Digital microfluidic device and method of determining the presence or absence of droplet using the same

Abstract

This invention discloses a method and apparatus for sensing the presence of water droplets on a digital microfluidic device.

Description

Technical Field

[0001] This invention discloses a method and apparatus for sensing the presence of water droplets on a digital microfluidic device. Prior Technology

[0002] Digital microfluidic devices utilize independently controllable electrodes to agitate, split, and connect droplets in confined environments, thereby providing a "lab-on-a-chip" experience. Digital microfluidic devices are also known as dielectric electrowetting (EWoD) devices to distinguish them from competing microfluidic systems that rely on electrophoresis and / or micropumps. (From Wheeler in "Digital Microfluidics") A 2012 review of electrowetting techniques is provided in [Annu. Rev. Anal. Chem.] 2012, 5:413-40. This technique allows for sample preparation, analysis, and synthetic chemistry using small amounts of both samples and reagents. In recent years, the use of electrowetting to control droplet manipulation in microfluidic units has become commercially viable; and products are now available from large life science companies such as Oxford Nanopore.

[0003] Most literature on EWoD reports involves so-called "segmented" devices, in which a limited number (usually ten to twenty) of electrodes are directly driven by a controller. Although segmented devices are easy to fabricate, the number of electrodes is limited by space and driving constraints. Therefore, large-scale parallel analysis, reactions, etc., are not possible in segmented devices.

[0004] In contrast, known "active matrix" devices (also known as active matrix EWoD, or AM-EWoD) can have thousands, hundreds of thousands, or even millions of addressable electrodes. These active matrix EWoD devices are conceptually similar to active matrix liquid crystal and electrophoretic displays and typically include a first substrate or backplane carrying a two-dimensional array of first electrodes arranged in multiple columns and rows. Each first electrode has an associated transistor, typically a thin-film transistor (TFT), where the first electrode is connected to the drain of the transistor (this arrangement will be assumed herein, however it is essentially arbitrary and the first electrode can be connected to the source of the transistor). The sources of all transistors in each row are connected to a single source line (also called a row or data line), and the gates of all transistors in each column are connected to a single column line. The various column lines are connected to column drivers, which essentially ensure that at any given time only one column is selected; that is, a voltage is applied to the selected column line so that all transistors in the selected column are conductive, while a voltage is applied to all other column lines so that the transistors are not conductive. Various source lines are connected to source drivers, which apply the voltage required to drive the first electrode in the selected column to the source lines; since only transistors in the selected column conduct electricity, the voltage on the source lines is only transmitted to the first electrode in the selected column. After a preselection time interval called the "column address time," the selected column is deselected, the next column is selected, and the voltage on the source lines changes to the next row of the display addressed. This process is repeated to address the entire device column by column.

[0005] The active matrix EWoD device further includes a second or front substrate containing at least one second electrode; typically, only one second electrode extends across the entire device. The first and second substrates remain parallel and have a small cavity ("microfluidic region") between them, which is typically filled with a first fluid, although it can be filled with gas. When the device is operated, the cavity also contains droplets of a second fluid that is immiscible with the first fluid; in practice, the second fluid is typically aqueous and the first fluid system is non-aqueous, and is typically oil. As described below with reference to Figure 1, droplets can be laterally moved and split and merged by appropriately manipulating the potential of the first electrode. Therefore, the AM-EWoD device can be used as a general-purpose device that allows for free control of multiple droplets and simultaneous execution of analytical processes.

[0006] One of the main challenges in operating an AM-EWoD device is detecting the position of a droplet. For example, if a droplet splits in two, it is necessary to confirm that the original droplet is in a suitable position for splitting and to confirm the presence of two droplets after the split. While this may seem easily achievable optically at first glance, the small size of the droplet (approximately 100 µm) and, in some cases, the lack of contrast between the droplet and the surrounding oil can make optical confirmation difficult. Optical confirmation also requires clear visibility of the droplet, which can be difficult, for example, if the device needs to be masked due to the presence of toxic or harmful biological materials. Therefore, electrical detection of the droplet position is generally preferred. U.S. Patent No. 10,882,042 describes a method for detecting the position of a droplet that relies on the capacitance change due to the difference in dielectric constant between the droplet and the surrounding oil. However, this method requires adding a TFT array to a second substrate and providing a second set of drivers, both of which significantly increase the cost and complexity of the second substrate and raise alignment issues, as the TFT array in the second substrate must be precisely aligned with the first electrode in the first substrate.

[0007] U.S. Patent No. 8,653,832 describes an array element in which each element provides writing circuitry and impedance measurement circuitry. This significantly increases the cost and complexity of the device due to the addition of complex circuitry at each element of the array, thus substantially increasing the manufacturing cost of TFTs and the cost of the numerous drivers required to control the circuitry added at each pixel, multiplied by the driver cost of thousands of drive lines.

[0008] Therefore, there is a need for improved methods to detect the position of droplets in an AM-EWoD device without substantially modifying the second substrate or adding additional circuitry to each pixel of the TFT array, and the present invention seeks to provide such methods and apparatus suitable for carrying them out. Summary of the Invention

[0009] Therefore, the present invention provides a digital microfluidic device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and A component for introducing fluid between the first and second substrates and creating a microfluidic region between the first and second substrates. At least one of the source lines is equipped with a capacitance measuring element, which is arranged to measure the capacitance between at least one of the first electrodes connected thereto and the at least one second electrode, thereby determining the presence or absence of a fluid droplet between the at least one of the first electrodes and the at least one second electrode.

