Abstract:
An apparatus and method for micropumping is disclosed. The apparatus ( 100 ) comprises a virtual or physical microchannel ( 106 ) and at least two hydrophobic electrode patches ( 104 A- 104 G) on a surface ( 102 ) or two electrode patches disposed underneath a dielectric film proximate to the channel ( 106 ). Each of the electrode patches modifies a surface property between hydrophobic and hydrophilic states in response to an electrical potential applied between the liquid and the electrode, and the electrical potential is provided to digitize the liquid, drive the liquid segments, and mix different liquids.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of co-pending and commonly assigned U.S. Provisional Patent Application Serial No. 60/220,803, filed on Jul. 25, 2000, by Chang-Jin Kim and Junghoon Lee, entitled “ELECTROWETTING-DRIVEN MICROPUMPING,” and attorney&#39;s docket number 30435.95USP1, which application is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] This invention was made with Government support under Grant No. 4-442560-21552, awarded by the National Science Foundation. The Government has certain rights in this invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to methods and systems for pumping and manipulating liquids in microscale, and particularly to methods and systems based on electrowetting principles.  
           [0005]    2. Description of the Related Art  
           [0006]    Mechanical micropumping systems may use various actuation mechanisms including piezoelectric, thermal, shape memory alloy, and electrostatic actuation systems. Such methods and systems employ moving parts which result in a system of diminished reliability, increased power consumption, or high operating voltage.  
           [0007]    Direct micropumping methods and systems may employ actuation based upon principles such as electrophoresis, electroosmosis, magnetohydrodynamic, and thermally-induced surface tension. An electrophoresis based system is limited because it requires that the operating liquid contains ionic particulates. In addition, electrophoresis and electroosmosis approaches require high voltage an a high energy dissipation and operate at low speed. Magnetohydrodynamic and thermally-induced surface tension mechanisms require relatively high power to operate as well.  
           [0008]    The area of micropumping is directed at moving very small volumes of liquid, typically within a single conduit. Ideally, a more advanced micropumping method and system would allow more complex manipulation of the liquid, such as “digitizing” a liquid (separating and manipulating increments of liquid) in very small volumes (as small as sub-picoliter). Complex manipulation would allow for mixing and moving liquid in a microfluidic matrix or circuit. However, none of the developed techniques for micropumping allow for such complex picoliter manipulation of the operating liquid.  
           [0009]    Electrowetting is a principle whereby the surface wetting property can be modified between the hydrophobic and hydrophilic states using an electric field. The surface can be hydrophobic conductive layer or conductive layer covered with hydrophobic dielectric film. A droplet of liquid can bead or spread on the surface depending on the surface state. A liquid lens with an adjustable focus has been developed using this principle, however, there has been no application to micropumping.  
           [0010]    There is a need for micropumping methods and systems which operate without moving mechanical parts. There is further a need for such systems and methods which operate at relatively low power and voltage and achieve a relatively high speed. There is still further a need for such methods and systems which function without requiring ionic particulates in the operating liquid. Finally, there is a need for methods and systems which will enable complex manipulation of sub-microliter liquid volumes. The present invention meets all of these needs.  
         SUMMARY OF THE INVENTION  
         [0011]    An apparatus and method for micropumping is disclosed. The apparatus comprises a a virtual or physical microchannel and at least two hydrophobic electrode patches disposed proximate to the channel. Each of the electrode patches modifies a surface property of itself or of the dielectric film covering it between hydrophobic and hydrophilic states in response to an electrical potential applied between the electrode and the liquid. The electrical potential is provided to selected electrode patches to drive the liquid along the channel, digitize the liquid, drive the liquid segments, and mix different liquids.  
           [0012]    One embodiment of the invention comprises a structure including one or more channels for a liquid and at least two hydrophobic electrode patches proximate to the channels. Each of the electrode patches modifies a surface property of the structure between hydrophobic and hydrophilic states in response to an electrical potential applied between the liquid and the electrode patches. A further embodiment comprises a means for grounding a first one of the electrode patches to produce a hydrophobic state on a first region of the structure proximate the first one of the electrode patches and means for applying the electric potential to a second one of the electrode patches to produce a hydrophilic state on a second region of the structure proximate the second one of the electrode patches.  
           [0013]    In another embodiment, a column of the liquid spans the first and second one of the electrode patches and a third one of the electrode patches proximate a middle section of the column of liquid is dewetted such that a liquid segment separates from the column of the liquid. One or more channels can include a breathing path to admit air into the channels to allow the liquid to flow freely in the channels an aid such segmenting. A segment of the liquid may also be driven by applying the electrical potential and wetting a first one of the electrodes at a first end of the segment while grounding and dewetting a second one of the electrodes at a second end of the segment. Conveniently, the electrode patches can be arranged in an array and the electrode patches can be arranged along the one or more channels. Furthermore, the operating liquid may be an electrolyte.  
