Improvements in or relating to a system and methods for facilitating manipulation of microdroplets

Abstract

A system for manipulating microdroplets an optical assembly (112, figure 1); a microfluidic chip (102, figure 1) including a microfluidic space in which a microdroplet array 506 can be formed; an inte

Description

FIELD OF THE INVENTION The present invention relates to a system and method for facilitating the manipulation of microdroplets. In particular, but not exclusively, the invention relates to a system and method for populating a microfluidic chip with one or more microdroplets. BACKGROUND Electrowetting-on-dielectric (EWOD) is a well-known effect in which an electric field applied between a liquid and a substrate makes the liquid more wetting on the surface than the natural state. The effect of electrowetting can be used to manipulate microdroplets (e.g., controlling the movement, merging, splitting or changing shape of microdroplets) by applying a series of spatially varying electrical fields on a substrate to increase the surface wettability following the spatial variations in a sequence. Droplets manipulated in electrowetting-based devices are typically sandwiched between two parallel plates and actuated by digital electrodes. The size of pixelated electrodes limits the minimum droplet size that can be manipulated as well as the rate and scale at which droplets can be processed in parallel. A variant of this approach uses optically-mediated electrowetting forces, known in the art as optoelectrowetting, to provide the motive force in a device for manipulating microdroplets. In this optically mediated electrowetting (oEWOD) device, the microdroplets are translocated through a microfluidic space defined by containing walls; for example a pair of parallel plates having the microfluidic space sandwiched there between. At least one of the containing walls includes what are hereinafter referred to as ‘virtual’ electrowetting electrode locations which are generated by selectively illuminating an area of a semiconductor layer buried within. By selective illumination of the layer with light from a separate light source, controlled by an optical assembly, a virtual pathway of virtual electrowetting electrode locations can be generated transiently along which the microdroplets can be caused to move. Thus conductive cells are dispensed with and permanent droplet-receiving locations are abandoned in favour of a homogeneous dielectric surface on which the droplet-receiving locations are generated ephemerally by selective and varying illumination of points on the photoconductive layer using, for example, a pixelated light source. This enables highly localised electrowetting fields capable of moving the microdroplets on the surface by induced capillary-type forces to be established anywhere on the dielectric layer; optionally in association with any directional microfluidic flow of the carrier medium in which the microdroplets are dispersed; for example by emulsification. In one example, an application of EWOD and oEWOD devices is in the pharmaceutical industry, in the fields of cell line development and antibody development In these fields there is a need to allow for initial screening of a large number of biological agents (up to millions) to enable the reduction of the number of agents down to sensible numbers (thousands). To achieve an efficient workflow this initial screen needs to take place across a large number of biological agents in a multiplexed fashion. A further difficulty with current systems is that populations of droplets loaded into an EWOD or oEWOD device may contain a substantial fraction of droplets which are unsuitable to be assayed. For example, the droplet may have an undesirable size which makes it difficult to select and manipulate using EWOD or oEWOD forces. Further, the content of the droplet may be undesirable. For example, in an assay which requires a starting point of a single cell per droplet, any droplets which are empty or contain multiple cells are undesirable. It is against this background that the present invention has arisen. SUMMARY OF INVENTION According to the present invention there is provided a system for manipulating microdroplets, the system comprising: an optical assembly; a microfluidic chip including a microfluidic space in which a microdroplet array can be formed comprising a plurality of columns and a plurality of rows, substantially orthogonal to the columns; an interrogation component configured to inspect each microdroplet to determine whether one or more predetermined conditions are met; and a controller configured to: selectively control the optical assembly to modulate a plurality of sprites such that each microdroplet received at the inlet is attached to a single sprite; selectively control the optical assembly to modulate the plurality of sprites such that each microdroplet moves along a virtual pathway comprising one or more of the rows and columns in order to form the microdroplet array; modulate a subset of the plurality of sprites corresponding to two or more rows of a predetermined column of the microdroplet array to rotate thereby presenting microdroplets of the subset sequentially to a virtual dispensing pathway associated with an adjacent column of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are transported around the chip via the virtual dispensing pathway. The interrogation component may be further configured to associate a unique identifier with each microdroplet and sprite pair. Additionally, the interrogation component may be configured to image at least one microdroplet and to determine the size and / or presence of content within the microdroplet based on at least one image of the microdroplet. The controller may be configured to determine the size and / or content of the microdroplet based on a plurality of images and / or to use a machine learning algorithm to determine the size and / or content of the microdroplet. The subset comprises six, eight, ten, twelve or more sprites. The controller may be further configured to: concurrently modulate a different subset of the plurality of sprites corresponding to two or more different rows of the predetermined column of the microdroplet array, thereby presenting each microdroplet of the different subset sequentially to the virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are dispensed via the virtual dispensing pathway. The controller may be further configured to: concurrently modulate a further subset of the plurality of sprites corresponding to two or more rows of a different predetermined column of the microdroplet array, thereby presenting each microdroplet of the further subset sequentially to the virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are ejected from the chip. Furthermore, according to the present invention there is provided a computer implemented method of manipulating microdroplets comprising: modulating a subset of a plurality of sprites corresponding to two rows of a predetermined column of a microdroplet array to rotate thereby presenting microdroplets of the subset sequentially to a virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are ejected from the chip. Additionally, according to the invention there may be provided a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the aforementioned method. Furthermore, according to the present invention there is provided a system for manipulating microdroplets, the system comprising: an optical assembly; a microfluidic chip comprising a microfluidic space and an inlet for receiving one or more microdroplets; and a controller configured to: selectively control the optical assembly to modulate a plurality of sprites to translocate microdroplets received at the inlet along at least one of a plurality of virtual pathways between the inlet and a (temporary) position in a microdroplet array in the microfluidic space; and form a pattern of illumination spanning at least two of the plurality of virtual pathways, wherein the pattern of illumination causes microdroplets received at the inlet to align with one of the plurality of virtual pathways. The pattern of illumination may alter the rate of population of each virtual pathway of the plurality of virtual pathways. The pattern of illumination may form an electrowetting field. The pattern of illumination may comprise a respective illuminated region associated with each of the two virtual pathways and a further illuminated region between the two virtual pathways, wherein the further illuminated region may be offset from the respective illuminated regions associated with each of the two virtual pathways. Furthermore according to the present invention there is provided a computer implemented method of manipulating microdroplets comprising: modulating a plurality of sprites to translocate microdroplets received at an inlet along at least one of a plurality of virtual pathways between the inlet and a (temporary) position in a microdroplet array in a microfluidic space; and forming a pattern of illumination spanning at least two of the plurality of virtual pathways, wherein the pattern of illumination causes microdroplets received at the inlet to align with one of the plurality of virtual pathways. Additionally, according to the invention there may be provided a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the aforementioned method. Furthermore, according to the present invention there is provided a system for manipulating microdroplets, the system comprising: an optical assembly; a microfluidic chip including a microfluidic space in which a substantially rectilinear microdroplet array can be formed comprising a plurality of columns and a plurality of rows, substantially orthogonal to the columns; an interrogation component configured to inspect each microdroplet to determine whether one or more predetermined conditions are met; and a controller configured to: selectively control the optical assembly to modulate a plurality of sprites such that microdroplets received at the inlet move along a virtual pathway comprising one or more of the rows and columns in order to form the microdroplet array; modulate a subset of the plurality of sprites corresponding to a predetermined column of the microdroplet array in a first direction to create a further virtual pathway between an identified microdroplet that meets at least one of the predetermined conditions and a discharge region; and transport the identified microdroplet to the discharge region via the further virtual pathway in a second direction, different from the first direction. The first direction may be substantially orthogonal to the second direction. The controller may be further configured to simultaneously modulate a further subset of the plurality of sprites corresponding to a further predetermined column of the microdroplet array in the first direction to create a yet further virtual pathway between a further identified microdroplet that meets at least one of the predetermined conditions and the discharge region; and the system may be further configured to dispense the further identified microdroplet via the yet further virtual pathway in the second direction. The discharge region may be an area of the microfluidic chip from which microdroplets are ejected from the chip to waste or dispensed from the chip for further analysis or processing. The subset of the plurality of sprites corresponding to a predetermined column of the microdroplet array may be modulated into a virtual pathway to a discharge region. Furthermore, according to the present invention there is provided a method of manipulating microdroplets comprising: inspecting microdroplets in a microdroplet array comprising a plurality of columns and a plurality of rows substantially orthogonal to the columns to determine whether one or more predetermined conditions are met; selectively controlling the optical assembly to modulate a plurality of sprites such that microdroplets received at an inlet move along a virtual pathway comprising one or more of the rows and columns in order to form the microdroplet array; modulating a subset of the plurality of sprites corresponding to a predetermined column of the microdroplet array in a first direction to create a further virtual pathway between an identified microdroplet that meets at least one of the predetermined conditions and a discharge region; and transporting of the identified microdroplet to the discharge region via the further virtual pathway in a second direction, different to the first direction. The method may further comprise the step of further processing and / or analysing the identified microdroplet. Additionally, according to the invention there may be provided a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the aforementioned method. According to an aspect of the present invention, there is provided a species screened by the device, apparatus or method as disclosed herein. According to an aspect of the present invention, there is provided a species selected by the device, apparatus or method as disclosed herein. According to an aspect of the present invention, there is provided a species isolated by the device, apparatus or method as disclosed herein. According to an aspect of the present invention, there is provided a species made by the device, apparatus or method as disclosed herein. The species may be chemical, biochemical, or biological in nature. For example, the present invention may provide an agonist / antagonist to an entity as identified by the screening, selection and / or isolation method disclosed herein. The present invention may provide an agonist / antagonist to an entity as identified by the screening, selection and / or isolation method disclosed herein, for use in therapy. The entity may be chemical, biochemical, or biological in nature. According to an aspect of the present invention, there is provided a use of the device, apparatus, method or species as disclosed herein. According to an aspect of the present invention, there is provided a use of the device, apparatus, method or species as disclosed herein in therapy. The present invention may provide for a use of the device, apparatus, method or species as disclosed herein in making a product. The product made may be chemical, biochemical, or biological in nature. The use may be peptide synthesis. The use may be synthetic biology. The use may be cell line engineering or development. The use may be cell therapy. The use may be drug discovery. The use may be antibody discovery. According to an aspect of the present invention, there is provided a use of the device, apparatus, method or species as disclosed herein in analysis. The analysis may be physical, chemical, or biological. The use may be sub-cellular imaging. The use may be high content imaging. The use may be diagnostics. The use may be a biological assay. The biological assay may be high throughput screening. The biological assay may be ELISA. The use may be cell secretion. The use may be QC safety profiling. BRIEF DESCRIPTION OF THE FIGURES The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a system for manipulating microdroplets in a microfluidic chip; Figure 2A shows a configuration of a microfluidic chip; Figures 2B to 2L show different configurations