BALANCED ASYMMETRIC ELECTRONIC PULSE PATTERNS FOR OPERATING ELECTRODE-BASED PUMPS

Abstract: Provided is a method of operating an electrode-based pump to pump a liquid, the method comprising periodically reversing the voltage polarity applied to the electrodes of the pump while maintaining a net flow of liquid in a desired direction. Further provided is an apparatus for pumping liquid with an electrode-based pump comprising: a channel of capillary dimensions; a pump comprising at least two electrodes inserted into the channel; a controller for controlling the voltages applied to the electrodes such that the pumps operate under a sequentially repeated pattern of polarity cycles, and such that over the period of the repeated pattern either (a) a first ratio of a voltage-integrated area (A1) associated with a first polarity to a voltage-integrated area (A2) associated with the other polarity or (b) a second ratio of a charge (q1) carried by the current associated with a first polarity to a charge (q2) carried by the current associated with the other polarity is between about 1:1/2 and about 1/2:1. Additionally provided are apparatuses and methods for electrophoresis.

Description: - 1 - Balanced Asymmetric Electronic Pulse Patterns for Operating Electrode-based Pumps The present invention is directed to a method of operating an electrode-based pump, which can be an electrohydrodynamic pump, or operating an electrophoresis apparatus, using an asymmetric electrical pulse pattern. A number of related applications have been filed on liquid distribution systems that use electrode-based pumps including US Application Nos.: 08/228,703, filed November 10, 1994 (DSRC 1 1402), 08/455,016, filed May 31, 1995 (DSRC 1 1402A); 08/454,771, filed May 31, 1995 (DSRC 1 1402B); 08/454,781, filed May 31, 1995 (DSRC 11402C); 08/454,774, filed May 31, 1995 (DSRC 1 1402D); 08/454,772, filed May 31, 1995 (DSRC 1 1402E); 08/454,768, filed May 31, 1995 (DSRC 1 1402F); 08/556,036, filed May 31, 1995 (DSRC 11402G); 08/469,238, June 6, 1995 (DSRC 11717); 08/556,423, November 9, 1995 (DSRC 11717A); 08/645,966, May 10, 1996 (DSRC 11717B); 08/483,331, June 7, 1995 (DSRC 11740); 08/730,636, October 1 1 , 1996 (DSRC 12385); and 08/744,386, November 7, 1996 (DSRC 12385A). These applications are hereby incorporated herein by reference in their entirety. Electrode-based pumps have no moving parts. Such systems, can be used for example, to relay liquids in very small devices, to conduct multiple parallel, but non- equivalent, small-scale syntheses, or to conduct multiple small-scale analytical reactions. In the course of operating such electrode-based pumping devices, the present applicant observed that some of the liquids selected to be pumped were susceptible to having unwanted electrochemical reactions occur at the pumping electrodes. Where this electrochemistry leads to substantial bubble formation at the pumping electrodes, pumping efficiency is diminished, or even stops. Accordingly, the invention provides methods for reducing or eliminating such bubbling at electrodes. In addition to improving electrode-based pumping, the invention is applicable in electrophoresis applications, where bubbling can also interfere with process efficiency. The process of the invention utilizes periodic reversals in the polarity applied to electrodes while maintaining a sufficient electronic impetus for pumping or electrophoresis in a desired direction. It is believed that, in the context of slab gel electrophoresis, particularly "submarine" gels that are electrophoresed when submerged under a layer of buffer, so-called - 2 - "pulsed field" methods have been used that apply modulations in field orientation to orient very large molecules during the electrophoresis process. See Schwartz and Cantor, Cell 37: 67-75, 1984. These are modulations are not believed to be reversals in voltage polarity applied to two reference electrodes. The range of pulse lengths typically used, 1 second to 90 minutes, corresponds to a frequency range of 1 to about 2 x 10 "4 Hz. It is not believed that there has been any motivation or suggestion for using such field modulation methods in a capillary environment, and indeed the goal of shifting the field lines between orientations where they obliquely intersect the direction of electrophoresis would be difficult to achieve in a capillary environment. Further, such methods have not been applied so as to minimize or eliminate bubbling. Complex arrays of electrodes have been applied in "traveling wave" pumping protocols with the intention of pumping liquids of higher conductivity. See, Fuhr et al., J. Microelectromechanical Systems 1 : 141-147, 1992. These methods apply traveling waves of voltage amplitude changes, but do not apply polarity reversals as in the methods of the invention. Further, while the methods of the invention can be applied to complex arrangements of electrodes, they can also, in contrast to traveling wave methods, be applied to very simple two-electrode pumps. Accordingly, pumps of the invention, but not