Patent Description:
Electrowetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by the application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in "<NPL>).

<FIG> shows a part of a conventional EWOD device in cross section. The device includes a lower substrate <NUM>, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes <NUM> (e.g., 12A and 12B in <FIG>) are realized. The electrode of a given array element may be termed the element electrode <NUM>. A liquid droplet <NUM>, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate <NUM> and a top substrate <NUM>. A suitable gap between the two substrates may be realized by means of a spacer <NUM>, and a non-polar surround fluid <NUM> (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet <NUM>. An insulator layer <NUM> disposed upon the lower substrate <NUM> separates the conductive element electrodes 12A, 12B from a first hydrophobic coating <NUM> upon which the liquid droplet <NUM> sits with a contact angle <NUM> represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).

On the top substrate <NUM> is a second hydrophobic coating <NUM> with which the liquid droplet <NUM> may come into contact. Interposed between the top substrate <NUM> and the second hydrophobic coating <NUM> is a reference electrode <NUM>.

The contact angle θ is defined as shown in <FIG>, and is determined by the balancing of the surface tension components between the solid-to liquid (γSL), the liquid-to non-polar surrounding fluid (γLG) and the solid to non-polar surrounding fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by: <MAT>.

In operation, voltages termed the EW drive voltages, (e.g. VT, V<NUM> and V<NUM> in <FIG>) may be externally applied to different electrodes (e.g. reference electrode <NUM>, element electrodes <NUM>, 12A and 12B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating <NUM>. By arranging for different EW drive voltages (e.g. V<NUM> and V<NUM>) to be applied to different element electrodes (e.g. 12A and 12B), the liquid droplet <NUM> may be moved in the lateral plane between the two substrates <NUM> and <NUM>.

Example configurations and operation of EWOD devices are described in the following. <CIT>) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions. <CIT>) further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials. <CIT>) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.

The approach of <CIT> may be termed "Active Matrix Electrowetting on Dielectric" (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:.

To perform various droplet operations in an AM-EWOD device, it can be desirable to be able to sense a droplet property, such as droplet size or location on the array of elements. <CIT>) describes a method, circuit and apparatus for detecting capacitance on a droplet actuator, inter alia, for determining the presence, partial presence or absence of a droplet at an electrode. <CIT>) describes how an impedance (capacitance) sensing function can be incorporated into the array element circuit of each array element of an AM-EWOD device. The impedance sensor circuit may be used for determining the presence and size of liquid droplets present at each electrode in the array.

Upon adequate sensing, droplet operations may then be performed, such as for example holding a droplet position, moving a droplet across the device, splitting a droplet into multiple droplets, mixing different droplets, and others. These various operations may be performed by actuating a suitable pattern of elements on the AM-EWOD device. For example, <CIT>) describes the use of element actuation to hold a droplet isolated from other droplets. Sturmer referenced above describes the use of capacitance detection as real time feedback to determine whether a droplet operation has been successful. <CIT>) describes the use of capacitance detection as real time feedback to control the volume of a droplet being dispensed or split from a reservoir. Other actuation methods for performing various droplet operations are known.

<CIT> proposes a droplet actuator with a droplet formation electrode configuration associated with a droplet operations surface, wherein the electrode configuration comprises one or more electrodes configured to control volume of a droplet during formation of a sub-droplet on the droplet operations surface. Methods of making and using the droplet actuator are also provided.

<CIT> proposes a method for reducing or preventing droplet pinning as the droplet is transported across a boundary between a ground electrode region and a non-ground electrode region on a droplet actuator. The invention also provides a method for reducing or preventing droplet super-movement as the droplet is transported across a boundary between a ground electrode region and a non-ground electrode region on a droplet actuator.

<CIT> proposes a method for splitting a droplet into two or more droplets includes providing a starting droplet on a surface comprising an array of electrodes and a substantially co-planar array of reference elements. The electrode array comprises at least three electrodes comprising a first outer electrode, a medial electrode adjacent to the first outer electrode, and a second outer electrode adjacent to medial electrode. The starting droplet is initially disposed on at least one of the three electrodes and at least partially overlaps at least one other of the three electrodes. The method further includes activating each of the three electrodes to spread the starting droplet across the three electrodes, and de-activating the medial electrode to split the starting droplet into first and second split droplets. The first split droplet is thereby disposed on the first outer electrode and the second split droplet is disposed on the second outer electrode.

<CIT> describes an active matrix electrowetting on dielectric device includes a plurality of array elements configured to manipulate one or more droplets of fluid on an array, each of the array elements including a corresponding array element circuit. Each array element circuit includes write circuitry configured to write data to the corresponding array element for controlling the manipulation of the droplets of fluid, and sensor circuitry configured to sense an impedance present at the corresponding array element. The sensor circuitry is configured to operate in one of a normal mode of sensitivity for detection of a droplet, or a high mode of sensitivity to detect an electric property of an array element hydrophobic surface. The sensor circuitry includes an active element, such as an active capacitor or active transistor, and a capacitance across the active element is different in the normal sensitivity mode as compared to the high sensitivity mode.

The inventors have found that the electric fields generated from excessive or prolonged actuation of the EWOD or AM-EWOD elements can be damaging to both the subject droplet and to components of the device itself. The protocols performed on EWOD platforms may use reagents which are delicate and may be adversely affected by actuation. Damage to reagents and other functional chemicals contained within a droplet can result in undesirable bubbles forming within the droplet (for example due to the release of gas dissolved in the droplet or the surrounding oil). In addition, excessive or prolonged actuation may reduce the lifetime of the EWOD device as the electric fields involved can have deleterious effects on components of the device. For example, the insulator layers and the hydrophobic coatings have been found in particular to be susceptible to damage from the electric fields that result from prolonged actuation of the EWOD elements.

The present invention solves this problem through enhanced control of the actuation patterns of the EWOD elements. In particular, the control system and related control methods of the present invention operate to minimize the time over which EWOD elements are actuated while still effectively performing requisite droplet operations. By minimizing actuation time of the EWOD elements, which minimizes exposure to the generated electric fields, the propensity to damage the subject droplets or device components is reduced.

The present invention pertains to enhanced control systems and methods for the actuation of array elements in an EWOD device, and AM-EWOD devices in particular. The control system implements a method of driving the array elements by which intermittent actuation of pertinent array elements is employed to maintain a droplet in a desired state. By employing intermittent actuation patterns, the control system minimizes actuation time of the EWOD elements, which in turn minimizes exposure to the generated electric fields and the resultant damage to the subject droplets or device components.

