Micrometer-scaled gradient surfaces generated using contact printing of octadecyltrichlorosilane

Gradient surfaces are widely employed in biological studies for protein adsorption and cell attachment and growth. They also offer great potential in the areas of fluid flow and combinatorial experimental design for obtaining material properties and behaviors. Gradient surfaces are created with organosilanes by using the diffusion techniques proposed by Ewling and modified by Chaudhury. However, these techniques have limitations. They either generate a significant amount of organic waste or require well-controlled deposition conditions. In this paper, we propose a fast, convenient, reproducible, and inexpensive method that generates gradient surfaces with minimum waste generation. Particularly, we have adopted Whitesides's contact-printing technique to achieve a gradient by gradually varying the contact time over the contacted area using octadecyltrichlorosilane. Elastomeric stamps with different geometries and various radii of curvature are used to generate gradient surfaces at both millimeter and micrometer scales. With this approach, we are able to generate micrometer-scaled gradient surfaces with a gradient steepness 1-3 orders of magnitude higher than those generated using diffusion-based techniques. The energy gradient on these surfaces is verified by the dewetting of polymer (e.g., polystyrene) thin films and by the movement of picoliter and nanoliter water droplets on the surface devoid of other driving forces such as gravity, temperature gradients, or a pressure drop.