Fluid measurement device

Abstract

A microfluidic device 1 for determining the impedance of a fluid F comprises a printed circuit board 20 comprising a pair of conductive tracks 21, 22 arranged within a plane of the PCB and a microflui

Description

The invention relates to a liquid measurement device for obtaining measurements on a fluid and particles within a fluid, in particular a microfluidic device for determining the impedance of the fluid flowing through the microfluidic channel. BACKGROUND Cell enumeration and identification are essential processes in modem medical research and diagnostics. Accordingly, methods for the analysis of bodily fluids using various technologies have become a growing area of interest to the scientific community. Existing automated cell counting devices can be categorised into three main groups based on their method of detection: optical analysis, image analysis, and electrical impedance. Such devices have many applications including haematological analysis, semen analysis, and urine analysis. In particular, a complete blood count (CBC) test, a type of haematological analysis, provides an estimated cell count for each blood cell type in a given sample. CBC tests are one of the most commonly used diagnostic tools in hospitals today to monitor the blood count of patients undergoing chemotherapy. Chemotherapy aims to destroy cancer cells, however in the process, it affects the growth and replication of normal cells, such as blood cells produced in the bone marrow. Abnormal counts of red blood cells, white blood cells, or platelets can lead to various health complications and therefore needs to be identified early on. Such information is vital to a medical professional in order to develop an informed treatment plan based on the patient’s current health status. CBC tests may additionally provide a measure of the number of each type of white blood cell present in a sample, also known as a white blood cell differential count. This information is especially important with the observed rise in antibiotic resistant bacteria caused by the overprescription of antibiotics by doctors. Consequently, when a patient develops a bacterial infection, it needs to be identified quickly. Currently, hospitals use sophisticated machinery to employ methods of optical analysis, electrical impedance, or centrifugal separation to carry out the blood tests discussed above. Tests of this kind are carried out using large and complex equipment, often with hazardous moving parts, restricted to use in hospitals or laboratories. Further, the equipment used to carry out blood tests is inflexible and may comprise excessive equipment unnecessary for the desired test at hand. For example, a patient may only need a CBC without a differential white blood cell count, such that specific components of the equipment are redundant. This complicates the process and may result in unnecessary labour and / or power consumption when running such tests. Therefore, there is a need for a more compact device to replace the bulky and expensive equipment, and a device that is, ideally, suitable for at-home use without the need of a trained professional. In particular, impedance flow cytometry is a technique used in cell biology and clinical diagnostics to analyse the properties of cells (i.e. the physical and electrical properties) as they flow in a through a microfluidic channel. In this technique, electrodes are arranged proximal to the microfluidic channel (such as underneath the channel). When a cell passes between the electrodes, the electric field generated between the electrodes is disrupted, which results in a change in impedance. The impedance signal can be analysed to determine different cell type. However, arranging the electrodes relative to the channel may be challenging due to the small scale of the device. Misalignment of the electrodes can cause variations in the impedance measurements which reduces the reliability of the measurements. Therefore, it is desirable to provide a device which simplifies fabrication while improving the accuracy of measurements obtained using an opposed pair of electrodes. There is also a need for such improvements to devices for measuring the impedance of a fluid and its content more broadly, for example for identifying other biological particles within a fluid, such as platelets, as well as non-biological particles such as microplastics, as well as bubbles or other variations in the composition or structure of a fluid detectable by measuring the impedance of the fluid. Due to the above-mentioned problems, there exists a need for a device that can return accurate results, is cost-effective, simple in design, and appropriate for point-of-care and at-home fluid analysis without the need for operation by a trained professional. SUMMARY OF INVENTION It is an object of the present invention to provide a cell counting device which makes progress in solving some of the problems identified above. According to a first aspect of the present invention, there is provided a microfluidic device for determining the impedance of a fluid, the microfluidic device comprising: a printed circuit board (PCB) comprising a pair of conductive tracks arranged within a plane of the PCB and a microfluidic channel configured to conduct a flow of a particle-containing fluid; wherein the conductive tracks are arranged to form two opposed electrodes either side of the microfluidic channel such that a impedance signal from the electrodes can be used to determine the impedance of the fluid flowing through the microfluidic channel. In some examples, the fluid is a particle-containing fluid and wherein the impedance signal from the electrodes can be used to determine the presence of a target particle flowing through the microfluidic channel. A microfluidic device is used to manipulate small volumes of fluid. Typically, microfluidic devices comprise a microfluidic channel configured to conduct a fluid. Often, these microfluidic devices are fabricated using materials like polymers, glass, or silicon. In contrast to these typical materials, the microfluidic device of the present invention comprises a PCB. A PCB (or a “printed wiring board”) is often used to connect components to one another in a circuit. A typical PCB has a laminated sandwich structure of conductive and insulating layers. A PCB often comprises a substrate for providing the board with mechanical support, at least a conductive layer (such as copper) having a pattern of traces, planes and other features, and at least a protective layer over the conductive layer. Electrical components are fixed to conductive pads on the outer layers of the PCB in a shape designed to accept the terminals of the components. By including a PCB in the microfluidic device of the present invention, electronic components can be easily integrated into the system. For example, various sensors and / or circuitry can be integrated into the device using the PCB. Further, by using a PCB, the device is simplified in design and provides a more compact system. The present invention further includes a pair of conductive tracks arranged within a plane of the PCB, where the conductive tracks are arranged to form two opposed electrodes either side of the microfluidic channel. The term "within the plane of the PCB" refers to the electrodes being oriented along the same plane as the PCB itself. In this way, the electrodes may be situated in or on the surface of the PCB board. In existing microfluidic devices that employ electrodes to obtain measurements of the fluid / particles in the fluid, the electrodes are arranged under a microfluidic channel, which is not within the plane of the PCB. Therefore, the electrodes of existing devices are not within the plane of the PCB. Arranging the electrodes within the plane of the PCB simplifies the fabrication of the device while providing more accurate measurements of impendence. The two opposed electrodes are arranged either side of the microfluidic channel. By arranging the electrodes in this way, they can be used to generate a non-uniform electric field across the microfluidic channel. This is advantageous as the electrodes can be used to measure the electrical impedance of a fluid or cells in the fluid. Changes in impedance can be used to detect and characterise cells, particles, or molecules present in the fluid. By measuring the impedance of cells flowing through the microfluidic channel, the presence of a target particle or cell can be determined. The term “impedance signal” refers to a signal that carries information about the impedance characteristics of an electrical circuit. It therefore provides information about how an electrical circuit or material resists and / or reacts to an alternating current. In this way, impedance signal may refer to measurements of frequencies of voltage and / or current. In some examples, the impedance signal comprises a measured voltage change, i.e. a change in voltage measured across the electrodes due to the fluid, or particles within the fluid, passing between the electrodes. In some examples, to obtain an impedance signal using the electrodes, an electric current, such as an alternating current (AC) is applied across the pair of opposed electrodes. The electrodes are made of a conductive material. The pair of opposed electrodes are arranged so as to create an electric field across the microfluidic channel. By measuring the voltage change and current, an impedance signal can be obtained. The particle or cell type can be determined based on this measured impedance since different cells have different physical and chemical properties which affect the cells ability to impede electrical current. Moreover, the size, shape membrane and intracellular contents of the cell also affect the how the cells impede electrical current. The impedance may be measured at one or more frequencies of AC signal. The target particle may be any particles or molecules within a solution or fluid. In some examples, the target particle may refer to an air bubble in the microfluidic channel. In some examples, the particle is a biological particle, molecule or cell of the body found in a bodily fluid. In some examples, the target particle may be a