In exemplary embodiments, the control system operates to apply suitable actuation voltages to pertinent array elements at a predetermined time, rate, and duration in accordance with a specified or preset duty cycle, regardless of the actual real time properties of the droplet. In other exemplary embodiments, the EWOD or AM-EWOD device incorporates one or more sensors, such as for example sensor circuitry within each array element circuit, which provides information and feedback regarding a droplet state. In embodiments employing sensor circuitry or other sensors, the control system operates to apply suitable actuation voltages only when an intervention is necessary to maintain the droplet in a desired state (e.g., to maintain droplet position and stop a droplet drifting out of position, maintain a particular droplet shape or aspect ratio, maintain a particular droplet size, prevent droplet collision with a second object or droplet, or the like).

The present invention provides for an enhanced microfluidic system including an electro-wetting on dielectric (EWOD) device and a control system, and a related control method. The EWOD device includes an element array configured to receive one or more fluid droplets, the element array comprising a plurality of individual array elements. The control system is configured to control actuation voltages applied to the element array to perform manipulation operations as to the fluid droplets. The control system is configured to apply a sequence of actuation voltages to a portion of the array elements associated with a droplet, the portion comprising a plurality of array elements and being fixed over time, and is configured to apply the sequence of actuation voltages commonly to the plurality of array elements to restrict deviation of the droplet from a desired droplet state, the droplet state comprising droplet position or droplet location. The sequence of actuation voltages includes actuation-on periods in which the plurality of array elements associated with the droplet is commonly actuated and actuation-off periods in which the plurality of array elements associated with the droplet is commonly not actuated, and the actuation-off periods are non-zero. The actuation-on periods alternate with the actuation-off periods.

In exemplary embodiments, the control system may be configured to apply the sequence of actuation voltages in accordance with a predetermined duty cycle including a predetermined time, rate and duration of actuation voltages to the plurality of array elements associated with the droplet. In exemplary embodiments, the system further may include a sensor for sensing a droplet state. With sensor based control, the control system may be configured to: receive droplet state information from the sensor; determine whether a droplet is in a state that deviates from the desired droplet state in accordance with predetermined criteria based on the droplet state information; and apply actuation voltages to the plurality of array elements associated with the droplet when the control system determines that the droplet state satisfies the predetermined criteria to return the droplet to the desired droplet state.

These and further features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto.

The control system and related control methods of the present invention operate to provide intermittent actuation of the array elements to minimize the time over which EWOD or AM-EWOD elements are actuated while still effectively performing requisite droplet operations (e.g. move, merge, split, dispense and hold). By minimizing actuation time of the EWOD or AM-EWOD elements, which minimizes exposure to the generated electric fields, the propensity to damage the subject droplets or device components is reduced. Intermittent actuation thus advantageously limits the duty cycle or time period over which the array elements and associated droplets are actuated. This improves the device reliability and/or prevents damage to chemically or biologically fragile reagents within the droplet.

<FIG> is a drawing depicting an exemplary EWOD based microfluidic system according to embodiments of the present invention. In the example of <FIG>, the measurement system includes a reader <NUM> and a cartridge <NUM>. The cartridge <NUM> may contain a microfluidic device, such as an EWOD or AM-EWOD device <NUM>, as well as (not shown) fluid input ports into the device and an electrical connection as are conventional. The fluid input ports may perform the function of inputting fluid into the AM-EWOD device <NUM> and generating droplets within the device, for example by dispensing from input reservoirs as controlled by electro-wetting. As further detailed below, the microfluidic device includes an electrode array configured to receive the inputted fluid droplets.

The microfluidic system further may include a control system configured to control actuation voltages applied to the electrode array of the microfluidic device to perform manipulation operations to the fluid droplets. For example, the reader <NUM> may contain such a control system configured as control electronics <NUM> and a storage device <NUM> that may store any application software any data associated with the system. The control electronics <NUM> may include suitable circuitry and/or processing devices that are configured to carry out various control operations relating to control of the AM-EWOD device <NUM>, such as a CPU, microcontroller or microprocessor.

Among their functions, to implement the features of the present invention, the control electronics may comprise a part of the overall control system that may execute program code embodied as a control application within the storage device <NUM>. It will be apparent to a person having ordinary skill in the art of computer programming, and specifically in application programming for electronic control devices, how to program the control system to operate and carry out logical functions associated with the stored control application. Accordingly, details as to specific programming code have been left out for the sake of brevity. The storage device <NUM> may be configured as a non-transitory computer readable medium, such as random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), or any other suitable medium. Also, while the code may be executed by control electronics <NUM> in accordance with an exemplary embodiment, such control system functionality could also be carried out via dedicated hardware, firmware, software, or combinations thereof, without departing from the scope of the invention.

The control system may be configured to perform some or all of the following functions:.

In the example of <FIG>, an external sensor module <NUM> is provided for sensing droplet properties. For example, optical sensors as are known in the art may be employed as external sensors for sensing droplet properties. Suitable optical sensors include camera devices, light sensors, charged coupled devices (CCDs) and image similar image sensors, and the like. As further detailed below, a sensor alternatively may be configured as internal sensor circuitry incorporated as part of the drive circuitry in each array element. Such sensor circuitry may sense droplet properties by the detection of an electrical property at the array element, such as impedance or capacitance.

The control system, such as via the control electronics <NUM>, may supply and control the actuation voltages applied to the electrode array of the microfluidics device <NUM>, such as required voltage and timing signals to perform droplet manipulation operations and sense liquid droplets on the AM-EWOD device <NUM>. The control electronics further may execute the application software to generate and output control voltages for droplet sensing and performing sensing operations. The reader <NUM> and cartridge <NUM> may be electrically connected together while in use, for example by a cable of connecting wires <NUM>, although various other methods (e.g. wireless connection) of providing electrical communication may be used as are known to those of ordinary skill in the art.

<FIG> is a drawing depicting additional details of the exemplary AM-EWOD device <NUM> in schematic perspective in accordance with embodiments of the present invention. The AM-EWOD device <NUM> has a lower substrate <NUM> with thin film electronics <NUM> disposed upon the lower substrate <NUM>. The thin film electronics <NUM> are arranged to drive array element electrodes <NUM>. A plurality of array element electrodes <NUM> are arranged in an electrode or element array <NUM>, having X by Y array elements where X and Y may be any integer. A liquid droplet <NUM> which may include any polar liquid and which typically may be aqueous, is enclosed between the lower substrate <NUM> and a top substrate <NUM> separated by a spacer <NUM>, although it will be appreciated that multiple liquid droplets <NUM> can be present.