of the microfluidic chip of Figure 2A, illustrating the manipulation of microdroplets in a microfluidic chip; Figure 3 shows a populated array of a microfluidic chip; Figures 4A to 4E show systems with exemplary sprite and microdroplet distributions; Figures 5A to 5D show carousel configurations in a system for manipulating microdroplets in a microfluidic chip; Figures 6A to 6D show dynamic configurations in a system for manipulating microdroplets in a microfluidic chip; and Figures 7A to 7D show a system for manipulating microdroplets in a microfluidic chip. DETAILED DESCRIPTION OF FIGURES Figure 1 shows a system 100 for manipulating microdroplets in a microfluidic chip. There is shown a microfluidic chip 102 comprising an inlet 104, a microfluidic space 106 and a discharge section 108. Microdroplets are introduced into the microfluidic chip 102 via the inlet 104. The microdroplets in the microfluidic chip 102 are analysed and manipulated by an optical assembly 112 that is in communication with a controller 116 via a communication pathway 110A. The microdroplets are manipulated in order to form an array in the microfluidic space 106 of the microfluidic chip 102. The controller 116 comprises a processor 118 and a memory 120. The optical assembly 112 comprises a light source 113 and an interrogation component 114. The controller 116 and the optical assembly 112 are in communication with one another directly via the communication pathway 110A. Alternatively, or additionally, the controller 116 and the optical assembly are in communication with a network 122 of one or more computing devices, via further communication pathways 110B, 110C. The communication pathways 110A, 110B, 110C are optionally wired or wireless connections and include corresponding interfaces to enable communication between different parts of the system 100. The controller 116 controls the light output of a light source 113 of the optical assembly 112 in order to manipulate microdroplets in the microfluidic chip 102 using an optoelectrowetting technique, as described herein. In particular, the controller 116 controls the light source 113 of the optical assembly 112 in order to selectively illuminate a semiconductor layer of the microfluidic chip 102. The semiconductor layer of the microfluidic chip at least partially defines the microfluidic space 106, as well as at least part of the inlet 104 and discharge section 108. As described herein, the microfluidic space 106 is sandwiched between two parallel plates, one of which comprises the semiconductor layer such that illumination of the semiconductor layer causes the formation of virtual electrowetting electrode locations. The virtual electrowettting electrode locations are generated transiently along virtual pathways through selective control of the optical assembly in order to modulate optically mediated electrowetting on dielectric (oEWOD) device traps, which are also known as sprites. A sprite is a software-defined pattern of pixels which creates an ephemeral electrowetting location when it is projected on to the photoactive layer of an oEWOD device. This provides a motive force causing microdroplets to follow the virtual pathways. Therefore, permanent droplet-receiving locations are abandoned in favour of a homogeneous dielectric surface on which the droplet-receiving locations are generated ephemerally by selective and varying illumination of points on the photoconductive layer using for example the light source 113. This enables highly localised electrowetting fields capable of moving the microdroplets on the surface by induced capillary-type forces to be established anywhere on the dielectric layer; optionally in association with any directional microfluidic flow of the carrier. Using this technique, microdroplets received at the inlet portion 102 are translocated to the microfluidic space 104 directed to form an array within the microfluidic space 102, such as an array formed substantially of orthogonal rows and columns. Although the dielectric surface is intended to be entirely homogenous, it is possible, though undesirable, for point defects to exist on the surface. Where such defects exist, the pathway that the microdroplet / sprite pair takes will be modified by the defect. The defect in the surface can cause a microdroplet / sprite pair to become separated at the location of the defect or pinning site. This interrupts the intended trajectory of the microdroplet and decouples it from the sprite to which it was previously paired. The light source 113 used is a pixelated light source, including, but not limited to an LED light source. The optical assembly 112 also includes an interrogation component 114. The interrogation component 114 is used to analyse microdroplets in the microfluidic chip 102. Data measured and provided by the interrogation component 114 is used in order to make decisions regarding the microdroplets within the microfluidic chip 102. For example, if the interrogation component 114 provides data indicating that a microdroplet is undesirable as it does not meet one or more predetermined conditions, or desirable as it does meet one or more predetermined conditions, the microdroplet can be transported out of an array in the microfluidic chip 102, for example by translocation from the microfluidic space 106 to the discharge section 108. The discharge section 108 is a holding area in which microdroplets that meet one or more predetermined conditions are rearranged prior to being transported out of the microfluidic space 106. In an example, the discharge section 108 comprises an outlet through which desirable microdroplets are dispensed from the microfluidic chip 102, to facilitate further processing and / or analysis of the desirable microdroplets. In an alternative example, the discharge section 108 can be used to collate for ejection down-selected microdroplets. Further, microdroplets in the microfluidic chip 102 are tracked using the interrogation component 114, which sends data to the controller 116, which in turn modulates the optical assembly 112 in order to selectively manipulate the tracked microdroplets. Data generated by the interrogation component 114, which may be a microscope configured to capture image data of microdroplets, is sent to a suitable device for processing. In an example, the data is processed at the processor 118 of the controller 116. In further examples, the data is processed locally at the optical assembly 112, or remotely within the network 122. The controller 116 is in communication with the optical assembly 112, which includes the light source or projector 113 and the interrogation component 114. Accordingly, the controller 116 is provided with information from the interrogation component 114 which is used to modulate sprites created using the optical assembly 112. The interrogation component 114 is used to capture image data from the microfluidic chip 102. The rate at which image data is captured is configurable to provide information to the controller 116 that enables decisions to be made by the processor 118. For example, image data is used in order to assign unique identifiers to individual microdroplets in the microfluidic chip 102 and to track and control the progress of individual microdroplets within the microfluidic chip 102. The unique identifier assigned to a microdroplet is associated with a particular sprite / microdroplet pair. This means the progress of a particular sprite / microdroplet pair is modulated in order to move the pair. The rate at which image data is captured is sufficient to identify when the movement of any particular microdroplet has been influenced by a surface defect such that the microdroplet has been de-coupled from its corresponding sprite. A surface defect, sometimes called a pinning site, can result in the coalescence of one or more microdroplets at a location other than their intended position in the array. The image data captured by the interrogation component 114 can identify such sites and the controller 116 can be configured to modify the trajectories of other sprite / microdroplet pairs to avoid the pinning site to prevent further coalescence of microdroplets. Furthermore, once a pinning site has been identified, the processor 118 can take this into account by reducing the array capacity and altering the array layout to ensure that the trajectories of the microdroplets do not pass sufficiently close to the pinning site to cause further coalescence. The pinning site is also made unavailable as a location for microdroplet operations such as incubation and merging. Under some circumstances, the processor 118 re-calibrates the assay to compensate for the reduced capacity and changed layout of the array so that the assay still completes, albeit with diminished output. The optical assembly 112 in conjunction with the controller 116 and / or other computing devices uses light or optical spectroscopy, such as fluorescence spectroscopy, in order to generate data relating to microdroplets. The interrogation component 114 comprises means to inspect microdroplets, for example, a light source and detector, such as a fluorescence detector. Image data is used to determine the size and / or content of microdroplets in the microfluidic chip 102. One or more frames of image data is used to establish the size and / or content of microdroplets. Advantageously, 20 to 30 image frames are used in order to determine the size and / or content of microdroplets. This is advantageous as microdroplets moving through the microfluidic chip 102 can present to an interrogation component in different forms at different times and statistical analysis improves the reliability of the determination. In an example, a machine learning algorithm is used to determine the size and / or content of one or more microdroplets in the microfluidic chip 102. Advantageously, the use of a machine learning algorithm enables improved reliability in the analysis of microdroplets in microfluidic chips even when changes in conditions and / or samples result in otherwise unknown variations that alter the reliability of sample assessment compared with previously assessed samples. The microfluidic chip 102 is an oEWOD device, and the oEWOD structures are comprised of: a first composite wall comprised of a first substrate, which can be made out of glass, a first conductor layer on the substrate, the first conductor layer having a thickness in the range 70 to 250nm; a photoactive layer activated by electromagnetic radiation in the wavelength range 400-850nm on the conductor layer, the photoactive layer having a thickness in the range 300-1500nm and a first dielectric layer on the photoactive layer. The first dielectric layer is formed as a continuous layer that has a thickness of less than 20nm. The lower bound for the thickness of the layer will be dictated, at least in part, by the methodology of providing such a thin layer that must be continuous. However, theoretically it could have a thickness of between 0.1 nm to 20 nm. In an example, the first conductor is transparent. Alternatively, the photoactive layer in the case of the first wall and optionally the conducting layer in the case of the second wall are coated with a dielectric layer which is typically in the thickness range from 120 to 160nm. The dielectric properties of this layer preferably include a high dielectric strength of >10A7 V / m and a dielectric constant of >3. It is as thin as possible consistent with avoiding dielectric breakdown. In one embodiment, the dielectric layer is selected from high purity alumina or silica, hafnia or a thin nonconducting polymer film. The device also comprises a second composite wall comprising: a second substrate, which can be made out of glass and a second conductor layer on the substrate. In an example, the second conductor is transparent and / or the second conductor layer has a thickness in the range 70 to 250nm. A second dielectric layer may be on the second conductor layer, where the second dielectric layer has a thickness of less than 20nm. As with the first dielectric layer, the second dielectric layer must be continuous and the practical lower bound for the thickness is dictated by manufacturing constraints although it could be between 0.1 nm to 20 nm. The exposed surfaces of the first and second continuous dielectric layers are disposed 20 to 180pm apart to define a microfluidic space 106 adapted to contain microdroplets. By providing the first and / or second dielectric layers with a continuous layer of thickness of less than 20 nm results in the droplets being more stable and therefore, the droplets are stationary on the substrate. In contrast, the inventors have found that increasing the first and / or second dielectric layers to a thickness of above 20 nm can result in more uncontrolled droplet movement on the substrate and therefore, droplets are more likely to exhibit uncontrolled motion deviating from the illuminated regions. As a consequence, uncontrolled droplets can make it more difficult for accurate and efficient oEWOD operations for example, merging or splitting of droplets. In some embodiments, the first and / or second dielectric layer may be a thickness of between 1 nm to 20 nm, or it may be 2 nm to 20 nm, 3 nm to 20 nm, 4 nm to 20 nm, 5 nm to 20 nm, 6 nm to 20 nm, 7 nm to 20 nm, 8 nm to 20 nm, 9 nm to 20 nm, 10 nm to 20 nm, 12 m to 20 nm, 14 nm to 20 nm, 15 nm to 20 m or 18 nm to 20 nm. It may also be 1 to 15 nm, 1 to 10 nm, 1 to 5 nm, 5 to 10 nm, 5 to 15 nm or 10 to 15 nm. The photoactive layer is made out of amorphous silicon. The first and second conductor layers are made out of ITO. An interstitial binding layer is provided on the first dielectric layer and can also be provided on the second dielectric layer. The thickness of the interstitial layer may be between 0.1 nm to 5 nm. The thickness of the interstitial layer can be more than 0.1, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4 or 4.5 nm, or it may be less than 5 nm, 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.75, 0.5 or 0.25 nm. The advantage of the interstitial layer is that it can be used as a binding layer for an anti-fouling or non-fouling layer, which may be hydrophobic. In some examples, the interstitial binding layer may be omitted. In such examples, the hydrophobic layer is applied directly to the first dielectric layer. A hydrophobic layer is provided on the interstitial binding layer. An example of a hydrophobic layer could be a fluorosilane or fluorosiloxane. The interstitial binding layers are optional and the channel walls can be laser cut PSA. Alternatively, the channel walls can be formed from a photo resistant material such as SU8, or it may be part of the glass structure. The interstitial layer is provided between the dielectric layer and the hydrophobic layer. The first and second substrates are made of a material which is mechanically strong. For example, the first and second substrates are formed from glass, metal or an engineering plastic. In examples, the substrates have a degree of flexibility. In some embodiments, the first and second substrates have a thickness that is at least 100pm. In some embodiments, the thickness of first and second substrates is more than 2500pm, for example 3000pm, 3500pm or 4000pm. In some