[0010] This invention provides a digital microfluidic device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and The microfluidic region between the first and second substrates, At least one of the source electrode lines is arranged to measure the capacitance between at least one of the first electrodes connected thereto and the at least one second electrode, thereby determining the presence or absence of fluid droplets between the first electrodes and the at least one second electrode.

[0011] The digital microfluidic device of the present invention may further include a component for applying an alternating voltage to at least one second electrode. A microfluidic region between the first and second substrates may be filled with a first fluid, and a fluid introduction component is arranged to introduce a second fluid immiscible with the first fluid. Typically, one fluid system is aqueous and the other is non-aqueous; in some embodiments of the invention, the second fluid system is aqueous and the first fluid system is non-aqueous. Each of the first and second dielectric layers may be hydrophobic itself, or may have a hydrophobic layer superimposed thereon. A capacitance measurement component may include a capacitor connected to a source line and a component for measuring the voltage drop across the capacitor. One or more of the source lines may contain capacitors. Each source line may contain a capacitor.

[0012] The present invention also provides a method for determining the presence or absence of a droplet adjacent to a specific first electrode in a digital microfluidic device, the device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and A component for introducing fluid between the first and second substrates and creating a microfluidic region between the first and second substrates. The method includes measuring the capacitance between the particular first electrode and the at least one second electrode.

[0013] The present invention also provides a method for determining the presence or absence of a droplet adjacent to a specific first electrode in a digital microfluidic device, the device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and The microfluidic region between the first and second substrates, The method includes measuring the capacitance between the particular first electrode and the at least one second electrode.

[0014] In these methods and apparatuses, the capacitance between the specific first electrode and at least one second electrode can be measured by applying an alternating voltage to the at least one second electrode, connecting a capacitor to the specific first electrode, and measuring the voltage drop across the capacitor. Wherein, if in a typical configuration the plurality of source lines are arranged parallel to each other, the method may further include driving the two source lines closest to the source line connected to the specific first electrode at the same voltage as the source lines connected to the specific first electrode. Simple Explanation of the Diagram

[0015] Figure 1 in the accompanying drawings is a top plan view of a typical prior art EWoD device.

[0016] Figure 2 is a cross-sectional schematic diagram of a portion of the EWoD device shown in Figure 1, illustrating the movement of droplets within the device.

[0017] Figure 3 is a schematic circuit diagram of a portion of the EWoD device shown in Figure 1, illustrating the application and maintenance of potentials for various pixel electrodes of the device.

[0018] Figure 4 is a schematic circuit diagram of the EWoD device of the present invention.

[0019] Figure 5 is a schematic circuit diagram of the source driver, which can replace the EWoD device shown in Figure 4.

[0020] Figure 6 is a schematic circuit diagram of a source line of the source driver shown in Figure 5 and the capacitance measurement component of its associated pixel electrode.

[0021] Figure 7 is a schematic circuit diagram of another EWoD device of the present invention having a capacitance measuring component different from that shown in Figure 6.

[0022] Figure 8 is a schematic circuit diagram of a modified version of the capacitance measurement component in Figure 6, which is used to reduce the number of analog-to-digital converters required in the EWoD device.

[0023] Figures 9, 10, and 11 are schematic circuit diagrams of other EWoD devices of the present invention having capacitance measurement components different from those shown in Figures 6 and 7. Implementation

[0024] As indicated above, the present invention provides a digital microfluidic (EWoD) device in which at least one source line is equipped with a capacitance measurement component, which is arranged to measure the capacitance between at least one of the first (or pixel) electrodes connected to the source line and a second (or common) electrode, thereby determining the presence or absence of a fluid droplet between the pixel electrode or the electrode and the common electrode.

[0025] The basic structure of the EWoD device will first be described with reference to Figures 1 to 3. As shown in Figure 1, the general architecture of the EWoD device (generally designated as 100) is similar to that of an active matrix electro-optical display, wherein a planar rectangular matrix of first or pixel electrodes 205 is parallel to but spaced from a single second or common electrode 206 (not shown in Figure 1, but best seen in Figure 2). Repositories R1 to R7 are arranged around the periphery of the matrix of pixel electrodes 205, and are arranged to introduce droplets 204 between the pixel electrodes 205 and the common electrode 206 (Figure 2). Depending on the intended application of the EWoD device, the reservoirs R1 to R7 may contain biological samples (e.g., body fluids), reagents, raw materials, catalysts, solvents and co-solvents, or any other materials required for chemical reactions or tests performed by the device. In Figure 1, reservoirs R1 to R7 are shown arranged in three groups along two different edges of the matrix of electrodes 205, but this is for illustrative purposes only, and the number of reservoirs, their grouping, and their placement may vary as needed.

