Classical Soft Lithography
Several techniques to produce microfluidic devices exist (photolithography, etching in silicon and glass...). We chose to use devices made of PDMS because it is optically transparent and malleable, it can seal easily and irreversibly (if exposed to plasma treatment) to itself or to glass. The fabrication of microfluidic devices in Poly(Dimethylsiloxane) (PDMS) is based on the techniques of soft lithography, and replica molding (1) (Figure. B). We first create the desired design with a computer-aided design software (Clewin for instance). The desired design is then printed on a transparency (for “large” features between 10 and 200 μm) or engraved in a chrome mask (for small features below 10 μm) by a high-resolution commercial image setter. This transparency (or chrome mask) serves as the photomask in contact photolithography.
The soft lithography step consists in producing a positive relief of photoresist (SU-8) on a silicon wafer. The speed of the spin-coater sets the thickness of this layer of photoresist, controlling the height of the channels. Then, we pre-bake the wafer for several minutes at 65oC and 95oC (the exact time depends on the brand of the photoresist and the desired thickness) in order to initiate the polymerization of the SU-8. A UV source is used to expose the silicon wafer which is covered with the photomask. Another baking step finishes to cure the photoresist. Dissolving away the non illuminated - and so the non polymerized - photoresist leaves a positive relief that serves as a master.
The PDMS channels are formed by replica molding (ridges on the master appear as valleys in the replica). We mix a solution of silicone elastomer with a curating agent (Sylgard 184 kit silicone elastomer, Dow Corning) in a ratio 9/1 and homogenize the mixture. After pouring the solution into a petri dish over the master, we degas under a vacuum in order to get ride of any bubbles. The whole preparation is cured in an oven at 65°C for 1 hour. The replica is then peeled from the master and access holes for the channels are punched out of the cured layer by using a truncated needle.
The PDMS device can be sealed to a cover glass or to another blank piece of PDMS (1). To do so, the surfaces of the two units (PDMS and PDMS/glass) are activated by a plasma treatment during 90 seconds in a plasma cleaner (Plasma cleaner PDC-32G, Harrick plasma). Immediately after the treatment, the two pieces are put in contact to let them stick together. The device is then stored at 65°C over night to allow the strengthening of bonding between the two blocks.
(1) MacDonald, et al., Fabrication of microfluidic systems in poly(dimethylsiloxane, 21, 27-44 (2000)