embodiments, the first and second substrates have a thickness in the range of 100 to 2500pm. In some embodiments, the first and second substrate have a thickness of more than 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300 or 2400pm. In some embodiments, the first and second substrate have a thickness of less than 2500, 2400, 2300, 2200, 2100, 2000, 1900, 1800, 1700, 1600, 1500, 1400, 1300, 1200, 1100, 1000, 900, 800, 700, 600, 500, 400, 300 or 200pm. In some embodiments, the first substrate has a thickness of approximately 1100pm and the second substrate has a thickness of approximately 700pm. In another embodiment, the first and second substrates have a thickness of 800 microns. In some embodiments, the first substrate is Silicon, fused silica or glass. In some embodiments, the second substrate is fused silica and / or glass. The glass may be, but is not limited to, a soda lime glass or a float glass. The first and second conductor layers are located on one surface of the first and second substrates and typically have a thickness in the range 70 to 250nm, preferably 70 to 150nm. At least one of these layers is made of a transparent conductive material such as Indium Tin Oxide (ITO), a very thin film of conductive metal such as silver or a conducting polymer such as PEDOT or the like. These layers may be formed as a continuous sheet or a series of discrete structures such as wires. Alternatively, the conductor layer may be a mesh of conductive material with the electromagnetic radiation being directed between the interstices of the mesh. The photoactive layer is formed from a semiconductor material which can generate localised areas of charge in response to stimulation by the source of electromagnetic radiation. Examples include hydrogenated amorphous silicon layers having a thickness in the range 300 to 1500nm. In some embodiments, the photoactive layer is activated by the use of visible light. The dielectric properties of this layer preferably include a high dielectric strength of >10A7 V / m and a dielectric constant of >3. In some embodiments, the dielectric layer is selected from alumina, silica, hafnia or a thin non-conducting polymer film. Alternatively, at least the first dielectric layer, preferably both, is coated with an anti-fouling layer to assist in establishing the desired microdroplet / carrier fluid / surface contact angle at the various virtual electrowetting electrode locations. The anti-fouling layer is intended additionally to prevent the contents of the microdroplets adhering to the surface and being diminished as the microdroplet is moved through the chip. For optimum performance, the anti-fouling layer should assist in establishing a microdroplet / carrier fluid / surface contact angle that should be in the range 50° to 180° when measured as an air-liquid-surface three-point interface at 25°C. In some embodiments, these layer(s) have a thickness of less than 10nm and are typically formed as a monomolecular layer. Alternatively, these layers are comprised of a polymer of an acrylate ester such as methyl methacrylate or a derivative thereof substituted with hydrophobic groups; e.g. alkoxysilyl. Either or both of the anti-fouling layers are hydrophobic to ensure optimum performance. In some embodiments, an interstitial layer of silica of thickness less than 20nm are interposed between the anti-fouling coating and the dielectric layer in order to provide a chemically compatible bridge. The first and second dielectric layers, and therefore the first and second walls, define a microfluidic space 106 which is at least 10pm, and preferably in the range of 20 to 180pm, in which the microdroplets are contained in use. Preferably, before they are contained, the microdroplets themselves have an intrinsic diameter, which is 10% greater or 20% greater, than the width of the microfluidic space 106. Thus, on entering the chip 102 the microdroplets 106 are caused to undergo compression leading to deformation of the spherical microdroplet that leads to enhanced electrowetting performance through e.g. a better microdroplet splitting capability. In some instances, the first and second dielectric layers are coated with a hydrophobic coating such a fluorosilane. In some embodiments, the microfluidic space 106 includes one or more spacers for holding the first and second walls apart by a predetermined amount. Options for spacers include beads or pillars, ridges created from an intermediate resist layer which has been produced by photo-patterning. Alternatively, deposited material such as silicon oxide or silicon nitride are used to create the spacers. Alternatively layers of film, including flexible plastic films with or without an adhesive coating, are used to form a spacer layer. Various spacer geometries can be used to form narrow channels, tapered channels or partially enclosed channels which are defined by lines of pillars. By careful design, it is possible to use these spacers to aid in the deformation of the microdroplets, subsequently perform microdroplet splitting and effect operations on the deformed microdroplets. Similarly these spacers can be used to physically separate zones of the chip to prevent crosscontamination between droplet populations, and to promote the flow of droplets in the correct direction when loading the chip under hydraulic pressure. The first and second walls are biased using a source of A / C power attached to the conductor layers to provide a voltage potential difference therebetween; suitably in the range 0 to 50 volts. These oEWOD structures are typically employed in association with a source of electromagnetic radiation having a wavelength in the range 400-850nm, for example 550, 620 and 660 nm and an energy that exceeds the bandgap of the photoactive layer. Suitably, the photoactive layer will be activated at the virtual electrowetting electrode locations where the incident intensity of the radiation employed is in the range 0.005 to 0.1 Wcnr2. The source of electromagnetic radiation is at a level of 0.005 to 0.1 Wcnr2, or it could be more than 0.005, 0.0075, 0.01, 0.025, 0.05 or 0.075 Wcnr2. In some embodiments, the source of electromagnetic radiation is at a level of less than 0.1, 0.075, 0.05, 0.025, 0.01, 0.0075, 0.005 or 0.0025 Wcnr2. Where the sources of electromagnetic radiation are pixelated they are suitably supplied either directly or indirectly using a reflective screen such as a digital micromirror device (DMD) illuminated by light from LEDs or other lamps. This enables highly complex patterns of virtual electrowetting electrode locations to be rapidly created and destroyed on the first dielectric layer thereby enabling the microdroplets to be precisely steered along essentially any virtual pathway using closely-controlled electrowetting forces. Such electrowetting pathways can be viewed as being constructed from a continuum of virtual electrowetting electrode locations upon the first dielectric layer. The first and the second dielectric layers are composed of a single dielectric material or are a composite of two or more dielectric materials. The dielectric layers are made from, but are not limited to, AI2O3 and SiO2. A structure may be provided between the first and second dielectric layers. The structure between the first and second dielectric layers is made of, but is not limited to, epoxy, polymer, silicon or glass, or mixtures or composites thereof, with straight, angled, curved or micro-structured walls / faces. The structure between the first and second dielectric layers may be connected to the top and bottom composite walls to create a sealed microfluidic device and define the channels and regions within the device. The structure may occupy the gap between the two composite walls. Alternatively, or additionally, the conductor and dielectrics may be deposited on a shaped substrate which already has walls. Alternatively, the spacer layer is not formed from a separate material, but is formed as part of a structure within the first (active) substrate. The sub layers of the oEWOD device formed from the first conductor layer, the photoactive layer, the first dielectric layer, interstitial binding layer and hydrophobic layer may partially or completely cover the walls of the spacer structure. A further embodiment is an alternative configuration of the device, in which the spacer layer is formed by structuring of the second (passive) substrate. In some cases, the spacer may be formed by structuring both the first and / or second substrates, or by using a combination of structures in the first and / or second substrates and an interposing material such as a channel wall. In examples of the systems and methods provided herein, one or more microdroplets contain a biological or chemical material different from the microdroplet medium. In some examples, the microdroplet medium is cell media and is selected from: F12 growth media, RPMI medium, DMEM, and Opti-MEM or EMEM. In examples of the systems and methods provided herein, the biological or chemical material is selected from: a biological cell, cell media, a chemical compound or composition, a drug, an enzyme, a bead with material optionally bound to its surface or a microsphere. In some examples, polystyrene or magnetic beads are bound through Biotin-Strepdavidin bonding to antigens, antibodies or small molecules. In some examples, oligos are bound as DNA tags. In some examples, small molecules or dye molecules are bound with or without UV cleavable linkers. In examples of the systems and methods provided herein, the biological cells are mammalian, bacterial, fungi, yeast, macrophage or hybridoma, and are selected from, but are not limited to: CHO, Jurkat, CAMA, HeLa, B-cell, T-cell, MCF-7, MDAMB-231, E. coli and Salmonella. In some examples of the systems and methods provided herein, the chemical compound or composition include enzymes, assay reagents, antibodies, antigens, drugs, antibiotics, lysis reagents, surfactants, dyes or cell stain. In examples of the systems and methods provided herein, the biological or chemical material are DNA oligos, nucleotides, beads / microspheres loaded or unloaded, fluorescent reporters, nanoparticles, nanowires or magnetic particles. Figures 2A to 2L show various configurations of a microfluidic chip, such as the microfluidic chip 102 described with reference to Figure 1. Figures 2A to 2L illustrate virtual pathways. As described above, the virtual pathways are created by modulating optically mediated electrowetting on dielectric (oEWOD) device traps, which are also known as sprites. This provides a motive force causing microdroplets to follow the virtual pathways. Figure 2A shows a configuration 200A of a microfluidic chip 102 that enables manipulation of microdroplets, as described with reference to Figures 2B to 2L. The configuration 200A includes an inlet portion 202, a fan portion 204 and an array portion 206 of a microfluidic chip. The array portion 206 is a portion of a microfluidic space, such as the microfluidic space 106 described with reference to Figure 1, in which an array is formable. There is shown a plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 from the inlet portion 202 to the array portion 206 of the microfluidic chip. Each of the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 is shown to have a number of branches within the array portion 206 of the microfluidic chip 102. For example, the first virtual pathway 208 from the inlet portion 202 to the array portion 206 is shown to have ten branches 208A, 208B, 209C, 208D, 208E, 208F, 208G, 208H, 208I, 208J. The first virtual pathway 208 provides a route from the inlet portion 202 to the array portion 206 and continues to a discharge section 108 of the microfluidic chip 102. As described herein, microdroplets are controlled to follow the virtual pathway 208 from the inlet portion 202 to the array portion 206 and are selectively directed to the discharge section 108 of the microfluidic chip 102. Microdroplets are beneficially held in the array portion 206, where large quantities of microdroplets can be simultaneously analysed. The discharge section 108 comprises one or more outlets for receiving and arranging microdroplets. Advantageously, microdroplets from the array portion 206 are selectively moved to the discharge section 108 from whence the microdroplets can either be ejected from the chip as down-selected microdroplets or dispensed from the chip for further postprocessing including processing including merging, culturing and / or further analysis. In order to accommodate the branches 208A to 208J within the array portion 206, the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 fan apart at inflection points between the inlet portion 202 and the fan portion 204 such that substantially parallel virtual pathways are provided through the inlet portion 202 and through the array portion 206 to the discharge section 208, with different distances between corresponding points on the parallel virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 in the inlet portion 202 and the array portion 206. Whilst the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 are shown to fan apart in a fan portion 204, in further examples, different configurations of the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 are used in order to enable the translocation of microdroplets from the inlet portion 204 to the array portion 206. It will be appreciated that the fanning out of the virtual pathways is schematic only in all of the accompanying figures. The trace as shown is idealised and actually comprises a series of orthogonal steps which, taken together, provide the diagonal trace illustrated. Each of the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 is shown to have branches extending in a substantially perpendicular direction from the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 in the array portion 206. The branches of the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 provide locations within the array portion 206 at which microdroplets can be held in temporary positions, in order to facilitate analysis of the microdroplets. Whilst ten electrowetting virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 are shown, in further examples, any number of virtual pathways are implemented. In an example, the number of electrowetting virtual pathways present is 2 to 250. In further examples, the number of electrowetting virtual pathways present is between 40 and 180. In further examples, the number of electrowetting virtual pathways is even up to 200-250. In an example, the number of electrowetting pathways present is more than 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46 or 48, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 210, 220, 230 or 240. In some examples, the number of electrowetting pathways present is less than 250, 240, 230, 220, 210, 200, 180, 160, 140, 120, 100, 80, 60, 50, 48, 46, 44, 42, 40, 38, 36, 34, 32, 30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 10, 8, 6, or 4. Upon interrogation, such as by using the apparatus 100 described with reference to Figure 1, a microdroplet may be determined to be undesirable if a measured value of the microdroplet is associated with certain characteristics that are equal to, above or below the one or