rexresearch.com



Richard CROOKS, et al.

Electro-Desalination









http://www.utexas.edu/news/2013/06/27/chemists-work-to-desalt-the-ocean-for-drinking-water-one-nanoliter-at-a-time/
June 27, 2013

Chemists Work to Desalt the Ocean for Drinking Water, One Nanoliter at a Time



A prototype "water chip" developed by researchers at The University of Texas at Austin in collaboration with a startup company.

By creating a small electrical field that removes salts from seawater, chemists at The University of Texas at Austin and the University of Marburg in Germany have introduced a new method for the desalination of seawater that consumes less energy and is dramatically simpler than conventional techniques. The new method requires so little energy that it can run on a store-bought battery.

The process evades the problems confronting current desalination methods by eliminating the need for a membrane and by separating salt from water at a microscale.

The technique, called electrochemically mediated seawater desalination, was described last week in the journal Angewandte Chemie. The research team was led by Richard Crooks of The University of Texas at Austin and Ulrich Tallarek of the University of Marburg. It’s patent-pending and is in commercial development by startup company Okeanos Technologies.

“The availability of water for drinking and crop irrigation is one of the most basic requirements for maintaining and improving human health,” said Crooks, the Robert A. Welch Chair in Chemistry in the College of Natural Sciences. “Seawater desalination is one way to address this need, but most current methods for desalinating water rely on expensive and easily contaminated membranes. The membrane-free method we’ve developed still needs to be refined and scaled up, but if we can succeed at that, then one day it might be possible to provide fresh water on a massive scale using a simple, even portable, system.”

This new method holds particular promise for the water-stressed areas in which about a third of the planet’s inhabitants live. Many of these regions have access to abundant seawater but not to the energy infrastructure or money necessary to desalt water using conventional technology. As a result, millions of deaths per year in these regions are attributed to water-related causes.

“People are dying because of a lack of freshwater,” said Tony Frudakis, founder and CEO of Okeanos Technologies. “And they’ll continue to do so until there is some kind of breakthrough, and that is what we are hoping our technology will represent.”
desalination

The left panel shows the salt (which is tagged with a fluorescent tracer) flowing upward after a voltage is applied by an electrode (the dark rectangle) jutting into the channel at just the point where it branches. In the right panel no voltage is being applied.

To achieve desalination, the researchers apply a small voltage (3.0 volts) to a plastic chip filled with seawater. The chip contains a microchannel with two branches. At the junction of the channel an embedded electrode neutralizes some of the chloride ions in seawater to create an “ion depletion zone” that increases the local electric field compared with the rest of the channel. This change in the electric field is sufficient to redirect salts into one branch, allowing desalinated water to pass through the other branch.

“The neutralization reaction occurring at the electrode is key to removing the salts in seawater,” said Kyle Knust, a graduate student in Crooks’ lab and first author on the paper.

Like a troll at the foot of the bridge, the ion depletion zone prevents salt from passing through, resulting in the production of freshwater.

Thus far Crooks and his colleagues have achieved 25 percent desalination. Although drinking water requires 99 percent desalination, they are confident that goal can be achieved.

“This was a proof of principle,” said Knust. “We’ve made comparable performance improvements while developing other applications based on the formation of an ion depletion zone. That suggests that 99 percent desalination is not beyond our reach.”

The other major challenge is to scale up the process. Right now the microchannels, about the size of a human hair, produce about 40 nanoliters of desalted water per minute. To make this technique practical for individual or communal use, a device would have to produce liters of water per day. The authors are confident that this can be achieved as well.

If these engineering challenges are surmounted, they foresee a future in which the technology is deployed at different scales to meet different needs.

“You could build a disaster relief array or a municipal-scale unit,” said Frudakis. “Okeanos has even contemplated building a small system that would look like a Coke machine and would operate in a standalone fashion to produce enough water for a small village.”

The fundamental scientific breakthroughs that led to this advance were primarily supported by the Office of Basic Energy Sciences in the U.S. Department of Energy. Okeanos Technologies is funded by venture capital and grants from the U.S. Environmental Protection Agency. The intellectual property is owned by The University of Texas at Austin through the Office of Technology Commercialization (OTC). In the event of eventual profits, patent holders, including Crooks and Knust, will be paid according to the OTC’s standard licensing agreement. Okeanos Technologies is also currently supporting Knust’s stipend and tuition via a gift to UT.

For more information, contact:

Daniel Oppenheimer, College of Natural Sciences, 512 745 3353;

Richard Crooks, 512-475-8639, crooks@cm.utexas.edu.



http://www.okeanostech.com/

Okeanos was formed in 2010 to develop a next-generation, ultra-efficient desalination technology to address our planets chronic and increasingly alarming fresh water shortages. Our WaterChip™ platform represents a “solid-state” alternative to present-day approaches for creating fresh water from seawater and brackish aquifer sources.
 
Our Technology

OKEANOS TECHNOLOGIES WAS FOUNDED ON THE BELIEF THAT THE WORLD NEEDS A NEW CLEAN-ENERGY DESALINATION FUTURE AND THAT ARRIVING AT THIS FUTURE WILL REQUIRE MASSIVELY PARALLEL DESALINATION (MPD)].

We believe the evolution of desalination technology requires a completely new way of thinking. Rather than reflexive, incremental improvements to existing but fundamentally inefficient technology platforms, we need to re-conceptualize the entire problem from different scales of perspective, using entirely new approaches and alternative modes of engineering.

Okeanos Technologies is the first to have done this.

The Okeanos WaterChip™ is a solid-state, Massively Paralleled Desalination (MPD) platform that employs a newly characterized, patent-pending micro-electrochemical process to desalinate with radical energy-efficiency. We desalinate millionths of a liter at a time – that is, we “Microdesalinate” – and we do this using tiny microstructures, which are then massively paralleled to produce useful water flows. Desalination in tiny volumes allows us exploit a form of energy that simply cannot be generated in “macroscale” (e.g. liters of space). This form of energy is called an “electrochemical field gradient” and using it to do the “work” of desalination is advantageous because this work is limited by electron rather than ion-transfer kinetics, making it far more efficient than those processes that function in the dimensions of space you are used to thinking about - such as Reverse Osmosis, Electrodialysis or heat/evaporative based methods.

To put all of this another way, we are in essence exploiting the power of the electron to harness and redirect the built-in, corrosive energy of seawater towards the useful task of desalination, and we do this in very tiny volumes.

BENEFITS OF WATERCHIP™ TECHNOLOGY:

COST-EFFECTIVENESS – World-record desalination efficiency = dramatic reduction in operational expenses.
RELEVANCE – Cost-effective desalination will provide profound economic, political, environmental and humanitarian benefits across the globe.
ELEGANCE – Operation without extreme hydraulic pressures, massive electrical currents or intense heat sources make for simple, compact installations with small system footprint.
CLEANLINESS – Operation on alternative energy sources, and/or reduced burden on dirty, coal-powered grid energy results in direct environmental benefits.
FEASIBILITY-- Pretreatment. No membranes means minimal pretreatment required other than basic sedimentation. Elimination of the need for chemicals, filters, treatment ponds etc. results in massive capital and effective operational savings. Post treatment. No post treatment required. Complete disinfection, de-boronation, and heavy metals removal without the use of chemicals, which have to be added and then removed with other technologies in expensive (capex/opex) multi-stage processes. Maintenance. No membranes also means no fouling and the minimization of maintenance capital and labor.
SCALABILITY -- Operating expense scales linearly with output rates – unlike other technologies which require massive operational economies of scale and capital accessories (e.g. energy recovery systems) to reach what might loosely be called “economic competitiveness”. Highly modular framework – systems to address from small to large scale (e.g. municipal) needs.
FLEXIBILITY – On demand programmability for seawater vs. brackish waters means no need for source water dependent configuration/equipment, further enhancing effective operating efficiencies.
SAFETY -- Bacteria, viruses and particulates are removed as efficiently as salt, without the use of expensive and cumbersome coagulants or chemicals -- On demand monitoring of system performance in real time
DURABILITY – no moving parts, no extreme operating conditions (e.g. hydraulic pressures or voltages).
MANUFACTURABILITY – recent advances in microelectronics fabrication and proprietary designs translate into capital (e.g. system purchase) expenses on par with today’s technologies.

Contact

Tony Frudakis, Ph.D.
Chief Executive Officer
tfrudakis@okeanostech.com
Okeanos Technologies, LLC
2012 Callie Way, Suite 201
Union, KY 41091
1-866-991-1446 toll free



http://onlinelibrary.wiley.com/doi/10.1002/anie.201302577/abstract
Angewandte Chemie International Edition, Vol. 52 Issue 27
DOI: 10.1002/anie.201302577
19 JUN 2013
 
Electrochemically Mediated Seawater Desalination

Kyle N. Knust, Dr. Dzmitry Hlushkou, Dr. Robbyn K. Anand, Prof. Ulrich Tallarek, Prof. Richard M. Crooks
   
Membraneless desalination: A simple power supply is used to apply a 3.0 V potential bias across a microelectrochemical cell comprising two microchannels spanned by a single bipolar electrode (BPE) to drive chloride oxidation and water electrolysis at the BPE poles. The resulting ion depletion zone and associated electric field gradient direct ions into a branching microchannel, consequently producing desalted water. Gnd=ground.




US2014183046
MEMBRANELESS SEAWATER DESALINATION

    
CROOKS RICHARD A [US]
KNUST KYLE N [US]

Disclosed are microfluidic devices and systems for the desalination of water. The devices and systems can include an electrode configured to generate an electric field gradient in proximity to an intersection formed by the divergence of two microfluidic channels from an inlet channel. Under an applied bias and in the presence of a pressure driven flow of saltwater, the electric field gradient can preferentially direct ions in saltwater into one of the diverging microfluidic channels, while desalted water flows into second diverging channel. Also provided are methods of using the devices and systems described herein to decrease the salinity of water.

[0001] This application claims benefit of U.S. Provisional Application No. 61/740,780, filed Dec. 21, 2012, which is hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government Support under Agreement DE-FG02-06ER15758 awarded by the U.S. Department of Energy, and Contract EP-D-12-026 awarded by the U.S. Environmental Protection Agency. The Government has certain rights in the invention.

TECHNICAL FIELD

[0003] This application relates generally to devices, systems, and methods for the desalination of water.

BACKGROUND

[0004] The global demand for freshwater is growing rapidly. Many conventional sources of freshwater, including lakes, rivers, and aquifers, are rapidly becoming depleted. As a consequence, freshwater is becoming a limited resource in many regions. In fact, the United Nations estimates two-thirds of the world's population could be living in water stressed regions by 2025.

[0005] Currently, approximately 97% of the world's water supply is present as seawater. Desalination-the process by which salinated water (e.g., seawater) is converted to fresh water-offers the potential to provide dependable supplies of freshwater suitable for human consumption or irrigation. Unfortunately, existing desalination processes, including distillation and reverse osmosis, require both large amounts of energy and specialized, expensive infrastructure. As a consequence, desalination is currently expensive compared to most conventional sources of water, and often prohibitively expensive in developing regions of the world. Therefore, only a small fraction of total human water use is currently satisfied by desalination. More energy efficient methods for water desalination offer the potential to address the increasing demands for freshwater, particularly in water stressed regions.

SUMMARY

[0006] Disclosed are microfluidic devices and systems for the desalination of water.

[0007] Microfluidic devices for the desalination of water can comprise a desalination unit. The desalination unit can comprise an inlet channel fluidly connected to a dilute outlet channel and a concentrated outlet channel. The dilute outlet channel and the concentrated outlet channel can diverge from the inlet channel at an intersection. The desalination unit can further comprise an electrode in electrochemical contact with the desalination unit. The electrode can be configured to generate an electric field gradient in proximity to the intersection where dilute outlet channel and concentrated outlet channel diverge from the inlet channel. Under an applied bias and in the presence of a flow of saltwater, the electric field gradient can preferentially direct ions in the saltwater into concentrated outlet channel, while desalted water flows into the dilute outlet channel.

[0008] In some embodiments, the microfluidic device can further include an auxiliary channel fluidly isolated from the desalination unit. The auxiliary channel can be electrochemically connected to the desalination unit via a bipolar electrode. In these cases, the bipolar electrode can be configured to be in electrochemical contact with both the desalination unit and the auxiliary channel. Under an applied bias across the auxiliary channel and the desalination unit and in the presence of a flow of saltwater, the electric field gradient can preferentially direct ions in the saltwater into concentrated outlet channel of the desalination unit, while desalted water flows into the dilute outlet channel.

[0009] In some embodiments, the auxiliary channel comprises a desalination unit. In these embodiments, the microfluidic device can comprise two desalination units, which can be of identical or different structure. The first desalination unit can be electrochemically connected to the second desalination unit by a bipolar electrode. Under an applied bias across the first desalination unit and the second desalination unit and in the presence of a pressure driven flow of saltwater, the electric field gradient can preferentially direct ions in the saltwater into concentrated outlet channels of the first and second desalination units, while desalted water flows into the dilute outlet channels of the first and second desalination units.

[0010] A plurality of the microfluidic devices described herein can be combined to form a water purification system. The system can comprise a plurality of the devices described herein arranged in parallel or fluidly connected in series. The systems can also comprise a plurality of devices both arranged in parallel and fluidly connected in series. For example, the device can include a first pair of devices fluidly connected in series which are arranged in parallel with a second pair of devices fluidly connected in series. In such systems, the plurality of devices can be fabricated in a single plane (i.e., as a 2-dimensional system) or in three dimensions.

[0011] Also provided are methods of using the devices and systems described herein to decrease the salinity of water.


DESCRIPTION OF DRAWINGS

[0012] FIG. 1A is a schematic drawing illustrating a microfluidic device for the desalination of water.

[0013] FIG. 1B is a schematic drawing illustrating an enlarged portion of the microfluidic device shown in FIG. 1A.

