INTEGRATED MICRO-PUMP AND ELECTRO-SPRAY

RELATED APPLICATION

[0001] This application claims priority to co-pending U.S. Provisional Patent Application No. 60/655,437, entitled "Integrated DC pump/electro-spray," filed on February 24, 2005, and is hereby incorporated by reference in its entirety.

GOVERNMENT INTEREST STATEMENT

[0002] The United States Government has rights in this invention pursuant to Contract No. DAAB 07-03-3-K414 with the United States Army.

FIELD OF THE DISCLOSURE

[0003] This disclosure relates generally to micro-fluidic pumping and spraying devices, and, more particularly, to an integrated DC micro-pump and electro-spray and methods for manufacturing the same.

BACKGROUND

[0004] A Total Analytical System (TAS) is a chemical analysis system that automates all necessary steps for analysis of a chemical substance (e.g. sampling, transport, filtration, dilution, chemical reactions, separation and detection). Considerable effort in analytical chemistry has been directed toward the miniaturization of these systems to enable rapid, portable, and automated analyses of small-volume samples. Ideally, a micro-TAS (μ-TAS) integrates all function units necessary to analyze a chemical sample on a single micro-fluidic substrate, sometimes referred to as a "lab-on-a-chip." Because the flow velocity in a micro-channel scales as the channel radius squared,

scaling down the system by a factor of n requires an n ² increase in the driving pressure to maintain the same velocity. As such, a notable component of a μ- TAS is a powerful micro-fluidic pump capable of generating high pressure. Moreover, this pump may be integrated into the entire system on the same substrate because fluid transfer from an external pump may defeat many of the advantages of μ-TAS and may require tedious tubing connection for each run. Constant high-pressure but low flow rates for micro- and nano-liter samples and especially pulsation-free flows are often the primary pump requirements for micro-flow injection analysis (μ-FIA), micro-column liquid chromatography (μ-LC), and other μ-TAS.

[0005] One micro-pump that has been proposed for use with μ-TAS is the electroosmotic pump (EOP), which uses electric current to cause a bulk fluid movement through a system. EOPs typically suffer from several major problems. One possible problem is that, with open channel or capillary EOPs, there is a low stall pressure and, therefore, these EOPs are generally not used in systems with high-pressure loads. High-pressure build-up can be achieved if the pump channel is smaller or if a dense packing material is used to produce large hydrodynamic resistance. Unlike mechanical pumps, which generate a local high pressure and for which hydrodynamic resistance in the pump would reduce this driving pressure, pure electroosmotic flow does not produce a pressure field, but instead relies on hydrodynamic resistance to reduce the flow and build a high pressure along the pump channel. Hence, in a counter-intuitive manner, EOP pump channels need to be as small as

possible. However, a single pump channel cannot produce enough flow and a large bundle of small micro-channels is needed for the EOP. [0006] Another potential problem is electrolytic bubble generation, because of the large current in the open channel. In aqueous solutions, when the applied electrode potential exceeds a threshold approximately 1.1 V, significant electrolysis and other electrode reactions may occur, producing ions that contaminate the sample and generate bubbles, which block the micro- channels. To eliminate this blockage, a bubble-releasing device may be used downstream of the pump, or alternatively, the electrodes may be placed in isolated open reservoirs such that bubbles can escape and the ions cannot invade the flow channel. However, the reservoir housing should be a conductor to enable electric field penetration. The traditional solution to the reaction problem is to reduce the current by using dense packing. Depending on the type of packing used, too dense a packing may be undesirable because it can create or further aggravate clogging problems.

[0007] Because both the low-pressure and electrode reaction disadvantages of EOPs can be reduced by dense packing within the pump channel, considerable effort has been devoted to fabrication of multiple micro-channels by lithography or internal packing with high surface charge density that still allows electroosmotic flow. One strategy is to pack the pump channel with small particles.

