 mm outer diameter (OD) tubing. If there is dust or dirt on the PDMS, it may be removed with Scotch tape. It is recommended that steps 4, 5, and 6 be performed in quick sussession to minimize dirt buildup on the device before it is sealed up. Avoid touching the replica surface.

PDMS must be made hydrophilic in order to be bonded to glass and for acqueous solution to flow through its channels. This can be accomplished by applying an oxygen plasma via Reactive Ion Etching (RIE). If RIE is not available, a corona treater may be used to treat the bonding surfaces of a glass slide and the PDMS replica (the side of the PDMS with the microfluidic features). 20 - 60 seconds of exposure using a Electro-Technic BD-20AC handheld corona treater with the field effect electrode should suffice. Perform this step in a well ventilated area, as the corona treater emits a small amount of ozone.

You can test the impact of hydrophilic treatment by placing a drop of water on the untreated and treated PDMS. Before treatment, water will bead up on the surface of PDMS. After plasma treatment, the water droplet will spread out over the PDMS surface. If desired, perform this test on a scrap piece of PDMS, not the actual device to be bonded.

As soon as the hydrophilic treatment has been completed, place the plasma-treated sides of the glass and PDMS together and press lightly. The PDMS should adhere to the glass immediately. To improve the adhesion further, place a flat weight (such as a piece of sheet metal or glass) on top of the PDMS and return to the oven or hot plate for 1 to several hours, even overnight.

Insert tubing into the inlets and outlets of the device and hook up to any necessary external equipment. The picture below shows a simple microfluidic channel with two colored liquids being mixed.