more stored threshold values set by a user. Alternatively, a microdroplet may be determined to be desirable if a measured value of the microdroplet is associated with certain characteristics that are equal to, above or below the one or more stored threshold values set by a user. In further examples, the desirability of a droplet is determined from a combination of characteristics, the time varying analysis of these characteristics or an average measurement of these characteristics. Undesired and / or desired microdroplets may, at different times, be directed to the discharge section 108 of a microfluidic chip 102. The undesired and / or desired microdroplets are transported to the discharge section 108 via discharge electrowetting virtual pathways which extend from a first region of the device, such as the array portion 206 of the array configuration 200A of Figure 2A, to outlets of the microfluidic chip 102. In some configurations, one or more discharge electrowetting virtual pathways may stem from one or more electrowetting virtual pathways. The discharge electrowetting virtual pathways facilitate the removal of undesired and / or desired droplets from the electrowetting virtual pathways, and remove the undesired and / or desired droplets from the array portion 206 in the microfluidic space 106. The controller 116 is configured to select one or more microdroplets having a measured dataset based on data provided by the optical assembly 112 that is associated with an undesired, or desired, characteristic and cause the one or more selected microdroplets to move into one or more discharge electrowetting virtual pathways. In further examples, the controller 116 is configured to select one or more microdroplets having measured datasets associated with multiple undesired, or multiple desired, characteristics. For example, the controller 112 is configured to select one or more microdroplets which are determined to be both undersized and / or empty which may be ejected from the chip 102 to waste. In further examples, the controller 112 is configured to select one or more microdroplets carrying matter of particular interest, which may be dispensed from the chip for further processing and / or analysis. The configuration 200A of Figure 2A enables effective population of a microfluidic space 106 of a microfluidic chip 102 with microdroplets in an array formation in an array portion 206 of the microfluidic chip 102. Population of an array in the array portion 206 is described with reference to Figures 2B to 2L. Figure 2B shows a configuration 200B of a microfluidic chip. The configuration 200B is the same as the configuration 200A described with reference to Figure 2A, with microdroplets 228 shown in the inlet portion 202, such as a first microdroplet 228A. The microdroplets 228 are introduced into the inlet portion 202 of the microfluidic chip 102. The microdroplets 228 are introduced into the inlet portion 202 from a microdroplet source, such as a reservoir for holding microdroplets or a droplet generator, such as an emulsifier device for generating droplets. The microdroplets 228 are introduced into the inlet portion 202 using a pressure source, such as a pump configured to apply a negative pressure at an outlet of the microfluidic chip 102 and / or a positive pressure at the microdroplet source. As microdroplets 228 are introduced into the inlet portion 202 of the microfluidic chip 102 they can be manipulated using optoelectrical wetting techniques. Microdroplets 228 introduced into the inlet portion 202 of the microfluidic chip are interrogated using an interrogation component 114 of an optical assembly 116, as described above with reference to Figure 1. Accordingly, each of the microdroplets 228 is assigned a unique identifier. This enables each individual microdroplet to be assessed and tracked as it is transported through the microfluidic chip 102. Such assessment provides data that can be used in order to determine whether or not a microdroplet should be transported to a temporary position in an array for further analysis, or to the discharge section 108. Figure 2C shows a configuration 200C that is the configuration 200B described with reference to Figure 2B with a number of sprites 230, 232, 234, 236, 238 shown on the eighth virtual pathway 220. The sprites 230, 232, 234, 236, 238 are oEWOD traps that trap microdroplets 228. Similar sprites are shown on the third virtual pathway 212, the fourth virtual pathway 214, the fifth virtual pathway 216, the sixth virtual pathway 218 and the seventh virtual pathway 220. The sprites 230, 232, 234, 236, 238 form part of a pattern of a plurality of sprites that is dynamically selectively controlled by modulation of the optical assembly 112 by the controller 116. Each individual sprite is controlled so that its temporal evolution through the microfluidic chip 102 is tracked. Each individual sprite can be associated with a uniquely identified microdroplet forming a microdroplet / sprite pair that can be manipulated in a controlled manner such that it can be transported within the microfluidic chip 102. Figure 2D shows a configuration 200D that is an evolution of the configuration 200C of Figure 2C by modulation of a plurality of sprites. The plurality of sprites is shown to have moved from the inlet portion 202 towards the fan portion 204 of the microfluidic chip 102. As shown at Figure 2D, the first microdroplet 228A is associated with a sprite 230 on the eighth virtual pathway 222 forming a sprite / microdroplet pair. Similarly, microdroplets 228 are shown to have associated with sprites on the third virtual pathway 212, the fourth virtual pathway 214, the fifth virtual pathway 216, the sixth virtual pathway 218 and the seventh virtual pathway 220, forming a leading front of microdroplets 228 that is substantially perpendicular to the direction of travel of the microdroplets 228 from the inlet portion 202 towards the fan portion 204 of the microfluidic chip 102. As the sprite pattern progresses such that sprites 230, 232, 234, 236, 238, 240 and their corresponding paired microdroplets follow the eighth virtual pathway 222 and sprites simultaneously follow their associated virtual pathway 212, 214, 216, 218, 220, more microdroplets 228 in the inlet portion 202 associate with a sprite. Figure 2E shows this progression, where a configuration 200E illustrates the sprite pattern in the configuration 200D shown at Figure 2D where the sprites have moved along their respective virtual pathway, trapping and moving microdroplets 228 such that one microdroplet is associated with one sprite forming a microdroplet / sprite pair. The evolution of the sprite pattern from the inlet portion 202, via the fan portion 204 to the array portion 206 is further shown at the configurations 200E, 200F, 200G, 200H, 2001 and 200J of Figures 2E to 2J. At Figures 2E to 2J there is shown a sprite pattern with six sprites on each of the third 212, fourth 214, fifth 216, sixth 218, seventh 220 and eighth 222 virtual pathways, illustrating the extraction of thirty six microdroplets 228 from the inlet portion 202 to the array portion 206 of the microfluidic chip 102. As shown at the configuration 200K of Figure 2K, the sprite pattern is manipulated in order to move microdroplets 228 from the main virtual pathways 212, 214, 216, 218, 220, 222 to branches of the virtual pathways 212, 214, 26, 218, 220, 222, where microdroplets 228 are held in an array formation, beneficially facilitating large scale, simultaneous, analysis of microdroplets. For example, a first microdroplet 228A associated with a sprite 230 is moved from the main eighth virtual pathway 222 along which it has transitioned, to a branch of the eighth virtual pathway. Similarly, microdroplets 228 associated with other sprites of the sprite pattern are controlled such that they move along branches of the virtual pathways 212, 214, 216, 218, 220, 222. Such manipulation enables the array portion 202 to be populated with microdroplets 228. This is shown at Figure 2L, where the configuration 200L illustrates the temporary positions of the thirty six microdroplets 228 that were illustrated in their manipulated transit from the inlet portion 202 to the array portion 206, as shown at Figures 2B to 2J. The thirty six microdroplets 228 are temporarily held in positions along branches of the virtual pathways 212, 214, 216, 218, 220, 222 such that analysis of the microdroplets 228 can be performed. Whilst the progression of thirty six specifically identified microdroplet / sprite pairs is shown at Figures 2C to 2L, the process of manipulating microdroplets is dynamic and in further examples different numbers of pairs are used and / or further sprites are introduced in order to manipulate microdroplets 228 received at the inlet portion 204. Different patterns of sprites, moving at different speeds and intervals are used in further examples. Whilst the sprites described with reference to Figures 2C to 2L are shown to follow uniform progression and to have a uniform distribution throughout the microfluidic chip 102, in further examples, different and / or non-uniform progression of sprites and their associated microdroplets is used in the population of an array of microdroplets. In further examples, the sprite number in a given pathway can be of any suitable number and may change with time, as sprites can be added or deleted from the electrowetting pathway. This enables continuous growth of the sprite patterns. The virtual pathways of the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 are at least double the average microdroplet diameter of microdroplets intended for analysis in the microfluidic chip 102. This is advantageous as it allows a single microdroplet to pass between two other microdroplets. This is used in order to enable droplets which are not controlled by sprites to fall between the gaps of the microdroplets which are controlled by an oEWOD force of a sprite. This provides a sieving effect for self-organising droplets. The sieving effect is optimised by moving the droplets at a maximum speed possible so that only droplets that have an optimum spritedroplet overlap are retained, thus causing each sprite to control a single droplet, thereby driving self organisation in an alignment portion of the inlet portion 202. The process is improved by varying the incident electromagnetic radiation used for the oEWOD sprites. The reduction in holding quality is particularly useful when applied to a self-assembling / alignment region because it limits the time droplets spend moving close to their maximum speed to just this region, thus allowing droplets to move comfortably below their maximum speed along the rest of the path without changing speed. This aids droplet retention and loading rates. A decrease in droplet velocity after self-assembly may cause droplets to be lost, or droplet-droplet collisions or changes to the droplet holding potential. For oEWOD the illumination intensity in the self-assembly region could be between 0.01 and 0.99 of the intensity utilised along the rest of the pathway. In very high quality devices, corresponding to low probability of accidental droplet loss, a higher initial light intensity such as between 0.75 and 0.99, for example 0.8, can be used. This allows a higher loading speed to be used. In lower quality devices, which have a correspondingly increased probability of droplet losses, a lower ratio of light intensities must be used, such as 0.01 to 0.5, this allows the risk of droplet losses to be minimised further but impairs the maximum loading speed. In other devices it may be optimum to utilise a light intensity ratio between 0.5 and 0.75. Additionally or alternatively, the gap provided between the electrowetting pathways can help reduce or minimise the risk of droplets from different electrowetting pathways coming into contact with each other. The spacing between the electrowetting pathways may enable droplets to efficiently and continuously move along the pathways until the droplets are screened and selected by a user or by an automated software controller for manipulation. The operation is particularly effective when dealing with a large number of microdroplets in a series of multiple electrowetting pathways, and facilitates the efficient organisation of droplets from disordered droplets. The average spherical microdroplet diameter is 20 to 200 nm. In examples, the average spherical microdroplet is 50 to 100nm. In some embodiments of the chip provided herein, the velocity of microdroplets in the electrowetting pathways can be 25 to 5000 pm / s. In some embodiments, the velocity of microdroplets in the electrowetting pathways can be more than 25, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450, 1500, 1550, 1600, 1650, 1700, 1750, 1800, 1850, 1900 or 1950, 2000, 2200, 2500, 2700, 3000, 3200, 3500, 3700, 4000, 4200, 4500, 4700 pm / s. In some embodiments, the velocity of microdroplets in the electrowetting pathways can be less than 5000, 4700, 4500, 4200, 4000, 3700, 3500, 3200 3000, 2700, 2500, 2200, 2000, 1950, 1900, 1850, 1800, 1750, 1700, 1650, 1600, 1550, 1500, 1450, 1400, 1350, 1300, 1250, 1200, 1150, 1100, 1050, 1000, 950, 900, 850, 800, 750, 700, 650, 600, 550, 500, 450, 400, 350, 300, 250, 200, 150, 100 or 50 pm / s. This enables effective manipulation of the droplets by EWOD or oEWOD force, and facilitates the self-organisation of the droplets into an ordered array. Figure 3 shows a populated array 300, where the array portion 206 of a microfluidic chip 102 has been populated by the modulation of sprites in order to trap and move microdroplets 228 from the inlet portion 202 of the microfluidic chip 102 along a plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 224, 226 to the array portion 206 of the microfluidic chip 102, as described above with reference to Figures 2A to 2L. The branches associated with the plurality of virtual pathways 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 are shown to be populated with three microdroplets along each branch. For example, the first branch of the tenth virtual pathway 226 populated with three microdroplets 302, 304, 306 and the second branch of the tenth virtual pathway 226 is populated with a further three microdroplets 301, 303, 305. The three microdroplets 302, 304, 306 shown on the first branch of the tenth virtual pathway 226 form part of a row of the array 300 and the three droplets 301, 303, 305 on the second branch of the tenth virtual pathway 226 form part of a different row of the array 300. The microdroplets on different branches of the different virtual pathways are formed substantially in columns. For example, the first microdroplet 302 of the first branch of the tenth virtual pathway 226 forms part of a same column of the array 300 as the first microdroplet 301 of the second branch of the tenth virtual pathway 226 of the array 300. Whilst an array 300 with ten branches associated with each of the plurality of virtual pathways in an array portion 206 is shown, in further examples the number of branches associated with each virtual pathway is different. In further examples, a different number of microdroplets are held along each of the branches of the plurality of virtual pathways and / or there is a different number of virtual pathways