[0008] An attempt at this is shown in FIG. 1, which illustrates a portion of conventional micro-pump 100. The conventional pump 100 is formed in a substrate 102. The substrate has a channel 104 that includes an inlet 106 and

an outlet 108. An electrolyte flows through the inlet past a packing 110 to the outlet 108. The packing 110 is made of a plurality of micro-beads 112. The micro-beads 112 are packed into the channel 104 through the inlet 106. The channel 104 further includes a filter 114 that holds the micro-beads 112 together as the packing 110 because the opening of the filter 114 has a dimension smaller than the length of the diameter of the micro-beads 112. [0009] The presence of the micro-beads 112 increases the pressure in the channel 104, which assists in the operation of the pump 100, as described above. However, in addition to clogging problems that a dense packing can create, the installation of the micro-beads 112 is oftentimes extremely tedious, time-consuming and expensive.

[0010] In other conventional pumps, a high pressure is created by etching small channels into a substrate. Etched channels can reach dimensions on the micrometer scale, but the etching process is also oftentimes tedious and expensive. Further, a simply-etched channel would not contain a porous material. Thus, several etched channels may need to be formed in a substrate to achieve optimal pressure in the pump. Repeating the etching process directly affects the key metrics of the manufacturing process, i.e., time and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic illustration of a prior art micro-pump with micro- bead packing.

[0012] FIG. 2 is a schematic diagram of an example integrated micro-pump and electro-spray.

[0013] FIG. 3 is a photograph of a magnified example monolithic matrix in an example capillary.

[0014] FIG. 4 is a graph plotting pump curves (pressure versus flow rate) for the example micro-pump of FIG. 1 at different voltages using acetonitrile as the pumping fluid.

[0015] FIG. 5 is a photograph of a magnified Taylor cone emitting from the example electro-spray of FIG. 1.

[0016] FIG. 6 is a schematic diagram of the example integrated micro-pump and electro-spray as a front-end to a mass spectrometer. [0017] FIG. 7 is an example mass spectrum of example amino acid peaks generated by the example mass spectrometer with front-end integrated micro- pump and electro-spray of FIG. 6

DETAILED DESCRIPTION

[0018] An example micro-pump of the illustrated examples includes a substrate that has at least one channel, and the substrate integrates a micro- pump and electro-spray. Further, the example apparatus includes a porous matrix that is formed from a chemical process, as described herein. [0019] FIG. 2 is a schematic diagram of an example analytical apparatus 200, which generally includes a micro-pump 210 and an electro-spray 212, as described in greater detail below. The analytical apparatus 200 may be utilized in a μ-TAS as described below. The analytical apparatus 200 includes a substrate 202 through which at least one channel 204 extends. The substrate 202 may be formed of any suitable material, including, for example, an inexpensive material such as fused silica. The channel 204 has an inlet 206

and an outlet 208. The micro-pump 210 is coupled to the channel 204 toward or at the inlet 206, and the electro-spray 212 is coupled to the outlet 208. In some example designs, the micro-pump 210 and electro-spray 212 may occupy distinct portions of the substrate 202 or, as illustrated in FIG. 2, the micro-pump 210 and electro-spray 212 may overlap. Fluid such as, for example, a solvent, electrolyte or biological sample flow through the channel 204.

[0020] The micro-pump 210 includes a power supply 214, which may include, for instance, batteries and micro-amplifiers, and wires 216 that connect the power supply to electrodes 218, 219. The electrodes 218, 219 are coupled to the substrate 202, either directly, via reservoirs or other materials, as described in greater detail below. In the example micro-pump 210, which operates as an electroosmotic micro-pump, the power supply 214 provides a high voltage and a direct current (DC). The power supply 214 is used to create an electric field across the substrate 202. The walls 220 of the channel or capillary 204 have electric charges, and an electric double layer of counter ions spontaneously forms at the walls 220, where the counter ions may be driven by the power supply. When the tangential electric field is applied, ions in the double layer suffer a Coulombic force, called the Maxwell force, and move toward the electrode of opposite polarity. This creates motion in the fluid near the walls 220. The motion at the walls 220 transfers momentum via viscous forces into the bulk of the fluid in the channel 204, creating flow from inlet 206 to outlet 208.

[0021] The illustrated electroosmotic micro-pump 210 has no moving parts and is advantageously relatively inexpensive to manufacture. Accordingly, if desired, the electroosmotic micro-pump 210 may be disposable with the rest of the analytical apparatus 200.

[0022] Further, because the fluid in the channel 204 is charged, the flow from the micro-pump 210, driven by the applied electric field, carries most of the current. Consequently, the flow rate and current are strongly correlated. Precise flow control can be achieved with a simple current or voltage- controlled circuit (not shown). In addition, for the example micro-pump 210, the flow carried current (convective current) may exceed the usual current due the applied electric field.