microplastic in, for example, a water solution. In some examples, the particlecontaining fluid may be prepared prior to its passage through the microfluidic channel. The terms particle and cell are used interchangeably throughout this specification. The present invention may be used to obtain red blood cell counts and / or white blood cell counts and / or a complete blood cell count. Further, the present invention has applications in pollen counting, milk compositions and microplastics in water. In some examples, the microfluidic device further comprises a control unit configured to apply an alternating current across the opposed electrodes; and a first detection unit configured to receive an impedance signal from the opposed electrodes and determine the presence of a target particle flowing through the microfluidic channel based on the measured impedance. In some examples, the first detection unit is configured to receive an impedance signal from the opposed electrodes and determine the presence or type of the fluid flowing through the microfluidic channel based on the measured impedance. Advantageously, by including a control unit in the microfluidic device, different frequencies of alternating current can be applied across the pair of electrodes using the control unit thereby improving the range of measurements that can be obtained using the device. By including a first detection unit the impedance signal can be analysed to determine a presence of the target particle. In some examples, the first detection unit may be further configured to determine a type of target particle from a plurality of target particle types. In some examples, the opposed electrodes are formed within a layer of the PCB. Preferably, the layer is a conductive layer of the PCB. Preferably, the electrodes are formed by etching a conductive layer of the PCB. Preferably the conductive layer is etched using a laser. Preferably, the conductive layer is a copper layer. By arranging the opposed electrodes within a layer of the PCB, the impedance signal that can be obtained by the device is improved since this arrangement provides better signal integrity. In existing microfluidic devices including electrodes, the electrodes are external to the microfluidic device and will require fixing to or arranging proximal to the microfluidic device. Therefore, forming the electrodes within a conductive layer of the PCB simplifies fabrication of the device while still allowing accurate measurements to be obtained using the device. In some examples, the electrodes are fabricated in the device so as to provide the walls of the microfluidic channel. In this way, the electrodes are in contact with the particle-containing fluid and can therefore obtain more accurate measurements. In some examples, a gap between the opposed electrodes has a minimum width dimension of 1pm -200 pm, preferably 1pm -100 pm, preferably 1pm - 50 pm. In examples where the opposed electrodes provide the walls of the microfluidic channel, the microfluidic channel therefore also has a minimum width dimension of 1pm -200 pm, preferably 1pm -100 pm, preferably 1pm - 50 pm. In some examples, a minimum width dimension of the microfluidic channel is less than the diameter of a target cell or particle type within the fluid when in a relaxed state, such that the target particle type is deformed from its relaxed shape as it flows through the microfluidic channel. The term “minimum width dimension” is used to refer to the smallest distance between the walls of the microfluidic channel. In particular, the width dimension may be measured in a direction across the channel, perpendicular to the flow direction. In some examples, this means that the minimum width dimension between the opposed electrodes is less than the diameter of a target particle or cell type within the fluid when in a relaxed state. The term “relaxed state” is used to refer to a cell in its natural state with no external forces causing it to deform or change in shape from its natural dimensions. In this embodiment, the target cell type will deform as it flows through the microfluidic channel based on its mechanical and elastic properties. When deformed, the target cell is in contact with the walls of the microfluidic channel such that frictional forces acting on the target cell cause it to slow down. A different cell type will deform by a different degree to the target cell type due to its different mechanical and elastic properties. In some examples, the microfluidic device further comprises a two or more pairs of electrodes, where each pair arranged to form opposing electrodes arranged either side of the microfluidic channel. In some examples, the microfluidic device further comprises a plurality of pairs of electrodes, where each pair arranged to form opposing electrodes arranged either side of the microfluidic channel. Each pair of opposed electrodes may be arranged at different locations along and / or around the microfluidic channel. In some examples, the plurality of pairs of electrodes are arranged at a plurality of positions along the longitudinal axis of the channel. Advantageously, including a plurality of pairs of electrodes in the device allows the device to obtain more than one impedance signal / measurement using the various pairs of opposed electrodes, thereby improving the accuracy in measurements. In some examples, one or more pairs of electrodes of the plurality of pairs of electrodes can be configured to obtain a reference impedance signal. A reference impedance signal is a signal obtained when no cell is passing between the pair of electrodes. A one or more different pairs of electrodes of the plurality of pairs of electrodes may then be used to obtain an impedance signal measurement when the target cell passes through these electrodes. The impedance signal can then be compared to the reference signal to determine the presence of a target cell type between the different pair of electrodes. By comparing these signals, an accurate measurement results can be obtained that is used to determine the presence and / or type of target cell. This method is particularly advantageous for smaller cells as these produce a smaller change in impedance thereby providing a less noticeable and distinct impedance signal. In some examples, the device comprises one or more sets of electrodes, where each set of electrodes comprises an electrode positioned on one side of the microfluidic channel and an opposed electrode on the other side of the microfluidic channel. Each set of electrodes may comprise an additional electrode, such that there are two or more electrodes on one side of the microfluidic channel and a single electrode on the opposing side of the microfluidic channel. In this way, each of the two or more electrodes may provide an impedance signal together with the opposing electrode, where one of the impedance signals may act as a reference for the other. In some examples two or more of the electrodes within a set may be vertically offset from each other (i.e. offset in a direction normal to the plane of the PCB). In some examples there may be a plurality of sets of electrodes arranged along the length of the microfluidic channel. In these examples, electrodes from different sets, that are on the same side of the microfluidic channel, may be vertically offset from one another. Preferably, the electrodes on the same side of the microfluidic channel, within neighbouring sets of electrodes are vertically offset from each other. With these examples, a greater volume of the channel may be measured. Vertically offset electrodes may be formed by fabricating the electrodes within different conductive layers of a multi-layer PCB. In some examples, the frequency of the alternating current applied may differ between the one or more of the electrode pairs of the plurality of electrode pairs such that impedance measurements can be obtained at more than one frequency. This further improves the accuracy of obtained measurements. In a specific example, at least a first pair of the plurality of pairs of electrodes may be configured to obtain a reference signal, at least a second pair of the plurality of pairs of electrodes may be configured to obtain an impedance signal at a first frequency and at least a third pair of the plurality of pairs of electrodes may be configured to obtain an impedance signal at a second frequency. Additional pairs of electrodes may also be included in the device to obtain impedance signals at other frequencies. In some examples, the microfluidic channel is formed within a layer of the PCB. The layer of the PCB in which the microfluidic channel is formed may be the same or different to the layer in which the opposed electrodes are formed. By forming the channel in the PCB there is an improved use of space and reduced complexity with fabricating the device. When the microfluidic channel is formed may be the same or different to the layer in which the opposed electrodes, this further improves the optimisation of space. In some examples, the microfluidic channel is formed within the solder mask layer or a pressure sensitive adhesive, PSA, layer of the PCB. The solder mask layer is a protective layer applied to the surface of the PCB. In this way, the microfluidic channel will be protected while simplifying fabrication and reducing complexity of the device. In some examples, the PCB comprises a fluid processing feature positioned within the microfluidic channel upstream of a measurement point between the opposed electrodes. In examples where there is a plurality of electrodes, there may be a measurement point between each pair of electrodes. Therefore, in this example, the fluid processing feature is upstream of all these measurement points. The fluid processing feature is a feature which is used to manipulate and / or control the movement of fluid and / or particles in the particle containing fluid upstream of the measurement point. For example, the fluid processing feature may be used to sort the target cells from other particles in the fluid such that by the time the fluid reaches the measurement point, only the target cells remain in the fluid (and have not been separated out). This improves the accuracy of