Publications

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Single-cell analysis offers a promising method of studying cellular functions including investigation of mechanisms of host-pathogen interaction. We are developing a microfluidic platform that integrates single-cell capture along with an optimized interface for high-resolution fluorescence microscopy. The goal is to monitor, using fluorescent reporter constructs and labeled antibodies, the early events in signal transduction in innate immunity pathways of macrophages and other immune cells. The work presented discusses the development of the single-cell capture device, the iCellator chip, that isolates, captures, and exposes cells to pathogenic insults. We have successfully monitored the translocation of NF-κB, a transcription factor, from the cytoplasm to the nucleus after lipopolysaccharide (LPS) stimulation of RAW264.7 macrophages.

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Accurate, periodic calibration is required to operate IMS, GC, and portal security systems with maximum efficiency, surety, and operator confidence. To this end, we are presently developing a microfabricated device for on-demand calibration of fleldable contraband detection instruments. Using robotically-assisted picoliter dispensing methods, precise nanogram or larger amounts of calibration compound(s) are placed on micron-sized bridge structures. Resistively heating these structures delivers precisely quantified low-levels of calibration compounds to the detector systems. This provides reliable calibration for identification and quantification of explosives, narcotics, and other contraband. © 2007 IEEE.

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Here, self-assembled monolayers (SAMS) have been investigated for their ability to confine the absorption of the motor protein kinesin and direct the movement of microtubule shuttles (MTs) within channels of a lithographically patterned microfluidic device. Channels were made from gold films deposited on a silicon wafer to provide chemically distinct surfaces for the selective formation of a range of alkane thiol monolayers on channel walls. Devices were then exposed to solutions containing casein and kinesin to develop protein monolayers capable of propelling microtubules in the presence of adenosine triphosphate (ATP) fuel. Fluorescence microscopy images were used to observe the attachment of MTs to chemically distinct regions and to evaluate the ability of the various monolayer coatings to confine the movement of MTs within the channel system. Ellipsometry was used to characterize the protein adsorption characteristics of SAMS terminated with different functional groups to help establish confinement mechanisms. Finally, both anti-fouling and cationic monolayers were found to be effective in confining MT movement within the channels by controlling the adsorption or orientation of the casein buffer layers that mediate motor protein attachment and functionality.

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A microflame-based detector suit has been developed for sensing of a broad range of chemical analytes. This detector combines calorimetry, flame ionization detection (FID), nitrogen-phosphorous detection (NPD) and flame photometric detection (FPD) modes into one convenient platform based on a microcombustor. The microcombustor consists in a micromachined microhotplate with a catalyst or low-work function material added to its surface. For the NPD mode a low work function material selectively ionizes chemical analytes; for all other modes a supported catalyst such as platinum/alumina is used. The microcombustor design permits rapid, efficient heating of the deposited film at low power. To perform calorimetric detection of analytes, the change in power required to maintain the resistive microhotplate heater at a constant temperature is measured. For FID and NPD modes, electrodes are placed around the microcombustor flame zone and an electrometer circuit measures the production of ions. For FPD, the flame zone is optically interrogated to search for light emission indicative of deexcitation of flame-produced analyte compounds. The calorimetric and FID modes respond generally to all hydrocarbons, while sulfur compounds only alarm in the calorimetric mode, providing speciation. The NPD mode provides 10,000:1 selectivity of nitrogen and phosphorous compounds over hydrocarbons. The FPD can distinguish between sulfur and phosphorous compounds. Importantly all detection modes can be established on one convenient microcombustor platform, in fact the calorimetric, FID and FPD modes can be achieved simultaneously on only one microcombustor. Therefore, it is possible to make a very universal chemical detector array with as little as two microcombustor elements. A demonstration of the performance of the microcombustor in each of the detection modes is provided herein.

The natural gas industry seeks inexpensive sensors and instrumentation to rapidly measure gas heating value in widely distributed locations. For gas pipelines, this will improve gas quality during transfer and blending, and will expedite accurate financial accounting. Industrial endusers will benefit through continuous feedback of physical gas properties to improve combustion efficiency during use. To meet this need, Sandia has developed a natural gas heating value monitoring instrument using existing and modified microfabricated components. The instrument consists of a silicon micro-fabricated gas chromatography column in conjunction with a catalytic micro-calorimeter sensor. A reference thermal conductivity sensor provides diagnostics and surety. This combination allows for continuous calorimetric determination with a 1 minute analysis time and 1.5 minute cycle time using air as a carrier gas. This system will find application at remote natural gas mining stations, pipeline switching and metering stations, turbine generators, and other industrial user sites. Microfabrication techniques will allow the analytical components to be manufactured in production quantities at a low per-unit cost.

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This work describes the design, computational prototyping, fabrication, and characterization of a microfabricated thermal conductivity detector ({mu}TCD) to analyze the effluent from a micro-gas chromatograph column ({mu}GC) and to complement the detection efficacy of a surface acoustic wave detector in the micro-ChemLab{trademark} system. To maximize the detection sensitivity, we designed a four-filament Wheatstone bridge circuit where the resistors are suspended by a thin silicon nitride membrane in pyramidal or trapezoidal shaped flow cells. The geometry optimization was carried out by simulation of the heat transfer in the devices, utilizing a boundary element algorithm. Within microfabrication constraints, we determined and fabricated nine sensitivity-optimized geometries of the {mu}TCD. The nine optimal geometries were tested with two different flow patterns. We demonstrated that the perpendicular flow, where the gas directly impinged upon the membrane, yielded a sensitivity that is three times greater than the parallel flow, where the gas passed over the membrane. The functionality of the {mu}TCD was validated with the theoretical prediction and showed a consistent linear response to effluent concentrations, with a detection sensitivity of 1 ppm, utilizing less than 1 W of power.