[0026] In one example embodiment, the presence of the analyte to be analyzed and the sample droplets at the desired concentration are diluted by combining them with one or more solvent droplets, and the dilution step can be repeated until the desired analyte concentration range is obtained. Then, the diluted sample droplets are mixed with one or more reactant droplets that form a detectable and quantifiable product with the analyte. Subsequently, these sample droplets can be transferred to other locations for detecting and measuring the concentration of the analyte. Example detection and measurement techniques include spectrophotometry in the visible, UV, and IR ranges, time-resolved spectroscopy, fluorescence spectroscopy, Raman spectroscopy, phosphorescence spectroscopy, and potentiodynamic electrochemical measurements (such as cyclic voltammetry (CV)). In the case of analytes that are diagnostic biomarkers (e.g., proteins associated with a given disease or ailment), the sample droplets can be mixed with solution droplets containing antibodies against the protein to be measured. In enzyme-linked immunosorbent assay (ELISA), the antibody system is linked to an enzyme and another droplet, while a substance containing the enzyme's acceptor is added. Subsequent reactions generate detectable signals, most typically detectable and measurable color changes. To enable the aforementioned type of spectral analysis (including color measurement), the common electrode 206 is typically radiotransmissive and may be formed of, for example, indium tin oxide. Clearly, the radiotransmissive properties of the common electrode 206 must be selected taking into account the wavelength of the spectral analysis to be performed. In one embodiment, the common electrode includes a light-transmitting region (e.g., an area of ​​10 mm²) to enable visual or spectrophotometric monitoring of fluid droplets within the device (not shown).

[0027] The device 100 further includes a gate (or column) driver 102 and a source (or data) driver 104, both of which function in a manner similar to the corresponding drivers in an active matrix electro-optic display, as discussed below with reference to FIG3. For ease of explanation, the connections between these drivers 102 and 104 and the pixel electrode 205 are omitted from FIG1.

[0028] As shown in Figure 2, the gap between pixel electrode 205 and common electrode 206 is filled with a layer of oil (or other hydrophobic fluid) 202 containing at least one water droplet 204 initially dispensed from one of the reservoirs R1 to R7 (Figure 1). The use of water droplets in water-immiscible matrices is common in EWoD devices because most reactions of interest take place in an aqueous medium. However, it should be understood that EWoD devices can use water-immiscible droplets in aqueous matrices, or any combination of two immiscible fluids with substantially different dielectric constants (to allow for the driving of the droplets as described below). The cell gap between these electrodes 205 and 206 is typically in the range of 50 to 200 µm, but this gap can be larger or smaller. Figure 2 illustrates a portion of three adjacent pixel electrodes (also referred to as "push electrodes") 205 and common electrode 206. A hydrophobic coating 207 is provided covering electrodes 205 and 206 and in contact with the oil layer 202. The dielectric layer 208 is inserted between the electrode 205 and the adjacent hydrophobic coating 207. (Although not shown in Figure 2, the dielectric coating may also be inserted between the common electrode 206 and its adjacent hydrophobic coating 207). While theoretically a single layer can simultaneously perform dielectric and hydrophobic functions, such layers typically require a thick inorganic layer with a resulting low dielectric constant (to prevent pinholes), thus requiring droplet movement exceeding 100 V. For low-voltage actuation, it is generally preferable to have a thin, pinhole-free inorganic layer with a thin organic hydrophobic layer on top for high capacitance. With this combination, electrowetting operation is possible within a voltage range of ±10 to ±50 V, which is within the range available from conventional TFT analysis.

[0029] The dielectric layer 208 should be thin enough to have a dielectric constant compatible with low-voltage AC drives, such as those available from conventional image controllers used in LCD displays. For example, the dielectric layer may comprise a silicon dioxide layer of approximately 20 to 40 nm top-coated with plasma-deposited silicon nitride of 200 to 400 nm thickness. Alternatively, the dielectric layer may comprise an atomically deposited aluminum oxide layer of 5 to 500 nm thickness, preferably 150 to 350 nm thickness.

[0030] The hydrophobic coating 207 can be constructed from one or more fluoropolymers, such as PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene propylene), PVF (polyvinylidene fluoride), PVDF (polyvinylidene fluoride), PCTFE (polychlorotrifluoroethylene), PFA (perfluoroalkoxy polymer), ETFE (polyethylene tetrafluoroethylene), and ECTFE (polyvinyl chloride trifluoroethylene). Commercially available fluoropolymers include Teflon AF (Sigma-Aldrich, Milwaukee, WI; "Teflon is a registered trademark") and FluoroPel from Cytonix (Beltsville, MD). TM coatings, which are spin-coated onto dielectric layer 208. The advantage of fluoropolymer films is their high inertness and ability to retain hydrophobicity even after exposure to oxidative treatments (such as corona treatment and plasma oxidation). Coatings with high contact angles can be made from one or more superhydrophobic materials. Contact angles on superhydrophobic materials typically exceed 150°, meaning only a small percentage of the droplet matrix contacts the surface. This results in a near-spherical droplet shape. Certain superhydrophobic materials formed from fluorinated silanes, perfluoroalkyl groups, perfluoropolyethers, and RF plasmas have been found to be used as coatings in electrowetting applications, making them relatively easier to slide along the surface. Some types of composite materials are characterized by chemically heterogeneous surfaces, where one component provides roughness and the other provides low surface energy to facilitate the production of coatings with superhydrophobic properties. Biomimetic superhydrophobic coatings rely on fine micron or nanostructures to achieve their hydrophobicity; however, it should be noted that such structures tend to be susceptible to wear or cleaning damage.