those using complex electrode arrangements, can be readily integrated into high-density microfabricated fluid distributing systems or electrophoresis systems. By the present invention, methods of operating electrode-based pumps while minimizing bubbling due to electrolysis and other electrochemical reactions have been identified. The method operates by periodically reversing the polarity applied to the electrodes, but doing this in a manner that results in bulk liquid flow (pumping) in the direction desired. Summary of the Invention In general, the present invention provides a method of and apparatus for operating an electrode-based pump by inducing transfer of particles responsive to a preselected asymmetric motive force. The preselected motive force can be electrical, magnetic, mechanical, chemical, or a selected combination thereof. In a first embodiment, the invention provides a method of operating an electrode-based pump for the purpose of pumping a liquid, the method comprising periodically reversing the voltage polarity applied to the electrodes of the pump, while maintaining a net flow of liquid in a desired direction. Preferably, the method - 3 - further comprises selecting a pattern of polarity reversals and an associated voltage amplitude profile to reduce or eliminate the nucleation of gas at the electrodes. Preferably, the voltage polarity reversals are applied by repeating a defined pattern with a defined frequency, wherein the defined pattern is made up of a voltage amplitude profile of a first polarity, which first polarity causes pumping in the desired direction, and a voltage amplitude profile of a second polarity opposite that of the first. Preferably, the frequency is at least about 10 Hz. More preferably, the frequency is from about 10 Hz to about 100 MHz. Even more preferably the frequency can be between about 100 Hz to about 10 kHz,; and yet still more preferably from about 100 Hz to about 1 kHz. Preferably, the maximum applied voltage of the first polarity is greater than the maximum applied voltage of the second polarity. Preferably, the maximum voltage of the first polarity is at least about 10 N; more preferably, at least about 100 N; and yet more preferably, at least about 500 N. Preferably, the maximum voltage of the second polarity is no more than about 50% of the maximum voltage of the first polarity; more preferably no more than about 40%; yet more preferably no more than about 30%. Of course, the particular voltage applied will be related, for example, to factors such as the geometry of the pumping electrodes, the geometry of the associated fluid channel, and the susceptibility of the pumped liquid to dielectric breakdown. These preferred voltages reflect, among other things, a preference for voltages that are readily driven in high-density arrangements with off-the-shelf electronics. Preferably, over an operating period of time encompassing at least one polarity cycle either (a) a first ratio of a voltage-integrated area Ai associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity or (b) a second ratio of a charge qi carried by the current associated with a first polarity to a charge q 2 carried by the current associated with the other polarity is between about 1 : Vi and about Vz : 1. The voltage-integrated areas are the voltage profiles integrated over the relevant time period. Preferably, the operation of the pump satisfies one of the ratio parameters when the pump is operated over a period of time of at least about 10 seconds without generating a sufficient rate of bubbling to stop liquid flow. More preferably, the first ratio or the second ratio is between about 1 : 0.8 and about 0.8 : 1, yet more preferably the first or second ratio is between about 1 : 0.9 and about 0.9 : 1 , still more preferably is between about 1 : 0.95 and about 0.95 : 1 , yet still more preferably is between about 1 : 0.98 and about 0.98 : 1. In a highly preferred - 4 - embodiment, the apparatus is operated pursuant to a control mechanism set such that one of these ratios is equal to one. Of course, the limitations of the control mechanism will imply some operational variance from this control target, but in this latter embodiment the effective ratio should remain within about 20% of unity. In one aspect of the first embodiment, the electrode-based pump comprises three or more electrodes, and wherein the voltage monitored at two of the electrodes displays the periodically reversing voltage. Preferably, the pump is operated to pump a liquid with a time-averaged pressure of P, and with essentially no gas bubbles observable, for example, by eye or by the aid of a microscope, and wherein, if the electrodes were driven by a constant DC voltage effective to pump the liquid with pressure P, the liquid would generate observable gas bubbles. Preferably, the pump is operated to pump a liquid with essentially no observable gas bubbles, wherein the liquid has a conductivity of at least about 10 "4 S/m. In another aspect of the invention, the pump can be operated without bubble generation