WO2024058780A1

WO2024058780A1 - Digital microfluidics

Info

Publication number: WO2024058780A1
Application number: PCT/US2022/043597
Authority: WO
Inventors: Viktor Shkolnikov; Napoleon J. Leoni; Michael W. CUMBIE
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2022-09-15
Filing date: 2022-09-15
Publication date: 2024-03-21

Abstract

A driving instrument for digital microfluidics can include an instrument substrate, a plurality of driving electrodes at a driving surface of the instrument substrate, and switching electronics to independently send voltage to individual or subsets of driving electrodes of the plurality of electrode. The driving instrument can also include an anisotropic conductive layer positioned on the array of driving electrodes. The anisotropic conductive layer can be more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface. Furthermore, the driving instrument is shaped to receive a droplet control cartridge that is not part of the driving instrument.

Description

DIGITAL MICROFLUIDICS

BACKGROUND

[0001] Digital microfluidics (DMF) relate to the manipulation and control of fluids in the form of small droplets at quantities ranging from nanoliters to several milliliters. In some examples, DMF can be a platform for lab-on-a-chip systems where microdroplets in a chamber may be electrically manipulated, e.g., dispensed, moved, mixed, reacted, thermoscycled, etc., using driving electrodes that are selectively energized to apply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] FIGS. 1A-1C schematically illustrate an example anisotropic conductive layer preparation and conductive particle alignment in accordance with the present disclosure;

[0003] FIG. 2 is a side schematic cross-sectional view of an example digital microfluidic system, including various example fluid processing and analytical components in accordance with the present disclosure;

[0004] FIG. 3 is a top schematic view of an example digital microfluidic system in accordance with the present disclosure;

[0005] FIG. 4 is a top schematic view of another example digital microfluidic system in accordance with the present disclosure;

[0006] FIGS. 5-12 are side schematic cross-sectional views of various example digital microfluidic systems, including portions of both a driving instrument and a droplet control cartridge in accordance with the present disclosure; and

[0007] FIG. 13 is a flowchart illustrating an example method of manipulating droplets in accordance with the present disclosure. DETAILED DESCRIPTION

[0008] Digital microfluidics can be designed in a variety of ways. In many examples, digital microfluidic devices can be capable of moving multiple discrete liquid droplets across their electrowetting surfaces. In some cases, the movement of many droplets can be controlled independently, which can allow the individual droplets to be directed to locations, combined with other droplets, split to form smaller droplets, and so on. Some digital microfluidic systems include an array of electrodes located at (on or just under) a layer of dielectric material. A voltage can be applied to an individual electrode to cause a liquid droplet to move to the surface over the individual electrode. By individually controlling the voltage of the electrodes in the array, such devices can control the movement of multiple liquid droplets across the hydrophobic surface. These devices can be used for a variety of applications, such as dividing a quantity of liquid into multiple droplets having a known volume, or separating specific species from other species in a liquid, or combining droplets containing different reactants to cause chemical reactions, or other applications. In accordance with the present disclosure, digital microfluidics that utilize a driving instrument and droplet control cartridges (that may be disposable) provide a way to reuse more expensive instrument equipment while the less expensive but reliable cartridges can be used where fluids interact with one another and/or secondary devices, e.g., heaters, sensors, etc. Furthermore, driving instruments and droplet control cartridges can be reliably electrically joined together while protecting the electronics that may otherwise be subjected to damage when inserting and/or removing the droplet control cartridges from the driving instrument.

[0009] In accordance with this, the present disclosure is drawn to digital microfluidics, including driving instruments for use/connection with droplet control cartridges, digital microfluidic systems, and methods of manipulating liquid droplets. In accordance with examples thereof, a driving instrument for digital microfluidics includes an instrument substrate that is electrically insulating or dielectric. Example driving instruments include a plurality of driving electrodes at a driving surface of the instrument substrate, as well as switching electronics to independently send voltage to individual or subsets of driving electrodes of the plurality of electrodes. The driving instruments also include an anisotropic conductive layer positioned on the array of driving electrodes. The anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface. In further detail, the driving instrument is shaped to receive a droplet control cartridge that is not part of the driving instrument. In some examples, the anisotropic conductive layer can include electrically conductive particles aligned as filaments in the direction normal to the driving surface. The electrically conductive particles can be fixed and embedded in a polymer matrix selected from the group consisting of an epoxy, a polyurethane, a polyacrylate, a silicone, a polyhedral oligomeric silsesquioxane, a phenolic resin, a cyanate ester resin, or a combination thereof; and wherein the electrically conductive ferromagnetic particles comprise iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, and a composite thereof. In further detail, the anisotropic conductive layer can include electrically conductive ferromagnetic particles aligned in a plurality of conductive paths that are spaced apart laterally and extend through a thickness of the anisotropic conductive layer. The driving instrument can further include a current spreader electrically coupled to the anisotropic conductive layer, but not contacting the plurality of driving electrodes. Furthermore, the driving instrument can likewise include or be electrically coupled to a signal generator and a voltage amplifier to supply selective voltage to the switching electronics. Delivery of the selective voltage to individual or subsets of the driving electrodes can provide current sufficient interact with fluid when a droplet control cartridge when electrically attached to the driving instrument. In some examples, a secondary device can be present to process liquid droplets of a droplet control cartridge when attached to the driving instrument. The secondary device can be selected from the group consisting of an optical sensor, a chemical sensor, a mechanical sensor, an electrical sensor, a magnet, an optical energizer, an optical filter, and a combination thereof, for example.

[0010] In another example, a digital microfluidic system includes a driving instrument with an anisotropic conductive layer positioned on an array of driving electrodes along a driving surface. The anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface. The digital microfluidic system also includes a droplet control cartridge including a dielectric substrate having an electrical interaction surface, a second anisotropic conductive layer positioned on the electrical interaction surface, a ground electrode to electrically communicate with the array of driving electrodes when the droplet control cartridge is connected to the driving instrument, and a droplet control chamber between a second surface facing opposite the electrical interaction surface of the dielectric substrate and the ground electrode. The second anisotropic conductive layer in this example is more electrically conductive in a direction normal to the electrical interaction surface than in planar directions parallel to the electrical interaction surface. In some examples, the first anisotropic conductive layer, the second anisotropic conductive layer, or both can include electrically conductive ferromagnetic particles aligned as filaments in the direction normal to the driving surface when the driving instrument and the droplet control cartridge are connected. The electrically conductive particles can be fixed and embedded in a polymer matrix, for example. In other examples, the anisotropic conductive layer, the second anisotropic conductive layer, or both can be electrically coupled to a current spreader. For example, the current spreader can be positioned between the electrical interaction surface and the second anisotropic conducive layer, among other locations. In other examples, the droplet control chamber can be defined at least in part by a low contact angle hysteresis surface including polytetrafluoroethylene, fluorosilane, fluoroalkylsilane, polytetrafluoroethylene- coated polyimide films, amorphous fluoropolymer, 1 H,1 H, 2H, 2H- perfluorodecyltriethoxysilane, trichloro(1 H,1 H, 2H, 2H-perfluorooctyl)silane, or a combination thereof. In further detail, the droplet control chamber can be filled with liquid droplets along with an immiscible medium relative to liquid droplets.

[0011] In other examples, a method of manipulating liquid droplets includes electrically coupling a first anisotropic conductive layer of a driving instrument with a second anisotropic conductive layer of a droplet control cartridge. The first anisotropic conductive layer in this example is positioned on an array of driving electrodes along a driving surface. Furthermore, the first anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface, and the first anisotropic conductive layer and the second anisotropic conductive layer are in electrical alignment. The method also includes selectively applying voltage to driving electrodes of the driving instrument to generate current through the first anisotropic conductive layer and the second anisotropic conductive layer. In this example, changes in current manipulate liquid droplets in a droplet control chamber of the droplet control cartridge. In some examples, the method includes passing the current through current spreaders positioned in electrical contact with the anisotropic conductive layer, the second anisotropic conductive layer, or both. In other examples, the changes in current include modifying a location of the current by selectively changing which driving electrodes receive voltage.

[0012] It is noted that when discussing the driving instrument of the digital microfluidics, the digital microfluidic systems, or methods of manipulating liquid droplets, these discussions are considered applicable to other examples whether or not they are explicitly discussed in the context of that example unless expressly indicated otherwise. Thus, for example, when discussing a certain type of conductive particle in an anisotropic conductive layer in the context of the driving instrument, such disclosure is also relevant to and directly supported in context of other coated dielectric layers, such as may be present on the droplet control cartridge of the digital microfluidic systems and/or methods of manipulating liquid droplets, and vice versa. Furthermore, for simplicity and illustrative purposes, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent however, that the present disclosure can be practiced without limitation to some of these specific details. In other instances, certain methods, compounds, compositions, and structures have not been described in detail so as not to obscure the present disclosure.

[0013] Thus, as described above, the present disclosure utilizes anisotropic conductive layers on driving instruments of various digital microfluidic systems. In some examples, droplet control cartridges may also include a second anisotropic conductive layer to be electrically connected (and electrically aligned) with the anisotropic conductive layer of the driving instrument. An example driving instrument is shown and described in greater detail in FIG. 2 at 110. Likewise, an example droplet control cartridge is shown and described in more detail in FIG. 2 at 140.

