Single-Sided Apparatus For Manipulating Droplets By Electrowetting-On-Dielectric Techniques
A single-sided electrowetting-on-dielectric apparatus, which is useful for microfluidic laboratory applications, is disclosed. The apparatus comprises a substrate, an array of control electrode elements disposed on the substrate, a first dielectric film disposed on, and overlaying, the substrate and tie array of control electrode elements, at least one ground electrode element disposed on the first dielectric film, a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element, and an electrowetting-compatible surface film disposed on the second dielectric film. A method of making the apparatus is also disclosed.
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1. Field of the Invention
The present invention relates generally to an apparatus for droplet-based liquid handling in laboratory-on-a-chip applications, and more specifically to a single-sided apparatus for manipulating droplets by electrowetting-on-dielectric techniques.
2. Description of the Related Art
In recent years, the principle of electrowetting-on-dielectric has attracted considerable interest for droplet-based liquid handling in laboratory-on-a-chip applications. Electrowetting-on-dielectric involving aqueous samples requires that a droplet rest on a surface or in a channel coated with a hydrophobic material. The surface is modified from hydrophobic to hydrophilic by applying a voltage between the liquid droplet and an electrode residing under a dielectric film surface layer. Charge accumulates at the liquid-solid interface, leading to an increase in surface wettability and a concomitant decrease in the liquid-solid contact angle. By changing the wettability of each of the electrodes patterned on a substrate, liquid drops can be shaped and driven along a series of adjacent electrodes, making microscale liquid handling extremely simple both with respect to device fabrication and operation (see, e.g., Washizu, M., IEEE Trans. on Industry Apps. (1998) 34(4), 732-737; Pollack, M. G., Fair, R. B. and Shenderov, A. D., Applied Physics Letters (2000) 77(11), 1725-1726; and Lee, J., Moon, H., Fowler, J., Schoellhammer, T. and Kim, C.-J., Sensors and Actuators A (2002) 95, 259-268).
As compared to conventional microfluidic approaches, electrowetting-on-dielectric offers the following advantages: (1) electrowetting-on-dielectric does not require that soluble or particulate analytes be charged or have large polarizabilities; (2) the power required to transport liquid droplets is much lower than in micropumping or electrophoresis-based devices; (3) electrowetting-on-dielectric devices can be reconfigured simply by reprogramming the sequence of applied potentials; and (4) electrowetting-on-dielectric devices require no moving parts. Furthermore, because the liquid is not usually in direct contact with the electrodes, electrolysis and analyte oxidation-reduction reactions are avoided.
Initially, development of electrowetting-on-dielectric devices focused primarily on configurations wherein liquid droplets are confined in a gap of uniform spacing between upper and lower substrates (see, e.g., Pollack, M. G., Fair, R. B. and Shenderov, A. D. Applied Physics Letters (2000) 77(11), 1725-26; Moon, H., Cho, S. K., Garrell, R. L. and Kim, C.-J. J. Applied Physics (2002) 92(7), 4080-87; Pollack, M. G., Shenderov, A. D. and Fair, R. B. Lab Chip (2002) 2, 96-101; Lee, J., Moon, H., Fowler, J., Schoellhammer, T. and Kim, C.-J. Sensors and Actuators A (2002) 95, 259-268; Cho, S. K., Moon, H. and Kim, C.-J., J. MEM Systems (2003) 12(1), 70-80). In most instances, the electrode elements that control electrowetting are located in the lower substrate and the upper substrate is comprised of a single ground electrode covered by a hydrophobic thin surface film. However, in alternate configurations, the control electrode elements may be located in both the lower and upper substrates.
Exemplary electrowetting-on-dielectric devices for liquid droplet manipulation are disclosed in U.S. Pat. No. 6,565,727, issued May 20, 2003; U.S. patent application Ser. No. 09/943,675, published Apr. 18, 2002 as U.S. Patent Application Publication No. 2002/0043463; U.S. patent application Ser. No. 10/343,261, published Nov. 6, 2003 as U.S. Patent Application Publication No. 2003/0205632; U.S. patent application Ser. No. 10/430,816, published Feb. 19, 2004 as U.S. Patent Application Publication No. 2004/0031688; U.S. patent application Ser. No. 10/253,342, published Mar. 25, 2004 as U.S. Patent Application Publication No. 2004/0058450; and U.S. patent application Ser. No. 10/253,372, published Mar. 25, 2004 as U.S. Patent Application Publication No. 2004/0055536; each of which is incorporated herein by reference in its entirety.