measurements obtained by the device as it removes noise in the signal that may be caused by other particles. Therefore, this improves the measurement conditions at the measurement point. In some examples, the fluid processing feature comprises: an inlet for receiving a particle-containing fluid comprising a plurality of particles types; a particle separator configured to receive the particle-containing sample and separate a target particle type from the plurality of particle types so as to output a particle containing fluid comprising the target particle type. The particle separator comprises one or more of: a lysing module configured to apply a lysing solution to the particle-containing fluid; an inertial microfluidics module configured to separate particle types using inertial microfluidics; a magnetic separation module configured to separate particles bonded to a magnetic label. Advantageously, including a means for separating the target cells as described above allows the target particle type to be separated from other cells in the fluid before they reach the measurement point. The particle separator may separate the target particles using a passive and / or an active process. The process may be a continuous separation process. When a passive process is used, it does not require additional energy to be provided to the system to carry out the separation process. In some examples the fluid processing feature comprises a metering device configured to obtain measurement of fluids in the form of flow rate and / or fluid volume and / or control the flow rate and / or fluid volume prior to the fluid entering the microfluidic channel. In this way, the fluid processing feature ensures that the correct amount of fluid is delivered to the microfluidic channel so as to aid in obtaining accurate measurements of the fluid. In some examples, the PCB comprises a micropillar array formed within the microfluidic channel and arranged to separate particles within the particle containing fluid by deterministic lateral displacement (DLD). In some examples, the micropillar array comprises one or more micropillars configured for displacing a target particle type within the particle-containing fluid. The term “micropillar” refers to a structure on the microscale. This structure may include one or more substantially vertical columns or pillars. The structure of the micropillar may be cylindrical, triangular or any suitable shape. The term “micropillar array” refers to one or more of these micropillars arranged to form an array, for example, they may be arranged in a specific pattern. The micropillars may be fabricated using etching and / or may be cut into a layer of the PCB using a laser. DLD occurs as the particle-containing fluid flows past the one or more micropillars of the micropillar array. The cells and particles may collide with the one or more micropillars, causing them to be displaced. Smaller particles will continue to flow along the streamlines of the fluid flow, without notable displacement, while larger particles will undergo a larger displacement due to repeat collisions with the one or more micropillars, resulting in lateral displacement relative to the direction of flow. Continual collisions result in the larger particles being separated from the smaller particles in the cell-containing fluid. Separating the target particles from the other particles in the particle containing fluid improves the accuracy of results obtained at the measurement points in the microfluidic channel. In some examples, the micropillar array may further include at least a portion of the microfluidic channel walls / inner walls of the microfluidic device. These walls may be proximal to the micropillar structures. The specific arrangement of the micropillars in the micropillar array may be based on the target particle type. A separation between the micropillars in the microfluidic array may be based on the target particle type. In specific examples, the separation between micropillars may be greaterfor larger particles, and lesser for smaller particles. An angle between columns and / or rows of the micropillars may be selected based on the target particle type. In some examples, the micropillar array is formed within a layer of the PCB. By including the one or more micropillars in the PCB, the manufacturing process is simplified by avoiding complicated methods usually associated with the fabrication of micropillars while still providing separation of cells as required. In some examples, the micropillars are conductive. In some examples, a dielectrophoretic (DEP) voltage is applied to at least a portion of the micropillar array so as to separate particles within the particle containing fluid. Therefore, a DEP voltage may be applied to one or more of the micropillar structures and / or an inner wall of the microfluidic device. Dielectrophoresis (DEP) employs a non-uniform electric field to exert forces on particles in the particle containing fluid. Since most particles have a level of electrical polarisation, when a non-uniform electric field is applied to the particles, they will develop electric dipoles. These induced dipoles experience a force when subject to a non-uniform electric field, causes the particles to travel in a given direction. By altering the strength of this field, the direction of the particles can be manipulated. The strength of the filed may be altered based on the target particle type. Using both DLD and DED provides effective separation of the target particle type from the particle-containing fluid. The micropillars may be formed in the copper layer. A DEP voltage may be applied to individual micropillars or to a plurality of the micropillars in the micropillar array. The plurality of micropillars may be position in a given portion of the micropillar array, for example, the plurality of micropillars may be located on one side of the array. In this way, the micropillar array is further configured to provide effective separation of cells. Advantageously, by altering the voltage applied to the micropillar array, the direction of the particles can be manipulated and the target cells separated from the other particles in the particle-containing fluid. In some examples, one or more of: a magnitude of the DEP voltage and a frequency of the DEP voltage is based on the target particle type such that the target particle type is effectively sorted and separated from the particle-containing fluid. In some examples, the micropillar array is arranged in a pattern for separating the target particle type. Preferably, the pattern is based on a size of the target particle type. The micropillar array may be arranged in a specific pattern. For example, the pattern may be a periodic arrangement of micropillars, comprising for example, one or more rows of micropillars. In examples where the micropillars are arranged in rows, the rows may be arranged parallel and offset to one another at a predetermined angle. In some examples, a length of the micropillar array is based on the target particle type. The length is measured in the direction of fluid travel. In some examples, the microfluidic channel comprises a measurement channel and a sorting region, wherein the measurement channel is arranged between the opposed electrodes and the micropillar array is provided in the sorting region. Preferably the sorting region is upstream of the measurement channel. By using DLD to sort the cells upstream of the particle fluid channel, the target particle type can be separated from other cells in the fluid before flowing through the measurement channel. This improves the measurements of the particles taken at the measurement point since noise in the signal caused by non-target particles is removed. In some examples, the microfluidic device further comprises a second detection unit. The second detection unit comprises a light source arranged so as to illuminate cells flowing through the microfluidic channel; a photodetector arranged to receive light that has passed through the microfluidic channel at a second measurement point; and a processing unit configured to receive a light signal from the photodetector and determine the presence of a target particle passing the second measurement point based the signal intensity received from the photodetector. In some examples, the light source comprises one or more of a waveguide and an optic fibre. By including a second detection unit in the microfluidic device additional measurements can be obtained by the device thereby improving the reliability and accuracy. For example, the two detection units can be used to cross-check and validate the results, thereby ensuring greater accuracy in measurements. Further, using two different detection means allows for greater specificity in distinguishing between different signals thereby reducing the chances of interference in the signal caused by other particles. The second detection unit uses light to determine the presence of a particle type in the microfluidic device while the first detection unit uses an impedance signal to determine the presence of a target particle. In some examples, the device can use algorithms to filter out noise by combining the signals from the two different detection methods thereby improving the quality of the signal and measurements. Further, including a different detection unit, different types of measurements can be obtained, such that different properties and details about the target particle type can be determined. In some examples, the first measurement point and second measurement point are the same measurement point. In this way, both the first and the second measurement unit, take measurements at the same point in the microfluidic device. This means both unitswill be obtaining measurements of the same target particle when the target particle is at the measurement point. This improves the data obtained by the device. In some examples, the opposed electrodes are arranged along a first longitudinal axis, the light source and the photodetector are arranged along a second longitudinal axis, wherein the first longitudinal axis is at an angle relative to the second longitudinal axis. In some examples, the first and second longitudinal axis intersect