Abstract:
A valve actuator based on a conductive polymer gel is disclosed. A nonconductive housing is provided having two separate chambers separated by a porous frit. The conductive polymer is held in one chamber and an electrolyte solution, used as a source of charged ions, is held in the second chamber. The ends of the housing a sealed with a flexible elastomer. The polymer gel is further provide with electrodes with which to apply an electrical potential across the gel in order to initiate an oxidation reaction which in turn drives anions across the porous frit and into the polymer gel, swelling the volume of the gel and simultaneously contracting the volume of the electrolyte solution. Because the two end chambers are sealed the flexible elastomer expands or contracts with the chamber volume change. By manipulating the potential across the gel the motion of the elastomer can be controlled to act as a “gate” to open or close a fluid channel and thereby control flow through that channel.

Description:
[0001] The United States Government has rights in this invention pursuant to Contract No. DE-AC04-94AL85000 between the United States Department of Energy and Sandia Corporation for the operation of Sandia National Laboratories. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a conductive polymer gel and its application for driving a miniature actuator. In particular the present invention is drawn to a polythiophene-based conductive polymer gel used to open and close an actuator valve.  
           [0004]    2. Background and Related Art  
           [0005]    Current trends in the designs of analytical instrumentation, especially where fluids are used to carry anayltes to various instrument stations for processing (such as separation, mixing, species detection, and the like), are increasingly tending toward integrated and miniaturized systems. However, it is generally recognized that these designs will only be possible with the development of fast and efficient micro-actuators that can be operated as pumps and valves. As analytical equipment is scaled down the mechanical structures comprising these devices must also decrease in size. As is often the case, however, such macroscopic designs are not easily scaled down and replicated as microdevices and while still providing acceptable function. Thus there is a need for development of microscopic “machines” having useful performance characteristics. In this regard conductive polymers lend themselves well to such designs.  
           [0006]    The basic premise of the present invention is to use a conductive polymer gel having the property of changing dimension upon application of a small electrical potential, typically less than 1 volt, as a value in an actuator. The expansion and contraction of a body comprising the polymer gel results from a potential induced change in the oxidation state of the polymer and the associated diffusion of charge balancing ions and their coordinating solvent into and out of the gel (FIG. 1). As the polymer is oxidized, it forms positive charges that reside on the “backbone” of the polymer chain. As solvent coordinated anions diffuse into the polymer to balance the charge, the dimensions of the sample increase, due to the osmotic pressure developed during the influx of the two species, and result in an effect that can be harnessed to produce useful work.  
           [0007]    While a large body of prior art exists describing electrochemically driven swelling/de-swelling of conductive polymers none of the work in the literature has reported the direct measurement of the mechanical response of these materials to electrochemical stimulation. Neither is there any mention of the use of the work-producing effect of the response of these polymer gels to the application of small electrical potentials. Presumably, therefore, none have recognized the potential use of these materials as control members in micro-actuators.  
           [0008]    Beginning in the late 1980&#39;s (Murthy, et al.,  J. Chem. Phys.  (1987), v. 87, 2346) and more recently with Li and Aoki,  J. Electroanal. Chem. ( 1998), v. 453, 107; Kaneko and Kaneto,  Synth. Met.  (1999), v. 102, 1350; Madden, et al.,  Synth. Met.  (1999), v. 105, 61; and Otero and Bengoechea,  Langmuir  (1999), v. 15, 1323). Our interest in these materials stems from their potential use as small-scale actuators, valves or pumps in microsystems applications. A much smaller body of art has reported measurements of the tensile forces (see, for instance, Chen, et al.,  Macromolecules  (2000), v. 33, 1232) and shear modulus (Chen and Inganäs,  Synth. Met.  (1995) v. 74, 159) generated in pre-loaded thin films.  
         SUMMARY OF THE INVENTION  
         [0009]    In the present invention, the free extension, or closure forces (pressures), generated by these materials when they are confined, is characterized in an unambiguous manner. This work describes the synthesis of a polythiophene-based conductive polymer gel actuators. Measurements of gel extension and force in one axis under an applied square wave electrical stimulating pulse is shown.  
           [0010]    It is therefore an object of this invention to provide a conductive polymer gel responsive to low electrical potentials.  