In the last few years, the development of “open” or “single-sided” electrowetting-on-dielectric devices has attracted considerable interest owing to the conceptual simplicity of such devices. For example, as such devices eliminate the need for an upper substrate, the need to maintain uniform gap spacing between upper and lower substrates is also eliminated. Such simplifications result in reductions in manufacturing complexity and cost. In addition, sample introduction into single-sided devices is greatly simplified as compared to closed devices and interfacing with existing laboratory liquid-handling robotics is facilitated. Furthermore, viscous drag (which is proportional to the total droplet surface contact area) is reduced significantly, resulting in increased droplet speed and/or reduced voltage requirements.
Conceptually, single-sided electrowetting-on dielectric devices can be enabled by one of three general approaches: (1) selectively biasing pairs of adjacent control electrodes elements to function as either drive or reference electrodes while allowing the potential of all immediately surrounding electrodes to float; (2) incorporating one or more conducting ground electrode lines into the substrate which carries the control electrode elements; or (3) allowing the potential of the liquid droplets to float on the surface above the control electrode elements. The first approach is disclosed in U.S. patent application Ser. Nos. 10/115,336 and 10/253,368, which published on Oct. 2, 2003 and Mar. 25, 2004 as U.S. Patent Application Publication Nos. 2003/0183525 and 2004/0055891, respectively; the second approach is also disclosed in U.S. patent application Ser. No. 10/253,368 and in U.S. patent application Ser. No. 10/688,835, filed Oct. 16, 2003; and the third approach is disclosed in U.S. patent application Ser. No. 10/305,429, published Sep. 4, 2003 as U.S. Patent Application Publication No. 2003/0164295; each of which is incorporated herein by reference in its entirety.
In practice, the fabrication of reliable single-sided electrowetting-on-dielectric devices having the configurations disclosed in the foregoing references has proven far more difficult than anticipated. For example, each of the disclosed single-sided configurations comprising a ground electrode line either requires that the ground electrode lines reside above the electrowetting-compatible hydrophobic surface film or that the electrowetting-compatible hydrophobic surface film covering the ground electrode lines be so thin and/or porous so as to enable electrical contact between the droplet and the ground electrode line. This requirement has proven problematic in reduction to practice, in that maintaining adhesion between a fluoropolymer hydrophobic coating and both the surface of a dielectric film and the surface of metallic ground electrode lines (which are not covered by the dielectric film) is particularly difficult. More specifically, this problem results from the properties of fluoropolymers which limit their adhesion with respect to most metals. Moreover, the presence of ground electrode lines, either on the surface of, or immediately adjacent to, the dielectric film covering a control electrode element, significantly alters the morphology of the external hydrophobic surface, which is preferably smooth. This issue is further exacerbated when conformal hydrophobic coatings are employed, in that microscopic ridges on the surface of such a device negatively impact smooth droplet transport and often result in sites of dielectric breakdown and electrolysis.
Accordingly, although there have been advances in the field, there remains a need for improved single-sided electrowetting-on-dielectric devices which overcome the limitations enumerated above. The present invention addresses these needs and provides further related advantages.
BRIEF SUMMARY OF THE INVENTIONIn brief, the present invention relates to a single-sided electrowetting-on-dielectric apparatus useful for microfluidic laboratory applications. In particular, the present invention relates to a single-sided electrowetting-on-dielectric apparatus that utilizes a second dielectric film to overcome certain limitations associated with prior art devices.
In a first embodiment, the present invention provides a single-sided electrowetting-on-dielectric apparatus comprising: (1) a substrate; (2) an array of control electrode elements disposed on the substrate; (3) a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; (4) at least one ground electrode element disposed on the first dielectric film; (5) a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and (6) an electrowetting-compatible surface film disposed on the second dielectric film.
In certain further embodiments, the at least one ground electrode element overlays the array of control electrode elements. In other further embodiments, the at least one ground electrode element runs between adjacent control electrode elements.
In yet other further embodiments, the array of control electrode elements is a planar two-dimensional matrix. In such further embodiments, the apparatus may comprise (1) at least two parallel ground electrode elements, or (2) at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements.
In a second embodiment, the present invention provides a method of making an single-sided electrowetting-on-dielectric apparatus comprising: (1) providing a substrate; (2) forming an array of control electrode elements disposed on the substrate; (3) forming a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements; (4) forming at least one ground electrode element disposed on the first dielectric film; (5) forming a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and (6) forming an electrowetting-compatible surface film disposed on the second dielectric film.
