Gasless and gas bubble-free electrodes
Gas bubble-free electrodes are necessary for stable long-term operation of millimeter-scale electrokinetic (EK) pumps when currents exceed 10-50 μA. An accompanying Technical Advance describes EK pumps that draw 1-3 mA. We have developed gasless and gas bubble-free electrodes that can run millimeter-scale (and smaller) EK pumps continuously at high current densities. Two types of gasless electrodes based on porous carbon and ruthenium/tantalum-on-titanium oxides have been developed that are supercapacitors which store ions from a fluid electrolyte. The gas bubble-free electrodes isolate gas generated by water electrolysis of the pump fluid from the fluid channels by means of an electrically-conductive polymer. Nafion® tubing is a cationic-selective polymer that is used to pass currents and water for electrolysis at titanium and platinum surfaces. The gas bubble-free electrodes are easy to fabricate and can operate well even with typical, low-conductivity electrolytes. The gas bubble-free cathode seals to 1500 psi for high-pressure microhydraulic actuation
This invention was made with Government support under government contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention, including a paid-up license and the right, in limited circumstances, to require the owner of any patent issuing in this invention to license others on reasonable terms.
TECHNICAL FIELDThe present invention relates to an embodiment comprising a miniature microfluidic transducer, and particularly to an actuator driven by an electrokinetic pump, wherein the hydraulic pressure is used to drive a piston or bellows.
BACKGROUNDMiniature pumps and valves have been a topic of increasing interest in recent years within the field of chemical analysis, especially in those applications where a variety of functions including pumping, mixing, metering, and species separation are necessary. In particular, there has been interest in integrating miniature pumps and valves with silicon and glass chip-based analysis systems designed to detect and identify trace amounts of chemical or biological material.
To meet these needs efforts have been made to develop and refine micro-scale pumps that rely on the well-known electroosmotic effect, so-called electrokinetic (“EK”) pumps, and related control and valving mechanisms for these devices. The phenomenon of electroosmosis, in which the application of an electric field to an electrolyte in contact with a dielectric surface produces a net force on a fluid and thus a net flow of fluid, has been known since the nineteenth century. The physics and mathematics defining electroosmosis and the associated phenomenon of streaming potential have been extensively explored in “Introduction to Electrochemistry,” by Glasstone, (1942) pp. 521-529 and by Rastogi, (J. Sci. and Industrial Res., v.28, (1969) p. 284). In like manner, electrophoresis, the movement of charged particles through a stationary medium under the influence of an electric field, has been extensively studied and employed in the separation and purification arts.
The use of electroosmotic flow for fluid transport in packed-bed capillary chromatography was first documented by Pretorius, et. al. (J. Chromatography, v.99, (1974) pp. 23-30). Although the possibility of using this phenomenon for fluid transport has long been recognized, its application to perform useful mechanical work has been addressed only indirectly. The present embodiment describes gasless and gas bubble-free electrodes for actuators based on EK pumps.
EK pumps are typically composed of a nanoporous packing or monolith (pore diameters from 10 to 500 nm) and a pair of high-voltage electrodes. For example, silica acquires a negative surface charge composed of deprotonated silanol groups (SiOH9 SiO−+H+) when an electrolyte with pH>4 is introduced. As illustrated in
Conversely, external pressure-driven flows in these systems will generate electric fields that may be used to perform electrical work.
Many different microfluidic transducers have been implemented by micromachining of silicon and glass substrates. Transducers with pneumatic, thermo-pneumatic, piezoelectric, thermal-electric, shape memory alloy, and a variety of other actuation mechanisms have been realized with this technology. However, only the thermo-pneumatic and shape memory alloy designs have been incorporated in commercially-available products. Unfortunately, transducers utilizing the aforementioned actuation mechanisms are only able to generate modest actuation pressures and are therefore of limited utility.
What is needed is a transducer that can be used for microfluidic systems that can exert larger actuation pressures over longer distances (i.e., more work per stroke) than can be presently developed by conventional (non-explosive) transducer and provides both rapid “on” and “off” actuation. Millimeter-scale electrokinetic pumps are capable of such actuation, but require gasless or gas bubble-free electrodes in order to operate for more than a few seconds, as is explained in subsequent sections.
SUMMARYEK pumps are known to exhibit a linear pressure-flowrate operating envelope for a given electric field. This linearity is due to the linearity of superposing linear electroosmotic and pressure-driven flows (ignoring property changes due to viscous heating or electrolyte composition). Because hydraulic power is the product of pressure and flowrate, the most efficient operating point for a given electric field is half the maximum pressure and half the maximum flowrate. The maximum power output increases linearly with electric field up to the point where property changes occur. For example, viscous heating at high electric fields decreases the viscosity which, in turn, increases the current draw and the power output.