           [0011]    It is another object of the invention to provide an actuator comprising said conductive polymer gel.  
           [0012]    Yet another object of this invention to provide a polythiophene polymer gel which is responsive to electrical potentials of less than ±1 volt.  
           [0013]    Still another object of the invention is to provide a polymer gel actuator generating an axial pressure of about 2 pounds-force per square-inch under the influence of a +0.8 volt potential. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows a general depiction of the morphology of the fracture surface of a frozen layer of the polymer gel of the present invention.  
         [0015]    [0015]FIG. 2 illustrates the swell response of the polythiophene gel of this invention as a function of time for a 20 minute period, under the influence of a +0.8/−0.5V applied square wave potential.  
         [0016]    [0016]FIG. 3A shows a detailed view of the gel expansion and cell current vs. time for a portion of the test shown in FIG. 2.  
         [0017]    [0017]FIG. 3B shows the applied potential waveform for the results shown in FIGS. 2 and 3A.  
         [0018]    [0018]FIG. 4 illustrates the relationship between gel extension and cell current for one oxidation pulse at approximately 300 minutes into the test shown in FIG. 2.  
         [0019]    [0019]FIG. 5 illustrates the axial pressure generated by the polythiophene gel as a function of time for a 20 minute period under the influence of a +0.8/−0.5V applied square wave potential.  
         [0020]    [0020]FIG. 6A illustrates the conductive gel plug assembled together with top and bottom porous electrodes.  
         [0021]    [0021]FIG. 6B illustrates one of porous electrodes and attached lead wire.  
         [0022]    [0022]FIG. 7 illustrates the rigid, nonconductive housing with the porous membrane between the two interior recessed portions provided to hold the gel plug assembly and the electrolyte solution.  
         [0023]    [0023]FIG. 8 illustrates the nonconductive housing into which the gel plug assembly and the electrolyte solution have been placed.  
         [0024]    [0024]FIG. 9 illustrates the assembled housing covered by top and bottom flexible layers to provide the actuator assembly.  
         [0025]    [0025]FIG. 10A illustrates the actuator assembly mounted onto a plenum in a first relationship so as to cover one or more fluid channels.  
         [0026]    [0026]FIG. 10B illustrates the actuator assembly mounted onto a plenum in the first relationship and actuated by applying an electrical potential to the porous electrodes attached to the conductive gel plug so as to open a passage between the fluid channels.  
         [0027]    [0027]FIG. 10C illustrates the actuator assembly mounted onto a plenum in a second relationship and actuated so as to cover one or more fluid channels. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    Interest in conductive polymer gels stems from their potential use as small-scale actuators, valves or pumps in microsystems applications. Synthesis of the gels was performed by the process below and has been generally described by others although the synthesis of the cross-linking agent has not been generally described.  
         [0029]    Synthesis of the Gel:  
         [0030]    The gels of the present invention were prepared as described in a process developed by Pei and lnganäts, (see  Synth. Met  (1993), v. 55-57, pp. 3724-3729). Ferric chloride powder, at a concentration of 2.3 equivalents, was placed in a reaction vessel and cold chloroform was slowly added in order to minimize disruption of the FeCl 3 . The vessel was then chilled to −30° C. After 2 hours, a solution of 3-octylthiophene (“OT”), 1,6-bis(2-thienyl)hexane (hereinafter “BTH”), and CHCl 3  was added and the vessel was placed in a refrigerator (4-5° C.) for 4 days. The supernant was then decanted from the resulting polymer mass. Methanol was added to the vessel to contract the chloroform-swollen gels. The polymer mass was then removed from the mold, placed in a Soxhlet® thimble and extracted with hot methanol for 24 hours. The resulting material contained less than 0.4% iron as determined by elemental analysis and inductively coupled plasma mass spectroscopy (ICP-MS). The measurement is an indication of the effectiveness of this purification method (It is believed that the purity of the gel is important to ensure its long term chemical and mechanical stability.). It is also important to note that only methanol, a mild reducing agent, was necessary.  