These and other aspects of the invention will be apparent upon reference to the attached figures and following detailed description.
In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.
As used herein, the following terms have the meanings set forth below:
“Substrate” refers to a solid material having a flat or substantially flat surface. A substrate may be comprised of any of a wide variety of materials, such as, for example, aluminum, ceramic, inorganic glasses, plastics, silica or silica-based materials, stainless steel, and the like.
“Film” refers to a structure that is typically, but not necessarily, planar or substantially planar, and is typically deposited on, formed on, coats, treats, or is otherwise disposed on another structure.
“Array” refers to an arrangement of a plurality of elements such as a plurality of control electrode elements.
“Planar array” refers to an array that is arranged in a plane such as on the surface of a planar substrate.
“Dielectric” refers to a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields, such as, for example, silicon nitride, silicon dioxide, silicon oxynitride, Parylene C, Teflon AF (DuPont), barium strontium titanate (BST) and the like. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy, as does a capacitor.
“Planar surface” refers to a generally two-dimensional structure on a solid substrate, which is usually, but not necessarily, rigid and not necessarily flat. The surface may be comprised of any of a wide variety of materials, for example, polymers, plastics, resins, silica or silica-based materials, carbon, metals, inorganic glasses, and the like.
“Electrowetting-compatible surface film” refers to a material having both hydrophobic and/or oleophobic properties and either dielectric or insulative properties that inhibit dielectric breakdown and electrolysis, such as, for example, Teflon AF (DuPont), CYTOP (Asahi Glass), and other plasma deposited fluropolymers and polysiloxanes.
Before addressing the present invention, an overview of the approaches taken in the prior art regarding the fabrication of single-sided electrowetting-on-dielectric devices is helpful. In this regard,
With reference to
With reference to
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As noted above, the present invention relates to a single-sided electrowetting-on-dielectric apparatus that utilizes a second dielectric film to overcome the foregoing limitations of the prior art devices. In particular, the use of such a second dielectric film eliminates the surface morphology and adhesion issues associated with the prior art devices. More specifically, the present invention provides a single-sided electrowetting-on-dielectric apparatus comprising (1) a substrate, (2) an array of control electrode elements disposed on the substrate, (3) a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements, (4) at least one ground electrode element disposed on the first dielectric film, (5) a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element, and (6) an electrowetting-compatible surface film disposed on the second dielectric film.
With reference to
In order to transfer a droplet 8 from a particular control electrode element 3 to an adjacent control electrode element 3 of apparatus 20, droplet 8 must be of a diameter such that the edges of the droplet overlap the edges of the adjacent control electrode element 3 as well as a ground electrode element 5, as shown in
In the embodiment shown in
Ground electrode elements 5 may overlay each column of control electrode elements 3 at any relative position (e.g., along the center-line or off-set to the right or left of center), provided that the droplet to be transferred overlays one of the ground electrode elements 5 and one or more of the adjacent control electrode elements 3. In the embodiment illustrated, ground electrode elements 5 are located along the center-line of the columns of control electrode elements 3. In certain embodiments, the width of each ground electrode element 5 is approximately 40% of the width of the associated control electrode elements 3. In other embodiments, the width of the ground electrode elements 5 may be less than or equal to 20% of the width of the associated control electrode elements 3.
With reference to
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In the embodiment shown in
Each of the ground electrode elements 5 may be utilized in conjunction with two columns of control electrode elements 3, provided that the droplet to be transferred must overlay at least one of the ground electrode elements 5 and one of the adjacent control electrode elements 3. As noted above, in certain embodiments, the width of each ground electrode element 5 is approximately 40% of the width of the associated control electrode elements 3. In other embodiments, the width of the ground electrode elements 5 may be less than or equal to 20% of the width of the associated control electrode elements 3.
With reference to both
With reference to
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A representative single-sided electrowetting-on-dielectric apparatus was fabricated according to the following method.