           [0014]    In yet another embodiment, the apparatus further comprises a reservoir connected to one or more of the channels, wherein the electrode patches are positioned in the channels and draw liquid from the reservoir into the channels.  
           [0015]    In other embodiments, the apparatus may comprise one or more junctions, each of the junctions connecting more than two of the channels. Electrode patches can be positioned at each of the junctions and the liquid can be mixed with a second liquid at the junctions. The channels and one or more junctions can form a fluidic circuit which can be an m×n grid of the channels and the junctions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0017]    [0017]FIG. 1 illustrates the principle of drawing a liquid into channel by electrowetting in the invention;  
         [0018]    [0018]FIG. 2 illustrates digitization of a liquid element and driving it by electrowetting in the invention;  
         [0019]    [0019]FIG. 3 illustrates a fluidic circuit grid embodiment of the invention; and  
         [0020]    FIGS.  4 A- 4 C illustrates liquid mixing at a junction of a fluidic circuit in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0022]    Overview  
         [0023]    The invention uses the principle of electrowetting, which includes the principle of electrowetting on dielectric film (EWOD), in driving or pumping liquid segments on a surface or in a channel. In a typical embodiment, an array of electrodes is patterned and coated with a thin hydrophobic dielectric layer. An aqueous liquid fills up to the edge of the hydrophobic region. Upon applying electric potential between the liquid and the buried electrode underneath the hydrophobic coating, i.e. the dielectric film, the hydrophobic region becomes hydrophilic, making the liquid advance and wet the region of dielectric film above the activated electrodes.  
         [0024]    The present invention can be realized through microfibrication technologies. Low voltage and low power on-chip liquid-handling microsystems of high complexity become possible with the present invention. The micro liquid handling system of the present invention finds its utility in many microdevices such as miniature chemical analysis systems, biomedical devices and micro-optical devices.  
         [0025]    Driving Liquid by Electrowetting  
         [0026]    [0026]FIG. 1 illustrates the principle of drawing a liquid into the channel by electrowetting in the invention. In the example EWOD device  100 , a wetted region of a generally non-wetting surface  102  has more than two electrode patches  104 A- 104 G. Channels  106  may be used to direct and contain an operating liquid  108 . An input operating liquid  108  (such as an electrolyte) may be initially contained in a channel  106  forming a reservoir  110  which surrounds a first patch  104 A. An electric potential is formed between the operating liquid  108  and driving electrodes by grounding (shown as positive charge) a first electrode patch  104 A at one end of the operating liquid  108  and activating (shown as negative charge) the three electrode patches  104 B- 104 D. The hydrophobic surface  102  obtains a hydrophilic state where any of the patches  104 A- 104 G is turned on. If any one of the patches  104 A- 104 G is turned off or grounded, the area of the hydrophobic film  102  proximate the patch  104 A- 104 G becomes hydrophobic.  
         [0027]    It should be noted that the hydrophobic electrode patches themselves may be hydrophobic or they may be made hydrophobic by being covered with hydrophobic dielectric film.  
         [0028]    In some areas, such as reservoirs  110 , direct contact between the operating liquid  108  and the electrode patch  104 A may be desirable to avoid driving the liquid  108  completely away. By positioning the electrode patches  104 A- 104 G along the channel  106  (typically in an array) and sequentially charging the patches  104 B- 104 G, a liquid  108  may be drawn into the channel.  
         [0029]    The channel  106  may also include a plurality of paths  112  from a junction  114 . By selectively charging the patches  104 D- 104 F as the liquid  108  passes the junction  114 , the liquid  108  may be directed down the different paths  112 . In addition, using the invention, a liquid may also be cut or digitized into separate liquid segments.  
         [0030]    Liquid Digitization and Driving  
         [0031]    [0031]FIG. 2 illustrates digitization of a liquid element and driving it. In digitization, a long liquid element drawn in by activating the electrode patches  104 B- 104 G is cut into a separate liquid segment  116  by turning off the electrode patch  104 C and returning it to hydrophobic state. An important aspect of the invention to create separated segments  116  and allow it to move along the channel is the use of breathing paths  118  which allow the liquid  108  to flow freely in the channel  106 . A breathing path  118  is especially important when the liquid  108  is digitized as air must flow in order for the gap to be formed. Breathing paths  118  may be part of the channel  106  for manipulating the liquid  108  or specially formed. Or, the channel  106  can be even virtual, in which case there is no physical barrier that defines the channel. When the channel is open and liquid path is prescribed by activating a series of electrodes, there is no need for the breathing path  108 . Once a liquid segment  116  is created, it can be driven by activating and wetting the electrode  104 G at one end of the segment while grounding and dewetting the electrode  104 C at the other end.  
