Microstructured Silicon Research Articles

Controlled Modification of Microstructured Silicon Surfaces for Confinement of Biological Macromolecules and Liquid Crystals

We report new methods of surface modifications for confining and aligning biological macromolecules and liquid crystals on microstructured surfaces. Microcontact printing and polyelectrolyte adsorption were used to pattern and control surface properties of silicon microchannels fabricated by photolithography and etching. We show that the wettability inside and on top of the microstructures can be independently varied by selective deposition of a hydrophobic monolayer using microcontact printing, whereas the surface charge, reactivity, and biocompatibility in the microchannels can be adjusted by adsorbing polyelectrolytes to the surface. A near ideal contrast in surface properties was achieved by microcontact printing on preadsorbed polyelectrolyte layers. Three-dimensional laser scanning confocal microscopy was used to characterize the wetting behavior of biological macromolecules (lipids, DNA, microtubules) confined in the microstructures. DNA molecules in concentrated solutions were observed to orient along the microchannels, as a result of surface confinement, when their contour length approached the width of the microchannels. We demonstrate that the surface microstructures may be used to control the mesoscopic defect structures and defect sizes of liquid crystals by studying the defect structure of 8CB (4‘-n-octyl-4-cyanobiphenyl) as a function of the widths and depths of the microchannels. The order induced due to microchannel confinement of biological molecules has the potential of resulting in unique structure characterization of highly oriented biological macromolecules using synchrotron X-ray microdiffraction methods.

Single bead parallel synthesis and screening.

Microstructured silicon wafers were employed as miniaturized solid-phase reaction vessels as well as miniaturized micro titer plates. Employing piezoelectric drop-on-demand liquid jets, a combinatorial library of 256 Peptides was synthesized on single beads. The synthesis protocol was associated to the location in the silicon nano-well arrangement. Products were photolytically cleaved in the same well that was used for synthesis and subsequently interrogated for thrombin inhibition in a homogeneous competition assay. The assay procedure was based on drop-on-demand liquid delivery and laser induced fluorescence imaging. The novel format proved useful for the integration of both synthesis and screening into one platform, a prerequisite for an iterative, evolutionary approach towards drug discovery.

Multi-electrode substrate for selectivity enhancement in air monitoring

A new microstructured silicon substrate for thin gas sensitive layers has been developed composing a thermoresistor at the chip surface and a buried polysilicon heater. The process flow of the largely MOS-compatible fabrication is described. The two pairs of planarized interdigitated electrodes with both a width and spacing of 2 microns were manufactured by a special lift-off process. A second interdigitated electrode is placed above the sensing tin oxide layer and can also serve as a measuring electrode. As a further possibility, we suggest the application of a transverse electric field to the sensing layer by using the upper interdigitated electrode as a gate electrode and the heater as a counter electrode. This additional electric field can influence the effective signal values caused by desorption and adsorption of gas specimen thus enhancing the selectivity of resistive gas sensors. In order to detect oxidizing gases like nitrogen oxides with a metal oxide layer we have developed a reactive sputter process for tin oxide with a homogenous unstoichiometry across the complete grains. Measuring the d.c. resistance of these strongly reduced layers at relatively low temperatures of about 175°C we have achieved a sensitivity in the sub-ppm range with regard to nitrogen dioxide in dry air.