Low-contact-angle polydimethyl siloxane (PDMS) membranes for fabricating micro-bioarrays

In efforts to design improved biological microarrays, we use a polydimethyl siloxane (PDMS) membrane as a template that allows biological solutions of interest to interact with the surface only at discrete locations (the array sites). In current techniques, in which solutions are "spotted" on an unpatterned surface, cross-contamination and limited feature size (>50μm) cause limitations in the information density available on the microarray. Because of these issues and the need for mechanical loading in non-DNA applications (enzymes, other proteins, cells, polysaccharides, catalysts), in which other methods are not appropriate, microarrays with superior information density on a variety of substrates are desirable. PDMS membranes provide the means for doing so. The membrane can be applied to any surface without a chemical patterning methodology. When a surface with a patterned membrane affixed to it is exposed to an aqueous solution, the biological species in the solution attach only to the specific regions of the surface defined by the holes in the membrane. However, the high hydrophobicity of PDMS membranes becomes an obstacle when loading aqueous solutions into very small openings. The fluid is rejected even though the array elements are hydrophilic. Exposure of the PDMS membrane to an oxygen plasma, a common practice, provides only a temporary reduction in contact angle. To produce a permanent change, we have used a novel RF-plasma process, in which O₂-plasma-treated PDMS surfaces are exposed to a second plasma, which generates functionalities on the PDMS surfaces that are converted in the presence of moisture into low-mobility, large, -Si(OH)_x groups. This process stabilizes the PDMS surface and produces a permanent reduction in contact angle. Tuneability in the con tact angle is possible by varying the process. We present XPS, FTIR, contact angle, and surface roughness measurements to support these conclusions. The process we describe for permanent modification of the wettability of PDMS can easily be extended to other applications of PDMS and to other polymeric substrates