[0014] FIG. 1C is a schematic drawing illustrating a microfluidic device for the desalination of water in combination with a power supply configured to apply a potential bias across the desalination unit.

[0015] FIG. 2 is a schematic drawing illustrating a microfluidic device for the desalination of water. The device includes a desalination unit and an auxiliary channel electrochemically connected by a bipolar electrode.

[0016] FIG. 3 is a schematic drawing illustrating a microfluidic device for the desalination of water. The device includes two desalination units electrochemically connected by a bipolar electrode.

[0017] FIG. 4 is a schematic drawing of a water purification system for the desalination of water. The system includes multiple desalination units configured to operate in parallel.

[0018] FIG. 5 is a schematic drawing of a water purification system for the desalination of water. The system includes multiple desalination units configured to operate in parallel.

[0019] FIG. 6 is a schematic drawing of a water purification system for the desalination of water. The device includes multiple desalination units configured to operate in series.

[0020] FIGS. 7A-7B are fluorescence micrographs illustrating the flow of a solution of Ru(bpy) <2+> (a fluorescent cationic tracer) in saltwater through the device illustrated in FIG. 2. FIGS. 7A is a fluorescence micrograph of the device taken before application of a potential bias. FIGS. 7B is a fluorescence micrograph of the device taken upon application of a potential bias.

[0021] FIG. 8 is a fluorescence micrograph illustrating the flow of a solution of Ru(bpy) <2+> (a fluorescent cationic tracer) in Na 2SO 4 through the device illustrated in FIG. 2 upon application of a potential bias.

[0022] FIG. 9 is a graph of total current flowing through the device illustrated in FIG. 2 (i tot, plotted in nanoamperes) as a function of time (in seconds) during operation.

         


DETAILED DESCRIPTION

[0023] Disclosed are microfluidic devices and systems for the desalination of water.

[0024] Microfluidic devices for the desalination of water can comprise a desalination unit. The desalination unit can comprise an inlet channel fluidly connected to a dilute outlet channel and a concentrated outlet channel. The dilute outlet channel and the concentrated outlet channel can diverge from the inlet channel at an intersection. The desalination unit can also comprise an electrode in electrochemical contact with the desalination unit. The electrode can be configured to generate an electric field gradient in proximity to the intersection where dilute outlet channel and concentrated outlet channel diverge from the inlet channel.

[0025] An example device comprising a desalination unit ( 100) is schematically illustrated in FIG. 1A. The desalination unit includes an inlet channel ( 102) fluidly connected to a dilute outlet channel ( 104) and a concentrated outlet channel ( 106). The dilute outlet channel ( 104) and the concentrated outlet channel ( 106) diverge from the inlet channel ( 102) at an intersection ( 107). An electrode ( 108) is positioned in proximity to the intersection ( 107). The electrode ( 108) is configured to form an ion depletion zone ( 109) at and downstream of the electrode during device operation, resulting in the formation of an electric field gradient in proximity to the intersection. The example device further includes a fluid reservoir ( 110) fluidly connected to the upstream terminus of the inlet channel ( 102), a fluid reservoir ( 114) fluidly connected to the downstream terminus of the dilute outlet channel ( 104), and a fluid reservoir ( 112) fluidly connected to the downstream terminus of the concentrated outlet channel ( 106).

[0026] The dimensions of the microfluidic channels in the desalination unit ( 100) (e.g., the inlet channel ( 102), the dilute outlet channel ( 104), and the concentrated outlet channel ( 106)) can individually and/or in combination be selected in view of a number of factors, including the size and position of the electrode relative to the microfluidic channels in the desalination unit, the desired device flow rate, salinity of the saltwater being treated using the device, and the desired degree of salinity reduction.

[0027] In some instances, the dimensions of the inlet channel ( 102), the dilute outlet channel ( 104), and the concentrated outlet channel ( 106) are selected such that the sum of the area of a cross-section of dilute outlet channel and the area of a cross-section of the concentrated outlet channel is substantially equal to the area of a cross-section of the inlet channel. In this context, substantially equal can mean that the sum of the area of a cross-section of dilute outlet channel and the area of a cross-section of the concentrated outlet channel is with for example, 15%, of the area of a cross-section of the inlet channel (e.g., within 10% of the area of a cross-section of the inlet channel, or within 5% of the area of a cross-section of the inlet channel). In some embodiments, the dilute outlet channel, and the concentrated outlet channel have substantially equivalent cross-sectional dimensions, meaning that the height and width of the dilute outlet channel are substantially equivalent (e.g., within 15%, within 10%, or within 5%) to the height and width of the concentrated outlet channel.

[0028] The dimensions of the microfluidic channels in the desalination unit ( 100) (e.g., the inlet channel ( 102), the dilute outlet channel ( 104), and the concentrated outlet channel ( 106)) can be fabricated so as to have a variety of cross-sectional shapes. In some embodiments, the microfluidic channels in the desalination unit (e.g., the inlet channel, the dilute outlet channel, and the concentrated outlet channel) have a substantially square or rectangular cross-sectional shape.

[0029] In some embodiments, the inlet channel ( 102) has a width of about 1000 microns or less (e.g., about 900 microns or less, about 800 microns or less, about 750 microns or less, about 700 microns or less, about 600 microns or less, about 500 microns or less, about 400 microns or less, about 300 microns or less, about 250 microns or less, about 200 microns or less, about 150 microns or less, about 100 microns or less, about 75 microns or less, or about 50 microns or less). In some embodiments, the inlet channel ( 102) has a width of at least about 1 micron (e.g., at least about 5 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 150 microns, at least about 200 microns, at least about 250 microns, at least about 300 microns, at least about 400 microns, at least about 500 microns, at least about 600 microns, at least about 700 microns, at least about 750 microns, at least about 800 microns, at least about 900 microns, or at least about 1000 microns).

[0030] The inlet channel ( 102) can have a width that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the inlet channel ( 102) can have a width that ranges from about 1000 microns to about 1 micron (e.g., from about 750 microns to about 5 microns, from about 500 microns to about 10 microns, from about 250 microns to about 20 microns, or from about 150 microns to about 25 microns).

[0031] In some embodiments, the inlet channel ( 102) has a height of about 50 microns or less (e.g., about 45 microns or less, about 40 microns or less, about 35 microns or less, about 30 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 9 microns or less, about 8 microns or less, about 7.5 microns or less, about 7 microns or less, about 6 microns or less, about 5 microns or less, about 4 microns or less, about 3 microns or less, about 2.5 microns or less, or about 2 microns or less). In some embodiments, the inlet channel ( 102) has a height of at least about 1 micron (e.g., at least about 2 microns, at least about 2.5 microns, at least about 3 microns, at least about 4 microns, at least about 5 microns, at least about 6 microns, at least about 7 microns, at least about 7.5 microns, at least about 8 microns, at least about 9 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 30 microns, at least about 35 microns, at least about 40 microns, or at least about 45 microns).

[0032] The inlet channel ( 102) can have a height that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the inlet channel ( 102) can have a height that ranges from about 50 microns to about 1 micron (e.g., from about 45 microns to about 1 micron, from about 40 microns to about 1 micron, from about 35 microns to about 1 micron, from about 30 microns to about 1 micron, from about 25 microns to about 1 micron, or from about 20 microns to about 1 micron).

[0033] In some embodiments, the dilute outlet channel ( 104) has a width of about 500 microns or less (e.g., about 450 microns or less, about 400 microns or less, about 350 microns or less, about 300 microns or less, about 250 microns or less, about 200 microns or less, about 150 microns or less, about 125 microns or less, about 100 microns or less, about 75 microns or less, about 50 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 5 microns or less, or about 1 micron or less). In some embodiments, the dilute outlet channel ( 104) has a width of at least about 0.5 microns (e.g., at least about 1 micron, at least about 2.5 microns, at least about 5 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 150 microns, at least about 200 microns, at least about 250 microns, at least about 300 microns, at least about 400 microns, or at least about 450 microns).

[0034] The dilute outlet channel ( 104) can have a width that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the dilute outlet channel ( 104) can have a width that ranges from about 500 microns to about 0.5 microns (e.g., from about 400 microns to about 1 micron, from about 250 microns to about 1 micron, from about 150 microns to about 5 microns, or from about 80 microns to about 10 microns).

[0035] In some embodiments, the dilute outlet channel ( 104) has a height of about 50 microns or less (e.g., about 45 microns or less, about 40 microns or less, about 35 microns or less, about 30 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 9 microns or less, about 8 microns or less, about 7.5 microns or less, about 7 microns or less, about 6 microns or less, about 5 microns or less, about 4 microns or less, about 3 microns or less, about 2.5 microns or less, or about 2 microns or less). In some embodiments, the dilute outlet channel ( 104) has a height of at least about 1 micron (e.g., at least about 2 microns, at least about 2.5 microns, at least about 3 microns, at least about 4 microns, at least about 5 microns, at least about 6 microns, at least about 7 microns, at least about 7.5 microns, at least about 8 microns, at least about 9 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 30 microns, at least about 35 microns, at least about 40 microns, or at least about 45 microns).

[0036] The dilute outlet channel ( 104) can have a height that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the dilute outlet channel ( 104) can have a height that ranges from about 50 microns to about 1 micron (e.g., from about 45 microns to about 1 micron, from about 40 microns to about 1 micron, from about 35 microns to about 1 micron, from about 30 microns to about 1 micron, from about 25 microns to about 1 micron, or from about 20 microns to about 1 micron).

[0037] In some embodiments, the concentrated outlet channel ( 106) has a width of about 500 microns or less (e.g., about 450 microns or less, about 400 microns or less, about 350 microns or less, about 300 microns or less, about 250 microns or less, about 200 microns or less, about 150 microns or less, about 125 microns or less, about 100 microns or less, about 75 microns or less, about 50 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 5 microns or less, or about 1 micron or less). In some embodiments, the concentrated outlet channel ( 106) has a width of at least about 0.5 microns (e.g., at least about 1 micron, at least about 2.5 microns, at least about 5 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 150 microns, at least about 200 microns, at least about 250 microns, at least about 300 microns, at least about 400 microns, or at least about 450 microns).

[0038] The concentrated outlet channel ( 106) can have a width that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the concentrated outlet channel ( 106) can have a width that ranges from about 500 microns to about 0.5 microns (e.g., from about 400 microns to about 1 micron, from about 250 microns to about 1 micron, from about 150 microns to about 5 microns, or from about 80 microns to about 10 microns).

[0039] In some embodiments, the concentrated outlet channel ( 106) has a height of about 50 microns or less (e.g., about 45 microns or less, about 40 microns or less, about 35 microns or less, about 30 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 9 microns or less, about 8 microns or less, about 7.5 microns or less, about 7 microns or less, about 6 microns or less, about 5 microns or less, about 4 microns or less, about 3 microns or less, about 2.5 microns or less, or about 2 microns or less). In some embodiments, the concentrated outlet channel ( 106) has a height of at least about 1 micron (e.g., at least about 2 microns, at least about 2.5 microns, at least about 3 microns, at least about 4 microns, at least about 5 microns, at least about 6 microns, at least about 7 microns, at least about 7.5 microns, at least about 8 microns, at least about 9 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 30 microns, at least about 35 microns, at least about 40 microns, or at least about 45 microns).

[0040] The concentrated outlet channel ( 106) can have a height that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the concentrated outlet channel ( 106) can have a height that ranges from about 50 microns to about 1 micron (e.g., from about 45 microns to about 1 micron, from about 40 microns to about 1 micron, from about 35 microns to about 1 micron, from about 30 microns to about 1 micron, from about 25 microns to about 1 micron, or from about 20 microns to about 1 micron).

[0041] The length of the microfluidic channels in the desalination unit ( 100) (e.g., the inlet channel ( 102), the dilute outlet channel ( 104), and the concentrated outlet channel ( 106)) can vary. The length of the microfluidic channels in the desalination unit can individually be selected in view of a number of the overall device design and other operational considerations. In some embodiments, the inlet channel ( 102), the dilute outlet channel ( 104), and the concentrated outlet channel ( 106) each have a length of at least about 0.1 cm (e.g., at least about 0.2 cm, at least about 0.3 cm, at least about 0.4 cm, at least about 0.5 cm, at least about 0.6 cm, at least about 0.7 cm, at least about 0.8 cm, at least about 0.9 cm, at least about 1 cm, at least about 2 cm, at least about 2.5 cm, at least about 3 cm, at least about 4 cm, at least about 5 cm, or longer). The microfluidic channels in the desalination unit can be substantially linear in shape, or they can possess one or more non-linear regions (e.g., a curved region, a spiral region, an angular region, or combinations thereof) along the length of their fluid flow path.

[0042] With reference again to FIG. 1A, the dilute outlet channel ( 104) and the concentrated outlet channel ( 106) diverge from the inlet channel ( 102) at an intersection ( 107). The orientation of the dilute outlet channel ( 104) and the concentrated outlet channel ( 106) with respect to one another at the intersection can be varied. The angle formed between the dilute outlet channel ( 104) and the concentrated outlet channel ( 106) in a device can be selected in view of a number of parameters, including the size and position of the electrode relative to the microfluidic channels in the desalination unit, the desired device flow rate, salinity of the saltwater being treated using the device, and the desired degree of salinity reduction.

[0043] In some cases, the angle formed between the dilute outlet channel ( 104) and the concentrated outlet channel ( 106) at the intersection ( 107) is about 60 degrees or less (e.g., about 55 degrees or less, about 50 degrees or less, about 45 degrees or less, about 40 degrees or less, about 35 degrees or less, about 30 degrees or less, about 25 degrees or less, about 20 degrees or less, about 15 degrees or less, or less).

