Feedback system for parallel droplet control in a digital microfluidic device

Abstract

Digital microfluidics apparatuses (e.g., devices and systems) configured to determine provide feedback on the location, rate of movement, rate of evaporation and/or size (or other physical characteristic) of one or more, and preferably more than one, droplet in the gap region of a digital microfluidics (DMF) apparatus.

Claims

1. A digital microfluidic (DMF) apparatus, the apparatus comprising: a plurality of actuation electrodes configured to move one or more droplets when actuated; a ground electrode; a voltage source coupled to the ground electrode; a plurality of sensing circuits, each of the plurality of sensing circuits comprising a charging circuit and a discharging circuit, wherein each sensing circuit is electrically connected to a corresponding actuation electrode of the plurality of actuation electrodes, and wherein each sensing circuit is configured to detect a charged voltage of a capacitor in the charging circuit of the sensing circuit; and a controller configured to alternately provide voltage from the voltage source to the ground electrode and one or more actuation electrodes of the plurality of actuation electrodes to move the one or more droplets, further wherein the controller is configured to sense, in parallel, one or more properties of the one or more droplets based on input from the plurality of sensing circuits when applying voltage to the ground electrode.

2. The apparatus of claim 1, wherein the sensed one or more properties include at least one of: a location of the one or more droplets relative to the plurality of actuation electrodes, a rate of movement of the one or more droplets, a rate of evaporation of the one or more droplets, or a size of the one or more droplets.

3. The apparatus of claim 1, wherein the discharging circuit comprises a transistor and a ground.

4. The apparatus of claim 1, wherein the charging circuit comprises a capacitor and a diode.

5. The apparatus of claim 1, further comprising an analog-to-digital converter (ADC) wherein the ADC is configured to detect the charged voltage of the charging circuit.

6. The apparatus of claim 5, wherein the controller is configured to sequentially activate the discharging circuit, then the charging circuit, and to receive the charged voltage of the charging circuit from the ADC in parallel for all of the sensing circuits of the plurality of sensing circuits.

7. The apparatus of claim 1, further comprising a forward/reverse switch connected between the voltage source, the ground electrode, and the plurality of actuation electrodes, wherein the controller is configured to operate the forward/reverse switch to switch between providing voltage to one or more of the plurality of electrodes and the ground electrode.

8. The apparatus of claim 1, further comprising a plurality of electrode switches, wherein each electrode switch of the plurality of electrode switches is connected to an actuation electrode of the plurality of actuation electrodes and is controlled by the controller through a switch controller to apply voltage from the voltage source to the actuation electrode.

9. The apparatus of claim 1, wherein the controller is configured to compare a voltage sensed by each of the plurality of sensing circuits to a threshold voltage value to determine the property of the one or more droplets.

10. The apparatus of claim 1, wherein the controller is configured to compare a voltage sensed by each of the plurality of sensing circuits to a predetermined voltage value or range of voltage values to determine the property of the one or more droplets wherein the property comprises a size of one or more droplets.

11. A method of simultaneously determining one or more properties of multiple drops in a digital microfluidics (DMF) apparatus, the method comprising: applying voltage to a plurality of actuation electrodes to move one or more droplets within a gap between the plurality of actuation electrodes and one or more ground electrodes; applying voltage to one or more of the one or more ground electrodes; concurrently sensing, in a plurality of sensing circuits, wherein each sensing circuit of the plurality of sensing circuits is associated with an actuation electrode of the plurality of actuation electrodes, a charging voltage while applying voltage to the one or more ground electrodes; and determining the one or more properties of the one or more droplets based on the sensed charging voltages by comparing the sensed charging voltages to a predetermined value or range of values, wherein the one or more properties includes one or more of: a location of the one or more droplets relative to the plurality of actuation electrodes, a rate of movement of the one or more droplets, a rate of evaporation of the one or more droplets, or a size of the one or more droplets.

12. The method of claim 11, wherein applying voltage to the plurality of actuation electrodes and applying voltage to the one or more ground electrodes comprises applying voltage from the same high voltage source.

13. The method of claim 11, wherein applying voltage to the plurality of actuation electrodes comprises sequentially applying voltage to adjacent actuation electrodes.

14. The method of claim 11, further comprising re-applying voltage to one or more of the plurality of actuation electrodes based on a determined location of the one or more droplets.

