Thompson Tang

Electrokinetic control of fluid flow in native poly(dimethylsiloxane) capillary electrophoresis devices.

Capillary zone electrophoresis (CZE) devices fabricated in poly(dimethylsiloxane) (PDMS) require continuous voltage control of all intersecting channels in the fluidic network in order to avoid catastrophic leakage at the intersections. This contrasts with the behavior of similar flow channel designs fabricated in glass substrates. When the injection plugs are shaped by voltage control and leakage from side channels is controlled by the application of pushback voltages during separation, fluorescein samples give 64 200 theoretical plates (7000 V separation voltage, E = 1340 V/cm). Native PDMS devices exhibit stable retention times (± 8.6% RSD) over a period of five days when filled with water. Contact angles were unchanged (± 1.9% RSD) over a period of 16 weeks of dry storage, in contrast to the known behavior of plasma‐oxidized PDMS surfaces. Electroosmotic flow (EOF) was observed in the direction of the cathode for the buffer systems studied (phosphate, pH 3—10.5), in the presence or absence of hydrophobic ions such as tetrabutylammonium or dodecyl sulfate. Electroosmotic mobilities of 1.49 × 10—5 and 5.84 × 10—4 cm2/Vs were observed on average at pH 3 and 10.5, respectively, the variation strongly suggesting that silica fillers in the polymer dominate the zeta potential of the material. Hydrophobic compounds such as dodecyl sulfate and BODIPY® 493/503 adsorbed strongly to the PDMS, indicating the hydrophobicity of the channel walls is clearly problematic for CZE analysis of hydrophobic analytes. A method to stack multiple channel layers in PDMS is also described.

Optimization of confocal epifluorescence microscopy for microchip-based miniaturized total analysis systems

A confocal epifluorescence detection scheme, optimized to give sub-picomolar detection within planar glass substrates etched to a 30 µm depth, is described. A ×40, 0.6 numerical aperture (N.A.) lens with a 3.7 mm working distance was used to create a focused laser spot about 12 µm in diameter, by under-filling the lens aperture to give an effective, measured N.A. of 0.22 for the laser beam. The sectioning power (optical axis field of view) of various pinholes and the corresponding detector probe volumes (overlap of the excitation and observation volumes) were: (pinhole diameter, sectioning power, probe volume): 100 µm, 18 µm, 0.1 pl; 200 µm, 20 µm, 0.4 pl; 400 µm, 26 µm, 1.7 pl; and 600 µm, 36 µm, 2.4 pl. A log–log plot of fluorescence intensity versus fluorescein concentration, measured in continuous-flow mode using the optimum 400 µm pinhole, showed a correlation coefficient of 0.996 and a slope of 0.85. In this mode, 300 fM fluorescein gave a signal of 34.6 ± 8.1 mV over background with an S/N of 6.1, representing the lowest measured fluorescein dye concentration reported on-chip. Capillary zone electrophoresis of 1 pM fluorescein resulted in a mean S/N of 5.8. The injection plug, estimated to be about 5470 molecules, corresponds to 570 detected molecules on average. The design and use of quick-fit, flangeless fittings for interfacing tubing, fused-silica capillaries or pressurized systems to microfluidic channels etched in planar glass chips is briefly presented.

Design of an interface to allow microfluidic electrophoresis chips to drink from the fire hose of the external environment.

An interface design is presented that facilitates automated sample introduction into an electrokinetic microchip, without perturbing the liquids within the microfluidic device. The design utilizes an interface flow channel with a volume flow resistance that is 0.54—4.1 × 106 times lower than the volume flow resistance of the electrokinetic fluid manifold used for mixing, reaction, separation, and analysis. A channel, 300 μm deep, 1 mm wide and 15—20 mm long, was etched in glass substrates to create the sample introduction channel (SIC) for a manifold of electrokinetic flow channels in the range of 10—13 μm depth and 36—275 μm width. Volume flow rates of up to 1 mL/min were pumped through the SIC without perturbing the solutions within the electrokinetic channel manifold. Calculations support this observation, suggesting a leakage flow to electroosmotic flow ratio of 0.1:1% in the electrokinetic channels, arising from 66—700 μL/min pressure‐driven flow rates in the SIC. Peak heights for capillary electrophoresis separations in the electrokinetic flow manifold showed no dependence on whether the SIC pump was on or off. On‐chip mixing, reaction and separation of anti‐ovalbumin and ovalbumin could be performed with good quantitative results, independent of the SIC pump operation. Reproducibility of injection performance, estimated from peak height variations, ranged from 1.5—4%, depending upon the device design and the sample composition.

Micromachinng chemical and biochemical analysis and reaction systems on glass substrates

Microfluidic systems micromachined in glass chips serve as systems for chemical analysis or sensing. Using electroosmotic pumping, applied voltages control the direction of fluid flow without the need for valves. Mixing of reagent solutions, chemical reactions and separation of compounds in mixtures can be achieved. Demonstration of pre-separation mixing of chemical reagents for reaction on-chip, post-separation fluorescent labelling on-chip, and immunological assays on-chip is presented.

Micromachining Chemical And Biochemical Analysis And Reaction Systems On Glass Substrates

Microfluidic systems micromachined in glass chips serve as systems for chemical analysis or sensing. Using electroosmotic pumping, applied voltages control the direction of fluid flow without the need for valves. Mixing of reagent solutions, chemical reactions and separation of compounds in mixtures can be, achieved. Demonstration of pre-separation mixing of chemical reagents for reaction on-chip, post-separation fluorescent labelling on-chip, and immunological assays on-chip is presented.

An Integrated System for Gene Detection Using Cycling Probe Technology

We have developed an integrated µTAS for DNA analysis that has the potential to be used for clinical diagnostics or for detection of bacterial pathogens in the environment [1]. The system performs on-chip mixing, reaction and separation using capillary electrophoresis (CE). The DNA technology used is a signal amplification technique called Cycling Probe Technology. In this work we demonstrate excellent quantitative data with a methicillin resistant Staphylococcus aureus detection system where earlier baseline drift problems have been eliminated. To test the µTAS for detection of bacteria in the environment, a genomic DNA sample derived from Erwinia herbicola was used.

Automated Microchip Platform for Biochemical Analysis

The development of the components and subsystems of an automated microchipbased platform for bioanalysis by immunoassay and gene probe assay is described. The device uses micromachined glass plates for the fabrication of channel networks and combines electroosmotic pumping and capillary electrophoresis for fluid transport and separation. The complete system enables the on-chip integration of the key elements in analytical processing: injection, mixing, separation, detection and elimination. The analysis platform is being designed for use with an on-line aerosol collector for environmental monitoring.