[0023] The channel 204 of the micro-pump 210 also include a packing such as a matrix 222 that, in addition to overcoming low pressure and chemical reactions at the electrodes in the channel 204, separates and further transports the fluid sample. In this example, the matrix 222 that may occupy up to two- thirds of the channel 204, but it will be appreciated that the matrix 222 may occupy any percentage of the channel 204 as desired. The skeleton or matrix 222 may be, for example, a monolithic matrix and/or a silica-based matrix. Silica has an intrinsic ability to generate a strong electroosmotic flow due to the presence of ionizable silanol groups at the surface of the matrix 222. In fact, a silica monolith matrix has a bimodal pore distribution whose nano- porous structure, which as discussed in more detail below, produces a higher charge density than regular fused-silica glasses, i.e., the substrate 202 alone, or non-porous silica particles. The silanol groups at the fused-silica capillary

walls 220 can be easily cross-linked to the matrix 222 to assure secure linkage. Thus, support structures, such as frits, are unnecessary to hold the matrix 222 in place. Because frits may cause pressure drops to occur, the efficiency of the micro-pump 210 increases with the use of the matrix 222 and the absence of frits.

[0024] It may be desirable to have high pressure and large flow rate in the channel 204. These metrics are competing principles because the high hydrodynamic resistance offered by a small pore size is responsible for high pressure, while large pore-size with large cross-sectional areas is desirable for a large flow rate. The example analytical apparatus 200 uses a bundle of parallel micro-channels with small pore size to optimize performance based on the competing principles. However, the pore size should not be smaller than the double layer thickness, as polarization, Maxwell force and flow would all dimmish significantly. Hence, an example advantageous structure is a porous medium with parallel pores whose radius is comparable to the double layer thickness. In fact, one optimum channel dimension is roughly the double layer thickness of the pumping fluid, which ranges from 10 nanometers to 1 micron. At this dimension, the fluid within the channel is fully charged with counter- ions up to its maximum capacitance. The matrix 222 offers such a structure with a small pore radius that approaches the double layer thickness of some polar organic liquids

[0025] In an exemplary embodiment, the matrix 222 is formed by a chemical process such as, for example, a sol-gel process. To form the matrix 222, it may first be desirable to clean the capillary 204. To that end, the fused-silica

capillary 204 may be flushed with a cleansing agent such as, for example, acetone and then baked to remove all liquids inside. This pretreatment substantially eliminates impurities inside the capillary 204. After, or despite a cleansing, a chemical mixture is then inserted into the channel 204. In one example, the mixture may include 0.5mL of 0.01 M acetic acid, 54 mg of polyethylene glycol and 0.2 niL of tetramethoxysilane that are mixed in a micro-liter size vial bottle and stirred for about 30 minutes in an ice- water bath (0° C). The matrix pore size can be controlled by adjusting the composition of the solution. In some cases, sub-micron silica beads are inserted in the solution to promote subsequent precipitation of more beads and to control the size of the pore size. When all the polyethylene glycol is dissolved and a transparent single-phased solution is observed, the solution is then introduced inside the channel 204. Then mixture is then heated. Eventually, the heated mixture transforms, at least in part, into silica precipitate. The precipitated beads are fused together by the heat. An example heating process includes heating the solution-filled channel 204 in an oven at 40° C for 12 hours. Then the temperature is increased from 40° C to 300° C at a rate of 1 ° C/minute, soaking the capillary 204 for 4 hours at each of 80°, 120°, 180°, and 300° C. Finally, the capillary 204, with the silica precipitate, is cooled to room temperature at a rate of 1 ° C/minute. Accordingly, the crystallization of the silica prepared by the sol-gel process forms the continuous matrix 222 that has micrometer-sized through pores 224, the size of which can be adjusted by the synthesis procedure from approximately 10 nanometers to approximately 10 microns. This range is approximately equal to the double layer thickness of

many solvents. The matrix 222 also has nano-porous surfaces 226 (FIG. 3). In this embodiment, the matrix 222 is naturally coupled to the walls 220 of the capillary 204. The silica-based matrix 222 is generally superior to polymeric monoliths in its mechanical strength and high stability in both aqueous and organic solutions.