[0014] To illustrate the formation of an anisotropic conductive layer that can be applied to the driving instrument and/or the droplet control cartridge, FIG. 1A shows an example composition 10 that includes electrically conductive ferromagnetic particles 20 in a polymer matrix 15. Initially, the polymer matrix is in an uncured state and the electrically conductive ferromagnetic particles have not yet been aligned. Thus, the ferromagnetic particles are carried by a polymer matrix that is in a condition that allows for the ferromagnetic particles to move around therein. FIG. 1 B is a side cross-sectional view of an anisotropic conductive layer 30 after the electrically conductive ferromagnetic particles have been aligned in a magnetic field. When the magnetic field is applied to the particles, the particles line up and form columns 25 of particles that are touching or close together. The ferromagnetic particles can then be held in this configuration or orientation as the polymer matrix becomes cured. Thus, electrically, the columns of ferromagnetic particles held in place by cured polymer matrix can act as individual electrically isolated (or semi-isolated) filaments with little to no electrical cross-talk. FIG. 1C is a top-down view of the anisotropic conductive layer. The tops of the conductive columns may be visible along a top surface of the anisotropic conductive layer when formed and applied to a surface, e.g., a surface of the driving instrument and/or a surface of the droplet control cartridge.

[0015] The term “anisotropic” refers to materials that have a property that is different when measured in different directions. The anisotropic conductive layers described herein can be anisotropic with respect to electrical resistivity. In other words, the coatings can have a low resistivity (higher conductivity) in the direction through the thickness of the coating layer that is normal to the driving surface (or other surface) to which it is applied. This may be otherwise referred to as the “z-axis” direction in some examples. However, the coatings can have a high resistivity in the lateral directions parallel to the driving surface (or other surface) to which it is applied. In some examples, these directions can be otherwise referred to as the x-axis and y-axis directions or correlate to planes parallel to the driving surface (or other surface) to which it is applied. This 1 anisotropic resistivity can be enabled by small conductive particles aligned in columns, e.g., as conductive filaments, embedded in a solidified insulated or dialectic material, e.g., a cured polymer matrix, and which lead through the thickness of the anisotropic conductive layer. These conductive particle columns or filaments are typically separated from one another within the polymer matrix, which again can be dielectric or resistive. This allows electrical charge to flow along the conductive paths in the z-axis direction (normal to the driving surface or other surface to which the material is applied), while at the same time, charge transfer is blocked in the lateral x-axis and y-axis directions (parallel to the plane of the driving surface or other surface to which the material is applied).

[0016] As mentioned above, some digital microfluidic systems can include a layer of dielectric material with this type of anisotropic conductive layer that is conductive in a direction normal to the surface applied and resistive laterally. Additional detail regarding the anisotropic conductive layers is provided hereinafter. Examples of how these types of layers can be useable with digital microfluidic systems is provided in greater detail in FIGS. 2-12 below.

[0017] Referring now to FIG. 2, a digital microfluidic system 100 is shown by way of example that includes a driving instrument 110 and a droplet control cartridge 140. The two structures can be physically and electrically joined together to operate as a lab-on-a-chip device, for example, with the droplet control cartridge being disposable and/or replaceable in some instances, and the driving instrument being usable time and time again.

[0018] In the context of the present disclosure, a “driving instrument” refers to the structure or device, e.g., microchip, including a plurality of driving electrodes that are protected and electrically coupled to an anisotropic conductive layer. The driving electrodes can be operated by switching electronics, which selectively provides electrical signal to the driving electrodes in a manner that manipulates droplets carried by an attached droplet control cartridge. The switching electronics used two selectively energize single or subsets of electrodes for movement of liquid droplets in an attached droplet control cartridge can be electrically associated with a signal generator to generate electrical signal and/or a voltage amplifier to generate appropriate voltage to send to individual or subsets of driving electrodes, for example. The switching electronics and related components may be integrated together or connected as separate components, each of these components may or may not be part of or integrated with the driving instrument, though in some examples the switching electronics and other electrical component may be partially or fully integrated with the driving instrument.

[0019] A “droplet control cartridge” refers to the structure or device that is electrically (and typically physically) connectable or connected to the anisotropic conductive layer (sometimes referred to herin as a “first anisotropic layer”) of the driving instrument. The droplet control cartridge may or may not include its own anisotropic conductive layer (sometimes referred to herein as a “second anisotropic layer”). The droplet control cartridge may be disposable and is the portion of the digital microfluidic system where fluidic work occurs, e.g., where liquid droplets are manipulated by the electrical current provided by the driving instrument, passing current through a droplet control chamber to a counter electrode, for example.

[0020] The digital microfluidic systems 100 of the present disclosure can be associated with any of a number of secondary devices for processing liquid droplets 200 in addition to the droplet manipulation that occurs within the droplet control cartridge when attached to the driving instrument and then operated accordingly. The liquid droplets may be carried by a continuous fluid medium that is immiscible with the liquid droplets, such as an oil medium with polar liquid droplets, air (or other gas) medium with liquid droplets, etc. Furthermore, the term “liquid droplets” does not infer that the entire droplet is liquid, as the liquid droplets may carry a fine dispersion of solid particles in some instances. For example, some biological material and/or reagent may include a liquid droplet carrier that carries dispersed biological materials or reagent chemicals. However, in other examples, the liquid droplet may carry dissolved components.

[0021] Secondary devices can be used to process these liquid droplets in preparation for mixing, after mixing, or when liquid droplets of different types are not mixed. Example secondary devices used for processing liquid droplets can include various heaters, which may include thermal interface material, sensors, motors (m), etc., and may be external to the driving instrument as shown at 220, or may be within the instrument substrate 112 of the driving instrument, as shown by an embedded resistive heater at 230. The heaters can be used for thermocycling of biological material, for example. Other example secondary devices include magnets 240, which may be movable, electronically energized and/or turned off, etc., using a motor (m) and/or electronics. Still other examples include optics 250, which can include optical devices to energize a sample or optical sensor. In the example shown, there are optics associated with the instrument substrate, which can be optically transparent, for example. However, there can also be optics associated with the droplet control cartridge which may communicate through optically transparent layers, as shown. In this example, those optics are exemplified by an LED 260 and exciter filter 262, which are optically coupled (upon interaction with a sample fluid droplet) with an optical sensor 270, e.g., a CMOS camera) and an emission filter 272.

[0022] Regarding the manipulation of the liquid droplets 200 of fluid contained in the droplet control cartridge 140, the electrical components in both the driving instrument and the droplet control cartridge can be used to manipulate liquid droplets on an electrowetting surface. “Electrowetting” refers to a change in contact angle between a liquid and a solid surface when an electric field is applied between the liquid and the solid surface. In some cases, an electrowetting surface can include a relatively hydrophobic surface that is in contact with the liquid droplet. Thus, the surface can have a relatively large contact angle with the liquid droplet, such as greater than 90° in some examples. However, applying an electric field can effectively make the surface more wettable. In other words, the surface and the liquid droplet can behave as if the surface is more hydrophilic when the electric field is applied. This effect can be due to a combination of ferees including surface tension and electric forces. This electrowetting effect can be used, in some examples, to cause liquid droplets to move across the electrowetting surface. For example, an electric field can be applied to an area of the surface near or adjacent to the location of a liquid droplet. The liquid can have a smaller contact angle with the surface in the area of the electric field than in the area outside the electric field. This can cause the liquid to preferentially wet the surface in the adjacent area where the electric field is applied. Thus, the liquid droplet can physically move into the area where the electric field is applied as the liquid wets the surface in this area, while leaving the more hydrophobic area of the surface outside the eiectric field. For clarity , the anisotropic conductive layers are not applied as the electrowetting surface(s) for contact with the liquid droplets, but rather are used to channel electrical current in a direction through the liquid droplets where electrowetting occurs within a droplet control chamber 175 of the droplet control cartridge.

[0023] The liquid droplets 200 that are manipulated by electrowetting may contain biological material therein, e.g., biological samples, biologically active reagents, etc. There may be multiple types of liquid droplets included, such as for mixing or other processing, as shown by example where a droplet containing biological material A is mixed with a droplet containing biological material B. These and other liquid droplets in the droplet control cartridge may be moved and/or otherwise manipulated using electronics from the driving instrument, for example. The secondary devices as described above can be used to process/interact with the liquid droplets 200, e.g., thermally, magnetically, optically, chemically, mechanically, etc. To illustrate by way of example, many assays are performed with biological fluids such as blood, saliva, or other biological material. Because these biological materials can be hazardous, testing equipment that comes in contact with biological material can be discarded after use or sterilized before a subsequent use. It is often more practical to discard a used digital microfluidic system instead of sterilizing the device. Many digital microfluidic systems are designed with a single unit that includes a dielectric surface, which directly contacts liquid droplets including biological material, and a driving instrument including an array of driving electrodes for driving movement of the liquid droplets. Therefore, the entire unit is typically discarded after one use. However, in accordance with the present disclosure, by separating the driving instrument with the array of driving electrodes from a cartridge that contains the biological material that can be discarded, cost savings can occur as the driving instrument portion can be used over and over again while the droplet control cartridge can be discarded. Furthermore, by applying an anisotropic conductive layer to the driving surface of a driving instrument (over the driving electrodes), the driving electrodes can be protected from damage due to insertion, removal, reinsertion, removal, etc., of multiple droplet control cartridges. Furthermore, application of the anisotropic conductive layer to the driving surface (and driving electrodes) of the driving instrument can also provide for good electrical contact with the droplet control cartridge (which may also have an anisotropic conductive layer thereon) and can also provide good directional current control that is normal to the driving surface.

