Electrokinetic Pump Research Articles

Integration of electropreconcentration and electrospray ionization in a microchip

A microfluidic glass chip for performing sensitive mass spectrometry (MS) is developed by integrating nanochannel-based electropreconcentration with electrospray ionization (ESI). Cation analytes in acidic buffer conditions (∼pH 3) were preconcentrated at a micro-nano-microchannel junction where a positively charged polymer, polyE-323, is coated to reverse the electroosmotic flows (anodic EOF) and prevent sample adsorption. The preconcentrated cation analytes were delivered to a monolithic spray tip using an integrated field-free electrokinetic pump, enabling a stable positive ESI with negligible dead volume. The sample preconcentration-ESI-MS achieved one order of magnitude signal enhancement in MS signal with a peptide mixture.

Induced charge electroosmosis micropumps using arrays of Janus micropillars.

We report on a microfluidic AC-driven electrokinetic pump that uses Induced Charge Electro-Osmosis (ICEO) to generate on-chip pressures. ICEO flows occur when a bulk electric field polarizes a metal object to induce double layer formation, then drives electroosmotic flow. A microfabricated array of metal-dielectric Janus micropillars breaks the symmetry of ICEO flow, so that an AC electric field applied across the array drives ICEO flow along the length of the pump. When pumping against an external load, a pressure gradient forms along the pump length. The design was analyzed theoretically with the reciprocal theorem. The analysis reveals a maximum pressure and flow rate that depend on the ICEO slip velocity and micropillar geometry. We then fabricate and test the pump, validating our design concept by demonstrating non-local pressure driven flow using local ICEO slip flows. We varied the voltage, frequency, and electrolyte composition, measuring pump pressures of 15-150 Pa. We use the pump to drive flows through a high-resistance microfluidic channel. We conclude by discussing optimization routes suggested by our theoretical analysis to enhance the pump pressure.

High figure of merit for electrokinetic energy conversion in Nafion membranes

Based on measurements of the streaming potential coefficient, hydraulic permeability and ion conductivity the electrokinetic energy conversion of nanoporous ion conductive Nafion®117 polymer membrane has been evaluated. A high figure-of-merit of 1.1 ± 0.2 has been found in 0.03 M aqueous LiCl solution that corresponds to an electrokinetic energy conversion efficiency of approximately 18 ± 2%. To the best of our knowledge, this is the first study in the literature designated to the electrokinetic energy conversion in materials where the pores have high surface charge density. The initial results are promising with respect to future electrokinetic generator or pump applications and highly charged polymer membranes might be a future pathway for low-cost high-efficiency electrokinetic energy conversion.

A numerical hydrodynamic and thermal characterization of an inter-strata liquid cooling solution for 3D ICs

A novel integrated thermal management solution is proposed to alleviate hot spots in a contemporary 3D IC architecture. The solution employs a series of integrated microchannels, interconnected through each stratum by through silicon fluidic vias (TSFVs), and permits the transfer of heat, via a coolant, from hot to cold zones. This microfluidic system is driven by an integrated AC electrokinetic pump embedded in the channel walls. Recent advancements in electrokinetic micropump technology have allowed greater increases in fluid velocity (mm/s) while operating within the voltage constraints of a 3D IC. This paper presents a 2D simulation of an electrokinetic micropump operating at Vpp = 1.5 V in a 40 μm channel and examines its velocity profile for six frequencies in the range 100 ≤ ω ≤ 100 MHz. An optimum frequency of 100 kHz was established within this range and this was further examined with a constant heat flux of 186 W/cm² imposed on the wall for an inlet fluid temperature of 40°C. Temperature profiles are presented at the channel-silicon interface and compared with theory.

Liquid glass electrodes for nanofluidics

Nanofluidic devices exploit molecular-level forces and phenomena to increase their density, speed and accuracy1. However, fabrication is challenging because dissimilar materials need to be integrated in three dimensions with nanoscale precision. Here we report a three-dimensional nanoscale liquid glass electrode (NLGE) made from monolithic substrates without conductive materials by femtosecond laser nanomachining. The electrode consists of a nanochannel terminating at a nanoscale glass tip that becomes a conductor in the presence of high electric fields through dielectric breakdown, and returns to an insulator when this field is removed. This reversibility relies on control of nanoampere breakdown currents and extremely fast heat dissipation at nanoscale volumes. We use the NLGE to fabricate a nano-injector that includes an electrokinetic pump, 4 µm across with 0.6 µm channels, and capable of well-controlled flow rates below 1 fL/s. The electrode can be easily integrated into other nanodevices and fluidic systems, including actuators and sensors.

Electrothermal ac electro-osmosis

Two ac polarization mechanisms, charge accumulation due to electrode double layer charging and bulk permittivity/conductivity gradients generated by Joule heating, are combined in the double layer by introducing zwitterions to produce a new ac electrokinetic pump with the largest velocity (>1 mm/s) and flow penetration depth (100 μm) reported for low-conductivity fluids. The large fluid velocity is due to a quartic scaling with respect to voltage, as is true of electrothermal flow, but exhibits a clear maximum at a frequency corresponding to the electrode double layer inverse RC time.