with a liquid having a conductivity of at least about 10 "3 S/m, or at least about 10 "2 S/m. In a second embodiment, the invention provides an apparatus for pumping liquid with an electrode-based pump having a liquid flow pathway, the apparatus comprising: a channel of capillary dimensions forming at least part of the flow pathway; a pump comprising at least two electrodes inserted into the flow pathway; a controller for controlling the voltages applied to the electrodes such that the pumps operate under a sequentially repeated pattern of polarity cycles, and such that over the period of the repeated pattern either (a) a first ratio of a voltage- integrated area Ai associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity or (b) a second ratio of a charge qi carried by the current associated with a first polarity to a charge q 2 carried by the current associated with the other polarity is between about 1 : Vi and about Vi : 1. Preferably, the controller operates pursuant to a programmable microprocessor, and wherein the microprocessor is programmed to sequentially and repetitively operate the pattern of polarity cycles. Preferably, either (i) the electrodes are inserted into the channel or (ii) the flow pathway comprises a reservoir for feeding liquid to the channel, one or more electrodes of the pump are inserted into the reservoir, and one or more of the electrodes are inserted into the channel. - 5 - In a first aspect of a third embodiment, the invention provides a method of operating a capillary electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway and operating with an electrophoresis liquid, the method comprising: periodically reversing the voltage polarity applied to the electrodes while maintaining a net electrophoretic migration of one or more solutes in a desired direction. Preferably, the method further comprises selecting a pattern of polarity reversals and an associated voltage amplitude profile reduce or eliminate the nucleation of gas at the electrodes. Preferably, the voltage polarity reversals are applied by repeating a defined pattern with a defined frequency, wherein the defined pattern comprises a voltage amplitude profile of a first polarity, which first polarity causes electrophoresis of the one or more solutes in the desired direction, and a voltage amplitude profile of a second polarity opposite that of the first. Preferably, the frequency is at least about 10 Hz. Preferably, the frequency is from about 10 Hz to about 100 MHz, more preferably from about 100 Hz to about 10 kHz, yet more preferably from about 100 Hz to about 1 kHz. As above, preferably the maximum voltage applied of the first polarity is greater than the maximum applied voltage of the second polarity. Preferably, the maximum voltage of the second polarity is no more than about 50% of the maximum voltage of the first polarity, more preferably no more than about 40%, yet more preferably no more than about 30%. Preferably, over an operating period of time encompassing at least one polarity cycle either (a) a first ratio of a voltage-integrated area Ai associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity or (b) a second ratio of a charge qi carried by the current associated with a first polarity to a charge q carried by the current associated with the other polarity is between about 1 : Vi and about Vi : 1. Preferably, the operation of the electrodes satisfies one of the ratio parameters when the pump is operated over a period of time of at least about 10 seconds without generating a sufficient rate of bubbling to retard electrophoretic migration. Preferably, the first ratio or the second ratio is between about 1 : 0.8 and about 0.8 : 1, more preferably the first or second ratio is between about 1 : 0.9 and about 0.9 : 1, yet more preferably is between about 1 : 0.95 and about 0.95 : 1 , yet still more preferably is between about 1 : 0.98 and about 0.98 : 1. In a highly preferred embodiment, the apparatus is operated pursuant to a control mechanism set such that one of these ratios is equal to one. Of course, the limitations of the control mechanism will imply some operational - 6 - variance from this control target, but in this latter embodiment the effective ratio should remain within about 20% of unity (1). Preferably, the electrodes are operated to move a the solute with a time averaged mobility of M and with no gas nucleations observable by eye, and wherein the liquid that would, if the electrodes were driven by a constant DC voltage effective to move the one solute with a mobility of M, generate gas nucleations that would be observable by eye. Preferably, the electrodes are operated to move one solute with no gas nucleations observable by eye, wherein the liquid has a conductivity of at least about 10 ~2 S/m. In a second aspect of the third embodiment, the invention provides a method of operating an electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway and operating with an electrophoresis liquid, the method comprising periodically reversing the voltage polarity