WO2005058500A1

WO2005058500A1 - System

Info

Publication number: WO2005058500A1
Application number: PCT/GB2004/005334
Authority: WO
Inventors: Simon Burnell; Victor Manneh
Original assignee: Inverness Medical Switzerland Gmbh
Priority date: 2003-12-17
Filing date: 2004-12-17
Publication date: 2005-06-30
Also published as: JP2007519896A; CA2549094A1; AU2004299341A1; EP1691925A1

Abstract

A microfluidic separation system (1) for separating fluid sample medium from cells provided in a sample, for example, a sample of bodily fluid which is to be subjected to further analysis. The system may include a microfluidic structure (2) and a cell aggregation agent. The microfluidic structure may include one or more microfluidic channels (3, 4, 5) operable to separate aggregated cells from fluid sample medium by size exclusion.

Description

System

The present invention relates to a microfluidic separation system for separating particles from a fluid sample, and to an assay system for detecting the amount and/or presence of an analyte in a fluid sample, or for the determination of a property of a fluid sample. In particular this invention relates to the separation of cells from a sample, for example the separation of red blood cells from a sample of whole blood.

Diagnostic assay devices for the measurement of the presence and/or amount of analytes present in a fluid sample have been developed for use at the point of care or in the home setting. Such devices may be used by the healthcare professional or non- specialist personnel alike, such as the patient for self -monitoring. Consequently, such devices are designed to be easy to use, to require small volumes of fluid sample and perform the measurement rapidly. Small volume samples are desirable as they may be less painful to collect from the patient, for example when obtaining a sample of capillary blood by application of a lancet or finger-stick to the skin. Typically it will be a one-step device, the user having simply to apply a fluid sample to the device without the need to perform any further sample manipulation steps in order to obtain a result. Such systems will typically be portable and have minimal or no moving parts. Typically, a microfluidic channel or a porous carrier is employed to move sample into and/or through the device by capillary action avoiding the need to actively move the fluid sample within the device.

When performing a diagnostic assay measurement on a fluid sample, it may be desirable or necessary to remove components from the sample that may interfere with the assay. For example, it may be necessary to remove red-blood cells from whole blood where the testing regimen requires a sample or plasma. Red-blood cells may also interfere with the assay measurement, for example by absorbing light of a particular wavelength. Red-blood cells are conventionally removed from whole blood by centrifugation or by allowing the red-blood cells to settle. However such methods are ill-suited for the purposes of conducting a rapid diagnostic assay on a fluid sample of a volume ranging from typically lOOμl to less than lμl. The separation of particles from a fluid medium may be effected by the use of a filter such as a non-woven fabric with the appropriate pore-size. However, for the purposes of red-blood cell separation from whole blood, the use of such a filter is inappropriate, due to the tendency of the filter to block which results in low yields of plasma filtrate.

Also, the time for plasma yield to take place increases. This is clearly disadvantageous in a flow system such as a microfluidic device that may be reliant only on capillary forces to drive the fluid sample.

There therefore exists a need to provide a separation system that results in rapid separation of small volumes of sample, with high levels of yield.

According to a first aspect of the present invention there is provided a microfluidic separation system for separating fluid sample medium from cells provided in a sample, the system comprising a microfluidic structure and a cell aggregation agent, the microfluidic structure comprising one or more microfluidic channels operable to separate aggregated cells from fluid sample medium by size exclusion.

The term aggregation agent is intended to encompass any agent that can cause aggregation and/or agglutination of cells, as well as agents that can cause rosetting of cells such as red blood cells and/or promote the formation of roleaux in cells such as red blood cells.

The provision of an aggregation agent causes cells in the sample to aggregate together thereby assisting the separation of the sample medium from cells. The aggregation agent may be provided adjacent to or remote to the microfluidic structure. The aggregation agent may be mixed with a reagent that aids its release from an internal surface of the microfluidic system into the fluid sample. A plurality of aggregation agents may be provided.

The microfluidic structure may comprise a size exclusion element having one or more size exclusion spacings which enable the aggregated cells to be filtered from the sample medium. The size exclusion spacing thereby enables the microfluidic channels to separate aggregated cells from fluid sample medium. The size of the exclusion spacing may be chosen from any suitable size that enables aggregated cells to be separated or substantially separated from the sample medium and will be determined upon the dimensions of the system, the amount of time that the aggregation agent interacts with the fluid sample and the nature of the sample itself. A suitable size of exclusion spacing may be determined by routine experimentation. The optimum sizes of exclusion spacings may be determined on the basis of the efficiency of aggregate separation and are also most easily determined by routine experimentation. Where a plurality of size exclusion spacings are used, they may be of the same size or of differing sizes. The upper limit of the exclusion spacings may be determined by the size of the aggregates and the lower limit determined by the speed and efficiency of separation.