15. The method of claim 11, wherein applying voltage to one or more ground electrodes comprises applying voltage to the one or more ground electrodes without applying voltage to the plurality of actuation electrodes.

16. The method of claim 11, further comprising discharging voltage in each of the sensing circuits prior to applying voltage to the one or more ground electrodes.

17. The method of claim 11, further comprising charging a capacitor in each of the sensing circuits of the plurality of sensing circuits when applying voltage to the one or more ground electrodes.

18. The method of claim 11, further comprising discharging voltage in each of the sensing circuits prior to applying voltage to the one or more ground electrodes and then charging a capacitor in each of the sensing circuits in the plurality of sensing circuits when applying voltage to the one or more ground electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which:

(2) FIG. 1A is a schematic of one example of a digital microfluidic (DMF) apparatus, from a top perspective view.

(3) FIG. 1B shows an enlarged view through a section through a portion of the DMF apparatus shown in FIG. 1A, taken through a thermally regulated region (thermal zone).

(4) FIG. 1C shows an enlarged view through a second section of a region of the (in this example, air-matrix) DMF apparatus of FIG. 1A; this region includes an aperture through the bottom plate and an actuation electrode, and is configured so that a replenishing droplet may be delivered into the air gap of the air-matrix DMF apparatus from the aperture (which connects to the reservoir of solvent, in this example shown as an attached syringe).

(5) FIGS. 1D and 1E illustrate schematics of a prior art droplet control system. FIG. 1D shows an overview schematic of a droplet control system, showing the relationships between the PC, the function generator and amplifier, the relay box, the DMF device, and the measurement circuit. FIG. 1E illustrates a detailed schematic and circuit model of a DMG device and the measurement/feedback circuit, adapted from Shih, S. C. C.; Fobel, R.; Kumar, P.; Wheeler, A. R. A, Feedback Control System for High-Fidelity Digital Microfluidics. Lab Chip 2011 (11), 535-540.

(6) FIG. 2A is an example of a DMF apparatus as described herein, configured to determine (in parallel) the location of one or more droplets in the gap between the plates, e.g., relative to the actuation electrodes.

(7) FIG. 2B is another schematic illustration of a DMF apparatus with parallel droplet detection as described herein, illustrating in particular a control system for manipulation of droplets on the DMF apparatus.

(8) FIG. 3 shows a schematic illustration of another variation of a digital microfluidic device design including concurrent (e.g., parallel) determination of the locations of multiple droplets in a DMF apparatus.

(9) FIG. 4 illustrates droplet actuation using a digital microfluidic device with corresponding photoMOS relay operations.

(10) FIG. 5 illustrates one example of a switch controller configuration; in this example, the switches include photoMOS switches, and the sensing circuit includes a discharging and a charging block. In this example the sensing circuit may also include an analog-to-digital converter (ADC).

(11) FIG. 6 is one example of a method for forward streaming (which may be embodied, for example, as an algorithm) for droplet motion control and reverse stream algorithm for droplet feedback (e.g., sensing).

(12) FIG. 7 illustrates charging and discharging timing diagrams based on an apparatus as described herein.

(13) FIG. 8 shows a schematic of an electrical circuit for the ‘Forward Stream’ mode for actuating a droplet by an electrode.

(14) FIG. 9 is a schematic of one example of an electrical circuit for the ‘Reverse Stream’ mode for detecting the presence of a droplet on an electrode. Switch controller reads different ADC values for the two scenarios: 1) a droplet present on an electrode and 2) a droplet missing from an electrode.

(15) FIG. 10 illustrates one method of detecting voltage value depends on the size of the droplet occupying the electrode pad.

DETAILED DESCRIPTION

(16) Described herein are Digital Mircrofluidics (DMF) apparatuses (e.g., devices and systems) that may be used for multiplexed processing and routing of samples and reagents to and from channel-based microfluidic modules that are specialized to carry out all other needed functions. These DMF apparatuses may be air-matrix (e.g., open air), enclosed and/or oil-matrix DMF apparatuses and methods of using them. In particular, described herein are DMF apparatuses and methods of using them for concurrent, e.g., simultaneous, parallel, etc., determining of droplet properties (such as location relative to the apparatus, rate of movement of the droplet, rate of evaporation of the droplet, size of the droplet, etc.). This is possible because the apparatus may include a plurality of individual sensing circuits, each connected to a particular actuating electrode, and a controller that switches between applying voltage to the actuating electrodes, and subsequently applying voltage to the ground electrode(s) opposite from the plurality of actuating electrodes (and sensing circuits). The controller may also receive the sensing circuit data and compare the results (e.g., charging voltage data) to predetermined values or ranges of values to infer the location, size, rate of movement, etc. of droplets. Because of the arrangement of elements described herein, which may be incorporated into any of a variety of DMF apparatuses, the resulting data may be used for feedback, including real-time feedback, for controlling and monitoring the operation of a DMF apparatus.