<FIG> is a drawing depicting a cross section through some of the array elements of the exemplary AM-EWOD <NUM> device of <FIG>. In the portion of the AM-EWOD device depicted in <FIG>, the device includes a pair of the array element electrodes 48A and 48B that are shown in cross section that may be utilized in the electrode or element array <NUM> of the AM-EWOD device <NUM> of <FIG>. The device configuration is similar to the conventional configuration shown in <FIG>, with the AM-EWOD device <NUM> further incorporating the thin-film electronics <NUM> disposed on the lower substrate <NUM>, which is separated from the upper substrate <NUM> by the spacer <NUM>. The uppermost layer of the lower substrate <NUM> (which may be considered a part of the thin film electronics layer <NUM>) is patterned so that a plurality of the array element electrodes <NUM> (e.g. specific examples of array element electrodes are 48A and 48B in <FIG>) are realized. The term element electrode <NUM> may be taken in what follows to refer both to the physical electrode structure <NUM> associated with a particular array element, and also to the node of an electrical circuit directly connected to this physical structure. A reference electrode <NUM> is shown in <FIG> disposed upon the top substrate <NUM>, but the reference electrode alternatively may be disposed upon the lower substrate <NUM> to realize an in-plane reference electrode geometry. The term reference electrode <NUM> may also be taken in what follows to refer to both or either of the physical electrode structure and also to the node of an electrical circuit directly connected to this physical structure.

Also similarly to the conventional structure of <FIG>, in the AM-EWOD device <NUM>, a non-polar fluid <NUM> (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet <NUM>. An insulator layer <NUM> may be disposed upon the lower substrate <NUM> that separates the conductive element electrodes 48A and 48B from a first hydrophobic coating <NUM> upon which the liquid droplet <NUM> sits with a contact angle <NUM> represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer). On the top substrate <NUM> is a second hydrophobic coating <NUM> with which the liquid droplet <NUM> may come into contact. The reference electrode <NUM> is interposed between the top substrate <NUM> and the second hydrophobic coating <NUM>.

<FIG> shows a circuit representation of the electrical load 70A between the element electrode <NUM> and the reference electrode <NUM> in the case where a liquid droplet <NUM> is present. The liquid droplet <NUM> can usually be modeled as a resistor and capacitor in parallel. Typically, the resistance of the droplet will be relatively low (e.g. if the droplet contains ions) and the capacitance of the droplet will be relatively high (e.g. because the relative permittivity of polar liquids is relatively high, e.g. ~<NUM> if the liquid droplet is aqueous). In many situations the droplet resistance is relatively small, such that at the frequencies of interest for electro-wetting, the liquid droplet <NUM> may function effectively as an electrical short circuit. The hydrophobic coatings <NUM> and <NUM> have electrical characteristics that may be modelled as capacitors, and the insulator <NUM> may also be modelled as a capacitor. The overall impedance between the element electrode <NUM> and the reference electrode <NUM> may be approximated by a capacitor whose value is typically dominated by the contribution of the insulator <NUM> and hydrophobic coatings <NUM> and <NUM> contributions, and which for typical layer thicknesses and materials may be on the order of a pico-Farad in value.

<FIG> shows a circuit representation of the electrical load 70B between the element electrode <NUM> and the reference electrode <NUM> in the case where no liquid droplet is present. In this case the liquid droplet components are replaced by a capacitor representing the capacitance of the non-polar fluid <NUM> which occupies the space between the top and lower substrates. In this case the overall impedance between the element electrode <NUM> and the reference electrode <NUM> may be approximated by a capacitor whose value is dominated by the capacitance of the non-polar fluid and which is typically small, of the order of femto-Farads.

For the purposes of driving and sensing the array elements, the electrical load 70A/70B overall functions in effect as a capacitor, whose value depends on whether a liquid droplet <NUM> is present or not at a given element electrode <NUM>. In the case where a droplet is present, the capacitance is relatively high (typically of order pico-Farads), whereas if there is no liquid droplet present the capacitance is low (typically of order femto-Farads). If a droplet partially covers a given electrode <NUM> then the capacitance may approximately represent the extent of coverage of the element electrode <NUM> by the liquid droplet <NUM>.

<FIG> is a drawing depicting an exemplary arrangement of thin film electronics <NUM> in the exemplary AM-EWOD device <NUM> of <FIG> in accordance with embodiments of the present invention. The thin film electronics <NUM> is located upon the lower substrate <NUM>. Each array element <NUM> of the array of elements <NUM> contains an array element circuit <NUM> for controlling the electrode potential of a corresponding element electrode <NUM>. Integrated row driver <NUM> and column driver <NUM> circuits are also implemented in thin film electronics <NUM> to supply control signals to the array element circuit <NUM>. The array element circuit <NUM> may also contain a sensing capability for detecting the presence or absence of a liquid droplet in the location of the array element. Integrated sensor row addressing <NUM> and column detection circuits <NUM> may further be implemented in thin film electronics for the addressing and readout of the sensor circuitry in each array element.

A serial interface <NUM> may also be provided to process a serial input data stream and facilitate the programming of the required voltages to the element electrodes <NUM> in the array <NUM>. A voltage supply interface <NUM> provides the corresponding supply voltages, top substrate drive voltages, and other requisite voltage inputs as further described herein. A number of connecting wires <NUM> between the lower substrate <NUM> and external control electronics, power supplies and any other components can be made relatively few, even for large array sizes. Optionally, the serial data input may be partially parallelized. For example, if two data input lines are used the first may supply data for columns <NUM> to X/<NUM>, and the second for columns (<NUM>+X/<NUM>) to M with minor modifications to the column driver circuits <NUM>. In this way the rate at which data can be programmed to the array is increased, which is a standard technique used in Liquid Crystal Display driving circuitry.

Generally, an exemplary AM-EWOD device <NUM> that includes thin film electronics <NUM> may be configured as follows. The AM-EWOD device <NUM> includes the reference electrode <NUM> mentioned above (which, optionally, could be an in-plane reference electrode) and a plurality of individual array elements <NUM> on the array of elements <NUM>, each array element <NUM> including an array element electrode <NUM> and array element circuitry <NUM>. Relatedly, the AM-EWOD device <NUM> may be configured to perform a method of actuating the array elements to manipulate liquid droplets on the array by controlling an electro-wetting voltage to be applied to a plurality of array elements. The applied voltages may be provided by operation of the control system described as to <FIG>, including the control electronics <NUM> and applications and data stored on the storage device <NUM>. The electro-wetting voltage at each array element <NUM> is defined by a potential difference between the array element electrode <NUM> and the reference electrode <NUM>. The method of controlling the electro-wetting voltage at a given array element typically includes the steps of supplying a voltage to the array element electrode <NUM>, and supplying a voltage to the reference electrode <NUM>, by operation of the control system.