in the microfluidic chip 102. Microdroplets in the populated array 300 are inspected using an optical assembly 112 in communication with a controller 116, as described above with reference to Figure 1. Whilst Figure 3 shows an evenly populated array of microdroplets in a microfluidic chip 102, it will be appreciated that the dynamic nature of microdroplets introduced at an inlet portion 202 of a microfluidic chip 102 is such that the distribution of microdroplets 228 is variable and the rate of population of microdroplets 228 per virtual pathway of the plurality of virtual pathways alters in accordance with the particular conditions at a given time. Figures 4A to 4E show systems with exemplary sprites. Figure 4A shows a system 400A with exemplary sprites that is analogous to the configuration 200A shown at Figure 2A. There is shown a plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426 from an inlet portion 402, through a fan portion 404 to an array portion 406 of a microfluidic chip, such as the microfluidic chip 102 shown at Figure 1. In addition to microdroplets 228 in the inlet portion 402, there is shown a distribution of sprites 432, 434, 436, 438, 440, 442, 444, 446, 452, 454, 456, 458, 460, 462, 464, 466, 472, 474, 476, 478, 480, 482, 484, 486 with their respectively associated microdroplets, such as microdroplets 228 drawn from the inlet portion 402 of a microfluidic chip 102. Each of the second virtual pathway 410 to the ninth virtual pathway 424 shows the presence of three microdroplet / sprite associations within part of the array portion 406. The system 400A is representative of a sprite pattern that is manipulated through control of an optical assembly 112 by a controller 116 controlling a light source 113 and an interrogation component 114 as described above with reference to Figures 1 to 3. Whilst a limited number of sprites are shown to be associated with microdroplets that have been transported through the system 400A, the skilled person understands that in further examples a different number of microdroplets is transported through the system 400A. Further, the skilled person understands that the movement of microdroplets 400A through the system 400A is a dynamic process and that the extraction and redistribution of microdroplets from an inlet portion 402 to an array portion 406 is an effectively continuous process to efficiently populate the array portion 406 for further analysis of microdroplets. Figure 4A represents a first, even, microdroplet distribution of the rate of population of virtual pathways across eight exemplary virtual pathways 410, 412, 414, 416, 418, 420, 422, 424, where the number of microdroplets being transported from the inlet portion 402 to the array portion 406 is substantially the same across each of the virtual pathways. However, in practice, the distribution of the rate of population varies depending on factors such as the way in which microdroplets are introduced into the inlet portion 402 and the size of microdroplets, for example. Figure 4B shows a system 400B showing a second microdroplet distribution, different from the first microdroplet distribution shown at Figure 4A. The distribution is representative of a snapshot of microdroplet / sprite pairs in the process of being transported to the array portion 406. A front of leading sprites 432, 434, 436, 438, 440, 442, 444, 446 is associated with the second 410, third 412, fourth 414, fifth 416, sixth 418, seventh 420, eighth 422 and ninth 424 virtual pathways respectively. Each of the leading sprites 432, 434, 436, 438, 440, 442, 444, 446 is associated with a microdroplet. In contrast to the distribution shown at Figure 4A, one sprite 452 and the associated microdroplet are shown to have been transported along the third virtual pathway 412 from the inlet portion 402 to the array portion 406, rather than along the second virtual pathway 410, as shown in the example of Figure 4A. Further, three sprites 454, 464, 476 have moved microdroplets along the fourth virtual pathway 414, four sprites 456, 466, 478, 484 have moved microdroplets along the fifth virtual pathway 416, four sprites 458, 472, 480, 486 have moved microdroplets along the sixth virtual pathway 418, three sprites 460, 474, 482 have moved microdroplets along the seventh virtual pathway 420 and one sprite 462 has moved a microdroplet along the eighth virtual pathway 422. The resultant distribution shows a successive deviation, along each virtual pathway, away from a front of sprites and microdroplets that is substantially perpendicular to the direction of travel within an array portion of a microfluidic chip, where the array is populating more readily in a central portion of the array compared with a peripheral portion of the array. The variation in distribution of population of microdroplets per virtual pathway is affected by the different conditions under which microdroplets are present and their individual properties. Figure 4C shows a system 400C showing a third microdroplet distribution, which is different to the first and second microdroplet distributions shown at Figures 4A and 4B, respectively. The distribution is representative of a snapshot of microdroplets that have associated with sprites in the inlet portion 402 and are in the process of being transported to the array portion 406. The distribution of microdroplets shown at Figure 4C is such that the array portion 406 fills more quickly towards the central pathways and less quickly towards the peripheral virtual pathways. A front of leading sprites 432, 434, 436, 438, 440, 442, 444, 446 is associated with the second 410, third 412, fourth 414, fifth 416, sixth 418, seventh 420, eighth 422 and ninth 424 virtual pathways respectively. Each of the leading sprites 432, 434, 436, 438, 440, 442, 444, 446 has formed a microdroplet / sprite pair with an associated microdroplet. However, in contrast to Figures 4A and 4B, three sprites 452, 472, 484 translocate associated microdroplets along the second virtual pathway 410, two sprites 454, 474 translocate associated microdroplets along the third virtual pathway 412, two sprites 456, 476 translocate associated microdroplets along the fourth virtual pathway 414 and a sprite 458 transports an associated microdroplet along the fifth virtual pathway 416. Similarly, a sprite 460 transports an associated microdroplet along the sixth virtual pathway 418, two sprites 462, 478 translocate associated microdroplets along the seventh virtual pathway 420, two sprites 464, 480 translocate associated microdroplets along the eighth virtual pathway 422 and three sprites 466, 482, 486 translocate associated microdroplets along the ninth virtual pathway 424. Following the introduction of microdroplets into the inlet portion 402 of a microfluidic chip, an optical assembly 112 is used in order to interrogate the microdroplets using an interrogation component 114. Information is passed to a controller 116, which is in communication with the optical assembly 112 in order to selectively control a light source 113 of the optical assembly 112, as described above with reference to Figures 1 to 3. Sprites are controlled through the inlet portion 402 of the microfluidic chip such that microdroplets associate with a single sprite and are moved to the array portion 406, or to the discharge section 108 of the microfluidic chip 102. Sprites are directed along virtual pathways of the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 424, 426 by modulating the sprites at a first power. The modulation at the first power is performed at a configurable rate such that the sprite effectively moves along a virtual pathway at a configurable speed, with the sprite being operated at the first power. Modulation of a plurality of sprites at the first power and at the same speed from the inlet portion 402 to the array portion 406 enables a plurality of microdroplets to associate with the sprites. Altering the first power to a second power ensures that a single microdroplet is associated with a single sprite and facilitates alignment of microdroplets associated with sprites as part of a pattern of sprites translocating along the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 424, 426 in a microfluidic chip. For example, improved alignment of a front of sprites and respectively associated microdroplets is provided with an increase of power of modulated sprites from a first power to a second power, wherein the first power is 50% of the second power. In further examples, improvements in alignment are provided with an increase from a first power to a second power, wherein the first power is 10% to 90% of the second power. The rate at which sprites are directed along virtual pathways in a microfluidic chip determines, at least in part, the distribution of the rate of population of the plurality of virtual pathways and hence the population distribution in an array. Therefore, selectively modulating a plurality of sprites at different, configurable, speeds through a microfluidic chip enables the distribution of the rate of population of virtual pathways to be controlled. For example, where the central portion of an array is populating more rapidly than the peripheral region, as shown in Figure 4B, altering the speed at which sprites (and thus microdroplets) move along virtual pathways from the inlet portion 402 to the array portion 402 is used to provide a more even distribution, as shown at Figure 4A. Similarly, where the peripheral portion of an array is populating more rapidly than a central portion, as shown at Figure 4C, altering the speed at which sprites (and thus microdroplets) move along virtual pathways from the inlet portion 402 to the array portion 402 provides a more even distribution, as shown at Figure 4A. Figure 4D shows a system 400D with a mechanism for efficiently identifying imbalances in the distribution of microdroplets populating an array portion and a responsive change to address the imbalance. Figure 4D shows the third microdroplet distribution of Figure 4C. The distribution of microdroplets shown at Figure 4D is such that the array portion 406 fills more quickly towards the peripheral virtual pathways and less quickly towards the central pathways. The second virtual pathway 410, third virtual pathway 412, fourth virtual pathway 414, fifth virtual pathway 416, sixth virtual pathway 418, seventh virtual pathway 420, eighth virtual pathway 422 and ninth virtual pathway 424 form a plurality of virtual pathways of a central band of the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426, whilst the first virtual pathway 408 and the tenth virtual pathway 426 each form a peripheral band of the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426. Sprite patterns that are modulated in order to trap microdroplets in the inlet portion 402 and transition them to the array portion 406 operate along each of the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426. However, detection of a microdroplet associating with a sprite in one of the peripheral bands can trigger an alert indicating that the distribution of the microdroplets across the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426 is sub-optimal with respect to the distribution of the rate of population of microdroplets per virtual pathway amongst the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426. In response to detection of a stray microdroplet in one of the peripheral bands, the modulation of sprites is changed in order to change the distribution of the rate of population of microdroplets per virtual pathway amongst the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426. For example where a microdroplet strays into the first virtual pathway 408, a sprite 492 directs the stray microdroplet through the array portion 406 either to the discharge portion 108, or to a position in the array portion 406 for further analysis. In either case, the detection of the stray microdroplet in the first virtual pathway 408 in its capacity as part of a peripheral band, causes a response in the modulation of sprites in order to change the distribution of the rate of population per virtual pathway to provide a more even distribution. For example, the modulation of sprites is changed in order to reduce the speed at which sprites move along the virtual pathways, or to increase the speed at which sprites move along the virtual pathways. Controlling modulation in this way enables the distribution of microdroplets across the plurality of virtual pathways to alter. Whilst the first virtual pathway 408 and the tenth virtual pathway 426 are each peripheral virtual pathways forming part of a peripheral band, in further examples, the number of peripheral virtual pathways in each peripheral band and in the central band is configurable. In further examples, the responsive action to determining the presence of a microdroplet in a peripheral band is dependent on the particular location, for example, the particular peripheral virtual pathway of the peripheral band in which the microdroplet is detected. For example, the power and / or speed at which all sprites in a sprite pattern are modulated is dependent on the particular detection of a microdroplet in a particular peripheral virtual pathway or peripheral band. Figure 4E shows a system 400E, which is the system 400D of Figure 4D with a stray droplet 494 in the first virtual pathway 408 in the inlet portion 402. The microdroplet is manipulated in the same manner as described above with respect to Figure 4D. The translocation of microdroplets received at the inlet portion 402 to the array portion 406 via a fan portion 404 results in the effective alignment of microdroplets across the plurality of virtual pathways 408, 410, 412, 414, 416, 418, 420, 422, 424, 426. Such alignment occurs in an alignment portion of the inlet portion 402. In further examples, alignment occurs within at least part of the fan portion 404 and / or the array portion 406. Figures 7A to 7C show further systems with exemplary sprites. Figure 7A shows a system 700A with exemplary sprites that is analogous to the configuration 200A shown at Figure 2A. There is shown a plurality of virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726 from an inlet portion 702, through a fan portion 704 to an array portion 706 of a microfluidic chip, such as the microfluidic chip 102 shown at Figure 1. Additionally, there is shown a light bar arrangement 730. The light bar arrangement 730 is effectively a mega-sprite of patterned illumination, which is controlled, as described above, in a similar manner to the virtual pathways that are created by modulating optically mediated electrowetting on dielectric (oEWOD) device traps. The light bar arrangement 730 is formed from alternating pairs of illuminated and nonilluminated regions, which are aligned with virtual pathways. For example, there is shown a non-illuminated region 732 and an illuminated region 734 forming a pair associated with the first virtual pathway 708. There is shown a further non-illuminated region 738 and an illuminated region 736 forming a pair adjacent to the pair associated with the first virtual pathway 708. There is shown a further non-illuminated region 740 and an illuminated region 742 associated with the second virtual pathway 710. The arrangement