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A Nonmechanical, Membrane-Based Liquid Pressurization System Christine E. Evans,† Richard D. Noble,‡ and Carl A. Koval† Department of Chemistry and Biochemistry, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309
Nonmechanical pumping of liquids is of key importance for applications ranging from biomedical lab-ona-chip systems to morphing mechanical structures. In this paper, we report a new, reversible micropumping and pressurization system, with no moving parts, that uses only modest external power. This new “e-pump” operates via a type of electro-osmosis (EO) in which a charge imbalance is created electrochemically across a cation-selective membrane, cations migrate to balance the charge, and solvent is transported across the membrane, along with the mobile cation. No gas is produced in this electrokinetic pumping system, so only liquids are involved in pumping and pressurization. In this proof-of-concept study, an aprotic solvent (dimethylformamide) was chosen to select the specific cation that is migrating (tetrapropylammonium ion). To date, pressures up to 23 atm have been successfully demonstrated. Introduction The pressurization and manipulation of fluids on the nanoscale and microscale are required for a wide range of microfluidics applications, including analytical and synthetic “labon-a-chip”, ultrasmall particle handling, and microspray/nanospray systems.1,2 Identical demands are key for smart structures and morphing technologies that incorporate plantlike nastic structures and/or individually addressable cells.3,4 Such adaptive structures require the high energy density and local shape control that is possible using microscale pressurized fluids. In such systems, mechanical pumps are too heavy for useful implementation and have frictional limitations on this small size scale; however, existing nonmechanical pumping systems often are dependent on high voltages or cannot achieve significant pressures. Although a full review of the literature is beyond the scope of this paper, a recent review of micropumps2 indicates that electro-osmotic (EO) micropumps demonstrate the best pressurization behavior; two references have been cited as producing maximum pressures over 3 atm.5,6 The maximum pressures achieved in these cases are quite high (20 atm, from Zeng,5 and 340 atm, from Paul et al.6); however, both reported values rely on a very high applied voltage (in the kilovolt range), resulting in values of 0.01 and 0.03 atm/V, respectively. We report here a membrane-based EO pumping strategy that produces pressures of >20 atm at 4.5 atm/V, in a simple, scaleable design. Significant pressurization is demonstrated under very modest applied potential (5 V) and current (20 mA) conditions. Although the phenomenon of EO flow across membranes has been known for decades,7 to our knowledge, this represents the first reversible, membrane-based EO pumping system to achieve such high pressures. Experimental Methods The “e-pumping” methodology, which is illustrated in Figure 1a, incorporates liquid-filled chambers that are separated by a nanoporous, cation permeable membrane. The two-compartment cell was fabricated from the high-performance polymers polyetheretherketone (PEEK)swith one side open to the atmosphere and the other side closed and fitted with a pressure * To whom correspondence should be addressed. Te.: 303-492-6100, -7517. Fax: 303-492-4637. E-mail:
[email protected]. † Department of Chemistry and Biochemistry. ‡ Department of Chemical and Biological Engineering.
Figure 1. (A) Schematic diagram of e-pump system (P denotes a pressure transducer). (B) High-pressure performance of e-pump with forward pumping (current of i ) +20 mA), backflow (current of i ) 0), and reverse pumping (current of i ) -20 mA).