[0031] The hydrophobic coating 207 prevents droplets from wetting the surface. When no electric field is applied to the droplet, it will remain spherical to minimize contact with the hydrophobic surfaces of the oil layer and the hydrophobic layer. Because the droplets do not wet the hydrophobic surfaces, they are unlikely to contaminate the surfaces or interact with other droplets unless such behavior is required. Therefore, as known in the art, individual droplets can be manipulated around an array of pixel electrodes and mixed, split, and combined.

[0032] However, as illustrated in Figure 2, when a voltage difference is applied between adjacent pixel electrodes 205, the voltage on one electrode attracts the opposite charge in the droplet 204 at the dielectric-droplet interface, and the droplet moves toward this electrode, as indicated by the arrow in Figure 2. The acceptable voltage required for droplet propulsion depends on the properties of the dielectric and hydrophobic layers. AC drive is typically used to reduce degradation of the droplet, dielectric, and electrodes due to various electrochemical reactions. Operating frequencies for EWoD devices can range from 100 Hz to 1 MHz, but frequencies of 1 kHz or lower are preferred for TFTs with limited operating speeds.

[0033] As shown in Figure 2, the common electrode 206 is a single conductive layer, typically set to a common voltage value (VCOM) of zero volts or near zero to account for the offset voltage on electrode 205 caused by capacitive backlash or gate voltage drop of the TFT used to switch the voltage on these electrodes (discussed below with reference to Figure 3). Any references to "top" and "bottom" herein are merely convention, as the positions of the two electrodes can be switched, and the device can be oriented in various ways; for example, the top and bottom electrodes can be substantially parallel, while the entire device is oriented such that the substrate is perpendicular to the working surface. The top electrode may also have a square wave applied to increase the voltage across the liquid. This arrangement allows for a lower push voltage to be used for the push electrode 205 of the TFT connection, since the top plate voltage 206 is an additional voltage provided by the TFT.

[0034] Figure 3 is a schematic circuit diagram showing the driving method of pixel electrode 205. The driving circuit shown in Figure 3 is an active matrix type and similar to the operation of an active matrix electro-optical display. Each pixel electrode 205 is connected to the drain of an associated transistor 306, and the source of transistor 306 is connected to data or source line 302, wherein the sources of all transistors 306 in a row of the matrix are connected to the same source line. The gate of transistor 306 is connected to gate or select line 304, and the gates of all transistors 306 in a column of the display are connected to the same select line. The drain of transistor 306 is also connected to one electrode of capacitor 308, and the other electrode is connected to capacitor line 310. The lower electrodes of all capacitors in a row of the display (shown in Figure 3) are connected to the same capacitor line 310. (Capacitor 308 is used to present crosstalk from parasitic capacitance, such as those discussed below). Alternatively, capacitor line 310 can be eliminated and the electrode below capacitor 308 can be connected to the select line adjacent to the select line connected to the associated transistor 306. All select lines 304 are connected to gate driver 102 (FIG. 1), and all source lines 302 are connected to source driver 104 (FIG. 1). These capacitor lines 310 are maintained at the same potential as the common electrode 206.

[0035] In a conventional active-matrix electro-optical display, source driver 104 applies a voltage to source line 302 for a column of feed electrodes 205 to be transmitted for electrowetting operation. Gate driver 102 then applies a voltage to a gate line 304 that opens the gates of all transistors in that column, thereby transmitting the voltage from source line 302 to the feed electrode 205 in the selected column. These steps are repeated such that by scanning the device row by row, 0 V is applied to the (non-target) feed electrode if no movement is required, or if the droplet intends to move away from the feed electrode. If the droplet intends to move towards the feed electrode, an AC voltage is applied to the (target) feed electrode.

[0036] Figure 4 is a schematic circuit diagram of the EWoD device (generally designated as 400) of the present invention. The common electrode 206, pixel electrode 205, transistor 306, capacitor 308, and gate driver 102 are substantially the same as the corresponding parts of the devices shown in Figures 1 to 3, and therefore will not be described in detail. The common electrode 206 and pixel electrode 205 are equipped with hydrophobic and dielectric layers similar to those shown in Figure 2, but these layers are omitted in Figure 4 and subsequent figures for ease of explanation. (Note that the device 400 shown in Figure 4 is rotated 90° relative to the device shown in Figure 1, so that the selection line system is vertical and the source line system is horizontal in Figure 4.) Although only three gate lines 304 and three source lines 302 are shown in Figure 4, in practice, a considerable number of each will be used (possibly 200). The on / off switchable AC excitation voltage indicated at 402 is fed to the common electrode 206 via amplifier 404.

[0037] The source driver of device 400 is distinctly different from that shown in Figure 1 and includes all the circuitry within the dashed box 420. The first part of this source driver (including shift register 422 and a series of switches 424 (each switch 424 is associated with a source line 302)) is a conventional three-stage source driver of a type familiar to anyone skilled in electro-optical display technology. Under the control of a clock signal (not shown in Figure 4), data is received in series on data bus 426 and latched by means of the shift register 422, thereby connecting each source line 302 by its associated switch 424 to one of the three inputs that deliver -V, 0, and +V voltages, where V is the operating voltage of the pixel electrode 205.

[0038] In conventional source drivers, the source line 302 leading from switch 424 is directly connected to the source of transistor 306. However, in the device 400 shown in FIG4, the source line 302 first passes through a capacitance measurement assembly 430 having a capacitance measurement circuit 432 associated with each source line 302. As described below, referring to FIG6, the function of the capacitance measurement circuit 432 is to measure the capacitance associated with each of the pixel electrodes connected to the circuit 432 via transistor 306.