of the pairs of non-illuminated and illuminated regions is such that the non-illuminated and illuminated regions of adjacent pairs are inverted with respect to each other, such that a zig-zag of illuminated regions is formed substantially perpendicular to the direction of travel of sprites along the virtual pathways and such that illuminated regions are formed to coincide with the virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726 on the array portion 706 side of the light bar arrangement 730. There is shown a plurality of microdroplets 744 in the inlet portion 402, as well as a pattern of electrowetting traps forming a pattern of sprites 750 that is controlled to follow the virtual pathways from the inlet portion 702, through the fan portion 704 to the array portion 706. The light bar arrangement 730 is a semi-permanent illumination in the region where the inlet portion 702 intersects the fan portion 704. As the pattern of sprites 750 is controlled to pass through the inlet portion 702 where the plurality of microdroplets 744 is found, microdroplets associate with sprites and are caused to move along the plurality of virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726. Beneficially, the use of the light bar arrangement 730 facilitates alignment of microdroplets with the plurality of virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726 and hence improves the population of the array portion 706. The progress of microdroplets in the system 700A in response to directing a pattern of sprites 750 through the plurality of microdroplets 744 is shown in the system 700B of Figure 7B. At Figure 7B, it is shown that where the plurality of microdroplets 744 associate with the pattern of sprites 750, microdroplets follow the individual virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726 to the array portion 706. The plurality of microdroplets 744 in the inlet portion 702 are received in the inlet portion in an unordered manner. Therefore, initially, the association of individual microdroplets with individual sprites may not be uniform. Advantageously, as the microdroplets pass through the light bar arrangement 730 to follow the virtual pathways 708, 710, 712, 714, 716, 718, 720, 722, 724, 726, the uniformity of the front of microdroplets entering the array portion 706 is improved and the plurality of microdroplets 744 transition from an unordered distribution to an ordered distribution. This is shown in the system 700C of Figure 7C. Figure 7C shows the evolution of microdroplets passing from the inlet portion 702, through the fan portion 704 to the array portion 706. In the array portion 706 there is shown microdroplets 752, 754, 756, 758, 760, 762, 764, 766 forming a front of microdroplets that is substantially perpendicular to the direction of travel of the microdroplets. The use of the light bar arrangement 730 is such that the uniformity of filling of the fan portion 704 and array portion 706 is improved. Whilst the use of the light bar arrangement 730 supplements the uniformity provided by the pattern of sprites 750, in further examples the light bar arrangement 730 acts to filter microdroplets 744 received in the inlet 702 such that a pattern of sprites is implemented on the array-side of the light bar arrangement in order to translocate microdroplets in the array portion 706. Advantageously, microdroplets received in the inlet portion 702 are received with a force that causes them to move towards the light bar arrangement 730, where they are effectively ordered and filtered such that microdroplets are manipulated with sprites only after passing through the light bar arrangement 730. Figure 7D shows an alternative configuration of a light bar 780. The light bar 780 shown in Figure 7D is configured to form a series of tapered portions each of which is aligned with the intended location of an individual virtual pathway. The light bar 780 creates an even front of droplets across the extent of the light bar 780. Furthermore, the sawtooth configuration of the light bar 780 assists the alignment of the sprites with the microdroplets as the microdroplets move past the light bar 780 and enter the fan section 704. The sawtooth configuration of the light bar 780 provides a tapered portion every two or three pixels so that the saw tooth configuration is relatively subtle, but is just sufficient to guide the microdroplets onto the sprites 750. The light bar arrangement 730 is provided using the optical assembly 112, which is controlled using the controller 116, as described with reference to Figures 1 to 6. In further examples, additional or alternative apparatus is used in order to provide the light bar arrangement 730. Whilst the light bar arrangement 730 is shown with alternating squares of illuminated and non-illuminated regions to form pairs of illuminated and nonilluminated regions associated with each virtual pathway, in further examples, alternative patterns are used in order to provide an improved distribution of filling an array portion 706 of a microfluidic chip. For example, the pairs of non-illuminated and illuminated regions associated with the virtual pathways are the same size and shape as the adjacent alternating pairs of non-illuminated and illuminated regions in between virtual pathways. In further examples, the size and area of the alternating pairs differs. Whilst the alternating pairs of non-illuminated and illuminated regions are shown such that the nonilluminated region is on the inlet side of the virtual pathway and the illuminated region is on the array side of the virtual pathway, in further examples a different configuration can be provided such that a repeated pattern of illuminated regions is formed with laterally offset portions substantially perpendicular to the direction of travel of microdroplets in the array portion 706. In the configuration shown in Figure 7A, the light bar arrangement 730 is positioned at the intersection of the inlet portion 702 and fan portion 704. In further examples, not illustrated in the accompanying drawings, more than one light bar arrangement 730 may be used in order to alter the distribution of microdroplets that are transported from the inlet portion 702 to the array portion 704. Advantageously, the use of a light bar arrangement 730 means that rogue microdroplets that are not following a virtual pathway in the array portion 706 do not crash into microdroplets that are following a virtual pathway in the array portion 706. Figure 5A shows a system 500A with a carousel configuration. The carousel configuration provides an efficient and effective way to increase the quality of microdroplet arrays through the identification and removal of unwanted droplets. Figure 5A shows a system 500A that is analogous to the populated array 300 shown at Figure 3. There is shown a plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 from an inlet portion 502 to an array portion 506 via a fan portion 504. The plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 each include a number of branches that provide locations for microdroplets to be held for analysis. The branches of the plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 are arranged to form a substantially rectilinear microdroplet array comprising a plurality of rows and a plurality of columns, such that the rows and the columns are substantially orthogonal. For example, as shown in Figure 5A, the branches of adjacent virtual pathways of the plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 are substantially aligned rows and branches associated with each virtual pathway of the plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 enable microdroplets to be held in columns. The microdroplets that are held in the array are each associated with a single sprite. Although the illustrated embodiment shows a substantially rectilinear array, any suitable shape could be used to form the microarray including, but not limited to a trapezoid, triangle, pentagon or hexagon. In relation to the rectilinear example shown in Figure 5A, the plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 extend from the array portion 506 towards a discharge section 108. Accordingly, the plurality of virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 also act as dispensing virtual pathways 508, 510, 512, 514, 516, 518, 520, 522, 524, 526 for the purpose of removing unwanted microdroplets from the microfluidic chip 102. The configuration 500A of Figure 5A shows a first subset 530 and a second subset 550. The first subset 530 relates to a number of microdroplets held by a respective number of sprites across two branches of the eighth virtual pathway 522. Each branch within the first subset 530 is shown to hold three microdroplets in temporary positions in the array portion 506 of a microfluidic chip 102 for analysis. The branches of the eighth virtual pathway 522 are populated through the manipulation of microdroplets by controlling sprites introduced into an inlet portion 502 of a microfluidic chip 102, for example, microdroplets introduced at the inlet portion 502 are trapped by a sprite and moved along the eighth virtual pathway 522 and along branches sequentially, such that microdroplet 542 is the first microdroplet to be moved along a branch within the first subset 530, followed by a second microdroplet 540, followed by a third microdroplet 538. The next branch of the first subset 530 is then populated with a first microdroplet 532, followed by a second microdroplet 534, followed by a third microdroplet 536. The second subset 550 relates to a number of microdroplets held by a respective number of sprites across two different branches of the eighth virtual pathway 522, compared to two branches of the eighth virtual pathway 522 described with reference to the first subset 530. Each branch within the second subset 550 is shown to hold three microdroplets in temporary positions in the array portion 506 of a microfluidic chip 102 for analysis. An interrogation component 114 of an optical assembly 112, as described with reference to Figures 1 to 4, is used to inspect each microdroplet in order to determine whether one or more predetermined conditions are met. If a predetermined condition is not met, the microdroplet may be unwanted in the array of the microfluidic chip 102 and therefore it would be beneficial to remove it from the array such that the space can be used more effectively by populating it with a microdroplet that does meet the one or more predetermined conditions. In a further example, if a predetermined condition is met, the microdroplet may have particularly desirable qualities and it may be wanted outside of the microfluidic chip 102 for further processing and / or analysis. Therefore it would be beneficial to remove it from the array. In the case where the microdroplet 532 is moved from the inlet portion 502 along the eighth virtual pathway 522 to the end of a branch in the first subset 530 and subsequently further microdroplets 534, 536 are positioned along the same branch of the subset 530, if the microdroplet 532 is determined not to meet one or more predetermined conditions, or determined to meet one or more predetermined conditions such that its extraction from the arrays is beneficial, it is not straightforwardly extracted from its position, as it is effectively blocked from moving back along the branch and along the eighth virtual pathway 522 to the discharge section 108. Sprites within the first subset 530 are controlled such that the microdroplet / sprite pairs can be moved between different branches of the first subset (and hence between rows of a substantially rectilinear microdroplet array). For example, a first microdroplet 532 is identified as being an unwanted microdroplet as it does not meet one or more predetermined conditions, or a desired droplet as it does meet one or more predetermined conditions. The microdroplets 532, 534, ,536, 538, 540, 542 of the first subset 530 are rotated such that the microdroplets 532, 534, ,536, 538, 540, 542 are sequentially presented to the main portion of the eighth virtual pathway 522 that is directed to the discharge section 108 and therefore effectively acts as a virtual dispensing pathway 522. The sequential presentation of microdroplets 532, 534, 536, 538, 540, 542 is shown at Figures 5B and 5C. The system 500B of Figure 5B shows sprites moving microdroplets within the first subset 530. The identified microdroplet 532 moves to the position where a different microdroplet 534 was previously situated, as shown in Figure 5A. Similarly, a second microdroplet 534 moves to where a third microdroplet 536 was, which in turn moves to where a fourth microdroplet 538 was, which fourth microdroplet 538 moves to where a fifth microdroplet 540 was positioned, which fifth microdroplet 540 moves to where a sixth microdroplet 542 was situated, which sixth microdroplet 542 moves to where the first, unwanted, microdroplet 532 was previously situated. Therefore, the microdroplets 532, 534, 536, 538, 540, 542 have effectively moved in a carousel configuration, maintaining the remaining microdroplets 534, 536, 538, 540, 542, whilst moving the identified microdroplet 532 closer to the virtual dispensing pathway 522. In rotating the carousel of the first subset 530, the third microdroplet 536 moves from a first row of a first branch of the subset 530 to a second row of a second branch of the first subset 530, whilst the sixth microdroplet 542 moves from the second row of the second branch of the subset 530 to the first row of the first branch of the first subset 530. The system 500C of Figure 5C shows sprites moving microdroplets within the first subset 530. The identified microdroplet 532 moves to the position where the second microdroplet 534 was previously situated, as shown in Figure 5B. Similarly, the second microdroplet 534 moves to where a third microdroplet 536 was, which in turn moves to where a fourth microdroplet 538 was, which fourth microdroplet 538 moves to where a fifth microdroplet 540 was positioned, which fifth microdroplet 540 moves to where a sixth microdroplet 542 was situated, which sixth microdroplet 542 moves to where the first, unwanted, microdroplet 532 was previously situated. Therefore, the microdroplets 532, 534, 536, 538, 540, 542 have effectively moved in a carousel configuration, maintaining remaining microdroplets 534, 536, 538, 540, 542, whilst moving the identified microdroplet 532 closer to the virtual dispensing pathway 522. In rotating the carousel of the first subset 530, the second microdroplet 534 moves from the first row of the first branch of the subset 530 to the second row of the second branch of the first subset 530, whilst the fifth microdroplet 540 moves from the second row of the second branch of the subset 530 to the first row of the first branch of the first subset 530. Following this further rotation of the carousel defined by the first subset 530, the identified first microdroplet 532 is presented to the virtual dispensing pathway 522 portion of the eighth virtual pathway 522. Therefore the identified first microdroplet 532 is removed from an otherwise inaccessible part of the array of microdroplets and can be directed to the discharge section 108 of the microfluidic chip 102. Further rotation of