The microfluidic structure may define a capillary pathway. The capillary pathway may define one or more size exclusion spacings to separate aggregated cells from fluid sample medium.

The dimensions of the microfluidic channel(s) of the microfluidic structure may correspond to a size exclusion spacing. The microfluidic channel(s) may be of a dimension which varies to define the size exclusion spacing. The microfluidic channel(s) may be in fluidic communication with other microfluidic channels of different or varying dimensions.

The microfluidic structure may comprise at least one first microfluidic channel, the at least one first channel having a base with extending side walls, the channel being in fluid communication along a longitudinal side thereof with one or more passages having a depth less than the depth of the channel, the depth of the passage defining a size exclusion spacing. The depth of the passage(s) defining the size exclusion spacing is such that sample medium, can pass into the passage from the first channel. Therefore aggregated cells can be separated from the sample medium by size exclusion.

The, or each passage may be in further fluid communication with a further microfluidic channel. Therefore the, or each passage may operate as a connecting region between the first and further channels which defines a size exclusion spacing. The passage(s) may be provided with one or more step formations to vary the size exclusion spacing. The, or each step formation may provide the passage with a size exclusion spacing.

The passage(s) and/or further channel(s) may be in fluid communication with a sample medium collecting region, wherein the sample medium separated from aggregated cells flows to said collecting region. The collecting region may be a further conduit or may comprise a chamber in the microfluidic structure.

Alternatively, the capillary pathway may comprise one or more microfluidic channels in which is provided one or more microstructures that define gaps corresponding to the desired size exclusion spacing(s). The microstructures may be configured to separate aggregated cells from the sample. The microstructures may define size exclusion spacings in the region of about lμm to about 50μm. A plurality of groups of microstructures maybe provided of the same size or of different sizes and may be arranged in any particular configuration with respect to each other. Each group may define size exclusion spacings different to other group(s). The groups of microstructures may be in an ordered configuration so that the sample flows through the group defining larger size exclusion spacings before flowing through the group(s) having smaller size exclusion spacings, wherein the sample medium is separated from aggregated cells. A collecting region may be provided downstream of the microstructures, the sample medium flowing to the collecting region after flowing through the size exclusion spacings and being separated. The microstructures may be in the form of grooved surfaces, pillars, or any form that defines size exclusion spacings.

The dimensions of the size exclusion spacing(s) in the capillary pathway may be less than or equal to about 50μm, less than or equal to about 40μm, less than or equal to about 30μm, less than or equal to about 20μm, less than or equal to about 15μm, less than or equal to about lOμm, less than or equal to about 5μm, or may be less than or equal to about 2μm.

The capillary pathway may define a tortuous path.

The system may further comprise a conduit in fluid communication with the microfluidic structure to supply the microfluidic structure with the sample. The supply conduit may also supply the aggregation agent to the microfluidic structure. The aggregation agent may be provided on one or more surfaces of the conduit and/or to one or more surfaces of the capillary pathway of the microfluidic structure upstream of the size exclusion spacing(s).

The conduit may have a Reynolds number of less than 3000. Alternatively the conduit may have a Reynolds number of less than 100. The conduit is preferably a capillary. Reynolds number can be calculated using the formula:

Re = pVd/η

Where Re = Reynolds number, p = Fluid density, N = Fluid velocity, d = length scale, η = dynamic density. A Reynolds number of 2000 or less will cause the conduit (which may be considered to be a microstructure or microchannel) to be filled passively by surface tension (capillarity) alone.

The sample may be applied to the conduit. The sample may be applied via a sample inlet port in fluidic connection with the conduit. Suitable non-limiting examples of aggregation agents are those that cause aggregation of red blood cells, such as dextran. Alternatively the aggregation agent may cause agglutination of red blood cells, such as lectin. As yet a further alternative, the aggregation agent may comprise one or more antibodies to red blood cells. Alternatively still, the aggregation agent may promote rosetting of red blood cells, or may promote the formation of roleaux in red blood cells.

The microfluidic system may comprise further microfluidic elements such as internal microstructures, time gates, fluid mixing chambers, sample collections chambers, wells, channels, baffles, constrictions, a sample application port, etc. The microfluidic elements may be of a regular or irregular shape and may be in the same plane or in different planes. The microfluidic channel(s) may be of a capillary dimension which varies along its length, and may be in fluidic communication with other microfluidic elements of different or varying capillary dimensions.