[0024] Referring now more specifically to the driving instrument 110, in this example, the driving instrument includes an instrument substrate 112, which can be a substrate of any of a number of dielectric or insulating materials. Example instrument substrate materials can include glass, PCB of a variety of types, ceramic material, sapphire, silicon, or the like. The instrument substrate includes a driving surface 114 and alignment posts 116 for attaching to alignment cavities 152 of the droplet control cartridge 140 when the cartridge is attached to the driving instrument. Along the driving surface (which is positioned beneath the anisotropic conductive layer 130) is a plurality of driving electrodes 120. The driving electrodes can be selectively energized via switching by selectively and sequentially applying voltage thereto to cause movement/manipulation of liquid droplets 200 in the attached droplet control cartridge 140. Selective appellation of voltage can be to individual driving electrode or subsets of driving electrodes via switching electronics 210. Thus, voltage can be applied by the switching electronics, or the switching electronics may be separate from the voltage applicator (not shown).

[0025] The driving electrodes 120 are essentially electrically isolated from one another laterally, even though there is an anisotropic conductive layer 130 electrically coupled to the driving surface (and thus, also the driving electrodes). This is because the anisotropic conductive layer is more electrically conductive in a direction normal 136 to the driving surface than in planar directions parallel to the driving surface. For example, the anisotropic conductive layer may include a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. The electrically conductive ferromagnetic particles may be aligned in a plurality of conductive paths that are spaced apart laterally and extend through a thickness of the anisotropic conductive layer. Thus, electrical communication between the driving electrodes to the counter electrode 180 (found in the droplet control cartridge 140) occurs essentially in the direction normal to the driving surface, and cross talk between adjacent driving electrodes is minimized or eliminated. Additional detail regarding the anisotropic conductive layer is provided in greater detail below.

[0026] The driving electrodes 120 at the driving surface 114 of the driving instrument 110, e.g. beneath the anisotropic conductive layer 130, can be formed of a conductive material, such as metal, a conductive ceramic, or other conductive materials. Some specific examples can include copper, copper plated with gold, gold, platinum, silver, aluminum, graphene, graphitic materials, indium tin oxide, zinc tin oxide, and others. In some examples, the driving instrument can also include conductive traces that lead to the individual electrodes, and the conductive traces can be connectable to a power source and/or an electronic controller to allow individual electrodes to be powered. In some examples, the conductive electrodes and traces can be deposited using a suitable deposition process, such as physical vapor deposition, chemical vapor deposition, electroplating, electroless plating, conductive ink printing, photo-etching, or combinations thereof. The thickness of the electrodes can be from about 50 nm to about 100 μm, or from about 100 nm to about 10 μm, or from about 100 nm to about 1 μm, in some examples. In certain examples, the driving instrument can be a commercially available electrode array such as an electrode array from an OPENDROP™ cartridge available from GaudiLabs (Switzerland).

[0027] Referring now more specifically to the droplet control cartridge 140 that is connected to the driving instrument 110, in some examples, the droplet control cartridge can be disposable or a single use cartridge. In other examples, the droplet control cartridge can be used for multiple fluid manipulations or processing. By way of example, the droplet control cartridge shown includes a dielectric substrate 150 with a second anisotropic conductive layer 160 positioned thereof that contacts the anisotropic conductive layer 130 of the driving instrument when the cartridge is attached or joined therewith. The second anisotropic conductive layer can be of the same material and/or density as the anisotropic conductive layer, or it can be of a different material and/or a different density.

[0028] Regarding the dielectric substrate 150 (and in some cases where a dielectric material is used for the substrate of the driving instrument), a variety of materials can be used to form this layer. In some examples, the dielectric substrate can include a polymer such as polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polyolefin, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene-propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene-propylene, perfluoropolyether, perfluorosulfonic acid, B-staged bisbenzocyclobutene, polybenzoxazole, parylene, or a combination thereof. Inorganic materials can also be included, such as alumina, silica, aluminum nitride, or a combination thereof. In some examples, the substrate can include a polyimide material such as a KAPTON® material obtainable from DuPont de Nemours, Inc. (USA) or UPILEX® films from UBE Industries (Japan). In further examples, the substrate can include a polyetherimide (PEI) material.

[0029] In some examples, the dielectric material can have a dielectric strength of 50 V/μm to 500 V/μm, while in some examples, the dielectric strength may be from 100 V/μm to 500 V/μm. In some examples, the dielectric strength can be from 200 V/μm to 400 V/μm. In some examples, the dielectric strength can be from 300 V/μm to 500 V/μm.

[0030] In further detail, the droplet control cartridge 140 includes a droplet control chamber 175, and in some instances, the droplet control chamber can be partially or fully defined by an electrowetting surface(s) 170. The electrowetting surfaces that define that come in contact with the immiscible fluid 204 carrying the liquid droplets 200 can be of any material that provides for movement of discontinuous liquid droplets within the continuous immiscible fluid. For example, the liquid droplets may be polar in nature and the immiscible fluid may be air or another gas, or may be a hydrophobic medium. Aqueous liquid droplets can have a high contact angle on hydrophobic surfaces. Thus, in some examples, the electrowetting surface may be a hydrophobic surface. Applying electric field across the droplet control chamber can reduce the contact angle of the liquid droplet contained therein, and thus, the liquid droplet can be moved by applying an electric field adjacent to the location of the liquid droplet. This can cause the liquid droplet to move into the area of the electric field where the contact angle is lower. [0031 ] in further detail regarding the electrowetting surface 170, in some more specific examples, the electrowetting surface may include a low contact angle hysteresis surface, which may be provided by a low contact angle hysteresis layer. A “low contact angle hysteresis layer,” for example, is defined as a material layer where the contact angle hysteresis at the surface thereof is low. “Contact angle hysteresis” is defined as the difference between the advancing and receding contact angles. For example, if a fluid is dropped on an inclined plane, the contact angle on the uphill side is referred to as the receding contact angle and the contact angle on the downhill side is referred to as the advancing contact angle. Thus, in accordance with the present disclosure, a low contact hysteresis can be further defined as a surface having a contact angle from about 0° to about 20°. In the example shown at FIG. 2, the droplet control chamber is essentially fully enclosed by an electrowetting surface (e.g., a low contact angle hysteresis layer), but there is a small portion where there is an adhesive 172 present to hold the two layers together within the droplet control chamber. Example materials that may provide a low contact hysteresis surface include polytetrafluoroethylene, e.g., TEFLON™ AF 1600 and AF 2400 available from The Chemours Company (USA), fluorosilane, fluoroalkylsilane, polytetrafluoroethylene-coated polyimide films, e.g., Kapton FN form DuPont (USA), amorphous fluoropolymer, e.g., CYTOP™ from AGC Chemicals (USA), 1 H,1 H, 2H, 2H-perfluorodecyltriethoxysilane, trichloro(1 H, 1 H, 2H, 2H- perfluorooctyl)silane, or a combination thereof.

[0032] In other examples, the surface of the droplet control chamber can be defined by a hydrophobic monolayer coating can be applied on surfaces defining the droplet control chamber. Examples of such hydrophobic monolayer coatings include FLUOROPEL™ hydrophobic coatings, available from CYTONIX (USA); RAIN-X® coatings, available from ITW Global Brands (USA); AQUAPEL™ coatings, available from PGW Auto Glass, LLC (USA); octadecyltrichlorosilane; dodecyltrichlorosilane; and others.

[0033] In still other examples, other types of hydrophobic surfaces can be provided via a layer of a bulk hydrophobic material, e.g., a bulk polymer or a bulk ceramic material. Notably, the low contact hysteresis layer described previously can be applied also as a bulk hysteresis layer. The terms “bulk” or “layer” refers to a thicker layer of a solid homogenous material, as opposed to a monolayer coating. Some additional examples of bulk polymers that can provide a hydrophobic surface (which overlaps to some degree with the example list of low contact angle hysteresis materials) include TEFLON™ AF 1600 and AF 2400 available from The Chemours Company (USA), CYTOP® fluoropolymer available from AGO chemicals Company (USA), NOVEC™ 1700 available from 3M (USA), or others. Examples of bulk ceramic materials that can be used include silicon oxycarbide, cerium oxide, and others. Other examples of hydrophobic surfaces include nanoceramic coatings. Nanoceramic coatings can include ceramic nanoparticles bound together by a polymeric binder. As used herein, “nanoparticles” can refer to particles that are from about 1 nm to about 1 ,000 nm in size. In some particular examples, the nanoceramic nanoparticles used in the coating can have an average particle size from about 1 nm to about 200 nm, or from about 5 nm to about 100 nm, or from about 10 nm to about 60 nm, or from about 60 nm to about 150 nm.