Our prior efforts have demonstrated electrokinetic pumps in glass capillaries (100 μm I.D./360 μm O.D., length 3-cm to 30-cm) that are capable of pressure gradients of 250-500 psi/mm and average fluid velocities of 2 mm/s. The present embodiment describes advances in EK pump fabrication for developing larger-diameter porous monoliths and their application to mechanisms for performing mechanical work. In particular, the pumps described herein have been fabricated with diameters of 2.9-mm, and lengths from 6-mm to 10-mm. Moreover, while these pumps produce pressure gradients that are similar to those of their smaller diameter counterparts they also produce much larger flowrates, e.g., 200 μL/min for the present embodiments vs. 5 μL/min for prior-art EK pumps.
The force and stroke (i.e., work per stroke) delivered by the EK actuators of the present embodiments exceed the output of solenoids, stepper motors, and DC motors of similar size, despite the low electric-to-hydraulic power conversion efficiency of EK pumps (1-6%). Piezoelectric actuators of similar size can deliver much larger forces (e.g., 200 lbf), but their displacements are very small (e.g., 50-μm). The pump and electrodes contain no moving parts and operate silently, which is beneficial for applications requiring actuation with low noise and vibration levels.
The objective of the present invention is to provide gasless and gas bubble-free electrodes for high-pressure electrokinetic pumping requiring up to 3 mA of current in a millimeter-scale package.
These and other objectives and advantages of the present invention may be clearly understood from the detailed description by referring to the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Electroosmotic flow is not an efficient method of converting electrical work to mechanical work because the mechanism is based on viscous coupling of ion motion to fluid motion in the nanometer-scale electric double layer, which results in high shear stress and corresponding viscous dissipation. EK pumps are therefore inefficient (our pumps have demonstrated efficiencies between about 1% and about 6%) and draw substantial current densities when large electric fields are applied (e.g. 100 mA/cm2 for 1000 V/cm). Moreover, for capillary EK pumps with 0.1-mm O.D. porous monoliths, typical currents of 5-10 μA result in current densities at the electrode surfaces that are insufficient to nucleate bubbles (for 0.38-mm-diameter platinum wires), and the electrolysis gases simply dissolve into solution. However, increasing the pump cross-section to a diameter of 2.5-mm results in currents up to 3 mA, which is sufficient to generate visible bubble growth in a few seconds. These gas bubbles cause the current to fluctuate and decrease to a trivial magnitude (nonzero due to water films around the bubbles). Hence, gas bubble-free electrodes are necessary for stable long-term operation.
Fabrication of Bubble-Free Electrodes:
The electrodes achieve gas bubble-free operation by using Nafion® tubing to isolate electrolysis gas, generated at the surface of the metal electrode, from the pump fluid. Nafion® is a cation-selective polymer (i.e., a charge-selective salt bridge) that permits diffusive and electrophoretic transport of cations and water.
Isolation of electrolysis gas from the fluid stream is achieved with Nafion® tubing in two high-current, high-pressure, compact electrodes: a flow-through titanium/Nafion® cathode and a flexible platinum/Nafion® electrode. The cathode is shown in
Fabrication of the titanium/Nafion® cathode starts with drilling a hole in the frit with a diameter that provides a close fit to the desired Nafion® tube outer diameter. A small length of Nafion® tubing (2-cm to 3-cm) is pushed through the drilled hole, and epoxy is painted around the interface. The epoxy is cured for 20 minutes at 90° C. The excess tubing length is trimmed, and the faces are ground and/or sanded smooth. The finished cathode of
The second electrode is shown in
The Nafion® tube has a thin wall (0.001″) that results in rapid water diffusion to the platinum, despite the opposing electrophoretic transport of hydrated protons. Platinum is the preferred metal because it is electrochemically inert for both anodic and cathodic operation, i.e., it does not passivate or corrode. Small diameter (0.005″) platinum wire is ductile and relatively affordable. Testing has shown that this anode configuration can supply more than 2 mA per cm of length (23 mA/cm2 and 31 mA/cm2 based on the tube O.D. and I.D., respectively) in 5 mM TRIS-HCl at pH 8.5. However, during high-current testing (2 mA/cm) a small number of bubbles appeared during startup and shutdown. Bubbles did not appear to be evolving during constant current operation over a period of minutes. Testing with EK pumps has shown the electrodes to be bubble free.
Examples of Gasless Electrodes Used in EK Pump Actuators:
A class of gasless electrodes has been developed for reciprocating EK actuation.