The time which a sample is allowed to interact with the aggregation agent before reaching the size exclusion spacing(s) may be influenced for example by where in the system the aggregation agent is positioned, the dimensions of the upstream fluid conduit and speed with which the fluid sample travels along the fluid conduit. Where necessary, means to ensure that the aggregation agent has interacted for a sufficiently long enough time with the fluid sample may be provided, such as a chamber or time gate which serves to slow down the rate of passage of fluid sample between the supply conduit and the size exclusion spacing(s).

Typical dimensions of the microfluidic structure elements are those having a cross- sectional dimension, such as a cross-sectional diameter, of between 0.1 and 500μm, more typically having a cross-sectional dimension of between 1 and lOOμm. Preferably the cross-sectional dimensions are chosen to be of a size such that fluid is able to be transported along, through or into the various elements of the system by capillary action. As an alternative or in addition, fluid may be transported through one or more of the elements of the system by external forces such as by electrokinetic pumping. In such cases the cross-sectional dimension may exceed the capillary dimension.

The substrate from which the system is prepared may be any suitable such as polycarbonate. In the case where the substrate is hydrophobic, the surfaces may be treated to render them hydrophilic by methods known in the art, such as for example by treatment with an oxygen plasma. As well as providing hydrophilic surfaces, other types of reagents, immobilised or otherwise may be provided on the internal surfaces.

The system may be prepared for example by providing a planar first substrate layer onto which are provided walls which serve to define the depth of the microfluidic channels in the structure, followed by provision of a second planar substrate layer which is disposed onto the upper surfaces of the walls. Suitable adhering means such as adhesives may be used to join the various structures as appropriate. Other means of preparing the structure are for example screen-printing, or the provision of multi- laminated systems having an upper and lower laminated surface which serve as upper and lower surfaces of the system, as well as intermediate laminates having structures which serve to define the elements of the microfluidic structure.

The fluid sample medium to be separated is preferably whole blood. The system may therefore enable aggregated red blood cells to be separated from the sample medium.

The microfluidic separation system may be disposable.

According to a second aspect of the present invention there is provided an assay system for conducting an assay on a fluid sample, the assay system comprising a microfluidic separation system according to the first aspect in fluid communication with an analyte detection zone

The assay system may comprise a sample entry port for the application of fluid sample in fluid connection with the microfluidic structure of the separation system. The detection zone may be provided downstream from the microfluidic structure, into which fluid sample may flow from the microfluidic structure after separation. Reagents either specific or non-specific to the analyte of interest may be provided within the assay system and may be provided on the inner surface of the detection zone. The assay system may further comprise an interferent zone that serves to neutralise or remove molecules from the sample that may interfere with the binding interactions in the assay system, or with the signal generation and detection. Yet further zones may be provided within the assay system in fluidic communication with one or more of the other zones and may comprise wash zones, time gates, pre-mixing zones, reaction zones and the like.

The assay may be chosen from any that is able to determine the presence and/or amount of an analyte of interest. The assay may be a binding assay, such as a specific binding assay in which a specific binding event takes place between a specific binding pair, one of the binding partners being the analyte of interest, the other being chosen from any compound or composition capable of recognising a particular spatial or polar orientation of a molecule, e. g., epitopic or determinant site. Examples of suitable binding pairs are an antibody and antigen, biotin and avidin, carbohydrates and lectins, complementary nucleotide sequences, complementary peptide sequences, effector and receptor molecules, enzyme cof actors and enzymes, enzyme inhibitors and enzymes, a peptide sequence and an antibody specific for the sequence or the entire protein, polymeric acids and bases, dyes and protein binders, peptides and specific protein binders and so on. Where the binding partner is an antibody it may be monoclonal or polyclonal, or it may be a fragment thereof. Fragments thereof may include Fab, Fv and F(ab') 2, Fab', and the like. The assay may involve a specific reaction which takes place between the analyte and an enzyme, Suitable enzymes are ones which employ FAD/FADH₂, NAD/NADH₂ or NADP/NADPH₂ systems.

One of the specific binding partners may be provided with a detectable label. "Label" refers to any substance that is capable of producing a signal that is detectable by visual or instrumental means. Examples of suitable labels include enzymes and substrates, chromogens, catalysts, fluorescent compounds, chemiluminescent compounds, radioactive labels as well as particulate colloidal metallic particles such as gold, or particulate dyed organic substances such as polyurethane.

The binding assay may be either heterogeneous or homogeneous.