at the measurement point. In this way, both the first and second detection unit are arranged to obtain measurements of at the same measurement point. This improves the accuracy of the measurements obtained by the microfluidic device. In some examples, the fluid channel comprises at least one bend. In some examples, the microfluidic channel comprises at least one curve. By having a channel that bends and / or curves, the detection area in the microfluidic channel is larger. Positioning a light source, photodetector and electrodes proximal to the microfluidic channel so as to obtain accurate measurements can be difficult. For instance, if the opposed electrodes are either side of the microfluidic channel, the light source and detector cannot be positioned at the same location. Therefore, it is difficult to further include a light source and detector in the device at an appropriate location so as to obtain accurate measurements. Therefore, the channel advantageously bends such to allow for the light source and detector to be positioned at an appropriate location relative to the microfluidic channel. In some examples, when the first longitudinal axis is approximately perpendicular to the microfluidic channel, the channel may bend or “twist” about the first longitudinal axis. The light source and photodetector can then be positioned about the channel along a second longitudinal axis at an acute angle to the first longitudinal axis. Positioning the detection units in this way means that the measurements can be obtained by the first and second detection unit at the same time, since both detection units can be positioned proximal to the channel as there is a larger detection area. In some examples, at least one bend is configured such that one or more of: the opposed electrodes, and the light source and detector can be positioned for obtaining measurements at the first measurement point and second measurement point respectively. In some examples, the bend is a kink. Therefore the at least on bend may be a sharp twist or curve. The microfluidic channel may have an abrupt change in direction. In some examples, at least a portion of an inner surface of the microfluidic channel is modified so as to reduce clogging of particles in the particle-containing fluid. Preferably, the whole inner surface is modified. In this way, there is a reduced likelihood of the cells sticking to inner surfaces of the channel. This is particularly important as electrodes arranged in this way more prone to fouling from the biological sample, leading to signal degradation over time. Therefore, it is important to reduce the risk of clogging at the electrodes if in the microfluidic channel. In some examples, oxygen plasma is used for surface hydrophilization. Oxygen reacts with the surface, introducing polar functional groups, making it more hydrophilic. Further, oxygen plasma can break bonds and replace non-polar groups with oxygen-containing polar groups, thereby reducing the hydrophobicity of the surface. Plasma-treated surfaces allow for better fluid movement through the microfluidic channels since they reduces clogging of particles. In some examples, at least an inner surface of the microfluidic channel is coated with a material configured to reduce clogging. The material used for coating must therefore be suitable for reducing clogging of cells and / or fluid in the particle fluid channel. In some examples, the material is a Functionalised FR-4 PCB material. In some examples, the device further comprises a processing unit configured to input impedance data into trained machine learning model trained to identify a target particle type based on input data. This improves the accuracy in determining the target particle type. The input impedance data may be one or more of the impedance signals from the electrodes and / or the light signal from the photodetector. In some examples, the processing unit is configured to provide denoising of the one or more of the impendence signals from the electrodes and the light signal from the photodetector. In some examples, the opposed electrodes either side of the microfluidic channel are vertically offset from one another within the plane of the PCB. As described herein, the term "within the plane of the PCB" refers to the electrodes being oriented along the same plane as the PCB itself, i.e. fabricated within a layer of the PCB. Therefore, in this example, the electrodes are still orientated along the same plane as the PCB itself, within the volume of the PCB, but the electrodes are offset within this volume. Therefore, the opposed electrodes have a different vertical elevation to one another within the volume of the PCB, i.e. they may be offset in a direction normal to the plane of the PCB. The pair of opposed electrodes may be fabricated in the same layer of the PCB, but at different vertical elevations, or may be fabricated in different layers of a multi-layerPCB. Advantageous, this example provides compensation for "blind spots", allowing measurement of impedance across a greater amount of the microfluidic channel. Therefore, this example can provide improved detection of target particles in the microfluidic channel. In some examples, the microfluid channel is fabricated in a layer of the PCB. In particular, the microfluidic channel can be fabricated in a substrate layer (a fibreglass layer, such as an FR4 layer) rather than, for example, in the solder mask. The microfluidic channel may be formed by etching processes to form a valley in the substrate (FR4) layer which have been applied to the PCB. Instead of the conductive tracks being on top of the layer, one electrode of the pair of electrodes is on top of the cut FR4 layer and the other of the electrodes is on the bottom of the cut FR4 layer. Therefore, the pair of electrodes have a different vertical elevation to one another on either side of the microfluidic channel. In some examples, under this layer there are other layers of PCB. A top layer may be fabricated over this arrangement. More specifically, the PCB may comprise a multiple layer PCB with multiple substrate (FR4) layers and multiple conductive (copper) layers. A microfluidic channel may be formed in a substrate layer positioned between two conductive layers, with electrodes formed by etching the conductive layers above and below the microfluidic channel substrate layer. In this way, electrodes are formed with differing vertical offset relative to the microfluidic channel. In some examples, the pair of electrodes may include additional electrodes. In other words, the electrodes do not have a one to one ratio. Instead, the pair of electrodes may comprise a first electrode on one side of the microfluidic channel and a second and third electrode on the other side of the microfluidic channel. In some examples the microfluidic device comprises two or more sets of opposed electrodes wherein: in each set, opposed electrodes either side of the microfluidic channel are vertically offset from one another within the plane of the PCB; and electrodes from different sets, that on the same side of the microfluidic channel, are vertically offset from one another within the plane of the PCB. Preferably there is a vertical offset between electrodes on the same side of the channel in neighbouring sets of electrodes along the length of the channel. According to a second embodiment of the present invention, there is provided a method of manufacturing a microfluidic device comprising: fabricating a pair of conductive tracks within a plane of a printed circuit board (PCB); fabricating a microfluidic channel in the PCB configured to conduct a flow of a fluid; and wherein the conductive tracks are arranged to form two opposed electrodes either side of the microfluidic channel such that an impedance signal from the electrodes can be used to determine the impedance of the fluid flowing through the microfluidic channel. In some examples, the method includes fabricating one or more pairs of conductive tracks within the plane of the PCB, wherein each of the pairs of conductive tracks are arranged to form opposed electrodes either side of the microfluidic channel. In some examples, the fluid is a particle-containing fluid and wherein the impedance signal from the electrodes can be used to determine the presence of a target particle flowing through the microfluidic channel. By using this manufacturing method, the device can be fabricated easily and consistently, enabling reproducible production. The process is scalable, as it avoids complex manufacturing techniques like microfluidic channel construction or intricate electrode alignment proximal to the channel. In some examples, the method of manufacturing further comprises configuring a control unit to apply an alternating current across the opposed electrodes; and configuring a first detection unit to receive an impedance signal from the electrodes and determine the presence of a target particle type flowing through the particle containing fluid channel based on the measured impedance. In some examples, the method of manufacturing a microfluidic device further comprises configuring the microfluidic channel to conduct a flow of a cellcontaining fluid, and arranging the electrodes such that an impedance signal from the electrodes can be used to determine the presence of a target cell flowing through the microfluidic channel. In some examples, fabricating the opposed electrodes comprises focusing a laser on at least a portion of a conductive layer of the PCB so as to cut said portion of the conductive layer. Advantageously by using a laser to cut the PCB, the PCB can be cut with precise control. Therefore, the PCB is cut with minimal tolerance errors. This results in opposed electrodes being fabricated with high accuracy and therefore there is consistent quality across all microfluidic devices fabricated in this way. Using a laser allows for much smaller portion of the PCB to be cut than when using conventional fabrication techniques. In this way, the opposed electrodes may be cut so as to have a small, precise gap between them. In some examples, fabricating the opposed electrodes within the plane of the PCB comprises etching at least a portion of a conductive layer of