[0034] The droplet control chamber 175 can be in the form of a space or channel (or of some other configuration) between the dielectric substrate 150 and a ground electrode 180 opposite the dielectric substrate. In some examples, the distance between the dielectric substrate and the ground electrode can be from 50 μm to 500 μm, from 100 μm to 150 μm, or from 150 μm to 250 μm. I this example, the ground electrode is shown separately relative to the adjacent electrowetting layer 170; however, it is noted that the ground electrode could be integrated as part of this layer as well with the ground electrode component being embedded therein. The ground electrode provides the electrical communication between the driving electrodes 120 of the driving instrument 110 when the droplet control cartridge is attached with or loaded onto the driving instrument. As mentioned, the anisotropic conductive layer 130 and the second anisotropic conductive layer 160 are aligned electrically when joined so that the driving electrodes that close the circuit with the ground electrode have an electrical influence on the liquid droplets 200 carried within the droplet control chamber. Furthermore, in this example, the droplet control cartridge further includes a lid 190, which may be optically transparent in some examples, to provide structure to the droplet control chamber of the droplet control cartridge. [0035] in further exampies, liquid droplets in the droplet control chamber can have a droplet volume from 10 pL to 30 pL. Liquid droplets in the droplet control chamber can be surrounded by air in some examples, while in other examples the droplet control chamber can be filled with a dielectric oil and the liquid droplets can be an aqueous liquid that does not mix with the dielectric oil. In some examples, the dielectric oil can affect electrowetting forces on the aqueous liquid droplets, and/or resist evaporation of the aqueous liquid droplets, and/or facilitate sliding of the liquid droplets and maintaining droplet integrity. Oils that can be used to fill the gap include silicone oil, fluorocarbon oil, engineered fluids, and others. Some specific examples can include 2 centistoke silicone oil, 5 centistoke silicone oil, FLUOROINERT™ FC-40 and FC-75 available from Sigma Aldrich (USA), NOVEC™ HFE 7100, HFE 7300, and HFE 7500 available from 3M (USA).

[0036] FIGS. 2 and 3 illustrate top schematic views of two different example digital microfluidic systems 100, with emphasis on a driving surface 114 of the driving instrument 110. Some features of the droplet control cartridge are not shown, but the liquid droplets are shown to illustrate operation of an assembled digital microfluidic system, even though they would actually reside in the droplet control cartridge.

[0037] Referring specifically to FIG. 3, a driving surface 114 of a driving instrument 110 is shown that is partially coated with an anisotropic conductive layer 130. The driving surface includes a plurality of driving electrodes 120 that are laterally electrically insulated from one another, but which can conduct electrically through the anisotropic conductive layer in the Z-direction, or in the direction normal to the planar surface of the driving surface.

[0038] Two different types of liquid droplets 200A and 200B are shown with arrows depicting the travel paths or movement from driving electrode 120 to adjacent driving electrode and so forth until ultimately the two liquid droplets are sent to the same location where they are admixed to form a third fluid droplet 200C. Again, the liquid droplets shown would be present in the droplet control cartridge, which is not shown in this FIG. Rather, the liquid droplets are shown in this example to illustrate the operation of the driving instrument 110 when coupled with the droplet control cartridge. [0039] On a surface opposite the driving surface 114, which is not shown, there can be positioned secondary devices to interact with the liquid droplets. For example, a heater (not shown) can provide a thermal zone 222 to heat the liquid droplets, thermocycle liquid droplets, etc. This can occur prior to mixing, after mixing, or even when the fluid droplet is not mixed with any other fluid. As another example, a magnet (not shown) may be used to provide a magnetic zone 242 to interact with a fluid droplet at that location. Other zones may include optical detection zones (or even spots), electrical interaction zones, chemical interactions zones, etc.

[0040] Referring now to FIG. 4, an alternative arrangement of a driving surface 114 of a driving instrument 110 is shown that is partially coated with an anisotropic conductive layer 130. In the example, the driving electrodes 120 are arranged in more defined pathways than that of FIG. 3. Similar features are shown in FIG. 4, however, along with some additional details. For example, in addition to the driving surface 114, the driving electrodes, and the anisotropic conductive coating, this example depicts electrical contacts 212 and electrical traces 214 that can be used to connect electrical components, such as the switching electronics 210, which is not shown. In some respects, the electrical traces and contacts can be considered to be part of the switching electronics, as they work with the controlling portions to provide selective voltage to the driving electrodes. Again, in this example, heating regions 222, magnetic regions 242, and optical detection regions 252 are shown, which can be controlled by various secondary devices. Furthermore, though the liquid droplets are not shown in this example, certain other features of the droplet control cartridge (which is not part of the driving instrument) are shown, namely a loading port 152, a waste port or receptacle, and a vent 156. In some examples, the driving instrument can be joined with the droplet control cartridge using a gasket 118 or other fluid resistant spacer, for example.

[0041] Turning now to FIG. 5, a partial cross-sectional schematic view of an example digital microfluidic system 100 is shown, which depicts both portions of a driving instrument 110 and a droplet control cartridge 140. The driving instrument includes an instrument substrate 112, a driving surface 114, driving electrodes 120, and an anisotropic conductive layer 130. Likewise, the droplet control cartridge includes a dielectric substrate, a second anisotropic conductive layer, two electrowetting layers 170 (such as low contact angle hysteresis layers), a ground electrode 180 (which may be a layer or merged with another layer), a lid 190, a droplet control chamber 175, an immiscible fluid 204, and liquid droplets of two different types 200A and/or 200B.

[0042] The features shown and described in the context of FIG. 5 are also included in FIGS. 6-12, and thus, are not re-described to avoid redundancy. It is noted that all of these features may or may not be present in all examples, but rather are provided by way of example to illustrate various digital microfluidic systems, driving instruments, and methods of the present disclosure.

[0043] With more specific reference to FIG. 6, notably the anisotropic conductive layer 130 is shown having a higher density relative to the density of the second anisotropic conductive layer 160 that is present on the droplet control cartridge. By increasing the density of the anisotropic conductive layer (or alternatively the second anisotropic conductive layer), there may in some instances be a better electrical connection between the two layers when the droplet control cartridge 140 is connected to the driving instrument 110.

[0044] Referring to FIG. 7, in this example, in addition to the driving electrodes, there are a plurality of current spreaders 132 included. In accordance with the present disclosure, a “current spreader” is a conductive or semi- conductive element that is electrically coupled to any anisotropic conductive layer described herein that is capable of receiving electrical current channeled directionally thereto, such as via filaments of an anisotropic conductive layer (or in some instances an immediately adjacent current spreader from another structure), and redistributing that current to align more closely with a footprint of the current spreader. Even though it is not an electrode perse, it can take on the properties of a secondary electrode by receiving current indirectly from an electrode through an anisotropic conductive layer and then redistributing the electrical current to pass along to the next structure it is in electrical communication with.

[0045] A current spreader 132 can provide many benefits when electrically coupling an anisotropic conductive layer 130 of a driving instrument 110 with a second anisotropic conductive layer 160 of a droplet control cartridge 140. For example, a current spreader can be used to couple current from an array of driving electrodes 120 through the filaments, e.g., columns of ferromagnetic particles as shown in more detail in FIG. 1 B at 25, of the anisotropic conductive layer where the current is collected from the filaments in contact therewith and spread essentially evenly therein. In other words, the presence of a current spreader can take the collimated electrical current from the individual filaments and then spread that current out within the current spreader to reproducibly apply an electric field thereabove at a dimension approximating the dimension of the current spreader. In this example, the size of the individual current spreaders are about the same size as the size of the driving electrodes, which may provide a more predictable and reproducible electric field to the droplets 200A, 200B in the droplet control cartridge 140. This arrangement also protects the driving electrodes of the array, e.g., TFT, from user handling that goes with insertion of and removal of the droplet control cartridge as well as may occur during driving instrument maintenance. In other words, in this example, the driving electrodes would not be exposed directly to contact with the droplet control cartridge, handling, and/or user maintenance, as there would instead be a robust anisotropic conductive layer and current spreaders at the surface of the driving instrument, while the current spreaders would provide a similar electric field footprint relative to a driving instrument where the driving electrodes would be contacted directly with the droplet control cartridge. In further detail, in some examples, current spreaders 132 can provide enhanced electrical communication between the anisotropic conductive layer 130 of the driving instrument 110 and the second anisotropic conductive layer 160 of the droplet control cartridge 140, even if there are manufacturing errors, dust or other unwanted particle present, e.g., these systems may not be operating in a clean room, etc. For example, if there is dust some other particle that finds it way between the connecting surfaces of the driving instrument and the droplet control cartridge, or if there is a manufacturing defect of some type at otherwise connecting surfaces, the presence of these current spreaders can introduce some electrical tolerance at some of these locations that may otherwise act as dead spots, redistributing the electrical current appropriately for proper operation of the digital microfluidic system. [0046] Current spreaders 132 can be made of any material that is electrically conductive or semi-conductive, such as a metal oxide or other similar conductive oxide material. Examples include elemental metals, metal alloys, oxides, or other conductive compounds that my include materials such as copper, copper plated with gold, gold, platinum, silver, aluminum, graphene, graphitic materials, indium tin oxide, zinc tin oxide, and others.