Where the assay is homogeneous it is desirable that the label, which is attached to the specific binding partner, is able to undergo some detectable physical or chemical change upon binding to form a specific binding pair such that the bound species may be distinguished from the unbound species. Examples of such binding assays which involve an energy transfer or result in a change in wavelength are given in

US5705622 and US6215560.

The sample medium of interest may be whole blood, but may be chosen from any sample medium where separation of particulate matter that is able to be aggregated by an aggregation agent is required, such as white blood cells.

Analytes of interest include, but are not limited to, toxins, organic compounds, proteins, peptides, microorganisms, bacteria, viruses, amino acids, nucleic acids, carbohydrates, hormones, steroids, vitamins, drugs of abuse, pollutants, pesticides, and metabolites of or antibodies to any of the above substances. Specific examples thereof are specific cardiac markers including troponin T and troponin I, CKMB, C~ reactive protein (CRP), natriuretic peptides such as ANP and BNP as well as their N- terminal fragments. Other analytes of interest include human chorionic gonadotrophin (hCG), luteinising hormone (LH) and follicle stimulating hormone (FSH) as well as markers of bone resorption.

The assay may be a binding assay, which has an optical emission and may be a luminescent oxygen channelling immunoassay. Such an immunoassay can comprise: a. donor particles able to generate singlet oxygen when irradiated with light; b. acceptor particles containing emission means activated by the singlet oxygen to emit detectable light; c. the donor and acceptor particles being adapted to recognise and bind to the analyte, wherein on binding of both the donor and acceptor particles to the analyte, generated singlet oxygen activates the emission means on the acceptor particles to emit detectable light; and d. a detector to detect light emitted by the acceptor particle.

The donor and acceptor particles recognise the analyte through antibodies provided on the surfaces of the particles. The emission means comprise a dissolved dye that can be activated by the singlet oxygen to produce chemiluminescent emission. The chemiluminescent emission activates fluorophores in the acceptor particle causing emission of light. The light may be emitted at 520-620nm. Singlet oxygen may be emitted when irradiated with light of wavelength 680nm. The donor particles may comprise dissolved phthalocyanine, the phthalocyanine generating the singlet oxygen when irradiated.

The assay system may further comprise a transduction system. The transduction system may be optical, magnetic, electrochemical, radiological or may involve measurement of a change of mass, frequency or energy state, depending upon the signal of interest to be detected. Where the signal is an optical one, it may be of any particular detectable wavelength or wavelength range and includes fluorescent and chemiluminescent signals.

The assay system may alternatively or in addition be used to determine a particular property of a fluid sample such as the coagulation time or prothrombin time.

The detection zone may comprise a microfluidic channel or may comprise a well. Detection means may be provided as an integral part of the detection zone. An excitation means may also be integrally provided as part of this zone. Examples of an excitation means and a detection means are respectively a light emitting diode and a photodetector. There may be one of more detection zones and one or more excitation and detector means. The detector means and excitation means may be chosen to be a size and shape as is convenient and will preferably be chosen such as to maximise the capture efficiency of the signal to be measured. The excitation means and detector means will typically be located on a surface exterior to the fluid sample in the vicinity of the detection zone.

The substrate of the detection zone may be chosen from any suitable material or materials depending upon the purpose. Examples of suitable substrates are plastics such as polycarbonate. In the region of the detection zone where the signal to be detected is an optical signal, the substrate may chosen from one that is able to transmit the optical signal such as a suitable optically transparent plastics material.

Additionally or alternatively the plastics material may incorporate a filter to remove light of undesired wavelength. As yet a further alternative, the filter may be present on a surface of the assay substrate. The optically transparent substrate may also be a lens, either converging or diverging onto which an excitation source or detector may be positioned. Light may then be either converged or diverged as desired. The substrate may also be partially transparent or have a surface roughness thus allowing for a diffuse source of light.

The assay system may be configured so that the sample flows into the assay detection zone after the interferent zone. The interferent zone may comprise one or more agents to influence the pH of the fluid sample or to remove or solubilise particular species such as lipids by for example solubilising them with surfactants, or by selectively binding them with a lipid-binding agent. The interferent zone may be time-gated so that only fully treated sample passes into the assay.

The assay system may measure analyte levels in bodily fluid which may be whole blood. Plasma may have been separated from aggregated red blood cells by the microfluidic separation system of the assay system.

The assay system may be disposable and may be designed to be used in combination with a meter that is able to display the results of the assay. The meter may comprise a display, a power source, and the appropriate circuitry. The meter may also comprise a light source and detector. Alternatively the assay system and meter may be wholly integrated into a disposable system. The assay system may be integrated with a fluid sampling and collection system such as a lancet, such that transfer of sample from the site of collection to the assay system may be avoided.