the PCB. In this way, the electrodes are fabricated directed onto the channel layer of the PCB, thereby simplifying the fabrication process. The PCB may be etched using a laser to improve the accuracy of the etching. In some examples, fabricating the opposed electrodes within the plane of the PCB comprises mounting a conducting layer comprising the opposed electrodes to the PCB. In some examples, fabricating the microfluidic channel comprises focusing a laser on at least a portion of a layer of the PCB so as to cut said portion of the layer. Laser cutting produces smooth and clean edges, thereby reducing the need for post-processing of the channel to achieve this effect. Therefore, clogging in the channel is further reduced when the microfluidic channel is fabricated using a laser. Advantageously by using a laser to cut the PCB, a much smaller channel can be fabricated then when using conventional fabrication techniques. In this way, the microfluidic channel can be fabricated to have a minimum width dimension of less than 100 micrometres. Moreover, lasers can be used to cut intricate patterns such that non-conventional microfluidic channels can be cut. In some examples, fabricating the microfluidic channel comprises forming the microfluidic channel within a solder mask layer or a pressure sensitive adhesive (PSA) layer. BRIEF DESCRIPTION OF DRAWINGS Figure 1 schematically illustrates a front view of a microfluidic device according to an embodiment of the present invention; Figure 2 schematically illustrates a embodiment of the present invention; Figure 3A schematically illustrates a embodiment of the present invention; Figure 3B schematically illustrates a embodiment of the present invention; Figure 4 schematically illustrates a embodiment of the present invention; Figure 5 schematically illustrates a embodiment of the present invention; Figure 6 schematically illustrates a embodiment of the present invention; microfluidic device according to an microfluidic device according to an microfluidic device according to an microfluidic device according to an microfluidic device according to an microfluidic device according to an Figure 7 schematically illustrates a method of manufacturing of a microfluidic device according to an embodiment of the present invention; Figure 8 schematically illustrates a microfluidic device according to an embodiment of the present invention. DETAILED DESCRIPTION Figures 1 and 2 schematically illustrate a microfluidic device 1 for measuring the impedance of a fluid F according to the present invention. The following specific description focussed on the application of detecting the impedance of a particle-containing fluid to determine the presence of, and preferably count, target particles flowing within the fluid, such as cells within a biological fluid. However, the present invention can be applied more broadly to the application of measuring the impedance of a fluid to determine its contents. As illustrated on Figures 1 and 2, the microfluidic device 1 comprises a printed circuit board (PCB) 20 comprising a pair of conductive tracks 21 and 22 arranged within a plane of the PCB 20. The device 1 further comprises a microfluidic channel 10 configured to conduct a flow of a particle-containing fluid F. The particle-containing fluid is a solution or fluid containing any particles or molecules within. The target particle 40 is any particle, molecule, or bubble within this fluid F. As shown in Figures 1 and 2, the conductive tracks are arranged to form two opposed electrodes 21 22 either side of the microfluidic channel 10. Typically, the microfluidic channel 10 has a minimum width dimension of 1 pm -200 pm, 1 pm -100 pm, preferably 1 pm - 50 pm. The dimensions of the channel can be based on the target particle type intended to be detected. For example, a relatively larger channel may be more suited to larger particles such as white blood cells, while a relatively small channel may be more suited for red blood cells. The electrodes 21 22 are arranged to obtain an impedance signal which can be used to determine the presence of a target particle 40 flowing through the microfluidic channel 10. In this way, a fluid impedance measuring device may be formed using PCB fabrication techniques, providing precision engineering of the required dimensions in a reproducible and low cost format. The electrodes are formed from the conductive layer of the PCB with the microfluidic channel formed within the substrate, conductive layer or solder mask, between the electrodes. Although not shown in Figure 1, the device 1 further comprises a control unit configured to apply an alternating current (AC) across the electrodes 21, 22. The electrodes 21, 22 are made of a conductive material. The pair of opposed electrodes 21, 22 are configured so as to create an electric field across the microfluidic channel 10 when the current is applied to one or more of the electrodes 21, 22. By measuring the voltage change and current across the electrodes 21,22, an impedance signal can be determined. The device 1 further comprises a first detection unit (not shown) configured to receive the impedance signal from the electrodes 21, 22 and determine the presence of a target cell type 40 flowing through the cell fluid channel 10 based on the impedance signal. The cell type 40 can be determined based on this impedance signal because different cells have different physical and chemical properties which affect how the cells impede electrical current. Moreover, the size and shape of the cell also affect the how the cells impede electrical current. The detection unit may be further configured to determine the size and shape of the cell based on the impendence signal. When a cell is not passing between the electrodes 21,22, a first impedance value may be obtained. This can be referred to as the reference impedance. As a cell 40 passes through the electrodes 21,22, a cell impedance value may be obtained. The reference impedance when no cell is passing between the pairs of electrodes will be different to the cell impedance. Therefore, this change in impedance implies the presence of a cell between the electrodes 21,22. The cell impedance itself or the difference between the reference impedance and the cell impedance can be further analysed by the detection unit to determine the cell type. The cell impedance may be measured at one or more frequencies of AC signal. The arrangement shown in Figures 1 and 2 is particularly advantageous for relatively large particles and / or cells. Since these larger particles change the impendence measurement by a relatively large amount, it is easier to detect and / or determine the cell type using only one pair of electrodes. To determine the cell type from the impedance signal, the first detection unit may comprise a database of known impedance values or ranges for a number of cells within a particular fluid at a given AC frequency. By comparing the measured cell impendence value to values in this database, the cell type can be determined. The cell impendence value may be normalized using the reference impedance. In this example, the electrodes 21,22 provide the walls of the microfluidic channel 10 such that the electrodes 21,22 are in contact with the particle-containing fluid. In other words, at least a first surface of each of the electrodes 21, 22 directly forms a portion of the inner walls of the microfluidic channel 21, 22. In this example, the microfluidic channel is prepared within the conductive layer and solder mask of the PCB such that the walls of the channel are formed by both the etched conductive tracks, forming the electrodes, and the surrounding solder mask. Preferably, as discussed below, a plurality of electrode pairs may be implemented to provide a plurality of measurement points. By recording additional measurements of the impedance, more data is collected to measure the impedance and determine a target particle with a greater degree of certainty. For example the impedance can be measured at a plurality of points along the channel and particles can be tracked as they flow along the channel. This can help in situations where there might be a number of different particles types flowing, possibly at different flow rates, along the channel where measuring the impedance at a plurality of points can help differentiate between the different particles flowing along the channel. In these examples, measurements from one or more of the electrode pairs can be used as a reference measurement. Figure 3A schematically illustrates a microfluidic device 1 comprising a plurality of pairs of electrodes 21-26. Each pair of electrodes 21 22 and 23 24 and 25 26 are arranged to form opposing electrodes arranged either side of the microfluidic channel 10. Each pair of electrodes 21-26 is arranged at a different location in the longitudinal axis of the microfluidic channel 10. In other examples to that shown in Figure 3A, there may be any number of pairs of opposed electrodes for a given microfluidic channel 10. For example, a microfluidic device 1 according to the present invention may include a microfluidic channel 10 having two pairs of electrodes, while in another example the device 1 may include a microfluidic channel 10 having four pairs of electrodes etc. In other examples, the device 1 may comprise a plurality of microfluidic channels 10. Each of the channels 10 may be configured to obtain impedance measurements in any of the ways described herein. Having a plurality of opposed electrodes 20-26 is particularly advantageous when measuring the impedance of small cells or particles, since change in impedance when these cells pass through the measurement point is usually relatively small. For example, a device 1 having two pairs of opposed electrodes may have the same frequency applied to each with one pair of electrodes being the reference electrodes used for obtaining reference impedance measurements and the other pair of electrodes may be used for obtaining impedance measurements. In this way, the reference electrodes provide a reference impendence when a target cell is not passing through the reference electrodes against which the impedance of other pair of electrodes can be compared. This makes it