[0039] The device 400 shown in Figure 4 is designed to minimize the amount of custom circuitry that must be manufactured; since the shift register 422 and switch 424 are known (and can be purchased as integrated circuits), only the capacitance measurement assembly 430 needs to be custom manufactured. However, device 400 does require a connection between each source line 302 and the switch 424 and the capacitance measurement assembly 430, and the resulting large number of wired connections can cause reliability issues. Therefore, if the number of units to be manufactured justifies the cost of custom manufacturing, the source driver 420 shown in Figure 4 can be replaced by the single-chip source driver 520 shown in Figure 5. In the source driver 520, the shift register 422, switch 424, and capacitance measurement circuit 432 are all provided on a single chip, thus eliminating a large number of wired connections and also reducing the number of external control signal connections required by the source driver.

[0040] The capacitance measurement circuit 432 will now be described with reference to Figure 6, measuring the capacitance of all pixel electrodes that can be connected by transistor 306. However, it should first be understood that since the apparatus and method of the present invention use the same set of source lines for driving the display and for measuring capacitance, the apparatus must have both a driving mode and a measurement mode, and can only operate in one mode at any given time. The relative time spent in its driving and measurement modes can vary widely depending primarily on the intended use of the apparatus. For example, when developing a scheme for a new reaction or test, it may be necessary to set the apparatus to measurement mode after each generation, movement, or splitting of a droplet, or a combination of two or more droplets, to check that everything is proceeding as planned. It has been found that capacitance measurement during this measurement mode can be fast enough not to unduly slow down the operation of the EWoD apparatus. On the other hand, when using the apparatus for a large number of repetitive routine tests according to a predetermined scheme, the driving mode can be used to continuously move a large number of droplets, while the measurement mode is used only at relatively long intervals to confirm certain critical steps.

[0041] The driving mode of the device 400 shown in Figure 4 operates in a manner substantially the same as that of prior art EWoD devices. The excitation voltage 402 is turned off, and droplet movement is achieved by scanning the matrix of pixel electrodes 205 in a conventional column-by-column manner as previously described with reference to Figure 2. (This invention does not preclude the possibility that the potential of the common electrode 206 during this driving mode could be changed by means other than the excitation voltage 402 known in the art to increase the driving voltage across the display.)

[0042] The measurement mode of the device 400 shown in Figure 4 will now be described with reference to Figure 6. As mentioned, the function of this measurement mode is to determine the position of the droplet 204 on the matrix of pixel electrodes 205, or in other words, to determine whether the droplet 204 is present between any particular pixel electrode 205 and the adjacent portion of the common electrode 206. As is evident from Figure 6, the pixel electrodes and the common electrode form a parallel-plate capacitor whose capacitance depends on the dielectric constant of the material between the two electrodes. As previously noted, EWoD devices typically manipulate water droplets containing solutions of many different chemical components (such as surfactants, salts, enzymes, proteins, DNA, and other solutes). Therefore, if this water droplet is present between a particular pixel electrode and the common electrode, the dielectric constant between these electrodes will be the dielectric constant of the water droplet, which will be close to the dielectric constant of water (approximately 80 at room temperature). If no droplet is present near a particular pixel electrode, the dielectric constant between these electrodes will be the dielectric constant of the oil or other non-aqueous fluid used, which will typically not exceed approximately 5. For a 200 µm square pixel and assuming a spacing of approximately 50 µm between the common electrode and the pixel electrode, the capacitance will be approximately 0.2 pF when a water droplet is present between these electrodes and approximately 0.01 pF when oil is present between them. Therefore, the capacitance will differ by approximately an order of magnitude depending on the presence or absence of a water droplet near a particular pixel electrode, and conversely, the capacitance impedance will differ by an order of magnitude in both cases. (If the EWoD device manipulates non-water droplets in the water medium, these differences will still occur, although they will, of course, reverse, and in the absence of the non-water droplet, the water medium will exhibit higher capacitance and lower impedance). This capacitance difference is sufficient to detect the presence or absence of a water droplet using the circuitry described below.

[0043] As shown in Figure 6, the capacitor formed by the pixel electrodes and the common electrode has a capacitance specified by "Cs" and an impedance specified by "Zsense". In the measurement mode illustrated in Figure 6, the excitation voltage Vex is applied to the common electrode 206 via amplifier 404; this excitation voltage (which may be periodic or sinusoidal) should be small enough to avoid interfering with any droplets near the pixel electrodes. The excitation voltage Vex is also supplied from amplifier 404 to one plate of capacitor 308 via line 310. The source of transistor 306 (within capacitance measurement circuit 432) is connected to the input of high-impedance sensing amplifier 440 via closed switch 434, and the output of high-impedance sensing amplifier 440 is fed to an analog-to-digital converter ("ADC", not shown in Figure 6). The voltage at the input of amplifier 440 is denoted as "Vsense", and amplifier 440 is used only to scale this voltage to the range of the ADC, typically 0 to 5 V. The source of transistor 306 is also connected to capacitor 442 via closed switch 436. Capacitor 442 has a capacitance designated as "Zscale," and the other side of capacitor 442 is grounded. (A resistor can be used instead of capacitor 442 if necessary, as its function is only to provide a known impedance in a half-bridge, as discussed below.) Switches 434 and 436 are closed when the device is in its measurement mode, but open when the device is in its drive mode to prevent the capacitance measurement circuit 432 from interfering with the drive mode. Switches 434 and 436 can be replaced by a single switch if necessary. The plate of capacitor 442 is also connected to the input of current-sensing amplifier 444, the output of which is fed to an analog-to-digital converter (not shown in Figure 6). Amplifier 444 measures the alternating current flowing through the pixel electrode to ensure it is low enough not to interfere with any droplets. Not every source line needs to be equipped with amplifier 444.

