MARANGONI STRESS-DRIVEN DROPLET MANIPULATION ON SMART POLYMERS FOR ULTRA-LOW VOLTAGE DIGITAL MICROFLUIDICS

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

The present invention relates to a microfluidic system for the manipulation of liquid droplets, and, more particularly, to a digital microfluidic system for the ultra-low voltage manipulation of liquid droplets for microfluidic applications.

BACKGROUND OF THE INVENTION

Digital microfluidic systems have been developed in the past decade to generate and manipulate discrete droplets of liquids for biomedical applications. By manipulating liquids at a droplet scale, these systems can handle samples and reagents with lower cost and shorter time for analysis by using smaller devices. At the microscale, droplet behavior is dominated by surface forces (e.g., surface tension or Laplace pressure) rather than body forces (e.g., gravity) due to high surface area-to-volume ratios. For example, droplet manipulation has been demonstrated on individual addressable control electrodes using the electrowetting-on-dielectric effect (“EWOD”) to generate net electromechanical force. An example of such droplet manipulation is described in the journal article by Sung Kwon Cho, Hyejin Moon, and Chang-Jim Kim, titled Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits, published in the Journal of Microelectromechanical Systems, Vol. 12, No. 1, February 2003 (hereinafter, the “Cho et al. Article”), which is incorporated herein by reference in its entirety.

Electrostatic actuation schemes such as those described in the Cho et al. Article typically require relatively high driving voltages (e.g., 15-80 V) to manipulate liquid droplets. Such high voltage requirements have been major obstacles for clinical applications that demand portability and rapid diagnosis, and where lower voltages are desirable to promote efficient EWOD applications. In addition, the high electric fields used to achieve the electrowetting effect can cause electrolysis of the working fluid in lab-on-a-chip applications. Even using high-κ dielectric materials and conductors such as indium tin oxide (“ITO”), driving voltages in the range of tens of volts are still required to effect the manipulation of fluid droplets. Therefore, it would be desirable to have lab-on-a-chip devices that are operable at, for example, the voltages which can be obtained from commercial-standard 1.5 V AA batteries.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides ultra-low voltage microfluidic devices for manipulating droplets of liquid using voltages having a magnitude of about 1 V or less. The microfluidic device includes a patterned substrate having a plurality of smart-polymer electrodes, each of which has a film of smart polymer exposed at its surface. In some embodiments, the smart-polymer electrodes are proximate one another and separated by electrical insulators. In some embodiments, the smart polymer is made so that its surface will become hydrophobic or hydrophilic in response to different electromagnetic potentials applied to the smart-polymer film. In some such embodiments, the smart polymer is reversibly oxidized by applying an electrical potential such that the smart polymer acquires a positive electrical charge. In some such embodiments, the oxidized smart polymer is reduced by applying a different electrical potential such that it loses its positive electrical charge. In some such embodiments, the smart polymer is doped with an amphiphilic chemical compound having a negatively-charged end and a long-chain hydrophobic tail. In some such embodiments, the smart polymer is a polypyrrole and the dopant is a dodecylbenzene sulfonate.

In some embodiments, the microfluidic device includes a plurality of individually-addressable electrically-conductive control electrodes, each of which is in electrical communication with at least one of the smart-polymer electrodes. In such embodiments, the control electrodes are arranged such that applying an electrical potential to a control electrode causes the electrical potential to be applied to the smart-polymer film. In some such embodiments, the control electrode is substantially coextensive with the smart-film, and separated from other control electrodes by an insulator.

In some embodiments, the microfluidic device includes means for selectively and individually applying electrical potentials to the control electrodes. In some such embodiments, the means includes a voltage source, electrical connectors to the control electrodes, and switching means for selectively connecting the voltage source to the electrical connectors, and thus to the control electrodes.

In another aspect, the present invention provides methods for manipulating droplets by inducing Marangoni stress in the individual droplets using microfluidic devices of the same general type described above. In some embodiments, the method includes the steps of placing a droplet on a surface of a smart-polymer film, then applying an electrical potential to the smart-polymer film to create a surface tension gradient across the contact line between the droplet and the smart polymer film, thus inducing Marangoni stress in the droplet. By manipulating the electrical potential, and thus the Marangoni stress, the droplet can be transported along adjacent smart-polymer films. By selectively and sequentially applying electrical potentials to the smart-polymer electrodes, droplets may be transported, cut into smaller droplets, or mixed with each other. In some such embodiments of the method, applying the electrical potential oxidizes the smart polymer, such that it acquires a positive charge. In some such embodiments, a dopant in the polymer, having a negatively-charged end and a long-chain hydrocarbon tail, orients such that the hydrocarbon tail is directed to the surface of the smart-polymer film, causing the surface to become hydrophobic. In some such embodiments, a second electrical potential reduces the oxidized smart-polymer, such that it loses its positive charge. Such a change in the state of the smart polymer causes the dopant to orient itself with the negatively-charged end near the surface of the smart-polymer film, causing the surface to become hydrophilic.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is made to the following detailed description of an exemplary embodiment considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an actuation device for droplet manipulation via Marangoni stress according to an embodiment of the present invention, in which a polymer surface is initially set in an oxidized state;