[0044] The electrode ( 108) can be fabricated from any suitable conductive material, such as a metal (e.g., gold), metal alloy, metal oxide, or conductive carbon. The electrode ( 108) is configured so as to be in electrochemical contact with the desalination unit ( 100), meaning that the electrode ( 108) can participate in a faradaic reaction with one or more components of a solution present in a microfluidic channel of the desalination unit. For example, the electrode ( 108) can be configured such that a surface of the electrode is in direct contact with fluid present in a microfluidic channel of the desalination unit. The device can be configured such that the electrode ( 108) can function as either an anode, cathode, or anode and cathode during device operation.

[0045] The position and dimensions of the electrode ( 108) relative to the desalination unit can be selected in view of a number of factors, including the size and configuration of the microfluidic channels in the desalination unit, the desired device flow rate, salinity of the saltwater being treated using the device, and the desired degree of salinity reduction. The electrode ( 108) can have a variety of 2-dimensional or 3-dimensional shapes, provided that the electrode ( 108) can be integrated into the device, and is compatible with the formation of an electric field gradient suitable to direct ions flowing through the inlet channel ( 102) preferentially into the concentrated outlet channel ( 106). In certain embodiments, the electrode ( 108) is a conductive surface (e.g., a line, a rectangular pad, or a square pad) substantially co-planar with the floor of the inlet channel ( 102), and integrated into the floor of the inlet channel in proximity to the intersection ( 107). In other embodiments, the electrode ( 108) is a conductive surface (e.g., a line, a rectangular pad, or a square pad) that is fabricated onto/into the floor of the inlet channel in proximity to the intersection ( 107), and which extends from the floor of the inlet channel into the inlet channel. In these embodiments, the electrode can be said to have a height, measured as the distance from the floor of the inlet channel to the surface or edge of the electrode within the inlet channel positioned at greatest distance from the floor of the inlet channel.

[0046] With reference again to FIG. 1A, the electrode ( 108) can be positioned in proximity to the intersection ( 107) so as to form an ion depletion zone ( 109) at and downstream of the electrode ( 108), and extending into the dilute outlet channel ( 104) during device operation. The ion depletion zone ( 109) can optionally extend into a portion of the concentrated inlet channel ( 106). In some embodiments, the electrode ( 108) is positioned within the floor of the inlet channel ( 102) upstream of the opening of the dilute outlet channel ( 104).

[0047] By way of exemplification, FIG. 1B illustrates an enlarged view of the intersection ( 107) of the device shown in FIG. 1A. The electrode ( 108) is positioned within the floor of the inlet channel ( 102). The surface of the electrode ( 108) in electrochemical contact with the desalination unit is positioned approximately +-50 microns (measured as the distance from the opening of the dilute outlet channel to the downstream edge of the electrode, 130) upstream or downstream of the opening of the dilute outlet channel ( 104).

[0048] In certain embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit is positioned upstream of the opening of the dilute outlet channel ( 104), and within about 500 microns of the opening of the dilute outlet channel (e.g., within about 400 microns, within about 300 microns, within about 250 microns, within about 200 microns, within about 150 microns, within about 100 microns, within about 90 microns, within about 80 microns, within about 75 microns, within about 70 microns, within about 60 microns, within about 50 microns, within about 40 microns, within about 30 microns, within about 25 microns, within about 20 microns, or within about 10 microns).

[0049] In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit is positioned downstream of the opening of the dilute outlet channel ( 104), and within about 100 microns of the opening of the dilute outlet channel (e.g., within about 90 microns, within about 80 microns, within about 75 microns, within about 70 microns, within about 60 microns, within about 50 microns, within about 40 microns, within about 30 microns, within about 25 microns, within about 20 microns, within about 10 microns, or within about 5 microns). When the surface of the electrode ( 108) in electrochemical contact with the desalination unit is positioned downstream of the opening of the dilute outlet channel ( 104), the length of the electrode (as discussed below) must be sufficient such that at least a portion of the electrode ( 108) in electrochemical contact with the desalination unit extends beyond the opening of the dilute outlet channel ( 104), and into the inlet channel (i.e., a portion of the electrode must be located upstream of the dilute outlet channel)

[0050] Again referring to FIG. 1B, the surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132, measured as the distance from one side of the surface of the electrode to the other side of the surface of the electrode along an axis perpendicular to the direction of fluid flow through the inlet channel) and a length ( 134, measured as the distance from one side of the surface of the electrode to the other side of the surface of the electrode along an axis parallel to the direction of fluid flow through the inlet channel). By way of exemplification, in the example device to FIG. 1B, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is about equal to the width of the dilute outlet channel ( 104) (50 microns), and a length ( 134) of about 100 microns.

[0051] In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) of at least about 50% of the width of the dilute outlet channel ( 104) (e.g., at least about 60% of the width of the dilute outlet channel, at least about 70% of the width of the dilute outlet channel, at least about 75% of the width of the dilute outlet channel, at least about 80% of the width of the dilute outlet channel, at least about 90% of the width of the dilute outlet channel, at least about 90% of the width of the dilute outlet channel, at least the width of the dilute outlet channel, at least about 105% of the width of the dilute outlet channel, or at least about 110% of the width of the dilute outlet channel). In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is less than about 150% of the width of the dilute outlet channel ( 104) (e.g., less than about 140% of the width of the dilute outlet channel, less than about 130% of the width of the dilute outlet channel, less than about 125% of the width of the dilute outlet channel, less than about 120% of the width of the dilute outlet channel, less than about 110% of the width of the dilute outlet channel, less than about 105% of the width of the dilute outlet channel, or less than the width of the dilute outlet channel).

[0052] The surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from about 50% of the width of the dilute outlet channel ( 104) to about 150% of the width of the dilute outlet channel (e.g., from about 75% of the width of the dilute outlet channel to about 125% of the width of the dilute outlet channel, from about 90% of the width of the dilute outlet channel to about 110% of the width of the dilute outlet channel, or from about 95% of the width of the dilute outlet channel to about 105% of the width of the dilute outlet channel). In certain embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is about equal to the width of the dilute outlet channel ( 104).

[0053] In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is at least about 25% of the width of the inlet channel ( 102) (e.g., at least about 30% of the width of the inlet channel, at least about 40% of the width of the inlet channel, at least about 45% of the width of the inlet channel, at least about 50% of the width of the inlet channel, at least about 55% of the width of the inlet channel, or at least about 60% of the width of the inlet channel). In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is less than about 75% of the width of the inlet channel ( 102) (e.g., less than about 60% of the width of the inlet channel, less than about 55% of the width of the inlet channel, less than about 50% of the width of the inlet channel, less than about 45% of the width of the inlet channel, or less than about 40% of the width of the inlet channel).

[0054] The surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from about 25% of the width of the inlet channel ( 102) to about 75% of the width of the inlet channel (e.g., from about 30% of the width of the dilute outlet channel to about 70% of the width of the dilute outlet channel, from about 40% of the width of the dilute outlet channel to about 60% of the width of the dilute outlet channel, or from about 45% of the width of the dilute outlet channel to about 55% of the width of the dilute outlet channel). In certain embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) that is about 50% of the width of the inlet channel ( 102).

[0055] In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) of about 600 microns or less (e.g., about 500 microns or less, about 450 microns or less, about 400 microns or less, about 350 microns or less, about 300 microns or less, about 250 microns or less, about 200 microns or less, about 150 microns or less, about 125 microns or less, about 100 microns or less, about 75 microns or less, about 50 microns or less, about 25 microns or less, about 20 microns or less, about 15 microns or less, about 10 microns or less, about 5 microns or less, or about 1 micron or less). In some embodiments, the surface of the electrode ( 108) in electrochemical contact with the desalination unit has a width ( 132) of at least about 0.5 microns (e.g., at least about 1 micron, at least about 2.5 microns, at least about 5 microns, at least about 10 microns, at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 150 microns, at least about 200 microns, at least about 250 microns, at least about 300 microns, at least about 400 microns, at least about 450 microns, or at least about 500 microns).

[0056] The surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a width ( 132) that ranges from about 600 microns to about 0.5 microns (e.g., from about 400 microns to about 1 micron, from about 250 microns to about 1 micron, from about 150 microns to about 5 microns, or from about 80 microns to about 10 microns).

[0057] The length ( 134) of the surface of the electrode ( 108) in electrochemical contact with the desalination unit can be varied. In some embodiments the surface of the electrode ( 108) has a length ( 134) of at least about 10 microns (e.g., at least about 15 microns, at least about 20 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 150 microns, at least about 200 microns, at least about 250 microns, at least about 300 microns, at least about 400 microns, at least about 450 microns, or at least about 450 microns). In some embodiments, the surface of the electrode ( 108) has a length ( 134) of less than about 500 microns (e.g., less than about 400 microns, less than about 300 microns, less than about 250 microns, less than about 200 microns, or less than about 100 microns).

[0058] The surface of the electrode ( 108) in electrochemical contact with the desalination unit can have a length ( 134) that ranges from any of the minimum dimensions to any of the maximum dimensions described above. For example, the surface of the electrode ( 108) can have a length ( 134) that ranges from about 10 microns to about 500 microns (e.g., from about 25 microns to about 250 microns, or from about 50 microns to about 150 microns).

[0059] The height of the electrode ( 108) in electrochemical contact with the desalination unit can also be varied. The height of the electrode ( 108) can be selected in view of a number of factors, including the height of the microfluidic channels in the desalination unit. In some cases, the height of the electrode ( 108) is approximately zero (i.e., the electrode is substantially co-planar with the floor of the inlet channel). In some embodiments, the height of the electrode ( 108) is less than about 1 micron (e.g., less than about 900 nm, less than about 800 nm, less than about 750 nm, less than about 700 nm, less than about 600 nm, less than about 500 nm, less than about 400 nm, less than about 300 nm, less than about 250 nm, less than about 200 nm, or less than about 100 nm).

[0060] As shown in FIG. 1C, a power supply ( 140) can be configured to apply a potential bias across the desalination unit. A flow of saltwater ( 120) can be initiated from the inlet channel ( 102) to the dilute outlet channel ( 104) and the concentrated outlet channel ( 106). Upon application of a potential bias, an ion depletion zone ( 109) and subsequent electric field gradient are formed near the electrode ( 108) in proximity to the intersection ( 107). As a consequence, ions in the saltwater are preferentially directed into the concentrated outlet channel ( 106), resulting in a brine ( 122) flowing through the concentrated outlet channel. Desalted water (i.e., water containing less salt that the saltwater introduced into the inlet channel; 124) flows into the dilute outlet channel ( 104).

[0061] In some embodiments, the microfluidic device can further include an auxiliary channel fluidly isolated from the desalination unit. An example device comprising a desalination unit ( 100) and an auxiliary channel ( 202) is schematically illustrated in FIG. 2. The desalination unit includes an inlet channel ( 102) fluidly connected to a dilute outlet channel ( 104) and a concentrated outlet channel ( 106). The dilute outlet channel ( 104) and the concentrated outlet channel ( 106) diverge from the inlet channel ( 102) at an intersection ( 107). The device also includes and an auxiliary channel ( 202) which is fluidly isolated from the desalination unit ( 100).

[0062] The auxiliary channel ( 202) can comprise, for example, a single microfluidic channel. In these embodiments, dimensions of the auxiliary channel (e.g., height, width, and length) can vary. The dimensions of the auxiliary channel ( 202) can individually be selected in view of a number of the overall device design and other operational considerations. The auxiliary channel ( 202) can be substantially linear in shape, or it can possess one or more non-linear regions (e.g., a curved region, a spiral region, an angular region, or combinations thereof) along the length of their fluid flow path. The auxiliary channel ( 202) can optionally possess one or more branch points. The auxiliary channel ( 202) can further include additional elements, such as electrodes, fluid inlets, fluid outlets, fluid reservoirs, valves, pumps, and combinations thereof, connected to the auxiliary channel to facilitate device operation.

[0063] The auxiliary channel ( 202) can be electrochemically connected to the desalination unit ( 100) via a bipolar electrode. In these embodiments, the bipolar electrode is configured so as to be in electrochemical contact with both the desalination unit ( 100) and the auxiliary channel ( 202), meaning that a first surface of the bipolar electrode can participate in a faradaic reaction with one or more components of a solution present in a microfluidic channel of the desalination unit, and a second surface of the bipolar electrode can participate in a faradaic reaction with one or more components of a solution present in the auxiliary channel. The device can be configured such that the bipolar electrode comprises an anode in electrochemical contact with the desalination unit and a cathode in electrochemical contact with the auxiliary channel during device operation. Alternatively, the device can be configured such that the bipolar electrode comprises a cathode in electrochemical contact with the desalination unit and an anode in electrochemical contact with the auxiliary channel during device operation.

[0064] By way of exemplification, referring again to the example device illustrated in FIG. 2, a bipolar electrode ( 204) electrochemically connects the auxiliary channel ( 202) and the desalination unit ( 100). A first surface of the bipolar electrode ( 206) is in electrochemical contact with the desalination unit ( 100), and is positioned in proximity to the intersection ( 107). The first surface of the bipolar electrode ( 206) is configured to form an ion depletion zone ( 109) at and downstream of the surface of the bipolar electrode during device operation, resulting in the formation of an electric field gradient in proximity to the intersection. A second surface of the bipolar electrode ( 208) is in electrochemical contact with the auxiliary channel ( 202).

[0065] The first surface of the bipolar electrode ( 206) can occupy the same position within the desalination unit, and have the same dimensions as the surface of electrode ( 108) described above with respect to the first desalination unit.

[0066] Referring again to FIG. 2, the example device further includes a fluid reservoir ( 110) fluidly connected to the upstream terminus of the inlet channel ( 102), a fluid reservoir ( 114) fluidly connected to the downstream terminus of the dilute outlet channel ( 104), a fluid reservoir ( 112) fluidly connected to the downstream terminus of the concentrated outlet channel ( 106), and fluid reservoirs ( 210 and 212) fluidly connected to the termini of the auxiliary channel ( 202).

