Layered microfluidic devices integrated with porous polycarbonate or polyester membranes have been widely used for mass transport control, immunoassays, and blood cell sorting. The placement of a semi-porous membrane at the interface of two channel layers is crucial to minimize unwanted crossover of fluid flows between microchannels while allowing diffusive mixing of reagents. Several methods have been reported to seal off the crevices inevitably generated because of the thickness of the membrane. For example, the application of PDMS pre-polymer as a mortar layer could prevent the leakage along the membrane. This method provides robust and reliable bonding between two PDMS layers. However, in the case of thicker membranes and/or narrower channels, the mortar layer method can clog the channels easily.
We introduce an alternative strategy of directly using PDMS as a porous membrane itself to fabricate monolithic microfluidic devices. In this case, the integration of a porous PDMS membrane can be completed without clogging microchannels by plasma oxidation or by utilizing different mixing ratios for curing PDMS polymers. To prepare porous PDMS membranes, SU-8 posts with different sizes are created on a silicon wafer. A thin film of PDMS is prepared by spin coating the wafer. The resulting thin PDMS with holes is lifted cleanly off the wafer using a specially designed cured PDMS frame. This method allows varying sizes of pores on a single membrane, compared to commercially available porous membranes with a fixed pore size. We demonstrate the use of this method to fabricate a particle sorter where different sizes of microbeads can be filtered through different sizes of pores of the membranes.
For more information on how we make microfluidic devices in the Zarelab, please see our guide (PPT or PDF).
Two different sizes of pores (10 and 35 Ám) in a PDMS membrane embedded into a microfluidic device