(17) For example, a DMF may integrate channel-based microfluidic modules. The apparatuses (including systems and devices) described herein may include any of the features or elements of previously described DMF apparatuses, such as actuating electrodes, thermal regulators, wells, reaction regions, lower (base or first) plates, upper (second) plates, ground(s), etc.

(18) As used herein, the term, “thermal regulator” (or in some instances, thermoelectric module or TE regulator) may refer to thermoelectric coolers or Peltier coolers and are semi-conductor based electronic component that functions as a small heat pump. By applying a low voltage DC power to a TE regulator, heat will be moved through the structure from one side to the other. One face of the thermal regulator may thereby be cooled while the opposite face is simultaneously heated. A thermal regulator may be used for both heating and cooling, making it highly suitable for precise temperature control applications. Other thermal regulators that may be used include resistive heating and/or recirculating heating/cooling (in which water, air or other fluid thermal medium is recirculated through a channel having a thermal exchange region in thermal communication with all or a region of the air gap, e.g., through a plate forming the air gap).

(19) As used herein, the term “temperature sensor” may include resistive temperature detectors (RTD) and includes any sensor that may be used to measure temperature. An RTD may measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The RTD element may be made from a pure material, typically platinum, nickel or copper or an alloy for which the thermal properties have been characterized. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature.

(20) As used herein, the term “digital microfluidics” may refer to a “lab on a chip” system based on micromanipulation of discrete droplets. Digital microfluidic processing is performed on discrete packets of fluids (reagents, reaction components) which may be transported, stored, mixed, reacted, heated, and/or analyzed on the apparatus. Digital microfluidics may employ a higher degree of automation and typically uses less physical components such as pumps, tubing, valves, etc.

(21) As used herein, the term “cycle threshold” may refer to the number of cycles in a polymerase chain reaction (PCR) assay required for a fluorescence signal to cross over a threshold level (i.e. exceeds background signal) such that it may be detected.

(22) The DMF apparatuses described herein may be constructed from layers of material, which may include printed circuit boards (PCBs), plastics, glass, etc. Multilayer PCBs may be advantageous over conventional single-layer devices (e.g., chrome or ITO on glass) in that electrical connections can occupy a separate layer from the actuation electrodes, affording more real estate for droplet actuation and simplifying on-chip integration of electronic components.

(23) A DMF apparatus may be any dimension or shape that is suitable for the particular reaction steps of interest. Furthermore, the layout and the particular components of the DMF device may also vary depending on the reaction of interest. While the DMF apparatuses described herein may primarily describe sample and reagent reservoirs situated on one plane (that may be the same as the plane of the air gap in which the droplets move), it is conceivable that the sample and/or reagent reservoirs may be on different layers relative to each other and/or the air gap, and that they may be in fluid communication with one another.

(24) FIG. 1A shows an example of the layout of a typical DMF apparatus 100. In general, this air-matrix DMF apparatus includes a plurality of unit cells 191 that are adjacent to each other and defined by having a single actuation electrode 106 opposite from a second-plate ground electrode 102; each unit cell may any appropriate shape, but may generally have the same approximate surface area. In FIG. 1A, the unit cells are rectangular. The droplets (e.g., reaction droplets) fit within the air gap between the first 153 and second 151 plates (shown in FIGS. 1A-1C as top and bottom plates). The overall air-matrix DMF apparatus may have any appropriate shape, and thickness. FIG. 1B is an enlarged view of a section through a thermal zone of the air-matrix DMF shown in FIG. 1A, showing layers of the DMF device (e.g., layers forming the bottom plate). In general, the DMF device (e.g., bottom plate) includes several layers, which may include layers formed on printed circuit board (PCB) material; these layers may include protective covering layers, insulating layers, and/or support layers (e.g., glass layer, ground electrode layer, hydrophobic layer; hydrophobic layer, dielectric layer, actuation electrode layer, PCB, thermal control layer, etc.). The air-matrix DMF apparatuses described herein also include both sample and reagent reservoirs, as well as a mechanism for replenishing reagents.