the subset 530 carousel is shown at Figure 5D, where system 500D shows further evolution of the system 500C shown at Figure 5C. The system 500D of Figure 5D shows sprites moving microdroplets within the first subset 530. The identified microdroplet 532 moves to the virtual dispensing pathway 522. The second microdroplet 534 moves to where a third microdroplet 536 was, which in turn moves to where a fourth microdroplet 538 was, which fourth microdroplet 538 moves to where a fifth microdroplet 540 was positioned, which fifth microdroplet 540 moves to where a sixth microdroplet 542 was situated, which sixth microdroplet 542 moves to where the first, identified, microdroplet 532 was previously situated. Therefore, the retained microdroplets 534, 536, 538, 540, 542 have effectively moved in a carousel configuration, whilst the identified microdroplet 532 is extracted via the eighth virtual pathway 522 acting as a virtual dispensing pathway 522. In rotating the carousel of the first subset 530, the fourth microdroplet 538 moves from the second row of the second branch of the first subset 530. Following rotation of the microdroplets within the first subset 530 and removal of the identified first microdroplet 532, a space is created in the array, which can be populated with a further microdroplet that is directed from the inlet portion 502 of the microfluidic chip 102. The second subset 550 is independently controlled with respect to the first subset 530 such that microdroplets held within the second subset 550 are interrogated and moved in order to identify and discard microdroplets that do, or do not meet a predetermined condition in a manner analogous to the first subset 530. The second subset 550 is modulated in order to sequentially present microdroplets to the eighth virtual pathway 522, acting as a virtual dispensing pathway 522. The simultaneous control of multiple, different subsets, such as the first subset 530 and the second subset 550 enables microdroplets to be identified and removed from portions within an array that would otherwise be inaccessible in a time efficient manner, meaning that improved packing within an array, with a higher yield of microdroplets meeting predetermined conditions is provided. This is particularly advantageous where microdroplets contain materials that have a limited lifetime for processing and / or analysis, either within the microfluidic chip 102, or outside the microfluidic chip 102. Whilst the first subset 530 and second subset 550 are each shown to have six sprites associated with six microdroplets, in further examples the subsets each comprise a different number of sprites. For example, the subsets comprise eight, ten, twelve or more sprites, arranged across multiple columns in an array. Advantageously, by including more sprites in a subset that effectively rotate in order to sequentially present microdroplets to a virtual dispensing pathway, fewer virtual dispensing pathways are required, thereby increasing the available space within an array portion. Whilst the first subset 530 and the second subset 550 are each shown to have the same number of sprites, in further examples, different subsets include different numbers of sprites. Beneficially, using different sized subsets enables the accommodation of features and / or differently shaped arrays to be implemented. Whilst the array described at Figures 5A to 5D is loaded with microdroplets in accordance with the methods explained with reference to Figures 1 to 4, in further examples, arrays of microdroplets held within an array portion 502 are loaded in any suitable manner whilst facilitating the carousel rotation of subsets of the array sequentially to present identified microdroplets to dispensing virtual pathways such that multiple columns of an array are accessible to the same dispensing virtual pathway. For example, whilst the carousel rotation in Figures 5A to 5D is shown making a counter-clockwise rotation of each subset of six microdroplet / sprite pairs, it will be apparent that, depending on the initial location of the down selected microdroplet / sprite pair, then a clockwise rotation of the carousel may enable the down selected pair to be more efficiently presented to the virtual pathway, thus facilitating its removal from the array to the discharge section 108. Figure 6A shows a system 600A with a configuration for translocating microdroplets within an array. The configuration provides an efficient and effective way to discharge unwanted microdroplets and / or wanted microdroplets for further processing and / or analysis, through the identification and removal of droplets, for example droplets that either meet one or more predetermined conditions, or droplets that do not meet one or more predetermined conditions. The configuration provides a way of retaining desirable microdroplets meeting one or more predetermined conditions, as well as a way of selectively disposing of remaining microdroplets. The system and method described with reference to Figure 6 is implementable in addition, or as an alternative to other systems and methods, such as the system and method described with reference to Figure 5. Figure 6A shows a system 600A that is analogous to the populated array 300 shown at Figure 3. There is shown a plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 from an inlet portion 602 to an array portion 606 via a fan portion 604. The plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 each include a number of branches that provide locations for microdroplets to be held for analysis. The branches of the plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 are arranged to form a substantially rectilinear microdroplet array comprising a plurality of rows and a plurality of columns, such that the rows and the columns are substantially orthogonal. For example, as shown in Figure 6A, the branches of adjacent virtual pathways of the plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 are substantially aligned rows and branches associated with each virtual pathway of the plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 to enable microdroplets to be held in a grid-like array. The microdroplets that are held in the array are each associated with a single sprite. The plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 extend from the array portion 606 towards a discharge section 108. Optionally, the discharge section comprises a further space 690, which may comprise an outlet from the microfluidic chip, such as the microfluidic chip 102 described with reference to Figures 1 to 5. Accordingly, the plurality of virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 also act as dispensing virtual pathways 608, 610, 612, 614, 616, 618, 620, 622, 624, 626 for the purpose of removing microdroplets from the microfluidic chip 102. In the example of Figure 6A, the eighth virtual pathway 622 is shown to branch within the array such that microdroplets 632, 634, 633 are temporarily positioned along a first branch of the eighth virtual pathway 622. Further microdroplets 642, 644, 646 are temporarily positioned along a second branch of the eighth virtual pathway 622. Further microdroplets 652, 654, 656 are temporarily positioned along a third branch of the eighth virtual pathway 622. Further microdroplets 662, 664, 666 are temporarily positioned along a fourth branch of the eighth virtual pathway 622. Further microdroplets 672, 674, 676 are temporarily positioned along a fifth branch of the eighth virtual pathway 622. Further, unlabelled, branches of the eighth virtual pathway 622 are shown. In further examples, the number of branches and / or the number of microdroplet positions along each branch varies. An interrogation component 114 of an optical assembly 112, as described with reference to Figures 1 to 5, is used to inspect each microdroplet in order to determine whether one or more predetermined conditions are met. If a predetermined condition is met, the microdroplet may be of particular interest. The identified droplet may be required for further analysis and / or processing outside of the array. Therefore it would be beneficial to remove it from the array. In the example of Figure 6A, microdroplets 646, 654 and 672 are identified as microdroplets meeting particular predetermined conditions, through interrogation of the microdroplets in the array portion 606. In order to extract the identified microdroplets 646, 654, 672, a system for shuffling microdroplets and systematically creating new virtual dispensing pathways is described with reference to Figures 6A to 6D. In a first step, the first microdroplet along each of the branches of the eighth virtual pathway between the identified microdroplet 646 and the discharge section 108 are moved to the eighth virtual pathway 622. For example, the first microdroplet 656, 666, 676 along the third, fourth and fifth branches of the eighth virtual pathway 622 are moved into the eighth virtual pathway 622, along with the corresponding microdroplets positioned along the sixth to tenth branches of the eighth virtual pathway. Once these microdroplets within the array portion 606 have been shuffled, a space between the unwanted microdroplet 646 of the second branch of the eighth virtual pathway 622 and the discharge section 108 is presented. The microdroplets that have been moved to the eighth virtual pathway 622 can be translocated to the discharge section 108. For example, advantageously, where the microdroplets are not desired, they can be collected in the discharge section 108 prior to being ejected from the microfluidic chip 102. Clearing microdroplets from the array portion 606 frees up space for the microfluidic chip 102 to be reused for the further analysis of the remaining, desirable microdroplets. The system 600B of Figure 6B shows the implementation of a further electrowetting virtual pathway 621 for the identified microdroplet 646 of the second branch of the eighth virtual pathway 622 to be transported directly to the discharge section 108. The identified microdroplet 646 can be dispensed from the microfluidic chip 102 for further analysis / processing, or retained in a different space in the microfluidic chip 102 for further analysis / processing. The system 600C of Figure 6C shows the arrangement of Figure 6B where the identified microdroplet 646 of the second branch of the eighth virtual pathway 622 has been removed towards the discharge section 108. Additionally, microdroplets on the branches in the array portion 606 between the identified microdroplet 654 of the third branch of the eighth virtual pathway 622 and the discharge section 108 have been transported to the eighth virtual pathway 622, where they are directed to the discharge section 108 for ejection from the chip to waste. Following the microdroplets shuffling into the eighth virtual pathway 622, a space between the identified microdroplet 654 of the third branch of the eighth virtual pathway 622 and the discharge section 108 is provided. A yet further virtual pathway 619 is formed between the identified microdroplet 654 and the discharge section 108, such that the identified microdroplet 654 is transported via the yet further virtual pathway 619 to the discharge section 108 from whence the desired microdroplet 654 can be dispensed from the chip 102 for further analysis / processing. The system 600D of Figure 6D shows the arrangement of Figure 6C where the identified microdroplet 654 of the third branch of the eighth virtual pathway 622 has been removed toward the discharge section 108. Additionally, microdroplets on the branches in the array portion 606 between the identified microdroplet 672 of the fifth branch of the eighth virtual pathway 622 and the discharge section 108 have been transported to the eighth virtual pathway 622 that was used to dispense the identified microdroplet 654 of the third branch. Following the microdroplets shuffling into the eighth virtual pathway 622 a space between the identified microdroplet 672 of the fifth branch of the eighth virtual pathway 622 and the discharge section 108 is provided. Another yet further virtual pathway 617 is formed between the identified microdroplet 672 and the discharge section 108, such that the identified microdroplet 672 is transported to the discharge section 108, via the yet further virtual pathway 617, where the identified microdroplet 672 is optionally dispensed via an outlet of the microfluidic chip 102 for further analysis / processing, or retained in a further space 690 for further analysis / processing. Whilst Figures 6A to 6D show the sequential movement of columns or rows of microdroplets to the eighth virtual pathway 622 to be ejected from the chip, with the interleaved extraction of identified microdroplets, in further examples, all of microdroplets hindering the creation of virtual pathways, and direct extraction of identified microdroplets, are sent to the eighth virtual pathway 622 and subsequently to the discharge section 108 prior to the creation and extraction of the identified microdroplets to a different dispensing mechanism of the discharge section 108. Beneficially, the column by column (or row by row) ejection of selected microdroplets not meeting one or more predetermined conditions, from the chip, is followed by the movement of one or more selected microdroplets meeting one or more predetermined conditions, to dispense, for further analysis / processing. Advantageously, the sequential movement of a subset of microdroplets in a direction to a virtual dispensing pathway and the subsequent creation of a different dispensing pathway in a different direction to the virtual dispensing pathway means that identified microdroplets are efficiently and elegantly extracted from an array, whilst the remaining microdroplets are removed to a different section, for example, to a waste section. Beneficially, shuffling microdroplets in this manner is faster than other configurations for extracting identified microdroplets, as redundant movement is reduced by the direct movement of multiple microdroplets to create virtual dispensing pathways. Further in contrast to the carousel configuration described with reference to Figure 5, the configuration of orthogonal movement of columns / rows in Figure 6 means that microdroplets are not directed in patterns with corners / tight angles which are more suited to widely spaced arrays. Whilst the discharge section 108 described with respect to Figures 1 to 7 is shown at a location on an opposite side of an array portion in a microfluidic space 106 of a microfluidic chip 102 to an inlet 104 and associated inlet portion, in further examples the discharge section 108 is arranged in any suitable manner to receive, sequentially both wanted and unwanted microdroplets. In some embodiments the discharge section 108 may be adjacent to the inlet portion. Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. 5 “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein. 10 Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described. It will further be appreciated by those skilled in the art that although the invention has been described by way of example with reference to several embodiments, it is not limited 15 to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined in the appended claims.