[0047] The current spreaders may be any dimension that makes sense with respect to the digital microfiuidic system 100 being operated. For example, as mentioned, the current spreader can be sized similar to that of the driving electrodes 120 of the driving instrument 110. The current spreader can likewise be larger or smaller than the driving instrument and/or may have the same or a different shape relative to the size and/or shape of the driving electrodes. In some examples, the thickness of the current spreaders can be from about 20 nm to about 100 μm, from about 50 nm to about 10 μm, or from about 100 nm to about 1 μm, from about 20 nm to 500 nm, or from about 20 nm to about 300 nm, though thicknesses outside of these ranges can be used. The thickness of the current spreaders is typically oriented in the z-direction, or the direction of current flow through the anisotropic conductive layer, e.g. normal relative to the driving surface 114 of the driving instrument.

[0048] FIG. 8 is similar to that shown in FIG. 7, except that the plurality of current spreaders 132 are located as part of the droplet control cartridge 140 rather than as part of the driving instrument 110. This arrangement can protect against dust or other particles that may be present when the driving instrument 110 and the droplet control cartridge 140 are connected. However, unlike the example shown at FIG. 7 which may protect against inoperability with manufacturer defects that may occur when fabricating the driving instrument, this arrangement would protect against potential inoperability with a manufacturer defect when fabricating the droplet control cartridge.

[0049] Regarding FIG. 9, in addition to the current spreaders 132 shown as part of the droplet control cartridge 140, this digital microfluidic system also includes a vacuum 280 with vacuum ports 282 for aligning and/or attaching the droplet control cartridge with the driving instrument 110. Similar to the posts and alignment cavities shown in FIG. 2, a vacuum system can be used for alignment and attachment of the two structures together for operation. Notably, vacuum ports can be just about anywhere through the instrument substrate 112, even though the driving electrodes 120, particularly since the current spreader is included for evening out the current at a location nearer the droplet control chamber 175.

[0050] FIG. 10 is similar to that shown in FIG. 8, except that the plurality of current spreaders 132 are located adjacent to the dielectric substrate 150 of the droplet control cartridge 140 rather than between where anisotropic conductive layer 130 of the driving instrument 110 is joined electrically with the second anisotropic conductive layer 160 of the droplet control cartridge. This approach can protect to some degree against potential inoperability in the presence of dust or other particles and/or manufacturing defects of the droplet control cartridge. However, more relevant in this example, the location of the current spreaders nearer the immiscible fluid 204 and liquid droplets 200A and 200B can provide the advantage of distributing the voltage at a location that is most proximal to the liquid droplets that are to be manipulated within the droplet control chamber 175. In other words, this arrangement can provide the benefit of evening out the electrical current at these locations to compensate for bad electrical connections that may otherwise be present.

[0051] FIG. 11 is similar to that shown in FIG. 10, except that there are second current spreaders 134 in addition to the (first) current spreaders 132 located adjacent to the dielectric substrate 150. In some instances, this can provide the advantage of distributing the voltage at a location that is most proximal to the droplet control chambers, and furthermore, can provide a better point of contact where the (first) anisotropic conductive layer 130 is joined electrically with the second anisotropic conductive layer 160. Additionally, it is noted that the current spreaders have a smaller footprint than the second current spreaders in this example. This can provide more focused current during transmission which is expanded at a location nearest the droplet control chamber. Furthermore, by utilizing multiple current spreaders at different locations, the work of the individual current spreaders can be split up between the various current spreaders positioned along the current flow passed through the anisotropic conductive layer(s). [0052] FIG. 12 is similar to that shown in FIG. 10 as well, but there is also a driving instrument ground 128, e.g., ground pad, ground ring, etc., that is electrically isolated from the anisotropic conductive layer 130. This can provide a pathway for current to travel if there is a malfunction or some other cause of electrical charge that might otherwise damage the driving instrument 110. For example, a user might pick up static charge that could be discharged at or near the digital microfluidic system 100. Thus, a driving instrument ground could help prevent machine static buildup or could provide a non-damaging electrical pathway in the event of a static discharge.

[0053] It is noted that the above FIGS, may not be drawn to scale. For example, in practice, the substrate 112 can be thicker or thinner relative to the thickness of the driving electrodes 120, the dielectric substrate 150 may have a much smaller thickness compared to their planar footprint, and the two anisotropic conductive layers shown 130 and 160 may not be as thick or thin as depicted, to name a few examples. Additionally, the driving instrument 110 is shown in the FIGS, include just a few electrodes for the sake of simplicity. However, in practice the driving instrument is more likely to include many more electrodes, such as an array having from 20 to 10,000 electrodes in some examples. It is also noted that in some examples, additional layers or fewer layers of materials can be added or subtracted compared to that shown, and some layers can be combined together. As an example, in some cases, adhesive layers can be used between some of the material layers shown in the FIGS.

[0054] Referring now to FIG. 13, a method of manipulating liquid droplets can include manipulating 310 liquid droplets includes electrically coupling a first anisotropic conductive layer of a driving instrument with a second anisotropic conductive layer of a droplet control cartridge. The first anisotropic conductive layer in this example can be positioned on an array of driving electrodes along a driving surface. Furthermore, the first anisotropic conductive layer can be more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface, and the first anisotropic conductive layer and the second anisotropic conductive layer are in electrical alignment. The method can also include selectively applying 320 voltage to driving electrodes of the driving instrument to generate current through the first anisotropic conductive layer and the second anisotropic conductive iayer. in some exampies, the method inciudes passing the current through current spreaders positioned in electricai contact with the anisotropic conductive iayer, the second anisotropic conductive iayer, or both, in other exampies, the changes in current inciude modifying a iocation of the current by selectively changing which driving eiectrodes receive voitage.

[0055] As mentioned in the context of FiGS. 1A-1C above, the anisotropic conductive iayers can be made by mixing eiectricaiiy conductive particies in an uncured polymer. The eiectricaiiy conductive particies can be ferromagnetic, meaning that the particies respond strongiy to an applied magnetic field. While the polymer is uncured, the polymer can be a liquid or otherwise have a sufficiently low viscosity to allow the conductive particies to move through the polymer. The mixture of uncured polymer and eiectricaiiy conductive ferromagnetic particles can be placed in a magnetic field. This can cause the particles to move and line up with the magnetic field. The result can be a plurality of conductive paths made up of aligned conductive particles. Within an individual conductive path, the particles may be touching one another or very close to one another so that the electrical resistance through the conductive paths is low. However, the conductive paths can be separated one from another by a greater distance, where the resistive polymer occupies the space between the paths. The polymer can be cured while the particles are aligned by the magnetic field.

[0056] With this example in mind, there are other formulations and methods that can be used in preparing the anisotropic conductive layers in accordance with the present disclosure. As mentioned, in some examples, an anisotropic conductive composition can include a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. The term “embedded” refers to the electrically conductive ferromagnetic particles being mixed into and surrounded by the polymer matrix. The anisotropic conductive composition can be in a cured state or in an uncured state. If the composition is in an uncured state, then the electrically conductive ferromagnetic particles can be in an aligned state or an unaiigned state. As explained above, the anisotropic electrical resistivity properties of the anisotropic conductive layers result from the conductive paths formed by aligning the particles. Thus, the term “anisotropic conductive composition” does not imply that the composition already has its anisotropic electrical resistivity properties since the particles may or may not be in an aligned state. The term “anisotropic conductive composition” can be understood as a composition that may be used to make an anisotropic conductive layer by aligning the electrically conductive ferromagnetic particles in a magnetic field and then solidifying the polymer matrix, such as by curing.

[0057] Regarding the polymer matrix, a variety of polymers can be used. The polymer matrix can have an initial uncured state in which the polymer matrix is a liquid, paste, or has a sufficiently low viscosity to allow ferromagnetic particles to move through the polymer matrix in order to align with a magnetic field. In some examples, the viscosity of the polymer matrix in the uncured state can be less than 50,000 cP, or less than 30,000 cP, or less than 20,000 cP, or less than 10,000 cP. In certain examples, the uncured polymer matrix can include monomers that can react to form polymer chains when cured. Thus, the term “polymer matrix” can include monomers that have not formed a polymer yet when in their uncured state. In other examples, the uncured polymer matrix can include polymer chains that have already been formed, but which are dispersed or dissolved in a liquid and which can become a solid polymer upon curing. A curing process for the polymer matrix can include polymerizing monomers to form polymer chains, cross-linking polymer chains, removing solvents by evaporation, or a combination thereof.