According to a further aspect of the present invention there is provided a method of separating fluid sample medium from cells provided in a fluid sample, comprising applying a fluid sample to a microfluidic separation system according to the first aspect of the present invention.

A sample of less than or equal to lOOμl may be applied.

The method may be for separating plasma from red blood cells in whole blood.

The sample may be mixed with a cell aggregation agent before addition to the microfluidic structure of the microfluidic separation system.

According to a further aspect of the present invention there is provided a method of detecting an analyte in a fluid sample, the method comprising applying a fluid sample to an assay system according to the second aspect of the present invention.

Preferred features of each aspect of the present invention are as for each other aspect mutatis mutandis.

The present invention will now be described by way of example only, and with reference to the accompanying figures, in which :

Figure 1 illustrates a microfluidic separation system of the present invention; Figure 2 illustrates an assay of the present invention incorporating a microfluidic separation system of the present invention;

Figure 3 illustrates further the assay system of Figure 2: Figure 3a is a plan view of the system; Figure 3b is a cross section taken along line X-X in Figure 3a; Figure 3c is an enlargement of the region A in Figure 3b; and Figure 3d is an enlargement of the region B in Figure 3c;

Figure 4 further illustrates the microfluidic separation system in the assay system of Figures 2 and 3: Figure 4a is a perspective view of the system; and Figure 4b is an enlargement of the region A illustrating the microstructures;

Figure 5 illustrates a further assay system in which an alternative microfluidic separation system is provided: Figure 5a is a perspective view of the assay system; and Figure 5b is an enlargement of the region A illustrating the separation system;

Figure 6 provides further illustration of the alternative separation system in Figure 5: Figure 6a is a plan view of the assay system of Figure 5; Figure 6b is a cross section taken along line X-X in Figure 6a; Figure 6c is an enlargement of the region A in Figure 6b; and Figure 6d is an enlargement of the region B in Figure 6a illustrating the separation system;

Figure 7a and b illustrate an assay system;

Figure 8 illustrates the sensitivity of the assay system;

Figure 9 also illustrates the sensitivity of the assay system; and

Figure 10 illustrates a standard curve relating to the assay system. The present invention relates to a microfluidic separation system for separating sample medium from cells, an assay system for detecting and measuring an analyte in a sample, and/or for measuring analyte in a sample.

A microfluidic separation system 1 is illustrated in figure 1. The system 1 includes a microfluidic structure 2 and a cell aggregation agent (not shown). The microfluidic structure 2 defines a capillary pathway. The capillary pathway includes a channel 3. The dimensions of the channel 3 varies along the length thereof. In particular at region 4 the channel narrows to provide a size exclusion spacing having dimensions less than lOμm. This size exclusion spacing is designed to prevent cells aggregated by the aggregation agent from flowing further downstream from this region 4. The aggregation agent can be provided in the channel 3 upstream of the size exclusion spacing or may be applied to the sample before the sample is applied to the microfluidic structure 2. Alternatively the aggregation agent can be immobilised onto one or more surfaces of the channel upstream of the size exclusion spacing. The channel widens at region 5 to provide a sample collecting region into which sample medium without cells can flow. The system is also provided with a lid (not shown) defining an upper surface of the channel 3. The sample flows through the capillary pathway by capillary forces. To assist this flow, the surfaces of the capillary pathway are coated with a hydrophilic coating.

Figure 2 of the accompanying drawings illustrates an assay system 10. The system 10 incorporates a microfluidic separation system 12 and an assay detection zone 18. The system 10 also includes an interferent zone 14, a pre-treatment zone 16, and a sample application region 20. The assay system 10 is for use in the detection of an analyte in a sample applied to the system. A sample of less than lOOμl can be applied to the region 20 before flowing down a conduit 24 towards the microfluidic separation system 12. The conduit 24 can be a microchannel and could have a Reynolds number of less than 3000, or less than 100 (in the case of a capillary). The sample flows passively by capillary action, or actively through the application of a pressure differential to the system 10. The assay system 10 is arranged so that the sample travels uni-directionally. The microfluidic separation system 12 comprises a microfluidic structure 13 defining a capillary pathway. The structure is arranged to separate sample medium from cells provided in the sample by size exclusion. Before the sample passes through the structure 13, the sample is mixed with a cell aggregation agent of the system to cause aggregation of cells. The cell aggregation agent can be added to the sample before applying the sample to region 20. Alternatively the agent can be provided in the region 20, or in the conduit 24. The agent can be provided on at least one surface of the region 20 or conduit 24.