         [0031]    Synthesis of the Cross-Linking Agent BTH:  
         [0032]    While the synthesis of the polythiophene-based polymer gels has been published, the supporting references (Chen and Inganäs,  Synth. Met.  (1995) v. 74, pp. 159; Mangold, et al.,  Electrochimica Acta  (1999), v. 44, pp. 1865) give only a general description of the synthesis of the cross-linking agent BTH (1,6-bis(2-thienyl)hexane). BTH was synthesized using a Ni-catalyst to cross-couple 3-bromothiophene and 1,6-diiodohexane in a technique described by Rasmussen, et al., ( J. Heterocyclic Chem.  (1997), v. 34, pp. 285). 1,6-diiodohexane was dissolved in ether containing 2.1 equivalents of magnesium turnings under an argon purge. Two-tenths (0.2) equivalent of 1,2-dibromoethane in ether was added slowly to allow control of the ether reflux. When the spontaneous reflux subsided, the reaction mixture was heated at reflux for 1 hour. The reaction mixture was then cooled with an ice bath and slowly transferred with the use of a polytetrafluoroethylene (PTFE) cannula to a chilled flask containing 3 equivalents of 3-bromothiophene, and 0.09 equivalents of [1,3-bis(diphenylphosphino)propane]-dichloro-nickel (II) (i.e., Ni(dppp)Cl 2 ) in ether, such that the temperature did not rise above 5° C. After the transfer was completed, the reaction was stirred at room temperature for 24 hours. The mixture was poured into a ten-fold excess of ice/1M HCl. The organic layer was washed with 1M HCl and water and dried over anhydrous NaHCO 3 . The mixture was filtered, and the ether was removed by reduced pressure distillation. The remaining red oil was quickly distilled under vacuum to yield three fractions. The samples were refrigerated over 48 hours to yielding white crystals in two of the three fractions. The solid was collected by suction filtration and recrystallized to yield BTH. The structure was verified by  1 H and  13 C NMR and GCMS.  
         [0033]    The samples synthesized by this method were found to have varying morphology as shown in a freeze-fracture sample seen in FIG. 1. The bottom of the as-cast material, where gel deposition initiates, has a large aggregate structure and fewer pores, the top has a very fine structure with much smaller pores. The density of solid polythiophene is about 1 g/cm 3 . The measured bulk density of these gels is ˜0.3 g/cm 3  indicating that they have a large free volume. This free volume is important because it will determine, in part, the rate at which solvent and ions will be able to flow into the gel.  
         [0034]    Measurement of the Gel Displacement Behavior:  
         [0035]    The load and displacement characteristics of this polymer gel have been characterized. Specimens for these measurements consisted of right cylinders cut from a larger sheet of the gel. These cylinders were nominally 3 mm in diameter and 2 mm tall. For each test, a cylinder was inserted into a porous polypropylene frit that had a machined bore slightly larger than the diameter of the specimen. The frit constrained the sides and bottom of the polymer sample while allowing for the influx of solvent and ion. A variety of working electrode geometries have been used. Most typically, a thin platinum wire (0.33 mm in diameter) was inserted into the bottom of the specimen along its cylindrical axis. Alternatively, a fine, wire mesh basket formed to surround the specimen, yields equivalent response. The frit/specimen assembly was then placed in the custom electrochemical cell consisting of a glass cylinder and a PTFE (polytetrafluoroethylene) spacer that was used to locate the polypropylene frit in a fixed position at the bottom of the cylinder. The spacer also had milled slots to locate the reference and counter electrodes. The testing apparatus allowed for either the measurement of the free displacement of the gel (that is, the unconstrained axial extension) or the measurement of force generation when the gel was fully constrained. The entire electrochemical cell assembly was then placed in an argon-purged boxed for testing. The samples were subjected to a square wave function with an oxidation pulse of +0.8 V vs. an Ag/Ag +  standard electrode for one, five, or ten minutes, and a subsequent reduction pulse of −0.5V vs. the Ag/Ag +  standard electrode for the same time resulting in a two, ten, or twenty minute period.  