The surface of a 4″ silicon wafer was exposed to wet O2/N2 at 1045° C. for 45 min to prepare a thermal oxide (2500 Å) insulator film thereon. A first metal conductive layer (i.e., control electrode elements and interconnects), comprised of 60 Å of Ti/W, 300 Å of Au and 60 Å of Ti/W was then sputtered onto the thermal oxide insulator film surface. A first photoresist was then spin-coated and patterned by contact printing to define the electrode pattern. The first metal conductive layer was then wet etched at room temperature employing the following sequence: (1) 30% H2O2 in TFA for 90 sec; (2) 30% H2O2 for 30 sec; and (3) 30% H2O2 in TFA for 90 sec. The first photoresist layer was then stripped using reagent EKC830 for 10 min followed by reagent AZ300 for 5 min. The resulting wafer was rinsed in deionized water and dried in a vacuum spinner. Unstressed silicon nitride dielectric (1000 Å) was then deposited by PECVD (plasma enhanced chemical vapor deposition) at 350° C. on the surface of the wafer and a second photoresist layer was spin-coated and patterned by contact printing to expose contacts (connectors) and vias. The silicon nitride dielectric layer was dry etched through the second photoresist mask by reactive ion etching (RIE) with sulfur hexafluoride gas. A second metal conductive layer (i.e., ground electrode lines), comprised of 300 Å of Au and 60 Å of Ti/W, was then sputtered onto the silicon nitride surface. To provide adequate gold depth at the contacts, an additional 1000 Å of Au was deposited on the contacts by shadow masking. A third photoresist layer was then spin-coated and patterned by contact printing to define the upper ground electrode, affinity capture site and contact pattern. The metal conductive film was wet etched at room temperature with 30% H2O2 in TFA for 90 sec and 30% H2O2 for 30 sec. Silicon dioxide dielectric (250 Å) was then deposited by PECVD at 350° C. on the surface of the wafer, and then the resulting wafer was protected with a fourth photoresist layer and diced into chips. The photoresist was then stripped using reagent EKC830 for 10 min followed by reagent AZ300 for 5 min and the wafers were rinsed in deionized water and dried in a vacuum spinner. Finally, a solution of CYTOP Amorphous Fluorocarbon Polymer (1.1% in CYTOP proprietary solvent) was spin-coated at 2500 rpm and dried at 120° C. for 10 min; 150° C. for 10 min; and 180° C. for 10 min to yield the desired apparatus.
Example 2 Use of a Representative Single-Sided Electrowetting-on-Dielectric ApparatusAll of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification are incorporated herein by reference, in their entirety.
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
Claims
1. A single-sided electrowetting-on-dielectric apparatus comprising:
- a substrate;
- an array of control electrode elements disposed on the substrate;
- a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements;
- at least one ground electrode element disposed on the first dielectric film;
- a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and
- an electrowetting-compatible surface film disposed on the second dielectric film.
2. The apparatus of claim 1, farther comprising a thin insulator film disposed between the substrate and the array of control electrode elements and first dielectric film.
3. The apparatus of claim 1 wherein the at least one ground electrode element overlays the array of control electrode elements.
4. The apparatus of claim 1 wherein the at least one ground electrode element runs between adjacent control electrode elements.
5. The apparatus of claim 1 wherein the array of control electrode elements is a planar two-dimensional matrix.
6. The apparatus of claim 5 wherein the apparatus comprises at least two parallel ground electrode elements.
7. The apparatus of claim 5 wherein the apparatus comprises at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements.
8. A method of making an single-sided electrowetting-on-dielectric apparatus comprising the steps of:
- providing a substrate;
- forming an array of control electrode elements disposed on the substrate;
- forming a first dielectric film disposed on, and overlaying, the substrate and the array of control electrode elements;
- forming at least one ground electrode element disposed on the first dielectric film;
- forming a second dielectric film disposed on, and overlaying, the first dielectric film and the at least one ground electrode element; and
- forming an electrowetting-compatible surface film disposed on the second dielectric film.
9. The method of claim 8, further comprising the step of forming a thin insulator film disposed between the substrate and the array of control electrode elements and first dielectric film.
10. The method of claim 8 wherein the at least one ground electrode element overlays the array of control electrode elements.
11. The method of claim 8 wherein the at least one ground electrode element runs between adjacent control electrode elements.
12. The method of claim 8 wherein the array of control electrode elements is a planar two-dimensional matrix.
13. The method of claim 12 wherein the apparatus comprises at least two parallel ground electrode elements.
14. The method of claim 12 wherein the apparatus comprises at least two parallel ground electrode elements and at least one additional ground electrode element arranged perpendicular to the at least two parallel ground electrode elements.
Type: Application
Filed: Oct 18, 2005
Publication Date: Jul 17, 2008
Applicant: STRATOS BIOSYSTEMS, LLC (Seattle, WA)
Inventors: Robert N. McRuer (Mercer Island, WA), Mark L. Stolowitz (Pleasanton, CA)
Application Number: 11/577,386
International Classification: C25B 11/00 (20060101);