(25) In the example shown in FIGS. 1A-1C, a top plate 101, in this case a glass or other top plate material provides support and protects the layers beneath from outside particulates as well as providing some amount of insulation for the reaction occurring within the DMF device. The top plate may therefore confine/sandwich a droplet between the plates, which may strengthen the electrical field when compared to an open air-matrix DMF apparatus (without a plate). The upper plate (the second plate in this example) may include the ground electrode and may be transparent or translucent; for example, the substrate of the first plate may be formed of glass and/or clear plastic. Adjacent to and beneath the substrate (e.g., glass) is a ground electrode for the DMF circuitry (ground electrode layer 102). In some instances, the ground electrode is a continuous coating; alternatively multiple, e.g., adjacent, ground electrodes may be used. Beneath the grounding electrode layer is a hydrophobic layer 103. The hydrophobic layer 103 acts to reduce the wetting of the surfaces and aids with maintaining the reaction droplet in one cohesive unit.

(26) The first plate, shown as a lower or bottom plate 151 in FIGS. 1A-1C, may include the actuation electrodes defining the unit cells. In this example, as with the first plate, the outermost layer facing the air gap 104 between the plates also includes a hydrophobic layer 103. The material forming the hydrophobic layer may be the same on both plates, or it may be a different hydrophobic material. The air gap 104 provides the space in which the reaction droplet is initially contained within a sample reservoir and moved for running the reaction step or steps as well as for maintaining various reagents for the various reaction steps. Adjacent to the hydrophobic layer 103 on the second plate is a dielectric layer 105 that may increase the capacitance between droplets and electrodes. Adjacent to and beneath the dielectric layer 105 is a PCB layer containing actuation electrodes (actuation electrodes layer 106). As mentioned, the actuation electrodes may form each unit cell. The actuation electrodes may be energized to move the droplets within the DMF device to different regions so that various reaction steps may be carried out under different conditions (e.g., temperature, combining with different reagents, etc.). A support substrate 107 (e.g., PCB) may be adjacent to and beneath (in FIGS. 1B and 1C) the actuation electrode layer 106 to provide support and electrical connection for these components, including the actuation electrodes, traces connecting them (which may be insulated), and/or additional control elements, including the thermal regulator 155 (shown as a TEC), temperature sensors, optical sensor(s), etc. One or more controllers 195 for controlling operation of the actuation electrodes and/or controlling the application of replenishing droplets to reaction droplets may be connected but separate from the first 153 and second plates 151, or it may be formed on and/or supported by the second plate. In FIGS. 1A-1C the first plate is shown as a top plate and the second plate is a bottom plate; this orientation may be reversed. A source or reservoir 197 of solvent (replenishing fluid) is also shown connected to an aperture in the second plate by tubing 198.

(27) As mentioned, the air gap 104 provides the space where the reaction steps may occur, providing areas where reagents may be held and may be treated, e.g., by mixing, heating/cooling, combining with reagents (enzymes, labels, etc.). In FIG. 1A the air gap 104 includes a sample reservoir 110 and a series of reagent reservoirs 111. The sample reservoir may further include a sample loading feature for introducing the initial reaction droplet into the DMF device. Sample loading may be loaded from above, from below, or from the side and may be unique based on the needs of the reaction being performed. The sample DMF device shown in FIG. 1A includes six sample reagent reservoirs where each includes an opening or port for introducing each reagent into the respective reservoirs. The number of reagent reservoirs may be variable depending on the reaction being performed. The sample reservoir 110 and the reagent reservoirs 111 are in fluid communication through a reaction zone 112. The reaction zone 112 is in electrical communication with actuation electrode layer 106 where the actuation electrode layer 106 site beneath the reaction zone 112.

(28) The actuation electrodes 106 are depicted in FIG. 1A as a grid or unit cells. In other examples, the actuation electrodes may be in an entirely different pattern or arrangement based on the needs of the reaction. The actuation electrodes are configured to move droplets from one region to another region or regions of the DMF device. The motion and to some degree the shape of the droplets may be controlled by switching the voltage of the actuation electrodes. One or more droplets may be moved along the path of actuation electrodes by sequentially energizing and de-energizing the electrodes in a controlled manner In the example of the DMF apparatus shown, a hundred actuation electrodes (forming approximately a hundred unit cells) are connected with the seven reservoirs (one sample and six reagent reservoirs). Actuation electrodes may be fabricated from any appropriate conductive material, such as copper, nickel, gold, or a combination thereof.