FIG. 2 is a schematic diagram of the actuation device of FIG. 1, wherein a reductive potential is applied to an electrode, thereby inducing Marangoni stress on a droplet;

FIG. 3 is a schematic diagram illustrating charge neutralization of an electrically-oxidized dodecylbenzene sulfonate-doped polypyrrole film (PPy(DBS));

FIG. 4 is a schematic diagram of the effect of electrical reduction of a PPy(DBS) film on a water droplet on a surface of the film;

FIG. 5 is a schematic diagram of the effect of electrical oxidation of the PPy(DBS) film of FIG. 4 on the water droplet of FIG. 4;

FIG. 6 is a schematic diagram of the effect of electrical reduction of a PPy(DBS) film on a dichloromethane droplet on a surface of the film;

FIG. 7 is a schematic diagram of the effect of electrical oxidation of the PPy(DBS) film of FIG. 6 on the dichloromethane droplet of FIG. 6;

FIG. 8 is a schematic diagram illustrating the effect of electrical reduction of a PPy(DBS) film on surface tension gradient and Marangoni stress relative to a dichloromethane droplet;

FIG. 9 is a schematic diagram illustrating the effect of electrical oxidation of a PPy(DBS) film on Marangoni stress relative to a dichloromethane droplet;

FIG. 10 is a schematic drawing of an ultra-low-voltage digital microfluidic system based on a tunable electrochemical reaction of PPy(DBS);

FIG. 11 is a schematic drawing of a liquid lens having an electrical potential applied thereto, such that the liquid lens is in a first state; and

FIG. 12 is a schematic drawing of a liquid lens having no electrical potential applied thereto, such that the liquid lens is in a second state.

DETAILED DESCRIPTION OF THE INVENTION

The Marangoni effect is the mass transfer along an interface between two fluids (e.g., an electrolytic bath and a droplet of immiscible liquid within the bath) due to a surface tension gradient. Since a liquid with high surface tension pulls more strongly on the surrounding liquid than one with a low surface tension, the presence of a gradient in surface tension causes the liquid to move away from a region of low surface tension to a region of high surface tension. The induced force at the liquid-liquid interface is the so-called Marangoni stress.

The present invention provides a microfluidic system that enables the operation of microfluidic devices at low voltages, such as those which can be provided by commercial-standard 1.5 V batteries, using the Marangoni effect to induce Marangoni stress between adjacent electrodes comprising a smart polymer. Certain embodiments of the present invention can be substituted for those employing the existing electrowetting-on-dielectric (EDOW) technique, as well as for other conventional microfluidic systems. Such embodiments of the present invention provide controlled manipulation of liquid droplets by inducing Marangoni stress through local reduction of the smart polymer. In contrast, prior art devices, such those described in the Cho et al. Article (see Background of the Invention, above) use polymers, such as Teflon®, that are difficult to reduce locally, or may not be reduced at all at the low voltages employed in embodiments of the present invention.

For the purpose of the present disclosure, a “smart polymer” is a high-performance polymer that reversibly changes its properties according to its environment. For example, polypyrrole (PPy), which is the exemplary smart polymer discussed herein, is sensitive to an electrical field and can respond in various ways, such as by reversibly oxidizing or by altering its color, volume and/or surface wettability.

The exemplary PPy electrodes discussed herein are doped with dodecylbenzenesulfonate (DBS) to form a PPy(DBS) complex, which may be locally-reduced at low voltages to change the surface energy of the electrodes. In an embodiment of the present invention, PPy can be formed by oxidation of a pyrrole monomer at a suitable anode within an electrolyte environment, where DBS is the electrolyte. Upon application of a positive potential, an insoluble, electrically-conducting polymer material (i.e., PPy(DBS)) is deposited at the anode. Since the PPy is oxidized, the DBS anions in the electrolyte are incorporated into the film to maintain charge neutrality. Thus, the PPy is “doped” with DBS. In other embodiments of the present invention, other amphiphilic compounds having hydrocarbon tails may be used in place of DBS. In embodiments where the smart polymer acquires a negative charge, a dopant having a positively-charged end may be used in place of a dopant having a negatively-charged end.