         [0032]    The smallest amount of the digitized liquid is limited by lithography technology; sub-picoliter digitization is possible with 3 micron lithography. The digitized liquid segment can be driven along a channel  108  by a sequential potential application applied to the electrode patches  104 A- 104 G. The channel  106  may be any configuration of paths  112  on the hydrophobic  102  surface, defined during the mask design process.  
         [0033]    Integrated Microfluidic Circuit  
         [0034]    [0034]FIG. 3 illustrates a fluidic circuit  300  of the invention. Employing the features and elements described, a complex fluidic circuit may be designed including a plurality of electrode patches  104 , paths  112 , junctions  114  and reservoirs  110 . Any circuit architecture is possible and more than one liquid may be used in the circuit. In the example of FIG. 3, the circuit  300  is laid out in an m×n grid of m vertical paths  302  and n horizontal paths  304  with junctions  306  at each vertical and horizontal path  302 ,  304  intersection. Liquid segments  308 ,  310  are introduced to the circuit  300  at the reservoirs  312  so they may be manipulated within the circuit  300 . In a circuit  300  manipulating more than one liquid type, two liquid segments  308 ,  310  may be mixed at a junction  306 A to produce a mixed liquid segment  314 .  
         [0035]    Based on the two functions detailed above, separation of a liquid into droplets, i.e., digitization, and driving of individual droplets, mixing of droplets is made possible on a chip-scale microdevice. This digitized fluidic circuit plays the key role for several applications.  
         [0036]    FIGS.  4 A- 4 C illustrates liquid mixing at a junction of a fluidic circuit in the present invention. In FIG. 4A first liquid  402  and a second liquid  404  are both driven to a junction  406  by appropriate charges on the electrode patches  408 A,  408 B,  408 D,  408 F,  408 G. In FIG. 4B, the first and second liquids  402 ,  404  are mixed at the junction  406  to form a mixed liquid  410 . Appropriate charges on the electrode patches  408 B,  408 C,  408 D,  408 E,  408 G begin to drive the liquid  410  from the junction. Finally, in FIG. 4C, the mixed liquid  410  is driven along the path by charges applied to patches  408 D,  408 E,  408 F.  
         [0037]    The invention can improve many microdevices which handle liquids in microscale, and is not bound by specific applications. Micro fluid pumps, micro total analysis system (μ-TAS), micro optical (especially integrated waveguide) switching networks, and micro RF switches are some of the possible applications where the present invention can be used for more effective fluid handling in microscale. Due to the self-contained nature of pumping and low power consumption, the invention is especially attractive for hand-held or implanted on-chip micro fluidic systems.  
         [0038]    Surface tension, a dominant force in the microscale, is used as a driving force to move liquids in the present invention. As a result, liquids can be pumped or transferred very effectively, requiring extremely low power consumption compared to thermal or magnetic micropumping mechanisms. Low voltage, low energy dissipation, and high speed are achievable with the present invention when compared with electrophoresis and electroosmosis driven systems. Also, the pumping media of the present invention does not need to have ionic particulates winch are necessary for electrophoresis type systems. The present invention also has no moving mechanical parts, because the liquid segment is driven by its own surface tension. The elimination of moving mechanical parts gives the present invention longer life and better reliability compared to mechanical micropumping systems.  
         [0039]    The present invention also allows liquids to be digitized even in sub-pico-liter volumes, which is impossible with any other existing techniques. Further, there is no cross talk between individual movement of liquid segments. Many (n×m) liquid segments can be moved in a parallel fashion with the present invention. This function is unattainable with any other existing micropumping mechanism. Lithography-based microfabrication makes the microactuation in a complex matrix of channels possible and easily achieved.  
         [0040]    The mechanism of the present invention has a large impact on many areas, including biomedical, chemical, and telecommunication applications. This technique enables very large scale (e.g., 1000×1000) networking of liquid with minimal energy consumption, which are not feasible with existing techniques. Extreme miniaturization can be achieved for the suggested devices and liquid handling system. The fabrication process is relatively simple, requiring no mechanically moving parts or complex packaging procedures, resulting in a dramatic reduction of product cost. The invention is expected to make disposable bio- and/or chemical lab-on-chips commercially attractive.  
         [0041]    References  
         [0042]    The following references are incorporated by reference herein:  
         [0043]    Wettability, edited by John C. Berg, Marcel Dekker, Inc., New York, 1993, pp. 162-164.  
         [0044]    U.S. Pat. No. 4,583,824, Electrocapillary Devices, Apt 22, 1986.  
         [0045]    European Patent No. 9519461.9, Apr., 1995.  
       CONCLUSION  
       [0046]    This concludes the description including the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching.  
         [0047]    It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the apparatus and method of the invention. Since many embodiments of the invention can be made without departing from the scope of the invention, the invention resides in the claims hereinafter appended.