applied to the electrodes while maintaining a net electrophoretic migration of one or more solutes in a desired direction, wherein a pattern of polarity reversals and an associated voltage amplitude profile are selected to reduce or eliminate the nucleation of gas at the electrodes. Preferably, the voltage polarity reversals are applied by repeating a defined pattern with a defined frequency of at least about 10 Hz. In a fourth embodiment of the invention, the invention provides an electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway comprising: two electrodes situated such that the distance between them encompasses the electrophoresis pathway, in which pathway electrophoretic separation is anticipated to occur; a controller for controlling the voltages applied to the electrodes such that the electrodes operate under a sequentially repeated pattern of polarity cycles, and such that over the period of the repeated pattern either (a) a first ratio of a voltage-integrated area Ai associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity or (b) a second ratio of a charge qi carried by the current associated with a first polarity to a charge q 2 carried by the current associated with the other polarity is between about 1 : Vi and about Vi : 1. Preferably, the controller operates pursuant to a programmable microprocessor, and wherein the microprocessor is programmed to sequentially and repetitively operate the pattern of polarity cycles. In an embodiment, the apparatus incorporates for the electrophoresis pathway additional electrodes at one or both ends. - 7 - Brief Description of the Drawings Figure 1 displays a square wave voltage driving pattern. Figure 2 shows a schematic of electrical drivers that can be used with the invention. Definitions The following terms shall have, for the purposes of this application, the meaning set forth below. In particular, for the purpose of interpreting the claims, the term definitions shall control over any assertion of a contrary meaning based on other text found herein: • capillary dimensions "Capillary dimensions" are dimensions that favor capillary flow of a liquid. Typically, channels of capillary dimensions are no wider than about 1.5 mm. Preferably channels are no wider than about 500 μm, yet more preferably no wider than about 250 μm, still more preferably no wider than about 150 μm. • polarity cycle A polarity cycle is the whole of (a) a continuous period operating with one of the polarities, and (b) an immediately following continuous period operating with the opposite polarity. Detailed Description of the Invention Electrode-based Pumping The present invention had its origin in the observation that in electrode-based pumping processes, particularly in electrohydrodynamic pumping processes, a number of liquids exhibited excessive bubbling. These are typically liquids with relatively high conductivity such as 0J M butylamine in dimethylformamide (conductivity 0.007 S/m), but include other liquids of lower conductivity. It has now been observed that net fluid movement in the desired direction could be maintained while minimizing electrolysis, other electrochemical processes, or like bubble-generating events at the electrodes by periodically reversing the polarity applied to the electrodes. In perhaps its most simple form, the invention uses a repeat pattern of a square wave of voltage of a first polarity which drives pumping in the desired direction followed by a square wave of lower amplitude and longer duration, as illustrated in Figure 1. The initial observation was that optimization of bubble minimization appeared to occur when the product of the amplitude of the first square wave times its duration was approximately equal to the corresponding product for the second square wave. In the generalized statement of this principle, one compares the areas defined by integrating voltage - 8 - amplitude over time. Without being bound by theory, it is believed that these areas serve as a surrogate for the more important parameters, which are the net charges carried by the respective voltage polarities. However, this area measurement is generally the most readily measured parameter and, at least over the voltage ranges most useful for operating the electrode-based devices of the invention, is highly correlated with the net charge parameter. This correlation is due to the current, over the relevant voltage range, being substantially linearly related to the voltage. In Figure 1 , a square wave of voltage of a polarity that causes the desired pumping is applied over a period of t + = (τ - τ ) and has an amplitude of ξ + . Immediately thereafter, an opposite polarity voltage of amplitude ξ_ is applied for a period of t. = (τ - τ 2 ). ξ + is larger than ξ_, such that ξ + = bξ_, where b is larger than 1. t + is shorter than t_, such that (t + )=c(t_), where c is smaller than 1. These values are selected such that (ξ + )(t + )=(ξ . )(tj. Applicant believes that the theoretical basis for why net fluid movement occurs even when the net charges carried