[0044] During the measurement mode of device 400, gate driver 102 scans the pixel electrode array in the same manner as during drive mode (although the scan rates may differ between the two modes) such that only one array of transistors 306 is conductive at any given time. Therefore, only one pixel electrode 205 is connected to its associated capacitance measurement circuit 432 at any given time. The capacitor formed by pixel electrode 205, droplet 204 (or oil dielectric), and common electrode 206, and capacitor 442 form a half-bridge or capacitive voltage divider. Based on the fundamental principle: Vsense = Vex.Zscale.(Zsense + Zscale) -1 In order to accurately measure Zsense, and therefore Cs, Zscale must be of the same order of magnitude as Zsense.

[0045] As can be seen from the aforementioned equations, Zsense can, in principle, be calculated using either the voltage measured solely by amplifier 440 or the current sensed solely by amplifier 444. However, in practice, the presence of both amplifiers is highly desirable because the phase angle and / or time difference between the voltage and current of the signal is typically known, and obtaining two independent measurements from each amplifier allows for this. Having two amplifiers also alleviates the problem of needing to know the accurate sense impedance in the circuit.

[0046] It should be noted that the aforementioned equations do not account for the drain-source impedance of transistor 306, which is connected in series with Zsense and Zscale. This drain-source impedance reduces the device's sensitivity to detecting droplets between the pixel electrode and the common electrode, but this effect is unlikely to affect droplet detection. The trend of TFT impedance changing over time (over hundreds of operating hours) complicates matters further, especially if the transistors are cyclically operated at high percentage loads, such as in adaptive gate drive configurations. If necessary, small sample sizes of the TFT can be equipped with impedance measurement circuitry to provide an average drain-source impedance value, and the Zsense calculation can be modified accordingly.

[0047] However, the reality is not so simple. To accurately measure Zsense, it is necessary to eliminate all capacitances except Zscale from the actively sensed source line. Stray capacitances affecting Zsense measurement include (i) the capacitance between the sensed source line and two adjacent parallel source lines; and (ii) the gate-source capacitance of all non-conductive transistors on the actively sensed source line (although the gate-source capacitance of any individual transistor is very small, the combined effect of hundreds of non-conductive transistors on the actively sensed source line is (typically) not negligible).

[0048] Figure 7 illustrates an EWoD device (generally designated 700) similar to the device 400 shown in Figure 4, but modified to eliminate the effect of source-to-source capacitance. For ease of explanation, amplifier 440 is omitted from Figure 7. As shown in Figure 7, in addition to the connections previously described, each source line is connected to a three-way switch assembly 750. If the source line is currently undergoing capacitance measurement (source line 302B in Figure 7), the center switch of the switch assembly 750 is closed and the two external switches are open, thus source line 302 is connected to one input of buffer amplifier 752. The output of amplifier 752 is fed to (i) its own second input; (ii) via line 756 as a virtual ground signal to gate driver 102; and (iii) via its switch assembly 750 through appropriately closed external switches to adjacent source lines 302A and 302C. It should be noted that the amplifier 752 associated with source lines 302A and 302C is disconnected from its source lines, so that the voltages of source lines 302A and 302C are controlled only by the amplifier 752 associated with source line 302B. Source lines 302A and 302C thereby maintain the same voltage as source line 302B, thus eliminating any source-to-source capacitance between the sensed source line 302B and the adjacent parallel source lines 302A and 302C.

[0049] In the device 700 shown in Figure 7, only one-third of the pixel electrodes in the selected column can have their capacitance measured at any time. In a typical display with hundreds of source lines, the pattern of source lines 302A, 302B, and 302C will be repeated in each group of three adjacent source lines, and only the pixels connected to source line 302B will have their capacitance measured. Note in Figure 7 that the switch 436 provided between each source line and its associated capacitor (or resistor) 442 is closed only for that source line 302B; the switches for source lines 302A and 302C remain open to prevent unwanted voltage changes on those source lines 302A and 302C. After the capacitance on source line 302B has been measured, the position of the switch assembly 750 is changed so that the capacitance of source line 302C can be measured, and then the capacitance of source line 302A is measured last. Since three separate capacitance measurements must be performed on each column of pixels, a slower scan rate may be required in measurement mode than in the drive mode of the device.

[0050] The device 700 shown in Figure 7 also allows for the reduction or elimination of the gate-source capacitance effect in non-conductive transistors. As noted, the output from amplifier 752 is fed to gate driver 102 via line 756 as a gate driver virtual ground. This virtual ground is then applied to the gates of all transistors except those in the selected column, thereby placing the gates of non-conductive transistors connected to source line 302B at the same voltage as the sources of the same transistors, thus eliminating the gate-source capacitance effect in these non-conductive transistors. Adding a virtual ground to the gate driver integrated circuit does increase some complexity when making digital data connections. Some level shifting or isolation circuitry should be added to the gate driver 102 to allow its ground signal to be driven by the buffer amplifier 752, as illustrated in Figure 7.