[0058] In some examples, the polymer matrix can include a heat curable polymer. Heat curable polymers can be cured by raising the temperature of the anisotropic conductive composition to a curing temperature for a curing time. In some cases, the curing time can be shortened by increasing the curing temperature, or a lower curing temperature can be used for a longer curing time. The curing temperature can be from 60 °C to 300 °C, or from 60 °C to 250 °C, or from 70 °C to 200 °C, or from 80 °C to 180 °C, in some examples. The curing time can be from 30 minutes to 6 hours, or from 30 minutes to 4 hours, or from 30 minutes to 3 hours, or from 1 hour to 3 hours, in several examples. Examples of heat curing polymers can include epoxies, polyurethanes, polyacrylates, silicones, hybrid polymers, polyhedral oligomeric silsesquioxanes, phenolic resins, cyanate ester resins, and combinations thereof. [0059] The polymer matrix can be cured in a magnetic field generated by magnets in order to align the electrically conductive ferromagnetic particles. However, the magnets may be damaged by high temperatures or high temperatures may interfere with aligning the ferromagnetic particles. If a polymer matrix has a high curing temperature, it can be useful to perform pre-curing at a first temperature that is relatively low, and then remove the partially cured anisotropic conductive layer from the magnet and cure again at a higher curing temperature. For example, the anisotropic conductive layer can be pre-cured at a temperature from 60 °C to 100 °C, and then removed from the magnet and cured again at a temperature from 120 °C to 300 °C.

[0060] In further examples, the polymer matrix can include a two-part curing polymer. This type of polymer can be made up of two or more different chemical compositions that are initially kept separate, but which cause a curing reaction when mixed together. The term “two-part curing polymer” can also encompass polymers that are made by mixing three chemical compositions, four chemical compositions, or more. The separate chemical compositions that are mixed together can include monomers, polymers in a liquid form, crosslinking agents, polymerization initiators, catalysts, or combinations thereof. Examples of two-part curing polymers can include epoxies, polyurethanes, polyacrylates, silicones, hybrid polymers, polyhedral oligomeric silsesquioxanes, phenolic resins, cyanate ester resins, and combinations thereof.

[0061] If the polymer matrix includes a two-part curing polymer, then the composition may have a limited pot life after mixing the two parts together. In some examples, the electrically conductive ferromagnetic particles can be mixed into one of the parts before the two parts are combined together. In other examples, the two parts of the polymer matrix can be mixed and then the particles can be mixed in afterward.

[0062] The polymer matrix can include an ultraviolet (UV) curing polymer. These polymers can be cured by the application of a particular wavelength of UV light, such as wavelengths from 200 nm to 400 nm. In further examples, the UV curing polymer can be cured with a wavelength from 250 nm to 385 nm, or from 300 to 375 nm, or from 320 to 365 nm. In some examples, the UV curing polymer can be a single-component polymer. [0063] Dual-curing polymers can indude polymers that can be cured in multiple ways. For example, some polymers can be cured by UV light or by heat. Other polymers can be cured by mixing two reactive parts in a two-part curing polymer, or by heating. Some polymers may also be cured using two or more of these curing methods applied together. For example, a polymer can be cured by UV light and heat together, or by mixing a two-part system and by heating together.

[0064] Non-limiting examples of polymers that can be used in the polymer matrix include UV22DC80-1 , UV15DC80ND, UV15DC80LV, UV25, EP45HTAN, EP3SP5FL, EP17HT-3, and MASTERSIL® 152, from MASTERBOND® (USA); POLYTEK® 74-45, POLYTEK® 74-30, POLYT EK® 74-20, POLYTEK® 75-70, POLYTEK® 75-60, PLATSIL® 73-25, PLATSIL® 73-60, and PLATSIL® 73-20 from Polytek Development Corp. (USA); SYLGARD™ 184, SYLGARD™ 182, SYLGARD™ 186, and SYLGARD™ 170, from Dow (USA); EPON™ Resin 863 and EPON™ Resin 828 from Hexion (USA).

[0065] In further examples, monomers that can be included in the polymer matrix in an uncured state can include bisphenol-A, bisphenol-F, diglycidyl ether of bisphenol-F, epichlorohydrin, methyltetrahydrophthalic anhydride, isocyanates, polyols, acrylic acid, methacrylic acid, acrylates, methacrylates, carbonates, amides, imines, lactones, unsaturated olefins, and others.

[0066] As mentioned, the anisotropic conductive compositions can also include electrically conductive ferromagnetic particles embedded in the polymer matrix. The term “ferromagnetic” refers to materials that have a high susceptibility to magnetism, the strength of which depends on the strength of the applied magnetic field. Ferromagnetism is the mechanism by which iron metal is attracted to magnets. Ferromagnetic materials can include iron, nickel, cobalt, and alloys thereof. Additional ferromagnetic materials can include manganese-bismuth alloys, manganese-antimony alloys, chromium dioxide, manganese-arsenic alloys, gadolinium, terbium, dysprosium, and europium oxide. The particles used in the anisotropic conductive layers described herein can include a ferromagnetic material, or multiple ferromagnetic materials. In some examples, the ferromagnetic material can be electrically conductive. In other examples, the particles can include a ferromagnetic material and a conductive material. As used herein, “electrically conductive” refers to materials that conduct electrical current. These materials can have a resistivity of 1 Ω cm or less in some examples. In further examples, the electrically conductive material can have a resistivity of 1 x 10^-8 Ω cm to 1 Ω cm, or from 1 x 10^-8 Ω cm to 0.1 Ω cm, or from 1 x 10^-8 Ω cm to 0.001 Ω cm. Some specific examples of materials that can be included in the electrically conductive ferromagnetic particles include: magnetite, silver, nickel, graphite, iron, gold, and combinations thereof.

[0067] In certain examples, the electrically conductive ferromagnetic particles can include a composite of multiple materials. This means that individual particles include more than one material in the individual particles, not merely that multiple types of particles are included. In some examples, the composite of multiple materials can be in the form of a core of a first material and a shell of a second material. These particles include a core made of a core material and a shell made of a shell material. In certain examples, the shell material can be more electrically conductive than the core material. In particular, the shell material can have a lower resistivity than the core material. In further examples, the core material can be ferromagnetic while the shell material can be nonferromagnetic. The reverse can also be true. In other examples the shell material can be ferromagnetic while the core material can be non-ferromagnetic. In such core-shell particles, the core and shell can be formed by any suitable process, such as plating a core of one material with a shell material through electroplating, electroless plating, physical vapor deposition, chemical vapor deposition, or others. In other examples, the core-shell particles can be aggregates of a core particle with smaller particles of shell material adhered, sintered, or otherwise attached to the core particle. Composite particles can also have other arrangements of the two materials, such as having multiple cores of one material encapsulated in a shell of another material, or having multiple layers of different materials such as alternating layers of two different materials, or being formed from a collection of smaller particles of two materials mixed together. Some specific examples of composite particles that can be used in the anisotropic conductive compositions described herein include silver-coated nickel powder, silver-coated iron powder, nickel-coated graphite powder, gold-coated nickel powder, gold-coated iron power, and others. [0068] The electrically conductive ferromagnetic particles can have a suitable particle size to form aligned conductive paths through the anisotropic conductive layer (or the second anisotropic layer). As such, in some examples the particle size can be less than half of the thickness of the anisotropic conductive layer, or less than one tenth, or less than one twentieth. In certain examples, the particle size can be from 1/1000^th of the thickness to 1/10^th of the thickness, or from 1/1000^th of the thickness to 1/50^th of the thickness, or from 1/1000^th of the thickness to 1/100^th of the thickness. The number average particle size of the electrically conductive ferromagnetic particles can be from 1 μm to 50 μm, or from 1 μm to 25 μm, or from 1 μm to 15 μm, or from 1 μm to 10 μm, or from 5 μm to 50 μm, or from 5 μm to 25 μm.

[0069] The electrically conductive ferromagnetic particles can have a spherical or nearly spherical shape in some examples, while in other examples the particles can have a high aspect ratio, such as an aspect ratio greater than 1.1. In certain examples, the particles can be shaped as flakes, platelets, rods, fibers, crystals, or other shapes. As mentioned above, the average particle size of a non-spherical particle can be the number average of the volume equivalent sphere diameter as measured using a particle analyzer such as the MASTERSIZER™ 3000 available from Malvern Panalytical (United Kingdom). In particular examples, the electrically conductive ferromagnetic particles can include nickel flakes, iron flakes, or cobalt flakes.

[0070] The concentration of electrically conductive ferromagnetic particles can be sufficient to form conductive paths through the anisotropic conductive layer when the particles are aligned by a magnetic field. In some examples, the concentration of particles can be sufficient to form conductive pathways that occupy from 25% to 50% of the surface area of the top, bottom, or a horizontal slice of the anisotropic conductive layer (referring to a layer in which the conductive paths are oriented from bottom to top). The concentration of the electrically conductive ferromagnetic particles in the anisotropic conductive composition can be from 5 wt% to 30 wt% with respect to the total weight of the final cured anisotropic conductive layer (i.e. , not including any solvents that may be in the coating composition but which are not present in the final cured anisotropic conductive layer). In further examples, the electrically conductive ferromagnetic partides can be present at a concentration from 5 wt% to 20 wt%, or from 5 wt% to 15 wt%, or from 5 wt% to 10 wt%, or from 10 wt% to 20 wt%, or from 15 wt% to 20 wt%, or from 15 wt% to 30 wt% with respect to the total weight of the anisotropic conductive layer.