by the two opposing voltages are equal has been identified. This theory is presented below, though of course the invention is not restricted to the theory. With simple square wave voltage driving patterns, the experimental observation is that net liquid movement is dictated by the larger amplitude square wave. For more complex patterns, simple empirical examinations can identify the net direction of flow, or resort can be made to the guidance provided by the theory presented below. The theory implies that pumping pressure is related to the square of the voltage amplitude. Accordingly, the ratio of pressure generated in a driving portion of the voltage driving pattern to that generated in the reverse portion can be much greater than the ratio of the associated voltages, resulting in greater flow in the desired direction. Without being bound by any particular theory, possible theoretical considerations in electrode-based pumping are set forth in detail in U.S. Application No. 08/556,423, filed November 9, 1995 (DSRC 1 1717A). At least two types of such electrode-based pumping, i.e., electrokinetic pumping, have been described, typically under the names "electrohydrodynamic pumping" (EHD) and "electroosmosis" (EO). EHD pumping has been described by Bart et al., "Microfabricated Electrohydrodynamic Pumps," Sensors and Actuators, A21-A23: 193-197, 1990 and Richter et al., "A Micromachined Electrohydrodynamic Pump," Sensors and Actuators, A29: 159-168, 1991. EO pumps have been described by Dasgupta et al., - 9 - "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analysis," Anal. Chem., 66: 1792-1798, 1994. EO pumping is believed to take advantage of the principle that the surfaces of many solids, including quartz, glass and the like, become charged, negatively or positively, in the presence of ionic materials, such as salts, acids or bases. The charged surfaces will attract oppositely charged counter ions in solutions of suitable conductivity. The application of a voltage to such a solution results in a migration of the counter ions to the oppositely charged electrode, and moves the bulk of the fluid as well. The volume flow rate is proportional to the current, and the volume flow generated in the fluid is also proportional to the applied voltage. Typically, in channels of capillary dimensions, the electrodes that cause flow can be spaced further apart than in EHD pumping, since the electrodes are only involved in applying force, and not, as appears to apply in EHD, in creating charges on which the force will act. EO pumping is generally perceived as a method appropriate for pumping conductive solutions. The present invention is believed to be applicable to all forms of electrode-based pumping, which pumping is also referred to herein as "electrokinetic" pumping. The invention is most preferably applied to electrode-based pumping where the field strength directly acts on liquid components to create pressure, as in EHD and EO. The invention is also applicable to other electrode-based methods, such as traveling wave methods, that are believed to operate by creating heat convection forces. The pumps applied in the present invention can be made of simple wire electrodes. Alternatively, where high density arrangements of electrode-based pumps are anticipated, reference can be made to U.S. Application No. 08/554,887, filed November 9, 1995 (DSRC 1 1948), which describes methods of mass producing high density microelectrodes using microfabrication techniques. This application is hereby incorporated into this disclosure by reference in its entirety. These electrodes are formed on plates of dielectric material such as glass, and each such plate is bonded to a plate in which channels have been etched. See U.S. Application No. 08/745/766, filed November 8, 1996 (DSRC 1 1865A) for plate bonding methodology, which application is hereby incorporated into this disclosure by reference in its entirety. References are made herein to pumping (or electrophoresis) parameters that minimize the gas nucleations observable by eye. It should be recognized that, in circumstances - 10 - where the electrodes cannot be viewed directly, this reference is to conditions which, if repeated in a comparable device where the electrodes can be viewed, shows an appropriate level of gas nucleations. It should be recognized that the invention can be applied in several contexts beyond simple two-electrode pumps, and that these contexts are intended to be within the scope of the claims. For example, the invention can be applied where several electrodes are incorporated into a channel to allow flexibility in selecting electrodes most appropriate for pumping a particular fluid. In complex electrode-based pumping protocols, such as traveling wave protocols, the invention is being utilized if it is used with respect to the voltage patterns applied to two electrodes in the array of electrodes. Theoretical Considerations Without being limited to theory, it is believed that the following conditions have to be satisfied in order to have a net