[0051] The sensing amplifiers 440 and 444 (Figure 6) and the buffer amplifier 752 (Figure 7) should have low parasitic current and high bandwidth to perform measurements at a better frequency in the 100 kHz range without using parasitic current to interfere with capacitance measurements.

[0052] One problem with the devices shown in Figures 6 and 7 is that they require each source line to have two separate ADCs, thus requiring 400 ADCs for a typical 200-line display. The number of ADCs can be effectively reduced by connecting the outputs of multiple amplifiers to a single ADC, as illustrated in Figure 8, where the output from amplifier 440 is fed to one input of analog switch 802 and the output from amplifier 444 is fed to one input of analog switch 804. It should be understood that other amplifiers 440 and 444 are connected to other inputs of switches 802 and 804. The outputs from switches 802 and 804 are fed to individual ADCs 806 and 808, respectively.

[0053] Those skilled in circuit design will recognize that the number of components required for the device of this invention can be further reduced by the "upstream" of the multiplexing amplifiers 440 and 444 rather than the "downstream" of these amplifiers, as shown in FIG8, or in other words, by inserting an analog switch between the source line and the amplifier 440 and between the capacitor 442 and the amplifier 444, thus allowing a single amplifier 440 and 444 to serve multiple source lines. However, the resulting cost savings in amplifiers must be balanced against the resulting speed loss and potential loss of accuracy.

[0054] Figure 9 shows a device similar to the device 700 shown in Figure 7 (generally designated 900), but this device lacks the line 756 connecting the buffer amplifier 752 to the gate driver 102. Device 900 operates in a very similar manner to device 700 and thus effectively eliminates errors caused by source-to-source capacitance, but it cannot handle the effect of gate-to-source capacitance in non-conductive transistors with sensed source lines. Therefore, device 900 can be useful when gate-to-source capacitance in such non-conductive transistors is not a major problem, as it does not require modification of the conventional gate driver 102.

[0055] Figure 10 shows a device similar to device 900 shown in Figure 9 (generally designated as 1000), but in which the three-way switch 750 shown in Figure 9 is replaced by a four-way switch 1050, wherein the fourth switch grounds the associated source line. Device 1000 operates in a similar manner to device 900, and the sensed source line 302B is connected to capacitor 442 and amplifier 444 via switch 754. However, adjacent source lines 302A and 302C are grounded via switch 1050 instead of being maintained at the same voltage as source line 302B. Grounding adjacent source lines in this manner reduces the sensitivity of device 1000 compared to device 900, because this sensitivity, or scaling, is altered by the parasitic capacitance between the sensed source line 302B on one hand and the grounded adjacent source lines 302A and 302C on the other hand. However, if the adjacent source lines 302A and 302C are only allowed to float at high impedance, the device 1000 is still much more accurate.

[0056] In the operating mode shown in Figure 10, since adjacent source lines 302A and 302C are grounded, buffer amplifier 752 is obviously redundant. However, by providing such amplifiers and connecting the output of one buffer amplifier 752 to a portion of the switch 1050 of the adjacent source lines, the device 1000 can operate with the adjacent source lines 302A and 302C grounded, or in the same manner as device 900, wherein the adjacent source lines 302A and 302C are maintained at the same voltage as the sensed source line 302B.

[0057] Finally, Figure 11 shows a device (generally designated as 1100) that is again similar to the device 900 shown in Figure 9, but in which each source line is equipped with both a current sensing amplifier 444 and a voltage sensing amplifier 440, both of which operate in the same manner as described above with reference to Figure 6.

[0058] As can be seen from the foregoing, the present invention provides an apparatus and method for detecting the position of droplets in an EWoD device without substantially modifying the second substrate or adding additional circuitry at each pixel of the TFT array. The present invention allows the use of conventional backplanes in EWoD devices, and such backplanes can be manufactured using amorphous silicon technology. The only component requiring modification to implement the present invention in an EWoD device is the source driver (and possibly the gate driver if a gate driver is to be used for virtual grounding, as described above with reference to FIG. 7), and the corresponding integrated circuitry is typically produced using advanced silicon methods. Modifications to the driver circuitry required to implement the present invention are routine for those skilled in the art of integrated circuitry.

[0059] Terms and conditions 1. A digital microfluidic device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and A component for introducing fluid between the first and second substrates and creating a microfluidic region between the first and second substrates. At least one of the source lines is equipped with a capacitance measuring element, which is arranged to measure the capacitance between at least one of the first electrodes connected thereto and the at least one second electrode, thereby determining the presence or absence of a fluid droplet between the at least one of the first electrodes and the at least one second electrode.

[0060] 2. The digital microfluidic device of clause 1, further comprising a component for applying an alternating voltage to the at least one second electrode.

[0061] 3. The digital microfluidic device of clause 1, wherein the microfluidic region between the first and second substrates is filled with a first fluid and the fluid introduction member is arranged to introduce a second fluid that is immiscible with the first fluid.

[0062] 4. The digital microfluidic device as described in clause 3, wherein the second fluid system is aqueous and the first fluid system is non-aqueous.

