Selective surface modification in silicon microfluidic channels for micromanipulation of biological macromolecules

Interactions between biological macromolecules and micrometer- and sub-micrometer-scale surface structures are directly influenced by the surface wettability, chemical reactivity and surface charge. Understanding these interactions is crucial for developing integrated microsystems for biological and biomedical processing and analysis. We report development of selective surface modification techniques based on microcontact printing and polyelectrolyte adsorption. These techniques were applied to lithographically patterned silicon microfluidic channels and flat silicon substrates to create surface microstructures with contrasting wetting properties and surface charges. These controls enabled us to devise various techniques for controlled loading and processing of biomaterials in the channels. Solutions containing long chain biological macromolecules DNA and microtubules were directly loaded into the microchannels by using a micromanipulator/microinjector system. Structural arrangements of these linear macromolecules, which were probed by using fluorescence and laser scanning confocal microscopy, were found to be quite different from bulk solutions. As expected, the filamentous molecules were observed to align linearly along the channels, with the degree of alignment dependent on channel width as well as the length of the molecule. This molecular alignment, which is induced by both the surface confinement effect and capillary flow during sample loading, may be used to enhance processing of biological materials in silicon biomedical microdevices. It also opens up the possibility of carrying out direct combinatorial structural characterization of proteins in the microchannels utilizing X-ray diffraction, which so far has not been possible.