The electrodes are supercapacitors that store charge by intercalation and adsorption to the surface. One version of these electrodes is a co-precipitated binary oxide of ruthenium and tantalum on titanium shown in
Bubble-free electrodes have been developed that permit water hydrolysis but which isolate the gases that develop at the surfaces of the electrodes from the electrolyte. Compact, high current electrodes have been developed using a perfluorosulfonic acid co-polymer tubing produced by Perma Pure LLC (Toms River, N.J.) under the DuPont trade name Nafion®, a cation-conductive co-polymer, i.e., a charge-selective salt bridge, that permits diffusive and electrophoretic transport of cations and water.
The first of these electrodes is the high-pressure flow-through cathode shown in
The second electrode is again the flexible anode bonded inside a modified syringe barrel shown in
The Nafion® tube has a thin wall (25-40 μM) that results in rapid water diffusion to the platinum, despite the opposing electrophoretic transport of hydrated protons.
Platinum is the preferred anode metal because it does not passivate (oxidize) while receiving electrons and is chemically inert in strong acids and bases. Testing has shown that this anode configuration can supply more than 2 mA/cm (23 mA/cm2 and 31 mA/cm2 based on the tube O.D. and I.D., respectively) in 5 mM Tris(hydroxymethyl) aminomethane-hydrochloride (TRIS-HCl) at pH 8.5.
As an example,
High-pressure microhydraulic actuation, therefore, has been demonstrated with gas bubble-free electrodes, an EK pump, and syringes with different plunger areas. Using the actuator shown in
High-pressure microhydraulic actuation driven by millimeter-scale electrokinetic pumps with gas bubble-free electrodes has been demonstrated. High performance porous polymer and sintered silica monoliths have been developed that give 1% and 3% electric-to-hydraulic work conversion efficiencies, respectively. Flowrates up to 200 μL/min, pressures up to 1500 psi (8.3 MPa), and hydraulic powers up to 17 mW have been observed. Electrokinetic pressures of 3 psi/Volt (21 kPa/V) and 8 psi/Volt (6.9 kPa/V) have been demonstrated. Gas bubble-free electrodes have been developed that permit extended hermetic operation. EK actuators are capable of delivering more work per stroke than electromechanical actuators of similar size.
Claims
1. An electrode assembly, comprising:
- an electronically conducting body, said conducting body having one or more exterior surfaces;
- a conductor attached to and making electrical contact with said conducting body; and
- a cation-selectable polymer jacket covering some portion of said one or more exterior surfaces, wherein an interface between said covered portion is open to an ambient exterior atmosphere
2. The electrode assembly of claim 1, wherein the conducting body comprises a metal foil or a metal wire.
3. The electrode assembly of claim 2, wherein the metal foil or a metal wire is titanium or tantalum.
4. The electrode assembly of claim 1, wherein the conducting body comprises a metal frit,
5. The electrode assembly of claim 4, wherein the metal frit comprises titanium or tantalum.
6. An electrode assembly, comprising:
- an electronically conducting collector substrate;
- an ionically conducting porous layer disposed on one or more surfaces of said collector substrate; and
- an electronic conductor making electrical contact with said collector substrate.
7. The electrode assembly of claim 6, wherein said ionically conductive porous layer is an oxide layer.
8. The electrode assembly of claim 7, wherein the oxide layer is a co-precipitated binary oxide.
9. The electrode assembly of claim 8, wherein the co-precipitated binary oxide are oxides of ruthenium and tantalum.
10. The electrode assembly of claim 6, wherein the metal collector substrate is an expanded metal screen or a metal foil.
11. The electrode assembly of claim 10, wherein the expanded metal screen or metal foil is titanium.
12. The electrode assembly of claim 6, wherein said ionically conductive porous layer is a porous carbon layer.
13. The electrode assembly of claim 12, wherein the porous carbon layer is a carbon xerogel or a carbon paste suspension.
14. The electrode assembly of claim 13, wherein the collector substrate is an expanded aluminum screen.
15. A flow-through electrode assembly, comprising:
- a metal body, comprising generally parallel first and second surfaces and a central opening therethrough, said first and second surfaces and said central opening covered by a cation-selective polymer jacket surrounding and isolating said surfaces and said central opening, wherein an interface between said surfaces and said central opening is open to an ambient exterior atmosphere; and
- a conductor attached to said metal body.
Type: Application
Filed: May 17, 2004
Publication Date: Nov 17, 2005
Inventors: Bruce Mosier (San Francisco, CA), Robert Crocker (Fremont, CA)
Application Number: 10/848,196