[0071] The electrically conductive ferromagnetic particles can be mixed with the polymer matrix using a suitable mixing or dispersing process. In some examples, the particles can be mixed with the polymer matrix in a liquid form using a high-shear mixer, a three-roll mill, a dual asymmetric centrifugal mixer, or a combination thereof. In specific examples, the particles can be mixed with the polymer in a high shear mixer at a speed of 6,000 rpm to 60,000 rpm, or from 10,000 rpm to 40,000 rpm, or from 20,000 rpm to 30,000 rpm. The mixing can be performed for a mixing time from 5 minutes to 30 minutes, or from 10 minutes to 30 minutes, or from 15 minutes to 30 minutes, or from 10 minutes to 20 minutes. If the polymer matrix includes a two-part cured polymer, the electrically conductive ferromagnetic particles can be mixed into one part of the two-part system before the other part of the polymer matrix is added, or the two parts of the polymer matrix can be mixed first and then the particles can be mixed in afterward. After the particles have been mixed into the polymer matrix, and the parts of the polymer matrix have also been mixed together in the case of a two- part polymer, then the coating composition can be coated onto the driving surface of the driving instrument and/or the dielectric substrate of the droplet control cartridge, then placed in a magnetic field, and the polymer matrix can be cured.

[0072] Dispersants can be used to help disperse the electrically conductive ferromagnetic particles in the polymer matrix. Some types of particles may be capable of being dispersed well without an additional dispersant, while other types of particles can be dispersed better when a dispersant is used. The dispersant can be a compound that is separate from the electrically conductive ferromagnetic particles, but which can associate with the particles by covalent bonding, by adsorption, or by another mechanism. The dispersant is in the form of molecules that associate with the surface of the particles. The dispersant can help to prevent the particles from coming into direct contact one with another, which can prevent clumping of the particles. The dispersant can also include organic groups that mix well with the polymer matrix, and/or polymerizable groups that can bond to the polymer matrix when the polymer matrix is cured.

[0073] Non-limiting examples of dispersants for use in the anisotropic conductive compositions include trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, silane coupling agents, titanate coupling agents, zirconate coupling agents, aluminate coupling agents, and combinations thereof. In some examples, the dispersant can be included at a concentration from 0.01 wt% to 5 wt%, or from 0.01 wt% to 3 wt%, or from 0.1 wt% to 3 wt%, or from 0.1 wt% to 2 wt%, or from 0.1 wt% to 1 wt%. Some types of electrically conductive ferromagnetic particles can be coated with a dispersant before the particles are mixed with the polymer matrix. For example, particles can be coated with trisilanol isooctyl polyhedral oligomeric silsesquioxane or trisilanol phenyl polyhedral oligomeric silsesquioxane prior to mixing the particles with the polymer matrix. The particles can be mixed with the dispersant and a solvent such as isopropyl alcohol and then mixed in a high shear mixer to coat the particles with the dispersant. The solvent can then be removed by decanting, evaporation, or another method. This can yield particles coated with the dispersant, which can then be mixed in the polymer matrix. In certain examples, magnetite particles can be coated with a dispersant in this way.

[0074] In further detail regarding the anisotropic conductive layer and/or the second anisotropic conductive layer described herein, additional details include those related to thickness, lateral resistivity, rigidity, elastomeric properties, etc. For example, the resistivity of the anisotropic conductive layer can be significantly lower in the thickness direction (z-axis) normal to the surface to which it is applied compared to in the in-plane directions (x-axis and y-axis). In some examples, the resistivity in the thickness direction can be less than the resistivity in the in-plane direction by a factor of 100 to 10,000,000, or by a factor of 100 to 100,000, or by a factor of 100 to 10,000, or by a factor of 100 to 1 ,000, or by a factor of 10,000 to 10,000,000, or by a factor of 100,000 to 10,000,000. In certain examples, the anisotropic conductive layer can have a resistivity in the in- piane direction that is greater than 1.0 x 10¹² Ω cm. For example, the in-plane resistivity can be from 1.0 x 10¹² Ω cm to 1.0 x 10¹⁶ Ω cm, or from 1.0 x 10¹² Ω-cm to 1.0 x 10¹⁵ (km, or from 1.0 x 10¹² Ω cm to 1.0 x 10¹⁴ Ω cm, or from 1.0 x 10¹² Ω cm to 1.0 x 10¹³ Ω-cm.

[0075] The ability of the anisotropic conductive layer to conduct electrical charge in the thickness direction can be expressed as a resistance-times-area. The actual resistance of the coating layer is related to the area of the coating layer, with the resistance decreasing as the total area increases. Therefore, the value of the resistance of the coating layer multiplied by the area of the coating layer can be a constant. In some examples the anisotropic conductive layer can have a resistance-times-area that is less than 7.0 x 10⁵ Ω cm² in the thickness direction along which the conductive paths are aligned. In further examples, the resistance-times-area can be from 1 Ω-cm² to 7.0 x 10⁵ Ω cm², or from 1 .0 x 10² Ω cm² to 7.0 x 10⁵ Ω cm², or from 1.0 x 10³ Ω cm² to 7.0 x 10⁵ Ω-cm², or from 1.0 x 10⁴ Ω cm² to 7.0 x 10⁵ Ω cm², or from 1 .0 x 10⁵ Ω-cm² to 7.0 x 10⁵ Ω-cm². The resistivity value of the anisotropic conductive layer in the thickness direction can be found by dividing the resistance-times-area value by the thickness of the coating layer in centimeters. In some examples, the resistivity in the thickness direction can be from 1.0 x 10² Ω cm to 1.0 x 10⁷ Ω-cm, or from 1.0 x 10³ Ω-cm to 1.0 x 10⁷ Ω-cm, or from 1.0 x 10⁴ Ω-cm to 1.0 x 10⁷ Ω-cm, or from 1.0 x 10⁵ Ω-cm to 1.0 x 10⁷ Ω-cm, or from 1.0 x 10⁶ Ω cm to 1.0 x 10⁷ Ω-cm.

[0076] The anisotropic conductive layer can be relatively thin to provide an acceptable resistance in the thickness direction. In some examples, the anisotropic conductive layer (or the second anisotropic conductive layer) can have a thickness from 50 μm to 2000 μm. In further examples, the thickness can be from 200 μm to 2000 μm, or from 200 μm to 1500 μm, or from 300 μm to 1000 μm, or from 500 μm to 1000 μm.

[0077] The substrate of the driving instrument and/or the dielectric substrate of the droplet control cartridge can be any thickness that is appropriate for the function thereof. For example, the substrate of the driving instrument may be any thickness suitable for providing support to the driving electrodes and carry the electronics suitable for operating the driving instrument. On the other hand, the dielectric substrate of the droplet control cartridge may be relatively thin. In some examples, the dielectric substrate can have a thickness from 100 μm to 3 mm. In further examples, the thickness can be from 100 μm to 2 mm, or from 100 μm to 1 mm, or from 100 μm to 500 μm, or from 500 μm to 3 mm, or from 500 μm to 2 mm, or from 500 μm to 1 mm.

[0078] The resistivity in the thickness direction and in the in-piane directions can be affected by the number, size, and spacing of the conductive paths, or columns, of aligned particles in the anisotropic conductive layer. These can also be related to a fraction of the surface area of the anisotropic conductive layer that is occupied by conductive paths vs. the surface area that is resistive polymer matrix. For example, the coating can be viewed from the top or bottom, or a cross-section can be taken at a certain height along a plane parallel to the x- y plane. The area of the cross-section that is occupied by conductive pathways can be divided by the total geometric area of the coating layer to yield the fractional area of conductive paths. In some examples, the fraction of the geometric area that is occupied by conductive paths can be from 25% to 50%.

[0079] In some examples, the anisotropic conductive layer can be elastomeric. As explained above, an elastomeric coating can be useful as a mating surface when the driving instrument is mated with the droplet control cartridge. The elastomeric coating can provide good contact if one or both of the structures are not acceptably flat. If the anisotropic conductive layer is elastomeric, then the anisotropic conductive layer can be compliant enough to "fill in” any small gaps that would otherwise be caused by such imperfections. In some examples, elastomeric anisotropic conductive layers can have a Young’s modulus of less than 1 GPa. For example, the Young’s modulus can be from 0.0001 GPa to 1 GPa, or from 0.001 GPa to 0.5 GPa, or from 0.001 to 0.1 GPa.

[0080] As the anisotropic conductive layers of the present disclosure may be elastomeric, this can also allow the anisotropic to conform to the surface to which it is applied, e g., on the driving instrument driving surface and/or on the dielectric substrate of the droplet control cartridge. Furthermore, anisotropic conductive layers can provide continuous physical contact between the droplet control cartridge and driving instrument across the entire interface. This can be useful because the presence of air gaps - even very small air gaps - between the two structures can increase the electric resistance between the electrodes and the dielectric substrate. Small air gaps may be introduced by such things as the presence of small particles, e.g., dust, which may be present when connecting the droplet control cartridge with the driving instrument. Such an increase in resistance can interfere with the operation of the digital microfluidic systems and may make some electrodes in the array inoperable for controlling liquid droplets. In fact, any dielectric gap causes an increase in impedance and therefore benefit from higher voltage. An air gap is one case of this, but dust can be a dielectric particle as well that adds resistance. This added resistance can be ameliorated to some degree using the anisotropic conductive layers that are directionally conductive, and in some instances, the presence of current spreaders can be used to spread current out to a footprint that may be more useful in moving liquid droplets around within the droplet control chamber of the droplet control cartridge, as previously described.