(29) All or some of the unit cells formed by the actuation electrodes may be in thermal communication with at least one thermal regulator (e.g., TEC 155) and at least one temperature detector/sensor (RTD 157). In addition, each of the actuation electrodes shown may also include a sensing circuit for providing feedback and on droplet properties (including location, size, etc.) at times during the operation of the apparatus.

(30) For example, FIGS. 2A and 2B illustrate examples of an apparatus providing simultaneous analysis of droplet properties. In this example, a new feedback system has been developed to monitor the position and the size of droplets on a digital microfluidic device.

(31) For example, FIG. 2A illustrates an apparatus configured as a digital microfluidic (DMF) apparatus with parallel droplet detection. The apparatus in this example includes a first plate (lower plate 209) having a first hydrophobic layer and a second plate 207 having a second hydrophobic layer. The generic example show in FIG. 2A also includes a plurality of actuation electrodes 213 in the first plate (any number of actuation electrodes may be included). As mentioned, these electrodes may be formed in or under the first plate, e.g., may be part of this first plate, which may include different layers and/or regions. The example system shown in FIG. 2A also includes one or more ground electrodes in the second plate. For example, a single second-plate ground electrode may be opposite and across the gap, e.g., air gap) from the actuation electrodes. In FIG. 2A the controller 201 is connected to (and controls) a voltage source 205 and may be connected to (and control) forward/reverse switch 203 that is connected to a ground, the voltage source 205, the one or more second-plate ground electrodes, and the plurality of actuation electrodes. The forward/reverse switch 203 may be configured to switch a connection between the voltage source and either the one or more second-plate ground electrodes or the plurality of actuation electrodes. The controller 201 may also be connected to (and control) a switch controller 202, which may regulate one or more switches, including (but not limited to): a plurality of electrode switches (223, 224, 225, 226, 227, etc.), and in some variations, a transistor in each of the sensing units 233, 234, 235, 236, 237, etc. The apparatus shown in FIG. 2A also includes a plurality of sensing circuits (233, 234, 235, 236, 237, etc.), and a sensing circuit from this plurality of sensing circuits may be connected between each electrode and the electrode switch. The plurality of electrode switches (223, 224, 225, 226, 227, etc.) may be connected to the switch controller 202 (controlling their open/close state) and to the voltage source through the forward/reverse switch. Thus, each actuation electrode may be configured to allow an application of voltage from the voltage source.

(32) As mentioned, the controller 201 and the switch controller 202 in FIG. 2A may be configured to control the forward/reverse switch and the plurality of electrode switches to move one or more droplets within a gap between the first plate and the second plate when the forward/reverse switch connects the voltage source to the plurality of electrodes, and further configured to determine the location (or other property) of one or more droplets relative to the plurality of actuation electrodes based on input from each of the sensing circuits when the forward/reverse switch connects the voltage source to the one or more second-plate ground electrodes.

(33) Droplet motion is generated and controlled by a DMF control system, shown in FIG. 2B, which may comprise: high voltage generator to generate high voltage (HV) actuation signals; switch controller that controls photoMOS relay switches and directs actuation signals to individual electrodes; DMF device.

(34) The DMF controller is the main processor that controls DMF devices and sub-controllers like switch controller and high-voltage generator. In a standard operation mode, a user creates commands in the main controller software to be released to the sub-controllers. Examples of such commands are ON/OFF commands to photoMOS relays, high voltage control commands to the high voltage generator, e.g. signal frequency, waveform (square or sinusoidal), etc. Upon execution, the processor reports the results back to the user including set voltage, frequency, droplet position, electrode pads state, etc. Software for the controller is provided on a host computer, a computer integrated with the controller, or wirelessly.

(35) A DMF device is comprised of two insulating substrates (FIG. 3)—bottom substrate with patterned electrode pads (typically Printed Circuit Board (PCB) with copper electrode pads) and a top substrate with at least one electrically conductive pad (typically floated glass coated with Indium Tin Oxide (ITO)). In a standard design, the conductive pad on the top substrate serves as a ground electrode while the high voltage is provided to the bottom electrodes. The bottom substrate and electrode pads are coated with a dielectric layer on top of which a hydrophobic layer like Teflon is deposited. Similarly, the top substrate is coated with a hydrophobic layer. A droplet is sandwiched between the two substrates that are a few hundred micrometers apart.