[0067] A power supply can be configured to apply a potential bias across the auxiliary channel ( 202) and the desalination unit ( 100). A flow of saltwater ( 120) can be initiated from the inlet channel ( 102) to the dilute outlet channel ( 104) and the concentrated outlet channel ( 106). Upon application of a potential bias, an ion depletion zone ( 109) and subsequent electric field gradient are formed near the first surface of the bipolar electrode ( 206) in proximity to the intersection ( 107). As a consequence, ions in the saltwater are preferentially directed into the concentrated outlet channel ( 106), resulting in a brine ( 122) flowing through the concentrated outlet channel. Desalted water ( 124) flows into the dilute outlet channel ( 104).

[0068] In some embodiments, the auxiliary channel can comprise a desalination unit. In these embodiments, the microfluidic device can comprise two desalination units, which can be of identical or different structure. An example device comprising two desalination units is illustrated in FIG. 3. The device includes a first desalination unit ( 100) electrochemically connected to a second desalination unit ( 302) by a bipolar electrode ( 310). The first desalination unit ( 100) is fluidly isolated from the second desalination unit ( 302).

[0069] The first desalination unit ( 100) includes an inlet channel ( 102) fluidly connected to a dilute outlet channel ( 104) and a concentrated outlet channel ( 106). The dilute outlet channel ( 104) and the concentrated outlet channel ( 106) diverge from the inlet channel ( 102) at an intersection ( 107). The second desalination unit ( 302) includes an inlet channel ( 304) fluidly connected to a dilute outlet channel ( 306) and a concentrated outlet channel ( 308). The dilute outlet channel ( 306) and the concentrated outlet channel ( 308) diverge from the inlet channel ( 304) at an intersection ( 307).

[0070] A bipolar electrode ( 310) electrochemically connects the first desalination unit ( 100) and the second desalination unit ( 302). A first surface of the bipolar electrode ( 312) is in electrochemical contact with the first desalination unit ( 100), and is positioned in proximity to the intersection ( 107). The first surface of the bipolar electrode ( 312) is configured to form an ion depletion zone ( 109) downstream of the surface of the bipolar electrode during device operation, resulting in the formation of an electric field gradient in proximity to the intersection of the first desalination unit. A second surface of the bipolar electrode ( 314) is in electrochemical contact with the second desalination unit ( 302), and is positioned in proximity to the intersection of the second desalination unit ( 307). The second surface of the bipolar electrode ( 314) is configured to form an ion depletion zone ( 309) downstream of the surface of the bipolar electrode during device operation, resulting in the formation of an electric field gradient in proximity to the intersection of the second desalination unit. The example device further includes fluid reservoirs ( 110 and 320) fluidly connected to the upstream termini of the inlet channels of the first and second desalination units, fluid reservoirs ( 114 and 322) fluidly connected to the downstream termini of the dilute outlet channels of the first and second desalination units, and fluid reservoirs ( 112 and 324) fluidly connected to the downstream termini of the concentrated outlet channels of the first and second desalination units.

[0071] The second desalination unit ( 302), as well as all of the elements making up the second desalination unit (e.g., the inlet channel ( 304), the dilute outlet channel ( 306), and the concentrated outlet channel ( 308)) can have the same dimensions and relative configurations as those described above with respect to the first desalination unit. The first surface of the bipolar electrode ( 312) and the second surface of the bipolar electrode ( 314) can occupy the same positions within their respective desalination units, and have the same dimensions as the surface of electrode ( 108) described above with respect to the first desalination unit.

[0072] A power supply can be configured to apply a potential bias across the first desalination unit ( 100) and the second desalination unit ( 302). A flow of saltwater ( 120 and 330) can be initiated from the inlet channels of the first and second desalination units to the dilute outlet channels and the concentrated outlet channels of the first and second desalination units. Upon application of a potential bias, ion depletion zones ( 109 and 309) and subsequent electric field gradients are formed near the first surface of the bipolar electrode ( 312) in proximity to the intersection ( 107) of the first desalination unit, and near the second surface of the bipolar electrode ( 314) in proximity to the intersection ( 307) of the second desalination unit. As a consequence, ions in the saltwater are preferentially directed into the concentrated outlet channels of the first and second desalination units ( 106 and 308), resulting in a brine ( 122 and 334) flowing through the concentrated outlet channels of the first and second desalination units. Desalted water ( 124 and 332) flows into the dilute outlet channels ( 104 and 306) of the first and second desalination units.

[0073] The microfluidic devices described herein can further include one or more additional components (e.g., pressure gauges, valves, pressure inlets, pumps, fluid reservoirs, sensors, electrodes, power supplies, and combinations thereof) to facilitate device function. In some embodiments, the devices include a pump, valve, fluid reservoir, or combination thereof configured to regulate fluid flow into the inlet channel of the device.

[0074] The devices can include a salinometer configured to measure the salinity of fluid flowing through one or more of the microfluidic channels of the device. For example, in some cases, the devices can include a salinometer configured to measure the salinity of fluid flowing through the dilute outlet channel. The salinometer can measure the salinity of the fluid via any suitable means. For example, the salinometer can measure the fluid's electrical conductivity, specific gravity, index of refraction, or combinations thereof.

[0075] In certain embodiments, the devices include a salinometer configured to measure the salinity of fluid flowing through the dilute outlet channel, and a pump, valve, fluid reservoir, or combination thereof configured to regulate fluid flow into the inlet channel of the device. The devices can further include signal processing circuitry or a processor configured to operate the pump and/or valve connected to the inlet channel so as to adjust fluid flow into the inlet channel of the device in response to the salinity of fluid flowing through the dilute outlet channel.

[0076] Systems

[0077] A plurality of the microfluidic devices described herein can be combined to form a water purification system.

[0078] Water purification systems can comprise any number of the devices described herein. The number of devices incorporated within the water purification system can be selected in view of a number of factors, including the overall system design, the desired throughput of the system, salinity of the saltwater being treated using the system, and the desired degree of salinity reduction.

[0079] In some cases, the inlet channels of two or more of the devices in the system are fluidly connected to a common water inlet, so as to facilitate the flow of saltwater into the inlet channels of multiple devices in the system. Similarly, the dilute outlet channels of two or more of the devices in the system can be fluidly connected to a common water outlet, so as to facilitate the collection of desalted water from the dilute outlet channels of multiple devices in the system.

[0080] The system can comprise a plurality of the devices described herein arranged in parallel. Within the context of the systems described herein, two devices can be described as being arranged in parallel within a system when fluid flowing from either the dilute outlet channel or the concentrated outlet channel of the first device in the system does not subsequently flow into the inlet channel of the second device in the system.

[0081] By way of example, FIG. 4 is a schematic drawing of a water purification system ( 400) that includes a first desalination unit ( 402) and a second desalination unit ( 404) arranged in parallel. The example device further includes an auxiliary channel ( 406) which is fluidly isolated from both the first and second desalination unit. A first bipolar electrode ( 408) electrochemically connects the auxiliary channel ( 406) and the first desalination unit ( 402). A second bipolar electrode ( 410) electrochemically connects the auxiliary channel ( 406) and the second desalination unit ( 404). The example system can be operated by applying a potential bias between the auxiliary channel and the first and second desalination units.

[0082] FIG. 5 illustrates a second example water purification system ( 500) that includes two devices arranged in parallel. The system ( 500) comprises a first device which includes a first desalination unit ( 502) electrochemically connected to a first auxiliary channel ( 504) by a first bipolar electrode ( 506). The system ( 500) further comprises a second device which is arranged in parallel with respect to the first device, and which includes a second desalination unit ( 508) electrochemically connected to a second auxiliary channel ( 510) by a second bipolar electrode ( 512). As illustrated in FIG. 5, a power supply can be configured to apply a potential bias across both the first auxiliary channel ( 504) and desalination unit ( 502) and the second auxiliary channel ( 510) and desalination unit ( 508).

[0083] The system can comprise a plurality of the devices described herein fluidly connected in series. Within the context of the systems described herein, two devices can be described as being fluidly connected in series within a system when fluid flowing from either the dilute outlet channel or the concentrated outlet channel of the first device in the system subsequently flows into the inlet channel of the second device in the system.

[0084] By way of example, FIG. 6 is a schematic drawing of a water purification system ( 600) that includes two devices fluidly connected in series. The system ( 600) includes a first desalination unit ( 602) and a second desalination unit ( 604) fluidly connected in series, such that the dilute outlet channel of the first desalination unit is fluidly connected to the inlet channel of the second desalination unit. The example device further includes an auxiliary channel ( 606) which is fluidly isolated from both the first and second desalination unit. A first bipolar electrode ( 608) electrochemically connects the auxiliary channel ( 606) and the first desalination unit ( 602). A second bipolar electrode ( 610) electrochemically connects the auxiliary channel ( 606) and the second desalination unit ( 604). The example system can be operated by applying a potential bias between the auxiliary channel and the first and second desalination units.

[0085] If desired, the systems can contain a plurality of devices both arranged in parallel and fluidly connected in series. For example, the device can include a first pair devices fluidly connected in series which are arranged in parallel with a second pair of devices fluidly connected in series.

[0086] Methods of Making

[0087] The microfluidic devices and systems described herein can be fabricated from any substrate material which is non-conductive, and suitable for the flow of aqueous solutions through the microfluidic channels of the device or system. For example, the device or system can be fabricated, in whole or in part, from glass, silicon, or combinations thereof. The device or system can also be fabricated, in whole or in part, from a polymer and/or plastic, such as a polyester (e.g., polyethylene terephthalate; PET) polyurethane, polycarbonate, halogenated polymer (e.g., polyvinyl chloride and/or fluorinated polymer such as polytetrafluoroethylene (PTFE)), polyacrylate and/or poly methacrylate (e.g., polymethyl methacrylate; PMMA), silicone (e.g., polydimethylsiloxane; PDMS), a thermosetting resin (e.g., Bakelite), or a copolymer, blend, and/or combination thereof. The device or system can also be fabricated, in whole or in part, from a ceramic (e.g., silicon nitride, silicon carbide, titania, alumina, silica, etc.).

[0088] In certain embodiments, the device or system is fabricated, in whole or in part, from a photocurable epoxy. In certain embodiments, the device or system is fabricated, in whole or in part, from PDMS.

[0089] The microfluidic devices and systems described herein can be fabricated using a variety of microfabrication techniques known in the art. Suitable methods for the microfabrication of microfluidic devices include, for example, lithography, etching, embossing, roll-to-roll manufacturing, lamination, printing, and molding of polymeric substrates. The microfabrication process can involve one or more of the processes described below (or similar processes). Different portions of the device or system can be fabricated using different methods, and subsequently assembled or bonded together to form the final microfluidic device or system. Suitable fabrication methods can be selected in view of a number of factors, including the nature of the substrate(s) used to form the device or system, performance requirements, and the dimensions of the microfluidic features making up the device or system.

[0090] Lithography involves use of light or other form of energy such as electron beam to selectively alter a substrate material. Typically, a polymeric material or precursor (e.g., photoresist, a light-resistant material) is coated on a substrate and is selectively exposed to light or other form of energy. Depending on the photoresist, exposed regions of the photoresist either remain or are dissolved in subsequent processing steps known generally as "developing." This process results in a pattern of the photoresist on the substrate. In some embodiments, the photoresist is used as a master in a molding process. In some embodiments, a polymeric precursor is poured on the substrate with photoresist, polymerized (i.e., cured) and peeled off. The resulting polymer is bonded or glued to another flat substrate after drilling holes for inlets and outlets.

[0091] In some embodiments, the photoresist is used as a mask for an etching process. For example, after patterning photoresist on a silicon substrate, channels can be etched into the substrate using a deep reactive ion etch (DRIE) process or other chemical etching process known in the art (e.g., plasma etch, KOH etch, HF etch, etc.). The photoresist can then be removed, and the substrate can be bonded to another substrate using one of any bonding procedures known in the art (e.g., anodic bonding, adhesive bonding, direct bonding, eutectic bonding, etc.). Multiple lithographic and etching steps and machining steps such as drilling can be included. Carbon electrodes may be fabricated in place by means of photoresist pyrolysis.

[0092] In some embodiments, a polymeric substrate, such as PMMA, can be heated and pressed against a master mold for an embossing process. The master mold can be formed by a variety of processes, including lithography and machining. The polymeric substrate can then be bonded with another substrate to form a microfluidic device or system. Machining processes can be included if necessary.

[0093] Devices and systems can also be fabricated using an injection molding process. In an injection molding process, a molten polymer or metal or alloy is injected into a suitable mold and allowed to cool and solidify. The mold typically consists of two parts that allow the molded component to be removed. Parts thus manufactured can be bonded to result in the device or system.

[0094] In some embodiments, sacrificial etch can be used to form the device or system. Lithographic techniques can be used to pattern a material on a substrate. This material can then be covered by another material of different chemical nature. This material can undergo lithography and etch processes, or another suitable machining process. The substrate can then be exposed to a chemical agent that selectively removes the first material. In this way, channels can be formed in the second material, leaving voids where the first material was present before the etch process.

[0095] In some embodiments, microchannels can be directly machined into a substrate by laser machining or CNC machining. If desired, several layers can be machined, and subsequently bonded together to obtain the final device or system.

[0096] Electrodes as well as other electrical device components can be fabricated within the devices and systems by patterning suitable conductive materials on and/or within substrate materials using a number of suitable methods known in the art.

[0097] In one or more embodiments, the conductive material includes one or more metals. Non-limiting examples of suitable metals include Sn, Zn, Au, Ag, Ni, Pt, Pd, Al, In, Cu, or a combination thereof. Other suitable conductive materials include metal oxides and conductive non-metals (e.g., carbon derivatives such as graphite). Conductive materials can be deposited using a vacuum deposition process (e.g., cathodic arc deposition, electron beam physical vapor deposition, evaporative deposition, pulsed laser deposition, or sputter deposition). Conductive material can also be provided in the form of a conductive ink which can be screen printed, ink-jet printed, or otherwise deposited onto the surface of the substrate material to form an electrical device component. Conductive inks are typically formed by blending resins or adhesives with one or more powdered conductive materials such as Sn, Zn, Au, Ag, Ni, Pt, Pd, Al, In, Cu, graphite powder, carbon black, or other conductive metals or metal alloys. Examples include carbon-based inks, silver inks, and aluminum inks.