transducer (Micron Instruments, Model MP40A-300A). Studies were conducted using a cation-selective ionomer membrane composed of a perfluorosulfonic acid/polytetrafluoroethylene (PTFE) copolymer (Nafion 117, DuPont) that was 180 µm thick and operated with an exposed area of 0.55 cm2. The membrane was suspended between two ethylene propylene O-rings, with a perforated membrane holder (PEEK) and nylon mesh filter (pore diameter of 100 µm, Millipore No. NY1H) providing additional mechanical support. An electrical connection was accomplished using platinum mesh electrodes in each equalvolume compartment (Vright ) Vleft ) 1.25 mL). Each electrode (diameter of ∼1 cm) was positioned to be coplanar with the membrane and separated from it by ∼7 mm of electrolyte solution. Using equivalent ionic conductivities for the tetrabutylammonium ion and iodide ion in dimethyl formamide (DMF) (26.7 and 51.1 S cm2/mol,8 respectively), and the cell geometry, the resistance associated with the electrolyte was calculated to be ∼50 Ω. The overall cell resistance was 250 Ω; thus, ∼80% of the applied voltage drop (5 V) fell across the Nafion 117 membrane. The pump was operated in constantcurrent mode, using a galvanostat (Princeton Applied Research,
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Model 273A) with computer data acquisition (10 Hz acquisition rate; National Instruments DAQcard-6036E with LabView software). During pump operation, each compartment was filled with identical solutions of tetrapropylammonium iodide (0.5 M; Sigma-Aldrich, >98% purity) and iodine (0.25 M, SigmaAldrich, 99.8% purity) in DMF (Sigma-Aldrich). The factors used in the selection of this electrolyte system have been reported elsewhere.9-11 All chemicals were used without further purification. The initial conditions ensure that no concentration difference is observed between compartments, effectively eliminating direct osmotic flow contributions. Membranes were conditioned in this solution for 24-48 h and placed directly into the cell. In preliminary studies, assembled e-pump cells showed no change in behavior for periods of at least one week. Operating in the constant-current mode, a current of 20 mA applied to the cell resulted in an essentially constant voltage of ∼5 V across the membrane. In all studies, the operating conditions have been chosen such that specific ion migration is the only mechanistic pathway within the system to balance the charge. Both oxidized and reduced forms of an electrochemical redox couple are present in both compartments, facilitating readily reversible operation. In this proof-of-concept study, sustained electrolysis of the reversible redox reaction I2 + 2e- T 2I- is used to cause the migration of the positively charged tetrapropylammonium counterion (TPA+) through the membrane. DMF, which is an aprotic solvent, was chosen to provide high solubility of the electrolyte and iodine, and to eliminate the possibility of H+ ions contributing to charge transport. Although demonstrated here for DMF solvent and cation transport, this pumping system can be easily extended to incorporate other solvent systems and anion transport.10,11 Results and Discussion In the present e-pump configuration, a current (i) is applied between the two compartments, creating a charge imbalance across the membrane, and the TPA+ cations respond by directionally migrating through the membrane to balance the charge. The efficacy of this system in promoting solvent transport through the membrane was recently demonstrated by our group with no pressure gradient,9 i.e., EO flow as distinct from EO pumping.2 The goals of the present work focus on advancing structural morphing technologies, which require the creation of controllable and reversible localized high pressures under modest applied current/voltage conditions. Pressure and flow control are accomplished by varying the current applied to (flowing through) the cell. As illustrated in Figure 1, in the forward e-pumping mode, reduction in the right chamber leads to cation transport into the right compartment to balance charge. An increase in pressure is observed, resulting from solvent flow through the membrane (Figure 1b). After a short induction time, linear pressure behavior is observed with a net forward rate in the presence of backflow of 2.2 atm/min or 2.0 atm/C, where C represents the coulombs of charge transferred across the membrane. The exact nature of this induction time is unclear, but small pressure-induced deformation of the membrane is likely to be the primary contributor. For this reason, we are exploring several strategies for increasing the mechanical rigidity of the membranes to mitigate this effect. It is important to note that the induction time is not due to the time response associated with the electrochemical cell itself. Voltages that are required to maintain constant current are established within a few seconds, indicating that the cell resistance, which is determined