The microfluidic structure 13 is best illustrated in Figure 3c and Figure 4b . As can been seen the conduit 24 splits into a capillary pathway having a number of channels 26. Each of the channels 26 is in fluid communication along a longitudinal side thereof with a respective further channel in the form of a passage 28, which has a depth that is smaller than the depth of the respective channel 26. The depth of the passage 28 is configured so that aggregates of cells are unable to pass there through whereas the sample medium can, due to size exclusion. Therefore the passage depth provides a size exclusion spacing. The channels 26 and passages 28 have an upper surface defined by a lid 22 (see Figure 3 c).

The passages 28 can be provided with step formations 30 that define a shallower depth. This is illustrated in Figure 3d. The provision of step formations 30 provides for the initial passage depth acting as a pre-filter with the step formations 30 defining a size exclusion spacing functioning as the main filter separating the sample medium from the aggregated cells. The step formations 30 can be in the form of pillars or raised surfaces.

The depth of the channels 26 is in the region of lOOμm. The depth of the passages 28 is in the region of 20μm, with the depth provided by the step formations 30 being in the region of lOμm. The cell medium passing into the passages 28 is channelled towards a collecting region 29 of the capillary pathway in the microfluidic structure 13.

An alternative microfluidic separation system 34 is provided in the assay system 32 illustrated in Figures 5 and 6. Figure 5a provides a perspective view of the system 32.

The system 32 includes an interferent zone 36, a pre-treatment zone 38, an assay system 40, and a sample application region 42.

The microfluidic separation system 34 has a aggregation agent and also a microfluidic structure that defines a capillary pathway. The capillary pathway comprises a first channel 48 in fluid communication with a second channel 50. A number of microstructures in the form of pillars 46 are provided between the channels 48, 50. The pillars 46 define a number of gaps 44. The gaps 44 are of a size to enable sample medium to flow there through whereas aggregated cells cannot. The gaps are size exclusion spacings. The second channel 50 is provided with an outlet 54 to a pre- treatment zone 38. As with the system 10, a lid 52 is provided.

The aggregation agent is provided in the first channel 48 and it may be immobilised on one or more surfaces of the channel. Alternatively, the aggregation agent is contacted with the sample before the sample flows into the capillary pathway of the microfluidic structure of the separation system 34.

The channel 48 is deeper than the channel 50. Indeed the channel 48 has a depth of lOOμm and the channel 50 has a depth of between 10 - 20μm. The gaps defined by the pillars 46 are in the region of lOμm.

As with the microfluidic system 10, the system can work actively (for example using a pressure differential or electrokinetic pumping),or passively (through capillary action and surface tension). The microfluidic separation systems 1, 12, 34 described above can be used to separate plasma from red blood cells in whole blood. In this example, a whole blood sample after application flows towards the respective microfluidic structures of the separation systems 1, 12, 34 . A red blood cell aggregation agent is provided upstream of the microfluidic structures of system 1, 12, 34. Alternatively the agent can be provided in the microfluidic structure of the system 1, 12, 34 and may be immobilised onto one or more surfaces upstream of the size exclusion spacing(s). The microfluidic structures of the separation systems 1,12, 34 are configured to separate the plasma from the aggregated red blood cells using size exclusion. The size exclusion spacings defined in the structures allow the plasma to flow there through.

The assay systems 10, 32 also include homogeneous assay/detection systems 24, 40 to detect and measure analyte in a sample of less than or equal to 50μl. An example of an assay system is illustrated in Figures 7a and 7b. This system depends on a combination of latex agglutination and chemi-luminescent signal. In this regard, an analyte molecule brings together two beads producing a cascade of chemical reactions to greatly amplify the signal such that in principle attomolar concentrations of analyte can be detected. The system provides a highly sensitive homogenous immunoassay that takes place in a highly efficient light capturing detection chamber. Incubation of the sample with assay components is achieved through a time-gated structure.

The assay illustrated in Figure 7a and 7b is a luminescent oxygen channelling immunoassay. In more detail, photosensitiser particles (Donor particles) containing dissolved phthalocyanine generate singlet oxygen when irradiated with light of wavelength 680nm, Figure 7a. The singlet oxygen produced has a very short half -life, circa 4 microseconds and hence decays rapidly to a ground state. As such it can only diffuse to a distance of a few hundred microns from the surface of the particles before it decays to ground state. However, it can survive long enough to enter any paired adjacent particle, Figure 7b. The paired adjacent particles (Acceptor particles) contain a dissolved dye that is activated by the singlet oxygen received to produce chemiluminescent emission. This chemiluminescent emission activates further fluorophores contained in the same bead, subsequently causing emission of light at 520-620 nm. The reagents can be lyophilised in a well with an optimised geometry and low optical absorbance to ensure maximum excitation and light capturing efficiency. The donor and acceptor particles can recognise the analyte through antibodies provided on the surfaces of the particles, such that the particles are brought together by the analyte.