<FIG> is a drawing depicting an exemplary arrangement of the array element circuit <NUM> present in each array element <NUM>, in accordance with embodiments of the present invention. The array element circuit <NUM> may contain an actuation circuit <NUM>, having inputs ENABLE, DATA and ACTUATE, and an output which is connected to an element electrode <NUM>. The array element circuit <NUM> also may contain a droplet sensor circuit <NUM>, which may be in electrical communication with the element electrode <NUM>. Typically, the read-out of the droplet sensor circuit <NUM> may be controlled by one or more addressing lines (e.g. RW) that may be common to elements in the same row of the array, and may also have one or more outputs, e.g. OUT, which may be common to all elements in the same column of the array.

The array element circuit <NUM> may typically perform the functions of:.

Exemplary configurations of array element circuits <NUM> including impedance sensor circuitry are known in the art, and for example are described in detail in <CIT> referenced in the background art section, and commonly assigned UK application <CIT>, both of which are incorporated here by reference. These patent documents include descriptions of how the droplet may be actuated (by means of electro-wetting) and how the droplet may be sensed by capacitive or impedance sensing means. Typically, capacitive and impedance sensing may be analogue and may be performed simultaneously, or near simultaneously, at every element in the array. By processing the returned information from such a sensor (for example in the application software in the storage device <NUM> of the reader <NUM>), the control system described above can determine in real-time, or almost real-time the position, size, centroid and perimeter of each liquid droplet present in the array of elements <NUM>. As referenced in connection with <FIG>, an alternative to sensor circuitry is to provide an external sensor (e.g., sensor <NUM>), such as an optical sensor that can be used to sense droplet properties.

The present invention pertains to an enhanced control system and control methods for the actuation of array elements in an EWOD device, including more specifically an AM-EWOD device. The control system implements a method of driving the array elements by which intermittent actuation of pertinent array elements is employed to maintain a droplet in a desired state. By employing an intermittent actuation, the control system minimizes actuation time of the AM-EWOD elements, which in turn minimizes exposure to the generated electric fields and the resultant damage to the subject droplets or device components.

Certain terms as used in the specification are defined as follows. The "state" of a droplet may refer to any property of the droplet such as size, shape, centroid position, aspect ratio, edge position, location on the device array, proximity to other objects on the device array, and the like. A "desired droplet state" may refer to a droplet state (i.e., any droplet property as above), that otherwise may be measured relative to a reference item, such as for example various components of the device such as the array elements, input or output ports and other physical structures in the device, other droplets in the device, and the like. A desired state typically may be thought of as a preferred droplet state as determined with reference to particular circumstances. A "sensor measurement" may refer to any means of measuring the droplet properties such as impedance measurements performed by an impedance sensor circuit in the array element circuitry, or optical measurements performed by an external optical sensor device (e.g., camera, light sensor, CCD and the like) as are known in the art. "Array elements associated with a droplet" are a subset of the array elements on an EWOD device that when actuated can control or affect a droplet state or otherwise manipulate a droplet.

Intermittent actuation is intended to provide actuation of array elements associated with a droplet for a sufficient amount of time to maintain the state of a droplet in a desired droplet state, while minimizing actuation time to substantially reduce the propensity to damage droplet constituents or device components.

Damage to the device may occur due to the exposure of the insulator <NUM> and hydrophobic coatings (<NUM> and <NUM>) to a high electric field, a situation that occurs when a droplet is actuated. In particular, the hydrophobic coatings are commonly formed from an electret material and thus have a tendency to trap charge, within them or at their interfaces. Trapped charge may have the effect of screening the applied electric field and reducing the actuation force applied to the droplet. Additionally, the high electric field may result in polarization of the insulator / hydrophobic coatings, which may also reduce the actuation strength. Additionally, the high electric field may occasionally result in defect paths forming through the insulator / hydrophobic coatings resulting in current flow, electrolysis and failure of the device.

Damage to the liquid due to exposure to the electric field may occur if the liquid contains components that may be denatured by the electric field, e.g. proteins, enzymes, cells or nucleic acids. The extent of the damage may be a function of the accumulated exposure time to the electric field.

When actuation voltages are turned off, small non-uniformities that may be present in droplet shape and thickness can result in migration or movement of the droplet. The result can be deviations of the droplet state away from a desired droplet state. For example, droplet size, shape, centroid position, edge position, location on the device array, and the like may change overtime in an undesirable manner that can undermine the purpose and use of the droplet, and perhaps undesirably result in the droplet coming in contact with or otherwise interfering with other droplets in close proximity. When such deviations occur, intermittent actuation can be performed to return the droplet from a current droplet state to a desired droplet state. The following are non-limiting example reasons why a droplet may deviate over time:.

In general, in accordance with principles of the present invention, a sequence of actuation of array elements associated with a droplet in an EWOD device is performed wherein a ratio of the actuation-on period to actuation-off period is less than infinite, i.e., the actuation-off period is non-zero meaning that the actuation voltages are not applied for at least a portion of the sequence of actuation. In this regard, the inventors have observed that droplet changes in state tend to occur very gradually over time (and sometimes not at all if the device and/or droplet is highly uniform). Accordingly, an actuation-off time period in which actuation voltages are not applied can be significantly greater than an actuation-on period in which the actuation voltages are applied to the array elements associated with a droplet. The figures described below illustrate different sequences of actuation of array elements associated with a droplet in accordance with various control methods in accordance with the principles of the present invention.

The present invention provides for an enhance microfluidic system including an electro-wetting on dielectric (EWOD) device and a control system, and a related control method. The EWOD device includes an element array configured to receive one or more fluid droplets, the element array comprising a plurality of individual array elements. The control system is configured to control actuation voltages applied to the element array to perform manipulation operations as to the fluid droplets. In exemplary embodiments, the control system is configured to apply a sequence of actuation voltages to a portion of the array elements associated with a droplet to maintain the droplet in a desired droplet state corresponding to a predetermined droplet property. The sequence of actuation voltages includes an actuation-on period in which the portion of the array elements associated with the droplet is actuated and an actuation-off period in which the portion of the array elements associated with the droplet is not actuated, and the actuation-off period is non-zero. In exemplary embodiments, the control system may be configured to apply a sequence of actuation voltages comprising a predetermined duty cycle, and/or the actuation voltages may be applied in accordance with a sensor based intervention.

