Brian J. Kirby

Increasing the performance of high-pressure, high-efficiency electrokinetic micropumps using zwitterionic solute additives

A zwitterionic additive is used to improve the performance of electrokinetic micropumps (EK pumps), which use voltage applied across a porous matrix to generate electroosmotic pressure and flow in microfluidic systems. Modeling of EK pump systems predicts that the additive, trimethylammoniopropane sulfonate (TMAPS), will result in up to a 3.3-fold increase in pumping efficiency and up to a 2.5-fold increase in the generated pressure. These predictive relations compare well with experimental results for flow, pressure and efficiency. With these improvements, pressures up to 156 kPa/V (22 psi/V) and efficiency up to 5.6% are demonstrated. Similar improvements can be expected from a wide range of zwitterionic species that exhibit large dipole moments and positive linear dielectric increments. These improvements lead to a reduction in voltage and power requirements and will facilitate miniaturization of micro-total-analysis systems (μTAS) and microfluidically driven actuators.

Voltage-addressable on/off microvalves for high-pressure microchip separations

We present a microchip-based, voltage-addressable on/off valve architecture that is fundamentally consistent with the pressures and solvents employed for high-pressure liquid chromatography. Laser photopatterning of polymer monoliths inside glass microchannels is used to fabricate mobile fluid control elements, which are opened and closed by electrokinetic pressures. The glass substrates and crosslinked polymer monoliths operate in water-acetonitrile mixtures and have been shown to hold off pressures as high as 350 bar (5000 p.s.i.). Open/closed flow ratios of 10(4) to 10(6) have been demonstrated over the pressure range 1.5-70 bar (20-1000 p.s.i.), and the pressure-leak relationship shows the potential for valving control of flow through packed or monolithic chromatography columns. We expect that this valve platform will enable multiplexing of multiple chromatographic separations on single microchips.

Microfluidic routing of aqueous and organic flows at high pressures: fabrication and characterization of integrated polymer microvalve elements

This paper presents the first systematic engineering study of the impact of chemical formulation and surface functionalization on the performace of free-standing microfluidic polymer elements used for high-pressure fluid control in glass microsystems. System design, chemical wet-etch processes, and laser-induced polymerization techniques are described, and parametric studies illustrate the effects of polymer formulation, glass surface modification, and geometric constraints on system performance parameters. In particular, this study shows that highly crosslinked and fluorinated polymers can overcome deficiencies in previously-reported microvalve architectures, particularly limited solvent compatibility. Substrate surface modification is shown effective in reducing the friction of the polymer-glass interface and thereby facilitating valve actuation. A microchip one-way valve constructed using this architecture shows a 2 x 10(8) ratio of forward and backward flow rates at 7 MPa. This valve architecture is integrated on chip with minimal dead volumes (70 pl), and should be applicable to systems (including chromatography and chemical synthesis devices) requiring high pressures and solvents of varying polarity.

Linear excitation schemes for IR planar-induced fluorescence imaging of CO and CO 2

A detailed discussion of linear excitation schemes for IR planar-induced fluorescence (PLIF) imaging of CO and CO2 is presented. These excitation schemes are designed to avoid laser scattering, absorption interferences, and background luminosity while an easily interpreted PLIF signal is generated. The output of a tunable optical parametric amplifier excites combination or overtone transitions in these species, and InSb IR cameras collect fluorescence from fundamental transitions. An analysis of the dynamics of pulsed laser excitation demonstrates that rotational energy transfer is prominent; hence the excitation remains in the linear regime, and standard PLIF postprocessing techniques may be used to correct for laser sheet inhomogeneities. Analysis of the vibrational energy-transfer processes for CO show that microsecond-scale integration times effectively freeze the vibrational populations, and the fluorescence quantum yield following nanosecond-pulse excitation can be made nearly independent of the collisional environment. Sensitivity calculations show that the single-shot imaging of nascent CO in flames is possible. Signal interpretation for CO2 is more complicated, owing to strongly temperature-dependent absorption cross sections and strongly collider-dependent fluorescence quantum yield. These complications limit linear CO2 IR PLIF imaging schemes to qualitative visualization but indicate that increased signal level and improved quantitative accuracy can be achieved through consideration of laser-saturated excitation schemes.