The sensitivity of detection of the assay was determined in two ways, Figure 8 and Figure 9. A first experiment in a 384 well microtiter plate, Figure 8, demonstrated the linear behaviour between the total number of Unibeads and assay output signal.

Unibeads are a pre-conjugated single bead that incorporates both the acceptor and donor particles. As such the quantum efficiency of the transfer of the singlet oxygen can be neglected due to the proximity of the particles. In this experiment the total number of beads was varied from 100 to 100000.

In Figure 9, reducing the volume to that likely to be used in the final chip configuration, 2μl, the signal can be seen to flattens off as the concentration is reduced. This is due to the auto-fluorescence of the unibeads. The minimum number of particles that can be reliably detected is approximately 400 in 2μl of sample. In developing the assay, Figure 10, a standard curve obtained from a biotinylated - DIG assay using 384 microtiter plate with 25 μl assay volume was obtained.

A sensitive homogeneous assay has been successfully designed and manufactured based on a micro fluidic system. The assay has been successfully transferred from a microtiter plate to a chip format and has generated a dose response curve whereby 1.6 x 10^"10 Molar of analyte can be detected in 2 μl of plasma.

The assay system does not necessarily have to be in a form of a luminescent oxygen channelling immuno-assay.

As discussed above the assay systems 10, 32 also comprise an interference zone 14, 36 for solid phase extraction of molecules that can interfere with the binding interactions in the detection zones 18, 40, or with a signal generation and detection. The interferent zone 14, 36, may be provided before the sample encounters the cell separation system 12, 34, or before the sample flows into the assay detection zone 25, 41. The zone 14, 36 includes a number of agents to neutralise or remove molecules from the sample. The interferent zones 14, 36 can be time gated to ensure that only fully treated sample passes into the assay system 18, 40.

Example

A sample of whole blood from a human patient (45% hematocrit) was drawn into an

EDTA tube and 4μl of the blood sample was mixed in a ratio of 1 : 1 with 2μl of Lectin PHA-E (Sigma) dissolved in phosphate buffer solution (PBS) at a concentration of 5 mgs/ ml. The sample was subsequently incubated at room temperature for 1 minute to allow the red-blood cells to agglomerate prior to application to the microfluidic separation system described above with reference to Figures 2, 3 and 4.

The conduit 24 has dimensions of 200μm (width) by lOOμm (height). The system was prepared from a polycarbonate substrate by injection moulding of a base substrate to form the base or lower part of the device as well as the microfluidic elements followed by ultrasonic welding of a one-piece injection moulded substrate to the base substrate to form the lid or upper surface of the system. Prior to assembly, the substrates were treated with an oxygen plasma to render the surfaces hydrophilic. The degree of hydrophilicity of the surfaces was measured and the surface contact angle found to be 20 degrees.

Fluid sample (4μl) was applied to the sample application region 20 and subsequently moved towards the microfluidic structure 13. The agglomerated cells were substantially unable to pass through the microfluidic structure thus resulting in separation of the plasma buffer filtrate from the agglomerated red cells. The total plasma/ buffer extracted was 200nl in a time of less than 10 minutes. The efficiency of the plasma separation was calculated as being 11% of the total available plasma.

Claims

1. A microfluidic separation system for separating fluid sample medium from cells provided in a sample, the system comprising a microfluidic structure and a cell aggregation agent, the microfluidic structure comprising one or more microfluidic channels operable to separate aggregated cells from fluid sample medium by size exclusion.

2. A system as claimed in claim 1, wherein the aggregation agent is provided adjacent to or remote to the microfluidic structure.

3. A system as claimed in claim 1 or 2, wherein the microfluidic structure defines a capillary pathway.

4. A system as claimed in any one of claims 1 to 3, wherein the microfluidic structure comprises a size exclusion element having one or more size exclusion spacings.

5. A system as claimed in claim 4, wherein the capillary pathway defines one or more size exclusion spacings to separate aggregated cells from fluid sample medium.

6. A system as claimed in claim 5, wherein the microfluidic channel (s) are of a dimension which varies to define the size exclusion spacing.