<FIG> are drawings depicting sequences of electrode actuation of array elements associated with a droplet in an EWOD device, the sequences being performed for maintaining or establishing a desired state of a droplet in the EWOD device. Generally, such figures depict an array of elements <NUM> that is comparable to the array of elements <NUM> described above. The additional details of the EWOD device are omitted from these figures for convenience of illustration, but an EWOD device employing the array of elements <NUM> may be configured as described above with respect to <FIG>. Accordingly, the array of elements <NUM> may include individual array elements <NUM> each comparable to the array element <NUM>, and each for example thus including electrodes <NUM> and <NUM>, and the corresponding array element circuitry <NUM> which may include a droplet sensor circuit <NUM>, such as for example an impedance or capacitive sensing circuit. As defined above, array elements associated with a droplet refers to a portion of the array elements on an EWOD device that when actuated can control of affect a droplet state or otherwise manipulate a droplet. In the examples of <FIG>, array elements 102a are designated as being associated with a droplet <NUM>, and remaining array elements 102b are in regions of the array in which the array elements are not associated with the droplet <NUM>, i.e., the array elements 102b are not involved in controlling or affecting the droplet state.

In exemplary embodiments, the control system operates to apply suitable actuation voltages to perform a sequence of electrode actuation of array elements associated with a droplet at a predetermined time, rate, and duration in accordance with a specified or preset duty cycle, regardless of the actual real time properties constituting the state of the droplet. In general, a predetermined duty cycle may be configured whereby a ratio of the actuation-on period to actuation-off period is less than infinite, i.e., again, the actuation-off period is non-zero meaning that the actuation voltages are not applied for at least a portion of the duty cycle. As referenced above, droplet changes in state may occur very gradually over time which permits the actuation-off period to be significantly greater than the actuation-on period. Based on such observation, a suitable duty cycle may be characterized by approximately <NUM>% actuation-on / <NUM>% actuation-off, i.e., a ratio of actuation-on period to actuation-off period is less than or equal to <NUM>:<NUM>. Under such parameters, an example of a suitable duty cycle may be to apply actuation voltages to electrodes of array elements associated with a droplet, with the actuation voltage maintained at high level for <NUM> seconds (high level time) once every <NUM> seconds (the total cycle time). Other suitable duty cycles may be employed as may be appropriate to particular circumstances or applications.

In other circumstances, droplet changes in state may occur more rapidly over time, which requires that the actuation-off period not be as significantly greater than the actuation-on period. For example, during loading of the droplet into the EWOD device, its size changes relatively rapidly and in order to maintain a desirable droplet state (such as, but not limited to, a desirable droplet centroid position or droplet aspect ratio) a suitable duty cycle may be characterized by a ratio of actuation-on period to actuation-off period greater than or equal to <NUM>:<NUM>. Under such parameters, an example of a suitable duty cycle may be to apply actuation voltages to electrodes of array elements associated with a droplet of <NUM> seconds every <NUM> seconds. Again, any suitable duty cycles may be employed as may be appropriate to particular circumstances or applications.

<FIG> are drawings depicting sequences of electrode actuation comprising a predetermined duty cycle for maintaining a desired state of a droplet on an associated array of elements in an EWOD device. In the example of <FIG>, a sequence of electrode actuation comprising a predetermined duty cycle is applied for maintaining droplet position on the array of elements <NUM>. The steps of the sequence of actuation in <FIG> respectively are labeled A, B, C, and D. Further in the example of <FIG>, there are nine array elements 102a associated with the droplet <NUM>, which can be actuated to maintain a current position of the droplet <NUM> on the array <NUM>. It will be appreciated that the number and position of elements associated with the droplet can be varied as suitable for any particular circumstances. The remaining elements 102b are in regions of the array that are not associated with the droplet <NUM>, i.e., the droplets 102b are not involved in maintaining the droplet current position and thus remain unactuated during the entire actuation sequence A-D.

The sequence of actuation of <FIG> illustrates a duty cycle of alternating actuation-off periods, sequence steps A and C, with actuation-on periods, sequence steps B and D. As shown in <FIG>, during the actuation-off periods, the droplet <NUM> is located at a particular position on the array <NUM>. To ensure that the droplet <NUM> maintains this position, periodically the control system applies actuation voltages to the portion of array elements 102a associated with the droplet. When actuated, the electrical field draws the droplet to the array element electrodes. As shown in sequence steps B and D, the boundary of droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. When the actuation voltages are removed, the droplet <NUM> resumes its unactuated state as shown, for example, from the progression of sequence step B to step C.

By applying intermittent actuation in accordance with the predetermined duty cycle, the droplet <NUM> generally maintains its desired state and does not move substantially from the initial position of sequence step A. Again, because droplet changes in state occur very gradually over time, the actuation-off period may be significantly greater than the actuation-on period (e.g., ratio actuation-on period to actuation-off period is no greater than <NUM>:<NUM>).

<FIG> is a variation in which a sequence of electrode actuation comprises a predetermined duty cycle for maintaining a desired shape or aspect ratio of a droplet. The steps of the sequence of actuation in <FIG> similarly are respectively labeled A, B, C, and D. Further in the example of <FIG>, there are <NUM> array elements 102a associated with the droplet <NUM> due to the elongated or ovular shape of the droplet <NUM> in the desired state shown in sequence step A. The array elements 102a can be actuated to maintain such shape and aspect ratio of the droplet <NUM> on the array <NUM>. The sequence of actuation of <FIG>, therefore, illustrates a duty cycle of alternating actuation-off periods, sequence steps A and C, with actuation-on periods, sequence steps B and D. During the actuation-off periods, the droplet <NUM> maintains the desired ovular and elongated shape on the array <NUM>. To ensure that the droplet <NUM> maintains this shape, periodically the control system applies actuation voltages to the portion of array elements 102a associated with the droplet. When actuated, the electrical field draws the droplet to the array element electrodes. As shown in sequence steps B and D, the boundary of droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. When the actuation voltages are removed, the droplet <NUM> resumes its unactuated state as shown, for example, from the progression of sequence step B to step C. A comparable duty cycle may be employed in the sequence of <FIG> as in <FIG>.

<FIG> is a variation in which a sequence of electrode actuation comprises a predetermined duty cycle for maintaining a desired size of a droplet. The steps of the sequence of actuation in <FIG> similarly are respectively labeled A-F. Further in the example of <FIG>, there are nine array elements 102a associated with the droplet <NUM> that may be actuated to maintain the desired size of the droplet <NUM> shown in sequence step A. The array elements 102a can be actuated to maintain such size of the droplet <NUM> on the array <NUM>. The sequence of actuation of <FIG>, therefore, illustrates a duty cycle of alternating actuation-off periods, sequence steps A-B and D-E, with actuation-on periods, sequence steps C and F. A comparable duty cycle may be employed in the sequence of <FIG> as in <FIG>.