[0026] The surface charge of the matrix 222 and/or the dimension of the micro-pores 224 and nano-pores 226 can be adjusted by chemically functionalizing the surface through silica chemistry, i.e., by adjusting the composition of the sol-gel solution during synthesis. Thus, the structure of the matrix 222 can be adjusted without additional equipment. This allows for easy adjustments to meet special requirements such as at low pH conditions. It also allows the matrix to act as a chromatograph packing or a catalytic surface.

[0027] Because of the disassociation of silanol groups on the nano-porous matrix 222, the surface of the matrix 222 has high charge density. This charge density, in combination with the low conductivity and micrometer-sized pores 224 of the matrix 222, result in large hydrodynamic resistance. Thus, the matrices 222 are ideal for electroosmotic micro-pumps, such as the micro- pump 210. FIG. 3 illustrates one example of the structure of the resultant matrix 222 formed by the above example process. As shown in FIG. 3, the illustrated matrix 222 is a very porous structure, both in terms of the micropores 224 between branches of the matrix 222 and the nano-pores 226 on the surface of the matrix 222. These pores range anywhere from about 10

nanometers to 10 microns in width; however, the pores may be larger or smaller depending upon the desired application. [0028] The analytical apparatus 200 may also include a nonpermeable membrane such as, for example, a NAFION ^® membrane 224, at the downstream electrode reservoir 219 as shown in FIG. 2. In the illustrated example of FIG. 2, the NAFION ^® membrane 224 is used with the cathode. In other examples, the NAFION ^® membrane 224 may be used with both electrodes 218, 219. The NAFION ^® membrane 224 is a conduction membrane but, in this embodiment, is not permeable to fluid flow. Thus, the cathode is hydrodynamically isolated from the pump channel 204 such that there is no flow exchange between the electrode reservoir 219 and the channel 204. The NAFION ^® membrane 224 hence offers even more hydrodynamic resistance and further enhances pump pressure.

[0029] The NAFION ^® membrane 224 allows the electric field and specific ions to penetrate but not the ions responsible for electro-chemical reactions at the electrode 219. As a result, a large voltage (e.g., greater than several kilovolts) can be applied through the capillary 204 of the matrix 222 without producing or introducing electrolytic bubbles or ions in the capillary 204. Thus, in addition to minimizing fluid leakage and pressure loss, the ion- specific NAFION ^® membrane 224 minimizes bubble generation. Bubbles may be disadvantageous where they block the micro-channels 224 and/or contaminate the sample. Ions also have the potential to contaminate the matrix 222. Although there may be negligible bubble generation because of the low current through the low-conducting matrix 222, i.e., the minimum

current of less than 100 μA has a reduced bubble generation to such an extent that any bubbles would dissolve in the fluid. Further, because the NAFION ^® membrane 224 is impermeable to fluid flow including ions responsible for electro-chemical reactions, the NAFION ^® membrane 224 also minimizes the problems caused by pH generation at the electrode 219. [0030] As described above, high pressure is desirable and the example micro- pump 210 is capable of sustaining high pressures. In fact, in one example, the micro-pump 210 can sustain pressures greater than 4 atmospheres, depending upon the fluid use as a sample, while maintaining precise low flow rates such as, for example, less than a microliter/minute. The low flow rate allows the micro-pump 210 to successfully operate with small samples. Pressure as high as several atmospheres may be achieved within a 100 micron capillary 203. A maximum pressure exceeding 20 atmospheres may be achieved with larger capillaries and the smallest through pores 224.

[0031] FIG. 4 illustrates an exemplary pump curve for the DC electroosmotic micro-pump 210 of FIG. 2 at different voltages and with acetonitrile as the pumping fluid. Organic solvents like methanol, ethanol, and acetone may also be used. Low-conductivity electrolytes such as, for example, de-ionized water can also be used. It can be seen from the curve that the micro-pump 210 with the matrix 222 can generate pressure as high as approximately 1.2 atmospheres when a potential of 6kV is applied and acetonitrile is used as the test fluid. Thus, the electroosmotic micro-pump 210 with the matrix 222, with its high pressure and low current, is ideal to dive flow against large loads in micro-systems such as μ-LC, μ-FIA, and micro-chip sensors. Other pressures

are achievable when other fluids are used in the channel 204, depending on the properties of the fluid sample used.