fluid flow in one direction without gas nucleation in the electrodes: Al) Symmetry condition: v(t)dt Q (1) * The values % \ , τ 2 and τ 3 designate, respectively, the start time of a polarity cycle, the end time for the first continuous period operating under a first polarity, and the end time for the following continuous period operating under the second polarity, as illustrated in Figure 1. This condition indicates that the area of the positive part Aj of the voltage waveform is approximately equal to the area of the negative part A 2 of the voltage waveform. This condition can be expressed as A, = \v(t)dt A 2 = jv(t)dt (2) Λ ~ A2 A2) Asymmetry condition: - 1 1 - , where ξ + and ξ_ are, respectively, applied voltages during the first continuous period of the polarity cycle and the second continuous period. This condition indicates that in order to obtain a net fluid flow the amplitude of the positive part of the voltage waveform has to be larger than the amplitude of the negative part of the voltage waveform Experimental results suggest that, in preferred operations of the pumps, a balance of charges exists during positive and negative pulses. Consider a pulse as shown in fig 1, where ξ + is the positive amplitude , ξ. is the negative amplitude, t + (= τ 2 - τ, ) is the duration of the positive pulse, and t_ (= τ 3 - τ,) is the duration of the negative pulse. Considering a resistive system, the charge during the positive pulse 1 1 and the charge during the negative pulse qi can be written as q_ = \i(t)dt (3) and q 1 2 2 = \i(t)dt (4) where i(t) is the cuirent as a function of time. The experimental observation suggests that the bubble formation or gas nucleation at the electrodes is minimized and in many cases eliminated when the charge in the positive pulse is comparable to the charge in the negative pulse. Q\ Q2 (5) For a square pulse excitation like figure 1 and for a resistive system, the charge balance condition gives the following expression ξ + t + = ξj_ (6) Assuming that the positive voltage amplitude is b times the negative voltage amplitude and that the positive voltage duration is c times the negative voltage duration. (7) t + = ct_ Then, we have to satisfy the following condition be = 1 (8) B) Net fluid flow: - 12 - Experimental observations indicate that a net fluid flow is obtained. From the theoretical point of view, the following seeks to determine the relationship between the mass transferred during the positive and negative pulses. According to Stuetzer (W.F. Pickard, J. Appl. Phys., 34: 246, 1963), the pressure developed by ion drag pumping like EHD pumping is given by: = |fiE β 2 (i7> (9) where *. = -- (10) , where / is the distance between electrodes, V is the potential and <η> is a dimensionless parameter. Therefore, the pressures generated by the positive and negative pulses are given by Py (")η ' (11) The pressure P is given by P = RQ (12) where Vol Q = Υ (13) R is the fluid flow resistance, Q is the fluid flow, Vol is the volume and t is the time Using equations (11) and (12), we can write ξ 2 Q — = — (14) ?- Q - Using equations (13), (14) and (7), we obtain Vol + = b 2 c Vol_ (15) Where Vol + is the volume displaced during the positive cycle in one direction and Vol. is the volume displaced during the negative circle in the opposite direction. Multiplying both sides by the density (fluid density, d = m + /Nol + = m_/Vol_, where m + and m_ are the masses of the volumes moved by the electrode-based pumps), we obtain: m + = b 2 cm_ (16) Applying the energy condition bc= 1 , we have: - 13 - m + - bm_ (17) Thus, if b ≠ 1 , meaning that the two voltage amplitudes are asymmetric, there will be a net mass transfer of fluid. Electrophoresis The same types of operational guidelines as apply for electrode-based pumping apply for electrophoresis. The major difference is that when seeking to electrophorese a certain class of molecules voltage parameters will be selected based on the voltages typically used, in contexts outside of the present invention, with comparable electrophoresis devices, and these voltages may not be similar to the voltages used with like fluids in the electrode-based pumping context. Drivers Driving circuits are set forth in U.S. Application No. 08/469,238, filed June 6, 1995 (DSRC 1 1717) and U.S. Application No. 08/556,423, filed November 9, 1995 (DSRC 11717A), which applications are incorporated herein in their entirety. A preferred circuit is set forth in Figure 2. Box 1 houses control electronics, which can be interfaced with a computer. Solid state switch A (for example a transistor or an operational amplifier) is used to generate a positive cycle (or if the driving polarity is negative, a negative cycle). Solid state switch B is used to generate a negative cycle (or if the driving polarity is negative, a positive cycle). The voltage applied to the pump 2 is a combination of the two waveforms. The waveforms generated by the two illustrated switches A and B, and the resulting waveform, are represented in the figure, with an indication of a reference