by ion migration in the electrolyte solution and in the membrane, is constant throughout the experiments. As shown in Figure 1b, with only backflow (at i ) 0), a reverse rate of 2.3 atm/min is measured. This result indicates an ion-driven forward rate of approximately twice the pressure-driven back rate (at Pright ) 23 atm and Pleft ) 1 atm). The maximum pressure for this system has not yet been achieved, because the time that current can be passed in one direction without gas evolution is limited by the amount of I2 present in each compartment. To reverse the pumping direction, the current is simply reversed to induce reduction in the left chamber, effectively regenerating the redox species within each chamber. Without gas generation, high pressures have been generated using this pumping methodology. The feasibility of reversible cycling has been demonstrated and initial studies show no diminution in observed pressure for at least 10 cycles. Because of the complicated microstructure of Nafion membranes,12 it is difficult make direct comparisons to models that predict the maximum pressurization for a given applied potential (∆pmax/E) that should be achievable with the system described here. Using equations for traditional EO flow2 and estimated values or ranges of the critical parameters Debye shielding length (λD ) 0.29 nm), zeta potential (ζ ) 10-100 mV), and pore radius (a ) 1-4 nm), values for ∆pmax/E ranging from 2500 to 16 atm/V are obtained. The fact that the value reported here (4.5 atm/V) falls slightly below the lower end of this range is probably due to many factors. First, our experimental system only allowed a lower limit for ∆pmax to be determined. Second, it is well-known from a variety of studies that Nafion does not contain contiguous pores with a fixed radius.12 Third, although the ionic strength of the electrolyte solution (0.5 M) was used to estimate the critical parameters given previously, the actual ionic strength in Nafion membranes is significantly higher, which implies that the estimated values of λD and ζ may be too high. Fourth, pressure-induced deformation of the membrane has a tendency to reduce the rate at which pressure is generated in the cathode compartment. Two types of efficiency are important for micropumping and micropressurization systems: thermodynamic efficiency and energy conversion efficiency. The estimated thermodynamic efficiency (η) for micropumps is described by2
η(est) )
0.25Qmax∆pmax iE
(1)
where Qmax is the flow rate at ∆p ) 0 and ∆pmax is the maximum pressure achieved at a given current i and potential E. The value of η(est) for the present configuration of 0.03 is similar to values reported for other electrokinetic micropumps.2,5,6,13-17 This value represents a lower limit to the thermodynamic efficiency because the present system has not been optimized and the maximum pressure has not yet been realized. As a result, the ultimate thermodynamic efficiency is expected to be significantly greater. For morphing or pressurization applications, one of the key factors in assessing pump performance is the energy conversion efficiency. In the limit of maximum pressurization (i.e., no flow-rate output) demonstrated here, this efficiency can be directly determined from
energy conversion efficiency )
V∆P iEt
(2)
where the compartment volume (V) and increase in pressure (∆P) determine the mechanical energy stored in a pressurized cell (potential energy) and the current i, potential E, and time
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Table 1. Energy Conversion Efficienciesa current, i (A)
potential, E (V)
pressure increase, ∆P (atm)
time interval, t (s)
efficiency (%)
0.005 0.010 0.015 0.020 0.025
1.3 2.6 3.8 5.1 6.2
1.36 3.13 4.35 6.40 7.14
180 180 180 180 150
15 8.5 5.4 4.4 3.9
a
All measurements performed on a pre-pressurized system.