The tunable wetting of PPy(DBS) in embodiments of the present invention permits liquid droplet manipulation at very low voltages (e.g., in a range of about −0.9V to 0.6V). The surface energy of PPy(DBS) is changed via re-orientation of DBS in PPy(DBS) through the application of reductive electrical potentials. When a reductive potential is applied to a first electrode made of a smart polymer that is adjacent to a second electrode of the smart polymer upon which a liquid droplet resides, a dissimilar surface state is created between the first and second electrodes to induce a surface tension gradient (i.e., Marangoni stress). The electrically-triggered surface tension gradient is utilized to manipulate liquid droplets. The actuation mechanism utilized to manipulate liquid droplets is described in detail hereinbelow:

Hereinafter, the exemplary electrodes comprising smart polymer will be referred to as PPy(DBS) electrodes to distinguish them from other types of electrodes that may be included in the device. For example, in microfluidic devices made and operated according to embodiments of the present invention, the PPy(DBS) electrodes are in electrical communication with other electrodes that transmit electrical force from the voltage source to the PPy(DBS) electrodes. Such other electrodes are referred to hereinafter as addressable control electrodes.

FIGS. 1 and 2 provide schematic illustrations of an actuation mechanism 10 constructed in accordance with an embodiment of the present invention. A PPy(DBS) patterned substrate 12 comprising the PPy(DBS) electrodes 14, 16 is contained within an electrolyte solution 18 to manipulate a droplet 20 of dichloromethane (DCM). The mechanism 10 further comprises addressable control electrodes 22, 24, made of electrically-conductive materials such as gold or platinum, on a substrate 26 made of an electrically-insulating material such as silicon dioxide. Each addressable control electrode 22, 24 is in contact with one of the PPy(DBS) electrodes 14, 16. Electrical insulators, such as electrical insulator 28, extend from the substrate 26 to insulate adjacent addressable control electrodes 22, 24 from each other and adjacent PPy(DBS) electrodes 14, 16 from each other. Two voltage sources 30, 32 are provided: an oxidizing voltage source 30, which provides a positive potential, and a reducing voltage source 32, which provides a negative potential, both potentials being relative to a reference electrode 34 in contact with the electrolyte 18. A switching mechanism 36 and electrical connectors 38, 40 are also provided, being arranged such that the voltage sources 30, 32 may be selectively applied to the individual control electrodes 24, 26, and thus to the respective PPy(DBS) electrodes 14, 16.

FIG. 1 depicts a positive potential being applied to the actuation mechanism 19 to oxidize the surfaces 42, 44 of the PPy(DBS) electrodes 14, 16. FIG. 2 depicts a negative potential being applied to one PPy(DBS) electrode (i.e., PPy(DBS) electrode 16) to create localized reduction of a portion 45 of the PPy(DBS) electrode surface 44 to induce Marangoni stress. The creation of such stress moves the contact line 46 of the DCM droplet 20, which in turn moves the DCM droplet 20 in a direction indicated by the arrows.

Intrinsic Wetting Property of Reduced and Oxidized PPy(DBS)

Referring to FIGS. 3-5, the surface states of PPy(DBS) are bi-directionally ‘tuned’ from hydrophilic to hydrophobic via re-orientation of its surfactant dopant molecules, DBS. More particularly, with specific reference to FIG. 3, when an oxidative potential is applied to PPy(DBS) 48, sodium (Na⁺) ions are repelled from the PPy(DBS) surface 50 into the adjacent electrolyte solution (not shown) for charge neutralization, leaving behind immobilized DBS⁻anions 52. Likewise, Na⁺ ions enter into the PPy(DBS) 48 for charge neutralization upon reduction. As shown in FIG. 3, the oxidized zone 54 of the PPy(DBS) 48 encroaches on the reduced zone 56 as oxidation proceeds, with charge neutralization occurring simultaneously.

Referring to FIG. 4, when PPy(DBS) is in its reduced state 58, the oxidized PPy chains 60 do not carry a positive charge. DBS⁻anions 62 in the reduced PPy(DBS) 58 are oriented with their hydrophobic tails 64 in the reduced PPy(DBS) 58 and the polar, electrically-negative sulfonate groups 66 at the surface 68 of the reduced PPy(DBS) 58, rendering the surface 68 of the reduced PPy(DBS) hydrophilic and attractive to the water droplet 70 shown.