(36) To manipulate droplets on the grid of electrodes, the switch controller controls photoMOS relays assigning a high voltage signal to an electrode pad in the vicinity of a droplet. Due to electrostatic forces, the droplet moves to the energized electrode. FIG. 4 shows the photoMOS relay operations, for the movement of a droplet across three electrodes. In the first step (1), a droplet is positioned on an energized electrode. In the second step (2), a user selects a neighboring electrode to which a HV will be assigned with the corresponding photoMOS ON position while the first pad/photoMOS will be OFF. This will result in the droplet movement from the first pad to the second pad. Applying similar steps, selecting the third pad ON and the second pad OFF, the droplet will move from the second pad to the third one.

(37) The present invention, Reverse Stream feedback system, is enabled by adding charging and discharging blocks and the analog to digital converter (ADC) to the circuits between each photoMOS relay and the corresponding electric pad. Discharging block consist of a transistor and a ground, and the charging block comprises a capacitor and diode, as FIG. 5 shows. The transistor is turned ON for discharging and OFF for charging the capacitor. With this configuration our system can work either in Forward Stream mode for moving the droplets or in Reverse Stream mode for detecting droplet position and size. An algorithm encompassing both modes is presented in FIG. 6.

(38) In Forward Stream mode, electrodes are energized for droplet actuation as the main processor sends droplet moving command to switch controller and assigns high voltage to electrode pads through photoMOS relays. During this mode, high voltage ground (HV GND) is connected to the system ground, as shown in FIG. 8. During the Forward Stream, neither charging block nor discharging block is engaged.

(39) After the droplet actuation and the Forward Stream mode, switch controller disables all photoMOS relays and there is no high voltage signal between photoMOS relay and device. The transistor in the discharging block is turned ON to discharge the high voltage lines and the unwanted capacitance on the capacitor. This constitutes discharging time as shown in FIG. 7.

(40) The discharging time is followed by the Reverse Stream mode, when the main controller sends high voltage signal through the glass-ITO to the charging block. During this charging time, the photoMOS and the transistor are OFF so that the sent high voltage can charge the capacitor. If the droplet is present in the air gap the signal/voltage travels through the droplet, and the capacitor will be charged more than when the signal travels through air only in the absence of a droplet, resulting in the higher charged voltage. This is due to the droplet having higher conductivity than air. The switch controller detects the charged voltage through an analog to digital converter (ADC). For example, in the Reverse Stream mode in FIG. 9 two different charged voltage values are reported: a higher value of 2.4V-2.8V for a droplet present in the gap and a lower value of 1.4V-2.0V for an air gap only/absent electrode. After the Reverse Stream is completed, main processor enables high voltage switching and reconnects the high voltage ground (HV GND) and system ground (GND) bringing the system back into the Forward Stream mode for further droplet actuation.

(41) Previously reported DMF feedback systems can only measure one charged voltage (or another electrical parameter) at a single time point. In these systems, there is one common measurement circuit and capacitor for all pads—the charging HV signal is sent through a pad (or multiple pads) to the top substrate and to the capacitor reporting only one feedback value. Even if multiple pads are engaged and measured there is only one voltage output. To obtain multiple pad reading the resulting charged voltage has to be measured for each pad sequentially making the DMF operations slow and inefficient. On contrary, Reverse Stream can read charged signals from different pads at a single time point and hence detect multiple droplets simultaneously as each pad is supplied with its own charging block, capacitor and the ADC. This makes Reverse Stream feedback system more advantageous over the prior art as digital microfluidic devices are typically used to miniaturize complex biochemistry protocols that require multiple, parallel droplet manipulations.

(42) Applications of the ‘Reverse Stream’ Feedback System

(43) The Reverse Stream feedback system reports a voltage value dependent on a droplet presence on an electrode pad. If a droplet occupies an electrode pad through which the measuring signal is sent through, the capacitor gets charged more and the reported voltage is significantly higher than in the case of an absent droplet when the measuring signal is sent though the air gap. This is due to the difference between the conductivities of the two media—air and water.

(44) We have also observed that the reported voltage value varies with the droplet base area size covering the electrode pad—the more area has been covered by a droplet, the higher the voltage reading is (FIG. 10). The sensitivity of our feedback system allows not only simple Yes/No answer to the question of a droplet presence on an electrode pad but can also help determine how much of an area is occupied by a droplet.