interval t determine the electrical energy input. In contrast with the thermodynamic efficiency η, this value focuses on the potential energy created by the pump upon pressurization. For the pressurization demonstrated in Figure 1b, the energy efficiency for the full pressurization range is 3.5%, and for the pre-pressurized range above 3.5 min, the energy efficiency is 4.1%. Mechanisms contributing to the loss of energy conversion efficiency include membrane deformation, backflow through the membrane, and Joule heating. Preliminary analysis of cycling behavior at different input currents shows that the energy efficiency for pressurization has an apparent current dependence (Table 1). The origin of this dependence is under investigation, but preliminary indications are that membrane deformation and backflow are likely to be dominant. During these initial proofof-concept studies, we have made no attempt to optimize the cell design or system parameters. Although performance is quite good, considerable parameter space remains for optimization, including solvent, electrolyte, membrane material, cell geometry, etc. At this point, it is interesting to compare this new e-pumping system with EO micropumps that have been reported in the literature.1,2,5,6,13-17 Both systems use an electric field to drive fluid flow through a porous medium, and the electric field gradients for the e-pump are of the same order of magnitude as EO pumping systems. In contrast, the porous separator in the e-pumping system is a membrane with nanoscale channel diameters, creating a flow mechanism that is not likely to be well-described by traditional models of EO.18 That is, for EO that occurs in well-defined pores such as glass capillaries, the EO mechanism that drives fluid transport is generated from a counterion gradient that forms at a charged wall (electrical double layer), together with an electric field applied parallel to the wall. Mobile counterions, driven by the applied field, migrate parallel to the wall and drag bulk fluid with them. EO micropumps reported in the literature are primarily silica-based packed capillaries or micromachined structures with channel diameters of 0.1-1 µm.1,2,5,6,13-17 Furthermore, studies of pressure-driven flow for hydrocarbon liquids through straight pores in mica with radii as small as 3.2 nm indicate that bulk viscosity values can be used at nanometer length scales.19 In the e-pumping system reported here, the ion-exchange membrane Nafion is used. The inhomogeneous structure and the nature of solvents in these membranes is the subject of considerable debate, and if open channels are present, they are similar to molecular dimensions (1-4 nm in diameter).12 Because the fixed negative charges associated with the ionexchange sites in Nafion are also distributed nonuniformly, it is questionable whether the traditional EO mechanism for solvent transport in straight pores can be applied straightforwardly to the e-pump described in this paper. Electrokinetically driven migration (ion transport in an electric field) certainly has an important role; however, the impact of the high-electricfield environment within flexible channels that contain fixed charges remains unresolved. Ultimately, pressure generation requires that the ion-driven solvent transport rate is greater than
the pressure-driven backflow. What happens when the channel diameter is on the molecular size scale? For the e-pumping system, sulfonate groups that line membrane channels 1-4 nm in diameter, with a membrane thickness of ∼180 µm at a pressure of 23 atm can be generated, as shown in Figure 1b. All of this leads to a remarkably high pressure gradient across the membrane of 1200 atm/cm! As shown in Figure 1b, the back rate is finite but unexpectedly small for this impressive pressure gradient. This observation is even more surprising, considering that the membrane absorbs 56% of its dry weight in solvent.9 That is, solvent constitutes more than one-third of this 180-µm-thick membrane, and yet the membrane still has a high resistance to pressure-driven flow. Finally, for the e-pump to generate pressures of this magnitude, pressure-driven backflow must remain minimal concurrent with ion-driven migration in the opposite direction. Specifically, mechanisms for iondriven solvent transport in this case must not open significant pathways for pressure-driven solvent backflow. Optimization of these counteracting processes is the fundamental key to the practical application