Electrokinetic Pumping Research Articles

Miniature flow-injection analysis manifold created by micromilling

The miniature flow injection analysis (μFIA) system is based on a mechanically engraved manifold in a PMMA-substrate. The channels have a trapezoidal cross-section with a depth of 150 μm and a width between 180 and 360 μm and the reactor volume is 3.75 μl. A light-emitting diode (LED) is used as radiation source for absorbance detection and optical fibres are used for transmitting light to and from the manifold. Computer controlled valves are used for directing the liquid flows maintained with syringe pumps. A sampling rate of approximately five determinations per minute was achieved with a reagent consumption of 10 μl min −1. Four applications were implemented and tested; the performance was found to be comparable to that obtained with conventional FIA-set-ups despite the drastically reduced optical pathlength. Electrokinetic pumping was extensively evaluated but found to generally be very restricted in scope because the reagent solutions cannot usually be adapted to the requirements for generating the flow.

Bubble-free electrokinetic pumping

Bubble-free electroosmotic flow (fr-EOF) of aqueous electrolytes in microfluidic channels with integrated electrodes is demonstrated. Undesirable electrolytic bubble formation is avoided by applying a periodic, zero net charge current to generate a nonzero average potential between the electrodes. Electrokinetic pressure generated in this active segment of the microchannel drives now upstream and downstream where electric field is absent. Flow rates commensurate with theoretical predictions for EOF driven by a dc voltage equivalent to the average net potential have been measured. By significantly reducing driving potentials and liquid exposure time to strong electric fields, fr-EOF opens the way for fully integrated, versatile micro total analysis systems (/spl mu/TAS).

High-pressure microfluidic control in lab-on-a-chip devices using mobile polymer monoliths.

We have developed a nonstick polymer formulation for creating moving parts inside of microfluidic channels and have applied the technique to create piston-based devices that overcome several microfluidic flow control challenges. The parts were created bycompletely filling the channels of a glass microfluidic chip with the monomer/ solvent/initiator components of a nonstick photopolymer and then selectively exposing the chip to UV light in order to define mobile pistons (or other quasi-two-dimensional shapes) inside the channels. Stops defined in the substrate prevent the part from flushing out of the device but also provide sealing surfaces so that valves and other flow control devices are possible. Sealing against pressures greater than 30 MPa (4,500 psi) and actuation times less than 33 ms are observed. An on-chip check valve, a diverter valve, and a 10-nL pipet are demonstrated. This valving technology, coupled with high-pressure electrokinetic pumps, should make it possible to create a completely integrated HPLC system on a chip.

Microfluidic chips for clinical and forensic analysis.

This review gives an overview of developments in the field of microchip analysis for clinical diagnostic and forensic applications. The approach chosen to review the literature is different from that in most microchip reviews to date, in that the information is presented in terms of analytes tested rather than microchip method. Analyte categories for which examples are presented include (i) drugs (quality control, seizures) and explosives residues, (ii) drugs and endogenous small molecules and ions in biofluids, (iii) proteins and peptides, and (iv) analysis of nucleic acids and oligonucleotides. Few cases of microchip analysis of physiological samples or other "real-world" matrices were found. However, many of the examples presented have potential application for these samples, especially with ongoing parallel developments involving integration of sample pretreatment onto chips and the use of fluid propulsion mechanisms other than electrokinetic pumping.

Fabrication of microfluidic devices for AC electrokinetic fluid pumping

Two techniques for the fabrication of novel microfluidic devices for electrokinetic fluid pumping are presented. Both consist of forming a micro-channel and reservoirs on a transparent cover sheet and patterning an array of interdigitated asymmetric micro-electrodes on a flat substrate. The two techniques differ from each other in their building materials as well as the pattern replication technique used for the cover sheet fabrication. In the first approach, we used a glass substrate for the electrode fabrication and casting of an elastomer for the cover sheet. In the second approach, both cover sheet and substrate are obtained by imprinting plastic pellets on a pre-patterned or flat mold. In assembled devices, pumping has been demonstrated and characterized by optical microscopy. The observed dependence of the pumping velocity on applied voltage and frequency are in line with theoretical predictions.

Integrated microfluidic electrophoresis system for analysis of genetic materials using signal amplification methods.

An isothermal signal amplification technique for specific DNA sequences, known as cycling probe technology (CPT), was performed within a microfluidic chip. The presence of DNA from methicillin-resistant Staphylococcus aureus was determined by signal amplification of a specific DNA sequence. The microfluidic device consisted of four channels intersecting to mix the sample and reagents within 55 s, as they were directed toward the reactor coil by electrokinetic pumping. The 160-nL CPT reactor occupied approximately 220 mm2. Gel-free capillary electrophoresis separation of the biotin- and fluorescein-labeled probe from the probe fragments was performed on-chip following the on-chip reaction. An off-chip CPT reaction, with on-chip separation gave a detection limit of 2 fM (0.03 amol) target DNA and an amplification factor of 85,000. Calibration curves, linear at <5% probe fragmentation, obeyed a power law relationship with an argument of 0.5 [target] at higher target DNA concentrations for both on-chip and off-chip CPT reaction and analysis. An amplification factor of 42,000 at 250 fM target (25,000 target molecules) was observed on-chip, but the reaction was approximately 4 times less sensitive than off-chip under the conditions used. Relative SD values for on-chip CPT were 0.8% for the peak migration times, 9% for the area of intact probe peak, and 8% for the fragment/probe peak area ratio.

NUMERICAL SIMULATION OF MIXED ELECTROOSMOTIC/PRESSURE DRIVEN MICROFLOWS

A spectral element algorithm is developed to analyze mixed electroosmotic/pressure driven flows in complex two-dimensional geometries. The new algorithm exhibits spectral accuracy in resolving thin electric double layers. Mixed electroosmotic/pressure driven flows are simulated in straight channels. Electrokinetic pumping and the means of producing large pressure gradients in microchannels are explored. Finally, electroosmotic flow in a T-junction geometry is analyzed under various external electric field strengths. Flowrate in the T-junction is shown to vary linearly with the electroosmotic body forces in the Stokes flow regime.

Plastic Microfluidic Systems for High-Throughput Genomic Analysis and Drug Screening

Plastic Microfluidic Systems for High-Throughput Genomic Analysis and Drug Screening

Nonaqueous packed capillary electrokinetic chromatographic separations of large polycyclic aromatic hydrocarbons and fullerenes.

Polycyclic aromatic hydrocarbons (PAH) and a fullerene mixture (C60/C70) were separated with capillary columns (50 microns ID) packed with a reversed-phase packing (octadecylsilica, 3 microns diameter) using electrokinetic pumping. Nonaqueous (acetonitrile modified with methylene chloride or tetrahydrofuran, THF) mobile phases were used for these experiments. The effects of mobile phase composition on such factors as electroosmotic flow, plate height, and capacity factor (k') are reported. The less polar solvents methylene chloride and THF produced predictable reductions in k' when used to modify an acetonitrile mobile phase. Large amounts of the less polar modifiers (50% v/v) also resulted in a fourfold decrease in flow rate. This meant that even with a decrease in k', the retention time increased. Nonaqueous capillary electrokinetic chromatographic (CEC) separations gave efficiencies as high as 160,000 plates/m. Use of nonaqueous mobile phases provided small currents which in turn diminished the role of heating effects on efficiency. The nonaqueous system also provided greater solubility for the hydrophobic solutes. A Van Deemter plot for an acetonitrile mobile phase was obtained that exhibited expected trends in plate height with flow rate and k'. Solvent rinses with water and THF are shown to have only small effects on retention and flow rate.