[0032] Returning to FIG. 2, the outlet 208 of the channel 204 is coupled to an electro-spray 212. The electro-spray 212, or nano-spray, generates small ions or droplets 230 that are easily ionized over a broad range of flow rates for any general sample. For example, the electro-spray 212 may produce a Taylor cone 228, which is a cone of fluid that emits the small charged droplets 230. Multiple Taylor cones may form at the outlet 208 if there are multiple paths through the matrix 222. An example Taylor cone 228 is shown in FIG. 5. The electro-spray 212 is able to form the stable Taylor cone 228 because of the high pressure the micro-pump 210 can deliver. Further, in this embodiment, the micro-pump 210 can maintain these high pressures with less than a 100 micro-liter sample. In addition the integration of a DC micro-pump with the matrix 222 into the device 200 with the electro-spray 212 allows for highly controllable sample flow-rates at as little as approximately a pico-liter per second. This allows easy sample delivery of small samples from the substrate 202 to another analytical device such as, for example, a mass spectrometer, or any other suitable device that could not be integrated into the same substrate 202, which is discussed in more detail below.

[0033] The use of the porous matrix 222 in combination with the electro-spray 212 may have several distinct advantages over most commercially available nano-sprays that were designed specifically to process small sample volumes. The multiplicity of flow paths in the matrix 222 results in fewer clogging problems even when used with organic or normal-phase samples, an issue that

plagues most existing nano-sprays. Other benefits have been described herein such as, for example, the ability to pump and spray normal-phase sample, such as organic fluids.

[0034] There is a focused electric field at the Taylor cone 228 that causes charges (typically ions of the same polarity as the walls 220 at the outlet 208) to concentrate at the tip of the Taylor cone 228. When the charge density becomes excessive, sub-micron charged droplets 230 are emitted from the Taylor cone 228. With evaporation of the solvent in flight, the charge density of the droplets 230 increases because the solvent within the droplet 230 evaporates but the droplet 230 remains charged. Because the volume of the droplet 230 decreases rapidly, the surface tension of the droplet 230 is unable to oppose the electrostatic repulsive forces of the charges within the droplet 230. This causes the droplet 230 to explode into many smaller droplets 230. This is known as Rayleigh fission. The final droplets 230 are nanometer in dimension and contains charged (ionized) molecules that will be identified in any adjoined μ-TAS. In FIG. 6, the example adjoined μ-TAS is a mass spectrometer 300. The electro-spray 212 may be used as a front-end or interface with many other μ-TAS or other analytical devices as well including, for example, biochips, diagnostic equipment, an implantable drug delivery device, a circuit cooling device, a device used for micro-fuel cell applications, and/or a capillary electrophoresis chromatograph.

[0035] Referring now to FIG. 6, mass spectrometers are commonly known, particularly for use with the detection of trace quantities of contaminants or other toxins, and will not be discussed here in great detail. However, in brief:

in the mass spectrometer 300 of FIG. 6, the droplets 230 formed by the analytical apparatus 200 accelerate through an ion analyzer 302, which in this example is a quadruple ion analyzer 302. The ion analyzer 302 uses positive and negative voltages to control the paths of the droplets 230. The droplets 230 travel down the path based on their mass to charge ratio; therefore, the path of a droplet 230 will depend on its mass. The droplet 230 eventually hits a detector 304. The detector 304 works by producing an electric signal when struck by a charged droplet 230. The detector 304 determines which droplets 230 have struck the detector 304 and a spectrum is produced. An example spectrum is shown in FIG. 7, which accurately identifies peptide and amino acids.

[0036] To generate a cone 228 that can be used as a firont-end to a mass spectrometer, considerable pressure typically needs to be sustained at the orifice where the cone 228 is situated. This usually requires large, expensive pumps. However, as described above, the micro-pump 210 with the matrix 222 is capable of producing and sustaining adequate high pressure for the electro-spray 212 to be properly used as the firont-end of the mass spectrometer 300. Further, the micro-pump 210 is integrated with the electro- spray 212 together forming a analytical apparatus that is cheap to manufacture, disposable so it does not need to be cleaned or sterilized and portable so it can be used in the field.

[0037] Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods,

apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.