Referring to FIG. 5, with PPy(DBS) in its oxidized state 74, DBS⁻anions 62 are coupled with PPy chains 60 via electrostatic attraction between the oxidized, electrically positive PPy chains 60 and the polar, electrically-negative sulfonate groups 66 of the DBS anions 62, allowing their hydrophobic tails 64 to thrust away from the PPy chains 60. Since the strongly hydrophilic polar sulfonate groups 66 are attracted to the PPy chains 60 and the hydrophobic hydrocarbon tails 64 are directed outward to the surface 76 of the oxidized PPy(DBS) 74, the surface 76 becomes hydrophobic, decreasing the area of contact between the water droplet 70 and the surface 76. This would cause the contact angle (not indicated) of the water droplet 70 to increase relative to the contact angle (not indicated) of the water droplet 70 when the PPy(DBS) is in its reduced state 58 (see FIG. 4).

Referring to FIGS. 6 and 7, a droplet 78 of a non-polar liquid such as dichloromethane (DCM) shows opposite wetting states to those of water. With the PPy(DBS) in its reduced state 58 (see FIG. 6), the DCM droplet 78 has a smaller area in contact with the hydrophilic surface 68 of the reduced PPy(DBS) 58 than the DCM droplet 78 would have with the hydrophobic surface 76 of the oxidized PPy(DBS) 74 (see FIG. 7). Thus, the DCM droplet 78 would have a greater contact angle (not shown) on the surface 68 of the reduced PPy(DBS) 58 relative to the lower contact angle (note shown) on the surface 76 of the oxidized PPy(DBS) 74. The contact angle of the DCM droplet 78 experimentally measured on the surface 68 of the reduced PPy(DBS) 58 and the surface 76 of the oxidized PPy(DBS) 74 were measured as θ_red˜133° and θ_oxi˜107°, respectively, within 0.1 M NaNO₃aqueous solution.

Theory of Droplet Actuation Upon Continuous Reduction and Oxidation Reactions

In contrast to the separate measurement of the intrinsic wetting states of DCM droplets for each of the above redox states 58, 74, “continuous” electrochemical tuning is performed by applying a square pulse potential to the PPy(DBS) substrate to instigate DCM droplet behavior. Without being bound by theory, droplet actuation according to embodiments of the present invention is believed to proceed as described hereinbelow.

Referring to FIG. 8, when a PPy(DBS) substrate 80 is reduced, the portion 82 of the substrate 80 underneath the DCM droplet 84 remains in the oxidized state 74, while the remainder of the substrate 80 is in the reduced state 58. The contact line 88 (i.e., the outermost limit of contact between the DCM droplet 84 and the surface 90 of the PPy(DBS) substrate 80) moves outward (see contact line 89 and arrows (M) indicating its direction of movement) due to Marangoni stress and the DCM droplet 84 is flattened to a disk-like shape 92 (indicated by a dashed line). Since DBS⁻anions 94 are relatively immobilized, the charge neutralization during PPy(DBS) redox is dominated by the transportation of cations (Na⁺) in electrolyte 96. For complete reduction of PPy(DBS) film 80, sodium ions (Na⁺) in the electrolyte 96 need to transport into PPy(DBS) 80 for charge neutralization.

Continuing to refer to FIG. 8, no such ion is available at the ‘contact zone’ 82 (i.e., portion 82 of the substrate 80 that is covered by the DCM droplet 84) when the reductive potential is applied. The blockage of the PPy(DBS) reduction at the contact zone 82 creates heterogeneous surface states (or localized reduction), across the droplet contact line 88. The surface tension gradient created across the contact line 88 upon a localized reduction of the PPy(DBS) 80 thereby induces Marangoni stress. The Marangoni stress causes the liquid in the DCM droplet 84 to move away from a region of low surface tension (i.e., contact zone 82) towards a region of high surface tension (i.e., regions 98, 100), and the DCM droplet 84 indicated in FIG. 8 as DCM droplet 102.

It should be noted that PPy(DBS) changes color upon reduction and oxidation, providing further evidence of the above theory. A thin PPy(DBS) film (<1 μm) on a gold substrate has a brown color in the reduced state while it is dim/dark in the oxidized state. During reduction of a PPy(DBS) film (not shown) having a DCM droplet thereupon, the PPy(DBS) film was observed to change color across the contact line. This indicated that the circular area of PPy(DBS) underneath the DCM droplet was in the oxidized state while the PPy(DBS) outside of the contact line was in the reduced state. Since reduced PPy(DBS) possesses higher surface energy, this observation of color change clearly illustrated the surface tension gradient across the contact line.