The example of <FIG> illustrates the potential for migration of droplet material to change the state of the droplet from the desired state during the actuation-off period. In this particular example of droplet size, during the actuation-off period spanning steps A-B, the droplet has spread out and the size has changed in the sense that the droplet covers more of the array <NUM> in step B than in the desired state of step A. When actuated, the electrical field draws the droplet to the actuated array element electrodes. As shown in sequence step C, the boundary of droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. When the actuation voltage is removed, the droplet <NUM> resumes its unactuated state as shown, for example, from the progression of sequence steps C to D. In this example, there actually is no droplet migration during the actuation-off period spanning sequence steps D-E. However, because actuation occurs in accordance with a predetermined or preset duty cycle in this embodiment, the electrodes 102a are actuated at sequence step F even though the droplet state has not changed, insofar as the predetermined duty cycle is independent of the real-time state of the droplet. Migration can also change the state of the droplet from the desired state with respect to location (<FIG>) or shape (<FIG>), or other droplet properties, with actuation in accordance with the duty cycle then returning the droplet to the state corresponding to the desired state of the droplet.

In exemplary embodiments, the control system may store and execute any number of predetermined or preset duty cycles as executable program code as part of the control application. For example, the control application may include executable program code for any number of duty cycles for different operational modes of the device, which may be stored in the storage device <NUM> and executed by processor devices of the control electronics <NUM> (see <FIG>). Initiation of a particular duty cycle may be initiated by user selection through interface devices of the control electronics, or otherwise selected automatically as part of the control application. In addition, the control application may be executed to apply different duty cycles to different portions of the array of elements.

In other exemplary embodiments, the AM-EWOD device incorporates one or more sensors, such as for example sensor circuitry <NUM> within each array element, or external sensors <NUM>, that provide information and feedback regarding a droplet state. In embodiments employing sensor circuitry or other sensors, the control system operates to apply suitable actuation voltages only when an intervention is necessary to maintain the droplet in a desired state based on data gathered by the sensors (e.g., to maintain droplet position and stop a droplet drifting out of position, maintain a particular droplet shape, maintain a particular droplet size, or the like). For example, the control system may determine when a droplet state has deviated from a desired state by a predetermined amount or other predetermined criteria, and apply actuation voltages to array elements associated with the droplet to return the droplet to the desired state.

In exemplary embodiments of the invention, therefore, the microfluidic system further may include a sensor for sensing a droplet state. The control system may be configured to: receive droplet state information from the sensor; determine whether a droplet is in a state that deviates from the desired droplet state in accordance with predetermined criteria based on the droplet state information; and apply actuation voltages to the portion of the array elements associated with the droplet when the control system determines that the droplet state satisfies the predetermined criteria to return the droplet to the desired droplet state.

<FIG> are drawings depicting a sequence of electrode actuation comprising a sensor-based intervention for maintaining a desired state of a droplet on an associated array of elements in an EWOD device. In the example of <FIG>, a sequence of electrode actuation comprising a sensor-based intervention is applied for maintaining droplet position on the array of elements <NUM>. The steps of the sequence of actuation in <FIG> respectively are labeled A-E. Further in the example of <FIG>, there are nine array elements 102a associated with the droplet <NUM>, which can be actuated to maintain a current position of the droplet <NUM> on the array <NUM>. Again, the number and position of elements associated with the droplet can be varied as suitable for any particular circumstances. The remaining droplets 102b are in regions of the array that are not associated with the droplet <NUM>, i.e., the droplets 102b are not involved in maintaining the droplet current position and thus remain unactuated during the entire actuation sequence A-F.

As shown in <FIG>, sequence step A is an actuation-off period in which the droplet <NUM> is located at a particular position on the array <NUM>. In sequence step B, the droplet has drifted from the desired state position of sequence step A. The droplet position may be tracked by the sensor until the droplet has deviated from the desired position by a predetermined amount or other predetermined criteria. In the example of <FIG>, the droplet position drifts further until at sequence step C, the droplet <NUM> has deviated from the desired position by a predetermined amount. Accordingly, at sequence step C the control system applies actuation voltages to the portion of array elements 102a associated with the droplet. When actuated, the electrical field draws the droplet to the array element electrodes, and as shown in sequence step D, the droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. The result is the droplet <NUM> is drawn back to the position of the desired state, and when the actuation voltages are removed as shown in sequence step E, the droplet <NUM> resumes its unactuated state located back at the desired position commensurate with sequence step A.

By applying intermittent actuation in accordance with the sensor information, the droplet <NUM> generally maintains its desired state and any deviations from the initial position of sequence step A beyond predetermined criteria are eliminated. Again, because droplet changes in state occur very gradually over time, the actuation-off period may be significantly greater than the actuation-on period. In addition, by using sensor-based actuation, the actuation-off period is minimized insofar as actuation voltages are applied only as needed to return the droplet to the desired state (the desired position in the example of <FIG>). Comparable actuation control may be applied to properties associated with any suitable desired state, as described with respect to the examples of the additional figures below.

<FIG> is a variation in which a sequence of electrode actuation comprises a sensor-based intervention for maintaining a desired shape or aspect ratio of a droplet. The steps of the sequence of actuation in <FIG> similarly are respectively labeled A-E. Sequence step A is an actuation-off period in which the droplet <NUM> has a particular shape and aspect ratio on the array <NUM>. Further in the example of <FIG>, there are <NUM> array elements 102a associated with the droplet <NUM> due to the elongated or ovular shape of the droplet <NUM> in the desired state shown in sequence step A. In sequence step B, the droplet has deviated from the desired state shape of sequence step A, flattening out to become more circular. The droplet shape and aspect ratio may be tracked by the sensor until the droplet has deviated from the desired shape of sequence step A by a predetermined amount or other predetermined criteria. Accordingly, at sequence step C the control system applies actuation voltages to the portion of array elements 102a associated with the droplet. When actuated, the electrical field draws the droplet to the array element electrodes, and as shown in sequence step D, the droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. The result is the droplet <NUM> is drawn back to the shape and aspect ratio of the desired state, and when the actuation voltages are removed as shown in sequence step E, the droplet <NUM> resumes its unactuated state having the desired shape and aspect ratio commensurate with sequence step A.