easier to determine a small change in impedance and therefore makes it easier to determine when a small particle is passing through the microfluidic channel. In the example device 1 shown in Figure 3A, a first pair of electrodes 21 22 may be used as the reference electrodes. In other examples, the reference electrodes may be any of the pairs of the electrodes 21-26 shown in Figure 3A. The second pair of electrodes 23 24 may be used to obtain a first impedance signal. A first frequency of AC current may be applied to the first pair of electrodes, so as to obtain a first impedance signal at the first frequency. The third pair of electrodes 25 26 may be used to obtain a second impedance signal. A second frequency of AC current may be applied to the second pair of electrodes, so as to obtain a second impedance signal at the second frequency. Therefore, the arrangement of electrodes 21-26 shown in Figure 3A can be used to obtain a reference signal, a first impedance signal and a second impedance signal. In this way, by obtaining two impedance signals, the determination of the target cell type is more accurate. In some examples, the microfluidic channel 10 comprises a measurement channel and a sorting region, wherein the measurement channel is arranged between the opposed electrodes and a micropillar array 50 is provided in the sorting region. Figure 3B and Figure 3C schematically illustrate a microfluidic device 1 comprising a plurality of sets of electrodes in an alternative arrangement to that of Figure 3A. Figure 3B shows a side view of the PCB having at least three layers and Figure 3C shows the same PCB to that of Figure 3B from a different perspective. As shown, the opposed electrodes either side of the microfluidic channel 10 are vertically offset from one another within the plane of the PCB. In this specific example, electrodes 022 and 023 are arranged in layer 1 of the PCB, the microfluidic channel 10 is arranged in layer 2 of the PCB and electrode 021 is arranged in layer 3 of the PCB. Similarly, electrode 026 is arranged in layer 1 of the PCB, the microfluidic channel 10 is arranged in layer 2 of the PCB and electrodes 024 and 025 are arranged in layer 3 of the PCB. By arranging the electrodes in this way, the opposed electrodes are at different heights within the volume of the PCB. In this example, vertical offset has been achieved by arranging the electrodes in different layers of the PCB, but in other examples the pair of electrodes may be offset within the same layer. In some examples, the microfluidic channel 10 has been etched into a FR4 layer (layer 2). Electrodes 022, 023, 024, 025 have been arranged on top of the FR4 layer. The conductive tracks forming these electrodes may have been fabricated using any of the techniques described herein. Electrodes 021 and 026 have been arranged below the FR4 layer and may have been fabricated using any of the techniques described herein. In some examples, the PCB comprises other layers above and / or below layers 1, 2 and 3. In some examples, the pair of electrodes may include additional electrodes to be a set of electrodes. In other words, the electrodes do not have a one-to-one ratio. Instead, the set of electrodes may comprise a first electrode 021 on one side of the microfluidic channel 10 and a second and third electrode 022, 023 on the other side of the microfluidic channel 10. As shown in Figure 3C, the first set of electrodes comprises electrode 021 on one side of the microfluidic channel 10 and electrodes 022 and 023 on the other side of the microfluidic channel 10. Similarly, the second set of electrodes comprises electrodes 024 and 025 on one side of the microfluidic channel 10 and electrode 021 on the other side of the microfluidic channel 10. Including the first set of electrodes 021, 022 and 023 in a mirroring arrangement to the second set of electrodes 024, 025 and 026 as shown in Figure 3C reduces “blind spots” when detecting particles in the particle-containing fluid. Since particles will be at different heights and locations in the channel, this arrangement improves the volume of the channel that is “seen” by the electrodes. Therefore, this arrangement increases the detection area of the channel 10, providing improved detection of particles, molecules or bubbles in the channel 10. The two-to-one ratio of electrodes shown in Figure 3C may be used to apply the same AC frequency to both electrodes 022 and 023. A single electrode 021 can be used to measure the voltage change created by the AC current. Alternatively, the single electrode may be used to apply an AC current across the electrodes and the two electrodes measure the voltage change at two points across the microfluidic channel 10. The two-to-one ratio of electrodes shown in Figure 3C is particularly advantageous at obtaining measurements at different frequencies. For example, 022 may have a first frequency of AC voltage applied to it while 023 has a second frequency of AC voltage applied. Electrode 021 may then be used to measure the voltage at these two frequencies. Therefore, measurements at different frequencies can be obtained using the electrode arrangements. The impedance can be determined from this. Figure 4 shows a portion of a microfluidic device 1 according to the present invention comprising a measurement channel (not shown) and a sorting region. The PCB comprises a sorting region having one or more micropillars 51 formed within the microfluidic channel 10, the one or more micropillars 51 configured for displacing a target cell type 40 within the cell-containing fluid F. In Figure 4, there is a micropillar portion 50 in the microfluidic channel 10 comprising a plurality of micropillars 51. In other examples, the micropillar portion 50 may additionally or alternatively comprise at least a portion of the inner walls of the microfluidic channel 10. The micropillars 51 are arranged so as to separate particles within the particle containing fluid F by deterministic lateral displacement. Deterministic lateral displacement (DLD) can be used to separate out a target cell type 40 from the particle-containing fluid using one or more micropillars 51 in the micropillar array. The arrangement of the micropillars 51 is based on the target particle type. Figure 4 shows the flow path A of a first particle type 40. The first particle type 40 is the target particle type 40 and is bigger of the two particle types shown in this example. The first particle type 40 collides with the micropillars 51 as a it flows through the micropillar array 50, causing it to be displaced from its natural flow path through the channel 10. Path A shows that the first particle type 40 moves towards the upper outlet of the channel 10 due to this displacement. The micropillar array 50 has been arranged in a specific pattern to allow for this displacement to occur. The arrangement is spaced such that the first particle type 10 interacts sufficiently with the micropillars 51 but has space to move in a lateral direction. In this way, these particles 40 are laterally displacement relative to the flow direction. Path B shows the flow path of a second particles type in the particle containing fluid. Path B continue to flow along the streamlines of the fluid flow, without notable displacement. Therefore, the second smaller particles follow path B towards the lower outlet. In this way, the two particle types are separated and exit via separate outlets. A length of the micropillar array 50 may be based on the target cell type, to allow for this separation to occur. In preferable examples, the micropillar array 50 is formed within a layer of the PCB. For example, the micropillar array 50 is formed within a conductive layer of the PCB, wherein the conductive layer is etched to create the micropillars. In this way, cell sorting features, in addition to the microfluidic channel and electrodes may all be formed on a single PCB chip through standard PCB fabrication techniques. In some examples, to aid in the particle separation, a dielectrophoretic (DEP) voltage is applied to at least a portion of the micropillar array 50 so as to separate particles within the particle containing fluid F. Dielectrophoresis (DEP) is a technique used to manipulate particles or cells in a fluid using a non-uniform electric field. DEP includes applying an alternating current (AC) electric field to a fluid containing particles or cells. This field polarises the particles or cells. The non-uniformity of the electric field generates a force on the polarized particles or cells causing them to be displaced. In this way, DEP can be used to separate particles or cells based on their dielectric properties. In this device 1, particles with different dielectric properties can be directed to different regions of the device by applying a voltage to the micropillar array. For example, with reference to Figure 4, the first particle type 50 may have different dielectric properties to that of the second particle type. Therefore, by applying a different voltage to different micropillars in the micropillar array, non-uniform electric fields can be generated in the microfluidic array resulting on forces acting on the different particle types based on their dielectric properties. In Figure 4, different voltage can be applied to the micropillars 51 such that the first particle type 40 follows path A i.e. these particles move towards the upper outlet, while the second particle type follow a similar path of path B and move towards the lower outlet. Cell movement can be manipulated by varying a magnitude of the DEP voltage and a frequency of the DEP voltage is based on the cells desired to be sorted. In some examples, the micropillars 51 themselves can be used to obtain impedance measurements. In these examples, the micropillar array 50 is formed within a conductive layer of the PCB. In this example, the micropillar array 50 may be arranged in the measurement channel of the device 1. The control unit can be configured to apply an AC current can be applied to one or more of the micropillars 51 and / or the inner walls of the micropillars array 50. The first detection unit is further configured to receive an impedance signal from the electrodes and determine the presence of