CO 2 imaging with saturated planar laser-induced vibrational fluorescence

We present new vibrational (infrared) planar laser-induced fluorescence (PLIF) imaging techniques for CO(2) that use a simple, inexpensive, high-pulse-energy transversely excited atmospheric CO(2) laser to saturate a CO(2) absorption transition at 10.6 mum. Strong excitation by use of a CO(2) laser provides increased signal levels at flame temperatures and simplifies image interpretation. Because rotational energy transfer and intramodal vibrational energy transfer are fast, vibrational distributions can be approximated by use of a simple three-temperature model. Imaging results from a 425 K unsteady transverse CO(2) jet and a laminar coflowing CO/H(2) diffusion flame with temperatures near 1500 K are presented. If needed, temperature-insensitive signal levels can be generated with a two-laser technique. These results illustrate the potential for saturated infrared PLIF in a variety of flows.

Effects of ammonioalkyl sulfonate internal salts on electrokinetic micropump performance and reversed-phase high-performance liquid chromatographic separations

Ammonioalkyl sulfonate internal salts are explored owing to their potential for improving electrokinetic pumps used to perform miniaturized HPLC separations. The internal salts investigated can be added at high molarity since they are net-neutral, and furthermore show potential for increasing electroosmotic pumping owing to their large positive dielectric increment. Streaming potential measurements of buffered aqueous systems with varying concentrations of ammonioalkyl sulfonate internal salts have been used to measure these dielectric increments, which increase with the length of the alkyl linker. Due to their positive dielectric increments and their decremental effect on solution conductivity, all of the measured species are predicted to improve the pressure generation (up to 85%) and efficiency performance (up to 140%) of electrokinetic pumps when added at 1 M concentration. RP-HPLC separations with an ammonioalkyl sulfonate (TMAPS) have been performed and indicate that separation performance is essentially unaffected by these species. These results indicate the potential for a variety of ammonioalkyl sulfonates to be used to improve electrokinetic pump performance for miniaturized HPLC.

In-situ Fabrication of Dialysis Membranes in Glass Microchannels Using Laser-induced Phase-Separation Polymerization

Laser-induced phase-separation polymerization of a porous acrylate polymer is used for in-situ fabrication of dialysis membranes inside glass microchannels. A shaped 355 nm laser beam is used to define polymer membranes of 4–14 μm thickness, which bond to the glass microchannel and form a semipermeable membrane. Differential diffusion through the membrane is observed for fluorescein molecules versus 200 nm latex microspheres, showing the potential for this technique to integrate sample cleanup into chip-based analysis systems.

CATH.A NEURON CELL ANALYSIS ON A CHIP WITH MICELLAR ELECTROKINETIC CHROMATOGRAPHY

An on-chip electrophoresis analysis of the contents of CATH.a neuron cells is presented. A novel mixed-micelle MEKC separation was required to resolve twelve amino acids and neurotransmitters within five minutes.

A Novel Miniaturized Protein Preconcentrator Based on Electric Field-Addressable Retention and Release

We report a novel technique for concentration of proteins from dilute solutions using electrokinetic trapping [1] where charged macromolecules are reversibly trapped in a microchannel packed with porous silica particles under an applied electric field. Electrokinetic trapping is electric field-addressable and reversible; the trapped proteins can be recovered quantitatively by removing the electric field. The concentration of proteins is carried out in two steps -1) electrokinetic injection of proteins in channels packed with porous silica particles, and 2) elution by applying pressure (using a mechanical pump) in the absence of applied voltage (Figure 1). A model protein, ovalbumin, could be concentrated by over two orders of magnitude using electrokinetic trapping. Electrokinetic trapping provides a simple, on-column method of preconcentration of proteins and other charged macromolecules that can be easily integrated with a miniaturized separation device. Furthermore, it does not require change of solvents (or buffers) or change of flow direction for elution of concentrated protein and hence, has advantages over filtration-based, electrolyte bridge-based [2] or adsorption-based [3] miniaturized concentrators. We demonstrate that the electrokinetic trapping of proteins is quantitative, reproducible and works for several proteins. Electrokinetic trapping can also be used for applications including selective modification of trapped proteins, using trapped proteins as immobilized catalysts, and concentration of DNA.

Microvalve Architectures for High-Pressure Hydraulic and Electrokinetic Fluid Control in Microchips

Microvalve architectures employing mobile polymer seating elements in glass substrates are presented. These valves can operate on chip with pressures above 3000 psi, at voltages above 1 kV, and in solvents including water, acetonitrile, and acetone. The valves open and close in milliseconds, and can be used to control, inject, and rout fluids. Closing a valve in a microchannel can cause a billion-fold reduction in the flow through that microchannel; this leak rate performance, combined with pressure and voltage compatibility, makes these valves useful for controlling and integrating practical high-pressure chromatographic or electrokinetic separations on microchips.