of this new e-pumping strategy. In this study, we have illustrated the proof-of-concept with cation transport in an aprotic solvent. However, several aspects of this pumping mechanism through nanoporous membranes are expected to be generalizable. Thus, analogous applications for anion transport, other solvent systems, and other nanoporous membrane materials are feasible and will open new optimization pathways.10,11 Conclusion In this report, the significant pressurization capability of a new e-pumping system has been successfully demonstrated. Pressures up to 23 atm have been realized for this nonmechanical pumping system, using only modest applied potential and current conditions. This report, which used dimethylformamide as the solvent, shows that this e-pump is not limited to aprotic solvents and has been recently demonstrated with water; other protic solvents and mixed-solvent systems are under investigation. Similarly, other reversible redox couples and anion transport membrane systems should be feasible, creating a large, accessible parameter space for performance optimization. Finally, the macroscale simplicity of the e-pump lends itself to application in a wide range of microfluidic and nanofluidic applications, as well as microstructural morphing technologies. Acknowledgment This research was supported by the U.S. Army Research Office and the U.S. Army Research Laboratory (Contract No. DAAD19-03-1-0053). The authors thank Dennis Curtin (DuPont, Fluoroproducts Division) for donation of the membrane material. Literature Cited (1) Nguyen, N.-T.; Huang, X.; Chuan, T. K. MEMS-Micropumps: A Review. J. Fluids Eng. 2002, 124, 384. (2) Laser, D. J.; Santiago, J. G. A Review of Micropumps. J. Micromech. Microeng. 2004, 14, R35. (3) Chopra, I. Review of State of Art of Small Structures and Integrated Systems. AIAA J. 2002, 40, 2145. (4) Loewy, R. G. Recent Developments in smart structures with aeronautical applications. Smart Mater. Struct. 1997, 6, R11. (5) Zeng, S.; Chen, C.-H.; Mikkelsen, J. C., Jr.; Santiago, J. G. Fabrication and Characterization of Electoosmotic Micropumps. Sens. Actuators B 2001, 79, 107. (6) Paul, P. H.; Rakestraw, D. J.; Arnold, D. W.; Hencken, K. R.; Schoeniger, J. S.; Neyer, D. W. Electrokinetic high pressure hydraulic system. U.S. Patent No. 6,572,749, June 3, 2003 (and associated patents).
Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 475 (7) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; Chapter 8. (8) Izutsu, K. Electrochemistry in Nonaqueous Solutions; WileyVCH: Weinheim, Germany, 2002. (9) Norman, M.; Noble, R. D.; Koval, C. A. Electrochemical pumping of DMF electrolyte solutions across membranes. J. Electrochem. Soc. 2004, 151, E364. (10) Norman, M. A. Pumping Without Mechanical Parts: Development of a Low Voltage Electrochemical Fluid Pump Utilizing Membrane Transport and Redox Chemistry, Ph.D. Thesis, University of Colorado at Boulder, Boulder, CO, 2005. (11) Norman, M. A.; Evans, C. E.; Fuoco, A. R.; Noble, R. D.; Koval, C. A. Characterization of a Membrane-Based, Electrochemically Driven Pumping System Using Aqueous Electrolyte Solutions. Anal. Chem. 2005, 77, 6374. (12) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. ReV. 2004, 104, 4535. (13) Yao, S.; Santiago, J. G. Porous Glass Electroosmotic Pumps: Theory. J. Colloid Interface Sci. 2003, 268, 133. (14) Yao, S.; Hertzog, D. E.; Zeng, S.; Mikkelsen, J. C., Jr.; Santiago, J. C. Porous Glass Electroosmotic Pumps: Design and Experiment. J. Colloid Interface Sci. 2003, 268, 143.
(15) Reichmuth, D. S.; Chirica, G. S.; Kirby, B. J. Increasing the Performance of High-Pressure, High-Efficiency Electrokinetic Micropumps Using Zwitterionic Solute Additives. Sens. Actuators B 2003, 92, 37. (16) Chen, L.; Ma, J.; Tan, F.; Guan, Y. Generating High-Pressure SubMicroliter Flow Rate in Packed Microchannel by Electroosmotic Force: Potential Application in Microfluidic Systems. Sens. Actuators B 2003, 88, 260. (17) Min, J. Y.; Hasselbrink, E. F.; Kim, S. J. On the Efficiency of Electrokinetic Pumping of Liquids Through Nanoscale Channels. Sens. Actuators B 2004, 98, 368. (18) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th Edition; Wiley: New York, 1997. (19) Knudstrup, T. K.; Bitsanis, I. A.; Westermann-Clark, G. B. Pressure-Driven Flow Experiments in Molecularly Narrow, Straight Nucleopores. Langmuir 1995, 11, 893.
ReceiVed for reView April 19, 2005 ReVised manuscript receiVed November 2, 2005 Accepted November 22, 2005 IE0504708