a target particle flowing through the microfluidic channel based on the measured impedance obtained by applying a current to one or more of the micropillars 51 and / or the inner walls of the micropillars array 50. In any of the described examples, the PCB of the device may comprise a fluid processing feature positioned within the microfluidic channel 10. The fluid processing feature may be upstream of a measurement point between the opposed electrodes. Similar to the micropillar array 50, the fluid processing feature may be configured to separate particles or cells in the particle containing fluid F. The fluid processing feature comprises an inlet for receiving the particle containing fluid a cell separator configured to receive the particle containing sample and separate a target particle type from the plurality of particle types so as to output a particle containing fluid comprising the target cell type. In some examples, the cell separator comprises an inertial microfluidics module configured to separate cell types using inertial microfluidics, as shown in Figure 5. Inertial focusing uses hydrodynamic forces generated by laminar flow in microfluidic channel to align and streamline cells. In other words, inertial microfluidics relies on fluid inertia to manipulate the migration of cells in a channel. As cells flow through a channel, due to an inertial lift force and wall-induced lift force, the cells are forced to align along the centre of the channel. By adjusting a flow rate and the shape and / or size of the channel, cells of different sizes and shapes can be selectively focused and separated into different outlets. As such, the inertial microfluidics module may include a straight, a spiral, a sinusoidal and / or an expansion-contraction microfluidic channel. The choice of channel may be based on the cell type to be separated. The outlet of the inertial microfluidics module may lead onto an inlet of the measurement region of the microfluidic channel. In some examples, the cell separator comprises a magnetic separation module configured to separate cells bonded to a magnetic label. In some examples, the cell separator comprises a lysing module configured to apply a lysing solution to the particle containing fluid, lysing module configured to apply a lysing solution to the cell-containing fluid F. In some examples, the lysing solution is configured to break down target cell type in order to isolate a particular cellular component. Figure 6 shows the microfluidic device 1 further comprising a second detection unit. The second detection unit comprises a light source 61 arranged so as to illuminate cells flowing through the cell fluid channel. The light source may comprise a waveguide and / or an optic fibre. The second detection unit further comprises a photodetector 62 arranged to receive light that has passed through the cell fluid channel at a second measurement point, and a processing unit configured to receive a light signal from the photodetector and determine the presence of a cell type passing a measurement point based the signal intensity received from the photodetector. As shown in Figure 6 the two pairs opposed electrodes 21-24 are arranged along a first longitudinal axis. In this example, there is a first pair of electrodes 21 22 and a second pair of electrodes 23 24, but in other examples of the device 1, there be any number of pairs of electrodes 21-24. The light source 61 and the photodetector 62 are arranged along a second longitudinal axis, wherein the first longitudinal axis is at an angle relative to the second longitudinal axis. In this example, the second longitudinal axis is at approximately a 45-degree angle relative to the first longitudinal axis due to the specific curvature of the microfluidic channel 10 of this specific device 1. In other examples, the first and second longitudinal axis may be arranged at any suitable angle relative to one another that enables both detection devices to obtain measurements. In order to facilitate the arrangement of components shown in Figure 6, the microfluidic channel 10 comprises at least one bend configured such that the opposed electrodes 21-24 and / or the light source 61 and / or the detector 62 can be positioned for obtaining measurements at the first measurement point and second measurement point respectively. As shown in this specific example the channel “twists” around the measurement components so as to not interfere with obtaining measurements. There is a first kink in the channel 10 proximal to the light source 61 wherein the channel 10 bends away from the light source 61 towards the electrodes 21-24, and second kink in the channel proximal to the photodetector 62 wherein the channel 10 bends away from the electrodes 21-24 and away from the photodetector 62. This microfluidic channel 10 arrangement provides a larger detection area in the channel 10 therefore enabling the gathering of accurate measurements using both detection devices. In some examples the processing unit is configured to input impedance data into trained machine learning model trained to identify a target cell type 40 based on input data. The processing unit is configured to provide denoising of the one or more of the impendence signals from the electrodes 21-26 and the light signal from the photodetector 62. The denoising may be done by signal processing and / or machine learning techniques that filter the noise from raw data. In any of the described embodiments of the invention, a portion of an inner surface of the microfluidic channel 10 may be modified so as to reduce clogging of cells 40 in the cell-containing fluid F. By coating a portion of the inner surface, there is a reduced likelihood of the cells sticking to inner surfaces of the channel 10. The coating of the channels 10 may be provided by pre-treating the channels 10. In this way, the channel walls are coated before the microfluidic device itself is fabricated. Alternatively, a fluid which provides the modification can be flushed into the channel 10 once the device is fabricated. Any coating that is suitable for reducing clogging in the channel 10 can be used. One exemplary way of modifying the inner surface is to modify the surface such that it is provided with hydrophobic or hydrophilic properties. In such examples, the surface may be covered with a hydrophobic coating, whereby the coating will repel water, or the surface may be covered with a hydrophilic coating whereby it mixes well with water. The inner surfaces may be treated using oxygen plasma treatment. The reactive oxygen in the plasma interacts with the inner surface. This causes the breaking of chemical bonds and introducing new functional groups. These groups increase the polarity of the surface, making it more hydrophilic. The inner surface may be treated before or after it is fabricated into the surface of the microfluidic channel. In other examples, a material may be fabricated to the inner surface of the microfluidic channel. In some examples, the material configured to reduce clogging is a Functionalised FR-4 PCB material. Any of the devices 1 described herein may be manufactured using the following method. The method comprises obtaining a printed circuit board and fabricating a pair of conductive tracks 21 22 within a plane of the PCB. PCBs typically comprise of an insulating substrate (for example an FR4 layer) and at least one conductive copper layer. Standard PCB fabrication techniques can be used. The copper layer of a PCB may be etched to create a series of conductive tracks, providing two conductive tracks with a gap between them for a microfluidic channel 10. Then, a solder mask layer may be applied to the copper layer and a microfluidic channel may be etched in the solder mask providing a microfluidic channel between the electrodes. The solder mask may be applied using UV light exposure through a mask to define where solder goes, or the solder mask layer is sealed with a pressure sensitive adhesive (PSA) layer. Alternatively, the microfluidic channel is etched in the substrate layer so the electrodes are either side. Alternatively, a separately formed microfluidic channel may be attached. In some examples, for example multilayer PCBs, multiple layers of copper and substrate are laminated together. The layers are stacked, and heat is applied to pressure to bond them into a single structure. A coating may be applied to protect the PCB structure. A laser can be used in the process of fabricating the pair of conductive tracks, as shown in Figure 7. For example, the opposed electrodes 21 22 can be fabricated directly into the PCB by focusing a laser on at least a portion of a conductive layer (e.g. the copper layer) of the PCB to be cut. The microfluidic channel 10 may also be fabricated in this manner in the PCB. This may be fabricated separately when the channel 10 and the electrodes 21 22 are in different layers of the PCB, or it may be fabricated simultaneously when the electrodes 21 22 provide at least a portion of the walls of the channel 10. Any type of suitable laser may be used including a CO2 laser, a UV laser etc. In a specific example, the cutting design of the PCB is prepared using Computer Aided Design (CAD) software. The PCB to be cut is placed on a bed of a laser cutter. The laser is focused on the PCB so as to achieve the desired depth and location of the cut. The laser beam follows the cutting path of the CAD design to cut the PCB. Therefore, to fabricate the opposed electrodes the device 1 is designed using CAD software which is then used to control the laser. Using lasers in the method of fabrication is advantageous as the laser is able to cut a microfluidic channel 10 with a width less than the typical microfluidic channel 10. A laser can be used to achieve a width of 50 micrometres. Figure 8 shows a PCB wherein a portion of the device 1 has been fabricated by 5 focusing a laser on the conductive layer to cut a portion of said layer. As shown, the conductive tracks are arranged to form two opposed electrodes 21 22 either side of the microfluidic channel 10. An impedance signal from the electrodes 21 22 can be used to determine the presence of a target particle 40 flowing through the microfluidic channel 10. 10

