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Integrated micro-pump and electro-spray

Integrated micro-pump and electro-spray description/claims

This application claims priority to U.S. Provisional Patent Application No. 60/655,437, entitled “Integrated DC pump/electro-spray,” filed on Feb. 24, 2005, and PCT Application No. PCT/US06/06457, entitled “Integrated Micro-Pump and Electro-Spray,” filed on Feb. 24, 2006, both of which are hereby incorporated by reference in their entireties.

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

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.

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 (t-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 n2 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 (P-LC), and other t-TAS.

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.

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.

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.

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.

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.

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.

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

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