Soft and rigid two-level microfluidic networks for patterning surfaces

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We describe the microfabrication and use of elastomeric and rigid two-level microfluidic networks (μFNs), made of poly(dimethylsiloxane) (PDMS) or Si, for patterning surfaces. The first level corresponds to microchannels and the second to via holes through the μFNs serving as filling and venting ports. μFNs in PDMS are manufactured using a 'sandwich' replication from a microfabricated four inch mold structured with SU-8 photoresist, which is planarized by mechanical polishing. μFNs in Si are microfabricated using deep reactive ion etching. Both types of μFNs can be positioned onto a substrate, creating sealed microchannels, filled with different liquids, flushed, removed and reused. These two-level μFNs allow us to access the ports from the rear, minimize interchannel crosstalk, and are economic of solutions. The channels are made wettable so that the liquids can flow spontaneously into the conduits, but stop at the venting ports. The sealing of the conduits usually requires that either the μFN or the substrate be soft. A strategy for using hard two-level μFNs, in Si, for patterning hard substrates is presented: despite voids in-between the μFN and the substrate, a water-based solution can be guided by hydrophilic microchannels over a hydrophobic surface. Adjusting the wetting properties of the various surfaces is key to preventing undesired spreading of solutions. We illustrate our concepts by micromolding colored photocurable polymers on glass and patterning proteins as lines on a polystyrene surface.