[0098] When forming an electrical device component, such as an electrode, in the devices or systems described herein, one or more conductive materials will preferably be deposited or applied as a thin film. In certain embodiments, the conductive layers are thin metallic or carbon films which are about 50 microns in thickness or less (e.g., about 40 microns in thickness or less, about 30 microns in thickness or less, about 25 microns in thickness or less, about 20 microns in thickness or less, about 15 microns in thickness or less, about 10 microns in thickness or less, about 5 microns in thickness or less, about 1 micron in thickness or less, about 900 nm in thickness or less, about 800 nm in thickness or less, about 750 nm in thickness or less, about 700 nm in thickness or less, about 600 nm in thickness or less, about 500 nm in thickness or less, about 400 nm in thickness or less, about 300 nm in thickness or less, or about 250 nm in thickness or less).

[0099] Methods of Using

[0100] The microfluidic devices and systems described herein can be used to decrease the salinity of water. The salinity of water can be decreased by flowing saltwater through the desalination unit of a device or system described herein, and performing a faradaic reaction at the electrode positioned in proximity to the intersection of the desalination unit. The faradaic reaction generates an electric field gradient that directs ions in the saltwater away from the dilute outlet channel of the desalination unit, and towards the concentrated outlet channel of the desalination unit. As a result, the salinity of water which flows into the dilute outlet channel is lower than the salinity of the saltwater flowing into the inlet channel.

[0101] In some embodiments, methods of decreasing the salinity of water include providing a flow of saltwater through the inlet channel of a device described herein or the water inlet of a system described herein, applying a potential bias to generate an electric field gradient that influences the flow of ions in the saltwater through the desalination unit of the device or the desalination units of the system, and collecting water from the dilute outlet channel of the device or the water outlet of the system. In these methods, the water collected from the dilute outlet channel of the device or the water outlet of the system can have a lower electrical conductivity than the saltwater flowed through the inlet channel of the device or the water inlet of the system.

[0102] In some embodiments, the potential bias applied to generate an electric field gradient is greater than about 1 volt (e.g., greater than about 2 volts, greater than about 2.5 volts, greater than about 3 volts, greater than about 4 volts, greater than about 5 volts, greater than about 6 volts, greater than about 7 volts, greater than about 8 volts, or greater than about 9 volts). In some embodiments, the potential bias applied to generate an electric field gradient is less than about 10 volts (e.g., less than about 9 volts, less than about 9 volts, less than about 8 volts, less than about 7 volts, less than about 6 volts, less than about 5 volts, less than about 4 volts, less than about 3 volts, less than about 2.5 volts, or less than about 2 volts).

[0103] The potential bias applied to generate an electric field gradient can range from any of the minimum voltages to any of the maximum voltages described above. In some embodiments, the potential bias applied to generate an electric field gradient ranges from about 1 volt to about 10 volts (e.g., from about 1 volt to about 7 volts, from about 2 volts to about 7 volts, or from about 2.5 to about 5 volts).

[0104] In some embodiments, the flow rate of the saltwater through the desalination unit of the device or the flow rate of the saltwater through each desalination unit of the system ranges from about 0.01 to about 1 microliter per minute (e.g., from about 0.05 to about 0.5 microliters per minute, or from about 0.1 to about 0.5 microliters per minute). Suitable flow rates can be selected in view of a variety of factors including the architecture of the device or system, the salinity of the saltwater being treated using the device or system, and the desired degree of salinity reduction.

[0105] The devices, systems, and methods described herein can be used to decrease the salinity of saltwater having any measurable concentration of dissolved sodium chloride. The saltwater can be seawater (e.g., saltwater having a conductivity of between about 4 S/m and about 6 S/m). The saltwater can be brackish water (e.g., saltwater having a conductivity of between about 0.05 S/m and about 4 S/m). In certain embodiments, the saltwater has a conductivity of greater than about 0.05 S/m (e.g., greater than about 0.1 S/m, greater than about 0.5 S/m, greater than about 1.0 S/m, greater than about 2.0 S/m, greater than about 2.5 S/m, greater than about 3.0 S/m, greater than about 3.5 S/m, greater than about 4.0 S/m, greater than about 4.5 S/m, greater than about 5.0 S/m, or greater than about 5.5 S/m).

[0106] The devices, systems, and methods described herein can be used to decrease the salinity of saltwater by varying degrees. The degree of salinity reduction can depend on a number of factors, including the architecture of the device or system, and the salinity of the saltwater being treated using the device or system.

[0107] In some embodiments, the conductivity of the water desalinated using the devices, systems, and methods described herein (e.g., the water collected from the dilute outlet channel of the device or the water outlet of the system) does not exceed about 90% of the conductivity of the saltwater flowed into the device or system (e.g., it does not exceed about 80% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 75% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 70% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 60% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 50% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 40% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 30% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 25% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 20% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 10% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 5% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 1% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 0.5% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 0.1% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 0.05% of the conductivity of the saltwater flowed into the device or system, it does not exceed about 0.01% of the conductivity of the saltwater flowed into the device or system, or less).

[0108] In some cases, water desalinated using the devices, systems, and methods described herein (e.g., water collected from the dilute outlet channel of the device or the water outlet of the system) has a conductivity of less than about 2.0 S/m (e.g., less than about 1.75 S/m, less than about 1.5 S/m, less than about 1.25 S/m, less than about 1.0 S/m, less than about 0.75 S/m, less than about 0.5 S/m, less than about 0.25 S/m, less than about 0.1 S/m, less than about 0.05 S/m, less than about 0.01 S/m, less than about 0.005 S/m, less than about 0.001 S/m, less than about 5.0*10 <-4 >S/m, less than about 1.0*10 <-4 >S/m, less than about 5.0*10 <-5 >S/m, less than about 1.0*10 <-5 >S/m, or less).

[0109] In some embodiments, the water desalinated using the devices, systems, and methods described herein (e.g., water collected from the dilute outlet channel of the device or the water outlet of the system) is drinking water (e.g., the water has a conductivity of from about 0.05 S/m to about 0.005 S/m). In some embodiments, the water desalinated using the devices, systems, and methods described herein (e.g., water collected from the dilute outlet channel of the device or the water outlet of the system) is ultrapure water (e.g., the water has a conductivity of from about 0.005 S/m to about 5.5*10 <-6 >S/m).

[0110] If desired, water can be treated multiple times using the devices, systems, and methods described herein to achieve a desired decrease in the salinity of the saltwater.

[0111] The devices and systems described herein can be used to desalinate water with greater energy efficiency than conventional desalination methods. In some cases, the devices and systems described herein can be used to desalinate water with at an energy efficiency of less than about 1000 mWh/L (e.g., at least about 900 mWh/L, at least about 800 mWh/L, at least about 750 mWh/L, at least about 700 mWh/L, at least about 600 mWh/L, at least about 500 mWh/L, at least about 400 mWh/L, at least about 300 mWh/L, at least about 250 mWh/L, at least about 200 mWh/L, at least about 100 mWh/L, at least about 90 mWh/L, at least about 80 mWh/L, at least about 75 mWh/L, at least about 70 mWh/L, at least about 60 mWh/L, at least about 50 mWh/L, at least about 40 mWh/L, at least about 30 mWh/L, at least about 25 mWh/L, at least about 20 mWh/L, at least about 15 mWh/L, or at least about 10 mWh/L, or at least about 5 mWh/L). In some embodiments, the devices and systems described herein can be used to desalinate water with at an energy efficiency ranging from any of the minimum values above to about 1 mWh/L (e.g., from at least about 1000 mWh/L to about 1 mWh/L, from at least about 500 mWh/L to about 1 mWh/L, from at least about 100 mWh/L to about 1 mWh/L, from at least about 75 mWh/L to about 1 mWh/L, or from at least about 50 mWh/L to about 1 mWh/L).

[0112] In some cases, the saltwater is not pre-treated prior to desalination with the devices and systems described herein. In other embodiments, the saltwater can be treated prior to desalination. For example, the removal of multivalent cations (e.g., Ca <2+>, Mg <2+>, or combinations thereof) from saltwater prior to desalination could reduce precipitate formation within the device or system over long operation times. Accordingly, in some embodiments, the saltwater can be pre-treated to reduce the level of dissolved multivalent cations in solution, for example, by contacting the saltwater with a suitable ion exchange resin. If necessary, saltwater can also be pre-treated to remove debris, for example, by sedimentation and/or filtration. If desired, saltwater can also be disinfected prior to desalination.

[0113] If desired for a particular end use, water can be further treated following desalination with the devices and systems described herein. For example, water can be fluoridated by addition of a suitable fluoride salt, such as sodium fluoride, fluorosilicic acid, or sodium fluorosilicate. Water can also be passed through an ion exchange resin and/or treated to adjust pH following desalination with the devices and systems described herein.

EXAMPLES

Example 1

Desalination Using a Microfluidic Device

[0114] A microelectrochemical cell comprising a desalination unit and an auxiliary channel spanned by a single bipolar electrode (BPE) was used to desalinate seawater along a locally generated electric field gradient in the presence of pressure driven flow (PDF). Seawater desalination was achieved by applying a potential bias between a parallel desalination unit and auxiliary channel to drive the oxidation of chloride at the anodic pole of the bipolar electrode. At the cathodic pole, water reduction occurs to support current flow.

[0115] The oxidation of chloride at the anodic pole of the BPE results in an ion depletion zone and subsequent electric field gradient. The electric field gradient directed ions flowing through the desalination unit into a branching microchannel, creating a brine stream, while desalted water continued to flow forward when the rate of pressure driven flow was controlled. Seawater desalination could thus be achieved by controlling the rate of pressure driven flow to create both a salted and desalted stream.

[0116] Materials and Methods

[0117] Fabrication of Microfluidic Device

[0118] A PDMS/quartz hybrid microfluidic device was prepared using microfabrication methods known in the art. The structure of the microfluidic device is schematically illustrated in FIG. 2. The device comprises a desalination unit and an auxiliary channel spanned by a single bipolar electrode.

[0119] A pyrolyzed photoresist carbon electrode was fabricated on a quartz slide (1 in*1 in). Photoresist was spin coated onto the slide at 3500 rpm for 45 seconds, and then soft baked on a hot plate at 100[deg.] C. for 1 minute to remove excess solvent. The device was then exposed to a UV lamp with patterned mask above to reveal the electrode (100 [mu]m wide by 6.3 mm long) design. The excess photoresist was then removed by development. The devices were then placed in a quartz tube furnace with a forming gas of 5% H 2 and 95% N 2 continuously flowing at 100 standard cubic centimeters per minute to allow the photoresist to pyrolyze. After pyrolysis, the device was cooled to room temperate.

[0120] A PDMS desalination unit (5.0 mm long and 22 [mu]m tall) with a 100 [mu]m wide inlet channel and 50 [mu]m wide dilute outlet channel and concentrated outlet channel was fabricated parallel to an auxiliary channel (5.0 mm long, 22 [mu]m tall, 100 [mu]m wide) using a SU-8 photoresist mold patterned on a silicon wafer. The separation between the desalination unit and the auxiliary channel was 6.0 mm (center-to-center). The PDMS channels were rinsed with ethanol and dried under N 2, then the PDMS and quartz/electrode surfaces were exposed to an air plasma for 15 seconds, and finally the two parts were bound together with the BPE aligned at the intersection where the dilute outlet channel and concentrated outlet channel diverge from the inlet channel. The PDMS/quartz microfluidic device was then placed in an oven at 65[deg.] C. for 5 min to promote irreversible bonding.

[0121] Evaluation of Desalination

[0122] Seawater collected from Port Aransas, Tex. was used to evaluate desalination. To prevent obstruction of the microfluidic channel, the seawater samples were allowed to undergo a simple sedimentation process before sample collection. The seawater was spiked with a cationic (20 [mu]M Ru(bpy) <2+>) tracer to fluorescently monitor the movement of ions through the desalination unit during desalination.

[0123] A solution height differential was created between the fluid reservoir fluidly connected to the inlet channel ( 110; V 1) and the fluid reservoirs fluidly connected to the concentrated outlet channel ( 112; V 2) and fluidly connected to the dilute outlet channel ( 114, V 3). In this way, a pressure driven flow (PDF) from right to left was initiated.

[0124] Results

[0125] Using Au driving electrodes, E tot=2.5 V was applied to reservoirs 212 and 210 while fluid reservoirs 110, 112, and 114 were grounded. The potential bias created a sufficiently large potential difference between the poles of the BPE ( 204) to drive water oxidation and reduction at the BPE anode ( 206) and cathode ( 208). See Eqn. 1 and 2, respectively. Moreover, chloride oxidation occurred at the BPE anode ( 206; Eqn. 3) directly resulting in an ion depletion zone near the BPE as chlorine was generated.

[0000]
2H 2O-4 e <-> [image]O 2+4H <+> (Eqn. 1)

[0000]
2H 2O+2 e <-> [image]H 2+2OH <-> (Eqn. 2)

[0000]
2C 1 <->-2 e <-> [image]Cl 2 (2) (Eqn. 3)

[0126] In addition, H <+> electrogenerated by water oxidation (Eqn. 1) can neutralize bicarbonate and borate that can be present in seawater, further contributing to the strength of the ion depletion zone ( 109) and subsequently formed electric field gradient. With PDF from right to left, seawater, and thus the ions present is seawater, were transported toward the electric field gradient formed at intersection where the dilute outlet channel ( 104) and concentrated outlet channel ( 106) diverge from the inlet channel ( 102).