[0063] 5. The digital microfluidic device of clause 4, wherein the first dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

[0064] 6. The digital microfluidic device of clause 4, wherein the second dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

[0065] 7. The digital microfluidic device of clause 2, wherein the capacitance measurement component includes a capacitor connected to a source line and a component for measuring the voltage drop across the capacitor.

[0066] 8. A method for determining the presence or absence of a droplet adjacent to a specific first electrode in a digital microfluidic device, the device comprising: A first substrate includes a plurality of first electrodes, each having an associated transistor, the first substrate further includes a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; A second substrate, which is spaced apart from the first substrate and includes at least one second electrode and a second dielectric layer covering the second electrode; and A component for introducing fluid between the first and second substrates and creating a microfluidic region between the first and second substrates. The method includes measuring the capacitance between the particular first electrode and the at least one second electrode.

[0067] 9. The method of clause 8, wherein the capacitance is measured by applying an alternating voltage to the at least one second electrode, connecting the capacitor to the particular first electrode, and measuring the voltage drop across the capacitor.

[0068] 10. The method of clause 8, wherein the plurality of source lines are arranged in parallel to each other and the method further includes driving the two source lines closest to the source line connected to the particular first electrode at the same voltage as the source line connected to the particular first electrode.

[0069] 100: EWoD device 102: Gate (or column) driver 104: Source (or data) driver 202: Oil (or other hydrophobic fluid) / oil layer 204: Droplets / Water Droplets 205: First or pixel electrode 206: Second or common electrode 207: Hydrophobic coating 208: Dielectric layer 302: Data line / Source line 302A: Source Line 302B: Source line 302C: Source Line 304: Gate Line / Selection Line 306: Transistor 308: Capacitor 310: Capacitor wire / line 400: EWoD Device 402: Excitation Voltage 404: Amplifier 420: Source Driver 422: Shift register 424: Switch 426: Data Bus 430: Capacitance Measurement Assembly 432: Capacitance Measurement Circuit 434: Switch 436: Switch 440: High Impedance Sensing Amplifier / Amplifier / Sensing Amplifier / Voltage Sensing Amplifier 442: Capacitor 444: Current sensing amplifier / amplifier / sensing amplifier 520: Single-chip source driver 700: Device 750: Three-way switch assembly / switch assembly / three-way switch 752: Buffer Amplifier / Amplifier 756: Line 802: Analog Switch 804: Analog Switch 806: Single ADC 808: Single ADC 900: Device 1000: Device 1050: Four-way switch / switch 1100: Device R1: Storage R2: Storage R3: Storage R4: Storage R5: Storage R6: Storage R7: Storage R8: Storage

Claims

1. A digital microfluidic device comprising: a first substrate including a plurality of first electrodes, each having an associated transistor, the first substrate further including a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; a second substrate spaced apart from the first substrate and including at least one second electrode and a second dielectric layer covering the second electrode; and a microfluidic region between the first and second substrates, wherein the at least one source line is arranged to measure the capacitance between at least one of the first electrodes connected thereto and the at least one second electrode, thereby determining the presence or absence of fluid droplets between the first electrodes and the at least one second electrode.

2. The digital microfluidic device of claim 1, wherein an alternating voltage is applied to the at least one second electrode.

3. The digital microfluidic device of claim 1 or 2, wherein each first electrode is attached to a capacitor via the transistor.

4. The digital microfluidic device of claim 1 or 2, wherein the microfluidic region between the first and second substrates is filled with a first fluid and contains droplets of a second fluid that is immiscible with the first fluid.

5. The digital microfluidic device of claim 4, wherein the second fluid system is aqueous and the first fluid system is non-aqueous.

6. The digital microfluidic device of claim 1 or 2, wherein the first dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

7. The digital microfluidic device of claim 1 or 2, wherein the second dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

8. The digital microfluidic device as claimed in claim 1 or 2, which includes a capacitor connected to the source line.

9. A method for determining the presence or absence of a droplet adjacent to a specific first electrode in a digital microfluidic device, the device comprising: a first substrate including a plurality of first electrodes, each having an associated transistor, the first substrate further including a plurality of source lines connected to the plurality of first electrodes via transistors associated with the plurality of first electrodes, and a first dielectric layer covering the first electrodes and their associated transistors; a second substrate spaced apart from the first substrate and including at least one second electrode and a second dielectric layer covering the second electrode; and a microfluidic region between the first and second substrates, the method comprising measuring the capacitance between the specific first electrode and the at least one second electrode using the source lines connected to the specific first electrode.

10. The method of claim 9, wherein the capacitance is measured by applying an alternating voltage to the at least one second electrode, connecting the capacitor to the particular first electrode, and measuring the voltage drop across the capacitor.

11. The method of claim 9 or 10, wherein the plurality of source lines are arranged in parallel to each other and the method further includes driving the two source lines closest to the source line connected to the particular first electrode at the same voltage as the source line connected to the particular first electrode.

12. The method of claim 9 or 10, wherein the microfluidic region between the first and second substrates is filled with a first fluid and contains droplets of a second fluid that is immiscible with the first fluid.

13. The method of claim 12, wherein the second flow system is aqueous and the first flow system is non-aqueous.

14. The method of claim 9 or 10, wherein the first dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

15. The method of claim 9 or 10, wherein the second dielectric layer is hydrophobic or has a hydrophobic layer superimposed thereon.

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