7. A system as claimed in claim 6, wherein the microfluidic channels may be in fluidic communication with other microfluidic channels of different or varying dimensions

8. A system as claimed in claim 7, wherein the capillary pathway of the microfluidic structure comprises at least one first microfluidic channel, the at least one first channel having a base with extending side walls, the channel being in fluid communication along a longitudinal side thereof with one or more passages having a depth less than the depth of the channel, the depth of the passage defining a size exclusion spacing.

9. A system as claimed in claim 8, wherein the, or each passage is in further fluid communication with a further microfluidic channel.

10. A system as claimed in claim 8 or 9, wherein the passage(s) are provided with one or more step formations to vary the size exclusion spacing.

11. A system as claimed in claim 9 or 10, wherein the passage(s) and/or further channel(s) are in fluid communication with a sample medium collecting region, wherein the sample medium separated from aggregated cells flows to said collecting region

12. A system as claimed in claim 7, wherein the capillary pathway comprises one or more microfluidic channels in which is provided one or more microstructures that define gaps corresponding to the desired size exclusion spacing(s)

13. A system as claimed in claim 12, wherein a plurality of groups of microstructures are provided of the same size or of different sizes and may be arranged in any particular configuration with respect to each other.

14. A system as claimed in claim 13, wherein each group defines size exclusion spacings different to other group(s).

15. A system as claimed in claim 14, wherein the groups of microstructures are in an ordered configuration so that the sample flows through the group defining larger size exclusion spacings before flowing through the group(s) having smaller size exclusion spacings, wherein the sample medium is separated from aggregated cells.

16. A system as claimed in any one of claims 12 to 15, wherein a collecting region is provided downstream of the microstructures, the sample medium flowing to the collecting region after flowing through the size exclusion spacings and being separated.

17. A system as claimed in any one of claims 4 to 16, wherein the dimensions of the size exclusion spacing(s) in the capillary pathway are less than or equal to about 50 μm, less than or equal to about 40 μm, less than or equal to about 30 μm, less than or equal to about 20 μm, less than or equal to about 15 μm, less than or equal to about 10 μm, less than or equal to about 5 μm, or less than or equal to about 2 μm.

18. A system as claimed in any one of the preceding claims, wherein the system further comprises a conduit in fluid communication with the microfluidic structure to supply the microfluidic structure with the sample.

19. A system as claimed in claim 18, wherein the supply conduit also supplies the aggregation agent to the microfluidic structure.

20. A system as claimed in claims 18 or 19, wherein the aggregation agent is provided on one or more surfaces of the conduit, and/or to one or more surfaces of the capillary pathway of the microfluidic structure upstream of the size exclusion spacing(s)

21. A system as claimed in any one of the preceding claims, wherein the fluid sample medium to be separated is whole blood.

22. A system as claimed in claim 21, wherein the aggregation agent is dextran or lectin.

23. An assay system for conducting an assay on a fluid sample, the assay system comprising a microfluidic separation system according to any one of the preceding claims, in fluid communication with an analyte detection zone.

24. An assay system as claimed in claim 23, wherein the assay system comprises a sample entry port for the application of fluid sample in fluid connection with the microfluidic structure of the separation system.

25. An assay system as claimed in claim 23 or 24, wherein the detection zone is provided downstream from the microfluidic structure, into which fluid sample flows from the microfluidic structure after separation

26. An assay system as claimed in claim 25, wherein reagents either specific or non-specific to the analyte of interest are provided within the assay system and are provided on an inner surface of the detection zone

27. An assay system as claimed in claim 26, wherein the assay system further comprises an interferent zone that serves to neutralise or remove molecules from the sample that may interfere with the binding interactions in the assay system, or with the signal generation and detection

28. An assay system as claimed in any one of claims 23 to 27, wherein the assay is a binding assay.

29. An assay system as claimed in claim 28, wherein the binding assay is heterogeneous or homogeneous.

30. An assay system as claimed in any one of claims 23 to 29, wherein the assay system further comprises a transduction system.

31. A method of separating fluid sample medium from cells provided in a fluid sample, comprising applying a fluid sample to a microfluidic separation system as claimed in any one of claims 1 to 22.

32. A method as claimed in claim 31, wherein the sample is mixed with a cell aggregation agent before addition to the microfluidic structure of the microfluidic separation system.

33. A method of detecting an analyte in a fluid sample, the method comprising applying a fluid sample to an assay system according to any one of claims 23 to 30.

34. A method as claimed in claim 33, wherein the sample is mixed with a cell aggregation agent before addition to the microfluidic structure of the microfluidic separation system.