[0081] In some cases, it may be useful to include a rigid anisotropic conductive layer. A rigid anisotropic conductive layer can provide more structural support than an elastomeric anisotropic conductive layer. In certain examples, the anisotropic conductive layer can be a double layer that includes a rigid layer and an elastomeric layer. In other examples, coated substrates of either structure can include a rigid anisotropic conductive layer without an elastomeric anisotropic conductive layer. One use for such a coated substrate can be with a non-contact ion head, which can deposit ions on the rigid anisotropic conductive layer. This type of device is described in more detail below. If the anisotropic conductive layer is rigid, in some cases the anisotropic conductive layer can have a Young’s modulus of 1 GPa or greater. In certain examples, the rigid anisotropic conductive layer can have a Young’s modulus from 1 GPa to 50 GPa, or from 1 GPa to 10 GPa, or from 1 GPa to 5 GPa.

[0082] The anisotropic conductive layers can be applied by forming a layer of an anisotropic conductive composition in an uncured state on a substrate. The substrate and the uncured coating can then be placed in a magnetic field to align the electrically conductive ferromagnetic particles. The polymer matrix can be cured while the particles are aligned. In various examples, the anisotropic conductive composition can be applied to the substrate material by spray coating, dip coating, spin coating, transfer coating, roller coating, extrusion coating, wipe- on coating, screen printing, ink-jetting, or other processes. [0083] in other exampies, the anisotropic conductive layer can be formed separate from the substrate to which it is to be applied. A layer of the anisotropic conductive composition can be placed in a magnetic field to align the particles and then the polymer matrix can be cured. The cured anisotropic conductive layer can then be transferred and adhered to the substrate. Alternately, the substrate can be applied in an uncured state to the anisotropic conductive layer and then substrate can be cured.

[0084] It is noted that many of the examples described above include aqueous liquid droplets on the electrowetting surface of the droplet control chamber. The electrowetting effect can be particularly useful with aqueous liquids, especially with aqueous liquids that include electrolytes. However, nonaqueous liquids can also be manipulated on the electrowetting surface. Some examples of non-aqueous liquids that can be manipulated with the electrowetting surface include formamide, formic acid, dimethyl sulfoxide, N,N- dimethylformamide, acetonitrile, methanol, ethanol, acetone, piperidine, 1- pentanol, 1 -hexanol, dichloromethane, dibromomethane, tetra hydrofuran, m- dichlorobenzene, chloroform, 4-methyl-3-heptanol, and others. Some nonaqueous fluids may move across the electrowetting surface when a more intense electric field is used, such as using a higher voltage or smaller gap distance between the top electrodes and bottom electrodes. In other examples, aqueous liquids can be moved using a less intense electric field. In certain examples, the voltage applied to the electrodes can be from about 100 V to about 400 V, or from about 200 V to about 400 V, or from about 200 V to about 300 V.

[0085] It is to be understood that this disclosure is not limited to the particular processes and materials disclosed herein because such processes and materials may vary somewhat. It is also to be understood that the terminology used herein is used for the purpose of describing particular examples. The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited by the appended claims and equivalents thereof.

[0086] It is noted that, as used in this specification and the appended claims, the singular forms ”a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

[0087] As used herein, the term “substantial” or “substantially” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context.

[0088] As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable and determined based on the associated description herein.

[0089] As used herein, “average particle size” refers to a number average of the diameter of the particles for spherical particles, or a number average of the volume equivalent sphere diameter for non-spherical particles. The volume equivalent sphere diameter is the diameter of a sphere having the same volume as the particle. Average particle size can be measured using a particle analyzer such as the MASTERSIZER™ 3000 available from Malvern Panalytical (United Kingdom). The particle analyzer can measure particle size using laser diffraction. A laser beam can pass through a sample of particles and the angular variation in intensity of light scattered by the particles can be measured. Larger particles scatter light at smaller angles, while smaller particles scatter light at larger angles. The particle analyzer can then analyze the angular scattering data to calculate the size of the particles using the Mie theory of light scattering. The particle size can be reported as a volume equivalent sphere diameter.

[0090] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though the members of the list are individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0091] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, and also to include individual numerical values or sub-ranges encompassed within that range as if the numerical values and sub-ranges are explicitly recited. As an illustration, a numerical range of “about 1 wt% to about 5 wt%” should be interpreted to include the explicitly recited values of about 1 wt% to about 5 wt%, and also to include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting a single numerical value.

Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described.

Claims

CLAIMS What is claimed is:

1. A driving instrument for digital microfluidics, comprising: an instrument substrate that is electrically insulating or dielectric; a plurality of driving electrodes at a driving surface of the instrument substrate; switching electronics to independently send voltage to individual or subsets of driving electrodes of the plurality of electrode; and an anisotropic conductive layer positioned on the array of driving electrodes, wherein the anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface, wherein the driving instrument is shaped to receive a droplet control cartridge that is not part of the driving instrument.

2. The driving instrument of claim 1 , wherein the anisotropic conductive layer includes electrically conductive particles aligned as filaments in the direction normal to the driving surface, wherein the electrically conductive particles are fixed and embedded in a polymer matrix selected from the group consisting of an epoxy, a polyurethane, a polyacrylate, a silicone, a polyhedral oligomeric silsesquioxane, a phenolic resin, a cyanate ester resin, or a combination thereof; and wherein the electrically conductive ferromagnetic particles comprise iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, and a composite thereof.

3. The driving instrument of claim 1 , wherein the anisotropic conductive layer includes electrically conductive ferromagnetic particles aligned in a plurality of conductive paths that are spaced apart laterally and extend through a thickness of the anisotropic conductive layer.

4. The driving instrument of ciaim 1 , further comprising a current spreader electrically coupled to the anisotropic conductive layer, but not contacting the plurality of driving electrodes.

5. The driving instrument of claim 1 , further comprising a signal generator and a voltage amplifier to supply selective voltage to the switching electronics, wherein delivery of the selective voltage to individual or subsets of the driving electrodes provides current sufficient interact with fluid when a droplet control cartridge is electrically attached to the driving instrument.

6. The driving instrument of claim 1 , further comprising a secondary device to process liquid droplets of a droplet control cartridge when attached to the driving instrument, wherein the secondary device is selected from the group consisting of an optical sensor, a chemical sensor, a mechanical sensor, an electrical sensor, a magnet, an optical energizer, an optical filter, and a combination thereof.

7. A digital microfluidic system, comprising: a driving instrument, including a first anisotropic conductive layer positioned on an array of driving electrodes along a driving surface, wherein the first anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface; and a droplet control cartridge, including: a dielectric substrate having an electrical interaction surface, a second anisotropic conductive layer positioned on the electrical interaction surface, wherein the second anisotropic conductive layer is more electrically conductive in a direction normal to the electrical interaction surface than in planar directions parallel to the electrical interaction surface, a ground electrode to electrically communicate with the array of driving electrodes when the droplet control cartridge is connected to the driving instrument, and a droplet contral chamber between a second surface facing opposite the electrical interaction surface of dielectric substrate and the ground electrode.

8. The digital microfluidic system of claim 7, wherein the first anisotropic conductive layer, the second anisotropic conductive layer, or both includes electrically conductive ferromagnetic particles aligned as filaments in the direction normal to the driving surface when the driving instrument and the droplet control cartridge are connected, wherein the electrically conductive particles are fixed and embedded in a polymer matrix.

9. The digital microfluidic system of claim 7, wherein the first anisotropic conductive layer, the second anisotropic conductive layer, or both are electrically coupled to a current spreader.

10. The digital microfluidic system of claim 9, wherein the current spreader is positioned between the electrical interaction surface and the second anisotropic conducive layer.

11 . The digital microfluidic system of claim 7, wherein the droplet control chamber is defined at least in part by a low contact angle hysteresis surface selected from the group consisting of polytetrafluoroethylene, fluorosilane, fluoroalkylsilane, polytetrafluoroethylene-coated polyimide films, amorphous fluoropolymer, 1 H,1 H,2H,2H-perfluorodecyltriethoxysilane, trichloro(1 H,1 H,2H,2H-perfluorooctyl)silane, and a combination thereof.

12. The digital microfluidic system of claim 7, wherein the droplet control chamber is filled with liquid droplets and an immiscible medium relative to liquid droplets.

13. A method of manipulating liquid droplets, comprising: electrically coupling a first anisotropic conductive layer of a driving instrument with a second anisotropic conductive layer of a droplet control cartridge, wherein the first anisotropic conductive iayer positioned on an array of driving electrodes along a driving surface, wherein the first anisotropic conductive layer is more electrically conductive in a direction normal to the driving surface than in planar directions parallel to the driving surface, and wherein the first anisotropic conductive layer and the second anisotropic conductive layer are in electrical alignment; and selectively applying voltage to driving electrodes of the driving instrument to generate current through the first anisotropic conductive layer and the second anisotropic conductive layer, wherein changes in current manipulate liquid droplets in a droplet control chamber of the droplet control cartridge.

14. The method of claim 13, further comprising passing the current through current spreaders positioned in contact with the first anisotropic conductive layer, the second anisotropic conductive layer, or both.

15. The method of claim 13, wherein the changes in current include modifying a location of the current by selectively changing which driving electrodes receive voltage.