Fabrication of nanoporous membranes for tuning microbial interactions and biochemical reactions

New strategies for combining conventional photo- and soft-lithographic techniques with high-resolution patterning and etching strategies are needed in order to produce multiscale fluidic platforms that address the full range of functional scales seen in complex biological and chemical systems. The smallest resolution required for an application often dictates the fabrication method used. Micromachining and micropowder blasting yield higher throughput, but lack the resolution needed to fully address biological and chemical systems at the cellular and molecular scales. In contrast, techniques such as electron beam lithography or nanoimprinting allow nanoscale resolution, but are traditionally considered costly and slow. Other techniques such as photolithography or soft lithography have characteristics between these extremes. Combining these techniques to fabricate multiscale or hybrid fluidics allows fundamental biological and chemical questions to be answered. In this study, a combination of photolithography and electron beam lithography are used to produce two multiscale fluidic devices that incorporate porous membranes into complex fluidic networks in order to control the flow of energy, information, and materials in chemical form. In the first device, materials and energy were used to support chemical reactions. A nanoporous membrane fabricated with e-beam lithography separates two parallel, serpentine channels. Photolithography was used to pattern microfluidic channels around the membrane. The pores were written at 150-nm and reduced in size with silicon dioxide deposition from plasma enhanced chemical vapor deposition and atomic layer deposition. Using this method, the molecular weight cutoff of the membrane can be adapted to the system of interest. In the second approach, photolithography was used to fabricate 200-nm thin pores. The pores confined microbes and allowed energy replenishment from a media perfusion channel. The same device can be used for study of intercellular communication via the secretion and uptake of signal molecules. Pore size was tested with 750-nm fluorescent polystyrene beads and fluorescein dye. The 200-nm polydimethylsiloxane pores were shown to be robust enough to hold 750-nm beads while under pressure, but allow fluorescein to diffuse across the barrier. Further testing showed that extended culture of bacteria within the chambers was possible. These two examples show how lithographically defined porous membranes can be adapted to two unique situations and used to tune the flow of chemical energy, materials, and information within a microfluidic network.