<FIG> shows a variation in which a sequence of electrode actuation comprises a sensor-based intervention for maintaining a desired size of a droplet. The steps of the sequence of actuation in <FIG> similarly are respectively labeled A-E. Sequence step A is an actuation-off period in which the droplet <NUM> has a particular size on the array <NUM>. In sequence step B, the droplet has deviated from the desired state size of sequence step A, widening out into a larger circle. The droplet size may be tracked by the sensor until the droplet has deviated from the desired size by a predetermined amount or other predetermined criteria. In the example of <FIG>, the droplet size deviates further until at sequence step C, the droplet <NUM> has deviated from the desired size by a predetermined amount. Accordingly, at sequence step C the control system applies actuation voltages to the portion of array elements 102a associated with the droplet. When actuated, the electric field draws the droplet to the array element electrodes, and as shown in sequence step D, the droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. The result is the droplet <NUM> is drawn back to the size of the desired state, and when the actuation voltages are removed as shown in sequence step E, the droplet <NUM> resumes its unactuated state with the desired size commensurate with sequence step A.

<FIG> shows another variation in which a sequence of electrode actuation comprises a sensor-based intervention for maintaining a desired position of a droplet, similar to <FIG>. The steps of the sequence of actuation in <FIG> similarly are respectively labeled A-E. In this particular example, the droplet <NUM> also is positioned in proximity to a second object whose position is known and with which a collision is non-desirable. In the specific example of <FIG>, the second object is a second droplet <NUM>, whose position also may be sensed by any suitable sensor (e.g., impedance sensor circuit or suitable external sensor). The second object, however, may be an object other than another droplet, such as for example a physical barrier within the device, like a device wall or spacer.

As shown in the example of <FIG>, sequence step A is an actuation-off period in which the first droplet <NUM> is located at a particular position on the array <NUM> with suitable spacing apart from the second object (e.g., second droplet) <NUM>. In sequence step B, the first droplet <NUM> has drifted from the desired state position of sequence step A. The droplet position may be tracked by the sensor until the first droplet <NUM> has deviated from the desired position by a predetermined amount or other predetermined criteria. In this example, the predetermined criteria may be a threshold proximity to the second object (e.g., second droplet <NUM>), which can present a potential for collision. In the example of <FIG>, the first droplet position drifts further until at sequence step C, the first droplet <NUM> has deviated from the desired position in accordance with the predetermined criteria being in close proximity to the second droplet <NUM>. Accordingly, at sequence step C the control system applies actuation voltages to the portion of array elements 102a associated with the first droplet <NUM>. When actuated, the electrical field draws the first droplet to the array element electrodes, and as shown in sequence step D, the first droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. The result is the first droplet <NUM> is drawn back to the position of the desired state, and when the actuation voltages are removed as shown in sequence step E, the first droplet <NUM> resumes its unactuated state located back at the desired position suitably spaced apart from the second droplet <NUM> commensurate with sequence step A.

<FIG> show another variation in which a sequence of electrode actuation comprises a sensor-based intervention for maintaining a desired position of a droplet, similar to <FIG> except the second droplet <NUM> is being moved across the array <NUM> in a direction indicated by the arrows in the sub-figures. As referenced above, the position of the second droplet <NUM> also may be sensed by any suitable sensor (e.g., impedance sensor circuit or suitable external sensor), and the movement of the second droplet <NUM> may be achieved by a sequential actuation of another portion of the array elements 102c associated with moving the second droplet <NUM> as is known in the art. A collision of the two droplets is undesirable, and the steps of the sequence of actuation in <FIG> to avoid such a collision is shown similarly in sequence steps A-E.

As shown in the example of <FIG>, sequence step A is an actuation-off period in which the first droplet <NUM> is located at a particular position on the array <NUM> without suitable spacing apart from a path of the moving second droplet <NUM> indicated by the arrow. At such position, there is a potential for collision with the moving second droplet <NUM>. As in previous embodiments, the droplet position may be tracked by the sensor to determine if the droplet position is different from a desired position by a predetermined amount or other predetermined criteria. In this example, the predetermined criteria may be a threshold proximity to the path of the moving second droplet <NUM>, which can present a potential for collision. In this example, in view of the path of the moving second droplet <NUM>, the droplet state of sequence step A can be considered a non-desirable state even though such position previously may have been a desirable state. Accordingly, at sequence step B the control system applies actuation voltages to the portion of array elements 102a associated with the first droplet <NUM>. When actuated, the electrical field draws the first droplet to the array element electrodes, and as shown in sequence step C, the first droplet <NUM> is drawn by the electric field to be commensurate with the actuated array element boundaries. The result is the first droplet <NUM> is drawn to the position of a desired state suitably spaced apart from the path of the moving droplet <NUM>. This actuation may be maintained as shown in sequence step D as the second droplet <NUM> passes the first droplet <NUM>, thereby precluding any collision. When the actuation voltages are removed as shown in sequence step E, the first droplet <NUM> resumes its unactuated state located now at the desired position suitably spaced apart from the second droplet <NUM>. Alternatively, after the second droplet <NUM> has passed the first droplet <NUM>, the control system may apply actuation voltages to a portion of array elements as appropriate to return the droplet to the initial position of sequence step A.

The control system and related control methods of the present invention, therefore, operate to provide intermittent actuation of the array elements to minimize the time over which EWOD or AM-EWOD elements are actuated while still effectively performing requisite droplet operations. By minimizing actuation time of the array elements, which minimizes exposure to the generated electric fields, the propensity to damage the subject droplets or device components is reduced. Intermittent actuation thus advantageously limits the time period over which the array elements and associated droplet are actuated. This improves the device reliability and/or prevents damage to chemically or biologically fragile reagents within the droplet.

Because the sensor-based intervention is targeted to the droplet state, more optimized actuation time periods are achieved, and therefore sensor-based intervention is preferred in EWOD systems that employ sensors. The presence of sensors, however, is not beneficial or feasible in all EWOD applications or technologies. For example, sensors typically are not employed in passive EWOD devices, and in some EWOD devices the pixel size may be too small to incorporate effective sensor circuitry or other sensors. For such EWOD devices that do not employ sensors, intermittent actuation by a preset or predetermined duty cycle is advantageous.