Claims

1. A system for manipulating microdroplets, the system comprising:an optical assembly;a microfluidic chip including a microfluidic space in which a microdroplet array can be formed comprising a plurality of columns and a plurality of rows, substantially orthogonal to the columns;an interrogation component configured to inspect each microdroplet to determine whether one or more predetermined conditions are met; anda controller configured to:selectively control the optical assembly to modulate a plurality of sprites such that each microdroplet received at the inlet is attached to a single sprite;selectively control the optical assembly to modulate the plurality of sprites such that each microdroplet moves along a virtual pathway comprising one or more of the rows and columns in order to form the microdroplet array;modulate a subset of the plurality of sprites corresponding to two rows of a predetermined column of the microdroplet array to rotate thereby presenting microdroplets of the subset sequentially to a virtual dispensing pathway associated with an adjacent column of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are transported around the chip via the virtual dispensing pathway.

2. The system according to claim 1, wherein the interrogation component is further configured to associate a unique identifier with each microdroplet and sprite pair.

3. The system according to claim 2, wherein the interrogation component is further configured to image at least one microdroplet and to determine the size and / or content of the microdroplet based on at least one image of the microdroplet.

4. The system according to claim 3, wherein the controller is configured to determine the size and / or content of the microdroplet based on a plurality of images.

5. The system according to claim 3, wherein the controller is configured to use a machine learning algorithm to determine the size and / or content of the microdroplet.

6. The system according to any preceding claim, wherein the subset comprises six, eight, ten, twelve or more sprites.

7. The system according to any preceding claim, wherein the controller is further configured to: concurrently modulate a different subset of the plurality of sprites corresponding to two or more different rows of the predetermined column of the microdroplet array, thereby presenting each microdroplet of the different subset sequentially to the virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are dispensed via the virtual dispensing pathway.

8. The system according to any preceding claim, wherein the controller is further configured to: concurrently modulate a further subset of the plurality of sprites corresponding to two or more rows of a different predetermined column of the microdroplet array, thereby presenting each microdroplet of the further subset sequentially to the virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are ejected from the chip.

9. A computer implemented method of manipulating microdroplets comprising: modulating a subset of a plurality of sprites corresponding to two rows of a predetermined column of a microdroplet array to rotate thereby presenting microdroplets of the subset sequentially to a virtual dispensing pathway associated with two adjacent columns of the microdroplet array such that those microdroplets that do not meet at least one of the predetermined conditions are ejected from the chip.

10. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of claim 9.

Images