voltage. Microfluidics Devices United States Patent Application No. 08/556,036, filed May 31, 1995 (DSRC 11402G), No. 08/483,331, filed June 7, 1995 (DSRC 1 1740), No. 08/730,636, filed October 11, 1996 (DSRC 12385), and No. 08/744,386, November 7, 1996 (DSRC 12385A) describe liquid distribution systems. Typically, these systems form, in sandwiched layers of a substrate which is preferably glass, reservoirs and connected complex networks of channels. The liquids in the reservoirs are typically maintained feeds from external containers. By selectively activating electrode-based pumps, liquids from a reservoir can be selectively distributed to reaction cells via the channels. Often, the electrodes are inserted into the channels. In some - 14 - embodiments, however, particularly where a reservoir contains liquids of relatively high conductivity, one or more of the driving electrodes can be placed in the reservoir so as to provide the relatively greater spacing between the driving electrodes that favors the EO mechanism of electrode-based pumping. Typically, where a reservoir has such electrodes, the electrode-based pumps that selectively drive liquid through the channels each will include a separate set of one or more electrodes inserted into each channel connected to the reservoir, but a plurality of such pumps will share electrodes located in the connected reservoir. The following examples further illustrate the present invention, but of course, should not be construed as in any way limiting its scope. Example For the example, a planar pump in a channel 240 μm wide and 80 μm deep was used. The pump was two electrodes inserted into the channel with a 250 μm spacing between them. 1 ) A 0.1 M solution of butylamine in dimethylformamide was pumped under DC conditions and separately pursuant to the invention. The DC conditions were the application of 40 N, which resulted in immediate bubbling. Pumping stopped as a result of the bubbling. Two sets of conditions pursuant to the invention were applied as follows: Frequency Voltages Periods A 1 kHz ξ + = 500 N τ + = 0.2 ξ. = -125 N τ. = 0.8 ~ B 500 Hz ξ + = 500 N τ + = 0.4 ξ. = -125 N τ. = 1.6 Under both conditions A and B, fluid flow is generated by the pump without gas nucleation or bubbling. 2) Dimethylformamide, containing a trace of water, was pumped under DC conditions and separately pursuant to the invention. The DC conditions were the application of 40 N, which resulted in bubbling at both electrodes. Conditions pursuant to the invention were applied as follows: 15 - Frequency Voltages Periods 1 kHz ς + = 300 N τ + = 0.2 ms ξ- = -75 N τ_ = 0.8 ms Conditions C avoided the formation of bubbles. While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations in the preferred devices and methods may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the claims that follow.

Claims: - 16 - What is Claimed: 1. A method of operating an electrode-based pump, the method comprising inducing transfer of particles responsive to a preselected asymmetric motive force, wherein the preselected asymmetric motive force is one of an electrical force, a magnetic force, a mechanical force, a chemical force, and a selected combination thereof. 2. The method of claim 1 wherein the preselected motive force includes an electrical force, the pump is operated to pump a liquid, the pump having a voltage polarity applied between electrodes, the method comprising periodically reversing the voltage polarity applied to the electrodes of the pump while maintaining a net flow of liquid in a desired direction. 3. The method of claim 2, further comprising at least one of: a. selecting a pattern of polarity reversals and an associated voltage amplitude profile to substantially reduce the nucleation of a gas at the electrodes; and b. applying the voltage polarity reversals by repeating a defined pattern with a defined frequency, wherein the defined pattern comprises a voltage amplitude profile of a first polarity, which first polarity causes pumping in the desired direction, and a voltage amplitude profile of a second polarity generally opposite that of the first polarity. 4. The method of claim 2, wherein one of: a. the voltage polarity reversals are applied by repeating a defined pattern with a defined frequency, wherein the defined pattern comprises a voltage amplitude profile of a first polarity, which first polarity causes pumping in the desired direction, and a voltage amplitude profile of a second polarity opposite that of the first; b. during an operating period of time encompassing a polarity cycle of voltage reversals at the electrodes, in which at least one of: ( 1 ) the voltage polarity reversals are applied by repeating a defined pattern of polarity reversal with a defined frequency; (2) a first ratio and a second ratio have a magnitude of between about 1 : V. and about '/ 2 : 1, the first ratio being of a voltage-integrated area A f associated with a - 17 - first polarity to a voltage-integrated area A 2 associated with the other polarity, and the second ratio being of a charge qi carried by current associated with a first polarity to a charge q carried by current associated with the other polarity; (3) a frequency corresponding to the polarity cycle is at least about 10 Hertz; (4) a maximum first polarity applied voltage magnitude of the first polarity is generally greater than a maximum second polarity applied voltage magnitude; and (5) a selected combination of 5(b)(1), 5(b)(2), 5(b)(3), and 5(b)(4); wherein gas nucleation at the electrodes is substantially reduced thereby. 