Claims

1. A microfluidic device for determining the impedance of a fluid, the microfluidic device comprising:a printed circuit board, PCB, comprising a pair of conductive tracks arranged within a plane of the PCB and a microfluidic channel configured to conduct a flow of a fluid;wherein the conductive tracks are arranged to form two opposed electrodes either side of the microfluidic channel such that an impedance signal from the electrodes can be used to determine the impedance of the fluid flowing through the microfluidic channel.

2. The microfluidic device of claim 1, wherein the fluid is a particle-containing fluid and wherein the impedance signal from the electrodes can be used to determine the presence of a target particle flowing through the microfluidic channel.

3. The microfluidic device of claim 1 or claim 2 further comprising:a control unit configured to apply an alternating current across the opposed electrodes; anda first detection unit configured to receive an impedance signal from the opposed electrodes and determine the presence of a target particle flowing through the microfluidic channel based on the measured impedance.

4. The microfluidic device of any preceding claim wherein the opposed electrodes are formed within a conductive layer of the PCB.

5. The microfluidic device of any preceding claim wherein a gap between the opposed electrodes has a minimum width dimension of 1pm -100 pm, preferably 1 pm - 50 pm.

6. The microfluidic device of any preceding claim further comprising a plurality of pairs of electrodes, each pair arranged to form opposing electrodes arranged either side of the microfluidic channel.

7. The microfluidic device of any preceding claim wherein the microfluidic channel is formed within a layer of the PCB, preferably wherein the microfluidic channel is formed within one or more of: a substrate layer, a solder mask layer or a pressure sensitive adhesive, PSA, layer of the PCB.

8. The microfluidic device of any preceding claim wherein the PCB comprises a fluid processing feature positioned within the microfluidic channel upstream of a measurement point between the opposed electrodes.

9. The microfluidic device of any preceding claim wherein the PCB comprises a micropillar array formed within the microfluidic channel and arranged to separate particles within the particle containing fluid by deterministic lateral displacement, DLD.

10. The microfluidic device of claim 9 wherein the micropillar array is formed within a layer of the PCB.

11. The microfluidic device of claim 10 wherein the micropillar array is formed within a conductive layer of the PCB.

12. The microfluidic device of claim 11 wherein a dielectrophoretic, DEP, voltage is applied to at least a portion of the micropillar array so as to separate particles within the particle containing fluid.

13. The microfluidic device of any of claims 9 to 12 wherein the microfluidic channel comprises a measurement channel and a sorting region, wherein the measurement channel is arranged between the opposed electrodes and the micropillar array is provided in the sorting region.

14. The microfluidic device of any preceding claim further comprising a second detection unit comprising:a light source arranged so as to illuminate particles flowing through the microfluidic channel;a photodetector arranged to receive light that has passed through the microfluidic channel at a second measurement point; anda processing unit configured to receive a light signal from the photodetector and determine the presence of a target particle passing the second measurement point based the signal intensity received from the photodetector.

15. The microfluidic device of claim 14 wherein the opposed electrodes are arranged along a first longitudinal axis, the light source and the photodetector are arranged along a second longitudinal axis, wherein the first longitudinal axis is at an angle relative to the second longitudinal axis.

16. The microfluidic device of claim 14 or claim 15 wherein the microfluidic fluid channel comprises at least one bend.

17. The microfluidic device of claim 16 wherein the at least one bend is configured such that one or more of: the opposed electrodes, and the light source and detector can be positioned for obtaining measurements at the first measurement point and second measurement point respectively.

18. The microfluidic device of any preceding claim wherein at least a portion of an inner surface of the microfluidic channel is modified so as to reduce clogging of particles in the particle-containing fluid; wherein at least an inner surface of the microfluidic channel is coated with a Functionalised FR-4 PCB material configured to reduce clogging.

19. The microfluidic device of any proceeding claim wherein the opposed electrodes either side of the microfluidic channel are vertically offset from one another within the plane of the PCB.

20. The microfluidic device of claim 19 wherein the device comprises two or more sets of opposed electrodes wherein:in each set, opposed electrodes either side of the microfluidic channel are vertically offset from one another within the plane of the PCB; andelectrodes from different sets, that on the same side of the microfluidic channel, are vertically offset from one another within the plane of the PCB.

21. A method of manufacturing a microfluidic device comprising:fabricating a pair of conductive tracks within a plane of a printed circuit board ,PCB;fabricating a microfluidic channel in the PCB configured to conduct a flow of a fluid; andwherein the conductive tracks are arranged to form two opposed electrodes either side of the microfluidic channel such that an impedance signal from the electrodes can be used to determine the impedance of the fluid flowing through the microfluidic channel.

22. The method of manufacturing a microfluidic device of claim 21 wherein fabricating the opposed electrodes comprises focusing a laser on at least a portion of a conductive layer of the PCB so as to cut said portion of the conductive layer.

23. The method of manufacturing a microfluidic device of claim 21 or 22 wherein fabricating the opposed electrodes within the plane of the PCB comprises etching at least a portion of a conductive layer of the PCB.

24. The method of manufacturing a microfluidic device of any of claims 21 to 23 wherein fabricating the opposed electrodes within the plane of the PCB comprising mounting a conducting layer comprising the opposed electrodes to the PCB.

25. The method of manufacturing a microfluidic device of any of claims 21 to 24 wherein fabricating the microfluidic channel comprises focusing a laser on at least a portion of a layer of the PCB so as to cut said portion of the layer.s

Images