[0127] The electrophoretic velocity ([mu] ep) of a charged analyte is governed by Eqn. 4, where [mu] ep is the analyte's electrophoretic mobility and V 1 is the local electric field strength.

[0000]
[mu] ep=[mu] epV 1 (Eqn. 4)

[0000] In all regions of the device depicted in FIG. 2, except near the ion depletion zone formed by the anode of the bipolar electrode in proximity to the intersection where dilute outlet channel and concentrated outlet channel diverge from the inlet channel, the transport of water and all dissolved species is controlled by PDF. As a consequence, all neutrals and ions to move generally in the direction of fluid flow (i.e., from right to left) throughout the device. However, as ions approach the local electric field gradient formed by the electrode in proximity to the intersection, they experience an increasing [mu] ep as the electric field strength increases. In the case of cations, this gradient causes them to redirect toward the grounded reservoir in the brine stream as a result of the local electrophoretic velocity of the ions ([mu] ep) exceeding the mean convective velocity of the fluid (PDF). To maintain electroneutrality with the microchannel, anions are also redirected into the brine stream.

[0128] The flow of ionic species through the microchannels of the device was monitored by observing the flow of Ru(bpy) <2+> (a fluorescent cationic tracer) through the device. FIGS. 7A and 7B are fluorescence micrographs of the device taken before ( FIG. 7A) and after ( FIG. 7B) application of a potential bias. As shown in FIG. 7A, when no potential bias was applied, ions flowed through the inlet channel ( 702), and into both the dilute outlet channel ( 704) and the concentrated outlet channel ( 706). Upon application of a potential bias, an ion depletion zone and subsequent electric field gradient are formed near the BPE anode ( 708) in proximity to the intersection ( 710) of the dilute outlet channel ( 704) and the concentrated outlet channel ( 706; FIG. 7B). As a consequence, ions, including the fluorescent cationic tracer Ru(bpy) <2+>, are directed into the concentrated outlet channel ( 706). Desalted water (which is non-fluorescent in the micrograph due to the absence of fluorescent cationic tracer Ru(bpy) <2+>) flows into the dilute outlet channel ( 704). These results demonstrate that both cations and anions flow into the concentrated outlet channel ( 706). The initial application of 2.5 V creates an oxidizing environment near the BPE anode which causes partial dissolution of the Au anode.

[0129] To confirm that the formation of an ion depletion zone resulted in the deionization of the fluid flowing into the device, a similar experiment was conducted using a solution lacking chloride ions. If all chloride ions are eliminated from solution, one would not expect the BPE anode to induce formation of an ion depletion zone and local electric field gradient (as in the case of saltwater containing chloride ions). In the control experiment, a solution of Na 2SO 4 was flowed through the device. As shown in FIG. 8, upon application of 2.5 V, no decrease in fluorescence intensity near the BPE anode was observed. This finding was consistent with the seawater desalination being the result of an ion depletion zone formed near the BPE anode.

[0130] FIG. 9 shows a representative plot of total current flowing through the device (i tot) vs. time. The steady-state operating current of the device was 65 nA. With a 2.5 V potential bias driving the desalination process, the device operated at a power consumption of only 162.5 nW.

[0131] Fluid flow rates through the dilute outlet channel could be measured using non-charged beads. Fluid flow rates through the dilute outlet channel could also be measured by tracking the movement of fluorescent tracer after the 2.5 V driving potential was turned off, in which case all mass transport was due to PDF.

[0132] The average operating fluid flow rate of the devices was ~400 [mu]m/s. At higher fluid flow rates, the ion depletion zone does not extend as far into the dilute outlet channel. Consequently, the desalination process becomes less efficient, and ions begin to flow into the dilute outlet channel during device operation.

[0133] Using the device operating at 162.5 nW, 34 mWh/L energy efficiencies were achieved. This energy efficiency is orders of magnitude higher than the current state-of-the-art seawater desalination technologies. For example, reverse osmosis is typically performed at energy efficiencies of approximately 5 Wh/L, and has only achieved maximum energy efficiencies of approximately 1.8 Wh/L. This superior efficiency of the microfluidic device relative to reverse osmosis is particularly notable when considering that these reverse osmosis energy efficiencies correspond to the efficiencies of industrial desalination facilities (which are often higher than efficiencies observed for the same process conducted on a smaller scale).

[0134] A reduction in device scale typically results in a decrease in energy efficiency. As a consequence, these devices appear to be extremely competitive for small-scale desalination use. Moreover, because little equipment is required, and device operation only requires a 2.5 V power supply, these devices can be used in water stresses regions. In addition, because BPEs do not require a direct electrical connection, it is possible to simultaneously operate numerous devices in parallel using a simple power supply.

[0135] The devices, systems, and methods of the appended claims are not limited in scope by the specific devices, systems, and methods described herein, which are intended as illustrations of a few aspects of the claims. Any devices, systems, and methods that are functionally equivalent are intended to fall within the scope of the claims. Various modifications of the devices, systems, and methods in addition to those shown and described herein are intended to fall within the scope of the appended claims. Further, while only certain representative devices, systems, and method steps disclosed herein are specifically described, other combinations of the devices, systems, and method steps also are intended to fall within the scope of the appended claims, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein or less, however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.

[0136] The term "comprising" and variations thereof as used herein is used synonymously with the term "including" and variations thereof and are open, non-limiting terms. Although the terms "comprising" and "including" have been used herein to describe various embodiments, the terms "consisting essentially of" and "consisting of" can be used in place of "comprising" and "including" to provide for more specific embodiments of the invention and are also disclosed. Other than where noted, all numbers expressing geometries, dimensions, and so forth used in the specification and claims are to be understood at the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, to be construed in light of the number of significant digits and ordinary rounding approaches.

[0137] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.



WO2005032717
SYSTEM AND METHOD FOR ELECTROKINETIC TRAPPING AND CONCENTRATION ENRICHMENT OF ANALYTES IN A MICROFLUIDIC CHANNEL

    
Inventor:
CROOKS RICHARD M
ITO TAKASHI

Applicant:
TEXAS A & M UNIV SYS
CROOKS RICHARD M 

The present invention relates generally to the field of microfluidics-based analysis and, more particularly, to system and method for electrokinetic trapping and concentration enrichment of analytes in a microfluidic channel.

BACKGROUND OF THE INVENTION

Sample enrichment or preconcentration plays an important role in chemical separation and analysis. In general, if molecules to be detected (the analyte) exist in a chemical sample in high concentrations, then it is easier to detect their presence. However, many important analytes, especially biomoleculesa such as DNA, proteins, antibodies, antigens, and polysaccharides, often exist in minute quantities in real, unprocessed chemical samples.

Microfluidic devies typically consist of a network of channels, which have cross- sectional dimensions of tens to hundreds of microns, terminated with reservoirs the contain analytes. Various approaches have been used to move analytes out of the reservoirs and into the channel network, where the analytes are separated and detected. Because the total fluid volume of a microfluidic device is very small, the analyte must exist in a sufficiently high concentration for there to be enough molecules present to be detectable.

Some methods are based on controlling the electrokinetic properties of at least two plugs (zones of solution of limited or defined length within a microchannel) of an electrolyte solution. These methods, which include field-amplification stacking, isotachophoresis, and micelle sweeping, require that neighboring electrolyte plugs have different compositions.

However, this is a difficult condition to realize in a commercially viable system. Other methods are based on the principle of size exclusion, which is essentially a filtration method.

In this case, the analyte is larger than the pores of a filtration membrane barrier and, as such, the analyte is retained by the filter while small molecules (solvent and electrolyte) pass through.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method for chemical analysis includes providing a device having a drain region, a source region, and a gate region disposed therebetween, associating a buffer solution with the drain region and the source region, causing a potential difference between the drain region and the source region until a stable current is reached, replacing the buffer solution in the source region with a solution containing an analyte, and applying a negative potential to the source region to create a forward bias.

Embodiments of the invention provide a number of technical advantages.

Embodiments of the invention may include all, some, or none of these advantages. Some embodiments of the invention provide devices and methods for the manipulation of charged analytes, such as molecules, particles, beads, and the like, such that the charged analytes may be trapped, filtered, fractionated, or locally enriched in concentration. Having been so manipulated, the charged analytes may be available for detection, reaction, collection, or other processing, within the same device or upon removal to another device or instrument.

The portion of the device responsible for trapping, filtering, fractionating or enriching the concentration of the charged analyte may be the sole functional aspect of the device, although generally this portion will be just one component of an integrated device, i. e. , a microfluidic device that is capable of performing other operations in combination with, either preceding or following, the operation disclosed herein as the subject invention. Examples of other operations performed within microfluidic devices include mixing, metering, binding, incubating, thermocycling, reacting, electrophoresing, absorbing or adsorbing and desorbing, extracting, etc. , employing structures such as valves, channel networks, pressure actuators, thermal sources and sinks, and pH, conductivity, temperature or pressure sensors, which are known by and familiar to those skilled in the art.

Some embodiments may provide a method for concentrating charged analytes prior to further processing within an integrated microfluidic device. Accordingly, one embodiment provides an improved method of electrophoretic separation analysis of charged analytes based on concentrating analytes as herein described and then releasing the concentrated species into a separation channel. Alternatively, further processing may involve releasing the concentrated analytes from a first gate region and passing the concentrated species via a fluidic network to another, second gate region wherein the species may be again concentrated, collected, detected or fractionated.

Some embodiments may provide a linear series of gate regions in combination in a manner such that each gate region acts to concentrate a subset of the charged analytes adjacent thereto, while another subset passes through the gate channels of that gate region and towards the next, wherein each successive gate region presents a different threshold for passage based on electrophoretic mobility and thus causing a different subset of charged analytes to concentrate adjacent thereto.

Various embodiments of the invention may find general application in microfluidic devices for a variety of purposes because the method relies upon the balancing of physical forces and motions. As such, the invention does not require, for example, molecule-specific binding interactions in order to provide trapping, filtering, fractionation or enrichment of the analyte. Thus, the devices and methods are broadly applicable to a wide range of analytes that are charged, or may be associated with charge-bearing species, such as molecules or particles.

Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic of a system that illustrates a principle of electrokinetic trapping and concentration enrichment of analytes in a microfluidic channel according to an embodiment of the invention;



FIGURE 2 illustrates a system for electrokinetic trapping and concentration enrichment of analytes in microfluidic systems according to one embodiment of the invention;



FIGURES 3A through 3C are fluorescence micrographs of a fluidic device used to demonstrate one embodiment of the invention;



FIGURE 4 is a graph illustrating fluorescence intensity as a function of time allotted for concentration enrichment of DNA in an experiment used to demonstrate one embodiment of the invention;



FIGURE 5 illustrates a system for electrokinetic trapping and concentration enrichment of analytes in microfluidic systems according to one embodiment of the invention in which an enriched band of a mixture may be utilized as an injection plug for electrokinetic separation;



FIGURES 6A and 6B illustrate systems for electrokinetic trapping and concentration enrichment of analytes in microfluidic systems according to embodiments of the invention in which the location of an enriched band may be controlled by applying a sequence of bias voltages with well-defined temporal control; and





FIGURE 7 illustrates a system for electrokinetic trapping and concentration enrichment of analytes in microfluidic systems according to one embodiment of the invention in which a mixture of charged molecules or objects may be trapped, enriched, and separated using a series of gate channels.




DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide methods and/or devices for the trapping and enrichment of charged analytes and, as described herein, is based on a fundamentally new principle that is broadly applicable to a wide range of uses. However, the present invention contemplates that the principle is general and versatile and should be applicable to any suitably charged molecule or object, and to many other suitable forms of a device in which the principle operates.

In the following detailed description, the term"analyte"is used in a broad sense. On one hand, analyte means a discrete substance, molecule, aggregate, polymer, bead, particle, cell or subcellular component, bearing a charge, that is to be subjected to manipulation or processing in a device or by a method. Analytes include, but are not limited to, peptides, proteins, polynucleotides, polypeptides, oligonucleotides, organic molecules, haptens, epitopes, cells or parts of biological cells, posttranslational modifications of proteins, receptors, complex sugars, vitamins, hormones, and the like. Analytes may also be inorganic compounds or particles, semiconductor particles, such as quantum dots, polymeric beads or particles, etc. , provided the material in question is small enough to be able to physically pass through gate channels of a particular device (as described in greater detail below) and thus be capable of performing according to inventive methods described herein. For any of the above-listed exemplary analytes, they must either have a net anionic or cationic charge; though if not intrinsically charged, the particular analyte may be modified or otherwise associated with some other charged species such that it bears a charge when used in embodiments of the invention. However, the term"analyte"is not intended to refer to the ions that make up the supporting electrolyte used in the buffer solutions.

On the other hand, and according to the context, "analyte"is also used to mean a multicomponent sample used according to embodiments of the invention. Used in this sense, the analyte contains a plurality of distinct charged species, and each of the species will respond differently in the operation of a device. Some of the species may become concentrated, while others may not under a given set of conditions.

FIGURE 1 illustrates a system 100 for electrokinetic trapping and concentration enrichment of an analyte 111 according to one embodiment of the invention. FIGURE 1 is meant to illustrate the general principle of the invention, which relies on exerting spatial control over the electrokinetic velocity of analyte 111. In the illustrated embodiment, system 100 includes a drain region or channel 102 having a first electrode 103, a source region or channel 104 having a second electrode 105, a gate region 106 providing fluid communication between drain channel 102 and source channel 104, a buffer solution 108 associated with drain channel 102, a second solution 110 containing analyte 111 disposed in source channel 104, and a power supply 112 operable to impart a potential difference, or bias, between first electrode 103 and second electrode 105.

In the illustrated embodiment, gate region 106 is defined by a plurality of gate channels 107 that span between drain channel 102 and source channel 104. The gate channel openings are pores that are filled with electrolyte buffer solution 108. The size of any one gate channel 107 (or pore opening) should be smaller than the cross-sectional area of the channel section that gate channel 107 opens into (i. e. , the area of the gate channel/channel section interface) in order to have a functional device. In one embodiment, the combined area of the openings of all gate channels 107 is prescribed to be smaller than the cross- sectional area of the interface between gate region 106 and the drain or source channels, as further described below. The area of the interface is in some embodiments equivalent to the cross-section of a channel section as is generally the case when the channel sections lie within the same plane.