5. The method of claim 3 is at least 10 Hertz, and the maximum applied voltage of the first polarity is greater than the maximum applied voltage of the second polarity. 6. The method of claim 3, wherein at least one of: a. the operation of the pump satisfies a ratio in 5(a)(2) when the pump is operated over a period of time of at least about 10 seconds without generating sufficient gas bubbles to substantially impair liquid flow; and b. the pump is operated to pump a liquid with a time-averaged pressure of about P and with no gas formations observable by eye, the liquid having a property that , if the electrodes were driven by a constant DC voltage effective to pump the liquid with pressure P, gas bubbles would be observable. 7. An apparatus for pumping liquid with an electrode-based pump having a liquid flow pathway, the apparatus comprising: a. a channel of capillary dimensions forming at least part of the flow pathway; b. a pump comprising at least two electrodes inserted into the flow pathway; c. a controller for controlling the voltages applied to the electrodes such that the pumps operate under a sequentially repeated pattern of polarity cycles, and such that over the period of the repeated pattern, one of: (1) a first ratio of a voltage-integrated area Ai associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity is between about 1 : V. and about Vi : 1.; and - 18 - (2) a second ratio of a charge qi carried by the current associated with a first polarity to a charge qi carried by the current associated with the other polarity is between about 1 : Vi and about Vi : 1. 8. A method of operating one of: a. a capillary electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway and operating with an electrophoresis liquid, the method comprising periodically reversing the voltage polarity applied to the electrodes while maintaining a net electrophoretic migration of one or more solutes in a desired direction, wherein the pattern of polarity reversals and an associated voltage amplitude profile are selected to substantially reduce the formation of gas bubbles at the electrodes; and b. an electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway and operating with an electrophoresis liquid, the method comprising periodically reversing the voltage polarity applied to the electrodes while maintaining a net electrophoretic migration of one or more solutes in a desired direction, wherein the pattern of polarity reversals and an associated voltage amplitude profile are selected to reduce or eliminate the formation of gas at the electrodes. 9. The method of claim 8, wherein at least one of: (a) forming one of a first ratio and a second ratio over an operating period of time encompassing at least one polarity cycle such that the first ratio is of a voltage-integrated area Ai associated with a first polarity to a voltage-integrated area A associated with the other polarity and the second ratio is of a charge qi carried by the current associated with a first polarity to a charge q 2 carried by the current associated with the other polarity, at least one of the first and second ratios being between one of about 1 : Vi and about Vi : 1 ; and (b) the electrodes are operated to move a solute with a time-averaged mobility of about M and produce essentially no observable gas bubbles, and wherein, if the electrodes were driven by a constant DC voltage effective to move the solute with a mobility of about M, the liquid would generate observable gas bubbles. - 19 - 10. An electrophoresis apparatus having at least one electrode at each end of an electrophoresis pathway comprising: two electrodes situated such that the distance between them encompasses the electrophoretic pathway, in which pathway electrophoretic separation is anticipated to occur; a controller for controlling the voltages applied to the electrodes such that the electrodes operate under a sequentially repeated pattern of polarity cycles, and such that over the period of the repeated pattern either (a) a first ratio of a voltage-integrated area A] associated with a first polarity to a voltage-integrated area A 2 associated with the other polarity or (b) a second ratio of a charge q f carried by the current associated with a first polarity to a charge q 2 carried by the current associated with the other polarity is between about 1 : Vi and about Vi : 1.

Inventors: MCBRIDE STERLING E, MCBRIDE, STERLING, E.
Applicants: SARNOFF CORP, SARNOFF CORPORATION
Application year: 1998
Main classification: G01N27
IPC codes: G01N27