         [0036]    The extension of the gel along the cylindrical axis was measured using a precision linear displacement transducer manufactured by Microstrain Inc. (Burlington, Vt.). Displacement in axial dimension was normalized to the initial height of the gel cylinder and the absolute change in height is reported as “Extension (%)”. FIG. 2 shows the typical extension vs. time behavior over a long period of time for a gel specimen driven under a +0.8/−0.5 V square wave function having a 20 minute period. Over each cycle (i.e., one oxidation pulse followed by one reduction pulse, the axial change in dimension is approximately 1.5%. The extension is not fully recovered at the end of each reduction pulse and therefore there is a net increase in the axial length of the specimen over the &gt;24 hours of total testing. The discontinuities in the evolution of the specimen length correspond to minor variations in the ambient temperature. It is believed that this effect is mostly an artifact resulting from thermal expansion and contraction of the test apparatus. The local temperature trace is also shown in FIG. 2. FIGS. 3A and 3B shows a portion of this test beginning at 300 minutes and continuing to 400 minutes, in greater detail. FIG. 3A shows both the axial extension as well as the applied voltage waveform. It is clear from this figure that, within each oxidation pulse, the axial extension is not linear with time. Rather, it falls monotonically with increasing pulse time. It can also be seen that the extension of the specimen does not commence instantaneously upon the onset of the oxidation pulse. Instead, there is an approximately 90 second lag time before the specimen begins to swell axially. FIG. 3B shows the current generated in the cell over the same time period. The current waveform is quite reproducible, peaking at ≈+8 mA at the onset of the reduction pulse and gradually approaching a minimum value of ≈−2 mA early in the oxidation pulse. Note should be taken that, unlike the extension, the current switches instantaneously with the applied voltage waveform. FIG. 4 shows the relationship between the change in the axial gel dimension and current flow in the cell during one arbitrary oxidation pulse. The trace represents a linear best fit through the data. Since the current is a measure of electron flow (and therefore counter ion flow in the cell) it is clear that there is a direct relationship between ion flow, uptake into the specimen and the resulting dimensional change of the gel.  
         [0037]    Lastly, the axial pressure generated by the expansion of the gel against a fixed surface was directly measured using a precision force transducer provided by Sensotec, Inc., (Columbus, Ohio); results are shown in FIG. 5. Such a measurement is equivalent to the closure pressures that would be generated if the gel were used as a valve to seal an orifice. Given the large free volume of the gel, the pressure generation is surprisingly high, especially given a specimen mass of only about 6 mg. The measured force is about 14 kPa (about 0.15 kg f /cm 2  or about 2 psi) at steady state.  
         [0038]    While the force generated in the first few cycles is minimal, as the gel expands to completely contact the transducer probe, after about three cycles the generation of a mean pressure pulse reflects the hysteresis in the expansion/contraction behavior illustrated in FIGS. 2, 3A and  3 B.  
         [0039]    Preferred Embodiments and Best Mode  
         [0040]    The gel of the present invention finds utility as a seal or diaphragm to regulate or control flow through miniature actuator assemblies. In one embodiment, illustrated in FIGS.  6 - 10 , a small cylindrical plug  12  of the gel is formed in a mold or cut from a sheet of the polymer. Electrodes  15 , formed by attaching a disk of a conductive porous mesh  14  to an electrode wire  11 , as shown in FIG. 6B, are attached at top and bottom surfaces of plug  12 , as shown in FIG. 6A, to form plug assembly  10 . Alternatively, disk  14  may be a thin metal layer deposited directly onto a portion of the surface of gel plug  12 , either by vapor, chemical or particle deposition, or low temperature sputtering.  
         [0041]    Once formed, plug assembly  10  is immersed in an electrolyte solution, comprising charge balancing ions and an associated coordinated solvent species molecule. Although other solvent/salt systems are possible, the particular electrolyte system used in the present invention comprised a quaternary salt dissolved in an aprotic solvent, specifically 0.1M tetra-n-butyl ammonium perchlorate (CAS #1923-70-2) dissolved in acetonitrile. An electrical potential is then applied across electrode  15  until the polymer plug is stabilized and fully doped with solvent coordinated anions. The doped plug is then pressed into one side of a ring-shaped structure, or housing  20 , having top and bottom chambers  26  and  28 , seen in FIG. 7. Housing  20 , has a porous member  24  situated between chambers  26  and  28  that separates and isolates each side of the housing but which allows for the passage of charged ion species with little or no interference. Housing  20 , may be fabricated from materials such as polytetrafluoroethylenes, crystalline homopolymer acetal resins, polysulfones, polyurethanes, polyimides, polycarbonates, polymethylmethacrylates and similar polymers, moldable or machinable glasses, ceramics, silicon wafers and any other material that is, or can be, rendered nonconductive, rigid, and chemically inert. Porous member  24  may be a glass frit, a porous polymer ,such as for example polypropylene, or a porous non-corroding metal, such as for example nickel.  