On the other hand, the present invention also calls for the pore openings to be larger than the size of the analyte that is to be trapped, filtered, or concentrated using the device. As such, pore openings, or gate channel widths, ranging in scale from about 2 nm to 2 llm, or even 5 pm are contemplated. A range of 10 nm to 1000 nm may be more typical.

Accordingly, for a device with channel sizes on the order of 10 pm to 1 mm, the relative width of the pore opening to the width of the channel sections is at least a ratio of about 1 to 10, and may be as small as 1 to 1,000 or 1 to 100,000. The size of opening chosen depends on a variety of factors, such as the size, the charge and mobility of the analyte, the viscosity and conductivity of the buffer solution, the chemical nature of the gate region material, and the anticipated rate of convective flow required through the pores.

The latter consideration, the rate of convective flow, is a significant factor in the operation of a device. Electroosmotic (eo) flow is generated within a gate channel 107, as indicated by arrow 109, when an electric field exists along that channel as a result of there being a permanent charge on the wall of the channel. Counterions predominate in the solution double layer at the wall interface, and the electrokinetic movement of these counterions in response to the electric field causes a net flow of the bulk solution. In one operation of the device, convective flow is required through gate channels 107 and, as such, channel walls bearing a charge are a characteristic of the device. At least one of either drain channel 102, source channel 104 or gate channels 107 of gate region 106 will have a charge- bearing surface. With an eo flow generated in at least one of these sections there will be a net convective flow (a combination of eo flow and pressure-driven flow caused by the eo flow) of buffer solution 108 in gate channels 107, due to the requirements of mass balance, subject to considerations such as the particular geometry of the particular channel network in a device.

Electrodes 103,105 are said to be associated with their respective channel sections 102,104. By this it is intended that a particular electrode sets the electrical potential distribution in the solution that is contained in its respective channel section, proximal to gate region 106. The electrode may reside within the channel, or within a suitable port or reservoir that communicates with the channel. In the case of a device being comprised of a network of channels, the electrode may be physically distant, and may even be located on the other side of a different gate region. However, for the gate region in question, the electrode is considered associated with a channel section as long as the channel section that the electrode sets the potential of is proximal with respect to the gate.

With reference to FIGURE 1, an operation of one embodiment of system 100 as it embodies a method for concentrating a charged analyte is now considered. Drain channel 102 and source channel 104 of the microfluidic device are filled with electrolyte buffer solution 108. The electrolyte provides ion conductors within the fluid and in response to a potential bias supplied between electrodes 103,105 supports the establishment of an electric field along the channels. Analyte 111, specifically a negatively charged analyte, such as a DNA molecule, is initially located in source channel 104, typically in a port or reservoir that communicates with the channel section, thereby associating it with the channel section. A potential bias is imparted to electrodes 103,105 to produce an electric field therebetween, wherein in this example the source electrode 105 is negative and drain electrode 103 is positive.

Accordingly, the electrokinetic motion of the DNA molecule under the influence of the electric field is towards drain electrode 103, and thus towards the gate region 106. This motion is characterized by an electrokinetic velocity that is the vector sum of the intrinsic electrophoretic (ep) velocity of analyte 111 and the convective velocity of buffer solution 108. Again, convective motion of buffer solution 108 refers to motion that is either electroosmotic flow or pressure-driven flow induced by the eo flow, or a combination thereof. By providing wall sections of any or all of the channels that bear a negative charge, the convective velocity of buffer solution 108 within gate region 106 is opposite in direction to the ep velocity of DNA in source channel 104. By having a convective velocity that is larger than the ep velocity, the DNA is not able to enter gate region 106 and instead accumulates and concentrates at a location in source channel 104 adjacent gate region 106.

The location of the concentrated band that forms is typically near the interface of gate region 106 with source channel 104.

As shown in FIGURE 1, the first requirement for trapping and enriching DNA (i. e., analyte 111) is that the ep velocity of DNA in source channel 104, which may be polydimethylsiloxane (PDMS), must be larger than the eo velocity of buffer solution 108 in source channel 104. That is to say, analyte 111 has a net electrokinetic velocity in source channel 104 towards gate region 106. The second requirement for DNA trapping and enrichment is that the local convective velocity of buffer solution 108 in the gate channels 107 is greater than and opposite in direction to the intrinsic ep velocity of analyte 111. Thus, fluid emanating from the pores, or gate channels 107, exemplified by, for example, a polyethylene terephthalate (PETE) membrane, effectively prevents analytes 111 from entering. Accordingly, analyte 111 accumulate near gate region 106 due to the opposing forces.

Because the eo velocity is equal to the product of the eo mobility and the electric field, thus it follows that the eo velocity may be modulated by changing the magnitude of the electric field. Furthermore, the local eo velocity may be varied according to position within a microfluidic network by simply changing the cross-sectional area at that position. For example, having a gate region comprised of gate channels having a smaller total cross- sectional area compared to the interface of the gate region and the source channel results in a higher local eo velocity through the pores. Thus, changing the cross-sectional area ratio provides a means for modulating the eo (convective) flow within a channel. This flow rate modulation selectively traps those analytes having an intrinsic ep mobility smaller than the local velocity of the eo"jets"emanating from the gate channel, while passing those analytes that have a ep mobility higher than the eo jet velocity.

Referring to FIGURE 1, in the case of analyte 111 being positively charged, the particular parameters given in the above example would need to be changed appropriately.

For example, the bias imparted would be reversed, with source electrode 105 being positive and drain electrode 103 being negative, and the walls would necessarily have a net positive charge in order that an opposing convective flow be established through gate channels 107.

Gate region 106 and gate channels 107 associated therewith are an important part of the subject invention. In one embodiment, the use of a porous membrane, more usually referred to as a nanoporous membrane, is contemplated. Nanoporous membrane materials are an area of interest, particularly for applications in filtration and sensing. As a result, methods for controlling the pore size, the composition, and the functional groups exposed within the pores, and the use of materials that range from organic polymers to semiconductors, ceramics and organic/inorganic composites, such as epoxy-embedded carbon nanotubes are known in the art. Depending on the material used, those skilled in the art will be familiar with the various techniques used to prepare such membranes, such as ion-track or ion-beam etching, anodic etching, microlithography in combination with etching, laser ablation, templated chemical assembly, sol-gel techniques, and the like. Nanoporous membranes for use in some embodiments of the invention may be comprised of at least one of the following: a polyester polymer such as PETE, polyimide, cellulose, polycarbonate, carbon or carbon nanotubes, semiconductors such as silicon or insulators such as silicon nitride, silica, alumina, or other inorganic ceramic materials such as titanates. Another class of nanoporous material, the hydrogel, is also useful in the invention and is described in further detail below.

FIGURE 2 illustrates a microfluidic system 200 chemical analysis and separation in accordance with one embodiment of the invention. In the illustrated embodiment, system 200 includes a drain reservoir 202 having a first electrode 203, a drain channel 204, a source reservoir 206 having a second electrode 207, a source channel 208, gate channel 210, a power supply 212, and an imaging device 214. Drain reservoir 202 may be any suitable size and shape and contains a buffer solution 216. Drain reservoir 202 couples to drain channel 204 in any suitable manner. Drain channel 204 may be any suitable size and shape; however, in one embodiment drain channel 204 as well as source channel 208 are polydimethylsiloxane (PDMS) channels having an approximate cross-section of 100 um x 20 u. m. Source reservoir 206 also may be any suitable size and shape and contains a second solution 218 containing one or more analytes 211. Gate channel 210 may be any suitable size and shape; however, in the illustrated embodiment, gate channel 210 is a nanoporous polyester membrane. For example, in a particular embodiment of the invention, gate channel 210 is a polyester membrane formed from PETE and having a 200 nm pore diameter, 10 llm thick, and 3x108 pores/cm2 manufactured by Osmonics. In other embodiments, gate channel 210 may be a porous hydrogel polymer network.

First electrode 203 and second electrode 207 may be any suitable electrodes and are immersed within their respective reservoir in order to create an electric field inside drain channel 204 and source channel 208. Power supply 212, which may be any suitable power supply, is operable to apply a potential difference between first electrode 203 and second electrode 207.

Imaging device 214 may be any suitable device that is operable to create an image of analyte 211 within system 200. For example, imaging device 214 may be an inverted fluorescence microscope equipped with an imaging CCD camera. In one embodiment, imaging device 214 is operable to obtain fluorescence micrographs of analyte 211.

In operation of one embodiment of system 200, drain reservoir 202, drain channel 204, source reservoir 206, and source channel 208 are filled with buffer solution 216. Buffer solution 216 may be any suitable buffer solution and, in a particular embodiment, buffer solution 216 is a TBE buffer solution (89mM TRIS base + 89mM boric acid + 2mM EDTA, pH 8.4) at reduced pressure. Buffer solution 216 is conditioned by applying 100 V between first electrode 203 and second electrode 207 until a stable current is reached. That portion of buffer solution 216 in source reservoir 206 is then replaced with second solution 218 containing analyte 211. For example, in a particular embodiment of the invention, analyte 211 is 10 llg/mL DNA (a 20mer ssDNA, 5'-labeled with fluorescein, obtained from IDT of Coralville, IA). Although analyte 211 is labeled with fluorescein in this example, any suitable label may be used to label analyte 211.

After obtaining a fluorescence micrograph 300 (FIGURE 3A), a forward bias (negative potential in source reservoir 206) is applied between first electrode 203 and second electrode 207. The resulting motion of analyte 211 (i. e. , DNA) is recorded using imaging device 214. This eventually results in the formation of an enriched band 220 of analyte 211 within source channel 208. Using the conditions noted above, enrichment of DNA is apparent within approximately thirty seconds and reaches an enrichment factor of eleven within approximately sixty-eight seconds, as shown in the fluorescence micrograph 302 of FIGURE 3B. The minimum width of enriched band 220 is approximately 100 um. The formation of enriched band 220 is a result of the general principle of the invention, as outlined and described in FIGURE 1.

When the forward bias is reversed, analyte 211 is immediately transported through gate channel 210, which indicates that enrichment is not a consequence of size exclusion induced by gate channel 210 (in this example the pores of the nanoporous membrane).

Transportation of analyte 211 through gate channel 210 is illustrated by the fluorescence micrograph 304 of FIGURE 3C. The transported analyte 211 is then trapped in drain channel 204 by the same balance of ep and eo velocities that was initially responsible for the formation of enrichment band 220 in source channel 208.

FIGURE 4 is a graph 400 illustrating fluorescence intensity as a function of time allotted for concentration enrichment of DNA in an experiment used to demonstrate one embodiment of the invention. As illustrated in FIGURE 4, the magnitude of DNA concentration reaches a limiting value in the center of the enriched band within a finite time (tconc). At t < tconc the enriched band has a nearly constant length, but when t > tConc this enriched band begins to expand longitudinally in the direction of the source reservoir. Using the conditions noted above, for an initial concentration of 10 llg/mL DNA and a 100 V forward bias, tronc is approximately five minutes and the enrichment factor, calculated from the relative fluorescence intensity is approximately thirty. However, when the DNA concentration in the source reservoir is reduced to approximately 1 llg/mL and 0. 1, ug/mL, the enrichment factor is increased to 300 and 800, respectively. Considering the simplicity and compactness of a microfluidic system, such as system 200, these enrichment factors are significant.

FIGURE 5 illustrates a system 500 for implementing another embodiment of trapping and enrichment of a negatively charged molecule or object according to the teachings of the invention. In the illustrated embodiment, system 500 includes a microfluidic device having a primary microchannel 502 that has a section 504 with an enlarged width at the center, and a first reservoir 506 and a second reservoir 508 at either primary microchannel 502 terminus.

Side channels 510,512 are also shown intersecting with first and second channel sections 502a, 502b of the primary channel 502, although these are optional and not critical to the concentrating function of the device. The enlarged width section 504 is used to hold a hydrogel plug 514 that serves as the gate region of the device.

A PDMS-glass microfluidic device similar to system 500 was fabricated for testing using standard rapid prototyping procedures and techniques. The microfluidic channel network formed had primary microchannel 502 about 7 mm long whose ends were connected to reservoirs 506,508, each 3 mm in diameter. This primary microchannel 502 was approximately 100 um wide and 25 um deep, except at center section 504 where it was approximately 200 um wide for a length of about 400 urn. On either side of section 504, that was to be the gate region, there was side channels 510,512 connected to primary microchannel 502 at arbitrary angles (~ 45 ). The side channels 510, 512 terminate in reservoirs 520,522, each about 0.5 mm in diameter. The cross-sectional dimensions of side channels 510,512 were the same as those for primary microchannel 502.

The gate region hydrogel plug 514 was prepared in situ by first placing a hydrogel precursor solution comprising 1: 4 molar ratio of acrylic acid (AA) to 2-hydroxyethyl methacrylate (HEMA), 5 wt % of a crosslinker, ethylene glycol dimethacrylate (EGDMA) and 3 wt % of a photoinitiator, 2,2-dimethoxy-2-phenyl acetophenone (DMPAP) (all from
Sigma-Aldrich, Inc. St. Louis, MO), into primary microchannel 502 by capillary action. A UV beam of 365 nm (300 mW/cm2, EFOS Lite E3000, Ontario, Canada) was projected onto the gate region for about 200 s from the side port of a microscope (DIAPHOT 300, Nikon) and through a lOx objective. A chrome mask was placed at the confocal plane at the side port so that a well-defined pattern of UV beam was created prior to projection and reduction through the microscope optics. Unreacted precursor solution was flushed out by introducing 10 mM Tris-HCl buffer solution (pH = 8.3) at a rate of 10 pL/min through reservoirs 506, 508 for more than 10 min. As the anionic hydrogel plug 514 comes in contact with the basic buffer solution, it expands and pushes itself against the walls of microchannel 502.