As aspect of the invention, therefore, is an enhanced microfluidic system including an electro-wetting on dielectric (EWOD) device comprising an element array configured to receive one or more fluid droplets, the element array comprising a plurality of individual array elements, and a control system configured to control actuation voltages applied to the element array to perform manipulation operations as to the fluid droplets. In exemplary embodiments, the control system is configured to apply a sequence of actuation voltages to a portion of the array elements associated with a droplet to maintain the droplet in a desired droplet state corresponding to a predetermined droplet property. The sequence of actuation voltages includes an actuation-on period in which the portion of the array elements associated with the droplet is actuated and an actuation-off period in which the portion of the array elements associated with the droplet is not actuated, and the actuation-off period is non-zero. The microfluidic system may include one or more of the following features, either individually or in combination.

In an exemplary embodiment of the microfluidic system, the control system is configured to apply a sequence of actuation voltages comprising a predetermined duty cycle including a predetermined time, rate and duration of actuation voltages to the portion of the array elements associated with the droplet.

In an exemplary embodiment of the microfluidic system, the actuation-off period of the duty cycle is greater than the actuation-on period of the duty cycle.

In an exemplary embodiment of the microfluidic system, a ratio of the actuation-on period of the duty cycle to the actuation-off period of the duty cycle is less than or equal to <NUM>:<NUM>.

In an exemplary embodiment of the microfluidic system, the system further includes a sensor for sensing a droplet state, and the control system is configured to: receive droplet state information from the sensor; determine whether a droplet is in a state that deviates from the desired droplet state in accordance with predetermined criteria based on the droplet state information; and apply actuation voltages to the portion of the array elements associated with the droplet when the control system determines that the droplet state satisfies the predetermined criteria to return the droplet to the desired droplet state.

In an exemplary embodiment of the microfluidic system, the sensor comprises sensor circuitry incorporated into one or more array elements.

In an exemplary embodiment of the microfluidic system, the predetermined droplet property of the desired droplet state is based on at least one of droplet position on the element array, droplet shape or aspect ratio, droplet size, proximity of the droplet to a second object or second droplet on the element array, and proximity of the droplet to a path of a second droplet moving along the array.

Another aspect of the invention is a related control method for controlling actuation voltages applied to array elements of an element array on an electro-wetting on dielectric (EWOD) device. In exemplary embodiments, the control method includes the steps of: receiving one or more fluid droplets on the element array; and applying a sequence of actuation voltages to a portion of the array elements associated with a droplet to maintain the droplet in a desired droplet state corresponding to a predetermined droplet property; wherein the sequence of actuation voltages includes an actuation-on period in which the portion of the array elements associated with the droplet is actuated and an actuation-off period in which the portion of the array elements associated with the droplet is not actuated, and the actuation-off period is non-zero. The microfluidic system may include one or more of the following features, either individually or in combination.

In an exemplary embodiment of the control method, applying a sequence of actuation voltages comprises applying actuation voltages in accordance with a predetermined duty cycle including a predetermined time, rate and duration of actuation voltages to the portion of the array elements associated with the droplet.

In an exemplary embodiment of the control method, the actuation-off period of the duty cycle is greater than the actuation-on period of the duty cycle.

In an exemplary embodiment of the control method, a ratio of the actuation-on period of the duty cycle to the actuation-off period of the duty cycle is less than or equal to <NUM>:<NUM>.

In an exemplary embodiment of the control method, the duty cycle comprises applying actuation voltages to the array elements associated with a droplet for <NUM> seconds once every <NUM> seconds.

In an exemplary embodiment of the control method, the control method further includes: sensing a droplet state with a sensor; determining whether the sensed droplet state is a state that deviates from the desired droplet state in accordance with predetermined criteria; and applying actuation voltages to the portion of the array elements associated with the droplet when it is determined that the droplet state satisfies the predetermined criteria to return the droplet state to the desired droplet state.

In an exemplary embodiment of the control method, sensing a droplet state with the sensor comprises sensing a droplet position on the element array, the control method further comprising: determining whether the sensed droplet state is a state in which the droplet position deviates from a desired droplet state position in accordance with the predetermined criteria; and applying actuation voltages to the portion of the array elements associated with the droplet when it is determined that the droplet state satisfies the predetermined criteria to return the droplet state to the desired droplet state position.

In an exemplary embodiment of the control method, the predetermined criteria includes whether the droplet is at a position within a preset proximity to a second object on the element array.

In an exemplary embodiment of the control method, the predetermined criteria includes whether the droplet is at a position within a preset proximity to a path of a moving second droplet on the element array.

In an exemplary embodiment of the control method, sensing a droplet state with the sensor comprises sensing a droplet shape or aspect ratio, the control method further comprising: determining whether the sensed droplet state is a state in which the droplet shape or aspect ratio deviates from a desired droplet state shape or aspect ratio in accordance with predetermined criteria; and applying actuation voltages to the portion of the array elements associated with the droplet when it is determined that the droplet state satisfies the predetermined criteria to return the droplet state to the desired droplet state shape or aspect ratio.

In an exemplary embodiment of the control method, sensing a droplet state with the sensor comprises sensing a droplet size, the control method further comprising: determining whether the sensed droplet state is a state in which the droplet size deviates from a desired droplet state size in accordance with predetermined criteria; and applying actuation voltages to the portion of the array elements associated with the droplet when it is determined that the droplet state satisfies the predetermined criteria to return the droplet state to the desired droplet state size.

Another aspect of the invention is a non-transitory computer-readable medium storing program code which is executed by a processing device for controlling actuation voltages applied to array elements of an element array of an electro-wetting on dielectric (EWOD) device for performing droplet manipulations on droplets on the element array. The program code is executable by the processing device to perform the steps of the control method.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application, without departing from the scope of the claims.

Claim 1:
A microfluidic system comprising:
an electro-wetting on dielectric (EWOD) device (<NUM>) comprising an element array (<NUM>) configured to receive one or more fluid droplets (<NUM>), the element array comprising a plurality of individual array elements (<NUM>); and
a control system (<NUM>,<NUM>) configured to control actuation voltages applied to the element array to perform manipulation operations as to the fluid droplets;
wherein:
the control system is configured to apply a sequence of actuation voltages to a portion (102a) of the array elements (<NUM>) associated with a droplet (<NUM>), the portion comprising a plurality of array elements and being fixed over time;
the control system (<NUM>,<NUM>) is configured to apply the sequence of actuation voltages commonly to the plurality (102a) of array elements (<NUM>) to restrict deviation of the droplet from a desired droplet state, the desired droplet state comprising droplet position or droplet shape; and
the sequence of actuation voltages includes actuation-on periods in which the plurality (<NUM>) of array elements associated with the droplet is commonly actuated and actuation-off periods in which the plurality (102a) of array elements associated with the droplet is commonly not actuated, and the actuation-off periods are non-zero, the actuation-on periods alternating with the actuation-off periods.