         [0042]    After inserting plug assembly  10 , the opposite chamber of housing  20  is filled with a small quantity of electrolyte solution  35  (again 0.1M tetra-n-butyl ammonium perchlorate dissolved in acetonitrile) and the entire assembly is sealed, or encapsulated, with a thin flexible conformal coating of an inert elastomer such Silicoat®, or with a thin layer of a silicone adhesive, such as room temperature-vulcanizing polydimethylsiloxane, to provide actuator assembly  40 . Alternatively, as is shown in FIG. 9, housing  20  may be closed and sealed by gluing or otherwise bonding overlaying disks  41  and  42  of an elastomeric membrane, such as a polyimide, a silicon-based elastomer, or some other flexible membrane, onto the end surfaces  11  of the housing. An adhesive  43 , such a cyanoacrylates, cyanosilicones, cyanosiloxanes, polyurethanes, epoxies, and epoxy-polyamides, may be used to bond disks  41  and  42  to housing  10 , however, the choice of materials used to fabricate the housing may also require a surface preparation process in order to achieve proper adhesion. It is important that the ends of housing  20  are tightly sealed in order to prevent leakage of the electrolyte.  
         [0043]    As shown in FIG. 10B, assembly  40  can now be made to function as an valve assembly by again bonding assembly  40 , along a region directly adjacent to surface  11 , over plenum  45 , wherein inlet and outlet channels  46  and  47  are situated. Alternatively, assembly  40  may be mechanically held against plenum  45  by a clamping means (not shown). Using a source  50  of electrical energy, typically a battery, a charged capacitor, or low voltage power supply, valve actuation is performed by applying a small voltage potential (about ±1V) across electrodes  15  of plug assembly  10 . As shown in FIG. 10B, when an electrical potential is applied the polymer gel expands under the influence of migrating charge species (and their coordinating solvent molecules) diffusing into the gel body  12  from the reservoir of electrolyte  35  contained in chamber opposite the gel body. The result is a net transfer of material from the electrolyte reservoir chamber into the gel chamber resulting in a net volume expansion of the gel body and a net volume contraction of the electrolyte reservoir. Because these chambers are sealed this is outwardly manifested by an outward flexing of the disk  42  covering gel assembly  40 , and an inward flexing of the disk  41  covering electrolyte  35 . Disk  41 , therefore, act as a diaphragm which can be made to open or close orifices  46   a  and  47   a  allowing flowing between channels  46  and  47  by simply manipulation of the electrical potential applied across electrodes  11  attached to gel body  12 .  
         [0044]    Finally, the technique can be further modified by placing the diaphragm on the inside of a valve housing, or raised above plenum  45  (as shown in FIG. 10C) by a ring or spaced assembly  60  such that diaphragm disk  41  over gel plug  12  can be made to expand against one or more orifices to block or regulate fluid flow, or contract to allow unrestricted flow through the valve. Such an assembly also could easily be made to operate as a rudimentary pump with the addition of a set of opposing check valve (flaps) over selected groups of fluid channels through plenum  45  so that as gel plug  12  changes state under the influence of the applied voltage one valve is drawn open while the opposite valve is drawn shut. The pump would therefore operate by drawing a fluid into the space between the plenum, the spacer ring, and the gel plug, through one set of channels opening as the plug contracts and then disgorging the trapped fluid as the plug is expanded.  
         [0045]    The foregoing list of examples are intended, therefore, to describe only a few of the possible embodiment. Other embodiments will be realized by those skilled in the art as each read the description of the invention and the appended claims. The list of embodiments recited by the present invention, therefore, should not be construed to be exhaustive, nor should is limit the scope of the utility of this invention in any way.