Flow reversal at low voltage and low frequency in a microfabricated ac electrokinetic pump

Microfluidic chips have been fabricated in Pyrex glass to study electrokinetic pumping generated by a low-voltage ac bias applied to an in-channel asymmetric metallic electrode array. A measurement procedure has been established and followed carefully resulting in a high degree of reproducibility of the measurements over several days. A large coverage fraction of the electrode array in the microfluidic channels has led to an increased sensitivity allowing for pumping measurements at low bias voltages. Depending on the ionic concentration a hitherto unobserved reversal of the pumping direction has been measured in a regime, where both the applied voltage and the frequency are low, V(rms)<1.5 V and f<20 kHz , compared to previously investigated parameter ranges. The impedance spectrum has been thoroughly measured and analyzed in terms of an equivalent circuit diagram to rule out trivial circuit explanations of our findings. Our observations agree qualitatively, but not quantitatively, with theoretical electrokinetic models published in the literature.

Liquid Cooling System for Advanced Microelectronics

Heat removal from advanced microelectronic devices is becoming a major packaging challenge. Thermal management solutions such as microchannel liquid cooling are now becoming commercially available. The use of a liquid cooling system is attractive because of higher heat transfer coefficients or lower thermal resistance as compared to traditional heat pipe or heat sink solutions. In this paper we present a description of the key features of Cooligy's closed loop Liquid Cooling System (LCS). Two key components of the LCS, namely a microheat exchanger and an electrokinetic pump are discussed in detail. Published literature on advanced thermal interface materials and nanofluids are briefly reviewed. Wherever applicable, the importance and impact of electrochemical processing and materials aspects are highlighted.

The microfabricated electrokinetic pump: a potential promising drug delivery technique

Directly delivering liquid using direct current power was a dream for 40 years. Now, the electrokinetic pump can accomplish it. This review reports recent developments in microfabricated electrokinetic pump technologies and their applications, and also discusses the electrophoresis pump and elctroosmotic pump, as well as the materials and fabrication methods frequently used for the production of these pump devices. Electrokinetics pumps can be used to deliver pure water, pure polar organic solvents, inorganic buffer and biomacromolecules, with wide applications for the delivery of liquids. These devices have the potential to become drug delivery systems, for the precise, timed and/or targeted delivery of drugs. Future trends, limitations and possibilities are also discussed.

Electrokinetic pumping system based on nanochannel membrane for liquid delivery

Nonmechanical pumping of liquids is of key importance for applications from the biomedical microfluidic chip to drug delivery systems. In this paper, a new electrokinetic pump (EOP) system with polycarbonate nanochannel membrane sandwiched between two membrane holders was constructed. The pump was tested with water and phosphate buffer at 1–6 V applied voltage, the maximum pressure and flow rate are 0.32 MPa (3.2 atm) and 4.2 μL/min for phosphate buffer, respectively. This proof-of-concept pump shows its potential use for drugs or chemical agents delivery by the usage of different membrane materials.

An integrated AC electrokinetic pump in a microfluidic loop for fast and tunable flow control.

We have built a dedicated lab on a chip to study the performance of an integrated electrokinetic micropump, driven by a low voltage AC signal. This micropump consists of an array of interdigitated electrodes and is here integrated in a microfluidic loop. We demonstrate that this device can pump continuously and reproducibly electrolyte solutions of low to moderate ionic strength. The pumping speed reaches up to 500 [micro sign]m s(-1) in 20 [micro sign]m deep and 100 [micro sign]m wide channels with a driving signal in the 1-10 kHz range and an amplitude of only a few volts. In addition, we have observed an interesting reversal of the pumping direction at higher frequencies (50-100 kHz). Our device permits a systematic and automated exploration of the influence of the ionic strength thanks to an integrated micromixer.

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.

Spatially averaged time-resolved particle-tracking velocimetry in microspace considering Brownian motion of submicron fluorescent particles

A time-series measurement method is proposed to detect velocity fields in a microchannel taking into account Brownian motion of submicron tracer particles. The present study proposes spatially averaged time-resolved particle-tracking velocimetry (SAT–PTV), which can detect temporal variations of fluid flow and eliminate errors associated with Brownian motion without losing temporal resolution. Velocity vectors of tracer particles obtained by PTV are spatially averaged in each interrogation window of particle-image velocimetry, yielding full velocity field information with temporal resolution. Synthetic particle images, which include Brownian motion of submicron fluorescent particles in flow fields with linear velocity gradients, are generated to validate the ability of SAT–PTV to track particles. SAT–PTV correctly captures the velocity gradient profiles. The spatial resolution based on the size of the first interrogation window and the measurement depth of the microscope system is 6.7 μm×6.7 μm×1.9 μm, within which several vectors are averaged. SAT–PTV is shown to measure the velocity field of a pulsating flow generated by an electrokinetic pump.

Liquid flow in capillary (electro)chromatography: Generation and control of micro‐ and nanoliter volumes

AbstractWe describe and discuss instrumental developments in capillary (electro)chromatography which are of particular relevance for generating (and controlling) required volumetric flow rates in the micro‐ and nanoliter range through packed capillaries. Both isocratic and gradient elution are considered. For capillary HPLC this practically involves only commercial instrumentation, with systems based on syringe or piston pumps, but it also realizes the innovative concept of a high‐pressure electrokinetic pump. The numerous systems that have been used to generate electroosmotic flow through chromatographic beds are classified under the following headings: i) basically commercial capillary electrophoresis instruments (adapted for electrochromatography); ii) home‐built configurations; and iii) commercial capillary electrochromatography systems. Concerning the reviewed instrumentation, emphasis is placed on feasibility, automation, as well as system‐inherent delay times and dead volumes.