(45) The main use of the feedback system is to correct droplet motion. If the detected voltage indicates is below the threshold value, indicating not fully covered electrode, the high voltage signal can be reapplied until the threshold voltage has been reached. The threshold voltage indicates full coverage of the electrode and successful droplet actuation.

(46) Additionally, the information about the area covered by a droplet can be used to determine evaporation rate of a stationary droplet. With evaporation, the base area of the droplet reduces and hence the detected voltage. The measured evaporation rate can be used to trigger evaporation management methods like droplet replenishment. For example, if the feedback voltage readout indicates that 70% of the electrode area is covered by a droplet, i.e. 30% of the droplet has evaporated, a supplementing droplet may be actuated to merge with the evaporating droplet to correct for the volume loss.

(47) In another embodiment, Reverse Stream system can be used to determine the composition of a droplet. The conductivity of a droplet depends on its constituents and can affect the charged voltage. With enough sensitivity, the system could potentially differentiate solutions of different conductivities and compositions.

(48) When a feature or element is herein referred to as being “on” another feature or element, it can be directly on the other feature or element or intervening features and/or elements may also be present. In contrast, when a feature or element is referred to as being “directly on” another feature or element, there are no intervening features or elements present. It will also be understood that, when a feature or element is referred to as being “connected”, “attached” or “coupled” to another feature or element, it can be directly connected, attached or coupled to the other feature or element or intervening features or elements may be present. In contrast, when a feature or element is referred to as being “directly connected”, “directly attached” or “directly coupled” to another feature or element, there are no intervening features or elements present. Although described or shown with respect to one embodiment, the features and elements so described or shown can apply to other embodiments. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature.

(49) Terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. For example, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”.

(50) Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms “upwardly”, “downwardly”, “vertical”, “horizontal” and the like are used herein for the purpose of explanation only unless specifically indicated otherwise.

(51) Although the terms “first” and “second” may be used herein to describe various features/elements (including steps), these features/elements should not be limited by these terms, unless the context indicates otherwise. These terms may be used to distinguish one feature/element from another feature/element. Thus, a first feature/element discussed below could be termed a second feature/element, and similarly, a second feature/element discussed below could be termed a first feature/element without departing from the teachings of the present invention.

(52) Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising” means various components can be co-jointly employed in the methods and articles (e.g., compositions and apparatuses including device and methods). For example, the term “comprising” will be understood to imply the inclusion of any stated elements or steps but not the exclusion of any other elements or steps.

(53) As used herein in the specification and claims, including as used in the examples and unless otherwise expressly specified, all numbers may be read as if prefaced by the word “about” or “approximately,” even if the term does not expressly appear. The phrase “about” or “approximately” may be used when describing magnitude and/or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/−0.1% of the stated value (or range of values), +/−1% of the stated value (or range of values), +/−2% of the stated value (or range of values), +/−5% of the stated value (or range of values), +/−10% of the stated value (or range of values), etc. Any numerical values given herein should also be understood to include about or approximately that value, unless the context indicates otherwise. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Any numerical range recited herein is intended to include all sub-ranges subsumed therein. It is also understood that when a value is disclosed that “less than or equal to” the value, “greater than or equal to the value” and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value “X” is disclosed the “less than or equal to X” as well as “greater than or equal to X” (e.g., where X is a numerical value) is also disclosed. It is also understood that the throughout the application, data is provided in a number of different formats, and that this data, represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point “10” and a particular data point “15” are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

(54) Although various illustrative embodiments are described above, any of a number of changes may be made to various embodiments without departing from the scope of the invention as described by the claims. For example, the order in which various described method steps are performed may often be changed in alternative embodiments, and in other alternative embodiments one or more method steps may be skipped altogether. Optional features of various device and system embodiments may be included in some embodiments and not in others. Therefore, the foregoing description is provided primarily for exemplary purposes and should not be interpreted to limit the scope of the invention as it is set forth in the claims.

(55) The examples and illustrations included herein show, by way of illustration and not of limitation, specific embodiments in which the subject matter may be practiced. As mentioned, other embodiments may be utilized and derived there from, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. Such embodiments of the inventive subject matter may be referred to herein individually or collectively by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept, if more than one is, in fact, disclosed. Thus, although specific embodiments have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description.