Referring to FIG. 9, the Marangoni stress vanishes when the PPy(DBS) film 80 is oxidized, rendering the surface 90 hydrophobic. The DCM droplet 102 reverts to its spherical shape, indicated in FIG. 9 as DCM droplet 84 (indicated in FIG. 9 by a dashed line), by an internal Laplace pressure gradient to minimize its surface energy.

It should be noted that the present invention can have numerous modifications and variations. For instance, smart polymer-based droplet manipulation can benefit any device designed to utilize digital microfluidics techniques at ultra-low voltages. Besides the exemplary applications described hereinbelow, smart polymer-based droplet manipulation provides the potential for many novel device applications involving tunable wetting properties.

Lab-on-a-Chip Device

Referring to FIG. 10, digital microfluidics using PPy(DBS) electrodes enables extremely flexible “lab-on-a-chip” devices that can be configured in software to execute virtually any assay protocol. For example, a lab-on-a-chip device 104 according to an embodiment of the present invention may comprise a series of actuation mechanisms similar to those discussed above with respect to FIGS. 1 and 2. A PPy(DBS) patterned substrate 106 comprises a plurality of PPy(DBS) electrodes 108, indicated by squares in FIG. 10, mounted on an electrically-insulating substrate 110 and contained within an electrolyte solution (not shown). The device 104 further comprises electrically-conductive addressable control electrodes (not shown) underlying the PPy(DBS) electrodes 108, between the PPy(DBS) electrodes 108 and the substrate 110. Electrical insulators, such as electrical insulators 112, extend from the substrate 110 to insulate adjacent control electrodes (note shown) from each other and adjacent PPy(DBS) electrodes 108 from each other. Power to operate the device 104 may be provided by a 1.5 V direct current source 114 (for example, a commercial AA battery), using switching mechanisms (not shown) and electrical connectors (not shown) in arrangements such as those discussed with respect to FIGS. 1 and 2, to selectively provide oxidizing and reducing voltages to individual PPy(DBS) electrodes 108.

Continuing to refer to FIG. 10, the specimen droplet 116 and reagent droplet 118 can be cut to form smaller specimen droplets 120 and reagent droplets 122, to merge the smaller droplets 120, 122 to form mixed droplets 124, and to transport mixed droplets 124 to a detection site 126 for testing. These operations are performed by selectively applying oxidative or reductive voltages to successive PPy(DBS) electrodes 108. The droplet manipulation using conventional EWOD typically requires about 15-80V to manipulate liquid droplets. The present invention can manipulate droplets at ultra-low voltages (i.e., from about −0.9V to about 0.6V), thereby enabling the fabrication of practical and portable microfluidic devices. Cutting, merging, and transporting the droplets 116, 118, 120, 122, 124 can be performed using the same general techniques described in the Cho et al. Article, which discusses them in the context of conventional EWOD. Thus, the electrically-triggered Marangoni stress and local reduction of a smart polymer can be utilized in any digital microfluidic systems where low voltage input is required.

Liquid Lens for Autofocus

Referring to FIGS. 11 and 12, certain conventional liquid lens manipulation techniques are based on the electrowetting phenomenon. A conventional liquid lens 128 may include an oil layer 130 and a water layer 132 confined between two transparent windows 134, 136 and in contact with thin insulating layers 138, 140 over metal substrates 142, 144. The voltage applied to the substrate modifies the contact angle α of the liquid drop. Referring to FIG. 11, the applied voltage moves the water/oil interface 146 along the insulating layers 138, 140 to control the curvature of the interface 146, and thus the focal length (not shown) of the lens 128, such that, for example, light rays (represented by arrows 148) converge toward the optical axis 150 (shown as a dashed line). Referring to FIG. 12, the variation of voltage leads to a change of curvature of the liquid-liquid interface 146 and the focal length (not shown) of the lens 128, such that, for example, the light rays 148 diverge about the optical axis 150. The present invention can be efficiently utilized for the same application. A PPy(DBS) film (not shown) can be substituted for the insulating layers 138, 140 so that contact angle α of the organic fluid (i.e., the oil layer 130) can be tuned upon reduction or oxidation of the PPy(DBS). An aqueous electrolyte solution (not shown) would be substituted for the water layer 132 to act as the ion provider for the PPy(DBS). An advantage of utilizing the present invention is that the required actuation voltage in this application may be reduced from about 20V to less than 1V.

It will be understood that the embodiment described herein is merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. For instance, all such variations and modifications are intended to be included within the scope of the invention, as described in the claims presented below.

MARANGONI STRESS-DRIVEN DROPLET MANIPULATION ON SMART POLYMERS FOR ULTRA-LOW VOLTAGE DIGITAL MICROFLUIDICS

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