Vapor-assisted micropatterning in replica structures: A solventless approach towards topologically and chemically designable surfaces

Methods for creating microor nanostructured surfaces are of fundamental importance to a range of applications including electronics and biotechnology. For instance, bioanalytical diagnostic devices, such as DNA or protein arrays, artificial substrates to mitigate cell-growth regulation, or next-generation micro/nanofluidic systems will require precisely structured surfaces. It can be expected that further progress in these and other areas of biotechnology will continue to fuel the need for simple and scalable surface structuring methods that result in both, topologically and chemically defined surfaces. Herein, we report on a simple method for fabricating both, chemically and topologically designable surfaces based on chemical vapor deposition polymerization via softlithographic stencils or replica structures. Over the past decade, extensive efforts have been made to create topological surface patterns using either lithographical methods on the basis of light, electron and ion beams, X-rays, or atoms as well as printing methods, such as dip/ pen lithography, patterning via scanning probes, imprinting lithographies, or soft lithography. The latter comprises an arsenal of methods, such as microcontact printing, replica moulding, microtransfer moulding, micromoulding in capillaries, solvent-assisted micromoulding, capillary force lithography, which all rely on the use of elastomeric stamps or replica structures to transfer material from a solution onto a surface. Patterned substrates created using shadow masks included a range of different materials, such as semiconductors, organic metals, polymers, biomaterials or cells. During the last few years, we have developed a generically applicable surface modification approach based on chemical vapor deposition (CVD) polymerization, which relies on the deposition of reactive coatings made of functionalized poly-p-xylylenes. These coatings have been deposited on a range of different substrate materials and geometries, including the luminal surfaces of microchannels. Once reactive coatings with appropriate chemical groups are deposited on a substrate, microcontact printing or photolithography can be used for surface structuring. Reactive coatings have been useful for spatially defined immobilization of biomolecules, planar cell and protein patterning, and for patterning of polymer brushes. On the basis of our preliminary results as well as results obtained by others, we hypothesize that chemical and topological surface microstructures can be obtained by masking certain areas of the substrate during chemical vapor deposition polymerization and then depositing the reactive coatings only within the exposed areas. Such an approach towards microstructures surfaces, although of intriguing conceptual simplicity, comes with a series of challenges: i) In CVD polymerization, the deposition of polymers is transport-limited and it is unclear whether the polymer can be deposited within replica structures with micrometer-scale capillaries. ii) Even though polymer films may be deposited with sufficient homogeneity, reactive groups may not be freely accessible at the interface for further surface modification. iii) The range of available microstructures may be limited to discontinuous, low-coverage features obtainable by conventional shadow masking approaches. iv) Even in these low coverage patterns, the deposition of discontinuous polymer films could interfere with substrate encapsulation by the CVD-deposited polymer film thereby negatively affecting the adhesion between reactive coating and substrate. v) Potential defects during polymer deposition or lift-off may further contribute to decreased pattern fidelity. Throughout this study, we have focused on a specific reactive coating, poly(4-pentafluoropropionyl-p-xylylene-co-p-xylylene) 3 deposited via chemical vapor deposition (CVD) polymerization to proof the concept of vapor-assisted micropatterning in replica structures (VAMPIR, Scheme 1). However, the same principles should hold for other previously established functionalized poly-p-xylylenes containing a wide range of different functional groups, such as amines, aldehydes, anhydrides, active esters, or alkynes. Prior to CVD polymerization, polydimethylsiloxane (PDMS) based replica structures or stencils designed to generate a desired surface pattern were reversibly sealed onto the substrate. The masked substrate was then placed onto a temperature-conC O M M U N IC A IO N