Comparatively less swelling is observed in case of uncharged hydrogel plug; still, it is enough to ensure that plug 514 remains stationary even under the influence of electric field.

The enlarged width section 504 in the primary channel 502 is advantageous for securing the position of hydrogel plug 514, and the prevention of any movement or slippage of the plug along the channel is desired. It is preferable to stabilize hydrogel plug 514 against the influence of high electric fields or other forces, such as hydrodynamic flow either during device fabrication or usage of the device. Alternatively, or in addition to the flange provided by enlarged width section 504, the channel walls may be treated to have a reactive chemical group (e. g. , a crosslinking agent) that can chemically bond to hydrogel plug 514 in order to secure its position.

Suitable hydrogel polymer plugs may be prepared from a wide variety of monomers, crosslinking agents, and initiators. The various components may be chosen for their degree of hydrophilicity, net charge, ability to enhance or reduce swelling or specific functional groups they may possess. However, the gel should provide two basic properties: (1) an ability to act as a porous ion conductor, and (2) the structural strength to withstand the forces of the electric field and any pressure-driven flow to which it will be exposed. In this regard, the amount of crosslinker used is important. When the percentage amount is too low the gel will not be rigid enough. Conversely, too high an amount of crosslinker limits the ability to swell and form a porous network. Thus, a percentage of at least about 5%, and no more than about 10% is preferred. Likewise, the amount of initiator used determines the properties of the gel produced. Initiator present in the range of-1-5 wt % generally produces hydrogels suitable for the present invention.

The hydrogel plug 514 may be preformed and added to the microfluidic channel network, or it may be formed in situ. The formation of the polymer may be photoinitiated or thermally initiated. Photoirradiated regions may be determined by interposing a mask or using a directed light source. On the other hand, the location of the gel precursor solution may be defined and thus determine the location and size of hydrogel plug 514. These methods are also well known in the art. Another exemplary method has been reported by Yu, et al. , in Anal. Chem. , 2002,73, p. 5088-5096.

FIGURES 6A through 6D illustrate the preconcentration phenomena observed in a microfluidic channel incorporating the anionic hydrogel plug 514, illustrated by system 500, as imaged using the microscope system previously described. After conditioning the channel by applying a potential bias of 100 V for 10 min, the potential was switched off and a 5 uM fluorescein solution was introduced through reservoir 508 to replace the buffer solution in channels 502b and 512. The solutions in all the reservoirs 506,520, 508,522 are kept at the same level to nullify any hydrodynamic flows inside the channel. Two platinum electrodes were inserted in reservoirs 506,508. A potential of 100 V was applied between the electrodes using a power source (range 0-1067 V) operated by a suitable in-house computer program.

FIGURE 6A shows the fluorescence micrograph obtained before the application of any potential bias. After applying a forward 100 V bias (reservoir 506 at positive potential), the negatively charged fluorescein ions migrate rapidly from reservoir 508 to reservoir 506.

However, hydrogel plug 514 at the gate region acts as a barrier to this movement, resulting in the concentration of fluorescein near the hydrogel-solution interface. This is apparent from FIGURE 6B, where concentration factors of 16 and 10 were achieved just inside the hydrogel and in the solution just outside the hydrogel, respectively, within 90 s. During forward bias, some fluorescein was lost as a fraction of fluorescein solution (about 2-fold concentrated) is directed towards floating reservoir 522. When the potential bias is reversed (FIGURE 6C), fluorescein is rapidly transported back towards reservoir 508. A minute amount of fluorescein (about 0. 75-fold) was trapped inside hydrogel plug 514 even after the application of reverse bias for 120 s (FIGURE 6D).

As shown by a graph 650 of concentration vs. time in FIGURE 6E, a higher preconcentration factor is observed inside hydrogel plug 514 (ROI 1) than in the solution (ROI 2). The enrichment factors reach a limiting value of 16 and 10 respectively within 100s. At t = 160 s, in the absence of potential bias, fluorescein concentrated inside hydrogel plug 514 starts to diffuse back to the solution, which in turn increases the concentration at ROI 2.

The above observation is likely caused by electrostatic repulsions between the hydrogel backbone and the fluorescein molecules, which become significant when the external bias voltage is turned off. On applying a reverse bias at t = 180 s, fluorescein is immediately transported back towards reservoir 508.

In a recent study of protein interaction and diffusion in HEMA-co-AA hydrogels, it has been reported that negatively charged protein Bovine Serum Albumin (Molecular weight 66 kDa and hydrodynamic radius of 3.4 nm at 4 C) is able to diffuse (diffusion coefficient of the order of 10-8 cm2/s) through the hydrogel at swelling ratios of less than 2. This suggests that the hydrogel pore size is greater than 3.4 nm. The calculated Debye length for 10 mM buffer solution is about 3 nm. Thus, the hydrogel pore size is sufficiently larger than the Debye length, giving rise to considerable electroosmotic flow inside the pores. The observed concentration phenomenon is consistent with the explanation that an electroosmotic flow opposes the electrokinetic transport of fluorescein ions, with a balance being achieved just inside the hydrogel boundary where sample stacking, i. e. , the formation of the band of fluorescein, is observed. Electrostatic repulsion between the charged hydrogel and the anionic analyte is also present, although experiments using uncharged hydrogel display the same stacking, or concentration, phenomenon.

The trapping and/or enrichment principles illustrated and described above in conjunction with FIGURES 1 through 6 may be important in many applications. Some of these applications are described in the embodiments illustrated in FIGURES 7 through 9.

FIGURE 7 illustrates a system 700 for electrokinetic trapping and concentration enrichment in microfluidic systems according to one embodiment of the present invention in which an enriched band of a mixture may be utilized as an injection plug for capillary electrophoretic (CE) separation. System 700 may be considered to be similar to system 200 of FIGURE 2; however, in the embodiment illustrated in FIGURE 7, system 700 also includes a separation region 702 having a separation reservoir 704, a separation electrode 706, and a separation channel 708, which is coupled to a source channel 710. Assuming that an analyte 711 is a negatively charged DNA molecule, a forward bias (negative potential in a source reservoir 712 and a positive potential in drain reservoir 714) is applied for a particular period of time. As a result, an enriched band 716 forms in a source channel 720 adjacent a gate channel 718, in accordance with the principle described above in conjunction with FIGURE 1.

When that bias is turned off, a bias is applied between a source electrode 713 and separation electrode 706 immediately thereafter, such that enriched band 716 is driven toward separation reservoir 704. Other embodiments for channel networks and the manner in which separation channel 708 communicates with gate channel 718 are also included in the subject invention. The analyte 711 concentrated at gate channel 718 may be processed in a variety of manners, such as, for example, to create a defined injection plug, prior to being driven into separation channel 708. Methods and devices for injecting sample plugs into microfluidic electrophoretic separation channels are well known in the art; see for example U. S. Patent Nos. 5,599, 432,5, 750,015, 5,900, 130,6, 007,690, 6,699, 377, which are herein incorporated by reference.

The separated analytes 711 may be collected at the end of separation channel 708, further injected into a separate instrument, or a detector may be positioned along or at the end of separation channel 708 to record the passage of analytes 711 along separation channel 708.

Detection may be optical (absorbance, fluorescence, shadowing), electrochemical (amperometric, potentiometric), or by other suitable methods. Note that separation channel 708 may be modified to obtain a particular benefit; for example, the channel surface may be treated to modulate and control eo flow, or the entire channel may be filled with a separation medium, such as a sieving polymeric matrix for the case of DNA separation, to improve resolution.

FIGURES 8A and 8B illustrate a system 800 for electrokinetic trapping and concentration enrichment in microfluidic systems according to an embodiment of the invention in which the location of an enriched band 802 may be controlled by applying a sequence of bias voltages with well-defined temporal control. In the illustrated embodiment, system 800 includes a source reservoir 804 having a source electrode 806, a source channel 808, a first drain reservoir 810 having a first drain electrode 812, a first drain channel 814, a second drain reservoir 816 having a second drain electrode 818, a second drain channel 820, a first gate channel 822, and a second gate channel 824.

In this embodiment of the invention, enriched band 802 forms by applying a forward bias voltage between source electrode 806, and first drain electrode 812. This forward bias is then switched off and another forward bias is immediately applied between source electrode 806 and second drain electrode 818. Enriched band 802 thus moves from a location adjacent first gate channel 822 to a location adjacent second gate channel 824, as indicated in FIGURE 8B. Enriched band 802 moves to this new location via ep transport. This type of manipulation of enriched bands containing highly concentrated chemical reagents may be utilized in other suitable applications. For example, using the illustrated technique to bring two reactive reagents together to initiate a chemical reaction or to bring together assay reagents for binding, interacting, or reacting is contemplated by the present invention.

FIGURE 9 illustrates a system 900 for electrokinetic trapping and concentration enrichment in microfluidic systems according to one embodiment of the invention in which a mixtures of charged molecules may be trapped, enriched, and separated using a series of gate channels. As such, system 900 includes a source reservoir 902 having a source electrode 904, a first source channel 906, a second source channel 908, a third source channel 910, a drain reservoir 912 having a drain electrode 914, a first gate channel 916, a second gate channel 918, and a third gate channel 920. Gate channels 916,918, and 920 have successively smaller cross-sectional areas with respect to their adjacent source channel to allow trapping, enrichment, and separation of a mixture of charged chemicals or objects based on differences in their ep mobilities. For example, based on the principles discussed above in conjunction with FIGURE 2, after separation a first enriched band forms in first source channel 906 adjacent first gate channel 916 and contains chemicals with the lowest average ep mobility, whereas an enriched band 924 forms within third source channel 910 adjacent third gate channel 920 and contains chemicals with the highest average ep mobility. In addition, an enriched band 926 forms in second source channel 908 adjacent second gate channel 918 and contains chemicals with an average ep mobility between that of enriched band 922 and enriched band 924. A combination of the pore size, number of pores, total area of pore openings and pore surface charge of gate channels 916,918, and 920 may be varied to achieve the proper balance of convective flow rate per pore for each gate channel.

Device Fabrication

A microfluidic device generally has a thickness of 0.2 to 10 mm, and a length and width that vary greatly depending on the purpose, but are generally in the range of 2 to 20 cm, and 2 to 10 cm, respectively. Devices for electrophoretic separations may be longer. In some embodiments, devices fabricated wholly using thin films may be used as devices and be as thin as about 0.1 mm

Devices according to the present invention may be fabricated in any suitable manner.

A device body may be formed in plastic off of a micromachined/etched positive rendition of the channels, chambers, reservoirs and any other fluidic features. Suitable plastics include acrylics, polycarbonates, polyolefins, polystyrenes and other polymers suitable for microfluidic or electrokinetic applications. A backing is preferably made of a nonconducting film or body attached to the surface of the device body containing the channels. One suitable material for a backing is a polymethylmethacrylate (PMMA) film. The backing is preferably attached to the body by chemical, thermal, or mechanical bonding. Ultrasonic welding (an example of mechanical welding) may also be employed to fuse various parts together. Of course, the body of microfluidic devices may be produced directly by etching the intended structures in a substrate. In such instances, a cover including wells or reservoir openings is preferably placed over channels or trenches in the substrate to complete the device.

Alternatively, the channel features may be formed in the cover (or film) by, e. g. , embossing, the film thereafter being attached to a substrate that may feature wells. Other materials are also contemplated, such as the exemplified material poly (dimethylsiloxane) (PDMS), silicon, and glass. Further details as to device construction may be appreciated by those skilled in the art.

In some embodiments, channels have a rectangular, trapezoidal, or"D"-shaped cross- section. However, other cross-sectional geometries may be employed. Preferably, the channels have a substantially constant or uniform cross-section. It is also preferred that channels have a surface finish that does not result in irregular flow effects.

Devices may also be constructed in a multilayer fashion, wherein the channels comprising the microfluidic network of the device do not all lie within the same plane. One advantage of a multilayer design is that thin porous polymer films or monoliths may be used as the gate region, which may be incorporated into the device as a layer interposed between substrate lamella in which channels, reservoirs, etc. are included. In other words, the device may be assembled by aligning, and sealing together, a channel substrate layer, a gate region layer, and a second channel substrate layer. U. S. Patent No. 6,623, 860 to Hu describes fabrication methods and designs for multilevel flow structures useful for microfluidic operations.

Material is generally added to or removed from devices at ports or reservoirs. A reservoir is preferably sized to contain sufficient material for performing a desired test or experiment and also may be used for insertion of an electrode so that an external electrode may be submerged into any fluid solution used in the device and used to apply an electric field within portions of the solution, e. g. , for applications requiring electrokinetic motion (i. e. , employing either or both electrophoretic and electroosmotic phenomena). Suitable materials for the electrodes include platinum or other suitable conducting materials, particularly those resistant to corrosion. The electrodes may be connected to a programmable voltage controller for applying desired voltage differentials across the channels.

Alternatively, electrodes may be integrated by directly fabricating electrodes on the surface of the device. See, for example, the disclosure of Zhao et al. in U. S. Patent Application No. 20020079219, for a discussion of forming electrodes on plastic microfluidic devices.

Methods for forming electrodes from metals or conducting inks on glass or polyimide substrates are also well known in the art.

As is known in the art, voltages may be used to drive the device. For example, U. S. Patent No. 6,010, 607 to Ramsey describes information on the manner in which voltages may be applied in order to operate the device. Although the present invention relies upon electrokinetic phenomena, any device using the invention need not rely exclusively on electrokinetic motivation throughout the device. As is apparent to one with skill in the art, features as described herein may be used in connection other components or modules within the microfluidic device that is at least partially pressure driven or otherwise motivated.

Although embodiments of the invention and their advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention.
 




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